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Beyer EC, Mathias RT, Berthoud VM. Loss of fiber cell communication may contribute to the development of cataracts of many different etiologies. Front Physiol 2022; 13:989524. [PMID: 36171977 PMCID: PMC9511111 DOI: 10.3389/fphys.2022.989524] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2022] [Accepted: 08/22/2022] [Indexed: 11/13/2022] Open
Abstract
The lens is an avascular organ that is supported by an internal circulation of water and solutes. This circulation is driven by ion pumps, channels and transporters in epithelial cells and by ion channels in fiber cells and is maintained by fiber-fiber and fiber-epithelial cell communication. Gap junctional intercellular channels formed of connexin46 and connexin50 are critical components of this circulation as demonstrated by studies of connexin null mice and connexin mutant mice. Moreover, connexin mutants are one of the most common causes of autosomal dominant congenital cataracts. However, alterations of the lens circulation and coupling between lens fiber cells are much more prevalent, beyond the connexin mutant lenses. Intercellular coupling and levels of connexins are decreased with aging. Gap junction-mediated intercellular communication decreases in mice expressing mutant forms of several different lens proteins and in some mouse models of lens protein damage. These observations suggest that disruption of ionic homeostasis due to reduction of the lens circulation is a common component of the development of many different types of cataracts. The decrease in the lens circulation often reflects low levels of lens fiber cell connexins and/or functional gap junction channels.
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Affiliation(s)
- Eric C. Beyer
- Department of Pediatrics, University of Chicago, Chicago, IL, United States
- *Correspondence: Eric C. Beyer,
| | - Richard T. Mathias
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, NY, United States
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2
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Li Z, Quan Y, Gu S, Jiang JX. Beyond the Channels: Adhesion Functions of Aquaporin 0 and Connexin 50 in Lens Development. Front Cell Dev Biol 2022; 10:866980. [PMID: 35465319 PMCID: PMC9022433 DOI: 10.3389/fcell.2022.866980] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2022] [Accepted: 03/18/2022] [Indexed: 12/04/2022] Open
Abstract
Lens, an avascular tissue involved in light transmission, generates an internal microcirculatory system to promote ion and fluid circulation, thus providing nutrients to internal lens cells and excreting the waste. This unique system makes up for the lack of vasculature and distinctively maintains lens homeostasis and lens fiber cell survival through channels of connexins and other transporters. Aquaporins (AQP) and connexins (Cx) comprise the majority of channels in the lens microcirculation system and are, thus, essential for lens development and transparency. Mutations of AQPs and Cxs result in abnormal channel function and cataract formation. Interestingly, in the last decade or so, increasing evidence has emerged suggesting that in addition to their well-established channel functions, AQP0 and Cx50 play pivotal roles through channel-independent actions in lens development and transparency. Specifically, AQP0 and Cx50 have been shown to have a unique cell adhesion function that mediates lens development and transparency. Precise regulation of cell-matrix and cell-cell adhesion is necessary for cell migration, a critical process during lens development. This review will provide recent advances in basic research of cell adhesion mediated by AQP0 and Cx50.
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Affiliation(s)
- Zhen Li
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, United States
- State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou, China
| | - Yumeng Quan
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, United States
| | - Sumin Gu
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, United States
| | - Jean X. Jiang
- Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX, United States
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3
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Berthoud VM, Gao J, Minogue PJ, Jara O, Mathias RT, Beyer EC. Connexin Mutants Compromise the Lens Circulation and Cause Cataracts through Biomineralization. Int J Mol Sci 2020; 21:E5822. [PMID: 32823750 PMCID: PMC7461132 DOI: 10.3390/ijms21165822] [Citation(s) in RCA: 26] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Revised: 08/03/2020] [Accepted: 08/06/2020] [Indexed: 12/18/2022] Open
Abstract
Gap junction-mediated intercellular communication facilitates the circulation of ions, small molecules, and metabolites in the avascular eye lens. Mutants of the lens fiber cell gap junction proteins, connexin46 (Cx46) and connexin50 (Cx50), cause cataracts in people and in mice. Studies in mouse models have begun to elucidate the mechanisms by which these mutants lead to cataracts. The expression of the dominant mutants causes severe decreases in connexin levels, reducing the gap junctional communication between lens fiber cells and compromising the lens circulation. The impairment of the lens circulation results in several changes, including the accumulation of Ca2+ in central lens regions, leading to the formation of precipitates that stain with Alizarin red. The cataract morphology and the distribution of Alizarin red-stained material are similar, suggesting that the cataracts result from biomineralization within the organ. In this review, we suggest that this may be a general process for the formation of cataracts of different etiologies.
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Affiliation(s)
- Viviana M. Berthoud
- Department of Pediatrics, University of Chicago, Chicago, IL 60637, USA; (P.J.M.); (O.J.); (E.C.B.)
| | - Junyuan Gao
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, NY 11794, USA; (J.G.); (R.T.M.)
| | - Peter J. Minogue
- Department of Pediatrics, University of Chicago, Chicago, IL 60637, USA; (P.J.M.); (O.J.); (E.C.B.)
| | - Oscar Jara
- Department of Pediatrics, University of Chicago, Chicago, IL 60637, USA; (P.J.M.); (O.J.); (E.C.B.)
| | - Richard T. Mathias
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, NY 11794, USA; (J.G.); (R.T.M.)
| | - Eric C. Beyer
- Department of Pediatrics, University of Chicago, Chicago, IL 60637, USA; (P.J.M.); (O.J.); (E.C.B.)
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4
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Schey KL, Wang Z, Friedrich MG, Garland DL, Truscott RJW. Spatiotemporal changes in the human lens proteome: Critical insights into long-lived proteins. Prog Retin Eye Res 2019; 76:100802. [PMID: 31704338 DOI: 10.1016/j.preteyeres.2019.100802] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2019] [Revised: 10/24/2019] [Accepted: 10/30/2019] [Indexed: 12/15/2022]
Abstract
The ocular lens is a unique tissue that contains an age gradient of cells and proteins ranging from newly differentiated cells containing newly synthesized proteins to cells and proteins that are as old as the organism. Thus, the ocular lens is an excellent model for studying long-lived proteins (LLPs) and the effects of aging and post-translational modifications on protein structure and function. Given the architecture of the lens, with young fiber cells in the outer cortex and the oldest cells in the lens nucleus, spatially-resolved studies provide information on age-specific protein changes. In this review, experimental strategies and proteomic methods that have been used to examine age-related and cataract-specific changes to the human lens proteome are described. Measured spatio-temporal changes in the human lens proteome are summarized and reveal a highly consistent, time-dependent set of modifications observed in transparent human lenses. Such measurements have led to the discovery of cataract-specific modifications and the realization that many animal systems are unsuitable to study many of these modifications. Mechanisms of protein modifications such as deamidation, racemization, truncation, and protein-protein crosslinking are presented and the implications of such mechanisms for other long-lived proteins in other tissues are discussed in the context of age-related neurological diseases. A comprehensive understanding of LLP modifications will enhance our ability to develop new therapies for the delay, prevention or reversal of age-related diseases.
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Affiliation(s)
- Kevin L Schey
- Department of Biochemistry, Vanderbilt University, USA.
| | - Zhen Wang
- Department of Biochemistry, Vanderbilt University, USA
| | - Michael G Friedrich
- Illawarra Health and Medical Research Institute, University of Wollongong, Australia
| | | | - Roger J W Truscott
- Illawarra Health and Medical Research Institute, University of Wollongong, Australia
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5
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Varadaraj K, Kumari S. Deletion of Seventeen Amino Acids at the C-Terminal End of Aquaporin 0 Causes Distortion Aberration and Cataract in the Lenses of AQP0ΔC/ΔC Mice. Invest Ophthalmol Vis Sci 2019; 60:858-867. [PMID: 30821811 PMCID: PMC6397018 DOI: 10.1167/iovs.18-26378] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
Purpose Investigate the effects of the absence of 17 amino acids at the C-terminal end of Aquaporin 0 (AQP0) on lens transparency, focusing property, and homeostasis. Methods A knockin (KI) mouse model (AQP0ΔC/ΔC) was developed to express AQP0 only as the end-cleaved form in the lens. For this, AQP0 was genetically engineered as C-terminally end-cleaved with amino acids 1 to 246, instead of the full length 1 to 263 of the wild type (WT). After verifying the KI integration into the genome and its expression, the mouse model was bred for several generations. AQP0 KI homozygous (AQP0ΔC/ΔC) and heterozygous (AQP0+/ΔC) lenses were imaged and analyzed at different developmental stages for transparency. Correspondingly, aberrations in the lens were characterized using the standard metal grid focusing method. Data were compared with age-matched WT, AQP0 knockout (AQP0-/-), and AQP0 heterozygous (AQP0+/-) lenses. Results AQP0ΔC/ΔC lenses were transparent throughout the embryonic development and until postnatal day 15 (P15) in contrast to age-matched AQP0-/- lenses, which developed cataract at embryonic stage itself. However, there was distortion aberration in AQP0ΔC/ΔC lens at P5; after P15, cataract began to develop and progressed faster surpassing that of age-matched AQP0-/- lenses. AQP0+/ΔC lenses were transparent even at the age of 1 year in contrast to AQP0+/- lenses; however, there was distortion aberration starting at P15. Conclusions A specific distribution profile of intact and end-cleaved AQP0 from the outer cortex to the inner nucleus is required in the lens for establishing refractive index gradient to enable proper focusing without aberrations and for maintaining transparency.
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Affiliation(s)
- Kulandaiappan Varadaraj
- Department of Physiology and Biophysics, Renaissance School of Medicine, Stony Brook University, Stony Brook, New York, United States
| | - Sindhu Kumari
- Department of Physiology and Biophysics, Renaissance School of Medicine, Stony Brook University, Stony Brook, New York, United States
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Kumari SS, Varadaraj K. A predominant form of C-terminally end-cleaved AQP0 functions as an open water channel and an adhesion protein in AQP0 ΔC/ΔC mouse lens. Biochem Biophys Res Commun 2019; 511:626-630. [PMID: 30826060 DOI: 10.1016/j.bbrc.2019.02.098] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2019] [Accepted: 02/19/2019] [Indexed: 01/14/2023]
Abstract
The purpose of this investigation was to find out whether C-terminally end-cleaved aquaporin 0 (AQP0), that is present predominantly in the lens mature fiber cells of the WT, functions as a water channel and a cell-to-cell adhesion (CTCA) protein in a knockin (KI) mouse model (AQP0ΔC/ΔC) that does not express intact AQP0. A genetically engineered KI mouse model, AQP0ΔC/ΔC, expressing only end-cleaved AQP0 was developed. This model expresses 1-246 amino acids of AQP0, instead of the full length 1-263 amino acids. Lens transparency of postnatal day 10 (P10) was analyzed qualitatively by dark field imaging. WT, AQP0+/- and AQP0+/ΔC lenses were transparent; AQP0-/- and AQP0ΔC/ΔC mouse lenses displayed loss of transparency. Lens fiber cell membrane vesicles (FCMVs) were prepared from wild type (WT), AQP0 heterozygous (AQP0+/-), AQP0 knockout (AQP0-/-), AQP0+/ΔC and AQP0ΔC/ΔC; water permeability (Pf) was measured using the osmotic shrinking method. CTCA assay was performed using adhesion-deficient L-cells and FCMVs prepared from the abovementioned genotypes. FCMVs of AQP0+/- and AQP0-/- showed a statistically significant reduction (P < 0.001) in Pf and CTCA compared to those of WT. AQP0+/ΔC and AQP0ΔC/ΔC FCMVs exhibited no statistically significant alteration (P > 0.05) in Pf compared to those of WT. However, CTCA of AQP0+/ΔC AQP0ΔC/ΔC FCMVs was significantly higher (P < 0.001) than that of WT FCMVs. Our experiments clearly show that C-terminally end-cleaved AQP0 can function both as a water channel and a CTCA molecule in the lens fiber cell membranes. Also, end-truncation plays an important role in increasing the CTCA between fiber cells.
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Affiliation(s)
| | - Kulandaiappan Varadaraj
- Physiology and Biophysics, Stony Brook University, NY, USA; SUNY Eye Institute, Syracuse, NY, USA.
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7
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Kumari S, Gao J, Mathias RT, Sun X, Eswaramoorthy A, Browne N, Zhang N, Varadaraj K. Aquaporin 0 Modulates Lens Gap Junctions in the Presence of Lens-Specific Beaded Filament Proteins. Invest Ophthalmol Vis Sci 2017; 58:6006-6019. [PMID: 29196765 PMCID: PMC5710632 DOI: 10.1167/iovs.17-22153] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
Purpose The objective of this study was to understand the molecular and physiologic mechanisms behind the lens cataract differences in Aquaporin 0-knockout-Heterozygous (AQP0-Htz) mice developed in C57 and FVB (lacks beaded filaments [BFs]) strains. Methods Lens transparency was studied using dark field light microscopy. Water permeability (Pf) was measured in fiber cell membrane vesicles. Western blotting/immunostaining was performed to verify expression of BF proteins and connexins. Microelectrode-based intact lens intracellular impedance was measured to determine gap junction (GJ) coupling resistance. Lens intracellular hydrostatic pressure (HP) was determined using a microelectrode/manometer system. Results Lens opacity and spherical aberration were more distinct in AQP0-Htz lenses from FVB than C57 strains. In either background, compared to wild type (WT), AQP0-Htz lenses showed decreased Pf (approximately 50%), which was restored by transgenic expression of AQP1 (TgAQP1/AQP0-Htz), but the opacities and differences between FVB and C57 persisted. Western blotting revealed no change in connexin expression levels. However, in C57 AQP0-Htz and TgAQP1/AQP0-Htz lenses, GJ coupling resistance decreased approximately 2.8-fold and the HP gradient decreased approximately 1.9-fold. Increased Pf in TgAQP1/AQP0-Htz did not alter GJ coupling resistance or HP. Conclusions In C57 AQP0-Htz lenses, GJ coupling resistance decreased. HP reduction was smaller than the coupling resistance reduction, a reflection of an increase in fluid circulation, which is one reason for the less severe cataract in C57 than FVB. Overall, our results suggest that AQP0 modulates GJs in the presence of BF proteins to maintain lens transparency and homeostasis.
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Affiliation(s)
- Sindhu Kumari
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, New York, United States
| | - Junyuan Gao
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, New York, United States
| | - Richard T Mathias
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, New York, United States.,SUNY Eye Institute, Syracuse, New York, United States
| | - Xiurong Sun
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, New York, United States
| | - Amizhdini Eswaramoorthy
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, New York, United States
| | - Nicholas Browne
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, New York, United States
| | - Nigel Zhang
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, New York, United States
| | - Kulandaiappan Varadaraj
- Department of Physiology and Biophysics, Stony Brook University, Stony Brook, New York, United States.,SUNY Eye Institute, Syracuse, New York, United States
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8
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Abstract
The lens is an avascular organ composed of an anterior epithelial cell layer and fiber cells that form the bulk of the organ. The lens expresses connexin43 (Cx43), connexin46 (Cx46) and connexin50 (Cx50). Epithelial Cx50 has critical roles in cell proliferation and differentiation, likely involving growth factor-dependent signaling pathways. Both Cx46 and Cx50 are crucial for lens transparency; mutations in their genes have been linked to congenital and age-related cataracts. Congenital cataract-associated connexin mutants can affect protein trafficking, stability and/or function, and the functional effects may differ between gap junction channels and hemichannels. Dominantly inherited cataracts may result from effects of the connexin mutant on its wild type isotype, the other co-expressed wild type connexin and/or its interaction with other cellular components.
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Affiliation(s)
| | - Anaclet Ngezahayo
- Institute of Biophysics, Leibniz University Hannover, Hannover, Germany.
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9
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Slavi N, Wang Z, Harvey L, Schey KL, Srinivas M. Identification and Functional Assessment of Age-Dependent Truncations to Cx46 and Cx50 in the Human Lens. Invest Ophthalmol Vis Sci 2016; 57:5714-5722. [PMID: 27787559 PMCID: PMC5089213 DOI: 10.1167/iovs.16-19698] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2016] [Accepted: 08/21/2016] [Indexed: 11/24/2022] Open
Abstract
PURPOSE Many proteins in the lens undergo extensive posttranslational modifications (PTMs) with age, leading to alterations in their function. The extent to which lens gap junction proteins, Cx46 and Cx50, accumulate PTMs with aging is not known. In this study, we identified truncations in Cx46 and Cx50 in the human lens using mass spectrometry. We also examined the effect of truncations on channel function using electrophysiological measurements. METHODS Human lenses were dissected into cortex, outer nucleus, and nucleus regions, and fiber cell membranes were subjected to trypsin digestion. Tryptic peptides were analyzed by liquid chromatography (LC)-electrospray tandem mass spectrometry (ESI/MS/MS). Effects of truncations on channel conductance, permeability, and gating were assessed in transfected cells. RESULTS Cleavage sites were identified in the C-terminus, the cytoplasmic loop, and the N-terminus of Cx46 and Cx50. Levels of C-terminal truncations, which were found at residues 238 to 251 in Cx46 and at residues 238 to 253 and 274 to 284 in Cx50, were similar in different lens regions. In contrast, levels of truncations in cytoplasmic loop and N-terminal domains of Cx46 and Cx50 increased dramatically from outer cortex to nucleus. Most of the C-terminally truncated proteins were functional, whereas truncations in the cytoplasmic loop did not result in the formation of functional channels. CONCLUSIONS Accumulation of cytoplasmic loop and N-terminal truncations in the core might lead to decreases in coupling with age. This reduction is expected to lead to an increase in intracellular calcium and a decrease in levels of glutathione in the nucleus. These changes may ultimately lead to age-related nuclear cataracts.
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Affiliation(s)
- Nefeli Slavi
- Department of Biological and Vision Sciences and the Graduate Center for Vision Research, SUNY College of Optometry, New York, New York, United States
| | - Zhen Wang
- Department of Biochemistry and Mass Spectrometry Research Center Vanderbilt University School of Medicine, Nashville, Tennessee, United States
| | - Lucas Harvey
- Department of Biological and Vision Sciences and the Graduate Center for Vision Research, SUNY College of Optometry, New York, New York, United States
| | - Kevin L. Schey
- Department of Biochemistry and Mass Spectrometry Research Center Vanderbilt University School of Medicine, Nashville, Tennessee, United States
| | - Miduturu Srinivas
- Department of Biological and Vision Sciences and the Graduate Center for Vision Research, SUNY College of Optometry, New York, New York, United States
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10
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Yoshikawa S, Vila A, Segelken J, Lin YP, Mitchell CK, Nguyen D, O'Brien J. Zebrafish connexin 79.8 (Gja8a): A lens connexin used as an electrical synapse in some neurons. Dev Neurobiol 2016; 77:548-561. [PMID: 27402207 DOI: 10.1002/dneu.22418] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2016] [Revised: 06/23/2016] [Accepted: 07/05/2016] [Indexed: 11/07/2022]
Abstract
In the mammalian central nervous system, a remarkably small number of connexins is used in electrical synapses, with the majority formed from Cx36. A larger number has been detected in teleosts, with some seeming to serve restricted roles. Here, we report the discovery of a new connexin expressed in the zebrafish lens and a limited set of neurons. Zebrafish cx79.8 (gja8a), previously annotated incorrectly as cx50.5 based on a partial cDNA sequence, is a homologue of mammalian Cx50 (Gja8). We examined its expression through transgenic promoter-reporter constructs, in situ hybridization, and immunolabeling, and examined regulation of coupling in transfected HeLa cells. cx79.8 was expressed most strongly in the lens, but expression was also found in several groups of neurons in the cerebellum and related areas at the midbrain-hindbrain boundary, in cone photoreceptors, and in neurons in the retinal inner nuclear and ganglion cell layers. Labeling in the retina with antibodies against two C-terminal epitopes revealed numerous small punctate spots in the inner plexiform layer and along the somata of cones. Abundant gap junctions were labeled in the outer 1/3 of the lens, but were absent from the center, suggesting that the epitopes or the entire protein was absent from the center. Cx79.8 tracer coupling was strongly regulated by phosphorylation, and was extremely low in control conditions in HeLa cells due to protein phosphatase 2A activity. These properties allow coupling to be strongly restricted in situ, a frequently observed property for electrical synapses. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 548-561, 2017.
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Affiliation(s)
- Shunichi Yoshikawa
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas
| | - Alejandro Vila
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas.,University of Texas Graduate School of Biomedical Sciences at Houston, Houston, Texas
| | - Jasmin Segelken
- Visual Neuroscience, Faculty of Medicine and Health Sciences, University of Oldenburg, Oldenburg, Germany
| | - Ya-Ping Lin
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas
| | - Cheryl K Mitchell
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas
| | - Duc Nguyen
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas
| | - John O'Brien
- Richard S. Ruiz M.D., Department of Ophthalmology and Visual Science, University of Texas Health Science Center at Houston, Houston, Texas.,University of Texas Graduate School of Biomedical Sciences at Houston, Houston, Texas
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11
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Gutierrez DB, Garland DL, Schwacke JH, Hachey DL, Schey KL. Spatial distributions of phosphorylated membrane proteins aquaporin 0 and MP20 across young and aged human lenses. Exp Eye Res 2016; 149:59-65. [PMID: 27339748 DOI: 10.1016/j.exer.2016.06.015] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2016] [Revised: 06/16/2016] [Accepted: 06/17/2016] [Indexed: 11/26/2022]
Abstract
In the human ocular lens it is now realized that post-translational modifications can alter protein function and/or localization in fiber cells that no longer synthesize proteins. The specific sites of post-translational modification to the abundant ocular lens membrane proteins AQP0 and MP20 have been previously identified and their functional effects are emerging. To further understand how changes in protein function and/or localization induced by these modifications alter lens homeostasis, it is necessary to determine the spatial distributions of these modifications across the lens. In this study, a quantitative LC-MS approach was used to determine the spatial distributions of phosphorylated AQP0 and MP20 peptides from manually dissected, concentric layers of fiber cells from young and aged human lenses. The absolute amounts of phosphorylation were determined for AQP0 Ser235 and Ser229 and for MP20 Ser170 in fiber cells from the lens periphery to the lens center. Phosphorylation of AQP0 Ser229 represented a minor portion of the total phosphorylated AQP0. Changes in spatial distributions of phosphorylated APQ0 Ser235 and MP20 Ser170 correlated with regions of physiological interest in aged lenses, specifically, where barriers to water transport and extracellular diffusion form.
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Affiliation(s)
- Danielle B Gutierrez
- Department of Cell and Molecular Pharmacology and Experimental Therapeutics, Medical University of South Carolina, BSB 358 MSC 509, 173 Ashley Ave., Charleston, SC 29425, USA
| | - Donita L Garland
- National Eye Institute, National Institutes of Health, Bethesda, MD 20892, USA
| | - John H Schwacke
- Department of Biostatistics and Epidemiology, Medical University of South Carolina, Cannon Place 303C, 135 Cannon St., Charleston, SC 29425, USA
| | - David L Hachey
- Mass Spectrometry Research Center, Vanderbilt University School of Medicine, Suite 9160 MRBIII, 465 21st Ave. So., Nashville, TN 37240-7916, USA
| | - Kevin L Schey
- Mass Spectrometry Research Center, Vanderbilt University School of Medicine, Suite 9160 MRBIII, 465 21st Ave. So., Nashville, TN 37240-7916, USA.
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12
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Retamal MA, León-Paravic CG, Ezquer M, Ezquer F, Rio RD, Pupo A, Martínez AD, González C. Carbon monoxide: A new player in the redox regulation of connexin hemichannels. IUBMB Life 2015; 67:428-37. [DOI: 10.1002/iub.1388] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2015] [Accepted: 05/05/2015] [Indexed: 01/23/2023]
Affiliation(s)
- Mauricio A. Retamal
- Centro de Fisiología Celular e Integrativa, Facultad de Medicina; Clínica Alemana Universidad del Desarrollo; Santiago Chile
| | - Carmen G. León-Paravic
- Centro de Fisiología Celular e Integrativa, Facultad de Medicina; Clínica Alemana Universidad del Desarrollo; Santiago Chile
| | - Marcelo Ezquer
- Centro de Medicina Regenerativa, Facultad de Medicina; Clínica Alemana Universidad del Desarrollo; Santiago Chile
| | - Fernando Ezquer
- Centro de Medicina Regenerativa, Facultad de Medicina; Clínica Alemana Universidad del Desarrollo; Santiago Chile
| | - Rodrigo Del Rio
- Centro de Investigación Biomédica; Universidad Autónoma de Chile; Santiago Chile
| | - Amaury Pupo
- Centro Interdisciplinario de Neurociencia de Valparaíso, Facultad de Ciencias; Instituto de Neurociencia; Universidad de Valparaíso; Valparaíso Chile
| | - Agustín D. Martínez
- Centro Interdisciplinario de Neurociencia de Valparaíso, Facultad de Ciencias; Instituto de Neurociencia; Universidad de Valparaíso; Valparaíso Chile
| | - Carlos González
- Centro Interdisciplinario de Neurociencia de Valparaíso, Facultad de Ciencias; Instituto de Neurociencia; Universidad de Valparaíso; Valparaíso Chile
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13
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Intact and N- or C-terminal end truncated AQP0 function as open water channels and cell-to-cell adhesion proteins: end truncation could be a prelude for adjusting the refractive index of the lens to prevent spherical aberration. Biochim Biophys Acta Gen Subj 2014; 1840:2862-77. [PMID: 24821012 DOI: 10.1016/j.bbagen.2014.05.002] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2013] [Revised: 04/30/2014] [Accepted: 05/02/2014] [Indexed: 11/20/2022]
Abstract
BACKGROUND Investigate the impact of natural N- or C-terminal post-translational truncations of lens mature fiber cell Aquaporin 0 (AQP0) on water permeability (Pw) and cell-to-cell adhesion (CTCA) functions. METHODS The following deletions/truncations were created by site-directed mutagenesis (designations in parentheses): Amino acid residues (AA) 2-6 (AQP0-N-del-2-6), AA235-263 (AQP0-1-234), AA239-263 (AQP0-1-238), AA244-263 (AQP0-1-243), AA247-263 (AQP0-1-246), AA250-263 (AQP0-1-249) and AA260-263 (AQP0-1-259). Protein expression was studied using immunostaining, fluorescent tags and organelle-specific markers. Pw was tested by expressing the respective complementary ribonucleic acid (cRNA) in Xenopus oocytes and conducting osmotic swelling assay. CTCA was assessed by transfecting intact or mutant AQP0 into adhesion-deficient L-cells and performing cell aggregation and adhesion assays. RESULTS AQP0-1-234 and AQP0-1-238 did not traffic to the plasma membrane. Trafficking of AQP0-N-del-2-6 and AQP0-1-243 was reduced causing decreased membrane Pw and CTCA. AQP0-1-246, AQP0-1-249 and AQP0-1-259 mutants trafficked properly and functioned normally. Pw and CTCA functions of the mutants were directly proportional to the respective amount of AQP0 expressed at the plasma membrane and remained comparable to those of intact AQP0 (AQP0-1-263). CONCLUSIONS Post-translational truncation of N- or C-terminal end amino acids does not alter the basal water permeability of AQP0 or its adhesive functions. AQP0 may play a role in adjusting the refractive index to prevent spherical aberration in the constantly growing lens. GENERAL SIGNIFICANCE Similar studies can be extended to other lens proteins which undergo post-translational truncations to find out how they assist the lens to maintain transparency and homeostasis for proper focusing of objects on to the retina.
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Minogue PJ, Beyer EC, Berthoud VM. A connexin50 mutant, CX50fs, that causes cataracts is unstable, but is rescued by a proteasomal inhibitor. J Biol Chem 2013; 288:20427-34. [PMID: 23720739 DOI: 10.1074/jbc.m113.452847] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
The mechanisms by which mutant connexins lead to disease are diverse, including those of connexin50 (CX50) encoded by the GJA8 gene. We investigated the cellular and functional behavior of CX50fs, a mutant CX50 that has a frameshift after amino acid 255 and causes recessive congenital cataracts. Cellular levels of CX50fs were much lower than those of wild type CX50 in stably transfected HeLa cells. Whereas CX50 localized at distinct gap junction plaques and supported extensive intercellular transfer of Neurobiotin, CX50fs gap junctions were rare, and their support of Neurobiotin transfer was reduced by >90%. After inhibition of new protein synthesis with cycloheximide, CX50fs disappeared much more rapidly than CX50, suggesting increased degradation of the mutant. Treatment of cells with epoxomicin (a proteasomal inhibitor) led to a dramatic increase in CX50fs levels and in the abundance of gap junctions. Epoxomicin treatment also rescued intercellular transfer of Neurobiotin to levels similar to those in cells expressing the wild type protein. Treatment with eeyarestatin I (an inhibitor of p97-dependent protein degradation) resulted in many abundant slowly migrating CX50 and CX50fs bands consistent with polyubiquitination of the proteins. These results demonstrate that the CX50fs mutant is rapidly degraded by endoplasmic reticulum-associated degradation in mammalian cells. This accelerated degradation reduces the abundance of gap junctions and the extent of intercellular communication, potentially explaining the pathogenesis of cataracts linked to this mutant. The efficacy of epoxomicin in restoring function suggests that protease inhibition might have therapeutic value for this and other diseases caused by mutants with similar defects.
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Affiliation(s)
- Peter J Minogue
- Department of Pediatrics, University of Chicago, Chicago, Illinois 60637, USA
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15
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Beyer EC, Ebihara L, Berthoud VM. Connexin mutants and cataracts. Front Pharmacol 2013; 4:43. [PMID: 23596416 PMCID: PMC3625720 DOI: 10.3389/fphar.2013.00043] [Citation(s) in RCA: 72] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2013] [Accepted: 03/26/2013] [Indexed: 10/27/2022] Open
Abstract
The lens is a multicellular, but avascular tissue that must stay transparent to allow normal transmission of light and focusing of it on the retina. Damage to lens cells and/or proteins can cause cataracts, opacities that disrupt these processes. The normal survival of the lens is facilitated by an extensive network of gap junctions formed predominantly of connexin46 and connexin50. Mutations of the genes that encode these connexins (GJA3 and GJA8) have been identified and linked to inheritance of cataracts in human families and mouse lines. In vitro expression studies of several of these mutants have shown that they exhibit abnormalities that may lead to disease. Many of the mutants reduce or modify intercellular communication due to channel alterations (including loss of function or altered gating) or due to impaired cellular trafficking which reduces the number of gap junction channels within the plasma membrane. However, the abnormalities detected in studies of other mutants suggest that they cause cataracts through other mechanisms including gain of hemichannel function (leading to cell injury and death) and formation of cytoplasmic accumulations (that may act as light scattering particles). These observations and the anticipated results of ongoing studies should elucidate the mechanisms of cataract development due to mutations of lens connexins and abnormalities of other lens proteins. They may also contribute to our understanding of the mechanisms of disease due to connexin mutations in other tissues.
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Affiliation(s)
- Eric C Beyer
- Department of Pediatrics, University of Chicago Chicago, IL, USA
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16
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Chai Z, Goodenough DA, Paul DL. Cx50 requires an intact PDZ-binding motif and ZO-1 for the formation of functional intercellular channels. Mol Biol Cell 2011; 22:4503-12. [PMID: 21965293 PMCID: PMC3226470 DOI: 10.1091/mbc.e11-05-0438] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2011] [Revised: 09/09/2011] [Accepted: 09/21/2011] [Indexed: 11/20/2022] Open
Abstract
The three connexins expressed in the ocular lens each contain PDZ domain-binding motifs directing a physical association with the scaffolding protein ZO-1, but the significance of the interaction is unknown. We found that Cx50 with PDZ-binding motif mutations did not form gap junction plaques or induce cell-cell communication in HeLa cells, whereas the addition of a seven-amino acid PDZ-binding motif restored normal function to Cx50 lacking its entire C-terminal cytoplasmic domain. C-Terminal deletion had a similar although weaker effect on Cx46 but little if any effect on targeting and function of Cx43. Furthermore, small interfering RNA knockdown of ZO-1 completely inhibited the formation of gap junctions by wild-type Cx50 in HeLa cells. Thus both a PDZ-binding motif and ZO-1 are necessary for Cx50 intercellular channel formation in HeLa cells. Knock-in mice expressing Cx50 with a PDZ-binding motif mutation phenocopied Cx50 knockouts. Furthermore, differentiating lens fibers in the knock-in displayed extensive intracellular Cx50, whereas plaques in mature fibers contained only Cx46. Thus normal Cx50 function in vivo also requires an intact PDZ domain-binding motif. This is the first demonstration of a connexin-specific requirement for a connexin-interacting protein in gap junction assembly.
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Affiliation(s)
- Zhifang Chai
- Department of Neurobiology, Harvard Medical School, Boston, MA 02115
| | | | - David L. Paul
- Department of Neurobiology, Harvard Medical School, Boston, MA 02115
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17
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Gutierrez DB, Garland D, Schey KL. Spatial analysis of human lens aquaporin-0 post-translational modifications by MALDI mass spectrometry tissue profiling. Exp Eye Res 2011; 93:912-20. [PMID: 22036630 DOI: 10.1016/j.exer.2011.10.007] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2011] [Revised: 10/17/2011] [Accepted: 10/17/2011] [Indexed: 10/16/2022]
Abstract
Aquaporin-0 (AQP0), the major integral membrane protein in lens fiber cells, becomes highly modified with increasing age. The functional consequences of these modifications are being revealed, and the next step is to determine how these modifications affect the ocular lens, which is directly related to their abundances and spatial distributions. The aim of this study was to utilize matrix-assisted laser desorption ionization (MALDI) direct tissue profiling methods, which produce spatially-resolved protein profiles, to map and quantify AQP0 post-translational modifications (PTMs). Direct tissue profiling was performed using frozen, equatorial human lens sections of various ages prepared by conditions optimized for MALDI mass spectrometry profiling of membrane proteins. Modified forms of AQP0 were identified and further investigated using liquid chromatography tandem mass spectrometry (LC-MS/MS). The distributions of unmodified, truncated, and oleoylated forms of AQP0 were examined with a maximum spatial resolution of 500 μm. Direct tissue profiling of intact human lens sections provided high quality, spatially-resolved, relative quantitative information of AQP0 and its modified forms indicating that 50% of AQP0 is truncated at a fiber cell age of 24 ± 1 year in all lenses examined. Furthermore, direct tissue profiling also revealed previously unidentified AQP0 modifications including N-terminal acetylation and carbamylation. N-terminal acetylation appears to provide a protective effect against N-terminal truncation.
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Affiliation(s)
- Danielle B Gutierrez
- Laboratory of Retinal Cell and Molecular Biology, National Eye Institute, National Institutes of Health, MD, USA
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18
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Schey KL, Gutierrez DB, Wang Z, Wei J, Grey AC. Novel fatty acid acylation of lens integral membrane protein aquaporin-0. Biochemistry 2010; 49:9858-65. [PMID: 20942504 DOI: 10.1021/bi101415w] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Fatty acid acylation of proteins is a well-studied co- or posttranslational modification typically conferring membrane trafficking signals or membrane anchoring properties to proteins. Commonly observed examples of protein acylation include N-terminal myristoylation and palmitoylation of cysteine residues. In the present study, direct tissue profiling mass spectrometry of bovine and human lens sections revealed an abundant signal tentatively assigned as a lipid-modified form of aquaporin-0. LC/MS/MS proteomic analysis of hydrophobic tryptic peptides from lens membrane proteins revealed both N-terminal and C-terminal peptides modified by 238 and 264 Da which were subsequently assigned by accurate mass measurement as palmitoylation and oleoylation, respectively. Specific sites of modification were the N-terminal methionine residue and lysine 238 revealing, for the first time, an oleic acid modification via an amide linkage to a lysine residue. The specific fatty acids involved reflect their abundance in the lens fiber cell plasma membrane. Imaging mass spectrometry indicated abundant acylated AQP0 in the inner cortical region of both bovine and human lenses and acylated truncation products in the lens nucleus. Additional analyses revealed that the lipid-modified forms partitioned exclusively to a detergent-resistant membrane fraction, suggesting a role in membrane domain targeting.
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Affiliation(s)
- Kevin L Schey
- Department of Biochemistry, Vanderbilt University, Nashville, Tennessee 37232, United States.
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19
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Chepelinsky AB. Structural function of MIP/aquaporin 0 in the eye lens; genetic defects lead to congenital inherited cataracts. Handb Exp Pharmacol 2008:265-97. [PMID: 19096783 DOI: 10.1007/978-3-540-79885-9_14] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
Abstract
Aquaporin 0 (AQP0) was originally characterized as a membrane intrinsic protein, specifically expressed in the lens fibers of the ocular lens and designated MIP, for major intrinsic protein of the lens. Once the gene was cloned, an internal repeat was identified, encoding for the amino acids Asp-Pro-Ala, the NPA repeat. Shortly, the MIP gene family was emerging, with members being characterized in mammals, insects, and plants. Once Peter Agre's laboratory developed a functional assay for water channels, the MIP family became the aquaporin family and MIP became known as aquaporin 0. Besides functioning as a water channel, aquaporin 0 also plays a structural role, being required for maintaining the transparency and optical accommodation of the ocular lens. Mutations in the AQP0 gene in human and mice result in genetic cataracts; deletion of the MIP/AQP0 gene in mice results in lack of suture formation required for maintenance of the lens fiber architecture, resulting in perturbed accommodation and focus properties of the ocular lens. Crystallography studies support the notion of the double function of aquaporin 0 as a water channel (open configuration) or adhesion molecule (closed configuration) in the ocular lens fibers. The functions of MIP/AQP0, both as a water channel and an adhesive molecule in the lens fibers, contribute to the narrow intercellular space of the lens fibers that is required for lens transparency and accommodation.
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Affiliation(s)
- Ana B Chepelinsky
- National Institutes of Health, National Eye Institute, Bldg. 31, Room 6A-32, Bethesda, MD, 20892-2510, USA.
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20
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Differentiation-dependent modification and subcellular distribution of aquaporin-0 suggests multiple functional roles in the rat lens. Differentiation 2008; 77:70-83. [PMID: 19281766 DOI: 10.1016/j.diff.2008.09.003] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2008] [Revised: 08/05/2008] [Accepted: 08/05/2008] [Indexed: 11/21/2022]
Abstract
Using immunohistochemistry and mass spectrometry, differentiation-dependent changes in the subcellular distribution and processing of aquaporin-0 (AQP0) have been mapped in the rat lens. Sections labelled with C-terminal tail AQP0 antibodies yielded two concentric rings of labelling with minimal signal in the lens core. The rings were separated by a transient zone of decreased labelling located prior to the transition of differentiating fiber (DF) cells into mature denucleated fiber (MF) cells. Mass spectrometry showed that the loss of core labelling was due to AQP0 cleavage, while the transient loss of labelling was more likely caused by masking of the antibody epitope. AQP0 subcellular distribution changed with radial distance into the lens. In peripheral DF cells, AQP0 was found throughout both broad and narrow side membranes. In deeper-lying DF cells, AQP0 aggregated into plaque-like structures located on the broad sides. This shift occurred prior to the transient loss of AQP0 signal, and coincided with formation of broad-side membrane invaginations between adjacent fiber cells to which filensin, a known binding partner of AQP0, was also localized. After nuclei loss, AQP0 was once again distributed throughout MF cell membranes. In the absence of protein synthesis, the observed subcellular redistribution of AQP0 in DF and subsequent cleavage of AQP0 in MF are suggestive of a switch in the function of AQP0 from a water channel to a junctional protein.
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21
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Yin X, Liu J, Jiang JX. Lens fiber connexin turnover and caspase-3-mediated cleavage are regulated alternately by phosphorylation. ACTA ACUST UNITED AC 2008; 15:1-11. [PMID: 18649174 DOI: 10.1080/15419060802253663] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
Abstract
Lens connexins are phosphorylated in vivo; however, the function and regulation of the phosphorylation remain largely unknown. We have previously identified an in vivo phosphorylation site, Ser(364), at the COOH terminus of lens connexin (Cx) Cx45.6 and phosphorylation appears to regulate connexin protein turnover. To assess the specific mechanism of Ser(364) phosphorylation in Cx45.6, exogenous wild type and Ser(364) mutant Cx45.6 were expressed in primary lens cultures through retroviral infection. Cx45.6 turnover was attenuated primarily by proteasomal inhibitors and to a lesser extent by lysosomal inhibitors. Furthermore, the level of Cx45.6 protein in ubiquitin co-expressed cells was significantly reduced as compared to the cells expressing Cx45.6 alone. Moreover, overexpression of ubiquitin led to a more significant decrease in wild type Cx45.6 than Cx45.6(S364A), a mutant deficient of phosphorylation site at Ser(364), although we did not detect any difference in the levels of ubiquitination between wild type and mutant Cx45.6. Interestingly, the mutant mimicking constitutive phosphorylation, Cx45.6(S364D), partially prevented the cleavage of Cx45.6 by caspase-3. Together, our data suggest that phosphorylation of Cx45.6 at Ser(364) appears to stimulate Cx45.6 turnover primarily through proteasome pathway and this phosphorylation inhibits the cleavage of Cx45.6 by caspase-3. These findings provide further insights into regulatory mechanism of the specific phosphorylation of connexins in the lens.
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Affiliation(s)
- Xinye Yin
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas, USA
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22
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González D, Gómez-Hernández JM, Barrio LC. Molecular basis of voltage dependence of connexin channels: An integrative appraisal. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2007; 94:66-106. [PMID: 17470374 DOI: 10.1016/j.pbiomolbio.2007.03.007] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.
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Affiliation(s)
- Daniel González
- Research Department, Unit of Experimental Neurology, Ramón y Cajal Hospital, Carretera de Colmenar Viejo km 9, Madrid, Spain
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23
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O'Brien JJ, Li W, Pan F, Keung J, O'Brien J, Massey SC. Coupling between A-type horizontal cells is mediated by connexin 50 gap junctions in the rabbit retina. J Neurosci 2006; 26:11624-36. [PMID: 17093084 PMCID: PMC6674794 DOI: 10.1523/jneurosci.2296-06.2006] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
There are many examples of neuronal coupling via gap junctions in the retina. Of these, perhaps the best known is the extensive coupling between horizontal cells. In the rabbit retina, there are two types of horizontal cells, A-type and B-type, both of which are independently coupled. Connexin 50 (Cx50) cDNA, encoding a 440 aa protein, was successfully isolated from rabbit retina RNA. Cx50 was also obtained from isolated A-type horizontal cells (A-type HCs) by single-cell RT-PCR. A-type HCs were visualized by intracellular dye injection or with an antibody against calbindin. Confocal analysis revealed all Cx50 labeling occurred on the A-type HC matrix, typically at dendritic intersections. The Cx50 plaques varied in size, from punctate signals in which fine dendrites cofasciculated, to giant plaques, >50 microm(2), in which large dendrites crossed. The numerous Cx50 plaques between A-type HCs may adequately account for the remarkable coupling observed in this network. We could not detect Cx50 staining on the tips of horizontal cell dendrites within the cone pedicle invagination. This distribution does not support a role for Cx50 in hemichannel-mediated feedback. In addition, the absence of Cx50 in B-type HCs suggests the presence of a different connexin for this cell type. In summary, these results suggest that gap junctions in the A-type horizontal cell matrix are composed from Cx50. Multiple neuronal connexins are expressed in the mammalian retina and different cell types express specific connexins.
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Affiliation(s)
- Jennifer J. O'Brien
- Department of Ophthalmology and Visual Science, University of Texas Medical School at Houston, Houston, Texas 77030
| | - Wei Li
- Department of Ophthalmology and Visual Science, University of Texas Medical School at Houston, Houston, Texas 77030
| | - Feng Pan
- Department of Ophthalmology and Visual Science, University of Texas Medical School at Houston, Houston, Texas 77030
| | - Joyce Keung
- Department of Ophthalmology and Visual Science, University of Texas Medical School at Houston, Houston, Texas 77030
| | - John O'Brien
- Department of Ophthalmology and Visual Science, University of Texas Medical School at Houston, Houston, Texas 77030
| | - Stephen C. Massey
- Department of Ophthalmology and Visual Science, University of Texas Medical School at Houston, Houston, Texas 77030
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DeRosa AM, Mui R, Srinivas M, White TW. Functional characterization of a naturally occurring Cx50 truncation. Invest Ophthalmol Vis Sci 2006; 47:4474-81. [PMID: 17003442 PMCID: PMC1780262 DOI: 10.1167/iovs.05-1582] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
PURPOSE Lens connexins undergo proteolytic cleavage of their C termini during fiber maturation. Although the functional significance of this is unknown, cleavage has been correlated with changes in channel-gating properties. This study evaluates the functional consequences of this endogenous truncation by characterizing the properties of a C-terminal truncated Cx50 protein. METHODS Murine and human Cx50 were truncated at amino acids 290 and 294, respectively, before expression in paired Xenopus oocytes or mammalian cells. Protein expression was evaluated by immunocytochemistry. Dual whole-cell voltage clamp techniques were used to analyze macroscopic and single-channel conductance, voltage-gating properties, and kinetics; pH gating sensitivity was measured by superfusion with 100% CO2-saturated media. RESULTS Cx50tr290 channels exhibited an 86% to 89% reduction in mean macroscopic conductance compared with full-length Cx50. Heterotypic channels formed functional gap junctions, displayed an intermediate level of coupling, and exhibited unaltered voltage-gating properties. C-terminal truncation did not alter single-channel gating characteristics or unitary conductance. Interestingly, truncated and full-length Cx50 channel conductances were reversibly blocked by cytoplasmic acidification. CONCLUSIONS C-terminal truncation of Cx50 did not inhibit the formation of homotypic or heterotypic channels. However, a significant decrease in conductance was observed for truncated channels, a phenomenon independent of alterations in voltage-gating sensitivity, kinetics, or chemical gating. These results provide a plausible explanation for the 50% decrease in junctional coupling observed during lens fiber maturation.
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Affiliation(s)
- Adam M. DeRosa
- From the Graduate Program in Genetics and the State University of New York, Stony Brook, New York
| | - Rickie Mui
- From the Department of Physiology and Biophysics, State University of New York, Stony Brook, New York
| | - Miduturu Srinivas
- From the Department of Biological Sciences, State University of New York College of Optometry, New York, New York
| | - Thomas W. White
- From the Graduate Program in Genetics and the State University of New York, Stony Brook, New York
- From the Department of Physiology and Biophysics, State University of New York, Stony Brook, New York
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25
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Dunia I, Cibert C, Gong X, Xia CH, Recouvreur M, Levy E, Kumar N, Bloemendal H, Benedetti EL. Structural and immunocytochemical alterations in eye lens fiber cells from Cx46 and Cx50 knockout mice. Eur J Cell Biol 2006; 85:729-52. [PMID: 16740340 DOI: 10.1016/j.ejcb.2006.03.006] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2005] [Revised: 03/02/2006] [Accepted: 03/07/2006] [Indexed: 11/28/2022] Open
Abstract
In the current study we describe the changes of overall organization of lens fiber cells in connexin 46 (Cx46) and connexin 50 (Cx50) knockout mice. Morphometric analyses and the application of immunocytochemical techniques revealed that in Cx46 knockout lens (Cx46 -/-), where Cx50 is expressed alone, the postnatal differentiation of secondary fiber cells proceeds faster and is characterized by an increased number of smaller fiber cells. Conversely, in Cx50 knockout mice (Cx50 -/-), the lenticular mass is considerably reduced and characterized by a small number of fiber cells added during the postnatal period. The process of terminal differentiation was impaired and generated larger fiber cells still possessing cytoplasmic organelles. Freeze-fracture and fracture labeling revealed that the junctional assembly, packing organization and topographic interactions between connexons and MP26 differed when Cx46 and Cx50 were co-assembled in the wild-type or expressed separately in the two distinct knockout phenotypes. Filipin cytochemistry provided indirect evidence that Cx46 and Cx50 expressed alone are recruited into different lipid environments. Our results represent the structural proof that interaction of connexins and MP26 contributes to the overall organization of the fiber cells.
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Affiliation(s)
- Irene Dunia
- Biologie Cellulaire, Institut Jacques Monod, CNRS, Universités Paris 6-Paris 7, 2, place Jussieu, F-75251 Paris Cedex 5, France.
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26
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Peracchia C, Peracchia LL. Inversion of both gating polarity and CO2 sensitivity of voltage gating with D3N mutation of Cx50. Am J Physiol Cell Physiol 2005; 288:C1381-9. [PMID: 15677379 DOI: 10.1152/ajpcell.00348.2004] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The effect of CO2-induced acidification on transjunctional voltage ( V j) gating was studied by dual voltage-clamp in oocytes expressing mouse connexin 50 (Cx50) or a Cx50 mutant (Cx50-D3N), in which the third residue, aspartate (D), was mutated to asparagine (N). This mutation inverted the gating polarity of Cx50 from positive to negative. CO2 application greatly decreased the V j sensitivity of Cx50 channels, and increased that of Cx50-D3N channels. CO2 also affected the kinetics of V j dependent inactivation of junctional current ( I j), decreasing the gating speed of Cx50 channels and increasing that of Cx50-D3N channels. In addition, the D3N mutation increased the CO2 sensitivity of chemical gating such that even CO2 concentrations as low as 2.5% significantly lowered junctional conductance ( G j). With Cx50 channels G j dropped by 78% with a drop in intracellular pH (pHi) to 6.83, whereas with Cx50-D3N channels G j dropped by 95% with a drop in pHi to just 7.19. We have previously hypothesized that the way in which V j gating reacts to CO2 might be related to connexin’s gating polarity. This hypothesis is confirmed here by evidence that the D3N mutation inverts the gating polarity as well as the effect of CO2 on V j gating sensitivity and speed.
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Affiliation(s)
- Camillo Peracchia
- Department of Pharmacology and Physiology, University of Rochester School of Medicine, 601 Elmwood Ave., Rochester, NY 14642-8711, USA.
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27
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Truscott RJW. Age-related nuclear cataract—oxidation is the key. Exp Eye Res 2005; 80:709-25. [PMID: 15862178 DOI: 10.1016/j.exer.2004.12.007] [Citation(s) in RCA: 561] [Impact Index Per Article: 29.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2004] [Revised: 11/22/2004] [Accepted: 12/03/2004] [Indexed: 11/26/2022]
Abstract
Age is by far the biggest risk factor for cataract, and it is sometimes assumed that cataract is simply an amplification of this aging process. This appears not to be the case, since the lens changes associated with aging and cataract are distinct. Oxidation is the hallmark of age-related nuclear (ARN) cataract. Loss of protein sulfhydryl groups, and the oxidation of methionine residues, are progressive and increase as the cataract worsens until >90% of cysteine and half the methionine residues are oxidised in the most advanced form. By contrast, there may be no significant oxidation of proteins in the centre of the lens with advancing age, even past age 80. The key factor in preventing oxidation seems to be the concentration of nuclear glutathione (GSH). Provided that nuclear GSH levels can be maintained above 2 mm, it appears that significant protein oxidation and posttranslational modification by reactive small molecules, such as ascorbate or UV filter degradation products, is not observed. Adequate coupling of the metabolically-active cortex, the source of antioxidants such as GSH, to the quiescent nucleus, is crucial especially since it would appear that the cortex remains viable in old lenses, and even possibly in ARN cataract lenses. Therefore it is vital to understand the reason for the onset of the lens barrier. This barrier, which becomes apparent in middle age, acts to impede the flow of small molecules between the cortex and the nucleus. The barrier, rather than nuclear compaction (which is not observed in human lenses), may contribute to the lowered concentration of GSH in the lens nucleus after middle age. By extending the residence time within the lens centre, the barrier also facilitates the decomposition of intrinsically unstable metabolites and may exacerbate the formation of H(2)O(2) in the nucleus. This hypothesis, which is based on the generation of reactive oxygen species and reactive molecules within the nucleus itself, shifts the focus away from theories for cataract that postulated a primary role for oxidants generated outside of the lens. Unfortunately, due to marked variability in the lenses of different species, there appears at present to be no ideal animal model system for studying human ARN cataract.
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Affiliation(s)
- Roger J W Truscott
- Australian Cataract Research Foundation, University of Wollongong, Northfields Ave, Wollongong, NSW 2522, Australia.
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Donaldson PJ, Grey AC, Merriman-Smith BR, Sisley AMG, Soeller C, Cannell MB, Jacobs MD. Functional imaging: New views on lens structure and function. Clin Exp Pharmacol Physiol 2004; 31:890-5. [PMID: 15659055 DOI: 10.1111/j.1440-1681.2004.04099.x] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
1. We have developed an experimental imaging approach that allows the distribution of lens membrane proteins to be mapped with subcellular resolution over large distances as a function of fibre cell differentiation. 2. Using this approach in the rat lens, we have localized precisely histological sites of connexin 46 cleavage, quantitatively mapped changes in gap junction distribution and fibre cell morphology and correlated these changes to differences in intercellular dye transfer. 3. Profiling of glucose transporter isoform expression showed that lens epithelial cells express GLUT1, whereas deeper cortical fibre cells express the higher-affinity GLUT3 isoform. Near the lens periphery, GLUT3 was located in the cytoplasm of fibre cells, but it underwent a differentiation-dependent membrane insertion. 4. Similarly, the putative adhesion protein membrane protein 20 is inserted into fibre cell membranes at the stage when the cells lose their nuclei. This redistribution is strikingly rapid in terms of fibre cell differentiation and correlates with a barrier to extracellular diffusion. 5. Our imaging-orientated approach has facilitated new insights into the relationships between fibre cell differentiation and lens function. Taken together, our results indicate that a number of strategies are used by the lens during the course of normal differentiation to change the subcellular distribution, gross spatial location and functional properties of key membrane transport proteins.
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Affiliation(s)
- Paul J Donaldson
- Department of Physiology, School of Medical Sciences, The University of Auckland, Auckland, New Zealand.
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29
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Gonen T, Cheng Y, Kistler J, Walz T. Aquaporin-0 Membrane Junctions Form Upon Proteolytic Cleavage. J Mol Biol 2004; 342:1337-45. [PMID: 15351655 DOI: 10.1016/j.jmb.2004.07.076] [Citation(s) in RCA: 98] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2004] [Revised: 07/08/2004] [Accepted: 07/15/2004] [Indexed: 11/27/2022]
Abstract
Aquaporin-0 (AQP0), previously known as major intrinsic protein (MIP), is the only water pore protein expressed in lens fiber cells. AQP0 is highly specific to lens fiber cells and constitutes the most abundant intrinsic membrane protein in these cells. The protein is initially expressed as a full-length protein in young fiber cells in the lens cortex, but becomes increasingly cleaved in the lens core region. Reconstitution of AQP0 isolated from the core of sheep lenses containing a proportion of truncated protein, produced double-layered two-dimensional (2D) crystals, which displayed the same dimensions as the thin 11 nm lens fiber cell junctions, which are prominent in the lens core. In contrast reconstitution of full-length AQP0 isolated from the lens cortex reproducibly yielded single-layered 2D crystals. We present electron diffraction patterns and projection maps of both crystal types. We show that cleavage of the intracellular C terminus enhances the adhesive properties of the extracellular surface of AQP0, indicating a conformational change in the molecule. This change of function of AQP0 from a water pore in the cortex to an adhesion molecule in the lens core constitutes another manifestation of the gene sharing concept originally proposed on the basis of the dual function of crystallins.
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Affiliation(s)
- Tamir Gonen
- Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA
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Grey AC, Jacobs MD, Gonen T, Kistler J, Donaldson PJ. Insertion of MP20 into lens fibre cell plasma membranes correlates with the formation of an extracellular diffusion barrier. Exp Eye Res 2003; 77:567-74. [PMID: 14550398 DOI: 10.1016/s0014-4835(03)00192-1] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
It is known that during lens differentiation a number of fibre cell specific membrane proteins change their expression profiles. In this study we have investigated how the profiles of the two most abundant fibre cell membrane proteins AQP0 (formerly known as Major Intrinsic Protein, MIP) and MP20 change as a function of fibre cell differentiation. While AQP0 was always found associated with fibre cell membranes, MP20 was initially found in the cytoplasm of peripheral fibre cells before becoming inserted into the membranes of deeper fibre cells. To determine at what stage in fibre cell differentiation MP20 becomes inserted into the membrane, sections were double-labelled with an antibody against MP20, and propidium iodide, a marker of cell nuclei. This showed that membrane insertion of MP20 occurs in a discrete transition zone that coincided with the degradation of cell nuclei. To test the significance of the membrane insertion of MP20 to overall lens function, whole lenses were incubated for varying times in a solution containing either Texas Red-dextran or Lucifer yellow as markers of extracellular space. Lenses were fixed and then processed for immunocytochemistry. Analysis of these sections showed that both tracer dyes were excluded from the extracellular space in an area that coincided with insertion of MP20 into the plasma membrane. Our results suggest that the insertion of MP20 into fibre cell membranes coincides with the creation of a barrier that restricts the diffusion of molecules into the lens core via the extracellular space.
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Affiliation(s)
- Angus C Grey
- Department of Physiology, School of Medical Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand
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31
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Abstract
The mammalian lens consists of an aged core of quiescent cells enveloped by a layer of synthetically active cells. Abundant gap junctions within and between these cell populations ensure that the lens functions as an electrical syncytium and facilitates the exchange of small molecules between surface and core cells. In the present study, we utilized an in vivo mouse model to characterize the properties of an additional pathway, permeable to macromolecules, which co-exists with gap-junction-mediated communication in the lens core. The TgN(GFPU)5Nagy strain of mice carries a green fluorescent protein (GFP) transgene. In the lenses of hemizyous animals, GFP was expressed in a variegated fashion, allowing diffusion of GFP to be visualized directly. Early in development, GFP expression in scattered fiber cells resulted in a checkerboard fluorescence pattern in the lens. However, at E15 and later, the centrally located fiber cells became uniformly fluorescent. In the adult lens, a superficial layer of cells, approximately 100 microm thick, retained the original mosaic fluorescence pattern, but the remainder, and majority, of the tissue was uniformly fluorescent. We reasoned that at the border between the two distinct labeling patterns, a macromolecule-permeable intercellular pathway was established. To test this hypothesis, we microinjected 10 kDa fluorescent dextran into individual fiber cells and followed its diffusion by time-lapse microscopy. Injections at depths of >100 microm resulted in intercellular diffusion of dextran from injected cells. By contrast, when injections were made into superficial fiber cells, the injected cell invariably retained the dextran. Together, these data suggest that, in addition to being coupled by gap junctions, cells in the lens core are interconnected by a macromolecule-permeable pathway. At all ages examined, a significant proportion of the nucleated fiber cell population of the lens was located within this region of the lens.
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Affiliation(s)
- Valery I Shestopalov
- Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, Saint Louis, MO 63110, USA.
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Xu X, Berthoud VM, Beyer EC, Ebihara L. Functional role of the carboxyl terminal domain of human connexin 50 in gap junctional channels. J Membr Biol 2002; 186:101-12. [PMID: 11944087 PMCID: PMC2744361 DOI: 10.1007/s00232-001-0139-5] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2001] [Indexed: 11/28/2022]
Abstract
Gap junction channels formed by connexin 50 (Cx50) are critical for maintenance of lens transparency. Because the C-terminus of Cx50 can be cleaved post-translationally, we hypothesized that channels formed by the truncated Cx50 exhibit altered properties or regulation. We used the dual whole-cell patch-clamp technique to investigate the macroscopic and single-channel properties of gap junctional channels formed by wild-type human Cx50 and a truncation mutant (Cx50A294stop) after transfection of N2A cells. Our results show that wild-type Cx50 formed functional gap junctional channels. The macroscopic Gjss-Vj relationship was well described by a Boltzmann equation with A of 0.10, V0 of 43.8 mV and Gjmin of 0.23. The single-channel conductance was 212 +/- 5 pS. Multiple long-lasting substates were observed with conductances ranging between 31 and 80 pS. Wild-type Cx50 gap junctional channels were reversibly blocked when pHi was reduced to 6.3. Truncating the C-terminus at amino acid 294 caused a loss of pHi sensitivity, but there were no significant changes in single-channel current amplitude or Gjss-Vj relationship. These results suggest that the C-terminus of human Cx50 is involved in pHi sensitivity, but has little influence over single-channel conductance, voltage dependence, or gating kinetics.
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Affiliation(s)
- X Xu
- Department of Physiology and Biophysics, Finch University of Health Science/The Chicago Medical School, 3333 Green Bay Rd, North Chicago, IL 60064, USA
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Baldo GJ, Gong X, Martinez-Wittinghan FJ, Kumar NM, Gilula NB, Mathias RT. Gap junctional coupling in lenses from alpha(8) connexin knockout mice. J Gen Physiol 2001; 118:447-56. [PMID: 11696604 PMCID: PMC2233836 DOI: 10.1085/jgp.118.5.447] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
Abstract
Lens fiber cell gap junctions contain alpha(3) (Cx46) and alpha(8) (Cx50) connexins. To examine the roles of the two different connexins in lens physiology, we have genetically engineered mice lacking either alpha(3) or alpha(8) connexin. Intracellular impedance studies of these lenses were used to measure junctional conductance and its sensitivity to intracellular pH. In Gong et al. 1998, we described results from alpha(3) connexin knockout lenses. Here, we present original data from alpha(8) connexin knockout lenses and a comparison with the previous results. The lens has two functionally distinct domains of fiber cell coupling. In wild-type mouse lenses, the outer shell of differentiating fibers (see 1, DF) has an average coupling conductance per area of cell-cell contact of approximately 1 S/cm(2), which falls to near zero when the cytoplasm is acidified. In the inner core of mature fibers (see 1, MF), the average coupling conductance is approximately 0.4 S/cm(2), and is insensitive to acidification of the cytoplasm. Both connexin isoforms appear to contribute about equally in the DF since the coupling conductance for either heterozygous knockout (+/-) was approximately 70% of normal and 30-40% of the normal for both -/- lenses. However, their contribution to the MF was different. About 50% of the normal coupling conductance was found in the MF of alpha(3) +/- lenses. In contrast, the coupling of MF in the alpha(8) +/- lenses was the same as normal. Moreover, no coupling was detected in the MF of alpha(3) -/- lenses. Together, these results suggest that alpha(3) connexin alone is responsible for coupling MF. The pH- sensitive gating of DF junctions was about the same in wild-type and alpha(3) connexin -/- lenses. However, in alpha(8) -/- lenses, the pure alpha(3) connexin junctions did not gate closed in the response to acidification. Since alpha(3) connexin contributes about half the coupling conductance in DF of wild-type lenses, and that conductance goes to zero when the cytoplasmic pH drops, it appears alpha(8) connexin regulates the gating of alpha(3) connexin. Both connexins are clearly important to lens physiology as lenses null for either connexin lose transparency. Gap junctions in the MF survive for the lifetime of the organism without protein turnover. It appears that alpha(3) connexin provides the long-term communication in MF. Gap junctions in DF may be physiologically regulated since they are capable of gating when the cytoplasm is acidified. It appears alpha(8) connexin is required for gating in DF.
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Affiliation(s)
- George J. Baldo
- Department of Physiology and Biophysics, State University of New York, Stony Brook, NY 11794
| | - Xiaohua Gong
- Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037
| | | | - Nalin M. Kumar
- Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037
| | - Norton B. Gilula
- Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037
| | - Richard T. Mathias
- Department of Physiology and Biophysics, State University of New York, Stony Brook, NY 11794
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Yahuaca P, Ek-Vitorin JF, Rush P, Delmar M, Taffet SM. Identification of a protein kinase activity that phosphorylates connexin43 in a pH-dependent manner. Braz J Med Biol Res 2000; 33:399-406. [PMID: 10775304 DOI: 10.1590/s0100-879x2000000400005] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The carboxyl-terminal (CT) domain of connexin43 (Cx43) has been implicated in both hormonal and pH-dependent gating of the gap junction channel. An in vitro assay was utilized to determine whether the acidification of cell extracts results in the activation of a protein kinase that can phosphorylate the CT domain. A glutathione S-transferase (GST)-fusion protein was bound to Sephadex beads and used as a target for protein kinase phosphorylation. A protein extract produced from sheep heart was allowed to bind to the fusion protein-coated beads. The bound proteins were washed and then incubated with 32P-ATP. Phosphorylation was assessed after the proteins were resolved by SDS-PAGE. Incubation at pH 7.5 resulted in a minimal amount of phosphorylation while incubation at pH 6.5 resulted in significant phosphorylation reaction. Maximal activity was achieved when both the binding and kinase reactions were performed at pH 6.5. The protein kinase activity was stronger when the incubations were performed with manganese rather than magnesium. Mutants of Cx43 which lack the serines between amino acids 364-374 could not be phosphorylated in the in vitro kinase reaction, indicating that this is a likely target of this reaction. These results indicate that there is a protein kinase activity in cells that becomes more active at lower pH and can phosphorylate Cx43.
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Affiliation(s)
- P Yahuaca
- Department of Pharmacology, SUNY Health Science Center at Syracuse, Syracuse, NY 13210, USA
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Yin X, Jedrzejewski PT, Jiang JX. Casein kinase II phosphorylates lens connexin 45.6 and is involved in its degradation. J Biol Chem 2000; 275:6850-6. [PMID: 10702244 DOI: 10.1074/jbc.275.10.6850] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Connexin (Cx) 45.6, an avian counterpart of rodent Cx50, is phosphorylated in vivo, but the sites and function of the phosphorylation have not been elucidated. Our peptide mapping experiments showed that the Ser(363) site in the carboxyl (COOH) terminus of Cx45.6 was phosphorylated and that this site is within casein kinase (CK) II consensus sequence, although showing some similarity to CKI sequence. The peptide containing Ser(363) could be phosphorylated in vitro by CKII, but not by CKI. Furthermore, CKII phosphorylated Cx45.6 in embryonic lens membrane and the fusion protein containing the COOH terminus of Cx45.6. Two-dimensional peptide mapping experiments showed that one of the Cx45.6 peptides phosphorylated in vivo migrated to the same spot as one of those phosphorylated by CKII in vitro. Furthermore, CKII activity could be detected in lens lysates. To assess the function of this phosphorylation event, exogenous wild type and mutant Cx45.6 (Ser(363) --> Ala) were expressed in lens primary cultures by retroviral infection. The mutant Cx45.6 was shown to be more stable having a longer half-life compared with wild type Cx45.6. Together, the evidence suggests that CKII is likely a kinase responsible for the Ser(363) phosphorylation, leading to the destablization and degradation of Cx45.6. The connexin degradation induced by phosphorylation has a broad functional significance in the regulation of gap junctions in vivo.
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Affiliation(s)
- X Yin
- Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78284-7760, USA
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36
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Srinivas M, Costa M, Gao Y, Fort A, Fishman GI, Spray DC. Voltage dependence of macroscopic and unitary currents of gap junction channels formed by mouse connexin50 expressed in rat neuroblastoma cells. J Physiol 1999; 517 ( Pt 3):673-89. [PMID: 10358109 PMCID: PMC2269370 DOI: 10.1111/j.1469-7793.1999.0673s.x] [Citation(s) in RCA: 80] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
Abstract
1. The macroscopic and single channel gating characteristics of connexin (Cx) 50 gap junction channels between pairs of N2A neuroblastoma cells transfected with mouse Cx50 DNA were investigated using the dual whole-cell voltage clamp technique. 2. The macroscopic junctional current (Ij) of Cx50-transfected cells decayed exponentially with time in response to transjunctional voltage (Vj) steps (time constant (tau) of approximately 4 s at a Vj of 30-40 mV and 100-200 ms at a Vj of 80-100 mV). The steady-state junctional conductance (gj) was well described by a two-state Boltzmann equation. The half-inactivation voltage (V0), the ratio of minimal to maximal gj (gmin/gmax) and the equivalent gating charge were +/- 37 mV, 0.21 and 4, respectively. 3. The conductance of single Cx50 channels measured using patch pipettes containing 130 mM CsCl was 220 +/- 13.1 pS (12 cell pairs). A prominent residual or subconductance state corresponding to 43 +/- 4. 2 pS (10 cell pairs) was also observed at large Vj s. 4. The relationship between channel open probability (Po) and Vj was well described by a Boltzmann relationship with parameters similar to those obtained for macroscopic gj (V0 = 34 mV, gating charge = 4.25, maximum P= 0.98). The ensemble average of single channel currents at Vj = 50 mV declined in a monoexponential manner (tau = 905 ms), a value similar to the decline of the macroscopic Ij of Cx50 channels at the same voltage. 5. Ion substitution experiments indicated that Cx50 channels have a lower permeability to anions than to cations (transjunctional conductance of KCl vs. potassium glutamate (gammaj, KCl/gammaj,KGlut), 1.2; 6 cell pairs). 6. The results have important implications for understanding the role of connexins in tissues where Cx50 is a major gap junction component, including the lens.
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Affiliation(s)
- M Srinivas
- Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, USA
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Stergiopoulos K, Alvarado JL, Mastroianni M, Ek-Vitorin JF, Taffet SM, Delmar M. Hetero-domain interactions as a mechanism for the regulation of connexin channels. Circ Res 1999; 84:1144-55. [PMID: 10347089 DOI: 10.1161/01.res.84.10.1144] [Citation(s) in RCA: 93] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Previous studies have shown that chemical regulation of connexin43 (Cx43) depends on the presence of the carboxyl terminal (CT) domain. A particle-receptor (or "ball-and-chain") model has been proposed to explain the mechanism of gating. We tested whether the CT region behaved as a functional domain for other members of the connexin family. The pH sensitivity of wild-type and Ct-truncated connexins was quantified by use of electrophysiological and optical techniques and the Xenopus oocyte system. The CT domain of Cx45 had no role in pH regulation, although a partial role was shown for Cx37 and Cx50. A prominent effect was observed for Cx40 and Cx43. In addition, we found that the CT domain of Cx40 that was expressed as a separate fragment rescued the pH sensitivity of the truncated Cx40 (Cx40tr), which was in agreement with a particle-receptor model. Because Cx40 and Cx43 often colocalize and possibly heteromerize, we tested the pH sensitivity of Cx40tr when coexpressed with the CT domain of Cx43 (hetero-domain interactions). We found that the CT domain of Cx43 enhanced the pH sensitivity of Cx40tr; similarly, the CT domain of Cx40 restored the pH sensitivity of the truncated Cx43. In addition, the CT domain of Cx43 granted insulin sensitivity to the otherwise insulin-insensitive Cx26 or Cx32 channels. These data show that the particle-receptor model is preserved in Cx40 and the regulatory domain of one connexin can specifically interact with a channel formed by another connexin. Hetero-domain interactions could be critical for the regulation of heteromeric channels.
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Affiliation(s)
- K Stergiopoulos
- Department of Pharmacology, SUNY Health Science Center, Syracuse, NY 13210, USA
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Spray D, Kojima T, Scemes E, Suadicani S, Gao Y, Zhao S, Fort A. Chapter 23: “Negative” Physiology: What Connexin-Deficient Mice Reveal about the Functional Roles of Individual Gap Junction Proteins. CURRENT TOPICS IN MEMBRANES 1999. [DOI: 10.1016/s0070-2161(08)61027-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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