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

Effect of hydrogen peroxide on lens transparency, intracellular pH, gap junction coupling, hydrostatic pressure and membrane water permeability

Clouding of the eye lens or cataract is an age-related anomaly that affects middle-aged humans. Exploration of the etiology points to a great extent to oxidative stress due to different forms of reactive oxygen species/metabolites such as Hydrogen peroxide (H2O2) that are generated due to intracellular metabolism and environmental factors like radiation. If accumulated and left unchecked, the imbalance between the production and degradation of H2O2 in the lens could lead to cataracts. Our objective was to explore ex vivo the effects of H2O2 on lens physiology. We investigated transparency, intracellular pH (pHi), intercellular gap junction coupling (GJC), hydrostatic pressure (HP) and membrane water permeability after subjecting two-month-old C57 wild-type (WT) mouse lenses for 3 h or 8 h in lens saline containing 50 μM H2O2; the results were compared with control lenses incubated in the saline without H2O2. There was a significant decrease in lens transparency in H2O2-treated lenses. In control lenses, pHi decreases from ∼7.34 in the surface fiber cells to 6.64 in the center. Experimental lenses exposed to H2O2 for 8 h showed a significant decrease in surface pH (from 7.34 to 6.86) and central pH (from 6.64 to 6.56), compared to the controls. There was a significant increase in GJC resistance in the differentiating (12-fold) and mature (1.4-fold) fiber cells compared to the control. Experimental lenses also showed a significant increase in HP which was ∼2-fold higher at the junction between the differentiating and mature fiber cells and ∼1.5-fold higher at the center compared to these locations in control lenses; HP at the surface was 0 mm Hg in either type lens. Fiber cell membrane water permeability significantly increased in H2O2-exposed lenses compared to controls. Our data demonstrate that elevated levels of lens intracellular H2O2 caused a decrease in intracellular pH and led to acidosis which most likely uncoupled GJs, and increased AQP0-dependent membrane water permeability causing a consequent rise in HP. We infer that an abnormal increase in intracellular H2O2 could induce acidosis, cause oxidative stress, alter lens microcirculation, and lead to the development of accelerated lens opacity and age-related cataracts.

A novel single-base deletional mutation of MIP impairs protein distribution and cell-to-cell adhesion in autosomal dominant cataracts in a Chinese family

Congenital cataracts are the leading cause of irreversible visual disability in children, and genetic factors play an important role in their development. In this study, targeted exome sequencing revealed a novel single-base deletional mutation of MIP (c.301delG; p.Ala101Profs*16) segregated with congenital punctate cataract in a Chinese family. The hydrophobic properties, and secondary and tertiary structures for truncated MIP were predicted to affect the function of protein by bioinformatics analysis. When MIP-WT and MIP-Ala101fs expression constructs were singly transfected into HeLa cells, it was found that the mRNA level showed no significant difference, while the protein level of the mutant was remarkably reduced compared to that of the wild-type MIP. Immunofluorescence images showed that the MIP-WT was principally localized to the plasma membrane, whereas the MIP-Ala101fs protein was aberrantly trapped in the cytoplasm. Furthermore, the cell-to-cell adhesion capability and the cell-to-cell communication property were both significantly reduced for MIP-Ala101fs compared to the MIP-WT (all *p < 0.05). This is the first report of the c.301delG mutation in the MIP gene associated with autosomal dominant congenital cataracts. We propose that the cataract is caused by the decreased protein expression and reduced cell-to-cell adhesion by the mutant MIP. The impaired trafficking or instability of the mutant protein, as well as compromised intercellular communication is probably a concurrent result of the mutation. The results expand the genetic and phenotypic spectra of MIP and help to better understand the molecular basis of congenital cataracts.

Verification of the gene and protein expression of the aquaglyceroporin AQP3 in the mammalian lens

Transport of water is critical for maintaining the transparency of the avascular lens, and the lens is known to express at least five distinctly different water channels from the Aquaporin (AQP) family of proteins. In this study we report on the identification of a sixth lens AQP, AQP3 an aquaglyceroporin, which in addition to water also transports glycerol and H2O2. AQP3 was identified at the transcript level and protein levels using RT-PCR and Western blotting, respectively, in the mouse, rat, bovine and human lens, showing that its expression is conserved in the mammalian lens. Western blotting showed AQP3 in the lens exists as 25 kDa non-glycosylated and 37 kDa glycosylated monomeric forms in all lens species. To identify the regions in the lens where AQP3 is expressed Western blotting was repeated using epithelial, outer cortical and inner cortical/core fractions isolated from the mouse lens. AQP3 was found in all lens regions, with the highest signal of non-glycosylated AQP3 being found in the epithelium. While in the inner cortex/core region AQP3 signal was not only lower but was predominately from the glycosylated form of AQP3. Immunolabelling of lens sections with AQP3 antibodies confirmed that AQP3 is found in all regions of the adult mouse, and also revealed that the subcellular distribution of AQP3 changes as a function of fiber cell differentiation. In epithelial and peripheral fiber cells of the outer cortex AQP3 labelling was predominately associated with membrane vesicles in the cytoplasm, but in the deeper regions of the lens AQP3 labelling was associated with the plasma membranes of fiber cells located in the inner cortex and core of the lens. To determine how this adult pattern of AQP3 subcellular distribution was established, immunolabelling for AQP3 was performed on embryonic and postnatal lenses. AQP3 expression was first detected on embryonic day (E) 11 in the membranes of primary fiber cells that have started to elongate and fill the lumen of the lens vesicle, while later at E16 the AQP3 labelling in the primary fiber cells had shifted to a predominately cytoplasmic location. In the following postnatal (P) stages of lens growth at P3 and P6, AQP3 labelling remained cytoplasmic across all regions of the lens and it was not until P15 when the pattern of localisation of AQP3 changed to an adult distribution with cytoplasmic labelling detected in the outer cortex and membrane localisation detected in the inner cortex and core of the lens. Comparison of the AQP3 labelling pattern to those obtained previously for AQP0 and AQP5 showed that the subcellular distribution was more similar to AQP5 than AQP0, but there were still significant differences that suggest AQP3 may have unique roles in the maintenance of lens transparency.

Aquaporin water channels: roles beyond renal water handling

Aquaporin (AQP) water channels are pivotal to renal water handling and therefore in the regulation of body water homeostasis. However, beyond the kidney, AQPs facilitate water reabsorption and secretion in other cells and tissues, including sweat and salivary glands and the gastrointestinal tract. A growing body of evidence has also revealed that AQPs not only facilitate the transport of water but also the transport of several small molecules and gases such as glycerol, H2O2, ions and CO2. Moreover, AQPs are increasingly understood to contribute to various cellular processes, including cellular migration, adhesion and polarity, and to act upstream of several intracellular and intercellular signalling pathways to regulate processes such as cell proliferation, apoptosis and cell invasiveness. Of note, several AQPs are highly expressed in multiple cancers, where their expression can correlate with the spread of cancerous cells to lymph nodes and alter the response of cancers to conventional chemotherapeutics. These data suggest that AQPs have diverse roles in various homeostatic and physiological systems and may be exploited for prognostics and therapeutic interventions.

Insight into the Mammalian Aquaporin Interactome

Aquaporins (AQPs) are a family of transmembrane water channels expressed in all living organisms. AQPs facilitate osmotically driven water flux across biological membranes and, in some cases, the movement of small molecules (such as glycerol, urea, CO₂, NH₃, H₂O₂). Protein-protein interactions play essential roles in protein regulation and function. This review provides a comprehensive overview of the current knowledge of the AQP interactomes and addresses the molecular basis and functional significance of these protein-protein interactions in health and diseases. Targeting AQP interactomes may offer new therapeutic avenues as targeting individual AQPs remains challenging despite intense efforts.

Beyond the Channels: Adhesion Functions of Aquaporin 0 and Connexin 50 in Lens Development

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.

Development of a potent embryonic chick lens model for studying congenital cataracts in vivo

Congenital cataracts are associated with gene mutations, yet the underlying mechanism remains largely unknown. Here we reported an embryonic chick lens model that closely recapitulates the process of cataract formation. We adopted dominant-negative site mutations that cause congenital cataracts, connexin, Cx50E48K, aquaporin 0, AQP0R33C, αA-crystallin, CRYAA R12C and R54C. The recombinant retroviruses containing these mutants were microinjected into the occlusive lumen of chick lenses at early embryonic development. Cx50E48K expression developed cataracts associated with disorganized nuclei and enlarged extracellular spaces. Expression of AQP0R33C resulted in cortical cataracts, enlarged extracellular spaces and distorted fiber cell organization. αA crystallin mutations distorted lens light transmission and increased crystalline protein aggregation. Together, retroviral expression of congenital mutant genes in embryonic chick lenses closely mimics characteristics of human congenital cataracts. This model will provide an effective, reliable in vivo system to investigate the development and underlying mechanism of cataracts and other genetic diseases.

A novel missense mutation in the gene encoding major intrinsic protein (MIP) in a Giant panda with unilateral cataract formation

Background

Cataracts are defects of the lens that cause progressive visual impairment and ultimately blindness in many vertebrate species. Most cataracts are age-related, but up to one third have an underlying genetic cause. Cataracts are common in captive zoo animals, but it is often unclear whether these are congenital or acquired (age-related) lesions.

Results

Here we used a functional candidate gene screening approach to identify mutations associated with cataracts in a captive giant panda (Ailuropoda melanoleuca). We screened 11 genes often associated with human cataracts and identified a novel missense mutation (c.686G > A) in the MIP gene encoding major intrinsic protein. This is expressed in the lens and normally accumulates in the plasma membrane of lens fiber cells, where it plays an important role in fluid transport and cell adhesion. The mutation causes the replacement of serine with asparagine (p.S229N) in the C-terminal tail of the protein, and modeling predicts that the mutation induces conformational changes that may interfere with lens permeability and cell–cell interactions.

Conclusion

The c.686G > A mutation was found in a captive giant panda with a unilateral cataract but not in 18 controls from diverse regions in China, suggesting it is most likely a genuine disease-associated mutation rather than a single-nucleotide polymorphism. The mutation could therefore serve as a new genetic marker to predict the risk of congenital cataracts in captive giant pandas.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07386-8.

Lens aquaporins function as peroxiporins to facilitate membrane transport of hydrogen peroxide

High levels of reactive oxygen species such as hydrogen peroxide (H₂O₂) cause oxidative stress in the lens and lead to cataractogenesis. The present investigation was undertaken to find out whether the mammalian lens aquaporins (AQPs) 0, 1, and 5 perform H₂O₂ transport across the plasma membrane to reduce oxidative stress. Our in vitro cell culture and ex vivo lens experiments demonstrated that in addition to the established water transport role, mouse AQP0, AQP1 and AQP5 facilitate transmembrane H₂O₂ transport and function as peroxiporins. Human lens epithelial cells expressing AQP1, AQP5 and AQP8, when treated with 50 μM HgCl₂ water channel inhibitor showed a significant reduction in H₂O₂ transport. Data obtained from the experiments involving H₂O₂-degrading enzyme glutathione peroxidase 1 (GPX1) knockout lenses showed H₂O₂ accumulation, suggesting H₂O₂ transport level by AQPs in the lens is regulated by GPX1. Under hyperglycemic conditions, there was an increased loss of transparency, and enhanced production and retention of H₂O₂ in AQP5^-/- lenses compared to similarly-treated WT lenses. Overall, the results show that lens AQPs function as peroxiporins and cooperate with GPX1 to maintain lens H₂O₂ homeostasis to prevent oxidative stress, highlighting AQPs and GPX1 as promising therapeutic drug targets to delay/treat/prevent age-related lens cataracts.

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

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.

C-Terminal End of Aquaporin 0 Regulates Lens Gap Junction Channel Function

Purpose

We reported previously that aquaporin 0 (AQP0) modulates lens fiber cell gap junction (GJ) channel function. The present study was conducted to find out whether the C-terminal end of AQP0 is involved in this regulation.

Methods

A mouse model, AQP0ΔC/ΔC, was genetically engineered to express AQP0 with 1-246 amino acids, without the normal intact AQP0 (1-263 amino acids) in the lens. Transparency and focusing of the lens were assessed. Intracellular impedance was measured to determine GJ coupling resistance. Intracellular hydrostatic pressure (HP) was also determined. Western blotting was performed to determine connexin (Cx46 and Cx50) expression levels.

Results

At postnatal day 10, AQP0ΔC/ΔC mouse lenses relative to age-matched wild-type lenses showed loss of transparency and abnormal optical distortion; GJ coupling resistance increased in the differentiating (1.6-fold) and mature (8-fold) fiber cells; lens HP increased approximately 1.5-fold at the junction between the differentiating and mature fiber cells and approximately 2.0-fold in the center; there was no significant change (P > 0.05) in expression levels of Cx46 or Cx50.

Conclusions

The increase in GJ coupling resistance was not associated with reduced connexin expression, suggesting either a reduction in the open probability or some physical change in plaque location. The increase in resistance was significantly greater than the increase in HP, suggesting less pressure-driven water flow through each open GJ channel. These changes may lead to a loss of transparency and abnormal optical distortion. Overall, our data demonstrate the C-terminal end of AQP0 is involved in modulating GJ coupling to maintain lens transparency and homeostasis.

Positively charged amino acid residues in the extracellular loops A and C of lens aquaporin 0 interact with the negative charges in the plasma membrane to facilitate cell-to-cell adhesion

This investigation was undertaken to find out whether the positive charges in the Extracellular Loops A (ELA) and C (ELC) of Aquaporin 0 (AQP0) are involved in lens fiber cell-to-cell adhesion (CTCA), and the possible mechanism of CTCA. AQP0 ELA or ELC was substituted with the corresponding AQP1 loop via Polymerase Chain Reaction. Positively charged arginine (R) and histidine (H) of mouse AQP0 ELA and ELC were substituted individually with glutamine (Q) to create R33Q, H40Q, R113Q and H122Q by mutagenesis. cRNA expression, immunostaining, Förster Resonance Energy Transfer (FRET) studies and protein analyses showed localization of all mutants except AQP0-AQP1ELC chimera (AQP0 ELC substituted with AQP1 ELC) at the plasma membrane. Osmotic Swelling Assay revealed comparable water permeability (P_f) among AQP0-AQP1ELA, R33Q, R113Q, and WT. CTCA assay demonstrated a significant reduction in adhesion in all mutants compared to the WT (14-73%) suggesting the importance of the conserved positively charged residues of ELA and ELC for adhesion. Studies involving AQP0-transfected L-cells, and lipid vesicles indicated that CTCA was due to the electrostatic interaction between the positively charged amino acids of AQP0 extracellular loops and the negative charges of the plasma membrane. Schematic models are provided to illustrate the mechanism.

A predominant form of C-terminally end-cleaved AQP0 functions as an open water channel and an adhesion protein in AQP0ΔC/ΔC mouse lens

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 (AQP0^ΔC/ΔC) that does not express intact AQP0. A genetically engineered KI mouse model, AQP0^ΔC/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 AQP0^ΔC/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 AQP0^ΔC/ΔC; water permeability (P_f) 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 P_f and CTCA compared to those of WT. AQP0^+/ΔC and AQP0^ΔC/ΔC FCMVs exhibited no statistically significant alteration (P > 0.05) in P_f compared to those of WT. However, CTCA of AQP0^+/ΔC AQP0^ΔC/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.

Molecular mechanism of Aquaporin 0-induced fiber cell to fiber cell adhesion in the eye lens

Cell-to-cell adhesion (CTCA), which is key for establishing lens transparency, is a critical function of Aquaporin 0 (AQP0). The aim of this investigation was to find out the possible mechanism by which AQP0 exerts CTCA between fiber cells, since there are two proposals currently, either an AQP0-AQP0 interaction or an AQP0-lipid interaction. We studied the mechanism of AQP0-induced CTCA in intact AQP0 and C-terminally cleaved AQP0 (CTC-AQP0). Assays showed CTCA between L-cells transfected with intact AQP0 or CTC-AQP0 and parental L-cells indicating AQP0-membrane interaction. Both forms of AQP0 significantly (P < 0.001) promoted adhesion to negatively charged l-α-phosphatidylserine lipid vesicles signifying AQP0-lipid interaction. AQP0-expressing L-cells also promoted adhesion of WT and AQP0-KO mouse lens fiber cell membrane vesicles (FCMVs) significantly (P < 0.001). However, when FCMVs of WT or AQP0-KO were plated over parental L-cells, only WT vesicles adhered significantly, corroborating AQP0-membrane interaction. After incubating with extracellular domain-specific AQP0 antibody, L-cells expressing intact AQP0 or CTC-AQP0 showed a significant reduction (P < 0.001) in the adhesion of AQP0-KO FCMVs indicating extracellular loop involvement in CTCA. WT FCMVs from outer cortex and inner cortex promoted adhesion to parental L-cells, without any statistically significant difference in adhesion efficiency (P > 0.05). Ultrastructure studies of WT, AQP0-KO and transgenic lenses showed AQP0 is critical for fiber CTCA and compaction. The data collected clearly demonstrate that the positively charged amino acids in the AQP0 extracellular loop domains interact with the negatively charged lipids in the plasma membrane to promote CTCA for compaction of fiber cells.

Identification of a novel MIP frameshift mutation associated with congenital cataract in a Chinese family by whole-exome sequencing and functional analysis

Purpose

To detect the underlying pathogenesis of congenital cataract in a four-generation Chinese family.

Methods

Whole-exome sequencing (WES) of family members (III:4, IV:4, and IV:6) was performed. Sanger sequencing and bioinformatics analysis were subsequently conducted. Full-length WT-MIP or K228fs-MIP fused to HA markers at the N-terminal was transfected into HeLa cells. Next, quantitative real-time PCR, western blotting and immunofluorescence confocal laser scanning were performed.

Results

The age of onset for nonsyndromic cataracts in male patients was by 1-year old, earlier than for female patients, who exhibited onset at adulthood. A novel c.682_683delAA (p.K228fs230X) mutation in main intrinsic protein (MIP) cosegregated with the cataract phenotype. The instability index and unfolded states for truncated MIP were predicted to increase by bioinformatics analysis. The mRNA transcription level of K228fs-MIP was reduced compared with that of WT-MIP, and K228fs-MIP protein expression was also lower than that of WT-MIP. Immunofluorescence images showed that WT-MIP principally localized to the plasma membrane, whereas the mutant protein was trapped in the cytoplasm.

Conclusions

Our study generated genetic and primary functional evidence for a novel c.682_683delAA mutation in MIP that expands the variant spectrum of MIP and help us better understand the molecular basis of cataract.

The Role of Aquaporins in Ocular Lens Homeostasis

Abstract: Aquaporins (AQPs), by playing essential roles in the maintenance of ocular lens homeostasis, contribute to the establishment and maintenance of the overall optical properties of the lens over many decades of life. Three aquaporins, AQP0, AQP1 and AQP5, each with distinctly different functional properties, are abundantly and differentially expressed in the different regions of the ocular lens. Furthermore, the diversity of AQP functionality is increased in the absence of protein turnover by age-related modifications to lens AQPs that are proposed to alter AQP function in the different regions of the lens. These regional differences in AQP functionality are proposed to contribute to the generation and directionality of the lens internal microcirculation; a system of circulating ionic and fluid fluxes that delivers nutrients to and removes wastes from the lens faster than could be achieved by passive diffusion alone. In this review, we present how regional differences in lens AQP isoforms potentially contribute to this microcirculation system by highlighting current areas of investigation and emphasizing areas where future work is required.

Aquaporin 0 Modulates Lens Gap Junctions in the Presence of Lens-Specific Beaded Filament Proteins

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 (P_f) 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 P_f (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 P_f 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.

β1-Integrin Deletion From the Lens Activates Cellular Stress Responses Leading to Apoptosis and Fibrosis

Purpose

Previous research showed that the absence of β1-integrin from the mouse lens after embryonic day (E) 13.5 (β1MLR10) leads to the perinatal apoptosis of lens epithelial cells (LECs) resulting in severe microphthalmia. This study focuses on elucidating the molecular connections between β1-integrin deletion and this phenotype.

Methods

RNA sequencing was performed to identify differentially regulated genes (DRGs) in β1MLR10 lenses at E15.5. By using bioinformatics analysis and literature searching, Egr1 (early growth response 1) was selected for further study. The activation status of certain signaling pathways (focal adhesion kinase [FAK]/Erk, TGF-β, and Akt signaling) was studied via Western blot and immunohistochemistry. Mice lacking both β1-integrin and Egr1 genes from the lenses were created (β1MLR10/Egr1-/-) to study their relationship.

Results

RNA sequencing identified 120 DRGs that include candidates involved in the cellular stress response, fibrosis, and/or apoptosis. Egr1 was investigated in detail, as it mediates cellular stress responses in various cell types, and is recognized as an upstream regulator of numerous other β1MLR10 lens DRGs. In β1MLR10 mice, Egr1 levels are elevated shortly after β1-integrin loss from the lens. Further, pErk1/2 and pAkt are elevated in β1MLR10 LECs, thus providing the potential signaling mechanism that causes Egr1 upregulation in the mutant. Indeed, deletion of Egr1 from β1MLR10 lenses partially rescues the microphthalmia phenotype.

Conclusions

β1-integrin regulates the appropriate levels of Erk1/2 and Akt phosphorylation in LECs, whereas its deficiency results in the overexpression of Egr1, culminating in reduced cell survival. These findings provide insight into the molecular mechanism underlying the microphthalmia observed in β1MLR10 mice.

Applying bimolecular fluorescence complementation to screen and purify aquaporin protein:protein complexes

Protein:protein interactions play key functional roles in the molecular machinery of the cell. A major challenge for structural biology is to gain high‐resolution structural insight into how membrane protein function is regulated by protein:protein interactions. To this end we present a method to express, detect, and purify stable membrane protein complexes that are suitable for further structural characterization. Our approach utilizes bimolecular fluorescence complementation (BiFC), whereby each protein of an interaction pair is fused to nonfluorescent fragments of yellow fluorescent protein (YFP) that combine and mature as the complex is formed. YFP thus facilitates the visualization of protein:protein interactions in vivo, stabilizes the assembled complex, and provides a fluorescent marker during purification. This technique is validated by observing the formation of stable homotetramers of human aquaporin 0 (AQP0). The method's broader applicability is demonstrated by visualizing the interactions of AQP0 and human aquaporin 1 (AQP1) with the cytoplasmic regulatory protein calmodulin (CaM). The dependence of the AQP0‐CaM complex on the AQP0 C‐terminus is also demonstrated since the C‐terminal truncated construct provides a negative control. This screening approach may therefore facilitate the production and purification of membrane protein:protein complexes for later structural studies by X‐ray crystallography or single particle electron microscopy.

MALDI Imaging Mass Spectrometry Spatially Maps Age-Related Deamidation and Truncation of Human Lens Aquaporin-0

PURPOSE

To spatially map human lens Aquaporin-0 (AQP0) protein modifications, including lipidation, truncation, and deamidation, from birth through middle age using matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS).

METHODS

Human lens sections were water-washed to facilitate detection of membrane protein AQP0. We acquired MALDI images from eight human lenses ranging in age from 2 months to 63 years. In situ tryptic digestion was used to generate peptides of AQP0 and peptide images were acquired on a 15T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Peptide extracts were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and database searched to identify peptides observed in MALDI imaging experiments.

RESULTS

Unmodified, truncated, and fatty acid-acylated forms of AQP0 were detected in protein imaging experiments. Full-length AQP0 was fatty acid acylated in the core and cortex of young (2- and 4-month) lenses. Acylated and unmodified AQP0 were C-terminally truncated in older lens cores. Deamidated tryptic peptides (+0.9847 Da) were mass resolved from unmodified peptides by FTICR MS. Peptide images revealed differential localization of un-, singly-, and doubly-deamidated AQP0 C-terminal peptide (239-263). Deamidation was present at 4 months and increases with age. Liquid chromatography-MS/MS results indicated N246 undergoes deamidation more rapidly than N259.

CONCLUSIONS

Results indicated AQP0 fatty acid acylation and deamidation occur during early development. Progressive age-related AQP0 processing, including deamidation and truncation, was mapped in human lenses as a function of age. The localization of these modified AQP0 forms suggests where AQP0 functions may change throughout lens development and aging.

Spreading of porous vesicles subjected to osmotic shocks: the role of aquaporins

Aquaporin 0 (AQP0) is a transmembrane protein specific to the eye lens, involved as a water carrier across the lipid membranes. During eye lens maturation, AQP0s are truncated by proteolytic cleavage. We investigate in this work the capability of truncated AQP0 to conduct water across membranes. We developed a method to accurately determine water permeability across lipid membranes and across proteins from the deflation under osmotic pressure of giant unilamellar vesicles (GUVs) deposited on an adhesive substrate. Using reflection interference contrast microscopy (RICM), we measure the spreading area of GUVs during deswelling. We interpret these results using a model based on hydrodynamic, binder diffusion towards the contact zone, and Helfrich's law for the membrane tension, which allows us to relate the spread area to the vesicle internal volume. We first study the specific adhesion of vesicles coated with biotin spreading on a streptavidin substrate. We then determine the permeability of a single functional AQP0 and demonstrate that truncated AQP0 is no more a water channel.

Beyond water homeostasis: Diverse functional roles of mammalian aquaporins

BACKGROUND

Aquaporin (AQP) water channels are best known as passive transporters of water that are vital for water homeostasis.

SCOPE OF REVIEW

AQP knockout studies in whole animals and cultured cells, along with naturally occurring human mutations suggest that the transport of neutral solutes through AQPs has important physiological roles. Emerging biophysical evidence suggests that AQPs may also facilitate gas (CO2) and cation transport. AQPs may be involved in cell signalling for volume regulation and controlling the subcellular localization of other proteins by forming macromolecular complexes. This review examines the evidence for these diverse functions of AQPs as well their physiological relevance.

MAJOR CONCLUSIONS

As well as being crucial for water homeostasis, AQPs are involved in physiologically important transport of molecules other than water, regulation of surface expression of other membrane proteins, cell adhesion, and signalling in cell volume regulation.

GENERAL SIGNIFICANCE

Elucidating the full range of functional roles of AQPs beyond the passive conduction of water will improve our understanding of mammalian physiology in health and disease. The functional variety of AQPs makes them an exciting drug target and could provide routes to a range of novel therapies.

Role of Aquaporin 0 in lens biomechanics

Maintenance of proper biomechanics of the eye lens is important for its structural integrity and for the process of accommodation to focus near and far objects. Several studies have shown that specialized cytoskeletal systems such as the beaded filament (BF) and spectrin-actin networks contribute to mammalian lens biomechanics; mutations or deletion in these proteins alters lens biomechanics. Aquaporin 0 (AQP0), which constitutes ∼45% of the total membrane proteins of lens fiber cells, has been shown to function as a water channel and a structural cell-to-cell adhesion (CTCA) protein. Our recent ex vivo study on AQP0 knockout (AQP0 KO) mouse lenses showed the CTCA function of AQP0 could be crucial for establishing the refractive index gradient. However, biomechanical studies on the role of AQP0 are lacking. The present investigation used wild type (WT), AQP5 KO (AQP5(-/-)), AQP0 KO (heterozygous KO: AQP0(+/-); homozygous KO: AQP0(-/-); all in C57BL/6J) and WT-FVB/N mouse lenses to learn more about the role of fiber cell AQPs in lens biomechanics. Electron microscopic images exhibited decreases in lens fiber cell compaction and increases in extracellular space due to deletion of even one allele of AQP0. Biomechanical assay revealed that loss of one or both alleles of AQP0 caused a significant reduction in the compressive load-bearing capacity of the lenses compared to WT lenses. Conversely, loss of AQP5 did not alter the lens load-bearing ability. Compressive load-bearing at the suture area of AQP0(+/-) lenses showed easy separation while WT lens suture remained intact. These data from KO mouse lenses in conjunction with previous studies on lens-specific BF proteins (CP49 and filensin) suggest that AQP0 and BF proteins could act co-operatively in establishing normal lens biomechanics. We hypothesize that AQP0, with its prolific expression at the fiber cell membrane, could provide anchorage for cytoskeletal structures like BFs and together they help to confer fiber cell shape, architecture and integrity. To our knowledge, this is the first report identifying the involvement of an aquaporin in lens biomechanics. Since accommodation is required in human lenses for proper focusing, alteration in the adhesion and/or water channel functions of AQP0 could contribute to presbyopia.

Spatial distributions of AQP5 and AQP0 in embryonic and postnatal mouse lens development

The expression of the water channel protein aquaporin (AQP)-5 in adult rodent and human lenses was recently reported using immunohistochemistry, molecular biology, and mass spectrometry techniques, confirming a second transmembrane water channel that is present in lens fibre cells in addition to the abundant AQP0 protein. Interestingly, the sub-cellular distribution and level of post-translational modification of both proteins changes with fibre cell differentiation and location in the adult rodent lens. This study compares the sub-cellular distribution of AQP0 and AQP5 during embryonic and postnatal fibre cell development in the mouse lens to understand how the immunolabelling patterns for both AQPs observed in adult lens are first established. Immunohistochemistry was used to map the cellular and sub-cellular distribution of AQP5 and AQP0 throughout the lens in cryosections from adult (6 weeks-8 months) and postnatal (0-2 weeks) mouse lenses and in sections from paraffin embedded mouse embryos (E10-E19). All sections were imaged by fluorescence confocal microscopy. Using antibodies directed against the C-terminus of each AQP, AQP5 was abundantly expressed early in development, being found in the cytoplasm of cells of the lens vesicle and surrounding tissues (E10), while AQP0 was detected later (E11), and only in the membranes of elongating primary fibre cells. During the course of subsequent embryonic and postnatal development the pattern of cytoplasmic AQP5 and membranous AQP0 labelling was maintained until postnatal day 6 (P6). From P6 AQP5 labelling became progressively more membranous initially in the lens nucleus and then later in all regions of the lens, while AQP0 labelling was abruptly lost in the lens nucleus due to C-terminal truncation. Our results show that the spatial distribution patterns of AQP0 and AQP5 observed in the adult lens are established during a narrow window of postnatal development (P6-P15) that precedes eye opening and coincides with regression of the hyaloid vascular system. Our results support the hypothesis that, in the older fibre cells, insertion of AQP5 into the fibre cell membrane may compensate for any change in the functionality of AQP0 induced by truncation of its C-terminal tail.

Aquaporin 0 plays a pivotal role in refractive index gradient development in mammalian eye lens to prevent spherical aberration

Aquaporin 0 (AQP0) is a transmembrane channel that constitutes ∼45% of the total membrane protein of the fiber cells in mammalian lens. It is critical for lens transparency and homeostasis as mutations and knockout cause autosomal dominant lens cataract. AQP0 functions as a water channel and as a cell-to-cell adhesion (CTCA) molecule in the lens. Our recent in vitro studies showed that the CTCA function of AQP0 could be crucial to establish lens refractive index gradient (RING). However, there is a lack of in vivo data to corroborate the role of AQP0 as a fiber CTCA molecule which is critical for creating lens RING. The present investigation is undertaken to gather in vivo evidence for the involvement of AQP0 in developing lens RING. Lenses of wild type (WT) mouse, AQP0 knockout (heterozygous, AQP0(+/-)) and AQP0 knockout lens transgenically expressing AQP1 (heterozygous AQP0(+/)(-)/AQP1(+/)(-)) mouse models were used for the study. Data on AQP0 protein profile of intact and N- and/or C-terminal cleaved AQP0 in the lens by MALDI-TOF mass spectrometry and SDS-PAGE revealed that outer cortex fiber cells have only intact AQP0 of ∼28kDa, inner cortical and outer nuclear fiber cells have both intact and cleaved forms, and inner nuclear fiber cells have only cleaved forms (∼26-24kDa). Knocking out of 50% of AQP0 protein caused light scattering, spherical aberration (SA) and cataract. Restoring the lost fiber cell membrane water permeability (Pf) by transgene AQP1 did not reinstate complete lens transparency and the mouse lenses showed light scattering and SA. Transmission and scanning electron micrographs of lenses of both mouse models showed increased extracellular space between fiber cells. Water content determination study showed increase in water in the lenses of these mouse models. In summary, lens transparency, CTCA and compact packing of fiber cells were affected due to the loss of 50% AQP0 leading to larger extracellular space, more water content and SA, possibly due to alteration in RING. To our knowledge, this is the first report identifying the role of AQP0 in RING development to ward off lens SA during focusing.

A novel MIP gene mutation analysis in a Chinese family affected with congenital progressive punctate cataract

Congenital cataracts are one of the leading causes of visual impairment and blindness in children, and genetic factors play an important role in their development. This study aimed to identify the genetic defects associated with autosomal dominant congenital progressive punctate cataracts in a Chinese family and to explore the potential pathogenesis. Detailed family history and clinical data were recorded, and all the family members’ blood samples were collected for DNA extraction. Linkage analysis was performed by microsatellite markers that are associated with punctate cataracts, and logarithm (base 10) of odds (LOD) scores were calculated using the LINKAGE program. Positive two-point LOD scores were obtained at markers D12S1622 (Z_max = 2.71 at θ = 0.0), D12S1724 (Z_max = 2.71 at θ = 0.0), and D12S90 (Z_max = 2.71 at θ = 0.0), which flank the major intrinsic protein of lens fiber (MIP) gene on chromosomal region 12q13. Direct sequencing of the encoding region of the MIP gene revealed a novel mutation (G>D) in exon 4 at nucleotide 644, which caused a substitution of glycine to aspartic acid at codon 215 (p.G215D) for the MIP protein. The mutation cosegregated with all patients with congenital progressive punctate cataracts, but it was absent in the healthy members. Bioinformatics analysis predicted that the mutation affects the function of the MIP protein. The wild type (WT) and G215D mutant of MIP were transfected with green fluorescent protein (GFP) into Hela cells separately, and it was found that the G215D mutant was aberrantly located in the cytoplasm instead of in the plasma membrane. In summary, our study presented genetic and functional evidence linking the new MIP mutation of G215D to autosomal dominant congenital cataracts, which adds to the list of MIP mutations linked to congenital progressive punctate cataracts.