Expression of aquaporins in the rat ocular tissue

Computational approaches for identifications of altered ion channels in keratoconus

BACKGROUND

Keratoconus is an etiologically complex, degenerative corneal disease that eventually leads to loss of corneal integrity. Cells in corneal epithelium and endothelium express various types of ion channels that play important roles in ocular pathology. This emphasizes the need of understanding alterations of ion channels in keratoconus.

METHOD

Differential gene expression analysis was performed to identify deregulated ion channels in keratoconus patients using transcriptomic data. Thereafter correlation analysis of ion channel expression was performed to obtain the changed correlation between ion channels' expression in keratoconus patients versus control samples. Moreover, Protein-protein interaction networks and a pathway map was constructed to identify cellular processes altered due to the deregulation of ion channels. Furthermore, drugs interacting with deregulated ion channels were identified.

RESULTS

Total 75 ion channels were found to be deregulated in keratoconus, of which 12 were upregulated and 63 were downregulated. Correlations between ion channel expressions found to be different in control and keratoconus samples. Thereafter, protein-protein interactions network was generated to identify hub ion channels in network. Furthermore, the pathway map was constructed to depict calcium signalling, MAPK signalling, synthesis and secretion of cortisol, and cAMP signalling. The 19 FDA- approved drugs that interact with the 6 deregulated ion channels were identified.

CONCLUSION

Down-regulation of voltage-gated calcium channels can be attributed to reduced cell proliferation and differentiation. Additionally, deregulated ion channels in 3',5'- cyclic adenosine monophosphate signalling may be responsible for elevated cortisol level in progressive keratoconus patients.

Aquaporins in the Cornea

The cornea is an avascular, transparent tissue that allows light to enter the visual system. Accurate vision requires proper maintenance of the cornea's integrity and structure. Due to its exposure to the external environment, the cornea is prone to injury and must undergo proper wound healing to restore vision. Aquaporins (AQPs) are a family of water channels important for passive water transport and, in some family members, the transport of other small molecules; AQPs are expressed in all layers of the cornea. Although their functions as water channels are well established, the direct function of AQPs in the cornea is still being determined and is the focus of this review. AQPs, primarily AQP1, AQP3, and AQP5, have been found to play an important role in maintaining water homeostasis, the corneal structure in relation to proper hydration, and stress responses, as well as wound healing in all layers of the cornea. Due to their many functions in the cornea, the identification of drug targets that modulate the expression of AQPs in the cornea could be beneficial to promote corneal wound healing and restore proper function of this tissue crucial for vision.

Aquaporin 5 in the eye: Expression, function, and roles in ocular diseases

As a water channel protein, aquaporin 5 (AQP5) is essential for the maintenance of the normal physiological functions of ocular tissues. This review provides an overview of the expression and function of AQP5 in the eye and discusses their role in related eye diseases. Although AQP5 plays a vital role in ocular functions, such as maintaining corneal and lens transparency, regulating water movement, and maintaining homeostasis, some of its functions in ocular tissues are still unclear. Based on the key role of AQP5 in eye function, this review suggests that in the future, eye diseases may be treated by regulating the expression of aquaporin.

Role of aquaporins in corneal healing post chemical injury

Aquaporins (AQPs) are transmembrane water channel proteins that regulate the movement of water through the plasma membrane in various tissues including cornea. The cornea is avascular and has specialized microcirculatory mechanisms for homeostasis. AQPs regulate corneal hydration and transparency for normal vision. Currently, there are 13 known isoforms of AQPs that can be subclassified as orthodox AQPs, aquaglyceroporins (AQGPs), or supraquaporins (SAQPs)/unorthodox AQPs. AQPs are implicated in keratocyte function, inflammation, edema, angiogenesis, microvessel proliferation, and the wound-healing process in the cornea. AQPs play an important role in wound healing by facilitating the movement of corneal stromal keratocytes by squeezing through tight stromal matrix and narrow extracellular spaces to the wound site. Deficiency of AQPs can cause reduced concentration of hepatocyte growth factor (HGF) leading to reduced epithelial proliferation, reduced/impaired keratocyte migration, reduced number of keratocytes in the injury site, delayed and abnormal wound healing process. Dysregulated AQPs cause dysfunction in osmolar homeostasis as well as wound healing mechanisms. The cornea is a transparent avascular tissue that constitutes the anterior aspect of the outer covering of the eye and aids in two-thirds of visual light refraction. Being the outermost layer of the eye, the cornea is prone to injury. Of the 13 AQP isoforms, AQP1 is expressed in the stromal keratocytes and endothelial cells, and AQP3 and AQP5 are expressed in epithelial cells in the human cornea. AQPs can facilitate wound healing through aid in cellular migration, proliferation, migration, extracellular matrix (ECM) remodeling and autophagy mechanism. Corneal wound healing post-chemical injury requires an integrative and coordinated activity of the epithelium, stromal keratocytes, endothelium, ECM, and a battery of cytokines and growth factors to restore corneal transparency. If the chemical injury is mild, the cornea will heal with normal clarity, but severe injuries can lead to partial and/or permanent loss of corneal functions. Currently, the role of AQPs in corneal wound healing is poorly understood in the context of chemical injury. This review discusses the current literature and the role of AQPs in corneal homeostasis, wound repair, and potential therapeutic target for acute and chronic corneal injuries.

Aquaporin 5 maintains lens transparency by regulating the lysosomal pathway using circRNA

The lens is transparent, non-vascular, elastic and wrapped in a transparent capsule. The lens oppacity of AQP5^-/- mice was increased more than that of wild-type (AQP5^+/+ ) mice. In this study, we explored the potential functional role of circular RNAs (circRNAs) and transcription factor HSF4 in lens opacity in aquaporin 5 (AQP5) knockout (AQP5^-/- ) mice. Autophagy was impaired in the lens tissues of AQP5^-/- mice. Autophagic lysosomes in lens epithelial cells of AQP5^-/- mice were increased compared with AQP5^+/+ mice, based on analysis by transmission electron microscopy. The genetic information of the mice lens was obtained by high-throughput sequencing, and then the downstream genes were analysed. A circRNA-miRNA-mRNA network related to lysosomal pathway was constructed by the bioinformatics analysis of the differentially expressed circRNAs. Based on the prediction of the TargetScan website and the validation by dual luciferase reporter assay and RNA immunoprecipitation-qPCR, we found that circRNA (Chr16: 33421321-33468183+) inhibited the function of HSF4 by sponging microRNA (miR-149-5p), and it downregulated the normal expression of lysosome-related mRNAs. The accumulation of autophagic lysosome may be one of the reasons for the abnormal development of the lens in AQP5^-/- mice.

Neuromyelitis Optica Spectrum Disorder and Uveitis

BACKGROUND

Neuromyelitis Optica Spectrum Disorder (NMOSD) is a rare disease marked by CNS demyelination with a predilection for the optic nerve and spinal cord often resulting in severe vision loss. We aimed to characterize uveitis occurring in the setting of NMOSD.

METHODS

Retrospective chart review.

RESULTS

Of 572 NMOSD patients, 1% were found to have uveitis with a relative risk of 6.2 (95% confidence interval 3-14, p < .001) compared to the general population. The mean age of uveitis onset was 50 years, and that of NMOSD onset was 52 years. Bilateral anterior uveitis was the most common subtype and most patients were treated with rituximab for their NMOSD. A uveitis attack preceded onset of demyelination attacks in 67% of patients. Eyes without optic neuritis had a mean visual acuity at last follow-up of 20/22.

CONCLUSION

Uveitis is a rare complication of NMOSD, bilateral anterior uveitis was the most common subtype.

The Water Transport System in Astrocytes–Aquaporins

Highlights

(AQPs) are transmembrane proteins responsible for fast water movement across cell membranes, including those of astrocytes.

The expression and subcellular localization of AQPs in astrocytes are highly dynamic under physiological and pathological conditions.

Besides their primary function in water homeostasis, AQPs participate in many ancillary functions including glutamate clearance in tripartite synapses and cell migration.

Abstract

Astrocytes have distinctive morphological and functional characteristics, and are found throughout the central nervous system. Astrocytes are now known to be far more than just housekeeping cells in the brain. Their functions include contributing to the formation of the blood–brain barrier, physically and metabolically supporting and communicating with neurons, regulating the formation and functions of synapses, and maintaining water homeostasis and the microenvironment in the brain. Aquaporins (AQPs) are transmembrane proteins responsible for fast water movement across cell membranes. Various subtypes of AQPs (AQP1, AQP3, AQP4, AQP5, AQP8 and AQP9) have been reported to be expressed in astrocytes, and the expressions and subcellular localizations of AQPs in astrocytes are highly correlated with both their physiological and pathophysiological functions. This review describes and summarizes the recent advances in our understanding of astrocytes and AQPs in regard to controlling water homeostasis in the brain. Findings regarding the features of different AQP subtypes, such as their expression, subcellular localization, physiological functions, and the pathophysiological roles of astrocytes are presented, with brain edema and glioma serving as two representative AQP-associated pathological conditions. The aim is to provide a better insight into the elaborate “water distribution” system in cells, exemplified by astrocytes, under normal and pathological conditions.

Aquaporin 4 is not present in normal porcine and human lamina cribrosa

Aquaporin 4 is absent from astrocytes in the rodent optic nerve head, despite high expression in the retina and myelinated optic nerve. The purpose of this study was to quantify regional aquaporin channel expression in astrocytes of the porcine and human mouse optic nerve (ON). Ocular tissue sections were immunolabeled for aquaporins 1(AQP1), 4(AQP4), and 9(AQP9), myelin basic protein (MBP), glial fibrillary acidic protein (GFAP) and alpha-dystroglycan (αDG) for their presence in retina, lamina, myelin transition zone (MTZ, region just posterior to lamina) and myelinated ON (MON). Semi- quantification of AQP4 labeling & real-time quantitative PCR (qPCR) data were analyzed in retina and ON tissue. Porcine and control human eyes had abundant AQP4 in Müller cells, retinal astrocytes, and myelinated ON (MON), but minimal expression in the lamina cribrosa. AQP1 and AQP9 were present in retina, but not in the lamina. Immunolabeling of GFAP and αDG was similar in lamina, myelin transition zone (MTZ) and MON regions. Semi-quantitative AQP4 labeling was at background level in lamina, increasing in the MTZ, and highest in the MON (lamina vs MTZ, MON; p≤0.05, p≤0.01, respectively). Expression of AQP4 mRNA was minimal in lamina and substantial in MTZ and MON, while GFAP mRNA expression was uniform among the lamina, MTZ, and MON regions. Western blot assay showed AQP4 protein expression in the MON samples, but none was detected in the lamina tissue. The minimal presence of AQP4 in the lamina is a specific regional phenotype of astrocytes in the mammalian optic nerve head.

Lens Aquaporins in Health and Disease: Location is Everything!

Cataract and presbyopia are the leading cause of vision loss and impaired vision, respectively, worldwide. Changes in lens biochemistry and physiology with age are responsible for vision impairment, yet the specific molecular changes that underpin such changes are not entirely understood. In order to preserve transparency over decades of life, the lens establishes and maintains a microcirculation system (MCS) that, through spatially localized ion pumps, induces circulation of water and nutrients into (influx) and metabolites out of (outflow and efflux) the lens. Aquaporins (AQPs) are predicted to play important roles in the establishment and maintenance of local and global water flow throughout the lens. This review discusses the structure and function of lens AQPs and, importantly, their spatial localization that is likely key to proper water flow through the MCS. Moreover, age-related changes are detailed and their predicted effects on the MCS are discussed leading to an updated MCS model. Lastly, the potential therapeutic targeting of AQPs for prevention or treatment of cataract and presbyopia is discussed.

AQP5 regulates vimentin expression via miR-124-3p.1 to protect lens transparency

The pathogenesis of congenital cataract (CC), a major disease associated with blindness in infants, is complex and diverse. Aquaporin 5 (AQP5) represents an essential membrane water channel. In the present study, whole exome sequencing revealed a novel heterozygous missense mutation of AQP5 (c.152 T > C, p. L51P) in the four generations of the autosomal dominant CC (adCC) family. By constructing a mouse model of AQP5 knockout (KO) using the CRISPR/Cas9 technology, we observed that the lens of AQP5-KO mice showed mild opacity at approximately six months of age. miR-124-3p.1 expression was identified to be downregulated in the lens of AQP5-KO mice as evidenced by qRT-PCR analysis. A dual luciferase reporter assay confirmed that vimentin was a target gene of miR-124-3p.1. Organ-cultured AQP5-KO mouse lenses were showed increased opacity compared to those of WT mice, and vimentin expression was upregulated as determined by RT-PCR, western blotting, and immunofluorescence staining. After miR-124-3p.1 agomir was added, the lens opacity in WT mice and AQP5-KO mice decreased, accompanied by the downregulation of vimentin. AQP5-L51P increased vimentin expression of in human lens epithelial cells. Therefore, a missense mutation in AQP5 (c.152 T > C, p. L51P) was associated with adCC, and AQP5 could participate in the maintenance of lens transparency by regulating vimentin expression via miR-124-3p.1.

The role of aquaporin-4 in optic nerve head astrocytes in experimental glaucoma

PURPOSE

To study aquaporin channel expression in astrocytes of the mouse optic nerve (ON) and the response to IOP elevation in mice lacking aquaporin 4 (AQP4 null).

METHODS

C57BL/6 (B6) and AQP4 null mice were exposed to bead-induced IOP elevation for 3 days (3D-IOP), 1 and 6 weeks. Mouse ocular tissue sections were immunolabeled against aquaporins 1(AQP1), 4(AQP4), and 9(AQP9). Ocular tissue was imaged to identify normal AQP distribution, ON changes, and axon loss after IOP elevation. Ultrastructure examination, cell proliferation, gene expression, and transport block were also analyzed.

RESULTS

B6 mice had abundant AQP4 expression in Müller cells, astrocytes of retina and myelinated ON (MON), but minimal AQP4in prelaminar and unmyelinated ON (UON). MON of AQP4 nulls had smaller ON area, smaller axon diameter, higher axon density, and larger proportionate axon area than B6 (all p≤0.05). Bead-injection led to comparable 3D-IOP elevation (p = 0.42) and axonal transport blockade in both strains. In B6, AQP4 distribution was unchanged after 3D-IOP. At baseline, AQP1 and AQP9 were present in retina, but not in UON and this was unaffected after IOP elevation in both strains. In 3D-IOP mice, ON astrocytes and microglia proliferated, more in B6 than AQP4 null. After 6 week IOP elevation, axon loss occurred equally in the two mouse types (24.6%, AQP4 null vs. 23.3%, B6).

CONCLUSION

Lack of AQP4 was neither protective nor detrimental to the effects of IOP elevation. The minimal presence of AQP4 in UON may be a vital aspect of the regionally specific phenotype of astrocytes in the mouse optic nerve head.

Altered Iris Aquaporin Expression and Aqueous Humor Osmolality in Glaucoma

Purpose

Aquaporins (AQPs) facilitate transmembrane osmotic water transport and may play a role in iris fluid conductivity, which is implicated in the pathophysiology of glaucoma. In this study, we compared the iris expression of AQPs and aqueous osmolality between primary angle closure glaucoma (PACG), primary open-angle glaucoma (POAG), and nonglaucoma eyes.

Methods

AQP1-5 transcripts from a cohort of 36 PACG, 34 POAG and 26 nonglaucoma irises were measured by quantitative real-time PCR. Osmolality of aqueous humor from another cohort of 49 PACG, 50 POAG, and 50 nonglaucoma eyes were measured using an osmometer. The localization of AQP1 in both glaucoma and nonglaucoma irises was determined by immunofluorescent analysis.

Results

Of the five AQP genes evaluated, AQP1 and AQP2 transcripts were significantly upregulated in both PACG (3.48- and 8.07-fold, respectively) and POAG (3.12- and 11.58-fold, respectively) irises relative to nonglaucoma counterparts. The aqueous osmolalities of PACG (303.68 mmol/kg) and POAG (300.79 mmol/kg) eyes were significantly lower compared to nonglaucoma eyes (312.6 mmol/kg). There was no significant difference in expression of AQP transcripts or aqueous osmolality between PACG and POAG eyes.

Conclusions

PACG and POAG eyes featured significant increase in AQP1 and AQP2 expression in the iris and reduced aqueous osmolality compared to nonglaucoma eyes. These findings suggest that the iris may be involved in altered aqueous humor dynamics in glaucoma pathophysiology. Because PACG did not differ from POAG in both properties studied, it is likely that they are common to glaucoma disease in general.

Bumetanide Suppression of Angiogenesis in a Rat Model of Oxygen-Induced Retinopathy

Aquaporins (AQPs) are involved in hypoxia-induced angiogenesis and retinal damage. Bumetanide is a diuretic agent, Na⁺/K⁺/Cl⁻ cotransporter (NKCC1), and AQP 1–4 inhibitor. We tested the hypothesis that early postnatal treatment with bumetanide suppresses biomarkers of angiogenesis and decreases severe retinopathy oxygen-induced retinopathy (OIR). Neonatal rats were exposed at birth (P0) to either (1) room air (RA); (2) hyperoxia (50% O₂); or (3) intermittent hypoxia (IH) consisting of 50% O₂ with brief, clustered episodes of 12% O₂ from P0 to postnatal day 14 (P14), during which they were treated intraperitoneally (IP) with bumetanide (0.1 mg/kg/day) or an equivalent volume of saline, on P0–P2. Pups were examined at P14 or allowed to recover in RA from P14–P21. Retinal angiogenesis, morphometry, pathology, AQPs, and angiogenesis biomarkers were determined at P14 and P21. Bumetanide reduced vascular abnormalities associated with severe OIR. This was associated with reductions in AQP-4 and VEGF. Bumetanide suppressed sVEGFR-1 in the serum and vitreous fluid, but levels were increased in the ocular tissues during recovery. Similar responses were noted for IGF-I. In this model, early systemic bumetanide administration reduces severe OIR, the benefits of which appear to be mediated via suppression of AQP-4 and VEGF. Further studies are needed to determine whether bumetanide at the right doses may be considered a potential pharmacologic agent to treat retinal neovascularization.

Relationship between Aquaporin-1 Protein Expression and Choroidal Thickness during the Recovery of Form-deprivation Myopia in Guinea Pigs

Purpose: The purpose of this study was to investigate the relationship between aquaporin-1 (AQP-1) protein expression in the choroid and choroid thickness (CT) during the recovery of form-deprivation (FD) myopia in guinea pigs.Materials and Methods: Seventy-two guinea pigs were randomly assigned to the normal control (NC) group, FD 21 group (animals wore a latex facemask in the right eye for 21 days to induce FD myopia) and four recovery (REC) groups. Guinea pigs in the REC groups also wore the facemask for 21 days to induce myopia; then, the facemask was removed, and the eye was re-exposed to the normal environment for 12 hours (REC ½ group), 1 day (REC 1 group), 2 days (REC 2 group), and 7 days (REC 7 group). All animals underwent biometric measurements (refraction, axial length, and CT), and the protein expression of AQP-1 in the choroid was determined using western blotting.Results: The protein expression of AQP-1 and CT were significantly decreased in the FD 21 group as compared with those in the NC group (p = .007 and p < .001). Both AQP-1 protein expression and CT gradually increased and peaked in the REC 2 group. Additionally, there were significant differences in AQP-1 protein expression and CT between the REC 2 group and all other groups (all p < .05). We observed a complete recovery in the in REC 7 group as compared with the NC group (p > .05). AQP-1 protein expression was significantly associated with CT (p = .001) in all groups; however, there was a significant negative correlation (p = .029) between AQP-1 protein expression and axial length in the REC groups.Conclusions: AQP-1 protein expression in the choroid was upregulated following recovery of FD myopia in guinea pigs, and these changes correlated with alterations in CT and axial length.

Aquaporin 5 promotes corneal wound healing

Aquaporins (AQPs), ordinarily regarded as water channels, have recently been shown to participate in other cellular functions such as cell-to-cell adhesion, cell migration, cell proliferation etc. The current investigation was undertaken to find out whether AQP5 water channel plays a role in corneal epithelial wound healing. Expression of AQP5 in mouse cornea and transfected Madin-Darby canine kidney (MDCK) cells was detected using immunofluorescence or EGFP tag. Cell migration and proliferation, the two major events in wound healing, were studied in vitro using cell culture scratch-wound healing model and cell proliferation assay, in vivo by conducting wound healing experiments on corneas of wild-type and AQP5 knockout mouse model and ex vivo on corneal epithelial cells isolated from wild type and AQP5 knockout mice. MDCK cells stably expressing AQP5 showed significantly higher levels of cell migration and proliferation compared to control cells. Likewise, corneal epithelial cells of wild type mouse with innate AQP5 exhibited faster wound healing than those of AQP5 knockout in vivo and under ex vivo culture conditions. In vitro, in vivo and ex vivo studies showed that presence of AQP5 improved cell migration, proliferation and wound healing. The data collected suggest that AQP5 plays a significant role in corneal epithelial wound healing.

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.

Dynamic functional contribution of the water channel AQP5 to the water permeability of peripheral lens fiber cells

Although the functionality of the lens water channels aquaporin 1 (AQP1; epithelium) and AQP0 (fiber cells) is well established, less is known about the role of AQP5 in the lens. Since in other tissues AQP5 functions as a regulated water channel with a water permeability (P_H2O) some 20 times higher than AQP0, AQP5 could function to modulate P_H2O in lens fiber cells. To test this possibility, a fluorescence dye dilution assay was used to calculate the relative P_H2O of epithelial cells and fiber membrane vesicles isolated from either the mouse or rat lens, in the absence and presence of HgCl₂, an inhibitor of AQP1 and AQP5. Immunolabeling of lens sections and fiber membrane vesicles from mouse and rat lenses revealed differences in the subcellular distributions of AQP5 in the outer cortex between species, with AQP5 being predominantly membranous in the mouse but predominantly cytoplasmic in the rat. In contrast, AQP0 labeling was always membranous in both species. This species-specific heterogeneity in AQP5 membrane localization was mirrored in measurements of P_H2O, with only fiber membrane vesicles isolated from the mouse lens, exhibiting a significant Hg²⁺-sensitive contribution to P_H2O. When rat lenses were first organ cultured, immunolabeling revealed an insertion of AQP5 into cortical fiber cells, and a significant increase in Hg²⁺-sensitive P_H2O was detected in membrane vesicles. Our results show that AQP5 forms functional water channels in the rodent lens, and they suggest that dynamic membrane insertion of AQP5 may regulate water fluxes in the lens by modulating P_H2O in the outer cortex.

Müller cells and diabetic retinopathy

Müller cells are one of the primary glial cell types found in the retina and play a significant role in maintaining retinal function and health. Since Müller cells are the only cell type to span the entire width of the retina and have contact to almost every cell type in the retina they are uniquely positioned to perform a wide variety of functions necessary to maintaining retinal homeostasis. In the healthy retina, Müller cells recycle neurotransmitters, prevent glutamate toxicity, redistribute ions by spatial buffering, participate in the retinoid cycle, and regulate nutrient supplies by multiple mechanisms. Any disturbance to the retinal environment is going to influence proper Müller cell function and well being which in turn will affect the entire retina. This is evident in a disease like diabetic retinopathy where Müller cells contribute to neuronal dysfunction, the production of pro-angiogenic factors leading to neovascularization, the set up of a chronic inflammatory retinal environment, and eventual cell death. In this review, we highlight the importance of Müller cells in maintaining a healthy and functioning retina and discuss various pathological events of diabetic retinopathy in which Müller cells seem to play a crucial role. The beneficial and detrimental effects of cytokine and growth factor production by Müller cells on the microvasculature and retinal neuronal tissue will be outlined. Understanding Müller cell functions within the retina and restoring such function in diabetic retinopathy should become a cornerstone for developing effective therapies to treat diabetic retinopathy.

Immunohistochemical evalulation of activated Ras and Rac1 as potential downstream effectors of aquaporin-5 in breast cancer in vivo

Aberrant levels of aquaporin-5 (AQP5) expression have been observed in several types of cancer, including breast cancer, where AQP5 overexpression is associated with metastasis and poor prognosis. In cultured cancer cells, AQP5 facilitates cell migration and activates Ras signaling. Both increased cell migration and Ras activation are associated with cancer metastasis, but so far it is unknown if AQP5 also affects these processes in vivo. Therefore, we investigated if high AQP5 expression in breast cancer tissue correlated with increased activation of Ras and of Rac1, which is a GTPase also involved in cell migration. This was accomplished by immunohistochemical analysis of invasive ductal carcinoma of breast tissue sections from human patients, followed by qualitative and quantitative correlation analysis between AQP5 and activated Ras and Rac1. Immunohistochemistry revealed that activation of Ras and Rac1 was positively correlated. There was, however, no correlation between high AQP5 expression and activation of Ras, whereas a nonsignificant, but positive, tendency between the levels of AQP5 and activated Rac1 levels was observed. In summary, this is the first report that correlates AQP5 expression levels to downstream signaling partners in breast cancer tissue sections. The results suggest Rac1 as a potential downstream signaling partner of AQP5 in vivo.

Aquaporins: Novel Targets for Age-Related Ocular Disorders

Aquaporins (AQPs), a large family of membrane protein channels that facilitate transport of water and other small solutes, play important roles in physiological functions and human diseases. Up till now, 13 types of AQPs, numbered 0 through 12, have been identified in various mammalian tissues. Homologous genes for AQPs in amphibians, insects, and bacteria highlight the evolutionary conservation and, thus, the importance of these membrane channels. Many members of the AQP family are expressed in the eye. AQP1, which is a water-selective channel, is expressed in the anterior chamber (cornea, ciliary body, trabecular meshwork) and posterior chamber (retina and microvessels in choroid), controlling the fluid homeostasis in the eye. Mice knockout studies have indicated that AQP1 plays an important function in the eye by suggesting its role in aqueous humor dynamics and retina angiogenesis. This review will focus on the role of AQP1 as a novel target for ocular disorders such as glaucoma and age-related macular degeneration, and it will discuss challenges and advances in identifying modulators of AQP1 function that could be useful in clinical applications.

The role of aquaporin-5 in cancer cell migration: A potential active participant

Emerging data identifies the water channel aquaporin-5 as a major player in multiple cancers. Over-expression of aquaporin-5 has been associated with increased metastasis and poor prognosis, suggesting that aquaporin-5 may enhance cancer cell migration. This review aims to highlight the current knowledge and hypothesis regarding downstream signaling partners of aquaporin-5 in relation to cancer cell migration. The molecular mechanisms that link aquaporin-5 to cell migration are not completely understood. Aquaporin-5 may promote cell movement by increasing water uptake into the front of the cell allowing local swelling. Aquaporin-5 may also activate extracellular-regulated kinases, increasing proliferation and potentially stimulating the migration machinery. Thus, further studies are warranted to identify the underlying mechanisms and signaling pathways. This will reveal whether aquaporin-5 and downstream effectors could be targets for developing new cancer therapeutics.

Expression of Aquaporin-6 in Rat Retinal Ganglion Cells

Several aquaporins (AQPs) have been identified to be present in the eyes, and it has been suggested that they are involved in the movement of water and small solutes. AQP6, which has low water permeability and transports mainly anions, was recently discovered in the eyes. In the present study, we investigate the localization of AQP6 in the rat retina and show that AQP6 is selectively localized to the ganglion cell layer and the outer plexiform layer. Along with the gradual decrease in retinal ganglion cells after a crushing injury of optic nerve, immunofluorescence signals of AQP6 gradually decreased. Confocal microscope images confirmed AQP6 expression in retinal ganglion cells and Müller cells in vitro. Therefore, AQP6 might participate in water and anion transport in these cells.

Expression and Distribution Pattern of Aquaporin 4, 5 and 11 in Retinas of 15 Different Species

Aquaporins (AQPs) are small integral membrane proteins with 13 members in mammals and are essential for water transport across membranes. They are found in many different tissues and cells. Currently, there are conflicting results regarding retinal aquaporin expression and subcellular localization between genome and protein analyses and among various species. AQP4, 7, 9 and 11 were described in the retina of men; whereas AQP6, 8 and 10 were earlier identified in rat retinas and AQP4, 5 and 11 in horses. Since there is a lack of knowledge regarding AQP expression on protein level in retinas of different animal models, we decided to analyze retinal cellular expression of AQP4, 5 and 11 in situ with immunohistochemistry. AQP4 was detected in all 15 explored species, AQP5 and AQP11 in 14 out of 15. Interestingly, AQP4 was unambiguously expressed in Muller glial cells, whereas AQP5 was differentially allocated among the species analyzed. AQP11 expression was Muller glial cell-specific in 50% of the animals, whereas in the others, AQP11 was detected in ganglion cell layer and at photoreceptor outer segments. Our data indicate a disparity in aquaporin distribution in retinas of various animals, especially for AQP5 and 11.

Aquaporin expression and localization in the rabbit eye

Aquaporins (AQPs) are important for ocular homeostasis and function. While AQP expression has been investigated in ocular tissues of human, mouse, rat and dog, comprehensive data in rabbits are missing. As rabbits are frequently used model organisms in ophthalmic research, the aim of this study was to analyze mRNA expression and to localize AQPs in the rabbit eye. The results were compared with the data published for other species. In cross sections of New Zealand White rabbit eyes AQP0 to AQP5 were labeled by immunohistology and analyzed by confocal microscopy. Immunohistological findings were compared to mRNA expression levels, which were analyzed by quantitative reverse transcription real time polymerase chain reaction (qRT-PCR). The primers used were homologous against conserved regions of AQPs. In the rabbit eye, AQP0 protein expression was restricted to the lens, while AQP1 was present in the cornea, the chamber angle, the iris, the ciliary body, the retina and, to a lower extent, in optic nerve vessels. AQP3 and AQP5 showed immunopositivity in the cornea. AQP3 was also present in the conjunctiva, which could not be confirmed for AQP5. However, at a low level AQP5 was also traceable in the lens. AQP4 protein was detected in the ciliary non-pigmented epithelium (NPE), the retina, optic nerve astrocytes and extraocular muscle fibers. For most tissues the qRT-PCR data confirmed the immunohistology results and vice versa. Although species differences exist, the AQP protein expression pattern in the rabbit eye shows that, especially in the anterior section, the AQP distribution is very similar to human, mouse, rat and dog. Depending on the ocular regions investigated in rabbit, different protein and mRNA expression results were obtained. This might be caused by complex gene regulatory mechanisms, post-translational protein modifications or technical limitations. However, in conclusion the data suggest that the rabbit is a useful in-vivo model to study AQP function and the effects of direct and indirect intervention strategies to investigate e. g. mechanisms for intraocular pressure modulation or cornea transparency regulation.

Is the main lacrimal gland indispensable? Contributions of the corneal and conjunctival epithelia

The ocular surface system is responsible for ensuring that the precorneal tear film is sufficient in both quality and quantity to preserve optimal vision. Tear secretion is a complex, multifactorial process, and dysfunction of any component of the ocular surface system can result in tear film instability and hyperosmolarity with resultant dry eye disease. The tear film is primarily composed of lipids, aqueous, and mucins, with aqueous accounting for most of its thickness. The aqueous is produced by the main lacrimal gland, accessory lacrimal glands, and corneal and conjunctival epithelia. Although the main lacrimal gland has long been considered an indispensable source of the aqueous component of tears, there is evidence that adequate tear secretion can exist in the absence of the main lacrimal gland. We review and discuss the basics of tear secretion, the tear secretory capacity of the ocular surface, and emerging treatments for dry eye disease.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

Differential Expression of AQP1 and AQP4 in Avascular Chick Retina Exposed to Moderate Light of Variable Photoperiods

Aquaporins (AQPs) are integral membrane proteins which maintain cellular water and ion homeostasis. Alterations in AQP expression have been reported in rod-dominated rodent retinas exposed to light. In rodents and also in birds, light of moderate intensities (700-2000 lux) damages the retina, though detailed changes were not examined in birds. The aim of our study was to see if light affects cone dominated retinas, which would be reflected in expression levels of AQPs. We examined AQP1 and AQP4 expressions in chick retina exposed to 2000 lux under 12 h light:12 h dark (12L:12D; normal photoperiod), 18L:6D (prolonged photoperiod) and 24L:0D (constant light). Additionally, morphological changes, apoptosis (by TUNEL) and levels of glutamate and GFAP (a marker of injury) in the retina were examined to correlate these with AQP expressions. Constant light caused damage in outer and inner nuclear layer (ONL, INL) and ganglion cell layer (GCL). Also, there were associated increases in GFAP and glutamate levels in retinal extracts. In normal photoperiod, AQP1 was expressed in GCL, outer part of INL and photoreceptor inner segments of. AQP4 was additionally expressed in nerve fiber layer. Immunohistochemistry and Western blotting revealed over all decreased AQP1 and AQP4 expression in constant light condition compared to those in other two groups. The elevated GFAP and glutamate levels might be involved in the reduction of AQPs in constant light group. Such decreases in AQP expressions are perhaps linked with retinal cell damage seen in constant light condition, while their relatively enhanced expression in two other conditions may help in maintaining a normal retinal architecture, indicating their neuroprotective potential.

The Neuroepithelium Disruption Could Generate Autoantibodies against AQP4 and Cause Neuromyelitis Optica and Hydrocephalus

Neuromyelitis optica is an inflammatory disease characterized by neuritis and myelitis of the optic nerve. Its physiopathology is connected with the aquaporin-4 water channel, since antibodies against aquaporin-4 have been found in the cerebrospinal fluid and blood of neuromyelitis optica patients. The seropositivity for aquaporin-4 antibodies is used for the diagnosis of neuromyelitis optica or neuromyelitis optica spectrum disease. On the other hand, aquaporin-4 is expressed in astrocyte feet in the brain-blood barrier and subventricular zones of the brain ventricles. Aquaporin-4 expression is high in cerebrospinal fluid in hydrocephalus. Furthermore, neuroepithelial denudation precedes noncommunicating hydrocephalus and this neuroepithelial disruption could allow aquaporin-4 to reach anomalous brain areas where it is unrecognized and induce the generation of aquaporin-4 antibodies which could cause the neuromyelitis optica and certain types of hydrocephalus.

Distribution of aquaporin genes and selection of individual reference genes for quantitative real-time RT-PCR analysis in multiple tissues of the mouse

Aquaporins (AQPs) are a family of water-transporting proteins that are selectively expressed in epithelial, endothelial, and many other cell types of various tissues, where they play important physiological functions. However, the accurate distribution of AQP gene expression has not yet been examined in various tissues of the mouse. We first evaluated the tissue distribution of AQP gene expression using tongue, nasal epithelium, bronchus, trachea, lung, esophagus, stomach, ileum, transverse colon, liver, pancreas, whole blood, thigh muscle, spinal cord, brain, thoracic aorta, heart, kidney, thymus, spleen, skin, eye, and testis of the mouse. Furthermore, for a quantitative analysis, we selected appropriate reference genes for normalized qRT-PCR data in various tissues. The stability of the reference genes was assessed using NormFinder. The stably expressed genes identified in the present study were 18s rRNA. When 18s rRNA was used, as the best reference gene in the present study, the genes for AQPs 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, and 12 were notably expressed in the eye, lung, testis, eye, spinal cord, trachea, kidney, testis, testis, testis, testis, and pancreas. These results, regarding the distribution of AQPs, suggest that AQPs may be involved in various physiological and pathophysiological processes.

Water permeability of the mammalian cochlea: functional features of an aquaporin-facilitated water shunt at the perilymph-endolymph barrier

The cochlear duct epithelium (CDE) constitutes a tight barrier that effectively separates the inner ear fluids, endolymph and perilymph, thereby maintaining distinct ionic and osmotic gradients that are essential for auditory function. However, in vivo experiments have demonstrated that the CDE allows for rapid water exchange between fluid compartments. The molecular mechanism governing water permeation across the CDE remains elusive. We computationally determined the diffusional (PD) and osmotic (Pf) water permeability coefficients for the mammalian CDE based on in silico simulations of cochlear water dynamics integrating previously derived in vivo experimental data on fluid flow with expression sites of molecular water channels (aquaporins, AQPs). The PD of the entire CDE (PD = 8.18 × 10(-5) cm s(-1)) and its individual partitions including Reissner's membrane (PD = 12.06 × 10(-5) cm s(-1)) and the organ of Corti (PD = 10.2 × 10(-5) cm s(-1)) were similar to other epithelia with AQP-facilitated water permeation. The Pf of the CDE (Pf = 6.15 × 10(-4) cm s(-1)) was also in the range of other epithelia while an exceptionally high Pf was determined for an epithelial subdomain of outer sulcus cells in the cochlear apex co-expressing AQP4 and AQP5 (OSCs; Pf = 156.90 × 10(-3) cm s(-1)). The Pf/PD ratios of the CDE (Pf/PD = 7.52) and OSCs (Pf/PD = 242.02) indicate an aqueous pore-facilitated water exchange and reveal a high-transfer region or "water shunt" in the cochlear apex. This "water shunt" explains experimentally determined phenomena of endolymphatic longitudinal flow towards the cochlear apex. The water permeability coefficients of the CDE emphasise the physiological and pathophysiological relevance of water dynamics in the cochlea in particular for endolymphatic hydrops and Ménière's disease.

Aquaporins in the eye: expression, function, and roles in ocular disease

BACKGROUND

All thirteen known mammalian aquaporins have been detected in the eye. Moreover, aquaporins have been identified as playing essential roles in ocular functions ranging from maintenance of lens and corneal transparency to production of aqueous humor to maintenance of cellular homeostasis and regulation of signal transduction in the retina.

SCOPE OF REVIEW

This review summarizes the expression and known functions of ocular aquaporins and discusses their known and potential roles in ocular diseases.

MAJOR CONCLUSIONS

Aquaporins play essential roles in all ocular tissues. Remarkably, not all aquaporin function as a water permeable channel and the functions of many aquaporins in ocular tissues remain unknown. Given their vital roles in maintaining ocular function and their roles in disease, aquaporins represent potential targets for future therapeutic development.

GENERAL SIGNIFICANCE

Since aquaporins play key roles in ocular physiology, an understanding of these functions is important to improving ocular health and treating diseases of the eye. It is likely that future therapies for ocular diseases will rely on modulation of aquaporin expression and/or function. This article is part of a Special Issue entitled Aquaporins.

Aquaporin 5 knockout mouse lens develops hyperglycemic cataract

The scope of this investigation was to understand the role of aquaporin 5 (AQP5) for maintaining lens transparency and homeostasis. Studies were conducted using lenses of wild-type (WT) and AQP5 knockout (AQP5-KO) mice. Immunofluorescent staining verified AQP5 expression in WT lens sections and lack of expression in the knockout. In vivo and ex vivo, AQP5-KO lenses resembled WT lenses in morphology and transparency. Therefore, we subjected the lenses ex vivo under normal (5.6mM glucose) and hyperglycemic (55.6mM glucose) conditions to test for cataract formation. Twenty-four hours after incubation in hyperglycemic culture medium, AQP5-KO lenses showed mild opacification which was accelerated several fold at 48 h; in contrast, WT lenses remained clear even after 48 h of hyperglycemic treatment. AQP5-KO lenses displayed osmotic swelling due to increase in water content. Cellular contents began to leak into the culture medium after 48 h. We reason that water influx through glucose transporters and glucose cotransporters into the cells could mainly be responsible for creating hyperglycemic osmotic swelling; absence of AQP5 in fiber cells appears to cause lack of required water efflux, challenging cell volume regulation and adding to osmotic swelling. This study reveals that AQP5 could play a critical role in lens microcirculation for maintaining transparency and homeostasis, especially by providing protection under stressful conditions. To the best of our knowledge, this is the first report providing evidence that AQP5 facilitates maintenance of lens transparency and homeostasis by regulating osmotic swelling caused by glucose transporters and cotransporters under hyperglycemic stressful conditions.

Age-related changes of aquaporin expression patterns in the postnatal rat retina

Previous studies revealed that the rat retina contains numerous membrane-located water channels, the aquaporins (AQPs). Protein expression patterns of AQP1-4, 6 and 9 were examined by immunohistochemistry. In the present study, we investigated the immunolocalization of AQP1-4, 6 and 9 during postnatal development in the rat retina and examined the effect of age on the tissue distribution of these channels. AQP1, 3, 4, 6 and 9 showed gradually increased expression in rat retinas from postnatal week 1 to week 12, and decreased in the 40-week-old rat retinas. AQP2 expression was barely seen in the first week in rat retinas and displayed a significant increase from week 1 to week 4, however no significant alteration of AQP2 was observed after 4weeks of development. AQP1 and 4 immunoreactivities were present in the inner limiting membrane (ILM), the ganglion cell layer (GCL), inner nuclear layer (INL) and retinal pigment epithelium (RPE) in the 4-, 12- and 40-week-old rat retinas. The RPE, OLM and ILM showed a remarkable expression of AQP1-4, 6 and 9 in the 4, 12 and 40-week-old rat retinas. The reduced expression of AQPs in aged rat retinas may indicate the involvement of AQPs in the pathogenesis of age-related retinal diseases.

Verification and spatial localization of aquaporin-5 in the ocular lens

Until recently, the lens was thought to express only two aquaporin (AQP) water channels, AQP1 and AQP0. In this study we confirm lenticular AQP5 protein expression by Western blotting and mass spectrometry in lenses from a variety of species. In addition, confocal microscopy was used to map cellular distributions of AQP5 in mouse, rat and human lenses. Tandem mass spectrometry of a human lens membrane preparation revealed extensive sequence coverage (56.2%) of AQP5. Western blotting performed on total fiber cell membranes from mouse, rat, bovine and human lenses confirmed AQP5 protein expression is conserved amongst species. Western blotting of dissected lens fractions suggests that AQP5 is processed in the lens core by C-terminal truncation. Immunohistochemistry showed that AQP5 signal was most abundant in the lens outer cortex and decreased in intensity in the lens core. Furthermore, AQP5 undergoes differentiation-dependent changes in subcellular location from an intracellular localization in differentiating fiber cells to the plasma membrane of mature fiber cells upon the loss of fiber cell nuclei. Our results show that AQP5 is a significant component of lens fiber cell membranes, representing the second most abundant water channel in these cells. Together, the changes to AQP5 distribution and structure are likely to modulate the functional role of AQP5 in different regions of the lens.

A novel look at astrocytes: aquaporins, ionic homeostasis, and the role of the microenvironment for regeneration in the CNS

Aquaporin-4 (AQP4) water channels are located at the basolateral membrane domain of many epithelial cells involved in ion transport and secretion. These epithelial cells separate fluid compartments by forming apical tight junctions. In the brain, AQP4 is located on astrocytes in a polarized distribution: At the border to blood vessels or the pial surface, its density is very high. During ontogeny and phylogeny, astroglial cells go through a stage of expressing tight junctions, separating fluid compartments differently than in adult mammals. In adult mammals, this barrier is formed by arachnoid, choroid plexus, and endothelial cells. The ontogenetic and phylogenetic barrier transition from glial to endothelial cells correlates with the regenerative capacity of neuronal structures: Glial cells forming tight junctions, and expressing no or unpolarized AQP4 are found in the fish optic nerve and the olfactory nerve in mammals both known for their regenerative ability. It is hypothesized that highly polarized AQP4 expression and the lack of tight junctions on astrocytes increase ionic homeostasis, thus improving neuronal performance possibly at the expense of restraining neurogenesis and regeneration.

The role of Eph receptors in lens function and disease

Cataract is the single largest contributor to blindness in the world, with the disease having a strong genetic component. In recent years the Eph family of receptor tyrosine kinases has been identified as a key regulator in lens clarity. In this review we discuss the roles of the Eph receptors in lens biology and cataract development.

Electrical signaling in control of ocular cell behaviors

Epithelia of the cornea, lens and retina contain a vast array of ion channels and pumps. Together they produce a polarized flow of ions in and out of cells, as well as across the epithelia. These naturally occurring ion fluxes are essential to the hydration and metabolism of the ocular tissues, especially for the avascular cornea and lens. The directional transport of ions generates electric fields and currents in those tissues. Applied electric fields affect migration, division and proliferation of ocular cells which are important in homeostasis and healing of the ocular tissues. Abnormalities in any of those aspects may underlie many ocular diseases, for example chronic corneal ulcers, posterior capsule opacity after cataract surgery, and retinopathies. Electric field-inducing cellular responses, termed electrical signaling here, therefore may be an unexpected yet powerful mechanism in regulating ocular cell behavior. Both endogenous electric fields and applied electric fields could be exploited to regulate ocular cells. We aim to briefly describe the physiology of the naturally occurring electrical activities in the corneal, lens, and retinal epithelia, to provide experimental evidence of the effects of electric fields on ocular cell behaviors, and to suggest possible clinical implications.

Unique and analogous functions of aquaporin 0 for fiber cell architecture and ocular lens transparency

Aquaporin (AQP) 1 and AQP0 water channels are expressed in lens epithelial and fiber cells, respectively, facilitating fluid circulation for nourishing the avascular lens to maintain transparency. Even though AQP0 water permeability is 40-fold less than AQP1, AQP0 is selectively expressed in the fibers. Delimited AQP0 fiber expression is attributed to a unique structural role as an adhesion protein. To validate this notion, we determined if wild type (WT) lens ultrastructure and fiber cell adhesion are different in AQP0(-/-), and TgAQP1(+/+)/AQP0(-/-) mice that transgenically express AQP1 (TgAQP1) in fiber cells without AQP0 (AQP0(-/-)). In WT, lenses were transparent with 'Y' sutures. Fibers contained opposite end curvature, lateral interdigitations, hexagonal shape, and were arranged as concentric growth shells. AQP0(-/-) lenses were cataractous, lacked 'Y' sutures, ordered packing and well-defined lateral interdigitations. TgAQP1(+/+)/AQP0(-/-) lenses showed improvement in transparency and lateral interdigitations in the outer cortex while inner cortex and nuclear fibers were severely disintegrated. Transmission electron micrographs exhibited tightly packed fiber cells in WT whereas AQP0(-/-) and TgAQP1(+/+)/AQP0(-/-) lenses had wide extracellular spaces. Fibers were easily separable by teasing in AQP0(-/-) and TgAQP1(+/+)/AQP0(-/-) lenses compared to WT. Our data suggest that the increased water permeability through AQP1 does not compensate for loss of AQP0 expression in TgAQP1(+/+)/AQP0(-/-) mice. Fiber cell AQP0 expression is required to maintain their organization, which is a requisite for lens transparency. AQP0 appears necessary for cell-to-cell adhesion and thereby to minimize light scattering since in the AQP0(-/-) and TgAQP1(+/+)/AQP0(-/-) lenses, fiber cell disorganization was evident.

Patterns of aquaporin expression in the canine eye

Aquaporins (AQPs) function as water channels in many types of cells involved in fluid transport. More than 10 isoforms have been identified, and these are differentially expressed in many types of mammalian cells in the body. Six AQPs (AQP0, AQP1, AQP3, AQP4, AQP5, and AQP9) have been identified in the eyes of humans and/or rodents. The unique permeability characteristics and distribution of AQPs indicate their diverse roles in the regulation of water homeostasis in the eye. The aim of this study was to investigate the localisation of AQPs in normal canine eyes, with AQP0 protein expressed in the crystalline lens and retina. Although AQP1 mRNA was detected in various areas of the canine eye, its protein expression was limited to the cornea, iris and ciliary body. AQP4 was identified in the iris, retina and optic nerve. AQP3 and AQP5 were found in the cornea and conjunctiva, and their expression was particularly high in the limbus. AQP3 and AQP5 were present in the nictitating membrane indicating that they play a role in water transport within the membrane. The observations suggested that several subtypes of the AQP family are involved in the regulation of water homeostasis in the canine eye.

Expression profile of aquaporins in human nasolacrimal duct epithelium

PURPOSE

To determine whether the epithelium of the human nasolacrimal ducts contains aquaporins (AQPs), a family of membrane proteins that function as selective pores and are able to transport water, glycerol, and other small solutes across the cell plasma membrane.

METHODS

Expression of AQPs 0 and 1-10 in human nasolacrimal duct tissue was determined by reverse transcription polymerase chain reaction (RT-PCR). Positive PCR amplification products were verified by direct cDNA sequencing. Western blot analysis was used to detect AQPs 3-5. Antisera specific for AQPs were used in immunohistochemical analysis to determine the presence and distribution of ten AQPs (AQP 0 and 1-9) in epithelia and subepithelial glands of the nasolacrimal ducts. All samples investigated originated from human postmortem tissue.

RESULTS

In human nasolacrimal duct samples, AQPs 1, 3, 4, 5, 7, 8, 9, and 10 were identified by RT-PCR. No RT-PCR products were detected for AQPs 0, 2, and 6. All AQPs, which were detected by RT-PCR, were also confirmed by direct sequencing of the cDNA. Immunohistochemical analyses revealed AQPs 1, 3, 5, 7, 8, and 9 in human nasolacrimal duct epithelium and were detected in different cells. Expression of AQP 4 could not be detected immunohistochemically but by Western blot analysis. Protein detection of AQP 10 could not be performed due to the unavailability of an appropriate antibody.

CONCLUSIONS

The results suggest specific roles for AQPs in water transport through the epithelium of the nasolacrimal ducts and underline the presumption that tear fluid components are selectively absorbed in the nasolacrimal passage.

Lens gap junctions in growth, differentiation, and homeostasis

The cells of most mammalian organs are connected by groups of cell-to-cell channels called gap junctions. Gap junction channels are made from the connexin (Cx) family of proteins. There are at least 20 isoforms of connexins, and most tissues express more than 1 isoform. The lens is no exception, as it expresses three isoforms: Cx43, Cx46, and Cx50. A common role for all gap junctions, regardless of their Cx composition, is to provide a conduit for ion flow between cells, thus creating a syncytial tissue with regard to intracellular voltage and ion concentrations. Given this rather simple role of gap junctions, a persistent question has been: Why are there so many Cx isoforms and why do tissues express more than one isoform? Recent studies of lens Cx knockout (KO) and knock in (KI) lenses have begun to answer these questions. To understand these roles, one must first understand the physiological requirements of the lens. We therefore first review the development and structure of the lens, its numerous transport systems, how these systems are integrated to generate the lens circulation, the roles of the circulation in lens homeostasis, and finally the roles of lens connexins in growth, development, and the lens circulation.

Vasopressin-induced differential stimulation of AQP4 splice variants regulates the in-membrane assembly of orthogonal arrays

Aquaporin-4 (AQP4) is a basolateral water channel in collecting duct principal cells and assembles into orthogonal array particles (OAPs), the size of which appears to depend on relative expression levels of AQP4 splice variants. Because the higher-order organization of AQP4 was perturbed by vasopressin in Brattleboro rats and phosphorylation sites have been identified on AQP4, we investigated whether vasopressin and forskolin (Fk) affect AQP4 assembly and/or expression in LLC-PK(1) cells stably transfected with the AQP4 splice variant M23, which is responsible for formation of OAPs, and/or the splice variant M1, which does not form OAPs. Our data show that [lys(8)]-vasopressin (LVP) and Fk treatment led to differential increases in expression levels of M23-AQP4 and M1-AQP4 that varied as a function of incubation time. At early time points (day 1) expression of M1 was significantly stimulated (4.5-fold), over that of M23 (1.6-fold), but after 3 days the expression of M23 became predominant (4.1-fold) over that of M1 (1.9-fold). This pattern of stimulation was dependent on an intact AQP4 residue serine 111 and required protein synthesis. In cells expressing both M1 and M23 (M1/M23 approximately 1), with small sized OAPs at the membrane, the LVP/Fk-induced stimulation of M23 was modified and mimicked that of M1 when expressed alone, suggesting a dominant role for M1. In Brattleboro kidney inner medulla, an 8-day chronic exposure to the vasopressin agonist (dDAVP) led to reduction in M1 and a significant increase in M23 immunoblot staining (M1/M23 = 2/3 --> 1/4). These results indicate that AQP4 organization and expression are regulated by vasopressin in vivo and in vitro and demonstrate that the dominant role for M1 is restricted to a one-to-one interaction between AQP4 splice variants that regulates the membrane expression of OAPs.

A role for aquaporin-4 in fluid regulation in the inner retina

Many diverse retinal disorders are characterized by retinal edema; yet, little experimental attention has been given to understanding the fundamental mechanisms underlying and contributing to these fluid-based disorders. Water transport in and out of cells is achieved by specialized membrane channels, with most rapid water transport regulated by transmembrane water channels known as aquaporins (AQPs). The predominant AQP in the mammalian retina is AQP4, which is expressed on the Müller glial cells. Müller cells have previously been shown to modulate neuronal activity by modifying the concentrations of ions, neurotransmitters, and other neuroactive substances within the extracellular space between the inner and the outer limiting membrane. In doing so, Müller cells maintain extracellular homeostasis, especially with regard to the spatial buffering of extracellular potassium (K+) via inward rectifying K+ channels (Kir channels). Recent studies of water transport and the spatial buffering of K+ through glial cells have highlighted the involvement of both AQP4 and Kir channels in regulating the extracellular environment in the brain and retina. As both glial functions are associated with neuronal activation, controversy exists in the literature as to whether the relationship is functionally dependent. It is argued in this review that as AQP4 channels are likely to be the conduit for facilitating fluid homeostasis in the inner retina during light activation, AQP4 channels are also likely to play a consequent role in the regulation of ocular volume and growth. Recent research has already shown that the level of AQP4 expression is associated with environmentally driven manipulations of light activity on the retina and the development of myopia.

In situ fluorescence measurement of tear film [Na+], [K+], [Cl-], and pH in mice shows marked hypertonicity in aquaporin-5 deficiency

PURPOSE

Tear film composition depends on water and ion transport across ocular surface epithelia and on fluid secretion by lacrimal glands. The purpose of this study was to establish in situ fluorescence methods to measure tear film ionic concentrations and pH in mice and to determine whether tear film composition is sensitive to deficiency of the major ocular surface aquaporin water channels.

METHODS

Tear film ionic concentrations and pH were measured in anesthetized mice by ratio imaging fluorescence microscopy after topical application of ion/pH-sensing, dual-wavelength fluorescent indicators. [Na(+)], [K(+)], and [Cl(-)] were measured with membrane-impermeant indicators developed by our laboratory, and pH was measured with bis(carboxyethyl)-carboxyfluorescein fluorescence-conjugated dextran. Measurements were performed on wild-type mice and on knockout mice lacking aquaporins AQP1, AQP3, and AQP5.

RESULTS

In wild-type mice, tear film [Na(+)] was 139 +/- 8 mM, [K(+)] was 48 +/- 1 mM, [Cl(-)] was 127 +/- 4 mM, and pH was 7.59 +/- 0.2 (SE; n = 5-8). pH did not differ significantly in the AQP knockout mice. [Na(+)] was increased by approximately twofold in AQP5 null mice (230 +/- 20 mM) and was greatly reduced after exposure of the ocular surface to a humidified atmosphere. [K(+)] was mildly reduced in AQP1 null mice.

CONCLUSIONS

These results establish an in situ optical methodology to measure tear film [Na(+)], [K(+)], [Cl(-)], and pH in living mice, without the need for fluid sampling. Tear film hypertonicity in AQP5 deficiency is likely caused by reduced transcorneal water secretion in response to evaporative water loss.

Functional characterization of a human aquaporin 0 mutation that leads to a congenital dominant lens cataract

The aquaporin (AQP) transmembrane proteins facilitate the movement of water across the plasma membrane. In the lens, AQP0 is expressed in fiber cells and AQP1 in the epithelium. Recently, two individuals were identified with congenital polymorphic autosomal dominant cataract, due to a single nucleotide base deletion mutation in the lens AQP0. The deletion modified the reading frame resulting in the addition of a premature stop codon. In the present study, we examined the water permeability properties, trafficking and dominant negative effects as well as cytotoxicity due to the mutant AQP0 (Delta213-AQP0) protein. The membrane water permeability (P(w)) of Delta213-AQP0 expressing oocytes (14+/-1 microm/s) was significantly lower than those expressing WT-AQP0 (25+/-3 microm/s). P(w) of water injected control oocytes was 13+/-2 microm/s. Co-expression of WT-AQP0 with Delta213-AQP0 significantly lowered the P(w) (18+/-3 microm/s) compared to WT-AQP0. With or without the EGFP tag, WT-AQP0 protein localized in the plasma membranes of oocytes and cultured cells whereas Delta213-AQP0 was retained in the ER. Forster Resonance Energy Transfer (FRET) showed that WT-AQP0 partly localized with the co-expressed Delta213-AQP0. Co-localization studies suggest that the mutant AQP0 gained its dominant function by trapping the WT-AQP0 in the ER through hetero-oligomerization. Incubating the cells with chemical chaperones, namely, TMAO and DMSO, did not correct the folding/trafficking defects. Cell death in the Delta213-AQP0 expressing cells was due to necrosis caused by the accumulation of Delta213-AQP0 protein in the ER in cytotoxic proportions. The data show that replacement of the distal end of the 6th TM domain and the C-terminal domain of AQP0 due to the deletion mutation resulted in the impairment of cell membrane P(w), localization of the mutant protein in the ER without trafficking to the plasma membrane, and cytotoxicity due to the accumulation of the mutant protein. Cataracts in patients with this mutation might have resulted from the above mentioned consequences.

Functions of aquaporins in the eye

The aquaporins (AQPs) are integral membrane proteins whose main function is to transport water across cell membranes in response to osmotic gradients. At the ocular surface, AQP1 is expressed in corneal endothelium, AQP3 and AQP5 in corneal epithelium, and AQP3 in conjunctival epithelium. AQPs are also expressed in lens fiber cells (AQP0), lens epithelium (AQP1), ciliary epithelium (AQP1, AQP4) and retinal Müller cells (AQP4). Mutations in AQP0 produce congenital cataracts in humans. Analysis of knockout mice lacking individual AQPs suggests their involvement in maintenance of corneal and lens transparency, corneal epithelial repair, intraocular pressure (IOP) regulation, retinal signal transduction and retinal swelling following injury. The mouse phenotype findings implicate AQPs as potential drug targets for therapy of elevated IOP and ocular disorders involving the cornea, lens and retina. However, much research remains in defining cell-level mechanisms for the ocular AQP functions, in establishing the relevance to human eye disease of conclusions from knockout mice, and in developing AQP-modulating drugs.