The Significance of TRPV4 Channels and Hemichannels in the Lens and Ciliary Epithelium

TRPV: An emerging target in glaucoma and optic nerve damage

Transient receptor potential vanilloid (TRPV) channels are members of the TRP channel superfamily, which are ion channels that sense mechanical and osmotic stimuli and participate in Ca2+ signalling across the cell membrane. TRPV channels play important roles in maintaining the normal functions of an organism, and defects or abnormalities in TRPV channel function cause a range of diseases, including cardiovascular, neurological and urological disorders. Glaucoma is a group of chronic progressive optic nerve diseases with pathological changes that can occur in the tissues of the anterior and posterior segments of the eye, including the ciliary body, trabecular meshwork, Schlemm's canal, and retina. TRPV channels are expressed in these tissues and play various roles in glaucoma. In this article, we review various aspects of the pathogenesis of glaucoma, the structure and function of TRPV channels, the relationship between TRPV channels and systemic diseases, and the relationship between TRPV channels and ocular diseases, especially glaucoma, and we suggest future research directions. This information will help to further our understanding of TRPV channels and provide new ideas and targets for the treatment of glaucoma and optic nerve damage.

Patch clamp studies on TRPV4-dependent hemichannel activation in lens epithelium

ATP release from the lens via hemichannels has been explained as a response to TRPV4 activation when the lens is subjected to osmotic swelling. To explore the apparent linkage between TRPV4 activation and connexin hemichannel opening we performed patch-clamp recordings on cultured mouse lens epithelial cells exposed to the TRPV4 agonist GSK1016790A (GSK) in the presence or absence of the TRPV4 antagonist HC067047 (HC). GSK was found to cause a fast, variable and generally large non-selective increase of whole cell membrane conductance evident as a larger membrane current (Im) over a wide voltage range. The response was prevented by HC. The GSK-induced Im increase was proportionally larger at negative voltages and coincided with fast depolarization and the simultaneous disappearance of an outward current, likely a K⁺ current. The presence of this outward current in control conditions appeared to be a reliable predictor of a cell's response to GSK treatment. In some studies, recordings were obtained from single cells by combining cell-attached and whole-cell patch clamp configurations. This approach revealed events with a channel conductance 180-270 pS following GSK application through the patch pipette on the cell-attached side. The findings are consistent with TRPV4-dependent opening of Cx43 hemichannels.

Mechanical Load and Piezo1 Channel Regulated Myosin II Activity in Mouse Lenses

The cytoarchitecture and tensile characteristics of ocular lenses play a crucial role in maintaining their transparency and deformability, respectively, which are properties required for the light focusing function of ocular lens. Calcium-dependent myosin-II-regulated contractile characteristics and mechanosensitive ion channel activities are presumed to influence lens shape change and clarity. Here, we investigated the effects of load-induced force and the activity of Piezo channels on mouse lens myosin II activity. Expression of the Piezo1 channel was evident in the mouse lens based on immunoblot and immufluorescence analyses and with the use of a Piezo1-tdT transgenic mouse model. Under ex vivo conditions, change in lens shape induced by the load decreased myosin light chain (MLC) phosphorylation. While the activation of Piezo1 by Yoda1 for one hour led to an increase in the levels of phosphorylated MLC, Yoda1 treatment for an extended period led to opacification in association with increased calpain activity and degradation of membrane proteins in ex vivo mouse lenses. In contrast, inhibition of Piezo1 by GsMTx4 decreased MLC phosphorylation but did not affect the lens tensile properties. This exploratory study reveals a role for the mechanical load and Piezo1 channel activity in the regulation of myosin II activity in lens, which could be relevant to lens shape change during accommodation.

TRPV4 Stimulation Level Regulates Ca2+-Dependent Control of Human Corneal Endothelial Cell Viability and Survival

The functional contribution of transient receptor potential vanilloid 4 (TRPV4) expression in maintaining human corneal endothelial cells (HCEC) homeostasis is unclear. Accordingly, we determined the effects of TRPV4 gene and protein overexpression on responses modulating the viability and survival of HCEC. Q-PCR, Western blot, FACS analyses and fluorescence single-cell calcium imaging confirmed TRPV4 gene and protein overexpression in lentivirally transduced 12V4 cells derived from their parent HCEC-12 line. Although TRPV4 overexpression did not alter the baseline transendothelial electrical resistance (TEER), its cellular capacitance (Ccl) was larger than that in its parent. Scanning electron microscopy revealed that only the 12V4 cells developed densely packed villus-like protrusions. Stimulation of TRPV4 activity with GSK1016790A (GSK101, 10 µmol/L) induced larger Ca2+ transients in the 12V4 cells than those in the parental HCEC-12. One to ten nmol/L GSK101 decreased 12V4 viability, increased cell death rates and reduced the TEER, whereas 1 µmol/L GSK101 was required to induce similar effects in the HCEC-12. However, the TRPV4 channel blocker RN1734 (1 to 30 µmol/L) failed to alter HCEC-12 and 12V4 morphology, cell viability and metabolic activity. Taken together, TRPV4 overexpression altered both the HCEC morphology and markedly lowered the GSK101 dosages required to stimulate its channel activity.

Ion Transport Regulation by TRPV4 and TRPV1 in Lens and Ciliary Epithelium

Aside from a monolayer of epithelium at the anterior surface, the lens is formed by tightly compressed multilayers of fiber cells, most of which are highly differentiated and have a limited capacity for ion transport. Only the anterior monolayer of epithelial cells has high Na, K-ATPase activity. Because the cells are extensively coupled, the lens resembles a syncytium and sodium-potassium homeostasis of the entire structure is largely dependent on ion transport by the epithelium. Here we describe recent studies that suggest TRPV4 and TRPV1 ion channels activate signaling pathways that play an important role in matching epithelial ion transport activity with needs of the lens cell mass. A TRPV4 feedback loop senses swelling in the fiber mass and increases Na, K-ATPase activity to compensate. TRPV4 channel activation in the epithelium triggers opening of connexin hemichannels, allowing the release of ATP that stimulates purinergic receptors in the epithelium and results in the activation of Src family tyrosine kinases (SFKs) and SFK-dependent increase of Na, K-ATPase activity. A separate TRPV1 feedback loop senses shrinkage in the fiber mass and increases NKCC1 activity to compensate. TRPV1 activation causes calcium-dependent activation of a signaling cascade in the lens epithelium that involves PI3 kinase, ERK, Akt and WNK. TRPV4 and TRPV1 channels are also evident in the ciliary body where Na, K-ATPase is localized on one side of a bilayer in which two different cell types, non-pigmented and pigmented ciliary epithelium, function in a coordinated manner to secrete aqueous humor. TRPV4 and TRPV1 may have a role in maintenance of cell volume homeostasis as ions and water move through the bilayer.

TRPV4: Cell type-specific activation, regulation and function in the vertebrate eye

The architecture of the vertebrate eye is optimized for efficient delivery and transduction of photons and processing of signaling cascades downstream from phototransduction. The cornea, lens, retina, vasculature, ciliary body, ciliary muscle, iris and sclera have specialized functions in ocular protection, transparency, accommodation, fluid regulation, metabolism and inflammatory signaling, which are required to enable function of the retina-light sensitive tissue in the posterior eye that transmits visual signals to relay centers in the midbrain. This process can be profoundly impacted by non-visual stimuli such as mechanical (tension, compression, shear), thermal, nociceptive, immune and chemical stimuli, which target these eye regions to induce pain and precipitate vision loss in glaucoma, diabetic retinopathy, retinal dystrophies, retinal detachment, cataract, corneal dysfunction, ocular trauma and dry eye disease. TRPV4, a polymodal nonselective cation channel, integrate non-visual inputs with homeostatic and signaling functions of the eye. The TRPV4 gene is expressed in most if not all ocular tissues, which vary widely with respect to the mechanisms of TRPV4 channel activation, modulation, oligomerization, and participation in protein- and lipid interactions. Under- and overactivation of TRPV4 may affect intraocular pressure, maintenance of blood-retina barriers, lens accommodation, neuronal function and neuroinflammation. Because TRPV4 dysregulation precipitates many pathologies across the anterior and posterior eye, the channel could be targeted to mitigate vision loss.

A possible follow-up method for diabetic heart failure patients

INTRODUCTION

Plasma osmolarity is maintained through various mechanisms. The osmolarity of the aqueous humor around the crystalline lens is correlated with plasma osmolarity. A vacuole can be formed in the lens upon changes in osmolarity. The sodium-glucose cotransporter 2 inhibitors (SGLT2i) are new in the treatment of heart failure. They can cause osmotic diuresis but do not affect plasma osmolarity.

OBJECTIVE

It is unclear if the presence or absence of lens vacuole changes can monitor diabetic heart failure and SGLT2i treatment efficacy.

METHODS

Web of Science, PubMed and Scopus databases were searched for relevant articles about osmolarity, diabetes, transient receptor potential vanilloid channel, diabetic heart failure, lens vacuoles up to May 2021.

MAIN MESSAGE

The effect of SGLT2i on osmosis underlies its benefit to heart failure, but this in turn affects many other mechanisms. Failure to experience osmolarity changes will reduce the negative changes in terms of heart failure affected by osmolarity. A practical observable method is needed.

CONCLUSIONS

There is a possibility of using lens vacuoles in the follow-up of diabetic heart failure patients.

Effect of Alpha-Glucosyl-Hesperidin Consumption on Lens Sclerosis and Presbyopia

Presbyopia is characterized by a decline in the ability to accommodate the lens. The most commonly accepted theory for the onset of presbyopia is an age-related increase in the stiffness of the lens. However, the cause of lens sclerosis remains unclear. With age, water microcirculation in the lens could change because of an increase in intracellular pressure. In the lens, the intracellular pressure is controlled by the Transient Receptor Potential Vanilloid (TRPV) 1 and TRPV4 feedback pathways. In this study, we tried to elucidate that administration of α-glucosyl-hesperidin (G-Hsd), previously reported to prevent nuclear cataract formation, affects lens elasticity and the distribution of TRPV channels and Aquaporin (AQP) channels to meet the requirement of intracellular pressure. As a result, the mouse control lens was significantly toughened compared to both the 1% and 2% G-Hsd mouse lens treatments. The anti-oxidant levels in the lens and plasma decreased with age; however, this decrease could be nullified with either 1% or 2% G-Hsd treatment in a concentration- and exposure time-dependent manner. Moreover, G-Hsd treatment affected the TRPV4 distribution, but not TRPV1, AQP0, and AQP5, in the peripheral area and could maintain intracellular pressure. These findings suggest that G-Hsd has great potential as a compound to prevent presbyopia and/or cataract formation.

Biochemical and biomechanical characteristics of dystrophin-deficient mdx3cv mouse lens

The molecular and cellular basis for cataract development in mice lacking dystrophin, a scaffolding protein that links the cytoskeleton to the extracellular matrix, is poorly understood. In this study, we characterized lenses derived from the dystrophin-deficient mdx3cv mouse model. Expression of Dp71, a predominant isoform of dystrophin in the lens, was induced during lens fiber cell differentiation. Dp71 was found to co-distribute with dystroglycan, connexin-50 and 46, aquaporin-0, and NrCAM as a large cluster at the center of long arms of the hexagonal fibers. Although mdx3cv mouse lenses exhibited dramatically reduced levels of Dp71, only older lenses revealed punctate nuclear opacities compared to littermate wild type (WT) lenses. The levels of dystroglycan, syntrophin, and dystrobrevin which comprise the dystrophin-associated protein complex (DAPC), and NrCAM, connexin-50, and aquaporin-0, were significantly lower in the lens membrane fraction of adult mdx3cv mice compared to WT mice. Additionally, decreases were observed in myosin light chain phosphorylation and lens stiffness together with a significant elevation in the levels of utrophin, a functional homolog of dystrophin in mdx3cv mouse lenses compared to WT lenses. The levels of perlecan and laminin (ligands of α-dystroglycan) remained normal in dystrophin-deficient lens fibers. Taken together, although mdx3cv mouse lenses exhibit only minor defects in lens clarity possibly due to a compensatory increase in utrophin, the noted disruptions of DAPC, stability, and organization of membrane integral proteins of fibers, and stiffness of mdx3cv lenses reveal the importance of dystrophin and DAPC in maintaining lens clarity and function.

Practical Method for Salt Intake Follow-Up in Hypertensive Patients

Background: Obese and hypertensive (HT) patients should restrict salt intake. In excessive salt intake, ouabain-like compounds inhibit Na/K-ATPase (Na+ pump), which increases intracellular Na+ and Ca2+. Ca2+ has a vasotonic effect on arteries and an inotropic effect on the heart and may cause cortical opacities in the lens. To our knowledge, there is still no practical method for salt intake follow-up. This study tested whether salt intake follow-up can be performed with the help of opacity tracking. Methods: In total, 400 HT patients (age 30-69 years) with cortical lens opacities were included in the study. Changes in opacities based on biomicroscopic examination at baseline and after 3 months were recorded digitally with the help of imaging software. Salt intake at 1 and 3 months was evaluated with a 24-hr urine Na assay. Changes in opacities were compared among group 1 (∼50% salt reduction), group 2 (∼10% salt reduction), and group 3 (∼15% salt increase). Results: Age and changes in small opacity diameter (SOD) and large opacity diameter (LOD) were the most important determinants of the 50% salt reduction in the third month. For changes in LOD, the sensitivity was 88.5% [confidence interval (95% CI) 85.2-91.7] and specificity was 95.5% (95% CI 93.1-98.7) for predicting a 50% salt restriction during the 3-month period. For SOD, the values were 85% (95% CI 82.5-87) and 95% (95% CI 92.3-97.5), respectively. Conclusions: Opacity changes are a practical method for predicting a 50% reduction in salt intake over a 3-month period in 30- to 59-year-old HT patients.

The role of TRPV4 channels in ocular function and pathologies

Transient potential receptor vanilloid 4 (TRPV4) is an ion channel responsible for sensing osmotic and mechanical signals, which in turn regulates calcium signaling across cell membranes. TRPV4 is widely expressed throughout the body, and plays an important role in normal physiological function, as well as different pathologies, however, its role in the eye is not well known. In the eye, TRPV4 is expressed in various tissues, such as the retina, corneal epithelium, ciliary body, and the lens. In this review, we provide an overview on TRPV4 structure, activation, mutations, and summarize the current knowledge of TRPV4 function and signaling mechanisms in various locations throughout the eye, as well as its role in ocular diseases, such as glaucoma and diabetic retinopathy. Based on the available data, we highlight the therapeutic potential of TRPV4 as well as the shortcomings of current research. Finally, we provide future perspectives on the implications of targeting TRPV4 to treat various ocular pathologies.

Polymodal Sensory Transduction in Mouse Corneal Epithelial Cells

Purpose

Contact lenses, osmotic stressors, and chemical burns may trigger severe discomfort and vision loss by damaging the cornea, but the signaling mechanisms used by corneal epithelial cells (CECs) to sense extrinsic stressors are not well understood. We therefore investigated the mechanisms of swelling, temperature, strain, and chemical transduction in mouse CECs.

Methods

Intracellular calcium imaging in conjunction with electrophysiology, pharmacology, transcript analysis, immunohistochemistry, and bioluminescence assays of adenosine triphosphate (ATP) release were used to track mechanotransduction in dissociated CECs and epithelial sheets isolated from the mouse cornea.

Results

The transient receptor potential vanilloid (TRPV) transcriptome in the mouse corneal epithelium is dominated by Trpv4, followed by Trpv2, Trpv3, and low levels of Trpv1 mRNAs. TRPV4 protein was localized to basal and intermediate epithelial strata, keratocytes, and the endothelium in contrast to the cognate TRPV1, which was confined to intraepithelial afferents and a sparse subset of CECs. The TRPV4 agonist GSK1016790A induced cation influx and calcium elevations, which were abolished by the selective blocker HC067047. Hypotonic solutions, membrane strain, and moderate heat elevated [Ca²⁺]_CEC with swelling- and temperature-, but not strain-evoked signals, sensitive to HC067047. GSK1016790A and swelling evoked calcium-dependent ATP release, which was suppressed by HC067027 and the hemichannel blocker probenecid.

Conclusions

These results demonstrate that cation influx via TRPV4 transduces osmotic and thermal but not strain inputs to CECs and promotes hemichannel-dependent ATP release. The TRPV4-hemichannel-ATP signaling axis might modulate corneal pain induced by excessive mechanical, osmotic, and chemical stimulation.

TRPV1 activation stimulates NKCC1 and increases hydrostatic pressure in the mouse lens

The porcine lens response to a hyperosmotic stimulus involves an increase in the activity of an ion cotransporter sodium-potassium/two-chloride cotransporter 1 (NKCC1). Recent studies with agonists and antagonists pointed to a mechanism that appears to depend on activation of transient receptor potential vanilloid 1 (TRPV1) ion channels. Here, we compare responses in lenses and cultured lens epithelium obtained from TRPV1^-/- and wild type (WT) mice. Hydrostatic pressure (HP) in lens surface cells was determined using a manometer-coupled microelectrode approach. The TRPV1 agonist capsaicin (100 nM) caused a transient HP increase in WT lenses that peaked after ∼30 min and then returned toward baseline. Capsaicin did not cause a detectable change of HP in TRPV1^-/- lenses. The NKCC inhibitor bumetanide prevented the HP response to capsaicin in WT lenses. Potassium transport was examined by measuring Rb⁺ uptake. Capsaicin increased Rb⁺ uptake in cultured WT lens epithelial cells but not in TRPV1^-/- cells. Bumetanide, A889425, and the Akt inhibitor Akti prevented the Rb⁺ uptake response to capsaicin. The bumetanide-sensitive (NKCC-dependent) component of Rb⁺ uptake more than doubled in response to capsaicin. Capsaicin also elicited rapid (<2 min) NKCC1 phosphorylation in WT but not TRPV1^-/- cells. HP recovery was shown to be absent in TRPV1^-/- lenses exposed to hyperosmotic solution. Bumetanide and Akti prevented HP recovery in WT lenses exposed to hyperosmotic solution. Taken together, responses to capsaicin and hyperosmotic solution point to a functional role for TRPV1 channels in mouse lens. Lack of NKCC1 phosphorylation and Rb⁺ uptake responses in TRPV1^-/- mouse epithelium reinforces the notion that a hyperosmotic challenge causes TRPV1-dependent NKCC1 activation. The results are consistent with a role for the TRPV1-activated signaling pathway leading to NKCC1 stimulation in lens osmotic homeostasis.

Verification and spatial mapping of TRPV1 and TRPV4 expression in the embryonic and adult mouse lens

The transient receptor protein vanilloid channels, TRPV1 and TRPV4, have recently been shown to be mechanosensors in the ocular lens that act to transduce physical changes in lens volume and internal hydrostatic pressure into the activation of signalling pathways in lens epithelial cells. These pathways in turn regulate ion and water transport to ensure that the optical properties of the lens remain constant. Despite the functional evidence that implicate the roles of TRPV1 and TRPV4 in the lens, their respective cellular expression patterns in the different regions of the lens has to date not been fully characterised. Using Western blotting we have confirmed that TRPV1 and TRPV4 are expressed throughout all regions (epithelium, outer cortex, inner cortex/core) of the adult mouse lens. Subsequent immunolabeling of lens cryosections confirmed that TRPV1 and TRPV4 are expressed throughout all regions of the lens, but revealed differentiation-dependent differences in the subcellular expression of the two channels in the different regions. In the epithelium and outer cortex, intense TRPV1 and TRPV4 labeling was predominately associated with the cytoplasm. In a discrete zone in the inner cortex, labeling for both proteins was greatly diminished, but could be enhanced by incubating sections with the detergent Triton X-100 to reveal TRPV1 and TRPV4 labelling that was associated with the membrane. This suggests that in this region of the lens there is a potential interacting protein that masks the binding of the TRPV1 and TRPV4 antibodies to their respective epitopes in the lens inner cortex. In the core of the lens, which contains the embryonic nucleus, TRPV1 and TRPV4 labelling was associated exclusively with fibre cell membranes. This labelling in the lens core of the adult mouse lens appeared to originate in early development as a similar membrane labelling was observed at embryonic day 10 (E10) of the cells in the lens vesicle that subsequently forms the embryonic nucleus in the adult lens. During subsequent stages of embryonic development TRPV1 and TRPV4 remained membranous in the inner cortex and core, while showing labelling that was associated with the cytoplasm in the superficial outer cortical region. The extent of cytoplasmic labelling for TRPV4, but not TRPV1, in this cortical region could however be dynamically regulated by cutting the zonules that normally attach the lens to the ciliary body. We have shown an early onset and continuous expression of TRPV1 and TRPV4 across all lens regions, and that TRPV4 can be dynamically trafficked into the membranes of differentiating fibre cells, results that suggests that these mechanosensitive channels may also be functionally active in lens fibre cells.

Salt, Hypertension, and the Lens

BACKGROUND

Salt is important in the pathogenesis of hypertension (HT). Salt-sensitive hypertension (SSH) accounts for about half of all HT cases. In SSH, sodium/potassium adenosine triphosphatase (Na⁺/K⁺-ATPase) activity is impaired. Impaired Na⁺/K⁺-ATPase activity in the lens epithelium results in cortical opacities in the peripheral equator of the lens. We investigated the sensitivity of cortical lens opacities in detecting SSH.

METHODS

The study included 191 SSH and 159 non-SSH, salt-resistant HT (SRH) patients (350 HT patients total), aged 40-80 years. One hundred twenty-four sex- and age-matched patients without a HT diagnosis made up the control group. Daily salt intake of all groups was calculated from 24-hr urinary Na excretion. SSH was diagnosed when the difference in mean arterial blood pressure values obtained during high- and low-Na diets was ≥10%. Non-SSH, SRH was diagnosed when the difference was <10%.Two researchers examined the presence of cortical lens opacities biomicroscopically using the diffuse, direct, Scheimpflug, and retroillumination from fundus methods.

RESULTS

Total lens opacity was predictive of SSH among all cases (P < 0.001), with a sensitivity and specificity of 75.4% [95% confidence interval (CI): 68.6-81.3] and 83.6% (95% CI: 77.0-89.0), respectively. Its positive and negative predictive values were 84.7% (95% CI: 79.4-88.8) and 73.9% (95% CI: 68.6-78.5), respectively.

CONCLUSIONS

Lens opacities can be used as a finding that can be easily observed in the detection of SSH and excess salt intake.

TRPV1-dependent ERK1/2 activation in porcine lens epithelium

Recently we determined that the Transient Receptor Potential Vanilloid 4 ion channel (TRPV4) has a crucial signaling role in a pathway that regulates various aspects of lens epithelium function. Here, we report on a different TRPV channel, TRPV1, in porcine lens. The presence of TRPV1 in the lens was evident from RT-PCR studies and Western blot analysis of MAPK signaling pathway activation caused by the TRPV1 agonist capsaicin. TRPV1 mRNA was detected in the epithelium of porcine as well as human lens. Transient ERK1/2 and p38 MAPK phosphorylation was detected within 1 min in the epithelium isolated from intact porcine lenses exposed to capsaicin (100 nM), a selective TRPV1 agonist, and the response was significantly inhibited by A889245 (1.0 μM), a TRPV1 antagonist. A similar ERK 1/2 and p38 response in the epithelium, also inhibitable by A889245, was evident in lenses treated with hyperosmotic solution (350 vs 300 mOsm). Lenses pre-treated with either the cytosolic Ca²⁺ chelator BAPTA-AM or the PKC inhibitor sotrastaurin (1.0 μM) had a diminished ERK1/2 activation response to capsaicin and hyperosmotic solution. Taken together the findings support the notion that TRPV1 functions as a plasma membrane ion channel that, when activated, permits the entry of extracellular calcium into the lens epithelium, leading to activation of PKC, ERK1/2 and p38 MAPK. It is significant that the findings confirm earlier proposals that hyperosmotic stress is linked to TRPV1 channel activation in the mouse lens. Further studies are ongoing to determine what functional changes are triggered by the TRPV1-linked signaling pathways and how they might relate to lens volume homeostasis.