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Glaucoma Dialogue IGR 22-1
Comment
Comment by Darryl Overby & Daniel Stamer on:
92713 Impaired TRPV4-eNOS signaling in trabecular meshwork elevates intraocular pressure in glaucoma, Patel PD; Chen YL; Kasetti RB et al., Proceedings of the National Academy of Sciences of the United States of America, 2021; 118:
See also comment(s) by Heather McGowan & Louis Pasquale • Gulab Zode & Swapnil Sonkusare & Pinkal Patel •The physiology of intraocular pressure (IOP) regulation is important for understanding and treating glaucoma. Patel et al. investigate a mechanism of IOP mechanosensation, identifying the interactive role for TRPV4 ion channels and endothelial nitric oxide synthase (eNOS), both of which are known to modulate IOP and outflow facility. 1-3 Patel et al. show that supra-pharmacological activation of TRPV4 lowers IOP and increases outflow facility in mice, and that low levels of shear stress stimulate TRPV4 activity in TM cells to increase intracellular calcium. Direct TRPV4 activation leads to eNOS phosphorylation in cultured TM cells/tissue to increase nitric oxide production. Interestingly, the IOP reduction observed in response to TRPV4 activation was lost in mice depleted of eNOS, which suggest that TRPV4 mediated effects on IOP and outflow act via eNOS. These experiments add to our growing knowledge about eNOS-mediated IOP mechanosensation by implicating TRPV4 in the process.
There are two putative mechanisms to explain IOP mechanosensation. The first involves IOP-induced expansion of the TM that stretches TM cells and drives stretch-induced signaling via focal adhesions, mechanosensitive ion channels or other mechanoreceptors. The second is that the IOP-induced expansion of the TM leads to narrowing of the SC lumen, which increases the shear stress acting on SC endothelium to drive shear-induced mechanosensation via sensors such as VE-cadherin/PECAM-1/VEGF-R2 or, as demonstrated by Patel et al., TRP4V. These two mechanisms provide complementary mechanosensory cues because each are sensitive to a different range of IOP perturbations.4
Patel et al. propose a third mechanism involving flow/shear mechanosensation by TM cells, which they describe as a 'key physiological pathway responsible for homeostatic regulation of IOP in normal human beings, which is impaired in glaucoma patients.' Their mechanism, however, fails to fit with current knowledge. Firstly, the outflow rate or filtration velocity of aqueous humor remains constant (or decreases) during ocular hypertension or untreated glaucoma, and shear stress is directly proportional to flow rate or velocity. Secondly, elevated IOP causes expansion of the TM and widening the flow pathways, which lowers the shear stress in the TM. Regardless of the precise molecular pathway, any physiological mechanisms for IOP mechanosensation should presumably act to increase outflow facility in response to elevated IOP, to oppose IOP perturbations and maintain IOP homeostasis. Hence, the mechanism proposed by Patel seems inconsistent with basic physiological knowledge of outflow, and it remains unclear how TRPV4-eNOS could be involved in IOP mechanosensation by the TM.
Although not acknowledged, the mechanism Patel et al. identified more likely implicates TRPV4-eNOS mechanosensation by SC cells, which are of vascular origin. Indeed, TRP4V is already known to mediate shear-induced vasodilation via nitric oxide and eNOS in other vascular endothelia.5 Moreover, evidence from three different laboratories indicate little to no expression of eNOS in TM cells, but high expression in SC cells based on single cell sequencing studies5,6 and GFP reporter studies.7 Thus, in light of the discrepancies in eNOS expression by outflow cells between labs and the seemingly self-contradictory mechanosensory mechanism proposed by Patel et al, it appears that TRVP4-mediated IOP mechanosensation is more likely occurring within SC, and not the TM. Regardless, Patel et al. identified a key role for TRPV4-eNOS signaling in IOP homeostasis, yet their mechanistic interpretation appears to be flawed and further work is required to resolve this important question about the role of TRPV4 and eNOS in the mechanosensation and homeostasis of IOP.
In light of the discrepancies in eNOS expression by outflow cells between labs and the seemingly self-contradictory mechanosensory mechanism proposed by Patel et al, it appears that TRVP4-mediated IOP mechanosensation is more likely occurring within SC, and not the TM
References
- Reina-Torres E, De Ieso ML, Pasquale LR, et al. The vital role for nitric oxide in intraocular pressure homeostasis.. Prog Retin Eye Res. 2021;83:100922.
- Ryskamp DA, Frye AM, Tam T, et al. TRPV4 regulates calcium homeostasis, cytoskeletal remodeling, conventional outflow and intraocular pressure in the mammalian eye. Sci Rep. 2016;6:30583.
- Luo N, Conwell MD, Chen X, et al. Primary cilia signaling mediates intraocular pressure sensation. Proc Natl Acad Sci U S A. 2014;111:12871-12876.
- Sherwood JM, Stamer WD, Overby DR. A model of the oscillatory mechanical forces in the conventional outflow pathway. J R Soc Interface. 2019;16(150):20180652.
- Mendoza SA, Fang J, Gutterman DD, et al. TRPV4-mediated endothelial Ca2+ influx and vasodilation in response to shear stress. Am J Physiol Heart Circ Physiol. 2010;298(2):H466-476.
- Patel G, Fury W, Yang H, et al. Molecular taxonomy of human ocular outflow tissues defined by single-cell transcriptomics. Proc Natl Acad Sci U S A. 2020;117(23):12856-12867.
- van Zyl T, Yan W, McAdams A, et al. Cell atlas of aqueous humor outflow pathways in eyes of humans and four model species provides insight into glaucoma pathogenesis. Proc Natl Acad Sci U S A. 2020;12;117(19):10339-10349.
- Chang JY, Stamer WD, Bertrand J, Read AT, Marando CM, Ethier CR, Overby DR. Role of nitric oxide in murine conventional outflow physiology. Am J Physiol Cell Physiol. 2015;309(4):C205-14.
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