VEGF as a Paracrine Regulator of Conventional Outflow Facility

Abstract

PurposeVascular endothelial growth factor (VEGF) regulates microvascular endothelial permeability, and the permeability of Schlemm's canal (SC) endothelium influences conventional aqueous humor outflow. We hypothesize that VEGF signaling regulates outflow facility.MethodsWe measured outflow facility (C) in enucleated mouse eyes perfused with VEGF-A164a, VEGF-A165b, VEGF-D, or inhibitors to VEGF receptor 2 (VEGFR-2). We monitored VEGF-A secretion from human trabecular meshwork (TM) cells by ELISA after 24 hours of static culture or cyclic stretch. We used immunofluorescence microscopy to localize VEGF-A protein within the TM of mice.ResultsVEGF-A164a increased C in enucleated mouse eyes. Cyclic stretch increased VEGF-A secretion by human TM cells, which corresponded to VEGF-A localization in the TM of mice. Blockade of VEGFR-2 decreased C, using either of the inhibitors SU5416 or Ki8751 or the inactive splice variant VEGF-A165b. VEGF-D increased C, which could be blocked by Ki8751.ConclusionsVEGF is a paracrine regulator of conventional outflow facility that is secreted by TM cells in response to mechanical stress. VEGF affects facility via VEGFR-2 likely at the level of SC endothelium. Disruption of VEGF signaling in the TM may explain why anti-VEGF therapy is associated with decreased outflow facility and sustained ocular hypertension.

<p>Incidence of Glaucoma or Ocular Hypertension After Repeated Anti-Vascular Endothelial Growth Factor Injections for Macular Degeneration</p>

“…The fact that pseudophakia proved greatly protective may begin to suggest the disease mechanism, and in our analysis, this finding, more than any other, supports a mechanical cause for the disease. Potential medicationinduced explanations for cases of post-injection glaucoma or sustained OHT have included clogging of the trabecular meshwork with silicone oil droplets 27 or aggregated proteins, 28 altered nitric oxide metabolism, 29 or a direct effect of VEGF blockage on trabecular endothelial permeability, 30 as reviewed by Aref. 31 However, it is unclear why these effects, which presumably would operate at the level of the trabecular meshwork, would be greater in phakic eyes than in pseudophakic eyes, as clinical observation suggests that substances injected into the vitreous cavity are more likely to migrate to the anterior chamber, or are likely to migrate faster, in a pseudophakic eye than in a phakic one.…”

Direct measurement of pressure-independent aqueous humour flow using iPerfusion

“…The perfusate used in both cannulae was Dulbecco’s PBS with 5.5 mM glucose passed through a sterile 0.22 μm filter prior to use. The flow rate from the iPerfusion system into the eye, Q , at pressure P was measured as described previously (Chandrawati et al, 2017; Li et al, 2016; O’Callaghan et al, 2017; Reina-Torres et al, 2017; Sherwood et al, 2016; Tam et al, 2017; Wang et al, 2017). As shown in Fig.…”

The β4-Subunit of the Large-Conductance Potassium Ion Channel KCa1.1 Regulates Outflow Facility in Mice

“…Stretch induces secretion of several compounds from TM cells that increase outflow facility, such as adenosine triphosphate, 56 metalloproteinases, 57 and vascular endothelial growth factor. 58 As K Ca 1.1 appears to be involved in the stretch response in TM cells, it is possible that blocking K Ca 1.1 may suppress stretch-induced release of these compounds to oppose the facility increase. K Ca 1.1 is also involved in homeostatic regulation of cell volume, as occurs during the regulatory volume decrease following hypoosmotic shock.…”