Enhanced Inflow and Outflow Rates Despite Lower IOP in Bestrophin-2-Deficient Mice

Abstract

Purpose Bestrophin-2 (Best2), a putative Cl− channel is expressed in the nonpigmented epithelium (NPE). Disruption of Best2 in mice results in a diminished intraocular pressure (IOP). Aqueous humor dynamics were compared in Best2+/+ and Best2−/− mice, to better understand the contribution of Best2 to IOP. Methods Measurements of IOP, episcleral venous pressure (EVP), conventional outflow facility (Ct), aqueous humor production (Fa), and anterior chamber volume (Va) were made using anterior chamber cannulation. Conventional (Fc) and uveoscleral outflow (Fu), and rate of aqueous humor turnover, were calculated from measured data. The anterior chamber was examined in live mice by optical coherence tomography (OCT) and postmortem by light microscopy. Results IOP in Best2−/− mice was lower compared with Best2+/+ littermates. EVP was unchanged. Since Best2 is expressed in NPE cells, the hypothesis was that Best2 is involved in generating aqueous flow. However, Fa in Best2−/− mice was increased by ~73% compared with Best2+/+ mice. This was accompanied by increases in Fc and Fu. Aqueous humor turnover was enhanced more than twofold in Best2−/− mice. No evidence of developmental structural changes was noted. Conclusions Best2 appears to antagonize the formation of aqueous humor and cause an inhibition of both Fc and Fu, despite being expressed only in NPE cells. These data support the hypothesis that the inflow and outflow pathways communicate via soluble agents present in the aqueous humor and implicate Best2 as a critical mediator of that communication.

Regulation of Anterior Chamber Drainage by Bicarbonate-sensitive Soluble Adenylyl Cyclase in the Ciliary Body

“…3B) (11,12). The effect of knocking out Best2 was to decrease IOP despite an increase in F a (11,13). Measurement of aqueous dynamics in those mice demonstrated that both C t and C u were increased to more than compensate for the increase in F a .…”

“…IOP and aqueous dynamics measurements were performed 3-4 h later and always between 2:00 and 6:00 p.m. to avoid diurnal pressure variations. Measurement of IOP, episcleral venous pressure (EVP), F a , and C t were made by cannulation of the anterior chamber with borosilicate glass microneedles as described previously (13,21) in mice anesthetized with Avertin (300 mg/kg injected intraperitoneally). C u was calculated using measured parameters according to the modified Goldmann equation:…”

“…In the eye, Best2 is uniquely localized to the basal membrane of NPE cells (11,12). Best2 Ϫ/Ϫ mice have a significantly lower IOP than their wild-type (WT) littermates (11,13). Interestingly, this lower IOP occurs despite an increase in aqueous flow (F a ).…”

“…The lower IOP results from overcompensation in conventional (C t ) and uveoscleral (C u ) drainage. The absence of Best2 in outflow tissues suggests the existence of pressure-independent, biochemical communication between the inflow and outflow pathways (13).…”

Control of Outflow Resistance by Soluble Adenylyl Cyclase

“…Although we worked under the hypothesis that absence of Best2 would cause increased IOP due to an increase in intracellular bicarbonate in NPE cells, we found the opposite; that Best2 -/ -mice have a significantly lower IOP than their wild-type (WT) littermates. 17,18 A more detailed examination of aqueous dynamics in the mice revealed that they do indeed produce more aqueous humor than their Best2 + / + littermates in a given span of time. 18 However, C t and F u are increased sufficiently to overcompensate for the increase in aqueous production.…”