Dysfunctional cGMP Signaling Leads to Age-Related Retinal Vascular Alterations and Astrocyte Remodeling in Mice

Abstract

The nitric oxide–guanylyl cyclase-1–cyclic guanylate monophosphate (NO–GC-1–cGMP) pathway is integral to the control of vascular tone and morphology. Mice lacking the alpha catalytic domain of guanylate cyclase (GC1−/−) develop retinal ganglion cell (RGC) degeneration with age, with only modest fluctuations in intraocular pressure (IOP). Increasing the bioavailability of cGMP in GC1−/− mice prevents neurodegeneration independently of IOP, suggesting alternative mechanisms of retinal neurodegeneration. In continuation to these studies, we explored the hypothesis that dysfunctional cGMP signaling leads to changes in the neurovascular unit that may contribute to RGC degeneration. We assessed retinal vasculature and astrocyte morphology in young and aged GC1−/− and wild type mice. GC1−/− mice exhibit increased peripheral retinal vessel dilation and shorter retinal vessel branching with increasing age compared to Wt mice. Astrocyte cell morphology is aberrant, and glial fibrillary acidic protein (GFAP) density is increased in young and aged GC1−/− mice, with areas of dense astrocyte matting around blood vessels. Our results suggest that proper cGMP signaling is essential to retinal vessel morphology with increasing age. Vascular changed are preceded by alterations in astrocyte morphology which may together contribute to retinal neurodegeneration and loss of visual acuity observed in GC1−/− mice.

cGMP Signaling in the Neurovascular Unit—Implications for Retinal Ganglion Cell Survival in Glaucoma

“…Mice lacking the alpha catalytic subunit of guanylate cyclase ( GC1 −/− ) were considered a novel murine model of POAG; the mice exhibit degeneration of RGCs with age, without large increases in IOP and an open irideocorneal angle [ 26 ]. GC1 −/− mice also develop retinal vascular dysfunction and aberrant retinal vascular morphology with age, and astrocyte morphology alterations precede vascular abnormalities [ 29 ]. These results suggest that NO-cGMP signaling functions to maintain homeostasis in cells of the NVU, and thus the NO-cGMP pathway remains an attractive area of investigation for glaucoma.…”

“…In focal brain injury, inhibiting PDE increased cGMP and altered the glial inflammatory response, decreased oxidative stress and cell death, and increased angiogenesis ( Figure 3 ) [ 84 ]. A potential role for cGMP signaling in retinal astrocyte physiology has been reported in a study using a germline GC1 alpha subunit knockout mouse [ 29 ]. In the study, GC1 −/− mice that developed RGC degeneration with age [ 26 , 28 ] were shown to have an altered retinal vascular phenotype; blood vessels in the peripheral retinal vasculature were dilated and exhibited shorter capillary branching [ 9 , 29 ].…”

“…A potential role for cGMP signaling in retinal astrocyte physiology has been reported in a study using a germline GC1 alpha subunit knockout mouse [ 29 ]. In the study, GC1 −/− mice that developed RGC degeneration with age [ 26 , 28 ] were shown to have an altered retinal vascular phenotype; blood vessels in the peripheral retinal vasculature were dilated and exhibited shorter capillary branching [ 9 , 29 ]. Interestingly, vascular changes were preceded by an increase in glial fibrillary acidic protein (GFAP) expression in astrocytes and changes in astrocyte morphology, specifically astrocytic matting around blood vessels and enlarged bulbous astrocytic endfeet [ 29 ].…”

“…In the study, GC1 −/− mice that developed RGC degeneration with age [ 26 , 28 ] were shown to have an altered retinal vascular phenotype; blood vessels in the peripheral retinal vasculature were dilated and exhibited shorter capillary branching [ 9 , 29 ]. Interestingly, vascular changes were preceded by an increase in glial fibrillary acidic protein (GFAP) expression in astrocytes and changes in astrocyte morphology, specifically astrocytic matting around blood vessels and enlarged bulbous astrocytic endfeet [ 29 ]. The effects in astrocytes may be due to reduced cGMP signaling in the astrocytes themselves; in human and rat cultured astrocytes, cGMP-PKG activity directly increases GFAP expression and RhoA inhibition, which are responsible for astrocyte morphological changes [ 97 , 99 ].…”

“…The effects in astrocytes may be due to reduced cGMP signaling in the astrocytes themselves; in human and rat cultured astrocytes, cGMP-PKG activity directly increases GFAP expression and RhoA inhibition, which are responsible for astrocyte morphological changes [ 97 , 99 ]. Moreover, changes in astrocyte connexin expression have also been observed in GC1 −/− mice [ 29 ]. Cx43 is the most prevalent connexin in astrocytes, and GC1 −/− mice exhibit a greater decrease in connexin-43 levels with age [ 29 , 67 ].…”