Retinal G-substrate, potential downstream component of NO/cGMP/PKG pathway, is located in subtype of retinal ganglion cells and amacrine cells with protein phosphatases

Abstract

The aim of this study was to determine the distribution and function of G-substrate, a specific substrate of the nitric oxide (NO)–cyclic guanosine monophosphate (cGMP)–cGMP-dependent protein kinase (PKG) signaling pathway, in normal rat retina and in G-substrate knockout mice. The retinas of adult wild-type rats and mice and G-substrate knockout mice were studied immunohistologically to characterize the upstream and downstream components of the NO–cGMP–PKG pathway. Immunoblot analysis showed that the molecular weight of retinal G-substrate was similar to that of cerebellar G-substrate. In adult rats and mice, retinal G-substrate was located in a subpopulation of amacrine cells and in C38-positive retinal ganglion cells (RGCs) but not in alpha RGCs. In addition, retinal G-substrate was co-expressed with other upstream and downstream signaling components of the NO–cGMP–PKG–G-substrate–phosphatase pathway in the adult retina. Electroretinographic (ERG) analysis demonstrated that there was no significant difference between the ERGs of wild-type and G-substrate knockout mice. These results suggest that retinal G-substrate plays a role as a downstream component of the NO–cGMP–PKG pathway. The co-localization of retinal G-substrate with protein Ser/Thr phosphatases suggests that it acts as an endogenous protein phosphatase inhibitor as in the cerebellum.