Muller Glia Activation in Response to Inherited Retinal Degeneration Is Highly Varied and Disease-Specific

Despite different aetiologies, most inherited retinal disorders culminate in photoreceptor loss, which induces concomitant changes in the neural retina, one of the most striking being reactive gliosis by Muller cells. It is typically assumed that photoreceptor loss leads to an upregulation of glial fibrilliary acidic protein (Gfap) and other intermediate filament proteins, together with other gliosis-related changes, including loss of integrity of the outer limiting membrane (OLM) and deposition of proteoglycans. However, this is based on a mix of both injury-induced and genetic causes of photoreceptor loss. There are very few longitudinal studies of gliosis in the retina and none comparing these changes across models over time. Here, we present a comprehensive spatiotemporal assessment of features of gliosis in the degenerating murine retina that involves Muller glia. Specifically, we assessed Gfap, vimentin and chondroitin sulphate proteoglycan (CSPG) levels and outer limiting membrane (OLM) integrity over time in four murine models of inherited photoreceptor degeneration that encompass a range of disease severities (Crb1(rd8/rd8), Prph2(+/Delta 307), Rho(-/-), Pde6b(rd1/rd1)). These features underwent very different changes, depending upon the disease-causing mutation, and that these changes are not correlated with disease severity. Intermediate filament expression did indeed increase with disease progression in Crb1(rd8/rd8) and Prph2(+/Delta 307), but decreased in the Prph2(+/Delta 307) and Pde6b(rd1/rd1) models. CSPG deposition usually, but not always, followed the trends in intermediate filament expression. The OLM adherens junctions underwent significant remodelling in all models, but with differences in the composition of the resulting junctions; in Rho(-/-) mice, the adherens junctions maintained the typical rod-Muller glia interactions, while in the Pde6b(rd1/rd1) model they formed predominantly between Muller cells in late stage of degeneration. Together, these results show that gliosis and its associated processes are variable and disease-dependent.