Abstract
Degeneration of photoreceptors in the retina is a major cause of blindness in humans. Often retinal degeneration is due to inheritance of mutations in genes important in photoreceptor (PR) function, but can also be induced by other events including retinal trauma, microvascular disease, virus infection or prion infection. The onset of apoptosis and degeneration of PR neurons correlates with invasion of the PR cellular areas by microglia or monocytes, suggesting a causal role for these cells in pathogenesis of PR degenerative disease. To study the role of microglia in prion-induced retinal disease, we fed prion-infected mice a CSF-1 receptor blocking drug, PLX5622, to eliminate microglia in vivo, and the effects on retinal degeneration were analyzed over time. In mice not receiving drug, the main inflammatory cells invading the degenerating PR areas were microglia, not monocytes. Administration of PLX5622 was highly effective at ablating microglia in retina. However, lack of microglia during prion infection did not prevent degeneration of PR cells. Therefore, microglia were not required for the PR damage process during prion infection. Indeed, mice lacking microglia had slightly faster onset of PR damage. Similar results were seen in C57BL/10 mice and transgenic mice expressing GFP or RFP on microglia and monocytes, respectively. These results were supported by experiments using prion-infected Cx3cr1 knockout mice without PLX5622 treatment, where microglial expansion in retina was delayed, but PR degeneration was not. Contrary to predictions, microglia were not a causative factor in retinal damage by prion infection. Instead, newly generated PrPSc accumulated around the inner segment region of the PR cells and appeared to correlate with initiation of the pathogenic process in the absence of microglia.
Highlights
Prion diseases are transmissible neurodegenerative diseases of the CNS which occur in humans and animals
Prion-induced retinal degeneration associated with gliosis and destruction of photoreceptor cells In the present experiments, we studied the retinal degeneration process in mice after i.c. infection with scrapie strain 79A
In both C57BL/10 and tgGFP/red fluorescent protein (RFP) mice, signs of retinal damage were initially noted at 118 to 128 dpi when thinning of the outer nuclear layer (ONL) was first observed, photoreceptors appeared to be disorganized, and mononuclear cells were seen in the PR areas
Summary
Prion diseases are transmissible neurodegenerative diseases of the CNS which occur in humans and animals. These diseases are caused by an infectious agent composed primarily of a misfolded host-derived protein known as prion protein (PrP or PrPC). In the current paper, we studied retinal degeneration caused by prion infection, which has been known for many years [1, 5]. Due to the devastating nature of the brain degeneration seen in typical human prion diseases, retinal damage is not considered a major clinical issue. The unique spatial organization of neurons and other cells in retina provides an opportunity to study prion-induced neuropathogenesis in a slightly less
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