The injury inflammatory response mediated by the innate immune system is an important contributor to neurodegeneration in the central nervous system (CNS) and retina. A major branch of the innate immune system is regulated by the
Toll-like receptors (TLRs), which are receptors for endogenous damage associated molecules released from injured cells as well as pathogen-derived molecules, and
interleukin-1 receptors (IL-1R), which are activated by IL-1α, IL-1β and
IL-18 cytokines. TLRs and IL-1R are expressed on immune and non-immune cell types and act as first responders to cell damage, which results in tissue repair, or
inflammation and apoptosis. Both TLR and IL-1R require the adaptor
protein myeloid differentiation primary response gene 88 (MyD88) for signaling. Although
inflammation is implicated in neuronal death in the retina, the role of MyD88-dependent TLR and IL-1R signaling in
retinal degeneration is unknown. Therefore, the purpose of this study was to investigate the role of MyD88-mediated signaling in neuronal degeneration in the
retinal degeneration 1 (rd1) mouse model, which exhibits a phenotype of rapid photoreceptor death and
inflammation. To generate rd1 mice lacking the MyD88 gene, rd1 were bred with MyD88 knockout mice (MyD88(-/-)) for several generations to produce rd1/MyD88(+/+) and rd1/MyD88(-/-) genotypes.
Chemokine mRNA expression levels were analyzed by qRT-PCR, and recruitment of activated microglia was quantified by immunodetection of the IBA-1
protein.
Retinal outer nuclear layer cell counts were performed to quantify photoreceptor degeneration, and
retinal function was assessed using electroretinograms (ERG). Our results revealed that
retinal expression of Ccl2, Ccl4, Ccl7 and Cxcl10 was reduced by 2 to 8-fold in rd1/MyD88(-/-) mice compared with rd1/MyD88(+/+) mice (p<0.05), which coincided with attenuated microglial activation, higher numbers of photoreceptors and higher retina responses to photopic and scotopic stimuli. At later ages, rd1/MyD88(-/-) had reduced
chemokine expression and higher photopic responses but no change in microglial recruitment compared with rd1 mice with functional MyD88. In conclusion, lack of MyD88-mediated signaling increased photoreceptor survival and retina function in rd1 mice, which implicates MyD88-mediated innate immunity pathways as an important pathogenic factor during
retinal degeneration.