Microglial behaviour during photoreceptor degeneration
Inherited photoreceptor dystrophies are a leading cause of blindness and have no effective treatment. Despite advances in our understanding of the genetic mechanisms underlying this diverse group of conditions, the sequence of events leading from genetic miscoding to photoreceptor death by apoptosis are still unknown, although influences from other cells within the retina have been implicated. Microglia, the macrophages of the central nervous system, have previously been shown to increase in number and migrate to the photoreceptor layer to phagocytose apoptotic cell debris. However increasing awareness of the cytotoxic potential of microglia, has led me to undertake this work with the primary remit of investigating the possible involvement of microglia in photoreceptor apoptosis using the rds mouse model of inherited photoreceptor degeneration. Using immunohistochemical and immunofluorescent methods I have demonstrated increased microglial numbers in the degenerating rds retina, resulting from both in situ proliferation and recruitment from the blood, with migration to the outer retinal layers and sub-retinal space. However, by closely scrutinising the period of greatest disease activity we have demonstrated that the peak rate of photoreceptor apoptosis precedes the peak in microglial numbers by approximately five days, suggesting that microglia respond to, rather than cause photoreceptor death. In addition, evidence of oxidative damage (a major mechanism of microglial cytotoxicity) is absent and depletion of retinal microglia using macrophage-depleting clodronate liposomes did not lead to a reduction in the rate of photoreceptor death, providing further evidence that microglia are not involved in causing photoreceptor apoptosis. Additional studies with the neuroprotective tetracycline antibiotic, minocycline showed that this drug was able to delay the onset of photoreceptor apoptosis in the rds mouse, possibly through a direct inhibitory effect on apoptosis and the caspase cascade. Delayed apoptosis was associated with a corresponding delay in microglial migration to the outer retina.