Proliferation of Microglia, but not Photoreceptors, in the Outer Nuclear Layer of the rd-1 Mouse

To establish whether photoreceptor apoptosis in the rd-1 mouse is accompanied by cell cycle progression. Studies of cell cycle proteins in other models of neuronal death provide consistent evidence that a repertoire of proliferative markers accompanies apoptosis. The spatiotemporal progression of photoreceptor loss in rd-1 and control mice at postnatal days (PN)8, -10, -12, -15, and -18 was correlated with markers of G(1)- and S-phase progression. Photoreceptor death was detected by using morphology and terminal dUTP transferase nick end labeling (TUNEL). Cell-cycle-associated markers consisted of bromodeoxyuridine (BrdU) uptake, and immunolabeling for proliferating cell nuclear antigen (PCNA), Ki-67, and cyclin-dependent kinases-2 and -4. The identity of proliferating cells in the outer nuclear layer was established by double immunolabeling with PCNA and either F4/80 or recoverin. A population of proliferating cells in the outer nuclear layer accompanies photoreceptor death along a central to peripheral gradient in rd-1 retinas. Double immunolabeling for PCNA and F4/80 readily identified these as microglial cells originating from the inner retina. Cell cycle progression in photoreceptors could not be demonstrated. These findings confirm that in rd-1, a preexisting condition for cell cycle progression does not exist as it does in other neurodegenerative conditions. Therefore, in this model, evidence of photoreceptor cell cycle progression in retinas exposed to neurotrophic factors is likely to result from the therapy itself. In addition, the results confirmed that proliferating microglial cells are intimately associated with the degenerative process in rd-1.