Microglia and Progranulin Regulate Neurogenesis in the Developing Vertebrate Retina.

Microglia are classically thought of as the resident macrophages of the CNS. However, evidence of the neurogenic role microglia play in the developing and adult brain is increasing. The main goal of my thesis research is to use zebrafish retinal development as a model to test the hypothesis that microglia and the factors they express and secrete play a fundamental role in vertebrate neurogenesis. Microglia derive from primitive yolk sac macrophages that migrate to and colonize the developing CNS. I utilized genetic and pharmacological models that prevent microglial precursors from colonizing neuroepithelial tissues to determine the function of microglia and their secreted factors during developmental neurogenesis in the retina. The results showed that when microglia are excluded from the retina, neurogenesis is significantly altered. Retinal progenitors do not exit the cell cycle at the appropriate developmental time, and neuronal differentiation and early retinal growth is delayed. I also evaluated the expression and function of the microglia-specific growth factor, Pgrn-a. At 24hpf, pgrn-a is expressed throughout the forebrain, but by 48hpf becomes exclusively expressed by microglia and their precursors. Compared to the genetic and pharmacological models, knockdown of Pgrn-a using morpholinos resulted in a similar, albeit more severe retinal phenotype. Blocking Pgrn-a translation diminished the number of microglia in the retina and caused a significant lengthening of retinal progenitor cell cycle and a corresponding paucity of neuronal differentiation. These data demonstrate that microglial colonization of embryonic neuroepithelial tissues is required for early neurogenic events in the developing retina. Further, Pgrn-a functions to recruit microglial precursors to the embryonic CNS, govern cell cycle kinetics, cell cycle exit and neuronal differentiation. Therefore, microglia and the factors they secrete play a fundamental role in governing early neurogenic events and serve as cellular components that regulate the brain’s intrinsic timing of the cell cycle. To enable further study of the function of microglia and Pgrn-a in the developing, adult, and regenerating retina, I recombineered BAC constructs to generate transgenic pgrn-a reporter zebrafish lines that will enable conditional targeted genetic ablation of microglial cells, and used genome engineering technology to establish a pgrn-a mutant zebrafish line.PhDNeuroscienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133182/1/walshce_2.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133182/2/walshce_3.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133182/3/walshce_1.pd