Microglia Maintain Homeostatic Conditions in the Developing Rostral Migratory Stream (P10-9.005)

Abstract

<h3>Objective:</h3> This work examines how microglia maintain the homeostatic conditions permissive to neuroblast migration in the rostral migratory stream (RMS) during the early postnatal period. <h3>Background:</h3> Microglia phagocytosis is essential during critical periods of development to clear the results of over-exuberant neurogenesis, including defective and apoptotic cells. The importance of microglia in mediating homeostasis during neurodevelopment was underscored by the case of a child born without microglia, who exhibited neuronal heterotopias and ventriculomegaly (Oosterhof et al., 2019). <h3>Design/Methods:</h3> To illustrate microglia phagocytosis of neuroblasts in the postnatal RMS, P7 <i>CX3CR-1</i><i>^GFP/+</i><i>;DCX</i><i>^DsRed/+</i> mice were stained with primary antibodies against CD68, CLEC7A, and MERTK. Fluorescent liposomes were injected into the cerebral lateral ventricles of P1 <i>CX3CR-1</i><i>^GFP/+</i> mice to demonstrate microglia phagocytic capacity. To selectively deplete microglia <i>in vivo, Cx3cr1</i><i>^CreER/+</i><i>; ROSA26</i><i>^eGFP-DTA/+</i> mice were injected with tamoxifen i.p. at P0 and were either sacrificed at P3 or injected with tamoxifen every 4 days until sacrifice at P14. To label microglia, neuroblasts and apoptotic cells, immunohistochemistry was performed with primary antibodies against Iba1, doublecortin (DCX) and cleaved caspase-3 (CC3), respectively. <h3>Results:</h3> Microglia express phagocytic markers, engulf extracellular material and “wrap” migrating neuroblasts in the early postnatal RMS. Microglia depletion was associated with a significant increase in the density of CC3+ apoptotic neuroblasts in the RMS start and RMS descent at both P3 and P14. All five mice that expressed the DTA subunit showed patent olfactory ventricles following 14 days of tamoxifen injection. <h3>Conclusions:</h3> Microglia expressing phagocytic markers closely interact with migrating neuroblasts in the developing RMS, suggesting phagocytosis of neuroblasts. Microglia ablation compromised the homeostasis of the early postnatal RMS, causing an accumulation of apoptotic neuroblasts that broadened the RMS domain and an extended patency of the lateral ventricle extension. Microglia may thus regulate neuroblast migratory corridors through the elimination of surplus neuroblasts and phagocytosis of apoptotic debris. <b>Disclosure:</b> Dr. Meller has received research support from National Institutes of Health Grant NIDCD F31DC018469-01. The institution of Prof. GREER has received research support from NIH.