The role of centriole biogenesis in control of brain size

Abstract

Mutations in several genes that encode centrosomal proteins dramatically decrease the size of the human brain, which is the largest in the primate lineage, but how the proteins encoded by these microcephaly (‘small brain’) genes interact in a cellular process is poorly understood. The centrosome is the main microtubule-nucleating organelle in animal cells and consists of two centrioles, which duplicate once per cell cycle. Asymmetric inheritance of centrosomes may be critical to the maintenance of stem cells but the mechanism is controversial. In this dissertation, I characterize the functions of ASPM and WDR62, the two most common genetic causes of primary microcephaly, in centriole biogenesis and neocortical development, using in vivo loss-of-function studies in the mouse, combined with biochemical and cell biological approaches.

Here, I show that WDR62 and ASPM encode proteins that localize to the mother centriole, interact physically, control a common cellular function, and interact genetically to control brain size in mice. Whereas mice lacking either Wdr62 or Aspm are microcephalic but viable, mice lacking both are embryonically lethal, and heterozygous mutation in either gene enhances the phenotype of mutations in the other, suggesting a genetic interaction between Wdr62 and Aspm. Mass spectrometry analysis of the WDR62 interactome identified ASPM as a binding partner of WDR62, which was confirmed by co-immunoprecipitation. Mouse embryonic fibroblasts (MEFs) deficient in Wdr62, Aspm or both show similar defects in centriole duplication, with the severity of the cellular defect proportional to the severity of the microcephaly. This defect in centriole duplication was also confirmed in situ in the developing mouse brain by crossbreeding with a GFP-Centrin reporter line. WDR62 is required for centrosomal localization of ASPM and other microcephaly-associated proteins. Loss of Wdr62 causes a reduction in centrosomes and cilia, as well as a premature dissociation of ciliary membrane remnants from centrosomes during neurogenesis, resulting in a precocious generation of basal progenitors at the expense of apical progenitors.

Together, these results reveal previously unknown functions of, and interactions between, WDR62 and ASPM in centriole biogenesis and neocortical development. Microcephaly genes may thus cooperate in ensuring centriole duplication, maintaining adequate numbers of centrosomes, cilia and apical progenitors during neurogenesis, and regulating brain size.