Hypomorphic wdr mutant mice have reduced brain size

Hypomorphic wdr62 mutant mice have reduced order Wnt agonist 1 size, with associated mitotic defects and an overall decrease in neural progenitor cells (Chen et al., 2014). In Drosophila, spindle orientation defects following wdr62 loss of function likely underlie the G2 delay and increased mitotic figures in NBs (this work and Nair et al., 2016). This phenotype is also reminiscent of the cleavage plane misorientation observed in NSCs in wdr62-depleted rat brains (Xu et al., 2014). In Drosophila NBs, WDR62 regulates the interphase localization of Centrosomin (CNN, mammalian CDK5RAP2) to the apical centrosome, and thus centrosomal maturation and positioning (Nair et al., 2016). Interestingly, CNN is also an AURKA target that governs spindle orientation independently from cortical polarity establishment during mitosis (Bowman et al., 2006; Lee et al., 2006). Similar to the phenotype observed for wdr62 depletion in NBs, cnn loss of function is associated with spindle orientation defects and reduced NB number (Bowman et al., 2006). Thus, it is tempting to speculate that WDR62 and CNN function in the same AURKA-dependent signaling complex during mitosis. Ex vivo studies have demonstrated that AURKA phosphorylation of WDR62 promotes spindle pole localization during mitosis (Lim et al., 2015, 2016). Mouse models suggest AURKA and WDR62 interact in vivo to control brain growth (Chen et al., 2014). Compound heterozygous wdr62;aurka mice have a much smaller body size than single heterozygotes but, although the mitotic index of the cerebral cortex was significantly increased and NSCs were reduced, consistent with a mitotic delay radial glia, potential changes to brain volume were not measured (Chen et al., 2014). Here, we demonstrate that the brain overgrowth associated with aurka depletion specifically in NBs was suppressed by co-depletion of wdr62, bringing brain volume to within the control range. In contrast, the small brain phenotypes, due to glial-specific depletion of either aurka or wdr62, were further reduced by co-knockdown. Thus, in the context of normal brain development, AURKA likely acts to promote WDR62-dependent glial proliferation, but antagonizes WDR62 function in the NB lineage. Our findings indicate that WDR62 likely functions in AURKA-mediated regulation of spindle orientation but not in the establishment of cortical polarity. One reason for the differential output of AURKA regulation of WDR62 (between NB and glia) could stem from the symmetrical nature of glial division, where there is no evidence for cortical polarization. In contrast to the in vivo mammalian studies and previous Drosophila studies, which employed global depletion strategies for wdr62, our studies have enabled dissection of the relative contribution of wdr62 loss of- function from each of the major brain lineages. In particular, the observation that depletion in either the NB or glial lineage is associated with reduced cell number, but an overall reduction in brain size was only observed when wdr62 was reduced in glia, places great interest in examining the relative contribution of glial-specific depletion of wdr62 in mice to brain size. Moreover, future studies of the pathogenic wdr62 mutations, and identified AURKA phosphorylation sites on WDR62, in the glial lineage are likely to inform on the contribution of this lineage to impaired brain growth in microcephaly.
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Author Contributions
Introduction Age-related macular degeneration (AMD) is a leading cause of blindness in the developed world (Wong et al., 2014). There are two major subtypes of AMD: non-exudative or dry AMD, which is characterized by subretinal accumulation of extracellular lipid-protein deposits termed drusen accompanied by RPE cell atrophy, and later stage exudative or wet AMD that occurs after ingrowth of the underlying choroidal vasculature into the retina (Gass, 1997). Dry AMD constitutes about 90% of AMD cases in the US, and there is currently no effective disease-altering therapy for this highly prevalent disorder.