Mouse Inscuteable Induces Apical-Basal Spindle Orientation to Facilitate Intermediate Progenitor Generation in the Developing Neocortex

Abstract

SummaryNeurons in the mammalian neocortex arise from asymmetric divisions of progenitors residing in the ventricular zone. While in most progenitor divisions, the mitotic spindle is parallel to the ventricular surface, some progenitors reorient the spindle and divide in oblique orientations. Here, we use conditional deletion and overexpression of mouse Inscuteable (mInsc) to analyze the relevance of spindle reorientation in cortical progenitors. Mutating mInsc almost abolishes oblique and vertical mitotic spindles, while mInsc overexpression has the opposite effect. Our data suggest that oblique divisions are essential for generating the correct numbers of neurons in all cortical layers. Using clonal analysis, we demonstrate that spindle orientation affects the rate of indirect neurogenesis, a process where progenitors give rise to basal progenitors, which in turn divide symmetrically into two differentiating neurons. Our results indicate that the orientation of progenitor cell divisions is important for correct lineage specification in the developing mammalian brain.

Analysis and modeling of mitotic spindle orientations in three dimensions

“…To facilitate the measurement of spindle orientation in three dimensions, we developed an algorithm that allows the calculation of spindle angles from confocal image stacks (15). The angle of the mitotic spindle is typically determined relative to a reference surface, for example the ventricular surface in the mouse cortex.…”

“…We therefore used our modeling approach to ask whether mInsc blocks the machinery for horizontal spindle orientation and shifts the distribution of spindle orientations to a more random-like distribution or whether mInsc actively promotes vertical spindle orientation during mouse corticogenesis. We modeled the distribution of spindle angles for apical progenitors from wild-type, mInsc knock-out, and mInsc overexpressing mice (15) and determined the enrichment parameters λ h and λ v . Fig.…”

“…At later stages, asymmetric divisions of the same progenitor cells generate both self-renewing daughter cells and also cells that give rise to transit-amplifying cells or neurons (8)(9)(10)(11)(12)(13)). The precise control of spindle orientation is vital for determining the different cell fate decisions during the course of cortical development (14)(15)(16).…”

Drosophila neuroblasts: a model for stem cell biology

“…Overexpression of mouse Insc, by contrast, has the opposite effect and increases the number of IPCs (Postiglione et al, 2011). Strikingly, mouse Insc overexpression or expression of a dominant-negative version of Lgn also increases the number of oRG cells, indicating that nonplanar divisions are more likely to generate the non-apical progenitor cells (Konno et al, 2008;Postiglione et al, 2011;Shitamukai et al, 2011;Wang et al, 2011a). As the number of oRG cells is dramatically increased in primates, this opens up exciting evolutionary perspectives for the spindle orientation field.…”

“…The mammalian Mud homolog NuMA (a nuclear protein that associates with the mitotic apparatus; Numa1 -Mouse Genome Informatics) has a role in the establishment and maintenance of spindle poles (Sun and Schatten, 2006;Silk et al, 2009). Finally, the single vertebrate homolog of Drosophila Inscuteable is required and sufficient for inducing non-planar spindle orientation (Zigman et al, 2005;Konno et al, 2008;Postiglione et al, 2011). Thus, conserved molecular machinery regulates the orientation of progenitor divisions.…”