Live Imaging at the Onset of Cortical Neurogenesis Reveals Differential Appearance of the Neuronal Phenotype in Apical versus Basal Progenitor Progeny

The neurons of the mammalian brain are generated by progenitors dividing either at the apical surface of the ventricular zone (neuroepithelial and radial glial cells, collectively referred to as apical progenitors) or at its basal side (basal progenitors, also called intermediate progenitors). For apical progenitors, the orientation of the cleavage plane relative to their apical-basal axis is thought to be of critical importance for the fate of the daughter cells. For basal progenitors, the relationship between cell polarity, cleavage plane orientation and the fate of daughter cells is unknown. Here, we have investigated these issues at the very onset of cortical neurogenesis. To directly observe the generation of neurons from apical and basal progenitors, we established a novel transgenic mouse line in which membrane GFP is expressed from the beta-III-tubulin promoter, an early pan-neuronal marker, and crossed this line with a previously described knock-in line in which nuclear GFP is expressed from the Tis21 promoter, a pan-neurogenic progenitor marker. Mitotic Tis21-positive basal progenitors nearly always divided symmetrically, generating two neurons, but, in contrast to symmetrically dividing apical progenitors, lacked apical-basal polarity and showed a nearly randomized cleavage plane orientation. Moreover, the appearance of beta-III-tubulin–driven GFP fluorescence in basal progenitor-derived neurons, in contrast to that in apical progenitor-derived neurons, was so rapid that it suggested the initiation of the neuronal phenotype already in the progenitor. Our observations imply that (i) the loss of apical-basal polarity restricts neuronal progenitors to the symmetric mode of cell division, and that (ii) basal progenitors initiate the expression of neuronal phenotype already before mitosis, in contrast to apical progenitors

ABHD4-dependent developmental anoikis safeguards the embryonic brain

During embryonic development, neural progenitor cells undergo numerous cell divisions. Here, the authors show that ABHD4-mediated developmental anoikis distinguishes the physiological delamination and the pathological detachment of progenitor cells with relevance to fetal alcohol-induced apoptosis

The Nf2 tumor suppressor regulates cell–cell adhesion during tissue fusion

Tissue fusion, the morphogenic process by which epithelial sheets are drawn together and sealed, has been extensively studied in Drosophila. However, there are unique features of mammalian tissue fusion that remain poorly understood. Notably, detachment and apoptosis occur at the leading front in mammals but not in invertebrates. We found that in the mouse embryo, expression of the Nf2 tumor suppressor, merlin, is dynamically regulated during tissue fusion: Nf2 expression is low at the leading front before fusion and high across the fused tissue bridge. Mosaic Nf2 mutants exhibit a global defect in tissue fusion characterized by ectopic detachment and increased detachment-induced apoptosis (anoikis). By contrast with core components of the junctional complex, we find that merlin is required specifically for the assembly but not the maintenance of the junctional complex. Our work reveals that regulation of Nf2 expression is a previously unrecognized means of controlling adhesion at the leading front, thereby ensuring successful tissue fusion

Nedd1 expression as a marker of dynamic centrosomal localization during mouse embryonic development

The original publication can be found at www.springerlink.comAs the primary microtubule-organizing centre of the mammalian cell, the centrosome plays many important roles during cell growth and organization. This is evident across a broad range of cell types and processes, such as the proliferation, differentiation and polarity of neural cells. Additionally, given its localization and function, there are likely to be many more processes that rely on the centrosome that have not yet been characterized. Currently, little is known about centrosomal dynamics during mammalian development. In this study, we have analyzed Nedd1 protein expression to characterize the localization of the centrosome during some aspects of mouse embryogenesis. Using a Nedd1 antibody we have demonstrated the colocalization of Nedd1 with centrosomal markers. We found strong expression of Nedd1, and therefore the centrosome, in highly proliferating cells during neural development. Additionally, Nedd1 was found to have high expression in the cytoplasm of a subset of cells in the dorsal root ganglia. We have also shown a distinct, polarized centrosomal localization of Nedd1 in the developing lens, retina and other polarized tissues. This study reveals the localization of Nedd1 and the centrosome during important processes in mouse embryogenesis and provides a basis for further study into its role in development.Jantina A. Manning, Paul A. Colussi, Simon A. Koblar and Sharad Kuma

Mitotic spindle orientation predicts outer radial glial cell generation in human neocortex

The human neocortex is increased in size and complexity as compared to most other species. Neocortical expansion has recently been attributed to protracted neurogenesis by outer radial glial (oRG) cells in the outer subventricular zone (oSVZ), a region present in humans but not in rodents. The mechanisms of human oRG cell generation are unknown, but are proposed to involve division of ventricular radial glial (vRG) cells; neural stem cells present in all developing mammals. Here we show that human vRG cells produce oRG cells and seed formation of the oSVZ via horizontal divisions, which occur more frequently in humans than in rodents. We further find that oRG cell mitotic behavior is cell intrinsic, and that the basal fiber, inherited by oRG cells after vRG division, determines cleavage angle. Our results suggest that altered regulation of mitotic spindle orientation increased oRG cell numbers, and ultimately neuronal numbers, during human brain evolution

Recessive Mutations in the Gene Encoding the Tight Junction Protein Occludin Cause Band-like Calcification with Simplified Gyration and Polymicrogyria

Band-like calcification with simplified gyration and polymicrogyria (BLC-PMG) is a rare autosomal-recessive neurological disorder showing highly characteristic clinical and neuroradiological features. Affected individuals demonstrate early-onset seizures, severe microcephaly, and developmental arrest with bilateral, symmetrical polymicrogyria (PMG) and a band of gray matter calcification on brain imaging; as such, the disorder can be considered as a “pseudo-TORCH” syndrome. By using autozygosity mapping and copy number analysis we identified intragenic deletions and mutations in OCLN in nine patients from six families with BLC-PMG. The OCLN gene encodes occludin, an integral component of tight junctions. Neuropathological analysis of an affected individual showed similarity to the mouse model of occludin deficiency with calcification predominantly associated with blood vessels. Both intracranial calcification and PMG are heterogeneous in etiology. Neuropathological and clinical studies of PMG have suggested that in utero ischemic or vascular insults may contribute to this common cortical abnormality. Tight junctions are functional in cerebral blood vessels early in fetal development and continue to play a vital role in maintenance of the blood-brain barrier during postnatal life. We provide evidence that the tight junction protein occludin (encoded by the OCLN gene) is involved in the pathogenesis of malformations of cortical development