Radial glia cells perform a dual function in the developing nervous system as precursor cells and guides for migrating neurons. We show here that during forebrain neurogenesis, the transcription factor Pax6 is specifically localized in radial glia cells of the cortex but not of the basal telencephalon. In Pax6-deficient mice, cortical radial glia cells were altered in their morphology, number, tenascin-C (TN-C) expression, and cell cycle. We show that some of these alterations are cell-autonomous, whereas others were rescued by coculturing with wild-type cortical cells. Our results suggest that Pax6 plays an essential role in the differentiation of cortical radial glia. Thus, despite their widespread distribution, radial glia cells are regionally specified in the developing CNS.

We have examined the transcript distribution of six members of the murine paired box-containing gene family (Pax-gene family) in midgestation embryo and adult brain using in situ hybridization analysis. The expression domains of several Pax-genes in the embryo brain were found to correspond with anatomical boundaries that coincide with neuromere landmarks and therefore respect former neuromere territories in the forebrain. The results are consistent with the concept of brain segmentation and suggest a role for Pax-genes in the brain regionalization. In the adult brain the expression of Pax-genes was observed in discreet areas, with a caudal to rostral restriction in the number of the expressed genes. In general the distribution of transcripts along the anterior-posterior axis was similar to that found in midgestation embryo brain, suggesting a role for Pax-genes in the commitment of the precursor cells to different neuronal cell fates and in the maintenance of specific brain cell subtypes. In the cerebellar cortex, the granular cell layer was found to express high levels of the Pax-6 gene, while putative Bergmann glia and cells surrounding the Purkinje cells contained Pax-3 transcripts. The main adult brain structures that expressed distinct Pax-mRNAs were the periglomerular and granular cell layer of olfactory bulb, nuclei of the septum, amygdala, and isthmus, which suggests a role for the Pax-gene family in the specification of the subcortical domains of the evolutionary old limbic system.

Autophagy is a self-degradative process involved both in basal turnover of cellular components and in response to nutrient starvation or organelle damage in a wide range of eukaryotes. During autophagy, portions of the cytoplasm are sequestered by double-membraned vesicles called autophagosomes, and are degraded after fusion with lysosomes for subsequent recycling. In vertebrates, this process acts as a pro-survival or pro-death mechanism in different physiological and pathological conditions, such as neurodegeneration and cancer; however, the roles of autophagy during embryonic development are still largely uncharacterized. Beclin1 (Becn1; coiled-coil, myosin-like BCL2-interacting protein) is a principal regulator in autophagosome formation, and its deficiency results in early embryonic lethality. Here we show that Ambra1 (activating molecule in Beclin1-regulated autophagy), a large, previously unknown protein bearing a WD40 domain at its amino terminus, regulates autophagy and has a crucial role in embryogenesis. We found that Ambra1 is a positive regulator of the Becn1-dependent programme of autophagy, as revealed by its overexpression and by RNA interference experiments in vitro. Notably, Ambra1 functional deficiency in mouse embryos leads to severe neural tube defects associated with autophagy impairment, accumulation of ubiquitinated proteins, unbalanced cell proliferation and excessive apoptotic cell death. In addition to identifying a new and essential element regulating the autophagy programme, our results provide in vivo evidence supporting the existence of a complex interplay between autophagy, cell growth and cell death required for neural development in mammals.

The Pax6 gene encodes a transcription factor with a restricted expression in the ventricular zone of the pallium and subpallium. We tested whether the function of Pax6 is necessary for the correct patterning and morphogenesis of the vertebrate telencephalon. Homozygous embryos of the Pax6/Small eye mutant lack functional PAX6 protein because of a point mutation of the gene. In the mutant Small eye embryos we detected a ventralization of the molecular patterning of the telencephalon at two borders, the pallium/subpallium and the lateral/medial ganglionic eminence. The results indicate that Pax6 controls the lateral limit of the expression of Nkx2.1, Shh, and Lhx6 in the prechordal neural tube, the telencephalon. This finding is in agreement with previous studies and supports a model for a common genetic mechanism for modulation of the dorsoventral patterning of the prechordal and epichordal CNS. The pattern defects caused by the loss of Pax6 function result in multiple morphological abnormalities in the Small eye brain: dysgenesis of the piriform, insular, and lateral cortices, the claustrum-endopiriform nucleus, and a failure in the differentiation of a subpopulation of the cortical precursors. Together the results demonstrate that Pax6 has an essential role for the modulation of the dorsoventral patterning of the embryonic telencephalon, influencing thereby the forebrain morphogenesis.

Pax6 is a regulatory gene with restricted expression and essential functions in the developing eye and pancreas and distinct domains of the CNS. In this study we report the identification of three conserved transcription start sites (P0, P1, alpha) in the murine Pax6 locus. Furthermore, using transgenic mouse technology we localized independent cis-regulatory elements controlling the tissue-specific expression of Pax6. Specifically, a 107-bp enhancer and a 1.1-kb sequence within the 4.6-kb untranslated region upstream of exon 0 are required to mediate Pax6 expression in the lens, cornea, lacrimal gland, conjunctiva, or pancreas, respectively. Another 530-bp enhancer fragment located downstream of the Pax6 translational start site is required for expression in the neural retina, the pigment layer of the retina, and the iris. Finally, a 5-kb fragment located between the promoters P0 and P1 can mediate expression into the dorsal telencephalon, the hindbrain, and the spinal cord. The identified Pax6/cis-essential elements are highly conserved in pufferfish, mouse, and human DNA and contain binding sites for several transcription factors indicative of the cascade of control events. Corresponding regulatory elements from pufferfish are able to mimic the reporter expression in transgenic mice. Thus, the results indicate a structural and functional conservation of the Pax6 regulatory elements in the vertebrate genome.

Increased cortical size is essential to the enhanced intellectual capacity of primates during mammalian evolution. The mechanisms that control cortical size are largely unknown. Here, we show that mammalian BAF170, a subunit of the chromatin remodeling complex mSWI/SNF, is an intrinsic factor that controls cortical size. We find that conditional deletion of BAF170 promotes indirect neurogenesis by increasing the pool of intermediate progenitors (IPs) and results in an enlarged cortex, whereas cortex-specific BAF170 overexpression results in the opposite phenotype. Mechanistically, BAF170 competes with BAF155 subunit in the BAF complex, affecting euchromatin structure and thereby modulating the binding efficiency of the Pax6/REST-corepressor complex to Pax6 target genes that regulate the generation of IPs and late cortical progenitors. Our findings reveal a molecular mechanism mediated by the mSWI/SNF chromatin-remodeling complex that controls cortical architecture.

This review aims to provide examples of how both comparative and genetic analyses contribute to our understanding of the rules for cortical development and evolution. Genetic studies have helped us to realize the evolutionary rules of telencephalic organization in vertebrates. The control of the establishment of conserved telencephalic subdivisions and the formation of boundaries between these subdivisions has been examined and the very specific alterations at the striatocortical junction have been revealed. Comparative studies and genetic analyses both demonstrate the differential origin and migratory pattern of the two basic neuron types of the cerebral cortex. GABAergic interneurons are mostly generated in the subpallium and a common mechanism governs their migration to the dorsal cortex in both mammals and sauropsids. The pyramidal neurons are generated within the cortical germinal zone and migrate radially, the earliest generated cell layers comprising preplate cells. Reelin-positive Cajal-Retzius cells are a general feature of all vertebrates studied so far; however, there is a considerable amplification of the Reelin signalling with cortical complexity, which might have contributed to the establishment of the basic mammalian pattern of cortical development. Based on numerous recent observations we shall present the argument that specialization of the mitotic compartments may constitute a major drive behind the evolution of the mammalian cortex. Comparative developmental studies have revealed distinct features in the early compartments of the developing macaque brain, drawing our attention to the limitations of some of the current model systems for understanding human developmental abnormalities of the cortex. Comparative and genetic aspects of cortical development both reveal the workings of evolution.

The amygdaloid complex is a group of nuclei that are thought to originate from multiple sites of the dorsal and ventral telencephalic neuroepithelium. The mechanisms that regulate their development are essentially unknown. We studied the role of Pax6 and Emx2, two transcription factors that regulate regional specification and growth of the telencephalon, in the morphogenesis of the amygdaloid complex. We used a set of specific marker genes that identify distinct amygdaloid nuclei to analyze Pax6/Small eye and Emx2 knock-out mutant mouse brains. We found that there is a selective requirement for Pax6, but not Emx2, in the formation a subset of nuclei within the amygdaloid complex. Specifically, structures that were not previously considered to be developmentally linked, the nucleus of the lateral olfactory tract and the lateral, basolateral, and basomedial nuclei, all appear to have a common requirement for Pax6. Together, our findings provide new insights into the origins and mechanisms underlying the development of the amygdaloid complex.