Emx1 Is Required for Neocortical Area Patterning.

Establishing appropriate area patterning in the neocortex is a critical developmental event, and transcription factors whose expression is graded across the developing neural axes have been implicated in this process. While previous reports suggested that the transcription factor Emx1 does not contribute to neocortical area patterning, those studies were performed at perinatal ages prior to the emergence of primary areas. We therefore examined two different Emx1 deletion mouse lines once primary areas possess mature features. Following the deletion of Emx1, the frontal and motor areas were expanded while the primary visual area was reduced, and overall the areas shifted posterio-medially. This patterning phenotype was consistent between the two Emx1 deletion strategies. The present study demonstrates that Emx1 is an area patterning transcription factor and is required for the specification of the primary visual area.

Genetic mechanisms control the linear scaling between related cortical primary and higher order sensory areas.

In mammals, the neocortical layout consists of few modality-specific primary sensory areas and a multitude of higher order ones. Abnormal layout of cortical areas may disrupt sensory function and behavior. Developmental genetic mechanisms specify primary areas, but mechanisms influencing higher order area properties are unknown. By exploiting gain-of and loss-of function mouse models of the transcription factor Emx2, we have generated bi-directional changes in primary visual cortex size in vivo and have used it as a model to show a novel and prominent function for genetic mechanisms regulating primary visual area size and also proportionally dictating the sizes of surrounding higher order visual areas. This finding redefines the role for intrinsic genetic mechanisms to concomitantly specify and scale primary and related higher order sensory areas in a linear fashion.

The role of adherens junctions in the developing neocortex.

The disproportional enlargement of the neocortex through evolution has been instrumental in the success of vertebrates, in particular mammals. The neocortex is a multilayered sheet of neurons generated from a simple proliferative neuroepithelium through a myriad of mechanisms with substantial evolutionary conservation. This developing neuroepithelium is populated by progenitors that can generate additional progenitors as well as post-mitotic neurons. Subtle alterations in the production of progenitors vs. differentiated cells during development can result in dramatic differences in neocortical size. This review article will examine how cadherin adhesion proteins, in particular α-catenin and N-cadherin, function in regulating the neural progenitor microenvironment, cell proliferation, and differentiation in cortical development.

Cortical neural precursors inhibit their own differentiation via N-cadherin maintenance of beta-catenin signaling.

Little is known about the architecture of cellular microenvironments that support stem and precursor cells during tissue development. Although adult stem cell niches are organized by specialized supporting cells, in the developing cerebral cortex, neural stem/precursor cells reside in a neurogenic niche lacking distinct supporting cells. Here, we find that neural precursors themselves comprise the niche and regulate their own development. Precursor-precursor contact regulates beta-catenin signaling and cell fate. In vivo knockdown of N-cadherin reduces beta-catenin signaling, migration from the niche, and neuronal differentiation in vivo. N-cadherin engagement activates beta-catenin signaling via Akt, suggesting a mechanism through which cells in tissues can regulate their development. These results suggest that neural precursor cell interactions can generate a self-supportive niche to regulate their own number.

Activity of the beta-catenin phosphodestruction complex at cell-cell contacts is enhanced by cadherin-based adhesion.

It is well established that cadherin protein levels impact canonical Wnt signaling through binding and sequestering beta-catenin (beta-cat) from T-cell factor family transcription factors. Whether changes in intercellular adhesion can affect beta-cat signaling and the mechanism through which this occurs has remained unresolved. We show that axin, APC2, GSK-3beta and N-terminally phosphorylated forms of beta-cat can localize to cell-cell contacts in a complex that is molecularly distinct from the cadherin-catenin adhesive complex. Nonetheless, cadherins can promote the N-terminal phosphorylation of beta-cat, and cell-cell adhesion increases the turnover of cytosolic beta-cat. Together, these data suggest that cadherin-based cell-cell adhesion limits Wnt signals by promoting the activity of a junction-localized beta-cat phosphodestruction complex, which may be relevant to tissue morphogenesis and cell fate decisions during development.

Focal reduction of alphaE-catenin causes premature differentiation and reduction of beta-catenin signaling during cortical development.

Cerebral cortical precursor cells reside in a neuroepithelial cell layer that regulates their proliferation and differentiation. Global disruptions in epithelial architecture induced by loss of the adherens junction component alphaE-catenin lead to hyperproliferation. Here we show that cell autonomous reduction of alphaE-catenin in the background of normal precursors in vivo causes cells to prematurely exit the cell cycle, differentiate into neurons, and migrate to the cortical plate, while normal neighboring precursors are unaffected. Mechanistically, alphaE-catenin likely regulates cortical precursor differentiation by maintaining beta-catenin signaling, as reduction of alphaE-catenin leads to reduction of beta-catenin signaling in vivo. These results demonstrate that, at the cellular level, alphaE-catenin serves to maintain precursors in the proliferative ventricular zone, and suggest an unexpected function for alphaE-catenin in preserving beta-catenin signaling during cortical development.

Differential expression of alpha-E-catenin and alpha-N-catenin in the developing cerebral cortex.

Alpha (alpha) catenin proteins can regulate both cell adhesion and cell proliferation. Here, we characterize the expression of two forms of alpha-catenin, alphaE-catenin and alphaN-catenin, in the developing cerebral cortex and primary cortical cultures. In situ hybridization and immunofluorescence studies reveal that alphaE-catenin is highly expressed in neuroepithelial precursor cells in the developing cortical ventricular zone, with markedly reduced expression in the cortical plate; in contrast, alphaN-catenin expression is low in the ventricular zone and high in the developing cortical plate. In the ventricular zone, immunoreactivity for both alphaE-catenin and alphaN-catenin is enriched in rings at the lumenal surface, reflecting localization at adherens junctions together with beta-catenin. Expression of alphaE-catenin in primary cortical precursor cultures is initially robust, but declines as neural precursors differentiate into neurons. Reflecting its expression pattern in vivo, alphaN-catenin is expressed in both neural precursors as well in neurons differentiated in culture. These differential expression patterns of alphaE-catenin and alphaN-catenin suggest both distinct and overlapping functions during cortical development.