Dynamic regulation of adhesion complexes is required for cell migration and has therefore emerged as a key issue in the study of cell motility. Recent progress has been made in defining some of the molecular mechanisms by which adhesion disassembly is regulated, including the contributions of adhesion adaptor proteins and tyrosine kinases. However, little is known about the potential contribution of proteolytic mechanisms to the regulation of adhesion complex dynamics. Here, we show that proteolysis of talin by the intracellular calcium-dependent protease calpain is critical for focal adhesion disassembly. We have generated a single point mutation in talin that renders it resistant to proteolysis by calpain. Quantification of adhesion assembly and disassembly rates demonstrates that calpain-mediated talin proteolysis is a rate-limiting step during adhesion turnover. Furthermore, we demonstrate that disassembly of other adhesion components, including paxillin, vinculin and zyxin, is also dependent on the ability of calpain to cleave talin, suggesting a general role for talin proteolysis in regulating adhesion turnover. Together, these findings identify calpain-mediated proteolysis of talin as a mechanism by which adhesion dynamics are regulated.

SUMMARY Neuronal migration is critical for establishing neocortical cell layers and migration defects can cause neurological and psychiatric diseases. Recent studies show that radially migrating neocortical neurons use glia-dependent and glia-independent modes of migration, but the signaling pathways that control different migration modes and the transitions between them are poorly defined. Here, we show that Dab1, an essential component of the reelin pathway, is required in radially migrating neurons for glia-independent somal translocation, but not for glia-guided locomotion. During migration, Dab1 acts in translocating neurons to stabilize their leading processes in a Rap1-dependent manner. Rap1, in turn, controls cadherin function to regulate somal translocation. Furthermore, cell-autonomous neuronal deficits in somal translocation are sufficient to cause severe neocortical lamination defects. Thus, we define the cellular mechanism of reelin function during radial migration, elucidate the molecular pathway downstream of Dab1 during somal translocation, and establish the importance of glia-independent motility in neocortical development.

The calpain family of proteases has been implicated in cellular processes such as apoptosis, proliferation and cell migration. Calpains are involved in several key aspects of migration, including: adhesion and spreading; detachment of the rear; integrin- and growth-factor-mediated signaling; and membrane protrusion. Our understanding of how calpains are activated and regulated during cell migration has increased as studies have identified roles for calcium and phospholipid binding, autolysis, phosphorylation and inhibition by calpastatin in the modulation of calpain activity. Knockout and knockdown approaches have also contributed significantly to our knowledge of calpain biology, particularly with respect to the specific functions of different calpain isoforms. The mechanisms by which calpain-mediated proteolysis of individual substrates contributes to cell motility have begun to be addressed, and these efforts have revealed roles for proteolysis of specific substrates in integrin activation, adhesion complex turnover and membrane protrusion dynamics. Understanding these mechanisms should provide avenues for novel therapeutic strategies to treat pathological processes such as tumor metastasis and chronic inflammatory disease.

During development of the mammalian cerebral cortex, radial glial cells (RGCs) generate layer-specific subtypes of excitatory neurons in a defined temporal sequence, in which lower layer neurons are formed before upper layer neurons. It has been proposed that neuronal subtype fate is determined by birthdate via progressive restriction of the neurogenic potential of a common RGC progenitor. We now demonstrate that the murine cerebral cortex contains RGC sublineages with distinct fate potentials. Using in vivo genetic fate mapping and in vitro clonal analysis, we identify an RGC lineage that is intrinsically specified to generate only upper layer neurons, independently of niche and birthdate. Since upper cortical layers were expanded during primate evolution, amplification of this RGC pool may have facilitated human brain evolution.

An astonishing number of extracellular matrix glycoproteins are expressed in dynamic patterns in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. Functional studies in vitro and genetic studies in mice have provided evidence that the extracellular matrix affects virtually all aspects of nervous system development and function. Here we will summarize recent findings that have shed light on the specific functions of defined extracellular matrix molecules on such diverse processes as neural stem cell differentiation, neuronal migration, the formation of axonal tracts, and the maturation and function of synapses in the peripheral and central nervous system.

The neural circuits of the mammalian neocortex are crucial for perception, complex thought, cognition and consciousness. This circuitry is assembled from many different neuronal subtypes with divergent properties and functions. Here, we review recent studies that have begun to clarify the mechanisms of cell-type specification in the neocortex, focusing on the lineage relationships between neocortical progenitors and subclasses of excitatory projection neurons. These studies reveal an unanticipated diversity in the progenitor pool that requires a revised view of prevailing models of cell-type specification in the neocortex. We propose a “sequential progenitor-diversification model” that integrates current knowledge to explain how projection neuron diversity is achieved by mechanisms acting on proliferating progenitors and their postmitotic offspring. We discuss the implications of this model for our understanding of brain evolution and pathological states of the neocortex.

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Cajal-Retzius (CR) cells are a transient cell population of the CNS that is critical for brain development. In the neocortex, CR cells secrete reelin to instruct the radial migration of projection neurons. It has remained unexplored, however, whether CR cells provide additional molecular cues important for brain development. Here, we show that CR cells express the immunoglobulin-like adhesion molecule nectin1, whereas neocortical projection neurons express its preferred binding partner, nectin3. We demonstrate that nectin1/3-mediated interactions between CR cells and the leading processes of migrating neurons are critical for radial migration. Furthermore, reelin signaling to Rap1 promotes Cdh2 function in neurons via nectin3 and afadin, thus directing the broadly expressed homophilic cell adhesion molecule Cdh2 towards mediating heterotypic cell-cell interactions between neurons and CR cells. Our findings identify nectins/afadin as components of the reelin signaling pathway and demonstrate that coincidence signaling between CR cell-derived secreted and short-range guidance-cues direct neuronal migration.