Gap junctions: multifaceted regulators of embryonic cortical development

Abstract

The morphological development of the cerebral cortex from a primitive neuroepithelium into a complex laminar structure underlying higher cognition must rely on a network of intercellular signaling. Gap junctions are widely expressed during embryonic development and provide a means of cell-cell contact and communication. We review the roles of gap junctions in regulating the proliferation of neural progenitors as well as the migration and differentiation of young neurons in the embryonic cerebral cortex. There is substantial evidence that although gap junctions act in the classical manner coupling neural progenitors, they also act as hemichannels mediating the spread of calcium waves across progenitor cell populations and as adhesive molecules aiding neuronal migration. Gap junctions are thus emerging as multifaceted regulators of cortical development playing diverse roles in intercellular communication.

Figure I

Functional properties of connexins, hemichannels and gap junctions.

Figure 1

The role of gap junction coupling and hemichannels in radial glial cell proliferation. Radial glial coupling and hemichannel activity are regulated during the course of neurogenesis and within each cell cycle. Radial glial cell gap junction coupling is greatest during mid-neurogenesis and decreases in late neurogenesis [10]. During the cell cycle, cells uncouple from clusters during M phase and recouple during S phase in mid-neurogenesis or late S or G2 phase in late neurogenesis [11]. During late neurogenesis, hemichannels on S phase radial glia initiate Ca²⁺ waves by releasing ATP which binds to P2Y1 receptors on adjacent cells inducing an IP3-mediated release of Ca²⁺ from intracellular stores [20]. Additionally, during each cell cycle, the levels of Cx26 and Cx43 fluctuate such that Cx26 and Cx43 levels are at their peak during M phase and S phase, respectively [17]. Pharmacologically blocking coupling or Ca²⁺ waves inhibits entry into S phase of the cell cycle [10,20].

Figure I

bFGF-induced proliferation is dependent on Cx43 upregulation.

Figure 2

The role of gap junction adhesion in neuronal migration. (a) Several studies suggest that gap junctions mediate the radial migration of neurons to the cortical plate [16,32,33]. One approach to studying this phenomenon is through the use of in utero injection and electroporation of shRNAs targeting Cxs (Cx-shRNA) into the VZ of the developing cortex [16]. Control electroporated neurons are able to migrate to the cortical plate (CP), whereas those expressing Cx26-shRNA or Cx43-shRNA are unable to migrate to the CP and remain in the intermediate zone (IZ). The shRNA-induced migration defect can be rescued by co-expression of Cx43 or Cx26 with conservative mutations (CM) that confer resistance to shRNA knockdown. Interestingly, selective rescue experiments using a variety of Cx mutants that selectively impair the channel, adhesion or the C terminus suggest that adhesion, but not the channel or C terminus, is necessary for migration [16]. SVZ = sub-ventricular zone. (b) Time-lapse imaging of Cx-shRNA-expressing neurons as well as the dynamics of Cx puncta localization in migrating neurons suggests that gap junction adhesions play two major roles in neuronal migration. (i) Gap junction adhesions, especially Cx43 but also Cx26, localize to the dominant but not the transient branch of bifurcated migrating neurons, thereby stabilizing the dominant leading process (yellow puncta) [16]. (ii) Gap junction adhesions, especially Cx26 but also Cx43, localize to the cell body of migrating neurons, specifically co-localizing with the centrosome, and move into the dilatation in the leading process before the translocation of the nucleus, possibly stabilizing the centrosome during nuclear translocation (orange punctum) [16]. Actin puncta also co-localize with gap junction adhesions and might provide a link to the internal cytoskeleton [16]. (Adapted from [16].)