Function and mechanism of cell coupling during neocortical neurogenesis in the mouse

The majority of neurons that populate the adult mammalian neocortex are generated through a discrete number of cell cycles within the ventricular zone (VZ) of the developing brain. The cellular and molecular mechanisms that regulate the production of neurons from the VZ are poorly understood; however cell to cell interactions are believed to play an essential role. A mechanism of cellular interaction that may operate to regulate neurogenesis is cell coupling through gap junction channels. Within the VZ, cells specifically couple together into clusters through gapjunction proteins and then uncouple as neurons are generated. In order to determine more specifically how coupling may regulate neurogenesis, experiments were performed to determine (i) which cell types in the VZ couple together, (ii) if coupling changes through neurogenesis, (iii) if gap junction protein expression changes through neurogenesis, and (iv) if cell coupling influences the number of VZ cells that enter the cell cycle. The results demonstrate that VZ clusters contain radial glial cells as well as neural precursors. M phase cells and post-mitotic neurons are not contained in clusters and coupling between cycling VZ cells changes through the cell cycle. VZ cells couple maximally by G2 phase of the cell cycle, uncouple during M phase, and re-couple by the next S phase of the cell cycle. If VZ cells are pharmacologically prevented from re-coupling in S phase, then they are inhibited from re-entering the cell cycle. Gap junction protein expression also changes through the cell cycle. The percentage of cells expressing the gap junction protein connexin 26 (Cx26) increases from S through G2 phase, while the percentage of cells expressing the gapjunction protein connexin 43 (Cx43) decreases from S to G1. Together, these results show that coupling and connexin expression change systematically through the cell cycle and suggest that cell to cell interactions between neural precursors in specific phases of the cell cycle radial glia shape neurogenesis.