| Summary: | During brain development, young neurons closely associate with radial glia while migrating from the ventricular zone (VZ) to the cortical plate (CP) of the neocortex. It has previously been shown that gap junctions are needed for this migration to occur properly, but the precise mechanism responsible is unclear. Using reverse transcription-polymerase chain reaction, western blot analysis, and immunohistochemistry, we found that among the family of gap junction proteins, connexin (Cx) 26, Cx36, Cx37, Cx43, and Cx45 were expressed in the mouse neocortex. In addition Cx43 and Cx26 were highly expressed in the radial glia and migrating neurons. Therefore, to examine the role of Cx43 in neuronal migration, we used Cre recombinase, driven by the nestin promoter, to conditionally knock-out a floxed coding DNA of the Cx43 gene in mice. Radial glia in the VZ normally express Cx43. They undergo divisions that produce neurons, and serve as migratory guides for the daughter cells that they produce. Based on histological analysis, we proposed that removing Cx43 from radial glia alters the normal lamination of the mouse neocortex. To monitor newborn neurons, we introduced a plasmid containing green fluorescent protein driven by a neuronal (Tα1 tubulin) promoter into the embryonic neocortex using in utero electroporation. The majority of transfected migrating neurons remained in the VZ/intermediate zone (IZ) of the Cx43 conditional knock-out (Cx43cKO) animals, whereas in Cx43fl/fl mice, neurons migrated through the IZ into the CP, indicating that deletion of Cx43 from nestin-positive cells disrupts neuronal migration. We were able to rescue migration of Cx43cKO neurons by electroporating a cytomegalovirus-Cx43 expression plasmid into the embryonic cortex. In contrast, a C-terminal truncated form of Cx43 failed to rescue migration. In addition, Cx43K²⁵⁸stop/- mice, in which Cx43 lacks the last 125 amino acid residues of the C-terminal tail, gave results similar to those seen with the Cx43cKO mice. Furthermore, we conducted experiments with more specific deletions within the C-terminal tail, and found that a narrow region of 47 amino acid residues is required in directing neuronal migration. This study illustrates that full length Cx43 is required for neuronal migration in the neocortex. === Medicine, Faculty of === Graduate
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