N-cadherin-mediated cell adhesion regulates proliferation and differentiation in the developing zebrafish central nervous system

Cadherins play an important role in morphogenesis and have recently been implicated in the regulation of cell proliferation, however the mechanisms by which they function are poorly understood. In the vertebrate CNS, loss of N-cadherin (N-cad) results in impaired neuroepithelial integrity. Zebrafish N-cad null mutants also exhibit a transient increase in neurons and in cell proliferation in the neural tube. Here, we investigate the cellular and molecular basis for this phenotype, using multiple N-cad alleles with distinct molecular properties. We confirm that cell proliferation is enhanced in N-cad mutants, but contrary to previous findings, we observe that the increase is sustained over multiple stages of development. At the cellular level, loss of N-cad results in a shorter cell cycle. Furthermore, we demonstrate that hyperproliferation is not linked to abnormal beta-catenin localization, suggesting that Wnt signaling is not increased. Our findings indicate that components of Shh signaling namely, ptc-1 and gli1 are upregulated in the dorsal region of N-cad mutants. In addition, blocking Shh signaling rescues the hyperproliferation phenotype observed in N-cad mutants suggesting that N-cad regulates cell division by limiting the range of Shh signaling. Despite an increase in the total number of neurons, the neuronal differentiation index is in fact reduced in N-cad mutants, suggesting that the supernumerary neurons are produced as a result of overproliferation rather than enhanced differentiation. Consistent with this observation, the rate of cell cycle exit is unchanged. Intriguingly, we also observe that a small subset of N-cad mutant cells are double positive for mitotic and neuronal differentiation markers, suggesting a role for N-cad in coupling cell cycle exit and differentiation. These findings highlight the role of N-cad in regulating cell proliferation in the developing nervous system and reveal a novel function for N-cad in coupling cell cycle exit and differentiation.