Disruption of zebrafish cyclin G-associated kinase (GAK) function impairs the expression of Notch-dependent genes during neurogenesis and causes defects in neuronal development

<p>Abstract</p> <p>Background</p> <p>The J-domain-containing protein auxilin, a critical regulator in clathrin-mediated transport, has been implicated in <it>Drosophila </it>Notch signaling. To ask if this role of auxilin is conserved and whether auxilin has additional roles in development, we have investigated the functions of auxilin orthologs in zebrafish.</p> <p>Results</p> <p>Like mammals, zebrafish has two distinct auxilin-like molecules, auxilin and cyclin <b>G-a</b>ssociated <b>k</b>inase (GAK), differing in their domain structures and expression patterns. Both zebrafish auxilin and GAK can functionally substitute for the <it>Drosophila </it>auxilin, suggesting that they have overlapping molecular functions. Still, they are not completely redundant, as morpholino-mediated knockdown of the ubiquitously expressed GAK alone can increase the specification of neuronal cells, a known Notch-dependent process, and decrease the expression of <it>Her4</it>, a Notch target gene. Furthermore, inhibition of GAK function caused an elevated level of apoptosis in neural tissues, resulting in severe degeneration of neural structures.</p> <p>Conclusion</p> <p>In support of the notion that endocytosis plays important roles in Notch signaling, inhibition of zebrafish GAK function affects embryonic neuronal cell specification and <it>Her4 </it>expression. In addition, our analysis suggests that zebrafish GAK has at least two functions during the development of neural tissues: an early Notch-dependent role in neuronal patterning and a late role in maintaining the survival of neural cells.</p