Disruption of zebrafish cyclin G-associated kinase (GAK) function impairs the expression of Notch-dependent genes during neurogenesis and causes defects in neuronal development.
BACKGROUND
The J-domain-containing protein auxilin, a critical regulator in clathrin-mediated transport, has been implicated in Drosophila 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.


RESULTS
Like mammals, zebrafish has two distinct auxilin-like molecules, auxilin and cyclin G-associated kinase (GAK), differing in their domain structures and expression patterns. Both zebrafish auxilin and GAK can functionally substitute for the Drosophila 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 Her4, 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.


CONCLUSIONS
In support of the notion that endocytosis plays important roles in Notch signaling, inhibition of zebrafish GAK function affects embryonic neuronal cell specification and Her4 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.
Bai
Ting
T
Department of Biological Sciences, Purdue University, 915 W, State St,, West Lafayette, Indiana 47907-2054, USA.
Seebald
Jamie L
JL
Kim
Kyu-Eui
KE
Ding
Hong-Mei
HM
Szeto
Daniel P
DP
Chang
Henry C
HC
D000595:Amino Acid Sequence; D000818:Animals; D034021:Auxilins / Q000235:genetics; D001921:Brain / Q000196:embryology / Q000378:metabolism; D016923:Cell Death; D002966:Clathrin / Q000378:metabolism; D015870:Gene Expression; D018398:Homeodomain Proteins / Q000378:metabolism; D006801:Humans; D008969:Molecular Sequence Data; D055495:Neurogenesis; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D015027:Zebrafish / Q000196:embryology* / Q000378:metabolism; D029961:Zebrafish Proteins / Q000378:metabolism*
D034021:Auxilins; D002966:Clathrin; D018398:Homeodomain Proteins; D029961:Zebrafish Proteins; C548161:GAK protein, zebrafish; D017346:Protein Serine-Threonine Kinases