Scube2 mediates Hedgehog signalling in the zebrafish embryo

Abstract

The Hedgehog family of secreted morphogens specifies the fate of a large number of different cell types within invertebrate and vertebrate embryos, including the muscle cell precursors of the embryonic myotome of zebrafish. Formation of Hedgehog-sensitive muscle fates is disrupted within homozygous zebrafish mutants of the "you"-type class, the majority of which disrupt components of the Hedgehog (HH) signal transduction pathway. We have undertaken a phenotypic and molecular characterisation of one of these mutants, you, which we show results from mutations within the zebrafish orthologue of the mammalian gene scube2. This gene encodes a member of the Scube family of proteins, which is characterised by several protein motifs including EGF and CUB domains. Epistatic and molecular analyses position Scube2 function upstream of Smoothened (Smoh), the signalling component of the HH receptor complex, suggesting that Scube2 may act during HH signal transduction prior to, or during, receipt of the HH signal at the plasma membrane. In support of this model we show that scube2 has homology to cubilin, which encodes an endocytic receptor involved in protein trafficking suggesting a possible mode of function for Scube2 during HH signal transduction.

Scube2 enhances proteolytic Shh processing from the surface of Shh-producing cells

“…This is supported by the fact that Scube2 activity is a positive component of the Hh pathway in vivo (Creanga et al, 2012;Tsai et al, 2009). It is known that the loss of Scube2 activity in the zebrafish embryo leads to mild defects in Hh signal transduction (Hollway et al, 2006), and that simultaneous knockdown of all three Scube genes in triple-morpholino embryos phenocopies a complete loss of embryonic Hh function (Johnson et al, 2012). Notably, impaired Scube function in vivo was bypassed by increased ligand expression; the injection of shh mRNA into scube triple-morpholino embryos rescued you-mutant phenotypes, and a moderate (fourfold) increase in shh mRNA induced wildtype-like ectopic expression of Shh target genes in triplemorpholino embryos (Johnson et al, 2012).…”

Pattern formation in the vertebrate neural tube: a sonic hedgehog morphogen-regulated transcriptional network

“…This outcome might compensate for fluctuations in ligand availability, rendering a degree of robustness to the process of gradient formation. Additional molecules, such as Scube2 (Hollway et al, 2006;Kawakami et al, 2005), have been implicated in regulating the spread or activity of Shh; however, their function remains to be verified. Moreover, as no systematic effort has been made to identify all Shh binding partners, it is likely that more remain to be discovered.…”

Region-specific requirement for cholesterol modification of sonic hedgehog in patterning the telencephalon and spinal cord

“…Another endocytic receptor, cubilin (Cubn -Mouse Genome Informatics), an EGF-CUB protein, has a nearly identical apical surface neuroepithelial expression pattern to megalin (Assemat et al, 2005). Interestingly, a cubulin-related protein encoded by the zebrafish scube2 gene has been shown to mediate Shh signaling in embryos (Hollway et al, 2006;Woods and Talbot, 2005). Therefore, it is possible that these two proteins act synergistically in promoting Shh signaling during mammalian development.…”