Function and Regulation of the Conserved Wnt Target sp5 in the Vertebrate Nervous System
Wnt/β-catenin signaling culminates in the transcriptional regulation of target genes. Target genes mediate Wnt function during different developmental processes. A dysfunctional Wnt/β-catenin signaling results in aberrant regulation of the target genes that often causes developmental malformations and cancer. In order to understand how Wnt signaling regulates development and diseases, identification of its downstream network of transcriptional targets and their subsequent roles is required. sp5 gene is a target of Wnt signaling in several vertebrates and invertebrates including humans, Xenopus, Mouse, zebrafish, Hydra and amphioxus. Despite the evidence of its involvement in Wnt mediated processes in several species, its regulation and exact function during Wnt mediated nervous system patterning is not well studied. Here we show that during neural patterning, Wnt/β-catenin signaling regulates the transcription of 3 Sp-family genes in zebrafish: sp5a, sp5-like (sp5l) and sp8b. These 3 genes are expressed in the neural plate during embryogenesis. I generated single and double loss of function genetic mutants for sp5a and sp5l that do not exhibit any observable phenotypic defects during development. However, sp5a, sp5l and sp8b triple knockdown embryos display hindbrain and posterior body defects, indicating that sp8b acts redundantly with the sp5a and sp5l to regulate early development and patterning. I examined the regulation of sp5a by Wnt and FGF in transgenic reporter lines to show that the promoter of sp5a is responsive to modulation in Wnt and FGF signaling levels. De-repression by Wnt followed by activation by FGF signaling is required for the expression of sp5a in the neural plate during embryonic development. Furthermore, I showed that sp5a is expressed in the adult habenula and regulates the expression of the habenular progenitor marker gene dbx1b. RNA sequencing reveals significant downregulation of dbx1b in sp5a loss of function mutants. Comparison of wild-type and mutant sp5a reporter lines suggests that the sp5a expression in the habenula is regulated by Wnt signaling. This data suggests a potential role of sp5a in habenula neurogenesis, downstream of Wnt signaling, through the regulation of dbx1b.
sp5 gene, nervous system patterning, Wnt/β-catenin signaling
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