Zygotic hypoxia-inducible factor alpha regulates spicule elongation in the sea urchin embryo

Sea urchin larval skeletons are produced by skeletogenic primary mesenchyme cells (PMCs), which migrate to form two ventrolateral clusters (VLCs) at the sites where biomineralization is initiated. Both PMC migration and biomineralization are controlled by VEGF signals emitted from lateral ectodermal cells. In mammals, VEGF signaling can be activated by hypoxia-inducible factor alpha (HIF alpha), an oxygen-sensitive transcription factor. Our previous study showed that the sea urchin maternal HIF alpha is involved in regulating gene expression along the dorsoventral axis. In this study, we discovered that zygotic hif alpha is expressed in PMCs, and at the late gastrula stage, hif alpha transcripts display a graded pattern, with stronger signal in the ventral PMCs than in the dorsal PMCs. We further showed that PMCs are hypoxic, which is a condition typically required for HIF alpha function. In embryos injected with a splice-blocking morpholino against hif alpha, elongation of the skeleton was impaired, and expression of vegfr-10-Ig (encodes VEGF receptor; VEGFR) was significantly reduced. This morpholino-caused defect could be partially rescued by injection of vegfr-10-Ig mRNA. Expression patterns of transcription factor and biomineralization genes, such as alx1, tbr, msp130, and the sm30 family, were affected when HIF alpha was knocked down or when VEGF signaling was inhibited. These results suggest that zygotic HIF alpha acts upstream or in parallel with VEGF signaling to regulate skeletogenic gene expression and participate in spicule elongation. Our study therefore links HIF alpha with the known role of VEGF signaling in sea urchin biomineralization.

Automated summary

Sea urchin larval skeleton formation is regulated by HIF alpha and VEGF signaling, with HIF alpha expressed in PMCs and involved in gene expression regulation and spicule elongation.