Possible co-option of a VEGF-driven tubulogenesis program for biomineralization in echinoderms

Abstract

Biomineralization is the process in which living organisms use minerals to form hard structures that protect and support them. Biomineralization is believed to have evolved rapidly and independently in different phyla utilizing existing components used for other purposes. The mechanistic understanding of the regulatory networks that drive biomineralization and their evolution is far from clear. The sea urchin skeletogenesis is an excellent model system for studying both gene regulation and mineral uptake and deposition. The sea urchin calcite spicules are formed within a tubular cavity generated by the skeletogenic cells under the control the vascular endothelial growth factor (VEGF) signaling. The VEGF pathway controls tubulogenesis and vascularization across metazoans while its regulation of biomineralization was only observed in echinoderms. Despite the critical role of VEGF signaling in sea urchin spiculogenesis, the downstream program it activates was largely unknown. Here we study the cellular and molecular machinery activated by the VEGF pathway during sea urchin spiculogenesis and reveal multiple parallels to the regulation of tubulogenesis during vertebrate vascularization. Human VEGF rescues sea urchin VEGF knock-down; VEGF-dependent vesicle deposition plays a significant role in both systems and sea urchin VEGF signaling activates hundreds of genes including biomineralization and vascularization genes. Five upstream transcription factors and three signaling genes active in spiculogenesis are homologous to vertebrate factors that regulate vascularization. Overall, our findings suggest that sea urchin spiculogenesis and vertebrate vascularization diverged from a common ancestral tubulogenesis program, broadly adapted for vascularization and specifically co-opted for biomineralization in the echinoderm phylum. Significance statementThe sea urchin calcite spicules and vertebrate blood vessels are quite distinct in their function, yet both have a tubular structure and are controlled by the vascular endothelial growth factor (VEGF) pathway. Here we study the downstream program by which VEGF pathway drives sea urchin spiculogenesis and find remarkable similarities to the control of vertebrate vascularization. The similarities are observed both in the upstream gene regulatory network, in the downstream effector genes and the cellular processes that VEGF signaling controls at the site of the calcite spicule formation. We speculate that sea urchin spiculogenesis and vertebrate vascularization diverged from a common ancestral tubulogenesis program that was co-opted for biomineralization in the echinoderm phylum.