The Kinase PIG-1/MELK is a Conserved Regulator of C. elegans Tubulogenesis and Vertebrate Angiogenesis

Biological tube formation, or tubulogenesis, is a complex process, requiring precise cytoskeletal regulation, that plays an integral role in vascular development and physiology. Kinases are key regulators of cellular functions and signal transduction pathways, and their well-defined mechanism of action makes them attractive targets for therapeutic development. Therefore, finding kinase regulators of vascular tubulogenesis is a promising translational research approach. The C. elegans excretory canal (ExCa) cell, which forms a large unicellular tube, provides a tractable model to study tubulogenesis, and vertebrate orthologs of genes that regulate ExCa tubulogenesis are involved in vascular development and disease. Through an RNAi screen, confirmed with mutant alleles, for conserved kinases whose loss affected ExCa tubulogenesis we identified PIG-1, and found that its vertebrate ortholog, MELK, is expressed in human umbilical vein endothelial cells (HUVEC), consistent with a possible role for this kinase in angiogenesis. Genetic studies in C. elegans allowed us to identify a novel mode of PIG-1/MELK regulation unique from other AMPK family members. Through structure- function analysis we confirmed the kinase and KA1 domains, as well as the catalytic lysine and putative autophosphorylation site of PIG-1. are required for ExCa tubulogenesis. We defined a cytoskeletal role for PIG-1 in regulating the INF2 formins EXC-6 and INFT-2 and F-actin accumulation in the ExCa. We also found that MELK is required for cell migration and formation of angiogenic sprouting of HUVEC in culture, suggesting that PIG-1/MELK is a conserved regulator of tubulogenesis and angiogenesis. Moreover, we also find that CRISPR-mediated deletion of Zebrafish Melk causes angiogenic defects. Therefore, we propose a novel conserved PIG-1/MELK and INF2 pathway that regulates the cytoskeleton in tubulogenesis and angiogenesis. Notably, MELK is upregulated in various aggressive cancers and although inhibitory compounds are undergoing clinical trials, little is known about MELK’s physiological function. Thus, our work represents an important step in understanding the function of this clinically relevant kinase.