Chemical and Genetic Analysis of AXL Receptor Tyrosine Kinase Signaling in Neural Crest EMT, Cell Migration, and Metastasis

Improved understandings of the molecular mechanisms that promote metastasis are needed to develop therapies that target this deadly disease. The stages of metastasis closely resemble events that occur during formation of the vertebrate neural crest (NC), and it is therefore not surprising that genes controlling NC development are aberrantly reactivated during metastasis. The AXL receptor tyrosine kinase (RTK) is a potential therapeutic target for cancer treatment because it can promote cancer cell growth, migration and metastasis. Overexpression of AXL is observed in several cancer cell lines including NC-derived neuroblastoma and melanoma. We are using the zebrafish model to study the role of AXL and its critical downstream mediators in NC EMT, migration, and, metastasis. By using genetic and pharmacological approaches to knockdown AXL in zebrafish embryos we have found that inhibition of AXL impairs EMT and migration of NC cells and NC-derived melanocytes. Consistent with this, treatment of zebrafish embryos with specific AXL inhibitors significantly decreases Snail1b expression, a key regulator of EMT during NC development. The expression pattern of AXL and its ligand Gas6 in specific segments of the hindbrain that pattern cranial NC migration suggest that Gas6 is secreted to stimulate activation of AXL which may induce EMT and migration of NC cells off the neural tube. Our studies suggest that AXL is an essential regulator of EMT during both NC development and cancer progression. We are currently testing whether overexpression of AXL can promote tumor onset, invasion, and/or metastasis in zebrafish models of melanoma and neuroblastoma.