Analyzing PTK7/RACK1 interaction in neural morphogenesis

Abstract

One of the most critical events during embryonic development is the formation of the nervous system. This process requires morphogenetic movements like the formation of the neural tube and the migration of neural crest cells. Failure of these cell movements results in severe phenotypes like open neural tubes and neurocristopathies. Currently the understandings of the signaling mechanisms controlling neural morphogenesis are limited. One set of genes that is known to affect neural tube closure and neural crest migration are regulators of planar cell polarity (PCP). Protein tyrosine kinase 7 (PTK7) is a regulator of planar cell polarity that is required for neural crest cell migration and neural tube closure. Analyzing the signaling mechanism of PTK7, it was found that PTK7 interacts with RACK1 (receptor of activated protein kinase C 1). In the following work the in vivo function of the PTK7/RACK1 interaction was analyzed in neural tube closure and neural crest migration. Like PTK7, RACK1 is required for Xenopus neural convergent extension, a cell movement crucial for neural tube closure. Furthermore, RACK1 loss-of-function leads to similar neural crest migration defects as PTK7 loss-of-function. On a molecular level, PTK7 recruits RACK1 to the plasma membrane and is required for the PTK7-mediated membrane localization of DSH in ectodermal explants. RACK1 facilitates the PTK7-DSH interaction by recruiting PKC1, a known effector of DSH membrane translocation and convergent extension movements. These data place RACK1 in a novel signaling cascade that translocates DSH to the plasma membrane and regulates vertebrate neural morphogenesis movements. Interestingly, RACK1 and PTK7 also co-localize in migrating neural crest cells at cell-cell contact sides suggesting that the PTK7/RACK1 interaction also plays a role for neural crest migration. Furthermore, life-cell imaging shows a dynamic localization of PTK7 during neural crest migration. PTK7 was enriched at distinct membrane sections during collective cell migration. Moreover, PTK7 accumulates at cell-cell contacts during the process of contact inhibition of locomotion (CIL) in single migrating cells. Loss-of-function data support the notion that PTK7 but not RACK1 is required for CIL. This suggests that PTK7 may mediate CIL via a RACK1-independent mechanism. In summary this work presents new evidences for the important role of RACK1 and PTK7 in the regulation of neural morphogenesis movements.