Coupling Receptor Activation to Cytoskeleton Changes

Abstract

Receptors translate extracellular signals into appropriate cellular responses through mediating cytoskeleton changes. Understanding receptor regulation and the mechanism of modulating cytoskeleton is important since dysregulation of receptor often leads to diseases. I focused on ACK1 (Activated Cdc42-associated kinase 1) and CRMP (Collapsin response mediator protein), signaling proteins involved downstream of certain receptor tyrosine kinases (RTK) or semaphorin signaling respectively. Both are closely linked to Rho GTPases: ACK1 is an effector of Cdc42 whereas CRMP2 is a substrate of ROK (RhoA effector) and a modulator of chimaerin (RacGAP). ACK1, a cytoplasmic tyrosine kinase, is implicated in promoting cancer cell migration, downstream of EGFR and a number of other important RTKs. It binds these through an unusual kinase binding site in the C-terminal region conserved only in one other protein termed MIG6. I have shown that multiple RTKs including Axl, LTK and ALK can bind to this region but often the interaction is potentiated through a common adaptor protein Grb2. Focusing on Axl, I compared the role of ACK1 in Axl versus and EGFR signaling. While activated-EGFR promotes ACK1 turnover, activated-Axl does not: interestingly ACK1 promotes the down-regulation of both EGFR and Axl upon activation. I have shown that ACK1 is required for proper trafficking of activated-Axl via endosomes. Suppression of ACK1 has an effect on membrane ruffling and cell migration in DU145 cells. These studies are consistent with ACK1 promoting metastatis via enhanced Rac1-type activities. The CRMPs are ubiquitous proteins related to dihydropyrimidinases and critical for axonal specification, axonal guidance, and dendritic function. Despite knock-out data from several organisms which clearly links CRMPs to the cytoskeleton, their mechanism has never been established. I show that CRMPs associate with mitotic microtubules; a function requiring the C-terminal region but not the dihydropyrimidinase domain as previously suggested. In vivo and in vitro microtubule association by CRMPs is reduced by taxol or epothilone B, indicating that the proteins are sensitive to core microtubule conformation. This binding allows CRMPs to stabilize microtubules in cultured cells, allowing us to map their activity. Remarkably, the C-terminus of CRMP1 is sufficient to stabilize microtubules, particularly in combination with GSK3ß inhibition. This function is inhibited by phosphorylation within the serine/threonine-rich C-terminus, consistent with known functional antagonism by CDK5, GSK3ß and other kinases. Mutational analysis of CRMP1 C-terminus points to Ser537 as a critical site for regulating microtubule stability, which I showed is modified and conserved across all CRMPs. Knock-down of CRMP2 diminishes anaphase astral microtubules and the spindle-to-pole distance. CRMP represents a new class of microtubule-associated protein that binds through a unique sequence not found in other proteins. This activity likely explains the multiple phenotypes associated with CRMP knock-out, and it is notable the hyper-phosphorylation in tauopathies might occur in parallel with CRMP modification. The connection between CRMP and ROK is particularly interesting. The basis for CRMPs interaction with ROK involves the dihydropyrimidinase domain. Whether ROK is recruited to MTs via CRMP is yet to be established. I showed that CRMP2 can cosediment with F-actin and is localized to membrane ruffles in an active Rac1-dependent manner. Thus CRMP2 is associated with actin dynamics in addition to MT dynamics.