Rewiring ret : PTB-adaptor regulated signaling and cell biology

Abstract

The Ret receptor belongs to a group of transmembrane tyrosine kinase receptors that serve to transduce environmental signals into cellular responses. Upon extracellular ligand activation the receptors dimerize and their structural conformation is altered into one that allows for interaction with proteins in the cell cytosol. Among the most proximal interactors are PTBadaptor molecules that dock to Ret tyrosine residues. As several PTB-adaptors compete for interaction with the same receptor-sequence the molecule that successfully binds to Ret excludes binding of any other PTB-adaptor at that time. Hence, the resulting signals are dependent on which adaptor that successfully engaged the receptor. In this work, Ret has been rewired to preferentially bind one PTB-adaptor on the expense of others to tyrosine 1062. This makes it possible to experimentally assign biochemical events as well as cell biological outcomes to one specific adaptor and so increasingly reveal how one common receptor may serve a plethora of functions depending on the subcellular milieu in which it operates.

In paper I the important residues for Ret interaction with the Shc and Frs2alpha adaptors were investigated. Based on data from the effect on adaptor affinity for Ret by specifically substituting amino acids around tyrosine 1062, mutants could be established with selective recruitment of either Shc or Frs2alpha to this tyrosine. Ret interaction with Shc resulted in capacity for the receptor to mediate ligand dependent survival in the setting of apoptotic stimuli or severe starvation while Frs2alpha mediated signaling was insufficient to do so.

In paper II the Frs2alpha adaptor docked to Ret were found to be essential for chemotactic directional migration towards Ret ligands. The molecular basis for Ret promoted migration depended on the membrane associated Src family of kinases that bind to Ret at a different tyrosine than Frs2alpha such that these two residues are cooperatively involved in assembling the molecular framework required to execute a migratory response downstream of Ret.

In paper III the Dok adaptors, which were recently found to interact with Ret, were investigated. Dok selective Ret mutants could be created and were expressed in neuronally derived cells. Dok binding resulted in prolonged phosphorylation of MAP kinases and allowed for Cdc42 activation. The cellular response was enhanced microspike formation as determined morphologically.

In paper IV local membranous Ret interaction with Shc and Frs2alpha were investigated. While Frs2alpha overexpression recruited Ret to distinct lipid raft like partitions of the plasma membrane Shc expression led to Ret being found outside these fractions. A Shc molecule with an appendage forcing its association to lipid rafts resembled Frs2alpha characteristics in terms of downstream biochemical profile and dependence of ordered cholesterol species in the membrane for signaling, suggesting the importance of adaptors for appropriate relocation of Ret. Moreover, the Frs2alpha dependent migration was indeed disrupted by cholesterol oxidation while the survival response promoted via Shc showed little dependence on disruption of membrane architecture.