Analysis of PDGF receptor internalization and signaling using proximity ligation assays

Sammanfattning: Cell signaling is mediated by signaling proteins that relay the signal in an intricate network of interactions before the signal is translated into a biological response. Short linear motifs (SLiMs) in intrinsically disordered regions of proteins serve as docking sites for protein interaction in all aspects of cell regulation including signal transduction. SLiM-mediated interactions are transient and low affinity and can be hijacked by virus. Only a small fraction of SLiMs have been described, but many more exist. Platelet derived growth factor receptor β (PDGFRβ) belongs to the family of receptor tyrosine kinases (RTKs) and controls cell growth, proliferation and migration. Dysregulation of PDGFRβ mediated signaling pathways is seen in many cancer types. To discover and characterize protein interactions, large scale high through-put methods are needed in concert with low through-put methods, that can characterize the interaction in a cellular context. The aim of this thesis has been to study protein-protein interactions in internalization and signaling of PDGFRβ and motif-mediated host-virus interactions through the use of in situ proximity ligation assay (PLA).  Signaling via PDGFRβ is compartmentalized and depends on receptor internalization. In paper I we investigated the effects of dynamin inhibition for activation and signaling of PDGFRβ, and found that dynamin inhibition leads to impaired dimerization of the PDGFRβ. The results indicate that membrane localization of PDGFRβ is affected by dynamin. In paper II we developed a new method, Molboolean, for localized simultaneous detection of both free protein and protein in complex in cells. Molboolean is based on the principles from PLA with a fluorescent read out detectable with fluorescence microscopy.       In paper III we mapped SLiM based host-virus interactions and explored their mechanisms. Using proteomic peptide phage display, we identified 1712 potential virus-host interactions by screening a library covering intrinsically disordered regions of the proteome for 229 RNA viruses. Clathrin mediated endocytosis was found to be a common target for viral hijacking, and viral binding of clathrin impaired PDGFRβ internalization.       Some RTKs are proteolytically cleaved following ligand activation. In paper IV we characterized the Ca2+-dependent proteolytic cleavage of PDGFRβ. The cleavage resulted in two PDGFRβ fragments and was dependent on receptor internalization. Inhibition of the proteasome with bortezomib prevented internalization and cleavage and resulted in increased activation of PLCγ and STAT3. This thesis provides insight in the regulation of PDGFRβ signaling and internalization, and highlights contributions of both large-scale screenings and low through-put methods for studying protein-protein interactions.