Identification and characterization of critical determinants in plexin signaling
Semaphorins, a class of soluble and membrane-bound ligands, and their corresponding receptors, plexins, play a pivotal role in organogenesis, physiology of different tissues and organ systems, as well as in the development of diseases. Plexins exert diverse signaling functions via their intracellula...
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|Semaphorins, a class of soluble and membrane-bound ligands, and their corresponding receptors, plexins, play a pivotal role in organogenesis, physiology of different tissues and organ systems, as well as in the development of diseases. Plexins exert diverse signaling functions via their intracellular domain. One of these functions is the deactivation of R-Ras, a non-classical isoform of the oncogene Ras. So far, the exact mechanism of this plexin-mediated R-Ras deactivation is unknown.
In this thesis, the yet unidentified mechanism is elucidated, recently discovered players in the semaphorin-plexin system are investigated and mutations of plexins are characterized for their role in ligand-binding and downstream signaling. For this purpose, molecular biological, biochemical, cell biological and pharmacological methods were combined.
It could be shown that mutations of Plexin-B2 occurring in patients with congenital anomalies of the kidney and urinary tract impair the plexin-mediated signaling ability. Moreover, TMEM260, a protein with so far unknown function, serves as an O-mannosyltransferase for plexins and a role for these protein-mediated Plexin-B2 mannosylations was identified in membrane targeting and proteolytic processing.
Additionally, Rasal1 was validated and characterized as a GTPase activating protein downstream of plexins, that deactivates R-Ras. In this mechanism plexins bind active R-Ras, without influencing its activity. The binding of semaphorins to plexins induces the release of R-Ras from plexin binding and R-Ras then gets deactivated by Rasal1.
Finally, these findings were set in a translational context by identifying a role of the semaphorin-plexin-Rasal1-R-Ras signaling axis in the negative regulation of gastrin gene expression in vitro, a peptide hormone that stimulates gastric acid production.
Concluding, these findings illuminate new mechanistic insights into plexin signaling, uncover new molecular functions of plexins in pathophysiology and clear up the controversy about the function of the plexin GTPase activating protein domain in the regulation of R-Ras activity.