RET as a novel receptor tyrosine kinase of mast cells: autocrine and paracrine activation of RET and its operation in mast cells within the cutaneous microenvironment

• Applicant: Dr. Magda Babina
• Subject Area: Dermatology ; Immunology
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 323618061

Mast cells (MCs) contribute to physiological processes and can be the root of disease, particularly of allergic-inflammatory origin or mastocytosis. IL-33 and TSLP, two factors released by skin resident cells under inflammation, can remodel and numerically strengthen skin MCs, as demonstrated in the first period. However, while MCs experience pathological increases, they are also abundant in the unperturbed skin. The protective niches, that maintain the MC population and shield it from harm are only rudimentarily understood and likely differ between homeostatic and inflammatory conditions.Exactly as hypothesized, our findings revealed that the GDNF/RET network provides crucial support to the MC population of healthy skin. Accordingly, RET ablation drastically impaired skin MC survival, attenuated expression of lineage genes, and suppressed the unique phenotypic and functional programs of the cells. Intriguingly, RET activation occurred in an autocrine fashion by MC-derived GDNF.In support of the latter, RET expression was confined to MCs in the skin, while GDNF was substantially enriched in MCs by immunofluorescence. Endogenous GDNF was crucial in a complex dermal environment, where its inactivation slashed the resident MC population. Inhibition of the SCF/KIT axis, considered most influential in the lineage, likewise reduced the MC fraction yet less drastically probably due to a paucity of SCF in the skin micromilieu.Correspondingly, exogenous GDNF interfered with apoptotic death, supported mitogenesis, and promoted signature characteristics and functional programs like degranulation and cytokine production. A comprehensive phospho-proteomics endeavor uncovered clusters and modules triggered specifically by GDNF/RET, while changes in the phospho-proteomic landscape elicited by SCF/KIT showed overlaps with GDNF yet differed substantially in other aspects.Provided this can be confirmed in the second period, our findings point to a vital, non-redundant role of the RET system, through which post-maturation MCs are orchestrated and functionally stabilized by their microenvironment. Over the new funding period we aim to finalize outstanding issues, explore the routes by which GDNF interferes with MC apoptosis, delve deeper into the signaling networks of both SCF/KIT and GDNF/RET, and examine whether aberrant RET function may underlie diseases like urticaria and cutaneous mastocytosis.Investigation of the KIT-signaling network confirmed expected pathways and also revealed a number of surprising events and programs. We therefore want to provide a comprehensive view of the KIT-induced phosphoproteomic landscape, elaborate the significance of distinct modules and finally compare the two systems RET and KIT regarding key differences.After its completion, this project will have substantially advanced our understanding of the MC supportive network operative in both healthy and diseased skin and unearthed the associated signaling programs.

• DFG Programme: Research Grants