Dissecting the role of Stonin1 in focal adhesion dynamics and tumor suppression

• Applicant: Professorin Dr. Tanja Maritzen
• Subject Area: Cell Biology
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 316456949

Numerous cellular processes, among them focal adhesion (FA) dynamics and tumor growth, are orchestrated by extracellular signals cells receive via cell surface receptors. Cellular responses to these signals are often fine-tuned by the endocytic regulation of the surface receptors. Defects in endocytosis have dramatic consequences ranging from impaired cell migration to oncogenic signalling. In fact, several hallmarks of cancer have been attributed to an aberrant propensity of malignant cells to endocytotically disassemble signalling and adhesion complexes. However, how precisely endocytosis regulates the turnover of distinct cell surface receptors and thereby influences FA dynamics and tumor growth remains in most cases unclear. We have recently identified Stonin1, an endocytic protein of hitherto unknown function, as dedicated adaptor for the internalization of the proteoglycan NG2, a co-receptor of integrins and platelet-derived growth factor receptor (PDGFR), which promotes cellular motility. NG2 is an established oncogene, which is expressed by various tumors and has been targeted in preclinical models of glioblastoma. In fact, NG2 promotes tumor growth, and its expression has been closely associated with increased malignancy of aggressive brain tumors. Embryonic fibroblasts obtained from Stonin1-deficient mice display a dramatic surface accumulation of NG2, increased cellular signalling and defective FA turnover as well as altered cellular motility. These results established Stonin1 not only as adaptor for NG2, but also as an important factor for signalling, FA dynamics and cell migration. While we provided first insights into the function of Stonin1, fundamental questions regarding its regulation, its action in FA dynamics, and, most importantly, its relevance for tumor growth have not been answered yet and will be tackled in the proposed project. We hypothesize that Stonin1 is crucial for limiting tumor growth by keeping NG2s oncogenic potential in check via its removal from the plasma membrane. Preliminary experiments in fact indicate that loss of Stonin1 promotes tumor growth. We will substantiate these results and use complementary cell biological and in vivo approaches to dissect Stonin1´s actual mechanism of action. Furthermore, we will investigate the molecular basis for Stonin1´s impact on FA maturation and disassembly. Finally, the critical function of Stonin1 as endocytic adaptor of an oncogenic signalling protein and as modulator of FA dynamics likely requires its elaborate regulation. In line with this hypothesis, preliminary analysis revealed that Stonin1 is a multi-phosphoprotein. We will analyze the mechanisms regulating Stonin1 in order to understand how its function is coordinated in the context of FA dynamics and NG2 signalling. Overall, the proposed studies will allow us to gain important insights into how endocytic regulation fine-tunes FA dynamics, cell migration and signalling, and thereby suppresses tumor growth.

• DFG Programme: Research Grants