CXCL12-dependent development of neuronal structures under the control of the atypical chemokine receptor CXCR7

• Applicant: Professor Dr. Ralf Stumm
• Subject Area: Developmental Neurobiology ; Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2012 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 225008604

GABAergic interneurons play a critical role in activity modulation of neuronal circuits in the cerebral cortex. Defects in the development of these cells have been linked to neurological and mental disorders. Precursors of cortical interneurons cover large distances in the embryonic cortex by tangential migration. Even in the small cortex of mice, individual interneurons migrate several days to become evenly dispersed. Work of the applicant led to the current notion that the chemokine CXCL12 and its receptors CXCR4 and CXCR7 form the only chemoattractant signaling module guiding interneurons on their main migration routes in the cortex. In this system, the conventional receptor CXCR4 mediates chemotactic signals through inhibitory G proteins. The applicant´s recent work showed that CXCR7 acts as an atypical chemokine receptor that sequesters CXCL12 to prevent excessive stimulation, down-regulation and desensitization of CXCR4 during the long-lasting tangential migration process. According to a second theory, an unidentified CXCR7 signal, probably ß-Arrestin (ßArr)-mediated MAP Kinase activation, influences interneuron migration. In fact, there is ample evidence with recombinant expressed heptahelical receptors that agonist-induced phosphorylation at C-terminal Ser/Thr sites stabilizes receptor/ßArr interactions, uncouples receptors from G proteins and promotes receptor internalization as well as ßArr-dependent MAP Kinase activation. To date, the influence of these processes on neuronal migration has not been investigated in mammalian organisms. This proposal addresses how receptor phosphorylation and ßArr regulate CXCL12-guided interneuron migration. To this end, the applicant has generated unique genetically engineered mouse models: (1.) A knockin of a Cxcr7 allele (Cxcr7STA) encoding for a receptor in which all potential C-terminal Ser and Thr phosphorylation sites are eliminated by Ala replacement, (2.) a colony of mice to breed embryos lacking one or both ßArr isoforms and (3.) Cxcr7STA and ßArr-deficient mice carrying transgenes that drive expression of a CXCL12-RFP fusion protein under control of the Cxcl12 promoter for visualization of CXCL12 and expression of eGFP under the Cxcr7 promoter for visualization of interneurons. The applicant proposes to study interneuron migration, receptor-mediated CXCL12 uptake, as well as phosphorylation, trafficking and down-regulation of CXCL12 receptors in preparations from Cxcr7STA/STA and ßArr-deficient embryos using histological, neuronal culture and imaging techniques. This project elucidates in a relevant in vivo system, how phosphorylation and ßArr regulate the interplay of a conventional and an atypical chemokine receptor necessary for cell migration.

• DFG Programme: Research Grants