Conventional and atypical chemokine receptors: different mechanisms of function and common responses

Cell migration is intrinsic to any higher organisms. Starting early in development cell migration can be observed during all ages. The mechanisms of cell migration can be seen from two edges: the attracting cues and the molecular mechanism allowing a single cell to move. The chemokine system has emerged as a major regulator of these processes and is best studies for its involvement in leukocyte trafficking. The system is comprised on one side by the chemokines, small 8-10 kD peptides, that are ubiquitously produced and form chemotactic gradients by binding to cell surfaces or matrix proteins, on the other side are the chemokine receptors, all structurally closely related to rhodopsin-like G-protein coupled receptors. Conventional chemokine receptors are selectively expressed on different leukocytes (and also non-hematopoietic cells) and respond to distinct chemokines activating intracellular signaling cascades via pertussis-toxin sensitive Gi-proteins. Recently a subset of chemokine receptors has emerged, which do not trigger typical intracellular signaling, but have in common the ability to internalize, degrade, or transport their ligands. This function enables them to modify gradients and create functional chemokine patterns in tissues. A member of the atypical chemokine receptors is CXCR7, a receptor that was deorphanized in our laboratory as second receptor for the chemokine CXCL12. Is critical involvement in the development of mice is manifested by the fact that targeted deletion is lethal. CXCR7 has received much attention in cancer studies, being expressed on many tumors, but compelling evidence exist showing its role in leukocyte traffic, e.g. for invasion of leukocytes into the CNS. Despite that several studies from different laboratories describe CXCR7 expression in leukocytes, one laboratory has claimed that the receptor in not expressed in leukocytes. Recently, we provided compelling evidence showing the expression of the receptor in B cells (submitted manuscript). Thus, we propose her to further investigate the function of CXCR7 in the B cell compartment. Our initial studies (2006) pointed at an important role of CXCR7 in B cell differentiation to plasma cells. Based on preliminary data we hypothesize that CXCR7 expression on plasmablast is critical for developing long lasting immune responses. The well accepted scavenging activity of CXCR7 is associated with trafficking of the receptor though endosomal pathways deploying the bound CXCL12 for degradation while recycling to the cell surface. Using fluorescence microscopy in the first place we propose to investigate the endocytic route of CXCR7. We suggest to search for structural determinants that regulate the pathway and are responsible for the scavenging activity and are not comprised in the sequence of conventional chemokine receptors. The data will reveal mechanisms of receptor recycling, which is important for the understanding of the proposed role of the receptor in regulating cell trafficking.Chemotaxis of leukocytes mediated by conventional receptors. For efficient migration, receptor responsiveness must be maintained whilst the cells crawl on cell surfaces or on matrices along the attracting gradient towards increasing concentrations of agonist. On the other hand agonist-induced desensitization and internalization is a general paradigm for chemokine receptors which is inconsistent with the prolonged migratory capacity. Using time-lapse video microscopy and fluorescent labeled receptors and chemokines we propose to study the localization and endocytic behavior of the chemotaxis-mediating receptors. Using genetically encoded calcium indicators we will test receptor responsiveness and potential desensitization during migration. With present project we give a new direction to our long standing endeavor to elucidate the role of chemokine receptors in regulating leukocyte trafficking. Investigating the properties of conventional and atypical receptors in shall unveil common and distinct mechanisms in this process.