During the development of a multicellular organism the members of a few distinctive families of extracellular proteins interact with specific cell surface receptors to guide cells to their correct location. Cell guidance functions continue to be essential throughout life to maintain healthy tissues. These functions depend on a number of cell guidance family members working in unison to provide exquisitely detailed instructions in an appropriately timely and location dependent manner. Such orchestration requires multiple, switchable, levels of control over the signalling outputs. Semaphorins use plexins as their main receptors and the semaphorin and plexin families together constitute one of the largest and functionally diverse of the cell guidance systems. Co-receptors, such as the neuropilins, have been implicated in context dependent switching of semaphorin-plexin signalling. To date we know very little about the molecular mechanisms that act at the cell surface to determine divergent signalling outputs despite their vital role in controlling timely, location-dependent functions. To address this lack of knowledge we are investigating the mechanisms that control switches in signalling output for the semaphorin-neuropilin-plexin system.
Although there have been enormous advances revealing semaphorin-neuropilin-plexin function at the level of genetic and cellular experiments, our knowledge of the molecular level mechanisms which deliver and discriminate between the numerous biological outcomes is still sparse. The myriad physiological functions of the semaphorin-neuropilin-plexin system are now known to span angiogenesis, bone maintenance, heart development, and regulation of immune responses, as well as their plethora of roles in the nervous system. Conversely, these proteins have been found to be involved in a broad range of pathologies, including cancer, and neurogenetic disorders. For this reason, semaphorins, plexins and neuropilins have emerged as potential points for clinical intervention. Already insights into the determinants of ligand-receptor binding, generated by us and others, are being applied in the design of novel therapeutic agents. To better guide such efforts, we need to be able to pinpoint the factors that control signalling outcomes. Our aim is to provide the understanding and tools required to manipulate semaphorin-neuropilin-plexin signalling in more tightly targeted and functionally specific modes than currently available.
Some of the most fundamental gaps in our knowledge concern the mechanisms controlling the signalling outputs of the secreted class 3 semaphorins (Sema3s), their PlexinA1-4 and PlexinD1 receptors and their co-receptors, Neuropilin 1 and 2 (Nrp1, Nrp2). The overarching objective of FLEXINGPLEXIN is to discover the mechanisms by which Sema3 variants, plexin ectodomain conformation and interactions with neuropilin result in multiple different signalling outcomes. We have subdivided this objective into three aims:
AIM 1. The structural determinants and mechanisms of action by which class 3 semaphorins exert differing effects on signalling.
AIM 2. The conformational state of the plexin ectodomain in different contexts and its contribution to signal outcome.
AIM 3. The mechanisms by which neuropilin binding can switch the outcomes of plexin signalling.
