Mechanism of protocadherin-mediated self-avoidance

项目总结

Project summary Dendrite self-avoidance is a critical aspect of development in many neuronal systems and is just now beginning to be appreciated for its significance in neurodevelopmental disorders such as autism. Self- avoidance is mediated by receptors on the neuronal surface that convey a “code” that signals to a contacting same-cell dendrite that the two processes originate from the same cell. In both vertebrates as well as Drosophilia, the code is generated by stochastic alternative splicing of cell adhesion receptors that give a unique homophilic adhesive identity to each cell. In Drosophila, this is mediated by the Ig superfamily adhesion molecule Dscam, while in vertebrates, it is mediated by the clustered protocadherins (Pcdhs), which are similar to the strongly adhesive classical cadherins. Defective self-avoidance was reflected in the abnormal persistence of “dendritic bridges” between sister dendrites in Drosophila. Our data show that Pcdhs are precisely located at dendritic bridge contact points in mammalian neurons. Significant progress has been made on identifying these families of molecules and effects of their knockout or perturbations on nervous system development in vivo. Despite this, what has not yet been answered is: how do apparently adhesive-like molecules cause the avoidance of membranes when they bind? The answer could shed significant light on how self-avoidance might go wrong in neurodevelopmental disorders. We have studied the cell biological activity of the Pcdhs with the goal of answering this question. It is clear that the Pcdhs are very different in terms of their intracellular trafficking to the endolysosome system from the related strongly adhesive classical cadherins, which lack this trafficking. Our published work and new preliminary data suggest that the Pcdhs could cause the detachment of same-cell membranes, after initial adhesive clustering, by triggering endocytosis at the adhesive site. We found that a novel endocytosis regulator, FCHSD2, is enriched in Pcdh complexes and could be the trigger for endocytosis upon Pcdh adhesion. Such endocytosis might target other pro-adhesive molecules for degradation. In this R15 proposal, we plan to develop a novel assay for self- avoidance in culture, so that many mutant Pcdhs and FCHSD2 can evaluated in a structure-function approach. We will use transfected Pcdh-GFP constructs, which are fully functional in vivo, to mark dendritic bridge contacts and study their dynamics and fate. We will also knock down Pcdhs and FCHSD2 and study effects on dendritic bridges and self-avoidance. Once these assays are established, we will mutate the endocytosis and/or trafficking motifs in Pcdhs and determine the effects of these mutations on self-avoidance and dendritic bridges. Alternative signaling pathways for Pcdhs in self-avoidance will be considered as well. These studies will shed new light on the cellular mechanism of self-avoidance. They will also expand the capacity to train undergraduate students in cellular and molecular neurobiology at the College of Staten Island.