Project

Discipline

Neural Development; Synapse Formation; Cell adhesion; Mouse genetics; Neuronal signaling mechanisms; nervous system; cerebellum; autism

Lead
Lay summary
During the development of the brain, which occurs in the embryo but also after birth, neuronal cells extend protrusions called axons and dendrites. Axons can span long distances from millimeters to meters. Once an axon reaches its destination it will form connections on the dendrites of other cells. These connections are called "synapses" and enable the two cells to communicate with each other. Synapses are assembled with exquisite selectivity. Only specific neuronal partners form synapses with each other and the location and number of synaptic connections is highly reproducible. This is remarkable if one considers that an axon passes hundreds or thousands of potential partners and selectively recognizes the appropriate partners for synapse formation. In our work, we are examining this question and hope to define the cellular interactions and signaling events that control formation of specific synapses.In previous studies, we have discovered that a complex of proteins with adhesive properties, called the neuroligin-neurexin complex, has remarkable synapse-organizing activities. That means neuroligin and neurexin proteins are presented at the opposing surfaces of two neuronal cells and can orchestrate the conversion of these cell contacts into functional synaptic junctions. A central goal of the work described in this proposal is to understand the function of the neuroligin-neurexin adhesion complex in the central nervous system. Notably, neuroligins and neurexins are encoded by gene families that produce multiple different protein isoforms. This means in the case of neurexins there are over 3,000 variants that slightly differ in their protein sequence and their ability to interact with the neuroligin molecules. To examine the molecular mechanisms underlying the assembly of functional neuronal networks, we are mapping neurexin isoform expression in genetically identified cells and test the mechanisms of synaptic specificity in the cerebellum, a brain structure that controls the coordination of motor and sensory information.Mutations in neuroligin and neurexin genes in humans are associated with autism-spectrum disorders. Therefore, we hope that understanding the basic function of these proteins in the nervous system will also shed some light on the functional abnormalities underlying autistic disorders.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Self-organised

Number	Title	Start	Funding scheme

The assembly of functional neuronal circuits during development of the central nervous system requires an array of selective cell-cell interactions. These interactions direct cell migration, targeted growth and branching of axonal and dendritic processes, recognition of appropriate target cells, differentiation of pre- and postsynaptic structures, and recruitment of synapse-specific release machinery and neurotransmitter receptors. The aim of our studies is to understand the molecular mechanisms underlying the formation of specific connections between neurons in the central nervous system. In particular, we are examining the trans-synaptic signals that coordinate the choice of synaptic partners, assembly of synaptic junctions and stabilization of appropriate contacts. To address these questions, we are using a combination of functional in vitro assays that facilitate the identification of new signaling mechanisms and in vivo analysis of neuronal circuits in the mouse cerebellum. The cerebellum is an excellent model system for the analysis of synaptic specificity due to it’s highly organized and well understood mature connectivity. Moreover, we have established a collection of genetic labeling and marking techniques that further facilitate dissection of synaptic specificity in a vertebrate in vivo system. Over the past years, we characterized the function of a family of neuronal cell adhesion molecules, called neuroligins and neurexins that form a heterophilic cell adhesion complex and that have potent synapse-organizing activities. These activities are regulated through extensive alternative splicing in sequences encoding the extracellular domains of these proteins. In our ongoing projects we are applying a combination of cell biological, biochemical, genetic and anatomical approaches to explore the molecular mechanisms of isoform-specific functions. Specifically, we will test the hypothesis that alternative splicing of neuroligin and neurexin isoforms underlies cell type- and synapse-specific trans-synaptic interactions in neuronal circuits of the mouse cerebellum. In further studies on the mechanisms underlying synaptic specificity in the mouse cerebellum we will examine a novel role for Bone Morphogenetic Proteins and their receptors in trans-synaptic signaling.The following projects will be the main focus of the research in my laboratory for the coming years and are described in detail in the Research Plan.(1) Coupling of Postsynaptic Neurotransmitter Complexes to Synaptic Adhesion Molecules: We previously observed highly selective association and function of specific neuroligin isoforms at GABAergic and glutamatergic synapses in vitro. In this project we will determine the structural and functional basis for these isoform-specific functions.(2) Molecular Diversity of Neurexins: With over 3,000 isoforms, the Neurexins represent one of the molecularly most diverse families of neuronal cell surface proteins in vertebrates. Our previous studies suggest that neurexin splice isoform diversity underlies a synapse-specific adhesive code. In this project we will perform single cell and population studies in genetically identified cerebellar neurons to examine the contribution of neurexin variants to neuronal identity and the selective wiring of cerebellar circuits.(3) Regulation of Alternative Splicing of the Neurexin Gene Family: In this project we are analyzing the molecular machinery that regulates neurexin splice isoform choice. We are particularly focusing on the selection of exon 20, an alternative exon that has determines the biochemical interactions between neurexins and specific neuroligin isoforms. To this end, we have identified an alternative splicing factor that associates with neurexin-1 mRNA and that is essential for the regulation of neurexin alternative splicing in vivo.(4) Emergence of synaptic specificity in the ponto-cerebellar projection system: We will use a combination of conditional mutant mice and gene transfer by in utero electroporation to examine the molecular mechanisms underlying selective axon-target interactions of cerebellar mossy fibers.