The Background for our Research
Neurons communicate with each other through synapses. These highly specialized intercellular junctions are formed in a coordinated manner between a pre- and a post-synaptic neuron. During development of the central nervous system (CNS), countless new synapses are established, and new synapses continue to form throughout life. When new synapses are generated, the surfaces of the pre- and postsynaptic neuron are first aligned in close proximity. Following this axo-dendritic contact, specialized membrane domains are established at both the juxtaposed neuronal plasma membranes. In this process, the membrane area destined to become the presynaptic plasma membrane forms the active zone, holding the synaptic vesicles that are ready for exocytosis, and the matching neurotransmitter receptors are recruited and retained at the postsynaptic membrane.
Recent progress
The first proteins have now been identified that initiate the formation of synapses. One is SynCAM 1, a synaptic cell adhesion molecule that connects pre- and postsynaptic sides. SynCAM 1 is the founding member of a family of immunoglobulin proteins with three extracellular Ig domains, a single transmembrane region, and a short cytosolic tail that includes interaction motifs with cytoskeletal; regulators and scaffolding proteins. SynCAMs are highly enriched in brain and are prominent components of purified synaptic plasma membranes. They can engage in homophilic interactions, bridging across two neurons, but can also interact with other SynCAM molecules in select heterophilic combinations. Importantly, SynCAM proteins alters neurotransmission and act across the nascent synaptic cleft to induce new, fully functional presynaptic terminals. This activity allowed us to reconstitute excitatory synaptic transmission for the first time (see Figure). Our recent studies of SynCAM 1 in mouse models have demonstrated that this protein not only drives synapse formation in vivo but also maintains the increase in synapses that it induces. In addition, SynCAM 1 regulates synaptic plasticity and up- and down-regulates the ability to learn.
Our current goals
We aim to determine the molecular mechanisms of synapse formation in the developing CNS of vertebrates. Our goal is to identify and characterize the events that initiate synapse formation, and the steps that lead to its completion. In addition, we are interested how these mechanisms act at mature synapses to provide for their structural and functional plasticity.
To achieve these goals, we pursue three aims. First, we biochemically identify the protein complexes of adhesion molecules and their intracellular partners that organize developing synapses. Second, we analyze the properties of these synapse-organizing complexes in cultured neurons, including structure/function studies. Third, we determine the roles of synapse-organizing molecules in the developing brain, using transgenic and knock-out mouse models. We utilize a comprehensive set of techniques to attain our objectives, including protein biochemistry, proteomics, live imaging of neuronal development and synapse formation in cultured cells, and studies of synaptic structure and function in mouse models.
Why we do it
Alterations in synapse formation affect synaptic plasticity, which is associated with changes in human behavior, learning and memory, and addiction. Furthermore, deficits of synapse organization and synaptic loss have been demonstrated in neurodevelopmental and neurodegenerative diseases. Determining the molecular mechanisms of synapse formation will therefore allow to better understand to which extent these processes regulate neuronal network dynamics, and how alterations in synapse organization are linked to disorders of the human brain. Last, but not least, it is an intriguing challenge for membrane protein biochemists to unravel how pre- and postsynaptic membranes can be so rapidly organized from a limited number of components at precisely defined neuronal contact sites, resulting in synaptic sites of very well defined architecture and function.
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Induction of presynaptic terminals and reconstitution of synaptic transmission. A. Illustration of the co-culture assay. A neuron (black) contacts a non-neuronal cell (green) that expresses SynCAM 1, a homophilic synaptic cell adhesion molecule. B. Model of the induction of presynaptic terminals in this assay. Extracellular interaction of neurons with SynCAM 1 triggers an unknown intracellular signal that leads to the formation of presynaptic terminals. These terminals can release neurotransmitter, which is detected by glutamate receptors (GluR2) expressed on the non-neuronal cell together with SynCAM 1.C. Image of a non-neuronal HEK 293 cell (silver, transmitted light image) in co-culture with hippocampal neurons (not shown), with presynaptic terminals on its surface (red, synaptophysin immunostaining).