In vivo imaging demonstrates dendritic spine stabilization by SynCAM 1

Dynamic changes of synaptic spines are associated with cognitive functions such as learning and memory. The formation and maintenance of synaptic spines is critically dependent on synaptic adhesion molecules1. According to the so called filopodia model, synapse formation can be divided into three main phases2,3 in which synaptic cell adhesion molecules are involved. First, a thin protrusion of the dendrite emerges and seeks contact to a nearby axon requiring target cell recognition. Importantly, this first phase is likely preceded by assembling a molecular machinery required to realize the structural changes of the dendrite to form a protrusion. In the second phase, the initial contact is stabilized by physically linking the membranes of the two different cells by the extracellular interaction of synaptic adhesion molecules. In parallel, further proteins are recruited to the developing pre- and postsynapse mediated by intracellular protein-protein interaction domains of synaptic adhesion molecules. In the third phase synaptic adhesion molecules maintain synaptic connections by extracellular interaction and providing intracellular binding sites for other synaptic proteins. Although synaptic adhesion molecules have been intensely studied, it is still unclear at which stage of the spine lifecycle the different synaptic adhesion molecules operate. Various synaptic adhesion molecules induce synapse formation when overexpressed in heterologous cells4,5; however, this might not represent their physiological function, as synaptic adhesion molecules also function in later phases of synapse development and maintenance. Members of the neuroligin, SynCAM and EphB receptor families are involved in the morphologic and functional differentiation of synapses4,5,6,7,8. Synapse disorganization and imbalanced neuronal excitation and inhibition lead to neurological disorders9. Consistent with the physiological relevance of synapse-organizing molecules, neuroligin, neurexin, SynCAM 1 and cadherins have been linked to neurological disorders such as Alzheimer’s disease, autism spectrum disorders and schizophrenia10,11,12,13,14,15. Here we focus on the function of SynCAM 1 in spine formation and maintenance. SynCAM 1 belongs to the immunoglobulin superfamily (IgSF). The SynCAM family comprises four members in mammals (SynCAM 1–4)16 that are localized at pre- and postsynaptic terminals. In the central nervous system (CNS) mainly the heterophilic interaction of SynCAM 1 and SynCAM 2 occurs17,18. We reported earlier that overexpression of SynCAM 1flag leads to an increase in synapse density and the loss of SynCAM 1 causes a decrease in synapse density in hippocampal CA1 pyramidal cells19. Overexpressing or deleting SynCAM 1 had neither an effect on presynaptic release nor on postsynaptic receptors; however, long-term depression (LTD) was impaired in SynCAM 1 overexpressing mice and facilitated in SynCAM 1 knockout animals. These findings indicate that SynCAM 1 rather acts on the structural properties of the synapse. Identifying the physiological role of a synaptic adhesion molecule is still a challenging question. The time domains of synapse formation and maturation are quite diverse; while forming a spine might take some minutes to hours, spines can be stable for several months. To cover the extended time domain we tracked individual spines in living animals by two-photon in vivo microscopy of layer V pyramidal neurons in the visual cortex. We used Thy1-GFP mice that express the green fluorescent protein (GFP) in a sparse subset of cortical neurons20. Here, we demonstrate that SynCAM 1 directly stabilizes newly formed contacts, thereby impacting on the maturation state of spines. Furthermore, SynCAM 1 improves the stability of mature spines. Inducing the overexpression of SynCAM 1 increases the spine density within a few days. Interestingly, the spine density does not rapidly return to control levels after turning the overexpression of SynCAM 1flag off. However, our data do not support the idea, that SynCAM 1 directly induces the formation of nascent protrusions. In summary, we show that SynCAM 1 is a key regulator of the spine stability.