The synapsins: Key actors of synapse function and plasticity

The synapsins are a family of neuronal phosphoproteins evolutionarily conserved in invertebrate and vertebrate organisms. Their best-characterised function is to modulate neurotransmitter release at the pre-synaptic terminal, by reversibly tethering synaptic vesicles (SVs) to the actin cytoskeleton. However, many recent data have suggested novel functions for synapsins in other aspects of the pre-synaptic physiology, such as SV docking, fusion and recycling. Synapsin activity is tightly regulated by several protein kinases and phosphatases, which modulate the association of synapsins to SVs as well as their interaction with actin filaments and other synaptic proteins. In this context, synapsins act as a link between extracellular stimuli and the intracellular signalling events activated upon neuronal stimulation. Genetic manipulation of synapsins in various in vivo models has revealed that, although not essential for the basic development and functioning of neuronal networks, these proteins are extremely important in the fine-tuning of neuronal plasticity, as shown by the epileptic phenotype and behavioural abnormalities characterising mouse lines lacking one or more synapsin isoforms. In this review, we summarise the current knowledge about how the various members of the synapsin family are involved in the modulation of the pre-synaptic physiology. We give a comprehensive description of the molecular basis of synapsin function, as well as an overview of the more recent evidence linking mutations in the synapsin proteins to the onset of severe central nervous system diseases such as epilepsy and schizophrenia.