Developmental expression and gene organization of protochordate synapsin: highlights on basic functions.

Synapsins belong to a family of neuron-specific phosphoproteins and are involved in several functions correlated with neurotransmitter release and synaptogenesis (reviewed in Hilfiker et al. 1999, Phil.Trans. R. Soc. Lond. B 354: 269-279). The comprehension of the basal role of the synapsin family is hampered in vertebrates by the presence of multiple members. Then, the study of homologous genes in protochordates, such as ascidians and amphioxus, lacking the whole genome duplications characteristic of vertebrates, could help to better understand the complex functions of synapsins. Therefore, we have cloned and analyzed two synapsin genes, Ci-Syn in the ascidian Ciona intestinalis and AmphiSyn in the amphioxus Branchiostoma floridae. Our results show the presence of a single synapsin gene in both organisms, and the occurrence of a second transcript by alternative splicing in B. floridae. Our genomic analysis reveals an high homology degree with vertebrate synapsins, in particular with synapsin III, the occurrence of three conserved domains A, C and E, and the presence of the classical nested organization of a TIMP (tissue inhibitor of metalloproteinase) gene in the intronic sequence. Moreover, we have localized by in situ hybridization Ci-Syn and AmphiSyn transcripts exclusively at neuronal levels and in particular in a relevant portion of developing neurons, suggesting that the basic role of synapsins as regulators of neurotransmission and synaptogenesys has been conserved during evolution.