EXPRESSION OF SYNAPSIN-I CORRELATES WITH MATURATION OF THE NEUROMUSCULAR SYNAPSE

Synapsins are a family of neuron-specific phosphoproteins that are loc alized within the presynaptic terminals in adult brain. Previous work has demonstrated that introduction of exogenous synapsins I(a + b) or IIa into Xenopus spinal neurons promoted maturation of the neuromuscul ar synapse in a nerve-muscle co-culture system. We have now studied th e expression of endogenous Xenopus synapsin I during synaptic maturati on in vivo and in culture, using a polyclonal antibody raised against Xenopus synapsin I. Immunoprecipitation experiments indicated that syn apsin I was not detectable during the early phase of synaptogenesis in vivo, and exhibited a marked increase during the period of synaptic m aturation. In contrast, the expression of synaptophysin, another synap tic vesicle protein, was detected at the start of nervous system forma tion, and remained at a high level thereafter. Similar expression prof iles for the two proteins were also observed in immunocytochemical stu dies of Xenopus spinal neurons in culture: intense staining of synapto physin was found on the first day, while synapsin I was not detected u ntil after three days in culture. The expression of synapsin I correla ted very well with the appearance of a bell-shaped amplitude distribut ion of spontaneous synaptic currents, a physiological parameter which reflects functional maturation of the neuromuscular synapse. In one-da y-old cultures grown in the absence of laminin, an extracellular matri x protein known to be present at the neuromuscular junction, the ampli tude distribution of virtually all synapses was skewed towards smaller values. In contrast, when laminin was used as a culture substrate, ma ny synapses exhibited a bell-shaped amplitude distribution. Laminin tr eatment also induced synapsin I expression in one-day-old cultures. Th ese results suggest that the expression of endogenous synapsin I may r egulate synaptic maturation at neuromuscular synapses. Copyright (C) 1 996 IBRO. Published by Elsevier Science Ltd.