TARGET CONTACT REGULATES EXPRESSION OF SYNAPTOTAGMIN GENES IN SPINAL MOTOR-NEURONS IN-VIVO

During neuromuscular development, neuronal contact with peripheral tar gets is associated with an increase in synaptic vesicle protein (SVP) gene expression, suggesting that target contact and upregulation of SV P genes are causally related. To test this idea, we analyzed the devel opmental expression pattern of synaptotagmin (syt) mRNAs in the chick lateral motor column (LMC) using in situ hybridization. Syt I mRNA in the LMC is upregulated from Embryonic Day 4.5 (E4.5) to E5.5, coincide nt with the time these neurons begin to make contact with their muscle targets. In contrast, levels of mRNA for neurofilament do not change during this time. Extirpation of the limb bud prior to motor axon outg rowth eliminates the increase in syt I mRNA ipsilaterally. Later in de velopment, there is a switch in syt isoform abundance in the LMC, with syt II mRNA being upregulated between E15 and E20 and syt I mRNA bein g downregulated. Our results suggest that contact with targets upregul ates syt I gene expression during neuromuscular synapse formation in v ivo, and that a later stage of synaptic maturation involves changes in SVP isoform abundance.