The downregulation of growth-associated proteins in motoneurons at the onset of synapse elimination is controlled by muscle activity and IGF1

Axonal growth during development and regeneration coincides with the expression of growth-associated proteins (GAPs), including GAP-43 and tubulin-alpha 1. Following contact with the target region and synaptogenesis, GAPs are downregulated. However, high levels of GAP-43 are expressed in some neurons in the adult CNS, indicating that its function may not be restricted to axonal growth. To define the type of signals that regulate GAP levels during development, we have determined whether GAP downregulation coincides with a defined phase of synapse development and whether it is controlled by the postsynaptic target. Levels of GAP-43 mRNA in spinal motoneurons and protein at the neuromuscular junction were analyzed in the developing neuromuscular system of chick and rat. In both species, GAP-43 mRNA declined rapidly at a time corresponding to the onset of synapse elimination, and nerve terminal GAP-43 immunoreactivity became undetectable with a delay of 2- 3 d. In rat motoneurons, GAP-43 and tubulin-alpha 1 mRNA levels declined with a similar time course, indicating that at least two GAPs are downregulated at the same time. Blockade of neuromuscular transmission with either a pre- or a postsynaptically acting toxin prevented GAP downregulation, indicating that target activity affects GAP mRNA levels in motoneurons. Finally, counteracting the activity- dependent decline of muscle insulin-like growth factors during synapse elimination by local subcutaneous injections prevented motoneuron GAP downregulation, suggesting that these growth factors may be involved in the regulation of motoneuron GAPs by the muscle. These results demonstrate that motoneurons undergo target-sensitive changes in GAPs at the onset of synapse elimination. In addition, these results raise the possibility that termination of a presynaptic growth mode may be a prerequisite for synapse elimination.