Rate of neurite outgrowth in sympathetic neurons is highly resistant to suppression of protein synthesis: role of protein degradation/synthesis coupling.

Neurites projecting to their target tissues during embryogenesis are subject to many perturbations that could influence their rate of growth. For example, environmental influences such as supply of neurotrophic factor or electrical activity profoundly influence the rate of neuronal protein synthesis. Because accumulation of protein is necessary for outgrowth to proceed normally, a perturbation in protein synthesis could cause a net change in the rate of accumulation of proteins with the result that neurite outgrowth rate increases or decreases. That neurite outgrowth does not normally seem to be subject to such perturbations suggests involvement of a homeostatic system controlling the rate of outgrowth. Consistent with this hypothesis, we show here that the rate of growth of neurites of sympathetic neurons is highly resistant to decreased rates of protein synthesis. Chronic suppression of protein synthesis by 60% had no significant effect on neurite outgrowth over a 2-day period while complete suppression halted it almost immediately. By the 3rd day of exposure, 60% suppression slowed outgrowth. Sustained suppression of protein synthesis rate by 33% had no effect on rate of outgrowth even after 7 days. We show that the ability of the growing neurites to resist protein synthesis suppression appears to be caused, at least in part, by a parallel decrease in the rate of protein degradation. The result of this coupling between degradation and synthesis is that proteins can continue to accumulate even when protein synthesis rate decreases, allowing normal rates of neurite outgrowth.