Potential developmental stop signals for GAP-43 expression during corticospinal tract growth

Abstract

Developmental (and regenerative) axon growth is accompanied by high expression of the growth-associated protein GAP-43, which is then down-regulated late in development (or following regenerative success). This thesis evaluate this hypothesis by examining the temporal pattern of GAP-43 expression in relation to myelin formation in the postnatal corticospinal tract and ascending sensory axons in the spinal dorsal column. It further investigates the effects of CNS myelin on neurite outgrowth from embryonic and postnatal central neurons in culture in order to more directly determine whether myelin induced inhibition of axon growth involves GAP-43 repression. My findings demonstrate that in vivo, high levels of GAP-43 protein persists long after the appearance of myelin in the spinal dorsal column and that in vitro, contact with CNS myelin does not cause GAP-43 down-regulation in correlation with inhibition of neurite outgrowth. I next examined a second hypothesis that retrograde signals from target tissue may play a role in developmental down regulation of GAP-43 mRNA in corticospinal neurons. I used in situ hybridization to quantify relative changes in GAP 43 mRNA in corticospinal tract neurons identified by Fast Blue retrograde labeling. I also used anterograde transport of biotinylated dextran amine to study the invasion of target area by corticospinal axons. I found that developmental down-regulation of GAP-43 mRNA shows a biphasic pattern. The first phase of down-regulation of GAP-43 in corticospinal neurons is coincident with initial target contact, while the second phase is coincident with final maturation of terminal arborization. Interestingly only the second decline in GAP-43 mRNA is accompanied by a decline in GAP-43 protein, as assessed by immunocytochemistry. To investigate whether expression of GAP-43 mRNA is regulated by retrograde signals, I injected colchicine into the corticospinal tract to block retrograde axonal transport during a time when GAP-43 is normally declining in corticospinal neurons. Colchicine caused a prolongation of high GAP-43 mRNA expression in neurons located in layer V (but not other layers) of sensorimotor cortex. (Abstract shortened by UMI.)