| The aim of this thesis has been to examine how the growth and stability of terminal axon branches is regulated at the neuromuscular junction. I compared the effect of axotomy and target cell death on terminal axon branches in vivo. Following death of an individual postsynaptic target muscle fiber, the terminal axon branch projecting to the lost target underwent both an acute phase of terminal loss over minutes followed by a more gradual retraction over days that resembled branch withdrawal seen during naturally occurring synapse elimination. In particular, retracting branches gradually atrophied and terminated in a bulb shaped structure. Generally, the changes progress in a distal to proximal gradient with the most distal branches within a neuromuscular junction retracting first. In contrast, less than a day after crushing a fluorescently labeled axon several millimeters proximal to its termination, its terminal branches lost all fluorescence over a period of 45 minutes. This loss of neuromuscular junction staining was followed shortly by fragmentation of the preterminal axon. These changes were initiated in a proximo-distal direction. The different responses to interruption of signals from the cell body and the postsynaptic target suggest that the support axons receive from these two sources is different but both kinds of trophic signaling are necessary for the maintenance of axonal branches and synaptic terminals.; One possible maintenance signal for axons is glial cell line-derived neurotrophic factor (GDNF). To explore the local effect of this factor at the neuromuscular junction, I expressed GDNF in single adult mouse muscle fibers in vivo. Muscle fiber transfection with GDNF had no effect on intact neuromuscular junctions. In contrast, when growing axons reinnervated GDNF transfected fibers following axotomy, a profusion of axonal sprouts were produced by axons in the vicinity of the transfected fiber. The significance of this potent effect of GDNF on growing axons may be related to our finding that GDNF expression increases in Schwann cells distal to the site of nerve injury. Therefore GDNF may be an important mediator of motor axon regeneration and branching during regeneration after injury. |
