Molecular mechanisms underlying functional regeneration following nerve injury.

摘要

Peripheral nerves have a robust capacity to regenerate following injury. Nevertheless, recovery of motor function after nerve injury is often sub-optimal due to the mistargeting of regenerating motor axons to incorrect muscles or skin. Consequently, therapies aimed at improving the accuracy of regenerating motor axons are required. Since the limits of mechanical nerve repair were reached decades ago, the next generation of therapies must arise from exploiting the molecular mechanisms underlying nerve regeneration. Therefore, the major goal of this thesis was to explore the roles of Neural Cell Adhesion Molecule (NCAM) and its Polysialic Acid (PSA) moiety in neuromuscular recovery after nerve injury. Previous studies had shown that these molecules were required for the appropriate targeting of embryonic motor axons to muscle, as well as the normal development and function of the neuromuscular junction (NMJ). Furthermore, PSA and NCAM are known to be up-regulated after nerve injury. I more closely examined PSA up-regulation after nerve injury and found that it was an intrinsic property of neurons because distinct motor and sensory neurons expressed it differently. To assess the functional consequence of this, I applied an established model of nerve regeneration where motor axons preferentially regenerate to muscle over skin despite having equal access to both. I discovered that the ability to preferentially regenerate to muscle requires PSA since its genetic or enzymatic ablation abolished this ability. Furthermore, I found that PSA levels and regeneration accuracy were both enhanced by brief electrical stimulation applied to cut motor axons. The benefit of electrical stimulation was directly related to increased PSA levels because it was completely blocked by PSA removal. Finally, I studied the role of NCAM in NMJ formation, maintenance and function following nerve injury. In the absence of NCAM, reinnervated muscle initially recovered normal function, but then experienced a dramatic loss of muscle strength. Immunohistochemical analysis of the muscle revealed a severe reduction in muscle area that was due to a selective loss of fast muscle fibers. In summary, PSA and NCAM are crucial for neuromuscular recovery after nerve injury and present promising targets for future molecular based nerve repair strategies.