While the mature central nervous system has an overall diminished capacity for regrowth compared to an embryonic nervous system, axonal regeneration of some populations is possible when they are provided with a potently growth-permissive environment, such as one conferred by a peripheral nerve graft (PNG). Still, axons that are capable of regenerating into the graft fail to grow beyond the PNG to reinnervate spinal cord, at least partly because of the presence of the inhibitory glial scar that is present at the graft-host interface. Digesting the inhibitory extracellular matrix within the scar with chondroitinase ABC (ChABC) allows some axons to traverse the distal interface, synapse upon distal neurons, and mediate some behavioral recovery. Nonetheless, most of the axons remain in the graft. Thus it is important to develop strategies that enhance the ability of axons to regenerate out of a graft in order to increase their potential to improve function. We have preliminary data indicating that expressing constitutively active Rheb (caRheb) - a GTPase that is a critical regulatory component of protein synthesis - in adult neurons results in robust axonal outgrowth in an in vitro glial scar model and in vivo after SCI.
In Aim 1, we will test the hypothesis that expressing caRheb after SCI will enhance regeneration into and beyond a PNG. We will use histological, physiological, and behavioral analyses to examine if caRheb augments axonal regeneration - especially following ChABC treatment of the distal graft interface - and if these regenerated axons form functional synapses that facilitate behavioral recovery. Any regeneration is irrelevant if these axons do not integrate into circuits. Though ChABC enhances plasticity, ChABC-induced sprouted/regenerated fibers do not always form synapses to impact function. Thus, it is also important to devise strategies to improve integration of these axons (i.. synaptogenesis) in order to exploit regrowth that we encourage. We recently demonstrated the proof-of-principle that providing a single exogenous factor enhances the integration of axons that regenerate out of a PNG. Recent work indicates that secreted glypican is sufficient to promote functional, excitatory synapse formation in vitro and behavioral recovery in a cerebral ischemia model in vivo.
In Aim 2, we will test the hypothesis that providing exogenous glypican will enhance synaptogenesis of regenerated axons following SCI. We will graft a PNG into a SCI site that has been treated with ChABC to promote axonal regeneration. We will determine if infusing exogenous glypican into tissue surrounding the SCI site enhances synaptogenesis of these regenerated axons upon neurons distal to the PNG. We will gauge improvements in functional synapse formation physiologically and histologically by examining the colocalization of pre- and postsynaptic markers. We will also assess if enhanced synapse formation correlates with behavioral recovery. Because the strategies to be tested in Aims 1 and 2 take different approaches - enhancing the growth response and synaptogenesis, respectively - to promote functional repair after SCI, it is possible that even if the results in the previous Aims are incremental, combining the two approaches will lead to greater recovery than either alone.
In Aim 3, we will use a novel, multipronged strategy to fill the lesion cavity with a growth-promoting substrate (PNG), enhance the growth response (caRheb), modulate the inhibitory properties of the glial scar (ChABC) and promote synaptogenesis (glypican). We will use histological and physiological analyses to examine if glypican enhances the functional integration of axonal regeneration beyond a ChABC-treated PNG interface that is induced by expressing caRheb and if this results in more robust behavioral recovery than what occurs with either caRheb or glypican treatment separately. These data will identify new therapeutic strategies that may eventually be translated to clinical use to improve the quality of life of persons with SCI.
Spinal cord injury (SCI) affects 1.3 million Americans. Because adult central nervous system axons fail to regenerate following injury, severed axons are permanently disconnected from their target neurons, resulting in the lasting loss of motor and/or sensory function. The proposed experiments in this application are relevant to public health because they will test whether a unique multi-faceted approach that simultaneously provides a growth-permissive substrate, mitigates the hostile inhibitory environment of the glial scar, enhances the axonal growth response, and promotes synaptogenesis facilitates functional axonal regeneration. The strategies have the potential to be translated to clinical application to improve the quality of life for persons who sustained SCI.