Targeting Sensory Axon Regeneration in Adult Spinal Cord

Abstract

Extensive regeneration of sensory axons into the spinal cord can be achieved experimentally after dorsal root injury, but no effort has been made to target regenerating axons and restore a normal lamina-specific projection pattern. Ectopic axon growth is potentially associated with functional disorders such as chronic pain and autonomic dysreflexia. This study was designed to target regenerating axons to normal synaptic locations in the spinal cord by combining positive and negative guidance molecules. Previously, we observed that, after dorsal rhizotomy, overexpression of NGF leads to robust regeneration and sprouting of calcitonin gene-related peptide (CGRP)-positive nociceptive axons throughout dorsal horn and ventral horns. To restrict these axons within superficial laminas, adenovirus expressing semaphorin 3A was injected into the ventral spinal cord 3 d after NGF virus injection. Semaphorin 3A expression was observed in deep dorsal and ventral cord regions and limited axon growth to laminas I and II, shaping axonal regeneration toward the normal distribution pattern. NGF and semaphorin 3A treatment also targeted the regeneration of substance P-positive nociceptive axons but had no effect on injured isolectin B4-binding nociceptive axons. Axon regeneration led to functional restoration of nociception in both NGF-and NGF/semaphorin 3A-treated rats. Although no significant difference in behavior was found between these two groups, confocal microscopy illustrated ectopic synaptic formations in deeper laminas in NGF/green fluorescent protein-treated rats. The results suggested that antagonistic guidance cues can be used to induce and refine regeneration within the CNS, which is important for longterm, optimal functional recovery.

Molecular/genetic manipulation of extrinsic axon guidance factors for CNS repair and regeneration

“…Thus, strategic placement of a chemorepulsive factors can potentially be used to control injury induced sprouting that leads to detrimental effects of the lesion. This model has also been used successfully to target regenerating nociceptive axons to their appropriate laminae (Tang et al, 2007). Numerous studies have shown that neurotrophins aid regeneration of many systems, and could be thought of as providing attractive guidance cues (Ramer et al, 2000;Romero et al, 2001;Taylor et al, 2006).…”

“…Viral over-expression of NGF in the dorsal horn led to robust regeneration specifically of CGRP+ axons; however, these axons failed to terminate within laminae I and II, and instead grew abnormally throughout the entire dorsal horn Tang et al, 2004a), most likely due to the loss of semaphorin 3A expression in the adult. To target regenerating nociceptive axons and recapitulate their developmental lamina specific patterning, slightly overlapping gradients of NGF and semaphorin 3A were established in the dorsal horn (Tang et al, 2007). For these experiments, recombinant adenovirus was used to control both the temporal and spatial aspects of the two gradients, which were important to obtain appropriate termination within laminae I and II.…”

Axon growth across a lesion site along a preformed guidance pathway in the brain

“…Previous studies in our lab demonstrated that injecting adenovirus encoding NGF in the dorsal spinal cord induced extensive sprouting of nociceptive fibers into the dorsal horn (Romero et al, 2000). Furthermore, overexpression of NGF induced lesioned sensory axons to re-grow into the dorsal horn, which would normally not happen after dorsal root injury (Romero et al, 2001;Tang et al, 2004;Tang et al, 2007). FGF-2 is also known to be a neurotrophic factor that supports neuronal survival (Morrison et al, 1986) and promotes axonal regeneration (Sapieha et al, 2003).…”

Modulating Sema3A signal with a L1 mimetic peptide is not sufficient to promote motor recovery and axon regeneration after spinal cord injury

“…Indeed, levels of cAMP and cGMP are known to modulate the response of growth cones to extracellular cues such as Sema3A and MAG (Song et al, 1997). In the same way, neurotrophins such as nerve growth factor (NGF) have the ability to enhance regenerative capacities of axons (Chaudhry et al, 2006) and in this context, Sema3A is able to restrict NGF-induced axon growth (Tang et al, 2007). This suggests that in combination with growth promoting strategies, ASNKL treatment might be able to promote axon regeneration.…”