Two-Tiered Inhibition of Axon Regeneration at the Dorsal Root Entry Zone

Abstract

Glial-derived inhibitory molecules and a weak cell-body response prevent sensory axon regeneration into the spinal cord after dorsal root injury. Neurotrophic factors, particularly neurotrophin-3 (NT-3), may increase the regenerative capacity of sensory neurons after dorsal rhizotomy, allowing regeneration across the dorsal root entry zone (DREZ). Intrathecal NT-3, delivered at the time of injury, promoted an upregulation of the growth-associated protein GAP-43 primarily in large-diameter sensory profiles (which did not occur after rhizotomy alone), as well as regeneration of cholera toxin B-labeled sensory axons across the DREZ and deep into the dorsal horn. However, delaying treatment for 1 week compromised regeneration: although axons still penetrated the DREZ, growth within white matter was qualitatively and quantitatively restricted. This was not associated with an impaired cell-body response (GAP-43 upregulation was equivalent for both immediate and delayed treatments), or with astrogliosis at the DREZ, which begins almost immediately after rhizotomy, but with the delayed appearance of mature ED1-expressing phagocytes in the dorsal white matter between 1 and 2 weeks after lesion, marking the beginning of myelin breakdown. After rhizotomy with immediate NT-3 treatment, regeneration continues beyond 2 weeks, but in the dorsal gray matter rather than in the degenerating dorsal columns. The ability of NT-3 to promote regeneration across the DREZ, but not after the beginning of degeneration after delayed treatment reveals a hierarchy of inhibitory influences: the astrogliotic, but not the degenerative barrier is surmountable by NT-3 treatment. Key words: neurotrophin-3; regeneration; degeneration; astrocytes; oligodendrocytes; myelin; dorsal root ganglionThe differential abilities of peripheral and central nervous tissue to support regeneration is exemplified at the dorsal root entry zone (DREZ), which marks the entry point of primary afferent axons into the spinal cord. Here, the environment changes abruptly from consisting of growth-permissive Schwann cells, to astrocytes, oligodendrocytes, and microglia, which may all inhibit regeneration (Fawcett and Asher, 1999). On contact with the DREZ, regenerating axons form club-like end bulbs or synapselike structures, (Ramon y Cajal, 1928;Carlstedt, 1985), but because of the prohibitive environment of the CNS and a paltry regenerative response of sensory neurons to rhizotomy, they never re-enter the adult cord.One strategy to encourage regeneration is to enhance the growth response of the rhizotomized neurons. GAP-43 is induced in nearly all neurons during peripheral nerve regeneration (Verge et al., 1990b), but in few neurons after rhizotomy (Schreyer and Skene, 1993), unless the root is severed very close to the DRG (Chong et al., 1996). Dorsal root axonal growth occurs at about half the rate of peripheral axons (Wujek and Lasek, 1983;Oblinger and Lasek, 1984). An experimental "conditioning" lesion to a peripheral nerve before dorsal rhizotomy doubles the rate of ...

Topographically specific regeneration of sensory axons in the spinal cord

“…Regeneration of sensory axons following DR crush also has been reported using several other neurotrophic factors, including nerve growth factor (NGF), neurotrophin 3 (NT3), and glialderived neurotrophic factor (GDNF) (14)(15)(16)(17); however, in all of these cases, the regeneration did not appear to be topographically specific (Discussion). A possible reason for the absence of specificity is that these factors were applied directly to the spinal cord either by viral expression within the cord or by intrathecal infusion, suggesting that the method of application is critical for topographically specific regeneration.…”

Degeneración de los terminales aferentes primarios de rata luego de lesión extensa por avulsión del plexo braquial

“…According to data by Bradbury et al, nearly 95% of neurons that express the P2X3 receptor belong to the C fiber group which binds the IB4 lectin (28). The P2X3-positive fibers normally project to lamina II of the dorsal horn (4,28,49). Immunoreactivity is exclusively observed in the primary afferents and no intrinsic spinal cord cells with immunoreactivity to the purinergic receptor have been identified (28).…”

“…Lesions produced by brachial plexus avulsion are frequently caused in adult humans during motorcycle, sports and industrial accidents, and in children by hyperstretching of the angle between the head and neck. The experimental lesion of dorsal rhizotomy and avulsion models in rats reproduce the damage produced by human avulsion (4)(5)(6). Depending on the extent of the lesion, consequences include severe motor and sensory deficit and upper limb paralysis, reflex loss, hypersensitivity, acute pain and allodynia and, in the most dramatic cases, self-mutilation (7).…”

Setting the stage for functional repair of spinal cord injuries: a cast of thousands

“…Astrogliosis begins within days of a dorsal rhizotomy, whereas frank Wallerian degeneration does not begin until 1 week later. 73,74 It is clear that activated astrocytes, whether part of a scar following SCI or as a component of the reactive DREZ following rhizotomy, deter regeneration. Following SCI, axons retract from the lesion site and are surrounded by CNS myelin.…”

“…Following a dorsal rhizotomy, neurotrophin-3 (NT-3) application promotes functional centripetal regeneration as well as GAP-43 upregulation. 74,143,144 Following dorsolateral funiculus lesions of the spinal cord, brain-derived neurotrophic factor application at the level of the red nucleus increases GAP-43 expression and improves regeneration of rubrospinal axons into a peripheral nerve graft. 27 Like RAG expression, cAMP also appears to play an important role in regenerative potential.…”