Targeting myelin to optimize plasticity of spared spinal axons

Endogenous neurotrophins and plasticity following spinal deafferentation

Neurons intrinsic to the spinal cord dorsal horn receive input from various classes of long-distance projection systems. Two of the best known of these are primary afferent and descending monoaminergic axons. Together with intrinsic interneurons, activity in these axonal populations shapes the early part of the sensory experience before it is transmitted to supraspinal structures via ascending projection axons. Injury to dorsal roots, which contain the centrally projecting branches of primary afferent axons, results in their permanent disconnection from the spinal cord, as well as sensory dysfunction such as pain. In animals, experimental dorsal root injuries affecting a small number of roots produce dynamic behavioural changes, providing evidence for the now familiar concept that sensory processing at the level of the spinal cord is not hard-wired. Changes in behaviour following rhizotomy suggest changes in spinal sensory circuitry, and we and others have shown that the density of spinal serotonergic axons as well as processes of inhibitory interneurons increases following rhizotomy. Intact primary afferent axons are less apt to sprout into denervated territory. Recent work from our group has asked (1) what is the stimulus that induces sprouting of serotonergic (and other) axons and (2) what prevents spared primary afferent axons from occupying the territory of those lost to injury. This article will review the evidence that a single factor upregulated by dorsal root injury, brain-derived neurotrophic factor (BDNF), underpins both serotonergic sprouting and a lack of primary afferent plasticity. BDNF also differentially modulates some of the behavioural consequences of dorsal root injury: antagonizing endogenous BDNF improves spontaneous mechanosensory recovery but prevents recovery from rhizotomy-induced hypersensitivity to cold. These findings reinforce the notion that in disease states as complex and variable as spinal cord injury, single pharmacological interventions are unlikely to produce meaningful results. However, understanding the differences in capacity for plasticity among different systems, as well as their triggers, should allow for more patient-tailored therapies.

Endogenous TrkB ligands suppress functional mechanosensory plasticity in the deafferented spinal cord

Dorsal root injury (DRI) disrupts the flow of sensory information to the spinal cord. Although primary afferents do not regenerate to their original targets, spontaneous recovery can, by unknown mechanisms, occur after DRI. Here, we show that brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), but not nerve growth factor or neurotrophin-4, are upregulated in the spinal gray matter after DRI. Because endogenous BDNF and NT-3 have well established roles in synaptic and axonal plasticity, we hypothesized that they contributed to spontaneous recovery after DRI. We first developed a model of DRI-induced mechanosensory dysfunction: rat C7/8 DRI produced a deficit in low-threshold cutaneous mechanosensation that spontaneously improved within 10 d but did not recover completely. To determine the effects of endogenous BDNF and NT-3, we administered TrkB-Fc or TrkC-Fc fusion proteins throughout the recovery period. To our surprise, TrkB-Fc stimulated complete recovery of mechanosensation by 6 d after DRI. It also stimulated mechanosensory axon sprouting but prevented deafferentation-induced serotonergic sprouting. TrkC-Fc had no effect on low-threshold mechanosensory behavior or axonal plasticity. There was no mechanosensory improvement with single-bolus TrkB-Fc infusions at 10 d after DRI (despite significantly reducing rhizotomy-induced cold pain), indicating that neuromodulatory effects of BDNF did not underlie mechanosensory recovery. Continuous infusion of the pan-neurotrophin antagonist K252a also stimulated behavioral and anatomical plasticity, indicating that these effects of TrkB-Fc treatment occurred independent of signaling by other neurotrophins. These results illustrate a novel, plasticity-suppressing effect of endogenous TrkB ligands on mechanosensation and mechanosensory primary afferent axons after spinal deafferentation.

An overview of tissue engineering approaches for management of spinal cord injuries

Severe spinal cord injury (SCI) leads to devastating neurological deficits and disabilities, which necessitates spending a great deal of health budget for psychological and healthcare problems of these patients and their relatives. This justifies the cost of research into the new modalities for treatment of spinal cord injuries, even in developing countries. Apart from surgical management and nerve grafting, several other approaches have been adopted for management of this condition including pharmacologic and gene therapy, cell therapy, and use of different cell-free or cell-seeded bioscaffolds. In current paper, the recent developments for therapeutic delivery of stem and non-stem cells to the site of injury, and application of cell-free and cell-seeded natural and synthetic scaffolds have been reviewed.

Spinally upregulated noggin suppresses axonal and dendritic plasticity following dorsal rhizotomy

Bone morphogenetic proteins (BMPs) and their antagonists, including noggin, are required for nervous system development, but their potential roles in the reactions of the adult central nervous system to injury are unknown. Here we have examined the expression of noggin and BMPs in the spinal cord following dorsal rhizotomy. Through the use of a function-blocking antibody, we have also investigated the role of endogenous noggin in the neuritic plasticity which follows rhizotomy. Dorsal rhizotomy resulted in the upregulation of BMPs 2/4, 7 and noggin in the superficial white matter and in the dorsal neuropil of the spinal cord. These co-localized with glial fibrillary acidic protein, indicating their expression by astrocytes. Because BMPs induce dendritic sprouting and synaptogenesis in some neuronal populations in vitro, we hypothesized that administration of a noggin function-blocking antibody (FbAb) in vivo would augment rhizotomy-induced sprouting in the spinal cord. Topical application of noggin-FbAb to the dorsal surface of the spinal cord following rhizotomy resulted in significant increases in the density of microtubule-associated protein 2 (MAP-2) and substance P (SP)-positive processes within the lateral spinal nucleus. In the deafferented dorsal horn, noggin-FbAb treatment induced significant increases in the density of SP, calcitonin gene-related peptide (CGRP)- and 5-hydroxytryptamine (5-HT)-positive axons. These results suggest a novel mechanism by which endogenous plasticity of spared axons is suppressed following dorsal rhizotomy, and which might be exploited to improve the outcome of spinal cord injury and other CNS trauma.