Limited functional recovery can be achieved through rehabilitation after incomplete spinal cord injury. Eliminating the function of a repulsive Wnt receptor, Ryk, in mice and rats by either ...conditional knockout in the motor cortex or monoclonal antibody infusion resulted in increased corticospinal axon collateral branches with presynaptic puncta in the spinal cord and enhanced recovery of forelimb reaching and grasping function following a cervical dorsal column lesion. Using optical stimulation, we observed that motor cortical output maps underwent massive changes after injury and that hindlimb cortical areas were recruited to control the forelimb over time. Furthermore, a greater cortical area was dedicated to controlling the forelimb in Ryk conditional knockout mice than in controls (wild-type or heterozygotes). In the absence of weekly task-specific training, recruitment of ectopic cortical areas was greatly reduced and there was no significant functional recovery even in Ryk conditional knockout mice. Our study provides evidence that maximal circuit reorganization and functional recovery can be achieved by combining molecular manipulation and targeted rehabilitation.
…once the development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably.
Santiago Ramón y Cajal
Cajal’s neurotropic theory postulates that the ...complexity of the nervous system arises from the collaboration of neurotropic signals from neuronal and non-neuronal cells and that once development has ended, a paucity of neurotropic signals means that the pathways of the central nervous system are “fixed, ended, immutable”. While the capacity for regeneration and plasticity of the central nervous system may not be quite as paltry as Cajal proposed, regeneration is severely limited in scope as there is no spontaneous regeneration of long-distance projections in mammals and therefore limited opportunity for functional recovery following spinal cord injury. It is not a far stretch from Cajal to hypothesize that reappropriation of the neurotropic programs of development may be an appropriate strategy for reconstitution of injured circuits. It has become clear, however, that a significant number of the molecular cues governing circuit development become re-active after injury and many assume roles that paradoxically obstruct the functional re-wiring of severed neural connections. Therefore, the problem to address is how individual neural circuits respond to specific molecular cues following injury, and what strategies will be necessary for instigating functional repair or remodeling of the injured spinal cord.
A principal objective of spinal cord injury (SCI) research is the restoration of axonal connectivity to denervated targets. We tested the hypothesis that chemotropic mechanisms would guide ...regenerating spinal cord axons to appropriate brainstem targets. We subjected rats to cervical level 1 (C1) lesions and combinatorial treatments to elicit axonal bridging into and beyond lesion sites. Lentiviral vectors expressing neurotrophin-3 (NT-3) were then injected into an appropriate brainstem target, the nucleus gracilis, and an inappropriate target, the reticular formation. NT-3 expression in the correct target led to reinnervation of the nucleus gracilis in a dose-related fashion, whereas NT-3 expression in the reticular formation led to mistargeting of regenerating axons. Axons regenerating into the nucleus gracilis formed axodendritic synapses containing rounded vesicles, reflective of pre-injury synaptic architecture. Thus, we report for the first time, to the best of our knowledge, the reinnervation of brainstem targets after SCI and an essential role for chemotropic axon guidance in target selection.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The plasticity of sensorimotor systems in mammals underlies the capacity for motor learning as well as the ability to relearn following injury. Spinal cord injury, which both deprives afferent input ...and interrupts efferent output, results in a disruption of cortical somatotopy. While changes in corticospinal axons proximal to the lesion are proposed to support the reorganization of cortical motor maps after spinal cord injury, intracortical horizontal connections are also likely to be critical substrates for rehabilitation-mediated recovery. Intrinsic connections have been shown to dictate the reorganization of cortical maps that occurs in response to skilled motor learning as well as after peripheral injury. Cortical networks incorporate changes in motor and sensory circuits at subcortical or spinal levels to induce map remodeling in the neocortex. This review focuses on the reorganization of cortical networks observed after injury and posits a role of intracortical circuits in recovery.
Restoring corticospinal function after spinal cord injury is a significant challenge as the corticospinal tract elicits no substantive, spontaneous regeneration, and its interruption leaves a ...permanent deficit. The corticospinal circuit serves multiple motor and sensory functions within the mammalian nervous system as the direct link between isocortex and spinal cord. Maturation of the corticospinal circuit involves the refinement of projections within the spinal cord and a subsequent refinement of motor maps within the cortex. The plasticity of these cortical motor maps mirrors the acquisition of skilled motor learning, and both the maps and motor skills are disrupted following injury to the corticospinal tract. The motor cortex exhibits the capacity to incorporate changes in corticospinal projections induced by both spontaneous and therapeutic-mediated plasticity of corticospinal axons through appropriate rehabilitation. An understanding of the mechanisms of corticospinal plasticity in motor learning will undoubtedly help inform strategies to improve motor rehabilitation after spinal cord injury.
Several experimental manipulations of the CNS environment successfully elicit regeneration of sensory and bulbospinal motor axons but fail to elicit regeneration of corticospinal axons, suggesting ...that cell-intrinsic mechanisms limit the regeneration of this critical class of motor neurons. We hypothesized that enhancement of intrinsic neuronal growth mechanisms would enable adult corticospinal motor axon regeneration. Lentiviral vectors were used to overexpress the BDNF receptor trkB in layer V corticospinal motor neurons. After subcortical axotomy, trkB transduction induced corticospinal axon regeneration into subcortical lesion sites expressing BDNF. In the absence of trkB overexpression, no regeneration occurred. Selective deletion of canonical, trkB-mediated neurite outgrowth signaling by mutation of the Shc/FRS-2 activation domain prohibited Erk activation and eliminated regeneration. These findings support the hypothesis that the refractory regenerative state of adult corticospinal axons can be attributed at least in part to neuron-intrinsic mechanisms, and that activation of ERK signaling can elicit corticospinal tract regeneration.
Guidance molecules in axon regeneration Giger, Roman J; Hollis, 2nd, Edmund R; Tuszynski, Mark H
Cold Spring Harbor perspectives in biology
2, Številka:
7
Journal Article
Recenzirano
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The regenerative capacity of injured adult mammalian central nervous system (CNS) tissue is very limited. Disease or injury that causes destruction or damage to neuronal networks typically results in ...permanent neurological deficits. Injury to the spinal cord, for example, interrupts vital ascending and descending fiber tracts of spinally projecting neurons. Because neuronal structures located proximal or distal to the injury site remain largely intact, a major goal of spinal cord injury research is to develop strategies to reestablish innervation lost as a consequence of injury. The growth inhibitory nature of injured adult CNS tissue is a major barrier to regenerative axonal growth and sprouting. An increasing complexity of molecular players is being recognized. CNS inhibitors fall into three general classes: members of canonical axon guidance molecules (e.g., semaphorins, ephrins, netrins), prototypic myelin inhibitors (Nogo, MAG, and OMgp) and chondroitin sulfate proteoglycans (lecticans, NG2). On the other end of the spectrum are molecules that promote neuronal growth and sprouting. These include growth promoting extracellular matrix molecules, cell adhesion molecules, and neurotrophic factors. In addition to environmental (extrinsic) growth regulatory cues, cell intrinsic regulatory mechanisms exist that greatly influence injury-induced neuronal growth. Various degrees of growth and sprouting of injured CNS neurons have been achieved by lowering extrinsic inhibitory cues, increasing extrinsic growth promoting cues, or by activation of cell intrinsic growth programs. More recently, combination therapies that activate growth promoting programs and at the same time attenuate growth inhibitory pathways have met with some success. In experimental animal models of spinal cord injury (SCI), mono and combination therapies have been shown to promote neuronal growth and sprouting. Anatomical growth often correlates with improved behavioral outcomes. Challenges ahead include testing whether some of the most promising treatment strategies in animal models are also beneficial for human patients suffering from SCI.
Conditioning lesion of the peripheral branch of dorsal column axons is a well-known paradigm enabling the central branch to regenerate after injury to the spinal cord. However, only a small number of ...regenerating axons enter grafted substrates, and they do not grow beyond the lesion. We found that conditioning lesion induces, in addition to growth-stimulating genes, related to receptor tyrosine kinase (Ryk), a potent repulsive receptor for Wnts. Wnts are expressed around the site of spinal cord injury, and we found that grafted bone marrow stromal cells secreting the Wnt inhibitors secreted frizzled-related protein 2 or Wnt inhibitory factor 1 enhanced regeneration of the central branch after peripheral conditioning lesion. Furthermore, we found that Wnt4-expressing grafts caused dramatic long-range retraction of the injured central branch of conditioned dorsal root ganglion neurons. Macrophages accumulate along the path of receding axons but not around Wnt4-expressing cells, suggesting that the retraction of dorsal column axons is not a secondary effect of increased macrophages attracted by Wnt4. Therefore, Wnt-Ryk signaling is an inhibitory force co-induced with growth-stimulating factors after conditioning lesion. Overcoming Wnt inhibition may further enhance therapies being designed on the basis of the conditioning-lesion paradigm.
Spinal cord injury permanently disrupts neuroanatomical circuitry and can result in severe functional deficits. These functional deficits, however, are not immutable and spontaneous recovery occurs ...in some patients. It is highly likely that this recovery is dependent upon spared tissue and the endogenous plasticity of the central nervous system. Neurotrophic factors are mediators of neuronal plasticity throughout development and into adulthood, affecting proliferation of neuronal precursors, neuronal survival, axonal growth, dendritic arborization and synapse formation. Neurotrophic factors are therefore excellent candidates for enhancing axonal plasticity and regeneration after spinal cord injury. Understanding growth factor effects on axonal growth and utilizing them to alter the intrinsic limitations on regenerative growth will provide potent tools for the development of translational therapeutic interventions for spinal cord injury.
Adeno-associated virus (AAV) is frequently used for gene transfer into the central nervous system (CNS). Similar to adenovirus and rabies virus, AAV can be taken up by axons and retrogradely ...transported, resulting in neuronal gene expression distant from the injection site. We investigated the retrograde transport properties of self-complementary AAV (scAAV) serotypes 1–6 following peripheral injection. Injection of scAAV into either rat extensor carpi muscle or sciatic nerve resulted in detectable retrograde vector transport and reporter gene expression in spinal cord motor neurons (MNs). Serotype 1 resulted in the highest level of retrograde transport, with 4.1 ± 0.3% of cervical MNs projecting to the extensor carpi transduced following intramuscular injection, and 7.5 ± 3.1% of lumbar MNs transduced after sciatic nerve injection. In contrast to scAAV1, retrograde transduction with scAAV2 was undetectable following intramuscular injection, and was detected in only 0.81 ± 0.15% of MNs projecting to the sciatic nerve following intranerve injection. Furthermore, sciatic injection of single-stranded AAV1 required injection of tenfold higher numbers of viral particles for detectable transgene expression compared to scAAV1, and then only 0.91 ± 0.24% of lumbar MNs were transduced. Our data provide the basis for increased retrograde transduction efficiency using peripheral injections of scAAV1 vectors for therapeutic gene delivery to the spinal cord.
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