Edmund R. Hollis

Guidance Molecules in Axon Regeneration

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.

Chemotropic Guidance Facilitates Axonal Regeneration and Synapse Formation after Spinal Cord Injury

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.

Induction of corticospinal regeneration by lentiviral trkB-induced Erk activation

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.

Efficient retrograde neuronal transduction utilizing self-complementary AAV1.

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.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.

IGF-I gene delivery promotes corticospinal neuronal survival but not regeneration after adult CNS injury.

An unmet challenge of spinal cord injury research is the identification of mechanisms that promote regeneration of corticospinal motor axons. Recently it was reported that IGF-I promotes corticospinal axon growth during nervous system development. We therefore investigated whether IGF-I also promotes regeneration or survival of adult lesioned corticospinal neurons. Adult Fischer 344 rats underwent C3 dorsal column transections followed by grafts of IGF-I-secreting marrow stromal cell grafts into the lesion cavity. IGF-I secreting cell grafts promoted growth of raphespinal and cerulospinal axons, but not corticospinal axons, into the lesion/graft site. We then examined whether IGF-I-secreting cell grafts promote corticospinal motor neuron survival or axon growth in a subcortical axotomy model. IGF-I expression coupled with infusion of the IGF binding protein inhibitor NBI-31772 significantly prevented corticospinal motor neuron death (93% cell survival compared to 49% in controls, P<0.05), but did not promote corticospinal axon regeneration. Coincident with observed effects of IGF-I on corticospinal survival but not growth, expression of IGF-I receptors was restricted to the somal compartment and not the axon of adult corticospinal motor neurons. Thus, whereas IGF-I influences corticospinal axonal growth during development, its application to sites of adult spinal cord injury or subcortical axotomy fails to promote corticospinal axonal regeneration under conditions that are sufficient to prevent corticospinal cell death and promote the growth of other supraspinal axons. We conclude that developmental patterns of growth factor responsiveness are not simply recapitulated after adult injury, potentially due to post-natal shifts in patterns of IGF-I receptor expression.

Neurotrophins: Potential Therapeutic Tools for the Treatment of Spinal Cord Injury

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.

Reinduced Wnt signaling limits regenerative potential of sensory axons in the spinal cord following conditioning lesion

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.

Axon guidance and injury – lessons from Wnts and Wnt signaling

Many studies in the past decade have revealed the role and mechanisms of Wnt signaling in axon guidance during development and the reinduction of Wnt signaling in adult central nervous system axons upon traumatic injury, which has profound influences on axon regeneration. With 19 Wnts and 14 known receptors (10 Frizzleds (Fzds), Ryk, Ror1/2 and PTK7), the Wnt family signaling proteins contribute significantly to the wiring specificity of the complex brain and spinal cord circuitry. Subsequent investigation into the signaling mechanisms showed that conserved cell polarity pathways mediate growth cone steering. These cell polarity pathways may unveil general principles of growth cone guidance. The reappeared Wnt signaling system after spinal cord injury limits the regrowth of both descending and ascending motor and sensory axons. Therefore, the knowledge of Wnt signaling mechanisms learned from axon development can be applied to axon repair in adulthood.

Expression of the Wnt signaling system in central nervous system axon guidance and regeneration

Wnt signaling is essential for axon wiring throughout the development of the nervous system in vertebrates and invertebrates. In rodents, Wnts are expressed in gradients that span the entire anterior-posterior (A-P) axis in the spinal cord and the medial-lateral axis in the superior colliculus. In the brainstem, Wnts are expressed in more complex gradients along the A-P axis. These gradients provide directional information for axon pathfinding and positional information for topographic mapping and are detected by cell polarity signaling pathways in the growth cone. The gradient expression of Wnts and the coordinated expression of Wnt signaling systems are regulated by mechanisms which are currently unknown. Injury to the adult spinal cord results in the re-induction of Wnts in multiple cell types around the lesion site and their signaling system in injured axons. The re-induced Wnts form gradients around the lesion site, with the lesion site being the peak. The re-inducedWnts may be responsible for the well-known retraction of descending motor axons through the receptor Ryk (related receptor tyrosine kinases). Wnt signaling is an appealing new therapeutic target for CNS repair. The mechanisms regulating the re-induction are unknown but will be informative for therapeutic design.

Ryk controls remapping of motor cortex during functional recovery after spinal cord injury.

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.