Partial functional recovery of paraplegic rat by adenovirus-mediated gene delivery of constitutively active MEK1

The potential of gene therapies for spinal cord injury repair: a systematic review and meta-analysis of pre-clinical studies

Currently, there is no cure for traumatic spinal cord injury but one therapeutic approach showing promise is gene therapy. In this systematic review and meta-analysis, we aim to assess the efficacy of gene therapies in pre-clinical models of spinal cord injury and the risk of bias. In this meta-analysis, registered at PROSPERO (Registration ID: CRD42020185008), we identified relevant controlled in vivo studies published in English by searching the PubMed, Web of Science, and Embase databases. No restrictions of the year of publication were applied and the last literature search was conducted on August 3, 2020. We then conducted a random-effects meta-analysis using the restricted maximum likelihood estimator. A total of 71 studies met our inclusion criteria and were included in the systematic review. Our results showed that overall, gene therapies were associated with improvements in locomotor score (standardized mean difference [SMD]: 2.07, 95% confidence interval [CI]:1.68–2.47, Tau² = 2.13, I² = 83.6%) and axonal regrowth (SMD: 2.78, 95%CI: 1.92–3.65, Tau² = 4.13, I² = 85.5%). There was significant asymmetry in the funnel plots of both outcome measures indicating the presence of publication bias. We used a modified CAMARADES (Collaborative Approach to Meta-Analysis and Review of Animal Data in Experimental Studies) checklist to assess the risk of bias, finding that the median score was 4 (IQR:3–5). In particular, reports of allocation concealment and sample size calculations were lacking. In conclusion, gene therapies are showing promise as therapies for spinal cord injury repair, but there is no consensus on which gene or genes should be targeted.

Recombinant Adenoviruses for Delivery of Therapeutics Following Spinal Cord Injury

The regeneration of nerve tissue after spinal cord injury is a complex and poorly understood process. Medication and surgery are not very effective treatments for patients with spinal cord injuries. Gene therapy is a popular approach for the treatment of such patients. The delivery of therapeutic genes is carried out in a variety of ways, such as direct injection of therapeutic vectors at the site of injury, retrograde delivery of vectors, and ex vivo therapy using various cells. Recombinant adenoviruses are often used as vectors for gene transfer. This review discusses the advantages, limitations and prospects of adenovectors in spinal cord injury therapy.

5-Nonyloxytryptamine oxalate-embedded collagen-laminin scaffolds augment functional recovery after spinal cord injury in mice

Polysialic acid (PSA) is crucial for the induction and maintenance of nervous system plasticity and repair after injury. In order to exploit the immense therapeutic potential of PSA, previous studies have focused on the identification and development of peptide-based or synthetic PSA mimetics. 5-Nonyloxytryptamine (5-NOT) has been previously reported as a PSA-mimicking compound for promoting functional recovery after spinal cord injury in mice. In order to explore the neuroregeneration potential of 5-NOT, the current study was based on a biomaterial approach using collagen-laminin (C/L) scaffolds. In in vitro studies, 5-NOT was observed to promote neurite outgrowth, migration, and fasciculation in cerebellar neuronal cells, whereas in 3D cell cultures it showed more ramification and complex Sholl profiles. 5-NOT promoted the survival and neurite length of cortical neurons when cocultured with glutamate-challenged astrocytes. In in vivo studies, spinal cord compression injury mice were used with immediate application of C/L hydrogels impregnated with 5-NOT. C/L + 5-NOT-treated mice demonstrated ∼75% of motor recovery 14 days after injury. Furthermore, this effect was shown to be dependent on the ERK-MAPK pathway and augmentation of cell survival. Thus, based on a biomaterial approach, our current study provides new insight for 5-NOT-containing hydrogels as a promising candidate to speed up recovery after central nervous system injuries.

Quercetin suppresses the chymotrypsin-like activity of proteasome via inhibition of MEK1/ERK1/2 signaling pathway in hepatocellular carcinoma HepG2 cells

The proteasomal system is a promising target for cancer treatment. Quercetin (Que), a flavonoid compound with antitumor ability, displays the inhibitory effect on proteasome activity. However, the underlying molecular mechanisms are ill defined. The present study found that Que treatment significantly reduced the chymotrypsin-like protease activity of proteasome whereas the trypsin- and caspase-like protease activities remained unchanged in HepG2 cancer cells, along with activation of p38 MAPK and JNK and reduction of ERK1/2 phosphorylation. Que-reduced proteasome activity could not be reverted by inhibition of p38 MAPK and JNK signaling pathway. In addition, MEK1 overexpression or knockdown upregulated or downregulated the chymotrypsin-like protease activity of proteasome, respectively. Both Que and MEK1/ERK1/2 inhibitor attenuated the expression levels of proteasome β subunits. These results indicate that Que-induced suppression of MEK1/ERK1/2 signaling and subsequent reduction of proteasome β subunits is responsible for its inhibitory impacts on proteasome activity.

CNS axon regeneration inhibitors stimulate an immediate early gene response via MAP kinase-SRF signaling

Background

CNS axon regeneration inhibitors such as Nogo and CSPGs (Chondroitin Sulfate Proteoglycans) are major extrinsic factors limiting outgrowth of severed nerve fibers. However, knowledge on intracellular signaling cascades and gene expression programs activated by these inhibitors in neurons is sparse. Herein we studied intracellular signaling cascades activated by total myelin, Nogo and CSPGs in primary mouse CNS neurons.

Results

Total myelin, Nogo and CSPGs stimulated gene expression activity of the serum response factor (SRF), a central gene regulator of immediate early (IEG) and actin cytoskeletal gene transcription. As demonstrated by pharmacological interference, SRF-mediated IEG activation by myelin, Nogo or CSPGs depended on MAP kinase, to a lesser extent on Rho-GTPase but not on PKA signaling. Stimulation of neurons with all three axon growth inhibitors activated the MAP kinase ERK. In addition to ERK activation, myelin activated the IEG c-Fos, an important checkpoint of neuronal survival vs. apoptosis. Employing Srf deficient neurons revealed that myelin-induced IEG activation requires SRF. This suggests an SRF function in mediating neuronal signaling evoked by axon regeneration associated inhibitors. Besides being a signaling target of axon growth inhibitors, we show that constitutively-active SRF-VP16 can be employed to circumvent neurite growth inhibition imposed by myelin, Nogo and CSPGs.

Conclusion

In sum, our data demonstrate that axon regeneration inhibitors such as Nogo trigger gene expression programs including an SRF-dependent IEG response via MAP kinases and Rho-GTPases.

Differential regulation of proliferation and neuronal differentiation in adult rat spinal cord neural stem/progenitors by ERK1/2, Akt, and PLCγ

Proliferation of endogenous neural stem/progenitor cells (NSPCs) has been identified in both normal and injured adult mammalian spinal cord. Yet the signaling mechanisms underlying the regulation of adult spinal cord NSPCs proliferation and commitment toward a neuronal lineage remain undefined. In this study, the role of three growth factor-mediated signaling pathways in proliferation and neuronal differentiation was examined. Adult spinal cord NSPCs were enriched in the presence of fibroblast growth factor 2 (FGF2). We observed an increase in the number of cells expressing the microtubule-associated protein 2 (MAP2) over time, indicating neuronal differentiation in the culture. Inhibition of the mitogen-activated protein kinase or extracellular signal-regulated kinase (ERK) kinase 1 and 2/ERK 1 and 2 (MEK/ERK1/2) or the phosphoinositide 3-kinase (PI3K)/Akt pathways suppressed active proliferation in adult spinal cord NSPC cultures; whereas neuronal differentiation was negatively affected only when the ERK1/2 pathway was inhibited. Inhibition of the phospholipase Cγ (PLCγ) pathway did not affect proliferation or neuronal differentiation. Finally, we demonstrated that the blockade of either the ERK1/2 or PLCγ signaling pathways reduced neurite branching of MAP2+ cells derived from the NSPC cultures. Many of the MAP2+ cells expressed synaptophysin and had a glutamatergic phenotype, indicating that over time adult spinal cord NSPCs had differentiated into mostly glutamatergic neurons. Our work provides new information regarding the contribution of these pathways to the proliferation and neuronal differentiation of NSPCs derived from adult spinal cord cultures, and emphasizes that the contribution of these pathways is dependent on the origin of the NSPCs.

Preferential and bidirectional labeling of the rubrospinal tract with adenovirus-GFP for monitoring normal and injured axons

The rodent rubrospinal tract (RST) has been studied extensively to investigate regeneration and remodeling of central nervous system (CNS) axons. Currently no retrograde tracers can specifically label rubrospinal axons and neurons (RSNs). The RST can be anterogradely labeled by injecting tracers into the red nucleus (RN), but accurately locating the RN is a technical challenge. Here we developed a recombinant adenovirus carrying a green fluorescent protein reporter gene (Adv-GFP) which can preferentially, intensely, and bi-directionally label the RST. When Adv-GFP was injected into the second lumbar spinal cord, the GFP was specifically transported throughout the entire RST, with peak labeling seen at 2 weeks post-injection. When Adv-GFP was injected directly into the RN, GFP was anterogradely transported throughout the RST. Following spinal cord injury (SCI), injection of Adv-GFP resulted in visualization of GFP in transected, spared, or sprouted RST axons bi-directionally. Thus Adv-GFP could be used as a novel tool for monitoring and evaluating strategies designed to maximize RST axonal regeneration and remodeling following SCI.

Phosphorylation by extracellular signal-regulated kinase of a multidomain adaptor protein, vinexin, at synapses

Vinexin is an adaptor protein that is supposed to play pivotal roles in cell adhesion, cytoskeletal organization and signaling. At least three splice variants, vinexinalpha, beta and gamma, have so far been reported. In spite of the possible importance of vinexin, the properties and functions of vinexin in neuronal cells are almost unknown. Here we show that vinexin isoforms are expressed in rat brain in a developmental stage-dependent manner, and that vinexinalpha is relatively abundant in the telencephalon regions of the adult rat brain. An immunohistochemical study showed the localization of vinexinalpha in neurons and glia in the rat brain. In primary cultured rat hippocampal neurons, vinexin was found to be present at synapses and filopodia in growth cones by immunofluorescent analyses. Biochemical fractionation revealed the distribution of vinexin in synaptosomes. Nerve terminal localization of vinexin was confirmed by electron microscopy. Vinexinbeta is reported to be phosphorylated by extracellular signal-regulated kinase (ERK) at Ser189, which is equivalent to Ser593 of vinexinalpha. We thus constructed a site- and phosphorylation state-specific antibody to monitor the ERK-mediated phosphorylation of vinexin. In immunofluorescent analyses, the phosphorylation was observed at synapses formed among cultured rat hippocampal neurons and it was reduced by treatment of the cells with PD98059. In an immunoelectron microscopic examination, the phosphorylation signal was mainly detected on the postsynaptic side of synapses in the rat hippocampal neurons. As active ERK was co-localized with vinexin in synapses, the ERK signal is likely to be involved in the regulation of vinexin-dependent cellular processes in synapses. On the other hand, the phosphorylation was hardly detected in neurons cultured for 3 days, suggesting the presence of a yet unidentified regulatory mechanism of vinexin at the growth cone.

Direct Gene Therapy for Repair of the Spinal Cord

For regrowth of injured nerve fibers following spinal cord injury (SCI), the environment must be favorable for axonal growth. The delivery of a therapeutic gene, beneficial for axonal growth, into the central nervous system for repair can be accomplished in many ways. Perhaps the most simple and elegant strategy is the so-called direct gene therapy approach that uses a single injection for delivery of a gene therapy vehicle. Among the vectors that have been used to transduce neural tissue in vivo are non-viral, herpes simplex viral, adeno-associated viral, adenoviral, and lentiviral vectors, each with their own merits and limitations. Many studies have been undertaken using direct gene therapy, ranging from strategies for neuroprotection to axonal growth promotion at the injury site, dorsal root injury repair, and initiation of a growth-supporting genetic program. The limitations and successes of direct gene transfer for spinal cord repair are discussed in this review.

Actions of neurotrophic factors and their signaling pathways in neuronal survival and axonal regeneration

Adult axons in the mammalian central nervous system do not elicit spontaneous regeneration after injury, although many affected neurons have survived the neurotrauma. However, axonal regeneration does occur under certain conditions. These conditions include: (a) modification of regrowth environment, such as supply of peripheral nerve bridges and transplantation of Schwann cells or olfactory ensheathing glia to the injury site; (b) application of neurotrophic factors at the cell soma and axon tips; (c) blockade of growth-inhibitory molecules such as Nogo-A, myelin-associated glycoprotein, and oligodendrocyte-myelin glycoprotein; (d) prevention of chondroitin-sulfate-proteoglycans-related scar tissue formation at the injury site using chondroitinase ABC; and (e) elevation of intrinsic growth potential of injured neurons via increasing intracellular cyclic adenosine monophosphate level. A large body of evidence suggests that these conditions achieve enhanced neuronal survival and axonal regeneration through sometimes overlapping and sometimes distinct signal transduction mechanisms, depending on the targeted neuronal populations and intervention circumstances. This article reviews the available information on signal transduction pathways underlying neurotrophic-factor-mediated neuronal survival and neurite outgrowth/axonal regeneration. Better understanding of signaling transduction is important in helping us develop practical therapeutic approaches for encouraging neuronal survival and axonal regeneration after traumatic injury in clinical context.

Protein kinase signaling cascades in CNS trauma

Advances in our understanding of the signaling pathways and cellular functions regulated by protein kinase cascades have paved the way to study their role in the response of brain and spinal cord to traumatic injury. Mechanical forces imparted by trauma stimulate mitogen-activated protein kinases and protein kinase B/Akt as well as cause changes in the state of phosphorylation of glycogen synthase kinase-3beta. Extracellular ATP released by mechanical strain stimulates P2 purinergic receptors that are coupled to these protein kinase signaling pathways. These kinases regulate gene expression, cell survival, proliferation, differentiation, growth arrest, and apoptosis, thereby affecting cell fate, repair and plasticity after trauma. Elucidation of the molecular responses of protein kinase cascades to mechanical strain and the genes regulated by these signaling pathways may lead to therapeutic opportunities to minimize losses in motor skills and cognitive function caused by trauma to the central nervous system.

MEKK1 controls neurite regrowth after experimental injury by balancing ERK1/2 and JNK2 signaling

After injury, peripheral neuronal cells initiate complex signaling cascades to promote survival and regeneration. In the present study, we have identified the mitogen-activated protein kinase (MAPK) isoforms which are necessary for nerve growth factor (NGF)-induced neurite regrowth after injury of differentiated PC12 cells. Extracellular signal-regulated kinases 1 and 2 (ERK1/2) and the usually pro-apoptotic c-Jun N-terminal kinase 2 (JNK2) are crucial for neurite regrowth, while p38 plays no role in this context. Surprisingly, the MEK1 inhibitors PD 98059 and U 0126 blocked both ERK1/2 and JNK phosphorylation, indicating a novel form of balancing MAPK cascade cross-talk. Results from RNAi experiments excluded direct ERK/JNK interactions. We identified the upstream kinase MEKK1 as an activator of both the ERK1/2 and JNK2 pathways, whereby the ERK1/2 kinase MEK1 and the JNK kinase MKK7 bind to MEKK1 in a competing fashion. Our findings suggest an important role of JNK2 and MAPK pathway cross-talk in neurite regeneration.

Extracellular signal-regulated kinase 1/2 mediates survival, but not axon regeneration, of adult injured central nervous system neurons in vivo

Neurotrophins play important roles in the response of adult neurons to injury. The intracellular signaling mechanisms used by neurotrophins to regulate survival and axon growth in the mature CNS in vivo are not well understood. The goal of this study was to define the role of the extracellular signal-regulated kinases 1/2 (Erk1/2) pathway in the survival and axon regeneration of adult rat retinal ganglion cells (RGCs), a prototypical central neuron population. We used recombinant adeno-associated virus (AAV) to selectively transduce RGCs with genes encoding constitutively active or wild-type mitogen-activated protein kinase kinase 1 (MEK1), the upstream activator of Erk1/2. In combination with anterograde and retrograde tracing techniques, we monitored neuronal survival and axon regeneration in vivo. MEK1 gene delivery led to robust and selective transgene expression in multiple RGC compartments including cell bodies, dendrites, axons and targets in the brain. Furthermore, MEK1 activation induced in vivo phosphorylation of Erk1/2 in RGC bodies and axons. Quantitative analysis of cell survival demonstrated that Erk1/2 activation promoted robust RGC neuroprotection after optic nerve injury. In contrast, stimulation of the Erk1/2 pathway was not sufficient to induce RGC axon growth beyond the lesion site. We conclude that the Erk1/2 pathway plays a key role in the survival of axotomized mammalian RGCs in vivo, and that activation of other signaling components is required for axon regeneration in the growth inhibitory CNS environment.

Rit contributes to nerve growth factor-induced neuronal differentiation via activation of B-Raf-extracellular signal-regulated kinase and p38 mitogen-activated protein kinase cascades

Rit is one of the original members of a novel Ras GTPase subfamily that uses distinct effector pathways to transform NIH 3T3 cells and induce pheochromocytoma cell (PC6) differentiation. In this study, we find that stimulation of PC6 cells by growth factors, including nerve growth factor (NGF), results in rapid and prolonged Rit activation. Ectopic expression of active Rit promotes PC6 neurite outgrowth that is morphologically distinct from that promoted by oncogenic Ras (evidenced by increased neurite branching) and stimulates activation of both the extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinase signaling pathways. Furthermore, Rit-induced differentiation is dependent upon both MAP kinase cascades, since MEK inhibition blocked Rit-induced neurite outgrowth, while p38 blockade inhibited neurite elongation and branching but not neurite initiation. Surprisingly, while Rit was unable to stimulate ERK activity in NIH 3T3 cells, it potently activated ERK in PC6 cells. This cell type specificity is explained by the finding that Rit was unable to activate C-Raf, while it bound and stimulated the neuronal Raf isoform, B-Raf. Importantly, selective down-regulation of Rit gene expression in PC6 cells significantly altered NGF-dependent MAP kinase cascade responses, inhibiting both p38 and ERK kinase activation. Moreover, the ability of NGF to promote neuronal differentiation was attenuated by Rit knockdown. Thus, Rit is implicated in a novel pathway of neuronal development and regeneration by coupling specific trophic factor signals to sustained activation of the B-Raf/ERK and p38 MAP kinase cascades.

Opposing extracellular signal-regulated kinase and Akt pathways control Schwann cell myelination

Schwann cells are the myelinating glia of the peripheral nervous system, and their development is regulated by various growth factors, such as neuregulin, platelet-derived growth factor (PDGF), and insulin-like growth factor-I (IGF-I). However, the mechanism of intracellular signaling pathways following these ligand stimuli in Schwann cell differentiation remains elusive. Here, we demonstrate that in cultured Schwann cells, neuregulin and PDGF suppressed the expression of myelin-associated protein markers, whereas IGF-I promoted it. Although these ligands activated common downstream signaling pathways [i.e., extracellular signal-regulated kinase (Erk) and phosphatidylinositol-3-kinase (PI3K)-Akt pathways], the profiles of activation varied among ligands. To elucidate the function of these pathways and the mechanisms underlying Schwann cell differentiation, we used adenoviral vectors to selectively activate or inactivate these pathways. We found that the selective activation of Erk pathways suppressed Schwann cell differentiation, whereas that of PI3K pathways promoted it. Furthermore, lithium chloride, a modulator of glycogen synthase kinase-3beta (GSK-3beta) promoted Schwann cell differentiation, suggesting the involvement of GSK-3beta as a downstream molecule of PI3K-Akt pathways. Selective activation of PI3K pathways in Schwann cells by gene transfer also demonstrated increased myelination in in vitro Schwann cell-DRG neuron cocultures and in vivo allogenic nerve graft experiments. We conclude that signals mediated by PI3K-Akt are crucial for initiation of myelination and that the effects of growth factors are primarily dependent on the balance between Erk and PI3K-Akt activation. Our results also propose the possibility of augmenting Schwann cell functions by modulating intracellular signals in light of future cell therapies.

Changes in the axonal membrane potential and Ca2+ concentration associated with peripheral nerve grafting after spinal cord injury

We performed peripheral nerve allografting in rats with spinal cord injury, and measured motor function and axonal membrane potential as well as Ca(2+) concentration of the nerve grafting spinal cord area by using a behavior observation system and a confocal laser-scanning microscope, respectively. In our experiments, we produced a model of peripheral nerve grafting after spinal cord injury by peripheral nerve allografting (sciatic nerve) in rats with spinal cord injury (thoracic cord hemisection). The group with spinal cord injury that underwent peripheral nerve grafting showed improvement in motor function, a significant increase in the axonal action potential, and a slight increase in the Ca(2+) concentration compared with the group that did not undergo nerve grafting.

Extracellular signal-regulated kinases regulate dendritic growth in rat sympathetic neurons

NGF activates several signaling cascades in sympathetic neurons. We examined how activation of one of these cascades, the ERK/MAP (extracellular signal-regulated kinase/mitogen-activated protein) kinase pathway, affects dendritic growth in these cells. Dendritic growth was induced by exposure to NGF and BMP-7 (bone morphogenetic protein-7). Exposure to NGF increased phosphorylation of ERK1/2. Unexpectedly, two MEK (MAP kinase kinase) inhibitors (PD 98059 and U 0126) enhanced dendritic growth, and a ligand, basic FGF, that activates the ERK pathway inhibited the growth of these processes. The enhancement of dendritic growth by PD 98059 was associated with an increase in the number of axo-dendritic synapses, and it appeared to represent a specific morphogenic effect because neither axonal growth nor cell survival was affected. In addition, increased dendritic growth was not observed after exposure to inhibitors of other signaling pathways, including the phosphatidylinositol-3-kinase inhibitor LY 294002. Dendritic growth was also increased in cells transfected with dominant-negative mutants of MEK1 and ERK2 but not with dominant-negative mutants of MEK5 and ERK5, suggesting that ERK1/2 is the primary mediator of this effect. Exposure to BMP-7 induces nuclear translocation of Smad1 (Sma- and Mad-related protein 1), and PD 98059 treatment potentiated nuclear accumulation of Smad-1 induced by BMP-7 in sympathetic neurons, suggesting a direct enhancement of BMP signaling in cells treated with an MEK inhibitor. These observations indicate that one of the signaling cascades activated by NGF can act in an antagonistic manner in sympathetic neurons and reduce the dendritic growth induced by other NGF-sensitive pathways.

Adenovirus Vector-Mediated In Vivo Gene Transfer of Brain-Derived Neurotrophic Factor (BDNF) Promotes Rubrospinal Axonal Regeneration and Functional Recovery after Complete Transection of the Adult Rat Spinal Cord

Neurotrophins have been shown to promote axonal regeneration, but the techniques available for delivering neurotrophins have limited effectiveness. The aim of this study was to evaluate the effect of adenovirus vector mediated gene transfer of brain-derived neurotrophic factor (BDNF) on axonal regeneration after spinal cord injury. We prepared adenovirus vectors encoding either beta-galactosidase (AxCALacZ) or BDNF (AxCABDNF). AxCALacZ was used to assess infection levels of the adenovirus BDNF produced by AxCABDNF was detected by Western blotting and its bioactivity was confirmed by bioassay. As a model of spinal cord injury, the rat spinal cord was completely transected at the T8 level. Immediately after transection, the vectors were injected into both stumps of the spinal cord. Axonal regeneration after transection was assessed by retrograde and anterograde tracing. In AxCALacZ-injected rats, adenovirus-infected cells were observed not only at the injected site but also in brainstem nuclei, as shown by LacZ expression. After the injection of the retrograde tracer fluorogold (FG) distal portion to the transection, AxCABDNF-injected rats showed FG-labeled neurons in the red nucleus. The anterograde tracer biotinylated dextran amine (BDA) injected into the red nucleus was also found in regenerating rubrospinal fibers distal to the transection. These tracing experiments demonstrated the regeneration of descending axons. In addition, rats of the AxCABDNF group showed significant locomotor recovery of hindlimb function, which was completely abolished by re-transection. These results indicate that the recovery was caused by regeneration of rubrospinal axons, not by simple enhancement of the central pattern generator.

Transient neuronal but persistent astroglial activation of ERK/MAP kinase after focal brain injury in mice

Astrogliosis is a nearly ubiquitous response to a variety of insults to the central nervous system (CNS). This reaction is triggered rapidly, but can persist for years after the initial trauma. Little is known about the signaling mechanisms responsible for this activation and its chronic maintenance. Extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) activation is implicated in several functions important to the reactive glial phenotype such as cellular proliferation and motility. Here we utilize immunohistochemistry with a phosphorylation state-specific antibody (pERK) to characterize the temporal and spatial pattern of ERK/MAPK activation in neurons and glia following a forebrain stab lesion (FSL) in mice. Early activation (1 h) was primarily in perilesional neuronal elements, particularly of the hippocampus. Occasional perilesional glia were also positive for pERK. Additionally, ependymal cells bilaterally stained prominently for pERK. These patterns of pERK immunoreactivity at 1 h were abolished by pretreatment with the selective MEK inhibitor, SL327. ERK/MAPK activation at later time points between 1 day (d) and 30 d was primarily restricted to perilesional astrocytes with maximum labeling at 3 d. However, pERK-positive astrocytes represented only a subset of total GFAP-positive cells and were found more proximal to the lesion suggesting specific functional activation of these cells. Finally, immunostaining for the phosphorylated form of cAMP response element-binding (CREB) protein, a downstream target of the ERK/MAPK cascade, was increased in perilesional glia 7 d after FSL. Sustained activation of the ERK/MAPK signaling pathway in perilesional reactive glia suggests a critical role for this cascade in astrogliosis.

Recovery of neurologic function after spinal cord injury in Israel

STUDY DESIGN

A retrospective cohort study was conducted.

OBJECTIVE

To assess neurologic recovery and the manner in which it is affected by the severity of the neurologic damage after spinal cord injury.

SUMMARY OF BACKGROUND DATA

Studies from various countries, but not from Israel, have shown considerable potential for recovery of the damaged human spinal cord.

METHODS

The study sample included 250 patients with a traumatic spinal cord lesion treated between 1962 and 1992 at the major referral hospital for rehabilitation in Israel. Demographic and clinical data were collected from the hospital charts. The degree of neurologic recovery in each patient was determined by comparing the Frankel grade of neurologic deficit at first admission for rehabilitation with the grade at discharge from that hospitalization.

RESULTS

There was median delay of 36 days between injury and admission for rehabilitation. During rehabilitation, full or substantial neurologic recovery (upgrade to Frankel Grade D or E) occurred in 27% of all the patients who were Grade A, B, or C on admission, and in 54% of those who were Grade C. The neurologic recovery was negatively associated with severity of the neurologic deficit.

CONCLUSIONS

The outcome findings are similar to those reported from spinal rehabilitation units in other countries. The study is a further demonstration of the considerable potential for neurologic recovery after spinal cord injury, when posttraumatic or postsurgical management is focused on prevention of complications and maximal use of functional ability.

Gene therapy for spinal applications

Gene therapy is a promising drug delivery mechanism for the treatment of spinal disorders. Currently, the technique has been most useful in enhancing growth factor therapy for spinal fusion, intervertebral disc regeneration, and spinal cord injury healing. Gene therapy allows for the high-level local production of growth factors, obviating the need for slow release carriers or continuous infusion pumps that are otherwise necessary because of the short half-lives of most peptide growth factors. Although continuous expression is desirable, growth factor therapy is usually intended to be transient. The typical expression profile of Ad vectors--at a high level over 2 weeks or so--has been ideal, leading to its widespread use in these applications. Despite the ability of Ad to deliver genes directly in vivo, however, the cell-based ex vivo approach has been used widely in spinal applications. In spinal cord injury, cells such as peripheral nerve or Schwann cells may provide a permissive substrate for axonal growth [51]. For spinal fusion and IVD regeneration, ex vivo manipulation of cells facilitates gene transfer, because bone and IVD tissue are too dense to be penetrated by injection of Ad or other vectors. The use of cells may be advantageous in these applications in which new tissue formation is the goal. Finally, the use of genetically modified cells may decrease the inflammatory reaction induced by Ad vectors. Although gene therapy for spinal disorders has been centered around Ad-mediated transfer of single growth factor genes, the options for candidate genes and vectors are growing rapidly. Ad vectors are being improved by decreasing their immunogenicity and altering their tropism [2]. Vectors based on other viruses (such as herpes, adeno-associated virus, and lentivirus) are being developed, also with lower immunogenicity and with longer durations of expression [26,67]. Regulated expression, such as with the tetracycline regulated promoter, is being developed so that genes can be turned on or off as needed. Such regulation may be sensitive even to physiologic cues in the future [68,69]. Finally, the high throughput technologies, such as the gene chip, are elucidating thousands of genes that may be good candidates for the enhancement of bone healing and IVD and spinal cord regeneration. Genes whose products not only support bone, fibrocartilage, or axon growth but also neutralize natural inhibitors or promote tissue remodeling and maturation may be good future candidates. In the future, a series of vectors with multiple genes that are regulated by physiologic cues might be used to enhance spinal fusion, restore IVD tissue, or support spinal cord healing.

Transcription factor expression and Notch-dependent regulation of neural progenitors in the adult rat spinal cord

Recent studies have demonstrated that neural stem cells and other progenitors are present in the adult CNS. Details of their properties, however, remain poorly understood. Here we examined the properties and control mechanisms of neural progenitors in the adult rat spinal cord at the molecular level. Adult and embryonic progenitors commonly expressed various homeodomain-type (Pax6, Pax7, Nkx2.2, and Prox1) and basic helix-loop-helix (bHLH)-type (Ngn2, Mash1, NeuroD1, and Olig2) transcriptional regulatory factors in vitro. Unlike their embryonic counterparts, however, adult progenitors could not generate specific neurons that expressed markers appropriate for spinal motoneurons or interneurons, including Islet1, Lim1, Lim3, and HB9. Cells expressing the homeodomain factors Pax6, Pax7, and Nkx2.2 also emerged in vivo in response to injury and were distributed in unique patterns in the lesioned spinal cord. However, neither the expression of the neurogenic bHLH factors including Ngn2, Mash1, and NeuroD1 nor subsequent generation of new neurons could be detected in injured tissue. Our results suggest that signaling through the cell-surface receptor Notch is involved in this restriction. The expression of Notch1 in vivo was enhanced in response to injury. Furthermore, activation of Notch signaling in vitro inhibited differentiation of adult progenitors, whereas attenuation of Notch signals and forced expression of Ngn2 significantly enhanced neurogenesis. These results suggest that both the intrinsic properties of adult progenitors and local environmental signals, including Notch signaling, account for the limited regenerative potential of the adult spinal cord.

Gene therapy against neurological insults: sparing neurons versus sparing function

Increasing knowledge of neuron death mediators has led to gene therapy techniques for neuroprotection. Overexpression of numerous genes enhances survival after necrotic or neurodegenerative damage. Nonetheless, although encouraging, little is accomplished if a neuron is spared from death, but not from dysfunction. This article reviews neuroprotection experiments that include some measure of function, and synthesizes basic principles relating to its maintenance. Variations in gene delivery systems, including virus-type and latency between damage onset and vector delivery, probably impact the therapeutic outcome. Additionally, functional sparing might depend on factors related to insult severity, neuron type involved or the step in the death cascade that is targeted.

Proliferation of parenchymal neural progenitors in response to injury in the adult rat spinal cord

It has long been believed that the fully developed mammalian central nervous system (CNS) lacks significant regenerative capacity. Recent advances have revealed, however, that many regions of the adult CNS contain neural progenitors that have the ability to generate new neurons and glia. Although the periventricular area has been identified as a rich source of these progenitors, their precise location in each region and details of their properties in vivo still remain poorly understood. Here we provide evidence that in the adult rat spinal cord, a significant number of neural progenitors are present, not only in the periventricular area, but also in other regions of the parenchyma. These progenitors could proliferate in vitro as neurosphere-like cell aggregates in the presence of growth factors and also gave rise to neurons and glia under appropriate conditions. We further demonstrate that these parenchymal neural progenitors were capable of proliferating in vivo in response to injury. Immunohistochemical studies suggested that proliferative progenitors emerged throughout the gray and white matter in the lesioned spinal cord. Consistently, an increased number of neurosphere-forming cells could be isolated from injured tissues, and they were able to differentiate into neurons in vitro. The widespread occurrence of neural progenitors in the parenchyma expands the possibility of repairing damaged tissue by activating the latent regenerative potential of the adult spinal cord.