Targeting a dominant negative rho kinase to neurons promotes axonal outgrowth and partial functional recovery after rat rubrospinal tract lesion

Macrophage-specific RhoA knockout delays Wallerian degeneration after peripheral nerve injury in mice

Background

Plenty of macrophages are recruited to the injured nerve to play key roles in the immunoreaction and engulf the debris of degenerated axons and myelin during Wallerian degeneration, thus creating a conducive microenvironment for nerve regeneration. Recently, drugs targeting the RhoA pathway have been widely used to promote peripheral axonal regeneration. However, the role of RhoA in macrophage during Wallerian degeneration and nerve regeneration after peripheral nerve injury is still unknown. Herein, we come up with the hypothesis that RhoA might influence Wallerian degeneration and nerve regeneration by affecting the migration and phagocytosis of macrophages after peripheral nerve injury.

Methods

Immunohistochemistry, Western blotting, H&E staining, and electrophysiology were performed to access the Wallerian degeneration and axonal regeneration after sciatic nerve transection and crush injury in the Lyz^Cre+/−; RhoA^flox/flox (cKO) mice or Lyz2^Cre+/− (Cre) mice, regardless of sex. Macrophages’ migration and phagocytosis were detected in the injured nerves and the cultured macrophages. Moreover, the expression and potential roles of ROCK and MLCK were also evaluated in the cultured macrophages.

Results

1. RhoA was specifically knocked out in macrophages of the cKO mice; 2. The segmentation of axons and myelin, the axonal regeneration, and nerve conduction in the injured nerve were significantly impeded while the myoatrophy was more severe in the cKO mice compared with those in Cre mice; 3. RhoA knockout attenuated the migration and phagocytosis of macrophages in vivo and in vitro; 4. ROCK and MLCK were downregulated in the cKO macrophages while inhibition of ROCK and MLCK could weaken the migration and phagocytosis of macrophages.

Conclusions

Our findings suggest that RhoA depletion in macrophages exerts a detrimental effect on Wallerian degeneration and nerve regeneration, which is most likely due to the impaired migration and phagocytosis of macrophages resulted from disrupted RhoA/ROCK/MLCK pathway. Since previous research has proved RhoA inhibition in neurons was favoring for axonal regeneration, the present study reminds us of that the cellular specificity of RhoA-targeted drugs is needed to be considered in the future application for treating peripheral nerve injury.

Secretion of a mammalian chondroitinase ABC aids glial integration at PNS/CNS boundaries

Schwann cell grafts support axonal growth following spinal cord injury, but a boundary forms between the implanted cells and host astrocytes. Axons are reluctant to exit the graft tissue in large part due to the surrounding inhibitory environment containing chondroitin sulphate proteoglycans (CSPGs). We use a lentiviral chondroitinase ABC, capable of being secreted from mammalian cells (mChABC), to examine the repercussions of CSPG digestion upon Schwann cell behaviour in vitro. We show that mChABC transduced Schwann cells robustly secrete substantial quantities of the enzyme causing large-scale CSPG digestion, facilitating the migration and adhesion of Schwann cells on inhibitory aggrecan and astrocytic substrates. Importantly, we show that secretion of the engineered enzyme can aid the intermingling of cells at the Schwann cell-astrocyte boundary, enabling growth of neurites over the putative graft/host interface. These data were echoed in vivo. This study demonstrates the profound effect of the enzyme on cellular motility, growth and migration. This provides a cellular mechanism for mChABC induced functional and behavioural recovery shown in in vivo studies. Importantly, we provide in vitro evidence that mChABC gene therapy is equally or more effective at producing these effects as a one-time application of commercially available ChABC.

The Rise of Molecules Able To Regenerate the Central Nervous System

Injury to the adult central nervous system (CNS) usually leads to permanent deficits of cognitive, sensory, and/or motor functions. The failure of axonal regeneration in the damaged CNS limits functional recovery. The lack of information concerning the biological mechanism of axonal regeneration and its complexity has delayed the process of drug discovery for many years compared to other drug classes. Starting in the early 2000s, the ability of many molecules to stimulate axonal regrowth was evaluated through automated screening techniques; many hits and some new mechanisms involved in axonal regeneration were identified. In this Perspective, we discuss the rise of the CNS regenerative drugs, the main biological techniques used to test these drug candidates, some of the most important screens performed so far, and the main challenges following the identification of a drug that is able to induce axonal regeneration in vivo.

Chemotherapy-Induced Neuropathy and Drug Discovery Platform Using Human Sensory Neurons Converted Directly from Adult Peripheral Blood

Chemotherapy‐induced peripheral neuropathy (PN) is a disorder damaging the peripheral nervous system (PNS) and represents one of the most common side effects of chemotherapy, negatively impacting the quality of life of patients to the extent of withdrawing life‐saving chemotherapy dose or duration. Unfortunately, the pathophysiological effects of PN are poorly understood, in part due to the lack of availability of large numbers of human sensory neurons (SNs) for study. Previous reports have demonstrated that human SNs can be directly converted from primitive CD34⁺ hematopoietic cells, but was limited to a small‐scale product of SNs and derived exclusively from less abundant allogenic sources of cord or drug mobilized peripheral blood (PB). To address this shortcoming, we have developed and report detailed procedures toward the generation of human SN directly converted from conventionally drawn PB of adults that can be used in a high‐content screening platform for discovery‐based studies of chemotherapy agents on neuronal biology. In the absence of mobilization drugs, cryogenically preserved adult human PB could be induced to (i)SN via development through expandable neural precursor differentiation. iSNs could be transferable to high‐throughput procedures suitable for high‐content screening applicable to neuropathy for example, alterations in neurite morphology in response to chemotherapeutics. Our study provides the first reported platform using adult PB‐derived iSNs to study peripheral nervous system‐related neuropathies as well as target and drug screening potential for the ability to prevent, block, or repair chemotherapy‐induced PN damage. stem cells translational medicine2019;8:1180–1191

Intraneural Injection of ATP Stimulates Regeneration of Primary Sensory Axons in the Spinal Cord

Injury to the peripheral axons of sensory neurons strongly enhances the regeneration of their central axons in the spinal cord. It remains unclear on what molecules that initiate such conditioning effect. Because ATP is released extracellularly by nerve and other tissue injury, we hypothesize that injection of ATP into a peripheral nerve might mimic the stimulatory effect of nerve injury on the regenerative state of the primary sensory neurons. We found that a single injection of 6 μl of 150 μm ATP into female rat sciatic nerve quadrupled the number of axons growing into a lesion epicenter in spinal cord after a concomitant dorsal column transection. A second boost ATP injection 1 week after the first one markedly reinforced the stimulatory effect of a single injection. Single ATP injection increased expression of phospho-STAT3 and GAP43, two markers of regenerative activity, in sensory neurons. Double ATP injections sustained the activation of phospho-STAT3 and GAP43, which may account for the marked axonal growth across the lesion epicenter. Similar studies performed on P2X7 or P2Y2 receptor knock-out mice indicate P2Y2 receptors are involved in the activation of STAT3 after ATP injection or conditioning lesion, whereas P2X7 receptors are not. Injection of ATP at 150 μm caused little Wallerian degeneration and behavioral tests showed no significant long-term adverse effects on sciatic nerve functions. The results in this study reveal possible mechanisms underlying the stimulation of regenerative programs and suggest a practical strategy for stimulating axonal regeneration following spinal cord injury.

SIGNIFICANCE STATEMENT Injury of peripheral axons of sensory neurons has been known to strongly enhance the regeneration of their central axons in the spinal cord. In this study, we found that injection of ATP into a peripheral nerve can mimic the effect of peripheral nerve injury and significantly increase the number of sensory axons growing across lesion epicenter in the spinal cord. ATP injection increased expression of several markers for regenerative activity in sensory neurons, including phospho-STAT3 and GAP43. ATP injection did not cause significant long-term adverse effects on the functions of the injected nerve. These results may lead to clinically applicable strategies for enhancing neuronal responses that support regeneration of injured axons.

Gene Manipulation Strategies to Identify Molecular Regulators of Axon Regeneration in the Central Nervous System

Limited axon regeneration in the injured adult mammalian central nervous system (CNS) usually results in irreversible functional deficits. Both the presence of extrinsic inhibitory molecules at the injury site and the intrinsically low capacity of adult neurons to grow axons are responsible for the diminished capacity of regeneration in the adult CNS. Conversely, in the embryonic CNS, neurons show a high regenerative capacity, mostly due to the expression of genes that positively control axon growth and downregulation of genes that inhibit axon growth. A better understanding of the role of these key genes controlling pro-regenerative mechanisms is pivotal to develop strategies to promote robust axon regeneration following adult CNS injury. Genetic manipulation techniques have been widely used to investigate the role of specific genes or a combination of different genes in axon regrowth. This review summarizes a myriad of studies that used genetic manipulations to promote axon growth in the injured CNS. We also review the roles of some of these genes during CNS development and suggest possible approaches to identify new candidate genes. Finally, we critically address the main advantages and pitfalls of gene-manipulation techniques, and discuss new strategies to promote robust axon regeneration in the mature CNS.

SCISSOR-Spinal Cord Injury Study on Small molecule-derived Rho inhibition: a clinical study protocol

INTRODUCTION

The approved analgesic and anti-inflammatory drugs ibuprofen and indometacin block the small GTPase RhoA, a key enzyme that impedes axonal sprouting after axonal damage. Inhibition of the Rho pathway in a central nervous system-effective manner requires higher dosages compared with orthodox cyclooxygenase-blocking effects. Preclinical studies on spinal cord injury (SCI) imply improved motor recovery after ibuprofen/indometacin-mediated Rho inhibition. This has been reassessed by a meta-analysis of the underlying experimental evidence, which indicates an overall effect size of 20.2% regarding motor outcome achieved after ibuprofen/indometacin treatment compared with vehicle controls. In addition, ibuprofen/indometacin may also limit sickness behaviour, non-neurogenic systemic inflammatory response syndrome (SIRS), neuropathic pain and heterotopic ossifications after SCI. Consequently, 'small molecule'-mediated Rho inhibition after acute SCI warrants clinical investigation.

METHODS AND ANALYSIS

Protocol of an investigator-initiated clinical open-label pilot trial on high-dose ibuprofen treatment after acute traumatic, motor-complete SCI. A sample of n=12 patients will be enrolled in two cohorts treated with 2400 mg/day ibuprofen for 4 or 12 weeks, respectively. The primary safety end point is an occurrence of serious adverse events, primarily gastroduodenal bleedings. Secondary end points are pharmacokinetics, feasibility and preliminary effects on neurological recovery, neuropathic pain and heterotopic ossifications. The primary safety analysis is based on the incidence of severe gastrointestinal bleedings. Additional analyses will be mainly descriptive and casuistic.

ETHICS AND DISSEMINATION

The clinical trial protocol was approved by the responsible German state Ethics Board, and the Federal Institute for Drugs and Medical Devices. The study complies with the Declaration of Helsinki, the principles of Good Clinical Practice and all further applicable regulations. This safety and pharmacokinetics trial informs the planning of a subsequent randomised controlled trial. Regardless of the result of the primary and secondary outcome assessments, the clinical trial will be reported as a publication in a peer-reviewed journal.

TRIAL REGISTRATION NUMBER

NCT02096913; Pre-results.

Survival of corticostriatal neurons by Rho/Rho-kinase signaling pathway

Developing cortical neurons undergo a number of sequential developmental events including neuronal survival/apoptosis, and the molecular mechanism underlying each characteristic process has been studied in detail. However, the survival pathway of cortical neurons at mature stages remains largely uninvestigated. We herein focused on mature corticostriatal neurons because of their important roles in various higher brain functions and the spectrum of neurological and neuropsychiatric disorders. The small GTPase Rho is known to control diverse and essential cellular functions through some effector molecules, including Rho-kinase, during neural development. In the present study, we investigated the role of Rho signaling through Rho-kinase in the survival of corticostriatal neurons. We performed the conditional expression of Clostridium botulinum C3 ADP-ribosyltransferase (C3 transferase) or dominant-negative form for Rho-kinase (Rho-K DN), a well-known inhibitor of Rho or Rho-kinase, respectively, in corticostriatal neurons using a dual viral vector approach combining a neuron-specific retrograde gene transfer lentiviral vector and an adeno-associated virus vector. C3 transferase markedly decreased the number of corticostriatal neurons, which was attributed to caspase-3-dependent enhanced apoptosis. In addition, Rho-K DN produced phenotypic defects similar to those caused by C3 transferase. These results indicate that the Rho/Rho-kinase signaling pathway plays a crucial role in the survival of corticostriatal neurons.

Citron Rho-interacting kinase mediates arsenite-induced decrease in endothelial nitric oxide synthase activity by increasing phosphorylation at threonine 497: Mechanism underlying arsenite-induced vascular dysfunction

We reported that arsenite causes an acute decrease in nitric oxide (NO) production by increasing phosphorylation of endothelial NO synthase at threonine 497 (eNOS-Thr(497)); however, the detailed mechanism has not yet been clarified. Here, we investigated the kinase involving in arsenite-stimulated eNOS-Thr(497) phosphorylation. Although treatment with H-89, a known protein kinase A (PKA) inhibitor, inhibited arsenite-stimulated eNOS-Thr(497) phosphorylation, no inhibition was found in cells treated with other PKA inhibitors, including Rp-8-Br-cAMPS or PKI. Based on previous reports, we also tested whether RhoA mediates arsenite-stimulated eNOS-Thr(497) phosphorylation and found that arsenite causes an acute increase in RhoA activity. Ectopic expression of dominant negative (DN)-RhoA significantly reversed arsenite-stimulated eNOS-Thr(497) phosphorylation. An in vitro phosphorylation assay also revealed that the well-known Rho effectors, Rho-associated protein kinase 1/2 (ROCK1/2), directly phosphorylate eNOS-Thr(497). Y27632, a selective ROCK inhibitor, reversed arsenite-stimulated eNOS-Thr(497) phosphorylation. However, overexpression of a small interfering RNA (siRNA) against ROCK1/2 or DN-ROCK did not reverse arsenite-stimulated eNOS-Thr(497) phosphorylation, thereby providing no conclusive evidence of a role for ROCK1/2. Knockdown of PKC-related protein kinase 1/2, another Rho effector, also did not reverse arsenite-stimulated eNOS-Thr(497) phosphorylation. In contrast, we found that transfection with an siRNA against citron Rho-interacting kinase (CRIK), the other downstream effector of Rho, significantly reversed the arsenite-induced eNOS-Thr(497) phosphorylation that was accompanied by restoration of eNOS enzymatic activity repressed by arsenite. Moreover, CRIK directly phosphorylated eNOS-Thr(497)in vitro. Finally, we also found that arsenite increased eNOS-Thr(497) phosphorylation and decreased acetylcholine-induced vessel relaxation in rat aortas. In conclusion, we demonstrate that arsenite acutely inhibits eNOS enzymatic activity and vessel relaxation in part by increasing the RhoA/CRIK/eNOS-Thr(497) phosphorylation signaling axis, which provides a molecular mechanism underlying arsenite-induced impaired vascular diseases.

Human amniotic epithelial cells combined with silk fibroin scaffold in the repair of spinal cord injury

Treatment and functional reconstruction after central nervous system injury is a major medical and social challenge. An increasing number of researchers are attempting to use neural stem cells combined with artificial scaffold materials, such as fibroin, for nerve repair. However, such approaches are challenged by ethical and practical issues. Amniotic tissue, a clinical waste product, is abundant, and amniotic epithelial cells are pluripotent, have low immunogenicity, and are not the subject of ethical debate. We hypothesized that amniotic epithelial cells combined with silk fibroin scaffolds would be conducive to the repair of spinal cord injury. To test this, we isolated and cultured amniotic epithelial cells, and constructed complexes of these cells and silk fibroin scaffolds. Implantation of the cell-scaffold complex into a rat model of spinal cord injury resulted in a smaller glial scar in the damaged cord tissue than in model rats that received a blank scaffold, or amniotic epithelial cells alone. In addition to a milder local immunological reaction, the rats showed less inflammatory cell infiltration at the transplant site, milder host-versus-graft reaction, and a marked improvement in motor function. These findings confirm that the transplantation of amniotic epithelial cells combined with silk fibroin scaffold can promote the repair of spinal cord injury. Silk fibroin scaffold can provide a good nerve regeneration microenvironment for amniotic epithelial cells.

Enhanced neurite outgrowth of human model (NT2) neurons by small-molecule inhibitors of Rho/ROCK signaling

Axonal injury in the adult human central nervous system often results in loss of sensation and motor functions. Promoting regeneration of severed axons requires the inactivation of growth inhibitory influences from the tissue environment and stimulation of the neuron intrinsic growth potential. Especially glial cell derived factors, such as chondroitin sulfate proteoglycans, Nogo-A, myelin-associated glycoprotein, and myelin in general, prevent axon regeneration. Most of the glial growth inhibiting factors converge onto the Rho/ROCK signaling pathway in neurons. Although conditions in the injured nervous system are clearly different from those during neurite outgrowth in vitro, here we use a chemical approach to manipulate Rho/ROCK signalling with small-molecule agents to encourage neurite outgrowth in cell culture. The development of therapeutic treatments requires drug testing not only on neurons of experimental animals, but also on human neurons. Using human NT2 model neurons, we demonstrate that the pain reliever Ibuprofen decreases RhoA (Ras homolog gene family, member A GTPase) activation and promotes neurite growth. Inhibition of the downstream effector Rho kinase by the drug Y-27632 results in a strong increase in neurite outgrowth. Conversely, activation of the Rho pathway by lysophosphatidic acid results in growth cone collapse and eventually to neurite retraction. Finally, we show that blocking of Rho kinase, but not RhoA results in an increase in neurons bearing neurites. Due to its anti-inflammatory and neurite growth promoting action, the use of a pharmacological treatment of damaged neural tissue with Ibuprofen should be explored.

Rho family GTPases: key players in neuronal development, neuronal survival, and neurodegeneration

The Rho family of GTPases belongs to the Ras superfamily of low molecular weight (∼21 kDa) guanine nucleotide binding proteins. The most extensively studied members are RhoA, Rac1, and Cdc42. In the last few decades, studies have demonstrated that Rho family GTPases are important regulatory molecules that link surface receptors to the organization of the actin and microtubule cytoskeletons. Indeed, Rho GTPases mediate many diverse critical cellular processes, such as gene transcription, cell–cell adhesion, and cell cycle progression. However, Rho GTPases also play an essential role in regulating neuronal morphology. In particular, Rho GTPases regulate dendritic arborization, spine morphogenesis, growth cone development, and axon guidance. In addition, more recent efforts have underscored an important function for Rho GTPases in regulating neuronal survival and death. Interestingly, Rho GTPases can exert either a pro-survival or pro-death signal in neurons depending upon both the cell type and neurotoxic insult involved. This review summarizes key findings delineating the involvement of Rho GTPases and their effectors in the regulation of neuronal survival and death. Collectively, these results suggest that dysregulation of Rho family GTPases may potentially underscore the etiology of some forms of neurodegenerative disease such as amyotrophic lateral sclerosis.

Directed neurite growth of rat dorsal root ganglion neurons and increased colocalization with Schwann cells on aligned poly(methyl methacrylate) electrospun nanofibers

Electrospun nanofibers are promising scaffolds for peripheral and central nervous system repair. The aim of this study was to examine the details of neurite growth of rat dorsal root ganglion neurons (DRGn) on randomly oriented and aligned poly(methyl methacrylate) (PMMA) nanofibers and the relationship between neurites and nanofibers on each substrate. Our substrate design involved electrospinning PMMA nanofibers directly onto bare glass coverslips with acceptable biocompatibility. We cocultured DRGn and Schwann cells on PMMA nanofibers and evaluated their response to each substrate. Compared with neurons cultured on PMMA film and randomly oriented nanofibers, DRGn on aligned PMMA nanofibers formed longer, parallel neurites in accordance with the orientation of the substrate nanofibers, although the average neurite number did not differ among the three groups. Regarding the relationship between neurites and nanofibers, the neurites of DRGn were in close contact with the substrate nanofibers, and the neurites seemed to follow aligned nanofibers more than randomly oriented nanofibers. Coculturing DRGn and Schwann cells on PMMA nanofibers revealed that on aligned nanofibers, neurites and Schwann cells had a higher chance of colocalization than on randomly oriented nanofibers or film; this colocalization may be beneficial during the process of myelination that follows. The results of this study enhance our understanding of the ability of aligned electrospun nanofibers to provide contact guidance to neural cells and strengthen the rationale for future in vivo studies.

The assessment of adeno-associated vectors as potential intrinsic treatments for brainstem axon regeneration

BACKGROUND

Adeno-associated virus (AAV) vector-mediated transgene expression is a promising therapeutic to change the intrinsic state of neurons and promote repair after central nervous system injury. Given that numerous transgenes have been identified as potential candidates, the present study demonstrates how to determine whether their expression by AAV has a direct intrinsic effect on axon regeneration.

METHODS

Serotype 2 AAV-enhanced green fluorescent protein (EGFP) was stereotaxically injected into the brainstem of adult rats, followed by a complete transection of the thoracic spinal cord and Schwann cell (SC) bridge implantation.

RESULTS

The expression of EGFP in brainstem neurons labeled numerous axons in the thoracic spinal cord and that regenerated into the SC bridge. The number of EGFP-labeled axons rostral to the bridge directly correlated with the number of EGFP-labeled axons that regenerated into the bridge. Animals with a greater number of EGFP-labeled axons rostral to the bridge exhibited an increased percentage of those axons found near the distal end of the bridge compared to animals with a lesser number. This suggested that EGFP may accumulate distally in the axon with time, enabling easier visualization. By labeling brainstem axons with EGFP before injury, numerous axon remnants undergoing Wallerian degeneration may be identified distal to the complete transection up to 6 weeks after injury.

CONCLUSIONS

Serotype 2 AAV-EGFP enabled easy visualization of brainstem axon regeneration. Rigorous models of axonal injury (i.e. complete transection and cell implantation) should be used in combination with AAV-EGFP to directly assess AAV-mediated expression of therapeutic transgenes as intrinsic treatments to improve axonal regeneration.

IL-6 promotes regeneration and functional recovery after cortical spinal tract injury by reactivating intrinsic growth program of neurons and enhancing synapse formation

Most neurons in adult mammalian central nervous system (CNS) fail to regenerate their axons after injury. Peripherally conditioned primary sensory neurons have an increased capacity to regenerate their central processes. Recent studies demonstrate that a conditioning lesion increased intrinsic growth capability is associated with the up-regulation of a group of growth-associated genes, one of the most established is interleukin-6 (IL-6). However, the cellular and molecular mechanisms by which IL-6 exerts its beneficial effect on axonal regeneration and functional recovery remain to be elucidated. The purpose of this study is to further investigate the molecular mechanisms of IL-6 in promoting regeneration and functional recovery after spinal cord injury (SCI). Here, we demonstrate that in vitro administration of IL-6 enhances neurite outgrowth of neurons on an inhibitory substrate myelin proteins, accompanied by increased expression of growth-associated genes GAP-43, SPRR1A and Arginase I. In vivo, intrathecal delivery of IL-6 for 7 days after cortical spinal tract injury induces synaptic rearrangements of sprouting axons and increases the expression of mTOR in neurons surrounding the lesion site, accompanied by improved functional recovery. In conclusion, our results show that IL-6 increases the expression of growth-associated genes and induces the expression of mTOR in lesion adjacent neurons, resulting in reactivating the intrinsic growth program of neurons to promote axonal regrowth and functional recovery after SCI.

Small-molecule-induced Rho-inhibition: NSAIDs after spinal cord injury

Limited axonal plasticity within the central nervous system (CNS) is a major restriction for functional recovery after CNS injury. The small GTPase RhoA is a key molecule of the converging downstream cascade that leads to the inhibition of axonal re-growth. The Rho-pathway integrates growth inhibitory signals derived from extracellular cues, such as chondroitin sulfate proteoglycans, Nogo-A, myelin-associated glycoprotein, oligodendrocyte-myelin glycoprotein, Ephrins and repulsive guidance molecule-A, into the damaged axon. Consequently, the activation of RhoA results in growth cone collapse and finally outgrowth failure. In turn, the inhibition of RhoA-activation blinds the injured axon to its growth inhibitory environment resulting in enhanced axonal sprouting and plasticity. This has been demonstrated in various CNS-injury models for direct RhoA-inhibition and for downstream/upstream blockade of the RhoA-associated pathway. In addition, RhoA-inhibition reduces apoptotic cell death and secondary damage and improves locomotor recovery in clinically relevant models after experimental spinal cord injury (SCI). Unexpectedly, a subset of "small molecules" from the group of non-steroid anti-inflammatory drugs, particularly the FDA-approved ibuprofen, has recently been identified as (1) inhibiting RhoA-activation, (2) enhancing axonal sprouting/regeneration, (3) protecting "tissue at risk" (neuroprotection) and (4) improving motor recovery confined to realistic therapeutical time-frames in clinically relevant SCI models. Here, we survey the effect of small-molecule-induced RhoA-inhibition on axonal plasticity and neurofunctional outcome in CNS injury paradigms. Furthermore, we discuss the body of preclinical evidence for a possible clinical translation with a focus on ibuprofen and illustrate putative risks and benefits for the treatment of acute SCI.

Lentiviral vector mediates exogenous gene expression in adult rat DRG following peripheral nerve remote delivery

The primary sensory neurons with cell bodies in the dorsal root ganglion (DRG) have been extensively used as models in neurobiology and provide a useful model to study the mechanism of neural regeneration. Therefore, efficient and stable gene delivery to these postmitotic cells has significant therapeutic potential. Various studies involving the viral vector systems capable of neuronal transduction have been extensively evaluated in the cultured DRG neurons by adeno-associated virus. In the present study, we investigated the transduction performance of the lentiviral vector that mediates the catalytic subunit of protein kinase A (PKAc) and green fluorescent protein (GFP) expression in the DRG by sciatic nerve retrograde transport and tested whether PKAc expression in the DRG could inhibit the activation of RhoA after spinal cord injury. Five days after sciatic nerve remote delivery of lentiviral vector (LV)/PKAc-internal ribosome entry site (IRES)-GFP or LV/GFP, the L4-L6 DRGs were dissected for primary culture or immunostaining to observe the exogenous gene expression, or transecting the dorsal part of lumbar enlargement was performed, and 16 h later, the function of the exogenous gene was tested by RhoA pull-down analysis. The results showed that the lentiviral vector could mediate exogenous gene PKAc expression in the DRG and then inhibit spinal cord injury-induced RhoA activation by remote delivery of LV/PKAc-IRES-GFP through the sciatic nerve.

Combination therapies

Spinal cord injury (SCI) has multiple consequences, ranging from molecular imbalances to glial scar formation to functional impairments. It is logical to think that a combination of single treatments implemented in the right order and at the right time will be required to repair the spinal cord. However, the single treatments that compose the combination therapy will need to be chosen with caution as many have multiple outcomes that may or may not be synergistic. Single treatments may also elicit unwanted side-effects and/or effects that would decrease the repair potential of other components and/or the entire combination therapy. In this chapter a number of single treatments are discussed with respect to their multiplicity of action. These include strategies to boost growth and survival (such as neurotrophins and cyclic AMP) and strategies to reduce inhibitory factors (such as antimyelin-associated growth inhibitors and digestion of glial scar-associated inhibitors). We also present an overview of combination therapies that have successfully or unsuccessfully been tested in the laboratory using animal models. To effectively design a combination therapy a number of considerations need to be made such as the nature and timing of the treatments and the method for delivery. This chapter discusses these issues as well as considerations related to chronic SCI and the logistics of bringing combination therapies to the clinic.

ROCKing Regeneration: Rho Kinase Inhibition as Molecular Target for Neurorestoration

Regenerative failure in the CNS largely depends on pronounced growth inhibitory signaling and reduced cellular survival after a lesion stimulus. One key mediator of growth inhibitory signaling is Rho-associated kinase (ROCK), which has been shown to modulate growth cone stability by regulation of actin dynamics. Recently, there is accumulating evidence the ROCK also plays a deleterious role for cellular survival. In this manuscript we illustrate that ROCK is involved in a variety of intracellular signaling pathways that comprise far more than those involved in neurite growth inhibition alone. Although ROCK function is currently studied in many different disease contexts, our review focuses on neurorestorative approaches in the CNS, especially in models of neurotrauma. Promising strategies to target ROCK by pharmacological small molecule inhibitors and RNAi approaches are evaluated for their outcome on regenerative growth and cellular protection both in preclinical and in clinical studies.

Enhancing intrinsic growth capacity promotes adult CNS regeneration

In the adult mammalian central nervous system (CNS), the axons do not spontaneously regenerate after injury due to the inhibitory extrinsic environment and a diminished intrinsic regenerative capability. Many previous studies focus largely on characterizing the hostile growth inhibitory molecules in the CNS. In fact, blocking such inhibitory activities by either genetic or pharmacological approaches only allows limited sprouting, and majority of the adult neurons fail to regenerate their axons even provided with permissive substrates. Upon the neural circuits established during development, the intrinsic neuronal growth activity is gradually repressed. Little is known to the mechanisms for transition from the robust growth mode of the immature neurons to the poor growth mode of the mature neurons and the way to reactivate the intrinsic growth capacity after injury. The primary sensory neurons with cell bodies in the dorsal root ganglion (DRG) provide a useful model to develop strategies to enhance the intrinsic growth capacity of neurons. The centrally projecting axons in the adult spinal cord do not regenerate, while the peripheral branches regenerate robustly after injury. Regeneration of the central branches can be significantly enhanced after a prior peripheral branch injury, which is defined as conditioning lesion. We reviewed the mode of conditioning lesion reactivating the intrinsic growth program. Importantly, we summarized the intrinsic neuronal determinants for neurite growth such as cAMP, PTEN/mTOR, APC-Cdh1, KLF4, etc., the mechanisms underlying development-dependent decline of CNS neurons growth ability, and procedures to enhance the intrinsic growth potential.

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.

Expression of a dominant-negative Rho-kinase promotes neurite outgrowth in a microenvironment mimicking injured central nervous system

AIM

To investigate whether lentiviral vector (LV)-mediated expression of a dominant negative mutant Rho-kinase (DNROCK) could inhibit activation of the Rho/ROCK signaling pathway and promote neurite outgrowth in a hostile microenvironment mimicking the injured central nervous system (CNS) in vitro.

METHODS

Lentiviral stock was produced using the three-plasmid system by transfecting HEK293 cells. Myelin prepared from rat brain was purified by two rounds of discontinuous density gradient centrifugation and osmotic disintegration. Differentiated PC12 cells and dissociated adult rat dorsal root ganglion (DRG) neurons were transduced with either LV/DNROCK or LV/green fluorescent protein (GFP) and seeded on solubilized myelin proteins. The effect of DNROCK on growth cone morphology was tested by rhodamine-conjugated phalloidin staining. Expression of DNROCK was determined by immunoblotting. The length of the longest neurite, the percentage of neurite-bearing neurons, or the total process outgrowth for all transduced neurons were measured by using the Scion image analysis program.

RESULTS

Transduction of DNROCK inhibited serum-induced stress fiber formation in NIH 3T3 cells and induced enlargement of cell bodies and decreased the phosphorylation levels of MYPT1 in HeLa cells. LV/DNROCK blocked myelin-induced increase in ROCK translocation from cytosol to membrane in LV/GFP-treated PC12 cells. DNROCK promotes neurite outgrowth of differentiated PC12 cells and DRG neurons on myelin protein. LV/DNROCK-transduced PC12 cells had longer neurites than LV/GFP-transduced cells (39.18+/-2.19 microm vs 29.32+/-1.7 microm, P<0.01) on myelin-coated coverslips. Furthermore, a significantly higher percentage of LV/DNROCK-transduced cells had extended neurites than LV/GFP-transduced cells (63.75%+/-8.03% vs 16.3%+/-3.70%, P<0.01). LV/DNROCK-transduced DRG neurons had longer neurite length (325.22+/-10.8 microm vs 202.47+/-9.3 microm, P<0.01) and more primary neurites per cell than those in LV/GFP-transduced cells plated on myelin and laminin (7.8+/-1.25 vs 4.84+/-1.45, P<0.01) or on laminin alone (5.2+/-1.88). LV/DNROCK-transduced cells had significantly larger growth cones (33.12+/-1.06 microm(2)) than LV/GFP-pretreated cells (23.72+/-1.22 microm(2)).

CONCLUSION

These results indicate that blocking the RhoA/ROCK signaling pathway by expression of DNROCK is effective in facilitating neurite outgrowth in a microenvironment mimicking injury of central nervous system.