Differential effects of two ROCK inhibitors, Fasudil and Y-27632, on optic nerve regeneration in adult cats

Compounds co-targeting kinases in axon regulatory pathways promote regeneration and behavioral recovery after spinal cord injury in mice

Recovery from spinal cord injury (SCI) and other central nervous system (CNS) trauma is hampered by limits on axonal regeneration in the CNS. Regeneration is restricted by the lack of neuron-intrinsic regenerative capacity and by the repressive microenvironment confronting damaged axons. To address this challenge, we have developed a therapeutic strategy that co-targets kinases involved in both extrinsic and intrinsic regulatory pathways. Prior work identified a kinase inhibitor (RO48) with advantageous polypharmacology (co-inhibition of targets including ROCK2 and S6K1), which promoted CNS axon growth in vitro and corticospinal tract (CST) sprouting in a mouse pyramidotomy model. We now show that RO48 promotes neurite growth from sensory neurons and a variety of CNS neurons in vitro, and promotes CST sprouting and/or regeneration in multiple mouse models of spinal cord injury. Notably, these in vivo effects of RO48 were seen in several independent experimental series performed in distinct laboratories at different times. Finally, in a cervical dorsal hemisection model, RO48 not only promoted growth of CST axons beyond the lesion, but also improved behavioral recovery in the rotarod, gridwalk, and pellet retrieval tasks. Our results provide strong evidence for RO48 as an effective compound to promote axon growth and regeneration. Further, they point to strategies for increasing robustness of interventions in pre-clinical models.

Remodeling of the Lamina Cribrosa: Mechanisms and Potential Therapeutic Approaches for Glaucoma

Glaucomatous optic neuropathy is the leading cause of irreversible blindness in the world. The chronic disease is characterized by optic nerve degeneration and vision field loss. The reduction of intraocular pressure remains the only proven glaucoma treatment, but it does not prevent further neurodegeneration. There are three major classes of cells in the human optic nerve head (ONH): lamina cribrosa (LC) cells, glial cells, and scleral fibroblasts. These cells provide support for the LC which is essential to maintain healthy retinal ganglion cell (RGC) axons. All these cells demonstrate responses to glaucomatous conditions through extracellular matrix remodeling. Therefore, investigations into alternative therapies that alter the characteristic remodeling response of the ONH to enhance the survival of RGC axons are prevalent. Understanding major remodeling pathways in the ONH may be key to developing targeted therapies that reduce deleterious remodeling.

Combination of Drugs and Cell Transplantation: More Beneficial Stem Cell-Based Regenerative Therapies Targeting Neurological Disorders

Cell transplantation therapy using pluripotent/multipotent stem cells has gained attention as a novel therapeutic strategy for treating neurodegenerative diseases, including Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, ischemic stroke, and spinal cord injury. To fully realize the potential of cell transplantation therapy, new therapeutic options that increase cell engraftments must be developed, either through modifications to the grafted cells themselves or through changes in the microenvironment surrounding the grafted region. Together these developments could potentially restore lost neuronal function by better supporting grafted cells. In addition, drug administration can improve the outcome of cell transplantation therapy through better accessibility and delivery to the target region following cell transplantation. Here we introduce examples of drug repurposing approaches for more successful transplantation therapies based on preclinical experiments with clinically approved drugs. Drug repurposing is an advantageous drug development strategy because drugs that have already been clinically approved can be repurposed to treat other diseases faster and at lower cost. Therefore, drug repurposing is a reasonable approach to enhance the outcomes of cell transplantation therapies for neurological diseases. Ideal repurposing candidates would result in more efficient cell transplantation therapies and provide a new and beneficial therapeutic combination.

Rho-associated kinases contribute to the regulation of tau phosphorylation and amyloid metabolism during neuronal plasticity

BACKGROUND

Neural plasticity under physiological condition develops together with normal tau phosphorylation and amyloid precursor protein (APP) processing. Since restoration of PI3-kinase signaling has therapeutic potential in Alzheimer's disease, we investigated plasticity-related changes in tau and APP metabolism by the selective Rho-kinase inhibitor fasudil.

METHODS

Field potentials composed of a field excitatory post-synaptic potential (fEPSP) and a population spike (PS) were recorded from a granule cell layer of the dentate gyrus. Plasticity of synaptic strength and neuronal function was induced by strong tetanic stimulation (HFS) and low-frequency stimulation (LFS) patterns. Infusions of saline or fasudil were given for 1 h starting from the application of the induction protocols. Total and phosphorylated tau levels and soluble APPα levels were measured in the hippocampus, which was removed after at least 1 h post-induction period.

RESULTS

Fasudil infusion resulted in attenuation of fEPSP slope and PS amplitude in response to both HFS and LFS. Fasudil reduced total tau and phosphorylated tau at residue Thr¹⁸¹ in the HFS-stimulated hippocampus, while Thr²³¹ phosphorylation was reduced by fasudil treatment in the LFS-stimulated hippocampus. Ser⁴¹⁶ phosphorylation was increased by fasudil treatment in both HFS- and LFS-stimulated hippocampus. Fasudil significantly increased soluble APPα in LFS-stimulated hippocampus, but not in HFS-stimulated hippocampus.

CONCLUSION

In light of our findings, we suggest that increased activity of Rho kinase could trigger a mechanism that goes awry during synaptic plasticity which is reversed by a Rho-kinase inhibitor. Thus, Rho-kinase inhibition might be a therapeutic target in cognitive disorders.

[Rho kinase inhibitors as new local therapy option in primary open angle glaucoma]

BACKGROUND

In 2014 in Japan and 2017 in the USA, the Rho-kinase inhibitors were approved as a new antiglaucomatous substance group and will now be launched in Europe.

OBJECTIVE

On this occasion the current state of knowledge on Rho-kinase inhibitors is presented.

METHODS

In intensive search in PubMed the relevant experimental and clinical literature on the Rho-kinase inhibitors ripasudil and netarsudil and the combination of netarsudil and latanoprost were selected and compiled for this review.

RESULTS

The intraocular pressure lowering efficacy of ripasudil and netarsudil is in the range of the beta blocker timolol and the prostaglandin analogue latanoprost. In the fixed combination netarsudil/latanoprost the intraocular pressure reduction is greater than that of the single components and reaches a target pressure of below 15 mm Hg in 32%. Conjunctival hyperemia with 53-65% is the most common local side effect. Systemic side effects are very rare and so far there are no contraindications.

CONCLUSION

The Rho-kinase inhibitors are an interesting new introduction for glaucoma therapy, as each new pressure-lowering therapy represents an additional chance to reach the individually defined target pressure level in a glaucoma patient with local therapy; however, many of the pleiotropic effects associated with Rho-kinase inhibitors have so far only been found experimentally and will require clinical confirmation in the future.

Fasudil attenuates glial cell-mediated neuroinflammation via ERK1/2 and AKT signaling pathways after optic nerve crush

To investigate the functional role of fasudil in optic nerve crush (ONC), and further explore its possible molecular mechanism. After ONC injury, the rats were injected intraperitoneally either with fasudil or normal saline once a day until euthanized. RGCs survival was assessed by retrograde labeling with FluoroGold. Retinal glial cells activation and population changes (GFAP, iba-1) were measured by immunofluorescence. The expressions of cleaved caspase 3 and 9, p-ERK1/2 and p-AKT were detected by western blot. The levels of the pro-inflammatory cytokines were determined using real-time polymerase chain reaction. Fasudil treatment inhibited RGCs apoptosis and reduced RGCs loss demonstrated by the decreased apoptosis-associated proteins expression and the increased fluorogold labeling of RGCs after ONC, respectively. In addition, the ONC + fasudil group compared had a significantly lower expression of GFAP and iba1 compared with the ONC group. The levels of pro-inflammatory cytokines were significantly reduced in the ONC + fasudil group than in the ONC group. Furthermore, the phosphorylation levels of ERK1/2 and AKT (p-ERK1/2 and p-AKT) were obviously elevated by the fasudil treatment. Our study demonstrated that fasudil attenuated glial cell-mediated neuroinflammation by up-regulating the ERK1/2 and AKT signaling pathways in rats ONC models. We conclude that fasudil may be a novel treatment for traumatic optic neuropathy.

Topical ripasudil stimulates neuroprotection and axon regeneration in adult mice following optic nerve injury

Optic nerve injury induces optic nerve degeneration and retinal ganglion cell (RGC) death that lead to visual disturbance. In this study, we examined if topical ripasudil has therapeutic potential in adult mice after optic nerve crush (ONC). Topical ripasudil suppressed ONC-induced phosphorylation of p38 mitogen-activated protein kinase and ameliorated RGC death. In addition, topical ripasudil significantly suppressed the phosphorylation of collapsin response mediator protein 2 and cofilin, and promoted optic nerve regeneration. These results suggest that topical ripasudil promotes RGC protection and optic nerve regeneration by modulating multiple signaling pathways associated with neural cell death, microtubule assembly and actin polymerization.

Rho-kinase (ROCK) Inhibitors - A Neuroprotective Therapeutic Paradigm with a Focus on Ocular Utility

BACKGROUND

Glaucoma is a progressive optic neuropathy causing visual impairment and Retinal Ganglionic Cells (RGCs) death gradually posing a need for neuroprotective strategies to minimize the loss of RGCs and visual field. It is recognized as a multifactorial disease, Intraocular Pressure (IOP) being the foremost risk factor. ROCK inhibitors have been probed for various possible indications, such as myocardial ischemia, hypertension, kidney diseases. Their role in neuroprotection and neuronal regeneration has been suggested to be of value in the treatment of neurological diseases, like spinal-cord injury, Alzheimer's disease and multiple sclerosis but recently Rho-associated Kinase inhibitors have been recognized as potential antiglaucoma agents.

EVIDENCE SYNTHESIS

Rho-Kinase is a serine/threonine kinase with a kinase domain which is constitutively active and is involved in the regulation of smooth muscle contraction and stress fibre formation. Two isoforms of Rho-Kinase, ROCK-I (ROCK β) and ROCK-II (ROCK α) have been identified. ROCK II plays a pathophysiological role in glaucoma and hence the inhibitors of ROCK may be beneficial to ameliorate the vision loss. These inhibitors decrease the intraocular pressure in the glaucomatous eye by increasing the aqueous humour outflow through the trabecular meshwork pathway. They also act as anti-scarring agents and hence prevent post-operative scarring after the glaucoma filtration surgery. Their major role involves axon regeneration by increasing the optic nerve blood flow which may be useful in treating the damaged optic neurons. These drugs act directly on the neurons in the central visual pathway, interrupting the RGC apoptosis and therefore serve as a novel pharmacological approach for glaucoma neuroprotection.

CONCLUSION

Based on the results of high-throughput screening, several Rho kinase inhibitors have been designed and developed comprising of diverse scaffolds exhibiting Rho kinase inhibitory activity from micromolar to subnanomolar ranges. This diversity in the scaffolds with inhibitory potential against the kinase and their SAR development will be intricated in the present review. Ripasudil is the only Rho kinase inhibitor marketed to date for the treatment of glaucoma. Another ROCK inhibitor AR-13324 has recently passed the clinical trials whereas AMA0076, K115, PG324, Y39983 and RKI-983 are still under trials. In view of this, a detailed and updated account of ROCK II inhibitors as the next generation therapeutic agents for glaucoma will be discussed in this review.

Bile canaliculi remodeling activates YAP via the actin cytoskeleton during liver regeneration

The mechanisms of organ size control remain poorly understood. A key question is how cells collectively sense the overall status of a tissue. We addressed this problem focusing on mouse liver regeneration. Using digital tissue reconstruction and quantitative image analysis, we found that the apical surface of hepatocytes forming the bile canalicular network expands concomitant with an increase in F-actin and phospho-myosin, to compensate an overload of bile acids. These changes are sensed by the Hippo transcriptional co-activator YAP, which localizes to apical F-actin-rich regions and translocates to the nucleus in dependence of the integrity of the actin cytoskeleton. This mechanism tolerates moderate bile acid fluctuations under tissue homeostasis, but activates YAP in response to sustained bile acid overload. Using an integrated biophysical-biochemical model of bile pressure and Hippo signaling, we explained this behavior by the existence of a mechano-sensory mechanism that activates YAP in a switch-like manner. We propose that the apical surface of hepatocytes acts as a self-regulatory mechano-sensory system that responds to critical levels of bile acids as readout of tissue status.

ROCK inhibitors for the treatment of ocular diseases

The Rho-kinase/ROCK (Rho-associated coiled-coil-containing protein kinase) pathway is involved in the pathogenesis of multiple ocular and systemic disorders. Recently, ROCK inhibitors have been suggested as novel treatments for various ocular diseases. Several in vitro, in vivo and clinical studies have demonstrated the safety and efficacy of ROCK inhibitors in the management of ocular disorders such as corneal epithelial and endothelial damage, glaucoma, retinal and choroidal neovascularisation, diabetic macular oedema and optic nerve disorders. In this review, these studies are explored with focus on the relevant clinical investigations.

Comparative Study of the Effects of Trabecular Meshwork Outflow Drugs on the Permeability and Nitric Oxide Production in Trabecular Meshwork Cells

PURPOSE

To compare the effects of the barrier function in human trabecular meshwork (TM) cells monolayer and the production of nitric oxide (NO) between trabecular outflow drugs, Rho-associated kinase (ROCK) inhibitors, adenosine, and statin.

METHODS

Primary cultured TM cells were exposed to 10 or 25 μM Y-27632, 0.1 or 1 μM N6-cyclohexyladenosine (CHA), or 15 or 30 μM simvastatin for 24 hours. NO production and expression of endothelial nitric oxide synthase mRNA were measured by Griess assay and reverse transcription polymerase chain reaction, respectively. Barrier functions of the TM cell monolayer were measured by carboxyfluorescein and trans-endothelial electrical resistance. The expression of matrix metalloproteinase-2 mRNA was assessed with reverse transcription polymerase chain reaction.

RESULTS

In TM cells, treatment with each drug increased endothelial nitric oxide synthase mRNA expression. Treatment with 25 μM Y-27632 and 1.0 μM CHA increased NO production significantly (p = 0.035 and p = 0.043, respectively). Treatment with each drug increased the permeability (all p = 0.001) and decreased the trans-endothelial electron resistance of the TM cell monolayer. Treatment with 0.1 μM and 1.0 μM CHA significantly increased matrix metalloproteinase-2 mRNA expression, but simvastatin inhibited its expression.

CONCLUSIONS

Since treatment with ROCK inhibitor more greatly increased NO production and permeability than did adenosine or statin, ROCK inhibitor seems to be more effective for lowering intraocular pressure.

Prospects for mTOR-mediated functional repair after central nervous system trauma

Recent research has suggested that the growth of central nervous system (CNS) axons during development is mediated through the PI3K/Akt/mammalian target of rapamycin (mTOR) intracellular signalling axis and that suppression of activity in this pathway occurs during maturity as levels of the phosphatase and tensin homologue (PTEN) rise and inhibit PI3K activation of mTOR, accounting for the failure of axon regeneration in the injured adult CNS. This hypothesis is supported by findings confirming that suppression of PTEN in experimental adult animals promotes impressive axon regeneration in the injured visual and corticospinal motor systems. This review focuses on these recent developments, discussing the therapeutic potential of a mTOR-based treatment aimed at promoting functional recovery in CNS trauma patients, recognising that to fulfil this ambition, the new therapy should aim to promote not only axon regeneration but also remyelination of regenerated axons, neuronal survival and re-innervation of denervated targets through accurate axonal guidance and synaptogenesis, all with minimal adverse effects. The translational challenges presented by the implementation of this new axogenic therapy are also discussed.

Towards axonal regeneration and neuroprotection in glaucoma: Rho kinase inhibitors as promising therapeutics

Due to a prolonged life expectancy worldwide, the incidence of age-related neurodegenerative disorders such as glaucoma is increasing. Glaucoma is the second cause of blindness, resulting from a slow and progressive loss of retinal ganglion cells (RGCs) and their axons. Up to now, intraocular pressure (IOP) reduction is the only treatment modality by which ophthalmologists attempt to control disease progression. However, not all patients benefit from this therapy, and the pathophysiology of glaucoma is not always associated with an elevated IOP. These limitations, together with the multifactorial etiology of glaucoma, urge the pressing medical need for novel and alternative treatment strategies. Such new therapies should focus on preventing or retarding RGC death, but also on repair of injured axons, to ultimately preserve or improve structural and functional connectivity. In this respect, Rho-associated coiled-coil forming protein kinase (ROCK) inhibitors hold a promising potential to become very prominent drugs for future glaucoma treatment. Their field of action in the eye does not seem to be restricted to IOP reduction by targeting the trabecular meshwork or improving filtration surgery outcome. Indeed, over the past years, important progress has been made in elucidating their ability to improve ocular blood flow, to prevent RGC death/increase RGC survival and to retard axonal degeneration or induce proper axonal regeneration. Within this review, we aim to highlight the currently known capacity of ROCK inhibition to promote neuroprotection and regeneration in several in vitro, ex vivo and in vivo experimental glaucoma models.

Transcriptomic analysis and 3D bioengineering of astrocytes indicate ROCK inhibition produces cytotrophic astrogliosis

Astrocytes provide trophic, structural and metabolic support to neurons, and are considered genuine targets in regenerative neurobiology, as their phenotype arbitrates brain integrity during injury. Inhibitors of Rho kinase (ROCK) cause stellation of cultured 2D astrocytes, increased L-glutamate transport, augmented G-actin, and elevated expression of BDNF and anti-oxidant genes. Here we further explored the signposts of a cytotrophic, “healthy” phenotype by data-mining of our astrocytic transcriptome in the presence of Fasudil. Gene expression profiles of motor and autophagic cellular cascades and inflammatory/angiogenic responses were all inhibited, favoring adoption of an anti-migratory phenotype. Like ROCK inhibition, tissue engineered bioscaffolds can influence the extracellular matrix. We built upon our evidence that astrocytes maintained on 3D poly-ε-caprolactone (PCL) electrospun scaffolds adopt a cytotrophic phenotype similar to that produced by Fasudil. Using these procedures, employing mature 3D cultured astrocytes, Fasudil (100 μM) or Y27632 (30 μM) added for the last 72 h of culture altered arborization, which featured numerous additional minor processes as shown by GFAP and AHNAK immunolabelling. Both ROCK inhibitors decreased F-actin, but increased G-actin labeling, indicative of disassembly of actin stress fibers. ROCK inhibitors provide additional beneficial effects for bioengineered 3D astrocytes, including enlargement of the overall arbor. Potentially, the combined strategy of bio-compatible scaffolds with ROCK inhibition offers unique advantages for the management of glial scarring. Overall these data emphasize that manipulation of the astrocyte phenotype to achieve a “healthy biology” offers new hope for the management of inflammation in neuropathologies.

Rho kinase inhibitors: a patent review (2012 - 2013)

INTRODUCTION

The Rho kinase/ROCK is critical in vital signal transduction pathways central to many essential cellular activities. Since ROCK possess multiple substrates, modulation of ROCK activity is useful for treatment of many diseases.

AREAS COVERED

Significant progress has been made in the development of ROCK inhibitors over the past two years (Jan 2012 to Aug 2013). Patent search in this review was based on FPO IP Research and Communities and Espacenet Patent Search. In this review, patent applications will be classified into four groups for discussions. The grouping is mainly based on structures or scaffolds (groups 1 and 2) and biological functions of ROCK inhibitors (groups 3 and 4). These four groups are i) ROCK inhibitors based on classical structural elements for ROCK inhibition; ii) ROCK inhibitors based on new scaffolds; iii) bis-functional ROCK inhibitors; and iv) novel applications of ROCK inhibitors.

EXPERT OPINION

Although currently only one ROCK inhibitor (fasudil) is used as a drug, more drugs based on ROCK inhibition are expected to be advanced into market in the near future. Several directions should be considered for future development of ROCK inhibitors, such as soft ROCK inhibitors, bis-functional ROCK inhibitors, ROCK2 isoform-selective inhibitors, and ROCK inhibitors as antiproliferation agents.

Regeneration of optic nerve fibers with unoprostone, a prostaglandin-related antiglaucoma drug, in adult cats

PURPOSE

We investigated the effects of unoprostone on neurite extension of cultured retinal pieces and axonal regeneration of retinal ganglion cells in the crushed optic nerve of adult cats.

METHODS

The retinal pieces were cultured with unoprostone or its primary metabolite, M1, dissolved in DMSO or polysorbate for 14 days, and the number and length of Tau-1-positive neurites and glial processes labeled with anti-glial fibrillary acidic protein antibodies were examined. After the optic nerve was crushed, unoprostone was injected into the vitreous body and the crushed site. On day 12, wheat germ agglutinin-conjugated horseradish peroxidase was injected into the vitreous body to anterogradely label the regenerated axons. On day 14, the optic nerve was excised and longitudinally sectioned. After peroxidase reaction, the number of axons regenerating beyond the crush site was examined.

RESULTS

The greatest number of neurites protruded from the cultured retinal pieces in 3 μM unoprostone and 3 μM M1. The neurite length was also the longest at 3 μM unoprostone and 3 μM M1, in which no glial processes were detected. After injections of 3 μM unoprostone, the final concentration in the vitreous humor, into the vitreous body and the crush site, the optic nerve fibers regenerated and extended beyond the crush site. In contrast, almost no fibers extended beyond the crush site after injection of phosphate-buffered saline.

CONCLUSIONS

The results indicate that intravitreal injection of unoprostone promotes regeneration of crushed optic nerve fibers in adult cats.

Inhibition of Rho-kinase improves erectile function, increases nitric oxide signaling and decreases penile apoptosis in a rat model of cavernous nerve injury

PURPOSE

Bilateral cavernous nerve injury results in up-regulation of ROCK signaling in the penis. This is linked to erectile dysfunction in an animal model of post-prostatectomy erectile dysfunction. We evaluated whether daily treatment with the ROCK inhibitor Y-27632 (Tocris Bioscience, Ellisville, Missouri) would prevent erectile dysfunction in a rat model of bilateral cavernous nerve injury.

MATERIALS AND METHODS

Sprague-Dawley® rats underwent surgery to create sham (14) or bilateral (27) cavernous nerve injury. In the injury group 13 rats received treatment with Y-27632 (5 mg/kg twice daily) and 14 received vehicle. At 14 days after injury, rats underwent cavernous nerve stimulation to determine erectile function. Penes were assessed for neuronal and nitric oxide synthase membrane-endothelial nitric oxide synthase. ROCK2 was assessed by Western blot. Cyclic guanosine monophosphate was determined by enzyme-linked immunosorbent assay. Cavernous homogenates were tested for ROCK and protein kinase G enzymatic activity. Penile apoptosis was evaluated using the Apostain technique (Alexis, San Diego, California). Data were analyzed on ROCK using ANOVA and the t test.

RESULTS

While erectile function was decreased in rats with bilateral cavernous nerve injury, daily administration of Y-27632 improved erectile responses. Injury decreased neuronal and nitric oxide synthase membrane-endothelial nitric oxide synthase but ROCK2 was significantly increased. Y-27632 treatment restored neuronal nitric oxide synthase, nitric oxide synthase membrane-endothelial nitric oxide synthase and cyclic guanosine monophosphate levels, and protein kinase G activity. Treatment significantly decreased ROCK2 protein and ROCK activity. There were significantly fewer apoptotic cells after treatment than in injured controls.

CONCLUSIONS

These results provide evidence for up-regulation of the RhoA/ROCK signaling pathway with detrimental effects on erectile function after bilateral cavernous nerve injury. ROCK inhibition improved erectile dysfunction associated with bilateral cavernous nerve injury by preserving penile nitric oxide bioavailability and decreasing penile apoptosis.

Four steps to optic nerve regeneration

The failure of the optic nerve to regenerate after injury or in neurodegenerative disease remains a major clinical and scientific problem. Retinal ganglion cell (RGC) axons course through the optic nerve and carry all the visual information to the brain, but after injury, they fail to regrow through the optic nerve and RGC cell bodies typically die, leading to permanent loss of vision. There are at least 4 hurdles to overcome in preserving RGCs and regenerating their axons: 1) increase RGC survival, 2) overcome the inhibitory environment of the optic nerve, 3) enhance RGC intrinsic axon growth potential, and 4) optimize the mapping of RGC connections back into their targets in the brain.

Citron kinase regulates axon growth through a pathway that converges on cofilin downstream of RhoA

Axon regeneration in the adult central nervous system (CNS) is prevented by inhibitory molecules present in myelin, which bind to a receptor complex that leads to downstream RhoGTP activation and axon growth cone collapse. Here, we compared expression of Citron kinase (Citron-K), a target molecule of RhoGTP in non-regenerating dorsal root ganglion neurons (DRGN) after dorsal column (DC) injury, and in regenerating DRGN after either sciatic nerve (SN) injury or preconditioning SN+DC lesion models. We show by microarray that Citron-K mRNA levels in DRGN of a non-regenerating DC injury model were elevated 2-fold compared to those of intact control DRGN. Conversely, Citron-K levels were reduced by 2 and 2.4-fold at 10 days post lesion in the regenerating SN and preconditioning SN+DC lesion models, respectively, compared to levels in control intact DRGN. Western blotting and immunohistochemistry confirmed these observations and localised Citron-K immunostaining to both DRGN and satellite glia. In dissociated, adult rat DRG cell cultures, 80% knockdown of Citron-K, in the presence of inhibitory concentrations of CNS myelin extract (CME), promoted significant disinhibited DRGN neurite outgrowth, only when cells were stimulated with neurotrophic factors. The levels of RhoGTP remained unchanged after Citron-K knockdown in the presence of CME while enhanced cofilin levels correlated with disinhibited DRGN neurite outgrowth. This observation suggests that Citron-K plays a role in axon growth downstream of Rho activation. We conclude that Citron-K regulates actin polymerisation downstream of RhoA and may offer a potentially novel therapeutic approach for promoting CNS axon regeneration.

Epidermal growth factor receptor antagonists and CNS axon regeneration: mechanisms and controversies

The reasons for the failure of central nervous system (CNS) axons to regenerate include the presence of myelin- and non-myelin derived inhibitory molecules, neuronal apoptosis and the absence of a potent neurotrophic stimulus. Transactivation of the epidermal growth factor receptor (EGFR) has been implicated in signalling inhibition of axon growth in the CNS. Small molecule EGFR inhibitors such as AG1478 and PD168393 promote CNS axon growth after optic nerve transection despite the presence of inhibitory molecules in the environment of the regenerating axon. However, our results demonstrate that phosphorylated EGFR (pEGFR) is not present on regenerating axons and that the majority of pEGFR is present in glia, suggesting that EGFR cannot play a direct intra-axonal role in signalling inhibition and thus disinhibited CNS axon growth must be indirectly mediated by glia. We argue that EGFR may not have a role in signalling axon growth inhibition since AG1478 and PD168393 promotes neuronal neurite outgrowth in CNS myelin-inhibited cultures after EGFR knockdown. This review discusses the current evidences for and against the involvement of EGFR in signalling myelin inhibition.