Staurosporine induces dopaminergic neurite outgrowth through AMP-activated protein kinase/mammalian target of rapamycin signaling pathway

Interleukin-4 protects retinal ganglion cells and promotes axon regeneration

BACKGROUND

The preservation of retinal ganglion cells (RGCs) and the facilitation of axon regeneration are crucial considerations in the management of various vision-threatening disorders. Therefore, we investigate the efficacy of interleukin-4 (IL-4), a potential therapeutic agent, in promoting neuroprotection and axon regeneration of retinal ganglion cells (RGCs) as identified through whole transcriptome sequencing in an in vitro axon growth model.

METHODS

A low concentration of staurosporine (STS) was employed to induce in vitro axon growth. Whole transcriptome sequencing was utilized to identify key target factors involved in the molecular mechanism underlying axon growth. The efficacy of recombinant IL-4 protein on promoting RGC axon growth was validated through in vitro experiments. The protective effect of recombinant IL-4 protein on somas of RGCs was assessed using RBPMS-specific immunofluorescent staining in mouse models with optic nerve crush (ONC) and N-methyl-D-aspartic acid (NMDA) injury. The protective effect on RGC axons was evaluated by anterograde labeling of cholera toxin subunit B (CTB), while the promotion of RGC axon regeneration was assessed through both anterograde labeling of CTB and immunofluorescent staining for growth associated protein-43 (GAP43).

RESULTS

Whole-transcriptome sequencing of staurosporine-treated 661 W cells revealed a significant upregulation in intracellular IL-4 transcription levels during the process of axon regeneration. In vitro experiments demonstrated that recombinant IL-4 protein effectively stimulated axon outgrowth. Subsequent immunostaining with RBPMS revealed a significantly higher survival rate of RGCs in the rIL-4 group compared to the vehicle group in both NMDA and ONC injury models. Axonal tracing with CTB confirmed that recombinant IL-4 protein preserved long-distance projection of RGC axons, and there was a notably higher number of surviving axons in the rIL-4 group compared to the vehicle group following NMDA-induced injury. Moreover, intravitreal delivery of recombinant IL-4 protein substantially facilitated RGC axon regeneration after ONC injury.

CONCLUSION

The recombinant IL-4 protein exhibits the potential to enhance the survival rate of RGCs, protect RGC axons against NMDA-induced injury, and facilitate axon regeneration following ONC. This study provides an experimental foundation for further investigation and development of therapeutic agents aimed at protecting the optic nerve and promoting axon regeneration.

Promotion of axon regeneration and protection on injured retinal ganglion cells by rCXCL2

BACKGROUND

In addition to rescuing injured retinal ganglion cells (RGCs) by stimulating the intrinsic growth ability of damaged RGCs in various retinal/optic neuropathies, increasing evidence has shown that the external microenvironmental factors also play a crucial role in restoring the survival of RGCs by promoting the regrowth of RGC axons, especially inflammatory factors. In this study, we aimed to screen out the underlying inflammatory factor involved in the signaling of staurosporine (STS)-induced axon regeneration and verify its role in the protection of RGCs and the promotion of axon regrowth.

METHODS

We performed transcriptome RNA sequencing for STS induction models in vitro and analyzed the differentially expressed genes. After targeting the key gene, we verified the role of the candidate factor in RGC protection and promotion of axon regeneration in vivo with two RGC-injured animal models (optic nerve crush, ONC; retinal N-methyl-D-aspartate, NMDA damage) by using cholera toxin subunit B anterograde axon tracing and specific immunostaining of RGCs.

RESULTS

We found that a series of inflammatory genes expressed upregulated in the signaling of STS-induced axon regrowth and we targeted the candidate CXCL2 gene since the level of the chemokine CXCL2 gene elevated significantly among the top upregulated genes. We further demonstrated that intravitreal injection of rCXCL2 robustly promoted axon regeneration and significantly improved RGC survival in ONC-injured mice in vivo. However, different from its role in ONC model, the intravitreal injection of rCXCL2 was able to simply protect RGCs against NMDA-induced excitotoxicity in mouse retina and maintain the long-distance projection of RGC axons, yet failed to promote significant axon regeneration.

CONCLUSIONS

We provide the first in vivo evidence that CXCL2, as an inflammatory factor, is a key regulator in the axon regeneration and neuroprotection of RGCs. Our comparative study may facilitate deciphering the exact molecular mechanisms of RGC axon regeneration and developing high-potency targeted drugs.

mTOR (Mammalian Target of Rapamycin): Hitting the Bull's Eye for Enhancing Neurogenesis After Cerebral Ischemia?

Ischemic stroke remains a leading cause of morbidity and disability around the world. The sequelae of serious neurological damage are irreversible due to body's own limited repair capacity. However, endogenous neurogenesis induced by cerebral ischemia plays a critical role in the repair and regeneration of impaired neural cells after ischemic brain injury. mTOR (mammalian target of rapamycin) kinase has been suggested to regulate neural stem cells ability to self-renew and differentiate into proliferative daughter cells, thus leading to improved cell growth, proliferation, and survival. In this review, we summarized the current evidence to support that mTOR signaling pathways may enhance neurogenesis, angiogenesis, and synaptic plasticity following cerebral ischemia, which could highlight the potential of mTOR to be a viable therapeutic target for the treatment of ischemic brain injury.

Multifunctional role of natural products for the treatment of Parkinson's disease: At a glance

Natural substances originating from plants have long been used to treat neurodegenerative disorders (NDs). Parkinson's disease (PD) is a ND. The deterioration and subsequent cognitive impairments of the midbrain nigral dopaminergic neurons distinguish by this characteristic. Various pathogenic mechanisms and critical components have been reported, despite the fact that the origin is unknown, such as protein aggregation, iron buildup, mitochondrial dysfunction, neuroinflammation and oxidative stress. Anti-Parkinson drugs like dopamine (DA) agonists, levodopa, carbidopa, monoamine oxidase type B inhibitors and anticholinergics are used to replace DA in the current treatment model. Surgery is advised in cases where drug therapy is ineffective. Unfortunately, the current conventional treatments for PD have a number of harmful side effects and are expensive. As a result, new therapeutic strategies that control the mechanisms that contribute to neuronal death and dysfunction must be addressed. Natural resources have long been a useful source of possible treatments. PD can be treated with a variety of natural therapies made from medicinal herbs, fruits, and vegetables. In addition to their well-known anti-oxidative and anti-inflammatory capabilities, these natural products also play inhibitory roles in iron buildup, protein misfolding, the maintenance of proteasomal breakdown, mitochondrial homeostasis, and other neuroprotective processes. The goal of this research is to systematically characterize the currently available medications for Parkinson's and their therapeutic effects, which target diverse pathways. Overall, this analysis looks at the kinds of natural things that could be used in the future to treat PD in new ways or as supplements to existing treatments. We looked at the medicinal plants that can be used to treat PD. The use of natural remedies, especially those derived from plants, to treat PD has been on the rise. This article examines the fundamental characteristics of medicinal plants and the bioactive substances found in them that may be utilized to treat PD.

High-throughput kinase inhibitor screening reveals roles for Aurora and Nuak kinases in neurite initiation and dendritic branching

Kinases are essential regulators of a variety of cellular signaling processes, including neurite formation—a foundational step in neurodevelopment. Aberrant axonal sprouting and failed regeneration of injured axons are associated with conditions like traumatic injury, neurodegenerative disease, and seizures. Investigating the mechanisms underlying neurite formation will allow for identification of potential therapeutics. We used a kinase inhibitor library to screen 493 kinase inhibitors and observed that 45% impacted neuritogenesis in Neuro2a (N-2a) cells. Based on the screening, we further investigated the roles of Aurora kinases A, B, and C and Nuak kinases 1 and 2. The roles of Aurora and Nuak kinases have not been thoroughly studied in the nervous system. Inhibition or overexpression of Aurora and Nuak kinases in primary cortical neurons resulted in various neuromorphological defects, with Aurora A regulating neurite initiation, Aurora B and C regulating neurite initiation and elongation, all Aurora kinases regulating arborization, and all Nuak kinases regulating neurite initiation and elongation and arborization. Our high-throughput screening and analysis of Aurora and Nuak kinases revealed their functions and may contribute to the identification of therapeutics.

Protein kinases: master regulators of neuritogenesis and therapeutic targets for axon regeneration

Proper neurite formation is essential for appropriate neuronal morphology to develop and defects at this early foundational stage have serious implications for overall neuronal function. Neuritogenesis is tightly regulated by various signaling mechanisms that control the timing and placement of neurite initiation, as well as the various processes necessary for neurite elongation to occur. Kinases are integral components of these regulatory pathways that control the activation and inactivation of their targets. This review provides a comprehensive summary of the kinases that are notably involved in regulating neurite formation, which is a complex process that involves cytoskeletal rearrangements, addition of plasma membrane to increase neuronal surface area, coupling of cytoskeleton/plasma membrane, metabolic regulation, and regulation of neuronal differentiation. Since kinases are key regulators of these functions during neuromorphogenesis, they have high potential for use as therapeutic targets for axon regeneration after injury or disease where neurite formation is disrupted.

Marine-Derived Natural Compounds for the Treatment of Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative disorder caused by the loss of dopaminergic neurons, leading to the motor dysfunctions of patients. Although the etiology of PD is still unclear, the death of dopaminergic neurons during PD progress was revealed to be associated with the abnormal aggregation of α-synuclein, the elevation of oxidative stress, the dysfunction of mitochondrial functions, and the increase of neuroinflammation. However, current anti-PD therapies could only produce symptom-relieving effects, because they could not provide neuroprotective effects, stop or delay the degeneration of dopaminergic neurons. Marine-derived natural compounds, with their novel chemical structures and unique biological activities, may provide anti-PD neuroprotective effects. In this study, we have summarized anti-PD marine-derived natural products which have shown pharmacological activities by acting on various PD targets, such as α-synuclein, monoamine oxidase B, and reactive oxygen species. Moreover, marine-derived natural compounds currently evaluated in the clinical trials for the treatment of PD are also discussed.

Silica nanoparticle-exposure during neuronal differentiation modulates dopaminergic and cholinergic phenotypes in SH-SY5Y cells

BACKGROUND

Silica-ε-polycaprolactone-nanoparticles (SiPCL-NPs) represent a promising tool for laser-tissue soldering in the brain. After release of the SiPCL-NPs in the brain, neuronal differentiation might be modulated. The present study was performed to determine effects of SiPCL-NP-exposure at different stages of neuronal differentiation in neuron-like SH-SY5Y cells. The resulting phenotypes were analyzed quantitatively and signaling pathways involved in neuronal differentiation and degeneration were studied. SH-SY5Y cells were differentiated with all-trans retinoic acid or staurosporine to obtain predominantly cholinergic or dopaminergic neurons. The resulting phenotype was analyzed at the end of differentiation with and without the SiPCL-NPs given at various times during differentiation.

RESULTS

Exposure to SiPCL-NPs before and during differentiation led to a decreased cell viability of SH-SY5Y cells depending on the differentiation protocol used. SiPCL-NPs co-localized with the neuronal marker β-3-tubulin but did not alter the morphology of these cells. A significant decrease in the number of tyrosine hydroxylase (TH) immunoreactive neurons was found in staurosporine-differentiated cells when SiPCL-NPs were added at the end of the differentiation. TH-protein expression was also significantly downregulated when SiPCL-NPs were applied in the middle of differentiation. Protein expression of the marker for the dopamine active transporter (DAT) was not affected by SiPCL-NPs. SiPCL-NP-exposure predominantly decreased the expression of the high-affinity choline transporter 1 (CHT1) when the NPs were given before the differentiation. Pathways involved in neuronal differentiation, namely Akt, MAP-K, MAP-2 and the neurodegeneration-related markers β-catenin and GSK-3β were not altered by NP-exposure.

CONCLUSIONS

The decrease in the number of dopaminergic and cholinergic cells may implicate neuronal dysfunction, but the data do not provide evidence that pathways relevant for differentiation and related to neurodegeneration are impaired.

[Establishment of a novel evaluation system for dopaminergic axonal outgrowth and its regulatory factor]

The nigrostriatal dopaminergic pathway is implicated with Parkinson's disease. Elucidation of this projection mechanism is not only important for considering developmental brain formation, but also contributes to the development of a therapy for regenerating the lost neural circuit. Although several axon guidance cues have been reported to induce dopaminergic axons from the substantia nigra to the striatum, the mechanisms by which the dopaminergic axons extend in the striatum remain unclear. An excellent culture system is necessary for studying the formation process of a neural circuit. Therefore, we tried to establish an in vitro model for the quantitative analysis of dopaminergic innervation of striatal neurons using primary dissociated cells. Mesencephalic cells prepared from rat embryos were seeded on the opposite side to striatal cells with the isolation wall in between. When the isolation wall was removed, the dopaminergic axons extended toward the striatal cell region and formed synapses with striatal neurons. The dopaminergic innervation of striatal neurons was suppressed by inhibiting integrin α5β1 expressed on dopaminergic neurons. Furthermore, dopaminergic neurons overexpressing integrin α5 exhibited a longer neurite outgrowth on striatal cells than normal dopaminergic neurons did. Because this evaluation system using dissociated cell culture has relatively high throughput and is easy to be pharmacologically and genetically manipulated, it is considered to be a useful tool in the study of neural circuit formation. In addition, as a result, we found integrin α5β1 as a molecule promoting striatal innervation by dopaminergic neuron, which is expected to contribute to regeneration of the nigrostriatal dopaminergic projection.

AMP-activated protein kinase activation in mediating phenylalanine-induced neurotoxicity in experimental models of phenylketonuria

Phenylketonuria (PKU), one of the most prevalent autosomal recessive disorders of amino acid metabolism, is characterized by abnormal accumulation of phenylalanine, which can lead to intellectual disability. The main pathologic changes in the central nervous system of untreated phenylketonuric patients are reductions in the number of axons, dendrites, and synapses in the brain. Such alterations are thought to be mainly associated with the toxic effects caused by phenylalanine. However, the underlying molecular mechanisms have not been fully elucidated. The present study shows that a high concentration of phenylalanine remarkably inhibited neuronal neurite formation in vitro. Interestingly, AMP-activated protein kinase (AMPK), the energy status sensor, was activated in cultured cerebral cortical neurons upon phenylalanine treatment. Pretreatment with an AMPK inhibitor ameliorated the reduction of neurite formation caused by phenylalanine. In addition, the levels of the phosphorylated AMPK, the active form of AMPK, were significantly higher in the cerebral cortices of PKU mice with elevated phenylalanine levels in this brain region compared to those in wild-type control mice, whereas the density of dendritic spines on basal secondary dendrites of pyramidal neurons in prefrontal cortices of PKU mice was significantly decreased. Collectively, these findings indicate that AMPK activation is a key event in impaired neuronal dendritic development in PKU and consequently, a potential therapeutic target for developing neuroprotective strategies against phenylalanine-evoked brain injury in PKU.

Marine-derived protein kinase inhibitors for neuroinflammatory diseases

Neuroinflammation is primarily characterized by overexpression of proinflammatory mediators produced by glial activation or immune cell infiltration. Several kinases have been shown to be critical mediators in neuroinflammation. One of the largest groups of kinases is protein kinases, which have been the second most studied group of drug targets after G-protein-coupled receptors. Thus far, most of the approved kinase inhibitor drugs are adenosine triphosphate-competitive inhibitors with various off-target liabilities because of cross-reactivities; however, marine-derived compounds provide opportunities for discovering allosteric kinase inhibitors. This review summarizes the potential of marine-derived protein kinase inhibitors in the field of neuroinflammatory diseases, such as Parkinson disease, Alzheimer disease, multiple sclerosis, and pain. The previous studies from 1990 to 2017 in this review have shown that marine-derived protein kinase inhibitors have great potential to elicit anti-neuroinflammatory or neuroprotective responses in in vitro and in vivo models of neuroinflammatory diseases. This suggests that further exploration and investigation of these marine-derived protein kinase inhibitors on neuroinflammatory diseases are warranted. Therefore, this review may inspire further discovery of new protein kinase inhibitors from a marine origin and additional neuroscience studies focusing on these valuable marine-derived protein kinase inhibitors.

Emerging preclinical pharmacological targets for Parkinson's disease

Parkinson's disease (PD) is a progressive neurological condition caused by the degeneration of dopaminergic neurons in the basal ganglia. It is the most prevalent form of Parkinsonism, categorized by cardinal features such as bradykinesia, rigidity, tremors, and postural instability. Due to the multicentric pathology of PD involving inflammation, oxidative stress, excitotoxicity, apoptosis, and protein aggregation, it has become difficult to pin-point a single therapeutic target and evaluate its potential application. Currently available drugs for treating PD provide only symptomatic relief and do not decrease or avert disease progression resulting in poor patient satisfaction and compliance. Significant amount of understanding concerning the pathophysiology of PD has offered a range of potential targets for PD. Several emerging targets including AAV-hAADC gene therapy, phosphodiesterase-4, potassium channels, myeloperoxidase, acetylcholinesterase, MAO-B, dopamine, A2A, mGlu5, and 5-HT-1A/1B receptors are in different stages of clinical development. Additionally, alternative interventions such as deep brain stimulation, thalamotomy, transcranial magnetic stimulation, and gamma knife surgery, are also being developed for patients with advanced PD. As much as these therapeutic targets hold potential to delay the onset and reverse the disease, more targets and alternative interventions need to be examined in different stages of PD. In this review, we discuss various emerging preclinical pharmacological targets that may serve as a new promising neuroprotective strategy that could actually help alleviate PD and its symptoms.

mTOR-Dependent Cell Proliferation in the Brain

The mammalian Target of Rapamycin (mTOR) is a molecular complex equipped with kinase activity which controls cell viability being key in the PI3K/PTEN/Akt pathway. mTOR acts by integrating a number of environmental stimuli to regulate cell growth, proliferation, autophagy, and protein synthesis. These effects are based on the modulation of different metabolic pathways. Upregulation of mTOR associates with various pathological conditions, such as obesity, neurodegeneration, and brain tumors. This is the case of high-grade gliomas with a high propensity to proliferation and tissue invasion. Glioblastoma Multiforme (GBM) is a WHO grade IV malignant, aggressive, and lethal glioma. To date, a few treatments are available although the outcome of GBM patients remains poor. Experimental and pathological findings suggest that mTOR upregulation plays a major role in determining an aggressive phenotype, thus determining relapse and chemoresistance. Among several activities, mTOR-induced autophagy suppression is key in GBM malignancy. In this article, we discuss recent evidence about mTOR signaling and its role in normal brain development and pathological conditions, with a special emphasis on its role in GBM.

Microtubule affinity-regulating kinases are potential druggable targets for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects normal functions of the brain. Currently, AD is one of the leading causes of death in developed countries and the only one of the top ten diseases without a means to prevent, cure, or significantly slow down its progression. Therefore, newer therapeutic concepts are urgently needed to improve survival and the quality of life of AD patients. Microtubule affinity-regulating kinases (MARKs) regulate tau-microtubule binding and play a crucial role in neurons. However, their role in hyperphosphorylation of tau makes them potential druggable target for AD therapy. Despite the relevance of MARKs in AD pathogenesis, only a few small molecules are known to have anti-MARK activity and not much has been done to progress these compounds into therapeutic candidates. But given the diverse role of MARKs, the specificity of novel inhibitors is imperative for their successful translation from bench to bedside. In this regard, a recent co-crystal structure of MARK4 in association with a pyrazolopyrimidine-based inhibitor offers a potential scaffold for the development of more specific MARK inhibitors. In this manuscript, we review the biological role of MARKs in health and disease, and draw attention to the largely unexplored area of MARK inhibitors for AD.

Activation of Transient Receptor Potential Vanilloid 4 Impairs the Dendritic Arborization of Newborn Neurons in the Hippocampal Dentate Gyrus through the AMPK and Akt Signaling Pathways

Neurite growth is an important process for the adult hippocampal neurogenesis which is regulated by a specific range of the intracellular free Ca²⁺ concentration ([Ca²⁺]_i). Transient receptor potential vanilloid 4 (TRPV4) is a calcium-permeable channel and activation of it causes an increase in [Ca²⁺]_i. We recently reported that TRPV4 activation promotes the proliferation of stem cells in the adult hippocampal dentate gyrus (DG). The present study aimed to examine the effect of TRPV4 activation on the dendrite morphology of newborn neurons in the adult hippocampal DG. Here, we report that intracerebroventricular injection of the TRPV4 agonist GSK1016790A for 5 days (GSK1016790A-injected mice) reduced the number of doublecortin immunopositive (DCX⁺) cells and DCX⁺ fibers in the hippocampal DG, showing the impaired dendritic arborization of newborn neurons. The phosphorylated AMP-activated protein kinase (p-AMPK) protein level increased from 30 min to 2 h, and then decreased from 1 to 5 days after GSK1016790A injection. The phosphorylated protein kinase B (p-Akt) protein level decreased from 30 min to 5 days after GSK1016790A injection; this decrease was markedly attenuated by the AMPK antagonist compound C (CC), but not by the AMPK agonist AICAR. Moreover, the phosphorylated mammalian target of rapamycin (mTOR) and p70 ribosomal S6 kinase (p70S6k) protein levels were decreased by GSK1016790A; these changes were sensitive to 740 Y-P and CC. The phosphorylation of glycogen synthase kinase 3β (GSK3β) at Y²¹⁶ was increased by GSK1016790A, and this change was accompanied by increased phosphorylation of microtubule-associated protein 2 (MAP2) and collapsin response mediator protein-2 (CRMP-2). These changes were markedly blocked by 740 Y-P and CC. Finally, GSK1016790A-induced decrease of DCX⁺ cells and DCX⁺ fibers was markedly attenuated by 740 Y-P and CC, but was unaffected by AICAR. We conclude that TRPV4 activation impairs the dendritic arborization of newborn neurons through increasing AMPK and inhibiting Akt to inhibit the mTOR-p70S6k pathway, activate GSK3β and thereby result in the inhibition of MAP2 and CRMP-2 function.

Gold nanoparticles enhance the differentiation of embryonic stem cells into dopaminergic neurons via mTOR/p70S6K pathway

AIM

This study aimed to investigate the effect of gold nanoparticles (AuNPs) on differentiation of mouse embryonic stem cells (ESCs) into dopaminergic (DA) neurons and explore the possible underlying molecular mechanisms.

MATERIALS & METHODS

The efficiency of AuNPs on DA neuron differentiation was evaluated by observing fluorescence in TH promoter-engineered GFP-reporter ESCs, western blot and real-time PCR. The possible signal pathway was determined by western blot.

RESULTS

Compared with feeder-free control condition, AuNPs are able to enhance fate specification of ESCs into DA neurons. Moreover, mTOR/p70S6K signaling pathway was found involving in this AuNPs-mediated DA neuron differentiation.

CONCLUSION

Our findings may lead future insight investigation into the underlying mechanisms and potential application of AuNPs in stem cell research.

Integrin α5β1 expression on dopaminergic neurons is involved in dopaminergic neurite outgrowth on striatal neurons

During development, dopaminergic neurons born in the substantia nigra extend their axons toward the striatum. However, the mechanisms by which the dopaminergic axons extend the striatum to innervate their targets remain unclear. We previously showed that paired-cultivation of mesencephalic cells containing dopaminergic neurons with striatal cells leads to the extension of dopaminergic neurites from the mesencephalic cell region to the striatal cell region. The present study shows that dopaminergic neurites extended along striatal neurons in the paired-cultures of mesencephalic cells with striatal cells. The extension of dopaminergic neurites was suppressed by the pharmacological inhibition of integrin α5β1. Using lentiviral vectors, short hairpin RNA (shRNA)-mediated knockdown of integrin α5 in dopaminergic neurons suppressed the neurite outgrowth to the striatal cell region. In contrast, the knockdown of integrin α5 in non-dopaminergic mesencephalic and striatal cells had no effect. Furthermore, overexpression of integrin α5 in dopaminergic neurons differentiated from embryonic stem cells enhanced their neurite outgrowth on striatal cells. These results indicate that integrin α5β1 expression on dopaminergic neurons plays an important role in the dopaminergic neurite outgrowth on striatal neurons.

Effects of Rho Kinase Inhibitors on Grafts of Dopaminergic Cell Precursors in a Rat Model of Parkinson's Disease

In models of Parkinson's disease (PD), Rho kinase (ROCK) inhibitors have antiapoptotic and axon-stabilizing effects on damaged neurons, decrease the neuroinflammatory response, and protect against dopaminergic neuron death and axonal retraction. ROCK inhibitors have also shown protective effects against apoptosis induced by handling and dissociation of several types of stem cells. However, the effect of ROCK inhibitors on dopaminergic cell grafts has not been investigated. In the present study, treatment of dopaminergic cell suspension with ROCK inhibitors yielded significant decreases in the number of surviving dopaminergic neurons, in the density of graft-derived dopaminergic fibers, and in graft vascularization. Dopaminergic neuron death also markedly increased in primary mesencephalic cultures when the cell suspension was treated with ROCK inhibitors before plating, which suggests that decreased angiogenesis is not the only factor leading to cell death in grafts. Interestingly, treatment of the host 6-hydroxydopamine-lesioned rats with ROCK inhibitors induced a slight, nonsignificant increase in the number of surviving neurons, as well as marked increases in the density of graft-derived dopaminergic fibers and the size of the striatal reinnervated area. The study findings discourage treatment of cell suspensions before grafting. However, treatment of the host induces a marked increase in graft-derived striatal reinnervation. Because ROCK inhibitors have also exerted neuroprotective effects in several models of PD, treatment of the host with ROCK inhibitors, currently used against vascular diseases in clinical practice, before and after grafting may be a useful adjuvant to cell therapy in PD.

SIGNIFICANCE

Cell-replacement therapy is one promising therapy for Parkinson's disease (PD). However, many questions must be addressed before widespread application. Rho kinase (ROCK) inhibitors have been used in a variety of applications associated with stem cell research and may be an excellent strategy for improving survival of grafted neurons and graft-derived dopaminergic innervation. The present results discourage the treatment of suspensions of dopaminergic precursors with ROCK inhibitors in the pregrafting period. However, treatment of the host (patients with PD) with ROCK inhibitors, currently used against vascular diseases, may be a useful adjuvant to cell therapy in PD.

Rho Kinase and Dopaminergic Degeneration

The small GTP-binding protein Rho plays an important role in several cellular functions. RhoA, which is a member of the Rho family, initiates cellular processes that act on its direct downstream effector Rho-associated kinase (ROCK). ROCK inhibition protects against dopaminergic cell death induced by dopaminergic neurotoxins. It has been suggested that ROCK inhibition activates neuroprotective survival cascades in dopaminergic neurons. Axon-stabilizing effects in damaged neurons may represent another mechanism of neuroprotection of dopaminergic neurons by ROCK inhibition. However, it has been shown that microglial cells play a crucial role in neuroprotection by ROCK inhibition and that activation of microglial ROCK mediates major components of the microglial inflammatory response. Additional mechanisms such as interaction with autophagy may also contribute to the neuroprotective effects of ROCK inhibition. Interestingly, ROCK interacts with several brain factors that play a major role in dopaminergic neuron vulnerability such as NADPH-oxidase, angiotensin, and estrogen. ROCK inhibition may provide a new neuroprotective strategy for Parkinson’s disease. This is of particular interest because ROCK inhibitors are currently used against vascular diseases in clinical practice. However, it is necessary to develop more potent and selective ROCK inhibitors to reduce side effects and enhance the efficacy.

Inhibition of the microglial response is essential for the neuroprotective effects of Rho-kinase inhibitors on MPTP-induced dopaminergic cell death

Several recent studies have shown that activation of the RhoA/Rho-associated kinase (ROCK) pathway is involved in the MPTP-induced dopaminergic cell degeneration and possibly in Parkinson's disease. ROCK inhibitors have been suggested as candidate neuroprotective drugs for Parkinson's disease. However, the mechanism responsible for the increased survival of dopaminergic neurons after treatment with ROCK inhibitors is not clear. We exposed primary (neuron-glia) mesencephalic cultures, cultures of the MES 23.5 dopaminergic neuron cell line and primary mesencephalic cultures lacking microglial cells to the dopaminergic neurotoxin MPP+ and the ROCK inhibitor Y-27632 in order to study the effects of ROCK inhibition on dopaminergic cell loss and the length of neurites of surviving dopaminergic neurons. In primary (neuron-glia) cultures, simultaneous treatment with MPP+ and the ROCK inhibitor significantly reduced the loss of dopaminergic neurons. In the absence of microglia, treatment with the ROCK inhibitor did not induce a significant reduction in the dopaminergic cell loss. Treatment with the ROCK inhibitor induced a significant decrease in axonal retraction in primary cultures with and without microglia and in cultures of the MES 23.5 neuron cell line. In conclusion, inhibition of microglial ROCK is essential for the neuroprotective effects of ROCK inhibitors against cell death induced by the dopaminergic neurotoxin MPP+. In addition, ROCK inhibition induced a direct effect against axonal retraction in surviving neurons. However, the latter effect was not sufficient to cause a significant increase in the survival of dopaminergic neurons after treatment with MPP+.