Rolipram promotes remyelination possibly via MEK-ERK signal pathway in cuprizone-induced demyelination mouse. Exp Neurol. 2012;237:304-311. [PMID: 22836144 DOI: 10.1016/j.expneurol.2012.07.011]

Beyond PDE4 inhibition: A comprehensive review on downstream cAMP signaling in the central nervous system

Cyclic adenosine monophosphate (cAMP) is a key second messenger that regulates signal transduction pathways pivotal for numerous biological functions. Intracellular cAMP levels are spatiotemporally regulated by their hydrolyzing enzymes called phosphodiesterases (PDEs). It has been shown that increased cAMP levels in the central nervous system (CNS) promote neuroplasticity, neurotransmission, neuronal survival, and myelination while suppressing neuroinflammation. Thus, elevating cAMP levels through PDE inhibition provides a therapeutic approach for multiple CNS disorders, including multiple sclerosis, stroke, spinal cord injury, amyotrophic lateral sclerosis, traumatic brain injury, and Alzheimer's disease. In particular, inhibition of the cAMP-specific PDE4 subfamily is widely studied because of its high expression in the CNS. So far, the clinical translation of full PDE4 inhibitors has been hampered because of dose-limiting side effects. Hence, focusing on signaling cascades downstream activated upon PDE4 inhibition presents a promising strategy, offering novel and pharmacologically safe targets for treating CNS disorders. Yet, the underlying downstream signaling pathways activated upon PDE(4) inhibition remain partially elusive. This review provides a comprehensive overview of the existing knowledge regarding downstream mediators of cAMP signaling induced by PDE4 inhibition or cAMP stimulators. Furthermore, we highlight existing gaps and future perspectives that may incentivize additional downstream research concerning PDE(4) inhibition, thereby providing novel therapeutic approaches for CNS disorders.

Potential biomaterials and experimental animal models for inventing new drug delivery approaches in the neurodegenerative disorder: Multiple sclerosis

The tight junction of endothelial cells in the central nervous system (CNS) has an ideal characteristic, acting as a biological barrier that can securely regulate the movement of molecules in the brain. Tightly closed astrocyte cell junctions on blood capillaries are the blood-brain barrier (BBB). This biological barrier prohibits the entry of polar drugs, cells, and ions, which protect the brain from harmful toxins. However, delivering any therapeutic agent to the brain in neurodegenerative disorders (i.e., schizophrenia, multiple sclerosis, etc.) is extremely difficult. Active immune responses such as microglia, astrocytes, and lymphocytes cross the BBB and attack the nerve cells, which causes the demyelination of neurons. Therefore, there is a hindrance in transmitting electrical signals properly, resulting in blindness, paralysis, and neuropsychiatric problems. The main objective of this article is to shed light on the performance of biomaterials, which will help researchers to create nanocarriers that can cross the blood-brain barrier and achieve a therapeutic concentration of drugs in the CNS of patients with multiple sclerosis (MS). The present review focuses on the importance of biomaterials with diagnostic and therapeutic efficacy that can help enhance multiple sclerosis therapeutic potential. Currently, the development of MS in animal models is limited by immune responses, which prevent MS induction in healthy animals. Therefore, this article also showcases animal models currently used for treating MS. A future advance in developing a novel effective strategy for treating MS is now a potential area of research.

Phosphodiesterase (PDE) 4 inhibition boosts Schwann cell myelination in a 3D regeneration model

Phosphodiesterase 4 (PDE4) inhibitors have been extensively researched for their anti-inflammatory and neuroregenerative properties. Despite the known neuroplastic and myelin regenerative properties of nonselective PDE4 inhibitors on the central nervous system, the direct impact on peripheral remyelination and subsequent neuroregeneration has not yet been investigated. Therefore, to examine the possible therapeutic effect of PDE4 inhibition on peripheral glia, we assessed the differentiation of primary rat Schwann cells exposed in vitro to the PDE4 inhibitor roflumilast. To further investigate the differentiation promoting effects of roflumilast, we developed a 3D model of rat Schwann cell myelination that closely resembles the in vivo situation. Using these in vitro models, we demonstrated that pan-PDE4 inhibition using roflumilast significantly promoted differentiation of Schwann cells towards a myelinating phenotype, as indicated by the upregulation of myelin proteins, including MBP and MAG. Additionally, we created a unique regenerative model comprised of a 3D co-culture of rat Schwann cells and human iPSC-derived neurons. Schwann cells treated with roflumilast enhanced axonal outgrowth of iPSC-derived nociceptive neurons, which was accompanied by an accelerated myelination speed, thereby showing not only phenotypic but also functional changes of roflumilast-treated Schwann cells. Taken together, the PDE4 inhibitor roflumilast possesses a therapeutic benefit to stimulate Schwann cell differentiation and, subsequently myelination, as demonstrated in the biologically relevant in vitro platform used in this study. These results can aid in the development of novel PDE4 inhibition-based therapies in the advancement of peripheral regenerative medicine.

Artesunate inhibits PDE4 leading to intracellular cAMP accumulation, reduced ERK/MAPK signaling, and blockade of influenza A virus vRNP nuclear export

Influenza A viruses (IAV) have been a major cause of mortality. Given the potential for future deadly pandemics, effective drugs are needed for the treatment of severe influenzas, such as those caused by H5N1 IAV. The anti-malaria drugs artemisinin and its derivates, including artesunate (AS), have been reported to have broad antiviral activities. Here, we showed AS's antiviral activity against H5N1, H1N1, H3N2 and oseltamivir-resistant influenza A(H1N1)virus in vitro. Moreover, we showed that AS treatment significantly protected mice from lethal challenges with H1N1 and H5N1 IAV. Strikingly, the combination of AS and peramivir treatment significantly improved survival outcomes compared to their monotherapy with either AS or peramivir. Furthermore, we demonstrated mechanistically that AS affected the later stages of IAV replication and limited nuclear export of viral ribonucleoprotein (vRNP) complexes. In A549 cells, we demonstrated for the first time that AS treatment induced cAMP accumulation via inhibiting PDE4, and consequently reduced ERK phosphorylation and blocked IAV vRNP export, and thus suppressed IAV replication. These AS's effects were reversed by the pre-treatment with a cAMP inhibitor SQ22536. Our findings suggest that AS could serve as a novel IAV inhibitor by interfering vRNP nuclear export to prevent and treat IAV infection.

Selective PDE4 subtype inhibition provides new opportunities to intervene in neuroinflammatory versus myelin damaging hallmarks of multiple sclerosis

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS) characterized by focal inflammatory lesions and prominent demyelination. Even though the currently available therapies are effective in treating the initial stages of disease, they are unable to halt or reverse disease progression into the chronic progressive stage. Thus far, no repair-inducing treatments are available for progressive MS patients. Hence, there is an urgent need for the development of new therapeutic strategies either targeting the destructive immunological demyelination or boosting endogenous repair mechanisms. Using in vitro, ex vivo, and in vivo models, we demonstrate that selective inhibition of phosphodiesterase 4 (PDE4), a family of enzymes that hydrolyzes and inactivates cyclic adenosine monophosphate (cAMP), reduces inflammation and promotes myelin repair. More specifically, we segregated the myelination-promoting and anti-inflammatory effects into a PDE4D- and PDE4B-dependent process respectively. We show that inhibition of PDE4D boosts oligodendrocyte progenitor cells (OPC) differentiation and enhances (re)myelination of both murine OPCs and human iPSC-derived OPCs. In addition, PDE4D inhibition promotes in vivo remyelination in the cuprizone model, which is accompanied by improved spatial memory and reduced visual evoked potential latency times. We further identified that PDE4B-specific inhibition exerts anti-inflammatory effects since it lowers in vitro monocytic nitric oxide (NO) production and improves in vivo neurological scores during the early phase of experimental autoimmune encephalomyelitis (EAE). In contrast to the pan PDE4 inhibitor roflumilast, the therapeutic dose of both the PDE4B-specific inhibitor A33 and the PDE4D-specific inhibitor Gebr32a did not trigger emesis-like side effects in rodents. Finally, we report distinct PDE4D isoform expression patterns in human area postrema neurons and human oligodendroglia lineage cells. Using the CRISPR-Cas9 system, we confirmed that pde4d1/2 and pde4d6 are the key targets to induce OPC differentiation. Collectively, these data demonstrate that gene specific PDE4 inhibitors have potential as novel therapeutic agents for targeting the distinct disease processes of MS.

Multifaceted Roles of cAMP Signaling in the Repair Process of Spinal Cord Injury and Related Combination Treatments

Spinal cord injury (SCI) results in multiple pathophysiological processes, including blood–spinal cord barrier disruption, hemorrhage/ischemia, oxidative stress, neuroinflammation, scar formation, and demyelination. These responses eventually lead to severe tissue destruction and an inhibitory environment for neural regeneration.cAMP signaling is vital for neurite outgrowth and axonal guidance. Stimulating intracellular cAMP activity significantly promotes neuronal survival and axonal regrowth after SCI.However, neuronal cAMP levels in adult CNS are relatively low and will further decrease after injury. Targeting cAMP signaling has become a promising strategy for neural regeneration over the past two decades. Furthermore, studies have revealed that cAMP signaling is involved in the regulation of glial cell function in the microenvironment of SCI, including macrophages/microglia, reactive astrocytes, and oligodendrocytes. cAMP-elevating agents in the post-injury milieu increase the cAMP levels in both neurons and glial cells and facilitate injury repair through the interplay between neurons and glial cells and ultimately contribute to better morphological and functional outcomes. In recent years, combination treatments associated with cAMP signaling have been shown to exert synergistic effects on the recovery of SCI. Agents carried by nanoparticles exhibit increased water solubility and capacity to cross the blood–spinal cord barrier. Implanted bioscaffolds and injected hydrogels are potential carriers to release agents locally to avoid systemic side effects. Cell transplantation may provide permissive matrices to synergize with the cAMP-enhanced growth capacity of neurons. cAMP can also induce the oriented differentiation of transplanted neural stem/progenitor cells into neurons and increase the survival rate of cell grafts. Emerging progress focused on cAMP compartmentation provides researchers with new perspectives to understand the complexity of downstream signaling, which may facilitate the clinical translation of strategies targeting cAMP signaling for SCI repair.

Reversing Alzheimer's disease dementia with clemastine, fingolimod, or rolipram, plus anti-amyloid therapy

A few anti-amyloid trials offer a slight possibility of preventing progression of cognitive loss, but none has reversed the process. A possible reason is that amyloid may be necessary but insufficient in the pathogenesis of AD, and other causal factors may need addressing in addition to amyloid. It is argued here that drugs addressing myelination and synaptogenesis are the optimum partners for anti-amyloid drugs, since there is much evidence that early in the process that leads to AD, both neural circuits and synaptic activity are dysfunctional. Evidence to support this argument is presented. Evidence is also presented that clemastine, fingolimod, and rolipram, benefit both myelination and synaptogenesis. It is suggested that a regimen that includes one of them plus an anti-amyloid drug, could reverse AD.

Regulation of common neurological disorders by gut microbial metabolites

The gut is connected to the CNS by immunological mediators, lymphocytes, neurotransmitters, microbes and microbial metabolites. A mounting body of evidence indicates that the microbiome exerts significant effects on immune cells and CNS cells. These effects frequently result in the suppression or exacerbation of inflammatory responses, the latter of which can lead to severe tissue damage, altered synapse formation and disrupted maintenance of the CNS. Herein, we review recent progress in research on the microbial regulation of CNS diseases with a focus on major gut microbial metabolites, such as short-chain fatty acids, tryptophan metabolites, and secondary bile acids. Pathological changes in the CNS are associated with dysbiosis and altered levels of microbial metabolites, which can further exacerbate various neurological disorders. The cellular and molecular mechanisms by which these gut microbial metabolites regulate inflammatory diseases in the CNS are discussed. We highlight the similarities and differences in the impact on four major CNS diseases, i.e., multiple sclerosis, Parkinson's disease, Alzheimer's disease, and autism spectrum disorder, to identify common cellular and molecular networks governing the regulation of cellular constituents and pathogenesis in the CNS by microbial metabolites.

Clemastine Enhances Myelination, Delays Axonal Loss and Promotes Functional Recovery in Spinal Cord Injury

Recent evidence has shown that demyelination occurs along with axonal degeneration in spinal cord injury (SCI) during the secondary injury phase. Oligodendrocyte precursor cells (OPC) are present in the lesions but fail to differentiate into mature oligodendrocytes and form new myelin. Given the limited recovery of neuronal functions after SCI in adults without effective treatment available so far, it remains unknown whether enhancing OPC differentiation and myelination could benefit the recovery of SCI. To show the significance of myelin regeneration after SCI, the injury was treated with clemastine in the rat model. Clemastine is an FDA-approved drug that is potent in promoting oligodendrocyte differentiation and myelination in vivo, for four weeks following SCI. Motor function was assessed using sloping boards and grid walking tests and scored according to the Basso, Beattie, and Bresnahan protocol. The myelin integrity and protein expression were evaluated using transmission electron microscopy and immunofluorescence, respectively. The results indicated that clemastine treatment preserves myelin integrity, decreases loss of axons and improves functional recovery in the rat SCI model. The presented data suggest that myelination-enhancing strategies may serve as a potential therapeutic approach for the functional recovery in SCI.

The Effects of PDE Inhibitors on Multiple Sclerosis: a Review of in vitro and in vivo Models

BACKGROUND

Multiple sclerosis (MS) is a chronic inflammatory and immune-mediated disease, whose current therapeutic means are mostly effective in the relapsing-remitting form of MS, where inflammation is still prominent, but fall short of preventing long term impairment. However, apart from inflammationmediated demyelination, autoimmune mechanisms play a major role in MS pathophysiology, constituting a promising pharmacological target. Phosphodiesterase (PDE) inhibitors have been approved for clinical use in psoriasis and have undergone trials suggesting their neuroprotective effects, rendering them eligible as an option for accessory MS therapy.

OBJECTIVE

In this review, we discuss the potential role of PDE inhibitors as a complementary MS therapy.

METHODS

We conducted a literature search through which we screened and comparatively assessed papers on the effects of PDE inhibitor use, both in vitro and in animal models of MS, taking into account a number of inclusion and exclusion criteria.

RESULTS

In vitro studies indicated that PDE inhibitors promote remyelination and axonal sustenance, while curbing inflammatory cell infiltration, hindering oligodendrocyte and neuronal loss and suppressing cytokine production. In vivo studies underlined that these agents alleviate symptoms and reduce disease scores in MS animal models.

CONCLUSION

PDE inhibitors proved to be effective in addressing various aspects of MS pathogenesis both in vitro and in vivo models. Given the latest clinical trials proving that the PDE4 inhibitor Ibudilast exerts neuroprotective effects in patients with progressive MS, research on this field should be intensified and selective PDE4 inhibitors with enhanced safety features should be seriously considered as prospective complementary MS therapy.

Mechanisms and repair strategies for white matter degeneration in CNS injury and diseases

White matter degeneration is an important pathophysiological event of the central nervous system that is collectively characterized by demyelination, oligodendrocyte loss, axonal degeneration and parenchymal changes that can result in sensory, motor, autonomic and cognitive impairments. White matter degeneration can occur due to a variety of causes including trauma, neurotoxic exposure, insufficient blood flow, neuroinflammation, and developmental and inherited neuropathies. Regardless of the etiology, the degeneration processes share similar pathologic features. In recent years, a plethora of cellular and molecular mechanisms have been identified for axon and oligodendrocyte degeneration including oxidative damage, calcium overload, neuroinflammatory events, activation of proteases, depletion of adenosine triphosphate and energy supply. Extensive efforts have been also made to develop neuroprotective and neuroregenerative approaches for white matter repair. However, less progress has been achieved in this area mainly due to the complexity and multifactorial nature of the degeneration processes. Here, we will provide a timely review on the current understanding of the cellular and molecular mechanisms of white matter degeneration and will also discuss recent pharmacological and cellular therapeutic approaches for white matter protection as well as axonal regeneration, oligodendrogenesis and remyelination.

PDE inhibition in distinct cell types to reclaim the balance of synaptic plasticity

Synapses are the functional units of the brain. They form specific contact points that drive neuronal communication and are highly plastic in their strength, density, and shape. A carefully orchestrated balance between synaptogenesis and synaptic pruning, i.e., the elimination of weak or redundant synapses, ensures adequate synaptic density. An imbalance between these two processes lies at the basis of multiple neuropathologies. Recent evidence has highlighted the importance of glia-neuron interactions in the synaptic unit, emphasized by glial phagocytosis of synapses and local excretion of inflammatory mediators. These findings warrant a closer look into the molecular basis of cell-signaling pathways in the different brain cells that are related to synaptic plasticity. In neurons, intracellular second messengers, such as cyclic guanosine or adenosine monophosphate (cGMP and cAMP, respectively), are known mediators of synaptic homeostasis and plasticity. Increased levels of these second messengers in glial cells slow down inflammation and neurodegenerative processes. These multi-faceted effects provide the opportunity to counteract excessive synapse loss by targeting cGMP and cAMP pathways in multiple cell types. Phosphodiesterases (PDEs) are specialized degraders of these second messengers, rendering them attractive targets to combat the detrimental effects of neurological disorders. Cellular and subcellular compartmentalization of the specific isoforms of PDEs leads to divergent downstream effects for these enzymes in the various central nervous system resident cell types. This review provides a detailed overview on the role of PDEs and their inhibition in the context of glia-neuron interactions in different neuropathologies characterized by synapse loss. In doing so, it provides a framework to support future research towards finding combinational therapy for specific neuropathologies.

Rolipram Protects Mice from Gram-negative Bacterium Escherichia coli-induced Inflammation and Septic Shock

Sepsis is typically triggered by an overwhelming systemic inflammatory response to pathogens, and may lead to severe organ dysfunction and/or death. Sepsis consequently has a high mortality rate and a high rate of complications for survivors, despite modern medical advances. Therefore, drug identification and validation for the treatment of sepsis is of the utmost importance. As a selective phosphodiesterase-4 inhibitor, rolipram also exhibits the abilities of inhibiting multiple pro-inflammatory cytokines production in macrophages and toxin-induced inflammation in mice. However, this drug has never been studied as a sepsis treatment method. We found that rolipram significantly improves survival in mice challenged with gram-negative bacterium E. coli, CLP, or E. coli derived lipopolysaccharide. We have also found that rolipram inhibits organ damage, pro-inflammatory cytokine production, and intracellular migration of early-stage inflammatory elements. Our results also show that rolipram increases anti-inflammatory cytokine production. The protective effects of rolipram on septic mice may result from inhibition of the MAP kinase and NF-κB signaling pathways. Rolipram may therefore be a potential novel sepsis treatment, one that would bypass the time-consuming and costly drug-discovery process.

Mechanical Stretch of High Magnitude Provokes Axonal Injury, Elongation of Paranodal Junctions, and Signaling Alterations in Oligodendrocytes

Increasing findings suggest that demyelination may play an important role in the pathophysiology of brain injury, but the exact mechanisms underlying such damage are not well known. Mechanical tensile strain of brain tissue occurs during traumatic brain injury. Several studies have investigated the cellular and molecular events following a static tensile strain of physiological magnitude on individual cells such as oligodendrocytes. However, the pathobiological impact of high-magnitude mechanical strain on oligodendrocytes and myelinated fibers remains under investigated. In this study, we reported that an applied mechanical tensile strain of 30% on mouse organotypic culture of cerebellar slices induced axonal injury and elongation of paranodal junctions, two hallmarks of brain trauma. It was also able to activate MAPK-ERK1/2 signaling, a stretch-induced responsive pathway. The same tensile strain applied to mouse oligodendrocytes in primary culture induced a profound damage to cell morphology, partial cell loss, and a decrease of myelin protein expression. The lower tensile strain of 20% also caused cell loss and the remaining oligodendrocytes appeared retracted with decreased myelin protein expression. Finally, high-magnitude tensile strain applied to 158N oligodendroglial cells altered myelin protein expression, dampened MAPK-ERK1/2 and MAPK-p38 signaling, and enhanced the production of reactive oxygen species. The latter was accompanied by increased protein oxidation and an alteration of anti-oxidant defense that was strain magnitude-dependent. In conclusion, mechanical stretch of high magnitude provokes axonal injury with significant alterations in oligodendrocyte biology that could initiate demyelination.

High cholesterol triggers white matter alterations and cognitive deficits in a mouse model of cerebrovascular disease: benefits of simvastatin

Transgenic mice overexpressing transforming growth factor-β1 (TGF mice) display impaired cerebrovascular reactivity, cerebral hypoperfusion and neurovascular uncoupling, but no overt cognitive deficits until old age. Cardiovascular diseases are a major risk factor for vascular cognitive impairment and dementia (VCID). We investigated the impact of a high cholesterol diet (HCD) on cerebrovascular and cognitive function in adult (6 months) and aged (12 months) TGF mice, together with the potential benefit of simvastatin (SV), an anti-cholesterol drug with pleiotropic effects, in adult mice. HCD increased blood, but not brain, cholesterol levels in treated mice, which SV did not reduce. In WT mice, HCD induced small, albeit significant, impairment in endothelium-dependent dilatory function. In TGF mice, HCD worsened the established brain vessel dilatory dysfunction in an age-dependent manner and increased the number of string vessels in the white matter (WM), alterations respectively normalized and significantly countered by SV. HCD triggered cognitive decline only in TGF mice at both ages, a deficit prevented by SV. Concurrently, HCD upregulated galectin−3 immunoreactivity in WM microglial cells, a response significantly reduced in SV-treated TGF mice. Grey matter astrogliosis and microgliosis were not affected by HCD or SV. In the subventricular zone of adult HCD-treated TGF mice, SV promoted oligogenesis and migration of oligodendrocyte progenitor cells. The results demonstrate that an underlying cerebrovascular pathology increases vulnerability to cognitive failure when combined to another risk factor for dementia, and that WM alterations are associated with this loss of function. The results further indicate that myelin repair mechanisms, as triggered by SV, may bear promise in preventing or delaying cognitive decline related to VCID.

Adenosine Promotes the Recovery of Mice from the Cuprizone-Induced Behavioral and Morphological Changes while Effecting on Microglia and Inflammatory Cytokines in the Brain

Recent studies have shown that multiple sclerosis (MS) and schizophrenia share similarities in some respects, including white matter damage and neuroinflammation. On the other hand, adenosine was reported to promote oligodendrocyte precursor maturation and remyelinating while influencing microglia activation. The aim of the present study was to examine possible beneficial effects of adenosine on the recovery of cuprizone (CPZ)-exposed mouse which has been used as an animal model of MS and schizophrenia as the CPZ-exposed mouse presents demyelination, oligodendrocyte loss, microglia accumulation, as well as behavioral changes. As reported previously, C57BL/6 mice, after fed CPZ for 5 weeks, showed salient demyelination and oligodendrocyte loss in the cerebral cortex (CTX) and hippocampus, in addition to displaying anxiety-like behavior, spatial working memory deficit, and social interaction impairment. Administration of adenosine for 7 days during the recovery period after CPZ withdrawal promoted the behavioral recovery of CPZ-exposed mice and accelerated the remyelinating process in the brains of mice after CPZ withdrawal in a dose-dependent manner. In addition, the effective dose (10 mg/kg) of adenosine inhibited microglia activation and suppressed abnormal elevation of the pro-inflammatory cytokines IL-1β and TNF-α in CTX and hippocampus, but increased levels of the anti-inflammatory cytokines IL-4 or IL-10 in the same brain regions during the remyelinating process. These results provided an evidence-based rationale for the application of adenosine or its analogues as add-on therapy for schizophrenia.

N-Cadherin is Involved in Neuronal Activity-Dependent Regulation of Myelinating Capacity of Zebrafish Individual Oligodendrocytes In Vivo

Stimulating neuronal activity increases myelin sheath formation by individual oligodendrocytes, but how myelination is regulated by neuronal activity in vivo is still not fully understood. While in vitro studies have revealed the important role of N-cadherin in myelination, our understanding in vivo remains quite limited. To obtain the role of N-cadherin during activity-dependent regulation of myelinating capacity of individual oligodendrocytes, we successfully built an in vivo dynamic imaging model of the Mauthner cell at the subcellular structure level in the zebrafish central nervous system. Enhanced green fluorescent protein (EGFP)-tagged N-cadherin was used to visualize the stable accumulations and mobile transports of N-cadherin by single-cell electroporation at the single-cell level. We found that pentylenetetrazol (PTZ) significantly enhanced the accumulation of N-cadherin in Mauthner axons, a response that was paralleled by enhanced sheath number per oligodendrocytes. By offsetting this phenotype using oligopeptide (AHAVD) which blocks the function of N-cadherin, we showed that PTZ regulates myelination in an N-cadherin-dependent manner. What is more, we further suggested that PTZ influences N-cadherin and myelination via a cAMP pathway. Consequently, our data indicated that N-cadherin is involved in neuronal activity-dependent regulation of myelinating capacity of zebrafish individual oligodendrocytes in vivo.

G protein-coupled receptor 37 is a negative regulator of oligodendrocyte differentiation and myelination

While the formation of myelin by oligodendrocytes is critical for the function of the central nervous system, the molecular mechanism controlling oligodendrocyte differentiation remains largely unknown. Here we identify G protein-coupled receptor 37 (GPR37) as an inhibitor of late-stage oligodendrocyte differentiation and myelination. GPR37 is enriched in oligodendrocytes and its expression increases during their differentiation into myelin forming cells. Genetic deletion of Gpr37 does not affect the number of oligodendrocyte precursor cells, but results in precocious oligodendrocyte differentiation and hypermyelination. The inhibition of oligodendrocyte differentiation by GPR37 is mediated by suppression of an exchange protein activated by cAMP (EPAC)-dependent activation of Raf-MAPK-ERK1/2 module and nuclear translocation of ERK1/2. Our data suggest that GPR37 regulates central nervous system myelination by controlling the transition from early-differentiated to mature oligodendrocytes.

The molecular mechanism controlling oligodendrocyte differentiation is not fully understood. Here the authors show that G protein coupled receptor 37 acts as a negative regulator of CNS myelination, and this effect is mediated by suppression of ERK signalling.

Intracellular signaling pathway regulation of myelination and remyelination in the CNS

The restoration of myelin sheaths on demyelinated axons remains a major obstacle in the treatment of multiple sclerosis (MS). Currently approved therapies work by modulating the immune system to reduce the number and rate of lesion formation but are only partially effective since they are not able to restore lost myelin. In the healthy CNS, myelin continues to be generated throughout life and spontaneous remyelination occurs readily in response to insults. In patients with MS, however, remyelination eventually fails, at least in part as a result of a failure of oligodendrocyte precursor cell (OPC) differentiation and the subsequent production of new myelin. A better understanding of the molecular mechanisms and signaling pathways that drive the process of myelin sheath formation is therefore important in order to speed the development of novel therapeutics designed to target remyelination. Here we review data supporting critical roles for three highly conserved intracellular signaling pathways: Wnt/β-catenin, PI3K/AKT/mTOR, and ERK/MAPK in the regulation of OPC differentiation and myelination both during development and in remyelination. Potential points of crosstalk between the three pathways and important areas for future research are also discussed.

The axon-glia unit in white matter stroke: mechanisms of damage and recovery

Approximately one quarter of all strokes in humans occur in white matter, and the progressive nature of white matter lesions often results in severe physical and mental disability. Unlike cortical grey matter stroke, the pathology of white matter stroke revolves around disrupted connectivity and injured axons and glial cells, rather than neuronal cell bodies. Consequently, the mechanisms behind ischemic damage to white matter elements, the regenerative responses of glial cells and their signaling pathways, all differ significantly from those in grey matter. Development of effective therapies for white matter stroke would require an enhanced understanding of the complex cellular and molecular interactions within the white matter, leading to the identification of new therapeutic targets. This review will address the unique properties of the axon-glia unit during white matter stroke, describe the challenging process of promoting effective white matter repair, and discuss recently-identified signaling pathways which may hold potential targets for repair in this disease. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

Translational control of myelin basic protein expression by ERK2 MAP kinase regulates timely remyelination in the adult brain

Successful myelin repair in the adult CNS requires the robust and timely production of myelin proteins to generate new myelin sheaths. The underlying regulatory mechanisms and complex molecular basis of myelin regeneration, however, remain poorly understood. Here, we investigate the role of ERK MAP kinase signaling in this process. Conditional deletion of Erk2 from cells of the oligodendrocyte lineage resulted in delayed remyelination following demyelinating injury to the adult mouse corpus callosum. The delayed repair occurred as a result of a specific deficit in the translation of the major myelin protein, MBP. In the absence of ERK2, activation of the ribosomal protein S6 kinase (p70S6K) and its downstream target, ribosomal protein S6 (S6RP), was impaired at a critical time when premyelinating oligodendrocytes were transitioning to mature cells capable of generating new myelin sheaths. Thus, we have described an important link between the ERK MAP kinase signaling cascade and the translational machinery specifically in remyelinating oligodendrocytes in vivo. These results suggest an important role for ERK2 in the translational control of MBP, a myelin protein that appears critical for ensuring the timely generation of new myelin sheaths following demyelinating injury in the adult CNS.

The roles of extracellular related-kinases 1 and 2 signaling in CNS myelination

Substantial progress has been made in identifying the intracellular signaling pathways that regulate central nervous system myelination. Recently, the mitogen activated protein kinase pathway, in particular the extracellular signal-related kinase 1 (Erk1) and Erk2, have been identified as critically important in mediating the effects of several growth factors that regulate oligodendroglial development and myelination. Here we will review the recent studies that identify the key role that Erk1/2 signaling plays in regulating oligodendroglial development, myelination and remyelination, discuss the potential mechanisms that Erk1/2 may utilize to influence myelination, and highlight some questions for further research. This article is part of the Special Issue entitled 'Oligodendrocytes in Health and Disease'.

Targeting remyelination treatment for multiple sclerosis

Since disability in multiple sclerosis (MS) is a product of neurodegeneration and deficient remyelination, the ability to enhance neuroregeneration and myelin regeneration in MS is an enticing goal for MS drug development. In particular, remyelination treatments could promote return of neurological function and also prevent further axonal loss and neurodegeneration in MS due to trophic effects of myelin. The study of remyelination has advanced dramatically in the last several years such that a number of pathways inhibiting remyelination have been discovered, including those involving LINGO-1, Notch-1, hyaluronan, retinoid X receptor, and wnt/ß-catenin. Other approaches such as high throughput drug screening for remyelination drugs have caught fire, with identification of dozens of known drugs with oligodendrocyte maturation stimulatory effects. Several drugs identified through screens and other mechanisms are in the process of being further evaluated for remyelination in MS and MS models. We discuss the potential molecular targets and the variety of mechanisms towards drug identification and development in remyelination for MS.

Epac and the high affinity rolipram binding conformer of PDE4 modulate neurite outgrowth and myelination using an in vitro spinal cord injury model

Background and Purpose

cAMP and pharmacological inhibition of PDE4, which degrades it, are promising therapeutic targets for the treatment of spinal cord injury (SCI). Using our previously described in vitro SCI model, we studied the mechanisms by which cAMP modulators promote neurite outgrowth and myelination using enantiomers of the PDE4-specific inhibitor rolipram and other modulators of downstream signalling effectors.

Experimental Approach

Rat mixed neural cell myelinating cultures were cut with a scalpel and treated with enantiomers of the PDE4-specific inhibitor rolipram, Epac agonists and PKA antagonists. Neurite outgrowth, density and myelination were assessed by immunocytochemistry and cytokine levels analysed by qPCR.

Key Results

Inhibition of the high-affinity rolipram-binding state (HARBS), rather than the low-affinity rolipram binding state (LARBS) PDE4 conformer promoted neurite outgrowth and myelination. These effects were mediated through the activation of Epac and not through PKA. Expression of the chemokine CXCL10, known to inhibit myelination, was markedly elevated in astrocytes after Rho inhibition and this was blocked by inhibition of Rho kinase or PDE4.

Conclusions and Implications

PDE4 inhibitors targeted at the HARBS conformer or Epac agonists may provide promising novel targets for the treatment of SCI. Our study demonstrates the differential mechanisms of action of these compounds, as well as the benefit of a combined pharmacological approach and highlighting potential promising targets for the treatment of SCI. These findings need to be confirmed in vivo.

Protective Effect of a cAMP Analogue on Behavioral Deficits and Neuropathological Changes in Cuprizone Model of Demyelination

Multiple sclerosis (MS) is an inflammatory demyelinating disease that leads to neuronal cell loss. Cyclic AMP and its analogs are well known to decrease inflammation and apoptosis. In the present study, we examined the effects of bucladesine, a cell-permeable analogue of cyclic adenosine monophosphate (cAMP), on myelin proteins (PLP, PMP-22), inflammation, and apoptotic, as well as anti-apoptotic factors in cuprizone model of demyelination. C57BL/6J mice were fed with chow containing 0.2% copper chelator cuprizone or vehicle by daily oral gavage for 5 weeks to induce reversible demyelination predominantly of the corpus callosum. Bucladesine was administered intraperitoneally at different doses (0.24, 0.48, or 0.7 μg/kg body weight) during the last 7 days of 5-week cuprizone treatment. Bucladesine exhibited a protective effect on myelination. Furthermore, bucladesine significantly decreased the production of interleukin-6 pro-inflammatory mediator as well as nuclear factor-κB activation and reduced the mean number of apoptotic cells compared to cuprizone-treated mice. Bucladesine also decreased production of caspase-3 as well as Bax and increased Bcl-2 levels. Our data revealed that enhancement of intracellular cAMP prevents demyelination and plays anti-inflammatory and anti-apoptotic properties in mice cuprizone model of demyelination. This suggests the modulation of intracellular cAMP as a potential target for treatment of MS.

Cyclic phosphatidic acid treatment suppress cuprizone-induced demyelination and motor dysfunction in mice

Multiple sclerosis is a chronic demyelinating disease of the central nervous system leading to progressive cognitive and motor dysfunction, which is characterized by neuroinflammation, demyelination, astrogliosis, loss of oligodendrocytes, and axonal pathologies. Cyclic phosphatidic acid (cPA) is a naturally occurring phospholipid mediator with a unique cyclic phosphate ring structure at the sn-2 and sn-3 positions of the glycerol backbone. cPA elicits a neurotrophin-like action and protects hippocampal neurons from ischemia-induced delayed neuronal death. In this study, we investigated the effects of cPA on cuprizone-induced demyelination, which is a model of multiple sclerosis. Mice were fed a diet containing 0.2% cuprizone for 5 weeks, which induces severe demyelination, astrocyte and microglial activation, and motor dysfunction. Simultaneous administration of cPA effectively attenuated cuprizone-induced demyelination, glial activation, and motor dysfunction. These data indicate that cPA may be a useful treatment to reduce the extent of demyelination and the severity of motor dysfunction in multiple sclerosis. cPA is a potential lead compound in the development of drugs for the treatment of this devastating disease.

Phosphodiesterase type 4 inhibitor rolipram improves survival of spiral ganglion neurons in vitro

Sensorineural deafness is caused by damage of hair cells followed by degeneration of the spiral ganglion neurons and can be moderated by cochlear implants. However, the benefit of the cochlear implant depends on the excitability of the spiral ganglion neurons. Therefore, current research focuses on the identification of agents that will preserve their degeneration. In this project we investigated the neuroprotective effect of Rolipram as a promising agent to improve the viability of the auditory neurons. It is a pharmaceutical agent that acts by selective inhibition of the phosphodiesterase 4 leading to an increase in cyclic AMP. Different studies reported a neuroprotective effect of Rolipram. However, its significance for the survival of SGN has not been reported so far. Thus, we isolated spiral ganglion cells of neonatal rats for cultivation with different Rolipram concentrations and determined the neuronal survival rate. Furthermore, we examined immunocytologically distinct proteins that might be involved in the neuroprotective signalling pathway of Rolipram and determined endogenous BDNF by ELISA. When applied at a concentration of 0.1 nM, Rolipram improved the survival of SGN in vitro. According to previous studies, our immunocytological data showed that Rolipram application induces the phosphorylation and thereby activation of the transcription factor CREB. This activation can be mediated by the cAMP-PKA-signalling pathway as well as via ERK as a part of the MAP-kinase pathway. However, only in cultures pre-treated with BDNF, an endogenous increase of BDNF was detected. We conclude that Rolipram has the potential to improve the vitality of neonatal auditory nerve cells in vitro. Further investigations are necessary to prove the effect of Rolipram in vivo in the adult organism after lesion of the hair cells and insertion of cochlear implants.

Prostacyclin promotes oligodendrocyte precursor recruitment and remyelination after spinal cord demyelination

Adult oligodendrocyte precursor cells (OPCs) are located adjacent to demyelinated lesion and contribute to myelin repair. The crucial step in remyelination is the migration of OPCs to the demyelinated area; however, the mechanism of OPC migration remains to be fully elucidated. Here we show that prostacyclin (prostaglandin I2, PGI2) promotes OPC migration, thereby promoting remyelination and functional recovery in mice after demyelination induced by injecting lysophosphatidylcholine (LPC) into the spinal cord. Prostacyclin analogs enhanced OPC migration via a protein kinase A (PKA)-dependent mechanism, and prostacyclin synthase expression was increased in the spinal cord after LPC injection. Notably, pharmacological inhibition of prostacyclin receptor (IP receptor) impaired remyelination and motor recovery, whereas the administration of a prostacyclin analog promoted remyelination and motor recovery after LPC injection. Our results suggest that prostacyclin could be a key molecule for facilitating the migration of OPCs that are essential for repairing demyelinated areas, and it may be useful in treating disorders characterized by demyelination.

Inhibition of endogenous phosphodiesterase 7 promotes oligodendrocyte precursor differentiation and survival

During the development of the central nervous system (CNS), oligodendrocyte precursors (OPCs) are generated in specific sites within the neural tube and then migrate to colonize the entire CNS, where they differentiate into myelin-forming oligodendrocytes. Demyelinating diseases such as multiple sclerosis (MS) are characterized by the death of these cells. The CNS reacts to demyelination and by promoting spontaneous remyelination, an effect mediated by endogenous OPCs, cells that represent approximately 5-7 % of the cells in the adult brain. Numerous factors influence oligodendrogliogenesis and oligodendrocyte differentiation, including morphogens, growth factors, chemotropic molecules, extracellular matrix proteins, and intracellular cAMP levels. Here, we show that during development and in early adulthood, OPCs in the murine cerebral cortex contain phosphodiesterase-7 (PDE7) that metabolizes cAMP. We investigated the effects of different PDE7 inhibitors (the well-known BRL-50481 and two new ones, TC3.6 and VP1.15) on OPC proliferation, survival, and differentiation. While none of the PDE7 inhibitors analyzed altered OPC proliferation, TC3.6 and VP1.15 enhanced OPC survival and differentiation, processes in which ERK intracellular signaling played a key role. PDE7 expression was also observed in OPCs isolated from adult human brains and the differentiation of these OPCs into more mature oligodendroglial phenotypes was accelerated by treatment with both new PDE7 inhibitors. These findings reveal new roles for PDE7 in regulating OPC survival and differentiation during brain development and in adulthood, and they may further our understanding of myelination and facilitate the development of therapeutic remyelination strategies for the treatment of MS.

Cuprizone-induced demyelination as a tool to study remyelination and axonal protection

In the brain of multiple sclerosis (MS) patients, the conduction block of axons due to demyelination and inflammation underlies early neurological symptoms, whereas axonal transection accounts for permanent deficits occurring during later disease stages. The beneficial function of myelin for the protection of the axonal compartment and network stability between neurons has been shown in numerous studies. Thus, rapid and adequate remyelination is an important factor for axonal patronage during neuroinflammatory conditions. In this review article, we discuss frequently used experimental in vivo and in vitro animal models to examine remyelination and repair in MS. The focus of the discussion is the relevance of the toxin model 'cuprizone' to study the pathology of demyelination and the physiology of remyelination. This also includes recent findings in this animal model which implicate that axonal damage is an ongoing process independent of the initiation of endogenous remyelination.

Neuronal activity promotes myelination via a cAMP pathway

Neuronal activity promotes myelination in vivo and in vitro. However, the molecular events that mediate activity-dependent myelination are not completely understood. Seven, daily 1 h sessions of patterned electrical stimulation (ESTIM) promoted myelin segment formation in mixed cultures of dorsal root ganglion (DRG) neurons and oligodendrocytes (OLs); the increase in myelination was frequency-dependent. Myelin segment formation was also enhanced following exposure of DRGs to ESTIM prior to OL addition, suggesting that ESTIM promotes myelination in a manner involving neuron-specific signaling. Cyclic adenosine monophosphate (cAMP) levels in DRGs were increased three-fold following ESTIM, and artificially increasing cAMP mimicked the ability of ESTIM to promote myelination. Alternatively, inhibiting the cAMP pathway suppressed ESTIM-induced myelination. We used compartmentalized, microfluidic platforms to isolate DRG soma from OLs and assessed cell-type specific effects of ESTIM on myelination. A selective increase or decrease in DRG cAMP levels resulted in enhanced or suppressed myelination, respectively. This work describes a novel role for the cAMP pathway in neurons that results in enhanced myelination.

Myelin management by the 18.5-kDa and 21.5-kDa classic myelin basic protein isoforms

The classic myelin basic protein (MBP) splice isoforms range in nominal molecular mass from 14 to 21.5 kDa, and arise from the gene in the oligodendrocyte lineage (Golli) in maturing oligodendrocytes. The 18.5-kDa isoform that predominates in adult myelin adheres the cytosolic surfaces of oligodendrocyte membranes together, and forms a two-dimensional molecular sieve restricting protein diffusion into compact myelin. However, this protein has additional roles including cytoskeletal assembly and membrane extension, binding to SH3-domains, participation in Fyn-mediated signaling pathways, sequestration of phosphoinositides, and maintenance of calcium homeostasis. Of the diverse post-translational modifications of this isoform, phosphorylation is the most dynamic, and modulates 18.5-kDa MBP's protein-membrane and protein-protein interactions, indicative of a rich repertoire of functions. In developing and mature myelin, phosphorylation can result in microdomain or even nuclear targeting of the protein, supporting the conclusion that 18.5-kDa MBP has significant roles beyond membrane adhesion. The full-length, early-developmental 21.5-kDa splice isoform is predominantly karyophilic due to a non-traditional P-Y nuclear localization signal, with effects such as promotion of oligodendrocyte proliferation. We discuss in vitro and recent in vivo evidence for multifunctionality of these classic basic proteins of myelin, and argue for a systematic evaluation of the temporal and spatial distributions of these protein isoforms, and their modified variants, during oligodendrocyte differentiation.

Dibutyryl cyclic AMP inhibits the progression of experimental autoimmune encephalomyelitis and potentiates recruitment of endogenous neural stem cells

Multiple sclerosis is a chronic inflammatory demyelinating disease of the central nervous system. Cyclic AMP and its analogs enhance regeneration of adult mammalian central nervous system (CNS). Endogenous neural stem cells (NSCs) play a pivotal role in CNS regeneration, producing new neuron and glial cells. Here, we examined the effect of dibutyryl cyclic AMP (dbcAMP) on experimental autoimmune encephalomyelitis (EAE) symptoms, endogenous remyelination, and recruitment of NSCs. EAE was induced by immunizing mice using myelin oligodendrocyte glycoprotein peptide and pertussis toxin. Proliferative cells within CNS were labeled using repetitive systemic injections of 5-bromo-2-deoxyuridine (BrdU) before EAE induction. Myelin staining was performed using Luxol fast blue. The number of nestin(+) and BrdU(+) cells in subventricular zone (SVZ) and olfactory bulb (OB) was evaluated using immunohistochemistry. dbcAMP suppressed EAE progression and decreased the extent of demyelinated plaques in the lumbar spinal cord. EAE induction reduced the number of proliferative cells in SVZ and increased their population in OB. EAE also increased the number of nestin(+) cells in OB. We also found that dbcAMP increased the recruitment of NSCs into the OB and brain parenchyma of EAE mice. Our results suggest dbcAMP as a potential therapy for inducing myelin repair in the context of demyelinating diseases like multiple sclerosis. Its positive effect seems to be mediated, at least partially, by endogenous neural stem cells and their increased recruitment.

Nucleus-localized 21.5-kDa myelin basic protein promotes oligodendrocyte proliferation and enhances neurite outgrowth in coculture, unlike the plasma membrane-associated 18.5-kDa isoform

The classic myelin basic protein (MBP) family of central nervous system (CNS) myelin arises from transcription start site 3 of the Golli (gene of oligodendrocyte lineage) complex and comprises splice isoforms ranging in nominal molecular mass from 14 kDa to (full-length) 21.5 kDa. We have determined here a number of distinct functional differences between the major 18.5-kDa and minor 21.5-kDa isoforms of classic MBP with respect to oligodendrocyte (OLG) proliferation. We have found that, in contrast to 18.5-kDa MBP, 21.5-kDa MBP increases proliferation of early developmental immortalized N19-OLGs by elevating the levels of phosphorylated ERK1/2 and Akt1 kinases and of ribosomal protein S6. Coculture of N2a neuronal cells with N19-OLGs transfected with the 21.5-kDa isoform (or conditioned medium from), but not the 18.5-kDa isoform, caused the N2a cells to have increased neurite outgrowth and process branching complexity. These roles were dependent on subcellular localization of 21.5-kDa MBP to the nucleus and on the exon II-encoded segment, suggesting that the nuclear localization of early minor isoforms of MBP may play a crucial role in regulating and/or initiating myelin and neuronal development in the mammalian CNS.