Inhibition of endogenous phosphodiesterase 7 promotes oligodendrocyte precursor differentiation and survival

Mapping the glial transcriptome in Huntington's disease using snRNAseq: selective disruption of glial signatures across brain regions

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease with a fatal outcome. There is accumulating evidence of a prominent role of glia in the pathology of HD, and we investigated this by conducting single nuclear RNA sequencing (snRNAseq) of human post mortem brain in four differentially affected regions; caudate nucleus, frontal cortex, hippocampus and cerebellum. Across 127,205 nuclei from donors with HD and age/sex matched controls, we found heterogeneity of glia which is altered in HD. We describe prominent changes in the abundance of certain subtypes of astrocytes, microglia, oligodendrocyte precursor cells and oligodendrocytes between HD and control samples, and these differences are widespread across brain regions. Furthermore, we highlight possible mechanisms that characterise the glial contribution to HD pathology including depletion of myelinating oligodendrocytes, an oligodendrocyte-specific upregulation of the calmodulin-dependent 3',5'-cyclic nucleotide phosphodiesterase 1 A (PDE1A) and an upregulation of molecular chaperones as a cross-glial signature and a potential adaptive response to the accumulation of mutant huntingtin (mHTT). Our results support the hypothesis that glia have an important role in the pathology of HD, and show that all types of glia are affected in the disease.

Upregulation of Phosphodiesterase 7A Contributes to Concurrent Pain and Depression via Inhibition of cAMP-PKA-CREB-BDNF Signaling and Neuroinflammation in the Hippocampus of Mice

BACKGROUND

Phosphodiesterases (PDEs) are enzymes that catalyze the hydrolysis of cyclic adenosine monophosphate AMP (cAMP) and/or cyclic guanosine monophosphate (cGMP). PDE inhibitors can mitigate chronic pain and depression when these disorders occur individually; however, there is limited understanding of their role in concurrent chronic pain and depression. We aimed to evaluate the mechanisms of action of PDE using 2 mouse models of concurrent chronic pain and depression.

METHODS

C57BL/6J mice were subjected to partial sciatic nerve ligation (PSNL) to induce chronic neuropathic pain or injected with complete Freund's adjuvant (CFA) to induce inflammatory pain, and both animals showed depression-like behavior. First, we determined the change in PDE expression in both animal models. Next, we determined the effect of PDE7 inhibitor BRL50481 or hippocampal PDE7A knockdown on PSNL- or CFA-induced chronic pain and depression-like behavior. We also investigated the role of cAMP-protein kinase A (PKA)-cAMP response element binding protein (CREB)-brain-derived neurotrophic factor (BDNF) signaling and neuroinflammation in the effect of PDE7A inhibition on PSNL- or CFA-induced chronic pain and depression-like behavior.

RESULTS

This induction of chronic pain and depression in the 2 animal models upregulated hippocampal PDE7A. Oral administration of PDE7 inhibitor, BRL50481, or hippocampal PDE7A knockdown significantly reduced mechanical hypersensitivity and depression-like behavior. Hippocampal PDE7 inhibition reversed PSNL- or CFA-induced downregulation of cAMP and BDNF and the phosphorylation of PKA, CREB, and p65. cAMP agonist forskolin reversed these changes and caused milder behavioral symptoms of pain and depression. BRL50481 reversed neuroinflammation in the hippocampus in PSNL mice.

CONCLUSIONS

Hippocampal PDE7A mediated concurrent chronic pain and depression in both mouse models by inhibiting cAMP-PKA-CREB-BDNF signaling. Inhibiting PDE7A or activating cAMP-PKA-CREB-BDNF signaling are potential strategies to treat concurrent chronic pain and depression.

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.

Glycogen Synthase Kinase-3β Inhibitor VP3.15 Ameliorates Neurogenesis, Neuronal Loss and Cognitive Impairment in a Model of Germinal Matrix-intraventricular Hemorrhage of the Preterm Newborn

Advances in neonatology have significantly reduced mortality rates due to prematurity. However, complications of prematurity have barely changed in recent decades. Germinal matrix-intraventricular hemorrhage (GM-IVH) is one of the most severe complications of prematurity, and these children are prone to suffer short- and long-term sequelae, including cerebral palsy, cognitive and motor impairments, or neuropsychiatric disorders. Nevertheless, GM-IVH has no successful treatment. VP3.15 is a small, heterocyclic molecule of the 5-imino-1,2,4-thiadiazole family with a dual action as a phosphodiesterase 7 and glycogen synthase kinase-3β (GSK-3β) inhibitor. VP3.15 reduces neuroinflammation and neuronal loss in other neurodegenerative disorders and might ameliorate complications associated with GM-IVH. We administered VP3.15 to a mouse model of GM-IVH. VP3.15 reduces the presence of hemorrhages and microglia in the short (P14) and long (P110) term. It ameliorates brain atrophy and ventricle enlargement while limiting tau hyperphosphorylation and neuronal and myelin basic protein loss. VP3.15 also improves proliferation and neurogenesis as well as cognition after the insult. Interestingly, plasma gelsolin levels, a feasible biomarker of brain damage, improved after VP3.15 treatment. Altogether, our data support the beneficial effects of VP3.15 in GM-IVH by ameliorating brain neuroinflammatory, vascular and white matter damage, ultimately improving cognitive impairment associated with GM-IVH.

Efficacy of a benzothiazole-based LRRK2 inhibitor in oligodendrocyte precursor cells and in a murine model of multiple sclerosis

AIMS

Multiple sclerosis (MS) is a chronic neurological disease that currently lacks effective curative treatments. There is a need to find effective therapies, especially to reverse the progressive demyelination and neuronal damage. Oligodendrocytes form the myelin sheath around axons in the central nervous system (CNS) and oligodendrocyte precursor cells (OPCs) undergo mechanisms that enable spontaneously the partial repair of damaged lesions. The aim of this study was to discover small molecules with potential effects in demyelinating diseases, including (re)myelinating properties.

METHODS

Recently, it has been shown how LRRK2 inhibition promotes oligodendrogliogenesis and therefore an efficient repair or myelin damaged lesions. Here we explored small molecules inhibiting LRRK2 as potential enhancers of primary OPCs proliferation and differentiation, and their potential impact on the clinical score of experimental autoimmune encephalomyelitys (EAE) mice, a validated model of the most frequent clinical form of MS, relapsing-remitting MS.

RESULTS

One of the LRRK2 inhibitors presented in this study promoted the proliferation and differentiation of OPC primary cultures. When tested in the EAE murine model of MS, it exerted a statistically significant reduction of the clinical burden of the animals, and histological evidence revealed how the treated animals presented a reduced lesion area in the spinal cord.

CONCLUSIONS

For the first time, a small molecule with LRRK2 inhibition properties presented (re)myelinating properties in primary OPCs cultures and potentially in the in vivo murine model. This study provides an in vivo proof of concept for a LRRK2 inhibitor, confirming its potential for the treatment of MS.

Neuronal activity and remyelination: new insights into the molecular mechanisms and therapeutic advancements

This article reviews the role of neuronal activity in myelin regeneration and the related neural signaling pathways. The article points out that neuronal activity can stimulate the formation and regeneration of myelin, significantly improve its conduction speed and neural signal processing ability, maintain axonal integrity, and support axonal nutrition. However, myelin damage is common in various clinical diseases such as multiple sclerosis, stroke, dementia, and schizophrenia. Although myelin regeneration exists in these diseases, it is often incomplete and cannot promote functional recovery. Therefore, seeking other ways to improve myelin regeneration in clinical trials in recent years is of great significance. Research has shown that controlling neuronal excitability may become a new intervention method for the clinical treatment of demyelinating diseases. The article discusses the latest research progress of neuronal activity on myelin regeneration, including direct or indirect stimulation methods, and the related neural signaling pathways, including glutamatergic, GABAergic, cholinergic, histaminergic, purinergic and voltage-gated ion channel signaling pathways, revealing that seeking treatment strategies to promote myelin regeneration through precise regulation of neuronal activity has broad prospects.

Phosphodiesterase 7 as a therapeutic target - Where are we now?

Cyclic nucleotide phosphodiesterases (PDEs) are a superfamily of enzymes that hydrolyse the intracellular second messengers cAMP and cGMP to their inactive forms 5'AMP and 5'GMP. Some members of the PDE family display specificity towards a single cyclic nucleotide messenger, and PDE4, PDE7, and PDE8 specifically hydrolyse cAMP. While the role of PDE4 and its use as a therapeutic target have been well studied, less is known about PDE7 and PDE8. This review aims to collate the present knowledge on human PDE7 and outline its potential use as a therapeutic target. Human PDE7 exists as two isoforms PDE7A and PDE7B that display different expression patterns but are predominantly found in the central nervous system, immune cells, and lymphoid tissue. As a result, PDE7 is thought to play a role in T cell activation and proliferation, inflammation, and regulate several physiological processes in the central nervous system, such as neurogenesis, synaptogenesis, and long-term memory formation. Increased expression and activity of PDE7 has been detected in several disease states, including neurodegenerative diseases such as Parkinson's, Alzheimer's and Huntington's disease, autoimmune diseases such as multiple sclerosis and COPD, and several types of cancer. Early studies have shown that administration of PDE7 inhibitors may ameliorate the clinical state of these diseases. Targeting PDE7 may therefore provide a novel therapeutic strategy for targeting a broad range of disease and possibly provide a complementary alternative to inhibitors of other cAMP-selective PDEs, such as PDE4, which are severely limited by their side-effects.

Advances in the development of phosphodiesterase 7 inhibitors

Phosphodiesterase 7 (PDE7) specifically hydrolyzes cyclic adenosine monophosphate (cAMP), a second messenger that plays essential roles in cell signaling and physiological processes. Many PDE7 inhibitors used to investigate the role of PDE7 have displayed efficacy in the treatment of a wide range of diseases, such as asthma and central nervous system (CNS) disorders. Although PDE7 inhibitors are developed more slowly than PDE4 inhibitors, there is increasing recognition of PDE7 inhibitors as potential therapeutics for no nausea and vomiting secondary. Herein, we summarized the advances in PDE7 inhibitors over the past decade, focusing on their crystal structures, key pharmacophores, subfamily selectivity, and therapeutic potential. Hopefully, this summary will lead to a better understanding of PDE7 inhibitors and provide strategies for developing novel therapies targeting PDE7.

Inhibition of CXCR2 enhances CNS remyelination via modulating PDE10A/cAMP signaling pathway

CXC chemokine receptor 2 (CXCR2) plays an important role in demyelinating diseases, but the detailed mechanisms were not yet clarified. In the present study, we mainly investigated the critical function and the potential molecular mechanisms of CXCR2 on oligodendrocyte precursor cell (OPC) differentiation and remyelination. The present study demonstrated that inhibiting CXCR2 significantly enhanced OPC differentiation and remyelination in primary cultured OPCs and ethidium bromide (EB)-intoxicated rats by facilitating the formation of myelin proteins, including PDGFRα, MBP, MAG, MOG, and Caspr. Further investigation identified phosphodiesterase 10A (PDE10A) as a main downstream protein of CXCR2, interacting with the receptor to regulate OPC differentiation, in that inhibition of CXCR2 reduced PDE10A expression while suppression of PDE10A did not affect CXCR2. Furthermore, inhibition of PDE10A promoted OPC differentiation, whereas overexpression of PDE10A down-regulated OPC differentiation. Our data also revealed that inhibition of CXCR2/PDE10A activated the cAMP/ERK1/2 signaling pathway, and up-regulated the expression of key transcription factors, including SOX10, OLIG2, MYRF, and ZFP24, that ultimately promoted remyelination and myelin protein biosynthesis. In conclusion, our findings suggested that inhibition of CXCR2 promoted OPC differentiation and enhanced remyelination by regulating PDE10A/cAMP/ERK1/2 signaling pathway. The present data also highlighted that CXCR2 may serve as a potential target for the treatment of demyelination diseases.

Neuronal lack of PDE7a disrupted working memory, spatial learning, and memory but facilitated cued fear memory in mice

BACKGROUND

PDEs regulate cAMP levels which is critical for PKA activity-dependent activation of CREB-mediated transcription in learning and memory. Inhibitors of PDEs like PDE4 and Pde7 improve learning and memory in rodents. However, the role of PDE7 in cognition or learning and memory has not been reported yet.

METHODS

Therefore, we aimed to explore the cognitive effects of a PDE7 subtype, PDE7a, using combined pharmacological and genetic approaches.

RESULTS

PDE7a-nko mice showed deficient working memory, impaired novel object recognition, deficient spatial learning & memory, and contextual fear memory, contrary to enhanced cued fear memory, highlighting the potential opposite role of PDE7a in the hippocampal neurons. Further, pharmacological inhibition of PDE7 by AGF2.20 selectively strengthens cued fear memory in C57BL/6 J mice, decreasing its extinction but did not affect cognitive processes assessed in other behavioral tests. The further biochemical analysis detected deficient cAMP in neural cell culture with genetic excision of the PDE7a gene, as well as in the hippocampus of PDE7a-nko mice in vivo. Importantly, we found overexpression of PKA-R and the reduced level of pPKA-C in the hippocampus of PDE7a-nko mice, suggesting a novel mechanism of the cAMP regulation by PDE7a. Consequently, the decreased phosphorylation of CREB, CAMKII, eif2a, ERK, and AMPK, and reduced total level of NR2A have been found in the brain of PDE7a-nko animals. Notably, genetic excision of PDE7a in neurons was not able to change the expression of NR2B, BDNF, synapsin1, synaptophysin, or snap25.

CONCLUSION

Altogether, our current findings demonstrated, for the first time, the role of PDE7a in cognitive processes. Future studies will untangle PDE7a-dependent neurobiological and molecular-cellular mechanisms related to cAMP-associated disorders.

The Dual PDE7-GSK3β Inhibitor, VP3.15, as Neuroprotective Disease-Modifying Treatment in a Model of Primary Progressive Multiple Sclerosis

Multiple sclerosis (MS) is a chronic, inflammatory, autoimmune and degenerative disease with axonal damage and demyelination as its main features. Its dual neurological and autoimmune nature makes it a disease that is difficult to treat. Treatments that simultaneously stop the immune response while protecting and repairing the nervous system are urgent. That is of utmost importance for the primary progressive multiple sclerosis (PPMS), a rare and severe variant of MS, characterized by worsening neurological function from the onset of symptoms. In this sense, inhibitors of glycogen synthase kinase 3β (GSK3β) and phosphodiesterase 7 (PDE7) have recently shown great therapeutic potential for the treatment of demyelinating diseases. Here we investigated a dual inhibitor of these two targets, the small molecule VP3.15, in a preclinical model, which resembles primary-progressive MS (PPMS), the Theiler's mouse encephalomyelitis virus-induced demyelinated disease (TMEV-IDD). In our study, VP3.15 ameliorates the disease course improving motor deficits of infected mice. Chronic treatment with VP3.15 also showed significant efficacy in the immunomodulation process, as well as in the proliferation and differentiation of oligodendroglial precursors, improving the preservation of myelin and axonal integrity. Therefore, our results support a treatment with the safe VP3.15 as an integrative therapeutic strategy for the treatment of PPMS.

cAMP-PKA cascade: An outdated topic for depression?

Depression is a common neuropsychiatric disorder characterized by persistent depressed mood and causes serious socioeconomic burden worldwide. Hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, deficiency of monoamine transmitters, neuroinflammation and abnormalities of the gut flora are strongly associated with the onset of depression. The cyclic AMP (cAMP)/protein kinase A (PKA) cascade, a major cross-species cellular signaling pathway, is supposed as important player and regulator of depression onset by controlling synaptic plasticity, cytokinesis, transcriptional regulation and HPA axis. In the central nervous system, the cAMP-PKA cascade can dynamically shape neural circuits by enhancing synaptic plasticity, and affect K⁺ channels by phosphorylating Kir4.1, thereby regulating neuronal excitation. The reduction of cAMP-PKA cascade affects neuronal excitation as well as synaptic plasticity, ultimately leading to pathological outcome of depression, while activation of cAMP-PKA cascade would provide a rapid antidepressant effect. In this review, we proposed to reconsider the function of cAMP-PKA cascade, especially in the rapid antidepressant effect. Local activation or indirect activation of PKA through adjusting anchor proteins would provide new idea for acute treatment of depression.

Potential role of IP3/Ca2+ signaling and phosphodiesterases: Relevance to neurodegeneration in Alzheimer's disease and possible therapeutic strategies

Despite large investments by industry and governments, no disease-modifying medications for the treatment of patients with Alzheimer's disease (AD) have been found. The failures of various clinical trials indicate the need for a more in-depth understanding of the pathophysiology of AD and for innovative therapeutic strategies for its treatment. Here, we review the rational for targeting IP3 signaling, cytosolic calcium dysregulation, phosphodiesterases (PDEs), and secondary messengers like cGMP and cAMP, as well as their correlations with the pathophysiology of AD. Various drugs targeting these signaling cascades are still in pre-clinical and clinical trials which support the ideas presented in this article. Further, we describe different molecular mechanisms and medications currently being used in various pre-clinical and clinical trials involving IP3/Ca⁺² signaling. We also highlight various isoforms, as well as the functions and pharmacology of the PDEs broadly expressed in different parts of the brain and attempt to unravel the potential benefits of PDE inhibitors for use as novel medications to alleviate the pathogenesis of AD.

Phosphodiesterase 7(PDE7): A unique drug target for central nervous system diseases

Phosphodiesterase 7 (PDE7), one of the 11 phosphodiesterase (PDE) families, specifically hydrolyzes cyclic 3', 5'-adenosine monophosphate (cAMP). PDE7 is involved in many important functional processes in physiology and pathology by regulating intracellular cAMP signaling. Studies have demonstrated that PDE7 is widely expressed in the central nervous system (CNS) and potentially related to pathogenesis of many CNS diseases. Here, we summarized the classification and distribution of PDE7 in the brain and its functional roles in the mediation of CNS diseases such as Parkinson's disease (PD), Alzheimer's disease (AD), multiple sclerosis (MS), and schizophrenia. It is expected that the findings collected here will not only lead to a better understanding of the mechanisms by which PDE7 mediates CNS function and diseases, but also aid in the development of novel drugs targeting PDE7 for treatment of CNS diseases.

Diabetic Theory in Anti-Alzheimer's Drug Research and Development. Part 2: Therapeutic Potential of cAMP-Specific Phosphodiesterase Inhibitors

Alzheimer's disease (AD) is one of the most prevalent age-related neurodegenerative disease that affects the cognition, behavior, and daily activities of individuals. Studies indicate that this disease is characterized by several pathological mechanisms, including the accumulation of amyloid-beta peptide, hyperphosphorylation of tau protein, impairment of cholinergic neurotransmission, and increase in inflammatory responses within the central nervous system. Chronic neuroinflammation associated with AD is closely related to disturbances in metabolic processes, including insulin release and glucose metabolism. As AD is also called type III diabetes, diverse compounds having antidiabetic effects have been investigated as potential drugs for its symptomatic and disease-modifying treatment. In addition to insulin and oral antidiabetic drugs, scientific attention has been paid to cyclic-3',5'-adenosine monophosphate (cAMP)-specific phosphodiesterase (PDE) inhibitors that can modulate the concentration of glucose and related hormones and exert beneficial effects on memory, mood, and emotional processing. In this review, we present the most recent reports focusing on the involvement of cAMP-specific PDE4, PDE7, and PDE8 in glycemic and inflammatory response controls as well as the potential utility of the PDE inhibitors in the treatment of AD. Besides the results of in vitro and in vivo studies, the review also presents recent reports from clinical trials.

Advances in Cyclic Nucleotide Phosphodiesterase-Targeted PET Imaging and Drug Discovery

Cyclic nucleotide phosphodiesterases (PDEs) control the intracellular concentrations of cAMP and cGMP in virtually all mammalian cells. Accordingly, the PDE family regulates a myriad of physiological functions, including cell proliferation, differentiation and apoptosis, gene expression, central nervous system function, and muscle contraction. Along this line, dysfunction of PDEs has been implicated in neurodegenerative disorders, coronary artery diseases, chronic obstructive pulmonary disease, and cancer development. To date, 11 PDE families have been identified; however, their distinct roles in the various pathologies are largely unexplored and subject to contemporary research efforts. Indeed, there is growing interest for the development of isoform-selective PDE inhibitors as potential therapeutic agents. Similarly, the evolving knowledge on the various PDE isoforms has channeled the identification of new PET probes, allowing isoform-selective imaging. This review highlights recent advances in PDE-targeted PET tracer development, thereby focusing on efforts to assess disease-related PDE pathophysiology and to support isoform-selective drug discovery.

Ginkgolide B promotes oligodendrocyte precursor cell differentiation and survival via Akt/CREB/bcl-2 signaling pathway after white matter lesion

White matter lesion (WML) is caused by chronic cerebral hypoperfusion, which are usually associated with cognitive impairment. Evidence from recent studies has shown that ginkgolide B has a neuroprotective effect that could be beneficial for the treatment of ischemia; however, it is not clear whether ginkgolide B has a protective effect on WML. Our data show that ginkgolide B can promote the differentiation of oligodendrocyte precursor cell (OPC) into oligodendrocytes and promote oligodendrocyte survival following a WML. Ginkgolide B (5, 10, 20 mg/kg) or saline is administered intraperitoneally every day after WML. After 4 weeks, the data of Morris water maze suggested that rats' memory and learning abilities were impaired, and the administration of ginkgolide B enhanced behavioral achievement. Also, treatment with ginkgolide B significantly attenuated this loss of myelin. Our result suggests that ginkgolide B promotes the differentiation of OPC into oligodendrocytes. We also found that ginkgolide B ameliorates oligodendrocytes apoptosis. Furthermore, ginkgolide B enhanced the expression of phosphorylated Akt and CREB. In conclusion, our data firstly show that ginkgolide B promotes oligodendrocyte genesis and oligodendrocyte myelin following a WML, possibly involving the Akt and CREB pathways.

Dynamics of Central Remyelination and Treatment Evolution in a Model of Multiple Sclerosis with Optic Coherence Tomography

The need for remyelinating drugs is essential for healing disabling diseases such as multiple sclerosis (MS). One of the reasons for the lack of this class of therapies is the impossibility to monitor remyelination in vivo, which is of utmost importance to perform effective clinical trials. Here, we show how optical coherence tomography (OCT), a cheap and non-invasive technique commonly used in ophthalmology, may be used to assess remyelination in vivo in MS patients. Our pioneer approach validates OCT as a technique to study remyelination of the optic nerve and reflects what is occurring in non-accessible central nervous system (CNS) structures, like the spinal cord. In this study we used the orally bioavailable small molecule VP3.15, confirming its therapeutical potential as a neuroprotective, anti-inflammatory, and probably remyelinating drug for MS. Altogether, our results confirm the usefulness of OCT to monitor the efficacy of remyelinating therapies in vivo and underscore the relevance of VP3.15 as a potential disease modifying drug for MS therapy.

Pharmacological inhibition of phosphodiesterase 7 enhances consolidation processes of spatial memory

Augmentation of cAMP signaling through inhibition of phosphodiesterases (PDE) is known to enhance plasticity and memory. Inhibition of PDE4 enhances consolidation into memory, but less is known about the role of other cAMP specific PDEs. Here, we tested the effects of oral treatment with a selective inhibitor of PDE7 of nanomolar potency on spatial and contextual memory. In an object location task, doses of 0.3-3 mg/kg administered 3 h after training dose-dependently attenuated time-dependent forgetting in rats. Significant enhancement of memory occurred at a dose of 3 mg/kg with corresponding brain levels consistent with PDE7 inhibition. The same dose given prior to training augmented contextual fear conditioning. In mice, daily dosing before training enhanced spatial memory in two different incremental learning paradigms in the Barnes Maze. Drug treated mice made significantly less errors locating the escape in a probe-test 24 h after the end of training, and they exhibited hippocampal-dependent spatial search strategies more frequently than controls, which tended to show serial sampling of escape locations. Acquisition and short-term memory, in contrast, were unaffected. Our data provide evidence for a role of PDE7 in the consolidation of hippocampal-dependent memory. We suggest that targeting PDE7 for memory enhancement may provide an alternative to PDE4 inhibitors, which tend to have undesirable gastrointestinal side-effects.

Sphingosine-1-Phosphate Receptor Modulators and Oligodendroglial Cells: Beyond Immunomodulation

Multiple sclerosis (MS) is an autoimmune inflammatory disease characterized by demyelination, axonal loss, and synaptic impairment in the central nervous system (CNS). The available therapies aim to reduce the severity of the pathology during the early inflammatory stages, but they are not effective in the chronic stage of the disease. In this phase, failure in endogenous remyelination is associated with the impairment of oligodendrocytes progenitor cells (OPCs) to migrate and differentiate into mature myelinating oligodendrocytes. Therefore, stimulating differentiation of OPCs into myelinating oligodendrocytes has become one of the main goals of new therapeutic approaches for MS. Different disease-modifying therapies targeting sphingosine-1-phosphate receptors (S1PRs) have been approved or are being developed to treat MS. Besides their immunomodulatory effects, growing evidence suggests that targeting S1PRs modulates mechanisms beyond immunomodulation, such as remyelination. In this context, this review focuses on the current understanding of S1PR modulators and their direct effect on OPCs and oligodendrocytes.

Functional Heterogeneity of Mouse and Human Brain OPCs: Relevance for Preclinical Studies in Multiple Sclerosis

Besides giving rise to oligodendrocytes (the only myelin-forming cell in the Central Nervous System (CNS) in physiological conditions), Oligodendrocyte Precursor Cells (OPCs) are responsible for spontaneous remyelination after a demyelinating lesion. They are present along the mouse and human CNS, both during development and in adulthood, yet how OPC physiological behavior is modified throughout life is not fully understood. The activity of adult human OPCs is still particularly unexplored. Significantly, most of the molecules involved in OPC-mediated remyelination are also involved in their development, a phenomenon that may be clinically relevant. In the present article, we have compared the intrinsic properties of OPCs isolated from the cerebral cortex of neonatal, postnatal and adult mice, as well as those recovered from neurosurgical adult human cerebral cortex tissue. By analyzing intact OPCs for the first time with 1H High Resolution Magic Angle Spinning Nuclear Magnetic Resonance (1H HR-MAS NMR) spectroscopy, we show that these cells behave distinctly and that they have different metabolic patterns in function for their stage of maturity. Moreover, their response to Fibroblast Growth Gactor-2 (FGF-2) and anosmin-1 (two molecules that have known effects on OPC biology during development and that are overexpressed in individuals with Multiple Sclerosis (MS)) differs in relation to their developmental stage and in the function of the species. Our data reveal that the behavior of adult human and mouse OPCs differs in a very dynamic way that should be very relevant when testing drugs and for the proper design of effective pharmacological and/or cell therapies for MS.

PDE-7 Inhibitor BRL-50481 Reduces Neurodegeneration and Long-Term Memory Deficits in Mice Following Sevoflurane Exposure

Sevoflurane, one of the most commonly used anesthetic agents, has been demonstrated to induce widespread neurodegeneration in the developing brain. We aimed to evaluate the protective effects of a PDE-7 inhibitor (BRL-50481) against the neurotoxic effects of sevoflurane on the developing nervous system. Spatial learning and memory in sevoflurane-treated mice were examined using the Morris water maze test, and neuroprotective effects of PDE-7 inhibitor (BRL-50481) against sevoflurane-induced impairments were evaluated. Our results showed that sevoflurane treatment markedly induced neurodegeneration and impaired long-term memory in neonatal mice. Notably, BRL-50481 coadministration could significantly attenuate sevoflurane-induced learning and memory defects, prevent deterioration of recognition memory, and protect against neuron apoptosis. Mechanistically, BRL-50481 administration suppressed sevoflurane-induced neurodegenerative disorders through restoring cAMP and activating cAMP/CREB signaling in the hippocampus. PDE7 inhibitor may be a potential therapeutic agent for sevoflurane-induced neurodegeneration and long-term memory deficits.

Phosphodiesterase inhibitors say NO to Alzheimer's disease

Phosphodiesterases (PDEs) consisted of 11 subtypes (PDE1 to PDE11) and over 40 isoforms that regulate levels of cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP), the second messengers in cell functions. PDE inhibitors (PDEIs) have been attractive therapeutic targets due to their involvement in diverse medical conditions, e.g. cardiovascular diseases, autoimmune diseases, Alzheimer's disease (AD), etc. Among them; AD with a complex pathology is a progressive neurodegenerative disorder which affect mostly senile people in the world and only symptomatic treatment particularly using cholinesterase inhibitors in clinic is available at the moment for AD. Consequently, novel treatment strategies towards AD are still searched extensively. Since PDEs are broadly expressed in the brain, PDEIs are considered to modulate neurodegenerative conditions through regulating cAMP and cGMP in the brain. In this sense, several synthetic or natural molecules inhibiting various PDE subtypes such as rolipram and roflumilast (PDE4 inhibitors), vinpocetine (PDE1 inhibitor), cilostazol and milrinone (PDE3 inhibitors), sildenafil and tadalafil (PDE5 inhibitors), etc have been reported showing encouraging results for the treatment of AD. In this review, PDE superfamily will be scrutinized from the view point of structural features, isoforms, functions and pharmacology particularly attributed to PDEs as target for AD therapy.

Targeting Phosphodiesterases-Towards a Tailor-Made Approach in Multiple Sclerosis Treatment

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system (CNS) characterized by heterogeneous clinical symptoms including gradual muscle weakness, fatigue, and cognitive impairment. The disease course of MS can be classified into a relapsing-remitting (RR) phase defined by periods of neurological disabilities, and a progressive phase where neurological decline is persistent. Pathologically, MS is defined by a destructive immunological and neuro-degenerative interplay. Current treatments largely target the inflammatory processes and slow disease progression at best. Therefore, there is an urgent need to develop next-generation therapeutic strategies that target both neuroinflammatory and degenerative processes. It has been shown that elevating second messengers (cAMP and cGMP) is important for controlling inflammatory damage and inducing CNS repair. Phosphodiesterases (PDEs) have been studied extensively in a wide range of disorders as they breakdown these second messengers, rendering them crucial regulators. In this review, we provide an overview of the role of PDE inhibition in limiting pathological inflammation and stimulating regenerative processes in MS.

Amidic derivatives of valproic acid, valpromide and valnoctamide, inhibit HSV-1 infection in oligodendrocytes

Herpes simplex virus type 1 (HSV-1) is a ubiquitous infectious agent that can establish latency in neurons, and in some cases, viral retrograde transport results in infection of the central nervous system (CNS). Several antivirals have been identified with the ability to inhibit HSV-1 replication in human cells to a greater or lesser degree, most of which are nucleoside analogues that unfortunately exhibit teratogenic potential, embryotoxicity, carcinogenic or antiproliferative activities and resistances in immunocompromised patients, specially. In the present study, we assessed two amidic derivatives of valproic acid (VPA) - valpromide (VPD) and valnoctamide (VCD) - which are already used in clinic treatments, as feasible HSV-1 antivirals in glial cells. Both VPD and VCD have exhibited increased efficacy in bipolar disorders and as anticonvulsant drugs compared to VPA, while being less teratogenic and hepatotoxic. Cytotoxicity assays carried out in our laboratory showed that VPD and VCD were not toxic in a human oligodendroglioma cell line (HOG), at least at the concentrations established for human treatments. Infectivity assays showed a significant inhibition of HSV-1 infection in HOG cells after VPD and VCD treatment, being more pronounced in VPD-treated cells, comparable to the effects obtained with acyclovir. Furthermore, the same antiherpetic effects of VPD were observed in other oligodendrocytic cell lines and rat primary oligodendrocytes (OPCs), confirming the results obtained in HOG cells. Altogether, our results allow us to propose VPD as a potential antiherpetic drug that is able to act directly on oligodendrocytes of the CNS.

Oligodendrocyte precursor cells as a therapeutic target for demyelinating diseases

The mechanisms regulating differentiation of multipotent oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes (OLs) are critical to our understanding of myelination and remyelination. Following acute demyelination in the central nervous system, adult OPCs migrate to the injury site, differentiate into OLs and generate new myelin sheaths. A common feature of regenerative processes is the fact that remyelination efficiency declines with aging and, accounts for the observation that chronic demyelinating diseases like multiple sclerosis (MS) are characterized by an ineffective remyelination. Without doubt, impairment of OPC differentiation is an essential determinant of the aging effects in remyelination. However, spontaneous remyelination is limited in demyelinating diseases such as MS, owing in part to the failure of adult OPCs to differentiate into myelinating OLs. The inability to restore myelin after injury compromises axon integrity and renders them vulnerable to degeneration. Although the genes that regulate the proliferation and differentiation of OPCs during development have been intensively studied, relatively little is known about the molecular signals that regulate the function of adult OPCs after demyelination. Elucidating the mechanisms regulating OPC differentiation are key to identifying pharmacological targets for remyelination-enhancing therapy. This review will discuss OPC biology, myelination, and possible pharmacological targets for promoting the differentiation of OPCs as a strategy to enhance remyelination, including the potential for nanoscale delivery.

Sildenafil Inhibits Myelin Expression and Myelination of Oligodendroglial Precursor Cells

Phosphodiesterases (PDEs) have previously been implicated in oligodendrocyte maturation and myelination of central nervous system axons. Sildenafil citrate is a phosphodiesterase inhibitor known to block PDE5, which also reduces inflammation in the experimental autoimmune encephalomyelitis demyelinating model. To find out whether this inhibitor might exert beneficial effects on central nervous system myelin repair activities, we investigated to what degree sildenafil modulates differentiation and maturation of cultured primary rat oligodendroglial precursor cells (OPCs). To this end, gene and protein expression of 2',3'-cyclic-nucleotide 3'-phosphodiesterase, myelin basic protein, and myelin oligodendrocyte glycoprotein, as well as of negative regulators of myelin expression (Hes1, Hes5, Id2, Id4, Rock2, and p57Kip2) were measured in OPCs treated with sildenafil. Moreover, the subcellular distribution of the p57kip2 protein was determined after sildenafil treatment, as this revealed to be an early predictor of the oligodendroglial differentiation capacity. In vitro myelination assays were done to measure the myelination capacity of oligodendrocytes treated with sildenafil. We found that sildenafil significantly diminished myelin gene expression and protein expression. Moreover, sildenafil also increased the expression of Id2 and Id4 negative transcriptional regulators, and the degree of OPCs with cytoplasmic p57kip2 protein localization was reduced, providing evidence that the PDE blocker impaired the differentiation capacity. Finally, sildenafil also interfered with the establishment of internodes as revealed by in vitro myelination assays. We therefore conclude that blocking PDE5 activities exerts a negative impact on intrinsic oligodendroglial differentiation processes.

R-Ras1 and R-Ras2 Are Essential for Oligodendrocyte Differentiation and Survival for Correct Myelination in the Central Nervous System

Rapid and effective neural transmission of information requires correct axonal myelination. Modifications in myelination alter axonal capacity to transmit electric impulses and enable pathological conditions. In the CNS, oligodendrocytes (OLs) myelinate axons, a complex process involving various cellular interactions. However, we know little about the mechanisms that orchestrate correct myelination. Here, we demonstrate that OLs express R-Ras1 and R-Ras2. Using female and male mutant mice to delete these proteins, we found that activation of the PI3K/Akt and Erk1/2-MAPK pathways was weaker in mice lacking one or both of these GTPases, suggesting that both proteins coordinate the activity of these two pathways. Loss of R-Ras1 and/or R-Ras2 diminishes the number of OLs in major myelinated CNS tracts and increases the proportion of immature OLs. In R-Ras1^-/- and R-Ras2^-/--null mice, OLs show aberrant morphologies and fail to differentiate correctly into myelin-forming phenotypes. The smaller OL population and abnormal OL maturation induce severe hypomyelination, with shorter nodes of Ranvier in R-Ras1^-/- and/or R-Ras2^-/- mice. These defects explain the slower conduction velocity of myelinated axons that we observed in the absence of R-Ras1 and R-Ras2. Together, these results suggest that R-Ras1 and R-Ras2 are upstream elements that regulate the survival and differentiation of progenitors into OLs through the PI3K/Akt and Erk1/2-MAPK pathways for proper myelination.SIGNIFICANCE STATEMENT In this study, we show that R-Ras1 and R-Ras2 play essential roles in regulating myelination in vivo and control fundamental aspects of oligodendrocyte (OL) survival and differentiation through synergistic activation of PI3K/Akt and Erk1/2-MAPK signaling. Mice lacking R-Ras1 and/or R-Ras2 show a diminished OL population with a higher proportion of immature OLs, explaining the observed hypomyelination in main CNS tracts. In vivo electrophysiology recordings demonstrate a slower conduction velocity of nerve impulses in the absence of R-Ras1 and R-Ras2. Therefore, R-Ras1 and R-Ras2 are essential for proper axonal myelination and accurate neural transmission.

GD1a Overcomes Inhibition of Myelination by Fibronectin via Activation of Protein Kinase A: Implications for Multiple Sclerosis

Remyelination failure by oligodendrocytes contributes to the functional impairment that characterizes the demyelinating disease multiple sclerosis (MS). Since incomplete remyelination will irreversibly damage axonal connections, treatments effectively promoting remyelination are pivotal in halting disease progression. Our previous findings suggest that fibronectin aggregates, as an environmental factor, contribute to remyelination failure by perturbing oligodendrocyte progenitor cell (OPC) maturation. Here, we aim at elucidating whether exogenously added gangliosides (i.e., cell surface lipids with a potential to modulate signaling pathways) could counteract fibronectin-mediated inhibition of OPC maturation. Exclusive exposure of rat oligodendrocytes to GD1a, but not other gangliosides, overcomes aggregated fibronectin-induced inhibition of myelin membrane formation, in vitro, and OPC differentiation in fibronectin aggregate containing cuprizone-induced demyelinated lesions in male mice. GD1a exerts its effect on OPCs by inducing their proliferation and, at a late stage, by modulating OPC maturation. Kinase activity profiling revealed that GD1a activated a protein kinase A (PKA)-dependent signaling pathway and increased phosphorylation of the transcription factor cAMP response element-binding protein. Consistently, the effect of GD1a in restoring myelin membrane formation in the presence of fibronectin aggregates was abolished by the PKA inhibitor H89, whereas the effect of GD1a was mimicked by the PKA activator dibutyryl-cAMP. Together, GD1a overcomes the inhibiting effect of aggregated fibronectin on OPC maturation by activating a PKA-dependent signaling pathway. Given the persistent presence of fibronectin aggregates in MS lesions, ganglioside GD1a might act as a potential novel therapeutic tool to selectively modulate the detrimental signaling environment that precludes remyelination.SIGNIFICANCE STATEMENT As an environmental factor, aggregates of the extracellular matrix protein fibronectin perturb the maturation of oligodendrocyte progenitor cells (OPCs), thereby impeding remyelination, in the demyelinating disease multiple sclerosis (MS). Here we demonstrate that exogenous addition of ganglioside GD1a overcomes the inhibiting effect of aggregated fibronectin on OPC maturation, both in vitro and in vivo, by activating a PKA-dependent signaling pathway. We propose that targeted delivery of GD1a to MS lesions may act as a potential novel molecular tool to boost maturation of resident OPCs to overcome remyelination failure and halt disease progression.

Phosphodiesterase Inhibitors as a Therapeutic Approach to Neuroprotection and Repair

A wide diversity of perturbations of the central nervous system (CNS) result in structural damage to the neuroarchitecture and cellular defects, which in turn are accompanied by neurological dysfunction and abortive endogenous neurorepair. Altering intracellular signaling pathways involved in inflammation and immune regulation, neural cell death, axon plasticity and remyelination has shown therapeutic benefit in experimental models of neurological disease and trauma. The second messengers, cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP), are two such intracellular signaling targets, the elevation of which has produced beneficial cellular effects within a range of CNS pathologies. The only known negative regulators of cyclic nucleotides are a family of enzymes called phosphodiesterases (PDEs) that hydrolyze cyclic nucleotides into adenosine monophosphate (AMP) or guanylate monophosphate (GMP). Herein, we discuss the structure and physiological function as well as the roles PDEs play in pathological processes of the diseased or injured CNS. Further we review the approaches that have been employed therapeutically in experimental paradigms to block PDE expression or activity and in turn elevate cyclic nucleotide levels to mediate neuroprotection or neurorepair as well as discuss both the translational pathway and current limitations in moving new PDE-targeted therapies to the clinic.

Promoting in vivo remyelination with small molecules: a neuroreparative pharmacological treatment for Multiple Sclerosis

Multiple Sclerosis (MS) is a neurodegenerative disease where immune-driven demyelination occurs with inefficient remyelination, but therapies are limited, especially those to enhance repair. Here, we show that the dual phosphodiesterase (PDE)7- glycogen synthase kinase (GSK)3 inhibitor, VP3.15, a heterocyclic small molecule with good pharmacokinetic properties and safety profile, improves in vivo remyelination in mouse and increases both adult mouse and adult human oligodendrocyte progenitor cell (OPC) differentiation, in addition to its immune regulatory action. The dual inhibition is synergistic, as increasing intracellular levels of cAMP by cyclic nucleotide PDE inhibition both suppresses the immune response and increases remyelination, and in addition, inhibition of GSK3 limits experimental autoimmune encephalomyelitis in mice. This combination of an advantageous effect on the immune response and an enhancement of repair, plus demonstration of its activity on adult human OPCs, leads us to propose dual PDE7-GSK3 inhibition, and specifically VP3.15, as a neuroprotective and neuroreparative disease-modifying treatment for MS.

The Orphan G Protein-coupled Receptor GPR17 Negatively Regulates Oligodendrocyte Differentiation via Gαi/o and Its Downstream Effector Molecules

Recent studies have recognized G protein-coupled receptors as important regulators of oligodendrocyte development. GPR17, in particular, is an orphan G protein-coupled receptor that has been identified as oligodendroglial maturation inhibitor because its stimulation arrests primary mouse oligodendrocytes at a less differentiated stage. However, the intracellular signaling effectors transducing its activation remain poorly understood. Here, we use Oli-neu cells, an immortalized cell line derived from primary murine oligodendrocytes, and primary rat oligodendrocyte cultures as model systems to identify molecular targets that link cell surface GPR17 to oligodendrocyte maturation blockade. We demonstrate that stimulation of GPR17 by the small molecule agonist MDL29,951 (2-carboxy-4,6-dichloro-1H-indole-3-propionic acid) decreases myelin basic protein expression levels mainly by triggering the Gαi/o signaling pathway, which in turn leads to reduced activity of the downstream cascade adenylyl cyclase-cAMP-PKA-cAMP response element-binding protein (CREB). In addition, we show that GPR17 activation also diminishes myelin basic protein abundance by lessening stimulation of the exchange protein directly activated by cAMP (EPAC), thus uncovering a previously unrecognized role for EPAC to regulate oligodendrocyte differentiation. Together, our data establish PKA and EPAC as key downstream effectors of GPR17 that inhibit oligodendrocyte maturation. We envisage that treatments augmenting PKA and/or EPAC activity represent a beneficial approach for therapeutic enhancement of remyelination in those demyelinating diseases where GPR17 is highly expressed, such as multiple sclerosis.

Interferon-Beta Therapy of Multiple Sclerosis Patients Improves the Responsiveness of T Cells for Immune Suppression by Regulatory T Cells

Multiple sclerosis (MS) is an inflammatory autoimmune disease characterized by imbalanced immune regulatory networks, and MS patient-derived T effector cells are inefficiently suppressed through regulatory T cells (Treg), a phenomenon known as Treg resistance. In the current study we investigated T cell function in MS patients before and after interferon-beta therapy. We compared cytokine profile, responsiveness for Treg-mediated suppression ex vivo and evaluated reactivity of T cells in vivo using a humanized mouse model. We found that CD4⁺ and CD8⁺ T cells of therapy-naive MS patients were resistant to Treg-mediated suppression. Treg resistance is associated with an augmented IL-6 production, enhanced IL-6 receptor expression, and increased PKB/c-Akt phosphorylation. These parameters as well as responsiveness of T cells to Treg-mediated suppression were restored after interferon-beta therapy of MS patients. Following transfer into immunodeficient mice, MS T cells induced a lethal graft versus host disease (GvHD) and in contrast to T cells of healthy volunteers, this aggressive T cell response could not be controlled by Treg, but was abolished by anti-IL-6 receptor antibodies. However, magnitude and lethality of GvHD induced by MS T cells was significantly decreased after interferon-beta therapy and the reaction was prevented by Treg activation in vivo. Our data reveals that interferon-beta therapy improves the immunoregulation of autoaggressive T effector cells in MS patients by changing the IL-6 signal transduction pathway, thus restoring their sensitivity to Treg-mediated suppression.

PDE7 inhibitor TC3.6 ameliorates symptomatology in a model of primary progressive multiple sclerosis

BACKGROUND AND PURPOSE

cAMP plays an important role in the transduction of signalling pathways involved in neuroprotection and immune regulation. Control of the levels of this nucleotide by inhibition of cAMP-specific PDEs such as PDE7 may affect the pathological processes of neuroinflammatory diseases like multiple sclerosis (MS). In the present study, we evaluated the therapeutic potential of the selective PDE7 inhibitor, TC3.6, in a model of primary progressive multiple sclerosis (PPMS), a rare and severe variant of MS.

EXPERIMENTAL APPROACH

Theiler's murine encephalomyelitis virus-induced demyelinated disease (TMEV-IDD) is one of the models used to validate the therapeutic efficacy of new drugs in MS. As recent studies have analysed the effect of PDE7 inhibitors in the EAE model of MS, here the TMEV-IDD model was used to test their efficacy in a progressive variant of MS. Mice were subjected to two protocols of TC3.6 administration: on the pre-symptomatic phase and once the disease was established.

KEY RESULTS

Treatment with TC3.6 ameliorated the disease course and improved motor deficits of infected mice. This was associated with down-regulation of microglial activation and reduced cellular infiltrates. Decreased expression of pro-inflammatory mediators such as COX-2 and the cytokines, IL-1β, TNF-α, IFN-γ and IL-6 in the spinal cord of TMEV-infected mice was also observed after TC3.6 administration.

CONCLUSION

These findings support the importance of PDE7 inhibitors, and specifically TC3.6, as a novel class of agents with therapeutic potential for PPMS. Preclinical studies are needed to determine whether their effects translate into durable clinical benefits.

Anosmin-1 over-expression regulates oligodendrocyte precursor cell proliferation, migration and myelin sheath thickness

During development of the central nervous system, anosmin-1 (A1) works as a chemotropic cue contributing to axonal outgrowth and collateralization, as well as modulating the migration of different cell types, fibroblast growth factor receptor 1 (FGFR1) being the main receptor involved in all these events. To further understand the role of A1 during development, we have analysed the over-expression of human A1 in a transgenic mouse line. Compared with control mice during development and in early adulthood, A1 over-expressing transgenic mice showed an enhanced oligodendrocyte precursor cell (OPC) proliferation and a higher number of OPCs in the subventricular zone and in the corpus callosum (CC). The migratory capacity of OPCs from the transgenic mice is increased in vitro due to a higher basal activation of ERK1/2 mediated through FGFR1 and they also produced more myelin basic protein (MBP). In vivo, the over-expression of A1 resulted in an elevated number of mature oligodendrocytes with higher levels of MBP mRNA and protein, as well as increased levels of activation of the ERK1/2 proteins, while electron microscopy revealed thicker myelin sheaths around the axons of the CC in adulthood. Also in the mature CC, the nodes of Ranvier were significantly longer and the conduction velocity of the nerve impulse in vivo was significantly increased in the CC of A1 over-expressing transgenic mice. Altogether, these data confirmed the involvement of A1 in oligodendrogliogenesis and its relevance for myelination.

Activation of endogenous neural stem cells for multiple sclerosis therapy

Multiple sclerosis (MS) is a chronic inflammatory disorder of the central nervous system, leading to severe neurological deficits. Current MS treatment regimens, consist of immunomodulatory agents aiming to reduce the rate of relapses. However, these agents are usually insufficient to treat chronic neurological disability. A promising perspective for future therapy of MS is the regeneration of lesions with replacement of the damaged oligodendrocytes or neurons. Therapies targeting to the enhancement of endogenous remyelination, aim to promote the activation of either the parenchymal oligodendrocyte progenitor cells or the subventricular zone-derived neural stem cells (NSCs). Less studied but highly potent, is the strategy of neuronal regeneration with endogenous NSCs that although being linked to numerous limitations, is anticipated to ameliorate cognitive disability in MS. Focusing on the forebrain, this review highlights the role of NSCs in the regeneration of MS lesions.

Pivotal role of choline metabolites in remyelination

Neuroprotective approaches for central nervous system regeneration have not been successful in clinical practice so far and compounds that enhance remyelination are still not available for patients with multiple sclerosis. The objective of this study was to determine potential regenerative effects of the substance cytidine-5'-diphospho (CDP)-choline in two different murine animal models of multiple sclerosis. The effects of exogenously applied CDP-choline were tested in murine myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis. In addition, the cuprizone-induced mouse model of de- and remyelination was used to specifically test the hypothesis that CDP-choline directly increases remyelination. We found that CDP-choline ameliorated the disease course of experimental autoimmune encephalomyelitis and exerted beneficial effects on myelin, oligodendrocytes and axons. After cuprizone-induced demyelination, CDP-choline effectively enhanced myelin regeneration and reversed motor coordination deficits. The increased remyelination arose from an increase in the numbers of proliferating oligodendrocyte precursor cells and oligodendrocytes. Further in vitro studies suggest that this process is regulated by protein kinase C. We thus identified a new mechanism to enhance central nervous system remyelination via the choline pathway. Due to its regenerative action combined with an excellent safety profile, CDP-choline could become a promising substance for patients with multiple sclerosis as an add-on therapy.

Changes of CXCL12, CXCL14 and PDGF levels in the brain of patients with idiopathic demyelinating optic neuritis and neuromyelitis optica

The CXC chemokines (CXC-motif ligand 12 and CXC-motif ligand 14) and platelet-derived growth factor are suggested to modulate remyelination in the course of many demyelinating diseases. The present study compared the difference in the brain levels of these chemokines between patients with idiopathic demyelinating optic neuritis (IDON) and neuromyelitis optica (NMO) by measuring their concentrations in the cerebrospinal fluid using an enzyme linked immunosorbent assay. Our data indicate that the prognosis of neuritis depends on the remyelinating process that is impaired due to decreased chemokines. The much lower levels of chemokines would specifically indicate the severe neuritis, such as NMO.

cAMP-specific phosphodiesterase inhibitors: promising drugs for inflammatory and neurological diseases

INTRODUCTION

PDEs are key enzymes in the adenosine and guanosine cyclic nucleotides (cAMP and cGMP) signaling cascade. Their inhibition increases cyclic nucleotide levels inside the cell. Thus, pharmacological modulation of PDE activity can have profound effects on the function of cells and organ systems throughout the body.

AREAS COVERED

Among the large PDE families, only PDE4, PDE7 and PDE8 are cAMP-specific hydrolyzing enzymes. cAMP is an important second messenger not only by its involvement in a vast number of physiological processes but also by activation of protein kinase A, exchange protein activated by cAMP (Epac) and cAMP response element-binding (CREB) or cyclic nucleotide-gated channels. Clearly, such enzymes represent ideal drug targets for the pharmacological treatment of many pathologies. The discovery and development of small molecules targeting cAMP-specific PDEs reported in the last 5 years is the focus of the present review.

EXPERT OPINION

The first PDE4 inhibitors recently reached the market, having avoided, by different strategies, their dose-limiting side effects (after more than two decades of drug development). Meanwhile, new cAMP-specific PDE7 and PDE8 inhibitors emerged as effective and safe drugs for severe unmet diseases. The therapeutic potential of these inhibitors will be tested in the near future, as many of these drug candidates are ready to start clinical trials.

Comparative assessment of PDE 4 and 7 inhibitors as therapeutic agents in experimental autoimmune encephalomyelitis

BACKGROUND AND PURPOSE

PDE4 inhibition suppresses experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). However, side effects hinder PDE4 inhibitors clinical use. PDE7 inhibition might constitute an alternative therapeutic strategy, but few data about the anti-inflammatory potential of PDE7 inhibitors are currently available. We have used the EAE model to perform a comparative evaluation of PDE4 and PDE7 inhibition as strategies for MS treatment.

EXPERIMENTAL APPROACH

Two PDE7 inhibitors, the sulfonamide derivative BRL50481 and the recently described quinazoline compound TC3.6, were assayed to modulate EAE in SJL mice, in comparison with the well-known PDE4 inhibitor Rolipram. We evaluated clinical signs, presence of inflammatory infiltrates in CNS and anti-inflammatory markers. We also analysed the effect of these inhibitors on the inflammatory profile of spleen cells in vitro.

KEY RESULTS

TC3.6 prevented EAE with efficacy similar to Rolipram, while BRL50481 had no effect on the disease. Differences between both PDE7 inhibitors are discussed. Data from Rolipram and TC3.6 showed that PDE4 and PDE7 inhibition work through both common and distinct pathways. Rolipram administration caused an increase in IL-10 and IL-27 expression which was not found after TC3.6 treatment. On the other hand, both inhibitors reduced IL-17 levels, prevented infiltration in CNS and increased the expression of the T regulator cell marker Foxp3.

CONCLUSIONS AND IMPLICATIONS

These results provide new information about the effects of Rolipram on EAE, underline PDE7 inhibition as a new therapeutic target for inflammatory diseases and show the value of TC3.6 to prevent EAE, with possible consequences for new therapeutic tools in MS.

Induction of oligodendrocyte differentiation and in vitro myelination by inhibition of rho-associated kinase

In inflammatory demyelinating diseases such as multiple sclerosis (MS), myelin degradation results in loss of axonal function and eventual axonal degeneration. Differentiation of resident oligodendrocyte precursor cells (OPCs) leading to remyelination of denuded axons occurs regularly in early stages of MS but halts as the pathology transitions into progressive MS. Pharmacological potentiation of endogenous OPC maturation and remyelination is now recognized as a promising therapeutic approach for MS. In this study, we analyzed the effects of modulating the Rho-A/Rho-associated kinase (ROCK) signaling pathway, by the use of selective inhibitors of ROCK, on the transformation of OPCs into mature, myelinating oligodendrocytes. Here we demonstrate, with the use of cellular cultures from rodent and human origin, that ROCK inhibition in OPCs results in a significant generation of branches and cell processes in early differentiation stages, followed by accelerated production of myelin protein as an indication of advanced maturation. Furthermore, inhibition of ROCK enhanced myelin formation in cocultures of human OPCs and neurons and remyelination in rat cerebellar tissue explants previously demyelinated with lysolecithin. Our findings indicate that by direct inhibition of this signaling molecule, the OPC differentiation program is activated resulting in morphological and functional cell maturation, myelin formation, and regeneration. Altogether, we show evidence of modulation of the Rho-A/ROCK signaling pathway as a viable target for the induction of remyelination in demyelinating pathologies.

The effect of cellular differentiation on HSV-1 infection of oligodendrocytic cells

Herpes simplex type 1 (HSV-1) is a neurotropic virus that infects many types of cells. Previous studies have demonstrated that oligodendrocytic cells are highly susceptible to HSV-1 infection. Here we analysed HSV-1 infection of a human oligodendrocytic cell line, HOG, and oligodendrocyte precursor cells (OPCs) cultured under growth or differentiation conditions. In addition to cell susceptibility, the role of the major cell receptors for viral entry was assessed. Our results revealed that OPCs and HOG cells cultured under differentiation conditions became more susceptible to HSV-1. On the other hand, viral infection induced morphological changes corresponding to differentiated cells, suggesting that HSV-1 might be inducing cell differentiation. We also observed colocalization of HVEM and nectin-1 with viral particles, suggesting that these two major HSV-1 receptors are functional in HOG cells. Finally, electron microscopy assays indicated that HSV-1 may be also entering OLs by macropinocytosis depending on their differentiation stage. In addition, vesicles containing intracellular enveloped virions observed in differentiated cells point to an endocytic mechanism of virus entry. All these data are indicative of diverse entry pathways dependent on the maturation stage of OLs.

Protocol to isolate a large amount of functional oligodendrocyte precursor cells from the cerebral cortex of adult mice and humans

During development, oligodendrocytes are generated from oligodendrocyte precursor cells (OPCs), a cell type that is a significant proportion of the total cells (3-8%) in the adult central nervous system (CNS) of both rodents and humans. Adult OPCs are responsible for the spontaneous remyelination that occurs in demyelinating diseases like Multiple Sclerosis (MS) and they constitute an interesting source of cells for regenerative therapy in such conditions. However, there is little data regarding the neurobiology of adult OPCs isolated from mice since an efficient method to isolate them has yet to be established. We have designed a protocol to obtain viable adult OPCs from the cerebral cortex of different mouse strains and we have compared its efficiency with other well-known methods. In addition, we show that this protocol is also useful to isolate functional OPCs from human brain biopsies. Using this method we can isolate primary cortical OPCs in sufficient quantities so as to be able to study their survival, maturation and function, and to facilitate an evaluation of their utility in myelin repair.