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.

Dynamic Role of Phospholipases A2 in Health and Diseases in the Central Nervous System.

Phospholipids are major components in the lipid bilayer of cell membranes. These molecules are comprised of two acyl or alkyl groups and different phospho-base groups linked to the glycerol backbone. Over the years, substantial interest has focused on metabolism of phospholipids by phospholipases and the role of their metabolic products in mediating cell functions. The high levels of polyunsaturated fatty acids (PUFA) in the central nervous system (CNS) have led to studies centered on phospholipases A2 (PLA2s), enzymes responsible for cleaving the acyl groups at the sn-2 position of the phospholipids and resulting in production of PUFA and lysophospholipids. Among the many subtypes of PLA2s, studies have centered on three major types of PLA2s, namely, the calcium-dependent cytosolic cPLA2, the calcium-independent iPLA2 and the secretory sPLA2. These PLA2s are different in their molecular structures, cellular localization and, thus, production of lipid mediators with diverse functions. In the past, studies on specific role of PLA2 on cells in the CNS are limited, partly because of the complex cellular make-up of the nervous tissue. However, understanding of the molecular actions of these PLA2s have improved with recent advances in techniques for separation and isolation of specific cell types in the brain tissue as well as development of sensitive molecular tools for analyses of proteins and lipids. A major goal here is to summarize recent studies on the characteristics and dynamic roles of the three major types of PLA2s and their oxidative products towards brain health and neurological disorders.

Inhibitors of Src Family Kinases, Inducible Nitric Oxide Synthase, and NADPH Oxidase as Potential CNS Drug Targets for Neurological Diseases.

Neurological diseases share common neuroinflammatory and oxidative stress pathways. Both phenotypic and molecular changes in microglia, astrocytes, and neurons contribute to the progression of disease and present potential targets for disease modification. Src family kinases (SFKs) are present in both neurons and glial cells and are upregulated following neurological insults in both human and animal models. In neurons, SFKs interact with post-synaptic protein domains to mediate hyperexcitability and neurotoxicity. SFKs are upstream of signaling cascades that lead to the modulation of neurotransmitter receptors and the transcription of pro-inflammatory cytokines as well as producers of free radicals through the activation of glia. Inducible nitric oxide synthase (iNOS/NOS-II) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2), the major mediators of reactive nitrogen/oxygen species (RNS/ROS) production in the brain, are also upregulated along with the pro-inflammatory cytokines following neurological insult and contribute to disease progression. Persistent neuronal hyperexcitability, RNS/ROS, and cytokines can exacerbate neurodegeneration, a common pathognomonic feature of the most prevalent neurological disorders such as Alzheimer's disease, Parkinson's disease, and epilepsy. Using a wide variety of preclinical disease models, inhibitors of the SFK-iNOS-NOX2 signaling axis have been tested to cure or modify disease progression. In this review, we discuss the SFK-iNOS-NOX2 signaling pathway and their inhibitors as potential CNS targets for major neurological diseases.

Nerve cell death, oxidative stress, and inflammation of the brain are the most common pathological processes of many neurological diseases. These processes are mediated through changes in glial cells, the supporting cells in the brain, via several molecular pathways. Some of these pathways are potential drug targets for the mitigation of brain pathology. In this review, we focus on pathways involving Src family kinases, inducible nitric oxide synthase and nicotinamide adenine dinucleotide phosphate oxidase, and their inhibitors, which are promising agents for modifying neurological diseases.

Efficient engraftment of genetically modified cells is necessary to ameliorate central nervous system involvement of murine model of mucopolysaccharidosis type II by hematopoietic stem cell targeted gene therapy.

Mucopolysaccharidosis type II (MPS II) is a lysosomal storage disease (LSD) caused by a deficiency of the iduronate-2-sulfatase (IDS) that catabolizes glycosaminoglycans (GAGs). Abnormal accumulations of GAGs in somatic cells lead to various manifestations including central nervous system (CNS) disease. Enzyme replacement therapy (ERT) and hematopoietic stem cell transplantation (HSCT) are the currently available therapy for MPS II, but both therapies fail to improve CNS manifestations. We previously showed that hematopoietic stem cell targeted gene therapy (HSC-GT) with lethal irradiation improved CNS involvement in a murine model of MPS II which lacks the gene coding for IDS. However, the strong preconditioning, with lethal irradiation, would cause a high rate of morbidity and mortality. Therefore, we tested milder preconditioning procedures with either low dose irradiation or low dose irradiation plus an anti c-kit monoclonal antibody (ACK2) to assess CNS effects in mice with MPS II after HSC-GT. Mice from all the HSC-GT groups displayed super-physiological levels of IDS enzyme activity and robust reduction of abnormally accumulated GAGs to the wild type mice levels in peripheral organs. However, only the mice treated with lethal irradiation showed significant cognitive function improvement as well as IDS elevation and GAG reduction in the brain. These results suggest that an efficient engraftment of genetically modified cells for HSC-GT requires strong preconditioning to ameliorate CNS involvement in cases with MPS II.

A Novel Inhibition Modality for Phosphodiesterase 2A.

Phosphodiesterase type 2A (PDE2A) has received considerable interest as a molecular target for treating central nervous system diseases that affect memory, learning, and cognition. In this paper, the authors present the discovery of small molecules that have a novel modality of PDE2A inhibition. PDE2A possesses GAF-A and GAF-B domains and is a dual-substrate enzyme capable of hydrolyzing both cGMP and cAMP, and activation occurs through cGMP binding to the GAF-B domain. Thus, positive feedback of the catalytic activity to hydrolyze cyclic nucleotides occurs in the presence of appropriate concentrations of cGMP, which binds to the GAF-B domain, resulting in a "brake" that attenuates downstream cyclic nucleotide signaling. Here, we studied the inhibitory effects of some previously reported PDE2A inhibitors, all of which showed impaired inhibitory effects at a lower concentration of cGMP (70 nM) than a concentration effective for the positive feedback (4 µM). This impairment depended on the presence of the GAF domains but was not attributed to binding of the inhibitors to these domains. Notably, we identified PDE2A inhibitors that did not exhibit this behavior; that is, the inhibitory effects of these inhibitors were as strong at the lower concentration of cGMP (70 nM) as they were at the higher concentration (4 µM). This suggests that such inhibitors are likely to be more effective than previously reported PDE2A inhibitors in tissues of patients with lower cGMP concentrations.

Checking NEKs: Overcoming a Bottleneck in Human Diseases.

In previous years, several kinases, such as phosphoinositide 3-kinase (PI3K), mammalian target of rapamycin (mTOR), and extracellular-signal-regulated kinase (ERK), have been linked to important human diseases, although some kinase families remain neglected in terms of research, hiding their relevance to therapeutic approaches. Here, a review regarding the NEK family is presented, shedding light on important information related to NEKs and human diseases. NEKs are a large group of homologous kinases with related functions and structures that participate in several cellular processes such as the cell cycle, cell division, cilia formation, and the DNA damage response. The review of the literature points to the pivotal participation of NEKs in important human diseases, like different types of cancer, diabetes, ciliopathies and central nervous system related and inflammatory-related diseases. The different known regulatory molecular mechanisms specific to each NEK are also presented, relating to their involvement in different diseases. In addition, important information about NEKs remains to be elucidated and is highlighted in this review, showing the need for other studies and research regarding this kinase family. Therefore, the NEK family represents an important group of kinases with potential applications in the therapy of human diseases.

Acute inhibition of the CNS-specific kinase TTBK1 significantly lowers tau phosphorylation at several disease relevant sites.

Hyperphosphorylated tau protein is a pathological hallmark of numerous neurodegenerative diseases and the level of tau pathology is correlated with the degree of cognitive impairment. Tau hyper-phosphorylation is thought to be an early initiating event in the cascade leading to tau toxicity and neuronal death. Inhibition of tau phosphorylation therefore represents an attractive therapeutic strategy. However, the widespread expression of most kinases and promiscuity of their substrates, along with poor selectivity of most kinase inhibitors, have resulted in systemic toxicities that have limited the advancement of tau kinase inhibitors into the clinic. We therefore focused on the CNS-specific tau kinase, TTBK1, and investigated whether selective inhibition of this kinase could represent a viable approach to targeting tau phosphorylation in disease. In the current study, we demonstrate that TTBK1 regulates tau phosphorylation using overexpression or knockdown of this kinase in heterologous cells and primary neurons. Importantly, we find that TTBK1-specific phosphorylation of tau leads to a loss of normal protein function including a decrease in tau-tubulin binding and deficits in tubulin polymerization. We then describe the use of a novel, selective small molecule antagonist, BIIB-TTBK1i, to study the acute effects of TTBK1 inhibition on tau phosphorylation in vivo. We demonstrate substantial lowering of tau phosphorylation at multiple sites implicated in disease, suggesting that TTBK1 inhibitors may represent an exciting new approach in the search for neurodegenerative disease therapies.

Regulating membrane lipid levels at the synapse by small-molecule inhibitors of monoacylglycerol lipase: new developments in therapeutic and PET imaging applications.

Monoacylglycerol lipase (MAGL) is a major endocannabinoid hydrolyzing enzyme and can be regulated to control endogenous lipid levels in the brain. This review highlights the pharmacological roles and in vivo PET imaging of MAGL in brain.

Matrix metalloproteinase-3 in brain physiology and neurodegeneration.

Structural and functional synapse reorganization is one of the key issues of learning and memory mechanisms. Specific proteases, called matrix metalloproteinases (MMPs), play a pivotal role during learning-related modification of neural circuits. Different types of MMPs modify the extracellular perisynaptic environment, leading to the plastic changes in the synapses. In recent years, there has been an increasing interest in the role played by matrix metalloproteinase-3 (MMP-3) in various processes occurring in the mammalian brain, both in physiological and pathological conditions. In this review, we discuss a crucial function of MMP-3 in synaptic plasticity, learning, neuronal development, as well as in neuroregeneration. We discuss the involvement of MMP-3 in synaptic long-term potentiation, which is likely to have a profound impact on experience-dependent learning. On the other hand, we also provide examples of deleterious actions of uncontrolled MMP-3 activity on the central nervous system (CNS) and its contribution to Alzheimer's and Parkinson's diseases (AD and PD). Since the molecular mechanisms controlled by MMP-3 have a profound and diverse impact on physiological and pathological brain functioning, their deep understanding may be crucial for the development of more specific methods for the treatment of neuropsychiatric diseases.

Phosphodiesterase Type 4 Inhibition in CNS Diseases.

Phosphodiesterases (PDEs) have been an interesting drug target for many diseases. Although a vast number of mainly preclinical studies demonstrates beneficial effects of PDE inhibitors for central nervous system (CNS) diseases, no drugs are currently available for CNS indications. In this review, we discuss the rationale of PDE4 inhibitors for different CNS diseases, including memory impairments, striatal disorders, multiple sclerosis (MS), and acquired brain injury (ABI). However, clinical development has been problematic due to mechanism-based adverse effects of these drugs in humans. Our increased understanding of factors influencing the conformational state of the PDE4 enzyme and of how to influence the binding affinity of PDE4 subtype inhibitors, holds promise for the successful development of novel selective PDE4 inhibitors with higher efficacy and fewer adverse effects.

Synthesis, CYP 450 evaluation, and docking simulation of novel 4-aminopyridine and coumarin derivatives.

Four series of novel compounds based on 4-aminopyridine, glatiramer acetate, pyrone, and coumarin backbones were sufficiently synthesized and identified by spectroscopic methods. CYP enzyme inhibition assays of five predominate human P450 isozymes indicate that all compounds, except for 4-hydrazide pyridine 1c, seem to be less toxic than 4-aminopyridine. Further investigation of the compounds using molecular docking experiments revealed different, the same, or stronger binding modes for most of the synthesized compounds, with both polar and hydrophobic interactions with the 1WDA and 1J95 receptors compared to benzoyl l-arginine amide and 4-aminopyridine, respectively. These results introduce the synthesized compounds as K+ channel blockers that could be considered for in vivo CNS disease studies.

DEGS1-associated aberrant sphingolipid metabolism impairs nervous system function in humans.

BACKGROUND: Sphingolipids are important components of cellular membranes and functionally associated with fundamental processes such as cell differentiation, neuronal signaling, and myelin sheath formation. Defects in the synthesis or degradation of sphingolipids leads to various neurological pathologies; however, the entire spectrum of sphingolipid metabolism disorders remains elusive. METHODS: A combined approach of genomics and lipidomics was applied to identify and characterize a human sphingolipid metabolism disorder. RESULTS: By whole-exome sequencing in a patient with a multisystem neurological disorder of both the central and peripheral nervous systems, we identified a homozygous p.Ala280Val variant in DEGS1, which catalyzes the last step in the ceramide synthesis pathway. The blood sphingolipid profile in the patient showed a significant increase in dihydro sphingolipid species that was further recapitulated in patient-derived fibroblasts, in CRISPR/Cas9-derived DEGS1-knockout cells, and by pharmacological inhibition of DEGS1. The enzymatic activity in patient fibroblasts was reduced by 80% compared with wild-type cells, which was in line with a reduced expression of mutant DEGS1 protein. Moreover, an atypical and potentially neurotoxic sphingosine isomer was identified in patient plasma and in cells expressing mutant DEGS1. CONCLUSION: We report DEGS1 dysfunction as the cause of a sphingolipid disorder with hypomyelination and degeneration of both the central and peripheral nervous systems. TRIAL REGISTRATION: Not applicable. FUNDING: Seventh Framework Program of the European Commission, Swiss National Foundation, Rare Disease Initiative Zurich.

Melatonin, a calpain inhibitor in the central nervous system: Current status and future perspectives.

Dysregulation of neuronal Ca2+ and oxidative stress plays an important role in the activation of cysteine proteases including calpains and caspases that contribute to neuronal death. In neurodegenerative diseases, traumatic brain injury, stroke, and neuropathic pain calpain activities are markedly increased. Melatonin is a beneficial supplement in the treatment of central nervous system (CNS) disorders. Melatonin is a potent antioxidant and works as a free-radical scavenger to regulate a large number of molecular pathways, including oxidative stress, inflammation, apoptosis, and cell death under different pathological conditions. However, limited studies have evaluated the inhibitory effect of melatonin on calpains. This review summarizes the current knowledge related to the effects of melatonin on calpains in some of the common CNS disorders.

Aminoacyl-tRNA synthetase deficiencies in search of common themes.

PURPOSE: Pathogenic variations in genes encoding aminoacyl-tRNA synthetases (ARSs) are increasingly associated with human disease. Clinical features of autosomal recessive ARS deficiencies appear very diverse and without apparent logic. We searched for common clinical patterns to improve disease recognition, insight into pathophysiology, and clinical care. METHODS: Symptoms were analyzed in all patients with recessive ARS deficiencies reported in literature, supplemented with unreported patients evaluated in our hospital. RESULTS: In literature, we identified 107 patients with AARS, DARS, GARS, HARS, IARS, KARS, LARS, MARS, RARS, SARS, VARS, YARS, and QARS deficiencies. Common symptoms (defined as present in ≥4/13 ARS deficiencies) included abnormalities of the central nervous system and/or senses (13/13), failure to thrive, gastrointestinal symptoms, dysmaturity, liver disease, and facial dysmorphisms. Deep phenotyping of 5 additional patients with unreported compound heterozygous pathogenic variations in IARS, LARS, KARS, and QARS extended the common phenotype with lung disease, hypoalbuminemia, anemia, and renal tubulopathy. CONCLUSION: We propose a common clinical phenotype for recessive ARS deficiencies, resulting from insufficient aminoacylation activity to meet translational demand in specific organs or periods of life. Assuming residual ARS activity, adequate protein/amino acid supply seems essential instead of the traditional replacement of protein by glucose in patients with metabolic diseases.

Role of GTPases in the Regulation of Mitochondrial Dynamics in Alzheimer's Disease and CNS-Related Disorders.

Data obtained from several studies have shown that mitochondria are involved and play a central role in the progression of several distinct pathological conditions. Morphological alterations and disruptions on the functionality of mitochondria may be related to metabolic and energy deficiency in neurons in a neurodegenerative disorder. Several recent studies demonstrate the linkage between neurodegeneration and mitochondrial dynamics in the spectrum of a promising era called precision mitochondrial medicine. In this review paper, an analysis of the correlation between mitochondria, Alzheimer's disease, and other central nervous system (CNS)-related disorders like the Parkinson's disease and the autism spectrum disorder is under discussion. The role of GTPases like the mfn1, mfn2, opa1, and dlp1 in mitochondrial fission and fusion is also under investigation, influencing mitochondrial population and leading to oxidative stress and neuronal damage.

Humble beginnings with big goals: Small molecule soluble epoxide hydrolase inhibitors for treating CNS disorders.

Soluble epoxide hydrolase (sEH) degrades epoxides of fatty acids including epoxyeicosatrienoic acid isomers (EETs), which are produced as metabolites of the cytochrome P450 branch of the arachidonic acid pathway. EETs exert a variety of largely beneficial effects in the context of inflammation and vascular regulation. sEH inhibition is shown to be therapeutic in several cardiovascular and renal disorders, as well as in peripheral analgesia, via the increased availability of anti-inflammatory EETs. The success of sEH inhibitors in peripheral systems suggests their potential in targeting inflammation in the central nervous system (CNS) disorders. Here, we describe the current roles of sEH in the pathology and treatment of CNS disorders such as stroke, traumatic brain injury, Parkinson's disease, epilepsy, cognitive impairment, dementia and depression. In view of the robust anti-inflammatory effects of stem cells, we also outlined the potency of stem cell treatment and sEH inhibitors as a combination therapy for these CNS disorders. This review highlights the gaps in current knowledge about the pathologic and therapeutic roles of sEH in CNS disorders, which should guide future basic science research towards translational and clinical applications of sEH inhibitors for treatment of neurological diseases.

Natural activators of adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) and their pharmacological activities.

Adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) is a heterotrimeric enzyme which monitors cellular energy status and regulates metabolism with energy balance. AMPK activation, as a master regulator of metabolism, plays role in key tissues like liver, skeletal muscles, and heart as well as central nervous system (CNS). Activation of the enzyme by indirect activators attracts scientific attentions to treat diabetes, obesity, cancer, and other related metabolic disorders like physiological and pathophysiological states in CNS. A number of hormones and pharmacological agents have been reported to activate AMPK including paroxetine, metformin, thiazolidinediones, adiponectin, leptin, interleukin-6, and etc. AMPK activity is prominent in regulation of glucose, lipid, and proteins metabolism as well as mitochondrial biogenesis and autophagy. Activation of AMPK in the liver decrease blood glucose and in skeletal muscles stimulates glucose uptake independently of insulin through modulation of activity of several downstream substrates. Activation of AMPK inhibits synthesis and induces oxidation of fatty acids, which may reduce ectopic lipid accumulation and improve insulin action. The enzyme activation promotes cardiovascular homeostasis by ensuring optimum redox balance of heart and vascular tissue. In addition, AMPK signaling may link to cancer development via regulation of checkpoints of cell cycle. Numerous of conventional drugs have been derived from natural resources, while the application of this fruitful source of chemical structures have not been explored in depth. A number of these compounds are discussed in this review that exhibit beneficial effects in metabolic disorders through AMPK activation.

Cladribine treatment for Erdheim-Chester disease involving the central nervous system and concomitant polycythemia vera: A case report.

Erdheim-Chester disease (ECD), a rare form of non-Langerhans cell histiocytosis, is characterized by the infiltration of foamy CD68+ and CD1a- histiocytes into multiple organ systems. Central nervous system (CNS) involvement has recently been reported to be a poor prognostic factor when treating ECD with interferon alpha. We report the case of a 66-year-old Japanese patient with ECD involving the CNS who harbored the BRAF V600E mutation and also concomitantly developed polycythemia vera with the JAK2 V617F mutation. We confirmed 2-chlorodeoxyadenosine (cladribine) therapy to be effective for the patient in this case.

Applications of carbonic anhydrases inhibitors in renal and central nervous system diseases.

INTRODUCTION: There are tissues and organs, among which kidneys and the central nervous system (CNS), rich in various isoforms of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1). Their role is to regulate pH, to provide bicarbonate or H+ ions for electrolyte secretion and possibly a metabolic one. Considering these two systems, CA inhibitors are clinically used mainly as diuretics and antiepileptics, but novel applications in the management of drug-induced renal injury, sleep apnea, migraine, lowering intracranial pressure, cognitive impairment, neuropathic pain, and cerebral ischemia have emerged. AREAS COVERED: The various classes of clinically used/investigational CA inhibitors and their applications in the management of renal and CNS - connected diseases is reviewed. A patent and literature review covering the period 2013-2018 is presented. EXPERT OPINION: Both kidneys and CNS are rich in many CA isoforms (CAIs), present also in high amounts. Their inhibition and activation has pharmacological applications, already exploited for diuretic and antiepileptic drugs for decades. New applications were demonstrated in the last years for the CAIs in the management of idiopathic intracranial hypertension, cerebral ischemia, neuropathic pain, avoiding the disruption of blood-brain barrier, and prevention/treatment of migraine, and for the activators for cognition enhancement and the possible treatment of posttraumatic shock and phobias.

miR-1303 regulates BBB permeability and promotes CNS lesions following CA16 infections by directly targeting MMP9.

Coxsackievirus A16 (CA16) is a member of the Picornaviridae family and causes mild and self-limiting hand, foot, and mouth disease (HFMD) in infants and young children. CA16 infection can also progress to central nervous system (CNS) complications; however, the underlying mechanism by which CA16 penetrates the blood-brain barrier (BBB) and then causes CNS damage remains unclear. This study aimed to explore the mechanism of CA16 neurotropic tropism by establishing an in vitro BBB model with CA16 infection and an in vivo CA16 rhesus monkey infant infection model. The results showed that CA16 infection induced increased permeability of the BBB accompanied by upregulation of matrix metalloproteinase 9 (MMP9) expression. Subsequently, high-throughput miRNA sequencing technology and bioinformatics analysis revealed that miR-1303 may regulate BBB permeability by targeting MMP9. Next, we used dual-luciferase, qRT-PCR, and western blot assays to provide evidence of MMP9 targeting by miR-1303. Further experiments revealed that CA16 infection promoted the degradation of junctional complexes (Claudin4, Claudin5, VE-Cadherin, and ZO-1), likely by downregulating miR-1303 and upregulating MMP9. Finally, EGFP-CA16 infection could enter the CNS by facilitating the degradation of junctional complexes, eventually causing neuroinflammation and injury to the CNS, which was confirmed using the in vivo rhesus monkey model. Our results indicate that CA16 might penetrate the BBB and then enter the CNS by downregulating miR-1303, which disrupts junctional complexes by directly regulating MMP9 and ultimately causing pathological CNS changes. These results provide new therapeutic targets in HFMD patients following CA16 infection.