CNS & Neurological Disorders - Drug Targets (Formerly Current Drug Targets - CNS & Neurological Disorders) - Volume 14, Issue 2, 2015

Patients with multiple sclerosis (MS) often suffer from cognitive dysfunction; the underlying mechanisms have not been fully elucidated. Recent studies suggest that MS leads to heightened synaptic transmission and plasticity in different brain areas, and therefore may contribute to the observed behavioral abnormalities. Recent findings demonstrate synaptic plasticity changes in MS, including evidence from animal models of experimental autoimmune encephalomyelitis and human MS patients.

Multiple sclerosis (MS) is a progressive inflammatory disease of the central nervous system (CNS) leading to severe neurological deficits. To date, no treatment is available that halts disease progression, but clinical symptoms can be generally improved by therapies involving anti-inflammatory and/or immune modulatory reagents, which may cause off-target effects. Therefore, there remains a high and unmet need for more selective treatment strategies in MS. An early event in MS is a diminished function of the blood-brain barrier (BBB) which consists of specialized brain endothelial cells (BECs) that are supported in their barrier function by surrounding glial cells. Leakage and inflammation of the BECs in MS patients facilitate the massive influx of leukocytes into the brain parenchyma, which in turn induces irreversible demyelination, tissue damage and axonal dysfunction. Identification of ways to restore BBB function and promote its immune quiescence may therefore lead to the development of novel therapeutic regimes that not only specifically reduce leukocyte entry into the central nervous system but also restore the disturbed brain homeostasis. However, the complex network of molecular players that leads to BBB dysfunction in MS is yet to be fully elucidated. Recent discoveries unravelled a critical role for microRNAs (miRNAs) in controlling the function of the barrier endothelium in the brain. Here we will review the current knowledge on the involvement of BBB dysfunction in MS and the central role that miRNAs play in maintaining BBB integrity under inflammatory conditions.

Individual microRNAs and/or microRNA signatures were associated with Alzheimer’s disease (AD). We report here the recent advances brought to the identification of microRNA changes in AD brain and their biological function in the molecular pathways associated with the disease. This field represents a fertile route to understand the pathogenic mechanisms underlying Alzheimer’s disease. In addition we review recent studies aimed to discover promising biomarkers for AD diagnosis by microRNA expression profiles of biofluids from AD patients.

MicroRNAs are small RNAs involved in gene silencing. They play important roles in transcriptional regulation and are selectively and abundantly expressed in the central nervous system. A considerable amount of the human genome is comprised of tandem repeating nucleotide streams. Several diseases are caused by above-threshold expansion of certain trinucleotide repeats occurring in a protein-coding or non-coding region. Though monogenic, CAG trinucleotide repeat expansion disorders have a complex pathogenesis, various combinations of multiple coexisting pathways resulting in one common final consequence: selective neurodegeneration. Mutant protein and mutant transcript gain of toxic function are considered to be the core pathogenic mechanisms. The profile of microRNAs in CAG trinucleotide repeat disorders is scarcely described, however microRNA dysregulation has been identified in these diseases and microRNA-related intereference with gene expression is considered to be involved in their pathogenesis. Better understanding of microRNAs functions and means of manipulation promises to offer further insights into the pathogenic pathways of CAG repeat expansion disorders, to point out new potential targets for drug intervention and to provide some of the much needed etiopathogenic therapeutic agents. A number of disease-modifying microRNA silencing strategies are under development, but several implementation impediments still have to be resolved. CAG targeting seems feasible and efficient in animal models and is an appealing approach for clinical practice. Preliminary human trials are just beginning.

Amyotrophic lateral sclerosis (ALS) causes neurodegeneration of both upper and lower motor neurons and progressive muscle impairment, atrophy and death within approximately five years from diagnosis. The aetiology is still not clear but evidence obtained in animal models of the disease indicates a non-cell-autonomous mechanism with the active contribution of non-neuronal cells such as microglia, astrocytes, muscle and T cells, which differently participate to the diverse phases of the disease. Clinically indistinguishable forms of ALS occur as sporadic disease in the absence of known mutation, or can be initiated by genetic mutations. About two-third of familial cases are triggered by mutations of four genes that are chromosome 9 open reading frame 72 (C9ORF72), Cu/Zn superoxide dismutase (SOD1), fused in sarcoma/translocated in liposarcoma (FUS/TLS), TAR-DNA binding protein 43 (TDP43). There is at present no succesfull treatment against ALS and the identification of novel signalling pathways, molecular mechanisms and cellular mediators are still a major task in the search for effective therapies. MiRNAs are conserved, endogenous, non-coding RNAs that post-transcriptionally regulate protein expression. Produced as long primary transcripts, they are exported to the cytoplasm and further modified to obtain the mature miRNAs, with each step of their biogenesis being a potential step of regulation. There are more than 1000 different known human miRNA sequences, and more than 20-30% of all human protein-coding genes are likely controlled by miRNAs. This earns to miRNAs the definition of fine regulators of genetic networks. The discovery of the involvement of ALS mutated proteins TDP43 and FUS/TLS in miRNAs biogenesis strongly suggests a role of miRNA dysregulation also in ALS and many efforts are thus directed toward understanding the role of these small RNA molecules in the pathogenesis of ALS. The overall objective of this review is thus to highlight the emerging involvement of miRNAs in ALS. After a brief description of miRNA biogenesis and function, we discuss the effects of miRNA dysregulation in cellular and molecular pathways that lead to ALS neuroinflammation and neurodegeneration. In the last part, we focus on the mechanistic insights of miRNAs that might have implications for the development of novel neuroprotective agents against ALS, and on recent attempts to establish new molecular miRNA-based therapies. Paving the way for more comparative studies on neuroinflammatory and neurodegenerative mechanisms, this strategy indeed promises a broader impact on ALS.

MicroRNAs (miRNAs) are ∼22 nucleotide non-coding RNAs that control gene expression post-transcriptionally by base pairing to mRNAs. MiRNAs are predicted to target ∼50% of all protein-coding genes and functional studies indicate that they participate in the regulation of almost every cellular process. They also play a key role in pathogenetic mechanisms underlying several diseases, e.g. cancer, cardiovascular diseases, autoimmune diseases, and neurodegenerative diseases. Several miRNAs are expressed in the human brain where they contribute to equilibrium between maintenance and differentiation of neural stem cells. MiRNAs specific mechanisms of action and their roles in brain development and synaptic plasticity resulted in a great interest in the analysis of their potential role in the pathogenesis and pathophysiology of neuropsychiatric disorders. Currently, schizophrenia is one of the fields in psychiatry where miRNAs have been most widely investigated. The understanding of miRNAs role in schizophrenia has been achieved through association, functional and expression profiling studies on post mortem brain and peripheral tissues. Several studies identified association between neuropsychiatric disorders and variants in miRNAs including variations in miRNA/primary-/precursor-miRNAs sequences, in miRNAs biogenesis machinery genes, in the 3’UTR of target genes and in miRNAs expression. In summary, there is growing evidence that miRNAs exert a crucial role in gene expression regulation in the central nervous system and are altered in the development, presentation and response to treatment of psychiatric disorders. In this review we summarize the most significant results of experimental studies aimed at highlighting the involvement of human miRNAs in schizophrenia.

Glioblastoma (GBM) is among the most lethal human cancers, being generally characterized by rapid diffuse and infiltrative growth and high level of cellular heterogeneity associated with therapeutic resistance. Despite remarkable advances in cancer theranostics, which resulted in significant improvement of clinical outcomes, patient survival remains under one year. In recent years, considerable progress has been made in understanding the role of small non-coding RNAs, designated microRNAs, in the pathogenesis of GBM. Indeed, microRNAs were found to play a critical role in multiple steps of the tumorigenic process, including cellular proliferation, apoptosis evasion, invasion, angiogenesis, and stemness. Moreover, the modulation of microRNA expression, using either antisense oligonucleotides or precursor/mimic sequences, revealed a tremendous potential for application in GBM-targeted therapeutic approaches, either per se or in combination with chemo- and/or radiotherapy. In this manuscript, we review the regulatory role of microRNAs in key cellular processes underlying GBM tumorigenesis, including migration and invasion, apoptosis evasion, angiogenesis and GBM stem-like cell proliferation/differentiation, and discuss the current knowledge on their potential as diagnostic, prognostic and predictive biomarkers in this disease. We also address the latest advances in microRNA-based therapeutic approaches for GBM, by summarizing the major achievements in in vitro and pre-clinical studies. The trends identified by these studies are highlighted in order to provide new prospects for future developments towards the successful treatment of GBM.

MicroRNAs (miRNAs) are small non-coding RNAs that function as translational repressors and represent an important element in tissue development as well as disease. MiRNA sequences and their target sites in mRNAs show an extensive degree of conservation across species. A single miRNA is now believed to recognize the 3’untranslated region of mRNAs in a sequence-specific manner to inhibit the protein expression of literally hundreds of targeted mRNAs. The human genome encodes more than 1000 miRNAs which target over 60% of mammalian and human transcripts. It is thus not surprising that expression changes in miRNAs can have a far-reaching impact on cellular functions, including the opioid system. Opioids, psychoactive chemicals that resemble morphine in their pharmacological effects are a class of potent analgesics used for treating various forms of acute and chronic pain. The mu opioid receptor is primarily responsible for opioid analgesia and anti-nociceptive tolerance. There is an ever-growing appreciation of miRNAs as important regulators of biological processes where opioids have an important impact, such as regulation of opioid receptors themselves. For example, a large number of splice variants of the primary transcript involve both 3' and 5' splicing of the mu opioid receptor mRNA, many of which can potentially be targeted by miRNAs. Conversely, miRNAs can be regulated by opioids. Two mu opioid receptor agonists, morphine and fentanyl, display differential mechanisms of signalling linked to specific miRNA expression. Moreover, miRNA – opioid connections impact on neuronal cell development, drug addiction, pain perception, neuroimmune system interaction and cell proliferation and tumorigenesis. These aspects will be the subject of this review.

Activated astrocytes, which can also be referred to as reactive astrocytes or astrogliosis, have been identified in affected regions of common neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis and multiple sclerosis. Activated astrocytes may be beneficial, promoting neuronal survival due to their production of growth factors and neurotrophins. Activated astrocytes can also be detrimental to neighboring neurons in neuroinflammatory processes. Astrocytes exposed to certain inflammatory stimulants in vitro have been shown to release potentially neurotoxic molecules, including inflammatory cytokines, glutamate, nitric oxide and reactive oxygen species. It has recently been shown that adult human astrocytes stimulated with interferon-γ, a common inflammatory cytokine evidently present in neuropathological brains, exert potent neurotoxicity in vitro. This interferon- γ-induced astrocytic neurotoxicity is mediated by the activation of the Janus kinase-signal transducer and activator of transcription (STAT) 3 pathway in the astrocytes, and involves intracellular phosphorylation of STAT3 at tyrosine-705 residue. Therefore, control of STAT3 activation in human astrocytes may be a promising new therapeutic strategy for a broad spectrum of neurodegenerative and neuroinflammatory disorders where activated astrocytes may contribute to the pathology.

Alzheimer’s disease (AD) is the most common cause of dementia, accounting for more than half of cases with cognitive impairment. With numbers of patients expected to rise sharply over the following years in parallel with the ageing of population, there is intense clinical interest in discovering modifiable risk factors that may contribute to the increasing prevalence of AD. Accumulating data from in vitro and epidemiological studies have highlighted the vascular component of AD and raised hope that treatment of vascular risk factors could eventually lead to primary prevention of AD. Among all the possible pathologic processes that have been tested for an association with AD, diabetes, hypertension and dyslipidemia are the most prominent. Here, we will briefly review the data highlighting a potential correlation of these diseases with AD. Then, we will present observational studies and clinical trials that assessed the impact of their respective approved medical therapies on AD incidence. We conclude by providing clinical information for the physician on potentially effective and non-effective medical treatments. Further research is ongoing and time will show whether AD will cease to be considered a pure, non-preventable neurodegenerative process or whether vascular risk factor management may also result in primary AD prevention.

Depression is an affective disorder characterized by hallucination, delusion and increased social risk and is estimated to affect approximately 20 % of the population at some point during the lifetime. As per World Health Organization (WHO) it is predicted to be the leading cause of burden of disease by 2030. Effects of currently available antidepressants have explained the monoamine hypothesis of depression, which proposes that impaired release of serotonin, noradrenaline and dopamine, are thought to be responsible for the development of depressive symptoms. However, these drugs are not specific for their action, as they also inhibit other enzymes; this explains the side effects/drug interactions associated with these agents. The present review will familiarize the readers with novel targets being identified for depression which will be certainly beneficial for researcher, academician for the development of drugs for the management of depression and related behavior.

Acute methamphetamine (METH) intoxication induces metabolic brain activation as well as multiple physiological and behavioral responses that could result in life-threatening health complications. Previously, we showed that METH (9 mg/kg) used in freely moving rats induces robust leakage of blood-brain barrier, acute glial activation, vasogenic edema, and structural abnormalities of brain cells. These changes were tightly correlated with drug-induced brain hyperthermia and were greatly potentiated when METH was used at warm ambient temperatures (29°C), inducing more robust and prolonged hyperthermia. Extending this line of research, here we show that METH also strongly increases the permeability of the blood-spinal cord barrier as evidenced by entry of Evans blue and albumin immunoreactivity in T9-12 segments of the spinal cord. Similar to the blood-brain barrier, leakage of bloodspinal cord barrier was associated with acute glial activation, alterations of ionic homeostasis, water tissue accumulation (edema), and structural abnormalities of spinal cord cells. Similar to that in the brain, all neurochemical alterations correlated tightly with drug-induced elevations in brain temperature and they were enhanced when the drug was used at 29°C and brain hyperthermia reached pathological levels (>40°C). We discuss common features and differences in neural responses between the brain and spinal cord, two inseparable parts of the central nervous system affected by METH exposure.

Aging is a process of progressive decline in physiological functions resulting in increased vulnerability to diseases and death. Aging results in increased rates of age related disorders like neurodegenerative diseases, cardiovascular diseases, diabetes, cancer, arthritis etc. Modulation of insulin signaling, protein aggregation, stress, free radical damage and inflammation are the major causes for deleterious changes resulting in aging. Many studies are being undertaken to find novel compounds which can improve a typical human life span and aid in healthy aging. We investigated the potential of one such compound silymarin for its anti-aging effect. Silymarin is a flavanone derivative extracted from the seeds of the milk thistle Silybum marianum. It is widely used for the treatment of liver diseases in clinical practice. We tested the anti-aging efficacy of silymarin using the Caenorhabditis elegans model system. Our results demonstrate that C. elegans treated with 25μM and 50μM silymarin concentration resulted in an increase in mean lifespan by 10.1% and 24.8% respectively compared to untreated control. Besides increased lifespan, silymarin treated aged animals showed better locomotion rate, higher response to stimuli and improved tolerance to stress compared to untreated control. We also checked the potential of silymarin to slow the progression of neurodegenerative disorder like Alzheimer's disease (AD) by using CL4176 C. elegans model for AD. C. elegans CL4176 transgenic animal induces expression of amyloid beta-protein (Aβ1-42) in muscle tissues when subjected to temperature of 23°C and above resulting in worm paralysis. CL4176 animals treated with silymarin showed delayed paralysis via enhancing resistance to oxidative stress. These results suggested that silymarin is a potential hormetin for preventing aging and age-related diseases.