CNS & Neurological Disorders - Drug Targets (Formerly Current Drug Targets - CNS & Neurological Disorders) - Volume 9, Issue 6, 2010

Research Highlights Hoever, P.; de Haas, S.; Winkler, J.; Schoemaker, R.C.; Chiossi, E.; van Gerven, J.; Dingemanse, J. Orexin receptor antagonism, a new sleep-promoting paradigm: an ascending single-dose study with almorexant. Clin Pharmacol Ther, 2010, 87, 593-600. Targeting the Orexin System: A New Approach for Treating Insomnia Sleep is an essential part of our everyday lives that matters more than most people realize. Regulation of the sleep-waking cycle is complex, involving multiple neurological circuits and diverse endogenous molecules. Interplay among assorted neuroanatomical and neurochemical systems maintain the waking state. Sleep-onset is governed by the interacting forces of the sleep drive, which steadily increases with duration of waking and circadian fluctuations. Epidemiological and clinical studies have established that sleep disorders are common and often untreated. Insomnia, one of the most common sleep disorders among adults, is a symptom of difficulty initiating sleep, maintaining sleep, or non-restorative sleep associated with impaired next-day functioning and an increased risk for accidents. Insomnia treatment continues to be dominated by benzodiazepines and related compounds, which are powerful sedatives that enhance the inhibitory action of GABA (γ-aminobutyric acid) in the brain. The hypocretins/orexins are recently-discovered peptides with a discrete localization in the lateral hypothalamus. A single gene encodes hypocretin, which is cleaved by proteolytic processing into two smaller peptides: hypocretin-1 (orexin A) and hypocretin-2 (orexin B). These neurons project throughout the entire brain and spinal cord, providing especially heavy innervation to forebrain and brainstem neuronal populations implicated in wakefulness. Hypocretin/orexin has been implicated in narcolepsy, a human sleep disorder. Two orexin receptor subtypes, OX1R and OX2R, have been cloned. They are serpentine G-protein coupled receptors that bind both orexins with low selectivity and are coupled functionally to Ca++ mobilization, probably through transient receptor potential channels. The hypocretin/orexin system plays an important role in the regulation of sleep and wakefulness, especially in the maintenance of long, consolidated awake periods. Increased hypocretin/orexin activity has been observed during waking and at the end of sleep. Almorexant, a tetrahydroisoquinoline derivative, is a reversible, selective, orally active non-selective OX1 and OX2 receptor antagonist that inhibits functional consequences of OX1 and OX2 receptor activation, such as intracellular Ca++ mobilization. Almorexant promotes sleep in a dose-dependent manner in both healthy volunteers and in patients with primary insomnia. In a new single-dose study, Almorexant was administered in an ascending single-dose (1-1,000 mg) to evaluate tolerability, pharmacokinetics, and pharmacodynamics. In this study, subjects showed no decrements on motor function or reactive time on the following day. No evidence of cataplexy or abnormal REM behavior was reported. However, new studies involving night-time administration in relation of Almorexan in patients with sleep disorders are necessary to explore the role of the hypocretin/orexin during the circadian cycle. Modulation of the sleep-wake cycle is very much a question of timing of dosing, so the development of dosage at the optimal time will be essential for this approach. On the other hand, most of the published compounds targeting the hypocretin/orexin system have been developed using radioliganddisplacement assays or simple functional assays (intracellular Ca++ release) in recombinant cells. Hypocretin/orexin signaling cascades operating in the central nervous system will allow the development of more specific assays and generate compounds to block or activate defined branches of the signaling cascades (e.g. MAP kinase cascades), potentially creating drugs with different efficacy profiles.

The recent biological revolution, spurred by the various genome projects, will lead to novel discoveries of the basic principles governing biological processes in the cell and, as a consequence, to a thorough understanding of life in general. The aging of the human population has uncovered a panoply of novel disorders which threaten the overall balance of modern societies. Among these diseases, neurodegenerative conditions play a central role because of their highly debilitating nature and the tremendous societal costs associated. Diseases such as Alzheimer's, Parkinson's, Huntington's, or amyotrophic lateral sclerosis are becoming more and more common. Currently, there are no effective therapeutic strategies to prevent the onset or to stop progression of these diseases, and symptomatic treatments are reasonably successful only in Parkinson's disease. While tremendous progress has been made in the understanding of the underlying mechanisms associated with these disorders, it is essential to devote more attention to their molecular basis, in the hope that we can open novel avenues for therapeutic intervention. The present issue focuses on the current status of drug discovery for central nervous system disorders with the goal of contributing to the closure of the gap between the bench and bedside. Numerous questions demand urgent answers: Is aging an irreversible process? Can we interfere with or prevent the accumulation of toxic protein intermediates? Can we restore the normal function of cellular quality control mechanisms? Can we identify novel targets and novel therapeutic opportunities? Although the answers to these questions remain elusive, recent findings at the molecular level in these different diseases are opening whole new avenues for therapeutic intervention that are only starting to be explored. The review articles included describe the new paradigms under consideration for targeting the broad spectrum of human neurodegenerative diseases. We believe that this issue represents a contemporary and comprehensive view on how different molecular aspects of disease can be used as targets for drug discovery, and we hope the broad readership of the journal find these articles informative and inspiring.

Genomic blood biomarkers hold great promise for development of novel clinical and therapeutic approaches in patients with neurodegenerative diseases. Such biomarkers could prove invaluable in early disease diagnosis, monitoring of disease progression, or assessment of response to therapy. More importantly, they could be helpful in search for disease-modifying new therapies which are very much needed in modern approaches to treatment of neurodegenerative diseases, serving as surrogate endpoints in clinical trials. However, when performing expression profiling experiments aimed at discovery of new biomarker genes, standard operating procedures regarding sample collection, microarray methodology, and statistical analysis need to be fully developed and strictly adhered to. Several studies performed on patients with multiple sclerosis, Huntington's, Parkinson's and Alzheimer's disease offer promise that such approaches might prove useful in clinical practice. Crucial for successful application of any genomic biomarker will be confirmation in multiple independent patient cohorts and correlation of the improvement in biomarker endpoint with clinical improvement in longitudinal patient studies.

Neurodegenerative diseases trigger neuronal cell death by a variety of endogenous suicide pathways. Although cell death may occur through highly heterogeneous processes, specific cell organelles and stress sensors have shown promise as potential therapeutic targets. The plasma membrane senses stress through residing receptors, which can directly or indirectly activate apoptosis. Importantly, several events involved in neuronal death also affect mitochondria homeostasis, leading to calcium uptake, opening of the permeability transition pore, and release of apoptogenic factors. In addition, nuclear DNA damage triggers cell death, where p53 is activated to modulate the expression of selected apoptosis target genes. Signaling proteins implicated in apoptosis pathways are enriched at the Golgi complex, including death receptors and the phosphoinositide 3-kinase. Finally, neurodegenerative diseases progress with accumulation of misfolded proteins, deficiently removed by intracellular proteases or chaperones, and transport abnormalities due to disturbance of cytoskeletal organization in degenerating neurons. The challenge is to decode the complex signaling network of inter-organellar crosstalk leading to cell death and identify therapeutic approaches for delaying or preventing neurodegenerative diseases.

Target-directed drug design, although a conceptually rational approach, is only one strategy for drug discovery. In the case of neurodegenerative diseases where molecular targets and disease mechanisms are unknown, even when specific genes are known to trigger the disease, phenotypic screening offers another approach. This review describes the establishment of phenotypic screening assays using primary neurons subjected to a disease-relevant pathophysiological stress and measuring the most important functional outcome, survival. Although a challenge both to screening teams to reproducibly produce the cells and chemists to interpret structure-activity relationships, such systems have historically identified or produced effective drugs. The primary screening assay is only the start; once hits are validated, they must be characterized using traditional target-directed or mechanism-based secondary assays to establish their selectivity, lack of side-effect liability, and eventually be shown to produce the desired effects in a preclinical animal model of the disease. These compounds then provide valuable pharmacological tools to identify neurodegenerative disease targets and mechanisms, whether or not they have all the properties required of a drug candidate.

The autophagy-lysosomal pathway is a major proteolytic pathway that in mammalian systems mainly comprises of macroautophagy and chaperone-mediated autophagy. The former is relatively non-selective and involves bulk degradation of proteins and organelles, whereas the latter is selective for certain cytosolic proteins. These autophagy pathways are important in development, differentiation, cellular remodeling and survival during nutrient starvation. Autophagy is crucial for neuronal homeostasis and acts as a local housekeeping process, since neurons are post-mitotic cells and require effective protein degradation to prevent accumulation of toxic aggregates. A growing body of evidence now suggests that dysfunction of autophagy causes accumulation of abnormal proteins and/or damaged organelles. Such accumulation has been linked to synaptic dysfunction, cellular stress and neuronal death. Abnormal autophagy may be involved in the pathology of both chronic nervous system disorders, such as proteinopathies (Alzheimer's, Parkinson's, Huntington's disease) and acute brain injuries. Although autophagy is generally beneficial, its aberrant activation may also exert a detrimental role in neurological diseases depending on the environment and the insult, leading to autophagic neuronal death. In this review we summarize the current knowledge regarding the role of autophagy-lysosomal pathway in the central nervous system and discuss the implication of autophagy dysregulation in human neurological diseases and animal models.

One of the most important contributions to our understanding of neurodegenerative diseases in the last decade has been the demonstration that several disorders have a common biochemical cause, involving aggregation and deposition of abnormal proteins. Abnormal protein deposition leads to neuronal degeneration with consequences to impaired brain function. Protein deposition can be extracellular (beta-amyloid peptide (Abeta), prion protein) or intracellular (Tau, alpha-synuclein, huntingtin). Individuals with Alzheimer's disease (AD) exhibit extracellular senile plaques (SPs) of aggregated Abeta and intracellular neurofibrillary tangles that contain hyperphosphorylated Tau protein (NFTs), and also an extensive loss in basal forebrain cholinergic neurons that innervate the hippocampus and neocortex. The SPs and NFTs contribute to neurodegeneration, although the mechanisms inducing basal forebrain cholinergic cell loss and cognitive impairment remain unclear. Furthermore, the pathophysiological relationship between NFTs and SPs remains undefined, and controversy still rages over which of the two hallmark pathologies of AD is the primary cause of neurodegeneration in the brain. However, consensus is beginning to develop that the two pathologies are not separate processes, and the Wnt signalling pathway may provide a pathological link between both. In fact, work in transgenic mice showed that Abeta or the amyloid precursor protein can influence the formation of Tau tangles in areas of the brain known to be affected in AD. Furthermore, Abeta can contribute to synaptic dysfunction. Thus, Abeta appears to be a recurring player affecting protein phosphorylation, signal transduction mechanisms, cytoskeletal organization, multiprotein complex formation, synaptotoxicity and ultimately culminating in protein aggregation. Consequently this peptide and the downstream signalling cascades are presently considered as potential therapeutic targets.

Tauopathies are neurodegenerative diseases characterized by insoluble hyperphosphorylated deposits of the microtubuleassociated protein tau in the central nervous system. In these disorders, tau is believed to cause neurodegeneration and neuronal loss due to the loss of function of the normal protein, and/or the gain of toxic properties by generating multimeric species. The obstacles found in amyloid-based therapies in Alzheimer's disease, the most common tauopathy, have stimulated the search for alternative targets, including tau. In this article, we review the strategies aimed at reducing tau phosphorylation and aggregation as a target for drug intervention in tauopathies.

Parkinson's disease is a neurodegenerative movement disorder that is caused, in part, by the loss of dopaminergic neurons within the substantia nigra pars compacta of the basal ganglia. The presence of intracellular protein aggregates, known as Lewy bodies and Lewy neurites, within the surviving nigral neurons is the defining neuropathological feature of the disease. Accordingly, the identification of specific genes mutated in families with Parkinson's disease and of genetic susceptibility variants for idiopathic Parkinson's disease has implicated abnormalities in proteostasis, or the handling and elimination of misfolded proteins, in the pathogenesis of this neurodegenerative disorder. Protein folding and the refolding of misfolded proteins are regulated by a network of interactive molecules, known as the chaperone system, which is composed of molecular chaperones and co-chaperones. The chaperone system is intimately associated with the ubiquitin-proteasome system and the autophagy-lysosomal pathway which are responsible for elimination of misfolded proteins and protein quality control. In addition to their role in proteostasis, some chaperone molecules are involved in the regulation of cell death pathways. Here we review the role of the molecular chaperones Hsp70 and Hsp90, and the cochaperones Hsp40, BAG family members such as BAG5, CHIP and Hip in modulating neuronal death with a focus on dopaminergic neurodegeneration in Parkinson's disease. We also review current progress in preclinical studies aimed at targetting the chaperone system to prevent neurodegeneration. Finally, we discuss potential future chaperone-based therapeutics for the symptomatic treatment and possible disease modification of Parkinson's disease.

Neurodegenerative disorders are devastating human diseases that include Parkinson's, Huntington's, Alzheimer's, amyotrophic lateral sclerosis, and the frontal temporal dementias. Although the clinical manifestations of these disorders have been known for quite some time, our understanding of the molecular underpinnings is only starting to emerge. Protein misfolding and aggregation is a common hallmark among these diseases, and produce a number of cellular and functional alterations. The loss of dopaminergic neurons in the substantia nigra justified the use of dopaminergic therapies in patients. However, these strategies do not appear to confer disease-modifying effects, and do not prevent progression. The idea that neurotrophic factors might promote cell survival is an attractive one. Existing evidence from clinical trials is currently inconclusive, but some patients display clear clinical benefits. Thus, the current challenge is to develop novel strategies that make the use of neurotrophic factors more consistent.

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease of the motor system. The diagnosis is clinical, but additional investigations such as electromyography, transcranial magnetic stimulation and neuroimaging have demonstrated their usefulness in supporting diagnosis. Exhaustive research for the identification of molecular markers in the cerebrospinal fluid and plasma of ALS patients have been made; however, at present, there are no validated biomarkers for the disease. Between 5 to 10% of the ALS cases have a positive familial history, up to now eleven genes have been identified as associated with the disease. The most studied gene encodes for cupper, zinc superoxide dismutase enzyme. The identified abnormal genes potentially allow the generation of experimental cell and animal models to study the mechanisms of the disease and to test potential therapeutic compounds. The pathological characteristics of ALS include protein aggregation, proteasome inhibition, impaired axonal transport, mitochondria damage and apoptosis, oxidative stress, glutamate induced excitotoxicity, neuroinflammation and transcriptional dysfunction. Many compounds targeted to one or more of these mechanisms have been tested in multiple clinical trials. Nonetheless, nowadays only one drug, riluzole, has demonstrated a positive effect in the disease progression, but a number of recent compounds are promising in ALS therapy.

Amyotrophic lateral sclerosis (ALS) is an incurable disease resulting from the deterioration of motor neurons. The onset of disease typically occurs in the fifth decade of life and progresses rapidly; death occurs for 75% of patients within 5 years. The only drug that is available to treat ALS is riluzole, which extends survival by just 2-3 months. Thus, new therapeutic directions are being sought to prolong the lifespan of ALS patients. Since the discovery of SOD1 as a genetic determinant of ALS in 1993, SOD1-models of ALS have been extensively employed for the development of ALS therapeutics. Novel genetic targets are now under investigation following the recent discoveries linking TDP-43, FUS/TLS, angiogenin, KIFAP3 and UNC13A to ALS. In this review, we present several of the genetic contributors to both sporadic and familial forms of ALS and discuss their potential as therapeutic targets for this devastating disease.

Huntington's disease (HD) is an adult onset neurodegenerative disease caused by a polyglutamine expansion in the huntingtin protein. Recent work has shown that perturbation of kynurenine pathway (KP) metabolism is a hallmark of HD pathology, and that changes in brain levels of KP metabolites may play a causative role in this disease. The KP contains three neuroactive metabolites, the neurotoxins 3-hydroxykynurenine (3-HK) and quinolinic acid (QUIN), and the neuroprotectant kynurenic acid (KYNA). In model systems in vitro and in vivo, 3-HK and QUIN have been shown to cause neurodegeneration via a combination of excitotoxic mechanisms and oxidative stress. Recent studies with HD patient samples and in HD model systems have supported the idea that a shift away from the synthesis of KYNA and towards the formation of 3-HK and QUIN may trigger the neuropathological features observed in HD. The enzyme kynurenine 3-monooxygenase (KMO) is located at a critical branching point in the KP such that inhibition of this enzyme by either pharmacological or genetic means shifts the flux in the pathway towards the formation of KYNA. This intervention ameliorates disease-relevant phenotypes in HD models. Here we review the work implicating the KP in HD pathology and discuss the potential of KMO as a therapeutic target for this disorder. As several neurodegenerative diseases exhibit alterations in KP metabolism, this concept has broader implications for the treatment of brain diseases.

Schwann cells, the myelin forming cells in the peripheral nervous system, play a key role in the pathology of various inflammatory, metabolic and hereditary polyneuropathies. Advances in identifying growth factors and signaling molecules that are expressed by Schwann cells have paved the way for development of new treatment strategies that are aimed to improve the protective and regenerative properties of Schwann cells in peripheral nerve disorders. These include the exogenous application of growth factors and neurohormones which have been advanced into clinical trials in humans, and transplantation paradigms that have been moved into late stage preclinical models. In this review we will discuss the latest developments in these therapeutic approaches with special regard to peripheral nerve disorders, in which progress in basic research has already been translated into clinical trials, including HIV-associated distal sensory polyneuropathy and diabetic neuropathy.

Recently, a pivotal role for neuroinflammation in the pathogenesis of several neurodegenerative diseases has been recognized. Once activated, glial cells produce pathological amounts of neurotoxic substances driving neurodegeneration into chronic progression through a self-propagating cycle. Nevertheless, mounting evidence suggests that also angiogenesis may importantly contribute to neurodegeneration, since activated glial cells may release also pro-angiogenic factors. A deregulation of the balance between pro- and anti-angiogenic mediators has been reported in in vivo and in vitro models of neuroinflammation. Indeed, in Alzheimer's disease brain, a significant increase in the expression of pro-angiogenic growth factors, such as Vvascular endothelial growth factor, was found strictly co-localized with senile plaques. In addition, converging results indicate that thalidomide and its derivatives, having newly discovered anti-inflammatory and anti-angiogenic properties, are useful in the prevention of several hallmarks of neurodegeneration occurring in experimental models of Parkinson's and Alzheimer's diseases. The present review primarily discusses about the possible roles, still under debate, of angiogenesis in neurodegeneration, and focuses on the identification of new possible anti-angiogenic compounds that could open new horizons in the treatment of neurodegenerative diseases where angiogenesis is detrimental.