CNS & Neurological Disorders - Drug Targets - Volume 10, Issue 2, 2011

Power Troubles in Parkinson's Disease Parkinson's disease (PD), which leads to motor and cognitive disabilities in 5 million people worldwide, is the second most common neurodegenerative disorder. Yet, a clear understanding of the mechanistic pathways and molecular machinery underlying the pathogenesis of this devastating disorder remains elusive, thus hampering the development of new therapeutic agents that are safer and more effective than levodopa (L-DOPA), a treatment introduced 40 years ago. The death of dopaminergic neurons in the substantia nigra and occurrence of α-synuclein-positive Lewy bodies in the brainstem and neocortex are neuropathological hallmarks of PD, explaining the classic motor symptoms of the disorder. Over the past decade, mutations in several genes have been shown to be associated with the early onset, Mendelian form of PD; similarly, polymorphisms in several loci have been established as risk factors for the common, non-Mendelian, late-onset sporadic PD. Together, these studies have clearly established the existence of a genetic component in the etiology of PD, which was long believed to be an environmental disease with no genetic component. Based on genetic data, dysfunctions in disparate cellular pathways have been postulated to play a causal role in PD pathogenesis. Prominent among them is the ‘mitochondrial dysfunction,’ which was prompted by the discovery of MPTP in 1976, followed by observations of signs of mitochondrial dysfunction in brain autopsies of subjects with PD, and mitochondrial location of three PD susceptibility gene products - Parkin, PINK1 and DJ-1. Evidence has since been mounting, suggesting that mitochondrial abnormalities could form part of PD neuropathology and result in impaired cellular energy production, increased free radical levels, or both. Now, adding weight to this idea, Zheng et al. (2010) report decreased expression of mitochondrial genes controlling cellular bioenergetics in PD. The researchers took a meta-GSEA (Gene Set Enrichment Analysis) approach to integrate 17 independent studies (with 221 PD patients and 221 control subjects) and analyze 522 gene sets to identify PD-associated molecular pathways. The analysis, conducted in three separate stages of replication, showed significant association of 10 gene sets (pathways). All 10 of these gene sets, which had never been linked to PD, were under-expressed at every stage. These gene sets implicate impairment of mitochondrial electron transport, mitochondrial biogenesis, glucose utilization, and glucose sensing early on in PD pathogenesis. Further, by performing a systems biology analysis, the authors demonstrated down-regulation of bio-energetic genes, including nuclear-encoded genes of the electron transport chain, whose expression is controlled by the master regulator PGC-1α (proliferator-activated receptor γ coactivator-1α). The role of PGC-1α in PD pathogenesis was validated using cellular disease models, wherein activation of PGC-1α resulted in increased expression of nuclear-encoded subunits of the mitochondrial respiratory chain and protected dopaminergic cells from the experimentally induced neurotoxic effect of two known risk factors of PD, i.e., rotenone and mutant (A53T) α-synuclein. This study is an elegant example of performing combined analysis of multiple gene expression studies using meta-GSEA to directly identify cellular pathways, in addition to genes, that are dysregulated in PD. The results, by associating PD with bio-energetic pathways that have not previously been linked, put the mitochondrial hypothesis of PD on a firmer footing and identify a new candidate therapeutic target (PGC-1α) for early intervention.

Stroke and arteriosclerosis with neurological consequences such as Alzheimer disease (AD) are two leading causes of age-associated disability, dementia, and death. The Center for Disease Control and Prevention and the National Center for Health Statistics recently reported that AD has surpassed diabetes as a leading cause of death. AD is now the sixth-leading cause of death in the United States. With our nation facing an unprecedented population shift of aging baby boomers--and AD poised to strike 10 million of them--it is clear this escalating epidemic must be addressed now with our help. Estimates for the US tell us that AD affects 4 million people (rising steeply from <1% of the population aged 65 to 40% of those aged 90) and costs $600 billion per year, which is equivalent to the total cost of stroke, heart disease, and cancer combined. Overall, there are no effective strategies for determining and controlling this devastating disease. Conventional wisdom for the last 20 years has decreed that AD is a ‘neurodegenerative’ disorder that is primarily caused by the abnormal deposition of a protein called ‘amyloid-beta’ (Aβ) in brain tissue. Neurodegenerative disorders are characterized by a loss of cognitive function and inappropriate death of nerve cells in areas of the brain that control such functions as memory and language. The trigger for nerve cell death is unknown in AD, as well as in other neurodegenerative conditions, in which memory decline is a prominent feature. The finding of Aβ deposition in AD brains after death led to the so-called “amyloid hypothesis”. For almost two decades now, the amyloid hypothesis has influenced and guided research in the field of AD dementia such that many researchers regard it as the gold standard of scientific investigation. Indeed, most of the literature claims that AD is caused by Aβ deposition within structures called senile plaques. The formation of these plaques are purported to lead to further abnormalities within the surrounding nerve cells, eventually killing them. However, there is little evidence to support this claim and ample evidence to question it. For example, the amyloid hypothesis has been criticized because research findings up to now have not generated any benefits in the clinical management and treatment of AD patients, nor have they advanced our understanding of how the elderly are preferentially affected. The three main flaws of the hypothesis appear to be that: (1) Aβ deposition has not been found to be toxic or to cause the damage and death of cerebrally located nerve cells in humans or animals; (2) the brains of many aged, but cognitively normal individuals show abundant Aβ-containing senile plaques but no clinical signs of AD; and (3) since there is general agreement that Aβ-containing senile plaques are the products of degenerating neurons, they can not be the cause, since it is axiomatic that a product is the result, not the cause of some activity. By contrast, there is now considerable and still growing evidence from the fields of epidemiology, pharmacology, neuroimaging, clinical medicine, microscopic anatomy/pathology, and molecular biology which indicate that non-genetic AD is an oxidative stress-induced mitochondrial disorder that affects cerebral vascular cells in addition to neurons and glia that exacerbates cerebral hypoperfusion. This evidence can be summarized as follows: (1) numerous epidemiologic studies link AD risk factors such as stroke, heart disease, hypertension, and atherosclerosis to reduced cerebral blood flow; (2) evidence that AD and vascular dementia (VaD), an acknowledged vascular disorder, share practically all the same risk factors and may benefit from the same treatments; (3) drug therapies reported to improve AD symptoms (including prescriptive drugs now available for AD) all increase blood flow to the brain; (4) people who are likely to develop AD but do not yet show signs of dementia can be identified by using brain blood flow measurements and brain positron emission tomography scans; (5) the clinical symptoms are very similar in most AD and VaD patients; (6) parallel abnormalities such as Aβ-laden plaques found in AD and VaD patients occur in both brain vessels and brain tissue; (7) low levels of brain blood flow in aged humans and animals can lead to abnormal cell metabolism, tissue damage, and memory problems independent of Aβ; (8) mild cognitive impairment (a term used to describe a preliminary stage leading to AD) can convert equally to AD or VaD; and (9) small vessel damage is present in the majority of AD brains after death. Based on these results, as well as a bare-bones examination of the literature, it is clear that no compelling evidence exists to conclude that Aβ deposition causes AD or that it results in significant damage to brain cells. Re-classification of AD from a neurodegenerative disorder to an oxidative stress-induced metabolic syndrome with mitochondrial failure that occurs within both cerebral blood vessels and neurons would steer researchers down the correct path and would be able to speed the development of truly beneficial treatments. It would be possible to improve patient management, provide earlier diagnoses, and reduce the number of AD cases in the future by aggressively treating the risk factors before they can turn into dementia. These initiating pathological events most likely occur several years before the clinical manifestation of the disease, implying that potential therapeutic interventions are currently being started too late to give beneficial results. Moreover, there is a growing body of evidence supporting the hypothesis that mild cognitive impairment is a prodromal phase of AD, since 80% of patients diagnosed with mild cognitive impairment develop the full blown manifestations of AD within 5-6 years. The articles compiled within this special issue highlight the most recent insights into the molecular mechanisms involved in the earliest stages of AD pathogenesis, namely that of oxidative stress-induced metabolic abnormalities, mitochondrial failure which initiates the energy crisis and vascular and cellular hypoperfusion, and lays out future potential treatment strategies based on these findings. Also discussed are the mutual pathogenic processes and antecedent biological markers shared between AD and other cardiovascular, cerebrovascular, and neurodegenerative diseases. I feel confident that elucidating the commonalities between these diseases will provide the necessary key to finally conquering AD.

Age-related dementias such as Alzheimer disease (AD) have been linked to vascular disorders like hypertension, diabetes and atherosclerosis. These risk factors cause ischemia, inflammation, oxidative damage and consequently reperfusion, which is largely due to reactive oxygen species (ROS) that are believed to induce mitochondrial damage. At higher concentrations, ROS can cause cell injury and death which occurs during the aging process, where oxidative stress is incremented due to an accelerated generation of ROS and a gradual decline in cellular antioxidant defense mechanisms. Neuronal mitochondria are especially vulnerable to oxidative stress due to their role in energy supply and use, causing a cascade of debilitating factors such as the production of giant and/or vulnerable young mitochondrion who's DNA has been compromised. Therefore, mitochondria specific antioxidants such as acetyl-L-carnitine and R-alphalipoic acid seem to be potential treatments for AD. They target the factors that damage mitochondria and reverse its effect, thus eliminating the imbalance seen in energy production and amyloid beta oxidation and making these antioxidants very powerful alternate strategies for the treatment of AD.

The two major neuropathologic hallmarks of AD are extracellular amyloid beta (Aβ) plaques and intracellular neurofibrillary tangles (NFTs). A number of additional pathogenic mechanisms, possibly overlapping with Aβ plaques and NFTs formation, have been described, including inflammation, oxidative damage, iron dysregulation, and alterations in cholesterol metabolism. In this review, all of these mechanisms will be discussed and treatments that are under development to interfere with these pathogenic steps will be presented. A primary goal of work in this area is identification of novel compounds that can block the course of the disease in early phases. For this reason they are currently termed “disease modifying” drugs. These drugs are designed to modify pathological steps leading to AD, thus acting on the evolution and progression of the disease. Some of these molecules are undergoing clinical testing whereas others are in preclinical phases of development. Several approaches have been considered, including mainly Aβ deposition interference by anti- Aβ aggregation agents, vaccination, γ-secretase inhibition or selective Aβ42-lowering agents (SALAs), tau deposition interference by methyl thioninium chloride (MTC), and methods for reduction of inflammation and oxidative damage.

Recent evidence has associated the aberrant, proximal re-expression of various cell cycle control elements with neuronal cell vulnerability in Alzheimer's and Parkinson's diseases, as a common chronic neurodegeneration. This phenomenon associated with oncogenic transduction pathways activation has attracted the interest of scientists all over the world for a few years now. The purpose of this paper is to outline areas of research related to oncogenic factors or medicines in the context of potential applications for future treatment of the above mentioned chronic and, largely, incurable diseases.

Flavonoids are natural, plant-derived compounds which exert diverse biological activities, also valuable neuroprotective actions within the brain and currently are intensively studied as agents able to modulate neuronal function and to prevent age-related neurodegeneration. Among them, flavones isolated from Scutellaria baicalensis root exhibit strong neuroprotective effects on the brain and are not toxic in the broad range of tested doses. Their neuroprotective potential has been shown in both oxidative stress-induced and amyloid-β and α-synuclein -induced neuronal death models. Baicalein, the main flavone present in Scutellaria baicalensis root, strongly inhibited aggregation of neuronal amyloidogenic proteins in vitro and induces dissolution of amyloid deposits. It exerts strong antioxidative and anti-inflammatory activities and also exhibits anti-convulsive, anxiolytic, and mild sedative actions. Importantly, baicalein, and also another flavone: oroxylin A, markedly enhanced cognitive and mnestic functions in animal models of aging brains and neurodegeneration. In the preliminary study, wogonin, another flavone from Scutellaria baicalensis root, has been shown to stimulate brain tissue regeneration, inducing differentiation of neuronal precursor cells. This concise review provides the main examples of neuroprotective activities of the flavones and reveals their potential in prevention and therapy of neurodegenerative diseases.

There is growing scientific agreement that antioxidants, particularly the polyphenolic forms, may help lower the incidence of disease, such as certain cancers, cardiovascular and neurodegenerative diseases, DNA damage, or even have anti-aging properties. On the other hand, questions remain as to whether some antioxidants or phytochemicals potentially could do more harm than good, as an increase in glycation-mediated protein damage (carbonyl stress) and some risk has been reported. Nevertheless, the quest for healthy aging has led to the use of antioxidants as a means to disrupt age-associated deterioration in physiological function, dysregulated metabolic processes or prevention of many age-related diseases. Although a diet rich in polyphenolic forms of antioxidants does seem to offer hope in delaying the onset of age-related disorders, it is still too early to define their exact clinical benefit for treating age-related disease. Regardless of where the debate will end, it is clear that any deficiency in antioxidant vitamins or adequate enzymatic antioxidant defenses can manifest in many disease states and shift the redox balance in some diseases. This review updated critically examines general antioxidant compounds in health, disease and aging with hope that a better understanding of the many mechanisms involved with these diverse compounds may lead to better health and novel treatment approaches for age-related diseases.

Few targets for neuroprotection have been defined in Alzheimer's disease (AD). Recent data from the role of Wnt, insulin-like growth factor-1 and estradiol pathways in AD suggest some therapeutic targets for disease treatment, and have led us to evaluate the “common factors” in these pathways as further candidate targets. These data have led us to propose that glycogen synthase kinase-3 (GSK-3) inhibition appears to be a common feature of these pathways. Besides, considering that GSK-3 activation seems to be correlated with neurodegeneration, its selection as a relevant target appears obvious. The capacity of different GSK-3 inhibitors to prevent amyloid β-peptide neurotoxicity and tau phosphorylation has been evaluated in order to develop novel clinical and therapeutic approaches. Different approaches could be used to search for new neuroprotective compounds. The most classical of these is to first define the target and then design a specific in vitro screening assay for it. Alternatively, a cell model of cell culture could be used as a “primary screen”. Following this rationale, we have used a combined approach in which we first used an in vitro system to select compounds able to inhibit recombinant GSK3β. Subsequently, we subjected the candidate compounds to three consecutive cell-based complementary screening assays. First, cell viability was assessed using a neuroblastoma cell line before assaying the capacity of the compounds to reduce tau phosphorylation. Finally, we designed a neuronal cell model of apoptosis using the phosphatidylinositol kinase-3 inhibitor LY294002. Finally, we summarize several new compounds with “neuroprotective” properties.

Therapeutic angiogenesis is a novel treatment for ischemic stroke, and vascular endothelial growth factor (VEGF) is a key angiogenic and neuroprotective pharmacological candidate for therapy. However, the greatest challenge of preclinical studies is demonstrating that VEGF-based therapeutic angiogenesis is safe and effective for ischemic stroke patients. This review presents the following crucial questions which must first be answered by preclinical studies before VEGF-based therapeutic angiogenesis advances to human stroke trials, (1) Does angiogenesis induced by VEGF monotherapy promote neuroprotection or further damage the nervous tissue? (2) Does angiogenesis by VEGF in combination with other agents (combination therapy) promote greater neuroprotection than monotherapy, and without additional side effects? (3) Which exogenous VEGF isoform best promotes angiogenesis and neuroprotection, with least adverse effects on other organs? (4) Does angiogenesis induced by exogenous VEGF produce similar results in different animal models of ischemic stroke, including variations in age, gender and coexisting chronic diseases? (5) Can angiogenesis be induced by exogenous VEGF without clinically-significant alterations of systemic hemodynamics? (6) Are gene therapy and stem cells more beneficial than recombinant protein for VEGF-based therapeutic angiogenesis? (7) What are the best routes, timing and duration for administering VEGF, and how do these parameters influence inflammation? (8) Does exogenous VEGF exacerbate inflammation when traumatic or other injuries are present with ischemia? (9) Are VEGF doses not causing tissue alterations at the light microscopy level associated with clinically-significant ultrastructural damages of the neurovascular unit? Both published and unpublished preclinical data from the author's laboratory are presented.

Hypoxia/ischemia brain damage (HIBD) is one of the most common central nervous system insults in newborns. Brain repair following HIBD is closely associated with cellular processes such as cell survival, angiogenesis, and neurogenesis. In recent years, many studies have suggested that ginsenoside Rg1, one of the major active ingredients of ginseng, may increase neural viability, promote angiogenesis, and induce neurogenesis. However, there are few reports on roles of Rg1 in HIBD repair, and the mechanisms involved are unclear. Recently, a Chinese drug consisting of Rg1 has been shown to be a potential regulator of hypoxia-inducible factor-1α expression in HIBD. Since it has been shown that HIF-1α is a key transcription factor involved in the neuroprotective response to HIBD, it is possible that Rg1 could facilitate the process of brain repair, possibly modulating cell survival, angiogenesis, and neurogenesis after HIBD by targeting HIF-1α.

Neurodegenerative disorders such as Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS) are currently incurable pathologies with huge social and economic impacts closely related to the increasing of life expectancy in modern times. Although the clinical and neuropathological aspects of these debilitating disorders are distinct, they share a pattern of neurodegeneration in anatomically or functionally related regions. For each disease, presently available treatments only address symptoms and do not alter the course or progression of the underlying diseases. In this context, the search for new effective chemical entities, capable of acting on diverse biochemical targets, with new mechanisms of action and low toxicity are genuine challenges to research groups and the pharmaceutical industry. This medical need has led to the reemerging of modern natural products chemistry that has yielded sophisticated and complex new lead molecules for drug discovery and development. In this review we discuss some of the main contributions of the natural products chemistry that covers multiple and varied plant species. Advances in the discovery of active constituents of plants, herbs, and extracts prescribed by traditional medicine practices for the treatment of senile neurodegenerative disorders, especially for PD, in the period after the 2000s is reviewed. The most important contributions from the 1990s are also discussed. The review also focuses on the pharmacological mechanisms of action that might underlie the purported beneficial improvements in memory and cognition, neurovascular function, and in neuroprotection. It is concluded that natural product chemistry brings tremendous diversity and historical precedent to a huge area of unmet medical need.

The present review is part II in a series (part I focuses on Parkinson's Disease) that addresses the value of natural product chemistry in the discovery of medicines for the treatment of neurodegenerative disorders. Data reviewed document that a host of products from plant species and derivatives have neuroprotectant effects in vitro and in vivo. In addition, besides neuroprotection, natural products also demonstrate biological effects that target biochemical pathways underlying associated symptoms of neurdegnerative disorders that include cognitive impairments, energy/fatigue, mood, and anxiety. This part of the review series focuses specifically upon Alzheimer's Disease (AD). AD is postulated to result from extracellular formation of amyloid plaques and intracellular deposits of neurofibrilary tangles in the hippocampus, cerebral cortex and other areas of the brain essential for cognitive function. Plaques are formed mostly from the deposition β-amyloid (Aβ), a peptide derived from the amyloid precursor protein (APP). Filamentous tangles are formed from paired helical filaments composed of neurofilament and hyperphosphorilated tau protein, a microtubule-associated protein. In addition, environmental factors can engender the production of cytokines that are closely related to the installation of an inflammatory process that contributes to neuronal death and the development and the progression of AD. In this review we focus on the recent main contribuitions of natural products chemistry to the discovery of new chemical entities usefull to the control and prevention of AD installation and progression. More than sixteen plant species, including Ginseng, Celastrus paniculatus, Centella asiatica, Curcuma longa, Ginkgo biloba, Huperzia serrata, Lycoris radiate, Galanthus nivalis, Magnolia officinalis, Polygala tenuifolia, Salvia lavandulaefolia, Salvia miltiorrhiza, Coptis chinensis, Crocus sativus, Evodia rutaecarpa, Sanguisorba officinalis, Veratrum grandiflorum and Picrorhiza kurvoa, are discussed as potential sources of active extracts. In addition, more than sixty secondary metabolites are under evaluation for their efficacy on controlling symptoms and to impede the development and progression of AD.

The urokinase receptor (uPAR) is a multifunctional glycosylphosphatidylinositol-anchored protein that regulates important processes such as gene expression, cell proliferation, adhesion, migration, invasion, and metastasis. uPAR is an essential component of the plasminogen activation cascade, a protease receptor that binds the urokinase-type plasminogen activator. uPAR is also an adhesionmodulating receptor, and a signalling receptor transmitting signals to the cell through lateral interactions with a wide array of membrane receptors. Altogether, the external ligands and membrane-bound partners of uPAR constitute a rich uPAR interactome. Recently, a new ligand of uPAR has been identified as the SRPX2 protein which is essential in language and cognitive development. SRPX2 is the second identified gene involved in language disorders. However, previous studies revealed cognitive disorders and defects in the development of the GABAergic interneurons in uPAR null mice. In addition, the expression of uPAR correlates with important human diseases such as epilepsy, autism, multiple sclerosis, Alzheimer's, AIDS dementia, cerebral malaria, and brain tumours. Therefore, uPAR has unexpectedly become a significant receptor in the central nervous system and made a few steps into philosophy. Language is indeed intimately linked to human culture. This in-depth review presents the structure and the sequences of uPAR that are essential for drug design and the generation of new inhibitors. In addition, we summarize all the inhibitors of uPAR that have been created so far. Finally, we discuss the functions of uPAR in the development, functioning, and pathology of the central nervous system.