Current Topics in Medicinal Chemistry - Volume 15, Issue 21, 2015

Recent data from epidemiologic studies have shown that the majority of the public health costs are related to age-related disorders, and most of these diseases can lead to neuronal death. The specific signaling mechanisms underpinning neurodegeneration and aging are incompletely understood. Much work has been directed to the search for the etiology of neurodegeneration and aging and to new therapeutic strategies, including not only drugs but also non-pharmacological approaches, such as physical exercise and low-calorie dietary intake. The most important processes in aging-associated conditions, including neurodegeneration, include the mammalian (or mechanistic) target of rapamycin (mTOR), sirtuin (SIRT) and insulin/insulin growth factor 1 signaling (IIS) pathways. These longevity pathways are involved in an array of different processes, including metabolism, cognition, stress response and brain plasticity. In this review we focus on the current advances involving the mTOR, SIRT and IIS longevity pathways during the course of healthy aging processes and neurodegenerative diseases, bringing new insights in the form of a better understanding of the signaling mechanisms underpinning neurodegeneration and how these differ from physiological normal aging processes. This also provides new targets for the therapeutic management and/or prevention of these devastating age-related disorders.

With the global aging population, Alzheimer’s disease, Parkinson’s disease and mild cognition impairment are increasing in prevalence. The success of rapamycin as an agent to extend lifespan in various organisms, including mice, brings hope that chronic mTOR inhibition could also refrain age-related neurodegeneration. Here we review the evidence suggesting that mTOR inhibition - mainly with rapamycin - is a valid intervention to delay age-related neurodegeneration. We discuss the potential mechanisms by which rapamycin may facilitate neurodegeneration prevention or restoration of cognitive function. We also discuss the known side effects of rapamycin and provide evidence to alleviate exaggerated concerns regarding its wider clinical use. We explore the small molecule alternatives to rapamycin and propose future directions for their development, mainly by exploring the possibility of targeting the downstream effectors of mTOR: S6K1 and especially S6K2. Finally, we discuss the strengths and weaknesses of the models used to determine intervention efficacy for neurodegeneration. We address the difficulties of interpreting data using the common way of investigating the efficacy of interventions to delay/prevent neurodegeneration by observing animal behavior while these animals are under treatment. We propose an experimental design that should isolate the variable of aging in the experimental design and resolve the ambiguity present in recent literature.

The pathogenesis of neurodegenerative diseases involves altered activity of proteolytic systems and accumulation of protein aggregates. Autophagy is an intracellular process in which damaged organelles and long-lived proteins are degraded and recycled for maintaining normal cellular homeostasis. Disruption of autophagic activity in neurons leads to modify the cellular homeostasis, causing deficient elimination of abnormal and toxic protein aggregates that promotes cellular stress and death. Therefore, induction of autophagy has been proposed as a reasonable strategy to help neurons to clear abnormal protein aggregates and survive. This review aims to give an overview of some of the main modulators of autophagy that are currently being studied as possible alternatives in the search of therapies that slow the progression of neurodegenerative diseases, which are incurable to date.

The adult brain of humans and other mammals continuously generates new neurons throughout life. However, this neurogenic capacity is limited to two brain areas, the dentate gyrus (DG) of the hippocampus and the subventricular zone (SVZ) of the lateral ventricle. Although the DG generates new neurons, its neurogenic capacity declines with age and neurodegenerative diseases such as Alzheimer’s disease (AD) and Huntington’s disease (HD). This review focuses on the role of newly-born neurons in cognitive processes, and discusses some of the strategies proposed in humans and animals to enhance neurogenesis and counteract age-related cognitive deficits, such as physical exercise and intake of natural products like omega-3 fatty acids, curcumin and flavanols.

Neurodegenerative diseases are characterized by a progressive deterioration of brain function, with a consequent significant decline in the quality of life of patients and their families. Due to the concurrent increase in life expectancy, the incidence of these diseases has been increasing over the last years and thus there is a growing interest in finding potential risk factors. This review focuses on the correlation between peripheral inflammatory diseases and neurodegeneration, in particular on the relationship between gastrointestinal disorders and Parkinson's disease, especially through the so called gut-brain axis.

As recombinant tissue plasminogen activator is the only drug approved for the clinical treatment of acute ischemic stroke, there is an urgent unmet need for novel stroke treatments. Endogenous defense mechanisms against stroke may hold the key to new therapies for stroke. A large number of studies suggest that nicotinamide phosphoribosyl-transferase (NAMPT) is an attractive candidate to improve post-stroke recovery. NAMPT is a multifunctional protein and plays important roles in immunity, metabolism, aging, inflammation, and stress responses. NAMPT exists in both the intracellular and extracellular space. As a rate-limiting enzyme, the intracellular form (iNAMPT) catalyzes the first step in the biosynthesis of nicotinamide adenine dinucleotide (NAD) from nicotinamide. iNAMPT closely regulates energy metabolism, enhancing the proliferation of endothelial cells, inhibiting apoptosis, regulating vascular tone, and stimulating autophagy in disease conditions such as stroke. Extracellular NAMPT (eNAMPT) is also known as visfatin (visceral fat–derived adipokine) and has pleotropic effects. It is widely believed that the diverse biological functions of eNAMPT are attributed to its NAMPT enzymatic activity. However, the effects of eNAMPT on ischemic injury are still controversial. Some authors have argued that eNAMPT exacerbates ischemic neuronal injury non-enzymatically by triggering the release of TNF-α from glial cells. In addition, NAMPT also participates in several pathophysiological processes such as hypertension, atherosclerosis, and ischemic heart disease. Thus, it remains unclear under what conditions NAMPT is beneficial or destructive. Recent work using in vitro and in vivo genetic/ pharmacologic manipulations, including our own studies, has greatly improved our understanding of NAMPT. This review focuses on the multifaceted and complex roles of NAMPT under both normal and ischemic conditions.

Extensive evidences has shown the promising effects of Ginkgo biloba consumption on several diseases such as Alzheimer's and Parkinson’s diseases, and ischemic stroke, etc. Several studies also have reported its beneficial role on motor activity and cognitive functions. This species contain a unique class of diterpenes, namely Ginkgolide B, which possess several pharmacological activities such as protective effect against cardiovascular disease; the most important causes of death worldwide. The promising effects of Ginkgolide B on stroke, both ischemic and hemorrhagic, are suggested by an overwhelming body of scientific evidences. Many studies have shown that the increase of sirt1 expression, the suppression of nuclear factor kappa-B (NF-kB), the inhibition of the PI3K/Akt pathway and toll-like receptor 4 (TLR4)/NF-kB, the up-regulation of heme oxygenase 1, erythropoietin secretion and anti-apoptotic protein expression, the inhibition of pro-apoptotic proteins expression, and the improvement of endothelial NO synthesis are the main molecular mechanisms involved in the protective effect of Ginkgolide B on ischemic stroke. In this review, all the available data on the chemistry, mechanisms of neuroprotection and clinical impacts of Ginkgolide B are critically discussed.

High uric acid (UA) levels have been correlated with a reduced risk of many neurodegenerative diseases through mechanisms involving chelating Fenton reaction transitional metals, antioxidant quenching of superoxide and hydroxyl free radicals, and as an electron donor that increases antioxidant enzyme activity (e.g. SOD). However, the clinical usefulness of UA is limited by its’ low water solubility and propensity to form inflammatory crystals at hyperuricemic levels. This review focuses on the role of UA in neuroprotection, as well as potential strategies aimed at increasing UA levels in the soluble range, and the potential therapeutic use of more water-soluble methyl-UA derivatives from the natural catabolic end-products of dietary caffeine, theophylline, and theobromine.

Secondary metabolites are plant products that occur usually in differentiated cells, generally not being necessary for the cells themselves, but likely useful for the plant as a whole. Neurodegeneration can be found in many different levels in the neurons, it always begins at the molecular level and progresses toward the systemic levels. Usually, alterations are observed such as decreasing cholinergic impulse, toxicity related to reactive oxygen species (ROS), inflammatory “amyloid plaque” related processes, catecholamine disequilibrium, etc. Computer aided drug design (CADD) has become relevant in the drug discovery process; technological advances in the areas of molecular structure characterization, computational science, and molecular biology have contributed to the planning of new drugs against neurodegenerative diseases. This review discusses scientific CADD studies of the secondary metabolites. Flavonoids, alkaloids, and xanthone compounds have been studied by various researchers (as inhibitory ligands) in molecular docking; mainly with three enzymes: acetylcholinesterase (AChE; EC 3.1.1.7), butyrylcholinesterase (BChE; EC 3.1.1.8), and monoamine oxidase (MAO; EC 1.4.3.4). In addition, we have applied ligand-based-virtual screening (using Random Forest), associated with structurebased- virtual screening (docking) of a small dataset of 469 alkaloids of the Apocynaceae family from an in-house data bank to select structures with potential inhibitory activity against human AChE. This computer-aided drug design study selected certain alkaloids that might be useful in further studies for the treatment of neurological disorders such as Alzheimer’s and Parkinson’s disease.