Effect of melatonin on calyculin A-induced tau hyperphosphorylation

Review on anti-alzheimer drug development: approaches, challenges and perspectives

Alzheimer is an irreversible progressive neurodegenerative disease that causes failure of cerebral neurons and disability of the affected person to practice normal daily life activities. There is no concrete evidence to identify the exact reason behind the disease, so several relevant hypotheses emerged, highlighting many possible therapeutic targets, such as acetylcholinesterase, cholinergic receptors, N-methyl d-aspartate receptors, phosphodiesterase, amyloid β protein, protein phosphatase 2A, glycogen synthase kinase-3 beta, β-secretase, γ-secretase, α-secretase, serotonergic receptors, glutaminyl cyclase, tumor necrosis factor-α, γ-aminobutyric acid receptors, and mitochondria. All of these targets have been involved in the design of new potential drugs. An extensive number of these drugs have been studied in clinical trials. However, only galantamine, donepezil, and rivastigmine (ChEIs), memantine (NMDA antagonist), and aducanumab and lecanemab (selective anti-Aβ monoclonal antibodies) have been approved for AD treatment. Many drugs failed in the clinical trials to such an extent that questions have been posed about the significance of some of the aforementioned targets. On the contrary, the data of other drugs were promising and shed light on the significance of their targets for the development of new potent anti-alzheimer drugs.

Phytotherapeutic targeting of the mitochondria in neurodegenerative disorders

Neurodegenerative diseases are characterized by degeneration or cellular atrophy within specific structures of the brain. Neurons are the major target of neurodegeneration. Neurons utilize 75-80% of the energy produced in the brain. This energy is either formed by utilizing the glucose provided by the cerebrovascular blood flow or by the in-house energy producers, mitochondria. Mitochondrial dysfunction has been associated with neurodegenerative diseases. But recently it has been noticed that neurodegenerative diseases are often associated with cerebrovascular diseases. Cerebral blood flow requires vasodilation which to an extent regulated by mitochondria. We hypothesize that when mitochondrial functioning is disrupted, it is not able to supply energy to the neurons. This disruption also affects cerebral blood flow, further reducing the possibilities of energy supply. Loss of sufficient energy leads to neuronal dysfunction, atrophy, and degeneration. In this chapter, we will discuss the metabolic modifications of mitochondria in aging-related neurological disorders and the potential of phytocompounds targeting them.

Oxidative Stress in Tauopathies: From Cause to Therapy

Oxidative stress (OS) is the result of an imbalance between the production of reactive oxygen species (ROS) and the antioxidant capacity of cells. Due to its high oxygen demand, the human brain is highly susceptible to OS and, thus, it is not a surprise that OS has emerged as an essential component of the pathophysiology of several neurodegenerative diseases, including tauopathies. Tauopathies are a heterogeneous group of age-related neurodegenerative disorders characterized by the deposition of abnormal tau protein in the affected neurons. With the worldwide population aging, the prevalence of tauopathies is increasing, but effective therapies have not yet been developed. Since OS seems to play a key role in tauopathies, it has been proposed that the use of antioxidants might be beneficial for tau-related neurodegenerative diseases. Although antioxidant therapies looked promising in preclinical studies performed in cellular and animal models, the antioxidant clinical trials performed in tauopathy patients have been disappointing. To develop effective antioxidant therapies, the molecular mechanisms underlying OS in tauopathies should be completely understood. Here, we review the link between OS and tauopathies, emphasizing the causes of OS in these diseases and the role of OS in tau pathogenesis. We also summarize the antioxidant therapies proposed as a potential treatment for tauopathies and discuss why they have not been completely translated to clinical trials. This review aims to provide an integrated perspective of the role of OS and antioxidant therapies in tauopathies. In doing so, we hope to enable a more comprehensive understanding of OS in tauopathies that will positively impact future studies.

Regulation of Melatonin and Neurotransmission in Alzheimer's Disease

Alzheimer’s disease is a neurodegenerative disorder associated with age, and is characterized by pathological markers such as amyloid-beta plaques and neurofibrillary tangles. Symptoms of AD include cognitive impairments, anxiety and depression. It has also been shown that individuals with AD have impaired neurotransmission, which may result from the accumulation of amyloid plaques and neurofibrillary tangles. Preclinical studies showed that melatonin, a monoaminergic neurotransmitter released from the pineal gland, is able to ameliorate AD pathologies and restore cognitive impairments. Theoretically, inhibition of the pathological progression of AD by melatonin treatment should also restore the impaired neurotransmission. This review aims to explore the impact of AD on neurotransmission, and whether and how melatonin can enhance neurotransmission via improving AD pathology.

Repurposing Licensed Drugs for Use Against Alzheimer's Disease

Substantial evidence, composed of drug mechanisms of action, in vivo testing, and epidemiological data, exists to support clinical testing of FDA-approved drugs for repurposing to the treatment of Alzheimer's disease (AD). Licensed compound investigation can often proceed at a faster and more cost-effective manner than un-approved compounds moving through the drug pipeline. As the prevalence of AD increases with life expectancy, the current rise in life expectancy amalgamated with the lack of an effective drug for the treatment of AD unnecessarily burdens our medical system and is an urgent public health concern. The unfounded reluctance to examine repurposing existing drugs for possible AD therapy further impedes the possibility of improving the quality of patient lives with a terminal disease. This review summarizes some evidence which exists to suggest certain already-approved drugs may be considered for the treatment of AD and will perhaps encourage physicians to off-label prescribe these safe therapeutics.

Cellular Mechanisms of Melatonin: Insight from Neurodegenerative Diseases

Neurodegenerative diseases are the second most common cause of death and characterized by progressive impairments in movement or mental functioning in the central or peripheral nervous system. The prevention of neurodegenerative disorders has become an emerging public health challenge for our society. Melatonin, a pineal hormone, has various physiological functions in the brain, including regulating circadian rhythms, clearing free radicals, inhibiting biomolecular oxidation, and suppressing neuroinflammation. Cumulative evidence indicates that melatonin has a wide range of neuroprotective roles by regulating pathophysiological mechanisms and signaling pathways. Moreover, melatonin levels are decreased in patients with neurodegenerative diseases. In this review, we summarize current knowledge on the regulation, molecular mechanisms and biological functions of melatonin in neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, vascular dementia and multiple sclerosis. We also discuss the clinical application of melatonin in neurodegenerative disorders. This information will lead to a better understanding of the regulation of melatonin in the brain and provide therapeutic options for the treatment of various neurodegenerative diseases.

The role of sleep deprivation and circadian rhythm disruption as risk factors of Alzheimer's disease

Emerging evidence suggests that sleep deprivation (SD) and circadian rhythm disruption (CRD) may interact and increase the risk for the development of Alzheimer's disease (AD). This review inspects different pathophysiological aspects of SD and CRD, and shows that the two may impair the glymphatic-vascular-lymphatic clearance of brain macromolecules (e.g., β-amyloid and microtubule associated protein tau), increase local brain oxidative stress and diminish circulatory melatonin levels. Lastly, this review looks into the potential association between sleep and circadian rhythm with stress granule formation, which might be a new mechanism along the AD pathogenic pathway. In summary, SD and CRD is likely to be associated with a positive risk in developing Alzheimer's disease in humans.

Multi-Faceted Role of Melatonin in Neuroprotection and Amelioration of Tau Aggregates in Alzheimer's Disease

Alzheimer's disease (AD) is one of the major age related neurodegenerative diseases whose pathology arises due to the presence of two distinct protein aggregates, viz., amyloid-β plaques in extracellular matrix and tau neurofibrillary tangles in neurons. Multiple factors play a role in AD pathology, which includes familial mutations, oxidative stress, and post-translational modifications. Melatonin is an endocrine hormone, secreted during darkness, derived from tryptophan, and produced mainly by the pineal gland. It is an amphipathic molecule, which makes it suitable to cross not only blood-brain barrier, but also to enter several other subcellular compartments like mitochondria and endoplasmic reticulum. In this context, the neuroprotective effect of melatonin may be attributed to its role as an antioxidant. Melatonin's pleiotropic function as an antioxidant and neuroprotective agent has been widely studied. However, its direct effect on the aggregation of tau and amyloid-β needs to be explored. Furthermore, an important aspect of its function is its ability to regulate the process of phosphorylation of tau by affecting the function of kinases and phosphatases. In this review, we are focusing on the pleiotropic function of melatonin on the aspect of its neuroprotective function in tau pathology, which includes antioxidant function, regulation of enzymes, including kinases and enzymes involved in free radical scavenging and mitochondrial protection.

Monoamines and their Derivatives on GPCRs: Potential Therapy for Alzheimer's Disease

Albeit cholinergic depletion remains the key event in Alzheimer's Disease (AD), recent information describes stronger links between monoamines (trace amines, catecholamines, histamine, serotonin, and melatonin) and AD than those known in the past century. Therefore, new drug design strategies focus efforts to translate the scope on these topics and to offer new drugs which can be applied as therapeutic tools in AD. In the present work, we reviewed the state-of-art regarding genetic, neuropathology and neurochemistry of AD involving monoamine systems. Then, we compiled the effects of monoamines found in the brain of mammals as well as the reported effects of their derivatives and some structure-activity relationships. Recent derivatives have triggered exciting effects and pharmacokinetic properties in both murine models and humans. In some cases, the mechanism of action is clear, essentially through the interaction on G-protein-coupled receptors as revised in this manuscript. Additional mechanisms are inhibition of enzymes for their biotransformation, regulation of free-radicals in the central nervous system and others for the effects on Tau phosphorylation or amyloid-beta accumulation. All these data make the monoamines and their derivatives attractive potential elements for AD therapy.

Role of immune-pineal axis in neurodegenerative diseases, unraveling novel hybrid dark hormone therapies

The anti-oxidant effects of melatonin and the immune-pineal axis are well established. However, how they play a role in the pathogenesis of neurodegenerative diseases is not well elucidated. A better understanding of this neuro-immuno-endocrinological link can help in the development of novel therapies with higher efficacy to alleviate symptomatology, slow disease progression and improve the quality of life. Recent studies have shown that the immune-pineal axis acts as an immunological buffer, neurohormonal switch and it also intricately links the pathogenesis of neurodegenerative diseases (like Multiple sclerosis, Alzheimer's disease, Parkinson's disease) and inflammation at a molecular level. Furthermore, alteration in circadian melatonin production is seen in neurodegenerative diseases. This review will summarise the mechanics by which the immune-pineal axis and neuro-immuno-endocrinological disturbances affect the pathogenesis and progression of neurodegenerative diseases. It will also explore, how this understanding will help in the development of novel hybrid melatonin hormone therapies for the treatment of neurodegenerative diseases.

Profiling donepezil template into multipotent hybrids with antioxidant properties

Alzheimer’s disease is debilitating neurodegenerative disorder in the elderly. Current therapy relies on administration of acetylcholinesterase inhibitors (AChEIs) -donepezil, rivastigmine, galantamine, and N-methyl-d-aspartate receptor antagonist memantine. However, their therapeutic effect is only short-term and stabilizes cognitive functions for up to 2 years. Given this drawback together with other pathological hallmarks of the disease taken into consideration, novel approaches have recently emerged to better cope with AD onset or its progression. One such strategy implies broadening the biological profile of AChEIs into so-called multi-target directed ligands (MTDLs). In this review article, we made comprehensive literature survey emphasising on donepezil template which was structurally converted into plethora of MTLDs preserving anti-cholinesterase effect and, at the same time, escalating the anti-oxidant potential, which was reported as a crucial role in the pathogenesis of the Alzheimer’s disease.

Pineal Calcification, Melatonin Production, Aging, Associated Health Consequences and Rejuvenation of the Pineal Gland

The pineal gland is a unique organ that synthesizes melatonin as the signaling molecule of natural photoperiodic environment and as a potent neuronal protective antioxidant. An intact and functional pineal gland is necessary for preserving optimal human health. Unfortunately, this gland has the highest calcification rate among all organs and tissues of the human body. Pineal calcification jeopardizes melatonin's synthetic capacity and is associated with a variety of neuronal diseases. In the current review, we summarized the potential mechanisms of how this process may occur under pathological conditions or during aging. We hypothesized that pineal calcification is an active process and resembles in some respects of bone formation. The mesenchymal stem cells and melatonin participate in this process. Finally, we suggest that preservation of pineal health can be achieved by retarding its premature calcification or even rejuvenating the calcified gland.

Mechanisms of Melatonin in Alleviating Alzheimer's Disease

Alzheimer's disease (AD) is a chronic, progressive and prevalent neurodegenerative disease characterized by the loss of higher cognitive functions and an associated loss of memory. The thus far "incurable" stigma for AD prevails because of variations in the success rates of different treatment protocols in animal and human studies. Among the classical hypotheses explaining AD pathogenesis, the amyloid hypothesis is currently being targeted for drug development. The underlying concept is to prevent the formation of these neurotoxic peptides which play a central role in AD pathology and trigger a multispectral cascade of neurodegenerative processes post-aggregation. This could possibly be achieved by pharmacological inhibition of β- or γ-secretase or stimulating the nonamyloidogenic α-secretase. Melatonin the pineal hormone is a multifunctioning indoleamine. Production of this amphiphilic molecule diminishes with advancing age and this decrease runs parallel with the progression of AD which itself explains the potential benefits of melatonin in line of development and devastating consequences of the disease progression. Our recent studies have revealed a novel mechanism by which melatonin stimulates the nonamyloidogenic processing and inhibits the amyloidogenic processing of β-amyloid precursor protein (βAPP) by stimulating α -secretases and consequently down regulating both β- and γ-secretases at the transcriptional level. In this review, we discuss and evaluate the neuroprotective functions of melatonin in AD pathogenesis, including its role in the classical hypotheses in cellular and animal models and clinical interventions in AD patients, and suggest that with early detection, melatonin treatment is qualified to be an anti-AD therapy.

Melatonin: A Potential Anti-Oxidant Therapeutic Agent for Mitochondrial Dysfunctions and Related Disorders

Mitochondria play a central role in cellular physiology. Besides their classic function of energy metabolism, mitochondria are involved in multiple cell functions, including energy distribution through the cell, energy/heat modulation, regulation of reactive oxygen species (ROS), calcium homeostasis, and control of apoptosis. Simultaneously, mitochondria are the main producer and target of ROS with the result that multiple mitochondrial diseases are related to ROS-induced mitochondrial injuries. Increased free radical generation, enhanced mitochondrial inducible nitric oxide synthase (iNOS) activity, enhanced nitric oxide (NO) production, decreased respiratory complex activity, impaired electron transport system, and opening of mitochondrial permeability transition pores have all been suggested as factors responsible for impaired mitochondrial function. Because of these, neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and aging, are caused by ROS-induced mitochondrial dysfunctions. Melatonin, the major hormone of the pineal gland, also acts as an anti-oxidant and as a regulator of mitochondrial bioenergetic function. Melatonin is selectively taken up by mitochondrial membranes, a function not shared by other anti-oxidants, and thus has emerged as a major potential therapeutic tool for treating neurodegenerative disorders. Multiple in vitro and in vivo experiments have shown the protective role of melatonin for preventing oxidative stress-induced mitochondrial dysfunction seen in experimental models of PD, AD, and HD. With these functions in mind, this article reviews the protective role of melatonin with mechanistic insights against mitochondrial diseases and suggests new avenues for safe and effective treatment modalities against these devastating neurodegenerative diseases. Future insights are also discussed.

The Incremental Induction of Neuroprotective Properties by Multiple Therapeutic Strategies for Primary and Secondary Neural Injury

Neural diseases including injury by endogenous factors, traumatic brain injury, and degenerative neural injury are eventually due to reactive oxygen species (ROS). Thus ROS generation in neural tissues is a hallmark feature of numerous forms of neural diseases. Neural degeneration and the neural damage process is complex, involving a vast array of tissue structure, transcriptional/translational, electrochemical, metabolic, and functional events within the intact neighbors surrounding injured neural tissues. During aging, multiple changes involving physical, chemical, and biochemical processes occur from the molecular to the morphological levels in neural tissues. Among many recommended therapeutic candidates, melatonin also plays a role in protecting the nervous system from anti-inflammation and efficiently safeguards neuronal cells via antioxidants and other endogenous/exogenous beneficial factors. Therefore, given the wide range of mechanisms responsible for neuronal damage, multi-action drugs or therapies for the treatment of neural injury that make use of two or more agents and target several pathways may have greater efficacy in promoting functional recovery than a single therapy alone.

Prophylactic melatonin significantly reduces Alzheimer's neuropathology and associated cognitive deficits independent of antioxidant pathways in AβPP(swe)/PS1 mice

Background

Alzheimer’s disease (AD) underlies dementia for millions of people worldwide, and its occurrence is set to double in the next 20 years. Currently, approved drugs for treating AD only marginally ameliorate cognitive deficits, and provide limited symptomatic relief, while newer substances under therapeutic development are potentially years away from benefiting patients. Melatonin (MEL) for insomnia has been proven safe with >15 years of over-the-counter access in the US. MEL exerts multiple complementary mechanisms of action against AD in animal models; thus it may be an excellent disease-modifying therapeutic. While presumed to provide neuroprotection via activation of known G-protein-coupled melatonin receptors (MTNRs), some data indicate MEL acts intracellularly to protect mitochondria and neurons by scavenging reactive oxygen species and reducing free radical formation. We examined whether genetic deletion of MTNRs abolishes MEL’s neuroprotective actions in the AβPP^swe/PSEN1dE9 mouse model of AD (2xAD). Beginning at 4 months of age, both AD and control mice either with or without both MTNRs were administered either MEL or vehicle in drinking water for 12 months.

Results

Behavioral and cognitive assessments of 15-month-old AD mice revealed receptor-dependent effects of MEL on spatial learning and memory (Barnes maze, Morris Water Maze), but receptor-independent neuroprotective actions of MEL on non-spatial cognitive performance (Novel Object Recognition Test). Similarly, amyloid plaque loads in hippocampus and frontal cortex, as well as plasma Aβ_1–42 levels, were significantly reduced by MEL in a receptor-independent manner, in contrast to MEL’s efficacy in reducing cortical antioxidant gene expression (Catalase, SOD1, Glutathione Peroxidase-1, Nrf2) only when receptors were present. Increased cytochrome c oxidase activity was seen in 16mo AD mice as compared to non-AD control mice. This increase was completely prevented by MEL treatment of 2xAD/MTNR+ mice, but only partially prevented in 2xAD/MTNR- mice, consistent with mixed receptor-dependent and independent effects of MEL on this measure of mitochondrial function.

Conclusions

These findings demonstrate that prophylactic MEL significantly reduces AD neuropathology and associated cognitive deficits in a manner that is independent of antioxidant pathways. Future identification of direct molecular targets for MEL action in the brain should open new vistas for development of better AD therapeutics.

The Ambiguous Relationship of Oxidative Stress, Tau Hyperphosphorylation, and Autophagy Dysfunction in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia. The pathological hallmarks of AD are amyloid plaques [aggregates of amyloid-beta (Aβ)] and neurofibrillary tangles (aggregates of tau). Growing evidence suggests that tau accumulation is pathologically more relevant to the development of neurodegeneration and cognitive decline in AD patients than Aβ plaques. Oxidative stress is a prominent early event in the pathogenesis of AD and is therefore believed to contribute to tau hyperphosphorylation. Several studies have shown that the autophagic pathway in neurons is important under physiological and pathological conditions. Therefore, this pathway plays a crucial role for the degradation of endogenous soluble tau. However, the relationship between oxidative stress, tau protein hyperphosphorylation, autophagy dysregulation, and neuronal cell death in AD remains unclear. Here, we review the latest progress in AD, with a special emphasis on oxidative stress, tau hyperphosphorylation, and autophagy. We also discuss the relationship of these three factors in AD.

Melatonin Therapy in Patients with Alzheimer's Disease

Alzheimer's disease (AD) is a major health problem and a growing recognition exists that efforts to prevent it must be undertaken by both governmental and non-governmental organizations. In this context, the pineal product, melatonin, has a promising significance because of its chronobiotic/cytoprotective properties potentially useful for a number of aspects of AD. One of the features of advancing age is the gradual decrease in circulating melatonin levels. A limited number of therapeutic trials have indicated that melatonin has a therapeutic value as a neuroprotective drug in the treatment of AD and minimal cognitive impairment (which may evolve to AD). Both in vitro and in vivo, melatonin prevented the neurodegeneration seen in experimental models of AD. For these effects to occur, doses of melatonin about two orders of magnitude higher than those required to affect sleep and circadian rhythmicity are needed. More recently, attention has been focused on the development of potent melatonin analogs with prolonged effects, which were employed in clinical trials in sleep-disturbed or depressed patients in doses considerably higher than those employed for melatonin. In view that the relative potencies of the analogs are higher than that of the natural compound, clinical trials employing melatonin in the range of 50-100 mg/day are urgently needed to assess its therapeutic validity in neurodegenerative disorders such as AD.

Melatonin in Alzheimer's disease

Alzheimer’s disease (AD), an age-related neurodegenerative disorder with progressive cognition deficit, is characterized by extracellular senile plaques (SP) of aggregated β-amyloid (Aβ) and intracellular neurofibrillary tangles, mainly containing the hyperphosphorylated microtubule-associated protein tau. Multiple factors contribute to the etiology of AD in terms of initiation and progression. Melatonin is an endogenously produced hormone in the brain and decreases during aging and in patients with AD. Data from clinical trials indicate that melatonin supplementation improves sleep, ameliorates sundowning and slows down the progression of cognitive impairment in AD patients. Melatonin efficiently protects neuronal cells from Aβ-mediated toxicity via antioxidant and anti-amyloid properties. It not only inhibits Aβ generation, but also arrests the formation of amyloid fibrils by a structure-dependent interaction with Aβ. Our studies have demonstrated that melatonin efficiently attenuates Alzheimer-like tau hyperphosphorylation. Although the exact mechanism is still not fully understood, a direct regulatory influence of melatonin on the activities of protein kinases and protein phosphatases is proposed. Additionally, melatonin also plays a role in protecting the cholinergic system and in anti-inflammation. The aim of this review is to stimulate interest in melatonin as a potentially useful agent in the prevention and treatment of AD.

Reciprocal interactions between sleep, circadian rhythms and Alzheimer's disease: focus on the role of hypocretin and melatonin

AD, sleep and circadian rhythm physiology display an intricate relationship. On the one hand, AD pathology leads to sleep and circadian disturbances, with a clear negative influence on quality of life. On the other hand, there is increasing evidence that both sleep and circadian regulating systems exert an influence on AD pathology. In this review we describe the impairments of both sleep regulating systems and circadian rhythms in AD and their link to clinical symptoms, as this may increase knowledge on appropriate diagnosis and adequate treatment of sleep problems in AD. Furthermore we discuss how sleep regulating systems, and especially neurotransmitters such as melatonin and hypocretin, may affect AD pathophysiology, as this may provide a role for lack of sleep and circadian rhythm deterioration in the onset of AD.

Antioxidant therapies for Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease featuring progressive impairments in memory, cognition, and behavior and ultimately leads to death. The histopathological changes of Alzheimer's disease include neuronal and synaptic loss, formation of extracellular senile plaques and intracellular neurofibrillary tangles in brain. Multiple lines of evidence indicate that oxidative stress not only strongly participates in an early stage of Alzheimer's disease prior to cytopathology, but plays an important role in inducing and activating multiple cell signaling pathways that contribute to the lesion formations of toxic substances and then promotes the development of Alzheimer's disease. Many years of studies show that antioxidant therapies have enjoyed general success in preclinical studies. Therefore, this paper mainly focuses on the recent developments of common used antioxidant therapies for Alzheimer's disease and thus provides indications for future potential antioxidant therapeutic strategies of neurodegenerative diseases.

Melatonin ameliorates Alzheimer-like pathological changes and spatial memory retention impairment induced by calyculin A

We have reported recently that inhibition of protein phosphatase (PP)-2A and PP-1 by calyculin A, a specific inhibitor of PP-2A and PP-1, induced Alzheimer-like hyperphosphorylation of tau and spatial memory retention impairment. In this study, we tested the in vivo effects of melatonin on these Alzheimer-like changes. We found that administration of melatonin intraperitoneally for 9 consecutive days before injection of calyculin A could prevent calyculin A-induced synaptophysin loss, memory retention deficits, as well as hyperphosphorylation of tau and neurofilaments. Furthermore, melatonin partially reversed the phosphorylation of the catalytic subunit of PP-2A at Tyrosine 307 (Y307), a crucial site negatively regulating the activity of PP-2A, and reduced the levels of malondialdehyde, a marker of oxidative stress, induced by calyculin A. These results suggest that melatonin could serve as a potential therapeutic agent for preventing Alzheimer-like pathological changes and behavioral abnormality via modulating the activity of PP-2A and oxidative stress.

Anti-amyloidogenic and anti-apoptotic role of melatonin in Alzheimer disease

Alzheimer disease (AD) is an age-related neurodegenerative disorder characterized by the presence of senile plaques, neurofibrillary tangles and neuronal loss. Amyloid-β protein (Aβ) deposition plays a critical role in the development of AD. It is now generally accepted that massive neuronal death due to apoptosis is a common characteristic in the brains of patients suffering from neurodegenerative diseases, and apoptotic cell death has been found in neurons and glial cells in AD. Melatonin is a secretory product of the pineal gland; melatonin is a potent antioxidant and free radical scavenger and may play an important role in aging and AD. Melatonin decreases during aging and patients with AD have a more profound reduction of this indoleamine. Additionally, the antioxidant properties, the anti-amyloidogenic properties and anti-apoptotic properties of melatonin in AD models have been studied. In this article, we review the anti-amyloidogenic and anti-apoptotic role of melatonin in AD

Melatonin reduces the impairment of axonal transport and axonopathy induced by calyculin A

Previous studies have reported that calyculin A (CA), a selective inhibitor of protein phosphatase (PP)-2A and PP-1, impairs axonal transport in neuroblastoma N2a cells. Melatonin prevents Alzheimer-like hyperphosphorylation of cytoskeletal proteins and the impairment of spatial memory retention induced by CA. In this study, we tested the effects of melatonin on the impairment of axonal transport induced by CA in neuroblastoma N2a cells. We found that melatonin protected the cells from CA-induced toxicity in metabolism and viability as well as hyperphosphorylation of tau and neurofilaments. Furthermore, melatonin partially reversed the CA-induced phosphorylation of the catalytic subunit of PP-2A at tyrosine 307, a crucial site that negatively regulates the activity of PP-2A, and reduced the levels of malondialdehyde and the activity of superoxide dismutase, which are markers of oxidative stress. Melatonin also significantly reversed the CA-induced impairment of axonal transport. These results suggest that melatonin may have a role in protecting against the CA-induced impairment of axonal transport by modulating the activity of PP-2A and oxidative stress.

Melatonin and its agonist ramelteon in Alzheimer's disease: possible therapeutic value

Alzheimer's disease (AD) is an age-associated neurodegenerative disease characterized by the progressive loss of cognitive function, loss of memory and insomnia, and abnormal behavioral signs and symptoms. Among the various theories that have been put forth to explain the pathophysiology of AD, the oxidative stress induced by amyloid β-protein (Aβ) deposition has received great attention. Studies undertaken on postmortem brain samples of AD patients have consistently shown extensive lipid, protein, and DNA oxidation. Presence of abnormal tau protein, mitochondrial dysfunction, and protein hyperphosphorylation all have been demonstrated in neural tissues of AD patients. Moreover, AD patients exhibit severe sleep/wake disturbances and insomnia and these are associated with more rapid cognitive decline and memory impairment. On this basis, the successful management of AD patients requires an ideal drug that besides antagonizing Aβ-induced neurotoxicity could also correct the disturbed sleep-wake rhythm and improve sleep quality. Melatonin is an effective chronobiotic agent and has significant neuroprotective properties preventing Aβ-induced neurotoxic effects in a number of animal experimental models. Since melatonin levels in AD patients are greatly reduced, melatonin replacement has the potential value to be used as a therapeutic agent for treating AD, particularly at the early phases of the disease and especially in those in whom the relevant melatonin receptors are intact. As sleep deprivation has been shown to produce oxidative damage, impaired mitochondrial function, neurodegenerative inflammation, and altered proteosomal processing with abnormal activation of enzymes, treatment of sleep disturbances may be a priority for arresting the progression of AD. In this context the newly introduced melatonin agonist ramelteon can be of much therapeutic value because of its highly selective action on melatonin MT₁/MT₂ receptors in promoting sleep.

Targeting tau protein in Alzheimer's disease

Alzheimer's disease (AD) is characterized histopathologically by numerous neurons with neurofibrillary tangles and neuritic (senile) amyloid-beta (Abeta) plaques, and clinically by progressive dementia. Although Abeta is the primary trigger of AD according to the amyloid cascade hypothesis, neurofibrillary degeneration of abnormally hyperphosphorylated tau is apparently required for the clinical expression of this disease. Furthermore, while approximately 30% of normal aged individuals have as much compact plaque burden in the neocortex as is seen in typical cases of AD, in several tauopathies, such as cortical basal degeneration and Pick's disease, neurofibrillary degeneration of abnormally hyperphosphorylated tau in the absence of Abeta plaques is associated with dementia. To date, all AD clinical trials based on Abeta as a therapeutic target have failed. In addition to the clinical pathological correlation of neurofibrillary degeneration with dementia in AD and related tauopathies, increasing evidence from in vitro and in vivo studies in experimental animal models provides a compelling case for this lesion as a promising therapeutic target. A number of rational approaches to inhibiting neurofibrillary degeneration include inhibition of one or more tau protein kinases, such as glycogen synthase kinase-3beta and cyclin-dependent protein kinase 5, activation of the major tau phosphatase protein phosphatase-2A, elevation of beta-N-acetylglucosamine modification of tau through inhibition of beta-N-acetylglucosaminidase or increase in brain glucose uptake, and promotion of the clearance of the abnormally hyperphosphorylated tau by autophagy or the ubiquitin proteasome system.

Antiinflammatory activity of melatonin in central nervous system

Melatonin is mainly produced in the mammalian pineal gland during the dark phase. Its secretion from the pineal gland has been classically associated with circadian and circanual rhythm regulation. However, melatonin production is not confined exclusively to the pineal gland, but other tissues including retina, Harderian glands, gut, ovary, testes, bone marrow and lens also produce it. Several studies have shown that melatonin reduces chronic and acute inflammation. The immunomodulatory properties of melatonin are well known; it acts on the immune system by regulating cytokine production of immunocompetent cells. Experimental and clinical data showing that melatonin reduces adhesion molecules and pro-inflammatory cytokines and modifies serum inflammatory parameters. As a consequence, melatonin improves the clinical course of illnesses which have an inflammatory etiology. Moreover, experimental evidence supports its actions as a direct and indirect antioxidant, scavenging free radicals, stimulating antioxidant enzymes, enhancing the activities of other antioxidants or protecting other antioxidant enzymes from oxidative damage. Several encouraging clinical studies suggest that melatonin is a neuroprotective molecule in neurodegenerative disorders where brain oxidative damage has been implicated as a common link. In this review, the authors examine the effect of melatonin on several neurological diseases with inflammatory components, including dementia, Alzheimer disease, Parkinson disease, multiple sclerosis, stroke, and brain ischemia/reperfusion but also in traumatic CNS injuries (traumatic brain and spinal cord injury).

Amyloid-beta neurotoxicity in organotypic culture is attenuated by melatonin: involvement of GSK-3beta, tau and neuroinflammation

Alzheimer's disease (AD) is a progressive neurodegenerative disorder marked by accumulation of extracellular deposits of amyloid-beta (Abeta) peptide in brain regions that are important for memory and cognition. The buildup of Abeta aggregates in the AD is followed by the formation of intracellular neurofibrillary tangles and activation of neuroinflammatory reactions. The present study investigated whether melatonin possesses a neuroprotective effect against Abeta-induced toxicity. For this purpose, organotypic hippocampal slices were cultured and exposed to 25 microm of Abeta(25-35) in the absence or in the presence of melatonin (25, 50, or 100 microm). In addition, the authors have investigated the involvement of GSK-3beta, tau protein, astroglial, and microglial activation, and cytokine levels in the melatonin protection against Abeta-induced neurotoxicity. Melatonin prevented the cell damage in hippocampus induced by the exposure to Abeta(25-35). In addition, melatonin significantly reduced the activation of GSK-3beta, the phosphorylation of tau protein, the glial activation and the Abeta-induced increase of TNF-alpha and IL-6 levels. On the basis of these findings, we speculate that melatonin may provide an effective therapeutic strategy for AD, by attenuating Abeta-induced phosphorylation of tau protein, and preventing GSK-3beta activation and neuroinflammation.

Calyculin A-induced neurite retraction is critically dependent on actomyosin activation but not on polymerization state of microtubules

Calyculin A (CL-A), a toxin isolated from the marine sponge Discodermia calyx, is a strong inhibitor of protein phosphatase 1 (PP1) and 2A (PP2A). Although CL-A is known to induce rapid neurite retraction in developing neurons, the cytoskeletal dynamics of this retraction have remained unclear. Here, we investigated the cytoskeletal dynamics during CL-A-induced neurite retraction in cultured rat hippocampal neurons, using fluorescence microscopy as well as polarized light microscopy, which can visualize the polymerization state of the cytoskeleton in living cells. We observed that MTs were bent while maintaining their polymerization state during the neurite retraction. In addition, we also found that CL-A still induced neurite retraction when MTs were depolymerized by nocodazole or stabilized by paclitaxel. These results imply a mechanism other than depolymerization of MTs for CL-A-induced neurite retraction. Our pharmacological studies showed that blebbistatin and cytochalasin D, an inhibitor of myosin II and a depolymerizer of actin, strongly inhibited CL-A-induced neurite retraction. Based on all these findings, we propose that CL-A generates strong contractile forces by actomyosin to induce rapid neurite retraction independently from MT depolymerization.

The antiapoptotic activity of melatonin in neurodegenerative diseases

Melatonin plays a neuroprotective role in models of neurodegenerative diseases. However, the molecular mechanisms underlying neuroprotection by melatonin are not well understood. Apoptotic cell death in the central nervous system is a feature of neurodegenerative diseases. The intrinsic and extrinsic apoptotic pathways and the antiapoptotic survival signal pathways play critical roles in neurodegeneration. This review summarizes the reports to date showing inhibition by melatonin of the intrinsic apoptotic pathways in neurodegenerative diseases including stroke, Alzheimer disease, Parkinson disease, Huntington disease, and amyotrophic lateral sclerosis. Furthermore, the activation of survival signal pathways by melatonin in neurodegenerative diseases is discussed.

Hyperphosphorylation of microtubule-associated protein tau: a promising therapeutic target for Alzheimer disease

Alzheimer disease (AD) is the most common cause of dementia in adults. The current therapy for AD has only moderate efficacy in controlling symptoms, and it does not cure the disease. Recent studies have suggested that abnormal hyperphosphorylation of tau in the brain plays a vital role in the molecular pathogenesis of AD and in neurodegeneration. This article reviews the current advances in understanding of tau protein, regulation of tau phosphorylation, and the role of its abnormal hyperphosphorylation in neurofibrillary degeneration. Furthermore, several therapeutic strategies for treating AD on the basis of the important role of tau hyperphosphorylation in the pathogenesis of the disease are described. These strategies include (1) inhibition of glycogen synthase kinase-3beta (GSK-3beta), cyclin-dependent kinase 5 (cdk5), and other tau kinases; (2) restoration of PP2A activity; and (3) targeting tau O-GlcNAcylation. Development of drugs on the basis of these strategies is likely to lead to disease-modifying therapies for AD.

Effect of melatonin and melatonylvalpromide on beta-amyloid and neurofilaments in N2a cells

In the present study, we have studied the effect of melatonin (Mt) and melatonin derivative, i.e., melatonylvalpromide (Mtv), on cell viability, beta-amyloid (Abeta) production, cell morphology, and expression and phosphorylation of neurofilament proteins in wild-type murine neuroblastoma N2a (N2a/wt) and N2a stably transfected with amyloid precursor protein (N2a/APP) cell lines. The study used MTT assay, Sandwich ELISA, immunocytochemistry and Western blots techniques. The results showed that both Mt and Mtv could increase cell viability, but Mtv did so more effectively. The N2a/APP showed shorter and less amount of cell processes than N2a/wt, and Mtv but not Mt slightly improved the morphological changes in N2A/APP. Both Mt and Mtv suppressed the Abeta level in cell lysates, but the effect of Mtv was stronger than Mt. The immunoreaction to the non-phosphorylated neurofilament proteins probed by SMI32 and SMI33 were remarkably weaker in N2a/APP than N2a/wt, while the immunoreaction to the phosphorylated neurofilament proteins at SMI34 epitopes was slightly stronger in N2a/APP than N2a/wt, suggesting higher phosphorylation level of neurofilament proteins in N2a/APP. Treatment of the cells with Mt and Mtv increased the immunoreaction at SMI32 and SMI33 epitopes, while only Mtv but not Mt decreased the staining at SMI34 epitope, suggesting both Mt and Mtv promote dephosphorylation of neurofilament at SMI32 and SMI33 epitopes, while Mtv stimulates dephosphorylation of neurofilament at SMI34 epitope. These results suggest that Mtv may be a better candidate in arresting the intracellular accumulation of Abeta and protecting the cells from Abeta-related toxicity.

Dehydroevodiamine attenuates calyculin A-induced tau hyperphosphorylation in rat brain slices

AIM

This study was to investigate the effect of dehydroevodiamine (DHED) on Alzheimer's disease (AD)-like tau hyperphosphorylation induced by calyculin A (CA), an inhibitor of protein phosphatase (PP)-2A and PP-1, and the involvement of PP-2A in metabolically competent rat brain slices.

METHODS

Rat brain slices were pre-incubated at 33 degree centigrade in the presence (10, 100, and 200 micromol/L, respectively) or absence of DHED for 1 h. Then, CA 0.1 micromol/L was added and the slices were treated for another 2 h. Western blotting and/or immunohistochemistry were used to measure the phosphorylation level of tau and PP-2A.

RESULTS

CA treatment could remarkably increase the immunoreactivity of pS262 and decrease the staining of Tau-1, representing tau hyperphosphorylation at Ser262 (pS262) and Ser198/ 199/202 (Tau-1, as the antibody reacts with unphosphorylated tau, therefore, decreased staining represents increased phosphorylation). Pre-incubation of the brain slices with DHED could efficiently attenuate the CA-induced tau hyperphosphorylation at the above AD-related sites. Additionally, DHED also decreased the basal phosphorylation level of tau at Ser396, although CA failed to induce tau hyperphosphorylation at this site. Furthermore, CA treatment induced an increased level of Tyr307-phosphorylated PP-2A, which represents inactivation of the phosphatase, whereas DHED arrested the elevation of the inhibitory modification of PP-2A.

CONCLUSION

DHED can attenuate CA-induced tau hyperphosphorylation at multiple AD-related sites in metabolically active rat brain slices. The underlying mechanism may involve a decreased inhibitory phosphorylation of PP-2A at Tyr307.

Effects of endogenous β-amyloid overproduction on tau phosphorylation in cell culture

Alzheimer's disease is characterized by beta-amyloid (Abeta) overproduction and tau hyperphosphorylation. Recent studies have shown that synthetic Abeta promotes tau phosphorylation in vitro. However, whether endogenously overproduced Abeta promotes tau phosphorylation and the underlying mechanisms remain unknown. Here, we used mouse neuroblastoma N2a stably expressing wild-type amyloid precursor protein (APPwt) or the Swedish mutant APP (APPswe) to determine the alterations of phosphorylated tau and the related protein kinases. We found that phosphorylation of tau at paired helical filament (PHF)-1, pSer396 and pThr231 epitopes was significantly increased in cells transfected with APPwt and APPswe, which produced higher levels of Abeta than cells transfected with vector or amyloid precursor-like protein 1. The activity of glycogen synthase kinase-3 (GSK-3) was up-regulated with a concomitant reduction in the inhibitory phosphorylation of GSK-3 at its N-terminal Ser9 residue. In contrast, the activity of cyclin-dependent kinase-5 (CDK-5) and protein kinase C (PKC) was down-regulated. Inhibition of GSK-3 by LiCl, but not inhibition of CDK-5 by roscovitine, arrested Abeta secretion and tau phosphorylation. Inhibition of PKC by GF-109203X activated GSK-3, whereas activation of PKC by phorbol-12,13-dibutyrate inhibited GSK-3. These results suggest that endogenously overproduced Abeta induces increased tau phosphorylation through activation of GSK-3, and that inactivation of PKC is at least one of the mechanisms involved in GSK-3 activation.

Melatonin in Alzheimer's disease and other neurodegenerative disorders

Increased oxidative stress and mitochondrial dysfunction have been identified as common pathophysiological phenomena associated with neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). As the age-related decline in the production of melatonin may contribute to increased levels of oxidative stress in the elderly, the role of this neuroprotective agent is attracting increasing attention. Melatonin has multiple actions as a regulator of antioxidant and prooxidant enzymes, radical scavenger and antagonist of mitochondrial radical formation. The ability of melatonin and its kynuramine metabolites to interact directly with the electron transport chain by increasing the electron flow and reducing electron leakage are unique features by which melatonin is able to increase the survival of neurons under enhanced oxidative stress. Moreover, antifibrillogenic actions have been demonstrated in vitro, also in the presence of profibrillogenic apoE4 or apoE3, and in vivo, in a transgenic mouse model. Amyloid-β toxicity is antagonized by melatonin and one of its kynuramine metabolites. Cytoskeletal disorganization and protein hyperphosphorylation, as induced in several cell-line models, have been attenuated by melatonin, effects comprising stress kinase downregulation and extending to neurotrophin expression. Various experimental models of AD, PD and HD indicate the usefulness of melatonin in antagonizing disease progression and/or mitigating some of the symptoms. Melatonin secretion has been found to be altered in AD and PD. Attempts to compensate for age- and disease-dependent melatonin deficiency have shown that administration of this compound can improve sleep efficiency in AD and PD and, to some extent, cognitive function in AD patients. Exogenous melatonin has also been reported to alleviate behavioral symptoms such as sundowning. Taken together, these findings suggest that melatonin, its analogues and kynuric metabolites may have potential value in prevention and treatment of AD and other neurodegenerative disorders.

Dysregulation of protein phosphorylation/dephosphorylation in Alzheimer's disease: a therapeutic target

Studies during the last two decades have provided new insights into the molecular mechanism of Alzheimer's disease (AD). One of the milestone findings in AD research was the demonstration that neurofibrillary degeneration characterized by tau pathology is central to the pathogenesis of AD and other tauopathies and that abnormal hyperphosphorylation of tau is pivotal to neurofibrillary degeneration. This article reviews the recent research advances in tau pathology and the underlying dysregulation of the protein phosphorylation/dephosphorylation system. An updated model of the mechanism of neurofibrillary degeneration is also presented, and a promising therapeutic target to treat AD by correcting dysregulation of protein phosphorylation/dephosphorylation is discussed.

Role of melatonin in Alzheimer-like neurodegeneration

Alzheimer disease (AD), an age-related neurodegenerative disorder with progressive loss of memory and deterioration of comprehensive cognition, is characterized by extracellular senile plaques of aggregated beta-amyloid (Abeta), and intracellular neurofibrillary tangles that contain hyperphosphorylated tau protein. Recent studies showed that melatonin, an indoleamine secreted by the pineal gland, may play an important role in aging and AD as an antioxidant and neuroprotector. Melatonin decreases during aging and patients with AD have a more profound reduction in this hormone. Data from clinical trials indicate that melatonin supplementation improves sleep, ameliorates sundowning, and slows down the progression of cognitive impairment in Alzheimer patients. Melatonin efficiently protects neuronal cells from Abeta-mediated toxicity via antioxidant and anti-amyloid properties: it not only inhibits Abeta generation, but also arrests the formation of amyloid fibrils by a structure-dependent interaction with Abeta. Our recent studies have demonstrated that melatonin efficiently attenuates Alzheimer-like tau hyperphosphorylation. Although the exact mechanism is still not fully understood, a direct regulatory influence of melatonin on the activities of protein kinases and protein phosphatases is proposed. Additionally, melatonin also plays a role in protecting cholinergic neurons and in anti-inflammation. Here, the neuroprotective effects of melatonin and the underlying mechanisms by which it exerts its effects are reviewed. The capacity of melatonin to prevent or ameliorate tau and Abeta pathology further enhances its potential in the prevention or treatment of AD.

Role of melatonin in neurodegenerative diseases

The pineal product melatonin has remarkable antioxidant properties. It scavenges hydroxyl, carbonate and various organic radicals, peroxynitrite and other reactive nitrogen species. Melatonyl radicals formed by scavenging combine with and, thereby, detoxify superoxide anions in processes terminating the radical reaction chains. Melatonin also enhances the antioxidant potential of the cell by stimulating the synthesis of antioxidant enzymes like superoxide dismutase, glutathione peroxidase and glutathione reductase, and by augmenting glutathione levels. The decline in melatonin production in aged individuals has been suggested as one of the primary contributing factors for the development of age-associated neurodegenerative diseases, e.g., Alzheimer's disease. Melatonin has been shown to be effective in arresting neurodegenerative phenomena seen in experimental models of Alzheimer's disease, Parkinsonism and ischemic stroke. Melatonin preserves mitochondrial homeostasis, reduces free radical generation, e.g., by enhancing mitochondrial glutathione levels, and safeguards proton potential and ATP synthesis by stimulating complex I and IV activities. Therapeutic trials with melatonin have been effective in slowing the progression of Alzheimer's disease but not of Parkinson's disease. Melatonin's efficacy in combating free radical damage in the brain suggests that it may be a valuable therapeutic agent in the treatment of cerebral edema after traumatic brain injury.