Journal of Alzheimer's Disease - Volume 57, issue 4

Authors: Reddy, P. Hemachandra

Article Type: Introduction

Abstract: The purpose of this mini-forum, “Neurotransmitters and Alzheimer’s Disease”, is to critically assess the current status of neurotransmitters in Alzheimer’s disease. Neurotransmitters are essential neurochemicals that maintain synaptic and cognitive functions in mammals, including humans, by sending signals across pre- to post-synaptic neurons. Authorities in the fields of synapses and neurotransmitters of Alzheimer’s disease summarize the current status of basic biology of synapses and neurotransmitters, and also update the current status of clinical trials of neurotransmitters in Alzheimer’s disease. This article discusses the prevalence, economic impact, and stages of Alzheimer’s dementia in humans.

Keywords: Amyloid-β, cognitive decline, mitochondrial dysfunction, synaptic pathology

DOI: 10.3233/JAD-170256

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 969-974, 2017

Authors: Rajmohan, Ravi | Reddy, P. Hemachandra

Article Type: Review Article

Abstract: Amyloid-beta (Aβ) and hyperphosphorylated tau are hallmark lesions of Alzheimer’s disease (AD). However, the loss of synapses and dysfunctions of neurotransmission are more directly tied to disease severity. The role of these lesions in the pathoetiological progression of the disease remains contested. Biochemical, cellular, molecular, and pathological studies provided several lines of evidence and improved our understanding of how Aβ and hyperphosphorylated tau accumulation may directly harm synapses and alter neurotransmission. In vitro evidence suggests that Aβ and hyperphosphorylated tau have both direct and indirect cytotoxic effects that affect neurotransmission, axonal transport, signaling cascades, organelle function, and immune response …in ways that lead to synaptic loss and dysfunctions in neurotransmitter release. Observations in preclinical models and autopsy studies support these findings, suggesting that while the pathoetiology of positive lesions remains elusive, their removal may reduce disease severity and progression. The purpose of this article is to highlight the need for further investigation of the role of tau in disease progression and its interactions with Aβ and neurotransmitters alike. Show more

Keywords: Amyloid, neurotransmitters, synapse, tau

DOI: 10.3233/JAD-160612

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 975-999, 2017

Authors: Bethea, Cynthia L. | Reddy, Arubala P. | Christian, Fernanda Lima

Article Type: Review Article

Abstract: Serotonin plays a key role in mood or affect, and dysfunction of the serotonin system has been linked to depression in humans and animal models. Depression appears prior to or coincident with overt symptoms of Alzheimer’s disease (AD) in about 50% of patients, and some experts consider it a risk factor for the development of AD. In addition, AD is more prevalent in women, who also show increased incidence of depression. Indeed, it has been proposed that mechanisms underlying depression overlap the mechanisms thought to hasten AD. Women undergo ovarian failure and cessation of ovarian steroid production in middle age …and the postmenopausal period correlates with an increase in the onset of depression and AD. This laboratory has examined the many actions of ovarian steroids in the serotonin system of non-human primates using a rhesus macaque model of surgical menopause with short or long-term estradiol (E) or estradiol plus progesterone (E+P) replacement therapy. In this mini-review, we present a brief synopsis of the relevant literature concerning AD, depression, and serotonin. We also present some of our data on serotonin neuron viability, the involvement of the caspase-independent pathway, and apoptosis-inducing factor in serotonin-neuron viability, as well as gene expression related to neurodegeneration and neuron viability in serotonin neurons from adult and aged surgical menopausal macaques. We show that ovarian steroids, particularly E, are crucial for serotonin neuron function and health. In the absence of E, serotonin neurons are endangered and deteriorating toward apoptosis. The possibility that this scenario may proceed or accompany AD in postmenopausal women seems likely. Show more

Keywords: Estrogen, macaque, menopause, neuroendangerment, serotonin

DOI: 10.3233/JAD-160601

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 1001-1015, 2017

Authors: Jha, Saurabh Kumar | Jha, Niraj Kumar | Kumar, Dhiraj | Sharma, Renu | Shrivastava, Abhishek | Ambasta, Rashmi K. | Kumar, Pravir

Article Type: Review Article

Abstract: The communication between neurons at synaptic junctions is an intriguing process that monitors the transmission of various electro-chemical signals in the central nervous system. Albeit any aberration in the mechanisms associated with transmission of these signals leads to loss of synaptic contacts in both the neocortex and hippocampus thereby causing insidious cognitive decline and memory dysfunction. Compelling evidence suggests that soluble amyloid-β (Aβ) and hyperphosphorylated tau serve as toxins in the dysfunction of synaptic plasticity and aberrant neurotransmitter (NT) release at synapses consequently causing a cognitive decline in Alzheimer’s disease (AD). Further, an imbalance between excitatory and inhibitory neurotransmission systems induced …by impaired redox signaling and altered mitochondrial integrity is also amenable for such abnormalities. Defective NT release at the synaptic junction causes several detrimental effects associated with altered activity of synaptic proteins, transcription factors, Ca2+ homeostasis, and other molecules critical for neuronal plasticity. These detrimental effects further disrupt the normal homeostasis of neuronal cells and thereby causing synaptic loss. Moreover, the precise mechanistic role played by impaired NTs and neuromodulators (NMs) and altered redox signaling in synaptic dysfunction remains mysterious, and their possible interlink still needs to be investigated. Therefore, this review elucidates the intricate role played by both defective NTs/NMs and altered redox signaling in synaptopathy. Further, the involvement of numerous pharmacological approaches to compensate neurotransmission imbalance has also been discussed, which may be considered as a potential therapeutic approach in synaptopathy associated with AD. Show more

Keywords: Amyloid-β, neurotransmitters/neuromodulators, redox signaling, synaptic dysfunction, tau, therapeutics

DOI: 10.3233/JAD-160623

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 1017-1039, 2017

Authors: Wang, Rui | Reddy, P. Hemachandra

Article Type: Review Article

Abstract: Excitatory glutamatergic neurotransmission via N-methyl-d-aspartate receptor (NMDAR) is critical for synaptic plasticity and survival of neurons. However, excessive NMDAR activity causes excitotoxicity and promotes cell death, underlying a potential mechanism of neurodegeneration occurred in Alzheimer’s disease (AD). Studies indicate that the distinct outcomes of NMDAR-mediated responses are induced by regionalized receptor activities, followed by different downstream signaling pathways. The activation of synaptic NMDARs initiates plasticity and stimulates cell survival. In contrast, the activation of extrasynaptic NMDARs promotes cell death and thus contributes to the etiology of AD, which can be blocked by an AD drug, memantine, an NMDAR antagonist that …selectively blocks the function of extrasynaptic NMDARs. Show more

Keywords: Alzheimer’s disease, excitotoxicity, extrasynaptic NMDA receptors, glutamate, memantine, NMDA receptors

DOI: 10.3233/JAD-160763

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 1041-1048, 2017

Authors: Kandimalla, Ramesh | Reddy, P. Hemachandra

Article Type: Review Article

Abstract: Alzheimer’s disease (AD) is a progressive neurodegenerative disease, characterized by the loss of memory, multiple cognitive impairments and changes in the personality and behavior. Several decades of intense research have revealed that multiple cellular changes are involved in disease process, including synaptic damage, mitochondrial abnormalities and inflammatory responses, in addition to formation and accumulation of amyloid-β (Aβ) and phosphorylated tau. Although tremendous progress has been made in understanding the impact of neurotransmitters in the progression and pathogenesis of AD, we still do not have a drug molecule associated with neurotransmitter(s) that can delay disease process in elderly individuals and/or restore …cognitive functions in AD patients. The purpose of our article is to assess the latest developments in neurotransmitters research using cell and mouse models of AD. We also updated the current status of clinical trials using neurotransmitters’ agonists/antagonists in AD. Show more

Keywords: Acetylcholine inhibitors, adenosine receptors, Alzheimer’s disease, histaminergic, N-methyl-D-aspartate, neurotransmitters

DOI: 10.3233/JAD-161118

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 1049-1069, 2017

Authors: Guo, Lan | Tian, Jing | Du, Heng

Article Type: Review Article

Abstract: Alzheimer’s disease (AD) is a chronic neurodegenerative disorder, in which multiple risk factors converge. Despite the complexity of the etiology of the disease, synaptic failure is the pathological basis of cognitive impairment, the cardinal sign of AD. Decreased synaptic density, compromised synaptic transmission, and defected synaptic plasticity are hallmark synaptic pathologies accompanying AD. However, the mechanisms by which synapses are injured in AD-related conditions have not been fully elucidated. Mitochondria are a critical organelle in neurons. The pivotal role of mitochondria in supporting synaptic function and the concomitant occurrence of mitochondrial dysfunction with synaptic stress in postmortem AD brains as …well as AD animal models seem to lend the credibility to the hypothesis that mitochondrial defects underlie synaptic failure in AD. This concept is further strengthened by the protective effect of mitochondrial medicine on synaptic function against the toxicity of amyloid-β, a key player in the pathogenesis of AD. In this review, we focus on the association between mitochondrial dysfunction and synaptic transmission deficits in AD. Impaired mitochondrial energy production, deregulated mitochondrial calcium handling, excess mitochondrial reactive oxygen species generation and release play a crucial role in mediating synaptic transmission deregulation in AD. The understanding of the role of mitochondrial dysfunction in synaptic stress may lead to novel therapeutic strategies for the treatment of AD through the protection of synaptic transmission by targeting to mitochondrial deficits. Show more

Keywords: Alzheimer’s disease, mitochondrial dysfunction, synaptic injury, synaptic mitochondria, synaptic transmission

DOI: 10.3233/JAD-160702

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 1071-1086, 2017

Authors: Cai, Qian | Tammineni, Prasad

Article Type: Review Article

Abstract: Alzheimer’s disease (AD) is characterized by brain deposition of amyloid plaques and tau neurofibrillary tangles along with steady cognitive decline. Synaptic damage, an early pathological event, correlates strongly with cognitive deficits and memory loss. Mitochondria are essential organelles for synaptic function. Neurons utilize specialized mechanisms to drive mitochondrial trafficking to synapses in which mitochondria buffer Ca2+ and serve as local energy sources by supplying ATP to sustain neurotransmitter release. Mitochondrial abnormalities are one of the earliest and prominent features in AD patient brains. Amyloid-β (Aβ) and tau both trigger mitochondrial alterations. Accumulating evidence suggests that mitochondrial perturbation acts as …a key factor that is involved in synaptic failure and degeneration in AD. The importance of mitochondria in supporting synaptic function has made them a promising target of new therapeutic strategies for AD. Here, we review the molecular mechanisms regulating mitochondrial function at synapses, highlight recent findings on the disturbance of mitochondrial dynamics and transport in AD, and discuss how these alterations impact synaptic vesicle release and thus contribute to synaptic pathology associated with AD. Show more

Keywords: Alzheimer’s disease, amyloid-β, ATP supply, axonal transport, mitochondrial trafficking, neurotransmitter, oxidative stress, synaptic pathology, synaptic vesicle release, tau

DOI: 10.3233/JAD-160726

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 1087-1103, 2017

Authors: Tönnies, Eric | Trushina, Eugenia

Article Type: Review Article

Abstract: Alzheimer’s disease (AD) is a devastating neurodegenerative disorder without a cure. Most AD cases are sporadic where age represents the greatest risk factor. Lack of understanding of the disease mechanism hinders the development of efficacious therapeutic approaches. The loss of synapses in the affected brain regions correlates best with cognitive impairment in AD patients and has been considered as the early mechanism that precedes neuronal loss. Oxidative stress has been recognized as a contributing factor in aging and in the progression of multiple neurodegenerative diseases including AD. Increased production of reactive oxygen species (ROS) associated with age- and disease-dependent loss …of mitochondrial function, altered metal homeostasis, and reduced antioxidant defense directly affect synaptic activity and neurotransmission in neurons leading to cognitive dysfunction. In addition, molecular targets affected by ROS include nuclear and mitochondrial DNA, lipids, proteins, calcium homeostasis, mitochondrial dynamics and function, cellular architecture, receptor trafficking and endocytosis, and energy homeostasis. Abnormal cellular metabolism in turn could affect the production and accumulation of amyloid-β (Aβ) and hyperphosphorylated Tau protein, which independently could exacerbate mitochondrial dysfunction and ROS production, thereby contributing to a vicious cycle. While mounting evidence implicates ROS in the AD etiology, clinical trials with antioxidant therapies have not produced consistent results. In this review, we will discuss the role of oxidative stress in synaptic dysfunction in AD, innovative therapeutic strategies evolved based on a better understanding of the complexity of molecular mechanisms of AD, and the dual role ROS play in health and disease. Show more

Keywords: Alzheimer’s disease, amyloid-β, antioxidants, caloric restriction, exercise, mitochondria, mitohormesis, neurotransmission, oxidative stress, synaptic function, tau protein

DOI: 10.3233/JAD-161088

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 1105-1121, 2017

Authors: Jiang, Shanya | Bhaskar, Kiran

Article Type: Review Article

Abstract: Alzheimer’s disease (AD) is the most common form of dementia affecting nearly 45 million people worldwide. However, the etiology of AD is still unclear. Accumulations of amyloid-β plaques and tau tangles, neuroinflammation, and synaptic and neuronal loss are the major neuropathological hallmarks of AD, with synaptic loss being the strongest correlating factor with memory and cognitive impairment in AD. Many of these pathological hallmarks influence each other during the onset and progression of the disease. Recent genetic evidence suggests the possibility of a causal link between altered immune pathways and synaptic dysfunction in AD. Emerging studies also suggest that immune …system-mediated synaptic pruning could initiate early-stage pathogenesis of AD. This comprehensive review is toward understanding the crosstalk of neuron-microglia-astrocyte and dynamics of complement, cytokine, and chemokine systems in the regulation of synaptic function and dysfunction relevant to AD. We start with summarizing several immune pathways, involving complements, MHC-I and CX3CL1, which mediate synaptic elimination during development and in AD. We then will discuss the potential of targeting these molecules as therapeutic interventions or as biomarkers for AD. Show more

Keywords: Alzheimer’s disease, astrocyte, complements, CX3CR1, cytokines, fractalkine, MHC-I, microglia, neurons, synaptic pruning

DOI: 10.3233/JAD-161123

Citation: Journal of Alzheimer's Disease, vol. 57, no. 4, pp. 1123-1135, 2017