Environmental factors in the development and progression of late-onset Alzheimer’s disease

Late-onset Alzheimer’s disease (LOAD) is an age-related neurodegenerative disorder characterized by gradual loss of synapses and neurons, but its pathogenesis remains to be clarified. Neurons live in an environment constituted by neurons themselves and glial cells. In this review, we propose that the neuronal degeneration in the AD brain is partially caused by diverse environmental factors. We first discuss various environmental stresses and the corresponding responses at different levels. Then we propose some mechanisms underlying the specific pathological changes, in particular, hypothalamic-pituitary adrenal axis dysfunction at the systemic level; cerebrovascular dysfunction, metal toxicity, glial activation, and Aβ toxicity at the intercellular level; and kinase-phosphatase imbalance and epigenetic modification at the intracellular level. Finally, we discuss the possibility of developing new strategies for the prevention and treatment of LOAD from the perspective of environmental stress. We conclude that environmental factors play a significant role in the development of LOAD through multiple pathological mechanisms.     

Tau hyperphosphorylation induces apoptotic escape and triggers neurodegeneration in Alzheimer＇s disease

Since abnormal post-translational modifications or gene mutations of tau have been detected in over twenty neurodegenerative disorders, tau has attracted widespread interest as a target protein. Among its various post-translational modifications, phosphorylation is the most extensively studied. It is recognized that tau hyperphosphorylation is the root cause of neurodegeneration in Alzheimer＇s disease （AD）; however, it is not clear how it causes neurodegeneration. Based on the findings that tau hyperphosphorylation leads to the escape of neurons from acute apoptosis and simultaneously impairs the function of neurons, we have proposed that the nature of AD neurodegeneration is the consequence of aborted apoptosis induced by tau phosphorylation. Therefore, proper manipulation of tau hyperphosphorylation could be promising for arresting AD neurodegeneration. In this review, the neuroprotective and neurodegenerative effects of tau hyperphosphorylation and our thoughts regarding their relationship are presented.     

Cerebrospinal fluid biomarkers of Alzheimer＇s disease

Alzheimer＇s disease （AD） is a fatal neurodegenerative disorder that takes about a decade to develop, making early diagnosis possible. Clinically, the diagnosis of AD is complicated, costly, and inaccurate, so it is urgent to find specific biomarkers. Due to its multifactorial nature, a panel of biomarkers for the multiple pathologies of AD, such as cerebral amyloidogenesis, neuronal dysfunction, synapse loss, oxidative stress, and inflammation, are most promising for accurate diagnosis. Highly sensitive and high-throughput proteomic techniques can be applied to develop a panel of novel biomarkers for AD. In this review, we discuss the metabolism and diagnostic performance of the well-established core candidate cerebrospinal fluid （CSF） biomarkers （β-amyloid, total tau, and hyperphosphorylated tau）. Meanwhile, novel promising CSF biomarkers, especially those identified by proteomics, updated in the last five years are also extensively discussed. Furthermore, we provide perspectives on how biomarker discovery for AD is evolving.     

Deregulation of brain insulin signaling in Alzheimer＇s disease

Contrary to the previous belief that insulin does not act in the brain, studies in the last three decades have demonstrated important roles of insulin and insulin signal transduction in various functions of the central nervous system. Deregulated brain insulin signaling and its role in molecular pathogenesis have recently been reported in Alzheimer＇s disease （AD）. In this article, we review the roles of brain insulin signaling in memory and cognition, the metabolism of amyloid 13 precursor protein, and tau phosphorylation. We further discuss deficiencies of brain insulin signaling and glucose metabolism, their roles in the development of AD, and recent studies that target the brain insulin signaling pathway for the treatment of AD. It is clear now that deregulation of brain insulin signaling plays an important role in the development of sporadic AD. The brain insulin signaling pathway also offers a promising therapeutic target for treating AD and probably other neurodegenerative disorders.     

APOE and APOC1 gene polymorphisms are associated with cognitive impairment progression in Chinese patients with late-onset Alzheimer’s disease

Current evidence shows that apolipoprotein E(APOE), apolipoprotein CI(APOC1) and low density lipoprotein receptor-related protein(LRP) variations are related to late-onset Alzheimer's disease. However, it remains unclear if genetic polymorphisms in these genes are associated with cognitive decline in late-onset Alzheimer's disease patients. We performed a 30-month longitudinal cohort study to investigate the relationship between Alzheimer's disease and APOE, APOC1, and LRP. In this study, 78 Chinese Han patients with late-onset Alzheimer's disease were recruited form Guangxi Zhuang Autonomous Region in China. APOE, APOC1, and LRP genotyping was performed using polymerase chain reaction-restriction fragment length polymorphisms. The Mini-Mental State Examination and Clinical Dementia Rating Scale were used to assess patients' cognitive function. After a 30-month follow-up period, we found a significant reduction in Mini-Mental State Examination total score, a higher proportion of patients fulfilling cognitive impairment progression criteria, and a higher proportion of APOC1 H2 carriers in APOE ε4 carriers compared with non-carriers. In addition, the APOE ε4 allele frequency was significantly higher in the cognitive impairment progression group compared with the non-cognitive impairment progression group. In conclusion, APOE ε4 plays an important role in augmenting cognitive decline, and APOC1 H2 may act synergistically with APOE ε4 in increasing the risk of cognitive decline in Chinese patients with late-onset Alzheimer's disease.     

Apolipoprotein E,amyloid-beta,and neuroinflammation in Alzheimer＇s disease

Alzheimer＇s disease （AD） is characterized by the accumulation and deposition of amyloid-beta （Aβ） peptides in the brain. Neuroinflammation occurs in the AD brain and plays a critical role in the neurodegenerative pathology. Particularly, Aβ evokes an inflammatory response that leads to synaptic dysfunction, neuronal death, and neurodegeneration. Apolipoprotein E （ApoE） proteins are involved in cholesterol transport, Aβ binding and clearance, and synaptic functions in the brain. The ApoE4 isoform is a key risk factor for AD, while the ApoE2 isoform has a neuroprotective effect. However, studies have reached different conclusions about the roles of the isoforms; some show that both ApoE3 and ApoE4 have anti-inflammatory effects, while others show that ApoE4 causes a predisposition to inflammation or promotes an inflammatory response following lipopolysaccharide treatment. These discrepancies may result from the differences in models, cell types, experimental conditions, and inflammatory stimuli used. Further, little was known about the role of ApoE isoforms in the Aβ-induced inflammatory response and in the neuroinflammation of AD. Our recent work showed that ApoE isoforms differentially regulate and modify the Aβ-induced inflammatory response in neural cells, with ApoE2 suppressing and ApoE4 promoting the response. In this article, we review the roles, mechanisms, and interrelations among Aβ, ApoE, and neuroinflammation in AD.     

M1 muscarinic acetylcholine receptor in Alzheimer＇s disease

The degeneration of cholinergic neurons and cholinergic hypofunction are pathologies associated with Alzheimer＇s disease （AD）. Muscarinic acetylcholine receptors （mAChRs） mediate acetylcholine-induced neurotransmission and five mAChR subtypes （M1-M5） have been identified. Among them, M1 mAChR is widely expressed in the central nervous system and has been implicated in many physiological and pathological brain functions. In addition, M1 mAChR is postulated to be an important therapeutic target for AD and several other neurodegenerative diseases. In this article, we review recent progress in understanding the functional involvement of M1 mAChR in AD pathology and in developing M1 mAChR agonists forAD treatment.     

Lipid metabolism in Alzheimer＇s disease

Lipids play crucial roles in cell signaling and various physiological processes, especially in the brain. Impaired lipid metabolism in the brain has been implicated in neurodegenerative diseases, such as Alzheimer＇s disease （AD）, and other central nervous system insults. The brain contains thousands of lipid species, but the complex lipid compositional diversity and the function of each of lipid species are currently poorly understood. This review integrates current knowledge about major lipid changes with the molecular mechanisms that underlie AD pathogenesis.     

Neuronal failure in Alzheimer＇s disease： a view through the oxidative stress looking-glass

Considerable debate and controversy surround the cause（s） of AIzheimer＇s disease （AD）. To date, several theories have gained notoriety, however none is universally accepted. In this review, we provide evidence for the oxidative stress-induced AD cascade that posits aged mitochondria as the critical origin of neurodegeneration in AD.     

Oxidative stress in Alzheimer＇s disease

Oxidative stress plays a significant role in the pathogenesis of Alzheimer＇s disease （AD）, a devastating disease of the elderly. The brain is more vulnerable than other organs to oxidative stress, and most of the components of neurons （lipids, proteins, and nucleic acids） can be oxidized in AD due to mitochondrial dysfunction, increased metal levels, inflammation, and β-amyloid （Aβ） peptides. Oxidative stress participates in the development of AD by promoting Aβ deposition, tau hyperphosphorylation, and the subsequent loss of synapses and neurons. The relationship between oxidative stress and AD suggests that oxidative stress is an essential part of the pathological process, and antioxidants may be useful for AD treatment.     

Clusterin in Alzheimer＇s disease： a player in the biological behavior of amyloid-beta

Alzheimer＇s disease （AD） remains a major killer, and although its pathogenesis varies, one dominant feature is an increase in the expression, formation, and sedimentation of senile plaques of amyloid-beta （Aβ） peptides in the brain. The chaperone protein clusterin has, since its first discovery at the end of the 20^th century, been labeled as a cytoprotector. However, epigenetic studies showing that clusterin is associated with the severity and risk of AD, especially in the hippocampus, triggered studies to clarify its role in the pathogenesis of AD. It is true that clusterin can inhibit the aggregation of Aβ and therefore prevent further formation of senile plaques in the AD brain, yet it induces the formation of soluble forms of Aβ which are toxic to neurons. Another problematic finding is that clusterin is involved in a pathway through which Aβ has neurodegenerative effects intracellularly. Although the role of clusterin in the pathogenesis of AD is still not clear, this review specifically discusses the interactions between clusterin and Aβ, to open up the possibility of a potential therapeutic approach for treating AD.     

Alzheimer＇s disease： from molecule to clinic

AIzheimer＇s disease （AD） is the most common neurodegenerative disorder and is characterized pathologically by the progressive intracellular accumulation of neurofibrillary tangles and extracellular deposition of plaques in the brain. Since hyperphosphorylated tau and ~-amyloid （A/3） are respectively the major components of the tangles and the plaques, recent studies have mainly focused on the alterations associated with these two molecules. Clinically, AD features memory deterioration. More than 6 million patients in China alone are suffering from this devastating disease, and this figure is increasing with the acceleration of population ageing. Although many specific aspects of AD have been documented, the disease is hard to diagnose in the early stage and the efficacy of current treatments is very limited. Therefore, AD is attracting worldwide attention.     

Shifting balance from neurodegeneration to regeneration of the brain： a novel therapeutic approach to Alzheimer＇s disease and related neurodegenerative conditions

<正>Neurodegeneration is one of the biggest public health problems in modern society.Age-associated neurodegeneration,which is accelerated several-fold in Alzheimer’s disease(AD)alone,is not only an enormous social and economic burden to the affected individuals and their families,but is also a great scientific challenge.     

Predictors of progression of cognitive decline in Alzheimer’s disease: the role of vascular and sociodemographic factors

Rates of disease progression differ among patients with Alzheimer’s disease, but little is known about prognostic predictors. The aim of the study was to assess whether sociodemographic factors, disease severity and duration, and vascular factors are prognostic predictors of cognitive decline in Alzheimer’s disease progression. We conducted a longitudinal clinical study in a specialized clinical unit for the diagnosis and treatment of dementia in Rome, Italy. A total of 154 persons with mild to moderate Alzheimer’s disease consecutively admitted to the dementia unit were included. All patients underwent extensive clinical examination by a physician at admittance and all follow-ups. We evaluated the time-dependent probability of a worsening in cognitive performance corresponding to a 5-point decrease in Mini-Mental State Examination (MMSE) score. Survival analysis was used to analyze risk of faster disease progression in relation to age, education, severity and duration of the disease, family history of dementia, hypertension, hypercholesterolemia, and type 2 diabetes. Younger and more educated persons were more likely to have faster Alzheimer’s disease progression. Vascular factors such as hypertension and hypercholesterolemia were not found to be significantly associated with disease progression. However, patients with diabetes had a 65% reduced risk of fast cognitive decline compared to Alzheimer patients without diabetes. Sociodemographic factors and diabetes predict disease progression in Alzheimer’s disease. Our findings suggest a slower disease progression in Alzheimer’s patients with diabetes. If confirmed, this result will contribute new insights into Alzheimer’s disease pathogenesis and lead to relevant suggestions for disease treatment.     

Dysfunctional autophagy in Alzheimer＇s disease： pathogenic roles and therapeutic implications

Neuronal autophagy is essential for neuronal survival and the maintenance of neuronal homeostasis. Increasing evidence has implicated autophagic dysfunction in the pathogenesis of Alzheimer＇s disease （AD）. The mechanisms underlying autophagic failure in AD involve several steps, from autophagosome formation to degradation. The effect of modulating autophagy is context-dependent. Stimulation of autophagy is not always beneficial. During the implementation of therapies that modulate autophagy, the nature of the autophagic defect, the timing of intervention, and the optimal level and duration of modulation should be fully considered.     

What is the new target inhibiting the progression of Alzheimer’s disease？

To stop the progression of Alzheimer＇s disease in the early stage, it is necessary to identify new therapeutic targets. We examined striatal-enriched phosphatase 61 expression in the brain tissues of 12-month-old APPswe/PSEN1dE9 transgenic mice. Immunohistochemistry showed that striatal-enriched phosphatase 61 protein expression was significantly increased but phosphorylated N-methyl-D-aspartate receptor 2B levels were significantly decreased in the cortex and hippocampus of APPswe/PSEN1dE9 transgenic mice. Western blotting of a cell model of Alzheimer＇s disease consisting of amyloid-beta peptide （1-42）-treated C57BL/6 mouse cortical neurons in vitro showed that valeric acid （AP5）, an N-methyl-D-aspartate receptor antagonist, significantly inhibited amyloidbeta 1-42-induced increased activity of striatal-enriched phosphatase 61. In addition, the phosphorylation of N-methyl-D-aspartate receptor 2B at Tyr1472 was impaired in amyloid-beta 1-42-treated cortical neurons, but knockdown of striatal-enriched phosphatase 61 enhanced the phosphorylation of N-methyl-D-aspartate receptor 2B. Collectively, these findings indicate that striatal-enriched phosphatase 61 can disturb N-methyl-D-aspartate receptor transport and inhibit the progression of learning and study disturbances induced by Alzheimer＇s disease. Thus, striatal-enriched phosphatase 61 may represent a new target for inhibiting the progression of Alzheimer＇s disease.     

T cells promote the regeneration of neural precursor cells in the hippocampus of Alzheimer＇s disease mice

Alzheimer's disease is closely associated with disorders of neurogenesis in the brain, and growing evidence supports the involvement of immunological mechanisms in the development of the disease. However, at present, the role of T cells in neuronal regeneration in the brain is unknown. We injected amyloid-beta 1–42 peptide into the hippocampus of six BALB/c wild-type mice and six BALB/c-nude mice with T-cell immunodeficiency to establish an animal model of Alzheimer's disease. A further six mice of each genotype were injected with same volume of normal saline. Immunohistochemistry revealed that the number of regenerated neural progenitor cells in the hippocampus of BALB/c wild-type mice was significantly higher than that in BALB/c-nude mice. Quantitative fluorescence PCR assay showed that the expression levels of peripheral T cell-associated cytokines(interleukin-2, interferon-γ) and hippocampal microglia-related cytokines(interleukin-1β, tumor necrosis factor-α) correlated with the number of regenerated neural progenitor cells in the hippocampus. These results indicate that T cells promote hippocampal neurogenesis in Alzheimer's disease and T-cell immunodeficiency restricts neuronal regeneration in the hippocampus. The mechanism underlying the promotion of neuronal regeneration by T cells is mediated by an increased expression of peripheral T cells and central microglial cytokines in Alzheimer's disease mice. Our findings provide an experimental basis for understanding the role of T cells in Alzheimer's disease.     

MiR-206 decreases brain-derived neurotrophic factor levels in a transgenic mouse model of Alzheimer＇s disease

MicroRNA alterations have been reported in patients with Alzheimer＇s disease （AD） and AD mouse models. We now report that miR-206 is upregulated in the hippocampal tissue, cerebrospinal fluid, and plasma of embryonic APP/PS1 transgenic mice. The increased miR-206 downregulates the expression of brain-derived neurotrophic factor （BDNF）. BDNF is neuroprotective against cell death after various insults, but in embryonic and newborn APP/PS1 mice it is decreased. Thus, a specific microRNA alteration may contribute to AD pathology by downregulating BDNF.     

Neurotrophic factors： from neurodevelopmental regulators to novel therapies for Parkinson＇s disease

Neuroprotection and neuroregeneration are two of the most promising disease-modifying ther- apies for the incurable and widespread Parkinson＇s disease. In Parkinson＇s disease, progressive degeneration of nigrostriatal dopaminergic neurons causes debilitating motor symptoms. Neurotrophic factors play important regulatory roles in the development, survival and maintenance of specific neuronal populations. These factors have the potential to slow down, halt or reverse the loss of nigrostriatal dopaminergic neurons in Parkinsoffs disease. Several neurotrophic fac- tors have been investigated in this regard. This review article discusses the neurodevelopmental roles and therapeutic potential of three dopaminergic neurotrophic factors： glial cell line-derived neurotrophic factor, neurturin and growth/differentiation factor 5.     

Two faces of Schwann cell dedifferentiation in peripheral neurodegenerative diseases：pro-demyelinating and axon-preservative functions

<正>Schwann cells are glial cells that are responsible for the synthesis and maintenance of the myelin sheath in the peripheral nerve system.Under pathological conditions,such as physical nerve injury and inflammatory neuropathies,Schwann cells undergo a substantial phenotype transformation that is not related to their intended function.For example,Schwann cells dedifferentiate into immature states and thereby cease to express myelin genes after nerve injury.Dedifferentiated Schwann cells activate