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The ε4 allele of the apolipoprotein E ( APOE ) gene is a strong risk factor for late-onset Alzheimer's disease (AD). Bu discusses the contribution of the various APOE isoforms and APOE receptors to the pathophysiology of AD and emerging therapeutic opportunities. Key Points Apolipoprotein E4 (APOE4) is the strongest risk factor for sporadic late-onset Alzheimer's disease (AD), which accounts for the vast majority of AD cases. APOE4 differs from APOE2 and APOE3 at amino acid positions 112 and 158 and has a unique conformation that influences its lipid- and receptor-binding properties. The cellular functions of APOE are mediated by APOE receptors, which are members of the low-density lipoprotein receptor (LDLR) family. LDLR-related protein 1 (LRP1) and the LDLRs are the two major types of APOE metabolic receptors in the brain. APOE receptors regulate amyloid precursor protein (APP) trafficking and processing to amyloid-β (Aβ). Some of these functions are further modified by particular APOE isoforms. APOE and APOE receptors have important roles in Aβ clearance both in the brain parenchyma and in the brain vasculature. APOE3 binds to Aβ more strongly than APOE4, and therefore it is more efficient at mediating Aβ clearance through APOE receptors. APOE fragments generated from APOE4 influence tau phosphorylation and mitochondrial function. However, the mechanisms of these events are poorly understood. The primary function of APOE is to transport lipids from astrocytes to neurons, an event that is crucial for synaptogenesis, synaptic repair, dendritic spine integrity and synaptic functions. APOE4 functions less efficiently than APOE3 in these processes. APOE and APOE receptors are new targets for AD therapy. Several strategies have been reported or proposed. The vast majority of Alzheimer's disease (AD) cases are late-onset and their development is probably influenced by both genetic and environmental risk factors. A strong genetic risk factor for late-onset AD is the presence of the ɛ4 allele of the apolipoprotein E ( APOE ) gene, which encodes a protein with crucial roles in cholesterol metabolism. There is mounting evidence that APOE4 contributes to AD pathogenesis by modulating the metabolism and aggregation of amyloid-β peptide and by directly regulating brain lipid metabolism and synaptic functions through APOE receptors. Emerging knowledge of the contribution of APOE to the pathophysiology of AD presents new opportunities for AD therapy.

Because Aβ deposition triggers or accelerates additional pathogenic events including microglia-mediated immune response and tau tangle spread, effective strategy targeting Aβ will likely be primary prevention prior to plaque development. [...]APOE genotype also has differential effects on age-related cognitive decline and Lewy body dementia parallel to their risk profile for AD. Preclinical studies using animal models and cellular models are needed to address specific effects of this variant on neuropathological features (i.e., amyloid plaques and tau tangles), immune response, lipid metabolism, vascular integrity and function, and other AD-related pathways. [...]the R251G mutation could potentially change the apoE4 structure such that it either reduces its harmful effects or enhances its physiological functions [1].

Vascular cognitive impairment and dementia (VCID) is commonly caused by vascular injuries in cerebral large and small vessels and is a key driver of age-related cognitive decline. Severe VCID includes post-stroke dementia, subcortical ischemic vascular dementia, multi-infarct dementia, and mixed dementia. While VCID is acknowledged as the second most common form of dementia after Alzheimer’s disease (AD) accounting for 20% of dementia cases, VCID and AD frequently coexist. In VCID, cerebral small vessel disease (cSVD) often affects arterioles, capillaries, and venules, where arteriolosclerosis and cerebral amyloid angiopathy (CAA) are major pathologies. White matter hyperintensities, recent small subcortical infarcts, lacunes of presumed vascular origin, enlarged perivascular space, microbleeds, and brain atrophy are neuroimaging hallmarks of cSVD. The current primary approach to cSVD treatment is to control vascular risk factors such as hypertension, dyslipidemia, diabetes, and smoking. However, causal therapeutic strategies have not been established partly due to the heterogeneous pathogenesis of cSVD. In this review, we summarize the pathophysiology of cSVD and discuss the probable etiological pathways by focusing on hypoperfusion/hypoxia, blood–brain barriers (BBB) dysregulation, brain fluid drainage disturbances, and vascular inflammation to define potential diagnostic and therapeutic targets for cSVD.

Investigations of apolipoprotein E ( APOE ) gene, the major genetic risk modifier for Alzheimer’s disease (AD), have yielded significant insights into the pathogenic mechanism. Among the three common coding variants, APOE*ε4 increases, whereas APOE*ε2 decreases the risk of late-onset AD compared with APOE*ε3 . Despite increased understanding of the detrimental effect of APOE*ε4 , it remains unclear how APOE*ε2 confers protection against AD. Accumulating evidence suggests that APOE*ε2 protects against AD through both amyloid-β (Aβ)-dependent and independent mechanisms. In addition, APOE*ε2 has been identified as a longevity gene, suggesting a systemic effect of APOE*ε2 on the aging process. However, APOE*ε2 is not entirely benign; APOE*ε2 carriers exhibit increased risk of certain cerebrovascular diseases and neurological disorders. Here, we review evidence from both human and animal studies demonstrating the protective effect of APOE*ε2 against AD and propose a working model depicting potential underlying mechanisms. Finally, we discuss potential therapeutic strategies designed to leverage the protective effect of APOE2 to treat AD.

Polymorphism in the apolipoprotein E (APOE) gene is a major genetic risk determinant of late-onset Alzheimer disease (AD), with the APOE*ε4 allele conferring an increased risk and the APOE*ε2 allele conferring a decreased risk relative to the common APOE*ε3 allele. Strong evidence from clinical and basic research suggests that a major pathway by which APOE4 increases the risk of AD is by driving earlier and more abundant amyloid pathology in the brains of APOE*ε4 carriers. The number of amyloid-β (Aβ)-dependent and Aβ-independent pathways that are known to be differentially modulated by APOE isoforms is increasing. For example, evidence is accumulating that APOE influences tau pathology, tau-mediated neurodegeneration and microglial responses to AD-related pathologies. In addition, APOE4 is either pathogenic or shows reduced efficiency in multiple brain homeostatic pathways, including lipid transport, synaptic integrity and plasticity, glucose metabolism and cerebrovascular function. Here, we review the recent progress in clinical and basic research into the role of APOE in AD pathogenesis. We also discuss how APOE can be targeted for AD therapy using a precision medicine approach.

APOE4 is the strongest genetic risk factor associated with late-onset Alzheimer’s disease (AD). To address the underlying mechanism, we develop cerebral organoid models using induced pluripotent stem cells (iPSCs) with APOE ε3/ε3 or ε4/ε4 genotype from individuals with either normal cognition or AD dementia. Cerebral organoids from AD patients carrying APOE ε4/ε4 show greater apoptosis and decreased synaptic integrity. While AD patient-derived cerebral organoids have increased levels of Aβ and phosphorylated tau compared to healthy subject-derived cerebral organoids, APOE4 exacerbates tau pathology in both healthy subject-derived and AD patient-derived organoids. Transcriptomics analysis by RNA-sequencing reveals that cerebral organoids from AD patients are associated with an enhancement of stress granules and disrupted RNA metabolism. Importantly, isogenic conversion of APOE4 to APOE3 attenuates the APOE4 -related phenotypes in cerebral organoids from AD patients. Together, our study using human iPSC-organoids recapitulates APOE4 -related phenotypes and suggests APOE4 -related degenerative pathways contributing to AD pathogenesis. APOE4 is a strong genetic risk factor for late-onset Alzheimer’s disease. Here, the authors show that APOE4 is associated with AD features in hiPSCs-derived cerebral organoids. Isogenic conversion of APOE4 to APOE3 attenuates the AD-associated phenotype.

The ε4 allele of the apolipoprotein E (APOE) gene is the strongest genetic risk factor for Alzheimer disease (AD). Guojun Bu and colleagues describe the pathogenic links between Apo-E4 and neurodegeneration, including amyloid-β-dependent mechanisms and impairment of neurovascular function. The authors suggest potential strategies to target Apo-E, which could provide important additions to therapeutic options for AD. Apolipoprotein E (Apo-E) is a major cholesterol carrier that supports lipid transport and injury repair in the brain. APOE polymorphic alleles are the main genetic determinants of Alzheimer disease (AD) risk: individuals carrying the ε4 allele are at increased risk of AD compared with those carrying the more common ε3 allele, whereas the ε2 allele decreases risk. Presence of the APOE ε4 allele is also associated with increased risk of cerebral amyloid angiopathy and age-related cognitive decline during normal ageing. Apo-E–lipoproteins bind to several cell-surface receptors to deliver lipids, and also to hydrophobic amyloid-β (Aβ) peptide, which is thought to initiate toxic events that lead to synaptic dysfunction and neurodegeneration in AD. Apo-E isoforms differentially regulate Aβ aggregation and clearance in the brain, and have distinct functions in regulating brain lipid transport, glucose metabolism, neuronal signalling, neuroinflammation, and mitochondrial function. In this Review, we describe current knowledge on Apo-E in the CNS, with a particular emphasis on the clinical and pathological features associated with carriers of different Apo-E isoforms. We also discuss Aβ-dependent and Aβ-independent mechanisms that link Apo-E4 status with AD risk, and consider how to design effective strategies for AD therapy by targeting Apo-E. Key Points The ε4 allele of the apolipo protein E ( APOE ) gene is the main genetic risk factor for Alzheimer disease (AD) APOE ε4 carriers have enhanced AD pathology, accelerated age-dependent cognitive decline and worse memory performance than do noncarriers Numerous structural and functional brain changes associated with AD pathogenesis are detected in APOE ε4 carriers before clinical symptoms become evident Apo-E affects amyloid-β (Aβ) clearance, aggregation and deposition in an isoform-dependent manner Apo-E4 also contributes to AD pathogenesis by Aβ-independent mechanisms that involve synaptic plasticity, cholesterol homeostasis, neurovascular functions, and neuroinflammation Apo-E-targeted AD therapy should focus on restoration of the physiological function of Apo-E through increased expression and lipidation, and inhibition of the detrimental effects of Apo-E4

Triggering receptor expressed on myeloid cells 2 (TREM2) is a microglial surface receptor genetically linked to the risk for Alzheimer’s disease (AD). A proteolytic product, soluble TREM2 (sTREM2), is abundant in the cerebrospinal fluid and its levels positively correlate with neuronal injury markers. To gain insights into the pathological roles of sTREM2, we studied sTREM2 in the brain of 5xFAD mice, a model of AD, by direct stereotaxic injection of recombinant sTREM2 protein or by adeno-associated virus (AAV)-mediated expression. We found that sTREM2 reduces amyloid plaque load and rescues functional deficits of spatial memory and long-term potentiation. Importantly, sTREM2 enhances microglial proliferation, migration, clustering in the vicinity of amyloid plaques and the uptake and degradation of Aβ. Depletion of microglia abolishes the neuroprotective effects of sTREM2. Our study demonstrates a protective role of sTREM2 against amyloid pathology and related toxicity and suggests that increasing sTREM2 can be explored for AD therapy. TREM2 is a genetic risk factor for Alzheimer’s disease, and soluble TREM2 (sTREM2) in the CSF correlates with AD progression. Here the authors study the role of sTREM2 in a mouse model of Alzheimer’s disease, and find it reduces amyloid accumulation and increases the numbers of plaque-associated microglia which correlates with improved behavioural function in the mice.

Alzheimer’s disease (AD) is the most common cause of dementia worldwide, and its prevalence is rapidly increasing due to extended lifespans. Among the increasing number of genetic risk factors identified, the apolipoprotein E ( APOE ) gene remains the strongest and most prevalent, impacting more than half of all AD cases. While the ε4 allele of the APOE gene significantly increases AD risk, the ε2 allele is protective relative to the common ε3 allele. These gene alleles encode three apoE protein isoforms that differ at two amino acid positions. The primary physiological function of apoE is to mediate lipid transport in the brain and periphery; however, additional functions of apoE in diverse biological functions have been recognized. Pathogenically, apoE seeds amyloid-β (Aβ) plaques in the brain with apoE4 driving earlier and more abundant amyloids. ApoE isoforms also have differential effects on multiple Aβ-related or Aβ-independent pathways. The complexity of apoE biology and pathobiology presents challenges to designing effective apoE-targeted therapeutic strategies. This review examines the key pathobiological pathways of apoE and related targeting strategies with a specific focus on the latest technological advances and tools.

Transactive response DNA binding protein of 43 kDa (TDP-43) is an intranuclear protein encoded by the TARDBP gene that is involved in RNA splicing, trafficking, stabilization, and thus, the regulation of gene expression. Cytoplasmic inclusion bodies containing phosphorylated and truncated forms of TDP-43 are hallmarks of amyotrophic lateral sclerosis (ALS) and a subset of frontotemporal lobar degeneration (FTLD). Additionally, TDP-43 inclusions have been found in up to 57% of Alzheimer’s disease (AD) cases, most often in a limbic distribution, with or without hippocampal sclerosis. In some cases, TDP-43 deposits are also found in neurons with neurofibrillary tangles. AD patients with TDP-43 pathology have increased severity of cognitive impairment compared to those without TDP-43 pathology. Furthermore, the most common genetic risk factor for AD, apolipoprotein E4 ( APOE4 ), is associated with increased frequency of TDP-43 pathology. These findings provide strong evidence that TDP-43 pathology is an integral part of multiple neurodegenerative conditions, including AD. Here, we review the biology and pathobiology of TDP-43 with a focus on its role in AD. We emphasize the need for studies on the mechanisms that lead to TDP-43 pathology, especially in the setting of age-related disorders such as AD.