Astrocytic and Neuronal Apolipoprotein E Isoforms Differentially Affect Neuronal Excitability

Multifaceted roles of APOE in Alzheimer disease

For the past three decades, apolipoprotein E (APOE) has been known as the single greatest genetic modulator of sporadic Alzheimer disease (AD) risk, influencing both the average age of onset and the lifetime risk of developing AD. The APOEε4 allele significantly increases AD risk, whereas the ε2 allele is protective relative to the most common ε3 allele. However, large differences in effect size exist across ethnoracial groups that are likely to depend on both global genetic ancestry and local genetic ancestry, as well as gene-environment interactions. Although early studies linked APOE to amyloid-β - one of the two culprit aggregation-prone proteins that define AD - in the past decade, mounting work has associated APOE with other neurodegenerative proteinopathies and broader ageing-related brain changes, such as neuroinflammation, energy metabolism failure, loss of myelin integrity and increased blood-brain barrier permeability, with potential implications for longevity and resilience to pathological protein aggregates. Novel mouse models and other technological advances have also enabled a number of therapeutic approaches aimed at either attenuating the APOEε4-linked increased AD risk or enhancing the APOEε2-linked AD protection. This Review summarizes this progress and highlights areas for future research towards the development of APOE-directed therapeutics.

Single-Cell Patch-Clamp/Proteomics of Human Alzheimer's Disease iPSC-Derived Excitatory Neurons Versus Isogenic Wild-Type Controls Suggests Novel Causation and Therapeutic Targets

Standard single-cell (sc) proteomics of disease states inferred from multicellular organs or organoids cannot currently be related to single-cell physiology. Here, a scPatch-Clamp/Proteomics platform is developed on single neurons generated from hiPSCs bearing an Alzheimer's disease (AD) genetic mutation and compares them to isogenic wild-type controls. This approach provides both current and voltage electrophysiological data plus detailed proteomics information on single-cells. With this new method, the authors are able to observe hyperelectrical activity in the AD hiPSC-neurons, similar to that observed in the human AD brain, and correlate it to ≈1400 proteins detected at the single neuron level. Using linear regression and mediation analyses to explore the relationship between the abundance of individual proteins and the neuron's mutational and electrophysiological status, this approach yields new information on therapeutic targets in excitatory neurons not attainable by traditional methods. This combined patch-proteomics technique creates a new proteogenetic-therapeutic strategy to correlate genotypic alterations to physiology with protein expression in single-cells.

A cell autonomous regulator of neuronal excitability modulates tau in Alzheimer's disease vulnerable neurons

Neurons from layer II of the entorhinal cortex (ECII) are the first to accumulate tau protein aggregates and degenerate during prodromal Alzheimer's disease. Gaining insight into the molecular mechanisms underlying this vulnerability will help reveal genes and pathways at play during incipient stages of the disease. Here, we use a data-driven functional genomics approach to model ECII neurons in silico and identify the proto-oncogene DEK as a regulator of tau pathology. We show that epigenetic changes caused by Dek silencing alter activity-induced transcription, with major effects on neuronal excitability. This is accompanied by the gradual accumulation of tau in the somatodendritic compartment of mouse ECII neurons in vivo, reactivity of surrounding microglia, and microglia-mediated neuron loss. These features are all characteristic of early Alzheimer's disease. The existence of a cell-autonomous mechanism linking Alzheimer's disease pathogenic mechanisms in the precise neuron type where the disease starts provides unique evidence that synaptic homeostasis dysregulation is of central importance in the onset of tau pathology in Alzheimer's disease.

Temporal Characterization of the Amyloidogenic APPswe/PS1dE9;hAPOE4 Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a devastating disorder with a global prevalence estimated at 55 million people. In clinical studies administering certain anti-beta-amyloid (Aβ) antibodies, amyloid-related imaging abnormalities (ARIAs) have emerged as major adverse events. The frequency of these events is higher among apolipoprotein ε4 allele carriers (APOE4) compared to non-carriers. To reflect patients most at risk for vascular complications of anti-Aβ immunotherapy, we selected an APPswe/PS1dE9 transgenic mouse model bearing the human APOE4 gene (APPPS1:E4) and compared it with the same APP/PS1 mouse model bearing the human APOE3 gene (APOE ε3 allele; APPPS1:E3). Using histological and biochemical analyses, we characterized mice at three ages: 8, 12, and 16 months. Female and male mice were assayed for general cerebral fibrillar and pyroglutamate (pGlu-3) Aβ deposition, cerebral amyloid angiopathy (CAA), microhemorrhages, apoE and cholesterol composition, astrocytes, microglia, inflammation, lysosomal dysfunction, and neuritic dystrophy. Amyloidosis, lipid deposition, and astrogliosis increased with age in APPPS1:E4 mice, while inflammation did not reveal significant changes with age. In general, APOE4 carriers showed elevated Aβ, apoE, reactive astrocytes, pro-inflammatory cytokines, microglial response, and neuritic dystrophy compared to APOE3 carriers at different ages. These results highlight the potential of the APPPS1:E4 mouse model as a valuable tool in investigating the vascular side effects associated with anti-amyloid immunotherapy.

Astrocytes at the intersection of ageing, obesity, and neurodegeneration

Once considered passive cells of the central nervous system (CNS), glia are now known to actively maintain the CNS parenchyma; in recent years, the evidence for glial functions in CNS physiology and pathophysiology has only grown. Astrocytes, a heterogeneous group of glial cells, play key roles in regulating the metabolic and inflammatory landscape of the CNS and have emerged as potential therapeutic targets for a variety of disorders. This review will outline astrocyte functions in the CNS in healthy ageing, obesity, and neurodegeneration, with a focus on the inflammatory responses and mitochondrial function, and will address therapeutic outlooks.

APOE3 Christchurch modulates β-catenin/Wnt signaling in iPS cell-derived cerebral organoids from Alzheimer's cases

A patient with the PSEN1 E280A mutation and homozygous for APOE3 Christchurch (APOE3Ch) displayed extreme resistance to Alzheimer's disease (AD) cognitive decline and tauopathy, despite having a high amyloid burden. To further investigate the differences in biological processes attributed to APOE3Ch, we generated induced pluripotent stem (iPS) cell-derived cerebral organoids from this resistant case and a non-protected control, using CRISPR/Cas9 gene editing to modulate APOE3Ch expression. In the APOE3Ch cerebral organoids, we observed a protective pattern from early tau phosphorylation. ScRNA sequencing revealed regulation of Cadherin and Wnt signaling pathways by APOE3Ch, with immunostaining indicating elevated β-catenin protein levels. Further in vitro reporter assays unexpectedly demonstrated that ApoE3Ch functions as a Wnt3a signaling enhancer. This work uncovered a neomorphic molecular mechanism of protection of ApoE3 Christchurch, which may serve as the foundation for the future development of protected case-inspired therapeutics targeting AD and tauopathies.

Andrographolide Improves ApoE4-Mediated Blood-Brain Barrier Injury by Alleviating Inflammation

The pathological and physiological studies of Alzheimer's disease (AD) have been in-depth, and apolipoprotein E4 (ApoE4) has been proven to be highly correlated with AD, and clinical and experimental data show that ApoE4 can cause blood-brain barrier (BBB) injury, and the change of BBB permeability is an important factor affecting the development of AD. Andrographolide (Andro), as the active component of the natural plant Andrographis paniculata, has been proven to have anti-inflammatory and antioxidant effects, which have potential neuroprotective effects. To verify the protective effect of Andro on BBB in a short term, our research group used atorvastatin (Atorva)-mediated zebrafish brain injury model and the ApoE4-mediated cell co-culture model of BBB injury to explore the protective effects and mechanisms of Andro on BBB injury. Studies have shown that Andro can inhibit the activation of CypA/NF-κB/MMP-9 signaling pathway and has achieved the effect of antagonizing the inhibition of ApoE4 on intercellular tight junction proteins (occludin, claudin-5, and ZO-1). At the same time, Andro can inhibit the secretion of cell adhesion molecules (VCAM-1 and ICAM-1) in cells, thereby delaying the occurrence and progression of neuroinflammation and playing a protective role in BBB. In conclusion, Andro is a potent natural product which can protect the blood-brain barrier.

Higher Neuronal Facilitation and Potentiation with APOE4 Suppressed by Angiotensin II

Progressive hippocampal degeneration is a key component of Alzheimer's disease (AD) progression. Therefore, identifying how hippocampal neuronal function is modulated early in AD is an important approach to eventually prevent degeneration. AD-risk factors and signaling molecules likely modulate neuronal function, including APOE genotype and angiotensin II. Compared to APOE3, APOE4 increases AD risk up to 12-fold, and high levels of angiotensin II are hypothesized to disrupt neuronal function in AD. However, the extent that APOE and angiotensin II modulates the hippocampal neuronal phenotype in AD-relevant models is unknown. To address this issue, we used electrophysiological techniques to assess the impact of APOE genotype and angiotensin II on basal synaptic transmission, presynaptic, and post-synaptic activity in mice that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce Aβ. We found that compared to E3FAD mice, E4FAD mice have lower synaptic activity, but higher levels of paired-pulse facilitation (PPF) and long-term potentiation (LTP) in the Schaffer Collateral Commissural Pathway (SCCP) of the hippocampus. We also found that exogenous angiotensin II has a profound inhibitory effect on hippocampal LTP in both E3FAD and E4FAD mice. Collectively, our data suggests that APOE4 and Aβ are associated with a hippocampal phenotype comprised of lower basal activity and higher responses to high-frequency stimulation, the latter of which is suppressed by angiotensin II. These novel data suggest a potential mechanistic link between hippocampal activity, APOE4 genotype, and angiotensin II in AD.

The Role of Impaired Receptor Trafficking in Mediating the Pathological Effects of APOE4 in Alzheimer's Disease

BACKGROUND

Apolipoprotein E4 (APOE4) is the most prevalent genetic risk factor of Alzheimer's disease. Several studies suggest that APOE4 binding to its receptors is associated with their internalization and accumulation in intracellular compartments. Importantly, this phenomenon also occurs with other, non-ApoE receptors. Based on these observations, we hypothesized that APOE4 pathological effects are mediated by impairment in the life cycle of distinct receptors (APOER2, LRP1, IR, VEGFR).

OBJECTIVE

To examine the effects of APOE genotype on receptors protein levels and compartmentalization.

METHODS

Primary mouse neurons were prepared from APOE3 or APOE4 targeted replacement mice, or APOE-KO mice. Specific receptors protein levels were evaluated in these neurons, utilizing immunofluorescent staining. Additionally, surface membrane protein levels of those receptors were assessed by cell surface biotinylation assay and ELISA. Receptors' colocalization with intracellular compartments was assessed by double staining and confocal microscopy, followed by colocalization analysis. Finally, LRP1 or APOER2 were knocked-down with CRISPR/Cas9 system to examine their role in mediating APOE4 effects on the receptors.

RESULTS

Our results revealed lower receptors' levels in APOE4, specifically on the membrane surface. Additionally, APOE4 affects the compartmentation of these receptors in two patterns: the first was observed with LRP1 and was associated with decreased receptor levels in numerous intracellular compartments. The second was obtained with the other receptors and was associated with their accumulation in early endosomes and their decrease in the late endosomes.

CONCLUSIONS

These results provide a unifying mechanism, in which APOE4 drives the down regulation of various receptors, which plays important roles in distinct APOE4 related pathological processes.

Circadian Regulation of Apolipoproteins in the Brain: Implications in Lipid Metabolism and Disease

The circadian rhythm is a 24 h internal clock within the body that regulates various factors, including sleep, body temperature, and hormone secretion. Circadian rhythm disruption is an important risk factor for many diseases including neurodegenerative illnesses. The central and peripheral oscillators' circadian clock network controls the circadian rhythm in mammals. The clock genes govern the central clock in the suprachiasmatic nucleus (SCN) of the brain. One function of the circadian clock is regulating lipid metabolism. However, investigations of the circadian regulation of lipid metabolism-associated apolipoprotein genes in the brain are lacking. This review summarizes the rhythmic expression of clock genes and lipid metabolism-associated apolipoprotein genes within the SCN in Mus musculus. Nine of the twenty apolipoprotein genes identified from searching the published database (SCNseq and CircaDB) are highly expressed in the SCN. Most apolipoprotein genes (ApoE, ApoC1, apoA1, ApoH, ApoM, and Cln) show rhythmic expression in the brain in mice and thus might be regulated by the master clock. Therefore, this review summarizes studies on lipid-associated apolipoprotein genes in the SCN and other brain locations, to understand how apolipoproteins associated with perturbed cerebral lipid metabolism cause multiple brain diseases and disorders. This review describes recent advancements in research, explores current questions, and identifies directions for future research.

Alzheimer's disease: an axonal injury disease?

Alzheimer's disease (AD) is the primary cause of dementia and is anticipated to impose a substantial economic burden in the future. Over a significant period, the widely accepted amyloid cascade hypothesis has guided research efforts, and the recent FDA approval of an anti- amyloid-beta (Aβ) protofibrils antibody, believed to decelerate AD progression, has further solidified its significance. However, the excessive emphasis placed on the amyloid cascade hypothesis has overshadowed the physiological nature of Aβ and tau proteins within axons. Axons, specialized neuronal structures, sustain damage during the early stages of AD, exerting a pivotal influence on disease progression. In this review, we present a comprehensive summary of the relationship between axonal damage and AD pathology, amalgamating the physiological roles of Aβ and tau proteins, along with the impact of AD risk genes such as APOE and TREM2. Furthermore, we underscore the exceptional significance of axonal damage in the context of AD.

Apolipoprotein E intersects with amyloid-β within neurons

Apolipoprotein E4 (ApoE4) is the most important genetic risk factor for Alzheimer's disease (AD). Among the earliest changes in AD is endosomal enlargement in neurons, which was reported as enhanced in ApoE4 carriers. ApoE is thought to be internalized into endosomes of neurons, whereas β-amyloid (Aβ) accumulates within neuronal endosomes early in AD. However, it remains unknown whether ApoE and Aβ intersect intracellularly. We show that internalized astrocytic ApoE localizes mostly to lysosomes in neuroblastoma cells and astrocytes, whereas in neurons, it preferentially localizes to endosomes-autophagosomes of neurites. In AD transgenic neurons, astrocyte-derived ApoE intersects intracellularly with amyloid precursor protein/Aβ. Moreover, ApoE4 increases the levels of endogenous and internalized Aβ42 in neurons. Taken together, we demonstrate differential localization of ApoE in neurons, astrocytes, and neuron-like cells, and show that internalized ApoE intersects with amyloid precursor protein/Aβ in neurons, which may be of considerable relevance to AD.

Cholesterol Metabolism in Aging and Age-Related Disorders

All mammalian cell membranes contain cholesterol to maintain membrane integrity. The transport of this hydrophobic lipid is mediated by lipoproteins. Cholesterol is especially enriched in the brain, particularly in synaptic and myelin membranes. Aging involves changes in sterol metabolism in peripheral organs and also in the brain. Some of those alterations have the potential to promote or to counteract the development of neurodegenerative diseases during aging. Here, we summarize the current knowledge of general principles of sterol metabolism in humans and mice, the most widely used model organism in biomedical research. We discuss changes in sterol metabolism that occur in the aged brain and highlight recent developments in cell type-specific cholesterol metabolism in the fast-growing research field of aging and age-related diseases, focusing on Alzheimer's disease. We propose that cell type-specific cholesterol handling and the interplay between cell types critically influence age-related disease processes.

The Potential Therapeutic Application of Simvastatin for Brain Complications and Mechanisms of Action

Statins are common drugs that are clinically used to reduce elevated plasma cholesterol levels. Based on their solubility, statins are considered to be either hydrophilic or lipophilic. Amongst them, simvastatin has the highest lipophilicity to facilitate its ability to cross the blood-brain barrier. Recent studies have suggested that simvastatin could be a promising therapeutic option for different brain complications and diseases ranging from brain tumors (i.e., medulloblastoma and glioblastoma) to neurological disorders (i.e., Alzheimer's disease, Parkinson's disease, and Huntington's disease). Specific mechanisms of disease amelioration, however, are still unclear. Independent studies suggest that simvastatin may reduce the risk of developing certain neurodegenerative disorders. Meanwhile, other studies point towards inducing cell death in brain tumor cell lines. In this review, we outline the potential therapeutic effects of simvastatin on brain complications and review the clinically relevant molecular mechanisms in different cases.

Neuronal ApoE4 in Alzheimer's disease and potential therapeutic targets

The most prevalent genetic risk factor for Alzheimer's disease (AD) is Apolipoprotein E (ApoE), a gene located on chromosome 19 that encodes three alleles (e2, e3, and e4) that give rise to the ApoE subtypes E2, E3, and E4, respectively. E2 and E4 have been linked to increased plasma triglyceride concentrations and are known to play a critical role in lipoprotein metabolism. The prominent pathological features of AD mainly include senile plaques formed by amyloid β (Aβ₄₂) aggregation and neuronal fibrous tangles (NFTs), and the deposited plaques are mainly composed of Aβ hyperphosphorylation and truncated head. In the central nervous system, the ApoE protein is primarily derived from astrocytes, but ApoE is also produced when neurons are stressed or affected by certain stress, injury, and aging conditions. ApoE4 in neurons induces Aβ and tau protein pathologies, leading to neuroinflammation and neuronal damage, impairing learning and memory functions. However, how neuronal ApoE4 mediates AD pathology remains unclear. Recent studies have shown that neuronal ApoE4 may lead to greater neurotoxicity, which increases the risk of AD development. This review focuses on the pathophysiology of neuronal ApoE4 and explains how neuronal ApoE4 mediates Aβ deposition, pathological mechanisms of tau protein hyperphosphorylation, and potential therapeutic targets.

Higher Neuronal Facilitation and Potentiation with APOE4 Suppressed by Angiotensin II

Progressive hippocampal degeneration is a key component of Alzheimer's disease (AD) progression. Therefore, identifying how hippocampal neuronal function is modulated early in AD is an important approach to eventually prevent degeneration. AD-risk factors and signaling molecules likely modulate neuronal function, including APOE genotype and angiotensin II. Compared to APOE3 , APOE4 increases AD risk up to 12-fold, and high levels of angiotensin II are hypothesized to disrupt neuronal function in AD. However, the extent that APOE and angiotensin II modulates the hippocampal neuronal phenotype in AD-relevant models is unknown. To address this issue, we used electrophysiological techniques to assess the impact of APOE genotype and angiotensin II on basal synaptic transmission, presynaptic and post-synaptic activity in mice that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce Aβ. We found that compared to E3FAD mice, E4FAD mice had lower basal synaptic activity, but higher levels of paired pulse facilitation (PPF) and Long-Term Potentiation (LTP) in the Schaffer Collateral Commissural Pathway (SCCP) of the hippocampus. We also found that exogenous angiotensin II has a profound inhibitory effect on hippocampal LTP in both E3FAD and E4FAD mice. Collectively, our data suggests that APOE4 and Aβ are associated with a hippocampal phenotype comprised of lower basal activity and higher responses to high frequency stimulation, the latter of which is suppressed by angiotensin II. These novel data suggest a potential mechanistic link between hippocampal activity, APOE4 genotype and angiotensin II in AD.

ApoE4-mediated Blood-Brain Barrier Damage in Alzheimer's Disease: Progress and Prospects

Late-onset Alzheimer's disease (AD), a neurodegenerative disease, is expected in the elderly population and adversely affects families and society. The extensive debate on the deposition of amyloid (Aβ), abnormal phosphorylation of Tau protein, and neuroinflammation hypothesis in the pathogenesis of AD has been recognized by many scholars. The blood-brain barrier (BBB) is an essential physical barrier that protects the brain from external material interference, and its integrity affects the process of AD. Apolipoprotein E4 (ApoE4) has shown a critical regulatory role in many studies and is a crucial protein that affects AD. Numerous current studies on ApoE4 are based on complementary hypotheses to the three hypotheses above, ignoring the effect of ApoE4 on BBB constitutive cells and the role of the BBB in AD. In this review, we summarize the findings of the role of ApoE4 in the composition of the BBB and the value of ApoE4 for maintaining BBB integrity, which may play an essential role in changing the progression of the disease.

Revealing cell vulnerability in Alzheimer's disease by single-cell transcriptomics

Alzheimer's disease (AD) is a neurodegenerative disorder that by affecting specific brain cell types and regions cause severe pathological and functional changes in memory neural circuits. A comprehensive knowledge of the pathogenic mechanisms underlying AD requires a deeper understanding of the cell-specific pathological responses through integrative molecular analyses. Recent application of high-throughput single-cell transcriptomics to postmortem tissue has proved powerful to unravel cell susceptibility and biological networks responding to amyloid and tau pathologies. Here, we review single-cell transcriptomic studies successfully applied to decipher cell-specific gene expression programs and pathways in the brain of AD patients. Transcriptional information reveals both specific and common gene signatures affecting the major cerebral cell types, including astrocytes, endothelial cells, microglia, neurons, and oligodendrocytes. Cell type-specific transcriptomes associated with AD pathology and clinical symptoms are related to common biological networks affecting, among others pathways, synaptic function, inflammation, proteostasis, cell death, oxidative stress, and myelination. The general picture that emerges from systems-level single-cell transcriptomics is a spatiotemporal pattern of cell diversity and biological pathways, and novel cell subpopulations affected in AD brain. We argue that broader implementation of cell transcriptomics in larger AD human cohorts using standardized protocols is fundamental for reliable assessment of temporal and regional cell-type gene profiling. The possibility of applying this methodology for personalized medicine in clinics is still challenging but opens new roads for future diagnosis and treatment in dementia.

Dissection and culturing of adult lateral entorhinal cortex layer II neurons from APP/PS1 Alzheimer model mice

BACKGROUND

Primary neuronal cultures enable cell-biological studies of Alzheimer's disease (AD), albeit typically non-neuron-specific. The first cortical neurons affected in AD reside in layer II of the lateralmost part of the entorhinal cortex, and they undergo early accumulation of intracellular amyloid-β, form subsequent tau pathology, and start degenerating pre-symptomatically. These vulnerable entorhinal neurons uniquely express the glycoprotein reelin and provide selective inputs to the hippocampal memory system. Gaining a more direct access to study these neurons is therefore highly relevant.

NEW METHOD

We demonstrate a methodological approach for dissection and long-term culturing of adult lateral entorhinal layer II-neurons from AD-model mice.

RESULTS

We maintain adult dissected lateralmost entorhinal layer II-neurons beyond two months in culture. We show that they express neuronal markers, and that they are electrophysiologically active by 15 days in vitro and continuing beyond 2 months.

COMPARISON WITH EXISTING METHODS

Primary neurons are typically harvested from embryonic or early postnatal brains because such neurons are easier to culture compared to adult neurons. Methods to culture adult primary neurons have been reported, however, to our knowledge, culturing of adult entorhinal neuron-type specific primary neurons from AD-model animals have not been reported.

CONCLUSIONS

Our methodological approach offers a window to study initial pathological changes in the AD disease-cascade. This includes the study of proteinopathy, single-neuron changes, and network-level dysfunction.

Astrocytes as a Therapeutic Target in Alzheimer's Disease-Comprehensive Review and Recent Developments

Alzheimer's disease (AD) is a frequent and disabling neurodegenerative disorder, in which astrocytes participate in several pathophysiological processes including neuroinflammation, excitotoxicity, oxidative stress and lipid metabolism (along with a critical role in apolipoprotein E function). Current evidence shows that astrocytes have both neuroprotective and neurotoxic effects depending on the disease stage and microenvironmental factors. Furthermore, astrocytes appear to be affected by the presence of amyloid-beta (Aβ), with alterations in calcium levels, gliotransmission and proinflammatory activity via RAGE-NF-κB pathway. In addition, astrocytes play an important role in the metabolism of tau and clearance of Aβ through the glymphatic system. In this review, we will discuss novel pharmacological and non-pharmacological treatments focused on astrocytes as therapeutic targets for AD. These interventions include effects on anti-inflammatory/antioxidant systems, glutamate activity, lipid metabolism, neurovascular coupling and glymphatic system, calcium dysregulation, and in the release of peptides which affects glial and neuronal function. According to the AD stage, these therapies may be of benefit in either preventing or delaying the progression of the disease.

A multi-hit hypothesis for an APOE4-dependent pathophysiological state

The APOE gene encoding the Apolipoprotein E protein is the single most significant genetic risk factor for late-onset Alzheimer's disease. The APOE4 genotype confers a significantly increased risk relative to the other two common genotypes APOE3 and APOE2. Intriguingly, APOE4 has been associated with neuropathological and cognitive deficits in the absence of Alzheimer's disease-related amyloid or tau pathology. Here, we review the extensive literature surrounding the impact of APOE genotype on central nervous system dysfunction, focussing on preclinical model systems and comparison of APOE3 and APOE4, given the low global prevalence of APOE2. A multi-hit hypothesis is proposed to explain how APOE4 shifts cerebral physiology towards pathophysiology through interconnected hits. These hits include the following: neurodegeneration, neurovascular dysfunction, neuroinflammation, oxidative stress, endosomal trafficking impairments, lipid and cellular metabolism disruption, impaired calcium homeostasis and altered transcriptional regulation. The hits, individually and in combination, leave the APOE4 brain in a vulnerable state where further cumulative insults will exacerbate degeneration and lead to cognitive deficits in the absence of Alzheimer's disease pathology and also a state in which such pathology may more easily take hold. We conclude that current evidence supports an APOE4 multi-hit hypothesis, which contributes to an APOE4 pathophysiological state. We highlight key areas where further study is required to elucidate the complex interplay between these individual mechanisms and downstream consequences, helping to frame the current landscape of existing APOE-centric literature.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer’s disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell–cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Biomarkers Related to Synaptic Dysfunction to Discriminate Alzheimer's Disease from Other Neurological Disorders

Recently, the synaptic proteins neurogranin (Ng) and α-synuclein (α-Syn) have attracted scientific interest as potential biomarkers for synaptic dysfunction in neurodegenerative diseases. In this study, we measured the CSF Ng and α-Syn concentrations in patients affected by AD (n = 69), non-AD neurodegenerative disorders (n-AD = 50) and non-degenerative disorders (n-ND, n = 98). The concentrations of CSF Ng and α-Syn were significantly higher in AD than in n-AD and n-ND. Moreover, the Aβ42/Ng and Aβ42/α-Syn ratios showed statistically significant differences between groups and discriminated AD patients from n-AD patients, better than Ng or α-Syn alone. Regression analyses showed an association of higher Ng concentrations with MMSE < 24, pathological Aβ 42/40 ratios, pTau, tTau and the ApoEε4 genotype. Aβ 42/Ng was associated with MMSE < 24, an AD-related FDG-PET pattern, the ApoEε4 genotype, pathological Aβ 42 levels and Aβ 42/40 ratios, pTau, and tTau. Moreover, APO-Eε4 carriers showed higher Ng concentrations than non-carriers. Our results support the idea that the Aβ 42/Ng ratio is a reliable index of synaptic dysfunction/degeneration able to discriminate AD from other neurological conditions.

Neuronal ApoE Regulates the Cell-to-Cell Transmission of α-Synuclein

The presence of protein inclusions, called Lewy bodies (LBs) and Lewy neurites (LNs), in the brain is the main feature of Parkinson’s disease (PD). Recent evidence that the prion-like propagation of α-synuclein (α-syn), as a major component of LBs and LNs, plays an important role in the progression of PD has gained much attention, although the molecular mechanism remains unclear. In this study, we evaluated whether neuronal ApoE regulates the cell-to-cell transmission of α-syn and explored its molecular mechanism using in vitro and in vivo model systems. We demonstrate that neuronal ApoE deficiency attenuates both α-syn uptake and release by downregulating LRP-1 and LDLR expression and enhancing chaperone-mediated autophagy activity, respectively, thereby contributing to α-syn propagation. In addition, we observed that α-syn propagation was attenuated in ApoE knockout mice injected with pre-formed mouse α-syn fibrils. This study will help our understanding of the molecular mechanisms underlying α-syn propagation.