Kevin P. Koster

APOE‐modulated Aβ‐induced neuroinflammation in Alzheimer's disease: current landscape, novel data, and future perspective

Chronic glial activation and neuroinflammation induced by the amyloid‐β peptide (Aβ) contribute to Alzheimers disease (AD) pathology. APOE4 is the greatest AD‐genetic risk factor; increasing risk up to 12‐fold compared to APOE3, with APOE4‐specific neuroinflammation an important component of this risk. This editorial review discusses the role of APOE in inflammation and AD, via a literature review, presentation of novel data on Aβ‐induced neuroinflammation, and discussion of future research directions. The complexity of chronic neuroinflammation, including multiple detrimental and beneficial effects occurring in a temporal and cell‐specific manner, has resulted in conflicting functional data for virtually every inflammatory mediator. Defining a neuroinflammatory phenotype (NIP) is one way to address this issue, focusing on profiling the changes in inflammatory mediator expression during disease progression. Although many studies have shown that APOE4 induces a detrimental NIP in peripheral inflammation and Aβ‐independent neuroinflammation, data for APOE‐modulated Aβ‐induced neuroinflammation are surprisingly limited. We present data supporting the hypothesis that impaired apoE4 function modulates Aβ‐induced effects on inflammatory receptor signaling, including amplification of detrimental (toll‐like receptor 4‐p38α) and suppression of beneficial (IL‐4R‐nuclear receptor) pathways. To ultimately develop APOE genotype‐specific therapeutics, it is critical that future studies define the dynamic NIP profile and pathways that underlie APOE‐modulated chronic neuroinflammation. In this editorial review, we present data supporting the hypothesis that impaired apoE4 function modulates Aβ‐induced effects on inflammatory receptor signaling, including amplification of detrimental (TLR4‐p38α) and suppression of beneficial (IL‐4R‐nuclear receptor) pathways, resulting in an adverse NIP that causes neuronal dysfunction. NIP, Neuroinflammatory phenotype; P.I., pro‐inflammatory; A.I., anti‐inflammatory.

Amyloid-β Pathology and APOE Genotype Modulate Retinoid X Receptor Agonist Activity in Vivo

Background: Human APOE effects on RXR agonist activity are unclear. Results: In RXR agonist-treated EFAD mice, beneficial effects in APOE4 hippocampus include ABCA1/ABCG1-induced apoE lipoprotein association/lipidation. Detrimental effects in APOE3 and APOE4 cortex might be from systemic hepatomegaly. Conclusion: Aβ pathology and APOE genotype modulate RXR agonist effects. Significance: Although promising for later stage Alzheimer disease, detrimental side effects limit translational application of RXR agonists. Previous data demonstrate that bexarotene (Bex), retinoid X receptor (RXR) agonist, reduces soluble and insoluble amyloid-β (Aβ) in Alzheimer disease (AD)-transgenic mice either by increasing the levels of mouse apolipoprotein E (apoE) or increasing ABCA1/ABCG1-induced apoE lipoprotein association/lipidation. However, although the mechanism of action of RXR agonists remains unclear, a major concern for their use is human (h)-APOE4, the greatest AD genetic risk factor. If APOE4 imparts a toxic gain-of-function, then increasing apoE4 may increase soluble Aβ, likely the proximal AD neurotoxin. If the APOE4 loss-of-function is lipidation of apoE4, then induction of ABCA1/ABCG1 may be beneficial. In novel EFAD-Tg mice (overexpressing h-Aβ42 with h-APOE), levels of soluble Aβ (Aβ42 and oligomeric Aβ) are highest in E4FAD hippocampus (HP) > E3FAD-HP > E4FAD cortex (CX) > E3FAD-CX, whereas levels of lipoprotein-associated/lipidated apoE have the opposite pattern (6 months). In E4FAD-HP, short-term RXR agonist treatment (Bex or LG100268; 5.75–6 months) increased ABCA1, apoE4 lipoprotein-association/lipidation, and apoE4/Aβ complex, decreased soluble Aβ, and increased PSD95. In addition, hydrogel delivery, which mimics low sustained release, was equally effective as gavage for Bex and LG100268. RXR agonists induced no beneficial effects in the E4FAD-HP in a prevention protocol (5–6 months) and actually increased soluble Aβ levels in E3FAD-CX and E4FAD-CX with the short-term protocol, possibly the result of systemic hepatomegaly. Thus, RXR agonists address the loss-of-function associated with APOE4 and exacerbated by Aβ pathology, i.e. low levels of apoE4 lipoprotein association/lipidation. Further studies are vital to address whether RXR agonists are an APOE4-specific AD therapeutic and the systemic side effects that limit translational application.

APOE4 enhances age-dependent decline in cognitive function by down-regulating an NMDA receptor pathway in EFAD-Tg mice

BackgroundAlzheimer’s disease (AD) causes progressive loss of memory and cognition, exacerbated by APOE4, the greatest genetic risk factor for AD. One proposed mechanism for apolipoprotein E (apoE) effects on cognition is via NMDAR-dependent signaling. APOE genotype-specific effects on this pathway were dissected using EFAD-transgenic (Tg) mice (5xFAD mice, that over-express human amyloid-beta (Aβ) via 5 familial-AD (FAD) mutations, and express human apoE), and 5xFAD/APOE-knockout (KO) mice. Previous data from EFAD-Tg mice demonstrate age-dependent (2-6 months), apoE-specific effects on the development of Aβ pathology. This study tests the hypothesis that apoE4 impairs cognition via modulation of NMDAR-dependent signaling, specifically via a loss of function by comparison of E4FAD mice with 5xFAD/APOE-KO mice, E3FAD and E2FAD mice.ResultsUsing female E2FAD, E3FAD, E4FAD and 5xFAD/APOE-KO mice aged 2-, 4-, and 6-months, the Y-maze and Morris water maze behavioral tests were combined with synaptic protein levels as markers of synaptic viability. The results demonstrate a greater age-induced deficit in cognition and reduction in PSD95, drebrin and NMDAR subunits in the E4FAD and 5xFAD/APOE-KO mice compared with E2FAD and E3FAD mice, consistent with an apoE4 loss of function. Interestingly, for NMDAR-mediated signaling, the levels of p-CaMK-II followed this same apoE-specific pattern as cognition, while the levels of p-CREB and BDNF demonstrate an apoE4 toxic gain of function: E2FAD > E3FAD > 5xFAD/APOE-KO > E4FAD.ConclusionThese findings suggest that compared with E2FAD and E3FAD, E4FAD and 5xFAD/APOE-KO mice exhibit enhanced age-induced reductions in cognition and key synaptic proteins via down-regulation of an NMDAR signaling pathway, consistent with an apoE4 loss of function. However, levels of p-CREB and BDNF, signaling factors common to multiple pathways, suggest a gain of toxic function. Publications in this field present contradictory results as to whether APOE4 imparts a loss or gain of function. As with the results reported herein, the overall effect of APOE4 on a given CNS-specific measure will be the product of multiple overlapping mechanisms. Thus, caution remains critical in determining whether APOE gene inactivation or therapies that correct the loss of positive function related to apoE4, are the appropriate therapeutic response.

The role of APOE in cerebrovascular dysfunction.

The ε4 allele of the apolipoprotein E gene (APOE4) is associated with cognitive decline during aging, is the greatest genetic risk factor for Alzheimer’s disease and has links to other neurodegenerative conditions that affect cognition. Increasing evidence indicates that APOE genotypes differentially modulate the function of the cerebrovasculature (CV), with apoE and its receptors expressed by different cell types at the CV interface (astrocytes, pericytes, smooth muscle cells, brain endothelial cells). However, research on the role of apoE in CV dysfunction has not advanced as quickly as other apoE-modulated pathways. This review will assess what aspects of the CV are modulated by APOE genotypes during aging and under disease states, discuss potential mechanisms, and summarize the therapeutic significance of the topic. We propose that APOE4 induces CV dysfunction through direct signaling at the CV, and indirectly via modulation of peripheral and central pathways. Further, that APOE4 predisposes the CV to damage by, and exacerbates the effects of, additional risk factors (such as sex, hypertension, and diabetes). ApoE4-induced detrimental CV changes include reduced cerebral blood flow (CBF), modified neuron-CBF coupling, increased blood–brain barrier leakiness, cerebral amyloid angiopathy, hemorrhages and disrupted transport of nutrients and toxins. The apoE4-induced detrimental changes may be linked to pericyte migration/activation, astrocyte activation, smooth muscle cell damage, basement membrane degradation and alterations in brain endothelial cells.

A multifunctional therapeutic approach to disease modification in multiple familial mouse models and a novel sporadic model of Alzheimer’s disease

BackgroundClinical failures singularly targeting amyloid-β pathology indicate a critical need for alternative Alzheimer’s disease (AD) therapeutic strategies. The mixed pathology reported in a large population of AD patients demands a multifunctional drug approach. Since activation of cAMP response element binding protein (CREB) plays a crucial role in synaptic strengthening and memory formation, we retooled a clinical drug with known neuroprotective and anti-inflammatory activity to activate CREB, and validated this novel multifunctional drug, NMZ, in 4 different mouse models of AD.ResultsNMZ was tested in three mouse models of familial AD and one model of sporadic AD. In 3 × Tg hippocampal slices, NMZ restored LTP. In vivo, memory was improved with NMZ in all animal models with robust cognitive deficits. NMZ treatment lowered neurotoxic forms of Aβ in both APP/PS1 and 3 × Tg transgenic mice while also restoring neuronal plasticity biomarkers in the 3 × Tg mice. In EFAD mice, incorporation of the major genetic AD risk factor, hAPOE4, did not mute the beneficial drug effects. In a novel sporadic mouse model that manifests AD-like pathology caused by accelerated oxidative stress in the absence of any familial AD mutation, oral administration of NMZ attenuated hallmark AD pathology and restored biomarkers of synaptic and neuronal function.ConclusionsThe multifunctional approach, embodied by NMZ, was successful in mouse models of AD incorporating Aβ pathology (APP/PS1), tau pathology (3xTg), and APOE4, the major human genetic risk factor for AD (EFAD). The efficacy observed in a novel model of sporadic AD (Aldh2−/−) demonstrates that the therapeutic approach is not limited to rare, familial AD genetic mutations. The multifunctional drug, NMZ, was not designed directly to target Aβ and tau pathology; however, the attenuation of this hallmark pathology suggests the approach to be a highly promising, disease-modifying strategy for AD and mixed pathology dementia.

Epidermal growth factor prevents oligomeric amyloid-β induced angiogenesis deficits in vitro:

Cerebrovascular dysfunction is a critical component of Alzheimer’s disease (AD) pathogenesis. Oligomeric amyloid-β42 (oAβ42) is considered a major contributor to AD progression. However, data are limited on the role of oAβ42 in brain endothelial cell vessel degeneration/angiogenesis, including the interaction with angiogenic mediators. Thus, the current study determined the effect of oAβ42 on angiogenesis in vitro, utilizing single brain endothelial cell cultures and triple cultures mimicking the microvascular unit (MVU: brain endothelial cells, astrocytes, and pericytes). oAβ42 dose-dependently reduced angiogenesis and induced vessel disruption. Critically, epidermal growth factor prevented oAβ42-induced deficits, implicating angiogenic pathways as potential therapeutics for AD.

Rexinoids as Therapeutics for Alzheimer’s Disease: Role of APOE

Alzheimers disease (AD) is a progressive neurodegenerative disease characterized by amyloid plaques, composed of amyloid-beta peptide (Aβ) and neurofibrillary tangles, composed of aberrantly phosphorylated tau. APOE4 is the greatest genetic risk factor for AD, increasing risk up to 12- fold with a double allele compared to APOE3. In contrast, APOE2 reduces AD risk ~2-fold per allele. Accumulating evidence demonstrates that apolipoprotein E4 (apoE4) plays a multifactorial role in AD pathogenesis, although the exact mechanisms remain unclear. Further data support roles for apoE4 as a toxic gain of function or loss of positive function in AD, a discrepancy that has significant implications for the future of apoE-directed therapeutics. However, recent evidence repurposing retinoid X receptor (RXR) agonists, or rexinoids, for the treatment of AD demonstrates conflicting, though potentially beneficial effects in familial AD-transgenic (FAD-Tg) mouse models. Of particular note is bexarotene (Targretin®), a selective rexinoid previously utilized in cancer treatment emerging as a viable candidate for AD clinical trials. However, the mechanism of action of bexarotene and similar rexinoids remains controversial, particularly in the context of human APOE. In addition, rexinoids demonstrate distinct adverse event profiles in humans that may have greater detrimental effects in an elderly AD population. Therefore, this special issue review discusses the implications for rexinoiddirected therapeutic strategies in AD, the potential mechanistic targets, and future directions for the improvement of rexinoid-based therapies in AD.

Peripheral Inflammation, Apolipoprotein E4, and Amyloid-β Interact to Induce Cognitive and Cerebrovascular Dysfunction:

Cerebrovascular dysfunction is rapidly reemerging as a major process of Alzheimer’s disease (AD). It is, therefore, crucial to delineate the roles of AD risk factors in cerebrovascular dysfunction. While apolipoprotein E4 (APOE4), Amyloid-β (Aβ), and peripheral inflammation independently induce cerebrovascular damage, their collective effects remain to be elucidated. The goal of this study was to determine the interactive effect of APOE4, Aβ, and chronic repeated peripheral inflammation on cerebrovascular and cognitive dysfunction in vivo. EFAD mice are a well-characterized mouse model that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce human Aβ42 via expression of 5 Familial Alzheimer’s disease (5xFAD) mutations. Here, we utilized EFAD carriers [5xFAD+/−/APOE+/+ (EFAD+)] and noncarriers [5xFAD−/−/APOE+/+ (EFAD−)] to compare the effects of peripheral inflammation in the presence or absence of human Aβ overproduction. Low-level, chronic repeated peripheral inflammation was induced in EFAD mice via systemic administration of lipopolysaccharide (LPS; 0.5 mg/kg/wk i.p.) from 4 to 6 months of age. In E4FAD+ mice, peripheral inflammation caused cognitive deficits and lowered post-synaptic protein levels. Importantly, cerebrovascular deficits were observed in LPS-challenged E4FAD+ mice, including cerebrovascular leakiness, lower vessel coverage, and cerebral amyloid angiopathy-like Aβ deposition. Thus, APOE4, Aβ, and peripheral inflammation interact to induce cerebrovascular damage and cognitive deficits.

NMDA Receptor Dysregulation by Defective Depalmitoylation in the Infantile Neuronal Ceroid Lipofuscinosis Mouse Model

Protein palmitoylation and depalmitoylation alter protein function. This post-translational modification is critical for synaptic transmission and plasticity. Mutation of the depalmitoylating enzyme palmitoyl-protein thioesterase 1 (PPT1) causes infantile neuronal ceroid lipofuscinosis (CLN1), a pediatric neurodegenerative disease. However, the role of protein depalmitoylation in synaptic maturation is unknown. Therefore, we studied synapse development in Ppt1-/- mouse visual cortex. We demonstrate the stagnation of the developmental N-methyl-D-aspartate receptor (NMDAR) subunit switch from GluN2B to GluN2A in Ppt1-/- mice. Correspondingly, GluN2A-mediated synaptic currents are diminished and Ppt1-/- dendritic spines maintain immature morphology in vivo. Further, GluN2B is hyperpalmitoylated in Ppt1-/- neurons and associated with extrasynaptic, diffuse calcium influxes and enhanced vulnerability to NMDA-induced excitotoxicity. Remarkably, Ppt1-/- neurons treated with palmitoylation inhibitors demonstrate normalized levels of palmitoylated GluN2B and Fyn kinase, reversing susceptibility to excitotoxic insult. Thus, depalmitoylation of GluN2B by PPT1 plays a critical role in postsynapse maturation and pathophysiology of neurodegenerative disease.

In Vitro Assays to Assess Blood-brain Barrier Mesh-like Vessel Formation and Disruption

Blood-brain barrier (BBB) coverage plays a central role in the homeostasis of the central nervous system (CNS). The BBB is dynamically maintained by astrocytes, pericytes and brain endothelial cells (BECs). Here, we detail methods to assess BBB coverage using single cultures of immortalized human BECs, single cultures of primary mouse BECs, and a humanized triple culture model (BECs, astrocytes and pericytes) of the BBB. To highlight the applicability of the assays to disease states, we describe the effect of oligomeric amyloid-β (oAβ), which is an important contributor to Alzheimers disease (AD) progression, on BBB coverage. Further, we utilize the epidermal growth factor (EGF) to illuminate the drug screening potential of the techniques. Our results show that single and triple cultured BECs form meshwork-like structures under basal conditions, and that oAβ disrupts this cell meshwork formation and degenerates the preformed mesh structures, but EGF blocks this disruption. Thus, the techniques described are important for dissecting fundamental and disease-relevant processes that modulate BBB coverage.