Apolipoprotein E-containing lipoproteins protect neurons from apoptosis via a signaling pathway involving low-density lipoprotein receptor-related protein-1

Role and action mechanisms of tPA in CRH-induced apoptosis of mouse oviductal epithelial and mural granulosa cells

Understanding how stress hormones induce apoptosis in oviductal epithelial cells (OECs) and mural granulosa cells (MGCs) can reveal the mechanisms by which female stress impairs embryonic development and oocyte competence. A recent study showed that tissue plasminogen activator (tPA) ameliorates corticosterone-induced apoptosis in MGCs and OECs by acting on its receptors low-density lipoprotein receptor-related protein 1 (LRP1) and Annexin A2 (ANXA2), respectively. However, whether tPA is involved in corticotropin-releasing hormone (CRH)-induced apoptosis and whether it uses the same or different receptors to inhibit apoptosis induced by different hormones in the same cell type remains unknown. This study showed that CRH triggered apoptosis in both OECs and MGCs and significantly downregulated tPA expression. Moreover, tPA inhibits CRH-induced apoptosis by acting on ANXA2 in both OECs and MGCs. While ANXA2 inhibits apoptosis via phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling, LRP1 reduces apoptosis via mitogen-activated protein kinase (MAPK) signaling. Thus, tPA used the same receptor to inhibit CRH-induced apoptosis in both OECs and MGCs, however used different receptors to inhibit corticosterone-induced apoptosis in MGCs and OECs. These data helps understand the mechanism by which female stress impairs embryo/oocyte competence and proapoptotic factors trigger apoptosis in different cell types.

Novel insights into the multifaceted and tissue-specific roles of the endocytic receptor LRP1

Receptor-mediated endocytosis provides a mechanism for the selective uptake of specific molecules thereby controlling the composition of the extracellular environment and biological processes. The low-density lipoprotein receptor-related protein 1 (LRP1) is a widely expressed endocytic receptor that regulates cellular events by modulating the levels of numerous extracellular molecules via rapid endocytic removal. LRP1 also participates in signalling pathways through this modulation as well as in the interaction with membrane receptors and cytoplasmic adaptor proteins. LRP1 SNPs are associated with several diseases and conditions such as migraines, aortic aneurysms, cardiopulmonary dysfunction, corneal clouding, and bone dysmorphology and mineral density. Studies using Lrp1 KO mice revealed a critical, nonredundant and tissue-specific role of LRP1 in regulating various physiological events. However, exactly how LRP1 functions to regulate so many distinct and specific processes is still not fully clear. Our recent proteomics studies have identified more than 300 secreted proteins that either directly interact with LRP1 or are modulated by LRP1 in various tissues. This review will highlight the remarkable ability of this receptor to regulate secreted molecules in a tissue-specific manner and discuss potential mechanisms underpinning such specificity. Uncovering the depth of these "hidden" specific interactions modulated by LRP1 will provide novel insights into a dynamic and complex extracellular environment that is involved in diverse biological and pathological processes.

Multiple Roles of Apolipoprotein E4 in Oxidative Lipid Metabolism and Ferroptosis During the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease worldwide and has a great socio-economic impact. Modified oxidative lipid metabolism and dysregulated iron homeostasis have been implicated in the pathogenesis of this disorder, but the detailed pathophysiological mechanisms still remain unclear. Apolipoprotein E (APOE) is a lipid-binding protein that occurs in large quantities in human blood plasma, and a polymorphism of the APOE gene locus has been identified as risk factors for AD. The human genome involves three major APOE alleles (APOE2, APOE3, APOE4), which encode for three subtly distinct apolipoprotein E isoforms (APOE2, APOE3, APOE4). The canonic function of these apolipoproteins is lipid transport in blood and brain, but APOE4 allele carriers have a much higher risk for AD. In fact, about 60% of clinically diagnosed AD patients carry at least one APOE4 allele in their genomes. Although the APOE4 protein has been implicated in pathophysiological key processes of AD, such as extracellular beta-amyloid (Aβ) aggregation, mitochondrial dysfunction, neuroinflammation, formation of neurofibrillary tangles, modified oxidative lipid metabolism, and ferroptotic cell death, the underlying molecular mechanisms are still not well understood. As for all mammalian cells, iron plays a crucial role in neuronal functions and dysregulation of iron homeostasis has also been implicated in the pathogenesis of AD. Imbalances in iron homeostasis and impairment of the hydroperoxy lipid-reducing capacity induce cellular dysfunction leading to neuronal ferroptosis. In this review, we summarize the current knowledge on APOE4-related oxidative lipid metabolism and the potential role of ferroptosis in the pathogenesis of AD. Pharmacological interference with these processes might offer innovative strategies for therapeutic interventions.

Coronal ApoE Protein Combines with LRP1 to Inactivate GSK3β That Mitigates Silica Nanoparticle-Induced Brain Lesion

Silica nanoparticles (SiO2 NPs) are widely used engineered materials that warrant their obvious environmental exposure risk. Our previous study has shown that different routes of SiO2 NP exposure on the glycogen synthase kinase 3 beta (GSK3β) activity were related to the serum proteins enriched on the surface of SiO2 NPs, which implied that a particular protein in the serum changed the inherent toxic behavior of SiO2 NPs and inhibited the activation of GSK3β by SiO2 NPs. Here, we identified that the SiO2 NP surface enriched a large amount of apolipoprotein E (ApoE), and the ApoE protein corona bound to the lipoprotein receptor-related protein 1 (LRP1) to inactivate GSK3β, thereby reducing the damage of SiO2 NPs to the brain. This work presented the first evidence that specific biocorona reduced the toxicity of SiO2 NPs at the molecular level, which helped to elucidate the role of specific corona components on nanotoxicity.

Role of APOE in glaucoma

Glaucoma is a chronic blinding eye disease caused by the progressive loss of retinal ganglion cells (RGCs). Currently, no clinically approved treatment can directly improve the survival rate of RGCs. The Apolipoprotein E (APOE) gene is closely related to the genetic risk of numerous neurodegenerative diseases and has become a hot topic in the field of neurodegenerative disease research in recent years. The optic nerve and retina are extensions of the brain's nervous system. The pathogenesis of retinal degenerative diseases is closely related to the degenerative diseases of the nerves in the brain. APOE consists of three alleles, ε4, ε3, and ε2, in a single locus. They have varying degrees of risk for glaucoma. APOE4 and the APOE gene deletion (APOE-/-) can reduce RGC loss. By contrast, APOE3 and the overall presence of APOE genes (APOE+/+) result in significant loss of RGC bodies and axons, increasing the risk of glaucoma RGCs death. Currently, there is no clear literature indicating that APOE2 is beneficial or harmful to glaucoma. This study summarises the mechanism of different APOE genes in glaucoma and speculates that APOE targeted intervention may be a promising method for protecting against RGCs loss in glaucoma.

Evidence that Alzheimer's Disease Is a Disease of Competitive Synaptic Plasticity Gone Awry

 Mounting evidence indicates that a physiological function of amyloid-β (Aβ) is to mediate neural activity-dependent homeostatic and competitive synaptic plasticity in the brain. I have previously summarized the lines of evidence supporting this hypothesis and highlighted the similarities between Aβ and anti-microbial peptides in mediating cell/synapse competition. In cell competition, anti-microbial peptides deploy a multitude of mechanisms to ensure both self-protection and competitor elimination. Here I review recent studies showing that similar mechanisms are at play in Aβ-mediated synapse competition and perturbations in these mechanisms underpin Alzheimer's disease (AD). Specifically, I discuss evidence that Aβ and ApoE, two crucial players in AD, co-operate in the regulation of synapse competition. Glial ApoE promotes self-protection by increasing the production of trophic monomeric Aβ and inhibiting its assembly into toxic oligomers. Conversely, Aβ oligomers, once assembled, promote the elimination of competitor synapses via direct toxic activity and amplification of "eat-me" signals promoting the elimination of weak synapses. I further summarize evidence that neuronal ApoE may be part of a gene regulatory network that normally promotes competitive plasticity, explaining the selective vulnerability of ApoE expressing neurons in AD brains. Lastly, I discuss evidence that sleep may be key to Aβ-orchestrated plasticity, in which sleep is not only induced by Aβ but is also required for Aβ-mediated plasticity, underlining the link between sleep and AD. Together, these results strongly argue that AD is a disease of competitive synaptic plasticity gone awry, a novel perspective that may promote AD research.

Apolipoprotein ε in Brain and Retinal Neurodegenerative Diseases

Alzheimer's disease (AD) is the most common form of dementia that remains incurable and has become a major medical, social, and economic challenge worldwide. AD is characterized by pathological hallmarks of senile plaques (SP) and neurofibrillary tangles (NFTs) that damage the brain up to twenty years before a clinical diagnosis is made. Interestingly these pathological features have also been observed in retinal neurodegenerative diseases including age related macular degeneration (ARMD), glaucoma and diabetic retinopathy (DR). An association of AD with these diseases has been suggested in epidemiological studies and several common pathological events and risk factors have been identified between these diseases. The E4 allele of Apolipoprotein E (APOE) is a well-established genetic risk factor for late onset AD. The ApoE ε4 allele is also associated with retinal neurodegenerative diseases however in contrast to AD, it is considered protective in AMD, likewise ApoE E2 allele, which is a protective factor for AD, has been implicated as a risk factor for AMD and glaucoma. This review summarizes the evidence on the effects of ApoE in retinal neurodegenerative diseases and discusses the overlapping molecular pathways in AD. The involvement of ApoE in regulating amyloid beta (Aβ) and tau pathology, inflammation, vascular integrity, glucose metabolism and vascular endothelial growth factor (VEGF) signaling is also discussed.

The recent progress of peptide regulators for the Wnt/β-catenin signaling pathway

Wnt signaling plays an important role in many biological processes such as stem cell self-renewal, cell proliferation, migration, and differentiation. The β-catenin-dependent signaling pathway mainly regulates cell proliferation, differentiation, and migration. In the Wnt/β-catenin signaling pathway, the Wnt family ligands transduce signals through LRP5/6 and Frizzled receptors to the Wnt/β-catenin signaling cascades. Wnt-targeted therapy has garnered extensive attention. The most commonly used approach in targeted therapy is small-molecule regulators. However, it is difficult for small-molecule regulators to make great progress due to their inherent defects. Therapeutic peptide regulators targeting the Wnt signaling pathway have become an alternative therapy, promising to fill the gaps in the clinical application of small-molecule regulators. In this review, we describe recent advances in peptide regulators for Wnt/β-catenin signaling.

The role of high cholesterol in SARS-CoV-2 infectivity

Coronavirus disease 2019 (COVID19) is a respiratory infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The virus binds to angiotensinogen converting enzyme 2 (ACE2) which mediates viral entry into mammalian cells. COVID19 is notably severe in elderly and those with underlying chronic conditions. The cause of selective severity is not well understood. Here we show cholesterol and the signaling lipid phosphatidyl-inositol 4,5 bisphosphate (PIP₂) regulate viral infectivity through the localization of ACE2's into nanoscopic (<200 nm) lipid clusters. Uptake of cholesterol into cell membranes (a condition common to chronic disease) causes ACE2 to move from PIP₂ lipids to endocytic ganglioside (GM1) lipids, where the virus is optimally located for viral entry. In mice, age, and high fat diet increase lung tissue cholesterol by up to 40%. And in smokers with chronic disease, cholesterol is elevated two-fold, a magnitude of change that dramatically increases infectivity of virus in cell culture. We conclude increasing the ACE2 location near endocytic lipids increases viral infectivity and may help explain the selective severity of COVID-19 in aged and diseased populations.

Role of tPA in Corticosterone-Induced Apoptosis of Mouse Mural Granulosa and Oviductal Epithelial Cells

Although studies indicate that female stress-increased secretion of glucocorticoids impairs oocyte competence and embryo development, by inducing apoptosis of ovarian and oviductal cells, respectively, the mechanisms by which glucocorticoids induce apoptosis of ovarian and oviductal cells are largely unclear. Tissue plasminogen activator (tPA) has been involved in apoptosis of different cell types. However, while some studies indicate that tPA is proapoptotic, others demonstrate its antiapoptotic effects. This study has explored the role and action mechanisms of tPA in corticosterone-induced apoptosis of mouse mural granulosa cells (MGCs) and oviductal epithelial cells (OECs). The results demonstrate that culture with corticosterone significantly increased apoptosis, while decreasing levels of tPA (Plat) mRNA and tPA protein in both MGCs and OECs. Culture with tPA ameliorated corticosterone-induced apoptosis of MGCs and OECs. Furthermore, while tPA protected MGCs from corticosterone-induced apoptosis by interacting with low-density lipoprotein receptor-related protein 1 (LRP1), it protected OECs from the apoptosis by acting on Annexin 2 (ANXA2). In conclusion, tPA is antiapoptotic in both MGCs and OECs, and it protects MGCs and OECs from corticosterone-induced apoptosis by interacting with LRP1 and ANXA2, respectively, suggesting that tPA may use different receptors to inhibit apoptosis in different cell types.

Aqueous Humor Apolipoprotein Concentration and Severity of Diabetic Retinopathy in Type 2 Diabetes

An imbalance of plasma apolipoproteins has been linked to diabetic retinopathy (DR); however, there is scarce information regarding their presence in the aqueous humor (AH) and their role in DR. Here, we aimed at analysing the relationship between apolipoprotein concentrations in human AH and the severity of DR. Concentrations of apolipoproteins were measured retrospectively in patients with type 2 diabetes mellitus (T2DM) without DR (n = 23), with mild to moderate nonproliferative DR (NPDR) (n = 13), and advanced NPDR/proliferative DR (PDR) (n = 14) using a multiplex immunoassay. Compared to the non-apparent DR group, the concentrations of seven apolipoproteins were elevated in advanced NPDR/PDR (Apo AI 5.8-fold, Apo AII 4.5-fold, Apo CI 3.3-fold, Apo CIII 6.8-fold, Apo D 3.3-fold, Apo E 2.4-fold, and Apo H 6.6-fold). No significant differences were observed in apolipoprotein concentrations between patients with non-apparent DR and healthy controls (n = 17). In conclusion, the AH concentrations of apolipoproteins AI, AII, CI, CIII, D, E, and H increased in advancing stages of DR, suggesting their role in the pathogenesis of DR, which deserves further examination.

The LRP1/CD91 ligands, tissue-type plasminogen activator, α2-macroglobulin, and soluble cellular prion protein have distinct co-receptor requirements for activation of cell-signaling

LDL Receptor-related Protein-1 (LRP1/CD91) binds diverse ligands, many of which activate cell-signaling. Herein, we compared three LRP1 ligands that inhibit inflammatory responses triggered by lipopolysaccharide (LPS), including: enzymatically-inactive tissue-type plasminogen activator (EI-tPA); activated α₂-macroglobulin (α₂M); and S-PrP, a soluble derivative of nonpathogenic cellular prion protein (PrP^C). In bone marrow-derived macrophages, the N-methyl-D-aspartate receptor was essential for all three LRP1 ligands to activate cell-signaling and inhibit LPS-induced cytokine expression. Intact lipid rafts also were essential. Only α₂M absolutely required LRP1. LRP1 decreased the EI-tPA concentration required to activate cell-signaling and antagonize LPS but was not essential, mimicking its role as a S-PrP co-receptor. Membrane-anchored PrP^C also functioned as a co-receptor for EI-tPA and α₂M, decreasing the ligand concentration required for cell-signaling and LPS antagonism; however, when the concentration of EI-tPA or α₂M was sufficiently increased, cell-signaling and LPS antagonism occurred independently of PrP^C. S-PrP is the only LRP1 ligand in this group that activated cell-signaling independently of membrane-anchored PrP^C. EI-tPA, α₂M, and S-PrP inhibited LPS-induced LRP1 shedding from macrophages, a process that converts LRP1 into a pro-inflammatory product. Differences in the co-receptors required for anti-inflammatory activity may explain why LRP1 ligands vary in ability to target macrophages in different differentiation states.

Low-Density Lipoprotein Receptor-Related Protein 8 at the Crossroad between Cancer and Neurodegeneration

The low-density-lipoprotein receptors represent a family of pleiotropic cell surface receptors involved in lipid homeostasis, cell migration, proliferation and differentiation. The family shares common structural features but also has significant differences mainly due to tissue-specific interactors and to peculiar proteolytic processing. Among the receptors in the family, recent studies place low-density lipoprotein receptor-related protein 8 (LRP8) at the center of both neurodegenerative and cancer-related pathways. From one side, its overexpression has been highlighted in many types of cancer including breast, gastric, prostate, lung and melanoma; from the other side, LRP8 has a potential role in neurodegeneration as apolipoprotein E (ApoE) and reelin receptor, which are, respectively, the major risk factor for developing Alzheimer’s disease (AD) and the main driver of neuronal migration, and as a γ-secretase substrate, the main enzyme responsible for amyloid formation in AD. The present review analyzes the contributions of LDL receptors, specifically of LRP8, in both cancer and neurodegeneration, pointing out that depending on various interactions and peculiar processing, the receptor can contribute to both proliferative and neurodegenerative processes.

Lipoproteins in the Central Nervous System: From Biology to Pathobiology

The brain, as one of the most lipid-rich organs, heavily relies on lipid transport and distribution to maintain homeostasis and neuronal function. Lipid transport mediated by lipoprotein particles, which are complex structures composed of apolipoproteins and lipids, has been thoroughly characterized in the periphery. Although lipoproteins in the central nervous system (CNS) were reported over half a century ago, the identification of APOE4 as the strongest genetic risk factor for Alzheimer's disease has accelerated investigation of the biology and pathobiology of lipoproteins in the CNS. This review provides an overview of the different components of lipoprotein particles, in particular apolipoproteins, and their involvements in both physiological functions and pathological mechanisms in the CNS.

Integrated Analysis of the miRNA-mRNA Regulatory Network Involved in HIV-Associated Neurocognitive Disorder

HIV-associated neurocognitive disorder (HAND) is an array of neurocognitive changes associated with HIV infection, and the roles of microRNAs in HAND have not yet been completely revealed. Based on published data and publicly available databases, we constructed an integrated miRNA-mRNA network involved in HAND. Bioinformatics analyses, including gene ontology, network analysis, and KEGG pathway analysis, were applied for further study of the network and the genes of the network. The axon guidance KEGG pathway, three genes NTNG1, EFNB2, CXCL12, and 17 miRNAs which regulate these genes are spotlighted in our study. This study provides new perspectives to the knowledge of miRNAs’ roles in the progression of HAND, and our findings provide potential therapeutic targets and clues of HAND.

Neuroserpin, a crucial regulator for axogenesis, synaptic modelling and cell-cell interactions in the pathophysiology of neurological disease

Neuroserpin is an axonally secreted serpin that is involved in regulating plasminogen and its enzyme activators, such as tissue plasminogen activator (tPA). The protein has been increasingly shown to play key roles in neuronal development, plasticity, maturation and synaptic refinement. The proteinase inhibitor may function both independently and through tPA-dependent mechanisms. Herein, we discuss the recent evidence regarding the role of neuroserpin in healthy and diseased conditions and highlight the participation of the serpin in various cellular signalling pathways. Several polymorphisms and mutations have also been identified in the protein that may affect the serpin conformation, leading to polymer formation and its intracellular accumulation. The current understanding of the involvement of neuroserpin in Alzheimer's disease, cancer, glaucoma, stroke, neuropsychiatric disorders and familial encephalopathy with neuroserpin inclusion bodies (FENIB) is presented. To truly understand the detrimental consequences of neuroserpin dysfunction and the effective therapeutic targeting of this molecule in pathological conditions, a cross-disciplinary understanding of neuroserpin alterations and its cellular signaling networks is essential.

Potential Tear Biomarkers for the Diagnosis of Parkinson’s Disease—A Pilot Study

Parkinson’s disease (PD) is the second most common neurodegenerative disease after Alzheimer’s disease. In this study, the tear proteome profile of patients with idiopathic PD (iPD, n = 24), carriers of the E46K-SNCA mutation (n = 3) and healthy control (CT, n = 27) subjects was analyzed to identify candidate biomarkers for the diagnosis of PD. An observational, prospective and case-control pilot study was carried out, analyzing the participants tear samples by nano-liquid chromatography–mass spectrometry (nLC–MS/MS) and assessing their neurological impairment. The proteomic data obtained are available at ProteomeXchange with identifier 10.6019/PXD028811. These analyses led to the identification of 560 tear proteins, some of which were deregulated in PD patients and that have been implicated in immune responses, inflammation, apoptosis, collagen degradation, protein synthesis, defense, lipid transport and altered lysosomal function. Of these proteins, six were related to neurodegenerative processes and showed a good capacity to classify patients and controls. These findings revealed that certain proteins were upregulated in the tears of PD patients, mainly proteins involved in lysosomal function. Thus, in this study, tear proteins were identified that are implicated in neurodegeneration and that may be related to an aggressive disease phenotype in PD patients.

Isoform-Specific Effects of Apolipoprotein E on Hydrogen Peroxide-Induced Apoptosis in Human Induced Pluripotent Stem Cell (iPSC)-Derived Cortical Neurons

Hydrogen peroxide (H₂O₂)-induced neuronal apoptosis is critical to the pathology of Alzheimer's disease (AD) as well as other neurodegenerative diseases. The neuroprotective effects of apolipoprotein (ApoE) isoforms against apoptosis and the underlying mechanism remains controversial. Here, we have generated human cortical neurons from iPSCs and induced apoptosis with H₂O₂. We show that ApoE2 and ApoE3 pretreatments significantly attenuate neuronal apoptosis, whereas ApoE4 has no neuroprotective effect and higher concentrations of ApoE4 even display toxic effect. We further identify that ApoE2 and ApoE3 regulate Akt/FoxO3a/Bim signaling pathway in the presence of H₂O₂. We propose that ApoE alleviates H₂O₂-induced apoptosis in human iPSC-derived neuronal culture in an isoform specific manner. Our results provide an alternative mechanistic explanation on how ApoE isoforms influence the risk of AD onset as well as a promising therapeutic target for diseases involving neuronal apoptosis in the central nervous system.

Apolipoprotein E-Containing Lipoproteins and LRP1 Protect From NMDA-Induced Excitotoxicity Associated With Reducing α2-Macroglobulin in Müller Glia

Purpose

Optic nerve damage leads to impairment of visual functions. We previously demonstrated that apolipoprotein E-containing lipoproteins (E-LPs) protect retinal ganglion cells (RGCs) from degeneration in a glaucoma model of glutamate/aspartate transporter-deficient mice. This study aimed to determine whether E-LPs protect RGCs from N-methyl-d-aspartate (NMDA)-induced excitotoxicity, and to investigate the details of an indirect neuroprotective mechanism of E-LPs by reducing α2-macroglobulin, which interferes with the neuroprotective effect of E-LPs, in Müller glia.

Methods

Excitotoxicity was caused by intravitreal injection of NMDA, and then retinae were subjected to immunoblotting or quantitative reverse transcription-PCR. Primary cultures of mouse mixed retinal cells and mouse Müller glia were used for evaluating the effects of E-LPs on the expression of α2-macroglobulin.

Results

Intravitreal injection of E-LPs protected the optic nerve from degeneration and attenuated the increase in α2-macroglobulin in aqueous humor and retina of rats. E-LPs directly decreased the expression and secretion of α2-macroglobulin in primary cultures of Müller glia; this decrease in production of α2-macroglobulin was blocked by knockdown of the low-density lipoprotein receptor-related protein 1 (LRP1) with small interfering RNA. E-LPs promoted the phosphorylation of STAT3, whereas Stattic, an inhibitor of STAT3, restored the expression of α2-macroglobulin decreased by E-LPs.

Conclusions

In addition to our previous findings of the protection of RGCs by E-LPs, the new observations in Müller glia indicate that a reduction of the intraocular α2-macroglobulin, regulated by the E-LP-LRP1-STAT3 pathway, might be an additional protective mechanism against excitotoxicity in the retina.

Advances in the Experimental Models of HIV-Associated Neurological Disorders

PURPOSE OF REVIEW

Involvement of the central nervous system (CNS) in HIV-1 infection is commonly associated with neurological disorders and cognitive impairment, commonly referred to as HIV-associated neurocognitive disorders (HAND). Severe and progressive neurocognitive impairment is rarely observed in the post-cART era; however, asymptomatic and mild neurocognitive disorders still exist, despite viral suppression. Additionally, comorbid conditions can also contribute to the pathogenesis of HAND.

RECENT FINDINGS

In this review, we summarize the characterization of HAND, factors contributing, and the functional impairments in both preclinical and clinical models. Specifically, we also discuss recent advances in the animal models of HAND and in in vitro cultures and the potential role of drugs of abuse in this model system of HAND. Potential peripheral biomarkers associated with HAND are also discussed. Overall, this review identifies some of the recent advances in the field of HAND in cell culture studies, animal models, clinical findings, and the limitations of each model system, which can play a key role in developing novel therapeutics in the field.

Regulation of beta-amyloid production in neurons by astrocyte-derived cholesterol

Alzheimer's disease (AD) is characterized by the presence of amyloid β (Aβ) plaques, tau tangles, inflammation, and loss of cognitive function. Genetic variation in a cholesterol transport protein, apolipoprotein E (apoE), is the most common genetic risk factor for sporadic AD. In vitro evidence suggests that apoE links to Aβ production through nanoscale lipid compartments (lipid clusters), but its regulation in vivo is unclear. Here, we use superresolution imaging in the mouse brain to show that apoE utilizes astrocyte-derived cholesterol to specifically traffic neuronal amyloid precursor protein (APP) in and out of lipid clusters, where it interacts with β- and γ-secretases to generate Aβ-peptide. We find that the targeted deletion of astrocyte cholesterol synthesis robustly reduces amyloid and tau burden in a mouse model of AD. Treatment with cholesterol-free apoE or knockdown of cholesterol synthesis in astrocytes decreases cholesterol levels in cultured neurons and causes APP to traffic out of lipid clusters, where it interacts with α-secretase and gives rise to soluble APP-α (sAPP-α), a neuronal protective product of APP. Changes in cellular cholesterol have no effect on α-, β-, and γ-secretase trafficking, suggesting that the ratio of Aβ to sAPP-α is regulated by the trafficking of the substrate, not the enzymes. We conclude that cholesterol is kept low in neurons, which inhibits Aβ accumulation and enables the astrocyte regulation of Aβ accumulation by cholesterol signaling.

Protective association of the ε2/ε3 heterozygote with Alzheimer's disease is strengthened by TOMM40-APOE variants in men

INTRODUCTION

Despite advances, understanding the protective role of the apolipoprotein E (APOE) ε2 allele in Alzheimer's disease (AD) remains elusive.

METHODS

We examined associations of variants comprised of the TOMM40 rs8106922 and APOE rs405509, rs440446, and ε2-encoding rs7412 polymorphisms with AD in a sample of 2862 AD-affected and 169,516 AD-unaffected non-carriers of the ε4 allele.

RESULTS

Association of the ε2/ε3 heterozygote of men with AD is 38% (P = 1.65 × 10^-2 ) more beneficial when it is accompanied by rs8106922 major allele homozygote and rs405509 and rs440446 heterozygotes than by rs8106922 heterozygote and rs405509 and rs440446 major allele homozygotes. No difference in the beneficial associations of these two most common ε2/ε3-bearing variants with AD was identified in women. The role of ε2/ε3 heterozygote may be affected by different immunomodulation functions of rs8106922, rs405509, and rs440446 variants in a sex-specific manner.

DISCUSSION

Combination of TOMM40 and APOE variants defines a more homogeneous AD-protective ε2/ε3-bearing profile in men.

The role of high cholesterol in age-related COVID19 lethality

Coronavirus disease 2019 (COVID19) is a respiratory infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) originating in Wuhan, China in 2019. The disease is notably severe in elderly and those with underlying chronic conditions. A molecular mechanism that explains why the elderly are vulnerable and why children are resistant is largely unknown. Here we show loading cells with cholesterol from blood serum using the cholesterol transport protein apolipoprotein E (apoE) enhances the entry of pseudotyped SARS-CoV-2 and the infectivity of the virion. Super resolution imaging of the SARS-CoV-2 entry point with high cholesterol shows almost twice the total number of endocytic entry points. Cholesterol concomitantly traffics angiotensinogen converting enzyme (ACE2) to the endocytic entry site where SARS-CoV-2 presumably docks to efficiently exploit entry into the cell. Furthermore, in cells producing virus, cholesterol optimally positions furin for priming SARS-CoV-2, producing a more infectious virion with improved binding to the ACE2 receptor. In vivo, age and high fat diet induces cholesterol loading by up to 40% and trafficking of ACE2 to endocytic entry sites in lung tissue from mice. We propose a component of COVID19 severity based on tissue cholesterol level and the sensitivity of ACE2 and furin to cholesterol. Molecules that reduce cholesterol or disrupt ACE2 localization with viral entry points or furin localization in the producer cells, may reduce the severity of COVID19 in obese patients.

Novel therapeutic approaches for Parkinson's disease by targeting brain cholesterol homeostasis

OBJECTIVES

Human brain is composed of 25% of the cholesterol & any dysfunction in brain cholesterol homeostasis contributes to neurodegenerative disorders such as Parkinson, Alzheimer's, Huntington's disease, etc. A growing literature indicates that alteration in neurotransmission & brain cholesterol metabolism takes place in the early stage of the disease. The current paper summarizes the role of cholesterol & its homeostasis in the pathophysiology of Parkinson's disease.

KEY FINDINGS

Literature findings suggest the possible role of lipids such as oxysterols, lipoproteins, etc. in Parkinson's disease pathophysiology. Cholesterol performs a diverse role in the brain but any deviation in its levels leads to neurodegeneration. Dysregulation of lipid caused by oxidative stress & inflammation leads to α-synuclein trafficking which contributes to Parkinson's disease progression. Also, α-synuclein by binding to membrane lipid forms lipid-protein complex & results in its aggregation. Different targets such as Phospholipase A2, Stearoyl-CoA desaturase enzyme, proprotein convertase subtilisin/kexin type 9, etc. have been identified as a potential novel approach for Parkinson's disease treatment.

SUMMARY

In the current review, we have discussed the possible molecular role of cholesterol homeostasis in Parkinson's disease progression. We also identified potential therapeutic targets that need to be evaluated clinically for the development of Parkinson's treatment.

APOE2: protective mechanism and therapeutic implications for Alzheimer's disease

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.

Effects of high-fat diet and Apoe deficiency on retinal structure and function in mice

To investigate the effects of a high-fat diet (HFD) and apolipoprotein E (Apoe) deficiency on retinal structure and function in mice. Apoe KO mice and wild-type C57BL/6J mice were given a low-fat diet (LFD) or a HFD for 32 weeks. Blood glucose, serum lipids, body weight and visceral fat weight were evaluated. Retinal sterol quantification was carried out by isotope dilution gas chromatography-mass spectrometry. The cholesterol metabolism related genes SCAP-SREBP expressions were detected by qRT-PCR. Retinal function was recorded using an electroretinogram. The thickness of each layer of the retina was measured by optical coherence tomography. Fundus fluorescein angiography was performed to detect retinal vasculature changes. Immunohistochemical staining was used to determine the expression of NF-κB, TNF-α and VEGFR2 in the retina among HFD, HFD Apoe^-/-, LFD Apoe^-/- and WT mice retinas. HFD feeding caused the mice to gain weight and develop hypercholesterinemia, while Apoe^-/- abnormalities also affected blood lipid metabolism. Both HFD and Apoe deficiency elevated retinal cholesterol, especially in the HFD Apoe^-/- mice. No up-regulated expression of SCAP-SREBP was observed as a negative regulator. Impaired retinal functions, thinning retinas and abnormal retinal vasculature were observed in the peripheral retinas of the HFD and Apoe^-/- mice compared with those in the normal chow group, particularly in the HFD Apoe^-/- mice. Moreover, the expression of NF-κB in the retinas of the HFD and Apoe^-/- mice was increased, together with upregulated TNF-α mRNA levels and TNF-α expression in the layer of retinal ganglion cells of the peripheral retina. At the same time, the expression level of VEGFR2 was elevated in the intervention groups, most notably in HFD Apoe^-/- mice. HFD or Apoe gene deletion had certain adverse effects on retinal function and structure, which were far below the combined factors and induced harm to the retina. Furthermore, HFD caused retinal ischemia and hypoxia. Additionally, Apoe abnormality increased susceptibility to ischemia. These changes upregulated NF-κB expression in ganglion cells and activated downstream TNF-α. Simultaneously, they activated VEGFR2, accelerating angiogenesis and vascular permeability. All of the aforementioned outcomes initiated inflammatory responses to trigger ganglion cell apoptosis and aggravate retinal neovascularization.

The multifaceted role of plasminogen in inflammation

A fine-tuned activation and deactivation of proteases and their inhibitors are involved in the execution of the inflammatory response. The zymogen/proenzyme plasminogen is converted to the serine protease plasmin, a key fibrinolytic factor by plasminogen activators including tissue-type plasminogen activator (tPA). Plasmin is part of an intricate protease network controlling proteins of initial hemostasis/coagulation, fibrinolytic and complement system. Activation of these protease cascades is required to mount a proper inflammatory response. Although best known for its ability to dissolve clots and cleave fibrin, recent studies point to the importance of fibrin-independent functions of plasmin during acute inflammation and inflammation resolution. In this review, we provide an up-to-date overview of the current knowledge of the enzymatic and cytokine-like effects of tPA and describe the role of tPA and plasminogen receptors in the regulation of the inflammatory response with emphasis on the cytokine storm syndrome such as observed during coronavirus disease 2019 or macrophage activation syndrome. We discuss tPA as a modulator of Toll like receptor signaling, plasmin as an activator of NFkB signaling, and summarize recent studies on the role of plasminogen receptors as controllers of the macrophage conversion into the M2 type and as mediators of efferocytosis during inflammation resolution.

Neuroprotective Effect of Antioxidants in the Brain

The brain is vulnerable to excessive oxidative insults because of its abundant lipid content, high energy requirements, and weak antioxidant capacity. Reactive oxygen species (ROS) increase susceptibility to neuronal damage and functional deficits, via oxidative changes in the brain in neurodegenerative diseases. Overabundance and abnormal levels of ROS and/or overload of metals are regulated by cellular defense mechanisms, intracellular signaling, and physiological functions of antioxidants in the brain. Single and/or complex antioxidant compounds targeting oxidative stress, redox metals, and neuronal cell death have been evaluated in multiple preclinical and clinical trials as a complementary therapeutic strategy for combating oxidative stress associated with neurodegenerative diseases. Herein, we present a general analysis and overview of various antioxidants and suggest potential courses of antioxidant treatments for the neuroprotection of the brain from oxidative injury. This review focuses on enzymatic and non-enzymatic antioxidant mechanisms in the brain and examines the relative advantages and methodological concerns when assessing antioxidant compounds for the treatment of neurodegenerative disorders.

Sex-dependent effect of APOE on Alzheimer's disease and other age-related neurodegenerative disorders

The importance of apolipoprotein E (APOE) in late-onset Alzheimer's disease (LOAD) has been firmly established, but the mechanisms through which it exerts its pathogenic effects remain elusive. In addition, the sex-dependent effects of APOE on LOAD risk and endophenotypes have yet to be explained. In this Review, we revisit the different aspects of APOE involvement in neurodegeneration and neurological diseases, with particular attention to sex differences in the contribution of APOE to LOAD susceptibility. We discuss the role of APOE in a broader range of age-related neurodegenerative diseases, and summarize the biological factors linking APOE to sex hormones, drawing on supportive findings from rodent models to identify major mechanistic themes underlying the exacerbation of LOAD-associated neurodegeneration and pathology in the female brain. Additionally, we list sex-by-genotype interactions identified across neurodegenerative diseases, proposing APOE variants as a shared etiology for sex differences in the manifestation of these diseases. Finally, we present recent advancements in 'omics' technologies, which provide a new platform for more in-depth investigations of how dysregulation of this gene affects the development and progression of neurodegenerative diseases. Collectively, the evidence summarized in this Review highlights the interplay between APOE and sex as a key factor in the etiology of LOAD and other age-related neurodegenerative diseases. We emphasize the importance of careful examination of sex as a contributing factor in studying the underpinning genetics of neurodegenerative diseases in general, but particularly for LOAD.

Metabolic correlates of prevalent mild cognitive impairment and Alzheimer's disease in adults with Down syndrome

INTRODUCTION

Disruption of metabolic function is a recognized feature of late onset Alzheimer's disease (LOAD). We sought to determine whether similar metabolic pathways are implicated in adults with Down syndrome (DS) who have increased risk for Alzheimer's disease (AD).

METHODS

We examined peripheral blood from 292 participants with DS who completed baseline assessments in the Alzheimer's Biomarkers Consortium-Down Syndrome (ABC-DS) using untargeted mass spectrometry (MS). Our sample included 38 individuals who met consensus criteria for AD (DS-AD), 43 who met criteria for mild cognitive impairment (DS-MCI), and 211 who were cognitively unaffected and stable (CS).

RESULTS

We measured relative abundance of 8,805 features using MS and 180 putative metabolites were differentially expressed (DE) among the groups at false discovery rate-corrected q< 0.05. From the DE features, a nine-feature classifier model classified the CS and DS-AD groups with receiver operating characteristic area under the curve (ROC AUC) of 0.86 and a two-feature model classified the DS-MCI and DS-AD groups with ROC AUC of 0.88. Metabolite set enrichment analysis across the three groups suggested alterations in fatty acid and carbohydrate metabolism.

DISCUSSION

Our results reveal metabolic alterations in DS-AD that are similar to those seen in LOAD. The pattern of results in this cross-sectional DS cohort suggests a dynamic time course of metabolic dysregulation which evolves with clinical progression from non-demented, to MCI, to AD. Metabolomic markers may be useful for staging progression of DS-AD.

A multimolecular signaling complex including PrPC and LRP1 is strictly dependent on lipid rafts and is essential for the function of tissue plasminogen activator

Prion protein (PrP^C ) localizes stably in lipid rafts microdomains and is able to recruit downstream signal transduction pathways by the interaction with promiscuous partners. Other proteins have the ability to occasionally be recruited to these specialized membrane areas, within multimolecular complexes. Among these, we highlight the presence of the low-density lipoprotein receptor-related protein 1 (LRP1), which was found localized transiently in lipid rafts, suggesting a different function of this receptor that through lipid raft becomes able to activate a signal transduction pathway triggered by specific ligands, including Tissue plasminogen activator (tPA). Since it has been reported that PrP^C participates in the tPA-mediated plasminogen activation, in this study, we describe the role of lipid rafts in the recruitment and activation of downstream signal transduction pathways mediated by the interaction among tPA, PrP^C and LRP1 in human neuroblastoma SK-N-BE2 cell line. Co-immunoprecipitation analysis reveals a consistent association between PrP^C and GM1, as well as between LRP1 and GM1, indicating the existence of a glycosphingolipid-enriched multimolecular complex. In our cell model, knocking-down PrP^C by siRNA impairs ERK phosphorylation induced by tPA. Moreover the alteration of the lipidic milieu of lipid rafts, perturbing the physical/functional interaction between PrP^C and LRP1, inhibits this response. We show that LRP1 and PrP^C , following tPA stimulation, may function as a system associated with lipid rafts, involved in receptor-mediated neuritogenic pathway. We suggest this as a multimolecular signaling complex, whose activity depends strictly on the integrity of lipid raft and is involved in the neuritogenic signaling.

Furin-mediated cleavage of LRP1 and increase in ICD of LRP1 after cerebral ischemia and after exposure of cultured neurons to NMDA

The N-methyl-D-aspartate (NMDA) receptor has been implicated in several neurodegenerative diseases, including stroke. Low-density lipoprotein receptor-related protein 1 (LRP1) plays pivotal roles in endocytosis and signaling in the cell. Immature LRP1 is processed by furin in the trans-Golgi network (TGN) and transported to the cell surface as its mature form. Activation of mature LRP1 exerts a protective effect against glutamate-induced degeneration of the rat retinal ganglion cells, as was shown in our previous study. However, the roles of LRP1 in the pathogenesis of excitotoxic neuronal injuries remain to be determined. The aim of this present study was to achieve further insight into the pathophysiologic roles of LRP1 after excitotoxic neuronal injuries. Our findings are the first to demonstrate that LRP1 was significantly cleaved by furin after cerebral ischemia in rats as well as after exposure of cultured cortical neurons to NMDA. It was noteworthy that the intracellular domain (ICD) of LRP1 was co-localized with TGN and furin. Furthermore, a furin inhibitor inhibited the cleavage of LRP1 and co-localization of LRP1-ICD with TGN or furin. Our findings suggest that furin-mediated cleavage of LRP1 and changes in the localization of LRP1-ICD were involved in the excitotoxic neuronal injury.

ApoE4: an emerging therapeutic target for Alzheimer's disease

BACKGROUND

The growing body of evidence indicating the heterogeneity of Alzheimer's disease (AD), coupled with disappointing clinical studies directed at a fit-for-all therapy, suggest that the development of a single magic cure suitable for all cases may not be possible. This calls for a shift in paradigm where targeted treatment is developed for specific AD subpopulations that share distinct genetic or pathological properties. Apolipoprotein E4 (apoE4), the most prevalent genetic risk factor of AD, is expressed in more than half of AD patients and is thus an important possible AD therapeutic target.

REVIEW

This review focuses initially on the pathological effects of apoE4 in AD, as well as on the corresponding cellular and animal models and the suggested cellular and molecular mechanisms which mediate them. The second part of the review focuses on recent apoE4-targeted (from the APOE gene to the apoE protein and its interactors) therapeutic approaches that have been developed in animal models and are ready to be translated to human. Further, the issue of whether the pathological effects of apoE4 are due to loss of protective function or due to gain of toxic function is discussed herein. It is possible that both mechanisms coexist, with certain constituents of the apoE4 molecule and/or its downstream signaling mediating a toxic effect, while others are associated with a loss of protective function.

CONCLUSION

ApoE4 is a promising AD therapeutic target that remains understudied. Recent studies are now paving the way for effective apoE4-directed AD treatment approaches.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

Vitamin E: Mechanism of transport and regulation in the CNS

Although vitamin E has been recognized as a critical micronutrient to neuronal health for more than half a century, vitamin E transport and regulation in the brain remain a mystery. Currently, the majority of what is known about vitamin E transport has been delineated in the liver. However, clues from the pathogenesis of neurological-related vitamin E deficient diseases point to compromised neuronal integrity and function, underlining the critical need to understand vitamin E regulation in the CNS. Additionally, most of the same molecular players involved in vitamin E transport in the liver are also found in CNS, including sterol SRB1, TTP, and ABCA/ABCG, suggesting similar intracellular pathways between these organ systems. Finally, based on chemical similarities, intracellular CNS shuttling of vitamin E likely resembles cholesterol's use of ApoE particles. Utilizing this information, this review will address what is currently known about trafficking vitamin E across the blood brain barrier in order to ensure an adequate supply of the essential nutrient to the brain. Although debatable, the health of the brain in relation to vitamin E levels has been demonstrated, most notably in oxidative stress-related conditions such as ataxias, Alzheimer's disease, and Parkinson's disease. Future vitamin E research is vital in understanding how the regulation of the vitamin can aid in the prevention, treatment, and curing of neurological diseases. © 2018 IUBMB Life, 71(4):424-429, 2019.

Administration of Downstream ApoE Attenuates the Adverse Effect of Brain ABCA1 Deficiency on Stroke

The ATP-binding cassette transporter member A1 (ABCA1) and apolipoprotein E (ApoE) are major cholesterol transporters that play important roles in cholesterol homeostasis in the brain. Previous research demonstrated that specific deletion of brain-ABCA1 (ABCA1^-B/-B) reduced brain grey matter (GM) and white matter (WM) density in the ischemic brain and decreased functional outcomes after stroke. However, the downstream molecular mechanism underlying brain ABCA1-deficiency-induced deficits after stroke is not fully understood. Adult male ABCA1^-B/-B and ABCA1-floxed control mice were subjected to distal middle-cerebral artery occlusion and were intraventricularly infused with artificial mouse cerebrospinal fluid as vehicle control or recombinant human ApoE2 into the ischemic brain starting 24 h after stroke for 14 days. The ApoE/apolipoprotein E receptor 2 (ApoER2)/high-density lipoprotein (HDL) levels and GM/WM remodeling and functional outcome were measured. Although ApoE2 increased brain ApoE/HDL levels and GM/WM density, negligible functional improvement was observed in ABCA1-floxed-stroke mice. ApoE2-administered ABCA1^-B/-B stroke mice exhibited elevated levels of brain ApoE/ApoER2/HDL, increased GM/WM density, and neurogenesis in both the ischemic ipsilateral and contralateral brain, as well as improved neurological function compared with the vehicle-control ABCA1^-B/-B stroke mice 14 days after stroke. Ischemic lesion volume was not significantly different between the two groups. In vitro supplementation of ApoE2 into primary cortical neurons and primary oligodendrocyte-progenitor cells (OPCs) significantly increased ApoER2 expression and enhanced cholesterol uptake. ApoE2 promoted neurite outgrowth after oxygen-glucose deprivation and axonal outgrowth of neurons, and increased proliferation/survival of OPCs derived from ABCA1^-B/-B mice. Our data indicate that administration of ApoE2 minimizes the adverse effects of ABCA1 deficiency after stroke, at least partially by promoting cholesterol traffic/redistribution and GM/WM remodeling via increasing the ApoE/HDL/ApoER2 signaling pathway.

P2X7 as a scavenger receptor for innate phagocytosis in the brain

The P2X7 receptor has been widely studied for its ATP-induced pro-inflammatory effect, but in the absence of a ligand, P2X7 has a second function as a scavenger receptor, which is active in the development of the human brain. The scavenger activity of P2X7 is only evident in the absence of serum but is fully active in cerebrospinal fluid. P2X7 on the cell surface is present as a membrane complex, and an attachment to non-muscle myosin of the cytoskeleton is required for particle engulfment. Selective antagonists of P2X7 pro-inflammatory function have little effect on phagocytosis, but inheritance of a variant haplotype spanning the P2RX7 and P2RX4 genes has been associated with loss of P2X7-mediated phagocytosis. Recent studies in mice suggest that the innate phagocytosis mediated by P2X7 receptors declines with ageing. Thus, defective P2X7-mediated phagocytosis may contribute to age-related neuro-degenerative diseases including Alzheimer's disease, age-related macular degeneration and primary progressive multiple sclerosis.

Apolipoprotein E Deficiency Exacerbates Spinal Cord Injury in Mice: Inflammatory Response and Oxidative Stress Mediated by NF-κB Signaling Pathway

Spinal cord injury (SCI) is a severe neurological trauma that involves complex pathological processes. Inflammatory response and oxidative stress are prevalent during the second injury and can influence the functional recovery of SCI. Specially, Apolipoprotein E (APOE) induces neuronal repair and nerve regeneration, and the deficiency of Apoe impairs spinal cord-blood-barrier and reduces functional recovery after SCI. However, the mechanism by which Apoe mediates signaling pathways of inflammatory response and oxidative stress in SCI remains largely elusive. This study was designed to investigate the signaling pathways that regulate Apoe deficiency-dependent inflammatory response and oxidative stress in the acute stage of SCI. In the present study, Apoe^−/− mice retarded functional recovery and had a larger lesion size when compared to wild-type mice after SCI. Moreover, deficiency of Apoe induced an exaggerated inflammatory response by increasing expression of interleukin-6 (IL-6) and interleukin-1β (IL-1β), and increased oxidative stress by reducing expression of Nrf2 and HO-1. Furthermore, lack of Apoe promoted neuronal apoptosis and decreased neuronal numbers in the anterior horn of the spinal cord after SCI. Mechanistically, we found that the absence of Apoe increased inflammation and oxidative stress through activation of NF-κB after SCI. In contrast, an inhibitor of nuclear factor-κB (NF-κB; Pyrrolidine dithiocarbamate) alleviates these changes. Collectively, these results indicate that a critical role for activation of NF-κB in regulating Apoe-deficiency dependent inflammation and oxidative stress is detrimental to recovery after SCI.

Detrimental Effects of Paraquat on Astrocytes-Regulating Synaptic Functions

Paraquat (PQ) exposure is known as a risk factor for developing neurodegenerative diseases. Astrocytes are implicated and affected in neurodegenerative diseases and brain injuries, so it is suspected that PQ may impose detrimental effects on astrocytes function. Here, we present a study of the effects of PQ on synaptic function of astrocytes. Human astrocytes (HA1800) were separately treated with PQ at 200, 400, and 800 µmol/L for 12, 24, and 48 hours, respectively. The concentrations of membrane cholesterol, intracellular glutamate, and adenosine triphosphate (ATP) were assessed by measuring changes with Kits after PQ treatment. Expression levels of glial fibrillary acidic protein (GFAP) and apolipoprotein E (ApoE) were detected using Western blot method. The results indicated that astrocytes were highly susceptible to PQ showing astrogliosis characterized by enhanced GFAP expression (P < .05). The decreased cholesterol level and ApoE expression were observed in PQ-treated astrocytes and so were the decreased levels of glutamates and ATP in PQ-treated astrocytes. These results suggested that PQ exerted the detrimental effects on synaptic function of astrocytes, which may be involved in the progressive process of neurodegenerative disorders.

Apolipoprotein E-Mimetic Peptide COG1410 Promotes Autophagy by Phosphorylating GSK-3β in Early Brain Injury Following Experimental Subarachnoid Hemorrhage

COG1410, a mimetic peptide derived from the apolipoprotein E (apoE) receptor binding region, exerts positive effect on neurological deficits in early brain injury (EBI) after experimental subarachnoid hemorrhage (SAH). Currently the neuroprotective effect of COG1410 includes inhibiting BBB disruption, reducing neuronal apoptosis, and neuroinflammation. However, the effect and mechanism of COG1410 to subcellular organelles disorder have not been fully investigated. As the main pathway for recycling long-lived proteins and damaged organelles, neuronal autophagy is activated in SAH and exhibits neuroprotective effects by reducing the insults of EBI. Pharmacologically elevated autophagy usually contributes to alleviated brain injury, while few of the agents achieved clinical transformation. In this study, we explored the activation of autophagy during EBI by measuring the Beclin-1 and LC3B-II protein levels. Administration of COG1410 notably elevated the autophagic markers expression in neurons, simultaneously reversed the neurological deficits. Furthermore, the up-regulated autophagy by COG1410 was further promoted by p-GSK-3β agonist, whereas decreased by p-GSK-3β inhibitor. Taken together, these data suggest that the COG1410 might be a promising therapeutic strategy for EBI via promoting autophagy in SAH.

Role of Apolipoprotein E in the tangled mystery of pain

Pain is one of the common and debilitating health manifestations associated with the majority of diseased conditions, thus making it a serious health concern worldwide. While trying to decipher the cryptic mechanism of pain in hope to provide better gene-based therapeutics, researchers have concluded pain to be of multigenic origin making it hard to cure. Apolipoprotein E is a protein coded by APOE gene containing 4 exons, located on chromosome 19q13.2. It is among the key regulators of various crucial body functions such as lipid transport, apoptosis, vitamin k pathway, and cognition, hence, it is highly suspected to play a pivotal role in the nociception process. However, very few studies have tried and succeeded to find a direct involvement of APOE in pain processing. The current article attempts to throw light on some of the major clinical research findings which strengthen the hypothesis stating that apolipoprotein E has a concealed yet deeply embedded association with the pain regulating pathways, through several underlying physiological, biochemical and neurological processes, that in turn, decide the fate of pain sensation in a complex manner.

Apolipoprotein E, Receptors, and Modulation of Alzheimer's Disease

Apolipoprotein E (apoE) is a lipid carrier in both the peripheral and the central nervous systems. Lipid-loaded apoE lipoprotein particles bind to several cell surface receptors to support membrane homeostasis and injury repair in the brain. Considering prevalence and relative risk magnitude, the ε4 allele of the APOE gene is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). ApoE4 contributes to AD pathogenesis by modulating multiple pathways, including but not limited to the metabolism, aggregation, and toxicity of amyloid-β peptide, tauopathy, synaptic plasticity, lipid transport, glucose metabolism, mitochondrial function, vascular integrity, and neuroinflammation. Emerging knowledge on apoE-related pathways in the pathophysiology of AD presents new opportunities for AD therapy. We describe the biochemical and biological features of apoE and apoE receptors in the central nervous system. We also discuss the evidence and mechanisms addressing differential effects of apoE isoforms and the role of apoE receptors in AD pathogenesis, with a particular emphasis on the clinical and preclinical studies related to amyloid-β pathology. Finally, we summarize the current strategies of AD therapy targeting apoE, and postulate that effective strategies require an apoE isoform-specific approach.

Role of Apolipoproteins and α-Synuclein in Parkinson's Disease

Parkinson's disease (PD) is a progressive brain disorder that interferes with activities of normal life. The main pathological feature of this disease is the loss of more than 80% of dopamine-producing neurons in the substantia nigra (SN). Dopaminergic neuronal cell death occurs when intraneuronal, insoluble, aggregated proteins start to form Lewy bodies (LBs), the most important component of which is a protein called α-synuclein (α-syn). α-Syn structurally contains hexameric repeats of 11 amino acids, which are characteristic of apolipoproteins and thus α-syn can also be considered an apolipoprotein. Moreover, apolipoproteins seem to be involved in the incidence and development of PD. Some apolipoproteins such as ApoD have a neuroprotective role in the brain. In PD, apoD levels increase in glial cells surrounding dopaminergic cells. However, elevated levels of some other apolipoproteins such as ApaA1 and ApoE are reported as a vulnerability factor of PD. At present, when a clinical diagnosis of PD is made, based on symptoms such as shaking, stiff muscles and slow movement, serious damage has already been done to nerve cells of the SN. The diagnosis of PD in its earlier stages, before this irreversible damage, would be of enormous benefit for future treatment strategies designed to slow or halt the progression of PD. This review presents the roles of apolipoproteins and α-syn in PD and how some of them could potentially be used as biomarkers for PD.

Exploring the potential of natural and synthetic neuroprotective steroids against neurodegenerative disorders: A literature review

Neurodegeneration is a complex process, which leads to progressive brain damage due to loss of neurons. Despite exhaustive research, the cause of neuronal loss in various degenerative disorders is not entirely understood. Neuroprotective steroids constitute an important line of attack, which could play a major role against the common mechanisms associated with various neurodegenerative disorders like Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Natural endogenous steroids induce the neuroprotection by protecting the nerve cells from neuronal injury through multiple mechanisms, therefore the structural modifications of the endogenous steroids could be helpful in the generation of new therapeutically useful neuroprotective agents. The review article will keep the readers apprised of the detailed description of natural as well as synthetic neuroprotective steroids from the medicinal chemistry point of view, which would be helpful in drug discovery efforts aimed toward neurodegenerative diseases.

LRP1-Dependent BMPER Signaling Regulates Lipopolysaccharide-Induced Vascular Inflammation

OBJECTIVE

Bacterial endotoxin (lipopolysaccharide)-mediated sepsis involves dysregulated systemic inflammation, which injures the lung and other organs, often fatally. Vascular endothelial cells act as both targets and mediators of lipopolysaccharide-induced inflammatory responses. Dysfunction of endothelium results in increases of proinflammatory cytokine production and permeability leakage. BMPER (bone morphogenetic protein-binding endothelial regulator), an extracellular modulator of bone morphogenetic protein signaling, has been identified as a vital component in chronic endothelial inflammatory responses and atherosclerosis. However, it is unclear whether BMPER also regulates inflammatory response in an acute setting such as sepsis. To address this question, we investigated the role of BMPER during lipopolysaccharide-induced acute lung injury.

APPROACH AND RESULTS

Mice missing 1 allele of BMPER (BMPER^+/- mice used in the place of BMPER^-/- mice that die at birth) were used for lipopolysaccharide challenge. Lipopolysaccharide-induced pulmonary inflammation and injury was reduced in BMPER^+/- mice as shown by several measures, including survival rate, infiltration of inflammatory cells, edema, and production of proinflammatory cytokines. Mechanistically, we have demonstrated that BMPER is required and sufficient for the activation of nuclear factor of activated T cells c1. This BMPER-induced nuclear factor of activated T cells activation is coordinated by multiple signaling pathways, including bone morphogenetic protein-independent low-density lipoprotein receptor-related protein 1-extracellular signal-regulated kinase activation, calcineurin signaling, and low-density lipoprotein receptor-related protein 1β-mediated nuclear factor 45 nuclear export in response to BMPER treatment.

CONCLUSIONS

We conclude that BMPER plays a pivotal role in pulmonary inflammatory response, which provides new therapeutic options against sepsis shock. The new signaling pathway initiated by BMPER/low-density lipoprotein receptor-related protein 1 axis broadens our understanding about BMPER's role in vascular homeostasis.

Low-Density Lipoprotein Receptor-Related Protein-1 Signaling in Angiogenesis

Low-density lipoprotein receptor-related protein-1 (LRP1) plays multifunctional roles in lipid homeostasis, signaling transduction, and endocytosis. It has been recognized as an endocytic receptor for many ligands and is involved in the signaling pathways of many growth factors or cytokines. Dysregulation of LRP1-dependent signaling events contributes to the development of pathophysiologic processes such as Alzheimer’s disease, atherosclerosis, inflammation, and coagulation. Interestingly, recent studies have linked LRP1 with endothelial function and angiogenesis, which has been underappreciated for a long time. During zebrafish embryonic development, LRP1 is required for the formation of vascular network, especially for the venous development. LRP1 depletion in the mouse embryo proper leads to angiogenic defects and disruption of endothelial integrity. Moreover, in a mouse oxygen-induced retinopathy model, specific depletion of LRP1 in endothelial cells results in abnormal development of neovessels. These loss-of-function studies suggest that LRP1 plays a pivotal role in angiogenesis. The review addresses the recent advances in the roles of LRP1-dependent signaling during angiogenesis.

Role of LRP1 in the pathogenesis of Alzheimer's disease: evidence from clinical and preclinical studies

Among the LDL receptor (LDLR) family members, the roles of LDLR-related protein (LRP)1 in the pathogenesis of Alzheimer's disease (AD), especially late-onset AD, have been the most studied by genetic, neuropathological, and biomarker analyses (clinical studies) or cellular and animal model systems (preclinical studies) over the last 25 years. Although there are some conflicting reports, accumulating evidence from preclinical studies indicates that LRP1 not only regulates the metabolism of amyloid-β peptides (Aβs) in the brain and periphery, but also maintains brain homeostasis, impairment of which likely contributes to AD development in Aβ-independent manners. Several preclinical studies have also demonstrated an involvement of LRP1 in regulating the pathogenic role of apoE, whose gene is the strongest genetic risk factor for AD. Nonetheless, evidence from clinical studies is not sufficient to conclude how LRP1 contributes to AD development. Thus, despite very promising results from preclinical studies, the role of LRP1 in AD pathogenesis remains to be further clarified. In this review, we discuss the potential mechanisms underlying how LRP1 affects AD pathogenesis through Aβ-dependent and -independent pathways by reviewing both clinical and preclinical studies. We also discuss potential therapeutic strategies for AD by targeting LRP1.

Neuroserpin Attenuates H2O2-Induced Oxidative Stress in Hippocampal Neurons via AKT and BCL-2 Signaling Pathways

Oxidative stress plays a critical role in neuronal injury and is associated with various neurological diseases. Here, we explored the potential protective effect of neuroserpin against oxidative stress in primary cultured hippocampal neurons. Our results show that neuroserpin inhibits H2O2-induced neurotoxicity in hippocampal cultures as measured by WST, LDH release, and TUNEL assays. We found that neuroserpin enhanced the activation of AKT in cultures subjected to oxidative stress and that the AKT inhibitor Ly294002 blocked this neuroprotective effect. Neuroserpin increased the expression of the anti-apoptotic protein BCL-2 and blocked the activation of caspase-3. Neuroserpin did not increase the level of neuroprotection over levels seen in neurons transduced with a BCL-2 expression vector, and an inhibitor of Trk receptors, K252a, did not block neuroserpin's effect. Taken together, our study demonstrates that neuroserpin protects against oxidative stress-induced dysfunction and death of primary cultured hippocampal neurons through the AKT-BCL-2 signaling pathway through a mechanism that does not involve the Trk receptors and leads to inhibition of caspase-3 activation.

The activities of LDL Receptor-related Protein-1 (LRP1) compartmentalize into distinct plasma membrane microdomains

LDL Receptor-related Protein-1 (LRP1) is an endocytic receptor for diverse ligands. In neurons and neuron-like cells, ligand-binding to LRP1 initiates cell-signaling. Herein, we show that in PC12 and N2a neuron-like cells, LRP1 distributes into lipid rafts and non-raft plasma membrane fractions. When lipid rafts were disrupted, using methyl-β-cyclodextrin or fumonisin B1, activation of Src family kinases and ERK1/2 by the LRP1 ligands, tissue-type plasminogen activator and activated α₂-macroglobulin, was blocked. Biological consequences of activated LRP1 signaling, including neurite outgrowth and cell growth, also were blocked. The effects of lipid raft disruption on ERK1/2 activation and neurite outgrowth, in response to LRP1 ligands, were reproduced in experiments with cerebellar granule neurons in primary culture. Because the reagents used to disrupt lipid rafts may have effects on the composition of the plasma membrane outside lipid rafts, we studied the effects of these reagents on LRP1 activities unrelated to cell-signaling. Lipid raft disruption did not affect the total ligand binding capacity of LRP1, the affinity of LRP1 for its ligands, or its endocytic activity. These results demonstrate that well described activities of LRP1 require localization of this receptor to distinct plasma membrane microdomains.