Proprotein Convertase Subtilisin/Kexin Type 9, Brain Cholesterol Homeostasis and Potential Implication for Alzheimer's Disease

Liver as a new target organ in Alzheimer's disease: insight from cholesterol metabolism and its role in amyloid-beta clearance

Alzheimer's disease, the primary cause of dementia, is characterized by neuropathologies, such as amyloid plaques, synaptic and neuronal degeneration, and neurofibrillary tangles. Although amyloid plaques are the primary characteristic of Alzheimer's disease in the central nervous system and peripheral organs, targeting amyloid-beta clearance in the central nervous system has shown limited clinical efficacy in Alzheimer's disease treatment. Metabolic abnormalities are commonly observed in patients with Alzheimer's disease. The liver is the primary peripheral organ involved in amyloid-beta metabolism, playing a crucial role in the pathophysiology of Alzheimer's disease. Notably, impaired cholesterol metabolism in the liver may exacerbate the development of Alzheimer's disease. In this review, we explore the underlying causes of Alzheimer's disease and elucidate the role of the liver in amyloid-beta clearance and cholesterol metabolism. Furthermore, we propose that restoring normal cholesterol metabolism in the liver could represent a promising therapeutic strategy for addressing Alzheimer's disease.

Targeting proprotein convertase subtilisin/kexin type 9 (PCSK9): from bench to bedside

Proprotein convertase subtilisin/kexin type 9 (PCSK9) has evolved as a pivotal enzyme in lipid metabolism and a revolutionary therapeutic target for hypercholesterolemia and its related cardiovascular diseases (CVD). This comprehensive review delineates the intricate roles and wide-ranging implications of PCSK9, extending beyond CVD to emphasize its significance in diverse physiological and pathological states, including liver diseases, infectious diseases, autoimmune disorders, and notably, cancer. Our exploration offers insights into the interaction between PCSK9 and low-density lipoprotein receptors (LDLRs), elucidating its substantial impact on cholesterol homeostasis and cardiovascular health. It also details the evolution of PCSK9-targeted therapies, translating foundational bench discoveries into bedside applications for optimized patient care. The advent and clinical approval of innovative PCSK9 inhibitory therapies (PCSK9-iTs), including three monoclonal antibodies (Evolocumab, Alirocumab, and Tafolecimab) and one small interfering RNA (siRNA, Inclisiran), have marked a significant breakthrough in cardiovascular medicine. These therapies have demonstrated unparalleled efficacy in mitigating hypercholesterolemia, reducing cardiovascular risks, and have showcased profound value in clinical applications, offering novel therapeutic avenues and a promising future in personalized medicine for cardiovascular disorders. Furthermore, emerging research, inclusive of our findings, unveils PCSK9's potential role as a pivotal indicator for cancer prognosis and its prospective application as a transformative target for cancer treatment. This review also highlights PCSK9's aberrant expression in various cancer forms, its association with cancer prognosis, and its crucial roles in carcinogenesis and cancer immunity. In conclusion, this synthesized review integrates existing knowledge and novel insights on PCSK9, providing a holistic perspective on its transformative impact in reshaping therapeutic paradigms across various disorders. It emphasizes the clinical value and effect of PCSK9-iT, underscoring its potential in advancing the landscape of biomedical research and its capabilities in heralding new eras in personalized medicine.

PCSK9 ablation attenuates Aβ pathology, neuroinflammation and cognitive dysfunctions in 5XFAD mice

BACKGROUND

Increasing evidence highlights the importance of novel players in Alzheimer's disease (AD) pathophysiology, including alterations of lipid metabolism and neuroinflammation. Indeed, a potential involvement of Proprotein convertase subtilisin/kexin type 9 (PCSK9) in AD has been recently postulated. Here, we first investigated the effects of PCSK9 on neuroinflammation in vitro. Then, we examined the impact of a genetic ablation of PCSK9 on cognitive performance in a severe mouse model of AD. Finally, in the same animals we evaluated the effect of PCSK9 loss on Aβ pathology, neuroinflammation, and brain lipids.

METHODS

For in vitro studies, U373 human astrocytoma cells were treated with Aβ fibrils and human recombinant PCSK9. mRNA expression of the proinflammatory cytokines and inflammasome-related genes were evaluated by q-PCR, while MCP-1 secretion was measured by ELISA. For in vivo studies, the cognitive performance of a newly generated mouse line - obtained by crossing 5XFADHet with PCSK9KO mice - was tested by the Morris water maze test. After sacrifice, immunohistochemical analyses were performed to evaluate Aβ plaque deposition, distribution and composition, BACE1 immunoreactivity, as well as microglia and astrocyte reactivity. Cholesterol and hydroxysterols levels in mouse brains were quantified by fluorometric and LC-MS/MS analyses, respectively. Statistical comparisons were performed according to one- or two-way ANOVA, two-way repeated measure ANOVA or Chi-square test.

RESULTS

In vitro, PCSK9 significantly increased IL6, IL1B and TNFΑ mRNA levels in Aβ fibrils-treated U373 cells, without influencing inflammasome gene expression, except for an increase in NLRC4 mRNA levels. In vivo, PCSK9 ablation in 5XFAD mice significantly improved the performance at the Morris water maze test; these changes were accompanied by a reduced corticohippocampal Aβ burden without affecting plaque spatial/regional distribution and composition or global BACE1 expression. Furthermore, PCSK9 loss in 5XFAD mice induced decreased microgliosis and astrocyte reactivity in several brain regions. Conversely, knocking out PCSK9 had minimal impact on brain cholesterol and hydroxysterol levels.

CONCLUSIONS

In vitro studies showed a pro-inflammatory effect of PCSK9. Consistently, in vivo data indicated a protective role of PCSK9 ablation against cognitive impairments, associated with improved Aβ pathology and attenuated neuroinflammation in a severe mouse model of AD. PCSK9 may thus be considered a novel pharmacological target for the treatment of AD.

PCSK9 and the nervous system: a no-brainer?

In the past 20 years, PCSK9 has been shown to play a pivotal role in LDL cholesterol metabolism and cardiovascular health by inducing the lysosomal degradation of the LDL receptor. PCSK9 was discovered by the cloning of genes up-regulated after apoptosis induced by serum deprivation in primary cerebellar neurons, but despite its initial identification in the brain, the precise role of PCSK9 in the nervous system remains to be clearly established. The present article is a comprehensive review of studies published or in print before July 2023 that have investigated the expression pattern of PCSK9, its effects on lipid metabolism as well as its putative roles specifically in the central and peripheral nervous systems, with a special focus on cerebrovascular and neurodegenerative diseases.

Resveratrol Attenuates the Disruption of Lipid Metabolism Observed in Amyloid Precursor Protein/Presenilin 1 Mouse Brains and Cultured Primary Neurons Exposed to Aβ

To examine whether resveratrol (RSV), an activator of silent mating-type information regulation 2 homolog 1 (SIRT1), can reverse the disruption of lipid metabolism caused by β-amyloid peptide (Aβ), APP/PS1 mice or cultured primary rat neurons were treated with RSV, suramin (inhibitor of SIRT1), ZLN005, a stimulator of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), or PGC-1α silencing RNA. In the brains of the APP/PS1 mice, expressions of SIRT1, PGC-1α, low-density lipoprotein receptor (LDLR) and very LDLR (VLDLR) were reduced at the protein and, in some cases, mRNA levels; while the levels of the proprotein convertase subtilisin/kexin type 9 (PCSK9), apolipoprotein E (ApoE), total cholesterol and LDL were all elevated. Interestingly, these changes were reversed by administration of RSV, while being aggravated by suramin. Furthermore, activation of PGC-1α, but inhibition of SIRT1, decreased the levels of PCSK9 and ApoE, while increased those of LDLR and VLDLR in the neurons exposed to Aβ, and silencing PGC-1α, but activation of SIRT1, did not influence the levels of any of these proteins. These findings indicate that RSV can attenuate the disruption of lipid metabolism observed in the brains of APP mice and in primary neurons exposed to Aβ by activating SIRT1, in which the mechanism may involve subsequently affecting PGC-1α.

Proteomics and Lipidomics to unveil the contribution of PCSK9 beyond cholesterol lowering: a narrative review

Proprotein convertase subtilisin/kexin type 9 (PCSK9), one of the key regulators of the low-density lipoprotein receptor (LDLR), can play a direct role in atheroma development. Although advances in the understandings of genetic PCSK9 polymorphisms have enabled to reveal the role of PCSK9 in the complex pathophysiology of cardiovascular diseases (CVDs), increasing lines of evidence support non-cholesterol-related processes mediated by PCSK9. Owing to major improvements in mass spectrometry-based technologies, multimarker proteomic and lipidomic panels hold the promise to identify novel lipids and proteins potentially related to PCSK9. Within this context, this narrative review aims to provide an overview of the most significant proteomics and lipidomics studies related to PCSK9 effects beyond cholesterol lowering. These approaches have enabled to unveil non-common targets of PCSK9, potentially leading to the development of novel statistical models for CVD risk prediction. Finally, in the era of precision medicine, we have reported the impact of PCSK9 on extracellular vesicles (EVs) composition, an effect that could contribute to an increased prothrombotic status in CVD patients. The possibility to modulate EVs release and cargo could help counteract the development and progression of the atherosclerotic process.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) in the central nervous system

The gene encoding proprotein convertase subtilisin/kexin type 9 (PCSK9) and its protein product have been widely studied for their role in cholesterol and lipid metabolism. PCSK9 increases the rate of metabolic degradation of low-density lipoprotein receptors, preventing the diffusion of low-density lipoprotein (LDL) from plasma into cells and contributes to high lipoprotein-bound cholesterol levels in the plasma. While most research has focused on the regulation and disease relevance of PCSK9 to the cardiovascular system and lipid metabolism, there is a growing body of evidence that PCSK9 plays a crucial role in pathogenic processes in other organ systems, including the central nervous system. PCSK9's impact on the brain is not yet fully understood, though several recent studies have sought to illuminate its impact on various neurodegenerative and psychiatric disorders, as well as its connection with ischemic stroke. Cerebral PCSK9 expression is low but is highly upregulated during disease states. Among others, PCSK9 is known to play a role in neurogenesis, neural cell differentiation, central LDL receptor metabolism, neural cell apoptosis, neuroinflammation, Alzheimer's Disease, Alcohol Use Disorder, and stroke. The PCSK9 gene contains several polymorphisms, including both gain-of-function and loss-of-function mutations which profoundly impact normal PCSK9 signaling and cholesterol metabolism. Gain-of-function mutations lead to persistent hypercholesterolemia and poor health outcomes, while loss-of-function mutations generally lead to hypocholesterolemia and may serve as a protective factor against diseases of the liver, cardiovascular system, and central nervous system. Recent genomic studies have sought to identify the end-organ effects of such mutations and continue to identify evidence of a much broader role for PCSK9 in extrahepatic organ systems. Despite this, there remain large gaps in our understanding of PCSK9, its regulation, and its effects on disease risk outside the liver. This review, which incorporates data from a wide range of scientific disciplines and experimental paradigms, is intended to describe PCSK9's role in the central nervous system as it relates to cerebral disease and neuropsychiatric disorders, and to examine the clinical potential of PCSK9 inhibitors and genetic variation in the PCSK9 gene on disease outcomes, including neurological and neuropsychiatric disease.

MS4A4A modifies the risk of Alzheimer disease by regulating lipid metabolism and immune response in a unique microglia state

Genome-wide association studies (GWAS) have identified many modifiers of Alzheimer disease (AD) risk enriched in microglia. Two of these modifiers are common variants in the MS4A locus (rs1582763: protective and rs6591561: risk) and serve as major regulators of CSF sTREM2 levels. To understand their functional impact on AD, we used single nucleus transcriptomics to profile brains from carriers of these variants. We discovered a "chemokine" microglial subpopulation that is altered in MS4A variant carriers and for which MS4A4A is the major regulator. The protective variant increases MS4A4A expression and shifts the chemokine microglia subpopulation to an interferon state, while the risk variant suppresses MS4A4A expression and reduces this subpopulation of microglia. Our findings provide a mechanistic explanation for the AD variants in the MS4A locus. Further, they pave the way for future mechanistic studies of AD variants and potential therapeutic strategies for enhancing microglia resilience in AD pathogenesis.

Recent Update on the Development of PCSK9 Inhibitors for Hypercholesterolemia Treatment

The proprotein convertase subtilisin/kexin-type 9 (PCSK9) binds to low-density lipoprotein receptors (LDLR), thereby trafficking them to lysosomes upon endocytosis and enhancing intracellular degradation to prevent their recycling. As a result, the levels of circulating LDL cholesterol (LDL-C) increase, which is a prominent risk factor for developing atherosclerotic cardiovascular diseases (ASCVD). Thus, PCSK9 has become a promising therapeutic target that offers a fertile testing ground for new drug modalities to regulate plasma LDL-C levels to prevent ASCVD. In this review, we have discussed the role of PCSK9 in lipid metabolism and briefly summarized the current clinical status of modalities targeting PCSK9. In particular, a detailed overview of peptide-based PCSK9 inhibitors is presented, which emphasizes their structural features and design, therapeutic effects on patients, and preclinical cardiovascular disease (CVD) models, along with PCSK9 modulation mechanisms. As a promising alternative to monoclonal antibodies (mAbs) for managing LDL-C, anti-PCSK9 peptides are emerging as a prospective next generation therapy.

PCSK9 Affects Astrocyte Cholesterol Metabolism and Reduces Neuron Cholesterol Supplying In Vitro: Potential Implications in Alzheimer's Disease

The Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) involvement in Alzheimer’s disease (AD) is poorly investigated. We evaluated the in vitro PCSK9 modulation of astrocyte cholesterol metabolism and neuronal cholesterol supplying, which is fundamental for neuronal functions. Moreover, we investigated PCSK9 neurotoxic effects. In human astrocytoma cells, PCSK9 reduced cholesterol content (−20%; p < 0.05), with a greater effect in presence of beta amyloid peptide (Aβ) (−37%; p < 0.01). PCSK9 increased cholesterol synthesis and reduced the uptake of apoE-HDL-derived cholesterol (−36%; p < 0.0001), as well as the LDL receptor (LDLR) and the apoE receptor 2 (ApoER2) expression (−66% and −31%, respectively; p < 0.01). PCSK9 did not modulate ABCA1- and ABCG1-cholesterol efflux, ABCA1 levels, or membrane cholesterol. Conversely, ABCA1 expression and activity, as well as membrane cholesterol, were reduced by Aβ (p < 0.05). In human neuronal cells, PCSK9 reduced apoE-HDL-derived cholesterol uptake (−41%; p < 0.001) and LDLR/apoER2 expression (p < 0.05). Reduced cholesterol internalization occurred also in PCSK9-overexpressing neurons exposed to an astrocyte-conditioned medium (−39%; p < 0.001). PCSK9 reduced neuronal cholesterol content overall (−29%; p < 0.05) and increased the Aβ-induced neurotoxicity (p < 0.0001). Our data revealed an interfering effect of PCSK9, in cooperation with Aβ, on brain cholesterol metabolism leading to neuronal cholesterol reduction, a potentially deleterious effect. PCSK9 also exerted a neurotoxic effect, and thus represents a potential pharmacological target in AD.

Relationship between Brain Metabolic Disorders and Cognitive Impairment: LDL Receptor Defect

The low-density-lipoprotein receptor (LDLr) removes low-density lipoprotein (LDL), an endovascular transporter that carries cholesterol from the bloodstream to peripheral tissues. The maintenance of cholesterol content in the brain, which is important to protect brain function, is affected by LDLr. LDLr co-localizes with the insulin receptor and complements the internalization of LDL. In LDLr deficiency, LDL blood levels and insulin resistance increase, leading to abnormal cholesterol control and cognitive deficits in atherosclerosis. Defects in brain cholesterol metabolism lead to neuroinflammation and blood–brain-barrier (BBB) degradation. Moreover, interactions between endoplasmic reticulum stress (ER stress) and mitochondria are induced by ox-LDL accumulation, apolipoprotein E (ApoE) regulates the levels of amyloid beta (Aβ) in the brain, and hypoxia is induced by apoptosis induced by the LDLr defect. This review summarizes the association between neurodegenerative brain disease and typical cognitive deficits.

Effects of PCSK9 inhibitors on HDL cholesterol efflux and serum cholesterol loading capacity in familial hypercholesterolemia subjects: a multi-lipid-center real-world evaluation

Proprotein convertase subtilisin/kexin type 9 (PCSK9), beyond regulating LDL cholesterol (LDL-c) plasma levels, exerts several pleiotropic effects by modulating lipid metabolism in extrahepatic cells such as macrophages. Macrophage cholesterol homeostasis depends on serum lipoprotein functions, including the HDL capacity to promote cell cholesterol efflux (CEC) and the serum capacity to promote cell cholesterol loading (CLC). The aim of this observational study was to investigate the effect of PCSK9 inhibitors (PCSK9-i) treatment on HDL-CEC and serum CLC in patients with familial hypercholesterolemia (FH). 31 genetically confirmed FH patients were recruited. Blood was collected and serum isolated at baseline and after 6 months of PCSK9-i treatment. HDL-CEC was evaluated through the main pathways with a radioisotopic cell-based assay. Serum CLC was assessed fluorimetrically in human THP-1 monocyte-derived macrophages. After treatment with PCSK9-i, total cholesterol and LDL-c significantly decreased (−41.6%, p < 0.0001 and −56.7%, p < 0.0001, respectively). Total HDL-CEC was not different between patients before and after treatment. Conversely, despite no changes in HDL-c levels between the groups, ABCG1 HDL-CEC significantly increased after treatment (+22.2%, p < 0.0001) as well as HDL-CEC by aqueous diffusion (+7.8%, p = 0.0008). Only a trend towards reduction of ABCA1 HDL-CEC was observed after treatment. PCSK9-i significantly decreased serum CLC (−6.6%, p = 0.0272). This effect was only partly related to the reduction of LDL-c levels. In conclusion, PCSK9-i treatment significantly increased HDL-CEC through ABCG1 and aqueous diffusion pathways and reduced the serum CLC in FH patients. The favorable effect of PCSK9-i on functional lipid profile could contribute to the cardiovascular benefit of these drugs in FH patients.

No association between APOE genotype and lipid lowering with cognitive function in a randomized controlled trial of evolocumab

APOE encodes a cholesterol transporter, and the ε4 allele is associated with higher circulating cholesterol levels, ß-amyloid burden, and risk of Alzheimer's disease. Prior studies demonstrated no significant differences in objective or subjective cognitive function for patients receiving the PCSK9 inhibitor evolocumab vs. placebo added to statin therapy. There is some evidence that cholesterol-lowering medications may confer greater cognitive benefits in APOE ε4 carriers. Thus, the purpose of this study was to determine whether APOE genotype moderates the relationships between evolocumab use and cognitive function. APOE-genotyped patients (N = 13,481; 28% ε4 carriers) from FOURIER, a randomized, placebo-controlled trial of evolocumab added to statin therapy in patients with stable atherosclerotic cardiovascular disease followed for a median of 2.2 years, completed the Everyday Cognition Scale (ECog) to self-report cognitive changes from the end of the trial compared to its beginning; a subset (N = 835) underwent objective cognitive testing using the Cambridge Neuropsychological Test Automated Battery as part of the EBBINGHAUS trial. There was a dose-dependent relationship between APOE ε4 genotype and patient-reported memory decline on the ECog in the placebo arm (p = .003 for trend across genotypes; ε4/ε4 carriers vs. non-carriers: OR = 1.46, 95% CI [1.03, 2.08]) but not in the evolocumab arm (p = .50, OR = 1.18, 95% CI [.83,1.66]). However, the genotype by treatment interaction was not significant (p = .30). In the subset of participants who underwent objective cognitive testing with the CANTAB, APOE genotype did not significantly modify the relationship between treatment arm and CANTAB performance after adjustment for demographic and medical covariates, (p's>.05). Although analyses were limited by the low population frequency of the ε4/ε4 genotype, this supports the cognitive safety of evolocumab among ε4 carriers, guiding future research on possible benefits of cholesterol-lowering medications in people at genetic risk for Alzheimer's disease.

The Emerging Roles of Intracellular PCSK9 and Their Implications in Endoplasmic Reticulum Stress and Metabolic Diseases

The importance of the proprotein convertase subtilisin/kexin type-9 (PCSK9) gene was quickly recognized by the scientific community as the third locus for familial hypercholesterolemia. By promoting the degradation of the low-density lipoprotein receptor (LDLR), secreted PCSK9 protein plays a vital role in the regulation of circulating cholesterol levels and cardiovascular disease risk. For this reason, the majority of published works have focused on the secreted form of PCSK9 since its initial characterization in 2003. In recent years, however, PCSK9 has been shown to play roles in a variety of cellular pathways and disease contexts in LDLR-dependent and -independent manners. This article examines the current body of literature that uncovers the intracellular and LDLR-independent roles of PCSK9 and also explores the many downstream implications in metabolic diseases.

Emerging role of HDL in brain cholesterol metabolism and neurodegenerative disorders

High-density lipoproteins (HDLs play a key role in cholesterol homeostasis maintenance in the central nervous system (CNS), by carrying newly synthesized cholesterol from astrocytes to neurons, to support their lipid-related physiological functions. As occurs for plasma HDLs, brain lipoproteins are assembled through the activity of membrane cholesterol transporters, undergo remodeling mediated by specific enzymes and transport proteins, and finally deliver cholesterol to neurons by a receptor-mediated internalization process. A growing number of evidences indicates a strong association between alterations of CNS cholesterol homeostasis and neurodegenerative disorders, in particular Alzheimer's disease (AD), and a possible role in this relationship may be played by defects in brain HDL metabolism. In the present review, we summarize and critically examine the current state of knowledge on major modifications of HDL and HDL-mediated brain cholesterol transport in AD, by taking into consideration the individual steps of this process. We also describe potential and encouraging HDL-based therapies that could represent new therapeutic strategies for AD treatment. Finally, we revise the main plasma and brain HDL modifications in other neurodegenerative disorders including Parkinson's disease (PD), Huntington's disease (HD), and frontotemporal dementia (FTD).

Lowering blood cholesterol does not affect neuroinflammation in experimental autoimmune encephalomyelitis

Background

Multiple sclerosis (MS) is a chronic disabling disease of the central nervous system (CNS) commonly affecting young adults. There is increasing evidence that environmental factors are important in the development and course of MS. The metabolic syndrome (MetS) which comprises dyslipidemia has been associated with a worse outcome in MS disease. Furthermore, the lipid-lowering drug class of statins has been proposed to improve MS disease course. However, cholesterol is also rate-limiting for myelin biogenesis and promotes remyelination in MS animal models. Thus, the impact of circulating blood cholesterol levels during the disease remains debated and controversial.

Methods

We assessed the role of circulating cholesterol on the murine model of MS, the experimental autoimmune encephalomyelitis (EAE) disease using two different approaches: (1) the mouse model of familial hypercholesterolemia induced by low-density lipoprotein receptor (LDLr) deficiency, and (2) the use of the monoclonal anti-PCSK9 neutralizing antibody alirocumab, which reduces LDLr degradation and consequently lowers blood levels of cholesterol.

Results

Elevated blood cholesterol levels induced by LDLr deficiency did not worsen clinical symptoms of mice during EAE. In addition, we observed that the anti-PCSK9 antibody alirocumab did not influence EAE disease course, nor modulate the immune response in EAE.

Conclusions

These findings suggest that blood cholesterol level has no direct role in neuro-inflammatory diseases and that the previously shown protective effects of statins in MS are not related to circulating cholesterol.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02409-x.

Development of small-molecule PCSK9 inhibitors for the treatment of hypercholesterolemia

When secreted into the circulation, proprotein convertase subtilisin kexin type 9 (PCSK9) blocks the low-density lipoprotein receptors (LDL-R) and, as a consequence, low-density lipoprotein cholesterol (LDL-C) levels increase. Therefore, PCSK9 has emerged as a potential therapeutic target for lowering LDL-C levels and preventing atherosclerosis. The US Food and Drug Administration (FDA) has approved two monoclonal antibodies (mAbs) against PCSK9, but the expensive manufacturing process limits their use. Subsequently, there have been tremendous efforts to develop cost-effective small molecules specific to PCSK9 over the past few years. These small molecules are promising therapeutics that act by preventing the synthesis of PCSK9, its secretion from cells, or the PCSK9-LDRL interaction. In this review, we summarize recent developments in the discovery of small-molecule PCSK9 inhibitors, focusing on their design, therapeutic effects, specific targets, and mechanisms of action.

Neurodegenerative Diseases and Cholesterol: Seeing the Field Through the Players

Neurodegenerative diseases, namely Alzheimer’s (AD), Parkinson’s (PD), and Huntington’s disease (HD) together with amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS), devastate millions of lives per year worldwide and impose an increasing socio-economic burden across nations. Consequently, these diseases occupy a considerable portion of biomedical research aiming to understand mechanisms of neurodegeneration and to develop efficient treatments. A potential culprit is cholesterol serving as an essential component of cellular membranes, as a cofactor of signaling pathways, and as a precursor for oxysterols and hormones. This article uncovers the workforce studying research on neurodegeneration and cholesterol using the TeamTree analysis. This new bibliometric approach reveals the history and dynamics of the teams and exposes key players based on citation-independent metrics. The team-centered view reveals the players on an important field of biomedical research.

Platelet-Derived PCSK9 Is Associated with LDL Metabolism and Modulates Atherothrombotic Mechanisms in Coronary Artery Disease

Platelets play a significant role in atherothrombosis. Proprotein convertase subtilisin/kexin type 9 (PCSK9) is critically involved in the regulation of LDL metabolism and interacts with platelet function. The effect of PCSK9 in platelet function is poorly understood. The authors of this article sought to characterize platelets as a major source of PCSK9 and PCSK9's role in atherothrombosis. In a large cohort of patients with coronary artery disease (CAD), platelet count, platelet reactivity, and platelet-derived PCSK9 release were analyzed. The role of platelet PCSK9 on platelet and monocyte function was investigated in vitro. Platelet count and hyper-reactivity correlated with plasma LDL in CAD. The circulating platelets express on their surface and release substantial amounts of PCSK9. Release of PCSK9 augmented platelet-dependent thrombosis, monocyte migration, and differentiation into macrophages/foam cells. Platelets and PCSK9 accumulated in tissue derived from atherosclerotic carotid arteries in areas of macrophages. PCSK9 inhibition reduced platelet activation and platelet-dependent thrombo-inflammation. The authors identified platelets as a source of PCSK9 in CAD, which may have an impact on LDL metabolism. Furthermore, platelet-derived PCSK9 contributes to atherothrombosis, and inhibition of PCSK9 attenuates thrombo-inflammation, which may contribute to the reported beneficial clinical effects.

Proprotein Convertase Subtilisin/Kexin Type 9: A View beyond the Canonical Cholesterol-Lowering Impact

Proprotein convertase subtilisin/kexin type 9 (PCSK9), mainly synthetized and released by the liver, represents one of the key regulators of low-density lipoprotein cholesterol. Although genetic and interventional studies have demonstrated that lowering PCSK9 levels corresponds to a cardiovascular benefit, identification of non-cholesterol-related processes has emerged since its discovery. Besides liver, PCSK9 is also expressed in many tissues (eg, intestine, endocrine pancreas, and brain). The aim of the present review is to describe and discuss PCSK9 pathophysiology and possible non-lipid-lowering effects whether already extensively characterized (eg, inflammatory burden of atherosclerosis, triglyceride-rich lipoprotein metabolism, and platelet activation), or to be unraveled (eg, in adipose tissue). The identification of novel transcriptional factors in the promoter region of human PCSK9 (eg, ChREBP) characterizes new mechanisms explaining how controlling intrahepatic glucose may be a therapeutic strategy to reduce cardiovascular risk in type 2 diabetes. Finally, the evidence describing PCSK9 as involved in cell proliferation and apoptosis raises the possibility of this protein being involved in cancer risk.

Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) inhibitors and the risk for neurocognitive adverse events: A systematic review, meta-analysis and meta-regression

BACKGROUND

It has been suggested that lipid lowering therapy causes impaired cognitive changes. The association between the use of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) inhibitors and the risk of neurocognitive adverse events remains unclear. This meta-analysis aims to assess neurocognitive safety of PCSK9 inhibitors in randomized controlled trials (RCTs).

METHODS AND RESULTS

The research was conducted according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). PubMed (MEDLINE), Embase and Cochrane library were searched through September 2019. Selection criteria included RCTs that addressed to neurocognitive adverse events of participants using Alirocumab, Evolocumab or Bococizumab, with a follow up duration of at least 6 months. The search results were screened by two independent reviewers. Safety data from included papers were extracted. Random effects meta-analysis was used to pool results, and meta-regression was utilized when applicable. Twenty-one studies were included. Among 59,733 patients, 31,611 were treated with PCSK9 inhibitors. The follow-up period ranged from 24 weeks to 48 months. No significant difference in the incidence of neurocognitive adverse effects between the groups was identified (RR = 1.01, 95% CI: 0.86-1.19, I² = 3%). Similar results were seen in subgroup analysis for each of the medications (alirocumab- RR = 0.88, 95% CI: 0.72-1.08, I² = 0%, evolocumab- RR = 1.42, 95% CI: 0.74-2.73, I² = 55%). A meta-regression analysis for evolocumab revealed that prolonged study duration was associated with decreased risk for neurocognitive adverse events (β_week = -0.0037, p-value = 0.03).

CONCLUSIONS

Pooled results of our meta-analysis and meta-regression show that exposure to PCSK9 inhibitors is not associated with an increased risk of neurocognitive adverse effects.

Maternal exposure to air pollutants, PCSK9 levels, fetal growth and gestational age - An Italian cohort

OBJECTIVE

Exposure to airborne pollutants during pregnancy appears to be associated with uterine growth restriction and adverse neonatal outcome. Proprotein convertase subtilisin/kexin type (PCSK9), the key modulator of low-density lipoprotein (LDL) metabolism, increases following particulate matter (PM₁₀) exposure. Because maternal cholesterol is required for fetal growth, PCSK9 levels could be used to evaluate the potential impact of airborne pollutants on fetal growth.

DESIGN

A cohort of 134 healthy women during early pregnancy (11-12 weeks of gestational age) was studied.

RESULTS

A significant association between circulating PCSK9 levels and three tested air pollutants (PM₁₀, PM_2.5, nitric oxide (NO₂)) was found. Of importance, gestational age at birth was reduced by approximately 1 week for each 100 ng/mL rise in circulating PCSK9 levels, an effect that became more significant at the highest quartile of PM_2.5 (with a 1.8 week advance in delivery date for every 100 ng/mL rise in circulating PCSK9; p for interaction = 0.026). This finding was supported by an elevation of the odds ratio for urgent cesarean delivery for each 100 ng/mL rise in PCSK9 (2.99, 95% CI, 1.22-6.57), similar trends being obtained for PM₁₀ and NO₂.

CONCLUSIONS

The association between exposure to air pollutants during pregnancy and elevation in PCSK9 advances our understanding of the unforeseen influences of environmental exposure in terms of pregnancy associated disorders.

Anti-PCSK 9 antibodies increase the ratios of the brain-specific oxysterol 24S-hydroxycholesterol to cholesterol and to 27-hydroxycholesterol in the serum

AIMS

The serum ratios of the brain-specific oxysterol 24S-hydroxycholesterol (24S-OHC) to cholesterol and to 27-OHC reflect brain cholesterol turnover. We studied the effect of proprotein convertase subtilisin/kexin type 9 monoclonal antibodies (PCSK9ab) that enhance low-density lipoprotein receptor activity on serum cholesterol and oxysterol concentrations.

METHODS

Twenty-eight hypercholesterolaemic patients (15 males and 13 females) responding insufficiently to maximally tolerated statin and/or ezetimibe therapy were additionally subcutanously treated biweekly with either the PCSK9ab alirocumab (150 mg, n = 13) or evolocumab (140 mg, n = 15). Fasting serum cholesterol was measured by gas chromatography and the oxysterols 24S-OHC and 27-OHC using gas chromatography-mass spectrometry before, after 1-month (n = 28) and after 3-month (n = 13) treatment.

RESULTS

As expected, PCSK9ab treatment lowered serum cholesterol and oxysterol levels after 1 month. The serum ratio of 24S-OHC to cholesterol increased after 1 month by 17 ± 28% (mean ± standard deviation; 95% confidence interval [CI]: 5.8 to 28%; P < .01) and 24S-OHC to 27-OHC by 15 ± 39% (95% CI: 0.2 to 30%; P < .01). Within 3 months, 24S-OHC to cholesterol increased by 2.8 μg g^-1 mo^-1 (95% CI: 2.1 to 3.6; P < .01) and 24S-OHC to 27-OHC by 0.019 mo^-1 (95% CI: 0.007 to 0.032; P < .01).

CONCLUSION

The serum ratios of 24S-OHC to cholesterol and to 27-OHC increased after treatment with PCSK9ab. We hypothesize that this is caused by a reduced entrance of 27-OHC into the brain, increased synthesis of brain cholesterol, increased production of 24S-OHC and its secretion across the blood-brain barrier.

The effect of transgender hormonal treatment on high density lipoprotein cholesterol efflux capacity

BACKGROUND AND AIMS

A decrease in high-density lipoprotein (HDL)-cholesterol concentrations during transgender hormone therapy has been shown. However, the ability of HDL to remove cholesterol from arterial wall macrophages, termed cholesterol efflux capacity (CEC), has proven to be a better predictor of cardiovascular disease (CVD) largely independently of HDL-concentrations. In addition, the serum capacity to load macrophages with cholesterol (cholesterol loading capacity, CLC) represents an index of pro-atherogenic potential. As transgender individuals are exposed to lifelong exogenous hormone therapy (HT), it becomes of interest to study whether HDL-CEC and serum CLC are affected by HT. HDL-CEC and serum CLC have been evaluated in 15 trans men treated with testosterone and in 15 trans women treated with estradiol and cyproterone acetate at baseline and after 12 months of HT.

METHODS

Total HDL-CEC from macrophages and its major contributors, the ATP-binding cassette transporters (ABC) A1 and ABCG1 HDL-CEC and HDL-CEC by aqueous diffusion were determined by a radioisotopic assay. CLC was evaluated in human THP-1 macrophages.

RESULTS

In trans women, total HDL-CEC decreased by 10.8% (95%CI: -14.3;-7.3; p < 0.001), ABCA1 HDL-CEC by 23.8% (-34.7; -12.9; p < 0.001) and aqueous diffusion HDL-CEC by 4.8% (-8.4;-1.1; p < 0.01). In trans men, only aqueous diffusion HDL-CEC decreased significantly, -9.8% (-15.7;-3.9; p < 0.01). ABCG1 HDL-CEC did not change in either group. Serum CLC and HDL subclass distribution were not modified by HT in both groups.

CONCLUSIONS

Total HDL-CEC decreased during HT in trans women, with a specific reduction in ABCA1 CEC. This finding might contribute to a higher CVD risk.

Sex-Specific Association of Endogenous PCSK9 With Memory Function in Elderly Subjects at High Cardiovascular Risk

Background: Growing evidence indicates that cognitive decline and cardiovascular diseases (CVDs) share common vascular risk factors. Protease proprotein convertase subtilisin/kexin type 9 (PCSK9) is associated with CV disease risk and has been also involved in neuronal differentiation.

Aim: Evaluate whether in patients at high CV risk cognitive function is related to PCSK9 levels.

Methods. One hundred sixty-six patients (67 female) were enrolled. A detailed neuropsychological (NP) assessment was performed. PCSK9 levels were measured with ELISA.

Results: Men had significantly higher short-term memory, executive function, and praxic and mental representation skills, as reflected by Forward Digit Span (FDS) (p = 0.005), Trail Making Test-A (TMT-A) (p = 0.047), Clock Drawing Test (CDT) (0.016). Endogenous PCSK9 levels were higher in female (p = 0.005). On linear regression analysis PCSK9 predicts short term memory only in females (Beta = 0.408, p = 0.001), with an interaction between PCSK9 and gender (p = 0.004 for interaction PCSK9 by sex). The association of PCSK9 with FDS in female was partially mediated by waist circumference (mediation effect 8.5%).

Conclusions: In patients at high CV risk short term memory was directly related to PCSK9 levels only in women, revealing the relevance of sex in this relationship. The association of PCSK9 with memory function may be mediated, at least in part, by waist circumference.

HMGB1/RAGE/TLR4 axis and glutamate as novel targets for PCSK9 inhibitor in high fat cholesterol diet induced cognitive impairment and amyloidosis

AIMS

Alzheimer's disease (AD) is a leading health problem in which increased amyloid β (Aβ) accumulation may occur due to abnormal Aβ precursor protein processing by β-secretase 1 (BACE1) enzyme. Lately, neuro-inflammation was recognized as a significant contributor to its pathogenesis. Although the causes of AD are not yet well understood, much evidence has suggested that dyslipidemia has harmful effects on cognitive function and is inextricably involved in AD pathogenesis. Cholesterol is a vital molecule involved in neuronal development. Alteration in neuronal cholesterol levels affects Aβ metabolism and results in neurodegeneration. Proprotein-convertase-subtilisin/kexin type-9 (PCSK9) was found to decrease neuronal cholesterol uptake by degradation of LDL-receptor related protein 1 (LRP-1) responsible for neuronal cholesterol uptake. Accordingly, this study was designed to evaluate the effect of PCSK9-inhibition by alirocumab (Aliro) in high-fat-cholesterol-diet (HFCD)-induced-AD-like condition.

MAIN METHODS

Wistar Rats were divided into six groups; control; HFCD; HFCD and Memantine; HFCD and Aliro (4, 8 and 16 mg/kg/week) to test for ability of Aliro to modulate cognitive impairment, amyloidosis, brain cholesterol homeostasis and neuro-inflammation in HFCD-induced-AD-like condition.

KEY FINDINGS

Our results demonstrated an association between PCSK9 inhibition by Aliro and amelioration of cognitive deficit, cholesterol hemostasis and reduction of neuro-inflammation. Aliro was able to alleviate hippocampal LRP-1expression levels and reduce brain cholesterol, hippocampal BACE1, Aβ₄₂, high-mobility-group-box-1 protein, receptor for advanced-glycation-end-products and toll like receptor-4 with subsequent decrease of different inflammatory mediators as nuclear-factor-kappa-B (NF-κB), tumor-necrosis-factor-alpha (TNF-α), interleukin-1beta (IL-1β) and IL-6.

SIGNIFICANCE

PCSK9-inhibition may represent a new therapeutic target in AD especially for HFCD-induced-AD-like condition.

Proteodynamics and aging of eukaryotic cells

All aspects of each protein existence in the eukaryotic cells, starting from the pre-translation events, through translation, multiple different post-translational modifications, functional life and eventual proteostatic removal after loss of functionality and changes in physico-chemical properties, can be collectively called the proteodynamics. With aging, passing of time as well as accumulating effects of exposures, interactions and wearing-off lead to problems at each of the above mentioned stages, eventually leading to general malfunction of the proteome. This work briefly reviews and summarizes current knowledge concerning this important topic.

PCSK9 Inhibitors and Neurocognitive Adverse Drug Reactions: Analysis of Individual Case Safety Reports from the Eudravigilance Database

Introduction

Proprotein convertase subtilisin/kexin type 9 inhibitors (PCSK9Is) were associated with a risk of neurocognitive adverse drug reactions (ADRs).

Objective

We aimed to investigate the occurrence of neuropsychiatric ADRs related to PCSK9Is.

Methods

We analyzed Individual Case Safety Reports (ICSRs) sent through the European pharmacovigilance database that reported alirocumab or evolocumab as the suspected drug and at least one neurological or psychiatric ADR. The reporting odds ratio (ROR) was computed to compare the probability of reporting ICSRs with neuropsychiatric ADRs between alirocumab, evolocumab and statins.

Results

Overall, 2041 ICSRs with alirocumab and/or evolocumab as the suspected drug described the occurrence of neuropsychiatric ADRs. The most reported preferred terms for both drugs were headache, insomnia and depression. No difference between alirocumab and evolocumab was observed for the RORs of ICSRs with ADRs belonging to the System Organ Classes (SOCs) ‘Nervous system disorders’ or ‘Psychiatric disorders’ (ROR 1.02, 95% confidence interval 0.91–1.14; and 1.12, 95% CI 0.94–1.34, respectively), while evolocumab and alirocumab had a higher reporting probability of ICSRs with ADRs belonging to the SOC ‘Nervous system disorders’ compared with atorvastatin and fluvastatin. A lower reporting probability was instead found for ICSRs with ADRs belonging to the SOC ‘Psychiatric disorders’ for evolocumab and alirocumab versus simvastatin, pravastatin and rosuvastatin.

Conclusion

Our results demonstrated that 22.7% of all ICSRs reporting alirocumab or evolocumab as suspect drugs described the occurrence of neuropsychiatric ADRs. The ROR showed that evolocumab and alirocumab had a higher reporting probability of neurological ADRs compared with statins. Further data from real-life contexts are needed.

Electronic supplementary material

The online version of this article (10.1007/s40264-020-01021-3) contains supplementary material, which is available to authorized users.

Coming Back to Physiology: Extra Hepatic Functions of Proprotein Convertase Subtilisin/Kexin Type 9

Neuronal apoptosis regulated convertase-1 (NARC-1), now mostly known as proprotein convertase subtilisin/kexin type 9 (PCSK9), has received a lot of attention due to the fact that it is a key regulator of the low-density lipoprotein (LDL) receptor (LDL-R) and is therefore involved in hepatic LDL clearance. Within a few years, therapies targeting PCSK9 have reached clinical practice and they offer an additional tool to reduce blood cholesterol concentrations. However, PCSK9 is almost ubiquitously expressed in the body but has less well-understood functions and target proteins in extra hepatic tissues. As such, PCSK9 is involved in the regulation of neuronal survival and protein degradation, it affects the expression of the epithelial sodium channel (ENaC) in the kidney, it interacts with white blood cells and with cells of the vascular wall, and it modifies contractile activity of cardiomyocytes, and contributes to the regulation of cholesterol uptake in the intestine. Moreover, under stress conditions, signals from the kidney and heart can affect hepatic expression and thereby the plasma concentration of PCSK9 which then in turn can affect other target organs. Therefore, there is an intense relationship between the local (autocrine) and systemic (endocrine) effects of PCSK9. Although, PCSK9 has been recognized as a ubiquitously expressed modifier of cellular function and signaling molecules, its physiological role in different organs is not well-understood. The current review summarizes these findings.

PCSK9 Inhibition: Insights From Clinical Trials and Future Prospects

In 2003, clinical observations led to the discovery of the involvement of proprotein convertase subtilisin/kexin type 9 (PCSK9) in lipid metabolism. Functional studies demonstrated that PCSK9 binds to the low-density lipoprotein (LDL) receptor directing it to its lysosomal degradation. Therefore, carriers of gain-of-function mutations in PCSK9 exhibit decreased expression of LDL receptors on the hepatocyte surface and have higher LDL cholesterol (LDL-C) levels. On the contrary, loss-of-function mutations in PCSK9 are associated with low LDL-C concentrations and significantly reduced lifetime risk of cardiovascular disease. These insights motivated the search for strategies to pharmacologically inhibit PCSK9. In an exemplary rapid development, fully human monoclonal antibodies against PCSK9 were developed and found to effectively reduce LDL-C. Administered subcutaneously every 2–4 weeks, the PCSK9 antibodies evolocumab and alirocumab reduce LDL-C by up to 60% in a broad range of populations either as monotherapy or in addition to statins. Two large cardiovascular outcome trials involving a total of ∼46,000 cardiovascular high-risk patients on guideline-recommended lipid-lowering therapy showed that treatment with evolocumab and alirocumab led to a relative reduction of cardiovascular risk by 15% after 2.2 and 2.8 years of treatment, respectively. These findings expanded the armamentarium of pharmacological approaches to address residual cardiovascular risk associated with LDL-C. Furthermore, the unprecedented low LDL-C concentrations achieved (e.g., 30 mg/dL in the FOURIER study) suggest that the relationship between LDL-C and cardiovascular risk is without a lower threshold, and without associated adverse events during the timeframe of the studies. The side effect profile of PCSK9 antibodies is favorable with few patients exhibiting injection-site reactions. Currently, the access to PCSK9 antibodies is limited by high treatment costs. The development of novel approaches to inhibit PCSK9 such as the use of small interfering RNA to inhibit PCSK9 synthesis seems promising and may soon become available.

Fisetin ameliorates atherosclerosis by regulating PCSK9 and LOX-1 in apoE-/- mice

The purpose of the current study was to investigate the mechanism by which fisetin improves atherosclerosis (AS) by regulating lipid metabolism and senescence in apolipoprotein E-deficient (apoE^-/-) mice. An AS model was established by feeding apoE^-/- mice a high-fat diet. Mice were randomly divided into the model group (n=18), the fisetin group (n=18) and the atorvastatin group (n=18). The control group (n=18) was composed of wild-type C57BL/6 mice of the same age and genetic background. The fisetin and atorvastatin groups were respectively treated with aqueous solutions of fisetin (12.5 mg/kg) and atorvastatin (2 mg/kg) via oral gavage daily for 12 weeks. The pathological morphology, lipid accumulation, collagen deposition of the aortic sinus were observed, serum lipids, superoxide dismutase (SOD) and malondialdehyde (MDA) levels and alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities were measured in the peripheral blood serum. Additionally, the expressions of proprotein convertase subtilisin/kexin type 9 (PCSK9), lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), tumor suppressor protein p53 (p53), cyclin-dependent kinase inhibitor 1A (p21) and multiple tumor suppressor-1 (p16) were analyzed in the aorta. The results of the current study indicated that compared with the control group, a large area of AS plaque in the aortic sinus that contained a large amount of red-stained lipids and decreased collagen fiber content were found in the model group, which exhibited higher total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), very low-density lipoprotein cholesterol (VLDL-C), oxidized low-density lipoprotein (ox-LDL) and MDA levels; higher ALT and AST activities, lower high-density lipoprotein cholesterol (HDL-C) and SOD levels and increased expression levels of PCSK9, LOX-1, p53, p21 and p16. Fisetin is a phytochemical and bioflavonoid that serves a potential role in chronic diseases including AS, obesity, diabetes and cancer due to its wide biological activities, such as regulating lipid metabolism and anti-aging, anti-oxidation and anti-inflammatory. Atorvastatin is recognized as a first-line treatment drug for AS; therefore it was used as a positive control in the current study. Following fisetin and atorvastatin treatment, both the AS plaque and the lipid accumulation in the aortic sinus were significantly reduced, and the expressions of PCSK9, LOX-1 and aging markers, including p53, p21 and p16 were downregulated.

Cholesterol homeostasis: Researching a dialogue between the brain and peripheral tissues

Cholesterol homeostasis is a highly regulated process in human body because of its several functions underlying the biology of cell membranes, the synthesis of all steroid hormones and bile acids and the need of trafficking lipids destined to cell metabolism. In particular, it has been recognized that peripheral and central nervous system cholesterol metabolism are separated by the blood brain barrier and are regulated independently; indeed, peripherally, it depends on the balance between dietary intake and hepatic synthesis on one hand and its degradation on the other, whereas in central nervous system it is synthetized de novo to ensure brain physiology. In view of this complex metabolism and its relevant functions in mammalian, impaired levels of cholesterol can induce severe cellular dysfunction leading to metabolic, cardiovascular and neurodegenerative diseases. The aim of this review is to clarify the role of cholesterol homeostasis in health and disease highlighting new intriguing aspects of the cross talk between its central and peripheral metabolism.

Novel therapeutic strategies for Alzheimer's disease targeting brain cholesterol homeostasis

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common cause of dementia. Aβ plaques and tauopathy are two major concerns associated with AD. Moreover, excessive Aβ accumulation can lead to other nonspecific metabolic brain abnormalities. There are various genetic, environmental, and other risk factors associated with AD. Identification of risk factors and its mechanisms by which these factors impart role in AD pathology would be helpful for the prevention of AD progression. Altered cholesterol homeostasis could be considered as a risk factor for AD progression. Brain cholesterol dysmetabolism is recognized as one of the crucial attributes for AD that affect major hallmarks of AD including neurodegeneration. To fill the gap between altered cholesterol levels in the brain and AD, the researchers started focusing on statins as re-purposing drugs for AD treatment. The various other hypothesis has been suggested due to a lack of beneficial results of statins in clinical trials, such as reduced brain cholesterol could underlie poor cognition. Unfortunately, it is still unclear, whether an increase or decrease in brain cholesterol levels responsible for Alzheimer's disease or not. Presently, scientists believed that managing the level of cholesterol in the brain may help as an alternative treatment strategy for AD. In this review, we focused on the therapeutic strategies for AD by targeting brain cholesterol levels.

Impact of Sex and Age on the Mevalonate Pathway in the Brain: A Focus on Effects Induced by Maternal Exposure to Exogenous Compounds

The mevalonate pathway produces cholesterol and other compounds crucial for numerous cellular processes. It is well known that age and sex modulate this pathway in the liver. Recently, similar effects were also noted in different brain areas, suggesting that alterations of the mevalonate pathway are at the root of marked sex-specific disparities in some neurodevelopmental disorders related to disturbed cholesterol homeostasis. Here, we show how the mevalonate pathway is modulated in a sex-, age- and region-specific manner, and how maternal exposure to exogenous compounds can disturb the regulation of this pathway in the brain, possibly inducing functional alterations.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitor exerts greater efficacy than atorvastatin on improvement of brain function and cognition in obese rats

The accumulation of lipid as a result of long-term consumption of a high-fat diet (HFD) may lead to metabolic and brain dysfunction. Atorvastatin, a recommended first-line lipid-lowering agent, has shown beneficial effects on metabolic and brain functions in several models. Recently, proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitor was approved as an effective therapeutic drug for dyslipidemia patients. However, few studies have reported on the effect of this PCSK9 inhibitor on brain function. In addition, the comparative efficacy on the improvement of metabolic and brain functions between PCSK9 inhibitor and atorvastatin in obese models have not been elucidated. We hypothesized that PCSK9 inhibitor improves metabolic and brain functions in an obese model to a greater extent than atorvastatin. Thirty-two female rats were fed with either a normal diet (ND) or HFD for 15 weeks. At week 13, ND rats were given normal saline and HFD rats were given either normal saline, atorvastatin (40 mg/kg/day) or PCSK9 inhibitor (4 mg/kg/day) for 3 weeks. Oxidative stress, blood brain barrier breakdown, microglial hyperactivity, synaptic dysplasticity, apoptosis, amyloid proteins production in the hippocampus and cognitive decline were found in HFD-fed rats. Atorvastatin and PCSK9 inhibitor therapies equally attenuated hippocampal apoptosis and amyloid protein production in HFD-fed rats. Interestingly, PCSK9 inhibitor had the greater efficacy than atorvastatin on the amelioration of hippocampal oxidative stress, blood brain barrier breakdown, microglial hyperactivity, synaptic dysplasticity in the hippocampus and cognitive decline. These findings suggest that PCSK9 inhibitor may be another drug of choice for improving brain function in the obese condition with discontinued statin therapy.

Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) in the Brain and Relevance for Neuropsychiatric Disorders

Proprotein convertase subtilisin/kexin type 9 (PCSK9) has long been studied in the liver due to its regulation of plasma low-density lipoprotein cholesterol (LDL-C) and its causal role in familial hypercholesterolemia. Although PCSK9 was first discovered in cerebellar neurons undergoing apoptosis, its function in the central nervous system (CNS) is less clear. PCSK9 has been shown to be involved in neuronal differentiation, LDL receptor family metabolism, apoptosis, and inflammation in the brain, but in vitro and in vivo studies offer contradictory findings. PCSK9 expression in the adult brain is low but is highly upregulated during disease states. Cerebral spinal fluid (CSF) PCSK9 concentrations are correlated with neural tube defects and neurodegenerative diseases in human patients. Epigenetic studies reveal that chronic alcohol use may modulate methylation of the PCSK9 gene and genetic studies show that patients with gain-of-function PCSK9 variants have higher LDL-C and an increased risk of ischemic stroke. Early safety studies of the PCSK9 inhibitors evolocumab and alirocumab, used to treat hypercholesterolemia, hinted that PCSK9 inhibition may negatively impact cognition but more recent, longer-term clinical trials found no adverse neurocognitive events. The purpose of this review is to elucidate the role of PCSK9 in the brain, particularly its role in disease pathogenesis.

Beyond cholesterol metabolism: The pleiotropic effects of proprotein convertase subtilisin/kexin type 9 (PCSK9). Genetics, mutations, expression, and perspective for long-term inhibition

Proprotein convertase subtilisin/kexin type 9 (PCSK9) has a crucial role in lipid metabolism, particularly due to its function in low-density lipoprotein receptor degradation. Gain-of-function genetic mutations of PCSK9 result in autosomal dominant familial hypercholesterolemia, characterized by high levels of low-density lipoprotein cholesterol (LDL-C) and clinical signs of early atherosclerosis. In recent years, PCSK9 has become an important therapeutic target for cholesterol-lowering therapy. Particularly, its inhibition with monoclonal antibodies has shown excellent efficacy in decreasing LDL-C and reducing cardiovascular events. However, PCSK9, first identified in the brain, seems to be a ubiquitous protein with different tissue-specific functions also independent of cholesterol metabolism. Accordingly, it appears to be involved in the immune response, haemostasis, glucose metabolism, neuronal survival, and several other biological functions. This review provides a comprehensive overview of the genetics, biochemical structure, expression, and function of PCSK9 and discusses the potential implications of its long-term pharmacological inhibition.

Evaluation of the Potential Role of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) in Niemann-Pick Disease, Type C1

Niemann–Pick disease, type C1, is a cholesterol storage disease where unesterified cholesterol accumulates intracellularly. In the cerebellum this causes neurodegeneration of the Purkinje neurons that die in an anterior-to-posterior and time-dependent manner. This results in cerebellar ataxia as one of the major outcomes of the disease. Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a significant role in the regulation of serum cholesterol levels by modulating LDL receptor levels on peripheral tissues. In the central nervous system, PCSK9 may have a similar effect on the closely related VLDL and ApoE2 receptors to regulate brain cholesterol. In addition, regulation of VLDLR and ApoER2 by PCSK9 may contribute to neuronal apoptotic pathways through Reelin, the primary ligand of VLDLR and ApoER2. Defects in reelin signaling results in cerebellar dysfunction leading to ataxia as seen in the Reeler mouse. Our recent findings that Pcsk9 is expressed ~8-fold higher in the anterior lobules of the cerebellum compared to the posterior lobule X, which is resistant to neurodegeneration, prompted us to ask whether PCSK9 could play a role in NPC1 disease progression. We addressed this question genetically, by characterizing NPC1 disease in the presence or absence of PCSK9. Analysis of double mutant Pcsk9^-/-/Npc1^-/- mice by disease severity scoring, motor assessments, lifespan, and cerebellar Purkinje cell staining, showed no obvious difference in NPC1 disease progression with that of Npc1^-/- mice. This suggests that PCSK9 does not play an apparent role in NPC1 disease progression.