Acid-dependent ligand dissociation and recycling of LDL receptor mediated by growth factor homology region

Recapitulating familial hypercholesterolemia in a mouse model by knock-in patient-specific LDLR mutation

Familial hypercholesterolemia (FH) is one of the most prevalent monogenetic disorders leading to cardiovascular disease (CVD) worldwide. Mutations in Ldlr, encoding a membrane-spanning protein, account for the majority of FH cases. No effective and safe clinical treatments are available for FH. Adenine base editor (ABE)-mediated molecular therapy is a promising therapeutic strategy to treat genetic diseases caused by point mutations, with evidence of successful treatment in mouse disease models. However, due to the differences in the genomes between mice and humans, ABE with specific sgRNA, a key gene correction component, cannot be directly used to treat FH patients. Thus, we generated a knock-in mouse model harboring the partial patient-specific fragment and including the Ldlr W490X mutation. LdlrW490X/W490X mice recapitulated cholesterol metabolic disorder and clinical manifestations of atherosclerosis associated with FH patients, including high plasma low-density lipoprotein cholesterol levels and lipid deposition in aortic vessels. Additionally, we showed that the mutant Ldlr gene could be repaired using ABE with the cellular model. Taken together, these results pave the way for ABE-mediated molecular therapy for FH.

Endocrine Regulation of Microvascular Receptor-Mediated Transcytosis and Its Therapeutic Opportunities: Insights by PCSK9-Mediated Regulation

Currently, many neurological disorders lack effective treatment options due to biological barriers that effectively separate the central nervous system (CNS) from the periphery. CNS homeostasis is maintained by a highly selective exchange of molecules, with tightly controlled ligand-specific transport systems at the blood-brain barrier (BBB) playing a key role. Exploiting or modifying these endogenous transport systems could provide a valuable tool for targeting insufficient drug delivery into the CNS or pathological changes in the microvasculature. However, little is known about how BBB transcytosis is continuously regulated to respond to temporal or chronic changes in the environment. The aim of this mini-review is to draw attention to the sensitivity of the BBB to circulating molecules derived from peripheral tissues, which may indicate a fundamental endocrine-operating regulatory system of receptor-mediated transcytosis at the BBB. We present our thoughts in the context of the recent observation that low-density lipoprotein receptor-related protein 1 (LRP1)-mediated clearance of brain amyloid-β (Aβ) across the BBB is negatively regulated by peripheral proprotein convertase subtilisin/kexin type 9 (PCSK9). We hope that our conclusions will inspire future investigations of the BBB as dynamic communication interface between the CNS and periphery, whose peripheral regulatory mechanisms could be easily exploited for therapeutic purposes.

Structures of LRP2 reveal a molecular machine for endocytosis

The low-density lipoprotein (LDL) receptor-related protein 2 (LRP2 or megalin) is representative of the phylogenetically conserved subfamily of giant LDL receptor-related proteins, which function in endocytosis and are implicated in diseases of the kidney and brain. Here, we report high-resolution cryoelectron microscopy structures of LRP2 isolated from mouse kidney, at extracellular and endosomal pH. The structures reveal LRP2 to be a molecular machine that adopts a conformation for ligand binding at the cell surface and for ligand shedding in the endosome. LRP2 forms a homodimer, the conformational transformation of which is governed by pH-sensitive sites at both homodimer and intra-protomer interfaces. A subset of LRP2 deleterious missense variants in humans appears to impair homodimer assembly. These observations lay the foundation for further understanding the function and mechanism of LDL receptors and implicate homodimerization as a conserved feature of the LRP receptor subfamily.

Low density lipoprotein receptor endocytosis in cardiovascular disease and the factors affecting LDL levels

Cardiovascular disease (CVD) is the one of major global health issues with approximately 30% of the mortality reported in the mid-income population. Low-density lipoprotein (LDL) plays a crucial role in development of CVD. High LDL along with others forms a plaque and blocks arteries, resulting in CVD. The present chapter deals with the mechanism of receptor-mediated endocytosis of LDL and its management by drugs such as statins and PCSK9 inhibitors along with dietary supplementation for health improvements.

Differential trafficking of ligands trogocytosed via CD28 versus CTLA4 promotes collective cellular control of co-stimulation

Intercellular communication is crucial for collective regulation of cellular behaviors. While clustering T cells have been shown to mutually control the production of key communication signals, it is unclear whether they also jointly regulate their availability and degradation. Here we use newly developed reporter systems, bioinformatic analyses, protein structure modeling and genetic perturbations to assess this. We find that T cells utilize trogocytosis by competing antagonistic receptors to differentially control the abundance of immunoregulatory ligands. Specifically, ligands trogocytosed via CD28 are shuttled to the T cell surface, enabling them to co-stimulate neighboring T cells. In contrast, CTLA4-mediated trogocytosis targets ligands for degradation. Mechanistically, this fate separation is controlled by different acid-sensitivities of receptor-ligand interactions and by the receptor intracellular domains. The ability of CD28 and CTLA4 to confer different fates to trogocytosed ligands reveals an additional layer of collective regulation of cellular behaviors and promotes the robustness of population dynamics.

The role of PCSK9 in NAFLD/NASH and therapeutic implications of PCSK9 inhibition

INTRODUCTION

There are inconsistent findings regarding the effect of lipid-lowering agents on nonalcoholic fatty liver disease (NAFLD). Proprotein Convertase Subtilisin/Kexin type 9 (PCSK9) is an important player in cholesterol homeostasis and intracellular lipogenesis, and PCSK9 inhibitors (PCSK9-i) have been found to be efficient for pharmacological management of hyperlipidemia.

AREAS COVERED

Whether PCSK9 (itself) or PCSK9-i affects NAFLD is still disputed. To address this question, we review published preclinical and clinical studies providing evidence for the role of PCSK9 in and the effect of PCSK9-I on the development and pathogenesis of NAFLD.

EXPERT OPINION

The current evidence from a landscape of preclinical and clinical studies examining the role of PCSK9 in NAFLD shows controversial results. Preclinical studies indicate that PCSK9 associates with NAFLD and nonalcoholic steatohepatitis (NASH) progression in opposite directions. In humans, it has been concluded that the severity of hepatic steatosis affects the correlation between circulating PCSK9 and liver fat content in humans, with a possible impact of circulating PCSK9 in the early stages of NAFLD, but not in the late stages. However, data from clinical trials with PCSK9-i reassure to the safety of these agents, although real-life long-term evidence is needed.

LDLR, LRP1, and Megalin redundantly participate in the uptake of Clostridium novyi alpha-toxin

Clostridium novyi alpha-toxin (Tcnα) is a potent exotoxin that induces severe symptoms including gas gangrene, myositis, necrotic hepatitis, and sepsis. Tcnα binds to sulfated glycosaminoglycans (sGAG) for cell-surface attachment and utilizes low-density lipoprotein receptor (LDLR) for rapid entry. However, it was also shown that Tcnα may use alternative entry receptors other than LDLR. Here, we define that LRP1 and Megalin can also facilitate the cellular entry of Tcnα by employing reconstitutive LDLR family proteins. LDLR, LRP1, and Megalin recognize Tcnα via their ligand-binding domains (also known as LDL receptor type A repeats). Notably, LDLR and LRP1 have contrasting expression levels in many different cells, thus the dominant entry receptor for Tcnα could be cell-type dependent. These findings together increase our knowledge of the Tcnα actions and further help to understand the pathogenesis of C. novyi infection-associated diseases.

Clostridium novyi alpha-toxin (Tcnα) also uses LRP1 and Megalin as cellular entry receptors besides LDLR, and this might be a response to cell-type dependent receptor availability for the exotoxin.

Megalin, a multi-ligand endocytic receptor, and its participation in renal function and diseases: A review

The endocytosis mechanism is a complicated system that is essential for cell signaling and survival. Megalin, a membrane-associated endocytic receptor, and its related proteins such as cubilin, the neonatal Fc receptor for IgG, and NaPi-IIa are important in receptors-mediated endocytosis. Physiologically, megalin uptakes plasma vitamins and proteins from primary urine, preventing their loss. It also facilitates tubular retrieval of solutes and endogenous components that may be involved in modulation and recovery from kidney injuries. Moreover, megalin is responsible for endocytosis of xenobiotics and drugs in renal tubules, increasing their half-life and/or their toxicity. Fluctuations in megalin expression and/or functionality due to changes in its regulatory mechanisms are associated with some sort of kidney injury. Also, it's an important component of several pathological conditions, including diabetic nephropathy and Dent disease. Thus, exploring the fundamental role of megalin in the kidney might help in the protection and/or treatment of multiple kidney-related diseases. Hence, this review aimed to explore the physiological roles of megalin in the kidney and their implications for kidney-related injuries.

A SURF4-to-proteoglycan relay mechanism that mediates the sorting and secretion of a tagged variant of sonic hedgehog

SignificanceSonic Hedgehog (Shh) is a key signaling molecule that plays important roles in embryonic patterning, cell differentiation, and organ development. Although fundamentally important, the molecular mechanisms that regulate secretion of newly synthesized Shh are still unclear. Our study reveals a role for the cargo receptor, SURF4, in facilitating export of Shh from the endoplasmic reticulum (ER) via a ER export signal. In addition, our study provides evidence suggesting that proteoglycans promote the dissociation of SURF4 from Shh at the Golgi, suggesting a SURF4-to-proteoglycan relay mechanism. These analyses provide insight into an important question in cell biology: how do cargo receptors capture their clients in one compartment, then disengage at their destination?

Molecular Identification and Functional Characterization of Vitellogenin Receptor From Harmonia axyridis (Coleoptera: Coccinellidae)

Vitellogenin receptors (VgRs) have vital roles in reproduction by mediating endocytosis of the vitellogenin (Vg) in oviparous insects. Same as most insect species that possess only one VgR, in this study, a single VgR mRNA (HmVgR) was identified in an important natural enemy ladybeetle, Harmonia axyridis (Pallas). The open reading frame of HmVgR was 5,340 bases encoding a protein of 1,779 amino acids. Bioinformatic analyses showed that HmVgR had conserved domain motifs of low-density lipoprotein receptor family. Based on phylogenetic analysis, HmVgR had highly homologous within the Coleoptera. The transcriptional level of HmVgR was initially detected in the newly emerged female adults, gradually increased from day 3 to day 9, peaked on day 13, and then sharply decreased on day 15. Similar to most insects, HmVgR was expressed specifically in ovarian tissue. Moreover, RNA interference (RNAi) clearly decreased the transcription levels of HmVgR, which blocked the deposition of yolk protein in the ovaries, shortened the ovarian length, and led to negative impacts on reproductive-related parameters (i.e., prolonged preoviposition periods, reduced spawning and depressed hatchability). In sum, these results indicated that HmVgR may be critical for yolk protein deposition of oocytes and can play a key role in reproduction of female adults of H. axyridis. Our results provide conclusive proof for the important roles of HmVgR in fecundity, and establish a basis for further research on its interaction with vitellogenin.

Endosomal escape of delivered mRNA from endosomal recycling tubules visualized at the nanoscale

Delivery of exogenous mRNA using lipid nanoparticles (LNPs) is a promising strategy for therapeutics. However, a bottleneck remains in the poor understanding of the parameters that correlate with endosomal escape versus cytotoxicity. To address this problem, we compared the endosomal distribution of six LNP-mRNA formulations of diverse chemical composition and efficacy, similar to those used in mRNA-based vaccines, in primary human adipocytes, fibroblasts, and HeLa cells. Surprisingly, we found that total uptake is not a sufficient predictor of delivery, and different LNPs vary considerably in endosomal distributions. Prolonged uptake impaired endosomal acidification, a sign of cytotoxicity, and caused mRNA to accumulate in compartments defective in cargo transport and unproductive for delivery. In contrast, early endocytic/recycling compartments have the highest probability for mRNA escape. By using super-resolution microscopy, we could resolve a single LNP-mRNA within subendosomal compartments and capture events of mRNA escape from endosomal recycling tubules. Our results change the view of the mechanisms of endosomal escape and define quantitative parameters to guide the development of mRNA formulations toward higher efficacy and lower cytotoxicity.

Low-Density Lipoprotein Internalization, Degradation and Receptor Recycling Along Membrane Contact Sites

Low-density lipoprotein (LDL) internalization, degradation, and receptor recycling is a fundamental process underlying hypercholesterolemia, a high blood cholesterol concentration, affecting more than 40% of the western population. Membrane contact sites influence endosomal dynamics, plasma membrane lipid composition, and cellular cholesterol distribution. However, if we focus on LDL-related trafficking events we mostly discuss them in an isolated fashion, without cellular context. It is our goal to change this perspective and to highlight that all steps from LDL internalization to receptor recycling are likely associated with dynamic membrane contact sites in which endosomes engage with the endoplasmic reticulum and other organelles.

PCSK9 inhibition-based therapeutic approaches: an immunotherapy perspective

Proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors (PCSK9-I) are novel therapeutic tools to decrease cardiovascular risk. These agents work by lowering the low-density lipoprotein cholesterol (LDL-C) in hypercholesterolemic patients who are statin resistant/intolerant. Current clinically approved and investigational PCSK9-I act generally by blocking PCSK9 activity in the plasma or suppressing its expression or secretion by hepatocytes. The most widely investigated method is the disruption of PCSK9/LDL receptor (LDLR) interaction by fully-humanized monoclonal antibodies (mAbs), evolocumab and alirocumab, which have been approved for the therapy of hypercholesterolemia and atherosclerotic cardiovascular disease (CVD). Besides, a small interfering RNA called inclisiran, which specifically suppresses PCSK9 expression in hepatocytes, is as effective as mAbs but with administration twice a year. Because of the high costs of such therapeutic approaches, several other PCSK9-I have been surveyed, including peptide-based anti-PCSK9 vaccines and small oral anti-PCSK9 molecules, which are under investigation in preclinical and phase I clinical studies. Interestingly, anti-PCSK9 vaccination has been found to serve as a more widely feasible and more cost-effective therapeutic tool over mAb PCSK9-I for managing hypercholesterolemia. The present review will discuss LDL-lowering and cardioprotective effects of PCSK9-I, mainly immunotherapy-based inhibitors including mAbs and vaccines, in preclinical and clinical studies.

NHE6 depletion corrects ApoE4-mediated synaptic impairments and reduces amyloid plaque load

Apolipoprotein E4 (ApoE4) is the most important and prevalent risk factor for late-onset Alzheimer’s disease (AD). The isoelectric point of ApoE4 matches the pH of the early endosome (EE), causing its delayed dissociation from ApoE receptors and hence impaired endolysosomal trafficking, disruption of synaptic homeostasis, and reduced amyloid clearance. We have shown that enhancing endosomal acidification by inhibiting the EE-specific sodium-hydrogen exchanger 6 (NHE6) restores vesicular trafficking and normalizes synaptic homeostasis. Remarkably and unexpectedly, loss of NHE6 (encoded by the gene Slc9a6) in mice effectively suppressed amyloid deposition even in the absence of ApoE4, suggesting that accelerated acidification of EEs caused by the absence of NHE6 occludes the effect of ApoE on amyloid plaque formation. NHE6 suppression or inhibition may thus be a universal, ApoE-independent approach to prevent amyloid buildup in the brain. These findings suggest a novel therapeutic approach for the prevention of AD by which partial NHE6 inhibition reverses the ApoE4-induced endolysosomal trafficking defect and reduces plaque load.

The vitellogenin receptor functionality of the migratory locust depends on its phosphorylation by juvenile hormone

Vitellogenin receptor (VgR) plays a pivotal role in ovarian vitellogenin (Vg) uptake and vertical transmission of pathogenic microbes and Wolbachia symbionts. However, the regulatory mechanisms of VgR action as an endocytic receptor and translocation from oocyte cytoplasm to the membrane remain poorly understood. Here, by using the migratory locust Locusta migratoria as a model system, we report that juvenile hormone (JH) promotes VgR phosphorylation at Ser¹³⁶¹ in the second EGF-precursor homology domain. A signaling cascade including GPCR, PLC, extracellular calcium, and PKC-ι is involved in JH-stimulated VgR phosphorylation. This posttranslational regulation is a prerequisite for VgR binding to Vg on the external surface of the oocyte membrane and subsequent VgR/Vg endocytosis. Acidification, a condition in endosomes, induces VgR dephosphorylation along with the dissociation of Vg from VgR. Phosphorylation modification is also required for VgR recycling from oocyte cytoplasm to the membrane. Additionally, VgR phosphorylation and its requirement for Vg uptake and VgR recycling are evolutionarily conserved in other representative insects including the cockroach Periplaneta americana and the cotton bollworm Helicoverpa armigera This study fills an important knowledge gap of low-density lipoprotein receptors in posttranslational regulation, endocytosis, and intracellular recycling.

Acanthaster planci Inhibits PCSK9 and Lowers Cholesterol Levels in Rats

Atherosclerosis is the main cause of cardiovascular diseases which in turn, lead to the highest number of mortalities globally. This pathophysiological condition is developed due to a constant elevated level of plasma cholesterols. Statin is currently the widely used treatment in reducing the level of cholesterols, however, it may cause adverse side effects. Therefore, there is an urgent need to search for new alternative treatment. PCSK9 is an enzyme responsible in directing LDL-receptor (LDL-R)/LDL-cholesterols (LDL-C) complex to lysosomal degradation, preventing the receptor from recycling back to the surface of liver cells. Therefore, PCSK9 offers a potential target to search for small molecule inhibitors which inhibit the function of this enzyme. In this study, a marine invertebrate Acanthaster planci, was used to investigate its potential in inhibiting PCSK9 and lowering the levels of cholesterols. Cytotoxicity activity of A. planci on human liver HepG2 cells was carried out using the MTS assay. It was found that methanolic extract and fractions did not exhibit cytotoxicity effect on HepG2 cell line with IC50 values of more than 30 µg/mL. A compound deoxythymidine also did not exert any cytotoxicity activity with IC50 value of more than 4 µg/mL. Transient transfection and luciferase assay were conducted to determine the effects of A. planci on the transcriptional activity of PCSK9 promoter. Methanolic extract and Fraction 2 (EF2) produced the lowest reduction in PCSK9 promoter activity to 70 and 20% of control at 12.5 and 6.25 μg/mL, respectively. In addition, deoxythymidine also decreased PCSK9 promoter activity to the lowest level of 60% control at 3.13 μM. An in vivo study using Sprague Dawley rats demonstrated that 50 and 100 mg/kg of A. planci methanolic extract reduced the total cholesterols and LDL-C levels to almost similar levels of untreated controls. The level of serum glutamate oxalate transaminase (SGOT) and serum glutamate pyruvate transaminase (SGPT) showed that the administration of the extract did not produce any toxicity effect and cause any damage to rat liver. The results strongly indicate that A. planci produced a significant inhibitory activity on PCSK9 gene expression in HepG2 cells which may be responsible for inducing the uptake of cholesterols by liver, thus, reducing the circulating levels of total cholesterols and LDL-C. Interestingly, A. planci also did show any adverse hepato-cytotoxicity and toxic effects on liver. Thus, this study strongly suggests that A. planci has a vast potential to be further developed as a new class of therapeutic agent in lowering the blood cholesterols and reducing the progression of atherosclerosis.

The emerging landscape of peptide-based inhibitors of PCSK9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a clinically validated target for treating cardiovascular disease (CVD) due to its involvement in cholesterol metabolism. Although approved monoclonal antibodies (alirocumab and evolocumab) that inhibit PCSK9 function are very effective in lowering cholesterol, their limitations, including high treatment costs, have so far prohibited widespread use. Accordingly, there is great interest in alternative drug modalities to antibodies. Like antibodies, peptides are valuable therapeutics due to their high target potency and specificity. Furthermore, being smaller than antibodies means they have access to more drug administration options, are less likely to induce adverse immunogenic responses, and are better suited to affordable production. This review surveys the current peptide-based landscape aimed towards PCSK9 inhibition, covering pre-clinical to patented drug candidates and comparing them to current cholesterol lowering therapeutics. Classes of peptides reported to be inhibitors include nature-inspired disulfide-rich peptides, combinatorially derived cyclic peptides, and peptidomimetics. Their functional activities have been validated in biophysical and cellular assays, and in some cases pre-clinical mouse models. Recent efforts report peptides with potent sub-nanomolar binding affinities to PCSK9, which highlights their potential to achieve antibody-like potency. Studies are beginning to address pharmacokinetic properties of PCSK9-targeting peptides in more detail. We conclude by highlighting opportunities to investigate their biological effects in pre-clinical models of cardiovascular disease. The anticipation concerning the PCSK9-targeting peptide landscape is accelerating and it seems likely that a peptide-based therapeutic for treating PCSK9-mediated hypercholesterolemia may be clinically available in the near future.

Generation of a homozygous LRP2 knockout human embryonic stem cell line (FDCHDPe010-A-56) by CRISPR/Cas9 system

LRP2 is mainly expressed in the cell membrane of epithelia, maintaining normal endocytosis of nutrients from the extracellular microenvironment and mediating growth factor signals. The deficiency of LRP2 can result in abnormal lysosomal and mitochondrial function as well as insufficient resistance to oxidative stress. LRP2-KO animals show enlarged eyes and malfunction of the retinal pigment epithelium (RPE). We were able to generate an LRP2-KO human embryonic stem (ES) cell line using CRISPR/Cas9 gene editing and differentiate the mutant ES cells into RPE cells. Thus, this LRP2-KO human ES line will facilitate studying cellular mechanisms of eye disease due to LRP2 deficiency.

Mutational Spectrum of LDLR and PCSK9 Genes Identified in Iranian Patients With Premature Coronary Artery Disease and Familial Hypercholesterolemia

Familial hypercholesterolemia (FH) is a common, yet underdiagnosed, genetic disorder characterized by lifelong elevated low-density lipoprotein cholesterol levels, which can increase the risk of early-onset coronary artery disease (CAD). In the present study, we screened the nucleotide variations of the LDLR and PCSK9 genes, as well as a part of the APOB gene, in Iranian patients with FH and premature CAD to find the genetic cause of the disorder. Fifteen unrelated individuals with a clinical diagnosis of FH and premature CAD were recruited. Direct DNA sequencing was applied to screen the whole coding exons and exon-intron boundaries of the LDLR and PCSK9 genes and the main parts of their introns, together with exon 26 of the APOB gene. The pathogenicity of the identified mutations was investigated via either segregation analyses in the family or in silico predictive software. Six different point mutations (p.Cys148Tyr, p.Cys216Tyr, p.Cys302Trp, p.Cys338Trp, p.Leu479Gln, and p.G593Afs∗72) in LDLR and a double mutation (p.Asp172His and p.Ala53Val) in both LDLR and PCSK9 genes were identified in seven families with clinically diagnosed FH (43%), whereas no pathogenic mutations were found in eight families with clinically diagnosed FH. This study is the first to identify 1 pathogenic mutation in the LDLR gene (c.1014C > G [p.Cys338Trp]) and to cosegregate it from the affected individual in the family. No mutations were found in the APOB gene, whereas several silent mutations/polymorphisms were identified in the LDLR and PCSK9 genes. Genetic testing and reports on nucleotide alterations in the Iranian population are still limited. Our findings not only further confirm the significant role of FH in the incidence of premature CAD but also enlarge the spectrum of LDLR and PCSK9 variations and exhibit the heterogeneity of FH in Iranians. In patients with no mutation in the examined genes, the disease could be begotten either by a polygenic cause or by gene defects occurring in other related genes and regions not targeted in this study.

Impact of PCSK9 Immunization on Glycemic Indices in Diabetic Rats

METHODS

To prepare the anti-PCSK9 vaccine, a peptide construct called Immunogenic Fused PCSK9-Tetanus (IFPT) was linked to the surface of nanoliposome carriers. Healthy rats received four subcutaneous injections of the vaccine at biweekly intervals. Two weeks after the last vaccination, anti-PCSK9 antibody titers, PCSK9 targeting, and inhibition of PCSK9-low-density lipoprotein receptor (LDLR) interaction were evaluated. After verification of antibody generation, the immunized rats were intraperitoneally treated with a single dose (45 mg/kg) of streptozotocin (STZ) to induce diabetes mellitus. The levels of fasting blood glucose (FBG) were measured, and the oral glucose tolerance test (OGTT) as well as the insulin tolerance test (ITT) were carried out to assess glycemic status. At the end of the study, the total cholesterol, low-density lipoprotein cholesterol (LDL-C), triglyceride, and high-density lipoprotein cholesterol concentrations were assayed. Histopathology examination of the liver and pancreas was also performed using the hematoxylin-eosin staining method.

RESULTS

The prepared nanoliposomal vaccine could strongly induce anti-PCSK9 antibodies in the vaccinated rats. Within one week following the STZ injection, the FBG level was lower in the vaccinated group vs. diabetic control group (49% (-171.7 ± 35 mg/dL, p < 0.001)). In the OGTT, the injected rats showed improved glucose tolerance as reflected by the reduction of blood glucose levels over 180 min, compared with the diabetic controls. Moreover, the ITT demonstrated that, after the insulin injection, blood glucose concentration declined by 49.3% in the vaccinated group vs. diabetic control group. Expectedly, the vaccinated rats exhibited lower (-26.65%, p = 0.03) plasma LDL-C levels compared with the diabetic controls. Histopathology examination of pancreas tissue demonstrated that the pancreatic islets of the vaccinated rats had a slight decline in the population of β-cells and few α-cells. Normal liver histology was also observed in the vaccinated rats.

CONCLUSION

PCSK9 inhibition through the liposomal IFPT vaccine can improve the glucose and insulin tolerance impairments as well as the lipid profile in diabetes.

From adsorption to covalent bonding: Apolipoprotein E functionalization of polymeric nanoparticles for drug delivery across the blood-brain barrier

The blood-brain barrier (BBB) is composed of brain endothelial cells, pericytes, and astrocytes, which build a tight cellular barrier. Therapeutic (macro)molecules are not able to transit through the BBB in their free form. This limitation is bypassed by apolipoprotein E (ApoE)-functionalized polymeric nanoparticles (NPs) that are able to transport drugs (e.g. dalargin, loperamide, doxorubicin, nerve growth factor) across the BBB via low density lipoprotein (LDL) receptor mediated transcytosis. Coating with polysorbate 80 or poloxamer 188 facilitates ApoE adsorption onto polymeric NPs enabling recognition by LDL receptors of brain endothelial cells. This effect is even enhanced when NPs are directly coated with ApoE without surfactant anchor. Similarly, covalent coupling of ApoE to NPs that bear reactive groups on their surface leads to significantly improved brain uptake while avoiding the use of surfactants. Several in vitro BBB models using brain endothelial cells or co-cultures with astrocytes/pericytes/glioma cells are described which provide first insights regarding the ability of a drug delivery system to cross this barrier. In vivo models are employed to simulate central nervous system-relevant diseases such as Alzheimer's or Parkinson's disease and cerebral cancer.

Advances in fatty acids nutrition in dairy cows: from gut to cells and effects on performance

High producing dairy cows generally receive in the diet up to 5-6% of fat. This is a relatively low amount of fat in the diet compared to diets in monogastrics; however, dietary fat is important for dairy cows as demonstrated by the benefits of supplementing cows with various fatty acids (FA). Several FA are highly bioactive, especially by affecting the transcriptome; thus, they have nutrigenomic effects. In the present review, we provide an up-to-date understanding of the utilization of FA by dairy cows including the main processes affecting FA in the rumen, molecular aspects of the absorption of FA by the gut, synthesis, secretion, and utilization of chylomicrons; uptake and metabolism of FA by peripheral tissues, with a main emphasis on the liver, and main transcription factors regulated by FA. Most of the advances in FA utilization by rumen microorganisms and intestinal absorption of FA in dairy cows were made before the end of the last century with little information generated afterwards. However, large advances on the molecular aspects of intestinal absorption and cellular uptake of FA were made on monogastric species in the last 20 years. We provide a model of FA utilization in dairy cows by using information generated in monogastrics and enriching it with data produced in dairy cows. We also reviewed the latest studies on the effects of dietary FA on milk yield, milk fatty acid composition, reproduction, and health in dairy cows. The reviewed data revealed a complex picture with the FA being active in each step of the way, starting from influencing rumen microbiota, regulating intestinal absorption, and affecting cellular uptake and utilization by peripheral tissues, making prediction on in vivo nutrigenomic effects of FA challenging.

Salt-sensitive hypertension in chronic kidney disease: distal tubular mechanisms

Chronic kidney disease (CKD) causes salt-sensitive hypertension that is often resistant to treatment and contributes to the progression of kidney injury and cardiovascular disease. A better understanding of the mechanisms contributing to salt-sensitive hypertension in CKD is essential to improve these outcomes. This review critically explores these mechanisms by focusing on how CKD affects distal nephron Na+ reabsorption. CKD causes glomerulotubular imbalance with reduced proximal Na+ reabsorption and increased distal Na+ delivery and reabsorption. Aldosterone secretion further contributes to distal Na+ reabsorption in CKD and is not only mediated by renin and K+ but also by metabolic acidosis, endothelin-1, and vasopressin. CKD also activates the intrarenal renin-angiotensin system, generating intratubular angiotensin II to promote distal Na+ reabsorption. High dietary Na+ intake in CKD contributes to Na+ retention by aldosterone-independent activation of the mineralocorticoid receptor mediated through Rac1. High dietary Na+ also produces an inflammatory response mediated by T helper 17 cells and cytokines increasing distal Na+ transport. CKD is often accompanied by proteinuria, which contains plasmin capable of activating the epithelial Na+ channel. Thus, CKD causes both local and systemic changes that together promote distal nephron Na+ reabsorption and salt-sensitive hypertension. Future studies should address remaining knowledge gaps, including the relative contribution of each mechanism, the influence of sex, differences between stages and etiologies of CKD, and the clinical relevance of experimentally identified mechanisms. Several pathways offer opportunities for intervention, including with dietary Na+ reduction, distal diuretics, renin-angiotensin system inhibitors, mineralocorticoid receptor antagonists, and K+ or H+ binders.

Beyond the tubule: pathological variants of LRP2, encoding the megalin receptor, result in glomerular loss and early progressive chronic kidney disease

Pathogenic variants in the LRP2 gene, encoding the multiligand receptor megalin, cause a rare autosomal recessive syndrome: Donnai-Barrow/Facio-Oculo-Acoustico-Renal (DB/FOAR) syndrome. Because of the rarity of the syndrome, the long-term consequences of the tubulopathy on human renal health have been difficult to ascertain, and the human clinical condition has hitherto been characterized as a benign tubular condition with asymptomatic low-molecular-weight proteinuria. We investigated renal function and morphology in a murine model of DB/FOAR syndrome and in patients with DB/FOAR. We analyzed glomerular filtration rate in mice by FITC-inulin clearance and clinically characterized six families, including nine patients with DB/FOAR and nine family members. Urine samples from patients were analyzed by Western blot analysis and biopsy materials were analyzed by histology. In the mouse model, we used histological methods to assess nephrogenesis and postnatal renal structure and contrast-enhanced magnetic resonance imaging to assess glomerular number. In megalin-deficient mice, we found a lower glomerular filtration rate and an increase in the abundance of injury markers, such as kidney injury molecule-1 and N-acetyl-β-d-glucosaminidase. Renal injury was validated in patients, who presented with increased urinary kidney injury molecule-1, classical markers of chronic kidney disease, and glomerular proteinuria early in life. Megalin-deficient mice had normal nephrogenesis, but they had 19% fewer nephrons in early adulthood and an increased fraction of nephrons with disconnected glomerulotubular junction. In conclusion, megalin dysfunction, as present in DB/FOAR syndrome, confers an increased risk of progression into chronic kidney disease.

Proteostasis Regulation in the Endoplasmic Reticulum: An Emerging Theme in the Molecular Pathology and Therapeutic Management of Familial Hypercholesterolemia

Familial hypercholesterolemia (FH) is an autosomal genetic disease characterized by high serum low-density lipoprotein (LDL) content leading to premature coronary artery disease. The main genetic and molecular causes of FH are mutations in low-density lipoprotein receptor gene (LDLR) resulting in the non-clearance of LDL from the blood by hepatocytes and consequently the formation of plaques. LDLR is synthesized and glycosylated in the endoplasmic reticulum (ER) and then transported to the plasma membrane via Golgi. It is estimated that more than 50% of reported FH-causing mutations in LDLR result in misfolded proteins that are transport-defective and hence retained in ER. ER accumulation of misfolded proteins causes ER-stress and activates unfolded protein response (UPR). UPR aids protein folding, blocks further protein synthesis, and eliminates misfolded proteins via ER-associated degradation (ERAD) to alleviate ER stress. Various studies demonstrated that ER-retained LDLR mutants are subjected to ERAD. Interestingly, chemical chaperones and genetic or pharmacological inhibition of ERAD have been reported to rescue the transport defective mutant LDLR alleles from ERAD and restore their ER-Golgi transport resulting in the expression of functional plasma membrane LDLR. This suggests the possibility of pharmacological modulation of proteostasis in the ER as a therapeutic strategy for FH. In this review, we picture a detailed analysis of UPR and the ERAD processes activated by ER-retained LDLR mutants associated with FH. In addition, we discuss and critically evaluate the potential role of chemical chaperones and ERAD modulators in the therapeutic management of FH.

Vaccines Targeting PSCK9 for the Treatment of Hyperlipidemia

Despite progress in both primary and secondary prevention, cardiovascular diseases (CVD) are still the largest group of ailments contributing to morbidity and mortality worldwide. Atherosclerotic changes, the primary pathological substrate for CVD, are closely related to hypercholesterolemia. Therefore, the treatment of hypercholesterolemia is a key therapeutic strategy for CVD management. Statins, as the gold standard in the treatment of hypercholesterolemia, have shown enhanced cardiac outcomes in many randomized clinical trials. However, often despite the maximum allowed and tolerated dosage of statins, we are not able to reach the target cholesterol levels, and thus patients persist at an increased cardiovascular risk. Recently, most of the large clinical studies in the field of preventive cardiology have focused on proprotein convertase subtilisin kexin type 9 (PCSK9) and its activity regulation. PCSK9 plays an essential role in the metabolism of LDL particles by inhibiting LDL receptor recirculation to the cell surface. Recent studies have shown that inhibition of PCSK9 by the administration of monoclonal antibodies is capable of significantly reducing LDL levels (up to an additional 60%) as well as reducing the incidence of CVD. However, this treatment procedure of administering the anti-PCSK9 antibodies, most frequently two times a month, has its limitations in terms of time, patient adherence, and nevertheless cost. Administration of active vaccination instead of passive immunization with anti-PCSK9 antibodies may be an effective way of controlling blood cholesterol levels. However, clinical data, as well as human testing, are still inadequate. This work aims to provide an overview of PCSK9 vaccines and their potential clinical benefit.

A tool with many applications: vesicular stomatitis virus in research and medicine

INTRODUCTION

Vesicular stomatitis virus (VSV) has long been a useful research tool in virology and recently become an essential part of medicinal products. Vesiculovirus research is growing quickly following its adaptation to clinical gene and cell therapy and oncolytic virotherapy.

AREAS COVERED

This article reviews the versatility of VSV as a research tool and biological reagent, its use as a viral and vaccine vector delivering therapeutic and immunogenic transgenes and an oncolytic virus aiding cancer treatment. Challenges such as the immune response against such advanced therapeutic medicinal products and manufacturing constraints are also discussed.

EXPERT OPINION

The field of in vivo gene and cell therapy is advancing rapidly with VSV used in many ways. Comparison of VSV's use as a versatile therapeutic reagent unveils further prospects and problems for each application. Overcoming immunological challenges to aid repeated administration of viral vectors and minimizing harmful host-vector interactions remains one of the major challenges. In the future, exploitation of reverse genetic tools may assist the creation of recombinant viral variants that have improved onco-selectivity and more efficient vaccine vector activity. This will add to the preferential features of VSV as an excellent advanced therapy medicinal product (ATMP) platform.

Vitellogenin Receptor (VgR) Mediates Oocyte Maturation and Ovarian Development in the Pacific White Shrimp (Litopenaeus vannamei)

Oocyte maturation and ovarian development are sequentially coordinated events critical to reproduction. In the ovaries of adult oviparous animals such as birds, bony fish, insects, and crustaceans, vitellogenin receptor (VgR) is a plasma membrane receptor that specifically mediates vitellogenin (Vg) transport into oocytes. Accumulation of Vg drives sexual maturation of the female crustaceans by acting as a pivotal regulator of nutritional accumulation within oocytes, a process known as vitellogenesis. However, the mechanisms by which VgR mediates vitellogenesis are still not fully understood. In this study, we first identified a unique VgR (Lv-VgR) and characterized its genomic organization and protein structural domains in Litopenaeus vannamei, a predominant cultured shrimp species worldwide. This newly identified Lv-VgR phylogenetically forms a group with VgRs from other crustacean species within the arthropod cluster. Duplicated LBD/EGFD regions are found exclusively among arthropod VgRs but not in paralogs from vertebrates and nematodes. In terms of expression patterns, Lv-VgR transcripts are specifically expressed in ovaries of female shrimps, which increases progressively during ovarian development, and rapidly declines toward embryonic development. The cellular and subcellular locations were For analyzed by in situ hybridization and immunofluorescence, respectively. The Lv-VgR mRNA was found to be expressed in the oocytes of ovaries, and Lv-VgR protein was found to localize in the cell membrane of maturing oocytes while accumulation of the ligand Vg protein assumed an even cytoplasmic distribution. Silencing of VgR transcript expression by RNAi was effective for stunting ovarian development. This present study has thus provided new insights into the regulatory roles of VgR in crustacean ovarian development.

The island of Gran Canaria: A genetic isolate for familial hypercholesterolemia

BACKGROUND

Genetic diagnosis of familial hypercholesterolemia (FH) has not been universally performed in the Canary Islands (Spain).

OBJECTIVES

This study aimed to genetically characterize a cohort of patients with FH in the island of Gran Canaria.

METHODS

Study subjects were 70 unrelated index cases attending a tertiary hospital in Gran Canaria, with a clinical diagnosis of FH, according to the criteria of the Dutch Lipid Clinic Network. Given that 7 of the first 10 cases with positive genetic study were carriers of a single mutation in the LDLR gene [p.(Tyr400_Phe402del)], a specific polymerase chain reaction-based assay was developed for the detection of this variant as a first screening step on the remaining subjects. In those without this mutation, molecular diagnosis was completed using a next-generation sequencing panel including LDLR, APOB, PCSK9, LDLRAP1, APOE, STAP1, and LIPA genes and incorporating copy number variation detection in LDLR.

RESULTS

On the whole, 44 subjects (62%) had a positive genetic study, of whom 30 (68%) were heterozygous carriers of the p.(Tyr400_Phe402del) variant. Eleven subjects carried other mutations in LDLR, including the novel mutation NM_000527.4: c.877dupG; NP_000518.1: p.(Asp293Glyfs*8). An unclassified PCSK9 gene variant was found in one subject [(NM_174936.3:c.1496G>A; NP_777596.2: p.(Arg499His)]. Other single patients had mutations in APOB (heterozygous) and in LIPA (homozygous). All identified variants co-segregated with the disease phenotype.

CONCLUSIONS

These findings suggest a founder effect for the p.(Tyr400_Phe402del) LDLR mutation in Gran Canaria. A cost-effective local screening strategy for genetic diagnosis of FH could be implemented in this region.

Lucky, times ten: A career in Texas science

On January 21, 2017, I received an E-mail from Herb Tabor that I had been simultaneously hoping for and dreading for several years: an invitation to write a "Reflections" article for the Journal of Biological Chemistry On the one hand, I was honored to receive an invitation from Herb, a man I have admired for over 40 years, known for 24 years, and worked with as a member of the Editorial Board and Associate Editor of the Journal of Biological Chemistry for 17 years. On the other hand, the invitation marked the waning of my career as an academic scientist. With these conflicting emotions, I wrote this article with the goals of recording my career history and recognizing the many mentors, trainees, and colleagues who have contributed to it and, perhaps with pretension, with the desire that students who are beginning a career in research will find inspiration in the path I have taken and appreciate the importance of luck.

Molecular modeling of LDLR aids interpretation of genomic variants

Abstract

Genetic variants in low-density lipoprotein receptor (LDLR) are known to cause familial hypercholesterolemia (FH), occurring in up to 1 in 200 people (Youngblom E. et al. 1993 and Nordestgaard BG et al. 34:3478–3490a, 2013) and leading to significant risk for heart disease. Clinical genomics testing using high-throughput sequencing is identifying novel genomic variants of uncertain significance (VUS) in individuals suspected of having FH, but for whom the causal link to the disease remains to be established (Nordestgaard BG et al. 34:3478–3490a, 2013). Unfortunately, experimental data about the atomic structure of the LDL binding domains of LDLR at extracellular pH does not exist. This leads to an inability to apply protein structure-based methods for assessing novel variants identified through genetic testing. Thus, the ambiguities in interpretation of LDLR variants are a barrier to achieving the expected clinical value for personalized genomics assays for management of FH. In this study, we integrated data from the literature and related cellular receptors to develop high-resolution models of full-length LDLR at extracellular conditions and use them to predict which VUS alter LDL binding. We believe that the functional effects of LDLR variants can be resolved using a combination of structural bioinformatics and functional assays, leading to a better correlation with clinical presentation. We have completed modeling of LDLR in two major physiologic conditions, generating detailed hypotheses for how each of the 1007 reported protein variants may affect function.

Key messages

• Hundreds of variants are observed in the LDLR, but most lack interpretation.

• Molecular modeling is aided by biochemical knowledge.

• We generated context-specific 3D protein models of LDLR.

• Our models allowed mechanistic interpretation of many variants.

• We interpreted both rare and common genomic variants in their physiologic context.

• Effects of genomic variants are often context-specific.

Electronic supplementary material

The online version of this article (10.1007/s00109-019-01755-3) contains supplementary material, which is available to authorized users.

Long-term generation of antiPCSK9 antibody using a nanoliposome-based vaccine delivery system

BACKGROUND AND AIMS

Proprotein convertase subtilisin kexin type 9 (PCSK9) is a liver secretory enzyme that controls plasma low-density lipoprotein cholesterol (LDL-C) levels through modulation of LDL receptor (LDLR). Inhibition of PCSK9 using monoclonal antibodies (mAbs) can efficiently lower plasma LDL-C. However, the relatively short half-life of mAbs necessitates frequent passive immunization, which is costly. These limitations can be circumvented by active immunization. Here, we evaluated the long-term antiPCSK9 antibody generation in BALB/c mice vaccinated with a nanoliposomal PCSK9-specific active vaccine.

METHODS

Negatively charged nanoliposomes were used as a vaccine delivery system and prepared via lipid-film hydration method. We constructed a peptide vaccine termed Immunogenic Fused PCSK9-Tetanus (IFPT) by linking a short PCSK9 peptide (as B cell epitope) to a tetanus peptide (as T cell epitope). The IFPT peptide was conjugated to the surface of nanoliposome carriers using a DSPE-PEG- Maleimide (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[maleimide(PEG)-2000]) linker. Nanoliposomal IFPT (L-IFPT) construct was formulated with alum vaccine adjuvant (L-IFPTA⁺). To evaluate induction of antiPCSK9 antibody in vivo, BALB/c mice were subcutaneously inoculated four times in bi-weekly intervals with prepared vaccine formulations, including L-IFPT, L-IFPTA⁺, IFPTA⁺, IFPT, and empty liposomes as negative control. The long-term efficacy of antiPCSK9 antibodies was evaluated over 48 weeks after prime inoculation. Specificity of generated antiPCSK9 antibodies was assessed using ELISA method. To evaluate immunogenic safety, production of IL-4 and IFN-γ, and population of CD8⁺ and CD4⁺ T cells in splenic cells isolated from the vaccinated mice were analyzed.

RESULTS

The L-IFPTA⁺ vaccine was found to elicit the highest IgG antibody response against PCSK9 peptide in the vaccinated mice, when compared with the other vaccine formulations. Antibody titer analyses over 48 weeks post-prime vaccination revealed that the L-IFPTA⁺ vaccine was able to stimulate a long-lasting humoral immune response against PCSK9 peptide, and thereby decrease plasma PCSK9. Generated antibodies could specifically target PCSK9 and thereby inhibit PCSK9-LDLR interaction. Analysis of splenic cells showed that the population of anti-inflammatory CD4⁺ Th2 cells and production and secretion of IL-4 cytokine were increased in mice vaccinated with the L-IFPTA⁺ vaccine, while population of inflammatory CD4⁺ Th1 cell and cytotoxic CD8⁺ T cells as well as production and secretion of IFN-γ were not altered.

CONCLUSIONS

The results indicate efficient activity of the tested nanoliposomal construct (L-IFPTA⁺) to induce humoral immune response against PCSK9 in BALB/c mice. L-IFPTA⁺ vaccine can induce immunogenic-safe and long-term generation of antiPCSK9 antibodies in BALB/c mice.

Improvement of pharmacokinetic properties of therapeutic antibodies by antibody engineering

Monoclonal antibodies (mAbs) have become an important therapeutic option for several diseases. Since several mAbs have shown promising efficacy in clinic, the competition to develop mAbs has become severe. In efforts to gain a competitive advantage over other mAbs and provide significant benefits to patients, innovations in antibody engineering have aimed at improving the pharmacokinetic properties of mAbs. Because engineering can provide therapeutics that are more convenient, safer, and more efficacious for patients in several disease areas, it is an attractive approach to provide significant benefits to patients. Further advances in engineering mAbs to modulate their pharmacokinetics were driven by the increase of total soluble target antigen concentration that is often observed after injecting a mAb, which then requires a high dosage to antagonize. To decrease the required dosage, several antibody engineering techniques have been invented that reduce the total concentration of soluble target antigen. Here, we review the various ways that antibody engineering can improve the pharmacokinetic properties of mAbs.

Spontaneous severe hypercholesterolemia and atherosclerosis lesions in rabbits with deficiency of low-density lipoprotein receptor (LDLR) on exon 7

Rabbits (Oryctolagus cuniculus) have been the very frequently used as animal models in the study of human lipid metabolism and atherosclerosis, because they have similar lipoprotein metabolism to humans. Most of hyperlipidemia and atherosclerosis rabbit models are produced by feeding rabbits a high-cholesterol diet. Gene editing or knockout (KO) offered another means of producing rabbit models for study of the metabolism of lipids and lipoproteins. Even so, apolipoprotein (Apo)E KO rabbits must be fed a high-cholesterol diet to induce hyperlipidemia. In this study, we used the CRISPR/Cas9 system anchored exon 7 of low-density lipoprotein receptor (LDLR) in an attempt to generate KO rabbits. We designed two sgRNA sequences located in E7:g.7055-7074 and E7:g.7102-7124 of rabbit LDLR gene, respectively. Seven LDLR-KO founder rabbits were generated, and all of them contained biallelic modifications. Various mutational LDLR amino acid sequences of the 7 founder rabbits were subjected to tertiary structure modeling with SWISS-MODEL, and results showed that the structure of EGF-A domain of each protein differs from the wild-type. All the founder rabbits spontaneously developed hypercholesterolemia and atherosclerosis on a normal chow (NC) diet. Analysis of their plasma lipids and lipoproteins at the age of 12 weeks revealed that all these KO rabbits exhibited markedly increased levels of plasma TC (the highest of which was 1013.15 mg/dl, 20-fold higher than wild-type rabbits), LDL-C (the highest of which was 730.00 mg/dl, 35-fold higher than wild-type rabbits) and TG accompanied by reduced HDL-C levels. Pathological examinations of a founder rabbit showed prominent aortic atherosclerosis lesions and coronary artery atherosclerosis.In conclusion, we have reported the generation LDLR-KO rabbit model for the study of spontaneous hypercholesterolemia and atherosclerosis on a NC diet. The LDLR-KO rabbits should be a useful rabbit model of human familial hypercholesterolemia (FH) for the simulations of human primary hypercholesterolemia and such models would allow more exact research into cardio-cerebrovascular disease.

Nanoparticles with High-Surface Negative-Charge Density Disturb the Metabolism of Low-Density Lipoprotein in Cells

Endocytosis is an important pathway to regulate the metabolism of low-density lipoprotein (LDL) in cells. At the same time, engineering nanoparticles (ENPs) enter the cell through endocytosis in biomedical applications. Therefore, a crucial question is whether the nanoparticles involved in endocytosis could impact the natural metabolism of LDL in cells. In this study, we fabricated a series of gold nanoparticles (AuNPs) (13.00 ± 0.69 nm) with varied surface charge densities. The internalized AuNPs with high-surface negative-charge densities (HSNCD) significantly reduced LDL uptake in HepG-2, HeLa, and SMMC-7721 cells compared with those cells in control group. Notably, the significant reduction of LDL uptake in cells correlates with the reduction of LDL receptors (LDL-R) on the cell surface, but there is no change in protein and mRNA of LDL-Rs. The cyclic utilization of LDL-R in cells is a crucial pathway to maintain the homoeostasis of LDL uptake. The release of LDL-Rs from LDL/LDL-R complexes in endosomes depended on reduction of the pH in the lumen. AuNPs with HSNCD hampered vacuolar-type H⁺-ATPase V1 (ATPaseV1) and ATPaseV0 binding on the endosome membrane, blocking protons to enter the endosome by the pump. Hence, fewer freed LDL-Rs were transported into recycling endosomes (REs) to be returned to cell surface for reuse, reducing the LDL uptake of cells by receptor-mediated endocytosis. The restrained LDL-Rs in the LDL/LDL-R complex were degraded in lysosomes.

Scavenger receptor BI promotes cytoplasmic accumulation of lipoproteins in clear-cell renal cell carcinoma

Clear-cell renal cell carcinomas (ccRCCs) are characterized by inactivation of the von Hippel-Lindau (VHL) gene and intracellular lipid accumulation by unknown pathomechanisms. The immunochemical analysis of 356 RCCs revealed high abundance of apoA-I and apoB, as well as scavenger receptor BI (SR-BI) in the ccRCC subtype. Given the characteristic loss of VHL function in ccRCC, we used VHL-defective and VHL-proficient cells to study the potential influence of VHL on lipoprotein uptake. VHL-defective patient-derived ccRCC cells and cell lines (786O and RCC4) showed enhanced uptake as well as less resecretion and degradation of radio-iodinated HDL and LDL (¹²⁵I-HDL and ¹²⁵I-LDL, respectively) compared with the VHL-proficient cells. The ccRCC cells showed enhanced vascular endothelial growth factor (VEGF) and SR-BI expression compared with normal kidney epithelial cells. Uptake of ¹²⁵I-HDL and ¹²⁵I-LDL by patient-derived normal kidney epithelial cells as well as the VHL-reexpressing ccRCC cell lines, 786-O-VHL and RCC4-O-VHL cells, was strongly enhanced by VEGF treatment. The knockdown of the VEGF coreceptor, neuropilin-1 (NRP1), as well as blocking of SR-BI significantly reduced the uptake of lipoproteins into ccRCC cells in vitro. LDL stimulated proliferation of 786-O cells more potently than 786-O-VHL cells in a NRP1- and SR-BI-dependent manner. In conclusion, enhanced lipoprotein uptake due to increased activities of VEGF/NRP1 and SR-BI promotes lipid accumulation and proliferation of VHL-defective ccRCC cells.

Novel compound heterozygous mutations in low density lipoprotein receptor gene causes a severe phenotype in a Chinese hypercholesterolemia family

Mutations in the low density lipoprotein receptor (LDLR) gene serve a causative role in the pathophysiology of familial hypercholesterolemia (FH), a common autosomal inherited disorder characterized by abnormal lipid metabolism. The aim of the present study was to investigate genetic defects in a Chinese family with FH. Clinical features and family histories were collected, as were the results of various laboratory tests, including determinations of serum lipid concentrations, ultrasonography and angiography results. Potential mutations in LDLR were screened using direct polymerase chain reaction (PCR) sequencing. Multiple sequence alignments, structure and hydrophobicity predictions were performed in silico. Novel compound heterozygote mutations in LDLR of the proband were identified, with a Trp577Term-bearing maternal allele and a Pro685Leu-bearing paternal allele. The proband, a 27-year-old male, had severe and diffuse coronary stenosis and non-ST segment elevation myocardial infarction, as well as multiple skin xanthomas and high serum lipid levels. The allele-dosage-dependent clinical features, including hypercholesterolemia and peripheral arterial atherosclerosis, were observed in the proband and the other heterozygous patients. Therefore, the coexistence of Pro685Leu and Trp577Term mutations in LDLR is a novel compound heterozygosis in Chinese patients and may lead to a severe FH phenotype. The explanation for the existence of compound heterozygous mutations instead of homozygous mutations in this particular family requires further study.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

Validation of LDLr Activity as a Tool to Improve Genetic Diagnosis of Familial Hypercholesterolemia: A Retrospective on Functional Characterization of LDLr Variants

Familial hypercholesterolemia (FH) is an autosomal dominant disorder characterized by high blood-cholesterol levels mostly caused by mutations in the low-density lipoprotein receptor (LDLr). With a prevalence as high as 1/200 in some populations, genetic screening for pathogenic LDLr mutations is a cost-effective approach in families classified as ‘definite’ or ‘probable’ FH and can help to early diagnosis. However, with over 2000 LDLr variants identified, distinguishing pathogenic mutations from benign mutations is a long-standing challenge in the field. In 1998, the World Health Organization (WHO) highlighted the importance of improving the diagnosis and prognosis of FH patients thus, identifying LDLr pathogenic variants is a longstanding challenge to provide an accurate genetic diagnosis and personalized treatments. In recent years, accessible methodologies have been developed to assess LDLr activity in vitro, providing experimental reproducibility between laboratories all over the world that ensures rigorous analysis of all functional studies. In this review we present a broad spectrum of functionally characterized missense LDLr variants identified in patients with FH, which is mandatory for a definite diagnosis of FH.

Potential Link Between Proprotein Convertase Subtilisin/Kexin Type 9 and Alzheimer's Disease

Alzheimer's disease [AD] is not only the most common neurodegenerative disease but is also currently incurable. Proprotein convertase subtilisin/kexin-9 [PCSK9] is an indirect regulator of plasma low density lipoprotein [LDL] levels controlling LDL receptor expression at the plasma membrane. PCSK9 also appears to regulate the development of glucose intolerance, insulin resistance, abdominal obesity, inflammation, and hypertension, conditions that have been identified as risk factors for AD. PCSK9 levels also depend on age, sex, and ethnic background, factors associated with AD. Herein, we will review indirect evidence that suggests a link between PCSK9 levels and AD.

How multi-scale structural biology elucidated context-dependent variability in ectodomain conformation along with the ligand capture and release cycle for LDLR family members

The low-density lipoprotein receptor (LDLR) and its homologs capture and internalize lipoproteins into the cell. Due to the fact that LDLR family members possess a modular ectodomain that undergoes dynamic conformational changes, multi-scale structural analysis has been performed so as to understand the ligand capture and release mechanism. For example, crystallographic analyses have provided models for both the entire ectodomain and high-resolution structures of individual modules. In addition, nuclear magnetic resonance spectroscopic analyses have shown the rigidity and flexibility of inter-module linkers to restrict the mobility of ectodomain. Accumulated structural data suggest that the ectodomains of LDLR family members are flexible at the cell surface and switch between two metastable conformations, that is, the extended and contracted conformations. Recent structural analysis of ApoER2, a close homolog of LDLR, raised the possibility that the receptor binds with the ligand in the contracted conformation. After transport to an endosome by endocytosis, the receptor undergoes a conformational change to the closed conformation for completion of ligand release. In contrast, LDLR has been reported to adopt the extended conformation when it binds with a inhibitory regulator that recruits LDLR toward the degradation pathway. These findings support a mechanism of different ectodomain conformations for binding the ligand versus binding the regulatory protein. In this review, I provide an overview of studies that analyze the structural and biophysical properties of the ectodomains of LDLR family members and discuss a hypothetical model for ligand uptake and receptor recycling that integrates the known ectodomain conformational variability.

Regulation of PCSK9 by nutraceuticals

PCSK9 (proprotein convertase subtilisin kexin type 9) is a liver secretory enzyme that regulates plasma low-density lipoprotein (LDL) cholesterol (LDL-C) levels through modulation of LDL receptor (LDLR) density on the surface of hepatocytes. Inhibition of PCSK9 using monoclonal antibodies can efficiently lower plasma LDL-C, non-high-density lipoprotein cholesterol and lipoprotein (a). PCSK9 inhibition is also an effective adjunct to statin therapy; however, the cost-effectiveness of currently available PCSK9 inhibitors is under question. Nutraceuticals offer a safe and cost-effective option for PCSK9 inhibition. Several nutraceuticals have been reported to modulate PCSK9 levels and exert LDL-lowering activity. Mechanistically, those nutraceuticals that inhibit PCSK9 through a SREBP (sterol-responsive element binding protein)-independent pathway can be more effective in lowering plasma LDL-C levels compared with those inhibiting PCSK9 through the SREBP pathway. The present review aims to collect available data on the nutraceuticals with PCSK9-inhibitory effect and the underlying mechanisms.