Megalin acts in concert with cubilin to mediate endocytosis of high density lipoproteins

Deciphering structural aspects of reverse cholesterol transport: mapping the knowns and unknowns

Atherosclerosis is a major contributor to the onset and progression of cardiovascular disease (CVD). Cholesterol-loaded foam cells play a pivotal role in forming atherosclerotic plaques. Induction of cholesterol efflux from these cells may be a promising approach in treating CVD. The reverse cholesterol transport (RCT) pathway delivers cholesteryl ester (CE) packaged in high-density lipoproteins (HDL) from non-hepatic cells to the liver, thereby minimising cholesterol load of peripheral cells. RCT takes place via a well-organised interplay amongst apolipoprotein A1 (ApoA1), lecithin cholesterol acyltransferase (LCAT), ATP binding cassette transporter A1 (ABCA1), scavenger receptor-B1 (SR-B1), and the amount of free cholesterol. Unfortunately, modulation of RCT for treating atherosclerosis has failed in clinical trials owing to our lack of understanding of the relationship between HDL function and RCT. The fate of non-hepatic CEs in HDL is dependent on their access to proteins involved in remodelling and can be regulated at the structural level. An inadequate understanding of this inhibits the design of rational strategies for therapeutic interventions. Herein we extensively review the structure-function relationships that are essential for RCT. We also focus on genetic mutations that disturb the structural stability of proteins involved in RCT, rendering them partially or completely non-functional. Further studies are necessary for understanding the structural aspects of RCT pathway completely, and this review highlights alternative theories and unanswered questions.

HDL and chronic kidney disease

Low HDL-cholesterol (HDL-C) concentrations are a typical trait of the dyslipidemia associated with chronic kidney disease (CKD). In this condition, plasma HDLs are characterized by alterations in structure and function, and these particles can lose their atheroprotective functions, e.g., the ability to promote cholesterol efflux from peripheral cells, anti-oxidant and anti-inflammatory proprieties and they can even become dysfunctional, i.e., exactly damaging. The reduction in plasma HDL-C levels appears to be the only lipid alteration clearly linked to the progression of renal disease in CKD patients. The association between the HDL system and CKD development and progression is also supported by the presence of genetic kidney alterations linked to HDL metabolism, including mutations in the APOA1, APOE, APOL and LCAT genes. Among these, renal disease associated with LCAT deficiency is well characterized and lipid abnormalities detected in LCAT deficiency carriers mirror the ones observed in CKD patients, being present also in acquired LCAT deficiency. This review summarizes the major alterations in HDL structure and function in CKD and how genetic alterations in HDL metabolism can be linked to kidney dysfunction. Finally, the possibility of targeting the HDL system as possible strategy to slow CKD progression is reviewed.

Reproductive toxicity in marine medaka (Oryzias melastigma) due to embryonic exposure to PCB 28 or 4'-OH-PCB 65

Previous studies have shown that juvenile or adult exposure to polychlorinated biphenyls (PCBs) induces alterations in reproductive functions (e.g., reduced fertilization rate) and behavior (e.g., reduced nest maintenance) in fish. Embryonic exposures to other endocrine disrupting chemicals have been reported to induce long-term reproductive toxicity in fish. However, the effects of embryonic exposure to PCBs or their metabolites, OH-PCBs, on long-term reproductive function in fish are unknown. In the present study, we used the marine medaka fish (Oryzias melastigma) as a model to assess the reproductive endpoints in response to embryonic exposure to either PCB 28 or 4'-OH-PCB 65. Our results showed that the sex ratio of marine medaka was feminized by exposure to 4'-OH-PCB 65. Fecundity was decreased in the medaka treated with either PCB 28 or 4'-OH-PCB 65, whereas the medaka from embryonic exposure to 4'-OH-PCB 65 additionally exhibited reduced fertilization and a reduction in the hatching success rate of offspring, as well as decreased sperm motility. Serum 11-KT concentrations were reduced in the PCB 28-treated medaka, and serum estradiol (E2)/testosterone (T) and E2/11-ketotestosterone (11-KT) ratios were decreased in the 4'-OH-PCB 65-treated medaka. To explain these observations at the molecular level, transcriptomic analysis of the gonads was performed. Bioinformatic analysis using Gene Ontology and Ingenuity Pathway Analysis revealed that genes involved in various pathways potentially involved in reproductive functions (e.g., steroid metabolism and cholesterol homeostasis) were differentially expressed in the testes and ovaries of either PCB- or OH-PCB-treated medaka. Thus, the long-term reproductive toxicity in fish due to embryonic exposure to PCB or OH-PCB should be considered for environmental risk assessment.

Identification of Potential Megalin/Cubilin Substrates Using Extensive Proteomics Quantification from Kidney Megalin-Knockdown Mice

Megalin and cubilin, endocytic proteins present in the proximal tubule of the kidney, are responsible for reabsorbing filtered proteins from urine. Our hypothesis was that potential substrates of megalin/cubilin could be identified by examining urinary protein differences between control (WT) mice and kidney-specific megalin knockdown (KD) mice. Using the IonStar proteomics approach, 877 potential megalin/cubilin substrates were discovered, with 23 of these compounds representing known megalin/cubilin substrates. Some of the proteins with the largest fold changes in the urine between KD and WT included the known megalin substrates retinol-binding protein and vitamin D-binding protein. Of the total proteins identified as novel substrates, about three-quarters of compounds had molecular weights (MWs) below 69 kDa, the MW of albumin, and the remaining had higher MWs, with about 5% of the proteins having MWs greater than 150 kDa. Sex differences in the number of identified substrates occurred, but this may be due to differences in kidney megalin expression between both male and female megalin KD and WT animals, with the ratio of megalin between WT and KD being 2.76 and 2.14 for female and male mice, respectively. The top three ingenuity canonical pathways based on the urinary proteins in both female and male KD mice were acute phase response signaling, liver X receptor/retinoid X receptor activation, and intrinsic prothrombin activation pathways. In conclusion, analysis of urine samples from kidney-specific megalin KD and WT mice was found to be useful for the identification of potential endogenous substrates for megalin and cubilin.

Cylindrospermopsin induces oocyte maturation and disrupts gene expression in zebrafish ovarian follicles

There is evidence that cylindrospermopsin (CYN) exerts reproductive toxicity in mice. However, little information is available concerning the toxicity of CYN in nonmammalian vertebrates. Here, we investigated the direct action of CYN on female reproduction by studying germinal vesicle breakdown, transcript abundance, caspase-3 activity, and testosterone production using cultured follicle-enclosed zebrafish oocytes as a model system. Treatment of follicles with 1,000 μg/L CYN significantly increased GVBD, Caspase-3 activity, and hCG-induced testosterone secretion. Exposure to CYN also reduced the abundance of 3βhsd as well as hCG-induced fshr and era transcripts and increased cyp19a1 mRNA levels. In summary, this study provides a framework for a better understanding of the adverse action of CYN on female reproduction in zebrafish and other vertebrate species. The findings are also relevant to developing valid biomarkers for CYN by measuring zebrafish oocyte maturation and gene expression.

HDL and Kidney Diseases

Serum lipid profiles, as well as HDL can be altered in patients with kidney diseases. There are various types of kidney diseases, including nephrotic syndrome and chronic kidney disease. In patients with nephrotic syndrome, plasma levels of HDL cholesterol and ApoA-I were within or below the normal limits. The HDL cholesterol: total cholesterol ratio decreased compared to healthy individuals. In patients with chronic kidney disease (CKD), reverse cholesterol transport function of HDL is impaired, and CKD also affects the composition and function of HDL. Cardiovascular disease (CVD) is the severe complication of CKD. Furthermore, HDL might also be a potential target for the prevention of cardiovascular complications associated with CKD.

The Distinct Role of the HDL Receptor SR-BI in Cholesterol Homeostasis of Human Placental Arterial and Venous Endothelial Cells

As opposed to adults, high-density lipoprotein (HDL) is the main cholesterol carrying lipoprotein in fetal circulation. The major HDL receptor, scavenger receptor class B type I (SR-BI), contributes to local cholesterol homeostasis. Arterial endothelial cells (ECA) from human placenta are enriched with cholesterol compared to venous endothelial cells (ECV). Moreover, umbilical venous and arterial plasma cholesterol levels differ markedly. We tested the hypothesis that the uptake of HDL-cholesteryl esters differs between ECA and ECV because of the differential expression of SR-BI. We aimed to identify the key regulators underlying these differences and the functional consequences. Immunohistochemistry was used for visualization of SR-BI in situ. ECA and ECV were isolated from the chorionic plate of human placenta and used for RT-qPCR, Western Blot, and HDL uptake assays with ³H- and ¹²⁵I-labeled HDL. DNA was extracted for the methylation profiling of the SR-BI promoter. SR-BI regulation was studied by exposing ECA and ECV to differential oxygen concentrations or shear stress. Our results show elevated SR-BI expression and protein abundance in ECA compared to ECV in situ and in vitro. Immunohistochemistry demonstrated that SR-BI is mainly expressed on the apical side of placental endothelial cells in situ, allowing interaction with mature HDL circulating in the fetal blood. This was functionally linked to a higher increase of selective cholesterol ester uptake from fetal HDL in ECA than in ECV, and resulted in increased cholesterol availability in ECA. SR-BI expression on ECV tended to decrease with shear stress, which, together with heterogeneous immunostaining, suggests that SR-BI expression is locally regulated in the placental vasculature. In addition, hypomethylation of several CpG sites within the SR-BI promoter region might contribute to differential expression of SR-BI between chorionic arteries and veins. Therefore, SR-BI contributes to a local cholesterol homeostasis in ECA and ECV of the human feto-placental vasculature.

Cubilin, the intrinsic factor-vitamin B12 receptor

Cubilin (CUBN), the intrinsic factor-vitamin B12 receptor is a large endocytic protein involved in various physiological functions: vitamin B12 uptake in the gut; reabsorption of albumin and maturation of vitamin D in the kidney; nutrient delivery during embryonic development. Cubilin is an atypical receptor, peripherally associated to the plasma membrane. The transmembrane proteins amnionless (AMN) and Lrp2/Megalin are the currently known molecular partners contributing to plasma membrane transport and internalization of Cubilin. The role of Cubilin/Amn complex in the handling of vitamin B12 in health and disease has extensively been studied and so is the role of the Cubilin-Lrp2 tandem in renal pathophysiology. Accumulating evidence strongly supports a role of Cubilin in some developmental defects including impaired closure of the neural tube. Are these defects primarily caused by the dysfunction of a specific Cubilin ligand or are they secondary to impaired vitamin B12 or protein uptake? We will present the established Cubilin functions, discuss the developmental data and provide an overview of the emerging implications of Cubilin in the field of cardiovascular disease and cancer pathogenesis.

A Specific Tubular ApoA-I Distribution Is Associated to FSGS Recurrence after Kidney Transplantation

A major complication of primary focal segmental glomerulosclerosis (FSGS) is its recurrence after kidney transplantation that happens in 30 to 40% of the patients. The diagnosis of these relapses is not always easy as the histological lesions are not highly specific and appear after the proteinuria increase. Currently, there are no accurate biomarkers to detect FSGS recurrence. Our group identified a modified form of Apolipoprotein A-I (ApoA-I), named ApoA-Ib, specifically present in the urine of recurrent FSGS patients after kidney transplantation. Aberrant forms of ApoA-I have also been described in the urine of native primary FSGS patients; this feature has been associated with prominent staining of ApoA-I at the apical membrane of the tubular cells. In this study, we aim to analyze the ApoA-I distribution in kidney allograft biopsies of recurrent FSGS patients. We detected ApoA-I by immunohistochemistry in kidney allograft biopsies of patients with FSGS relapse after kidney transplantation and in kidney allograft biopsies of patients with a disease different from FSGS in the native kidney (non-FSGS). In recurrent FSGS patients, ApoA-I was prominently localized at the brush border of the tubular cells, while in the non-FSGS patients, ApoA-I was found along the cytoplasm of the tubular cells. The localization of ApoA-I at the brush border of the tubular cells is a specific feature of primary FSGS in relapse. This suggests that ApoA-I staining in kidney biopsies, coupled with ApoA-Ib measurement in urine, could be used as a diagnostic tool of primary FSGS relapse after kidney transplantation due to its highly specific tubular distribution.

High-Density Lipoproteins and the Kidney

Dyslipidemia is a typical trait of patients with chronic kidney disease (CKD) and it is typically characterized by reduced high-density lipoprotein (HDL)-cholesterol(c) levels. The low HDL-c concentration is the only lipid alteration associated with the progression of renal disease in mild-to-moderate CKD patients. Plasma HDL levels are not only reduced but also characterized by alterations in composition and structure, which are responsible for the loss of atheroprotective functions, like the ability to promote cholesterol efflux from peripheral cells and antioxidant and anti-inflammatory proprieties. The interconnection between HDL and renal function is confirmed by the fact that genetic HDL defects can lead to kidney disease; in fact, mutations in apoA-I, apoE, apoL, and lecithin–cholesterol acyltransferase (LCAT) are associated with the development of renal damage. Genetic LCAT deficiency is the most emblematic case and represents a unique tool to evaluate the impact of alterations in the HDL system on the progression of renal disease. Lipid abnormalities detected in LCAT-deficient carriers mirror the ones observed in CKD patients, which indeed present an acquired LCAT deficiency. In this context, circulating LCAT levels predict CKD progression in individuals at early stages of renal dysfunction and in the general population. This review summarizes the main alterations of HDL in CKD, focusing on the latest update of acquired and genetic LCAT defects associated with the progression of renal disease.

Perioperative high density lipoproteins, oxidative stress, and kidney injury after cardiac surgery

Oxidative stress promotes acute kidney injury (AKI). Higher HDL cholesterol concentrations are associated with less AKI. To test the hypothesis that HDL antioxidant activity is associated with AKI after cardiac surgery, we quantified HDL particle (HDL-P) size and number, paraoxonase-1 (PON-1) activity, and isofuran concentrations in 75 patients who developed AKI and 75 matched control patients. Higher preoperative HDL-P was associated with less AKI (OR: 0.80; 95% CI, 0.71–0.91; P = 0.001), higher PON-1 activity ( P < 0.001), and lower plasma concentrations of isofurans immediately after surgery (P = 0.02). Similarly, higher preoperative small HDL-P was associated with less AKI, higher PON-1 activity, and lower isofuran concentrations. Higher intraoperative particle losses were associated with less AKI (OR: 0.79; 95% CI 0.67–0.93; P = 0.005), and with decreased postoperative isofuran concentrations (P = 0.04) . Additionally, higher preoperative small HDL-P and increased intraoperative small particle loss were associated with improved long-term renal function (P = 0.003, 0.01, respectively). In conclusion, a higher preoperative concentration of HDL-P, particularly small particles, is associated with lower oxidative damage and less AKI. Perioperative changes in HDL-P concentrations are also associated with AKI. Small HDL-P may represent a novel modifiable risk factor for AKI.

HDL cholesterol is associated with PBMC expression of genes involved in HDL metabolism and atherogenesis

Background

To reveal the association of plasma level of high density lipoprotein cholesterol (HDL-C) level with the transcript level of annotated genes in peripheral blood mononuclear cells (PBMC) and involved in HDL metabolism and atherogenesis at the absence of morphologically evident coronary stenosis.

Methods

Transcript levels of 63 genes in PBMC from 38 male patients 40-60 years without coronary atherosclerosis with widely varied HDL-C level were measured. The protein interactions were analyzed with STRING database.

Results

Among 22 HDL-related genes, the transcript levels for 10 genes (ABCA1, BMP1, CUBN, HDLBP, LCAT, LDLR, PRKACB, PRKACG, SCARB1 and ZDHHC8) negatively correlated with HDL-C, while positively for APOA1 gene. Among 41 atherosclerosis-prone genes, the transcript levels for 11 genes (CSF1R, CSF2RB, IL18R1, ITGAM, ITGB3, PRKCQ, SREBF1, TLR5, TLR8, TNFRSF1A and TNFRSF1B) negatively correlated with HDL-C only, not with LDL-C and plasma TG. The protein products efficiently interacted within each cluster while only two intersection nodes existed between clusters.

Conclusions

Coordinate regulation of cholesterol influx and efflux in PBMC in atherosclerosis-free subjects with widely varied HDL-C level is suggested. The decreased synthesis and transport of cholesteryl ester to the liver may contribute to hyperalphalipoproteinemia. HDL-C increase is associated with the decrease of expression of innate immunity and inflammation genes. Visualization of 22 responder genes is suggested to be useful in the validation of HDL functionality and atherogenesis even at the absence of morphologically evident coronary stenosis.

A cellular model of albumin endocytosis uncovers a link between membrane and nuclear proteins

Cubilin (CUBN) and amnionless (AMN), expressed in kidney and intestine, form a multiligand receptor complex called CUBAM that plays a crucial role in albumin absorption. To date, the mechanism of albumin endocytosis mediated by CUBAM remains to be elucidated. Here, we describe a quantitative assay to evaluate albumin uptake by CUBAM using cells expressing full-length CUBN and elucidate the crucial roles of the C-terminal part of CUBN and the endocytosis signal motifs of AMN in albumin endocytosis. We also demonstrate that nuclear valosin-containing protein-like 2 (NVL2), an interacting protein of AMN, is involved in this process. Although NVL2 was mainly localized in the nucleolus in cells without AMN expression, it was translocated to the extranuclear compartment when coexpressed with AMN. NVL2 knockdown significantly impaired internalization of the CUBN-albumin complex in cultured cells, demonstrating an involvement of NVL2 in endocytic regulation. These findings uncover a link between membrane and nucleolar proteins that is involved in endocytic processes.

A misprocessed form of Apolipoprotein A-I is specifically associated with recurrent Focal Segmental Glomerulosclerosis

Apolipoprotein A-Ib (ApoA-Ib) is a high molecular weight form of Apolipoprotein A-I (ApoA-I) found specifically in the urine of kidney-transplanted patients with recurrent idiopathic focal segmental glomerulosclerosis (FSGS). To determine the nature of the modification present in ApoA-Ib, we sequenced the whole APOA1 gene in ApoA-Ib positive and negative patients, and we also studied the protein primary structure using mass spectrometry. No genetic variations in the APOA1 gene were found in the ApoA-Ib positive patients that could explain the increase in its molecular mass. The mass spectrometry analysis revealed three extra amino acids at the N-Terminal end of ApoA-Ib that were not present in the standard plasmatic form of ApoA-I. These amino acids corresponded to half of the propeptide sequence of the immature form of ApoA-I (proApoA-I) indicating that ApoA-Ib is a misprocessed form of proApoA-I. The description of ApoA-Ib could be relevant not only because it can allow the automated analysis of this biomarker in the clinical practice but also because it has the potential to shed light into the molecular mechanisms that cause idiopathic FSGS, which is currently unknown.

Endothelial-specific deficiency of megalin in the brain protects mice against high-fat diet challenge

BACKGROUND

The increasing risk of obesity and diabetes among other metabolic disorders are the consequence of shifts in dietary patterns with high caloric-content food intake. We previously reported that megalin regulates energy homeostasis using blood-brain barrier (BBB) endothelial megalin-deficient (EMD) mice, since these animals developed obesity and metabolic syndrome upon normal chow diet administration. Obesity in mid-life appears to be related to greater dementia risk and represents an increasing global health issue. We demonstrated that EMD phenotype induced impaired learning ability and recognition memory, neurodegeneration, neuroinflammation, reduced neurogenesis, and mitochondrial deregulation associated with higher mitochondrial mass in cortical tissues.

METHODS

EMD mice were subjected to normal chow and high-fat diet (HFD) for 14 weeks and metabolic changes were evaluated.

RESULTS

Surprisingly, BBB megalin deficiency protected against HFD-induced obesity improving glucose tolerance and preventing hepatic steatosis. Compared to wild type (wt), the brain cortex in EMD mice showed increased levels of the mitochondrial biogenesis regulator, peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), and uncoupling protein 2 (UCP2), a thermogenic protein involved in the regulation of energy metabolism. This agreed with the previously found increased mitochondrial mass in the transgenic mice. Upon HFD challenge, we demonstrated these two proteins were found elevated in wt mice but reported no changes over the already increased levels in EMD animals.

CONCLUSION

We propose a protective role for megalin on diet-induce obesity, suggesting this could be related to metabolic disturbances found in dementia through brain endocrine system communications.

Urinary apolipoprotein AI in children with kidney disease

BACKGROUND

Although high-density lipoprotein (HDL) modulates many cell types in the cardiovascular system, little is known about HDL in the kidney. We assessed urinary excretion of apolipoprotein AI (apoAI), the main protein in HDL.

METHODS

We enrolled 228 children with various kidney disorders and 40 controls. Urinary apoAI, albumin, and other markers of kidney damage were measured using ELISA, apoAI isoforms with Western blot, and renal biopsies stained for apoAI.

RESULTS

Patients followed in nephrology clinic had elevated urinary apoAI vs. controls (median 0.074 μg/mg; interquartile range (IQR) 0.0160-0.560, vs. 0.019 μg/mg; IQR 0.004-0.118, p < 0.001). Patients with tubulopathies, renal dysplasia/congenital anomalies of the kidney and urogenital tract, glomerulonephritis, and nephrotic syndrome (NS) in relapse had the greatest elevations (p ≤ 0.01). Patients with NS in remission, nephrolithiasis, polycystic kidney disease, transplant, or hypertension were not different from controls. Although all NS in relapse had higher apoAI excretion than in remission (0.159 vs. 0.0355 μg/mg, p = 0.01), this was largely driven by patients with focal segmental glomerulosclerosis (FSGS). Many patients, especially with FSGS, had increased urinary apoAI isoforms. Biopsies from FSGS patients showed increased apoAI staining at proximal tubule brush border, compared to diffuse cytoplasmic distribution in minimal change disease.

CONCLUSIONS

Children with kidney disease have variably increased urinary apoAI depending on underlying disease. Urine apoAI is particularly elevated in diseases affecting proximal tubules. Kidney disease is also associated with high molecular weight (HMW) apoAI isoforms in urine, especially FSGS. Whether abnormal urinary apoAI is a marker or contributor to renal disease awaits further study.

Cubilin, the Intrinsic Factor-Vitamin B12 Receptor in Development and Disease

Gp280/Intrinsic factor-vitamin B12 receptor/Cubilin (CUBN) is a large endocytic receptor serving multiple functions in vitamin B12 homeostasis, renal reabsorption of protein or toxic substances including albumin, vitamin D-binding protein or cadmium. Cubilin is a peripheral membrane protein consisting of 8 Epidermal Growth Factor (EGF)-like repeats and 27 CUB (defined as Complement C1r/C1s, Uegf, BMP1) domains. This structurally unique protein interacts with at least two molecular partners, Amnionless (AMN) and Lrp2/Megalin. AMN is involved in appropriate plasma membrane transport of Cubilin whereas Lrp2 is essential for efficient internalization of Cubilin and its ligands. Observations gleaned from animal models with Cubn deficiency or human diseases demonstrate the importance of this protein. In this review addressed to basic research and medical scientists, we summarize currently available data on Cubilin and its implication in renal and intestinal biology. We also discuss the role of Cubilin as a modulator of Fgf8 signaling during embryonic development and propose that the Cubilin-Fgf8 interaction may be relevant in human pathology, including in cancer progression, heart or neural tube defects. We finally provide experimental elements suggesting that some aspects of Cubilin physiology might be relevant in drug design.

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.

Plasma proteomic analysis reveals altered protein abundances in cardiovascular disease

BACKGROUND

Cardiovascular disease (CVD) describes the pathological conditions of the heart and blood vessels. Despite the large number of studies on CVD and its etiology, its key modulators remain largely unknown. To this end, we performed a comprehensive proteomic analysis of blood plasma, with the scope to identify disease-associated changes after placing them in the context of existing knowledge, and generate a well characterized dataset for further use in CVD multi-omics integrative analysis.

METHODS

LC-MS/MS was employed to analyze plasma from 32 subjects (19 cases of various CVD phenotypes and 13 controls) in two steps: discovery (13 cases and 8 controls) and test (6 cases and 5 controls) set analysis. Following label-free quantification, the detected proteins were correlated to existing plasma proteomics datasets (plasma proteome database; PPD) and functionally annotated (Cytoscape, Ingenuity Pathway Analysis). Differential expression was defined based on identification confidence (≥ 2 peptides per protein), statistical significance (Mann-Whitney p value ≤ 0.05) and a minimum of twofold change.

RESULTS

Peptides detected in at least 50% of samples per group were considered, resulting in a total of 3796 identified proteins (838 proteins based on ≥ 2 peptides). Pathway annotation confirmed the functional relevance of the findings (representation of complement cascade, fibrin clot formation, platelet degranulation, etc.). Correlation of the relative abundance of the proteins identified in the discovery set with their reported concentrations in the PPD was significant, confirming the validity of the quantification method. The discovery set analysis revealed 100 differentially expressed proteins between cases and controls, 39 of which were verified (≥ twofold change) in the test set. These included proteins already studied in the context of CVD (such as apolipoprotein B, alpha-2-macroglobulin), as well as novel findings (such as low density lipoprotein receptor related protein 2 [LRP2], protein SZT2) for which a mechanism of action is suggested.

CONCLUSIONS

This proteomic study provides a comprehensive dataset to be used for integrative and functional studies in the field. The observed protein changes reflect known CVD-related processes (e.g. lipid uptake, inflammation) but also novel hypotheses for further investigation including a potential pleiotropic role of LPR2 but also links of SZT2 to CVD.

The association between high-density lipoproteins and estimated glomerular filtration rate in patients without severe kidney disease

PURPOSE

Several studies investigated the association between the estimated glomerular filtration rate (eGFR) and the concentration of high-density lipoproteins (HDL) in patients without severely damaged kidney function. As results of those studies were inconclusive and contradictory, we wanted to investigate this association in multiple cardiovascular disease (CVD) risk patients without severe kidney disease or renal failure.

METHODS

We enrolled a cohort of 187 patients with intermediate and high CVD risk without severe renal disease. We grouped them based on their eGFR into: group 1 (≥ 30 < 60 ml/min/1.73 m²), group 2 (≥ 60 < 90 ml/min/1.73 m²) and group 3 (≥ 90 ml/min/1.73 m²). We analyzed the difference between their HDL levels and assessed the association of HDL and eGFR in three regression models with the following predictors: model 1 (age and gender), model 2 (model 1 plus smoking status, hs-CRP and diabetes mellitus) and model 3 (model 2 plus excessive weight and obesity, hypertension, hypercholesterolemia, hypertriglyceridemia, family history of CVD and medications they used).

RESULTS

Patients with the lowest eGFR had the lowest HDL values (P = 0.013). In multiple linear regression, HDL was an independent predictor of eGFR (β = 0.189, P = 0.025) which was also shown in multinomial regression for all three models: model 1 [odds ratio (OR) 0.05; 95% confidence interval (CI) 0.007-0.331; P = 0.002], model 2 (OR 0.052; 95% CI 0.006-0.428; P = 0.006) and model 3 (OR 0.2; 95% CI 0.001-0.309; P = 0.005).

CONCLUSIONS

Low HDL is an independent predictor of lower eGFR in intermediate and high CVD risk patients without severe kidney disease. In such patients, low HDL could be one of the early indicators of renal failure.

Kidneys: key modulators of high-density lipoprotein levels and function

PURPOSE OF REVIEW

The review will examine advances in our understanding of the role kidneys play in high-density lipoprotein (HDL) metabolism and the effect on levels, composition, and function of HDL particles.

RECENT FINDINGS

Components of the HDL particles can cross the glomerular filtration barrier. Some of these components, including apolipoproteins and enzymes involved in lipid metabolism, are taken up by the proximal tubule and degraded, modified, salvaged/returned to the circulation, or lost in the urine. Injury of the glomerular capillaries or tubules can affect these intrarenal processes and modify HDL. Changes in the plasma and urine levels of HDL may be novel markers of kidney damage or mechanism(s) of kidney disease.

SUMMARY

The kidneys have a significant role in the metabolism of individual HDL components, which in turn modulate HDL levels, composition, and functionality of HDL particles. These intrarenal effects may be useful markers of kidney damage and have consequences on kidney-related perturbations in HDL.

Impact of individual acute phase serum amyloid A isoforms on HDL metabolism in mice

The acute phase (AP) reactant serum amyloid A (SAA), an HDL apolipoprotein, exhibits pro-inflammatory activities, but its physiological function(s) are poorly understood. Functional differences between SAA1.1 and SAA2.1, the two major SAA isoforms, are unclear. Mice deficient in either isoform were used to investigate plasma isoform effects on HDL structure, composition, and apolipoprotein catabolism. Lack of either isoform did not affect the size of HDL, normally enlarged in the AP, and did not significantly change HDL composition. Plasma clearance rates of HDL apolipoproteins were determined using native HDL particles. The fractional clearance rates (FCRs) of apoA-I, apoA-II, and SAA were distinct, indicating that HDL is not cleared as intact particles. The FCRs of SAA1.1 and SAA2.1 in AP mice were similar, suggesting that the selective deposition of SAA1.1 in amyloid plaques is not associated with a difference in the rates of plasma clearance of the isoforms. Although the clearance rate of SAA was reduced in the absence of the HDL receptor, scavenger receptor class B type I (SR-BI), it remained significantly faster compared with that of apoA-I and apoA-II, indicating a relatively minor role of SR-BI in SAA's rapid clearance. These studies enhance our understanding of SAA metabolism and SAA's effects on AP-HDL composition and catabolism.

PET Imaging of Tumor-Associated Macrophages with 89Zr-Labeled High-Density Lipoprotein Nanoparticles

Tumor-associated macrophages (TAMs) are increasingly investigated in cancer immunology and are considered a promising target for better and tailored treatment of malignant growth. Although TAMs also have high diagnostic and prognostic value, TAM imaging still remains largely unexplored. Here, we describe the development of reconstituted high-density lipoprotein (rHDL)-facilitated TAM PET imaging in a breast cancer model.

METHODS

Radiolabeled rHDL nanoparticles incorporating the long-lived positron-emitting nuclide (89)Zr were developed using 2 different approaches. The nanoparticles were composed of phospholipids and apolipoprotein A-I (apoA-I) in a 2.5:1 weight ratio. (89)Zr was complexed with deferoxamine (also known as desferrioxamine B, desferoxamine B), conjugated either to a phospholipid or to apoA-I to generate (89)Zr-PL-HDL and (89)Zr-AI-HDL, respectively. In vivo evaluation was performed in an orthotopic mouse model of breast cancer and included pharmacokinetic analysis, biodistribution studies, and PET imaging. Ex vivo histologic analysis of tumor tissues to assess regional distribution of (89)Zr radioactivity was also performed. Fluorescent analogs of the radiolabeled agents were used to determine cell-targeting specificity using flow cytometry.

RESULTS

The phospholipid- and apoA-I-labeled rHDL were produced at 79% ± 13% (n = 6) and 94% ± 6% (n = 6) radiochemical yield, respectively, with excellent radiochemical purity (>99%). Intravenous administration of both probes resulted in high tumor radioactivity accumulation (16.5 ± 2.8 and 8.6 ± 1.3 percentage injected dose per gram for apoA-I- and phospholipid-labeled rHDL, respectively) at 24 h after injection. Histologic analysis showed good colocalization of radioactivity with TAM-rich areas in tumor sections. Flow cytometry revealed high specificity of rHDL for TAMs, which had the highest uptake per cell (6.8-fold higher than tumor cells for both DiO@Zr-PL-HDL and DiO@Zr-AI-HDL) and accounted for 40.7% and 39.5% of the total cellular DiO@Zr-PL-HDL and DiO@Zr-AI-HDL in tumors, respectively.

CONCLUSION

We have developed (89)Zr-labeled TAM imaging agents based on the natural nanoparticle rHDL. In an orthotopic mouse model of breast cancer, we have demonstrated their specificity for macrophages, a result that was corroborated by flow cytometry. Quantitative macrophage PET imaging with our (89)Zr-rHDL imaging agents could be valuable for noninvasive monitoring of TAM immunology and targeted treatment.

Residual Cardiovascular Risk in Chronic Kidney Disease: Role of High-density Lipoprotein

Although reducing low-density lipoprotein-cholesterol (LDL-C) levels with lipid-lowering agents (statins) decreases cardiovascular disease (CVD) risk, a substantial residual risk (up to 70% of baseline) remains after treatment in most patient populations. High-density lipoprotein (HDL) is a potential contributor to residual risk, and low HDL-cholesterol (HDL-C) is an established risk factor for CVD. However, in contrast to conventional lipid-lowering therapies, recent studies show that pharmacologic increases in HDL-C levels do not bring about clinical benefits. These observations have given rise to the concept of dysfunctional HDL where increases in serum HDL-C may not be beneficial because HDL loss of function is not corrected by or even intensified by the therapy. Chronic kidney disease (CKD) increases CVD risk, and patients whose CKD progresses to end-stage renal disease (ESRD) requiring dialysis are at the highest CVD risk of any patient type studied. The ESRD population is also unique in its lack of significant benefit from standard lipid-lowering interventions. Recent studies indicate that HDL-C levels do not predict CVD in the CKD population. Moreover, CKD profoundly alters metabolism and composition of HDL particles and impairs their protective effects on functions such as cellular cholesterol efflux, endothelial protection, and control of inflammation and oxidation. Thus, CKD-induced perturbations in HDL may contribute to the excess CVD in CKD patients. Understanding the mechanisms of vascular protection in renal disease can present new therapeutic targets for intervention in this population.

Dab2, megalin, cubilin and amnionless receptor complex might mediate intestinal endocytosis in the suckling rat

We previously proposed that Dab2 participates in the endocytosis of milk macromolecules in rat small intestine. Here we investigate the receptors that may mediate this endocytosis by studying the effects of age and diet on megalin, VLDLR, and ApoER2 expression, and that of age on the expression of cubilin and amnionless. Of megalin, VLDLR and ApoER2, only the megalin expression pattern resembles that of Dab2 previously reported. Thus the mRNA and protein levels of megalin and Dab2 are high in the intestine of the suckling rat, down-regulated by age and up-regulated by milk diet, mainly in the ileum. Neither age nor diet affect ApoER2 mRNA levels. The effect of age on VLDLR mRNA levels depends on the epithelial cell tested but they are down-regulated by milk diet. In the suckling rat, the intestinal expressions of both cubilin and amnionless are similar to that of megalin and megalin, cubilin, amnionless and Dab2 co-localize at the microvilli and in the apical endocytic apparatus. Co-localization of Dab2 with ApoER2 and VLDLR at the microvilli and in the apical endocytic apparatus is also observed. This is the first report showing intestinal co-localization of: megalin/cubilin/amnionless/Dab2, VLDLR/Dab2 and ApoER2/Dab2. We conclude that the megalin/cubilin/amnionless/Dab2 complex/es participate in intestinal processes, mainly during the lactation period and that Dab2 may act as an adaptor in intestinal processes mediated by ApoER2 and VLDLR.

Megalin expression in human term and preterm placental villous tissues: effect of gestational age and sample processing and storage time

INTRODUCTION

The aims of this study were to characterize megalin expression in human term and preterm placental villous tissues and to assess the impact of gestational age and sample storage on receptor expression.

METHODS

Placental tissue samples were collected from pregnant women undergoing term and preterm Cesarean deliveries. Placental villous tissues were used to quantify megalin protein and mRNA expression by western blotting and quantitative polymerase chain reaction (q-PCR), respectively. Stability of megalin expression was also evaluated under various processing and storage conditions.

RESULTS

Megalin mRNA was detected in term and preterm placental villous tissues. Expression in early preterm samples was 6-fold higher than in late preterm and term samples. Refrigeration of processed term samples at 4°C for up to 18h had a slight impact on megalin mRNA expression with stored samples exhibiting mRNA levels approximately 1.5-fold lower than those frozen immediately after processing. A greater decrease in mRNA expression (up to 33-fold) was observed when processed samples were snap-frozen immediately and thawed at 4°C. Processing of samples prior to refrigeration also appeared to improve mRNA stability with significantly higher expression levels noted in processed vs. unprocessed samples at all points for up to 48h.

DISCUSSION

These data suggest that expression of megalin mRNA in term placental villous tissue is relatively stable for up to 18h when samples are processed immediately and refrigerated at 4°C prior to freezing. Processing prior to storage also appears to improve mRNA stability. This paper demonstrates the practical feasibility of analyzing stored tissue samples, thus, it will help with placental mRNA analysis. Additionally, megalin expression appears to vary inversely with gestational age with the greatest expression noted in the most premature samples. Age-dependent differences in placental megalin may therefore influence fetal exposure.

Fibrinogen alpha chain precursor and apolipoprotein A-I in urine as biomarkers for noninvasive diagnosis of calcium oxalate nephrolithiasis: a proteomics study

Calcium oxalate nephrolithiasis is the most common urological disease, but noninvasive and convenient methods of diagnosis are rarely available. Objective. The present study aimed to identify potential urine biomarkers for noninvasive diagnosis of CaOx nephrolithiasis. Methodology. Urine samples from 72 patients with CaOx nephrolithiasis and 30 healthy controls were collected and proteomics analysis was performed using matrix-assisted laser desorption/ionization-time of flight-mass spectrometer (MALDI-TOF-MS). Results. Thirteen proteins/peptides displayed statistically significant differences. The peptides of m/z 1207.23 and 2773.86 were selected by the genetic algorithm (GA) to build a possible diagnostic model. The area under the curve of m/z 1207.23 and 2773.86 was 0.936 and 0.987, respectively. The diagnostic model in distinguishing patients and healthy subjects showed 100% sensitivity and specificity. The peak at m/z 2773.86 was identified as fibrinogen alpha chain (FGA) with the sequence G.EGDFLAEGGGVR.G, and the peak at m/z 2773.86 was identified as apolipoprotein A-I (apoA-I) with the sequence L.PVLESFKVSFLSALEEYTKKLNTQ. Conclusion. The study results strongly suggested that urinary FGA and apoA-I are highly sensitive and specific biomarkers for noninvasive diagnosis of CaOx nephrolithiasis.

The kidneys play an important role in the clearance of rFVIIa in rats

INTRODUCTION

Previous distribution and histological studies have indicated that the kidneys and renal proximal tubular cells play a role in clearance of rFVIIa. However, the relative importance of the kidneys in clearance of rFVIIa has not previously been addressed. The objective of the present study was to evaluate the importance of the kidneys in the clearance process of rFVIIa after iv administration to rats using a nephrectomy model.

MATERIALS AND METHODS

A nephrectomized rat model was established and validated using inulin, a compound primarily cleared by the kidneys, as a test substance and several physiological parameters were monitored to ensure viability and robustness of the model. The model was then used for pharmacokinetic evaluation of renal clearance of rFVIIa. The pharmacokinetic parameters for rFVIIa were evaluated both by use of standard non-compartmental methods and by use of mixed effects methods, where a pharmacokinetic model was used to simultaneously model all data from healthy, sham operated, and nephrectomized rats.

RESULTS

Nephrectomized animals showed stable rectal temperature, SpO2 and pulse and as expected, clearance of inulin was essentially abolished compared to control animals (p<0.001). For rFVIIa, nephrectomy resulted in a clearance and terminal half-life of 34mL/h/kg and 2.8h compared to 68mL/h/kg and1.9h in rats exposed to sham surgery (p<0.0001 for both parameters).

CONCLUSION

The present data show that about 50% of the total clearance of rFVIIa from circulation in rats under isoflurane anaesthesia is due to renal clearance.

Regulation of high-density lipoprotein metabolism

There is compelling evidence from human population studies that plasma levels of high-density lipoprotein (HDL) cholesterol correlate inversely with cardiovascular risk. Identification of this relationship has stimulated research designed to understand how HDL metabolism is regulated. The ultimate goal of these studies has been to develop HDL-raising therapies that have the potential to decrease the morbidity and mortality associated with atherosclerotic cardiovascular disease. However, the situation has turned out to be much more complex than originally envisaged. This is partly because the HDL fraction consists of multiple subpopulations of particles that vary in terms of shape, size, composition, and surface charge, as well as in their potential cardioprotective properties. This heterogeneity is a consequence of the continual remodeling and interconversion of HDL subpopulations by multiple plasma factors. Evidence that the remodeling of HDLs may impact on their cardioprotective properties is beginning to emerge. This serves to highlight the importance of understanding not only how the remodeling and interconversion of HDL subpopulations is regulated but also how these processes are affected by agents that increase HDL levels. This review provides an overview of what is currently understood about HDL metabolism and how the subpopulation distribution of these lipoproteins is regulated.

Cubilin maintains blood levels of HDL and albumin

Cubilin is an endocytic receptor highly expressed in renal proximal tubules, where it mediates uptake of albumin and filtered forms of apoA-I/HDL. Cubilin deficiency leads to urinary loss of albumin and apoA-I; however, the consequences of cubilin loss on the homeostasis of blood albumin and apoA-I/HDL have not been studied. Using mice heterozygous for cubilin gene deletion (cubilin HT mice), we show that cubilin haploinsufficiency leads to reduced renal proximal tubular uptake of albumin and apoA-I and significantly increased urinary loss of albumin and apoA-I. Moreover, cubilin HT mice displayed significantly decreased blood levels of albumin, apoA-I, and HDL. The levels of albumin and apoA-I protein or mRNA expressed in the liver, kidney, or intestine of cubilin HT mice did not change significantly. The clearance rate of small HDL3 particles (density>1.13 g/ml) from the blood increased significantly in cubilin HT mice. In contrast, the rate of clearance of larger HDL2 particles from the blood did not change significantly, indicating a decreased half-life for HDL particles capable of filtering through the glomerulus. On the basis of these findings, we conclude that cubilin deficiency reduces renal salvage and delivery back to the blood of albumin and apoA-I, which decreases blood levels of albumin and apoA-I/HDL. These findings raise the possibility that therapeutic increase of renal cubilin expression might reduce proteinuria and increase blood levels of albumin and HDL.

Uptake and transport of B12-conjugated nanoparticles in airway epithelium

Non-invasive delivery of biotherapeutics, as an attractive alternative to injections, could potentially be achieved through the mucosal surfaces, utilizing nanoscale therapeutic carriers. However, nanoparticles do not readily cross the mucosal barriers, with the epithelium presenting a major barrier to their translocation. The transcytotic pathway of vitamin B12 has previously been shown to 'ferry' B12-decorated nanoparticles across intestinal epithelial (Caco-2) cells. However, such studies have not been reported for the airway epithelium. Furthermore, the presence in the airways of the cell machinery responsible for transepithelial trafficking of B12 is not widely reported. Using a combination of molecular biology and immunostaining techniques, our work demonstrates that the bronchial cell line, Calu-3, expresses the B12-intrinsic factor receptor, the transcobalamin II receptor and the transcobalamin II carrier protein. Importantly, the work showed that sub-200 nm model nanoparticles chemically conjugated to B12 were internalised and transported across the Calu-3 cell layers, with B12 conjugation not only enhancing cell uptake and transepithelial transport, but also influencing intracellular trafficking. Our work therefore demonstrates that the B12 endocytotic apparatus is not only present in this airway model, but also transports ligand-conjugated nanoparticles across polarised epithelial cells, indicating potential for B12-mediated delivery of nanoscale carriers of biotherapeutics across the airways.

Inhibition of proteolysis in histiotrophic nutrition pathways alters DNA methylation and one-carbon metabolism in the organogenesis-stage rat conceptus

Epigenetic modifications, including DNA methylation, contribute to the transcriptional regulation of developmental genes that control growth and differentiation during embryogenesis. The methyl donor, S-adenosylmethionine (SAM), is biosynthesized from methionine and adenosine triphosphate by methionine adenosyltransferase 2a (Mat2a) in the one-carbon (C1) metabolism pathway. SAM biosynthesis requires a steady supply of nutrients, vitamins and cofactors obtained by the developing conceptus through histiotrophic nutrition pathways (HNPs). The visceral yolk sac (VYS) captures proteins and their substrate cargos by receptor-mediated endocytosis and degrades them using lysosomal proteases. We hypothesize that leupeptin, a protease inhibitor, reduces the availability of methionine and C1 substrates, restricting SAM biosynthesis and altering patterns of DNA methylation. Rat conceptuses were exposed to 50 and 100 μM leupeptin in whole embryo culture for periods of 26 h from gestational day (GD) 10 or 6 h on GD11. After 6 h on GD11, the 100-μM leupeptin treatment significantly decreased methionine in embryo (EMB) and VYS, reduced Mat2a protein levels and inhibited Mat2a specific activity, all of which produced a significant 52% reduction of SAM in the VYS. The 50- and 100-μM leupeptin treatments significantly decreased global methylation levels by 6%-9% in EMB and by 11%-15% in VYS following both 6- and 26-h exposure periods. This study demonstrates that HNP disruption alters C1 activity and significantly reduces global DNA methylation during organogenesis. Because epigenetic reprogramming is crucial for normal differentiation and growth, these findings suggest a possible mechanism through which nutrients and environmental factors may alter early developmental regulation.

Cubilin expression is monoallelic and epigenetically augmented via PPARs

Background

Cubilin is an endocytic receptor that is necessary for renal and intestinal absorption of a range of ligands. Endocytosis mediated by cubilin and its co-receptor megalin is the principal mechanism for proximal tubule reabsorption of proteins from the glomerular filtrate. Cubilin is also required for intestinal endocytosis of intrinsic factor-vitamin B₁₂ complex. Despite its importance, little is known about the regulation of cubilin expression.

Results

Here we show that cubilin expression is under epigenetic regulation by at least two processes. The first process involves inactivation of expression of one of the cubilin alleles. This monoallelic expression state could not be transformed to biallelic by inhibiting DNA methylation or histone deacetylation. The second process involves transcriptional regulation of cubilin by peroxisome proliferator-activated receptor (PPAR) transcription factors that are themselves regulated by DNA methylation and histone deacetylation. This is supported by findings that inhibitors of DNA methylation and histone deacetylation, 5Aza and TSA, increase cubilin mRNA and protein in renal and intestinal cell lines. Not only was the expression of PPARα and γ inducible by 5Aza and TSA, but the positive effects of TSA and 5Aza on cubilin expression were also dependent on both increased PPAR transcription and activation. Additionally, 5Aza and TSA had similar effects on the expression of the cubilin co-receptor, megalin.

Conclusions

Together, these findings reveal that cubilin and megalin mRNA expression is under epigenetic control and thus point to new avenues for overcoming pathological suppression of these genes through targeting of epigenetic regulatory processes.

HDL-C: does it matter? An update on novel HDL-directed pharmaco-therapeutic strategies

It has long been recognized that elevated levels of low-density lipoprotein cholesterol (LDL-C) increase the risk of cardiovascular disease (CHD) and that pharmacologic therapy to decrease LDL-C significantly reduces cardiovascular events. Despite the effectiveness of statins for CHD risk reduction, even optimal LDL-lowering therapy alone fails to avert 60% to 70% of CHD cases. A low plasma concentration of high-density lipoprotein cholesterol (HDL-C) is also associated with increased risk of CHD. However, the convincing epidemiologic data linking HDL cholesterol (HDL-C) to CHD risk in an inverse correlation has not yet translated into clinical trial evidence supporting linearity between HDL-C increases and CHD risk reduction. It is becoming clear that a functional HDL is a more desirable target than simply increasing HDL-C levels. Discoveries in the past decade have shed light on the complex metabolic and antiatherosclerotic pathways of HDL. These insights, in turn, have fueled the development of new HDL-targeted drugs, which can be classified according to four different therapeutic approaches: directly augmenting the concentration of apolipoprotein A-I (apo A-I), the major protein constituent of HDL; indirectly augmenting the concentration of apo A-I and HDL cholesterol; mimicking the functionality of apo A-I and enhancing reverse cholesterol transport. This review discusses the latest in novel HDL directed therapeutic strategies.

Vitamin B12 transport from food to the body's cells--a sophisticated, multistep pathway

Vitamin B(12) (B(12); also known as cobalamin) is a cofactor in many metabolic processes; deficiency of this vitamin is associated with megaloblastic anaemia and various neurological disorders. In contrast to many prokaryotes, humans and other mammals are unable to synthesize B(12). Instead, a sophisticated pathway for specific uptake and transport of this molecule has evolved. Failure in the gastrointestinal part of this pathway is the most common cause of nondietary-induced B(12) deficiency disease. However, although less frequent, defects in cellular processing and further downstream steps in the transport pathway are also known culprits of functional B(12) deficiency. Biochemical and genetic approaches have identified novel proteins in the B(12) transport pathway--now known to involve more than 15 gene products--delineating a coherent pathway for B(12) trafficking from food to the body's cells. Some of these gene products are specifically dedicated to B(12) transport, whereas others embrace additional roles, which explains the heterogeneity in the clinical picture of the many genetic disorders causing B(12) deficiency. This Review describes basic and clinical features of this multistep pathway with emphasis on gastrointestinal transport of B(12) and its importance in clinical medicine.

Low-density lipoprotein receptor (LDLR) family orchestrates cholesterol homeostasis

The LDLR family of proteins is involved in lipoproteins trafficking. While the role of LDLR in cardiovascular disease has been widely studied, only recently the role of other members of the LDLR proteins in lipoprotein homeostasis and atherosclerosis has emerged. LDLR, VLDLR, and LRPs bind and internalize apoE- and apoB-containing lipoprotein, including LDL and VLDL, and regulate their cellular uptake. LRP6 is a unique member of this family for its function as a co-receptor for Wnt signal transduction. The work in our laboratory has shown that LRP6 also plays a key role in lipoprotein and TG clearance, glucose homoeostasis, and atherosclerosis. The role of these receptor proteins in pathogenesis of diverse metabolic risk factors is emerging, rendering them targets of novel therapeutics for metabolic syndrome and atherosclerosis. This manuscript reviews the physiological role of the LDLR family of proteins and describes its involvement in pathogenesis of hyperlipidemia and atherosclerosis.

Low-density lipoprotein receptor-related protein 2 gene is associated with egg-quality traits in dwarf layers

Some members of the low-density lipoprotein receptor (LDLR) family play important roles in the regulation of lipoprotein metabolism and egg quality traits. Low-density lipoprotein receptor-related protein 2 (LRP2) gene belongs to the LDLR super family, and widely expresses in many tissues. This work identified and genotyped 1 single-nucleotide polymorphism (SNP), T14347C, at 3'-UTR of the LRP2 using matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS), and analyzed the effects of the SNP (T14347C) on egg-quality traits in 544 dwarf hens from 44 sire families. Frequencies of this SNP in the studied population did not agree with the Hardy-Weinberg equilibrium (P < 0.0001). Egg weight, albumen weight, albumen height, and albumen ratio of the TT genotype were significantly higher than those of the CC genotype (P < 0.05), whereas eggshell ratio of the TT genotype was significantly lower than that of the CC genotype (P < 0.05). The relative expression level of the LRP2 gene in the magnum was determined by real-time quantitative PCR. The gene expression of genotype CC individuals was significantly higher than that of TT and CT birds (P < 0.05). By combining both genetic effects and expression analyses results, we propose that the LRP2 gene is a good candidate gene, exhibiting a key role in albumen formation processes.

Glomerular proteinuria: a complex interplay between unique players

Protein leak in the urine is a harbinger of disruption of the glomerular filtration barrier. It also correlates with disease progression and development of ESRD. At present, therapies are aimed at decreasing proteinuria to decrease further damage to the filter and as a marker of remission. Understanding the mechanism of molecular events that lead to protein leak is vital to developing new therapeutic interventions. There has been tremendous progress over the last decade in identifying gene defects which result in hereditary proteinuric defects. This has led to identifying pathways by which these genes regulate the structure and function of the components of the filtration barrier, namely the podocytes, mesangial cells, endothelial cells, and the basement membrane. Using gene knockout mouse models, a role of tubular cells in regulating proteinuria is also emerging. In this review, we have attempted to present some of the prevailing understanding of the underlying mechanisms and physiology of proteinuria.

Renal uptake of different radiolabelled peptides is mediated by megalin: SPECT and biodistribution studies in megalin-deficient mice

PURPOSE

Radiolabelled peptides used for peptide receptor radionuclide therapy are excreted mainly via the kidneys and are partly reabsorbed and retained in the proximal tubular cells. The resulting high renal radiation dose can cause nephrotoxicity, limiting the maximum activity dose and the effectiveness of peptide receptor radionuclide therapy. The mechanisms of kidney reabsorption of these peptides are incompletely understood, but the scavenger receptor megalin has been shown to play a role in the reabsorption of (111)In-octreotide. In this study, the role of megalin in the renal reabsorption of various relevant radiolabelled peptides was investigated.

METHODS

Groups of kidney-specific megalin-deficient mice and wild-type mice were injected with (111)In-labelled somatostatin, exendin, neurotensin or minigastrin analogues. Single photon emission computed tomographic (SPECT) images of the kidneys were acquired and analysed quantitatively, or the animals were killed 3 h after injection and the activity concentration in the kidneys was measured.

RESULTS

Megalin-deficient mice showed significantly lower uptake of all studied radiolabelled peptides in the kidneys, ranging from 22% ((111)In-octreotide) to 65% ((111)In-exendin) of uptake in wild-type kidneys. Quantitative analysis of renal uptake by SPECT and ex vivo measurements showed a very good correlation.

CONCLUSION

Megalin is involved in the renal reabsorption of radiolabelled octreotide, octreotate, exendin, neurotensin and minigastrin. This knowledge may help in the design of strategies to reduce this reabsorption and the resulting nephrotoxicity in peptide receptor radionuclide therapy, enabling more effective therapy. Small-animal SPECT is an accurate tool, allowing in vivo quantification of renal uptake and serial measurements in individual mice.

High-density lipoprotein therapeutics and cardiovascular prevention

The field of cardiovascular prevention has long anticipated the evolution of high-density lipoprotein (HDL) therapy from unproven metabolic tweaking to pillar of risk reduction on par with low-density lipoprotein control. However, the convincing epidemiologic data linking HDL cholesterol (HDL-C) and cardiovascular disease risk in an inverse correlation has not yet translated into clinical trial evidence supporting linearity between HDL-C increases and risk reduction, or identifying obvious goals of therapy. Although HDL-C-increasing lifestyle maneuvers and established HDL drugs such as niacin and fibrates are likely to protect the vasculature, the negative results obtained in trials of a cholesteryl ester transfer protein inhibitor remind us that HDL-C increases are not always beneficial. It is becoming clear that a functional HDL is a more desirable target than simply increasing HDL-C levels. The larger objective of improving HDL functionality (with or without HDL-C level changes) is bound to become the guiding principle for pharmaceutical research in this area. Several new compounds currently being tested bridge the classical aim of increasing HDL-C levels with the novel target of improving HDL function.

Lower HDL-C and apolipoprotein A-I are related to higher glomerular filtration rate in subjects without kidney disease

Animal experiments show that the kidney contributes to apolipoprotein (apo)A-I catabolism. We tested relationships of HDL cholesterol (HDL-C) and apo-I with kidney function in subjects without severe chronic kidney disease. Included was a random sample of the general population (part of the PREVEND cohort). Kidney function [estimated glomerular filtration rate (e-GFR) by two well-established equations and creatinine clearance], HDL-C, triglycerides, apoA-I and insulin resistance (HOMAir) were measured in 2,484 fasting subjects (e-GFR>or=45 ml/min/1.73 m2) without macroalbuminuria, cardiovascular disease, diabetes, or the use of anti-hypertensives and/or lipid-lowering agents. HDL-C (r=-0.056 to -0.102, P<0.01 to <0.001) and apo A-I (r=-0.096 to -0.126, P<0.001) were correlated inversely with both GFR estimates and creatinine clearance in univariate analyses. Multiple linear regression analyses also demonstrated inverse relationships of HDL-C and apoA-I with all measures of kidney function even after adjustment for age, sex, waist circumference, HOMAir, triglycerides, and urinary albumin excretion (P=0.053 to 0.004). In conclusion, HDL-C and apoA-I are inversely related to e-GFR and creatinine clearance in subjects without severely compromised kidney function, which fits the concept that the kidney contributes to apoA-I regulation in humans. High glomerular filtration rate may be an independent determinant of a pro-atherogenic lipoprotein profile.

Proteinuria and events beyond the slit

The origin of proteinuria is found in either the glomerular filtration device or the proximal tubular reabsorption machinery. During equilibrium, small amounts of predominantly low molecular weight proteins are filtered and reabsorbed by the receptor complex megalin/cubilin/amnionless. This results in a protein-free filtrate passing further down the tubule. During glomerular damage, the reabsorption machinery in the proximal tubule is challenged due to elevated amounts of proteins passing the glomerular filtration slits. Even though it is considered to be a high-capacity system, several conditions result in proteinuria, thus exposing the cells in the rest of the nephron to a protein-rich environment. The impact on cells in the more distal part of the nephron is uncertain, but studies support an involvement in fibrosis development. Protein accumulation in lysosomes of the proximal tubule, due to increased protein internalization, is thought to mediate inflammation and fibrosis, eventually leading to renal failure. In contrast, low molecular weight proteinuria develops when the endocytic machinery is malfunctioning either by direct or indirect causes such as in Imerslund-Gräsbeck syndrome (IGS) or Dent's disease, respectively. This review discusses the origin of proteinuria and describes the structural fundament for protein reabsorption in the proximal tubule as well as conditions resulting in low molecular weight proteinuria.

Cellular uptake of steroid carrier proteins--mechanisms and implications

Steroid hormones are believed to enter cells solely by free diffusion through the plasma membrane. However, recent studies suggest the existence of cellular uptake pathways for carrier-bound steroids. Similar to the clearance of cholesterol via lipoproteins, these pathways involve the recognition of carrier proteins by endocytic receptors on the surface of target cells, followed by internalization and cellular delivery of the bound sterols. Here, we discuss the emerging concept that steroid hormones can selectively enter steroidogenic tissues by receptor-mediated endocytosis, and we discuss the implications of these uptake pathways for steroid hormone metabolism and action in vivo.