Intracellular fate of LDL receptor family members depends on the cooperation between their ligand-binding and EGF domains

Vitellogenin receptor expression in ovaries controls innate immunity in the Chinese mitten crab (Eriocheir sinensis) by regulating vitellogenin accumulation in the hemolymph

The vitellogenin receptor (Vgr), which is specific for vitellogenin (Vtg), recognises and transports Vtg into the ovaries. Accumulating evidence suggests that Vtg also performs an immune defence function and plays critical roles in innate immunity in oviparous animals. However, whether Vgr is involved in innate immunity in the Chinese mitten crab (Eriocheir sinensis) is unknown. In this study, we obtained a 3009 nucleotide partial cDNA of the E. sinensis vitellogenin receptor gene (Es-vgr) encoding an open reading frame of 1003 amino acid residues. Bioinformatics analysis showed that the domains of Es-vgr were conserved during evolution. Quantitative real-time PCR and western blotting revealed that the highest Es-vgr expression levels occurred in the ovary, and expression was specific. Comparison of the expression levels of Es-vgr and the Vtg gene (Es-vtg1) at different ovary developmental stages suggested that there may be some regulatory relationship between them. Bacterial challenge induced high-level expression of antimicrobial peptide genes and reduced Es-vgr expression in ovaries, resulting in massive accumulation of Vtg in the hemolymph. The survival rate of crabs increased significantly after injection with recombinant Es-vtg1 protein following bacterial infection. Collectively, these results demonstrate that Es-vgr plays critical roles in antimicrobial function by regulating the accumulation of Vtg in the hemolymph.

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.

Target engagement and intracellular delivery of mono- and bivalent LDL receptor-binding peptide-cargo conjugates: Implications for the rational design of new targeted drug therapies

Targeted delivery to specific tissues and subcellular compartments is of paramount importance to optimize therapeutic or diagnostic interventions while minimizing side-effects. Using recently identified LDL receptor (LDLR) -targeting small synthetic peptide-vectors conjugated to model cargos of different nature and size, we investigated in LDLR-expressing cells the impact of vector-cargo molecular engineering and coupling valency, as well as the cellular exposure duration on their target engagement and intracellular trafficking and delivery profiles. All vector-cargo conjugates evaluated were found to be delivered to late compartments together with the natural ligand LDL, although to varying extents and with different kinetics. Partial recycling together with the LDLR was also consistently observed. Under continuous cellular exposure, the extent of intracellular vector-cargo delivery primarily relies on their endosomal unloading potential. In this condition, the highest intracellular delivery potential was observed with a monovalent conjugate displaying a rather high LDLR dissociation rate. On the contrary, under transient cellular exposure followed by chase, low dissociation-rate bivalent conjugates revealed a higher intracellular delivery potential than the monovalent conjugate. This was shown to rely on their ability to undergo multiple endocytosis-recycling rounds, with limited release in the ligand-free medium. The absence of reciprocal competition with the natural ligand LDL on their respective intracellular trafficking was also demonstrated, which is essential in terms of potential safety liabilities. These results demonstrate that not only molecular engineering of new therapeutic conjugates of interest, but also the cellular exposure mode used during in vitro evaluations are critical to anticipate and optimize their delivery potential.

Lipoprotein assembly and function in an evolutionary perspective

Circulatory fat transport in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). ApoB and apoLp-II/I, constituting the structural (non-exchangeable) basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride-transfer protein, another LLTP family member, and bind them by means of amphipathic α-helical and β-sheet structural motifs. Comparative research reveals that LLTPs evolved from the earliest animals and highlights the structural adaptations in these lipid-binding proteins. Thus, in contrast to apoB, apoLp-II/I is cleaved post-translationally by a furin, resulting in the appearance of two non-exchangeable apolipoproteins in the single circulatory lipoprotein in insects, high-density lipophorin (HDLp). The remarkable structural similarities between mammalian and insect lipoproteins notwithstanding important functional differences relate to the mechanism of lipid delivery. Whereas in mammals, partial delipidation of apoB-containing lipoproteins eventually results in endocytic uptake of their remnants, mediated by members of the low-density lipoprotein receptor (LDLR) family, and degradation in lysosomes, insect HDLp functions as a reusable lipid shuttle capable of alternate unloading and reloading of lipid. Also, during muscular efforts (flight activity), an HDLp-based lipoprotein shuttle provides for the transport of lipid for energy generation. Although a lipophorin receptor - a homolog of LDLR - was identified that mediates endocytic uptake of HDLp during specific developmental periods, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. These data highlight that the functional adaptations in the lipoprotein lipid carriers in mammals and insects also emerge with regard to the functioning of their cognate receptors.

Evaluation of the causality of the low-density lipoprotein receptor gene (LDLR) for serum lipids in pigs

A significant quantitative trait locus (QTL) for low-density lipoprotein cholesterol (LDL-C) and total cholesterol (TC) was identified around the LDLR gene on chromosome 2 (SSC2) in a White Duroc × Erhualian F2 resource population and Sutai pigs in our previous study. However, in previous reports, the causality of LDLR with serum lipids is controversial in pigs. To systematically assess the causality of LDLR with serum lipids, association analyses were successively performed in three populations: Sutai pigs, a White Duroc × Erhualian F2 resource population and a Duroc × (Landrace × Large White) population. We first performed a haplotype-based association study with 60K SNP genotyping data and evidenced the significant association with LDL-C and TC around the LDLR gene region. We also found that there is more than one QTL for LDL-C and TC on SSC2. Then, we evaluated the causalities of two missense mutations, c.1812C>T and c.1520A>G, with LDL-C and TC. We revealed that the c.1812C>T SNP showed the strongest association with LDL-C (P = 5.40 × 10(-11) ) and TC (P = 3.64 × 10(-8) ) and explained all the QTL effect in Sutai pigs. Haplotype analysis found that two missense SNPs locate within a 1.93-Mb haplotype block. One major haplotype showed the strongest significant association with LDL-C (P = 4.62 × 10(-18) ) and TC (P = 1.06 × 10(-9) ). However, the c.1812C>T SNP was not identified in the White Duroc × Erhualian intercross, and the association of c.1520A>G with both LDL-C and TC did not achieve significance in this F2 population, suggesting population heterogeneity. Both missense mutations were identified in the Duroc × (Landrace × Large White) population and showed significant associations with LDL-C and TC. Our data give evidence that the LDLR gene should be a candidate causative gene for LDL-C and TC in pigs, but heterogeneity exists in different populations.

New horizons for lipoprotein receptors: communication by β-propellers

The lipoprotein receptor (LR) family constitutes a large group of structurally closely related receptors with broad ligand-binding specificity. Traditionally, ligand binding to LRs has been anticipated to involve merely the complement type repeat (CR)-domains omnipresent in the family. Recently, this dogma has transformed with the observation that β-propellers of some LRs actively engage in complex formation too. Based on an in-depth decomposition of current structures and sequences, we suggest that exploitation of the β-propellers as binding targets depends on receptor subgroups. In particular, we highlight the shutter mechanism of β-propellers as a general recognition motif for NxI-containing ligands, and we present indications that the generalized β-propeller-induced ligand release mechanism is not applicable for the larger LRs. For the giant LR members, we present evidence that their β-propellers may also actively engage in ligand binding. We therefore advocate for an increased focus on solving the structure-function relationship of this group of important biological receptors.

Mechanism of LDL binding and release probed by structure-based mutagenesis of the LDL receptor

The LDL receptor (LDL-R) mediates cholesterol metabolism in humans by binding and internalizing cholesterol transported by LDL. Several different molecular mechanisms have been proposed for the binding of LDL to LDL-R at neutral plasma pH and for its release at acidic endosomal pH. The crystal structure of LDL-R at acidic pH shows that the receptor folds back on itself in a closed form, obscuring parts of the ligand binding domain with the epidermal growth factor (EGF)-precursor homology domain. We have used a structure-based site-directed mutagenesis approach to examine 12 residues in the extracellular domain of LDL-R for their effect on LDL binding and release. Our studies show that the interface between the ligand binding domain and the EGF-precursor homology domain seen at acidic pH buries residues mediating both LDL binding and release. Our results are consistent with an alternative model of LDL-R whereby multiple modules of the extracellular domain interact with LDL at neutral pH, concurrently positioning key residues so that at acidic pH the LDL-R:LDL interactions become unfavorable, triggering release. After LDL release, the closed form of LDL-R may target its return to the cell surface.

Circulatory lipid transport: lipoprotein assembly and function from an evolutionary perspective

Circulatory transport of neutral lipids (fat) in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). Latter proteins, which constitute the structural basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride transfer protein (MTP)--another LLTP family member--and bind them by means of amphipathic structures. Comparative research reveals that LLTPs have evolved from the earliest animals and additionally highlights the structural and functional adaptations in these lipid carriers. For instance, in contrast to mammalian apoB, the insect apoB homologue, apoLp-II/I, is post-translationally cleaved by a furin, resulting in their appearance of two non-exchangeable apolipoproteins in the insect low-density lipoprotein (LDL) homologue, high-density lipophorin (HDLp). An important difference between mammalian and insect lipoproteins relates to the mechanism of lipid delivery. Whereas in mammals, endocytic uptake of lipoprotein particles, mediated via members of the LDL receptor (LDLR) family, results in their degradation in lysosomes, the insect HDLp was shown to act as a reusable lipid shuttle which is capable of reloading lipid. Although the recent identification of a lipophorin receptor (LpR), a homologue of LDLR, reveals that endocytic uptake of HDLp may constitute an additional mechanism of lipid delivery, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. Binding studies indicate that the HDLp-LpR complex, in contrast to the LDL-LDLR complex, is resistant to dissociation at endosomal pH as well as by treatment with EDTA mimicking the drop in Ca(2+) concentration in the endosome. This remarkable stability of the ligand-receptor complex may provide a crucial key to the recycling mechanism. Based on the binding and dissociation capacities of mutant and hybrid receptors, the specific binding interaction of the ligand-binding domain of the receptor with HDLp was characterized. These structural similarities and functional adaptations of the lipid transport systems operative in mammals and insects are discussed from an evolutionary perspective.

Genome-wide expression analysis of cells expressing gain of function mutant D374Y-PCSK9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a key regulator of serum cholesterol. The possibility that PCSK9 also functions in other pathways needs to be addressed. We have transfected HepG2 cells with mutant D374Y-PCSK9 or control vector. Gene expression signatures were determined using the Affymetrix GeneChip technology, and the expression pattern of selected genes was confirmed by quantitative real-time polymerase chain reaction (qRT-PCR). Data was normalized and analyzed using a model-based background adjustment for oligonucleotide expression arrays, then filtered based upon expression within treatments group, and subjected to moderated t-statistics. Five hundred twenty transcripts had altered expression levels between D374Y-PCSK9 and control vector. Among the 520 probes on our top list, 312 were found to have an assigned Gene Ontology (GO) term, and 96 were found in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. Genome-wide expression profiling revealed that "steroid biosynthesis," "sterol metabolism," and "cholesterol biosynthsis" were affected by D374Y-PCSK9. Also, the GO biological process terms "response to stresss," "response to virus," "response to unfolded protein," and "immune response" were influenced by D374Y-PCSK9. Our results suggest that D374Y-PCSK9 results in up-regulation of genes involved in sterol biosynthesis and down-regulation of stress-response genes and specific inflammation pathways.

Functional duplication of ligand-binding domains within low-density lipoprotein receptor-related protein for interaction with receptor associated protein, alpha2-macroglobulin, factor IXa and factor VIII

The low-density lipoprotein receptor-related protein (LRP) binds a range of proteins including receptor associated protein (RAP), activated alpha2-macroglobulin (alpha2M*), factor IXa (FIXa), and factor VIII (FVIII) light chain. The binding is mediated by the complement-type repeats, which are clustered in four distinct regions within LRP. Cluster II of 8 repeats (CR3-10) and cluster IV of 11 repeats (CR21-31) have been implicated in ligand-binding. Previous studies have aimed to identify the cluster II repeats involved in binding alpha2M* and RAP. We now evaluated the binding to RAP, alpha2M*, FIXa and FVIII light chain of triplicate repeat-fragments of not only clusters II but also of cluster IV. Employing surface plasmon resonance analysis, we found that most efficient ligand-binding was displayed by the repeats within region CR4-8 of cluster II and within region CR24-28 of cluster IV. Whereas the binding to RAP could be attributed to two consecutive repeats (CR5-6, CR26-27), combinations of three repeats showed most efficient binding to FIXa (CR6-8, CR26-28), FVIII light chain (CR5-7, CR6-8, CR24-26), and alpha2M* (CR4-6, CR24-26). The results imply that there is an internal functional duplication of complement-type repeats within LRP resulting in two clusters that bind the same ligands.

Hitch-hiking between cells on lipoprotein particles

Cell surface proteins containing covalently linked lipids associate with specialized membrane domains. Morphogens like Hedgehog and Wnt use their lipid anchors to bind to lipoprotein particles and employ lipoproteins to travel through tissues. Removal of their lipid anchors or decreasing lipoprotein levels give rise to adverse Hedgehog and Wnt signaling. Some parasites can also transfer their glycosylphosphatidylinositol-anchored surface proteins to host lipoprotein particles. These antigen-loaded lipoproteins spread throughout the circulation, and probably hamper an adequate immune response by killing neutrophils. Together, these findings imply a widespread role for lipoproteins in intercellular transfer of lipid-anchored surface proteins, and may have various physiological consequences. Here, we discuss how lipid-modified proteins may be transferred to and from lipoproteins at the cellular level.