Functional domains of apolipoprotein E and apolipoprotein B

Dissecting the Extracellular Complexity of Neuromuscular Junction Organizers

Synapse formation is a very elaborate process dependent upon accurate coordination of pre and post-synaptic specialization, requiring multiple steps and a variety of receptors and signaling molecules. Due to its relative structural simplicity and the ease in manipulation and observation, the neuromuscular synapse or neuromuscular junction (NMJ)-the connection between motor neurons and skeletal muscle-represents the archetype junction system for studying synapse formation and conservation. This junction is essential for survival, as it controls our ability to move and breath. NMJ formation requires coordinated interactions between motor neurons and muscle fibers, which ultimately result in the formation of a highly specialized post-synaptic architecture and a highly differentiated nerve terminal. Furthermore, to ensure a fast and reliable synaptic transmission following neurotransmitter release, ligand-gated channels (acetylcholine receptors, AChRs) are clustered on the post-synaptic muscle cell at high concentrations in sites opposite the presynaptic active zone, supporting a direct role for nerves in the organization of the post-synaptic membrane architecture. This organized clustering process, essential for NMJ formation and for life, relies on key signaling molecules and receptors and is regulated by soluble extracellular molecules localized within the synaptic cleft. Notably, several mutations as well as auto-antibodies against components of these signaling complexes have been related to neuromuscular disorders. The recent years have witnessed strong progress in the understanding of molecular identities, architectures, and functions of NMJ macromolecules. Among these, prominent roles have been proposed for neural variants of the proteoglycan agrin, its receptor at NMJs composed of the lipoprotein receptor-related protein 4 (LRP4) and the muscle-specific kinase (MuSK), as well as the regulatory soluble synapse-specific protease Neurotrypsin. In this review we summarize the current state of the art regarding molecular structures and (agrin-dependent) canonical, as well as (agrin-independent) non-canonical, MuSK signaling mechanisms that underscore the formation of neuromuscular junctions, with the aim of providing a broad perspective to further stimulate molecular, cellular and tissue biology investigations on this fundamental intercellular contact.

Targeted siRNA Nanoparticles for Mammary Carcinoma Therapy

Non-viral, polymeric-based, siRNA nanoparticles (NPs) have been proposed as promising gene delivery systems. Encapsulating siRNA in targeted NPs could confer improved biological stability, extended half-life, enhanced permeability, effective tumor accumulation, and therapy. In this work, a peptide derived from apolipoprotein B100 (ApoB-P), the protein moiety of low-density lipoprotein, was used to target siRNA-loaded PEGylated NPs to the extracellular matrix/proteoglycans (ECM/PGs) of a mammary carcinoma tumor. siRNA against osteopontin (siOPN), a protein involved in breast cancer development and progression, was encapsulated into PEGylated poly(d,l-lactic-co-glycolic acid) (PLGA) NPs using the double emulsion solvent diffusion technique. The NPs obtained possessed desired physicochemical properties including ~200 nm size, a neutral surface charge, and high siOPN loading of ~5 µg/mg. ApoB-P-targeted NPs exhibited both enhanced binding to isolated ECM and internalization by MDA-MB-231 human mammary carcinoma cells, in comparison to non-targeted NPs. Increased accumulation of the targeted NPs was achieved in the primary mammary tumor of mice xenografted with MDA-MB-231 mammary carcinoma cells as well as in the lungs, one of the main sites affected by metastases. siOPN NPs treatment resulted in significant inhibition of tumor growth (similar bioactivity of both formulations), accompanied with significant reduction of OPN mRNA levels (~40% knockdown of mRNA levels). We demonstrated that targeted NPs possessed enhanced tumor accumulation with increased therapeutic potential in mice models of mammary carcinoma.

Association of apolipoprotein E (ApoE) polymorphisms with risk of primary hyperuricemia in Uygur men, Xinjiang, China

BACKGROUND

Apolipoprotein E (ApoE) participates in lipoprotein metabolism and immune regulation. This study assessed association between ApoE polymorphisms with hyperuricemia and uric acid metabolism in Uygur men, Xinjiang, China.

METHODS

A total of 474 hyperuricemia patients and 518 healthy male controls were recruited from the Health Screening Center, Uygur region of Xinjiang, China and subjected to ApoE genotyping using a multiplex amplification refractory mutation system PCR.

RESULTS

Apolipoprotein E3/3 genotype was the predominant type with a frequency of 67.7%, while E2/2 was lower than E4/4 in Uygur males. The frequencies of ApoE2, E3, and E4 alleles were 8.5%, 80.1% and 11.4%, respectively. Distribution of ApoE genotypes was significantly different in hyperuricemia patients from the healthy controls (p<0.001). Particularly, the frequency of ApoE E3/3 was 71.7%, E2/3 9.3%, E3/4 9.3%, E4/4 3.2%, E2/4 2.3%, and E2/2 0.2% in patients vs. 68.1%, 4.6%, 2.9%, 12%, 0.6%, and 4.6% in controls, respectively. Moreover, frequency of ApoE E2 allele was greater in the healthy controls than in patients (p<0.001) and the highest level of uric acid occurred in those with ApoE2/4 and E3/4 genotypes, whereas the lowest uric acid level occurred in those with ApoE E2/2 genotype. In addition, the subjects with the ApoE2 allele had a lower uric acid and LDL-C level than those with the ApoE3 allele and ApoE4 allele (p<0.05). The risk of developing hyperuricemia in subjects without the ApoE2 allele was 1.7 fold higher than those subjects with the ApoE2 allele.

CONCLUSIONS

This study revealed frequencies and distributions of ApoE alleles and genotypes in Uygur males, which are different from Han Chinese. ApoE E4 was associated with a slightly higher risk of primary hyperuricemia, whereas ApoE E2 was associated with reduced risk of primary hyperuricemia and LDL-C level.

Rerouting lipoprotein nanoparticles to selected alternate receptors for the targeted delivery of cancer diagnostic and therapeutic agents

We report that a lipoprotein-based nanoplatform generated by conjugating tumor-homing molecules to the protein components of naturally occurring lipoproteins reroutes them from their normal lipoprotein receptors to other selected cancer-associated receptors. Multiple copies of these targeting moieties may be attached to the same nanoparticle, or a variety of different targeting moieties can be attached. Such a diverse set of tumor-homing molecules could be used to create a variety of conjugated lipoproteins as multifunctional, biocompatible nanoplatforms with a broad application to both cancer imaging and treatment. The same principle can be applied to imaging and treatment of other diseases and for monitoring specific tissues. To validate this concept, we prepared a low-density lipoprotein (LDL)-based folate receptor (FR)-targeted agent by conjugating folic acid to the Lys residues of the apolipoprotein B (apoB)-100 protein. To demonstrate the ability of the lipoprotein-based nanoplatform to deliver surface-loaded and core-loaded payloads, the particles were labeled either with the optical reporter 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine that was intercalated in the phospholipid monolayer or with the lipophilic photodynamic therapy agent, tetra-t-butyl-silicon phthalocyanine bisoleate, that was reconstituted into the lipid core. Cellular localization of the labeled LDL was monitored by confocal microscopy and flow cytometry in FR-overexpressing KB cells, in FR-nonexpressing CHO and HT-1080 cells, and in LDL receptor-overexpressing HepG2 cells. These studies demonstrate that the folic acid conjugation to the Lys side-chain amino groups blocks binding to the normal LDL receptor and reroutes the resulting conjugate to cancer cells through their FRs.

Progress toward identification of protease activity involved in proteolysis of apolipoprotein e in human brain

Apolipoprotein E (apoE) genotype is the single most important genetic risk factor for the most common (sporadic) form of Alzheimer's disease (AD). Increasing evidence supports the hypothesis that the presence of the E4 isoform of this cholesterol-binding protein contributes directly to disease risk, age of onset, and severity of the neuropathology. For example, studies in transgenic mice demonstrate that apoE is necessary for the formation of plaques with neuritic pathology. The precise mechanism by which apoE contributes to the disease remains unknown. However, several lines of investigation from a number of laboratories now point to a role for proteolytic fragments of apoE in the formation of both plaques and tangles, the two pathological hallmarks of the disease. In particular, the C-terminal portion of apoE has been implicated in binding to amyloid and is localized to plaques. The N-terminal domain, on the other hand, is neurotoxic in culture and has been localized to, and implicated in the formation of, neurofibrillary tangles. These results suggest that inhibition of apoE proteolysis is a potential therapeutic strategy for AD. Using human brain homogenates, we have determined that proteolysis of apoE is greatest at acidic pH and can be inhibited by compounds targeting aspartic proteases. The feasibility of screening candidate inhibitors is supported by both ELISA and immunoblotting methods. Future studies will use a combination of in vitro and in vivo assays to test the efficacy of the most effective compounds for their ability to inhibit apoE proteolysis in human brain and apoE transgenic mouse brain tissue.

Nucleic acid-binding properties of low-density lipoproteins: LDL as a natural gene vector

The role of apo B-100 as a transcription factor is indicated by the presence of regions in its primary structure that are similar to the DNA-binding domains of the transcription factors ISGF3gamma, STATs, IRFs, and SREBPs as well as by the presence of 11 RNA-binding KH domains. The Apo B-100 sequence also contains numerous bipartite nuclear localization sequences (NLS). A modified gel shift assay was used to show binding of highly purified preparations of human LDLs to fragmented genomic DNA, plasmid DNA, synthetic oligonucleotides (ISRE, 5'-GGGAAACCGAAACTG and E/C, E-box motif and CCAAT, adipocyte-specific genes promoter site), and total RNA from human liver. LDL was observed to bind preferentially to plasmid DNA containing the hCMV IE2 promoter region. In experiments using human liver total RNA, RNA for five different genes was recovered from LDL and VLDL bands. Gene transfection experiments using human skin fibroblast cells were used to study the gene transfer capacity of LDL. Cells transfected with a pEGFP-N1 plasmid DNA and LDL expressed functional GFP, as indicated by fluorescence, at approximately 3 hrs after transfection. Our results strongly support an alternative role for apo B-100, in toto or perhaps as functional fragments, in the control of gene expression and as gene transfer vector.

Expression, secretion, and lipid-binding characterization of the N-terminal 17% of apolipoprotein B

The N-terminal 17% of human apolipoprotein B (apoB-17) was expressed in murine C127 cells following transfection with a bovine papilloma virus-based expression vector. A permanent cell line overexpressing the expected 89-kDa protein was selected and characterized. Pulse-chase experiments showed that the depletion of intracellular apoB-17 follows an apparent first-order kinetics with t1/2 = 51 min. Under conditions of continuous labeling, greater than 60% of the total synthesized apoB-17 was secreted in a soluble form, approximately 98% lipid-poor and approximately 2% lipid-bound. Inclusion of 1.2 mM oleate resulted in 5- and 2.5-fold increases in the amount of labeled apoB-17 in the p less than 1.063 g/ml and 1.063 less than p less than 1.21 g/ml fractions, respectively, which was coordinated with increased secretion of radiolabeled core lipids, triacylglycerols, and cholesteryl esters. Thus under conditions in which lipid pools are enriched a greater fraction of apoB-17 may be secreted on lipoprotein-like particles. The lipid-poor apoB-17 present in p greater than 1.21 g/ml readily associates with exogenously added dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles to form discoidal particles. Discs formed with DMPC/apoB-17, 7:1 (wt/wt), are 239 +/- 43 A in diameter and 61 +/- 4 A thick and contain approximately 2 molecules of apoB-17 and 2250 molecules of DMPC per disc. Based on volume calculations we conclude that apoB-17 forms an annulus about one bilayer high and 10 A thick surrounding the DMPC disc. Circular dichroic spectra of apoB-17 on DMPC discs showed apoB-17 to contain 39% alpha-helix, 36% beta-sheet, 9% beta-turn, and 16% random coil. To be consistent with this model greater than 70% of apoB-17 on DMPC discs must bind to lipid. These data suggest that the N-terminal 17% of apoB-100 can bind lipid and may contribute to some extent to the stabilization of triglyceride-rich lipoproteins.

Familial defective apolipoprotein B-100: low density lipoproteins with abnormal receptor binding

Previous in vivo turnover studies suggested that retarded clearance of low density lipoproteins (LDL) from the plasma of some hypercholesterolemic patients is due to LDL with defective receptor binding. The present study examined this postulate directly by receptor binding experiments. The LDL from a hypercholesterolemic patient (G.R.) displayed a reduced ability to bind to the LDL receptors on normal human fibroblasts. The G.R. LDL possessed 32% of normal receptor binding activity (approximately equal to 9.3 micrograms of G.R. LDL per ml were required to displace 50% of 125I-labeled normal LDL, vs. approximately equal to 3.0 micrograms of normal LDL per ml). Likewise, the G.R. LDL were much less effective than normal LDL in competing with 125I-labeled normal LDL for cellular uptake and degradation and in stimulating intracellular cholesteryl ester synthesis. The defect in LDL binding appears to be due to a genetic abnormality of apolipoprotein B-100: two brothers of the proband possess LDL defective in receptor binding, whereas a third brother and the proband's son have normally binding LDL. Further, the defect in receptor binding does not appear to be associated with an abnormal lipid composition or structure of the LDL: the chemical and physical properties of the particles were normal, and partial delipidation of the LDL did not alter receptor binding activity. Normal and abnormal LDL subpopulations were partially separated from plasma of two subjects by density-gradient ultracentrifugation, a finding consistent with the presence of a normal and a mutant allele. The affected family members appear to be heterozygous for this disorder, which has been designated familial defective apolipoprotein B-100. These studies indicate that the defective receptor binding results in inefficient clearance of LDL and the hypercholesterolemia observed in these patients.