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GPIHBP1 plays an important role in the hydrolysis of triglyceride (TG) lipoproteins by lipoprotein lipases (LPLs). However, Gpihbp1 knockout mice did not develop hypertriglyceridemia (HTG) during the suckling period but developed severe HTG after weaning on a chow diet. It has been postulated that LPL expression in the liver of suckling mice may be involved. To determine whether hepatic LPL expression could correct severe HTG in Gpihbp1 deficiency, liver-targeted LPL expression was achieved via intravenous administration of the adeno-associated virus (AAV)-human LPL gene, and the effects of AAV-LPL on HTG and HTG-related acute pancreatitis (HTG-AP) were observed. Suckling Gpihbp1 -/- mice with high hepatic LPL expression did not develop HTG, whereas Gpihbp1 -/- rat pups without hepatic LPL expression developed severe HTG. AAV-mediated liver-targeted LPL expression dose-dependently decreased plasma TG levels in Gpihbp1 -/- mice and rats, increased post-heparin plasma LPL mass and activity, decreased mortality in Gpihbp1 -/- rat pups, and reduced the susceptibility and severity of both Gpihbp1 -/- animals to HTG-AP. However, the muscle expression of AAV-LPL had no significant effect on HTG. Targeted expression of LPL in the liver showed no obvious adverse reactions. Thus, liver-targeted LPL expression may be a new therapeutic approach for HTG-AP caused by GPIHBP1 deficiency., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Lipolysis-stimulated lipoprotein receptor (LSR) is a multi-functional protein that is best known for its roles in assembly of epithelial tricellular tight junctions and hepatic clearance of lipoproteins. Here, we investigated whether LSR contributes to intestinal epithelium homeostasis and pathogenesis of intestinal disease. By using multiple conditional deletion mouse models and ex vivo cultured organoids, we find that LSR elimination in intestinal stem cells results in the disappearance of Paneth cells without affecting the differentiation of other cell lineages. Mechanistic studies reveal that LSR deficiency increases abundance of YAP by modulating its phosphorylation and proteasomal degradation. Using gain- and loss-of-function studies, we show that LSR protects against necrotizing enterocolitis through enhancement of Paneth cell differentiation in small-intestinal epithelium. Thus, this study identifies LSR as an upstream negative regulator of YAP activity, an essential factor for Paneth cell differentiation, and a potential therapeutic target for necrotizing enterocolitis., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

In response to cold exposure, thermogenic adipocytes internalize large amounts of fatty acids after lipoprotein lipase-mediated hydrolysis of triglyceride-rich lipoproteins (TRL) in the capillary lumen of brown adipose tissue (BAT) and white adipose tissue (WAT). Here, we show that in cold-exposed mice, vascular endothelial cells in adipose tissues endocytose substantial amounts of entire TRL particles. These lipoproteins subsequently follow the endosomal-lysosomal pathway, where they undergo lysosomal acid lipase (LAL)-mediated processing. Endothelial cell-specific LAL deficiency results in impaired thermogenic capacity as a consequence of reduced recruitment of brown and brite/beige adipocytes. Mechanistically, TRL processing by LAL induces proliferation of endothelial cells and adipocyte precursors via beta-oxidation-dependent production of reactive oxygen species, which in turn stimulates hypoxia-inducible factor-1α-dependent proliferative responses. In conclusion, this study demonstrates a physiological role for TRL particle uptake into BAT and WAT and establishes endothelial lipoprotein processing as an important determinant of adipose tissue remodeling during thermogenic adaptation., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Molecular interactions at the cellular interface mediate organized assembly of single cells into tissues and, thus, govern the development and physiology of multicellular organisms. Here, we developed a cell-type-specific, spatiotemporally resolved approach to profile cell-surface proteomes in intact tissues. Quantitative profiling of cell-surface proteomes of Drosophila olfactory projection neurons (PNs) in pupae and adults revealed global downregulation of wiring molecules and upregulation of synaptic molecules in the transition from developing to mature PNs. A proteome-instructed in vivo screen identified 20 cell-surface molecules regulating neural circuit assembly, many of which belong to evolutionarily conserved protein families not previously linked to neural development. Genetic analysis further revealed that the lipoprotein receptor LRP1 cell-autonomously controls PN dendrite targeting, contributing to the formation of a precise olfactory map. These findings highlight the power of temporally resolved in situ cell-surface proteomic profiling in discovering regulators of brain wiring., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Lipoprotein lipase (LPL), identified in the 1950s, has been studied intensively by biochemists, physiologists, and clinical investigators. These efforts uncovered a central role for LPL in plasma triglyceride metabolism and identified LPL mutations as a cause of hypertriglyceridemia. By the 1990s, with an outline for plasma triglyceride metabolism established, interest in triglyceride metabolism waned. In recent years, however, interest in plasma triglyceride metabolism has awakened, in part because of the discovery of new molecules governing triglyceride metabolism. One such protein-and the focus of this review-is GPIHBP1, a protein of capillary endothelial cells. GPIHBP1 is LPL's essential partner: it binds LPL and transports it to the capillary lumen; it is essential for lipoprotein margination along capillaries, allowing lipolysis to proceed; and it preserves LPL's structure and activity. Recently, GPIHBP1 was the key to solving the structure of LPL. These developments have transformed the models for intravascular triglyceride metabolism., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Triglyceride-rich lipoproteins (TRLs) undergo lipolysis by lipoprotein lipase (LPL), an enzyme that is transported to the capillary lumen by an endothelial cell protein, GPIHBP1. For LPL-mediated lipolysis to occur, TRLs must bind to the lumen of capillaries. This process is often assumed to involve heparan sulfate proteoglycans (HSPGs), but we suspected that TRL margination might instead require GPIHBP1. Indeed, TRLs marginate along the heart capillaries of wild-type but not Gpihbp1⁻/⁻ mice, as judged by fluorescence microscopy, quantitative assays with infrared-dye-labeled lipoproteins, and EM tomography. Both cell-culture and in vivo studies showed that TRL margination depends on LPL bound to GPIHBP1. Notably, the expression of LPL by endothelial cells in Gpihbp1⁻/⁻ mice did not restore defective TRL margination, implying that the binding of LPL to HSPGs is ineffective in promoting TRL margination. Our studies show that GPIHBP1-bound LPL is the main determinant of TRL margination., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Familial hypercholesterolemia (FH) is a genetic disorder characterized by extremely high levels of plasma low-density lipoprotein (LDL), due to defective LDL receptor-apolipoprotein B (APOB) binding. Current therapies such as statins or LDL apheresis for homozygous FH are insufficiently efficacious at lowering LDL cholesterol or are expensive. Treatments that target APOB100, the structural protein of LDL particles, are potential therapies for FH. We have developed a series of APOB-directed splice-switching oligonucleotides (SSOs) that cause the expression of APOB87, a truncated isoform of APOB100. APOB87, like similarly truncated isoforms expressed in patients with a different condition, familial hypobetalipoproteinemia, lowers LDL cholesterol by inhibiting very low-density lipoprotein (VLDL) assembly and increasing LDL clearance. We demonstrate that these "APO-skip " SSOs induce high levels of exon skipping and expression of the APOB87 isoform, but do not substantially inhibit APOB48 expression in cell lines. A single injection of an optimized APO-skip SSO into mice transgenic for human APOB resulted in abundant exon skipping that persists for >6 days. Weekly treatments generated a sustained reduction in LDL cholesterol levels of 34-51% in these mice, superior to pravastatin in a head-to-head comparison. These results validate APO-skip SSOs as a candidate therapy for FH.

Inflammatory cytokines are well-recognized mediators of atherosclerosis. Depending on the pathological context, type I interferons (IFNs; IFNalpha and IFNbeta) exert either pro- or anti-inflammatory immune functions, but their exact role in atherogenesis has not been clarified. Here, we demonstrate that IFNbeta enhances macrophage-endothelial cell adhesion and promotes leukocyte attraction to atherosclerosis-prone sites in mice in a chemokine-dependent manner. Moreover, IFNbeta treatment accelerates lesion formation in two different mouse models of atherosclerosis and increases macrophage accumulation in the plaques. Concomitantly, absence of endogenous type I IFN signaling in myeloid cells inhibits lesion development, protects against lesional accumulation of macrophages, and prevents necrotic core formation. Finally, we show that type I IFN signaling is upregulated in ruptured human atherosclerotic plaques. Hereby, we identify type I IFNs as proatherosclerotic cytokines that may serve as additional targets for prevention or treatment., (Copyright 2010 Elsevier Inc. All rights reserved.)

The lipolytic processing of triglyceride-rich lipoproteins by lipoprotein lipase (LPL) is the central event in plasma lipid metabolism, providing lipids for storage in adipose tissue and fuel for vital organs such as the heart. LPL is synthesized and secreted by myocytes and adipocytes, but then finds its way into the lumen of capillaries, where it hydrolyzes lipoprotein triglycerides. The mechanism by which LPL reaches the lumen of capillaries has remained an unresolved problem of plasma lipid metabolism. Here, we show that GPIHBP1 is responsible for the transport of LPL into capillaries. In Gpihbp1-deficient mice, LPL is mislocalized to the interstitial spaces surrounding myocytes and adipocytes. Also, we show that GPIHBP1 is located at the basolateral surface of capillary endothelial cells and actively transports LPL across endothelial cells. Our experiments define the function of GPIHBP1 in triglyceride metabolism and provide a mechanism for the transport of LPL into capillaries., (Copyright 2010 Elsevier Inc. All rights reserved.)

Lipoprotein receptors of the LDLR family serve as clearance receptors for beta2GPI and as signaling receptors for the beta2GPI/antibody complexes in antiphospholipid syndrome. We compared four ligand-binding LA modules from LDLR and ApoER2 for their ability to bind domain V of beta2GPI (beta2GPI-DV). We found that the LA modules capable of binding beta2GPI-DV interact with the same region on beta2GPI-DV using residues at their calcium-coordination site. The structure of a complex between beta2GPI-DV and LA4 of LDLR, solved by molecular docking guided by NMR-derived restraints and extensively validated, represents the general mode of interaction between beta2GPI and lipoprotein receptors. We have shown that beta2GPI-DV cannot simultaneously bind to lipoprotein receptors and anionic phospholipids, suggesting that the association of beta2GPI/anti-beta2GPI antibody complexes with anionic phospholipids will interfere with lipoprotein receptors' signaling in APS.

Apolipoprotein E receptor 2 (ApoER2) and very-low-density lipoprotein receptor, members of the low-density lipoprotein receptor (LDLR) protein family, function as neuronal receptors for a secreted glycoprotein reelin during brain development. In both receptors, the first LDLR class A (LA1) module is sufficient to bind reelin. Analysis of a 2.6 A crystal structure of the reelin receptor-binding fragment in complex with the LA1 of ApoER2 revealed that Lys2467 of reelin is recognized by both a conserved Trp residue and calcium-coordinating acidic residues from LA1, which together with Lys2360 plays a critical role in the interaction. This "double-Lys" recognition mode is, in fact, shared among other LDLR family proteins in ligand binding. The interface between reelin and LA1 covers a small surface area of approximately 350 A(2) on each side, which ensures a stable complex formation under physiological conditions. An examination of structure-guided mutagenesis on interface residues revealed key features of this interaction.

The triglycerides in chylomicrons are hydrolyzed by lipoprotein lipase (LpL) along the luminal surface of the capillaries. However, the endothelial cell molecule that facilitates chylomicron processing by LpL has not yet been defined. Here, we show that glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) plays a critical role in the lipolytic processing of chylomicrons. Gpihbp1-deficient mice exhibit a striking accumulation of chylomicrons in the plasma, even on a low-fat diet, resulting in milky plasma and plasma triglyceride levels as high as 5000 mg/dl. Normally, Gpihbp1 is expressed highly in heart and adipose tissue, the same tissues that express high levels of LpL. In these tissues, GPIHBP1 is located on the luminal face of the capillary endothelium. Expression of GPIHBP1 in cultured cells confers the ability to bind both LpL and chylomicrons. These studies strongly suggest that GPIHBP1 is an important platform for the LpL-mediated processing of chylomicrons in capillaries.

At the endothelial cell surface, binding of chylomicrons and lipoprotein lipase (LpL), the major enzyme involved in the processing of these triglyceride-rich lipoproteins, is thought to involve electrostatic interactions with glycosaminoglycans. A new study published in this issue of Cell Metabolism (Beigneux et al., 2007) provides evidence for a specific chylomicron/LpL receptor, which may serve as a platform for LpL-mediated processing of chylomicrons on the capillary endothelium.

The extracellular protein Reelin is crucial for neuronal positioning during brain development, but its expression persists long after cell migration is completed. In this issue of Neuron, Beffert et al. demonstrate that Reelin exerts an additional function in the mature brain, to modulate synaptic plasticity and to favor memory formation. This activity is carried out exquisitely by the Apoer2 receptor and critically requires the presence of an alternatively spliced exon. This exon encodes an intracellular domain that interacts with postsynaptic proteins and promotes binding and phosphorylation of NMDA receptors.

Up until now, the precise mechanism for endostatin's antiangiogenesis action was not known. In a recent report, have taken advantage of gene array and proteomic analysis to map the antiangiogenic pathways turned on by endostatin. This study resolves some of the controversies surrounding endostatin's biology, and provides a new direction to help dissect the molecular pathways involved in endostatin's selective tumor antiangiogenic effects.

Reelin is a secreted glycoprotein that regulates neuronal positioning in cortical brain structures through the VLDLR and ApoER2 receptors and the adaptor protein Dab1. In addition to cellular disorganization, dendrite abnormalities are present in the brain of reeler mice lacking Reelin. It is unclear whether these defects are due primarily to cellular ectopia or the absence of Reelin. Here we examined dendrite development in the hippocampus of normal and mutant mice and in dissociated cultures. We found that dendrite complexity is severely reduced in homozygous mice deficient in Reelin signaling both in vivo and in vitro, and it is also reduced in heterozygous mice in the absence of cellular ectopia. Addition of Reelin interfering antibodies, receptor antagonists, and Dab1 phosphorylation inhibitors prevented dendrite outgrowth from normal neurons, whereas addition of recombinant Reelin rescued the deficit in reeler cultures. Thus, the same signaling pathway controls both neuronal migration and dendrite maturation.

Background: Reelin is a large signaling molecule that regulates the positioning of neurons in the mammalian brain. Transmission of the Reelin signal to migrating embryonic neurons requires binding to the very-low-density lipoprotein receptor (VLDLR) and the apolipoprotein E receptor-2 (apoER2). This induces tyrosine phosphorylation of the adaptor protein Disabled-1 (Dab1), which interacts with a shared sequence motif in the cytoplasmic tails of both receptors. However, the kinases that mediate Dab1 tyrosine phosphorylation and the intracellular pathways that are triggered by this event remain unknown., Results: We show that Reelin activates members of the Src family of non-receptor tyrosine kinases (SFKs) and that this activation is dependent on the Reelin receptors apoER2 and VLDLR and the adaptor protein Dab1. Dab1 is tyrosine phosphorylated by SFKs, and the kinases themselves can be further activated by phosphorylated Dab1. Increased Dab1 protein expression in fyn-deficient mice implies a response to impaired Reelin signaling that is also observed in mice lacking Reelin or its receptors. However, fyn deficiency alone does not compound the neuronal positioning defect of vldlr- or apoer2-deficient mice, and this finding suggests functional compensation by other SFKs., Conclusions: Our results show that Dab1 is a physiological substrate as well as an activator of SFKs in neurons. Based on genetic evidence gained from multiple strains of mutant mice with defects in Reelin signaling, we conclude that activation of SFKs is a normal part of the cellular Reelin response.

The large extracellular matrix protein Reelin is produced by Cajal-Retzius neurons in specific regions of the developing brain, where it controls neuronal migration and positioning. Genetic evidence suggests that interpretation of the Reelin signal by migrating neurons involves two neuronal cell surface proteins, the very low density lipoprotein receptor (VLDLR) and the apoE receptor 2 (ApoER2) as well as a cytosolic adaptor protein, Disabled-1 (Dab1). We show that Reelin binds directly and specifically to the ectodomains of VLDLR and ApoER2 in vitro and that blockade of VLDLR and ApoER2 correlates with loss of Reelin-induced tyrosine phosphorylation of Disabled-1 in cultured primary embryonic neurons. Furthermore, mice that lack either Reelin or both VLDLR and ApoER2 exhibit hyperphosphorylation of the microtubule-stabilizing protein tau. Taken together, these findings suggest that Reelin acts via VLDLR and ApoER2 to regulate Disabled-1 tyrosine phosphorylation and microtubule function in neurons.

A signaling pathway involving the extracellular protein Reelin and the intracellular adaptor protein Disabled-1 (Dab1) controls cell positioning during mammalian brain development. Here, we demonstrate that Reelin binds directly to lipoprotein receptors, preferably the very low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2). Binding requires calcium, and it is inhibited in the presence of apoE. Furthermore, the CR-50 monoclonal antibody, which inhibits Reelin function, blocks the association of Reelin with VLDLR. After binding to VLDLR on the cell surface, Reelin is internalized into vesicles. In dissociated neurons, apoE reduces the level of Reelin-induced tyrosine phosphorylation of Dab1. These data suggest that Reelin directs neuronal migration by binding to VLDLR and ApoER2.

Layering of neurons in the cerebral cortex and cerebellum requires Reelin, an extracellular matrix protein, and mammalian Disabled (mDab1), a cytosolic protein that activates tyrosine kinases. Here, we report the requirement for two other proteins, cell surface receptors termed very low density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2). Both receptors can bind mDab1 on their cytoplasmic tails and are expressed in cortical and cerebellar layers adjacent to layers that express Reelin. mDab1 expression is upregulated in knockout mice that lack both VLDLR and ApoER2. Inversion of cortical layers and absence of cerebellar foliation in these animals precisely mimic the phenotype of mice lacking Reelin or mDab1. These findings suggest that VLDLR and ApoER2 participate in transmitting the extracellular Reelin signal to intracellular signaling processes initiated by mDab1.

The secreted form of beta-amyloid precursor protein (APP) containing the Kunitz proteinase inhibitor (KPI) domain, also called protease nexin II, is internalized and degraded by cells. We show that the low density lipoprotein (LDL) receptor-related protein (LRP) is responsible for the endocytosis of secreted APP. APPs770 degradation is inhibited by an LRP antagonist called the receptor-associated protein (RAP) and by LRP antibodies and is greatly diminished in fibroblasts genetically deficient in LRP. APPs695, which lacks the KPI domain, is a poor LRP ligand. Since LRP also binds apolipoprotein E (apoE)-enriched lipoproteins and inheritance of the epsilon 4 allele of the apoE gene is a risk factor for Alzheimer's disease (AD), these data link in a single metabolic pathway two molecules strongly implicated in the pathophysiology of AD.