LDL receptor-related protein internalizes and degrades uPA-PAI-1 complexes and is essential for embryo implantation

Establishment of tumor-associated immunity requires interaction of heat shock proteins with CD91

Host antitumor adaptive immune responses are generated as a result of the body's immunosurveillance mechanisms. How the antitumor immune response is initially primed remains unclear, given that soluble tumor antigens generally are quantitatively insufficient for cross-priming and tumors generally lack the classical pathogen-associated molecular patterns to activate costimulation and initiate cross-priming. We explored the interaction of the tumor-derived heat shock proteins (HSP) with their common receptor (CD91) on antigen-presenting cells (APC) as a mechanism for host-priming of T-cell-mediated antitumor immunity. Using targeted genetic disruption of the interaction between HSPs and CD91, we demonstrated that specific ablation of CD91 in APCs prevented the establishment of antitumor immunity. The antitumor immunity was also inhibited when the transfer of tumor-derived HSPs to APCs was prevented using an endogenous inhibitor of CD91. Inhibition was manifested in a reduction of cross-presentation of tumor-derived antigenic peptides in the lymph nodes, providing a molecular basis for the observed immunity associated with tumor development. Our findings demonstrate that early in tumor development, the HSP-CD91 pathway is critical for the establishment of antitumor immunity.

Low-density lipoprotein receptors in liver: old acquaintances and a newcomer

The lipoprotein receptors low-density lipoprotein receptor (LDLR), the low-density lipoprotein receptor-related protein 1 (LRP1) and megalin/LRP2 share characteristic structural elements. In addition to their well-known roles in endocytosis of lipoproteins and systemic lipid homeostasis, it has been established that LRP1 mediates the endocytotic clearance of a multitude of extracellular ligands and regulates diverse signaling processes such as growth factor signaling, inflammatory signaling pathways, apoptosis, and phagocytosis in liver. Here, possible functions of LRP1 expression in hepatocytes and non-parenchymal cells in healthy and injured liver are discussed. Recent studies indicate the expression of megalin (LRP2) by hepatic stellate cells, myofibroblasts and Kupffer cells and hypothesize that LRP2 might represent another potential regulator of hepatic inflammatory processes. These observations provide the experimental framework for the systematic and dynamic analysis of the LDLR family during chronic liver injury and fibrogenesis.

Comprehensive quantitative comparison of the membrane proteome, phosphoproteome, and sialiome of human embryonic and neural stem cells

Human embryonic stem cells (hESCs) can differentiate into neural stem cells (NSCs), which can further be differentiated into neurons and glia cells. Therefore, these cells have huge potential as source for treatment of neurological diseases. Membrane-associated proteins are very important in cellular signaling and recognition, and their function and activity are frequently regulated by post-translational modifications such as phosphorylation and glycosylation. To obtain information about membrane-associated proteins and their modified amino acids potentially involved in changes of hESCs and NSCs as well as to investigate potential new markers for these two cell stages, we performed large-scale quantitative membrane-proteomic of hESCs and NSCs. This approach employed membrane purification followed by peptide dimethyl labeling and peptide enrichment to study the membrane subproteome as well as changes in phosphorylation and sialylation between hESCs and NSCs. Combining proteomics and modification specific proteomics we identified a total of 5105 proteins whereof 57% contained transmembrane domains or signal peptides. The enrichment strategy yielded a total of 10,087 phosphorylated peptides in which 78% of phosphopeptides were identified with ≥99% confidence in site assignment and 1810 unique formerly sialylated N-linked glycopeptides. Several proteins were identified as significantly regulated in hESCs and NSC, including proteins involved in the early embryonic and neural development. In the latter group of proteins, we could identify potential NSC markers as Crumbs 2 and several novel proteins. A motif analysis of the altered phosphosites showed a sequence consensus motif (R-X-XpS/T) significantly up-regulated in NSC. This motif is among other kinases recognized by the calmodulin-dependent protein kinase-2, emphasizing a possible importance of this kinase for this cell stage. Collectively, this data represent the most diverse set of post-translational modifications reported for hESCs and NSCs. This study revealed potential markers to distinguish NSCs from hESCs and will contribute to improve our understanding on the differentiation process.

Extracellular heat shock protein 90 signals through subdomain II and the NPVY motif of LRP-1 receptor to Akt1 and Akt2: a circuit essential for promoting skin cell migration in vitro and wound healing in vivo

Normal cells secrete heat shock protein 90 alpha (Hsp90α) in response to tissue injury. Tumor cells have managed to constitutively secrete Hsp90α during invasion and metastasis. The sole function of extracellular Hsp90α (eHsp90α) is to promote cell motility, a critical event for both wound healing and tumor progression. The mechanism of promotility action by eHsp90α, however, has remained elusive. A key issue is whether eHsp90α still acts as a chaperone outside the cells or is a new and bona fide signaling molecule. Here, we have provided evidence that eHsp90α utilizes a unique transmembrane signaling mechanism to promote cell motility and wound healing. First, subdomain II in the extracellular part of low-density lipoprotein receptor-related protein 1 (LRP-1) receives the eHsp90α signal. Then, the NPVY but not the NPTY motif in the cytoplasmic tail of LRP-1 connects eHsp90α signaling to serine 473 but not threonine 308 phosphorylation in Akt kinases. Individual knockdown of Akt1, Akt2, or Akt3 revealed the importance of Akt1 and Akt2 in eHsp90α-induced cell motility. Akt gene rescue experiments suggest that Akt1 and Akt2 work in concert, rather than independently, to mediate eHsp90α promotility signaling. Finally, Akt1 and Akt2 knockout mice showed impaired wound healing that cannot be corrected by topical application with the eHsp90α protein.

LDL receptor-related protein-1: a regulator of inflammation in atherosclerosis, cancer, and injury to the nervous system

Low-density lipoprotein receptor-related protein-1 (LRP1) is an endocytic receptor for numerous proteins that are both structurally and functionally diverse. In some cell types, LRP1-mediated endocytosis is coupled to activation of cell signaling. LRP1 also regulates the composition of the plasma membrane and may, thereby, indirectly regulate the activity of other cell-signaling receptors. Given the scope of LRP1 ligands and its multifunctional nature, it is not surprising that numerous biological activities have been attributed to this receptor. LRP1 gene deletion is embryonic-lethal in mice. However, elegant studies using Cre-LoxP recombination have helped elucidate the function of LRP1 in mature normal and pathological tissues. One major theme that has emerged is the role of LRP1 as a regulator of inflammation. In this review, we will describe evidence for LRP1 as a regulator of inflammation in atherosclerosis, cancer, and injury to the nervous system.

Low-density lipoprotein receptor related protein-1 (LRP1)-dependent cell signaling promotes neurotrophic activity in embryonic sensory neurons

Developing sensory neurons require neurotrophic support for survival, neurite outgrowth and myelination. The low-density lipoprotein receptor-related protein-1 (LRP1) transactivates Trk receptors and thereby functions as a putative neurotrophin. Herein, we show that LRP1 is abundantly expressed in developing dorsal root ganglia (DRG) and that LRP1-dependent cell signaling supports survival, neurite extension and receptivity to Schwann cells even in the absence of neurotrophins. Cultured embryonic DRG neurons (E15) were treated with previously characterized LRP1 ligands, LRP1-receptor binding domain of α2-macroglobulin (RBD), hemopexin domain of MMP-9 (PEX) or controls (GST) for two weeks. These structurally diverse LRP1 ligands significantly activated and sustained extracellular signal-regulated kinases (ERK1/2) 5-fold (p<0.05), increased expression of growth-associated protein-43(GAP43) 15-fold (P<0.01), and increased neurite outgrowth 20-fold (P<0.01). Primary sensory neurons treated with LRP1 ligands survived > 2 weeks in vitro, to an extent equaling NGF, a finding associated with canonical signaling mechanisms and blockade of caspase-3 cleavage. LRP1 ligand-induced survival and sprouting were blocked by co-incubation with the LRP1 antagonist, receptor associated protein (RAP), whereas RAP had no effect on NGF-induced activity. Site directed mutagenesis of the LRP1 ligand, RBD, in which Lys¹³⁷⁰ and Lys¹³⁷⁴ are converted to alanine to preclude LRP1 binding, were ineffective in promoting cell signaling, survival or inducing neurite extension in primary sensory neurons, confirming LRP1 specificity. Furthermore, LRP1-induced neurite sprouting was mediated by Src-family kinase (SFK) activation, suggesting transactivation of Trk receptors. Co-cultures of primary embryonic neurons and Schwann cells showed that LRP1 agonists promoted axonal receptivity to myelination to Schwann cells. Collectively, these findings identify LRP1 as a novel and perhaps essential trophic molecule for sensory neuronal survival and development.

Apolipoprotein E and apolipoprotein E receptors: normal biology and roles in Alzheimer disease

Apolipoprotein E (APOE) genotype is the major genetic risk factor for Alzheimer disease (AD); the ε4 allele increases risk and the ε2 allele is protective. In the central nervous system (CNS), apoE is produced by glial cells, is present in high-density-like lipoproteins, interacts with several receptors that are members of the low-density lipoprotein receptor (LDLR) family, and is a protein that binds to the amyloid-β (Aβ) peptide. There are a variety of mechanisms by which apoE isoform may influence risk for AD. There is substantial evidence that differential effects of apoE isoform on AD risk are influenced by the ability of apoE to affect Aβ aggregation and clearance in the brain. Other mechanisms are also likely to play a role in the ability of apoE to influence CNS function as well as AD, including effects on synaptic plasticity, cell signaling, lipid transport and metabolism, and neuroinflammation. ApoE receptors, including LDLRs, Apoer2, very low-density lipoprotein receptors (VLDLRs), and lipoprotein receptor-related protein 1 (LRP1) appear to influence both the CNS effects of apoE as well as Aβ metabolism and toxicity. Therapeutic strategies based on apoE and apoE receptors may include influencing apoE/Aβ interactions, apoE structure, apoE lipidation, LDLR receptor family member function, and signaling. Understanding the normal and disease-related biology connecting apoE, apoE receptors, and AD is likely to provide novel insights into AD pathogenesis and treatment.

LRP-1: a checkpoint for the extracellular matrix proteolysis

Low-density lipoprotein receptor-related protein-(LRP-1) is a large endocytic receptor that binds more than 35 ligands and exhibits signaling properties. Proteinases capable of degrading extracellular matrix (ECM), called matrix proteinases in this paper, are mainly serine proteinases: the activators of plasminogen into plasmin, tissue-type (tPA) and urokinase-type (uPA) plasminogen activators, and the members of the matrix metalloproteinase (MMP) family. LRP-1 is responsible for clearing matrix proteinases, complexed or not with inhibitors. This paper attempts to summarize some aspects on the cellular and molecular bases of endocytic and signaling functions of LRP-1 that modulate extra- and pericellular levels of matrix proteinases.

Vitamin D activation of functionally distinct regulatory miRNAs in primary human osteoblasts

When bound to the vitamin D receptor (VDR), the active form of vitamin D, 1,25-dihydroxyvitamin D (1,25D) is a potent regulator of osteoblast transcription. Less clear is the impact of 1,25D on posttranscriptional events in osteoblasts, such as the generation and action of microRNAs (miRNAs). Microarray analysis using replicate (n = 3) primary cultures of human osteoblasts (HOBs) identified human miRNAs that were differentially regulated by >1.5-fold following treatment with 1,25D (10 nM, 6 hours), which included miRNAs 637 and 1228. Quantitative reverse transcription PCR analyses showed that the host gene for miR-1228, low-density lipoprotein receptor-related protein 1 (LRP1), was coinduced with miR-1228 in a dose-dependent fashion following treatment with 1,25D (0.1-10 nM, 6 hours). By contrast, the endogenous host gene for miR-637, death-associated protein kinase 3 (DAPK3), was transcriptionally repressed by following treatment with 1,25D. Analysis of two potential targets for miR-637 and miR-1228 in HOB, type IV collagen (COL4A1) and bone morphogenic protein 2 kinase (BMP2K), respectively, showed that 1,25D-mediates suppression of these targets via distinct mechanisms. In the case of miR-637, suppression of COL4A1 appears to occur via decreased levels of COL4A1 mRNA. By contrast, suppression of BMP2K by miR-1228 appears to occur by inhibition of protein translation. In mature HOBs, small interfering RNA (siRNA) inactivation of miR-1228 alone was sufficient to abrogate 1,25D-mediated downregulation of BMP2K protein expression. This was associated with suppression of prodifferentiation responses to 1,25D in HOB, as represented by parallel decrease in osteocalcin and alkaline phosphatase expression. These data show for the first time that the effects of 1,25D on human bone cells are not restricted to classical VDR-mediated transcriptional responses but also involve miRNA-directed posttranscriptional mechanisms.

Schwann cell LRP1 regulates remak bundle ultrastructure and axonal interactions to prevent neuropathic pain

Trophic support and myelination of axons by Schwann cells in the PNS are essential for normal nerve function. Herein, we show that deletion of the LDL receptor-related protein-1 (LRP1) gene in Schwann cells (scLRP1(-/-)) induces abnormalities in axon myelination and in ensheathment of axons by nonmyelinating Schwann cells in Remak bundles. These anatomical changes in the PNS were associated with mechanical allodynia, even in the absence of nerve injury. In response to crush injury, sciatic nerves in scLRP1(-/-) mice showed accelerated degeneration and Schwann cell death. Remyelinated axons were evident 20 d after crush injury in control mice, yet were largely absent in scLRP1(-/-) mice. In the partial nerve ligation model, scLRP1(-/-) mice demonstrated significantly increased and sustained mechanical allodynia and loss of motor function. Evidence for central sensitization in pain processing included increased p38MAPK activation and activation of microglia in the spinal cord. These studies identify LRP1 as an essential mediator of normal Schwann cell-axonal interactions and as a pivotal regulator of the Schwann cell response to PNS injury in vivo. Mice in which LRP1 is deficient in Schwann cells represent a model for studying how abnormalities in Schwann cell physiology may facilitate and sustain chronic pain.

Myeloid cell receptor LRP1/CD91 regulates monocyte recruitment and angiogenesis in tumors

Recruitment of monocytes into sites of inflammation is essential in the immune response. In cancer, recruited monocytes promote invasion, metastasis, and possibly angiogenesis. LDL receptor-related protein (LRP1) is an endocytic and cell-signaling receptor that regulates cell migration. In this study, we isografted PanO2 pancreatic carcinoma cells into mice in which LRP1 was deleted in myeloid lineage cells. Recruitment of monocytes into orthotopic and subcutaneous tumors was significantly increased in these mice, compared with control mice. LRP1-deficient bone marrow-derived macrophages (BMDM) expressed higher levels of multiple chemokines, including, most prominently, macrophage inflammatory protein-1α/CCL3, which is known to amplify inflammation. Increased levels of CCL3 were detected in LRP1-deficient tumor-associated macrophages (TAM), isolated from PanO2 tumors, and in RAW 264.7 macrophage-like cells in which LRP1 was silenced. LRP1-deficient BMDMs migrated more rapidly than LRP1-expressing cells in vitro. The difference in migration was reversed by CCL3-neutralizing antibody, by CCR5-neutralizing antibody, and by inhibiting NF-κB with JSH-23. Inhibiting NF-κB reversed the increase in CCL3 expression associated with LRP1 gene silencing in RAW 264.7 cells. Tumors formed in mice with LRP1-deficient myeloid cells showed increased angiogenesis. Although VEGF mRNA expression was not increased in LRP1-deficient TAMs, at the single-cell level, the increase in TAM density in tumors with LRP1-deficient myeloid cells may have allowed these TAMs to contribute an increased amount of VEGF to the tumor microenvironment. Our results show that macrophage density in tumors is correlated with cancer angiogenesis in a novel model system. Myeloid cell LRP1 may be an important regulator of cancer progression.

The heparin-binding growth factor midkine: the biological activities and candidate receptors

The heparin-binding growth factor midkine (MK) comprises a family with pleiotrophin/heparin-binding growth-associated molecule. The biological phenomena in which MK is involved can be categorized into five areas: (i) cancer, (ii) inflammation/immunity, (iii) blood pressure, (iv) development and (v) tissue protection. The phenotypes are clear in vivo, but the mechanisms by which MK exerts these actions are not fully understood. Candidate receptors for MK include anaplastic lymphoma kinase, protein tyrosine phosphatase ζ, Notch2, LDL receptor-related protein 1, integrins and proteoglycans. Some physical associations between these candidate receptors are also known. Because of the striking in vivo phenotypes after manipulation of MK, MK could be an important molecular target for the treatment of various diseases. To this end, it will be important to pursue studies to fully understand the mechanisms of MK action.

A LewisX glycoprotein screen identifies the low density lipoprotein receptor-related protein 1 (LRP1) as a modulator of oligodendrogenesis in mice

In the developing and adult CNS multipotent neural stem cells reside in distinct niches. Specific carbohydrates and glycoproteins are expressed in these niche microenvironments which are important regulators of stem cell maintenance and differentiation fate. LewisX (LeX), also known as stage-specific embryonic antigen-1 or CD15, is a defined carbohydrate moiety expressed in niche microenvironments of the developing and adult CNS. LeX-glycans are involved in stem cell proliferation, migration, and stemness. A few LeX carrier proteins are known, but a systematic analysis of the targets of LeX glycosylation in vivo has not been performed so far. Using LeX glycosylation as a biomarker we aimed to discover new glycoproteins with a potential functional relevance for CNS development. By immunoaffinity chromatography we enriched LeX glycoproteins from embryonic and postnatal mouse brains and used one-dimensional nLC-ESI-MS/MS for their identification. We could validate phosphacan, tenascin-C, and L1-CAM as major LeX carrier proteins present in vivo. Furthermore, we identified LRP1, a member of the LDL receptor family, as a new LeX carrier protein expressed by mouse neural stem cells. Surprisingly, little is known about LRP1 function for neural stem cells. Thus, we generated Lrp1 knock-out neural stem cells by Cre-mediated recombination and investigated their properties. Here, we provide first evidence that LRP1 is necessary for the differentiation of neural stem cells toward oligodendrocytes. However, this function is independent of LeX glycosylation.

Endocytic receptor-mediated control of morphogen signaling

Receptor-mediated endocytosis provides a mechanism by which cells take up signaling molecules from the extracellular space. Recent studies have shown that one class of endocytic receptors, the low-density lipoprotein receptor-related proteins (LRPs), is of particular relevance for embryonic development. In this Primer, we describe how LRPs constitute central pathways that modulate morphogen presentation to target tissues and cellular signal reception, and how LRP dysfunction leads to developmental disturbances in many species.

Neurovascular defects and faulty amyloid-β vascular clearance in Alzheimer's disease

The evidence that neurovascular dysfunction is an integral part of Alzheimer's disease (AD) pathogenesis has continued to emerge in the last decade. Changes in the brain vasculature have been shown to contribute to the onset and progression of the pathological processes associated with AD, such as microvascular reductions, blood brain barrier (BBB) breakdown, and faulty clearance of amyloid β-peptide (Aβ) from the brain. Herein, we review the role of the neurovascular unit and molecular mechanisms in cerebral vascular cells behind the pathogenesis of AD. In particular, we focus on molecular pathways within cerebral vascular cells and the systemic circulation that contribute to BBB dysfunction, brain hypoperfusion, and impaired clearance of Aβ from the brain. We aim to provide a summary of recent research findings implicated in neurovascular defects and faulty Aβ vascular clearance contributing to AD pathogenesis.

Pyrosequencing of Haliotis diversicolor transcriptomes: insights into early developmental molluscan gene expression

Background

The abalone Haliotis diversicolor is a good model for study of the settlement and metamorphosis, which are widespread marine ecological phenomena. However, information on the global gene backgrounds and gene expression profiles for the early development of abalones is lacking.

Methodology/Principal Findings

In this study, eight non-normalized and multiplex barcode-labeled transcriptomes were sequenced using a 454 GS system to cover the early developmental stages of the abalone H. diversicolor. The assembly generated 35,415 unigenes, of which 7,566 were assigned GO terms. A global gene expression profile containing 636 scaffolds/contigs was constructed and was proven reliable using qPCR evaluation. It indicated that there may be existing dramatic phase transitions. Bioprocesses were proposed, including the ‘lock system’ in mature eggs, the collagen shells of the trochophore larvae and the development of chambered extracellular matrix (ECM) structures within the earliest postlarvae.

Conclusion

This study globally details the first 454 sequencing data for larval stages of H. diversicolor. A basic analysis of the larval transcriptomes and cluster of the gene expression profile indicates that each stage possesses a batch of specific genes that are indispensable during embryonic development, especially during the two-cell, trochophore and early postlarval stages. These data will provide a fundamental resource for future physiological works on abalones, revealing the mechanisms of settlement and metamorphosis at the molecular level.

VEGF-initiated angiogenesis and the uPA/uPAR system

Angiogenesis involves a series of tightly regulated cellular processes initiated primarily by the vascular endothelial growth factor (VEGF). The urokinase-type plasminogen activator system, consisting of the urokinase-type plasminogen activator (uPA), its cellular receptor uPAR and its inhibitor PAI-1, participates in the realization of these VEGF-induced processes by activating pericellular proteolysis, increasing vascular permeability and by supporting endothelial cell proliferation and migration.

LRP-1-mediated endocytosis regulates extracellular activity of ADAMTS-5 in articular cartilage

Aggrecan is a major matrix component of articular cartilage, and its degradation is a crucial event in the development of osteoarthritis (OA). Adamalysin-like metalloproteinase with thrombospondin motifs 5 (ADAMTS-5) is a major aggrecan-degrading enzyme in cartilage, but there is no clear correlation between ADAMTS-5 mRNA levels and OA progression. Here, we report that post-translational endocytosis of ADAMTS-5 by chondrocytes regulates its extracellular activity. We found 2- to 3-fold reduced aggrecanase activity when ADAMTS-5 was incubated with live porcine cartilage, resulting from its rapid endocytic clearance. Studies using receptor-associated protein (RAP), a ligand-binding antagonist for the low-density lipoprotein receptor-related proteins (LRPs), and siRNA-mediated gene silencing revealed that the receptor responsible for ADAMTS-5 clearance is LRP-1. Domain-deletion mutagenesis of ADAMTS-5 identified that the noncatalytic first thrombospondin and spacer domains mediate its endocytosis. The addition of RAP to porcine cartilage explants in culture increased the basal level of aggrecan degradation, as well as ADAMTS-5-induced aggrecan degradation. Notably, LRP-1-mediated endocytosis of ADAMTS-5 is impaired in chondrocytes of OA cartilage, with ∼90% reduction in protein levels of LRP-1 without changes in its mRNA levels. Thus, LRP-1 dictates physiological and pathological catabolism of aggrecan in cartilage as a key modulator of the extracellular activity of ADAMTS-5.

Brain region-specific immunolocalization of the lipolysis-stimulated lipoprotein receptor (LSR) and altered cholesterol distribution in aged LSR+/- mice

Brain lipid homeostasis is important for maintenance of brain cell function and synaptic communications, and is intimately linked to age-related cognitive decline. Because of the blood-brain barrier's limiting nature, this tissue relies on a complex system for the synthesis and receptor-mediated uptake of lipids between the different networks of neurons and glial cells. Using immunofluorescence, we describe the region-specific expression of the lipolysis-stimulated lipoprotein receptor (LSR), in the mouse hippocampus, cerebellum Purkinje cells, the ependymal cell interface between brain parenchyma and cerebrospinal fluid, and the choroid plexus. Colocalization with cell-specific markers revealed that LSR was expressed in neurons, but not astrocytes. Latency in arms of the Y-maze exhibited by young heterozygote LSR(+/-) mice was significantly different as compared to control LSR(+/+), and increased in older LSR(+/-) mice. Filipin and Nile red staining revealed membrane cholesterol content accumulation accompanied by significantly altered distribution of LSR in the membrane, and decreased intracellular lipid droplets in the cerebellum and hippocampus of old LSR(+/-) mice, as compared to control littermates as well as young LSR(+/-) animals. These data therefore suggest a potential role of LSR in brain cholesterol distribution, which is particularly important in preserving neuronal integrity and thereby cognitive functions during aging.

Reduction of low-density lipoprotein receptor-related protein (LRP1) in hippocampal neurons does not proportionately reduce, or otherwise alter, amyloid deposition in APPswe/PS1dE9 transgenic mice

INTRODUCTION

The low-density lipoprotein receptor-related protein (LRP1) and its family members have been implicated in the pathogenesis of Alzheimer's disease. Multiple susceptibility factors converge to metabolic pathways that involve LRP1, including modulation of the processing of amyloid precursor protein (APP) and the clearance of Aβ peptide.

METHODS

We used the Cre-lox system to lower LRP1 levels in hippocampal neurons of mice that develop Alzheimer-type amyloid by crosses between mice that express Cre recombinase under the transcriptional control of the GFAP promoter, mice that harbor loxp sites in the LRP1 gene, and the APPswe/PS1dE9 transgenic model. We compared amyloid plaque numbers in APPswe/PS1dE9 mice lacking LRP1 expression in hippocampus (n = 13) to mice with normal levels of LRP1 (n = 12). Student t-test was used to test whether there were significant differences in plaque numbers and amyloid levels between the groups. A regression model was used to fit two regression lines for these groups, and to compare the rates of Aβ accumulation.

RESULTS

Immunohistochemical analyses demonstrated efficient elimination of LRP1 expression in the CA fields and dentate gyrus of the hippocampus. Within hippocampus, we observed no effect on the severity of amyloid deposition, the rate of Aβ40/42 accumulation, or the architecture of amyloid plaques when LRP1 levels were reduced.

CONCLUSIONS

Expression of LRP1 by neurons in proximity to senile amyloid plaques does not appear to play a major role in modulating the formation of these proximal deposits or in the appearance of the associated neuritic pathology.

LRP1 mediates Hedgehog-induced endocytosis of the GPC3-Hedgehog complex

Glypican-3 (GPC3) is a heparan sulfate (HS) proteoglycan that is bound to the cell membrane through a glycosylphosphatidylinositol link. This glypican regulates embryonic growth by inhibiting the hedgehog (Hh) signaling pathway. GPC3 binds Hh and competes with Patched (Ptc), the Hh receptor, for Hh binding. The interaction of Hh with GPC3 triggers the endocytosis and degradation of the GPC3-Hh complex with the consequent reduction of Hh available for binding to Ptc. Currently, the molecular mechanisms by which the GPC3-Hh complex is internalized remains unknown. Here we show that the low-density-lipoprotein receptor-related protein-1 (LRP1) mediates the Hh-induced endocytosis of the GPC3-Hh complex, and that this endocytosis is necessary for the Hh-inhibitory activity of GPC3. Furthermore, we demonstrate that GPC3 binds through its HS chains to LRP1, and that this interaction causes the removal of GPC3 from the lipid rafts domains.

Angiogenesis in cancer: Anti-VEGF escape mechanisms

It is now widely accepted that tumor-angiogenesis plays a crucial role in tumor growth, tumor propagation and metastasis formation. Among several angiogenic activators, the vascular endothelial growth factor (VEGF) and its receptors represent one of the major inducers of tumor angiogenesis. Thus, this system has become the focus of therapeutic interventions, which led to the approval of the anti-VEGF blocking antibody bevacizumab and the VEGFR-2 pathway inhibitors pazopanib, sorafenib and sunitinib. However, not every cancer patient benefits from such treatment or finally becomes resistant to anti-VEGF approaches; others are suffering from adverse effects. Thus, there is an urgent need for a better understanding of VEGF-independent mechanisms leading to angiogenesis in cancer. This review focuses on anti-VEGF escape mechanisms of tumor cells and its microenvironment.

Role of low-density lipoprotein receptor related protein 1 (LRP1) in CCN2/connective tissue growth factor (CTGF) protein transport in chondrocytes

LRP1 is known to be a receptor for signal transmission and endocytosis. We formerly reported that LRP1 regulates WNT/β-catenin and protein kinase C signaling in chondrocytes and represses the hypertrophy of chondrocytes during endochondral ossification, and that LRP1 is co-localized with a ligand, CCN2, which conducts endochondral ossification, on chondrocytes. However, the role of LRP1 in endocytotic transport of CCN2 in chondrocytes is not yet understood. In the present study, we investigated the interaction between LRP1 and CCN2 during endocytotic trafficking.

RNAi-mediated knockdown of LRP1 in chondrocytic HCS-2/8 cells showed that the amount of exogenous CCN2 binding/incorporation was decreased in the LRP1 down-regulated cells. Importantly, we observed that CCN2 internalization in chondrocytes was dependent on clathrin and internalizated CCN2 was co-localized with an early or recycling endosome marker. Transcytosis of CCN2 through HCS-2/8 cells was confirmed by performing experiments with a trans-well apparatus, and the amount of transcytosed CCN2 was decreased by an LRP1 antagonist. These findings rule out possible leakage and confirm the critical involvement of LRP1 during experimental transcytosis. Moreover, under the hypoxic condition mimicking the cartilaginous microenvironment, the production level of LRP1 and the amount of transcytosed CCN2 were increased, which increases were neutralized by the LRP1 antagonist. The distribution of LRP1 and its antagonist in the growth plate in vivo was consistent with that of CCN2 therein, which was produced by and transported from the chondrocytes in the prehypertrophic layer.

These findings suggest that LRP1 mediates the transcytosis of CCN2, which may be a critical event that determines the distribution of CCN2 in cartilage.

Lipid metabolism and glial lipoproteins in the central nervous system

Lipoproteins in the central nervous system (CNS) are not incorporated from the blood but are formed mainly by glial cells within the CNS. In addition, cholesterol in the CNS is synthesized endogenously because the blood-brain barrier segregates the CNS from the peripheral circulation. Apolipoprotein (apo) E is a major apo in the CNS. In normal condition, apo E is secreted from glia, mainly from astrocytes, and forms cholesterol-rich lipoproteins by ATP-binding cassette transporters. Subsequently, apo E-containing glial lipoproteins supply cholesterol and other components to neurons via a receptor-mediated process. Recent findings demonstrated that receptors of the low density lipoprotein (LDL) receptor family not only internalize lipoproteins into the cells but also, like signaling receptors, transduce signals upon binding the ligands. In this review, the regulation of lipid homeostasis will be discussed as well as roles of lipoproteins and functions of receptors of LDL receptor family in the CNS. Furthermore, the relation between lipid metabolism and Alzheimer's disease (AD) is discussed.

Deficiency of receptor-associated protein attenuates angiotensin II-induced atherosclerosis in hypercholesterolemic mice without influencing abdominal aortic aneurysms

OBJECTIVE

Receptor-associated protein (RAP) was initially described as a regulator of low density lipoprotein receptor-related protein 1 (LRP1), but is now known to regulate many proteins. Since the direct effects of RAP on vascular pathologies have not been studied, this study determined whether RAP deficiency influenced angiotensin II (AngII)-induced atherosclerosis and abdominal aortic aneurysms (AAAs) in hypercholesterolemic mice.

METHODS AND RESULTS

Male LDL receptor -/- mice that were either RAP +/+ or -/- were infused with AngII (500 ng/kg/min) for 4 weeks while consuming a saturated fat-enriched diet. RAP deficiency had no effects on body weight or AngII-induced increases of systolic blood pressure. Despite increased plasma cholesterol concentrations, RAP deficiency reduced atherosclerotic lesion size in aortic arches, while having no effect on AngII-induced AAAs. RAP deficiency profoundly reduced LRP1 protein abundance in macrophages, but did not change its abundance in aortic smooth muscle cells. Also, RAP deficiency had no effects on mRNA abundance of LRP1 or lipoprotein lipase in macrophages. To determine whether RAP deficiency in leukocytes influenced AngII-induced atherosclerosis, irradiated male LDL receptor -/- mice were repopulated with bone marrow-derived cells from either RAP +/+ or -/- male mice. The chimeric mice were infused with AngII (500 ng/kg/min) for 4 weeks while fed the saturated fat-enriched diet. RAP deficiency in bone marrow-derived cells did not influence either plasma cholesterol concentrations or atherosclerotic lesion size.

CONCLUSIONS

Whole body RAP deficiency attenuated atherosclerosis without influencing AAAs in hypercholesterolemic mice infused with AngII. The anti-atherogenic effect was not attributable to RAP deficiency in bone marrow-derived cells.

Dietary fat impacts fetal growth and metabolism: uptake of chylomicron remnant core lipids by the placenta

The fetus requires significant energy for growth and development. Although glucose is a major source of energy for the fetus, other maternal nutrients also appear to promote growth. Thus, the goal of these studies was to determine whether triglyceride-rich remnants are taken up by the placenta and whether maternal dietary lipids, independently of adiposity, can impact fetal growth. To accomplish our first goal, chylomicron particles were duallly labeled with cholesteryl ester and triglycerides. The placenta took up remnant particles/core lipids at rates greater than adipose tissue and skeletal muscle but less than the liver. Although the placenta expresses apoE receptors, uptake of chylomicron remnants and/or core lipids can occur independently of apoE. To determine the impact of dietary lipid on fetal growth, independent of maternal adiposity, females were fed high-fat diets (HFD) for 1 mo; there was no change in adiposity or leptin levels prior to or during pregnancy of dams fed HFD. Fetal masses were greater in dams fed HFD, and mRNA levels of proteins involved in fatty acid oxidation (CPT I, PPARα), but not glucose oxidation (pyruvate kinase) or other regulatory processes (HNF-4α, LXR), were increased with maternal dietary fat. There was also no change in mRNA levels of proteins involved in placental glucose and fatty acid transport, and GLUT1 protein levels in microvillous membranes were similar in placentas of dams fed either diet. Thus, the ability of the placenta to take up chylomicron remnant core lipids likely contributes to accelerated fetal growth in females fed high fat diets.

Cell cholesterol modulates metalloproteinase-dependent shedding of low-density lipoprotein receptor-related protein-1 (LRP-1) and clearance function

Low-density lipoprotein receptor-related protein-1 (LRP-1) is a plasma membrane scavenger and signaling receptor, composed of a large ligand-binding subunit (515-kDa α-chain) linked to a shorter transmembrane subunit (85-kDa β-chain). LRP-1 cell-surface level and function are controlled by proteolytic shedding of its ectodomain. Here, we identified ectodomain sheddases in human HT1080 cells and demonstrated regulation of the cleavage by cholesterol by comparing the classical fibroblastoid type with a spontaneous epithelioid variant, enriched ∼ 2-fold in cholesterol. Two membrane-associated metalloproteinases were involved in LRP-1 shedding: a disintegrin and metalloproteinase-12 (ADAM-12) and membrane-type 1 matrix metalloproteinase (MT1-MMP). Although both variants expressed similar levels of LRP-1, ADAM-12, MT1-MMP, and specific tissue inhibitor of metalloproteinases-2 (TIMP-2), LRP-1 shedding from epithelioid cells was ∼4-fold lower than from fibroblastoid cells. Release of the ectodomain was triggered by cholesterol depletion in epithelioid cells and impaired by cholesterol overload in fibroblastoid cells. Modulation of LRP-1 shedding on clearance was reflected by accumulation of gelatinases (MMP-2 and MMP-9) in the medium. We conclude that cholesterol exerts an important control on LRP-1 levels and function at the plasma membrane by modulating shedding of its ectodomain, and therefore represents a novel regulator of extracellular proteolytic activities.

Neuronal LRP1 knockout in adult mice leads to impaired brain lipid metabolism and progressive, age-dependent synapse loss and neurodegeneration

The vast majority of Alzheimer's disease (AD) cases are late onset with progressive synapse loss and neurodegeneration. Although the amyloid hypothesis has generated great insights into the disease mechanism, several lines of evidence indicate that other risk factors might precondition the brain to amyloid toxicity. Here, we show that the deletion of a major lipoprotein receptor, low-density lipoprotein receptor-related protein 1 (LRP1), in forebrain neurons in mice leads to a global defect in brain lipid metabolism characterized by decreased brain levels of cholesterol, sulfatide, galactosylceramide, and triglyceride. These lipid deficits correlate with progressive, age-dependent dendritic spine degeneration, synapse loss, neuroinflammation, memory loss, and eventual neurodegeneration. We further show that the levels of glutamate receptor subunits NMDA receptor 1 and Glu receptor 1 are selectively reduced in LRP1 forebrain knock-out mice and in LRP1 knockdown neurons, which is partially rescued by restoring neuronal cholesterol. Together, these studies support a critical role for LRP1 in maintaining brain lipid homeostasis and associated synaptic and neuronal integrity, and provide important insights into the pathophysiological mechanisms in AD.

Density enhanced phosphatase-1 down-regulates urokinase receptor surface expression in confluent endothelial cells

VEGF(165), the major angiogenic growth factor, is known to activate various steps in proangiogenic endothelial cell behavior, such as endothelial cell migration and invasion, or endothelial cell survival. Thereby, the urokinase-type plasminogen activator (uPA) system has been shown to play an essential role not only by its proteolytic capacities, but also by induction of intracellular signal transduction. Therefore, expression of its cell surface receptor uPAR is thought to be an essential regulatory mechanism in angiogenesis. We found that uPAR expression on the surface of confluent endothelial cells was down-regulated compared with subconfluent proliferating endothelial cells. Regulation of uPAR expression was most probably affected by extracellular signal-regulated kinase 1/2 (ERK1/2) activation, a downstream signaling event of the VEGF/VEGF-receptor system. Consistently, the receptor-like protein tyrosine phosphatase DEP-1 (density enhanced phosphatase-1/CD148), which is abundantly expressed in confluent endothelial cells, inhibited the VEGF-dependent activation of ERK1/2, leading to down-regulation of uPAR expression. Overexpression of active ERK1 rescued the DEP-1 effect on uPAR. That DEP-1 plays a biologic role in angiogenic endothelial cell behavior was demonstrated in endothelial cell migration, proliferation, and capillary-like tube formation assays in vitro.

Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family

The evolutionarily ancient low-density lipoprotein (LDL) receptor gene family represents a class of widely expressed cell surface receptors. Since the dawn of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors have evolved. In accordance with the now obsolete 'one-gene-one-function' hypothesis, these cell surface receptors were originally perceived as mere transporters of lipoproteins, lipids, and nutrients or as scavenger receptors, which remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular environment and the cell surface. This picture has since undergone a fundamental change. Experimental evidence has replaced the perception that these receptors serve merely as cargo transporters. Instead it is now clear that the transport of macromolecules is inseparably intertwined with the molecular machinery by which cells communicate with each other. Lipoprotein receptors are essentially sensors of the extracellular environment that participate in a wide range of physiological processes by physically interacting and coevolving with primary signal transducers as co-regulators. Furthermore, lipoprotein receptors modulate cellular trafficking and localization of the amyloid precursor protein (APP) and the β-amyloid peptide (Aβ), suggesting a role in the pathogenesis of Alzheimer's disease. Moreover, compelling evidence shows that LDL receptor family members are involved in tumor development and progression.

Lipoprotein receptor LRP1 regulates leptin signaling and energy homeostasis in the adult central nervous system

Lipoprotein receptor LRP1 play critical roles in lipid metabolism, and this study reveals a novel role for LRP1 in controlling food intake and obesity in the central nervous system of the adult mouse.

Obesity is a growing epidemic characterized by excess fat storage in adipocytes. Although lipoprotein receptors play important roles in lipid uptake, their role in controlling food intake and obesity is not known. Here we show that the lipoprotein receptor LRP1 regulates leptin signaling and energy homeostasis. Conditional deletion of the Lrp1 gene in the brain resulted in an obese phenotype characterized by increased food intake, decreased energy consumption, and decreased leptin signaling. LRP1 directly binds to leptin and the leptin receptor complex and is required for leptin receptor phosphorylation and Stat3 activation. We further showed that deletion of the Lrp1 gene specifically in the hypothalamus by Cre lentivirus injection is sufficient to trigger accelerated weight gain. Together, our results demonstrate that the lipoprotein receptor LRP1, which is critical in lipid metabolism, also regulates food intake and energy homeostasis in the adult central nervous system.

The World Health Organization estimates that at least 1 in 10 adults worldwide are obese, and in some western countries, a far greater percentage (25% or more) is affected. Obesity is a serious concern because it increases the risk of cardiovascular disease, type 2 diabetes, and some cancers, among other health problems. Despite recent advances in understanding the disease mechanism, effective treatments are still lacking. Lipoprotein receptors play critical roles in lipid metabolism, but their potential roles in controlling food intake and obesity in the central nervous system have not been examined. Here we show that deletion of LRP1, a member of the LDL (low density lipoprotein) receptor family, in the adult mouse brain results in obese phenotype characterized by increased food intake, decreased energy consumption and decreased leptin signaling. We further show that deletion of the Lrp1 gene specifically in the hypothalamus (a region of the brain) by using Cre lentivirus injection is sufficient to trigger accelerated weight gain. Together, our results present a novel function of LRP1: the direct regulation of leptin signaling and energy balance in the adult central nervous system. Hence, LRP1 represents a very promising new therapeutic target for the design of innovative and more effective therapies for obesity.

Premature ligand-receptor interaction during biosynthesis limits the production of growth factor midkine and its receptor LDL receptor-related protein 1

Protein production within the secretory pathway is accomplished by complex but organized processes. Here, we demonstrate that the growth factor midkine interacts with LDL receptor-related protein 1 (LRP1) at high affinity (K(d) value, 2.7 nm) not only at the cell surface but also within the secretory pathway during biosynthesis. The latter premature ligand-receptor interaction resulted in aggregate formation and consequently suppressed midkine secretion and LRP1 maturation. We utilized an endoplasmic reticulum (ER) retrieval signal and an LRP1 fragment, which strongly bound to midkine and the LRP1-specialized chaperone receptor-associated protein (RAP), to construct an ER trapper. The ER trapper efficiently trapped midkine and RAP and mimicked the premature ligand-receptor interaction, i.e. suppressed maturation of the ligand and receptor. The ER trapper also diminished the inhibitory function of LRP1 on platelet-derived growth factor-mediated cell migration. Complementary to these results, an increased expression of RAP was closely associated with midkine expression in human colorectal carcinomas (33 of 39 cases examined). Our results suggest that the premature ligand-receptor interaction plays a role in protein production within the secretory pathway.

Plasminogen activator inhibitor type 2: still an enigmatic serpin but a model for gene regulation

Plasminogen activator inhibitor type-2 (PAI-2; SERPINB2) is an atypical member of the Ov-serpin family of serine protease inhibitors. While it is an undisputed inhibitor of urokinase and tissue-type plasminogen activator in the extracellular space and on the cell surface, the weight of circumstantial evidence suggests that PAI-2 also fulfills an intracellular role which is independent of plasminogen activator inhibition and indeed may not even involve protease inhibition at all. More and more data continue to implicate a role for PAI-2 in many settings, the most recent associating it as a modulator of the innate immune response. Further to the debates concerning its physiological role, there are few genes, if any, that display the regulation profile of the PAI-2 gene: PAI-2 protein and mRNA levels can be induced in the order of, not hundred-, but thousand-folds in a process that is controlled at many levels including gene transcription and mRNA stability while an epigenetic component is also likely. The ability of some cells, including monocytes, fibroblasts, and neurons to have the capacity to increase PAI-2 synthesis to such high levels is intriguing enough. So why do these cells have the capacity to synthesize so much of this protein? While tantalizing clues continue to be revealed to the field, an understanding of how this gene is regulated so profoundly has provided insights into the broader mechanics of gene expression and regulation.

Determination of the kinetic on- and off-rate of single virus-cell interactions

Human rhinoviruses are the causative agents of the common cold. The serotypes belonging to the minor receptor group attach to members of the low-density lipoprotein receptor family and enter the host cell via receptor-mediated endocytosis. Receptor binding, the very first step in infection, was characterized by force spectroscopy measurements at the single molecule level. We demonstrate how kinetic on- and off-rate constants can be derived from such experiments carried out with the atomic force microscope.

Chromosomal, epigenetic and microRNA-mediated inactivation of LRP1B, a modulator of the extracellular environment of thyroid cancer cells

The low-density lipoprotein receptor-related protein (LRP1B), encoding an endocytic LDL-family receptor, is among the 10 most significantly deleted genes across 3312 human cancer specimens. However, currently the apparently crucial role of this lipoprotein receptor in carcinogenesis is not clear. Here we show that LRP1B inactivation (by chromosomal, epigenetic and microRNA (miR)-mediated mechanisms) results in changes to the tumor environment that confer cancer cells an increased growth and invasive capacity. LRP1B displays frequent DNA copy number loss and CpG island methylation, resulting in mRNA underexpression. By using CpG island reporters methylated in vitro, we found that DNA methylation disrupts a functional binding site for the histone-acetyltransferase p300 located at intron 1. We identified and validated an miR targeting LRP1B (miR-548a-5p), which is overexpressed in cancer cell lines as a result of 8q22 DNA gains. Restoration of LRP1B impaired in vitro and in vivo tumor growth, inhibited cell invasion and led to a reduction of matrix metalloproteinase 2 in the extracellular medium. We emphasized the role of an endocytic receptor acting as a tumor suppressor by modulating the extracellular environment composition in a way that constrains the invasive behavior of the cancer cells.

MESD is essential for apical localization of megalin/LRP2 in the visceral endoderm

Deletion of the Mesd gene region blocks gastrulation and mesoderm differentiation in mice. MESD is a chaperone for the Wnt co-receptors: low-density lipoprotein receptor-related protein (LRP) 5 and 6 (LRP5/6). We hypothesized that loss of Wnt signaling is responsible for the polarity defects observed in Mesd-deficient embryos. However, because the Mesd-deficient embryo is considerably smaller than Lrp5/6 or Wnt3 mutants, we predicted that MESD function extends more broadly to the LRP family of receptors. Consistent with this prediction, we demonstrated that MESD function in vitro was essential for maturation of the β-propeller/EGF domain common to LRPs. To begin to understand the role of MESD in LRP maturation in vivo, we generated a targeted Mesd knockout and verified that loss of Mesd blocks WNT signaling in vivo. Mesd mutants continue to express the pluripotency markers Oct4, Nanog, and Sox2, suggesting that Wnt signaling is essential for differentiation of the epiblast. Moreover, we demonstrated that MESD was essential for the apical localization of the related LRP2 (Megalin/MEG) in the visceral endoderm, resulting in impaired endocytic function. Combined, our results provide evidence that MESD functions as a general LRP chaperone and suggest that the Mesd phenotype results from both signaling and endocytic defects resulting from misfolding of multiple LRP receptors.

LDL receptor-related protein 1 regulates the abundance of diverse cell-signaling proteins in the plasma membrane proteome

LDL receptor-related protein 1 (LRP1) is an endocytic receptor, reported to regulate the abundance of other receptors in the plasma membrane, including uPAR and tissue factor. The goal of this study was to identify novel plasma membrane proteins, involved in cell-signaling, that are regulated by LRP1. Membrane protein ectodomains were prepared from RAW 264.7 cells in which LRP1 was silenced and control cells using protease K. Peptides were identified by LC-MS/MS. By analysis of spectral counts, 31 transmembrane and secreted proteins were regulated in abundance at least 2-fold when LRP1 was silenced. Validation studies confirmed that semaphorin4D (Sema4D), plexin domain-containing protein-1 (Plxdc1), and neuropilin-1 were more abundant in the membranes of LRP1 gene-silenced cells. Regulation of Plxdc1 by LRP1 was confirmed in CHO cells, as a second model system. Plxdc1 coimmunoprecipitated with LRP1 from extracts of RAW 264.7 cells and mouse liver. Although Sema4D did not coimmunoprecipitate with LRP1, the cell-surface level of Sema4D was increased by RAP, which binds to LRP1 and inhibits binding of other ligands. These studies identify Plxdc1, Sema4D, and neuropilin-1 as novel LRP1-regulated cell-signaling proteins. Overall, LRP1 emerges as a generalized regulator of the plasma membrane proteome.

LRP1 mediates bidirectional transcytosis of amyloid-β across the blood-brain barrier

According to the "amyloid hypothesis", the amyloid-β (Aβ) peptide is the toxic intermediate driving Alzheimer's disease (AD) pathogenesis. Recent evidence suggests that the low density lipoprotein receptor-related protein 1 (LRP1) transcytoses Aβ out of the brain across the blood-brain barrier (BBB). To provide genetic evidence for LRP1-mediated transcytosis of Aβ across the BBB we analyzed Aβ transcytosis across primary mouse brain capillary endothelial cells (pMBCECs) derived from wild-type and LRP1 knock-in mice. Here, we show that pMBCECs in vitro express functionally active LRP1. Moreover, we demonstrate that LRP1 mediates transcytosis of [(125)I]-Aβ(1-40) across pMBCECs in both directions, whereas no role for LRP1-mediated Aβ degradation was detected. Analysis of [(125)I]-Aβ(1-40) transport across pMBCECs generated from mice harboring a knock-in mutation in the NPxYxxL endocytosis/sorting domain of endogenous LRP1 revealed a reduced Aβ clearance from brain-to-blood and blood-to-brain compared with wild-type derived pMBCECs. Therefore, for the first time, we present genetic evidence that LRP1 modulates the pathogenic actions of soluble Aβ in the brain by clearing Aβ across the BBB.

Inflammatory mediators promote production of shed LRP1/CD91, which regulates cell signaling and cytokine expression by macrophages

LRP1 is a type-1 transmembrane receptor that mediates the endocytosis of diverse ligands. LRP1 β-chain proteolysis results in release of sLRP1 that is present in human plasma. In this study, we show that LPS and IFN-γ induce shedding of LRP1 from RAW 264.7 cells and BMMs in vitro. ADAM17 was principally responsible for the increase in LRP1 shedding. sLRP1 was also increased in vivo in mouse plasma following injection of LPS and in plasma from human patients with RA or SLE. sLRP1, which was purified from human plasma, and full-length LRP1, purified from mouse liver, activated cell signaling when added to cultures of RAW 264.7 cells and BMMs. Robust activation of p38 MAPK and JNK was observed. The IKK-NF-κB pathway was transiently activated. Proteins that bind to the ligand-binding clusters in LRP1 failed to inhibit sLRP1-initiated cell signaling, however an antibody that targets the sLRP1 N terminus was effective. sLRP1 induced expression of regulatory cytokines by RAW 264.7 cells, including TNF-α, MCP-1/CCL2, and IL-10. These results demonstrate that sLRP1 is generated in inflammation and may regulate inflammation by its effects on macrophage physiology.

Macrophage LRP-1 controls plaque cellularity by regulating efferocytosis and Akt activation

OBJECTIVE

The balance between apoptosis susceptibility and efferocytosis of macrophages is central to plaque remodeling and inflammation. LRP-1 and its ligand, apolipoprotein E, have been implicated in efferocytosis and apoptosis in some cell types. We investigated the involvement of the macrophage LRP-1/apolipoprotein E axis in controlling plaque apoptosis and efferocytosis. Method and Results- LRP-1(-/-) macrophages displayed nearly 2-fold more TUNEL positivity compared to wild-type cells in the presence of DMEM alone or with either lipopolysaccharide or oxidized low-density lipoprotein. The survival kinase, phosphorylated Akt, was barely detectable in LRP-1(-/-) cells, causing decreased phosphorylated Bad and increased cleaved caspase-3. Regardless of the apoptotic stimulation and degree of cell death, LRP-1(-/-) macrophages displayed enhanced inflammation with increased IL-1 beta, IL-6, and tumor necrosis factor-alpha expression. Efferocytosis of apoptotic macrophages was reduced by 60% in LRP-1(-/-) vs wild-type macrophages despite increased apolipoprotein E expression by both LRP-1(-/-) phagocytes and wild-type apoptotic cells. Compared to wild-type macrophage lesions, LRP-1(-/-) lesions had 5.7-fold more necrotic core with more dead cells not associated with macrophages.

CONCLUSIONS

Macrophage LRP-1 deficiency increases cell death and inflammation by impairing phosphorylated Akt activation and efferocytosis. Increased apolipoprotein E expression in LRP-1(-/-) macrophages suggests that the LRP-1/apolipoprotein E axis regulates the balance between apoptosis and efferocytosis, thereby preventing necrotic core formation.

Plasminogen activator inhibitor-1 regulates myoendothelial junction formation

RATIONALE

Plasminogen activator inhibitor-1 (PAI-1) is a biomarker for several vascular disease states; however, its target of action within the vessel wall is undefined.

OBJECTIVE

Determine the ability of PAI-1 to regulate myoendothelial junction (MEJ) formation.

METHODS AND RESULTS

MEJs are found throughout the vasculature linking endothelial cells (ECs) and vascular smooth muscle cells. Using a vascular cell coculture we isolated MEJ fractions and performed two-dimensional differential gel electrophoresis. Mass spectrometry identified PAI-1 as being enriched within MEJ fractions, which we confirmed in vivo. In the vascular cell coculture, recombinant PAI-1 added to the EC monolayer significantly increased MEJs. Conversely, addition of a PAI-1 monoclonal antibody to the EC monolayer reduced the number of MEJs. This was also observed in vivo where mice fed a high fat diet had increased PAI-1 and MEJs and the number of MEJs in coronary arterioles of PAI-1(-/-) mice was significantly reduced when compared to C57Bl/6 mice. The presence of MEJs in PAI-1(-/-) coronary arterioles was restored when their hearts were transplanted into and exposed to the circulation of C57Bl/6 mice. Application of biotin-conjugated PAI-1 to the EC monolayer in vitro confirmed the ability of luminal PAI-1 to translocate to the MEJ. Functionally, phenylephrine-induced heterocellular calcium communication in the vascular cell coculture was temporally enhanced when recombinant PAI-1 was present, and prolonged when PAI-1 was absent.

CONCLUSION

Our data implicate circulating PAI-1 as a key regulator of MEJ formation and a potential target for pharmacological intervention in diseases with vascular abnormalities (eg, diabetes mellitus).

Extensive proteomic screening identifies the obesity-related NYGGF4 protein as a novel LRP1-interactor, showing reduced expression in early Alzheimer's disease

BACKGROUND

The low-density lipoprotein receptor related protein 1 (LRP1) has been implicated in Alzheimer's disease (AD) but its signalling has not been fully evaluated. There is good evidence that the cytoplasmic domain of LRP1 is involved in protein-protein interactions, important in the cell biology of LRP1.

RESULTS

We carried out three yeast two-hybrid screens to identify proteins that interact with the cytoplasmic domain of LRP1. The screens included both conventional screens as well as a novel, split-ubiquitin-based screen in which an LRP1 construct was expressed and screened as a transmembrane protein. The split-ubiquitin screen was validated in a screen using full-length amyloid protein precursor (APP), which successfully identified FE65 and FE65L2, as well as novel interactors (Rab3a, Napg, and ubiquitin b). Using both a conventional screen as well as the split-ubiquitin screen, we identified NYGGF4 as a novel LRP1 interactor. The interaction between LRP1 and NYGGF4 was validated using two-hybrid assays, coprecipitation and colocalization in mammalian cells. Mutation analysis demonstrated a specific interaction of NYGGF4 with an NPXY motif that required an intact tyrosine residue. Interestingly, while we confirmed that other LRP1 interactors we identified, including JIP1B and EB-1, were also able to bind to APP, NYGGF4 was unique in that it showed specific binding with LRP1. Expression of NYGGF4 decreased significantly in patients with AD as compared to age-matched controls, and showed decreasing expression with AD disease progression. Examination of Nyggf4 expression in mice with different alleles of the human APOE4 gene showed significant differences in Nyggf4 expression.

CONCLUSIONS

These results implicate NYGGF4 as a novel and specific interactor of LRP1. Decreased expression of LRP1 and NYGGF4 over disease, evident with the presence of even moderate numbers of neuritic plaques, suggests that LRP1-NYGGF4 is a system altered early in disease. Genetic and functional studies have implicated both LRP1 and NYGGF4 in obesity and cardiovascular disease and the physical association of these proteins may reflect a common mechanism. This is particularly interesting in light of the dual role of ApoE in both cardiovascular risk and AD. The results support further studies on the functional relationship between NYGGF4 and LRP1.

Hepatic heparan sulfate proteoglycans and endocytic clearance of triglyceride-rich lipoproteins

Hypertriglyceridemia, characterized by the accumulation of triglyceride-rich lipoproteins in the blood, affects 10-20% of the population in western countries and increases the risk of atherosclerosis, coronary artery disease, and pancreatitis. The etiology of hypertriglyceridemia is complex, and much interest exists in identifying and characterizing the biological and environmental factors that affect the synthesis and turnover of plasma triglycerides. Genetic studies in mice have recently identified that heparan sulfate proteoglycans are a class of receptors that mediate the clearance of triglyceride-rich lipoproteins in the liver. Heparan sulfate proteoglycans are expressed by endothelial cells that line the hepatic sinusoids and the underlying hepatocytes, and are present in the perisinusoidal space (space of Disse). This chapter discusses the dependence of lipoprotein binding on heparan sulfate structure and the identification of hepatocyte syndecan-1 as the primary proteoglycan that mediates triglyceride-rich lipoprotein clearance.

Inactivation of the proximal NPXY motif impairs early steps in LRP1 biosynthesis

The proximal NPXY and distal NPXYXXL motifs in the intracellular domain of LRP1 play an important role in regulation of the function of the receptor. The impact of single and double inactivating knock-in mutations of these motifs on receptor maturation, cell surface expression, and ligand internalization was analyzed in mutant and control wild-type mice and MEFs. Single inactivation of the proximal NPXY or in combination with inactivation of the distal NPXYXXL motif are both shown to be associated with an impaired maturation and premature proteasomal degradation of full-length LRP1. Therefore, only a small mature LRP1 pool is able to reach the cell surface resulting indirectly in severe impairment of ligand internalization. Single inactivation of the NPXYXXL motif revealed normal maturation, but direct impairment of ligand internalization. In conclusion, the proximal NPXY motif proves to be essential for early steps in the LRP1 biosynthesis, whereas NPXYXXL appears rather relevant for internalization.