The low-density lipoprotein receptor gene family: a cellular Swiss army knife?

Thrombospondin-1 inhibits VEGF levels in the ovary directly by binding and internalization via the low density lipoprotein receptor-related protein-1 (LRP-1)

VEGF is a potent pro-angiogenic factor whose effects are opposed by a host of anti-angiogenic proteins, including thrombospondin-1 (TSP-1). We have previously shown that VEGF has important extravascular roles in the ovary and that VEGF and TSP-1 are inversely expressed throughout the ovarian cycle. To date, however, a causal interaction between TSP-1 and VEGF has not been identified. Here, we show that TSP-1 has a direct inhibitory effect on VEGF by binding the growth factor and internalizing it via LRP-1. Mice lacking TSP-1 are subfertile and exhibited ovarian hypervascularization and altered ovarian morphology. Treatment of ovarian cells with TSP-1 decreased VEGF levels and rendered the cells more susceptible to TNFalpha-induced apoptosis. Knockdown of TSP-1, through RNA interference, resulted in overexpression of VEGF and reduced cytokine-induced apoptosis. In conclusion, we demonstrate a direct inhibitory effect of TSP-1 on VEGF in the ovary. TSP-1's regulation of VEGF appears to be an important mediator of ovarian angiogenesis and follicle development.

Human rhinovirus type 54 infection via heparan sulfate is less efficient and strictly dependent on low endosomal pH

K-type major-group human rhinoviruses (HRVs) (including HRV54) share a prominent lysine residue in the HI surface loop of VP1 with all minor-group HRVs. Despite the presence of this residue, they cannot use members of the low-density lipoprotein receptor family for productive infection. Reexamining all K-type viruses for receptor usage, we noticed that HRV54 is able to replicate in RD cells that lack the major-group receptor intercellular adhesion molecule 1 (ICAM-1). By using receptor blocking assays, inhibition of sulfation, enzymatic digestion, and proteoglycan-deficient cell lines, we show here that wild-type HRV54, without any adaptation, uses heparan sulfate (HS) proteoglycan as an alternate receptor. However, infection via HS is less efficient than infection via ICAM-1. Moreover, HRV54 has an acid lability profile similar to that of the minor-group virus HRV2. In ICAM-1-deficient cells its replication is completely blocked by the H(+)-ATPase inhibitor bafilomycin A1, whereas in ICAM-1-expressing cells it replicates in the presence of the drug. Thus, use of a "noncatalytic" receptor requires the virus to be highly unstable at low pH.

Expanded tropism and altered activation of a retroviral glycoprotein resistant to an entry inhibitor peptide

The envelope of class I viruses can be a target for potent viral inhibitors, such as the human immunodeficiency virus type 1 (HIV-1) inhibitor enfuvirtide, which are derived from the C-terminal heptad repeat (HR2) of the transmembrane (TM) subunit. Resistance to an HR2-based peptide inhibitor of a model retrovirus, subgroup A of the Avian Sarcoma and Leukosis Virus genus (ASLV-A), was studied by examining mutants derived by viral passage in the presence of inhibitor. Variants with reduced sensitivity to inhibitor were readily selected in vitro. Sensitivity determinants were identified for 13 different isolates, all of which mapped to the TM subunit. These determinants were identified in two regions: (i) the N-terminal heptad repeat (HR1) and (ii) the N-terminal segment of TM, between the subunit cleavage site and the fusion peptide. The latter class of mutants identified a region outside of the predicted HR2-binding site that can significantly alter sensitivity to inhibitor. A subset of the HR1 mutants displayed the unanticipated ability to infect nonavian cells. This expanded tropism was associated with increased efficiency of envelope triggering by soluble receptor at low temperatures, as measured by protease sensitivity of the surface subunit (SU) of envelope. In addition, expanded tropism was linked for the most readily triggered mutants with increased sensitivity to neutralization by SU-specific antiserum. These observations depict a class of HR2 peptide-selected mutations with a reduced activation threshold, thereby allowing the utilization of alternative receptors for viral entry.

Serpin A1 and CD91 as host instruments against HIV-1 infection: are extracellular antiviral peptides acting as intracellular messengers?

Serpin A1 (alpha1-antitrypsin, alpha1-proteinase inhibitor) has been shown to be a non-cytolytic antiviral factor present in blood and effective against HIV infection. The best known physiological role of serpin A1 is to inhibit neutrophil elastase, a proteinase which is secreted by neutrophils at sites of infection and inflammation. Decreased HIV-infectivity is associated with decreased density of membrane-associated elastase. The enzyme may facilitate binding of the HIV membrane protein gp120 to host cells, and it specifically cleaves SDF-1, the physiological ligand of the HIV-1 co-receptor CXCR4. It has been suggested that one of the actions of serpin A1 as antiviral agent is to reduce HIV infectivity, and this property could be due to elastase inhibition. However, the most dramatic effect of serpin A1 is inhibition of HIV production. In vitro experiments indicate that the C-terminal peptide of serpin A1, produced during the formation of the complex of serpin with serine proteinases, may be responsible for the inhibition of HIV-1 expression in infected cells. This peptide, an integral part of the serpin-enzyme complex, is internalized by several scavenger receptors. Peptides corresponding to the C-terminal section of serpin A1 inhibit HIV-1 long-terminal-repeat-driven transcription and interact with nuclear proteins, such as alpha1-fetoprotein transcription factor. LDL-receptor-related protein 1 (LRP1/CD91), the best known receptor for serpin-enzyme complexes, is up-regulated in monocytes of HIV-1-infected true non-progressors. CD91 could be one of the major players in host resistance against HIV-1. It has the capacity of internalizing antiviral peptides such as serpin C-terminal fragments and alpha-defensins, and is at the same time the receptor for heat-shock proteins in antigen-presenting cells, in which chaperoned viral peptides could lead to the induction of cytotoxic T-cell responses.

The LDL receptor-related protein (LRP) family: an old family of proteins with new physiological functions

The low-density lipoprotein (LDL) receptor is the founding member of a family of seven structurally closely related transmembrane proteins (LRP1, LRP1b, megalin/LRP2, LDL receptor, very low-density lipoprotein receptor, MEGF7/LRP4, LRP8/apolipoprotein E receptor2). These proteins participate in a wide range of physiological processes, including the regulation of lipid metabolism, protection against atherosclerosis, neurodevelopment, and transport of nutrients and vitamins. While currently available data suggest that the role of the LDL receptor is limited to the regulation of cholesterol homeostasis by receptor-mediated endocytosis of lipoprotein particles, there is growing experimental evidence that the other members of the gene family have additional physiological functions as signal transducers. In this review, we focus on the latest discovered functions of two major members of this family, LRP1 and megalin/LRP2, and on the newly elucidated physiological role of a third member of the family, MEGF7/LRP4, which can also function as a modulator of diverse signaling pathways during development.

Lipoprotein receptors in Alzheimer's disease

Lipoprotein receptors have important roles in pathological processes that lead to Alzheimer's disease (AD). Previously, they were believed to act mainly by modulating the neuronal metabolism of cholesterol and apolipoprotein E, major risk factors for spontaneous AD. However, recent findings point towards an unexpected new function for lipoprotein receptors in regulation of intracellular transport and processing of the amyloid precursor protein (APP) to give amyloid-beta peptide, the principal component of senile plaques. Here, we will discuss how lipoprotein receptors might modulate distinct steps in neuronal trafficking of APP, and how an intricate balance between opposing receptor activities might be a crucial determinant of APP processing, and of onset and progression of neurodegeneration.

Inhibition of plasminogen activator inhibitor-1 binding to endocytosis receptors of the low-density-lipoprotein receptor family by a peptide isolated from a phage display library

The functions of the serpin PAI-1 (plasminogen activator inhibitor-1) are based on molecular interactions with its target proteases uPA and tPA (urokinase-type and tissue-type plasminogen activator respectively), with vitronectin and with endocytosis receptors of the low-density-lipoprotein family. Understanding the significance of these interactions would be facilitated by the ability to block them individually. Using phage display, we have identified the disulfide-constrained peptide motif CFGWC with affinity for natural human PAI-1. The three-dimensional structure of a peptide containing this motif (DVPCFGWCQDA) was determined by liquid-state NMR spectroscopy. A binding site in the so-called flexible joint region of PAI-1 was suggested by molecular modelling and validated through binding studies with various competitors and site-directed mutagenesis of PAI-1. The peptide with an N-terminal biotin inhibited the binding of the uPA-PAI-1 complex to the endocytosis receptors low-density-lipoprotein-receptor-related protein 1A (LRP-1A) and very-low-density-lipoprotein receptor (VLDLR) in vitro and inhibited endocytosis of the uPA-PAI-1 complex in U937 cells. We conclude that the isolated peptide represents a novel approach to pharmacological interference with the functions of PAI-1 based on inhibition of one specific molecular interaction.

LR11/SorLA expression is reduced in sporadic Alzheimer disease but not in familial Alzheimer disease

LR11 is an ApoE receptor that is enriched in the brain. We have shown that LR11 is markedly downregulated in patients with sporadic Alzheimer disease (AD). This finding led us to explore whether reduced LR11 expression reflects a primary mechanism of disease or merely a secondary consequence of other AD-associated changes. Therefore, LR11 expression was assessed in a transgenic mouse model of AD and familial AD (FAD) brains. Immunohistochemistry and immunoblotting of LR11 in PS1/APP transgenic and wild-type mice indicated that LR11 levels are not affected by genotype or accumulation of amyloid pathology. LR11 expression was also evaluated based on immunoblotting and LR11 immunostaining intensity in human frontal cortex in controls, sporadic AD, and FAD, including cases with presenilin-1 (PS1) and presenilin-2 (PS2) mutations. Although LR11 was reduced in sporadic AD, there was no difference in protein level or staining intensity between control and FAD cases. The finding that LR11 expression is unaffected in both a mouse model of AD and autosomal-dominant forms of AD suggests that LR11 is not regulated by amyloid accumulation or other AD neuropathologic changes. We hypothesize that LR11 loss may be specific to sporadic AD and influence amyloid pathology through mechanisms independent of substrate-enzyme interactions regulated by FAD mutations.

Defective splicing of Megf7/Lrp4, a regulator of distal limb development, in autosomal recessive mulefoot disease

Mulefoot disease (MFD) is an autosomal recessive disorder of phenotypically variable expression that causes syndactyly in certain strains of cows. MFD maps to a narrow interval on bovine chromosome 15 that is syntenic to human chromosome 11p12-p11.2. This region contains MEGF7/LRP4 (approved gene symbol LRP4), a gene that encodes a member of the multifunctional low-density lipoprotein receptor gene family. Targeted and naturally occurring mutations in the murine Megf7/Lrp4 gene, a putative coreceptor in the Wnt signaling pathway, cause polysyndactyly in the rodent. Thus, Megf7/Lrp4 is a strong candidate for the MFD mutation. Using PCR analysis of tissue samples and sperm from confirmed homozygous MFD carriers, we have identified a functional single base pair mutation in the affected animals. We show that a G --> A transition at the first nucleotide in the splice donor site of intron 37 completely disables this splice site. The abnormal splicing that is caused by this mutation predicts the generation of a dysfunctional membrane-anchored receptor lacking the normal cytoplasmic domain. These findings confirm that autosomal recessive loss-of-function mutations in Megf7/Lrp4 result in phenotypically similar forms of syndactyly in different mammalian species and that such mutations are the cause of MFD in bovines.

The low density lipoprotein receptor-related protein functions as an endocytic receptor for decorin

Decorin is a small leucine-rich proteoglycan that modulates the activity of transforming growth factor type beta and other growth factors and thereby influences the processes of proliferation and differentiation in a wide array of physiological and pathological reactions. Hence, understanding the regulatory mechanisms of decorin activity has broad implications. Here we report that the extracellular levels of decorin are controlled by receptor-mediated catabolism, involving the low density lipoprotein receptor family member, low density lipoprotein receptor-related protein (LRP). We show that decorin is endocytosed and degraded by C2C12 myoblast cells and that both processes are blocked by suppressing LRP expression using short interfering RNA. The same occurs with CHO cells, but not with CHO cells genetically deficient in LRP. Finally, we show that LRP-null CHO cells, transfected to express mini-LRP polypeptides containing either the second or fourth LRP ligand-binding domains, carry out decorin endocytosis and lysosomal degradation. These findings point to LRP-mediated catabolism as a new control pathway for the biological activities of decorin, specifically for its ability to influence extracellular matrix signaling.

The Single Ligand-binding Repeat of Tva, a Low Density Lipoprotein Receptor-related Protein, Contains Two Ligand-binding Surfaces

The receptor for avian sarcoma/leukosis virus subtype A (ASLV-A), Tva, is the simplest member of the low density lipoprotein receptor family containing a single ligand-binding repeat (LBR). Most LBRs contain a central Trp (Trp33 in Tva) that is important for ligand binding and, for the low density lipoprotein receptor, is associated with familial hypercholesterolemia. The Tva ligand-binding module contains a second Trp (Trp48) that is part of a DEW motif present in a subset of LBRs. Trp48 is important for ASLV-A infectivity. A soluble Tva (sTva) ligand-binding module is sufficient for ASLV-A infectivity. Tva interacts with the viral glycoprotein, and a soluble receptor-binding domain (SUA) binds sTva with picomolar affinity. We investigated whether Tva, a retroviral receptor, could behave as a classic LBR by assessing sTva interactions with the universal receptor-associated protein (RAP) and comparing these interactions with those between sTva and its viral ligand (SUA). To address the role of the two Trp residues in Tva function, we prepared sTva harboring mutations of Trp33, Trp48, or both and determined the binding kinetics with RAP and SUA. We found that sTva behaved as a "normal" receptor toward RAP, requiring both calcium and Trp33 for binding. However, sTva binding to SUA required neither calcium nor Trp33. Furthermore, sTva could bind both RAP and SUA simultaneously. These results show that the single LBR of Tva has two ligand-binding sites, raising the possibility that other LBRs may also.

Structure of an LDLR-RAP complex reveals a general mode for ligand recognition by lipoprotein receptors

Proteins of the low-density lipoprotein receptor (LDLR) family are remarkable in their ability to bind an extremely diverse range of protein and lipoprotein ligands, yet the basis for ligand recognition is poorly understood. Here, we report the 1.26 A X-ray structure of a complex between a two-module region of the ligand binding domain of the LDLR and the third domain of RAP, an escort protein for LDLR family members. The RAP domain forms a three-helix bundle with two docking sites, one for each LDLR module. The mode of recognition at each site is virtually identical: three conserved, calcium-coordinating acidic residues from each LDLR module encircle a lysine side chain protruding from the second helix of RAP. This metal-dependent mode of electrostatic recognition, together with avidity effects resulting from the use of multiple sites, represents a general binding strategy likely to apply in the binding of other basic ligands to LDLR family proteins.

Mutations in the gene encoding the low-density lipoprotein receptor LRP4 cause abnormal limb development in the mouse

Positional cloning of two recessive mutations of the mouse that cause polysyndactyly (dan and mdig-Chr 2) confirmed that the gene encoding MEGF7/LRP4, a member of the low-density lipoprotein receptor family, plays an essential role in the process of digit differentiation. Pathologies observed in the mutant mice provide insight into understanding the function(s) of LRP4 as a negative regulator of the Wnt-beta-catenin signaling pathway and may help identify the genetic basis for common human disorders with similar phenotypes.

The C-terminal 26-residue peptide of serpin A1 is an inhibitor of HIV-1

Serpin A1 (alpha1-proteinase inhibitor) inhibits human immunodeficiency virus 1 (HIV-1) production by mechanisms which remain to be elucidated. The complex formation of serpin A1 with proteinases eliminates the proteolytic activity and generates a fragment corresponding to the serpin C-terminal 36-residue peptide. Here, we show that the C-terminal 26-residue peptide of serpin A1 (A1-C26) inhibits HIV-long terminal repeat (LTR)-driven transcription in epithelial cells transfected with HIV-1 LTR promoter-driven genes. A1-C26 increased STAT1 phosphorylation and strongly reduced viral expression in a monocytic cell line infected with HIV-1 NL4-3. This reduction of expression was also observed in HIV-1 infected, PHA-activated peripheral blood mononuclear cells. In HIV-1 infected cells, the inhibitory activity of HIV-1 caused by B9-C23 and C1-C26, the A1-C26 homologues corresponding to the C-terminal sections of serpin B9 and serpin C1, was much lower than that obtained with A1-C26. These serpin peptides represent a novel class of antiviral agents.

Characterization of low density lipoprotein receptor ligand interactions by fluorescence resonance energy transfer

The low density lipoprotein receptor (LDLR) is the prototype of a family of cell surface receptors involved in a wide range of biological processes. A soluble low density lipoprotein receptor (sLDLR) and a tryptophan (Trp)-deficient variant human apolipoprotein E3 (apoE3) N-terminal domain (NT) were used in binding studies. The sole cysteine in apoE3-NT was covalently modified with an extrinsic fluorescence probe, N-(iodoacetyl)-N'-(5-sulfo-1-napthyl)ethylenediamine (AEDANS), and the protein was complexed with lipid. Incubation of sLDLR with AEDANS-Trp-null apoE3-NT dimyristoylphosphatidylcholine (DMPC) disks, but not lipid-free AEDANS-apoE, induced an enhancement in AEDANS fluorescence emission intensity (excitation, 280 nm) consistent with intermolecular energy transfer from excited Trp in sLDLR to receptor-bound apoE. Ligand binding to sLDLR required calcium and was saturable. In competition binding assays, unlabeled apoE3-NT DMPC inhibited AEDANS-apoE DMPC binding to sLDLR more effectively than low density lipoprotein. Fluorescence changes in this system reflected pH-dependent ligand binding and release from sLDLR consistent with models derived from the X-ray crystal structure of the receptor at endosomal pH. Intermolecular energy transfer from excited Trp in LDLR family members to fluorescently tagged ligands represents a sensitive and convenient assay for the characterization of the myriad molecular interactions ascribed to this family of receptor.

Expression of low density lipoprotein receptor-related protein 4 (Lrp4) gene in the mouse germ cells

The low density lipoprotein receptor-related protein 4 gene (Lrp4) was identified by subtractive screening of cDNAs of the migratory primordial germ cells (PGCs) of E8.5-9.5 embryo and E3.5 blastocysts. Lrp4 is expressed in PGCs in the hindgut and the dorsal mesentery of E9.5 embryos, and in germ cells in the genital ridges of male and female E10.5-13.5 embryos. Lrp4 is also expressed in spermatogonia of the neonatal and adult testes and in the immature oocytes and follicular cells of the adult ovary. The absence of Lrp4 expression in the blastocyst, embryonic stem cells and embryonic germ cells suggests the Lrp4 is a molecular marker that distinguishes the germ cells from embryo-derived pluripotent stem cells.

A mutation in the first ligand-binding repeat of the human very-low-density lipoprotein receptor results in high-affinity binding of the single V1 module to human rhinovirus 2

Minor group human rhinoviruses (HRVs) bind members of the low-density lipoprotein receptor family for cell entry. The ligand-binding domains of these membrane proteins are composed of various numbers of direct repeats of about 40 amino acids in length. Residues involved in binding of module 3 (V3) of the very-low-density lipoprotein receptor (VLDLR) to HRV2 have been identified by X-ray crystallography (N. Verdaguer, I. Fita, M. Reithmayer, R. Moser, and D. Blaas, Nat. Struct. Mol. Biol. 11:429-434, 2004). Sequence comparisons of the eight repeats of VLDLR with respect to the residues implicated in the interaction between V3 and HRV2 suggested that (in addition to V3) V1, V2, V5, and V6 also fulfill the requirements for interacting with the virus. Using a highly sensitive binding assay employing phage display, we demonstrate that single modules V2, V3, and V5 indeed bind HRV2. However, V1 does not. A single mutation from threonine 17 to proline converted the nonbinding wild-type form of V1 into a very strong binder. We interpret the dramatic increase in affinity by the generation of a hydrophobic patch between virus and receptor; in the presence of threonine, the contact area might be disturbed. This demonstrates that the interaction between virus and its natural receptors can be strongly enhanced by mutation.

Activation of ERK signaling upon alternative protease nexin-1 internalization mediated by syndecan-1

Protease nexin-1 (PN-1), an inhibitor of serine proteases, contributes to tissue homeostasis and influences the behavior of some tumor cells. The internalization of PN-1 protease complexes is considered to be mediated by the low-density lipoprotein receptor related protein 1 (LRP1). In this study, both wild-type and LRP1-/- mouse embryonic fibroblasts (MEF) were shown to internalize PN-1. Receptor associated protein (RAP) interfered with PN-1 uptake only in wild-type MEF cells, indicating that another receptor mediates PN-1 uptake in the absence of LRP1. In LRP1-/- MEF cells, inhibitor sensitivity and kinetic values (t(1/2) at 45 min) of PN-1 uptake showed a similarity to syndecan-1-mediated endocytosis. In these cells, PN-1 uptake was increased by overexpression of full-length syndecan-1 and decreased by RNA interference targeting this proteoglycan. Most important, in contrast to PKA activation known to be triggered by LRP1-mediated internalization, our study shows that syndecan-1-mediated internalization of PN-1 stimulated the Ras-ERK signaling pathway.

Positive cooperativity in the cellular uptake of a boronated porphyrin

This work was undertaken to assess the kinetics of boronated porphyrin cellular uptake, which has been reported to occur by way of the low-density lipoprotein receptors. Because of current interest in the use of boronated porphyrins in boron neutron capture therapy of tumors, this pathway was investigated for the cellular uptake of a boronated porphyrin (tetrakis-carborane-carboxylate, esters of 2,4-bis (alpha,beta-dihydroxyethyl) deuteroporphyrin IX). Boron uptake occurred even without low-density lipoprotein in the culture medium. Pre-incubation of V-79 Chinese hamster cells for 24 h in medium containing delipidized fetal bovine serum markedly increased the subsequent uptake of boron when compared with cells pre-incubated with medium containing 10% fetal bovine serum. The increased uptake was characterized by greater affinity for boronated porphyrin, compared to cells pre-incubated in 10% fetal bovine serum. Twenty-four hour preincubation of cells with increasing concentrations of LDL added to delipidized medium suppressed the up-regulation of the boron level. In contrast, incubation with added acetylated LDL did not prevent the up-regulation of boron uptake. Positive cooperativity was demonstrated by Hill and Scatchard plots. It is concluded that uptake of boronated porphyrin is characterized by positive cooperativity, that its uptake is markedly enhanced when preincubated in delipidized serum, and that significant uptake occurs even in the absence of low density lipoprotein in the medium. These data suggest a novel way for enhancing uptake of boron (and perhaps other agents) into tissues using carrier porphyrins, by increasing the number and/or affinity of cellular LDL receptors.

Effects of parathyroid hormone on Wnt signaling pathway in bone

The Wnt signaling pathway has recently been demonstrated to play an important role in bone cell function. In previous studies using DNA microarray analyses, we observed a change in some of the molecular components of the canonical Wnt pathway namely, frizzled-1 (FZD-1) and axil, in response to continuous parathyroid hormone (PTH) treatment in rats. In the present study, we further explored other components of the Wnt signaling pathway in rat distal metaphyseal bone in vivo, and rat osteoblastic osteosarcoma cells (UMR 106) in culture. Several Wnt pathway components, including low-density lipoprotein-receptor-related protein 5 (LRP5), LRP6, FZD-1, Dickkopf-1 (Dkk-1), and Kremen-1 (KRM-1), were expressed in bone in vivo and in osteoblasts in vitro. Continuous exposure to PTH (1-38) both in vivo and in vitro upregulated the mRNA expression of LRP6 and FZD-1 and decreased LRP5 and Dkk-1. These effects in UMR 106 cells were associated with an increase in beta-catenin as measured by Western blots and resulted in functional activation (three to six-fold) of a downstream Wnt responsive TBE6-luciferase (TCF/LEF-binding element) reporter gene. Activation of the TBE6-luciferase reporter gene by PTH (1-38) in UMR 106 cells was inhibited by the protein kinase A (PKA) inhibitor, H89. Activation was mimicked by PTH (1-31), PTH-related protein (1-34), and forskolin, but both PTH (3-34) and (7-34) had no effect. These findings suggest that the effect of PTH on the canonical Wnt signaling pathway occurs at least in part via the cAMP-PKA pathway through the differential regulation of the receptor complex proteins (FZD-1/LRP5 or LRP6) and the antagonist (Dkk-1). Taken together, these results reveal a possible role for the Wnt signaling pathway in PTH actions in bone.

The Egg Surface LDL Receptor Repeat-Containing Proteins EGG-1 and EGG-2 Are Required for Fertilization in Caenorhabditis elegans

The molecular machinery that mediates sperm-egg interactions at fertilization is largely unknown. We identify two partially redundant egg surface LDL receptor repeat-containing proteins (EGG-1 and EGG-2) that are required for Caenorhabditis elegans fertility in hermaphrodites, but not males. Wild-type sperm cannot enter the morphologically normal oocytes produced by hermaphrodites that lack egg-1 and egg-2 function despite direct gamete contact. Furthermore, we find that levels of meiotic maturation/ovulation and sperm migratory behavior are altered in egg-1 mutants. These observations suggest an unexpected regulatory link between fertilization and other events necessary for reproductive success. egg-1 and egg-2 are the result of a gene duplication in the nematode lineage leading to C. elegans. The two closely related species C. briggsae and C. remanei encode only a single egg-1/egg-2 homolog that is required for hermaphrodite/female fertility. In addition to being the first identified egg components of the nematode fertilization machinery, the egg-1 and egg-2 gene duplication could be vital with regards to maximizing C. elegans fecundity and understanding the evolutionary differentiation of molecular function and speciation.

Abnormal development of the apical ectodermal ridge and polysyndactyly in Megf7-deficient mice

Megf7/Lrp4 is a member of the functionally diverse low-density lipoprotein receptor gene family, a class of ancient and highly conserved cell surface receptors with broad functions in cargo transport and cellular signaling. To gain insight into the as yet unknown biological role of Megf7/Lrp4, we have disrupted the gene in mice. Homozygous Megf7-deficient mice are growth-retarded, with fully penetrant polysyndactyly in their fore and hind limbs, and partially penetrant abnormalities of tooth development. The reason for this developmental abnormality is apparent as early as embryonic day 9.5 when the apical ectodermal ridge (AER), the principal site of Megf7 expression at the distal edge of the embryonic limb bud, forms abnormally in the absence of Megf7. Ectopic expression and aberrant signaling of several molecules involved in limb patterning, including Fgf8, Shh, Bmp2, Bmp4 and Wnt7a, as well as the Wnt- and Bmp-responsive transcription factors Lmx1b and Msx1, result in reduced apoptosis and symmetrical dorsal and ventral expansions of the AER. Abnormal signaling from the AER precedes ectopic chondrocyte condensation and subsequent fusion and duplication of digits in the Megf7 knockouts. Megf7 can antagonize canonical Wnt signaling in vitro. Taken together, these findings are consistent with a role of Megf7 as a modulator of cellular signaling pathways involving Wnts, Bmps, Fgfs and Shh. A similar autosomal recessive defect may also occur in man, where polysyndactyly, in combination with craniofacial abnormalities, is also part of a common genetic syndrome.

Structure and physiologic function of the low-density lipoprotein receptor

The low-density lipoprotein receptor (LDLR) is responsible for uptake of cholesterol-carrying lipoprotein particles into cells. The receptor binds lipoprotein particles at the cell surface and releases them in the low-pH environment of the endosome. The focus of the current review is on biochemical and structural studies of the LDLR and its ligands, emphasizing how structural features of the receptor dictate the binding of low-density lipoprotein (LDL) and beta-migrating forms of very low-density lipoprotein (beta-VLDL) particles, how the receptor releases bound ligands at low pH, and how the cytoplasmic tail of the LDLR interfaces with the endocytic machinery.

“Bound” to Work: The Free Hormone Hypothesis Revisited

Megalin is a member of the low-density lipoprotein receptor-related protein (LRP) family. The plasma membrane-anchored LRPs serve as receptors for a wide variety of extracellular ligands, promoting their entry into cells by endocytosis of the receptor-ligand complex. In this issue of Cell, Hammes et al. (2005) show that resistance (insensitivity) to sex steroid hormones is encountered in animals lacking megalin. These data provide important insights into an endocytic mechanism for the uptake of sex steroids by mammalian cells.

LRP1B functions as a receptor for Pseudomonas exotoxin

Pseudomonas aeruginosa is an opportunistic pathogen that produces several virulence factors, among them Pseudomonas Exotoxin A (PE). Previously, low-density lipoprotein receptor-related protein 1 (LRP 1) was shown to be the primary receptor for PE. In this report, we show that a close family member, LRP 1B, can also function as a receptor.

Variations of very low-density lipoprotein receptor subtype expression in gastrointestinal adenocarcinoma cells with various differentiations

AIM: This study is aimed at investigating the expression and possible significances of very low-density lipoprotein receptor (VLDLR) subtypes in gastroenteric adenocarcinoma tissues and cells with various differentiations.

METHODS: Thirty-one cases of gastroenteric carcinoma/adjacent normal tissues were enrolled in the study, which were diagnosed and classified by the clinicopathological diagnosis. The expression of VLDLR subtypes was detected in gastroenteric carcinoma/adjacent normal tissues and three various differentiated human gastric adenocarcinoma cell lines (MKN28, SGC7901 and MKN45) by reverse transcription polymerase chain reaction (RT-PCR) and Western blot analysis.

RESULTS: Two VLDLR subtypes, namely, type II VLDLR and type I VLDLR, were found to express changes in gastroenteric carcinoma tissues, their adjacent normal tissue, and gastric adenocarcinoma cell lines as well. Type II VLDLR is predominantly expressed in poorly- or moderately-differentiated gastroenteric carcinoma tissues and gastric adenocarcinoma cell lines, whereas type I VLDLR is mainly detected in well-differentiated intestinal carcinoma tissues and gastric adenocarcinoma cells compared with the adjacent normal tissues.

CONCLUSION: The results suggested that the variations of the VLDLR subtype expression might be correlated with the progress and differentiation of gastroenteric carcinoma.

Mutually dependent localization of megalin and Dab2 in the renal proximal tubule

Disabled-2 (Dab2) is a cytoplasmic adaptor protein that binds to the cytoplasmic tail of the multiligand endocytic receptor megalin, abundantly expressed in renal proximal tubules. Deletion of Dab2 induces a urinary increase in specific plasma proteins such as vitamin D binding protein and retinol binding protein (Morris SM, Tallquist MD, Rock CO, and Cooper JA. EMBO J 21: 1555-1564, 2002). However, the subcellular localization of Dab2 in the renal proximal tubule and its function have not been fully elucidated yet. Here, we report the characterization of Dab2 in the renal proximal tubule. Immunohistocytochemistry revealed colocalization with megalin in coated pits and vesicles but not in dense apical tubules and the brush border. Kidney-specific megalin knockout almost abolished Dab2 staining, indicating that Dab2 subcellular localization requires megalin in the proximal tubule. Reciprocally, knockout of Dab2 led to a redistribution of megalin from endosomes to microvilli. In addition, there was an overall decrease in levels of megalin protein observed by immunoblotting but no decrease in clathrin or alpha-adaptin protein levels or in megalin mRNA. In rat yolk sac epithelial BN16 cells, Dab2 was present apically and colocalized with megalin. Introduction of anti-Dab2 antibody into BN16 cells decreased the internalization of 125I-labeled receptor-associated protein, substantiating the role of Dab2 in megalin-mediated endocytosis. The present study shows that Dab2 is localized in the apical endocytic apparatus of the renal proximal tubule and that this localization requires megalin. Furthermore, the study suggests that the urinary loss of megalin ligands observed in Dab2 knockout mice is caused by suboptimal trafficking of megalin, leading to decreased megalin levels.

Potential cellular receptors involved in hepatitis C virus entry into cells

Hepatitis C virus (HCV) infects hepatocytes and leads to permanent, severe liver damage. Since the genomic sequence of HCV was determined, progress has been made towards understanding the functions of the HCV-encoded proteins and identifying the cellular receptor(s) responsible for adsorption and penetration of the virus particle into the target cells. Several cellular receptors for HCV have been proposed, all of which are associated with lipid and lipoprotein metabolism. This article reviews the cellular receptors for HCV and suggests a general model for HCV entry into cells, in which lipoproteins play a crucial role.

Neuronal migration and the role of reelin during early development of the cerebral cortex

During development, neurons migrate to the cortex radially from periventricular germinative zones as well as tangentially from ganglionic eminences. The vast majority of cortical neurons settle radially in the cortical plate. Neuronal migration requires an exquisite regulation of leading edge extension, nuclear translocation (nucleokinesis), and retraction of trailing processes. During the past few years, several genes and proteins have been identified that are implicated in neuronal migration. Many have been characterized by reference to known mechanisms of neuronal and non-neuronal cell migration in culture; however, probably the most interesting have been identified by gene inactivation or modification in mice and by positional cloning of brain malformation genes in humans and mice. Although it is impossible to provide a fully integrated view, some patterns clearly emerge and are the subject of this article. Specific emphasis is placed on three aspects: first, the role of the actin treadmill, with cyclic formation of filopodial and lamellipodial extensions, in relation to surface events that occur at the leading edge of radially migrating neurons; second, the regulation of microtubule dynamics, which seems to play a key role in nucleokinesis; and third, the mechanisms by which the extracellular protein Reelin regulates neuronal positioning at the end of migration.

Reelin receptors in developing laminated brain structures of mouse and human

Reelin is an extracellular matrix protein secreted by a variety of cell types throughout the developing brain. The target cells for reelin express the cytoplasmic adapter protein Dab1, which binds to the reelin receptors VLDLR and ApoER2. In the present work, we have studied the localization of both receptors in developing mouse and human cortex, olfactory bulb and cerebellum. In mouse, some Cajal-Retzius cells express reelin and VLDLR; in humans, all the components of the signalling pathway (Reelin, Dab1, VLDLR and ApoER2) are present in subsets of Cajal-Retzius cells. In the mouse cortical plate, VLDLR and ApoER2 are present from E15 to postnatal stages; in human cortical plate they are most prominent at approximately 20 gestational weeks. In mice, cerebellar Purkinje cells only express VLDLR whereas in humans they express both VLDLR and ApoER2. Mitral cells of the mouse olfactory bulb are ApoER2-positive and VLDLR-negative. In sum, the receptor expression patterns are similar in the human and mouse cortical plate but differ in Cajal-Retzius and Purkinje cells, which in humans express additional components of the reelin-Dab1 pathway.

The mosaic receptor sorLA/LR11 binds components of the plasminogen-activating system and platelet-derived growth factor-BB similarly to LRP1 (low-density lipoprotein receptor-related protein), but mediates slow internalization of bound ligand

The type-1 receptor sorLA/LR11, a member of the Vps10p-domain receptor family that also contains domains characterizing members of the LDL (low-density lipoprotein) receptor family, has been shown to induce increased uPAR (urokinase receptor) expression as well as enhanced migration and invasion activities in smooth muscle cells in the presence of PDGF-BB (platelet-derived growth factor-BB). Here we show that sorLA interacts with both components of the plasminogen activating system and PDGF-BB similarly to LRP1 (LDL receptor-related protein/alpha2-macroglobulin receptor), which is an important clearance receptor with established functions in controlling uPAR expression as well as PDGF-BB signalling. In contrast with LRP1, sorLA does not interact with alpha2-macroglobulin, which is a binding protein for several growth factors, including PDGF-BB. By using LRP1-deficient cells transfected with sorLA, we demonstrate that sorLA-bound ligand is internalized at a much lower rate than LRP1-bound ligand, and that sorLA is inefficient in regulating cell surface uPAR expression, which depends on rapid internalization of the ternary complex between urokinase-type plasminogen activator, its type-1 inhibitor, and uPAR. Thus, although overlapping with regard to binding profiles, sorLA is substantially less efficient as a clearance receptor than LRP1. We propose that sorLA can divert ligands away from LRP1 and thereby inhibit both their clearance and signalling events mediated by LRP1.

Kinetic analysis of plasminogen activator inhibitor type-2: urokinase complex formation and subsequent internalisation by carcinoma cell lines

The overexpression of urokinase (uPA), which plays a key role in tumour invasion and metastasis, is an established prognostic marker and potential therapeutic target. Plasminogen activator inhibitor type 2 (PAI-2), an efficient and specific inhibitor of uPA, has been shown to selectively deliver potent cytotoxins to tumour cells. However, a direct quantitative analysis of both the inhibition kinetics and subsequent fate of PAI-2 upon interaction with cell-surface uPA has not been previously undertaken. In this study, we analysed specific PAI-2 binding to receptor-bound uPA on human breast and prostate cancer cell lines to directly measure inhibition kinetics. Cell-surface uPA:PAI-2 complex formation, which is reflective of complete uPA inhibition, was found to be very efficient (inactivation constant [K(I)] = 60-80 pM, depending on cell line used) and rapid (inactivation rate constant [k(inact)] = 0.32-0.47 min(-1) at 37 degrees C, depending on cell line used). To directly quantify and visualise cellular internalisation and localisation, we developed a novel assay based on the use of PAI-2 labelled with Alexa(488) fluorochrome and a polyclonal antibody to quench Alexa(488) fluorescence. The efficient and rapid formation of uPA:PAI-2 complexes was thus shown to be associated with specific and rapid internalisation of PAI-2, which could be localised within endosomes and lysosomes. PAI-2 was subsequently degraded, presumably within lysosomes. This study is the first to provide definitive evidence for uPA/uPAR-mediated PAI-2 endocytosis.

Functional domains and temperature-sensitive mutations in SPE-9, an EGF repeat-containing protein required for fertility in Caenorhabditis elegans

The spe-9 gene is required for fertility in Caenorhabditis elegans and encodes a sperm transmembrane protein with an extracellular domain (ECD) that contains 10 epidermal growth factor (EGF) repeats. Deletion analysis reveals that the EGF repeats and the transmembrane domain are required for fertilization. In contrast, the cytoplasmic region of SPE-9 is not essential for fertilization. Individual point mutations in all 10 EGF motifs uncover a differential sensitivity of these sequences to alteration. Some EGF repeats cannot tolerate mutation leading to a complete lack of fertility. Other EGF repeats can be mutated to create animals with temperature-sensitive (ts) fertility phenotypes. All ts mutations were generated by changing either conserved cysteine or glycine residues in the EGF motifs. For two endogenous ts alleles of spe-9, loss of function at nonpermissive temperatures is not due to protein mislocalization or degradation. Additionally, the proper localization of SPE-9 in sperm is not altered in a genetically interacting fertility mutant (spe-13) or a mutant that affects sperm vesicle-plasma membrane fusion (fer-1). Like the EGF repeats in the Notch/LIN-12/GLP-1 receptors and their ligands, the EGF repeats in SPE-9 may carry out different functions. Because EGF motifs are found in many proteins in different species, similar experimental strategies could be used to generate useful temperature-sensitive mutations in other EGF motif-containing molecules.

Clearance of coagulation factor VIII in very low-density lipoprotein receptor knockout mice

Low-density lipoprotein receptor-related protein (LRP) contributes to factor VIII (FVIII) catabolism in vivo. Besides LRP, FVIII also interacts with very low-density lipoprotein receptor (VLDLR) in vitro. We investigated the physiological role of VLDLR in FVIII catabolism, using knockout mouse models for VLDLR and LRP, alone and in combination. VLDLR(-/-) mice displayed normal plasma FVIII, whereas VLDLR(-/-) LRP(-) double-knockout mice had slightly increased FVIII compared with LRP-deficient mice. Remarkably, VLDLR deficiency slightly accelerated FVIII clearance. Adenovirus-mediated overexpression of VLDLR did not lower plasma FVIII in LRP-deficient mice. We conclude that VLDLR does not act in concert with LRP in FVIII clearance in vivo.

Twelve receptor molecules attach per viral particle of human rhinovirus serotype 2 via multiple modules

The crystallographic T = 1 (pseudo T = 3) icosahedral symmetry of the human rhinovirus capsid dictates the presence of 60 identical, symmetry related surface structures that are available for antibody and receptor binding. X-ray crystallography has shown that 60 individual very-low density lipoprotein receptor (VLDLR) modules bind to HRV2. Their arrangement around the fivefold axes of the virion suggested that tandem oligomers of such modules could attach simultaneously to symmetry-related sites. By resolving virus particles carrying various numbers of artificial recombinant concatemers of VLDLR repeat 3 (V33333) by capillary electrophoresis and extrapolation of the measured mobilities to that at saturation of all binding sites, we present evidence for up to 12 molecules of the concatemer to bind one single virion.

Identification of the human rhinovirus serotype 1A binding site on the murine low-density lipoprotein receptor by using human-mouse receptor chimeras

Human rhinovirus serotype 1A (HRV1A) binds more strongly to the mouse low-density lipoprotein receptor (LDLR) than to the human homologue (M. Reithmayer, A. Reischl, L. Snyers, and D. Blaas, J. Virol. 76:6957-6965, 2002). Here, we used this fact to determine the binding site of HRV1A by replacing selected ligand binding modules of the human receptor with the corresponding ligand binding modules of the mouse receptor. The chimeric proteins were expressed in mouse fibroblasts deficient in endogenous LDLR and LDLR-related protein, both used by minor group HRVs for cell entry. Binding was assessed by virus overlay blots, by immunofluorescence microscopy, and by measuring cell attachment of radiolabeled virus. Replacement of ligand binding repeat 5 of the human LDLR with the corresponding mouse sequence resulted in a substantial increase in HRV1A binding, whereas substitution of repeats 3 and 4 was without effect. Replacement of human receptor repeats 1 and 2 with the murine homologues also increased virus binding. Finally, murine receptor modules 1, 2, and 5 simultaneously introduced into the human receptor resulted in HRV1A binding indistinguishable from mouse wild-type receptor. Thus, repeats 1 and/or 2 and repeat 5 are involved in HRV1A attachment. Changing CDGGPD in the acidic cluster of module 5 in the human receptor to CDGEAD present in the mouse receptor led to substantially increased binding of HRV1A, indicating an important role of the glutamate residue in HRV1A recognition.

Normal development and fertility of knockout mice lacking the tumor suppressor gene LRP1b suggest functional compensation by LRP1

LRP1b and the closely related LRP1 are large members of the low-density lipoprotein receptor family. At the protein level LRP1b is 55% identical to LRP1, a multifunctional and developmentally essential receptor with roles in cargo transport and cellular signaling. Somatic LRP1b mutations frequently occur in non-small cell lung cancer and urothelial cancers, suggesting a role in the modulation of cellular growth. In contrast to LRP1, LRP1b-deficient mice develop normally, most likely due to its restricted expression pattern and functional compensation by LRP1 or other receptors. LRP1b is expressed predominantly in the brain, and a differentially spliced form is present in the adrenal gland and in the testis. Despite the presence of a potential furin cleavage site and in contrast to LRP1, immunoblotting for LRP1b reveals the presence of a single 600-kDa polypeptide species. Using a yeast two-hybrid approach, we have identified two intracellular proteins, the postsynaptic density protein 95 and the aryl hydrocarbon receptor-interacting protein, that bind to the intracellular domain of LRP1b. In addition, we have found several potential ligands that bind to the extracellular domain. Analysis of LRP1b knockout mice may provide further insights into the role of LRP1b as a tumor suppressor and into the mechanisms of cancer development.

Potential role of the low-density lipoprotein receptor family as mediators of cellular drug uptake

We highlight the importance of the low-density lipoprotein (LDL) receptor family and its pharmaceutical implications in the field of drug delivery. The members of the LDL receptor family are a group of cell surface receptors that transport a number of macromolecules into cells through a process called receptor-mediated endocytosis. This process involves the receptor recognizing a ligand from the extracellular membrane (ECM), internalizing it through clathrin-coated pits and degrading it upon fusion with lysosomes. There are nine members of the receptor family, which include the LDL receptor, low-density lipoprotein-related protein (LRP), megalin, very low-density lipoprotein (VLDL) receptor, apoER2 and sorLA/LRP11, LRP1b, MEGF7, LRP5/6; the former six having been identified in humans. Each member is expressed in a number of different tissues and has a wide range of different ligands, not specific to the recognition of the LDL particle. Thus, rather than the original hypothesis that the receptor is only a mediator of cholesterol uptake, it may also be involved in a number of other physiological functions, including the progression of certain disease states and, potentially, cellular drug uptake. A number of studies have suggested that the LDL receptors are involved in endocytosis of drugs and drug formulations including aminoglycosides, anionic liposomes and cyclosporine A (CsA). This article reviews the importance of lipoproteins as a drug delivery system and how LDL receptors are relevant to the design and targeting of specific drugs.

Human rhinovirus type 2-antibody complexes enter and infect cells via Fc-gamma receptor IIB1

HeLa cells were stably transfected with a cDNA clone encoding the B1 isoform of the mouse FcgammaRII receptor (hereafter referred to as HeLa-FcRII cells). The receptor was expressed at high level at the plasma membrane in about 90% of the cells. These cells bound and internalized mouse monoclonal virus-neutralizing antibodies 8F5 and 3B10 of the subtype immunoglobulin G2a (IgG2a) and IgG1, respectively. Binding of the minor-group human rhinovirus type 2 (HRV2) to its natural receptors, members of the low-density lipoprotein receptor family, is dependent on the presence of Ca(2+) ions. Thus, chelating Ca(2+) ions with EDTA prevented HRV2 binding, entry, and infection. However, upon complex formation of (35)S-labeled HRV2 with 8F5 or 3B10, virus was bound, internalized, and degraded in HeLa-FcRII cells. Furthermore, challenge of these cells with HRV2-8F5 or HRV2-3B10 complexes resulted in de novo synthesis of viral proteins, as shown by indirect immunofluorescence microscopy. These data demonstrate that minor-group receptors can be replaced by surrogate receptors to mediate HRV2 cell entry, delivery into endosomal compartments, and productive uncoating. Consequently, the conformational change and uncoating of HRV2 appears to be solely triggered by the low-pH (pH </= 5.6) environment in these compartments.

X-ray structure of a minor group human rhinovirus bound to a fragment of its cellular receptor protein

Although many viral receptors have been identified, the ways in which they interact with their cognate viruses are not understood at the molecular level. We have determined the X-ray structure of a complex between calcium-containing modules of the very low-density lipoprotein receptor and the minor group human rhinovirus HRV2. The receptor binds close to the icosahedral five-fold vertex, with only one module per virus protomer. The binding face of this module is defined by acidic calcium-chelating residues and, in particular, by an exposed tryptophan that is highly conserved. The attachment site on the virus involves only residues from VP1, particularly a lysine strictly conserved in all minor group HRVs. The disposition of the attached ligand-binding repeats around the five-fold axis, together with the proximity of the N- and C-terminal ends of adjacent modules, suggests that more than one repeat in a single receptor molecule might attach simultaneously.

cDNA cloning and transcriptional regulation of the vitellogenin receptor from the imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae)

We describe the cloning of the first hymenopteran vitellogenin receptor (VgR) cDNA from the imported fire ant, Solenopsis invicta, an invasive pest. Using reverse transcription polymerase chain reaction and rapid amplification of cDNA ends, fragments encompassing the entire coding region of a putative VgR were cloned and sequenced. The complete 5764 bp cDNA encodes a 1782 residue protein with a predicted molecular mass of 201.3 kDa (=SiVgR). Northern blot analysis demonstrated that the 7.4 kb SiVgR transcript was present only in ovaries of reproductive females (virgin alates and queens). The temporal profile of transcriptional expression showed that SiVgR mRNA increased with age in virgin alate females and that this was up-regulated by methoprene, a juvenile hormone (JH) analogue. This suggests that the SiVgR gene is JH regulated.

Construction of a plasminogen activator inhibitor-1 variant without measurable affinity to vitronectin but otherwise normal

Vitronectin (VN) and plasminogen activator inhibitor-1 (PAI-1) have important functional interactions: VN stabilises the protease inhibitory activity of PAI-1 and PAI-1 inhibits binding of adhesion receptors to VN. Having previously mapped the PAI-1 binding area for VN, we have now constructed a PAI-1 variant, R103A-M112A-Q125A, without measurable affinity to VN, but with full protease inhibitory activity and endocytosis receptor binding. As a tool for evaluating the physiological and pathophysiological functions of the PAI-1-VN interaction, our new variant is far superior to the previously widely used PAI-1 variant Q125K, which we have found possesses an only about 10-fold reduced affinity to VN.

Squamous cell carcinoma antigen 1-mediated binding of hepatitis B virus to hepatocytes does not involve the hepatic serpin clearance system

The cellular receptor for hepatitis B virus (HBV) has not yet been identified. A recent candidate is a homologue of squamous cell carcinoma antigen 1 (SCCA1), a serpin. This study confirms that transfection of SCCA1 into mammalian cells (both hepatocyte-derived and of non-hepatocyte origin) results in increased HBV binding. Furthermore, virus bound to transfected cells is protected significantly from degradation by trypsin (75% compared with 30% in untransfected cells). The possibility that HBV enters cells via the hepatic clearance system for serpin-enzyme complexes was investigated by analysis of the reactive site loop of SCCA1. Functional and deletion mutants of SCCA1 were constructed by site-directed mutagenesis and compared with the wild type construct. In no case was virus binding reduced by functional alterations or deletions within the reactive site loop. A possible role for the low density lipoprotein receptor-related protein (LRP) in binding virus was investigated. SCCA1 transfection of Huh7 cells was shown to result in up-regulation of LRP expression, reaching levels observed in total liver. However, the use of receptor-associated protein (RAP), a competitive ligand for LRP, suggests than LRP up-regulation is not responsible for enhanced virus binding to SCCA1-transfected cells.

Lipoprotein receptors and a disabled family cytoplasmic adaptor protein regulate EGL-17/FGF export in C. elegans

Growth factors and morphogens need to be secreted to act on distant cells during development and in response to injury. Here, we report evidence that efficient export of a fibroblast growth factor (FGF), EGL-17, from the Caenorhabditis elegans developing vulva requires the lipoprotein receptor-related proteins Ce-LRP-1 and Ce-LRP-2 and a cytoplasmic adaptor protein, Ce-DAB-1 (Disabled). Lipoprotein receptors are transmembrane proteins best known for their roles in endocytosis. Ce-LRP-1 and Ce-LRP-2 possess a conserved intraluminal domain that can bind to EGL-17, as well as a cytosolic FXNPXY motif that can bind to Ce-DAB-1. Ce-DAB-1 contains signals that confer subcellular localization to Golgi-proximal vesicles. These results suggest a model in which Ce-DAB-1 coordinates selection of receptors and cargo, including EGL-17, for transport through the secretory pathway.

LDL receptor-related proteins in neurodevelopment

Low-density lipoprotein receptor-related proteins (LRPs) are evolutionarily ancient cell-surface receptors with diverse biological functions. All are expressed in the central nervous system and, for most receptors, animal models have shown that they are indispensable for successful neurodevelopment. The mechanisms by which they regulate the formation of the nervous system are varied and include the transduction of extracellular signals and the modulation of intracellular signal propagation, as well as cargo transport, the function most commonly attributed to this gene family. Here, we will summarize recent advances in our understanding of the molecular basis on which these receptors function during development.

Coaxing the LDL receptor family into the fold

Low-density-lipoprotein (LDL) receptor family members control diverse developmental and physiological pathways. In this issue of Cell, both Culi and Mann and Hsieh et al. report on Boca/MESD, a highly conserved chaperone required for transport of LDLR family proteins to the cell surface. Together with recent insights into the atomic structure of the LDL receptor, they shed new light on the synthesis and trafficking of this important class of multifunctional receptors.

Functional interaction of megalin with the megalinbinding protein (MegBP), a novel tetratrico peptide repeat-containing adaptor molecule

Megalin is a member of the LDL receptor gene family that plays an important role in forebrain development and in cellular vitamin D metabolism through endocytic uptake of vitamin D metabolites. Similar to other receptors in this gene family, megalin is believed to functionally interact with intracellular proteins through adaptors that bind to the receptor tail and regulate its endocytic and signal transducing activities. Using yeast two-hybrid screens, we identified a novel scaffold protein with tetratrico peptide repeats, the megalin-binding protein (MegBP) that associates with the receptor. The binding site of MegBP was mapped to an N-terminal region on the receptor tail harboring a proline-rich peptide element. MegBP binding did not block the endocytic activity of the receptor; however, overexpression resulted in cellular lethality. In further screens, we identified proteins that bound to MegBP and thus might be recruited to the megalin tail. MegBP-interacting partners included several transcriptional regulators such as the SKI-interacting protein (SKIP), a co-activator of the vitamin D receptor. These finding suggest a model whereby megalin directly participates in transcriptional regulation through controlled sequestration or release of transcription factors via MegBP.