Analysis of ligand binding to the alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein. Evidence that lipoprotein lipase and the carboxyl-terminal domain of the receptor-associated protein bind to the same site

The endo-lysosomal system of bEnd.3 and hCMEC/D3 brain endothelial cells

Background

Brain endothelial cell-based in vitro models are among the most versatile tools in blood–brain barrier research for testing drug penetration to the central nervous system. Transcytosis of large pharmaceuticals across the brain capillary endothelium involves the complex endo-lysosomal system. This system consists of several types of vesicle, such as early, late and recycling endosomes, retromer-positive structures, and lysosomes. Since the endo-lysosomal system in endothelial cell lines of in vitro blood–brain barrier models has not been investigated in detail, our aim was to characterize this system in different models.

Methods

For the investigation, we have chosen two widely-used models for in vitro drug transport studies: the bEnd.3 mouse and the hCMEC/D3 human brain endothelial cell line. We compared the structures and attributes of their endo-lysosomal system to that of primary porcine brain endothelial cells.

Results

We detected significant differences in the vesicular network regarding number, morphology, subcellular distribution and lysosomal activity. The retromer-positive vesicles of the primary cells were distinct in many ways from those of the cell lines. However, the cell lines showed higher lysosomal degradation activity than the primary cells. Additionally, the hCMEC/D3 possessed a strikingly unique ratio of recycling endosomes to late endosomes.

Conclusions

Taken together our data identify differences in the trafficking network of brain endothelial cells, essentially mapping the endo-lysosomal system of in vitro blood–brain barrier models. This knowledge is valuable for planning the optimal route across the blood–brain barrier and advancing drug delivery to the brain.

Electronic supplementary material

The online version of this article (10.1186/s12987-019-0134-9) contains supplementary material, which is available to authorized users.

Cluster III of low-density lipoprotein receptor-related protein 1 binds activated blood coagulation factor VIII

Low-density lipoprotein receptor-related protein 1 (LRP) mediates clearance of blood coagulation factor VIII (FVIII). In LRP, FVIII binds the complement-type repeats (CRs) of clusters II and IV, which also bind a majority of other LRP ligands. No ligand is known for LRP cluster I, and only three ligands, including the LRP chaperone alpha-2 macroglobulin receptor-associated protein (RAP), bind cluster III. Using surface plasmon resonance, we found that in addition to clusters II and IV, activated FVIII (FVIIIa) binds cluster III. The specificity of this interaction was confirmed using an anti-FVIII antibody fragment, which inhibited the binding. Recombinant fragments of cluster III and its site-directed mutagenesis were used to localize the cluster's site for binding FVIIIa to CR.14-19. The interactive site of FVIIIa was localized within its A1/A3'-C1-C2 heterodimer (HDa), which is a major physiological remnant of FVIIIa. In mice, the clearance of HDa was faster than that of FVIII and prolonged in the presence of RAP, which is known to inhibit interactions of LRP with its ligands. In accordance with this, the cluster III site for RAP (CR.15-19) was found to overlap that for FVIIIa. Altogether, our findings support the involvement of LRP in FVIIIa catabolism and suggest a greater significance of the biological role of cluster III compared to that previously known.

Differential regulation of extracellular tissue inhibitor of metalloproteinases-3 levels by cell membrane-bound and shed low density lipoprotein receptor-related protein 1

Tissue inhibitor of metalloproteinases-3 (TIMP-3) plays a key role in regulating extracellular matrix turnover by inhibiting matrix metalloproteinases (MMPs), adamalysins (ADAMs), and adamalysins with thrombospondin motifs (ADAMTSs). We demonstrate that levels of this physiologically important inhibitor can be regulated post-translationally by endocytosis. TIMP-3 was endocytosed and degraded by a number of cell types including chondrocytes, fibroblasts, and monocytes, and we found that the endocytic receptor low density lipoprotein receptor-related protein-1 (LRP-1) plays a major role in TIMP-3 internalization. However, the cellular uptake of TIMP-3 significantly slowed down after 10 h due to shedding of LRP-1 from the cell surface and formation of soluble LRP-1 (sLRP-1)-TIMP-3 complexes. Addition of TIMP-3 to HTB94 human chondrosarcoma cells increased the release of sLRP-1 fragments of 500, 215, 160, and 110 kDa into the medium in a concentration-dependent manner, and all of these fragments were able to bind to TIMP-3. TIMP-3 bound to sLRP-1, which was resistant to endocytosis, retained its inhibitory activity against metalloproteinases. Extracellular levels of sLRP-1 can thus increase the half-life of TIMP-3 in the extracellular space, controlling the bioavailability of TIMP-3 to inhibit metalloproteinases.

Inhibition of PDGF-BB by Factor VII-activating protease (FSAP) is neutralized by protease nexin-1, and the FSAP-inhibitor complexes are internalized via LRP

FSAP (Factor VII-activating protease) can inhibit neointima formation and VSMC (vascular smooth-muscle cell) proliferation by cleavage of PDGF-BB (platelet-derived growth factor-BB). Negatively charged polyanions lead to autoactivation of the FSAP, but no information is available concerning the potential regulation of FSAP activity and its metabolism in the vessel wall. In the present study, we demonstrate that the enzymatic activity of FSAP can be inhibited by the serine protease inhibitor, PN-1 (protease nexin-1), that is found in the vasculature. This leads to the loss of the inhibitory effect of FSAP on PDGF-BB-mediated DNA synthesis and mitogen-activated protein kinase phosphorylation in VSMCs. The FSAP-PN-1 complexes bind to the LRP (low-density lipoprotein receptor-related protein) and are subsequently internalized. This binding is inhibited by receptor-associated protein, an antagonist of LRP, as well as heparin. While PDGFbetaR (PDGFbeta receptor) is internalized by an LRP-dependent mechanism after stimulation of cells by PDGF-BB, the FSAP-PN-1 complex neither influenced PDGF-BB-mediated phosphorylation of PDGFbetaR nor its internalization via LRP. Hence, PN-1 inhibits the enzymatic activity of FSAP and neutralizes its effect on PDGF-BB-mediated VSMC proliferation. The FSAP-inhibitor complexes are internalized via LRP without influencing the PDGF-BB signal transduction pathway.

Low density lipoprotein receptor-related protein mediates endocytic clearance of pro-MMP-2.TIMP-2 complex through a thrombospondin-independent mechanism

The low density lipoprotein receptor-related protein (LRP) mediates the endocytic clearance of various proteinases and proteinase.inhibitor complexes, including thrombospondin (TSP)-dependent endocytosis of matrix metalloproteinase (MMP)-2 (or gelatinase A), a key effector of extracellular matrix remodeling and cancer progression. However, the zymogen of MMP-2 (pro-MMP-2) mostly occurs in tissues as a complex with the tissue inhibitor of MMPs (TIMP-2). Here we show that clearance of the pro-MMP-2.TIMP-2 complex is also mediated by LRP, because addition of receptor-associated protein (RAP), a natural LRP ligand antagonist, inhibited endocytosis and lysosomal degradation of (125)I-pro-MMP-2.TIMP-2. Both TIMP-2 and the pro-MMP-2 collagen-binding domain independently competed for endocytosis of (125)I-pro-MMP-2.TIMP-2 complex. Surface plasmon resonance studies indicated that pro-MMP-2, TIMP-2, and pro-MMP-2.TIMP-2 directly interact with LRP in the absence of TSP. LRP-mediated endocytic clearance of (125)I-pro-MMP-2 was inhibited by anti-TSP antibodies and accelerated upon complexing with TSP-1, but these treatments had no effect on (125)I-pro-MMP-2.TIMP-2 uptake. This implies that mechanisms of clearance by LRP of pro-MMP-2 and pro-MMP-2.TIMP-2 complex are different. Interestingly, RAP did not inhibit binding of (125)I-pro-MMP-2.TIMP-2 to the cell surface. We conclude that clearance of pro-MMP-2.TIMP-2 complex is a TSP-independent two-step process, involving (i) initial binding to the cell membrane in a RAP-insensitive manner and (ii) subsequent LRP-dependent (RAP-sensitive) internalization and degradation.

All three LDL receptor homology regions of the LDL receptor-related protein bind multiple ligands

The three complete human LDL receptor homology regions of the LDL receptor-related protein (sLRP2, sLRP3, and sLRP4) have been expressed in Pichia pastoris SMD1168 with constitutive coexpression of the receptor-associated protein (RAP). Each sLRP was purified to homogeneity after deglycosylation using a combination of anion-exchange and size exclusion chromatography. Mass spectrometry and N-terminal sequencing confirmed the identity of each fragment at purified yields of several milligrams per liter. Despite the large number of disulfide linkages and glycosylation sites in each LDL receptor homology region (sLRP), all were shown to be competent for binding to several LRP1 ligands. Each sLRP also bound human RAP, which is thought to be a generalized receptor antagonist, in solution-binding experiments. As expected, sLRP2 bound the receptor-binding domain of alpha(2)-macroglobulin (residues 1304-1451). All three sLRPs bound human apolipoprotein-enriched beta very low density lipoprotein, the canonical ligand for this receptor. All three sLRPs also bound lactoferrin and thrombin-protease nexin 1 complexes. Only sLRP4 bound thrombin-antithrombin III complexes. The results show that binding-competent LDL receptor homology regions (sLRPs) can be produced in high yield in P. pastoris and readily purified. Each sLRP has binding sites for multiple ligands, but not all ligand binding could be competed by RAP.

Low density lipoprotein receptor-related protein (LRP) is required for lactoferrin-enhanced collagen gel contractile activity of human fibroblasts

Fibroblasts plated on a type I collagen gel can reduce the size of the gel in a way that mimics the reorganization of the collagen matrix that accompanies the wound healing process. We demonstrated previously that lactoferrin (Lf) specifically binds to WI-38 human fibroblasts and enhances their collagen gel contractile activity. The effect of Lf correlated with the phosphorylation of myosin light chain (MLC), suggesting that Lf promotes fibroblast contractile activity by regulating MLC phosphorylation. We found here that the binding of Lf to WI-38 cells was inhibited by recombinant receptor-associated protein (RAP), a universal competitor for ligand binding to LRP (LDL receptor-related protein), and RAP can also promote the collagen gel contractile activity. These observations suggest that LRP is a receptor that mediates the Lf-induced enhancement of collagen gel contractile activity in WI-38 fibroblasts. To confirm the hypothesis, we utilized LRP antisense oligonucleotide, which was modified by morpholino linkage. Suppression of LRP expression abrogated the Lf-induced enhancement the contractile activity in fibroblasts. Treatment of fibroblasts with Lf enhanced the phosphorylation of ERK1/2 and the activation of MLC kinase (MLCK). These effects were attenuated by suppression of LRP expression. These findings suggest that LRP is involved in the Lf-enhanced collagen gel contractile activity of WI-38 fibroblasts by converting the Lf binding signal into the activation of ERK1/2 and MLCK.

The second and fourth cluster of class A cysteine-rich repeats of the low density lipoprotein receptor-related protein share ligand-binding properties

The low density lipoprotein receptor-related protein (LRP) is a multifunctional endocytic cell-surface receptor that binds and internalizes a diverse array of ligands. The receptor contains four putative ligand-binding domains, generally referred to as clusters I, II, III, and IV. In this study, soluble recombinant receptor fragments, representing each of the four individual clusters, were used to map the binding sites of a set of structurally and functionally distinct ligands. Using surface plasmon resonance, we studied the binding of these fragments to methylamine-activated alpha(2)-macroglobulin, pro-urokinase-type plasminogen activator, tissue-type plasminogen activator (t-PA), plasminogen activator inhibitor-1, t-PA.plasminogen activator inhibitor-1 complexes, lipoprotein lipase, apolipoprotein E, tissue factor pathway inhibitor, lactoferrin, the light chain of blood coagulation factor VIII, and the intracellular chaperone receptor-associated protein (RAP). No binding of the cluster I fragment to any of the tested ligands was observed. The cluster III fragment only bound to the anti-LRP monoclonal antibody alpha(2)MRalpha3 and weakly to RAP. Except for t-PA, we found that each of the ligands tested binds both to cluster II and to cluster IV. The affinity rate constants of ligand binding to clusters II and IV and to LRP were measured, showing that clusters II and IV display only minor differences in ligand-binding kinetics. Furthermore, we demonstrate that the subdomains C3-C7 of cluster II are essential for binding of ligands and that this segment partially overlaps with a RAP-binding site on cluster II. Finally, we show that one RAP molecule can bind to different clusters simultaneously, supporting a model in which RAP binding to LRP induces a conformational change in the receptor that is incompatible with ligand binding.

Ligand binding properties of the very low density lipoprotein receptor. Absence of the third complement-type repeat encoded by exon 4 is associated with reduced binding of Mr 40,000 receptor-associated protein

The very low density lipoprotein receptor (VLDLR) binds, among other ligands, the Mr 40,000 receptor-associated protein (RAP) and a variety of serine proteinase-serpin complexes, including complexes of the proteinase urokinase-type plasminogen activator (uPA) with the serpins plasminogen activator inhibitor-1 (PAI-1) and protease nexin-1 (PN-1). We have analyzed the binding of RAP, uPA.PAI-1, and uPA.PN-1 to two naturally occurring VLDLR variants, VLDLR-I, containing all eight complement-type repeats, and VLDLR-III, lacking the third complement-type repeat, encoded by exon 4. VLDLR-III displayed approximately 4-fold lower binding of RAP than VLDLR-I and approximately 10-fold lower binding of the most C-terminal one of the three domains of RAP. In contrast, the binding of uPA.PAI-1 and uPA.PN-1 to the two VLDLR variants was indistinguishable. Surprisingly, uPA.PN-1, but not uPA.PAI-1, competed RAP binding to both VLDLR variants. These observations show that the third complement-type repeat plays a crucial role in maintaining the contact sites needed for optimal recognition of RAP, but does not affect the proteinase-serpin complex contact sites, and that two ligands can show full cross-competition without sharing the same contacts with the receptor. These results elucidate the mechanisms of molecular recognition of ligands by receptors of the low density lipoprotein receptor family.

Sortilin/neurotensin receptor-3 binds and mediates degradation of lipoprotein lipase

Lipoprotein lipase and the receptor-associated protein (RAP) bind to overlapping sites on the low density lipoprotein receptor-related protein/alpha2-macroglobulin receptor (LRP). We have investigated if lipoprotein lipase interacts with the RAP binding but structurally distinct receptor sortilin/neurotensin receptor-3. We show, by chemical cross-linking and surface plasmon resonance analysis, that soluble sortilin binds lipoprotein lipase with an affinity similar to that of LRP. The binding was inhibited by heparin and RAP and by the newly discovered sortilin ligand neurotensin. In 35S-labeled 3T3-L1 adipocytes treated with the cross-linker dithiobis(succinimidyl propionate), lipoprotein lipase-containing complexes were isolated by anti-sortilin antibodies. To elucidate function in cells, sortilin-negative Chinese hamster ovary cells were transfected with full-length sortilin and shown to express about 8% of the receptors on the cell surface. These cells degraded 125I-labeled lipoprotein lipase much faster than the wild-type cells. The degradation was inhibited by unlabeled lipoprotein lipase, indicating a saturable pathway, and by RAP and heparin. Moreover, inhibition by the weak base chloroquine suggested that degradation occurs in an acidic vesicle compartment. The results demonstrate that sortilin is a multifunctional receptor that binds lipoprotein lipase and, when expressed on the cell surface, mediates its endocytosis and degradation.

Characterization of the calcium site in two complement-like domains from the low-density lipoprotein receptor-related protein (LRP) and comparison with a repeat from the low-density lipoprotein receptor

Calcium is required for the binding and endocytosis of protein ligands by the low-density lipoprotein receptor-related protein (LRP) and other members of the low-density lipoprotein (LDL) receptor family. Calcium binding sites are thought to be present in the complement-like repeats that occur in clusters in all members of this receptor family. We have expressed two such complement-like repeats, CR3 and CR8, from an alpha2-macroglobulin-proteinase ligand binding region of LRP, as well as repeat 1 from the LDL receptor and examined the metal binding properties and resulting structural changes of these three repeats using changes in tryptophan and terbium fluorescence and perturbation of [1H-15N]-HSQC NMR spectra of the 15N-labeled domains from LRP. We found that all three domains contain a tight calcium binding site at physiological pH and that calcium binding results in a major structural rigidification. Changes in tryptophan fluorescence and tryptophan-sensitized terbium fluorescence indicate that the calcium binding sites are located in homologous regions in all of the repeats. Differences in the details of the perturbations, as well as in the pH dependence of calcium binding, show, however, that each metal site is distinct.

Localization of basic residues required for receptor binding to the single alpha-helix of the receptor binding domain of human alpha2-macroglobulin

To better understand the structural basis for the binding of proteinase-transformed human alpha2-macroglobulin (alpha2M) to its receptor, we have used three-dimensional multinuclear NMR spectroscopy to determine the secondary structure of the receptor binding domain (RBD) of human alpha2M. Assignment of the backbone NMR resonances of RBD was made using 13C/15-N and 15N-enriched RBD expressed in Escherichia coli. The secondary structure of RBD was determined using 1H and 13C chemical shift indices and inter- and intrachain nuclear Overhauser enhancements. The secondary structure consists of eight strands in beta-conformation and one alpha-helix, which together comprise 44% of the protein. The beta-strands form three regions of antiparallel beta-sheet. The two lysines previously identified as being critical for receptor binding are located in (Lys1374), and immediately adjacent to (Lys1370) the alpha-helix, which also contains an (Arg1378). Secondary structure predictions of other alpha-macroglobulins show the conservation of this alpha-helix and suggest an important role for this helix and for basic residues within it for receptor binding.

Three Complement-Type Repeats of the Low-Density Lipoprotein Receptor-Related Protein Define a Common Binding Site for RAP, PAI-1, and Lactoferrin

The low-density lipoprotein receptor-related protein (LRP) is a 600-kD scavenger receptor that binds a number of protein ligands with high affinity. Although some ligands do not compete with each other, binding of all is uniformly blocked by the 39-kD receptor-associated protein (RAP). RAP is normally found in the endoplasmic reticulum and seems to function as a chaperone for LRP. To identify the binding sites for RAP, lactoferrin, and plasminogen activator inhibitor-1 (PAI-1), a bacterial expression system has been developed to produce soluble LRP fragments spanning residues 783-1399. These residues overlap most of the CNBr fragment containing the second cluster of complement-type repeats (C). Solid phase binding assays show that 125I-RAP binds to fragments containing three successive complement-type repeats: C5-C7. PAI-1 and lactoferrin bind to the same fragments. A fragment containing C5-C7 also blocks uptake and degradation of 125I-RAP by fibroblasts in a concentration-dependent manner. Binding competition experiments show that RAP, PAI-1, and lactoferrin each inhibit the binding of the others, suggesting that at this site in LRP, RAP acts as a competitive, rather than an allosteric, inhibitor of PAI-1 and lactoferrin binding.

© 1998 by The American Society of Hematology.

Three Complement-Type Repeats of the Low-Density Lipoprotein Receptor-Related Protein Define a Common Binding Site for RAP, PAI-1, and Lactoferrin

The low-density lipoprotein receptor-related protein (LRP) is a 600-kD scavenger receptor that binds a number of protein ligands with high affinity. Although some ligands do not compete with each other, binding of all is uniformly blocked by the 39-kD receptor-associated protein (RAP). RAP is normally found in the endoplasmic reticulum and seems to function as a chaperone for LRP. To identify the binding sites for RAP, lactoferrin, and plasminogen activator inhibitor-1 (PAI-1), a bacterial expression system has been developed to produce soluble LRP fragments spanning residues 783-1399. These residues overlap most of the CNBr fragment containing the second cluster of complement-type repeats (C). Solid phase binding assays show that 125I-RAP binds to fragments containing three successive complement-type repeats: C5-C7. PAI-1 and lactoferrin bind to the same fragments. A fragment containing C5-C7 also blocks uptake and degradation of 125I-RAP by fibroblasts in a concentration-dependent manner. Binding competition experiments show that RAP, PAI-1, and lactoferrin each inhibit the binding of the others, suggesting that at this site in LRP, RAP acts as a competitive, rather than an allosteric, inhibitor of PAI-1 and lactoferrin binding.

© 1998 by The American Society of Hematology.

Molecular mechanisms that underlie structural and functional changes at the postsynaptic membrane during synaptic plasticity

The synaptic plasticity that is addressed in this review follows neurodegeneration in the brain and thus has both structural as well as functional components. The model of neurodegeneration that has been selected is the kainic acid lesioned hippocampus. Degeneration of the CA3 pyramidal cells results in a loss of the Schaffer collateral afferents innervating the CA1 pyramidal cells. This is followed by a period of structural plasticity where new synapses are formed. These are associated with changes in the numbers and shapes of spines as well as changes in the morphometry of the dendrites. It is suggested that this synaptogenesis is responsible for an increase in the ratio of NMDA to AMPA receptors mediating excitatory synaptic transmission at these synapses. Changes in the temporal and spatial properties of these synapses resulted in an altered balance between LTP and LTD. These properties together with a reduction in the inhibitory drive increased the excitability of the surviving CA1 pyramidal cells which in turn triggered epileptiform bursting activity. In this review we discuss the insights that may be gained from studies of the underlying molecular machinery. Developments in one of the collections of the cogs in this machinery has been summarized through recent studies characterizing the roles of neural recognition molecules in synaptic plasticity in the adult nervous systems of vertebrates and invertebrates. Such investigations of neural cell adhesion molecules, cadherins and amyloid precursor protein have shown the involvement of these molecules on the morphogenetic level of synaptic changes, on the one hand, and signal transduction effects, on the other. Further complex cogs are found in the forms of the low-density lipoprotein receptor (LDL-R) family of genes and their ligands play pivotal roles in the brain development and in regulating the growth and remodelling of neurones. Evidence is discussed for their role in the maintenance of cognitive function as well as Alzheimer's. The molecular mechanisms responsible for the clustering and maintenance of transmitter receptors at postsynaptic sites are the final cogs in the machinery that we have reviewed. Postsynaptic densities (PSD) from excitatory synapses have yielded many cytoskeletal proteins including actin, spectrin, tubulin, microtubule-associated proteins and calcium/calmodulin-dependent protein kinase II. Isolated PSDs have also been shown to be enriched in AMPA, kainate and NMDA receptors. However, recently, a new family of proteins, the MAGUKs (for membrane-associated guanylate kinase) has emerged. The role of these proteins in clustering different NMDA receptor subunits is discussed. The MAGUK proteins are also thought to play a role in synaptic plasticity mediated by nitric oxide (NO). Both NMDA and non-NMDA receptors are highly clustered at excitatory postsynaptic sites in cortical and hippocampal neurones but have revealed differences in their choice of molecular components. Both GABAA and glycine (Gly) receptors mediate synaptic inhibition in the brain and spinal cord. Whilst little is known about how GABAA receptors are localized in the postsynaptic membrane, considerable progress has been made towards the elucidation of the molecular mechanisms underlying the formation of Gly receptors. It has been shown that the peripheral membrane protein gephyrin plays a pivotal role in the formation of Gly receptor clusters most likely by anchoring the receptor to the subsynaptic cytoskeleton. Evidence for the distribution as well as function of gephyrin and Gly receptors is discussed. Postsynaptic membrane specializations are complex molecular machinery subserving a multitude of functions in the proper communication between neurones. Despite the fact that only a few key players have been identified it will be a fascinating to watch the story as to how they contribute to structural and functional plasticity unfold.

The carboxy-terminal domain of the receptor-associated protein binds to the Vps10p domain of sortilin

Binding of the receptor-associated protein (RAP) to the newly identified putative sorting receptor, sortilin, was analyzed by surface plasmon resonance analysis of recombinant RAP and sortilin domains and compared with binding to megalin and low density lipoprotein receptor-related protein (LRP). The data show that the RAP-binding site in sortilin is localized in the cysteine-rich lumenal part homologous to yeast vacuolar protein-sorting 10 protein (Vps10p), and the sortilin-binding site in RAP is localized in the carboxy-terminal domain III of the three homologous domains in RAP. Whereas sortilin bound only RAP domain III, megalin and LRP bound all RAP domains with the functional affinity order: domain III >domain I > domain II.

Crystal structure of the receptor-binding domain of alpha 2-macroglobulin

BACKGROUND

The large plasma proteinase inhibitors of the alpha 2-macroglobulin superfamily inhibit proteinases by capturing them within a central cavity of the inhibitor molecule. After reaction with the proteinase, the alpha-macroglobulin-proteinase complex binds to the alpha-macroglobulin receptor, present in the liver and other tissues, and becomes endocytosed and rapidly removed from the circulation. The complex binds to the receptor via recognition sites located on a separate domain of approximately 138 residues positioned at the C terminus of the alpha-macroglobulin subunit.

RESULTS

The crystal structure of the receptor-binding domain of bovine alpha 2-macroglobulin (bRBD) has been determined at a resolution of 1.9 A. The domain primarily comprises a nine-strand beta structure with a jelly-roll topology, but also contains two small alpha helices.

CONCLUSIONS

The surface patch responsible for receptor recognition is thought to involve residues located on one of the two alpha helices of the bRBD as well as residues in two of the beta strands. Located on this alpha helix are two lysine residues that are important for receptor binding. The structure of bRBD is very similar to the approximately 100-residue C-terminal domain of factor XIII, a transglutaminase from the blood coagulation system.

Low density lipoprotein receptor-related protein (LRP) expression varies among Hep G2 cell lines

The multiligand receptor, low density lipoprotein receptor-related protein (LRP), is implicated in processes such as atherosclerosis and fibrinolysis through its mediation of the catabolism of lipoproteins, proteases, and protease inhibitor complexes. The hepatoma cell line Hep G2 expresses LRP and has been used widely to investigate the catabolism of LRP ligands including tissue-type plasminogen activator (tPA). However, the mechanism and degree by which tPA interacts with Hep G2 has been reported with some inconsistencies which may reflect variation in their level of LRP expression. To address this possibility we characterized, antigenically and functionally, LRP expression in high and low passage Hep G2 cells both from the parental line (ATCC sourced) and a cloned subline, a16. The LRP contribution to 125I-tPA binding varied from 65% for high passage a16 cells, to 20% for low passage parent cells as quantified by inhibition in the presence of 39-kD receptor associated protein (RAP) which prevents binding of all known LRP ligands. The same trend in LRP expression among Hep G2 sublines was further evident in their ability to degrade 125I-tPA and survive Pseudomonas exotoxin A challenge. These results imply wide variability in basal LRP expression among Hep G2 lines dependent on cell lineage and long-term culture conditions.

Segments in the C-terminal folding domain of lipoprotein lipase important for binding to the low density lipoprotein receptor-related protein and to heparan sulfate proteoglycans

Lipoprotein lipase (LpL) can mediate cellular uptake of chylomicron and VLDL remnants via binding to heparan sulfate proteoglycans (HSPG) and the endocytic alpha2-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha2MR/LRP). Whereas it is established that the C-terminal folding domain binds to alpha2MR/LRP, it remains uncertain whether it binds to heparin and to HSPG. To identify segments important for binding to alpha2MR/LRP and to clarify possible binding to heparin, we produced constructs of the human C-terminal folding domain, LpL-(313-448), and of the fragment LpL-(347-448) in Escherichia coli. In addition to binding to alpha2MR/LRP, LpL-(313-448) displayed binding to heparin with an affinity similar to that of the LpL monomer, whereas it bound poorly to lipoprotein particles. Moreover, LpL-(313-448) displayed heparin sensitive binding to normal, but not to HSPG deficient Chinese hamster ovary cells. LpL-(313-448) and LpL-(347-448) showed similar affinities for binding to both purified alpha2MR/LRP and to heparin. Deletion of LpL residues 380-384 abolished the binding to LRP, whereas binding to heparin was unperturbed. The binding to both heparin and alpha2MR/LRP was essentially abolished following deletion of residues 404-430, and pretreatment of CHO cells with the peptide comprising aa 402-423 inhibited the binding of LpL-(313-448). We conclude that the C-terminal folding domain of human LpL has a site for binding to heparin and to HSPG, presumably involving amino acids within residues 404-430. Two segments of the domain are necessary for efficient binding to alpha2MR/LRP, one comprising residues 380-384 and another overlapping the segment important for binding to heparin.

Molecular characterization of a novel human hybrid-type receptor that binds the alpha2-macroglobulin receptor-associated protein

The 39-40-kDa receptor-associated protein (RAP) binds to the members of the low density lipoprotein receptor gene family and functions as a specialized endoplasmic reticulum/Golgi chaperone. Using RAP affinity chromatography, we have purified a novel approximately 250-kDa brain protein and isolated the corresponding cDNA. The gene, designated SORL1, maps to chromosome 11q 23/24 and encodes a 2214-residue type 1 receptor containing a furin cleavage site immediately preceding the N terminus determined in the purified protein. The receptor, designated sorLA-1, has a short cytoplasmic tail containing a tyrosine-based internalization signal and a large external part containing (from the N-terminal): 1) a segment homologous to domains in the yeast vacuolar protein sorting 10 protein, Vps10p, that binds carboxypeptidase Y, 2) five tandemly arranged YWTD repeats and a cluster of 11 class A repeats characteristic of the low density lipoprotein receptor gene family receptors, and 3) six tandemly arranged fibronectin type III repeats also found in certain neural adhesion proteins. sorLA-1 may therefore be classified as a hybrid receptor. Northern blotting revealed specific mRNA transcripts in brain, spinal cord, and testis but not in several major organs. Both RAP and an antibody against a synthetic peptide derived from a sequence determined in the mature protein detected sorLA-1 in crude human brain extracts. The domain structure suggests that sorLA-1 is an endocytic receptor possibly implicated in the uptake of lipoproteins and of proteases.