Complement-type Repeats Research Articles

Characterization of interaction between blood coagulation factor VIII and LRP1 suggests dynamic binding by alternating complex contacts

BackgroundDeficiency in blood coagulation factor VIII (FVIII) results in life‐threating bleeding (hemophilia A) treated by infusions of FVIII concentrates. To improve disease treatment, FVIII has been modified to increase its plasma half‐life, which requires understanding mechanisms of FVIII catabolism. An important catabolic actor is hepatic low density lipoprotein receptor‐related protein 1 (LRP1), which also regulates many other clinically significant processes. Previous studies showed complexity of FVIII site for binding LRP1. ObjectivesTo characterize binding sites between FVIII and LRP1 and suggest a model of the interaction. MethodsA series of recombinant ligand‐binding complement‐type repeat (CR) fragments of LRP1 including mutated variants was generated in a baculovirus system and tested for FVIII interaction using surface plasmon resonance, tissue culture model, hydrogen–deuterium exchange mass spectrometry, and in silico. ResultsMultiple CR doublets within LRP1 clusters II and IV were identified as alternative FVIII‐binding sites. These interactions follow the canonical binding mode providing major binding energy, and additional weak interactions are contributed by adjacent CR domains. A representative CR doublet was shown to have multiple contact sites on FVIII. ConclusionsFVIII and LRP1 interact via formation of multiple complex contacts involving both canonical and non‐canonical binding combinations. We propose that FVIII‐LRP1 interaction occurs via switching such alternative binding combinations in a dynamic mode, and that this mechanism is relevant to other ligand interactions of the low‐density lipoprotein receptor family members including LRP1.

Von Willebrand Factor A1 Domain Binds to Clusters II and IV of Low-Density Lipoprotein Receptor-Related Protein 1

Introduction. Von Willebrand factor (VWF) is a large multimeric plasma glycoprotein involved in hemostasis and thrombosis. Genetic and in vivo studies have identified low-density lipoprotein receptor-related protein 1 (LRP) as the major clearance receptor of VWF. The ligand-binding moiety of LRP is represented by complement-type repeats grouped into four clusters, among which clusters II and IV are prominent in ligand binding. In in vitro studies under shear stress conditions, VWF acquires an unfolded conformation and interacts with LRP. More than 30 mutations of VWF have been reported to associate with its increased plasma clearance and result in type 2B von Willebrand disease. The majority of these mutations are located within the VWF A1 domain (A1), which is in agreement with previous reports that isolated A1 binds to LRP. However, the interactive site of LRP has not been identified.Objective. We aim to characterize the LRP binding site for VWF by investigating the interaction of individual LRP clusters II-IV to different A1 variants.Experimental Approach. An A1-containing large N-terminal homo-dimeric proteolytic fragment of VWF (SPIII) was produced upon its proteolytic digestion by V8-protease. Alternatively, we generated three variants of recombinant A1: (i) A1(residues 1261-1478), (ii) a VWD type 2B mutant, A1(V1316M) (residues 1261-1478) and (iii) an N-terminus truncated form, (residues 1269-1478). These proteins were tested for binding to immobilized LRP, purified from human placenta, and recombinant LRP clusters II and IV by surface plasmon resonance.Results. We did not find any interaction between the SPIII proteolytic fragment and LRP, suggesting that this fragment may still preserve a folded conformation of VWF that masked its LRP-binding site. In turn, all three forms of recombinant A1 were found to bind LRP and its clusters II and IV. Notably, A1(V1316M) and A1(1269-1478) had higher affinity for LRP and its clusters than A1. This result is consistent with the enhanced clearance of the VWF(V1316M) in vivo and better exposure of the A1 site in a truncated form of A1 to the platelet receptor GpIb-alpha reported previously.Conclusions. Our data demonstrate that LRP has at least two sites for binding to VWF, which are located within clusters II and IV. This finding is in accordance with previous observations that LRP clusters II and IV can serve as “functional duplicates” for binding selected ligands. DisclosuresNo relevant conflicts of interest to declare.

Complement C1q Interacts With LRP1 Clusters II and IV Through a Site Close but Different From the Binding Site of Its C1r and C1s-Associated Proteases.

LRP1 is a large endocytic modular receptor that plays a crucial role in the scavenging of apoptotic material through binding to pattern-recognition molecules. It is a membrane anchored receptor of the LDL receptor family with 4 extracellular clusters of ligand binding modules called cysteine rich complement-type repeats that are involved in the interaction of LRP1 with its numerous ligands. Complement C1q was shown to interact with LRP1 and to be implicated in the phagocytosis of apoptotic cells. The present work aimed at exploring how these two large molecules interact at the molecular level using a dissection strategy. For that purpose, recombinant LRP1 clusters II, III and IV were produced in mammalian HEK293F cells and their binding properties were investigated. Clusters II and IV were found to interact specifically and efficiently with C1q with KDs in the nanomolar range. The use of truncated C1q fragments and recombinant mutated C1q allowed to localize more precisely the binding site for LRP1 on the collagen-like regions of C1q (CLRs), nearby the site that is implicated in the interaction with the cognate protease tetramer C1r2s2. This site could be a common anchorage for other ligands of C1q CLRs such as sulfated proteoglycans and Complement receptor type 1. The use of a cellular model, consisting in CHO LRP1-null cells transfected with full-length LRP1 or a cluster IV minireceptor (mini IV) confirmed that mini IV interacts with C1q at the cell membrane as well as full-length LRP1. Further cellular interaction studies finally highlighted that mini IV can endorse the full-length LRP1 binding efficiency for apoptotic cells and that C1q has no impact on this interaction.

SorLA Complement-type Repeat Domains Protect the Amyloid Precursor Protein against Processing

SorLA is a neuronal sorting receptor that is genetically associated with Alzheimer disease. SorLA interacts directly with the amyloid precursor protein (APP) and affects the processing of the precursor, leading to a decreased generation of the amyloid-β peptide. The SorLA complement-type repeat (CR) domains associate in vitro with APP, but the precise molecular determinants of SorLA·APP complex formation and the mechanisms responsible for the effect of binding on APP processing have not yet been elucidated. Here, we have generated protein expression constructs for SorLA devoid of the 11 CR-domains and for two SorLA mutants harboring substitutions of the fingerprint residues in the central CR-domains. We generated SH-SY5Y cell lines that stably express these SorLA variants to study the binding and processing of APP using co-immunoprecipitation and Western blotting/ELISAs, respectively. We found that the SorLA CR-cluster is essential for interaction with APP and that deletion of the CR-cluster abolishes the protection against APP processing. Mutation of identified fingerprint residues in the SorLA CR-domains leads to changes in the O-linked glycosylation of APP when expressed in SH-SY5Y cells. Our results provide novel information on the mechanisms behind the influence of SorLA activity on APP metabolism by controlling post-translational glycosylation in the Golgi, suggesting new strategies against amyloidogenesis in Alzheimer disease.

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.

New horizons for lipoprotein receptors: communication by β-propellers

The lipoprotein receptor (LR) family constitutes a large group of structurally closely related receptors with broad ligand-binding specificity. Traditionally, ligand binding to LRs has been anticipated to involve merely the complement type repeat (CR)-domains omnipresent in the family. Recently, this dogma has transformed with the observation that β-propellers of some LRs actively engage in complex formation too. Based on an in-depth decomposition of current structures and sequences, we suggest that exploitation of the β-propellers as binding targets depends on receptor subgroups. In particular, we highlight the shutter mechanism of β-propellers as a general recognition motif for NxI-containing ligands, and we present indications that the generalized β-propeller-induced ligand release mechanism is not applicable for the larger LRs. For the giant LR members, we present evidence that their β-propellers may also actively engage in ligand binding. We therefore advocate for an increased focus on solving the structure-function relationship of this group of important biological receptors.

Mapping the Binding Region on the Low Density Lipoprotein Receptor for Blood Coagulation Factor VIII

Low density lipoprotein receptor (LDLR) was shown to mediate clearance of blood coagulation factor VIII (FVIII) from the circulation. To elucidate the mechanism of interaction of LDLR and FVIII, our objective was to identify the region of the receptor necessary for binding FVIII. Using surface plasmon resonance, we found that LDLR exodomain and its cluster of complement-type repeats (CRs) bind FVIII in the same mode. This indicated that the LDLR site for FVIII is located within the LDLR cluster. Similar results were obtained for another ligand of LDLR, α-2-macroglobulin receptor-associated protein (RAP), a common ligand of receptors from the LDLR family. We further generated a set of recombinant fragments of the LDLR cluster and assessed their structural integrity by binding to RAP and by circular dichroism. A number of fragments overlapping CR.2-5 of the cluster were positive for binding RAP and FVIII. The specificity of these interactions was tested by site-directed mutagenesis of conserved tryptophans within the LDLR fragments. For FVIII, the specificity was also tested using a single-chain variable antibody fragment directed against the FVIII light chain as a competitor. Both cases resulted in decreased binding, thus confirming its specificity. The mutagenic study also showed an importance of the conserved tryptophans in LDLR for both ligands, and the competitive binding results showed an involvement of the light chain of FVIII in its interaction with LDLR. In conclusion, the region of CR.2-5 of LDLR was defined as the binding site for FVIII and RAP.

Gentamicin Binds to the Megalin Receptor as a Competitive Inhibitor Using the Common Ligand Binding Motif of Complement Type Repeats: INSIGHT FROM THE NMR STRUCTURE OF THE 10TH COMPLEMENT TYPE REPEAT DOMAIN ALONE AND IN COMPLEX WITH GENTAMICIN

Gentamicin is an aminoglycoside widely used in treatments of, in particular, enterococcal, mycobacterial, and severe Gram-negative bacterial infections. Large doses of gentamicin cause nephrotoxicity and ototoxicity, entering the cell via the receptor megalin. Until now, no structural information has been available to describe the interaction with gentamicin in atomic detail, and neither have any three-dimensional structures of domains from the human megalin receptor been solved. To address this gap in our knowledge, we have solved the NMR structure of the 10th complement type repeat of human megalin and investigated its interaction with gentamicin. Using NMR titration data in HADDOCK, we have generated a three-dimensional model describing the complex between megalin and gentamicin. Gentamicin binds to megalin with low affinity and exploits the common ligand binding motif previously described (Jensen, G. A., Andersen, O. M., Bonvin, A. M., Bjerrum-Bohr, I., Etzerodt, M., Thogersen, H. C., O'Shea, C., Poulsen, F. M., and Kragelund, B. B. (2006) J. Mol. Biol. 362, 700-716) utilizing the indole side chain of Trp-1126 and the negatively charged residues Asp-1129, Asp-1131, and Asp-1133. Binding to megalin is highly similar to gentamicin binding to calreticulin. We discuss the impact of this novel insight for the future structure-based design of gentamicin antagonists.

Characterization of Molecular Interaction of Blood Coagulation Factor VIII and Its Clearance Receptor, Low-Density Lipoprotein Receptor.

AbstractAbstract 2206 Background.Clearance of blood coagulation factor VIII (FVIII) is mediated by several receptors (Saenko E. et al, 1999; Lenting P. et al, 1999 and Sarafanov A. et al, 2001) and one of them was proposed to be low-density lipoprotein receptor (LDLR) (Bovenschen N. et al, 2005). Besides FVIII, the major ligands of LDLR are plasma lipoproteins. The ligand-binding portion of LDLR is represented by a cluster of seven complement-type repeats. Each repeat forms a structurally autonomous domain, which are connected with flexible linkers. Two and five adjacent CRs were shown to form sites for the lipoproteins ApoE and ApoB, respectively, and the site for FVIII has not been determined. Objective.As a part of elucidation of mechanisms of FVIII clearance, our goal was to identify the FVIII-binding region on LDLR. Experimental Approach.Our experimental strategy was based on generation of recombinant CR-doublets that systematically overlap the CR cluster in LDLR (Fig. 1) and testing their interactions with FVIII. The proteins were expressed in insect cells using a baculovirus system and tested for structural integrity by circular dichroism. The functional properties of the fragments were assessed by their binding with alpha-2-macroglobulin receptor-associated protein (RAP), a universal ligand of a family of receptors that LDLR belongs to. The binding studies were performed using surface plasmon resonance technique. Using this assay, the LDLR fragments were further tested for interaction with different FVIII preparations, such as plasma-derived FVIII and recombinant FVIII: Advate (full-size FVIII) and Xyntha (B-domain deleted FVIII). The specificity of these interactions was tested in a competitive binding assay using an anti-FVIII single-chain variable antibody fragment KM33 known to inhibit binding FVIII to LDLR (Limburg V. et al, 2005). Also, the specificity of the interaction of LDLR fragments and FVIII was tested upon mutating the fragments by targeting conservative tryptophanes, one residue per selected CR. Upon expressing the mutants, their structural adequacy to the respective wild-type forms was confirmed by circular dichroism. Results.Three overlapping CR doublets were found active for binding with all FVIII preparations (Fig. 2). In presence of increased concentrations of KM33 and upon site-directed mutagenesis of the LDLR fragments, this binding was diminished confirming its specificity. Conclusions.The binding site of LDLR for FVIII is formed by a region spanned by the complement-type repeats from second to fifth. We proposed a tentative model of the interaction (Fig. 3), in which this region of the receptor contacts the A3 and C1 domains of FVIII. These portions of FVIII were previously shown to be involved in interaction with another FVIII clearance receptor known as LDLR-related protein (Bovenschen N. et al, 2003 and Meems H. et al, 2011). [Display omitted] [Display omitted] [Display omitted] Disclosures:No relevant conflicts of interest to declare.

A single lysine of the two-lysine recognition motif of the D3 domain of receptor-associated protein is sufficient to mediate endocytosis by low-density lipoprotein receptor-related protein

Ligand binding of the low-density lipoprotein (LDL) receptor family is mediated by complement-type repeats (CR) each comprising a binding pocket for a single basic amino acid residue. It has been proposed that at least two CRs are required for high-affinity interaction by utilising two spatially distinct lysine residues on the ligand surface. LDL receptor-related protein (LRP) mediates the cellular uptake of a multitude of ligands, some of which bind LRP with a relatively low affinity suggesting a suboptimal positioning of the two critical lysines. We now addressed the role of the two critical lysines not only in LRP binding but also in LRP-dependent endocytosis. Variants of the third domain (D3) of receptor-associated protein (RAP) were created carrying lysine to alanine or arginine replacements at the putative contact residues K253, K256 and K270. Surface plasmon resonance revealed that replacement of K253 did not affect high-affinity LRP binding at all, whereas replacement of either K256 or K270 markedly reduced the affinity by approximately 10-fold. Binding was abolished when both lysines were replaced. Substitution by either alanine or arginine exerted an almost identical effect on LRP binding. This suggests that despite their positive charge, arginine residues do not support receptor binding at all. Confocal microscopy and flow cytometry studies surprisingly revealed that the single mutants were still taken up and still competed for the uptake of full length RAP despite their receptor binding defect. We therefore propose that the presence of only one of the two critical lysines is sufficient to drive endocytosis.

A neuronal VLDLR variant lacking the third complement-type repeat exhibits high capacity binding of apoE containing lipoproteins

Very-low-density lipoprotein receptor (VLDLR) is a multi ligand apolipoprotein E (apoE) receptor and is involved in brain development through Reelin signaling. Different forms of VLDLR can be generated by alternative splicing. VLDLR-I contains all exons. VLDLR-II lacks an O-linked sugar domain encoded by exon 16, while VLDLR-III lacks the third complement-type repeat in the ligand binding domain encoded by exon 4. We quantitatively compared lipoprotein binding to human VLDLR variants and analyzed their mRNA expression in both human cerebellum and mouse brain. VLDLR-III exhibited the highest capacity in binding to apoE enriched β-VLDL in vitro and was more effective in removing apoE containing lipoproteins from the circulation than other variants in vivo. In human cerebellum, the major species was VLDLR-II, while the second most abundant species was a newly identified VLDLR-IV which lacks both exon 4 and 16. VLDLR-I was present at low levels. In adult mice, exon 4 skipping varied between 30 and 47% in different brain regions, while exon 16 skipping ranged by 51–76%. Significantly higher levels of VLDLR proteins were found in mouse cerebellum and cerebral cortex than other regions. The deletions of exon 4 and exon 16 frequently occurred in primary neurons, indicating that newly identified variant VLDLR-IV is abundant in neurons. In contrast, VLDLR mRNA lacking exon 4 was not detectable in primary astrocytes. Such cell type-specific splicing patterns were found in both mouse cerebellum and cerebral cortex. These results suggest that a VLDLR variant lacking the third complement-type repeat is generated by neuron-specific alternative splicing. Such differential splicing may result in different lipid uptake in neurons and astrocytes.

Functional mimicry of the LDL receptor-associated protein through multimerization of a minimized third domain

The LDL receptor-associated protein (RAP) is a ligand for the LDL receptor-related protein (LRP1). The first and third domains of RAP can each bind to one of many sequence-related pairs of complement-type repeats (CR) found within the LRP1 ectodomain. Multiple sites of interaction between the multivalent RAP ligand and the multivalent LRP1 receptor yield strong binding avidity for the complex. The third domain of RAP can be significantly truncated, with material retention of monovalent CR pair-binding affinity, provided that the minimized sequence is stabilized with an intramolecular disulfide bond. We demonstrate that the avidity of full-length RAP for LRP1 in vitro can be partially reconstituted by assembly of truncated, disulfide-linked RAP peptides on tetravalent streptavidin or bivalent immunoglobulin scaffolds. The peptide complex with streptavidin shows pronounced hepatotropism in vivo, replicating the biodistribution of full-length RAP.

Minimization of the third domain of the LDL receptor-associated protein (RAP)

The third domain of the low-density lipoprotein receptor-associated protein (RAP d3) binds with high-affinity to pairs of complement-type repeats (CR) within the LDLR family of receptors. Structural analyses have defined the contact surface between RAP d3 and a CR pair from the low-density lipoprotein receptor (LDLR). Much of the sequence of RAP d3 has been proposed to stabilize the receptor-binding region without participating directly in formation of the contact surface. We have developed a truncated version of RAP d3 in which these scaffolding regions are excised and replaced with a single, intramolecular disulfide bond. This substitution allows for deletion of as much as a third of the RAP d3 sequence with substantial retention of receptor-binding ability.

Functional duplication of ligand-binding domains within low-density lipoprotein receptor-related protein for interaction with receptor associated protein, α 2-macroglobulin, factor IXa and factor VIII

The low-density lipoprotein receptor-related protein (LRP) binds a range of proteins including receptor associated protein (RAP), activated α 2-macroglobulin (α 2M*), factor IXa (FIXa), and factor VIII (FVIII) light chain. The binding is mediated by the complement-type repeats, which are clustered in four distinct regions within LRP. Cluster II of 8 repeats (CR3–10) and cluster IV of 11 repeats (CR21–31) have been implicated in ligand-binding. Previous studies have aimed to identify the cluster II repeats involved in binding α 2M* and RAP. We now evaluated the binding to RAP, α 2M*, FIXa and FVIII light chain of triplicate repeat-fragments of not only clusters II but also of cluster IV. Employing surface plasmon resonance analysis, we found that most efficient ligand-binding was displayed by the repeats within region CR4–8 of cluster II and within region CR24–28 of cluster IV. Whereas the binding to RAP could be attributed to two consecutive repeats (CR5–6, CR26–27), combinations of three repeats showed most efficient binding to FIXa (CR6–8, CR26–28), FVIII light chain (CR5–7, CR6–8, CR24–26), and α 2M* (CR4–6, CR24–26). The results imply that there is an internal functional duplication of complement-type repeats within LRP resulting in two clusters that bind the same ligands.

Localization of the low-density lipoprotein receptor-related protein regions involved in binding to the A2 domain of coagulation factor VIII

Catabolism of coagulation factor VIII (FVIII) is mediated by low-density lipoprotein receptor-related protein (LRP). The ligand-binding sites of LRP are formed by complement-type repeats (CR), and CR clusters II and IV bind most of LRP ligands. FVIII contains two major LRP-binding sites located in the A2 and A3 domains. This study was aimed to identify specific complement-type repeats of LRP involved in interaction with the A2 site and to probe their functional importance in A2 catabolism. We generated individual LRP clusters II, III and IV, along with nine overlapping CR triplets encompassing clusters II and IV in a baculovirus expression system and studied their interaction with isolated A2. In surface plasmon resonance (SPR) assay, A2 bound to clusters II and IV with KDs 22 and 39 nM, respectively, and to the majority of CR triplets with affinities in the range of KDs 25-90 nM. Similar affinities were determined for A2 interaction with a panel of CR doublets overlapping cluster II (CR 3-4, 4-5, 5-6, 6-7 and 7-8). These LRP fragments inhibited the binding of 125I-A2 to LRP in solid-phase assay, LRP-mediated internalization of 125I-A2 in cell culture and 125I-A2 clearance from the mouse circulation. Point mutations of critical A2 residues of the LRPbinding site resulted in differential reduction or abolishment of its binding to LRP fragments. We conclude that A2 interacts with LRP via multiple binding sites spanning CR 3-8 in cluster II and CR 23-29 in cluster IV, and the minimal A2-binding unit of LRP is formed by two adjacent CR.

Binding Site Structure of One LRP–RAP Complex:Implications for a Common Ligand–Receptor Binding Motif

The low-density lipoprotein receptor-related protein (LRP) interacts with more than 30 ligands of different sizes and structures that can all be replaced by the receptor-associated protein (RAP). The double module of complement type repeats, CR56, of LRP binds many ligands including all three domains of RAP and α 2-macroglobulin, which promotes the catabolism of the Aβ-peptide implicated in Alzheimer's disease. To understand the receptor–ligand cross-talk, the NMR structure of CR56 has been solved and ligand binding experiments with RAP domain 1 (RAPd1) have been performed. From chemical shift perturbations of both binding partners upon complex formation, a HADDOCK model of the complex between CR56 and RAPd1 has been obtained. The binding residues are similar to a common binding motif suggested from α 2-macroglobulin binding studies and provide evidence for an understanding of their mutual cross-competition pattern. The present structural results convey a simultaneous description of both binding partners of an LRP–ligand complex and open a route to a broader understanding of the binding specificity of the LRP receptor, which may involve a general four-residue receptor–ligand recognition motif common to all LRP ligands. The present result may be beneficial in the design of antagonists of ligand binding to the LDL receptor family, and especially of drugs for treatment of Alzheimer's disease.

Molecular Dissection of the Interaction between Amyloid Precursor Protein and Its Neuronal Trafficking Receptor SorLA/LR11

SorLA/LR11 is a sorting receptor that regulates the intracellular transport and processing of the amyloid precursor protein (APP) in neurons. SorLA/LR11-mediated binding results in sequestration of APP in the Golgi and in protection from processing into the amyloid-beta peptide (Abeta), the principal component of senile plaques in Alzheimer's disease (AD). To gain insight into the molecular mechanisms governing sorLA and APP interaction, we have dissected the respective protein interacting domains. Using a fluorescence resonance energy transfer (FRET) based assay of protein proximity, we identified binding sites in the extracellular regions of both proteins. Fine mapping by surface plasmon resonance analysis and analytical ultracentrifugation of recombinant APP and sorLA fragments further narrowed down the binding domains to the cluster of complement-type repeats in sorLA that forms a 1:1 stoichiometric complex with the carbohydrate-linked domain of APP. These data shed new light on the molecular determinants of neuronal APP trafficking and processing and on possible targets for intervention with senile plaque formation in patients with AD.

Fragments of Activated Coagulation Factor VIII Bind to Multiple Sites within Low-Density Lipoprotein Receptor-Related Protein.

Catabolism of coagulation factor VIII (fVIII) is mediated by the hepatic multiligand receptor low-density lipoprotein receptor-related protein (LRP). The ligand-binding sites of LRP are formed by complement-type repeats (CRs) organized in four clusters, among which clusters II and IV bind most of LRP ligands. In turn, fVIII contains two major LRP-binding sites, located in A2 and A3 domains (Saenko et al, JBC 1999; Bovenschen et al, JBC 2003). In present work, we characterized binding sites in LRP for A2 domain (A2) and heterodimer A1/A3-C1-C2 (HD), the products of dissociation of activated fVIII. Using a baculovirus expression system, we generated CR clusters II, III and IV, along with eight overlapping CR triplets encompassing clusters II and IV. Surface plasmon resonance-based assays demonstrated that both A2 and HD bind to clusters II and IV, and to the same sets of their CR triplets with similar affinities (KDs 25–50 nM). The same kinetic parameters of interaction of both A2 and HD were observed for several CR doublets from cluster II, shown previously to be minimal binding sites for a classical ligand of LRP, receptor associated protein (RAP) (Andersen et al, JBC 2000). The specificity of A2 and HD interactions with all tested fragments of LRP was confirmed by the ability of RAP to inhibit these interactions, and by the ability of these fragments to inhibit binding of 125I-A2 and 125I-HD to immobilized LRP in a solid-phase assay, and LRP-mediated catabolism of 125I-A2 and 125I-HD in cell culture. Notably, some mutations of the LRP-binding site in A2 resulted in significant reduction or abolishment of its binding to certain fragments of LRP, while the binding to other LRP fragments was less affected. In summary, we demonstrated that i) A2 and HD interact with LRP via its multiple binding sites spanning CRs 3–8 in cluster II and CRs 24–29 in cluster IV, and ii) the elementary binding unit of LRP is formed by at least two adjacent CRs, similar to that shown for RAP. The above data also suggest that besides regulating fVIII levels, LRP also plays a role in clearance of the products of dissociation of activated fVIII.

Role of Low Density Lipoprotein Receptor in the Clearance of Coagulation Factor VIII In Vivo.

Low Density Lipoprotein Receptor (LDLR) is the archetype of the family of endocytic receptors that also includes Low-density lipoprotein Receptor-related Protein (LRP), Megalin, and Very Low Density Lipoprotein Receptor (VLDLR). While most of these receptors bind a variety of ligands, LDLR has restricted specifity. Its known ligands are apolipoprotein E (apoE) and apolipoprotein B100 (apoB100). Ligand binding to the LDL-receptor family is inhibited by the Receptor-Associated Protein (RAP). We have previously reported that RAP overexpression elevates plasma levels of factor VIII (FVIII) in both normal mice and mice with hepatic LRP deficiency [Bovenschen et al. (2003) Blood 101, 3933–3939]. This implies that LRP, but also another RAP-sensitive mechanism contributes to the regulation of FVIII in vivo. This study addresses the question whether LDLR, despite its restricted ligand specificity, binds FVIII and contributes to its clearance. In vitro binding was studied using a recombinant LDLR fragment spanning the extracellular domain of complement-type repeats 1–7. The purified fragment efficiently bound to immobilized ApoE and apoB-100 containing LDL. The immobilized LDLR fragment also bound human FVIII, with half-maximal binding at 156 nM FVIII, and binding was inhibited by RAP. Human von Willebrand Factor (VWF) or non-activated factor IX did not bind to the LDLR fragment. The relevance of the FVIII-LDLR interaction was assessed in vivo employing LDLR−/− mice, cre/loxP-mediated conditional LRP-deficient mice (LRP−), and mice with the combined deficiency. Plasma FVIII levels of controls, LDLR−/− and LRP− mice were 1.1, 0.9 and 1.8 U/ml, respectively. This suggests that LRP, but not LDLR regulates FVIII in plasma. Surprisingly, however, mice that combined LDLR deficiency with hepatic LRP deficiency displayed much higher FVIII levels (median value 4.6 U/ml) than mice lacking LRP alone. This suggests that LDLR does have the potential of regulating FVIII levels. LDLR−/− LRP− mice further displayed elevated levels of VWF (median value 3.3 U/ml), but not of factor V or factor IX. The possibility was considered that FVIII levels were elevated secondary to the profound changes in lipoprotein profiles. To this end, we also examined ApoE deficient mice, which have reduced LDL, and mice that overexpress ApoC1, which is associated with elevated levels of cholesterol- and triglyceride VLDL. ApoE−/− LDLR−/−LRP− mice had a median FVIII level of 4.2 U/ml, which is close to that of LDLR−/−LRP− mice. Mice that overexpressed human ApoC1 had elevated levels of cholesterol and triglycerides, but 0.5 U/ml FVIII. This demonstrates that elevated FVIII levels were independent of lipoprotein levels. The role of LDLR and LRP in FVIII clearance were further studied by analyzing the pharmacokinetics of human FVIII. In normal mice the Mean Residence Time (MRT) was 160 min [68% confidence intervals (CI) 117–218 min]. MRT was 200 [CI 154–259] min in LDLR−/− mice, and 263 [CI 206–336] min in LRP− mice. This confirms the previously described role of LRP in FVIII clearance. Strikingly, in LDLR−/−LRP− mice the MRT of FVIII was 760 [691–826] min, which is approximately 5-fold longer than in control mice. These data demonstrate that LRP and LDLR act in concert in regulating FVIII levels in plasma. In the absence of LDLR, LRP maintains normal FVIII levels, while hepatic LRP deficiency is largely compensated by LDLR. This regulatory role of LDLR represents a novel link between LDLR and the hemostatic system.

The Role of a Conserved Acidic Residue in Calcium-dependent Protein Folding for a Low Density Lipoprotein (LDL)-A Module: IMPLICATIONS IN STRUCTURE AND FUNCTION FOR THE LDL RECEPTOR SUPERFAMILY

One common feature of the more than 1,000 complement-type repeats (or low density lipoprotein (LDL)-A modules) found in LDL receptor and the other members of the LDL receptor superfamily is a cluster of five highly conserved acidic residues in the C-terminal region, DXXXDXXDXXDE. However, the role of the third conserved aspartate of these LDL-A modules in protein folding and ligand recognition has not been elucidated. In this report, using a model LDL-A module and several experimental approaches, we demonstrate that this acidic residue, like the other four conserved acidic residues, is involved in calcium-dependent protein folding. These results suggest an alternative calcium coordination conformation for the LDL-A modules. The proposed model provides a plausible explanation for the conservation of this acidic residue among the LDL-A modules. Furthermore, the model can explain why mutations of this residue in human LDL receptor cause familial hypercholesterolemia.