Elevated plasma factor VIII in a mouse model of low-density lipoprotein receptor-related protein deficiency

The aptamer BT200 blocks interaction of K1405-K1408 in the VWF-A1 domain with macrophage LRP1

ABSTRACT

Rondaptivon pegol (previously BT200) is a pegylated RNA aptamer that binds to the A1 domain of von Willebrand factor (VWF). Recent clinical trials demonstrated that BT200 significantly increased plasma VWF-factor VIII levels by attenuating VWF clearance. The biological mechanism(s) through which BT200 attenuates in vivo clearance of VWF has not been defined. We hypothesized that BT200 interaction with the VWF-A1 domain may increase plasma VWF levels by attenuating macrophage-mediated clearance. We observed that full-length and VWF-A1A2A3 binding to macrophages and VWF-A1 domain binding to lipoprotein receptor-related protein 1 (LRP1) cluster II and cluster IV were concentration-dependently inhibited by BT200. Additionally, full-length VWF binding to LRP1 expressed on HEK293T (HEK-LRP1) cells was also inhibited by BT200. Importantly, BT200 interacts with the VWF-A1 domain in proximity to a conserved cluster of 4 lysine residues (K1405, K1406, K1407, and K1408). Alanine mutagenesis of this K1405-K1408 cluster (VWF-4A) significantly (P < .001) attenuated binding of VWF to both LRP1 clusters II and IV. Furthermore, in vivo clearance of VWF-4A was significantly (P < .001) reduced than that of wild-type VWF. BT200 did not significantly inhibit binding of VWF-4A to LRP1 cluster IV or HEK-LRP1 cells. Finally, BT200 interaction with the VWF-A1 domain also inhibited binding to macrophage galactose lectin and the SR-AI scavenger receptor. Collectively, our findings demonstrate that BT200 prolongs VWF half-life by attenuating macrophage-mediated clearance and specifically the interaction of K1405-K1408 in the VWF-A1 domain with macrophage LRP1. These data support the concept that targeted inhibition of VWF clearance pathways represents a novel therapeutic approach for von Willebrand disease and hemophilia A.

Coagulation factor VIII regulates von Willebrand factor homeostasis invivo

BACKGROUND

Coagulation factor VIII (FVIII) and von Willebrand factor (VWF) circulate as a noncovalent complex, but each has its distinct functions. Binding of FVIII to VWF results in a prolongation of FVIII's half-life in circulation and modulates FVIII's immunogenicity during hemophilia therapy. However, the biological effect of FVIII and VWF interaction on VWF homeostasis is not fully understood.

OBJECTIVES

To determine the effect of FVIII in VWF proteolysis and homeostasis in vivo.

METHODS

Mouse models, recombinant FVIII infusion, and patients with hemophilia A on a high dose FVIII for immune tolerance induction therapy or emicizumab for bleeding symptoms were included to address this question.

RESULTS

An intravenous infusion of a recombinant B-domain less FVIII (BDD-FVIII) (40 and 160 μg/kg) into wild-type mice significantly reduced plasma VWF multimer sizes and its antigen levels; an infusion of a high but not low dose of BDD-FVIII into Adamts13+/- and Adamts13-/- mice also significantly reduced the size of VWF multimers. However, plasma levels of VWF antigen remained unchanged following administration of any dose BDD-FVIII into Adamts13-/- mice, suggesting partial ADAMTS-13 dependency in FVIII-augmented VWF degradation. Moreover, persistent expression of BDD-FVIII at ∼50 to 250 U/dL via AAV8 vector in hemophilia A mice also resulted in a significant reduction of plasma VWF multimer sizes and antigen levels. Finally, the sizes of plasma VWF multimers were significantly reduced in patients with hemophilia A who received a dose of recombinant or plasma-derived FVIII for immune tolerance induction therapy.

CONCLUSION

Our results demonstrate the pivotal role of FVIII as a cofactor regulating VWF proteolysis and homeostasis under various (patho)physiological conditions.

The contribution of the sinusoidal endothelial cell receptors CLEC4M, stabilin-2, and SCARA5 to VWF-FVIII clearance in thrombosis and hemostasis

Quantitative abnormalities in factor VIII (FVIII) and its binding partner, von Willebrand factor (VWF), are associated with an increased risk of bleeding or thrombosis, and pathways that regulate the clearance of VWF-FVIII can strongly influence their plasma levels. In 2010, the Cohorts for Heart and Aging Research in Genome Epidemiology (CHARGE) on genome-wide association study meta-analysis identified variants in the genes for the sinusoidal endothelial receptors C-type lectin domain family 4 member M (CLEC4M), stabilin-2, and scavenger receptor class A member 5 (SCARA5) as being associated with plasma levels of VWF and/or FVIII in normal individuals. The ability of these receptors to bind, internalize, and clear the VWF-FVIII complex from the circulation has now been reported in a series of studies using in vitro and in vivo models. The receptor stabilin-2 has also been shown to modulate the immune response to infused VWF-FVIII concentrates in a murine model. In addition, the influence of genetic variants in CLEC4M, STAB2, and SCARA5 on type 1 von Willebrand disease/low VWF phenotype, FVIII pharmacokinetics, and the risk of venous thromboembolism has been described in a number of patient-based studies. Understanding the role of these receptors in the regulation of VWF-FVIII clearance has led to significant insights into the genomic architecture that modulates plasma VWF and FVIII levels, improving the understanding of pathways that regulate VWF-FVIII clearance and the mechanistic basis of quantitative VWF-FVIII pathologies.

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

BACKGROUND

Deficiency 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.

OBJECTIVES

To characterize binding sites between FVIII and LRP1 and suggest a model of the interaction.

METHODS

A 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.

RESULTS

Multiple 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.

CONCLUSIONS

FVIII 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.

FVIII at the crossroad of coagulation, bone and immune biology: Emerging evidence of biological activities beyond hemostasis

Hemophilia A is an X-linked hereditary disorder that results from deficient coagulation factor VIII (FVIII) activity, leading to spontaneous bleeding episodes, particularly in joints and muscles. FVIII deficiency has been associated with altered bone remodeling, dysregulated macrophage polarization, and inflammatory processes that are associated with the neoformation of abnormal blood vessels. Treatment based on FVIII replacement can lead to the development of inhibitors that render FVIII concentrate infusion ineffective. In this context, hemophilia has entered a new therapeutic era with the development of new drugs, such as emicizumab, that seek to restore the hemostatic balance by bypassing pathologically acquired antibodies. We discuss the potential extrahemostatic functions of FVIII that may be crucial for defining future therapies in hemophilia.

PCSK9 Functions in Atherosclerosis Are Not Limited to Plasmatic LDL-Cholesterol Regulation

The relevance of PCSK9 in atherosclerosis progression is demonstrated by the benefits observed in patients that have followed PCSK9-targeted therapies. The impact of these therapies is attributed to the plasma lipid-lowering effect induced when LDLR hepatic expression levels are recovered after the suppression of soluble PCSK9. Different studies show that PCSK9 is involved in other mechanisms that take place at different stages during atherosclerosis development. Indeed, PCSK9 regulates the expression of key receptors expressed in macrophages that contribute to lipid-loading, foam cell formation and atherosclerotic plaque formation. PCSK9 is also a regulator of vascular inflammation and its expression correlates with pro-inflammatory cytokines release, inflammatory cell recruitment and plaque destabilization. Furthermore, anti-PCSK9 approaches have demonstrated that by inhibiting PCSK9 activity, the progression of atherosclerotic disease is diminished. PCSK9 also modulates thrombosis by modifying platelets steady-state, leukocyte recruitment and clot formation. In this review we evaluate recent findings on PCSK9 functions in cardiovascular diseases beyond LDL-cholesterol plasma levels regulation.

General concepts on hemophilia A and on women carrying the disease

Hemophilia A affects one in every 5000 live male births. As the disorder follows a hereditary X-linked recessive pattern, women who inherit the mutation become carriers of the disease. The exact prevalence of carriers of hemophilia A or B is unknown. A search of the literature identified only one study that provides an approximation. According to its authors, for every 100 male with hemophilia there are 277 potential carriers. We will review through this supplement carrier condition from reproductive to care giver and individual point of view.

LRP1 polymorphisms associated with warfarin stable dose in Chinese patients: a stepwise conditional analysis

Aim: The aim of this study was to investigate whether variability in warfarin stable dose (WSD) could be influenced by vitamin K-related polymorphisms in patients with heart valve replacement. Patients & methods: Twenty-nine vitamin K-related SNPs in 208 patients who initially took warfarin and achieved WSD were genotyped. Results: After conducting conditional analysis for both VKORC1 -1639G>A and CYP2C9*3, LRP1 rs1800139 and LRP1 rs1800154 were significantly associated with WSD (p = 0.007 and p = 0.015, respectively). Multivariate analysis showed that LRP1 rs1800139 accounted for 5.9% WSD variability. Conclusion: Our results suggest that a novel vitamin K-related gene, LRP1, exerts a relevant influence on WSD, independent of VKORC1 -1639G>A and CYP2C9*3.

LRP1 and APOA1 Polymorphisms: Impact on Warfarin International Normalized Ratio-Related Phenotypes

Warfarin international normalized ratio (INR)-related phenotypes such as the percentage of INR time in the therapeutic range (PTTR) and INR variability are associated with warfarin adverse reactions. However, INR-related phenotypes greatly vary among patients, and the underlying mechanism remains unclear. As a key cofactor for coagulation proteins, vitamin K can affect warfarin INR values. The aim of this study was to address the influence of vitamin K-related single-nucleotide polymorphisms (SNPs) on warfarin INR-related phenotypes. A total of 262 patients who were new recipients of warfarin therapy and followed up for 3 months were enrolled. Twenty-nine SNPs were genotyped by matrix-assisted laser desorption/ionization time-of-flight mass array. Sixteen warfarin INR-related phenotypes were observed. After association analysis, 11 SNPs were significantly associated with at least one INR-related phenotype, and 6 SNPs were associated with at least 2 INR-related phenotypes (P < 0.05). In these SNPs, rs1800139, rs1800154, rs1800141, and rs486020 were the most representative. rs1800139, rs1800154, and rs1800141 locate in LRP1 and were found to be correlated with 1-month and 2-month INR variability (P < 0.05). Besides, the APOA1 rs486020 was significantly associated with the first month PTTR (P = 0.009), and patients with C-allele had higher PTTR than those with G-alleles almost during the entire monitoring period. In conclusion, the study revealed that the polymorphisms of LRP1 and APOA1 gene may play important roles in the variation of warfarin INR-related phenotypes. Our results provide new information for improving warfarin anticoagulation management.

Factor VIII Fc Fusion Protein but not FVIII Drives Human Monocyte-Derived Dendritic Cell Activation via FcγRIIa

This study compares the effect of recombinant Factor VIII Fc fusion protein (rFVIII-Fc) with recombinant FVIII (rFVIII) on monocyte-derived dendritic cells (moDC's). Cells treated with rFVIII-Fc showed morphological changes typical for cell activation, had a significant up-regulation of cell activation markers and produced higher levels of pro-inflammatory cytokines. Even after stimulation with Lipopolysaccharides, the addition of rFVIII-Fc led to increased expression of activation markers, indicating that rFVIII-Fc is capable of amplifying the maturation signal. On the contrary, cultivation of moDC's with rFVIII did not alter cell morphology or increase surface activation marker expression and pro-inflammatory cytokine production. The binding of the Fc domain to the activating Fcγ receptor IIa (FcγRIIa) can cause cell activation. Therefore, the effect of rFVIII-Fc on FcγRIIa was analyzed in detail. Cultivation of moDC's with rFVIII-Fc led to increased phosphorylation of FcγRIIa, which was not detected for rFVIII. Blocking FcγRIIa prior to the cultivation with rFVIII-Fc significantly reduced the activating effect of rFVIII-Fc, indicating that rFVIII-Fc-induced moDC activation was caused by FcγRIIa. Moreover, rFVIII-Fc bound to FCGR2A-transfected human embryonic kidney 293 cells. Taken together, our data present a new mechanism of moDC activation by rFVIII-Fc via FcγRIIa.

Functional polymorphisms in the LDLR and pharmacokinetics of Factor VIII concentrates

BACKGROUND

Optimization of factor VIII (FVIII) infusion in hemophilia A would benefit from identification of FVIII pharmacokinetics (PK) determinants. The low-density lipoprotein receptor (LDLR) contains an FVIII-binding site and might influence FVIII clearance. Consistently, LDLR polymorphisms have been associated with FVIII levels.

OBJECTIVE

To investigate the relationships between individual FVIII PK and functional LDLR polymorphisms.

PATIENTS/METHODS

Thirty-three hemophilia A patients (FVIII coagulant activity [FVIII:C] ≤2 IU/dL) without inhibitors underwent 85 FVIII single-dose (21.4-51.8 IU/kg) PKs with different FVIII concentrates. Twenty patients underwent repeated PKs (2-6).

FVIII

C measured up to 72 hours was analyzed by two-compartment model. Parameters were evaluated in relation to F8 mutations, ABO blood-group and LDLR genotypes.

RESULTS

F8 mutation types were not associated with PK parameters. ABO and LDLR c.1773C/T polymorphism were associated with Alpha, Alpha HL, CLD2, K1-2, and K2-1 parameters, suggesting an influence on the FVIII initial distribution phase. Regression analysis showed an independent association of both ABO and LDLR c.1773C/T with PK parameters (Alpha, β-coefficient -0.311 vs 0.348; CLD2, β-coefficient -0.335 vs 0.318), giving rise to an additive effect in subjects stratified by combined phenotypes. Differently, the LDLR c.81C/T was associated with FVIII clearance and volume of distribution at steady state, which could be related to distinct effects of polymorphisms, potentially linked to LDLR intracellular distribution and FVIII binding behavior.

CONCLUSIONS

With the limitation of different FVIII concentrates and low number of patients, our data show plausible associations of LDLR polymorphisms with FVIII PK parameters, thus supporting their investigation as candidate functional determinants of FVIII PK.

Venous thromboembolism, factor VIII and chronic kidney disease

Chronic kidney disease (CKD) affects 30 million Americans and is associated with approximately a two-fold increased risk of venous thromboembolism (VTE). There is a graded increased risk of VTE across declining kidney function, as measured by estimated glomerular filtration rate (eGFR) and albuminuria. When patients with end-stage kidney disease (ESKD) experience VTE they are more likely than the general population to be hospitalized and they have a higher mortality. The incidence and consequences of VTE may also differ depending on the cause of kidney disease. In addition, kidney transplant patients with VTE are at a greater risk for death and graft loss than transplant patients without VTE. The reasons that patients with CKD are at increased risk of VTE are not well understood, but recent data suggest that factor VIII is a mediator. Factor VIII is an essential cofactor in the coagulation cascade and a strong risk factor for VTE in general. It is inversely correlated with eGFR and prospective studies demonstrate that factor VIII activity predicts incident CKD and rapid eGFR decline. The etiology of CKD may also influence factor VIII levels. This review summarizes the epidemiology VTE in CKD and reviews the biochemistry of factor VIII and determinants of its levels, including von Willebrand factor and ABO blood group. We explore mechanisms by which the complications of CKD might give rise to higher factor VIII and suggests future research directions to understand how factor VIII and CKD are linked.

Advancement in the treatment of haemophilia

Poor understanding of the pathophysiological mechanisms involved in Haemophilia is a major obstacle in accessing effective haemophilia disease management. Haemophilia is a life-frightening bleeding problem in which there are repeated bleeding episodes. Various approaches have been used, involves clotting factor replacement therapy for effective bleeding control in Haemophilia. Current advancements in the management of patients with haemophilia include altered pharmacokinetics clotting factor concentrates for better prophylaxis and management of haemophilia. This review sums up the prophylactic treatment, novel production techniques, other treatment techniques and the present position of gene therapy in the treatment of haemophilia.

Half-life-extended recombinant coagulation factor IX-albumin fusion protein is recycled via the FcRn-mediated pathway

The neonatal Fc receptor (FcRn) has a pivotal role in albumin and IgG homeostasis. Internalized IgG captured by FcRn under acidic endosomal conditions is recycled to the cell surface where exocytosis and a shift to neutral pH promote extracellular IgG release. Although a similar mechanism is proposed for FcRn-mediated albumin intracellular trafficking and recycling, this pathway is less well defined but is relevant to the development of therapeutics exploiting FcRn to extend the half-life of short-lived plasma proteins. Recently, a long-acting recombinant coagulation factor IX–albumin fusion protein (rIX-FP) has been approved for the management of hemophilia B. Fusion to albumin potentially enables internalized proteins to engage FcRn and escape lysosomal degradation. In this study, we present for the first time a detailed investigation of the FcRn-mediated recycling of albumin and the albumin fusion protein rIX-FP. We demonstrate that following internalization via FcRn at low pH, rIX-FP, like albumin, is detectable within the early endosome and rapidly (within 10–15 min) traffics into the Rab11+ recycling endosomes, from where it is exported from the cell. Similarly, rIX-FP and albumin taken up by fluid-phase endocytosis at physiological pH traffics into the Rab11+ recycling compartment in FcRn-positive cells but into the lysosomal compartment in FcRn-negative cells. As expected, recombinant factor IX (without albumin fusion) and an FcRn interaction–defective albumin variant localized to the lysosomal compartments of both FcRn-expressing and nonexpressing cells. These results indicate that FcRn-mediated recycling via the albumin moiety is a mechanism for the half-life extension of rIX-FP observed in clinical studies.

To serve and protect: The modulatory role of von Willebrand factor on factor VIII immunogenicity

Hemophilia A is a bleeding disorder characterized by the absence or dysfunction of blood coagulation factor VIII (FVIII). Patients are treated with regular infusions of FVIII concentrate. In response to treatment, approximately 30% of patients with severe hemophilia A develop inhibitory antibodies targeting FVIII. Both patient and treatment related risk factors for inhibitor development have been described. Multiple studies comparing the immunogenicity of recombinant and plasma-derived FVIII have yielded conflicting results. The randomized controlled SIPPET (Survey of Inhibitors in Plasma-Product Exposed Toddlers) trial demonstrated an increased risk of inhibitor development of recombinant FVIII when compared to von Willebrand factor (VWF)-containing plasma-derived FVIII. Presently, it is unclear which mechanism underlies the reduced immunogenicity of plasma-derived FVIII. In this review we address the potential role of VWF on FVIII immunogenicity and we discuss how VWF affects the immune recognition, processing and presentation of FVIII. We also briefly discuss the potential impact of glycan-composition on FVIII immunogenicity. It is well established that VWF shields the uptake of FVIII by antigen presenting cells. We have recently shown that VWF binds to the surface of dendritic cells. Here, we present a novel model in which surface bound FVIII-VWF complexes regulate the internalization of FVIII. Binding of FVIII to VWF is critically dependent on sulfation of Tyr1699 (HVGS numbering) in the light chain of FVIII. Incomplete sulfation of Tyr1699 has been suggested to occur in several recombinant FVIII products resulting in a loss of VWF binding. We hypothesize that this results in alternative pathways of FVIII internalization by antigen presenting cells which are not regulated by VWF. This hypothetical mechanism may explain the reduced immunogenicity of VWF containing plasma-derived FVIII concentrates as found in the SIPPET study.

Evidence That Factor VIII Forms a Bivalent Complex with the Low Density Lipoprotein (LDL) Receptor-related Protein 1 (LRP1): IDENTIFICATION OF CLUSTER IV ON LRP1 AS THE MAJOR BINDING SITE

Hemophilia A is a bleeding disorder caused by a deficiency in coagulation factor VIII (fVIII) that affects 1 in 5,000 males. Current prophylactic replacement therapy, although effective, is difficult to maintain due to the cost and frequency of injections. Hepatic clearance of fVIII is mediated by the LDL receptor-related protein 1 (LRP1), a member of the LDL receptor family. Although it is well established that fVIII binds LRP1, the molecular details of this interaction are unclear as most of the studies have been performed using fragments of fVIII and LRP1. In the current investigation, we examine the binding of intact fVIII to full-length LRP1 to gain insight into the molecular interaction. Chemical modification studies confirm the requirement for lysine residues in the interaction of fVIII with LRP1. Examination of the ionic strength dependence of the interaction of fVIII with LRP1 resulted in a Debye-Hückel plot with a slope of 1.8 ± 0.5, suggesting the involvement of two critical charged residues in the interaction of fVIII with LRP1. Kinetic studies utilizing surface plasmon resonance techniques reveal that the high affinity of fVIII for LRP1 results from avidity effects mediated by the interactions of two sites in fVIII with complementary sites on LRP1 to form a bivalent fVIII·LRP1 complex. Furthermore, although fVIII bound avidly to soluble forms of clusters II and IV from LRP1, only soluble cluster IV competed with the binding of fVIII to full-length LRP1, revealing that cluster IV represents the major fVIII binding site in LRP1.

Rapid hepatic clearance of full length CCN-2/CTGF: a putative role for LRP1-mediated endocytosis

CCN-2 (connective tissue growth factor; CTGF) is a key factor in fibrosis. Plasma CCN-2 has biomarker potential in numerous fibrotic disorders, but it is unknown which pathophysiological factors determine plasma CCN-2 levels. The proteolytic amino-terminal fragment of CCN-2 is primarily eliminated by the kidney. Here, we investigated elimination and distribution profiles of full length CCN-2 by intravenous administration of recombinant CCN-2 to rodents. After bolus injection in mice, we observed a large initial distribution volume (454 mL/kg) and a fast initial clearance (120 mL/kg/min). Immunosorbent assay and immunostaining showed that CCN-2 distributed mainly to the liver and was taken up by hepatocytes. Steady state clearance in rats, determined by continuous infusion of CCN-2, was fast (45 mL/kg/min). Renal CCN-2 clearance, determined by arterial and renal vein sampling, accounted for only 12 % of total clearance. Co-infusion of CCN-2 with receptor-associated protein (RAP), an antagonist of LDL-receptor family proteins, showed that RAP prolonged CCN-2 half-life and completely prevented CCN-2 internalization by hepatocytes. This suggests that hepatic uptake of CCN-2 is mediated by a RAP-sensitive mechanism most likely involving LRP1, a member of the LDL-receptor family involved in hepatic clearance of various plasma proteins. Surface plasmon resonance binding studies confirmed that CCN-2 is an LRP1 ligand. Co-infusion of CCN-2 with an excess of the heparan sulphate-binding protamine lowered the large initial distribution volume of CCN-2 by 88 % and reduced interstitial staining of CCN-2, suggesting binding of CCN-2 to heparan sulphate proteoglycans (HSPGs). Protamine did not affect clearance rate, indicating that RAP-sensitive clearance of CCN-2 is HSPG independent. In conclusion, unlike its amino-terminal fragment which is cleared by the kidney, full length CCN-2 is primarily eliminated by the liver via a fast RAP-sensitive, probably LRP1-dependent pathway.

N-linked glycans within the A2 domain of von Willebrand factor modulate macrophage-mediated clearance

Enhanced von Willebrand factor (VWF) clearance is important in the etiology of von Willebrand disease. However, the molecular mechanisms underlying VWF clearance remain poorly understood. In this study, we investigated the role of VWF domains and specific glycan moieties in regulating in vivo clearance. Our findings demonstrate that the A1 domain of VWF contains a receptor-recognition site that plays a key role in regulating the interaction of VWF with macrophages. In A1-A2-A3 and full-length VWF, this macrophage-binding site is cryptic but becomes exposed following exposure to shear or ristocetin. Previous studies have demonstrated that the N-linked glycans within the A2 domain play an important role in modulating susceptibility to ADAMTS13 proteolysis. We further demonstrate that these glycans presented at N1515 and N1574 also play a critical role in protecting VWF against macrophage binding and clearance. Indeed, loss of the N-glycan at N1515 resulted in markedly enhanced VWF clearance that was significantly faster than that observed with any previously described VWF mutations. In addition, A1-A2-A3 fragments containing the N1515Q or N1574Q substitutions also demonstrated significantly enhanced clearance. Importantly, clodronate-induced macrophage depletion significantly attenuated the increased clearance observed with N1515Q and N1574Q in both full-length VWF and A1-A2-A3. Finally, we further demonstrate that loss of these N-linked glycans does not enhance clearance in VWF in the presence of a structurally constrained A2 domain. Collectively, these novel findings support the hypothesis that conformation of the VWF A domains plays a critical role in modulating macrophage-mediated clearance of VWF in vivo.

To clear or to fear: An innate perspective on factor VIII immunity

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FVIII inhibitor development involves a combination of innate immune modulators.

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Clearance and immunity is influenced at 3 levels: the protein, cell, and location.

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Cells associated with FVIII half-life may influence the immune response against FVIII.

The enigma that is factor VIII immunogenicity remains ever pertinent in the treatment of hemophilia A. Development of neutralizing antibodies against the therapeutic protein in 25–30% of patients likely depends on the appropriate activation of the innate immune response shortly following antigen encounter. Our understanding of this important immunological synapse remains ill-defined. In this review, we examine the three distinct factors contributing to the fate of factor VIII almost immediately after infusion: the characteristics of the protein, the cell, and the microenvironment. We propose a continuum between clearance and antigen presentation that facilitates removal of FVIII from circulation leading to either tolerance or immunity.

Genetic variations in sites of affinity between FVIII and LRP1 are not associated with high FVIII levels in venous thromboembolism

Increased factor VIII (FVIII) levels are a prevalent and independent risk factor for venous thromboembolism (VTE). The low density lipoprotein receptor-related protein 1 (LRP1) has been associated with FVIII catabolism. After a median of 10 years of the first thrombotic episode, we evaluated FVIII activity levels in 75 patients with VTE and high FVIII levels and in 74 healthy controls. Subsequently, we evaluated the regions of F8 and LRP1 genes coding sites of affinity between these proteins, with the objective of determining genetic alterations associated with plasma FVIII levels. After a median time of 10 years after the VTE episode, FVIII levels were significantly higher in patients when compared to controls (158.6 IU/dL vs. 125.8 IU/dL; P ≤ 0.001]. Despite the fact that we found 14 genetic variations in F8 and LRP1 genes, no relationship was found between FVIII levels with these variations. We demonstrated a persistent increase of FVIII levels in patients with VTE, but in a much lower magnitude after 10 years when compared to 3-years after the episode. Moreover, we observed no relationship of genetic variations in the gene regions coding affinity sites between LRP1 and FVIII with FVIII levels.

Managing haemophilia for life: 4th Haemophilia Global Summit

The 4th Haemophilia Global Summit was held in Potsdam, Germany, in September 2013 and brought together an international faculty of haemophilia experts and delegates from multidisciplinary backgrounds. The programme was designed by an independent Scientific Steering Committee of haemophilia experts and explored global perspectives in haemophilia care, discussing practical approaches to the optimal management of haemophilia now and in the future. The topics outlined in this supplement were selected by the Scientific Steering Committee for their relevance and potential to influence haemophilia care globally. In this supplement from the meeting, Jan Astermark reviews current understanding of risk factors for the development of inhibitory antibodies and discusses whether this risk can be modulated and minimized. Factors key to the improvement of joint health in people with haemophilia are explored, with Carlo Martinoli and Víctor Jiménez-Yuste discussing the utility of ultrasound for the early detection of haemophilic arthropathy. Other aspects of care necessary for the prevention and management of joint disease in people with haemophilia are outlined by Thomas Hilberg and Sébastian Lobet, who highlight the therapeutic benefits of physiotherapy and sports therapy. Riitta Lassila and Carlo-Federico Perno describe current knowledge surrounding the risk of transmission of infectious agents via clotting factor concentrates. Finally, different types of extended half-life technology are evaluated by Mike Laffan, with a focus on the practicalities and challenges associated with these products.

Low-density lipoprotein receptor-related protein-1: role in the regulation of vascular integrity

Low-density lipoprotein receptor-related protein-1 (LRP1) is a large endocytic and signaling receptor that is widely expressed. In the liver, LRP1 plays an important role in regulating the plasma levels of blood coagulation factor VIII (fVIII) by mediating its uptake and subsequent degradation. fVIII is a key plasma protein that is deficient in hemophilia A and circulates in complex with von Willebrand factor. Because von Willebrand factor blocks binding of fVIII to LRP1, questions remain on the molecular mechanisms by which LRP1 removes fVIII from the circulation. LRP1 also regulates cell surface levels of tissue factor, a component of the extrinsic blood coagulation pathway. This occurs when tissue factor pathway inhibitor bridges the fVII/tissue factor complex to LRP1, resulting in rapid LRP1-mediated internalization and downregulation of coagulant activity. In the vasculature LRP1 also plays protective role from the development of aneurysms. Mice in which the lrp1 gene is selectively deleted in vascular smooth muscle cells develop a phenotype similar to the progression of aneurysm formation in human patient, revealing that these mice are ideal for investigating molecular mechanisms associated with aneurysm formation. Studies suggest that LRP1 protects against elastin fiber fragmentation by reducing excess protease activity in the vessel wall. These proteases include high-temperature requirement factor A1, matrix metalloproteinase 2, matrix metalloproteinase-9, and membrane associated type 1-matrix metalloproteinase. In addition, LRP1 regulates matrix deposition, in part, by modulating levels of connective tissue growth factor. Defining pathways modulated by LRP1 that lead to aneurysm formation and defining its role in thrombosis may allow for more effective intervention in patients.

KIF13B enhances the endocytosis of LRP1 by recruiting LRP1 to caveolae

The motor protein KIF13B has an unconventional role as a scaffold that recruits lipoprotein receptor–related protein 1 to caveolae, thereby enhancing its endocytosis.

Multifunctional low-density lipoprotein (LDL) receptor-related protein 1 (LRP1) recognizes and internalizes a large number of diverse ligands, including LDL and factor VIII. However, little is known about the regulation of LRP1 endocytosis. Here, we show that a microtubule-based motor protein, KIF13B, in an unexpected and unconventional function, enhances caveolin-dependent endocytosis of LRP1. KIF13B was highly expressed in the liver and was localized on the sinusoidal plasma membrane of hepatocytes. KIF13B knockout (KO) mice showed elevated levels of serum cholesterol and factor VIII, and KO MEFs showed decreased uptake of LDL. Exogenous KIF13B, initially localized on the plasma membrane with caveolae, was translocated to the vesicles in the cytoplasm with LRP1 and caveolin-1. KIF13B bound to hDLG1 and utrophin, which, in turn, bound to LRP1 and caveolae, respectively. These linkages were required for the KIF13B-enhanced endocytosis of LRP1. Thus, we propose that KIF13B, working as a scaffold, recruits LRP1 to caveolae via LRP1–hDLG1–KIF13B–utrophin–caveolae linkage and enhances the endocytosis of LRP1.

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

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

The regulation of ApoB metabolism by insulin

The leading cause of death in diabetic patients is cardiovascular disease. Apolipoprotein B (ApoB)-containing lipoprotein particles, which are secreted and cleared by the liver, are essential for the development of atherosclerosis. Insulin plays a key role in the regulation of ApoB. Insulin decreases ApoB secretion by promoting ApoB degradation in the hepatocyte. In parallel, insulin promotes clearance of circulating ApoB particles by the liver via the low-density lipoprotein receptor (LDLR), LDLR-related protein 1 (LRP1), and heparan sulfate proteoglycans (HSPGs). Consequently, the insulin-resistant state of type 2 diabetes (T2D) is associated with increased secretion and decreased clearance of ApoB. Here, we review the mechanisms by which insulin controls the secretion and uptake of ApoB in normal and diabetic livers.

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.

Clearance of von Willebrand factor

Quantitative deficiencies in von Willebrand factor (VWF) are associated with abnormal hemostasis that can manifest in bleeding or thrombotic complications. Consequently, many studies have endeavored to elucidate the mechanisms underlying the regulation of VWF plasma levels. This review focuses on the role of VWF clearance pathways. A summary of recent developments are provided, including results from genetic studies, the relationship between glycosylation and VWF clearance, the contribution of increased VWF clearance to the pathogenesis of von Willebrand disease and the identification of VWF clearance receptors. These different studies converge in their conclusion that VWF clearance is a complex phenomenon that involves multiple mechanisms. Deciphering how such different mechanisms coordinate their role in this process is but one of the remaining challenges. Nevertheless, a better insight into the complex clearance pathways of VWF may help us to better understand the clinical implications of aberrant clearance in the pathogenesis of von Willebrand disease and perhaps other disorders as well as aid in developing alternative therapeutic approaches.

LRP1/CD91 is up-regulated in monocytes from patients with haemophilia A: a single-centre analysis

The low-density lipoprotein receptor-related protein 1 (LRP1) is an ubiquitously expressed endocytic receptor that, among its several functions, is involved in the catabolism of coagulation factor VIII (FVIII) and in the regulation of its plasma concentrations. Although LRP1/CD91 polymorphisms have been associated with increased FVIII levels and a consequent thrombotic risk, no data are available on LRP1/CD91 expression in patients with inherited FVIII deficiency. With the aim of elucidating this issue, 45 consecutive patients with haemophilia A (HA) (18 severe, 5 moderate and 22 mild HA) were enrolled in this cross-sectional, single-centre survey. The LRP1/CD91 mean fluorescence intensity (MFI) in monocytes from HA patients was significantly higher than that detected in 90 healthy blood donors (105 vs. 67, P < 0.001). This over-expression was independent of hepatitis C virus infection status and varied according to the severity of the haemophilia, being higher in patients with more severe FVIII deficiency. In conclusion, our study documents for the first time that LRP1/CD91 is over-expressed on monocytes from HA patients, with the intensity of expression varying according to the severity of the FVIII deficiency. Further studies are needed to assess the clinical implications of these findings.

A novel B-domain O-glycoPEGylated FVIII (N8-GP) demonstrates full efficacy and prolonged effect in hemophilic mice models

Frequent infusions of intravenous factor VIII (FVIII) are required to prevent bleeding associated with hemophilia A. To reduce the treatment burden, recombinant FVIII with a longer half-life was developed without changing the protein structure. FVIII-polyethylene glycol (PEG) conjugates were prepared using an enzymatic process coupling PEG (ranging from 10 to 80 kDa) selectively to a unique O-linked glycan in the FVIII B-domain. Binding to von Willebrand factor (VWF) was maintained for all conjugates. Upon cleavage by thrombin, the B-domain and the associated PEG were released, generating activated FVIII (FVIIIa) with the same primary structure and specific activity as native FVIIIa. In both FVIII- and VWF-deficient mice, the half-life was found to increase with the size of PEG. In vivo potency and efficacy of FVIII conjugated with a 40-kDa PEG (N8-GP) and unmodified FVIII were not different. N8-GP had a longer duration of effect in FVIII-deficient mouse models, approximately a twofold prolonged half-life in mice, rabbits, and cynomolgus monkeys; however, the prolongation was less pronounced in rats. Binding capacity of N8-GP on human monocyte-derived dendritic cells was reduced compared with unmodified FVIII, resulting in several-fold reduced cellular uptake. In conclusion, N8-GP has the potential to offer efficacious prevention and treatment of bleeds in hemophilia A at reduced dosing frequency.

Low-density lipoprotein receptor-related protein 1: a physiological Aβ homeostatic mechanism with multiple therapeutic opportunities

Low-density lipoprotein receptor-related protein-1 (LRP1) is the main cell surface receptor involved in brain and systemic clearance of the Alzheimer's disease (AD) toxin amyloid-beta (Aβ). In plasma, a soluble form of LRP1 (sLRP1) is the major transport protein for peripheral Aβ. LRP1 in brain endothelium and mural cells mediates Aβ efflux from brain by providing a transport mechanism for Aβ across the blood-brain barrier (BBB). sLRP1 maintains a plasma 'sink' activity for Aβ through binding of peripheral Aβ which in turn inhibits re-entry of free plasma Aβ into the brain. LRP1 in the liver mediates systemic clearance of Aβ. In AD, LRP1 expression at the BBB is reduced and Aβ binding to circulating sLRP1 is compromised by oxidation. Cell surface LRP1 and circulating sLRP1 represent druggable targets which can be therapeutically modified to restore the physiological mechanisms of brain Aβ homeostasis. In this review, we discuss how increasing LRP1 expression at the BBB and liver with lifestyle changes, statins, plant-based active principles and/or gene therapy on one hand, and how replacing dysfunctional plasma sLRP1 on the other regulate Aβ clearance from brain ultimately controlling the onset and/or progression of AD.

Shear stress is required for the endocytic uptake of the factor VIII-von Willebrand factor complex by macrophages

BACKGROUND

Low-density lipoprotein (LDL) receptor family members contribute to the cellular uptake of factor VIII. How von Willebrand factor fits into this endocytic pathway has remained poorly understood.

OBJECTIVES

It has been suggested that macrophages contribute to the clearance of the factor VIII (FVIII)-von Willebrand factor (VWF) complex. We now assessed the mechanisms of uptake employing human monocyte-derived macrophages.

METHODS

A confocal microscopy study was employed to study the uptake by monocyte-derived macrophages of a functional green fluorescent FVIII-GFP derivative in the presence and absence of VWF.

RESULTS

The results revealed that FVIII-GFP is internalized by macrophages. We found that FVIII-GFP co-localizes with LDL receptor-related protein (LRP), and that the LRP antagonist Receptor Associated Protein (RAP) blocks the uptake of FVIII-GFP. However, FVIII-GFP was not detected in the macrophages in the presence of VWF, suggesting that the FVIII-VWF complex is not internalized by these cells at all. Apart from static conditions, we also investigated the effect of shear stress on the uptake of FVIII-GFP in presence of VWF. Immunofluorescence studies demonstrated that VWF does not block endocytosis of FVIII-GFP under flow conditions. Moreover, VWF itself was also internalized by the macrophages. Strikingly, in the presence of RAP, endocytosis of FVIII-GFP and VWF was inhibited.

CONCLUSION

The results show that shear stress is required for macrophages to internalize both constituents of the FVIII-VWF complex.

Factor VIII and von Willebrand factor are ligands for the carbohydrate-receptor Siglec-5

BACKGROUND

Factor VIII (FVIII) and von Willebrand factor (VWF) circulate in plasma in a tight non-covalent complex, being critical to hemostasis. Although structurally unrelated, both share the presence of sialylated glycan-structures, making them potential ligands for sialic-acid-binding-immunoglobulin-like-lectins (Siglecs).

DESIGN AND METHODS

We explored the potential interaction between FVIII/VWF and Siglec-5, a receptor expressed in macrophages using various experimental approaches, including binding experiments with purified proteins and cell-binding studies with Siglec-5 expressing cells. Finally, Siglec-5 was overexpressed in mice via hydrodynamic gene transfer.

RESULTS

In different systems using purified proteins, saturable, dose-dependent and reversible interactions between a soluble Siglec-5 fragment and both hemostatic proteins were found. Sialidase treatment of VWF resulted in a complete lack of Siglec-5 binding. In contrast, sialidase treatment left interactions between FVIII and Siglec-5 unaffected. FVIII and VWF also bound to cellsurface exposed Siglec-5, as was visualized by classical immunostaining as well as by Duolinkproximity ligation assays. Co-localization of FVIII and VWF with early endosomal markers further suggested that binding to Siglec-5 is followed by endocytosis of the proteins. Finally, overexpression of human Siglec-5 in murine hepatocytes following hydrodynamic gene transfer resulted in a significant decrease in plasma levels of FVIII and VWF in these mice.

CONCLUSIONS

Our data indicate that FVIII and VWF may act as a ligand for Siglec-5, and that Siglec-5 may contribute to the regulation of plasma levels of the FVIII/VWF complex.

Elevated factor VIII levels and risk of venous thrombosis

Modern thrombophilia testing fails to identify any underlying prothrombotic tendency in a significant number of patients presenting with objectively confirmed venous thromboemboembolism (VTE). This observation has led to a search for other novel inherited or acquired human thrombophilias. Although a number of putative mechanisms have been described, the evidence behind many of these candidates remains weak. In contrast, an increasing body of work supports the hypothesis that increased plasma factor VIII (FVIII) levels may be important in this context. An association between elevated plasma FVIII levels and VTE was first described in the Leiden Thrombophilia Study (LETS). Subsequently, these conclusions have been supported by an increasing number of independent case-control studies. Cumulatively, these studies have clearly demonstrated that high FVIII levels constitute a prevalent, dose-dependent risk factor for VTE. Furthermore, more recent studies have shown that the risk of recurrent venous thrombosis is also significantly increased in patients with high FVIII levels. In this review, we present the evidence supporting the hypothesis that elevated FVIII levels constitute a clinically important thrombophilia. In addition, we examine the biological mechanisms that may underlie persistently elevated FVIII levels, and the pathways through which high FVIII may serve to increase thrombotic risk.

Macrophage LRP1 contributes to the clearance of von Willebrand factor

The relationship between low-density lipoprotein receptor-related protein-1 (LRP1) and von Willebrand factor (VWF) has remained elusive for years. Indeed, despite a reported absence of interaction between both proteins, liver-specific deletion of LRP1 results in increased VWF levels. To investigate this discrepancy, we used mice with a macrophage-specific deficiency of LRP1 (macLRP1(-)) because we previously found that macrophages dominate VWF clearance. Basal VWF levels were increased in macLRP1(-) mice compared with control mice (1.6 ± 0.4 vs 1.0 ± 0.4 U/mL). Clearance experiments revealed that half-life of human VWF was significantly increased in macLRP1(-) mice. Ubiquitous blocking of LRP1 or additional lipoprotein receptors by overexpressing receptor-associated protein in macLRP1(-) mice did not result in further rise of VWF levels (0.1 ± 0.2 U/mL), in contrast to macLRP1(+) mice (rise in VWF, 0.8 ± 0.4 U/mL). This points to macLRP1 being the only lipoprotein receptor regulating VWF levels. When testing the mechanism(s) involved, we observed that VWF-coated beads adhered efficiently to LRP1 but only when exposed to shear forces exceeding 2.5 dyne/cm(2), implying the existence of shear stress-dependent interactions. Furthermore, a mechanism involving β2-integrins that binds both VWF and LRP1 also is implicated because inhibition of β2-integrins led to increased VWF levels in control (rise, 0.19 ± 0.16 U/mL) but not in macLRP1(-) mice (0.08 ± 0.15 U/mL).

Engineering Factor Viii for Hemophilia Gene Therapy

Current treatment of hemophilia A by intravenous infusion of factor VIII (fVIII) concentrates is very costly and has a potential adverse effect of developing inhibitors. Gene therapy, on the other hand, can potentially overcome these limitations associated with fVIII replacement therapy. Although hemophilia B gene therapy has achieved promising outcomes in human clinical trials, hemophilia A gene therapy lags far behind. Compared to factor IX, fVIII is a large protein which is difficult to express at sustaining therapeutic levels when delivered by either viral or non-viral vectors. To improve fVIII gene delivery, numerous strategies have been exploited to engineer the fVIII molecule and overcome the hurdles preventing long term and high level expression. Here we reviewed these strategies, and discussed their pros and cons in human gene therapy of hemophilia A.

Procoagulant activity induced by vascular injury determines contribution of elevated factor VIII to thrombosis and thrombus stability in mice

Studies have correlated elevated plasma factor VIII (FVIII) with thrombosis; however, it is unclear whether elevated FVIII is a proinflammatory biomarker, causative agent, or both. We raised FVIII levels in mice and measured the time to vessel occlusion (TTO) after ferric chloride-induced injury. Compared with control (saline-infused) mice, elevated FVIII had no effect after longer (3-minute) carotid artery injury, but it shortened the TTO after shorter (2-minute) injury (P < .008). After injury, circulating thrombin-antithrombin (TAT) complexes were lower after short versus long injury (P < .04), suggesting short treatment produced less coagulation activation. TAT levels in FVIII-infused mice were higher than in controls after short, but not longer, injury. Accordingly, elevated FVIII had no effect on in vitro thrombin generation or platelet aggregation triggered by high tissue factor, but it increased thrombin generation rate and peak (2.4- and 1.5-fold, respectively), and it accelerated platelet aggregation (up to 1.6-fold) when initiated by low tissue factor. Compared with control mice, elevated FVIII stabilized thrombi (fewer emboli) after short injury, but it had no effect after longer injury. TTO and emboli correlated with TATs. These results demonstrate dependence of FVIII activity on extent of vascular injury. We propose elevated plasma FVIII is an etiologic, prothrombotic agent after moderate but not extensive vascular damage.

Low-density lipoprotein receptor-related protein 1: new functions for an old molecule

The low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional cell surface receptor member of the low-density lipoprotein (LDL)-receptor family. As LRP1 plays an important role in endocytosis and regulation of signalling pathways, it is implicated in a number of physiologic processes, including the regulation of lipid metabolism, the proliferation of vascular smooth muscle cells and in neuro-development. More recently, LRP1 has been implicated in the catabolism of factor VIII and regulation of its plasma concentrations. The pathophysiology of the role of LRP1 in hemostasis will be summarized in this review.

Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family

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