Crystal structure of CD59: implications for molecular recognition of the complement proteins C8 and C9 in the membrane-attack complex

Bioinformatics-Guided Identification and Quantification of Biomarkers of Crotalus atrox Envenoming and its Neutralization by Antivenom

Quantitative mass spectrometry-based proteomics of extracellular vesicles (EVs) provides systems-level exploration for the analysis of snakebite envenoming (SBE) as the venom progresses, causing injuries such as hemorrhage, trauma, and death. Predicting EV biomarkers has become an essential aspect of this process, offering an avenue to explore the specific pathophysiological changes that occur after envenoming. As new omics approaches emerge to advance our understanding of SBE, further bioinformatics analyses are warranted to incorporate the use of antivenom or other therapeutics to observe their global impact on various biological processes. Herein, we used an in vivo BALB/c mouse model and proteomics approach to analyze the physiological impacts of SBE and antivenom neutralization in intact animals; this was followed by bioinformatics methods to predict potential EV biomarkers. Groups of mice (n=5) were intramuscularly injected with Saline or Crotalus atrox venom. After 30 minutes, the mice received saline or antivenom (ANTIVIPMYN®) by intravenous injection. After 24 hours, blood was collected to extract the plasma to analyze the EV content and determine the exposome of C. atrox venom as well as the neutralizing capabilities of the antivenom. The predicted biomarkers consistently and significantly sensitive to antivenom treatment are Slc25a4, Rps8, Akr1c6, Naa10, Sult1d1, Hadha, Mbl2, Zc3hav, Tgfb1, Prxl2a, Coro1c, Tnni1, Ryr3, C8b, Mycbp, and Cfhr4. These biomarkers pointed towards specific physiological alterations, causing significant metabolic changes in mitochondrial homeostasis, lipid metabolism, immunity, and cytolysis, indicating hallmarks of traumatic injury. Here, we present a more comprehensive view of murine plasma EV proteome and further identify significant changes in abundance for potential biomarkers associated with antivenom treatment. The predicted biomarkers have the potential to enhance current diagnostic tools for snakebite management, thereby contributing significantly to the evolution of treatment strategies in the diagnosis and prognosis of SBE.

Lipoprotein Lipase: Structure, Function, and Genetic Variation

Biallelic rare pathogenic loss-of-function (LOF) variants in lipoprotein lipase (LPL) cause familial chylomicronemia syndrome (FCS). Heterozygosity for these same variants is associated with a highly variable plasma triglyceride (TG) phenotype ranging from normal to severe hypertriglyceridemia (HTG), with longitudinal variation in phenotype severity seen often in a given carrier. Here, we provide an updated overview of genetic variation in LPL in the context of HTG, with a focus on disease-causing and/or disease-associated variants. We provide a curated list of 300 disease-causing variants discovered in LPL, as well as an exon-by-exon breakdown of the LPL gene and protein, highlighting the impact of variants and the various functional residues of domains of the LPL protein. We also provide a curated list of variants of unknown or uncertain significance, many of which may be upgraded to pathogenic/likely pathogenic classification should an additional case and/or segregation data be reported. Finally, we also review the association between benign/likely benign variants in LPL, many of which are common polymorphisms, and the TG phenotype.

Association of Blood Group Antigen CD59 with Disease

In 2014, the membrane-bound protein CD59 became a blood group antigen. CD59 has been known for decades as an inhibitor of the complement system, located on erythrocytes and on many other cell types. In paroxysmal nocturnal haemoglobinuria (PNH), a stem cell clone with acquired deficiency to express GPI-anchored molecules, including the complement inhibitor CD59, causes severe and life-threatening disease. The lack of CD59, which is the only membrane-bound inhibitor of the membrane attack complex, contributes a major part of the intravascular haemolysis observed in PNH patients. This crucial effect of CD59 in PNH disease prompted studies to investigate its role in other diseases. In this review, the role of CD59 in inflammation, rheumatic disease, and age-related macular degeneration is investigated. Further, the pivotal role of CD59 in PNH and congenital CD59 deficiency is reviewed.

Fish complement C8 evolution, functional network analyses, and the theoretical interaction between C8 alpha chain and CD59

Complement C8, as a main component of the membrane attack complex, has only been identified in vertebrates. C8 comprises three subunits encoded by individual genes: C8a (alpha chain), C8b (beta chain), and C8g (gamma chain). However, in fish, there have been limited studies on the evolutionary history and systematic function of C8. In the present study, phylogenetic analysis indicated the complete divergence of C8 genes in different fish species. Codon usage bias analysis revealed the evolutionary complexity of C8 genes. Selective pressure analysis found that C8 genes have been affected by negative selection during evolution. Sequence alignment identified the sites that are under selective pressure. The systematic functions of C8 were revealed by gene co-expression and protein-protein interaction (PPI) network analyses. Notably, gene ontology enrichment analysis suggested that C8 proteins in zebrafish function mainly in the neuroendocrine system. Protein structural comparisons showed that putative functional residues and domains were conserved between the C8 subunits of human and grass carp. A preliminary study on the theoretical interaction between C8a and CD59 was performed according to the simulated protein stereo structure. The first functionally-related site was absent in the simulated conformation of the grass carp (Ctenopharyngodon idella) C8a-CD59 protein complex. We speculated that Tyr63 is involved in the functional loss of CD59 binding. The docking of CD59 to four potential sites (Met390, Ser391, Leu392, and Val405) in grass carp C8a was analyzed. The results of the present study provide a deeper understanding of the evolution and function of fish complement C8.

Cholesterol-Dependent Cytolysins Produced by Vaginal Bacteria: Certainties and Controversies

Bacterial vaginosis (BV) is a vaginal anaerobic dysbiosis that affects women of reproductive age worldwide. BV is microbiologically characterized by the depletion of vaginal lactobacilli and the overgrowth of anaerobic bacterial species. Accumulated evidence suggests that Gardnerella spp. have a pivotal role among BV-associated bacteria in the initiation and development of BV. However, Gardnerella spp. often colonize healthy women. Lactobacillus iners is considered as a prevalent constituent of healthy vaginal microbiota, and is abundant in BV. Gardnerella spp. and L. iners secrete the toxins vaginolysin (VLY) and inerolysin (INY), which have structural and activity features attributed to cholesterol-dependent cytolysins (CDCs). CDCs are produced by many pathogenic bacteria as virulence factors that participate in various stages of disease progression by forming lytic and non-lytic pores in cell membranes or via pore-independent pathways. VLY is expressed in the majority of Gardnerella spp. isolates; less is known about the prevalence of the gene that encodes INY. INY is a classical CDC; membrane cholesterol acts a receptor for INY. VLY uses human CD59 as its receptor, although cholesterol remains indispensable for VLY pore-forming activity. INY-induced damage of artificial membranes is directly dependent on cholesterol concentration in the bilayer, whereas VLY-induced damage occurs with high levels of membrane cholesterol (>40 mol%). VLY primarily forms membrane-embedded complete rings in the synthetic bilayer, whereas INY forms arciform structures with smaller pore sizes. VLY activity is high at elevated pH, which is characteristic of BV, whereas INY activity is high at more acidic pH, which is specific for a healthy vagina. Increased VLY levels in vaginal mucosa in vivo were associated with clinical indicators of BV. However, experimental evidence is lacking for the specific roles of VLY and INY in BV. The interplay between vaginal bacterial species affects the expression of the gene encoding VLY, thereby modulating the virulence of Gardnerella spp. This review discusses the current evidence for VLY and INY cytolysins, including their structures and activities, factors affecting their expression, and their potential impacts on the progression of anaerobic dysbiosis.

Evolution and Medical Significance of LU Domain-Containing Proteins

Proteins containing Ly6/uPAR (LU) domains exhibit very diverse biological functions and have broad taxonomic distributions in eukaryotes. In general, they adopt a characteristic three-fingered folding topology with three long loops projecting from a disulfide-rich globular core. The majority of the members of this protein domain family contain only a single LU domain, which can be secreted, glycolipid anchored, or constitute the extracellular ligand binding domain of type-I membrane proteins. Nonetheless, a few proteins contain multiple LU domains, for example, the urokinase receptor uPAR, C4.4A, and Haldisin. In the current review, we will discuss evolutionary aspects of this protein domain family with special emphasis on variations in their consensus disulfide bond patterns. Furthermore, we will present selected cases where missense mutations in LU domain-containing proteins leads to dysfunctional proteins that are causally linked to genesis of human disease.

Lynx Prototoxins: Roles of Endogenous Mammalian Neurotoxin-Like Proteins in Modulating Nicotinic Acetylcholine Receptor Function to Influence Complex Biological Processes

The cholinergic system modulates many biological functions, due to the widespread distribution of cholinergic neuronal terminals, and the diffuse release of its neurotransmitter, acetylcholine. Several layers of regulation help to refine and control the scope of this excitatory neurotransmitter system. One such regulatory mechanism is imparted through endogenous toxin-like proteins, prototoxins, which largely control the function of nicotinic receptors of the cholinergic system. Prototoxins and neurotoxins share the distinct three finger toxin fold, highly effective as a receptor binding protein, and the former are expressed in the mammalian brain, immune system, epithelium, etc. Prototoxins and elapid snake neurotoxins appear to be related through gene duplication and divergence from a common ancestral gene. Protein modulators can provide a graded response of the cholinergic system, and within the brain, stabilize neural circuitry through direct interaction with nicotinic receptors. Understanding the roles of each prototoxin (e.g., lynx1, lynx2/lypd1, PSCA, SLURP1, SLURP2, Lypd6, lypd6b, lypdg6e, PATE-M, PATE-B, etc.), their binding specificity and unique expression profile, has the potential to uncover many fascinating cholinergic-dependent mechanisms in the brain. Each family member can provide a spatially restricted level of control over nAChR function based on its expression in the brain. Due to the difficulty in the pharmacological targeting of nicotinic receptors in the brain as a result of widespread expression patterns and similarities in receptor sequences, unique interfaces between prototoxin and nicotinic receptor could provide more specific targeting than nicotinic receptors alone. As such, this family is intriguing from a long-term therapeutic perspective.

Structure of the lipoprotein lipase-GPIHBP1 complex that mediates plasma triglyceride hydrolysis

Lipoprotein lipase (LPL) is responsible for the intravascular processing of triglyceride-rich lipoproteins. The LPL within capillaries is bound to GPIHBP1, an endothelial cell protein with a three-fingered LU domain and an N-terminal intrinsically disordered acidic domain. Loss-of-function mutations in LPL or GPIHBP1 cause severe hypertriglyceridemia (chylomicronemia), but structures for LPL and GPIHBP1 have remained elusive. Inspired by our recent discovery that GPIHBP1's acidic domain preserves LPL structure and activity, we crystallized an LPL-GPIHBP1 complex and solved its structure. GPIHBP1's LU domain binds to LPL's C-terminal domain, largely by hydrophobic interactions. Analysis of electrostatic surfaces revealed that LPL contains a large basic patch spanning its N- and C-terminal domains. GPIHBP1's acidic domain was not defined in the electron density map but was positioned to interact with LPL's large basic patch, providing a likely explanation for how GPIHBP1 stabilizes LPL. The LPL-GPIHBP1 structure provides insights into mutations causing chylomicronemia.

Complements are involved in alcoholic fatty liver disease, hepatitis and fibrosis

The complement system is a key component of the body’s immune system. When abnormally activated, this system can induce inflammation and damage to normal tissues and participate in the development and progression of a variety of diseases. In the past, many scholars believed that alcoholic liver disease (ALD) is induced by the stress of ethanol on liver cells, including oxidative stress and dysfunction of mitochondria and protease bodies, causing hepatocyte injury and apoptosis. Recent studies have shown that complement activation is also involved in the genesis and development of ALD. This review focuses on the roles of complement activation in ALD and of therapeutic intervention in complement-activation pathways. We intend to provide new ideas on the diagnosis and treatment of ALD.

Molecular pathogenesis of human CD59 deficiency

Objective

To characterize all 4 mutations described for CD59 congenital deficiency.

Methods

The 4 mutations, p.Cys64Tyr, p.Asp24Val, p.Asp24Valfs*, and p.Ala16Alafs*, were described in 13 individuals with CD59 malfunction. All 13 presented with recurrent Guillain-Barré syndrome or chronic inflammatory demyelinating polyneuropathy, recurrent strokes, and chronic hemolysis. Here, we track the molecular consequences of the 4 mutations and their effects on CD59 expression, localization, glycosylation, degradation, secretion, and function. Mutants were cloned and inserted into plasmids to analyze their expression, localization, and functionality.

Results

Immunolabeling of myc-tagged wild-type (WT) and mutant CD59 proteins revealed cell surface expression of p.Cys64Tyr and p.Asp24Val detected with the myc antibody, but no labeling by anti-CD59 antibodies. In contrast, frameshift mutants p.Asp24Valfs* and p.Ala16Alafs* were detected only intracellularly and did not reach the cell surface. Western blot analysis showed normal glycosylation but mutant-specific secretion patterns. All mutants significantly increased MAC-dependent cell lysis compared with WT. In contrast to CD59 knockout mice previously used to characterize phenotypic effects of CD59 perturbation, all 4 hCD59 mutations generate CD59 proteins that are expressed and may function intracellularly (4) or on the cell membrane (2). None of the 4 CD59 mutants are detected by known anti-CD59 antibodies, including the 2 variants present on the cell membrane. None of the 4 inhibits membrane attack complex (MAC) formation.

Conclusions

All 4 mutants generate nonfunctional CD59, 2 are expressed as cell surface proteins that may function in non-MAC-related interactions and 2 are expressed only intracellularly. Distinct secretion of soluble CD59 may have also a role in disease pathogenesis.

Organization, evolution and functions of the human and mouse Ly6/uPAR family genes

Members of the lymphocyte antigen-6 (Ly6)/urokinase-type plasminogen activator receptor (uPAR) superfamily of proteins are cysteine-rich proteins characterized by a distinct disulfide bridge pattern that creates the three-finger Ly6/uPAR (LU) domain. Although the Ly6/uPAR family proteins share a common structure, their expression patterns and functions vary. To date, 35 human and 61 mouse Ly6/uPAR family members have been identified. Based on their subcellular localization, these proteins are further classified as GPI-anchored on the cell membrane, or secreted. The genes encoding Ly6/uPAR family proteins are conserved across different species and are clustered in syntenic regions on human chromosomes 8, 19, 6 and 11, and mouse Chromosomes 15, 7, 17, and 9, respectively. Here, we review the human and mouse Ly6/uPAR family gene and protein structure and genomic organization, expression, functions, and evolution, and introduce new names for novel family members.

Congenital CD59 Deficiency

The severe clinical symptoms of inherited CD59 deficiency confirm the importance of CD59 as essential complement regulatory protein for protection of cells against complement attack, in particular protection of hematopoietic cells and human neuronal tissue. Targeted complement inhibition might become a treatment option as suggested by a case report. The easy diagnostic approach by flow cytometry and the advent of a new treatment option should increase the awareness of this rare differential diagnosis and lead to further studies on their pathophysiology.

A new blood group antigen is defined by anti-CD59, detected in a CD59-deficient patient

BACKGROUND

CD59 is a cell surface glycoprotein of approximately 20 kDa limiting the lytic activity of the terminal complement complex C5b-9. Although CD59 is known as a red blood cell (RBC) antigen defined by monoclonal antibodies, it so far has not been identified as a blood group antigen, since the description of a human alloantibody was missing. In this study we show the presence of an anti-CD59 in a patient affected by a homozygous CD59 deficiency.

STUDY DESIGN AND METHODS

RBC CD59 and CD55 were determined by flow cytometry or by the column agglutination technique using monoclonal antisera. Commercially available His-tagged recombinant soluble CD59 protein was used to inhibit anti-CD59.

RESULTS

Seven cases of an isolated CD59 deficiency due to three distinct null alleles of the CD59 gene have been published so far. Recently we described the CD59-null allele c.146delA in a young child of heterozygous parents. Her plasma contained an alloantibody directed against the high-prevalence RBC antigen CD59. The antibody specificity was identified using soluble recombinant human CD59 protein, which blocked the reactivity of the patient's antibody and of monoclonal anti-CD59 but not of monoclonal anti-CD55. In addition, RBC alloantibodies such as anti-K, anti-C, anti-c, or anti-Fy(a) remained unaffected. Therefore, inhibition by recombinant CD59 is a useful diagnostic tool to detect alloantibodies in the presence of anti-CD59.

CONCLUSION

This is the first demonstration of a human anti-CD59 alloantibody, which defines CD59 as an RBC blood group antigen. CD59 represents a candidate for a new blood group system.

Initiation and regulation of complement during hemolytic transfusion reactions

Hemolytic transfusion reactions represent one of the most common causes of transfusion-related mortality. Although many factors influence hemolytic transfusion reactions, complement activation represents one of the most common features associated with fatality. In this paper we will focus on the role of complement in initiating and regulating hemolytic transfusion reactions and will discuss potential strategies aimed at mitigating or favorably modulating complement during incompatible red blood cell transfusions.

Deletion of GPIHBP1 causing severe chylomicronemia

Lipoprotein lipase (LPL) is a hydrolase that cleaves circulating triglycerides to release fatty acids to the surrounding tissues. The enzyme is synthesized in parenchymal cells and is transported to its site of action on the capillary endothelium by glycophosphatidylinositol (GPI)-anchored high-density lipoprotein-binding protein 1 (GPIHBP1). Inactivating mutations in LPL; in its cofactor, apolipoprotein (Apo) C2; or in GPIHBP1 cause severe hypertriglyceridemia. Here we describe an individual with complete deficiency of GPIHBP1. The proband was an Asian Indian boy who had severe chylomicronemia at 2 months of age. Array-based copy-number analysis of his genomic DNA revealed homozygosity for a 17.5-kb deletion that included GPIHBP1. A 44-year-old aunt with a history of hypertriglyceridemia and pancreatitis was also homozygous for the deletion. A bolus of intravenously administered heparin caused a rapid increase in circulating LPL and decreased plasma triglyceride levels in control individuals but not in two GPIHBP1-deficient patients. Thus, short-term treatment with heparin failed to attenuate the hypertriglyceridemia in patients with GPIHBP1 deficiency. The increasing resolution of copy number microarrays and their widespread adoption for routine cytogenetic analysis is likely to reveal a greater role for submicroscopic deletions in Mendelian conditions. We describe the first neonate with complete GPIHBP1 deficiency due to homozygosity for a deletion of GPIHBP1.

GPIHBP1, an endothelial cell transporter for lipoprotein lipase

Interest in lipolysis and the metabolism of triglyceride-rich lipoproteins was recently reignited by the discovery of severe hypertriglyceridemia (chylomicronemia) in glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1)-deficient mice. GPIHBP1 is expressed exclusively in capillary endothelial cells and binds lipoprotein lipase (LPL) avidly. These findings prompted speculation that GPIHBP1 serves as a binding site for LPL in the capillary lumen, creating "a platform for lipolysis." More recent studies have identified a second and more intriguing role for GPIHBP1-picking up LPL in the subendothelial spaces and transporting it across endothelial cells to the capillary lumen. Here, we review the studies that revealed that GPIHBP1 is the LPL transporter and discuss which amino acid sequences are required for GPIHBP1-LPL interactions. We also discuss the human genetics of LPL transport, focusing on cases of chylomicronemia caused by GPIHBP1 mutations that abolish GPIHBP1's ability to bind LPL, and LPL mutations that prevent LPL binding to GPIHBP1.

The CD59 family member Leaky/Coiled is required for the establishment of the blood-brain barrier in Drosophila

The blood-brain barrier of Drosophila is established by the subperineurial glial cells that encase the CNS and PNS. The subperineurial glial cells are thin, highly interdigitated cells with epithelial character. The establishment of extensive septate junctions between these cells is crucial for the prevention of uncontrolled paracellular leakage of ions and solutes from the hemolymph into the nervous system. In the absence of septate junctions, macromolecules such as fluorescently labeled dextran can easily cross the blood-brain barrier. To identify additional components of the blood-brain barrier, we followed a genetic approach and injected Texas-Red-conjugated dextran into the hemolymph of embryos homozygous for chromosomal deficiencies. In this way, we identified the 153-aa-large protein Coiled, a new member of the Ly6 (leukocyte antigen 6) family, as being crucially required for septate junction formation and blood-brain barrier integrity. In coiled mutants, the normal distribution of septate junction markers such as NeurexinIV, Coracle, or Discs large is disturbed. EM analyses demonstrated that Coiled is required for the formation of septate junctions. We further show that Coiled is expressed by the subsperineurial glial cells in which it is anchored to the cell membrane via a glycosylphosphatidylinositol anchor and mediates adhesive properties. Clonal rescue studies indicate that the presence of Coiled is required symmetrically on both cells engaged in septate junction formation.

Application of a novel inhibitor of human CD59 for the enhancement of complement-dependent cytolysis on cancer cells

Many monoclonal antibodies (mAbs) have been extensively used in the clinic, such as rituximab to treat lymphoma. However, resistance and non-responsiveness to mAb treatment have been challenging for this line of therapy. Complement is one of the main mediators of antibody-based cancer therapy via the complement-dependent cytolysis (CDC) effect. CD59 plays a critical role in resistance to mAbs through the CDC effect. In this paper, we attempted to investigate whether the novel CD59 inhibitor, recombinant ILYd4, was effective in enhancing the rituximab-mediated CDC effect on rituximab-sensitive RL-7 lymphoma cells and rituximab-induced resistant RR51.2 cells. Meanwhile, the CDC effects, which were mediated by rituximab and anti-CD24 mAb, on the refractory multiple myeloma (MM) cell line ARH-77 and the solid tumor osteosarcoma cell line Saos-2, were respectively investigated. We found that rILYd4 rendered the refractory cells sensitive to the mAb-mediated CDC effect and that rILYd4 exhibited a synergistic effect with the mAb that resulted in tumor cells lysis. This effect on tumor cell lysis was apparent on both hematological tumors and solid tumors. Therefore, rILYd4 may serve as an adjuvant for mAb mediated-tumor immunotherapy.

GPIHBP1 and the processing of triglyceride-rich lipoproteins

GPIHBP1 is a new addition to a group of proteins required for the lipolysis of triglyceride-rich lipoproteins. GPIHBP1 contains an acidic domain and an Ly6 domain with ten cysteines. GPIHBP1 binds lipoprotein lipase (LPL) avidly and likely tethers LPL to the luminal surface of capillaries.Inactivation of Gpihbp1 in mice is associated with milky plasma and severe chylomicronemia, even on a low-fat chow diet. Recently, four missense mutations in GPIHBP1 were identified in humans with severe chylomicronemia (C65Y, C65S, C68G, and Q115P). All four mutations involve highly conserved residues within GPIHBP1's Ly6 domain.This review will provide an update on GPIHBP1's role in the processing of chylomicrons and the pathogenesis of chylomicronemia.

The good and evil of complement activation in HIV-1 infection

The complement system, a key component of innate immunity, is a first-line defender against foreign pathogens such as HIV-1. The role of the complement system in HIV-1 pathogenesis appears to be multifaceted. Although the complement system plays critical roles in clearing and neutralizing HIV-1 virions, it also represents a critical factor for the spread and maintenance of the virus in the infected host. In addition, complement regulators such as human CD59 present in the envelope of HIV-1 prevent complement-mediated lysis of HIV-1. Some novel approaches are proposed to combat HIV-1 infection through the enhancement of antibody-dependent complement activity against HIV-1. In this paper, we will review these diverse roles of complement in HIV-1 infection.

Chylomicronemia with low postheparin lipoprotein lipase levels in the setting of GPIHBP1 defects

BACKGROUND

Recent studies in mice have established that an endothelial cell protein, glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), is essential for the lipolytic processing of triglyceride-rich lipoproteins.

METHODS AND RESULTS

We report the discovery of a homozygous missense mutation in GPIHBP1 in a young boy with severe chylomicronemia. The mutation, p.C65Y, replaces a conserved cysteine in the GPIHBP1 lymphocyte antigen 6 domain with a tyrosine and is predicted to perturb protein structure by interfering with the formation of a disulfide bond. Studies with transfected Chinese hamster ovary cells showed that GPIHBP1-C65Y reaches the cell surface but has lost the ability to bind lipoprotein lipase (LPL). When the GPIHBP1-C65Y homozygote was given an intravenous bolus of heparin, only trace amounts of LPL entered the plasma. We also observed very low levels of LPL in the postheparin plasma of a subject with chylomicronemia who was homozygous for a different GPIHBP1 mutation (p.Q115P). When the GPIHBP1-Q115P homozygote was given a 6-hour infusion of heparin, a significant amount of LPL appeared in the plasma, resulting in a fall in the plasma triglyceride levels from 1780 to 120 mg/dL.

CONCLUSIONS

We identified a novel GPIHBP1 missense mutation (p.C65Y) associated with defective LPL binding in a young boy with severe chylomicronemia. We also show that homozygosity for the C65Y or Q115P mutations is associated with low levels of LPL in the postheparin plasma, demonstrating that GPIHBP1 is important for plasma triglyceride metabolism in humans.

Mutation of conserved cysteines in the Ly6 domain of GPIHBP1 in familial chylomicronemia

We investigated a family from northern Sweden in which three of four siblings have congenital chylomicronemia. LPL activity and mass in pre- and postheparin plasma were low, and LPL release into plasma after heparin injection was delayed. LPL activity and mass in adipose tissue biopsies appeared normal. [(35)S]Methionine incorporation studies on adipose tissue showed that newly synthesized LPL was normal in size and normally glycosylated. Breast milk from the affected female subjects contained normal to elevated LPL mass and activity levels. The milk had a lower than normal milk lipid content, and the fatty acid composition was compatible with the milk lipids being derived from de novo lipogenesis, rather than from the plasma lipoproteins. Given the delayed release of LPL into the plasma after heparin, we suspected that the chylomicronemia might be caused by mutations in GPIHBP1. Indeed, all three affected siblings were compound heterozygotes for missense mutations involving highly conserved cysteines in the Ly6 domain of GPIHBP1 (C65S and C68G). The mutant GPIHBP1 proteins reached the surface of transfected Chinese hamster ovary cells but were defective in their ability to bind LPL (as judged by both cell-based and cell-free LPL binding assays). Thus, the conserved cysteines in the Ly6 domain are crucial for GPIHBP1 function.

Highly conserved cysteines within the Ly6 domain of GPIHBP1 are crucial for the binding of lipoprotein lipase

GPIHBP1, a glycosylphosphatidylinositol-anchored endothelial cell protein of the lymphocyte antigen 6 (Ly6) family, binds lipoprotein lipase (LPL) avidly and is required for the lipolytic processing of triglyceride-rich lipoproteins. GPIHBP1 contains two key structural motifs, an acidic domain and an Ly6 motif (a three-fingered domain specified by 10 cysteines). The acidic domain is required for LPL binding, but the importance of the Ly6 domain is less clear. To explore that issue, we transfected cells with a wild-type GPIHBP1 expression vector or mutant GPIHBP1 vectors in which specific cysteines in the Ly6 domain were changed to alanine. The mutant GPIHBP1 proteins reached the cell surface, as judged by antibody binding studies and by the ability of a phosphatidylinositol-specific phospholipase C to release these proteins from the cell surface. However, cells expressing the cysteine mutants could not bind LPL. The acidic domain of the cysteine mutants appeared to remain accessible, as judged by binding studies with an antibody against the acidic domain. We also developed a cell-free assay of LPL binding. We created a rat monoclonal antibody against the carboxyl terminus of mouse GPIHBP1 and used that antibody to coat agarose beads. We then tested the ability of soluble forms of GPIHBP1 that had been immobilized on monoclonal antibody-coated beads to bind LPL. In this assay, wild-type soluble GPIHBP1 bound LPL avidly, but the cysteine mutants did not. Thus, our studies suggest that a structurally intact Ly6 domain (in addition to the acidic domain) is essential for LPL binding.

High-resolution structures of bacterially expressed soluble human CD59

CD59 is a membrane-bound glycoprotein that protects host cells from lysis by inhibiting the terminal pathway of complement, preventing the formation and insertion of the membrane attack complex (MAC). Crystals of bacterially expressed and nonglycosylated recombinant soluble human CD59 have been obtained from three crystallization conditions, each of which gave rise to a distinct crystal form. Each crystal form led to a crystal structure at high resolution (1.15, 1.35 and 1.8 A). In one of these structures the electron-density map shows an as yet unidentified small molecule in the predicted C8/C9-binding site. The presence/absence of this ligand is linked to alternate conformations of the amino acids implicated in C8/C9 binding.