Positive autoregulation shapes response timing and intensity in two-component signal transduction systems

Positive feedback loops are regulatory elements that can modulate expression output, kinetics and noise in genetic circuits. Transcriptional regulators participating in such loops are often expressed from two promoters, one constitutive and one autoregulated. Here, we investigate the interplay of promoter strengths and the intensity of the stimulus activating the transcriptional regulator in defining the output of a positively autoregulated genetic circuit. Using a mathematical model of two-component regulatory systems, which are present in all domains of life, we establish that positive feedback strongly affects the steady-state output levels at both low and high levels of stimulus if the constitutive promoter of the regulator is weak. By contrast, the effect of positive feedback is negligible when the constitutive promoter is sufficiently strong, unless the stimulus intensity is very high. Furthermore, we determine that positive feedback can affect both transient and steady state output levels even in the simplest genetic regulatory systems. We tested our modeling predictions by abolishing the positive feedback loop in the two-component regulatory system PhoP/PhoQ of Salmonella enterica, which resulted in diminished induction of PhoP-activated genes.

Evolution of a bacterial regulon controlling virulence and Mg(2+) homeostasis

Related organisms typically rely on orthologous regulatory proteins to respond to a given signal. However, the extent to which (or even if) the targets of shared regulatory proteins are maintained across species has remained largely unknown. This question is of particular significance in bacteria due to the widespread effects of horizontal gene transfer. Here, we address this question by investigating the regulons controlled by the DNA-binding PhoP protein, which governs virulence and Mg(2+) homeostasis in several bacterial species. We establish that the ancestral PhoP protein directs largely different gene sets in ten analyzed species of the family Enterobacteriaceae, reflecting both regulation of species-specific targets and transcriptional rewiring of shared genes. The two targets directly activated by PhoP in all ten species (the most distant of which diverged >200 million years ago), and coding for the most conserved proteins are the phoPQ operon itself and the lipoprotein-encoding slyB gene, which decreases PhoP protein activity. The Mg(2+)-responsive PhoP protein dictates expression of Mg(2+) transporters and of enzymes that modify Mg(2+)-binding sites in the cell envelope in most analyzed species. In contrast to the core PhoP regulon, which determines the amount of active PhoP and copes with the low Mg(2+) stress, the variable members of the regulon contribute species-specific traits, a property shared with regulons controlled by dissimilar regulatory proteins and responding to different signals.

Evolution and dynamics of regulatory architectures controlling polymyxin B resistance in enteric bacteria

Complex genetic networks consist of structural modules that determine the levels and timing of a cellular response. While the functional properties of the regulatory architectures that make up these modules have been extensively studied, the evolutionary history of regulatory architectures has remained largely unexplored. Here, we investigate the transition between direct and indirect regulatory pathways governing inducible resistance to the antibiotic polymyxin B in enteric bacteria. We identify a novel regulatory architecture—designated feedforward connector loop—that relies on a regulatory protein that connects signal transduction systems post-translationally, allowing one system to respond to a signal activating another system. The feedforward connector loop is characterized by rapid activation, slow deactivation, and elevated mRNA expression levels in comparison with the direct regulation circuit. Our results suggest that, both functionally and evolutionarily, the feedforward connector loop is the transitional stage between direct transcriptional control and indirect regulation.

A regulatory protein can activate the expression of a target gene either directly, i.e., by binding to the gene's promoter, or indirectly, i.e., by altering the expression of regulators, which, in turn, bind to the target gene's promoter and induce or inhibit its transcription. Indirect regulatory circuits can contain multiple components and functional elements, such as feedforward and feedback loops. The complex structure of indirect regulation raises the question of its evolutionary origins. Here, we study the dynamic and evolutionary properties of regulatory architectures that involve members of the recently emerged class of bacterial proteins termed connectors. Such proteins post-translationally modulate the activity of two-component systems and phosphorelays, which constitute the prevalent form of bacterial signal transduction. We describe a novel connector-mediated regulatory circuit that combines the structural and functional properties of direct and indirect regulation. Our results indicate that this architecture is the evolutionary link between direct and connector-dependent regulatory designs.

Temperature-sensitive paralytic mutants are enriched for those causing neurodegeneration in Drosophila

Age-dependent neurodegeneration is a pathological condition found in many metazoans. Despite the biological and medical significance of this condition, the cellular and molecular mechanisms underlying neurodegeneration are poorly understood. The availability of a large collection of mutants exhibiting neurodegeneration will provide a valuable resource to elucidate these mechanisms. We have developed an effective screen for isolating neurodegeneration mutants in Drosophila. This screen is based on the observation that neuronal dysfunction, which leads to observable behavioral phenotypes, is often associated with neurodegeneration. Thus, we used a secondary histological screen to examine a collection of mutants originally isolated on the basis of conditional paralytic phenotypes. Using this strategy, we have identified 15 mutations affecting at least nine loci that cause gross neurodegenerative pathology. Here, we present a genetic, behavioral, and anatomical analysis of vacuous (vacu), the first of these mutants to be characterized, and an overview of other mutants isolated in the screen. vacu is a recessive mutation located cytologically at 85D-E that causes locomotor defects in both larvae and adults as well as neuronal hyperactivity. In addition, vacu exhibits extensive age-dependent neurodegeneration throughout the central nervous system. We also identified mutations in at least eight other loci that showed significant levels of neurodegeneration with a diverse array of neuropathological phenotypes. These results demonstrate the effectiveness of our screen in identifying mutations causing neurodegeneration. Further studies of vacu and the other neurodegenerative mutants isolated should ultimately help dissect the biochemical pathways leading to neurodegeneration.

Up-regulation of Duffy antigen receptor expression in children with renal disease

BACKGROUND

The Duffy antigen chemokine receptor (DARC) is a promiscuous chemokine receptor that binds chemokines from the C-X-C and C-C families. DARC was initially described on red blood cells, but subsequent studies have demonstrated DARC protein expression on renal endothelial and epithelial cells, even in Duffy-negative individuals whose red cells lack DARC. Because approximately 68% of African Americans lack the Duffy/DARC on their red cells, we carried out experiments to identify the specific renal cells expressing DARC protein and mRNA in African American children and to define whether DARC expression was altered in renal inflammatory processes.

METHODS

Immunohistochemistry and in situ hybridization studies were done in 28 renal sections from children with each of the following diagnoses: HIV nephropathy (HIVAN), HIV-associated hemolytic uremic syndrome (HIV-HUS), HIV infection without renal disease, HIV-negative children without renal disease, and Argentinean children with classic HUS.

RESULTS

The predominant localization of DARC mRNA and protein was found in endothelial cells underlying postcapillary renal venules in all patients studied. However, DARC mRNA and protein were significantly up-regulated in peritubular and glomerular capillaries, collecting duct epithelial cells, and interstitial inflammatory cells in children with HIVAN, HIV-HUS, and classic HUS.

CONCLUSION

These findings support the notion that the renal DARC is linked to the inflammatory cascade and that African American children may be at risk of accumulating chemokines in renal tissues.

The quality of medical evidence in hematology-oncology

PURPOSE

The purpose of this study was to evaluate the quality of the medical evidence available to the clinician in the practice of hematology/oncology.

METHODS

We selected 14 neoplastic hematologic disorders and identified 154 clinically important patient management decision/interventions, ranging from initial treatment decisions to those made for the treatment of recurrent or refractory disease. We also performed a search of the scientific literature for the years 1966 through 1996 to identify all randomized controlled trials in hematology/oncology.

RESULTS

We identified 783 randomized controlled trials (level 1 evidence) pertaining to 37 (24%) of the decision/interventions. An additional 32 (21%) of the decision/interventions were supported by evidence from single arm prospective studies (level 2 evidence). However, only retrospective or anecdotal evidence (level 3 evidence) was available to support 55% of the identified decision/interventions. In a retrospective review of the decision/interventions made in the management of 255 consecutive patients, 78% of the initial decision/interventions in the management of newly diagnosed hematologic/oncologic disorders could have been based on level 1 evidence. However, more than half (52%) of all the decision/interventions made in the management of these 255 patients were supported only by level 2 or 3 evidence.

CONCLUSIONS

We conclude that level 1 evidence to support the development of practice guidelines is available primarily for initial decision/interventions of newly diagnosed diseases. Level 1 evidence to develop guidelines for the management of relapsed or refractory malignant diseases is currently lacking.

Expression of chemokine receptors by subsets of neurons in the central nervous system

IL-8 is expressed by activated and neoplastic astrocytes and enhances the survival of hippocampal neurons in vitro. Since mRNA encoding chemokine receptors have been demonstrated in brain, the expression of chemokine receptors by specific cell types in anatomic regions of the central nervous system (CNS) was investigated. Archival tissues from various regions of the CNS were stained with specific mAbs to the Duffy Ag/receptor for chemokines, a promiscuous receptor that binds selected chemokines; the specific receptor for IL-8 (CXCR1); and the receptor (CXCR2) shared by IL-8 and melanoma growth stimulatory activity. The Duffy Ag/receptor for chemokines was expressed exclusively by Purkinje cells in the cerebellum. Chemokine binding and radioligand cross-linking confirmed the presence of a high affinity, promiscuous chemokine receptor in the cerebellum. Although CXCR1 was not expressed in the CNS, CXCR2 was expressed at high levels by subsets of projection neurons in diverse regions of the brain and spinal cord, including the hippocampus, dentate nucleus, pontine nuclei, locus coeruleus, and paraventricular nucleus, and in the anterior horn, interomediolateral cell column, and Clarke's column of the spinal cord. Fibers that express CXCR2 included those in the superior cerebellar peduncle and the substantia gelatinosa. Immunohistochemical analysis of the involved brain tissues from patients with Alzheimer's disease revealed expression of CXCR2 in the neuritic portion of plaques surrounding deposits of amyloid. These data suggest that chemokines may play a role in reactive processes in normal neuronal function and neurodegenerative disorders.

Expression of chemokine receptors by subsets of neurons in the central nervous system

IL-8 is expressed by activated and neoplastic astrocytes and enhances the survival of hippocampal neurons in vitro. Since mRNA encoding chemokine receptors have been demonstrated in brain, the expression of chemokine receptors by specific cell types in anatomic regions of the central nervous system (CNS) was investigated. Archival tissues from various regions of the CNS were stained with specific mAbs to the Duffy Ag/receptor for chemokines, a promiscuous receptor that binds selected chemokines; the specific receptor for IL-8 (CXCR1); and the receptor (CXCR2) shared by IL-8 and melanoma growth stimulatory activity. The Duffy Ag/receptor for chemokines was expressed exclusively by Purkinje cells in the cerebellum. Chemokine binding and radioligand cross-linking confirmed the presence of a high affinity, promiscuous chemokine receptor in the cerebellum. Although CXCR1 was not expressed in the CNS, CXCR2 was expressed at high levels by subsets of projection neurons in diverse regions of the brain and spinal cord, including the hippocampus, dentate nucleus, pontine nuclei, locus coeruleus, and paraventricular nucleus, and in the anterior horn, interomediolateral cell column, and Clarke's column of the spinal cord. Fibers that express CXCR2 included those in the superior cerebellar peduncle and the substantia gelatinosa. Immunohistochemical analysis of the involved brain tissues from patients with Alzheimer's disease revealed expression of CXCR2 in the neuritic portion of plaques surrounding deposits of amyloid. These data suggest that chemokines may play a role in reactive processes in normal neuronal function and neurodegenerative disorders.

Identification of the Fy6 epitope recognized by two monoclonal antibodies in the N-terminal extracellular portion of the Duffy antigen receptor for chemokines

The epitope Fy6 recognized by two monoclonal antibodies (i3A and BG6), which inhibit binding of chemokines to the Duffy antigen, was characterized by means of peptides synthesized on pins (Epitope Scanning Kit) and deletion mutagenesis. Both antibodies showed very similar specificities. They recognized a linear epitope, the essential portion of which was the heptapeptide Gln-Leu-Asp-Phe-Glu-Asp-Val comprising amino acid residues 21-27, located between two glycosylation sites of the Duffy protein. All the amino acid residues of the epitope, except Glu, were essential for antibody binding, since they could not be replaced by any other amino acid residues or by only one or two. The Glu residue could be replaced by most other amino acid residues, and its replacement by 10 amino acid residues gave a distinct increase in the antibody binding. The results were in full agreement with the finding that the mutant of the Duffy antigen, lacking amino acid residues 23-25 (-Asp-Phe-Glu-), did not bind the i3A antibody, but bound the anti-Fy3 monoclonal antibody similarly to the wild type of the Duffy antigen. The apparent affinity constants of both anti-Fy6 antibodies were determined by surface plasmon resonance, using immunopurified Duffy protein as a ligand.

The promiscuous chemokine binding profile of the Duffy antigen/receptor for chemokines is primarily localized to sequences in the amino-terminal domain

The Duffy antigen (DARC) is a promiscuous chemokine receptor that also binds Plasmodium vivax. DARC belongs to a family of heptahelical chemokine receptors that includes specific (IL-8RA) and shared (IL-8RB) IL-8 receptors. Ligand binding specificity of IL-8 receptors was localized to the amino-terminal extracellular (E1) domain. To determine the basis for promiscuous chemokine binding by DARC, a chimeric receptor composed of the E1 domain of DARC and hydrophobic helices and loops from IL-8RB (DARCe1/IL-8RB) was constructed. Scatchard analysis of stable transfectants demonstrated that the DARCe1/IL-8RB chimeric receptor bound IL-8 and melanoma growth stimulating activity (MGSA) with KD values almost identical to the native receptors. The hybrid receptor also bound RANTES, MCP-1, and MGSA-E6A (which binds DARC, but not IL-8RB), but not MIP-1 alpha, similarly to DARC. Ligand binding to DARC transfectants was unaltered by anti-Fy3, but inhibited by Fy6, which binds an epitope in the E1 domain. The epitope recognized by Fy3 was localized to the third extracellular loop by analysis of insect cells expressing chimeric receptors composed of complementary portions of DARC and IL-8RB. These findings implicate the E1 domain of DARC in multispecific chemokine binding.

The Duffy antigen/receptor for chemokines (DARC) is expressed in endothelial cells of Duffy negative individuals who lack the erythrocyte receptor

The Duffy antigen/receptor for chemokines (DARC), first identified on erythrocytes, functions not only as a promiscuous chemokine receptor but also as a receptor for the malarial parasite, Plasmodium vivax. The recent finding that DARC is ubiquitously expressed by endothelial cells lining postcapillary venules provides a possible insight into the function of this receptor because this anatomic site is an active interface for leukocyte trafficking. However, the biological significance of DARC is questionable since it has not yet been determined whether individuals lacking the expression of this protein on their erythrocytes (Duffy negative individuals), who are apparently immunologically normal, express the receptor on endothelial cells. However, we report here that DARC is indeed expressed in endothelial cells lining postcapillary venules and splenic sinusoids in individuals who lack the erythrocyte receptor. These findings are based on immunohistochemical, biochemical, and molecular biological analysis of tissues from Duffy negative individuals. We also present data showing that, in contrast to erythrocyte DARC, cells transfected with DARC internalize radiolabeled ligand. We conclude that the DARC may play a critical role in mediating the effects of proinflammatory chemokines on the interactions between leukocyte and endothelial cells since the molecular pathology of the Duffy negative genotype maintains expression on the latter cell type.

HLA-DR53 protects against thrombotic thrombocytopenic purpura/adult hemolytic uremic syndrome

Class I and Class II HLA antigens were tested in patients treated for thrombotic thrombocytopenic purpura or hemolytic uremic syndrome (TTP/HUS) to determine whether there is a disease association. Based on the results of a pilot trial, retrospective HLA-typing of 18 patients with a diagnosis of TTP/HUS and prospective typing of 12 newly diagnosed patients with TTP/HUS were performed. Twenty-one patients were non-Hispanic Caucasians, 7 were African-Americans, and 2 were Hispanic-Caucasians. Of 30 patients tested, 28 were positive for DR52 (chi-squared = 5.14, P < 0.05), and only two were positive for DR53 compared to 57% of controls (chi-squared = 18.5, P < 0.0005). Diverse DR52 subtypes (DRB3*0101, DRB3*02, and DRB3*0301) were found by oligonucleotide testing in 15 patients, suggesting the association was not with DR52 but with absence of the DR53 antigen. The 2 patients with DR53 were not homozygous. This study suggests that the supertypic antigen, DR53, may govern susceptibility to TTP/HUS, since the relative risk of this disease among DR53 positives is reduced at 0.09 (95% confidence interval, 0.01-0.28). This finding indicates a possible immunogenetic component in the pathogenesis of TTP.

Postcapillary venule endothelial cells in kidney express a multispecific chemokine receptor that is structurally and functionally identical to the erythroid isoform, which is the Duffy blood group antigen

The human erythrocyte chemokine receptor has recently been shown to be identical to the Duffy blood group antigen and is expressed in multiple organs, including kidney. Here we have examined the molecular properties of the renal isoform. Immunoblot analysis of erythrocyte and kidney detergent lysates, with a monoclonal antibody (Fy6) to the Duffy antigen, revealed that the renal isoform had a molecular mass of 43-45 kD, which could be distinguished from that observed in erythroid cells (38-47 kD). Chemical cross-linking of kidney membranes to 125I-melanoma growth stimulatory activity (MGSA) indicated that the renal chemokine receptor had a molecular mass of 38-45 kD. Binding of 125I-labeled MGSA to kidney membranes was competitively inhibited by the addition of unlabeled MGSA, IL-8, regulated on activation, normal T expressed and secrted, and monocyte chemotactic protein-1. Scatchard analysis of MGSA binding showed that the chemokine receptor from renal tissues had a binding affinity of 3.5 nM similar to that observed for the erythroid isoform (5-10 nM). The primary structure of the renal chemokine receptor predicted from the nucleotide sequence of cDNA from renal tissues is identical to that reported for the erythroid isoform. Immunocytochemical staining of kidney with Fy6 localized expression to endothelial cells present in postcapillary venules. These studies implicate the Duffy antigen/chemokine receptor in the complex interactions between postcapillary endothelial cells and granulocytes, which are modulated by pro-inflammatory chemokines.

Receptor and ligand domains for invasion of erythrocytes by Plasmodium falciparum

A 175-kilodalton erythrocyte binding protein, EBA-175, of the parasite Plasmodium falciparum mediates the invasion of erythrocytes. The erythrocyte receptor for EBA-175 is dependent on sialic acid. The domain of EBA-175 that binds erythrocytes was identified as region II with the use of truncated portions of EBA-175 expressed on COS cells. Region II, which contains a cysteine-rich motif, and native EBA-175 bind specifically to glycophorin A, but not to glycophorin B, on the erythrocyte membrane. Erythrocyte recognition of EBA-175 requires both sialic acid and the peptide backbone of glycophorin A. The identification of both the receptor and ligand domains may suggest rational designs for receptor blockade and vaccines.

A receptor for the malarial parasite Plasmodium vivax: the erythrocyte chemokine receptor

Plasmodium vivax and P. falciparum are the major causes of human malaria, except in sub-Saharan Africa where people lack the Duffy blood group antigen, the erythrocyte receptor for P. vivax. Duffy negative human erythrocytes are resistant to invasion by P. vivax and the related monkey malaria, P. knowlesi. Several lines of evidence in the present study indicate that the Duffy blood group antigen is the erythrocyte receptor for the chemokines interleukin-8 (IL-8) and melanoma growth stimulatory activity (MGSA). First, IL-8 binds minimally to Duffy negative erythrocytes. Second, a monoclonal antibody to the Duffy blood group antigen blocked binding of IL-8 and other chemokines to Duffy positive erythrocytes. Third, both MGSA and IL-8 blocked the binding of the parasite ligand and the invasion of human erythrocytes by P. knowlesi, suggesting the possibility of receptor blockade for anti-malarial therapy.

Purification of a 28 kD non-aggregating tryptic peptide of the Duffy blood group protein

Duffy blood group antigenic epitopes are located on a 35-43 kD integral membrane protein of the erythrocyte membrane. This protein functions as a receptor for the human malaria parasite, Plasmodium vivax. The Duffy protein has been difficult to purify because of its tendency to form aggregates. Here we describe purification of a 28 kD tryptic fragment of the Duffy protein and purification of an 18 kD de-glycosylated form of the Duffy tryptic peptide using Thiopropyl Sepharose 6B chromatography and preparative SDS-PAGE. These Duffy-reactive peptides do not form aggregates and may prove amendable to protein sequencing.

Prolongation of isovolumetric relaxation time as assessed by Doppler echocardiography predicts doxorubicin-induced systolic dysfunction in humans

A reasonably sensitive and specific noninvasive test for doxorubicin cardiotoxicity is needed. In addition, few data exist on the short- and long-term effects of doxorubicin on diastolic filling. To determine if pulsed Doppler indexes of diastolic filling could predict doxorubicin-induced systolic dysfunction, 26 patients (mean age 48 +/- 12 years) were prospectively studied before receiving chemotherapy (control) and 3 weeks after obtaining cumulative doses of doxorubicin. In nine patients developing doxorubicin-induced systolic dysfunction (that is, a decrease in ejection fraction by greater than or equal to 10 ejection fraction units to less than 55%), the isovolumetric relaxation time was prolonged (from 66 +/- 18 to 84 +/- 24 ms, p less than 0.05) after a cumulative doxorubicin dose of 100 to 120 mg/m2. This prolongation preceded a significant decrease in ejection fraction. Other Doppler indexes of filling were impaired after doxorubicin therapy but occurred simultaneously with the decrease in ejection fraction. A greater than 37% increase in isovolumetric relaxation time was 78% (7 of 9) sensitive and 88% (15 of 17) specific for predicting the ultimate development of doxorubicin-induced systolic dysfunction. In 15 patients studied 1 h after the first treatment, doxorubicin enhanced Doppler indexes of filling and shortened isovolumetric relaxation time. In 22 patients, indexes of filling remained impaired and isovolumetric relaxation time was prolonged 3 months after the last doxorubicin dose. In conclusion, doxorubicin-induced systolic dysfunction is reliably predicted by prolongation of Doppler-derived isovolumetric relaxation time. Early after administration, doxorubicin enhances filling and isovolumetric relaxation time. The adverse effects of doxorubicin on both variables persist at least 3 months after cessation of treatment.

A 60-kDa Plasmodium falciparum protein at the moving junction formed between merozoite and erythrocyte during invasion

Invasion of erythrocytes by malaria merozoites requires the formation of a junction of attachment between erythrocyte and merozoite membranes. The attachment junction initially forms at the apical region of the merozoite. It then moves around to the posterior of the merozoite as invasion proceeds. A monoclonal antibody against a 60-kDa merozoite protein (termed MCP-1 for merozoite capping protein 1) of Plasmodium falciparum reacts in an immunofluorescence pattern resembling the moving junction. By two-color immunofluorescence, MCP-1 was located at the attachment site formed between the merozoite apical region and erythrocyte. During invasion, MCP-1 separated and migrated around merozoites at the orifice of the parasitophorous vacuole. In newly-invaded erythrocytes, MCP-1 persisted at the pole of the young parasite nearest the erythrocyte membrane, suggesting its anterior-to-posterior movement. MCP-1 exhibited no variability in molecular mass among the FCR-3, Camp and 7G8 strains of P. falciparum, and the epitope was invariant in the P. falciparum strains studied. We conclude that MCP-1 may participate in merozoite invasion of erythrocytes by facilitating attachment or movement of the junction along the parasite cytoskeletal network.

Receptor-like specificity of a Plasmodium knowlesi malarial protein that binds to Duffy antigen ligands on erythrocytes

A 135-kD parasite protein, a minor component of the Plasmodium knowlesi malaria radiolabeled proteins released into culture supernatant at the time of merozoite release and reinvasion, specifically bound to human erythrocytes that are invaded and carry a Duffy blood group determinant (Fya or Fyb), but did not bind to human erythrocytes that are not invaded and do not carry a Duffy determinant (FyFy). Specific anti-Duffy antibodies blocked the binding of the 135-kD protein to erythrocytes carrying that specific Duffy determinant. Purified 135-kD protein bound specifically to the 35-45-kD Duffy glycoprotein on a blot of electrophoretically separated membrane proteins from Fya and Fyb erythrocytes but not from FyFy erythrocytes. Binding of the 135-kD protein was consistently greater to Fyb than to Fya both on the blot and on intact erythrocytes. The 135-kD protein also bound to rhesus erythrocytes that are Fyb and are invaded, but not to rabbit or guinea pig erythrocytes that are Duffy-negative and are not invaded. Cleavage of the Duffy determinant by pretreating Fyb human erythrocytes with chymotrypsin greatly reduced both invasion and binding of the 135-kD protein, whereas pretreating Fyb erythrocytes with trypsin had little effect on the Duffy antigen, the 135-kD protein binding, or on invasion. However, instances of invasion of other enzyme-treated erythrocytes that are Duffy-negative and do not bind the 135-kD protein suggest that alternative pathways for invasion do exist.

Falciparum malaria parasites invade erythrocytes that lack glycophorin A and B (MkMk). Strain differences indicate receptor heterogeneity and two pathways for invasion

To determine the ligands on erythrocytes for invasion by Plasmodium falciparum, we tested invasion into MkMk erythrocytes that lack glycophorins A and B and enzyme-treated erythrocytes by parasites that differ in their requirement for erythrocyte sialic acid. The 7G8 strain invaded MkMk erythrocytes and neuraminidase-treated normal erythrocytes with greater than 50% the efficiency of normal erythrocytes. In contrast, the Camp strain invaded MkMk erythrocytes at 20% of control and neuraminidase-treated normal erythrocytes at only 1.8% of control. Invasion of MkMk erythrocytes by 7G8 parasites was unaffected by treatment with neuraminidase but was markedly reduced by treatment with trypsin. In contrast, invasion of MkMk cells by Camp parasites was markedly reduced by neuraminidase but was unaffected by trypsin. We conclude that the 7G8 and Camp strains differ in ligand requirements for invasion and that 7G8 requires a trypsin sensitive ligand distinct from glycophorins A and B.

Invasion of erythrocytes by malaria parasites: a cellular and molecular overview

Studies on the morphology, cell biology, and immunology of invasion have characterized events that are now being studied at the molecular level. The initial events of invasion are receptor-specific. A determinant associated with Duffy blood group antigens is involved in the invasion of human erythrocytes by P. knowlesi and P. vivax. The Duffy Fya antigen has recently been identified and further characterization of its role in reception and invasion should now be possible. P. falciparum utilizes erythrocyte ligands that differ from those of P. knowlesi and P. vivax. Sialic acid and a trypsin-sensitive erythrocyte membrane component are important for invasion by P. falciparum parasites. There is evidence that at least two ligands are involved in invasion. For P. knowlesi there is a ligand for attachment, common to both Duffy-negative and Duffy-positive human erythrocytes, and a second ligand for invasion, which is found only on Duffy-positive human erythrocytes. P. vivax also appears to utilize two ligands, a Duffy-associated ligand and a ligand specific for reticulocytes. P. falciparum binds to sialic acid-dependent and sialic acid-independent trypsin-sensitive ligands. P. falciparum merozoites require erythrocyte sialic acid to varying degrees in order to invade; this indicates heterogeneity of the receptor mechanism. Monoclonal antibodies and recombinant DNA technology have greatly facilitated the identification, isolation, and characterization of proteins that may be involved in invasion. Molecules that may have invasion-related functions include those whose antibodies block invasion, those that bind to erythrocyte ligands important for invasion, those that appear on the merozoite surface, and those that appear to be inserted into the erythrocyte membrane at the time of invasion. It has not been possible to identify a definite function for any of the molecules identified thus far. No monoclonal or polyclonal monospecific antibody has been identified that reacts specifically over the surface of the apical region of the merozoite where junction formation occurs. Identification of molecules responsible for apical attachment and junction formation will be important for our understanding of invasion. In terms of vaccine development, it is not yet known whether any of the molecules discussed here will prove to be effective immunogens. It is clear from the data obtained with the 140-kd protein of P. knowlesi that antigenic variation poses a potential problem for vaccine development. As the molecular events responsible for invasion become better understood, novel ways may be devised to interfere with the process and prevent the disease.

Factors influencing invasion of erythrocytes by Plasmodium falciparum parasites: the effects of an N-acetyl glucosamine neoglycoprotein and an anti-glycophorin A antibody

When schizont-infected erythrocytes were incubated with N-acetyl glucosamine coupled to bovine serum albumin (GluNAc-BSA), the number of new ring forms which appeared several hours later was reduced and the number of abnormal and unruptured schizont-infected erythrocytes was increased compared with controls, indicating that GluNAc-BSA prevents invasion by a toxic effect on schizonts rather than by receptor blockade. Invasion of erythrocytes by Plasmodium falciparum was inhibited by a monoclonal antibody against glycophorin A, but inhibition also occurred with P. knowlesi, a parasite that is known to invade independently of glycophorin A. Inhibition of invasion with anti-glycophorin A is unlikely to be related to receptor blockade and is probably related to decreased deformability of the erythrocyte membrane caused by the binding of this antibody. Previous studies suggesting that GluNAc-BSA and anti-glycophorin A antibodies inhibit invasion by receptor blockade should be reevaluated. Erythrocytes deficient in glycophorin C and band 4.1 were also resistant to invasion by both P. falciparum and P. knowlesi.

Invasion of erythrocytes by Plasmodium falciparum malaria parasites: evidence for receptor heterogeneity and two receptors

Plasmodium falciparum malaria parasites with different capabilities of invading sialic acid-deficient erythrocytes were identified. Thai-2 parasites cultured in Tn erythrocytes invaded neuraminidase-treated and Tn erythrocytes twice as efficiently as Thai-2 parasites cultured in normal erythrocytes and seven to ten times more efficiently than a cloned line of Camp parasites cultured in normal erythrocytes. All three parasite lines required sialic acid for optimal invasion, but Thai-2 parasites cultured in Tn erythrocytes invaded neuraminidase-treated erythrocytes with 45% efficiency whereas Camp parasites invaded neuraminidase-treated erythrocytes with less than 10% efficiency. P falciparum malaria parasites probably possess two receptors: one that binds to a sialic acid-dependent ligand and another that binds to a sialic acid-independent ligand. Parasites may differ in the quantity or affinity of their receptors for the sialic acid-independent ligand.

A Plasmodium falciparum antigen that binds to host erythrocytes and merozoites

Antigens that bind to erythrocytes were identified in the supernatant fluids of a cultured human malaria parasite (Plasmodium falciparum). A 175-kilodalton (175K) antigen bound only to erythrocytes susceptible to invasion. The 175K antigen from the Camp or the FCR-3 strain also bound to merozoites. However, the antigen did not bind to merozoites when merozoites and supernatant antigens were from different strains unless proteinase inhibitors were present. Moreover, erythrocytes coated with supernatant antigens from the Camp or FCR-3 strain were invaded normally by merozoites of the homologous strain but were partially resistant to invasion by merozoites of the heterologous strain. The 175K antigen may be a receptor acting as a "bridge" between erythrocytes and merozoites.

Immunization of monkeys with a 140 kilodalton merozoite surface protein of Plasmodium knowlesi malaria: appearance of alternate forms of this protein

The merozoite is the invasive stage of the malaria parasite which is released by rupture of the schizont-infected erythrocyte. A monoclonal antibody against a 140 kilodalton (kDa) merozoite surface antigen of Plasmodium knowlesi was used to characterize and to purify this antigen. It was shown by pulse-chase metabolic labeling of mature schizonts that the 140 kDa merozoite antigen was the processed product of a 143 kDa schizont component, and that processing occurred at the time of erythrocyte rupture. Antiserum, prepared by immunizing a rabbit with the 143/140 kDa antigen purified by immunoaffinity chromatography with the monoclonal antibody, strongly inhibited invasion of erythrocytes in vitro; Fab fragments prepared from purified rabbit IgG were inactive at blocking invasion, suggesting that agglutination of merozoites was the mechanism of invasion inhibition. The purified 143/140 kDa antigen was used in Freund's adjuvant to immunize four rhesus monkeys. Two of the immunized animals developed fulminating infections on challenge with 10(4) schizonts, as did the three control animals. The remaining two immunized animals controlled their infections and developed chronic low-grade parasitemias. The animals which were partially protected were those that had developed anti-143/140 kDa antibodies capable of blocking invasion in vitro. Parasites were isolated from the chronic stage of infection (V5 population) and were compared with the original parasite population used for challenge (P population). Inhibition of invasion, immunofluorescence, and immunoprecipitation with anti-143/140 kDa monoclonal antibody, with immune rabbit, and with monkey sera showed that the 143/140 kDa surface antigen had been replaced by multiple cross-reacting alternate antigenic forms of the molecule in the V population. Thus, specific immune response directed against a purified merozoite surface antigen resulted in the replacement of this antigen by variant or mutant forms.

Immunization of monkeys with a 140 kilodalton merozoite surface protein of Plasmodium knowlesi malaria: appearance of alternate forms of this protein

The merozoite is the invasive stage of the malaria parasite which is released by rupture of the schizont-infected erythrocyte. A monoclonal antibody against a 140 kilodalton (kDa) merozoite surface antigen of Plasmodium knowlesi was used to characterize and to purify this antigen. It was shown by pulse-chase metabolic labeling of mature schizonts that the 140 kDa merozoite antigen was the processed product of a 143 kDa schizont component, and that processing occurred at the time of erythrocyte rupture. Antiserum, prepared by immunizing a rabbit with the 143/140 kDa antigen purified by immunoaffinity chromatography with the monoclonal antibody, strongly inhibited invasion of erythrocytes in vitro; Fab fragments prepared from purified rabbit IgG were inactive at blocking invasion, suggesting that agglutination of merozoites was the mechanism of invasion inhibition. The purified 143/140 kDa antigen was used in Freund's adjuvant to immunize four rhesus monkeys. Two of the immunized animals developed fulminating infections on challenge with 10(4) schizonts, as did the three control animals. The remaining two immunized animals controlled their infections and developed chronic low-grade parasitemias. The animals which were partially protected were those that had developed anti-143/140 kDa antibodies capable of blocking invasion in vitro. Parasites were isolated from the chronic stage of infection (V5 population) and were compared with the original parasite population used for challenge (P population). Inhibition of invasion, immunofluorescence, and immunoprecipitation with anti-143/140 kDa monoclonal antibody, with immune rabbit, and with monkey sera showed that the 143/140 kDa surface antigen had been replaced by multiple cross-reacting alternate antigenic forms of the molecule in the V population. Thus, specific immune response directed against a purified merozoite surface antigen resulted in the replacement of this antigen by variant or mutant forms.

Perspectives for malaria vaccination

The need for vaccines to relieve the current global resurgence of malaria is apparent. Immunity is specific for each species of human malaria and for each stage in the life cycle. Once protective immunogens have been identified for one species, the homologous molecules in other species may lead to protection. The usefulness of a particular immunogen will be determined, in part, by its antigenic diversity in the population and the potential for boosting during natural infection. Successful immunization with malarial antigens may require adjuvants to induce effective, long-lived immunity. If different vaccines become available against each stage in the life cycle, then the composition of a particular vaccine may be tailored for different objectives: protection for short periods (for example, during epidemics and for tourists), decrease in disease and death, and malaria eradication.

Processing of a major parasite surface glycoprotein during the ultimate stages of differentiation in Plasmodium knowlesi

A monoclonal antibody (13C11) was used to investigate the processing of a Plasmodium knowlesi plasma membrane protein during the late stages of schizogony. 13C11 bound to the surface of merozoites, blocked invasion of erythrocytes and immunoprecipitated a 230 kDa glycoprotein from metabolically labelled schizonts. This protein was a major parasite surface component inserted into the membrane of immature schizonts as shown through the study of saponin-freed schizonts which bound 13C11 to their surface (indirect immunofluorescence and immunoelectron microscopy); in addition, the 230 kDa protein on saponin-freed schizonts was susceptible to trypsin treatment. Cleavage of the protein in pulse-chase experiments was followed by immunoprecipitation with 13C11. As schizogony proceeded, the 230 kDa protein was cleaved to 200, 145 and 110 kDa polypeptides. However, this cleavage did not reflect processing but occurred in vitro during detergent extraction and was due to a proteolytic activity which appeared in the parasite during the later stages of schizogony. As schizonts reached maturity and infected erythrocytes lysed, the 230 kDa protein was processed to 75, 57, 50 kDa and 43 kDa polypeptides which were the surface labelled components on purified merozoites immunoprecipitated by 13C11.

Identification of an erythrocyte component carrying the Duffy blood group Fya antigen

The erythrocyte component carrying the Duffy blood group antigen Fya has been identified as a 35- to 43-kilodalton protein. The protein is degraded by proteases, chymotrypsin, and Pronase, which destroy its antigenicity on intact erythrocytes. Its unusual property of aggregating on being boiled in 5 percent sodium dodecyl sulfate with 5 percent 2-mercaptoethanol distinguishes it from other erythrocyte membrane proteins described to date.

Monoclonal antibodies to a 140,000-m.w. protein on Plasmodium knowlesi merozoites inhibit their invasion of rhesus erythrocytes

Merozoites are the invasive stage of the malaria parasite, which are released from infected erythrocytes to invade other erythrocytes. Antibody to surface antigens on merozoites may prevent invasion by agglutinating merozoites as they are released from infected erythrocytes or by blocking receptors before contact of merozoites with the host erythrocyte. Monoclonal antibodies were produced to a 140,000-m.w. protein on the merozoite surface. The protein was synthesized by the mature intraerythrocytic parasite, the schizont, as a 143,000-m.w. protein and had a m.w. of 140,000 on the surface of free merozoites. The monoclonal antibodies were shown to bind to the surface of merozoites by immune electron microscopy. Ascitic fluid containing four of 11 anti-140,000 monoclonal antibodies partially blocked invasion of erythrocytes by merozoites released from schizont-infected cells. The low invasion rate was always associated with a high frequency of multiply infected erythrocytes (two or more rings per erythrocyte). Monoclonal antibodies purified by (NH4)2SO4 precipitation and diethylaminoethyl column fractionation also blocked invasion and caused multiple invasion of individual erythrocytes. The monoclonal antibodies, incubated with free merozoites, did not block invasion, indicating that the antibodies did not bind to merozoite receptors for erythrocytes. We propose that the reduced rate of invasion and the multiple invasion of erythrocytes, the characteristic of these monoclonal antibodies, was caused by weak agglutination of merozoites as they were released from infected erythrocytes.

Monoclonal antibodies to a 140,000-m.w. protein on Plasmodium knowlesi merozoites inhibit their invasion of rhesus erythrocytes

Merozoites are the invasive stage of the malaria parasite, which are released from infected erythrocytes to invade other erythrocytes. Antibody to surface antigens on merozoites may prevent invasion by agglutinating merozoites as they are released from infected erythrocytes or by blocking receptors before contact of merozoites with the host erythrocyte. Monoclonal antibodies were produced to a 140,000-m.w. protein on the merozoite surface. The protein was synthesized by the mature intraerythrocytic parasite, the schizont, as a 143,000-m.w. protein and had a m.w. of 140,000 on the surface of free merozoites. The monoclonal antibodies were shown to bind to the surface of merozoites by immune electron microscopy. Ascitic fluid containing four of 11 anti-140,000 monoclonal antibodies partially blocked invasion of erythrocytes by merozoites released from schizont-infected cells. The low invasion rate was always associated with a high frequency of multiply infected erythrocytes (two or more rings per erythrocyte). Monoclonal antibodies purified by (NH4)2SO4 precipitation and diethylaminoethyl column fractionation also blocked invasion and caused multiple invasion of individual erythrocytes. The monoclonal antibodies, incubated with free merozoites, did not block invasion, indicating that the antibodies did not bind to merozoite receptors for erythrocytes. We propose that the reduced rate of invasion and the multiple invasion of erythrocytes, the characteristic of these monoclonal antibodies, was caused by weak agglutination of merozoites as they were released from infected erythrocytes.

A comparison of knobby (K+) and knobless (K-) parasites from two strains of Plasmodium falciparum

Erythrocytes infected with Plasmodium falciparum develop knob-like protrusions on their membranes. Knobby (K+) parasites of the FCR-3 (Gambian) strain have been shown to possess a histidine-labelled protein of apparent molecular weight 80 000 which is absent from knobless (K-) variants of the same strain. Here we report similar findings with K+ and K- parasites of another strain, the Malayan Camp strain, and also with cloned K+ and K- parasites of the FCR-3 strain. A histidine-labelled protein unique to the two K+ parasites was identified as a broad band with an apparent molecular weight of 89 000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The presence of this protein in both K+ Malayan Camp parasites and K+ FCR-3 (Gambian) parasites and its absence from K- parasites of both strains is consistent with this protein being a major component of knobs.

A monoclonal antibody to rhesus erythrocyte band 3 inhibits invasion by malaria (Plasmodium knowlesi) merozoites

Receptors on erythrocytes and malaria parasites mediate specific attachment and junction formation between these cells that lead to invasion of the erythrocytes. We identified monoclonal antibody A9 and its subclone A9D3 that bound to rhesus erythrocytes and blocked invasion of the erythrocytes by Plasmodium knowlesi merozoites. The monoclonal antibodies did not block attachment, the initial step in invasion, although swelling and crenation of the erythrocyte, which normally occur after attachment, were rarely observed in the presence of antibody. The monoclonal antibody immunoprecipitated rhesus erythrocyte band 3. It bound to erythrocytes of another Old World monkey, the kra monkey, but not to erythrocytes of New World monkeys, chimpanzees, or man. Since the antibody did not bind to human erythrocytes, we could test for nonspecific toxicity to the parasite by studying the effect of the ascites and purified antibody on invasion of human erythrocytes. The antibody caused a minimal reduction in invasion of human erythrocytes, a reduction no greater than that seen with an unrelated monoclonal antibody. Further evidence that the inhibition was specific came from study of Fab fragments of A9D3. Column-purified Fab fragments reduced invasion of rhesus erythrocytes without affecting invasion of human erythrocytes. Fab fragments preabsorbed with rhesus erythrocytes did not inhibit invasion. From the above data, we conclude that band 3 is involved in a stage in the invasion process after initial recognition.