Membrane permeabilization by trypanosome lytic factor, a cytolytic human high density lipoprotein

How are Trypanosoma brucei receptors protected from host antibody-mediated attack?

Trypanosoma brucei is the causal agent of African Trypanosomiasis in humans and other animals. It maintains a long-term infection through an antigenic variation based population survival strategy. To proliferate in a mammal, T. brucei acquires iron and haem through the receptor mediated uptake of host transferrin and haptoglobin-hemoglobin respectively. The receptors are exposed to host antibodies but this does not lead to clearance of the infection. Here we discuss how the trypanosome avoids this fate in the context of recent findings on the structure and cell biology of the receptors.

Atypical human trypanosomosis: Potentially emerging disease with lack of understanding

Trypanosomes are the hemoflagellate kinetoplastid protozoan parasites affecting a wide range of vertebrate hosts having insufficient host specificity. Climatic change, deforestation, globalization, trade agreements, close association and genetic selection in links with environmental, vector, reservoir and potential susceptible hosts' parameters have led to emergence of atypical human trypanosomosis (a-HT). Poor recording of such neglected tropical disease, low awareness in health professions and farming community has approached a serious intimidation for mankind. Reports of animal Trypanosoma species are now gradually increasing in humans, and lack of any compiled literature has diluted the issue. In the present review, global reports of livestock and rodent trypanosomes reported from human beings are assembled and discrepancies with the available literature are discussed along with morphological features of Trypanosoma species. We have described 21 human cases from the published information. Majority of cases 10 (47%) are due to T. lewisi, followed by 5 (24%) cases of T. evansi, 4 (19%) cases of T. brucei and 1 (5%) case each of T. vivax and T. congolense. Indian subcontinent witnessed 13 cases of a-HT, of which 9 cases are reported from India, which includes 7 cases of T. lewisi and 2 cases of T. evansi. Apart from, a-HT case reports, epidemiological investigation and treatment aspects are also discussed. An attempt has been made to provide an overview of the current situation of atypical human trypanosomosis caused by salivarian animal Trypanosoma globally. The probable role of Trypanosoma lytic factors (TLF) present in normal human serum (NHS) in providing innate immunity against salivarian animal Trypanosoma species and the existing paradox in medical science after the finding on intact functional apolipoprotein L1 (ApoL1) in Vietnam T. evansi Type A case is also discussed to provide an update on all aspects of a-HT. Insufficient data and poor reporting in Asian and African countries are the major hurdle resulting in under-reporting of a-HT, which is a potential emerging threat. Therefore, concerted efforts must be directed to address attentiveness, preparedness and regular surveillance in suspected areas with training of field technicians, medical health professionals and veterinarians. Enhancing a one health approach is specifically important in case of trypanosomosis.

Kidney-disease-associated variants of Apolipoprotein L1 show gain of function in cation channel activity

Variants in Apolipoprotein L1 (ApoL1) are known to be responsible for increased risk of some progressive kidney diseases among people of African ancestry. ApoL1 is an amphitropic protein that can insert into phospholipid membranes and confer anion- or cation-selective permeability to phospholipid membranes depending on pH. Whether these activities differ among the variants or whether they contribute to disease pathogenesis is unknown. We used assays of voltage-driven ion flux from phospholipid vesicles and of stable membrane association to assess differences among ApoL1 isoforms. There is a significant (approximately twofold) increase in the cation-selective ion permease activity of the two kidney-disease-associated variants compared with the reference protein. In contrast, we find no difference in the anion-selective permease activity at low pH among the isoforms. Compared with the reference sequence, the two disease-associated variants show increased stable association with phospholipid vesicles under conditions that support the cation permease activity, suggesting that the increased activity may be due to more efficient membrane association and insertion. There is no difference in membrane association among isoforms under optimal conditions for the anion permease activity. These data support a model in which enhanced cation permeability may contribute to the progressive kidney diseases associated with high-risk ApoL1 alleles.

Serum amyloid A exhibits pH dependent antibacterial action and contributes to host defense against Staphylococcus aureus cutaneous infection

Serum amyloid A (SAA), one of the major highly conserved acute-phase proteins in most mammals, is predominantly produced by hepatocytes and also by a variety of cells in extrahepatic tissues. It is well-known that the expression of SAA is sharply increased in bacterial infections. However, the exact physiological function of SAA during bacterial infection remains unclear. Herein, we showed that SAA expression significantly increased in abscesses of Staphylococcus aureus cutaneous infected mice, which exert direct antibacterial effects by binding to the bacterial cell surface and disrupting the cell membrane in acidic conditions. Mechanically, SAA disrupts anionic liposomes by spontaneously forming small vesicles or micelles under acidic conditions. Especially, the N-terminal region of SAA is necessary for membrane disruption and bactericidal activity. Furthermore, we found that mice deficient in SAA1/2 were more susceptible to infection by S. aureus In addition, the expression of SAA in infected skin was regulated by interleukin-6. Taken together, these findings support a key role of the SAA in host defense and may provide a novel therapeutic strategy for cutaneous bacterial infection.

Alterations in plasma membrane ion channel structures stimulate NLRP3 inflammasome activation in APOL1 risk milieu

We evaluated alterations in the structural configurations of channels and activation of nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3 (NLRP3) inflammasome formation in apolipoprotein L1 (APOL1) risk and nonrisk milieus. APOL1G1- and APOL1G2-expressing podocytes (PD) displayed enhanced K⁺ efflux, induction of pyroptosis, and escalated transcription of interleukin (IL)-1β and IL-18. APOL1G1- and APOL1G2-expressing PD promoted the transcription as well as translation of proteins involved in the formation of inflammasomes. Since glyburide (a specific inhibitor of K⁺ efflux channels) inhibited the transcription of NLRP3, IL-1β, and IL-18, the role of K⁺ efflux in the activation of inflammasomes in APOL1 risk milieu was implicated. To evaluate the role of structural alterations in K⁺ channels in plasma membranes, bioinformatics studies, including molecular dynamic simulation, were carried out. Superimposition of bioinformatics reconstructions of APOL1G0, G1, and G2 showed several aligned regions. The analysis of pore-lining residues revealed that Ser342 and Tyr389 are involved in APOL1G0 pore formation and the altered conformations resulting from the Ser342Gly and Ile384Met mutation in the case of APOLG1 and deletion of the Tyr389 residue in the case of APOL1G2 are expected to alter pore characteristics, including K⁺ ion selectivity. Analysis of multiple membrane (lipid bilayer) models of interaction with the peripheral protein, integral membrane protein, and multimer protein revealed that for an APOL1 multimer model, APOL1G0 is not energetically favorable while the APOL1G1 and APOL1G2 moieties favor the insertion of multiple ion channels into the lipid bilayer. We conclude that altered pore configurations carry the potential to facilitate K⁺ ion transport in APOL1 risk milieu.

APOL1 and Kidney Disease: From Genetics to Biology

Genetic variants in the APOL1 gene, found only in individuals of recent African ancestry, greatly increase risk of multiple types of kidney disease. These APOL1 kidney risk alleles are a rare example of genetic variants that are common but also have a powerful effect on disease susceptibility. These alleles rose to high frequency in sub-Saharan Africa because they conferred protection against pathogenic trypanosomes that cause African sleeping sickness. We consider the genetic evidence supporting the association between APOL1 and kidney disease across the range of clinical phenotypes in the APOL1 nephropathy spectrum. We then explore the origins of the APOL1 risk variants and evolutionary struggle between humans and trypanosomes at both the molecular and population genetic level. Finally, we survey the rapidly growing literature investigating APOL1 biology as elucidated from experiments in cell-based systems, cell-free systems, mouse and lower organism models of disease, and through illuminating natural experiments in humans.

APOL1 and kidney cell function

The apolipoprotein L1 (APOL1) gene is unique to humans and gorillas and appeared ~33 million years ago. Since the majority of the mammals do not carry APOL1, it seems to be dispensable for kidney function. APOL1 renal risk variants (RRVs; G1 and G2) are associated with the development as well as progression of chronic kidney diseases (CKDs) at higher rates in populations with African ancestry. Cellular expression of two APOL1 RRVs has been demonstrated to induce cytotoxicity, including necrosis, apoptosis, and pyroptosis, in several cell types including podocytes; mechanistically, these toxicities were attributed to lysosomal swelling, K⁺ depletion, mitochondrial dysfunction, autophagy blockade, protein kinase receptor activation, ubiquitin D degradation, and endoplasmic reticulum stress; notably, these effects were found to be dose dependent and occurred only in overtly APOL1 RRV-expressing cells. However, cellular protein expressions as well as circulating blood levels of APOL1 RRVs were not elevated in patients suffering from APOL1 RRV-associated CKDs. Therefore, the question arises as to whether it is gain or loss of function on the part of APOL1 RRVs contributing to kidney cell injury. The question seems to be more pertinent after the recognition of the role of APOL1 nonrisk (G0) in the transition of parietal epithelial cells and preservation of the podocyte molecular phenotype through modulation of the APOL1-miR-193a axis. With this background, the present review analyzed the available literature in terms of the known function of APOL1 nonrisk and how the loss of these functions could have contributed to two APOL1 RRV-associated CKDs.

The Cell Biology of APOL1

The association of variants in the APOL1 gene, which encodes apolipoprotein L1 (APOL1), with progressive nondiabetic kidney diseases in African Americans has prompted intense investigation into the function(s) of APOL1. APOL1 is an innate immune effector that protects human beings from infection by some trypanosomal parasites. We review the data characterizing APOL1 trypanolytic function, which has been a basis for studies of APOL1 function in mammalian cells. Subsequently, we discuss the studies that use animal models, mammalian cell culture models, and kidney biopsy tissue to discover the mechanisms of variant APOL1-associated kidney diseases.

APOL1: The Balance Imposed by Infection, Selection, and Kidney Disease

Chronic kidney disease (CKD) affects millions of people and constitutes a major health and financial burden worldwide. People of African descent are at an increased risk of developing kidney disease, which is mostly explained by two variants in the Apolipoprotein L1 (APOL1) gene that are found only in people of west African origin. It is hypothesized that these variants were genetically selected due to the protection they afford against African sleeping sickness, caused by the parasite Trypanosoma brucei. Targeting mutant APOL1 could have substantial therapeutic potential for treating kidney disease. In this review, we will describe the intriguing interplay between microbiology, genetics, and kidney disease as revealed in APOL1-associated kidney disease, discuss APOL1-induced cytotoxicity and its therapeutic implications.

Decoding the network of Trypanosoma brucei proteins that determines sensitivity to apolipoprotein-L1

In contrast to Trypanosoma brucei gambiense and T. b. rhodesiense (the causative agents of human African trypanosomiasis), T. b. brucei is lysed by apolipoprotein-L1 (apoL1)-containing human serum trypanolytic factors (TLF), rendering it non-infectious to humans. While the mechanisms of TLF1 uptake, apoL1 membrane integration, and T. b. gambiense and T. b. rhodesiense apoL1-resistance have been extensively characterised, our understanding of the range of factors that drive apoL1 action in T. b. brucei is limited. Selecting our bloodstream-form T. b. brucei RNAi library with recombinant apoL1 identified an array of factors that supports the trypanocidal action of apoL1, including six putative ubiquitin modifiers and several proteins putatively involved in membrane trafficking; we also identified the known apoL1 sensitivity determinants, TbKIFC1 and the V-ATPase. Most prominent amongst the novel apoL1 sensitivity determinants was a putative ubiquitin ligase. Intriguingly, while loss of this ubiquitin ligase reduces parasite sensitivity to apoL1, its loss enhances parasite sensitivity to TLF1-dominated normal human serum, indicating that free and TLF1-bound apoL1 have contrasting modes-of-action. Indeed, loss of the known human serum sensitivity determinants, p67 (lysosomal associated membrane protein) and the cathepsin-L regulator, 'inhibitor of cysteine peptidase', had no effect on sensitivity to free apoL1. Our findings highlight a complex network of proteins that influences apoL1 action, with implications for our understanding of the anti-trypanosomal action of human serum.

Apolipoprotein L1 confers pH-switchable ion permeability to phospholipid vesicles

Apolipoprotein L1 (ApoL1) is a human serum protein conferring resistance to African trypanosomes, and certain ApoL1 variants increase susceptibility to some progressive kidney diseases. ApoL1 has been hypothesized to function like a pore-forming colicin and has been reported to have permeability effects on both intracellular and plasma membranes. Here, to gain insight into how ApoL1 may function in vivo, we used vesicle-based ion permeability, direct membrane association, and intrinsic fluorescence to study the activities of purified recombinant ApoL1. We found that ApoL1 confers chloride-selective permeability to preformed phospholipid vesicles and that this selectivity is strongly pH-sensitive, with maximal activity at pH 5 and little activity above pH 7. When ApoL1 and lipid were allowed to interact at low pH and were then brought to neutral pH, chloride permeability was suppressed, and potassium permeability was activated. Both chloride and potassium permeability linearly correlated with the mass of ApoL1 in the reaction mixture, and both exhibited lipid selectivity, requiring the presence of negatively charged lipids for activity. Potassium, but not chloride, permease activity required the presence of calcium ions in both the association and activation steps. Direct assessment of ApoL1-lipid associations confirmed that ApoL1 stably associates with phospholipid vesicles, requiring low pH and the presence of negatively charged phospholipids for maximal binding. Intrinsic fluorescence of ApoL1 supported the presence of a significant structural transition when ApoL1 is mixed with lipids at low pH. This pH-switchable ion-selective permeability may explain the different effects of ApoL1 reported in intracellular and plasma membrane environments.

The Changing Face of HDL and the Best Way to Measure It

BACKGROUND

HDL cholesterol (HDL-C) is a commonly used lipid biomarker for assessing cardiovascular health. While a central focus has been placed on the role of HDL in the reverse cholesterol transport (RCT) process, our appreciation for the other cardioprotective properties of HDL continues to expand with further investigation into the structure and function of HDL and its specific subfractions. The development of novel assays is empowering the research community to assess different aspects of HDL function, which at some point may evolve into new diagnostic tests.

CONTENT

This review discusses our current understanding of the formation and maturation of HDL particles via RCT, as well as the newly recognized roles of HDL outside RCT. The antioxidative, antiinflammatory, antiapoptotic, antithrombotic, antiinfective, and vasoprotective effects of HDL are all discussed, as are the related methodologies for assessing these different aspects of HDL function. We elaborate on the importance of protein and lipid composition of HDL in health and disease and highlight potential new diagnostic assays based on these parameters.

SUMMARY

Although multiple epidemiologic studies have confirmed that HDL-C is a strong negative risk marker for cardiovascular disease, several clinical and experimental studies have yielded inconsistent results on the direct role of HDL-C as an antiatherogenic factor. As of yet, our increased understanding of HDL biology has not been translated into successful new therapies, but will undoubtedly depend on the development of alternative ways for measuring HDL besides its cholesterol content.

A Primate APOL1 Variant That Kills Trypanosoma brucei gambiense

Humans are protected against infection from most African trypanosomes by lipoprotein complexes present in serum that contain the trypanolytic pore-forming protein, Apolipoprotein L1 (APOL1). The human-infective trypanosomes, Trypanosoma brucei rhodesiense in East Africa and T. b. gambiense in West Africa have separately evolved mechanisms that allow them to resist APOL1-mediated lysis and cause human African trypanosomiasis, or sleeping sickness, in man. Recently, APOL1 variants were identified from a subset of Old World monkeys, that are able to lyse East African T. b. rhodesiense, by virtue of C-terminal polymorphisms in the APOL1 protein that hinder that parasite’s resistance mechanism. Such variants have been proposed as candidates for developing therapeutic alternatives to the unsatisfactory anti-trypanosomal drugs currently in use. Here we demonstrate the in vitro lytic ability of serum and purified recombinant protein of an APOL1 ortholog from the West African Guinea baboon (Papio papio), which is able to lyse examples of all sub-species of T. brucei including T. b. gambiense group 1 parasites, the most common agent of human African trypanosomiasis. The identification of a variant of APOL1 with trypanolytic ability for both human-infective T. brucei sub-species could be a candidate for universal APOL1-based therapeutic strategies, targeted against all pathogenic African trypanosomes.

African trypanosomes are protozoan parasites that affect both humans and animals in poor rural areas of sub-Saharan Africa, and are a major constraint on health and agricultural development. Disease control is principally dependent on the administration of drugs, which are old and largely unsatisfactory. Humans are naturally resistant to infection by most African trypanosomes species because of a lytic protein component in their blood, called APOL1. However, human-infective trypanosomes, T. b. rhodesiense in East Africa, and T. b. gambiense in West Africa, have evolved separate mechanisms to disarm this lytic protein and cause disease. Recently, variants of APOL1 were discovered in some primates that are able to kill the East African human disease-causing sub-species. These APOL1 variants form the basis of current attempts to create novel therapeutic interventions that can kill both animal and human-infective trypanosomes. In this study, we show that another variant of the same protein from a West African baboon species is able to kill, not only East African human-infective trypanosomes, but also the West African parasites, which causes the majority of human African trypanosomiasis cases. This new APOL1 variant could be a potential candidate for anti-trypanosomal therapies targeted at all pathogenic trypanosome species.

Immune Evasion Strategies of Trypanosoma brucei within the Mammalian Host: Progression to Pathogenicity

The diseases caused by African trypanosomes (AT) are of both medical and veterinary importance and have adversely influenced the economic development of sub-Saharan Africa. Moreover, so far not a single field applicable vaccine exists, and chemotherapy is the only strategy available to treat the disease. These strictly extracellular protozoan parasites are confronted with different arms of the host's immune response (cellular as well as humoral) and via an elaborate and efficient (vector)-parasite-host interplay they have evolved efficient immune escape mechanisms to evade/manipulate the entire host immune response. This is of importance, since these parasites need to survive sufficiently long in their mammalian/vector host in order to complete their life cycle/transmission. Here, we will give an overview of the different mechanisms AT (i.e. T. brucei as a model organism) employ, comprising both tsetse fly saliva and parasite-derived components to modulate host innate immune responses thereby sculpturing an environment that allows survival and development within the mammalian host.

Trypanosome Lytic Factor-1 Initiates Oxidation-stimulated Osmotic Lysis of Trypanosoma brucei brucei

Human innate immunity against the veterinary pathogen Trypanosoma brucei brucei is conferred by trypanosome lytic factors (TLFs), against which human-infective T. brucei gambiense and T. brucei rhodesiense have evolved resistance. TLF-1 is a subclass of high density lipoprotein particles defined by two primate-specific apolipoproteins: the ion channel-forming toxin ApoL1 (apolipoprotein L1) and the hemoglobin (Hb) scavenger Hpr (haptoglobin-related protein). The role of oxidative stress in the TLF-1 lytic mechanism has been controversial. Here we show that oxidative processes are involved in TLF-1 killing of T. brucei brucei. The lipophilic antioxidant N,N'-diphenyl-p-phenylenediamine protected TLF-1-treated T. brucei brucei from lysis. Conversely, lysis of TLF-1-treated T. brucei brucei was increased by the addition of peroxides or thiol-conjugating agents. Previously, the Hpr-Hb complex was postulated to be a source of free radicals during TLF-1 lysis. However, we found that the iron-containing heme of the Hpr-Hb complex was not involved in TLF-1 lysis. Furthermore, neither high concentrations of transferrin nor knock-out of cytosolic lipid peroxidases prevented TLF-1 lysis. Instead, purified ApoL1 was sufficient to induce lysis, and ApoL1 lysis was inhibited by the antioxidant DPPD. Swelling of TLF-1-treated T. brucei brucei was reminiscent of swelling under hypotonic stress. Moreover, TLF-1-treated T. brucei brucei became rapidly susceptible to hypotonic lysis. T. brucei brucei cells exposed to peroxides or thiol-binding agents were also sensitized to hypotonic lysis in the absence of TLF-1. We postulate that ApoL1 initiates osmotic stress at the plasma membrane, which sensitizes T. brucei brucei to oxidation-stimulated osmotic lysis.

Large multiallelic copy number variations in humans

Thousands of genomic segments appear to be present in widely varying copy numbers in different human genomes. We developed ways to use increasingly abundant whole-genome sequence data to identify the copy numbers, alleles and haplotypes present at most large multiallelic CNVs (mCNVs). We analyzed 849 genomes sequenced by the 1000 Genomes Project to identify most large (>5-kb) mCNVs, including 3,878 duplications, of which 1,356 appear to have 3 or more segregating alleles. We find that mCNVs give rise to most human variation in gene dosage-seven times the combined contribution of deletions and biallelic duplications-and that this variation in gene dosage generates abundant variation in gene expression. We describe 'runaway duplication haplotypes' in which genes, including HPR and ORM1, have mutated to high copy number on specific haplotypes. We also describe partially successful initial strategies for analyzing mCNVs via imputation and provide an initial data resource to support such analyses.

A retained secretory signal peptide mediates high density lipoprotein (HDL) assembly and function of haptoglobin-related protein

Haptoglobin-related protein (Hpr) is a component of a minor subspecies of high density lipoproteins (HDL) that function in innate immunity. Here we show that assembly of Hpr into HDL is mediated by its retained N-terminal signal peptide, an unusual feature for a secreted protein and the major difference between Hpr and the soluble acute phase protein haptoglobin (Hp). The 18-amino acid signal peptide is necessary for binding to HDL and interacts directly with the hydrocarbon region of lipids. Utilizing model liposomes, we show that the rate of assembly and steady-state distribution of Hpr in lipid particles is mediated by the physical property of lipid fluidity. Dye release assays reveal that Hpr interacts more rapidly with fluid liposomes. Conversely, steady-state binding assays indicate that more rigid lipid compositions stabilize Hpr association. Lipid association also plays a role in facilitating hemoglobin binding by Hpr. Our data may offer an explanation for the distinct distribution of Hpr among HDL subspecies. Rather than protein-protein interactions mediating localization, direct interaction with phospholipids and sensitivity to lipid fluidity may be sufficient for localization of Hpr and may represent a mechanism of HDL subspeciation.

Trypanosome resistance to human innate immunity: targeting Achilles' heel

Trypanosome lytic factors (TLFs) are powerful, naturally occurring toxins in humans that provide sterile protection against infection by several African trypanosomes. These trypanocidal complexes predominantly enter the parasite by binding to the trypanosome haptoglobin/hemoglobin receptor (HpHbR), trafficking to the lysosome, causing membrane damage and, ultimately, cell lysis. Despite TLF-mediated immunity, the parasites that cause human African Trypanosomiasis (HAT), Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense, have developed independent mechanisms of resistance to TLF killing. In this review we describe the parasite defenses that allow trypanosome infections of humans and discuss how targeting these apparent strengths of the parasite may reveal their Achilles' heel, leading to new approaches in the treatment of HAT.

Human apolipoprotein L1 (ApoL1) in cancer and chronic kidney disease

Human apolipoprotein L1 (ApoL1) possesses both extra- and intra-cellular functions crucial in host defense and cellular homeostatic mechanisms. Alterations in ApoL1 function due to genetic, environmental, and lifestyle factors have been associated with African sleeping sickness, atherosclerosis, lipid disorders, obesity, schizophrenia, cancer, and chronic kidney disease (CKD). Importantly, two alleles of APOL1 carrying three coding-sequence variants have been linked to CKD, particularly in Sub-Saharan Africans and African Americans. Intracellularly, elevated ApoL1 can induce autophagy and autophagy-associated cell death, which may be critical in the maintenance of cellular homeostasis in the kidney. Similarly, ApoL1 may protect kidney cells against renal cell carcinoma (RCC). We summarize the role of ApoL1 in RCC and CKD, highlighting the critical function of ApoL1 in autophagy.

Antimicrobial peptide killing of African trypanosomes

The diseases caused by trypanosomes are medically and economically devastating to the population of Sub-Saharan Africa. Parasites of the genus Trypanosoma infect both humans, causing African sleeping sickness, and livestock, causing Nagana. The development of effective treatment strategies has suffered from severe side effects of approved drugs, resistance and major difficulties in delivering drugs. Antimicrobial peptides (AMPs) are ubiquitous components of immune defence and are being rigorously pursued as novel sources of new therapeutics for a variety of pathogens. Here, we review the role of AMPs in the innate immune response of the tsetse fly to African trypanosomes, catalogue trypanocidal AMPs from diverse organisms and highlight the susceptibility of bloodstream form African trypanosomes to killing by unconventional toxic peptides.

APOL1 localization in normal kidney and nondiabetic kidney disease

In patients of African ancestry, genetic variants in APOL1, which encodes apolipoprotein L1, associate with the nondiabetic kidney diseases, focal segmental glomerulosclerosis (FSGS), HIV-associated nephropathy (HIVAN), and hypertensive nephropathy. Understanding the renal localization of APOL1 may provide clues that will ultimately help elucidate the mechanisms by which APOL1 variants promote nephropathy. Here, we used immunohistology to examine APOL1 localization in normal human kidney sections and in biopsies demonstrating either FSGS (n = 8) or HIVAN (n = 2). Within normal glomeruli, APOL1 only localized to podocytes. Compared with normal glomeruli, fewer cells stained for APOL1 in FSGS and HIVAN glomeruli, even when expression of the podocyte markers GLEPP1 and synaptopodin appeared normal. APOL1 localized to proximal tubular epithelia in normal kidneys, FSGS, and HIVAN. We detected APOL1 in the arteriolar endothelium of normal and diseased kidney sections. Unexpectedly, in both FSGS and HIVAN but not normal kidneys, the media of medium artery and arterioles contained a subset of α-smooth muscle actin-positive cells that stained for APOL1. Comparing the renal distribution of APOL1 in nondiabetic kidney disease to normal kidney suggests that a previously unrecognized arteriopathy may contribute to disease pathogenesis in patients of African ancestry.

Mechanism of Trypanosoma brucei gambiense (group 1) resistance to human trypanosome lytic factor

Human innate immunity against most African trypanosomes, including Trypanosoma brucei brucei, is mediated by a minor subclass of toxic serum HDL, called trypanosome lytic factor-1 (TLF-1). This HDL contains two primate specific proteins, apolipoprotein L-1 and haptoglobin (Hp)-related protein, as well as apolipoprotein A-1. These assembled proteins provide a powerful defense against trypanosome infection. Trypanosoma brucei rhodesiense causes human African sleeping sickness because it has evolved an inhibitor of TLF-1, serum resistance-associated (SRA) protein. Trypanosoma brucei gambiense lacks the SRA gene, yet it infects humans. As transfection of T. b. gambiense (group 1) is not possible, we initially used in vitro-selected TLF-1-resistant T. b. brucei to examine SRA-independent mechanisms of TLF-1 resistance. Here we show that TLF-1 resistance in T. b. brucei is caused by reduced expression of the Hp/Hb receptor gene (TbbHpHbR). Importantly, T. b. gambiense (group 1) also showed a marked reduction in uptake of TLF-1 and a corresponding decrease in expression of T. b. gambiense Hp/Hb receptor (TbgHpHbR). Ectopic expression of TbbHpHbR in TLF-1-resistant T. b. brucei rescued TLF-1 uptake, demonstrating that decreased TbbHpHbR expression conferred TLF-1 resistance. Ectopic expression of TbgHpHbR in TLF-1-resistant T. b. brucei failed to rescue TLF-1 killing, suggesting that coding sequence changes altered Hp/Hb receptor binding affinity for TLF-1. We propose that the combination of coding sequence mutations and decreased expression of TbgHpHbR directly contribute to parasite evasion of human innate immunity and infectivity of group 1 T. b. gambiense.

The plasma membrane of bloodstream-form African trypanosomes confers susceptibility and specificity to killing by hydrophobic peptides

Trypanosoma brucei is the causative agent of both a veterinary wasting disease and human African trypanosomiasis, or sleeping sickness. The cell membrane of the developmental stage found within the mammalian host, the bloodstream form (BSF), is highly dynamic, exhibiting rapid rates of endocytosis and lateral flow of glycosylphosphatidylinositol-anchored proteins. Here, we show that the cell membrane of these organisms is a target for killing by small hydrophobic peptides that increase the rigidity of lipid bilayers. Specifically, we have derived trypanocidal peptides that are based upon the hydrophobic N-terminal signal sequences of human apolipoproteins. These peptides selectively partitioned into the plasma membrane of BSF trypanosomes, resulting in an increase in the rigidity of the bilayer, dramatic changes in cell motility, and subsequent cell death. No killing of the developmental stage found within the insect midgut, the procyclic form, was observed. Additionally, the peptides exhibited no toxicity toward mammalian cell lines and did not induce hemolysis. Studies with model liposomes indicated that bilayer fluidity dictates the susceptibility of membranes to manipulation by hydrophobic peptides. We suggest that the composition of the BSF trypanosome cell membrane confers a high degree of fluidity and unique susceptibility to killing by hydrophobic peptides and is therefore a target for the development of trypanocidal drugs.

The trypanolytic factor of human serum: many ways to enter the parasite, a single way to kill

Humans have developed a particular innate immunity system against African trypanosomes, and only two Trypanosoma brucei clones (T. b. gambiense, T. b. rhodesiense) can resist this defence and cause sleeping sickness. The main players of this immunity are the primate-specific apolipoprotein L-I (apoL1) and haptoglobin-related protein (Hpr). These proteins are both associated with two serum complexes, a minor subfraction of HDLs and an IgM/apolipoprotein A-I (apoA1) complex, respectively, termed trypanosome lytic factor (TLF) 1 and TLF2. Although the two complexes appear to lyse trypanosomes by the same mechanism, they enter the parasite through various modes of uptake. In case of TLF1 one uptake process was characterized. When released in the circulation, haemoglobin (Hb) binds to Hpr, hence to TLF1. In turn the TLF1-Hpr-Hb complex binds to the trypanosome haptoglobin (Hp)-Hb receptor, whose original function is to ensure haem uptake for optimal growth of the parasite. This binding triggers efficient uptake of TLF1 and subsequent trypanosome lysis. While Hpr is involved as TLF ligand, the lytic activity is due to apoL1, a Bcl-2-like pore-forming protein. We discuss the in vivo relevance of this uptake pathway in the context of other potentially redundant delivery routes.

Immunobiology of African trypanosomes: need of alternative interventions

Trypanosomiasis is one of the major parasitic diseases for which control is still far from reality. The vaccination approaches by using dominant surface proteins have not been successful, mainly due to antigenic variation of the parasite surface coat. On the other hand, the chemotherapeutic drugs in current use for the treatment of this disease are toxic and problems of resistance are increasing (see Kennedy (2004) and Legros et al. (2002)). Therefore, alternative approaches in both treatment and vaccination against trypanosomiasis are needed at this time. To be able to design and develop such alternatives, the biology of this parasite and the host response against the pathogen need to be studied. These two aspects of this disease with few examples of alternative approaches are discussed here.

Activity of trypanosome lytic factor: a novel component of innate immunity

Trypanosome lytic factors (TLFs) are high-density lipoproteins and components of primate innate immunity. TLFs are characterized by their ability to kill extracellular protozoon parasites of the genus Trypanosoma. Two subspecies of Trypanosoma brucei have evolved resistance to TLFs and can consequently infect humans, resulting in the disease African sleeping sickness. The unique protein components of TLFs are a hemoglobin-binding protein, haptoglobin-related protein and a pore-forming protein, apoL-I. The recent advances in our understanding of the roles that these proteins play in the mechanism of TLF-mediated lysis are highlighted in this article. In light of recent data, which demonstrate that TLFs can ameliorate infection by the intracellular pathogen Leishmania, we also discuss the broader function of TLFs as components of innate immunity.

Human innate immunity against African trypanosomes

Humans are naturally resistant to infection by the African trypanosome prototype Trypanosoma brucei brucei, and only two variant clones of this parasite can avoid this innate immunity and cause sleeping sickness. The resistance to T. brucei is due to serum complexes associating apolipoprotein A-1 (apoA1) with two primate-specific proteins, apolipoprotein L-1 (apoL1) and haptoglobin-related protein (Hpr). We discuss recent advances on the respective functions of apoL1 and Hpr in this system. ApoL1 was found to share structural and functional similarities with proteins of the apoptotic Bcl2 family, and to kill trypanosomes through anionic pore formation in the lysosomal membrane of the parasite. In association with hemoglobin (Hb), Hpr was found to promote the binding of the trypanolytic complexes to a haptoglobin (Hp)-Hb receptor of the trypanosome surface, hereby facilitating the internalization of apoL1. Hpr or apoL1 deficiency respectively leads to the reduction or abolishment of human protection against T. brucei.