What is pure hemozoin? A close look at the surface of the malaria pigment

Natural hemozoin and β-hematin induce tissue damage and apoptosis in human placental explants

Hemozoin (HZ) is a waste product of hemoglobin digestion by Plasmodium and has been implicated in several pathological processes, including inflammation, oxidative stress, endothelial dysfunction, and immune dysregulation. Studying the effects of HZ on the human placenta is essential to understanding the impact of malaria infection during pregnancy. The present study explored the impact of HZ produced by Plasmodium and β-hematin, referred to here as natural HZ (nHZ) and synthetic HZ (sHZ), respectively, on human placental explants exposed in vitro.

Methodology

nHZ was derived from Plasmodium falciparum cultures and isolated using magnetic MACS® Separation Columns (Miltenyi Biotec, Auburn, CA) [1]. sHZ was synthesized from hemin closure in an aqueous solution. Both nHZ and sHZ were characterized by infrared spectroscopy and scanning electron microscopy. Human placental explants (HPE) were exposed to 5 and 10 μg/mL of nHZ and sHZ for 24 h, and tissue integrity was studied using histological and immunohistochemical techniques.

Results

The studies have demonstrated that the exposition of both the nHZ and sHZ to placental tissue are comparable and cause effects in increased STB detachment, dysregulation of collagen distribution in the villous stroma, and increase in the frequency of cell apoptosis. This contributes to the understanding of the pathophysiology of malaria in pregnancy using synthetic products such as β-hematin.

Distinct Physical Properties of β-Hematin in Two Synthetic Media: Compelling Evidence

It is now widely accepted that detailed knowledge of the physicochemical characteristics of the β-hematin crystals, i.e., the synthetic versions of the natural hemozoin crystals, is important for understanding their formation, the design of antimalarial medicines, and malarial diagnosis. We report that the overall physical properties exhibited by β-hematins greatly depend on the synthetic media. Here, we synthesize β-hematin from hemin in aqueous-acetate and in aqueous-oily media and characterize their properties by several techniques. Infrared spectra clearly demonstrate the formation of β-hematin in both media. The β-hematin crystals prepared in aqueous-acetate are composed by needle-like particles with average lengths around 760-770 nm; their lattice parameters and unit cell volumes are larger than those reported in the literature. They are paramagnetic at 300 K and antiferromagnetic at very low temperatures. Their Mössbauer spectra at 298 K, 77 K, and 10 K are consistent with the presence of high-spin Fe (III) and are less asymmetric as a result of the occurrence of fast spin-spin relaxation time, and their surface composition is complex, showing the presence of a multiplicity of iron oxidation and spin states (although with a majority of high-spin Fe3+ ions). In comparison, the β-hematin crystals prepared in aqueous-oily medium have significantly smaller lengths (ca. 560 nm) and slightly larger unit cell volume in comparison to the previous sample. The magnetic measurements show that they are affected by superparamagnetism and paramagnetism at 298 K and the coexistence of weakly ferromagnetic or possibly ferrimagnetic and paramagnetic phases at 80 K. Their Mössbauer spectra at 298 K, 77 K, and 10 K, also consistent with the presence of high-spin Fe (III), show longer spin-spin relaxation times, and their surface composition is also complex containing less surface OH- groups and higher amounts of Fe (II) ions in low- and high-spin states. The observed differences are discussed in relation to the specific formation conditions present in the synthesis medium. The results reported here are of outmost importance for understanding how the physicochemical properties of β-hematins depend on the synthesis conditions.

Calcium signaling from damaged lysosomes induces cytoprotective stress granules

Lysosomal damage induces stress granule (SG) formation. However, the importance of SGs in determining cell fate and the precise mechanisms that mediate SG formation in response to lysosomal damage remain unclear. Here, we describe a novel calcium-dependent pathway controlling SG formation, which promotes cell survival during lysosomal damage. Mechanistically, the calcium-activated protein ALIX transduces lysosomal damage signals to SG formation by controlling eIF2α phosphorylation after sensing calcium leakage. ALIX enhances eIF2α phosphorylation by promoting the association between PKR and its activator PACT, with galectin-3 inhibiting this interaction; these regulatory events occur on damaged lysosomes. We further find that SG formation plays a crucial role in promoting cell survival upon lysosomal damage caused by factors such as SARS-CoV-2ORF3a, adenovirus, malarial pigment, proteopathic tau, or environmental hazards. Collectively, these data provide insights into the mechanism of SG formation upon lysosomal damage and implicate it in diseases associated with damaged lysosomes and SGs.

Cryo-tomography and 3D Electron Diffraction Reveal the Polar Habit and Chiral Structure of the Malaria Pigment Crystal Hemozoin

Detoxification of heme in Plasmodium depends on its crystallization into hemozoin. This pathway is a major target of antimalarial drugs. The crystalline structure of hemozoin was established by X-ray powder diffraction using a synthetic analog, β-hematin. Here, we apply emerging methods of in situ cryo-electron tomography and 3D electron diffraction to obtain a definitive structure of hemozoin directly from ruptured parasite cells. Biogenic hemozoin crystals take a striking polar morphology. Like β-hematin, the unit cell contains a heme dimer, which may form four distinct stereoisomers: two centrosymmetric and two chiral enantiomers. Diffraction analysis, supported by density functional theory analysis, reveals a selective mixture in the hemozoin lattice of one centrosymmetric and one chiral dimer. Absolute configuration has been determined by morphological analysis and confirmed by a novel method of exit-wave reconstruction from a focal series. Atomic disorder appears on specific facets asymmetrically, and the polar morphology can be understood in light of water binding. Structural modeling of the heme detoxification protein suggests a function as a chiral agent to bias the dimer formation in favor of rapid growth of a single crystalline phase. The refined structure of hemozoin should serve as a guide to new drug development.

Hemozoin-induced IFN-γ production mediates innate immune protection against sporozoite infection

Hemozoin is a crystal synthesized by Plasmodium parasites during hemoglobin digestion in the erythrocytic stage. The hemozoin released when the parasites egress from the red blood cell, which is complexed with parasite DNA, is cleared from the circulation by circulating and tissue-resident monocytes and macrophages, respectively. Recently, we reported that intravenous administration of purified hemozoin complexed with Plasmodium berghei DNA (HzPbDNA) resulted in an innate immune response that blocked liver stage development of sporozoites that was dose-dependent and time-limited. Here, we further characterize the organismal, cellular, and molecular events associated with this protective innate response in the liver and report that a large proportion of the IV administered HzPbDNA localized to F4/80+ cells in the liver and that the rapid and strong protection against liver-stage development waned quickly such that by 1 week post-HzPbDNA treatment animals were fully susceptible to infection. RNAseq of the liver after IV administration of HzPbDNA demonstrated that the rapid and robust induction of genes associated with the acute phase response, innate immune activation, cellular recruitment, and IFN-γ signaling observed at day 1 was largely absent at day 7. RNAseq analysis implicated NK cells as the major cellular source of IFN-γ. In vivo cell depletion and IFN-γ neutralization experiments supported the hypothesis that tissue-resident macrophages and NK cells are major contributors to the protective response and the NK cell-derived IFN-γ is key to induction of the mechanisms that block sporozoite development in the liver. These findings advance our understanding of the innate immune responses that prevent liver stage malaria infection.

A mechanism that transduces lysosomal damage signals to stress granule formation for cell survival

Lysosomal damage poses a significant threat to cell survival. Our previous work has reported that lysosomal damage induces stress granule (SG) formation. However, the importance of SG formation in determining cell fate and the precise mechanisms through which lysosomal damage triggers SG formation remains unclear. Here, we show that SG formation is initiated via a novel calcium-dependent pathway and plays a protective role in promoting cell survival in response to lysosomal damage. Mechanistically, we demonstrate that during lysosomal damage, ALIX, a calcium-activated protein, transduces lysosomal damage signals by sensing calcium leakage to induce SG formation by controlling the phosphorylation of eIF2α. ALIX modulates eIF2α phosphorylation by regulating the association between PKR and its activator PACT, with galectin-3 exerting a negative effect on this process. We also found this regulatory event of SG formation occur on damaged lysosomes. Collectively, these investigations reveal novel insights into the precise regulation of SG formation triggered by lysosomal damage, and shed light on the interaction between damaged lysosomes and SGs. Importantly, SG formation is significant for promoting cell survival in the physiological context of lysosomal damage inflicted by SARS-CoV-2 ORF3a, adenovirus infection, Malaria hemozoin, proteopathic tau as well as environmental hazard silica.

Plasmodium berghei Purified Hemozoin Associated with DNA Strongly Inhibits P. berghei Liver-Stage Development in BALB/c Mice after Intravenous Inoculation

In the acidic lysosome-like digestive vacuole, Plasmodium parasites crystallize heme from hemoglobin into hemozoin, or malaria pigment. Upon release of progeny merozoites, the residual hemozoin is phagocytized by macrophages principally in the liver and spleen where the heme crystals can persist for months to years, as heme oxygenase does not readily degrade the crystal. Previous studies demonstrated hemozoin modulation of monocytes and macrophages. Hemozoin modulates immune function activity of monocytes/macrophages. Here, we used purified/washed hemozoin (W-Hz) isolated from murine Plasmodium berghei infections and intravenously (i.v.) injected it back into naive mice. We characterized the modulating effect of W-Hz on liver-stage replication. Purified washed hemozoin decreases P. berghei liver levels both at 1 week and 1 month after i.v. injection in a dose and time dependent fashion. The injected hemozoin fully protected in nine out of 10 mice given a 50 sporozoite inoculum, and in 10 out of 10 mice against 2,000 sporozoites when they were infected an hour or a day after hemozoin inoculation. DNase treatment at the hemozoin reversed the observed liver load reduction. The liver load reduction was similar in mature B- and T-cell-deficient RAG-1 knockout (KO) mice suggesting an innate immune protection mechanism. This work indicates a role for residual hemozoin in down modulation of Plasmodium liver stages.

Hemozoin: a Complex Molecule with Complex Activities

Purpose of Review

Malaria is a disease caused by parasites that reside in host red blood cells and use hemoglobin as a nutrient source. Heme released by hemoglobin catabolism is modified by the parasite to produce hemozoin (HZ), which has toxic effects on the host. Experimentation aiming to elucidate how HZ contributes to malaria pathogenesis has utilized different preparations of this molecule, complicating interpretation and comparison of findings. We examine natural synthesis and isolation of HZ and highlight studies that have used multiple preparations, including synthetic forms, in a comparative fashion.

Recent Findings

Recent work utilizing sophisticated imaging and detection techniques reveals important molecular characteristics of HZ synthesis and biochemistry. Other recent studies further refine understanding of contributions of HZ to malaria pathogenesis yet highlight the continuing need to characterize HZ preparations and contextualize experimental conditions in the in vivo infection milieu.

Summary

This review highlights the necessity of collectively determining what is physiologically relevant HZ. Characterization of isolated natural HZ and use of multiple preparations in each study are recommended with application of in vivo studies whenever possible. Adoption of such practices is expected to improve reproducibility of results and elucidate the myriad of ways that HZ participates in malaria pathogenesis.

Potent antiplasmodial alkaloids from the rhizobacterium Pantoea agglomerans as hemozoin modulators

Global health concern regarding malaria has increased since the first report of artemisinin-resistant Plasmodium falciparum (Pf) two decades ago. The current therapies suffer various drawbacks such as low efficacy and significant side effects, alarming for an urgent need of more effective and less toxic drugs with higher patient compliance. Chemical entities with natural origins become progressively attractive as new drug leads due to their structural diversity and bio-compatibility. This study initially aimed at the targeted isolation of hydroxyquinoline derivatives following our published genomics and metabolomics study of Pantoea agglomerans (Pa). Fermentation of Pa on a pre-selected medium followed by chromatographic isolation, NMR and HRMS analyses led to the characterisation of one new hydroxyquinoline alkaloid together with another six known congeners and two known hydroxyquinolone derivatives. When screened for their antimalarial activity by high throughput screening against asexual blood-stage parasites, almost all compounds showed potent and selective sub-micromolar activities. Computational investigation was performed to identify the antiplasmodial potential targets. Ligand-based similarity search predicted the tested compounds to act as hemozoin inhibitors. Computational target identification results were further validated by competitive hemozoin inhibitory properties of hydroxyquinoline and hydroxyquinolone derivatives in vitro. The overall results suggest this natural scaffold is of potential to be developed as antimalarial drug lead.

Pyronaridine: An update of its pharmacological activities and mechanisms of action

Pyronaridine (PYR) is an erythrocytic schizonticide with a potent antimalarial activity against multidrug-resistant Plasmodium. The drug is used in combination with artesunate for the treatment of uncomplicated P. falciparum malaria, in adults and children. The present review briefly retraces the discovery of PYR and recent antimalarial studies which has led to the approval of PYR/artesunate combination (Pyramax) by the European Medicines Agency to treat uncomplicated malaria worldwide. PYR also presents a marked antitumor activity and has revealed efficacy for the treatment of other parasitic diseases (notably Babesia and Trypanosoma infections) and to mitigate the Ebola virus propagation. On the one hand, PYR functions has an inhibitor of hemozoin (biomineral malaria pigment, by-product of hemoglobin digestion) formation, blocking the biopolymerization of β-hematin and thus facilitating the accumulation of toxic hematin into the digestive vacuole of the parasite. On the other hand, PYR is a bona fide DNA-intercalating agent and an inhibitor of DNA topoisomerase 2, leading to DNA damages and cell death. Inhibition of hematin polymerization represents the prime mechanism at the origin of the antimalarial activity, whereas anticancer effects relies essentially on the interference with DNA metabolism, as with structurally related anticancer drugs like amsacrine and quinacrine. In addition, recent studies point to an immune modulatory activity of PYR and the implication of a mitochondrial oxidative pathway. An analogy with the mechanism of action of artemisinin drugs is underlined. In brief, the biological actions of pyronaridine are recapitulated to shed light on the diverse health benefits of this unsung drug.

Fluorinated scaffolds for antimalarial drug discovery

INTRODUCTION

The unique physicochemical properties and chemical diversity of organofluorine compounds have remarkably contributed for their wide utility in the area of pharmaceuticals, materials and agrochemicals. The noteworthy characteristics of fluorine include high electron affinity, lipophilicity and bioavailability, extending the half-life of the drugs. The incorporation of fluorine substituents, particularly trifluoromethyl groups, into organic molecules has led to their high potency against various diseases, including malaria. Hence, organofluorinated molecules offer valuable avenues for the design of new drug candidates against malaria.

AREAS COVERED

In this review, the authors discuss the importance of fluorine substituents present in the chemical compounds, and their potential applications for antimalarial drug discovery.

EXPERT OPINION

Fluorinated molecules represent a reliable strategy to develop new antimalarial drugs. Fluorine or fluorinated groups have been identified as a promising precursor, and their presence in approximately twenty-five percent of approved drugs is notable. Selective fluorination of chemical entities has the potential to be applied not only to improve the activity profile against the malaria parasite, but could be extrapolated for favorable pharmacological applications. Hazardous reagents such as HF, F₂ and SF₄ used for fluorination, are not considered as safe, and therefore, this process remains challenging, particularly for the pharmaceutical industry.

Inorganic ions on hemozoin surface provide a glimpse into Plasmodium biology

In malaria, Plasmodium parasites produce hemozoin (Hz) as a route to detoxify free heme released from the catabolism of hemoglobin. Hz isolated from the parasites is encapsulated in an organic layer constituted by parasite and host components. This organic coating may play a role in Hz formation and in the immunomodulatory properties attributed to Hz, and they may influence the mode of action of antimalarials that block Hz formation. In this work, we analyze the organic layer adhered to Hz, and find Na, Cl, Si, Ca and P present, in addition to organic material. Our results suggest that Na, Cl, and P adsorb during Hz release from the red blood cells, while Si and Ca derive from components present during Hz biomineralization within the digestive vacuole of the parasite. Overall, we show that inorganic elements associated with Hz surface provide insights into the biological functions of Plasmodium parasites.

The malaria toxin hemozoin induces apoptosis in human neurons and astrocytes: Potential role in the pathogenesis of cerebral malaria

Malaria, caused by an intracellular protozoan parasite of the genus Plasmodium, is one of the most important infectious diseases worldwide. In 2017, a total of 219 millions cases were reported with 435,000 deaths related to malaria. A major complication of malaria infection is cerebral malaria (CM), characterized by enhanced blood-brain barrier permeability, leukocyte infiltration and/or activation, and neuronal dropout resulting in coma and death in significant numbers of individuals, especially children. Despite the high incidence and mortality, the pathogenesis of cerebral malaria is not well characterized. Hemozoin (HMZ) or "malaria pigment," a by-product of intraerythrocytic parasite-mediated hemoglobin catabolism, is released into the bloodstream after lysis of the host infected erythrocyte. The effects of HMZ on brain cells has not been studied due to the contamination/adhesion/aggregation of the HMZ with host and toxic parasitic factors. We now demonstrate that extracellular purified HMZ is taken up by human neurons and astrocytes, resulting in cellular dysfunction and toxicity. These findings contribute to a better understanding of the neuropathogenesis of CM and provide evidence that HMZ accumulation in the bloodstream could result in CNS compromise. Thus, alternative approaches to reducing circulating HMZ could serve as a potential treatment.