Localization of Fe-S Biosynthesis Machinery in Cryptosporidium parvum Mitosome

Iron‑sulfur cluster biogenesis and function in Apicomplexa parasites

Iron‑sulfur cluster are ubiquitous and ancient protein cofactors that support a wide array of essential cellular functions. In eukaryotes, their assembly requires specific and dedicated machineries in each subcellular compartment. Apicomplexans are parasitic protists that are collectively responsible for a significant burden on the health of humans and other animals, and most of them harbor two organelles of endosymbiotic origin: a mitochondrion, and a plastid of high metabolic importance called the apicoplast. Consequently, apicomplexan parasites have distinct iron‑sulfur cluster assembly machineries located to their endosymbiotic organelles, as well as a cytosolic pathway. Recent findings have not only shown the importance of iron‑sulfur cluster assembly for the fitness of these parasites, but also highlighted parasite-specific features that may be promising for the development of targeted anti-parasitic strategies.

ATM1, an essential conserved transporter in Apicomplexa, bridges mitochondrial and cytosolic [Fe-S] biogenesis

The Apicomplexa phylum encompasses numerous obligate intracellular parasites, some associated with severe implications for human health, including Plasmodium, Cryptosporidium, and Toxoplasma gondii. The iron-sulfur cluster [Fe-S] biogenesis ISC pathway, localized within the mitochondrion or mitosome of these parasites, is vital for parasite survival and development. Previous work on T. gondii and Plasmodium falciparum provided insights into the mechanisms of [Fe-S] biogenesis within this phylum, while the transporter linking mitochondria-generated [Fe-S] with the cytosolic [Fe-S] assembly (CIA) pathway remained elusive. This critical step is catalyzed by a well-conserved ABC transporter, termed ATM1 in yeast, ATM3 in plants and ABCB7 in mammals. Here, we identify and characterize this transporter in two clinically relevant Apicomplexa. We demonstrate that depletion of TgATM1 does not specifically impair mitochondrial metabolism. Instead, proteomic analyses reveal that TgATM1 expression levels inversely correlate with the abundance of proteins that participate in the transfer of [Fe-S] to cytosolic proteins at the outer mitochondrial membrane. Further insights into the role of TgATM1 are gained through functional complementation with the well-characterized yeast homolog. Biochemical characterization of PfATM1 confirms its role as a functional ABC transporter, modulated by oxidized glutathione (GSSG) and [4Fe-4S].

Microbiota-produced indole metabolites disrupt mitochondrial function and inhibit Cryptosporidium parvum growth

Cryptosporidiosis is a leading cause of life-threatening diarrhea in young children in resource-poor settings. To explore microbial influences on susceptibility, we screened 85 microbiota-associated metabolites for their effects on Cryptosporidium parvum growth in vitro. We identify eight inhibitory metabolites in three main classes: secondary bile salts/acids, a vitamin B6 precursor, and indoles. Growth restriction of C. parvum by indoles does not depend on the host aryl hydrocarbon receptor (AhR) pathway. Instead, treatment impairs host mitochondrial function and reduces total cellular ATP, as well as directly reducing the membrane potential in the parasite mitosome, a degenerate mitochondria. Oral administration of indoles, or reconstitution of the gut microbiota with indole-producing bacteria, delays life cycle progression of the parasite in vitro and reduces the severity of C. parvum infection in mice. Collectively, these findings indicate that microbiota metabolites impair mitochondrial function and contribute to colonization resistance to Cryptosporidium infection.

Microbiota produced indole metabolites disrupt host cell mitochondrial energy production and inhibit Cryptosporidium parvum growth

Cryptosporidiosis is a leading cause of life-threatening diarrhea in young children in resource-poor settings. Susceptibility rapidly declines with age, associated with changes in the microbiota. To explore microbial influences on susceptibility, we screened 85 microbiota- associated metabolites enriched in the adult gut for their effects on C. parvum growth in vitro. We identified eight inhibitory metabolites in three main classes: secondary bile salts/acids, a vitamin B 6 precursor, and indoles. Growth restriction of C. parvum by indoles did not depend on the host aryl hydrocarbon receptor (AhR) pathway. Instead, treatment impaired host mitochondrial function and reduced total cellular ATP, as well as directly reduced the membrane potential in the parasite mitosome, a degenerate mitochondria. Oral administration of indoles, or reconstitution of the gut microbiota with indole producing bacteria, delayed life cycle progression of the parasite in vitro and reduced severity of C. parvum infection in mice. Collectively, these findings indicate that microbiota metabolites contribute to colonization resistance to Cryptosporidium infection.

An update on Cryptosporidium biology and therapeutic avenues

Cryptosporidium species has been identified as an important pediatric diarrheal pathogen in resource-limited countries, particularly in very young children (0–24 months). However, the only available drug (nitazoxanide) has limited efficacy and can only be prescribed in a medical setting to children older than one year. Many drug development projects have started to investigate new therapeutic avenues. Cryptosporidium’s unique biology is challenging for the traditional drug discovery pipeline and requires novel drug screening approaches. Notably, in recent years, new methods of oocyst generation, in vitro processing, and continuous three-dimensional cultivation capacities have been developed. This has enabled more physiologically pertinent research assays for inhibitor discovery. In a short time, many great strides have been made in the development of anti-Cryptosporidium drugs. These are expected to eventually turn into clinical candidates for cryptosporidiosis treatment in the future. This review describes the latest development in Cryptosporidium biology, genomics, transcriptomics of the parasite, assay development, and new drug discovery.

Oxygen levels are key to understanding "Anaerobic" protozoan pathogens with micro-aerophilic lifestyles

Publications abound on the physiology, biochemistry and molecular biology of "anaerobic" protozoal parasites as usually grown under "anaerobic" culture conditions. The media routinely used are poised at low redox potentials using techniques that remove O₂ to "undetectable" levels in sealed containers. However there is growing understanding that these culture conditions do not faithfully resemble the O₂ environments these organisms inhabit. Here we review for protists lacking oxidative energy metabolism, the oxygen cascade from atmospheric to intracellular concentrations and relevant methods of measurements of O₂, some well-studied parasitic or symbiotic protozoan lifestyles, their homeodynamic metabolic and redox balances, organism-drug-oxygen interactions, and the present and future prospects for improved drugs and treatment regimes.

[Fe-S] biogenesis and unusual assembly of the ISC scaffold complex in the Plasmodium falciparum mitochondrion

The malaria parasite harbors two [Fe-S] biogenesis pathways of prokaryotic origin-the SUF and ISC systems in the apicoplast and mitochondrion, respectively. While the SUF machinery has been delineated, there is little experimental evidence on the ISC pathway. We confirmed mitochondrial targeting of Plasmodium falciparum ISC proteins followed by analyses of cysteine desulfurase, scaffold, and [Fe-S]-carrier components. PfIscU functioned as the scaffold in complex with the PfIscS-PfIsd11 cysteine desulfurase and could directly assemble [4Fe-4S] without prior [2Fe-2S] formation seen in other homologs. Small angle X-ray scattering and spectral studies showed that PfIscU, a trimer, bound one [4Fe-4S]. In a deviation from reported complexes from other organisms, the P. falciparum desulfurase-scaffold complex assembled around a PfIscS tetramer instead of a dimer, resulting in a symmetric hetero-hexamer [2× (2PfIscS-2PfIsd11-2PfIscU)]. PfIscU directly transferred [4Fe-4S] to the apo-protein aconitase B thus abrogating the requirement of intermediary proteins for conversion of [2Fe-2S] to [4Fe-4S] before transfer to [4Fe-4S]-recipients. Among the putative cluster-carriers, PfIscA2 was more efficient than PfNifU-like protein; PfIscA1 primarily bound iron, suggesting its potential role as a Fe²⁺ carrier/donor. Our results identify the core P. falciparum ISC machinery and reveal unique features compared with those in bacteria or yeast and human mitochondria.

A key cytosolic iron-sulfur cluster synthesis protein localizes to the mitochondrion of Toxoplasma gondii

Iron-sulfur (Fe-S) clusters are prosthetic groups on proteins that function in a range of enzymatic and electron transfer reactions. Fe-S cluster synthesis is essential for the survival of all eukaryotes. Independent Fe-S cluster biosynthesis pathways occur in the mitochondrion, plastid, and cytosolic compartments of eukaryotic cells. Little is known about the cytosolic Fe-S cluster biosynthesis in apicomplexan parasites, the causative agents of diseases such as malaria and toxoplasmosis. NBP35 serves as a key scaffold protein on which cytosolic Fe-S clusters assemble, and has a cytosolic localization in most eukaryotes studied thus far. Unexpectedly, we found that the NBP35 homolog of the apicomplexan Toxoplasma gondii (TgNBP35) localizes to the outer mitochondrial membrane, with mitochondrial targeting mediated by an N-terminal transmembrane domain. We demonstrate that TgNBP35 is critical for parasite proliferation, but that, despite its mitochondrial localization, it is not required for Fe-S cluster synthesis in the mitochondrion. Instead, we establish that TgNBP35 is important for the biogenesis of cytosolic Fe-S proteins. Our data are consistent with TgNBP35 playing a central and specific role in cytosolic Fe-S cluster biosynthesis, and imply that the assembly of cytosolic Fe-S clusters occurs on the cytosolic face of the outer mitochondrial membrane in these parasites.

On the Origin of Iron/Sulfur Cluster Biosynthesis in Eukaryotes

Iron and sulfur are indispensable elements of every living cell, but on their own these elements are toxic and require dedicated machineries for the formation of iron/sulfur (Fe/S) clusters. In eukaryotes, proteins requiring Fe/S clusters (Fe/S proteins) are found in or associated with various organelles including the mitochondrion, endoplasmic reticulum, cytosol, and the nucleus. These proteins are involved in several pathways indispensable for the viability of each living cell including DNA maintenance, protein translation and metabolic pathways. Thus, the formation of Fe/S clusters and their delivery to these proteins has a fundamental role in the functions and the evolution of the eukaryotic cell. Currently, most eukaryotes harbor two (located in cytosol and mitochondrion) or three (located in plastid) machineries for the assembly of Fe/S clusters, but certain anaerobic microbial eukaryotes contain sulfur mobilization (SUF) machineries that were previously thought to be present only in archaeal linages. These machineries could not only stipulate which pathway was present in the last eukaryotic common ancestor (LECA), but they could also provide clues regarding presence of an Fe/S cluster machinery in the proto-eukaryote and evolution of Fe/S cluster assembly machineries in all eukaryotes.

NMR metabolomics reveals effects of Cryptosporidium infections on host cell metabolome

Background

Cryptosporidium is an important gut microbe whose contributions towards infant and immunocompromise patient mortality rates are steadily increasing. Over the last decade, we have seen the development of various tools and methods for studying Cryptosporidium infection and its interactions with their hosts. One area that is sorely overlooked is the effect infection has on host metabolic processes.

Results

Using a ¹H nuclear magnetic resonance approach to metabolomics, we have explored the nature of the mouse gut metabolome as well as providing the first insight into the metabolome of an infected cell line. Statistical analysis and predictive modelling demonstrated new understandings of the effects of a Cryptosporidium infection, while verifying the presence of known metabolic changes. Of note is the potential contribution of host derived taurine to the diarrhoeal aspects of the disease previously attributed to a solely parasite-based alteration of the gut environment, in addition to other metabolites involved with host cell catabolism.

Conclusion

This approach will spearhead our understanding of the Cryptosporidium-host metabolic exchange and provide novel targets for tackling this deadly parasite.

A Cell Culture Platform for the Cultivation of Cryptosporidium parvum

Cryptosporidium is a genus of ubiquitous unicellular parasites belonging to the phylum Apicomplexa. Cryptosporidium species are the second largest cause of childhood diarrhea and are associated with increased morbidity. Accompanying this is the low availability of treatment and lack of vaccines. The major barrier to developing effective treatment is the lack of reliable in vitro culture methods. Recently, our lab has successfully cultivated C. parvum in the esophageal cancer-derived cell line COLO-680N, and has been able to maintain infection for several weeks. The success of this cell line was assessed with a combination of various techniques including fluorescent microscopy and qPCR. In addition, to tackle the issue of long-term oocyst production in vitro, a simple, low-cost bioreactor system using the COLO-680N cell line was established, which produced infectious oocysts for 4 months. This chapter provides details on the methodologies used to culture, maintain, and assess Cryptosporidium infection and propagation in COLO-680N. © 2019 by John Wiley & Sons, Inc.

Past and future trends of Cryptosporidium in vitro research

Cryptosporidium is a genus of single celled parasites capable of infecting a wide range of animals including humans. Cryptosporidium species are members of the phylum apicomplexa, which includes well-known genera such as Plasmodium and Toxoplasma. Cryptosporidium parasites cause a severe gastro-intestinal disease known as cryptosporidiosis. They are one of the most common causes of childhood diarrhoea worldwide, and infection can have prolonged detrimental effects on the development of children, but also can be life threatening to HIV/AIDS patients and transplant recipients. A variety of hosts can act as reservoirs, and Cryptosporidium can persist in the environment for prolonged times as oocysts. While there has been substantial interest in these parasites, there is very little progress in terms of treatment development and understanding the majority of the life cycle of this unusual organism. In this review, we will provide an overview on the existing knowledge of the biology of the parasite and the current progress in developing in vitro cultivation systems. We will then describe a synopsis of current and next generation approaches that could spearhead further research in combating the parasite.