The metabolic pathways and transporters of the plastid organelle in Apicomplexa

Insights into the removal of antibiotics from livestock and aquaculture wastewater by algae-bacteria symbiosis systems

With the burgeoning growth of the livestock and aquaculture industries, antibiotic residues in treated wastewater have become a serious ecological threat. Traditional biological wastewater treatment technologies-while effective for removing conventional pollutants, such as organic carbon, ammonia and phosphate-struggle to eliminate emerging contaminants, notably antibiotics. Recently, the use of microalgae has emerged as a sustainable and promising approach for the removal of antibiotics due to their non-target status, rapid growth and carbon recovery capabilities. This review aims to analyse the current state of antibiotic removal from wastewater using algae-bacteria symbiosis systems and provide valuable recommendations for the development of livestock/aquaculture wastewater treatment technologies. It (1) summarises the biological removal mechanisms of typical antibiotics, including bioadsorption, bioaccumulation, biodegradation and co-metabolism; (2) discusses the roles of intracellular regulation, involving extracellular polymeric substances, pigments, antioxidant enzyme systems, signalling molecules and metabolic pathways; (3) analyses the role of treatment facilities in facilitating algae-bacteria symbiosis, such as sequencing batch reactors, stabilisation ponds, membrane bioreactors and bioelectrochemical systems; and (4) provides insights into bottlenecks and potential solutions. This review offers valuable information on the mechanisms and strategies involved in the removal of antibiotics from livestock/aquaculture wastewater through the symbiosis of microalgae and bacteria.

Phosphatidylserine synthase in the endoplasmic reticulum of Toxoplasma is essential for its lytic cycle in human cells

Glycerophospholipids have emerged as a significant contributor to the intracellular growth of pathogenic protist Toxoplasma gondii. Phosphatidylserine (PtdSer) is one such lipid, attributed to the locomotion and motility-dependent invasion and egress events in its acutely infectious tachyzoite stage. However, the de novo synthesis of PtdSer and the importance of the pathway in tachyzoites remain poorly understood. We show that a base-exchange-type PtdSer synthase (PSS) located in the parasite's endoplasmic reticulum produces PtdSer, which is rapidly converted to phosphatidylethanolamine (PtdEtn) by PtdSer decarboxylase (PSD) activity. The PSS-PSD pathway enables the synthesis of several lipid species, including PtdSer (16:0/18:1) and PtdEtn (18:2/20:4, 18:1/18:2 and 18:2/22:5). The PSS-depleted strain exhibited a lower abundance of the major ester-linked PtdEtn species and concurrent accrual of host-derived ether-PtdEtn species. Most phosphatidylthreonine (PtdThr) species-an exclusive natural analog of PtdSer, also made in the endoplasmic reticulum-were repressed. PtdSer species, however, remained largely unaltered, likely due to the serine-exchange reaction of PtdThr synthase in favor of PtdSer upon PSS depletion. Not least, the loss of PSS abrogated the lytic cycle of tachyzoites, impairing the cell division, motility, and egress. In a nutshell, our data demonstrate a critical role of PSS in the biogenesis of PtdSer and PtdEtn species and its physiologically essential repurposing for the asexual reproduction of a clinically relevant intracellular pathogen.

Apicoplast-Resident Processes: Exploiting the Chink in the Armour of Plasmodium falciparum Parasites

The discovery of a relict plastid, also known as an apicoplast (apicomplexan plastid), that houses housekeeping processes and metabolic pathways critical to Plasmodium parasites' survival has prompted increased research on identifying potent inhibitors that can impinge on apicoplast-localised processes. The apicoplast is absent in humans, yet it is proposed to originate from the eukaryote's secondary endosymbiosis of a primary symbiont. This symbiotic relationship provides a favourable microenvironment for metabolic processes such as haem biosynthesis, Fe-S cluster synthesis, isoprenoid biosynthesis, fatty acid synthesis, and housekeeping processes such as DNA replication, transcription, and translation, distinct from analogous mammalian processes. Recent advancements in comprehending the biology of the apicoplast reveal it as a vulnerable organelle for malaria parasites, offering numerous potential targets for effective antimalarial therapies. We provide an overview of the metabolic processes occurring in the apicoplast and discuss the organelle as a viable antimalarial target in light of current advances in drug discovery. We further highlighted the relevance of these metabolic processes to Plasmodium falciparum during the different stages of the lifecycle.

The Toxoplasma monocarboxylate transporters are involved in the metabolism within the apicoplast and are linked to parasite survival

The apicoplast is a four-membrane plastid found in the apicomplexans, which harbors biosynthesis and organelle housekeeping activities in the matrix. However, the mechanism driving the flux of metabolites, in and out, remains unknown. Here, we used TurboID and genome engineering to identify apicoplast transporters in Toxoplasma gondii. Among the many novel transporters, we show that one pair of apicomplexan monocarboxylate transporters (AMTs) appears to have evolved from a putative host cell that engulfed a red alga. Protein depletion showed that AMT1 and AMT2 are critical for parasite growth. Metabolite analyses supported the notion that AMT1 and AMT2 are associated with biosynthesis of isoprenoids and fatty acids. However, stronger phenotypic defects were observed for AMT2, including in the inability to establish T. gondii parasite virulence in mice. This study clarifies, significantly, the mystery of apicoplast transporter composition and reveals the importance of the pair of AMTs in maintaining the apicoplast activity in apicomplexans.

Novel Ion Channel Genes in Malaria Parasites

Ion channels serve many cellular functions including ion homeostasis, volume regulation, signaling, nutrient acquisition, and developmental progression. Although the complex life cycles of malaria parasites necessitate ion and solute flux across membranes, the whole-genome sequencing of the human pathogen Plasmodium falciparum revealed remarkably few orthologs of known ion channel genes. Contrasting with this, biochemical studies have implicated the channel-mediated flux of ions and nutritive solutes across several membranes in infected erythrocytes. Here, I review advances in the cellular and molecular biology of ion channels in malaria parasites. These studies have implicated novel parasite genes in the formation of at least two ion channels, with additional ion channels likely present in various membranes and parasite stages. Computational approaches that rely on homology to known channel genes from higher organisms will not be very helpful in identifying the molecular determinants of these activities. Given their unusual properties, novel molecular and structural features, and essential roles in pathogen survival and development, parasite channels should be promising targets for therapy development.

Complex Endosymbiosis II: The Nonphotosynthetic Plastid of Apicomplexa Parasites (The Apicoplast) and Its Integrated Metabolism

Chloroplasts are essential organelles that are responsible for photosynthesis in a wide range of organisms that have colonized all biotopes on Earth such as plants and unicellular algae. Interestingly, a secondary endosymbiotic event of a red algal ancestor gave rise to a group of organisms that have adopted an obligate parasitic lifestyle named Apicomplexa parasites. Apicomplexa parasites are some of the most widespread and poorly controlled pathogens in the world. These infectious agents are responsible for major human diseases such as toxoplasmosis, caused by Toxoplasma gondii, and malaria, caused by Plasmodium spp. Most of these parasites harbor this relict plastid named the apicoplast, which is essential for parasite survival. The apicoplast has lost photosynthetic capacities but is metabolically similar to plant and algal chloroplasts. The apicoplast is considered a novel and important drug target against Apicomplexa parasites. This chapter focuses on the apicoplast of apicomplexa parasites, its maintenance, and its metabolic pathways.

Toxoplasma gondii infection: novel emerging therapeutic targets

INTRODUCTION

Toxoplasmosis constitutes a challenge for public health, animal production, and welfare. So far, only a limited panel of drugs has been marketed for clinical applications. In addition to classical screening, the investigation of unique targets of the parasite may lead to the identification of novel drugs.

AREAS COVERED

Herein, the authors describe the methodology to identify novel drug targets in Toxoplasma gondii and review the literature with a focus on the last two decades.

EXPERT OPINION

Over the last two decades, the investigation of essential proteins of T. gondii as potential drug targets has fostered the hope of identifying novel compounds for the treatment of toxoplasmosis. Despite good efficacies in vitro, only a few classes of these compounds are effective in suitable rodent models, and none has cleared the hurdle to applications in humans. This shows that target-based drug discovery is in no way better than classical screening approaches. In both cases, off-target effects and adverse side effects in the hosts must be considered. Proteomics-driven analyses of parasite- and host-derived proteins that physically bind drug candidates may constitute a suitable tool to characterize drug targets, irrespectively of the drug discovery methods.

Reconstruction of Plastid Proteomes of Apicomplexans and Close Relatives Reveals the Major Evolutionary Outcomes of Cryptic Plastids

Apicomplexans and related lineages comprise many obligate symbionts of animals; some of which cause notorious diseases such as malaria. They evolved from photosynthetic ancestors and transitioned into a symbiotic lifestyle several times, giving rise to species with diverse non-photosynthetic plastids. Here, we sought to reconstruct the evolution of the cryptic plastids in the apicomplexans, chrompodellids, and squirmids (ACS clade) by generating five new single-cell transcriptomes from understudied gregarine lineages, constructing a robust phylogenomic tree incorporating all ACS clade sequencing datasets available, and using these to examine in detail, the evolutionary distribution of all 162 proteins recently shown to be in the apicoplast by spatial proteomics in Toxoplasma. This expanded homology-based reconstruction of plastid proteins found in the ACS clade confirms earlier work showing convergence in the overall metabolic pathways retained once photosynthesis is lost, but also reveals differences in the degrees of plastid reduction in specific lineages. We show that the loss of the plastid genome is common and unexpectedly find many lineage- and species-specific plastid proteins, suggesting the presence of evolutionary innovations and neofunctionalizations that may confer new functional and metabolic capabilities that are yet to be discovered in these enigmatic organelles.

The ferredoxin redox system - an essential electron distributing hub in the apicoplast of Apicomplexa

The apicoplast, a relict plastid found in most species of the phylum Apicomplexa, harbors the ferredoxin redox system which supplies electrons to enzymes of various metabolic pathways in this organelle. Recent reports in Toxoplasma gondii and Plasmodium falciparum have shown that the iron-sulfur cluster (FeS)-containing ferredoxin is essential in tachyzoite and blood-stage parasites, respectively. Here we review ferredoxin's crucial contribution to isoprenoid and lipoate biosynthesis as well as tRNA modification in the apicoplast, highlighting similarities and differences between the two species. We also discuss ferredoxin's potential role in the initial reductive steps required for FeS synthesis as well as recent evidence that offers an explanation for how NADPH required by the redox system might be generated in Plasmodium spp.

Toxoplasma metabolic flexibility in different growth conditions

Apicomplexan parasites have complex metabolic networks that coordinate acquisition of metabolites by de novo synthesis and by scavenging from the host. Toxoplasma gondii has a wide host range and may rely on the flexibility of this metabolic network. Currently, the literature categorizes genes as essential or dispensable according to their dispensability for parasite survival under nutrient-replete in vitro conditions. However, recent studies revealed correlations between medium composition and gene essentiality. Therefore, nutrient availability in the host environment likely determines the requirement of metabolic pathways, which may redefine priorities for drug target identification in a clinical setting. Here we review the recent work characterizing some of the major Toxoplasma metabolic pathways and their functional adaptation to host nutrient content.

PMRT1, a Plasmodium-Specific Parasite Plasma Membrane Transporter, Is Essential for Asexual and Sexual Blood Stage Development

Membrane transport proteins perform crucial roles in cell physiology. The obligate intracellular parasite Plasmodium falciparum, an agent of human malaria, relies on membrane transport proteins for the uptake of nutrients from the host, disposal of metabolic waste, exchange of metabolites between organelles, and generation and maintenance of transmembrane electrochemical gradients for its growth and replication within human erythrocytes. Despite their importance for Plasmodium cellular physiology, the functional roles of a number of membrane transport proteins remain unclear, which is particularly true for orphan membrane transporters that have no or limited sequence homology to transporter proteins in other evolutionary lineages. Therefore, in the current study, we applied endogenous tagging, targeted gene disruption, conditional knockdown, and knockout approaches to investigate the subcellular localization and essentiality of six membrane transporters during intraerythrocytic development of P. falciparum parasites. They are localized at different subcellular structures-the food vacuole, the apicoplast, and the parasite plasma membrane-and four out of the six membrane transporters are essential during asexual development. Additionally, the plasma membrane resident transporter 1 (PMRT1; PF3D7_1135300), a unique Plasmodium-specific plasma membrane transporter, was shown to be essential for gametocytogenesis and functionally conserved within the genus Plasmodium. Overall, we reveal the importance of four orphan transporters to blood stage P. falciparum development, which have diverse intracellular localizations and putative functions. IMPORTANCE Plasmodium falciparum-infected erythrocytes possess multiple compartments with designated membranes. Transporter proteins embedded in these membranes not only facilitate movement of nutrients, metabolites, and other molecules between these compartments, but also are common therapeutic targets and can confer antimalarial drug resistance. Orphan membrane transporters in P. falciparum without sequence homology to transporters in other evolutionary lineages and divergent from host transporters may constitute attractive targets for novel intervention approaches. Here, we localized six of these putative transporters at different subcellular compartments and probed their importance during asexual parasite growth by using reverse genetic approaches. In total, only two candidates turned out to be dispensable for the parasite, highlighting four candidates as putative targets for therapeutic interventions. This study reveals the importance of several orphan transporters to blood stage P. falciparum development.

Roles of the apicoplast across the life cycles of rodent and human malaria parasites

Malaria parasites are diheteroxenous, requiring two hosts-a vertebrate and a mosquito-to complete their life cycle. Mosquitoes are the definitive host where malaria parasite sex occurs, and vertebrates are the intermediate host, supporting asexual amplification and more significant geographic spread. In this review, we examine the roles of a single malaria parasite compartment, the relict plastid known as the apicoplast, at each life cycle stage. We focus mainly on two malaria parasite species-Plasmodium falciparum and P. berghei-comparing the changing, yet ever crucial, roles of their apicoplasts.

Inhibitors of the Plasmodium falciparum Hsp90 towards Selective Antimalarial Drug Design: The Past, Present and Future

Malaria is still one of the major killer parasitic diseases in tropical settings, posing a public health threat. The development of antimalarial drug resistance is reversing the gains made in attempts to control the disease. The parasite leads a complex life cycle that has adapted to outwit almost all known antimalarial drugs to date, including the first line of treatment, artesunate. There is a high unmet need to develop new strategies and identify novel therapeutics to reverse antimalarial drug resistance development. Among the strategies, here we focus and discuss the merits of the development of antimalarials targeting the Heat shock protein 90 (Hsp90) due to the central role it plays in protein quality control.