Acidocalcisome-Mitochondrion Membrane Contact Sites in Trypanosoma brucei

Polyphosphate Nanoparticles: Balancing Energy Requirements in Tissue Regeneration Processes

Nanoparticles of a particular, evolutionarily old inorganic polymer found across the biological kingdoms have attracted increasing interest in recent years not only because of their crucial role in metabolism but also their potential medical applicability: it is inorganic polyphosphate (polyP). This ubiquitous linear polymer is composed of 10-1000 phosphate residues linked by high-energy anhydride bonds. PolyP causes induction of gene activity, provides phosphate for bone mineralization, and serves as an energy supplier through enzymatic cleavage of its acid anhydride bonds and subsequent ATP formation. The biomedical breakthrough of polyP came with the development of a successful fabrication process, in depot form, as Ca- or Mg-polyP nanoparticles, or as the directly effective polymer, as soluble Na-polyP, for regenerative repair and healing processes, especially in tissue areas with insufficient blood supply. Physiologically, the platelets are the main vehicles for polyP nanoparticles in the circulating blood. To be biomedically active, these particles undergo coacervation. This review provides an overview of the properties of polyP and polyP nanoparticles for applications in the regeneration and repair of bone, cartilage, and skin. In addition to studies on animal models, the first successful proof-of-concept studies on humans for the healing of chronic wounds are outlined.

Advances in the cellular biology, biochemistry, and molecular biology of acidocalcisomes

SUMMARYAcidocalcisomes are organelles conserved during evolution and closely related to the so-called volutin granules of bacteria and archaea, to the acidocalcisome-like vacuoles of yeasts, and to the lysosome-related organelles of animal species. All these organelles have in common their acidity and high content of polyphosphate and calcium. They are characterized by a variety of functions from storage of phosphorus and calcium to roles in Ca2+ signaling, osmoregulation, blood coagulation, and inflammation. They interact with other organelles through membrane contact sites or by fusion, and have several enzymes, pumps, transporters, and channels.

Single and combination treatment of Toxoplasma gondii infections with a bumped kinase inhibitor and artemisone in vitro and with artemiside in experimentally infected mice

In previous studies, the artemisinin derivatives artemisone, its pro-drug artemiside and the bumped-kinase inhibitor BKI-1748 were effective against T. gondii via different modes of action. This suggests that they may act synergistically resulting in improved efficacies in vitro and in vivo. To test this hypothesis, the compounds were applied alone and in combination to T. gondii infected human fibroblast host cells in order to determine their inhibition constants and effects on cellular ultrastructure. In addition, the efficacy of either single- or combined treatments were assessed in an acute TgShSp1-oocyst infection model based on CD1 outbred mice. Whereas the IC50 of the compounds in combination (42 nM) was close to the IC50 of BKI-1748 alone (46 nM) and half of the IC50 of artemisone alone (92 nM), the IC90 of the combination was half of the values found with the single compounds (138 nM vs. ca. 270 nM). Another indication for synergistic effects in vitro were distinct alterations of the cellular ultrastructure of tachyzoites observed in combination, but not with the single compounds. These promising results could not be reproduced in vivo. There was no decrease in number of T. gondii positive brains by either treatment. However, the levels of infection in these brains, i. e. the number of tachyzoites, was significantly decreased upon BKI-1748 treatment alone, and the combination with artemiside did not produce any further decrease. The treatment with artemiside alone had no significant effects. A vertical transmission model could not be established since artemiside strongly interfered with pregnancy and caused abortion. These results show that is difficult to extrapolate from promising in vitro results to the situation in vivo.

Mapping diversity in African trypanosomes using high resolution spatial proteomics

African trypanosomes are dixenous eukaryotic parasites that impose a significant human and veterinary disease burden on sub-Saharan Africa. Diversity between species and life-cycle stages is concomitant with distinct host and tissue tropisms within this group. Here, the spatial proteomes of two African trypanosome species, Trypanosoma brucei and Trypanosoma congolense, are mapped across two life-stages. The four resulting datasets provide evidence of expression of approximately 5500 proteins per cell-type. Over 2500 proteins per cell-type are classified to specific subcellular compartments, providing four comprehensive spatial proteomes. Comparative analysis reveals key routes of parasitic adaptation to different biological niches and provides insight into the molecular basis for diversity within and between these pathogen species.

New insights into the role of acidocalcisomes in trypanosomatids

Acidocalcisomes are electron-dense organelles rich in polyphosphate and inorganic and organic cations that are acidified by proton pumps, and possess several channels, pumps, and transporters. They are present in bacteria and eukaryotes and have been studied in greater detail in trypanosomatids. Biogenesis studies of trypanosomatid acidocalcisomes found that they share properties with lysosome-related organelles of animal cells. In addition to their described roles in autophagy, cation and phosphorus storage, osmoregulation, pH homeostasis, and pathogenesis, recent studies have defined the role of these organelles in phosphate utilization, calcium ion (Ca2+ ) signaling, and bioenergetics, and will be the main subject of this review.

Protein control of membrane and organelle dynamics: Insights from the divergent eukaryote Toxoplasma gondii

Integral membrane protein complexes control key cellular functions in eukaryotes by defining membrane-bound spaces within organelles and mediating inter-organelles contacts. Despite the critical role of membrane complexes in cell biology, most of our knowledge is from a handful of model systems, primarily yeast and mammals, while a full functional and evolutionary understanding remains incomplete without the perspective from a broad range of divergent organisms.

Apicomplexan parasites are single-cell eukaryotes whose survival depends on organelle compartmentalisation and communication. Studies of a model apicomplexan, Toxoplasma gondii, reveal unexpected divergence in the composition and function of complexes previously considered broadly conserved, such as the mitochondrial ATP synthase and the tethers mediating ER–mitochondria membrane contact sites. Thus, Toxoplasma joins the repertoire of divergent model eukaryotes whose research completes our understanding of fundamental cell biology.

Mitochondrial Ca2+ and Reactive Oxygen Species in Trypanosomatids

Significance: Millions of people are infected with trypanosomatids and new therapeutic approaches are needed. Trypanosomatids possess one mitochondrion per cell and its study has led to discoveries of general biological interest. These mitochondria, as in their animal counterparts, generate reactive oxygen species (ROS) and have evolved enzymatic and nonenzymatic defenses against them. Mitochondrial calcium ion (Ca2+) overload leads to generation of ROS and its study could lead to relevant information on the biology of trypanosomatids and to novel drug targets. Recent Advances: Mitochondrial Ca2+ is normally involved in maintaining the bioenergetics of trypanosomes, but when Ca2+ overload occurs, it is associated with cell death. Trypanosomes lack key players in the mechanism of cell death described in mammalian cells, although mitochondrial Ca2+ overload results in collapse of their membrane potential, production of ROS, and cytochrome c release. They are also very resistant to mitochondrial permeability transition, and cell death after mitochondrial Ca2+ overload depends on generation of ROS. Critical Issues: In this review, we consider the mechanisms of mitochondrial oxidant generation and removal and the involvement of Ca2+ in trypanosome cell death. Future Directions: More studies are required to determine the reactions involved in generation of ROS by the mitochondria of trypanosomatids, their enzymatic and nonenzymatic defenses against ROS, and the occurrence and composition of a mitochondrial permeability transition pore. Antioxid. Redox Signal. 36, 969-983.

Transfer RNA-Derived Small RNAs in the Pathogenesis of Parasitic Protozoa

Transfer RNA (tRNA)-derived small RNAs (tsRNAs) are newly identified non-coding small RNAs that have recently attracted attention due to their functional significance in both prokaryotes and eukaryotes. tsRNAs originated from the cleavage of precursor or mature tRNAs by specific nucleases. According to the start and end sites, tsRNAs can be broadly divided into tRNA halves (31-40 nucleotides) and tRNA-derived fragments (tRFs, 14-30 nucleotides). tsRNAs have been reported in multiple organisms to be involved in gene expression regulation, protein synthesis, and signal transduction. As a novel regulator, tsRNAs have also been identified in various protozoan parasites. The conserved biogenesis of tsRNAs in early-branching eukaryotes strongly suggests the universality of this machinery, which requires future research on their shared and potentially disparate biological functions. Here, we reviewed the recent studies of tsRNAs in several representative protozoan parasites including their biogenesis and the roles in parasite biology and intercellular communication. Furthermore, we discussed the remaining questions and potential future works for tsRNAs in this group of organisms.

The Spliced Leader RNA Silencing (SLS) Pathway in Trypanosoma brucei Is Induced by Perturbations of Endoplasmic Reticulum, Golgi Complex, or Mitochondrial Protein Factors: Functional Analysis of SLS-Inducing Kinase PK3

In the parasite Trypanosoma brucei, the causative agent of human African sleeping sickness, all mRNAs are trans-spliced to generate a common 5′ exon derived from the spliced leader (SL) RNA. Perturbations of protein translocation across the endoplasmic reticulum (ER) induce the spliced leader RNA silencing (SLS) pathway. SLS activation is mediated by a serine-threonine kinase, PK3, which translocates from the cytosolic face of the ER to the nucleus, where it phosphorylates the TATA-binding protein TRF4, leading to the shutoff of SL RNA transcription, followed by induction of programmed cell death. Here, we demonstrate that SLS is also induced by depletion of the essential ER-resident chaperones BiP and calreticulin, ER oxidoreductin 1 (ERO1), and the Golgi complex-localized quiescin sulfhydryl oxidase (QSOX). Most strikingly, silencing of Rhomboid-like 1 (TIMRHOM1), involved in mitochondrial protein import, also induces SLS. The PK3 kinase, which integrates SLS signals, is modified by phosphorylation on multiple sites. To determine which of the phosphorylation events activate PK3, several individual mutations or their combination were generated. These mutations failed to completely eliminate the phosphorylation or translocation of the kinase to the nucleus. The structures of PK3 kinase and its ATP binding domain were therefore modeled. A conserved phenylalanine at position 771 was proposed to interact with ATP, and the PK3^F771L mutation completely eliminated phosphorylation under SLS, suggesting that the activation involves most if not all of the phosphorylation sites. The study suggests that the SLS occurs broadly in response to failures in protein sorting, folding, or modification across multiple compartments.

Interaction With the Extracellular Matrix Triggers Calcium Signaling in Trypanosoma cruzi Prior to Cell Invasion

Trypanosoma cruzi, the etiological agent of Chagas disease in humans, infects a wide variety of vertebrates. Trypomastigotes, the parasite infective forms, invade mammalian cells by a still poorly understood mechanism. Adhesion of tissue culture- derived trypomastigotes to the extracellular matrix (ECM) prior to cell invasion has been shown to be a relevant part of the process. Changes in phosphorylation, S-nitrosylation, and nitration levels of proteins, in the late phase of the interaction (2 h), leading to the reprogramming of both trypomastigotes metabolism and the DNA binding profile of modified histones, were described by our group. Here, the involvement of calcium signaling at a very early phase of parasite interaction with ECM is described. Increments in the intracellular calcium concentrations during trypomastigotes-ECM interaction depends on the Ca²⁺ uptake from the extracellular medium, since it is inhibited by EGTA or Nifedipine, an inhibitor of the L-type voltage gated Ca²⁺ channels and sphingosine-dependent plasma membrane Ca²⁺ channel, but not by Vanadate, an inhibitor of the plasma membrane Ca²⁺-ATPase. Furthermore, Nifedipine inhibits the invasion of host cells by tissue culture- derived trypomastigotes in a dose-dependent manner, reaching 95% inhibition at 100 µM Nifedipine. These data indicate the importance of both Ca²⁺ uptake from the medium and parasite-ECM interaction for host-cell invasion. Previous treatment of ECM with protease abolishes the Ca²⁺ uptake, further reinforcing the possibility that these events may be connected. The mitochondrion plays a relevant role in Ca²⁺ homeostasis in trypomastigotes during their interaction with ECM, as shown by the increment of the intracellular Ca²⁺ concentration in the presence of Antimycin A, in contrast to other calcium homeostasis disruptors, such as Cyclopiazonic acid for endoplasmic reticulum and Bafilomycin A for acidocalcisome. Total phosphatase activity in the parasite decreases in the presence of Nifedipine, EGTA, and Okadaic acid, implying a role of calcium in the phosphorylation level of proteins that are interacting with the ECM in tissue culture- derived trypomastigotes. In summary, we describe here the increment of Ca²⁺ at an early phase of the trypomastigotes interaction with ECM, implicating both nifedipine-sensitive Ca²⁺ channels in the influx of Ca²⁺ and the mitochondrion as the relevant organelle in Ca²⁺ homeostasis. The data unravel a complex sequence of events prior to host cell invasion itself.

TbVps41 regulates trafficking of endocytic but not biosynthetic cargo to lysosomes of bloodstream forms of Trypanosoma brucei

The bloodstream stage of Trypanosoma brucei, the causative agent of African trypanosomiasis, is characterized by its high rate of endocytosis, which is involved in remodeling of its surface coat. Here we present evidence that RNAi-mediated expression down-regulation of vacuolar protein sorting 41 (Vps41), a component of the homotypic fusion and vacuole protein sorting (HOPS) complex, leads to a strong inhibition of endocytosis, vesicle accumulation, enlargement of the flagellar pocket ("big eye" phenotype), and dramatic effect on cell growth. Unexpectedly, other functions described for Vps41 in mammalian cells and yeasts, such as delivery of proteins to lysosomes, and lysosome-related organelles (acidocalcisomes) were unaffected, indicating that in trypanosomes post-Golgi trafficking is distinct from that of mammalian cells and yeasts. The essentiality of TbVps41 suggests that it is a potential drug target.

Mitochondrial Ca2+ homeostasis in trypanosomes

Mitochondrial calcium ion (Ca2+) uptake is important for buffering cytosolic Ca2+ levels, for regulating cell bioenergetics, and for cell death and autophagy. Ca2+ uptake is mediated by a mitochondrial Ca2+ uniporter (MCU) and the discovery of this channel in trypanosomes has been critical for the identification of the molecular nature of the channel in all eukaryotes. However, the trypanosome uniporter, which has been studied in detail in Trypanosoma cruzi, the agent of Chagas disease, and T. brucei, the agent of human and animal African trypanosomiasis, has lineage-specific adaptations which include the lack of some homologues to mammalian subunits, and the presence of unique subunits. Here, we review newly emerging insights into the role of mitochondrial Ca2+ homeostasis in trypanosomes, the composition of the uniporter, its functional characterization, and its role in general physiology.

The IP3 receptor and Ca2+ signaling in trypanosomes

Trypanosoma cruzi, and the T. brucei group of parasites cause neglected diseases that affect millions of people around the world. These unicellular microorganisms have complex life cycles involving an insect vector and a mammalian host. Both groups of pathogens possess an inositol 1,4,5-trisphosphate (IP₃)/diacylglycerol (DAG) signaling pathway, and an IP₃ receptor, but with lineage-specific adaptations that make them different from their mammalian counterparts. The phospholipase C (PLC), which hydrolyzes phosphatidyl inositol 4,5-bisphosphate (PIP₂) to IP₃ is N-terminally myristoylated and palmitoylated. Acidocalcisomes, which are lysosome-related organelles rich in polyphosphate, are the main intracellular Ca²⁺ stores. The inositol 1,4,5-trisphosphate receptor (IP₃R) localizes to acidocalcisomes instead of the endoplasmic reticulum. The trypanosome IP₃R is stimulated by luminal phosphate and pyrophosphate, which are hydrolysis products of polyphosphate (polyP), and inhibited by tripolyphosphate (polyP₃), which is the most abundant polyP in acidocalcisomes. Ca²⁺ signaling is important for host cell invasion and differentiation and to maintain cellular bioenergetics.

Mitochondrial sphingosine-1-phosphate lyase is essential for phosphatidylethanolamine synthesis and survival of Trypanosoma brucei

Sphingosine-1-phosphate is a signaling molecule involved in the control of cell migration, differentiation, survival and other physiological processes. This sphingolipid metabolite can be degraded by the action of sphingosine-1-phosphate lyase (SPL) to form hexadecenal and ethanolamine phosphate. The importance of SPL-mediated ethanolamine phosphate formation has been characterized in only few cell types. We show that in the protozoan parasite Trypanosoma brucei, expression of TbSpl is essential for cell survival. Ablation of TbSpl expression increased sphingosine-1-phosphate levels and reduced de novo formation and steady-state levels of the glycerophospholipid phosphatidylethanolamine (PE). Growth of TbSpl-depleted parasites could be in part rescued by ethanolamine supplementation to the growth medium, indicating that the main function of TbSpl is to provide ethanolamine phosphate for PE synthesis. In contrast to most cell types analyzed, where SPL localizes to the endoplasmic reticulum, we found by high-resolution microscopy that TbSpl is a mitochondrial protein. In spite of its mitochondrial localization, TbSpl depletion had no apparent effect on mitochondrial morphology but resulted in aggregation of acidocalcisomes. Our results link mitochondria to sphingolipid metabolism and suggest possible roles for PE in acidocalcisome function.

Airyscan Superresolution Microscopy to Study Trypanosomatid Cell Biology

The recent introduction by Carl Zeiss Ltd. of the Airyscan detector module for their LSM880 confocal laser-scanning microscope has enabled routine superresolution microscopy to be combined with the advantages of confocal-based fluorescence imaging. Resulting enhanced spatial resolution in X, Y, and Z provides tractable opportunity to derive new insight into protein localization(s), organelle dynamics, and thence protein function within trypanosomatids or other organisms. Here, we describe methods for preparing slides, cells, and basic microscope setup for fluorescence imaging of trypanosomatids using the LSM-880 with Airyscan platform.

Inorganic Polyphosphates As Storage for and Generator of Metabolic Energy in the Extracellular Matrix

Inorganic polyphosphates (polyP) consist of linear chains of orthophosphate residues, linked by high-energy phosphoanhydride bonds. They are evolutionarily old biopolymers that are present from bacteria to man. No other molecule concentrates as much (bio)chemically usable energy as polyP. However, the function and metabolism of this long-neglected polymer are scarcely known, especially in higher eukaryotes. In recent years, interest in polyP experienced a renaissance, beginning with the discovery of polyP as phosphate source in bone mineralization. Later, two discoveries placed polyP into the focus of regenerative medicine applications. First, polyP shows morphogenetic activity, i.e., induces cell differentiation via gene induction, and, second, acts as an energy storage and donor in the extracellular space. Studies on acidocalcisomes and mitochondria provided first insights into the enzymatic basis of eukaryotic polyP formation. In addition, a concerted action of alkaline phosphatase and adenylate kinase proved crucial for ADP/ATP generation from polyP. PolyP added extracellularly to mammalian cells resulted in a 3-fold increase of ATP. The importance and mechanism of this phosphotransfer reaction for energy-consuming processes in the extracellular matrix are discussed. This review aims to give a critical overview about the formation and function of this unique polymer that is capable of storing (bio)chemically useful energy.

5-Diphosphoinositol pentakisphosphate (5-IP7) regulates phosphate release from acidocalcisomes and yeast vacuoles

Acidocalcisomes of Trypanosoma brucei and the acidocalcisome-like vacuoles of Saccharomyces cerevisiae are acidic calcium compartments that store polyphosphate (polyP). Both organelles possess a phosphate-sodium symporter (TbPho91 and Pho91p in T. brucei and yeast, respectively), but the roles of these transporters in growth and orthophosphate (P_i) transport are unclear. We found here that Tbpho91 ^-/- trypanosomes have a lower growth rate under phosphate starvation and contain larger acidocalcisomes that have increased P_i content. Heterologous expression of TbPHO91 in Xenopus oocytes followed by two-electrode voltage clamp recordings disclosed that myo-inositol polyphosphates stimulate both sodium-dependent depolarization of the oocyte membrane potential and P_i conductance. Deletion of the SPX domain in TbPho91 abolished this stimulation. Inositol pyrophosphates such as 5-diphosphoinositol pentakisphosphate generated outward currents in Na⁺/P_i-loaded giant vacuoles prepared from WT or from TbPHO91-expressing pho91Δ strains but not from the pho91Δ yeast strains or from the pho91Δ strains expressing PHO91 or TbPHO91 with mutated SPX domains. Our results indicate that TbPho91 and Pho91p are responsible for vacuolar P_i and Na⁺ efflux and that myo-inositol polyphosphates stimulate the Na⁺/P_i symporter activities through their SPX domains.

Membrane Proteins in Trypanosomatids Involved in Ca2+ Homeostasis and Signaling

Calcium ion (Ca2+) serves as a second messenger for a variety of cell functions in trypanosomes. Several proteins in the plasma membrane, acidocalcisomes, endoplasmic reticulum, and mitochondria are involved in its homeostasis and in cell signaling roles. The plasma membrane has a Ca2+ channel for its uptake and a plasma membrane-type Ca2+-ATPase (PMCA) for its efflux. A similar PMCA is also located in acidocalcisomes, acidic organelles that are the primary Ca2+ store and that possess an inositol 1,4,5-trisphosphate receptor (IP₃R) for Ca2+ efflux. Their mitochondria possess a mitochondrial calcium uniporter complex (MCUC) for Ca2+ uptake and a Ca2+/H⁺ exchanger for Ca2+ release. The endoplasmic reticulum has a sarcoplasmic-endoplasmic reticulum-type Ca2+-ATPase (SERCA) for Ca2+ uptake but no Ca2+ release mechanism has been identified. Additionally, the trypanosomatid genomes contain other membrane proteins that could potentially bind calcium and await further characterization.