A central role for phosphatidic acid as a lipid mediator of regulated exocytosis in apicomplexa

Phosphatidic acid: from biophysical properties to diverse functions

Phosphatidic acid (PA), the simplest phospholipid, acts as a key metabolic intermediate and second messenger that impacts diverse cellular and physiological processes across species ranging from microbes to plants and mammals. The cellular levels of PA dynamically change in response to stimuli, and multiple enzymatic reactions can mediate its production and degradation. PA acts as a signalling molecule and regulates various cellular processes via its effects on membrane tethering, enzymatic activities of target proteins, and vesicular trafficking. Because of its unique physicochemical properties compared to other phospholipids, PA has emerged as a class of new lipid mediators influencing membrane structure, dynamics, and protein interactions. This review summarizes the biosynthesis, dynamics, and cellular functions and properties of PA.

Phosphatidic acid signaling and function in nuclei

Membrane lipidomes are dynamic and their changes generate lipid mediators affecting various biological processes. Phosphatidic acid (PA) has emerged as an important class of lipid mediators involved in a wide range of cellular and physiological responses in plants, animals, and microbes. The regulatory functions of PA have been studied primarily outside the nuclei, but an increasing number of recent studies indicates that some of the PA effects result from its action in nuclei. PA levels in nuclei are dynamic in response to stimuli. Changes in nuclear PA levels can result from activities of enzymes associated with nuclei and/or from movements of PA generated extranuclearly. PA has also been found to interact with proteins involved in nuclear functions, such as transcription factors and proteins undergoing nuclear translocation in response to stimuli. The nuclear action of PA affects various aspects of plant growth, development, and response to stress and environmental changes.

Phosphoproteomic Comparison of Four Eimeria tenella Life Cycle Stages

Protein phosphorylation is an important post-translational modification (PTM) involved in diverse cellular functions. It is the most prevalent PTM in both Toxoplasma gondii and Plasmodium falciparum, but its status in Eimeria tenella has not been reported. Herein, we performed a comprehensive, quantitative phosphoproteomic profile analysis of four stages of the E. tenella life cycle: unsporulated oocysts (USO), partially sporulated (7 h) oocysts (SO7h), sporulated oocysts (SO), and sporozoites (S). A total of 15,247 phosphorylation sites on 9514 phosphopeptides corresponding to 2897 phosphoproteins were identified across the four stages. In addition, 456, 479, and 198 differentially expressed phosphoproteins (DEPPs) were identified in the comparisons SO7h vs. USO, SO vs. SO7h, and S vs. SO, respectively. Gene Ontology (GO) term and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of DEPPs suggested that they were involved in diverse functions. For SO7h vs. USO, DEPPs were mainly involved in cell division, actin cytoskeleton organization, positive regulation of transport, and pyruvate metabolism. For SO vs. SO7h, they were related to the peptide metabolic process, translation, and RNA transport. DEPPs in the S vs. SO comparison were associated with the tricarboxylic acid metabolic process, positive regulation of ATPase activity, and calcium ion binding. Time course sequencing data analysis (TCseq) identified six clusters with similar expression change characteristics related to carbohydrate metabolism, cytoskeleton organization, and calcium ion transport, demonstrating different regulatory profiles across the life cycle of E. tenella. The results revealed significant changes in the abundance of phosphoproteins during E. tenella development. The findings shed light on the key roles of protein phosphorylation and dephosphorylation in the E. tenella life cycle.

Distinct effects on the secretion of MTRAP and AMA1 in Plasmodium yoelii following deletion of acylated pleckstrin homology domain-containing protein

Plasmodium, the causative agents of malaria, are obligate intracellular organisms. In humans, pathogenesis is caused by the blood stage parasite, which multiplies within erythrocytes, thus erythrocyte invasion is an essential developmental step. Merozoite form parasites released into the blood stream coordinately secrets a panel of proteins from the microneme secretory organelles for gliding motility, establishment of a tight junction with a target naive erythrocyte, and subsequent internalization. A protein identified in Toxoplasma gondii facilitates microneme fusion with the plasma membrane for exocytosis; namely, acylated pleckstrin homology domain-containing protein (APH). To obtain insight into the differential microneme discharge by malaria parasites, in this study we analyzed the consequences of APH deletion in the rodent malaria model, Plasmodium yoelii, using a DiCre-based inducible knockout method. We found that APH deletion resulted in a reduction in parasite asexual growth and erythrocyte invasion, with some parasites retaining the ability to invade and grow without APH. APH deletion impaired the secretion of microneme proteins, MTRAP and AMA1, and upon contact with erythrocytes the secretion of MTRAP, but not AMA1, was observed. APH-deleted merozoites were able to attach to and deform erythrocytes, consistent with the observed MTRAP secretion. Tight junctions were formed, but echinocytosis after merozoite internalization into erythrocytes was significantly reduced, consistent with the observed absence of AMA1 secretion. Together with our observation that APH largely colocalized with MTRAP, but less with AMA1, we propose that APH is directly involved in MTRAP secretion; whereas any role of APH in AMA1 secretion is indirect in Plasmodium.

Diverse Roles of TgMIC1/4/6 in the Toxoplasma Infection

Toxoplasma gondii microneme is a specialized secretory organelle that discharges its contents at the apical tip of this apicomplexan parasite in a sequential and regulated manner. Increasing number of studies on microneme proteins (MICs) have shown them as a predominant and important role in host cell attachment, invasion, motility and pathogenesis. In this review, we summarize the research advances in one of the most important MICs complexes, TgMIC1/4/6, which will contribute to improve the understanding of the molecular mechanism of T. gondii infection and provide a theoretical basis for the effective control against T. gondii.

Ferlins and TgDOC2 in Toxoplasma Microneme, Rhoptry and Dense Granule Secretion

The host cell invasion process of apicomplexan parasites like Toxoplasma gondii is facilitated by sequential exocytosis of the microneme, rhoptry and dense granule organelles. Exocytosis is facilitated by a double C2 domain (DOC2) protein family. This class of C2 domains is derived from an ancestral calcium (Ca2+) binding archetype, although this feature is optional in extant C2 domains. DOC2 domains provide combinatorial power to the C2 domain, which is further enhanced in ferlins that harbor 5-7 C2 domains. Ca2+ conditionally engages the C2 domain with lipids, membranes, and/or proteins to facilitating vesicular trafficking and membrane fusion. The widely conserved T. gondii ferlins 1 (FER1) and 2 (FER2) are responsible for microneme and rhoptry exocytosis, respectively, whereas an unconventional TgDOC2 is essential for microneme exocytosis. The general role of ferlins in endolysosmal pathways is consistent with the repurposed apicomplexan endosomal pathways in lineage specific secretory organelles. Ferlins can facilitate membrane fusion without SNAREs, again pertinent to the Apicomplexa. How temporal raises in Ca2+ combined with spatiotemporally available membrane lipids and post-translational modifications mesh to facilitate sequential exocytosis events is discussed. In addition, new data on cross-talk between secretion events together with the identification of a new microneme protein, MIC21, is presented.

Plasma membrane phosphatidylinositol (4,5)-bisphosphate promotes Weibel-Palade body exocytosis

Phosphatidylinositol (4,5)-bisphosphate transiently accumulates at sites of Weibel–Palade body–plasma membrane fusion and promotes agonist-evoked exocytosis of endothelial von-Willebrand factor.

Weibel–Palade bodies (WPB) are specialized secretory organelles of endothelial cells that control vascular hemostasis by regulated, Ca²⁺-dependent exocytosis of the coagulation-promoting von-Willebrand factor. Some proteins of the WPB docking and fusion machinery have been identified but a role of membrane lipids in regulated WPB exocytosis has so far remained elusive. We show here that the plasma membrane phospholipid composition affects Ca²⁺-dependent WPB exocytosis and von-Willebrand factor release. Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂] becomes enriched at WPB–plasma membrane contact sites at the time of fusion, most likely downstream of phospholipase D1-mediated production of phosphatidic acid (PA) that activates phosphatidylinositol 4-phosphate (PI4P) 5-kinase γ. Depletion of plasma membrane PI(4,5)P₂ or down-regulation of PI4P 5-kinase γ interferes with histamine-evoked and Ca²⁺-dependent WPB exocytosis and a mutant PI4P 5-kinase γ incapable of binding PA affects WPB exocytosis in a dominant-negative manner. This indicates that a unique PI(4,5)P₂-rich environment in the plasma membrane governs WPB fusion possibly by providing interaction sites for WPB-associated docking factors.

Signaling Cascades Governing Entry into and Exit from Host Cells by Toxoplasma gondii

The Apicomplexa phylum includes a large group of obligate intracellular protozoan parasites responsible for important diseases in humans and animals. Toxoplasma gondii is a widespread parasite with considerable versatility, and it is capable of infecting virtually any warm-blooded animal, including humans. This outstanding success can be attributed at least in part to an efficient and continuous sensing of the environment, with a ready-to-adapt strategy. This review updates the current understanding of the signals governing the lytic cycle of T. gondii, with particular focus on egress from infected cells, a key step for balancing survival, multiplication, and spreading in the host. We cover the recent advances in the conceptual framework of regulation of microneme exocytosis that ensures egress, motility, and invasion. Particular emphasis is given to the trigger molecules and signaling cascades regulating exit from host cells.

Phosphatidic acid in membrane rearrangements

Phosphatidic acid (PA) is the simplest cellular glycerophospholipid characterized by unique biophysical properties: a small headgroup; negative charge; and a phosphomonoester group. Upon interaction with lysine or arginine, PA charge increases from -1 to -2 and this change stabilizes protein-lipid interactions. The biochemical properties of PA also allow interactions with lipids in several subcellular compartments. Based on this feature, PA is involved in the regulation and amplification of many cellular signalling pathways and functions, as well as in membrane rearrangements. Thereby, PA can influence membrane fusion and fission through four main mechanisms: it is a substrate for enzymes producing lipids (lysophosphatidic acid and diacylglycerol) that are involved in fission or fusion; it contributes to membrane rearrangements by generating negative membrane curvature; it interacts with proteins required for membrane fusion and fission; and it activates enzymes whose products are involved in membrane rearrangements. Here, we discuss the biophysical properties of PA in the context of the above four roles of PA in membrane fusion and fission.

The roles of Centrin 2 and Dynein Light Chain 8a in apical secretory organelles discharge of Toxoplasma gondii

To efficiently enter host cells, apicomplexan parasites such as Toxoplasma gondii rely on an apical complex composed of tubulin-based structures as well as two sets of secretory organelles named micronemes and rhoptries. The trafficking and docking of these organelles to the apical pole of the parasite is crucial for the discharge of their contents. Here, we describe two proteins typically associated with microtubules, Centrin 2 (CEN2) and Dynein Light Chain 8a (DLC8a), that are required for efficient host cell invasion. CEN2 localizes to four different compartments, and remarkably, conditional depletion of the protein occurs in stepwise manner, sequentially depleting the protein pools from each location. This phenomenon allowed us to discern the essential function of the apical pool of CEN2 for microneme secretion, motility, invasion and egress. DLC8a localizes to the conoid, and its depletion also perturbs microneme exocytosis in addition to the apical docking of the rhoptry organelles, causing a severe defect in host cell invasion. Phenotypic characterization of CEN2 and DLC8a indicates that while both proteins participate in microneme secretion, they likely act at different steps along the cascade of events leading to organelle exocytosis.

An apically located hybrid guanylate cyclase-ATPase is critical for the initiation of Ca2+ signaling and motility in Toxoplasma gondii

Protozoan parasites of the phylum Apicomplexa actively move through tissue to initiate and perpetuate infection. The regulation of parasite motility relies on cyclic nucleotide-dependent kinases, but how these kinases are activated remains unknown. Here, using an array of biochemical and cell biology approaches, we show that the apicomplexan parasite Toxoplasma gondii expresses a large guanylate cyclase (TgGC) protein, which contains several upstream ATPase transporter-like domains. We show that TgGC has a dynamic localization, being concentrated at the apical tip in extracellular parasites, which then relocates to a more cytosolic distribution during intracellular replication. Conditional TgGC knockdown revealed that this protein is essential for acute-stage tachyzoite growth, as TgGC-deficient parasites were defective in motility, host cell attachment, invasion, and subsequent host cell egress. We show that TgGC is critical for a rapid rise in cytosolic [Ca²⁺] and for secretion of microneme organelles upon stimulation with a cGMP agonist, but these deficiencies can be bypassed by direct activation of signaling by a Ca²⁺ ionophore. Furthermore, we found that TgGC is required for transducing changes in extracellular pH and [K⁺] to activate cytosolic [Ca²⁺] flux. Together, the results of our work implicate TgGC as a putative signal transducer that activates Ca²⁺ signaling and motility in Toxoplasma.

Biogenesis and secretion of micronemes in Toxoplasma gondii

One of the hallmarks of the parasitic phylum of Apicomplexa is the presence of highly specialised, apical secretory organelles, called the micronemes and rhoptries that play critical roles in ensuring survival and dissemination. Upon exocytosis, the micronemes release adhesin complexes, perforins, and proteases that are crucially implicated in egress from infected cells, gliding motility, migration across biological barriers, and host cell invasion. Recent studies on Toxoplasma gondii and Plasmodium species have shed more light on the signalling events and the machinery that trigger microneme secretion. Intracellular cyclic nucleotides, calcium level, and phosphatidic acid act as key mediators of microneme exocytosis, and several downstream effectors have been identified. Here, we review the key steps of microneme biogenesis and exocytosis, summarising the still fractal knowledge at the molecular level regarding the fusion event with the parasite plasma membrane.

Three F-actin assembly centers regulate organelle inheritance, cell-cell communication and motility in Toxoplasma gondii

Toxoplasma gondii possesses a limited set of actin-regulatory proteins and relies on only three formins (FRMs) to nucleate and polymerize actin. We combined filamentous actin (F-actin) chromobodies with gene disruption to assign specific populations of actin filaments to individual formins. FRM2 localizes to the apical juxtanuclear region and participates in apicoplast inheritance. Restricted to the residual body, FRM3 maintains the intravacuolar cell-cell communication. Conoidal FRM1 initiates a flux of F-actin crucial for motility, invasion and egress. This flux depends on myosins A and H and is controlled by phosphorylation via PKG (protein kinase G) and CDPK1 (calcium-dependent protein kinase 1) and by methylation via AKMT (apical lysine methyltransferase). This flux is independent of microneme secretion and persists in the absence of the glideosome-associated connector (GAC). This study offers a coherent model of the key players controlling actin polymerization, stressing the importance of well-timed post-translational modifications to power parasite motility.

Essential cGMP Signaling in Toxoplasma Is Initiated by a Hybrid P-Type ATPase-Guanylate Cyclase

Apicomplexan parasites rely on cyclic nucleotide-dependent kinases for host cell infection, yet the mechanisms that control their activation remain unknown. Here we show that an apically localized guanylate cyclase (GC) controls microneme secretion and lytic growth in the model apicomplexan Toxoplasma gondii. Cell-permeable cGMP reversed the block in microneme secretion seen in a knockdown of TgGC, linking its function to production of cGMP. TgGC possesses an N-terminal P-type ATPase domain fused to a C-terminal heterodimeric guanylate cyclase domain, an architecture found only in Apicomplexa and related protists. Complementation with a panel of mutants revealed a critical requirement for the P-type ATPase domain for maximum GC function. We further demonstrate that knockdown of TgGC in vivo protects mice from lethal infection by blocking parasite expansion and dissemination. Collectively, this work demonstrates that cGMP-mediated signaling in Toxoplasma relies on a multi-domain architecture, which may serve a conserved role in related parasites.

Lipid signaling to membrane proteins: From second messengers to membrane domains and adapter-free endocytosis

Hilgemann et al. explain how lipid signaling to membrane proteins involves a hierarchy of mechanisms from lipid binding to membrane domain coalescence.

Lipids influence powerfully the function of ion channels and transporters in two well-documented ways. A few lipids act as bona fide second messengers by binding to specific sites that control channel and transporter gating. Other lipids act nonspecifically by modifying the physical environment of channels and transporters, in particular the protein–membrane interface. In this short review, we first consider lipid signaling from this traditional viewpoint, highlighting innumerable Journal of General Physiology publications that have contributed to our present understanding. We then switch to our own emerging view that much important lipid signaling occurs via the formation of membrane domains that influence the function of channels and transporters within them, promote selected protein–protein interactions, and control the turnover of surface membrane.

The Recombinant Sea Urchin Immune Effector Protein, rSpTransformer-E1, Binds to Phosphatidic Acid and Deforms Membranes

The purple sea urchin, Strongylocentrotus purpuratus, possesses a sophisticated innate immune system that functions without adaptive capabilities and responds to pathogens effectively by expressing the highly diverse SpTransformer gene family (formerly the Sp185/333 gene family). The swift gene expression response and the sequence diversity of SpTransformer cDNAs suggest that the encoded proteins have immune functions. Individual sea urchins can express up to 260 distinct SpTransformer proteins, and their diversity suggests that different versions may have different functions. Although the deduced proteins are diverse, they share an overall structure of a hydrophobic leader, a glycine-rich N-terminal region, a histidine-rich region, and a C-terminal region. Circular dichroism analysis of a recombinant SpTransformer protein, rSpTransformer-E1 (rSpTrf-E1) demonstrates that it is intrinsically disordered and transforms to α helical in the presence of buffer additives and binding targets. Although native SpTrf proteins are associated with the membranes of perinuclear vesicles in the phagocyte class of coelomocytes and are present on the surface of small phagocytes, they have no predicted transmembrane region or conserved site for glycophosphatidylinositol linkage. To determine whether native SpTrf proteins associate with phagocyte membranes through interactions with lipids, when rSpTrf-E1 is incubated with lipid-embedded nylon strips, it binds to phosphatidic acid (PA) through both the glycine-rich region and the histidine-rich region. Synthetic liposomes composed of PA and phosphatidylcholine show binding between rSpTrf-E1 and PA by fluorescence resonance energy transfer, which is associated with leakage of luminal contents suggesting changes in lipid organization and perhaps liposome lysis. Interactions with liposomes also change membrane curvature leading to liposome budding, fusion, and invagination, which is associated with PA clustering induced by rSpTrf-E1 binding. Longer incubations result in the extraction of PA from the liposomes, which form disorganized clusters. CD shows that when rSpTrf-E1 binds to PA, it changes its secondary structure from disordered to α helical. These results provide evidence for how SpTransformer proteins may associate with molecules that have exposed phosphates including PA on cell membranes and how the characteristic of protein multimerization may drive changes in the organization of membrane lipids.

TgPL2, a patatin-like phospholipase domain-containing protein, is involved in the maintenance of apicoplast lipids homeostasis in Toxoplasma

Patatin-like phospholipases are involved in numerous cellular functions, including lipid metabolism and membranes remodeling. The patatin-like catalytic domain, whose phospholipase activity relies on a serine-aspartate dyad and an anion binding box, is widely spread among prokaryotes and eukaryotes. We describe TgPL2, a novel patatin-like phospholipase domain-containing protein from the parasitic protist Toxoplasma gondii. TgPL2 is a large protein, in which the key motifs for enzymatic activity are conserved in the patatin-like domain. Using immunofluorescence assays and immunoelectron microscopy analysis, we have shown that TgPL2 localizes to the apicoplast, a non-photosynthetic plastid found in most apicomplexan parasites. This plastid hosts several important biosynthetic pathways, which makes it an attractive organelle for identifying new potential drug targets. We thus addressed TgPL2 function by generating a conditional knockdown mutant and demonstrated it has an essential contribution for maintaining the integrity of the plastid. In absence of TgPL2, the organelle is rapidly lost and remaining apicoplasts appear enlarged, with an abnormal accumulation of membranous structures, suggesting a defect in lipids homeostasis. More precisely, analyses of lipid content upon TgPL2 depletion suggest this protein is important for maintaining levels of apicoplast-generated fatty acids, and also regulating phosphatidylcholine and lysophosphatidylcholine levels in the parasite.

Structural basis for catalysis at the membrane-water interface

The membrane-water interface forms a uniquely heterogeneous and geometrically constrained environment for enzymatic catalysis. Integral membrane enzymes sample three environments - the uniformly hydrophobic interior of the membrane, the aqueous extramembrane region, and the fuzzy, amphipathic interfacial region formed by the tightly packed headgroups of the components of the lipid bilayer. Depending on the nature of the substrates and the location of the site of chemical modification, catalysis may occur in each of these environments. The availability of structural information for alpha-helical enzyme families from each of these classes, as well as several beta-barrel enzymes from the bacterial outer membrane, has allowed us to review here the different ways in which each enzyme fold has adapted to the nature of the substrates, products, and the unique environment of the membrane. Our focus here is on enzymes that process lipidic substrates. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.