LC3-association with the parasitophorous vacuole membrane of Plasmodium berghei liver stages follows a noncanonical autophagy pathway

CSP ubiquitylation favours Plasmodium berghei survival during early liver stage infection

The circumsporozoite protein (CSP), an essential protein that covers the surface of the Plasmodium sporozoite, is a key player in multiple stages of the parasite development within the mosquito and during interactions between sporozoites and mammalian hepatocytes. Here, we identify a novel function of Plasmodium berghei CSP: preventing parasite elimination during the early stages of hepatic infection, through its ubiquitylation at two lysine (K) residues, K252 and K258, located in the C-terminal domain. A Plasmodium berghei transgenic line lacking these lysine residues exhibited a significant decrease in hepatic infectivity, with parasites being eliminated 4 h after infection. The reduced infectivity correlated with an increased association of host autophagy markers, LC3 and LAMP1, to the parasitophorous vacuole membrane of the liver stage parasite. Notably, inhibiting the host autophagy pathway fully rescued the mutant parasites from elimination. Collectively, we reveal a strategy employed by Plasmodium to evade early clearance during hepatic infection, which relies on the ubiquitylation of specific CSP lysine residues, that results in reduced parasite elimination via host autophagic and lysosomal activity.

Apicomplexan Espionage: Orchestrated Miscommunication at the Host-Parasite Interface

Intracellular parasites, including Toxoplasma and Plasmodium, are entirely reliant on the active scavenging of host-derived nutrients to fuel their replicative cycle, as they are confined within a specialized membrane-bound compartment, the parasitophorous vacuole (PV). Initial observations, based on the proximity of host vesicles to the parasitophorous vacuole membrane (PVM), suggested that parasites utilize host vesicles to obtain essential nutrients. However, mounting evidence has now unequivocally demonstrated that intracellular pathogens establish membrane contacts with host organelles, establishing control over host cellular machinery. These intimate interactions enable the parasites to gain unimpeded access to cytosolic resources critical for development while evading host immune responses. This review consolidates the latest advancements in understanding the molecular machinery driving these transkingdom contacts and their functional roles. Further investigation into these processes promises to revolutionize our understanding of organelle communication, with profound implications for identifying new therapeutic targets and strategies.

The role of host autophagy in intracellular protozoan parasites diseases

Intracellular protozoan parasites are the etiologic agents of important human diseases, like malaria, Chagas disease, toxoplasmosis, and leishmaniasis. Inside host cells, these parasites manipulate the host metabolism and intracellular trafficking for their own benefits and, inevitably, induce several stress response mechanisms. In this review, we discuss autophagy as a stress response mechanism that can be both (i) explored by these intracellular parasites to acquire nutrients and (ii) to restrict parasite proliferation and survival within host cells. We also discuss the immunomodulatory role of autophagy as a strategy to reduce inflammatory-mediated damage, an essential player in the pathophysiology of these parasitic diseases. At last, we propose and discuss several known autophagy modulators as possible pharmaceuticals for adjunctive therapies.

LC3B labeling of the parasitophorous vacuole membrane of Plasmodium berghei liver stage parasites depends on the V-ATPase and ATG16L1

The protozoan parasite Plasmodium, the causative agent of malaria, undergoes an obligatory stage of intra-hepatic development before initiating a blood-stage infection. Productive invasion of hepatocytes involves the formation of a parasitophorous vacuole (PV) generated by the invagination of the host cell plasma membrane. Surrounded by the PV membrane (PVM), the parasite undergoes extensive replication. During intracellular development in the hepatocyte, the parasites provoke the Plasmodium-associated autophagy-related (PAAR) response. This is characterized by a long-lasting association of the autophagy marker protein, and ATG8 family member, LC3B with the PVM. LC3B localization at the PVM does not follow the canonical autophagy pathway since upstream events specific to canonical autophagy are dispensable. Here, we describe that LC3B localization at the PVM of Plasmodium parasites requires the V-ATPase and its interaction with ATG16L1. The WD40 domain of ATG16L1 is crucial for its recruitment to the PVM. Thus, we provide new mechanistic insight into the previously described PAAR response targeting Plasmodium liver stage parasites.

Autophagy as an innate immunity response against pathogens: a Tango dance

Intracellular infections as well as changes in the cell nutritional environment are main events that trigger cellular stress responses. One crucial cell response to stress conditions is autophagy. During the last 30 years, several scenarios involving autophagy induction or inhibition over the course of an intracellular invasion by pathogens have been uncovered. In this review, we will present how this knowledge was gained by studying different microorganisms. We intend to discuss how the cell, via autophagy, tries to repel these attacks with the objective of destroying the intruder, but also how some pathogens have developed strategies to subvert this. These two fates can be compared with a Tango, a dance originated in Buenos Aires, Argentina, in which the partner dancers are in close connection. One of them is the leader, embracing and involving the partner, but the follower may respond escaping from the leader. This joint dance is indeed highly synchronized and controlled, perfectly reflecting the interaction between autophagy and microorganism.

Pathogenic mechanism of Eimeria tenella autophagy activation of chicken embryo cecal epithelial cells induced by Eimeria tenella

Eimeria tenella mainly invades and develops into cecal epithelial cells of chickens, resulting in cecal epithelial cell damage. Infectious intracellular pathogens possibly act by influencing the autophagy process after invading cells. The interaction between E. tenella and the autophagy of host cells was explored by infecting E. tenella with chick embryo cecal epithelial cells. Transmission electron microscopy, laser confocal microscopy, and Western blot analysis were used to demonstrate that E. tenella infection could induce autophagy in host cells. Results showed that infection with E. tenella induced the formation of autophagosomes in cells. The expression of ATG 5, Beclin-1, and LC3B-II proteins were significantly (P < 0.01) increased after E. tenella infected host cells. Expression of p62 protein levels were significantly (P < 0.01) decreased in host cells infected with E. tenella. Chloroquine (CQ) significantly (P < 0.01) increased the expression levels of LC3B-II and P62 in E. tenella-infected host cells. Rapamycin (RAPA) induced autophagy in host cells, thus reducing the intracellular infection of E. tenella. By contrast, the infection rate of E. tenella increased in cells treated with 3-Methyladenine (3-MA). Hence, E. tenella sporozoite infection could induce autophagy activation in chick embryo cecal epithelial cells, and enhanced autophagy could reduce the infection rate of E. tenella.

Autophagy in protists and their hosts: When, how and why?

Pathogenic protists are a group of organisms responsible for causing a variety of human diseases including malaria, sleeping sickness, Chagas disease, leishmaniasis, and toxoplasmosis, among others. These diseases, which affect more than one billion people globally, mainly the poorest populations, are characterized by severe chronic stages and the lack of effective antiparasitic treatment. Parasitic protists display complex life-cycles and go through different cellular transformations in order to adapt to the different hosts they live in. Autophagy, a highly conserved cellular degradation process, has emerged as a key mechanism required for these differentiation processes, as well as other functions that are crucial to parasite fitness. In contrast to yeasts and mammals, protist autophagy is characterized by a modest number of conserved autophagy-related proteins (ATGs) that, even though, can drive the autophagosome formation and degradation. In addition, during their intracellular cycle, the interaction of these pathogens with the host autophagy system plays a crucial role resulting in a beneficial or harmful effect that is important for the outcome of the infection. In this review, we summarize the current state of knowledge on autophagy and other related mechanisms in pathogenic protists and their hosts. We sought to emphasize when, how, and why this process takes place, and the effects it may have on the parasitic cycle. A better understanding of the significance of autophagy for the protist life-cycle will potentially be helpful to design novel anti-parasitic strategies.

Overview of Apoptosis, Autophagy, and Inflammatory Processes in Toxoplasma gondii Infected Cells

Toxoplasma gondii (T. gondii) is an obligate intracellular parasite. During the parasitic invasion, T. gondii creates a parasitophorous vacuole, which enables the modulation of cell functions, allowing its replication and host infection. It has effective strategies to escape the immune response and reach privileged immune sites and remain inactive in a controlled environment in tissue cysts. This current review presents the factors that affect host cells and the parasite, as well as changes in the immune system during host cell infection. The secretory organelles of T. gondii (dense granules, micronemes, and rhoptries) are responsible for these processes. They are involved with proteins secreted by micronemes and rhoptries (MIC, AMA, and RONs) that mediate the recognition and entry into host cells. Effector proteins (ROP and GRA) that modify the STAT signal or GTPases in immune cells determine their toxicity. Interference byhost autonomous cells during parasitic infection, gene expression, and production of microbicidal molecules such as reactive oxygen species (ROS) and nitric oxide (NO), result in the regulation of cell death. The high level of complexity in host cell mechanisms prevents cell death in its various pathways. Many of these abilities play an important role in escaping host immune responses, particularly by manipulating the expression of genes involved in apoptosis, necrosis, autophagy, and inflammation. Here we present recent works that define the mechanisms by which T. gondii interacts with these processes in infected host cells.

Control of infection by LC3-associated phagocytosis, CASM, and detection of raised vacuolar pH by the V-ATPase-ATG16L1 axis

The delivery of pathogens to lysosomes for degradation provides an important defense against infection. Degradation is enhanced when LC3 is conjugated to endosomes and phagosomes containing pathogens to facilitate fusion with lysosomes. In phagocytic cells, TLR signaling and Rubicon activate LC3-associated phagocytosis (LAP) where stabilization of the NADPH oxidase leads to sustained ROS production and raised vacuolar pH. Raised pH triggers the assembly of the vacuolar ATPase on the vacuole membrane where it binds ATG16L1 to recruit the core LC3 conjugation complex (ATG16L1:ATG5-12). This V-ATPase-ATG16L1 axis is also activated in nonphagocytic cells to conjugate LC3 to endosomes containing extracellular microbes. Pathogens provide additional signals for recruitment of LC3 when they raise vacuolar pH with pore-forming toxins and proteins, phospholipases, or specialized secretion systems. Many microbes secrete virulence factors to inhibit ROS production and/or the V-ATPase-ATG16L1 axis to slow LC3 recruitment and avoid degradation in lysosomes.

A genome-wide CRISPR-Cas9 screen identifies CENPJ as a host regulator of altered microtubule organization during Plasmodium liver infection

Prior to initiating symptomatic malaria, a single Plasmodium sporozoite infects a hepatocyte and develops into thousands of merozoites, in part by scavenging host resources, likely delivered by vesicles. Here, we demonstrate that host microtubules (MTs) dynamically reorganize around the developing liver stage (LS) parasite to facilitate vesicular transport to the parasite. Using a genome-wide CRISPR-Cas9 screen, we identified host regulators of cytoskeleton organization, vesicle trafficking, and ER/Golgi stress that regulate LS development. Foci of γ-tubulin localized to the parasite periphery; depletion of centromere protein J (CENPJ), a novel regulator identified in the screen, exacerbated this re-localization and increased infection. We demonstrate that the Golgi acts as a non-centrosomal MT organizing center (ncMTOC) by positioning γ-tubulin and stimulating MT nucleation at parasite periphery. Together, these data support a model where the Plasmodium LS recruits host Golgi to form MT-mediated conduits along which host organelles are recruited to PVM and support parasite development.

Plasmodium's fight for survival: escaping elimination while acquiring nutrients

Plasmodium parasites extensively alter their host hepatocyte to evade host detection and support an unprecedented replication rate. Host cell manipulation includes association with the host early and late endomembrane systems, where Plasmodium accesses nutrients while suppressing cellular immune processes. Early endomembrane organelles provide an opportunity to sequester an abundance of lipids and proteins, but the association with late endomembrane organelles also risks autophagy-mediated elimination. While not all parasites survive, those that do benefit from a plethora of nutrients provided through this pathway. In this review, we discuss recent advances in our understanding of how Plasmodium parasites balance the need for host nutrients while avoiding elimination during the liver stage.

Toxoplasma gondii CDPK3 Controls the Intracellular Proliferation of Parasites in Macrophages

Interferon-γ (IFN-γ)-activated macrophages restrain the replication of intracellular parasites and disrupt the integrity of vacuolar pathogens. The growth of the less virulent type II strain of Toxoplasma gondii (such as ME49) was strongly inhibited by IFN-γ-activated murine macrophages. However, the mechanism of resistance is poorly understood. Immunity-related GTPases (IRGs) as well as guanylate-binding proteins (GBPs) contributed to this antiparasitic effect. Previous studies showed the cassette of autophagy-related proteins including Atg7, Atg3, and Atg12-Atg5-Atg16L1 complex, plays crucial roles in the proper targeting of IFN-γ effectors onto the parasitophorous vacuole (PV) membrane of Toxoplasma gondii and subsequent control of parasites. TgCDPK3 is a calcium dependent protein kinase, located on the parasite periphery, plays a crucial role in parasite egress. Herein, we show that the less virulent strain CDPK3 (ME49, type II) can enhance autophagy activation and interacts with host autophagy proteins Atg3 and Atg5. Infection with CDPK3-deficient ME49 strain resulted in decreased localization of IRGs and GBPs around PV membrane. In vitro proliferation and plaque assays showed that CDPK3-deficient ME49 strain replicated significantly more quickly than wild-type parasites. These data suggested that TgCDPK3 interacts with the host Atg3 and Atg5 to promote the localization of IRGs and GBPs around PV membrane and inhibits the intracellular proliferation of parasites, which is beneficial to the less virulent strain of Toxoplasma gondii long-term latency in host cells.

Plasmodium berghei-Mediated NRF2 Activation in Infected Hepatocytes Enhances Parasite Survival

The protozoan parasite Plasmodium, causative agent of malaria, initially invades and develops in hepatocytes where it resides in a parasitophorous vacuole (PV). A single invaded parasite develops into thousands of daughter parasites. Survival of the host cell is crucial for successful completion of liver stage development. Nuclear factor erythroid-derived 2-related factor 2 (NRF2) is a transcription factor known to induce transcription of cytoprotective genes when activated. Here we show that NRF2 is activated in Plasmodium berghei-infected hepatocytes. We observed that this NRF2 activation depends on PV membrane resident p62 recruiting KEAP1, the negative regulator of NRF2. Disrupting the NRF2 gene results in reduced parasite survival, indicating that NRF2 signaling is an important event for parasite development in hepatocytes. Together, our observations uncovered a novel mechanism of how Plasmodium parasites ensure host cell survival during liver stage development.

Autophagy Pathways in the Genesis of Plasmodium-Derived Microvesicles: A Double-Edged Sword?

Malaria, caused by Plasmodium species (spp.), is a deadly parasitic disease that results in approximately 400,000 deaths per year globally. Autophagy pathways play a fundamental role in the developmental stages of the parasite within the mammalian host. They are also involved in the production of Plasmodium-derived extracellular vesicles (EVs), which play an important role in the infection process, either by providing nutrients for parasite growth or by contributing to the immunopathophysiology of the disease. For example, during the hepatic stage, Plasmodium-derived EVs contribute to parasite virulence by modulating the host immune response. EVs help in evading the different autophagy mechanisms deployed by the host for parasite clearance. During cerebral malaria, on the other hand, parasite-derived EVs promote an astrocyte-mediated inflammatory response, through the induction of a non-conventional host autophagy pathway. In this review, we will discuss the cross-talk between Plasmodium-derived microvesicles and autophagy, and how it influences the outcome of infection.

Mid-Liver Stage Arrest of Plasmodium falciparum Schizonts in Primary Porcine Hepatocytes

During co-evolution Plasmodium parasites and vertebrates went through a process of selection resulting in defined and preferred parasite-host combinations. As such, Plasmodium falciparum (Pf) sporozoites can infect human hepatocytes while seemingly incompatible with host cellular machinery of other species. The compatibility between parasite invasion ligands and their respective human hepatocyte receptors plays a key role in Pf host selectivity. However, it is unclear whether the ability of Pf sporozoites to mature in cross-species infection also plays a role in host tropism. Here we used fresh hepatocytes isolated from porcine livers to study permissiveness to Pf sporozoite invasion and development. We monitored intra-hepatic development via immunofluorescence using anti-HSP70, MSP1, EXP1, and EXP2 antibodies. Our data shows that Pf sporozoites can invade non-human hepatocytes and undergo partial maturation with a significant decrease in schizont numbers between day three and day five. A possible explanation is that Pf sporozoites fail to form a parasitophorous vacuolar membrane (PVM) during invasion. Indeed, the observed aberrant EXP1 and EXP2 staining supports the presence of an atypical PVM. Functions of the PVM include the transport of nutrients, export of waste, and offering a protective barrier against intracellular host effectors. Therefore, an atypical PVM likely results in deficiencies that may detrimentally impact parasite development at multiple levels. In summary, despite successful invasion of porcine hepatocytes, Pf development arrests at mid-stage, possibly due to an inability to mobilize critical nutrients across the PVM. These findings underscore the potential of a porcine liver model for understanding the importance of host factors required for Pf mid-liver stage development.

Characterization of the Tubovesicular Network in Plasmodium vivax Liver Stage Hypnozoites and Schizonts

Plasmodium is a genus of apicomplexan parasites which replicate in the liver before causing malaria. Plasmodium vivax can also persist in the liver as dormant hypnozoites and cause clinical relapse upon activation, but the molecular mechanisms leading to activation have yet to be discovered. In this study, we use high-resolution microscopy to characterize temporal changes of the P. vivax liver stage tubovesicular network (TVN), a parasitophorous vacuole membrane (PVM)-derived network within the host cytosol. We observe extended membrane clusters, tubules, and TVN-derived vesicles present throughout P. vivax liver stage development. Additionally, we demonstrate an unexpected presence of the TVN in hypnozoites and observe some association of this network to host nuclei. We also reveal that the host water and solute channel aquaporin-3 (AQP3) associates with TVN-derived vesicles and extended membrane clusters. AQP3 has been previously shown to localize to the PVM of P. vivax hypnozoites and liver schizonts but has not yet been shown in association to the TVN. Our results highlight host-parasite interactions occur in both dormant and replicating liver stage P. vivax forms and implicate AQP3 function during this time. Together, these findings enhance our understanding of P. vivax liver stage biology through characterization of the TVN with an emphasis on the presence of this network in dormant hypnozoites.

Hijacking of the host cell Golgi by Plasmodium berghei liver stage parasites

The intracellular lifestyle represents a challenge for the rapidly proliferating liver stage Plasmodium parasite. In order to scavenge host resources, Plasmodium has evolved the ability to target and manipulate host cell organelles. Using dynamic fluorescence-based imaging, we here show an interplay between the pre-erythrocytic stages of Plasmodium berghei and the host cell Golgi during liver stage development. Liver stage schizonts fragment the host cell Golgi into miniaturized stacks, which increases surface interactions with the parasitophorous vacuolar membrane of the parasite. Expression of specific dominant-negative Arf1 and Rab GTPases, which interfere with the host cell Golgi-linked vesicular machinery, results in developmental delay and diminished survival of liver stage parasites. Moreover, functional Rab11a is critical for the ability of the parasites to induce Golgi fragmentation. Altogether, we demonstrate that the structural integrity of the host cell Golgi and Golgi-associated vesicular traffic is important for optimal pre-erythrocytic development of P. berghei. The parasite hijacks the Golgi structure of the hepatocyte to optimize its own intracellular development. This article has an associated First Person interview with the first author of the paper.

Unraveling Cell Death Pathways during Malaria Infection: What Do We Know So Far?

Malaria is a parasitic disease (caused by different Plasmodium species) that affects millions of people worldwide. The lack of effective malaria drugs and a vaccine contributes to this disease, continuing to cause major public health and socioeconomic problems, especially in low-income countries. Cell death is implicated in malaria immune responses by eliminating infected cells, but it can also provoke an intense inflammatory response and lead to severe malaria outcomes. The study of the pathophysiological role of cell death in malaria in mammalians is key to understanding the parasite-host interactions and design prophylactic and therapeutic strategies for malaria. In this work, we review malaria-triggered cell death pathways (apoptosis, autophagy, necrosis, pyroptosis, NETosis, and ferroptosis) and we discuss their potential role in the development of new approaches for human malaria therapies.

Modeling Relapsing Malaria: Emerging Technologies to Study Parasite-Host Interactions in the Liver

Recent studies of liver stage malaria parasite-host interactions have provided exciting new insights on the cross-talk between parasite and its mammalian (predominantly rodent) host. We review the latest state of the art and and zoom in on new technologies that will provide the tools necessary to investigate host-parasite interactions of relapsing parasites. Interactions between hypnozoites and hepatocytes are particularly interesting because the parasite can remain in a quiescent state for prolonged periods of time and triggers for reactivation have not been irrefutably identified. If we learn more about the cross-talk between hypnozoite and host we may be able to identify factors that encourage waking up these dormant parasite reservoirs and help to achieve the total eradication of malaria.

Disrupting Plasmodium UIS3-host LC3 interaction with a small molecule causes parasite elimination from host cells

The malaria parasite Plasmodium obligatorily infects and replicates inside hepatocytes surrounded by a parasitophorous vacuole membrane (PVM), which is decorated by the host-cell derived autophagy protein LC3. We have previously shown that the parasite-derived, PVM-resident protein UIS3 sequesters LC3 to avoid parasite elimination by autophagy from hepatocytes. Here we show that a small molecule capable of disrupting this interaction triggers parasite elimination in a host cell autophagy-dependent manner. Molecular docking analysis of more than 20 million compounds combined with a phenotypic screen identified one molecule, C4 (4-{[4-(4-{5-[3-(trifluoromethyl) phenyl]-1,2,4-oxadiazol-3-yl}benzyl)piperazino]carbonyl}benzonitrile), capable of impairing infection. Using biophysical assays, we established that this impairment is due to the ability of C4 to disrupt UIS3-LC3 interaction, thus inhibiting the parasite's ability to evade the host autophagy response. C4 impacts infection in autophagy-sufficient cells without harming the normal autophagy pathway of the host cell. This study, by revealing the disruption of a critical host-parasite interaction without affecting the host's normal function, uncovers an efficient anti-malarial strategy to prevent this deadly disease.

The Host Autophagy During Toxoplasma Infection

Autophagy is an important homeostatic mechanism, in which lysosomes degrade and recycle cytosolic components. As a key defense mechanism against infections, autophagy is involved in the capture and elimination of intracellular parasites. However, intracellular parasites, such as Toxoplasma gondii, have developed several evasion mechanisms to manipulate the host cell autophagy for their growth and establish a chronic infection. This review provides an insight into the autophagy mechanism used by the host cells in the control of T. gondii and the host exploitation by the parasite. First, we summarize the mechanism of autophagy, xenophagy, and LC3-associated phagocytosis. Then, we illustrate the process of autophagy proteins-mediated T. gondii clearance. Furthermore, we discuss how the parasite blocks and exploits this process for its survival.

Plasmodium berghei sporozoites in nonreplicative vacuole are eliminated by a PI3P-mediated autophagy-independent pathway

The protozoan parasite Plasmodium, causative agent of malaria, invades hepatocytes by invaginating the host cell plasma membrane and forming a parasitophorous vacuole membrane (PVM). Surrounded by this PVM, the parasite undergoes extensive replication. Parasites inside a PVM provoke the Plasmodium‐associated autophagy‐related (PAAR) response. This is characterised by a long‐lasting association of the autophagy marker protein LC3 with the PVM, which is not preceded by phosphatidylinositol 3‐phosphate (PI3P)‐labelling. Prior to productive invasion, sporozoites transmigrate several cells and here we describe that a proportion of traversing sporozoites become trapped in a transient traversal vacuole, provoking a host cell response that clearly differs from the PAAR response. These trapped sporozoites provoke PI3P‐labelling of the surrounding vacuolar membrane immediately after cell entry, followed by transient LC3‐labelling and elimination of the parasite by lysosomal acidification. Our data suggest that this PI3P response is not only restricted to sporozoites trapped during transmigration but also affects invaded parasites residing in a compromised vacuole. Thus, host cells can employ a pathway distinct from the previously described PAAR response to efficiently recognise and eliminate Plasmodium parasites.

Autophagy in the control and pathogenesis of parasitic infections

Background

Autophagy has a crucial role in the defense against parasites. The interplay existing between host autophagy and parasites has varied outcomes due to the kind of host cell and microorganism. The presence of autophagic compartments disrupt a significant number of pathogens and are further cleared by xenophagy in an autolysosome. Another section of pathogens have the capacity to outwit the autophagic pathway to their own advantage.

Result

To comprehend the interaction between pathogens and the host cells, it is significant to distinguish between starvation-induced autophagy and other autophagic pathways. Subversion of host autophagy by parasites is likely due to differences in cellular pathways from those of ‘classical’ autophagy and that they are controlled by parasites in a peculiar way. In xenophagy clearance at the intracellular level, the pathogens are first ubiquitinated before autophagy receptors acknowledgement, followed by labeling with light chain 3 (LC3) protein. The LC3 in LC3-associated phagocytosis (LAP) is added directly into vacuole membrane and functions regardless of the ULK, an initiation complex. The activation of the ULK complex composed of ATG13, FIP200 and ATG101causes the initiation of host autophagic response. Again, the recognition of PAMPs by conserved PRRs marks the first line of defense against pathogens, involving Toll-like receptors (TLRs). These all important immune-related receptors have been reported recently to regulate autophagy.

Conclusion

In this review, we sum up recent advances in autophagy to acknowledge and understand the interplay between host and parasites, focusing on target proteins for the design of therapeutic drugs. The target host proteins on the initiation of the ULK complex and PRRs-mediated recognition of PAMPs may provide strong potential for the design of therapeutic drugs against parasitic infections.

The host targeting effect of chloroquine in malaria

Due to the rapid onset and spread of the COVID-19 pandemic, the treatment of COVID-19 patients by hydroxychloroquine alone or in combination with other drugs has captured a great deal of attention and triggered considerable debate. Historically, the worldwide use of quinoline based-drugs has led to a spectacular reduction in death from malaria. Unfortunately, scientists have been forced to seek alternative drugs to treat malaria due to the emergence of chloroquine-resistant parasites in the 1960s. The repurposing of hydroxychloroquine against viral infections, various types of cancer and autoimmune diseases has been ongoing for more than 70 years, with no clear understanding of its mechanism of action (MOA). Here, we closely examine the MOA of this old but influential drug in and beyond malaria. Better insights into how chloroquine targets the host's cellular and immune responses may help to develop applications against to new pathogens and diseases, and perhaps even restore the clinical utility of chloroquine against malaria.

Regulation and repurposing of nutrient sensing and autophagy in innate immunity

Nutrients not only act as building blocks but also as signaling molecules. Nutrient-availability promotes cell growth and proliferation and suppresses catabolic processes, such as macroautophagy/autophagy. These effects are mediated by checkpoint kinases such as MTOR (mechanistic target of rapamycin kinase), which is activated by amino acids and growth factors, and AMP-activated protein kinase (AMPK), which is activated by low levels of glucose or ATP. These kinases have wide-ranging activities that can be co-opted by immune cells upon exposure to danger signals, cytokines or pathogens. Here, we discuss recent insight into the regulation and repurposing of nutrient-sensing responses by the innate immune system during infection. Moreover, we examine how natural mutations and pathogen-mediated interventions can alter the balance between anabolic and autophagic pathways leading to a breakdown in tissue homeostasis and/or host defense.Abbreviations: AKT1/PKB: AKT serine/threonine kinase 1; ATG: autophagy related; BECN1: beclin 1; CGAS: cyclic GMP-AMP synthase; EIF2AK4/GCN2: eukaryotic translation initiation factor 2 alpha kinase 4; ER: endoplasmic reticulum; FFAR: free fatty acid receptor; GABARAP: GABA type A receptor-associated protein; IFN: interferon; IL: interleukin; LAP: LC3-associated phagocytosis; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MAP3K7/TAK1: mitogen-activated protein kinase kinase kinase 7; MAPK: mitogen-activated protein kinase; MTOR: mechanistic target of rapamycin kinase; NLR: NOD (nucleotide-binding oligomerization domain) and leucine-rich repeat containing proteins; PI3K, phosphoinositide 3-kinase; PRR: pattern-recognition receptor; PtdIns3K: phosphatidylinositol 3-kinase; RALB: RAS like proto-oncogene B; RHEB: Ras homolog, MTORC1 binding; RIPK1: receptor interacting serine/threonine kinase 1; RRAG: Ras related GTP binding; SQSTM1/p62: sequestosome 1; STING1/TMEM173: stimulator of interferon response cGAMP interactor 1; STK11/LKB1: serine/threonine kinase 11; TBK1: TANK binding kinase 1; TLR: toll like receptor; TNF: tumor necrosis factor; TRAF6: TNF receptor associated factor 6; TRIM: tripartite motif protein; ULK1: unc-51 like autophagy activating kinase 1; V-ATPase: vacuolar-type H⁺-proton-translocating ATPase.

Melanin processing by keratinocytes: A non-microbial type of host-pathogen interaction?

The mechanisms that regulate skin pigmentation have been the subject of intense research in recent decades. In contrast with melanin biogenesis and transport within melanocytes, little is known about how melanin is transferred and processed within keratinocytes. Several models have been proposed for how melanin is transferred, with strong evidence supporting coupled exo/endocytosis. Recently, two reports suggest that upon internalization, melanin is stored within keratinocytes in an arrested compartment, allowing the pigment to persist for long periods. In this commentary, we identify a striking parallelism between melanin processing within keratinocytes and the host-pathogen interaction with Plasmodium, opening new avenues to understand the complex molecular mechanisms that ensure skin pigmentation and photoprotection.

Intravital imaging of host-parasite interactions in organs of the thoracic and abdominopelvic cavities

Infections with protozoan and helminthic parasites affect multiple organs in the mammalian host. Imaging pathogens in their natural environment takes a more holistic view on biomedical aspects of parasitic infections. Here, we focus on selected organs of the thoracic and abdominopelvic cavities most commonly affected by parasites. Parasitic infections of these organs are often associated with severe medical complications or have health implications beyond the infected individual. Intravital imaging has provided a more dynamic picture of the host-parasite interplay and contributed not only to our understanding of the various disease pathologies, but has also provided fundamental insight into the biology of the parasites.

The plant-pathogen haustorial interface at a glance

Many filamentous pathogens invade plant cells through specialized hyphae called haustoria. These infection structures are enveloped by a newly synthesized plant-derived membrane called the extrahaustorial membrane (EHM). This specialized membrane is the ultimate interface between the plant and pathogen, and is key to the success or failure of infection. Strikingly, the EHM is reminiscent of host-derived membrane interfaces that engulf intracellular metazoan parasites. These perimicrobial interfaces are critical sites where pathogens facilitate nutrient uptake and deploy virulence factors to disarm cellular defenses mounted by their hosts. Although the mechanisms underlying the biogenesis and functions of these host-microbe interfaces are poorly understood, recent studies have provided new insights into the cellular and molecular mechanisms involved. In this Cell Science at a Glance and the accompanying poster, we summarize these recent advances with a specific focus on the haustorial interfaces associated with filamentous plant pathogens. We highlight the progress in the field that fundamentally underpin this research topic. Furthermore, we relate our knowledge of plant-filamentous pathogen interfaces to those generated by other plant-associated organisms. Finally, we compare the similarities between host-pathogen interfaces in plants and animals, and emphasize the key questions in this research area.

Clearing or subverting the enemy: Role of autophagy in protozoan infections

The protozoan parasites are evolutionarily divergent, unicellular eukaryotic pathogens representing one of the essential sources of parasitic diseases. These parasites significantly affect the economy and cause public health burdens globally. Protozoan parasites share many cellular features and pathways with their respective host cells. This includes autophagy, a process responsible for self-degradation of the cell's components. There is conservation of the central structural and functional machinery for autophagy in most of the eukaryotic phyla, however, Plasmodium and Toxoplasma possess a decreased number of recognizable autophagy-related proteins (ATG). Plasmodium noticeably lacks clear orthologs of the initiating kinase ATG1/ULK1/2, and both Plasmodium and Toxoplasma lack proteins involved in the nucleation of autophagosomes. These organisms have essential apicoplast, a plastid-like non-photosynthetic organelle, which is an adaptation that is used in penetrating the host cell. Furthermore, available evidence suggests that Leishmania, an intracellular protozoan parasite, induces autophagy in macrophages. The autophagic pathway in Trypanosoma cruzi is activated during metacyclogenesis, a process responsible for the infective forms of parasites. Therefore, numerous pathogens have developed strategies to impair the autophagic mechanism in phagocytes. Regulating autophagy is essential to maintain cellular health as adjustments in the autophagy pathway have been linked to the progression of several physiological and pathological conditions in humans. In this review, we report current advances in autophagy in parasites and their host cells, focusing on the ramifications of these studies in the design of potential anti-protozoan therapeutics.

Identification of Plasmodium falciparum proteoforms from liver stage models

BACKGROUND

Immunization with attenuated malaria sporozoites protects humans from experimental malaria challenge by mosquito bite. Protection in humans is strongly correlated with the production of T cells targeting a heterogeneous population of pre-erythrocyte antigen proteoforms, including liver stage antigens. Currently, few T cell epitopes derived from Plasmodium falciparum, the major aetiologic agent of malaria in humans are known.

METHODS

In this study both in vitro and in vivo malaria liver stage models were used to sequence host and pathogen proteoforms. Proteoforms from these diverse models were subjected to mild acid elution (of soluble forms), multi-dimensional fractionation, tandem mass spectrometry, and top-down bioinformatics analysis to identify proteoforms in their intact state.

RESULTS

These results identify a group of host and malaria liver stage proteoforms that meet a 5% false discovery rate threshold.

CONCLUSIONS

This work provides proof-of-concept for the validity of this mass spectrometry/bioinformatic approach for future studies seeking to reveal malaria liver stage antigens towards vaccine development.

Plasmodium Secretion Induces Hepatocyte Lysosome Exocytosis and Promotes Parasite Entry

The invasion of a suitable host hepatocyte by Plasmodium sporozoites is an essential step in malaria infection. We demonstrate that in infected hepatocytes, lysosomes are redistributed away from the nucleus, and surface exposure of lysosome-associated membrane protein 1 (LAMP1) is increased. Lysosome exocytosis in infected cells occurs independently of sporozoite traversal. Instead, a sporozoite-secreted factor is sufficient for the process. Knockdown of SNARE proteins involved in lysosome-plasma membrane fusion reduces lysosome exocytosis and Plasmodium infection. In contrast, promoting fusion between the lysosome and plasma membrane dramatically increases infection. Our work demonstrates parallels between Plasmodium sporozoite entry of hepatocytes and infection by the excavate pathogen Trypanosoma cruzi and raises the question of whether convergent evolution has shaped host cell invasion by divergent pathogens.

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Plasmodium sporozoites induce host lysosome exocytosis during invasion

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Hepatocyte lysosome exocytosis occurs in a SPECT2-independent manner

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Inhibition of lysosome-plasma membrane fusion inhibits sporozoite invasion

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Secreted parasite factors are sufficient to induce lysosome exocytosis

Activation of the macroautophagy pathway by Yersinia enterocolitica promotes intracellular multiplication and egress of yersiniae from epithelial cells

The virulence strategy of pathogenic Yersinia spp. involves cell-invasive as well as phagocytosis-preventing tactics to enable efficient colonisation of the host organism. Enteropathogenic yersiniae display an invasive phenotype in early infection stages, which facilitates penetration of the intestinal mucosa. Here we show that invasion of epithelial cells by Yersinia enterocolitica is followed by intracellular survival and multiplication of a subset of ingested bacteria. The replicating bacteria were enclosed in vacuoles with autophagy-related characteristics, showing phagophore formation, xenophagy, and recruitment of cytoplasmic autophagosomes to the bacteria-containing compartments. The subsequent fusion of these vacuoles with lysosomes and concomitant vesicle acidification were actively blocked by Yersinia. This resulted in increased intracellular proliferation and detectable egress of yersiniae from infected cells. Notably, deficiency of the core autophagy machinery component FIP200 impaired the development of autophagic features at Yersinia-containing vacuoles as well as intracellular replication and release of bacteria to the extracellular environment. These results suggest that Y. enterocolitica may take advantage of the macroautophagy pathway in epithelial cells to create an autophagosomal niche that supports intracellular bacterial survival, replication, and, eventually, spread of the bacteria from infected cells.

LC3-associated phagocytosis: host defense and microbial response

The innate immune system has evolved to recognize diverse microbes and destroy them. At the same time, microbial pathogens undermine immunity to cause disease. Here, we highlight recent advances in understanding an antimicrobial pathway called LC3-associated phagocytosis (LAP), which combines features of autophagy with phagocytosis. Upon phagocytosis, many microbes, including bacteria, fungi, and parasites, are sequestered in an LC3-positive, single-membrane bound compartment, a hallmark of LAP. LAP depends upon NADPH oxidase activity at the incipient phagosome and culminates in lysosomal trafficking and microbial degradation. Most often LAP is an effective host defense, but some pathogens evade LAP or replicate successfully in this microenvironment. Here, we review how LAP targets microbial pathogens and strategies pathogens employ to circumvent LAP.

Deciphering host lysosome-mediated elimination of Plasmodium berghei liver stage parasites

Liver stage Plasmodium parasites reside in a parasitophorous vacuole (PV) that associates with lysosomes. It has previously been shown that these organelles can have beneficial as well as harmful effects on the parasite. Yet it is not clear how the association of lysosomes with the parasite is controlled and how interactions with these organelles lead to the antagonistic outcomes. In this study we used advanced imaging techniques to characterize lysosomal interactions with the PV. In host cells harboring successfully developing parasites we observed that these interaction events reach an equilibrium at the PV membrane (PVM). In a population of arrested parasites, this equilibrium appeared to shift towards a strongly increased lysosomal fusion with the PVM witnessed by strong PVM labeling with the lysosomal marker protein LAMP1. This was followed by acidification of the PV and elimination of the parasite. To systematically investigate elimination of arrested parasites, we generated transgenic parasites that express the photosensitizer KillerRed, which leads to parasite killing after activation. Our work provides insights in cellular details of intracellular killing and lysosomal elimination of Plasmodium parasites independent of cells of the immune system.

The Interplay of Host Autophagy and Eukaryotic Pathogens

For intracellular pathogens, host cells provide a replicative niche, but are also armed with innate defense mechanisms to combat the intruder. Co-evolution of host and pathogens has produced a complex interplay of host-pathogen interactions during infection, with autophagy emerging as a key player in the recent years. Host autophagy as a degradative process is a significant hindrance to intracellular growth of the pathogens, but also can be subverted by the pathogens to provide support such as nutrients. While the role of host cell autophagy in the pathogenesis mechanisms of several bacterial and viral pathogens have been extensively studied, less is known for eukaryotic pathogens. In this review, we focus on the interplay of host autophagy with the eukaryotic pathogens Plasmodium spp, Toxoplasma, Leishmania spp and the fungal pathogens Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans. The differences between these eukaryotic pathogens in terms of the host cell types they infect, infective strategies and the host responses required to defend against them provide an interesting insight into how they respond to and interact with host cell autophagy. Due to the ability to infect multiple host species and cell types during the course of their usually complex lifestyles, autophagy plays divergent roles even for the same pathogen. The scenario is further compounded since many of the eukaryotic pathogens have their own sets of either complete or partial autophagy machinery. Eukaryotic pathogen-autophagy interplay is thus a complex relationship with many novel insights for the basic understanding of autophagy, and potential for clinical relevance.

Host cell cytosolic immune response during Plasmodium liver stage development

Recent years have witnessed a great gain in knowledge regarding parasite-host cell interactions during Plasmodium liver stage development. It is now an accepted fact that a large percentage of sporozoites invading hepatocytes fail to form infectious merozoites. There appears to be a delicate balance between parasite survival and elimination and we now start to understand why this is so. Plasmodium liver stage parasites replicate within the parasitophorous vacuole (PV), formed during invasion by invagination of the host cell plasma membrane. The main interface between the parasite and hepatocyte is the parasitophorous vacuole membrane (PVM) that surrounds the PV. Recently, it was shown that autophagy marker proteins decorate the PVM of Plasmodium liver stage parasites and eliminate a proportion of them by an autophagy-like mechanism. Successfully developing Plasmodium berghei parasites are initially also labeled but in the course of development, they are able to control this host defense mechanism by shedding PVM material into the tubovesicular network (TVN), an extension of the PVM that releases vesicles into the host cell cytoplasm. Better understanding of the molecular events at the PVM/TVN during parasite elimination could be the basis of new antimalarial measures.

BioID Reveals Novel Proteins of the Plasmodium Parasitophorous Vacuole Membrane

Intracellular pathogens are often surrounded by a host-cell derived membrane. This membrane is modified by the pathogens to their own needs and is crucial for their intracellular lifestyle. In Plasmodium parasites, this membrane is referred to as the PVM and only a limited number of its proteins are known so far. Here, we applied in rodent P. berghei parasites a method called BioID, which is based on biotinylation of proximal and interacting proteins by the promiscuous biotin ligase BirA*, and demonstrated its usefulness in identification of novel PVM proteins.

During their development within the vertebrate host, Plasmodium parasites infect hepatocytes and red blood cells. Within these cells, parasites are surrounded by a parasitophorous vacuole membrane (PVM). The PVM plays an essential role for the interaction of parasites with their host cells; however, only a limited number of proteins of this membrane have been identified so far. This is partially because systematic proteomic analysis of the protein content of the PVM has been difficult in the past, due to difficulties encountered in attempts to separate the PVM from other membranes such as the parasite plasma membrane. In this study, we adapted the BioID technique to in vitro-cultivated Plasmodium berghei blood stage parasites and utilized the promiscuous biotin ligase BirA* fused to PVM-resident exported protein 1 to biotinylate proteins of the PVM. These we further processed by affinity purification, liquid chromatography-tandem mass spectrometry (LC-MS/MS), and label-free quantitation, leading to a list of 61 known and candidate PVM proteins. Seven proteins were analyzed further during blood and liver stage development. This resulted in the identification of three novel PVM proteins, which were the serine/threonine protein phosphatase UIS2 (PlasmoDB accession no. PBANKA_1328000) and two conserved Plasmodium proteins with unknown functions (PBANKA_0519300 and PBANKA_0509000). In conclusion, our report expands the number of known PVM proteins and experimentally validates BioID as a powerful method to screen for novel constituents of specific cellular compartments in P. berghei.

IMPORTANCE Intracellular pathogens are often surrounded by a host-cell derived membrane. This membrane is modified by the pathogens to their own needs and is crucial for their intracellular lifestyle. In Plasmodium parasites, this membrane is referred to as the PVM and only a limited number of its proteins are known so far. Here, we applied in rodent P. berghei parasites a method called BioID, which is based on biotinylation of proximal and interacting proteins by the promiscuous biotin ligase BirA*, and demonstrated its usefulness in identification of novel PVM proteins.

Norcantharidin alone or in combination with crizotinib induces autophagic cell death in hepatocellular carcinoma by repressing c-Met-mTOR signaling

There is an urgent need for effective molecular therapies for hepatocellular carcinoma (HCC), the third-leading cause of cancer-related deaths worldwide. Norcantharidin (NCTD), a demethylated derivative of cantharidin, reportedly exhibits anticancer activity against various types of tumors, including HCC, though the mechanisms involved remain largely unknown. Here, we report that NCTD reduces viability of human MHCC-97H (97H) and HepG2 HCC cells, and induces cell death by triggering high levels of autophagy. Moreover, a significant attenuation of tumor growth was observed after NCTD treatment of HepG2 tumors in vivo, and this effect was enhanced by co-treatment with the c-Met inhibitor crizotinib. Interestingly, western blot analyses showed that the cytotoxic autophagy induced by NCTD correlates with a reduction in the phosphorylation status of both c-Met and m-TOR. These results suggest that cytotoxic autophagy resulting from inhibition of c-Met/mTOR signaling may be achieved in HCC by combined NCTD and crizotinib administration. Further studies to validate the therapeutic potential of this approach are warranted.

LC3-associated phagocytosis in microbial pathogenesis

Phagocytosis is essential for uptake and elimination of pathogenic microorganisms. Autophagy is a highly conserved mechanism for incorporation of cellular constituents to replenish nutrients by degradation. Recently, parts of the autophagy machinery - above all microtubule-associated protein 1 light chain 3 (LC3) - were found to be specifically recruited to phagosomal membranes resulting in phagosome-lysosome fusion and efficient degradation of internalized cargo in a process termed LC3-associated phagocytosis (LAP). Many pathogenic bacterial, fungal and parasitic microorganisms reside within LAP-targeted single-membrane phagosomes or vacuoles after infection of host cells. In this minireview we describe the state of knowledge on the interaction of pathogens with LAP or LAP-like pathways and report on various pathogens that have evolved strategies to circumvent degradation in LAP compartments.

Plasmodium UIS3 sequesters host LC3 to avoid elimination by autophagy in hepatocytes

The causative agent of malaria, Plasmodium, replicates inside a membrane-bound parasitophorous vacuole (PV), which shields this intracellular parasite from the cytosol of the host cell ¹ . One common threat for intracellular pathogens is the homeostatic process of autophagy, through which cells capture unwanted intracellular material for lysosomal degradation ² . During the liver stage of a malaria infection, Plasmodium parasites are targeted by the autophagy machinery of the host cell, and the PV membrane (PVM) becomes decorated with several autophagy markers, including LC3 (microtubule-associated protein 1 light chain 3) ^3,4 . Here we show that Plasmodium berghei parasites infecting hepatic cells rely on the PVM transmembrane protein UIS3 to avoid elimination by host-cell-mediated autophagy. We found that UIS3 binds host LC3 through a non-canonical interaction with a specialized surface on LC3 where host proteins with essential functions during autophagy also bind. UIS3 acts as a bona fide autophagy inhibitor by competing with host LC3-interacting proteins for LC3 binding. Our work identifies UIS3, one of the most promising candidates for a genetically attenuated vaccine against malaria ⁵ , as a unique and potent mediator of autophagy evasion in Plasmodium. We propose that the protein-protein interaction between UIS3 and host LC3 represents a target for antimalarial drug development.

The Plasmodium liver-stage parasitophorous vacuole: A front-line of communication between parasite and host

The intracellular development and differentiation of the Plasmodium parasite in the host liver is a prerequisite for the actual onset of malaria disease pathology. Since liver-stage infection is clinically silent and can be completely eliminated by sterilizing immune responses, it is a promising target for urgently needed innovative antimalarial drugs and/or vaccines. Discovered more than 65 years ago, these stages remain poorly understood regarding their molecular repertoire and interaction with their host cells in comparison to the pathogenic erythrocytic stages. The differentiating and replicative intrahepatic parasite resides in a membranous compartment called the parasitophorous vacuole, separating it from the host-cell cytoplasm. Here we outline seminal work that contributed to our present understanding of the fundamental dynamic cellular processes of the intrahepatic malarial parasite with both specific host-cell factors and compartments.

Inverted recruitment of autophagy proteins to the Plasmodium berghei parasitophorous vacuole membrane

Selective autophagy and related mechanisms can act as variable defense mechanisms against pathogens and can therefore be considered as intracellular immune responses. When in hepatocytes, Plasmodium parasites reside in a parasitophorous vacuole (PV) and the PV membrane (PVM) is the main contact site between host cell and parasite. Early in infection, the PVM is directly labeled with host cell autophagy proteins LC3B and p62 (nucleoporin 62). We investigated the recruitment of different selective autophagy receptors and could show that mainly p62 and NBR1 (neighbour of BRCA1 gene 1) and to a lesser extent NDP52 (nuclear dot protein 52) associate with the PVM. To investigate the recruitment of these receptors to the PVM in Plasmodium-infected cells, we generated LC3B knock out HeLa cells. In these cell lines, autophagosome formation and autophagic flux are not different to those in WT cells. Unexpectedly, p62 and NBR1 recruitment to the PVM was strongly impaired in LC3B-negative host cells, suggesting that LC3B recruits both receptors to the PVM of Plasmodium parasites. We also noticed that LC3B recruited ubiquitin to the PVM. This indicates that, in comparison to classical selective autophagy, in P. berghei-infected cells the order of membrane labeling with autophagy proteins appears to be inverted from canonical ubiquitin-receptor-LC3B recruitment to LC3B-receptor and possibly ubiquitin.