Daily Rhythms of TNFα Expression and Food Intake Regulate Synchrony of Plasmodium Stages with the Host Circadian Cycle

Circadian Control of the Response of Macrophages to Plasmodium Spp.-Infected Red Blood Cells

Malaria is a serious vector-borne disease characterized by periodic episodes of high fever and strong immune responses that are coordinated with the daily synchronized parasite replication cycle inside RBCs. As immune cells harbor an autonomous circadian clock that controls various aspects of the immune response, we sought to determine whether the intensity of the immune response to Plasmodium spp., the parasite causing malaria, depends on time of infection. To do this, we developed a culture model in which mouse bone marrow-derived macrophages are stimulated with RBCs infected with Plasmodium berghei ANKA (iRBCs). Lysed iRBCs, but not intact iRBCs or uninfected RBCs, triggered an inflammatory immune response in bone marrow-derived macrophages. By stimulating at four different circadian time points (16, 22, 28, or 34 h postsynchronization of the cells' clock), 24-h rhythms in reactive oxygen species and cytokines/chemokines were found. Furthermore, the analysis of the macrophage proteome and phosphoproteome revealed global changes in response to iRBCs that varied according to circadian time. This included many proteins and signaling pathways known to be involved in the response to Plasmodium infection. In summary, our findings show that the circadian clock within macrophages determines the magnitude of the inflammatory response upon stimulation with ruptured iRBCs, along with changes of the cell proteome and phosphoproteome.

Time of day and circadian disruption influence host response and parasite growth in a mouse model of cerebral malaria

Malaria is a disease caused by infection with parasite Plasmodium spp. We studied the circadian regulation of host responses to the parasite, in a mouse model of cerebral malaria. The course of the disease was markedly affected by time of infection, with decreased parasitemia and increased inflammation upon infection in the middle of the night. At this time, there were fewer reticulocytes, which are target cells of the parasites. We next investigated the effects of desynchronization of host clocks on the infection: after 10 weeks of recurrent jet lags, mice showed decreased parasite growth and lack of parasite load rhythmicity, paralleled by a loss of glucose rhythm. Accordingly, disrupting host metabolic rhythms impacted parasite load rhythmicity. In summary, our findings of a circadian modulation of malaria parasite growth and infection shed light on aspects of the disease relevant to human malaria and could contribute to new therapeutic or prophylactic measures.

Crosstalk between circadian clocks and pathogen niche

Circadian rhythms are intrinsic 24-hour oscillations found in nearly all life forms. They orchestrate key physiological and behavioral processes, allowing anticipation and response to daily environmental changes. These rhythms manifest across entire organisms, in various organs, and through intricate molecular feedback loops that govern cellular oscillations. Recent studies describe circadian regulation of pathogens, including parasites, bacteria, viruses, and fungi, some of which have their own circadian rhythms while others are influenced by the rhythmic environment of hosts. Pathogens target specific tissues and organs within the host to optimize their replication. Diverse cellular compositions and the interplay among various cell types create unique microenvironments in different tissues, and distinctive organs have unique circadian biology. Hence, residing pathogens are exposed to cyclic conditions, which can profoundly impact host-pathogen interactions. This review explores the influence of circadian rhythms and mammalian tissue-specific interactions on the dynamics of pathogen-host relationships. Overall, this demonstrates the intricate interplay between the body's internal timekeeping system and its susceptibility to pathogens, which has implications for the future of infectious disease research and treatment.

Itaconate impairs immune control of Plasmodium by enhancing mtDNA-mediated PD-L1 expression in monocyte-derived dendritic cells

Severe forms of malaria are associated with systemic inflammation and host metabolism disorders; however, the interplay between these outcomes is poorly understood. Using a Plasmodium chabaudi model of malaria, we demonstrate that interferon (IFN) γ boosts glycolysis in splenic monocyte-derived dendritic cells (MODCs), leading to itaconate accumulation and disruption in the TCA cycle. Increased itaconate levels reduce mitochondrial functionality, which associates with organellar nucleic acid release and MODC restraint. We hypothesize that dysfunctional mitochondria release degraded DNA into the cytosol. Once mitochondrial DNA is sensitized, the activation of IRF3 and IRF7 promotes the expression of IFN-stimulated genes and checkpoint markers. Indeed, depletion of the STING-IRF3/IRF7 axis reduces PD-L1 expression, enabling activation of CD8+ T cells that control parasite proliferation. In summary, mitochondrial disruption caused by itaconate in MODCs leads to a suppressive effect in CD8+ T cells, which enhances parasitemia. We provide evidence that ACOD1 and itaconate are potential targets for adjunct antimalarial therapy.

Meta-analysis of Diurnal Transcriptomics in Mouse Liver Reveals Low Repeatability of Rhythm Analyses

To assess the consistency of biological rhythms across studies, 57 public mouse liver tissue timeseries totaling 1096 RNA-seq samples were obtained and analyzed. Only the control groups of each study were included, to create comparable data. Technical factors in RNA-seq library preparation were the largest contributors to transcriptome-level differences, beyond biological or experiment-specific factors such as lighting conditions. Core clock genes were remarkably consistent in phase across all studies. Overlap of genes identified as rhythmic across studies was generally low, with no pair of studies having over 60% overlap. Distributions of phases of significant genes were remarkably inconsistent across studies, but the genes that consistently identified as rhythmic had acrophase clustering near ZT0 and ZT12. Despite the discrepancies between single-study analyses, cross-study analyses found substantial consistency. Running compareRhythms on each pair of studies identified a median of only 11% of the identified rhythmic genes as rhythmic in only 1 of the 2 studies. Data were integrated across studies in a joint and individual variance estimate (JIVE) analysis, which showed that the top 2 components of joint within-study variation are determined by time of day. A shape-invariant model with random effects was fit to the genes to identify the underlying shape of the rhythms, consistent across all studies, including identifying 72 genes with consistently multiple peaks.

The parasite intraerythrocytic cycle and human circadian cycle are coupled during malaria infection

During infections with the malaria parasites Plasmodium vivax, patients exhibit rhythmic fevers every 48 h. These fever cycles correspond with the time the parasites take to traverse the intraerythrocytic cycle (IEC). In other Plasmodium species that infect either humans or mice, the IEC is likely guided by a parasite-intrinsic clock [Rijo-Ferreiraet al., Science 368, 746-753 (2020); Smith et al., Science 368, 754-759 (2020)], suggesting that intrinsic clock mechanisms may be a fundamental feature of malaria parasites. Moreover, because Plasmodium cycle times are multiples of 24 h, the IECs may be coordinated with the host circadian clock(s). Such coordination could explain the synchronization of the parasite population in the host and enable alignment of IEC and circadian cycle phases. We utilized an ex vivo culture of whole blood from patients infected with P. vivax to examine the dynamics of the host circadian transcriptome and the parasite IEC transcriptome. Transcriptome dynamics revealed that the phases of the host circadian cycle and the parasite IEC are correlated across multiple patients, showing that the cycles are phase coupled. In mouse model systems, host-parasite cycle coupling appears to provide a selective advantage for the parasite. Thus, understanding how host and parasite cycles are coupled in humans could enable antimalarial therapies that disrupt this coupling.

Circadian orchestration of host and gut microbiota in infection

Circadian rhythms are present in almost every organism and regulate multiple aspects of biological and physiological processes (e.g. metabolism, immune responses, and microbial exposure). There exists a bidirectional circadian interaction between the host and its gut microbiota, and potential circadian orchestration of both host and gut microbiota in response to invading pathogens. In this review, we summarize what is known about these intestinal microbial oscillations and the relationships between host circadian clocks and various infectious agents (bacteria, fungi, parasites, and viruses), and discuss how host circadian clocks prime the immune system to fight pathogen infections as well as the direct effects of circadian clocks on viral activity (e.g. SARS-CoV-2 entry and replication). Finally, we consider strategies employed to realign normal circadian rhythmicity for host health, such as chronotherapy, dietary intervention, good sleep hygiene, and gut microbiota-targeted therapy. We propose that targeting circadian rhythmicity may provide therapeutic opportunities for the treatment of infectious diseases.

Sex Inclusion in Transcriptome Studies of Daily Rhythms

Biomedical research on mammals has traditionally neglected females, raising the concern that some scientific findings may generalize poorly to half the population. Although this lack of sex inclusion has been broadly documented, its extent within circadian genomics remains undescribed. To address this gap, we examined sex inclusion practices in a comprehensive collection of publicly available transcriptome studies on daily rhythms. Among 148 studies having samples from mammals in vivo, we found strong underrepresentation of females across organisms and tissues. Overall, only 23 of 123 studies in mice, 0 of 10 studies in rats, and 9 of 15 studies in humans included samples from females. In addition, studies having samples from both sexes tended to have more samples from males than from females. These trends appear to have changed little over time, including since 2016, when the US National Institutes of Health began requiring investigators to consider sex as a biological variable. Our findings highlight an opportunity to dramatically improve representation of females in circadian research and to explore sex differences in daily rhythms at the genome level.

Gluconeogenesis: Adding a new piece to the puzzle of malaria parasitism

Disorders of carbohydrate metabolism, including hypoglycemia and lactic acidosis, are common features of malaria. In this issue of Cell Metabolism, Ramos et al. report that regulation of gluconeogenesis and glycemia by the host glucose-6-phosphatase catalytic subunit 1 (G6Pc1) is a key metabolic step that affects both Plasmodium replication and clinical outcome of disease.

A hypometabolic defense strategy against malaria

Hypoglycemia is a clinical hallmark of severe malaria, the often-lethal outcome of Plasmodium falciparum infection. Here, we report that malaria-associated hypoglycemia emerges from a non-canonical resistance mechanism, whereby the infected host reduces glycemia to starve Plasmodium. This hypometabolic response is elicited by labile heme, a byproduct of hemolysis that induces illness-induced anorexia and represses hepatic glucose production. While transient repression of hepatic glucose production prevents unfettered immune-mediated inflammation, organ damage, and anemia, when sustained over time it leads to hypoglycemia, compromising host energy expenditure and adaptive thermoregulation. The latter arrests the development of asexual stages of Plasmodium via a mechanism associated with parasite mitochondrial dysfunction. In response, Plasmodium activates a transcriptional program associated with the reduction of virulence and sexual differentiation toward the generation of transmissible gametocytes. In conclusion, malaria-associated hypoglycemia represents a trade-off of a hypometabolic-based defense strategy that balances parasite virulence versus transmission.

Circadian rhythms in immunity and host-parasite interactions

The mammalian immune system adheres to a 24 h circadian schedule, exhibiting daily rhythmic patterns in homeostatic immune processes, such as immune cell trafficking, as well as the inflammatory response to infection. These diurnal rhythms are driven by endogenous molecular clocks within immune cells which are hierarchically coordinated by a light-entrained central clock in the suprachiasmatic nucleus of the hypothalamus and responsive to local rhythmic cues including temperature, hormones and feeding time. Circadian control of immunity may enable animals to anticipate daily pathogenic threat from parasites and gate the magnitude of the immune response, potentially enhancing fitness. However, parasites also strive for optimum fitness and some may have co-evolved to benefit from host circadian timing mechanisms, possibly via the parasites' own intrinsic molecular clocks. In this review, we summarize the current knowledge surrounding the influence of the circadian clock on the mammalian immune system and the host-parasitic interaction. We also discuss the potential for chronotherapeutic strategies in the treatment of parasitic diseases.

The involvement of host circadian clocks in the regulation of the immune response to parasitic infections in mammals

Circadian rhythms are recurring variations of physiology with a period of ~24 hours, generated by circadian clocks located throughout the body. Studies have shown a circadian regulation of many aspects of immunity. Immune cells have intrinsic clock mechanisms, and innate and adaptive immune responses - such as leukocyte migration, magnitude of inflammation, cytokine production and cell differentiation - are under circadian control. This circadian regulation has consequences for infections including parasitic infections. In the context of Leishmania infection, the circadian clock within host immune cells modulates the magnitude of the infection and the inflammatory response triggered by the parasite. As for malaria, rhythms within the immune system were shown to impact the developmental cycles of Plasmodium parasites within red blood cells. Further, host circadian rhythms impact infections by multicellular parasites; for example, infection with helminth Trichuris muris shows different kinetics of worm expulsion depending on time of day of infection, a variation that depends on the dendritic cell clock. Although the research on the circadian control of immunity in the context of parasitic infections is in its infancy, the research reviewed here suggests a crucial involvement of host circadian rhythms in immunity on the development and progression of parasitic infections.

Mistimed malaria parasites re-synchronize with host feeding-fasting rhythms by shortening the duration of intra-erythrocytic development

AIMS

Malaria parasites exhibit daily rhythms in the intra-erythrocytic development cycle (IDC) that underpins asexual replication in the blood. The IDC schedule is aligned with the timing of host feeding-fasting rhythms. When the IDC schedule is perturbed to become mismatched to host rhythms, it readily reschedules but it is not known how.

METHODS

We intensively follow four groups of infections that have different temporal alignments between host rhythms and the IDC schedule for 10 days, before and after the peak in asexual densities. We compare how the duration, synchrony and timing of the IDC differs between parasites in control infections and those forced to reschedule by 12 hours and ask whether the density of parasites affects the rescheduling process.

RESULTS AND CONCLUSIONS

Our experiments reveal parasites shorten the IDC duration by 2-3 hours to become realigned to host feeding-fasting rhythms with 5-6 days, in a density-independent manner. Furthermore, parasites are able to reschedule without significant fitness costs for them or their hosts. Understanding the extent of, and limits on, plasticity in the IDC schedule may reveal targets for novel interventions, such as drugs to disrupt IDC regulation and preventing IDC dormancy conferring tolerance to existing drugs.

Synchrony between daily rhythms of malaria parasites and hosts is driven by an essential amino acid

Background: Rapid asexual replication of blood stage malaria parasites is responsible for the severity of disease symptoms and fuels the production of transmission forms. Here, we demonstrate that a  Plasmodium chabaudi's schedule for asexual replication can be orchestrated by isoleucine, a metabolite provided to the parasite in a periodic manner due to the host's rhythmic intake of food. Methods: We infect female C57BL/6 and Per1/2-null mice which have a disrupted canonical (transcription translation feedback loop, TTFL) clock with 1×10 ⁵ red blood cells containing P. chabaudi (DK genotype). We perturb the timing of rhythms in asexual replication and host feeding-fasting cycles to identify nutrients with rhythms that match all combinations of host and parasite rhythms. We then test whether perturbing the availability of the best candidate nutrient in vitro changes the schedule for asexual development. Results: Our large-scale metabolomics experiment and follow up experiments reveal that only one metabolite - the amino acid isoleucine - fits criteria for a time-of-day cue used by parasites to set the schedule for replication. The response to isoleucine is a parasite strategy rather than solely the consequences of a constraint imposed by host rhythms, because unlike when parasites are deprived of other essential nutrients, they suffer no apparent costs from isoleucine withdrawal. Conclusions: Overall, our data suggest parasites can use the daily rhythmicity of blood-isoleucine concentration to synchronise asexual development with the availability of isoleucine, and potentially other resources, that arrive in the blood in a periodic manner due to the host's daily feeding-fasting cycle. Identifying both how and why parasites keep time opens avenues for interventions; interfering with the parasite's time-keeping mechanism may stall replication, increasing the efficacy of drugs and immune responses, and could also prevent parasites from entering dormancy to tolerate drugs.

Synchrony between daily rhythms of malaria parasites and hosts is driven by an essential amino acid

Background: Rapid asexual replication of blood stage malaria parasites is responsible for the severity of disease symptoms and fuels the production of transmission forms. Here, we demonstrate that the Plasmodium chabaudi's schedule for asexual replication can be orchestrated by isoleucine, a metabolite provided to the parasite in periodic manner due to the host's rhythmic intake of food. Methods: We infect female C57BL/6 and Per1/2-null TTFL clock-disrupted mice with 1×10 ⁵ red blood cells containing P. chabaudi (DK genotype). We perturb the timing of rhythms in asexual replication and host feeding-fasting cycles to identify nutrients with rhythms that match all combinations of host and parasite rhythms. We then test whether perturbing the availability of the best candidate nutrient in vitro elicits changes their schedule for asexual development. Results: Our large-scale metabolomics experiment and follow up experiments reveal that only one metabolite - the amino acid isoleucine - fits criteria for a time-of-day cue used by parasites to set the schedule for replication. The response to isoleucine is a parasite strategy rather than solely the consequences of a constraint imposed by host rhythms, because unlike when parasites are deprived of other essential nutrients, they suffer no apparent costs from isoleucine withdrawal. Conclusions: Overall, our data suggest parasites can use the daily rhythmicity of blood-isoleucine concentration to synchronise asexual development with the availability of isoleucine, and potentially other resources, that arrive in the blood in a periodic manner due to the host's daily feeding-fasting cycle. Identifying both how and why parasites keep time opens avenues for interventions; interfering with the parasite's time-keeping mechanism may stall replication, increasing the efficacy of drugs and immune responses, and could also prevent parasites from entering dormancy to tolerate drugs.

Molecular basis of synchronous replication of malaria parasites in the blood stage

The search for host factors that leads to malaria parasite synchronization has been the focus of several laboratories. The host hormone melatonin synchronizes Plasmodium falciparum in culture by increasing the number of mature parasite stages through a PLC-IP3 activation. Melatonin signaling is linked to crosstalk between Ca²⁺-cAMP that results in PKA activation. Two other kinases, PfPK7 and PfeIK1, and the nuclear protein PfMORC that lacks melatonin sensitivity in the inducible knock-down parasites are also identified as part of the hormone-signal transduction pathways. Melatonin also modulates P. falciparum mitochondrial fission genes FIS1, DYN1, and DYN2 in a stage-specific manner. How these multiple molecular mechanisms are orchestrated to lead to parasite synchronization is a fascinating and opened biological question.

Malaria parasite infection compromises colonization resistance to an enteric pathogen by reducing gastric acidity

Malaria parasite infection weakens colonization resistance against Salmonella enterica serovar (S.) Typhimurium. S. Typhimurium is a member of the Enterobacterales, a taxon that increases in abundance when the colonic microbiota is disrupted or when the colonic mucosa is inflamed. However, here, we show that infection of mice with Plasmodium yoelii enhances S. Typhimurium colonization by weakening host control in the upper GI tract. P. yoelii-infected mice had elevated gastric pH. Stimulation of gastric acid secretion during P. yoelii infection restored stomach acidity and colonization resistance, demonstrating that parasite-induced hypochlorhydria increases gastric survival of S. Typhimurium. Furthermore, blockade of P. yoelii-induced TNF-α signaling was sufficient to prevent elevation of gastric pH and enhance S. Typhimurium colonization during concurrent infection. Collectively, these data suggest that abundance in the fecal microbiota of facultative anaerobes, such as S. Typhimurium, can be increased by suppressing antibacterial defenses in the upper GI tract, such as gastric acid.

Challenges in forming inferences from limited data: a case study of malaria parasite maturation

Inferring biological processes from population dynamics is a common challenge in ecology, particularly when faced with incomplete data. This challenge extends to inferring parasite traits from within-host infection dynamics. We focus on rodent malaria infections (Plasmodium berghei), a system for which previous work inferred an immune-mediated extension in the length of the parasite development cycle within red blood cells. By developing a system of delay-differential equations to describe within-host infection dynamics and simulating data, we demonstrate the potential to obtain biased estimates of parasite (and host) traits when key biological processes are not considered. Despite generating infection dynamics using a fixed parasite developmental cycle length, we find that known sources of measurement bias in parasite stage and abundance data can affect estimates of parasite developmental duration, with stage misclassification driving inferences about extended cycle length. We discuss alternative protocols and statistical methods that can mitigate such misestimation.

Daily rhythms of both host and parasite affect antimalarial drug efficacy

BACKGROUND AND OBJECTIVES

Circadian rhythms contribute to treatment efficacy in several non-communicable diseases. However, chronotherapy (administering drugs at a particular time-of-day) against infectious diseases has been overlooked. Yet, the daily rhythms of both hosts and disease-causing agents can impact the efficacy of drug treatment. We use the rodent malaria parasite Plasmodium chabaudi, to test whether the daily rhythms of hosts, parasites and their interactions affect sensitivity to the key antimalarial, artemisinin.

METHODOLOGY

Asexual malaria parasites develop rhythmically in the host's blood, in a manner timed to coordinate with host daily rhythms. Our experiments coupled or decoupled the timing of parasite and host rhythms, and we administered artemisinin at different times of day to coincide with when parasites were either at an early (ring) or later (trophozoite) developmental stage. We quantified the impacts of parasite developmental stage, and alignment of parasite and host rhythms, on drug sensitivity.

RESULTS

We find that rings were less sensitive to artemisinin than trophozoites, and this difference was exacerbated when parasite and host rhythms were misaligned, with little direct contribution of host time-of-day on its own. Furthermore, the blood concentration of haem at the point of treatment correlated positively with artemisinin efficacy but only when parasite and host rhythms were aligned.

CONCLUSIONS AND IMPLICATIONS

Parasite rhythms influence drug sensitivity in vivo. The hitherto unknown modulation by alignment between parasite and host daily rhythms suggests that disrupting the timing of parasite development could be a novel chronotherapeutic approach.

LAY SUMMARY

We reveal that chronotherapy (providing medicines at a particular time-of-day) could improve treatment for malaria infections. Specifically, parasites' developmental stage at the time of treatment and the coordination of timing between parasite and host both affect how well antimalarial drug treatment works.

Dual RNA Sequencing Meta-analysis in Plasmodium Infection Identifies Host-Parasite Interactions

Dual RNA sequencing (RNA-Seq) is the simultaneous transcriptomic analysis of interacting symbionts, for example, in malaria. Potential cross-species interactions identified by correlated gene expression might highlight interlinked signaling, metabolic, or gene regulatory pathways in addition to physically interacting proteins. Often, malaria studies address one of the interacting organisms-host or parasite-rendering the other "contamination." Here we perform a meta-analysis using such studies for cross-species expression analysis. We screened experiments for gene expression from host and Plasmodium. Out of 171 studies in Homo sapiens, Macaca mulatta, and Mus musculus, we identified 63 potential studies containing host and parasite data. While 16 studies (1,950 samples) explicitly performed dual RNA-Seq, 47 (1,398 samples) originally focused on one organism. We found 915 experimental replicates from 20 blood studies to be suitable for coexpression analysis and used orthologs for meta-analysis across different host-parasite systems. Centrality metrics from the derived gene expression networks correlated with gene essentiality in the parasites. We found indications of host immune response to elements of the Plasmodium protein degradation system, an antimalarial drug target. We identified well-studied immune responses in the host with our coexpression networks, as our approach recovers known broad processes interlinked between hosts and parasites in addition to individual host and parasite protein associations. The set of core interactions represents commonalities between human malaria and its model systems for prioritization in laboratory experiments. Our approach might also allow insights into the transferability of model systems for different pathways in malaria studies.IMPORTANCE Malaria still causes about 400,000 deaths a year and is one of the most studied infectious diseases. The disease is studied in mice and monkeys as lab models to derive potential therapeutic intervention in human malaria. Interactions between Plasmodium spp. and its hosts are either conserved across different host-parasite systems or idiosyncratic to those systems. Here we use correlation of gene expression from different RNA-Seq studies to infer common host-parasite interactions across human, mouse, and monkey studies. First, we find a set of very conserved interactors, worth further scrutiny in focused laboratory experiments. Second, this work might help assess to which extent experiments and knowledge on different pathways can be transferred from models to humans for potential therapy.

Mammalian circadian systems: Organization and modern life challenges

Humans and other mammalian species possess an endogenous circadian clock system that has evolved in adaptation to periodically reoccurring environmental changes and drives rhythmic biological functions, as well as behavioural outputs with an approximately 24-hour period. In mammals, body clocks are hierarchically organized, encompassing a so-called pacemaker clock in the hypothalamic suprachiasmatic nucleus (SCN), non-SCN brain and peripheral clocks, as well as cell-autonomous oscillators within virtually every cell type. A functional clock machinery on the molecular level, alignment among body clocks, as well as synchronization between endogenous circadian and exogenous environmental cycles has been shown to be crucial for our health and well-being. Yet, modern life constantly poses widespread challenges to our internal clocks, for example artificial lighting, shift work and trans-meridian travel, potentially leading to circadian disruption or misalignment and the emergence of associated diseases. For instance many of us experience a mismatch between sleep timing on work and free days (social jetlag) in our everyday lives without being aware of health consequences that may arise from such chronic circadian misalignment, Hence, this review provides an overview of the organization and molecular built-up of the mammalian circadian system, its interactions with the outside world, as well as pathologies arising from circadian disruption and misalignment.

Ecology of asynchronous asexual replication: the intraerythrocytic development cycle of Plasmodium berghei is resistant to host rhythms

BACKGROUND

Daily periodicity in the diverse activities of parasites occurs across a broad taxonomic range. The rhythms exhibited by parasites are thought to be adaptations that allow parasites to cope with, or exploit, the consequences of host activities that follow daily rhythms. Malaria parasites (Plasmodium) are well-known for their synchronized cycles of replication within host red blood cells. Whilst most species of Plasmodium appear sensitive to the timing of the daily rhythms of hosts, and even vectors, some species present no detectable rhythms in blood-stage replication. Why the intraerythrocytic development cycle (IDC) of, for example Plasmodium chabaudi, is governed by host rhythms, yet seems completely independent of host rhythms in Plasmodium berghei, another rodent malaria species, is mysterious.

METHODS

This study reports a series of five experiments probing the relationships between the asynchronous IDC schedule of P. berghei and the rhythms of hosts and vectors by manipulating host time-of-day, photoperiod and feeding rhythms.

RESULTS

The results reveal that: (i) a lack coordination between host and parasite rhythms does not impose appreciable fitness costs on P. berghei; (ii) the IDC schedule of P. berghei is impervious to host rhythms, including altered photoperiod and host-feeding-related rhythms; (iii) there is weak evidence for daily rhythms in the density and activities of transmission stages; but (iv), these rhythms have little consequence for successful transmission to mosquitoes.

CONCLUSIONS

Overall, host rhythms do not affect the performance of P. berghei and its asynchronous IDC is resistant to the scheduling forces that underpin synchronous replication in closely related parasites. This suggests that natural variation in the IDC schedule across species represents different parasite strategies that maximize fitness. Thus, subtle differences in the ecological interactions between parasites and their hosts/vectors may select for the evolution of very different IDC schedules.

Interactions among microbes, the immune system, and the circadian clock

The circadian clock couples physiological processes and behaviors to environmental light cycles. This coupling ensures the synchronization of energetically expensive processes to the time of day at which an organism is most active, thus improving overall fitness. Host immunity is an energetically intensive process that requires the coordination of multiple immune cell types to sense, communicate, and respond to a variety of microorganisms. Interestingly the circadian clock entrains immune cell development, function, and trafficking to environmental light cycles. This entrainment results in the variation of host susceptibility to microbial pathogens across the day-night cycle. In addition, the circadian clock engages in bi-directional communication with the microbiota, resident microorganisms that reside in proximity to the epithelial surfaces of animals. This bi-directional interchange plays an essential role in regulating host immunity and is also pivotal for the circadian control of metabolism. Here, we review the role of the circadian clock in directing host immune programs and consider how commensal and pathogenic microbes impact circadian physiological processes.

Integrative analysis of microRNA and mRNA expression profiles of monocyte-derived dendritic cells differentiation during experimental cerebral malaria

Heterogeneity and high plasticity are common features of cells from the mononuclear phagocyte system: monocytes (MOs), macrophages, and dendritic cells (DCs). Upon activation by microbial agents, MO can differentiate into MO-derived DCs (MODCs). In previous work, we have shown that during acute infection with Plasmodium berghei ANKA (PbA), MODCs become, transiently, the main CD11b+ myeloid population in the spleen (SP) and once recruited to the brain play an important role in the development of experimental cerebral malaria (ECM). Here, we isolated 4 cell populations: bone marrow (BM) MOs (BM-MOs) and SP-MOs from uninfected mice; BM inflammatory MOs (BM-iMOs) and SP-MODCs from PbA-infected mice and used a system biology approach to a holistic transcriptomic comparison and provide an interactome analysis by integrating differentially expressed miRNAs (DEMs) and their differentially expressed gene targets (DEGs) data. The Jaccard index (JI) was used for gauging the similarity and diversity among these cell populations. Whereas BM-MOs, BM-iMOs, and SP-MOs presented high similarity of DEGs, SP-MODCs distinguished by showing a greater number of DEGs. Moreover, functional analysis identified an enrichment in canonical pathways, such as DC maturation, neuroinflammation, and IFN signaling. Upstream regulator analysis identified IFNγ as the potential upstream molecule that can explain the observed DEMs-Target DEGs intersections in SP-MODCs. Finally, directed target analysis and in vivo/ex vivo assays indicate that SP-MODCs differentiate in the SP and IFNγ is a main driver of this process.

Periodic Parasites and Daily Host Rhythms

Biological rhythms appear to be an elegant solution to the challenge of coordinating activities with the consequences of the Earth's daily and seasonal rotation. The genes and molecular mechanisms underpinning circadian clocks in multicellular organisms are well understood. In contrast, the regulatory mechanisms and fitness consequences of biological rhythms exhibited by parasites remain mysterious. Here, we explore how periodicity in parasite traits is generated and why daily rhythms matter for parasite fitness. We focus on malaria (Plasmodium) parasites which exhibit developmental rhythms during replication in the mammalian host's blood and in transmission to vectors. Rhythmic in-host parasite replication is responsible for eliciting inflammatory responses, the severity of disease symptoms, and fueling transmission, as well as conferring tolerance to anti-parasite drugs. Thus, understanding both how and why the timing and synchrony of parasites are connected to the daily rhythms of hosts and vectors may make treatment more effective and less toxic to hosts.

Host circadian clocks do not set the schedule for the within-host replication of malaria parasites

Circadian clocks coordinate organisms' activities with daily cycles in their environment. Parasites are subject to daily rhythms in the within-host environment, resulting from clock-control of host activities, including immune responses. Parasites also exhibit rhythms in their activities: the timing of within-host replication by malaria parasites is coordinated to host feeding rhythms. Precisely which host feeding-related rhythm(s) parasites align with and how this is achieved are unknown. Understanding rhythmic replication in malaria parasites matters because it underpins disease symptoms and fuels transmission investment. We test if rhythmicity in parasite replication is coordinated with the host's feeding-related rhythms and/or rhythms driven by the host's canonical circadian clock. We find that parasite rhythms coordinate with the time of day that hosts feed in both wild-type and clock-mutant hosts, whereas parasite rhythms become dampened in clock-mutant hosts that eat continuously. Our results hold whether infections are initiated with synchronous or with desynchronized parasites. We conclude that malaria parasite replication is coordinated to rhythmic host processes that are independent of the core-clock proteins PERIOD 1 and 2; most likely, a periodic nutrient made available when the host digests food. Thus, novel interventions could disrupt parasite rhythms to reduce their fitness, without interference by host clock-controlled homeostasis.

MCD diet-induced steatohepatitis generates a diurnal rhythm of associated biomarkers and worsens liver injury in Klf10 deficient mice

A large number of hepatic functions are regulated by the circadian clock and recent evidence suggests that clock disruption could be a risk factor for liver complications. The circadian transcription factor Krüppel like factor 10 (KLF10) has been involved in liver metabolism as well as cellular inflammatory and death pathways. Here, we show that hepatic steatosis and inflammation display diurnal rhythmicity in mice developing steatohepatitis upon feeding with a methionine and choline deficient diet (MCDD). Core clock gene mRNA oscillations remained mostly unaffected but rhythmic Klf10 expression was abolished in this model. We further show that Klf10 deficient mice display enhanced liver injury and fibrosis priming upon MCDD challenge. Silencing Klf10 also sensitized primary hepatocytes to apoptosis along with increased caspase 3 activation in response to TNFα. This data suggests that MCDD induced steatohepatitis barely affects the core clock mechanism but leads to a reprogramming of circadian gene expression in the liver in analogy to what is observed in other experimental disease paradigms. We further identify KLF10 as a component of this transcriptional reprogramming and a novel hepato-protective factor.

Use of a highly specific kinase inhibitor for rapid, simple and precise synchronization of Plasmodium falciparum and Plasmodium knowlesi asexual blood-stage parasites

During the course of the asexual erythrocytic stage of development, Plasmodium spp. parasites undergo a series of morphological changes and induce alterations in the host cell. At the end of this stage, the parasites egress from the infected cell, after which the progeny invade a new host cell. These processes are rapid and occur in a time-dependent manner. Of particular importance, egress and invasion of erythrocytes by the parasite are difficult to capture in an unsynchronized culture, or even a culture that has been synchronized within a window of one to several hours. Therefore, precise synchronization of parasite cultures is of paramount importance for the investigation of these processes. Here we describe a method for synchronizing Plasmodium falciparum and Plasmodium knowlesi asexual blood stage parasites with ML10, a highly specific inhibitor of the cGMP-dependent protein kinase (PKG) that arrests parasite growth approximately 15 minutes prior to egress. This inhibitor allows parasite cultures to be synchronized so that all parasites are within a window of development of several minutes, with a simple wash step. Furthermore, we show that parasites remain viable for several hours after becoming arrested by the compound and that ML10 has advantages, owing to its high specificity and low EC50, over the previously used PKG inhibitor Compound 2. Here, we demonstrate that ML10 is an invaluable tool for the study of Plasmodium spp. asexual blood stage biology and for the routine synchronization of P. falciparum and P. knowlesi cultures.

For Whom the Clock Ticks: Clinical Chronobiology for Infectious Diseases

The host defense against pathogens varies among individuals. Among the factors influencing host response, those associated with circadian disruptions are emerging. These latter depend on molecular clocks, which control the two partners of host defense: microbes and immune system. There is some evidence that infections are closely related to circadian rhythms in terms of susceptibility, clinical presentation and severity. In this review, we overview what is known about circadian rhythms in infectious diseases and update the knowledge about circadian rhythms in immune system, pathogens and vectors. This heuristic approach opens a new fascinating field of time-based personalized treatment of infected patients.

An intrinsic oscillator drives the blood stage cycle of the malaria parasite Plasmodium falciparum

The blood stage of the infection of the malaria parasite Plasmodium falciparum exhibits a 48-hour developmental cycle that culminates in the synchronous release of parasites from red blood cells, which triggers 48-hour fever cycles in the host. This cycle could be driven extrinsically by host circadian processes or by a parasite-intrinsic oscillator. To distinguish between these hypotheses, we examine the P. falciparum cycle in an in vitro culture system and show that the parasite has molecular signatures associated with circadian and cell cycle oscillators. Each of the four strains examined has a different period, which indicates strain-intrinsic period control. Finally, we demonstrate that parasites have low cell-to-cell variance in cycle period, on par with a circadian oscillator. We conclude that an intrinsic oscillator maintains Plasmodium's rhythmic life cycle.

Malaria parasites regulate intra-erythrocytic development duration via serpentine receptor 10 to coordinate with host rhythms

Malaria parasites complete their intra-erythrocytic developmental cycle (IDC) in multiples of 24 h suggesting a circadian basis, but the mechanism controlling this periodicity is unknown. Combining in vivo and in vitro approaches utilizing rodent and human malaria parasites, we reveal that: (i) 57% of Plasmodium chabaudi genes exhibit daily rhythms in transcription; (ii) 58% of these genes lose transcriptional rhythmicity when the IDC is out-of-synchrony with host rhythms; (iii) 6% of Plasmodium falciparum genes show 24 h rhythms in expression under free-running conditions; (iv) Serpentine receptor 10 (SR10) has a 24 h transcriptional rhythm and disrupting it in rodent malaria parasites shortens the IDC by 2-3 h; (v) Multiple processes including DNA replication, and the ubiquitin and proteasome pathways, are affected by loss of coordination with host rhythms and by disruption of SR10. Our results reveal malaria parasites are at least partly responsible for scheduling the IDC and coordinating their development with host daily rhythms.

The mechanism underlying periodicity of Plasmodium’s intra-erythrocytic developmental cycle (IDC) is unclear. Here, Subudhi et al. show that serpentine receptor 10 (SR10) plays a role in regulating the schedule of the IDC in line with the timing of host daily rhythms.

The malaria parasite has an intrinsic clock

Malarial rhythmic fevers are the consequence of the synchronous bursting of red blood cells (RBCs) on completion of the malaria parasite asexual cell cycle. Here, we hypothesized that an intrinsic clock in the parasite Plasmodium chabaudi underlies the 24-hour-based rhythms of RBC bursting in mice. We show that parasite rhythms are flexible and lengthen to match the rhythms of hosts with long circadian periods. We also show that malaria rhythms persist even when host food intake is evenly spread across 24 hours, suggesting that host feeding cues are not required for synchrony. Moreover, we find that the parasite population remains synchronous and rhythmic even in an arrhythmic clock mutant host. Thus, we propose that parasite rhythms are generated by the parasite, possibly to anticipate its circadian environment.

Circadian Regulation of Immunity Through Epigenetic Mechanisms

The circadian clock orchestrates daily rhythms in many physiological, behavioral and molecular processes, providing means to anticipate, and adapt to environmental changes. A specific role of the circadian clock is to coordinate functions of the immune system both at steady-state and in response to infectious threats. Hence, time-of-day dependent variables are found in the physiology of immune cells, host-parasite interactions, inflammatory processes, or adaptive immune responses. Interestingly, the molecular clock coordinates transcriptional-translational feedback loops which orchestrate daily oscillations in expression of many genes involved in cellular functions. This clock function is assisted by tightly controlled transitions in the chromatin fiber involving epigenetic mechanisms which determine how a when transcriptional oscillations occur. Immune cells are no exception, as they also present a functional clock dictating transcriptional rhythms. Hereby, the molecular clock and the chromatin regulators controlling rhythmicity represent a unique scaffold mediating the crosstalk between the circadian and the immune systems. Certain epigenetic regulators are shared between both systems and uncovering them and characterizing their dynamics can provide clues to design effective chronotherapeutic strategies for modulation of the immune system.

Circadian rhythms in adaptive immunity

The circadian clock provides organisms with the ability to track time of day, allowing them to predict and respond to cyclical changes in the external environment. In mammals this clock consists of multiple auto-regulatory feedback loops generated by a network of circadian clock proteins. This network provides the fundamental basis for rhythms in behaviour and physiology. This clockwork machinery exists in most cells, including those of the immune system. In recent years evidence has emerged highlighting the important role of molecular clocks in dictating the response of immune pathways. While initial work highlighted the effect of the clock in the 'first line of defence', the innate immune system, it has become increasingly apparent that it also plays a role in the more tailored, later-stage adaptive immune response. This review provides an overview of the role of the circadian cycle in the adaptive immune response. We interrogate the depth of knowledge on cell intrinsic clocks within adaptive immune cells and how these cells may be temporally directed by extrinsic rhythmic signals. We discuss the role of the circadian clock in diseases associated with adaptive immunity such as multiple sclerosis, asthma and parasitic infection. We also discuss the current knowledge on timing of vaccination, and the implications this may have on how we can harness and modulate temporal gating of the adaptive immune response in a clinical setting.

Perfect timing: circadian rhythms, sleep, and immunity - an NIH workshop summary

Recent discoveries demonstrate a critical role for circadian rhythms and sleep in immune system homeostasis. Both innate and adaptive immune responses - ranging from leukocyte mobilization, trafficking, and chemotaxis to cytokine release and T cell differentiation -are mediated in a time of day-dependent manner. The National Institutes of Health (NIH) recently sponsored an interdisciplinary workshop, "Sleep Insufficiency, Circadian Misalignment, and the Immune Response," to highlight new research linking sleep and circadian biology to immune function and to identify areas of high translational potential. This Review summarizes topics discussed and highlights immediate opportunities for delineating clinically relevant connections among biological rhythms, sleep, and immune regulation.

Testing possible causes of gametocyte reduction in temporally out-of-synch malaria infections

BACKGROUND

The intraerythrocytic development cycle (IDC) of the rodent malaria Plasmodium chabaudi is coordinated with host circadian rhythms. When this coordination is disrupted, parasites suffer a 50% reduction in both asexual stages and sexual stage gametocytes over the acute phase of infection. Reduced gametocyte density may not simply follow from a loss of asexuals because investment into gametocytes ("conversion rate") is a plastic trait; furthermore, the densities of both asexuals and gametocytes are highly dynamic during infection. Hence, the reasons for the reduction of gametocytes in infections that are out-of-synch with host circadian rhythms remain unclear. Here, two explanations are tested: first, whether out-of-synch parasites reduce their conversion rate to prioritize asexual replication via reproductive restraint; second, whether out-of-synch gametocytes experience elevated clearance by the host's circadian immune responses.

METHODS

First, conversion rate data were analysed from a previous experiment comparing infections of P. chabaudi that were in-synch or 12 h out-of-synch with host circadian rhythms. Second, three new experiments examined whether the inflammatory cytokine TNF varies in its gametocytocidal efficacy according to host time-of-day and gametocyte age.

RESULTS

There was no evidence that parasites reduce conversion or that their gametocytes become more vulnerable to TNF when out-of-synch with host circadian rhythms.

CONCLUSIONS

The factors causing the reduction of gametocytes in out-of-synch infections remain mysterious. Candidates for future investigation include alternative rhythmic factors involved in innate immune responses and the rhythmicity in essential resources required for gametocyte development. Explaining why it matters for gametocytes to be synchronized to host circadian rhythms might suggest novel approaches to blocking transmission.

The Complex Interplay of Parasites, Their Hosts, and Circadian Clocks

Parasites have evolved various mechanisms to favor infection of their hosts and enhance the success of the infection. In this respect, time-of-day effects were found during the course of parasitic infections, which can be caused or controlled by circadian rhythms in the physiology of their vertebrate hosts. These include circadian clock-controlled rhythms in metabolism and in immune responses. Conversely, parasites can also modulate their hosts' behavioral and cellular rhythms. Lastly, parasites themselves were in some cases shown to possess their own circadian clock mechanisms, which can influence their capacity to infect their hosts. A better knowledge of the circadian regulation of host-parasite interactions will help in designing new preventive and therapeutic strategies for parasitic diseases.

Host circadian rhythms are disrupted during malaria infection in parasite genotype-specific manners

Infection can dramatically alter behavioural and physiological traits as hosts become sick and subsequently return to health. Such “sickness behaviours” include disrupted circadian rhythms in both locomotor activity and body temperature. Host sickness behaviours vary in pathogen species-specific manners but the influence of pathogen intraspecific variation is rarely studied. We examine how infection with the murine malaria parasite, Plasmodium chabaudi, shapes sickness in terms of parasite genotype-specific effects on host circadian rhythms. We reveal that circadian rhythms in host locomotor activity patterns and body temperature become differentially disrupted and in parasite genotype-specific manners. Locomotor activity and body temperature in combination provide more sensitive measures of health than commonly used virulence metrics for malaria (e.g. anaemia). Moreover, patterns of host disruption cannot be explained simply by variation in replication rate across parasite genotypes or the severity of anaemia each parasite genotype causes. It is well known that disruption to circadian rhythms is associated with non-infectious diseases, including cancer, type 2 diabetes, and obesity. Our results reveal that disruption of host circadian rhythms is a genetically variable virulence trait of pathogens with implications for host health and disease tolerance.

Plasticity and genetic variation in traits underpinning asexual replication of the rodent malaria parasite, Plasmodium chabaudi

BACKGROUND

The ability of malaria (Plasmodium) parasites to adjust investment into sexual transmission stages versus asexually replicating stages is well known, but plasticity in other traits underpinning the replication rate of asexual stages in the blood has received less attention. Such traits include burst size (the number of merozoites produced per schizont), the duration of the asexual cycle, and invasion preference for different ages of red blood cell (RBC).

METHODS

Here, plasticity [environment (E) effects] and genetic variation [genotype (G) effects] in traits relating to asexual replication rate are examined for 4 genotypes of the rodent malaria parasite Plasmodium chabaudi. An experiment tested whether asexual dynamics differ between parasites infecting control versus anaemic hosts, and whether variation in replication rate can be explained by differences in burst size, asexual cycle, and invasion rates.

RESULTS

The within-host environment affected each trait to different extents but generally had similar impacts across genotypes. The dynamics of asexual densities exhibited a genotype by environment effect (G×E), in which one of the genotypes increased replication rate more than the others in anaemic hosts. Burst size and cycle duration varied between the genotypes (G), while burst size increased and cycle duration became longer in anaemic hosts (E). Variation in invasion rates of differently aged RBCs was not explained by environmental or genetic effects. Plasticity in burst size and genotype are the only traits making significant contributions to the increase in asexual densities observed in anaemic hosts, together explaining 46.4% of the variation in replication rate.

CONCLUSIONS

That host anaemia induces several species of malaria parasites to alter conversion rate is well documented. Here, previously unknown plasticity in other traits underpinning asexual replication is revealed. These findings contribute to mounting evidence that malaria parasites deploy a suite of sophisticated strategies to maximize fitness by coping with, or exploiting the opportunities provided by, the variable within-host conditions experienced during infections. That genetic variation and genotype by environment interactions also shape these traits highlights their evolutionary potential. Asexual replication rate is a major determinant of virulence and so, understanding the evolution of virulence requires knowledge of the ecological (within-host environment) and genetic drivers of variation among parasites.

Within-host modeling of blood-stage malaria

Malaria infection continues to be a major health problem worldwide and drug resistance in the major human parasite species, Plasmodium falciparum, is increasing in South East Asia. Control measures including novel drugs and vaccines are in development, and contributions to the rational design and optimal usage of these interventions are urgently needed. Infection involves the complex interaction of parasite dynamics, host immunity, and drug effects. The long life cycle (48 hours in the common human species) and synchronized replication cycle of the parasite population present significant challenges to modeling the dynamics of Plasmodium infection. Coupled with these, variation in immune recognition and drug action at different life cycle stages leads to further complexity. We review the development and progress of "within-host" models of Plasmodium infection, and how these have been applied to understanding and interpreting human infection and animal models of infection.

The Challenge of Quantifying Synchrony in Malaria Parasites

Malaria infection is often accompanied by periodic fevers, triggered by synchronous cycles of parasite replication within the host. The degree of synchrony in parasite development influences the efficacy of drugs and immune defenses and is therefore relevant to host health and infectiousness. Synchrony is thought to vary over the course of infection and across different host-parasite genotype or species combinations, but the evolutionary significance - if any - of this diversity remains elusive. Standardized methods are lacking, but the most common metric for quantifying synchrony is the percentage of parasites in a particular developmental stage. We use a heuristic model to show that this metric is often unacceptably biased. Methodological challenges must be addressed to characterize diverse patterns of synchrony and their consequences for disease severity and spread.

The evolutionary ecology of circadian rhythms in infection

Biological rhythms coordinate organisms' activities with daily rhythms in the environment. For parasites, this includes rhythms in both the external abiotic environment and the within-host biotic environment. Hosts exhibit rhythms in behaviours and physiologies, including immune responses, and parasites exhibit rhythms in traits underpinning virulence and transmission. Yet, the evolutionary and ecological drivers of rhythms in traits underpinning host defence and parasite offence are largely unknown. Here, we explore how hosts use rhythms to defend against infection, why parasites have rhythms and whether parasites can manipulate host clocks to their own ends. Harnessing host rhythms or disrupting parasite rhythms could be exploited for clinical benefit; we propose an interdisciplinary effort to drive this emerging field forward.

Parasite Recognition and Signaling Mechanisms in Innate Immune Responses to Malaria

Malaria caused by the Plasmodium family of parasites, especially P.falciparum and P. vivax, is a major health problem in many countries in the tropical and subtropical regions of the world. The disease presents a wide array of systemic clinical conditions and several life-threatening organ pathologies, including the dreaded cerebral malaria. Like many other infectious diseases, malaria is an inflammatory response-driven disease, and positive outcomes to infection depend on finely tuned regulation of immune responses that efficiently clear parasites and allow protective immunity to develop. Immune responses initiated by the innate immune system in response to parasites play key roles both in protective immunity development and pathogenesis. Initial pro-inflammatory responses are essential for clearing infection by promoting appropriate cell-mediated and humoral immunity. However, elevated and prolonged pro-inflammatory responses owing to inappropriate cellular programming contribute to disease conditions. A comprehensive knowledge of the molecular and cellular mechanisms that initiate immune responses and how these responses contribute to protective immunity development or pathogenesis is important for developing effective therapeutics and/or a vaccine. Historically, in efforts to develop a vaccine, immunity to malaria was extensively studied in the context of identifying protective humoral responses, targeting proteins involved in parasite invasion or clearance. The innate immune response was thought to be non-specific. However, during the past two decades, there has been a significant progress in understanding the molecular and cellular mechanisms of host-parasite interactions and the associated signaling in immune responses to malaria. Malaria infection occurs at two stages, initially in the liver through the bite of a mosquito, carrying sporozoites, and subsequently, in the blood through the invasion of red blood cells by merozoites released from the infected hepatocytes. Soon after infection, both the liver and blood stage parasites are sensed by various receptors of the host innate immune system resulting in the activation of signaling pathways and production of cytokines and chemokines. These immune responses play crucial roles in clearing parasites and regulating adaptive immunity. Here, we summarize the knowledge on molecular mechanisms that underlie the innate immune responses to malaria infection.

Adrenal hormones mediate disease tolerance in malaria

Malaria reduces host fitness and survival by pathogen-mediated damage and inflammation. Disease tolerance mechanisms counter these negative effects without decreasing pathogen load. Here, we demonstrate that in four different mouse models of malaria, adrenal hormones confer disease tolerance and protect against early death, independently of parasitemia. Surprisingly, adrenalectomy differentially affects malaria-induced inflammation by increasing circulating cytokines and inflammation in the brain but not in the liver or lung. Furthermore, without affecting the transcription of hepatic gluconeogenic enzymes, adrenalectomy causes exhaustion of hepatic glycogen and insulin-independent lethal hypoglycemia upon infection. This hypoglycemia is not prevented by glucose administration or TNF-α neutralization. In contrast, treatment with a synthetic glucocorticoid (dexamethasone) prevents the hypoglycemia, lowers cerebral cytokine expression and increases survival rates. Overall, we conclude that in malaria, adrenal hormones do not protect against lung and liver inflammation. Instead, they prevent excessive systemic and brain inflammation and severe hypoglycemia, thereby contributing to tolerance.

Disease tolerance mechanisms counter the negative effects of infection without decreasing the pathogen load. Here, the authors show that in mouse models of malaria, such disease tolerance can be conferred by adrenal hormones, by preventing excessive inflammation and hypoglycemia.

Adaptive periodicity in the infectivity of malaria gametocytes to mosquitoes

Daily rhythms in behaviour, physiology and molecular processes are expected to enable organisms to appropriately schedule activities according to consequences of the daily rotation of the Earth. For parasites, this includes capitalizing on periodicity in transmission opportunities and for hosts/vectors, this may select for rhythms in immune defence. We examine rhythms in the density and infectivity of transmission forms (gametocytes) of rodent malaria parasites in the host's blood, parasite development inside mosquito vectors and potential for onwards transmission. Furthermore, we simultaneously test whether mosquitoes exhibit rhythms in susceptibility. We reveal that at night, gametocytes are twice as infective, despite being less numerous in the blood. Enhanced infectiousness at night interacts with mosquito rhythms to increase sporozoite burdens fourfold when mosquitoes feed during their rest phase. Thus, changes in mosquito biting time (owing to bed nets) may render gametocytes less infective, but this is compensated for by the greater mosquito susceptibility.

Malaria Makes the Most of Mealtimes

Successive synchronized cycles of Plasmodium replication in the host's blood causes the symptoms of malaria and fuels disease transmission. In this issue of Cell Host & Microbe, Hirako et al. (2018) reveal that host circadian rhythms of inflammation and metabolism are responsible for the timing of cycles of parasite replication.

The Clock Keeps on Ticking: Emerging Roles for Circadian Regulation in the Control of Fungal Physiology and Pathogenesis

Tic-tac, tic-tac, the sound of time is familiar to us, yet, it also silently shapes daily biological processes conferring 24-hour rhythms in, among others, cellular and systemic signaling, gene expression, and metabolism. Indeed, circadian clocks are molecular machines that permit temporal control of a variety of processes in individuals, with a close to 24-hour period, optimizing cellular dynamics in synchrony with daily environmental cycles. For over three decades, the molecular bases of these clocks have been extensively described in the filamentous fungus Neurospora crassa, yet, there have been few molecular studies in fungi other than Neurospora, despite evidence of rhythmic phenomena in many fungal species, including pathogenic ones. This chapter will revise the mechanisms underlying clock regulation in the model fungus N. crassa, as well as recent findings obtained in several fungi. In particular, this chapter will review the effect of circadian regulation of virulence and organismal interactions, focusing on the phytopathogen Botrytis cinerea, as well as several entomopathogenic fungi, including the behavior-manipulating species Ophiocordyceps kimflemingiae and Entomophthora muscae. Finally, this review will comment current efforts in the study of mammalian pathogenic fungi, while highlighting recent circadian lessons from parasites such as Trypanosoma and Plasmodium. The clock keeps on ticking, whether we can hear it or not.