Parasite and Vector Circadian Clocks Mediate Efficient Malaria Transmission
Source: Bento et al., "Parasite and vector circadian clocks mediate efficient malaria transmission," Nature Microbiology, Published online 31 March 2025, https://doi.org/10.1038/s41564-025-01949-1
Date: Received: 03 September 2024 | Accepted: 26 March 2025 | Published: 04 April 2025
Key Themes:
This study uncovers a critical tripartite relationship between the Anopheles mosquito vector, the Plasmodium malaria parasite, and the mammalian host, highlighting the significant role of their respective circadian clocks in mediating efficient malaria transmission. The research demonstrates that both the mosquito salivary glands and the resident sporozoite parasite exhibit substantial circadian transcriptional activity, preparing them for the nocturnal blood-feeding behavior of the mosquito and subsequent host infection. The alignment of these circadian rhythms, particularly during nighttime, is shown to be crucial for maximizing transmission efficiency.
Most Important Ideas and Facts:
- Circadian Rhythms in Mosquito Salivary Glands:
- Approximately half of the Anopheles stephensi mosquito salivary gland transcriptome displays circadian expression.
- Genes essential for efficient bloodmeals, such as those encoding anti-blood clotting factors (e.g., anophelin/cEF, aegyptin), vasodilators (Peroxidase 5B), and anti-inflammatory proteins (D7 long form L2), exhibit rhythmic expression, peaking around the times when mosquitoes prefer to feed.
- The study found that mosquitoes prefer to feed and ingest more blood at nighttime, as demonstrated by higher hemoglobin levels in their midguts after nocturnal feeding ("measuring haemoglobin levels, we demonstrate that mosquitos prefer to feed and ingest more blood at nighttime").
- Circadian clock genes (Clock, Cycle, Period, Vrille) are also rhythmically expressed in the salivary glands, suggesting an internal clock regulating these transcriptional changes.
- This rhythmic expression in salivary glands occurs independently of light/dark cycles (LD vs. constant dark DD), indicating genuine circadian control ("Genes in the salivary glands of infected mosquitos cycled independently of the mosquito light/dark schedule (LD versus DD), suggesting that they are under circadian control and that the time of day (rather than light) is the main driver of transcriptional fluctuations in mosquito salivary glands").
- Proteomics analysis confirmed rhythmic abundance of some salivary gland proteins, including those involved in glycolysis.
- Circadian Rhythms in Salivary-Gland Sporozoites:
- A substantial subset (12-20%) of the Plasmodium berghei sporozoite transcriptome within the mosquito salivary glands also exhibits circadian cycling.
- This finding challenges the traditional view of salivary-gland sporozoites as transcriptionally quiescent ("Notably, we show a substantial subset of the salivary-gland-resident parasite transcriptome cycling throughout the day, indicating that this stage is not transcriptionally quiescent.").
- Sporozoite genes involved in motility, such as myosin A and thrombospondin-related sporozoite protein (TRSP), show rhythmic expression, potentially modulating their ability to initiate infection at different times of day ("Among the sporozoite genes undergoing rhythmic expression are those involved in parasite motility, potentially modulating the ability to initiate infection at different times of day.").
- Other cycling sporozoite genes include apical membrane antigen 1 (AMA1), circumsporozoite protein (CSP), and plasmepsin X, all implicated in invasion and host interaction.
- The rhythmic gene expression in sporozoites is not due to cell division within the salivary glands, as confirmed by EdU incorporation assays ("Taken together, our results show that the transcriptional daily rhythms identified in sporozoites are not a consequence of cell divi-sion but instead resemble a robust circadian rhythm.").
- Alignment of Rhythms for Efficient Transmission:
- The study proposes a "circadian tripartite relationship" between the vector, parasite, and mammalian host that modulates malaria transmission efficiency.
- Increased mosquito biting and blood ingestion occur at nighttime, aligning with the peak expression of bloodmeal-related genes in the salivary glands.
- Sporozoite motility-associated genes peak in the early morning, potentially preparing them for transmission during the mosquito's nighttime feeding.
- Experimental infections in mice demonstrated that parasite load in the liver was significantly higher when infection was initiated during the nighttime (using both nighttime sporozoites and mice) compared to daytime infections. This effect was abolished when the rhythms of sporozoites and mice were mismatched ("We observed a reduced parasite load in the livers of mice when infection was initiated during the daytime (using daytime sporozoites and mice) compared with nighttime infections (using nighttime sporozoites and mice; Fig. 4f). By contrast, this increase in parasite load was abolished when the biological timing of the sporozoites and that of the mice were mismatched...").
- In vitro experiments with hepatocytes showed that parasite infection was significantly increased at night when the host cell circadian rhythms were synchronized with the time of sporozoite addition ("When the hepatocyte population was synchronous and matched the time with sporozoites, there was a significant increase in infection at night...").
- Implications for Disease Intervention:
- Understanding this intricate circadian interplay could lead to novel strategies for combating malaria transmission, potentially by disrupting the synchronized rhythms.
- The findings may have broader implications for other vector-borne diseases like Zika and dengue, which might also exhibit daily rhythms in transmission.
- The timing of mosquito biting and the establishment of host infections are highlighted as important factors in malaria transmission biology.
Quotes Highlighting Key Findings:
- "Here we show that approximately half of the mosquito salivary gland transcriptome, particularly genes essential for efficient bloodmeals such as anti-blood clotting factors, exhibits circadian expression."
- "Notably, we show a substantial subset of the salivary-gland-resident parasite transcriptome cycling throughout the day, indicating that this stage is not transcriptionally quiescent."
- "Our findings suggest a circadian tripartite relationship between the vector, parasite and mammalian host that together modulates malaria transmission."
- "Together, our results suggest that the circadian regulation in both the mammalian host and the Anopheles mosquito vector has driven Plasmodium to evolve rhythms that align with both hosts, ensuring successful transmission."
Further Research Directions Identified:
- Dissecting the specific contribution of each player's physiological rhythms and clock mechanisms to the observed phenomenon.
- Investigating other layers of regulation (e.g., central brain-driven regulation) that shape the mosquito's feeding rhythm and influence salivary gland biology.
- Characterizing mosquito circadian rhythms across the lifespan and across multiple bloodmeals.
- Exploring the potential for targeting these circadian rhythms for intervention strategies against malaria and other vector-borne diseases.
This study provides compelling evidence for the importance of circadian clocks in the complex process of malaria transmission, opening new avenues for understanding and potentially disrupting this deadly disease cycle.