Abstract
Each year, approximately 230 million malaria cases and 400,00 malaria deaths are reported worldwide. Malaria is a life-threatening disease caused by Plasmodium parasites that are transmitted from one individual to another through the bites of infected female Anopheles mosquitoes. Malaria parasites replicate asexually in the human host, and, in each replication cycle, a portion of the asexual stages develops into sexual gametocytes that permit transmission. The proportion of infections that carries gametocytes and the infectivity of gametocytes are indicators of human-to-mosquito transmission potential. In P. falciparum, gametocytes appear 10–14 days after infection, whereas in P. vivax gametocytes appear simultaneously with asexual schizonts. Such difference in development not only increases the length of time that an individual is infectious, but also increases the likelihood of transmission before treatment. The conversion from asexual parasites to gametocytes is also highly variable between infections. Differences in age, host immune response, parasite genetic composition, density of red blood cells, presence of co-infecting parasite strains, and antimalarial drug use could affect gametocytes production. In P. vivax, the unique ability to produce hypnozoites, a dormant liver stage of the parasite, may allow gametocytes to be produced periodically from relapse and contribute to transmission. In this chapter, we will provide an overview of the biology of Plasmodium gametocytes, existing tools for gametocyte detection, and features of gametocyte genes. The biological insights and genetic findings are essential to developing better detection biomarkers and effective strategies to reduce transmission in malaria-endemic countries.
Keywords
- Plasmodium
- gametocyte
- epidemiology
- biomarkers
- transmission
1. Introduction
1.1 Malaria epidemiology
Malaria is a mosquito-borne disease. In humans, malaria is primarily caused by six different
Most
1.2 Development and transmission of Plasmodium
The lifecycle of
All six human malaria parasites require gametocytes to infect female
2. Gametocytogenesis
2.1 Gametocyte commitment and development
Gametocytogenesis is the commitment of a
To date, most of our knowledge on gametocyte commitment is derived from
2.2 Polymorphisms of gametocyte genes
Recently, advances in next generation sequencing provide new insights in our understanding of genetic variation and gene expression across different stages of
The innate immune response in the mosquito vector is mediated in most part by the hemocytes, which eliminate pathogens such as bacteria, fungi, and protozoa.
2.3 Expression of gametocyte genes
During the different stages of gametocyte development, various genes express differentially. For instance,
The female gametocyte specific gene
In both male and female gametocytes,
Recent clustering analyses indicated that there is a clear distinction in the expression levels between male and female gametocyte genes. Male and female gametocytes have been shown to be differentiated by their gene expression levels as early as stage III. The female gametes are generally separated by stage, indicating differentiating gene expression levels with respect to mid and late-stage female gametocytes [74]. For male gametocytes, PF3D7_1325200, a putative lactate dehydrogenase gene, is highly expressed [74]. Another male gametocyte gene PF3D7_1311100 is a putative meiosis-specific nuclear structure protein 1. This gene is essential for normal assembly of the sperm flagella in mice, suggesting that it may have a role in the male gamete development and exflagellation. Other genes including Pf3D7_1114000, Pf3D7_1122900, Pfg14–748, HAP2, and MAPK2 are found to be associated with male gametocytes, though both Pf3D7_1114000 and Pf3D7_1122900 are also expressed in a few female gametocytes [74].
3. Genetic markers for detection and prevention
3.1 Conventional genes used for gametocyte detection
Previous study has indicated that approximately 10% of
Molecular tests for diminutive amounts of gametocytes use reverse-transcription polymerase chain reaction (RT-PCR) to amplify RNA transcripts of gametocyte-specifically expressed genes. Compared to DNA-based assay, qRT-PCR of targeted RNA transcripts revealed higher sensitivity in detecting gametocytes of considerably low densities. For example, there are more than 106 copies of 18S rRNA transcript per cell but only 5 copies of the 18 s rRNA gene per genome [76]. The production of high transcript copies in parasite cells allow for greater detection limits. For
For
3.2 Novel gene candidates to improve detection sensitivity
Recently, the male-specific gene
A recent study of 26
3.3 Treatment and prevention
In
4. Conclusions
To date, gametocytogenesis and gametocyte transmission tracking remain largely uncharacterized due to low prevalence and technological hurdles. As several countries approach the elimination phase for malaria, the need for sensitive and reliable biomarkers for gametocyte detection is more urgent than ever. Microscopy has low detection limit to overcome low parasitemia loads and existing qPCR biomarkers fail to accurately detect both sexes of the parasites. Information on genetic polymorphisms and expression levels of gametocyte biomarkers enable researchers to develop a more sensitive and accurate diagnostic test for
Acknowledgments
We thank our colleagues at the University of North Carolina at Charlotte and Jimma University for the inspiration and discussion that lead to the conceptualization of this paper. We also thank the reviewers for their insightful comments.
Author contributions
Conceptualization, D.K., A.F., and E.L.; resources, D.Y. and E.L.; writing— original draft preparation, D.K., A.F., E.L., G.K., D.Y., and E.L.; writing—review and editing, D.K., B.A., A.B., S.D.S.I., D.Y., and E.L.; funding acquisition, E.L. All authors have read and agreed to the published version of the manuscript.
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