Department of Pharmacology and Toxicology

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Apicoplast Dynamics During Plasmodium Cell Cycle
(Frontiers Media, 2022-04-29) Elaagip, Arwa; Absalon, Sabrina; Florentin, Anat; Pharmacology and Toxicology, School of Medicine
The deadly malaria parasite, Plasmodium falciparum, contains a unique subcellular organelle termed the apicoplast, which is a clinically-proven antimalarial drug target. The apicoplast is a plastid with essential metabolic functions that evolved via secondary endosymbiosis. As an ancient endosymbiont, the apicoplast retained its own genome and it must be inherited by daughter cells during cell division. During the asexual replication of P. falciparum inside human red blood cells, both the parasite, and the apicoplast inside it, undergo massive morphological changes, including DNA replication and division. The apicoplast is an integral part of the cell and thus its development is tightly synchronized with the cell cycle. At the same time, certain aspects of its dynamics are independent of nuclear division, representing a degree of autonomy in organelle biogenesis. Here, we review the different aspects of organelle dynamics during P. falciparum intraerythrocytic replication, summarize our current understanding of these processes, and describe the many open questions in this area of parasite basic cell biology.
Atlas of Plasmodium falciparum intraerythrocytic development using expansion microscopy
(eLife Sciences, 2023-12-18) Liffner, Benjamin; Cepeda Diaz, Ana Karla; Blauwkamp, James; Anaguano, David; Frolich, Sonja; Muralidharan, Vasant; Wilson, Danny W.; Dvorin, Jeffrey D.; Absalon, Sabrina; Pharmacology and Toxicology, School of Medicine
Apicomplexan parasites exhibit tremendous diversity in much of their fundamental cell biology, but study of these organisms using light microscopy is often hindered by their small size. Ultrastructural expansion microscopy (U-ExM) is a microscopy preparation method that physically expands the sample by ~4.5×. Here, we apply U-ExM to the human malaria parasite Plasmodium falciparum during the asexual blood stage of its lifecycle to understand how this parasite is organized in three dimensions. Using a combination of dye-conjugated reagents and immunostaining, we have cataloged 13 different P. falciparum structures or organelles across the intraerythrocytic development of this parasite and made multiple observations about fundamental parasite cell biology. We describe that the outer centriolar plaque and its associated proteins anchor the nucleus to the parasite plasma membrane during mitosis. Furthermore, the rhoptries, Golgi, basal complex, and inner membrane complex, which form around this anchoring site while nuclei are still dividing, are concurrently segregated and maintain an association to the outer centriolar plaque until the start of segmentation. We also show that the mitochondrion and apicoplast undergo sequential fission events while maintaining an association with the outer centriolar plaque during cytokinesis. Collectively, this study represents the most detailed ultrastructural analysis of P. falciparum during its intraerythrocytic development to date and sheds light on multiple poorly understood aspects of its organelle biogenesis and fundamental cell biology.
Depletion of the mini-chromosome maintenance complex binding protein allows the progression of cytokinesis despite abnormal karyokinesis during the asexual development of Plasmodium falciparum
(Wiley, 2021) Absalon, Sabrina; Dvorin, Jeffrey D.; Pharmacology and Toxicology, School of Medicine
The eukaryotic cell cycle is typically divided into distinct phases with cytokinesis immediately following mitosis. To ensure proper cell division, each phase is tightly coordinated via feedback controls named checkpoints. During its asexual replication cycle, the malaria parasite Plasmodium falciparum undergoes multiple asynchronous rounds of mitosis with segregation of uncondensed chromosomes followed by nuclear division with intact nuclear envelope. The multi-nucleated schizont is then subjected to a single round of cytokinesis that produces dozens of daughter cells called merozoites. To date, no cell cycle checkpoints have been identified that regulate the Plasmodium spp. mode of division. Here, we identify the Plasmodium homologue of the Mini-Chromosome Maintenance Complex Binding Protein (PfMCMBP), which co-purified with the Mini-Chromosome Maintenance (MCM) complex, a replicative helicase required for genomic DNA replication. By conditionally depleting PfMCMBP, we disrupt nuclear morphology and parasite proliferation without causing a block in DNA replication. By immunofluorescence microscopy, we show that PfMCMBP depletion promotes the formation of mitotic spindle microtubules with extensions to more than one DNA focus and abnormal centrin distribution. Strikingly, PfMCMBP-deficient parasites complete cytokinesis and form aneuploid merozoites with variable cellular and nuclear sizes. Our study demonstrates that the parasite lacks a robust checkpoint response to prevent cytokinesis following aberrant karyokinesis.
Disruption of Plasmodium falciparum kinetochore proteins destabilises the nexus between the centrosome equivalent and the mitotic apparatus
(Springer Nature, 2024-07-10) Li, Jiahong; Shami, Gerald J.; Liffner, Benjamin; Cho, Ellie; Braet, Filip; Duraisingh, Manoj T.; Absalon, Sabrina; Dixon, Matthew W. A.; Tilley, Leann; Pharmacology and Toxicology, School of Medicine
Plasmodium falciparum is the causative agent of malaria and remains a pathogen of global importance. Asexual blood stage replication, via a process called schizogony, is an important target for the development of new antimalarials. Here we use ultrastructure-expansion microscopy to probe the organisation of the chromosome-capturing kinetochores in relation to the mitotic spindle, the centriolar plaque, the centromeres and the apical organelles during schizont development. Conditional disruption of the kinetochore components, PfNDC80 and PfNuf2, is associated with aberrant mitotic spindle organisation, disruption of the centromere marker, CENH3 and impaired karyokinesis. Surprisingly, kinetochore disruption also leads to disengagement of the centrosome equivalent from the nuclear envelope. Severing the connection between the nucleus and the apical complex leads to the formation of merozoites lacking nuclei. Here, we show that correct assembly of the kinetochore/spindle complex plays a previously unrecognised role in positioning the nascent apical complex in developing P. falciparum merozoites.
Expansion Microscopy Reveals Plasmodium falciparum Blood-Stage Parasites Undergo Anaphase with A Chromatin Bridge in the Absence of Mini-Chromosome Maintenance Complex Binding Protein
(MDPI, 2021-11) Liffner, Benjamin; Absalon, Sabrina; Pharmacology and Toxicology, School of Medicine
The malaria parasite Plasmodium falciparum undergoes closed mitosis, which occurs within an intact nuclear envelope, and differs significantly from its human host. Mitosis is underpinned by the dynamics of microtubules and the nuclear envelope. To date, our ability to study P. falciparum mitosis by microscopy has been hindered by the small size of the P. falciparum nuclei. Ultrastructure expansion microscopy (U-ExM) has recently been developed for P. falciparum, allowing the visualization of mitosis at the individual nucleus level. Using U-ExM, three intranuclear microtubule structures are observed: hemispindles, mitotic spindles, and interpolar spindles. A previous study demonstrated that the mini-chromosome maintenance complex binding-protein (MCMBP) depletion caused abnormal nuclear morphology and microtubule defects. To investigate the role of microtubules following MCMBP depletion and study the nuclear envelope in these parasites, we developed the first nuclear stain enabled by U-ExM in P. falciparum. MCMBP-deficient parasites show aberrant hemispindles and mitotic spindles. Moreover, anaphase chromatin bridges and individual nuclei containing multiple microtubule structures were observed following MCMBP knockdown. Collectively, this study refines our understanding of MCMBP-deficient parasites and highlights the utility of U-ExM coupled with a nuclear envelope stain for studying mitosis in P. falciparum.
Global Release of Translational Repression Across Plasmodium’s Host-to-Vector Transmission Event
(bioRxiv, 2024-03-16) Rios, Kelly T.; McGee, James P.; Sebastian, Aswathy; Moritz, Robert L.; Feric, Marina; Absalon, Sabrina; Swearingen, Kristian E.; Lindner, Scott E.; Pharmacology and Toxicology, School of Medicine
Malaria parasites must be able to respond quickly to changes in their environment, including during their transmission between mammalian hosts and mosquito vectors. Therefore, before transmission, female gametocytes proactively produce and translationally repress mRNAs that encode essential proteins that the zygote requires to establish a new infection. This essential regulatory control requires the orthologues of DDX6 (DOZI), LSM14a (CITH), and ALBA proteins to form a translationally repressive complex in female gametocytes that associates with many of the affected mRNAs. However, while the release of translational repression of individual mRNAs has been documented, the details of the global release of translational repression have not. Moreover, the changes in spatial arrangement and composition of the DOZI/CITH/ALBA complex that contribute to translational control are also not known. Therefore, we have conducted the first quantitative, comparative transcriptomics and DIA-MS proteomics of Plasmodium parasites across the host-to-vector transmission event to document the global release of translational repression. Using female gametocytes and zygotes of P. yoelii, we found that nearly 200 transcripts are released for translation soon after fertilization, including those with essential functions for the zygote. However, we also observed that some transcripts remain repressed beyond this point. In addition, we have used TurboID-based proximity proteomics to interrogate the spatial and compositional changes in the DOZI/CITH/ALBA complex across this transmission event. Consistent with recent models of translational control, proteins that associate with either the 5' or 3' end of mRNAs are in close proximity to one another during translational repression in female gametocytes and then dissociate upon release of repression in zygotes. This observation is cross-validated for several protein colocalizations in female gametocytes via ultrastructure expansion microscopy and structured illumination microscopy. Moreover, DOZI exchanges its interaction from NOT1-G in female gametocytes to the canonical NOT1 in zygotes, providing a model for a trigger for the release of mRNAs from DOZI. Finally, unenriched phosphoproteomics revealed the modification of key translational control proteins in the zygote. Together, these data provide a model for the essential translational control mechanisms used by malaria parasites to promote their efficient transmission from their mammalian host to their mosquito vector.
mSphere of Influence: the Dynamic Nature of the Nuclear Envelope during Mitosis of Malaria Parasites
(American Society for Microbiology, 2020-09-02) Absalon, Sabrina; Pharmacology and Toxicology, School of Medicine
Sabrina Absalon works in the field of cellular and molecular biology of Plasmodium falciparum, the most virulent parasite causing malaria in humans. In this mSphere of Influence article, she reflects on how the paper “3D nuclear architecture reveals coupled cell cycle dynamics of chromatin and nuclear pores in the malaria parasite Plasmodium falciparum” by Allon Weiner et al. (A. Weiner, N. Dahan-Pasternak, E. Shimoni, V. Shinder, et al., Cell Microbiol 13:967–977, 2011, https://doi.org/10.1111/j.1462-5822.2011.01592.x) triggered her aspiration to study the molecular mechanisms governing nuclear envelope assembly and integrity of P. falciparum throughout the intraerythrocytic development cycle.
Plasmodium RON11 triggers biogenesis of the merozoite rhoptry pair and is essential for erythrocyte invasion
(bioRxiv, 2024-02-13) Anaguano, David; Adewale-Fasoro, Opeoluwa; Vick, Grace S.; Yanik, Sean; Blauwkamp, James; Fierro, Manuel A.; Absalon, Sabrina; Srinivasan, Prakash; Muralidharan, Vasant; Pharmacology and Toxicology, School of Medicine
Malaria is a global and deadly human disease caused by the apicomplexan parasites of the genus Plasmodium. Parasite proliferation within human red blood cells (RBC) is associated with the clinical manifestations of the disease. This asexual expansion within human RBCs, begins with the invasion of RBCs by P. falciparum, which is mediated by the secretion of effectors from two specialized club-shaped secretory organelles in merozoite-stage parasites known as rhoptries. We investigated the function of the Rhoptry Neck Protein 11 (RON11), which contains seven transmembrane domains and calcium-binding EF-hand domains. We generated conditional mutants of the P. falciparum RON11. Knockdown of RON11 inhibits parasite growth by preventing merozoite invasion. The loss of RON11 did not lead to any defects in processing of rhoptry proteins but instead led to a decrease in the amount of rhoptry proteins. We utilized ultrastructure expansion microscopy (U-ExM) to determine the effect of RON11 knockdown on rhoptry biogenesis. Surprisingly, in the absence of RON11, fully developed merozoites had only one rhoptry each. The single rhoptry in RON11 deficient merozoites were morphologically typical with a bulb and a neck oriented into the apical polar ring. Moreover, rhoptry proteins are trafficked accurately to the single rhoptry in RON11 deficient parasites. These data show that in the absence of RON11, the first rhoptry is generated during schizogony but upon the start of cytokinesis, the second rhoptry never forms. Interestingly, these single-rhoptry merozoites were able to attach to host RBCs but are unable to invade RBCs. Instead, RON11 deficient merozoites continue to engage with RBC for prolonged periods eventually resulting in echinocytosis, a result of secreting the contents from the single rhoptry into the RBC. Together, our data show that RON11 triggers the de novo biogenesis of the second rhoptry and functions in RBC invasion.
γδ T cells suppress Plasmodium falciparum blood stage infection by direct killing and phagocytosis
(Springer Nature, 2021) Junqueira, Caroline; Polidoro, Rafael; Castro, Guilherme; Absalon, Sabrina; Liang, Zhitao; Santara, Sumit Sen; Crespo, Ângela; Pereira, Dhelio B.; Gazzinelli, Ricardo T.; Dvorin, Jeffrey D.; Lieberman, Judy; Pharmacology and Toxicology, School of Medicine
Activated Vγδ9Vδ2 (γδ2) T lymphocytes that sense parasite-produced phosphoantigens are expanded in Plasmodium falciparum-infected patients. Although previous studies suggested that γδ2 T cells help control erythrocytic malaria, whether γδ2 T cells recognize infected red blood cells (iRBCs) was uncertain. Here we show that iRBCs stained for the phosphoantigen sensor, butyrophilin 3A1 (BTN3A1). γδ2 T cells formed immune synapses and lysed iRBCs in a contact, phosphoantigen, BTN3A1 and degranulation-dependent manner, killing intracellular parasites. Granulysin released into the synapse lysed iRBCs and delivered death-inducing granzymes to the parasite. All intra-erythrocytic parasites were susceptible, but schizonts were most sensitive. A second protective γδ2 T cell mechanism was identified. In the presence of patient serum, γδ2 T cells phagocytosed and degraded opsonized iRBCs in a CD16-dependent manner, decreasing parasite multiplication. Thus, γδ2 T cells have two ways to control blood stage malaria – γδT cell antigen receptor (TCR)-mediated degranulation and phagocytosis of antibody-coated iRBCs.

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