Plasmodium schizogony, a chronology of the parasite's cell cycle in the blood stage.

Malaria remains a significant threat to global health, and despite concerted efforts to curb the disease, malaria-related morbidity and mortality increased in recent years. Malaria is caused by unicellular eukaryotes of the genus Plasmodium, and all clinical manifestations occur during asexual proliferation of the parasite inside host erythrocytes. In the blood stage, Plasmodium proliferates through an unusual cell cycle mode called schizogony. Contrary to most studied eukaryotes, which divide by binary fission, the parasite undergoes several rounds of DNA replication and nuclear division that are not directly followed by cytokinesis, resulting in multinucleated cells. Moreover, despite sharing a common cytoplasm, these nuclei multiply asynchronously. Schizogony challenges our current models of cell cycle regulation and, at the same time, offers targets for therapeutic interventions. Over the recent years, the adaptation of advanced molecular and cell biological techniques have given us deeper insight how DNA replication, nuclear division, and cytokinesis are coordinated. Here, we review our current understanding of the chronological events that characterize the unusual cell division cycle of P. falciparum in the clinically relevant blood stage of infection.

Malaria parasite centrins can assemble by Ca2+-inducible condensation.

Centrins are small calcium-binding proteins that have a variety of roles and are universally associated with eukaryotic centrosomes. Rapid proliferation of the malaria-causing parasite Plasmodium falciparum in the human blood depends on a particularly divergent and acentriolar centrosome, which incorporates several essential centrins. Their precise mode of action, however, remains unclear. In this study calcium-inducible liquid-liquid phase separation is revealed as an evolutionarily conserved principle of assembly for multiple centrins from P. falciparum and other species. Furthermore, the disordered N-terminus and calcium-binding motifs are defined as essential features for reversible biomolecular condensation, and we demonstrate that certain centrins can form co-condensates. In vivo analysis using live cell STED microscopy shows liquid-like dynamics of centrosomal centrin. Additionally, implementation of an inducible protein overexpression system reveals concentration-dependent formation of extra-centrosomal centrin assemblies with condensate-like properties. The timing of foci formation and dissolution suggests that centrin assembly is regulated. This study thereby provides a new model for centrin accumulation at eukaryotic centrosomes.

Microscopy-based assay for semi-quantitative detection of SARS-CoV-2 specific antibodies in human sera: A semi-quantitative, high throughput, microscopy-based assay expands existing approaches to measure SARS-CoV-2 specific antibody levels in human sera.

Emergence of the novel pathogenic coronavirus SARS-CoV-2 and its rapid pandemic spread presents challenges that demand immediate attention. Here, we describe the development of a semi-quantitative high-content microscopy-based assay for detection of three major classes (IgG, IgA, and IgM) of SARS-CoV-2 specific antibodies in human samples. The possibility to detect antibodies against the entire viral proteome together with a robust semi-automated image analysis workflow resulted in specific, sensitive and unbiased assay that complements the portfolio of SARS-CoV-2 serological assays. Sensitive, specific and quantitative serological assays are urgently needed for a better understanding of humoral immune response against the virus as a basis for developing public health strategies to control viral spread. The procedure described here has been used for clinical studies and provides a general framework for the application of quantitative high-throughput microscopy to rapidly develop serological assays for emerging virus infections.

Plasmodium vinckei genomes provide insights into the pan-genome and evolution of rodent malaria parasites.

BACKGROUND: Rodent malaria parasites (RMPs) serve as tractable tools to study malaria parasite biology and host-parasite-vector interactions. Among the four RMPs originally collected from wild thicket rats in sub-Saharan Central Africa and adapted to laboratory mice, Plasmodium vinckei is the most geographically widespread with isolates collected from five separate locations. However, there is a lack of extensive phenotype and genotype data associated with this species, thus hindering its use in experimental studies. RESULTS: We have generated a comprehensive genetic resource for P. vinckei comprising of five reference-quality genomes, one for each of its subspecies, blood-stage RNA sequencing data for five P. vinckei isolates, and genotypes and growth phenotypes for ten isolates. Additionally, we sequenced seven isolates of the RMP species Plasmodium chabaudi and Plasmodium yoelii, thus extending genotypic information for four additional subspecies enabling a re-evaluation of the genotypic diversity and evolutionary history of RMPs. The five subspecies of P. vinckei have diverged widely from their common ancestor and have undergone large-scale genome rearrangements. Comparing P. vinckei genotypes reveals region-specific selection pressures particularly on genes involved in mosquito transmission. Using phylogenetic analyses, we show that RMP multigene families have evolved differently across the vinckei and berghei groups of RMPs and that family-specific expansions in P. chabaudi and P. vinckei occurred in the common vinckei group ancestor prior to speciation. The erythrocyte membrane antigen 1 and fam-c families in particular show considerable expansions among the lowland forest-dwelling P. vinckei parasites. The subspecies from the highland forests of Katanga, P. v. vinckei, has a uniquely smaller genome, a reduced multigene family repertoire and is also amenable to transfection making it an ideal parasite for reverse genetics. We also show that P. vinckei parasites are amenable to genetic crosses. CONCLUSIONS: Plasmodium vinckei isolates display a large degree of phenotypic and genotypic diversity and could serve as a resource to study parasite virulence and immunogenicity. Inclusion of P. vinckei genomes provide new insights into the evolution of RMPs and their multigene families. Amenability to genetic crossing and transfection make them also suitable for classical and functional genetics to study Plasmodium biology.

Translational profiling of B cells infected with the Epstein-Barr virus reveals 5' leader ribosome recruitment through upstream open reading frames.

The Epstein-Barr virus (EBV) genome encodes several hundred transcripts. We have used ribosome profiling to characterize viral translation in infected cells and map new translation initiation sites. We show here that EBV transcripts are translated with highly variable efficiency, owing to variable transcription and translation rates, variable ribosome recruitment to the leader region and coverage by monosomes versus polysomes. Some transcripts were hardly translated, others mainly carried monosomes, showed ribosome accumulation in leader regions and most likely represent non-coding RNAs. A similar process was visible for a subset of lytic genes including the key transactivators BZLF1 and BRLF1 in cells infected with weakly replicating EBV strains. This suggests that ribosome trapping, particularly in the leader region, represents a new checkpoint for the repression of lytic replication. We could identify 25 upstream open reading frames (uORFs) located upstream of coding transcripts that displayed 5' leader ribosome trapping, six of which were located in the leader region shared by many latent transcripts. These uORFs repressed viral translation and are likely to play an important role in the regulation of EBV translation.

Plasmodium falciparum CRK4 links early mitotic events to the onset of S-phase during schizogony.

Plasmodium falciparum proliferates through schizogony in the clinically relevant blood stage of infection. During schizogony, consecutive rounds of DNA replication and nuclear division give rise to multinucleated stages before cellularization occurs. Although these nuclei reside in a shared cytoplasm, DNA replication and nuclear division occur asynchronously. Here, by mapping the proteomic context of the S-phase-promoting kinase PfCRK4, we show that it has a dual role for nuclear-cycle progression: PfCRK4 orchestrates not only DNA replication, but in parallel also the rearrangement of intranuclear microtubules from hemispindles into early mitotic spindles. Live-cell imaging of a reporter parasite showed that these microtubule rearrangements coincide with the onset of DNA replication. Together, our data render PfCRK4 a key factor for nuclear-cycle progression, linking entry into S-phase with the initiation of mitotic events. In part, such links may compensate for the absence of canonical cell cycle checkpoints in P. falciparum. IMPORTANCE The human malaria parasite Plasmodium falciparum proliferates in erythrocytes through schizogony, forming multinucleated stages before cellularization occurs. In marked contrast to the pattern of proliferation seen in most model organisms, P. falciparum nuclei multiply asynchronously despite residing in a shared cytoplasm. This divergent mode of replication is, thus, a good target for therapeutic interventions. To exploit this potential, we investigated a key regulator of the parasite's unusual cell cycle, the kinase PfCRK4 and found that this kinase regulated not only DNA replication but also in parallel the rearrangement of nuclear microtubules into early mitotic spindles. Since canonical cell cycle checkpoints have not been described in P. falciparum parasites, linking entry into S-phase and the initiation of mitotic events via a kinase, may be an alternative means to exert control, which is typically achieved by checkpoints.

Glypican-1 drives unconventional secretion of fibroblast growth factor 2.

Fibroblast growth factor 2 (FGF2) is a tumor cell survival factor that is transported into the extracellular space by an unconventional secretory mechanism. Cell surface heparan sulfate proteoglycans are known to play an essential role in this process. Unexpectedly, we found that among the diverse subclasses consisting of syndecans, perlecans, glypicans, and others, Glypican-1 (GPC1) is the principle and rate-limiting factor that drives unconventional secretion of FGF2. By contrast, we demonstrate GPC1 to be dispensable for FGF2 signaling into cells. We provide first insights into the structural basis for GPC1-dependent FGF2 secretion, identifying disaccharides with N-linked sulfate groups to be enriched in the heparan sulfate chains of GPC1 to which FGF2 binds with high affinity. Our findings have broad implications for the role of GPC1 as a key molecule in tumor progression.

Asynchronous nuclear cycles in multinucleated Plasmodium falciparum facilitate rapid proliferation.

Malaria-causing parasites proliferate within erythrocytes through schizogony, forming multinucleated stages before cellularization. Nuclear multiplication does not follow a strict geometric 2n progression, and each proliferative cycle produces a variable number of progeny. Here, by tracking nuclei and DNA replication, we show that individual nuclei replicate their DNA at different times, despite residing in a shared cytoplasm. Extrapolating from experimental data using mathematical modeling, we provide strong indication that a limiting factor exists, which slows down the nuclear multiplication rate. Consistent with this prediction, our data show that temporally overlapping DNA replication events were significantly slower than partially overlapping or nonoverlapping events. Our findings suggest the existence of evolutionary pressure that selects for asynchronous DNA replication, balancing available resources with rapid pathogen proliferation.

Protein abundance and folding rather than the redox state of Kelch13 determine the artemisinin susceptibility of Plasmodium falciparum.

Decreased susceptibilities of the human malaria parasite Plasmodium falciparum towards the endoperoxide antimalarial artemisinin are linked to mutations of residue C580 of PfKelch13, a homologue of the redox sensor Keap1 and other vertebrate BTB-Kelch proteins. Here, we addressed whether mutations alter the artemisinin susceptibility by modifying the redox properties of PfKelch13 or by compromising its native fold or abundance. Using selection-linked integration and the glmS ribozyme, efficient down-regulation of PfKelch13 resulted in ring-stage survival rates around 40%. While the loss of the thiol group of C469 or of the potential disulfide bond between residues C580 and C532 had no effect on the artemisinin susceptibility, the thiol group of C473 could not be replaced. Furthermore, we detected two different forms of PfKelch13 with distinct electrophoretic mobilities around 85 and 95 kDa, suggesting an unidentified post-translational modification. We also established a protocol for the production of recombinant PfKelch13 and produced an antibody against the protein. Recombinant PfKelch13 adopted alternative oligomeric states and only two of its seven cysteine residues, C469 and C473, reacted with Ellman's reagent. While common field mutations resulted in misfolded and completely insoluble recombinant PfKelch13, cysteine-to-serine replacements had no effect on the solubility except for residue C473. In summary, in contrast to residues C469, C532, and C580, the surface-exposed thiol group of residue C473 appears to be essential. However, not the redox properties but impaired folding of PfKelch13, resulting in a decreased PfKelch13 abundance, alters the artemisinin susceptibility and is the central parameter for mutant selection.