Izumi Kaneko

Identification of a transcription factor in the mosquito‐invasive stage of malaria parasites

Gene expression in Plasmodium parasites undergoes significant changes in each developmental stage, but the transcription factors (TFs) regulating these changes have not been identified. We report here a Plasmodium TF (AP2‐O) that activates gene expression in ookinetes, the mosquito‐invasive form, and has a DNA‐binding domain structurally related to that of a plant TF, Apetala2 (AP2). AP2‐O mRNA is pre‐synthesized by intraerythrocytic female gametocytes and translated later during ookinete development in the mosquito. The Plasmodium TF activates a set of genes, including all genes reported to be required for midgut invasion, by binding to specific six‐base sequences on the proximal promoter. These results indicate that AP2 family TFs have important roles in stage‐specific gene regulation in Plasmodium parasites.

Transcription factor AP2‐Sp and its target genes in malarial sporozoites

The malarial sporozoite is the stage that infects the liver, and genes expressed in this stage are potential targets for vaccine development. Here, we demonstrate that specific gene expression in this stage is regulated by an AP2‐related transcription factor, designated AP2‐Sp (APETALA2 in sporozoites), that is expressed from the late oocyst to the salivary gland sporozoite. Disruption of the AP2‐Sp gene did not affect parasite replication in the erythrocyte but resulted in loss of sporozoite formation. The electrophoretic mobility‐shift assay showed that the DNA‐binding domain of AP2‐Sp recognizes specific eight‐base sequences, beginning with TGCATG, which are present in the proximal promoter region of all known sporozoite‐specific genes. Promoter assays demonstrated that these sequences act as cis‐acting elements and are critical for the expression of sporozoite‐specific genes with different expression profiles. In transgenic parasites that express endogenous AP2‐O (APETALA2 in ookinetes), but whose AP2 domain had been swapped with that of AP2‐Sp, several target genes of AP2‐Sp were induced in the ookinete stage. These results indicate that AP2‐Sp is a major transcription factor that regulates gene expression in the sporozoite stage.

Identification of an AP2-family Protein That Is Critical for Malaria Liver Stage Development

Liver-stage malaria parasites are a promising target for drugs and vaccines against malaria infection. However, little is currently known about gene regulation in this stage. In this study, we used the rodent malaria parasite Plasmodium berghei and showed that an AP2-family transcription factor, designated AP2-L, plays a critical role in the liver-stage development of the parasite. AP2-L-depleted parasites proliferated normally in blood and in mosquitoes. However, the ability of these parasites to infect the liver was approximately 10,000 times lower than that of wild-type parasites. In vitro assays showed that the sporozoites of these parasites invaded hepatocytes normally but that their development stopped in the middle of the liver schizont stage. Expression profiling using transgenic P. berghei showed that fluorescent protein-tagged AP2-L increased rapidly during the liver schizont stage but suddenly disappeared with the formation of the mature liver schizont. DNA microarray analysis showed that the expression of several genes, including those of parasitophorous vacuole membrane proteins, was significantly decreased in the early liver stage of AP2-L-depleted parasites. Investigation of the targets of this transcription factor should greatly promote the exploration of liver-stage antigens and the elucidation of the mechanisms of hepatocyte infection by malaria parasites.

Liver-specific protein 2: a Plasmodium protein exported to the hepatocyte cytoplasm and required for merozoite formation.

The liver stage is the first stage of the malaria parasite that replicates in the vertebrate host. However, little is known about the interplay between the parasite liver stage and its host cell, the hepatocyte. In this study, we identified an exported protein that has a critical role in parasite development in host hepatocytes. Expressed sequence tag analysis of Plasmodium berghei liver‐stage parasites indicated that transcripts encoding a protein with an N‐terminal signal peptide, designated liver‐specific protein 2 (LISP2), are highly expressed in this stage. Expression of LISP2 was first observed 24 h after infection and rapidly increased during the liver‐stage schizogony. Immunofluorescent staining with anti‐LSP2 antibodies showed that LISP2 was carried to the parasitophorous vacuole and subsequently transported to the cytoplasm and nucleus of host hepatocytes. Gene targeting experiments demonstrated that majority of the LISP2‐mutant liver‐stage parasites arrested their development during formation of merozoites. These results indicate that exported LISP2 is involved in parasite–host interactions required for the development of liver‐stage parasites inside hepatocytes. This study demonstrated that mid‐to‐late liver‐stage malarial parasites have a system for exporting proteins to the host cell as intraerythrocytic stages do and presumably to use the proteins to modify the host cell and improve the environment.

Functional Identification of the Plasmodium Centromere and Generation of a Plasmodium Artificial Chromosome

Summary The artificial chromosome represents a useful tool for gene transfer, both as cloning vectors and in chromosome biology research. To generate a Plasmodium artificial chromosome (PAC), we had to first functionally identify and characterize the parasites centromere. A putative centromere (pbcen5) was cloned from chromosome 5 of the rodent parasite P. berghei based on a Plasmodium gene-synteny map. Plasmids containing pbcen5 were stably maintained in parasites during a blood-stage infection with high segregation efficiency, without drug pressure. pbcen5-containing plasmids were also stably maintained during parasite meiosis and mitosis in the mosquito. A linear PAC (L-PAC) was generated by integrating pbcen5 and telomere into a plasmid. The L-PAC segregated with a high efficiency and was stably maintained throughout the parasites life cycle, as either one or two copies. These results suggest that L-PAC behaves like a Plasmodium chromosome, which can be exploited as an experimental research tool.

Global transcriptional repression: An initial and essential step for Plasmodium sexual development

Significance In malaria parasites, gametocytogenesis is a process whereby gametocytes, the precursors of gametes, are generated from asexual blood-stage parasites. In this study, we report that the plasmodium transcriptional repressor AP2-G2 plays an essential role in this process. We found that AP2-G2 represses the genes that are required for the proliferation of the asexual stage and, thereby, supports the conversion from the asexual blood stage to the nonreplicative sexual stage. Similarly, during the initial phase of germ-line formation in metazoan animals (including insects), a master transcriptional repressor specifies primordial germ cells among embryonic cells. Thus, our findings suggest that the basic molecular mechanisms that are required to establish the germ line have been conserved in eukaryotes, despite their evolutionary divergence. Gametocytes are nonreplicative sexual forms that mediate malaria transmission to a mosquito vector. They are generated from asexual blood-stage parasites that proliferate in the circulation. However, little is known about how this transition is genetically regulated. Here, we report that an Apetala2 (AP2) family transcription factor, AP2-G2, regulates this transition as a transcriptional repressor. Disruption of AP2-G2 in the rodent malaria parasite Plasmodium berghei did not prevent commitment to the sexual stage but did halt development before the appearance of sex-specific morphologies. ChIP-seq analysis revealed that AP2-G2 targeted ∼1,500 genes and recognized a five-base motif in their promoters. Most of these target genes are required for asexual proliferation of the parasites in the blood, suggesting that AP2-G2 blocks the program that precedes asexual replication to promote conversion to the sexual stage. Microarray analysis showed that the identified targets constituted ∼70% of the up-regulated genes in AP2-G2–depleted parasites, suggesting that AP2-G2 actually functions as a repressor in gametocytes. A promoter assay using a centromere plasmid demonstrated that the binding motif functions as a cis-acting negative regulatory element. These results suggest that global transcriptional repression, which occurs during the initial phase of gametocytogenesis, is an essential step in Plasmodium sexual development.

Genome-Wide Identification of the Target Genes of AP2-O, a Plasmodium AP2-Family Transcription Factor

Stage-specific transcription is a fundamental biological process in the life cycle of the Plasmodium parasite. Proteins containing the AP2 DNA-binding domain are responsible for stage-specific transcriptional regulation and belong to the only known family of transcription factors in Plasmodium parasites. Comprehensive identification of their target genes will advance our understanding of the molecular basis of stage-specific transcriptional regulation and stage-specific parasite development. AP2-O is an AP2 family transcription factor that is expressed in the mosquito midgut-invading stage, called the ookinete, and is essential for normal morphogenesis of this stage. In this study, we identified the genome-wide target genes of AP2-O by chromatin immunoprecipitation-sequencing and elucidate how this AP2 family transcription factor contributes to the formation of this motile stage. The analysis revealed that AP2-O binds specifically to the upstream genomic regions of more than 500 genes, suggesting that approximately 10% of the parasite genome is directly regulated by AP2-O. These genes are involved in distinct biological processes such as morphogenesis, locomotion, midgut penetration, protection against mosquito immunity and preparation for subsequent oocyst development. This direct and global regulation by AP2-O provides a model for gene regulation in Plasmodium parasites and may explain how these parasites manage to control their complex life cycle using a small number of sequence-specific AP2 transcription factors.

Centromere plasmid: a new genetic tool for the study of Plasmodium falciparum.

The introduction of transgenes into Plasmodium falciparum, a highly virulent human malaria parasite, has been conducted either by single crossover recombination or by using episomal plasmids. However, these techniques remain insufficient because of the low transfection efficiency and the low frequency of recombination. To improve the genetic manipulation of P. falciparum, we developed the centromere plasmid as a new genetic tool. First, we attempted to clone all of the predicted centromeres from P. falciparum into E. coli cells but failed because of the high A/T contents of these sequences. To overcome this difficulty, we identified the common sequence features of the centromere of Plasmodium spp. and designed a small centromere that retained those features. The centromere plasmid constructed with the small centromere sequence, pFCEN, segregated into daughter parasites with approximately 99% efficiency, resulting in the stable maintenance of this plasmid in P. falciparum even in the absence of drug selection. This result demonstrated that the small centromere sequence harboured in pFCEN could function as an actual centromere in P. falciparum. In addition, transgenic parasites were more rapidly generated when using pFCEN than when using the control plasmid, which did not contain the centromere sequence. Furthermore, in contrast to the control plasmid, pFCEN did not form concatemers and, thus, was maintained as a single copy over multiple cell divisions. These unique properties of the pFCEN plasmid will solve the current technical limitations of the genetic manipulation of P. falciparum, and thus, this plasmid will become a standard genetic tool for the study of this parasite.

A highly infectious Plasmodium yoelii parasite, bearing Plasmodium falciparum circumsporozoite protein

BackgroundPlasmodium circumsporozoite protein (CSP) is a major surface antigen present in the sporozoite (Spz) stage of a malaria parasite. RTS, S vaccine, the most clinically advanced malaria vaccine, consists of a large portion of Plasmodium falciparum CSP (PfCSP). A highly infectious, recombinant rodent malaria, Plasmodium yoelii parasite bearing a full-length PfCSP, PfCSP/Py Spz, was needed as a tool to evaluate the role of PfCSP in mediating, protective, anti-malaria immunity in a mouse model.MethodsA transgenic parasite, PfCSP/Py Spz, was generated by inserting a construct expressing the PfCSP at the locus of the P. yoelii CSP gene by double cross-over homologous recombination. Then the biological and protective properties of PfCSP/Py Spz were determined.ResultsThis PfCSP/Py parasite produced up to 30,000 Spz in mosquito salivary glands, which is equal or even higher than the number of Spz produced by wild-type P. yoelii parasites. Five bites of PfCSP/Py-infected mosquitoes could induce blood infection in BALB/c mice.ConclusionsThe current study has demonstrated a successful establishment of a transgenic P. yoelii parasite clone that is able to express a full-length PfCSP, PfCSP/Py parasite. Importantly, this PfCSP/Py parasite can be as infectious as the wild-type P. yoelii parasite both in mosquito vector and in mouse, a mammalian host. A new transgenic parasite that expresses a full-length PfCSP may become a useful tool for researchers to investigate immunity against PfCSP in a mouse model.

Isolation and identification of gametogenesis-related genes in Porphyra yezoensis (Rhodophyta) using subtracted cDNA libraries

Gametogenesis of Porphyra yezoensis thalli is induced by ageing as well as by changing water temperature and photoperiod. Under laboratory conditions, thalli cultivated at 10 ∘C with a 10:14 h (light: dark) cycle develop vegetatively to adult form without gametogenesis. On the other hand, sexual reproduction, which involves differentiation of vegetative cells and subsequent gametogenesis, is induced by culturing at 15 ∘C with a 16: 8 h (light: dark) cycle. We have constructed subtracted cDNA libraries enriched for differentially expressed transcripts in vegetative and reproductive thalli, and randomly selected 1,152 cDNAs from each subtracted library. Results of the dot blot analyses used for identification of differentially expressed cDNAs indicated that mRNA levels of 176 and 138 cDNAs tended to increase in the vegetative and reproductive thalli, respectively. BLAST analysis of nucleotide and deduced amino acid sequences showed that the cDNAs represented 63 and 59 unique clones for the vegetative and reproductive cDNA enriched subtracted libraries, respectively. Interestingly, some of the cDNAs isolated from the reproductive subtracted library were homologous to genes encoding protein kinases, GTP-binding protein, and heat shock proteins involved in signal transduction and the molecular chaperon system.