THE CELL CYCLE OF THE MALARIA PARASITE PLASMODIUM FALCIPARUM: STRANGE WAYS OF A DISTANT RELATIVE

Christian Doerig1*, Debopam Chakrabarti2, Laurent Meijer3

  1. Inserm team, Wellcome Centre for Molecular Parasitology, University of Glasgow, Glasgow G11 6NU, Scotland, UK
  2. Dept. of Molecular Biology and Microbiology , University of Central Florida, Orlando, FL32816, USA
  3. CNRS, Station Biologique, 29682 Roscoff, France ,and Laboratory of Molecular & Cellular Neuroscience, The Rockefeller University, NEW YORK, NY 10021-6399 USA

*cdoer001@udcf.gla.ac.uk

Most eukaryotes used as models for investigations into cell cycle control, from yeast to man, belong to the Opisthokonta lineage.  This lineage represents only one branch of the eukaryotic tree, and several eukaryotes of high medical importance, such as malaria parasites or trypanosomes, belong to phylogenetic groups that are vastly distant from the Opisthokonta branch [1].  To what extent is the knowledge gained from the study of the cell cycle in yeast or mammalian cells valid to describe cell division in these organisms?

Malaria parasites have a complex life cycle.  Infection by Plasmodium falciparum, the species responsible for the lethal form of human malaria, begins by the bite of an infected Anopheles mosquito, which delivers sporozoites into the bloodstream.  These cells establish an infection inside hepatocytes, where they undergo an intense multiplication process called exo-erythrocytic schizogony.  The resulting merozoites invade erythrocytes, where they also multiply through schizogony.  Some merozoites, however, arrest the cell cycle and differentiate into male or female gametocytes, which are the developmental stages that are infective to the mosquito.  Once ingested by the insect, the gametocytes develop into gametes (which for the male cells involves three rapid rounds of cell division) and fuse into a zygote.  Further development in the mosquito involves a process of sporogony, producing sporozoites that accumulate in the salivary glands and are now ready to infect a new human host (see www.malaria.org for information on malaria).

The organisation of the cell cycle during erythrocytic schizogony (the stage responsible for malaria pathogensis) diverges widely from that in yeast or mammalian cells, and a correspondance between cellular events during schizogony and the G1, S, G2 and M phases of the typical cell cycle has not been clearly established.  Peculiar features of Plasmodium cell proliferation include asynchronous nuclear divisions within a given schizont,  specific mechanisms for organelle segregation, and morphogenesis of daughter merozoites [2].

Research in our laboratories has two broad objectives:  (1°) to understand the control of cell proliferation in P. falciparum, and (2°) to identify novel anti-malarials based on inhibitors of protein kinases involved in this process [3].   Research in this field has now entered the post-genomic era. 

We will present a condensed overview of data on:

  1. The biochemical characterisation of P. falciparum cell cycle regulators.  Seven protein kinases related to CDKs and four cyclin-like proteins have been identified so far in P. falciparum.  These elements display very atypical features when compared to their homologues in model eukaryotes, both at the structural (presence of extensions/insertions in the kinase catalytic domains, modified regulatory sites, presence of domains from distinct protein kinase families within a single polypeptide) and the functional (promiscuity of a CDK with regard to cyclin-dependent activation, autophosphorylation properties of several CDKs) levels [4].  Likewise, malarial homologues of proteins involved in the control of entry into S-phase, such as ORC and MCM family members, have been characterized;  available data suggest that the modalities of  initiation of DNA synthesis in malaria parasites diverge considerably from those in higher eukaryotes [2].
  2. The search for anti-malarials based on protein kinase inhibition.  Several malarial protein kinases, including putative cell cycle regulators such as PfPK5 ( a putative CDK1 homologue)[5], PfPK6 (which has similar levels of homology with CDK1/2 and MAPKs of the ERK1/ERK2 family)[6], Pfmrk (a putative CDK7 homologue)[7] and PfGSK3 (a GSK3ß homologue)[8] are active in vitro as recombinant enzymes.  A few malarial kinase activity assays have been adapted to medium throughput screening, and used to screen chemical libraries.

In parallel, a library of CDK inhibitor derivatives has been screened on P. falciparum cultures, and the potential target of a purvalanol B, a known CDK inhibitor that kills the parasite in vitro but has no effect on mammalian cells, has been identified by affinity chromatography to be the parasite’s casein kinase 1 homologue [9,10].  Work to develop this lead is in progress.

REFERENCES

  1. Baldauf, S. L. (2003) Science 300, 1703-1706
  2. Doerig, C., and Chakrabarti, D. in Plasmodium genomes and teheir molecular and cellular biology (Janse, A. W. W. a. C. J., ed) Vol. in press, Horizon Press
  3. Doerig, C., Meijer, L., and Mottram, J. C. (2002) Trends Parasitol 18, 366-371.
  4. Doerig, C., Endicott, J, Chakrabarti, D. (2002) Int. J. Parasitol.32, 1575-1585
  5. Le Roch, K., Sestier, C., Dorin, D., Waters, N., Kappes, B., Chakrabarti, D., Meijer, L., and Doerig, C. (2000) J Biol Chem 275, 8952-8958.
  6. Bracchi-Ricard, V., Barik, S., Delvecchio, C., Doerig, C., Chakrabarti, R., and Chakrabarti, D. (2000) Biochem J 347 Pt 1, 255-263.
  7. Waters, N. C., Woodard, C. L., and Prigge, S. T. (2000) Mol Biochem Parasitol 107, 45-55.
  8. Droucheau, E., Primot, A., Mattei, D., Knockaert, M., Richardson, C., Sallicandro, P., Alano, P., Jafarshad, A., Baratte, B., Kunick, C., Parzy, D., Pearl, L., Doerig, C., and Meijer, L. (2003) Biochim. Biophys. Acta in press.
  9. Knockaert, M., Gray, N., Damiens, E., Chang, Y. T., Grellier, P., Grant, K., Fergusson, D., Mottram, J., Soete, M., Dubremetz, J. F., Le Roch, K., Doerig, C., Schultz, P., and Meijer, L. (2000) Chem Biol 7, 411-422.
  10. Harmse, L., van Zyl, R., Gray, N., Schultz, P., Leclerc, S., Meijer, L., Doerig, C., and Havlik, I. (2001) Biochem Pharmacol 62, 341-348.