SummaryComponents of mitogen-activated signal transduction pathways have been shown to be involved in flagellum biogenesis and maintenance. A mitogenactivated protein kinase homologue, designated LmxMPK9 from Leishmania mexicana , has been recently identified in a homology screen and its mRNA found to be present in all life stages. Three different splice-addition sites were used for mRNA maturation in trans -splicing in the different life stages. However, here we show that LmxMPK9 protein is exclusively found in the promastigote stage. Recombinant expression of LmxMPK9 in Escherichia coli and kinase assays revealed a temperature optimum at 27 ∞ ∞ ∞ ∞ C, the optimal growth temperature for L. mexicana promastigotes, and a preference for manganese to promote substrate phosphorylation of myelin basic protein. A deletion mutant for the singlecopy gene revealed significantly elongated flagella, whereas overexpression led to a subpopulation with rather short to no flagella suggesting a role for LmxMPK9 in flagellar morphogenesis.

During its life cycle, the parasitic protozoon Leishmania mexicana differentiates from a flagellated form, the promastigote, to an amastigote form carrying a rudimentary flagellum. Besides biochemical changes, this process involves a change in overall cell morphology including flagellar shortening. A mitogen-activated protein kinase kinase homologue designated LmxMKK was identified in a homology screening and found to be critically involved in the regulation of flagellar assembly and cell size. LmxMKK is exclusively expressed in the promastigote stage and is likely to be regulated by posttranslational mechanisms such as phosphorylation. A deletion mutant for the single-copy gene revealed motile flagella dramatically reduced in length and lacking the paraflagellar rod, a structure adjacent to the axoneme in kinetoplastid flagella. Moreover, a fraction of the cells showed perturbance of the axonemal structure. Complementation of the deletion mutant with the wild-type gene restored typical promastigote morphology. We propose that LmxMKK influences anterograde intraflagellar transport to maintain flagellar length in Leishmania promastigotes; as such, it is the first protein kinase known to be involved in organellar assembly.Protein kinases are key regulatory molecules in the proliferation, differentiation, motility, and stress response of all eukaryotic cells. Together with their antagonists, the protein phosphatases, they are organized in complex networks to guarantee proper regulation of cellular processes according to environmental changes and intercellular communication. There is a wealth of information present on protein kinases in higher eukaryotes and Saccharomyces cerevisiae (18,45). Information on signal transduction processes for other unicellular organisms such as the sporozoa (Plasmodium, Toxoplasma, and Theileria) (22) and the kinetoplastida (Trypanosoma and Leishmania) (40, 50) as causative agents of infectious disease or the green alga Chlamydomonas (48) and the slime mold Dictyostelium discoideum (2) as model organisms for flagellar assembly and differentiation, respectively, is relatively scarce. It is likely that in Leishmania, as in other organisms, signal transduction pathways culminate in altered gene expression. As transcription factors and RNA polymerase II promoters are absent in these parasites, it is generally thought that regulation occurs posttranscriptionally on the level of mRNA maturation and turnover, efficiency of translation, and protein stability (10, 40). However, the regulatory molecules involved in these processes have not been identified.Leishmania parasites undergo a digenetic life cycle, differentiating from the promastigote form in the insect vector, the phlebotomine sand fly, to the amastigote form in the lysosomal compartment of the macrophages of mammals. Promastigotes are spindle-shaped cells, 11 to 20 m in length and 2 m in diameter, carrying a single flagellum of at least the length of the cell body at their anterior pole, which pulls the cell forward but also mediates...

SummaryThe adhesion of infected red blood cells (iRBCs) to human endothelium is considered a key event in the pathogenesis of cerebral malaria and other life‐threatening complications caused by the most prevalent malaria parasite Plasmodium falciparum. In the past 30 years, 14 endothelial receptors for iRBCs have been identified. Exposing 10 additional surface proteins of endothelial cells to a mixture of P. falciparum isolates from three Ghanaian malaria patients, we identified seven new iRBC receptors, all expressed in brain vessels. This finding strongly suggests that endothelial binding of P. falciparum iRBCs is promiscuous and may use a combination of endothelial surface moieties.

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Repeated exposure to psychostimulant drugs can result in behavioral sensitization, an amplified response in locomotor activity and stereotypy, which is used to model aspects of drug addiction. The expression of behavioral sensitization, induced by i.p. injections of nicotine once daily for 5 days, was examined in 450-day-old male rats exposed prenatally on GD 8-20 to one of the following conditions: (1) low nicotine: 2.5 mg/kg/day nicotine [LN]; (2) high nicotine: 5.0 mg/kg/day nicotine [HN]; (3) low nicotine/high cocaine: 2.5 mg/kg/day nicotine plus 40 mg/kg/day cocaine [LN/HC]; (4) high nicotine/low cocaine: 5.0 mg/kg/day nicotine plus 20 mg/kg/day cocaine [HN/LC]; (5) pair-fed controls: food intake yoked to HC dams [PF]; and (6) saline controls: daily injections of 0.9% NaCl solution[SAL]. Initial injection of nicotine did not alter activity or stereotypy in comparison to saline injections, with offspring in all prenatal treatment groups showing a desensitization to nicotine. Five consecutive daily nicotine injections resulted in behavioral sensitization in HN and HN/LC prenatal drug groups. Offspring exhibited an increase in horizontal activity that was evident on day 3, and still present after a 1.0 mg/kg i.p. nicotine challenge 72 hours after the last injection (day 8). SAL offspring exhibited attenuated sensitization. In contrast, nicotine sensitization was not seen in the LN, HC/LN, and the PF offspring; activity remained at the level seen after the initial injection of nicotine. Moreover, nicotine significantly reduced total activity in the LN and PF groups in comparison with their saline-injected counterparts. These data suggest that gestational exposure to high-dose nicotine, either alone or in combination with cocaine, may carry a greater risk than low-dose nicotine exposure of stimulant abuse in later life.