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Malaria is today a disease of poverty and underdeveloped countries. In Africa, mortality remains high because there is limited access to treatment in the villages. We should follow in Pasteur's footsteps by using basic research to develop better tools for the control and cure of malaria. Insight into the complexity of malaria pathogenesis is vital for understanding the disease and will provide a major step towards controlling it. Those of us who work on pathogenesis must widen our approach and think in terms of new tools such as vaccines to reduce disease. The inability of many countries to fund expensive campaigns and antimalarial treatment requires these tools to be highly effective and affordable.

Two studies provide evidence that the protein complex PTEX is needed for export of malaria-parasite proteins into the cytoplasm of infected cells, and that such export is essential for parasite survival.

The factors determining disease severity in malaria are complex and include host polymorphisms, acquired immunity and parasite virulence1. Studies in Africa have shown that severe malaria is associated with the ability of erythrocytes infected with the parasite Plasmodium falciparum to bind uninfected erythrocytes and form rosettes2,3,4,5. The molecular basis of rosetting is not well understood, although a group of low-molecular-mass proteins called rosettins have been described as potential parasite ligands6. Infected erythrocytes also bind to endothelial cells, and this interaction is mediated by the parasite-derived variant erythrocyte membrane protein PfEMP1 (refs 7, 8), which is encoded by the var gene family9,10,11. Here we report that the parasite ligand for rosetting in a P. falciparum clone is PfEMP1, encoded by a specific var gene. We also report that complement-receptor 1 (CR1) on erythrocytes plays a role in the formation of rosettes and that erythrocytes with a common African CR1 polymorphism (Sl(a−))12 have reduced adhesion to the domain of PfEMP1 that binds normal erythrocytes. Thus we describe a new adhesive function for PfEMP1 and raise the possibility that CR1 polymorphisms in Africans that influence the interaction between erythrocytes and PfEMP1 may protect against severe malaria.

Two studies provide evidence that the protein complex PTEX is needed for export of malaria-parasite proteins into the cytoplasm of infected cells, and that such export is essential for parasite survival. See Letters p.587 & p.592 The malaria parasite, Plasmodium falciparum, infects and remodels the host's red blood cells, a process that requires the export of hundreds of proteins into the cytosol. This a considerable topological feat, as the parasite initially resides in a compartment known as the parasitophorous vacuole. A protein complex known as PTEX (Plasmodium translocon of exported proteins) was thought to be involved in this process, but evidence of function was circumstantial and indirect. Two reports in this issue of Nature use contrasting techniques to demonstrate that PTEX is essential for the export of malaria parasite proteins into the cytoplasm of infected cells, and that such export is essential for the parasite's life cycle. Brendan Elsworth et al. generated conditional mutants of PTEX components HSP101 and PTEX150, and show that when PTEX function is perturbed the export of proteins including the major virulence factor PfEMP1 is much reduced. Josh Beck et al. use an innovative dihydrofolate reductase based destabilization domain approach to inactivate the HSP101 and show that it is needed for the secretion of all classes of exported malarial proteins.

During periods of feeding, pancreatic islets secrete insulin to maintain glucose homeostasis — a rhythmic process that is disturbed in people with diabetes. Experiments in mice now show that the pancreatic islets contain their own biological clock, which orchestrates insulin secretion during the sleep–wake cycle. The transcription factors CLOCK and BMAL1 are vital for this process, and mice with defective copies of the genes Clock and Bmal1 develop hypoinsulinaemia and diabetes. By demonstrating that a local tissue clock integrates circadian and metabolic signals in pancreatic β-cells, this work suggests that circadian analyses are crucial for deeper understanding of metabolic phenotypes, as well as for the treatment of metabolic diseases such as type 2 diabetes. Circadian rhythms control many physiological functions. During periods of feeding, pancreatic islets secrete insulin to maintain glucose homeostasis — a rhythmic process that is disturbed in people with diabetes. These authors show that pancreatic islets contain their own clock: they have self-sustained circadian oscillations of CLOCK and BMAL1 genes and proteins, which are vital for the regulation of circadian rhythms. Without this clock, a cascade of cellular failure and pathology initiates the onset of diabetes mellitus. The molecular clock maintains energy constancy by producing circadian oscillations of rate-limiting enzymes involved in tissue metabolism across the day and night1,2,3. During periods of feeding, pancreatic islets secrete insulin to maintain glucose homeostasis, and although rhythmic control of insulin release is recognized to be dysregulated in humans with diabetes4, it is not known how the circadian clock may affect this process. Here we show that pancreatic islets possess self-sustained circadian gene and protein oscillations of the transcription factors CLOCK and BMAL1. The phase of oscillation of islet genes involved in growth, glucose metabolism and insulin signalling is delayed in circadian mutant mice, and both Clock5,6 and Bmal17 (also called Arntl) mutants show impaired glucose tolerance, reduced insulin secretion and defects in size and proliferation of pancreatic islets that worsen with age. Clock disruption leads to transcriptome-wide alterations in the expression of islet genes involved in growth, survival and synaptic vesicle assembly. Notably, conditional ablation of the pancreatic clock causes diabetes mellitus due to defective β-cell function at the very latest stage of stimulus–secretion coupling. These results demonstrate a role for the β-cell clock in coordinating insulin secretion with the sleep–wake cycle, and reveal that ablation of the pancreatic clock can trigger the onset of diabetes mellitus.

CELL growth and differentiation are usually mutually exclusive1. Transformation of myoblasts by retroviruses containing the myc oncogene inhibits differentiation, preventing cells from withdrawing from the cell cycle4,5. If cell-cycle withdrawal is a prerequisite for myoblast differentiation, it is probably an early event in terminal cell differentiation, but this has not yet been established6. MyoD1 regulates myogenesis7. It is expressed only in skeletal muscle8, but can convert other cells to muscle cells9,10. The MyoD1 protein, a nuclear phosphoprotein in part similar to the myc family of proteins, is a DNA-binding protein binding to the enhancer sequences of the muscle-specific creatine phosphokinase gene11. Thus, introduction of MyoD1 into cells provides a simple approach to study the effect of induction of differentiation on cell growth. In cultured NIH 3T3 cells, inhibition of cell proliferation occurs within 18 hours, and expression of myosin starts after 72 hours. Furthermore, injection of MyoD1 into quiescent NIH3T3 cells inhibits the serum-induced G0–S phase transition of the cell cycle. In CV1 cells, which are not induced to differentiate by MyoD1, DNA synthesis is also inhibited. Expression of MyoD1 can thus inhibit cell proliferation independently of induction of differentiation. Deletion of the myc-like domain in the MyoD1 gene12 eliminates the inhibition of DNA synthesis, but substitution of the basic domain with the analogous domain from the E12 transcription factor13 inhibits growth yet fails to induce differentiation. Inhibition of DNA synthesis, therefore, seems to be controlled separately from myogenic differentiation.

The southern pine beetle, Dendroctonus frontalis Zimmerman (SPB) is attacked by several species of hymenopterous parasites1,2 the most common of which also parasitise other bark beetles3–5 including the engraver beetles Ips grandicollis (Eichhoif), I. avulsus (Eichhoff), I. calligraphus (Germar), I. pini (Say); and the eastern juniper bark beetle, Phloeosinus dentatus (Say). The increasing importance of pest management has focused attention on the role of their natural enemies. However, the dynamics of bark beetle parasite populations is not generally known beyond estimates of mortality caused by parasites. The hymenopteran parasite guild of D. frontalis may kill up to 30% of a given brood2. Since D. frontalis is cyclic in most areas with epidemics followed by very low population densities1, other beetle hosts probably act as reservoirs for certain species of these parasites3. Conversely, during D. frontalis epidemics, other beetles, such as I. grandicollis and P. dentatus may compete as alternate hosts. Information on preferences for D. frontalis and other associated bark beetles is necessary to understand the interactions of this host–parasite complex. We report here the results of a study designed to determine if several known D. frontalis parasites display any host preferences among D. frontalis, I. grandicollis or P. dentatus.

Asexual proliferation of malaria parasites proceeds by multiplication of the parasites within red cells. Following rupture of the host cells the released merozoites re-invade other red cells. On re-invasion, a proportion of merozoites become, not asexual parasites but gametocytes, the sexual stages infective to the mosquito vectors. Conversion of asexual parasites to gametocytes occurs not only during natural infections but also in continuous in vitro culture as reported first by Trager and Jensen and by others. We showed previously that the proportion of early intra-erythrocytic stages (ring stages) of Plasmodium falciparum which developed into gametocytes in culture was influenced by culture conditions. Gametocyte formation was rare in conditions supporting rapid proliferation but frequent when parasite densisites were static. We now show that nearly 100% of ring stages develop into gametocytes in response to 1mM cyclic AMP in static cultures whereas in rapidly growing cultures few rings become gametocytes in response to cyclic AMP.

Malaria vaccines are being developed against different stages in the parasite's life cycle, each increasing the opportunity to control malaria in its diverse settings. Sporozoite vaccines are designed to prevent mosquito-induced infection; first generation recombinant or synthetic peptide vaccines have been tested in humans. Asexual erythrocytic stage vaccines, developed to prevent or reduce the severity of disease, have been tested in animals and in humans. A third strategy is to produce sexual stage vaccines that would induce antibodies which would prevent infection of mosquitoes when ingested in a bloodmeal containing sexual stage parasites. Although not directly protective, the sexual stage vaccine combined with a sporozoite or asexual stage vaccine (protective component) could prolong the useful life of the protective component by reducing transmission of resistant vaccine-induced mutants. In areas of low endemnicity, the sexual stage vaccine could reduce transmission below the critical threshold required to maintain the infected population, thereby assisting in the control or eradication of malaria. Transmission of Plasmodium falciparum, the major human malaria, can be blocked by monoclonal antibodies against three sexual stage-specific antigens. We have cloned the gene encoding the surface protein of relative molecular mass Mr 25,000 (25K; Pfs25), expressed on zygotes and ookinetes of P. falciparum. The deduced amino-acid sequence consists of a signal sequence, a hydrophobic C-terminus, and four tandem epidermal growth factor EGF-like domains.

THE specific receptors for several non-enveloped viruses are present on cells in only limited numbers (1 × 104–10 × 104), and can therefore be saturated with excess virus1–5. An excess of inactivated poliovirus type 1 inhibits the attachment of infectious poliovirus of all 3 serotypes, but not the attachment of other viruses, including B Coxsackie viruses6. Most of the B Coxsackie viruses probably share the same receptor7, but Coxsackie viruses B1 and B3 do not compete for the poliovirus receptor1. Adenovirus 2 and 5 also appear to share the same receptor, since their attachment to host cells is blocked by an excess of the adenovirus type 2 fibre protein, which seems to be the receptor-recognising portion of the virus particle2. A number of other adenoviruses may also belong to the same receptor family since soluble antigens block their attachment to erythrocytes8. We here report results which permit the construction of a ‘receptor map’ for a number of non-enveloped viruses.

Malaria is initiated by the inoculation of a susceptible host with sporozoites from an infected mosquito. The sporozoites enter hepatocytes and develop for a period as exoerythrocyte or hepatic stage parasites1. Vaccination with irradiated sporozoites can provide protective immunity1 and a recent study2 shows that this can also be conferred by immunization with a recombinant salmonella expressing only the circumsporozoite protein that normally covers the sporozoites. Protection against infection is likely to be mediated by cytotoxic CD8+ cells, as depletion of CD8+ T cells in a sporozoite-immunized animal can completely abrogate immunity3,4. Here we demonstrate directly the existence of CD8+ cytotoxic T lymphocytes (CTL) that recognize the circumsporozoite protein. B10.BR mice immunized with sporozoites or with recombinant vaccinia virus expressing the CS protein of Plasmodium falciparum contain CTL that specifically kill L cell fibroblasts transfected with the gene encoding the same CS protein. The peptide epitope from the CS protein that is recognized by CTL from this strain of mice is from a variant region of the protein.

An important feature of Plasmodium falciparum malaria which differentiates it from other human malarias is that erythrocytes infected with trophozoites and schizonts are not present in the peripheral blood but are sequestered along capillary and venular endothelium. Infected erythrocytes attach via parasite-induced ultrastructural modifications on the surface of the infected cells, called 'knobs'. This sequestration may be important for parasite survival because it prevents infected erythrocytes from circulating through the spleen where they could be eliminated. We have established an in vitro correlate of sequestration and used it to demonstrate that immune sera from repeatedly infected Aotus monkeys inhibit binding of infected erythrocytes to endothelial cells. We have investigated whether antiserum that blocks binding of one isolate of P. falciparum to target cells can block or reverse binding of other isolates. We report here that sera which block or reverse binding are strain-specific, indicating that the corresponding antigens on the surface of the infected erythrocytes are strain (isolate)-specific.

MUSCULAR weakness is one of the well-known clinical sequelae of exposure of mammals to ionizing radiation. The degree of myasthenia is determined by the radiation dose to which the individual has been exposed. The present report deals with observations which may give a clue to some of the biochemical mechanisms underlying this radiation-induced myopathy.

NORMAL mammalian serum of most species is capable of neutralizing bacteriophages of the Escherichia coli T series. This activity of normal serum has been attributed to the properdin system although a lack of correlation between properdin-levels and phage-neutralizing activity was noted with certain sera1. Recent work by Cowan2, moreover, in which several human sera were assayed for activity against T2, T6, and T7, indicated that the phage-neutralizing activity of a serum against one of the phage types was not correlated with its activity against the other two. These results suggested that the properdin system, essentially non-specific in its action, could not adequately account for the activity of normal serum against the three phage types. Cowan postulated, therefore, that properdin might be a family of cross-reacting antibodies capable of combining with zymosan. Similar considerations are applicable to other activities of normal serum. Our own findings in studies of the bactericidal action of normal serum against several species of Enterobacteriaceae indicated that bactericidal antibody of marked specificity plays the major part3.

IN the course of an investigation of the effect of partial body X-irradiation on collagen metabolism we used thorazine (chlorpromazine) as an aid in immobilizing rats1. We found quite unexpectedly that the incorporation of carbon-14-labelled amino-acids into collagen was markedly suppressed during the first month, and considerably enhanced 2–4 months after the injection of the drugs. The effect of thorazine on collagen metabolism was observed in all tissues, but was most pronounced in bone collagen. Because of the involvement of bone collagen, we feel that our findings should be brought to the attention of clinicians who may be treating fracture cases with, among other drugs, thorazine, a drug which may impair the healing process.

A comparison of the effects of x rays on the potential capabilities of livers of normal and partially hepatectomized rats to incorporate amino acids into liver porteins in relation to the dose and time of exposure to radiation after hepatectomy is presented. The intact rat liver differs qualitatively from the hepatic tissue remaining after partial hepatectomy as far as the effect of x irradiation on the incorporation of leucine-U-C/sup 14/ into liver proteins is concerned. Thus, x irradiation at 1000 or 2000 r is followed by an increase in the isotope concentration in the proteins of the nonhepatectomized rat liver, whereas in the case of regenerating liver exposed to 1000 r, a decrease in the isotope concentration in liver proteins is found if the animals are irradiated within 6 hr after hepatectomy. After this time period, the exposure of the hepatectomized rats to x rays is without effect on amino acid incorporation. It is concluded that the normal rat liver, with cells in the intermitotic phase, and regenerating liver, many cells of which are in the pre-mitotic phase, seem to respond differently to the action of ionizing radiation as far as their ability to incorporate C/sup 14/ labeled leucine intomore » protein is concerned.« less