Hijacking the Cell: Parasites in the Driver's Seat

As our knowledge of pathways affected by parasites increases, attention turns now to the process of identifying the parasite molecules that mediate the remarkable events transpiring in the host cell. This will be familiar to students of viral and bacterial pathogenesis as the search for parasite “virulence factors.” Already, application of the methods of biochemistry and molecular biology has yielded numerous attractive molecules for model-builders searching for virulence genes. Examples of these include a multitude of surface molecules well positioned to interface with the host, such as the malaria circumsporozoite protein which mediates attachment of the invading sporozoite to hepatocytes (Cerami et al. 1992xCerami, C, Frevert, U, Sinnis, P, Takacs, B, Clavijo, P, Santos, M.J, and Nussenzweig, V. Cell. 1992; 70: 1021–1033Abstract | Full Text PDF | PubMed | Scopus (238)See all ReferencesCerami et al. 1992), or any of a number of genes up-regulated in infective stages of parasites. The initiation of several parasite genome projects promises to expand this roster considerably.Accompanying this has been the development of critical genetic tools for use in parasites. In the groups where progress has been greatest (trypanosomes, Leishmania, and Toxoplasma), one can now readily express or knockout genes, and carry out functional genetic rescue of interesting mutants. A variety of “bells and whistles” such as regulatable expression and artificial chromosomes are also available. Recent successes with the introduction of DNA into Entamoeba histolytica, Giardia lamblia, and malaria will undoubtedly lead to similarly rapid deployments there.In 1988, Stanley Falkow, in recognition of the ability of molecular biology to provide an abundance of genes for which roles in pathogenesis could be envisioned, proposed a set of “molecular Koch's postulates” to serve as a guide in testing and assessing their role (Falkow 1988xFalkow, S. Rev. Infect. Dis. 1988; 10: S274–276Crossref | PubMedSee all ReferencesFalkow 1988). These may be summarized as follows: first, the property under study should be reasonably associated with pathogenicity or infectivity; second, that specific inactivation of a candidate gene should lead to a significant loss in virulence; and third, that restoration of gene function should fully restore pathogenicity. As many workers hopeful of knockout phenotypes in yeast and mammalian cells can testify, this stringent criterion can both satisfy and disappoint.Thus far, tests analogous to those proposed by Falkow have not been widely applied in parasitic systems due to the recency of the necessary methodology. One example is that elimination of the rhoptry protein encoded by Toxoplasma ROP1 failed to affect invasion (Kim et al. 1993xKim, K, Soldati, D, and Boothroyd, J.C. Science. 1993; 262: 911–914Crossref | PubMedSee all ReferencesKim et al. 1993). Of course, this may reflect the existence of compensatory or redundant pathways. In contrast, Leishmania mutants defective in the synthesis of the surface glycolipid lipophosphoglycan (LPG) show an inability to survive in both the insect vector and while establishing infections in the macrophage. Here, the combined efforts of several labs have shown that restoration of the defective gene can restore survival. Other examples will undoubtedly be forthcoming. The application of Falkow's tests in parasite systems is particularly important, as culture and genetic manipulations can lead to changes in virulence unrelated to the planned genetic modifications.The marriage of insights gained into the mechanisms and cell biology of parasitism, and the arrival of much needed genetic tools for identifying and dissecting the genes carrying out these processes, assures us that the next few years will be one of the most exciting periods in molecular parasitological research.