The process of erythrocyte invasion by merozoites of Plasmodium falciparum involves multiple steps, including the formation of a moving junction between parasite and host cell, and it is characterised by the redundancy of many of the receptor–ligand interactions involved. Several parasite proteins that interact with erythrocyte receptors or participate in other steps of invasion are encoded by small subtelomerically located gene families of four to seven members. We report here that members of the eba, rhoph1/clag, acbp, and pfRh multigene families exist in either an active or a silenced state. In the case of two members of the rhoph1/clag family, clag3.1 and clag3.2, expression was mutually exclusive. Silencing was clonally transmitted and occurred in the absence of detectable DNA alterations, suggesting that it is epigenetic. This was demonstrated for eba-140. Our data demonstrate that variant or mutually exclusive expression and epigenetic silencing in Plasmodium are not unique to genes such as var, which encode proteins that are exported to the surface of the erythrocyte, but also occur for genes involved in host cell invasion. Clonal variant expression of invasion-related ligands increases the flexibility of the parasite to adapt to its human host.

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SummaryThe first gene characterizing the clag (cytoadherence linked asexual gene) family of Plasmodium falciparum was identified on chromosome 9. The protein product (Clag9) was implicated in cytoadhesion, the binding of infected erythrocytes to host endothelial cells, but little information on the biochemical characteristics of this protein is available. Other genes related to clag9 have been identified on different chromosomes. These genes encode similar amino acid sequences, but clag9 shows least conservation. Clag9 was detected in schizonts, merozoites and ring-stage parasites after protease digestion and peptide analysis by mass spectrometry. Using antisera raised against unique regions of Clag9 and against RhopH2, a component of the RhopH high-molecular-mass protein complex of merozoites, immunofluorescence colocalized the two proteins to the apical region of merozoites. Immunoelectron microscopy co-localized Clag9 and RhopH2 exclusively to the basal bulb region of rhoptries rather than to their apical ducts. The same Clag9-specific antibodies bound the RhopH complex, and the protein was detected in the complex purified by antibodies to RhopH2. Clag9 protein was also shown to be present in ring-stage parasites, carried through from the previous cycle with the RhopH complex, in a location identical to that of RhopH2. Transcription of the clag9 gene was shown to occur at the same time as the genes for other members of the RhopH complex, rhoph2 and 3 . The results indicate that Clag9 is part of the RhopH complex and suggest that, within this complex, the protein previously designated RhopH1 is composed of more than one protein product of the clag gene family. The results cast doubt on a direct role for Clag9 in cytoadhesion; we suggest that the primary role of the RhopH complex is in remodelling the infected red blood cell after invasion by the merozoite. The complex may have multiple functions dependent on its exact composition, which may include, with respect to Clag9, a contribution to the mechanism of cytoadhesion.

Background: Methamphetamine is a drug of abuse that disrupts the blood-brain barrier.Results: Blocking actin nucleation protects against methamphetamine-induced occludin internalization and disruption of blood-brain barrier integrity.Conclusion: Methamphetamine-induced transendothelial breaches may result from actin-mediated redistribution of occludin.Significance: Actin cytoskeletal dynamics modulates redistribution of occludin and blood-brain barrier integrity.