Biomedical subjects

K Rangachari

Publications and source records attributed to K Rangachari.

23 records · Page 2Linked to original sources

PubMed1983

A cytoplasmic requirement of red cells for invasion by malarial parasites.

Human red cells, when lysed by dialysis at high haematocrit against a medium of low ionic strength and then dialysed back to physiological saline at 37 degrees C, give rise to resealed ghosts that are invaded with high efficiency by Plasmodium falciparum parasites. When the haematocrit is reduced, a critical concentration is reached, such that the resealed ghosts no longer support invasion. This indicates that a constituent of the cytoplasm becomes diluted to a concentration below a critical level. This constituent is evidently ATP, for when extraneous ATP is added to the diluent and the dialysate, the susceptibility to invasion is fully restored. This does not occur when the non-hydrolysable analogue, adenylyl-imidodiphosphate (AMP-PNP) is substituted for ATP, whereas the hydrolysable ATP analogue, adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S), which can be utilised by kinases, can partly replace ATP. Stimulation of invasion by the addition of 2,3-diphosphoglycerate was also associated with a perceptible rise in ATP concentration. The invasion process does not appear to involve intracellular calcium, for EGTA introduced into the resealed ghost has no detectable effect. Moreover, vanadate in the medium does not appreciably inhibit invasion, and it is thus unlikely that the requirement for ATP is linked to the activity of membrane ion-pump enzymes. An inhibitor of phosphorylation, adenosine, introduced into the cells at high concentration, causes significant inhibition of invasion. The results suggest that ATP is required for maintaining the turnover of phosphoryl groups of membrane-associated proteins, such as spectrin. A basic scheme for the mechanism of the invasion process is suggested. In addition to the effect of ATP, it is also shown that with greater dilution, and in the presence of ATP, there is an abrupt loss of susceptibility to invasion. It is inferred that this is due to the dilution of another essential cytoplasmic constituent to below a critical concentration. This second constituent has not yet been identified.

Adenosine↗

PubMed1983

Properties of red cell ghost preparations susceptible to invasion by malaria parasites.

Experimental procedures are described that lead to the formation of sealed red cell ghosts capable of efficient invasion by malaria parasites. Both human and monkey cells have been studied with respect to invasion by Plasmodium falciparum and P. knowlesi respectively. Resealed ghosts containing about half the normal concentration of haemoglobin are prepared by osmotic lysis and resealing at a haematocrit of 70%. When examined in the scanning electron microscope, populations of these ghosts contain few echinocytes, but have an abundance of stomatocytic forms. When undiluted haemolysate is substituted for the aqueous saline diluent, in which the cells are suspended for lysis and resealing, discocytes result, with a morphology very similar to that of the original cells. Invasion is somewhat, but not dramatically higher for this material. An investigation of the effect of intracellular potassium concentration revealed that this had no major effect on invasion. A small increase in invasion resulted from addition of 50 or 100 mg/ml of albumin to the medium in which the cells were resealed, but intra-erythrocytic protein content per se is evidently not a major factor in determining the efficiency of invasion. Augmentation of the cytoplasmic ATP concentration during lysis and resealing increased the level of parasitaemia significantly, and the parasites in these ghosts developed normally and gave rise to viable merozoites. It was found that albumin at certain concentrations passed freely into the lysed cells and reached equilibrium with that in the external medium. By contrast, the high molecular weight solute Blue Dextran 2000 was completely excluded from the lysed cells.

Adenosine Triphosphate↗

PubMed1983

Inhibition of malarial invasion of red cells by chemical and immunochemical linking of spectrin molecules.

The invasion of resealed human red cell ghosts by Plasmodium falciparum, and those from monkey cells by P. knowlesi, was strongly inhibited by anti-spectrin antibodies introduced into their cytoplasm. Univalent F(ab)1 fragments gave no such effect, but a combination of these fragments and goat anti-rabbit immunoglobulin, to restore bifunctionality, caused perceptible inhibition of invasion. Disulphide cross-links introduced between spectrin molecules in intact red cells by the membrane-permeant oxidizing agent, diamide, again led to inhibition of invasion. This effect was largely reversed by reduction of the cross-links. Gel electrophoresis was used to confirm that cross-linking was essentially confined to spectrin, and that extended covalent networks were not formed. It follows that local formation of bridges, whether by antibodies or oxidation of thiol groups, functions by inhibiting a local rearrangement of the cytoskeleton that forms a step in the invasion process.

Animals↗

PubMed1981

Entry of malaria parasites into resealed ghosts of human and simian erythrocytes.

Whereas "white' ghosts, conventionally prepared by hypotonic lysis and washing are resistant to invasion by malaria parasites, cells lysed at high haematocrit and resealed without separating the membranes from the cytosol are invaded with high efficiency (not less than one-third the frequency of intact cells). Similar results were obtained with human and monkey cells, exposed respectively to Plasmodium falciparum and P. knowlesi. Microspectrophotometry of individual cells was used to ascertain the the susceptibility to invasion of lysed, resealed cells was not related to its haemoglobin content.

Animals↗

PubMedDate not supplied

Control of malarial invasion by phosphorylation of the host cell membrane cytoskeleton.

It has been shown that the entry of the malaria parasite into the red blood cell requires the presence of ATP in the host cell cytoplasm. In red blood cell ghosts that contain no ATP the receptor on the extracellular surface remains in place and parasites will bind to the membrane, but will not enter. ATP is thus necessary for one of the steps in the invasion sequence that follows recognition and attachment. The process of entry appears to involve the active participation of the host cell membrane cytoskeleton. We have suggested that the function of the intracellular ATP may be to regulate phosphorylation of the cytoskeleton. We now present evidence that the activity of the membrane-associated cyclic AMP-independent kinase of the red blood cell is inseparable from invasion; the active substrate may be spectrin.

Animals↗