Differentiation of pathogenic amoebae: encystation and excystation of Acanthamoeba culbertsoni —A model.
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Date
1984-10
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Abstract
Differentiation into dormant cysts and vegetative trophozoites is an inherent
character intimately associated with the life cycle and infectivity of pathogenic amoebae. In the
case of human intestinal amoebiasis encystation and excystation are of immediate relevance to
the process of transmission of the disease from healthy carriers to susceptible individuals.
Using a pathogenic free living amoeba Acanthamoeba culbertsoni as a model, considerable
progress has been achieved in understanding the mechanism and control of the process of
differentiation. The turnover of the regulatory molecule cyclic 3: ‘5’ adenosine monophosphate
is responsible for triggering the process of encystation. Amoebae bind effector molecules
such as biogenic amines to a membrane localized receptor which itself resembles the β-
adrenergic receptor of mammalian organisms. The activation of adenylate cyclase or inhibition
of cyclic AMP phosphodiesterase maintain the dynamic intracellular cyclic AMP. The cytosol
fraction of amoebae has a cyclic AMP binding protein. During encystation, enzymes needed
for synthesis of cellulose and glycoproteins are induced. Control is exercised at transcriptional
level and the process is subject to catabolic repression.
Excystation of mature amoebic cysts is mediated by glutamic acid and certain other amino
acids by an as yet unelucidated mechanism. During excystation there is dormancy break,
induction of deploymerizing enzymes viz. two proteases, a cellulase and a chitinase. The empty
cysts or cyst walls are digested by these enzymes and their break down products are used for
cellular growth. By invoking a flip-flop mechanism of repression and derepression some
plausible explanation can be offered for the cascade of biochemical events that sets in when
amoeba is ‘turned on’ to encystation or excystation.
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Amoeba, encystation, excystation, cyclic AMP
Citation
Murti C R Krishna, Shukla O P. Differentiation of pathogenic amoebae: encystation and excystation of Acanthamoeba culbertsoni —A model. Journal of Biosciences. 1984 Oct; 6(4): 475-489.