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THERMOBIOLOGY AND HOST SWAPPING IN MALARIA
Stanford University
JStillman{at}stanford.edu
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Mechanisms of temperature adaptation in eukaryotes have been extensively studied during constant temperature acclimation, but relatively little is known about how organisms cope with life in fluctuating thermal environments. For free-living organisms, fluctuations may occur over time scales of hours to days or seasonally. Parasites can experience a change in their thermal habitat when they move between hosts during different life history stages. The malarial parasite, Plasmodium falciparum, is one such case. The first life stage occurs in the human host, which has a high body temperature, ranging from 37°C to 41°C during fever. The second two stages occur in the cooler mosquito host, which lives in habitats that commonly range from 20 to 30°C. In their Nature Brief Communication, Fang and McCutchan describe an investigation of the regulation of the transcription of a ribosomal RNA molecule in P. falciparum at different temperatures, which they hope will help us to understand how the parasite senses and reacts to it's changing environment.
The team followed how one RNA component of the ribosome (a part of the cellular protein synthesis apparatus) was expressed when the parasite's cells were cultured at different temperatures. They already knew that this component of the ribosome was expressed as different isoforms, called A and S, during different stages of the parasite's life cycle. As these different life stages mark transitions from one temperature environment to another, they decided to monitor how expression of this component was affected by rearing parasite cells at different temperatures. They tracked the transcription of the RNA component using real-time PCR at 26, 31, 37 or 42°C to see how the expression profiles of A and S varied with temperature.
At first they discovered that the A form of the RNA was expressed mostly during the first life stage, while the S form was expressed during the latter two life stages.
Then they monitored the transcription of two separate isoforms of A and S to see if the parasite regulated their expression at different points in the parasite's life cycle. They found that both forms of A were expressed during the first life stage, but the parasite expressed S1 during the first developmental stage in the mosquito host, and switched to S2 when it progressed on to the second stage.
When the team looked at the effect of temperature on the expression profiles of the four forms of the RNA subunit, they found that A expression levels were essentially the same at all four temperatures, but when they looked at the expression profiles of the S isoforms, they varied significantly at different temperatures. Neither form of S RNA was expressed at 42°C, the temperature found in a human with fever, but as they dropped the temperature, both forms began to be expressed at increasing levels. The S2 RNA was most dramatically affected; its expression level at 26°C was over 20 times the level at 37°C! The authors speculate that the cold-activation of S2 expression may be due to transcriptional control by a cold-induced promoter, although this hypothesis is untested. Whether the sequence variations in the ribosomal RNA confers changes in the ribosome's function that match each of the parasite's thermal habitats is yet to be determined.
References
Fang, J. and McCutchan, T. F. (2002). Thermoregulation in a parasite's life cycle. Nature 418, 742.[CrossRef][Medline]
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