Stress response in tardigrades: differential gene expression of molecular chaperones

Abstract

Semi-terrestrial tardigrades exhibit a remarkable tolerance to desiccation by entering a state called anhydrobiosis. In this state, they show a strong resistance against several kinds of physical extremes. Because of the probable importance of stress proteins during the phases of dehydration and rehydration, the relative abundance of transcripts coding for two alpha-crystallin heat-shock proteins (Mt-sHsp17.2 and Mt-sHsp19.5), as well for the heat-shock proteins Mt-sHsp10, Mt-Hsp60, Mt-Hsp70 and Mt-Hsp90, were analysed in active and anhydrobiotic tardigrades of the species Milnesium tardigradum. They were also analysed in the transitional stage (I) of dehydration, the transitional stage (II) of rehydration and in heat-shocked specimens. A variable pattern of expression was detected, with most candidates being downregulated. Gene transcripts of one Mt-hsp70 isoform in the transitional stage I and Mt-hsp90 in the anhydrobiotic stage were significantly upregulated. A high gene expression (778.6-fold) was found for the small alpha-crystallin heat-shock protein gene Mt-sHsp17.2 after heat shock. We discuss the limited role of the stress-gene expression in the transitional stages between the active and anhydrobiotic tardigrades and other mechanisms which allow tardigrades to survive desiccation.

Fig. 1

Alignment of two α-crystallin/small heat-shock proteins from M. tardigradum with α-crystallin/small heat-shock proteins from Ixodes scapularis (EEC06453), Acyrthosiphon pisum (XP_001949446), Bombyx mori (NP_001036985), Drosophila ananassae (XP_001963454) and Locusta migratoria (ABC84493). Black high homology, grey weak homology, blank no homology

Fig. 2

Relative expression of analysed stress-gene transcripts of M. tardigradum in different stages of anhydrobiosis and heat shock. The asterisks (P ≤ 0.05) indicate different expression of transcripts compared with control specimens, which underwent no treatment (displayed by the base line)

Fig. 3

a Domain analysis of Mt-shsp 17.2 shows that it contains an alpha-crystallin domain (residues 34–113) from the Hsps-p23-like superfamily. There is a putative dimer interface predicted, and residues 1 to 127 belong to COG0071/IbpA, molecular chaperon COG. Compared to other known metazoan proteins, this is a small single domain protein (most others have multidomain context). The closest neighbour by sequence comparison is the heat-shock protein 20.6 (putative) from I. scapularis (e-value 4e−13) but there are also the well-characterised ones, e.g. from B. mori similar over most of the sequence (13–131) with an e-value of 2e−12. b Domain analysis of Mt-shsp 19.5 Domain analysis shows that also this protein contains an alpha-crystallin domain (residues 76–154) from the Hsps-p23-like superfamily. At the N-terminal end of the domain, there is again a dimer interface predicted but somewhat weaker. There is highest similarity (1e−33 to heat-shock protein 20.6 isoform 2 from Nasonia vitripennis; residues 42–173) but again also to the B. mori version (residues 60–156) with 7e−33. Compared to other known metazoan proteins, this is again a small domain protein (most others have multidomain context). However, compared to the shorter version Mt-shsp 17.2, we have here a tardigrade-specific N-terminus (first 75 residues) not occurring in other organisms