Info

the sense that cold hardening does not generally improve survival of heat shock. Moreover, cold tolerance can be induced with as little as a 10 min exposure to 0oC in S. crassipalpis, whereas a 30 min exposure to 40oC is required to provide protection against injury at 45oC (Chen et al. 1991).

Furthermore, although heat shock proteins are synthesized rapidly in response to both cold shock and heat shock, there are considerable differences between these two responses. First, the time course of the response differs. That is, during heat shock, Hsps are synthesized during the stress, while in response to cold shock Hsps are only produced once the animals have been returned to a higher temperature (Goto and Kimura 1998; Rinehart et al. 2000a). This suggests that heat shock proteins do not contribute directly to rapid cold hardening, but provide protection against low temperature injury (Denlinger and Lee 1998, but see also Minois 2001). Second, the duration of the response differs dramatically between the two forms of shock. Usually, synthesis of Hsps in response to high temperature is brief and ceases almost immediately on cessation of the stress (Yocum and Denlinger 1992), while in response to low temperature, Hsp synthesis may continue for days (Yocum et al. 1991). Third, during heat shock, normal protein synthesis is almost entirely replaced by stress protein synthesis, whereas following a cold shock normal protein synthesis and the production of stress proteins occur concurrently.

The fact that the responses to cold and heat shock are different is also illustrated by a comparison of related species from geographically disjunct areas that have dissimilar climates. Although several such studies have been undertaken (e.g. Goto and Kimura 1998), the comparison of flesh flies from tropical and temperate areas made by Chen et al. (1990) is one of the most comprehensive. While all the species show an inducible tolerance to heat shock, only the species from temperate and alpine areas show rapid cold hardening (Fig. 5.16). As might be expected, basal tolerance of cold is greater in the temperate and alpine species than in the tropical ones, but this is true also of basal heat tolerance. Although this appears somewhat unusual, it should be kept in mind that mid-latitude areas are often characterized by very high temperatures

(Somme 1995), and that global variation of absolute maximum temperatures is much less than that of absolute minima (Section 5.4.2).

Clearly, the relationship between tolerance of temperature extremes and the synthesis of heat shock proteins is intricate (Goto et al. 1998). One of the reasons for this complexity undoubtedly emerges from the fact that stress proteins are involved in many different processes in the cell and are not synthesized only as a response to stress. In insects this has been most clearly illustrated in investigations of Hsp regulation during diapause (see Denlinger 2002 for a recent review). Expression of hsp70 and hsp23 certainly do contribute to ther-motolerance, as has been shown by their almost immediate expression on entry into diapause in S. crassipalpis, where cold tolerance and diapause are linked (Joplin et al. 1990), but delayed expression in L. dispar, where cold tolerance and diapause are developmentally separated (Denlinger et al. 1992). However, hsp70 is also upregulated on entry into diapause in S. crassipalpis in the absence of any stress (Rinehart et al. 2000a). Likewise, hsp23 is upregulated in response to heat and cold shock in non-diapausing S. crassipalpis, whereas in diapaus-ing individuals stress does not cause greater expression of hsp23 which is highly upregulated at the onset of diapause (Yocum et al. 1998). In contrast, hsp90 is downregulated during diapause, but continues to respond to heat and cold shock (Rinehart and Denlinger 2000).

Given that the continued expression of heat shock proteins is known to be deleterious (Section 5.2.2), their continued upregulation during diapause initially appears remarkable. However, cell cycle arrest plays an important role in diapause in S. crassipalpis. Therefore, if the majority of negative effects of Hsp expression have to do with reduced cellular growth and differentiation, Hsps may have little adverse effect during diapause and may even assist in the maintenance of diapause (Yocum et al. 1998; Rinehart et al. 2000a), as well as serving to protect diapausing individuals from thermal and other stresses. The downregulation of hsp90 at the onset of diapause, and its upregulation following diapause termination, or in response to heat or cold shock, is also readily comprehensible within this framework. Hsp90 keeps unstable proteins ready

Cold shock Heat shock

Cold shock Heat shock

Figure 5.16 The effects of exposure to —10°C or 45°C either following direct transfer from 25°C or following a 2-h treatment at 0°C and 40°C, respectively, on flesh flies from (a) temperate, (b) alpine, and (c) tropical environments.

Source: Chen et al. (1990). Journal of Comparative Physiology B 160, 543-547, Fig. 1. © Springer.

Figure 5.16 The effects of exposure to —10°C or 45°C either following direct transfer from 25°C or following a 2-h treatment at 0°C and 40°C, respectively, on flesh flies from (a) temperate, (b) alpine, and (c) tropical environments.

Source: Chen et al. (1990). Journal of Comparative Physiology B 160, 543-547, Fig. 1. © Springer.

for activation until they are stabilized during signal transduction (Rutherford and Lindquist 1998). Thus, given relative cell inactivity during diapause, Hsp90 is unlikely to be required, but because of its ability to stabilize proteins, it remains responsive to thermal stress. During the non-diapausing state this responsiveness to denatured proteins may also be the cause of the expression of phenocopies, or developmental abnormalities that resemble specific mutations (Denlinger and Yocum 1998), in response to heat shock. Rutherford and Lindquist (1998) have pointed out that thermal stress can divert Hsp90 from its normal functions to partially denatured proteins, thus compromising the ability of Hsp90 to buffer developmental variation. In consequence, the widespread variation in morpho-genetic pathways that is usually suppressed during development can now be expressed. Selection can subsequently lead to expression of these traits even when Hsp90 function is regained, thus leading to considerable evolutionary change in what are otherwise highly conserved developmental processes.

Was this article helpful?

0 0

Post a comment