Chill and freeze represent very different components of low temperature stress. Whilst the principal mechanisms of tissue damage and of acquired protection from freeze-induced effects are reasonably well established, those for chill damage and protection are not. Non-freeze cold exposure (i.e. chill) can lead to serious disruption to normal life processes, including disruption to energy metabolism, loss of membrane perm-selectivity and collapse of ion gradients, as well as loss of neuromuscular coordination. If the primary lesions are not relieved then the progressive functional debilitation can lead to death. Thus, identifying the underpinning molecular lesions can point to the means of building resistance to subsequent chill exposures. Researchers have focused on four specific lesions: (i) failure of neuromuscular coordination, (ii) perturbation of bio-membrane structure and adaptations due to altered lipid composition, (iii) protein unfolding, which might be mitigated by the induced expression of compatible osmolytes acting as 'chemical chaperones ', (iv) or the induced expression of protein chaperones along with the suppression of general protein synthesis. Progress in all these potential mechanisms has been ongoing but not substantial, due in part to an over-reliance on straightforward correlative approaches. Also, few studies have intervened by adoption of single gene ablation, which provides much more direct and compelling evidence for the role of specific genes, and thus processes, in adaptive phenotypes. Another difficulty is the existence of multiple mechanisms, which often act together, thus resulting in compensatory responses to gene manipulations, which may potentially mask disruptive effects on the chill tolerance phenotype. Consequently, there is little direct evidence of the underpinning regulatory mechanisms leading to induced resistance to chill injury. Here, we review recent advances mainly in lower vertebrates and in arthropods, but increasingly in genetic model species from a broader range of taxa.
KEY WORDS: Membrane fluidity, Proteostasis, Compatible solutes, Gene ablation
IntroductionEnvironmental cold poses multiple problems for all living organisms, especially poikilotherms (Cossins and Bowler, 1987). The effects of low temperature on performance are largely determined by the extent to which cooling reduces the rate of biochemical reactions -thus slowing down any rate-dependent processes (Hochachka and Somero, 2002). The impact on survival, however, is determined by the disruptive effects of extreme cold on *Author for correspondence (cossins@liverpool.ac.uk) the molecular processes underpinning normal cellular function, such as protein and membrane integrity, which in turn are closely associated with such fundamental cellular properties such as ion homeostasis and continued activity of excitable cells (Lee, 2010). Thus, at some point during both declining temperatures and diminishing performance, cold becomes stressful, and chilling injuries then accumulate over time, leading...
