HSP70 is upregulated after heat but not freezing stress in the freeze-tolerant cricket Gryllus veletis

Heat shock proteins (HSPs) are well known to prevent and repair protein damage caused by various abiotic stressors, but their role in low temperature and freezing stress is not well-characterized compared to other thermal challenges. Ice formation in and around cells is hypothesized to cause protein damage, yet many species of insects can survive freezing, suggesting HSPs may be an important mechanism in freeze tolerance. Here, we studied HSP70 in a freeze-tolerant cricket Gryllus veletis to better understand the role of HSPs in this phenomenon. We measured expression of one heat-inducible HSP70 isoform at the mRNA level (using RT-qPCR), as well as the relative abundance of total HSP70 protein (using semi-quantitative Western blotting), in five tissues from crickets exposed to a survivable heat treatment (2 h at 40°C), a 6-week fall-like acclimation that induces freeze tolerance, and a survivable freezing treatment (1.5 h at -8°C). While HSP70 expression was upregulated by heat at the mRNA or protein level in all tissues studied (fat body, Malphigian tubules, midgut, femur muscle, nervous system ganglia), no tissue exhibited HSP70 upregulation within 2 – 24 h following a survivable freezing stress. During fall-like acclimation to mild low temperatures, we only saw moderate upregulation of HSP70 at the protein level in muscle, and at the RNA level in fat body and nervous tissue. Although HSP70 is important for responding to a wide range of stressors, our work suggests that this chaperone may be less critical in the preparation for, and response to, moderate freezing stress.

Highlights

• Heat shock protein 70 (HSP70) may not contribute substantially to freeze tolerance

• Heat stress caused HSP70 mRNA and protein upregulation in the spring field cricket

• Acclimation prior to freezing was correlated with slight HSP70 upregulation

• HSP70 was not upregulated after freezing in this freeze-tolerant insect

• Further work is needed to determine whether freezing causes protein damage