Calcium signaling mediates cold sensing in insect tissues

Abstract

The ability to rapidly respond to changes in temperature is a critical adaptation for insects and other ectotherms living in thermally variable environments. In a process called rapid cold hardening (RCH), insects significantly enhance cold tolerance following brief (i.e., minutes to hours) exposure to nonlethal chilling. Although the ecological relevance of RCH is well-established, the underlying physiological mechanisms that trigger RCH are poorly understood. RCH can be elicited in isolated tissues ex vivo, suggesting coldsensing and downstream hardening pathways are governed by brain-independent signaling mechanisms. We previously provided preliminary evidence that calcium is involved in RCH, and here we firmly establish that calcium signaling mediates cold sensing in insect tissues. In tracheal cells of the freeze-tolerant goldenrod gall fly, Eurosta solidaginis, chilling to 0°C evoked a 40% increase in intracellular calcium concentration as determined by live-cell confocal imaging. Downstream of calcium entry, RCH conditions significantly increased the activity of calcium/calmodulin-dependent protein kinase II (CaMKII) while reducing phosphorylation of the inhibitory Thr306 residue. Pharmacological inhibitors of calcium entry, calmodulin activation, and CaMKII activity all prevented ex vivo RCH in midgut and salivary gland tissues, indicating that calcium signaling is required for RCH to occur. Similar results were obtained for a freeze-intolerant species, adults of the flesh fly, Sarcophaga bullata, suggesting that calcium-mediated cold sensing is a general feature of insects. Our results imply that insect tissues use calcium signaling to instantly detect decreases in temperature and trigger downstream cold-hardening mechanisms.ow temperature is one of the primary constraints for insects and other ectotherms living in temperate and polar regions (1). Although seasonal adaptations to cold stress, including environmentally programmed periods of dormancy called diapause, have been well-studied (2-5), physiological responses to sudden changes in temperature have received less attention. In a process termed rapid cold hardening (RCH), insects dramatically enhance their cold tolerance in a matter of minutes to hours (6). For example, in the flesh fly, Sarcophaga crassipalpis, the first species in which RCH was described, exposure to 0°C for as little as 30 min significantly enhances cold tolerance at −10°C (6). RCH has since been described in dozens of insect species (7), including both freeze-intolerant (insects in which internal ice formation is lethal) and freeze-tolerant species (insects that tolerate internal ice formation) (8, 9). Naturally occurring thermoperiods can elicit RCH (10), and RCH preserves essential functions such as courtship and mating (11,12), supporting the relevance of this process to natural populations.Although the ecological relevance of RCH has been established, the physiological mechanisms are poorly understood. RCH results in a slight increase in the cryoprotectant glycerol (6, 13),...

Cold-acclimation improves chill tolerance in the migratory locust through preservation of ion balance and membrane potential

“…This observation is consistent with earlier work by Yi and Lee (2003), who found that seasonal cold acclimation and rapid cold hardening improved cellular survival in the freeze-tolerant gall fly larvae Eurosta solidaginis. A similar trend has been observed in the flesh fly Sarcophaga crassipalpis (Yi and Lee, 2004;Teets et al, 2013). Yi and Lee (2004) further observed that improvement of cellular survival occurred even if the rapid cold hardening was given to isolated tissues in vitro (i.e.…”

“…The causal relationship between membrane depolarization and cellular viability has been proposed to stem from an imbalance in Ca 2+ balance such that constant severe depolarization of V m causes cellular apoptosis or necrosis through a catastrophic increase in intracellular [Ca 2+ ] (Hochachka, 1986;Boutilier, 2001). Teets et al (2013) demonstrated how small increases in intracellular [Ca 2+ ] occurred when temperature was gradually lowered and suggested that Ca 2+ balance could aid cold sensing in insects. The cell membrane depolarization that occurs when insects are gradually cooled could be responsible for this influx of Ca 2+ into the cytosol, which further corroborates the hypothesis of a certain threshold for membrane depolarization and intracellular [Ca 2+ ] needs to be passed before initiating cell death (Nicotera and Orrenius, 1998;Bortner et al, 2001).…”

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