INAUGURAL ARTICLE by a Recently Elected Academy Member:Up-regulation of heat shock proteins is essential for cold survival during insect diapause

Winter poses a major challenge for insects. To be successful in a highly seasonal, temperate zone environment, insects must restrict growth and reproduction to a few months during the summer and survive the remainder of the year without feeding while confronting the rigors of winter. The shortening of day length in late summer is widely exploited as an environmental token signaling the advent of winter. For most insects, short day length evokes a stage-specific developmental arrest, known as diapause (1–3). Diapause represents an alternative developmental pathway prompted by unique patterns of gene expression that result in the sequestration of nutrient reserves, suppression of metabolism, a halt or slowing of development, and the acquisition of increased tolerance to environmental stresses such as low temperature (4). Although a few insect species are freeze-tolerant, the majority cannot tolerate body freezing and, instead, have evolved mechanisms to avoid freezing (5). The contributors best known to prevent freezing include polyols such as glycerol and the insect blood sugar trehalose that function colligatively to depress the body's supercooling point and noncolligatively to stabilize proteins and cellular membranes (6). A few insects also have the capacity to synthesize antifreeze proteins, first described from Antarctic fish (7), that function noncolligatively in the hemolymph to decrease the insect's supercooling point (8). Cytoskeletal modifications, including alterations that enhance elasticity of the cell membrane to promote function at low temperature, are likewise widely exploited by overwintering insects to prevent cold-induced damage (9–12). In this study, we propose that another class of protective agents, the heat shock proteins (Hsps), also contribute significantly to the overwintering cold tolerance of insects. The Hsps are a group of remarkably well described proteins that are commonly expressed in response to environmental stress. Originally described from Drosophila as a response to high temperature, and hence their name, Hsps are up-regulated by diverse stresses, including cold shock, desiccation, anoxia, and exposure to a wide range of chemicals including heavy metals, ethanol, and other contaminants (13–15). They function as molecular chaperones during periods of stress by binding to other proteins, thereby ameliorating the detrimental effects of misfolding and then promoting the return of these proteins to their native conformations when favorable conditions again prevail. In most cases, the genes encoding these proteins are rapidly up-regulated at the onset of the stress, concurrent with the down-regulation of most other genes. When more favorable conditions return, the Hsps are again down-regulated, a feature that is essential because expression of Hsps during nonstress conditions can lead to deleterious effects, including retardation and cessation of development (16). For several years, we have known that Hsp23, a small Hsp (17), and Hsp70 (18) are developmentally up-regulated during the overwintering pupal diapause of the flesh fly, Sarcophaga crassipalpis, but several questions remain unanswered. Are additional Hsps involved in the diapause response? Does this link of Hsp expression with diapause occur in other species? What function do these Hsps serve during diapause? In this report, we review the linkage between diapause and Hsp expression in the flesh fly, present evidence that quite a few additional Hsps are diapause up-regulated, show that many additional insect species also up-regulate Hsps during diapause, and use RNAi to demonstrate that suppression of the Hsps results in a loss of cold tolerance during winter diapause. We thus argue that Hsp expression is a vital component of the overwintering defense strategy of many temperate zone insects.