Red-eared slider turtles, Trachemys scripta elegans, can survive for several weeks without
oxygen when submerged in cold water. We hypothesized that anaerobiosis is aided by adaptive up-regulation of the unfolded protein response (UPR), a stress-responsive pathway that is activated by accumulation of unfolded
proteins in the endoplasmic reticulum (ER) and functions to restore ER homeostasis. RT-PCR, western immunoblotting and
DNA-binding assays were used to quantify the responses and/or activation status of UPR-responsive genes and
proteins in turtle tissues after animal exposure to 5 or 20 h of anoxic submergence at 4 °C. The phosphorylation state of
protein kinase-like ER
kinase (PERK) (a UPR-regulated
kinase) and
eukaryotic initiation factor 2 (eIF2α) increased by 1.43-2.50 fold in response to
anoxia in turtle heart, kidney, and liver. Activation of the PERK-regulated
transcription factor,
activating transcription factor 4 (ATF4), during
anoxia was documented by elevated atf4 transcripts and total ATF4
protein (1.60-2.43 fold), increased nuclear ATF4 content, and increased
DNA-binding activity (1.44-2.32 fold). ATF3 and GADD34 (downstream targets of ATF4) also increased by 1.38-3.32 fold in heart and liver under
anoxia, and atf3 transcripts were also elevated in heart. Two characteristic chaperones of the UPR,
GRP78, and
GRP94, also responded positively to
anoxia with strong increases in both the transcript and
protein levels. The data demonstrate that the UPR is activated in turtle heart, kidney, and liver in response to
anoxia, suggesting that this pathway mediates an integrated stress response to protect tissues during
oxygen deprivation.