Miura replies:

We developed the endoplasmic reticulum stress-activated indicator (ERAI) system for monitoring endoplasmic reticulum (ER) stress in vivo. The system is based on the selective splicing of XBP1 mRNA by IRE1 during ER stress. We confirmed that XBP1 mRNA splicing occurs in several mouse tissues exposed to ER stressors, including tunicamycin and thapsigargin (unpublished data). At present, it is not known whether there are factors other than ER stress that can induce XBP1 mRNA splicing in vivo. We reported in our original paper that intense fluorescence signals are detected in the mature skeletal muscles of the ERAI mice, which coincide with the splicing of endogenous XBP1 mRNA. In their letter to the editor, Mao et al. point out that BiP mRNA is hardly detectable in the skeletal muscles of challenged animals, and suggest that the splicing of XBP1 mRNA may occur independently of ER stress in this tissue. Mammalian cells have three ER stress sensors, ATF6, IRE1 and PERK. The ERAI system detects the activation of IRE1 (or unknown RNase), but the endoplasmic reticulum stress element (ERSE)-LacZ system described by Mao et al. is expected to detect ERSE-mediated ER stress, which includes the production of activated ATF6 (and expression of BiP). But it is possible that endogenous BiP expression is transcriptionally regulated by other transcription factor(s) that can bind to the outside of the ERSE. Therefore, we are not surprised by the different expression pattern of reporters between the ERSE-LacZ mouse and the ERAI mouse. The discrepancy between the two studies clearly indicates that the ER stress response in mammals is more complicated than previously thought and some molecules known as essential components of the ER stress response pathway may be differentially used in vivo. Alternatively, some components of the ER response may be activated independently of ER stress in developmental and pathophysiological processes. We agree that ER stress in vivo should not only be assessed by the ERAI system but also by other systems, including the ERSE-LacZ mouse. Thus, further analysis using different in vivo reporter systems will provide insight into the complexity of the ER stress response in multicellular organisms.