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The endoplasmic reticulum (ER) is an important organelle in which newly synthesized secretory and membrane proteins are correctly folded and assembled. Numerous ER-resident molecular chaperones and catalysts are known to assist in this process. When cells are exposed to ER stress such as glucose starvation, disturbance of Ca2+ ion stores, and genetic mutations, unfolded and misfolded proteins accumulate in the ER. Accumulation of unfolded proteins in the ER is highly detrimental to the cell and the organism. To maintain ER homeostasis, cells induce at least three responses to overcome this deleterious condition (termed as UPR: unfolded protein response), all of which increase the capacity of protein folding in the ER: (1) transcriptional upregulation of UPR target genes, (2) translational attenuation of protein synthesis, and (3) ER associated protein degradation (ERAD) (Fig. 1). If this damage cannot be overcome, cells undergo apoptosis. Recent studies suggest that ER stress is a factor responsible for neurodegenerative disease, and that the regulation of this response plays an important role also in the development and differentiation of animals. We are conducting studies, with a goal of clarifying the quality control of protein folding in the ER and the physiological roles of UPR at each of the molecular, the cellular and the individual animal level. In other work in our lab, we have developed a simple and highly sensitive method for conditional cell ablation in transgenic mice, called "toxin receptor-mediated cell knockout (TRECK)". We have created mouse models of hepatitis and diabetes mellitus and are conducting studies on regenerative medicine, making use of these TRECK-Tg mice.
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- ER quality control and Unfolded Protein Response (UPR)
We are focusing on the following projects: i) how IRE1 senses the accumulation of unfolded proteins in the ER [3,4,10,11] (Fig. 2), ii) how IRE1 recognizes and cleaves the target RNA on ER membrane [8], iii) the analysis of the downstream of UPR in yeast and mammalian cultured cells [2], iv) the physiological role of UPR activation in animals using ERAI mice and IRE1 knockout mice [5](Fig. 3). Furthermore, active research is now under way concerning the molecular chaperones involved in protein folding in the ER[1,6].
- Regenerative medicine, using TRECK-Tg mice
Using our unique TRECK method, we have created mouse models of hepatitis and diabetes mellitus [7,9,12]. These TRECK-Tg mice are expected to be useful not only in developing new therapies but also in exploring tissue-stem cells of adult mice. We are attempting to apply these mouse models to regenerative medicine (isolation and identification of hepatic stem cells and pancreatic beta-stem cells) (Fig. 4).
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- Takeuchi M. et al., J. Biochem., 139, 597-605, 2006
- Kimata Y. et al., Genes Cells, 11, 59-69, 2006
- Oikawa D. et al., Biochem. J., 391, 135-142, 2005
- Kimata Y. et al., J. Cell Biol., 167, 445-456, 2004
- Iwawaki T. et al., Nature Med., 10, 98-102, 2004
- Hosoda A. et al., J. Biol. Chem., 278, 2669-2676, 2003
- Saito M. et al., Nature Biotechnol., 19, 746-750, 2001
- Iwawaki T. et al., Nature Cell Biol., 3, 158-164, 2001
- Saito M. & Kohno K. Bioscience and Industry, 63, 31-34, 2005
- Oikawa D. et al., Jikkenigaku, 23, 2327-2332, 2005
- Kimata Y. & Kohno K. Tanpakushitsu Kakusan Koso, 49, 998-1001, 2004
- Kohno K. et al., Patent application: PCT/JP2005/014476, "Diabetes model animal"
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Fig. 1 ER stress response (three major pathways for cell survival).
Fig. 2 A model depicting the activation of IRE1 by ER stress.
Fig. 3 Detection of the activation of UPR in an ERAI-GFP mouse.
The pancreas (P) with active secretion and protein synthesis shows activation of UPR even under physiological condition and emits fluorescence (right) [5].
Fig. 4 Liver cell transplantation and regeneration in a mouse model of hepatitis.
The donated liver cells (green) undergo regeneration throughout the recipient's liver (a mouse model of hepatitis). Left, a light image; right, a fluorescent image.
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