Role of endoplasmic reticulum chaperones GRP78 and GRP94 in regulating unfolded protein response and cell survival

Endoplasmic reticulum (ER) is a key cellular organelle responsible for membrane and secretary protein folding and maturation. When the ER folding capacity is overwhelmed by protein loading, the unfolded protein response is activated. As the master regulator of UPR, the role of GRP78 in cell survival has been studied intensively through cell culture systems. To further understand the in vivo role of GRP78, a novel conditional knockout mouse model with GRP78 specifically depleted in Purkinje cells (PCs) is created. The GRP78 null PCs activate UPR signaling pathways including the upregulation of GRP94 and PDI, the induction of CHOP and GADD34, the feedback suppression of eIF2alpha phosphorylation and apoptotic cell death. Cytosolic protein ubiquitination level is strikingly reduced, in contrast to many neurodegenerative mouse models where abnormal accumulation of ubiquitinated protein is prominent. Ultrastructural evaluation reveals that the ER shows prominent dilatation with focal accumulation of electron-dense material surrounded by ER membrane. Growth retardation, severe motor coordination ability defect by week 5 and cerebellar atrophy by week 13 were observed in the conditional knockout mice. Therefore GRP78 is required for neuronal cell survival and provides a novel link between GRP78 depletion and reduction of protein ubiquitination level, suggesting the important role of GRP78 in ER associated protein degradation. As another important ER chaperone, GRP94 is discovered to be dispensable for Purkinje cells, mouse embryonic stem cells (mESCs) and mammary epithelial cells. GRP94 null Purkinje cells survive and function as well as wildtype control. Under ER stress conditions, GRP94 depletion in mESCs leads to induction of specific ER chaperones like GRP78, calnexin (CNX) and calreticulin (CRT), but not PDI and reduction of XBP-1 splicing, a major downstream target of IRE1 signaling pathway. However, PERK signaling pathway is not affected in GRP94 null cells. In addition, specific knockout of Grp94 in mouse mammary epithelial cells reveals that GRP94 is not required for the survival of these cells or the normal development and function of mammary gland. Conditional depletion of GRP94 in the mammary gland epithelial cells may be able to inhibit HER2/Neu induced tumorigenesis. Therefore our study provides novel insights for the role of GRP78 and GRP94 in mouse development, cell survival and UPR activation by utilizing multiple mouse models.