Quality Control in the Early Secretory Pathway Plays Significant Roles in vivo

The endoplasmic reticulum (ER) provides a folding environment for newly synthesized secretory and membrane proteins (Ellgaard and Helenius, 2003). Secretory proteins are synthesized by ribosomes and translocated cotranslationally or posttranslationally to the ER. These newly synthesized proteins interact with ER molecular chaperones, such as immunoglobulin heavy chain-binding protein (BiP), calnexin, calreticulin, and protein disulfide isomerase, to become properly folded and assembled into a mature protein complex for transport along the secretory pathway. Aberrant protein folding, due to extracellular stimuli such as ischemia and oxidative stress, or genetic mutation leads to the accumulation of misfolded proteins in the ER, which in turn evokes the unfolded protein response (UPR) (Patil & Walter, 2001). The UPR reduces the amount of misfolded proteins (Ron & Walter, 2007) by inducing the production of ER chaperones that promote protein folding, reducing general protein synthesis, and enhancing the degradation of misfolded proteins via a ubiquitin-proteasome system, termed ER-associated degradation (ERAD) (Bonifacino & Weissman, 1998). The persistent accumulation of misfolded proteins beyond the capacity for ER quality control causes cellular dysfunction and cell death, leading to diverse human disorders, such as diabetes mellitus and neurodegenerative diseases (Zhao & Ackerman, 2006). Misfolded proteins had been believed to remain in the ER, but yeast genetic analyses have indicated that the UPR involves the whole secretory pathway (Travers et al., 2000), and that some misfolded proteins require transport between the ER and the Golgi complex for ERAD (Vashist et al., 2001). In addition, certain misfolded proteins in mammalian cells have also been reported to exit the ER and recycle between the ER and post-ER compartments, associating with ER chaperones. The KDEL receptor mediates this retrieval, suggesting that the secretion of misfolded proteins with ER chaperones from the ER and their retrieval may contribute to ER quality control (Hammond & Helenius, 1994; Yamamoto et al., 2001). We focus on the KDEL receotor-ER chaperone retrieval system in order to explore the physiological and pathological significance of ER quality control by using culture cells and mutnat mouse models. The impairment of the retrieval system in our mutant mice caused several developmental and aging dysfunctions, which reveals that quality control in the early secretory pathway plays significant roles in vivo.