Table 1 Endoplasmic reticulum stress and the unfolded protein response

Under unstressed conditions, the chief regulators of the tripartite UPR network, PERK, IRE1 and ATF6, interact with binding immunoglobulin protein (BiP; also known as GRP78 and HSP5A) through their luminal domains, which stabilises these transmembrane proteins in an inactive state. Subsequent to cellular insults such as perturbed calcium homeostasis, the ER folding capacity may become overwhelmed, prompting an accumulation of aberrant protein species in the ER lumen. The malfolded secretory cargo sequesters BiP thereby liberating activation of the UPR cascade and encouraging efforts towards proteostasis restoration [58]. PERK and IRE1 are type I transmembrane proteins with cytosolic serine/threonine kinase domains which perform kinase (PERK and IRE1) and endoribonuclease (IRE1) activities during ER stress relief endeavours. Emancipation from BiP releases PERK and IRE1 for homo-oligomerisation and trans-phosphorylation eliciting their activation. Activated PERK targets and phosphorylates eukaryotic initiation factor 2α (eIF2α) at serine-51, which diminishes the presence of eIF2-guanosine triphosphate-tRNA^methionine ternary complexes, precipitating attenuation of global translation [59]. On the other hand, IRE1 splices a 26 base pair intron from X-box binding protein 1 (Xbp1) mRNA, generating a transcriptionally active XBP1 protein that regulates genes involved in protein trafficking, folding and cell survival [60].

ER stress also induces disunion of BiP from ATF6 (type II transmembrane glycoprotein), resulting in the exposure of Golgi-localisation sequences on the ER-luminal domain of ATF6. Following translocation to the Golgi apparatus, ATF6 is cleaved sequentially by site-1-protease and site-2-protease, liberating a 50 kDa amino-terminal cytoplasmic fragment (ATF6f). ATF6f enters the nucleus and binds to ER stress-response elements which results in the expression of ER stress proteins including BiP and XBP1 (thereby augmenting the output of the IRE1 arm). The activation of the PERK and ATF6 axes is thought to precede IRE1 activation, consistent with the proposed signals transduced by these branches. Indeed, the PERK and ATF6 pathways primarily are implicated in adaptive responses to protein misfolding whereas the IRE1 arm is associated with a complex dichotomy of promoting both survival and pro-apoptotic programmes [61].