The endoplasmic reticulum (ER) is a multi-functional organelle vital for proper cell physiology. It consists of the rough ER (rER), which serves as a site for protein synthesis, folding and transport, and the smooth ER (sER), which is heavily involved in the biogenesis of lipids and steroids. When folded correctly, a protein’s cellular function occurs normally; however, the accumulation of unfolded or incorrectly folded proteins during stressed conditions in the ER leads to the instigation of the unfolded protein response (UPR). This aims to protect the cell from a cellular stress response known as ER stress, which coordinates adaptive and apoptotic responses in order to restore and maintain ER homeostasis. The pathogenesis of disease states such as neurodegenerative diseases, metabolic disorders, type II diabetes, and inflammatory diseases, however, has also been linked to prolonged and persistent ER stress – a critical understanding of which can lead to improved and effective diagnoses and treatment methods.
FUNCTIONING OF A HEALTHY ER
The ER is a crucial component in the process of producing proteins, playing a vital role in protein folding. The localisation of ribosomes to the cytosolic face of a healthy, properly-functioning ER allows for the synthesis of correctly-folded, operational proteins. As well, the co-translational attachment of the messenger RNA (mRNA) ribosomal complex to the membrane of the ER allows for the canonical pathway involved in the regulation of protein synthesis. It is in the cytosol that the translation of secretory or integral membrane proteins commences, and the ER where the translation continues. Via a sequence of signals that occurs within the amino terminus of the nascent polypeptide bound by the signal recognition particle (SRP), ribosomes containing the mRNAs from the protein translations in the cytosol are sent to the membrane of the ER. The complex formed from the binding of the mRNA ribosome, nascent polypeptide, which is derived from the ribosome-nascent chain complex (RNC), and SRP is sent to the ER where it binds to the SRP receptor. Upon the completion of the translation occurring in the ER, the emerging polypeptide is able to co-translationally enter the ER via the translocon, which spans the lipid bilayer and acts as a channel containing several Sec proteins. The protein must finally undergo proper folding and modifications through the interactions between the cytosolic regions of transmembrane proteins and chaperones, resulting in correctly-folded, functional proteins for cellular use.
MECHANISMS THAT LEAD TO ER STRESS
The lumen of the ER acts as a major site for the proper folding of proteins, containing molecular chaperones and several folding enzymes. Proteins that have been properly folded are exported to the Golgi apparatus, whilst those that have been incompletely folded are retained in the ER to either complete the folding process or to be delivered into the cytosol to undergo ER-associated degradation. An equilibrium is established between the protein load of the ER and its folding capacity under physiological conditions; however, due to potentially increased rates of protein synthesis or accumulation of incorrectly-folded proteins leading to alterations in ER homeostasis, a condition known as ER stress can arise. In order to effectively deal with this stress, an adaptive signalling pathway called the UPR has been developed by cells to alleviate ER stress and thus re-establish homeostasis. This can be accomplished via two mechanisms: increasing the folding capacity of proteins through the expression of protein-folding chaperones, or down-regulating the protein load of the ER through the inhibition of general protein translation and promoting of the degradation of misfolded or aggregated proteins. Prolonged or sever stress can, however, cause the initiation of apoptosis by the UPR, contributing to the onset of several disease states.
EVIDENCE AROUND POSSIBLE LINKS BETWEEN ER STRESS AND INFLAMMATION
Upon activation by ER stress, the most evolutionarily conserved UPR signalling pathway in mammalian cells, kinase/endoribonuclease inositol-requiring enzyme-1 (IRE1), results in the excision of a fragment from the mRNA encoding the transcription factor X-box-binding protein-1 (XBP1). This produces XBP1s, required for the expansion of the ER and the development of highly secretory cells, such as plasma cells and pancreatic epithelial cells. Intestinal epithelial cells (IECs) also express IRE1?, the deletion of which causes increased ER stress, as well as exacerbated dextran sodium sulfate (DSS)-induced colitis, an inflammatory disease.
According to a (2008) study conducted by Arthur Kaser, Ann-Hwee Lee, Andre Franke, Jonathon N. Glickman, Sebastian Zeissig, Herbert Tilg, Edward E.S. Nieuwenhuis, Darren E. Higgins, Stefan Schreiber, Laurie H. Glimcher, and Richard S. Blumberg, the ER stress links to intestinal inflammation by XBP1 in cells with secretory activity was investigated. In the experiment, XBP1 mice were bred with Villin (V)-Cre transgenic mice (within whom the mouse villin 1 promoter directs the expression of Cre recombinase), and the newborn offspring allowed to develop normally. The deletion of the XBP1 exon 2 within the intestinal epithelium illustrated elevated basal GRP78 (an ER chaperone and signalling regulator) levels in XBP1, signifying increased ER stress within the small intestinal epithelia. 61% of adult XBP1 mice demonstrated spontaneous small intestinal mucosal inflammation, associated with elevated ER stress, ad 31% of the heterozygous offspring mice displayed mild spontaneous small intestinal inflammation. The results obtained from the study highlight the inability of XBP1-deficient IECs to appropriately respond to inflammatory signals. Furthermore, numerous single nucleotide polymorphisms (SNPs) located within the XBP1 gene locus demonstrate a vulnerability to Inflammatory Bowel Disease (IBD), illuminating the ER stress pathway as a genetic contributor to inflammatory disorders diseases within the human population.