Endoplasmic reticulum stress and renal disease

Endoplasmic reticulum stress in proteinuric kidney disease

Endoplasmic reticulum (ER) stress refers to physiological or pathological states that result in accumulation of misfolded proteins in the ER. To handle misfolded proteins, the ER has in place quality control mechanisms, including the unfolded protein response and ER-associated degradation (ERAD). ER stress in renal pathophysiology is a relatively new area of research. Mice heterozygous for a mutation in the ER chaperone, BiP, develop glomerulosclerosis and tubulointerstitial disease. Induction of ER stress in glomerular cells has been described in experimental models of membranous nephropathy and membranoproliferative glomerulonephritis, and exogenous induction of ER stress ('preconditioning') reduced proteinuria. In human kidney biopsies, markers of ER stress in glomeruli have been identified in various noninflammatory and inflammatory glomerulopathies. A tubulointerstitial ER stress response, in some cases associated with tubular cell apoptosis, may occur in glomerular diseases associated with proteinuria, including puromycin aminonucleoside nephrosis, protein overload, and experimental and human diabetic nephropathy. Certain missense mutations in nephrin and podocin, as well as underglycosylation of nephrin, result in misfolding and retention in the ER, and eventually ERAD. Understanding the various aspects of ER stress will provide an opportunity for development of novel therapeutic strategies for proteinuric diseases.

The unfolded protein response in health and disease

The endoplasmic reticulum (ER) is the principal cellular organelle in which correct folding and maturation of transmembrane, secretory, and ER-resident proteins occur. Research over the past decade has demonstrated that mutations in proteins or agents/conditions that disrupt protein folding adversely affect ER homeostasis, leading to ER stress. This in turn initiates the unfolded protein response (UPR), an integrated intracellular signalling pathway that responds to ER stress by increasing the expression of ER-resident molecular chaperones, attenuating global protein translation and degrading unfolded proteins. Failure to relieve prolonged or acute ER stress causes the cell to undergo apoptotic cell death. Recent groundbreaking studies have provided compelling evidence that ER stress and UPR activation contribute to the development and progression of human disease, including neurodegenerative disorders, diabetes, obesity, cancer, and cardiovascular disease. Furthermore, the ability of the UPR to modulate oxidative stress, inflammation, and apoptosis provides important cellular clues as to how this evolutionarily conserved cellular-stress pathway maintains and responds to both normal physiologic and pathologic processes. In this Forum issue, many aspects of the UPR are reviewed in the context of how ER stress and UPR activation influence human disease. This current information provides a solid foundation for future investigations aimed at targeting the UPR in an attempt to reduce the risk of human disease.