The mitochondrial unfolded protein response (UPRmt) is a cellular stress response related to the mitochondria. The UPRmt results from unfolded or misfolded proteins in mitochondria beyond the capacity of chaperone proteins to handle them. [1] The UPRmt can occur either in the mitochondrial matrix or in the mitochondrial inner membrane. [1] In the UPRmt, the mitochondrion will either upregulate chaperone proteins or invoke proteases to degrade proteins that fail to fold properly. [1] UPRmt causes the sirtuin SIRT3 to activate antioxidant enzymes and mitophagy. [2]
Mitochondrial electron transport chain mutations that extend the life span of Caenorhabditis elegans (nematode worms) also activate the UPRmt. [3] Activation of the UPRmt in nematode worms by increasing NAD+ by supplementation with nicotinamide or nicotinamide riboside has been shown to extend lifespan. [4] Glial and germline mitochondria has been found to play a significant role in the signalling and regulation of UPRmt [5] [6] [7] have been shown to play a central role Nicotinamide riboside supplementation in mice has also been shown to activate the UPRmt. [8]
A majority of cellular proteins are translated and folded in the cytosol with the help of molecular chaperones. Just as proteins must be folded to function in the cytosol, proteins in organelles like the endoplasmic reticulum (ER) and mitochondria also must be folded to function. Consequently, specific cellular mechanisms exist that aim to detect cellular stress (causing misfolded/unfolded proteins to accumulate), transduce the signal to the nucleus, and mediate the restoration of protein homeostasis (proteostasis). In the cytosol, the heat shock response (HSR) manages the unfolded proteins through heat shock factor 1 (HSF1). HSF-1 is a transcription factor that, upon increases in unfolded cytosolic proteins, will trimerize and enter the nucleus to upregulate the expression of heat shock proteins (HSPs) that will act as protein folding chaperones. [9]
In organelles like the ER and mitochondria, the response is slightly more complex. Both UPR mechanisms are conceptually similar in that they are activated by the accumulation of misfolded/ unfolded proteins and induce the translational upregulation of molecular chaperones and proteases to process proteins and restore homeostasis. [10] Despite their names, the two pathways possess distinct initiating stimuli and signaling mechanisms that regulate the responses. The ER UPR is induced by a variety of cellular stressors that inhibit protein folding or exit of the ER. Within the ER GRP78, an ER lumen chaperone, is bound to ER membrane proteins. When unfolded proteins build up, it dissociates to from the membrane to aid in protein folding. GRP78 dissociation triggers the UPRER that restores protein homeostasis via three pathways (IRE1, PERK, and ATF6). [11] The UPRER restores proteostasis by selectively attenuation protein translation, upregulating protein folding chaperones, and degrading excess misfolded proteins via ER associated protein degradation (ERAD). Prolonged activation of the UPRER can result in apoptosis. [9]
The UPRmt progresses through the bZIP transcription factor ATFS-1 (in C. elegans; ATF5 in mammals). AFTS-1 is usually imported into the mitochondria where it is degraded by the LON protease. Mitochondrial dysfunction inhibits this process and allows ATFS-1 to accumulate in the cytosol and enter the nucleus where it can act as a transcription factor. This responses restores proteostasis by upregulating chaperones and proteases, increasing reactive oxygen species (ROS) detoxification, and increasing mitochondrial import machinery. [12] [9]
In mammals, UPRmt has mostly been studied using transfection with a truncated, dysfunctional mitochondrial enzyme (OTCΔ) that does not fold correctly after translocation into the mitochondrial matrix. [13] Using this approach, several components of the mammalian UPRmt have been identified including the mitochondrial chaperone heat shock protein 60 (Hsp60), the mitochondrial caseinolytic peptidase ClpP, the transcription factor Chop and the kinases c-Jun N-terminal kinase (JNK) and the interferon-induced, double-stranded RNA-activated protein kinase (Pkr). [13] [14] [15]
The appropriately named activating transcription factor associated with stress (ATFS-1) is one of the primary transcription factors required for UPRmt activation in worms. ATFS-1 has a nuclear localization sequence that allows it to be imported into the nucleus as well as an N-terminal mitochondrial targeting sequence (MTS) that allows for import into the mitochondria. [12] In healthy cells, ATFS-1 is preferentially targeted to the mitochondrial matrix where it is degraded by the Lon protease. The MTS on ATFS-1 is predicted by Mitofates [16] to be substantially weaker than most MTSs which would allow it to be sensitive to subtle mitochondrial dysfunction. [17] Following mitochondrial stress, ATFS-1 mitochondrial import efficiency is decreased resulting in a cytoplasmic accumulation of ATFS-1. Subsequently, ATFS-1 will enter the nucleus via its nuclear transport signal. In the nucleus, ATFS-1 has a broad transcriptional regulation as it will: attenuate OXPHOS gene expression in both the nucleus and mitochondria, upregulate chaperones and proteases to re-establish mitochondrial proteostasis, increase ROS detoxification, and increase mitochondrial import machinery. [10] [12]
Recent research has implicated the UPRmt in the transformation of cells in to cancer cells. Researchers have identified the SIRT3 axis of UPRmt as a marker to differentiate between metastatic and non-metastatic breast cancer. [18] As many cancers exhibit a metabolic shift from oxidative phosporylation-depentent energy production to aerobic glycolysis dependent energy production, also known as the Warburg effect, researchers suggest that cancer cells rely on the UPRmt to maintain the mitochondrial integrity. [19] Furthermore, multiple studies have shown that inhibition of UPRmt, specifically ATF5, selectively kills human and rat cancer cells rather than non-cancer cells. [19] [20] [21]
Inflammatory bowel diseases (Crohn´s disease and ulcerative colitis) have been associated with mitochondrial dysfunction in the intestinal epithelium. [15] In mouse models of intestinal inflammation and in IBD patients, signs of UPRmt -activation have been demonstrated. [15] [22] In particular, mitochondrial dysfunction and UPRmt -activation have been linked to intestinal stemness and Paneth cell (dys-)function. [22] [23]
The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER). The endoplasmic reticulum is found in most eukaryotic cells and forms an interconnected network of flattened, membrane-enclosed sacs known as cisternae, and tubular structures in the SER. The membranes of the ER are continuous with the outer nuclear membrane. The endoplasmic reticulum is not found in red blood cells, or spermatozoa.
In molecular biology, molecular chaperones are proteins that assist the conformational folding or unfolding of large proteins or macromolecular protein complexes. There are a number of classes of molecular chaperones, all of which function to assist large proteins in proper protein folding during or after synthesis, and after partial denaturation. Chaperones are also involved in the translocation of proteins for proteolysis.
In humans, clusterin (CLU) is encoded by the CLU gene on chromosome 8. CLU is an extracellular molecular chaperone which binds to misfolded proteins in body fluids to neutralise their toxicity and mediate their cellular uptake by receptor-mediated endocytosis. Once internalised by cells, complexes between CLU and misfolded proteins are trafficked to lysosomes where they are degraded. CLU is involved in many diseases including neurodegenerative diseases, cancers, inflammatory diseases, and aging.
GroEL is a protein which belongs to the chaperonin family of molecular chaperones, and is found in many bacteria. It is required for the proper folding of many proteins. To function properly, GroEL requires the lid-like cochaperonin protein complex GroES. In eukaryotes the organellar proteins Hsp60 and Hsp10 are structurally and functionally nearly identical to GroEL and GroES, respectively, due to their endosymbiotic origin.
The heat shock response (HSR) is a cell stress response that increases the number of molecular chaperones to combat the negative effects on proteins caused by stressors such as increased temperatures, oxidative stress, and heavy metals. In a normal cell, proteostasis must be maintained because proteins are the main functional units of the cell. Many proteins take on a defined configuration in a process known as protein folding in order to perform their biological functions. If these structures are altered, critical processes could be affected, leading to cell damage or death. The heat shock response can be employed under stress to induce the expression of heat shock proteins (HSP), many of which are molecular chaperones, that help prevent or reverse protein misfolding and provide an environment for proper folding.
The unfolded protein response (UPR) is a cellular stress response related to the endoplasmic reticulum (ER) stress. It has been found to be conserved between mammalian species, as well as yeast and worm organisms.
X-box binding protein 1, also known as XBP1, is a protein which in humans is encoded by the XBP1 gene. The XBP1 gene is located on chromosome 22 while a closely related pseudogene has been identified and localized to chromosome 5. The XBP1 protein is a transcription factor that regulates the expression of genes important to the proper functioning of the immune system and in the cellular stress response.
Activating transcription factor 6, also known as ATF6, is a protein that, in humans, is encoded by the ATF6 gene and is involved in the unfolded protein response.
Binding immunoglobulin protein (BiPS) also known as 78 kDa glucose-regulated protein (GRP-78) or heat shock 70 kDa protein 5 (HSPA5) is a protein that in humans is encoded by the HSPA5 gene.
DNA damage-inducible transcript 3, also known as C/EBP homologous protein (CHOP), is a pro-apoptotic transcription factor that is encoded by the DDIT3 gene. It is a member of the CCAAT/enhancer-binding protein (C/EBP) family of DNA-binding transcription factors. The protein functions as a dominant-negative inhibitor by forming heterodimers with other C/EBP members, preventing their DNA binding activity. The protein is implicated in adipogenesis and erythropoiesis and has an important role in the cell's stress response.
The serine/threonine-protein kinase/endoribonuclease inositol-requiring enzyme 1 α (IRE1α) is an enzyme that in humans is encoded by the ERN1 gene.
Mitochondrial antiviral-signaling protein (MAVS) is a protein that is essential for antiviral innate immunity. MAVS is located in the outer membrane of the mitochondria, peroxisomes, and mitochondrial-associated endoplasmic reticulum membrane (MAM). Upon viral infection, a group of cytosolic proteins will detect the presence of the virus and bind to MAVS, thereby activating MAVS. The activation of MAVS leads the virally infected cell to secrete cytokines. This induces an immune response which kills the host's virally infected cells, resulting in clearance of the virus.
CAMP responsive element binding protein-like 1, also known as CREBL1, is a protein which in humans is encoded by the CREBL1 gene.
Richard I. Morimoto is a Japanese American molecular biologist. He is the Bill and Gayle Cook Professor of Biology and Director of the Rice Institute for Biomedical Research at Northwestern University.
Mitophagy is the selective degradation of mitochondria by autophagy. It often occurs to defective mitochondria following damage or stress. The process of mitophagy was first described over a hundred years ago by Margaret Reed Lewis and Warren Harmon Lewis. Ashford and Porter used electron microscopy to observe mitochondrial fragments in liver lysosomes by 1962, and a 1977 report suggested that "mitochondria develop functional alterations which would activate autophagy." The term "mitophagy" was in use by 1998.
Mitochondrial biogenesis is the process by which cells increase mitochondrial numbers. It was first described by John Holloszy in the 1960s, when it was discovered that physical endurance training induced higher mitochondrial content levels, leading to greater glucose uptake by muscles. Mitochondrial biogenesis is activated by numerous different signals during times of cellular stress or in response to environmental stimuli, such as aerobic exercise.
Proteostasis is the dynamic regulation of a balanced, functional proteome. The proteostasis network includes competing and integrated biological pathways within cells that control the biogenesis, folding, trafficking, and degradation of proteins present within and outside the cell. Loss of proteostasis is central to understanding the cause of diseases associated with excessive protein misfolding and degradation leading to loss-of-function phenotypes, as well as aggregation-associated degenerative disorders. Therapeutic restoration of proteostasis may treat or resolve these pathologies.
Beta cells are heavily engaged in the synthesis and secretion of insulin. They are therefore particularly sensitive to endoplasmic reticulum (ER) stress and the subsequent unfolded protein response (UPR). Severe or prolonged episodes of ER stress can lead to the death of beta cells, which can contribute to the development of both type I and type II diabetes.
ATP-dependent Clp protease ATP-binding subunit clpX-like, mitochondrial is an enzyme that in humans is encoded by the CLPX gene. This protein is a member of the family of AAA Proteins and is to form the protein complex of Clp protease.
The mitochondrial theory of ageing has two varieties: free radical and non-free radical. The first is one of the variants of the free radical theory of ageing. It was formulated by J. Miquel and colleagues in 1980 and was developed in the works of Linnane and coworkers (1989). The second was proposed by A. N. Lobachev in 1978.