document: mitochondria produce reactive oxygen species ( ros ) during oxidative metabolism through the single - electron reduction of molecular oxygen ( o2 ) . 
   the resulting superoxide anion ( o2 ) , is converted to hydrogen peroxide ( h2o2 ) by cellular superoxide dismutases ( sods ) . 
   similar to phosphorylation , ubiquitination , and acetylation , oxidation of protein cysteine residues represents a form of reversible , posttranslational protein modification which can regulate signaling . 
   oxidation of cysteine thiol groups ( sh ) by h2o2 results in formation of sulphenic acid ( soh ) which quickly forms disulfide ( ss ) or sulfenyl amide ( sn ) bonds , changing protein function . 
   the cellular thioredoxin and glutathione reductase systems restore these oxidatively modified residues back to their reduced forms in a manner analogous to phosphatases , deubiquitinating enzymes , or histone deacetylases . 
   mitochondrial ros ( mros ) generation is required for the propagation of numerous cellular signaling pathways including those regulating tumorgenesis , immune responses , and cellular adaptation to stresses such as hypoxia . in most cases 
   it is unknown exactly what oxidative events are required for the effects of ros on these pathways and it is likely that multiple oxidative targets potentiate transduction through a given pathway . 
   increasing evidence suggests that mitochondria and mros generation play key roles in cellular differentiation programs . 
   cellular mitochondrial content and oxidative capacity are increased when mouse embryonic stem cells ( escs ) , induced pluripotent stem cells ( ipscs ) , or mesenchymal stem cells are induced to differentiate . 
   furthermore , increased cellular ros levels and oxidation products correlate with the differentiation of escs and ipscs as well as mesenchymal , neural , and epithelial stem cells . 
   hematopoietic stem cells ( hscs ) contain significantly lower levels of ros than do the more - differentiated common myeloid progenitor cells . 
   thus increased mitochondrial and oxidant content appears to correlate with stem cell differentiation while low mitochondrial mass and oxidant content correlates with stem cell maintenance . in our recent report 
   , we used the mammalian epidermis as a model system to test the hypothesis that mitochondrial metabolism and ros production promote signaling through pathways required for differentiation of mammalian stem cell populations in vivo . 
   the epidermis is a self - renewing stratified squamous epithelium and is thus regulated by stem cell populations . to maintain epidermal homeostasis , cells within the proliferative basal layer 
   withdraw from the cell cycle and differentiate as they process outward through the suprabasal layers , compensating for cellular loss via desquamanation from the outermost epidermal layer . 
   the epidermis also elaborates appendages such as hair follicles ( hfs ) which are themselves regulated by stem cell populations . 
   multiple transcriptional networks are associated with differentiation within the epidermis including notch , p63 , c / ebp , and ap2 ( interfollicular epidermis ) and -catenin ( hf ) . 
   it is incompletely understood how these various factors are regulated to promote epidermal homeostasis and what role cellular metabolism might play in this regulation . to test the hypothesis that mitochondria play an active role in the regulation of epidermal homeostasis , 
   we conditionally deleted tfam ( transcription factor a , mitochondrial ) in basal , undifferentiated epidermal keratinocytes . tfam is required for the replication and transcription of the mitochondrial genome , and cells lacking tfam are unable to conduct oxidative phosphorylation or produce mros . 
   mice conditionally lacking tfam in epidermis ( tfam cko mice ) lacked hair and developed an epidermal barrier defect which contributed to their perinatal mortality . 
   similar to in vivo , primary keratinocytes derived from tfam cko mice displayed impaired differentiation in vitro . 
   differentiation of wild - type keratinocytes was inhibited by antioxidant treatment , and differentiation marker expression in tfam cko cells could be partly restored by treatment with exogenously applied h2o2 , clearly demonstrating that oxidative signaling promotes keratinocyte differentiation . 
   we went on to demonstrate that mros generation is required for activation of the notch and -catenin transcriptional programs that promote epidermal differentiation and hair growth respectively . 
   together , our results show that mros act as pro - differentiation signals and are key upstream regulators of stem cell fate decisions . 
   while the role for mros in signal transduction is becoming increasingly accepted , the identities of the exact targets of oxidation which promote signaling remain unknown in most situations . 
   our work identified nucleoredoxin ( nxn ) as a putative target of mros - mediated wnt--catenin signaling . 
   nxn is a member of the thioredoxin family which has previously been demonstrated to regulate -catenin - dependent transcription in a redox - sensitive manner . 
   our results show that nxn becomes oxidized when wild - type keratinocytes are treated with the -catenin activator wnt-3a . 
   this did not occur in tfam cko keratinocytes , suggesting that mros - mediated nxn oxidation plays a causal role in the transduction of signals between wnt receptors and -catenin . 
   further studies will be required to determine how mros regulate notch transcriptional activity during keratinocyte differentiation . 
   this raises the interesting possibility that aberrant keratinocyte metabolism may be associated with , and presents targets for , the treatment of epidermal disease . 
   while the genetic causes of these diseases are increasingly studied , little is known about how disease - causing mutations might affect cellular metabolism . 
   more generally , the interactions between cellular differentiation programs and cellular metabolism will be increasingly studied in years to come . 
   the development of regenerative therapies depends on the ability to maximize the renewal capacity of stem cells , and subsequently , to promote complete differentiation into desired cell types . 
   cellular metabolism represents an intriguing  rheostat  which may be used to modulate these ends . 
   a further understanding of the mechanisms by which differentiation programs both regulate , and are regulated by metabolism will prove useful in this regard . 
   proper regulation is likely dependent on cell type - specific factors as well as environmental influences such as hypoxia . in keratinocytes , 
   as an epidermal calcium gradient exists in vivo , it is hypothesized that calcium concentrations regulate epidermal differentiation in vivo as well . 
   calcium uptake into mitochondria is known to stimulate the activity of tca cycle enzymes ( pyruvate- , isocitrate- , and oxoglutarate - dehydrogenases ) , promoting mitochondrial respiration . 
   we found that inhibition of mitochondrial calcium uptake prevented keratinocyte differentiation in vitro , suggesting that mitochondrial calcium uptake might promote the mitochondrial metabolism and ros production required for epidermal differentiation . 
   a recent study suggests that mitochondrial uncoupling proteins may play a role in regulating differentiation - dependent metabolism . 
   uncoupling protein 2 ( ucp2 ) is highly expressed in ipscs , with reduced expression in differentiated cells . 
   ectopic ucp2 expression inhibited oxidative metabolism of pyruvate , reduced cellular ros content , and prevented differentiation suggesting that this protein could act as a metabolic switch that controls stem cell differentiation . 
   how ucp2 expression is repressed during differentiation remains to be determined and it will also be of interest to determine if ucp2 regulates the metabolism of less - pluripotent , tissue - specific stem cell populations . beyond promotion of oxidant - dependent 
   signaling , the mechanisms by which mitochondrial metabolism promotes cellular differentiation remain poorly understood . some differentiated lineages may require the efficient atp production provided by oxidative phosphorylation ; however , another intriguing possibility involves the nutrient - dependent regulation of chromatin ( fig . 
   acetyl - coa , a key fuel for oxidative metabolism , is also a substrate used by histone acetyltransferases to acetylate histones and other cellular proteins . 
   global increases in histone acetylation are associated with differentiation of escs , 3t3-l1 preadipocytes , and keratinocytes . 
   treatment of primary human keratinocytes with histone deacetylase inhibitors is sufficient to induce cell cycle arrest and expression of several differentiation markers . 
   although differentiation programs likely regulate histone acetylation in a gene and time - dependent manner ; these reports suggest that cellular differentiation may depend on availability of acetyl - coa to carry out acetylation reactions . in mammalian cells , 
   most cytosolic acetyl - coa is produced through the cleavage of tca - cycle - derived citrate by the enzyme atp - citrate lyase ( acl ) . 
   inhibition of acl expression prevents global increases in histone acetylation and differentiation of 3t3-l1 cells . 
   electrons are transferred from tca cycle substrates to complexes i and ii of the electron transport chain via nadh and fadh2 respectively . 
   these electrons are transferred to complex iii ( via coenzyme q ) and subsequently to complex iv ( via cytochrome c ) where they are used to reduce molecular oxygen to water . 
   complexes i , ii , and iii of the respiratory chain contain sites in which electrons can prematurely react with oxygen , producing ros . 
   our recent report demonstrates that mitochondrial ros production is required for activation of notch and -catenin transcriptional programs which promote epidermal differentiation and hair development respectively . 
   mitochondrial metabolism may also promote keratinocyte differentiation by providing substrates for histone acetyltransferases ( hats ) and histone demethylases ( such as jmjd3 ) . 
   histone methylation status is regulated by histone methyltransferases and demethylases with various methylation  codes  associated with either transcriptional activation or repression . one particular methyl modification 
   trimethylation of histone 3 on lysine 27 ( h3k27me3)is associated with transcriptional repression of epidermal differentiation markers . 
   genes such as keratin 1 and involucrin display high levels of h3k27me3 in undifferentiated keratinocytes , and lose this mark as cells differentiate . 
   demethylation of h3k27me3 requires the demethylase jmjd3 and cells lacking jmjd3 expression are unable to differentiate in culture . 
   jmjd3 is a member of the jumanji - c domain containing family of demethylases which use oxygen , fe(ii ) , and the tca cycle intermediate -ketoglutarate as cofactors and are thus potentially sensitive to cellular metabolic state . 
   many other nutrient - sensitive mechanisms are likely to regulate stem cell maintenance and differentiation in both the epidermis and in other compartments . 
   cancer researchers are currently striving to identify cancer - specific metabolic signatures which can be therapeutically targeted . 
   metabolic study of cellular differentiation is an emerging area which , in concert with genetic approaches , has the potential to lead to strategies for targeted control of cell fate decisions and therapies for diseases associated with aging .
summary: mitochondrial metabolism has traditionally been thought of as a source of cellular energy in the form of atp . 
   the recent renaissance in the study of cellular metabolism , particularly in the cancer field , has highlighted the fact that mitochondria are also critical biosynthetic and signaling hubs , making these organelles key governors of cellular outcomes.1 - 5 using the epidermis as a model system , our recent study looked into the role that mitochondrial metabolism and ros production play in cellular differentiation in vivo.6 we showed that conditional deletion of the mitochondrial transcription factor , tfam within the basal cells of the epidermis results in loss of mitochondrial ros production and impairs epidermal differentiation and hair growth . 
   we demonstrated that mitochondrial ros generation is required for the propagation of notch and -catenin signals which promote epidermal differentiation and hair follicle development respectively . 
   this study bolsters accumulating evidence that oxidative mitochondrial metabolism plays a causal role in cellular differentiation programs . 
   it also provides insights into the role that mitochondrial oxidative signaling plays in a cell type - dependent manner .