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genome analysis and did not detect any significant changes. Fluorescence in situ hybridization of MSCs isolated from five controls and three renal failure patients indicated that more than 95% of the cells had a normal karyotype. However, a small number of tetraploid cells were detected after ten population doublings, an amount of expansion that is often required to obtain a sufficient number of cells for clinical applications. The studies by Roemeling-van Rhijn et al.3 demonstrated that the phenotype of adipose tissue–derived MSCs isolated from renal failure patients and control subjects is similar. While maintaining enthusiasm in using our own fat-tissue-derived MSCs for potential treatment of kidney disease, it is critical that we perform thorough testing in animal models before conducting human studies and continue long-term monitoring for safety and efficacy. DISCLOSURE The author declared no competing interests.
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Interaction of receptor for advanced glycation end products with advanced oxidation protein products induces podocyte injury Yasuhiko Yamamoto1 and Hiroshi Yamamoto1 Chronic kidney disease (CKD) is characterized by progressive decline in renal function. Podocyte dropouts contribute to the pathogenesis of diabetic and nondiabetic CKD. Zhou and colleagues demonstrate that the association between advanced oxidation protein products (AOPPs) and receptor for advanced glycation end products (RAGE) elicited podocyte injuries, using cultured podocytes and mice injected with AOPPs. This study suggests that the blockade of RAGE is preventive and therapeutic against podocyte apoptosis caused by oxidative stress-related AOPPs in CKD. Kidney International (2012) 82, 733–735. doi:10.1038/ki.2012.163
REFERENCES 1.
Friedenstein AJ, Chailakhjan RK, Lalykina KS. The development of fibroblast colonies in monolayer cultures of guinea-pig bone marrow and spleen cells. Cell Tissue Kinet 1970; 3: 393–403. 2. Dominici M, Le Blanc K, Mueller I et al. Minimal criteria for defining multipotent mesenchymal stromal cells. The International Society for Cellular Therapy position statement. Cytotherapy 2006; 8: 315–317. 3. Roemeling-van Rhijn M, Reinders MEJ, de Klein A et al. Mesenchymal stem cells derived from adipose tissue are not affected by renal disease. Kidney Int 2012; 82: 748–758. 4. Goligorsky MS, Kuo MC, Patschan D et al. Review article: Endothelial progenitor cells in renal disease. Nephrology (Carlton) 2009; 14: 291–297. 5. Tögel F, Weiss K, Yang Y et al. Vasculotropic, paracrine actions of infused mesenchymal stem cells are important to the recovery from acute kidney injury. Am J Physiol Renal Physiol 2007; 292: F1626–F1635. 6. Morigi M, Imberti B, Zoja C et al. Mesenchymal stem cells are renotropic, helping to repair the kidney and improve function in acute renal failure. J Am Soc Nephrol 2004; 15: 1794–1804. 7. Burst VR, Gillis M, Putsch F et al. Poor cell survival limits the beneficial impact of mesenchymal stem cell transplantation on acute kidney injury. Nephron Exp Nephrol 2010; 114: e107–e116. 8. Herrera MB, Bussolati B, Bruno S et al. Exogenous mesenchymal stem cells localize to the kidney by means of CD44 following acute tubular injury. Kidney Int 2007; 72: 430–431. 9. Perico N, Casiraghi F, Introna M et al. Autologous mesenchymal stromal cells and kidney transplantation: a pilot study of safety and clinical feasibility. Clin J Am Soc Nephrol 2011; 6: 412–422. 10. Kunter U, Rong S, Boor P et al. Mesenchymal stem cells prevent progressive experimental renal failure but maldifferentiate into glomerular adipocytes. J Am Soc Nephrol 2007; 18: 1754–1764. Kidney International (2012) 82
Chronic kidney disease (CKD) is a worldwide health problem, with a rapidly increasing prevalence. Podocyte dysfunctions, including podocyte loss and slit diaphragm disruptions, are closely linked to the pathogenesis of CKD.1 Progressive kidney disease leading to end-stage renal disease is characterized by the progression of glomerulosclerosis and interstitial fibrosis. The common critical pathways could be involved in almost all forms of CKD. Therefore, exploring the factors that can activate the pathways is important for the development of new strategies to prevent and attenuate the development of CKD. The numerous factors that have been implicated in the initiation of the cascades include angiotensin II, growth factors, cytokines, oxidative and nitrosative stress, endoplasmic reticulum stress, mechanical stress, advanced glycation end 1Department of Biochemistry and Molecular
Vascular Biology, Kanazawa University Graduate School of Medical Science, Kanazawa, Japan Correspondence: Yasuhiko Yamamoto, Department of Biochemistry and Molecular Vascular Biology, Kanazawa University Graduate School of Medical Science, 13-1 Takara-machi, Kanazawa 920-8640, Japan. E-mail:
[email protected]
products (AGEs), and advanced oxidation protein products (AOPPs). AGEs are products of non-enzymatic glycation and oxidation of proteins and lipids, and AGE formation increases in situations with hyperglycemia and oxidative stress, such as diabetes mellitus. Recent clinical studies demonstrated that AOPPs are closely related to the initiation and progression of renal dysfunction in patients with a variety of diabetic and nondiabetic CKD.2 In experimental animal studies, chronic administration of AOPPs accelerated renal fibrosis and deteriorated renal dysfunction.2 AOPPs are thus considered causative factors in CKD. Witko-Sarsat and colleagues3 first designated and described AOPPs in 1996. AOPPs abundantly contain dityrosines, which allow cross-linking through disulfide bridges and carbonyl groups. AOPPs are formed mainly from chlorinated oxidants, such as hypochloric acid and chloramines, partly through catalysis by myeloperoxidase. Neutrophils use the myeloperoxidase–H2O2–chloride system and form AOPPs and AGEs, such as Ncarboxymethyl-lysine and glycolaldehydepyridine. 4 Characteristic chemical structures of heterogeneous AOPPs need 733
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AGE S100 proteins
RAGE
V
AOPP C3a HMGB1
V
CD11b/ Mac1 PS
Amyloid β C
C
C
C
LPS
ROS
Ras/MAPK NF-κB JAK-STAT mDia-1 ERK PKC-ζ Rac/Cdc42
Figure 1 | Possible participation of the pattern-recognition receptor RAGE and its multiple ligands in the development of chronic kidney disease. AGE, advanced glycation end products; AOPP, advanced oxidation protein products; ERK, extracellular signal-regulated kinase; HMGB1, high-mobility group box protein 1; JAK-STAT, Janus kinase–signal transducers and activators of transcription; LPS, lipopolysaccharides; mDia-1, mammalian diaphanous-related formin-1; NF-B, nuclear factor-B; PKC-, protein kinase C-; PS, phosphatidylserine; Rac/Cdc42, Rac/cell division control protein 42 homolog; RAGE, receptor for AGEs; Ras/MAPK, Ras/mitogen-activated protein kinase; ROS, reactive oxygen species.
to be clarified. AOPPs have several physicochemical characteristics similar to those of AGEs. Apart from the common mechanism of formation leading to protein damages, they share some biological effects as well, including interaction with RAGE.5,6 The receptor for AGEs (RAGE), originally isolated as an AGE-binding receptor, belongs to the immunoglobulin superfamily and is now recognized as a member of the pattern-recognition receptors and a proinflammatory molecular device.7 Endogenous and exogenous RAGE ligands other than AGEs and AOPPs have been reported, including high-mobility group box protein 1 (HMGB1), the calcium-binding S100 protein group, 2integrin Mac/CD11b, amyloid peptide, -sheet fibrils, complement C3a, lipopolysaccharides, and phosphatidylserine on the surface of apoptotic cells7 (Figure 1). The ligand engagement of RAGE activates nuclear factor-B and other signaling pathways via the stimulation of extracellular signal-regulated kinase 1/2, p38 mitogen-activated protein kinase–c-Jun N-terminal kinase, Janus kinase–signal transducer and activator of transcription, 734
protein kinase C, and Rac–Cdc42.7 The ligation of RAGE leads to a positive feedforward loop in which inflammatory stimuli activate nuclear factor-B, which induces RAGE expression, followed again by nuclear factor-B activation. TIRAP (toll/interleukin-1 receptor domain-containing adaptor protein) and MyD88, which are adaptor proteins for toll-like receptors (TLRs) 2 and 4, bind to phosphorylated RAGE and transduce the signal to downstream molecules.8 This suggests a functional interaction between RAGE and TLRs, as well as the regulation of immune responses and inflammation in a coordinated fashion. Zhou and colleagues9 (this issue) report that interactions between AOPPs and RAGE elicited podocyte injuries, with less effect of the association of AOPPs with scavenger receptors class BI (SR-BI) and CD36, known as the receptor in other cell types. AOPP-mediated cellular derangements have already been reported in endothelial and mesangial cells. Direct physical association and the binding affinity of AOPPs with RAGE need to be clarified. Zhou and colleagues describe that the AOPP preparations used in the study
did not contain AGE components, such as N-carboxymethyl-lysine, pentosidine, pyridine, and glyoxal-, glycolaldehyde-, and glyceraldehyde-derived AGEs, suggesting that the observed effects were not associated with AGE-like structures.9 Endogenous secretory RAGE (esRAGE) is a major splice variant of RAGE and is expressed in a wide variety of cell types.10 The cleavage of the membrane-bound, full-length signal-transducing RAGE yields soluble RAGE (sRAGE).10 esRAGE and sRAGE are thought to act locally and systemically as decoy receptors. It is interesting to evaluate the expression of fulllength RAGE and esRAGE/sRAGE and to induce the esRAGE production and ectodomain shedding of full-length RAGE in podocytes. In summary, suppression of RAGE action may be beneficial for preventing and retarding the development of CKD. Future studies should focus on developing new devices and remedies for targeting RAGE in podocytes. DISCLOSURE The authors declared no competing interests. REFERENCES 1.
2.
3.
4.
5.
6.
7.
8.
Chiang CK, Inagi R. Glomerular diseases: genetic causes and future therapeutics. Nat Rev Nephrol 2010; 6: 539–554. Liu Y. Advanced oxidation protein products: a causative link between oxidative stress and podocyte depletion. Kidney Int 2009; 76: 1125–1127. Witko-Sarsat V, Friedlander M, Capeillère-Blandin C et al. Advanced oxidation protein products as a novel marker of oxidative stress in uremia. Kidney Int 1996; 49: 1304–1313. Anderson MM, Hazen SL, Hsu FF et al. Human neutrophils employ the myeloperoxidasehydrogen peroxide-chloride system to convert hydroxy-amino acids into glycolaldehyde, 2-hydroxypropanal, and acrolein. A mechanism for the generation of highly reactive alphahydroxy and alpha,beta-unsaturated aldehydes by phagocytes at sites of inflammation. J Clin Invest 1997; 99: 424–432. Marsche G, Semlitsch M, Hammer A et al. Hypochlorite-modified albumin colocalizes with RAGE in the artery wall and promotes MCP-1 expression via the RAGE-Erk1/2 MAP-kinase pathway. FASEB J 2007; 21: 1145–1152. Guo ZJ, Niu HX, Hou FF et al. Advanced oxidation protein products activate vascular endothelial cells via a RAGE-mediated signaling pathway. Antioxid Redox Signal 2008; 10: 1699–1712. Yamamoto Y, Yamamoto H. Controlling the receptor for advanced glycation end-products to conquer diabetic vascular complications. J Diabetes Invest 2012; 3: 107–114. Sakaguchi M, Murata H, Yamamoto K et al. TIRAP, an adaptor protein for TLR2/4, transduces a signal Kidney International (2012) 82
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9.
from RAGE phosphorylated upon ligand binding. PLoS One [online] 2011; 6: e23132. Zhou LL, Cao W, Xie C et al. The receptor of advanced glycation end products plays a central role in advanced oxidation protein products-
induced podocyte apoptosis. Kidney Int 2012; 82: 759–770. 10. Yamamoto Y, Miura J, Sakurai S et al. Assaying soluble forms of receptor for advanced glycation end products. Arterioscler Thromb Vasc Biol 2007; 27: 33–34.
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The mitochondrial SIRT1–PGC1␣ axis in podocyte injury Shuichi Tsuruoka1, Akira Hiwatashi1, Joichi Usui1 and Kunihiro Yamagata1 Dysfunction of mitochondria in podocytes is believed to be a trigger of injury and contributes to progressive glomerular sclerosis; however, the mechanisms had not been fully understood. Yuan et al. report involvement of SIRT1 (a homolog of the life-extending gene sir2 in mammals) and PPAR-␥ coactivator 1␣, a major regulator of oxidative metabolism, in mitochondria during podocyte injury. This information will be important in exploration of the mechanisms and future treatment of glomerular sclerosis. Kidney International (2012) 82, 735–736. doi:10.1038/ki.2012.182
The podocyte is one of the components of the glomerular filtration barrier and serves to prevent filtration of protein from the blood. This cell is terminally differentiated and highly specialized. Mature podocytes have limited ability to proliferate following injury, linked to high expression of the cyclin-dependent kinase inhibitors, which appear to be rate-limiting for podocytes to reenter the cell cycle.1 Podocyte injury causes detachment of the cell from the barrier. Once podocytes have detached from the glomerular basement membrane (GBM), the exposed outer surface of the GBM adheres to the inner surface of the Bowman’s capsule, and a focal segmental glomerulosclerosis (FSGS) lesion is established.1 This series of changes is commonly seen in various types of glomerular disease and causes progres1Department of Nephrology, Faculty of Medicine, University of Tsukuba, Tsukuba, Japan Correspondence: Kunihiro Yamagata, Department of Nephrology, Faculty of Medicine, University of Tsukuba, 1–1–1 Tennodai, Tsukuba, Ibaraki 305–8575, Japan. E-mail:
[email protected]
Kidney International (2012) 82
sion of chronic kidney disease. The mechanisms of the injury of this cell are not fully understood; however, several mechanisms, such as peroxisome proliferatoractivated receptor- (PPAR-)-mediated apoptosis and increase of reactive oxygen species, were reported in an animal model of puromycin aminonucleoside nephrosis with FSGS.1 Mitochondria are essential intracellular organelles that have a major role in energy production by adenosine triphosphate (ATP) synthesis through oxidative phosphorylation. Each mitochondrion has its own DNA, which is the only extranuclear genome in eukaryotes. Mitochondrial DNA (mtDNA) is prone to oxidative stress, as it apparently lacks histone-like coverage and is localized closely to the inner mitochondrial membrane, a major site of reactive oxygen species in cells.2 Mitochondrial dysfunction causes various types of diseases, such as myopathy and encephalopathy.3 With regard to the kidney, mitochondrial dysfunction was reported mainly in tubular cells. For example, acute kidney injury by various toxins and hypoxia causes the dysfunction
of mitochondria in proximal tubular cells.1,4 Although FSGS developed in children with mitochondrial cytopathies,5 information about mitochondrial dysfunction in glomerular disease was still limited. In a rat model of puromycin aminonucleoside nephrosis with FSGS, reduction of mtDNA copy number in podocytes was reported.2 Therefore, podocyte injury was triggered by mitochondrial dysfunction due to mtDNA depletion of the cell, which led to glomerular sclerosis, perhaps via an increase of oxidative stress and a relative shortage of ATP synthesis resulting from the energy demand of the podocyte; however, the mechanisms were not fully understood. It was previously reported that PPAR- coactivator 1 (PGC-1), a transcriptional activator of PPAR-, is a major regulator of mitochondrial oxidative metabolism, which specifically binds to the mitochondrial transcription factor A (TFAM) promoter, a direct regulator of mtDNA replication in non-kidney cells.1,6 It was also recently reported that SIRT1 (silent mating type information regulation 2 homolog 1, a homolog of the life-extending gene sir2 in mammals) regulates PGC-1 activity and energy metabolism of mitochondria by its deacetylation in different tissues.7 With regard to the kidney, modification of cisplatin-induced acute kidney injury by transgenic overexpression of SIRT1 in S1- and S2-segment proximal tubules was reported.8 It was also reported that calorie restriction enhances cell adaptation to hypoxia through SIRT1-dependent mitochondrial autophagy in mouse renal tubules.9 Yuan et al.10 (this issue) report several lines of evidence that the activation of the SIRT1–PGC-1 axis in mitochondria ameliorates aldosterone-induced podocyte injury in vitro and in vivo. They found that aldosterone suppressed SIRT1–PGC-1 activation in the cultured podocytes. They further showed that overexpression of SIRT1 or PGC-1 inhibited the aldosterone-induced mitochondrial dysfunction and podocyte injury. Resveratrol, an activator of SIRT1, protected mitochondrial function against podocyte injury in aldosterone-treated mice. The authors also showed that podocyte injury by aldosterone was mediated 735
