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* Division of Matrix Biology, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School,
Department of Biological Chemistry and Molecular Pharmacology, and
Harvard-MIT Division of Health Sciences and Technology, Boston, Massachusetts
Correspondence: Dr. Raghu Kalluri, Department of Medicine, Harvard Medical School, Division of Matrix Biology, Department of Medicine, CLS 11086, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, 3 Blackfan Circle, Boston, MA 02215. Phone: 617-735-4601; Fax: 617-735-4602; E-mail: rkalluri{at}bidmc.harvard.edu
Received for publication May 19, 2008. Accepted for publication August 18, 2008.
| Abstract |
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-smooth muscle actin. Endothelial lineage tracing using Tie2-Cre;R26R-stop-EYFP transgenic mice further confirmed the presence of EndMT-derived fibroblasts. Collectively, our results demonstrate that EndMT contributes to the accumulation of activated fibroblasts and myofibroblasts in kidney fibrosis and suggest that targeting EndMT might have therapeutic potential. | Introduction |
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Kidney fibrosis is a good model system for studying the source of activated fibroblasts. Although for many years a common notion was that activated fibroblasts arise primarily from resident fibroblasts, recent evidence has demonstrated that during fibrosis, activated fibroblasts can also arise from epithelial cells via epithelial-to-mesenchymal transition (EMT) and can be recruited from the bone marrow.6 In addition to these mechanisms of recruiting activated fibroblasts, we recently demonstrated that endothelial-to-mesenchymal transition (EndMT) plays a significant role in cardiac fibrosis and also in the recruitment of carcinoma-associated fibroblasts.7,8 EndMT was first described during embryonic heart development, where mesenchymal cells of the endocardial cushion (a tissue that later gives rise to the cardiac septa and valves) arise from endothelial cells of the endocardium.9 Here we addressed the question of whether EndMT contributes to fibroblast accumulation in kidney fibrosis.
In this study, we explored the contribution of EndMT to renal fibrosis in three independent mouse models of CKD: (1) A mouse model of unilateral ureteral obstruction (UUO; a model of obstructive nephropathy), (2) a model of streptozotocin-induced diabetic nephropathy, and (3) mice that systemically lack the collagen IV
3 chain (COL4A3 KO; a mouse model for Alport disease). Here we report that in all three mouse models, a considerable portion of activated fibroblasts coexpress the endothelial marker CD31, indicating that these fibroblasts likely carry an endothelial imprint. In the UUO model, endothelial lineage tracing using Tie2-Cre;R26R-stop-EYFP transgenic mice revealed yellow fluorescence protein (YFP) expression in a substantial portion of activated fibroblasts, thereby confirming the endothelial origin of these fibroblasts. To our knowledge, this is the first report demonstrating that EndMT is a possible contributor to the accumulation of activated fibroblasts in kidney fibrosis. These findings have far-reaching implications, raising the possibility that inhibiting EndMT may be an effective therapy for delaying the progression of fibrosis associated with CKD.
Using genetic lineage tracing of endothelial cells and double labeling of tissue for endothelial and fibroblast markers, we previously demonstrated that co-labeling of tissue with the endothelial marker CD31 and the fibroblast markers
-smooth muscle actin (
-SMA) and fibroblast-specific protein 1 (FSP1) reliably identifies fibroblasts derived via EndMT.7 Furthermore, FSP1 and
-SMA identify distinct fibroblast populations with only few fibroblasts carrying both markers at the same time.7,8 Here we performed endothelial lineage tracing and FSP1/CD31 and
-SMA/CD31 double-labeling experiments to gain insights into possible EndMT in renal fibrogenesis. We first analyzed EndMT in the setting of UUO, a model of obstructive nephropathy.10 This model is well studied with regard to the origin of fibroblasts during fibrosis.6 Furthermore, TGF-β has been implicated in the development of renal fibrosis induced by UUO and is a known inducer of EndMT in cardiac endothelial cells.11 Smad3 deficiency has been shown to attenuate renal fibrosis after UUO, and in this regard Smad3 deficiency has also been shown to attenuate EndMT in the context of cardiac fibrosis.11 In addition to the proliferation of fibroblasts, injury to the peritubular capillary network of the kidney is considered to be a key factor in the resulting pathology after UUO.12 On the basis of these studies, we sought to test our hypothesis that the UUO model of kidney fibrosis is associated with EndMT.
We performed UUO in CD1 mice and confirmed mild interstitial fibrosis by Masson Trichrome staining (MTS) 1 wk after disease induction (Figure 1A). Both the FSP1-positive and
-SMA–positive fibroblast populations were substantially increased in kidneys with UUO as compared with sham-operated controls (Figure 1, B and C). To identify fibroblasts of endothelial origin, we performed double stainings for FSP1 and the endothelial marker CD31 and also for
-SMA and CD31. Although there was variability in the number of double-positive cells among different UUO mice, an average of 36% of all FSP1-positive fibroblasts and 25% of all
-SMA–positive fibroblasts coexpressed CD31. (Figure 1, B and C). To confirm the endothelial origin of these fibroblasts, we further performed UUO in Tie2-Cre;R26R-stop-EYFP mice. In these mice, cells of endothelial origin are irreversibly labeled with YFP, regardless of any subsequent phenotypic changes these cells may undergo. Staining with FSP1 and
-SMA revealed coexpression of YFP in a substantial portion of FSP1-positive fibroblasts and, albeit to a lesser extent, in
-SMA–positive fibroblasts (Figure 1D). These findings suggest that EndMT may account for a considerable portion of the fibroblasts in the mouse model of obstructive nephropathy.
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-SMA–positive fibroblasts and found a substantial increase of both FSP1-positive fibroblasts and of
-SMA–positive fibroblasts in STZ-treated kidneys compared with untreated controls (Figure 2, B and C). Double staining of these fibroblasts with CD31 further revealed that approximately 40% of all FSP1-positive cells and 50% of the
-SMA–positive cells in STZ kidneys were also CD31 positive, suggesting that these fibroblasts are likely of endothelial origin (Figure 2, B and C).
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-SMA–positive and FSP1-positive fibroblasts were considerably higher compared with wild-type mice, corresponding to the robust fibrosis (Figure 3, B and C). Coexpression of CD31 could be observed in 45% of all
-SMA–positive fibroblasts and 60% of all FSP1-positive fibroblasts, suggesting that EndMT may substantially contribute to the accumulation of fibroblasts in kidney fibrosis associated with COL4A3 KO mice (Figure 3, B and C).
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| CONCISE METHODS |
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Method of UUO
UUO was performed on the left kidney of CD1 mice as described previously.19 Mice were killed humanely at day 7 after UUO surgery and kidney samples were collected.
STZ Administration to CD1 Mice
We made CD1 mice diabetic by single intraperitoneal injection of STZ at 200 mg/kg in 10 mM citrate buffer (pH 4.5) at the age of 8 wk.14 We injected citrate buffer as a control. Mice were killed at 6 mo after the injection of STZ.
Immunofluorescence Labeling
We cut frozen tissue into 10-µm-thick cross-sections that were fixed in 100% acetone at –20°C for 10 min. We incubated the sections with primary antibodies at 4°C overnight. The primary antibodies were rat anti-CD31 (clone MEC13.3; BD Pharmingen, San Diego, CA), rabbit anti-FSP1 (polyclonal; research gift from Eric G. Neilson, Vanderbilt University, Nashville, TN), and mouse anti–
-SMA (Sigma [St. Louis, MO] or Abcam [Cambridge, MA]). We used Alexa Fluor 488–, 568–, and 594–conjugated secondary antibodies (Invitrogen, Carlsbad, CA). We counterstained the nuclei with 4',6-diamidino-2-phenylindole (Vectashield; Vector Laboratories, Burlingame, CA). Staining was analyzed independently by two investigators using a Zeiss LSM 510 Meta scanning confocal microscope. Ten visual fields per kidney were analyzed for co-localization of endothelial and fibroblast markers. Results are expressed as means ± SEM. With regard to the evaluation of
-SMA–positive fibroblasts, we considered only single cells that were not associated with larger vessels. To preserve the YFP signal in the Tie2-Cre;R26R-stop-EYFP mice, we fixed tissue in 4% PFA for 2 h, then cryoprotected it in 30% sucrose in PBS at 4°C overnight and snap-froze it in OCT. FSP1 and
-SMA staining was then performed as described already.
Quantitative Evaluation of Fibrosis
Masson Trichrome Staining was performed by the BIDMC Histology Core Facility on paraffin-embedded tissue to detect collagen fibers. The amount of collagen deposition (blue area) was then digitally quantified using the Image-Pro Plus 6.2 (Media Cybernetics, Bethesda, MD).
| DISCLOSURES |
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| Acknowledgments |
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| Footnotes |
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E.M.Z. and S.E.P. contributed equally to this work and should be considered co-first authors.
See related editorial, "How Many Different Roads May a Cell Walk down in Order to Become a Fibroblast?" on pages 2246–2248.
| REFERENCES |
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