During renal fibrosis epithelial cells undergo a partial epithelial-to-mesenchymal transition that can be targeted to reverse established disease. Kidney fibrosis is marked by an epithelial-to-mesenchymal transition (EMT) of tubular epithelial cells (TECs). Here we find that, during renal fibrosis, TECs acquire a partial EMT program during which they remain associated with their basement membrane and express markers of both epithelial and mesenchymal cells. The functional consequence of the EMT program during fibrotic injury is an arrest in the G2 phase of the cell cycle and lower expression of several solute and solvent transporters in TECs. We also found that transgenic expression of either Twist1 (encoding twist family bHLH transcription factor 1, known as Twist) or Snai1 (encoding snail family zinc finger 1, known as Snail) expression is sufficient to promote prolonged TGF-β1–induced G2 arrest of TECs, limiting the cells' potential for repair and regeneration. In mouse models of experimentally induced renal fibrosis, conditional deletion of Twist1 or Snai1 in proximal TECs resulted in inhibition of the EMT program and the maintenance of TEC integrity, while also restoring cell proliferation, dedifferentiation-associated repair and regeneration of the kidney parenchyma and attenuating interstitial fibrosis. Thus, inhibition of the EMT program in TECs during chronic renal injury represents a potential anti-fibrosis therapy.

BMP7 has been previously shown to protect against renal fibrosis. Raghu Kalluri and his colleagues have now identified activin-like kinase 3 (Alk3) as the key co-receptor for BMP7 in the kidney and have identified an orally available, small-peptide agonist of Alk3 that reduces established fibrosis in five animal models of kidney injury. Molecules associated with the transforming growth factor β (TGF-β) superfamily, such as bone morphogenic proteins (BMPs) and TGF-β, are key regulators of inflammation, apoptosis and cellular transitions. Here we show that the BMP receptor activin-like kinase 3 (Alk3) is elevated early in diseased kidneys after injury. We also found that its deletion in the tubular epithelium leads to enhanced TGF-β1–Smad family member 3 (Smad3) signaling, epithelial damage and fibrosis, suggesting a protective role for Alk3-mediated signaling in the kidney. A structure-function analysis of the BMP-Alk3–BMP receptor, type 2 (BMPR2) ligand-receptor complex, along with synthetic organic chemistry, led us to construct a library of small peptide agonists of BMP signaling that function through the Alk3 receptor. One such peptide agonist, THR-123, suppressed inflammation, apoptosis and the epithelial-to-mesenchymal transition program and reversed established fibrosis in five mouse models of acute and chronic renal injury. THR-123 acts specifically through Alk3 signaling, as mice with a targeted deletion for Alk3 in their tubular epithelium did not respond to therapy with THR-123. Combining THR-123 and the angiotensin-converting enzyme inhibitor captopril had an additive therapeutic benefit in controlling renal fibrosis. Our studies show that BMP signaling agonists constitute a new line of therapeutic agents with potential utility in the clinic to induce regeneration, repair and reverse established fibrosis.

EndMT was originally described in heart development, where the endocardial endothelial cells which line the AV canal undergo an endothelial to mesenchymal transition (EndMT) to form the endocardial mesenchymal cushion that later gives rise to the septum and mitral and tricuspid valves. In the post-natal heart specifically endothelial cells that originate from the endocardium maintain increased susceptibility to undergo EndMT as remnants from their embryonic origin. Such EndMT involving adult coronary endothelial cells contributes to microvascular rarefication and subsequent chronification of hypoxia in the injured heart, ultimately leading to cardiac fibrosis. While in most endothelial beds hypoxia induces tip cell formation and sprouting angiogenesis we here demonstrate that hypoxia is a stimulus for human coronary endothelial cells (HCAEC) to undergo phenotypic changes reminiscent of EndMT via a mechanism involving HIF1-α-induced activation of the EndMT master regulatory transcription factor Snail. Our study adds further evidence for the unique susceptibility of endocardium-derived endothelial cells to undergo EndMT and provides novel insights how hypoxia contributes to progression of cardiac fibrosis. Additional studies may be required to dissect between distinct sprouting angiogenesis and EndMT responses of different endothelial cells populations.

Methylation of CpG island promoters is an epigenetic event that can effectively silence transcription over multiple cell generations. Hypermethylation of the Rasal1 promoter contributes to activation of fibroblasts and progression of kidney fibrosis. Here, we explored whether such causative hypermethylation could be reversed through endogenous mechanisms and whether such reversal of hypermethylation is a constituent of the antifibrotic activity of bone morphogenic protein 7 (BMP7). We show that successful inhibition of experimental kidney fibrosis through administration of BMP7 associates with normalization of Rasal1 promoter hypermethylation. Furthermore, this reversal of pathologic hypermethylation was achieved specifically through Tet3-mediated hydroxymethylation. Collectively, our findings reveal a new mechanism that may be exploited to facilitate therapeutic DNA demethylation to reverse kidney fibrosis.

Rationale:: Endocardial fibroelastosis (EFE) is a unique form of fibrosis, which forms a de novo subendocardial tissue layer encapsulating the myocardium and stunting its growth, and which is typically associated with congenital heart diseases of heterogeneous origin, such as hypoplastic left heart syndrome. Relevance of EFE was only recently highlighted through the establishment of staged biventricular repair surgery in infant patients with hypoplastic left heart syndrome, where surgical removal of EFE tissue has resulted in improvement in the restrictive physiology leading to the growth of the left ventricle in parallel with somatic growth. However, pathomechanisms underlying EFE formation are still scarce, and specific therapeutic targets are not yet known. Objective:: Here, we aimed to investigate the cellular origins of EFE tissue and to gain insights into the underlying molecular mechanisms to ultimately develop novel therapeutic strategies. Methods and Results:: By utilizing a novel EFE model of heterotopic transplantation of hearts from newborn reporter mice and by analyzing human EFE tissue, we demonstrate for the first time that fibrogenic cells within EFE tissue originate from endocardial endothelial cells via aberrant endothelial to mesenchymal transition. We further demonstrate that such aberrant endothelial to mesenchymal transition involving endocardial endothelial cells is caused by dysregulated transforming growth factor beta/bone morphogenetic proteins signaling and that this imbalance is at least in part caused by aberrant promoter methylation and subsequent transcriptional suppression of bone morphogenetic proteins 5 and 7. Finally, we provide evidence that supplementation of exogenous recombinant bone morphogenetic proteins 7 effectively ameliorates endothelial to mesenchymal transition and experimental EFE in rats. Conclusions:: In summary, our data point to aberrant endothelial to mesenchymal transition as a common denominator of infant EFE development in heterogeneous, congenital heart diseases, and to bone morphogenetic proteins 7 as an effective treatment for EFE and its restriction of heart growth.