Flávia G. Machado

Peritubular capillary rarefaction is hypothesized to contribute to the increased risk of future CKD after AKI. Here, we directly tested the role of Gli1 + kidney pericytes in the maintenance of peritubular capillary health, and the consequences of pericyte loss during injury. Using bigenic Gli1-CreER t2 ; R26tdTomato reporter mice, we observed increased distance between Gli1 + pericytes and endothelial cells after AKI (mean±SEM: 3.3±0.1 µ m before injury versus 12.5±0.2 µ m after injury; P <0.001). Using a genetic ablation model, we asked whether pericyte loss alone is sufficient for capillary destabilization. Ten days after pericyte ablation, we observed endothelial cell damage by electron microscopy. Furthermore, pericyte loss led to significantly reduced capillary number at later time points (mean±SEM capillaries/high-power field: 67.6±4.7 in control versus 44.1±4.8 at 56 days; P <0.05) and increased cross-sectional area (mean±SEM: 21.9±0.4 µ m 2 in control versus 24.1±0.6 µ m 2 at 10 days; P <0.01 and 24.6±0.6 µ m 2 at 56 days; P <0.001). Pericyte ablation also led to hypoxic focal and subclinical tubular injury, reflected by transient expression of Kim1 and vimentin in scattered proximal tubule segments. This analysis provides direct evidence that AKI causes pericyte detachment from capillaries, and that pericyte loss is sufficient to trigger transient tubular injury and permanent peritubular capillary rarefaction.

Fibrosis is the common final pathway of virtually all chronic injury to the kidney. While it is well accepted that myofibroblasts are the scar-producing cells in the kidney, their cellular origin is still hotly debated. The relative contribution of proximal tubular epithelium and circulating cells, including mesenchymal stem cells, macrophages, and fibrocytes, to the myofibroblast pool remains highly controversial. Using inducible genetic fate tracing of proximal tubular epithelium, we confirm that the proximal tubule does not contribute to the myofibroblast pool. However, in parabiosis models in which one parabiont is genetically labeled and the other is unlabeled and undergoes kidney fibrosis, we demonstrate that a small fraction of genetically labeled renal myofibroblasts derive from the circulation. Single-cell RNA sequencing confirms this finding but indicates that these cells are circulating monocytes, express few extracellular matrix or other myofibroblast genes, and express many proinflammatory cytokines. We conclude that this small circulating myofibroblast progenitor population contributes to renal fibrosis by paracrine rather than direct mechanisms.