Kidney tubular epithelium is restored without replacement with bone marrow-derived cells during repair after ischemic injury

Abstract

The kidney has the ability to restore the structural and functional integrity of the proximal tubule, which undergoes extensive epithelial cell death after prolonged exposure to ischemia. In order to study the role that adult bone marrow-derived stem cells might play in kidney remodeling after injury, we employed a murine model of ischemia/reperfusion (I/R) injury in which the degree of injury, dysfunction, repair, tubular cell proliferation and functional recovery have been characterized [Park KM, et al, J Biol Chem 276:11870-11876, 2001]. We generated chimeric mice using marrow from mice expressing the bacterial LacZ gene, or the enhanced green fluorescence protein (eGFP) gene, or from male mice transplanted into female mice. The establishment of chimerism was confirmed at 6 weeks following transplantation in each case. I/R injury was induced in chimeric mice by occluding the renal arteries and veins with microaneurysm clamps for 30 minutes. After functional recovery in the eGFP chimeras, although there were many interstitial cells, no tubular cells were derived from bone marrow cells. In the bacterial beta-galactosidase (beta-gal) chimeric mice we found evidence of mammalian (endogenous) beta-gal by 5-bomo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-gal) staining, but not bacterial beta-gal in tubule cells. Detection of the Y chromosome by fluorescence in situ hybridization (FISH) in the postischemic kidneys of gender-mismatched chimeras revealed Y chromosome positivity only in the nuclei of interstitial cells, when scrutinized by deconvolution microscopy. In our model of I/R injury there was a large amount of proliferation of surviving, injured tubular cells indicating that the injured tubule is repopulated by daughter cells of surviving tubular cells. Analysis of the phenotype of interstitial and vascular cells following I/R injury revealed small numbers of peritubular endothelial cells to be derived from bone marrow cells that may serve in the repair process.

Fig. 1. Proximal tubule cell proliferation is widespread in the outer medulla following ischemic kidney injury

(A) Tubulin immunolabeling of sections from rat kidney (×400) 2 days following ischemic injury showing mitotic spindles of injured proximal tubules cells (thick arrows). Note also four consecutive cells, sectioned above or below the chromosomes, but showing spindle tips or ground glass cytoplasm (fine arrows), all indicative of mitosis. (B) Periodic acid-Schiff (PAS)–stained section of mouse kidney (×400) 2 days following ischemic injury showing injured proximal tubule cells in different stages of mitosis (thick arrows). Note also (insert) a dedifferentiated cell in telophase lining an injured tubule filled with PAS-positive debris.

Fig. 2. In enhanced green fluorescence protein (eGFP) bone marrow chimeras, ischemic kidney injury recruits bone marrow–derived cells to the interstitium, not the renal tubule

(A) Section of contalateral kidney (×200) from eGFP chimeric mouse showing small numbers of eGFP-positive cells in the interstitium, but no eGFP cells in the tubules. Note tubules are weakly visible due to autofluorescence. (B) Section of outer medulla (bottom left) and cortex of kidney (×200) 7 days postischemia showing increased eGFP-positive cells in between the tubules, but no evidence of eGFP-positive cells in the tubules. (C) Section of spleen from eGFP chimera (×200) showing red pulp surrounding white pulp (bottom right). Note many cells expressing eGFP in both areas of the spleen. (D) Section of kidney (×400) from eGFP bone marrow donor mouse. Tubules and cells between the tubules express eGFP. (E) Flow cytometric histograms of purified peripheral blood leukocytes showing background levels of green fluorescence in leukocytes of control recipient mouse (lefthand panel) and a very high percentage of eGFP and leukocytes in eGFP chimeric mouse (righthand panel).

Fig. 3. In gender-mismatched chimeras, ischemic kidney injury recruits bone marrow–derived cells to the interstitium, not the renal tubule

(A) Section showing the outer medulla of kidney 15 days postischemia. Section has been hybridized with fluorescent probe (green) to detect Y chromosome, counterstained with lotus lectin (red) to highlight proximal tubule cells and counterstained with DAPI (blue) to show nuclei. Note intratubular casts in some tubules. Note also that many cells between tubules have the Y chromosome within their nuclei (arrows), but none of the tubule cells have the Y chromosome. (B) Section of spleen (X320) from the chimeric mouse showing many cells with the Y chromosome, confirming chimerism. (C) Two tubule cells (×600) appear to have positive hybridization in their nuclei as viewed by standard epifluorescence microscopy suggesting they are derived from bone marrow cells. (D) Deconvolution of the same area of section viewed in (C) reveals that the green probe aggregates are clearly lying adjacent to the tubule nuclei not within the nuclei, indicating that these “positive” cells are artifacts.