Tubular regeneration and the role of bone marrow cells: ‘stem cell therapy’ – a panacea?

Marc E. De Broe

Department of Nephrology, University of Antwerp, Belgium

Correspondence and offprint requests to: Marc E. De Broe, M.D., PhD, Department of Nephrology, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk (Antwerpen), Belgium. Email: marc.debroe{at}ua.ac.be

The kidney possesses an impressive regeneration capacity and is the most performant among all tissues in the body. Yet ischaemic injury to the kidney produces acute tubular necrosis and apoptosis followed by tubular regeneration and recovery of renal function. Although mitotic cells are present in the tubules of postischaemic kidneys, the origins of the proliferating cells were considered until recently as the result of this regeneration capacity of the kidney, whereby the surviving remaining tubular cells start proliferating and migrating along the denuded basement membrane.

In both humans and mice, the proof-of-principle clinical observation has been obtained in support of the ability of cells emanating from the bone marrow (BM) to become renal epithelial cells. Several groups have reported the presence of Y-chromosome-positive renal tubular cells in kidneys of male recipients who received a renal transplant from a female donor, which suggests that cells from outside the kidney can populate the renal tubule (0.6–6.8% of the tubular cells) [1–3]. The lineage-negative (primitive stem cells) fraction of bone marrow contains multiple cell types, including rare subpopulations of haematopoietic stem cells and marrow stromal cells, alternatively referred to as mesenchymal stem cells (MSCs).

Protection of the kidney against ischaemia/reperfusion injury after injection of in vitro expanded skeletal muscle-derived stem cells, differentiated along the endothelial lineage (not after injection of non-differentiated stem cells) was reported. After intrarenal injection, engraftment of the differentiated cells into the renal microvasculature that accompanied protected renal function after ischaemia, was documented [4]. In a cisplatin model of acute renal failure, injection of MSCs of bone marrow origin, but not injection of haematopoietic cells, protected syngeneic female mice against severe tubular injury and renal functional impairment. MSCs markedly accelerated tubular proliferation and partial repopulation of the tubule by Y-chromosome-containing cells [5]. Lin et al. [6] showed that haematopoietic stem cells contributed to the regeneration of renal tubular epithelial cells. In the kidneys of non-transgenic female recipients that had been subjected to unilateral ischaemia, these authors showed that still after 4 weeks ß-galactosidase-expressing Y-chromosome-positive cells from transgenic male donors were detected. It was suggested that these findings are accounted for by ‘transdifferentiation’, i.e. phenotypic conversion of pluripotent somatic stem cells of one tissue type to another tissue type [2,3]. One cannot, however, exclude cell hybridization, i.e. the possibility that bone-marrow-derived cells adapted the phenotype of other cell lineages by ‘fusion’ [7,8].

Oliver and colleagues [9] demonstrated that the adult kidney contained numerous (resident) stem cells in the renal papilla, which proliferate and disappear from the papilla during the repair phase of ischaemia. In other words, the adult kidney possesses a ‘reservoir of kidney stem cells’.

Three recent papers published almost simultaneously by three research teams, independently came to the conclusion that the majority of the cells that were dividing to repair the injured tubules comes from an endogenous cell population rather than from bone marrow-derived cells [10–12]. They clearly demonstrate that the majority of tubule repair does not occur via proliferation of bone marrow-derived cells. They even suggested that in vivo differentiation of BMC cells into renal tubular cells may not occur at all, or is at most a minor component of the repair process after ischaemic injury. In contrast, it seems to be that the paracrine effects of the incoming or resident stem cells caused downregulation of pro-inflammatory and upregulation of anti-inflammatory cytokines contributing substantially to the repair process.

In the study by Tögel et al. [10], MSCs were isolated from rat femurs, and well characterized by isolation as adherent cells, their specificity being validated by their ability to differentiate into osteocytes and adipocytes. The renal arteries were clamped for 40 min and thereafter 106 MSCs (or equal numbers of fibroblasts) were injected into the carotid artery. Intracarotid administration of the cells immediately after ischaemia/reperfusion or after 24 h resulted in protection of renal function, higher proliferative and lower apoptotic indices, and lower morphological injury compared with injection of vehicle or syngeneic fibroblasts. Using different techniques including genetic markers (Y-chromosome), the fluorescence-labelled MSCs could be transiently detected in control and postischaemic kidneys, mostly in the glomerular capillaries, and some were attached to peritubular capillaries. After 24 and 72 h, however, MSCs were no longer demonstrable in the kidneys using different techniques for detection. Measurement of gene expression of pro- and anti-inflammatory cytokines in the kidney provided further support for the hypothesis of paracrine actions.



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Fig. 1. Different mechanisms of renal repair and potential roles of bone marrow-derived cells after acute ischaemic/toxic injury. MSCs, mesenchymal stem cells. Adapted from: Krause D and Cantley LG: J Clin Invest 115: 1705–8, 2005.

 
Duffield et al. studied kidney repair in chimeric mice expressing green fluorescent protein (GFP) or bacterial ß-gal or harbouring the male Y-chromosome exclusively in bone marrow-derived cells [12]. In GFP chimeras, some interstitial cells, mainly leukocytes, were GFP positive. They demonstrate the ability of purified MSCs to improve renal function in an I/R model of acute tubular injury in the mouse. They found that the effects of transplanted MSCs were independent of any direct contribution to the renal parenchyma, either tubular or endothelial in nature. Furthermore, using three separate approaches for tracking donor-derived cells (GFP, Y-chromosome, and ß-gal), they found that animals undergoing whole bone marrow transplantation followed by acute renal injury also failed to exhibit any tubular cells derived from donor bone marrow. In addition, they claim that the ß-gal-positive tubular cells reported in prior studies may have been detected as a result of increased intrinsic ß-gal activity in the injured tubule rather than the presence of ß-gal-positive bone marrow cells. They also suggest that the Y-chromosome-positive tubular cells detected in previous studies of female mice, following transplantation of bone marrow from male donor mice, are likely artefacts of imaging caused by the staining technique or the superimposition of a tubular cell and an infiltrating bone marrow-derived (leukocyte) cell.

Importantly, in agreement with the study by Duffield et al. [12], Lin and colleagues [11] demonstrated that the majority of the cells that were dividing to repair the injured tubules came from an endogenous cell population, rather than from bone marrow-derived cells.

Lin et al. [11] produced transgenic mice that expressed enhanced GFP (EGFP) specifically and permanently in mature renal tubular epithelial cells. Following ischaemia/reperfusion injury (IRI), EGFP-positive cells showed incorporation of BrdU and expression of vimentin, which provides direct evidence that the cells composing regenerating tubules are derived from renal tubular epithelial cells. In BMC-transplanted mice, 89% of the proliferating epithelial cells originated from host cells, and 11% originated from donor BMCs. Twenty-eight days after IRI, the kidneys contained 8% donor-derived cells, mostly (81%) interstitial cells.

These three studies [10–12], a independently support the fact that during regeneration after injury intrarenal cells are the main source and that intrinsic tubular cells proliferation accounts for renal repair. In vivo transdifferentiation, of BM stem cells into renal tubular cells and angiogenesis may not occur at all, or is at most a minor component of the repair process after toxic-ischaemic injury.

The contribution of cell hybridization, i.e. the capacity of bone marrow-derived cells to adopt the phenotype of other cells lineages by fusion, in the renal repair remains to be determined. Since most bone marrow-derived cells after renal injury differentiated into renal interstitial cells (myofibroblasts), the involvement of BMCs in fibrosis has to be considered. Haemopoietic stem cell mobilization-associated granulocytosis can even worsen acute renal failure in mice [13].

Altogether, renal repair after acute ischaemic toxic injury still seems to be mainly an affair of the kidney itself mobilizing growth factors, secreted by infiltrating cells, stimulating the proliferation/migration of the surviving dedifferentiated cells, upregulation of protective proteins and recruiting stem cell localized deep in the kidney. Research looking after intervention with medication (e.g. erythropoietin) [14] or other ways of stimulating these repair processes should be our priority.

The different studies cited in this article call for caution in premature clinical use of bone marrow cells in the treatment of renal failure.

Conflict of interest statement. None declared.



   References
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  2. Poulsom R, Forbes SJ, Hodivala-Dilke K et al. Bone marrow contributes to renal parenchymal turnover and regeneration. J Pathol 2001; 195: 229–235[CrossRef][ISI][Medline]
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