The Skate Weighs in on Kidney Regeneration

Research into kidney organogenesis has seen dramatic growth in recent years, fueled not only by new data on epithelial cell differentiation and the function of genes expressed in the kidney but also by a renewed interest in stem cell biology and its potential application to treating disease. Since the pioneering work of Grobstein in the middle 1950s (1), it has been known that the formation of new nephrons in the developing mammalian kidney is dependent on an interaction between a mass of fibroblastic cells, the metanephric mesenchyme or renal progenitor cells, and the branching epithelium derived from the ureteric bud, the future collecting system. In the intervening years, the search for the identity of signals mediating the conversion of mesenchymal progenitor cells to epithelium and nephron formation has taken many forms; from mouse gene knockout experiments to biochemical analyses of factors secreted by embryonic kidney cells and cell lines (2). Underlying this search are these questions: What cellular processes are required for continued growth and differentiation of kidney tissue? How are these processes regulated at the molecular level to generate the elaborate architecture of the mature kidney and the functional integration of its varied cell types? How are growth and proliferation of renal progenitor cells coordinated with nephron differentiation to result in uniformly sized kidneys? In addition to illuminating the origins of the kidney, the answers to these questions could have broad implications for treatment of kidney injury and end-stage renal disease. Unlike the liver, the kidney has a limited regenerative capacity. Kidney resection in mammals causes compensatory contralateral hypertrophy but no new nephron formation. The response to tubular damage in the mammalian kidney is an important aspect of injury repair but is largely limited to local reproliferation of epithelial cells to cover the denuded basement membrane (3). Damage to glomeruli seen in glomerulosclerosis is for the most part irreversible and leads finally to nephron degeneration (4). Translating basic discoveries in nephrogenesis to therapies that might enhance kidney regeneration after injury remains an attractive goal. In a parallel vein, recent discoveries detailing the unexpected plasticity of tissue and bone marrow stem cells is generating optimism that some aspects of development may be recapitulated in the context of the adult organ and allow new avenues for tissue repair. In this issue of JASN, Elger et al. (5) demonstrate that unlike the mammalian kidney, partial nephrectomy of the elasmobranch kidney (Leucoraja erinacea; the skate) is followed by regrowth of new nephrons, or neo-nephrogenesis, most likely from kidney progenitor cells that are resident in the adult skate kidney. The authors have identified a nephrogenic zone in the adult skate kidney where new nephrons form during growth and in response to injury. They show that injury-induced neo-nephrogenesis occurs not only on the ipsilateral injured kidney but also on the contralateral, unoperated side. One of the implications of these studies is that kidney progenitor cells or renal “stem cells” persist in the adult kidneys of lower vertebrates and elasmobranchs. Similar studies of the freshwater teleost kidney (6) have shown that kidney regeneration after chemical injury also occurs via the formation of new nephrons as opposed to repopulation of tubules by proliferating epithelial cells. In the skate, injury results in an enhanced proliferation of progenitor cells resembling the embryonic metanephric mesenchyme and an increase in the number of nephron intermediates resembling mammalian metanephric S-shaped bodies. In some respects, these studies present a model that is more readily translatable to the mammalian kidney than the previous teleost kidney neo-nephrogenesis studies, because the skate kidney includes a countercurrent system not present in the freshwater teleost and the skate nephrogenic mesenchyme appears to be adjacent to the branching tips of the collecting system, much as it is in the developing mammalian kidney. Although the mammalian kidney does not exhibit neo-nephrogenesis after injury, the search for resident kidney stem cells in higher vertebrates is being actively pursued. Iwatani et al. (7) have recently reported on FACS-sorted adult rat kidney cells using a protocol originally optimized to identify bone marrow cells with stem cell-like properties (8). Bone marrow cells that actively efflux the DNA binding dye Hoechst 33342 generate a dark “side-population” (SP cells) on FACS-sorting and have been shown to be pluripotent and capable of repopulating various injured tissues, including the heart, when injected into the circulation. When injected into irradiated rats, donor kidney SP cells were shown to contribute to hepatocytes, skeletal muscle, and hematopoietic lineages but not to renal epithelia or mesangial cells (7). In a different approach, Ito et al. (9) tested the idea that the bone marrow may serve as a reservoir of organ stem cells in adult animals that could participate in organ repair. They used green fluorescent protein (GFP)-expressing bone marrow cells to repopulate the marrow of an irradiated host and examined kidneys for GFP expression after anti-Thy1 antibody-mediated glomerulonephritis. GFP-positive cells were observed in the mesangium soon after injury, replacing damaged host cells with donor-derived, macrophage-like cells. Using the Y chromosome as a marker of male host extrarenal cells in transplanted female kidneys, Poulsom et al. (10) showed that host cells could contribute to renal tubular epithelia and podocytes in mice and humans. Similarly, Krause et al. (11) demonstrated a contribution of donor male bone marrow cells to host female renal tubular cells. In the context of acute tubular necrosis, Gupta et al. (12) demonstrate that 1% of tubules in female donor kidneys into male recipients were positive for the Y chromosome and, when present, Y chromosome-positive cells were clustered, suggesting local clonal expansion of what were extrarenal cells. While encouraging and provocative, some of these studies may need to be reinterpreted in light of new findings that introduced marrow stem cells can fuse with host cells and adopt the fate and behavior of the resident host cell (13). Current objectives of mammalian stem cell work include (1) developing culture conditions that allow the expansion of stem cell populations without loss of their pluripotency and (2) devising methods of regulating stem cell differentiation so that the fate of organ or bone marrow-derived stem cells can be directed to match the injured tissue. Both of these objectives are well served by further studies of basic developmental mechanisms in model organisms. It has long been appreciated in embryology that the environment in which an embryonic cell lives has a major influence on its behavior and eventual fate. Proliferative and anti-apoptotic effects of secreted growth factors such as FGF and BMP on embryonic kidney nephrogenic mesenchyme (14) may well be applicable to keeping adult organ stem cells alive in culture. Pluripotent stem cells may be nudged in a kidney direction by the application of other factors such as the Wnts, which have been shown to play key early roles in the transformation of kidney mesenchyme to epithelia (15). As we continue to pursue basic studies of kidney development in fish, frogs, chicken, mice, and now the skate, we stand to learn much more about the earliest steps in the specification of nephrogenic mesoderm, i.e., how kidney progenitor cells are established in the first place, and how stem cells are mobilized after injury. It is fair to say that any newly discovered data on factors influencing stem cell survival and fate will be greeted with enthusiasm by those who hope to one day treat damaged kidneys with potentially life-saving cell-based therapies.