Kidney Stem Cell / Renal Progenitors
(A Scientific Review)

Kidney Stem Cells – Introduction

In the late 1990s, the understanding of bone marrow (BM) stem cells, including hematopoietic stem cells (HSC), mesenchymal stem cells (MSC), and embryonic stem cells (ESC), reached a point where this knowledge could be adapted to the study of kidney cells, including stem cell influence on the recovery after ischemic injuries to the kidney (Poulsom 2003, Kale 2003).  During this period, scientists began to look into the origin of endothelial differentiation during nephrogenesis hoping to find the location of kidney progenitor cells.

The location and identification of a kidney stem cell is arguable among researchers. Opinions on the locations and identification markers have been controversial between authors.  The search has revealed the existence of Renal Progenitor Cells (RPCs) located in the human adult kidney (Zerbini 2006).  RPCs can differentiate in vitro into epithelial and endothelial cells within the kidney. RPCs are clinically useful as they should be available immediately to repair the kidney as a treatment for acute renal injuries (Zerbini 2006).

Among the literature, progenitor cells located within the kidney are also referred to as multipotent renal progenitor cells (MRPCs) (Gupta 2006).  MRPCs retain the ability to induce the expression of cells containing endothelial, hepatocyte, and neural markers (Gupta 2006).  It is proposed that MRPCs participate in a regenerative repair pathway in the kidney (Gupta 2006).

Long-term efforts towards kidney regeneration from stem cells are being undertaken with skepticism with regard to the ability of true de novo kidney regeneration.  Oliver et al suggest that regeneration of kidney from stem cells may be limited (Oliver 2004).  It is unclear if there is single kidney stem cell or multiple types needed for different renal cell types (Oliver 2004).  Zerbini and colleagues share the theory that there may be several renal progenitor types that each contribute to the repair of different sections of the kidney (Zerbini 2006).

Development

Interest in the formation and generation of the kidney started with the pioneer works of Grobstein in 1955 on the development of the mouse mesenephros (Grobstein 1955).  From the 1960s to the mid 1990s, researchers conducted in vivo experiments to better understand the biochemical processes and mechanisms of kidneys, including its ontogeny (Bose 1965) and response to injury (Leahy 2005, Rosenberg 1991).  Researchers also studied the impact of growth factors, as well as the interactions between these factors and the migration and repair of kidney epithelial cells (Zhang 1991, Kartha 1992).  All of these studies point to the possibility of cell populations in the kidney that are capable of kidney repair when needed and thus suggesting the presence of adult renal stem cells.

Renal embryogenesis begins when the dorsal mesoderm gives rise to aorta-gonad-mesonephros (AGM) and the intermediate mesoderm (Al-Awqati 2002, Challen 2004).  It is the AGM, the intermediate mesoderm, or the interaction of these two that eventually form the metanephric mesenchyme and the Wolffian duct.  The Wolffian duct gives rise to the ureteric bud, which invades the metanephric mesenchyme.  Reciprocal interactions between the metanephric mesenchyme and the ureteric bud result in growth and branching of the duct, while the mesenchyme grows and converts to epithelia.  The ureteric bud cells give rise to the collecting tubules, the pelvic, and the ureteric epithelia. The metanephric mesenchymal cells give rise to the epithelia of the rest of the nephron (Al-Awqati 2002).  Fetal renal stem cells have been identified as those of metanephric mesenchymal, not ureteric bud (Gupta 2006) (Figure 1).

Follow-up research discovered that the Pax-2 gene (Rothenpieler 1993), OP-1 (Vukicevic 1996), WT-1 (Donovan 1999), BMP-4 (Raatikainen-Ahokas 2000), Wnt-4 (Stark 2002), Sonic hedgehog (Yu 2003), Six1 (Xu 2003), Sall1, and JNK pathways (Osafune 2006) are all involved in mesenchymal differentiation and proliferation in kidney development.  In addition, some of these genes can become markers, which can be useful for identifying fetal renal stem cells.

Although the genes and various cytokines affecting the metanephric mesenchyme have been documented in detail, the markers of metanephric mesenchyme are still being studied.  Using cDNA microarrays in a recent study, Challen and collaborators indicated that metanephric mesenchyme expresses Ewing sarcoma homolog, 14-3-3, retinoic acid receptor-alpha, stearoyl-CoA desaturase 2, CD24, and cadherin-11 (Challen 2004).  Another study also found that Side Population (SP) cells from the kidney are CD24a+, c-kit–, and Sca-1+ (Challen 2006).  SP cells in the kidney are theorized to represent a progenitor population.

Location/Identification of Kidney Stem Cells

The location of adult kidney stem cells remains elusive.  The discovery of fetal kidney progenitor cells intensified the search for adult kidney progenitor cells.  The difficulty in identifying the growth factors, repair mechanisms, and adult kidney progenitor's location primarily stemmed from the limited regenerative capability of adult kidney cells (Mene 2003).  Nonetheless, efforts were made to identify the specific adult SP cells capable of regeneration in the fetal kidney.  In the mid 1990s, most studies involved inducing acute renal failure or ischemia in animals and then monitoring how various growth factors impact the recovery of renal cells (Kawaida 1994).  Researchers also found that new renal cells were formed by undergoing mitosis after ischemic injury in renal cells (Witzgall 1994).  The link was finally established in 2004, when Oliver et al. administered nucleotide bromodeoxyuridine (BrdU) to a population of low-cycling cells and found that the retention of BrdU occurs mostly in the renal papilla revealing the location of adult kidney stem cells (Oliver 2004).  Additionally, Oliver et al. found that the renal papilla is the only area where apoptosis could not be detected after ischemia, thus further suggesting that the renal papilla serves as a niche for kidney stem cells (Oliver 2004) (Figure 2).

Although BM stem cells do improve renal cell function and regeneration after ischemic injury via transdifferentiation, is believed that the kidney stem cells possess the potential to perform the majority of renal cell regeneration (Lin 2003, Kale 2003).  Moreover, new nephrons are produced after toxic treatment (Salice 2001) or when there is a partial nephrectomy (Elger 2003).  This evidence further strengthens the hypothesis for the location and role of kidney stem cells.  Other possible locations of adult renal progenitor cells have also been found recently, including the label-retaining cells (LRC) in the renal epithelial tubular cells (Maeshima 2003), the rKS56 cells in the S3 segment of the nephron (Kitamura 2005), the multipotent progenitor cells from the Bowman's capsules (Sagrinati 2006), and the kidney side population (SP) primarily locates in proximal tubule (Challen 2006) (Figure 2).

Today, there is no consensus or definitive markers for adult kidney stem cells.  Research on the identification of kidney stem cell markers is currently an ongoing process.  Recent in vitro studies focusing on the identification of kidney stem cells have shown that CD133+ cells derived from normal adult human kidney differentiate into epithelial cells expressing aminopeptidase A and NaCl co-transporter (markers of renal proximal and distal epithelium) (Bussolatti 2005).   These cells are expected to represent a MPRC population in the human kidney (Bussolatti 2005).  This MRPC population lacked hematopoietic marker, expressed Pax- 2 (a renal embryonic marker), performed limited self-renewal, and homed to injury site in the kidney (Bussolatti 2005).

The current proposed adult kidney stem cells markers include CD133+, Pax-2 (Benedetta 2005), CD45–, CD24a+, c-kit–, and Sca-1+ (Challen 2006).  Specifically for kidney SP, the markers are mostly Sca-1 + and CD24a+ and has low level of CD45, CD34, CD31, and c-kit. For rKS56 cells, they have Pax-2, Wnt-4, Wt-1, aquaporin-1, and aquaporin-2. (Kitamura 2005).  In addition, there is a group of renal stem cells known as multipotent renal progenitor cells (MRPC) that expresses vimentin, CD90 (thy1.1), Pax-2, and Oct4, but not cytokeratin, MHC class I or II, or other markers of more differentiated cells (Gupta 2006).  Vimentin marker can also be found on the LRC cells in the renal epithelial tubular cell (Maeshima 2003).

Homing/Function of Kidney Stem Cells

Studies have located resident renal progenitor cells in the kidney. There is also evidence that bone marrow-derived renal progenitor cells may participate in kidney repair and remodeling (Zerbini 2006).  Both populations have been found during ischemic episodes to release cytokine in the area of injury that mobilize progenitor cells to the site of injury (Schachinger 2005).  Bone marrow derived cells in kidney have been shown to replace specialized adult renal cells in humans and animals, thus requiring homing and differentiation capability (Ricardo 2005).  Furthermore, BM derived cells have been reported in the renal vasculature, tubules and glomeruli after kidney transplant and in experimental animal studies (Ricardo 2005). 

The definitive renal progenitor cell (RPC) has not been established.  It is theorized that it should possess the capability for expansion and have the potential for self-renewal (Bussolati 2005).  RPCs can respond to local instructive signals, differentiate into epithelial and endothelial cells, expressed PAX-2, migrated into injured cells, and when injected into SCID mice were able to generate in vivo blood vessels repairing renal vascular injury. RPCs also expressed CD44, CD29, and CD73 (potential mesenchymal origin) (Bussolati 2005).

Disease and the Use of Kidney Stem Cells in Medicine

Kidney cells can form from both metanephric mesenchymal and adult kidney cells. Metanephric mesenchymal can only form during the fetal dorsal mesoderm and AGM period (Al-Awqati 2002), and these can only generate from ESCs.  Due to ESC plasticity, scientists hope to someday reprogram them to develop into kidney cells or an entire organ (Dekel 2004).  In the United States, the current political hurdles for stem cell research and the lack of new ESC lines have made the development of potential ESC based therapies difficult.  Also, while MSCs and HSCs in the BM have been shown to either improve kidney cell regeneration or kidney function (Morigi 2004, Poulsom 2003), they are adult stem cell types, and thus, subject to limited plasticity (Wagers 2004). 

Acute renal failure (ARL) has a mortality rate between 50%-80%, and this disease is increasingly common among the older population (Schrier 2004).  While much of the pathological causes and mechanisms have been identified, the mortality rate has not improved.  Current treatment for ARL is primarily dialysis (Lin 2006).  Thus, stem cells offer an alternative way of treating ARL.

It is possible that kidney stem cells may offer a benefit for renal autoimmune diseases such as glomerulonephritis.  Although this disease afflicts a small portion of the population (Simon 1986), glomerulonephritis has complex pathological mechanisms (Nassar 1998) and has treatments that are often inadequate and can result in toxicity (Schlondorff 1995).  Furthermore, failure to treat this disease will lead to kidney failure. 

Other major causes of kidney dysfunction include ischemic acute renal injury and ischemic chronic renal injury.  Ignoring or improper treatments will lead to renal failure and subsequently renal fibrosis.  While medicine cannot rescue a failing kidney, someday stem cells will be able to help.

When the kidney reaches an end-stage failure, the two most viable options are using dialysis and transplantation.  Dialysis treatment is painful for patients, and kidney transplant offers the solution to end that pain.  In order to transplant, however, a match has to be made, or else an allogeneic immune reaction takes place due to allograft rejection.  The chance of finding a correct match is therefore slim.  Patients often take immunosuppressant drugs to prevent rejection.  Furthermore, studies shown chronic or acute renal failure can take place after transplantation of a non-renal organ (Ojo et al. 2003), such as liver or BM transplantation.  Thus, if it is possible to someday bioengineer a functional kidney from stem cells, then it will be possible to eliminate the need of matching organs for transplantation.  Additionally, kidney-focused stem cell therapies may eliminate transplantation waiting lists, pain from dialysis, viral infections and sepsis, and the financial cost placed upon the healthcare system and patients (Brodie 2005).

In vivo studies showed that CD24+ CD133+ parietal epithelial cells injected via IV into SCID mice with acute renal failure developed tubular structures in different sections of the nephron and reduced kidney damage, thus indicating potential regenerative treatment for patients with renal diseases (Sagrinati 2006).  While kidney stem cells and MPRCs hold a definitive potential for therapeutic usage, so far they have been found to be less then effective.  Research has been conducted to define the molecular pathways and events associated with the renal repair process, and strategies have been instrumented to test in animal and human models.  These studies concluded that the contribution of MPRCs to regenerative renal repair/ response is minimal to none (Gupta 2006).  Furthermore, injection of MPRCs to the arterial of the kidney were found to cause some potential adverse consequences such as some cells being lodged in the glomerulus and some forming tubular casts (Gupta 2006). 

Future of Kidney Stem Cells

The study of renal progenitor cells is still in its infancy, but promises to provide future benefit through further research.  In the near future, further marker studies will be established to isolate definitive kidney stem cells that can help with research for potential therapeutic values. Other strategies include the study of the renal papilla’s response to injury, as these efforts may offer insight into the kidney’s response to other renal diseases (Oliver 2004).  Additionally, more studies are needed to have a clearer understanding of renal embryogenesis' pathway and differentiation, especially from the dorsal mesoderm to metanephric mesenchymal period.

Regardless of the hurdles involved with future renal progenitor cell research, it is greatly believed that research will impact the future treatment of renal diseases, including ischemic renal disease, regeneration of glomeruli, and transplant vasculopathy (Schachinger 2005).  Already, tissue engineering efforts have resulted in reconstituted nephron segments using seeded implants in bovine that resulted in defined nephron structures that secreted urine (Ricardo 2005).  Adult kidney stem cells may present therapeutic value for the treatment of acute renal failure and renal autoimmune diseases also.

Chronic renal failure is a leading cause of mortality and morbidity in Western countries.  Furthermore, the long-term costs associated with the treatment of end-stage renal disease is greater than the direct costs of cancer.  Thus, successful developments of stem cell therapy for treatment of kidney disease may meet a significant clinical impact (Sagrinati 2006).

Figure 1: Proposed mechanism of nephrogenesis: renal embryogenesis begins when the dorsal mesoderm gives rise to aorta-gonad-mesonephros (AGM) and the intermediate mesoderm.  The AGM and the intermediate mesoderm may give rise to the metanephric mesenchyme.  The ureteric bud cells generate the collecting tubules, the pelvic, and the ureteric epithelia.  The metanephric mesenchymal cells give rise to the epithelia of the rest of the nephron.

Figure 2: Anatomy of kidney and suspected location of kidney stem cells

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Acknowledgements

This review was prepared by the following graduate students in the Fall 2006 Stem Cell Biology Class, Graduate School of Biomedical Sciences, University of Medicine and Dentistry of New Jersey:
Jason Aptaker, Fenwick Garvey, John Le (in alphabetical order).
Teaching Assistant:  Krista Buono