|
Gut Stem Cell
Scientific Review
Introduction
Epithelial surfaces in the gut are constantly being replaced by new epithelial cells. Turnover of these cells occurs rapidly with an average turnover time of 2 – 7 days (1). Homeostasis of the epithelial surface of the gastrointestinal tract therefore depends on cell proliferation, differentiation, and cell death (apoptosis). It is believed that the gut stem cell (GSC) is responsible for all of the aforementioned cellular events, making it one of the most important elements of the gastrointestinal system (1, 2).
Definition of the Gut Stem Cell
The gut is composed of several organs that extend from the mouth to the anus, but is generally thought of as the stomach, small intestine, and large intestine. Little is known about the GSC, and until recently, its existence has been up to debate. In the 1970s, it was believed that cells in the gut were not derived from GSCs, but instead from migrating neuroendocrine stem cells from the neural crest (3). In the1980s, it was suggested that the endoderm is colonized by ‘neuroendocrine-programmed stem cells’, which give rise to GSCs (1). The type and function of each cell varies throughout the different organs in the gut, but many now believe that each cell is derived from a common GSC, originating in the endoderm during embryogenesis (4). Like many other multipotent stem cells, the GSC has the ability to differentiate into various cell lineages, and also has the ability for self renewal (5). The GSC is capable of differentiating into the four main cell lineages of the intestinal epithelium: columnar cells, mucin-secreting cells, endocrine cells, and paneth cells (Fig. 1) (1). It is also believed to be responsible for differentiating into several different lineages of the stomach epithelial lining. including the parietal cells, gastrin-producing G-cells, intestinal subepithelial myofibroblasts (ISEMF), and the interstitial cells of Cajal (ICC) (1, 6).
Cell Markers
Currently, there are few cell markers for the GSCs, but these markers serve to be helpful in providing insight into GSC behavior. Musashi-1 (Msi-1) and Hairy and Enhancer of Split homologue-1 (Hes-1) proteins were shown to be co-expressed in the small intestine crypts of mice near the paneth cells, making both potential markers for GSCs (7, 8). Msi-1 may play a role in keeping GSCs in an undifferentiated state during post-transcriptional gene regulation by repressing m-Numb mRNA (9). Msi-1 up-regulates expression of Hes-1, which has also been shown to be a marker for neural stem cell self renewal and suppression of differentiation (8). Hes-1, a transcriptional factor regulated by Notch signaling, represses commitment of neural stem cells to neural lineages (10) and may play a similar role with GSCs. Studies have shown that Hes-1 expression is reduced for epithelial cells migrating out of the crypt towards the villi tips, making it possible that Hes-1 expression alone is a marker for GSCs that have begun cell migration and therefore are committed to differentiation (Fig. 2) (7).
Location and Stem Cell Niche
After the identification of cell markers for GSCs, the location of these stem cells was hypothesized from the well-known migration patterns of the corresponding differentiated cells (1, 2, 10). In the intestines, the majority of epithelial cells originate in the base of the crypts formed by the villi, and it is therefore believed that GSCs reside there as well (Fig. 2). Specifically, GSCs in the large intestine are located in the crypt base surrounded by intestinal subepithelial myofibroblasts (ISEMFs) (3), while GSCs in the small intestine are thought to reside higher up in the crypts above the paneth cells at approximately cell position 4-5 (8). In the stomach, cells migrate to both the lumen and the lower region of the tubular glands, leading to the theory that GSCs in the stomach originate in the neck/isthmus region of the tubular glands (2). Recent studies have had success in staining murine GSCs with bromo-deoxyuridine and 3H-thymidine in order to confirm the cell locations mentioned above (11). It is believed that the intestinal crypts and tubular glands contain myo-epithelial fibroblasts that help provide the necessary niche for GSCs by secreting various mediators and growth factors such as hepatocyte growth factor (Hgf), transforming growth factor beta (Tgf-β) and keratinocyte growth factor (Kgf) (2, 12). These mediators and growth factors play an important role in GSCs through facilitating mesenchymal-epithelial crosstalk, although the underlying mechanisms are still not understood (5).
Relevant Pathology
Colon cancer is the second most common cause of cancer-related death (13). The lifetime risk of developing colon cancer has been estimated at 5% (14), with about 50% of the western population developing a benign tumor (12). The rapid and constant differentiation of GSCs in the intestine is thought to be the reason behind the intestinal epithelium’s high susceptibility to cancer (12). Disruptions and mutations that cause errors in GSC signaling pathways have been shown to lead to several forms of gastrointestinal cancers, including stomach and colon cancers (15). Once a mutation occurs and a GSC starts differentiating into malignant cells, the constant renewal of epithelial cells increases the chances of future mutations. Differentiation of GSCs and gut development is controlled through two signaling pathways; one is the TGF-β pathway and the other is the Wnt pathway (16, 17). Mutations that activate the Wnt pathway have been shown to be common in the majority of colon cancers (18). Similarly, mutations in the TGF- β pathway causing inactivation are also very common in colon cancers (19). Activation of cancer-inactivated TGF- β pathways has been suggested as a potential treatment option for gastrointestinal cancers (16). For instance, the development of compounds designed to activate the TGF- β pathway has led to several new anti-cancer drugs such as cetuximab (Erbitux) and bevacizumab (Avastin), both of which have been approved to treat colon cancer.
Conclusion
Gut epithelial cells are in a continuous state of proliferation, differentiation and apoptosis leading to a rapid turnover of the epithelium. This rapid turnover of the epithelium makes the gut significantly more susceptible to developing gastrointestinal cancers. The GSC is believed to play a key role in the development of the epithelium because of its ability to self renew and differentiate into the lineages of the gut. Researchers have been able to determine GSC locations and cell markers, which have helped in identifying signaling pathways responsible for GSC cancer-inducing mutations. The pharmaceutical industry is now utilizing this research to develop cancer drugs that prevent mutations in GSCs. Despite the advances in research in the field of GSCs, there are many areas that require further investigation. Research involving in vitro models of GSCs has been lacking due to inadequate culture conditions (20). Development of short-term cultures derived from intact and dispersed crypt cells are being used to develop colony forming assays (21, 22). The use of such assays, along with determining better culture conditions may help us expand GSCs in vitro. The cells can then be used clinically for mucosal implantation to help restore epithelial cells damaged during chemotherapy (20).
Figure 1: GSC hierarchy showing asymmetrical stem cell division.
Fig. 2 GSCs in the intestine reside at the base of the crypt; once GSCs begin to differentiate the cells rise to the top. Hes-1 and Msi-1 together are believed to be markers of GSCs, while Hes-1 alone may be a marker of GSCs that have begun to differentiate.
References
- Brittan M, Wright NA. Gastrointestinal stem cells. J Pathol 2002;197:492-509.
- Brittan M, Wright NA. The gastrointestinal stem cell. Cell Prolif 2004;37:35-53.
- Pearse AG, Takor T. Neuroendocrine embryology and the APUD concept. Clin Endocrinol 1976;5:299-44.
- Maunoury R, Robine S, Pringault E et. al. Villin expressin in the visceral endoderm and in the gut anlage during early mouse embryogenesis. EMBO J 1998;7:3321-9.
- Rizvi AZ, Hunter JG, Wong MH. Gut-derived stem cells. Surgery 2005;137:585-90.
- Joyce NC, Haire MF, Palade GE. Morphologic and biochemical evidence for a contractile cell network within the rat intestinal mucosa. Gastroenterol 1987;92:68-81.
- Kayahara T, Sawada M, Takaishi S et. al. Candidate markers for stem and progenitor cells, Muasashi-1 and Hes1, are expressed in crypt base columnar cells of mouse and small intestine. FEBS Lett 2003;535:131-5.
- Nakamura Y, Sakakibara S, Miyata T et. al. The bHLH gene hes1 as a repressor of the neuronal commitment of CNS stem cells. J Neurosci 2000;20:283 -93.
- Okano h, Imai T, Okabe M. Musashi: a translational regulator of cell fate. J Cell Sci 2002;115:1355-59.
- Sante Barbara P. de, van de Brink G R, Roberts D J. Development and differentiation of the intestinal epithelium. CMLS 2003;60:1322-32.
- Karam SM. Lineage commitment and maturation of epithelial cells in the gut. Front Biosci 1999;4:D286.
- Radtke F, Clevers H. Self-Renewal and Cancer of the Gut: Two Sides of a Coin. Science 2005;307:1904-9.
- Nelson H, Petrelli N, Carlin A et. al. Guidelines 2000 for colon and rectal cancer surgery. J Natl Cancer Inst 2001;93:583-96.
- Jemal A, Thomas A, Murray T, Thun M. Cancer Statistics 2002. CA Cancer J Clin 2002;52:23-47.
- Mishra L, Shetty K, Tang Y, Stuart A, Byers S W. The role of TGF-β and Wnt signaling in gastrointestinal stem cells and cancer. Oncogene 2005;24,5775-89.
- Wells JM, Melton DA. Vertebrate endoderm development. Annu Rev Cell Dev Biol 1999;15:393-410.
- Liu D, Kang JS, Derynck R. TGF-β-activated Smad3 represses MEF2-dependent transcription in myogenic differentiation. EMBO J 2004;23:1557–66.
- Kinzler K W, Vogelstein B. Landscaping the cancer terrain. Science 1998;280:1036-37.
- Munoz NM, Upton M, Rojas A et. al. Transforming growth factor beta receptor type II inactivation induces the malignant transformation of intestinal neoplasms initiated by Apc mutation. Cancer Res 2006;66:9837-44.
- Booth C, Potten CS. Gut instincts: thoughts on intestinal epithelial stem cells. J Clin Inv 2000;105:1493-99.
- Booth C, O’Shea JA, Potten CS. Maintenance of functional stem cells in isolated and cultured intestinal epithelium. Exp Cell Res 1999;249:359-66.
- Whitehead RH, Demmler K, Rockman SP, Watson NK. Clonogenic growth of epithelial cells from normal colonic mucosa from both mice and humans. Gastroenterol 1999;117:858-65.
Summarized by: Manocchio J, Rana R, Reddy B, Stem Cell Graduate Course, Fall 2006
TA: Shakti Ramkissoon
|
