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Rao JN, Wang JY. Regulation of Gastrointestinal Mucosal Growth. San Rafael (CA): Morgan & Claypool Life Sciences; 2010.

Cover of Regulation of Gastrointestinal Mucosal Growth

Regulation of Gastrointestinal Mucosal Growth.

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Intestinal Stem Cells

The continuous renewal and repair of adult intestinal mucosal epithelium after injury depend on resident specialized stem cells. Stem cells are cells that are capable of self-renewal and consistently maintain themselves over long periods of time, producing all undifferentiated cell types of that tissue. In various tissue types, stem cells serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the organism is alive [3942]. An increasing body of evidence shows that each adult tissue contains its own unique type(s) of dedicated stem cells; there are many different types of stem cells in mammals. To date, it remains largely unknown if shared molecular and cell biology principles underlie the behavior of various tissue-specific stem cells. Generally, stem cells were thought to come from two main sources, embryos formed during the blastocyst phase of embryological development (embryonic stem cells) and nonembryonic “somatic” or “adult.” Both types of stem cells are typically characterized by their potency or potential to differentiate into another type of cells with more specialized functions, such as muscle cells, red blood cells, brain cells, or epithelial cells. Stem cells are distinguished from other cell types by two important characteristic features: (1) stem cells are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of inactive state and (2) stem cells can be induced to become tissue- or organ-specific cells with specialized biological functions under certain physiological or experimental conditions. In several organs, such as gut mucosa and bone marrow, stem cells regularly divide to repair and replace worn out or damaged cells, while in some other organs, including pancreas, muscle, brain and the heart, discrete populations of adult stem cells generate for replacements of cells that are lost through normal wear and tear, injury, or disease. Given their unique regenerative capabilities, stem cells are able to offer novel and new potential therapeutic approaches for treating different diseases, such as diabetes and heart disease.

The intestinal epithelial villus/crypt structure, its surrounding pericryptal fibroblasts, and mesenchyme form an anatomical unit that generates four cell lineages, namely absorptive enterocytes and goblet, enteroendocrine, and Paneth cells of the secretory lineage. The crypt is a contiguous pocket of epithelial cells at the base of the villus, in which intestinal stem cells (ISCs) are periodically activated to produce progenitor or transit amplifying (TA) cells which are committed to produce mature cell lineages [43,44,45]. In normal conditions, newly formed TA cells reside within the crypts for 2–3 days and undergo up to six rounds of cell division. When these newly divided cells reach the crypt–villus junction, they rapidly differentiate into each of the four terminally differentiated cell types (Figure 3). The crypt is mainly occupied by undifferentiated cells; but differentiated Paneth cells that secrete antibacterial peptides into the crypt are also unusually located at the base of the crypt area and escape their upward migration [45,46]. It is likely that ISCs divide asymmetrically, thus giving rise to one daughter stem cell and one TA cell, which differentiate toward mature epithelial cells. In the damaged gut mucosa, after exposure to irradiation or chemotherapy, ISCs are undergoing symmetrical division that yield into two stem cells to replace damaged ISCs. It has been shown that there are several intrinsic and extrinsic cellular mechanisms that regulate ISC self-renewal and differentiation of newly divided cells.

FIGURE 3. A scheme of adult stem cell-driven tissue renewal in organs such as intestine and stomach.

FIGURE 3

A scheme of adult stem cell-driven tissue renewal in organs such as intestine and stomach. (A) Stem cells concomitantly self-renew and generate, rapidly dividing transit-amplifying (TA) daughter cells via asymmetric cell division. The TA cells undergo (more...)

ISCs AND THEIR NICHES

The ISC niche is an anatomical structure that is composed of stem cells, their progeny, and elements of their microenvironment, and it coordinates the normal homeostatic production of functional mature cells [38,47,48]. ISC niches provide a sheltering environment that sequesters ISCs from various stimuli, such as differentiation and apoptosis and are also safeguarded against excessive ISC production, thus reducing the risk of cancers. The epithelial homeostasis of the intestine is based on a delicate balance between self-renewal and differentiation. Intestinal pericryptal fibroblasts (also named as subepithelial myofibroblasts) are also implicated in the formation of ISC niches, and they are believed to secrete various putative growth factors and cytokines that promote epithelial proliferation and enhance production of differentiated cell lineages, including enterocytes, goblet, enteroendocrine, and Paneth cells (Figure 4). There are two models to explain how pluripotent stem cells fuel the proliferative activity of crypts [45,49,50]. One concept is the “+4 position” model from the crypt bottom and the other is called the “stem cell zone” located below the +4 position (Figure 4). Each of the crypts is commonly believed to contain approximately four to six independent stem cells. BrdU-labeling studies suggest that label-retaining cells reside specifically at the +4 position relative to the crypt bottom, with the first three positions occupied by the terminally differentiated Paneth cells and that +4 cells are extremely sensitive to radiation, a property that functionally protects the stem cell compartment from genetic damage. More support for the +4 model also results from recent studies on lineage tracing experiments utilizing a newly generated Bmi-Cre-ER knock-in allele [51,52]. After induction for 24 h, the cells expressing Cre-reporter are located at the +4 position, directly above the Paneth cells. The model of “stem cell zone” was originally proposed by Cheng and Leblond [53], based on the identification of crypt base columnar (CBC) cells (Figure 4). These CBC cells are small undifferentiated cycling cells hidden between the Paneth cells [45]. Recently, Barker et al. further identified the Wnt target genes, Prominin and Lgr5/GPR49, as markers that specifically label CBC stem cells in the mouse small intestine [54,55]. It has been shown that CBC cells are capable of long-term maintenance of intestinal epithelial self-renewal and generate differentiated mature intestinal epithelial cells (Figure 4). Furthermore, lineage-tracing experiments show that both CBC and +4 cells produce offspring within days and persist for up to a year and that they are multipotent stem cells and exhibit different cycling kinetics and molecular features [45,56]. It is possible that these two types of stem cells coordinately regulate intestinal epithelial tissue homeostasis and regeneration under physiological and various pathological conditions.

FIGURE 4. Two opposing models showing the identity of the intestinal stem cells.

FIGURE 4

Two opposing models showing the identity of the intestinal stem cells. The exact identity of the intestinal stem cells has proven controversial over the last 30 years, with two opposing models dominating the literature. Top: “+4 position” (more...)

SIGNALING PATHWAYS REGULATING ISCs

Several studies using transgenic and knock-out animal models suggest that different signaling pathways, such as Wnt, bone morphogenic protein (BMP), Notch, Ephrin, JAK/STAT1, PTEN, AKT, and PI3K, play an important role in the regulation of intestinal epithelial renewal, particularly ISC function [43,46,51,57,58]. Recently, microRNAs (miRNA) are also shown to modulate ISC cell proliferation and differentiation [59,60]. Disruption of these pathways alters gut mucosal growth and could lead to intestinal mucosal tumorigenesis.

Wnt Signaling Pathway

The Wnt signaling pathway has a unique and central role in the regulation of intestinal epithelial renewal under biological conditions. It is not only the dominant force behind the proliferative activity in the crypts but also the principal cause of colon cancer after its mutational activation. The Wnt signaling is the first pathway that is shown to regulate ISC functions. Activation of the Wnt pathway is crucial for the maintenance of stem/progenitor cell division and for newly divided cell migration along the crypt–villus axis [6163]. Several lines of in vivo evidence show that TA cell proliferation in the crypt is strictly dependent on the continuous stimulation of canonical Wnt signaling pathway, thus activating nuclear β-catenin/T cell factor (TCF) transcriptional activity [61]. In the absence of Wnt signals, free cytosolic β-catenin is sequestered and targeted for degradation via the β-catenin destruction complex (Figure 5).

FIGURE 5. Signaling pathways within the crypt-ISC activation.

FIGURE 5

Signaling pathways within the crypt-ISC activation. Outline of Notch, Wnt, PI3K, and BMP pathways and their potential points of interaction. Normally, +4 LRCs are maintained in a quiescent state through canonical BMP signaling via the transcriptional (more...)

It has been shown that crypt epithelial cells highly express Wnt receptors and co-receptors, such as frizzled and LRPs, and are targets of Wnt signals. Genetic experiments suggest that Wnt signals pattern the physical structure of ISC niches by generating opposing and complementary gradients of ephrins and their tyrosine kinase receptors [61,63,64]. Silencing TCF4 and β-catenin or inhibition of Wnt activity by overexpression of its natural inhibitor Dickkopf homologue-1 (Dkk-1) inhibits ISC proliferation in the intestinal epithelium, whereas mutations of APC (a negative regulator of Wnt) or expression of oncogenic forms of β-catenin cause hyperproliferation (Figure 5). Besides mitogenic activity, Wnt signaling pathways are also involved in the regulation of differentiation of Paneth cells at the crypt bottom [65]. After the cells are generated near or at the crypt base, they migrate toward the villus while undergoing the process of maturation and differentiation. Ephrin molecules that play a role in the maintenance of cellular boundaries and migratory paths are identified as target genes of Wnt and are shown to segregate cells along the crypt–villus axis [64,65]. Wnt activation enhances expression of Ephrin B receptors and ligands through increasing β-catenin/TCF transcription complex within the intestinal epithelium.

Several recent studies conducted by Clevers and colleagues identify the specific markers of ISCs [38,51,65]. Although a great majority of the genes are expressed throughout the proliferative crypt compartment, the Lgr5/Gpr49 gene is expressed in a particularly unique fashion. The Lgr5 gene encodes an orphan G protein-coupled receptor (GPCR), characterized by a large leucine-rich extracellular domain; it is closely related to GPCRs with glycoprotein ligands, such as TSH, FSH, and LH receptors. Lgr5 is highly expressed in the stem cells of another Wnt-driven self-renewing structure, the hair follicle. APCmin mouse exhibit the expression of Lgr5 in a limited number of crypt bottom cells as well as in adenomas. It has been reported that single Lgr5+ ISC regenerates the self-renewing and functional crypt-like structures in vitro when it is exposed to appropriate signaling factors and extracellular matrix [51]. Similar results are also observed by using Bmi, Prominin-1 and both Prominin-1 and Lgr5 [43,52,54,67]. Other lineage-tracing experiments also reveal that Lgr5+ cells represent actively dividing and multipotent ISCs that contribute to the long-term renewal of the entire gut epithelium. The Lgr5 marker now allows identification of stem cells not only in the intestine but also in other tissues, such as hair follicle, mammary gland, and stomach epithelium. All results obtained from different tissues support the notion that Lgr5+ cells represent a more general marker of adult stem cells [43,51,54,68]. In the mammalian intestinal stem cell system, several markers, such as Lgr5, Prominin 1, and Bmi, are currently available to trace stem cell lineage and stem cell identities. It has been noticed that colon cancer stem cells can be identified through these specific markers, including Lgr5, CD133 glycoprotein, Musashi-1, CD29, and CD24 and Lgr5 and can be used for targets for novel therapies in the future [51,69,70].

BMP Pathway

BMP is a member of transforming growth factor-β family and specifically binds to BMP receptors. BMP signaling plays an important role in the regulation of intestinal development and adult epithelial tissue homeostasis. BMP2 and BMP4 isoforms are expressed in the mesenchyme, and their receptors are identified in the epithelium [46,71,72]. Generally, BMP functions as a negative regulator of intestinal epithelial cell proliferation in the crypts; and conditional deletion of BMP receptor 1A results in cell hyperproliferation. The BMP antagonist “noggin” is also expressed in the submucosal region adjacent to the crypt base as well as dynamically in +4 cells, whereas overexpression of noggin causes ectopic crypt formation [46,73]. In addition, BMP-2 and BMP-4 null mice are embryonically lethal. Studies using conditional ablation of BMP receptor 1A suggest that BMP signals serve to antagonize the crypt formation and ISC self-renewal (Figure 5) [73,74]. Inactivation of BMP receptor-1A or overexpression of Noggin increases β-catenin nuclear translocation, suggesting that there is a cross-talk between BMP and Wnt pathways.

Notch Pathway

Like Wnt and BMP signals, the Notch pathway is also essential for maintaining the crypt compartment at undifferentiated and proliferative states during gut development [43,7577]. Notch genes encode single transmembrane receptors that regulate a broad spectrum of cell fate decisions. Components of Notch pathway are expressed in the crypt base. Functions of Notch signaling in the regulation of ISC activity are revealed by studies using knock-out animal models. Activation of Notch signaling in the intestinal epithelium increases cell proliferation, but inhibition of Notch reduces secretory cells (Figure 5). Although Notch signaling plays a role in ISC proliferation, current literature suggests that it functions in the TA compartment controlling absorptive (enterocytes) rather than secretory (enteroendocrine, goblet, and Paneth cells) fate decisions in the intestinal epithelium [43,66].

Little is known about the involvement of other signaling pathways including PTEN, PI3K, JAK/STAT in the regulation of ISC proliferation, although direct or indirect evidence exists in the literature (Figure 5). Due to space limitations, this information is not mentioned here, but can be found in several recent excellent review articles [46,48,58,7880].

Copyright © 2011 by Morgan & Claypool Life Sciences.
Bookshelf ID: NBK54089
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