Heterogeneity in histone 2B-green fluorescent protein-retaining putative small intestinal stem cells at cell position 4 and their absence in the colon

Establishment of a 3D organoid culture model for the investigation of adult slow-cycling putative intestinal stem cells

The study of intestinal stem cells is a prerequisite for the development of therapies aimed at regenerating the gut. To enable investigation of adult slow-cycling H2B-GFP-retaining putative small intestinal (SI) stem cells in vitro, we have developed a three-dimensional (3D) SI organoid culture model based on the Tet-Op histone 2 B (H2B)-green fluorescent protein (GFP) fusion protein (Tet-Op-H2B-GFP) transgenic mouse. SI crypts were isolated from 6- to 12-week-old Tet-Op-H2B-GFP transgenic mice and cultured with appropriate growth factors and an animal-derived matrix (Matrigel). For in vitro transgene expression, doxycycline was added to the culture medium for 24 h. By pulse-chase experiments, H2B-GFP expression and retention were assessed through direct GFP fluorescence observations, both by confocal and fluorescence microscopy and by immunohistochemistry. The percentages of H2B-GFP-retaining putative SI stem cells and H2B-GFP-retaining Paneth cells persisting in organoids were determined by scoring relevant GFP-positive cells. Our results indicate that 24 h exposure to doxycycline (pulse) induced ubiquitous expression of H2B-GFP in the SI organoids. During subsequent culture, in the absence of doxycycline (chase), there was a gradual loss (due to cell division) of H2B-GFP. At 6-day chase, slow-cycling H2B-GFP-retaining putative SI stem cells and H2B-GFP-retaining Paneth cells were detected in the SI organoids. The developed culture model allows detection of slow-cycling H2B-GFP-retaining putative SI stem cells and will enable the study of self-renewal and regeneration for further characterization of these cells.

Identification of Quiescent LGR5+ Stem Cells in the Human Colon

BACKGROUND & AIMS

In the mouse intestinal epithelium, Lgr5⁺ stem cells are vulnerable to injury, owing to their predominantly cycling nature, and their progenies de-differentiate to replenish the stem cell pool. However, how human colonic stem cells behave in homeostasis and during regeneration remains unknown.

METHODS

Transcriptional heterogeneity among colonic epithelial cells was analyzed by means of single-cell RNA sequencing analysis of human and mouse colonic epithelial cells. To trace the fate of human colonic stem or differentiated cells, we generated LGR5-tdTomato, LGR5-iCasase9-tdTomato, LGR5-split-Cre, and KRT20-ERCreER knock-in human colon organoids via genome engineering. p27⁺ dormant cells were further visualized with the p27-mVenus reporter. To analyze the dynamics of human colonic stem cells in vivo, we orthotopically xenotransplanted fluorescence-labeled human colon organoids into immune-deficient mice. The cell cycle dynamics in xenograft cells were evaluated using 5-ethynyl-2'-deoxyuridine pulse-chase analysis. The clonogenic capacity of slow-cycling human stem cells or differentiated cells was analyzed in the context of homeostasis, LGR5 ablation, and 5-fluorouracil-induced mucosal injury.

RESULTS

Single-cell RNA sequencing analysis illuminated the presence of nondividing LGR5⁺ stem cells in the human colon. Visualization and lineage tracing of slow-cycling LGR5⁺p27⁺ cells and orthotopic xenotransplantation validated their homeostatic lineage-forming capability in vivo, which was augmented by 5-FU-induced mucosal damage. Transforming growth factor-β signaling regulated the quiescent state of LGR5⁺ cells. Despite the plasticity of differentiated KRT20⁺ cells, they did not display clonal growth after 5-FU-induced injury, suggesting that occupation of the niche environment by LGR5⁺p27⁺ cells prevented neighboring differentiated cells from de-differentiating.

CONCLUSIONS

Our results highlight the quiescent nature of human LGR5⁺ colonic stem cells and their contribution to post-injury regeneration.

Following the adverse outcome pathway from micronucleus to cancer using H2B-eGFP transgenic healthy stem cells

In vitro assessment of genotoxicity as an early warning tool for carcinogenicity mainly relies on recording cytogenetic damages (micronuclei, nucleoplasmic bridges) in tumour-derived mammalian cell lines like V79 or CHO. The forecasting power of the corresponding standardised test is based on epidemiological evidence between micronuclei frequencies and cancer incidence. As an alternative to destructive staining of nuclear structures a fish stem cell line transgenic for a fusion protein of histone 2B (H2B) and enhanced green fluorescent protein (eGFP) was established. The cells are derived from koi carp brain (KCB) and distinguish from mammalian culturable cells by non-tumour-driven self-renewal. This technology enables the analysis of genotoxic- and malign downstream effects in situ in a combined approach. In proof-of concept-experiments, we used known carcinogens (4-Nitroquinoline 1-oxide, colchicine, diethylstilbestrol, ethyl methanesulfonate) and observed a significant increase in micronuclei (MNi) frequencies in a dose-dependent manner. The concentration ranges for MNi induction were comparable to human/mammalian cells (i.e. VH-16, CHL and HepG2). Cannabidiol caused the same specific cytogenetic damage pattern as observed in human cells, in particular nucleoplasmic bridges. Metabolic activation of aflatoxin B1 and cyclophosphamide could be demonstrated by pre-incubation of the test compounds using either conventional rat derived S9 mix as well as an in vitro generated biotechnological alternative product ewoS9R. The presented high throughput live H2B-eGFP imaging technology using non-transformed stem cells opens new perspectives in the field of in vitro toxicology. The technology offers experimental access to investigate the effects of carcinogens on cell cycle control, gene expression pattern and motility in the course of malign transformation. The new technology enables the definition of Adverse Outcome Pathways leading to malign cell transformation and contributes to the replacement of animal testing. Summary: Complementation of genotoxicity testing by addressing initiating events leading to malign transformation is suggested. A vertebrate cell model showing "healthy" stemness is recommended, in contrast to malign transformed cells used in toxicology/oncocology.

Reduced enamel epithelium-derived cell niche in the junctional epithelium is maintained for a long time in mice

BACKGROUND

Although numerous reports have demonstrated that the junctional epithelium (JE) is derived from the reduced enamel epithelium (REE), the fate of the REE-derived JE remains controversial. The present study elucidated the fate of the REE-derived JE and the cell dynamics of stem/progenitor cells in the JE following tooth eruption.

METHODS

Mandibular first molar germs (embryonic days 15 to postnatal 1-day-old) were transplanted into the socket of 2-week-old mice after extraction of the upper first molars of B6 wild-type (WT) and green fluorescent protein (GFP) transgenic mice. After analysis by µ-CT, paraffin sections were processed for immunohistochemistry for Nestin, Ki67 and GFP. We also performed chasing analysis using BrdU-administered TetOP-H2B-GFP mice.

RESULTS

Amelogenesis progressed normally in the cervical areas, and the structure of the JE was like that in normal tooth development. The JE was GFP-negative in transplantations using GFP transgenic mice as the host, and the oral epithelium (OE) showed a positive reaction. By contrast, the JE remained GFP-positive throughout the experimental period in transplantations using GFP transgenic mice as the donor. Chasing analysis revealed that H2B-GFP- and 5-Bromo-2'-deoxyuridine (BrdU)-labeled cells in the basal side of the JE translocated to oral side of the JE as the chasing time increased. Some label-retaining cells remained at the supra-basal cell layer in the JE.

CONCLUSION

These results suggest that REE-derived cell niche in the JE is maintained for a long time following tooth eruption. Therefore, the JE may be available as the source of the odontogenic epithelium.

The putative role of insulin-like growth factor (IGF)-binding protein 5 independent of IGF in the maintenance of pulpal homeostasis in mice

Although insulin-like growth factor binding protein 5 (IGFBP5) may play a crucial role in activating the functions of periodontal and bone marrow stem cells, the factors responsible for regulating the maintenance of dental pulp stem cells (DPSCs) remain to be clarified. This study aimed to elucidate the role of IGFBP5 in maintaining pulpal homeostasis during tooth development and pulpal healing after tooth injury in doxycycline-inducible TetOP-histone 2B (H2B)-green fluorescent protein (GFP) transgenic mice (GFP expression was induced at E14.5 or E15.5) by using TUNEL assay, RT-PCR, in situ hybridization for Igfbp5, and immunohistochemistry for IGFBP5, Nestin, and GFP. To observe the pulpal response to exogenous stimuli, the roots of the maxillary first molars were resected, and the coronal portion was autografted into the sublingual region. Intense IGFBP5/Igfbp5 expression was observed in cells from the center of the pulp tissue and the subodontoblastic layer in developing teeth during postnatal Week 4. Intense H2B-GFP-expressing label-retaining cells (LRCs) were localized in the subodontoblastic layer in addition to the center of the pulp tissue, suggesting that slowly dividing cell populations reside in these areas. During postoperative days 3–7, the LRCs were maintained in the dental pulp, showed an IGFBP5-positve reaction in their nuclei, and lacked a TUNEL-positive reaction. In situ hybridization and RT-PCR analyses confirmed the expression of Igfbp5 in the dental pulp. These findings suggest that IGFBP5 play a pivotal role in regulating the survival and apoptosis of DPSCs during both tooth development and pulpal healing following tooth injury.

Mature enteroendocrine cells contribute to basal and pathological stem cell dynamics in the small intestine

Lgr5-expressing intestinal stem cells (ISCs) maintain continuous and rapid generation of the intestinal epithelium. Here, we present evidence that dedifferentiation of committed enteroendocrine cells (EECs) contributes to maintenance of the epithelium under both basal conditions and in response to injury. Lineage-tracing studies identified a subset of EECs that reside at +4 position for more than 2 wk, most of which were BrdU-label-retaining cells. Under basal conditions, cells derived from these EECs grow from the bottom of the crypt to generate intestinal epithelium according to neutral drift kinetics that is consistent with dedifferentiation of mature EECs to ISCs. The lineage tracing of EECs demonstrated reserve stem cell properties in response to radiation-induced injury with the generation of reparative EEC-derived epithelial patches. Finally, the enterochromaffin (EC) cell was the predominant EEC type participating in these stem cell dynamics. These results provide novel insights into the +4 reserve ISC hypothesis, stem cell dynamics of the intestinal epithelium, and in the development of EC-derived small intestinal tumors. NEW & NOTEWORTHY The current manuscript demonstrating that a subset of mature enteroendocrine cells (EECs), predominantly enterochromaffin cells, dedifferentiates to fully functional intestinal stem cells (ISCs) is novel, timely, and important. These cells dedifferentiate to ISCs not only in response to injury but also under basal homeostatic conditions. These novel findings provide a mechanism in which a specified cell can dedifferentiate and contribute to normal tissue plasticity as well as the development of EEC-derived intestinal tumors under pathologic conditions.

Determination of Histone 2B-Green Fluorescent Protein (GFP) Retention in Intestinal Stem Cells

The epithelium of the gastrointestinal tract represents the interface between the luminal contents of the gut and that of the host tissues and plays a central role not only in regulating absorption of dietary nutrients but also in providing a barrier to prevent the entry of bacteria and other pathogens. Repair and replacement of damaged aging cells within the epithelium is modulated by stem cells, which are located in the intestinal crypts of the small intestine.Two distinct populations of intestinal stem cells have been described in the literature, one population at the very base of the crypt and a second population of long-lived stem cells located just above the Paneth cell zone. Herein, we describe a method to label this population of long-lived GFP label retaining cells. This method is free from confounding factors of previous methodologies based on radioactive tracers and also enables functional studies not previously possible using the radioactive tracer techniques described in the literature.

Nonrandom Germline Transmission of Mouse Spermatogonial Stem Cells

Genes are thought to be transmitted to offspring by random fertilization of a small number of oocytes with numerous spermatozoa. Here we analyzed the dynamics of male germline transmission by genetic marking and transplantation of spermatogonial stem cells (SSCs). We found that offspring deriving from a small number of specific SSCs appear within a limited time. Interestingly, the same SSC clones reappear later with an average functional lifespan of ∼124.4 days. Cyclic offspring production from SSCs was not caused by changes in SSC self-renewal activity because lineage-tracing analyses suggested that all SSCs actively proliferated. Selection appears to occur during the differentiating spermatogonia stage, when extensive apoptosis was observed. The pattern of germline transmission could be predicted using a mathematical model in which SSCs repeat cycles of transient spermatogenic burst and refractory periods. Thus, spermatogenesis is a regulated process whereby specific SSC clones are repeatedly recruited for fertilization with long-term cycles.

Mouse Label-Retaining Cells Are Molecularly and Functionally Distinct From Reserve Intestinal Stem Cells

BACKGROUND & AIMS

Intestinal homeostasis and regeneration after injury are controlled by 2 different types of cells: slow cycling, injury-resistant reserve intestinal stem cells (ISCs) and actively proliferative ISCs. Putative reserve ISCs have been identified using a variety of methods, including CreER insertions at Hopx or Bmi1 loci in mice and DNA label retention. Label-retaining cells (LRCs) include dormant stem cells in several tissues; in the intestine, LRCs appear to share some properties with reserve ISCs, which can be marked by reporter alleles. We investigated the relationships between these populations.

METHODS

Studies were performed in Lgr5-EGFP-IRESCreERT2, Bmi1-CreERT2, Hopx-CreERT2, and TRE-H2BGFP::Hopx-CreERT2::lox-stop-lox-tdTomato mice. Intestinal epithelial cell populations were purified; we compared reporter allele-marked reserve ISCs and several LRC populations (marked by H2B-GFP retention) using histologic flow cytometry and functional and single-cell gene expression assays.

RESULTS

LRCs were dynamic and their cellular composition changed with time. Short-term LRCs had properties of secretory progenitor cells undergoing commitment to the Paneth or enteroendocrine lineages, while retaining some stem cell activity. Long-term LRCs lost stem cell activity and were a homogenous population of terminally differentiated Paneth cells. Reserve ISCs marked with HopxCreER were primarily quiescent (in G0), with inactive Wnt signaling and robust stem cell activity. In contrast, most LRCs were in G1 arrest and expressed genes that are regulated by the Wnt pathway or are in the secretory lineage.

CONCLUSIONS

LRCs are molecularly and functionally distinct from reporter-marked reserve ISCs. This information provides an important basis for future studies of relationships among ISC populations.

The role of adipose stem cells in inflammatory bowel disease: From biology to novel therapeutic strategies

Inflammatory bowel diseases are an increasing phenomenon in western countries and in growing populations. The physiopathology of these conditions is linked to intestinal stem cells homeostasis and regenerative potential in a chronic inflammatory microenvironment. Patients with IBD present an increased risk of developing colorectal cancer (CRC), or colitis associated cancer (CAC). Conventional treatment for IBD target the inflammatory process (and include anti-inflammatory and immunosuppressive drugs) with biological agents emerging as a therapeutic approach for non-responders to traditional therapy. Conventional treatment provides scarce results and present severe complications. The intestinal environment may host incoming stem cells, able to engraft in the epithelial damaged sites and differentiate. Therefore, stem cell therapies represent an emerging alternative in inflammatory bowel diseases, with current investigations on the use of haematopoietic and mesenchymal stem cells, in particular adipose stem cells, apparently fundamental as regenerators and as immune-modulators. Here, we discuss stem cells in intestinal homeostasis and as therapeutic agents for the treatment of inflammatory bowel diseases.

GI stem cells - new insights into roles in physiology and pathophysiology

This overview gives a brief historical summary of key discoveries regarding stem cells of the small intestine. The current concept is that there are two pools of intestinal stem cells (ISCs): an actively cycling pool that is marked by Lgr5, is relatively homogeneous and is responsible for daily turnover of the epithelium; and a slowly cycling or quiescent pool that functions as reserve ISCs. The latter pool appears to be quite heterogeneous and may include partially differentiated epithelial lineages that can reacquire stem cell characteristics following injury to the intestine. Markers and methods of isolation for active and quiescent ISC populations are described as well as the numerous important advances that have been made in approaches to the in vitro culture of ISCs and crypts. Factors regulating ISC biology are briefly summarized and both known and unknown aspects of the ISC niche are discussed. Although most of our current knowledge regarding ISC physiology and pathophysiology has come from studies with mice, recent work with human tissue highlights the potential translational applications arising from this field of research. Many of these topics are further elaborated in the following articles.

Side population sorting separates subfractions of cycling and non-cycling intestinal stem cells

We report here that side population (SP) sorting allows for the simultaneous isolation of two intestinal stem cell (ISC) subsets from wild-type (WT) mice which are phenotypically different and represent cycling and non-cycling pools of cells. Following 5-ethynyl-2'-deoxyuridine (EdU) injection, in the upper side population (USP) the percentage of EdU+ was 36% showing this fraction to be highly proliferative. In the lower side population (LSP), only 0.4% of cells were EdU+, indicating this fraction to be predominantly non-cycling. Using Lgr5-EGFP mice, we show that Lgr5-EGFP(hi) cells, representing actively cycling ISCs, are essentially exclusive to the USP. In contrast, using histone 2B-YFP mice, SP analysis revealed YFP label retaining cells (LRCs) in both the USP and the LSP. Correspondingly, evaluation of the SP fractions for mRNA markers by qRT-PCR showed that the USP was enriched in transcripts associated with both quiescent and active ISCs. In contrast, the LSP expressed mRNA markers of quiescent ISCs while being de-enriched for those of the active ISC. Both the USP and LSP are capable of generating enteroids in culture which include the four intestinal lineages. We conclude that sorting of USP and LSP fractions represents a novel isolation of cycling and non-cycling ISCs from WT mice.

Low dose effects of ionizing radiation on normal tissue stem cells

In recent years, there has been growing evidence for the involvement of stem cells in cancer initiation. As a result of their long life span, stem cells may have an increased propensity to accumulate genetic damage relative to differentiated cells. Therefore, stem cells of normal tissues may be important targets for radiation-induced carcinogenesis. Knowledge of the effects of ionizing radiation (IR) on normal stem cells and on the processes involved in carcinogenesis is very limited. The influence of high doses of IR (>5Gy) on proliferation, cell cycle and induction of senescence has been demonstrated in stem cells. There have been limited studies of the effects of moderate (0.5-5Gy) and low doses (<0.5Gy) of IR on stem cells however, the effect of low dose IR (LD-IR) on normal stem cells as possible targets for radiation-induced carcinogenesis has not been studied in any depth. There may also be important parallels between stem cell responses and those of cancer stem cells, which may highlight potential key common mechanisms of their response and radiosensitivity. This review will provide an overview of the current knowledge of radiation-induced effects on normal stem cells, with particular focus on low and moderate doses of IR.

Location-specific effect of microbiota and MyD88-dependent signaling on Wnt/β-catenin pathway and intestinal stem cells

Intestinal homeostasis depends on the proper activity of the intestinal stem cells (ISCs) and an appropriate host response to the normal resident microbiota. The question on the effect of microbiota on ISCs behavior has not been addressed yet. Canonical Wnt pathway and ISC gene expression signature was compared in germfree vs. conventional and MyD88(-/-) vs. Myd88(+/+) mice based on publicly available gene expression data sets. Microbiota and MyD88-dependent signaling have distinct effects on the Wnt pathway and ISC at gene expression level. In addition, the effect of microbiota and MyD88-dependent signaling on Wnt pathway and ISCs show regional variation. The net effect of microbiota on Wnt pathway and ISCs cannot be inferred from the available data. Nonetheless, the data are suggestive of a potential regulatory mechanism of the Wnt pathway by the microbiota and plausibly by any alteration in the microbiota composition.

Host-microbiota interaction and intestinal stem cells in chronic inflammation and colorectal cancer

Inflammatory bowel disease (IBD) and colorectal cancer (CRC) are the major diseases of the lower gastrointestinal tract. The intestinal epithelium plays a critical role in the host's interactions with the large communities of resident luminal bacteria. Epithelial cells recognize the bacterial components via pattern-recognition receptors. Toll-like receptors (TLRs) are a major class of pattern-recognition receptors that are present on intestinal epithelial cells, including putative stem cells. Stem cells are responsible for tissue homeostasis and regeneration after injury including IBD. Stem cells are also implicated in the pathogenesis of CRC. In susceptible individuals, disruption of normal homeostatic balance between the host's mucosal cells and enteric microflora is believed to result in aberrant immune responses against the resident commensal bacteria, leading to IBD. Microbiological analyses have revealed that the composition and localization of microbiota is altered in CRC and IBD. It is plausible that stem cells directly sense and respond to microbiota. This review aims to summarize the current knowledge on the effect of microbiota and TLR signaling on intestinal stem cells. It also describes how TLR signaling could affect the stem cell regulatory pathways.

Random chromosome segregation in mouse intestinal epithelial stem cells

The mammalian intestinal epithelium is endowed with a high cell turnover sustained by a few stem cells located in the bottoms of millions of crypts. Until recently, it was generally assumed that the extreme sensitivity to DNA damaging agents leading to cell death and the asymmetric mode of chromosome segregation of intestinal epithelial stem cells prevented the illicit survival of mutated stem cells and guarded against mistakes leading to aneuploidy and neoplastic transformation. Recent evidence points instead to a pool of mutipotent self-renewing stem cells capable of repairing DNA by homologous recombination significantly more efficiently than other crypt cells. Furthermore, the equilibrium between cell division and differentiation is achieved at the level of the cell population obeying to a random mode of chromosome segregation and a predominantly symmetric mode of cell division. This review summarizes the experimental findings on the mode of cell division adopted by intestinal epithelial stem cells.

Small intestinal stem cells

PURPOSE OF REVIEW

To summarize our current understanding of small intestinal stem cell biology and the current tools available for studying intestinal stem cells (ISCs).

RECENT FINDINGS

Recent reviews and original reports point toward the presence of two distinct populations of stem cells (ISCs) within the intestinal crypts. Until recently, the study of these two populations has been hindered by the lack of biomarkers available for isolation and characterization of ISCs and the absence of suitable culture conditions for expansion of ISCs in vitro. With the accumulation of various surface markers and transgenic mouse models, we have been able to gain a better understanding of the genetic signature of ISCs. In addition, these tools have provided opportunities to begin to study how ISCs are influenced by the various components of the ISC niche, including fibroblasts, bacteria, lymphoid cells, and Paneth cells. Advances in culture conditions now allow for the establishment of in-vitro studies of ISC function and dynamics.

SUMMARY

This brief review provides a general historical perspective of our understanding of the delineation of the two ISC populations. Furthermore, it discusses the known ISC markers and how these markers have been used to isolate and characterize ISC populations.