Label-Retaining, Putative Mesenchymal Stem Cells Contribute to Murine Myometrial Repair During Uterine Involution

Uterine remodeling during pregnancy is a fundamental, dynamic process required for successful propagation of eutherian species. The uterus can increase in size up to 40-fold during pregnancy, which is largely attributed to expansion of the myometrium by hyperplasia and hypertrophy. After pregnancy, the uterus repairs the remodeled or "damaged" tissue during uterine involution (INV). Little is known about this repair process, particularly the role of mesenchymal stem/progenitor cells. The objective of this study was to identify and characterize putative mesenchymal stem/progenitor cells in the murine myometrium using a combination of label retention and mesenchymal stem cell (MSC) marker expression and a pregnancy and uterine INV model. Tet-off transgenic mice with the Cre-lox system were used to specifically label mesenchymal cells (ie, myometrial and endometrial stromal cells) within the uterus while avoiding other cell types (eg, epithelial, immune, and endothelial cells) to identify slowly dividing cells and assess their stem cell qualities. We identified myometrial label-retaining cells (LRCs) that persisted for at least 3 months, expressed CD146 and CD140b (MSC markers), and proliferated at a higher rate during uterine INV compared with nonlabeled cells. The LRCs did not appear to express either estrogen receptor alpha or progesterone receptor, nor did the number of LRCs change at different estrous stages or in response to exogenous estradiol or progesterone administration, suggesting that LRCs were not involved in normal estrous cycling. The results from this study provide important insight into putative stem/progenitor cells in the myometrium and their possible role in uterine physiology.

Histone 2B-GFP Label-Retaining Prostate Luminal Cells Possess Progenitor Cell Properties and Are Intrinsically Resistant to Castration

The existence of slow-cycling luminal cells in the prostate has been suggested, but their identity and functional properties remain unknown. Using a bigenic mouse model to earmark, isolate, and characterize the quiescent stem-like cells, we identify a label-retaining cell (LRC) population in the luminal cell layer as luminal progenitors. Molecular and biological characterizations show that these luminal LRCs are significantly enriched in the mouse proximal prostate, exhibit relative dormancy, display bipotency in both in vitro and in vivo assays, and express a stem/progenitor gene signature with resemblance to aggressive prostate cancer. Importantly, these LRCs, compared with bulk luminal cells, maintain a lower level of androgen receptor (AR) expression and are less androgen dependent and also castration resistant in vivo. Finally, analysis of phenotypic markers reveals heterogeneity within the luminal progenitor cell pool. Our study establishes luminal LRCs as progenitors that may serve as a cellular origin for castration-resistant prostate cancer.

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A bigenic mouse model to study prostatic slow-cycling luminal epithelial cells

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Prostate label-retaining cells (LRCs) exhibit stem/progenitor cell activities

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Luminal LRCs are developmentally bipotent and display a progenitor gene signature

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Luminal LRCs resist castration and molecularly resemble aggressive prostate cancer

Distinct populations of label-retaining cells in the adult kidney are defined temporally and exhibit divergent regional distributions

DNA label-retention, or retention of a thymidine analog, is a characteristic of slow cycling cells and has been used to identify stem cells in several organ systems. Recent findings have demonstrated inconsistent localization of label-retaining cells (LRCs) in the kidney. Differences in the dose and timing of administration of deoxyuridine, the length of the chase period, and the species of animal used have made understanding the distinctions between these findings difficult. In the present studies, we utilized a dual loading scheme in the same animal to demonstrate that the cells labeled at different ages identified independent populations of LRC that distributed globally in an anti-parallel manner in the kidney. Loading with a DNA label in neonates identified LRC more often in the papilla, while administering the DNA label in adult mice identified LRC prominently in the cortex and the outer medulla. Furthermore, the tissue compartment distribution (epithelial-endothelial-interstitial) as well as the specific distribution within the nephron epithelia differed for these populations. These findings highlighted the complexity of the dynamics of cell proliferation in the kidney throughout the postnatal and adult period and call attention to the confusion associated with the term "label-retaining cells" for different timings of the loading and chase periods. This study indicated that the results of previous studies should be viewed as nonoverlapping and that further studies are needed to ascertain the role of each of these populations in the steady-state maintenance and injury recovery of the kidney.

Label-retaining, quiescent globose basal cells are found in the olfactory epithelium

The vertebrate olfactory epithelium (OE) is known for its ability to renew itself throughout life as well as to reconstitute after injury. Although this remarkable capacity demonstrates the persistence of stem cells and multipotent progenitor cells, their nature in the OE remains undefined and controversial, as both horizontal basal cells (HBCs) and globose basal cells (GBCs) have features in common with each other and with stem cells in other tissues. Here, we investigate whether some among the population of GBCs satisfy a key feature of stem cells, i.e., mitotic quiescence with retention of thymidine analogue label and activation by injury. Accordingly, we demonstrate that some GBCs express p27(Kip1) , a member of the Kip/Cip family of cyclin-dependent kinase inhibitors. In addition, some GBCs retain bromodeoxyuridine or ethynyldeoxyuridine for an extended period when the pulse is administered in neonates followed by a 1-month chase. Their identity as GBCs was confirmed by electron microscopy. All spared GBCs express Ki-67 in the methyl bromide (MeBr)-lesioned OE initially after lesion, indicating that the label-retaining (LR) GBCs are activated in response to injury. LR-GBCs reappear during the acute recovery period following MeBr exposure, as demonstrated with 2- or 4-week chase periods after labeling. Taken together, our data demonstrate the existence of LR-GBCs that are seemingly activated in response to epithelial injury and then re-established after the initial phase of recovery is completed. In this regard, some among the GBCs satisfy a common criterion for functioning like stem cells.

Long-term label retaining cells localize to distinct regions within the female reproductive epithelium

The uterus is an extremely plastic organ that undergoes cyclical remodeling including endometrial regeneration during the menstrual cycle. Endometrial remodeling and regeneration also occur during pregnancy and following parturition, particularly in hemochorial implanting species. The mechanisms of endometrial regeneration are not well understood. Endometrial stem/progenitor cells are proposed to contribute to endometrial regeneration in both humans and mice. BrdU label retention has been used to identify potential stem/progenitor cells in mouse endometrium. However, methods are not available to isolate BrdU label-retaining cells (LRC) for functional analyses. Therefore, we employed a transgenic mouse model to identify H2B-GFP LRCs throughout the female reproductive tract with particular interest on the endometrium. We hypothesized that the female reproductive tract contains a population of long-term LRCs that persist even following pregnancy and endometrial regeneration. Endometrial cells were labeled (pulsed) either transplacentally/translactationally or peripubertally. When mice were pulsed transplacentally/translactationally, the label was not retained in the uterus. However, LRCs were concentrated to the distal oviduct and endocervical transition zone (TZ) following natural (i.e., pregnancy/parturition induced) and mechanically induced endometrial regeneration. LRCs in the distal oviduct and endocervical TZ expressed stem cell markers and did not express ERα or PGR, implying the undifferentiated phenotype of these cells. Oviduct and endocervical TZ LRCs did not proliferate during endometrial re-epithelialization, suggesting that they do not contribute to the endometrium in a stem/progenitor cell capacity. In contrast, when mice were pulsed peripubertally long-term LRCs were identified in the endometrial glandular compartment in mice as far out as 9 months post-pulse. These findings suggest that epithelial tissue of the female reproductive tract contains 3 distinct populations of epithelial cells that exhibit stem/progenitor cell qualities. Distinct stem/progenitor-like cells localize to the oviduct, endometrium, and cervix.

Wnt signaling in adult intestinal stem cells and cancer

Signaling initiated by secreted glycoproteins of the Wnt family regulates many aspects of embryonic development and it is involved in homeostasis of adult tissues. In the gastrointestinal (GI) tract the Wnt pathway maintains the self-renewal capacity of epithelial stem cells. The stem cell attributes are conferred by mutual interactions of the stem cell with its local microenvironment, the stem cell niche. The niche ensures that the threshold of Wnt signaling in the stem cell is kept in physiological range. In addition, the Wnt pathway involves various feedback loops that balance the opposing processes of cell proliferation and differentiation. Today, we have compelling evidence that mutations causing aberrant activation of the Wnt pathway promote expansion of undifferentiated progenitors and lead to cancer. The review summarizes recent advances in characterization of adult epithelial stem cells in the gut. We mainly focus on discoveries related to molecular mechanisms regulating the output of the Wnt pathway. Moreover, we present novel experimental approaches utilized to investigate the epithelial cell signaling circuitry in vivo and in vitro. Pivotal aspects of tissue homeostasis are often deduced from studies of tumor cells; therefore, we also discuss some latest results gleaned from the deep genome sequencing studies of human carcinomas of the colon and rectum.