Continuous clonal labeling reveals small numbers of functional stem cells in intestinal crypts and adenomas

Mammary stem cells and the differentiation hierarchy: current status and perspectives

Based on transplantation and lineage tracing studies, a hierarchy of stem and progenitor cells has been shown to exist among the mammary epithelium. In this review, Visvader and Stingl integrate recent data on the mammary stem cell differentiation hierarchy and its control at the transcriptional and epigenetic levels. They also discuss the relevance of the evolving hierarchy to the identification of “cells of origin” of breast cancer.

The mammary epithelium is highly responsive to local and systemic signals, which orchestrate morphogenesis of the ductal tree during puberty and pregnancy. Based on transplantation and lineage tracing studies, a hierarchy of stem and progenitor cells has been shown to exist among the mammary epithelium. Lineage tracing has highlighted the existence of bipotent mammary stem cells (MaSCs) in situ as well as long-lived unipotent cells that drive morphogenesis and homeostasis of the ductal tree. Moreover, there is accumulating evidence for a heterogeneous MaSC compartment comprising fetal MaSCs, slow-cycling cells, and both long-term and short-term repopulating cells. In parallel, diverse luminal progenitor subtypes have been identified in mouse and human mammary tissue. Elucidation of the normal cellular hierarchy is an important step toward understanding the “cells of origin” and molecular perturbations that drive breast cancer.

Stem cell dynamics in homeostasis and cancer of the intestine

Intestinal stem cells (ISCs) and colorectal cancer (CRC) biology are tightly linked in many aspects. It is generally thought that ISCs are the cells of origin for a large proportion of CRCs and crucial ISC-associated signalling pathways are often affected in CRCs. Moreover, CRCs are thought to retain a cellular hierarchy that is reminiscent of the intestinal epithelium. Recent studies offer quantitative insights into the dynamics of ISC behaviour that govern homeostasis and thereby provide the necessary baseline parameters to begin to apply these analyses during the various stages of tumour development.

Stem cell plasticity. Plasticity of epithelial stem cells in tissue regeneration

Tissues rely upon stem cells for homeostasis and repair. Recent studies show that the fate and multilineage potential of epithelial stem cells can change depending on whether a stem cell exists within its resident niche and responds to normal tissue homeostasis, whether it is mobilized to repair a wound, or whether it is taken from its niche and challenged to de novo tissue morphogenesis after transplantation. In this Review, we discuss how different populations of naturally lineage-restricted stem cells and committed progenitors can display remarkable plasticity and reversibility and reacquire long-term self-renewing capacities and multilineage differentiation potential during physiological and regenerative conditions. We also discuss the implications of cellular plasticity for regenerative medicine and for cancer.

Rethinking differentiation: stem cells, regeneration, and plasticity

Cell differentiation is an essential process for the development, growth, reproduction, and longevity of all multicellular organisms, and its regulation has been the focus of intense investigation for the past four decades. The study of natural and induced stem cells has ushered an age of re-examination of what it means to be a stem or a differentiated cell. Past and recent discoveries in plants and animals, as well as novel experimental manipulations, are beginning to erode many of these established concepts and are forcing a re-evaluation of the experimental systems and paradigms presently being used to explore these and other biological process.

Metastatic stem cells: sources, niches, and vital pathways

Metastasis is powered by disseminated cancer cells that re-create a full-fledged tumor in unwelcoming tissues, away from the primary site. How cancer cells moving from a tumor into the circulation manage to infiltrate distant organs and initiate metastatic growth is of interest to cancer biologists and clinical oncologists alike. Recent findings have started to define the sources, phenotypic properties, hosting niches, and signaling pathways that support the survival, self-renewal, dormancy, and reactivation of cancer cells that initiate metastasis: metastatic stem cells. By dissecting the biology of this process, vulnerabilities are being exposed that could be exploited to prevent metastasis.

Unraveling intestinal stem cell behavior with models of crypt dynamics

The definition, regulation and function of intestinal stem cells (ISCs) has been hotly debated. Recent discoveries have started to clarify the nature of ISCs, but many questions remain. This review discusses the current advances and controversies of ISC biology as well as theoretical compartmental models that have been coupled with in vivo experimentation to investigate the mechanisms of ISC dynamics during homeostasis, tumorigenesis, repair and development. We conclude our review by discussing the key lingering questions in the field and proposing how many of these questions can be addressed using both compartmental models and experimental techniques.

Epithelial stem cells and intestinal cancer

The mammalian intestine is comprised of an epithelial layer that serves multiple functions in order to maintain digestive activity as well as intestinal homeostasis. This epithelial layer contains highly proliferative stem cells which facilitate its characteristic rapid regeneration. How these stem cells contribute to tissue repair and normal homeostasis are actively studied, and while we have a greater understanding of the molecular mechanisms and cellular locations that underlie stem cell regulation in this tissue, much still remains undiscovered. This review describes epithelial stem cells in both intestinal and non-intestinal tissues, as well as the strategies that have been used to further characterize the cells. Through a discussion of the current understanding of intestinal self-renewal and tissue regeneration in response to injury, we focus on how dysregulation of critical signaling pathways results in potentially oncogenic aberrations, and highlight issues that should be addressed in order for effective intestinal cancer therapies to be devised.

Intestinal crypt homeostasis revealed at single-stem-cell level by in vivo live imaging

The rapid turnover of the mammalian intestinal epithelium is supported by stem cells located around the base of the crypt¹. Alongside Lgr5, intestinal stem cells have been associated with various markers, which are expressed heterogeneously within the crypt base region^1-6. Previous quantitative clonal fate analyses have proposed that homeostasis occurs as the consequence of neutral competition between dividing stem cells^7-9. However, the short-term behaviour of individual Lgr5⁺ cells positioned at different locations within the crypt base compartment has not been resolved. Here, we established the short-term dynamics of intestinal stem cells using a novel approach of continuous intravital imaging of Lgr5-Confetti mice. We find that Lgr5⁺ cells in the upper part of the niche (termed ‘border cells’) can be passively displaced into the transit-amplifying (TA) domain, following division of proximate cells, implying that determination of stem cell fate can be uncoupled from division. Through the quantitative analysis of individual clonal lineages, we show that stem cells at the crypt base, termed ‘central cells’, experience a survival advantage over border stem cells. However, through the transfer of stem cells between the border and central regions, all Lgr5⁺ cells are endowed with long-term self-renewal potential. These findings establish a novel paradigm for stem cell maintenance in which a dynamically heterogeneous cell population is able to function long-term as a single stem cell pool.

Biased competition between Lgr5 intestinal stem cells driven by oncogenic mutation induces clonal expansion

The concept of ‘field cancerization’ describes the clonal expansion of genetically altered, but morphologically normal cells that predisposes a tissue to cancer development. Here, we demonstrate that biased stem cell competition in the mouse small intestine can initiate the expansion of such clones. We quantitatively analyze how the activation of oncogenic K-ras in individual Lgr5⁺ stem cells accelerates their cell division rate and creates a biased drift towards crypt clonality. K-ras mutant crypts then clonally expand within the epithelium through enhanced crypt fission, which distributes the existing Paneth cell niche over the two new crypts. Thus, an unequal competition between wild-type and mutant intestinal stem cells initiates a biased drift that leads to the clonal expansion of crypts carrying oncogenic mutations.

Adult intestinal stem cells: critical drivers of epithelial homeostasis and regeneration

Small populations of adult stem cells are responsible for the remarkable ability of the epithelial lining of the intestine to be efficiently renewed and repaired throughout life. The recent discovery of specific markers for these stem cells, together with the development of new technologies to track endogenous stem cell activity in vivo and to exploit their ability to generate new epithelia ex vivo, has greatly improved our understanding of stem cell-driven homeostasis, regeneration and cancer in the intestine. These exciting new insights into the biology of intestinal stem cells have the potential to accelerate the development of stem cell-based therapies and ameliorate cancer treatments.

A calibrated agent-based computer model of stochastic cell dynamics in normal human colon crypts useful for in silico experiments

BACKGROUND

Normal colon crypts consist of stem cells, proliferating cells, and differentiated cells. Abnormal rates of proliferation and differentiation can initiate colon cancer. We have measured the variation in the number of each of these cell types in multiple crypts in normal human biopsy specimens. This has provided the opportunity to produce a calibrated computational model that simulates cell dynamics in normal human crypts, and by changing model parameter values, to simulate the initiation and treatment of colon cancer.

RESULTS

An agent-based model of stochastic cell dynamics in human colon crypts was developed in the multi-platform open-source application NetLogo. It was assumed that each cell's probability of proliferation and probability of death is determined by its position in two gradients along the crypt axis, a divide gradient and in a die gradient. A cell's type is not intrinsic, but rather is determined by its position in the divide gradient. Cell types are dynamic, plastic, and inter-convertible. Parameter values were determined for the shape of each of the gradients, and for a cell's response to the gradients. This was done by parameter sweeps that indicated the values that reproduced the measured number and variation of each cell type, and produced quasi-stationary stochastic dynamics. The behavior of the model was verified by its ability to reproduce the experimentally observed monocolonal conversion by neutral drift, the formation of adenomas resulting from mutations either at the top or bottom of the crypt, and by the robust ability of crypts to recover from perturbation by cytotoxic agents. One use of the virtual crypt model was demonstrated by evaluating different cancer chemotherapy and radiation scheduling protocols.

CONCLUSIONS

A virtual crypt has been developed that simulates the quasi-stationary stochastic cell dynamics of normal human colon crypts. It is unique in that it has been calibrated with measurements of human biopsy specimens, and it can simulate the variation of cell types in addition to the average number of each cell type. The utility of the model was demonstrated with in silico experiments that evaluated cancer therapy protocols. The model is available for others to conduct additional experiments.