Defining hierarchies of stemness in the intestine: evidence from biomarkers and regulatory pathways

Abstract

For decades, the rapid proliferation and well-defined cellular lineages of the small intestinal epithelium have driven an interest in the biology of the intestinal stem cells (ISCs) and progenitors that produce the functional cells of the epithelium. Recent and significant advances in ISC biomarker discovery have established the small intestinal epithelium as a powerful model system for studying general paradigms in somatic stem cell biology and facilitated elegant genetic and functional studies of stemness in the intestine. However, this newfound wealth of ISC biomarkers raises important questions of marker specificity. Furthermore, the ISC field must now begin to reconcile biomarker status with functional stemness, a challenge that is made more complex by emerging evidence that cellular hierarchies in the intestinal epithelium are more plastic than previously imagined, with some progenitor populations capable of dedifferentiating and functioning as ISCs following damage. In this review, we discuss the state of the ISC field in terms of biomarkers, tissue dynamics, and cellular hierarchies, and how these processes might be informed by earlier studies into signaling networks in the small intestine.

Keywords: Bmp signaling; Wnt signaling; cell fate; differentiation; intestinal stem cells.

Fig. 1.

Structure and anatomy of the intestinal epithelium. A: the intestinal lumen is lined by a monolayer of columnar epithelial cells, the intestinal epithelium, which is responsible for critical nutrient processing and barrier function. B: the intestine is a complex, multilayered tissue composed of epithelium and its supporting structures, the lamina propria and muscularis, all derived from distinct cellular origins. C: the intestinal epithelium forms repeating microanatomical units consisting of crypts and villi. New epithelial cells are produced by ISCs in the base of the intestinal crypts at a rapid rate under normal conditions. These cells then undergo multiple rounds of differentiation and lineage specification as they migrate toward the villus tip, where they are sloughed off after ∼7–10 days and undergo programmed cell death, called anoikis.

Fig. 2.

The intestinal epithelium consists of absorptive and secretory lineages. A: intestinal stem cells (ISCs) give rise to absorptive progenitor cells, which are characterized by an active Notch-pathway and produce functional absorptive enterocytes, which are identified by their microvillus brush borders and expression of digestive enzymes, such as sucrase isomaltase (Sim) and lactase (Lct). B: alternatively, progenitor cells can adopt an early, unspecified secretory state and undergo multiple rounds of division before a reduction in Notch-signaling promotes differentiation into Paneth, enteroendocrine, or goblet cells. The origin of Tuft and M cell lineages remains controversial, but the former is thought to arise from secretory progenitors, and both are known to derive from ISCs (orange, dashed arrows). While traditional models of cellular hierarchies in the intestine propose a linear model (in which multipotency is exclusive to the ISCs and lineage potential becomes more restricted as cells pass through progenitor stages), emerging evidence suggests a considerable degree of plasticity in secretory progenitors, which can dedifferentiate to function as ISCs, and even between early absorptive and secretory progenitors, which can switch lineage commitment based on trans-acting factors (blue arrows).

Fig. 3.

Evolution of ISC models in the intestinal crypt. A: the intestinal crypt was originally thought to contain one or two populations of cells with ISC potential: the cycling crypt base columnar cell (CBC) population, and the label-retaining +4 population. B: advances in ISC biomarker discovery and the development of mouse models for in vivo lineage tracing led to a model where Lgr5^High CBCs function as active ISCs, and Bmi1 expressing +4 cells function as reserve ISCs. C: however, emerging data suggest that potency is shared more broadly across stem and progenitor populations in the crypt, with some secretory progenitor populations, identified by high levels of Dll1 or Sox9 expression, or label retention, capable of functioning as facultative ISCs that convert to active ISCs (gray arrows) following damage.

Fig. 4.

Invariant asymmetry and population asymmetry as models of ISC-driven tissue dynamics. A: conceptually, one can model clonal behavior of ISCs by imagining each ISC and its progeny being distinctly labeled using multicolor fluorescent lineage tracing technology, which is facilitated by R26-Confetti reporter mice. B: under the classical model of invariant asymmetry, each ISC produces one differentiated daughter cell and one ISC with every round of division. In this model, clones and their progeny (represented by a single color) expand at an equal rate and occupy roughly the same tissue area. Homeostasis is maintained in the absence of mutations, because all ISCs behave the same in regard to proliferation and differentiation. C: under population asymmetry, asymmetric or symmetric cell fate decisions by ISCs occur on a stochastic basis. Because some ISCs produce two ISC or two differentiated daughters, clone numbers decrease over time due to the loss of self-renewing ISCs to a symmetric division event that results in two progenitors. The impact of this is that clone sizes become increasingly divergent over time, with crypts eventually drifting to clonality. In this model, homeostasis is maintained in the absence of mutations due to uncharacterized mechanisms that balance overall proliferation and differentiation at the tissue level.