Identifying key regulators of the intestinal stem cell niche

Abstract

The intestinal tract is lined by a single layer of epithelium that is one of the fastest regenerating tissues in the body and which therefore requires a very active and exquisitely controlled stem cell population. Rapid renewal of the epithelium is necessary to provide a continuous physical barrier from the intestinal luminal microenvironment that contains abundant microorganisms, whilst also ensuring an efficient surface for the absorption of dietary components. Specialised epithelial cell populations are important for the maintenance of intestinal homeostasis and are derived from adult intestinal stem cells (ISCs). Actively cycling ISCs divide by a neutral drift mechanism yielding either ISCs or transit-amplifying epithelial cells, the latter of which differentiate to become either absorptive lineages or to produce secretory factors that contribute further to intestinal barrier maintenance or signal to other cellular compartments. The mechanisms controlling ISC abundance, longevity and activity are regulated by several different cell populations and signalling pathways in the intestinal lamina propria which together form the ISC niche. However, the complexity of the ISC niche and communication mechanisms between its different components are only now starting to be unravelled with the assistance of intestinal organoid/enteroid/colonoid and single-cell imaging and sequencing technologies. This review explores the interaction between well-established and emerging ISC niche components, their impact on the intestinal epithelium in health and in the context of intestinal injury and highlights future directions and implications for this rapidly developing field.

Keywords: epithelial dynamics; gastrointestinal physiology; intestinal epithelial cells; intestinal homeostasis; intestinal stem cell niche; intestinal stem cells.

Figure 1.. The small intestinal epithelium forms crypt and villus structures overlying the lamina propria (left) whereas the colonic epithelium only has crypts (right).

Active cycling intestinal stem cells (ISCs) are located within the crypt base (red). Paneth cells (yellow) are only found in the small intestine. Under homeostatic conditions, ISCs divide via a neutral drift process to generate either two ISCs or two cells that enter the transit-amplifying population (purple) which maintain a limited capacity to divide prior to maturation into secretory or absorptive lineages [5,19]. Cells (other than ISCs and Paneth cells which are longer lived and remain in the crypt base) undergo a process of migration and differentiation along the crypt-villus axis (black arrow). Once epithelial cells reach the villus tip in the small intestine or inter-crypt table in the colon, they undergo apoptosis and are shed into the lumen. Neighbouring cells reform tight junctions beneath the shedding cell to ensure the intestinal epithelial barrier is maintained throughout the extrusion process [5,92,108]. The process from ISC division at the crypt base to apoptosis and cell shedding at the villus tip takes 3–5 days depending on species and location along the cephalocaudal axis [109].

Figure 2.. Mesenchymal cells form a continuous network along the crypt-villus axis.

Trophocyte, telocyte and sub-epithelial myofibroblasts (SEMF) are key cell populations that regulate signalling gradients along the crypt-villus axis to promote the ISC niche at the crypt base and differentiation programmes at higher cell positions. Trophocytes express CD81 and predominate towards the crypt base (blue) and telocytes/SEMFs span the crypt base to villus tip with higher abundance observed at the crypt-villus junction and villus tip (red/green). The expression patters of telocytes along the crypt-villus axis is non-uniform and due to the heterogeneity of this cell population identified from single cell sequencing, discrete sub-populations are currently difficult to identify. Telocytes express higher amounts of PDGFRα than trophocytes and a sub-population of Lgr5 expressing telocytes are located within the villus tip (red). Signalling gradients that contribute to ISC niche regulation are indicated.

Figure 3.. Canonical Wnt signalling is critical for ISC niche maintenance and is potentiated by Lgr5.

In the absence of Wnt ligands, β-catenin is held in a destruction complex that includes Dvl, Axin, Ck1, Gsk3 and Apc which results in β-catenin being targeted to the proteasome for degradation (A). When present, canonical Wnt ligands (eg Wnt3a) bind to frizzled receptors (Fzd) on proliferation-permissible cells which then recruit LRP6 and Dvl and release β-catenin from the destruction complex allowing β-catenin stabilisation and cytoplasmic accumulation. β-catenin can then translocate to the nucleus and binds to the T-cell factor/lymphoid enhancer factor (Tcf/Lef) transcription factor which results in the activation of a panel of genes critical for cell division [110]. R-spondins (Rspo) are important in potentiating this pathway by binding to Lgr5 and recruiting the Rnf43 ubiquitin ligase which targets the Lgr5–Rspo–Rnf43 complex for lysosomal degradation thus preventing Rnf43 from removing Fzd from the cell membrane and stabilising Fzd to enhance its availability for Wnt ligands [62] (B).

Figure 4.. Crosstalk between canonical and non-canonical Wnt, Hippo and Notch signalling pathways.

Non-canonical Wnt (eg Wnt 5a) bridges frizzled receptors with Ror to activate Rho kinase that prevents Lats1/2 inhibition of Yap1 resulting in the suppression of β-catenin nuclear translocation and canonical Wnt pathway inhibition. Hippo signalling is modulated by G protein coupled receptors (GPCR), receptor tyrosine kinases (RTK) and by cell-cell adhesion. When Hippo signalling is active, cytosolic Yap1 is also high and inhibits β-catenin translocation. When Hippo signalling is supressed, Yap1 can translocate to the nucleus and binds to TEAD transcription factors which results in the production of non-canonical Wnt 5a, Bmp4 and Dkk1 and also stimulates the nuclear translocation of β-catenin, thus activating canonical Wnt signalling. Nuclear Yap1 also stimulates the activation of Notch signalling by promoting the cleavage of the Notch intracellular domain (NICD) which can bind the CSL transcription factor in the nucleus resulting in production of Hes1. Hes1 represses Atoh1 and modulates whether cells become secretory or absorptive populations.