Wnt pathway regulation of intestinal stem cells

Abstract

Wnt signalling is involved in multiple aspects of embryonic development and adult tissue homeostasis, notably via controlling cellular proliferation and differentiation. Wnt signalling is subject to stringent positive and negative regulation to promote proper development and homeostasis yet avoid aberrant growth. Such multi-layer regulation includes post-translational modification and processing of Wnt proteins themselves, R-spondin (Rspo) amplification of Wnt signalling, diverse receptor families, and intracellular and extracellular antagonists and destruction and transcription complexes. In the gastrointestinal tract, Wnt signalling is crucial for development and renewal of the intestinal epithelium. Intestinal stem cells (ISCs) undergo symmetric division and neutral drift dynamics to renew the intestinal epithelium. Sources of Wnts and Wnt amplifers such as R-spondins are beginning to be elucidated as well as their functional contribution to intestinal homeostasis. In this review we focus on regulation of ISCs and intestinal homeostasis by the Wnt/Rspo pathway, the potential cellular sources of Wnt signalling regulators and highlight potential future areas of study.

Figure 1. Overview of the canonical Wnt pathway and points of regulation

Wnts bind to their receptors Fzd/Lrp5/6, recruit Dvl and block the destruction complex from degrading cytosolic β‐catenin. Stabilized β‐catenin translocates to the nucleus where it interacts with LEF/TCF to activate Wnt target genes. Wnt antagonists such as Dkk, sFRPs, WIF1 and sclerostin block Wnt signalling by binding and blocking Wnt ligands or their receptors. Norrin and R‐spondins promote Wnt activity by binding or stabilizing membrane availability of Fzd/Lrp5/6. Pygo promotes Wnt signalling by activating transcription via interactions with the complex of proteins bound to the LEF/TCF transcription factors.

Figure 2. The intestinal epithelium contains two functionally distinct pools of intestinal stem cells (ISCs)

Crypt base columnar ISCs (green) and +4 quiescent ISCs (red) co‐exist in the intestinal crypt and differ in their cycling kinetics, sensitivity to extracellular Wnt pathway manipulations and radiation.

Figure 3. R‐spondins potently amplify Wnt signalling

The transmembrane E3 ubiquitin ligases Rnf43/Znrf3 negatively regulate Wnt signalling by ubiquitinating the cytoplasmic tails of Fzds to promote their membrane clearance by endocytosis and degradation, ultimately downregulating Wnt signalling. When R‐spondins (Rspo1–4) are present, Rnf43/Znrf3 mediated degradation of Fzds is inhibited and Fzd/Lrp accumulation on the plasma membrane upregulates Wnt signalling. In an independent mechanism, Rnf43 present on the nuclear membrane interacts with and prevents LEF/TCF‐mediated transcription.