It would be interesting to know how stem cells receive niche signals and effectively integrate these signals into homeostasis. On the other hand, there are also physical signals that might help regulate intestinal stem cell (ISC) behavior including biochemical signals inherent to the stem cell niche. Stem cells sense mechanical signals and respond to them via the concerted activation of various signaling pathways. This mechano-transduction phenomenon leads to cell shape, gene expression, and even cell fate changes.
The two most important mechanosensitive signaling receptors for PIEZO ion channels are PIEZO1 and PIEZO2. These channels then open in response to mechanical stimuli to allow calcium entry into the cell and activate downstream signaling pathways. Piezo1 and Piezo2 are expressed very highly in mammalian intestinal epithelial cells, making PIEZO channels apparent candidates for mechano-transduction of extrinsic signals in ISCs.
To determine the specific roles of PIEZO channels in ISCs, authors conducted conditional genetic ablation of Piezo genes in the intestine in vivo. Double knockout of Piezo1 and Piezo2 resulted in stem loss and subsequent rapid lethality in mice. Single-cell transcriptome analysis of intestinal crypts isolated from Piezo knockout mice showed that ISCs lose stemness and acquire a highly proliferative phenotype that leads to their depletion. At the same time, pre-existing multipotent transit amplifying cells preferentially differentiate into absorptive progenitors causing secretory cell deficiency.
For clarification of the underlying mechanisms, authors employed single-cell RNA sequencing analysis combined with 3D organoid culture, which led to the observation that PIEZO activity blocking causes hyperactivation of NOTCH and suppression of WNT signaling. They defined different upstream extracellular mechanical stimuli activating PIEZO channels in ISCs.
Atomic force microscopy measurements of basement membrane stiffness in mouse small intestine showed that, at the bottom of the crypt, ISCs experience a more rigid microenvironment compared with cells at the top. 2D organoid monolayer culture on bioengineered substrates with varying rigidities supports that stiffness regulates ISC behavior through PIEZO. Similarly dimensional monolayers mounted on stretching devices to modulate tissue tensions revealed stretching as an enhancer of ISC number.
With this research, the author introduces a comprehensive model that deals with the PIEZO mechanosensing of the ISC niche. Conditional epithelial cell knockout of PIEZO is sufficient to cause defects in self-renewal and lineage specification, eventually resulting in the loss of ISCs and secretory cells. Mechanistically, PIEZO activation because of the niche stiffness and/or tension brings about an intracellular influx of Ca2+, eventuating in repression of the NOTCH pathway to induce secretory cell differentiation, and modulating WNT signaling to maintain an adequate balance between self-renewal and proliferation. Thus, these data show that the internal microenvironment conditions ISCs if the level of difficulty and availability of nutrients changes through PIEZO mechanosensitive channels to self-renew and lineage decisions about biomechanical properties of the stem cell niche.
Reference: Baghdadi MB, Houtekamer RM, Perrin L, et al. PIEZO-dependent mechanosensing is essential for intestinal stem cell fate decision and maintenance. Science. 2024;386(6725):eadj761. doi:10.1126/science.adj761
