Subepithelial Stromal Cells: Their Roles and Interactions with Intestinal Epithelial Cells during Gut Mucosal Homeostasis and Regeneration

Abstract

Intestinal epithelial cell activities during homeostasis and regeneration are well described, but their potential interactions with stromal cells remain unresolved. Exploring the functions of these heterogeneous intestinal mesenchymal stromal cells (iMSCs) remains challenging. This difficulty is due to the lack of specific markers for most functionally homogenous subpopulations. In recent years, however, novel clustering techniques such as single-cell RNA sequencing (scRNA-seq), fluorescence-activated cell sorting (FACS), confocal microscope, and computational remodeling of intestinal anatomy have helped identify and characterize some specific iMSC subsets. These methods help researchers learn more about the localization and functions of iMSC populations during intestinal morphogenic and homeostatic conditions. Consequently, it is imperative to understand the cellular pathways that regulate their activation and how they interact with surrounding cellular components, particularly during intestinal epithelial regeneration after mucosal injury. This review provides insights into the spatial distribution and functions of identified iMSC subtypes. It focuses on their involvement in intestinal morphogenesis, homeostasis, and regeneration. We reviewed related signaling mechanisms implicated during epithelial and subepithelial stromal cell crosstalk. Future research should focus on elucidating the molecular intermediates of these regulatory pathways to open a new frontier for potential therapeutic targets that can alleviate intestinal mucosa-related injuries.

Keywords: epithelial–mesenchymal interactions; intestinal stem-cell niche; mesenchymal stromal cells; subepithelial gradient factors.

Figure 1

Multiple signaling pathways regulating intestinal epithelial–mesenchymal crosstalk in the crypt. The diagram provides an overview of the intricate mechanisms involved in the interactions between crypt-based epithelial cells and the neighboring iMSCs. This interplay controls intestinal stem cell (ISC) homeostasis and epithelial differentiation. The activation of Wnt-promoting pathways and inhibition of Bmp/Bmpr binding orchestrate ISC stemness in the crypt base. Paneth cells secrete Wnt, Notch, and EGF ligands that induce Wnt target gene transcription in ISC. Wnt ligands secreted by subcryptal iMSCs bind FZD and LRPs co-receptors. The ligand–receptor binding stabilized β-catenin in the Wnt signaling cascade to promote ISC-related gene transcription. Rspos binding LGR family receptors stabilized FZD expressions, contributing to WNT pathway activation. In addition, subcryptal iMSCs secrete Bmp antagonists such as Gremlin to maintain Wnt activities in the ISC niche.

Figure 2

Intestinal villus base epithelial-secreted ligands activate hedgehog signaling in neighboring mesenchymal cells. These epithelial–mesenchymal interactions at the villus base stimulate intestinal epithelial cell terminal differentiation and coordinate the migration of differentiated cells toward the villus tip. Hedgehog ligands produced by epithelial cells bind on the Pitch1 receptor of PDGFRα^hi cells located at the villus base. This binding triggers the transcription of hedgehog target genes. The genes transcribed by the Gli family of PDGFRα^hi cells include Bmps. Secreted Bmp ligands from these iMSCs bind with the Bmp receptor (Bmpr) on adjacent epithelial cells. The Bmp and Bmpr binding subsequently phosphorylate the Smad family transcription factor, which in turn induces the differentiation of villus epithelial cells.

Figure 3

Schematic representation of intestinal mesenchymal stromal cell subsets and their location. The illustration here describes the subpopulations of PDGFRα-expressing cells, including telocytes, CD81⁻ cells, and trophocytes. The trophocyte cluster group, CD81⁺, is confined to the subcryptal domain, beneath the muscularis mucosa, to secrete Wnt-promoting factors that support the ISC niche. Other PDGFRα^lo subsets, including CD55^hi and Fgfr2⁺ cells, localize in the lamina propria and extend upward to the TA domain/villi trunk to initiate terminal differentiation. They switch the signal gradients from Wnt-promoting factors to Bmp agonists. PDGFRα^hi subsets form the subepithelial stromal populations that are localized in the villi core in the small intestine and the colon top [38,39].