BEST4⁺ cells: a potential hub of intestinal ion transport and diarrhea manipulation.

BEST4⁺ cells are a recently identified, functionally specialized intestinal epithelial cell population characterized through single-cell and spatial transcriptomic analyses in humans, pigs, rats, and other vertebrates. These cells characteristically express BEST4, OTOP2, CA7, and GUCY2C, and exhibit particularly high CFTR expression in the small intestine, supporting their roles in luminal pH sensing, fluid-electrolyte homeostasis, and mucus hydration. Emerging evidence from organoid and cross-species studies supports a predominant role for the NOTCH-SPIB signaling axis in driving BEST4⁺ cell differentiation, but their lineage identity and regulatory mechanisms remain incompletely resolved. This review focuses on intestinal epithelial BEST4⁺ cells as a critical cellular hub that connects ion transport physiology with the pathophysiology of secretory diarrhea. We summarize the molecular identity, regional distribution, and cross-species conservation of BEST4⁺ cells. We then evaluate their roles in luminal pH regulation, electrogenic fluid secretion, and mucus barrier integrity. We detail how bacterial enterotoxins activate the GC-C/cGMP/CFTR and cAMP/PKA/CFTR pathways in BEST4⁺ cells to drive pathological fluid hypersecretion, and discuss how viral infections may indirectly engage or amplify BEST4⁺ cell-associated ion-transport pathways. We also outline their relevance to viral diarrhea, inflammatory bowel disease, and cystic fibrosis-associated intestinal dysfunction. Finally, we assess established antidiarrheal approaches and investigational strategies that modulate BEST4⁺ cell-associated pathways, and emphasize the need for cell-type-specific validation in physiologically relevant in vivo models.