EECs

EECs are a heterogeneous group of specialised epithelial cells in the gastrointestinal tract. They are commonly classified according to their predominant hormone secretion and physiological function. Landmark studies, such as Haber et al., revealed a continuum of EEC transcriptional states, supporting a “hormone gradient” model rather than the traditional dogma of discrete subtypes [1-3], although the traditional nomenclature remains useful for functional classification and communication.

Transcriptomic Atlases of EECs

Single-cell RNA sequencing (scRNA-seq) has revolutionized our understanding of EECs, moving beyond traditional immunohistochemistry, which is limited in detecting hormone co-expression. Human scRNA-seq atlases highlight key principles: (i) EEC identity is best described by transcriptional gradients along crypt-villus and proximal-distal axes; (ii) mouse and human EECs share broad features but show species-specific differences in subtype distribution and hormone co-expression; (iii) EECs are rare (~1% of epithelium), requiring deep or enriched sequencing; (iv) EEC identity appears dynamic, influenced by nutrients, inflammation, and microbiota-derived metabolites. These atlases provide a crucial framework for functional studies and therapeutic target discovery.

Key Model Systems

Mechanistic studies of EEC biology rely on models that capture epithelial diversity, hormone secretion, and the intestinal environment [4]:

• Intestinal organoids: 3D structures from Lgr5⁺ stem cells containing all epithelial types. Can be directed via Notch/Wnt pathways to enrich EEC subtypes. Standard organoids have closed lumens, limiting apical access; microinjection or monolayers are often needed. Microfluidic and organ-on-chip systems allow controlled luminal flow, apical–basolateral access, and integration of additional cell types.
• Immortalised EEC lines: STC-1 (multi-hormone), GLUTag (colonic L-cells), and NCI-H716 (human L-cell surrogate). Easy to culture and suitable for high-throughput assays, but tumour-derived lines differ from primary EECs.
• iPSC-derived intestinal models: Patient-specific iPSCs differentiated into intestinal tissue containing EECs. Useful for disease modelling and genetic studies, scalable and reproducible but often immature.
• Ex vivo tissue (using chambers & explants): Fresh intestinal tissue preserves polarity, regional identity, and microenvironment, allowing controlled stimulation and hormone collection. Limited by short viability, requirement for fresh material, and lower throughput.
• Primary isolated EECs: Directly sorted from intestinal tissue. Fully mature and physiologically representative but low yield, short lifespan, and dependent on fresh tissue.
• Co-culture systems: EECs combined with neurons, immune cells, or other epithelial cells to study paracrine and neural interactions. Can be integrated with organoids or monolayers.
• Transgenic reporter animals: Mice expressing fluorescent/luminescent reporters under EEC promoters (e.g., GLP-1, CCK) for in vivo tracking, lineage tracing,calcium flux secretion assays and validation of in vitro

Gut-Brain Axis

It has recently been recognised that most major EEC subtypes present neuron-like extrusions known as neuropods. These neuropods express synaptic proteins and neurotransmitters, extending EEC function beyond classical hormone secretion and enabling them to serve as local signalling and neuroepithelial communication hubs [5]. Through neuropod-mediated signalling, EECs can influence neighbouring mucosal cells, including other EECs and enterocytes, while also transmitting information directly to the nervous system. By synapsing with vagal afferents, neuropod cells connect the gut lumen to the brainstem, and transduce sensory stimuli from sugars using glutamate as a neurotransmitter [6]. Enterochromaffin cells use sensory receptors to detect irritants, metabolites, and catecholamines and modulate sensory nerves via serotonin secretion [7]. Moreover, neuropod cells can also modulate visceral pain and may constitute an important non-opioid target [8].

Gaps and Challenges

Research on EECs has advanced rapidly with the advent of modern high-throughput and single-cell technologies, yet several important gaps and challenges remain:

• Classification: Single-cell studies reveal that traditional functional classification (e.g., L, K, I cells) is now oversimplified, with many EECs co-expressing multiple hormones. Functional redundancy and overlapping signalling pathways complicate this.
• Low abundance and dispersed distribution: EECs are rare and scattered throughout the epithelium of the whole gastrointestinal tract, making them difficult to isolate, image, and manipulate in vivo. Capturing real-time signalling dynamics, particularly along the gut–brain axis, remains technically challenging.
• Interspecies differences: Much of our current understanding is derived from mouse models. Translating these findings to human physiology is challenging due to species differences in EEC distribution, signalling, and receptor expression.
• Sex differences: Males and females have distinct metabolic demands and consequently exhibit widespread differences in metabolic regulation. Further research is needed to elucidate sex differences in EEC and further our understanding how hormonal and genetic factors can shape EEC development, distribution, and secretory function.
• Microbiome–EEC interactions: EECs respond to microbial metabolites and byproducts, but the mechanisms linking specific microbes to EEC activation are still poorly understood. Moreover, links between specific microbial activity and systemic outcomes (e.g., appetite, insulin secretion, or behaviour) remain largely correlative rather than mechanistically defined.
• Senescence: Our understanding of EEC function during the natural ageing process is currently limited. Elucidating how these pathways change over the life course and influence age-related diseases may reveal promising therapeutic avenues.
• Pathology: Altered EEC number and secretion are linked to infection and inflammation, but their contribution to diseases such as IBD remains unclear. Defining EEC–immune cross-talk will inform current weight-loss therapies and highlight the immunoendocrine axis as a potential therapeutic target in intestinal disease.
• Latrogenicity: The impact of concomitant medications on EEC function has been largely overlooked, yet these agents may significantly influence secretory activity and associated pathways, and their effects warrant further investigation, particularly when studying human senescence and pathology, in which polypharmacy is common.

Text authored by Amisha A Modasia¹, Marta Camacho², John Worthington³

¹ Quadram Institute, ² University of Cambridge, ³ Lancaster University

 

References

1. Haber AL et al., (2017) DOI: 1038/nature24489
2. Beumer et al., (2020) DOI: 1016/j.cell.2020.04.036
3. Beumer et al., (2024) DOI: 1126/science.adl1460
4. Goldspink et al., (2018) DOI: 1210/en.2018-00672
5. Liddle RA (2019)., DOI: 1016/j.jcmgh.2019.01.006
6. Kaelberer MM et al., (2018) DOI: 1126/science.aat5236
7. Bellono NW et al., (2017) DOI: 1016/j.cell.2017.05.034
8. Barton JR et al., (2023) DOI: 1172/JCI165578