The understanding of paracrine interactions between intestinal stem cells (ISCs) and their associated niche is crucial to characterise stem cell regulatory mechanisms. Defining these signals may enable stem cell pools to be manipulated in degenerative disease and pathologies.
Paneth cells are a differentiated secretory epithelial population interspersed between intestinal Lgr5+ve stem cells at the base of the crypts that have been recently suggested to be a key component of the intestinal stem cell niche. These cells secrete crucial paracrine signals that activate specific signalling pathways within ISCs. Neuregulin 1 (Nrg1) is a member of the EGF ligand family that binds to the receptor tyrosine kinases ErbB3 and ErbB4 utilising ErbB2 as a co-receptor. Nrg1 is involved in multiple developmental processes but also has key roles in numerous adult tissues, including the nervous system and heart. However, the function of Nrg1 in the intestinal epithelium and its potential stem cell regulatory activity has not previously been examined.
Our studies show that Nrg1 is expressed in the intestinal epithelium with strong cytoplasmic and membranous expression in Paneth cells. Fluorescent Activated Cell Sorting (FACS) isolation of murine cell populations enriched for Paneth cells or Lgr5+ve stem cells combined with qRT-PCR have confirmed that Nrg1 is highly expressed in Paneth cells with little expression in Lgr5+ve stem cells. In contrast, the Neuregulin receptors ErbB2 and ErbB3 are present in Lgr5+ve ISCs but are at low levels in Paneth cells. We investigated the functional activity of Nrg1 using the intestinal “organoid” crypt culture method. This involves the isolation and culture of intestinal crypts in matrigel where organoids form with crypt domains that contail ISCs and villus domains that contain differentiated cell types. Nrg1-treated organoids were larger in size, with multiple proliferating crypt units per organoid compared to control. The number of Lgr5+ stem cells was also elevated in Nrg1-treated organoids.
In conclusion, these results demonstrate that Paneth cells produce Nrg1 that is likely to act locally on ErbB3 expressing Lgr5+ve stem cells to promote cell proliferation. Future work will define whether loss-of-function of Nrg1 impairs intestinal homeostasis.