Paneth cells secrete antimicrobial proteins including lysozyme via secretory autophagy as part of the mucosal protective response. The ELAV like RNA-binding protein 1 (ELAVL1, also called HuR) regulates stability and translation of messenger RNAs (mRNAs) and many aspects of mucosal physiology. We studied the posttranscriptional mechanisms by which HuR regulates Paneth cell function.

Intestinal mucosal tissues were collected from mice with intestinal epithelium (IE)-specific disruption of HuR (IE-HuR^–/–), HuR^fl/fl-Cre^– mice (controls), and patients with inflammatory bowel diseases and analyzed by histology and immunohistochemistry. Paneth cell functions were determined by lysozyme-immunostaining assays. We isolated primary enterocytes from IE-HuR^–/– and control mice and derived intestinal organoids. HuR and the chaperone CNPY3 were overexpressed from transgenes in intestinal epithelial cells (IECs) or knocked down with small interfering RNAs. We performed RNA pulldown assays to investigate interactions between HuR and its target mRNAs.

Intestinal tissues from IE-HuR^–/– mice had reduced numbers of Paneth cells, and Paneth cells had fewer lysozyme granules per cell, compared with tissues from control mice, but there were no effects on Goblet cells or enterocytes. Intestinal mucosa from patients with inflammatory bowel diseases had reduced levels of HuR and fewer Paneth cells. IE-HuR^–/– mice did not have the apical distribution of TLR2 in the intestinal mucosa as observed in control mice. IECs from IE-HuR^–/– mice expressed lower levels of CNPY3. Intestinal organoids from IE-HuR^–/– mice were smaller and contained fewer buds compared with those generated from controls, and had fewer lysozyme-positive cells. In IECs, knockdown of HuR decreased levels of the autophagy proteins LC3-I and LC3-II, compared with control cells, and prevented rapamycin-induced autophagy. We found HuR to interact directly with the Cnpy3 mRNA coding region and increase levels of CNPY3 by increasing the stability and translation of Cnpy3 mRNA. CNPY3 bound TLR2, and cells with knockdown of CNPY3 or HuR lost membrane localization of TLR2, but increased cytoplasmic levels of TLR2.

In studies of mice, IECs, and human tissues, we found HuR to increase expression of CNPY3 at the posttranscriptional level. CNPY3 is required for membrane localization of TLR2 and Paneth cell function.