Distinct Alterations in the Guanylate Cyclase-C (GC-C)/Cyclic GMP (cGMP) Pathway Are Evident Across Different Subtypes of Irritable Bowel Syndrome (IBS) Patients

Abstract

Background and Aims: Linaclotide, a GC-C agonist, reduces abdominal pain and improves constipation in patients with IBS with constipation (IBS-C).1 We have recently shown that linaclotide activates GC-C expressed on intestinal epithelial cells, resulting in the production and release of cGMP, which accelerates gastrointestinal transit and inhibits colonic nocicep- tors.1 We have shown that key components of the GC-C/cGMP signalling pathway are expressed within human colonic mucosa. However, it remains to be determined if compo- nents of this pathway are differentially expressed in different IBS patient subtypes. Methods: Recto-sigmoid mucosal biopsies were obtained from healthy subjects (N=10) and IBS patients (N=14), as per Rome II criteria. We compared IBS patients with mixed (constipation and diarrhea) bowel habits (IBS-M; N=7) and patients with IBS-C (N=7). RNA was extracted from biopsies and Taqman qRT-PCR used to assess mRNA expression of GC-C (GUCY2C); the endogenous GC-C agonists, guanylin (GUCA2A) and uroguanylin (GUCA2B); and the cGMP transporters MRP4 (ABCC4) and MRP5 (ABCC5). Expression of these targets was determined relative to the housekeeping genes 18sRNA and GAPDH. In separate biopsies, immunohistochemistry determined localization of GC-C/cGMP signalling pathway compo- nents to cellular structures. Results: In mucosal biopsies from healthy controls, guanylin was the most abundantly expressed component of the GC-C/cGMP signalling pathway, followed sequentially by uroguanylin (P<0.01), GC-C (P<0.001), MRP5 (P<0.001) and MRP4 (P<0.001), respectively. In IBS-M biopsies both of the endogenous GC-C agonists, guanylin and uroguanylin, were significantly reduced compared with healthy controls (P<0.05). By contrast, in IBS-C patient biopsies, MRP4 was significantly down-regulated compared with expression in biopsies from healthy controls (P<0.001). No significant change in either MRP5 or GC-C expression was observed between IBS patient subtypes and healthy controls. Immunohistochemistry revealed MRP4 expression on the apical side of colonic epithelial cells, whilst MRP5 displayed basolateral expression. Conclusions: Distinct alterations in the GC-C/cGMP pathway are evident between different subtypes of IBS patients and may contrib- ute to the pathophysiology of IBS. In IBS-M, reduced expression of the endogenous hormones guanylin and uroguanylin may contribute to alternating bowel habits. In IBS-C, a reduction in apically expressed MRP4 may result in reduced release of cGMP into the colonic lumen. Overall, these changes may help to explain some aspects of the pathophysiology associated with IBS and the differential stool frequency and symptom patterns between IBS subtypes, which are under further investigation.