An investigation into the role of short-chain fatty acids and the enteroendocrine system in colonic pathophysiology

BACKGROUND Despite the ability of enteroendocrine cells (EECs) to secrete proliferative and pro-angiogenic factors and their influence on gut innervation, the role of EECs and their interaction with short-chain fatty acids (SCFAs) in colonic carcinogenesis and afferent neuronal sensation is under-investigated. Recent work suggests that SCFA butyrate may exert its anti-neoplastic effect via modulation of co-receptor neuropilin-1 (NRP-1). The aim of this thesis is to investigate alteration of EEC expression in neoplastic colonic epithelium and the influence of SCFAs on EEC populations, to identify the subset of colonic EEC that express NRP-1 and to investigate the influence of SCFAs on colonic sensation. METHODOLOGY I performed IHC colocalisation analysis of human colon biopsies stained with NRP-1 and EEC markers. EEC populations were compared in normal, adenoma and adenocarcinoma colonic biopsies and correlated with SCFA concentrations. An in vitro murine model was developed to record the effects of luminally applied SCFAs on colonic afferent nerve firing. Calcium-imaging of DRGs was performed to determine if SCFAs have a direct influence of afferent neuronal excitability. RESULTS I determined that NRP-1 is expressed by a subset of colonic EECs and demonstrated altered EEC expression in neoplastic epithelium and alterations in EEC population in the local lesion field with pan-colonic field effects. Afferent nerve recordings reveal that SCFAs have an inhibitory effect on colonic afferent mechanosensitivity, appearing to modulate this indirect effect via the high threshold afferents. CONCLUSIONS Altered EEC expression in the neoplastic epithelium and altered relationships with SCFA levels provide additional evidence for a role of EECs and SCFAs in colonic carcinogenesis. Expression of NRP-1 by colonic GLP-1 expressing EECs provides additional insight into the biological role of NRP-1 in the colonic epithelium. Inhibition of colonic afferent mechanosensitivity in a murine model supports the findings observed in human subjects that suggest analgesic properties of SCFAs.