RTCB deficiency triggers colitis in mice by influencing the NF-κB and Wnt/β-catenin signaling pathways

Abstract

RNA terminal phosphorylase B (RTCB) has been shown to play a significant role in multiple physiological processes. However, the specific role of RTCB in the mouse colon remains unclear. In this study, we employ a conditional knockout mouse model to investigate the effects of RTCB depletion on the colon and the potential molecular mechanisms. We assess the efficiency and phenotype of Rtcb knockout using PCR, western blot analysis, histological staining, and immunohistochemistry. Compared with the control mice, the Rtcb-knockout mice exhibit compromised colonic barrier integrity and prominent inflammatory cell infiltration. In the colonic tissues of Rtcb-knockout mice, the protein levels of TNF-α, IL-8, and p-p65 are increased, whereas the levels of IKKβ and IκBα are decreased. Moreover, the level of GSK3β is increased, whereas the levels of Wnt3a, β-catenin, and LGR5 are decreased. Collectively, our findings unveil a close association between RTCB and colonic tissue homeostasis and demonstrate that RTCB deficiency can lead to dysregulation of both the NF-κB and Wnt/β-catenin signaling pathways in colonic cells.

1. Introduction

RNA terminal phosphate cyclase B (RTCB), also known as FAAP, HSPC117, C22orf28, and D10Wsu52e, is a ligase that specifically connects the 2′,3′-cyclic phosphate and 5′-hydroxyl terminus of RNA [1,2]. RTCB is a highly conserved protein with a unique structure. It is present in archaea, bacteria, and animals but absent in plants and fungi [3]. In archaea and mammals, RTCB appears to serve as a catalyst in tRNA splicing [2,4]. In eukaryotes, RTCB is also involved in the unfolded protein response (UPR) by influencing the splicing of Xbp1 mRNA [5].

RTCB has been shown to play a significant role in multiple physiological processes. In embryonic development, inhibition of RTCB in mouse embryos results in embryonic death and placental dysplasia [6]. In reproductive function, while worms can grow to adulthood but fail to generate oocytes after knockdown of Rtcb [7], mice have premature ovarian failure after conditional knockout of Rtcb [8], and RTCB is specifically highly expressed in the initial segment of the mouse epididymis [9]. In immune function, specific knockout of Rtcb in mouse B cells leads to abnormal cell proliferation and differentiation and disrupts the structure of the endoplasmic reticulum (ER), ultimately affecting the secretion of some antibodies [5]. In neuroprotection, RTCB could ameliorate damage to dopaminergic neurons in a worm model of Parkinson’s disease [10]. A previous study showed that RTCB is widely expressed in intestinal tissues [11]; however, its function is unknown.

Colitis is an inflammatory disease of colonic tissues induced by various factors. Inflammation extends from the rectum to the proximal colon from onset, causing persistent superficial mucosal inflammation of varying degrees [12]. Inflammatory bowel disease (IBD) includes ulcerative colitis (UC) and Crohn’s disease (CD). UC is most common in industrialized countries, and its incidence has been increasing in Asia [13,14]. The total incidence and prevalence of UC are reported to be 0.0012%‒0.0203% and 0.0076%‒0.245% per year, respectively [15]. The differences in incidence among races are more related to environment, diet, and lifestyle. There is no significant sex difference in UC patients [16]. The onset time of UC has a bimodal distribution, with the first peak at 20‒30 years old and the second peak at 50‒80 years old [17]. The MAPK, NF-κB and Wnt/β-catenin signaling pathways play pivotal roles in the pathogenesis of colitis and its associated disorders [18,19]. Alterations in these signaling pathways are commonly concurrent with the onset of colitis [20,21].