A Novel Antagonist Peptide Reveals a Physiological Role of Insulin-Like Peptide 5 in Control of Colorectal Function

Localisation of the relaxin-family peptide 3 receptor to enteroendocrine cells of the intestine in RXFP3-Cre/tdTomato mice

The relaxin-family peptide 3 receptor (RXFP3) and its native ligand, relaxin-3, are expressed in specific populations of brain neurons, and research on this system has focused on its role in the central nervous system. However, some studies have indicated that relaxin-3 and RXFP3 are also expressed in peripheral organs, including the gut. In this study, we characterised the identity of RXFP3-expressing cells in the gastrointestinal tract, using RXFP3-Cre/tdTomato reporter mice. We identified RXFP3-tdTomato expression in neurons throughout the small and large intestine, in cells in the lamina propria of the colon, and in enteroendocrine cells in the small intestine. We characterised the frequency and phenotype of the RXFP3-tdTomato + enteroendocrine cells in both the duodenum and distal ileum and discovered that the reporter was expressed in populations of cells that co-express 5-hydroxytryptamine (5-HT), cholecystokinin (CCK), secretin, peptide YY (PYY), oxyntomodulin, neurotensin, ghrelin, or glucose-dependent insulinotropic polypeptide (GIP). Faithful co-expression of Cre and RXFP3 mRNA was confirmed in RXFP3-Cre mice using multiplex, fluorescence in situ hybridisation (via RNAscope™). Our results indicate that RXFP3 is expressed by the LIN, X, K, Onecut3, and EC enteroendocrine cell types. In light of the key physiological roles of these cells, this study highlights the potential for relaxin-3 signalling via RXFP3 in enteroendocrine cells to modulate digestion, metabolism, food intake, and inflammatory processes.

Stratification of enterochromaffin cells by single-cell expression analysis

Dynamic interactions between gut mucosal cells and the external environment are essential to maintain gut homeostasis. Enterochromaffin (EC) cells transduce both chemical and mechanical signals and produce 5-hydroxytryptamine (5-HT) to mediate disparate physiological responses. However, the molecular and cellular basis for functional diversity of ECs remains to be adequately defined. Here, we integrated single-cell transcriptomics with spatial image analysis to identify fourteen EC clusters that are topographically organized along the gut. Subtypes predicted to be sensitive to the chemical environment and mechanical forces were identified that express distinct transcription factors and hormones. A Piezo2 + population in the distal colon was endowed with a distinctive neuronal signature. Using a combination of genetic, chemogenetic and pharmacological approaches, we demonstrated Piezo2 + ECs are required for normal colon motility. Our study constructs a molecular map for ECs and offers a framework for deconvoluting EC cells with pleiotropic functions.

Engineering of Novel Analogues That Are More Receptor-Selective and Potent than the Native Hormone, Insulin-like Peptide 5 (INSL5)

Insulin-like peptide 5 (INSL5) targets the G protein-coupled receptor, relaxin family peptide receptor 4 (RXFP4), predominantly coexpressed in the colorectum. While INSL5 also binds to the related receptor RXFP3, it does not activate it. The INSL5/RXFP4 axis is a promising target for treating gastrointestinal disorders such as constipation. Despite its therapeutic potential, the clinical application of INSL5 has been hindered by synthesis complexities, necessitating the need for more accessible yet potent mimetics. In this study, we engineered an INSL5 analogue A13:B7-24-GG, featuring a simplified two-chain, two-disulfide scaffold with 32 amino acids, as opposed to the 45 amino acids found in native INSL5 (two-chain, three-disulfide), improving the synthesis yield by 19.5-fold. Additionally, A13:B7-24-GG demonstrates ∼4-fold higher potency (EC50 = 1.17 nM vs 4.57 nM) and ∼11 times greater selectivity than native INSL5, with significantly reduced RXFP3 binding affinity, positioning it as a promising new therapeutic candidate for the treatment of constipation.

Improving Differentiation of Crohn's Disease and Ulcerative Colitis Proteomes through Protein-Wide Association Study Feature Selection in Machine Learning

Background and Aims

Diagnostic differentiation between Crohn's disease (CD) and ulcerative colitis (UC) is crucial for timely and suitable therapeutic measures. The current gold standard for differentiating between CD and UC involves endoscopy and histology, which are invasive and costly. We aimed to identify blood plasma proteomic signatures using a Protein-Wide Association Study (PWAS) approach to differentiate CD from UC and evaluate the efficacy of these signatures as features in machine learning (ML) classifiers.

Methods

Among participants (n=1,106; nCD=636; nUC=470) of the Study of a Prospective Adult Research Cohort with IBD (SPARC), plasma protein (n=2,920) levels were estimated using Olink proteomics. A PWAS with Bonferroni correction for multiple testing was used to identify proteins associated with disease states after controlling for age, sex, and disease severity. ML classifiers examined the diagnostic utility of these models. Feature importance was determined via SHapley Additive exPlanations (SHAP) analysis.

Results

Thirteen proteins which were significantly differentially abundant in CD vs UC (all |β|s > 0.22, all adjusted p values < 8.42E-06). Random forest models of proteins differentiated between CD and UC with models trained only on PWAS identified proteins (Average ROC-AUC 0.73) outperforming models trained of the full proteome (Average ROC-AUC 0.62). SHAP analysis revealed that Granzyme B, insulin-like peptide 5 (INSL5), and interleukin-12 subunit beta (IL-12B) were the most important features.

Conclusions

Our findings demonstrate that PWAS-based feature selection approaches are a powerful method to identify features in complex, noisy datasets. Importantly, we have identified novel peptide based biomarkers such as INSL5, that can be potentially used to complement existing strategies to differentiate between CD and UC.

Developing insulin-like peptide 5-based antagonists for the G protein-coupled receptor, RXFP4

Human insulin-like peptide 5 (INSL5) is a gut hormone produced by colonic L-cells, and its biological functions are mediated by Relaxin Family Peptide Receptor 4 (RXFP4). Our preliminary data indicated that RXFP4 agonists are potential drug leads for the treatment of constipation. More recently, we designed and developed a novel RXFP4 antagonist, A13-nR that was shown to block agonist-induced activity in cells and animal models. We showed that A13-nR was able to block agonist-induced increases in colon motility in mice of both genders that express the receptor, RXFP4. Our data also showed that colorectal propulsion induced by intracolonic administration of short-chain fatty acids was antagonized by A13-nR. Therefore, A13-nR is an important research tool and potential drug lead for the treatment of colon motility disorders, such as bacterial diarrhea. However, A13-nR acted as a partial agonist at high concentrations in vitro and demonstrated modest antagonist potency (∼35 nM). Consequently, the primary objective of this study is to pinpoint novel modifications to A13-nR that eliminate partial agonist effects while preserving or augmenting antagonist potency. In this work, we detail the creation of a series of A13-nR-modified analogues, among which analogues 3, 4, and 6 demonstrated significantly improved RXFP4 affinity (∼3 nM) with reduced partial agonist activity, enhanced antagonist potency (∼10 nM) and maximum agonist inhibition (∼80 %) when compared with A13-nR. These compounds have potential as candidates for further preclinical evaluations, marking a significant stride toward innovative therapeutics for colon motility disorders.

Development of a synthetic relaxin-3/INSL5 chimeric peptide ligand for NanoBiT complementation binding assays

INSL5 and relaxin-3 are relaxin family peptides with important roles in gut and brain function, respectively. They mediate their actions through the class A GPCRs RXFP4 and RXFP3. RXFP4 has been proposed to be a therapeutic target for colon motility disorders whereas RXFP3 targeting could be effective for neurological conditions such as anxiety. Validation of these targets has been limited by the lack of specific ligands and the availability of robust ligand-binding assays for their development. In this study, we have utilized NanoBiT complementation to develop a SmBiT-conjugated tracer for use with LgBiT-fused RXFP3 and RXFP4. The low affinity between LgBiT:SmBiT should result in a low non-specific luminescence signal and enable the quantification of binding without the tedious separation of non-bound ligands. We used solid-phase peptide synthesis to produce a SmBiT-labelled RXFP3/4 agonist, R3/I5, where SmBiT was conjugated to the B-chain N-terminus via a PEG12 linker. Both SmBiT-R3/I5 and R3/I5 were synthesized and purified in high purity and yield. Stable HEK293T cell lines expressing LgBiT-RXFP3 and LgBiT-RXFP4 were produced and demonstrated normal signaling in response to the synthetic R3/I5 peptide. Binding was first characterized in whole-cell binding kinetic assays validating that the SmBiT-R3/I5 bound to both cell lines with nanomolar affinity with minimal non-specific binding without bound and free SmBiT-R3/I5 separation. We then optimized membrane binding assays, demonstrating easy and robust analysis of both saturation and competition binding from frozen membranes. These assays therefore provide an appropriate rigorous binding assay for the high-throughput analysis of RXFP3 and RXFP4 ligands.

History of key regulatory peptide systems and perspectives for future research

Throughout the 20th Century, regulatory peptide discovery advanced from the identification of gut hormones to the extraction and characterization of hypothalamic hypophysiotropic factors, and to the isolation and cloning of multiple brain neuropeptides. These discoveries were followed by the discovery of G-protein-coupled and other membrane receptors for these peptides. Subsequently, the systems physiology associated with some of these multiple regulatory peptides and receptors has been comprehensively elucidated and has led to improved therapeutics and diagnostics and their approval by the US Food and Drug Administration. In light of this wealth of information and further potential, it is truly a time of renaissance for regulatory peptides. In this perspective, we review what we have learned from the pioneers in exemplified fields of gut peptides, such as cholecystokinin, enterochromaffin-like-cell peptides, and glucagon, from the trailblazing studies on the key stress hormone, corticotropin-releasing factor, as well as from more recently characterized relaxin-family peptides and receptors. The historical viewpoints are based on our understanding of these topics in light of the earliest phases of research and on subsequent studies and the evolution of knowledge, aiming to sharpen our vision of the current state-of-the-art and those studies that should be prioritized in the future.

Ligand recognition mechanism of the human relaxin family peptide receptor 4 (RXFP4)

Members of the insulin superfamily regulate pleiotropic biological processes through two types of target-specific but structurally conserved peptides, insulin/insulin-like growth factors and relaxin/insulin-like peptides. The latter bind to the human relaxin family peptide receptors (RXFPs). Here, we report three cryo-electron microscopy structures of RXFP4-G_i protein complexes in the presence of the endogenous ligand insulin-like peptide 5 (INSL5) or one of the two small molecule agonists, compound 4 and DC591053. The B chain of INSL5 adopts a single α-helix that penetrates into the orthosteric pocket, while the A chain sits above the orthosteric pocket, revealing a peptide-binding mode previously unknown. Together with mutagenesis and functional analyses, the key determinants responsible for the peptidomimetic agonism and subtype selectivity were identified. Our findings not only provide insights into ligand recognition and subtype selectivity among class A G protein-coupled receptors, but also expand the knowledge of signaling mechanisms in the insulin superfamily.

Expression of the relaxin family peptide 4 receptor by enterochromaffin cells of the mouse large intestine

The gastrointestinal hormone, insulin-like peptide 5 (INSL5), is found in large intestinal enteroendocrine cells (EEC). One of its functions is to stimulate nerve circuits that increase propulsive activity of the colon through its receptor, the relaxin family peptide 4 receptor (RXFP4). To investigate the mechanisms that link INSL5 to stimulation of propulsion, we have determined the localisation of cells expressing Rxfp4 in the mouse colon, using a reporter mouse to locate cells expressing the gene. The fluorescent signal indicating the location of Rxfp4 expression was in EEC, the greatest overlap of Rxfp4-dependent labelling being with cells containing 5-HT. In fact, > 90% of 5-HT cells were positive for Rxfp4 labelling. A small proportion of cells with Rxfp4-dependent labelling was 5-HT-negative, 11-15% in the distal colon and rectum, and 35% in the proximal colon. Of these, some were identified as L-cells by immunoreactivity for oxyntomodulin. Rxfp4-dependent fluorescence was also found in a sparse population of nerve endings, where it was colocalised with CGRP. We used the RXFP4 agonist, INSL5-A13, to activate the receptor and probe the role of the 5-HT cells in which it is expressed. INSL5-A13 administered by i.p. injection to conscious mice caused an increase in colorectal propulsion that was antagonised by the 5-HT₃ receptor blocker, alosetron, also given i.p. We conclude that stimuli that excite INSL5-containing colonic L-cells release INSL5 that, through RXFP4, excites 5-HT release from neighbouring endocrine cells, which in turn acts on 5-HT₃ receptors of enteric sensory neurons to elicit propulsive reflexes.