Chemical synthesis of human trefoil factor 1 (TFF1) and its homodimer provides novel insights into their mechanisms of action

Foreign Mucins Alter the Properties of Reconstituted Gastric Mucus

During the course of evolution, distinct mucin subtypes have evolved, that predominantly occur in specific mucus variants of the body. A loss of this clear regional assignment is often associated with pathophysiological conditions such as asthma or gastric cancer. We here reconstitute mucus from different mucin subtypes to elucidate the influence of MUC5B/MUC2 contaminations on physiologically relevant properties of acidic MUC5AC gels as found in the stomach. Our findings indicate that these properties may be critically altered by the presence of an atypical mucin species. A weak integration of a contaminating mucin subtype into the host network yields weak viscoelastic gels with increased barrier capabilities. Unravelling the complex properties of mucosal barriers under disease conditions is crucial for the understanding of mucosal disease progression and for developing drug-carriers to traverse this biological barrier. Here, our results provide useful insights into mechanistic principles governing the physical properties of gastro-intestinal mucus.

Advances in the chemical synthesis of human proteoforms

Access to structurally-defined human proteoforms is essential to the biochemical studies on human health and medicine. Chemical protein synthesis provides a bottom-up and atomic-resolution approach for the preparation of homogeneous proteoforms bearing any number of post-translational modifications of any structure, at any position, and in any combination. In this review, we summarize the development of chemical protein synthesis, focusing on the recent advances in synthetic methods, product characterizations, and biomedical applications. By analyzing the chemical protein synthesis studies on human proteoforms reported to date, this review demonstrates the significant methodological improvements that have taken place in the field of human proteoform synthesis, especially in the last decade. Our analysis shows that although further method development is needed, all the human proteoforms could be within reach in a cost-effective manner through a divide-and-conquer chemical protein synthesis strategy. The synthetic proteoforms have been increasingly used to support biomedical research, including spatial-temporal studies and interaction network analysis, activity quantification and mechanism elucidation, and the development and evaluation of diagnostics and therapeutics.

The structural integrity of human TFF1 under reducing conditions

The trefoil factor family (TFF) comprises three secretory peptides (TFF1, TFF2, TFF3) that regulate diverse physiological processes to maintain gastrointestinal mucosal integrity and homeostasis. The TFF domain is stabilized by six conserved cysteine residues forming three intramolecular disulfide bonds. In this work, we investigated human TFF1 domain stability against increasing concentrations of the reducing agent tris(2-carboxyethyl)phosphine (TCEP). Experiments revealed high resistance of the disulfide bonds within the TFF1 domain to reduction compared to two reference peptides with similar three-disulfide frameworks, namely the bovine pancreatic trypsin inhibitor (BPTI) and the peptide drug linaclotide. Full reduction of TFF1 was only achieved with a large excess of TCEP (150-fold), and no partially reduced intermediates were observed, supporting a compact TFF1 domain. This TFF1 domain stability was supported by extensive all-atom molecular dynamics simulations for a total of 24 μs of all possible combinatorial states of disulfide bond reduction. Despite minor structural and conformational changes observed upon reduction, the domain substantially retained its overall compactness and solvent exposure when only one or two disulfide bonds were removed. The reduced cysteine residues did not undergo large structural rearrangements and remained buried. The loss of covalent disulfide bonds upon reduction was counterbalanced through persistent non-covalent interactions. These molecular simulations explained why TFF1 could not be partially reduced and alkylated during the experiments despite titrating different TCEP concentrations in the presence of alkylating agents. Our findings provide the first insights into the remarkable stability of the human TFF domain under reducing conditions, supporting its functional resilience upon expression and secretion throughout the human body.

Self-Renewal and Cancers of the Gastric Epithelium: An Update and the Role of the Lectin TFF1 as an Antral Tumor Suppressor

In 2020, gastric cancer was the fourth leading cause of cancer deaths globally. About 90% of gastric cancers are sporadic and the vast majority are correlated with Helicobacter pylori infection; whereas familial clustering is observed in about 10% of cases. Gastric cancer is now considered to be a disease originating from dysregulated self-renewal of the gastric glands in the setting of an inflammatory environment. The human stomach contains two types of gastric units, which show bi-directional self-renewal from a complex variety of stem cells. This review focuses on recent progress concerning the characterization of the different stem cell populations and the mainly mesenchymal signals triggering their stepwise differentiation as well as the genesis of pre-cancerous lesions and carcinogenesis. Furthermore, a model is presented (Lectin-triggered Receptor Blocking Hypothesis) explaining the role of the lectin TFF1 as an antral tumor suppressor possibly regulating Lgr5⁺ antral stem cells in a paracrine or maybe autocrine fashion, with neighboring antral gland cells having a role as niche cells.

Gastrokine 2 Regulates the Antitumor Effect of JAK2/STAT3 Pathway in Gastric Cancer

GKN2 (gastrokine 2) mainly plays a regulatory role in gastric mucosal defense and cell protection mechanisms, and its role in gastric cancer has not been thoroughly elucidated. Immunohistochemistry was used to detect GKN2 and TFF1 expressions in 90 gastric cancer tissues, 48 neoplastic resection margins, and 22 normal gastric mucosa epithelia. It showed that the downregulation of GKN2 and TFF1 expressions in gastric cancer tissues was significantly different from that in adjacent normal gastric tissues and distal gastric mucosal tissues. Nevertheless, correlation analysis showed that GKN2 expression in gastric cancer tissues was independent of TFF1 expression. After overexpression of GKN2 was constructed in human gastric cancer cell line MKN28 with the Ad-GFP-GKN2 transfected, cell viability was measured by CCK-8 assay, and migration and invasion ability were analyzed by transwell migration assay and transwell invasion assay. It indicated that overexpression of GKN2 significantly reduced the viability of MKN28 and SGC7901 cells. Overexpression of GKN2 could also inhibit the migration and invasion ability in MKN28 and SGC7901 cells. In addition, upregulation of GKN2 can inactivate the JAK2/STAT3 pathway. Our data suggest that GKN2 and TFF1 play the antitumor role in gastric carcinoma, and TFF1 may not interact or cooperate with GKN2. GKN2 overexpression can inhibit the growth and metastasis by downregulating the JAK2/STAT3 pathway in gastric cancer cells.

Chemical Synthesis of TFF3 Reveals Novel Mechanistic Insights and a Gut-Stable Metabolite

TFF3 regulates essential gastro- and neuroprotective functions, but its molecular mode of action remains poorly understood. Synthetic intractability and lack of reliable bioassays and validated receptors are bottlenecks for mechanistic and structure-activity relationship studies. Here, we report the chemical synthesis of TFF3 and its homodimer via native chemical ligation followed by oxidative folding. Correct folding was confirmed by NMR and circular dichroism, and TFF3 and its homodimer were not cytotoxic or hemolytic. TFF3, its homodimer, and the trefoil domain (TFF310-50) were susceptible to gastrointestinal degradation, revealing a gut-stable metabolite (TFF37-54; t1/2 > 24 h) that retained its trefoil structure and antiapoptotic bioactivity. We tried to validate the putative TFF3 receptors CXCR4 and LINGO2, but neither TFF3 nor its homodimer displayed any activity up to 10 μM. The discovery of a gut-stable bioactive metabolite and reliable synthetic accessibility to TFF3 and its analogues are cornerstones for future molecular probe development and structure-activity relationship studies.

Improving the Gastrointestinal Stability of Linaclotide

High susceptibility to proteolytic degradation in the gastrointestinal tract limits the therapeutic application of peptide drugs in gastrointestinal disorders. Linaclotide is an orally administered peptide drug for the treatment of irritable bowel syndrome with constipation (IBS-C) and abdominal pain. Linaclotide is however degraded in the intestinal environment within 1 h, and improvements in gastrointestinal stability might enhance its therapeutic application. We therefore designed and synthesized a series of linaclotide analogues employing a variety of strategic modifications and evaluated their gastrointestinal stability and pharmacological activity at its target receptor guanylate cyclase-C. All analogues had substantial improvements in gastrointestinal half-lives (>8 h vs linaclotide 48 min), and most remained active at low nanomolar concentrations. This work highlights strategic approaches for the development of gut-stable peptides toward the next generation of orally administered peptide drugs for the treatment of gastrointestinal disorders.

Trefoil Factor Family (TFF) Peptides and Their Links to Inflammation: A Re-evaluation and New Medical Perspectives

Trefoil factor family peptides (TFF1, TFF2, TFF3), together with mucins, are typical exocrine products of mucous epithelia. Here, they act as a gastric tumor suppressor (TFF1) or they play different roles in mucosal innate immune defense (TFF2, TFF3). Minute amounts are also secreted as endocrine, e.g., by the immune and central nervous systems. As a hallmark, TFF peptides have different lectin activities, best characterized for TFF2, but also TFF1. Pathologically, ectopic expression occurs during inflammation and in various tumors. In this review, the role of TFF peptides during inflammation is discussed on two levels. On the one hand, the expression of TFF1-3 is regulated by inflammatory signals in different ways (upstream links). On the other hand, TFF peptides influence inflammatory processes (downstream links). The latter are recognized best in various Tff-deficient mice, which have completely different phenotypes. In particular, TFF2 is secreted by myeloid cells (e.g., macrophages) and lymphocytes (e.g., memory T cells), where it modulates immune reactions triggering inflammation. As a new concept, in addition to lectin-triggered activation, a hypothetical lectin-triggered inhibition of glycosylated transmembrane receptors by TFF peptides is discussed. Thus, TFFs are promising players in the field of glycoimmunology, such as galectins and C-type lectins.

Trefoil Factor Family (TFF) Peptides and Their Diverse Molecular Functions in Mucus Barrier Protection and More: Changing the Paradigm

Trefoil factor family peptides (TFF1, TFF2, TFF3) are typically co-secreted together with mucins. Tff1 represents a gastric tumor suppressor gene in mice. TFFs are also synthesized in minute amounts in the immune and central nervous systems. In mucous epithelia, they support rapid repair by enhancing cell migration ("restitution") via their weak chemotactic and anti-apoptotic effects. For a long time, as a paradigm, this was considered as their major biological function. Within recent years, the formation of disulfide-linked heterodimers was documented for TFF1 and TFF3, e.g., with gastrokine-2 and IgG Fc binding protein (FCGBP). Furthermore, lectin activities were recognized as enabling binding to a lipopolysaccharide of Helicobacter pylori (TFF1, TFF3) or to a carbohydrate moiety of the mucin MUC6 (TFF2). Only recently, gastric TFF1 was demonstrated to occur predominantly in monomeric forms with an unusual free thiol group. Thus, a new picture emerged, pointing to diverse molecular functions for TFFs. Monomeric TFF1 might protect the gastric mucosa as a scavenger for extracellular reactive oxygen/nitrogen species. Whereas, the TFF2/MUC6 complex stabilizes the inner layer of the gastric mucus. In contrast, the TFF3-FCGBP heterodimer (and also TFF1-FCGBP) are likely part of the innate immune defense of mucous epithelia, preventing the infiltration of microorganisms.

Structure, Function, and Therapeutic Potential of the Trefoil Factor Family in the Gastrointestinal Tract

Trefoil factor family peptides (TFF1, TFF2, and TFF3) are key players in protecting, maintaining, and repairing the gastrointestinal tract. Accordingly, they have the therapeutic potential to treat and prevent a variety of gastrointestinal disorders associated with mucosal damage. TFF peptides share a conserved motif, including three disulfide bonds that stabilize a well-defined three-loop-structure reminiscent of a trefoil. Although multiple functions have been described for TFF peptides, their mechanisms at the molecular level remain poorly understood. This review presents the status quo of TFF research relating to gastrointestinal disorders. Putative TFF receptors and protein partners are described and critically evaluated. The therapeutic potential of these peptides in gastrointestinal disorders where altered mucosal biology plays a crucial role in the underlying etiology is discussed. Finally, areas of investigation that require further research are addressed. Thus, this review provides a comprehensive update on TFF literature as well as guidance toward future research to better understand this peptide family and its therapeutic potential for the treatment of gastrointestinal disorders.