High-resolution structure of intramolecularly proteolyzed human mucin-1 SEA domain

The structure of the second CysD domain of MUC2 and role in mucin organization by transglutaminase-based cross-linking

The MUC2 mucin protects the colonic epithelium by a two-layered mucus with an inner attached bacteria-free layer and an outer layer harboring commensal bacteria. CysD domains are 100 amino-acid-long sequences containing 10 cysteines that separate highly O-glycosylated proline, threonine, serine (PTS) regions in mucins. The structure of the second CysD, CysD2, of MUC2 is now solved by nuclear magnetic resonance. CysD2 shows a stable stalk region predicted to be partly covered by adjacent O-glycans attached to neighboring PTS sequences, whereas the CysD2 tip with three flexible loops is suggested to be well exposed. It shows transient dimer interactions at acidic pH, weakened at physiological pH. This transient interaction can be stabilized in vitro and in vivo by transglutaminase 3-catalyzed isopeptide bonds, preferring a specific glutamine residue on one flexible loop. This covalent dimer is modeled suggesting that CysD domains act as connecting hubs for covalent stabilization of mucins to form a protective mucus.

MeltSerts technology (brinzolamide ocular inserts via hot-melt extrusion): QbD-steered development, molecular dynamics, in vitro, ex vivo and in vivo studies

The research work aimed to develop a robust sustained release biocompatible brinzolamide (BRZ)-loaded ocular inserts (MeltSerts) using hot-melt extrusion technology with enhanced solubility for glaucoma management. A 32 rotatable central composite design was employed for the optimization of the MeltSerts to achieve sustained release. The effect of two independent factors was examined: Metolose® SR 90SH-100000SR (HPMC, hydroxypropyl methyl cellulose) and Kolliphor® P 407 (Poloxamer 407, P407). The drug release (DR) of BRZ at 0.5 h and 8 h were adopted as dependent responses. The factorial analysis resulted in an optimum composition of 50.00 % w/w of HPMC and 15.00 % w/w of P407 which gave % DR of 9.11 at 0.5 h and 69.10 at 8 h. Furthermore, molecular dynamic simulations were performed to elucidate various interactions between BRZ, and other formulation components and it was observed that BRZ showed maximum interactions with HPC and HPMC with an occupancy of 92.82 and 52.87 %, respectively. Additionally, molecular docking studies were performed to understand the interactions between BRZ and mucoadhesive polymers with ocular mucin (MUC-1). The results indicated a docking score of only -5.368 for BRZ alone, whereas a significantly higher docking score was observed for the optimized Meltserts -6.977, suggesting enhanced retention time of the optimized MeltSerts. SEM images displayed irregular surfaces, while EDS analysis validated uniform BRZ distribution in the optimized formulation. The results of the ocular irritancy studies both ex vivo and in vivo demonstrated that MeltSerts are safe for ocular use. The results indicate that the developed MeltSerts Technology has the potential to manufacture ocular inserts with cost-effectiveness, one-step processability, and enhanced product quality. Nonetheless, it also offers a once-daily regimen, consequently decreasing the dosing frequency, preservative exposure, and ultimately better glaucoma management.

Bacterial SEAL domains undergo autoproteolysis and function in regulated intramembrane proteolysis

Gram-positive bacteria use SigI/RsgI-family sigma factor/anti-sigma factor pairs to sense and respond to cell wall defects and plant polysaccharides. In Bacillus subtilis, this signal transduction pathway involves regulated intramembrane proteolysis (RIP) of the membrane-anchored anti-sigma factor RsgI. However, unlike most RIP signaling pathways, site-1 cleavage of RsgI on the extracytoplasmic side of the membrane is constitutive and the cleavage products remain stably associated, preventing intramembrane proteolysis. The regulated step in this pathway is their dissociation, which is hypothesized to involve mechanical force. Release of the ectodomain enables intramembrane cleavage by the RasP site-2 protease and activation of SigI. The constitutive site-1 protease has not been identified for any RsgI homolog. Here, we report that RsgI's extracytoplasmic domain has structural and functional similarities to eukaryotic SEA domains that undergo autoproteolysis and have been implicated in mechanotransduction. We show that site-1 proteolysis in B. subtilis and Clostridial RsgI family members is mediated by enzyme-independent autoproteolysis of these SEA-like domains. Importantly, the site of proteolysis enables retention of the ectodomain through an undisrupted β-sheet that spans the two cleavage products. Autoproteolysis can be abrogated by relief of conformational strain in the scissile loop, in a mechanism analogous to eukaryotic SEA domains. Collectively, our data support the model that RsgI-SigI signaling is mediated by mechanotransduction in a manner that has striking parallels with eukaryotic mechanotransducive signaling pathways.

Bacterial SEAL domains undergo autoproteolysis and function in regulated intramembrane proteolysis

Gram-positive bacteria use SigI/RsgI-family sigma factor/anti-sigma factor pairs to sense and respond to cell wall defects and plant polysaccharides. In Bacillus subtilis this signal transduction pathway involves regulated intramembrane proteolysis (RIP) of the membrane-anchored anti-sigma factor RsgI. However, unlike most RIP signaling pathways, site-1 cleavage of RsgI on the extracytoplasmic side of the membrane is constitutive and the cleavage products remain stably associated, preventing intramembrane proteolysis. The regulated step in this pathway is their dissociation, which is hypothesized to involve mechanical force. Release of the ectodomain enables intramembrane cleavage by the RasP site-2 protease and activation of SigI. The constitutive site-1 protease has not been identified for any RsgI homolog. Here, we report that RsgI's extracytoplasmic domain has structural and functional similarities to eukaryotic SEA domains that undergo autoproteolysis and have been implicated in mechanotransduction. We show that site-1 proteolysis in B. subtilis and Clostridial RsgI family members is mediated by enzyme-independent autoproteolysis of these SEA-like (SEAL) domains. Importantly, the site of proteolysis enables retention of the ectodomain through an undisrupted ß-sheet that spans the two cleavage products. Autoproteolysis can be abrogated by relief of conformational strain in the scissile loop, in a mechanism analogous to eukaryotic SEA domains. Collectively, our data support the model that RsgI-SigI signaling is mediated by mechanotransduction in a manner that has striking parallels with eukaryotic mechanotransducive signaling pathways.

SIGNIFICANCE

SEA domains are broadly conserved among eukaryotes but absent in bacteria. They are present on diverse membrane-anchored proteins some of which have been implicated in mechanotransducive signaling pathways. Many of these domains have been found to undergo autoproteolysis and remain noncovalently associated following cleavage. Their dissociation requires mechanical force. Here, we identify a family of bacterial SEA-like (SEAL) domains that arose independently from their eukaryotic counterparts but have structural and functional similarities. We show these SEAL domains autocleave and the cleavage products remain stably associated. Importantly, these domains are present on membrane-anchored anti-sigma factors that have been implicated in mechanotransduction pathways analogous to those in eukaryotes. Our findings suggest that bacterial and eukaryotic signaling systems have evolved a similar mechanism to transduce mechanical stimuli across the lipid bilayer.

Development of an Optimal In vitro Release and Permeation Testing Method for Rectal Suppositories

Currently there are no compendial assays for testing drug release from rectal suppositories. It is therefore essential to study different in vitro release testing (IVRT) and in vitro permeation testing (IVPT) methods for identifying a suitable technique to compare in vitro drug release and to predict in vivo performance of rectal suppositories. In the present study, three different rectal suppository formulations of mesalamine (CANASA, Generic, and In-house) were studied for in vitro bioequivalence. All the different suppository products were characterized by performing weight variation, content uniformity, hardness, melting time, and pH tests. Viscoelastic behavior of the suppositories was also tested both in presence and absence of mucin. Four different IVRT techniques such as Dialysis, Horizontal Ussing Chamber, Vertical Franz cell, and USP apparatus 4. IVPT studies were performed using Horizontal Ussing chamber and Vertical Franz cell methods. Q1/Q2 equivalent products (CANASA, Generic) and a half-strength product were studied to understand the reproducibility, bio relevance, and discriminatory ability of the IVRT and IVPT methods. This study is the first of its kind where molecular docking studies were performed to determine the potential interactions of drug (mesalamine) with mucin, IVRT studies were conducted with and without the presence of mucin, and porcine rectal mucosa was used to perform IVPT tests. The USP 4 method and Horizontal Ussing chamber methods were found to be suitable IVRT and IVPT techniques, respectfully, for rectal suppositories. RLD (Reference Listed Drug) and Generic rectal suppositories were found to exhibit similar release rate and permeation profiles obtained from USP 4, and the IVPT studies, respectfully. Wilcoxon Rank Sum/Mann-Whitney rank test, conducted for the IVRT profiles obtained using USP 4 method, proved the sameness of RLD and Generic suppository products.

Interphotoreceptor matrix proteoglycans IMPG1 and IMPG2 proteolyze in the SEA domain and reveal localization mutual dependency

The interphotoreceptor matrix (IPM) is a specialized extracellular mesh of molecules surrounding the inner and outer segments of photoreceptor neurons. Interphotoreceptor matrix proteoglycan 1 and 2 (IMPG1 and IMPG2) are major components of the IPM. Both proteoglycans possess SEA (sperm protein, enterokinase and agrin) domains, which may support proteolysis. Interestingly, mutations in the SEA domains of IMPG1 and IMPG2 are associated with vision disease in humans. However, if SEA domains in IMPG molecules undergo proteolysis, and how this contributes to vision pathology is unknown. Therefore, we investigated SEA-mediated proteolysis of IMPG1 and IMPG2 and its significance to IPM physiology. Immunoblot analysis confirmed proteolysis of IMPG1 and IMPG2 in the retinas of wildtype mice. Point mutations mimicking human mutations in the SEA domain of IMPG1 that are associated with vision disease inhibited proteolysis. These findings demonstrate that proteolysis is part of the maturation of IMPG1 and IMPG2, in which deficits are associated with vision diseases. Further, immunohistochemical assays showed that proteolysis of IMPG2 generated two subunits, a membrane-attached peptide and an extracellular peptide. Notably, the extracellular portion of IMPG2 trafficked from the IPM around the inner segment toward the outer segment IPM by an IMPG1-dependent mechanism. This result provides the first evidence of a trafficking system that shuttles IMPG1 and IMPG2 from the inner to outer IPM in a co-dependent manner. In addition, these results suggest an interaction between IMPG1-IMPG2 and propose that mutations affecting one IMPG could affect the localization of the normal IMPG partner, contributing to the disease mechanism of vision diseases associated with defective IMPG molecules.

Crystal structure of a human MUC16 SEA domain reveals insight into the nature of the CA125 tumor marker

MUC16 is a membrane bound glycoprotein involved in the progression and metastasis of pancreatic and ovarian cancer. The protein is shed into the serum and the resulting cancer antigen 125 (CA125) can be detected by immunoassays. The CA125 epitope is used for monitoring ovarian cancer treatment progression, and has emerged as a potential target for antibody mediated immunotherapy. The extracellular tandem repeat domain of the protein is composed of repeating segments of heavily glycosylated sequence intermixed with homologous SEA (Sperm protein, Enterokinase and Agrin) domains. Here we report the purification and the first X-ray structure of a human MUC16 SEA domain. The structure was solved by molecular replacement using a Rosetta generated structure as a search model. The SEA domain reacted with three different MUC16 therapeutic antibodies, confirming that the CA125 epitope is localized to the SEA domain. The structure revealed a canonical ferredoxin-like fold, and contained a conserved disulfide bond. Analysis of the relative solvent accessibility of side chains within the SEA domain clarified the assignment of N-linked and O-linked glycosylation sites within the domain. A model of the glycosylated SEA domain revealed two major accessible faces, which likely represent the binding sites of CA125 specific antibodies. The results presented here will serve to accelerate future work to understand the functional role of MUC16 SEA domains and antibody recognition of the CA125 epitope.

The Relationship between Mucins and Ulcerative Colitis: A Systematic Review

Mucins are a family of glycosylated proteins which are the primary constituents of mucus and play a dynamic role in the regulation of the protective mucosal barriers throughout the human body. Ulcerative colitis (UC) is an Inflammatory Bowel Disease (IBD) characterised by continuous inflammation of the inner layer of the large intestine, and in this systematic review we analyse currently available data to determine whether alterations exist in mucin activity in the colonic mucosa of UC patients. Database searches were conducted to identify studies published between 1990 and 2020 that assess the role of mucins in cohorts of UC patients, where biopsy specimens were resected for analysis and control groups were included for comparison. 5497 articles were initially identified and of these 14 studies were systematically selected for analysis, a further 2 articles were identified through citation chaining. Therefore, 16 studies were critically reviewed. 13 of these studies assessed the role of MUC2 in UC and the majority of articles indicated that alterations in MUC2 structure or synthesis had an impact on the colonic mucosa, although conflicting results were presented regarding MUC2 expression. This review highlights the importance of further research to enhance our understanding of mucin regulation in UC and summarises data that may inform future studies.