Mutation in transforming growth factor beta induced protein associated with granular corneal dystrophy type 1 reduces the proteolytic susceptibility through local structural stabilization

Classic lattice corneal dystrophy: a brief review and summary of treatment modalities

PURPOSE

To provide a brief summary and comparison of the most recent literature on available and theorized treatment modalities for classic lattice corneal dystrophy (LCD). This paper aims to support practitioners in their management of this disease.

METHODS

A search was carried out on available literature through PubMed and Google Scholar of English language articles up to January 2023 that relate to the treatment of LCD. Due to scarcity of literature regarding specific novel therapies for LCD, results from other corneal pathologies (granular corneal dystrophy, corneal scarring) are sometimes included for contrast, which is clearly denoted.

RESULTS

LCD is a slowly progressive disease that leads to recurrent epithelial corneal erosions, stromal haze, corneal opacification, substantial discomfort, and visual impairment. Due to its autosomal-dominant inheritance pattern, this disease can persist throughout ancestral lines and requires consistent treatment and follow-up. An optimal management plan is necessary to (1) prolong years of life with best achievable visual acuity; (2) treat painful recurrent corneal erosions as they occur; (3) ensure proper follow-up throughout the life of a patient, as well as monitor at-risk offspring; and (4) monitor efficacy of treatment.

CONCLUSIONS

This paper addresses (1) treatment for early disease including corneal epithelial debridement, photo therapeutic keratectomy (PTK), femtosecond laser-assisted lamellar keratectomy (FLK), and others; (2) treatment for late disease including full thickness keratoplasties and anterior lamellar keratoplasties; and (3) potential future treatment considerations including a wide variety of topical/systemic, genetic, and regenerative approaches.

Targeted Expression of TGFBIp Peptides in Mouse and Human Tissue by MALDI-Mass Spectrometry Imaging

Stromal corneal dystrophies are a group of hereditary disorders caused by mutations in the TGFBI gene. The mutant TGFBIp is prone to protein aggregation and the mutant protein gets deposited in the cornea, leading to severe visual impairment. The mutations lead to a corneal specific protein aggregation suggesting the involvement of tissue-specific factors. The exact molecular mechanism of the process of tissue-specific protein aggregation remains to be elucidated. Differential proteolysis of mutant TGFBIp is a critical component of the disease pathology. The differential proteolysis gives rise to shorter peptides that are highly aggregation-prone and initiate the aggregation cascade. Analyzing the proteolytic processing of the different TGFBIp mutant may provide insight to aid in understanding the amyloid aggregation mechanism. We developed a MALDI-MSI methodology to identify expression and spatial localization of TGFBIp peptides in the cornea. Corneal tissue samples were collected from both control and dystrophic patients (with 2 different mutations), embedded in OCT and sectioned. The sections were trypsin digested and subjected to mass spectrometry imaging using a targeted approach to detect TGFBIp. MALDI-MSI identified peptides from TGFBIp that co-localized with the amyloid corneal deposits. In addition to the relative abundance data, the specific location of the peptides across the corneal sections as molecular signatures was also identified. Spatial distribution and intensity of the TGFBIp peptides showed differences between diseased and control models but also between the two LCD phenotypes. The TGFBIp peptide with m/z of 787.474 and m/z of 1179.579 showed increased expression in both LCD mutants compared to the controls. The peptide with m/z of 929.5 showed increased expression in the LCD phenotype with H626R mutation while the peptide with m/z of 1315.802 was abundant in the sample with R124C mutation. This initial report of 2D spatial protein signature and localization of TGFBIp may be expanded to other mutations to understand the proteolytic patterns of TGFBIp in different mutations.

Periostin Short Fragment with Exon 17 via Aberrant Alternative Splicing Is Required for Breast Cancer Growth and Metastasis

BACKGROUND

Periostin (POSTN) is a 93 kDa matrix protein that helps to regulate collagen gene expression in the extracellular matrix. POSTN overexpression is a prognostic factor in malignant cancers; however, some researchers have observed it in the stroma, whereas others have reported it on tumors.

OBJECTIVE

This study aimed to investigate the function of POSTN on tumors.

METHODS AND RESULTS

We found that POSTN in cancer cells can be detected by using an antibody against the POSTN C-terminal region exon 17 (Ex17 antibody), but not with an antibody against the POSTN N-terminal region exon 12 (Ex12 antibody) in patients with breast cancer. In a fraction secreted from fibroblasts, LC-MS/MS analysis revealed a short fragment of POSTN of approximately 40 kDa with exon 17. In addition, molecular interaction analysis showed that POSTN with exon 17, but not POSTN without exon 17, bound specifically to wnt3a, and the Ex17 antibody inhibited the binding.

CONCLUSION

A short fragment of POSTN with exon 17, which originates in the fibroblasts, is transported to cancer cells, whereas POSTN fragments without exon 17 are retained in the stroma. The Ex17 antibody inhibits the binding between POSTN exon 17 and wnt3a.

Potential mechanisms of action of celastrol against rheumatoid arthritis: Transcriptomic and proteomic analysis

The clinical efficacy for treating of celastrol rheumatoid arthritis (RA) has been well-documented, but its mechanism of action remains unclear. Here we explored through what proteins and processes celastrol may act in activated fibroblast-like synoviocytes (FLS) from RA patients. Differential expression of genes and proteins after celastrol treatment of FLS was examined using RNA sequencing, label-free relatively quantitative proteomics and molecular docking. In this paper, expression of 26,565 genes and 3,372 proteins was analyzed. Celastrol was associated with significant changes in genes that respond to oxidative stress and oxygen levels, as well as genes that stabilize or synthesize components of the extracellular matrix. These results identify several potential mechanisms through which celastrol may inhibit inflammation in RA.

Pharmaceutical modulation of the proteolytic profile of Transforming Growth Factor Beta induced protein (TGFBIp) offers a new avenue for treatment of TGFBI-corneal dystrophy

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Corneal stromal dystrophies are a group of hereditary disorders caused by mutations in the TGFBI gene and affect the corneal stroma and epithelium.

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The disease is characterized by the accumulation of insoluble deposits of the mutant TGFBIp leading to poor visual acuity in patients.

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Mutations are hypothesized to disrupt the protein folding and stability, leading oligomerization of the mutant protein.

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Current treatment relies on surgical intervention, either tissue removal or substitution, both of which are associated with disease recurrence.

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The lead compounds reported here prevent/delay the atypical proteolysis of the mutant protein and the generation of amyloidogenic fragments.

Corneal dystrophies are a group of genetically inherited disorders with mutations in the TGFBI gene affecting the Bowman’s membrane and the corneal stroma. The mutant TGFBIp is highly aggregation-prone and is deposited in the cornea. Depending on the type of mutation the protein deposits may vary (amyloid, amorphous powdery aggregate or a mixed form of both), making the cornea opaque and thereby decreases visual acuity. The aggregation of the mutant protein is found to be specific with a unique aggregation mechanism distinct to the cornea. The proteolytic processing of the mutant protein is reported to be different compared to the WT protein. The proteolytic processing of mutant protein gives rise to highly amyloidogenic peptide fragments. The current treatment option, available for patients, is tissue replacement surgery that is associated with high recurrence rates. The clinical need for a simple treatment option for corneal dystrophy patients has become highly essential either to prevent the protein aggregation or to dissolve the preformed aggregates. Here, we report the screening of 2500 compounds from the Maybridge RO3 fragment library using weak affinity chromatography (WAC). The primary hits from WAC were validated by ¹⁵N-HSQC NMR assays and specific regions of binding were identified. The recombinant mutant proteins (4th FAS-1 domain of R555W and H572R) were subjected to limited proteolysis by trypsin together with the lead compounds identified by NMR assays. The lead compounds (MO07617, RJF00203 and, BTB05094) were effective to delay/prevent the generation of amyloidogenic peptides in the R555W mutant and compounds (RJF00203 and BTB05094) were effective to delay/prevent the generation of amyloidogenic peptides in the H572R mutant. Thus the lead compounds reported here upon further validation and/or modification might be proposed as a potential treatment option to prevent/delay aggregation by inhibiting the formation of amyloidogenic peptides in TGFBI-corneal dystrophy.

Investigation of TGFBI (transforming growth factor beta-induced) Gene Mutations in Families with Granular Corneal Dystrophy Type 1 in the Konya Region

Objectives:

Granular corneal dystrophies (GCD) are characterized by small, discrete, sharp-edged, grayish-white opacities in the corneal stroma. Among the genes responsible for the development of GCD, the most strongly related gene is transforming growth factor beta-induced (TGFBI), located in the 5q31.1 locus. Studies show that R124H in exon 4 and R555W in exon 12 are hot-spot mutations in the TGFBI gene that lead to GCD development. In this study, we aimed to investigate these two hot-spot mutations in exons 4 and 12 of the TGFBI gene and other possible mutations in the same regions, which code important functional regions of the protein, in Turkish families with GCD1 and to determine the relationship between the mutations and disease and related phenotypes.

Materials and Methods:

The study included, 16 individuals diagnosed with GCD type 1 (GCD1), 11 of these patients’ healthy relatives, and 28 unrelated healthy individuals. DNA was obtained from peripheral blood samples taken from each individual and polymerase chain reaction was used to amplify target gene regions. Genotyping studies were done by sequence analysis.

Results:

The R124S mutation in exon 4 of TGFBI was not detected in the patients or healthy individuals in our study. However, all individuals diagnosed as having GCD1 were found to be heterozygous carriers of the R555W mutation in exon 12 of TGFBI. This mutation was not detected in healthy family members or control individuals unrelated to these families. In addition, we detected the silent mutation F540F in exon 12 and c.32924 G>A substitution in an intronic region of the gene in a few patients and healthy individuals.

Conclusion:

Our study strongly supports the association of GCD1 with R555W mutation in exon 12 region of the TGFBI gene, as reported in the literature.

The serine protease HtrA1 cleaves misfolded transforming growth factor β-induced protein (TGFBIp) and induces amyloid formation

The serine protease high-temperature requirement protein A1 (HtrA1) is associated with protein-misfolding disorders such as Alzheimer's disease and transforming growth factor β-induced protein (TGFBIp)-linked corneal dystrophy. In this study, using several biochemical and biophysical approaches, including recombinant protein expression, LC-MS/MS and 2DE analyses, and thioflavin T (ThT) fluorescence assays for amyloid fibril detection, and FTIR assays, we investigated the role of HtrA1 both in normal TGFBIp turnover and in corneal amyloid formation. We show that HtrA1 can cleave WT TGFBIp but prefers amyloidogenic variants. Corneal TGFBIp is extensively processed in healthy people, resulting in C-terminal degradation products spanning the FAS1-4 domain of TGFBIp. We show here that HtrA1 cleaves the WT FAS1-4 domain only inefficiently, whereas the amyloidogenic FAS1-4 mutations transform this domain into a considerably better HTRA1 substrate. Moreover, HtrA1 cleavage of the mutant FAS1-4 domains generated peptides capable of forming in vitro amyloid aggregates. Significantly, these peptides have been previously identified in amyloid deposits in vivo, supporting the idea that HtrA1 is a causative agent for TGFBIp-associated amyloidosis in corneal dystrophy. In summary, our results indicate that TGFBIp is an HtrA1 substrate and that some mutations in the gene encoding TGFBIp cause aberrant HtrA1-mediated processing that results in amyloidogenesis in corneal dystrophies.

Impaired Autophagic Degradation of Transforming Growth Factor-β-Induced Protein by Macrophages in Lattice Corneal Dystrophy

Purpose

Lattice corneal dystrophy (LCD) is related to the denaturation of transforming growth factor-β-induced protein (TGFBIp). Autophagic degradation of the denatured proteins by macrophages is one pathway to remove the denatured proteins. Thus, we investigated the role of autophagy in the degradation of mutant (MU) TGFBIp in macrophages.

Methods

Corneas from participants were observed by slit-lamp photography and subjected to histopathologic and genetic analysis. Wild-type (WT) and MU TGFBIp were recombined and expressed. Macrophages from MU participants were isolated and cocultured with the recombinant TGFBIp. Colocalization of the two molecules was observed by immunofluorescent microscopy. Enzyme-linked immunosorbent assay, Western blotting, and flow cytometry were used to detect changes in molecule expression related to the phenotype and autophagy process.

Results

Fourteen members from a family of 25 were identified as LCD sufferers. Significant TGFBIp aggregates and macrophage infiltration were found only in the corneas of LCD sufferers. Marker accumulation of TGFBIp was found in macrophages exposed to MU TGFBIp even at 5 hours after MU TGFBIp was withdrawn. High expressions of CD68 and CD36 were found in macrophages exposed to WT TGFBIp, but not to MU TGFBIp. Impaired autophagic flux due to defective autophagosome fusion to lysosomes was found in macrophages exposed to MU TGFBIp. Blockage of the autophagic process suppressed the expression of CD68 and CD36 in macrophages exposed to WT TGFBIp to levels similar to those found in macrophages exposed to MU TGFBIp.

Conclusions

Our results suggested that reversion of the defective autophagic process in macrophages may be a therapeutic strategy for patients with LCD.

Identification of a Heterozygous Mutation in the TGFBI Gene in a Hui-Chinese Family with Corneal Dystrophy

Background/Aims

Corneal dystrophies (CDs) belong to a group of hereditary heterogeneous corneal diseases which result in visual impairment due to the progressive accumulation of deposits in different corneal layers. So far, mutations in several genes have been responsible for various CDs. The purpose of this study is to identify gene mutations in a three-generation Hui-Chinese family associated with granular corneal dystrophy type I (GCD1).

Methods

A three-generation Hui-Chinese pedigree with GCD1 was recruited for this study. Slit-lamp biomicroscopy, optical coherence tomography, and confocal microscopy were performed to determine the clinical features of available members. Whole exome sequencing was performed on two patients to screen for potential disease-causing variants in the family. Sanger sequencing was used to test the variant in the family members.

Results

Clinical examinations demonstrated bilaterally abundant multiple grayish-white opacities in the basal epithelial and superficial stroma layers of corneas of the two patients. Whole exome sequencing revealed that a heterozygous missense mutation (c.1663C > T, p.Arg555Trp) in the transforming growth factor beta-induced gene (TGFBI) was shared by the two patients, and it cosegregated with this disease in the family confirmed by Sanger sequencing.

Conclusions

The results suggested that the heterozygous TGFBI c.1663C > T (p.Arg555Trp) mutation was responsible for GCD1 in the Hui-Chinese family, which should be of great help in genetic counseling for this family.

Fascinating Fasciclins: A Surprisingly Widespread Family of Proteins that Mediate Interactions between the Cell Exterior and the Cell Surface

The Fasciclin 1 (FAS1) domain is an ancient structural motif in extracellular proteins present in all kingdoms of life and particularly abundant in plants. The FAS1 domain accommodates multiple interaction surfaces, enabling it to bind different ligands. The frequently observed tandem FAS1 arrangement might both positively and negatively regulate ligand binding. Additional protein domains and post-translational modifications are partially conserved between different evolutionary clades. Human FAS1 family members are associated with multiple aspects of health and disease. At the cellular level, mammalian FAS1 proteins are implicated in extracellular matrix structure, cell to extracellular matrix and cell to cell adhesion, paracrine signaling, intracellular trafficking and endocytosis. Mammalian FAS1 proteins bind to the integrin family of receptors and to protein and carbohydrate components of the extracellular matrix. FAS1 protein encoding plant genes exert effects on cellulosic and non-cellulosic cell wall structure and cellular signaling but to establish the modes of action for any plant FAS1 protein still requires biochemical experimentation. In fungi, eubacteria and archaea, the differential presence of FAS1 proteins in closely related organisms and isolated biochemical data suggest functions in pathogenicity and symbiosis. The inter-kingdom comparison of FAS1 proteins suggests that molecular mechanisms mediating interactions between cells and their environment may have evolved at the earliest known stages of evolution.

Structural characterizations of human periostin dimerization and cysteinylation

Human periostin plays a multifaceted role in remodeling the extracellular matrix milieu by interacting with other proteins and itself in both a heterophilic and homophilic manner. However, the structural mechanism for its extensive interactions has remained elusive. Here, we report the crystal structures of human periostin (EMI-Fas1^I-IV ) and its Cys60Ala mutant. In combination with multi-angle light-scattering analysis and biochemical assays, the crystal structures reveal that periostin mainly exists as a dimer in solution and its homophilic interaction is mainly mediated by the EMI domain. Furthermore, Cys60 undergoes cysteinylation as confirmed by mass spectroscopy, and this site hardly affects the homophilic interaction. Also, the structures yield insights into how periostin forms heterophilic interactions with other proteins under physiological or pathological conditions.

Mutation-Induced Deamidation of Corneal Dystrophy-Related Transforming Growth Factor β-Induced Protein

Mutations in the transforming growth factor β-induced protein (TGFBIp) cause phenotypically diverse corneal dystrophies, where protein aggregation in the cornea leads to severe visual impairment. Previous studies have shown a relationship between mutant-specific corneal dystrophy phenotypes and the thermodynamic stability of TGFBIp. Using liquid chromatography-tandem mass spectrometry and nuclear magnetic resonance (NMR), we investigated correlations between the structural integrity of disease-related mutants of the fourth FAS1 domain (FAS1-4) and deamidation of TGFBIp residue Asn622. We observed a high rate of Asn622 deamidation in the A546D and A546D/P551Q FAS1-4 mutants that were both largely unstructured as determined by NMR. Conversely, the more structurally organized A546T and V624M FAS1-4 mutants had reduced deamidation rates, suggesting that a folded and stable FAS1-4 domain precludes Asn622 deamidation. Wild-type, R555Q, and R555W FAS1-4 mutants displayed very slow deamidation, which agrees with their similar and ordered NMR structures, where Asn622 is in a locked conformation. We confirmed the FAS1-4 mutational effect on deamidation rates in full-length TGFBIp mutants and found a similar ranking compared to that of the FAS1-4 domain alone. Consequently, the deamidation rate of Asn622 can be used to predict the structural effect of the many destabilizing and/or stabilizing mutations reported for TGFBIp. In addition, the deamidation of Asn622 may influence the pathophysiology of TGFBIp-induced corneal dystrophies.

Structural and Functional Implications of Human Transforming Growth Factor β-Induced Protein, TGFBIp, in Corneal Dystrophies

A major cause of visual impairment, corneal dystrophies result from accumulation of protein deposits in the cornea. One of the proteins involved is transforming growth factor β-induced protein (TGFBIp), an extracellular matrix component that interacts with integrins but also produces corneal deposits when mutated. Human TGFBIp is a multi-domain 683-residue protein, which contains one CROPT domain and four FAS1 domains. Its structure spans ∼120 Å and reveals that vicinal domains FAS1-1/FAS1-2 and FAS1-3/FAS1-4 tightly interact in an equivalent manner. The FAS1 domains are sandwiches of two orthogonal four-stranded β sheets decorated with two three-helix insertions. The N-terminal FAS1 dimer forms a compact moiety with the structurally novel CROPT domain, which is a five-stranded all-β cysteine-knot solely found in TGFBIp and periostin. The overall TGFBIp architecture discloses regions for integrin binding and that most dystrophic mutations cluster at both molecule ends, within domains FAS1-1 and FAS1-4.

Proteomic Analysis of Amyloid Corneal Aggregates from TGFBI-H626R Lattice Corneal Dystrophy Patient Implicates Serine-Protease HTRA1 in Mutation-Specific Pathogenesis of TGFBIp

TGFBI-associated corneal dystrophies are inherited disorders caused by TGFBI gene variants that promote deposition of mutant protein (TGFBIp) as insoluble aggregates in the cornea. Depending on the type and position of amino acid substitution, the aggregates may be amyloid fibrillar, amorphous globular or both, but the molecular mechanisms that drive these different patterns of aggregation are not fully understood. In the current study, we report the protein composition of amyloid corneal aggregates from lattice corneal dystrophy patients of Asian origin with H626R and R124C mutation and compared it with healthy corneal tissues via LC-MS/MS. We identified several amyloidogenic, nonfibrillar amyloid associated proteins and TGFBIp as the major components of the deposits. Our data indicates that apolipoprotein A-IV, apolipoprotein E, and serine protease HTRA1 were significantly enriched in patient deposits compared to healthy controls. HTRA1 was also found to be 7-fold enriched in the amyloid deposits of patients compared to the controls. Peptides sequences (G511DNRFSMLVAAIQSAGLTETLNR533 and Y571HIGDEILVSGGIGALVR588) derived from the fourth FAS-1 domain of TGFBIp were enriched in the corneal aggregates in a mutation-specific manner. Biophysical studies of these two enriched sequences revealed high propensity to form amyloid fibrils under physiological conditions. Our data suggests a possible proteolytic processing mechanism of mutant TGFBIp by HTRA1 and peptides generated by mutant protein may form the β-amyloid core of corneal aggregates in dystrophic patients.

Disulfide Bond Pattern of Transforming Growth Factor β-Induced Protein

Transforming growth factor β-induced protein (TGFBIp) is an extracellular matrix protein composed of an NH₂-terminal cysteine-rich domain (CRD) annotated as an emilin (EMI) domain and four fasciclin-1 (FAS1-1-FAS1-4) domains. Mutations in the gene cause corneal dystrophies, a group of debilitating protein misfolding diseases that lead to severe visual impairment. Previous studies have shown that TGFBIp in the cornea is cross-linked to type XII collagen through a reducible bond. TGFBIp contains 11 cysteine residues and is thus able to form five intramolecule disulfide bonds, leaving a single cysteine residue available for the collagen cross-link. The structures of TGFBIp and its homologues are unknown. We here present the disulfide bridge pattern of TGFBIp, which was determined by generating specific peptides. These were separated by ion exchange followed by reversed-phase high-performance liquid chromatography and analyzed by mass spectrometry and Edman degradation. The NH₂-terminal CRD contains six cysteine residues, and one of these (Cys65) was identified as the candidate for the reducible cross-link between TGFBIp and type XII collagen. In addition, the CRD contained two intradomain disulfide bridges (Cys49-Cys85 and Cys84-Cys97) and one interdomain disulfide bridge to FAS1-2 (Cys74-Cys339). Significantly, this arrangement violates the predicted disulfide bridge pattern of an EMI domain. The cysteine residues in FAS1-3 (Cys473 and Cys478) were shown to form an intradomain disulfide bridge. Finally, an interdomain disulfide bridge between FAS1-1 and FAS1-2 (Cys214-Cys317) was identified. The interdomain disulfide bonds indicate that the NH₂ terminus of TGFBIp (CRD, FAS1-1, and FAS1-2) adopts a compact globular fold, leaving FAS1-3 and FAS1-4 exposed.

Characteristics of corneal dystrophies: a review from clinical, histological and genetic perspectives

Corneal dystrophy is a common type of hereditary corneal diseases. It includes many types, which have varied pathology, histology and clinical manifestations. Recently, the examination techniques of ophthalmology and gene sequencing advance greatly, which do benefit to our understanding of these diseases. However, many aspects remain still unknown. And due to the poor knowledge of these diseases, the results of the treatments are not satisfoctory. The purpose of this review was to summarize the clinical, histological and genetic characteristics of different types of corneal dystrophies.

pH Induced Conformational Transitions in the Transforming Growth Factor β-Induced Protein (TGFβIp) Associated Corneal Dystrophy Mutants

Most stromal corneal dystrophies are associated with aggregation and deposition of the mutated transforming growth factor-β induced protein (TGFβIp). The 4^th_FAS1 domain of TGFβIp harbors ~80% of the mutations that forms amyloidogenic and non-amyloidogenic aggregates. To understand the mechanism of aggregation and the differences between the amyloidogenic and non-amyloidogenic phenotypes, we expressed the 4^th_FAS1 domains of TGFβIp carrying the mutations R555W (non-amyloidogenic) and H572R (amyloidogenic) along with the wild-type (WT). R555W was more susceptible to acidic pH compared to H572R and displayed varying chemical stabilities with decreasing pH. Thermal denaturation studies at acidic pH showed that while WT did not undergo any conformational transition, the mutants exhibited a clear pH-dependent irreversible conversion from αβ conformation to β-sheet oligomers. The β-oligomers of both mutants were stable at physiological temperature and pH. Electron microscopy and dynamic light scattering studies showed that β-oligomers of H572R were larger compared to R555W. The β-oligomers of both mutants were cytotoxic to primary human corneal stromal fibroblast (pHCSF) cells. The β-oligomers of both mutants exhibit variations in their morphologies, sizes, thermal and chemical stabilities, aggregation patterns and cytotoxicities.

Pathogenesis and treatments of TGFBI corneal dystrophies

Transforming growth factor beta-induced (TGFBI) corneal dystrophies are a group of inherited progressive corneal diseases. Accumulation of transforming growth factor beta-induced protein (TGFBIp) is involved in the pathogenesis of TGFBI corneal dystrophies; however, the exact molecular mechanisms are not fully elucidated. In this review article, we summarize the current knowledge of TGFBI corneal dystrophies including clinical manifestations, epidemiology, most common and recently reported associated mutations for each disease, and treatment modalities. We review our current understanding of the molecular mechanisms of granular corneal dystrophy type 2 (GCD2) and studies of other TGFBI corneal dystrophies. In GCD2 corneal fibroblasts, alterations of morphological characteristics of corneal fibroblasts, increased susceptibility to intracellular oxidative stress, dysfunctional and fragmented mitochondria, defective autophagy, and alterations of cell cycle were observed. Other studies of mutated TGFBIp show changes in conformational structure, stability and proteolytic properties in lattice and granular corneal dystrophies. Future research should be directed toward elucidation of the biochemical mechanism of deposit formation, the relationship between the mutated TGFBIp and the other materials in the extracellular matrix, and the development of gene therapy and pharmaceutical agents.

Early Events in the Amyloid Formation of the A546T Mutant of Transforming Growth Factor β-Induced Protein in Corneal Dystrophies Compared to the Nonfibrillating R555W and R555Q Mutants

The human transforming growth factor β-induced protein (TGFBIp) is involved in several types of corneal dystrophies where protein aggregation and amyloid fibril formation severely impair vision. Most disease-causing mutations are located in the last of four homologous fasciclin-1 (FAS1) domains of the protein, and it has been shown that when isolated, the fourth FAS1 domain (FAS1-4) mimics the behavior of full-length TGFBIp. In this study, we use molecular dynamics simulations and principal component analysis to study the wild-type FAS1-4 domain along with three disease-causing mutations (R555W, R555Q, and A546T) to decipher any internal difference in dynamical properties of the domains that may explain their varied stabilities and aggregation properties. In addition, we use a protein-protein docking method in combination with chemical cross-linking experiments and mass spectrometry of the cross-linked species to obtain information about interaction faces between identical FAS1-4 domains. The results show that the pathogenic mutations A546T and R555W affect the packing in the hydrophobic core of FAS1-4 in different directions. We further show that the FAS1-4 monomers associate using their β-rich regions, consistent with peptides observed to be part of the amyloid fibril core in lattice corneal dystrophy patients.

Absence of Vitamin K-Dependent γ-Carboxylation in Human Periostin Extracted from Fibrotic Lung or Secreted from a Cell Line Engineered to Optimize γ-Carboxylation

Periostin (PN, gene name POSTN) is an extracellular matrix protein that is up-regulated in bronchial epithelial cells and lung fibroblasts by TH-2 cytokines. Its paralog, TGF-β-induced protein (βig-h3, gene name TGFBI), is also expressed in the lung and up-regulated in bronchial myofibroblasts by TGF-β. PN and βig-h3 contain fasciclin 1 modules that harbor putative recognition sequences for γ-glutamyl carboxylase and are annotated in UniProt as undergoing vitamin K-dependent γ-carboxylation of multiple glutamic acid residues. γ-carboxylation profoundly alters activities of other proteins subject to the modification, e.g., blood coagulation factors, and would be expected to alter the structure and function of PN and βig-h3. To analyze for the presence of γ-carboxylation, proteins extracted from fibrotic lung were reacted with monoclonal antibodies specific for PN, βig-h3, or modification with γ-carboxyglutamic acid (Gla). In Western blots of 1-dimensional gels, bands stained with anti-PN or -βig-h3 did not match those stained with anti-Gla. In 2-dimensional gels, anti-PN-positive spots had pIs of 7.0 to >8, as expected for the unmodified protein, and there was no overlap between anti-PN-positive and anti-Gla-positive spots. Recombinant PN and blood coagulation factor VII were produced in HEK293 cells that had been transfected with vitamin K 2, 3-epoxide reductase C1 to optimize γ-carboxylation. Recombinant PN secreted from these cells did not react with anti-Gla antibody and had pIs similar to that found in extracts of fibrotic lung whereas secreted factor VII reacted strongly with anti-Gla antibody. Over 67% coverage of recombinant PN was achieved by mass spectrometry, including peptides with 19 of the 24 glutamates considered targets of γ-carboxylation, but analysis revealed no modification. Over 86% sequence coverage and three modified glutamic acid residues were identified in recombinant fVII. These data indicate that PN and βig-h3 are not subject to vitamin K-dependent γ-carboxylation.

Periostin and TGF-β-induced protein: Two peas in a pod?

Periostin (PN) and TGF-β-induced protein (βig-h3) are paralogs that contain a single emilin and four fasciclin-1 modules and are secreted from cells. PN receives attention because of its up-regulation in cancer and degenerative and allergic diseases. βig-h3 is highly enriched in cornea and best known for harboring mutations in humans associated with corneal dystrophies. Both proteins are expressed widely, and many functions, some over-lapping, have been attributed to PN and βig-h3 based on biochemical, cell culture, and whole animal experiments. We attempt to organize this knowledge so as to facilitate research on these interesting and incompletely understood proteins. We focus particularly on whether PN and βig-h3 are modified by vitamin K-dependent γ-glutamyl carboxylation, a question of considerable importance given the profound effects of γ-carboxylation on structure and function of other proteins. We consider the roles of PN and βig-h3 in formation of extracellular matrix and as ligands for integrin receptors. We attempt to reconcile the contradictory results that have arisen concerning the role of PN, which has emerged as a marker of TH2 immunity, in murine models of allergic asthma. Finally, when possible we compare and contrast the structures and functions of the two proteins.

Fibril Core of Transforming Growth Factor Beta-Induced Protein (TGFBIp) Facilitates Aggregation of Corneal TGFBIp

Mutations in the transforming growth factor beta-induced (TGFBI) gene result in a group of hereditary diseases of the cornea that are collectively known as TGFBI corneal dystrophies. These mutations translate into amino acid substitutions mainly within the fourth fasciclin 1 domain (FAS1-4) of the transforming growth factor beta-induced protein (TGFBIp) and cause either amyloid or nonamyloid protein aggregates in the anterior and central parts of the cornea, depending on the mutation. The A546T substitution in TGFBIp causes lattice corneal dystrophy (LCD), which manifests as amyloid-type aggregates in the corneal stroma. We previously showed that the A546T substitution renders TGFBIp and the FAS1-4 domain thermodynamically less stable compared with the wild-type (WT) protein, and the mutant FAS1-4 is prone to amyloid formation in vitro. In the present study, we identified the core of A546T FAS1-4 amyloid fibrils. Significantly, we identified the Y571-R588 region of TGFBIp, which we previously found to be enriched in amyloid deposits in LCD patients. We further found that the Y571-R588 peptide seeded fibrillation of A546T FAS1-4, and, more importantly, we demonstrated that native TGFBIp aggregates in the presence of fibrils formed by the core peptide. Collectively, these data suggest an involvement of the Y571-R588 peptide in LCD pathophysiology.

In silico analyses of missense mutations in coagulation factor VIII: identification of severity determinants of haemophilia A

Factor VIII (FVIII) mutations cause haemophilia A (HA), an X-linked recessive coagulation disorder. Over 1000 missense mutations in FVIII are known and they lead to variable clinical phenotypes (severe, moderate and mild). The exact molecular basis of this phenotypic heterogeneity by FVIII missense mutations is elusive to date. In this study, we aimed to identify the severity determinants that cause phenotypic heterogeneity of HA. We compiled and curated a data set of 766 missense mutations from the repertoire of missense mutations in FVIII. We analysed these mutations by computational programs (e.g. Swiss-PdbViewer) and different mutation analysis servers (e.g. SIFT, PROVEAN, CUPSAT, PolyPhen2, MutPred); and various sequence- and structure-based parameters were assessed for any significant distribution bias among different HA phenotypes. Our analyses suggest that 'mutations in evolutionary conserved residues', 'mutations in buried residues', mutation-induced 'steric clash' and 'surface electrostatic potential alteration' act as risk factors towards severe HA. We have developed a grading system for FVIII mutations combining the severity determinants, and the grading pattern correlates with HA phenotype. This study will help to correctly associate the HA phenotype with a mutation and aid early characterization of novel variants.

Structure-function correlation analysis of connexin50 missense mutations causing congenital cataract: electrostatic potential alteration could determine intracellular trafficking fate of mutants

Connexin50 (Cx50) mutations are reported to cause congenital cataract probably through the disruption of intercellular transport in the lens. Cx50 mutants that undergo mistrafficking have generally been associated with failure to form functional gap junction channels; however, sometimes even properly trafficked mutants were found to undergo similar consequences. We hereby wanted to elucidate any structural bases of the varied functional consequences of Cx50 missense mutations through in silico approach. Computational studies have been done based on a Cx50 homology model to assess conservation, solvent accessibility, and 3-dimensional localization of mutated residues as well as mutation-induced changes in surface electrostatic potential, H-bonding, and steric clash. This was supplemented with meta-analysis of published literature on the functional properties of connexin missense mutations. Analyses revealed that the mutation-induced critical alterations of surface electrostatic potential in Cx50 mutants could determine their fate in intracellular trafficking. A similar pattern was observed in case of mutations involving corresponding conserved residues in other connexins also. Based on these results the trafficking fates of 10 uncharacterized Cx50 mutations have been predicted. Further experimental analyses are needed to validate the observed correlation.