Xylosyltransferase-deficient human HEK293 cells show a strongly reduced proliferation capacity and viability

A novel SPE-UPLC-MS/MS-based assay for the selective, simultaneous quantification of xylosyltransferase-I and -II activity

Xylosyltransferase-I and -II (XT-I, -II) possess a central role during the glycosylation of proteoglycans (PGs). They catalyze the formation of an O-glycosidic bond between the xylosyl residue of uridinediphosphate-xylose and the core protein of a PG. Thereafter, three following glycosyltransferases lead to the generation of a tetrasaccharide linker, which connects the PG core protein to the respective glycosaminoglycan. The selective quantification of XT-I and XT-II activity is of biological and clinical interest due to their association with fibrotic processes and skeletal dysplasia. There is no assay available to date that simultaneously determines the activity of the two XT isoforms. Although an XT-I selective UPLC MS/MS-based assay was published by Fischer et al., in 2021, the determination of XT-II activity can only be performed simultaneously by the improved assay presented here. To establish the assay, two synthetic peptides, selectively xylosylated by the respective isoform, were identified and the associated measurement parameters for the mass spectrometer were optimized. In addition, the quantitative range of the xylosylated peptides were validated, and the incubation time of the enzyme reaction was optimized for cell culture samples and human sera. The specific enzyme kinetics (KM and Vmax) of the respective XT isoform for the two peptides were also determined. Subsequently, a mathematical model was developed, allowing the simultaneous determination of XT-I and XT-II activity from the chromatographic results. Summarized, a mass spectrometric method suitable for the simultaneous analysis of XT-I and XT-II activity in cell culture lysates, supernatants and human sera was successfully developed.

Human Xylosyltransferase I-An Important Linker between Acute Senescence and Fibrogenesis

The human xylosyltransferase isoform XT-I catalyzes the initial step in proteoglycan biosynthesis and represents a biomarker of myofibroblast differentiation. Furthermore, XT-I overexpression is associated with fibrosis, whereby a fibrotic process initially develops from a dysregulated wound healing. In a physiologically wound healing process, extracellular matrix-producing myofibroblasts enter acute senescence to protect against fibrosis. The aim of this study was to determine the role of XT-I in acute senescent proto-myofibroblasts. Normal human dermal fibroblasts were seeded in a low cell density to promote myofibroblast differentiation and treated with H₂O₂ to induce acute senescence. Initiation of the acute senescence program in human proto-myofibroblasts resulted in a suppression of XYLT mRNA expression compared to the control, whereby the isoform XYLT1 was more affected than XYLT2. Moreover, the XT-I protein expression and enzyme activity were also reduced in H₂O₂-treated cells compared to the control. The examination of extracellular matrix remodeling revealed reduced expression of collagen I, fibronectin and decorin. In summary, acute senescent proto-myofibroblasts formed an anti-fibrotic phenotype, and suppression of XT-I during the induction process of acute senescence significantly contributed to subsequent ECM remodeling. XT-I therefore plays an important role in the switch between physiological and pathological wound healing.

First Characterization of Human Dermal Fibroblasts Showing a Decreased Xylosyltransferase-I Expression Induced by the CRISPR/Cas9 System

BACKGROUND

Xylosyltransferases-I and II (XT-I and XT-II) catalyze the initial and rate limiting step of the proteoglycan (PG) biosynthesis and therefore have an import impact on the homeostasis of the extracellular matrix (ECM). The reason for the occurrence of two XT-isoforms in all higher organisms remains unknown and targeted genome-editing strategies could shed light on this issue.

METHODS

XT-I deficient neonatal normal human dermal fibroblasts were generated by using the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated proteins (Cas) 9 system. We analyzed if a reduced XT-I activity leads to abnormalities regarding ECM-composition, myofibroblast differentiation, cellular senescence and skeletal and cartilage tissue homeostasis.

RESULTS

We successfully introduced compound heterozygous deletions within exon 9 of the XYLT1 gene. Beside XYLT1, we detected altered gene-expression levels of further, inter alia ECM-related, genes. Our data further reveal a dramatically reduced XT-I protein activity. Abnormal myofibroblast-differentiation was demonstrated by elevated alpha-smooth muscle actin expression on both, mRNA- and protein level. In addition, wound-healing capability was slightly delayed. Furthermore, we observed an increased cellular-senescence of knockout cells and an altered expression of target genes knowing to be involved in skeletonization.

CONCLUSION

Our data show the tremendous relevance of the XT-I isoform concerning myofibroblast-differentiation and ECM-homeostasis as well as the pathophysiology of skeletal disorders.