Posttranslational Modifications of Biglycan Orchestrate Inflammation in Calcific Aortic Valve Disease

Background: Inflammation is a major trigger for the onset of calcific aortic valve disease (CAVD). Biglycan (BGN) is a proteoglycan responsible for the stability of the valvular extracellular matrix. It is released upon mechanical stress and acts as danger-associated molecular pattern activating the innate immune system. We aimed to (1) characterize the mechanism of innate immune activation and (2) explore it as potential therapeutic target.

Method: Ligand receptor interactions were characterized via immunoprecipitation. Mice were subjected to high-fat diet to induce CAVD. Aortic valve function was analyzed via echocardiography and histology. The bone phenotype of Bgn−/− mice was characterized through µ-CT. Selective digestion of BGN side chains using specific enzymes was performed. Patient samples and aged valvular interstitial cells (VICs) were analyzed for gene- and protein expression via qPCR and western blot. Two clinical cohorts (GERA, n = 55,192 with 3,469 aortic stenosis cases; UK Biobank, n = 257,231 with 2,213 aortic stenosis cases) were examined for genetic variation at 12 genes implicated in the BGN/TLR3 signaling pathway.

Results: Biglycan is a proteoglycan occurring ubiquitously in the extracellular matrix. Biological activity of BGN was primarily dependent on the glycosylation of its serine residues. While a direct interaction was found, the studies revealed that post-translational modifications (PTMs) of BGN by xylosyltransferase 1 (XYLT1) strongly modified its potential to activate innate immune receptor Toll-like receptor 3 (TLR3). Levels of XYLT1 were increased in human samples of stenotic valves and aged VICs. Both Tlr3−/− and Bgn−/− mice were protected from a CAVD phenotype, showing no signs of morphological valve thickening or hemodynamic signs of CAVD in transthoracic echocardiographies. Finally, we observed 294 variants which were nominally significant (p ≤ 0.05) in two clinical cohorts of aortic stenosis. Notably, 14 variants in genes within the BGN/TLR3 pathway demonstrated strong associations (p ≤ 1 × 10–3) and/or 2-fold or greater (up to 5.86-fold) odds of aortic stenosis.

Conclusion: Our results uncover the XYLT1-BGN-TLR3 axis as a novel pathway to endogenous regulation of inflammation posing a promising target for future development of therapeutic intervention or biomarkers to tackle CAVD.

Publikationsverlauf

Artikel online veröffentlicht:
28. Januar 2023