Proteomic characterization of type I collagen N-terminal crosslinked peptides

Collagen cross-links mediated by the lysyl oxidase and lysyl hydroxylase families of enzymes significantly contribute to the biomechanical strength and rigidity of tissues, influencing cell signaling and the downstream cell phenotype. In the clinic, the proteolytically liberated N-terminal cross-linked peptide of collagen I (NTX) is used as a biomarker of bone and connective tissue turnover, which is altered in several disease processes. Despite the clinical utility of these collagen breakdown products, the majority of the cross-linked peptide species have not been identified in proteomic datasets. Here, we evaluate several parameters for the preparation and identification of these peptides from the collagen I-rich Achilles tendon. Our refined approach, which involves chemical digestion for protein solubilization coupled with mass spectrometry, enables the identification of NTX cross-links in a range of modification states. We then applied a spectral library approach to identify differences in collagen cross-links in bovine pulmonary hypertension. The presented method offers unique opportunities to understand extracellular matrix remodeling events in development, aging, wound healing, and fibrotic disease that modulate collagen architecture through lysyl hydroxylase and lysyl oxidase enzymes.