Identification of latexin by a proteomic analysis in rat normal articular cartilage

In this study, 147 proteins were visualized, and 47 proteins were identified by MS. A significant proportion of proteins are involved in metabolic processes and energy (32%), as well as participating in different biological functions including structural organization (19%), signal transduction and molecular signaling (11%), redox homeostasis (9%), transcription and protein synthesis (6%), and transport (6%). The identified proteins were assigned to one or more subcellular compartments.Among the identified proteins, we found some proteins already recognized in other studies such as OA-associated proteins. Interestingly, we identified LXN, an inhibitor of mammalian carboxypeptidases, which had not been described in articular cartilage. Immunolabeling assays for LXN showed a granular distribution pattern in the cytoplasm of most chondrocytes of the middle, deep and calcified zones of normal articular cartilage as well as in subchondral bone. In osteoarthritic cartilage, LXN was observed in superficial and deep zones.This study provides the first proteomic analysis of normal articular cartilage of rat. We identified LXN, whose location was demonstrated by immunolabeling in the chondrocytes from the middle, deep and calcified zones of normal articular cartilage, and superficial and deep zones of osteoarthritic cartilage.Articular cartilage is a specialized connective tissue and its principal functions are reduction of friction in the joint, resistance to compressive forces and distribution of load [1]. Macromolecules that constitute the extracellular matrix (ECM) of cartilage such as collagens and proteoglycans (PGs) make the reproducibility of protein profiles difficult, interfering with isoelectric focusing (IEF) and masking minor cell proteins [2]. Previous studies reported the use of different selective extraction techniques in human and animal cartilage models, which allowed the rapid and efficient detection of protein expression patterns [3-5].Proteomic technology