Proteolytic Regulation of Proteoglycan 4 in Inflammation
Creators: name: Das, Nabangshu Shekhar
Publisher: name: Graduate Studies
Year: 2023
Subjects: subject: Proteoglycan 4
subject: Lubricin
subject: Inflammation
subject: Education--Sciences

description: Proteoglycan 4 (PRG4) is an extracellular macromolecule that is synthesized and secreted by cells lining surfaces of multiple tissues. While it has traditionally been studied and described as a boundary lubricant, recent evidence suggests that it can bind to and affect downstream signaling of several cell surface receptors, including toll-like receptors (TLRs) that are involved in regulating inflammatory responses. Although previous studies have shown that proteolysis of PRG4 reduces its boundary lubricating ability in vitro compared to intact PRG4, the effect of proteolysis on inflammatory signaling remained uncharacterized. Furthermore, while differential levels of PRG4 expression have been associated with various inflammatory conditions, such as osteoarthritis, the role of PRG4 in maintaining inflammatory signaling during normal aging under homeostatic conditions is still unclear. The objectives of this thesis were to 1- Determine expression of PRG4 in global proteomes during inflammation, and 2- Characterize of proteolytic processing of PRG4 and the mechanism and effects in OA and 3- Examine the effects of PRG4 on the global proteomes during age-related joint inflammation. Global proteome analysis revealed PRG4 expression levels changed locally during inflammation as demonstrated by a reduction in the PRG4 expression in the tears while an increase in the saliva of Sjogren’s syndrome patients comparing to healthy individuals. These differential levels of PRG4 expression were associated with differential expression profiles of different signaling molecules (i.e., proteases) associated with multiple inflammatory pathways including neutrophil degranulation both of the tissue environment. With respect to proteolytic susceptibility of PRG4 during inflammation, this study revealed that the proinflammatory serine protease, tryptase β, cleaves PRG4 altering its functional properties with respect to lubrication and inflammation: it reduces the ability to provide boundary lubrication and increases the ability to activate NF-κB-mediated inflammation through the TLR pathway. In a destabilization of medial meniscus (DMM) model of osteoarthritis (OA) in rat knee joint, differential colocalization of tryptase β and PRG4 in knee joints was associated with the development of OA and disease progression. Treatment with an intra-articular injection of exogenous PRG4 was able to resolve joint inflammation and OA phenotypes in the post-DMM rat knee by restoring the expression of PRG4 on articular cartilage and tryptase β by cartilage chondrocytes. In the absence Prg4, chondrocytes in murine knee cartilage express elevated level of proteases, particularly tryptase β and Htra-1, along with concurrent development of neutrophil-like phenotypes during aging. The neutrophil-like phenotype of chondrocytes is associated with loss of joint integrity in Prg4-/- mice. Overall, this thesis demonstrates the importance of expression levels and structural integrity of PRG4 in maintaining homeostasis through regulating inflammation, provides greater insights into the complex interplay between PRG4, proteases and inflammation, and provide the foundation and motivation for the development of new treatments for inflammatory diseases.