Cathepsin S: Modulating Osteoimmunology

During the process of aging bone mass declines and the hematopoietic bone marrow of young subjects is replaced with fat marrow. At a cellular level, the number of adipocytes increases at the cost of osteoblasts, consequently leading to porous bone. Adipocytes and osteoblasts arise from a common progenitor cell and various factors such as PPARg have been shown to positively regulate adipocyte differentiation while concomitantly decreasing osteoblastogenesis. Recently, the cysteine protease cathepsin S has been identified as a positive regulator of preadipocyte differentiation but its role in osteoblast differentiation has not been determined yet. Besides the role of cathepsin S in adipocyte differentiation and matrix degradation this molecule plays an important role in the maturation of MHC II molecules in professional antigen presenting cells (APC). The inactivation of cathepsin S has been demonstrated to lead to resistance of autoimmune diseases suggesting an imperative function for cathepsin S in autoimmune responses. Some cell types, including osteoblasts, act as non-professional APCs after stimulation with bacterial cell products or inflammatory cytokines. However, the role of cathepsin S has not been determined in the maturation process of MHC II and MHC I-like molecules in osteoblasts. Therefore, we hypothesize that cathepsin S negatively regulates osteoblast differentiation and that the effects will be reflected in the lowered expression of osteoblast markers. We believe that the shift from osteoblasts to adipocytes, thus, the higher expression of cathepsin S, will be best reflected in aged animals. Further, we hypothesize that cathepsin S is necessary for the expression of MHC class II and CD1d molecules on cytokine- challenged osteoblasts and that absence of cathepsin S will lead to a decreased T cell activation by osteoblasts. To test the first hypothesis we will assess the bone and fat phenotype of cathepsin S null mice by histomorphometric analysis and immunohistochemistry. Moreover, we plan to study osteoblast differentiation by marker expression analyses at mRNA and protein level in osteoblasts isolated from cathepsin S null and wild-type mice in time-course experiments. Moreover, the aspect of aging will be taken into consideration by investigating the phenotype and marker analysis in aged animals as well. The second hypothesis will be tested determining the expression of MHC class II and CD1d molecules on the surface of cytokine-challenged cathepsin S null and wild- type osteoblasts by FCM. Finally, MAC sorted T cells will be co-cultured with cytokine-stimulated cathepsin S null and wild-type osteoblasts to determine the effects of cathepsin S in T cell activation by osteoblasts. The results obtained from our study are expected to reveal novel insights into the role of cathepsin S in osteoblast differentiation. If cathepsin S appears to be a regulator between adipocyte and osteoblast differentiation, it might be an interesting target gene for shifting the ratio towards osteoblast differentiation in patients suffering from age- related osteoporosis. Due to the importance of cathepsin S in the antigen presentation in autoimmune diseases it would be a great benefit to identify its role in MHC class II and CD1d-mediated antigen presentation by osteoblasts, in order to find new, selective therapeutic strategies for autoimmune diseases, such as rheumatoid arthritis (RA).

During the process of aging bone mass declines and the hematopoietic bone marrow of young subjects is replaced with fat marrow. At a cellular level, the number of adipocytes increases at the cost of osteoblasts, consequently leading to porous bone. Adipocytes and osteoblasts arise from a common progenitor cell and various factors such as PPARg have been shown to positively regulate adipocyte differentiation while concomitantly decreasing osteoblastogenesis. Recently, the cysteine protease cathepsin S has been identified as a positive regulator of preadipocyte differentiation but its role in osteoblast differentiation has not been determined yet. Besides the role of cathepsin S in adipocyte differentiation and matrix degradation this molecule plays an important role in the maturation of MHC II molecules in professional antigen presenting cells (APC). The inactivation of cathepsin S has been demonstrated to lead to resistance of autoimmune diseases suggesting an imperative function for cathepsin S in autoimmune responses. Some cell types, including osteoblasts, act as non-professional APCs after stimulation with bacterial cell products or inflammatory cytokines. However, the role of cathepsin S has not been determined in the maturation process of MHC II and MHC I-like molecules in osteoblasts. Therefore, we hypothesize that cathepsin S negatively regulates osteoblast differentiation and that the effects will be reflected in the lowered expression of osteoblast markers. We believe that the shift from osteoblasts to adipocytes, thus, the higher expression of cathepsin S, will be best reflected in aged animals. Further, we hypothesize that cathepsin S is necessary for the expression of MHC class II and CD1d molecules on cytokine- challenged osteoblasts and that absence of cathepsin S will lead to a decreased T cell activation by osteoblasts. To test the first hypothesis we will assess the bone and fat phenotype of cathepsin S null mice by histomorphometric analysis and immunohistochemistry. Moreover, we plan to study osteoblast differentiation by marker expression analyses at mRNA and protein level in osteoblasts isolated from cathepsin S null and wild-type mice in time-course experiments. Moreover, the aspect of aging will be taken into consideration by investigating the phenotype and marker analysis in aged animals as well. The second hypothesis will be tested determining the expression of MHC class II and CD1d molecules on the surface of cytokine-challenged cathepsin S null and wild- type osteoblasts by FCM. Finally, MAC sorted T cells will be co-cultured with cytokine-stimulated cathepsin S null and wild-type osteoblasts to determine the effects of cathepsin S in T cell activation by osteoblasts. The results obtained from our study are expected to reveal novel insights into the role of cathepsin S in osteoblast differentiation. If cathepsin S appears to be a regulator between adipocyte and osteoblast differentiation, it might be an interesting target gene for shifting the ratio towards osteoblast differentiation in patients suffering from age- related osteoporosis. Due to the importance of cathepsin S in the antigen presentation in autoimmune diseases it would be a great benefit to identify its role in MHC class II and CD1d-mediated antigen presentation by osteoblasts, in order to find new, selective therapeutic strategies for autoimmune diseases, such as rheumatoid arthritis (RA).