Background The Fibroblast Activation Protein alpha (FAPα) is an integral membrane serine protease that plays a major role in migration, wound healing, and metastasis. Based on recent studies that have implicated membrane-bound serine proteases in osteoclast migration, we studied the expression of FAPα in rheumatoid arthritis (RA) and analysed its role in osteoclast development under inflammatory conditions.
Methods FAPα expression in vivo was determined by immunohistochemistry in human synovial tissues of patients with RA and osteoarthritis (OA) as well as in hind paws of tumour necrosis factor-alpha transgenic (hTNFtg) mice, which develop a RA-like destructive arthritis. In vitro expression and regulation of FAPα was analysed in differentiated osteoclasts cocultured with osteoblasts, RA or OA synovial fibroblasts (SF) by PCR. To scrutinise the role of FAPα in osteoclastogenesis, we analysed the in vitro differentiation of osteoclasts from wildtype (WT) and FAP-/- mice (Oncology Research, Boehringer Ingelheim RCV, Vienna) by TRAP staining. In order to assess the role of FAP in arthritis severity, we crossed FAP-/- and hTNFtg mice and performed TRAP staining.
Results RA synovial tissues demonstrated a high expression of FAPα throughout the tissue whereas in OA samples FAPα was expressed only in the lining layer. In vitro, no expression of FAPα was found in differentiated preosteoclasts and osteoclasts, but coculture experiments showed that RASF, but not OASF or osteoblasts, induce the expression of FAPα in preosteoclasts and osteoclasts. Consistent with the selective induction of FAPα in osteoclasts by RASF, FAPα expression was detected in osteoclasts at the invasion front of the hyperplastic synovial tissues in joints of hTNFtg mice. FAP-/- mice show a severely diminished osteoclast formation compared to WT. We also found a lesser amount of osteoclasts in the hind paws of FAP-/-hTNFtg compared to hTNFtg.
Conclusions The disease-dependent expression of FAPα by osteoclasts in human RA and hTNFtg mice suggests an important role of FAPα in joint destruction in RA. The selective induction of FAPα in preosteoclasts and osteoclasts by RASF indicate that FAPα may be regulated through the interaction with the pannus tissue. The fact, that under inflammatory conditions the loss of FAP led to a reduced number of osteoclasts in the hind paws and FAP deficient bone marrow derived macrophages showed a reduced osteoclast formation, suggest a role of this serine protease in macrophage/osteoclast precursor migration and differentiation.