Background In rheumatoid arthritis (RA) the attachment of synovial fibroblasts to articular cartilage is an important prerequisite in the process of cartilage degradation. The actin-associated protein Lasp-1 is involved in processes of actin organisation and polymerization and focal adhesion turnover, respectively. Therefore, we investigated its role in regulating cell-cell contacts and ECM interactions of synovial fibroblasts in RA.
Methods Lasp-1 expression was analysed in human RA tissue and in arthritic mouse models such as the hTNFtg and G6PI mouse model by using WB and immunohistochemistry. Lasp-1 expression and its subcellular distribution was investigated in isolated synovial fibroblasts by immunofluorescence in all genotypes. Furthermore, the formation of cell-cell and -ECM contacts was investigated in an electrical cell/substrate impedance sensing assay (ECIS) as well as by immunofluorescence. Additionally, we performed an in vitro three-dimensional organ culture system to identify genotype-specific differences of cellular distribution and formation of the artificial synovial lining layer.
Results An increased Lasp-1 expression was observed in synovial tissue from RA patients as well as in RASF. Isolated SF from hTNFtg mice also produced higher levels of Lasp-1 compared to wild type (wt) SF. Interestingly, this was completely reproducible in the G6PI model. Lasp-1 deficient mice that had been crossed into the hTNFtg background exhibited less cartilage degradation (−4% vs. hTNFtg) and less attachment of synovial tissue to the cartilage (−30% vs. hTNFtg) compared to hTNFtg mice. Functional analyses indicated that Lasp-1-/- hTNFtg SF form closer (+20% vs hTNFtg SF) and more stable cell-cell contacts in comparison to hTNFtg SF. Furthermore, we detected Lasp-1 to be located at structures of adherens junction complexes. The loss of Lasp-1 led to alterations of these structures. Histological analysis of 3D matrices showed that Lasp-1 deletion in the hTNFtg background resulted in an organised cellular lining layer at the interface between the matrix and fluid phases comparable with wild type SF. In contrast, hTNFtg SF formed unorganised cellular condensations with no synovial architecture evident.
Conclusions Lasp-1 regulates the migratory behaviour of synovial fibroblasts and their invasion into cartilage matrix in rheumatoid arthritis by controlling the dynamics of cell-cell contacts.