Background/Purpose: In rheumatoid arthritis (RA) the attachment of synovial fibroblasts (SF) 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 organization 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 tissue from RA patients, in hind paws of arthritic mouse models such as the hTNFtg and G6PI mouse model by using WB and immunohistochemistry. Furthermore, Lasp-1^-/- mice were interbred with hTNFtg mice and offsprings were analysed for the progression of joint destruction by clinical evaluation and histopathology. Cell spreading as well as migration characteristics of SF derived from wild type (wt), Lasp-1^-/-, hTNFtg and Lasp1^-/-hTNFtg mice were analysed by live cell imaging and TEM. Cell-matrix interactions as well as cell-cell contacts of isolated SF from all different genotypes were investigated in an electrical cell/substrate impedance sensing assay (ECIS). Additionally, we used an in vitro 3D organ culture system for functional analyses.

Results: Lasp-1 expression levels were significantly increased in human and murine RA tissue as well as in arthritic SF in comparison to healthy controls. Evaluation of Lasp-1^-/-hTNFtg mice revealed a milder arthritis score, less cartilage degradation and reduced SF attachment to articular cartilage compared to hTNFtg mice. Results of cell spreading and migration analyses demonstrated alterations in spreading morphology and cell-cell contacts and a significantly reduced migration rate of Lasp-1^-/- SF and Lasp-1^-/-hTNFtg SF compared to healthy controls. Immunofluorescence showed an unstructured and irregular cytoskeleton in cells with Lasp-1 deletion compared to other cells, confirmed by TEM. ECIS analysis demonstrated increased cell-cell contact formation in Lasp-1^-/- compared to wt SF (+22% vs wt SF) and prolonged cell-cell interactions of Lasp-1^-/-hTNFtg SF in comparison to hTNFtg SF. Histological sections of the 3D matrices demonstrated that wt SF formed an organised synovial structure comparable with healthy synovial tissue in vivo whereas in matrices with hTNFtg SF, this synovial architecture was not seen. Interestingly, Lasp-1 deletion in the hTNFtg background resulted in an organised cellular lining layer comparable with wt SF matrices.

Conclusion: Lasp-1 regulates the migratory behaviour of synovial fibroblasts and their invasion into cartilage matrix in rheumatoid arthritis by controlling the dynamics of cell-matrix and cell-cell contacts.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/lasp-1-regulates-cell-matrix-and-cell-cell-contacts-in-arthritic-mouse-models/