Background/Purpose: RA synovial fibroblasts (SF) have been suggested to contribute to the spreading of disease through their ability to leave cartilage destruction sites, migrate via the bloodstream and re-initiate the destructive process at distant articular cartilage surfaces. The underlying mechanisms are unclear, but the actin-crosslinking protein Lasp-1 is of interest in this context, because it localizes to the leading edges of migrating cells and is of importance for the metastatic dissemination of different tumors. Therefore, it is particularly important to investigate the role of Lasp-1 in synovial fibroblast migration and its effects on RA.

Methods: The expression of Lasp-1 in the hind paws of wt and hTNFtg mice, an established model for human RA, was investigated by immunohistochemistry. Western-blot analyses and immunofluorescence was performed to analyze Lasp-1 expression and sub-cellular distribution in SFs from wt and hTNFtg mice. The migratory capacity of SFs derived from wild-type, Lasp-1^-/-and hTNFtg mice was studied in a modified scratch assay as well as in a transmigration assay using murine endothelioma cells (bEnd.5) as an endothelial barrier and TNF-alpha as a stimulus. Furthermore, we used Lasp-1 knockout mice for interbreeding studies with hTNFtg mice.

Results:  Immunostainings and Western Blot analyses showed a prominent expression of Lasp-1 in synovial fibroblasts obtained from hTNFtg mice with a characteristic sub-cellular distribution localizing Lasp-1 to structures of cell adhesion and invasion. In the scratch assay, Lasp-1^-/- SFs exhibited a significantly reduced migration rate after 2 days (-30% vs. wt, p<0.05). Although the migratory capacity of unstimulated wt SFs through the endothelial monolayer was generally low (-32% vs. hTNFtg, p<0.05), this was virtually abolished by the knock out of Lasp-1 (-78% vs. wt, p<0.05). TNF-alpha enhanced the migratory potential of wild-type SFs to a significantly higher extent than of Lasp-1 null SFs (+84%, p<0.05). Interestingly, interbred Lasp1^-/-/hTNFtg mice presented milder clinical symptoms and analyses of histopathology revealed less cartilage degradation than hTNFtg mice at an age of 14 weeks. 

Conclusion: Our data provide first evidence that Lasp-1 regulates the migratory capacity of synovial fibroblasts and influences the severity of arthritis in hTNFtg mice. While the mechanisms of trans-endothelial migration of SFs are largely undiscovered, our data suggest that these cells – when activated – migrate through the formation of invasive and adhesive membrane structures such as invadopodia, where Lasp-1 is prominently localized. Thus, targeting Lasp-1 may be a promising strategy modulate the invasive and migratory behavior of synovial fibroblasts in RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/synovial-fibroblast-migration-is-modulated-by-the-focal-contact-protein-lasp-1/