Collagen <scp>XII</scp> Plays a More Prominent Cell‐Mediated Role in Tendon Organization Compared to Matrix Assembly During Postnatal Development

Tendon structural and mechanical integrity is essential for overall joint function. Establishment of tendon structure-function is regulated by coordinated processes between cells and the extracellular matrix. However, the cell-cell and cell-matrix interactions necessary for re-creating the native tendon structure following injury remain unelucidated. Collagen XII is a fibril-associated collagen with key structural roles in modulating collagen fibrillogenesis, matrix protein interactions, and forming bridges between fibrils. Collagen XII has also been shown to regulate cell structure and organization, suggesting that collagen XII coordinates cell- and matrix-mediated processes necessary for proper tendon formation. Therefore, the study objective was to define the temporal roles of collagen XII in regulating cell arrangement and matrix assembly during tendon development. To investigate the cumulative effects of manipulating collagen XII expression in tendon with respect to cell organization, scleraxis-Cre collagen XII knockout mice were evaluated for patellar tendon morphology, cell organization, matrix structure and function, and gene expression. At postnatal day 0, disruptions to cell and matrix organization due to collagen XII knockout were profound. F-actin organization in knockout tendons lacked parallelism with areas of high density, and cell membrane protrusions did not make contact with neighboring cells. Disrupted matrix structure was also observed at later postnatal ages, indicating a critical cell-mediated role of collagen XII. Collagen XII knockout also inhibited the formation of a proper tibial attachment, resulting in altered collagen organization and mechanical properties at the insertion site. To investigate the specific matrix assembly role of collagen XII, knockdown was induced at postnatal day 0. Mechanical changes at the tibial insertion site were similar, but overall effects on matrix organization and structure were minimal. Together, our findings indicate a more substantial role of collagen XII for regulating cell arrangement rather than matrix assembly in the establishment of tendon structure-function.