Critical role of collagen XII in cell- and matrix-mediated mechanisms regulating acquisition of tendon structure and function in development and the injury response
Establishment of tendon hierarchical structure is critical to mechanical function. This tightly controlled process requires coordinated cell-cell and cell-matrix communication. During embryogenesis, tendon progenitors organize into linear arrays and establish cell-cell communication prior to assembling ECM, suggesting that cells dictate ECM organization. Cells also clonally expand within linear arrays, suggesting that the ECM also dictates cell organization. Collagen XII is known to regulate collagen fibril assembly by forming bridges between fibrils, and our recent data show that collagen XII-deficient tendons exhibit reduced fibril packing and loss of distinct fiber domains. Interestingly, we also found that these tendons have disordered tenocyte arrangement and gap junction organization, indicating a novel role for collagen XII in cell organization, cell communication, and establishing an organized tenocyte network. However, the extent to which disrupted tendon hierarchical structure due to collagen XII deficiency is driven by disordered cellular arrangement and communication or by the deposition of disorganized ECM remains unelucidated. Therefore, our overarching goal is to establish the temporal roles of collagen XII in regulating tendon cell organization, hierarchical structure, and mechanical function during tendon development and healing. Our global hypothesis is that, in addition to ECM fibril assembly, collagen XII regulates cellular arrangement and communication prior to ECM deposition during development and healing, which is pivotal to establishing normal tendon structure-function. We will use novel tissue-targeted and inducible Col12a1 knockout mouse models to specifically target tendons during development and healing. These mouse models will be used with an innovative multiscale approach to assess tissue level mechanics, cell organization and communication, fiber alignment, and fibril size/organization. We will define the temporal roles of collagen XII in regulating cell arrangement and ECM assembly during tendon growth and development through targeted knockdown of Col12a1 throughout tendon development or following establishment of cell organization. We will also define the temporal roles of collagen XII in regulating cell arrangement and ECM assembly during and after the proliferative phase of tendon healing. We will utilize sophisticated and rigorous measures of hierarchical structure/function to define the interplay between cell and ECM assembly in tendon formation through establishment of temporal roles for collagen XII in these processes. These innovative studies will provide guidance for future therapies.