Defining the Role of Type III Collagen in Neonatal Tendon Development and Healing

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Project Abstract Tendons can withstand large forces due to a highly aligned, dense collagen matrix. However, their low cellularity and relative inability to recruit reparative cells post-injury, as well as susceptibility to excessive scarring, results in loss of tendon structure and mechanical function. Type I collagen (Col1) is the primary collagen of healthy tendon and type III collagen (Col3) is a minor constituent that increases in response to injury. Persistently increased Col3 contributes to persistent fibrovascular scarring and structural and functional deficits in the healing tendon. In perinatal tendons, Col3 is increased, similar to the injured state, compared to healthy mature tendons. Unlike the healing response, the process of neonatal tendon development yields a structurally and functionally superior tendon with a highly aligned Col1-dense matrix. Moreover, neonatal developing tendon demonstrates improved efficiency and quality of healing compared to healing mature tendon. Understanding the role of Col3 in the developmental and healing processes of the neonatal tendon will increase our ability to recapitulate tendon development with tissue engineering and improve tendon injury treatment. Therefore, our overall objective is to delineate the contribution of Col3 to development and healing in the neonatal tendon through modulation of matrix properties and cellular activity. Specifically, we will test the hypothesis that Col3 is crucial for early neonatal development but contributes less to regulation of development at later time points as relative Col3 in the tendon decreases. We also hypothesize that the neonatal tendon has enhanced capacity for a robust proliferative response to tendon injury which creates a Col3-dense healing matrix favorable for tendon progenitor migration and differentiation to ultimately deposit aligned, Col1 fibrils which restore tendon structure and function. To test these hypotheses, we generated a novel, inducible Col3 deficient mouse (i.e. Col3a1F/F) to temporally control Col3 reduction. The study aims are: Aim 1: Define the temporal dynamics of the regulatory function(s) of Col3 during phases of neonatal tendon development and Aim 2: Define the regulatory function(s) of Col3 during phases of neonatal healing. Viscoelastic mechanical testing, transmission electron microscopy, immunohistochemistry, gene expression, proteomics, and 11 integrin analyses will be used to assess the structural, mechanical, and compositional properties of tendons in both aims. Insights gleaned from this work will be relevant to a variety of conditions that reduce Col3 expression including vascular Ehlers Danlos syndrome, aging, smoking and menopause and will highlight therapeutic targets for enhancing tendon injury treatment. The proposed work will be carried out in a world- class training environment at the University of Pennsylvania’s McKay Orthopaedic Research Laboratory. This environment combined with an expert sponsorship team, including experts in studies of tendon structure and function as well as matrix biology, will fully support completion of this proposal and facilitate development into a competent independent investigator capable of producing rigorous and reproducible clinically relevant work.