Collagen III differential roles in temporal regulation of tendon healing across ages

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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. In other Col1-rich tissues such as skin and bone, Col3 directs reparative cell activities by regulating early cellular infiltration to promote healing, as well as collagen deposition, architecture and crosslink formation, supporting an early critical role of Col3 in wound healing, which has not been studied in tendon. Adding to the importance of understanding a role for Col3 in tendon, Col3 levels in aging tissues are reduced and in aged tendon, we have shown inferior healing which raises the possibility that age-induced Col3 loss increases susceptibility to poor tendon healing in aging populations. While Col3 may orchestrate cellular activities and fate that are critical for an optimal reparative response post-injury at early stages in tendon, its persistent expression in the remodeling phase may compromise the desired healing response. Therefore, our overall objective is to delineate mechanisms by which the temporal expression of Col3 modulates the injury response throughout tendon healing, as well as its differential effect throughout aging. Specifically, we will test the hypothesis that Col3 is crucial for early tendon healing, but that its continued expression during remodeling is detrimental. To test this, we generated a novel inducible Col3 deficient (i.e., Col3a1F/+, Col3a1F/F) mouse model to determine the dose-dependent effects of Col3 by temporal targeting of Col3. Using this approach, we will define the regulatory roles of Col3 throughout tendon healing at the time of injury, during the early proliferative phase, and during remodeling. In addition, by knockdown of Col3 in young, middle-aged, and old animals, we will evaluate the effects of aging on tendon healing with altered Col3. Understanding the role of Col3 throughout healing will lead to clinically relevant insights to improve outcomes following tendon injury in patients of varying ages. The study aims are: Aim 1: To define the age-sensitive mechanistic role(s) of Col3 throughout tendon healing by knocking down Col3 at the time of tendon injury. Aim 2: To elucidate the age- sensitive mechanistic role(s) of Col3 in directing reparative cell activities and prolonged effects of knocking down Col3 during the proliferative phase of tendon healing. Aim 3: To define the age-sensitive mechanistic role(s) of Col3 in tendon repair by knocking down Col3 during the remodeling phase of tendon healing. This study will define the critical temporal roles of Col3 in response to tendon injury throughout aging. We will utilize a novel mouse model coupled with rigorous hierarchical structure/function assays to define the role of Col3 in directing interplay between cellular activities and matrix assembly/organization in tendon healing.