Knee injury causes a loss of stability in the joint and frequently leads to secondary degeneration of the cartilage in the months and years after injury, which can further impair joint function and cause pain and disability. Moreover, the secondary damage to the joint appears to be worse for females compared to males. One factor that has been widely proposed to affect outcomes but has been largely neglected in study designs is the influence of sex hormones. Even though sex hormones are known to regulate inflammation, which is triggered in the knee by injury, hormonal influences have not been examined in the injured knee joint. Thus, we sought to investigate the mechanisms whereby sex hormones may alter the secondary damage to the cartilage after injury, focusing on two interrelated biological processes: hormonal modulation of post-injury inflammation in the knee and degradation of collagen, a key load-bearing structure in cartilage. First, we used a systems biology computational approach to examine the post-injury inflammatory response in the knee joint, since sex hormones have been widely documented as modulators of inflammatory processes throughout the body. Hormonal effects on inflammation in the knee may be particularly important in the context of tissue damage because inflammatory molecules regulate the production of matrix metalloproteinases (MMPs), which cleave and denature collagen fibrils. Further, sex hormones are present in the knee joint at concentrations similar to serum, which could allow them to modulate production of inflammatory factors and MMPs by the cells in the joint. To examine hormonal influences on inflammation and MMP production, we specified three hormone conditions: high estrogen with low testosterone (“Female High E”); low estrogen with low testosterone (“Female Low E”); and low estrogen with high testosterone (“Male”). The “Female High E” condition led to the highest concentrations of collagenase, a type of MMP, the “Male” condition led to the lowest collagenase production, and the “Female Low E” condition led to an intermediate concentration of collagenase. The collagenase concentrations differed significantly between each condition at nearly every day during the twenty-day simulation period. Thus, collagenase production did not just depend on whether male or female hormone concentrations were present in the knee joint, but also depended on estrogen concentrations among the female hormonal levels. After investigating hormonal effects on the post-injury inflammatory process, we examined the process of collagen fibril degradation using a Metropolis Monte Carlo algorithm and examined the subsequent loss of collagen mechanical integrity using a coarse-grained steered molecular dynamics approach. In the degradation simulations, we investigated the combined influences of two types of MMPs, collagenases and gelatinases, which work in tandem to degrade collagen structures; collagenases cleave intact tropocollagen molecules, while gelatinases cleave tropocollagen molecules that are partially denatured due to collagenase action. The simulated fibril was subjected degradation by both MMP types until it lost 1.1% of its mass. Subsequently, the fibril was subjected to tensile testing using steered molecular dynamics. These simulations revealed that toughness and ultimate tensile strength were lowest when the relative amount of collagenase was highest. That is, as the relative number of collagenases increased, the mechanical integrity decreased for a specified amount of lost fibril mass. However, these mechanical properties were unaffected by the total number of enzymes on the fibril when tensile testing occurred after a fixed amount of degradation. Together, our approaches begin to uncover a mechanism whereby female concentrations of sex hormones may enhance degradation of the collagenous structures in the injured knee joint. The results of our systems biology simulations suggest that post-injury collagenase concentration is highest when estrogen concentrations are highest and lowest for male hormone concentrations. Further, the simulations of fibril degradation and mechanics revealed that mechanical integrity decreased as the relative amount of collagenase increased after the fibril had lost a specified amount of mass. Taken together, these results hint at one mechanism by which estrogen may increase collagenase production and cause a reduction of collagen mechanical integrity. This possible mechanism could begin to explain sex differences in post-injury outcomes and may inform future observational studies. Further, the results of this work could eventually be used to develop treatment strategies that reduce or prevent the loss of cartilage mechanical integrity while accounting for the hormonal effects that may have the potential to worsen post-injury outcomes for females.

Creator

• Powell, Bethany

DOI

• https://doi.org/10.21985/n2-fx5r-1b90

Subject

• Bioengineering

Language

• en

Alternate Identifier

• etdadmin_upload_636837
• http://dissertations.umi.com/northwestern:14504

Date created

• 2019-01-01

Resource type

• Dissertation

Rights statement

• In Copyright