CD44 deficiency improves healing tendon mechanics and increases matrix and cytokine expression in a mouse patellar tendon injury model

Abstract

CD44 plays an important role in inflammation and healing. Previous studies investigated its role in inflammatory diseases and skin wounds; however, the role of CD44 in tendon healing is unknown. Therefore, we investigated the effect of CD44 in the healing of the patellar tendon in a knockout mouse model. We hypothesized that in comparison to wild-type counterparts, CD44 knockout mice would have decreased material parameters, increased organization, decreased expression of proinflammatory cytokines, and increased expression of matrix components during healing. These hypotheses were tested through an in vivo surgical model and mechanical, organizational, and gene expression analyses. Material strength and tissue organization were significantly improved in the CD44 knockout mouse. This could be attributed to increased expression of cytokines and matrix components that are also elevated in regenerative healing. Our study showed that the absence of CD44 in a mouse patellar tendon injury creates an environment that is conducive to regenerative healing through altered gene expression, resulting in superior material properties and reduced cross-sectional area. Therefore, limiting the role of CD44 may improve healing parameters in adult tendon injury.

Figure 1

Photos that depict the partial transection created in the mouse patellar tendon. A plastic coated backing is placed underneath the tendon to provide support (Fig. 1(1)), and a biopsy punch is used to create a full thickness partial transection. The backing is removed, leaving a distinct and reproducible injury (Figs. 1(2)).

Figure 2

Cross-sectional area compared across genotype. Cross-sectional area was significantly reduced in knockouts (KO) compared to wild types (WT) at both 3 and 6 weeks post injury.

Figure 3

Material properties compared across genotype. Maximum stress and strain energy density were significantly increased in knockouts (KO) compared to wild type (WT) at both 3 and 6 weeks post injury. Similarly, knockout animals demonstrated at trend toward greater failure strain compared to wildtype. No significant changes were seen across genotype in modulus. *: p ≤ 0.05; #: p ≤ 0.1.

Figure 4

Angular deviation compared across time. Appreciable differences were seen in the knockout (KO) tendons across time. While the wild type (WT) tendons remain disorganized, the KO tendons approach uninjured values at 6 weeks post injury.

Figure 5

Gene expression in a patellar tendon injury in wild type (WT) and CD44 knockout (KO) represented as relative quantity fold changes (2^−ΔΔCT) with error bars representing 2^{(−ΔΔCT-s)} where s is the standard deviation of the ΔΔCT value. All post injury time points are normalized to uninjured tendons (day 0) which is represented by a value of one.