Cycle-dependent matrix remodeling gene expression response in fatigue-loaded rat patellar tendons

Abstract

Expression profiling of selected matrix remodeling genes was conducted to evaluate differences in molecular response to low-cycle (100) and high-cycle (7,200) sub-failure-fatigue loading of patellar tendons. Using our previously developed in vivo patellar tendon model, tendons were loaded for 100 or 7,200 cycles and expression of selected metalloproteinases (MMPs), tissue inhibitors of metalloproteinases (TIMPs), and collagens were quantified by real-time RT-PCR at 1- and 7-day post-loading. Expression profiles were also obtained from lacerated tendons as an acute injury model. The high-cycle group showed upregulation of TIMP-1, -2, Col3a1, and Col5a1, and downregulation TIMP-4 at both time points, upregulation of MMP-2 at 7-day post-loading and downregulation of MMP-13 and -14 at 1-day post-loading, suggesting overall repair/remodeling. In contrast, the low-cycle loaded group showed upregulation of MMP-2, -3, -13, and Col12a1 at both time points, upregulation of TIMP-1, -2, -3, Col3a1, and integrin β1 and downregulation of integrin α11 at 1-day post-loading and upregulation of Col1a1 at 7-day post-loading, consistent with a hypertrophic (adaptive) pattern. Lacerated tendons showed a typical acute wound response with upregulation of all examined remodeling genes. Differences found in tendon response to high- and low-cycle loading are suggestive of the underlying mechanisms associated with a healthy or damaging response.

Figure 1

(A,B,C) At 7-day post-loading, high-cycle loaded and low-cycle fatigue-loaded tendons have distinct expression signatures. (A) MMP expression, (B) TIMP expression, and (C) Col expression relative to pooled sham-operated and naive control. A significant difference of p <0.05 is denoted by *.

Figure 2

(A,B,C) At 1-day post-loading, high-cycle loaded and low-cycle fatigue-loaded tendons have distinct expression signatures. (A) MMP expression, (B) TIMP expression, and (C) Col expression relative to pooled sham-operated and naive control. A significant difference of p <0.05 is denoted by *.

Figure 3

mRNA expression of integrin α11 and β1 for low-cycle and high-cycle loaded groups at 1- and 7-day post-loading.

Figure 4

Representative images of the tendon midsubstance acquired using SHG at 7 days post-fatigue for (A) control, (B) 100-cycle loaded group, and (C) 7,200-cycle loaded group.