Modulation of Gene Expression in Response to Mechanical Load in Rabbit Flexor Tendons
The goal of this dissertation was to investigate how mechanical loading affected gene expression in tendons. A better understanding of which genes are affected by load may help identify proteins which are involved in the initial response to tendon overuse. Characterizing the gene expression response to load may help identify threshold limits to avoid overuse injuries.
Gene expression levels for eight matrix molecules were quantified for tendon tissue obtained from rabbits exposed to cyclic load. No significant differences in expression levels were found. Long term repetitive loading may not lead to up-regulation at the transcriptional level of these eight genes or localized changes were missed due to homogenization of the tissue samples. RT-PCR may be an inappropriate technique for measuring localized changes in gene expression.
A novel in vitro tissue loading system was designed capable of simultaneously applying specific loading patterns to six tissues under force or displacement control. Loading parameters such as amplitude, rate and frequency can be controlled while biomechanical factors such as creep, force relaxation and modulus can be affected and monitored.
Employing the in vitro tissue loading system, rabbit flexor tendons were exposed to static loads of 0 (stress-shielded), 2 (low), 4 (moderate) or 6 (high) MPa for 20 hours. Gene expression levels for collagenase-1 (MMP-1), interlukin-1b (IL-1b) and cyclooxegenase-2 (COX-2) were measured. High static loads up-regulated MMP-1 expression compared to moderate loads. Correlations found between creep and the expression of MMP-1 and IL-1B suggests that the pathway for their expression, leading eventually to tendon degeneration, may be regulated by the biomechanical factor creep.
These findings indicate in vitro culture systems can be a useful tool in examining the effects of loading on connective tissues, allowing investigators to measure the initial cellular response to mechanical load. Changes in gene expression may indicate possible threshold limits where loading leads to a remodeling response.