Our use of inherited hypercholesterolemia models throughout the project have allowed us to conduct a series of experiments to investigate the link between hypercholesterolemia and tendon health. The results from this project demonstrate that tendon health is impacted physiologically and biomechanically by lipid presence.
Animal Studies (1-3)
We bred a cohort of control and genetically modified rats in-house to keep genetic variations minimal. At 12 weeks old, rats received an injury to the right patellar tendon. After 3, 14 or 42 days, animals were euthanized and the patellar, Achilles and tail tendons collected for various investigations. Blood samples were used to quantify cholesterol levels.
Human Studies (3-5)
Using a collaboration with an FH network, we have accessed patients from which we have conducted very relevant and clinically helpful studies. In using this cohort, we have also been able to minimize factors associated with secondary hypercholesterolemia, which may have confused our results or made recruitment difficult with exclusion criteria.
1. The patellar tendon was used to examine the effect of high cholesterol on tendon healing after injury. There was little histological evidence of lipid content in the tendons of either group and no obvious differences in healing speed between groups. No differences in tendon size, strength or collagen content were found between the injured tendons of the control and FH rats. However, there were differences in the gene expression of injured patellar tendon tissue between groups that suggest high cholesterol may modulate tendon inflammation and healing, even with a mild phenotype. This work was recently presented at the Experimental Biology biennial conference and is currently being written up for publication.
2. Achilles tendons were used to investigate regional lipid presence. We found histological evidence of lipid accumulation in the Achilles tendons of both groups that appeared more intense in the experimental vs. control rats. Moreover, lipid staining was almost exclusively found in the matrix between Achilles sub-tendons, rather than between tendon fibres. We quantified the biomechanical properties of the matrix using sub-tendon sliding, an important feature of AT function, and found higher strains in the experimental group. Higher strains = higher injury risk. This work is currently being written up for publication.
3. Rat tail and human semitendinosus tendons were used for extracting tendon cells. In both species we found that cells exposed to cholesterol were less able to move into an injury site and expressed different gene patterns in unstretched and stretched conditions; the differences were greater with increasing cholesterol concentration. These results link cholesterol exposure to inferior wound healing and greater inflammation and tissue turnover, which may lead to increased risk of tendon injury. The human work was recently presented at the American Society for Matrix Biology annual conference and has been submitted for publication; the rat work is a work in progress.
4. 16 FH patients and 16 control participants were recruited for studying the impact of tendon xanthomas on tendon function during walking. Participants walked on a treadmill while movement kinematics and Achilles tendon displacements were recorded. This data allowed us to visualize tendon function and compute certain tendon properties that relate to strength. We found tendon function and some tendon properties differed in FH, which may indicate an increased risk of injury in this population. This work was presented at international conferences and has now been published (doi: 10.1371/journal.pone.0257269).
5. We capitalized on our access to FH patients and investigated tendon xanthoma diagnosis, which are typically diagnosed by eye and feel, crude techniques that do not allow them to be distinguished from conditions with similar physical symptoms e.g. tendinopathy. 30 participants (10 FH, 10 tendinopathy, 10 controls) had their Achilles tendons scanned using non-invasive imaging techniques; scans were analysed for fat and water content, collagen organization and tendon thickness. FH individuals demonstrated a higher water content in combination with higher lipid content. This information could be used to modify existing criteria for clinicians to identify FH and differentiate from other conditions with similar symptoms. This work has now been published (doi: 10.1186/s12891-021-04494-0).
