The effects of biophysical and biochemical cues on tendon cells: a study of underlying mechanisms in tendinopathy

Abstract

Background: Tendinopathy is a commonly diagnosed musculoskeletal disorders and is associated with the tendon microenvironmental changes, including disordered matrix structures, reduced stiffness and altered biochemical signalling. The effect of these microenvironmental cues on tendon cells and tendinopathy progression is poorly understood. Hypothesis: The overarching hypothesis of the thesis is that altered microenvironmental cues in tendinopathy negatively affect cells and facilitate disease progression. Methods: Bovine tendons were decellularised and enzymatically modified to mimic healthy and tendinopathic tendons. Human tendon-derived cells (hTDCs) were cultured on these and morphology and gene expression was assayed. Polydimethylsiloxane (PDMS) substrates mimicking healthy and tendinopathic stiffness was prepared. hTDCs viability, and hTDC and THP-1 macrophage gene expression were characterised. Rat-TDC (rTDC) were treated with palmitic acid (PA), with cell viability, gene expression and reactive oxygen species (ROS) production characterised. Genipin treatment was hypothesised to restore biomechanical properties in collagenase-induced tendinopathy in rat Achilles tendons. Healing outcomes were characterised using biomechanical and histological measurements. hTDCs were cultured on plastic or physiological stiffness PDMS substrate and treated with PA, and cell viability was measured. Tendon imprints with healthy and tendinopathic architecture were prepared, and hTDC morphology was characterised. Results: hTDCs on tendinopathic matrices had altered cell morphology and gene expression profile. On PDMS matrices mimicking tendinopathy stiffness, hTDCs had decreased SCX expression and increased proliferation, and THP-1 macrophages had increased IL-1β expression. PA treatment induced rTDC cell death, decreased SCX expression, increased MMP expression, and increased ROS production. In rats, collagenase-induced Achilles tendinopathy caused hypercellularity and reduced tissue modulus, but genipin treatment did not improve healing outcomes. PA treatment caused hTDCs cell death on plastic substrates but not on the physiological substrates. hTDCs on healthy tendon imprints had an elongated morphology and directional alignment, while on tendinopathic imprints, had no preferential directionality and were more rounded. Conclusion: Overall, our study indicates that the altered microenvironment cues in tendinopathy can contribute to the pathological state of tendon cells, further contributing to the progression of tendinopathy and ultimately inhibiting tendon healing. Understanding microenvironmental cues and the underlying mechanisms are important to direct future treatments to improve tendinopathic healing outcomes.