Loss of Homeostatic Strain Alters Mechanostat “Set Point” of Tendon Cells In Vitro

Abstract

Tendon cells respond to mechanical loads. The character (anabolic or catabolic) and sensitivity of this response is determined by the mechanostat set point of the cell, which is governed by the cytoskeleton and its interaction with the extracellular matrix. To determine if loss of cytoskeletal tension following stress deprivation decreases the mechanoresponsiveness of tendon cells, we cultured rat tail tendons under stress-deprived conditions for 48 hours and then cyclically loaded them for 24 hours at 1%, 3%, or 6% strain at 0.17 Hz. Stress deprivation upregulated MMP-13 mRNA expression and caused progressive loss of cellmatrix contact compared to fresh controls. The application of 1% strain to fresh tendons for 24 hours inhibited MMP-13 mRNA expression compared to stress-deprived tendons over the same period. However, when tendons were stressdeprived for 48 hours and then subjected to the same loading regime, the inhibition of MMP-13 mRNA expression was decreased. In stress-deprived tendons, it was necessary to increase the strain magnitude to 3% to achieve the same level of MMP-13 mRNA inhibition seen in fresh tendons exercised at 1% strain. The data suggest loss of cytoskeletal tension alters the mechanostat set point and decreases the mechanoresponsiveness of tendon cells.

Effect of in vitro stress‐deprivation and cyclic loading on the length of tendon cell cilia in situ

“…Previous studies from our lab have shown that the loading scheme utilized in the current study (3% strain at 0.17 Hz) were able to maintain tendon cell homeostasis with respect to catabolic gene expression 27 or decrease catabolic gene expression in 24 h SD tendons. 34 While these load induced alterations in gene expression in tendon cells cannot be directly attributed to a mechanosensory function of the primary cilium, the affects of loading have been associated with changes in cilia length. 9,18,21,31,32 Tensile loading of rat tail tendons has been shown to result in fluid flow, 25,26 and cellular deformations.…”

“…9 The decrease in cilia length in response to these stimuli has been attributed to the cell no longer requiring the cilia to ''fine tune'' the sensory information being transmitted to the loaded cell 20 or as an adaptive mechanism which minimizes cellular exposure to overt or prolonged sensory signals. 9 The ability of tendon cells to sense and respond to load is central to the concept of mechanotransduction and the subsequent maintenance of tissue homeostasis 27,[34][35][36][37][38] and there is increasing evidence that primary cilia may play a role in the mechanosensory apparatus of tendon cells. 5,9,39 While the results of the current study provide some insight as to the ability of tendon cell cilia to adapt their length in response to changing loading environments, the precise mechanism(s) by which cilia may contribute to the mechanosensory apparatus of tendon cells is unclear.…”

“…27 A previous study has demonstrated this loading regime to be effective in reversing alterations in gene expression caused by SD in rat tail tendons. 34 To determine the ability of CL to reverse (shorten) tendon cell cilia lengthening in response to SD, RTTfs from three animals were placed into three groups; 0 day (fresh control), 1 day SD, and 1 day SD þ 1 day CL. CL was maintained at 3% strain 0.17 Hz for 24 h under tissue culture conditions.…”

Tendon injury and repair – A perspective on the basic mechanisms of tendon disease and future clinical therapy

“…Rat and mouse tail tendons thus have historically played an important role in studies of tendon structure-function [74]. Tail tendon explant models arguably provide the most reproducible and human-relevant in vitro models of tendon physiology that are available [75][76][77][78][79]. Regarding mechanical properties, rodent fascicles range in elastic modulus from several hundred MPa to over 1 GPa, depending on the anatomical location of tissue harvest, as well as the age, breed, sex, and/or diet of the animal [80][81][82][83][84].…”

“…Such adjustments affect not only the loading of mechanosensory elements of the cell, but also affect sensory proteins within the cell membrane and nucleus that are mechanically coupled [114]. In the longer term, cells cope with transient mechanical perturbations by coordinating the structure and composition of the extracellular matrix until their mechanical environment reaches homeostasis [75]. In connective tissues like tendon this is primarily achieved by modulating the filamentous composition and structure of collagen networks [115].…”