Loss of Homeostatic Tension Induces Apoptosis in Tendon Cells: An In Vitro Study

Abstract

Apoptosis (programmed cell death) has been identified as a histopathologic feature of tendinopathy. While the precise mechanism(s) that triggers the apoptotic cascade in tendon cells has not been identified, it has been theorized that loss of cellular homeostatic tension following microscopic damage to individual tendon fibrils could be the stimulus for initiating the pathologic events associated with tendinopathy. To determine if loss of homeostatic tension following stress deprivation could induce apoptosis in tendon cells, rat tail tendons were stress-deprived or cyclically loaded (3% strain at 0.17 Hz) for 24 hours under tissue culture conditions. Caspase-3 (an upstream mediator of apoptosis) mRNA expression was evaluated using quantitative polymerase chain reaction and caspase-3 protein synthesis was identified using immunohistochemistry. Apoptotic cells were identified histologically using an antibody for single-stranded DNA. Stress deprivation for 24 hours resulted in an increase in caspase-3 mRNA expression when compared to fresh controls or cyclically loaded tendons. Stress deprivation also increased the percentage of apoptotic cells (10.59% +/- 2.80) compared to controls (1.87% +/- 1.07) or cyclically loaded tendons (3.73% +/- 0.87). These data suggest loss of homeostatic tension following stress deprivation induces apoptosis in rat tail tendon cells.

Substrate topography: A valuable in vitro tool, but a clinical red herring for in vivo tenogenesis

“…This observation indicates that although both topography and mechanical stretching can induce similar bidirectional cell elongation, only mechanical loading, in excessive form, can activate apoptotic pathways [61][62][63][64].…”

The Role of Mechanical Loading in Tendon Development, Maintenance, Injury, and Repair

“…Excessive loading events lead to microtear formation in the tendon 48 , which, if not repaired properly, may lead to the initiation of inflammatory and degenerative responses. This results in an overall weakened structure and increased propensity for tendon rupture [48][49][50] . In an in vivo tendinopathy model, Nakama et al found that when the New Zealand White rabbit flexor digitorum profundus muscle was stimulated repetitively for eighty hours, microtears were found in all tendon regions and were significantly greater (p < 0.0001) in the loaded limb compared with the unloaded limb 50 .…”

“…In an in vivo tendinopathy model, Nakama et al found that when the New Zealand White rabbit flexor digitorum profundus muscle was stimulated repetitively for eighty hours, microtears were found in all tendon regions and were significantly greater (p < 0.0001) in the loaded limb compared with the unloaded limb 50 . Other investigators have claimed that underuse of a damaged segment of a tendon may be the source of the chronic impairment 49 . Egerbacher et al found that a loss of homeostatic tension following stress deprivation correlated with increased cell apoptosis in a rat tail tendon model 49 .…”

“…Other investigators have claimed that underuse of a damaged segment of a tendon may be the source of the chronic impairment 49 . Egerbacher et al found that a loss of homeostatic tension following stress deprivation correlated with increased cell apoptosis in a rat tail tendon model 49 . Ultimately, tendinopathy may result from a combination of overuse and underuse mechanisms, with overloading creating microtears leading to decreased loading of the cell population resident on the damaged collagen fibers.…”

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

“…Indeed, in vitro experimental studies have demonstrated that loss of homeostatic tendon strain can produce a pattern of catabolic gene expression [5,[33][34][35][36]41], apoptosis [3,17], and degenerative histological changes [5,21] similar to those seen in clinical cases of tendinopathy [1,2,5,19,24,26,28,30,31,38,39,40,45,50,54,60,61]. However, the effect of these catabolic changes on the subsequent mechanoresponsiveness of tendon cells is unknown.…”