Tendinosis develops from age‐ and oxygen tension‐dependent modulation of Rac1 activity

Abstract

Age‐related tendon degeneration (tendinosis) is characterized by a phenotypic change in which tenocytes display characteristics of fibrochondrocytes and mineralized fibrochondrocytes. As tendon degeneration has been noted in vivo in areas of decreased tendon vascularity, we hypothesized that hypoxia is responsible for the development of the tendinosis phenotype, and that these effects are more pronounced in aged tenocytes. Hypoxic (1% O2) culture of aged, tendinotic, and young human tenocytes resulted in a mineralized fibrochondrocyte phenotype in aged tenocytes, and a fibrochondrocyte phenotype in young and tendinotic tenocytes. Investigation of the molecular mechanism responsible for this phenotype change revealed that the fibrochondrocyte phenotype in aged tenocytes occurs with decreased Rac1 activity in response to hypoxia. In young hypoxic tenocytes, however, the fibrochondrocyte phenotype occurs with concomitant decreased Rac1 activity coupled with increased RhoA activity. Using pharmacologic and adenoviral manipulation, we confirmed that these hypoxic effects on the tenocyte phenotype are linked directly to the activity of RhoA/Rac1 GTPase in in vitro human cell culture and tendon explants. These results demonstrate that hypoxia drives tenocyte phenotypic changes, and provide a molecular insight into the development of human tendinosis that occurs with aging.

Chemo-Mechanical Cues Modulate Nano-Scale Chromatin Organization in Healthy and Diseased Connective Tissue Cells

“…Given our results on the impact of biophysical cues on the nano-scale chromatin organization in vitro , we next asked how tissue aging and/or degeneration may alter chromatin organization at the nano-scale in native tissue cells. We isolated tenocytes from human tendons from donors that were young and healthy, young and diagnosed with tendinosis (which is a common tendon pathology, associated with repetitive use and mechanical overload 10,34 ), or aged and healthy. These human tenocytes were cultured on glass substrates for 48 hours before fixation and super-resolution imaging.…”

“…Since we observed different nano-scale chromatin organization in human tenocytes with tissue degeneration or aging, we hypothesized that cues from the degenerative chemo-physical environment (e.g., soft ECM 9 , hypoxia 10 , and inflammation 35 ) might result in nano-scale chromatin remodeling, and cause young healthy tenocytes to take on features that are comparable to that seen in aged (Aged) or degenerative (tendinosis) tenocytes. To test this hypothesis, we first cultured human tenocytes isolated from young healthy donors (Young) on glass, stiff (30kPa), or soft (3kPa) substrates for 2 days in basal growth media 4,9 .…”

“…To extend this work, here we took advantage of these recent advances to determine how biophysical cues regulate spatial nano-scale chromatin organization during progenitor cell differentiation and native tissue degeneration. For this, we explored human mesenchymal stem cell (hMSC) and human tenocyte (hTC) chromatin nano-material architecture, and investigated how chromatin nano-structure changed in response to physiologic and pathologic chemo-physical cues (e.g., soft substrates 9 or hypoxic conditions 10 ) using super-resolution microscopy. Our findings show that exogenous chemo-physical perturbations rapidly and reversibly regulate spatial chromatin organization and localization at the nano-scale level in both hMSCs and hTCs through changes in histone modifications.…”

Rotator Cuff Tenocytes Differentiate into Hypertrophic Chondrocyte-Like Cells to Produce Calcium Deposits in an Alkaline Phosphatase-Dependent Manner

“…Donor 1 was the oldest of our donors (86 years), but did not present any other specific traits. A higher age could have influenced the results as shown by Mc Beath et al Indeed, in their study, aged tenocytes had a higher propensity than young tenocytes to produce mineral deposits under hypoxic culture conditions [44].…”

Bringing tendon biology to heel: Leveraging mechanisms of tendon development, healing, and regeneration to advance therapeutic strategies

“…Although the principal causes of tendinopathy are generally accepted as overuse, aging, and genetics, 152,154 the molecular mechanisms driving disease initiation and progression remain poorly understood. Most patient biopsy samples taken for research are typically obtained from advanced stages of the disease when surgical intervention is necessary, making study of early stages of human tendinopathy difficult.…”