Tracking chondrocyte-to-fibroblast transformation via changes in cell electrophysiology

Cultured chondrocytes have potential for cartilage regeneration to treat degenerative diseases. However, when explanted chondrocytes are cultured in monolayer, they are known to dedifferentiate over time, adopting a more fibroblastic phenotype. This greatly impacts both research and potential clinical applications; cell-based cartilage repair therapies require significant in vitro expansion to obtain sufficient chondrocyte numbers for reimplantation, as chondrocytes adopt a more fibroblastic phenotype causing up to 70% of patients to develop fibrocartilaginous fill. We used dielectrophoresis (DEP) to observe changes in the electrophysiological properties of primary bovine chondrocytes over time. Using a multi-conductivity approach, we demonstrate that monitoring the cytoplasmic conductivity is a reliable method of observing cell changes over 100 days in culture. Results show statistically significant changes in both membrane capacitance (p = 0.0039) and cytoplasm conductivity (p =  < 0.0001) when tested at multiple conductivities. Analysis of cytoplasmic vs. medium conductivity allowed simple tracking of chondrocyte membrane potential, which exhibited transitions between three stable values of V_m which correspond to patch-clamp-derived literature values as they transition from chondrocytes (- 13 to - 18 mV) to proliferating fibroblasts (- 32 to - 43 mV) and ultimately to non-proliferating fibroblasts (- 55 to - 71 mV), transitions occurring around days 40 and 80 respectively.