Changes in Fluorescence Recovery After Photobleaching (FRAP) as an indicator of SOX9 transcription factor activity.

Cells respond to their environment via an intricate cellular signaling network, directing cell fate. Changes in cell fate are characterized by changes in gene transcription, dictated by (master) transcription factor activity. SOX9 is the master transcription factor for chondrocyte development. Its impaired function is implicated in osteoarthritis and growth disorders, such as dwarfism. However, the factors regulating SOX9 transcriptional activity are not yet fully mapped. Current methods to study transcription factor activity are indirect and largely limited to quantification of SOX9 target gene and protein expression levels after several hours or days of stimulation, leading to poor temporal resolution. We used Fluorescence Recovery After Photobleaching (FRAP) to study the mobility of SOX9 and correlated the changes in mobility to changes in its transcriptional activity by cross-validating with chromatin immunoprecipitation and qPCR. We show that using FRAP, we can quantify the changes in SOX9 mobility on short time scales as an indication of transcriptional activity, which correlated to changes of SOX9 DNA-binding and long-term target gene expression.