Changes in lncRNA NORAD expression reflect stiffness-associated phenotypic alterations in human chondrocytes

Extracellular matrix-derived mechanical signals play an important role in maintaining chondrocyte homeostasis, and their disruption has been implicated in osteoarthritis progression. Here, we investigated whether the long non-coding RNA NORAD reflects substrate stiffness-associated phenotypic alterations in human chondrocytes. Immortalized and primary human chondrocytes were cultured on substrates with defined stiffnesses ranging from 0.2 to 64 kPa or on tissue culture plastic as a rigid control. NORAD expression was differentially regulated according to substrate stiffness and chondrocyte phenotypic conditions. Primary human chondrocytes cultured on intermediate-stiffness substrates at 32 and 64 kPa showed better preservation of the chondrogenic phenotype, reduced phenotypic instability, and attenuated TNF-α-induced inflammatory and hypertrophic responses at the mRNA level. In late-passage dedifferentiated chondrocytes, stiffness modulation partially rescued the loss of chondrogenic marker expression and was accompanied by altered NORAD expression. These findings suggest that NORAD expression reflects stiffness-associated changes in chondrocyte phenotype and may help evaluate mechanically defined culture conditions for chondrocyte maintenance.