Superficial and deep zone articular chondrocytes exhibit differences in actin polymerization status and actin-associated molecules in vitro

OBJECTIVE: The actin cytoskeleton regulates cell shape and plays a role in regulating chondrocyte phenotype. Most studies investigating regulation of the chondrocyte phenotype by the actin cytoskeleton use chondrocytes isolated from full-thickness (FT) cartilage, which has a heterogeneous cell population. Superficial zone chondrocytes (SZC) have an elongated morphology and account for 10–20% of chondrocytes, while the remaining chondrocytes in the deeper zones appear more rounded. This study characterizes the actin cytoskeleton and expression of actin-associated molecules in SZC and deep zone (DZ) chondrocytes (DZC) in vitro in order to identify molecules differentially expressed by SZC and DZC that may contribute to the observed differences in zonal chondrocyte shapes. DESIGN: SZ, DZ, and FT chondrocytes isolated from bovine metacarpal-phalangeal joints were cultured in monolayer for 48 h. Macroscopic morphology, actin polymerization status, and expression of select actin-associated molecules (adseverin, cofilin, transgelin, vinculin, MRTF-A, and YAP/TAZ) were determined. RESULTS: SZC appeared more elongated and have more filamentous actin compared to DZC, as determined by quantifying cell circularity and G-/F-actin ratio. MRTF-A gene and protein levels were significantly higher in SZC compared to DZC while DZC more highly expressed transgelin and TAZ. Although there was differential gene expression, no significant differences in adseverin, cofilin, vinculin, or YAP protein levels were observed between the two cell populations. CONCLUSIONS: This study identifies differences in actin polymerization status and expression of actin-associated molecules in primary SZC and DZC in vitro. These findings further our understanding of candidate actin-related pathways that may be regulating zonal chondrocyte phenotype.