Matrix Protein mRNA Levels in Canine Meniscus Cellsin Vitro

The resident cells of the meniscus synthesize a fibrocartilaginous extracellular matrixin vivocomposed predominantly of type I collagen fibers. To increase our understanding of matrix biosynthesis by meniscus cellsin vitro,we examined matrix protein mRNA levels in cultured meniscus cells isolated from skeletally mature dogs. The mRNA levels of five matrix protein genes (COL1A1, COL2A1, aggrecan, COL6A1, and fibronectin) were measured in meniscus cells by Northern blotting and compared with those of patellar tendon fibroblasts and femoral articular cartilage chondrocytes. In freshly isolated cells (Day 0 cells), COL1A1, COL2A1, and aggrecan mRNA levels were low or undetectable in both meniscus cells and tendon fibroblasts. In intact meniscus tissue, COL1A1 mRNA levels were also low or undetectable. COL2A1 and aggrecan mRNA transcripts were readily observed, however, in Day 0 articular chondrocytes. The levels of expression of COL6A1 and fibronectin mRNA transcripts in Day 0 meniscus cells were intermediate between higher articular chondrocyte levels and lower tendon fibroblast levels. After 1 week in monolayer culture (Day 7 cells), meniscus cells expressed readily detectable levels of COL1A1 mRNA transcripts, similar to that observed for cultured tendon fibroblasts. COL1A1 mRNA transcripts were either not detected or detected at very low levels in monolayer cultures of articular chondrocytes. COL2A1 and aggrecan mRNA transcripts were readily detected in cultured articular chondrocytes but not in meniscus cells or in tendon fibroblasts. All three types of cells continued to express COL6A1 and fibronectin mRNA transcripts after 1 week in culture. These results demonstrate that the patterns of expression of COL1A1 and COL2A1 mRNA transcripts by meniscus cells are similar to those of tendon fibroblasts and dissimilar to those of articular chondrocytes both in freshly isolated cells and in monolayer cultured cells. This mRNA expression pattern supports the idea that monolayer culture of meniscus cells results in the expression of a predominantly fibroblastic phenotype.