Expression and production of stathmin in growth plate chondrocytes is cell-maturation dependent

Growth plate cartilage is comprised of linear columns of chondrocytes with the least differentiated cells at one end and the terminally differentiated cells at the other end. Rat costochondral chondrocytes can be divided into the resting cell zone (reserve cell zone), which contains relatively immature chondrocytes (RC cells), and the phenotypically more mature prehypertrophic and upper hypertrophic cell zones, which together may be termed the growth zone chondrocytes (GC cells). When grown separately in monolayer culture, they continue to express their zone-specific phenotype, providing a model for assessing cell-maturation-dependent expression of molecules associated with differentiation. Stathmin (also called prosolin, Op18, p19, 19K, and others) is a highly conserved, phosphorylated cytosolic protein with apparent ubiquitous expression. Although its exact function is unknown, stathmin is considered to be a messenger phosphorylated protein, it plays a role in tubulin stability, and it may participate in both general and specific regulatory pathways. One uniform observation is that the expression of stathmin protein decreases in all cells as they become more terminally differentiated in culture. There have been no published data regarding stathmin expression and production in chondrocytes. This study was based on the hypothesis that stathmin exists in chondrocytes and that the mRNA and protein levels decline in the GC cell with respect to the RC cell. Stathmin mRNA levels were determined and quantitated by reverse transcription-polymerase chain reaction (RT-PCR) and northern blots. Protein levels were determined using immunoblots. It was found that stathmin exists in chondrocytes and that RC cells express approximately twice the level of mRNA and protein to that found in GC cells. The results support the hypothesis and suggest that the level of stathmin expression and production in culture is related to the level of differentiation of RC and GC cells in vivo.