Developmental Expression of Creatine Kinase Isoenzymes in Chicken Growth Cartilage

We have shown previously that creatine kinase (CK) activity is required for normal development and mineralization of chicken growth cartilage and that expression of the cytosolic isoforms of CK is related to the biosynthetic and energy status of the chondrocyte. In this study, we have characterized changes in isoenzyme activity and mRNA levels of CK (muscle‐specific CK, M‐CK; brain‐type CK, B‐CK; and mitochondrial CK subunits, MiaCK and MibCK) in the growth plate in situ and in chondrocyte culture systems that model the development/maturation program of the cartilage. The in vitro culture systems analyzed were as follows: tibial chondrocytes, which undergo hypertrophy; embryonic cephalic and caudal sternal chondrocytes, which differ from each other in their mineralization response to retinoic acid; and long‐term micromass cultures of embryonic limb mesenchymal cells, which recapitulate the chondrocyte differentiation program. In all systems analyzed, B‐CK was found to be the predominant isoform. In the growth plate, B‐CK expression was highest in the most calcified regions, and M‐CK was less abundant than B‐CK in all regions of the growth plate. In tibial chondrocytes, an increase in B‐CK expression was seen when the cells became hypertrophic. Expression of B‐CK increased slightly over 15 days in mineralizing, retinoic acid–treated cephalic chondrocytes, but it decreased in nonmineralizing caudal chondrocytes, while there was little expression of M‐CK. Interestingly, in limb mesenchyme cultures, significant M‐CK expression was detected during chondrogenesis (days 2–7), whereas hypertrophic cells expressed only B‐CK. Finally, expression of MiaCK and MibCK was low both in situ and in vitro. These observations suggest that the CK genes are differentially regulated during cartilage development and maturation and that an increase in CK expression is important in initiating chondrocyte maturation.