IN-SITU EXPRESSION OF COLLAGEN AND PROTEOGLYCAN GENES IN NOTOCHORD AND DURING SKELETAL DEVELOPMENT AND GROWTH

Cartilage is an important tissue in skeletogenesis, in the growth of l ong bones, and as a flexible component of the mature skeleton. The ext racellular matrix proteins type II collagen and aggrecan comprise 90% of the matrix and are characteristic of cartilage. Type II collagen pr ovides structural integrity to the tissue, while aggrecan confers resi liency. The quantity of type II procollagen is controlled at the level of transcription of mRNA from the COL2A1 gene. In addition, type II p rocollagen can be expressed in two isoforms by differential splicing o f the primary gene transcript, a post-transcriptional control mechanis m. The two mRNAs either include exon 2 (type IIA) or exclude exon 2 (t ype IIB) which encodes the major portion of the amino (NH2)propeptide [Ryan and Sandell (1990), J. Biol. Chem., 265:10334-10339]. The aggrec an gene also encodes alternative splice forms that may be developmenta lly expressed. The regulation of aggrecan splicing or transcription ha s not been studied in detail. To determine the spatial and temporal pa tterns of expression of extracellular matrix in the development of car tilage, we have examined the expression of type II collagen and aggrec an during chondrogenesis in the vertebral column and during elongation of a newborn growth plate. Our results indicate that there is a devel opmental sequence of type II collagen splice form expression during ch ondrogenesis with type IIA expressed in prechondrocytes and type IIB e xpressed in chondrocytes. During elongation of the growth plate, matur e chondrocytes express type IIB procollagen and then differentiate int o hypertrophic chondrocytes and initiate expression of type X collagen . In all cases, aggrecan was coordinately expressed with type IIB proc ollagen. As cartilage-like proteins have been observed in more primiti ve structures such as notochord, the expression of type II collagen mR NAs was also examined in the notochordal remnants of the vertebral col umn. In the notochord, the predominant collagen expressed was the type IIA collagen prechondrocyte isoform. Notochordal cells also expressed mRNAs more characteristic of fibroblasts such as versican and decorin : low expression of type I collagen, type IIB collagen, and aggrecan w ere observed. (C) 1994 Wiley-Liss, Inc.