Microarray analysis of proliferative and hypertrophic growth plate zones identifies differentiation markers and signal pathways

Abstract

Longitudinal bone growth results from coordination of proliferation and hypertrophy of chondrocytes, calcification of the matrix, vascular invasion, and completion of endochondral bone formation in the growth plate. Although proliferative and hypertrophic chondrocytes are well characterized histomorphologically, the understanding of factors governing this transition is not fully explained. Our hypothesis was that significant differential gene expression exists between proliferative and hypertrophic chondrocytes that may provide clues to the regulation of this transition at the transcriptional level. Normal Sprague-Dawley rat growth plate chondrocytes from the proliferative zone (PZ) and hypertrophic zone (HZ) were isolated by laser capture microdissection and then subjected to microarray analysis. Confirmation of the differential expression of selected genes was done by in situ hybridization and quantitative reverse transcription (RT) polymerase chain reaction (PCR). A total of 40 transcripts showed at least twofold greater expression in the PZ compared to HZ at both 6 and 7 weeks of age, while 52 transcripts showed twofold greater expression in the HZ compared to PZ at these time points. Many of the differentially expressed genes in each zone had very high levels of expression and thus were classified as "enriched transcripts" for that zone. The PZ-enriched transcripts included fibromodulin, proline arginine-rich end leucine-rich repeat protein, lactate dehydrogenase, and enolase 1 alpha. In contrast, HZ-enriched transcripts included collagen I, protein kinase (lysine deficient 4), proteasome (prosome, macropain) activator subunit 4, prostaglandin I2 synthase, and integrin-binding sialoprotein, matrix metalloproteinase 13 (MMP13), and collagen X. Other genes were highly expressed in cells from both zones, including collagen II, aggrecan, cartilage oligomeric protein, cartilage link protein, laminin receptor, and eukaryotic translocation elongation factor. Functional classification of the PZ-enriched transcripts showed an increased percentage of genes expressed in nuclear cell cycle and transcription functions. In contrast, the HZ-enriched transcripts were more involved in extracellular structure and membrane receptor and transporter functions. Pathway analysis indicated that transforming growth factor beta and parathyroid hormone-related protein (PTHrP) pathways were important in both zones, and bone morphogenic protein pathway played a role in the HZ. It is likely that these differentially expressed genes are involved in regulation of the transition from proliferation to differentiation functions in the growth plate.