Advances in the understanding of hypophosphatemic disorders have identified a novel group of molecules (FGF23, PHEX, MEPE, DMP1) that have been implicated in osteoblast mineralisation directly.The specific binding of PHEX to MEPE regulates the release of ASARM peptides which have an inhibitory role. Current concepts are speculative and the functional role of MEPE in chondrocyte mineralisation remains largely undefined.
Proximal tibiae from 3-week old wild-type mice were analysed by i) in situ hybridisation and ii) growth plate microdissection for Mepe quantification by RT-qPCR. Gene expression by the ATDC5 chondrogenic cell-line was examined by RT-qPCR over a 20 day culture period under calcifying conditions (ascorbic acid and 10 mM β-glycerophosphate). Twenty μM phosphorylated (pASARM) and non-phosphorylated ASARM (npASARM) peptides were added to ATDC5 cultures, and to 17-day-old embryonic metatarsal explants.
Mepe expression was abundant throughout the growth plate as shown by in situ hybridisation. The accuracy of growth plate microdissection was validated by assessment of collagen X expression and data confirmed an increased expression of Mepe in the hypertrophic chondrocytes (9-fold increase compared to proliferative chondrocytes, P<0.05). ATDC5 cells showed an initial decrease in Mepe expression at day-10 of culture, after which expression increased at day-15. Treatment of ATDC5 cells with pASARM peptide caused an inhibition of mineralisation, as assessed by Alizarin Red staining (compared to control, P<0.01). Treatment with npASARM promoted mineralization (compared to control, P<0.01). In embryonic metatarsals, the percentage change in total length was not affected by treatment with 20 μM pASARM or npASARM peptides. However, the growth of the central diaphyseal mineralisation zone was inhibited in bones treated with 20 μM pASARM (compared to control, P<0.001).
Our findings indicate that Mepe is expressed by growth plate chondrocytes and that it is likely to have a developmental role in the control of cartilage matrix mineralisation during endochondral ossification.