Bone Abstracts

Interleukin- 34 (IL34) is a new, challenging cytokine discovered in 2008. It promotes the proliferation, survival and differentiation of the monocyte/macrophage lineage with almost the same efficiency as the macrophage-colony stimulating factor (M-CSF). IL34 has already been described to play a key role in various musculoskeletal bone diseases such as bone giant cell tumors or in rheumatoid arthritis. These ‘twin’ cytokines share a lot of functional similarities which were explained by their ability to bind and activate the M-CSF receptor (M-CSFR). The aim of this work was to search for an alternative binding of IL34 on cells.

M-CSFR expression was analyzed by flow-cytometry, and intracellular pathways were studied by western blot. Signaling was also investigated after enzymatic treatment of cells. To characterize IL34 binding to various cell lines (myeloid cell lines, ostesarcoma and embryonic cells), Scatchard and binding inhibition assays were carried out using ¹²⁵I radiolabelled IL34 and M-CSF. Molecular interactions were also studied using surface plasmon resonance (Biacore).

M-CSF and IL34 induced a different pattern of phopshorylations in cells overexpressing the M-CSFR, hence suggesting the existence of an alternative receptor for IL34. Biacore and scatchard experiments confirmed the binding of IL34 on cells in which the M-CSFR was lacking. Enzymatic treatments of the cells and binding assays of glycosaminoglycans on immobilized IL34 identified the chondroitin sulfate chains as co-receptors for IL34.

This work evidences an alternative binding of IL34 to chondroitin sulfate chains, on cells lacking the known IL34 receptors, the M-CSFR and the receptor protein tyrosine phoshatase β/ζ (RPTP β/ζ). In fact, recent data show that RPTP β/ζ is another receptor for IL34 in the central nervous system, and that this receptor needs its chondroitin sulfate chains to work. Consequently, these glycosaminoglycans could control IL34 bioavailability at the cell surface, thereby modulating the M-CSFR activations.