EXPRESSION OF THE HEPARIN-BINDING CYTOKINES, MIDKINE (MK) AND HB-GAM (PLEIOTROPHIN) IS ASSOCIATED WITH EPITHELIAL-MESENCHYMAL INTERACTIONS DURING FETAL DEVELOPMENT AND ORGANOGENESIS

Midkine (MK) and heparin binding-growth associated molecule (HB-GAM or pleiotrophin), constitute a new family of heparin-binding proteins im plicated in the regulation of growth and differentiation (T. Muramatsu (1993) Int. J. Dev. Biol. 37, 183-188). We used affinity-purified ant ibodies against MK and HB-GAM to analyze their distribution during mou se embryonic development. From 9 to 14.5 day post-coitum (dpc), both p roteins were detected in central and peripheral nervous systems, facia l processes, limb buds, sense organs, respiratory, digestive, urogenit al, and skeletal systems, MK and HB-GAM were often localized on the su rface of differentiating cells and in basement membranes of organs und ergoing epithelial-mesenchymal interactions. The levels of MK protein decreased considerably in the 16.5 dpc embryo, whereas HB-GAM staining persisted in many tissues. Our in situ hybridization results revealed a widespread expression of MK transcripts that was not always consist ent with the distribution of MK protein in developing tissues. In many epithelio-mesenchymal organs MK and HB-GAM were codistributed with sy ndecan-1, a cell surface proteoglycan. In limb buds and facial process es, MK, HB-GAM, and syndecan-1 were localized to the apical epithelium and the adjacent proliferating mesenchyme. Both MK and HB-GAM bound s yndecan-1 in solid-phase assays in a heparan sulfate-dependent manner. The biological effects of MK and HB-GAM I on limb and facial mesenchy me were studied in vitro by application of beads preloaded with the pr oteins, Neither MK nor HB-GAM stimulated mesenchymal cell proliferatio n or induced syndecan-1 expression. Taken together these results indic ate that MK and HB-GAM may play regulatory roles in differentiation an d morphogenesis of the vertebrate embryo, particularly in epitheliomes enchymal organs, and suggest molecular interactions with syndecan-1.