Differential expression of Dlk-1 in bovine adipose tissue depots.

Dlk-1, a type 1 membrane glycoprotein, is a member of the Epidermal Growth Factor-like family of homeotic proteins that are typically involved in cell fate decisions and in mice it has been implicated in the control of differentiation of adipocytes. The aim of this study was to determine whether there were tissue-specific expression patterns of Dlk-1 splice variants in bovine tissues. Only the Dlk-1-C2 variant was expressed in adult bovine tissues while both Dlk-1-C2 and Dlk-1-A variants were expressed in foetal tissues. Quantitative real-time PCR revealed large differences in the relative levels of expression of the Dlk-1-C2 variant in adult adipose tissue depots with no expression in subcutaneous and brisket adipose tissues. Expression was also demonstrated in three adult skeletal muscle samples. The large variation in the level of expression of Dlk-1-C2 in different adult tissues may reflect the relative preadipocyte content of those tissues and consequently their potential for generating new adipocytes. A low abundance soluble glycoprotein (bFA1) was purified from bovine amniotic fluid. Analyses of its amino acid sequence revealed that it corresponded to most of the extracellular domain of bovine Dlk-1 and was derived by proteolytic processing from the full-length Dlk-1 protein encoded by the Dlk-1-A variant. The tissues expressing the Dlk-1-A variant have not been identified but are likely to be foetal in origin. Splice variants of Dlk-1 may have varied functional roles with the foetal Dlk-1-A form capable of generating a protein that undergoes proteolytic processing to release a soluble ecto-domain of Dlk-1. In contrast the Dlk-1-C2 splice variant codes for a protein lacking this processing site and therefore it probably remains bound to the cell membrane.