Insulin-like growth factor-II/mannose-6-phosphate receptor expression during early heart development

Expression of the insulin-like growth factor-II/mannose-6-phosphate (ICF-II/ M6P) receptor was examined during the major stages of heart morphogenesis in the chicken embryo. By using an affinity-purified antibody, Western blot analysis of total embryonic proteins from stages 5-24 revealed little if any IGF-II/M6P receptor protein until stage 7, approximately 8 hours prior to the appearance of the rudimentary myocardial tubes. Thereafter, receptor accumulation increased until stage 14, after which receptor protein levels remained constant, up to 7 days in ovo. Immunohistochemical localization revealed that, among all embryonic tissues at stages 10-24, the predominant site of receptor expression was the developing myocardium. Receptor expression was also immunohistochemically evaluated in a defined in vitro model of cardiogenesis in which explanted precardiac mesoderm is induced to undergo differentiation by co-explanted endoderm. In this system, as in vivo, IGF-II/M6P receptors were only detected after precardiac mesoderm had differentiated into a synchronously contractile multilayer which expressed cardiac alpha-actin. These findings indicate that the IGF-II/M6P receptor has an important role during early heart development.