Expression of the two mannose 6-phosphate receptors is spatially and temporally different during mouse embryogenesis.

Mammalian cells express two mannose 6-phosphate receptors, MPR46 and MPR300, both of which mediate the targeting of lysosomal enzymes to lysosomes. Additionally the receptors mediate the secretion (MPR46) and the endocytosis (MPR300) of lysosomal enzymes and the binding of IGFII (MPR300). We have analyzed the distribution of MPR46 and MPR300 transcripts during mouse embryogenesis by in situ hybridization. Up to day 15.5 of embryonic development we found a non-overlapping distribution of the transcripts for the two receptors. High expression of MPR46 was observed at sites of hemopoiesis and in the thymus while MPR300 was highly expressed in the cardiovascular system. Late in embryogenesis (day 17.5) a wide variety of tissues expressed the receptors, but still the expression pattern was almost non-overlapping. This unexpected complementary expression pattern points to specific functions of the two mannose 6-phosphate receptors during mouse embryogenesis.