ESOP-1, a secreted protein expressed in the hematopoietic, nervous, and reproductive systems of embryonic and adult mice.

To isolate soluble factors expressed in early phases of hematopoietic differentiation, we applied the signal sequence trap method to the in vitro murine hematopoietic differentiation system, in which ES cells are cocultured with OP-9 stroma cells. This strategy allowed us to isolate cDNA for a secreted protein, ESOP-1, of 160 amino acids, the sequence of which shows 64% identity with human ESOP-1/MD-2. ESOP-1 mRNA was highly expressed in the mouse embryos at 7.5 days after coitus. Expression of the ESOP-1 mRNA and protein was shown in the embryonic and adult hematopoietic system. In addition, the ESOP-1 protein was found in the yolk sac-blood islands, the developing nervous system, and the adult reproductive system. These results suggest that ESOP-1 may play some roles in the development or maintenance of hematopoietic, nervous, and reproductive systems.