MESP1 - A NOVEL BASIC HELIX-LOOP-HELIX PROTEIN EXPRESSED IN THE NASCENT MESODERMAL CELLS DURING MOUSE GASTRULATION

A subtractive hybridization strategy was used to isolate putative gene s involved in the development of mouse primordial germ cells (PGC). Co mplimentary DNA was amplified on RNA isolated from the base of the all antois where PGC are located in the 7.5 days post coitum (dpc) mouse e mbryo, It was then subtracted by hybridization with cDNA amplified on RNA of the anterior region where PGC are absent, A novel gene thus iso lated is designated as Mesp1 and encodes a possible transcription fact or MesP1 containing a basic helix-loop-helix motif. Its earliest expre ssion was observed at the onset of gastrulation, as early as 6.5 dpc, in the nascent mesodermal cells that first ingressed at the end of the primitive streak, These expressing cells in the lateral and extraembr yonic mesoderm showed a wing-shaped distribution, Its initial expressi on was soon down-regulated at 7.5 dpc before the completion of gastrul ation, except at the proximal end of the primitive streak which includ ed the extraembryonic mesoderm and the base of allantois, At 8 dpc, th e expression at the base of the allantois moved laterally, This distri bution between 7.0 and 8.0 dpc was similar to that of PGC detected by the alkaline phosphatase activity. However, the expression of Mesp1 wa s down-regulated thereafter, when PGC entered in the migration stage. After birth, Mesp1 expression was detected only in mature testes, but in a different isoform from that expressed in the embryo, Mesp1 was ma pped to the mid region of chromosome 7, near the mesodermal deficiency gene (mesd). However, a Southern hybridization study clearly showed t hat Mesp1 was distinctly different from mesd. The amino acid sequence and its expression pattern suggest that MesP1 plays an important role in the development of the nascent mesoderm including PGC.