BMP signaling regulates PGC numbers and motility in organ culture

Abstract

Members of the bone morphogenetic protein (BMP) family play diverse roles in multiple developmental processes. However, in the mouse, mutations in many BMPs, BMP receptors and signaling components result in early embryonic lethality making it difficult to analyze the role of these factors during organogenesis or tissue homeostasis in the adult. To bypass this early lethality, we used an organ culture system to study the role of BMPs during primordial germ cell (PGC) migration. PGCs are the embryonic precursors of the sperm and eggs. BMPs induce formation of primordial germ cells within the proximal epiblast of embryonic day 7.5 (E7.5) mouse embryos. PGCs then migrate via the gut to arrive at the developing gonads by E10.5. Addition of BMP4 or the BMP-antagonist Noggin to transverse slices dissected from E9.5 embryos elevated PGC numbers or reduced PGC numbers, respectively. Noggin treatment also slowed and randomized PGC movements, resulting in a failure of PGCs to colonize the urogenital ridges (UGRs). Based on p-Smad1/5/8 staining, migratory PGCs do not respond to endogenous BMPs. Instead, the somatic cells of the urogenital ridges exhibit elevated p-Smad1/5/8 staining revealing active BMP signaling within the UGRs. Noggin treatment abrogated p-Smad staining within the UGRs and blocked localized expression of Kitl, a cytokine known to regulate the survival and motility of PGCs and Id1, a transcription factor expressed within the UGRs. We propose that BMP signaling regulates PGC migration by controlling gene expression within the somatic cells along the migration route and within the genital ridges.

Foetal germ cells: striking the balance between pluripotency and differentiation

“…Consistent with this, loss of function of the receptor for FGF7, FGFR2-3b results in reduced germ cell numbers (Takeuchi et al, 2005). In addition, BMPs regulate PGC numbers and motility in culture (Dudley et al, 2007;Ross et al, 2007). BMPs are also required for regulation of pluripotency in ES cells (Ying et al, 2003) and for recruitment of PGCs from the proximal epiblast during germ line specification (Lawson et al, 1999;de Sousa Lopes et al, 2004).…”

Alk3 controls nephron number and androgen production via lineage-specific effects in intermediate mesoderm

“…The murine testis is formed from genital swellings ventral to the mesonephros, which are populated by migrating primordial germ cells at E10.5 (Wilhelm and Koopman, 2006). BMP signaling regulates primordial germ cell number, proliferation and migration, yet no role had been defined in testicular somatic cells (Hu et al, 2004;Puglisi et al, 2004;Dudley et al, 2007). No germ cell defects were observed in Alk3 IMP null mice, consistent with lack of expression of Rarb2-Cre in these cells.…”

Conditional knockdown of Nanog induces apoptotic cell death in mouse migrating primordial germ cells

“…Id1 is downstream of the BMP/phosphorylated Smad pathway and functions as a dominant-negative binding factor for HLH genes (Norton et al, 1998). However, phosphorylated Smad1, 5 and 8 are not detected in migrating PGCs (Dudley et al, 2007). Thus, Id1 has to be upregulated by another pathway.…”

“…S7A in the supplementary material). Although the mechanism of transcriptional regulation of Id1, which bypasses the BMP/phosphorylated Smad pathway (Dudley et al, 2007), is unclear, Id1 might be directly downstream of Nanog in PGCs, as shown by the binding of Nanog to Id1 in ESCs (Kim et al, 2008).…”