BMP signaling mediated by ALK2 in the visceral endoderm is necessary for the generation of primordial germ cells in the mouse embryo

Abstract

Deletion of various bone morphogenetic proteins (BMPs) and their downstream Smads in mice have clearlyshown that BMP signaling is essential for the formation of primordial germ cells (PGCs). However, the molecular mechanism through which this takes place is still unclear. Here, we demonstrate that BMP4 produced in the extraembryonic ectoderm signals through ALK2, a type I BMP receptor, in the visceral endoderm (VE) to induce formation of PGCs from the epiblast. Firstly, embryonic day 5.5-6.0 (E5.5-E6.0) embryos cultured on fibronectin formed PGCs in the presence of VE, but not in its absence. Secondly, Alk2-deficient embryos completely lacked PGCs and the heterozygotes had reduced numbers, resembling Bmp4-deficient phenotypes. Thirdly, expression of constitutively active ALK2 in the VE, but not in the epiblast, was sufficient to rescue the PGC phenotype in Bmp4-deficient embryos. In addition, we show that the requirement for the VE at E5.5-E6.0 can be replaced by culturing embryos stripped of VE on STO cells, indicating that STO cells provide or transduce signals necessary for PGC formation that are normally transmitted by the VE. We propose a model in which direct signaling to proximal epiblast is supplemented by an obligatory indirect BMP-dependent signal via the VE.

Caracterización y Marcación de Células Germinales Primordiales

“…Formación de células germinales primordiales: La señali-zación mediante BMP es esencial para la formación de las CGP (de Sousa Lopes et al, 2004). La activación de la cascada de señalización a través de BMP4 producida en el ectodermo extraembrionario, es necesario para la generación de células germinales primordiales, mientras que otros BMPs tales como BMP8b (producido por el ectodermo extraembrionario) y BMP2 (producido por el endodermo visceral), han demostrado su importancia en el establecimiento del número normal de estas células.…”

“…La activación de la cascada de señalización a través de BMP4 producida en el ectodermo extraembrionario, es necesario para la generación de células germinales primordiales, mientras que otros BMPs tales como BMP8b (producido por el ectodermo extraembrionario) y BMP2 (producido por el endodermo visceral), han demostrado su importancia en el establecimiento del número normal de estas células. En esta misma etapa, Fragilis es detectado a nivel del epiblasto proximal entre los días 6,25 y 6,5 dpc (de Sousa Lopes et al, 2004).…”

“…Se ha demostrado que tanto la activación de Smad1 como de Smad5 son importantes para la inducción de las CGP (de Sousa Lopes et al, 2004).…”

Genetics of germ cell development

“…The additional BMP family members BMP8b and BMP2, coming from the extraembryonic ectoderm, oppose the inhibitory anterior signal, setting up a gradient of inhibition and supporting induction of germ cells in the posterior epiblast. 20,[24][25][26] During this time, the pluripotency-associated genes Oct4, Sox2, and Nanog are progressively restricted to the germline and become germ cell specific at E8.0. [27][28][29] The means by which a limited subset of cells is initially selected from among the larger number of competent cells in the correct position of the posterior epiblast is unclear.…”

Mouse models for identifying genes modulating fertility parameters

“…Among those, transforming growth factor beta (TGFb) family members appear heavily over-represented. This is the case for ligands such as Bmp4 (Lawson et al, 1999), Bmp8b (Ying et al, 2000) and Bmp2 (Ying and Zhao, 2001) as well as for their receptors (de Sousa Lopes et al, 2004). Defects in TGFb signalling pathway, by intracellular effectors such as Smad1 and Smad5, also have a reducing effect on the PGC number (Chang and Matzuk, 2001;Tremblay et al, 2001).…”