Characterisation of the transcriptional dynamics underpinning the function, fate, and migration of the mouse Anterior Visceral Endoderm

During early post-implantation development of the mouse embryo, the Anterior Visceral Endoderm (AVE) differs from surrounding visceral endoderm (VE) in its migratory behaviour and ability to restrict primitive streak formation to the opposite side of the egg cylinder. In order to characterise the molecular basis for the unique properties of the AVE, we combined single-cell RNA-sequencing of the VE prior to and during AVE migration, with high-resolution imaging, short-term lineage labelling, phosphoproteomics and pharmacological intervention. This revealed the transient nature of the AVE, the emergence of heterogeneities in AVE transcriptional states relative to position of cells, and its prominence in establishing gene expression asymmetries within the spatial constraints of the embryo. We identified a previously unknown requirement of Ephrin- and Semaphorin-signalling for AVE migration. These findings point to a tight coupling of transcriptional state and position in the AVE and reveal molecular heterogeneities underpinning its migratory behaviour and function.

MLL4 is required for the first embryonic collective cell migration whereas MLL3 is not required until birth

Methylation of histone 3 lysine 4 (H3K4) is a major epigenetic system associated with gene expression. In mammals there are six H3K4 methyltransferases related to yeast Set1 and fly Trithorax, including two orthologs of fly Trithorax-related: MLL3 and MLL4. Exome sequencing has documented high frequencies of Mll3 and Mll4 mutations in many types of human cancer. Despite this emerging importance, the requirements of these sister genes in mammalian development have only been incompletely reported. Here we examined the null phenotypes to establish that MLL3 is first required for lung maturation whereas MLL4 is first required for migration of the anterior visceral endoderm (AVE) that initiates gastrulation and is the first collective cell migration in development. This migration is preceded by a columnar to squamous transition in visceral endoderm cells that depends on MLL4. Furthermore, Mll4 mutants display incompletely penetrant, sex distorted, embryonic haploinsufficiency and adult heterozygous mutants show aspects of Kabuki syndrome, indicating that MLL4 action, unlike MLL3, is dosage dependent. The highly specific and discordant functions of these sister genes argues against their action as general enhancer factors.Summary statementThe H3K4 methyltransferases MLL3 and MLL4 have strikingly different null phenotypes during mouse development; MLL3 is required for lung maturation whereas MLL4 is required for anterior visceral endoderm migration.

Heading forwards: anterior visceral endoderm migration in patterning the mouse embryo

The elaboration of anterior–posterior (A–P) pattern is one of the earliest events during development and requires the precisely coordinated action of several players at the level of molecules, cells and tissues. In mammals, it is controlled by a specialized population of migratory extraembryonic epithelial cells, the anterior visceral endoderm (AVE). The AVE is a signalling centre that is responsible for several important patterning events during early development, including specifying the orientation of the A–P axis and the position of the heart with respect to the brain. AVE cells undergo a characteristic stereotypical migration which is crucial to their functions.