Lactate Promotes the Second Cell Fate Decision in Blastocysts by Prompting Primitive Endoderm Formation Through an Intercellular Positive Feedback Loop That Couples Paracrine <scp>FGF</scp> Signalling

Lactate has been widely recognised as an energy source and metabolic by-product, but increasing evidence supports its critical role as a signalling molecule or epigenetic substrate. During early embryogenesis, lactate production increases during the transition from early to late blastocyst, coinciding with the differentiation of inner mass cell (ICM) into epiblast (EPI) and primitive endoderm (PrE), termed the second cell fate decision. However, the role of this hallmark metabolic change in the second cell fate segregation remains unknown. Herein, using in vitro and in vivo models, we found lactate production is preferentially increased in PrE cells and is essential for ICM differentiation into PrE. Mechanically, increased lactate in PrE precursor cells and FGF signalling in EPI precursor cells reciprocally activate each other and synergise to prompt PrE specification, forming an intercellular positive feedback loop essential for this lineage commitment. Additionally, lactate enhanced histone lactylation levels during differentiation into PrE fate. Thus, our findings construct a complex multilayer model in which intracellular metabolite in PrE cooperates with intercellular growth factor signalling from EPI to regulate early embryonic lineage commitment. Highlighting the multifaceted lactate's function, our findings also advance the current knowledge that bridges epigenetic reprogramming and metabolic remodelling during early embryonic development.