The transcription factor Prox1 acts as a mitotic bookmark in mouse hippocampal development, preserving dentate gyrus neuronal lineage identity across cell divisions by remaining bound to the key cornu ammonis identity gene Fezf2 during mitosis and promoting timely restoration of PRC2-mediated H3K27me3 repressive marks, with specific disruption of Prox1's mitotic retention ability causing severe developmental defects in conditional knock-in mice.

Mitosis poses a daunting challenge to transgenerational inheritance of cell identity memory. How neuronal lineage identity is faithfully transmitted across mitosis remains largely unexplored. Here we report that, in mouse hippocampus, the transcription factor Prox1 acts as a mitotic bookmark for safeguarding neuronal lineage identity across cell divisions. Prox1 exhibits mitotic retention in dentate gyrus (DG) neural stem cells and defines DG lineage identity by suppressing the alternative cornu ammonis (CA) identity. Mitotic bookmarking by Prox1 at key bivalent CA identity genes promotes timely and precise restoration of Polycomb repressive complex 2 (PRC2)-mediated H3K27me3 deposition to avoid ectopic expression of CA identity genes and lineage identity crisis. Remarkably, specific mitotic retention-deficient Prox1 conditional knock-in mice produce severe DG developmental defects. Thus, mitotic bookmarking imprints neuronal lineage identity in mouse hippocampus, which is likely to represent a fundamental principle underlying the preservation of lineage identity memory in mammalian brain development.