Intestinal crypt homeostasis results from neutral competition between symmetrically dividing Lgr5 stem cells.

Intestinal stem cells, characterized by high Lgr5 expression, reside between Paneth cells at the small intestinal crypt base and divide every day. We have carried out fate mapping of individual stem cells by generating a multicolor Cre-reporter. As a population, Lgr5(hi) stem cells persist life-long, yet crypts drift toward clonality within a period of 1-6 months. We have collected short- and long-term clonal tracing data of individual Lgr5(hi) cells. These reveal that most Lgr5(hi) cell divisions occur symmetrically and do not support a model in which two daughter cells resulting from an Lgr5(hi) cell division adopt divergent fates (i.e., one Lgr5(hi) cell and one transit-amplifying [TA] cell per division). The cellular dynamics are consistent with a model in which the resident stem cells double their numbers each day and stochastically adopt stem or TA fates. Quantitative analysis shows that stem cell turnover follows a pattern of neutral drift dynamics.

Antagonism between H3K27me3 and genome-lamina association drives atypical spatial genome organization in the totipotent embryo.

In mammals, early embryonic development exhibits highly unusual spatial positioning of genomic regions at the nuclear lamina, but the mechanisms underpinning this atypical genome organization remain elusive. Here, we generated single-cell profiles of lamina-associated domains (LADs) coupled with transcriptomics, which revealed a striking overlap between preimplantation-specific LAD dissociation and noncanonical broad domains of H3K27me3. Loss of H3K27me3 resulted in a restoration of canonical LAD profiles, suggesting an antagonistic relationship between lamina association and H3K27me3. Tethering of H3K27me3 to the nuclear periphery showed that the resultant relocalization is partially dependent on the underlying DNA sequence. Collectively, our results suggest that the atypical organization of LADs in early developmental stages is the result of a tug-of-war between intrinsic affinity for the nuclear lamina and H3K27me3, constrained by the available space at the nuclear periphery. This study provides detailed insight into the molecular mechanisms regulating nuclear organization during early mammalian development.