Background

Adult stem cells continuously divide to repair damaged tissues and maintain organ homeostasis. The intestinal epithelium, in particular, is characterized by a high cell turnover rate. A long-standing question in biology is the mechanism by which daughter cells differentiate into either absorptive cells (enterocytes, ECs) or hormone-secreting enteroendocrine cells (EEs) during this process. For a long time, the scientific community has focused on signals from the stem cell niche. However, external signals alone cannot fully explain the precise regulation that maintains a constant cell ratio within the tissue. This has led to the hypothesis that stem cells may possess an internal timer.

Key Findings

The research team used a Drosophila adult intestinal model to analyze the changes that intestinal stem cells (ISCs) undergo during division at the genomic level. They employed single-cell RNA sequencing and chromatin accessibility analysis. This revealed a unique epigenetic pattern in which the histone methylation status of specific gene regions changes stepwise with each division. In essence, a chemical counter is at work. From the first to the eighth division, ISCs activate a genetic program that directs daughter cells to differentiate into ECs. At the ninth division, the situation changes. Precisely at the ninth division, the accumulated mark reaches a threshold, triggering the expression of genes involved in enteroendocrine mother cell (EMC) differentiation. The barrier to transcription factor expression is finally broken. Consequently, daughter cells generated after the ninth division differentiate into EMCs rather than ECs. The research team confirmed this causal relationship using CRISPR technology. Blocking the gene encoding the enzyme that mediates this epigenetic accumulation process caused ISCs to lose track of the number of divisions, resulting in the continuous production of ECs over many divisions. Conversely, overexpressing the enzyme accelerated the clock, leading to premature differentiation, with EMCs being generated after only a few divisions.

Significance and Prospects

This discovery establishes a new milestone, demonstrating that stem cell differentiation is determined not only by signals from the microenvironment but also by the cell's internal division history. It provides evidence for an independent time-control system within cells. The cell-counting mechanism proposed by the researchers is likely to operate not only in Drosophila but also in higher organisms, including mammals. Of course, there are interspecies differences. Since the physical structure and division cycle of stem cells differ between Drosophila and mammals, further research is needed to verify whether the same mechanism exists in humans. Despite these differences, the impact of this research is expected to be significant. In particular, the fact that the accumulation of division counts in adult stem cells may be directly related to cellular aging and cancer development is stimulating to many researchers. This is because it could lead to technologies that can turn back the clock. In the future, if technologies can be developed to regulate or reset the division-counting speed within stem cells, it will be of great benefit in the treatment of degenerative diseases.