Coordinated regulation of gene activity by transcriptional and translational mechanisms poise stem cells for a timely cell-state transition during differentiation. Although important for all stemness-to-differentiation transitions, mechanistic understanding of the fine-tuning of gene transcription is lacking due to the compensatory effect of translational control. We used intermediate neural progenitor (INP) identity commitment to define the mechanisms that fine-tune stemness gene transcription in fly neural stem cells (neuroblasts). We demonstrate that the transcription factor FruitlessC (FruC) binds cis-regulatory elements of most genes uniquely transcribed in neuroblasts. Loss of fruC function alone has no effect on INP commitment but drives INP dedifferentiation when translational control is reduced. FruC negatively regulates gene expression by promoting low-level enrichment of the repressive histone mark H3K27me3 in gene cis-regulatory regions. Identical to fruC loss-of-function, reducing Polycomb Repressive Complex 2 activity increases stemness gene activity. We propose low-level H3K27me3 enrichment fine-tunes gene transcription in stem cells, a mechanism likely conserved from flies to humans.
Keywords: D. melanogaster; Drosophila; Notch; asymmetric division; developmental biology; fine-tuning; neuroblast; polycomb repressive complex 2; regenerative medicine; stem cells.
From neurons to sperm, our bodies are formed of a range of cells tailored to perform a unique role. However, organisms also host small reservoirs of unspecialized ‘stem cells’ that retain the ability to become different kinds of cells. When these stem cells divide, one daughter cell remains a stem cell while the other undergoes a series of changes that allows it to mature into a specific cell type. This ‘differentiation’ process involves quickly switching off the stem cell programme, the set of genes that give a cell the ability to keep dividing while maintaining an unspecialized state. Failure to do so can result in the differentiating cell reverting towards its initial state and multiplying uncontrollably, which can lead to tumours and other health problems. While scientists have a good understanding of how the stem cell programme is turned off during differentiation, controlling these genes is a balancing act that starts even before division: if the program is over-active in the ‘mother’ stem cell, for instance, the systems that switch it off in its daughter can become overwhelmed. The mechanisms presiding over these steps are less well-understood. To address this knowledge gap, Rajan, Anhezini et al. set out to determine how stem cells present in the brains of fruit flies could control the level of activity of their own stem cell programme. RNA sequencing and other genetic analyses revealed that a protein unique to these cells, called Fruitless, was responsible for decreasing the activity of the programme. Biochemical experiments then showed that Fruitless performed this role by attaching a small amount of chemical modifications (called methyl groups) to the proteins that ‘package’ the DNA near genes involved in the stem cell programme. High levels of methyl groups present near a gene will switch off this sequence completely; however, the amount of methyl groups that Fruitless helped to deposit is multiple folds lower. Consequently, Fruitless ‘fine-tunes’ the activity of the stem cell programme instead, dampening it just enough to stop it from overpowering the ‘off’ mechanism that would take place later in the daughter cell. These results shed new light on how stem cells behave – and how our bodies stop them from proliferating uncontrollably. In the future, Rajan, Anhezini et al. hope that this work will help to understand and treat diseases caused by defective stem cell differentiation.
© 2023, Rajan, Anhezini et al.