Fd4 (cyan) and Fd5 (magenta) are coexpressed in the ventral nerve cord during embryogenesis of a fruit fly, as shown in a stage 13 embryo (lateral view, dorsal upwards, ventral downwards). Image credit: Lai and Doe (CC BY-4.0).
The brain contains a diverse range of neurons that form networks responsible for our thoughts, movements and behaviours. All neurons originate from neural stem cells, the earliest cells of the nervous system. However, neural stem cells are not identical; each is programmed to generate specific neuron types.
During early development, stem cells receive brief signals that provide temporary instructions about which neurons to produce. A key challenge is that these signals are short-lived, whereas brain development occurs over a much longer period. To overcome this, neural stem cells must “remember” their identity, converting transient signals into long-lasting cellular memory that ensures consistent neuron production.
Lai and Doe investigated how short-lived developmental signals are transformed into durable memory within neural stem cells. They asked whether a specific “memory molecule” preserves stem-cell identity and whether transferring this molecule to another stem cell could redirect it to produce a different neuron type. To find out how stem cells determine their fate and generate the correct neurons at the appropriate times, the researchers used genetically modified fruit flies.
The experiments identified the transcription factor Fd4 as a key molecule that maintains long-lasting cellular memory in neural stem cells. This protein is known to help control which genes are turned on and off inside a cell and helps determine what type of cell it becomes. In the flies, Fd4 was activated by developmental signals and remained expressed in specific stem cell lineages. When Fd4 (and its partner Fd5) was removed, the stem cells failed to produce the neurons they normally generate. Conversely, forcing other stem cells to express Fd4 redirected them to produce neurons typically associated with Fd4-expressing lineages. These results demonstrate that Fd4 preserves the identity of stem cells over time and guides the production of the correct neuron types throughout development.
These findings have important implications for research in brain development and neurological disorders, particularly for efforts to guide stem cells to generate specific neuron types. In the long term, this knowledge could contribute to regenerative therapies aimed at replacing damaged neurons. However, further research is needed to determine whether this mechanism exists in humans, including identifying equivalent genes and evaluating safe ways to manipulate them before clinical application.
