Connecting temporal identity to mitosis: the regulation of Hunchback in Drosophila neuroblast lineages.

Both in vertebrates and invertebrates, neural stem cells generate different cell types at different times during development. It has been suggested that this process depends on temporal identity transitions of neural progenitors, but the underlying mechanism has not been resolved, yet. Recently, Drosophila neuroblasts (NBs) have been shown to be an excellent model system to investigate this subject. Here, changes in temporal identity are regulated by sequential and transient expression of transcription factors in the NB, such as Hunchback (Hb) and Krüppel (Kr). The temporal expression profile is maintained in the progeny. Hb is expressed first and thus defines the earliest identity in a given lineage. Transition to Kr requires the termination of hb expression, which occurs in response to mitosis, and is mediated by Seven-up (Svp). Recent results provided evidence that the dependency of Svp activity on mitosis could be due to an inhibition of the nuclear export of svp mRNA. Furthermore, the maintenance of hb expression in the GMC is regulated by the activity of Prospero (Pros), a transcription factor which asymmetrically segregates into the GMC during mitosis and inhibits Svp activity on both, the transcriptional and posttranscriptional level. These results give first insights as to how temporal cell fate specification can be correlated with mitosis.