Specification of unique neuronal sub-types by integration of positional and temporal cues

Abstract: The nervous system contains vast numbers of neuronal sub-types, generated at specific time points, in the proper location, and in proper numbers. One of the fundamental issues in neurobiology is to understand the molecular genetic mechanisms that underlie the generation of this daunting neuronal diversity.To help shed light upon these fundamental questions, my PhD project has addressed the generation and specification of a certain group of neurons, the Ap cluster. This group of four neurons is found only in thoracic segments within the Drosophila melanogaster central nervous system, and consists of three different cell types. Mapping of the neuroblast (stem cell) that generates the Ap cluster neurons, neuroblast 5-6, and the highly restricted appearance of this cluster allowed me to address the following questions: How does NB 5-6 change its temporal competence over time to generate the Ap cluster neurons late in the lineage, and how is temporal competence altered to ensure diversity among the Ap neurons? What are the mechanisms that allow these Ap cluster neurons to emerge only in the thoracic segments?My studies have helped identify a number of mechanisms acting to specify the Ap cluster neurons. One type of mechanism involves several of different feed-forward loops that play out during NB 5-6 lineage development. These are triggered within the stem cell, where the temporal gene castor activates a number of genes. These castor targets are subsequently involved in several regulatory feed-forward loops, that ultimately result in the unique combinatorial expression of cell fate determinants in the different Ap neurons, which in turn ultimately lead to the activation of unique terminal differentiation genes. In addition, I have identified three different mechanisms by which the NB 5-6 lineage is modulated along the neuroaxis. In the abdomen I find that an early cell cycle exit is initiated by the Bx-C gene members and Pbx/Meis cofactors, which result in the truncation of the NB 5-6 lineage, preventing the Ap cluster neurons from being generated. In thoracic segments Hox, Pbx/Meisand temporal genes act in concert to specify Ap cluster neurons, by integrating with the castor temporal gene. In anterior segments, improper Hox and temporal coding results in a failure to specify bona fide Ap cluster neurons, even though equivalents of Ap cluster neurons are generated.In summary, my thesis work has helped identify a number of mechanisms acting to specify this unique neuronal sub-type, including: feed-forward combinatorial coding, opposing feed-forward loops and integrated temporal/Hox mediated specification throughout different axial levels. I suggest that these mechanisms may be widely used within the animal kingdom, hence contributing to the great cellular diversity observed within the central nervous system of most animal species.