Comparative developmental transcriptomics of Drosophila mushroom body neurons highlights the mevalonate pathway as a regulator of axon growth

The ability of neurons to extend axons is governed by tightly regulated genetic programs that vary across developmental stages and cell types. Understanding the molecular features that control axon growth potential is critical for uncovering how neural circuits form, mature, and respond to injury or disease. The Drosophila mushroom body (MB) offers a powerful model to dissect axon growth programs, as lineage-related Kenyon cells (KCs) undergo different developmental events under shared spatiotemporal conditions. During metamorphosis, γ-KCs undergo axon pruning, followed by developmental regrowth at the same time-frame as α/β-KCs initiate axon growth - thus providing a unique opportunity to compare these distinct growth paradigms. To uncover the underlying genetic programs, we performed RNA-sequencing of α/β- and γ-KCs during their initial growth and developmental regrowth, respectively, revealing dynamic transcriptional changes and identifying 300 shared genes upregulated during both growth states. A targeted loss-of-function screen revealed genes specifically required for either α/β initial growth, γ regrowth, or both. Focusing on one such candidate, Pmvk, we found that it plays a crucial role in axon regrowth by acting within the mevalonate pathway. Notably, other enzymes in this pathway were also required, suggesting that the entire metabolic pathway is essential for supporting regrowth. Using mutant analyses and rescue exepriements, we demosntrate that Pmvk likely controls axon regrwoth via prenylation of Rheb, an effector of the TOR pathway, which we previously found to be required for regrowth. Our developmental transcriptomic atlas not only advances understanding of intrinsic axon growth programs, but also provides candidate genes and a valuable framework for future studies aimed at enhancing axon regeneration in the adult nervous system.