Cellular NAD+ availability constrains developmental speed in the Drosophila eye

The cellular and biochemical processes that limit the rates at which embryos develop, tissues form, and cells differentiate remain largely unknown. Using the speed of progression of a differentiation front in the developing eye of Drosophila as a proxy for developmental speed, we identified genetic perturbations that slowed down the progression of this front. We found that inhibition of the Electron Transport Chain (ETC), and more generally of energy production in mitochondria, reduced developmental speed, led to increased NADH/ NAD+ ratio but did not change ATP levels due to a compensatory increase in glycolysis. Using targeted perturbations, we further showed that the metabolic state of cells located at and ahead of the differentiation front determined the speed of this front. Additionally, increasing the rate of regeneration of the NAD+ pool suppressed the defects resulting from the inhibition of the ETC whereas inhibition of the NAD+ salvage pathway led to reduced developmental speed. Thus, developmental speed appeared to be limited by the demand for NAD+, at least upon reduced energy supply. Overall design: A mirr-Gal4 driver was used to direct gene-based perturbation in the eye imaginal disc. Here, brain complexes from control OreR larvae were compared with those from mirr-Gal4 UAS-ND-42RNAi (BL-28894) larvae (grown at 25°C).

制约胚胎发育、组织形成及细胞分化速率的细胞与生化过程，目前仍在很大程度上未被阐明。本研究以果蝇发育眼中分化前沿的推进速率作为发育速度的替代指标，筛选出了可减缓该前沿推进的遗传扰动因子。我们发现，抑制电子传递链（Electron Transport Chain, ETC），更广泛地说抑制线粒体能量产生过程，会降低发育速度，导致NADH/NAD+比值升高，但由于糖酵解的代偿性增强，ATP水平并未发生改变。通过靶向扰动实验，我们进一步证实，分化前沿及其前方细胞的代谢状态决定了该前沿的推进速率。此外，提高NAD+库的再生速率可抑制由ETC抑制引发的缺陷，而抑制NAD+补救合成途径则会降低发育速度。由此可见，至少在能量供应不足的情况下，发育速度似乎受到NAD+需求的限制。

整体实验设计：本研究使用mirr-Gal4驱动因子在果蝇眼成虫盘内介导基于基因的遗传扰动。将野生型对照OreR幼虫的脑-眼复合体，与在25℃条件下培养的mirr-Gal4 UAS-ND-42RNAi（BL-28894）幼虫的脑-眼复合体进行对比分析。