Kavli Affiliate: Mark Ellisman
| Authors: Juan I. Ispizua, Micaela Rodriguez-Caron, Francisco J. Tassara, Kim Keun-young, Catalina Insusarry Perkins, Milagros Barzi, Christian Carpio-Romero, Celia N. Hansen, Julian Gargiulo, Ezio Rosato, Horacio de la iglesia, Mark Ellisman and M. Fernanda Ceriani
| Summary:
The central clock in the Drosophila’s brain consists of approximately 250 neurons organized into a relatively complex network. Within this network, four small Lateral ventral Neurons (s-LNvs) in each hemisphere are rhythmically loaded with the neuropeptide pigment dispersing factor (PDF), which plays a crucial role in synchronizing the calcium rhythms across the network. Additionally, there are daily variations in markers of active synaptic sites at their terminals, along with changes in synaptic partners. The s-LNv axo-dendritic arbor undergoes characteristic remodeling in sync with these events, a process known as circadian structural plasticity. The relationship between these various plastic changes remains unclear, largely due to a lack of techniques that offer precise, comprehensive insights into both connectivity and structure. In this study, we genetically labeled the mitochondrial matrix of the s-LNvs using APEX2 peroxidase expression, enabling us to generate three Serial Block-face Scanning Electron Microscopy (SBEM) volumes where the s-LNvs were recognizable. Each volume represents a distinct time point in the circadian remodeling of the s-LNv terminals: ZT22 (two hours before dawn), ZT2 (two hours after dawn), and ZT14 (two hours after dusk). By tracing the labeled mitochondria, we identified and segmented the terminals of four s-LNvs in each brain. We then manually segmented dense core vesicles, both free-floating and those fusing to the membrane, as well as pre-synaptic sites and mitochondria. We discovered that dense core vesicles (DCVs) are accumulated and released from the s-LNv varicosities with a peak in the morning. Additionally, we observed nearly 50% more presynaptic sites in the morning compared to nighttime. These changes are accompanied by a marked shift in mitochondrial shape and volume, indicating functional differences between day and night. Finally, these adjustments are coordinated with changes in neurite length and the number of varicosities. We propose that structural plasticity helps organize daily changes in functional units, dynamizing the impact of these neurons on the network.