Daily ultrastructural remodeling of clock neurons

Area of Science:

• Neuroscience
• Chronobiology
• Cell Biology

Background:

• Animals possess endogenous biological clocks that regulate physiological processes in response to environmental cycles.
• Circadian rhythmicity is orchestrated by neural networks, with some clock neurons exhibiting daily morphological changes known as circadian structural plasticity.
• The subcellular details of circadian structural plasticity and its impact on neuronal connectivity remain largely undefined.

Purpose of the Study:

• To investigate the ultrastructural changes in the terminals of core clock neurons during the day-night cycle.
• To identify the subcellular units responsible for circadian structural plasticity.
• To understand how these structural changes influence neuronal function and connectivity within the circadian network.

Main Methods:

• Generation of 3D electron microscopy reconstructions of adult Drosophila brains.
• Analysis of ultrastructural changes in core clock neuron terminals at three key time points.
• Quantification of changes in neuronal varicosities, active zones, dense-core vesicles, and mitochondria.

Main Results:

• Neuronal varicosities were identified as key organizational units for structural changes, containing elements for communication and metabolic support.
• Varicosity number, active zone counts, dense-core vesicle accumulation, and mitochondrial morphology/quantity varied across the day-night cycle.
• Evidence suggests an interplay between structural and functional plasticity at the subcellular level.

Conclusions:

• Circadian plasticity of presynaptic varicosities modulates the influence of clock neurons on the circadian network.
• These ultrastructural changes likely contribute to daily shifts in neuronal connectivity.
• The findings may have implications for understanding timekeeping mechanisms across different species due to conserved biological clock components.