Homeostatic Depression Shows Heightened Sensitivity to Synaptic Calcium

Area of Science:

• Neuroscience
• Synaptic Plasticity
• Molecular Biology

Background:

• Homeostatic regulation is essential for synaptic function and information transmission.
• The Drosophila melanogaster neuromuscular junction (NMJ) exhibits robust homeostatic control, primarily studied through presynaptic homeostatic potentiation (PHP).
• Presynaptic homeostatic depression (PHD) is a less-understood counterpart to PHP, involving decreased quantal content in response to challenges like VGlut overexpression.

Purpose of the Study:

• To identify molecules involved in both presynaptic homeostatic potentiation (PHP) and presynaptic homeostatic depression (PHD).
• To investigate the role of intracellular calcium release channels in PHD.
• To understand the sensitivity of homeostatic signaling to presynaptic calcium levels.

Main Methods:

• Electrophysiology-based genetic screen in Drosophila melanogaster to identify blocks in PHD.
• Assessment of evoked neurotransmitter release and synaptic function under various genetic and pharmacological conditions.
• Genetic manipulation of inositol trisphosphate receptor (IP3R) and ryanodine receptor (RyR) function, and pharmacological inhibition.

Main Results:

• Loss-of-function mutations in IP3R and RyR genes, when combined with VGlut overexpression, caused a significant deficit in evoked synaptic amplitude, indicating a failure in PHD.
• Pharmacological agents targeting IP3R and RyR mimicked the genetic findings.
• Reduced extracellular calcium levels also impaired the homeostatic response, suggesting a broader sensitivity to calcium availability.

Conclusions:

• The homeostatic signaling pathway underlying PHD is highly sensitive to presynaptic calcium levels.
• IP3Rs and RyRs play a critical role in mediating PHD by regulating calcium release from intracellular stores.
• Disruption of calcium homeostasis significantly impairs a synapse's ability to maintain stable function under stress.