Reviewer #2 (Public Review):
In this paper, the authors discover that postsynaptic mitochondria in C. elegans govern glutamate receptor trafficking dynamics. The core results are two-fold. For one, they find that loss or inhibition of mcu-1 - the C. elegans mitochondrial calcium uniporter - increases GLR-1 glutamate receptor accumulation at the postsynaptic dendritic sites and enhances its trafficking dynamics. The authors hypothesize that this effect on glutamate receptors may have something to do with mitochondrial ROS production. This is because ROS is a by-product of normal oxidative phosphorylation, downstream of calcium import. Indeed, the generation of artificially high amounts of mitochondrial ROS has the opposite effect of mcu-1 loss: decreased glutamate receptor subunit accumulation. Collectively, the results support the idea that mitochondrial function can control receptor dynamics at synaptic sites. This is interesting because tight control of synaptic function likely combines several mitochondrial functions: energy production, calcium buffering, and (here) ROS signaling.
STRENGTHS
• The C. elegans genetic model is a strength because the authors are able to make refined conclusions by classical loss-of-function mutants (e.g., mcu-1) along with an impressive cytological toolkit to examine GLR-1 dynamics.
• The use of pharmacology as a second means to test those genetic conclusions is a strength.
• The authors' careful reagent verification of reporters (Ca2+, ROS, etc.) is a strength.
• The ability to link fundamental mitochondrial processes to GLR-1 exocytosis will expand how the field thinks about mitochondrial synapse function.
WEAKNESSES
For the most part, the data in the paper support the conclusions, and the authors were careful to try experiments in multiple ways. But please see below:
• (Main Point) The data are good, but they fall short of mechanism (e.g., Line 322). Figure 6 is accurate as drawn. But calcium and ROS are not abstract signals. They are likely exerting affirmative actions on specific targets. The Discussion does acknowledge this in terms of ROS and it speculates on possible targets.
The general idea seems to be that mitochondria import calcium through MCU-1 (and interacting factors). As a result, oxidative phosphorylation successfully occurs and mitochondrial ROS is a signaling by-product that signals glutamate receptors not to undergo exocytosis. But there are other interpretations of what might happen in between. In fact, if OXPHOS is disrupted, it is known that this can generate a lot more mitochondrial ROS than the normal by-product levels.
This reviewer wonders if excess ROS would cause an extreme response. Or alternatively, if scavenging ROS via pharmacological scavengers or SOD expression would reverse the effects.
Small Points
• 33.3 mHz - just making sure, do the authors mean once every 30 seconds? That would be more straightforward.
• Figure 2 is confusing. The text says that the mcu-1 mutants have a GLR-1::GFP FRAP rate that is comparable to controls (Lines 165-167). But Figure 2E suggests that it is markedly less, which is the opposite result of the slight increase in rate resulting from Ru360 treatment. And is the explanation why the GLR-1::GFP results differ from the SEP::GLR-1 results a difference between total GFP vs. surface GFP?
• I could not watch Video 2 (not sure if it is the file or just the copy I downloaded).
• It is good that the authors tried both optical stimulation and mechanical stimulation (dropping culture plates to stimulate the worms, Figure 3). Why was the mechanical stimulation set aside for further tests in the paper?
• Does this process affect all kinds of transport, or is it just the glutamate receptors? Was anything else examined?