Mitochondrial origins of the pressure to sleep, 2025, Raffaele Sarnataro et al

[Mij]

Abstract ​

To gain a comprehensive, unbiased perspective on molecular changes in the brain that may underlie the need for sleep, we have characterized the transcriptomes of single cells isolated from rested and sleep-deprived flies. Here we report that transcripts upregulated after sleep deprivation, in sleep-control neurons projecting to the dorsal fan-shaped body (dFBNs) but not ubiquitously in the brain, encode almost exclusively proteins with roles in mitochondrial respiration and ATP synthesis.

These gene expression changes are accompanied by mitochondrial fragmentation, enhanced mitophagy and an increase in the number of contacts between mitochondria and the endoplasmic reticulum, creating conduits for the replenishment of peroxidized lipids . The morphological changes are reversible after recovery sleep and blunted by the installation of an electron overflow in the respiratory chain. Inducing or preventing mitochondrial fission or fusion in dFBNs alters sleep and the electrical properties of sleep-control cells in opposite directions: hyperfused mitochondria increase, whereas fragmented mitochondria decrease, neuronal excitability and sleep. ATP concentrations in dFBNs rise after enforced waking because of diminished ATP consumption during the arousal-mediated inhibition of these neurons which augments their mitochondrial electron leak.

Consistent with this view, uncoupling electron flux from ATP synthesis relieves the pressure to sleep, while exacerbating mismatches between electron supply and ATP demand (by powering ATP synthesis with a light-driven proton pump) precipitates sleep. Sleep, like ageing , may be an inescapable consequence of aerobic metabolism.
LINK

 

[Creekside]

So, the mitochondria in a few brain cells get tired and drag the rest of the body along?

 

[jnmaciuch]

Fascinating, this aligns very well with research in rats from some labs at my university showing that genetically altering redox metabolism interferes with circadian rhythm. A very thorough paper at first glance (though the wet lab techniques are outside my expertise and I can only judge the obvious logic). I have no major complaints about the single cell analysis so far either, which is refreshing. And I appreciate a paper that includes cautions about over-interpreting data in the body of its own results text.

So, the mitochondria in a few brain cells get tired and drag the rest of the body along?

Pretty much

 

[Utsikt]

Would it be possible to test for any of this in humans?

 

[jnmaciuch]

Explain like I'm brain foggy:

This study looked at a particular cluster of neurons [edit: in the brains of flies] which are strongly associated with sleep regulation. Compared to the rest of the brain, the gene transcription of these neurons is dominated by genes relating to cellular metabolism/mitochondrial function.

They found that sleep deprivation induces build up of reactive oxygen species (ROS) in these neurons (a byproduct of ATP production), and that these neurons have unique proteins enabling sensitive detection of these levels. Additionally, they identified that the balance of mitochondrial fusion and fission in these neurons--shape/size changes that are closely linked to mitochondrial capacity--are responsible for maintaining the link between metabolism and sleep behavior.

Altering the production of ROS, lowering the excitability of these neurons (therefore decreasing ATP production and consumption), and inhibiting mitochondrial fission/fusion all substantially changed the sleep behavior in flies.

The takeaway is that these neurons have evolved to be very metabolically active when we are [edit: asleep, such that they will] exhibit signs of metabolic "stress" the longer you are awake. [Edit: Sleep changes the metabolic activity of these neurons such that they are generating less ROS]. The authors propose that this cluster of neurons essentially serves as an early warning system for the metabolism of the brain as a whole, inducing rest and the associated changes in synaptic firing patterns before any profound damage occurs in the rest of the brain.

[Edited to correct a mis-reading about increased metabolic activity during sleep vs wakefulness]

 

[Utsikt]

Thank you for the ELIBF, @jnmaciuch !

Is it worth adding that the study is on flies?
Edit: I missed that it’s mentioned at the end of the third paragraph.

 

[jnmaciuch]

Would it be possible to test for any of this in humans?

Depends what exactly you're trying to confirm. Theoretically it might be possible to evaluate some indirect measures of metabolic activity in this region of the brain in humans corresponding with sleep deprivation--I'm not sure if something like that has already been done. But that's probably the most detailed you could get in humans.

I assume some more targeted studies in mice are underway to confirm the findings. We have plenty of evidence that metabolism is involved in circadian rhythm regulation there as well, though it's possible that there are differences in the exact proteins between species. Fundamental systems like this tend to be overwhelmingly evolutionarily conserved.

Is it worth adding that the study is on flies?
Edit: I missed that it’s mentioned at the end of the third paragraph.

I'll edit to make that more obvious

 

[jnmaciuch]

Ah my bad everyone, I initially misread a part of the paper. The metabolic activity of these neurons actually increases substantially during sleep (not decreases like I originally said), so much so that it actually decreases ROS production. Although ROS production is typically talked about in relation to "overactivity", it is actually the efficiency of electron flow that determines whether ROS are produced.

During wakefulness, caloric intake is high but these neurons are not active, meaning that electron flow is less "efficient" (i.e. allows electron leak) than it would be during the extremely high metabolic demand during sleep. So it's the "reduced" metabolic activity during wakefulness that serves as a proxy for the metabolic activity of the rest of the brain, even though the neurons are less active. The correct relationship is even more confusing, I know--apologies for the mix-up!

I’ve edited the ELIBF accordingly

 

[jnmaciuch]

This is what the text says (edited to remove citations which were being pasted with the full hypertext formatting):

When the demands of ATP synthesis are high, the vast majority of electrons reach O2 in an enzymatic reaction catalysed by cytochrome c oxidase (complex IV); only a small minority leak prematurely from the upstream mobile carrier coenzyme Q (CoQ), producing superoxide and other reactive oxygen species (ROS) (Fig. 2a). The probability of these non-enzymatic single-electron reductions of O2 increases sharply under conditions that overfill the CoQ pool as a consequence of increased supply (high NADH to NAD+ratio) or reduced demand (large protonmotive force (∆p) and high ATP to ADP ratio) (Fig. 2a). The mitochondria of dFBNs are prone to this mode of operation during waking, when caloric intake is high but the neurons’ electrical activity is reduced, leaving their ATP reserves full. Indeed, measurements with the genetically encoded ATP sensors iATPSnFR and ATeam showed approximately 1.2-fold higher ATP concentrations in dFBNs, but not projection neurons, after a night of sleep deprivation than at rest (Fig. 2b,c and Extended Data Fig. 4a,b). ATP concentrations rose acutely when dFBNs were inhibited by an arousing heat stimulus, which releases dopamine onto their dendrites (Fig. 2a,d), and fell below baseline when dFBNs themselves were stimulated, mimicking sleep.

As always in metabolism (and life), things are intuitive up until they aren’t.