The latest manuscript from Ciaran which was recently published in Nature Communications, titled “Stress gates a synaptic energy reservoir to impair synaptic plasticity”.

In this work we set out to investigate whether astrocytes contribute to the frequently observed phenomenon that stress impairs synaptic plasticity.

Starting with RNAseq, we showed that stress has widespread effects on astrocyte gene expression profiles. This was done using the Ribotag transgenic mice, allowing us to isolate ribosomes specifically from astrocytes, and found that just over 100 genes were altered by 20-minute swim stress. The full RNAseq dataset can be found here.

Other stress-induced modifications included cellular hypertrophy, as well as increased duration of spontaneous calcium events in astrocyte microdomains.

Using a live-imaging approach to study astrocyte-astrocyte coupling in real time using patch-clamp and two-photon microscopy we demonstrated that stress hormones (glucocorticoids) impair coupling between astrocytes. Importantly, stress-induced impairment of coupling slowed the flux of energetic substrates between astrocytes (we used the fluorescent glucose analogue 2-NBDG).

We next asked the question whether this impaired coupling could be important for synaptic function?

To answer this, we developed a genetic strategy to block astrocyte coupling without affecting other properties of astrocytes, such as morphology. This approach involved modifying connexin 43 protein, one of the major astrocytic gap junction channel proteins, so that the channel pore was no longer functional, but could still form gap junction adhesion structures. This approach effectively blocked both gap junction channels and hemichannels.

Expressing this modified connexin 43 (dnCx43) in astrocytes completely occluded synaptic plasticity at nearby synapses.

Questioning why this could be the case, we took inspiration from the abundant literature on the astrocyte-neuron L-lactate shuttle hypothesis. This hypothesis states that neurons rely on L-lactate from astrocytes as an energy source for synaptic processes such as long-term potentiation. We found that supplementing dnCx43-expressing astrocytes (i.e. astrocytes that have been uncoupled from their network) with L-lactate, we were able to rescue synaptic plasticity to control levels. This shows, that disconnecting astrocytes from their network impaired their capacity to ‘feed’ neurons L-lactate, but that they could still carry out this vital function when supplemented with L-lactate through the patch pipette.

Now we get to ask the question whether these changes in astrocyte function are responsible for stress-induced impairment of of synaptic plasticity?

Our data above suggests that decreased coupling induced by stress could impair the capacity of astrocytes to supply neurons with L-lactate. To see if this was the case, we used enzymatic L-lactate sensors to measure the capacity astrocytes to release L-lactate following synaptic stimulation and found that stress impaired this vital function.

Similar to above in dnCx43 experiments, we investigated whether L-lactate was a limiting factor in LTP expression following stress, and found that we were able to rescue the effects on synaptic plasticity by supplementing a patched astrocyte with L-lactate. This experiment effectively overcomes the astrocyte network deficit induced by stress, using the patch pipette as the energy reservoir rather than the network of coupled cells. The rescue was blocked when we impaired neuronal uptake of L-lactate, demonstrating that this L-lactate needs to be shuttled from astrocytes to neurons for energetic consumption.

This work highlights the importance of astrocytes in metabolic support of neuronal function, showing that astrocyte networks can act as an energy reservoir and that this process is compromised after stress. This also demonstrates the sensitivity of astrocytes to stress hormones, with a single bout of acute stress impacting astrocyte-astrocyte coupling on a rapid timescale. This suggests that astrocytes play play a key role in mediating the effects of stress in the brain. Astrocytes, which physically bridge the synapses with the vasculature, are uniquely positioned to integrate synaptic signals with blood borne signalling molecules such as stress hormones.

One of the research questions we wish to tackle in the lab is understanding how astrocytes integrate these distinct signals. Also, whether these cellular changes persist in chronic stress, which more faithfully reproduce the cellular changes occurring in stress disorders, remains to be seen.

This project was led by Ciaran during his postdoc at Dr. Jaideep Bains & Grant Gordon’s labs, was a collaborative effort between teams in Canada - Roger Thompson lab U. Calgary, and the USA - Baljit Khakh lab UCLA, David Spray lab Albert Einstein U., and Randy Stout lab NYIT.

We are extremely grateful to all of our collaborators who made this exciting work possible.

Our lab intends to build upon these ideas and observations, to investigate the stress-induced changes in astrocyte structure and function and how this might influence memory in vivo.

Full text open access to the paper can be found on the lab website in the publications section or at Nature Communications.