Real-time images of microglia nibbling on brain synapses have been captured for the first time, by European Molecular Biology Laboratory researchers. Their findings show that the special glial cells help synapses grow and rearrange, demonstrating the essential role of microglia in brain development.

Around one in ten cells in your brain are microglia. Cousins of macrophages, they act as the first and main contact in the central nervous system’s active immune defense. They also guide healthy brain development.

Researchers have proposed that microglia pluck off and eat synapses - connections between brain cells - as an essential step in the pruning of connections during early circuit refinement. But until now, no one had seen them do it.

That is why Laetitia Weinhard, from the Gross group at EMBL Rome, set out on a massive imaging study to actually see this process in action in the mouse brain, in collaboration with the Schwab team at EMBL Heidelberg.

“Our findings suggest that microglia are nibbling synapses as a way to make them stronger, rather than weaker,"

said Cornelius Gross, who led the work.

Reach Out And Touch

The team saw that around half of the time that microglia contact a synapse, the synapse head sends out thin projections or filopodia to greet them.

[caption id=“attachment_94954” align=“aligncenter” width=“680”]CLEM analysis of microglia-spine interactions. CLEM analysis of microglia-spine interactions.
(a) Schematic of correlative light and electron microscopy (CLEM) workflow.
(b) Confocal orthogonal view of a region of interest (ROI, dotted line in a) containing a spine encapsulated by microglia.
(c) Segmentation of the ROI containing the encapsulation from the corresponding electron microscopy dataset, side view.
(d) Top view of the ROI revealed that the spine was not encapsulated, with (e) no sign of elimination.
(f) Quantification showed that the majority of the encapsulations observed by confocal microscopy were simple appositions by electron microscopy (n = 13 contacts analyzed from two animals). Scale bars: 5 µm for a, 0.5 µm for b–e
Credit: Laetitia Weinhard, et al. CC-BY[/caption]

In one particularly dramatic case - as seen in the below image - fifteen synapse heads extended filopodia toward a single microglia as it picked on a synapse.

[caption id=“attachment_94955” align=“aligncenter” width=“680”]Multiple synapse heads send out filopodia Multiple synapse heads send out filopodia (green) converging on one microglia (red), as seen by focused ion beam scanning electron microscopy (FIB-SEM).
Credit: L. Weinhard, EMBL Rome[/caption]

“As we were trying to see how microglia eliminate synapses, we realised that microglia actually induce their growth most of the time,"

Laetitia Weinhard explains.

It turns out that microglia might underly the formation of double synapses, where the terminal end of a neuron releases neurotransmitters onto two neighboring partners instead of one. This process can support effective connectivity between neurons.

“This shows that microglia are broadly involved in structural plasticity and might induce the rearrangement of synapses, a mechanism underlying learning and memory,"

said Weinhard.

Microglia Eating Synapses

Since this was the first attempt to visualise this process in the brain, the current paper entails five years of technological development. The team tried three different state-of-the-art imaging systems before they succeeded.

Finally, by combining correlative light and electron microscopy (CLEM) and light sheet fluorescence microscopy - a technique developed at EMBL - they were able to make the first movie of microglia eating synapses.

Live imaging of the microglia and synapses in hippocampal slice cultures was done using a Zeiss Z1 light sheet microscope. The imaging chamber was set at 35 °C and 5% CO2, and filled with imaging medium 2 hours before the imaging session, to allow the system to equilibrate and the medium to reach pH7.

“This is what neuroscientists fantasised about for years, but nobody had ever seen before,” says Cornelius Gross. “These findings allow us to propose a mechanism for the role of microglia in the remodeling and evolution of brain circuits during development."

In the future, he plans to investigate the role of microglia in brain development during adolescence and the possible link to the onset of schizophrenia and depression.

Funding for the work came from EMBL, ERC Advanced Grant COREFEAR, and the People Programme of the European Union’s Seventh Framework Programme.

Laetitia Weinhard, Giulia di Bartolomei, Giulia Bolasco, Pedro Machado, Nicole L. Schieber, Urte Neniskyte, Melanie Exiga, Auguste Vadisiute, Angelo Raggioli, Andreas Schertel, Yannick Schwab & Cornelius T. Gross Microglia remodel synapses by presynaptic trogocytosis and spine head filopodia induction Nature Communications volume 9, Article number: 1228 (2018) doi:10.1038/s41467-018-03566-5

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