Powerful, super-resolution imaging could uncover the nanoscale ‘wiring diagram’ of brain memory neurons
The brain is the most sophisticated part of the human body: it contains around 100 billion neurons, just as the number of stars in the Milky Way, and a quadrillion synaptic connections that form a giant network that gives us consciousness.
Peering at tree-like structures
How our memory emerges from this neuron swarm remains a mystery. The hippocampus is a deeply embedded region in the mammalian brain that has long been considered the archetypal centre for memory formation. The aim of the EU-funded IVSTED project was to investigate the connections between hippocampal neurons in vivo while maintaining a sub-cellular spatial resolution. Hippocampal neurons process information by engaging a myriad of synaptic inputs via dendritic spines. Researchers concentrated on the dynamic changes of these tiny neuronal compartments. Changes in their spine structure, known as structural plasticity, enable synapses to modulate their connection strength. “Dendritic spines are highly dynamic, and their plasticity (or synaptic strength) is now deemed an important structural correlate of the memory trace,” explains Stéphane Bancelin, IVSTED coordinator. However, owing to their nanometre size and high density, it is extremely challenging to experimentally study and reveal their fine structure in a realistic environment.
Unprecedented spatial resolution
The project developed a microscope that offers sufficient spatial resolution to observe the fine details of dendritic spines. Importantly, this microscope was applied to monitor in vivo the synapses in the brain of an anaesthetised mouse. Until now, conventional optical microscopy has failed to properly resolve the fine morphological details of these postsynaptic structures, while electron microscopy has provided only snapshots from fixed brain sections. This has limited neuroscientists’ view of spine dynamics and their understanding of the synaptic mechanisms underlying brain physiology and behaviour. IVSTED researchers capitalised on the potential of stimulated emission depletion microscopy, a super-resolution microscopy technique that bypasses the diffraction resolution limit to increase resolution. For deeper tissue inspection, they also used spatial light modulation, an adaptive optics technology that allows spatial shaping of the light beam to cancel distortions (aberrations) induced by tissue at large depths. “In a first, we succeeded in visualising postsynaptic structures in the hippocampus of a living animal, with spatial resolution below 50 nm,” notes Bancelin.
Stable memory with unstable synapses
Researchers investigated not only the nanoscale morphology of dendritic spines, but also their turnover – the formation and termination of these transient postsynaptic structures. “We noticed an extremely high synaptic turnover: approximately 40 % of the observed dendritic spines were replaced in 4 days. Our observations are in line with a recent study that revealed that the fleeting nature of a mammalian hippocampus reflects the turnover dynamics of its synapses,” observes Bancelin. The ensuing question is whether a stable population of synapses exist in the hippocampus. Researchers are keen to continue work on this synaptic rewiring, to reveal how the brain responds to changing behavioural conditions and how it forms new memories. IVSTED has already opened up new avenues for investigating the structural mechanisms of memory formation. “Understanding the workings and anatomy of the brain’s intricate networks could provide clues to the underlying cellular mechanisms of neurological disorders. For example, a large number of studies suggest that memory loss and other cognitive deficits in Alzheimer’s disease primarily result from impaired synapses,” concludes Bancelin.
Keywords
IVSTED, dendritic spines, memory formation, postsynaptic structures, super-resolution imaging, stimulated emission depletion microscopy, adaptive optics, hippocampus, synapses
