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Biophysics and circuit function of a giant cortical glutamatergic synapse

Periodic Reporting for period 3 - GIANTSYN (Biophysics and circuit function of a giant cortical glutamatergic synapse)

Reporting period: 2020-03-01 to 2021-08-31

The highly ambitious goal of the project GIANTSYN is to understand the hippocampal mossy fiber synapse, a key synapse in the hippocampal microcircuit, at all levels of complexity. At the subcellular level, we want to unravel the mechanisms of transmission and plasticity in the same biophysical depth as previously achieved at the neuromuscular junction or the calyx of Held. At the circuit level, we want to understand the connectivity of this synapse and the contribution to learning and memory. Thus, by the end of this project the hippocampal mossy fiber synapse could become the first synapse in the history of neuroscience where we reach complete insight into both synaptic biophysics and contribution to higher network computations. In the long run, the results may open new perspectives for the diagnosis and treatment of brain diseases in which mossy fiber transmission, plasticity, or connectivity are impaired.
In the course of the GIANTSYN project, we made substantial progress towards reaching these highly ambitious goals. Most importantly, we published the first paper on functional electron microscopy (“flash and freeze”) in acute brain slices (Borges-Merjane et al., 2020, Neuron). Using the novel flash-and-freeze technique, we were able to show that optogenetic stimulation of hippocampal mossy fibers depleted the pool of docked vesicles in active zones. This suggests that docked and readily releasable pool are overlapping, as often assumed, but never directly shown. The methods paper also provides a detailed recipe how structure–function analysis can be performed at other synapses. Second, using paired recordings between mossy fiber terminals and postsynaptic CA3 pyramidal cells, we found that posttetanic potentiation (PTP), a major form of presynaptic plasticity at this synapse, is not primarily caused by increased release probability, as previously assumed, but rather by an augmented size of the readily releasable vesicle pool (Vandael et al., 2020, Neuron). In parallel, flash-and-freeze experiments revealed that the docked vesicle pool not only recovered back to the control value, but rather became larger in comparison to control conditions. This pool overfilling may, to a large extent, explain the PTP observed in our paired recording experiments. Based on these observations, we suggest a new mechanism of short-term memory in the hippocampus, in which information is stored as a “pool engram”. Third, we found that PTP has a uniquely low induction threshold and anti-associative induction properties (Vandael et al., 2021, Nature Communications). These results indicate that the mossy fiber synapse may act as a “smart teacher”, in which the teacher function is reduced when the target neurons are already firing. More generally, the results identify a new transsynaptic mechanism, in which the postsynaptic neuron signals back to the presynaptic terminal. Fourth, we characterized the activity of granule cells in vivo in awake mice during a spatial navigation task. We discovered that granule cells fire action potentials only very sparsely, but if they fire, they often generate bursts and higher order activity patterns, termed “superbursts”. These superbursts are sufficient to induce PTP at hippocampal mossy fiber synapses, corroborating the physiological significance of PTP. To quantitatively analyze the underlying synaptic activity in granule cells, we developed a new method for detection of EPSPs based on the principles of machine learning and optimal filtering (Zhang et al., 2021, J. Neuroscience Methods). In comparison to conventional methods, the new method was up to 3-fold more powerful. Surprisingly, we found that both active and silent cells received spatially tuned synaptic input. Fourier analysis indicated that the input showed place-like tuning, grid-like tuning, or complex mixtures (Zhang et al, 2020, Neuron). These results demonstrate that single granule cells are capable of higher-order computations, contributing to the conversion of grid codes in the entorhinal cortex into place codes in the hippocampus. Fifth, we used transsynaptic rabies labeling to examine the converging input on dentate gyrus granule cells and CA3 pyramidal neurons. We found that hippocampal neurons not only received input from superficial layers of the entorhinal cortex, but also from neurons in the entorhinal subplate (Ben-Simon et al., in revision). Based on these results, we propose a revision of the trisynaptic circuit model of the hippocampal formation. Finally, we established a full-scale model of the entorhinal cortex–dentate gyrus–CA3 network. We found that a winner-takes-all mechanism mediated by lateral inhibition in the dentate gyrus contributes to pattern separation (Guzman et al., 2021, Nature Computational Science, in press). In contrast, both detonation properties of the hippocampal mossy fiber synapse and Hebbian synaptic plasticity at perforant path input synapses effectively shifted the network from a pattern separation into a pattern completion regime. Taken together, the results provide outstanding examples of how we are beginning to understand the ways in which synaptic properties shape higher-order computations in the brain.
The project GIANTSYN addresses fundamental questions in the fields of synaptic transmission and circuit function. With the rapid progress in the establishment of the new techniques, including nanophysiology, flash-and-freeze electron microscopy, and transsynaptic labeling using rabies virus, we have already reached several major goals specified in the original application, and we are optimistic that we will soon be able to address the remaining questions. If the GIANTSYN project continues to be successful, it could provide a role model of how we are going to analyze the relation between structure and function in the brain and how we can close the huge gap between the cellular-synaptic level and circuit-behavioral level in the upcoming decade.