Ultrafast glutamate sensors resolve high-frequency release at Schaffer collateral synapses

Helassa, Nordine ORCID: 0000-0003-3743-1886, Dürst, Céline, Coates, Catherine, Kerruth, Silke, Arif, Urwa, Schulze, Christian ORCID: 0000-0002-1442-1987, Wiegert, Simon ORCID: 0000-0003-0893-9349, Geeves, Michael, Oertner, Thomas and Török, Katalin ORCID: 0000-0002-3107-4534 (2017) Ultrafast glutamate sensors resolve high-frequency release at Schaffer collateral synapses. BioRxiv. 233494-.

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Official URL: http://dx.doi.org/10.1101/233494

Abstract

<h4>ABSTRACT</h4> Glutamatergic synapses display a rich repertoire of plasticity mechanisms on many different time scales, involving dynamic changes in the efficacy of transmitter release as well as changes in the number and function of postsynaptic glutamate receptors. The genetically encoded glutamate sensor iGluSnFR enables visualization of glutamate release from presynaptic terminals at frequencies up to ∼10 Hz. However, to resolve glutamate dynamics during high frequency bursts, faster indicators are required. Here we report the development of fast (iGlu f ) and ultrafast (iGlu u ) variants with comparable brightness, but increased K d for glutamate (137 μM and 600 μM, respectively). Compared to iGluSnFR, iGlu u has a 6-fold faster dissociation rate in vitro and 5-fold faster kinetics in synapses. Fitting a three-state model to kinetic data, we identify the large conformational change after glutamate binding as the rate-limiting step. In rat hippocampal slice culture stimulated at 100 Hz, we find that iGlu u is sufficiently fast to resolve individual glutamate release events, revealing that glutamate is rapidly cleared from the synaptic cleft. Depression of iGlu u responses during 100 Hz trains correlates with depression of postsynaptic EPSPs, indicating that depression during high frequency stimulation is purely presynaptic in origin. At individual boutons, the recovery from depression could be predicted from the amount of glutamate released on the second pulse (paired pulse facilitation/depression), demonstrating differential frequency-dependent filtering of spike trains at Schaffer collateral boutons. <h4>Significance Statement</h4> Excitatory synapses convert presynaptic action potentials into chemical signals that are sensed by postsynaptic glutamate receptors. To eavesdrop on synaptic transmission, genetically encoded fluorescent sensors for glutamate have been developed. However, even the best available sensors lag behind the very fast glutamate dynamics in the synaptic cleft. Here we report the development of an ultrafast genetically encoded glutamate sensor, iGlu u , which allowed us to image glutamate clearance and synaptic depression during 100 Hz spike trains. We found that only boutons showing paired-pulse facilitation were able to rapidly recover from depression. Thus, presynaptic boutons act as frequency-specific filters to transmit select features of the spike train to specific postsynaptic cells.

Item Type:	Article
Uncontrolled Keywords:	Depression, Mental Health, Neurosciences, Bioengineering
Depositing User:	Symplectic Admin
Date Deposited:	19 Sep 2018 09:33
Last Modified:	15 Mar 2024 06:38
DOI:	10.1101/233494
Open Access URL:	https://www.biorxiv.org/content/early/2017/12/13/2...
Related URLs:	Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3026481