Biology subjects

Tzounopoulos, T.

Publications and source records attributed to Tzounopoulos, T..

6 recordsLinked to original sources

bioRxiv2025

hpGRISZ: a high-performance fluorescent biosensor for in vivo imaging of synaptic zinc dynamics

Despite the crucial role of synaptic Zn2+ in neurotransmission and neural processing, direct in vivo measurement of Zn2+ transients has remained challenging due to the limited responsiveness of existing fluorescent indicators. Here we report hpGRISZ (high-performance green indicator for synaptic Zn2+), an ultraresponsive turn-on green fluorescent biosensor engineered through iterative linker optimization, mutagenesis, and directed evolution. hpGRISZ exhibits exceptional brightness, thermostability, and a large fluorescence response (F/F0 {approx} 25) with micromolar affinity suitable for detecting extracellular synaptic Zn2+ release. We comprehensively characterized hpGRISZ in vitro, in mammalian cells, and in vivo. A membrane-anchored version of hpGRISZ traffics robustly to the cell surface under physiological conditions, where it retains strong responsiveness and supports wide-field, two-photon, and fiber photometry recordings in awake mice. These assays revealed synaptic Zn2+ dynamics across distinct brain regions and neuronal circuits. Together, our findings establish hpGRISZ as a powerful tool for dissecting zinc signaling in neural circuits and as a prototype for next-generation genetically encoded biosensors for in vivo imaging.

neuroscience↗

bioRxiv2025

Cell-type-specific plasticity in synaptic, intrinsic, and sound response properties of deep-layer auditory cortical neurons after noise trauma

Peripheral trauma, such as noise-induced hearing loss (NIHL), triggers compensatory plasticity in the auditory cortex (ACtx) to maintain auditory function. While cortical plasticity in superficial cortical layers has been relatively well studied, the plasticity mechanisms governing deep-layer excitatory projection neurons remain less understood. Here, we investigated the plasticity of layer (L)5 extratelencephalic (ETs) and L6 corticothalamic neurons (CTs) following NIHL. Using a combination of in vitro slice electrophysiology, optogenetics, and in vivo two-photon imaging in a mouse model of NIHL, we characterized changes in evoked thalamocortical (TC) synaptic input strength, intrinsic excitability, and sound response properties. We found that TC input was initially equivalent between ETs and CTs, then shifted to CT-dominant one day after noise exposure. This shift renormalized to equivalent seven days after noise exposure and was associated with a transient increase in both the quantal size (q) in TC[->]CT synapses and intrinsic CT suprathreshold excitability. ETs maintained stable intrinsic properties and showed minor changes in their TC input. In vivo imaging revealed that CTs displayed a persistent elevation in sound intensity thresholds, whereas ETs transiently shifted their best frequency representation and reduced their responsiveness to high-frequency tones one day after NIHL, followed by recovery at seven days. Together, our findings highlight cell-type-specific plasticity mechanisms in deep-layer cortical neurons, enhance our understanding of cortical adaptation to peripheral damage, and highlight targets for developing therapeutic strategies to mitigate hearing loss and related disorders such as tinnitus and hyperacusis. Short AbstractPeripheral damage drives auditory cortex (ACtx) plasticity, but the underlying synaptic and cellular mechanisms remain poorly understood. We used a combination of in vitro slice electrophysiology, optogenetics, and in vivo two-photon imaging to investigate layer 5 extratelencephalic (ET) and layer 6 corticothalamic (CT) neuronal plasticity in mice, following noise-induced hearing loss (NIHL). Thalamocortical (TC) input was initially balanced between CTs and ETs but shifted to CT-dominant one day post-NIHL and then normalized by day seven. This transient shift was accompanied by increased quantal size and suprathreshold excitability in CTs, with minimal changes in ETs. In vivo, CTs exhibited persistent elevation in sound intensity thresholds, while ETs showed a transient shift in frequency tuning and reduced high-frequency responsiveness that recovered within a week. These findings reveal distinct, cell-type-specific plasticity mechanisms in deep-layer ACtx neurons following peripheral damage and highlight potential targets for treating hearing loss-related disorders such as tinnitus and hyperacusis.

neuroscience↗

bioRxiv2025

Synaptic properties of layer 6 auditory corticothalamic inputs in normal hearing and noise-induced hearing loss

Layer 6 corticothalamic neurons (CTs) provide strong feedback input that is crucial to perception and cognition in normal and pathological states; however, the synaptic properties of this input remain largely unknown, especially in pathology. Here, we examined the synaptic properties of CT axon terminals in the medial geniculate body (MGB), the auditory thalamus, in normal hearing mice and in a mouse model of noise-induced hearing loss. In normal hearing mice, we found that the synaptic strength of CTs to the core-type ventral subdivision of the auditory thalamus (MGv), which mainly conveys rapid sensory information, is stronger than the synaptic strength of CTs to the matrix-type dorsal subdivision of the auditory thalamus (MGd), which likely conveys higher-order internal state information. This is due to increased functional release sites (n) in CT[->]MGv compared to CT[->]MGd synapses. After noise trauma, we observed enhanced short-term facilitation in CT[->]MGd but not CT[->]MGv synapses. Our findings reveal a previously unknown mechanism of short-term synaptic plasticity after noise-induced hearing loss via which CTs enhance the throughput of matrix-type thalamus, likely to improve perceptual recovery via higher-order contextual modulation.

neuroscience↗

bioRxiv2022

Cell-type-specific roles of inhibitory interneurons in the rehabilitation of auditory cortex after peripheral damage

Peripheral sensory organ damage leads to compensatory cortical plasticity that supports a remarkable recovery of perceptual capabilities. A major knowledge gap is the lack of precise mechanisms that explain how this plasticity is implemented and distributed over a diverse collection of excitatory and inhibitory cortical neurons. Here, we explored these mechanisms in mouse A1. After peripheral damage, we found recovered sound-evoked activity of excitatory principal neurons (PNs) and parvalbumin (PVs) interneurons (INs), reduced activity in somatostatin-INs (SOMs), and recovered activity in vasoactive intestinal peptide-INs (VIPs). Given the sequentially organized cortical network where VIPs inhibit INs, SOMs inhibit PVs and PNs, and PVs inhibit PNs, our results suggest that PVs contribute to PN stability, SOMs allow for increased PN and PV activity, and VIPs enable the PN and PV recovery by inhibiting SOMs. These results highlight a strategic, cooperative, and cell-type-specific plasticity program that restores cortical sound processing after peripheral damage.

neuroscience↗

bioRxiv2022

A Genetically Encoded Far-Red Fluorescent Indicator for Imaging Synaptically-Released Zn2+

Synaptic Zn2+ has emerged as a key neuromodulator in the brain. However, the lack of research tools for directly tracking synaptic Zn2+ in the brain in live animals hinders our rigorous understanding of the physiological and pathological roles of synaptic Zn2+. In this study, we developed a genetically encoded far-red fluorescent indicator for monitoring synaptic Zn2+ dynamics in the nervous system. Our engineered far-red fluorescent indicator for synaptic Zn2+ (FRISZ) displayed a substantial Zn2+-specific turn-on response and low micromolar affinity. We genetically anchored FRISZ to the mammalian extracellular membrane via a transmembrane -helix. We further successfully used membrane-tethered FRISZ (FRISZ-TM) to image synaptic Zn2+ dynamics in response to sound in the primary auditory cortex (A1) in awake mice. This study thus establishes a new technology for studying the roles of synaptic Zn2+ in the nervous system.

neuroscience↗

bioRxiv2018

Mechanisms of synaptic zinc plasticity at mouse dorsal cochlear nucleus glutamatergic synapses

In many excitatory synapses, synaptic zinc is co-released with glutamate to modulate neurotransmission. Synaptic zinc modulates the responsiveness of auditory cortex to sound, and synaptic zinc levels and signaling are modulated by sensory experience, termed zinc plasticity. The mechanisms underlying zinc plasticity remain unknown. We discovered that high- and low-frequency electrical stimulation of dorsal cochlear nucleus synapses reduces and increases synaptic zinc signaling, respectively. This bidirectional zinc plasticity is evidenced by changes in zinc inhibition of AMPA and NMDA receptor activity. Increases and decreases in zinc signaling require activation of Group 1 metabotropic glutamate receptors (mGluRs). Activation of Group 1 mGluRs with a higher agonist concentration increases presynaptic zinc levels and postsynaptic zinc signaling, whereas activation with a lower concentration reduces zinc levels and signaling. Sound-induced zinc plasticity also requires Group 1 mGluRs. Our results reveal the mechanisms underlying synaptic zinc plasticity, elicited by either synaptic activity or sound experience.

neuroscience↗