Andrea Barberis
@iit.it
Neuroscience Area
Fondazione Istituto Italiano di Tecnologia
RESEARCH INTERESTS
Synaptic physiology
Scopus Publications
Scopus Publications
Giulia Suarato, Samuel Pressi, Enzo Menna, Massimo Ruben, Enrica Maria Petrini, Andrea Barberis, Dalila Miele, Giuseppina Sandri, Marco Salerno, Andrea Schirato,et al.
American Chemical Society (ACS)
As is known, carbon nanotubes favor cell growth in vitro, although the underlying mechanisms are not yet fully elucidated. In this study, we explore the hypothesis that electrostatic fields generated at the interface between nonexcitable cells and appropriate scaffold might favor cell growth by tuning their membrane potential. We focused on primary human fibroblasts grown on electrospun polymer fibers (poly(lactic acid)─PLA) with embedded multiwall carbon nanotubes (MWCNTs). The MWCNTs were functionalized with either the p-methoxyphenyl (PhOME) or the p-acetylphenyl (PhCOMe) moiety, both of which allowed uniform dispersion in a solvent, good mixing with PLA and the consequent smooth and homogeneous electrospinning process. The inclusion of the electrically conductive MWCNTs in the insulating PLA matrix resulted in differences in the surface potential of the fibers. Both PLA and PLA/MWCNT fiber samples were found to be biocompatible. The main features of fibroblasts cultured on different substrates were characterized by scanning electron microscopy, immunocytochemistry, Rt-qPCR, and electrophysiology revealing that fibroblasts grown on PLA/MWCNT reached a healthier state as compared to pure PLA. In particular, we observed physiological spreading, attachment, and Vmem of fibroblasts on PLA/MWCNT. Interestingly, the electrical functionalization of the scaffold resulted in a more suitable extracellular environment for the correct biofunctionality of these nonexcitable cells. Finally, numerical simulations were also performed in order to understand the mechanism behind the different cell behavior when grown either on PLA or PLA/MWCNT samples. The results show a clear effect on the cell membrane potential, depending on the underlying substrate.
Amira El Merhie, Barbara Salis, Tiziana Ravasenga, Chiara Tramontano, Sahitya Kumar Avugadda, Giammarino Pugliese, Andrea Barberis, Teresa Pellegrino, and Silvia Dante
American Chemical Society (ACS)
Alessandro Rossetta, Eli Slenders, Mattia Donato, Sabrina Zappone, Francesco Fersini, Martina Bruno, Francesco Diotalevi, Luca Lanzanò, Sami Koho, Giorgio Tortarolo,et al.
Springer Science and Business Media LLC
AbstractFluorescence laser-scanning microscopy (LSM) is experiencing a revolution thanks to new single-photon (SP) array detectors, which give access to an entirely new set of single-photon information. Together with the blooming of new SP LSM techniques and the development of tailored SP array detectors, there is a growing need for (i) DAQ systems capable of handling the high-throughput and high-resolution photon information generated by these detectors, and (ii) incorporating these DAQ protocols in existing fluorescence LSMs. We developed an open-source, low-cost, multi-channel time-tagging module (TTM) based on a field-programmable gate array that can tag in parallel multiple single-photon events, with 30 ps precision, and multiple synchronisation events, with 4 ns precision. We use the TTM to demonstrate live-cell super-resolved fluorescence lifetime image scanning microscopy and fluorescence lifetime fluctuation spectroscopy. We expect that our BrightEyes-TTM will support the microscopy community in spreading SP-LSM in many life science laboratories.
Tamara Fernandez Cabada, Massimo Ruben, Amira El Merhie, Remo Proietti Zaccaria, Alessandro Alabastri, Enrica Maria Petrini, Andrea Barberis, Marco Salerno, Marco Crepaldi, Alexander Davis,et al.
Royal Society of Chemistry (RSC)
Glioblastoma cancer stem-like cells seeded on substrates exhibiting surface potential differences, undergo differentiation due to the forced hyperpolarization of the membrane potential at the cell/substrate interface.
Tiziana Ravasenga, Massimo Ruben, Vincenzo Regio, Alice Polenghi, Enrica Maria Petrini, and Andrea Barberis
Elsevier BV
Eli Slenders, Eleonora Perego, Mauro Buttafava, Giorgio Tortarolo, Enrico Conca, Sabrina Zappone, Agnieszka Pierzynska-Mach, Federica Villa, Enrica Maria Petrini, Andrea Barberis,et al.
Elsevier BV
Alexander Kuhlemann, Gerti Beliu, Dieter Janzen, Enrica Maria Petrini, Danush Taban, Dominic A. Helmerich, Sören Doose, Martina Bruno, Andrea Barberis, Carmen Villmann,et al.
Frontiers Media SA
Fluorescence labeling of difficult to access protein sites, e.g., in confined compartments, requires small fluorescent labels that can be covalently tethered at well-defined positions with high efficiency. Here, we report site-specific labeling of the extracellular domain of γ-aminobutyric acid type A (GABA-A) receptor subunits by genetic code expansion (GCE) with unnatural amino acids (ncAA) combined with bioorthogonal click-chemistry labeling with tetrazine dyes in HEK-293-T cells and primary cultured neurons. After optimization of GABA-A receptor expression and labeling efficiency, most effective variants were selected for super-resolution microscopy and functionality testing by whole-cell patch clamp. Our results show that GCE with ncAA and bioorthogonal click labeling with small tetrazine dyes represents a versatile method for highly efficient site-specific fluorescence labeling of proteins in a crowded environment, e.g., extracellular protein domains in confined compartments such as the synaptic cleft.
Grzegorz Wiera, Katarzyna Lebida, Anna Maria Lech, Patrycja Brzdąk, Inge Van Hove, Lies De Groef, Lieve Moons, Enrica Maria Petrini, Andrea Barberis, and Jerzy W. Mozrzymas
Springer Science and Business Media LLC
Abstract Learning and memory are known to depend on synaptic plasticity. Whereas the involvement of plastic changes at excitatory synapses is well established, plasticity mechanisms at inhibitory synapses only start to be discovered. Extracellular proteolysis is known to be a key factor in glutamatergic plasticity but nothing is known about its role at GABAergic synapses. We reveal that pharmacological inhibition of MMP3 activity or genetic knockout of the Mmp3 gene abolishes induction of postsynaptic iLTP. Moreover, the application of exogenous active MMP3 mimics major iLTP manifestations: increased mIPSCs amplitude, enlargement of synaptic gephyrin clusters, and a decrease in the diffusion coefficient of synaptic GABAA receptors that favors their entrapment within the synapse. Finally, we found that MMP3 deficient mice show faster spatial learning in Morris water maze and enhanced contextual fear conditioning. We conclude that MMP3 plays a key role in iLTP mechanisms and in the behaviors that presumably in part depend on GABAergic plasticity.
Rocco Pizzarelli, Marilena Griguoli, Paola Zacchi, Enrica Maria Petrini, Andrea Barberis, Antonino Cattaneo, and Enrico Cherubini
Elsevier BV
To be highly reliable, synaptic transmission needs postsynaptic receptors (Rs) in precise apposition to the presynaptic release sites. At inhibitory synapses, the postsynaptic protein gephyrin self-assembles to form a scaffold that anchors glycine and GABAARs to the cytoskeleton, thus ensuring the accurate accumulation of postsynaptic receptors at the right place. This protein undergoes several post-translational modifications which control protein–protein interaction and downstream signaling pathways. In addition, through the constant exchange of scaffolding elements and receptors in and out of synapses, gephyrin dynamically regulates synaptic strength and plasticity. The aim of the present review is to highlight recent findings on the functional role of gephyrin at GABAergic inhibitory synapses. We will discuss different approaches used to interfere with gephyrin in order to unveil its function. In addition, we will focus on the impact of gephyrin structure and distribution at the nanoscale level on the functional properties of inhibitory synapses as well as the implications of this scaffold protein in synaptic plasticity processes. Finally, we will emphasize how gephyrin genetic mutations or alterations in protein expression levels are implicated in several neuropathological disorders, including autism spectrum disorders, schizophrenia, temporal lobe epilepsy and Alzheimer's disease, all associated with severe deficits of GABAergic signaling. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.
Alice Polenghi, Thierry Nieus, Stefania Guazzi, Pau Gorostiza, Enrica Maria Petrini, and Andrea Barberis
Elsevier BV
Summary Kainate receptors (KARs) mediate postsynaptic currents with a key impact on neuronal excitability. However, the molecular determinants controlling KAR postsynaptic localization and stabilization are poorly understood. Here, we exploit optogenetic and single-particle tracking approaches to study the role of KAR conformational states induced by glutamate binding on KAR lateral mobility at synapses. We report that following glutamate binding, KARs are readily and reversibly trapped at glutamatergic synapses through increased interaction with the β-catenin/N-cadherin complex. We demonstrate that such activation-dependent synaptic immobilization of KARs is crucial for the modulation of short-term plasticity of glutamatergic synapses. Thus, the present study unveils the crosstalk between conformational states and lateral mobility of KARs, a mechanism regulating glutamatergic signaling, particularly in conditions of sustained synaptic activity.
Andrea Barberis
Elsevier BV
The flexibility of neuronal networks is believed to rely mainly on the plasticity of excitatory synapses. However, like their excitatory counterparts, inhibitory synapses also undergo several forms of synaptic plasticity. This review examines recent advances in the understanding of the molecular mechanisms leading to postsynaptic GABAergic plasticity. Specifically, modulation of GABAA receptor (GABAAR) number at postsynaptic sites plays a key role, with the interaction of GABAARs with the scaffold protein gephyrin and other postsynaptic scaffold/regulatory proteins having particular importance. Our understanding of these molecular interactions are progressing, based on recent insights into the processes of GABAAR lateral diffusion, gephyrin dynamics, and gephyrin nanoscale organization.
Chiayu Q. Chiu, Andrea Barberis, and Michael J. Higley
Springer Science and Business Media LLC
Cellular mechanisms that regulate the interplay of synaptic excitation and inhibition are thought to be central to the functional stability of healthy neuronal circuits. A growing body of literature demonstrates the capacity for inhibitory GABAergic synapses to exhibit long-term plasticity in response to changes in neuronal activity. Here, we review this expanding field of research, focusing on the diversity of mechanisms that link glutamatergic signalling, postsynaptic action potentials and inhibitory synaptic strength. Several lines of evidence indicate that multiple, parallel forms of plasticity serve to regulate activity at both the input and output domains of individual neurons. Overall, these varied phenomena serve to promote both stability and flexibility over the life of the organism.There is growing evidence for activity-dependent plasticity at inhibitory GABAergic synapses. In this Review, Chiu and colleagues propose that an array of molecular mechanisms promotes the parallel regulation of synapses formed by distinct presynaptic interneurons innervating perisomatic or dendritic targets.
Marta Orlando, Tiziana Ravasenga, Enrica Maria Petrini, Andrea Falqui, Roberto Marotta, and Andrea Barberis
Springer Science and Business Media LLC
Both excitatory and inhibitory synaptic contacts display activity dependent dynamic changes in their efficacy that are globally termed synaptic plasticity. Although the molecular mechanisms underlying glutamatergic synaptic plasticity have been extensively investigated and described, those responsible for inhibitory synaptic plasticity are only beginning to be unveiled. In this framework, the ultrastructural changes of the inhibitory synapses during plasticity have been poorly investigated. Here we combined confocal fluorescence microscopy (CFM) with high resolution scanning electron microscopy (HRSEM) to characterize the fine structural rearrangements of post-synaptic GABAA Receptors (GABAARs) at the nanometric scale during the induction of inhibitory long-term potentiation (iLTP). Additional electron tomography (ET) experiments on immunolabelled hippocampal neurons allowed the visualization of synaptic contacts and confirmed the reorganization of post-synaptic GABAAR clusters in response to chemical iLTP inducing protocol. Altogether, these approaches revealed that, following the induction of inhibitory synaptic potentiation, GABAAR clusters increase in size and number at the post-synaptic membrane with no other major structural changes of the pre- and post-synaptic elements.
Arianna Maffei, Cécile Charrier, Maddalena Delma Caiati, Andrea Barberis, Vivek Mahadevan, Melanie A. Woodin, and Shiva K. Tyagarajan
Society for Neuroscience
Inhibitory circuits are diverse, yet with a poorly understood cell biology. Functional characterization of distinct inhibitory neuron subtypes has not been sufficient to explain how GABAergic neurotransmission sculpts principal cell activity in a relevant fashion. Our Mini-Symposium brings together several emerging mechanisms that modulate GABAergic neurotransmission dynamically from either the presynaptic or the postsynaptic site. The first two talks discuss novel developmental and neuronal subtype-specific contributions to the excitatory/inhibitory balance and circuit maturation. The next three talks examine how interactions between cellular pathways, lateral diffusion of proteins between synapses, and chloride transporter function at excitatory and inhibitory synapses and facilitate inhibitory synapse adaptations. Finally, we address functional differences within GABAergic interneurons to highlight the importance of diverse, flexible, and versatile inputs that shape network function. Together, the selection of topics demonstrates how developmental and activity-dependent mechanisms coordinate inhibition in relation to the excitatory inputs and vice versa.
Silvia Dante, Alessia Petrelli, Enrica Maria Petrini, Roberto Marotta, Alessandro Maccione, Alessandro Alabastri, Alessandra Quarta, Francesco De Donato, Tiziana Ravasenga, Ayyappan Sathya,et al.
American Chemical Society (ACS)
Nanoparticles (NPs) are increasingly used in biomedical applications, but the factors that influence their interactions with living cells need to be elucidated. Here, we reveal the role of NP surface charge in determining their neuronal interactions and electrical responses. We discovered that negatively charged NPs administered at low concentration (10 nM) interact with the neuronal membrane and at the synaptic cleft, whereas positively and neutrally charged NPs never localize on neurons. This effect is shape and material independent. The presence of negatively charged NPs on neuronal cell membranes influences the excitability of neurons by causing an increase in the amplitude and frequency of spontaneous postsynaptic currents at the single cell level and an increase of both the spiking activity and synchronous firing at neural network level. The negatively charged NPs exclusively bind to excitable neuronal cells, and never to nonexcitable glial cells. This specific interaction was also confirmed by manipulating the electrophysiological activity of neuronal cells. Indeed, the interaction of negatively charged NPs with neurons is either promoted or hindered by pharmacological suppression or enhancement of the neuronal activity with tetrodotoxin or bicuculline, respectively. We further support our main experimental conclusions by using numerical simulations. This study demonstrates that negatively charged NPs modulate the excitability of neurons, revealing the potential use of NPs for controlling neuron activity.
Emanuela de Luca, Tiziana Ravasenga, Enrica Maria Petrini, Alice Polenghi, Thierry Nieus, Stefania Guazzi, and Andrea Barberis
Elsevier BV
Summary The lateral mobility of neurotransmitter receptors has been shown to tune synaptic signals. Here we report that GABAA receptors (GABAARs) can diffuse between adjacent dendritic GABAergic synapses in long-living desensitized states, thus laterally spreading “activation memories” between inhibitory synapses. Glutamatergic activity limits this inter-synaptic diffusion by trapping GABAARs at excitatory synapses. This novel form of activity-dependent hetero-synaptic interplay is likely to modulate dendritic synaptic signaling.
Giuseppe Sancataldo, Lorenzo Scipioni, Tiziana Ravasenga, Luca Lanzanò, Alberto Diaspro, Andrea Barberis, and Martí Duocastella
The Optical Society
The precise localization of nanometric objects in three dimensions is essential to identify functional diffusion mechanisms in complex systems at the cellular or molecular level. However, most optical methods can achieve high temporal resolution and high localization precision only in two dimensions or over a limited axial (z) range. Here we develop a novel wide-field detection system based on an electrically tunable lens that can track multiple individual nanoscale emitters in three dimensions over a tunable axial range with nanometric localization precision. The optical principle of the technique is based on the simultaneous acquisition of two images with an extended depth of field while encoding the z position of the emitters via a lateral shift between images. We provide a theoretical framework for this approach and demonstrate tracking of free diffusing beads and GABAA receptors in live neurons. This approach allows getting nanometric localization precision up to an axial range above 10 μm with a high numerical aperture lens—quadruple that of a typical 3D tracking system. Synchronization or complex fitting procedures are not requested here, which leads to a suitable architecture for localizing single molecules in four dimensions, namely, three dimensions in real-time.
Francesca Pennacchietti, Sebastiano Vascon, Thierry Nieus, Christian Rosillo, Sabyasachi Das, Shiva K. Tyagarajan, Alberto Diaspro, Alessio Del Bue, Enrica Maria Petrini, Andrea Barberis,et al.
Society for Neuroscience
Gephyrin is a key scaffold protein mediating the anchoring of GABAA receptors at inhibitory synapses. Here, we exploited superresolution techniques combined with proximity-based clustering analysis and model simulations to investigate the single-molecule gephyrin reorganization during plasticity of inhibitory synapses in mouse hippocampal cultured neurons. This approach revealed that, during the expression of inhibitory LTP, the increase of gephyrin density at postsynaptic sites is associated with the promoted formation of gephyrin nanodomains. We demonstrate that the gephyrin rearrangement in nanodomains stabilizes the amplitude of postsynaptic currents, indicating that, in addition to the number of synaptic GABAA receptors, the nanoscale distribution of GABAA receptors in the postsynaptic area is a crucial determinant for the expression of inhibitory synaptic plasticity. In addition, the methodology implemented here clears the way to the application of the graph-based theory to single-molecule data for the description and quantification of the spatial organization of the synapse at the single-molecule level. SIGNIFICANCE STATEMENT The mechanisms of inhibitory synaptic plasticity are poorly understood, mainly because the size of the synapse is below the diffraction limit, thus reducing the effectiveness of conventional optical and imaging techniques. Here, we exploited superresolution approaches combined with clustering analysis to study at unprecedented resolution the distribution of the inhibitory scaffold protein gephyrin in response to protocols inducing LTP of inhibitory synaptic responses (iLTP). We found that, during the expression of iLTP, the increase of synaptic gephyrin is associated with the fragmentation of gephyrin in subsynaptic nanodomains. We demonstrate that such synaptic gephyrin nanodomains stabilize the amplitude of inhibitory postsynaptic responses, thus identifying the nanoscale gephyrin rearrangement as a key determinant for inhibitory synaptic plasticity.
R. Antonelli, R. De Filippo, S. Middei, S. Stancheva, B. Pastore, M. Ammassari-Teule, A. Barberis, E. Cherubini, and P. Zacchi
Society for Neuroscience
Phosphorylation of serine/threonine residues preceding a proline regulates the fate of its targets through postphosphorylation conformational changes catalyzed by the peptidyl-prolyl cis-/trans isomerase Pin1. By flipping the substrate between two different functional conformations, this enzyme exerts a fine-tuning of phosphorylation signals. Pin1 has been detected in dendritic spines and shafts where it regulates protein synthesis required to sustain the late phase of long-term potentiation (LTP). Here, we demonstrate that Pin1 residing in postsynaptic structures can interact with postsynaptic density protein-95 (PSD-95), a key scaffold protein that anchors NMDA receptors (NMDARs) in PSD via GluN2-type receptor subunits. Pin1 recruitment by PSD-95 occurs at specific serine-threonine/proline consensus motifs localized in the linker region connecting PDZ2 to PDZ3 domains. Upon binding, Pin1 triggers structural changes in PSD-95, thus negatively affecting its ability to interact with NMDARs. In electrophysiological experiments, larger NMDA-mediated synaptic currents, evoked in CA1 principal cells by Schaffer collateral stimulation, were detected in hippocampal slices obtained from Pin1−/− mice compared with controls. Similar results were obtained in cultured hippocampal cells expressing a PSD-95 mutant unable to undergo prolyl-isomerization, thus indicating that the action of Pin1 on PSD-95 is critical for this effect. In addition, an enhancement in spine density and size was detected in CA1 principal cells of Pin1−/− or in Thy-1GFP mice treated with the pharmacological inhibitor of Pin1 catalytic activity PiB. Our data indicate that Pin1 controls synaptic content of NMDARs via PSD-95 prolyl-isomerization and the expression of dendritic spines, both required for LTP maintenance. SIGNIFICANCE STATEMENT PSD-95, a membrane-associated guanylate kinase, is the major scaffolding protein at excitatory postsynaptic densities and a potent regulator of synaptic strength and plasticity. The activity of PSD-95 is tightly controlled by several post-translational mechanisms including proline-directed phosphorylation. This signaling cascade regulates the fate of its targets through postphosphorylation conformational modifications catalyzed by the peptidyl-prolyl cis-/trans isomerase Pin1. Here, we uncover a new role of Pin1 in glutamatergic signaling. By interacting with PSD-95, Pin1 dampens PSD-95 ability to complex with NMDARs, thus negatively affecting NMDAR signaling and spine morphology. Our findings further emphasize the emerging role of Pin1 as a key modulator of synaptic transmission.
Hanako Tsushima, Marco Emanuele, Alice Polenghi, Alessandro Esposito, Massimo Vassalli, Andrea Barberis, Francesco Difato, and Evelina Chieregatti
Springer Science and Business Media LLC
Maintenance of neuronal polarity and regulation of cytoskeletal dynamics are vital during development and to uphold synaptic activity in neuronal networks. Here we show that soluble β-amyloid (Aβ) disrupts actin and microtubule (MT) dynamics via activation of RhoA and inhibition of histone deacetylase 6 (HDAC6) in cultured hippocampal neurons. The contact of Aβ with the extracellular membrane promotes RhoA activation, leading to growth cone collapse and neurite retraction, which might be responsible for hampered neuronal pathfinding and migration in Alzheimer's disease (AD). The inhibition of HDAC6 by Aβ increases the level of heterodimeric acetylated tubulin and acetylated tau, both of which have been found altered in AD. We also find that the loss of HDAC6 activity perturbs the integrity of axon initial segment (AIS), resulting in mislocalization of ankyrin G and increased MT instability in the AIS concomitant with loss of polarized localization of tau and impairment of action potential firing.
Andrea Barberis and Alberto Bacci
Frontiers Media SA
For long time, plasticity of brain circuits has been hypothesized to mainly rely on the flexibility of glutamatergic excitatory synapses, whereas inhibitory synapses have been assumed to be relatively invariant. Based on this view, inhibition should be exclusively modulated by the differential glutamatergic-driven activation of a highly diverse population of inhibitory interneurons displaying specific temporal dynamics and selective innervation patterns. However, it has been demonstrated that inhibitory synapses undergo several forms of plasticity, thus providing an additional source of versatility to the regulation of the neuronal network and the emergence of complex brain states.
The cellular and molecular mechanisms occurring at inhibitory synapses during the induction/expression of inhibitory short- and long-term synaptic plasticity are now beginning to be unraveled. At the presynaptic side, retrograde synaptic messengers modulate GABA release (Mendez and Bacci, 2011; Iremonger et al., 2013; Lourenco et al., 2014; Younts and Castillo, 2014), whereas postsynaptic plasticity typically involves changes in the number/gating properties of post-synaptic GABAA receptors (Kurotani et al., 2008; Houston et al., 2009; Luscher et al., 2011; Petrini et al., 2014; Flores et al., 2015). In addition, acute or chronic alterations of intracellular chloride concentration modulate the driving force of GABAergic currents and the subunit composition of GABAA receptors (Woodin et al., 2003; Raimondo et al., 2012; Succol et al., 2012).
The 14 articles presented in this ebook (including hypothesis and theory, minireviews, reviews, and original research articles) cover the mechanisms of inhibitory synaptic plasticity, at the molecular and microcircuit levels. Zacchi et al. (2014) focus on the signaling pathways controlling the phosphorylation state of gephyrin, a key scaffold protein at inhibitory synapses responsible for the synaptic clustering of both glycine and GABAA receptors. By considering the synapse as a highly dynamic element, Petrini and Barberis (2014), review the recent literature addressing the role of protein diffusion in the reorganization of the inhibitory postsynaptic density during inhibitory synaptic plasticity. A similar conceptual approach, based on the analysis of receptor dynamics, has been adopted by Muir and Kittler (2014) to investigate inhibitory plasticity in relation to GABAA receptor diffusion at inhibitory synapses located in the axon initial segment. This original research article reports that chronic depolarization increases the lateral mobility of GABAA receptors and reduces the size of post-synaptic GABAA receptor clusters, thus critically interfering with neuronal excitability. Hirano and Kawaguchi (2014) review another form of postsynaptic inhibitory plasticity observed at cerebellar synapses formed by stellate cells onto Purkinje cells. This inhibitory long-term potentiation involves the CaMKII-dependent increase of GABAA receptor signaling through direct GABAA receptor phosphorylation and/or promoted surface delivery via a GABARAP-dependent mechanism. In their original article, Gao et al. (2014) further address the molecular mechanisms of the aforementioned long-term inhibitory plasticity at cerebellar Purkinje cells. They report that the pathway of iLTP induction critically depends on the coordinated action of both αCaMKII and βCaMKII isoforms, and is modulated by the activation of GABAB receptors. Flores et al. (2015) provide a broad yet detailed analysis of the molecular organization of inhibitory post-synaptic density. In addition, they highlight the formation and elimination of GABAergic synapses as an important source of inhibitory synaptic plasticity. The mini review by Maguire (2014) examines the plasticity of inhibition in response to acute and chronic stress involving region-specific changes of GABAA receptor subunit expression and alterations of the chloride gradient. Moreover, Dr. Maguire reports that stress acts as a metaplastic switch by enabling iLTP at parvocellular neuroendocrine cells (PNCs). Mapelli et al. (2015) provide a comprehensive overview of diverse forms of plasticity at specific cerebellar sub-circuits, introducing the concept of the coordination between excitatory and inhibitory plasticity for correct circuit functioning. In their minireview, Chevaleyre and Piskorowski (2014) highlight the importance of short- and long-term changes of inhibitory synaptic strength in tuning the threshold for the induction of excitatory plasticity. In addition, they discuss how plasticity of glutamatergic synapses onto PV+ interneurons shapes inhibition at hippocampal microcircuits. Pallotto and Deprez (2014) analyze the influence of inhibition in adult neurogenesis in the olfactory bulb and dentate gyrus, by discussing the role of GABAergic signaling in the development and plasticity of adult-born neurons. In their comprehensive review Griffen and Maffei (2014) examine different forms of pre- and post-synaptic inhibitory plasticity occurring at diverse somatosensory cortex interneuron subtypes and discuss the role of such plasticity in sensory cortical activity.
Synaptic signaling does not only depend on pre- or post-synaptic determinants but is also shaped by the dynamics of neurotransmitter in the synaptic cleft. The minireview and the hypothesis and theory by Scimemi (2014a,b) propose the intriguing idea that changes of GABA transporters activity may modulate GABAergic responses. In particular, by exploiting a computer modeling approach, Dr. Scimemi validates the hypothesis that the density, distribution and lateral mobility of GABA transporters affect the GABA concentration sensed by postsynaptic GABAA receptors.
In addition to synaptic inhibition, tonic inhibition produced by the persistent activation of extrasynaptic GABAA receptors is crucial for the tuning of neuronal excitability. Recent evidence demonstrates that also tonic inhibition is plastic. The original article by Barth et al. (2014) illustrates that the ovarian cycle is associated with variations of expression of GABAA receptors containing the “tonic” δ-subunit, both in hippocampal principal cells and interneurons. Interestingly, such plasticity modulates γ-oscillations, thus representing a possible determinant for altered memory and cognitive performance observed during ovarian cycle.
The ability of inhibitory synapses to undergo plasticity emphasized in this ebook raises important questions. First, what are the specific molecular mechanisms of inhibitory plasticity at synapses formed by different interneuron subtypes? Second: how is plasticity orchestrated at both excitatory and inhibitory synapses? In keeping with this, how do different forms of excitatory and inhibitory plasticity co-exist? Do variations of both excitation and inhibition strength occur in parallel/homestatic (Froemke et al., 2007; Xue et al., 2014; Flores et al., 2015), independent (Lourenco et al., 2014), or opposite fashions (Petrini et al., 2014). Are these different “plasticity modes” dependent on the stimulus pattern, specific spatial distributions of synapses and/or time points after plasticity induction? What are the behavioral and cognitive correlates of these different forms of plasticity?
Answering these questions will contribute in redefining the excitation to inhibition balance (E/I) as a “dynamic” activity-dependent determinant for the functioning of brain microcircuits.
Enrica Maria Petrini and Andrea Barberis
Frontiers Media SA
The plasticity of inhibitory transmission is expected to play a key role in the modulation of neuronal excitability and network function. Over the last two decades, the investigation of the determinants of inhibitory synaptic plasticity has allowed distinguishing presynaptic and postsynaptic mechanisms. While there has been a remarkable progress in the characterization of presynaptically-expressed plasticity of inhibition, the postsynaptic mechanisms of inhibitory long-term synaptic plasticity only begin to be unraveled. At postsynaptic level, the expression of inhibitory synaptic plasticity involves the rearrangement of the postsynaptic molecular components of the GABAergic synapse, including GABAA receptors, scaffold proteins and structural molecules. This implies a dynamic modulation of receptor intracellular trafficking and receptor surface lateral diffusion, along with regulation of the availability and distribution of scaffold proteins. This Review will focus on the mechanisms of the multifaceted molecular reorganization of the inhibitory synapse during postsynaptic plasticity, with special emphasis on the key role of protein dynamics to ensure prompt and reliable activity-dependent adjustments of synaptic strength.
Enrica Maria Petrini, Tiziana Ravasenga, Torben J. Hausrat, Giuliano Iurilli, Umberto Olcese, Victor Racine, Jean-Baptiste Sibarita, Tija C. Jacob, Stephen J. Moss, Fabio Benfenati,et al.
Springer Science and Business Media LLC
Postsynaptic long-term potentiation of inhibition (iLTP) can rely on increased GABAA receptors (GABAARs) at synapses by promoted exocytosis. However, the molecular mechanisms that enhance the clustering of postsynaptic GABAARs during iLTP remain obscure. Here we demonstrate that during chemically induced iLTP (chem-iLTP), GABAARs are immobilized and confined at synapses, as revealed by single-particle tracking of individual GABAARs in cultured hippocampal neurons. Chem-iLTP expression requires synaptic recruitment of the scaffold protein gephyrin from extrasynaptic areas, which in turn is promoted by CaMKII-dependent phosphorylation of GABAAR-β3-Ser383. Impairment of gephyrin assembly prevents chem-iLTP and, in parallel, blocks the accumulation and immobilization of GABAARs at synapses. Importantly, an increase of gephyrin and GABAAR similar to those observed during chem-iLTP in cultures were found in the rat visual cortex following an experience-dependent plasticity protocol that potentiates inhibitory transmission in vivo. Thus, phospho-GABAAR-β3-dependent accumulation of gephyrin at synapses and receptor immobilization are crucial for iLTP expression and are likely to modulate network excitability.
Andrea Barberis and Fabio Benfenati
Springer International Publishing
The nervous system confers living organisms the ability to perceive and appropriately respond to external signals. At any given moment, a multitude of different stimuli are received, processed, and integrated by the brain. The correct handling of this huge amount of information requires the coordination of extraordinary number of different events at both cellular and network levels. The basis of this extremely complex regulation lays on synapses, the specific contact sites between neurons. At the presynaptic level, electrical stimuli are translated into chemical signals, i.e., the release of neurotransmitters, which are recognized and translated into an appropriate biological response (either electric or metabolic, or both) at the postsynaptic level. The combined action of synapses acting in distinct brain areas is ultimately responsible for the generation and shaping of higher brain functions such as learning and memory. The molecular mechanisms modulating synaptic function have been the subject of intense investigation since the earliest days of modern neuroscience. Initially, synapses were thought to be “static” structures where presynaptic stimuli are linearly converted into neurotransmitter release and action potentials. This idea has been now substituted by a more modern view, whereby synapses represent extremely dynamic sites whose activity can be modified by a vast array of signals coming from the presynaptic, postsynaptic, and extracellular compartments as well as by the previous history of the neuron. This new view of synaptic functioning has been obtained by the application of novel advanced techniques that allow interrogating the synapses in live neurons under various environmental conditions, among these are patch-clamp electrophysiology, dynamic electron microscopy, and innovative imaging and optogenetic techniques coupled with high-resolution and super-resolution live imaging approaches.
Enrica Maria Petrini and Andrea Barberis
Springer New York
The generation of a synaptic current at the postsynaptic element (PSCs) is the result of a dynamic sequence of events including the release of the neurotransmitter, its diffusion in the synaptic cleft, and the activation of neurotransmitter receptors located at the postsynaptic side. It is widely accepted that the amplitude and the duration of PSCs are largely dictated by the gating properties of postsynaptic receptors. However, the knowledge of the properties of postsynaptic receptors is mostly derived from steady-state analysis, a condition that is substantially different from the non-equilibrium activation of synaptic receptors imposed by submillisecond neurotransmitter exposures. Given the technical limitations to reproduce the brief "synaptic-like" agonist pulse durations, the functioning of postsynaptic receptors during synaptic transmission is not fully elucidated and the "on-demand" postsynaptic activation of synapses cannot be easily achieved. In this chapter, we review the diverse approaches to study receptor gating at times relevant for synaptic transmission and novel optical/optogenetic techniques for controlling synaptic activity at the postsynaptic level. In addition, we emphasize the role of non-equilibrium in unmasking specific features of synaptic receptor gating and the recent advances in photonics for the light-control of neuronal activity at the single-receptor level.