Pharmacological and optical activation of TrkB in Parvalbumin interneurons regulate intrinsic states to orchestrate cortical plasticity.

Elevated states of brain plasticity typical for critical periods of early postnatal life can be reinstated in the adult brain through interventions, such as antidepressant treatment and environmental enrichment, and induced plasticity may be critical for the antidepressant action. Parvalbumin-positive (PV) interneurons regulate the closure of developmental critical periods and can alternate between high and low plasticity states in response to experience in adulthood. We now show that PV plasticity states and cortical networks are regulated through the activation of TrkB neurotrophin receptors. Visual cortical plasticity induced by fluoxetine, a widely prescribed selective serotonin reuptake inhibitor (SSRI) antidepressant, was lost in mice with reduced expression of TrkB in PV interneurons. Conversely, optogenetic gain-of-function studies revealed that activation of an optically activatable TrkB (optoTrkB) specifically in PV interneurons switches adult cortical networks into a state of elevated plasticity within minutes by decreasing the intrinsic excitability of PV interneurons, recapitulating the effects of fluoxetine. TrkB activation shifted cortical networks towards a low PV configuration, promoting oscillatory synchrony, increased excitatory-inhibitory balance, and ocular dominance plasticity. OptoTrkB activation promotes the phosphorylation of Kv3.1 channels and reduces the expression of Kv3.2 mRNA providing a mechanism for the lower excitability. In addition, decreased expression and puncta of Synaptotagmin2 (Syt2), a presynaptic marker of PV interneurons involved in Ca2+-dependent neurotransmitter release, suggests lower inputs onto pyramidal neurons suppressing feed-forward inhibition. Together, the results provide mechanistic insights into how TrkB activation in PV interneurons orchestrates the activity of cortical networks and mediating antidepressant responses in the adult brain.

Elevated expression of endogenous glial cell line-derived neurotrophic factor impairs spatial memory performance and raises inhibitory tone in the hippocampus.

Parvalbumin-positive interneurons (PV+) are a key component of inhibitory networks in the brain and are known to modulate memory and learning by shaping network activity. The mechanisms of PV+ neuron generation and maintenance are not fully understood, yet current evidence suggests that signalling via the glial cell line-derived neurotrophic factor (GDNF) receptor GFRα1 positively modulates the migration and differentiation of PV+ interneurons in the cortex. Whether GDNF also regulates PV+ cells in the hippocampus is currently unknown. In this study, we utilized a Gdnf "hypermorph" mouse model where GDNF is overexpressed from the native gene locus, providing greatly increased spatial and temporal specificity of protein expression over established models of ectopic expression. Gdnfwt/hyper mice demonstrated impairments in long-term memory performance in the Morris water maze test and an increase in inhibitory tone in the hippocampus measured electrophysiologically in acute brain slice preparations. Increased PV+ cell number was confirmed immunohistochemically in the hippocampus and in discrete cortical areas and an increase in epileptic seizure threshold was observed in vivo. The data consolidate prior evidence for the actions of GDNF as a regulator of PV+ cell development in the cortex and demonstrate functional effects upon network excitability via modulation of functional GABAergic signalling and under epileptic challenge.

ALS and Parkinson's disease genes CHCHD10 and CHCHD2 modify synaptic transcriptomes in human iPSC-derived motor neurons.

Mitochondrial intermembrane space proteins CHCHD2 and CHCHD10 have roles in motor neuron diseases such as amyotrophic lateral sclerosis, spinal muscular atrophy and axonal neuropathy and in Parkinson's disease. They form a complex of unknown function. Here we address the importance of these two proteins in human motor neurons. We show that gene edited human induced pluripotent stem cells (iPSC) lacking either CHCHD2 or CHCHD10 are viable and can be differentiated into functional motor neurons that fire spontaneous and evoked action potentials. Mitochondria in knockout iPSC and motor neurons sustain ultrastructure but show increased proton leakage and respiration, and reciprocal compensatory increases in CHCHD2 or CHCHD10. Knockout motor neurons have largely overlapping transcriptome profiles compared to isogenic control line, in particular for synaptic gene expression. Our results show that the absence of either CHCHD2 or CHCHD10 alters mitochondrial respiration in human motor neurons, inducing similar compensatory responses. Thus, pathogenic mechanisms may involve loss of synaptic function resulting from defective energy metabolism.

GluA4 Dependent Plasticity Mechanisms Contribute to Developmental Synchronization of the CA3-CA1 Circuitry in the Hippocampus.

During the course of development, molecular mechanisms underlying activity-dependent synaptic plasticity change considerably. At immature CA3-CA1 synapses in the hippocampus, PKA-driven synaptic insertion of GluA4 AMPA receptors is the predominant mechanism for synaptic strengthening. However, the physiological significance of the developmentally restricted GluA4-dependent plasticity mechanisms is poorly understood. Here we have used microelectrode array (MEA) recordings in GluA4 deficient slice cultures to study the role of GluA4 in early development of the hippocampal circuit function. We find that during the first week in culture (DIV2-6) when GluA4 expression is restricted to pyramidal neurons, loss of GluA4 has no effect on the overall excitability of the immature network, but significantly impairs synchronization of the CA3 and CA1 neuronal populations. In the absence of GluA4, the temporal correlation of the population spiking activity between CA3-CA1 neurons was significantly lower as compared to wild-types at DIV6. Our data show that synapse-level defects in transmission and plasticity mechanisms are efficiently compensated for to normalize population firing rate at the immature hippocampal network. However, lack of the plasticity mechanisms typical for the immature synapses may perturb functional coupling between neuronal sub-populations, a defect frequently implicated in the context of developmentally originating neuropsychiatric disorders.

Actin Tyrosine-53-Phosphorylation in Neuronal Maturation and Synaptic Plasticity.

UNLABELLED: Rapid reorganization and stabilization of the actin cytoskeleton in dendritic spines enables cellular processes underlying learning, such as long-term potentiation (LTP). Dendritic spines are enriched in exceptionally short and dynamic actin filaments, but the studies so far have not revealed the molecular mechanisms underlying the high actin dynamics in dendritic spines. Here, we show that actin in dendritic spines is dynamically phosphorylated at tyrosine-53 (Y53) in rat hippocampal and cortical neurons. Our findings show that actin phosphorylation increases the turnover rate of actin filaments and promotes the short-term dynamics of dendritic spines. During neuronal maturation, actin phosphorylation peaks at the first weeks of morphogenesis, when dendritic spines form, and the amount of Y53-phosphorylated actin decreases when spines mature and stabilize. Induction of LTP transiently increases the amount of phosphorylated actin and LTP induction is deficient in neurons expressing mutant actin that mimics phosphorylation. Actin phosphorylation provides a molecular mechanism to maintain the high actin dynamics in dendritic spines during neuronal development and to induce fast reorganization of the actin cytoskeleton in synaptic plasticity. In turn, dephosphorylation of actin is required for the stabilization of actin filaments that is necessary for proper dendritic spine maturation and LTP maintenance. SIGNIFICANCE STATEMENT: Dendritic spines are small protrusions from neuronal dendrites where the postsynaptic components of most excitatory synapses reside. Precise control of dendritic spine morphology and density is critical for normal brain function. Accordingly, aberrant spine morphology is linked to many neurological diseases. The actin cytoskeleton is a structural element underlying the proper morphology of dendritic spines. Therefore, defects in the regulation of the actin cytoskeleton in neurons have been implicated in neurological diseases. Here, we revealed a novel mechanism for regulating neuronal actin cytoskeleton that explains the specific organization and dynamics of actin in spines. The better we understand the regulation of the dendritic spine morphology, the better we understand what goes wrong in neurological diseases.

Developmental switch in the kinase dependency of long-term potentiation depends on expression of GluA4 subunit-containing AMPA receptors.

The AMPA-receptor subunit GluA4 is expressed transiently in CA1 pyramidal neurons at the time synaptic connectivity is forming, but its physiological significance is unknown. Here we show that GluA4 expression is sufficient to alter the signaling requirements of long-term potentiation (LTP) and can fully explain the switch in the LTP kinase dependency from PKA to Ca2(+)/calmodulin-dependent protein kinase II during synapse maturation. At immature synapses, activation of PKA leads to a robust potentiation of AMPA-receptor function via the mobilization of GluA4. Analysis of GluA4-deficient mice indicates that this mechanism is critical for neonatal PKA-dependent LTP. Furthermore, lentiviral expression of GluA4 in CA1 neurons conferred a PKA-dependent synaptic potentiation and LTP regardless of the developmental stage. Thus, GluA4 defines the signaling requirements for LTP and silent synapse activation during a critical period of synapse development.

Activity-dependent upregulation of presynaptic kainate receptors at immature CA3-CA1 synapses.

Presynaptic kainate-type glutamate receptors (KARs) regulate glutamate release probability and short-term plasticity in various areas of the brain. Here we show that long-term depression (LTD) in the area CA1 of neonatal rodent hippocampus is associated with an upregulation of tonic inhibitory KAR activity, which contributes to synaptic depression and causes a pronounced increase in short-term facilitation of transmission. This increased KAR function was mediated by high-affinity receptors and required activation of NMDA receptors, nitric oxide (NO) synthetase, and postsynaptic calcium signaling. In contrast, KAR activity was irreversibly downregulated in response to induction of long-term potentiation in a manner that depended on activation of the TrkB-receptor of BDNF. Both tonic KAR activity and its plasticity were restricted to early stages of synapse development and were lost in parallel with maturation of the network due to ongoing BDNF-TrkB signaling. These data show that presynaptic KARs are targets for activity-dependent modulation via diffusible messengers NO and BDNF, which enhance and depress tonic KAR activity at immature synapses, respectively. The plasticity of presynaptic KARs in the developing network allows nascent synapses to shape their response to incoming activity. In particular, upregulation of KAR function after LTD allows the synapse to preferentially pass high-frequency afferent activity. This can provide a potential rescue from synapse elimination by uncorrelated activity and also increase the computational dynamics of the developing CA3-CA1 circuitry.

Ongoing intrinsic synchronous activity is required for the functional maturation of CA3-CA1 glutamatergic synapses.

Fine-tuning of synaptic connectivity during development is guided by intrinsic activity of the immature networks characteristically consisting of intermittent bursts of synchronous activity. However, the role of synchronous versus asynchronous activity in synapse maturation in the brain is unclear. Here, we have pharmacologically prevented generation of synchronous activity in the immature rat CA3-CA1 circuitry in a manner that preserves unitary activity. Long-term desynchronization of the network resulted in weakening of AMPA-receptor-mediated glutamatergic transmission in CA1 pyramidal cells. This weakening was dependent on protein phosphatases and mGluR activity, associated with an increase in the proportion of silent synapses and a decrease in the protein levels of GluA4 suggesting postsynaptic mechanisms of expression. The findings demonstrate that synchronous activity in the immature CA3-CA1 circuitry is critical for the induction and maintenance of glutamatergic synapses and underscores the importance of temporal activity patterns in shaping the synaptic circuitry during development.

Synaptic kainate receptors in CA1 interneurons gate the threshold of theta-frequency-induced long-term potentiation.

Theta oscillations (4-12 Hz) in neuronal networks are known to predispose the synapses involved to plastic changes and may underlie their association with learning behaviors. The lowered threshold for synaptic plasticity during theta oscillations is thought to be due to decreased GABAergic inhibition. Interneuronal kainate receptors (KARs) regulate GABAergic transmission and are implicated in theta activity; however, the physiological significance of this regulation is unknown. In rat hippocampus, we show that during theta activity, there is excitatory postsynaptic drive to CA1 interneurons mediated by KARs. This promotes feedforward inhibition of pyramidal neurons, raising the threshold for induction of theta-burst long-term potentiation. These results identify a novel mechanism whereby the activation of postsynaptic KARs in CA1 interneurons gate changes in synaptic efficacy to a physiologically relevant patterned stimulation.

AMIGO is an auxiliary subunit of the Kv2.1 potassium channel.

Kv2.1 is a potassium channel α-subunit abundantly expressed throughout the brain. It is a main component of delayed rectifier current (I(K)) in several neuronal types and a regulator of excitability during high-frequency firing. Here we identify AMIGO (amphoterin-induced gene and ORF), a neuronal adhesion protein with leucine-rich repeat and immunoglobin domains, as an integral part of the Kv2.1 channel complex. AMIGO shows extensive spatial and temporal colocalization and association with Kv2.1 in the mouse brain. The colocalization of AMIGO and Kv2.1 is retained even during stimulus-induced changes in Kv2.1 localization. AMIGO increases Kv2.1 conductance in a voltage-dependent manner in HEK cells. Accordingly, inhibition of endogenous AMIGO suppresses neuronal I(K) at negative membrane voltages. In conclusion, our data indicate AMIGO as a function-modulating auxiliary subunit for Kv2.1 and thus provide new insights into regulation of neuronal excitability.