We have identified several novel features related to functions and regulation of AMPA and kainate-type glutamate receptors at developing synapses. Our current research aims to understand in detail how these mechanisms contribute to development and fine-tuning of the neural circuits underlying behavior under physiological and aberrant conditions, such as early life stress (e.g. Englund et al., 2021).
Our experimental approach involves the use of electrophysiological techniques in combination with pharmacological and local genetic manipulation in various neuronal preparations and with behavioral testing.
Kainate receptors (KARs), together with AMPA and NMDA receptors, belong to the family of ionotropic glutamate receptors. KARs are composed of five subunits, designated GluK1 – 5 (Grik1-5) that may localize to various subcellular compartments to modulate neuronal excitability, synaptic transmission and plasticity. KARs are highly expressed in the developing brain where they are thought to regulate activity –dependent refinement of the circuitry (for review, see Lauri et al., 2021).
Below are some of our publications addressing the mechanisms by which KARs influence synaptic transmission, plasticity and synaptogenesis at the immature hippocampus.
During early development, transmission at hippocampal CA3-CA1 synapses is regulated by a high-affinity, G-protein dependent kainate receptor (KAR), which is endogenously activated by ambient glutamate. By tonically depressing glutamate release, this mechanism sets the dynamic properties of neonatal inputs to favour transmission during high frequency bursts of activity, typical for developing neuronal networks. In response to induction of LTP, the tonic activation of KAR is rapidly down regulated, causing an increase in Pr and profoundly changing the dynamic properties of transmission. Early development of the glutamatergic connectivity thus involves an activity-dependent loss of presynaptic KAR function producing maturation in the mode of excitatory transmission from CA3 to CA1 neurons.
KARs are strongly expressed in the amygdala during early postnatal development, temporally coinciding with rapid development of functional glutamatergic synapses. Similar to hippocampus, KARs in the newborn basolateral amygdala (BLA) are tonically activated to regulate glutamate release via a G-protein-dependent mechanism. Genetic manipulation of this endogenous KAR activity locally in the newborn LA perturbed development of glutamatergic input to CeA, identifying KARs as a physiological mechanism regulating formation of the glutamatergic circuitry in the amygdala. Together, these data identify tonic KAR activity as a mechanism modulating development of behaviorally relevant limbic circuits
Presynaptic KAR activity can be up- or down-regulated in response to different frequencies of neuronal activity at immature glutamatergic synapses. Long-term depression (LTD) in the area CA1 of neonatal rodent hippocampus is associated with an up-regulation of tonic inhibitory KAR activity, which contributs 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 synthetase (NOS) and postsynaptic calcium signalling. In contrast, KAR activity was irreversibly down-regulated in response to induction of long-term potentiation (LTP) 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 signalling.
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 contributes to LTD and LTP in a developmentally restricted period of synapse maturation. Unlike the purely postsynaptic changes in synapse strength, the KAR dependent changes in transmitter release allows nascent synapses to shape their dynamic response to incoming activity. In particular, up-regulation 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.
Presynaptic KARs tonically depress glutamatergic transmission during restricted period of synapse development; however, the molecular basis behind this effect is unknown. Here, we show that the developmental and cell-type specific expression pattern of a KAR subunit splice variant, GluK1c, corresponds to the immature-type KAR activity in the hippocampus. GluK1c localizes to dendritic contact sites at distal axons, the distal targeting being promoted by heteromerization with the subunit GluK4. Presynaptic expression of GluK1c strongly suppresses glutamatergic transmission in cell-pairs in vitro and mimics the immature-type KAR activity at CA3-CA1 synapses in vivo, at a developmental stage when the endogenous expression is already downregulated. These data support a central role for GluK1c in mediating tonic inhibition of glutamate release and the consequent effects on excitability and activity-dependent fine-tuning of the developing hippocampal circuitry.
Kainate type of glutamate receptors are highly expressed during early brain development and may influence refinement of the circuitry via modulating synaptic transmission and plasticity. KARs are also localized to axons, however, their exact roles in regulating presynaptic processes remain controversial. Here, we have used a microfluidic platform allowing specific manipulation of KARs in presynaptic neurons to study their role in synaptic development and function at high resolution in vitro. Our data reveals that the axonal calcium permeable KARs coordinate presynaptic differentiation to enhance the strength of upcoming synaptic connection.
NETO proteins act as auxiliary subunits for KARs; however, their functions at the immature circuit are not known. Using a combination of cell biology and electrophysiology in Neto1- and Neto2-deficient mouse models, we show that NETO proteins regulate axonal delivery of KARs. The deficit in axonal KAR at immature Neto1-/- neurons manifested as loss of presynaptic KAR activity and resulted in impaired synaptogenesis and perturbed development of the hippocampal CA3-CA1 circuit. These findings suggest novel roles for the NETO auxiliary subunits in orchestrating development and refinement of synaptic connectivity, a process increasingly implicated in developmentally originating neurologic disorders.
KARs are highly expressed in GABAergic interneurons early in postnatal development. We show that NETO1 is necessary for dendritic delivery of KAR subunits and, consequently, for formation of KAR containing synapses in GABAergic neurons. Interestingly, loss of NETO1/KAR signalling had no significant effect on development of AMPAR and NMDAR mediated glutamatergic inputs in GABAergic interneurons, contrasting the synaptogenic role proposed for KARs in principal cells. These results provide the first description of NETO1 dependent subcellular targeting of KAR subunits at GABAergic neurons. Furthermore, we show that NETO1 regulates both ionotropic and metabotropic KAR functions in the CA3 interneurons already during the first week of life, and is critical for KA induced modulation of network bursts and GABAergic transmission at the immature network.
Below , there are examples of our recent papers addressing the physiological significance and regulation of GluA4 subunit of AMPA receptors at the immature hippocampal synapses
Ionotropic glutamate receptors are critical for excitatory transmission and plasticity in the brain, and have also been implicated in several neurological diseases. The GluA4 subunit of the AMPA-type glutamate receptors is transiently expressed in hippocampal CA1 principal 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 mechanisms of synaptic plasticity and can fully explain the switch in the kinase dependency of long-term potentiation from PKA to CaMKII -dependent during synapse maturation. GluA4 expression at developing synapses confers a minimal mechanism for activity-dependent AMPA-receptor regulation, to facilitate silent synapse activation during early development of glutamatergic synapses.
Development of the neuronal circuitry involves both Hebbian and homeostatic plasticity mechanisms that orchestrate activity-dependent refinement of the synaptic connectivity. AMPA receptor subunit GluA4 is expressed in hippocampal pyramidal neurons during early postnatal period and is critical for neonatal long-term potentiation; however, its role in homeostatic plasticity is unknown. Here we show that GluA4 enables efficient homeostatic upscaling during the critical period of activity-dependent refinement of the circuitry. Our data suggest that expression of GluA4 at immature synapses is responsible for their greater sensitivity to activity dependent regulation.
In this paper we show that PKA activation leads to insertion of GluA4 to synaptic sites with initially weak or silent AMPAR-mediated transmission. This effect depends on a novel mechanism involving the extreme C-terminal end of GluA4, which interacts with the membrane proximal region of the C-terminal domain to control GluA4 trafficking. In the absence of GluA4, strengthening of AMPAR-mediated transmission during postnatal development was significantly delayed. These data suggest that the GluA4-mediated activation of silent synapses is a critical mechanism facilitating the functional maturation of glutamatergic circuitry during the critical period of experience-dependent fine-tuning.