NETO1 Regulates Postsynaptic Kainate Receptors in CA3 Interneurons During Circuit Maturation

Abstract

Kainate type ionotropic glutamate receptors (KARs) are expressed in hippocampal interneurons and regulate interneuron excitability and GABAergic transmission. Neuropilin tolloid-like proteins (NETO1 and NETO2) act as KAR auxiliary subunits; however, their significance for various functions of KARs in GABAergic interneurons is not fully understood. Here we show that NETO1, but not NETO2, is necessary for dendritic delivery of KAR subunits and, consequently, for formation of KAR-containing synapses in cultured GABAergic neurons. Accordingly, electrophysiological analysis of neonatal CA3 stratum radiatum interneurons revealed impaired postsynaptic and metabotropic KAR signaling in Neto1 knockouts, while a subpopulation of ionotropic KARs in the somatodendritic compartment remained functional. Loss of NETO1/KAR signaling had no significant effect on development of αamino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA)-receptor-mediated glutamatergic transmission in CA3 interneurons, contrasting the synaptogenic role proposed for KARs in principal cells. Furthermore, loss of NETO1 had no effect on excitability and characteristic spontaneous network bursts in the immature CA3 circuitry. However, we find that NETO1 is critical for kainate-dependent modulation of network bursts and GABAergic transmission in the hippocampus already during the first week of life. Our results provide the first description of NETO1-dependent subcellular targeting of KAR subunits in GABAergic neurons and indicate that endogenous NETO1 is required for formation of KAR-containing synapses in interneurons. Since aberrant KAR-mediated excitability is implicated in certain forms of epilepsy, NETO1 represents a potential therapeutic target for treatment of both adult and early life seizures.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

“…Knockout and overexpression studies have underscored the importance of Neto proteins to synaptic targeting of functional hippocampal kainate receptors (Copits et al, 2011;Sheng et al, 2015;Palacios-Filardo et al, 2016;Orav et al, 2017Orav et al, , 2019Wyeth et al, 2017), although biochemical studies have been less definitive. Ablation of Neto1 either reduced (Tang et al, 2011;Wyeth et al, 2014) or had no effect (Straub et al, 2011a) on GluK2 and GluK5 immunoreactivity in hippocampal postsynaptic densities.…”

“…. Neto1 is also required for postsynaptic targeting of kainate receptors in CA3 interneurons during development; genetic ablation does not alter basal levels of excitability but attenuates circuit activity in response to kainate receptor activation(Orav et al, 2019).…”

Two Signaling Modes Are Better than One: Flux-Independent Signaling by Ionotropic Glutamate Receptors Is Coming of Age

“…During development, KARs are uncoupled from SK Ca channels, and this regulation is lost in the adulthood, thereby strongly reducing interneuron firing [138]. Similarly, flux-independent KAR signaling suppresses I mAHP in immature mouse CA3 interneurons, where GluK2 is targeted to the postsynaptic membrane only in the presence of Neto1 [139].…”

Targeting Ionotropic Glutamate Receptors in the Treatment of Epilepsy

“…Orav et al . [ 227 ] described neuropilin tolloid-like protein 1 (NETO1) as a potential therapeutic target for the treatment of both adult and early life seizures: it acts as kainate receptor auxiliary protein necessary for dendritic delivery of kainate receptor subunits and formation of kainate receptor- containing synapses in cultured GABAergic neurons.…”