PKN1 promotes synapse maturation by inhibiting mGluR-dependent silencing through neuronal glutamate transporter activation

Hiroki Yasuda,Hiromi Morisaki,Masaaki Miyamoto,Hikaru Yamamoto,Mona Mehruba,Toshio Kawamata,Yoshitaka Ono,Hideyuki Mukai,Kenji Hanamura,Tomoaki Shirao,Shinji Takada

doi:10.1038/s42003-020-01435-w

Abstract

Abnormal metabotropic glutamate receptor (mGluR) activity could cause brain disorders; however, its regulation has not yet been fully understood. Here, we report that protein kinase N1 (PKN1), a protein kinase expressed predominantly in neurons in the brain, normalizes group 1 mGluR function by upregulating a neuronal glutamate transporter, excitatory amino acid transporter 3 (EAAT3), and supports silent synapse activation. Knocking out PKN1a, the dominant PKN1 subtype in the brain, unmasked abnormal input-nonspecific mGluR-dependent long-term depression (mGluR-LTD) and AMPA receptor (AMPAR) silencing in the developing hippocampus. mGluR-LTD was mimicked by inhibiting glutamate transporters in wild-type mice. Knocking out PKN1a decreased hippocampal EAAT3 expression and PKN1 inhibition reduced glutamate uptake through EAAT3. Also, synaptic transmission was immature; there were more silent synapses and fewer spines with shorter postsynaptic densities in PKN1a knockout mice than in wild-type mice. Thus, PKN1 plays a critical role in regulation of synaptic maturation by upregulating EAAT3 expression.

Highlights

Abnormal metabotropic glutamate receptor activity could cause brain disorders; its regulation has not yet been fully understood
We identified two variant protein kinase N1 (PKN1) by molecular cloning
We developed PKN1a KO mice and found that deleting PKN1a unmasks group 1 metabotropic glutamate receptor (mGluR)-dependent input-nonspecific LTD in the developing CA1 region

Summary

Introduction

Abnormal metabotropic glutamate receptor (mGluR) activity could cause brain disorders; its regulation has not yet been fully understood. We report that protein kinase N1 (PKN1), a protein kinase expressed predominantly in neurons in the brain, normalizes group 1 mGluR function by upregulating a neuronal glutamate transporter, excitatory amino acid transporter 3 (EAAT3), and supports silent synapse activation. Knocking out PKN1a, the dominant PKN1 subtype in the brain, unmasked abnormal input-nonspecific mGluR-dependent long-term depression (mGluR-LTD) and AMPA receptor (AMPAR) silencing in the developing hippocampus. Inhibition of NMDAR function using pharmacological tools or genetic manipulation in the immature brain causes a decrease[8] or an increase in expression of AMPARs5,9 at synapses, suggesting that developmental expression of synaptic AMPARs is regulated by an NMDAR-dependent long-term depression (NMDAR-LTD)-like mechanism, which is an activity-dependent decrease in synaptic efficacy and predominant in the immature hippocampus[10,11]. We propose that PKN1 is critical for normalizing mGluR activity and essential for normal brain development

Methods

Results

Conclusion