Auxiliary Subunits Control Function and Subcellular Distribution of AMPA Receptor Complexes in NG2 Glia of the Developing Hippocampus.

Stefan Hardt,Stefan Passlick,Dario Tascio,Gerald Seifert,Christian Steinhäuser,Aline Timmermann,Ronald Jabs

doi:10.3389/fncel.2021.669717

Abstract

Synaptic and axonal glutamatergic signaling to NG2 glia in white matter is critical for the cells’ differentiation and activity dependent myelination. However, in gray matter the impact of neuron-to-NG2 glia signaling is still elusive, because most of these cells keep their non-myelinating phenotype throughout live. Early in postnatal development, hippocampal NG2 glia express AMPA receptors with a significant Ca2+ permeability allowing for plasticity of the neuron-glia synapses, but whether this property changes by adulthood is not known. Moreover, it is unclear whether NG2 glia express auxiliary transmembrane AMPA receptor related proteins (TARPs), which modify AMPA receptor properties, including their Ca2+ permeability. Through combined molecular and functional analyses, here we show that hippocampal NG2 glia abundantly express TARPs γ4, γ7, and γ8 as well as cornichon (CNIH)-2. TARP γ8 undergoes profound downregulation during development. Receptors of adult NG2 glia showed an increased sensitivity to blockers of Ca2+ permeable AMPA receptors, but this increase mainly concerned receptors located close to the soma. Evoked synaptic currents of NG2 glia were also sensitive to blockers of Ca2+ permeable AMPA receptors. The presence of AMPA receptors with varying Ca2+ permeability during postnatal maturation may be important for the cells’ ability to sense and respond to local glutamatergic activity and for regulating process motility, differentiation, and proliferation.

Highlights

AMPA receptors co-assemble with auxiliary subunits, called transmembrane AMPA receptor regulatory protein (TARP), which modulates receptor function
In stellate cells of the cerebellum, TARP γ2 promotes synaptic expression of Ca2+ impermeable AMPA receptors while its deletion increased the proportion of extrasynaptic Ca2+ permeable AMPA receptors associated with TARP γ7 (Bats et al, 2012)
Fluorescent NG2 glial cells were identified by their EYFP fluorescence and sorted by a FACSAriaIII flow cytometer (70 μm nozzle, BD Biosciences, Heidelberg, Germany) into tubes containing Hanks’ balanced salt solution (HBSS, Ca2+- and Mg2+-free)

Summary

Introduction

AMPA receptors co-assemble with auxiliary subunits, called transmembrane AMPA receptor regulatory protein (TARP), which modulates receptor function. The subunit TARP γ2, called stargazin, is necessary for surface translocation and synaptic expression of AMPA receptors as demonstrated in cerebellar granule cells (Chen et al, 2000; Tomita et al, 2005). The preferred expression of TARP γ8 in the hippocampus initiated the search for inhibitors of this subunit to prevent hyperexcitation as observed in temporal lobe epilepsy or anxiety disorders (Maher et al, 2017). Another family of AMPA receptor-associated proteins is formed by the cornichons (CNIHs), among which CNIH-2 and CNIH-3 are abundantly expressed in the brain and assemble with GluA subunits (Schwenk et al, 2009). CNIH-2 has been shown to modulate receptor gating and pharmacology of AMPA receptorTARP complexes (Kato et al, 2010; Gill et al, 2011)

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