KCTD12 Auxiliary Proteins Modulate Kinetics of GABA_BReceptor-Mediated Inhibition in Cholecystokinin-Containing Interneurons

Abstract

Cholecystokinin-expressing interneurons (CCK-INs) mediate behavior state-dependent inhibition in cortical circuits and themselves receive strong GABAergic input. However, it remains unclear to what extent GABAB receptors (GABABRs) contribute to their inhibitory control. Using immunoelectron microscopy, we found that CCK-INs in the rat hippocampus possessed high levels of dendritic GABABRs and KCTD12 auxiliary proteins, whereas postsynaptic effector Kir3 channels were present at lower levels. Consistently, whole-cell recordings revealed slow GABABR-mediated inhibitory postsynaptic currents (IPSCs) in most CCK-INs. In spite of the higher surface density of GABABRs in CCK-INs than in CA1 principal cells, the amplitudes of IPSCs were comparable, suggesting that the expression of Kir3 channels is the limiting factor for the GABABR currents in these INs. Morphological analysis showed that CCK-INs were diverse, comprising perisomatic-targeting basket cells (BCs), as well as dendrite-targeting (DT) interneurons, including a previously undescribed DT type. GABABR-mediated IPSCs in CCK-INs were large in BCs, but small in DT subtypes. In response to prolonged activation, GABABR-mediated currents displayed strong desensitization, which was absent in KCTD12-deficient mice. This study highlights that GABABRs differentially control CCK-IN subtypes, and the kinetics and desensitization of GABABR-mediated currents are modulated by KCTD12 proteins.

KCTD Hetero-oligomers Confer Unique Kinetic Properties on Hippocampal GABABReceptor-Induced K+Currents

“…Cells were transfected at 70%-90% confluency in 6-or 24-well plates or in 10 cm dishes using Lipofectamine 2000 (Invitrogen), X-tremeGENE (Roche), or polyethylenimine (Sigma). Cultured hippocampal neurons were prepared as described previously (Brewer et al, 1993;Biermann et al, 2010). Briefly, embryonic day 16.5 mouse hippocampi were dissected, digested with 0.25% trypsin (Invitrogen) in 1 ϫ PBS solution for 15 min at 37°C, dissociated by trituration, and plated on glass coverslips coated with 1 mg/ml poly-L-lysine hydrobromide (Sigma) in 0.1 M borate buffer (boric acid/sodium tetraborate).…”

“…Hetero-oligomerization of KCTD8, KCTD12, and KCTD16 is mediated by the self-interacting T1 and H1 homology domains that also mediate homo-oligomerization (Schwenk et al, 2010;Correale et al, 2013). Hetero-oligomerization was observed within other KCTD clades (Sowa et al, 2009;De Smaele et al, 2011).…”

“…GABA B receptors are expressed throughout the brain and influence synaptic transmission by regulating the activity of ion channels and adenylate cyclases (Couve et al, 2000;Chalifoux and Carter, 2011;. Postsynaptic GABA B receptors activate inwardly rectifying K ϩ channels (Kir3; also known as G-proteinactivated inwardly rectifying K ϩ channels [GIRK]) that generate slow IPSCs (sIPSCs) (De Koninck and Mody, 1997;Lüscher et al, 1997;Lüscher and Slesinger, 2010;Booker et al, 2013). GABA B receptors comprise principal and auxiliary subunits that assemble into molecularly and functionally distinct receptor subtypes (Schwenk et al, 2010(Schwenk et al, , 2016.…”

“…The principal subunits GABA B1a , GABA B1b , and GABA B2 are seven-transmembrane proteins and generate fully functional heterodimeric GABA B(1a,2) and GABA B(1b,2) receptors (Möhler and Fritschy, 1999;Monnier et al, 2011). The auxiliary subunits KCTD8, KCTD12, KCTD12b, and KCTD16 (herein collectively referred to as KCTDs) are cytosolic proteins that bind as homo-tetramers (Schwenk et al, 2010;Correale et al, 2013) or homo-pentamers (Smaldone et al, 2016) to the GABA B2 subunit (Bartoi et al, 2010;Schwenk et al, 2010). KCTD8, KCTD12, KCTD12b, and KCTD16 comprise a clade of a larger family of KCTD proteins that all contain a T1 domain, which in voltage-gated K ϩ channels mediates tetramerization (Liu et al, 2013;Skoblov et al, 2013).…”

“…KCTD8, KCTD12, KCTD12b, and KCTD16 comprise a clade of a larger family of KCTD proteins that all contain a T1 domain, which in voltage-gated K ϩ channels mediates tetramerization (Liu et al, 2013;Skoblov et al, 2013). The T1 domain of the KCTDs interacts with the C-terminal intracellular domain of the GABA B2 subunit, in which tyrosine-902 (Y902) is mandatory for interaction (Schwenk et al, 2010;Correale et al, 2013). The KCTDs additionally contain a conserved H1 homology domain, which in KCTD12 induces a pronounced desensitization of the GABA B receptor response by uncoupling the ␤␥ subunits of the activated G-protein from the effector channel .…”