Developmental profile of SK2 channel expression and function in CA1 neurons

Abstract

We investigated the temporal and spatial expression of SK2 in the developing mouse hippocampus using molecular and biochemical techniques, quantitative immunogold electron microscopy and electrophysiology. The mRNA encoding SK2 was expressed in the developing and adult hippocampus. Western blotting and immunohistochemistry showed that SK2 protein increased with age. This was accompanied by a shift in subcellular localization. Early in development (P5), SK2 was predominantly localized to the endoplasmic reticulum in the pyramidal cell layer. But by P30 SK2 was almost exclusively expressed in the dendrites and spines. The level of SK2 at the postsynaptic density (PSD) also increased during development. In the adult, SK2 expression on the spine plasma membrane showed a proximal-to-distal gradient. Consistent with this redistribution and gradient of SK2, the selective SK channel blocker apamin increased evoked excitatory postsynaptic potentials (EPSPs) only in CA1 pyramidal neurons from mice older than P15. However, the effect of apamin on EPSPs was not different between synapses in proximal or distal stratum radiatum or stratum lacunosum-moleculare in adult. These results show a developmental increase and gradient in SK2-containing channel surface expression that underlie their influence on neurotransmission, and that may contribute to increased memory acquisition during early development.

Small-conductance calcium-activated potassium type 2 channels (SK2, KCa2.2) in human brain

“…Medium levels of SK2-LI were found in the stratum lacunosum moleculare, while the stratum pyramidale exposed only moderate levels of SK2-LI (Sailer et al 2002, 2004). These findings were supported by recent studies performed at electron microscopic level in mouse brain (Ballesteros-Merino et al 2012), and through whole-cell voltage-clamp and current-clamp recordings in rat CA1 pyramidal cells (Chen et al 2014). In human hippocampal regions, highest expression levels of SK2-LI were detected in strata oriens and radiatum of area CA1 and CA2.…”

“…SK2 channels have been shown to be located in the cell soma, as well as in presynaptic and postsynaptic compartments. In the presynaptic compartment of CA1 neurons, opening of SK2 channels can be associated with the control of neurotransmitter release (Ballesteros-Merino et al 2012). In the postsynaptic membrane of glutamatergic synapses, activation of SK2 channels modulates synaptic transmission and the induction and expression of synaptic plasticity (Adelman et al 2012).…”

Subcellular Localization of K+ Channels in Mammalian Brain Neurons: Remarkable Precision in the Midst of Extraordinary Complexity

“…Subpopulations of each of these dendritic KChs are found within spines, both within the PSD itself, and in extra-synaptic regions of the spine head membrane, as seen for Kv4.2 (Figure 2D) (Kerti et al, 2012), KCa2.2 (Figure 3B) (Allen et al, 2011; Ballesteros-Merino et al, 2012) and Kir3.2 (Figure 3C) (Drake et al, 1997; Fernandez-Alacid et al, 2011; Kirizs et al, 2014; Koyrakh et al, 2005). It should be noted that in general the PSD itself has been shown to be difficult to access with antibodies (Fukaya and Watanabe, 2000), such that the use of pre-embedding methods [LM-DAB, LM-IF and pre-embedding EM-DAB and EM-IG; (Masugi-Tokita and Shigemoto, 2007)], could yield an underestimate of true levels of expression within the PSD.…”

“…Scale bar: 0.2 μm. From (Ballesteros-Merino et al, 2012). C. EM-IG labeling of Kir3.2, GABA-B1 receptors, and PSD-95 in a SDS-FRL sample from rat CA1 showing colocalization of Kir3.2, GABA-B1 receptors in the extrasynaptic membrane of a dendritic spine (s) emerging from a dendrite (Den).…”

Localization and Targeting of GIRK Channels in Mammalian Central Neurons

“…It has been demonstrated that distinct glutamatergic inputs can act through different signaling proteins within a single neuron, contributing to input-specific functional requirements (reviewed by Luján, 2010). Such an input-dependent localization has been described for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and NMDA receptors (Nusser et al, 1998;Watanabe et al, 1998) and for small conductance Ca 2+ activated K + (SK) channels in the hippocampus (Ballesteros-Merino et al, 2012. The question then arises as to whether different GIRK channel subtypes can also be selectively associated with functionally distinct synaptic inputs.…”