Distinct axo-somato-dendritic distributions of three potassium channels in CA1 hippocampal pyramidal cells.

Tekla Kirizs,Zoltan Nusser,Katalin Kerti‐Szigeti,Andrea Lorincz

doi:10.1111/ejn.12526

Abstract

Potassium channels comprise the most diverse family of ion channels and play critical roles in a large variety of physiological and pathological processes. In addition to their molecular diversity, variations in their distributions and densities on the axo-somato-dendritic surface of neurons are key parameters in determining their functional impact. Despite extensive electrophysiological and anatomical investigations, the exact location and densities of most K+ channels in small subcellular compartments are still unknown. Here we aimed at providing a quantitative surface map of two delayed-rectifier (Kv1.1 and Kv2.1) and one G-protein-gated inwardly rectifying (Kir3.2) K+ channel subunits on hippocampal CA1 pyramidal cells (PCs). Freeze-fracture replica immunogold labelling was employed to determine the relative densities of these K+ channel subunits in 18 axo-somato-dendritic compartments. Significant densities of the Kv1.1 subunit were detected on axon initial segments (AISs) and axon terminals, with an approximately eight-fold lower density in the latter compartment. The Kv2.1 subunit was found in somatic, proximal dendritic and AIS plasma membranes at approximately the same densities. This subunit has a non-uniform plasma membrane distribution; Kv2.1 clusters are frequently adjacent to, but never overlap with, GABAergic synapses. A quasi-linear increase in the Kir3.2 subunit density along the dendrites of PCs was detected, showing no significant difference between apical dendritic shafts, oblique dendrites or dendritic spines at the same distance from the soma. Our results demonstrate that each subunit has a unique cell-surface distribution pattern, and predict their differential involvement in synaptic integration and output generation at distinct subcellular compartments.

Highlights

Potassium channels comprise the most diverse group of ion channels, with more than 100 subunits being identified and classified into different channel families based on their molecular and biophysical properties (Hille, 2001; Gutman et al, 2003)
In the present study we extend the current knowledge on the distribution of the Kv1.1, Kv2.1 and Kir3.2 subunits, and advance our quantitative molecular map of the surface of CA1 pyramidal cells (PC) by determining their densities in 18 well-defined subcellular compartments
The highly sensitive, high-resolution quantitative Electron microscopic (EM) immunogold method SDS-digested freeze-fracture replica immunolabelling technique (SDS-FRL) allowed us to study the distribution of these K+ channels in small subcellular compartments usually unresolvable and inaccessible with traditional light microscopic (LM) immunohistochemical methods

Summary

Introduction

Potassium channels comprise the most diverse group of ion channels, with more than 100 subunits being identified and classified into different channel families based on their molecular and biophysical properties (Hille, 2001; Gutman et al, 2003). Electrophysiological experiments have identified K+ currents in PC dendrites, where they regulate the integration of synaptic inputs, control action potential (AP) back-propagation and dendritic electrogenesis (Hoffman et al, 1997; Migliore et al, 1999; Cai et al, 2004; Losonczy et al, 2008). They have been found in axons, where they set the threshold and sculpt the shape of the Received 6 December 2013, revised 24 January 2014, accepted 27 January 2014

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