The Kv2.1 K+ channel targets to the axon initial segment of hippocampal and cortical neurons in culture and in situ.
BACKGROUND
The Kv2.1 delayed-rectifier K+ channel regulates membrane excitability in hippocampal neurons where it targets to dynamic cell surface clusters on the soma and proximal dendrites. In the past, Kv2.1 has been assumed to be absent from the axon initial segment.


RESULTS
Transfected and endogenous Kv2.1 is now demonstrated to preferentially accumulate within the axon initial segment (AIS) over other neurite processes; 87% of 14 DIV hippocampal neurons show endogenous channel concentrated at the AIS relative to the soma and proximal dendrites. In contrast to the localization observed in pyramidal cells, GAD positive inhibitory neurons within the hippocampal cultures did not show AIS targeting. Photoactivable-GFP-Kv2.1-containing clusters at the AIS were stable, moving <1 microm/hr with no channel turnover. Photobleach studies indicated individual channels within the cluster perimeter were highly mobile (FRAP tau=10.4+/-4.8 sec), supporting our model that Kv2.1 clusters are formed by the retention of mobile channels behind a diffusion-limiting perimeter. Demonstrating that the AIS targeting is not a tissue culture artifact, Kv2.1 was found in axon initial segments within both the adult rat hippocampal CA1, CA2, and CA3 layers and cortex.


CONCLUSIONS
In summary, Kv2.1 is associated with the axon initial segment both in vitro and in vivo where it may modulate action potential frequency and back propagation. Since transfected Kv2.1 initially localizes to the AIS before appearing on the soma, it is likely multiple mechanisms regulate Kv2.1 trafficking to the cell surface.
Sarmiere
Patrick D
PD
Program in Molecular, Cellular, and Developmental Neuroscience, Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado, USA. psarmiere@scs.cbeyond.com
Weigle
Cecile M
CM
Tamkun
Michael M
MM
D000818:Animals; D001369:Axons / Q000378:metabolism*; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology / Q000378:metabolism*; D036681:Fluorescence Recovery After Photobleaching; D006624:Hippocampus / Q000166:cytology / Q000378:metabolism*; D016501:Neurites / Q000378:metabolism; D009474:Neurons / Q000378:metabolism* / Q000648:ultrastructure; D021381:Protein Transport; D017966:Pyramidal Cells / Q000378:metabolism / Q000648:ultrastructure; D051381:Rats; D051669:Shab Potassium Channels / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
C496991:Kcnb1 protein, rat; D051669:Shab Potassium Channels