Dept. of Cell & Developmental Biology
Oregon Health & Science University
Small-conductance Ca2+ activated potassium channels (SK) channels play a fundamental role in all excitable cells. They are potassium selective, voltage-insensitive, and activated by increases in the level of intracellular Ca2+ ions and selectively blocked by peptide toxin apamin. Increases in the intracellular Ca2+ concentration activated SK channels, leading to hyperpolarization of membrane potential, which in turn reduces the Ca2+ transient in the cell. For example, in the spine of hippocampal CA1 pyramidal neurons and in lateral amygdala pyramidal neurons, Ca2+ influx through N-methyl-D-aspartate receptors (NMDARs) activates the SK2-containing channels, the efflux of K+ ions through SK channels repolarizes the spine, promoting the Mg2+ blockage of NMDARs, therefore reducing the Ca2+ transient in the cell. Thus, blocking SK2-containing channels with apamin facilitates the induction of long-term potentiation (LTP) by enhancing NMDAR-dependent Ca2+ signals within dendritic spines. The SK2 gene encodes two isoforms that differ only in the length of their N-terminal domains. SK2-long (SK2-L) and SK2-short (SK2-S) are co-expressed in CA1 pyramidal neurons and likely form heteromeric channels. In mice lacking SK2-L, SK2-S-containing channels are expressed in the extrasynaptic membrane, but are excluded from the postsynaptic density (PSD). The SK channel contribution to excitatory postsynaptic potentials (EPSP) is absent in SK2-S only mice and could be restored by SK2-L re-expression. The first aim of this thesis was to assess the contribution of SK-L isoform to the induction of LTP in hippocampal neurons. We found that blocking SK channels increased the amount of LTP induced in area CA1 in slices from wild-type mice but had no effect in slices from SK2-S only mice. Taken together with the SK2-S expression pattern and the effect of SK2-L on EPSP, these results indicate that SK2-L directs synaptic SK2-containing channels expression and is important for normal synaptic signaling and plasticity. There are many Ca2+ sources for activating SK channels in excitable cells, for example, Ca2+ influx through NMDAR or via voltage gated Ca2+ channels, or Ca2+ induced Ca2+ release from intracellular store. In hippocampal CA1 pyramidal neurons, synaptically evoked Ca2+ influx through NMDARs activates spine SK channels, reducing EPSPs and the associated spine head Ca2+ transient. However, results using glutamate uncaging implicated Ca2+ influx through SNX-482 sensitive R-type (Cav2.3) Ca2+ channels as the Ca2+ source for SK channel activation. The second aim of this thesis was to study the two Ca2+ sources, one from Ca2+ influx through NMDARs and the other from R-type Ca2+ channels, and their downstream targets of potassium channels. By using synaptic stimulation, we found that both apamin and SNX-482 boosted synaptically evoked EPSPs and their effects were not mutually exclusive. Ca2+ influx through R-type Ca2+ channels activates voltage dependent Kv4.2 containing potassium channels in a Kv channel interacting protein (KChIP) dependent manner. These results suggest two distinct Ca2+ signaling pathways within dendritic spine that link Ca2+ influx through NMDARs to SK channels and Ca2+ influx through R-type Ca2+ channels to Kv4.2-containing channels.
School of Medicine
Wang, Kang, "SK channels and signaling microdomains in dendritic spines" (2014). Scholar Archive. 3598.
Available for download on Friday, September 01, 2017