Molecular Neurobiology of Potassium Channels
Dr Martin Stocker
|Reader in Molecular and Cellular Neuroscience|
|Tel: 020 7679 7244|
Dr Martin Stocker graduated in Biochemistry in 1989 and received his PhD in 1991 at the University of Bochum. In 1992 he took a Postdoctoral Fellowship in Hamburg in the group of Prof. O. Pongs at the Centre for Molecular Neurobiology, Dept. of Neuronal Signal Transduction. In 1993 he joined the group of Prof. C. Miller at the Howard Hughes Medical Institute at Brandeis University, Dept. of Biochemistry. In 1994 he went to the Max-Planck Institute for Experimental Medicine in Göttingen. In the department of Prof. W. Stühmer, Molecular Biology of Neuronal Signals, he started working with his independent research group. Since 2000 he is a Wellcome Senior Research Fellow at the Laboratory for Molecular Pharmacology (LMP) at UCL and in 2006 he was made a Reader in Molecular & Cellular Neuroscience.
The non-uniform distribution of ion channels in neurones is an important parameter in shaping their signal processing properties. We are particularly interested in the molecular and cellular mechanisms responsible for targeting, clustering and regulation of calcium-activated potassium channels and associated molecules.
A single or a train of action potentials in CA1 neurones of the hippocampus is followed by three afterhyperpolarisations distinguished by their time-courses: f(ast)AHP, m(edium)AHP and s(low)AHP.
Large conductance, voltage- and calcium-activated potassium channels (BK) carry out at least two important functions in neurones. In the somata, they participate in action potential repolarisation and generate the fAHP, thus contributing to set the rate of action potential firing. In presynaptic terminals, they regulate the duration of the action potential and limit Ca2+ entry, thereby modulating neurotransmitter release.
The mAHP in CA1 neurons has at least two current components. One of them is the recently identified apamin-sensitive IAHP , which is generated by small conductance, purely calcium-activated potassium channels (SK). IAHP contributes to the early phase of spike frequency adaptation, demonstrating impressively how a subclass of potassium channels is able to influence signal processing in neurons.
The current underlying the slow AHP is sIAHP . When sIAHP is suppressed as a consequence of neurotransmitter-induced phosphorylation, the neurones are more excitable and will follow incoming signals more faithfully. This neuromodulatory effect can be regarded as a molecular correlate of paying attention. This current is generated by calcium-activated potassium channels of unknown molecular composition.
In the last decade, electrophysiological recordings performed in various systems have provided strong evidence that calcium channels, kinases and phosphatases must be in close proximity to calcium-activated potassium channels. Biochemical data about the organisation of these modulatory clusters are nevertheless not available to date.
We are using molecular biological techniques, in particular the yeast-two hybrid system, to identify interaction partners of BK and SK channels present in such modulatory clusters. Histochemical and electrophysiological techniques on transfected cultured cell lines and in native systems are additionally used to characterise and study the function of the interacting proteins. The molecular elucidation of these modulatory clusters will contribute to understanding how the non-uniformity of potassium channel distribution is generated and maintained, and how signal processing is modulated on the molecular level.
For more information on this topic see also the following web-page: "Pay attention to potassium channels!"
Potassium channels are structurally among the simplest in the superfamily of ion channels. Since the cloning of the first potassium channel subunit, more than 60 different mammalian genes have been identified. Voltage-gated potassium channels (Kv) are classified into 9 families. Assembly of 4 principal subunits results in functional homomeric potassium channels for members of the Kv1-4 families (e.g. four Kv1.1). Within the same family, different principal subunits can also assemble into functional heteromeric potassium channels (e.g. two Kv1.1 + two Kv1.4). Members of the Kv5, Kv6, Kv8 and Kv9 families, termed modulatory α-subunits, cannot assemble into functional homomeric potassium channels, but have the ability to co-assemble with members of the Kv2 family, thus crossing "family borders". The resulting heteromeric potassium channels show different kinetic properties compared to homomeric Kv2 channels.
Based on the emerging information about potassium channel structure, we are combining molecular biology with heterologous expression of mutated channels in X. oocytes to identify critical regions and, if possible, amino acids that are crucial for the different gating behaviour caused by the modulatory subunits. We are also interested in understanding the physiological importance of the modulatory α-subunits.
Potassium channel toxins
Toxins are an extremely valuable tool in studying ion channel structure, function and physiology. After cloning of the first potassium channel subunit, the scorpion toxin charybdotoxin has been extensively used to achieve insights on the structure of the outer pore of the potassium channel. Apamin, from the venom of the honey bee, is a potassium channel blocker specific for SK channels. The use of this toxin in combination with in-situ hybridisation enabled us to correlate specific SK channel subunits with calcium-activated potassium currents in the central nervous system. Furthermore, this highly specific toxin enabled us to reveal the physiological function of SK-mediated currents in different neurons. The animal kingdom is a rich source of poisonous creatures producing toxins valuable for research. Together with our collaborators, we are interested in finding highly specific new toxins, study their action and use them in native systems to characterise otherwise undistinguishable potassium channels.
- A. Nolting, T. Ferraro, D. D'hoedt & M. Stocker (2007) An amino acid outside the pore region influences apamin sensitivity in small conductance Ca2+-activated K+ channels. J. Biol. Chem. 282:3478-3486 Abstract (Link to PubMed) Full text (Link to Journal)
- P. Pedarzani, J. E. McCutcheon, G. Rogge, B. S. Jensen, P. Christophersen, C. Hougaard, D. Strobaek & M. Stocker (2005) Specific enhancement of SK channel activity selectively potentiates the afterhyperpolarizing current IAHP and modulates the firing properties of hippocampal pyramidal neurons. J. Biol. Chem. 280:41404-41411. Abstract (Link to PubMed) Full text (Link to Journal)
- D. Kerschensteiner, F. Soto & M. Stocker (2005) Fluorescence measurements reveal stoichiometry of K+ channels formed by modulatory and delayed rectifier α-subunits. Proc. Natl. Acad. Sci. USA. 102:6160-6165. Abstract (Link to PubMed) Full text (Link to Journal)
- J. Scuvee-Moreau, A. Boland, A. Graulich, L. Van Overmeire, D. D’hoedt, F. Graulich-Lorge, E. Thomas, A. Abras, M. Stocker, J.-F. Liegeois & V. Seutin (2004) Electrophysiological characterization of the SK channel blockers methyl-laudanosine and methyl-noscapine in cell lines and rat brain slices. Br. J. Pharmacol. 143:753-764. Abstract (Link to PubMed) Full text (Link to Journal)
- M. Stocker (2004) Ca2+-activated K+ channels: molecular determinants and function of the SK family. Nat. Rev. Neurosci. 5:758-770 Abstract (Link to PubMed) Full text (Link to Journal)
- M. Stocker, K. Hirzel, D. D'hoedt & P. Pedarzani (2004) Matching molecules to function: neuronal Ca2+-activated K+ channels and afterhyperpolarizations. Toxicon 43:933-949. Abstract (Link to PubMed) Full text (Link to Journal)
- D. D'hoedt, K. Hirzel, P. Pedarzani & M. Stocker (2004) Domain analysis of the calcium-activated potassium channel SK1 from rat brain. Functional expression and toxin sensitivity. J. Biol. Chem. 279:12088-12092. Abstract (Link to PubMed) Full text (Link to Journal)
- D. Kerschensteiner, F. Monje & M. Stocker (2003) Structural Determinants of the Regulation of the Voltage-gated Potassium Channel Kv2.1 by the Modulatory α-subunit Kv9.3. J. Biol. Chem. 278: 18154-18161. Abstract (Link to PubMed) Full text (Link to Journal)
- P. Pedarzani, D. D'hoedt, K. B. Doorty, J. D. Wadsworth, J. S. Joseph, K. Jeyaseelan, R. M. Kini, S. V. Gadre, S. M. Sapatnekar, M. Stocker & P. N. Strong (2002) Tamapin, a venom peptide from the Indian red scorpion (Mesobuthustamulus) that targets small conductance Ca2+-activated K+ channels and afterhyperpolarization currents in central neurons. J. Biol. Chem. 277: 46101-46109. Abstract (Link to PubMed) Full text (Link to Journal)
- L. A. Cingolani, M. Gymnopoulos, A. Boccaccio, M. Stocker & P. Pedarzani (2002) Developmental regulation of small-conductance Ca2+-activated K+ channel expression and function in rat Purkinje neurons. J. Neurosci. 22: 4456-67. Abstract (Link to PubMed) Full text (Link to Journal)
- P. Pedarzani, J. Mosbacher, A. Rivard, L. A. Cingolani, D. Oliver, M. Stocker, J. P. Adelman & B. Fakler (2001) Control of electrical activity in central neurons by modulating the gating of small conductance Ca2+-activated K+ channels. J. Biol. Chem. 276: 9762-9769. Abstract (Link to PubMed) Full text (Link to Journal)
- M. Stocker & P. Pedarzani (2000) Differential distribution of three Ca2+-activated K+ channel subunits, SK1, SK2, and SK3, in the adult rat central nervous system. Mol. Cell. Neuroscience 15: 476-493. Abstract (Link to PubMed) Full text (Link to Journal)
- M. Stocker, M. Krause & P. Pedarzani (1999) An apamin-sensitive Ca2+-activated K+ current in hippocampal pyramidal neurons. Proc. Natl. Acad. Sci. USA. 96: 4662-4667. Abstract (Link to PubMed) Full text (Link to Journal)