Potassium channels--multiplicity and challenges

Abstract

The development of our knowledge of the function, structure and pharmacology of K(+) channels is briefly outlined. This is the most diverse of all the ion channel families with at least 75 coding genes in mammals. Alternative splicing as well as variations in the channel subunits and accessory proteins that co-assemble to form the functional channel add to the multiplicity. Whereas diversity of this order suggests that it may be possible to develop new classes of drug, for example, for immunomodulation and some diseases of the central nervous system, the ubiquity of K(+) channels imposes stringent requirements for selectivity. Animal toxins from the snake, bee and scorpion provide useful leads, though only in a few instances (e.g. with apamin) it has been possible to produce non-peptidic analogues of high potency. The scale of the resources needed to identify, and characterize fully, specific K(+) channel as targets and then develop modulators with the required selectivity presents a challenge to both academic and applied pharmacologists.

Figure 1

Upper panel: A comparison of the effects of noradrenaline (Nor, 0.9 μM) and isoprenaline (Iso, 1.1 μM) on ⁴²K efflux from guinea-pig taenia caeci bathed in K⁺-rich (mainly K₂SO₄) solution. Adapted from Jenkinson & Morton (1965). Lower panel: The equivalent experiment but now carried out in a normal bathing solution and using the double sucrose gap technique to record both tension development (upper of each pair of traces) and the membrane potential and conductance (lower traces). The concentrations of noradrenaline (NA) and isoprenaline (Iso) were 0.8 and 1.0 μM, respectively. (a) Noradrenaline hyperpolarizes the membrane (downward movement of the baseline) and increases its conductance (as shown from the reduction in the amplitude of the deflections produced by regularly applied constant current pulses of alternating direction). These changes indicate an increase in P_K. (b) Isoprenaline reduces tension development with little effect on the electrical properties of the membrane. From Bülbring & Tomita (1969), with permission.

Figure 2

The ion conduction pore of a potassium channel (KcsA). Left panel: Two of the four channel subunits are shown with the extracellular side at the top. Each subunit contains an outer helix, an inner helix close to the pore, a pore helix (red) and a selectivity filter (gold). Blue mesh shows electron density for K⁺ ions (green) and water molecules (red atoms) along the pore. Right panel: Close-up of the selectivity filter with dehydrated K⁺ ions at positions 1–4 inclusive (external to internal) inside the filter and a hydrated K⁺ ion in the central cavity below the filter. Reproduced from MacKinnon (2003), with permission.

Figure 3

Detecting K⁺ channel mRNA and protein. Upper panel: The application of the in situ mRNA hybridization technique to show the distribution of the three main subtypes of SK_Ca (SK1-3) mRNA in coronal sections of adult rat brain. Regions are indicated by the abbreviations CA1 and CA3 (CA1 and CA3 regions of hippocampus), DG (dentate gyrus), DM (dorsomedial hypothalamic nucleus), ME (medial amygdaloid nucleus), Rt (reticular thalamic nucleus), CM (central medial thalamic nucleus), MHB (medial habenular nucleus), VMH (ventromedial hypothalamic nucleus). From Stocker & Pedarzani (2000), with permission. Middle panel: Immunostaining for the SK3 channel in transversely sectioned rat lumbar spinal cord. Left panel: Staining is most intense in laminas I, II and III of the dorsal horn (see adjacent diagram), particularly lamina II. There is a second area of staining in the ependymal region (lower right). Right panel: Ependymal SK3 staining at higher magnification. Width of field 230 μm. From Bahia et al. (2005), with permission. Lower panel: (a and b) Distribution of SK1 channels in the rat hippocampus as revealed by immunostaining. The regions are indicated as follows: so, stratum oriens; sp, stratum pyramidale; sr, stratum radiatum; slm, stratum lacunosum moleculare; ml, molecular layer of the dentate gyrus; gl, granule cell layer; mf, mossy fibre system. Scale bars for a and b, 200 and 40 μM, respectively. From Sailer et al. (2004), with permission.

Figure 4

Development of potent SK_Ca blockers based on apamin. Upper left panel: A possible spatial structure for apamin in solution (from Bystrov et al., 1980, with permission). Two arginines (13 and 14) carry positive charges and are crucial for the activity of the peptide. Upper right panel: Dequalinium (IC₅₀ 1.1 μM as a blocker of the SK3-mediated AHP in cultured rat superior cervical ganglion neurons). Lower panel: UCL 1684 and 1848 (IC₅₀ on the same response, 4.1 and 2.7 nM, respectively, as compared with 2.3 nM for apamin).