br Activators Many compounds are known
Activators Many compounds are known to influence the activity of Ca2+-activated K+ channels, and since hIK1 was cloned more insight has been gained on compounds that have the capacity to increase channel activity. Activation by divalent metal cations has been investigated, with Ca2+, Pb2+, Cd2+, Sr2+ and Ba2+ potentially activating the channel (in order of decreasing potency) while Co2+, Fe2+ and Zn2+ elicited no apparent K+ currents for concentrations up to 100μM (Cao & Houamed, 1999).
Structure The basic topology of the Gárdos channel is thought to be a homo-tetramer of six-transmembrane-domain polypeptides (Fig. 2). This situation is predicted from the amino USA sequence (Vandorpe et al., 1998). The region between the s5 and s6 membrane domains contains the pore (‘p’) region, analogous to that seen in the K+ channel from Streptomyces lividans, the structure of which is known (Doyle et al., 1998). The K+ selectivity is mediated by the carbonyl oxygen atoms within the selectivity filter protruding from the backbone of the polypeptides from each of the four identical subunits. Insight on the structure of Ca2+-gated K+ channels has come from the very recent publication of the first crystal structure of such a channel. Jiang et al. (2002) have solved the structure of the Ca2+-activated K+ channel from Methanobacterium thermoautotrophicum. This protein contains two membrane-spanning α-helices and one RCK (regulator of conductance of K+) domain per subunit. To form a channel, four subunits unite in the presence of four additional RCK domains from solution; this is apparently the product of alternative splicing of the mRNA derived from the primary gene. It appears that the binding of Ca2+ confers a conformational alteration in the cytoplasmic ‘gating ring’ of eight RCKs, which translates mechanically to bring about an alteration in the arrangement of α-helices in the pore thus opening the channel. This channel is different from the Gárdos channel in that the Ca2+-binding domain is part of the polypeptide sequence of the subunits. However, it is likely that binding of Ca2+ to the CaM that is tightly associated with the cytoplasmic domain of the Gárdos channel confers a similar mechanical alteration, that is responsible for pore opening.
Sickle cell anaemia Sickle cell anaemia (SSA) is characterised by painful episodes resulting from occlusion of blood vessels by rigidified sickle cells. Formation of dense sickle cells is associated with haemoglobin polymerisation, which is greatly accelerated by an increase in the haemoglobin concentration (or, equivalently, a reduction in cell volume). The decrease in cell volume, combined with an activation of a non-selective cation pathway (called Psickle), increases the concentration of Ca2+ inside the cell. Thus, a ‘positive feedback’ loop ensues with loss of K+, Cl− and H2O. As a major contributor to the dehydration of erythrocytes of SSA patients, the Gárdos channel has long been recognised as a therapeutic target for amelioration of the symptoms of this disease. Recent research has focussed on the extent to which the Gárdos channel contributes to cell dehydration in terms of subpopulations of cells responsible, and various cellular conditions and compounds that influence the Gárdos effect in normal and sickle cells. The human erythrocyte has four pathways that facilitate membrane transport of K+ ions: Na+, K+-ATPase, which uses energy derived directly from the hydrolysis of ATP to set-up cation gradients across the lipid bilayer (Garrahan & Glynn, 1967); the electroneutral Na+/K+/2Cl− cotransporter, implicated in hypertension (Canessa, 1989; Kracke et al., 1988, Tuck et al., 1987); the K+/Cl− cotransporter, sensitive to pH and cell volume (Lauf et al., 1992, Lauf and Adragna, 1998, Pellegrino et al., 1998, Brugnara et al., 1989); and the Gárdos channel (represented as transporters 2, 6, 4, and 3, respectively, in Fig. 1). The latter two pathways have been established as the major contributors to cell dehydration in SSA (Brugnara, 1995; Brugnara, Bunn, & Tosteson, 1986). A profile of erythrocyte densities from a sickle cell patient reveals cells that are heterogeneous with respect to their density distribution. Schwartz, Musto, Fabry, and Nagel (1998) defined intermediate (ID) and high density (HD) fractions of erythrocytes as 1.090–1.114g/ml and >1.114g/ml, respectively, and showed that the KCC pathway contributes more to ID cell formation whereas the Gárdos channel is largely responsible for HD cell formation. Shartava et al., 1999, Shartava et al., 2000 specifically identified the Gárdos channel (rather than the KCl cotransporter) as the primary route for dense cell formation from low density SS (homozygous for Hb-S) sickle cells, following addition of 1-chloro-2,4-dinitrobenzene (CDNB) to the cells. CDNB depletes cellular glutathione (GSH), and in doing so renders cytoplasmic and membrane proteins more susceptible to oxidative damage. Shartava et al., 1999, Shartava et al., 2000 demonstrated this by using specific blockers of KCl cotransport (((dihydroindenyl)oxy)alkanoic acid, DIOA) and the Gárdos channel (clotrimazole), and postulate that oxidative damage to the Gárdos channel protein (or a closely associated protein) induces K+ loss in these cells. Qualification of this finding with respect to the identity of the proteins involved will be an important development.