The human lens expresses calcium activated SK channels linking cataract, schizophrenia and myotonic dystrophy

Purpose: Apamin-sensitive, calcium-activated SK potassium channels have been implicated in schizophrenia and myotonic dystrophy (MD). Both of these conditions carry an increased risk of cataract and in MD a reduction in anterior epithelial cell density has been observed. Therefore, the presence and functional activity of SK channels were investigated in the human lens. Methods: The expression of all 3 members of the SK channel family was investigated in RNA samples extracted from donor lenses (age 50-80 yrs) by TaqMan RT-PCR using human primers for SK1, SK2 and SK3. The functional activity was investigated following G-protein and tyrosine kinase receptor activation using electrophysiological and calcium-imaging techniques. Lens voltage was monitored by inserting a single electrode into the intact human lens, perifused with artificial aqueous humour, and changes in intracellular calcium were recorded in either anterior or equatorial epithelial cells after fura-2 incorporation. Lens epithelial cell growth was quantified by imaging cell cover of the posterior capsule in an in vitro capsular bag model developed in this laboratory. Results: Expression of all 3 SK family members was detected in both anterior and equatorial lens epithelial cells. Application of either G-protein (e.g. ATP, 100 µM) or tyrosine kinase (EGF, 10 ng/ml) receptor agonists induce a rapid hyperpolarisation of lens voltage which was accompanied by a parallel increase in intracellular calcium. Application of the calcium ionophore ionomycin also induced a rapid hyperpolarisation. All of the hyperpolarising responses were abolished by the specific inhibitor apamin (10 nM) and by the antipsychotic trifluoperazine (100 µM). The hyperpolarising responses were accentuated by the SK channel activator 1-EBIO. The growth of epithelial cells on the posterior capsule monitored over a 28 day period was inhibited by apamin. Conclusions: This study shows that SK channels are an integral part of the G-protein and tyrosine kinase calcium signalling mechanisms in the human lens and inhibiting their activation leads to a decline in cell growth. This helps to explain why a change in their activity, whether by abnormal gene expression or by drug intervention, can lead to cataract.