REGULATION OF PHOSPHATASE CONTROL ION CHANNEL FUNCTION

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DESCRIPTION (adapted from abstract) Reversible phosphorylation is an important mechanism by which hormones and neurotransmitters modulate ion channels. Although much is known concerning the kinases that phosphorylate ion channels, far less is known about the phosphatases that reverse this process. The applicants' long term goal is to determine how the action of ser/thr phosphatases on ion channels is controlled. In this proposal, two regulatory mechanisms that control ion channel phosphatases will be investigated: phosphorylation and direct activation by a lipid second messenger. The applicants have found that in vitro phosphorylation of protein kinase C sites on rat brain sodium channels decreases the dephosphorylation of distinct cAMP-dependent phosphorylation sites on channels. In addition, activation of protein kinase C in synaptosomes leads to an increase in channel phosphorylation at cAMP-dependent phosphorylation sites. In synaptosomes, protein kinase C may potentially control channel dephosphorylation by its action on channels themselves or by direct phosphorylation and inhibition of phosphatases. They will determine the contribution of each of these processes regulating sodium channel dephosphorylation in synaptosomes. In the case of calcium-activated potassium channels, evidence suggests that the neuropeptide hormones atrial natriuretic factor and somatostatin each stimulate these channels by activating a ser/thr phosphatase. Atrial natriuretic factor acts via cGMP and cGMP-dependent protein kinase, whereas somatostatin acts via arachidonic acid release. These observations suggest the possibility that ser/thr phosphatases are controlled by the second messenger cGMP and arachidonic acid or one of its metabolites. The investigators will determine whether two protein ser/thr phosphatases, PP2A or PP5, mediate the activation of calcium-dependent potassium channels by atrial natriuretic factor or somatostatin, and whether these phosphatases are intracellular targets for cGMP-dependent phosphorylation or lipid activation. These studies together will increase our understanding of how the action of ser/thr phosphatases on ion channels is controlled. In addition, they will uncover important details concerning the modulation of two ion channels that play key roles in neuronal excitation and secretion.