Potassium channels are found in all mammalian cell types, and they perform many distinct functions in both excitable and non-excitable cells. These functions are subserved by several different families of potassium channels distinguishable by primary sequence features as well as by physiological characteristics. Of these families, the tandem pore domain potassium channels are a new and distinct class, primarily distinguished by the presence of two pore-forming domains within a single polypeptide chain. We have cloned a new member of this family, TWIK-2, from a human brain cDNA library. Primary sequence analysis of TWIK-2 shows that it is most closely related to TWIK-1, especially in the pore-forming domains. Northern blot analysis reveals the expression of TWIK-2 in all human tissues assayed except skeletal muscle. Human TWIK-2 expressed heterologously in Xenopus oocytes is a non-inactivating weak inward rectifier with channel properties similar to TWIK-1. Pharmacologically, TWIK-2 channels are distinct from TWIK-1 channels in their response to quinidine, quinine, and barium. TWIK-2 is inhibited by intracellular, but not extracellular, acidification. This new clone reveals the existence of a subfamily in the tandem pore domain potassium channel family with weak inward rectification properties.

Human KCNK5 is a tandem pore domain potassium channel exhibiting delayed activation and sensitivity to volatile anesthetics and may therefore have a role in suppressing cellular excitability during general anesthesia.

Production of amyloid ␤ peptides (A␤), followed by their deposition in the brain as amyloid plaques, contributes to the hallmark pathology of Alzheimer disease. The enzymes responsible for production of A␤, BACE1 and ␥-secretase, are therapeutic targets for treatment of Alzheimer disease. Two presenilin (PS) homologues, referred to as PS1 and PS2, comprise the catalytic core of ␥-secretase. In comparing presenilin selectivity of several classes of ␥-secretase inhibitors, we observed that sulfonamides in general tend to be more selective for inhibition of PS1-comprising ␥-secretase, as exemplified by ELN318463 and BMS299897. We employed a combination of chimeric constructs and point mutants to identify structural determinants for PS1-selective inhibition by ELN318463. Our studies identified amino acid residues Leu 172 , Thr 281 , and Leu 282 in PS1 as necessary for PS1-selective inhibition by ELN318463. These residues also contributed in part to the PS1-selective inhibition by BMS299897. Alanine scanning mutagenesis of areas flanking Leu 172 , Thr 281 , and Leu 282 identified additional amino acids that affect inhibitor potency of not only these sulfonamides but also nonsulfonamide inhibitors, without affecting A␤ production and presenilin endoproteolysis. Interestingly, many of these same residues have been identified previously to be important for ␥-secretase function. These findings implicate TM3 and a second region near the carboxyl terminus of PS1 aminoterminal fragment in mediating the activity of ␥-secretase inhibitors. Our observations demonstrate that PS-selective inhibitors of ␥-secretase are feasible, and such inhibitors may allow differential inhibition of A␤ peptide production and Notch signaling.

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