The amyloid beta peptide (Abeta) is a product of the sequential gamma- and beta-secretase cleavage of amyloid precursor protein. Inhibitors of secretase enzymes have been proposed as a potential therapeutic strategy in the treatment of Alzheimer's disease. Here, we investigate the effect of inhibiting these key enzymes on the viability of a range of cell types. Treatment of rat cortical neurons for 24 hr with secretase inhibitors or an antibody that binds Abeta resulted in a marked reduction in cell viability, as measured by MTT reduction. Incubation with secretase inhibitors caused similar effects on other neuronal cell types (rat cerebellar granule neurons and the human SH-SY5Y cell line). Interestingly, rat astrocytes and a number of non-neuronal cell lines investigated (HEK293, DDT1-FM2, and human teratorhabdoid tumor cells) were unaffected by incubation with secretase inhibitors. The coincubation of Abeta1-40 prevented the toxicity of secretase inhibitors in neuronal cells. Abeta1-40 was protective in a concentration-dependent manner, and its effects were significant at concentrations as low at 10 pm. Importantly, the protective effects of Abeta were Abeta size-form specific, with the Abeta1-42 size form affording limited protection and the Abeta25-35 size form having very little protective effect. The present study demonstrates that inhibition of beta-or gamma-secretase activity induces death in neuronal cells. Importantly, this toxicity, which our data suggest is a consequence of a decline in neuronal Abeta levels, was absent in non-neuronal cells. This study further supports a key physiological role for the enigmatic Abeta peptide.

1. Ca2+-activated K+ (KCa) channels in neonatal rat type I carotid body cells were studied using single channel patch clamp techniques. In outside-out patches, using symmetrical 120 mm [K+] solutions, channels were observed with a slope conductance of 190 pS and a reversal potential of 0 mV. Reducing [K+]0 to 5 mm shifted the reversal potential as expected for a K+-selective channel.2. With 100 nM Ca2+ bathing the cytosolic aspect of patches, channel activity (number of active channels in a patch x open probability, NP0) increased with depolarization. NPo also increased with increasing 'cytosolic' [Ca2+] at a fixed membrane potential (0 mV). Using outside-out patches, bath application of 20 or 100 nM charybdotoxin reduced NPo by >85 %. These data indicate the presence of K+a channels in type I cells.3. At 0 mV, using solutions of identical composition (1 /M Ca2+ bathing the cytosolic aspect of the channels), NPo was higher in outside-out patches than in inside-out patches. NPo was greatest in recordings using the perforated-vesicle technique. 4. Hypoxia and anoxia were without effect on Kta channels in outside-out patches, but caused significant, reversible reductions of NPo in channels recorded in perforated vesicles. 5. The whole-cell perforated-patch technique was used to record membrane potential at 35-37 'C. Hypoxia, anoxia and charybdotoxin all depolarized type I cells. 6. Our results suggest an important role for Ka channels in type I carotid body cells, and their activity in relation to a model for chemotransduction is discussed.

No abstract

Diphenyleneiodonium (DPI) blocks hypoxic vasoconstriction in the pulmonary vasculature. Because one of the actions of DPI is the inhibition of NADPH oxidase, this has led to the suggestion that NADPH oxidase acts as an oxygen tension sensor in pulmonary smooth muscle cells. We investigated the effects of DPI on potassium and calcium currents in freshly isolated pulmonary artery smooth muscle cells by using whole cell patch-clamp recordings, since these ionic currents are known to be involved in hypoxic pulmonary vasoconstriction. DPI (3 and 10 microM) reversibly inhibited potassium currents, and in its presence, residual currents appeared markedly more transient than under control conditions. The actions of DPI could not be reversed by 4.4 mM hydrogen peroxide, the product of NADPH oxidase. Calcium channel currents were also reversibly inhibited by 3 microM DPI. Thus DPI is a nonselective blocker of ionic channels in pulmonary smooth muscle cells, and its mechanism of action does not appear to involve inhibition of hydrogen peroxide formation. The ability of DPI to block calcium currents can explain its inhibition of hypoxic pulmonary vasoconstriction.

M-type (Kv7, KCNQ) potassium channels are proteins that control the excitability of neurons and muscle cells. Many physiological and pathological mechanisms of excitation operate via the suppression of M channel activity or expression. Conversely, pharmacological augmentation of M channel activity is a recognized strategy for the treatment of hyperexcitability disorders such as pain and epilepsy. However, physiological mechanisms resulting in M channel potentiation are rare. Here we report that intracellular free zinc directly and reversibly augments the activity of recombinant and native M channels. This effect is mechanistically distinct from the known redoxdependent KCNQ channel potentiation. Interestingly, the effect of zinc cannot be attributed to a single histidine-or cysteine-containing zinc-binding site within KCNQ channels. Instead, zinc dramatically reduces KCNQ channel dependence on its obligatory physiological activator, phosphatidylinositol 4,5-bisphosphate (PIP 2 ). We hypothesize that zinc facilitates interactions of the lipid-facing interface of a KCNQ protein with the inner leaflet of the plasma membrane in a way similar to that promoted by PIP 2 . Because zinc is increasingly recognized as a ubiquitous intracellular second messenger, this discovery might represent a hitherto unknown native pathway of M channel modulation and provide a fresh strategy for the design of M channel activators for therapeutic purposes.+ channels are a family of voltagegated K + channels with a very distinctive and robust role in the control of cellular excitability. The channels give rise to noninactivating K + currents with slow kinetics and a very negative activation threshold (−60 mV or even more negative). In combination, these features allow KCNQ channels to remain partially active at voltages near the resting membrane potential of a neuron or a muscle cell and thus strongly influence excitability (1, 2). Transient KCNQ channel inhibition leads to reversible increases in neuronal excitability, whereas long-term losses of KCNQ channel activity often result in debilitating excitability disorders (1, 2). Thus, loss-of-function mutations within KCNQ genes underlie some types of epilepsy, deafness, and arrhythmias, whereas transcriptional down-regulation in sensory nerves may result in chronic pain (2). Conversely, M channel enhancers ("openers") reduce excitability and are clinically used as antiepileptic drugs (e.g., retigabine) or analgesics (e.g., flupirtine) (3). The therapeutic utility of KCNQ channel openers extends to other disorders linked to deregulated excitability, such as anxiety, stroke, and smooth muscle disorders (2, 3); therefore a global quest for specific and selective KCNQ openers is currently underway (3).The KCNQ channel family contains five members, KCNQ1-5 (Kv7.1-Kv7.5). KCNQ1 is expressed mostly within the cardiovascular system, whereas the other members are predominantly neuronal (1, 2). The most abundant M-type channel within the nervous system is believed to be the heteromeric KCNQ2/3 chan...