Brain mitochondrial ATP-insensitive large conductance Ca⁺²-activated K⁺ channel properties are altered in a rat model of amyloid-ß neurotoxicity.

Mitochondrial dysfunction is a hallmark of amyloid-beta (Aß)-induced neuronal toxicity in Alzheimer's disease (AD). However, the underlying mechanism of how Aß affects mitochondrial function remains uncertain. Because mitochondrial potassium channels have been involved in several mitochondrial functions including cytoprotection, apoptosis and calcium homeostasis, a study was undertaken to investigate whether the gating behavior of the mitochondrial ATP- and ChTx-insensitive-IbTx-sensitive Ca(2+)-activated potassium channel (mitoBKCa) is altered in a rat model of Aß neurotoxicity. Aß1-42 (4 µg/µl) was intracerebroventricularly injected in male Wistar rats (220-250 g). Brain Aß accumulation was confirmed two weeks later on the basis of an immunohistochemistry staining assay, and physiological impacts measured in passive avoidance task cognitive performance experiments. Brain mitochondrial inner membranes were then extracted and membrane vesicles prepared for channel incorporation into bilayer lipid. Purity of the cell fraction was confirmed by Western blot using specific markers of mitochondria, plasma membrane, endoplasmic reticulum, and Golgi. Our results first provide evidence for differences in mitoBKCa ion permeation properties with channels coming from Aß vesicle preparations characterized by an inward rectifying I-V curve, in contrast to control mitoBKCa channels which showed a linear I-V relationship under the same ionic conditions (200 mM cis/50mM trans). More importantly the open probability of channels from Aß vesicles appeared 1.5 to 2.5 smaller compared to controls, the most significant decrease being observed at depolarizing potentials (30 mV to 50 mV). Because BKCa-ß4 subunit has been documented to shift the BKCa channel voltage dependence curve, a Western blot analysis was undertaken where expression of mitoBKCa α and ß4 subunits was estimated using anti-α and ß4 subunit antibodies. Our results indicated a significant increase in mitoBKCa-ß4 subunit expression coupled to a decrease in the expression of α subunit. Our results thus demonstrate a modification in the mitoBKCa channel gating properties in membrane preparations coming from a rat model of Aß neurotoxicity, an effect potentially linked to a change in mitoBKCa-ß4 and -α subunits expression or increased ROS production due to an enhanced Aß mitochondrial accumulation. Our results may provide new insights into the cellular mechanisms underlying mitochondrial dysfunctions in Aß neurotoxicity.

Impairment of brain mitochondrial charybdotoxin- and ATP-insensitive BK channel activities in diabetes.

Existing evidence indicates an impairment of mitochondrial functions and alterations in potassium channel activities in diabetes. Because mitochondrial potassium channels have been involved in several mitochondrial functions including cytoprotection, apoptosis and calcium homeostasis, a study was carried out to consider whether the gating behavior of the mitochondrial ATP- and ChTx-insensitive Ca(2+)-activated potassium channel (mitoBKCa) is altered in a streptozotocin (STZ) model of diabetes. Using ion channel incorporation of brain mitochondrial inner membrane into the bilayer lipid membrane, we provide in this work evidence for modifications of the mitoBKCa ion permeation properties with channels from vesicles preparations coming from diabetic rats characterized by a significant decrease in conductance. More importantly, the open probability of channels from diabetic rats was reduced 1.5-2.5 fold compared to control, the most significant decrease being observed at depolarizing potentials. Because BKCa ß4 subunit has been documented to left shift the BKCa channel voltage dependence curve in high Ca(2+) conditions, a Western blot analysis was undertaken where the expression of mitoBKCa α and ß4 subunits was estimated using of anti-α and ß4 subunit antibodies. Our results indicated a significant decrease in mitoBKCa ß4 subunit expression coupled to a decrease in the expression of α subunit, an observation compatible with the observed decrease in Ca(2+) sensitivity. Our results thus demonstrate a modification in the mitoBKCa channel gating properties in membrane preparations coming from STZ model of diabetic rats, an effect potentially linked to a change in mitoBKCa ß4 and α subunits expression and/or to an increase in reactive oxygen species production in high glucose conditions.