Differential distribution of Ca2+-activated potassium channel β4 subunit in rat brain: Immunolocalization in neuronal mitochondria

Large conductance Ca2+-activated potassium channels (BKCa channels) are expressed in the plasma membrane of various cell types. Interestingly, recent studies provided evidence for the existence of BKCa channels also in mitochondria. However, the molecular composition of these channels as well as their cellular and tissue distribution is still unknown. The goal of the present study was to find a candidate for the regulatory component of the mitochondrial large conductance calcium activated potassium (mitoBKCa) channel in neurons. A combined approach of Western blot analysis, high-resolution immunofluorescence and immunoelectron microscopy with the use of antibodies directed against four distinct β subunits demonstrated the presence of the BKCa channel β4 subunit (KCNMB4) in the inner membrane of neuronal mitochondria in the rat brain and cultured neurons. Within the cell, the expression of β4 subunit was restricted to a subpopulation of mitochondria. The analysis of β4 subunit distribution throughout the brain revealed that the highest expression levels occur in the thalamus and the brainstem. Our results suggest that β4 subunit is a regulatory component of mitochondrial BKCa channels in neurons. These findings may support the perspectives for the neuroprotective role of mitochondrial BKCa channel in specific brain structures.