Voltage-dependent inwardly rectifying potassium conductance in the outer membrane of neuronal mitochondria.

Potassium fluxes integrate mitochondria into cellular activities, controlling their volume homeostasis and structural integrity in many pathophysiological mechanisms. The outer mitochondrial membrane (OMM) is thought to play a passive role in this process because K(+) is believed to equilibrate freely between the cytosol and mitochondrial intermembrane space. By patch clamping mitochondria isolated from the central nervous systems of adult mitoCFP transgenic mice, we discovered the existence of I(OMMKi), a novel voltage-dependent inwardly rectifying K(+) conductance located in the OMM. I(OMMKi) is regulated by osmolarity, potentiated by cAMP, and activated at physiological negative potentials, allowing K(+) to enter the mitochondrial intermembrane space in a controlled regulated fashion. The identification of I(OMMKi) in the OMM supports the notion that a membrane potential could exist across this membrane in vivo and suggests that the OMM possesses regulated pathways for K(+) uptake.

Postnatal development of membrane excitability in taste cells of the mouse vallate papilla.

The mammalian peripheral taste system undergoes functional changes during postnatal development. These changes could reflect age-dependent alterations in the membrane properties of taste cells, which use a vast array of ion channels for transduction mechanisms. Yet, scarce information is available on the membrane events in developing taste cells. We have addressed this issue by studying voltage-dependent Na+, K+, and Cl- currents (I(Na), I(K), and I(Cl), respectively) in a subset of taste cells (the so-called "Na/OUT" cells, which are electrically excitable and thought to be sensory) from mouse vallate papilla. Voltage-dependent currents play a key role during taste transduction, especially in the generation of action potentials. Patch-clamp recordings revealed that I(Na), I(K), and I(Cl) were expressed early in postnatal development. However, only I(K) and I(Cl) densities increased significantly in developing Na/OUT cells. Consistent with the rise of I(K) density, we found that action potential waveform changed markedly, with an increased speed of repolarization that was accompanied by an enhanced capability of repetitive firing. In addition to membrane excitability changes in putative sensory cells, we observed a concomitant increase in the occurrence of glia-like taste cells (the so called "leaky" cells) among patched cells. Leaky cells are likely involved in dissipating the increase of extracellular K+ during action potential discharge in chemosensory cells. Thus, developing taste cells of the mouse vallate papilla undergo a significant electrophysiological maturation and diversification. These functional changes may have a profound impact on the transduction capabilities of taste buds during development.

Channels as taste receptors in vertebrates.

Taste reception is fundamental for proper selection of food and beverages. Chemicals detected as taste stimuli by vertebrates include a large variety of substances, ranging from inorganic ions (e.g., Na(+), H(+)) to more complex molecules (e.g., sucrose, amino acids, alkaloids). Specialized epithelial cells, called taste receptor cells (TRCs), express specific membrane proteins that function as receptors for taste stimuli. Classical view of the early events in chemical detection was based on the assumption that taste substances bind to membrane receptors in TRCs without permeating the tissue. Although this model is still valid for some chemicals, such as sucrose, it does not hold for small ions, such as Na(+), that actually diffuse inside the taste tissue through ion channels. Electrophysiological, pharmacological, biochemical, and molecular biological studies have provided evidence that indeed TRCs use ion channels to reveal the presence of certain substances in foodstuff. In this review, we focus on the functional and molecular properties of ion channels that serve as receptors in taste transduction.

Activity of the mitochondrial calcium uniporter varies greatly between tissues.

The mitochondrial calcium uniporter is a highly selective channel responsible for mitochondrial Ca(2+) uptake. The mitochondrial calcium uniporter shapes cytosolic Ca(2+) signals, controls mitochondrial ATP production, and is involved in cell death. Here using direct patch-clamp recording from the inner mitochondrial membrane, we compare mitochondrial calcium uniporter activity in mouse heart, skeletal muscle, liver, kidney and brown fat. Surprisingly, heart mitochondria show a dramatically lower mitochondrial calcium uniporter current density than the other tissues studied. Similarly, in Drosophila flight muscle, mitochondrial calcium uniporter activity is barely detectable compared with that in other fly tissues. As mitochondria occupy up to 40% of the cell volume in highly metabolically active heart and flight muscle, low mitochondrial calcium uniporter activity is likely essential to avoid cytosolic Ca(2+) sink due to excessive mitochondrial Ca(2+) uptake. Simultaneously, low mitochondrial calcium uniporter activity may also prevent mitochondrial Ca(2+) overload in such active tissues exposed to frequent cytosolic Ca(2+) activity.

Electrophysiological heterogeneity in a functional subset of mouse taste cells during postnatal development.

Taste cells in adult mammals are functionally heterogeneous as to the expression of ion channels. How these adult phenotypes are established during postnatal development, however, is not yet clear. We have addressed this issue by studying voltage-gated K(+) and Cl(-) currents (I(K) and I(Cl), respectively) in developing taste cells of the mouse vallate papilla. I(K) and I(Cl) underlie action potential waveform and firing properties, and play an important role in taste transduction. By using the patch clamp technique, we analyzed these currents in a specific group of cells, called Na/OUT cells and thought to be sensory. In adult mice, three different electrophysiological phenotypes of Na/OUT cells could be detected: cells with I(K) (K cells); cells with both I(K) and I(Cl) (K+Cl cells); and cells with I(Cl) (Cl cells). In contrast, at early developmental stages (2-4 postnatal days, PD) there were no Cl cells, which appeared at PD 8. Our findings indicate a mechanism that contributes to building-up the functional heterogeneity of mammalian taste cells during the postnatal development.

Apical and basal neurones isolated from the mouse vomeronasal organ differ for voltage-dependent currents.

The mammalian vomeronasal organ (VNO) contains specialized neurones that transduce the chemical information related to pheromones into discharge of action potentials to the brain. Molecular and biochemical studies have shown that specific components of the pheromonal transduction systems are segregated into two distinct subsets of vomeronasal neurones: apical neurones and basal neurones. However, it is still unknown whether these neuronal subsets also differ in other functional characteristics, such as their membrane properties. We addressed this issue by studying the electrophysiological properties of vomeronasal neurones isolated from mouse VNO. We used the patch-clamp technique to examine both the passive membrane properties and the voltage-gated Na+, K+ and Ca2+ currents. Apical neurones were distinguished from basal ones by the length of their dendrites and by their distinct immunoreactivity for the putative pheromone receptor V2R2. The analysis of passive properties revealed that there were no significant differences between the two neuronal subsets. Also, apical neurones were similar to basal neurones in their biophysical and pharmacological properties of voltage-gated Na+ and K+ currents. However, we found that the density of Na+ currents was about 2-3 times greater in apical neurones than in basal neurones. Consistently, in situ hybridization analysis revealed a higher expression of the Na+ channel subtype III in apical neurones than in basal ones. In contrast, basal neurones were endowed with Ca2+ currents (T-type) of greater magnitude than apical neurones. Our findings indicate that apical and basal neurones in the VNO exhibit distinct electrical properties. This might have a profound effect on the sensory processes occurring in the VNO during pheromone detection.

Ion conductances in supporting cells isolated from the mouse vomeronasal organ.

The vomeronasal organ (VNO) is a chemosensory structure involved in the detection of pheromones in most mammals. The VNO sensory epithelium contains both neurons and supporting cells. Data suggest that vomeronasal neurons represent the pheromonal transduction sites, whereas scarce information is available on the functional properties of supporting cells. To begin to understand their role in VNO physiology, we have characterized with patch-clamp recording techniques the electrophysiological properties of supporting cells isolated from the neuroepithelium of the mouse VNO. Supporting cells were distinguished from neurons by their typical morphology and by the lack of immunoreactivity for Ggamma8 and OMP, two specific markers for vomeronasal neurons. Unlike glial cells in other tissues, VNO supporting cells exhibited a depolarized resting potential (about -29 mV). A Goldman-Hodgkin-Katz analysis for resting ion permeabilities revealed indeed an unique ratio of P(K):P(Na):P(Cl) = 1:0.23:1.4. Supporting cells also possessed voltage-dependent K(+) and Na(+) conductances that differed significantly in their biophysical and pharmacological properties from those expressed by VNO neurons. Thus glial membranes in the VNO can sustain significant fluxes of K(+) and Na(+), as well as Cl(-). This functional property might allow supporting cells to mop-up and redistribute the excess of KCl and NaCl that often occurs in certain pheromone-delivering fluids, like urine, and that could blunt the sensitivity of VNO neurons to pheromones. Therefore vomeronasal supporting cells could affect chemosensory transduction in the VNO by regulating the ionic strength of the pheromone-containing medium.