Depolarization-induced mitochondrial Ca accumulation in sympathetic neurons: spatial and temporal characteristics

Specialized synapse-associated structures within the calyx of Held

The calyx of Held exhibits fast glutamatergic neurotransmission at high rates with low temporal jitter and has adapted specialized synaptic mechanisms to support its functional demands. We report the presence in calyces of an atypical arrangement of subcellular organelles, called the mitochondria-associated adherens complex (MAC). We demonstrate that MACs are located adjacent to synapses and contain membranous elements linked with coated and uncoated vesicles. Mitochondria that form MACs have more complex geometries than other mitochondria within the calyx and can extend between clusters of synapses. We estimate that the calyx contains 1600 MACs, 2400 synapses, and 6200 readily releasable vesicles. We also identify synaptic vesicle endocytotic regions close to MACs and synapses and hypothesize that calyces are composed of multiple activity modules, each containing machinery for vesicle release and recycling.

Limitations of cyclosporin A inhibition of the permeability transition in CNS mitochondria

Activation of the mitochondrial permeability transition may contribute to excitotoxic neuronal death (Ankarcrona et al., 1996; Dubinsky and Levi, 1998). However, cyclosporin A (CsA), a potent inhibitor of the permeability transition in liver mitochondria, only protects against neuronal injury by limited doses of glutamate and selected ischemic paradigms. The lack of consistent CsA inhibition of the mitochondrial permeability transition was analyzed with the use of isolated brain mitochondria. Changes in the permeability of the inner mitochondrial membrane were evaluated by monitoring mitochondrial membrane potential (Deltapsi), using the distribution of tetraphenylphosphonium, and by monitoring mitochondrial swelling, using light absorbance measurements. Metabolic impairments, large Ca(2+) loads, omission of external Mg(2+), or low doses of palmitic acid or the protonophore FCCP exacerbated Ca(2+)-induced sustained depolarizations and swelling and eliminated CsA inhibition. BSA restored CsA inhibition in mitochondria challenged with 50 microm Ca(2+), but not with 100 microm Ca(2+). CsA failed to prevent Ca(2+)-induced depolarization or to repolarize mitochondria when mitochondria were depolarized excessively. Similarly, CsA failed to prevent mitochondrial swelling or PEG-induced shrinkage after swelling when the Ca(2+) challenge produced a strong, sustained depolarization. Thus in brain mitochondria CsA may be effective only as an inhibitor of the permeability transition and the Ca(2+)-activated low permeability state under conditions of partial depolarization. In contrast, ADP plus oligomycin inhibited both permeabilities under all of the conditions that were tested. In situ, the neuroprotective action of CsA may be limited to glutamate challenges sufficiently toxic to induce the permeability transition but not so severe that mitochondrial depolarization exceeds threshold.

Pharmacological investigation of mitochondrial ca(2+) transport in central neurons: studies with CGP-37157, an inhibitor of the mitochondrial Na(+)-Ca(2+) exchanger

Mitochondria buffer large changes in [Ca(2+)](i)following an excitotoxic glutamate stimulus. Mitochondrial sequestration of [Ca(2+)](i)can beneficially stimulate oxidative metabolism and ATP production. However, Ca(2+)overload may have deleterious effects on mitochondrial function and cell survival, particularly Ca(2+)-dependent production of reactive oxygen species (ROS) by the mitochondria. We recently demonstrated that the mitochondrial Na(+)-Ca(2+)exchanger in neurons is selectively inhibited by CGP-37157, a benzothiazepine analogue of diltiazem. In the present series of experiments we investigated the effects of CGP-37157 on mitochondrial functions regulated by Ca(2+). Our data showed that 25 microM CGP-37157 quenches DCF fluorescence similar to 100 microM glutamate and this effect was enhanced when the two stimuli were applied together. CGP-37157 did not increase ROS generation and did not alter glutamate or 3mM hydrogen-peroxide-induced increases in ROS as measured by DHE fluorescence. CGP-37157 induces a slight decrease in intracellular pH, much less than that of glutamate. In addition, CGP-37157 does not enhance intracellular acidification induced by glutamate. Although it is possible that CGP-37157 can enhance mitochondrial respiration both by blocking Ca(2+)cycling and by elevating intramitochondrial Ca(2+), we did not observe any changes in ATP levels or toxicity either in the presence or absence of glutamate. Finally, mitochondrial Ca(2+)uptake during an excitotoxic glutamate stimulus was only slightly enhanced by inhibition of mitochondrial Ca(2+)efflux. Thus, although CGP-37157 alters mitochondrial Ca(2+)efflux in neurons, the inhibition of Na(+)-Ca(2+)exchange does not profoundly alter glutamate-mediated changes in mitochondrial function or mitochondrial Ca(2+)content.

Mitochondria as regulators of stimulus-evoked calcium signals in neurons

An important challenge in the study of Ca2+ signalling is to understand the dynamics of intracellular Ca2+ levels during and after physiological stimulation. While extensive information is available regarding the structural and biophysical properties of Ca2+ channels, pumps and exchangers that control cellular Ca2+ movements, little is known about the quantitative properties of the transporters that are expressed together in intact cells or about the way they operate as a system to orchestrate stimulus-induced Ca2+ signals. This lack of information is particularly striking given that many qualitative properties of Ca2+ signals (e.g. whether the Ca2+ concentration within a particular organelle rises or falls during stimulation) depend critically on quantitative properties of the underlying Ca2+ transporters (e.g. the rates of Ca2+ uptake and release by the organelle). This monograph describes the in situ characterization of Ca2+ transport pathways in sympathetic neurons, showing how mitochondrial Ca2+ uptake and release systems define the direction and rate of net Ca2+ transport by this organelle, and how the interplay between mitochondrial Ca2+ transport and Ca+2 transport across the plasma membrane contribute to depolarization-evoked Ca2+ signals in intact cells.

Stimulation-evoked increases in cytosolic [Ca(2+)] in mouse motor nerve terminals are limited by mitochondrial uptake and are temperature-dependent

Increases in cytosolic [Ca(2+)] evoked by trains of action potentials (20-100 Hz) were recorded from mouse and lizard motor nerve terminals filled with a low-affinity fluorescent indicator, Oregon Green BAPTA 5N. In mouse terminals at near-physiological temperatures (30-38 degrees C), trains of action potentials at 25-100 Hz elicited increases in cytosolic [Ca(2+)] that stabilized at plateau levels that increased with stimulation frequency. Depolarization of mitochondria with carbonylcyanide m-chlorophenylhydrazone (CCCP) or antimycin A1 caused cytosolic [Ca(2+)] to rise to much higher levels during stimulation. Thus, mitochondrial Ca(2+) uptake contributes importantly to limiting the rise of cytosolic [Ca(2+)] during repetitive stimulation. In mouse terminals, the stimulation-induced increase in cytosolic [Ca(2+)] was highly temperature-dependent over the range 18-38 degrees C, with greater increases at lower temperatures. At the lower temperatures, application of CCCP continued to depolarize mitochondria but produced a much smaller increase in the cytosolic [Ca(2+)] transient evoked by repetitive stimulation. This result suggests that the larger amplitude of the stimulation-induced cytosolic [Ca(2+)] transient at lower temperatures was attributable in part to reduced mitochondrial Ca(2+) uptake. In contrast, the stimulation-induced increases in cytosolic [Ca(2+)] measured in lizard motor terminals showed little or no temperature-dependence over the range 18-33 degrees C.

Mitochondria shape hormonally induced cytoplasmic calcium oscillations and modulate exocytosis

Pituitary gonadotropes transduce hormonal input into cytoplasmic calcium ([Ca(2+)](cyt)) oscillations that drive rhythmic exocytosis of gonadotropins. Using Calcium Green-1 and rhod-2 as optical measures of cytoplasmic and mitochondrial free Ca(2+), we show that mitochondria sequester Ca(2+) and tune the frequency of [Ca(2+)](cyt) oscillations in rat gonadotropes. Mitochondria accumulated Ca(2+) rapidly and in phase with elevations of [Ca(2+)](cyt) after GnRH stimulation or membrane depolarization. Inhibiting mitochondrial Ca(2+) uptake by the protonophore CCCP reduced the frequency of GnRH-induced [Ca(2+)](cyt) oscillations or, occasionally, stopped them. Much of the Ca(2+) that entered mitochondria is bound by intramitochondrial Ca(2+) buffering systems. The mitochondrial Ca(2+) binding ratio may be dynamic because [Ca(2+)](mit) appeared to reach a plateau as mitochondrial Ca(2+) accumulation continued. Entry of Ca(2+) into mitochondria was associated with a small drop in the mitochondrial membrane potential. Ca(2+) was extruded from mitochondria more slowly than it entered, and much of this efflux could be blocked by CGP-37157, a selective inhibitor of mitochondrial Na(+)-Ca(2+) exchange. Plasma membrane capacitance changes in response to depolarizing voltage trains were increased when CCCP was added, showing that mitochondria lower the local [Ca(2+)](cyt) near sites that trigger exocytosis. Thus, we demonstrate a central role for mitochondria in a significant physiological response.

Microheterogeneity of calcium signalling in dendrites

Transient changes in the intracellular concentration of free Ca2+ ([Ca2+]i) originating from voltage- or ligand-gated influx and by ligand- or Ca2+-gated release from intracellular stores, trigger or modulate many fundamental neuronal processes, including neurotransmitter release and synaptic plasticity. Of the intracellular compartments involved in Ca2+ clearance, the endoplasmic reticulum (ER) has received the most attention because it expresses Ca2+ pumps and Ca2+ channels, thus endowing it with the potential to act as both an intracellular calcium sink and store. We review here our ongoing work on the role of calcium sequestration into, and release from, ER cisterns and the role that this plays in the generation and termination of free [Ca2+]i transients in dendrites of pyramidal neurons in hippocampal slices during and after synaptic activity. These studies have been approached by combining parallel microfluorometric measurements of free cytosolic [Ca2+]i transients with energy-dispersive X-ray microanalytical measurements of total Ca content within specific dendritic compartments at the electron microscopy level. Our observations support the emerging realization that specific subsets of dendritic ER cisterns provide spatial and temporal microheterogeneity of Ca2+ signalling, acting not only as a major intracellular Ca sink involved in active clearance mechanisms after voltage- and ligand-gated Ca2+ influx, but also as an intracellular Ca2+ source that can be mobilized by a signal cascade originating at activated synapses.

Mitochondrial DNA-depleted neuroblastoma (Rho degrees) cells exhibit altered calcium signaling

To investigate the role of chronic mitochondrial dysfunction on intracellular calcium signaling, we studied basal and stimulated cytosolic calcium levels in SH-SY5Y cells and a derived cell line devoid of mitochondrial DNA (Rho degrees ). Basal cytosolic calcium levels were slightly but significantly reduced in Rho degrees cells. The impact of chronic depletion of mitochondrial DNA was more evident following exposure of cells to carbachol, a calcium mobilizing agent. Calcium transients generated in Rho degrees cells following application of carbachol were more rapid than those in SH-SY5Y cells. A plateau phase of calcium recovery during calcium transients was present in SH-SY5Y cells but absent in Rho degrees cells. The rapid calcium transients in Rho degrees cells were due, in part, to increased reliance on Na(+)/Ca(2+) exchange activity at the plasma membrane and the plateau phase in calcium recovery in SH-SY5Y cells was dependent on the presence of extracellular calcium. We also examined whether mitochondrial DNA depletion influenced calcium responses to release of intracellular calcium stores. Rho degrees cells showed reduced responses to the uncoupler, FCCP, and the sarcoplasmic reticulum calcium ATPase inhibitor, thapsigargin. Acute exposure of SH-SY5Y cells to mitochondrial inhibitors did not mimic the results seen in Rho degrees cells. These results suggest that cytosolic calcium homeostasis in this neuron-like cell line is significantly altered as a consequence of chronic depletion of mitochondrial DNA.

Subcellular localization of glutamate-stimulated intracellular magnesium concentration changes in cultured rat forebrain neurons using confocal microscopy

Glutamate can stimulate increases in intracellular magnesium concentration ([Mg2+]i) and induce neurotoxicity, both independent of Ca2+ changes. Although Mg2+ is essential within the cell, very little is known about how it is regulated, especially in neurons. Therefore we used the fluorescent indicator, magindo-1 and confocal microscopy to examine possible intracellular pools of Mg2+ in cultured neurons that can be dynamically regulated by glutamate. The magindo-1 fluorescence signal was present throughout the cell body and extends into the neuronal processes. The magindo-1 405 nm/490 nm ratio signal was similar in the cytoplasm and nucleus, suggesting that resting [Mg2+]i is uniform across the neuron. The addition of 100 microM glutamate/10 microM glycine in an extracellular Ca2+- and Na+-free buffer stimulated an increase in [Mg2+]i in both the nuclear and cytoplasmic regions of similar magnitude and duration. This glutamate exposure also stimulated a [Mg2+]i increase in neuronal processes which was inhibited by the N-methyl-D-aspartate receptor antagonist, MK-801 (10 microM). The glutamate-stimulated [Mg2+]i increase in both the cell body and neuronal processes was dependent on the extracellular Mg2+ concentration. These findings suggest glutamate-stimulated [Mg2+]i changes may not only impact cytoplasmic processes, but also directly trigger nuclear events involved, for example, in neuronal injury.

Ca(2+)-mediated mitochondrial dysfunction and the protective effects of Bcl-2

Mitochondrial Ca2+ sequestration likely contributes to cell death in excitotoxicity and ischemia reperfusion injury, and may also be involved in chronic forms of neurodegeneration in which a compromise in bioenergetic function alters cellular Ca2+ homeostasis. Bcl-2 overexpression is known to protect against Ca(2+)-mediated death; the mechanism of protection remains unresolved. Our data of the ability of Bcl-2 to potentiate mitochondrial Ca2+ uptake capacity and resistance to Ca(2+)-induced damage is discussed in light of current information on apoptotic signaling pathways.

Muscles express motor patterns of non-innervating neural networks by filtering broad-band input

We describe three slow muscles that responded to low-frequency modulation of a high-frequency neuronal input and, consequently, could express the motor patterns of neural networks whose neurons did not directly innervate the muscles. Two of these muscles responded to different frequency components present in the same input, and as a result each muscle expressed the motor pattern of a different, non-innervating, neural network. In an analogous manner, the distinct dynamics of the multiple intracellular processes that most cells possess may allow each process to respond to, and hence differentiate among, specific frequency ranges present in broad-band input.

Dissection of mitochondrial Ca2+ uptake and release fluxes in situ after depolarization-evoked [Ca2+](i) elevations in sympathetic neurons

We studied how mitochondrial Ca2+ transport influences [Ca2+](i) dynamics in sympathetic neurons. Cells were treated with thapsigargin to inhibit Ca2+ accumulation by SERCA pumps and depolarized to elevate [Ca2+(i); the recovery that followed repolarization was then examined. The total Ca2+ flux responsible for the [Ca2+](i) recovery was separated into mitochondrial and nonmitochondrial components based on sensitivity to the proton ionophore FCCP, a selective inhibitor of mitochondrial Ca2+ transport in these cells. The nonmitochondrial flux, representing net Ca2+ extrusion across the plasma membrane, has a simple dependence on [Ca2+](i), while the net mitochondrial flux (J(mito)) is biphasic, indicative of Ca+) accumulation during the initial phase of recovery when [Ca2+](i) is high, and net Ca2+ release during later phases of recovery. During each phase, mitochondrial Ca2+ transport has distinct effects on recovery kinetics. J(mito) was separated into components representing mitochondrial Ca2+ uptake and release based on sensitivity to the specific mitochondrial Na(+)/Ca2+ exchange inhibitor, CGP 37157 (CGP). The CGP-resistant (uptake) component of J(mito) increases steeply with [Ca2+](i), as expected for transport by the mitochondrial uniporter. The CGP-sensitive (release) component is inhibited by lowering the intracellular Na(+) concentration and depends on both intra- and extramitochondrial Ca2+ concentration, as expected for the Na(+)/Ca2+ exchanger. Above approximately 400 nM [Ca2+](i), net mitochondrial Ca2+ transport is dominated by uptake and is largely insensitive to CGP. When [Ca2+](i) is approximately 200-300 nM, the net mitochondrial flux is small but represents the sum of much larger uptake and release fluxes that largely cancel. Thus, mitochondrial Ca2+ transport occurs in situ at much lower concentrations than previously thought, and may provide a mechanism for quantitative control of ATP production after brief or low frequency stimuli that raise [Ca(2+)](i) to levels below approximately 500 nM.

Ca(2+)- and metabolism-related changes of mitochondrial potential in voltage-clamped CA1 pyramidal neurons in situ

In hippocampal slices from rats, dialysis with rhodamine-123 (Rh-123) and/or fura-2 via the patch electrode allowed monitoring of mitochondrial potential (DeltaPsi) changes and intracellular Ca(2+) ([Ca(2+)](i)) of CA1 pyramidal neurons. Plasmalemmal depolarization to 0 mV caused a mean [Ca(2+)](i) rise of 300 nM and increased Rh-123 fluorescence signal (RFS) by </=50% of control. The evoked RFS, indicating depolarization of DeltaPsi, and the [Ca(2+)](i) transient were abolished by Ca(2+)-free superfusate or exposure of Ni(2+)/Cd(2+). Simultaneous measurements of RFS and [Ca(2+)](i) showed that the kinetics of both the Ca(2+) rise and recovery were considerably faster than those of the DeltaPsi depolarization. The plasmalemmal Ca(2+)/H(+) pump blocker eosin-B potentiated the peak of the depolarization-induced RFS and delayed recovery of both the RFS and [Ca(2+)](i) transient. Thus the DeltaPsi depolarization due to plasmalemmal depolarization is related to mitochondrial Ca(2+) sequestration secondary to Ca(2+) influx through voltage-gated Ca(2+) channels. CN(-) elevated [Ca(2+)](i) by <50 nM but increased RFS by 221% as a result of extensive depolarization of DeltaPsi. Oligomycin decreased RFS by 52% without affecting [Ca(2+)](i). In the presence of oligomycin, CN(-) and p-trifluoromethoxy-phenylhydrazone (FCCP) elevated [Ca(2+)](i) by <50 nM and increased RFS by 285 and 290%, respectively. Accordingly, the metabolism-related DeltaPsi changes are independent of [Ca(2+)](i). Imaging techniques revealed that evoked [Ca(2+)](i) rises are distributed uniformly over the soma and primary dendrites, whereas corresponding changes in RFS occur more localized in subregions within the soma. The results show that microfluorometric measurement of the relation between mitochondrial function and intracellular Ca(2+) is feasible in whole cell recorded mammalian neurons in situ.

Quantitative analysis of mitochondrial Ca2+ uptake and release pathways in sympathetic neurons. Reconstruction of the recovery after depolarization-evoked [Ca2+]i elevations

Rate equations for mitochondrial Ca2+ uptake and release and plasma membrane Ca2+ transport were determined from the measured fluxes in the preceding study and incorporated into a model of Ca2+ dynamics. It was asked if the measured fluxes are sufficient to account for the [Ca2+]i recovery kinetics after depolarization-evoked [Ca2+]i elevations. Ca2+ transport across the plasma membrane was described by a parallel extrusion/leak system, while the rates of mitochondrial Ca2+ uptake and release were represented using equations like those describing Ca2+ transport by isolated mitochondria. Taken together, these rate descriptions account very well for the time course of recovery after [Ca2+]i elevations evoked by weak and strong depolarization and their differential sensitivity to FCCP, CGP 37157, and [Na+]i. The model also leads to three general conclusions about mitochondrial Ca2+ transport in intact cells: (1) mitochondria are expected to accumulate Ca2+ even in response to stimuli that raise [Ca2+]i only slightly above resting levels; (2) there are two qualitatively different stimulus regimes that parallel the buffering and non-buffering modes of Ca2+ transport by isolated mitochondria that have been described previously; (3) the impact of mitochondrial Ca2+ transport on intracellular calcium dynamics is strongly influenced by nonmitochondrial Ca2+ transport; in particular, the magnitude of the prolonged [Ca2+]i elevation that occurs during the plateau phase of recovery is related to the Ca2+ set-point described in studies of isolated mitochondria, but is a property of mitochondrial Ca2+ transport in a cellular context. Finally, the model resolves the paradoxical finding that stimulus-induced [Ca2+]i elevations as small as approximately 300 nM increase intramitochondrial total Ca2+ concentration, but the steady [Ca2+]i elevations evoked by such stimuli are not influenced by FCCP.

The role of mitochondria in presynaptic calcium handling at a ribbon synapse

Mitochondria are thought to be important in clearing calcium from synaptic terminals. It is unclear, however, whether the principal role of mitochondria in pre-synaptic calcium handling is to take up Ca2+ directly or to fuel Ca2+ removal by other mechanisms. We used patch clamp techniques and fluorescence imaging to examine calcium clearance mechanisms, including mitochondrial uptake, in single synaptic terminals of retinal bipolar neurons. We found that extrusion through the ATP-dependent Ca2+ pump of the plasma membrane is the dominant form of Ca2+ removal in the synaptic terminal. Calcium uptake into mitochondria was sometimes evident with large Ca2+ loads but was consistently observed only when plasma membrane extrusion was inhibited. We conclude that mitochondria act primarily as an energy source in clearance of Ca2+ from bipolar cell synaptic terminals.