High resolution ultrastructural mapping of total calcium: electron spectroscopic imaging/electron energy loss spectroscopy analysis of a physically/chemically processed nerve-muscle preparation

Effects of intravesicular loading of a Ca2+ chelator and depolymerization of actin fibers on neurotransmitter release in frog motor nerve terminals

Ca²⁺ -induced Ca²⁺ release (CICR) via type-3 ryanodine receptor enhances neurotransmitter release in frog motor nerve terminals. To test a possible role of synaptic vesicle in CICR, we examined the effects of loading of EGTA, a Ca²⁺ chelator, into synaptic vesicles and depolymerization of actin fibers. Intravesicular EGTA loading via endocytosis inhibited the ryanodine sensitive enhancement of transmitter release induced by tetanic stimulation and the associated rises in intracellular-free Ca²⁺ ([Ca²⁺ ]_i : Ca²⁺ transients). Latrunculin A, a depolymerizer of actin fibers, enhanced both spontaneous and stimulation-induced transmitter release, but inhibited the enhancement of transmitter release elicited by successive tetanic stimulation. The results suggest a possibility that the activation of CICR from mobilized synaptic vesicles caused the enhancement of neurotransmitter release.

Calcium-Sensing Receptor: A Key Target for Extracellular Calcium Signaling in Neurons

Though both clinicians and scientists have long recognized the influence of extracellular calcium on the function of muscle and nervous tissue, recent insights reveal that the mechanisms allowing changes in extracellular calcium to alter cellular excitability have been incompletely understood. For many years the effects of calcium on neuronal signaling were explained only in terms of calcium entry through voltage-gated calcium channels and biophysical charge screening. More recently however, it has been recognized that the calcium-sensing receptor is prevalent in the nervous system and regulates synaptic transmission and neuronal activity via multiple signaling pathways. Here we review the multiplicity of mechanisms by which changes in extracellular calcium alter neuronal signaling and propose that multiple mechanisms are required to describe the full range of experimental observations.

Iron sources used by the nonpathogenic lactic acid bacterium Lactobacillus sakei as revealed by electron energy loss spectroscopy and secondary-ion mass spectrometry

Lactobacillus sakei is a lactic acid bacterium naturally found on meat. Although it is generally acknowledged that lactic acid bacteria are rare species in the microbial world which do not have iron requirements, the genome sequence of L. sakei 23K has revealed quite complete genetic equipment dedicated to transport and use of this metal. Here, we aimed to investigate which iron sources could be used by this species as well as their role in the bacterium's physiology. Therefore, we developed a microscopy approach based on electron energy loss spectroscopy (EELS) analysis and nano-scale secondary-ion mass spectrometry (SIMS) in order to analyze the iron content of L. sakei cells. This revealed that L. sakei can use iron sources found in its natural ecosystem, myoglobin, hemoglobin, hematin, and transferrin, to ensure long-term survival during stationary phase. This study reveals that analytical image methods (EELS and SIMS) are powerful complementary tools for investigation of metal utilization by bacteria.

The relationship between calcium accumulation in osteoclast mitochondrial granules and bone resorption

In the process of bone resorption, calcium is considered to be transported within vesicles in osteoclasts and eventually released. We studied the ultramicromorphology of calcium (Ca) transport in osteoclasts by preparing samples of osteoclasts collected from rat femurs in which calcium was maximally preserved and subjected them to high-pressure quick-freezing and freeze-substitution. We then examined the localization of calcium by Electron Energy Loss Spectroscopy (EELS). The structures of cell membranes were preserved, suggesting the suitability of this high-pressure quick-freezing and freeze-substitution technique. Osteoclast mitochondria adjacent to the ruffled border were rich in mitochondrial granules and contained a large amount of Ca. In contrast, mitochondria in the basolateral region contained few granules. Moreover, by an osteoclast-culturing experiment, differences in the morphology of mitochondrial granules were noted between culturing on a dentin slice and that on a gold plate, i.e., few mitochondrial granules were noted in osteoclasts cultured on a non-dentin plate. These findings suggest an association between the morphology of mitochondrial granules in osteoclasts and bone resorption as well as a new transport route for Ca resorbed by osteoclasts. We propose that Ca accumulates in mitochondria granules to prevent increased Ca concentration in cytoplasm of osteoclasts during bone resorption.

In vivo low-density bone apposition on different implant surface materials

During osseointegration, new bone may be laid down on the implant surface and/or on the old bone surface; the former is known as contact osteogenesis and the latter as distance osteogenesis. Implant surface topography and material composition affect this process. The present study evaluates Ca and P apposition onto three different dental implant material surfaces (carbon monoxide (CO) ion implantation on Ti6Al4V, sand blasting and acid etching on commercially pure titanium and untreated Ti6Al4V) on the mandibles of beagles after healing periods of 3 and 6 months. Energy dispersive spectroscopy is useful for identifying low-density bone relative to surrounding mature bone, allowing for discrimination of the osteogenesis source. Low-density bone was only found at the apical end; there was none on the surface of untreated implants. Low-density bone arising from mature bone towards the implant at month 3 (i.e. distance osteogenesis) was only present on the CO ion implanted samples, due to the modification of the surface nano-topography and the chemistry and structure of the material.

How to measure Ca2+ in cellular organelles?

The review will aim to briefly summarise information on calcium measurements in cellular organelles with emphases on studies conducted in live cells using optical probes. When appropriate we will try to compare the effectiveness of different indicators for intraorganellar calcium measurements. We will consider calcium measurements in endoplasmic reticulum, Golgi apparatus, endosomes/lysosomes, nucleoplasm, nuclear envelope, mitochondria and secretory granules.

Calnuc binds to Alzheimer's beta-amyloid precursor protein and affects its biogenesis

Calnuc, a Golgi calcium binding protein, plays a key role in the constitution of calcium storage. Abnormal calcium homeostasis has been linked to Alzheimer's disease (AD). Excessive production and/or accumulation of beta-amyloid (Abeta) peptides that are proteolytically derived from the beta-amyloid precursor protein (APP) have been linked to the pathogenesis of AD. APP has also been indicated to play multiple physiological functions. In this study, we demonstrate that calnuc interacts with APP through direct binding to the carboxyl-terminal region of APP, possibly in a calcium-sensitive manner. Immunofluorescence study revealed that the two proteins co-localize in the Golgi in both cultured cells and mouse brains. Over-expression of calnuc in neuroblastoma cells significantly reduces the level of endogenous APP. Conversely, down-regulation of calnuc by siRNA increases cellular levels of APP. Additionally, we show that over-expression of calnuc down-regulates the APP mRNA level and inhibits APP biosynthesis, which in turn results in a parallel reduction of APP proteolytic metabolites, sAPP, CTFs and Abeta. Furthermore, we found that the level of calnuc was significantly decreased in the brain of AD patients as compared with that of age-matched non-AD controls. Our results suggest a novel function of calnuc in modulating the levels of APP and its proteolytic metabolites, which may further affect the patho/physiological functions of APP including AD pathogenesis.

Imaging and speciation of trace elements in biological environment

Mineral elements, often at the trace level, play a considerable role in physiology and pathology of biological systems. Metallogenomics, metalloproteomics, and metallomics are among the emerging disciplines which are critically dependent on spatially resolved concentration maps of trace elements in a cell or tissue, on information on chemical speciation, and on that on metal-binding coordination sites. The mini-review discusses recent progress in analytical techniques for element profiling on the genome scale, biological trace element imaging, and probing, identification and quantification of chemical species in the biological environment. Imaging of the element distribution in cells and tissue sections is becoming possible with sub-micrometer spatial resolution and picogram-level sensitivity owing to advances in laser ablation MS, ion beam and synchrotron radiation X-ray fluorescence microprobes. Progress in nanoflow chromatography and capillary electrophoresis coupled with element specific ICP MS and molecule-specific electrospray MS/MS and MALDI enables speciation of elements in microsamples in a complex biological environment. Laser ablation ICP MS, micro-SXRF, and micro-PIXE allow mapping of trace element distribution in 1D and 2D proteomics gels. The increasing sensitivity of EXAFS and XANES owing to the use of more intense synchrotron beams and efficient focusing optics provide information about oxidation state, fingerprint speciation of metal sites and metal-site structures.

The role of extracellular calcium in exo- and endocytosis of synaptic vesicles at the frog motor nerve terminals

In the present study we combined FM 1-43 imaging and electrophysiological recording of miniature end-plate currents (MEPCs) to determine the role of extracellular calcium in synaptic vesicle exo- and endocytosis at the frog motor nerve terminals. We replaced extracellular Ca2+ ions with other bivalent cations (Sr2+, Ba2+, Cd2+, Mg2+) or used a calcium-free solution and monitored fluorescent staining of the nerve terminals in the presence of caffeine, which promotes the release of Ca2+ from intracellular stores. Caffeine has induced FM1-43 internalization only in the presence of bivalent cations in the external solution. The exposure of the neuromuscular junction to caffeine in a calcium-free solution caused a reversible failure of FM 1-43 loading and an increase in the nerve terminal width. This effect of a calcium-free solution was not due to a decrease in exocytosis, because caffeine-induced FM1-43 unloading from the previously loaded nerve terminals, as well as a degree of the MEPCs frequency increase, was unchanged. We conclude that the presence of Ca2+ or other bivalent cations in extracellular space is necessary for endocytosis but not for exocytosis of synaptic vesicles, while transmitter release is promoted by efflux of Ca2+ from intracellular stores. The effect of extracellular Ca2+ on endocytosis might be driven by the non-specific interactions with membrane lipids.

A store-operated Ca(2+) influx pathway in the bag cell neurons of Aplysia

Although store-operated Ca(2+) influx has been well-studied in nonneuronal cells, an understanding of its nature in neurons remains poor. In the bag cell neurons of Aplysia californica, prior work has suggested that a Ca(2+) entry pathway can be activated by Ca(2+) store depletion. Using fura-based imaging of intracellular Ca(2+) in cultured bag cell neurons, we now characterize this pathway as store-operated Ca(2+) influx. In the absence of extracellular Ca(2+), the endoplasmic reticulum Ca(2+)-ATPase inhibitors, cyclopiazonic acid (CPA) or thapsigargin, depleted intracellular stores and elevated intracellular free Ca(2+). With the subsequent addition of extracellular Ca(2+), a prominent Ca(2+) influx was observed. The ryanodine receptor agonist, chloroethylphenol (CEP), also increased intracellular Ca(2+) but did not initiate store-operated Ca(2+) influx, despite overlap between CEP- and CPA-sensitive stores. Bafilomycin A, a vesicular H(+)-ATPase inhibitor, liberated intracellular Ca(2+) from acidic stores and attenuated subsequent Ca(2+) influx, presumably by replenishing CPA-depleted stores. Store-operated Ca(2+) influx was partially blocked by low concentrations of La(3+) or BTP2, and strongly inhibited by either 1-[b-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF-96365) or a high concentration of Ni(2+). Regarding IP(3) receptor blockers, 2-aminoethyldiphenyl borate, but not xestospongin C, prevented store-operated Ca(2+) influx. However, jasplakinolide, an actin stabilizer reported to inhibit this pathway in smooth muscle cell lines, was ineffective. The bag cell neurons initiate reproductive behavior through a prolonged afterdischarge associated with intracellular Ca(2+) release and neuropeptide secretion. Store-operated Ca(2+) influx may serve to replenish stores depleted during the afterdischarge or participate in the release of peptide that triggers behavior.

Type-3 ryanodine receptor involved in Ca2+-induced Ca2+ release and transmitter exocytosis at frog motor nerve terminals

Ca(2+)-induced Ca2+ release (CICR) occurs in frog motor nerve terminals after ryanodine receptors (RyRs) are primed for activation by conditioning large Ca2+ entry. We studied which type of RyR exists, whether CICR occurs without conditioning Ca2+ entry and how RyRs are primed. Immunohistochemistry revealed the existence of RyR3 in motor nerve terminals and axons and both RyR1 and RyR3 in muscle fibers. A blocker of RyR, 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) slightly decreased rises in intracellular Ca2+ ([Ca2+]i) induced by a short tetanus (50 Hz, 1-2s), but not after treatment with ryanodine. Repetitive tetani (50 Hz for 15s every 20s) produced repetitive rises in [Ca2+]i, whose amplitude overall waxed and waned. TMB-8 blocked the waxing and waning components. Ryanodine suppressed a slow increase in end-plate potentials (EPPs) induced by stimuli (33.3 Hz, 15s) in a low Ca2+, high Mg2+ solution. KN-62, a blocker of Ca(2+)/calmoduline-activated protein kinase II (CaMKII), slightly reduced short tetanus-induced rises in [Ca2+]i, but markedly the slow waxing and waning rises produced by repetitive tetani in both normal and low Ca2+, high Mg2+ solutions. Likewise, KN-62, but not KN-04, an inactive analog, suppressed slow increases in EPP amplitude and miniature EPP frequency during long tetanus. Thus, CICR normally occurs weakly via RyR3 activation by single impulse-induced Ca2+ entry in frog motor nerve terminals and greatly after the priming of RyR via CaMKII activation by conditioning Ca2+ entry, thus, facilitating transmitter exocytosis and its plasticity.

Sublethal dose of 4-hydroxynonenal reduces intracellular calcium in surviving motor neurons in vivo

4-Hydroxynonenal (4-HNE), a major lipid peroxidation product, induces oxidative stress, acts as an autonomous effector of cell death in motor neuron hybrid cell cultures, and is elevated in the cerebrospinal fluid (CSF) of patients with amyotrophic lateral sclerosis (ALS). Elevation of the total intracellular calcium has also been demonstrated in motor axon terminals of ALS patients as well as in spinal motor neurons of animal models of familial and sporadic ALS. Since the association of intracellular calcium and oxidative stress has been suggested in ALS, the in vivo effect of intrathecally administered 4-HNE on the motor neuronal calcium level was examined in the spinal cord of rats. After 12 days of treatment, total intracellular calcium was assayed by electron microscopic histochemistry using the oxalate-pyroantimonate method. Morphology of spinal motor neurons was characterized by light and electron microscopy. In rats, 4-HNE treatment induced a mild impairment of gait, elevation of 4-HNE in the CSF, loss of spinal motor neurons, and reduction of total calcium in the surviving, structurally intact motor neurons. 4-HNE could only cause a lesion if glutathione synthesis was concomitantly inhibited in the animals. The results suggest that upstream components of the oxidative injury in relation to lipid peroxidation are necessary to compromise the glutathione system in ALS, allowing an increase of 4-HNE in the CSF, which further aggravates the primary oxidative lesion. The reduced intracellular calcium in the surviving motor neurons with no morphological features of degeneration may reflect an impaired ionic homeostasis, which may indicate a residual damage of an incomplete degenerative process.

Direct visualization of intracellular calcium in rat osteoblasts by energy-filtering transmission electron microscopy

Osteoblasts are the highly specialized bone cells responsible for matrix mineralization. Mineralization is a complex, incompletely understood, process involving intracellular calcium homeostasis. Rapid changes in ionized calcium concentration ([Ca(2+)](i)) occur in these cells, but the intracellular distribution of total calcium, which may be involved in matrix mineralization, remains unknown. We have therefore investigated the distribution of total calcium in osteoblasts either ex vivo from rapidly mineralizing neonatal rat bones or in the same cells cultured to confluence before they had entered the mineralization phase, and without stimulation for mineralized matrix formation. All cells were examined bone-untreated (controls) or following the addition of the ionophore ionomycin that induced a large and sustained increase in [Ca(2+)](i). Cryomethods, quick-freezing and freeze-drying, and OsO(4) vapor fixation were employed to preserve the original calcium distribution, and the preservation was verified by secondary ion mass spectrometry (SIMS). Intracellular calcium distribution was identified by energy-filtering transmission electron microscopy (EELS). Scarce calcium signals were recorded from all osteoblasts maintained in buffer (controls). Ionomycin addition resulted in the accumulation of calcium in mitochondria, and more calcium was stored in the mitochondria of osteoblasts involved in mineralization than in those of osteoblasts before mineralization. Moreover, in the former, strong calcium signals were recorded around the junctions between mitochondria and the endoplasmic reticulum. Thus EELS allowed to obtain high-resolution total calcium maps in defined intracellular structures, but only at elevated calcium levels.

My favorite cell--Paramecium

A Paramecium cell has a stereotypically patterned surface, with regularly arranged cilia, dense-core secretory vesicles and subplasmalemmal calcium stores. Less strikingly, there is also a patterning of molecules; for instance, some ion channels are restricted to certain regions of the cell surface. This design may explain very effective and selective responses, such as that to Ca(2+) upon stimulation. It enables the cell to respond to a Ca(2+) signal precisely secretion (exocytosis) or by changing its ciliary activity. These responses depend on the location and/or type of signal, even though these two target structures co-exist side-by-side, and normally only limited overlap occurs between the different functions. Furthermore, the patterning of exocytotic sites and the possibility of synchronous exocytosis induction in the sub-second time range have considerably facilitated analyses, and thus led to new concepts of exocytotic membrane fusion. It has been possible to dissect complicated events like overlapping Ca(2+) fluxes produced from external sources and from internal stores. Since molecular genetic approaches have become available for Paramecium, many different gene products have been identified only some of which are known from "higher" eukaryotes. Although a variety of basic cellular functions are briefly addressed to demonstrate the uniqueness of this unicellular organism, this article focuses on exocytosis regulation.

Ca(2+) shuttling in vesicles during tip growth in Neurospora crassa

Tip-growing organisms maintain an apparently essential tip-high gradient of cytoplasmic Ca(2+). In the oomycete Saprolegnia ferax, in pollen tubes and root hairs, the gradient is produced by a tip-localized Ca(2+) influx from the external medium. Such a gradient is normally dispensable for Neurospora crassa hyphae, which may maintain their Ca(2+) gradient by some form of internal recycling. We localized Ca(2+) in N. crassa hyphae at the ultrastructural level using two techniques (a) electron spectroscopic imaging of freeze-dried hyphae and (b) pyroantimoniate precipitation. The results of both methods support the presence of Ca(2+) in the wall vesicles and Golgi body equivalents, providing a plausible mechanism for the generation and maintenance of the gradient by Ca(2+) shuttling in vesicles to the apex, without exogenous Ca(2+) influx. Ca(2+) sequestration into the vesicles seems to be dependent on Ca(2+)-ATPases since cyclopiazonic acid, a specific inhibitor of Ca(2+) pumps, eliminated all Ca(2+) deposits from the vesicles of N. crassa.

Ultra rapid calcium events in electrically stimulated frog nerve terminals

Fast calcium events occurring in cytoplasmic organelles after a single electrical stimulus were investigated by electron spectroscopic imaging (an electron microscope technique that reveals total calcium with high sensitivity and spatial resolution) in quick frozen presynaptic terminals of the frog neuromuscular junction. In resting preparations synaptic vesicles showed a prominent calcium signal whereas mitochondria were mostly negative and only some of the cisternae of the endoplasmic reticulum were clearly positive. In preparations quick frozen 10 ms after the application to the nerve of a single, supramaximal electric stimulus, no obvious change was observed in synaptic vesicles, while calcium levels rose to high values in the endoplasmic reticulum cisternae and in the matrix of mitochondria. Voltage-induced influx of Ca(2+) within synaptic terminals appears therefore to induce an extremely rapid uptake into selected organelles. The possible physiological role of this response is discussed.

Total calcium ultrastructure: advances in excitable cells

This is a review which is written on the basis of a cell calcium lecture delivered on 22 July 2000 at the European Research Meeting 'Calcium as a molecule of cellular integration'.

Differential regulation of transmitter release by presynaptic and glial Ca2+ internal stores at the neuromuscular synapse

The differential regulation of synaptic transmission by internal Ca(2+) stores of presynaptic terminals and perisynaptic Schwann cells (PSCs) was studied at the frog neuromuscular junction. Thapsigargin (tg), an inhibitor of Ca(2+)-ATPase pumps of internal stores, caused a transient Ca(2+) elevation in PSCs, whereas it had no effect on Ca(2+) stores of presynaptic terminals at rest. Tg prolonged presynaptic Ca(2+) responses evoked by single action potentials with no detectable increase in the resting Ca(2+) level in nerve terminals. However, Ca(2+) accumulation was observed during high frequency stimulation. Tg induced a rapid rise in endplate potential (EPP) amplitude, accompanied by a delayed and transient increase. The effects appeared presynaptic, as suggested by the lack of effects of tg on the amplitude and time course of miniature EPPs (MEPPs). However, MEPP frequency was increased when preparations were stimulated tonically (0.2 Hz). The delayed and transient increase in EPP amplitude was occluded by injections of the Ca(2+) chelator BAPTA into PSCs before tg application, whereas a rise in intracellular Ca(2+) in PSCs induced by inositol 1,4,5-triphosphate (IP(3)) injections potentiated transmitter release. Furthermore, increased Ca(2+) buffering capacity after BAPTA injection in PSCs resulted in a more pronounced synaptic depression induced by high frequency stimulation of the motor nerve (10 Hz/80 sec). It is concluded that presynaptic Ca(2+) stores act as a Ca(2+) clearance mechanism to limit the duration of transmitter release, whereas Ca(2+) release from glial stores initiates Ca(2+)-dependent potentiation of synaptic transmission.

Contribution of cryofixation and freeze-substitution to analytical microscopy: a study of Tritrichomonas foetus hydrogenosomes

The hydrogenosome, an organelle that produces molecular hydrogen and ATP from the oxidation of pyruvate or malate under anaerobic conditions, presents some characteristics common to mitochondria. It is found in several trichomonad species, protists living in oxygen-poor environments, as well as certain free-living ciliates, rumen ciliates, and some fungi. We performed a comparative microanalytical study (energy dispersive X-ray analysis and electron spectroscopic imaging) of different fixation methods for electron microscopy analyzing hydrogenosomes of the bovine parasite Tritrichomonas foetus. The study included the elemental composition and the mapping of calcium, phosphorus, and oxygen. A preparation of T. foetus cells, based on cryoimmobilization by high-pressure freezing and freeze-substitution, was compared to a second preparation based on chemical fixation followed by dehydration and routine processing. The ultrastructural preservation achieved by the cryotechnique was far superior to the chemical fixation, since it allowed the successful cryoimmobilization of intracellular ion contents. The detection of several cations (Al, Mg, Co, Ca, Fe) by X-ray microanalysis inside the peripheral vesicle of the hydrogenosome was only possible in cryofixed cells. The presence of aluminum and cobalt ion in the hydrogenosomal vesicle was established for the first time. Electron-spectroscopic images of calcium showed that this element, in addition to the vesicle compartment, is present in the hydrogenosome's membrane in varying concentrations, which might reflect changes in the physiology of this organelle.

Differential regulation of transmitter release by presynaptic and glial Ca2+ internal stores at the neuromuscular synapse

The differential regulation of synaptic transmission by internal Ca(2+) stores of presynaptic terminals and perisynaptic Schwann cells (PSCs) was studied at the frog neuromuscular junction. Thapsigargin (tg), an inhibitor of Ca(2+)-ATPase pumps of internal stores, caused a transient Ca(2+) elevation in PSCs, whereas it had no effect on Ca(2+) stores of presynaptic terminals at rest. Tg prolonged presynaptic Ca(2+) responses evoked by single action potentials with no detectable increase in the resting Ca(2+) level in nerve terminals. However, Ca(2+) accumulation was observed during high frequency stimulation. Tg induced a rapid rise in endplate potential (EPP) amplitude, accompanied by a delayed and transient increase. The effects appeared presynaptic, as suggested by the lack of effects of tg on the amplitude and time course of miniature EPPs (MEPPs). However, MEPP frequency was increased when preparations were stimulated tonically (0.2 Hz). The delayed and transient increase in EPP amplitude was occluded by injections of the Ca(2+) chelator BAPTA into PSCs before tg application, whereas a rise in intracellular Ca(2+) in PSCs induced by inositol 1,4,5-triphosphate (IP(3)) injections potentiated transmitter release. Furthermore, increased Ca(2+) buffering capacity after BAPTA injection in PSCs resulted in a more pronounced synaptic depression induced by high frequency stimulation of the motor nerve (10 Hz/80 sec). It is concluded that presynaptic Ca(2+) stores act as a Ca(2+) clearance mechanism to limit the duration of transmitter release, whereas Ca(2+) release from glial stores initiates Ca(2+)-dependent potentiation of synaptic transmission.

alpha-Latrotoxin releases calcium in frog motor nerve terminals

alpha-Latrotoxin (alpha-LTX) is a neurotoxin that accelerates spontaneous exocytosis independently of extracellular Ca(2+). Although alpha-LTX increases spontaneous transmitter release at synapses, the mechanism is unknown. We tested the hypothesis that alpha-LTX causes transmitter release by mobilizing intracellular Ca(2+) in frog motor nerve terminals. Transmitter release was measured electrophysiologically and with the vesicle marker FM1-43; presynaptic ion concentration dynamics were measured with fluorescent ion-imaging techniques. We report that alpha-LTX increases transmitter release after release of a physiologically relevant concentration of intracellular Ca(2+). Neither the blockade of Ca(2+) release nor the depletion of Ca(2+) from endoplasmic reticulum affected Ca(2+) signals produced by alpha-LTX. The Ca(2+) source is likely to be mitochondria, because the effects on Ca(2+) mobilization of CCCP (which depletes mitochondrial Ca(2+)) and of alpha-LTX are mutually occlusive. The release of mitochondrial Ca(2+) is partially attributable to an increase in intracellular Na(+), suggesting that the mitochondrial Na(+)/Ca(2+) exchanger is activated. Effects of alpha-LTX were not blocked when Ca(2+) increases were reduced greatly in saline lacking both Na(+) and Ca(2+) and by application of intracellular Ca(2+) chelators. Therefore, although increases in intracellular Ca(2+) may facilitate the effects of alpha-LTX on transmitter release, these increases do not appear to be necessary. The results show that investigations of Ca(2+)-independent alpha-LTX mechanisms or uses of alpha-LTX to probe exocytosis mechanisms would be complicated by the release of intracellular Ca(2+), which itself can trigger exocytosis.

alpha-Latrotoxin releases calcium in frog motor nerve terminals

alpha-Latrotoxin (alpha-LTX) is a neurotoxin that accelerates spontaneous exocytosis independently of extracellular Ca(2+). Although alpha-LTX increases spontaneous transmitter release at synapses, the mechanism is unknown. We tested the hypothesis that alpha-LTX causes transmitter release by mobilizing intracellular Ca(2+) in frog motor nerve terminals. Transmitter release was measured electrophysiologically and with the vesicle marker FM1-43; presynaptic ion concentration dynamics were measured with fluorescent ion-imaging techniques. We report that alpha-LTX increases transmitter release after release of a physiologically relevant concentration of intracellular Ca(2+). Neither the blockade of Ca(2+) release nor the depletion of Ca(2+) from endoplasmic reticulum affected Ca(2+) signals produced by alpha-LTX. The Ca(2+) source is likely to be mitochondria, because the effects on Ca(2+) mobilization of CCCP (which depletes mitochondrial Ca(2+)) and of alpha-LTX are mutually occlusive. The release of mitochondrial Ca(2+) is partially attributable to an increase in intracellular Na(+), suggesting that the mitochondrial Na(+)/Ca(2+) exchanger is activated. Effects of alpha-LTX were not blocked when Ca(2+) increases were reduced greatly in saline lacking both Na(+) and Ca(2+) and by application of intracellular Ca(2+) chelators. Therefore, although increases in intracellular Ca(2+) may facilitate the effects of alpha-LTX on transmitter release, these increases do not appear to be necessary. The results show that investigations of Ca(2+)-independent alpha-LTX mechanisms or uses of alpha-LTX to probe exocytosis mechanisms would be complicated by the release of intracellular Ca(2+), which itself can trigger exocytosis.

Inositol trisphosphate and cyclic adenosine diphosphate-ribose increase quantal transmitter release at frog motor nerve terminals: possible involvement of smooth endoplasmic reticulum

The release of chemical transmitter from nerve terminals is critically dependent on a transient increase in intracellular Ca2+. The increase in Ca2+ may be due to influx of Ca2+ from the extracellular fluid or release of Ca2+ from intracellular stores such as mitochondria. Whether Ca2+ utilized in transmitter release is liberated from organelles other than mitochondria is uncertain. Smooth endoplasmic reticulum is known to release Ca2+, e.g., on activation by inositol trisphosphate or cyclic adenosine diphosphate-ribose, so the possibility exists that Ca2+ from this source may be involved in the events leading to exocytosis. We examined this hypothesis by testing whether inositol trisphosphate and cyclic adenosine diphosphate-ribose modified transmitter release. We used liposomes to deliver these agents into the cytoplasmic compartment and binomial analysis to determine their effects on the quantal components of transmitter release. Administration of inositol trisphosphate (10(-4)M) caused a rapid, 25% increase in the number of quanta released. This was due to an increase in the number of functional release sites, as the other quantal parameters were unaffected. The effect was reversed with 40 min of wash. Virtually identical results were obtained with cyclic adenosine diphosphate-ribose (10(-4)M). Inositol trisphosphate caused a 10% increase in quantal size, whereas cyclic adenosine diphosphate-ribose had no effect. The results suggest that quantal transmitter release can be increased by Ca2+ released from smooth endoplasmic reticulum upon stimulation by inositol trisphosphate or cyclic adenosine diphosphate-ribose. This may involve priming of synaptic vesicles at the release sites or mobilization of vesicles to the active zone. Inositol trisphosphate may have an additional action to increase the content of transmitter within the vesicles. These findings raise the possibility of a role of endogenous inositol phosphate and smooth endoplasmic reticulum in the regulation of cytoplasmic Ca2+ and transmitter release.

Depletion of calcium in the synaptic cleft of a calyx-type synapse in the rat brainstem

1. A new form of synaptic depression of excitatory synaptic transmission was observed when making voltage-clamp recordings from large presynaptic terminals, the calyces of Held and postsynaptic cells, the principal cells of the medial nucleus of the trapezoid body (MNTB), in slices of the rat auditory brainstem. 2. A short (100 ms) depolarization of the postsynaptic cell to 0 mV reduced the amplitude of the EPSCs by 35 +/- 5 % (n = 7), measured at 10 ms following the depolarization. Recovery occurred within 0.5 s. 3. The reduction of the EPSCs was most probably due to reduced presynaptic calcium influx, since postsynaptic depolarization reduced presynaptic calcium or barium currents. Conversely, presynaptic depolarization also reduced postsynaptic calcium or barium influx, under conditions where transmitter release was minimal. 4. The calcium currents and the postsynaptic depolarization-induced suppression of synaptic transmission recovered with a similar time course, suggesting that this form of synaptic depression was, most probably, due to depletion of Ca2+ in the synaptic cleft. 5. We conclude that when the Ca2+ influx into the pre- or postsynaptic cell is large, extracellular Ca2+ is depleted. Under these conditions, the Ca2+ concentration in the synaptic cleft is a sensitive indicator of the level of synaptic activity. However, the synaptic cleft is less sensitive to Ca2+ depletion than predicted from its estimated volume.

Neurosecretory cells without neurosecretion: evidence of an independently regulated trait of the cell phenotype

Neurosecretion competence is a fundamental property that enables differentiated neurones and professional neurosecretory cells to store neurotransmitters and hormones in specialized organelles, the synaptic-like vesicles and dense granules, and to release them by regulated exocytosis. In our laboratory, the study of rat phaeochromocytoma (PC12) clones that fail to express the above organelles or any other components involved in neurosecretion, whilst maintaining most of the general markers of the parental population, has served to demonstrate that this trait is controlled independently from the rest of the phenotype. The present review focuses on recent advances in elucidating the molecular mechanisms governing neurosecretion competence. Moreover, the opportunities that such neurosecretion-defective PC12 clones offer for the investigation of new aspects of regulated exocytosis and the localization of its components are summarized.

Correlated measurements of free and total intracellular calcium concentration in central nervous system neurons

Transient changes in the intracellular concentration of free calcium ([Ca2+])i) act as a trigger or modulator for a large number of important neuronal processes. Such transients can originate from voltage- or ligand-gated fluxes of Ca2+ into the cytoplasm from the extracellular space, or by ligand- or Ca2+(-)gated release from intracellular stores. Characterizing the sources and spatio-temporal patterns of [Ca2+]i transients is critical for understanding the role of different neuronal compartments in dendritic integration and synaptic plasticity. Optical imaging of fluorescent indicators sensitive to free Ca2+ is especially suited to studying such phenomena because this approach offers simultaneous monitoring of large regions of the dendritic tree in individual living central nervous system neurons. In contrast, energy-dispersive X-ray (EDX) microanalysis provides quantitative information on the amount and location of intracellular total, i.e., free plus bound, calcium (Ca) within specific subcellular dendritic compartments as a function of the activity state of the neuron. When optical measurements of [Ca2+]i transients and parallel EDX measurements of Ca content are used in tandem, and correlated simultaneously with electrophysiological measurements of neuronal activity, the combined information provides a relatively general picture of spatio-temporal neuronal total Ca fluctuations. To illustrate the kinds of information available with this approach, we review here results from our ongoing work aimed at evaluating the role of various Ca uptake, release, sequestration, and extrusion mechanisms in the generation and termination of [Ca2+]i transients in dendrites of pyramidal neurons in hippocampal slices during and after synaptic activity. Our observations support the long-standing speculation that the dendritic endoplasmic reticulum acts not only as an intracellular Ca2+ source that can be mobilized by a signal cascade originating at activated synapses, but also as a major intracellular Ca sink involved in active clearance mechanisms after voltage- and ligand-gated Ca2+ influx.

Quantification of total calcium in terminal cisternae of skinned muscle fibers by imaging electron energy-loss spectroscopy

Skinned muscle fibers are ideal model preparations for the investigation of Ca2+ -regulatory mechanisms. Their internal ionic milieu can be easily controlled and distinct physiological states are well defined. We have measured the total Ca content in the terminal cisternae of such preparations using imaging electron energy-loss spectroscopy (Image-EELS) as a new approach for quantification of sub-cellular element distributions. Murine muscle fibers submitted to a standardized calcium-loading procedure were cryo-fixed with a combined solution exchanger/plunge freezing device. Energy-filtered image series were recorded from ultrathin freeze-dried cryosections of samples immobilized in either relaxed or caffeine-contracted state. From these image series, electron energy-loss spectra were extracted by digital image-processing and quantitatively processed by multiple-least-squares-fitting with reference spectra. The calculated fit coefficients were converted to Ca-concentrations by a calibration obtained from Ca-standards. Total Ca-contents in the terminal cisternae of skinned skeletal muscle fibers decreased upon caffeine-induced Ca-release from 123+/-159 (+/-11) to 73+/-102 (+/-8) mmol/kg d.w. (weighted mean +/- SD (+/-SEM)).

A combined solution exchange/plunge-freezing device for skinned muscle fibers

For many contractility studies, defined functional states of skinned muscle fiber preparations can be introduced by application of standardized perfusion protocols with large varieties of experimental solutions. Functionally important subcellular element distributions in the myoplasm and in the sarcoplasmic reticulum can be measured with high spatial resolution by electron microscopic microanalysis. Capturing these subcellular ion distributions requires their rapid immobilization by quick-freezing. We therefore combined a plunge-freezing device with a semiautomatic solution exchanger to reproducibly perfuse skinned muscle fiber bundles with multiple solutions. The isometric tension produced is simultaneously recorded as an indicator for the functional state. The samples can be quick-frozen at any chosen time of the tension transient. A cryoglueing technique finally delivers specimens suitable for cryoultramicrotomy.

Overexpression of CALNUC (nucleobindin) increases agonist and thapsigargin releasable Ca2+ storage in the Golgi

We previously demonstrated that CALNUC, a Ca2+-binding protein with two EF-hands, is the major Ca2+-binding protein in the Golgi by 45Ca2+ overlay (Lin, P., H. Le-Niculescu, R. Hofmeister, J.M. McCaffery, M. Jin, H. Henneman, T. McQuistan, L. De Vries, and M. Farquhar. 1998. J. Cell Biol. 141:1515-1527). In this study we investigated CALNUC's properties and the Golgi Ca2+ storage pool in vivo. CALNUC was found to be a highly abundant Golgi protein (3.8 microg CALNUC/mg Golgi protein, 2.5 x 10(5) CALNUC molecules/NRK cell) and to have a single high affinity, low capacity Ca2+-binding site (Kd = 6.6 microM, binding capacity = 1.1 micromol Ca2+/micromol CALNUC). 45Ca2+ storage was increased by 2.5- and 3-fold, respectively, in HeLa cells transiently overexpressing CALNUC-GFP and in EcR-CHO cells stably overexpressing CALNUC. Deletion of the first EF-hand alpha helix from CALNUC completely abolished its Ca2+-binding capability. CALNUC was correctly targeted to the Golgi in transfected cells as it colocalized and cosedimented with the Golgi marker, alpha-mannosidase II (Man II). Approximately 70% of the 45Ca2+ taken up by HeLa and CHO cells overexpressing CALNUC was released by treatment with thapsigargin, a sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) (Ca2+ pump) blocker. Stimulation of transfected cells with the agonist ATP or IP3 alone (permeabilized cells) also resulted in a significant increase in Ca2+ release from Golgi stores. By immunofluorescence, the IP3 receptor type 1 (IP3R-1) was distributed over the endoplasmic reticulum and codistributed with CALNUC in the Golgi. These results provide direct evidence that CALNUC binds Ca2+ in vivo and together with SERCA and IP3R is involved in establishment of the agonist-mobilizable Golgi Ca2+ store.

The frog neuromuscular junction revisited after quick-freezing-freeze-drying: ultrastructure, immunogold labelling and high resolution calcium mapping

Until now, most ultrastructural studies on the neuromuscular junction have been carried out on samples first exposed to chemical treatments--with fixatives and/or dehydration agents--that are known to induce, or to be inadequate to prevent, artefactual changes of the native state. We report here on the potential of a physical approach to the preparation of samples that combines quick-freezing and freeze-drying (with or without exposure to OsO4 vapours) followed by direct embedding of the samples in various resins. Thin sections from physically processed frog neuromuscular junctions, when compared to their chemically fixed counterparts, exhibit an overall excellent preservation, with the organelles retaining their native density and shape. These preparations were also investigated by electron spectroscopic imaging and electron energy loss spectroscopy, obtaining high resolution maps of native total calcium distribution within the nerve terminal. Finally, thin sections from analogously processed, however unfixed, preparations embedded in Lowicryl, were immunogold labelled before exposure to OsO4. Nerve-muscle preparations treated this way exhibited adequate preservation of ultrastructure and revealed the distribution of synaptophysin with high sensitivity and resolution. In conclusion, we provide an overview of the potential of the quick-freezing-freeze-drying approach in the study of the neuromuscular junction function.

Presynaptic mitochondria and the temporal pattern of neurotransmitter release

Mitochondria are critical for the function of nerve terminals as the cycling of synaptic vesicle membrane requires an efficient supply of ATP. In addition, the presynaptic mitochondria take part in functions such as Ca2+ buffering and neurotransmitter synthesis. To learn more about presynaptic mitochondria, we have examined their organization in two types of synapse in the lamprey, both of which are glutamatergic but are adapted to different temporal patterns of activity. The first is the giant lamprey reticulospinal synapse, which is specialized to transmit phasic signals (i.e. bursts of impulses). The second is the synapse established by sensory dorsal column axons, which is adapted to tonic activity. In both cases, the presynaptic axons were found to contain two distinct types of mitochondria; small 'synaptic' mitochondria, located near release sites, and larger mitochondria located in more central parts of the axon. The size of the synapse-associated mitochondria was similar in both types of synapse. However, their number differed considerably. Whereas the reticulospinal synapses contained only single mitochondria within 1 micron distance from the edge of the active zone (on average 1.2 per active zone, range of 1-3), the tonic dorsal column synapses were surrounded by clusters of mitochondria (4.5 per active zone, range of 3-6), with individual mitochondria sometimes apparently connected by intermitochondrial contacts. In conjunction with studies of crustacean neuromuscular junctions, these observations indicate that the temporal pattern of transmitter release is an important determinant of the organization of presynaptic mitochondria.

Multiple and diverse forms of regulated exocytosis in wild-type and defective PC12 cells

Regulated exocytosis triggered by the photolysis of a caged Ca2+ compound, DM-nitrophen, was investigated by patch-clamp capacitance measurements in two clones of PC12, the first wild-type and the second (PC12-27) defective of both types of classical secretory vesicles together with the neuronal-type receptors for the attachment proteins of the N-ethylmaleimide-sensitive fusion protein, the so called SNAREs. Moreover, the electrophysiological data were correlated with the ultrastructure of resting quick-frozen-freeze-dried cells of the two clones. Wild-type PC12 exhibited two-component capacitance responses, time constants of 30-100 ms and >10 s, that previous studies had suggested to reflect primarily the fusion of the small and large secretory vesicles, each contributing cell surface increases of approximately 10%. Both of these components were largely and specifically inhibited whether cells previously were microinjected with tetanus toxin light chain. In the defective clone, large responses also were recorded ( approximately 19% surface expansion; time constant, approximately 1 s) that, in contrast to those of the wild-type, were entirely resistant to the toxin. Although secretory organelles, i.e., large vesicles and also profiles of small vesicles, were abundant at the cell periphery and often docked to the plasmalemma of resting wild-type PC12, in the defective clone, no superficial accumulation of vesicles was observed. Our coordinate structural and functional results have revealed diversities between the two classical forms of regulated secretion in wild-type PC12 and have provided evidence of a toxin-insensitive form of Ca2+-induced exocytosis, prominent in the defective clone, that may play an important role(s) in cellular physiology.

Ca2+ depletion from granules inhibits exocytosis. A study with insulin-secreting cells

The secretory compartment is characterized by low luminal pH and high Ca2+ content. Previous studies in several cell types have shown that the size of the acidic Ca2+ pool, of which secretory granules represent a major portion, could be estimated by applying first a Ca2+ ionophore followed by agents that collapse acidic pH gradients. In the present study we have employed this protocol in the insulin-secreting cell line Ins-1 to determine whether the Ca2+ trapped in the secretory granules plays a role in exocytosis. The results demonstrate that a high proportion of ionophore-mobilizable Ca2+ in Ins-1 cells resides in the acidic compartment. The latter pool, however, does not significantly contribute to the [Ca2+]i changes elicited by thapsigargin and the inositol trisphosphate-producing agonist carbachol. By monitoring membrane capacitance at the single cell level or by measuring insulin release in cell populations, we show that Ca2+ mobilization from nonacidic Ca2+ pools causes a profound and long lasting increase in depolarization-induced secretion, whereas breakdown of granule pH had no significant effect. In contrast, releasing Ca2+ from the acidic pool markedly reduces secretion. It is suggested that a high Ca2+ concentration in the secretory compartment is needed to sustain optimal exocytosis.

The mammalian calcium-binding protein, nucleobindin (CALNUC), is a Golgi resident protein

We have identified CALNUC, an EF-hand, Ca2+-binding protein, as a Golgi resident protein. CALNUC corresponds to a previously identified EF-hand/calcium-binding protein known as nucleobindin. CALNUC interacts with Galphai3 subunits in the yeast two-hybrid system and in GST-CALNUC pull-down assays. Analysis of deletion mutants demonstrated that the EF-hand and intervening acidic regions are the site of CALNUC's interaction with Galphai3. CALNUC is found in both cytosolic and membrane fractions. The membrane pool is tightly associated with the luminal surface of Golgi membranes. CALNUC is widely expressed, as it is detected by immunofluorescence in the Golgi region of all tissues and cell lines examined. By immunoelectron microscopy, CALNUC is localized to cis-Golgi cisternae and the cis-Golgi network (CGN). CALNUC is the major Ca2+-binding protein detected by 45Ca2+-binding assay on Golgi fractions. The properties of CALNUC and its high homology to calreticulin suggest that it may play a key role in calcium homeostasis in the CGN and cis-Golgi cisternae.

High-resolution calcium mapping of the endoplasmic reticulum-Golgi-exocytic membrane system. Electron energy loss imaging analysis of quick frozen-freeze dried PC12 cells

The calcium pools segregated within the endoplasmic reticulum, Golgi complex, exocytic, and other organelles are believed to participate in the regulation of a variety of cell functions. Until now, however, the precise intracellular distribution of the element had not been established. Here, we report about the first high-resolution calcium mapping obtained in neurosecretory PC12 cells by the imaging mode of the electron energy loss spectroscopy technique. The preparation procedure used included quick freezing of cell monolayers, followed by freeze-drying, fixation with OSO4 vapors, resin embedding, and cutting of very thin sections. Conventional electron microscopy and high-resolution immunocytochemistry revealed a high degree of structural preservation, a condition in which inorganic elements are expected to maintain their native distribution. Within these cells, calcium signals of nucleus, cytosol, and most mitochondria remained below detection, whereas in other organelles specific patterns were identified. In the endoplasmic reticulum, the distribution was heterogeneous with strongly positive cisternae (more often the nuclear envelope and stacks of parallel elements that are frequent in quick frozen preparations) lying in the proximity of or even in direct continuity with other, apparently negative cisternae. The Golgi complexes were labeled strongly and uniformly in all cisternae and part of their vesicles, with no appreciable differences along the cis-trans axis. Weaker or negative signals were recorded from the trans-Golgi network elements and from scattered vesicles, whereas in contrast secretion granules were strongly positive for calcium. These results are discussed in relation to the existing knowledge about the mechanisms of calcium transport in the variations organelles, and about the processes and functions regulated by organelle lumenal calcium in eukaryotic cells.

Mitochondrial involvement in post-tetanic potentiation of synaptic transmission

Posttetanic potentiation (PTP) is an essential aspect of synaptic transmission that arises from a persistent presynaptic [Ca2+]i following tetanic stimulation. At crayfish neuromuscular junctions, several inhibitors of mitochondrial Ca2+ uptake and release (tetraphenylphosphonium or TPP+, carbonyl cyanide m-chlorophenylhydrazone or CCCP, and ruthenium red) blocked PTP and the persistence of presynaptic residual [Ca2+]i, while endoplasmic reticulum (ER) Ca2+ pump inhibitors and release channel activators (thapsigargin, 2,5-di-(tert-butyl)-1,4-benzohydroquinone or BHQ, and caffeine) had no effects. PTP apparently results from the slow efflux of tetanically accumulated mitochondrial Ca2+.

Dynamic properties of an inositol 1,4,5-trisphosphate- and thapsigargin-insensitive calcium pool in mammalian cell lines

The functional characteristics of a nonacidic, inositol 1,4,5-trisphosphate- and thapsigargin-insensitive Ca2+ pool have been characterized in mammalian cells derived from the rat pituitary gland (GH3, GC, and GH3B6), the adrenal tissue (PC12), and mast cells (RBL-1). This Ca2+ pool is released into the cytoplasm by the Ca2+ ionophores ionomycin or A23187 after the discharge of the inositol 1,4,5-trisphosphate-sensitive store with an agonist coupled to phospholipase C activation and/or thapsigargin. The amount of Ca2+ trapped within this pool increased significantly after a prolonged elevation of intracellular Ca2+ concentration elicited by activation of Ca2+ influx. This pool was affected neither by caffeine-ryanodine nor by mitochondrial uncouplers. Probing mitochondrial Ca2+ with recombinant aequorin confirmed that this pool did not coincide with mitochondria, whereas its homogeneous distribution across the cytosol, as revealed by confocal microscopy, and its insensitivity to brefeldin A make localization within the Golgi complex unlikely. A proton gradient as the driving mechanism for Ca2+ uptake was excluded since ionomycin is inefficient in releasing Ca2+ from acidic pools and Ca2+ accumulation/release in/from this store was unaffected by monensin or NH4Cl, drugs known to collapse organelle acidic pH gradients. Ca2+ sequestration inside this pool, thus, may occur through a low-affinity, high-capacity Ca2+-ATPase system, which is, however, distinct from classical endosarcoplasmic reticulum Ca2+-ATPases. The cytological nature and functional role of this Ca2+ storage compartment are discussed.