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

Effects of high-potassium solutions and caffeine on synaptic vesicle exoendocytosis processes in the frog neuromuscular junction

Studies on frog skin-pectoris muscle preparations using vital fluorescent microscopy showed that stimulation of transmitter secretion using high-potassium solutions with the endocytosis marker FM 1-43 induced bright spots in all motor nerve terminals, these representing accumulations of vesicles undergoing the exoendocytic cycle in the active zones of nerve endings. Stimulation of transmitter secretion with caffeine evoked bright spots only in some nerve terminals and only in some parts of the terminals. In summer, the number of bright spots on stimulation of transmitter secretion by caffeine increased sharply. Extracellular recording of spontaneous synaptic signals showed that high-potassium solutions, like caffeine, produced dose-dependent increases in the frequency of miniature endplate currents. However, while high-potassium solutions always increased the frequency, this occurred with caffeine in only a proportion of experiments. This leads to the conclusion that exoendocytosis processes can occur both because of the influx of Ca(2+) ions into nerve endings as a result of depolarization (high-potassium solutions) and because of the release of Ca(2+) ions from the endoplasmic reticulum (caffeine). The possible spatial localization of the endoplasmic reticulum in nerve endings is discussed. The endoplasmic reticulum is suggested to have a role in synapse remodeling processes.

Synaptic vesicles: test for a role in presynaptic calcium regulation

Membrane-bound organelles such as mitochondria and the endoplasmic reticulum play an important role in neuronal Ca(2+) homeostasis. Synaptic vesicles (SVs), the organelles responsible for exocytosis of neurotransmitters, occupy more of the volume of presynaptic nerve terminals than any other organelle and, under some conditions, can accumulate Ca(2+). They are also closely associated with voltage-gated Ca(2+) channels (VGCCs) that trigger transmitter release by admitting Ca(2+) into the nerve terminal in response to action potentials (APs). We tested the hypothesis that SVs can modulate Ca(2+) signals in the presynaptic terminal. This has been a difficult question to address because neither pharmacological nor genetic approaches to block Ca(2+) permeation of the SV membrane have been available. To investigate the possible role of SVs in Ca(2+) regulation, we used imaging techniques to compare Ca(2+) dynamics in motor nerve terminals before and after depletion of SVs. We used the temperature-sensitive Drosophila dynamin mutant shibire, in which SVs can be eliminated by stimulation. There was no difference in the amplitude or time course of Ca(2+) responses during high-frequency trains of APs, or single APs, in individual presynaptic boutons before and after depletion of SVs. SVs have a limited role, if any, in the rapid sequestration of Ca(2+) within the neuronal cytosol or the synaptic microdomain. We also conclude that SVs are not important for regulation of synaptic VGCCs.

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.

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.

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'.

Cytoskeletal and Ca2+ regulation of hyphal tip growth and initiation

Hyphal tip growth is a complex process involving finely regulated interactions between the synthesis and expansion of cell wall and plasma membrane, diverse intracellular movements, and turgor regulation. F-actin is a major regulator and integrator of these processes. It directly contributes to (a) tip morphogenesis, most likely by participation in an apical membrane skeleton that reinforces the apical plasma membrane, (b) the transport and exocytosis of vesicles that contribute plasma membrane and cell wall material to the hyphal tips, (c) the localization of plasma membrane proteins in the tips, and (d) cytoplasmic and organelle migration and positioning. The pattern of reorganization of F-actin prior to formation of new tips during branch initiation also indicates a critical role in early stages of assembly of the tip apparatus. One of the universal characteristics of all critically examined tip-growing cells, including fungal hyphae, is the obligatory presence of a tip-high gradient of cytoplasmic Ca2+ that probably regulates both actin and nonactin components of the apparatus, and the formation of which may also initiate new tips. This review discusses the diversity of evidence behind these concepts.

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.