Evidence that Ca2+-waves in Xenopus melanotropes depend on calcium-induced calcium release: a fluorescence correlation microscopy and linescanning study

Imaging of Endoplasmic Reticulum Ca2+ in the Intact Pituitary Gland of Transgenic Mice Expressing a Low Affinity Ca2+ Indicator

The adenohypophysis contains five secretory cell types (somatotrophs, lactotrophs, thyrotrophs, corticotrophs, and gonadotrophs), each secreting a different hormone, and controlled by different hypothalamic releasing hormones (HRHs). Exocytic secretion is regulated by cytosolic Ca²⁺ signals ([Ca²⁺]_C), which can be generated either by Ca²⁺ entry through the plasma membrane and/or by Ca²⁺ release from the endoplasmic reticulum (ER). In addition, Ca²⁺ entry signals can eventually be amplified by ER release via calcium-induced calcium release (CICR). We have investigated the contribution of ER Ca²⁺ release to the action of physiological agonists in pituitary gland. Changes of [Ca²⁺] in the ER ([Ca²⁺]_ER) were measured with the genetically encoded low-affinity Ca²⁺ sensor GAP3 targeted to the ER. We used a transgenic mouse strain that expressed erGAP3 driven by a ubiquitous promoter. Virtually all the pituitary cells were positive for the sensor. In order to mimick the physiological environment, intact pituitary glands or acute slices from the transgenic mouse were used to image [Ca²⁺]_ER. [Ca²⁺]_C was measured simultaneously with Rhod-2. Luteinizing hormone-releasing hormone (LHRH) or thyrotropin releasing hormone (TRH), two agonists known to elicit intracellular Ca²⁺ mobilization, provoked robust decreases of [Ca²⁺]_ER and concomitant rises of [Ca²⁺]_C. A smaller fraction of cells responded to thyrotropin releasing hormone (TRH). By contrast, depolarization with high K⁺ triggered a rise of [Ca²⁺]_C without a decrease of [Ca²⁺]_ER, indicating that the calcium-induced calcium-release (CICR) via ryanodine receptor amplification mechanism is not present in these cells. Our results show the potential of transgenic ER Ca²⁺ indicators as novel tools to explore intraorganellar Ca²⁺ dynamics in pituitary gland in situ.

BDNF stimulates Ca2+ oscillation frequency in melanotrope cells of Xenopus laevis: contribution of IP3-receptor-mediated release of intracellular Ca2+ to gene expression

Pituitary melanotrope cells of the amphibian Xenopus laevis are neuroendocrine cells regulating the animal's skin color adaptation through secretion of α-melanophore-stimulating hormone (α-MSH). To fulfill this function optimally, the melanotrope cell undergoes plastic changes in structure and secretory activity in response to changed background light conditions. Xenopus melanotrope cells display Ca(2+) oscillations that are thought to drive α-MSH secretion and gene expression. They also produce brain-derived neurotrophic factor (BDNF), which stimulates in an autocrine way the biosynthesis of the α-MSH precursor, pro-opiomelanocortin (POMC). We have used this physiological adaptation mechanism as a model to investigate the role of BDNF in the regulation of Ca(2+) kinetics and Ca(2+)-dependent gene expression. By dynamic video imaging of isolated cultured melanotropes we demonstrated that BDNF caused a dose-dependent increase in Ca(2+) oscillation frequency up to 64.7±2.3% of control level. BDNF also induced a transient Ca(2+) peak in Ca(2+)-free medium, which was absent when calcium stores were blocked by thapsigargin and 2-aminoethoxydiphenyl borate, indicating that BDNF stimulates acute release of Ca(2+) from IP(3)-sensitive intracellular Ca(2+) stores. Moreover, we show that thapsigargin inhibits the expression of BDNF transcript IV (by 61.1±28.8%) but does not affect POMC transcript. We conclude that BDNF mobilizes Ca(2+) from IP(3)-sensitive intracellular Ca(2+) stores and propose the possibility that the resulting Ca(2+) oscillations selectively stimulate expression of the BDNF gene.

About a snail, a toad, and rodents: animal models for adaptation research

Neural adaptation mechanisms have many similarities throughout the animal kingdom, enabling to study fundamentals of human adaptation in selected animal models with experimental approaches that are impossible to apply in man. This will be illustrated by reviewing research on three of such animal models, viz. (1) the egg-laying behavior of a snail, Lymnaea stagnalis: how one neuron type controls behavior, (2) adaptation to the ambient light condition by a toad, Xenopus laevis: how a neuroendocrine cell integrates complex external and neural inputs, and (3) stress, feeding, and depression in rodents: how a neuronal network co-ordinates different but related complex behaviors. Special attention is being paid to the actions of neurochemical messengers, such as neuropeptide Y, urocortin 1, and brain-derived neurotrophic factor. While awaiting new technological developments to study the living human brain at the cellular and molecular levels, continuing progress in the insight in the functioning of human adaptation mechanisms may be expected from neuroendocrine research using invertebrate and vertebrate animal models.

Chapter 16 The use of fluorescence correlation spectroscopy to probe mitochondrial mobility and intramatrix protein diffusion

Within cells, functional changes in mitochondrial metabolic state are associated with alterations in organelle mobility, shape, and configuration of the mitochondrial matrix. Fluorescence correlation spectroscopy (FCS) is a technique that measures intensity fluctuations caused by single fluorescent molecules moving through a small detection volume. By mathematically correlating these fluctuations, information can be obtained about the concentration and rate of diffusion of the fluorescent molecules. Here we present an FCS-based approach for determining the mobility of enhanced yellow fluorescent protein (mitoEYFP) in the mitochondrial matrix of primary human skin fibroblasts. This protocol allows simultaneous quantification of intramatrix EYFP concentration and its diffusion constant, as well as the fraction of moving mitochondria and their velocity.

Inherited complex I deficiency is associated with faster protein diffusion in the matrix of moving mitochondria

Mitochondria continuously change shape, position, and matrix configuration for optimal metabolite exchange. It is well established that changes in mitochondrial metabolism influence mitochondrial shape and matrix configuration. We demonstrated previously that inhibition of mitochondrial complex I (CI or NADH:ubiquinone oxidoreductase) by rotenone accelerated matrix protein diffusion and decreased the fraction and velocity of moving mitochondria. In the present study, we investigated the relationship between inherited CI deficiency, mitochondrial shape, mobility, and matrix protein diffusion. To this end, we analyzed fibroblasts of two children that represented opposite extremes in a cohort of 16 patients, with respect to their residual CI activity and mitochondrial shape. Fluorescence correlation spectroscopy (FCS) revealed no relationship between residual CI activity, mitochondrial shape, the fraction of moving mitochondria, their velocity, and the rate of matrix-targeted enhanced yellow fluorescent protein (mitoEYFP) diffusion. However, mitochondrial velocity and matrix protein diffusion in moving mitochondria were two to three times higher in patient cells than in control cells. Nocodazole inhibited mitochondrial movement without altering matrix EYFP diffusion, suggesting that both activities are mutually independent. Unexpectedly, electron microscopy analysis revealed no differences in mitochondrial ultrastructure between control and patient cells. It is discussed that the matrix of a moving mitochondrion in the CI-deficient state becomes less dense, allowing faster metabolite diffusion, and that fibroblasts of CI-deficient patients become more glycolytic, allowing a higher mitochondrial velocity.

Partial complex I inhibition decreases mitochondrial motility and increases matrix protein diffusion as revealed by fluorescence correlation spectroscopy

We previously reported that inhibition of mitochondrial complex I (CI) by rotenone induces marked increases in mitochondrial length and degree of branching, thus revealing a relationship between mitochondrial function and shape. We here describe the first time use of fluorescence correlation spectroscopy (FCS) to simultaneously probe mitochondrial mobility and intra-matrix protein diffusion, with the aim to investigate the effects of chronic CI inhibition on the latter two parameters. To this end, EYFP was expressed in the mitochondrial matrix of human skin fibroblasts (mitoEYFP) using baculoviral transduction and its diffusion monitored by FCS. This approach revealed the coexistence of moving and stationary mitochondria within the same cell and enabled simultaneous quantification of mitochondrial velocity and mitoEYFP diffusion. When CI activity was chronically reduced by 80% using rotenone treatment, the percentage of moving mitochondria and their velocity decreased by 30%. MitoEYFP diffusion did not differ between moving and stationary mitochondria but was increased 2-fold in both groups of mitochondria following rotenone treatment. We propose that the increase in matrix protein diffusion together with the increase in mitochondrial length and degree of branching constitutes part of an adaptive response which serves to compensate for the reduction in CI activity and mitochondrial motility.

Plasticity in the melanotrope neuroendocrine interface of Xenopus laevis

Melanotrope cells of the amphibian pituitary pars intermedia produce alpha-melanophore-stimulating hormone (alpha-MSH), a peptide which causes skin darkening during adaptation to a dark background. The secretory activity of the melanotrope of the South African clawed toad Xenopus laevis is regulated by multiple factors, both classical neurotransmitters and neuropeptides from the brain. This review concerns the plasticity displayed in this intermediate lobe neuroendocrine interface during physiological adaptation to the environment. The plasticity includes dramatic morphological plasticity in both pre- and post-synaptic elements of the interface. Inhibitory neurons in the suprachiasmatic nucleus, designated suprachiasmatic melanotrope-inhibiting neurons (SMINs), possess more and larger synapses on the melanotrope cells in white than in black-background adapted animals; in the latter animals the melanotropes are larger and produce more proopiomelanocortin (POMC), the precursor of alpha-MSH. On a white background, pre-synaptic SMIN plasticity is reflected by a higher expression of inhibitory neuropeptide Y (NPY) and is closely associated with postsynaptic melanotrope plasticity, namely a higher expression of the NPY Y1 receptor. Interestingly, melanotrope cells in such animals also display higher expression of the receptors for thyrotropin-releasing hormone (TRH) and urocortin 1, two neuropeptides that stimulate alpha-MSH secretion. Possibly, in white-adapted animals melanotropes are sensitized to neuropeptide stimulation so that, when the toad moves to a black background, they can immediately initiate alpha-MSH secretion to achieve rapid adaptation to the new background condition. The melanotrope cell also produces brain-derived neurotrophic factor (BDNF), which is co-sequestered with alpha-MSH in secretory granules within the cells. The neurotrophin seems to control melanotrope cell plasticity in an autocrine way and we speculate that it may also control presynaptic SMIN plasticity.

Simultaneous quantification of oxidative stress and cell spreading using 5-(and-6)-chloromethyl-2′,7′-dichlorofluorescein

BACKGROUND

Mitochondrial dysfunction may lead to increased oxidative stress and consequent changes in cell spreading. Here, we describe and validate a novel method for simultaneous quantification of these two parameters.

METHODS

Human skin fibroblasts were loaded with 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein (CM-H(2)DCF), and its oxidative conversion into CM-DCF was monitored as a function of time by video-rate confocal microscopy and real-time image averaging. Cell size was determined after binarization of the acquired images.

RESULTS

At the lowest practical laser output, CM-DCF formation occurred with zero order kinetics, indicating that [CM-H(2)DCF] was not rate-limiting and that the rate of [CM-DCF] formation (V(CM-DCF)) was a function of the cellular oxidant level. Analysis of fibroblasts of a healthy control subject and a patient with a deficiency of NADH:ubiquinone oxidoreductase, the first complex of the oxidative phosphorylation system, revealed a significant increase in cellular oxidant level in the latter cells that was, however, not accompanied by a change in cell spreading. Conversely, chronic treatment with 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), a derivative of vitamin E, markedly decreased the oxidant level and cell spreading in both control and patient fibroblasts.

CONCLUSIONS

We present a reliable method for simultaneous quantification of oxidant levels and cell spreading in living cells.

Global analysis of fluorescence fluctuation data

Over the last decade the number of applications of fluorescence correlation spectroscopy (FCS) has grown rapidly. Here we describe the development and application of a software package, FCS Data Processor, to analyse the acquired correlation curves. The algorithms combine strong analytical power with flexibility in use. It is possible to generate initial guesses, link and constrain fit parameters to improve the accuracy and speed of analysis. A global analysis approach, which is most effective in analysing autocorrelation curves determined from fluorescence fluctuations of complex biophysical systems, can also be implemented. The software contains a library of frequently used models that can be easily extended to include user-defined models. The use of the software is illustrated by analysis of different experimental fluorescence fluctuation data sets obtained with Rhodamine Green in aqueous solution and enhanced green fluorescent protein in vitro and in vivo.

Role of cortical filamentous actin in the melanotrope cell of Xenopus laevis

In secretory cells filamentous actin (f-actin) is mostly present subjacent to the plasma membrane, referred to as cortical actin. While the function of cortical actin in the secretory processes has been extensively studied, little attention has been given to the role of actin in signal transduction and intracellular second messenger dynamics. Analysis with the fluorescent f-actin probe Alexa-phalloidin shows that Xenopus laevis pituitary melanotrope cells possess a thick cortical actin ring. This cell is a good model to study the possible function(s) of f-actin in signal transduction processes. Regulation of the release of alpha-MSH from this cell involves a convergence of various receptor mechanisms to regulate the activity of voltage-operated Ca2+ channels. We have considered three potential functions for the cortical actin ring in the signaling process of the melanotrope: (1) it functions as a barrier for access of secretory granules to the membrane for exocytosis, (2) it is involved in anchoring components of the Ca2+ signalling machinery of the cell, and/or (3) it helps to form a scaffold for components of the signal transduction machinery used by the various neurotransmitters and neuropeptides that regulate the activity of the cell. To test these possibilities we have examined the effect of the f-actin depolymerising toxin latrunculin B on Ca2+ signaling, signal transduction and alpha-MSH secretion in the melanotrope. We show that while the toxin is effective in disrupting the cortical actin ring, this treatment has no effect on either Ca2+ signaling or the signal transduction processes studied. The toxin does induce an increase in alpha-MSH release, indicating that the cortical actin ring acts as a barrier for secretory granule access to the membrane.

Ca2+ oscillations in melanotropes of Xenopus laevis: their generation, propagation, and function

The melanotrope cell of the amphibian Xenopus laevis is a neuroendocrine transducer that converts neuronal input concerning the color of background into an endocrine output, the release of alpha-melanophore-stimulating hormone (alpha-MSH). The cell displays intracellular Ca(2+) oscillations that are thought to be the driving force for secretion as well as for the expression of genes important to the process of background adaptation. Here we review the functioning of the Xenopus melanotrope cell, with emphasis on the role of Ca(2+) oscillations in signal transduction in this cell. We start by giving a general overview of the evolution of Ca(2+) as an intracellular messenger molecule. This is followed by an examination of the melanotrope as a neuroendocrine integrator cell. Then, the evidence that Ca(2+) oscillations drive the secretion of alpha-MSH is reviewed, followed by a similar analysis of the evidence that the same oscillations regulate the expression of proopiomelanocortin (POMC), the precursor protein for alpha-MSH. Finally, the possible importance of the pattern of Ca(2+) signaling to melanotrope cell function is considered.

On the conservation of fast calcium wave speeds

Calcium waves were first seen about 25 years ago as the giant, 10 micro m/s wave or tsunami which crosses the cytoplasm of an activating medaka fish egg [J Cell Biol 76 (1978) 448]. By 1991, reports of such waves with approximately 10 micro m/s velocities through diverse, activating eggs and with approximately 30 micro m/s velocities through diverse, fully active systems had been compiled to form a class of what are now called fast calcium waves [Proc Natl Acad Sci USA 88 (1991) 9883; Bioessays 21 (1999) 657]. This compilation is now updated to include organisms from algae and sponges up to blowflies, squid and men and organizational levels from mammalian brains and hearts as well as chick embryos down to muscle, nerve, epithelial, blood and cancer cells and even cell-free extracts. Plots of these data confirm the narrow, 2-3-fold ranges of fast wave speeds through activating eggs and 3-4-fold ones through fully active systems at a given temperature. This also indicate Q(10)'s of 2.7-fold per 10 degrees C for both activating eggs and for fully activated cells.Speeds through some ultraflat preparations which are a few-fold above the conserved range are attributed to stretch propagated calcium entry (SPCE) rather than calcium-induced calcium release (CICR).

Local electric stimulation causes conducted calcium response in rat interlobular arteries

The purpose of the present study was to investigate the conducted Ca(2+) response to local electrical stimulation in isolated rat interlobular arteries. Interlobular arteries were isolated from young Sprague-Dawley rats, loaded with fura 2, and attached to pipettes in a chamber on an inverted microscope. Local electrical pulse stimulation (200 ms, 100 V) was administered by means of an NaCl-filled microelectrode (0.7-1 M(Omega)) juxtaposed to one end of the vessel. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured with an image system at a site approximately 500 microm from the location of the electrode. The expression of mRNA for pore-forming units Ca(V)3.1 and Ca(V)3.2 of voltage-sensitive T-type channels was investigated by using RT-PCR. Current stimulation elicited a conducted [Ca(2+)](i) response. A positive electrode (relative to ground) increased [Ca(2+)](i) to 145 +/- 7% of baseline, whereas the response was absent when the electrode was negative. This response was not dependent on perivascular nerves, because the conducted response was unaffected by TTX (1 microM). The conducted [Ca(2+)](i) response was abolished by an ambient Ca(2+) free solution and blunted by nifedipine (1 microM). Rat interlobular arteries exhibited conducted [Ca(2+)](i) response to current stimulation. This response was dependent on Ca(2+) entry. L-type Ca(2+) channels may play a role in this process.

TRH signal transduction in melanotrope cells of Xenopus laevis

TRH is a neuropeptide that activates phospholipase C and, when acting on secretory cells, usually induces a biphasic response consisting of a transitory increase in secretion (due to IP(3) mobilization of Ca(2+) from intracellular stores), followed by a sustained plateau phase of stimulated secretion (by protein kinase C-dependent influx of extracellular Ca(2+) through voltage-operated Ca(2+) channels). The melanotrope cell of the amphibian Xenopus laevis displays a unique secretory response to TRH, namely a broad transient but no sustained second phase, consistent with the observation that TRH induces a single Ca(2+) transient rather than the classic biphasic increase in [Ca(2+)](i). The purpose of the present study was to determine the signal transduction mechanism utilized by TRH in generating this Ca(2+) signaling response. Our hypothesis was that the transient reflects the operation of only one of the two signaling arms of the lipase (i.e., either IP(3)-induced mobilization of internal Ca(2+) or PKC-dependent influx of external Ca(2+)). Using video-imaging microscopy it is shown that the TRH-induced Ca(2+) transient is dramatically attenuated under Ca(2+)-free conditions and that thapsigargin has no noticeable effect on the TRH-induced transient. These observations indicate that an IP(3)-dependent mechanism plays no important role in the action of TRH. PKC also does not seem to be involved because an activator of PKC did not induce a Ca(2+) transient and an inhibitor of PKC did not affect the TRH response. Experiments with a bis-oxonol membrane potential probe showed that the TRH response also does not underlie a PKC-independent mechanism that would induce membrane depolarization. We conclude that the action of TRH on the Xenopus melanotrope does not rely on the classical phospholipase C-dependent mechanism.

Multiple control and dynamic response of the Xenopus melanotrope cell

Some amphibian brain-melanotrope cell systems are used to study how neuronal and (neuro)endocrine mechanisms convert environmental signals into physiological responses. Pituitary melanotropes release alpha-melanophore-stimulating hormone (alpha-MSH), which controls skin color in response to background light stimuli. Xenopus laevis suprachiasmatic neurons receive optic input and inhibit melanotrope activity by releasing neuropeptide Y (NPY), dopamine (DA) and gamma-aminobutyric acid (GABA) when animals are placed on a light background. Under this condition, they strengthen their synaptic contacts with the melanotropes and enhance their secretory machinery by upregulating exocytosis-related proteins (e.g. SNAP-25). The inhibitory transmitters converge on the adenylyl cyclase system, regulating Ca(2+) channel activity. Other messengers like thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH, from the magnocellular nucleus), noradrenalin (from the locus coeruleus), serotonin (from the raphe nucleus) and acetylcholine (from the melanotropes themselves) stimulate melanotrope activity. Ca(2+) enters the cell and the resulting Ca(2+) oscillations trigger alpha-MSH secretion. These intracellular Ca(2+) dynamics can be described by a mathematical model. The oscillations travel as a wave through the cytoplasm and enter the nucleus where they may induce the expression of genes involved in biosynthesis and processing (7B2, PC2) of pro-opiomelanocortin (POMC) and release (SNAP-25, munc18) of its end-products. We propose that various environmental factors (e.g. light and temperature) act via distinct brain centers in order to release various neuronal messengers that act on the melanotrope to control distinct subcellular events (e.g. hormone biosynthesis, processing and release) by specifically shaping the pattern of melanotrope Ca(2+) oscillations.

New aspects of signal transduction in the Xenopus laevis melanotrope cell

Light and temperature stimuli act via various brain centers and neurochemical messengers on the pituitary melanotrope cells of Xenopus laevis to control distinct subcellular activities such as the biosynthesis, processing, and release of alpha-melanophore-stimulating hormone (alphaMSH). The melanotrope signal transduction involves the action of a large repertoire of neurotransmitter and neuropeptide receptors and the second messengers cAMP and Ca(2+). Here we briefly review this signaling mechanism and then present new data on two aspects of this process, viz. the presence of a stimulatory beta-adrenergic receptor acting via cAMP and the egress of cAMP from the melanotrope upon a change of alphaMSH release activity.

Dynamics and plasticity of peptidergic control centres in the retino-brain-pituitary system of Xenopus laevis

This review deals particularly with the recent literature on the structural and functional aspects of the retino-brain-pituitary system that controls the physiological process of background adaptation in the aquatic toad Xenopus laevis. Taking together the large amount of multidisciplinary data, a consistent picture emerges of a highly plastic system that efficiently responds to changes in the environmental light condition by releasing POMC-derived peptides, such as the peptide alpha-melanophore-stimulating hormone (alpha-MSH), into the circulation. This plasticity is exhibited by both the central nervous system and the pituitary pars intermedia, at the level of molecules, subcellular structures, synapses, and cells. Signal transduction in the pars intermedia of the pituitary gland of Xenopus laevis appears to be a complex event, involving various environmental factors (e.g., light and temperature) that act via distinct brain centres and neuronal messengers converging on the melanotrope cells. In the melanotropes, these messages are translated by specific receptors and second messenger systems, in particular via Ca(2+) oscillations, controlling main secretory events such as gene transcription, POMC-precursor translation and processing, posttranslational peptide modifications, and release of a bouquet of POMC-derived peptides. In conclusion, the Xenopus hypothalamo-hypophyseal system involved in background adaptation reveals how neuronal plasticity at the molecular, cellular and organismal levels, enable an organism to respond adequately to the continuously changing environmental factors demanding physiological adaptation.

Membrane-initiated Ca(2+) signals are reshaped during propagation to subcellular regions

An important aspect of Ca(2+) signaling is the ability of cells to generate intracellular Ca(2+) waves. In this study we have analyzed the cellular and subcellular kinetics of Ca(2+) waves in a neuroendocrine transducer cell, the melanotrope of Xenopus laevis, using the ratiometric Ca(2+) probe indo-1 and video-rate UV confocal laser-scanning microscopy. The purpose of the present study was to investigate how local Ca(2+) changes contribute to a global Ca(2+) signal; subsequently we quantified how a Ca(2+) wave is kinetically reshaped as it is propagated through the cell. The combined kinetics of all subcellular Ca(2+) signals determined the shape of the total cellular Ca(2+) signal, but each subcellular contribution to the cellular signal was not constant in time. Near the plasma membrane, [Ca(2+)](i) increased and decreased rapidly, processes that can be described by a linear and exponential function, respectively. In more central parts of the cell slower kinetics were observed that were best described by a Hill equation. This reshaping of the Ca(2+) wave was modeled with an equation derived from a low-pass RC filter. We propose that the differences in spatial kinetics of the Ca(2+) signal serves as a mechanism by which the same cellular Ca(2+) signal carries different regulatory information to different subcellular regions of the cell, thus evoking differential cellular responses.