Intracellular Ca2+ contributes to K+-induced increase in renal kallikrein secretion

We have reported that natriuretic effects of K(+) are involved in enhancement of renal kallikrein-kinin system. The study was aimed to examine 1) comparison of augmentative effects of K(+) on urinary KK excretion with non-specific washout effects by trichlormethiazide (thiazide), polyethyleneglycol 200 (PEG) and rapid physiological saline infusion, 2) contribution of Ca(2+) on the K(+)-induced increase in renal kallikrein secretion. Renal kallikrein activities were measured as fluorescence activities of methylcoumarinylamide-labeled synthetic substrate of tissue kallikrein (TK). Increases in urinary TK excretion were simultaneously observed with diuresis caused by thiazide, PEG, and rapid saline infusion. K(+) infusion increased urinary TK excretion with a diuretic response same as the control. K(+), but not thiazide, showed an early increase in renal TK secretion dose dependently in the kidney slices. Increases in renal TK secretion persisted during treatment with K(+). Neither voltage-dependent Ca(2+)-channel blockers such as verapamil and nifedipine nor simultaneous treatment of EDTA affected on the K(+)-induced increase in renal TK secretion. While, EDTA decreased the K(+)-induced increases in renal TK secretion with time. Caffeine also had an early effect on the increase in renal TK secretion. K(+)-induced increases in renal TK secretion was demonstrated even after treatment with ryanodine or depletion of caffeine-sensitive intracellular Ca(2+) by thapsigargin. It was indicated that the increase in renal TK secretion by K(+) depends on the intracellular Ca(2+) and the caffeine-sensitive release of intracellular Ca(2+) may not be involved in this response. Mechanisms for the K(+)-induced increase in renal TK secretion needs to be further elucidated.

The bacterial signal molecule, ppGpp, mediates the environmental regulation of both the invasion and intracellular virulence gene programs of Salmonella

During infection of mammalian hosts, facultative intracellular pathogens have to adjust rapidly to different environmental conditions encountered during passage through the gastrointestinal tract and following uptake into epithelial cells and macrophages. Successful establishment within the host therefore requires the coordinated expression of a large number of virulence genes necessary for the adaptation between the extracellular and intracellular phases of infection. In this study we show that the bacterial signal molecule, ppGpp, plays a major role in mediating the environmental signals involved in the regulation of both the extracellular and intracellular virulence gene programs. Under oxygen limiting conditions, we observed a strong ppGpp dependence for invasion gene expression, the result of severe reductions in expression of the Salmonella pathogenicity island (SPI) 1 transcriptional regulator genes hilA, C, and D and invF. Overexpression of the non-SPI1-encoded regulator RtsA restored hilA expression in the absence of ppGpp. SPI2-encoded genes, required for intracellular proliferation in macrophages, were activated in the wild type strain under aerobic, late log phase growth conditions. The expression of SPI2 genes was also shown to be ppGpp-dependent under these conditions. The results from this study suggest a mechanism for the alternate regulation of the opposing extracellular and intracellular virulence gene programs and indicate a remarkable specificity for ppGpp in the regulation of genes involved in virulence compared with the rest of the genome. This is the first demonstration that this highly conserved regulatory system is involved in bacterial virulence gene expression on a global scale.

Differential effects of mitochondrial heat shock protein 60 and related molecular chaperones to prevent intracellular beta-amyloid-induced inhibition of complex IV and limit apoptosis

Defects in mitochondrial oxidative metabolism, in particular decreased activity of cytochrome c oxidase, have been reported in Alzheimer disease tissue and in cultured cells that overexpress amyloid precursor protein. Mitochondrial dysfunction contributes to neurodegeneration in Alzheimer disease partly through formation of reactive oxygen species and the release of sequestered molecules that initiate programmed cell death pathways. The heat shock proteins (HSP) are cytoprotective against a number of stressors, including accumulations of misfolded proteins and reactive oxygen species. We reported on the property of Hsp70 to protect cultured neurons from cell death caused by intraneuronal beta-amyloid. Here we demonstrate that Hsp60, Hsp70, and Hsp90 both alone and in combination provide differential protection against intracellular beta-amyloid stress through the maintenance of mitochondrial oxidative phosphorylation and functionality of tricarboxylic acid cycle enzymes. Notably, beta-amyloid was found to selectively inhibit complex IV activity, an effect selectively neutralized by Hsp60. The combined effect of HSPs was to reduce the free radical burden, preserve ATP generation, decrease cytochrome c release, and prevent caspase-9 activation, all important mediators of beta-amyloid-induced neuronal dysfunction and death.

Intracellular heterogeneity in adhesiveness of endothelium affects early steps in leukocyte adhesion

Endothelial cell junctions are thought to be preferential sites for transmigration. However, the factors that determine the site of transmigration are not well defined. Our data show that the preferential role of endothelial cell junctions is not limited to transmigration but extends to earlier steps of leukocyte recruitment, such as rolling and arrest. We used primary mouse neutrophils and mouse aortic endothelium in a flow chamber system to compare adhesive interactions near endothelial cell junctions to interactions over endothelial cell centers. We found differences in both rolling velocity and arrest frequency for neutrophils at endothelial cell junctions vs. more central areas of endothelial cells. Differences were governed by adhesion molecule interactions, not local topography. Interestingly, the role of particular adhesion molecules depended on their location on the endothelial cell surface. Although ICAM-1 stabilized and slowed rolling over central areas of the cell, it did not influence rolling velocity over endothelial cell junctions. P-selectin and VCAM-1 were more important for rolling near endothelial cell junctions than E-selectin. This demonstrates that adhesive properties of endothelial cell junctions influence early events in the adhesion cascade, which may help explain how leukocytes are localized to sites of eventual transmigration.

Regulation of antiviral responses by a direct and specific interaction between TRAF3 and Cardif

Upon recognition of viral infection, RIG-I and Helicard recruit a newly identified adapter termed Cardif, which induces type I interferon (IFN)-mediated antiviral responses through an unknown mechanism. Here, we demonstrate that TRAF3, like Cardif, is required for type I interferon production in response to intracellular double-stranded RNA. Cardif-mediated IFNalpha induction occurs through a direct interaction between the TRAF domain of TRAF3 and a TRAF-interaction motif (TIM) within Cardif. Interestingly, while the entire N-terminus of TRAF3 was functionally interchangeable with that of TRAF5, the TRAF domain of TRAF3 was not. Our data suggest that this distinction is due to an inability of the TRAF domain of TRAF5 to bind the TIM of Cardif. Finally, we show that preventing association of TRAF3 with this TIM by mutating two critical amino acids in the TRAF domain also abolishes TRAF3-dependent IFN production following viral infection. Thus, our findings suggest that the direct and specific interaction between the TRAF domain of TRAF3 and the TIM of Cardif is required for optimal Cardif-mediated antiviral responses.

Condition for intracellular adaptive dynamics for chemotaxis

Chemotaxis is ubiquitously performed by bacteria. They sense and move toward a region with a higher concentration of an attractive chemical by changing the rate of tumbling for random walk. We numerically studied several models with internal adaptive dynamics to examine the validity of the condition for chemotaxis proposed by Oosawa and Nakaoka [J. Theor. Biol. 66, 747 (1977)], which states that the time scale of tumbling frequency must be smaller than that of adaptation and greater than that of sensing. Suitable renormalization of the time scales showed that the condition holds for a variety of environments and for both short- and long-term behavior.

A basolateral sorting signal directs ADAM10 to adherens junctions and is required for its function in cell migration

ADAM10 (a disintegrin and metalloprotease) initiates regulated intramembrane proteolysis by shedding the ectodomain of a number of different substrates. Shedding is followed by subsequent intramembrane proteolysis leading to the liberation of intracellular domains capable of nuclear signaling. ADAM10 substrates have been found at cell-cell contacts and are apparently involved in cell-cell interaction and cell migration. Here we have investigated the cellular mechanism that guides ADAM10 to substrates at cell-cell contacts. We demonstrate that intracellular trafficking of ADAM10 critically requires a novel sorting signal within its cytoplasmic domain. Sequential deletion of the cytoplasmic domain and site-directed mutagenesis suggest that a potential Src homology 3-binding domain is essential for ADAM10 sorting. In a polarized epithelial cell line this motif not only targets ADAM10 to adherens junctions but is also strictly required for ADAM10 function in E-cadherin processing and cell migration.

Developmental regulation of intracellular calcium homeostasis in early cardiac myocytes

The proper intracellular Ca(2+) signaling is essential for normal cell functions and organ development, and the maintaining Ca(2+) homeostasis in cardiac myocytes is of functional importance for the intact heart. As the first functional organ in the vertebrate embryo, the heart is continuously remodeled and maintains its physiologic pumping function in response to increasing circulatory demands. The expressions of Ca(2+) handing proteins in the embryonic heart, however, are different from those in neonatal and adult hearts, which means that the regulation of Ca(2+) transients in embryonic cardiomyocytes is different from that in adult cardiac myocytes. Recent advances in molecular and cellular biology, as well as the application of embryonic stem cell differentiation system, have made progress in uncovering the regulation of Ca(2+) homeostasis during cardiomyogenesis. This paper briefly summarizes the Ca(2+) homeostasis during early development of cardiomyocytes and reviews current knowledge of the regulatory mechanisms controlling Ca(2+) homeostasis during cardiomyocyte development.

[Regulative effects of temperature, intracellular sodium, ATP and pH on I(Na/Ca) of cardiac myocytes]

The Na(+)-Ca(2+) exchange is a major pathway for removal of cytosolic Ca(2+) in cardiac myocytes. To explore the effects of temperature, intracellular Na(+), ATP and pH on Na(+)-Ca(2+) exchange currents (I(Na/Ca)) of intact guinea-pig myocytes, the whole-cell patch-clamp technique was used to record I(Na/Ca) in isolated guinea-pig ventricular myocytes. We found that I(Na/Ca) at 34 degrees C was four times higher than that at 22 degrees C. However, intracellular acidification had no obvious influence on bidirectional I(Na/Ca). At 22~24 degrees C , intracellular ATP depletion and intracellular acidification did not markedly affect bidirectional I(Na/Ca) either. At 34~37 degrees C , intracellular ATP depletion and intracellular acidification synergistically inhibited the outward and inward currents of I(Na/Ca), and blocked the inward currents of I(Na/Ca)more potently than the outward currents of I(Na/Ca). The effect of ATP on I(Na/Ca) is temperature-dependent. Intracellular higher sodium increased the outward currents of I(Na/Ca) however it did not increase, even sometimes decreased the inward currents of I(Na/Ca). These results suggest that intracellular ATP depletion and intracellular acidification synergistically impair Ca(2+) extrusion via forward mode Na(+)-Ca(2+) exchange, and intracellular sodium overload increases Ca(2+) influx via reverse mode Na(+)-Ca(2+) exchange, leading to calcium overload respectively.

Dichotomy of Ca2+ in the heart: contraction versus intracellular signaling

Ca2+ plays a pivotal role in both excitation-contraction coupling (ECC) and activation of Ca2+-dependent signaling pathways. One of the remaining questions in cardiac biology is how Ca2+-dependent signaling pathways are regulated under conditions of continual Ca2+ transients that mediate cardiac contraction during each heartbeat. Ca2+-calmodulin-dependent protein kinase II (CaMKII) activation and its ability to regulate histone deacetylase 5 (HDAC5) nuclear shuttling represent a critical Ca2+-dependent signaling circuit for controlling cardiac hypertrophy and heart failure, yet the mechanism of activation by Ca2+ is not known. In this issue of the JCI, Wu et al. convincingly demonstrate that the inositol 1,4,5-trisphosphate receptor (InsP3R) is involved in local control of Ca2+ for activating CaMKII in the nuclear envelope of adult ventricular cardiac myocytes (see the related article beginning on page 675). The overall paradigm that is demonstrated is the best example of a molecular mechanism whereby signaling is directly regulated by a local Ca2+ pool that is disparate or geometrically insensitive to cytosolic Ca2+ underlying each contractile cycle.

Intercellular waves propagation in an array of cells coupled through paracrine signaling: a computer simulation study

A linear chain of cells is considered in which calcium (Ca2+) fluctuations within a cell are described by a simple minimal model. Cells are coupled together by bidirectional paracrine signaling via calcium oscillations. Two typical zones of propagation are observed: a transition zone and a regular zone. The transition zone exhibits the same phenomena that can be observed in single cells, pairs or triplets of cells. Within the regular zone, simple periodic oscillations of calcium propagate and the Ca2+ signal is similar from one cell to another (same amplitude and same frequency). But, the signals are separated by a slight phase shift characterizing the propagation of Ca2+ waves due to the type of coupling used. We also consider the colonization of the lattice by the abnormal oscillations of sick cells.

Theory of spatial patterns of intracellular organelles

Here we report on a generalized theory of spatial patterns of intracellular organelles, which are controlled by cells using cytoskeleton-based movements powered by molecular motors. The theory reveals that organelles exhibit one of the four distinct, stable patterns, namely aggregation, hyperdispersion, radial dispersion, and areal dispersion. Existence of specific patterns is determined by the contributions from three transport mechanisms, characterized by two Peclet numbers. The predicted patterns compare well with experimental data. This study provides a firm theoretical ground for classification of spatial patterns of organelles and understanding their regulation by cells.

Cellular fluid mechanics and mechanotransduction

Mechanotransduction, the transformation of an applied mechanical force into a cellular biomolecular response, is briefly reviewed focusing on fluid shear stress and endothelial cells. Particular emphasis is placed on recent studies of the surface proteoglycan layer (glycocalyx) as a primary sensor of fluid shear stress that can transmit force to apical structures such as the plasma membrane or the actin cortical web where transduction can take place or to more remote regions of the cell such as intercellular junctions and basal adhesion plaques where transduction can also occur. All of these possibilities are reviewed from an integrated perspective.

Translocation of an endoproteolytically cleaved maxi-K channel isoform: mechanisms to induce human myometrial cell repolarization

Large conductance Ca(2+)- and voltage-activated K+ (maxi-K) channels modulate human myometrial smooth muscle cell (hMSMC) excitability; however, the role of individual alternatively spliced isoforms remains unclear. We have previously shown that the transcript of a human maxi-K channel isoform (mK44) is expressed predominantly in myometrial and aortic smooth muscle and forms a functional channel in heterologous expression systems. The mK44 isoform contains unique consensus motifs for both endoproteolytic cleavage and N-myristoylation, although the function of these post-translational modifications is unknown. The goal of these studies was to determine the role of post-translational modifications in regulating mK44 channel function in hMSMCs. An mK44-specific antibody indicated that this channel is localized intracellularly in hMSMCs and translocates to the cell membrane in response to increases in intracellular Ca(2+). Immunological analyses using an N-terminally myc-tagged mK44 construct demonstrated endoproteolytical cleavage of mK44 in hMSMCs resulting in membrane localization of the mK44 N-termini and intracellular retention of the pore-forming C-termini. Caffeine-induced Ca(2+) release from intracellular stores resulted in translocation of the C-termini of mK44 to the cell membrane and co-localization with its N-termini. Translocation of mK44 channels to the cell membrane was concomitant with repolarization of the hMSMCs. Endoproteolytic digest of mK44 did not occur in HEK293 cells or mouse fibroblasts. MK44 truncated at a putative N-myristoylation site did not produce current when expressed alone, but formed a functional channel when co-expressed with the N-terminus. These findings provide novel insight into cell-specific regulation of maxi-K channel function.

Generalizability of extracellular-to-intracellular fluid ratio using bio-impedance spectroscopy

Generalizability theory was used to investigate the score consistency of observed extracellular fluid/intracellular fluid (ECF/ICF) ratio measurements for both the global and leg-segmental bio-impedance spectroscopy methods. The test instrument used was a Xitron Hydra ECF/ICF Bio-Impedance Analyzer System, model 4200 (Xitron Technologies, San Diego, CA). Fifty able-bodied American men (17 to 72 years) and 50 able-bodied American women (17 to 76 years) volunteered as experimental subjects. Xitron continuous global and leg-segmental ECF/ICF procedures for testing were followed for assessing subjects in both the standing erect and lying supine postures. A two-facet, person-by-trial, completely crossed design was used, and all facets were treated as random. Data were independently analyzed for each method, each body posture and each sex group. The major findings of this study were: (1) the leg-segmental method was superior in producing the higher G-coefficients when compared to the global method regardless of gender or posture; (2) the global method resulted in higher G-coefficients in males compared to females regardless of posture; (3) when the global method was used, the relative and absolute error variances were higher for females while the opposite trend was observed when the segmental method was used and (4) when using the global method, the precision of the ECF/ICF ratio scores in females could be increased by simply using the mean of several trials (e.g., by using the mean of 5, 10 or more test trials).

Pediatric fluid and electrolyte balance: critical care case studies

The care of the critically ill infant or child often is complicated further by disruptions in fluid or electrolyte balance. Prompt recognition of these disruptions is essential to the care of these patients. This article provides an overview of the principles of fluid and electrolyte balance in the critically ill infant and child. Imbalances in fluid homeostasis and imbalances in sodium, potassium, and calcium homeostasis are presented in a case study format.

Regulation of Intracellular Calcium by Bupivacaine Isomers in Cardiac Myocytes from Wistar Rats

In this study we investigated the effects of a racemic mixture of bupivacaine (RS(+/-)bupivacaine) and its isomers (S(-)bupivacaine and R(+)bupivacaine) on the Ca2+ handling by ventricular myocytes from Wistar rats. Single ventricular myocytes were enzymatically isolated and loaded with the fluorescent Ca2+ indicator fura 2-am to estimate intracellular Ca2+ concentration during contraction and relaxation cycles. S(-)bupivacaine (10 muM) significantly increased peak amplitude and the rate of increase of Ca2+ transients in 155% +/- 54% (P < 0.05) and 194% +/- 94% (P < 0.01) of control. However, exposure to R(+)bupivacaine had no effect on either peak amplitude or rate of increase at any concentration tested. Saponin-skinned ventricular fibers were used to investigate the effect of bupivacaine on the intracellular Ca2+ regulation by sarcoplasmic reticulum (SR) and on the Ca2+ sensitivity of contractile system. S(-), R(+), and RS(+/-)bupivacaine induced Ca2+ release from SR (P < 0.01). In SR-disrupted skinned ventricular cells, bupivacaine and its isomers (5 mM) increased the sensitivity of contractile system to Ca(2+). S(-), RS(+/-), and R(+)bupivacaine significantly increased pCa50 from 5.8 +/- 0.1, 5.8 +/- 0.1, and 5.8 +/- 0.1, to 6.1 +/- 0.1 (P < 0.05), 6.0 +/- 0.1 (P < 0.05), and 6.1 +/- 0.1 (P < 0.05). Ca2+ release from SR through RyR2 activation could explain the increase of Ca2+ transients in cardiac cells. Increased intracellular Ca2+ in cardiac myocytes display a stereoselectivity to S(-)bupivacaine.

Intracellular targeting of phosphodiesterase-4 underpins compartmentalized cAMP signaling

The phosphodiesterase-4 (PDE4) enzyme belongs to a family of cAMP-dependent phosphodiesterases that provide the major means of hydrolyzing and, thereby, inactivating the key intracellular second messenger, cAMP. As such, PDE4s are central to the regulation of many diverse signaling processes that allow cells to respond to external stimuli. Four genes (4A, 4B, 4C, and 4D) encode around 20 distinct isoform members of the PDE4 family. Each isoform is characterized by a unique N-terminal region. PDE4s are multidomain metallohydrolases with each domain serving particular roles allowing them to be targeted to varying regions and organelles of intracellular space and regulated in distinct fashions by phosphorylation and protein-protein interaction. Although identical in catalytic function, each isoform locates to distinct regions within the cell so as to create and manage spatially distinct pools of cAMP. The multiplicity of partners associating with members of the four gene PDE4 family places these enzymes in key regulatory positions, permitting them to channel complex biological signals via fundamental signaling cohorts such as G-protein-coupled receptors (GPCRs), arrestins, A-kinase-anchoring proteins (AKAPs), and tyrosyl family kinases. The cAMP cascade has long been linked to cellular growth and embryogenesis and with this comes the implication that PDE4 may play considerable roles in the regulation of progeny development in maturing cells and tissues.

Redox equilibrium in mucosal T cells tunes the intestinal TCR signaling threshold

Mucosal immune tolerance in the healthy intestine is typified by lamina propria T cell (LPT) functional hyporesponsiveness after TCR engagement when compared with peripheral blood T cell (PBT). When LPT from an inflamed intestine are activated through TCR cross-linking, their responsiveness is stronger. LPT are thus capable of switching from a tolerant to a reactive state, toggling between high and low thresholds of activation. We demonstrate that in normal LPT global tyrosine phosphorylation upon TCR cross-linking or an increase in intracellular H2O2, an inhibitor of protein tyrosine phosphatases, is muted. Thus, we propose that LPT have a greater reducing capacity than PBT, shifting the balance between kinases and protein tyrosine phosphatases in favor of the latter. Surface gamma-glutamyl transpeptidase, an indirect indicator of redox potential, and glutathione are significantly elevated in LPT compared with PBT, suggesting that elevated glutathione detoxifies TCR-induced reactive oxygen species. When glutathione is depleted, TCR-induced LPT tyrosine phosphorylation rises to PBT levels. Conversely, increasing glutathione in PBT attenuates tyrosine phosphorylation. In LPT isolated from inflamed mucosa, TCR cross-linking induces greater phosphorylation, and gamma-glutamyl transpeptidase levels are reduced compared with those from autologous noninflamed tissue. We conclude that the high TCR signaling threshold of mucosal T cells is tuned by intracellular redox equilibrium, whose dysregulation may mediate intestinal inflammation.

Subdividing the embryo: a role for Notch signaling during germ layer patterning in Xenopus laevis

The development of all vertebrate embryos requires the establishment of a three-dimensional coordinate system in order to pattern embryonic structures and create the complex shape of the adult organism. During the process of gastrulation, the three primary germ layers are created under the guidance of numerous signaling pathways, allowing cells to communicate during development. Cell-cell communication, mediated by receptors of the Notch family, has been shown to be involved in mediating diverse cellular behaviors during development and has been implicated in the regulation of cell fate decisions in both vertebrate and invertebrate organisms. In order to investigate a role for Notch signaling during boundary formation between the mesoderm and endoderm during gastrulation, we manipulated Notch signaling in gastrula stage embryos and examined gene expression in resultant tissues and organs. Our findings demonstrate a much broader role for Notch signaling during germ layer determination than previously reported in a vertebrate organism. Activation of the Notch pathway, specifically in gastrula stage embryos, results in a dramatic decrease in the expression of genes necessary to create many different types of mesodermal tissues while causing a dramatic expansion of endodermal tissue markers. Conversely, temporally controlled suppression of this pathway results in a loss of endodermal cell types and an expansion of molecular markers of mesoderm. Thus, our data are consistent with and significantly extend the implications of prior observations suggesting roles for Notch signaling during germ layer formation and establish an evolutionarily conserved role for Notch signaling in mediating mesoderm-endoderm boundaries during early vertebrate development.

Intracellular interleukin-1 receptor antagonist type 1 antagonizes the stimulatory effect of interleukin-1α precursor on cell motility

Interleukin (IL)-1alpha, a proinflammatory cytokine, is produced as a 33 kDa protein precursor (preIL-1alpha) which is cleaved to generate the 17 kDa C-terminal mature IL-1alpha (mIL-1alpha) and the 16kDa N-terminal IL-1alpha propiece (NIL-1alpha). The biological effect of IL-1alpha is regulated by the IL-1 receptor antagonist (IL-1Ra), its naturally occurring inhibitor. Four different isoforms of the IL-1Ra have been described, one secreted (sIL-1Ra) and three intracellular (icIL-1Ra1, 2, 3). Whether the icIL-1Ra1 isoform can antagonize some of the biological effects of intracellular IL-1alpha is still unknown. The aim of this study is to investigate effects of preIL-1alpha and icIL-1Ra1 on cell motility in stably transfected ECV304 cells. We show that expression of preIL-1alpha in ECV304 cells significantly increases cell motility. Furthermore, transfection with NIL-1alpha propiece also increases cell motility whereas this stimulatory effect was not observed by addition of exogenous mIL-1alpha, suggesting an intracellular effect of preIL-1alpha mediated by NIL-1alpha propiece. Co-transfection of ECV304 cells with icIL-1Ra1 completely antagonizes the stimulatory effect of preIL-1alpha and NIL-1alpha propiece on cell motility. In conclusion, NIL-1alpha propiece increases ECV304 cell motility and icIL-1Ra1 exerts intracellular functions regulating this stimulatory effect.

To be or not to be ... nicotinic acid adenine dinucleotide phosphate (NAADP): a new intracellular second messenger?

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent activator of intracellular Ca2+ release in several vertebrate and invertebrate systems. The role of the NAADP system in physiological processes is being extensively investigated at the present time. The NAADP receptor and its associated Ca2+ pool have been hypothesized to be important in several physiological processes including fertilization, T cell activation, and pancreatic secretion. However, whether NAADP is a new second messenger or a tool for the discovery of a new Ca2+ channel is still an unanswered question. Research developed over the last two years has provided some important clues to whether NAADP is or not a physiological cellular messenger. In this short review, I will discuss some of these new findings that are helping us to find an answer to the important question: Is NAADP a second messenger or not?

Stabilizing role of calcium store-dependent plasma membrane calcium channels in action-potential firing and intracellular calcium oscillations

In many biological systems, cells display spontaneous calcium oscillations (CaOs) and repetitive action-potential firing. These phenomena have been described separately by models for intracellular inositol trisphosphate (IP3)-mediated CaOs and for plasma membrane excitability. In this study, we present an integrated model that combines an excitable membrane with an IP3-mediated intracellular calcium oscillator. The IP3 receptor is described as an endoplasmic reticulum (ER) calcium channel with open and close probabilities that depend on the cytoplasmic concentration of IP3 and Ca2+. We show that simply combining this ER model for intracellular CaOs with a model for membrane excitability of normal rat kidney (NRK) fibroblasts leads to instability of intracellular calcium dynamics. To ensure stable long-term periodic firing of action potentials and CaOs, it is essential to incorporate calcium transporters controlled by feedback of the ER store filling, for example, store-operated calcium channels in the plasma membrane. For low IP3 concentrations, our integrated NRK cell model is at rest at -70 mV. For higher IP3 concentrations, the CaOs become activated and trigger repetitive firing of action potentials. At high IP3 concentrations, the basal intracellular calcium concentration becomes elevated and the cell is depolarized near -20 mV. These predictions are in agreement with the different proliferative states of cultures of NRK fibroblasts. We postulate that the stabilizing role of calcium channels and/or other calcium transporters controlled by feedback from the ER store is essential for any cell in which calcium signaling by intracellular CaOs involves both ER and plasma membrane calcium fluxes.

Fibroblast growth factor 14 is an intracellular modulator of voltage-gated sodium channels

Genetic ablation of the fibroblast growth factor (Fgf) 14 gene in mice or a missense mutation in Fgf14 in humans causes ataxia and cognitive deficits. These phenotypes suggest that the neuronally expressed Fgf14 gene is essential for regulating normal neuronal activity. Here, we demonstrate that FGF14 interacts directly with multiple voltage-gated Na(+) (Nav) channel alpha subunits heterologously expressed in non-neuronal cells or natively expressed in a murine neuroblastoma cell line. Functional studies reveal that these interactions result in the potent inhibition of Nav channel currents (I(Na)) and in changes in the voltage dependence of channel activation and inactivation. Deletion of the unique amino terminus of the splice variant of Fgf14, Fgf14-1b, or expression of the splice variant Fgf14-1a modifies the modulatory effects on I(Na), suggesting an important role for the amino terminus domain of FGF14 in the regulation of Na(v) channels. To investigate the function of FGF14 in neurones, we directly expressed Fgf14 in freshly isolated primary rat hippocampal neurones. In these cells, the addition of FGF14-1a-GFP or FGF14-1b-GFP increased I(Na) density and shifted the voltage dependence of channel activation and inactivation. In fully differentiated neurones, FGF14-1a-GFP or FGF14-1b-GFP preferentially colocalized with endogenous Nav channels at the axonal initial segment, a critical region for action potential generation. Together, these findings implicate FGF14 as a unique modulator of Nav channel activity in the CNS and provide a possible mechanism to explain the neurological phenotypes observed in mice and humans with mutations in Fgf14.