D-Aspartate as a putative cell-cell signaling molecule in the Aplysia californica central nervous system

The content, synthesis and transport of d ‐aspartate (d ‐Asp) in the CNS of Aplysia californica is investigated using capillary electrophoresis (CE) with both laser‐induced fluorescence and radionuclide detection. Millimolar concentrations of d ‐Asp are found in various regions of the CNS. In the cerebral ganglion, three adjacent neuronal clusters have reproducibly different d ‐Asp levels; for example, in the F‐ and C‐clusters, up to 85% of the free Asp is present in the d ‐form. Heterogeneous distribution of d ‐Asp is also found in the individual identified neurons tested, including the optical ganglion top‐layer neurons, metacerebral cells, R2 neurons, and F‐, C‐ and G‐cluster neurons. The F‐cluster neurons have the highest percentage of d ‐Asp (～58% of the total Asp), whereas the lowest value of ～8% is found in R2 neurons. In pulse‐chase experiments with radiolabeled d ‐Asp, followed by CE with radionuclide detection, the synthesis of d ‐Asp from l ‐aspartate (l ‐Asp) is confirmed. Is d ‐Asp in the soma, or is it transported to distantly located release sites? d ‐Asp is clearly detected in the major nerves of A. californica, including the pleuroabdominal and cerebrobuccal connectives and the anterior tentacular nerves, suggesting it is transported long distances. In addition, both d ‐Asp and l ‐Asp are transported in the pleuroabdominal connectives in a colchicine‐dependent manner, whereas several other amino acids are not. Finally, d ‐Asp produces electrophysiological effects similar to those induced by l ‐Asp. These data are consistent with an active role for d ‐Asp in cell‐to‐cell communication.     

Purinergic signaling in neural development

Extracellular purine and pyrimidine compounds induce a multiplicity of cellular signal pathways that can induce multiple trophic functions. They interact with other low molecular weight messengers, growth factors, and extracellular matrix components. An increasing number of studies now provide evidence for a role of purinergic signaling in neural development, including progenitor cell proliferation, cell migration, neuronal and glial maturation and differentiation, and cell death and survival. This brief overview highlights recent developments supporting a contribution of purinergic signaling to embryonic and adult neurogenesis.     

The adhesion signaling molecule p190 RhoGAP is required for morphogenetic processes in neural development

Rho GTPases direct actin rearrangements in response to a variety of extracellular signals. P190 RhoGAP (GTPase activating protein) is a potent Rho regulator that mediates integrin-dependent adhesion signaling in cultured cells. We have determined that p190 RhoGAP is specifically expressed at high levels throughout the developing nervous system. Mice lacking functional p190 RhoGAP exhibit several defects in neural development that are reminiscent of those described in mice lacking certain mediators of neural cell adhesion. The defects reflect aberrant tissue morphogenesis and include abnormalities in forebrain hemisphere fusion, ventricle shape, optic cup formation, neural tube closure, and layering of the cerebral cortex. In cells of the neural tube floor plate of p190 RhoGAP mutant mice, polymerized actin accumulates excessively, suggesting a role for p190 RhoGAP in the regulation of +Rho-mediated actin assembly within the neuroepithelium. Significantly, several of the observed tissue fusion defects seen in the mutant mice are also found in mice lacking MARCKS, the major substrate of protein kinase C (PKC), and we have found that p190 RhoGAP is also a PKC substrate in vivo. Upon either direct activation of PKC or in response to integrin engagement, p190 RhoGAP is rapidly translocated to regions of membrane ruffling, where it colocalizes with polymerized actin. Together, these results suggest that upon activation of neural adhesion molecules, the action of PKC and p190 RhoGAP leads to a modulation of Rho GTPase activity to direct several actin-dependent morphogenetic processes required for normal neural development.     

Inorganic phosphate as a signaling molecule in osteoblast differentiation

The spatial and temporal coordination of the many events required for osteogenic cells to create a mineralized matrix are only partially understood. The complexity of this process, and the nature of the final product, demand that these cells have mechanisms to carefully monitor events in the extracellular environment and have the ability to respond through cellular and molecular changes. The generation of inorganic phosphate during the process of differentiation may be one such signal. In addition to the requirement of inorganic phosphate as a component of hydroxyapatite mineral, Ca(10)(PO(4))(6)(OH)(2), a number of studies have also suggested it is required in the events preceding mineralization. However, contrasting results, physiological relevance, and the lack of a clear mechanism(s) have created some debate as to the significance of elevated phosphate in the differentiation process. More recently, a number of studies have begun to shed light on possible cellular and molecular consequences of elevated intracellular inorganic phosphate. These results suggest a model in which the generation of inorganic phosphate during osteoblast differentiation may in and of itself represent a signal capable of facilitating the temporal coordination of expression and regulation of multiple factors necessary for mineralization. The regulation of protein function and gene expression by elevated inorganic phosphate during osteoblast differentiation may represent a mechanism by which mineralizing cells monitor and respond to the changing extracellular environment.     

Extracellular ATP as a signaling molecule for epithelial cells

The charge of this invited review is to present a convincing case for the fact that cells release their ATP for physiological reasons. Many of our "purinergic" colleagues as well as ourselves have experienced resistance to this concept, because it is teleologically counter-intuitive. This review serves to integrate the three main tenets of extracellular ATP signaling: ATP release from cells, ATP receptors on cells, and ATP receptor-driven signaling within cells to affect cell or tissue physiology. First principles will be discussed in the Introduction concerning extracellular ATP signaling. All possible cellular mechanisms of ATP release will then be presented. Use of nucleotide and nucleoside scavengers as well as broad-specificity purinergic receptor antagonists will be presented as a method of detecting endogenous ATP release affecting a biological endpoint. Innovative methods of detecting released ATP by adapting luciferase detection reagents or by using "biosensors" will be presented.Because our laboratory has been primarily interested in epithelial cell physiology and pathophysiology for several years, the role of extracellular ATP in regulation of epithelial cell function will be the focus of this review. For ATP release to be physiologically relevant, receptors for ATP are required at the cell surface. The families of P2Y G protein-coupled receptors and ATP-gated P2X receptor channels will be introduced. Particular attention will be paid to P2X receptor channels that mediate the fast actions of extracellular ATP signaling, much like neurotransmitter-gated channels versus metabotropic heptahelical neurotransmitter receptors that couple to G proteins. Finally, fascinating biological paradigms in which extracellular ATP signaling has been implicated will be highlighted. It is the goal of this review to convert and attract new scientists into the exploding field of extracellular nucleotide signaling and to convince the reader that extracellular ATP is indeed a signaling molecule.     

Mitochondria and calcium signaling in embryonic development

Calcium (Ca²⁺) is a simple but critical signal for controlling various cellular processes and is especially important in fertilization and embryonic development. The dynamic change of cellular Ca²⁺ concentration and homeostasis are tightly regulated. Cellular Ca²⁺ increases by way of Ca²⁺ influx from extracellular medium and Ca²⁺ release from cellular stores of the endoplasmic reticulum (ER) and sarcoplasmic reticulum (SR). The elevated Ca²⁺ is subsequently sequestered by expelling it out of the cell or by pumping back to the ER/SR. Mitochondria function as a power house for energy production via oxidative phosphorylation in most eukaryotes. In addition to this well-known function, mitochondria are also recognized to regulate Ca²⁺ homeostasis through different mechanisms. Although critical roles of Ca²⁺ signaling in fertilization and embryonic development are known, the involvement of mitochondria in these processes are not fully understood. This review is focused on the role of mitochondrial respiratory chain complex I in the regulation of Ca²⁺ signaling pathway and gene expression in embryonic development, especially on the new findings in the cardiac development of Xenopus embryos. The data demonstrate that mitochondria modulate Ca²⁺ signaling and the Ca²⁺-dependent NFAT pathway and its target gene which are essential for embryonic heart development.     

d-Aspartate acts as a signaling molecule in nervous and neuroendocrine systems

d-Aspartate (d-Asp) is an endogenous amino acid in the central nervous and reproductive systems of vertebrates and invertebrates. High concentrations of d-Asp are found in distinct anatomical locations, suggesting that it has specific physiological roles in animals. Many of the characteristics of d-Asp have been documented, including its tissue and cellular distribution, formation and degradation, as well as the responses elicited by d-Asp application. d-Asp performs important roles related to nervous system development and hormone regulation; in addition, it appears to act as a cell-to-cell signaling molecule. Recent studies have shown that d-Asp fulfills many, if not all, of the definitions of a classical neurotransmitter—that the molecule’s biosynthesis, degradation, uptake, and release take place within the presynaptic neuron, and that it triggers a response in the postsynaptic neuron after its release. Accumulating evidence suggests that these criteria are met by a heterogeneous distribution of enzymes for d-Asp’s biosynthesis and degradation, an appropriate uptake mechanism, localization within synaptic vesicles, and a postsynaptic response via an ionotropic receptor. Although d-Asp receptors remain to be characterized, the postsynaptic response of d-Asp has been studied and several l-glutamate receptors are known to respond to d-Asp. In this review, we discuss the current status of research on d-Asp in neuronal and neuroendocrine systems, and highlight results that support d-Asp’s role as a signaling molecule.     

Neogenin and repulsive guidance molecule signaling in the central nervous system

The repulsive guidance molecule (RGM) is a membrane-bound protein that was originally identified as an axon guidance molecule in the visual system. Functional studies have revealed that it has roles in axon guidance and laminar patterning in Xenopus and chick embryos, and in controlling cephalic neural tube closure in mouse embryos. The recent identification of neogenin as a receptor for RGM has provided evidence of the diverse functions of this ligand-receptor pair. Re-expression of RGM is observed after injury in the adult human and rat central nervous systems. Inhibition of RGM enhances growth of injured axons and promotes functional recovery after spinal cord injury in rats. Thus, re-expression of embryonic repulsive cues in adult tissues contributes to failure of axon regeneration in the central nervous system.     

Glutathione as a signaling molecule: Another challenge to pathogens

Plants harbor a variety of signaling molecules which are members of a vast array of signaling networks in maintaining their physiological balance. The well known members up till now are salicylic acid (SA), jasmonic acid (JA), ethylene (ET), abscissic acid (ABA) and reactive oxygen species (ROS) which are employed by plants for their adaptation to various environmental stresses in order to survive. GSH is gradually gaining importance and becoming a molecule of interest to a number of researchers especially in relation to plant defense to pathogens. Although the role of GSH in plant defense has long been known, a dearth of information still exists regarding the mechanism underlying this defense response. This review highlights on the progress made in the cross-communication of GSH with other established signaling molecules through which GSH acts in abating biotic stress.Key words: glutathione, salicylic acid, biotrophic pathogen, NPR1, crosstalk, signalingGlutathione, popularly known as the “master anioxidant” or “super defender,” is a nearly ubiquitous non-protein tripeptide thiol compound found in both prokaryotes and eukaryotes^1–3 except for some organisms which use different other thiol cofactors. This molecule helps to prevent or even reduce the effect of certain human diseases which are of major concern in today''s world including cancer, inflammation, kwashiorkor, Alzheimer''s disease, Parkinson''s disease, sickle cell anaemia, liver disease, cystic fibrosis, HIV, AIDS, infection, heart attack, stroke and diabetes. GSH, in animals, participates in the detoxification of ROS and xenobiotics, plays a major role in cell proliferation and death, DNA synthesis and repair, regulation of protein synthesis, prostaglandin synthesis, amino acid transport and enzyme activation, maintains essential thiol status, regulates immune functions, plays a role in spermatogenesis and sperm maturation and so on. In prokaryotes, GSH is one of the most abundant thiols as well and is present in cyanobacteria and proteobacteria. In bacteria, in addition to its key role in maintaining the proper oxidation state of protein thiols, GSH also serves a key function in protecting the cell from the action of low pH, chlorine compounds and oxidative as well as osmotic stresses.The well known functions performed by this molecule in plants are as a major player in redox chemistry, heavy metals and electrophilic xenobiotics elimination, serving as electron donor for biochemical reactions, long-distance transport of reduced sulfur, stress defense gene expression, posttranslational modifications through glutathionylation, role in biotic and abiotic stresses and so on.^4–13 Over the past three decades, GSH has been known to be involved in defense reactions against a variety of pathogens in addition to the induction of various defense genes. GSH when supplied at various concentrations to the cell suspension culture of bean induced several genes encoding enzymes that participate in the biosynthesis of lignin and phytoalexins.¹⁴ Early reports also found that GSH supplementation partly mimicked induction of chalcone synthase, the expression of which occurs as a result of fungal elicitor stimulation in soybean.¹⁵ Bean and soybean cells treated with fungal elicitor or GSH causes the rapid insolubilization of hydroxy-proline-rich structural proteins in the cell wall.¹⁶ Previous report also revealed that significant increase in GSH levels occurred as a result of enhanced resistance of melon and tomato roots against Fusarium oxysporum brought about by herbicides.¹⁷ In compatible barley-barley powdery mildew interactions the ascorbate-GSH cycle and other antioxidative enzymes (e.g., glutathione S-transferase) are activated and these processes might diminish the damaging effects of oxidative stress. However, in incompatible interactions these antioxidative reactions are not or are only slightly activated.¹⁸ A considerable accumulation of GSH and, in particular, oxidized glutathione (GSSG) has been observed in tomato cells carrying Cf-9 or Cf-2 resistance genes after treatment with race-specific elicitors of the fungus Cladosporium fulvum.¹⁹ Ball et al. reported that 32 stress-responsive genes were altered due to changed GSH metabolism in Arabidopsis rax1-1 and cad2-1, mutants of γ-ECS. Previous studies also reported that Arabidopsis pad2-1 mutant with only 22% of wild-type amounts of GSH were susceptible to Pseudomonas syringae as well as Phytophthora brassicae.^20–22 Additionally, according to a recent report, enhanced resistance of transgenic tobacco with enhanced level of GSH was observed against P. syringae.²³Recent reports have found the role of GSH in cell signaling and in signaling pathways induced by different phytohormones. This review was an effort to throw some light on understanding the role of GSH as a signaling molecule by discussing the interrelationship of this multi-faceted molecule with other established signaling molecules which have well documented signaling roles in plants against biotic and abiotic stresses.     

Ion fluxes and neurotransmitters signaling in neural development

The brain develops and functions in a complex ionic milieu, which is a prerequisite for neurotransmitter function and neuronal signaling. Neurotransmitters and ion fluxes are, however, important not only in neuronal signaling, but also in the control of neural differentiation, and in this review, we highlight the recent advances in our understanding of how the gamma-amino butyric acid (GABA) neurotransmitter and ion fluxes are relevant for cell cycle control and neural differentiation. Conversely, proteins previously associated with ion transport across membranes have been endowed with novel ion-independent functions, and we discuss this in the context of gap junctions in cell adhesion and of the neuron-specific K(+)-Cl(-) cotransporter KCC2 in dendritic spine development. Collectively, these findings provide a richer and more complex picture of when ion fluxes are needed in neural development and when they are not.     

Redox signaling in central neural regulation of cardiovascular function

One of the most prominent concepts to emerge in cardiovascular research over the past decade, especially in areas focused on angiotensin II (AngII), is that reactive oxygen species (ROS) are critical signaling molecules in a wide range of cellular processes. Many of the physiological effects of AngII are mediated by ROS, and alterations in AngII-mediated redox mechanisms are implicated in cardiovascular diseases such as hypertension and atherosclerosis. Although most investigations to date have focused on the vasculature as a key player, the nervous system has recently begun to gain attention in this field. Accumulating evidence suggests that ROS have important effects on central neural mechanisms involved in blood pressure regulation, volume homeostasis, and autonomic function, particularly those that involve AngII signaling. Furthermore, oxidant stress in the central nervous system is implicated in the neuro-dysregulation associated with some forms of hypertension and heart failure. The main objective of this review is to discuss the recent progress and prospects for this new field of central redox signaling in cardiovascular regulation, while also addressing the molecular tools that have spurred it forward.