The generation of localized calcium rises mediated by cell adhesion molecules and their role in neuronal growth cone motility

Modulation of Early Stage Neuronal Outgrowth through Out-of-Plane Graphene

The correct wiring of a neural network requires neuron to integrate an incredible repertoire of cues found in their extracellular environment. The astonishing efficiency of this process plays a pivotal role in the correct wiring of the brain during development and axon regeneration. Biologically inspired micro- and nanostructured substrates have been shown to regulate axonal outgrowth. In parallel, several studies investigated graphene's potential as a conductive neural interface, able to enhance cell adhesion, neurite sprouting and outgrowth. Here, we engineered a 3D single- to few-layer fuzzy graphene morphology (3DFG), 3DFG on a collapsed Si nanowire (SiNW) mesh template (NT-3DFGc), and 3DFG on a noncollapsed SiNW mesh template (NT-3DFGnc) as neural-instructive materials. The micrometric protruding features of the NWs templates dictated neuronal growth cone establishment, as well as influencing axon elongation and branching. Furthermore, neurons-to-graphene coupling was investigated with comprehensive view of integrin-mediated contact adhesion points and plasma membrane curvature processes.

IGFBPL1 Regulates Axon Growth through IGF-1-mediated Signaling Cascades

Activation of axonal growth program is a critical step in successful optic nerve regeneration following injury. Yet the molecular mechanisms that orchestrate this developmental transition are not fully understood. Here we identified a novel regulator, insulin-like growth factor binding protein-like 1 (IGFBPL1), for the growth of retinal ganglion cell (RGC) axons. Expression of IGFBPL1 correlates with RGC axon growth in development, and acute knockdown of IGFBPL1 with shRNA or IGFBPL1 knockout in vivo impaired RGC axon growth. In contrast, administration of IGFBPL1 promoted axon growth. Moreover, IGFBPL1 bound to insulin-like growth factor 1 (IGF-1) and subsequently induced calcium signaling and mammalian target of rapamycin (mTOR) phosphorylation to stimulate axon elongation. Blockage of IGF-1 signaling abolished IGFBPL1-mediated axon growth, and vice versa, IGF-1 required the presence of IGFBPL1 to promote RGC axon growth. These data reveal a novel element in the control of RGC axon growth and suggest an unknown signaling loop in the regulation of the pleiotropic functions of IGF-1. They suggest new therapeutic target for promoting optic nerve and axon regeneration and repair of the central nervous system.

Lactucopicrin potentiates neuritogenesis and neurotrophic effects by regulating Ca2+/CaMKII/ATF1 signaling pathway

ETHNO-PHARMACOLOGICAL RELEVANCE

Lactucopicrin is one of constitutes in Cichorium intybus L, which is commonly known as chicory in worldwide. It has been used for traditional usage such as antianalgesics, antidepressants and antihyperglycemics AIM OF STUDY: We investigated the neurotrophin-mediated neuroprotective effect of lactucopicrin in in vitro and examined for the underlying mechanism.

MATERIALS AND METHOD

To verify the neuroprotective effect of lactucopicrin, we investigated the inhibitory AChE activity, neurite outgrowth-related downstream signaling in murine neuroblastoma N2a and neurotrophins secretion in rat C6 glioma cells.

RESULTS

Lactucopicrin inhibited the AChE activity and increased intracellular Ca²⁺ levels with a substantial rise in muscarinic acetylcholine receptor M1 (CHRM1) expression in N2a cells. Moreover, lactucopicrin actively promoted neurite outgrowth via Ca²⁺-mediated activation of Ca²⁺/calmodulin-dependent protein kinase-II (CaMKII). It further activates transcription factor 1 (ATF1) along with modulating the levels of tropomyosin receptor kinase A, extracellular signal-regulated kinase 1 and 2, AKT, and synaptophysin 1 in N2a cells. Additionally, the levels of neurotrophins including NGF, BDNF, and NT3 were increased by treatment of lactucopicrin in C6 cells. The effects of lactucopicrin on NGF secretion and neuritogenesis were maintained even in the presence of phosphatidylinositol-3-kinase (PI3K) inhibitor LY294002, indicating that lactucopicrin exerts its effect on neuritogenesis in a PI3K-independent manner.

CONCLUSION

Our results suggest that the natural compound lactucopicrin may be a promising neurotrophin-mediated neuroprotective candidate for neurodegenerative diseases.

The natural product 4,10-aromadendranediol induces neuritogenesis in neuronal cells in vitro through activation of the ERK pathway

Recent studies focus on promoting neurite outgrowth to remodel the central nervous network after brain injury. Currently, however, there are few drugs treating brain diseases in the clinic by enhancing neurite outgrowth. In this study, we established an NGF-induced PC12 differentiation model to screen novel compounds that have the potential to induce neuronal differentiation, and further characterized 4,10-Aromadendranediol (ARDD) isolated from the dried twigs of the Baccharis gaudichaudiana plant, which exhibited the capability of promoting neurite outgrowth in neuronal cells in vitro. ARDD (1, 10 μmol/L) significantly enhanced neurite outgrowth in NGF-treated PC12 cells and N1E115 cells in a time-dependent manner. In cultured primary cortical neurons, ARDD (5, 10 μmol/L) not only significantly increased neurite outgrowth but also increased the number of neurites on the soma and the number of bifurcations. Further analyses showed that ARDD (10 μmol/L) significantly increased the phosphorylation of ERK1/2 and the downstream GSK-3β, subsequently induced β-catenin expression and up-regulated the gene expression of the Wnt ligands Fzd1 and Wnt3a in neuronal cells. The neurite outgrowth-promoting effect of ARDD in neuronal cells was abolished by pretreatment with the specific ERK1/2 inhibitor PD98059, but was partially reversed by XAV939, an inhibitor of the Wnt/β-catenin pathway. ARDD also increased the expression of BDNF, CREB and GAP-43 in N1E115 cells, which was reversed by pretreatment with PD98059. In N1E115 cells subjected to oxygen and glucose deprivation (OGD), pretreatment with ARDD (1-10 μmol/L) significantly enhanced the phosphorylation of ERK1/2 and induced neurite outgrowth. These results demonstrated that the natural product ARDD exhibits neurite outgrowth-inducing activity in neurons via activation of the ERK signaling pathway, which may be beneficial to the treatment of brain diseases.

Molecular mechanisms, biological actions, and neuropharmacology of the growth-associated protein GAP-43

GAP-43 is an intracellular growth-associated protein that appears to assist neuronal pathfinding and branching during development and regeneration, and may contribute to presynaptic membrane changes in the adult, leading to the phenomena of neurotransmitter release, endocytosis and synaptic vesicle recycling, long-term potentiation, spatial memory formation, and learning. GAP-43 becomes bound via palmitoylation and the presence of three basic residues to membranes of the early secretory pathway. It is then sorted onto vesicles at the late secretory pathway for fast axonal transport to the growth cone or presynaptic plasma membrane. The palmitate chains do not serve as permanent membrane anchors for GAP-43, because at steady-state most of the GAP-43 in a cell is membrane-bound but is not palmitoylated. Filopodial extension and branching take place when GAP-43 is phosphorylated at Ser-41 by protein kinase C, and this occurs following neurotrophin binding and the activation of numerous small GTPases. GAP-43 has been proposed to cluster the acidic phospholipid phosphatidylinositol 4,5-bisphosphate in plasma membrane rafts. Following GAP-43 phosphorylation, this phospholipid is released to promote local actin filament-membrane attachment. The phosphorylation also releases GAP-43 from calmodulin. The released GAP-43 may then act as a lateral stabilizer of actin filaments. N-terminal fragments of GAP-43, containing 10-20 amino acids, will activate heterotrimeric G proteins, direct GAP-43 to the membrane and lipid rafts, and cause the formation of filopodia, possibly by causing a change in membrane tension. This review will focus on new information regarding GAP-43, including its binding to membranes and its incorporation into lipid rafts, its mechanism of action, and how it affects and is affected by extracellular agents.

A role for local calcium signaling in rapid synaptic partner selection by dendritic filopodia

Synapse elimination is an important process underlying the establishment of functional neuronal networks during development. Here, we tested the idea that neurons select among potential synaptic partners already during initial contact formation between dendritic filopodia and axons-well before mature synapses are established. We show that filopodia frequently make contact with axons, and while some contacts are selectively stabilized, many are short-lived. More specifically, we demonstrate that contacts with a certain population of GABAergic axons never get stabilized, indicating that filopodia already early on select between different types of axons. Local dendritic calcium transients that are independent of glutamate occur within seconds after contact formation, and their frequency is high where contacts become stabilized and low at short-lived contacts. Thus, filopodia are capable of choosing between potential synaptic partners well before a mature synapse is established.

Peripheral regeneration

Whereas the central nervous system (CNS) usually cannot regenerate, peripheral nerves regenerate spontaneously after injury because of a permissive environment and activation of the intrinsic growth capacity of neurons. Functional regeneration requires axon regrowth and remyelination of the regenerated axons by Schwann cells. Multiple factors including neurotrophic factors, extracellular matrix (ECM) proteins, and hormones participate in Schwann cell dedifferentiation, proliferation, and remyelination. We describe the current understanding of peripheral axon regeneration and focus on the molecules and potential mechanisms involved in remyelination.

A calcium- and calmodulin-dependent kinase Ialpha/microtubule affinity regulating kinase 2 signaling cascade mediates calcium-dependent neurite outgrowth

Calcium is a critical regulator of neuronal differentiation and neurite outgrowth during development, as well as synaptic plasticity in adulthood. Calcium- and calmodulin-dependent kinase I (CaMKI) can regulate neurite outgrowth; however, the signal transduction cascades that lead to its physiological effects have not yet been elucidated. CaMKIalpha was therefore used as bait in a yeast two-hybrid assay and microtubule affinity regulating kinase 2 (MARK2)/Par-1b was identified as an interacting partner of CaMKI in three independent screens. The interaction between CaMKI and MARK2 was confirmed in vitro and in vivo by coimmunoprecipitation. CaMKI binds MARK2 within its kinase domain, but only if it is activated by calcium and calmodulin. Expression of CaMKI and MARK2 in Neuro-2A (N2a) cells and in primary hippocampal neurons promotes neurite outgrowth, an effect dependent on the catalytic activities of these enzymes. In addition, decreasing MARK2 activity blocks the ability of the calcium ionophore ionomycin to promote neurite outgrowth. Finally, CaMKI phosphorylates MARK2 on novel sites within its kinase domain. Mutation of these phosphorylation sites decreases both MARK2 kinase activity and its ability to promote neurite outgrowth. Interaction of MARK2 with CaMKI results in a novel, calcium-dependent pathway that plays an important role in neuronal differentiation.

Mechanosensitive channels in neurite outgrowth

This chapter focuses on the convergence of two areas of investigation in the past: the first is the study of the molecular basis for Ca(2+)-dependent axon pathfinding, and the second is the molecular and the functional characterization of mechanosensitive Ca(2+)-permeant cation channels (MscCa). The convergence of these two fields has reached a pivotal point when some ion channels belonging to the transient receptor potential (TRP) superfamily of proteins play essential roles in the growth cone guidance, and, independently, some of these channels are found to form MscCa of vertebrate cells. Various lines of evidence taken together make likely the idea that MscCa can substantially contribute to the spatial and temporal shaping of Ca(2+) responses in growing neurites. These findings are described and the possible contributions of MscCa to the neurite outgrowth are also discussed in the chapter.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

TRAIL gene reorganizes the cytoskeleton and decreases the motility of human leukemic Jurkat cells

TRAIL can selectively induce rapid apoptosis of various types of tumor cells. We induced the expression of TRAIL in Jurkat cells, and measured the adhesion of cells to human umbilical vein endothelial cells (HUVECs) and laminin (LN) in a parallel plate flow chamber system and by using a colorimetric method. The apoptosis percentage, cycle distribution, intracellular Ca(2+) concentration, and adhesion molecule expression of the cells were detected by flow cytometry. Cytoskeleton was observed with a laser confocal microscopy. The roles of adhesion molecules in the cell interaction was defined by their function blocking. The results showed that TRAIL attenuated the adhesion of Jurkat cells to HUVECs and LN, as well as their transendothelial migration. The increased apoptosis and G1-phase cell percentages, decreased intracellular Ca(2+) concentration, depolymerized actin and impaired cell deformability could contribute to the decreased adhesion of Jurkat cells caused by TRAIL. Furthermore, CD11a was found to play a more important role than CD62L in the adhesion of Jurkat cells to HUVECs. These findings contribute to the knowledge on the role of TRAIL in tumor metastasis and provide mechanistic basis for the clinical application of TRAIL and tumor therapy.

Growth-associated protein GAP-43 and L1 act synergistically to promote regenerative growth of Purkinje cell axons in vivo

Neuronal expression of growth-associated protein 43 (GAP-43) and the cell adhesion molecule L1 has been correlated with CNS axonal growth and regeneration, but it is not known whether expression of these molecules is necessary for axonal regeneration to occur. We have taken advantage of the fact that Purkinje cells do not express GAP-43 or L1 in adult mammals or regenerate axons into peripheral nerve grafts to test the importance of these molecules for axonal regeneration in vivo. Transgenic mice were generated in which Purkinje cells constitutively express L1 or both L1 and GAP-43 under the Purkinje cell-specific L7 promoter, and regeneration of Purkinje cell axons into peripheral nerve grafts implanted into the cerebellum was examined. Purkinje cells expressing GAP-43 or L1 showed minor enhancement of axonal sprouting. Purkinje cells expressing both GAP-43 and L1 showed more extensive axonal sprouting and axonal growth into the proximal portion of the graft. When a predegenerated nerve graft was implanted into double-transgenic mice, penetration of the graft by Purkinje cell axonal sprouts was strongly enhanced, and some axons grew along the entire intracerebral length of the graft (2.5-3.0 mm) and persisted for several months. The results demonstrate that GAP-43 and L1 coexpressed in Purkinje cells can act synergistically to switch these regeneration-incompetent CNS neurons into a regeneration-competent phenotype and show that coexpression of these molecules is a key regulator of the regenerative ability of intrinsic CNS neurons in vivo.

Molecular mechanisms in successful peripheral regeneration

Peripheral nerve injury is normally followed by a robust regenerative response. Here we describe the early changes associated with injury from the initial rise in intracellular calcium and the subsequent activation of transcription factors and cytokines leading to an inflammatory reaction, and the expression of growth factors, cytokines, neuropeptides, and other secreted molecules involved in cell-to-cell communication promoting regeneration and neurite outgrowth. The aim of this review is to summarize the molecular mechanisms that play a part in executing successful regeneration.

Blockage of voltage-gated calcium signaling impairs migration of glial cells in vivo

Migration of glial cells is an essential step in the development of the antennal lobe, the primary olfactory center of insects, to establish well-defined borders between olfactory glomeruli required for odor discrimination. In the present study, we used two-photon microscopy to visualize calcium signaling in developing antennal lobe glial cells of the sphinx moth Manduca sexta. We found a correlation between the upregulation of functional voltage-gated calcium channels and the onset of glial cell migration. In addition, glial cells migrating into the center of the antennal lobe express larger voltage-gated calcium transients than glial cells that remain at the periphery. Migration behavior and calcium signaling of glial cells in vivo were manipulated either by deafferentation, by injection of the calcium channel blockers diltiazem, verapamil, and flunarizine, or by injection of the calcium chelators BAPTA-AM and Fluo-4-AM. In deafferented antennal lobes, glial cells failed to express functional voltage-gated calcium channels and did not migrate. Calcium channel blockage or reducing glial calcium signals by calcium chelators prevented glial cell migration and resulted in antennal lobes lacking glial borders around glomeruli, indicating that voltage-gated calcium signaling is required for the migration of antennal lobe glial cells and the development of mature olfactory glomeruli.

A study of gene expression profiles of cultured embryonic rat neurons induced by phenylalanine

To have more insight into the mechanism of neuronal injury in phenylketonuria (PKU) patients, gene expression profiles were studied in cell culture of embryonic rat cortical neurons induced by phenylalanine. Randomly chose cortical cultured for 3 days were treated by 0.9-mM phenylalanine for 12 h. Control group of the same batch was treated with the same volume of medium. Total RNA was extracted and hybridized with the Affymetrix gene chip U34 according to the protocol provided by the Affymetrix Company. Real-time PCR was used to further confirm the result. We found that the hybridization signals of 167 genes were increased among the total 1323 probes plotted on the chip. The 167 increased genes could be functionally categorized into signal transduction, neuron related, cytoskeleton, metabolism, ion channels, transcription factors, cytokines, and apoptosis related. Signals of seven probes were decreased, which accounted to 0.5% of the total number. A series of genes that were not reported previously were upregulated by phenylalanine, including Ca2+/calmodulin-dependent protein kinase, Brain type II (CaMK II), ras, P38, L-voltage dependent calcium channel, some genes related to vesicle formation and transmitter release, some glutamate receptor subunits and glutamate transporters. According to the gene expression profiles, it is likely that multiprocesses are involved in the neuronal injury induced by phenylalanine, such as the activation on of the NMDR-Ca2+-CaMK II-Ras-P38 axis, the abnormality in neurotransmitter release. Our study also suggests that the excitatory neurotransmitter glutamate may play a role in the neural pathology of PKU.

Adapter protein SH2-B beta undergoes nucleocytoplasmic shuttling: implications for nerve growth factor induction of neuronal differentiation

The adapter protein SH2-B has been shown to bind to activated nerve growth factor (NGF) receptor TrkA and has been implicated in NGF-induced neuronal differentiation and the survival of sympathetic neurons. However, the mechanism by which SH2-B enhances and maintains neurite outgrowth is unclear. We examined the ability of truncation mutants to regulate neuronal differentiation and observed that certain truncation mutants localized in the nucleus rather than in the cytoplasm or at the plasma membrane as reported for wild-type SH2-B beta. Addition of the nuclear export inhibitor leptomycin B caused both overexpressed wild-type and endogenous SH2-B beta to accumulate in the nucleus of both PC12 cells and COS-7 cells as did deletion of a putative nuclear export sequence (amino acids 224 to 233) or mutation of two critical lysines in that sequence. Deleting or mutating the nuclear export signal caused SH2-B beta to lose its ability to enhance NGF-induced differentiation of PC12 cells. Neither the NGF-induced phosphorylation of ERKs 1 and 2 nor their subcellular distribution was altered in PC12 cells stably expressing the nuclear export-defective SH2-B beta(L231A, L233A). These data provide strong evidence that SH2-B beta shuttles constitutively between the nucleus and cytoplasm. However, SH2-B beta needs continuous access to the cytoplasm and/or plasma membrane to participate in NGF-induced neurite outgrowth. These data also suggest that the stimulatory effect of SH2-B beta on NGF-induced neurite outgrowth of PC12 cells is either downstream of ERKs or via some other pathway yet to be identified.

Ig superfamily cell adhesion molecules in the brain

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) were discovered 25 years ago based on their role in cell-cell adhesion. Ever since, they have played a major role in developmental neuroscience research. The elucidation of IgSF CAM structure and function has been tightly linked to the establishment of new areas of research. Over the years, our view of the role of the IgSF CAMs has changed. First, they were thought to provide "specific glue" segregating subtypes of cells in the nervous system. Soon it became clear that IgSF CAMs can do much more. The focus shifted from simple adhesion to CAM-associated signaling that was shown to be involved in the promotion of axon growth and the regulation of cell migration. From there it was a small step to axon guidance, a field that has been given a lot of attention during the last decade. More recently, the involvement of IgSF CAMs in synapse formation and maturation has been discovered, although this last step in the formation of neural circuits was thought to be the domain of other families of cell adhesion molecules, such as the neuroligins, the neurexins, and the cadherins. Certainly, the most striking discovery in the context of IgSF CAMs has been the diversity of signaling mechanisms that are associated with them. The versatility of signals and their complexity make IgSF CAMs a perfect tool for brain development.

Of rafts and moving water

The ability to respond rapidly to changes in tonicity is crucial for cellular and organismal survival. Sensors of osmotic stress are beginning to be discovered. For example, results from expression cloning in a heterologous system have implicated GAP43 as a component of a peripheral nervous system sensor of hypotonicity. These results and the role of lipid rafts, protein kinase C, and members of the phospholipase C-delta family are discussed in the context of cellular responses to osmotic stress. Calcium is also involved in the osmotic stress response, and both intracellular calcium released through inositol trisphosphate receptors and extracellular calcium transported through TRPV4 (a member of the transient receptor potential family) may contribute.

Monitoring neuronal calcium signalling using a new method for ratiometric confocal calcium imaging

Ca2+ signalling influences many processes in the adult and developing nervous system like exocytosis, synaptic plasticity, and growth cone motility. Optical techniques in combination with fluorescent Ca2+ indicators are the most frequently used methods to measure Ca2+ signalling in cells. In the present study, a new method for ratiometric confocal Ca2+ imaging was developed, and the usefulness of the system was tested with two different neuronal preparations. Developing Manduca sexta antennal lobe neurons were loaded with the Ca2+-sensitive dye Fura Red-AM, and the ratio of fluorescence excited at 457 and 488nm was measured with a confocal laser scanning microscope. During pupal stages 4-12, the antennal lobe neuropil is restructured which includes the ingrowth of olfactory receptor axons, dendritic outgrowth of antennal lobe neurons, and synaptogenesis. In antennal lobe neurons, application of the AChR agonist carbachol induced Ca2+ oscillations the amplitude and frequency of which changed during stages 4-9, while at the end of synaptogenesis, at stages 11 and 12, only single Ca2+ transients were elicited. The Ca2+ oscillations were blocked by D-tubocurarine and Cd2+, indicating that they were due to Ca2+ influx through voltage-gated Ca2+ channels, activated by nAChR-mediated membrane depolarization. To test whether single action potentials can induce Ca2+ transients detectable by Fura Red, individual leech Retzius neurons were injected iontophoretically with the Ca2+ indicator, and the membrane potential was recorded during Ca2+ imaging. Single action potentials induced transient increases in the Fura Red ratio measured in the axon, while trains of action potentials elicited Ca2+ transients that could also be recorded in the cell body and the nucleus. The results show that Fura Red can be used as a ratiometric Ca2+ indicator for confocal imaging.

A lipoxygenase product, hepoxilin A(3), enhances nerve growth factor-dependent neurite regeneration post-axotomy in rat superior cervical ganglion neurons in vitro

Hepoxilins are 12-lipoxygenase metabolites of arachidonic acid found in the CNS. They can modulate neuronal signaling but their functions are not known. We examined the effects of hepoxilin A(3) on neurite outgrowth post-axotomy in an in vitro model of spinal cord transection using superior cervical ganglion neurons. In the absence of nerve growth factor, hepoxilin A(3) did not support neuronal survival, or regeneration post-axotomy but did significantly enhance neurite regeneration in the presence of nerve growth factor. As early as 1 h post-injury hepoxilin A(3)-treated cultures (+nerve growth factor) had significantly more neurites than controls (nerve growth factor alone). Average hourly rates of outgrowth in hepoxilin A(3)-treated cultures were significantly higher than in controls for at least 12 h post-injury, suggesting that the effect of hepoxilin A(3) is maintained in vitro for several hours post-injury. In uninjured neurons hepoxilin A(3) caused a rapid but transient increase in intracellular calcium in the somata; by 2 min post-addition, calcium levels decreased to a new stable plateau significantly higher than pre treatment levels. In injured neurons, hepoxilin A(3) addition immediately post-transection caused a rapid transient increase in intracellular calcium in cell bodies; however, peak calcium levels were significantly lower than in uninjured neurons and the new baseline lower than in uninjured cells. In uninjured cells hepoxilin A(3) addition in zero calcium produced the same pattern, a transient elevation and subsequent decline to a new stable baseline significantly above rest but in injured cells levels fell rapidly to pretreatment values. Taken overall, these findings demonstrate a novel role for hepoxilins as a potentiator of neurite regeneration. They also provide the first evidence that this lipoxygenase metabolite can alter intracellular calcium in neurons by causing release of calcium from intracellular stores and modulating calcium influx mechanisms.

Intracellular signaling by the neural cell adhesion molecule

Cell adhesion molecules are known to play far more complex roles than mechanically attaching one cell to an adjacent cell or to components of the extracellular matrix. Thus, important roles for cell adhesion molecules in the regulation of intracellular signaling pathways have been revealed. In this review, we discuss the present knowledge about signaling pathways activated upon homophilic binding of the neural cell adhesion molecule (NCAM). Homophilic NCAM binding leads to activation of a signal transduction pathway involving Ca2+ through activation of the fibroblast growth factor receptor, and to activation of the mitogen-activated protein kinase pathway. In addition, cyclic adenosine monophosphate and protein kinase A are involved in NCAM-mediated signaling. Among these pathways the possibility exists of cross talk or convergence, of which different possible mediators have been suggested. Finally, several downstream effector molecules leading to NCAM-mediated cellular endpoints have been demonstrated, including transcription factors and regulators of the cytoskeleton.

Direct measurement of local raised subplasmalemmal calcium concentrations in growth cones advancing on an N-cadherin substrate

We have used the membrane-localized calcium probe fura-piperazine-C12H25 (FFP-18) to examine cytosolic calcium concentrations in a volume close to the plasmalemma. Although promotion of axon outgrowth by cell adhesion molecules requires extracellular calcium and is correlated with an opening of plasmalemmal channels, conventional indicators cannot detect a change in the calcium concentration in such stimulated growth cones. We have examined calcium signalling in chick retinal ganglion cell growth cones extending along stripes of N-cadherin. Subplasmalemmal calcium concentrations, reported by FFP-18, were significantly higher in these growth cones than in neighbouring growth cones on either fibronectin or polylysine. In contrast, the bulk cytosolic calcium concentration throughout the growth cone, as measured by Fura-2, was identical in growth cones on and off the N-cadherin stripes. Our results suggest that guidance cues can use extremely local calcium signals to control pathfinding decisions.

Dual action of estrogen on glutamate-induced calcium signaling: mechanisms requiring interaction between estrogen receptors and src/mitogen activated protein kinase pathway

Conjugated equine estrogens (CEE) is the most widely prescribed pharmaceutical estrogen replacement therapy (ERT) for postmenopausal women in the United States and is the ERT of the Women's Health Initiative. Previous studies from our laboratory have demonstrated that CEE exerts neurotrophic and neuroprotective effects in neurons involved in learning and memory, and which are affected in Alzheimer's disease. The present work demonstrates that CEE potentiated the rise in intracellular calcium ([Ca(2+)](i)) following exposure to physiological concentrations of glutamate. In contrast, the reverse effect occurred in the presence of excitotoxic levels of glutamate exposure, where CEE attenuated the rise in [Ca(2+)](i). Potentiation of the glutamate response was mediated by the NMDA receptor, as the NMDA receptor antagonist MK-801 blocked the CEE-induced potentiation, whereas the L-type calcium channel blocker nifedipine did not. Further, the CEE-potentiated glutamate response was mediated by a src tyrosine kinase, as the tyrosine kinase inhibitor PP2 blocked the potentiation induced by CEE and neurons treated with CEE displayed increased phosphorylated tyrosine. The inhibition by CEE of [Ca(2+)](i) rise in the presence of excitotoxic levels of glutamate was mediated by mitogen activated protein kinase (MAPK), as the protective effect of CEE was blocked by inhibiting MAPK activation with PD98059. These data provide potential mechanisms to explain the cognitive enhancing and neuroprotective effects exerted by ERT.