Stimulation of L-type Ca2+ channel in growth cones activates two independent signaling pathways

Cell adhesion and intracellular calcium signaling in neurons

Cell adhesion molecules (CAMs) play indispensable roles in the developing and mature brain by regulating neuronal migration and differentiation, neurite outgrowth, axonal fasciculation, synapse formation and synaptic plasticity. CAM-mediated changes in neuronal behavior depend on a number of intracellular signaling cascades including changes in various second messengers, among which CAM-dependent changes in intracellular Ca²⁺ levels play a prominent role. Ca²⁺ is an essential secondary intracellular signaling molecule that regulates fundamental cellular functions in various cell types, including neurons. We present a systematic review of the studies reporting changes in intracellular Ca²⁺ levels in response to activation of the immunoglobulin superfamily CAMs, cadherins and integrins in neurons. We also analyze current experimental evidence on the Ca²⁺ sources and channels involved in intracellular Ca²⁺ increases mediated by CAMs of these families, and systematically review the role of the voltage-dependent Ca²⁺ channels (VDCCs) in neurite outgrowth induced by activation of these CAMs. Molecular mechanisms linking CAMs to VDCCs and intracellular Ca²⁺ stores in neurons are discussed.

Mechanistic involvement of the calpain-calpastatin system in Alzheimer neuropathology

The mechanism by which amyloid-β peptide (Aβ) accumulation causes neurodegeneration in Alzheimer's disease (AD) remains unresolved. Given that Aβ perturbs calcium homeostasis in neurons, we investigated the possible involvement of calpain, a calcium-activated neutral protease. We first demonstrated close postsynaptic association of calpain activation with Aβ plaque formation in brains from both patients with AD and transgenic (Tg) mice overexpressing amyloid precursor protein (APP). Using a viral vector-based tracer, we then showed that axonal termini were dynamically misdirected to calpain activation-positive Aβ plaques. Consistently, cerebrospinal fluid from patients with AD contained a higher level of calpain-cleaved spectrin than that of controls. Genetic deficiency of calpastatin (CS), a calpain-specific inhibitor protein, augmented Aβ amyloidosis, tau phosphorylation, microgliosis, and somatodendritic dystrophy, and increased mortality in APP-Tg mice. In contrast, brain-specific CS overexpression had the opposite effect. These findings implicate that calpain activation plays a pivotal role in the Aβ-triggered pathological cascade, highlighting a target for pharmacological intervention in the treatment of AD.

Developmental changes in presynaptic Ca(2 +) clearance kinetics and synaptic plasticity in mouse Schaffer collateral terminals

Presynaptic Ca(2+) influx pathways, cytoplasmic Ca(2+) buffering proteins and Ca(2+) extrusion processes undergo considerable change during the first postnatal month in rodent neurons. These changes may be critical in establishing short-term plasticity at maturing presynaptic terminals where neurotransmitter release is directly dependent on the dynamics of cytoplasmic residual Ca(2+) ([Ca(2+)](res)). In particular, the robust paired-pulse facilitation characteristic of adult neurons is almost entirely lacking in newborns. To examine developmental changes in processes controlling [Ca(2+)](res), we measured the timecourse of [Ca(2+)](res) decay in presynaptic terminals of Schaffer collateral to CA1 synapses in acute hippocampal slices following single and paired orthodromic stimuli in the stratum radiatum. Developmental changes were observed in both the rise time and slow exponential decay components of the response to single stimuli such that this decay was larger and faster in the adult. Furthermore, we observed a greater caffeine-sensitive basal Ca(2+) store, which was differentially affected when active uptake into the endoplasmic reticulum was blocked, in the presynaptic fields of the Schaffer collateral to CA1 terminals of P6 and younger mice when compared to adults. These transitions in [Ca(2+)](res) dynamics occurred gradually over the first weeks of postnatal life and correlated with changes in short-term plasticity.

Physical stress differs from psychosocial stress in the pattern and time-course of behavioral responses, serum corticosterone and expression of plasticity-related genes in the rat

Stressors differ in their physiological and behavioral outcomes. One of the major mechanisms by which stressors affect the brain and behavior is alteration in neuronal plasticity. We investigated in the rat the effects of a single exposure to psychophysical (electrical foot shock) vs. psychological (social defeat) stressors on anxiety- and depression-related behaviors, serum levels of corticosterone and the expression of plasticity-related genes CAM-L1, CREB, GAP-43, and laminin in the prefrontal cortex (PFC), the amygdala and the hippocampus. Rats were examined for 24 h or 1 week after the exposure to stress. Footshocks enhanced anxiety-related behaviors, whereas social defeat induced depression-related behaviors at both time points and less pronounced anxiety 1 week post-exposure. Serum corticosterone concentrations were enhanced 24 h after shocks, but only 1 week after exposure to the social stressor. Moreover, the shock-stressed rats exhibited decreased CAM-L1 protein level in the hippocampus 24 h post-exposure and decreased GAP-43 protein level in the PFC 1 week post-exposure. By contrast, the social stressor enhanced expression of the plasticity-related proteins in the amygdala and the hippocampus, mostly 1 week after the exposure. These results indicate stressor-specific time-dependent changes in different neuronal pathways, and suggest consideration of a cause-specific approach to the treatment of stress-related disorders.

Lipoxin A4 counterregulates GM-CSF signaling in eosinophilic granulocytes

Eosinophils are granulated leukocytes that are involved in many inflammation-associated pathologies including airway inflammation in asthma. Resolution of eosinophilic inflammation and return to homeostasis is in part due to endogenous chemical mediators, for example, lipoxins, resolvins, and protectins. Lipoxins are endogenous eicosanoids that demonstrate antiinflammatory activity and are synthesized locally at sites of inflammation. In view of the importance of lipoxins (LXs) in resolving inflammation, we investigated the molecular basis of LXA(4) action on eosinophilic granulocytes stimulated with GM-CSF employing the eosinophilic leukemia cell line EoL-1 as well as peripheral blood eosinophils. We report herein that LXA(4) (1-100 nM) decreased protein tyrosine phosphorylation in EoL-1 cells stimulated with GM-CSF. Additionally, the expression of a number of GM-CSF-induced cytokines was inhibited by LXA(4) in a dose-dependent manner. Furthermore, using a proteomics approach involving mass spectrometry and immunoblot analysis we identified 11 proteins that were tyrosine phosphorylated after GM-CSF stimulation and whose phosphorylation was significantly inhibited by LXA(4) pretreatment. Included among these 11 proteins were alpha-fodrin (nonerythroid spectrin) and actin. Microscopic imaging showed that treatment of EoL-1 cells or blood eosinophils with GM-CSF resulted in the reorganization of actin and the translocation of alpha-fodrin from the cytoplasm to the plasma membrane. Importantly, alpha-fodrin translocation was prevented by LXA(4) but actin reorganization was not. Thus, the mechanism of LXA(4) action likely involves prevention of activation of eosinophilic granulocytes by GM-CSF through inhibition of protein tyrosine phosphorylation and modification of some cytoskeletal components.

NCAM induces CaMKIIalpha-mediated RPTPalpha phosphorylation to enhance its catalytic activity and neurite outgrowth

Receptor protein tyrosine phosphatase α (RPTPα) phosphatase activity is required for intracellular signaling cascades that are activated in motile cells and growing neurites. Little is known, however, about mechanisms that coordinate RPTPα activity with cell behavior. We show that clustering of neural cell adhesion molecule (NCAM) at the cell surface is coupled to an increase in serine phosphorylation and phosphatase activity of RPTPα. NCAM associates with T- and L-type voltage-dependent Ca²⁺ channels, and NCAM clustering at the cell surface results in Ca²⁺ influx via these channels and activation of NCAM-associated calmodulin-dependent protein kinase IIα (CaMKIIα). Clustering of NCAM promotes its redistribution to lipid rafts and the formation of a NCAM–RPTPα–CaMKIIα complex, resulting in serine phosphorylation of RPTPα by CaMKIIα. Overexpression of RPTPα with mutated Ser180 and Ser204 interferes with NCAM-induced neurite outgrowth, which indicates that neurite extension depends on NCAM-induced up-regulation of RPTPα activity. Thus, we reveal a novel function for a cell adhesion molecule in coordination of cell behavior with intracellular phosphatase activity.

Calpain activation impairs neuromuscular transmission in a mouse model of the slow-channel myasthenic syndrome

The slow-channel myasthenic syndrome (SCS) is a hereditary disorder of the acetylcholine receptor (AChR) of the neuromuscular junction (NMJ) that leads to prolonged AChR channel opening, Ca(2+) overload, and degeneration of the NMJ. We used an SCS transgenic mouse model to investigate the role of the calcium-activated protease calpain in the pathogenesis of synaptic dysfunction in SCS. Cleavage of a fluorogenic calpain substrate was increased at the NMJ of dissociated muscle fibers. Inhibition of calpain using a calpastatin (CS) transgene improved strength and neuromuscular transmission. CS caused a 2-fold increase in the frequency of miniature endplate currents (MEPCs) and an increase in NMJ size, but MEPC amplitudes remained reduced. Persistent degeneration of the NMJ was associated with localized activation of the non-calpain protease caspase-3. This study suggests that calpain may act presynaptically to impair NMJ function in SCS but further reveals a role for other cysteine proteases whose inhibition may be of additional therapeutic benefit in SCS and other excitotoxic disorders.

Nitric oxide regulates growth cone filopodial dynamics via ryanodine receptor-mediated calcium release

Nitric oxide (NO) is a gaseous intercellular messenger involved in numerous processes during development, including wiring of the nervous system. Neuronal growth cones are responsible for establishing the correct connectivity in the nervous system, but how NO might affect neuronal pathfinding is not fully understood. We have demonstrated in a previous study that local application of a NO donor, NOC-7, via micropipette onto individual growth cones from Helisoma trivolvis B5 neurons results in an increase in filopodial length, a decrease in filopodial number and an increase in the intracellular calcium concentration ([Ca(2+)](i)). Moreover, these NO-induced effects were demonstrated to be mediated via an intracellular cascade involving soluble guanylyl cyclase, protein kinase G (PKG) and cyclic adenosine diphosphate ribose (cADPR). We now demonstrate that the increase in the [Ca(2+)](i) that results from local NO application is mediated via release from ryanodine receptor (RyR)-sensitive intracellular stores. We also show that PKG and RyRs are localized within growth cones and microinjection of cADPR mimics the effects of NO, providing further support that the NO-induced effects are mediated via cADPR. Lastly, we provide evidence that calcium influx across the plasma membrane is a necessary component of the NO-induced calcium increase; however, this calcium influx is secondary to the RyR-induced calcium release from intracellular stores. This study details a signalling pathway by which NO can cause changes in growth cone morphology and thus provides a mechanism by which NO could affect neuronal wiring by acting locally on individual growth cones during the pathfinding process.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Differential targeting of the L-type Ca2+ channel alpha1C (CaV1.2) to synaptic and extrasynaptic compartments in hippocampal neurons

In central nervous system neurons L-type Ca2+ channels are involved in developmental processes, the integration and conduction of postsynaptic electric activity and synaptic plasticity. However, little is known about the channel isoforms underlying each of these functions or about the exact localization and targeting properties of the major L-type channel isoform alpha1C (CaV1.2) in neurons. We addressed these questions using high-resolution immunofluorescence analysis of the endogenous alpha1C and epitope-tagged recombinant channel isoforms expressed in mouse hippocampal neurons. Endogenous alpha1C and surface-expressed hemagglutinin (HA)-tagged alpha1C-HA were localized in small clusters distributed between the axon initial segment and the apical branches of the dendritic tree. The average cluster size was estimated to be eight channels per alpha1C-HA cluster. Analysis of the subcellular localization of alpha1C-HA clusters relative to known synaptic markers suggested the existence of two distinct populations of alpha1C clusters, extrasynaptic and synaptic, the latter associated with glutamatergic synapses in dendritic spines. Both glutamatergic and GABAergic neurons expressed alpha1C in the soma and dendrites. In contrast to the N-type channel GFP-alpha1B, GFP-alpha1C was excluded from distal axons and nerve terminals of mature neurons. In developing neurons, however, alpha1C and alpha1C-HA were robustly expressed in the growth cone, indicating that specific targeting properties of neuronal compartments change during differentiation. Synaptic and extrasynaptic localizations of alpha1C correspond to putative roles of L-type Ca2+ currents in synaptic modulation and in the propagation of dendritic Ca2+ spikes, respectively. The transiently expressed alpha1C in the growth cone may be involved in neurite extension and axonal pathfinding.

Inosine reverses the inhibitory effects of the L-type Ca2+ channel antagonist, DM-BODIPY-dihydropyridine, on neuritogenesis in an in vitro rat superior cervical ganglia axotomy model

It has recently been demonstrated that L-type calcium channel antagonism with the fluorescent dihydropyridine DM-BODIPY-dihydropyridine (DMBD) inhibits neurite regeneration in rat superior cervical ganglia (SCG). The neuritogenic effects of inosine have been described in various models and the mechanism is thought to be N-kinase dependent. Because of the final common pathway between calcium dependent and N-kinase dependent neurite regeneration it was hypothesized that inosine would increase regeneration in normally regenerating SCG and reverse the inhibitory effects of DMBD on regenerating SCG. An in vitro model of rat SCG injury, where mature neurites are transected and observed at 2 and 24 h, was used to assess the effects of inosine on DMBD treated neurons. Results demonstrate a significant inhibition of growth in DMBD treated cultures, significantly increased growth in the inosine + DMBD treated SCG over DMBD treated cells and significantly increased growth in the inosine alone treated group over control cells. There is also evidence that inosine + DMBD treatment promotes linear growth of neurites. The implications of the findings are discussed.

Pharmacological dissection of calcium channel subtype-related components of strontium inflow in large mossy fiber boutons of mouse hippocampus

Several subtypes of voltage-dependent calcium channels (VDCCs) are present in the presynaptic terminals. In the mammalian hippocampus, P/Q-, N-, and R- but not L-type VDCCs are involved in the fast transmitter release from large mossy fiber (MF) boutons, which are associated with CA3 pyramidal cell dendrites. We investigated whether L-type VDCCs are indeed absent in these large MF boutons. With the use of Sr2+ as the Ca2+ substitute, the stimulus-evoked Sr2+ increment (delta[Sr2+]pre) was evaluated fluorometrically. Delta[Sr2+]pre appeared to be proportional to Sr2+ inflow through VDCCs and was specifically attenuated by conventional VDCC subtype-selective antagonists. The P/Q-type selective omega-agatoxin IVA (AgTx(IVA)) blocked delta[Sr2+]pre with an IC50 of 28 nM and by 30-35% at its maximum effective concentration of 0.5 microM. The N-type selective omega-conotoxin GVIA (CgTx(GVIA)) blocked delta[Sr2+]pre with an IC50 of 15 nM and by 20-25% at its maximum effective concentration of 1 microM. The R-type selective SNX-482 blocked delta[Sr2+]pre with an IC50 of 79 nM and by 20-25% at its maximum effective concentration of 1 microM. The effects of these toxins did not overlap at their maximum effective concentrations and about 70-80% of delta[Sr +]pre was blocked by the simultaneous exposure to these toxins. delta[Sr2+]pre component that is resistant to AgTx(IVA), CgTx(IVA), and SNX-482 was significantly potentiated by an L-type agonist, (S)-(-)-Bay K8644, and attenuated by an L-type antagonist, nimodipine, suggesting that L-type VDCCs are present in large MF terminals. The L-type agonist, (+/-)-Bay K8644, also potentiated Sr2+ inflow into individual boutons identified as large MF boutons under confocal microscopy. Almost similar results were observed for Ca2+ inflow-dependent fluorescence increments. L-type VDCCs appear to be present in large MF boutons and mediate a substantial Ca2+ inflow into presynaptic terminals during action potentials.

Spinal cord regeneration induced by a voltage-gated calcium channel agonist

Regeneration in the central nervous system (CNS) is prohibitive. This is likely due to an interplay of cellular (gene expression, growth factors) and environmental (inhibition by CNS myelin) factors. Calcium supports various intracellular functions, and multiple in vitro studies have shown a role of calcium in axonal growth. In this study, we examine the role of a calcium agonist, S(-)-Bay K 8644, in promoting or impeding CNS growth in vivo, in an effort to understand further the relationship between the voltage-gated L type calcium channel and regeneration. Using a well-established rat spinal cord model of regeneration, we have injected various doses of S(-)-Bay K 8644 (30-240 M) around the injured spinal cord. Our results demonstrate that S(-)-Bay K 8644 enhances regeneration in a dose-dependent fashion. In addition, at very specific concentrations, the same agonist has no effect on or even inhibits regeneration. We conclude that spinal regeneration is highly dependent on intracellular calcium concentration. Furthermore, depending on the dose used, the effect of calcium agonist supplementation on spinal regeneration can be supportive or inhibitory.

Sustained phosphorylation of MARCKS in differentiating neurogenic regions during chick embryo development

MARCKS, a substrate for several kinases, has critical functions in morphogenetic processes involved in the development of the nervous system. We previously described the purification of MARCKS from chick embryo brain, using a monoclonal antibody (mAb 3C3), raised against embryonic neural retina. Here we show that mAb 3C3 is an antibody sensitive to phosphorylation state. We used it to explore the appearance and developmental progression of phospho-MARCKS (ph-MARCKS) during initial stages of neurogenesis in retina and spinal cord, and compared its distribution with total MARCKS. Before the onset of neural differentiation, MARCKS protein was already accumulated in neural and non-neural embryonic tissues, while ph-MARCKS immunoreactivity was weak, although ubiquitous too. A sudden increase of ph-MARCKS, paralleling a total MARCKS augmentation, was particularly noticeable in the earliest differentiating neurons in the neural retina. Ganglion cells displayed a high ph-MARCKS signal in the soma, as well as in the growing axon. A short time thereafter, a similar increase of ph-MARCKS was present across the entire width of the neural retina, where the differentiation of other neurons and photoreceptors occurs. The increase of ph-MARCKS in cells took place before the detection of the transcription factor Islet-1/2, an early neuronal differentiation molecular marker, in cells of the same region. Analogous phenomena were observed in cervical regions of the spinal cord, where motor neurons were differentiating. Neurogenic regions in the spinal cord contained higher amounts of ph-MARCKS than the floor plate. Taken together, these results strongly suggest that the appearance and relatively long-lasting presence of ph-MARCKS polypeptides are related to specific signaling pathways active during neurogenesis.

A cyclic AMP-regulated negative feedforward system for neuritogenesis revealed in a neuroblastomaxglioma hybrid cell line

We examined the role of second messengers during the neuritogenesis that accompanies neuronal differentiation in a neuroblastomaxglioma hybrid cell line (NG108-15). NG108-15 cells extended neurites after treatment with dibutyryl cyclic AMP. This dibutyryl cyclic AMP treatment evoked the synthesis of voltage-dependent Ca(2+) channel proteins in the cells. The number of neurites was decreased by Ca(2+) influx under condition of high K(+). Interestingly, the increase of neurites stimulated by dibutyryl cyclic AMP and the decrease of neurites caused by high K(+) were both reversible. This is the first study to demonstrate that cyclic AMP regulates a negative feedforward system for neuritogenesis, which links with Ca(2+) signaling. Such a dual role of cyclic AMP may play an important part in precise neurite targeting.

GABA expression dominates neuronal lineage progression in the embryonic rat neocortex and facilitates neurite outgrowth via GABA(A) autoreceptor/Cl- channels

GABA emerges as a trophic signal during rat neocortical development in which it modulates proliferation of neuronal progenitors in the ventricular/subventricular zone (VZ/SVZ) and mediates radial migration of neurons from the VZ/SVZ to the cortical plate/subplate (CP/SP) region. In this study we investigated the role of GABA in the earliest phases of neuronal differentiation in the CP/SP. GABAergic-signaling components emerging during neuronal lineage progression were comprehensively characterized using flow cytometry and immunophenotyping together with physiological indicator dyes. During migration from the VZ/SVZ to the CP/SP, differentiating cortical neurons became predominantly GABAergic, and their dominant GABA(A) receptor subunit expression pattern changed from alpha4beta1gamma1 to alpha3beta3gamma2gamma3 coincident with an increasing potency of GABA on GABA(A) receptor-mediated depolarization. GABA(A) autoreceptor/Cl(-) channel activity in cultured CP/SP neurons dominated their baseline potential and indirectly their cytosolic Ca(2+) (Ca(2+)c) levels via Ca(2+) entry through L-type Ca(2+) channels. Block of this autocrine circuit at the level of GABA synthesis, GABA(A) receptor activation, intracellular Cl(-) ion homeostasis, or L-type Ca(2+) channels attenuated neurite outgrowth in most GABAergic CP/SP neurons. In the absence of autocrine GABAergic signaling, neuritogenesis could be preserved by depolarizing cells and elevating Ca(2+)c. These results reveal a morphogenic role for GABA during embryonic neocortical neuron development that involves GABA(A) autoreceptors and L-type Ca(2+) channels.

Vasopressin-induced neurotrophism in cultured neurons of the cerebral cortex: dependency on calcium signaling and protein kinase C activity

Neuronal process outgrowth has been postulated to be one of the fundamental steps involved in neuronal development. To test whether vasopressin can influence neuronal development by acting on the outgrowth of neuronal processes, we determined the neurotrophic action of the memory-enhancing peptide, vasopressin, in neurons derived from the cerebral cortex, a site of integrative cognitive function and long-term memory. Exposure to V(1) receptor agonist significantly increased multiple features of nerve cell morphology, including neurite length, number of branches, branch length, number of branch bifurcation points and number of microspikes. The dose-response profile of V(1) receptor agonist-induced neurotrophism exhibited a biphasic function, with lower concentrations inducing a significant increase while higher concentrations generally induced no significant effect. The neurotrophic effect of V(1) receptor activation did not require growth factors present in serum. Analysis of the regional selectivity of the vasopressin-induced neurotrophic effect revealed significant V(1) receptor agonist-induced neurotrophism in occipital and parietal neurons, whereas frontal and temporal neurons were unresponsive. Results of experiments to determine the mechanism of vasopressin-induced neurotrophism demonstrated that vasopressin-induced neurotrophism is dependent on V(1)a receptor activation, requires L-type calcium channel activation and activation of both pathways of the phosphatidylinositol signaling cascade, inositol trisphosphate and protein kinase C. These studies are the first to describe a functional cellular response for vasopressin in the cerebral cortex. The findings are discussed with respect to their implications for understanding the role of vasopressin-induced neurotrophism, the associated signaling pathways required for this response, and the ability of vasopressin to enhance memory function.

Postmortem changes in the level of brain proteins

A systematic study on postmortem changes of brain proteins has not been performed so far and information is limited to basic principles of specific or nonspecific proteolysis or proteolysis of individual proteins. We studied protein level alterations in rat brain of animals kept at 23 degrees C for several postmortem times up to 72 h. Brain tissue protein extracts were analyzed by two-dimensional electrophoresis and the proteins with different levels were identified by matrix-assisted laser desorption ionization mass spectrometry. The changes observed mainly concerned structural proteins and enzymes. The levels of dihydropyrimidinase-related protein-2 decreased within 6 h and two new spots were detected representing shorter forms of the protein. Most of the other alterations appeared about 48 h postmortem. The most significant were reduced levels of neurofilament, alpha-internexin, synaptosomal-associated protein 25, glial fibrillary acidic protein, heat shock proteins, and dynamin-1; increased levels of 14-3-3 proteins and spectrin; and generation of shorter forms of certain proteins, such as tubulins, actin, and serum albumin. The results may be useful in neuropathology and brain protein studies.

Nitrous oxide, but not xenon, affects the signaling in the neuronal growth cone

1. Xenon (Xe) is an inert gas with the anesthetic property. To investigate whether Xe affects the neural network formation, the authors examined the biochemical characteristics of growth cones prepared from rat forebrains at different perinatal periods, in comparison with inhalation of N2O. 2. Fetal or neonatal rats were exposed to an atmosphere containing inhalational anesthetics (70% Xe or N2O) or the control atmosphere (30% O2 and 70% N2) for 6 h. After the exposure, isolated growth cone particles (IGCs) were prepared from their forebrains using a subcellular fractionation method. Protein composition, Ca2+-dependent protein phosphorylation, protein kinase C (PKC) activity, and degradation of PKC in the IGCs were compared among three groups. 3. No apparent change of protein composition in IGCs was observed by electrophoresis. Ca2+dependent phosphorylation of GAP-43 and MARCKS protein, and PKC activity in IGCs significantly decreased after exposure to N20. The degradation of PKC increased significantly after inhalation of N2O. 4. The authors concluded that Xe dose not change the above biochemical characteristic of the growth cones, suggesting that Xe is free from the teratogenic effect on the neuronal network formation and that Xe is a safe anesthetics for the perinatal neuronal development.

CAMs and axonal growth: a critical evaluation of the role of calcium and the MAPK cascade

Calcium has long been recognized as a key player in the control of axonal growth and guidance. Recent studies lend support to this pivotal role by showing that local changes in calcium can directly induce the formation of filopodia in vivo and turn a growth cone in vitro. Under normal growth conditions, the L1 adhesion molecule has now been shown to induce local rather than global changes in calcium in growth cones, and this suggests that cell adhesion molecules (CAMs) use localized calcium transients to stimulate axonal growth and guidance. A number of recent reports have demonstrated that the neurite outgrowth response stimulated by L1 and other adhesion molecules (NCAM, N-cadherin, laminin) also depends in part upon the integrity of the MAPK cascade in cells. In this review we consider the recent data and suggest that calcium and the MAPK cascade might be required for very distinct growth cone functions. Finally, we will consider the contentious issue of how the above CAMs activate signaling cascades in growth cones and review the recently available data that support the hypothesis that at least one of these CAMs (N-cadherin) might promote growth cone motility by directly interacting with the FGFR in growth cones.

The mechanism of the neurotransmitter release in growth cones

The growth cone is considered the precursor of the presynaptic terminal. To elucidate the minimal molecular machinery required for exocytosis, we examined the characteristics of alpha-latrotoxin-induced exocytosis in growth cones. In isolated growth cones (IGC), neurotransmitters were released in a SNARE-dependent manner, but rab3A cycling was blocked. By supplying rabphilin, a rab3A acceptor found in low levels in IGC, the IGC obtained as high an exocytotic efficiency as adult synaptosomes, and the complete GDP-GTP conversion of rab3A occurred on growth cone vesicles (GCV). GCVs bound SNAREs but not NSF or alpha-SNAP; whereas in the rabphilin-supplied IGC, GCVs recruited both NSF and alpha-SNAP, to form the SNARE-NSF-SNAP complex. These results suggest that rab3A cycling is dependent upon the accumulation of rabphilin and is completed later than the SNARE mechanism, and that rabphilin is involved in determining the efficiency of exocytosis by modifying the SNARE mechanism.

Brain dystrophin, neurogenetics and mental retardation

Duchenne muscular dystrophy (DMD) and the allelic disorder Becker muscular dystrophy (BMD) are common X-linked recessive neuromuscular disorders that are associated with a spectrum of genetically based developmental cognitive and behavioral disabilities. Seven promoters scattered throughout the huge DMD/BMD gene locus normally code for distinct isoforms of the gene product, dystrophin, that exhibit nervous system developmental, regional and cell-type specificity. Dystrophin is a complex plasmalemmal-cytoskeletal linker protein that possesses multiple functional domains, autosomal and X-linked homologs and associated binding proteins that form multiunit signaling complexes whose composition is unique to each cellular and developmental context. Through additional interactions with a variety of proteins of the extracellular matrix, plasma membrane, cytoskeleton and distinct intracellular compartments, brain dystrophin acquires the capability to participate in the modulatory actions of a large number of cellular signaling pathways. During neural development, dystrophin is expressed within the neural tube and selected areas of the embryonic and postnatal neuraxis, and may regulate distinct aspects of neurogenesis, neuronal migration and cellular differentiation. By contrast, in the mature brain, dystrophin is preferentially expressed by specific regional neuronal subpopulations within proximal somadendritic microdomains associated with synaptic terminal membranes. Increasing experimental evidence suggests that in adult life, dystrophin normally modulates synaptic terminal integrity, distinct forms of synaptic plasticity and regional cellular signal integration. At a systems level, dystrophin may regulate essential components of an integrated sensorimotor attentional network. Dystrophin deficiency in DMD/BMD patients and in the mdx mouse model appears to impair intracellular calcium homeostasis and to disrupt multiple protein-protein interactions that normally promote information transfer and signal integration from the extracellular environment to the nucleus within regulated microdomains. In DMD/BMD, the individual profiles of cognitive and behavioral deficits, mental retardation and other phenotypic variations appear to depend on complex profiles of transcriptional regulation associated with individual dystrophin mutations that result in the corresponding presence or absence of individual brain dystrophin isoforms that normally exhibit developmental, regional and cell-type-specific expression and functional regulation. This composite experimental model will allow fine-level mapping of cognitive-neurogenetic associations that encompass the interrelationships between molecular, cellular and systems levels of signal integration, and will further our understanding of complex gene-environmental interactions and the pathogenetic basis of developmental disorders associated with mental retardation.

Critical dependence of cAMP response element-binding protein phosphorylation on L-type calcium channels supports a selective response to EPSPs in preference to action potentials

Activity-dependent gene expression in neurons shows a remarkable ability to differentiate between different types of stimulation: orthodromic inputs that engage synaptic transmission are much more effective than antidromic stimuli that do not. We have studied the basis of such selectivity in cultured hippocampal neurons in which nuclear cAMP response element-binding protein (CREB) phosphorylation is induced by synaptic activity but not by action potential (AP) stimulation in the absence of EPSPs, although spikes by themselves generate large elevations in intracellular Ca(2+). Previous work has shown that Ca(2+) entry through L-type Ca(2+) channels plays a dominant role in triggering calmodulin mobilization and activation of calmodulin-dependent kinases that phosphorylate CREB, raising the possibility that L-type channels contribute to the selective response to EPSPs rather than APs. Accordingly, we performed voltage-clamp experiments to compare the currents carried by L-type channels during depolarizing waveforms that approximated APs or dendritic EPSPs. The integrated current generated by L-type channels was significantly less after mock APs than with EPSP-like depolarizations. The difference was traced to two distinct factors. Compared with other channels, L-type channels activated at relatively negative potentials, favoring their opening with EPSP stimulation; they also exhibited relatively slow activation kinetics, weighing against their contribution during an AP. The relative ineffectiveness of APs as a stimulus for CREB phosphorylation could be overcome by exposure to the agonist Bay K8644, which potentiated the AP-induced influx through L-type channels by approximately 10-fold. Under normal conditions, the unique biophysical properties of L-type channels allow them to act as a kinetic filter to support spike-EPSP discrimination.

Calcium influx alters actin bundle dynamics and retrograde flow in Helisoma growth cones

The ability of calcium (Ca(2+)) to effect changes in growth cone motility requires remodeling of the actin cytoskeleton. To understand the mechanisms involved, we evaluated the effect of elevated intracellular calcium ([Ca(2+)](i)) on actin bundle dynamics, organization, and retrograde flow in the large growth cones of identified Helisoma neurons. Depolarization with 15 mM KCl (high K(+)) for 30 min caused a rapid and sustained increase in [Ca(2+)](i) and resulted in longer filopodia, shorter actin ribs, and a decrease in lamellipodia width. Time-lapse microscopy revealed that increasing [Ca(2+)](i) affected actin bundle dynamics differently at the proximal and distal ends. Filopodial lengthening resulted from assembly-driven elongation of actin bundles whereas actin rib shortening resulted from a distal shift in the location of breakage. Buckling of ribs occurred before breakage, suggesting nonuniform forces were applied to ribs before shortening. Calcium (Ca(2+)) influx also resulted in a decrease in density of F-actin in bundles, as determined by contrast changes in ribs imaged by differential interference contrast microscopy and fluorescent intensity changes in rhodamine-labeled ribs. The velocity of retrograde flow decreased by 50% after elevation of [Ca(2+)](i). However, no significant change in retrograde flow occurred when the majority of changes in actin bundles were blocked by phalloidin. This suggests that inhibition of retrograde flow resulted from Ca(2+)-induced changes in the actin cytoskeleton. These results implicate Ca(2+) as a regulator of actin dynamics and, as such, provide a mechanism by which Ca(2+) can influence growth cone motility and behavior.