beta -Neuregulin-1 is required for the in vivo development of functional Ca2+-activated K+ channels in parasympathetic neurons

Neuregulin-1/ErbB4 signaling regulates Kv4.2-mediated transient outward K+ current through the Akt/mTOR pathway

Neuregulin-1 (NRG-1) is a member of a family of neurotrophic factors that is required for the differentiation, migration, and development of neurons. NRG-1 signaling is thought to contribute to both neuronal development and the neuropathology of schizophrenia, which is believed to be a neurodevelopmental disorder. However, few studies have investigated the role of NRG-1 on voltage-gated ion channels. In this study, we report that NRG-1 specifically increases the density of transient outward K(+) currents (IA) in rat cerebellar granule neurons (CGNs) in a time-dependent manner without modifying the activation or inactivation properties of IA channels. The increase in IA density is mediated by increased protein expression of Kv4.2, the main α-subunit of the IA channel, most likely by upregulation of translation. The effect of NRG-1 on IA density and Kv4.2 expression was only significant in immature neurons. Mechanistically, both Akt and mammalian target of rapamycin (mTOR) signaling pathways are required for the increased NRG-1-induced IA density and expression of Kv4.2. Moreover, pharmacological blockade of the ErbB4 receptor reduced the effect of NRG-1 on IA density and Kv4.2 induction. Our data reveal, for the first time, that stimulation of ErbB4 signaling by NRG-1 upregulates the expression of K(+) channel proteins via activation of the Akt/mTOR signaling pathway and plays an important role in neuronal development and maturation. NRG1 does not acutely change IA and delayed-rectifier outward (IK) of rat CGNs, suggesting that it may not alter excitability of immature neurons by altering potassium channel property.

Association study of neuregulin-1 gene polymorphisms in a North Indian schizophrenia sample

BACKGROUND

Neuregulin-1 (NRG1) gene polymorphisms have been proposed as risk factors for several common disorders. Associations with cognitive variation have also been tested. With regard to schizophrenia (SZ) risk, studies of Caucasian ancestry samples indicate associations more consistently than East Asian samples, suggesting heterogeneity. To exploit the differences in linkage disequilibrium (LD) structure across ethnic groups, we conducted a SZ case-control study (that included cognitive evaluations) in a sample from the north Indian population.

METHODS

NRG1 variants (n=35 SNPs, three microsatellite markers) were initially analyzed among cases (DSM IV criteria, n=1007) and controls (n=1019, drawn from two groups) who were drawn from the same geographical region in North India. Nominally significant associations with SZ were next analyzed in relation to neurocognitive measures estimated with a computerized neurocognitive battery in a subset of the sample (n=116 cases, n=170 controls).

RESULTS

Three variants and one microsatellite showed allelic association with SZ (rs35753505, rs4733263, rs6994992, and microsatellite 420M9-1395, p≤0.05 uncorrected for multiple comparisons). A six marker haplotype 221121 (rs35753505-rs6994992-rs1354336-rs10093107-rs3924999-rs11780123) showed (p=0.0004) association after Bonferroni corrections. Regression analyses with the neurocognitive measures showed nominal (uncorrected) associations with emotion processing and attention at rs35753505 and rs6994992, respectively.

CONCLUSIONS

Suggestive associations with SZ and SZ-related neurocognitive measures were detected with two SNPs from the NRG1 promoter region in a north Indian cohort. The functional role of the alleles merits further investigation.

Functional upregulation of Ca(2+)-activated K(+) channels in the development of substantia nigra dopamine neurons

Many connections in the basal ganglia are made around birth when animals are exposed to a host of new affective, cognitive, and sensori-motor stimuli. It is thought that dopamine modulates cortico-striatal synapses that result in the strengthening of those connections that lead to desired outcomes. We propose that there must be a time before which stimuli cannot be processed into functional connections, otherwise it would imply an effective link between stimulus, response, and reward in uterus. Consistent with these ideas, we present evidence that early in development dopamine neurons are electrically immature and do not produce high-frequency firing in response to salient stimuli. We ask first, what makes dopamine neurons immature? and second, what are the implications of this immaturity for the basal ganglia? As an answer to the first question, we find that at birth the outward current is small (3nS-V), insensitive to Ca(2+), TEA, BK, and SK blockers. Rapidly after birth, the outward current increases to 15nS-V and becomes sensitive to Ca(2+), TEA, BK, and SK blockers. We make a detailed analysis of the kinetics of the components of the outward currents and produce a model for BK and SK channels that we use to reproduce the outward current, and to infer the geometrical arrangement of BK and Ca(2+) channels in clusters. In the first cluster, T-type Ca(2+) and BK channels are coupled within distances of ~20 nm (200 Å). The second cluster consists of L-type Ca(2+) and BK channels that are spread over distances of at least 60 nm. As for the second question, we propose that early in development, the mechanism of action selection is in a "locked-in" state that would prevent dopamine neurons from reinforcing cortico-striatal synapses that do not have a functional experiential-based value.

Neph1 regulates steady-state surface expression of Slo1 Ca(2+)-activated K(+) channels: different effects in embryonic neurons and podocytes

Large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels encoded by the Slo1 gene are often components of large multiprotein complexes in excitable and nonexcitable cells. Here we show that Slo1 proteins interact with Neph1, a member of the immunoglobulin superfamily expressed in slit diaphragm domains of podocytes and in vertebrate and invertebrate nervous systems. This interaction was established by reciprocal coimmunoprecipitation of endogenous proteins from differentiated cells of a podocyte cell line, from parasympathetic neurons of the embryonic chick ciliary ganglion, and from HEK293T cells heterologously expressing both proteins. Neph1 can interact with all three extreme COOH-terminal variants of Slo1 (Slo1(VEDEC), Slo1(QEERL), and Slo1(EMVYR)) as ascertained by glutathione S-transferase (GST) pull-down assays and by coimmunoprecipitation. Neph1 is partially colocalized in intracellular compartments with endogenous Slo1 in podocytes and ciliary ganglion neurons. Coexpression in HEK293T cells of Neph1 with any of the Slo1 extreme COOH-terminal splice variants suppresses their steady-state expression on the cell surface, as assessed by cell surface biotinylation assays, confocal microscopy, and whole cell recordings. Consistent with this, small interfering RNA (siRNA) knockdown of endogenous Neph1 in embryonic day 10 ciliary ganglion neurons causes an increase in steady-state surface expression of Slo1 and an increase in whole cell Ca(2+)-dependent K(+) current. Surprisingly, a comparable Neph1 knockdown in podocytes causes a decrease in surface expression of Slo1 and a decrease in whole cell BK(Ca) currents. In podocytes, Neph1 siRNA also caused a decrease in nephrin, even though the Neph1 siRNA had no sequence homology with nephrin. However, we could not detect nephrin in ciliary ganglion neurons.

Neurotrophins and target interactions in the development and regulation of sympathetic neuron electrical and synaptic properties

The electrical and synaptic properties of neurons are essential for determining the function of the nervous system. Thus, understanding the mechanisms that control the appropriate developmental acquisition and maintenance of these properties is a critical problem in neuroscience. A great deal of our understanding of these developmental mechanisms comes from studies of soluble growth factor signaling between cells in the peripheral nervous system. The sympathetic nervous system has provided a model for studying the role of these factors both in early development and in the establishment of mature properties. In particular, neurotrophins produced by the targets of sympathetic innervation regulate the synaptic and electrophysiological properties of postnatal sympathetic neurons. In this review we examine the role of neurotrophin signaling in the regulation of synaptic strength, neurotransmitter phenotype, voltage-gated currents and repetitive firing properties of sympathetic neurons. Together, these properties determine the level of sympathetic drive to target organs such as the heart. Changes in this sympathetic drive, which may be linked to dysfunctions in neurotrophin signaling, are associated with devastating diseases such as high blood pressure, arrhythmias and heart attack. Neurotrophins appear to play similar roles in modulating the synaptic and electrical properties of other peripheral and central neuronal systems, suggesting that information provided from studies in the sympathetic nervous system will be widely applicable for understanding the neurotrophic regulation of neuronal function in other systems.

Xenopus ADAM19 is involved in neural, neural crest and muscle development

ADAM19 is a member of the meltrin subfamily of ADAM metalloproteases. In Xenopus, ADAM19 is present as a maternal transcript. Zygotic expression starts during gastrulation and is apparent in the dorsal blastopore lip. ADAM19 expression through neurulation and tailbud formation becomes enriched in dorsal structures such as the neural tube, the notochord and the somites. Using morpholino knock-down, we show that a reduction of ADAM19 protein in gastrula stage embryos results in a decrease of Brachyury expression in the notochord concomitant with an increase in the dorsal markers, Goosecoid and Chordin. These changes in gene expression are accompanied by a decrease in phosphorylated AKT, a downstream target of the EGF signaling pathway, and occur while the blastopore closes at the same rate as the control embryos. During neurulation and tailbud formation, ADAM19 knock-down induces a reduction of the neural markers N-tubulin and NRP1 but not Sox2. In the somitic mesoderm, the expression of MLC is also decreased while MyoD is not. ADAM19 knockdown also reduces neural crest markers prior to cell migration. Neural crest induction is also decreased in embryos treated with an EGF receptor inhibitor suggesting that this pathway is necessary for neural crest cell induction. Using targeted knock-down of ADAM19 we show that the reduction of neural and neural crest markers is cell autonomous and that the migration if the cranial neural crest is perturbed. We further show that ADAM19 protein reduction affects somite organization, reduces 12-101 expression and perturbs fibronectin localization at the intersomitic boundary.

Neuregulin 1 in neural development, synaptic plasticity and schizophrenia

Schizophrenia is a highly debilitating mental disorder that affects approximately 1% of the general population, yet it continues to be poorly understood. Recent studies have identified variations in several genes that are associated with this disorder in diverse populations, including those that encode neuregulin 1 (NRG1) and its receptor ErbB4. The past few years have witnessed exciting progress in our knowledge of NRG1 and ErbB4 functions and the biological basis of the increased risk for schizophrenia that is potentially conferred by polymorphisms in the two genes. An improved understanding of the mechanisms by which altered function of NRG1 and ErbB4 contributes to schizophrenia might eventually lead to the development of more effective therapeutics.

A novel actin-binding domain on Slo1 calcium-activated potassium channels is necessary for their expression in the plasma membrane

Large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels regulate the physiological properties of many cell types. The gating properties of BK(Ca) channels are Ca(2+)-, voltage- and stretch-sensitive, and stretch-sensitive gating of these channels requires interactions with actin microfilaments subjacent to the plasma membrane. Moreover, we have previously shown that trafficking of BK(Ca) channels to the plasma membrane is associated with processes that alter cytoskeletal dynamics. Here, we show that the Slo1 subunits of BK(Ca) channels contain a novel cytoplasmic actin-binding domain (ABD) close to the C terminus, considerably downstream from regions of the channel molecule that play a major role in determining channel-gating properties. Binding of actin to the ABD can occur in a binary mixture in the absence of other proteins. Coexpression of a small ABD-green fluorescent protein fusion protein that competes with full-length Slo1 channels for binding to actin markedly suppresses trafficking of full-length Slo1 channels to the plasma membrane. In addition, Slo1 channels containing deletions of the ABD that eliminate actin binding are retained in intracellular pools, and they are not expressed on the cell surface. At least one point mutation within the ABD (L1020A) reduces surface expression of Slo1 channels to approximately 25% of wild type, but it does not cause a marked effect on the gating of point mutant channels that reach the cell surface. These data suggest that Slo1-actin interactions are necessary for normal trafficking of BK(Ca) channels to the plasma membrane and that the mechanisms of this interaction may be different from those that underlie F-actin and stretch-sensitive gating.

Interactions with filamin A stimulate surface expression of large-conductance Ca2+-activated K+ channels in the absence of direct actin binding

Large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels play an important role in the regulation of cell physiology in a wide variety of excitable and nonexcitable tissues. Filamin A is a conserved and ubiquitous actin-binding protein that forms perpendicular actin cross-links and contributes to changes in cell shape, stiffness, and motility. A variety of membrane proteins bind to filamin A, which regulates their trafficking in and out of the plasma membrane. Filamin A is therefore believed to couple membrane dynamics with those of the underlying cytoskeleton. Filamin A was identified in a yeast two-hybrid screen of a neuronal transcriptome using a subunit of BK(Ca) channels as bait, and the interaction was confirmed by a variety of biochemical assays in native neuronal cells and in human embryonic kidney 293T cells expressing BK(Ca) channels. BK(Ca) channels do not traffic to the plasma membrane in M2 melanoma cells, which lack filamin A, but normal trafficking is seen in A7 cells, which express filamin A, or in M2 cells transiently transfected with filamin A. It is noteworthy that stimulation of plasma membrane expression of BK(Ca) channels also occurs when M2 cells are transfected with filamin A constructs that lack the actin binding domain and that do not bind actin in vivo or in vitro. Filamin A is necessary for normal trafficking of BK(Ca) channels to the plasma membrane, but this effect does not require interactions with actin microfilaments, and it is possible that other actions of the filamin family of scaffolding proteins are independent of effects on actin.

Alternatively spliced C-terminal domains regulate the surface expression of large conductance calcium-activated potassium channels

The Slo1 gene, also known as KCNMA1, encodes the pore-forming subunits of large-conductance Ca2+-activated K+ (BK(Ca)) channels. Products of this gene are widely expressed in vertebrate tissues, and occur in a large number (>or=20) of alternatively spliced variants that vary in their gating properties, susceptibility to modulation, and trafficking to the plasma membrane. Motifs in the large cytoplasmic C-terminal are especially important in determining the functional properties of BK(Ca) channels. Here we report that chick ciliary ganglion neurons express transcripts and proteins of two Slo1 splice variants that differ at the extreme C-terminal. We refer to these variants as VEDEC and QEDRL (or QEERL for the orthologous mammalian versions), after the five terminal amino acid residues in each isoform. Individual ciliary ganglion neurons preferentially express these variants in different subcellular compartments. Moreover, QEERL channels show markedly higher levels of constitutive expression on the plasma membrane than VEDEC channels in HEK293T and NG108-15 cells. However, growth factor treatment can stimulate surface expression of VEDEC channels to levels comparable to those seen with QEERL. In addition, we show that co-expression of a soluble protein composed of VEDEC C-terminal tail residues markedly increases cell surface expression of full-length VEDEC channels, suggesting that this region binds to proteins that cause retention of the these channels in intracellular stores.

Beta1-subunits increase surface expression of a large-conductance Ca2+-activated K+ channel isoform

Auxiliary (beta) subunits of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels regulate the gating properties of the functional channel complex. Here we show that an avian beta1-subunit also stimulates the trafficking of BK(Ca) channels to the plasma membrane in HEK293T cells and in a native population of developing vertebrate neurons. One C-terminal variant of BK(Ca) alpha-subunits, called the VEDEC isoform after its five last residues, is largely retained in intracellular compartments when it is heterologously expressed in HEK293T cells. A closely related splice variant, called QEERL, shows high levels of constitutive trafficking to the plasma membrane. Co-expression of beta1-subunits with the VEDEC isoform resulted in a large increase in surface BK(Ca) channels as assessed by cell-surface biotinylation assays, whole cell recordings of membrane current, and confocal microscopy in HEK293T cells. Co-expression of beta1-subunits slowed the gating kinetics of BK(Ca) channels, as reported previously. Consistent with this, overexpression of beta1-subunits in a native cell type that expresses intracellular VEDEC channels, embryonic day 9 chick ciliary ganglion neurons, resulted in a significant increase in macroscopic Ca(2+)-activated K(+) current. Both the cytoplasmic N- and C-terminal domains of avian beta1 are able to bind directly to VEDEC and QEERL channels. However, overexpression of the N-terminal domain by itself is sufficient to stimulate trafficking of VEDEC channels to the plasma membrane, whereas overexpression of either the cytoplasmic C-terminal domain or the extracellular loop domain did not affect surface expression of VEDEC.

Increased large conductance calcium-activated potassium (BK) channel expression accompanied by STREX variant downregulation in the developing mouse CNS

Background

Large conductance calcium- and voltage activated potassium (BK) channels are important determinants of neuronal excitability through effects on action potential duration, frequency and synaptic efficacy. The pore- forming subunits are encoded by a single gene, KCNMA1, which undergoes extensive alternative pre mRNA splicing. Different splice variants can confer distinct properties on BK channels. For example, insertion of the 58 amino acid stress-regulated exon (STREX) insert, that is conserved throughout vertebrate evolution, encodes channels with distinct calcium sensitivity and regulation by diverse signalling pathways compared to the insertless (ZERO) variant. Thus, expression of distinct splice variants may allow cells to differentially shape their electrical properties during development. However, whether differential splicing of BK channel variants occurs during development of the mammalian CNS has not been examined.

Results

Using quantitative real-time polymerase chain reaction (RT-PCR) Taqman™ assays, we demonstrate that total BK channel transcripts are up regulated throughout the murine CNS during embryonic and postnatal development with regional variation in transcript levels. This upregulation is associated with a decrease in STREX variant mRNA expression and an upregulation in ZERO variant expression.

Conclusion

As BK channel splice variants encode channels with distinct functional properties the switch in splicing from the STREX phenotype to ZERO phenotype during embryonic and postnatal CNS development may provide a mechanism to allow BK channels to control distinct functions at different times of mammalian brain development.

Nerve growth factor decreases potassium currents and alters repetitive firing in rat sympathetic neurons

The sympathetic nervous system is an essential regulator of the cardiovascular system and interactions with target tissue regulate sympathetic neuronal properties. The heart produces nerve growth factor (NGF), which promotes sympathetic noradrenergic innervation of cardiac tissue and affects sympathetic synaptic strength. Neurotrophins, including NGF, are important modulators of synaptic plasticity and membrane electrical properties. Here we show that acute application of NGF causes a change in the repetitive firing pattern of cultured sympathetic neurons of the rat superior cervical ganglion. Neurons fire fewer action potentials in NGF, but with increased frequency, demonstrating an NGF-dependent change from a tonic to a phasic firing pattern. Additionally, NGF decreases the spike time variance, making spikes more tightly time locked to stimulus onset. NGF causes a decrease in the amplitude of both calcium-dependent and -independent potassium currents, and inhibition of calcium-dependent potassium currents using CdCl(2) reproduces some, but not all, of the firing properties induced by NGF. This study suggests that NGF release from cardiac tissue may act to modulate the repetitive firing properties of sympathetic neurons to tune their output to meet the physiological needs of the organism.

Modulation of glioma BK channels via erbB2

Glioma cells show up-regulation and constitutive activation of erbB2, and its expression correlates positively with increased malignancy. A similar correlation has been demonstrated for the expression of gBK, a calcium-sensitive, large-conductance K(+) channel. We show here that glioma BK channels are a downstream target of erbB2/neuregulin signaling. Tyrphostin AG825 was able to disrupt the constituitive erbB2 activation in a dose-dependent manner, causing a 30-mV positive shift in gBK channel activation in cell-attached patches. Conversely, maximal stimulation of erbB2 with a recombinant neuregulin (NRG-1beta) caused a 12-mV shift in the opposite direction. RT-PCR studies reveal no change in the BK splice variants expressed in treated glioma cells. Furthermore, isolation of surface proteins through biotinylation did not show a change in gBK channel expression, and probing with phospho-specific antibodies showed no alteration in channel phosphorylation. However, fura-II Ca(2+) fluorescence imaging revealed a 35% decrease in the free intracellular Ca(2+) concentration after erbB2 inhibition and an increase in NRG-1beta-treated cells, suggesting that the observed changes most likely were due to alterations in [Ca(2+)](i). Consistent with this conclusion, neither tyrphostin AG825 nor NRG-1beta was able to modulate gBK channels under inside-out or whole-cell recording conditions when intracellular Ca(2+) was fixed. Thus, gBK channels are a downstream target for the abundantly expressed neuregulin-1 receptor erbB2 in glioma cells. However, unlike the case in other systems, this modulation appears to occur via changes in [Ca(2+)](i) without changes in channel expression or phosphorylation. The enhanced sensitivity of gBK channels in glioma cells to small, physiological Ca(2+) changes appears to be a prerequisite for this modulation.

Regulation of neuronal K(Ca) channels by beta-neuregulin-1 does not require activation of Ras-MEK-extracellular signal-regulated kinase signaling cascades

Endogenous beta-neuregulin-1 is required for the plasma membrane expression of large-conductance (BK-type) Ca2+-activated K+ channels in developing chick ciliary neurons of the chick ciliary ganglion. During normal development, beta-neuregulin-1 acts in concert with transforming growth factor-beta1 to stimulate movement of large-conductance Ca2+-activated K+ channels from intracellular stores into the plasma membrane, although these two growth factors preferentially act on different intracellular pools. We have previously shown that actions of transforming growth factor-beta1 on ciliary neurons require activation of phosphoinositol 3-kinase and Akt, as well as a parallel cascade composed of the small GTPase Ras and a mitogen-activated protein kinase (extracellular signal-regulated kinase). In addition, we have shown that the actions of beta-neuregulin-1 require activation of phosphoinositol 3-kinase and the protein kinase Akt. Here we examine whether beta-neuregulin-1-evoked mobilization of large-conductance Ca2+-activated K+ channels also requires activation of a Ras-extracellular signal-regulated kinase signaling cascade. We observed that application of beta-neuregulin-1 caused a robust and MEK1/2-dependent increase in extracellular signal-regulated kinase diphosphorylation that indicates activation of this signaling cascade in ciliary ganglion neurons, similar to what we have previously observed for transforming growth factor-beta1. However, activation of this cascade is not necessary for beta-neuregulin-1-evoked mobilization because stimulation of macroscopic large-conductance Ca2+-activated K+ channels persisted in cells treated with the MEK1/2 inhibitors PD98059 or U0126, in cells over-expressing dominant-negative forms of extracellular signal-regulated kinase, and in cells treated with the Ras inhibitor FTI-277. These results indicate that the mechanisms that underlie beta-neuregulin-1 and transforming growth factor-beta1 mobilization of large-conductance Ca2+-activated K+ channels are only partly overlapping, possibly because they cause recruitment of spatially distinct signaling complexes.

The p38 mitogen-activated protein kinase pathway negatively regulates Ca2+-activated K+ channel trafficking in developing parasympathetic neurons

The trafficking of large-conductance Ca2+-activated K+ channels (K(Ca)) in chick ciliary ganglion neurons is regulated by growth factors. Here we show that a canonical p38 cascade inhibits K(Ca) trafficking in ciliary ganglion neurons. Two different p38 inhibitors (SB202190 or SB203580) or over-expression of dominant-negative forms of several components of the p38 cascade increased K(Ca) in ciliary neurons. Inhibition of protein synthesis or Golgi processing had no effect on this phenomenon, suggesting that p38 is acting at a distal step of the trafficking pathway. Depolymerization of filamentous actin (F-actin) increased functional expression of K(Ca), whereas stabilization of F-actin inhibited the effect of SB202190 on K(Ca) trafficking. SB202190 also caused an immunochemically detectable increase in K(Ca) on the plasma membrane. Inhibition of p38 decreased the extent of cortical F-actin in ciliary neurons. Macroscopic K(Ca) is suppressed by transforming growth factor (TGF) beta3. Application of TGFbeta3 increased the phosphorylation of p38 in ciliary neurons and increased cortical F-actin. Thus, the p38 signaling cascade endogenously suppresses development of functional K(Ca), in part by stabilizing an F-actin barrier that prevents plasma membrane insertion of functional channel complexes. This cascade also appears to mediate inhibitory effects of TGFbeta3 on the expression of K(Ca).

Growth Factors Mobilize Multiple Pools ofKCaChannels in Developing Parasympathetic Neurons: Role of ADP-Ribosylation Factors and Related Proteins

In developing ciliary ganglion (CG) neurons, movement of functional large-conductance (BK type) Ca2+-activated K+( KCa) channels to the cell surface is stimulated by the endogenous growth factors TGFβ1 and β-neuregulin-1 (NRG1). Here we show that a brief NRG1 treatment (0.5–1.5 h) mobilizes KCachannels in a post-Golgi compartment, but longer treatments (>3.5 h) mobilize KCachannels located in the endoplasmic reticulum or Golgi apparatus. Specifically, the effects of 3.5 h NRG1 treatment were completely blocked by treatments that disrupt Golgi apparatus function. These include inhibition of microtubules, or inhibition of the ADP-ribosylation factor-1 (ARF1) system by brefeldin A, by over-expression of dominant-negative ARF1, or over-expression of an ARF1 GTPase-activating protein that blocks ARF1 cycling between GTP- and GDP-bound states. These treatments had no effect on stimulation of KCaevoked by 1.5 h treatment with NRG1, indicating that short-term responses to NRG1 do not require an intact Golgi apparatus. By contrast, both the acute and sustained effects of NRG1 were inhibited by treatments that block trafficking processes that occur close to the plasma membrane. Thus mobilization of KCawas blocked by treatments than inhibit ADP-ribosylation factor-6 (ARF6) signaling, including overexpression of dominant-negative ARF6, dominant-negative ARNO, or dominant-negative phospholipase D1. TGFβ1, the effects of which on KCaare much slower in onset, is unable to selectively mobilize channels in the post-Golgi pool, and its effects on KCaare completely blocked by inhibition of microtubules, Golgi function and also by plasma membrane ARF6 and phospholipase D1 signaling.

Akt Activation Is Necessary for Growth Factor–Induced Trafficking of Functional KCaChannels in Developing Parasympathetic Neurons

The protein kinase Akt is a crucial regulator of neuronal survival and apoptosis. Here we show that Akt activation is necessary for mobilization of large-conductance KCachannels in ciliary ganglion (CG) neurons evoked by β-neuregulin-1 (NRG1) and transforming growth factor-β1 (TGFβ1). Application of NRG1 to embryonic day 9 (E9) CG neurons increased Akt phosphorylation, as observed previously for TGFβ1. NRG1- and TGFβ1-evoked stimulation of KCais blocked by inhibitors of PI3K by overexpression of a dominant-negative form of Akt, by overexpression of CTMP, an endogenous negative regulator of Akt, and by application of the Akt inhibitor 1L-6-hydroxymethyl-chiro-inositol 2-( R)-2- O-methyl-3- O-octadecylcarbonate (HIMO). Conversely, overexpression of a constitutively-active form of Akt was sufficient by itself to increase mobilization of functional KCachannels. NRG1 and TGFβ1 evoked an Akt-dependent increase in cell-surface SLO α-subunits. These procedures have no effect on voltage-activated Ca2+currents. Thus Akt plays an essential role in the developmental regulation of excitability in CG neurons.

ErbB transmembrane tyrosine kinase receptors are expressed by sensory and motor neurons projecting into sciatic nerve

Adult spinal cord motor and dorsal root ganglion (DRG) sensory neurons express multiple neuregulin-1 (NRG-1) isoforms that act as axon-associated factors promoting neuromuscular junction formation and Schwann cell proliferation and differentiation. NRG-1 isoforms are also expressed by muscle and Schwann cells, suggesting that motor and sensory neurons are themselves acted on by NRG-1 isoforms produced by their peripheral targets. To test this hypothesis, we examined the expression of the NRG-1 receptor subunits erbB2, erbB3, and erbB4 in rat lumbar DRG and spinal cord. All three erbB receptors are expressed in these tissues. Sciatic nerve transection, an injury that induces Schwann cell expression of NRG-1, alters erbB expression in DRG and cord. Virtually all DRG neurons are erbB2- and erbB3-immunoreactive, with erbB4 also detectable in many neurons. In spinal cord white matter, erbB2 and erbB4 antibodies produce dense punctate staining, whereas the erbB3 antibody primarily labels glial cell bodies. Spinal cord dorsal and ventral horn neurons, including alpha-motor neurons, exhibit erbB2, erbB3, and erbB4 immunoreactivity. Spinal cord ventral horn also contains a population of small erbB3+/S100beta+/GFAP- cells (GFAP-negative astrocytes or oligodendrocytes). We conclude that sensory and motor neurons projecting into sciatic nerve express multiple erbB receptors and are potentially NRG-1 responsive.

A-current expression is regulated by activity but not by target tissues in developing lumbar motoneurons of the chick embryo

The functional expression of A-type K+ channels (IA) was examined in chick lumbar motoneurons (LMNs) at embryonic days 6 and 11 (E6 and E11). We observed a threefold increase in IA density between E6 and E11 in spinal cord slices and acutely dissociated LMNs. There was no change in current density, kinetics, or voltage dependence of IA in E11 homozygous limbless mutants or in E11 embryos in which hindlimbs were surgically removed at E6. Moreover, chronic in ovo administration of D-tubocurarine, which causes an increase in motoneuron branching on the surface of target muscles, had no effect on IA. Electrical activity played an important role in IA regulation in LMNs in vitro and in ovo. Blocking spontaneous electrical activity of LMNs by chronic in ovo application of mecamylamine or muscimol reduced IA by 80%. LMNs cultured in the presence of TTX also failed to express normal densities of IA, even when the cultures also contained target tissues. The portion of IA that remained after in ovo or in vitro blockade of activity inactivated more quickly than the IA of LMNs that were allowed to discharge spikes. The developmental expression of LMN IA increases significantly during development, and this increase is activity dependent but does not require interactions with target tissues. Ongoing activity also seems to regulate the kinetics of IA inactivation.

Postsynaptic protein kinase A reduces neuronal excitability in response to increased synaptic excitation in the Drosophila CNS

Previous work has identified a role for synaptic activity in the development of excitable properties of motoneurons in the Drosophila embryo. In this study the underlying mechanism that enables two such neurons, termed aCC and RP2, to respond to increased exposure to synaptic excitation is characterized. Synaptic excitation is increased in genetic backgrounds that lack either a cAMP-specific phosphodiesterase (EC:3.1.4, dunce) or acetylcholinesterase (EC:3.1.1.7, ace), the enzyme that terminates the endogenous cholinergic excitation of these motoneurons. Analysis of membrane excitability in aCC/RP2, in either background, shows that these neurons have a significantly reduced capability to fire action potentials (APs) in response to injection of depolarizing current. Analysis of underlying voltage-gated currents show that this effect is associated with a marked reduction in magnitude of the voltage-dependent inward Na+ current (INa). Partially blocking INa in these motoneurons, using low concentrations of TTX, demonstrates that a reduction of INa is, by itself, sufficient to reduce membrane excitability. An analysis of firing implicates an increased AP threshold to underlie the reduction in membrane excitability observed because of heightened exposure to synaptic excitation. Genetic or pharmacological manipulations that either elevate cAMP or increase protein kinase A (PKA) activity in wild-type aCC/RP2 mimic both the reductions in membrane excitability and INa. In comparison, increasing cAMP catabolism or inhibition of PKA activity is sufficient to block the induction of these activity-dependent changes. The induced changes in excitability can be rapid, occurring within 5 min of exposure to a membrane-permeable cAMP analog, indicative that threshold can be regulated in these neurons by a post-translational mechanism that is dependent on phosphorylation.

Neuregulin-1β modulates in vivo entorhinal–hippocampal synaptic transmission in adult rats

Neuregulin-1 (NRG-1) proteins and their erbB receptors are essential for neuronal development during embryogenesis and may contribute importantly to neuronal function in the adult brain. This study tests the hypothesis that NRG-1beta acts as a modulator of synaptic activity in the adult brain, specifically at hippocampal formation synapses. Adult, male Sprague-Dawley rats were anesthetized and a recording electrode with an attached stainless steel microinjector was stereotaxically positioned to record field potentials (fEPSP) in either the dentate gyrus or the cornu ammonis (CA) 1 field of the hippocampus. The entorhinal cortex was continuously stimulated via a paired stainless steel electrode. Microinjection of NRG-1beta significantly increased the slope of the fEPSP in the dentate gyrus in a dose-dependent manner. Compared with a low dose (20 nM), a high dose (100 nM) of NRG-1beta induced a shorter latency response that was of greater magnitude. Responses to NRG-1beta were abolished by pretreatment with a selective, reversible erbB tyrosine kinase inhibitor, PD158780 (100 microM). Further, PD158780 (100 microM) itself significantly decreased the entorhinal-dentate fESPS slope by about 15%. Neither equimolar (100 nM) nor hypermolar (100 microM) sucrose or heat-inactivated NRG-1beta (100 nM) significantly altered the entorhinal-dentate fEPSP slope. In contrast to its effect at the entorhinal-dentate synapse, NRG-1beta (100 nM) depressed, and PD158780 potentiated entorhinal-CA1 synaptic transmission. Thus, in adult rats NRG-1beta potentiates transmission at the entorhinal-dentate synapse but suppresses transmission at the entorhinal-CA1 synapse. These observations indicate that NRG-1 is not only a developmental growth factor, but also modifies synaptic transmission in adult rat brain.

Postsynaptic protein kinase A reduces neuronal excitability in response to increased synaptic excitation in the Drosophila CNS

Previous work has identified a role for synaptic activity in the development of excitable properties of motoneurons in the Drosophila embryo. In this study the underlying mechanism that enables two such neurons, termed aCC and RP2, to respond to increased exposure to synaptic excitation is characterized. Synaptic excitation is increased in genetic backgrounds that lack either a cAMP-specific phosphodiesterase (EC:3.1.4, dunce) or acetylcholinesterase (EC:3.1.1.7, ace), the enzyme that terminates the endogenous cholinergic excitation of these motoneurons. Analysis of membrane excitability in aCC/RP2, in either background, shows that these neurons have a significantly reduced capability to fire action potentials (APs) in response to injection of depolarizing current. Analysis of underlying voltage-gated currents show that this effect is associated with a marked reduction in magnitude of the voltage-dependent inward Na+ current (INa). Partially blocking INa in these motoneurons, using low concentrations of TTX, demonstrates that a reduction of INa is, by itself, sufficient to reduce membrane excitability. An analysis of firing implicates an increased AP threshold to underlie the reduction in membrane excitability observed because of heightened exposure to synaptic excitation. Genetic or pharmacological manipulations that either elevate cAMP or increase protein kinase A (PKA) activity in wild-type aCC/RP2 mimic both the reductions in membrane excitability and INa. In comparison, increasing cAMP catabolism or inhibition of PKA activity is sufficient to block the induction of these activity-dependent changes. The induced changes in excitability can be rapid, occurring within 5 min of exposure to a membrane-permeable cAMP analog, indicative that threshold can be regulated in these neurons by a post-translational mechanism that is dependent on phosphorylation.

Target-mediated control of neural differentiation

The development of the nervous system entails the coordination of the spatial and chemical development of both pre- and postsynaptic elements. This coordination is accomplished by signals passing between neurons and the target cells that they innervate. This review focuses on well-characterized examples of target-mediated neuronal differentiation in the central and peripheral nervous systems. These include control of neurogenesis in the leech by male genitalia, presynaptic differentiation induced by postsynaptic molecules expressed by skeletal muscle, postsynaptic adhesion molecules that induce presynaptic differentiation in the central nervous system (CNS), target-mediated control of neurotransmitter phenotype in peripheral neurons, and target-regulated control of neuronal nicotinic acetylcholine receptors (nAChRs) and large conductance calcium-activated potassium channels (BK). The detailed understanding of these processes will uncover signals critical for the directed differentiation of stem cells as well as identify future targets for therapies in neural regeneration that promote the reestablishment of functional connections.

Association of neuregulin 1 with schizophrenia confirmed in a Scottish population

Recently, we identified neuregulin 1 (NRG1) as a susceptibility gene for schizophrenia in the Icelandic population, by a combined linkage and association approach. Here, we report the first study evaluating the relevance of NRG1 to schizophrenia in a population outside Iceland. Markers representing a core at-risk haplotype found in Icelanders at the 5' end of the NRG1 gene were genotyped in 609 unrelated Scottish patients and 618 unrelated Scottish control individuals. This haplotype consisted of five SNP markers and two microsatellites, which all appear to be in strong linkage disequilibrium. For the Scottish patients and control subjects, haplotype frequencies were estimated by maximum likelihood, using the expectation-maximization algorithm. The frequency of the seven-marker haplotype among the Scottish patients was significantly greater than that among the control subjects (10.2% vs. 5.9%, P=.00031). The estimated risk ratio was 1.8, which is in keeping with our report of unrelated Icelandic patients (2.1). Three of the seven markers in the haplotype gave single-point P values ranging from .000064 to .0021 for the allele contributing to the at-risk haplotype. This direct replication of haplotype association in a second population further implicates NRG1 as a factor that contributes to the etiology of schizophrenia.

Neuregulin 1 and susceptibility to schizophrenia

The cause of schizophrenia is unknown, but it has a significant genetic component. Pharmacologic studies, studies of gene expression in man, and studies of mouse mutants suggest involvement of glutamate and dopamine neurotransmitter systems. However, so far, strong association has not been found between schizophrenia and variants of the genes encoding components of these systems. Here, we report the results of a genomewide scan of schizophrenia families in Iceland; these results support previous work, done in five populations, showing that schizophrenia maps to chromosome 8p. Extensive fine-mapping of the 8p locus and haplotype-association analysis, supplemented by a transmission/disequilibrium test, identifies neuregulin 1 (NRG1) as a candidate gene for schizophrenia. NRG1 is expressed at central nervous system synapses and has a clear role in the expression and activation of neurotransmitter receptors, including glutamate receptors. Mutant mice heterozygous for either NRG1 or its receptor, ErbB4, show a behavioral phenotype that overlaps with mouse models for schizophrenia. Furthermore, NRG1 hypomorphs have fewer functional NMDA receptors than wild-type mice. We also demonstrate that the behavioral phenotypes of the NRG1 hypomorphs are partially reversible with clozapine, an atypical antipsychotic drug used to treat schizophrenia.

SpADAM, a sea urchin ADAM, has conserved structure and expression

ADAMs are multidomain cell surface proteins that function in receptor-ligand processing, cell adhesion and fusion, and signaling. SpADAM, a single copy sea urchin ADAM gene with a 3072 bp open reading frame, is expressed during embryonic and larval development. The deduced SpADAM protein is 1023 amino acids long and includes all domains characteristic of ADAMs. Northern blots reveal the presence of 4.4 and 2.3 kb SpADAM transcripts throughout development. Predominant SpADAM proteins are 131 and 95 kDa. The deduced primary structure of SpADAM is closely related to vertebrate ADAMs 12, 13, and 19. SpADAM is expressed during cleavage on blastomere surfaces, and later by vegetal plate cells, migrating secondary mesenchyme, skeletogenic mesenchyme, muscles, and neurons within the ciliated band. Apparently, the structure and types of cells in which ADAM 12/13/19 orthologues are expressed are conserved in deuterostomes.

Developmental regulation of neuronal K(Ca) channels by TGFbeta1: an essential role for PI3 kinase signaling and membrane insertion

TGFbeta1 is a target-derived factor responsible for the developmental expression of large-conductance Ca(2+)-activated K(+) (K(Ca)) channels in ciliary neurons of the chick ciliary ganglion. The acute effects of TGFbeta1 on K(Ca) channels are mediated by posttranslational events and require activation of the MAP kinase Erk. Here we show that TGFbeta1 evokes robust phosphorylation of Akt/PKB, a protein kinase dependent on the products of phosphatidylinositol 3-OH kinase (PI3K). TGFbeta1-evoked stimulation of K(Ca) channels is blocked by the PI3K inhibitors wortmannin and LY294002. These drugs also inhibit TGFbeta1 effects on Akt/PKB phosphorylation but have no effect on TGFbeta1-evoked Erk activation. Application of the MEK1 inhibitor PD98059 blocked TGFbeta1 effects on Erk but had no effect on Akt/PKB phosphorylation. These results indicate that PI3K and Erk represent parallel signaling cascades activated by TGFbeta1 in ciliary neurons. The effects of TGFbeta1 on functional expression of K(Ca) are blocked by the microtubule inhibitors colchicine and nocodazole, by botulinum toxins A and E, and by brefeldin-A, an agent that disrupts the Golgi apparatus. These data indicate that translocation of a membrane protein, possibly Slowpoke (SLO), is required for the acute posttranslational effects of TGFbeta1 on K(Ca) channels. Confocal immunofluorescence studies with three different SLO antisera showed robust expression of SLO in multiple intracellular compartments of embryonic day 9-13 ciliary neurons, including the cell nucleus. These data suggest that TGFbeta1 evokes insertion of SLO channels into the plasma membrane as a result of signaling cascades that entail activation of Erk and PI3K.

The glial growth factors deficiency and synaptic destabilization hypothesis of schizophrenia

BACKGROUND

A systems approach to understanding the etiology of schizophrenia requires a theory which is able to integrate genetic as well as neurodevelopmental factors.

PRESENTATION OF THE HYPOTHESIS

Based on a co-localization of loci approach and a large amount of circumstantial evidence, we here propose that a functional deficiency of glial growth factors and of growth factors produced by glial cells are among the distal causes in the genotype-to-phenotype chain leading to the development of schizophrenia. These factors include neuregulin, insulin-like growth factor I, insulin, epidermal growth factor, neurotrophic growth factors, erbB receptors, phosphatidylinositol-3 kinase, growth arrest specific genes, neuritin, tumor necrosis factor alpha, glutamate, NMDA and cholinergic receptors. A genetically and epigenetically determined low baseline of glial growth factor signaling and synaptic strength is expected to increase the vulnerability for additional reductions (e.g., by viruses such as HHV-6 and JC virus infecting glial cells). This should lead to a weakening of the positive feedback loop between the presynaptic neuron and its targets, and below a certain threshold to synaptic destabilization and schizophrenia.

TESTING THE HYPOTHESIS

Supported by informed conjectures and empirical facts, the hypothesis makes an attractive case for a large number of further investigations.

IMPLICATIONS OF THE HYPOTHESIS

The hypothesis suggests glial cells as the locus of the genes-environment interactions in schizophrenia, with glial asthenia as an important factor for the genetic liability to the disorder, and an increase of prolactin and/or insulin as possible working mechanisms of traditional and atypical neuroleptic treatments.

Development of convergent synaptic inputs to subpopulations of autonomic neurons

Visceromotor neurons in mammalian prevertebral sympathetic ganglia receive convergent synaptic inputs from spinal preganglionic neurons and peripheral intestinofugal neurons projecting from the enteric plexuses. Vasomotor neurons in the same ganglia receive only preganglionic inputs. How this pathway-specific pattern of connectivity is established is unknown. We have used a combination of immunohistochemical, ultrastructural, and electrophysiological techniques to investigate the development of synaptic inputs onto visceromotor and vasomotor neurons in the celiac ganglion of guinea pigs. Functional synaptogenesis occurred primarily from early fetal (F30-F35) to midfetal (F36-F45) stages, after the neurochemical differentiation of vasomotor and visceromotor neurons but before establishment of their electrophysiological phenotypes. Intestinofugal inputs were detected only on presumptive visceromotor neurons located primarily in medial regions of the ganglion. The number of ultrastructurally identified synaptic profiles increased in parallel with functional synaptogenesis, especially in medial regions, where dendritic growth rates also were higher. However, the expression of immunoreactivity to choline acetyltransferase in the terminals of inputs was very low until late fetal stages, after functional transmission already had been established. These results show that peripheral intestinofugal neurons directly establish appropriate functional connections with their target visceromotor neurons simultaneously with the development of functional preganglionic inputs to both visceromotor and vasomotor neurons. It seems likely that synaptogenesis occurs independently of the neurochemical differentiation of the target neurons but is closely related to the pathway-specific dendritic development of those neurons.