Signaling mechanisms underlying dendrite formation

Microtubule stabilization by bone morphogenetic protein receptor-mediated scaffolding of c-Jun N-terminal kinase promotes dendrite formation

Neuronal outgrowth occurs via coordinated remodeling of the cytoskeleton involving both actin and microtubules. Microtubule stabilization drives the extending neurite, yet little is known of the molecular mechanisms whereby extracellular cues regulate microtubule dynamics. Bone morphogenetic proteins (BMPs) play an important role in neuronal differentiation and morphogenesis, and BMP7 in particular induces the formation of dendrites. Here, we show that BMP7 induces stabilization of microtubules in both a MAP2-dependent neuronal cell culture model and in dendrites of primary cortical neurons. BMP7 rapidly activates c-Jun N-terminal kinases (JNKs), known regulators of microtubule dynamics, and we show that JNKs associate with the carboxy terminus of the BMP receptor, BMPRII. Activation and binding of JNKs to BMPRII is required for BMP7-induced microtubule stabilization and for BMP7-mediated dendrite formation in primary cortical neurons. These data indicate that BMPRII acts as a scaffold to localize and coordinate cytoskeletal remodeling and thereby provides an efficient means for extracellular cues, such as BMPs, to control neuronal dendritogenesis.

Antillatoxin, a novel lipopeptide, enhances neurite outgrowth in immature cerebrocortical neurons through activation of voltage-gated sodium channels

Antillatoxin (ATX) is a structurally novel lipopeptide that activates voltage-gated sodium channels (VGSC) leading to sodium influx in cerebellar granule neurons and cerebrocortical neurons 8 to 9 days in vitro (Li et al., 2001; Cao et al., 2008). However, the precise recognition site for ATX on the VGSC remains to be defined. Inasmuch as elevation of intracellular sodium ([Na(+)](i)) may increase N-methyl-d-aspartate receptor (NMDAR)-mediated Ca(2+) influx, Na(+) may function as a signaling molecule. We hypothesized that ATX may enhance neurite outgrowth in cerebrocortical neurons by elevating [Na(+)](i) and augmenting NMDAR function. ATX (30-100 nM) robustly stimulated neurite outgrowth, and this enhancement was sensitive to the VGSC antagonist, tetrodotoxin. To unambiguously demonstrate the enhancement of NMDA receptor function by ATX, we recorded single-channel currents from cell-attached patches. ATX was found to increase the open probability of NMDA receptors. Na(+)-dependent up-regulation of NMDAR function has been shown to be regulated by Src family kinase (SFK) (Yu and Salter, 1998). The Src kinase inhibitor PP2 abrogated ATX-enhanced neurite outgrowth, suggesting a SFK involvement in this response. ATX-enhanced neurite outgrowth was also inhibited by the NMDAR antagonist, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), and the calmodulin-dependent kinase kinase (CaMKK) inhibitor, 1,8-naphthoylene benzimidazole-3-carboxylic acid (STO-609), demonstrating the requirement for NMDAR activation with subsequent downstream engagement of the Ca(2+)-dependent CaMKK pathway. These results with the structurally and mechanistically novel natural product, ATX, confirm and generalize our earlier results with a neurotoxin site 5 ligand. These data suggest that VGSC activators may represent a novel pharmacological strategy to regulate neuronal plasticity through NMDAR-dependent mechanisms.

MeCP2 functions largely cell-autonomously, but also non-cell-autonomously, in neuronal maturation and dendritic arborization of cortical pyramidal neurons

Rett syndrome is a human neurodevelopmental disorder presenting almost exclusively in female infants; it is the second most common cause of mental retardation in girls, after Down's syndrome. The identification in 1999 that mutation of the methyl-CpG-binding protein 2 (MECP2) gene on the X chromosome causes Rett syndrome has led to a rapid increase in understanding of the neurobiological basis of the disorder. However, much about the functional role of MeCP2, and the cellular phenotype of both patients with Rett syndrome and mutant Mecp2 mouse models, remains unclear. Building on prior work in which we demonstrated that cortical layer 2/3 pyramidal neurons (primarily interhemispheric "callosal projection neurons" (CPN)) have reduced dendritic complexity and smaller somata in Mecp2-null mice, here we investigate whether Mecp2 loss-of-function affects neuronal maturation cell-autonomously and/or non-cell-autonomously by creating physical chimeras. We transplanted Mecp2-null or wild-type (wt) E17-18 cortical neuroblasts and immature neurons from mice constitutively expressing enhanced green fluorescent protein (eGFP) into wt P2-3 mouse cortices to generate chimeric cortices. Mecp2-null layer 2/3 pyramidal neurons in both Mecp2-null and wt neonatal cortices exhibit equivalent reduction in dendritic complexity, and are smaller than transplanted wt neurons, independent of recipient environment. These results indicate that the phenotype of Mecp2-null pyramidal neurons results largely from cell-autonomous mechanisms, with additional non-cell-autonomous effects. Dysregulation of MeCP2 target genes in individual neuronal populations such as CPN is likely centrally involved in Rett syndrome pathogenesis. Our results indicating MeCP2 function in the centrally affected projection neuron population of CPN themselves provide a foundation and motivation for identification of transcriptionally regulated MeCP2 target genes in developing CPN.

Glucocorticoid attenuates brain-derived neurotrophic factor-dependent upregulation of glutamate receptors via the suppression of microRNA-132 expression

Brain-specific microRNAs (miRs) may be involved in synaptic plasticity through the control of target mRNA translation. Brain-derived neurotrophic factor (BDNF) also contributes to the regulation of synaptic function. However, the possible involvement of miRs in BDNF-regulated synaptic function is poorly understood. Importantly, an increase in glucocorticoid levels and the downregulation of BDNF are supposed to be involved in the pathophysiology of depressive disorders. Previously, we reported that glucocorticoid exposure inhibited BDNF-regulated synaptic function via weakening mitogen-activated protein kinase/extracellular signal-regulated kinase1/2 (MAPK/ERK) and/or phospholipase C-gamma (PLC-gamma) intracellular signaling in cultured neurons [Kumamaru et al (2008) Mol Endocrinol 22:546-558; Numakawa et al (2009) Proc Natl Acad Sci U S A 106:647-652]. Therefore, in this study, we investigate the possible influence of glucocorticoid on BDNF/miRs-stimulated biological responses in cultured cortical neurons. Significant upregulation of miR-132 was caused by BDNF, although miR-9, -124, -128a, -128b, -134, -138, and -16 were intact. Transfection of exogenous ds-miR-132 induced marked upregulation of glutamate receptors (NR2A, NR2B, and GluR1), suggesting that miR-132 has a positive effect on the increase in postsynaptic proteins levels. Consistently, transfection of antisense RNA to inhibit miR-132 function decreased the BDNF-dependent increase in the expression of postsynaptic proteins. U0126, an inhibitor of the MAPK/ERK pathway, suppressed the BDNF-increased miR-132, suggesting that BDNF upregulates miR-132 via the MAPK/ERK1/2 pathway. Interestingly, pretreatment with glucocorticoid (dexamethasone, DEX) reduced BDNF-increased ERK1/2 activation, miR-132 expression, and postsynaptic proteins. We demonstrate that the exposure of neurons to an excess glucocorticoid results in a decrease in the BDNF-dependent neuronal function via suppressing miR-132 expression.

Dendrite formation of cerebellar Purkinje cells

During postnatal cerebellar development, Purkinje cells form the most elaborate dendritic trees among neurons in the brain, which have been of great interest to many investigators. This article overviews various examples of cellular and molecular mechanisms of formation of Purkinje cell dendrites as well as the methodological aspects of investigating those mechanisms.

Withania somnifera prevents morphine withdrawal-induced decrease in spine density in nucleus accumbens shell of rats: a confocal laser scanning microscopy study

Opiate withdrawal is associated with morphological changes of dopamine neurons in the ventral tegmental area and with reduction of spine density of second-order dendrites of medium size spiny neurons in the nucleus accumbens shell but not core. Withania somnifera has long been used in the Middle East, Africa, and India as a remedy for different conditions and diseases and a growing body of evidence points to its beneficial effects on a number of experimental models of neurological disorders. Recently, many studies focused on the potential neuritic regeneration and synaptic reconstruction properties of its methanolic extract and its constituents (withanolides). This study investigates whether morphine withdrawal-induced spine reduction in the nucleus accumbens is affected by the administration of a Withania somnifera extract. To this end, rats were chronically treated with Withania somnifera extract along with morphine or saline and, upon spontaneous (1 and 3 days) or pharmacologically precipitated withdrawal, their brains were fixed in Golgi-Cox stain for confocal microscopic examination. In a separate group of animals, Withania somnifera extract was administered during three days of spontaneous withdrawal. Withania somnifera extract treatment reduced the severity of the withdrawal syndrome when given during chronic morphine but not during withdrawal. In addition, treatment with Withania somnifera extract during chronic morphine, but not during withdrawal, fully prevented the reduction of spine density in the nucleus accumbens shell in spontaneous and pharmacologically precipitated morphine withdrawal. These results indicate that pretreatment with Withania somnifera extract protects from the structural changes induced by morphine withdrawal potentially providing beneficial effects on the consequences related to this condition.

Neuronal hyperactivity sustains the basal dendrites of immature dentate granule cells: time-lapse confocal analysis using hippocampal slice cultures

Dendritic morphogenesis is an essential process for the establishment of proper neural circuitry. In the epileptic hippocampus, mature dentate granule cells (GCs) possess basal dendrites (BDs), which is abnormal and is assumed to contribute to seizure progression. However, there is a lack of direct time-lapse evidence showing that neuronal hyperactivity regulates the dendritic development of GCs. In the present study, we carried out time-lapse confocal analysis of the dendritic morphogenesis of GCs in hippocampal slice cultures that were prepared from postnatal 6-day-old (P6) rats. By electroporating membrane-targeted green fluorescent protein at 5 days in vitro (DIV), we found that most of the Prox1-positive and calbindin-negative immature GCs possessed several BDs and filopodia-rich apical dendrites at 7 DIV. BDs were gradually eliminated from 7 to 9 DIV, and they completely vanished at 14 DIV in all the GCs examined. However, most BDs failed to retract from 7 to 9 DIV, when the GABA(A) receptor antagonist picrotoxin was chronically applied to induce epileptic conditions in the cultures. These effects were blocked by coapplying tetrodotoxin, a sodium channel blocker, thus convincing us that neuronal hyperactivity contributes to the maintenance of BDs. Further, in the picrotoxin-treated cultures, most of the GCs persistently exhibited several BDs even after 14 DIV. In contrast, neither the progressive pruning of the filopodia nor the branch dynamics of the apical dendrites during the culture periods was affected by picrotoxin. These results, for the first time, provide us with direct evidence that neuronal hyperactivity contributes to the stability of pre-existing BDs.

Sodium channel activation augments NMDA receptor function and promotes neurite outgrowth in immature cerebrocortical neurons

A range of extrinsic signals, including afferent activity, affect neuronal growth and plasticity. Neuronal activity regulates intracellular Ca(2+), and activity-dependent calcium signaling has been shown to regulate dendritic growth and branching (Konur and Ghosh, 2005). NMDA receptor (NMDAR) stimulation of Ca(2+)/calmodulin-dependent protein kinase signaling cascades has, moreover, been demonstrated to regulate neurite/axonal outgrowth (Wayman et al., 2004). We used a sodium channel activator, brevetoxin (PbTx-2), to explore the relationship between intracellular [Na(+)] and NMDAR-dependent development. PbTx-2 alone, at a concentration of 30 nM, did not affect Ca(2+) dynamics in 2 d in vitro cerebrocortical neurons; however, this treatment robustly potentiated NMDA-induced Ca(2+) influx. The 30 nM PbTx-2 treatment produced a maximum [Na(+)](i) of 16.9 +/- 1.5 mM, representing an increment of 8.8 +/- 1.8 mM over basal. The corresponding membrane potential change produced by 30 nM PbTx-2 was modest and, therefore, insufficient to relieve the voltage-dependent Mg(2+) block of NMDARs. To unambiguously demonstrate the enhancement of NMDA receptor function by PbTx-2, we recorded single-channel currents from cell-attached patches. PbTx-2 treatment was found to increase both the mean open time and open probability of NMDA receptors. These effects of PbTx-2 on NMDA receptor function were dependent on extracellular Na(+) and activation of Src kinase. The functional consequences of PbTx-2-induced enhancement of NMDAR function were evaluated in immature cerebrocortical neurons. PbTx-2 concentrations between 3 and 300 nM enhanced neurite outgrowth. Voltage-gated sodium channel activators may accordingly represent a novel pharmacologic strategy to regulate neuronal plasticity through an NMDA receptor and Src family kinase-dependent mechanism.

Changes in apical dendritic structure correlate with sustained ERK1/2 phosphorylation in medial prefrontal cortex of a rat model of dopamine D1 receptor agonist sensitization

Rats lesioned with 6-hydroxydopamine (6-OHDA) as neonates exhibit behavioral and neurochemical abnormalities in adulthood that mimic Lesch-Nyhan disease, schizophrenia, and other developmental disorders of frontostriatal circuit dysfunction. In these animals a latent sensitivity to D1 agonists is maximally exposed by repeated administration of dopamine agonists in the postpubertal period (D1 priming). In neonate-lesioned, adult rats primed with SKF-38393, we found selective, persistent alterations in the morphology of pyramidal neuron apical dendrites in the prelimbic area of the medial prefrontal cortex (mPFC). In these animals, dendrite bundling patterns and the typically straight trajectories of primary dendritic shafts were disrupted, whereas the diameter of higher-order oblique branches was increased. Although not present in neonate-lesioned rats treated with saline, these morphological changes persisted at least 21 days after repeated dosing with SKF-38393, and were not accompanied by markers of neurodegenerative change. A sustained increase in phospho-ERK immunoreactivity in wavy dendritic shafts over the same period suggested a relationship between prolonged ERK phosphorylation and dendritic remodeling in D1-primed rats. In support of this hypothesis, pretreatment with the MEK1/2-ERK1/2 pathway inhibitors PD98059 or SL327, prior to each priming dose of SKF-38393, prevented the morphological changes associated with D1 priming. Together, these findings demonstrate that repeated stimulation of D1 receptors in adulthood interacts with the developmental loss of dopamine to profoundly and persistently modify neuronal signaling and dendrite morphology in the mature prefrontal cortex. Furthermore, sustained elevation of ERK activity in mPFC pyramidal neurons may play a role in guiding these morphological changes in vivo.

Rit signaling contributes to interferon-gamma-induced dendritic retraction via p38 mitogen-activated protein kinase activation

The proinflammatory cytokine interferon-gamma (IFNgamma) alters neuronal connectivity via selective regressive effects on dendrites but the signaling pathways that mediate this effect are poorly understood. We recently demonstrated that signaling by Rit, a member of the Ras family of GTPases, modulates dendritic growth in primary cultures of sympathetic and hippocampal neurons. In this study, we investigated a role for Rit signaling in IFNgamma-induced dendritic retraction. Expression of a dominant negative Rit mutant inhibited IFNgamma-induced dendritic retraction in cultured embryonic rat sympathetic and hippocampal neurons. In pheochromacytoma cells and hippocampal neurons, IFNgamma caused rapid Rit activation as indicated by increased GTP binding to Rit. Silencing of Rit by RNA interference suppressed IFNgamma-elicited activation of p38 MAPK in pheochromacytoma cells, and pharmacological inhibition of p38 MAPK significantly attenuated the dendrite-inhibiting effects of IFNgamma in cultured sympathetic and hippocampal neurons without altering signal transducer and activator of transcription 1 activation. These observations identify Rit as a downstream target of IFNgamma and suggest that a novel IFNgamma-Rit-p38 signaling pathway contributes to dendritic retraction and may, therefore, represent a potential therapeutic target in diseases with a significant neuroinflammatory component.

Brain-derived neurotrophic factor modulates expression of chemokine receptors in the brain

Chemokine receptors, and in particular CXCR4 and CCR5 play a key role in the neuropathogenesis of Human Immunodeficiency Virus-1 (HIV)4 associated dementia (HAD). Thus, new insight into the expression of CXCR4 in the central nervous system may help develop therapeutic compounds against HAD. Brain-derived neurotrophic factor (BDNF) is neuroprotective in vitro against two strains of the HIV envelope protein gp120 that binds to CXCR4 or CCR5. Therefore, we examined whether BDNF modulates chemokine receptor expression in vivo. The content of CXCR4 mRNA and proteins was determined in the cerebral cortex and hippocampus of 6-month-old BDNF heterozygous mice and wild type littermates by using polymerase chain reaction and immunohistochemistry, respectively. BDNF heterozygous mice exhibited an increase in CXCR4 mRNA compared to wild type. Histological analyses revealed an up-regulation of CXCR4 immunoreactivity mainly in neurons. Most of these neurons were positive for TrkB, the BDNF receptor with a tyrosine kinase activity. Increases in CXCR4 mRNA levels were observed in 18-month-old BDNF heterozygous mice but not in 7-day-old mice, suggesting that the modulatory role of BDNF occurs only in mature animals. To determine whether BDNF affects also CXCR4 internalization, SH-SY5Y neuroblastoma cells were exposed to BDNF and cell surface CXCR4 levels were measured at various times. BDNF induced CXCR4 internalization within minutes. Lastly, BDNF heterozygous mice showed higher levels of CCR5 and CXCR3 mRNA than wild type in the cerebral cortex, hippocampus and striatum. Our data indicate that BDNF may modulate the availability of chemokine receptors implicated in HIV infection.

Chemokine receptors and neurotrophic factors: potential therapy against aids dementia?

Chemokine receptors, in particular, CXCR4 and CCR5, mediate human immunodeficiency virus type 1 (HIV-1) infection of immunocompetent cells and the apoptosis of these cells. However, the virus does not infect neurons. Yet through a variety of mechanisms, HIV promotes glial cell activation, synaptodendritic alterations, and neuronal loss that ultimately lead to motor and cognitive impairment. Chemokines and chemokine receptors are abundant in the adult central nervous system and play a role in neuronal apoptosis evoked by HIV proteins. Thus, reducing the availability of chemokine receptors may prevent the neuronal degeneration seen in HIV-positive patients. In this article, we present and discuss a recent experimental approach aimed at testing effective neuroprotective therapies against HIV-mediated neuronal degeneration.

Calcium-permeable AMPA receptors containing Q/R-unedited GluR2 direct human neural progenitor cell differentiation to neurons

We identify calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on human neural progenitor cells (NPCs) and present a physiological role in neurogenesis. RNA editing of the GluR2 subunit at the Q/R site is responsible for making most AMPA receptors impermeable to calcium. Because a single-point mutation could eliminate the need for editing at the Q/R site and Q/R-unedited GluR2 exists during embryogenesis, the Q/R-unedited GluR2 subunit presumably has some important actions early in development. Using calcium imaging, we found that NPCs contain calcium-permeable AMPA receptors, whereas NPCs differentiated to neurons and astrocytes express calcium-impermeable AMPA receptors. We utilized reverse-transcription polymerase chain reaction and BbvI digestion to demonstrate that NPCs contain Q/R-unedited GluR2, and differentiated cells contain Q/R-edited GluR2 subunits. This is consistent with the observation that the nuclear enzyme responsible for Q/R-editing, adenosine deaminase (ADAR2), is increased during differentiation. Activation of calcium-permeable AMPA receptors induces NPCs to differentiate to the neuronal lineage and increases dendritic arbor formation in NPCs differentiated to neurons. AMPA-induced differentiation of NPCs to neurons is abrogated by overexpression of ADAR2 in NPCs. This elucidates the role of AMPA receptors as inductors of neurogenesis and provides a possible explanation for why the Q/R editing process exists.

Roles of l-serine and sphingolipid synthesis in brain development and neuronal survival

Sphingolipids represent a class of membrane lipids that contain a hydrophobic ceramide chain as its common backbone structure. Sphingolipid synthesis requires two simple components: l-serine and palmitoyl CoA. Although l-serine is classified as a non-essential amino acid, an external supply of l-serine is essential for the synthesis of sphingolipids and phosphatidylserine (PS) in particular types of central nervous system (CNS) neurons. l-Serine is also essential for these neurons to undergo neuritogenesis and to survive. Biochemical analysis has shown that l-serine is synthesized from glucose and released by astrocytes but not by neurons, which is the major reason why this amino acid is an essential amino acid for neurons. Biosynthesis of membrane lipids, such as sphingolipids, PS, and phosphatidylethanolamine (PE), in neurons is completely dependent on this astrocytic factor. Recent advances in lipid biology research using transgenic mice have demonstrated that synthesis of endogenous l-serine and neuronal sphingolipids is essential for brain development. In this review, we discuss the metabolic system that coordinates sphingolipid synthesis with the l-serine synthetic pathway between neurons and glia. We also discuss the crucial roles of the metabolic conversion of l-serine to sphingolipids in neuronal development and survival. Human diseases associated with serine and sphingolipid biosynthesis are also discussed.

Regulation of dendritic development by neuron-specific chromatin remodeling complexes

The diversity of dendritic patterns is one of the fundamental characteristics of neurons and is in part regulated by transcriptional programs initiated by electrical activity. We show that dendritic outgrowth requires a family of combinatorially assembled, neuron-specific chromatin remodeling complexes (nBAF complexes) distinguished by the actin-related protein BAF53b and based on the Brg/Brm ATPases. nBAF complexes bind tightly to the Ca(2+)-responsive dendritic regulator CREST and directly regulate genes essential for dendritic outgrowth. BAF53b is not required for nBAF complex assembly or the interaction with CREST, yet is required for their recruitment to the promoters of specific target genes. The highly homologous BAF53a protein, which is a component of neural progenitor and nonneural BAF complexes, cannot replace BAF53b's role in dendritic development. Remarkably, we find that this functional specificity is conferred by the actin fold subdomain 2 of BAF53b. These studies suggest that the genes encoding the individual subunits of BAF complexes function like letters in a ten-letter word to produce biologically specific meanings (in this case dendritic outgrowth) by combinatorial assembly of their products.

Role of mTOR in physiology and pathology of the nervous system

Mammalian target of rapamycin (mTOR) is a serine-threonine protein kinase that regulates several intracellular processes in response to extracellular signals, nutrient availability, energy status of the cell and stress. mTOR regulates survival, differentiation and development of neurons. Axon growth and navigation, dendritic arborization, as well as synaptogenesis, depend on proper mTOR activity. In adult brain mTOR is crucial for synaptic plasticity, learning and memory formation, and brain control of food uptake. Recent studies reveal that mTOR activity is modified in various pathologic states of the nervous system, including brain tumors, tuberous sclerosis, cortical displasia and neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases. This review presents current knowledge about the role of mTOR in the physiology and pathology of the nervous system, with special focus on molecular targets acting downstream of mTOR that potentially contribute to neuronal development, plasticity and neuropathology.

Glucocorticoid prevents brain-derived neurotrophic factor-mediated maturation of synaptic function in developing hippocampal neurons through reduction in the activity of mitogen-activated protein kinase

An increased level of glucocorticoid may be related to the pathophysiology of depressive disorder. The involvement of brain-derived neurotrophic factor (BDNF) in the antidepressive effect has also been suggested; however, the possible influence of glucocorticoid on the action of BDNF in the developing central nervous system has not been elucidated. In this study, we investigated the effect of glucocorticoid (dexamethasone, DEX) on synaptic maturation and function enhanced by BDNF in early developing hippocampal neurons. In the immature stage, BDNF increased the outgrowth of dendrites and the expression of synaptic proteins including glutamate receptors and presynaptic proteins. Pretreatment with DEX significantly inhibited the BDNF-dependent up-regulation of both dendritic outgrowth and synaptic proteins. In the more mature stage, the BDNF-reinforced postsynaptic Ca(2+) influx was decreased by DEX. BDNF-enhanced presynaptic glutamate release was also suppressed. RU486, a glucocorticoid receptor antagonist, canceled the DEX-dependent blocking effect on the action of BDNF. After down-regulation of glucocorticoid receptor by small interfering RNA application, no inhibitory effect of DEX on the BDNF-increased synaptic proteins was observed. Interestingly, the BDNF-activated MAPK/ERK pathway, which is an essential intracellular signaling pathway for the BDNF-increased synaptic proteins, was reduced by DEX. These results suggest that BDNF-mediated synaptic maturation is disturbed after neurons are exposed to high-level glucocorticoid in their development stage.

Very-KIND, a KIND domain containing RasGEF, controls dendrite growth by linking Ras small GTPases and MAP2

The regulation of cytoskeletal components in the dendritic shaft core is critical for dendrite elongation and branching. Here, we report that a brain-specific Ras guanine nucleotide exchange factor (RasGEF) carrying two kinase non-catalytic C-lobe domains (KINDs), very-KIND (v-KIND), regulates microtubule-associated protein 2 (MAP2). v-KIND is expressed in developing mouse brain, predominantly in the cerebellar granule cells. v-KIND not only activates Ras small GTPases via the C-terminal RasGEF domain, but also specifically binds to MAP2 via the second KIND domain (KIND2), leading to threonine phosphorylation of MAP2. v-KIND overexpression suppresses dendritic extension and branching of hippocampal neurons and cerebellar granule cells, whereas knockdown of endogenous v-KIND expression promotes dendrite growth. These findings suggest that v-KIND mediates a signaling pathway that links Ras and MAP2 to control dendrite growth.

GPCR-jacking: from a new route in RTK signalling to a new concept in GPCR activation

A large body of evidence indicates that agonists of some G protein-coupled receptors (GPCRs) can activate growth factor receptor tyrosine kinases (RTKs) in the absence of added growth factor. This phenomenon, called transactivation, is an important pathway that contributes to growth-promoting activity of many GPCR ligands. Reciprocally, recent advances indicate that RTKs utilize GPCR signalling molecules to transduce signals and that RTK ligands themselves can transactivate GPCRs. This novel transactivation process, which places GPCR signalling downstream of RTKs, either requires the production of a GPCR ligand of the transactivated GPCR or occurs in a ligand independent manner within an integrated signalling network. Here, we provide an overview of the molecular mechanisms involved in this novel cross-communication between GPCRs and RTKs and discuss its relevance in the specification of growth factor signalling and functions.

Mechanisms that regulate establishment, maintenance, and remodeling of dendritic fields

Although dendrite arborization patterns are hallmarks of neuronal type and critical determinants of neuronal function, how dendritic arbors take shape is still largely unknown. Transcription factors play important roles in specifying neuronal types and have a profound influence on dendritic arbor size and complexity. The space that a dendritic arbor occupies is determined largely by a combination of growth-promoting signals that regulate arbor size, chemotropic cues that steer dendrites into the appropriate space, and neurite-neurite contacts that ensure proper representation of the dendritic field and appropriate synaptic contacts. Dendritic arbors are largely maintained over the neuron's lifetime, but in some cases, dendritic arbors are refined, in large part as a result of neuronal activity. In this review, we summarize our current understanding of the cellular and molecular mechanisms that regulate dendritic field formation and influence the shaping of dendritic arbors.

Regulation of neurite growth by spontaneous Ca2+ oscillations in astrocytes

Astrocytes play a pivotal role in the regulation of neurite growth, but the intracellular signaling mechanism in astrocytes that mediates this regulation remains unclarified. We studied the relationship between spontaneous Ca(2+) oscillations in astrocytes and the astrocyte-mediated neurite growth. We generated Ca(2+) signal-deficient astrocytes in which spontaneous Ca(2+) oscillations were abolished by a chronic inhibition of IP(3) signaling. When hippocampal neurons were cultured on a monolayer of Ca(2+) signal-deficient astrocytes, the growth of dendrites and axons was inhibited. Time-lapse imaging of the advancement of axonal growth cones indicated the involvement of membrane-bound molecules for this inhibition. Among six candidate membrane-bound molecules that may modulate neuronal growth, N-cadherin was downregulated in Ca(2+) signal-deficient astrocytes. Although a blocking antibody to N-cadherin suppressed the axonal growth on control astrocytes, extrinsic N-cadherin expression rescued the suppressed axonal growth on Ca(2+) signal-deficient astrocytes. These findings suggest that spontaneous Ca(2+) oscillations regulate the astrocytic function to promote neurite growth by maintaining the expression of specific growth-enhancing proteins on their surface, and that N-cadherin is one of such molecules.

The growing role of mTOR in neuronal development and plasticity

Neuronal development and synaptic plasticity are highly regulated processes in which protein kinases play a key role. Recently, increasing attention has been paid to a serine/threonine protein kinase called mammalian target of rapamycin (mTOR) that has well-known functions in cell proliferation and growth. In neuronal cells, mTOR is implicated in multiple processes, including transcription, ubiquitin-dependent proteolysis, and microtubule and actin dynamics, all of which are crucial for neuronal development and long-term modification of synaptic strength. The aim of this article is to present our current understanding of mTOR functions in axon guidance, dendritic tree development, formation of dendritic spines, and in several forms of long-term synaptic plasticity. We also aim to present explanation for the mTOR effects on neurons at the level of mTORregulated genes and proteins.

Dendrite self-avoidance is controlled by Dscam

Dendrites distinguish between sister branches and those of other cells. Self-recognition can often lead to repulsion, a process termed "self-avoidance." Here we demonstrate that dendrite self-avoidance in Drosophila da sensory neurons requires cell-recognition molecules encoded by the Dscam locus. By alternative splicing, Dscam encodes a vast number of cell-surface proteins of the immunoglobulin superfamily. We demonstrate that interactions between identical Dscam isoforms on the cell surface underlie self-recognition, while the cytoplasmic tail converts this recognition to dendrite repulsion. Sister dendrites expressing the same isoforms engage in homophilic repulsion. By contrast, Dscam diversity ensures that inappropriate repulsive interactions between dendrites sharing the same receptive field do not occur. The selectivity of Dscam-mediated cell interactions is likely to be widely important in the developing fly nervous system, where processes of cells must distinguish between self and nonself during the construction of neural circuits.

Brain-derived neurotrophic factor promotes long-term potentiation-related cytoskeletal changes in adult hippocampus

Brain-derived neurotrophic factor (BDNF) is an extremely potent, positive modulator of theta burst induced long-term potentiation (LTP) in the adult hippocampus. The present studies tested whether the neurotrophin exerts its effects by facilitating cytoskeletal changes in dendritic spines. BDNF caused no changes in phalloidin labeling of filamentous actin (F-actin) when applied alone to rat hippocampal slices but markedly enhanced the number of densely labeled spines produced by a threshold level of theta burst stimulation. Conversely, the BDNF scavenger TrkB-Fc completely blocked increases in spine F-actin produced by suprathreshold levels of theta stimulation. TrkB-Fc also blocked LTP consolidation when applied 1-2 min, but not 10 min, after theta trains. Additional experiments confirmed that p21 activated kinase and cofilin, two actin-regulatory proteins implicated in spine morphogenesis, are concentrated in spines in mature hippocampus and further showed that both undergo rapid, dose-dependent phosphorylation after infusion of BDNF. These results demonstrate that the influence of BDNF on the actin cytoskeleton is retained into adulthood in which it serves to positively modulate the time-dependent LTP consolidation process.

Neuronal morphology and neuropil structure in the stomatogastric ganglion of the lobster, Homarus americanus

The stomatogastric nervous system (STNS) has long been used as a model system for the study of central pattern generation, neuromodulation, and network dynamics. Anatomical studies of the crustacean stomatogastric ganglion (STG) in different species have mostly been restricted to subsets of neurons and/or general structural features. For the first time, we describe the morphology of all STG neurons belonging to the two circuits that produce the well-described pyloric and gastric rhythms in the lobster, Homarus americanus. Somata sit on the dorsal and lateral surface of the STG and send a single primary neurite into the core of the neuropil, which is mostly made up of larger lower order branches. The perimeter of the neuropil consists mostly of finer higher order branches. Immunohistochemical labeling for synaptic proteins is associated with the small diameter branches. Somata positions are not constant but show preferred locations across individuals. The number of copies is constant for all neuron types except the PY and GM neurons (PY neuron number ranges from 3 to 7, and GM neuron number ranges from 6 to 9). Branch structure is largely nondichotomous, and branches can deviate substantially from cylindrical shape. Diameter changes at branch points can be as large as 20-fold. Clearly, the morphology of a specific neuron type can be quite variable from animal to animal.

A Critical Role for System A Amino Acid Transport in the Regulation of Dendritic Development by Brain-derived Neurotrophic Factor (BDNF)

Dendritic development is essential for the establishment of a functional nervous system. Among factors that control dendritic development, brain-derived neurotrophic factor (BDNF) has been shown to regulate dendritic length and complexity of cortical neurons. However, the cellular and molecular mechanisms that underlie these effects remain poorly understood. In this study, we examined the role of amino acid transport in mediating the effects of BDNF on dendritic development. We show that BDNF increases System A amino acid transport in cortical neurons by selective up-regulation of the sodium-coupled neutral amino acid transporter (SNAT)1. Up-regulation of SNAT1 expression and System A activity is required for the effects of BDNF on dendritic growth and branching of cortical neurons. Further analysis revealed that induction of SNAT1 expression and System A activity by BDNF is necessary in particular to enhance synthesis of tissue-type plasminogen activator, a protein that we demonstrate to be essential for the effects of BDNF on cortical dendritic morphology. Together, these data reveal that stimulation of neuronal differentiation by BDNF requires the up-regulation of SNAT1 expression and System A amino acid transport to meet the increased metabolic demand associated with the enhancement of dendritic growth and branching.

An essential role for the integrin-linked kinase-glycogen synthase kinase-3 beta pathway during dendrite initiation and growth

Multiple cues, including growth factors and circuit activity, signal to regulate the initiation and growth of mammalian dendrites. In this study, we have asked how these environmental cues regulate dendrite formation, and in particular, whether dendrite initiation and growth requires integrin-linked kinase (ILK) or its downstream effector, glycogen synthase kinase-3beta (GSK-3beta). In cultured sympathetic neurons, NGF and neuronal depolarization activated ILK and promoted dendrite initiation and growth, and inhibition of ILK (either pharmacologically, with a dominant-negative form of ILK, or by genetic knockdown) reduced depolarization-induced dendrite formation. In sympathetic neurons, ILK phosphorylated and inhibited GSK-3beta, and inhibition of GSK-3beta (either pharmacologically, with dominant-negative GSK-3beta, or by genetic knockdown) caused robust dendrite initiation. GSK-3beta inhibition also caused dendrite initiation in cultured cortical neurons and growth of hippocampal neurons in slice cultures. GSK-3beta functioned downstream of ILK to regulate dendrite formation, because inhibition of GSK-3beta promoted dendrite initiation even when ILK was simultaneously inhibited. Moreover, GSK-3beta promoted dendrite formation in sympathetic neurons by regulating the activity of a key dendrite formation effector, the MAP (microtubule-associated protein) kinase kinase (MEK)-extracellular signal-regulated protein kinase (ERK) pathway. Specifically, inhibition of GSK-3beta led to increased ERK phosphorylation, and inhibition of MEK completely blocked the effects of GSK-3beta inhibition on dendrite initiation and growth. Thus, the ILK-GSK-3beta pathway plays a key role in regulating dendrite formation in developing mammalian neurons.

Inositol 1,4,5-trisphosphate receptor type 1 in granule cells, not in Purkinje cells, regulates the dendritic morphology of Purkinje cells through brain-derived neurotrophic factor production

Here, we show that cultured Purkinje cells from inositol 1,4,5-trisphosphate receptor type 1 knock-out (IP3R1KO) mice exhibited abnormal dendritic morphology. Interestingly, despite the huge amount of IP3R1 expression in Purkinje cells, IP3R1 in granule cells, not in the Purkinje cells, was responsible for the shape of Purkinje cell dendrites. We also found that BDNF application rescued the dendritic abnormality of IP3R1KO Purkinje cells, and that the increase in BDNF expression in response to activation of AMPA receptor (AMPAR) and metabotropic glutamate receptor (mGluR) was impaired in IP3R1KO cerebellar granule cells. In addition, we observed abnormalities in the dendritic morphology of Purkinje cells and in the ultrastructure of parallel fiber-Purkinje cell (PF-PC) synapses in IP3R1KO mice in vivo. We concluded that activation of AMPAR and mGluR increases BDNF expression through IP3R1-mediated signaling in cerebellar granule cells, which contributes to the dendritic outgrowth of Purkinje cells intercellularly, possibly by modifying PF-PC synaptic efficacy.

Homer proteins control neuronal differentiation through IP(3) receptor signaling

Neurons expand, sustain or prune their dendritic trees during ontogenesis [Cline, H.T. (2001). Dendritic arbor development and synaptogenesis. Curr. Opin. Neurobiol. 11, 118-126; Wong, W.T. and Wong, R.O.L. (2000) Rapid dendritic movements during synapse formation and rearrangement. Curr. Opin. Neurobiol. 10, 118-124] which critically depends on neuronal activity [Wong, W.T., Faulkner-Jones, B.E., Sanes, J.R. and Wong, R.O.L. (2000) Rapid dendritic remodeling in the developing retina: dependence on neurotransmission and reciprocal regulation by Rac and Rho. J. Neurosci. 20, 5024-5036; Li, Z., Van Aelst, L. and Cline, H.T. (2000) Rho GTPases regulate distinct aspects of dendritic arbor growth in Xenopus central neurons in vivo. Nat. Neurosci. 3, 217-225; Wong, W.T. and Wong, R.O.L. (2001) Changing specificity of neurotransmitter regulation of rapid dendritic remodeling during synaptogenesis. Nat. Neurosci. 4, 351-352.] and sub-cellular Ca(2+) signals [Lohmann, C., Myhr, K.L. and Wong, R.O. (2002) Transmitter-evoked local calcium release stabilizes developing dendrites, Nature 418, 177-181.]. The role of synaptic clustering proteins connecting both processes is unclear. Here, we show that expression levels of Vesl-1/Homer 1 isoforms critically control properties of Ca(2+) release from intracellular stores and dendritic morphology of CNS neurons. Vesl-1L/Homer 1c, an isoform with a functional WH1 and coiled-coil domain, but not isoforms missing these features were capable of potentiating intracellular calcium signaling activity indicating that such regulatory interactions function as a general paradigm in cellular differentiation and are subject to changes in expression levels of Vesl/Homer isoforms.

Vasoactive Intestinal Peptide and PACAP38 Control N-Methyl-D-aspartic Acid-induced Dendrite Motility by Modifying the Activities of Rho GTPases and Phosphatidylinositol 3-Kinases

Dendrite morphogenesis is highly dynamic and characterized by the addition and elongation of processes and also by their selective maintenance, retraction, and elimination. Glutamate can influence these events via N-methyl-d-aspartic acid (NMDA) receptors. The neuropeptides vasoactive intestinal peptide and pituitary adenylyl cyclase-activating polypeptide-38 (PACAP38) affect neurogenesis and differentiation in the developing nervous system. We report here that the peptides and NMDA acted synergistically on dendrite and branch formation. In stage III hippocampal neurons, NMDA increased not only the addition but also the elimination of new dendrites and branches by activating Rac and Cdc42 and phosphatidylinositol 3-kinases, respectively. When applied alone, the neuropeptides did not influence dendrite or branch formation. However, they reduced the elimination of newly formed dendrites and branches caused by NMDA by preventing the NMDA-induced activation of phosphatidylinositol 3-kinases. This led to the formation of persistent dendrites and branches. Additional timelapse studies on the dynamics of dendrite elongation showed alternating periods of elongation and retraction. Phosphatidylinositol 3-kinases increased the velocities of dendrite elongation and retraction, whereas the neuropeptides prolonged the periods of elongation. By modifying NMDA-induced activation of Rho GTPases and phosphatidylinositol 3-kinases, vasoactive intestinal peptide and PACAP38 could play an important role in the control of dendrite growth and branching during development and in response to neuronal activity.

Activity-dependent dendritic arborization mediated by CaM-kinase I activation and enhanced CREB-dependent transcription of Wnt-2

Members of the Wnt signaling family are important mediators of numerous developmental events, including activity-dependent dendrite development, but the pathways regulating expression and secretion of Wnt in response to neuronal activity are poorly defined. Here, we identify an NMDA receptor-mediated, Ca2+-dependent signaling pathway that couples neuronal activity to dendritic arborization through enhanced Wnt synthesis and secretion. Activity-dependent dendritic outgrowth and branching in cultured hippocampal neurons and slices is mediated through activation by CaM-dependent protein kinase kinase (CaMKK) of the membrane-associated gamma isoform of CaMKI. Downstream effectors of CaMKI include the MAP-kinase pathway of Ras/MEK/ERK and the transcription factor CREB. A serial analysis of chromatin occupancy screen identified Wnt-2 as an activity-dependent CREB-responsive gene. Neuronal activity enhances CREB-dependent transcription of Wnt-2, and expression of Wnt-2 stimulates dendritic arborization. This novel signaling pathway contributes to dynamic remodeling of the dendritic architecture in response to neuronal activity during development.

Bi-directional control of motor neuron dendrite remodeling by the calcium permeability of AMPA receptors

Motor neurons express particularly high levels of the AMPA receptor subunit GluR1(Q)flip (GluR1(Q)i) during the period in early postnatal life when their dendritic tree grows and becomes more branched. To investigate how GluR1-containing AMPA receptors contribute to dendrite morphogenesis, we characterized a mutant form of GluR1 (containing a histidine in the Q/R editing site) with unique electrophysiological properties. Most notably, AMPA receptors assembled from GluR1(H)i display less calcium permeability than AMPA receptors assembled from GluR1(Q)i. Expression of GluR1(Q)i in vivo or in vitro led to an increase in dendrite branching with no net change in the overall tree size while GluR1(H)i led to a loss of branches and a net reduction in overall tree size. GluR1(H)i-dependent dendrite atrophy is mediated by protein phosphatase 2B. The results suggest that the electrophysiological properties of cell surface AMPA receptors, specifically their permeability to calcium, can be a central determinant of whether the dendrites undergo activity-dependent branching or atrophy.

Dendritic arbors of developing retinal ganglion cells are stabilized by beta 1-integrins

The architecture of dendritic arbors is a defining characteristic of neurons and is established through a sequential but overlapping series of events involving process outgrowth and branching, stabilization of the global pattern, and synapse formation. To investigate the roles of cadherins and beta1-integrins in maintaining the global architecture of the arbor, we used membrane permeable peptides and transfection with dominant-negative constructs to disrupt adhesion molecule function in intact chick neural retina at a stage when the architecture of the ganglion cell (RGC) arbor is established but synapse formation is just beginning. Inactivation of beta1-integrins induces rapid dendrite retraction, with loss of dynamic terminal filopodia followed by resorption of major branches. Disruption of N-cadherin-beta-catenin interactions has no effect; however, dendrites do retract following perturbation of the juxtamembrane region of N-cadherin, which disrupts N-cadherin-mediated adhesion and initiates a beta1-integrin inactivating signal. Thus, developing RGC dendritic arbors are stabilized by beta1-integrin-dependent processes.

Notch and NGF/p75NTR control dendrite morphology and the balance of excitatory/inhibitory synaptic input to hippocampal neurones through Neurogenin 3

We have previously shown that dendrite morphology of cultured hippocampal neurones is controlled by Notch receptor activation or binding of nerve growth factor (NGF) to its low affinity receptor p75NTR, i.e. processes that up-regulate the expression of the Homologue of enhancer of split 1 and 5. Thus, the increased expression of these genes decreases the number of dendrites, whereas abrogation of Homologue of enhancer of split 1/5 activity stimulates the outgrowth of new dendrites. Here, we show that Neurogenin 3 is a proneural gene that is negatively regulated by Homologue of enhancer of split 1/5. It also influences dendrite morphology. Hence, a deficit of Notch or NGF/p75NTR activation can lead to the production of high levels of Neurogenin 3, which stimulates the outgrowth of new dendrites. Conversely, activation of either Notch or p75NTR depressed Neurogenin 3 expression, which not only decreased the number of dendrites but also favoured inhibitory (GABAergic) synaptogenesis, thereby diminishing the ratios of excitatory/inhibitory inputs. NGF also augmented the levels of mRNA encoding the vesicular inhibitory amino acid transporter, but it did not affect the fraction of GAD65/67-positive neurones. Conversely, overexpression of Neurogenin 3 largely reduced the number of inhibitory synaptic contacts and, consequently, produced a strong increase in the ratios of excitatory/inhibitory synaptic terminals. Our results reveal a hitherto unknown contribution of NGF/p75NTR to dendritic and synaptic plasticity through Neurogenin 3 signalling.

The level and integrity of synaptic input regulates dendrite structure

How localized synaptic input regulates dendritic branch structure is not well understood. For these experiments, we used single-cell electroporation, live cell imaging, in vitro deafferentation, pharmacology, and electrophysiological stimulation to study how local alterations in synaptic input affect dendritic branch structure in nucleus laminaris (NL). We found that interrupting or modulating synaptic input to distinct sets of NL dendrites can regulate their structure on a very short timescale. Specifically, eliminating synaptic input by deafferenting only one set of the bitufted NL dendrites caused a selective reduction in the total dendritic branch length of the deafferented dendrites but relatively few changes in the normally innervated dendrites on the same cell. An analysis of individual dendritic branch changes demonstrated that both control and deafferented NL dendrites exhibit branch extension and retraction. However, the presence of intact synaptic inputs balanced these changes, maintaining the total dendritic branch length of control dendrites. When glutamate receptor signaling was blocked (DNQX and AP-5), NL neurons exhibited significant dendrite retraction, demonstrating that NL dendrite maintenance depends in part on presynaptic glutamatergic input. Electrophysiological experiments further confirmed that modulating the level of synaptic input regulates NL dendrite structure. Differential stimulation of the two sets of dendrites resulted in a selective reduction in the total dendritic branch length of the unstimulated dendrites and a selective increase in the total dendritic branch length of the stimulated dorsal dendrites. These results suggest that balanced activation of the two sets of NL dendrites is required to maintain the relative amount of dendritic surface area allotted to each input.

Effect of Ketamine on Dendritic Arbor Development and Survival of Immature GABAergic Neurons In Vitro

Ketamine, a noncompetitive antagonist of the N-methyl-D-aspartate type of glutamate receptors, was reported to induce neuronal cell death when administered to produce anesthesia in young rodents and monkeys. Subanesthetic doses of ketamine, as adjuvant to postoperative sedation and pain control, are also frequently administered to young children. However, the effects of these low concentrations of ketamine on neuronal development remain unknown. The present study was designed to evaluate the effects of increasing concentrations (0.01-40 microg/ml) and durations (1-96 h) of ketamine exposure on the differentiation and survival of immature gamma-aminobutyric acidergic (GABAergic) interneurons in culture. In line with previous studies (Scallet et al., 2004), we found that a 1-h-long exposure to ketamine at concentrations > or = 10 microg/ml was sufficient to trigger cell death. At lower concentrations of ketamine, cell loss was only observed when this drug was chronically (> 48 h) present in the culture medium. Most importantly, we found that a single episode of 4-h-long treatment with 5 microg/ml ketamine induced long-term alterations in dendritic growth, including a significant (p < 0.05) reduction in total dendritic length and in the number of branching points compared to control groups. Finally, long-term exposure (> 24 h) of neurons to ketamine at concentrations as low as 0.01 microg/ml also severely impaired dendritic arbor development. These results suggest that, in addition to its dose-dependent ability to induce cell death, even very low concentrations of ketamine could interfere with dendritic arbor development of immature GABAergic neurons and thus could potentially interfere with the development neural networks.

Regulation of dendritic morphogenesis by Ras-PI3K-Akt-mTOR and Ras-MAPK signaling pathways

Dendritic arborization and spine formation are critical for the functioning of neurons. Although many proteins have been identified recently as regulators of dendritic morphogenesis, the intracellular signaling pathways that control these processes are not well understood. Here we report that the Ras-phosphatidylinositol 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signaling pathway plays pivotal roles in the regulation of many aspects of dendrite formation. Whereas the PI3K-Akt-mTOR pathway alone controlled soma and dendrite size, a coordinated activation together with the Ras-mitogen-activated protein kinase signaling pathway was required for increasing dendritic complexity. Chronic inhibition of PI3K or mTOR reduced soma and dendrite size and dendritic complexity, as well as density of dendritic filopodia and spines, whereas a short-term inhibition promoted the formation of mushroom-shaped spines on cells expressing constitutively active mutants of Ras, PI3K, or Akt, or treated with the upstream activator BDNF. Together, our data underscore the central role of a spatiotemporally regulated key cell survival and growth pathway on trophic regulation of the coordinated development of dendrite size and shape.

Constitutively active cytoplasmic c-Jun N-terminal kinase 1 is a dominant regulator of dendritic architecture: role of microtubule-associated protein 2 as an effector

Normal functioning of the nervous system requires precise regulation of dendritic shape and synaptic connectivity. Here, we report a severe impairment of dendritic structures in the cerebellum and motor cortex of c-Jun N-terminal kinase 1 (JNK1)-deficient mice. Using an unbiased screen for candidate mediators, we identify the dendrite-specific high-molecular-weight microtubule-associated protein 2 (MAP2) as a JNK substrate in the brain. We subsequently show that MAP2 is phosphorylated by JNK in intact cells and that MAP2 proline-rich domain phosphorylation is decreased in JNK1-/- brain. We developed compartment-targeted JNK inhibitors to define whether a functional relationship exists between the physiologically active, cytosolic pool of JNK and dendritic architecture. Using these, we demonstrate that cytosolic, but not nuclear, JNK determines dendritic length and arbor complexity in cultured neurons. Moreover, we confirm that MAP2-dependent process elongation is enhanced after activation of JNK. Using JNK1-/- neurons, we reveal a dominant role for JNK1 over ERK in regulating dendritic arborization, whereas ERK only regulates dendrite shape under conditions in which JNK activity is low (JNK1-/- neurons). These results reveal a novel antagonism between JNK and ERK, potentially providing a mechanism for fine-tuning the dendritic arbor. Together, these data suggest that JNK phosphorylation of MAP2 plays an important role in defining dendritic architecture in the brain.

Abelson, enabled, and p120 catenin exert distinct effects on dendritic morphogenesis in Drosophila

Neurons exhibit diverse dendritic branching patterns that are important for their function. However, the signaling pathways that control the formation of different dendritic structures remain largely unknown. To address this issue in vivo, we use the peripheral nervous system (PNS) of Drosophila as a model system. Through both loss-of-function and gain-of-function analyses in vivo, we show here that the nonreceptor tyrosine kinase Abelson (Abl), an important regulator of cytoskeleton dynamics, inhibits dendritic branching of dendritic arborization (DA) sensory neurons in Drosophila. Enabled (Ena), a substrate for Abl, promotes the formation of both dendritic branches and actin-rich spine-like protrusions of DA neurons, an effect opposite to that of Abl. In contrast, p120 catenin (p120 ctn) primarily enhances the development of spine-like protrusions. These results suggest that Ena is a key regulator of dendritic branching and that different regulators of the actin cytoskeleton exert distinct effects on dendritic morphogenesis.

Dendritic morphogenesis of cerebellar Purkinje cells through extension and retraction revealed by long-term tracking of living cells in vitro

Cerebellar Purkinje cells have the most elaborate dendritic trees among the neurons in the CNS. To investigate the dynamic aspects of dendritic morphogenesis of Purkinje cells, we performed a long-term analysis of living cells in cerebellar cell cultures derived from glutamate decarboxylase 67-green fluorescent protein mice. Most Purkinje cells had several primary dendrites during the 25-day culture period. Repeated observation of green fluorescent protein-expressing Purkinje cells over a period of 10-25 days in vitro demonstrated that not only extension, but also retraction of primary dendrites occurred during this culture period. Interestingly, both extension and retraction of primary dendrites were active between 10 and 15 days in vitro, and retraction of a primary dendrite occurred concomitantly with elongation of other primary dendrites in the same cell. Analysis of the morphological characteristics of the retracted primary dendrites demonstrated that shorter and less branched primary dendrites tended to retract. Furthermore, treatment with an inhibitor of calcium/calmodulin-dependent protein kinase II reduced the number of primary dendrites specifically during 5-15 days in vitro, the culture period when the extension and retraction of primary dendrites occurred actively. Blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-type glutamate receptors also reduced the number of primary dendrites during the same culture period, while inhibition of glutamate transporters increased the number. These findings suggest that the final morphology of Purkinje cells is achieved not only through extension, but also through retraction of their dendrites, and that calcium/calmodulin-dependent protein kinase II and neuronal activity are involved in this dendritic morphogenesis.

Control of dendritic diversity

The dendritic trees of different neuronal types display an astonishing diversity in structure and function. How this diversity is generated remains incompletely understood. However, recent studies have revealed some of the underlying mechanisms by which intrinsic programs of cell-type specification and extrinsic factors exert their effects on the dendritic cytoskeleton to regulate patterns of growth and branching.

Brain-derived neurotrophic factor as a prototype neuroprotective factor against HIV-1-associated neuronal degeneration

Patients with human immunodeficiency virus type 1 (HIV-1) infection develop a broad spectrum of motor impairments and cognitive deficits, which follow or parallel cellular loss and atrophy in their brains. The viral envelope glycoprotein 120 (gp120) has been suggested to be a causal agent of neuronal loss. Therefore, reducing gp120 neurotoxicity may prevent neuronal degeneration seen in these patients. Here, we describe in vitro and in vivo experimental evidence that gp120 toxicity can be reduced by brain-derived neurotrophic factor (BDNF), a naturally occurring peptide that has been shown to block neurotoxin and trauma-induced neuronal injury. Moreover, we review the survival promoting properties of BDNF and the issues concerning its delivery into the brain, in an attempt to explain the rationale for exploring BDNF as a prototype trophic factor for a therapy to reduce neuronal cell death in HIV-1 infected patients.

Genetic dissection of trophic interactions in the larval optic neuropil of Drosophila melanogaster

The larval visual system of Drosophila melanogaster consists of two bilateral clusters of 12 photoreceptors, which express Rhodopsin 5 and 6 (Rh5 and Rh6) in a non-overlapping manner. These neurons send their axons in a fascicle, the larval optic nerve (LON), which terminates in the larval optic neuropil. The LON is required for the development of a serotonergic arborization originating in the central brain and for the development of the dendritic tree of the circadian pacemakers, the small ventral lateral neurons (LNv) [Malpel, S., Klarsfeld, A., Rouyer, F., 2002. Larval optic nerve and adult extra-retinal photoreceptors sequentially associate with clock neurons during Drosophila brain development. Development 129, 1443-1453; Mukhopadhyay, M., Campos, A.R., 1995. The larval optic nerve is required for the development of an identified serotonergic arborization in Drosophila melanogaster. Dev. Biol., 169, 629-643]. Here, we show that both Rh5- and Rh6-expressing fibers overlap equally with the 5-HT arborization and that it, in turn, also contacts the dendritic tree of the LNv. The experiments described here aimed at determining whether Rh5- or Rh6-expressing fibers, as well as the LNv, influence the development of this serotonergic arborization. We conclude that Rh6-expressing fibers play a unique role in providing a signal required for the outgrowth and branching of the serotonergic arborization. Moreover, the innervation of the larval optic neuropil by the 5-HT arborization depends on intact Rac function. A possible role for these serotonergic processes in modulating the larval circadian rhythmicity and photoreceptor function is discussed.

Growth and survival signals controlling sympathetic nervous system development

The precise coordination of the many events in nervous system development is absolutely critical for the correct establishment of functional circuits. The postganglionic sympathetic neuron has been an amenable model for studying peripheral nervous system formation. Factors that control several developmental events, including multiple stages of axon extension, neuron survival and death, dendritogenesis, synaptogenesis, and establishment of functional diversity, have been identified in this neuron type. This knowledge allows us to integrate the various intricate processes involved in the formation of a functional sympathetic nervous system and thereby create a paradigm for understanding neuronal development in general.

GRIP1 controls dendrite morphogenesis by regulating EphB receptor trafficking

The function of the multi-PDZ domain scaffold protein GRIP1 (glutamate receptor interacting protein 1) in neurons is unclear. To explore the function of GRIP1 in hippocampal neurons, we used RNA interference (RNAi) to knock down the expression of GRIP1. Knockdown of GRIP1 by small interfering RNA (siRNA) in cultured hippocampal neurons caused a loss of dendrites, associated with mislocalization of the GRIP-interacting proteins GIuR2 (AMPA receptor subunit), EphB2 (receptor tyrosine kinase) and KIF5 (also known as kinesin 1; microtubule motor). The loss of dendrites by GRIP1-siRNA was rescued by overexpression of the extracellular domain of EphB2, and was phenocopied by overexpression of the intracellular domain of EphB2 and extracellular application of ephrinB-Fc fusion proteins. Neurons from EphB1-EphB2-EphB3 triple knockout mice showed abnormal dendrite morphogenesis. Disruption of the KIF5-GRIP1 interaction inhibited EphB2 trafficking and strongly impaired dendritic growth. These results indicate an important role for GRIP1 in dendrite morphogenesis by serving as an adaptor protein for kinesin-dependent transport of EphB receptors to dendrites.

A CaMKII-NeuroD signaling pathway specifies dendritic morphogenesis

The elaboration of dendrites is fundamental to the establishment of neuronal polarity and connectivity, but the mechanisms that underlie dendritic morphogenesis are poorly understood. We found that the genetic knockdown of the transcription factor NeuroD in primary granule neurons including in organotypic cerebellar slices profoundly impaired the generation and maintenance of dendrites while sparing the development of axons. We also found that NeuroD mediated neuronal activity-dependent dendritogenesis. The activity-induced protein kinase CaMKII catalyzed the phosphorylation of NeuroD at distinct sites, including endogenous NeuroD at Ser336 in primary neurons, and thereby stimulated dendritic growth. These findings uncover an essential function for NeuroD in granule neuron dendritic morphogenesis. Our study also defines the CaMKII-NeuroD signaling pathway as a novel mechanism underlying activity-regulated dendritic growth that may play important roles in the developing and mature brain.

Regulation of dendritic development by neuronal activity

Proper development of dendrites is essential for the establishment of neuronal circuitry. The elaboration of the dendritic tree is a highly dynamic and regulated process, which involves the formation of new branches as well as the maintenance or elimination of pre-existing branches. This review describes recent advances in our understanding of the molecular mechanisms of activity-dependent dendritic development. Neuronal activity triggers calcium-mediated signaling events that affect the structural components of dendrites and adhesion molecules. These calcium-induced signaling pathways also target nuclear transcription factors thereby controlling expression of genes required for dendritic development. Thus, a coordinated response to calcium-regulated signaling pathways mediates activity-dependent dendritic development.