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

Calmodulin-dependent protein kinase II/cAMP response element-binding protein/Wnt/β-catenin signaling cascade regulates angiotensin II-induced podocyte injury and albuminuria

Angiotensin II (Ang II) plays a pivotal role in promoting podocyte dysfunction and albuminuria, however, the underlying mechanisms have not been fully delineated. In this study, we found that Ang II induced Wnt1 expression and β-catenin nuclear translocation in cultured mouse podocytes. Blocking Wnt signaling with Dickkopf-1 (Dkk1) or β-catenin siRNA attenuated Ang II-induced podocyte injury. Ang II could also induce the phosphorylation of calmodulin-dependent protein kinase (CaMK) II and cAMP response element-binding protein (CREB) in cultured podocytes. Blockade of this pathway with CK59 or CREB siRNA could significantly inhibit Ang II-induced Wnt/β-catenin signaling and podocyte injury. In in vivo studies, administration of Ang II promoted Wnt/β-catenin signaling, aggregated podocyte damage, and albuminuria in mice. CK59 could remarkably ameliorate Ang II-induced podocyte injury and albuminuria. Furthermore, ectopic expression of exogenous Dkk1 also attenuated Ang II-induced podocytopathy in mice. Taken together, this study demonstrates that the CaMK II/CREB/Wnt/β-catenin signaling cascade plays an important role in regulating Ang II-induced podocytopathy. Targeting this signaling pathway may offer renal protection against the development of proteinuric kidney diseases.

Wnt signaling in the regulation of adult hippocampal neurogenesis

In the adult brain new neurons are continuously generated mainly in two regions, the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) in the hippocampal dentate gyrus. In the SGZ, radial neural stem cells (NSCs) give rise to granule cells that integrate into the hippocampal circuitry and are relevant for the plasticity of the hippocampus. Loss of neurogenesis impairs learning and memory, suggesting that this process is important for adult hippocampal function. Adult neurogenesis is tightly regulated by multiple signaling pathways, including the canonical Wnt/β-catenin pathway. This pathway plays important roles during the development of neuronal circuits and in the adult brain it modulates synaptic transmission and plasticity. Here, we review current knowledge on the regulation of adult hippocampal neurogenesis by the Wnt/β-catenin signaling cascade and the potential mechanisms involved in this regulation. Also we discuss the evidence supporting that the canonical Wnt pathway is part of the signaling mechanisms involved in the regulation of neurogenesis in different physiological conditions. Finally, some unsolved questions regarding the Wnt-mediated regulation of neurogenesis are discussed.

A genome-wide screen of CREB occupancy identifies the RhoA inhibitors Par6C and Rnd3 as regulators of BDNF-induced synaptogenesis

Neurotrophin-regulated gene expression is believed to play a key role in long-term changes in synaptic structure and the formation of dendritic spines. Brain-derived neurotrophic factor (BDNF) has been shown to induce increases in dendritic spine formation, and this process is thought to function in part by stimulating CREB-dependent transcriptional changes. To identify CREB-regulated genes linked to BDNF-induced synaptogenesis, we profiled transcriptional occupancy of CREB in hippocampal neurons. Interestingly, de novo motif analysis of hippocampal ChIP-Seq data identified a non-canonical CRE motif (TGGCG) that was enriched at CREB target regions and conferred CREB-responsiveness. Because cytoskeletal remodeling is an essential element of the formation of dendritic spines, within our screens we focused our attention on genes previously identified as inhibitors of RhoA GTPase. Bioinformatic analyses identified dozens of candidate CREB target genes known to regulate synaptic architecture and function. We showed that two of these, the RhoA inhibitors Par6C (Pard6A) and Rnd3 (RhoE), are BDNF-induced CREB-regulated genes. Interestingly, CREB occupied a cluster of non-canonical CRE motifs in the Rnd3 promoter region. Lastly, we show that BDNF-stimulated synaptogenesis requires the expression of Par6C and Rnd3, and that overexpression of either protein is sufficient to increase synaptogenesis. Thus, we propose that BDNF can regulate formation of functional synapses by increasing the expression of the RhoA inhibitors, Par6C and Rnd3. This study shows that genome-wide analyses of CREB target genes can facilitate the discovery of new regulators of synaptogenesis.

CaMKII-dependent phosphorylation of the GTPase Rem2 is required to restrict dendritic complexity

The morphogenesis of the dendritic arbor is a critical aspect of neuronal development, ensuring that proper neural networks are formed. However, the molecular mechanisms that underlie this dendritic remodeling remain obscure. We previously established the activity-regulated GTPase Rem2 as a negative regulator of dendritic complexity. In this study, we identify a signaling pathway whereby Rem2 regulates dendritic arborization through interactions with Ca(2+)/calmodulin-dependent kinases (CaMKs) in rat hippocampal neurons. Specifically, we demonstrate that Rem2 functions downstream of CaMKII but upstream of CaMKIV in a pathway that restricts dendritic complexity. Furthermore, we show that Rem2 is a novel substrate of CaMKII and that phosphorylation of Rem2 by CaMKII regulates Rem2 function and subcellular localization. Overall, our results describe a unique signal transduction network through which Rem2 and CaMKs function to restrict dendritic complexity.

Synaptic plasticity in depression: molecular, cellular and functional correlates

Synaptic plasticity confers environmental adaptability through modification of the connectivity between neurons and neuronal circuits. This is achieved through changes to synapse-associated signaling systems and supported by complementary changes to cellular morphology and metabolism within the tripartite synapse. Mounting evidence suggests region-specific changes to synaptic form and function occur as a result of chronic stress and in depression. Within subregions of the prefrontal cortex (PFC) and hippocampus structural and synapse-related findings seem consistent with a deficit in long-term potentiation (LTP) and facilitation of long-term depression (LTD), particularly at excitatory pyramidal synapses. Other brain regions are less well-studied; however the amygdala may feature a somewhat opposite synaptic pathology including reduced inhibitory tone. Changes to synaptic plasticity in stress and depression may correlate those to several signal transduction pathways (e.g. NOS-NO, cAMP-PKA, Ras-ERK, PI3K-Akt, GSK-3, mTOR and CREB) and upstream receptors (e.g. NMDAR, TrkB and p75NTR). Deficits in synaptic plasticity may further correlate disrupted brain redox and bioenergetics. Finally, at a functional level region-specific changes to synaptic plasticity in depression may relate to maladapted neurocircuitry and parallel reduced cognitive control over negative emotion.

Wnt signaling: role in LTP, neural networks and memory

Wnt components are key regulators of a variety of developmental processes, including embryonic patterning, cell specification, and cell polarity. The Wnt signaling pathway participates in the development of the central nervous system and growing evidence indicates that Wnts also regulates the function of the adult nervous system. In fact, most of the key components including Wnts and Frizzled receptors are expressed in the adult brain. Wnt ligands have been implicated in the regulation of synaptic assembly as well as in neurotransmission and synaptic plasticity. Deregulation of Wnt signaling has been associated with several pathologies, and more recently has been related to neurodegenerative diseases and to mental and mood disorders. In this review, we focus our attention on the Wnt signaling cascade in postnatal life and we review in detail the presence of Wnt signaling components in pre- and postsynaptic regions. Due to the important role of Wnt proteins in wiring neural circuits, we discuss recent findings about the role of Wnt pathways both in basal spontaneous activities as well as in activity-dependent processes that underlie synaptic plasticity. Finally, we review the role of Wnt in vivo and we finish with the most recent data in literature that involves the effect of components of the Wnt signaling pathway in neurological and mental disorders, including a special emphasis on in vivo studies that relate behavioral abnormalities to deficiencies in Wnt signaling, as well as the data that support a neuroprotective role of Wnt proteins in relation to the pathogenesis of Alzheimer's disease.

Neurotrophin and Wnt signaling cooperatively regulate dendritic spine formation

Dendritic spines are major sites of excitatory synaptic transmission and changes in their numbers and morphology have been associated with neurodevelopmental and neurodegenerative disorders. Brain-derived Neurotrophic Factor (BDNF) is a secreted growth factor that influences hippocampal, striatal and neocortical pyramidal neuron dendritic spine density. However, the mechanisms by which BDNF regulates dendritic spines and how BDNF interacts with other regulators of spines remain unclear. We propose that one mechanism by which BDNF promotes dendritic spine formation is through an interaction with Wnt signaling. Here, we show that Wnt signaling inhibition in cultured cortical neurons disrupts dendritic spine development, reduces dendritic arbor size and complexity, and blocks BDNF-induced dendritic spine formation and maturation. Additionally, we show that BDNF regulates expression of Wnt2, and that Wnt2 is sufficient to promote cortical dendrite growth and dendritic spine formation. Together, these data suggest that BDNF and Wnt signaling cooperatively regulate dendritic spine formation.

Neuronal connectivity as a convergent target of gene × environment interactions that confer risk for Autism Spectrum Disorders

Evidence implicates environmental factors in the pathogenesis of Autism Spectrum Disorders (ASD). However, the identity of specific environmental chemicals that influence ASD risk, severity or treatment outcome remains elusive. The impact of any given environmental exposure likely varies across a population according to individual genetic substrates, and this increases the difficulty of identifying clear associations between exposure and ASD diagnoses. Heritable genetic vulnerabilities may amplify adverse effects triggered by environmental exposures if genetic and environmental factors converge to dysregulate the same signaling systems at critical times of development. Thus, one strategy for identifying environmental risk factors for ASD is to screen for environmental factors that modulate the same signaling pathways as ASD susceptibility genes. Recent advances in defining the molecular and cellular pathology of ASD point to altered patterns of neuronal connectivity in the developing brain as the neurobiological basis of these disorders. Studies of syndromic ASD and rare highly penetrant mutations or CNVs in ASD suggest that ASD risk genes converge on several major signaling pathways linked to altered neuronal connectivity in the developing brain. This review briefly summarizes the evidence implicating dysfunctional signaling via Ca(2+)-dependent mechanisms, extracellular signal-regulated kinases (ERK)/phosphatidylinositol-3-kinases (PI3K) and neuroligin-neurexin-SHANK as convergent molecular mechanisms in ASD, and then discusses examples of environmental chemicals for which there is emerging evidence of their potential to interfere with normal neuronal connectivity via perturbation of these signaling pathways.

Early postnatal lesion of the medial dorsal nucleus leads to loss of dendrites and spines in adult prefrontal cortex

Research suggests that the medial dorsal nucleus (MD) of the thalamus influences pyramidal cell development in the prefrontal cortex (PFC) in an activity-dependent manner. The MD is reciprocally connected to the PFC. Many psychiatric disorders, such as schizophrenia, affect the PFC, and one of the most consistent findings in schizophrenia is a decrease in volume and neuronal number in the MD. Therefore, understanding the role the MD plays in the development of the PFC is important and may help in understanding the progression of psychiatric disorders that have their root in development. Focusing on the interplay between the MD and the PFC, this study examined the hypothesis that the MD plays a role in the dendritic development of pyramidal cells in the PFC. Unilateral electrolytic lesions of the MD in Long-Evans rat pups were made on postnatal day 4 (P4), and the animals developed to P60. We then examined dendritic morphology by examining MAP2 immunostaining and by using Golgi techniques to determine basilar dendrite number and spine density. Additionally, we examined pyramidal cell density in cingulate area 1 (Cg1), prelimbic region, and dorsolateral anterior cortex, which receive afferents from the MD. Thalamic lesions caused a mean MD volume decrease of 12.4% which led to a significant decrease in MAP2 staining in both superficial and deep layers in all 3 cortical areas. The lesions also caused a significant decrease in spine density and in the number of primary and secondary basilar dendrites on superficial and deep layer pyramidal neurons in all 3 regions. No significant difference was observed in pyramidal cell density in any of the regions or layers, but a nonsignificant increase in cell density was observed in 2 regions. Our data are thus consistent with the hypothesis that the MD plays a role in the development of the PFC and, therefore, may be a good model to begin to examine neurodevelopmental disorders such as autism and schizophrenia.

Derepression of a neuronal inhibitor due to miRNA dysregulation in a schizophrenia-related microdeletion

22q11.2 microdeletions result in specific cognitive deficits and schizophrenia. Analysis of Df(16)A(+/-) mice, which model this microdeletion, revealed abnormalities in the formation of neuronal dendrites and spines, as well as altered brain microRNAs. Here, we show a drastic reduction of miR-185, which resides within the 22q11.2 locus, to levels more than expected by a hemizygous deletion, and we demonstrate that this reduction alters dendritic and spine development. miR-185 represses, through an evolutionarily conserved target site, a previously unknown inhibitor of these processes that resides in the Golgi apparatus and shows higher prenatal brain expression. Sustained derepression of this inhibitor after birth represents the most robust transcriptional disturbance in the brains of Df(16)A(+/-) mice and results in structural alterations in the hippocampus. Reduction of miR-185 also has milder age- and region-specific effects on the expression of some Golgi-related genes. Our findings illuminate the contribution of microRNAs in psychiatric disorders and cognitive dysfunction.

The molecular basis of experience-dependent motor system development

Neurons in the vertebrate nervous system acquire their mature features over an extended period in pre-natal and early post-natal life. The interaction of the organism with its environment (“experience”) has been shown to profoundly influence sensory neuron development. Over the past ~2 decades, it has become increasingly clear that motor system development is also experience-dependent. Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype have been implicated in both sensory and motor system experience-dependent development. An additional molecular mechanism involves the GluA1 subunit of the 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) subtype glutamate receptors. GluA1-dependent development operates in an NMDA-R independent manner and uses a distinct set of signaling molecules. The synapse associated protein of 97 kDa molecular weight (SAP97) is key. A deeper understanding of how experiences guides motor system development may lead to new ways to improve function after central nervous system insult.

Evaluation of metabolically stabilized angiotensin IV analogs as procognitive/antidementia agents

Angiotensin IV (AngIV: VYIHPF)-related peptides have long been recognized as procognitive agents with potential as antidementia therapeutics. Their development as useful therapeutics, however, has been limited by physiochemical properties that make them susceptible to metabolic degradation and impermeable to gut and blood-brain barriers. A previous study demonstrated that the core structural information required to impart the procognitive activity of the AngIV analog, norleucine(1)-angiotensin IV, resides in its three N-terminal amino acids, Nle-Tyr-Ile. The goal of this project was to chemically modify this tripeptide in such a way to enhance its metabolic stability and barrier permeability to produce a drug candidate with potential clinical utility. Initial results demonstrated that several N- and C-terminal modifications lead to dramatically improved stability while maintaining the capability to reverse scopolamine-induced deficits in Morris water maze performance and augment hippocampal synaptogenesis. Subsequent chemical modifications, which were designed to increase hydrophobicity and decrease hydrogen bonding, yielded an orally active, blood-barrier permeant, metabolically stabilized analog, N-hexanoic-Tyr-Ile-(6) aminohexanoic amide (dihexa), that exhibits excellent antidementia activity in the scopolamine and aged rat models and marked synaptogenic activity. These data suggest that dihexa may have therapeutic potential as a treatment of disorders, such as Alzheimer's disease, where augmented synaptic connectivity may be beneficial.

Molecular mechanisms of dendrite morphogenesis

Dendrites are key integrators of synaptic information in neurons and play vital roles in neuronal plasticity. Hence, it is necessary that dendrite arborization is precisely controlled and coordinated with synaptic activity to ensure appropriate functional neural network integrity. In the past several years, it has become increasingly clear that several cell intrinsic and extrinsic mechanisms contribute to dendritic arborization. In this review, we will discuss some of the molecular mechanisms that regulate dendrite morphogenesis, particularly in cortical and hippocampal pyramidal neurons and some of the implications of aberrant dendritic morphology for human disease. Finally, we will discuss the current challenges and future directions in the field.

Systematic identification of rhythmic genes reveals camk1gb as a new element in the circadian clockwork

A wide variety of biochemical, physiological, and molecular processes are known to have daily rhythms driven by an endogenous circadian clock. While extensive research has greatly improved our understanding of the molecular mechanisms that constitute the circadian clock, the links between this clock and dependent processes have remained elusive. To address this gap in our knowledge, we have used RNA sequencing (RNA–seq) and DNA microarrays to systematically identify clock-controlled genes in the zebrafish pineal gland. In addition to a comprehensive view of the expression pattern of known clock components within this master clock tissue, this approach has revealed novel potential elements of the circadian timing system. We have implicated one rhythmically expressed gene, camk1gb, in connecting the clock with downstream physiology of the pineal gland. Remarkably, knockdown of camk1gb disrupts locomotor activity in the whole larva, even though it is predominantly expressed within the pineal gland. Therefore, it appears that camk1gb plays a role in linking the pineal master clock with the periphery.

The circadian clock is a molecular pacemaker that drives rhythmic expression of genes with a ∼24-hour period. As a result, many physiological processes have daily rhythms. Many of the conserved elements that constitute the circadian clock are known, but the links between the clock and dependent processes have remained elusive. With its amenability to genetic manipulations and a variety of genetic tools, the zebrafish has become an attractive vertebrate model for the quest to identify and characterize novel clock components. Here, we take advantage of another attraction of the zebrafish, the fact that its pineal gland is the site of a central clock which directly receives light input and autonomously generates circadian rhythms that affect the physiology of the whole organism. We show that the systematic design and analysis of genome-wide experiments based on the zebrafish pineal gland can lead to the discovery of new clock elements. We have characterized one novel element, camk1gb, and show that this gene, predominantly expressed within the pineal gland and driven by the circadian clock, links circadian clock timing with locomotor activity in zebrafish larvae.

In vitro substrate phosphorylation by Ca²⁺/calmodulin-dependent protein kinase kinase using guanosine-5'-triphosphate as a phosphate donor

Background

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) phosphorylates and activates particular downstream protein kinases — including CaMKI, CaMKIV, and AMPK— to stimulate multiple Ca²⁺-signal transduction pathways. To identify previously unidentified CaMKK substrates, we used various nucleotides as phosphate donors to develop and characterize an in vitro phosphorylation assay for CaMKK.

Results

Here, we found that the recombinant CaMKK isoforms were capable of utilizing Mg-GTP as a phosphate donor to phosphorylate the Thr residue in the activation-loop of CaMKIα (Thr¹⁷⁷) and of AMPK (Thr¹⁷²) in vitro. Kinetic analysis indicated that the K_m values of CaMKK isoforms for GTP (400-500 μM) were significantly higher than those for ATP (~15 μM), and a 2- to 4-fold decrease in V_max was observed with GTP. We also confirmed that an ATP competitive CaMKK inhibitor, STO-609, also competes with GTP to inhibit the activities of CaMKK isoforms. In addition, to detect enhanced CaMKI phosphorylation in brain extracts with Mg-GTP and recombinant CaMKKs, we found potential CaMKK substrates of ~45 kDa and ~35 kDa whose Ca²⁺/CaM-induced phosphorylation was inhibited by STO-609.

Conclusions

These results indicated that screens that use STO-609 as a CaMKK inhibitor and Mg-GTP as a CaMKK-dependent phosphate donor might be useful to identify previously unidentified downstream target substrates of CaMKK.

Mitogen- and stress-activated kinases regulate progenitor cell proliferation and neuron development in the adult dentate gyrus

The neurogenic niche within the subgranular zone (SGZ) of the dentate gyrus is a source of new neurons throughout life. Interestingly, SGZ proliferative capacity is regulated by both physiological and pathophysiological conditions. One outstanding question involves the molecular mechanisms that regulate both basal and inducible adult neurogenesis. Here, we examined the role of the MAPK-regulated kinases, mitogen- and stress-activated kinase (MSK)1 and MSK2. as regulators of dentate gyrus SGZ progenitor cell proliferation and neurogenesis. Under basal conditions, MSK1/2 null mice exhibited significantly reduced progenitor cell proliferation capacity and a corollary reduction in the number of doublecortin (DCX)-positive immature neurons. Strikingly, seizure-induced progenitor proliferation was totally blocked in MSK1/2 null mice. This blunting of cell proliferation in MSK1/2 null mice was partially reversed by forskolin infusion, indicating that the inducible proliferative capacity of the progenitor cell population was intact. Furthermore, in MSK1/2 null mice, DCX-positive immature neurons exhibited reduced neurite arborization. Together, these data reveal a critical role for MSK1/2 as regulators of both basal and activity-dependent progenitor cell proliferation and morphological maturation in the SGZ.

Neuronal Per Arnt Sim (PAS) domain protein 4 (NPAS4) regulates neurite outgrowth and phosphorylation of synapsin I

Neuronal Per Arnt Sim domain protein 4 (NPAS4), a brain-specific basic helix-loop-helix transcription factor, has recently been shown to regulate the development of the GABAergic inhibitory synapses and transcription program for contextual memory formation in the hippocampus. We previously reported that chronic social isolation or restriction stress in mice resulted in an impairment in memory and emotional behavior, which was associated with a decrease in Npas4 mRNA levels. In this study, we investigated the role of NPAS4 in neuronal function in vitro and in vivo. Differentiation medium-induced neurite outgrowth was inhibited in Npas4 knockdown Neuro2a cells, whereas overexpression of NPAS4 accelerated the neurite outgrowth in Neuro2a cells. Furthermore, depolarization-induced neurite outgrowth was abolished in Npas4 KO hippocampal neurons. NPAS4 overexpression increased cyclin-dependent kinase 5 (CDK5)-dependent synapsin I phosphorylation in Neuro2a cells and primary cultured hippocampal neurons. A CDK5 inhibitor, roscovitine, inhibited the neurite outgrowth and the increase in phosphorylated synapsin I (p-SYN I) levels in Npas4-overexpressed Neuro2a cells. Interaction of NPAS4 with promoters of Cdk5 and NeuN genes was demonstrated by a chromatin immunoprecipitation assay. In an in vivo study, pentylenetetrazole-induced convulsions in mice resulted in an increase in NPAS4 and p-SYN I levels in the prefrontal cortex of wild-type mice, although no changes in p-SYN I levels were observed in Npas4 knock-out mice. These results suggest that NPAS4 plays an important role in the structural and functional plasticity of neurons.

Bidirectional influence of sodium channel activation on NMDA receptor-dependent cerebrocortical neuron structural plasticity

Neuronal activity regulates brain development and synaptic plasticity through N-methyl-D-aspartate receptors (NMDARs) and calcium-dependent signaling pathways. Intracellular sodium ([Na(+)](i)) also exerts a regulatory influence on NMDAR channel activity, and [Na(+)](i) may, therefore, function as a signaling molecule. In an attempt to mimic the influence of neuronal activity on synaptic plasticity, we used brevetoxin-2 (PbTx-2), a voltage-gated sodium channel (VGSC) gating modifier, to manipulate [Na(+)](i) in cerebrocortical neurons. The acute application of PbTx-2 produced concentration-dependent increments in both intracellular [Na(+)] and [Ca(2+)]. Pharmacological evaluation showed that PbTx-2-induced Ca(2+) influx primarily involved VGSC activation and NMDAR-mediated entry. Additionally, PbTx-2 robustly potentiated NMDA-induced Ca(2+) influx. PbTx-2-exposed neurons showed enhanced neurite outgrowth, increased dendritic arbor complexity, and increased dendritic filopodia density. The appearance of spontaneous calcium oscillations, reflecting synchronous neuronal activity, was accelerated by PbTx-2 treatment. Parallel to this response, PbTx-2 increased cerebrocortical neuron synaptic density. PbTx-2 stimulation of neurite outgrowth, dendritic arborization, and synaptogenesis all exhibited bidirectional concentration-response profiles. This profile paralleled that of NMDA, which also produced bidirectional concentration-response profiles for neurite outgrowth and synaptogenesis. These data are consistent with the hypothesis that PbTx-2-enhanced neuronal plasticity involves NMDAR-dependent signaling. Our results demonstrate that PbTx-2 mimics activity-dependent neuronal structural plasticity in cerebrocortical neurons through an increase in [Na(+)](i), up-regulation of NMDAR function, and engagement of downstream Ca(2+)-dependent signaling pathways. These data suggest that VGSC gating modifiers may represent a pharmacologic strategy to regulate neuronal plasticity through NMDAR-dependent mechanisms.

The role of Wnt/beta-catenin signaling in postsynaptic differentiation

Synapses are basic units that mediate the communication between neurons and their target cells. The formation of synapse is regulated by secreted factors, receptors, adhesion molecules and intracellular signaling molecules. The interplay between positive and negative factors determines synapse assembling, remodeling and elimination, resulting in the formation of precise synaptic connections. However, compared to the abundant identified positive factors, negative factors are largely unknown. We have recently shown that Wnt3a acts as a negative factor that inhibits postsynaptic differentiation at the neuromuscular junction (NMJ), the synapse formed between motor neurons and skeletal muscle fibers. The clustering of acetylcholine receptor (AChR) guarantees efficient and accurate neurotransmission and is a hallmark for postsynaptic differentiation at the NMJ. We found that treatment with Wnt3a or upregulation of beta-catenin inhibited the formation of AChR clusters. Furthermore, we investigated the underlying mechanism and found that Wnt/beta-catenin signaling negatively regulated AChR clustering by downregulating the expression of Rapsyn, an AChR-associated protein required for formation and stabilization of AChR clusters.

Wnt signaling in vertebrate neural development and function

Members of the Wnt family of secreted signaling proteins influence many aspects of neural development and function. Wnts are required from neural induction and axis formation to axon guidance and synapse development, and even help modulate synapse activity. Wnt proteins activate a variety of downstream signaling pathways and can induce a similar variety of cellular responses, including gene transcription changes and cytoskeletal rearrangements. This review provides an introduction to Wnt signaling pathways and discusses current research on their roles in vertebrate neural development and function.

Brain-derived neurotrophic factor activation of CaM-kinase kinase via transient receptor potential canonical channels induces the translation and synaptic incorporation of GluA1-containing calcium-permeable AMPA receptors

Glutamatergic synapses in early postnatal development transiently express calcium-permeable AMPA receptors (CP-AMPARs). Although these GluA2-lacking receptors are essential and are elevated in response to brain-derived neurotrophic factor (BDNF), little is known regarding molecular mechanisms that govern their expression and synaptic insertion. Here we show that BDNF-induced GluA1 translation in rat primary hippocampal neurons requires the activation of mammalian target of rapamycin (mTOR) via calcium calmodulin-dependent protein kinase kinase (CaMKK). Specifically, BDNF-mediated phosphorylation of threonine 308 (T308) in AKT, a known substrate of CaMKK and an upstream activator of mTOR-dependent translation, was prevented by (1) pharmacological inhibition of CaMKK with STO-609, (2) overexpression of a dominant-negative CaMKK, or (3) short hairpin-mediated knockdown of CaMKK. GluA1 surface expression induced by BDNF, as assessed by immunocytochemistry using an extracellular N-terminal GluA1 antibody or by surface biotinylation, was impaired following knockdown of CaMKK or treatment with STO-609. Activation of CaMKK by BDNF requires transient receptor potential canonical (TRPC) channels as SKF-96365, but not the NMDA receptor antagonist d-APV, prevented BDNF-induced GluA1 surface expression as well as phosphorylation of CaMKI, AKT(T308), and mTOR. Using siRNA we confirmed the involvement of TRPC5 and TRPC6 subunits in BDNF-induced AKT(T308) phosphorylation. The BDNF-induced increase in mEPSC was blocked by IEM-1460, a selected antagonist of CP-AMPARs, as well as by the specific repression of acute GluA1 translation via siRNA to GluA1 but not GluA2. Together these data support the conclusion that newly synthesized GluA1 subunits, induced by BDNF, are readily incorporated into synapses where they enhance the expression of CP-AMPARs and synaptic strength.

Crystal structures of human CaMKIα reveal insights into the regulation mechanism of CaMKI

Human calcium/calmodulin-dependent protein kinase I (CaMKI) plays pivotal roles in the nervous system. The activity of human CaMKI is regulated by a regulatory region including an autoinhibitory segment and a CaM-binding segment. We report here four structures of three CaMKIα truncates in apo form and in complexes with ATP. In an apo, autoinhibited structure, the activation segment adopts a unique helical conformation which together with the autoinhibitory segment constrains helices αC and αD in inactive conformations, sequesters Thr177 from being phosphorylated, and occludes the substrate-binding site. In an ATP-bound, inactive structure, the activation segment is largely disordered and the CaM-binding segment protrudes out ready for CaM binding. In an ATP-bound, active structure, the regulatory region is dissociated from the catalytic core and the catalytic site assumes an active conformation. Detailed structural analyses reveal the interplay of the regulatory region, the activation segment, and the nucleotide-binding site in the regulation of CaMKI.

TRPC5 channel is the mediator of neurotrophin-3 in regulating dendritic growth via CaMKIIα in rat hippocampal neurons

Neurotrophin-3 (NT-3) plays numerous important roles in the CNS and the elevation of intracellular Ca(2+) ([Ca(2+)](i)) is critical for these functions of NT-3. However, the mechanism by which NT-3 induces [Ca(2+)](i) elevation remains largely unknown. Here, we found that transient receptor potential canonical (TRPC) 5 protein and TrkC, the NT-3 receptor, exhibited a similar temporal expression in rat hippocampus and cellular colocalization in hippocampal neurons. Stimulation of the neurons by NT-3 induced a nonselective cation conductance and PLCγ-dependent [Ca(2+)](i) elevation, which were both blocked when TRPC5, but not TRPC6 channels, were inhibited. Moreover, the Ca(2+) influx through TRPC5 induced by NT-3 inhibited the neuronal dendritic growth through activation of calmodulin-dependent kinase (CaMK) IIα. In contrast, the Ca(2+) influx through TRPC6 induced by NT-4 promoted the dendritic growth. Thus, TRPC5 acts as a novel and specific mediator for NT-3 to regulate dendrite development through CaMKIIα.

NMDA receptor signaling mediates the expression of protein inhibitor of activated STAT1 (PIAS1) in rat hippocampus

Protein inhibitor of activated STAT1 (PIAS1) was shown to play an important role in inflammation and innate immune response, but how PIAS1 is regulated is not known. We have recently demonstrated that PIAS1 enhances spatial learning and memory performance in rats. In this study, we examined the signaling pathway and neural mechanism that regulate PIAS1 expression in the brain by using pharmacological and molecular approaches. Our results revealed that pias1 gene expression is rapidly induced upon NMDA receptor activation in rat hippocampus, but this effect is blocked by transfection of sub-threshold concentrations of ERK1 siRNA/ERK2 siRNA or CREB siRNA. Pias1 gene expression is similarly induced by overexpression of the ERK1/ERK2 plasmids in rat hippocampus, and this effect is also blocked by sub-threshold concentration of CREB siRNA transfection. On the other hand, transfection of ERK1 siRNA/ERK2 siRNA or CREB siRNA at a higher concentration is sufficient to down-regulate PIAS1 expression. Inhibition of PI-3 kinase signaling and CaMKII signaling, which both result in CREB inactivation, similarly decreases PIAS1 expression. But NMDA and MK-801 do not affect the expression of IL-6 and TNFα. NMDA also did not affect the expression of PIAS2, PIAS3 and PIAS4. Further, pias1 mRNA has a similar degradation rate to that of the zif268 gene. These results together suggest that pias1 may function as an immediate early gene in an activity-dependent manner and PIAS1 expression is regulated by the NMDA-MAPK/ERK-CREB signaling pathway implicated in neuronal plasticity.

Snapin recruits dynein to BDNF-TrkB signaling endosomes for retrograde axonal transport and is essential for dendrite growth of cortical neurons

Neurotrophin signaling is crucial for neuron growth. While the "signaling endosomes" hypothesis is one of the accepted models, the molecular machinery that drives retrograde axonal transport of TrkB signaling endosomes is largely unknown. In particular, mechanisms recruiting dynein to TrkB signaling endosomes have not been elucidated. Here, using snapin deficient mice and gene rescue experiments combined with compartmentalized cultures of live cortical neurons, we reveal that Snapin, as a dynein adaptor, mediates retrograde axonal transport of TrkB signaling endosomes. Such a role is essential for dendritic growth of cortical neurons. Deleting snapin or disrupting Snapin-dynein interaction abolishes TrkB retrograde transport, impairs BDNF-induced retrograde signaling from axonal terminals to the nucleus, and decreases dendritic growth. Such defects were rescued by reintroducing the snapin gene. Our study indicates that Snapin-dynein coupling is one of the primary mechanisms driving BDNF-TrkB retrograde transport, thus providing mechanistic insights into the regulation of neuronal growth and survival.

PCB-95 promotes dendritic growth via ryanodine receptor-dependent mechanisms

BACKGROUND

Aroclor 1254 (A1254) interferes with normal dendritic growth and plasticity in the developing rodent brain, but the mechanism(s) mediating this effect have yet to be established. Non-dioxin-like (NDL) polychlorinated biphenyls (PCBs) enhance the activity of ryanodine receptor (RyR) calcium ion (Ca(2+)) channels, which play a central role in regulating the spatiotemporal dynamics of intracellular Ca(2+) signaling. Ca(2+) signaling is a predominant factor in shaping dendritic arbors, but whether PCB potentiation of RyR activity influences dendritic growth is not known.

OBJECTIVE

We determined whether RyR activity is required for PCB effects on dendritic growth.

METHODS AND RESULTS

Golgi analysis of hippocampi from weanling rats confirmed that developmental exposure via the maternal diet to NDL PCB-95 (2,2',3,5'6-pentachlorobiphenyl), a potent RyR potentiator, phenocopies the dendrite-promoting effects of A1254. Dendritic growth in dissociated cultures of primary hippocampal neurons and in hippocampal slice cultures is similarly enhanced by PCB-95 but not by PCB-66 (2,3,4',4-tetrachlorobiphenyl), a congener with negligible effects on RyR activity. The dendrite-promoting effects of PCB-95 are evident at concentrations as low as 2 pM and are inhibited by either pharmacologic blockade or siRNA knockdown of RyRs.

CONCLUSIONS

Our findings demonstrate that environmentally relevant levels of NDL PCBs modulate neuronal connectivity via RyR-dependent effects on dendritic arborization. In addition, these findings identify RyR channel dysregulation as a novel mechanism contributing to dysmorphic dendritogenesis associated with heritable and environmentally triggered neurodevelopmental disorders.

PCB-95 modulates the calcium-dependent signaling pathway responsible for activity-dependent dendritic growth

BACKGROUND

Non-dioxin-like (NDL) polychlorinated biphenyls (PCBs) promote dendritic growth in hippocampal neurons via ryanodine receptor (RyR)-dependent mechanisms; however, downstream signaling events that link enhanced RyR activity to dendritic growth are unknown. Activity-dependent dendritic growth, which is a critical determinant of neuronal connectivity in the developing brain, is mediated by calcium ion (Ca(2+))-dependent activation of Ca(2+)/calmodulin kinase-I (CaMKI), which triggers cAMP response element binding protein (CREB)-dependent Wnt2 transcription. RyRs regulate the spatiotemporal dynamics of intracellular Ca(2+) signals, but whether RyRs promote dendritic growth via modulation of this signaling pathway is not known.

OBJECTIVE

We tested the hypothesis that the CaMKI-CREB-Wnt2 signaling pathway couples NDL PCB-enhanced RyR activity to dendritic arborization.

METHODS AND RESULTS

Ca(2+) imaging of dissociated cultures of primary rat hippocampal neurons indicated that PCB-95 (2,2',3,5'6-pentachlorobiphenyl; a potent RyR potentiator), enhanced synchronized Ca(2+) oscillations in somata and dendrites that were blocked by ryanodine. As determined by Western blotting and quantitative polymerase chain reaction, PCB-95 also activated CREB and up-regulated Wnt2. Blocking CaMKK, CaMKIα/γ, MEK/ERK, CREB, or Wnt2 prevented PCB-95-induced dendritic growth. Antagonism of γ-aminobutyric acid (GABA) receptors with bicuculline (BIC) phenocopied the dendrite-promoting effects of PCB-95, and pharmacological antagonism or siRNA knockdown of RyR blocked BIC-induced dendritic growth in dissociated and slice cultures of hippocampal neurons.

CONCLUSIONS

RyR activity contributes to dynamic remodeling of dendritic architecture in response to NDL PCBs via CaMKI-CREB-Wnt2 signaling in rats. Our findings identify PCBs as candidate environmental risk factors for neurodevelopmental disorders, especially in children with heritable deficits in calcium signaling associated with autism.

Regulation of neuronal mRNA translation by CaM-kinase I phosphorylation of eIF4GII

Ca²⁺/calmodulin-dependent kinases (CaMKs) are essential for neuronal development and plasticity, processes requiring de novo protein synthesis. Roles for CaMKs in modulating gene transcription are well established, but their involvement in mRNA translation is evolving. Here we report that activity-dependent translational initiation in cultured rat hippocampal neurons is enhanced by CaMKI-mediated phosphorylation of Ser1156 in eukaryotic initiation factor eIF4GII (4GII). Treatment with bicuculline or gabazine to enhance neuronal activity promotes recruitment of wild-type 4GII, but not the 4GII S1156A mutant or 4GI, to the heterotrimeric eIF4F (4F) complex that assembles at the 5' cap structure (m⁷GTP) of mRNA to initiate ribosomal scanning. Recruitment of 4GII to 4F is suppressed by pharmacological inhibition (STO-609) of CaM kinase kinase, the upstream activator of CaMKI. Post hoc in vitro CaMKI phosphorylation assays confirm that activity promotes phosphorylation of S1156 in transfected 4GII in neurons. Changes in cap-dependent and cap-independent translation were assessed using a bicistronic luciferase reporter transfected into neurons. Activity upregulates cap-dependent translation, and RNAi knockdown of CaMKIβ and γ isoforms, but not α or δ, led to its attenuation as did blockade of NMDA receptors. Furthermore, RNAi knockdown of 4GII attenuates cap-dependent translation and reduces density of dendritic filopodia and spine formation without effect on dendritic arborization. Together, our results provide a mechanistic link between Ca²⁺ influx due to neuronal activity and regulation of cap-dependent RNA translation via CaMKI activation and selective recruitment of phosphorylated 4GII to the 4F complex, which may function to regulate activity-dependent changes in spine density.

WNTs in synapse formation and neuronal circuitry

Wnt proteins play important roles in wiring neural circuits. Wnts regulate many aspects of neural circuit generation through their receptors and distinct signalling pathways. In this review, we discuss recent findings on the functions of Wnts in various aspects of neural circuit formation, including neuronal polarity, axon guidance, synapse formation, and synaptic plasticity in vertebrate and invertebrate nervous systems.

Activation of NMDA receptors upregulates a disintegrin and metalloproteinase 10 via a Wnt/MAPK signaling pathway

A disintegrin and metalloproteinase 10 (ADAM10) is the constitutive α-secretase that governs the nonamyloidogenic pathway of β-amyloid precursor protein processing and is an attractive drug target for treating Alzheimer's disease. To date, little is known about the mechanism by which ADAM10 is regulated in neurons. Using mouse primary cortical neurons, we show here that NMDA receptor (NMDAR) activation led to upregulation of the genes encoding ADAM10 and β-catenin proteins. Interestingly, the ADAM10 upregulation was abolished by inhibitors of Wnt/β-catenin signaling. Conversely, activation of the Wnt/β-catenin signaling pathway by recombinant Wnt3a stimulated ADAM10 expression. We further showed that both the NMDAR- and Wnt3a-induced ADAM10 upregulation was blocked by ERK inhibitors. We suggest that the NMDARs control ADAM10 expression via a Wnt/MAPK signaling pathway.

Neuronal D-serine regulates dendritic architecture in the somatosensory cortex

D-Serine, which is synthesized by the enzyme serine racemase (SR), is a co-agonist at the N-methyl-D-aspartate receptor (NMDAR). In an animal model of NMDAR hypofunction, the constitutive SR knockout (SR-/-) mouse, pyramidal neurons in primary somatosensory cortex (S1) have reductions in the complexity, total length, and spine density of apical and basal dendrites. We wondered whether the dendritic pathology required deprivation of D-serine throughout development or reflected the loss of D-serine only in adulthood. To address this question, we used mice homozygous for floxed SR in which we bred CaMKIICre2834, which is expressed in forebrain glutamatergic neurons starting at 3-4 weeks post-partum (nSR-/-). Our prior studies demonstrated that the majority of cortical SR is expressed in glutamatergic neurons. We found that similar to SR-/- mice, pyramidal neurons in S1 of nSR-/- also had significantly reduced dendritic arborization and spine density, albeit to a lesser degree. S1 neurons of nSR-/- mice had reduced total basal dendritic length that was accompanied by less complex arborization. These characteristics were unaltered in the apical dendritic compartment. In contrast, spine density on S1 neurons was significantly reduced on apical, but not basal dendrites of nSR-/- mice. These results demonstrate that in adulthood neuronally derived D-serine, which is required for optimal activation of post-synaptic NMDAR activity, regulates pyramidal neuron dendritic arborization and spine density. Moreover, they highlight the glycine modulatory site (GMS) of the NMDAR as a potential target for therapeutic intervention in diseases characterized by synaptic deficits, like schizophrenia.

Frizzled receptors in neurons: from growth cones to the synapse

The Wnt signaling pathway has been implicated in several different aspects of neural development and function, including dendrite morphogenesis, axonal growth and guidance, synaptogenesis and synaptic plasticity. Here, we studied several Frizzled Wnt receptors and determined their differential expression during hippocampal development. In cultured hippocampal neurons, the cellular distributions of Frizzleds vary greatly, some of them being localized at neurites, growth cones or synaptic sites. These findings suggest that the Wnt signaling pathway might be temporally and spatially fine tuned during the development of neuronal circuits through specific Frizzled receptors.

Curcumin enhances neurogenesis and cognition in aged rats: implications for transcriptional interactions related to growth and synaptic plasticity

Background

Curcumin has been demonstrated to have many neuroprotective properties, including improvement of cognition in humans and neurogenesis in animals, yet the mechanism of such effects remains unclear.

Methodology

We assessed behavioural performance and hippocampal cell proliferation in aged rats after 6- and 12-week curcumin-fortified diets. Curcumin enhanced non-spatial and spatial memory, as well as dentate gyrate cell proliferation as compared to control diet rats. We also investigated underlying mechanistic pathways that might link curcumin treatment to increased cognition and neurogenesis via exon array analysis of cortical and hippocampal mRNA transcription. The results revealed a transcriptional network interaction of genes involved in neurotransmission, neuronal development, signal transduction, and metabolism in response to the curcumin treatment.

Conclusions

The results suggest a neurogenesis- and cognition-enhancing potential of prolonged curcumin treatment in aged rats, which may be due to its diverse effects on genes related to growth and plasticity.

Wnt signaling in the vertebrate central nervous system: from axon guidance to synaptic function

Regulation of cell signaling by Wnt proteins is critical for the formation of neuronal circuits. Wnts modulate axon pathfinding, dendritic development, and synaptic assembly. Through different receptors, Wnts activate diverse signaling pathways that lead to local changes on the cytoskeleton or global cellular changes involving nuclear function. Recently, a link between neuronal activity, essential for the formation and refinement of neuronal connections, and Wnt signaling has been uncovered. Indeed, neuronal activity regulates the release of Wnt and the localization of their receptors. Wnts mediate synaptic structural changes induced by neuronal activity or experience. New emerging evidence suggests that dysfunction in Wnt signaling contributes to neurological disorders. In this article, the attention is focused on the function of Wnt signaling in the formation of neuronal circuits in the vertebrate central nervous system.

Dynamic O-GlcNAc modification regulates CREB-mediated gene expression and memory formation

The transcription factor CREB is a key regulator of many neuronal processes, including brain development, circadian rhythm, and long-term memory. Studies of CREB have focused on its phosphorylation, although the diversity of CREB functions in the brain suggests additional forms of regulation. Here we expand on a chemoenzymatic strategy for quantifying glycosylation stoichiometries to characterize the functional roles of CREB glycosylation in neurons. We show that CREB is dynamically O-GlcNAc-modified in response to neuronal activity and glycosylation represses CREB-dependent transcription by impairing its association with the co-activator CRTC/TORC. Blocking glycosylation of CREB altered cellular function and behavioral plasticity, enhancing both axonal and dendritic growth and long-term memory consolidation. Our findings demonstrate a new role for O-glycosylation in memory formation and provide a mechanistic understanding of how glycosylation contributes to critical neuronal functions. Moreover, we identify a previously unknown mechanism for the regulation of activity-dependent gene expression, neural development, and memory.

CRP1, a protein localized in filopodia of growth cones, is involved in dendritic growth

The cysteine-rich protein (CRP) family is a subgroup of LIM domain proteins. CRP1, which cross-links actin filaments to make actin bundles, is the only CRP family member expressed in the CNS with little known about its function in nerve cells. Here, we report that CRP1 colocalizes with actin in the filopodia of growth cones in cultured rat hippocampal neurons. Knockdown of CRP1 expression by short hairpin RNA interference results in inhibition of filopodia formation and dendritic growth in neurons. Overexpression of CRP1 increases filopodia formation and neurite branching, which require its actin-bundling activity. Expression of CRP1 with a constitutively active form of Cdc42, a GTPase involved in filopodia formation, increases filopodia formation in COS-7 cells, suggesting cooperation between the two proteins. Moreover, we demonstrate that neuronal activity upregulates CRP1 expression in hippocampal neurons via Ca²⁺ influx after depolarization. Ca²⁺/calmodulin-dependent protein kinase IV (CaMKIV) and cAMP response element binding protein mediate the Ca²⁺-induced upregulation of CRP1 expression. Furthermore, CRP1 is required for the dendritic growth induced by Ca²⁺ influx or CaMKIV. Together, these data are the first to demonstrate a role for CRP1 in dendritic growth.

From synapse to nucleus and back again--communication over distance within neurons

How do neurons integrate intracellular communication from synapse to nucleus and back? Here we briefly summarize aspects of this topic covered by a symposium at Neuroscience 2011. A rich repertoire of signaling mechanisms link both dendritic terminals and axon tips with neuronal soma and nucleus, using motor-dependent transport machineries to traverse the long intracellular distances along neuronal processes. Activation mechanisms at terminals include localized translation of dendritic or axonal RNA, proteolytic cleavage of receptors or second messengers, and differential phosphorylation of signaling moieties. Signaling complexes may be transported in endosomes, or as non-endosomal complexes associated with importins and dynein. Anterograde transport of RNA granules from the soma to neuronal processes, coupled with retrograde transport of proteins translated locally at terminals or within processes, may fuel ongoing bidirectional communication between soma and synapse to modulate synaptic plasticity as well as neuronal growth and survival decisions.

NMDA receptor activation stimulates transcription-independent rapid wnt5a protein synthesis via the MAPK signaling pathway

Wnt proteins are emerging key regulators of the plasticity and functions of adult brains. However, the mechanisms by which the expression of Wnt proteins is regulated in neurons are unclear. Using cortical primary cultures, we show here that activation of NMDA receptors (NMDARs) induces rapid Wnt5a protein synthesis and secretion. This NMDAR-regulated Wnt5a synthesis does not require transcription and is a result of activity-dependent translation. We also show that NMDAR-regulated Wnt5a translation depends on MAPK signaling but not mTOR signaling. Our findings suggest that the synaptic activity of CNS neurons activates NMDARs, which in turn stimulate translation from stored Wnt5a mRNA via the MAPK signaling pathway.

Sonic Hedgehog regulates Wnt activity during neural circuit formation

Gradients of secreted morphogens, such as Sonic hedgehog (Shh), Wnt, and TGFβ/Bmp, have classically been shown to control many aspects of early development by regulating cell proliferation and determining cell fate. However, recent studies demonstrate that these molecules also play important and evolutionarily conserved roles in later aspects of neural development. Depending on the context, these molecules can elicit gene transcription in the nucleus, or alternatively can provide instructive signals at the growth cone that induce local and rapid changes in cytoskeletal organization. Shh can activate different cellular transduction pathways via its binding to alternative coreceptor complexes or simply by adaptation of its "classical" signaling pathway. However, in most of its activities during neural development, Shh does not act alone but rather in concert with other morphogens, particularly the Wnts. This review provides an overview of the mechanisms by which Shh signaling acts in concert with Wnts to mediate a myriad of cellular processes that are required for neural circuit formation.

Developmental plasticity of the dendritic compartment: focus on the cytoskeleton

Plasticity, the ability to undergo lasting changes in response to a stimulus, is an important attribute of neurons. It allows proper development and underlies learning, memory, and the recovery of the nervous system after severe injuries. Often, an outcome of neuronal plasticity is a structural plasticity manifested as a change of neuronal morphology. In this chapter, we focus on the structural plasticity of dendritic arbors and spines during development. Dendrites receive and compute synaptic inputs from other neurons. The number of dendrites and their branching pattern are strictly correlated with the function of a particular neuron and the geometry of the connections it receives. The development of proper dendritic tree morphology depends on the interplay between genetic programming and extracellular signals. Spines are tiny actin-rich dendritic protrusions that harbor excitatory synapses. No consensus has been reached regarding how dendritic spines form, and several models of spine morphogenesis exist. Nevertheless, most researchers agree that spinogenesis is an important target for structural plasticity. In this chapter, we discuss examples of such plasticity and describe the principles and molecular mechanisms underlying this process, focusing mostly on the regulation of the cytoskeleton during dendrito- and spinogenesis.

The roles of neurotrophic factor and Wnt signaling in depression

Depression is a debilitating disease with a lifetime prevalence of ~16% in the American population. In addition to the monoamine hypothesis, altered expressions of neurotrophic factors, growth factors, and Wnt signaling are implicated in the pathophysiology and treatment of depression. This review focuses on intracellular signaling cascades that underlie depression and treatment response.

Activation of intracellular metabotropic glutamate receptor 5 in striatal neurons leads to up-regulation of genes associated with sustained synaptic transmission including Arc/Arg3.1 protein

The G-protein coupled receptor, metabotropic glutamate receptor 5 (mGluR5), is expressed on both cell surface and intracellular membranes in striatal neurons. Using pharmacological tools to differentiate membrane responses, we previously demonstrated that cell surface mGluR5 triggers rapid, transient cytoplasmic Ca(2+) rises, resulting in c-Jun N-terminal kinase, Ca(2+)/calmodulin-dependent protein kinase, and cyclic adenosine 3',5'-monophosphate-responsive element-binding protein (CREB) phosphorylation, whereas stimulation of intracellular mGluR5 induces long, sustained Ca(2+) responses leading to the phosphorylation of extracellular signal-regulated kinase (ERK1/2) and Elk-1 (Jong, Y. J., Kumar, V., and O'Malley, K. L. (2009) J. Biol. Chem. 284, 35827-35838). Using pharmacological, genetic, and bioinformatics approaches, the current findings show that both receptor populations up-regulate many immediate early genes involved in growth and differentiation. Activation of intracellular mGluR5 also up-regulates genes involved in synaptic plasticity including activity-regulated cytoskeletal-associated protein (Arc/Arg3.1). Mechanistically, intracellular mGluR5-mediated Arc induction is dependent upon extracellular and intracellular Ca(2+) and ERK1/2 as well as calmodulin-dependent kinases as known chelators, inhibitors, and a dominant negative Ca(2+)/calmodulin-dependent protein kinase II construct block Arc increases. Moreover, intracellular mGluR5-induced Arc expression requires the serum response transcription factor (SRF) as wild type but not SRF-deficient neurons show this response. Finally, increased Arc levels due to high K(+) depolarization is significantly reduced in response to a permeable but not an impermeable mGluR5 antagonist. Taken together, these data highlight the importance of intracellular mGluR5 in the cascade of events associated with sustained synaptic transmission.

Transcriptional regulation of neuronal polarity and morphogenesis in the mammalian brain

The highly specialized morphology of a neuron, typically consisting of a long axon and multiple branching dendrites, lies at the core of the principle of dynamic polarization, whereby information flows from dendrites toward the soma and to the axon. For more than a century, neuroscientists have been fascinated by how shape is important for neuronal function and how neurons acquire their characteristic morphology. During the past decade, substantial progress has been made in our understanding of the molecular underpinnings of neuronal polarity and morphogenesis. In these studies, transcription factors have emerged as key players governing multiple aspects of neuronal morphogenesis from neuronal polarization and migration to axon growth and pathfinding to dendrite growth and branching to synaptogenesis. In this review, we will highlight the role of transcription factors in shaping neuronal morphology with emphasis on recent literature in mammalian systems.

15N-labeled brain enables quantification of proteome and phosphoproteome in cultured primary neurons

Terminally differentiated primary cells represent a valuable in vitro model to study signaling events associated within a specific tissue. Quantitative proteomic methods using metabolic labeling in primary cells encounter labeling efficiency issues hindering the use of these cells. Here we developed a method to quantify the proteome and phosphoproteome of cultured neurons using (15)N-labeled brain tissue as an internal standard and applied this method to determine how an inhibitor of an excitatory neural transmitter receptor, phencyclidine (PCP), affects the global phosphoproteome of cortical neurons. We identified over 10,000 phosphopeptides and made accurate quantitative measurements of the neuronal phosphoproteome after neuronal inhibition. We show that short PCP treatments lead to changes in phosphorylation for 7% of neuronal phosphopeptides and that prolonged PCP treatment alters the total levels of several proteins essential for synaptic transmission and plasticity and leads to a massive reduction in the synaptic strength of inhibitory synapses. The results provide valuable insights into the dynamics of molecular networks implicated in PCP-mediated NMDA receptor inhibition and sensorimotor deficits.

Wnt signaling in amygdala-dependent learning and memory

In addition to its role in cellular development and proliferation, there are emerging in vitro data implicating the Wnt/β-catenin pathway in synaptic plasticity. Yet in vivo studies have not examined whether Wnt activity is required for learning and memory. In the amygdala during fear memory formation, we found that many Wnt-signaling genes were dynamically regulated, with an immediate decrease, followed by an eventual normalization during memory consolidation. This rapid decrease in Wnt mRNA was confirmed with individual quantitative PCR and in situ hybridization. We then manipulated Wnt signaling with a specific peptide antagonist (Dkk-1) or agonist (Wnt1) injected stereotaxically into the adult amygdala during fear learning. We found that neither manipulation had an effect on locomotion, anxiety, fear acquisition, or fear expression. However, both Wnt modulators prevented long-term fear memory consolidation without affecting short-term memory. Dkk-1 and Wnt infusions had destabilizing, but opposite, effects on the requisite β-catenin/cadherin dynamic interactions that occur during consolidation. These data suggest that dynamic modulation of Wnt/β-catenin signaling during consolidation is critical for the structural basis of long-term memory formation.

Raphe-mediated signals control the hippocampal response to SRI antidepressants via miR-16

Serotonin reuptake inhibitor (SRI) antidepressants such as fluoxetine (Prozac), promote hippocampal neurogenesis. They also increase the levels of the bcl-2 protein, whose overexpression in transgenic mice enhances adult hippocampal neurogenesis. However, the mechanisms underlying SRI-mediated neurogenesis are unclear. Recently, we identified the microRNA miR-16 as an important effector of SRI antidepressant action in serotonergic raphe and noradrenergic locus coeruleus (LC). We show here that miR-16 mediates adult neurogenesis in the mouse hippocampus. Fluoxetine, acting on serotonergic raphe neurons, decreases the amount of miR-16 in the hippocampus, which in turn increases the levels of the serotonin transporter (SERT), the target of SRI, and that of bcl-2 and the number of cells positive for Doublecortin, a marker of neuronal maturation. Neutralization of miR-16 in the hippocampus further exerts an antidepressant-like effect in behavioral tests. The fluoxetine-induced hippocampal response is relayed, in part, by the neurotrophic factor S100β, secreted by raphe and acting via the LC. Fluoxetine-exposed serotonergic neurons also secrete brain-derived neurotrophic factor, Wnt2 and 15-Deoxy-delta12,14-prostaglandin J2. These molecules are unable to mimic on their own the action of fluoxetine and we show that they act synergistically to regulate miR-16 at the hippocampus. Of note, these signaling molecules are increased in the cerebrospinal fluid of depressed patients upon fluoxetine treatment. Thus, our results demonstrate that miR-16 mediates the action of fluoxetine by acting as a micromanager of hippocampal neurogenesis. They further clarify the signals and the pathways involved in the hippocampal response to fluoxetine, which may help refine therapeutic strategies to alleviate depressive disorders.