Regulation of dendritogenesis via a lipid-raft-associated Ca2+/calmodulin-dependent protein kinase CLICK-III/CaMKIgamma

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.

GABA mediates the network activity-dependent facilitation of axonal outgrowth from the newborn granule cells in the early postnatal rat hippocampus

Neural network activity regulates the development of hippocampal newborn granule cells (GCs). Excitatory GABAergic input is known to be a key player in this regulation. Although calcium signaling is thought to be a downstream mediator of GABA, GABA-induced calcium signaling in newborn GCs is not well understood. We investigated Ca(2+) signaling and its regulatory role in axon and dendrite outgrowth in newborn GCs identified in the organotypic slice culture of early postnatal rat hippocampus. Here, we report that hippocampal network activity can induce calcium transients (CaTs) in newborn GCs during the first post-mitotic week via GABAergic inputs. The GABA-induced CaTs were mediated mainly by L-type Ca(2+) channels. Furthermore, we found that inhibiting any step in the signaling pathway, network activity → GABA → L-type Ca(2+) channels, selectively suppressed the axonal outgrowth and pruning of newborn GCs, but not dendritic outgrowth. The GABA(A) receptor blocker bicuculline significantly suppressed axonal outgrowth, despite increasing network activity, thus indicating an essential role of GABAergic inputs. Therefore, we conclude that network activity-dependent GABAergic inputs open L-type Ca(2+) channels and promote axonal outgrowth in newborn GC during the first post-mitotic week.

Temporal profiling of gene networks associated with the late phase of long-term potentiation in vivo

Long-term potentiation (LTP) is widely accepted as a cellular mechanism underlying memory processes. It is well established that LTP persistence is strongly dependent on activation of constitutive and inducible transcription factors, but there is limited information regarding the downstream gene networks and controlling elements that coalesce to stabilise LTP. To identify these gene networks, we used Affymetrix RAT230.2 microarrays to detect genes regulated 5 h and 24 h (n = 5) after LTP induction at perforant path synapses in the dentate gyrus of awake adult rats. The functional relationships of the differentially expressed genes were examined using DAVID and Ingenuity Pathway Analysis, and compared with our previous data derived 20 min post-LTP induction in vivo. This analysis showed that LTP-related genes are predominantly upregulated at 5 h but that there is pronounced downregulation of gene expression at 24 h after LTP induction. Analysis of the structure of the networks and canonical pathways predicted a regulation of calcium dynamics via G-protein coupled receptors, dendritogenesis and neurogenesis at the 5 h time-point. By 24 h neurotrophin-NFKB driven pathways of neuronal growth were identified. The temporal shift in gene expression appears to be mediated by regulation of protein synthesis, ubiquitination and time-dependent regulation of specific microRNA and histone deacetylase expression. Together this programme of genomic responses, marked by both homeostatic and growth pathways, is likely to be critical for the consolidation of LTP in vivo.

Inverse synaptic tagging of inactive synapses via dynamic interaction of Arc/Arg3.1 with CaMKIIβ

The Arc/Arg3.1 gene product is rapidly upregulated by strong synaptic activity and critically contributes to weakening synapses by promoting AMPA-R endocytosis. However, how activity-induced Arc is redistributed and determines the synapses to be weakened remains unclear. Here, we show targeting of Arc to inactive synapses via a high-affinity interaction with CaMKIIβ that is not bound to calmodulin. Synaptic Arc accumulates in inactive synapses that previously experienced strong activation and correlates with removal of surface GluA1 from individual synapses. A lack of CaMKIIβ either in vitro or in vivo resulted in loss of Arc upregulation in the silenced synapses. The discovery of Arc's role in "inverse" synaptic tagging that is specific for weaker synapses and prevents undesired enhancement of weak synapses in potentiated neurons reconciles essential roles of Arc both for the late phase of long-term plasticity and for reduction of surface AMPA-Rs in stimulated neurons.

Quantitative analysis of BDNF/TrkB protein and mRNA in cortical and striatal neurons using α-tubulin as a normalization factor

The neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase TrkB serve important regulatory roles for multiple aspects of the biology of neurons including cell death, survival, growth, differentiation, and plasticity. Regulation of the local availability of BDNF/TrkB at distinct subcellular domains such as soma, dendrites, axons, growth cones, nerve terminals, and spines appears to contribute to their specific functions. In view of the variance in size and shape of neurons and their compartments, previous quantitative studies of the BDNF/TrkB protein and mRNA lacked a robust normalization procedure. To overcome this problem, we have established methods that use immunofluorescence detection of α-tubulin as a normalization factor for the quantitative analysis of protein and mRNA in primary rat cortical and striatal neurons in culture. The efficacy of this approach is demonstrated by studying the dynamic distribution of proteins and mRNA at different growth stages or conditions. Treatment of cultured neurons with KCl resulted in increased levels of TrkB protein, reduced levels of BDNF mRNA (composite of multiple transcripts) and a slight reduction in BDNF protein levels in the dendrites from the cortex. The KCl treatment also lowered the percentage of BDNF and TrkB proteins in the soma indicative of protein transport. Finally, analysis of the rat cortical and striatal neurons demonstrated comparable or even higher levels of BDNF/TrkB protein and BDNF mRNA in the neurons from the striatum. Thus, in contrast to previous observations made in vivo, striatal neurons are capable of synthesizing BDNF mRNA when cultured in growth media in vitro. The analytical approach presented here provides a detailed understanding of BDNF/TrkB levels in response to a variety of neuronal activities. Our methods could be used broadly, including applications in cell and tissue cytometry, to yield accurate quantitative data of gene expression in cellular and subcellular contexts. © 2012 International Society for Advancement of Cytometry.

Deconstructing signal transduction pathways that regulate the actin cytoskeleton in dendritic spines

Dendritic spines are the sites of most excitatory synapses in the central nervous system. Recent studies have shown that spines function independently of each other, and they are currently the smallest known processing units in the brain. Spines exist in an array of morphologies, and spine structure helps dictate synaptic function. Dendritic spines are rich in actin, and actin rearrangements are critical regulators of spine morphology and density. In this review, we discuss the importance of actin in regulating dendritic spine morphogenesis, and discuss the upstream signal transduction pathways that either foster or inhibit actin polymerization. The understanding of actin regulatory pathways is best conceptualized as a hierarchical network in which molecules function in discrete levels defined by their molecular distance to actin. To this end, we focus on several classes of molecules, including guanine nucleotide exchange factors, small GTPases, small GTPase effectors, and actin binding proteins. We discuss how individual proteins in these molecular classes impact spine morphogenesis, and reveal the biochemical interactions in these networks that are responsible for shaping actin polymerization. Finally, we discuss the importance of these actin regulatory pathways in neuropsychiatric disorders.

Small GTPase Rab17 regulates dendritic morphogenesis and postsynaptic development of hippocampal neurons

Neurons are compartmentalized into two morphologically, molecularly, and functionally distinct domains: axons and dendrites, and precise targeting and localization of proteins within these domains are critical for proper neuronal functions. It has been reported that several members of the Rab family small GTPases that are key mediators of membrane trafficking, regulate axon-specific trafficking events, but little has been elucidated regarding the molecular mechanisms that underlie dendrite-specific membrane trafficking. Here we show that Rab17 regulates dendritic morphogenesis and postsynaptic development in mouse hippocampal neurons. Rab17 is localized at dendritic growth cones, shafts, filopodia, and mature spines, but it is mostly absent in axons. We also found that Rab17 mediates dendrite growth and branching and that it does not regulate axon growth or branching. Moreover, shRNA-mediated knockdown of Rab17 expression resulted in a dramatically reduced number of dendritic spines, probably because of impaired filopodia formation. These findings have revealed the first molecular link between membrane trafficking and dendritogenesis.

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.

Neurotrophin-mediated degradation of histone methyltransferase by S-nitrosylation cascade regulates neuronal differentiation

Epigenetic regulation of histones mediates neurotrophin actions with histone acetylation enhancing cAMP response element-binding (CREB)-associated transcription elicited by brain-derived neurotrophic factor (BDNF) and nerve-growth factor (NGF). Roles for histone methylation in CREB's transcriptional activity have not been well characterized. We show that depletion of the histone methyltransferase suppressor of variegation 3-9 homolog 1 (SUV39H1) selectively augments BDNF- and NGF-mediated neurite outgrowth. SUV39H1 is the principal enzyme responsible for trimethylation of histone H3 at lysine 9, a molecular mark associated with transcriptional silencing. BDNF and NGF act via a signaling cascade wherein degradation of SUV39H1 down-regulates trimethylation of H3K9 in a nitric oxide-dependent pathway. BDNF activates neuronal NOS with the nitrosylated GAPDH/seven in absentia (Siah) homolog complex translocating to the nucleus. Degradation of SUV39H1 by Siah facilitates histone H3 on lysine 9 acetylation, CREB binding to DNA, enhanced expression of CREB-regulated genes and neurite outgrowth.

Ca2+/Calmodulin-dependent protein kinase kinase beta is regulated by multisite phosphorylation

Ca(2+)/calmodulin-dependent protein kinase kinase β (CaMKKβ) is a serine/threonine-directed kinase that is activated following increases in intracellular Ca(2+). CaMKKβ activates Ca(2+)/calmodulin-dependent protein kinase I, Ca(2+)/calmodulin-dependent protein kinase IV, and the AMP-dependent protein kinase in a number of physiological pathways, including learning and memory formation, neuronal differentiation, and regulation of energy balance. Here, we report the novel regulation of CaMKKβ activity by multisite phosphorylation. We identify three phosphorylation sites in the N terminus of CaMKKβ, which regulate its Ca(2+)/calmodulin-independent autonomous activity. We then identify the kinases responsible for these phosphorylations as cyclin-dependent kinase 5 (CDK5) and glycogen synthase kinase 3 (GSK3). In addition to regulation of autonomous activity, we find that phosphorylation of CaMKKβ regulates its half-life. We find that cellular levels of CaMKKβ correlate with CDK5 activity and are regulated developmentally in neurons. Finally, we demonstrate that appropriate phosphorylation of CaMKKβ is critical for its role in neurite development. These results reveal a novel regulatory mechanism for CaMKKβ-dependent signaling cascades.

A deficiency of ceramide biosynthesis causes cerebellar purkinje cell neurodegeneration and lipofuscin accumulation

Sphingolipids, lipids with a common sphingoid base (also termed long chain base) backbone, play essential cellular structural and signaling functions. Alterations of sphingolipid levels have been implicated in many diseases, including neurodegenerative disorders. However, it remains largely unclear whether sphingolipid changes in these diseases are pathological events or homeostatic responses. Furthermore, how changes in sphingolipid homeostasis shape the progression of aging and neurodegeneration remains to be clarified. We identified two mouse strains, flincher (fln) and toppler (to), with spontaneous recessive mutations that cause cerebellar ataxia and Purkinje cell degeneration. Positional cloning demonstrated that these mutations reside in the Lass1 gene. Lass1 encodes (dihydro)ceramide synthase 1 (CerS1), which is highly expressed in neurons. Both fln and to mutations caused complete loss of CerS1 catalytic activity, which resulted in a reduction in sphingolipid biosynthesis in the brain and dramatic changes in steady-state levels of sphingolipids and sphingoid bases. In addition to Purkinje cell death, deficiency of CerS1 function also induced accumulation of lipofuscin with ubiquitylated proteins in many brain regions. Our results demonstrate clearly that ceramide biosynthesis deficiency can cause neurodegeneration and suggest a novel mechanism of lipofuscin formation, a common phenomenon that occurs during normal aging and in some neurodegenerative diseases.

Targeting ligand-operated chaperone sigma-1 receptors in the treatment of neuropsychiatric disorders

INTRODUCTION

Current drugs for the treatment of psychiatric or neurodegenerative disorders have limitations. Psychotherapeutic drugs such as typical and atypical antipsychotics, tricyclic antidepressants and selective monoamine reuptake inhibitors, aim to normalize the hyper- or hypo-neurotransmission of monoaminergic systems. Despite their contribution to the outcomes of psychiatric patients, these agents often exert severe side effects and require chronic treatments to promote amelioration of symptoms. Drugs available for the treatment of neurodegenerative disorders are severely limited.

AREAS COVERED

Recent evidence that has shed light on sigma-1 receptor ligands, which may serve as a new class of antidepressants or neuroprotective agents. Sigma-1 receptors are novel ligand-operated molecular chaperones regulating signal transduction, ER stress, cellular redox, cellular survival and synaptogenesis. Selective sigma-1 receptor ligands exert rapid antidepressant-like, anxiolytic, antinociceptive and robust neuroprotective actions in preclinical studies. Recent studies that suggest that reactive oxygen species might play a role as signal integrators downstream of Sig-1Rs are also covered.

EXPERT OPINION

The advances in sigma receptor research in the last decade have begun to elucidate the intracellular signal cascades upstream and downstream of sigma-1 receptors. The novel ligand-operated properties of the sigma-1 receptor chaperone may enable interventions by which stress-related cellular systems can be pharmacologically controlled.

Making memories last: the synaptic tagging and capture hypothesis

The synaptic tagging and capture hypothesis of protein synthesis-dependent long-term potentiation asserts that the induction of synaptic potentiation creates only the potential for a lasting change in synaptic efficacy, but not the commitment to such a change. Other neural activity, before or after induction, can also determine whether persistent change occurs. Recent findings, leading us to revise the original hypothesis, indicate that the induction of a local, synapse-specific 'tagged' state and the expression of long-term potentiation are dissociable. Additional observations suggest that there are major differences in the mechanisms of functional and structural plasticity. These advances call for a revised theory that incorporates the specific molecular and structural processes involved. Addressing the physiological relevance of previous in vitro findings, new behavioural studies have experimentally translated the hypothesis to learning and the consolidation of newly formed memories.

Single-cell optogenetic excitation drives homeostatic synaptic depression

Homeostatic processes have been proposed to explain the discrepancy between the dynamics of synaptic plasticity and the stability of brain function. Forms of synaptic plasticity such as long-term potentiation alter synaptic activity in a synapse- and cell-specific fashion. Although network-wide excitation triggers compensatory homeostatic changes, it is unknown whether neurons initiate homeostatic synaptic changes in response to cell-autonomous increases in excitation. Here we employ optogenetic tools to cell-autonomously excite CA1 pyramidal neurons and find that a compensatory postsynaptic depression of both AMPAR and NMDAR function results. Elevated calcium influx through L-type calcium channels leads to activation of a pathway involving CaM kinase kinase and CaM kinase 4 that induces synaptic depression of AMPAR and NMDAR responses. The synaptic depression of AMPARs but not of NMDARs requires protein synthesis and the GluA2 AMPAR subunit, indicating that downstream of CaM kinase activation divergent pathways regulate homeostatic AMPAR and NMDAR depression.

Calmodulin-kinases regulate basal and estrogen stimulated medulloblastoma migration via Rac1

Medulloblastoma is a highly prevalent pediatric central nervous system malignancy originating in the cerebellum, with a strong propensity for metastatic migration to the leptomeninges, which greatly increases mortality. While numerous investigations are focused on the molecular mechanisms of medulloblastoma histogenesis, the signaling pathways regulating migration are still poorly understood. Medulloblastoma likely arises from aberrant proliferative signaling in cerebellar granule precursor cells during development, and estrogen is a morphogen that promotes medulloblastoma cell migration. It has been previously shown that the calcium/calmodulin activated kinase kinase (CaMKK) pathway promotes cerebellar granule precursor migration and differentiation during normal cerebellar development via CaMKIV. Here we investigate the regulatory role of the CaMKK pathway in migration of the human medulloblastoma DAOY and cerebellar granule cells. Using pharmacological inhibitors and dominant negative approaches, we demonstrate that the CaMKK/CaMKI cascade regulates basal medulloblastoma cell migration via Rac1, in part by activation of the RacGEF, βPIX. Additionally, pharmacological inhibition of CaMKK blocks both the estrogen induced Rac1 activation and medulloblastoma migration. The CaMKK signaling module described here is one of the first reported calcium regulated pathways that modulates medulloblastoma migration. Since tumor dissemination requires cell migration to ectopic sites, this CaMKK pathway may be a putative therapeutic target to limit medulloblastoma metastasis.

Induced loss of ADAR2 engenders slow death of motor neurons from Q/R site-unedited GluR2

GluR2 is a subunit of the AMPA receptor, and the adenosine for the Q/R site of its pre-mRNA is converted to inosine (A-to-I conversion) by the enzyme called adenosine deaminase acting on RNA 2 (ADAR2). Failure of A-to-I conversion at this site affects multiple AMPA receptor properties, including the Ca(2+) permeability of the receptor-coupled ion channel, thereby inducing fatal epilepsy in mice (Brusa et al., 1995; Feldmeyer et al., 1999). In addition, inefficient GluR2 Q/R site editing is a disease-specific molecular dysfunction found in the motor neurons of sporadic amyotrophic lateral sclerosis (ALS) patients (Kawahara et al., 2004). Here, we generated genetically modified mice (designated as AR2) in which the ADAR2 gene was conditionally targeted in motor neurons using the Cre/loxP system. These AR2 mice showed a decline in motor function commensurate with the slow death of ADAR2-deficient motor neurons in the spinal cord and cranial motor nerve nuclei. Notably, neurons in nuclei of oculomotor nerves, which often escape degeneration in ALS, were not decreased in number despite a significant decrease in GluR2 Q/R site editing. All cellular and phenotypic changes in AR2 mice were prevented when the mice carried endogenous GluR2 alleles engineered to express edited GluR2 without ADAR2 activity (Higuchi et al., 2000). Thus, loss of ADAR2 activity causes AMPA receptor-mediated death of motor neurons.

Directing traffic in neural cells: determinants of receptor tyrosine kinase localization and cellular responses

The trafficking of receptor tyrosine kinases (RTKs) to distinct subcellular locations is essential for the specificity and fidelity of signal transduction and biological responses. This is particularly important in the PNS and CNS in which RTKs mediate key events in the development and maintenance of neurons and glia through a wide range of neural processes, including survival, proliferation, differentiation, neurite outgrowth, and synaptogenesis. The mechanisms that regulate the targeting of RTKs to their subcellular destinations for appropriate signal transduction, however, are still elusive. In this review, we discuss evidence for the spatial organization of signaling machinery into distinct subcellular compartments, as well as the role for ligand specificity, receptor sorting signals, and lipid raft microdomains in RTK targeting and the resultant cellular responses in neural cells.

Geranylgeranyltransferase I is essential for dendritic development of cerebellar Purkinje cells

Background

During cerebellar development, Purkinje cells (PCs) form the most elaborate dendritic trees among neurons in the brain, but the mechanism regulating PC arborization remains largely unknown. Geranylgeranyltransferase I (GGT) is a prenyltransferase that is responsible for lipid modification of several signaling proteins, such as Rho family small GTPase Rac1, which has been shown to be involved in neuronal morphogenesis. Here we show that GGT plays an important role in dendritic development of PCs.

Results

We found that GGT was abundantly expressed in the developing rat cerebellum, in particular molecular layer (ML), the region enriched with PC dendrites. Inhibition or down-regulation of GGT using small interference RNA (siRNA) inhibited dendritic development of PCs. In contrast, up-regulation of GGT promoted dendritic arborization of PCs. Furthermore, neuronal depolarization induced by high K⁺ or treatment with brain-derived neurotrophic factor (BDNF) promoted membrane association of Rac1 and dendritic development of PCs in cultured cerebellar slices. The effect of BDNF or high K⁺ was inhibited by inhibition or down-regulation of GGT.

Conclusion

Our results indicate that GGT plays an important role in Purkinje cell development, and suggest a novel role of GGT in neuronal morphogenesis in vivo.

Bcl11A/CTIP1 mediates the effect of the glutamate receptor on axon branching and dendrite outgrowth

While neuronal activity regulates neurite outgrowth and branching, the details of the underlying mechanisms are still largely unclear. This study investigated the effect of the glutamate receptors on neuritogenesis using cultured hippocampal neurons. At 3-4 days in vitro, NMDA treatment promoted axon branching but not primary axon extension. In contrast, blockade of the NMDA receptor (NMDAR) by AP5 treatment enhanced primary axon extension. NMDAR activation also increased dendrite number and total dendrite length. These results suggest that NMDAR controls axon and dendrite morphogenesis. A previous study demonstrated that knockdown of the zinc finger transcription factor B cell lymphoma 11A-long (Bcl11A-L) reduces deleted in colorectal cancer (DCC) and microtubule-associated protein (MAP) 1b expression, thereby promoting axon branching. Here, glutamate stimulation down-regulated the levels of the Bcl11A-L, DCC, MAP1b, and MAP2c proteins. Over-expression of either Bcl11A-L or DCC countered the effect of NMDA or glutamate on axon branching and dendrite outgrowth, indicating that the Bcl11A-L/DCC pathway is an important downstream effector of glutamate receptors in neurite arborization. Because knockdown of Bcl11A-L did not down-regulate MAP2c, our results suggest that glutamate receptors also use a Bcl11A-L-independent pathway to control dendrite outgrowth. To summarize, this study reveals novel pathways downstream of glutamate receptors that regulate axon and dendrite arborization.

GABAergic control of neurite outgrowth and remodeling during development and adult neurogenesis: general rules and differences in diverse systems

During development, Gamma-aminobutyric acidergic (GABAergic) neurons mature at early stages, long before excitatory neurons. Conversely, GABA reuptake transporters become operative later than glutamate transporters. GABA is therefore not removed efficiently from the extracellular domain and it can exert significant paracrine effects. Hence, GABA-mediated activity is a prominent source of overall neural activity in developing CNS networks, while neurons extend dendrites and axons, and establish synaptic connections. One of the unique features of GABAergic functional plasticity is that in early development, activation of GABA_A receptors results in depolarizing (mainly excitatory) responses and Ca²⁺ influx. Although there is strong evidence from several areas of the CNS that GABA plays a significant role in neurite growth not only during development but also during adult neurogenesis, surprisingly little effort has been made into putting all these observations into a common framework in an attempt to understand the general rules that regulate these basic and evolutionary well-conserved processes. In this review, we discuss the current knowledge in this important field. In order to decipher common, universal features and highlight differences between systems throughout development, we compare findings about dendritic proliferation and remodeling in different areas of the nervous system and species, and we also review recent evidence for a role in axonal elongation. In addition to early developmental aspects, we also consider the GABAergic role in dendritic growth during adult neurogenesis, extending our discussion to the roles played by GABA during dendritic proliferation in early developing networks versus adult, well established networks.

Regulation of Rap2A by the ubiquitin ligase Nedd4-1 controls neurite development

Nedd4-1 is a "neuronal precursor cell expressed and developmentally downregulated protein" and among the most abundant E3 ubiquitin ligases in mammalian neurons. In analyses of conventional and conditional Nedd4-1-deficient mice, we found that Nedd4-1 plays a critical role in dendrite formation. Nedd4-1, the serine/threonine kinase TNIK, and Rap2A form a complex that controls Nedd4-1-mediated ubiquitination of Rap2A. Ubiquitination by Nedd4-1 inhibits Rap2A function, which reduces the activity of Rap2 effector kinases of the TNIK family and promotes dendrite growth. We conclude that a Nedd4-1/Rap2A/TNIK signaling pathway regulates neurite growth and arborization in mammalian neurons.

Protein palmitoylation in neuronal development and synaptic plasticity

Protein palmitoylation, a classical and common lipid modification, regulates diverse aspects of neuronal protein trafficking and function. The reversible nature of palmitoylation provides a potential general mechanism for protein shuttling between intracellular compartments. The recent discovery of palmitoylating enzymes--a large DHHC (Asp-His-His-Cys) protein family--and the development of new proteomic and imaging methods have accelerated palmitoylation analysis. It is becoming clear that individual DHHC enzymes generate and maintain the specialized compartmentalization of substrates in polarized neurons. Here, we discuss the regulatory mechanisms for dynamic protein palmitoylation and the emerging roles of protein palmitoylation in various aspects of pathophysiology, including neuronal development and synaptic plasticity.

Splice variant specific increase in Ca2+/calmodulin-dependent protein kinase 1-gamma mRNA expression in response to acute pyrethroid exposure

In mammals, pyrethroids are neurotoxicants that interfere with ion channel function in excitable neuronal membranes. Previous work demonstrated increases in the expression of Ca(2+)/calmodulin-dependent protein kinase 1-gamma (Camk1g) mRNA following acute deltamethrin and permethrin exposure. In the rat, this gene is expressed as two distinct splice variants, Camk1g1 and Camk1g2. The present study tests the hypothesis that changes in Camk1g mRNA expression in the rat following acute pyrethroid exposure are due to a specific increase in the Camk1g1 splice variant and not the Camk1g2 splice variant. Long-Evans rats were acutely exposed to permethrin, deltamethrin, or corn oil vehicle. Frontal cortex was collected at 6 h postdosing. In addition, rats were exposed to permethrin (100 mg/kg) or deltamethrin (3 mg/kg), and frontal cortex was collected at 1, 3, 6, 9, 12, or 24 h along with time-matched vehicle controls. Expression of Camk1g1 and Camk1g2 mRNA was measured by quantitative real-time RT-PCR and quantified using the 2(-Delta Delta C)T method. Dose-dependent increases in Camk1g1 mRNA expression were observed for both pyrethroids at 6 h. In addition, a dose-dependent increase in Camk1g2 was observed at 6 h although it was very small in magnitude. The increases in Camk1g1 expression for deltamethrin and permethrin peak between 3 and 6 h postexposure and returns to control levels by 9 h. There was no increase in CAMK1G1 protein as measured with Western blots. The present data demonstrate that pyrethroid-induced changes in Camk1g are driven mainly by increased expression of the Camk1g1 splice variant.

Control of cortical axon elongation by a GABA-driven Ca2+/calmodulin-dependent protein kinase cascade

Ca(2+) signaling plays important roles during both axonal and dendritic growth. Yet whether and how Ca(2+) rises may trigger and contribute to the development of long-range cortical connections remains mostly unknown. Here, we demonstrate that two separate limbs of the Ca(2+)/calmodulin-dependent protein kinase kinase (CaMKK)-CaMKI cascades, CaMKK-CaMKIalpha and CaMKK-CaMKIgamma, critically coordinate axonal and dendritic morphogenesis of cortical neurons, respectively. The axon-specific morphological phenotype required a diffuse cytoplasmic localization and a strikingly alpha-isoform-specific kinase activity of CaMKI. Unexpectedly, treatment with muscimol, a GABA(A) receptor agonist, selectively stimulated elongation of axons but not of dendrites, and the CaMKK-CaMKIalpha cascade critically mediated this axonogenic effect. Consistent with these findings, during early brain development, in vivo knockdown of CaMKIalpha significantly impaired the terminal axonal extension and thereby perturbed the refinement of the interhemispheric callosal projections into the contralateral cortices. Our findings thus indicate a novel role for the GABA-driven CaMKK-CaMKIalpha cascade as a mechanism critical for accurate cortical axon pathfinding, an essential process that may contribute to fine-tuning the formation of interhemispheric connectivity during the perinatal development of the CNS.

Transient receptor potential canonical 5 channels activate Ca2+/calmodulin kinase Igamma to promote axon formation in hippocampal neurons

Functionality of neurons is dependent on their compartmentalized polarization of dendrites and an axon. The rapid and selective outgrowth of one neurite, relative to the others, to form the axon is critical in initiating neuronal polarity. Axonogenesis is regulated in part by an optimal intracellular calcium concentration. Our investigation of Ca(2+)-signaling pathways involved in axon formation using cultured hippocampal neurons demonstrates a role for Ca(2+)/calmodulin kinase kinase (CaMKK) and its downstream target Ca(2+)/calmodulin kinase I (CaMKI). Expression of constitutively active CaMKI induced formation of multiple axons, whereas blocking CaMKK or CaMKI activity with pharmacological, dominant-negative, or short hairpin RNA (shRNA) methods significantly inhibited axon formation. CaMKK signals via the gamma-isoform of CaMKI as shRNA to CaMKIgamma, but not the other CaMKI isoforms, inhibited axon formation. Furthermore, overexpression of wild-type CaMKIgamma, but not a mutant incapable of membrane association, accelerated the rate of axon formation. Pharmacological or small interfering RNA inhibition of transient receptor potential canonical 5 (TRPC5) channels, which are present in developing axonal growth cones, suppressed CaMKK-mediated activation of CaMKIgamma as well as axon formation. We demonstrate using biochemical fractionation and immunocytochemistry that CaMKIgamma and TRPC5 colocalize to lipid rafts. These results are consistent with a model in which highly localized calcium influx through the TRPC5 channels activates CaMKK and CaMKIgamma, which subsequently promote axon formation.

Ca2+/calmodulin-dependent protein kinase IV-mediated LIM kinase activation is critical for calcium signal-induced neurite outgrowth

Actin cytoskeletal remodeling is essential for neurite outgrowth. LIM kinase 1 (LIMK1) regulates actin cytoskeletal remodeling by phosphorylating and inactivating cofilin, an actin filament-disassembling factor. In this study, we investigated the role of LIMK1 in calcium signal-induced neurite outgrowth. The calcium ionophore ionomycin induced LIMK1 activation and cofilin phosphorylation in Neuro-2a neuroblastoma cells. Knockdown of LIMK1 or expression of a kinase-dead mutant of LIMK1 suppressed ionomycin-induced cofilin phosphorylation and neurite outgrowth in Neuro-2a cells. Ionomycin-induced cofilin phosphorylation and neurite outgrowth were also blocked by KN-93, an inhibitor of Ca(2+)/calmodulin-dependent protein kinases (CaMKs), and STO-609, an inhibitor of CaMK kinase. An active form of CaMKIV but not CaMKI enhanced Thr-508 phosphorylation of LIMK1 and increased the kinase activity of LIMK1. Moreover, the active form of CaMKIV induced cofilin phosphorylation and neurite outgrowth, and a dominant negative form of CaMKIV suppressed ionomycin-induced neurite outgrowth. Taken together, our results suggest that LIMK1-mediated cofilin phosphorylation is critical for ionomycin-induced neurite outgrowth and that CaMKIV mediates ionomycin-induced LIMK1 activation.

Protein kinase D controls the integrity of Golgi apparatus and the maintenance of dendritic arborization in hippocampal neurons

Protein kinase D (PKD) is known to participate in various cellular functions, including secretory vesicle fission from the Golgi and plasma membrane-directed transport. Here, we report on expression and function of PKD in hippocampal neurons. Expression of an enhanced green fluorescent protein (EGFP)-tagged PKD activity reporter in mouse embryonal hippocampal neurons revealed high endogenous PKD activity at the Golgi complex and in the dendrites, whereas PKD activity was excluded from the axon in parallel with axonal maturation. Expression of fluorescently tagged wild-type PKD1 and constitutively active PKD1(S738/742E) (caPKD1) in neurons revealed that both proteins were slightly enriched at the trans-Golgi network (TGN) and did not interfere with its thread-like morphology. By contrast, expression of dominant-negative kinase inactive PKD1(K612W) (kdPKD1) led to the disruption of the neuronal Golgi complex, with kdPKD1 strongly localized to the TGN fragments. Similar findings were obtained from transgenic mice with inducible, neuron-specific expression of kdPKD1-EGFP. As a prominent consequence of kdPKD1 expression, the dendritic tree of transfected neurons was reduced, whereas caPKD1 increased dendritic arborization. Our results thus provide direct evidence that PKD activity is selectively involved in the maintenance of dendritic arborization and Golgi structure of hippocampal neurons.

Synaptic activity-responsive element in the Arc/Arg3.1 promoter essential for synapse-to-nucleus signaling in activated neurons

The neuronal immediate early gene Arc/Arg-3.1 is widely used as one of the most reliable molecular markers for intense synaptic activity in vivo. However, the cis-acting elements responsible for such stringent activity dependence have not been firmly identified. Here we combined luciferase reporter assays in cultured cortical neurons and comparative genome mapping to identify the critical synaptic activity-responsive elements (SARE) of the Arc/Arg-3.1 gene. A major SARE was found as a unique approximately 100-bp element located at >5 kb upstream of the Arc/Arg-3.1 transcription initiation site in the mouse genome. This single element, when positioned immediately upstream of a minimal promoter, was necessary and sufficient to replicate crucial properties of endogenous Arc/Arg-3.1's transcriptional regulation, including rapid onset of transcription triggered by synaptic activity and low basal expression during synaptic inactivity. We identified the major determinants of SARE as a unique cluster of neuronal activity-dependent cis-regulatory elements consisting of closely localized binding sites for CREB, MEF2, and SRF. Consistently, a SARE reporter could readily trace and mark an ensemble of cells that have experienced intense activity in the recent past in vivo. Taken together, our work uncovers a novel transcriptional mechanism by which a critical 100-bp element, SARE, mediates a predominant component of the synapse-to-nucleus signaling in ensembles of Arc/Arg-3.1-positive activated neurons.

S-palmitoylation of gamma-secretase subunits nicastrin and APH-1

Proteolytic processing of amyloid precursor protein (APP) by beta- and gamma-secretases generates beta-amyloid (Abeta) peptides, which accumulate in the brains of individuals affected by Alzheimer disease. Detergent-resistant membrane microdomains (DRM) rich in cholesterol and sphingolipid, termed lipid rafts, have been implicated in Abeta production. Previously, we and others reported that the four integral subunits of the gamma-secretase associate with DRM. In this study we investigated the mechanisms underlying DRM association of gamma-secretase subunits. We report that in cultured cells and in brain the gamma-secretase subunits nicastrin and APH-1 undergo S-palmitoylation, the post-translational covalent attachment of the long chain fatty acid palmitate common in lipid raft-associated proteins. By mutagenesis we show that nicastrin is S-palmitoylated at Cys(689), and APH-1 is S-palmitoylated at Cys(182) and Cys(245). S-Palmitoylation-defective nicastrin and APH-1 form stable gamma-secretase complexes when expressed in knock-out fibroblasts lacking wild type subunits, suggesting that S-palmitoylation is not essential for gamma-secretase assembly. Nevertheless, fractionation studies show that S-palmitoylation contributes to DRM association of nicastrin and APH-1. Moreover, pulse-chase analyses reveal that S-palmitoylation is important for nascent polypeptide stability of both proteins. Co-expression of S-palmitoylation-deficient nicastrin and APH-1 in cultured cells neither affects Abeta40, Abeta42, and AICD production, nor intramembrane processing of Notch and N-cadherin. Our findings suggest that S-palmitoylation plays a role in stability and raft localization of nicastrin and APH-1, but does not directly modulate gamma-secretase processing of APP and other substrates.

Transcriptional response of rat frontal cortex following acute in vivo exposure to the pyrethroid insecticides permethrin and deltamethrin

Background

Pyrethroids are neurotoxic pesticides that interact with membrane bound ion channels in neurons and disrupt nerve function. The purpose of this study was to characterize and explore changes in gene expression that occur in the rat frontal cortex, an area of CNS affected by pyrethroids, following an acute low-dose exposure.

Results

Rats were acutely exposed to either deltamethrin (0.3 – 3 mg/kg) or permethrin (1 – 100 mg/kg) followed by collection of cortical tissue at 6 hours. The doses used range from those that cause minimal signs of intoxication at the behavioral level to doses well below apparent no effect levels in the whole animal. A statistical framework based on parallel linear (SAM) and isotonic regression (PIR) methods identified 95 and 53 probe sets as dose-responsive. The PIR analysis was most sensitive for detecting transcripts with changes in expression at the NOAEL dose. A sub-set of genes (Camk1g, Ddc, Gpd3, c-fos and Egr1) was then confirmed by qRT-PCR and examined in a time course study. Changes in mRNA levels were typically less than 3-fold in magnitude across all components of the study. The responses observed are consistent with pyrethroids producing increased neuronal excitation in the cortex following a low-dose in vivo exposure. In addition, Significance Analysis of Function and Expression (SAFE) identified significantly enriched gene categories common for both pyrethroids, including some relating to branching morphogenesis. Exposure of primary cortical cell cultures to both compounds resulted in an increase (~25%) in the number of neurite branch points, supporting the results of the SAFE analysis.

Conclusion

In the present study, pyrethroids induced changes in gene expression in the frontal cortex near the threshold for decreases in ambulatory motor activity in vivo. The penalized regression methods performed similarly in detecting dose-dependent changes in gene transcription. Finally, SAFE analysis of gene expression data identified branching morphogenesis as a biological process sensitive to pyrethroids and subsequent in vitro experiments confirmed this predicted effect. The novel findings regarding pyrethroid effects on branching morphogenesis indicate these compounds may act as developmental neurotoxicants that affect normal neuronal morphology.

Calmodulin-kinases: modulators of neuronal development and plasticity

In the nervous system, many intracellular responses to elevated calcium are mediated by CaM kinases (CaMKs), a family of protein kinases whose activities are initially modulated by binding Ca(2+)/calmodulin and subsequently by protein phosphorylation. One member of this family, CaMKII, is well-established for its effects on modulating synaptic plasticity and learning and memory. However, recent studies indicate that some actions on neuronal development and function attributed to CaMKII may instead or in addition be mediated by other members of the CaMK cascade, such as CaMKK, CaMKI, and CaMKIV. This review summarizes key neuronal functions of the CaMK cascade in signal transduction, gene transcription, synaptic development and plasticity, and behavior. The technical challenges of mapping cellular protein kinase signaling pathways are also discussed.

TrkB-mediated activation of geranylgeranyltransferase I promotes dendritic morphogenesis

Dendrite morphogenesis is regulated by neuronal activity or neurotrophins, which may function by activating intrinsic signaling proteins, including Rho family GTPases. Here we report that activity- and brain-derived neurotrophic factor (BDNF)-dependent dendritic morphogenesis requires activation of geranylgeranyltransferase I (GGT), a prenyltransferase that mediates lipid modification of Rho GTPases. Dendritic arborization in cultured hippocampal neurons was promoted by over-expression of GGT, and reduced by inhibition or down-regulation of GGT. Furthermore, GGT was activated by neuronal depolarization or BDNF, both of which promote dendritic arborization, in cultured hippocampal neurons. Moreover, exploration of a novel environment caused activation of GGT in the mice hippocampus, suggesting that neural activity activates GGT in vivo. Interestingly, GGT was physically associated with tropomyosin-related kinase B (TrkB), the receptor for BDNF, and this association was enhanced by depolarization. Disrupting the GGT-TrkB interaction or down-regulating GGT activity attenuated depolarization- or BDNF-induced dendrite development. Finally, the GGT effect on dendrite arborization was prevented by over-expressing Rac1 with the prenylation site deleted or mutated. Thus depolarization- or BDNF-dependent dendrite development may be mediated by GGT-induced prenylation of Rho GTPases.

Fibroblast growth factor-regulated palmitoylation of the neural cell adhesion molecule determines neuronal morphogenesis

During development of the nervous system, short- and long-range signals cooperate to promote axonal growth, guidance, and target innervation. Particularly, a short-range signal transducer, the neural cell adhesion molecule (NCAM), stimulates neurite outgrowth via mechanisms that require posttranslational modification of NCAM and signaling via receptors to a long-range messenger, the fibroblast growth factor (FGF). In the present study we further characterized a mechanism which regulates the functional interplay between NCAM and FGF receptor(s). We show that activation of FGF receptor(s) by FGF2 leads to palmitoylation of the two major transmembrane NCAM isoforms, NCAM140 and NCAM180, translocation of NCAM to GM1 ganglioside-containing lipid rafts, and stimulation of neurite outgrowth of hippocampal neurons. Ablation of NCAM, mutation of NCAM140 or NCAM180 palmitoylation sites, or pharmacological suppression of NCAM signaling inhibited FGF2-stimulated neurite outgrowth. Of the 23 members of the aspartate-histidine-histidine-cysteine (DHHC) domain containing proteins, DHHC-7 most strongly stimulated palmitoylation of NCAM, and enzyme activity was enhanced by FGF2. Thus, our study uncovers a molecular mechanism by which a growth factor regulates neuronal morphogenesis via activation of palmitoylation, which in turn modifies subcellular location and thus signaling via an adhesion molecule.

Synthetic activation of endogenous PI3K and Rac identifies an AND-gate switch for cell polarization and migration

Phosphatidylinositol 3-OH kinase (PI3K) has been widely studied as a principal regulator of cell polarization, migration, and chemotaxis [1], [2], [3], [4]. Surprisingly, recent studies showed that mammalian neutrophils and Dictyostelium discoideum cells can polarize and migrate in the absence of PI3K activity [5], [6], [7]. Here we directly probe the roles of PI3K and its downstream effector, Rac, in HL-60 neutrophils by using a chemical biology approach whereby the endogenously present enzymes are synthetically activated in less than one minute [8], [9], [10]. We show that uniform activation of endogenous PI3K is sufficient to polarize previously unpolarized neutrophils and trigger effective cell migration. After a delay following symmetrical phosphatidylinositol (3,4,5)-triphosphate (PIP₃) production, a polarized distribution of PIP₃ was induced by positive feedback requiring actin polymerization. Pharmacological studies argue that this process does not require receptor-coupled trimeric G proteins. Contrary to the current working model, rapid activation of endogenous Rac proteins triggered effective actin polymerization but failed to feed back to PI3K to generate PIP₃ or induce cell polarization. Thus, the increase in PIP₃ concentration at the leading edge is generated by positive feedback with an AND gate logic with a PI3K-Rac-actin polymerization pathway as a first input and a PI3K initiated non-Rac pathway as a second input. This AND-gate control for cell polarization can explain how Rac can be employed for both PI3K-dependent and -independent signaling pathways coexisting in the same cell.

Characterization of Plasmodium falciparum protein kinase 2

A sustained elevation of free Ca(2+) is observed on the rupture and release of merozoites of Plasmodium falciparum from the erythrocytes. The immunoelectron micrographs demonstrate that calmodulin is localized in merozoites. To elucidate the Ca(2+) signal of P. falciparum invasion, we attempted to characterize P. falciparum protein kinase 2 (PfPK2), which is homologous to human calcium calmodulin-dependent protein kinase (CaMK). PfPK2 was purified as a fusion protein that was labeled with [gamma-(32)P]ATP; this labeling was then eliminated by phosphatase. This phosphorylation was eliminated when the putative catalytic lysine residue of PfPK2 was replaced with alanine. PfPK2 phosphorylated histone II(AS) as a representative substrate in a Ca(2+)- and calmodulin-dependent manner. Calmodulin antagonists inhibited the phosphorylation of PfPK2 in vitro and markedly decreased the parasitemia of ring forms in an invasion assay, whereas CaMKII-specific inhibitors had no effect. PfPK2 was localized in the merozoites in the culture of P. falciparum. Thus, purified PfPK2 possesses protein kinase activity in a Ca(2+)- and calmodulin-dependent manner and the catalytic lysine of this protein was determined. These data suggest that PfPK2 is the Plasmodium protein kinase expressed in the merozoites during the invasion stage.

Convergent CaMK and RacGEF signals control dendritic structure and function

Structural plasticity of excitatory synapses is a vital component of neuronal development, synaptic plasticity and behavior, and its malfunction underlies many neurodevelopmental and psychiatric disorders. However, the molecular mechanisms that control dendritic spine morphogenesis have only recently emerged. We summarize recent work that has revealed an important connection between calcium/calmodulin-dependent kinases (CaMKs) and guanine-nucleotide-exchange factors (GEFs) that activate the small GTPase Rac (RacGEFs) in controlling dendritic spine morphogenesis. These two groups of molecules function in neurons as a unique signaling cassette that transduces calcium influx into small GTPase activity and, thence, actin reorganization and spine morphogenesis. Through this pathway, CaMKs and RacGEFs amplify calcium signals and translate them into spatially and temporally regulated structural remodeling of dendritic spines.

The beta-arrestin-2 scaffold protein promotes c-Jun N-terminal kinase-3 activation by binding to its nonconserved N terminus

The c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) signaling pathway mediates stress responses in cells. JNK activity is regulated by a protein kinase cascade consisting of a MAPK kinase (MKK) and a MAPK kinase kinase (MAPKKK). beta-Arrestin-2 acts as a scaffold by directly binding to the JNK3 isoform and also by recruiting MKK4 and the MAPKKK apoptosis-signaling kinase-1 (ASK1). In this study, we demonstrate by co-precipitation that the extended N-terminal region of JNK3 mediates binding to the C terminus of beta-arrestin-2 and that the N terminus of JNK3 is required for its activation via beta-arrestin-2. We have used site-specific mutagenesis to identify key residues within the N terminus of JNK3 that are essential for binding and demonstrate that this region represents an independent beta-arrestin-2 binding motif that can be fused to other MAPKs and permit their recruitment to the scaffold complex. In addition, we demonstrate that JNK3 recruits MKK4 to the beta-arrestin-2 scaffold complex by binding to the MAPK docking domain (D-domain) located within the N terminus of MKK4. These findings uncover molecular determinants of beta-arrestin-2 scaffold complex assembly and assign a previously unrecognized role for the unique extended N terminus of JNK3.

Activity-dependent synaptogenesis: regulation by a CaM-kinase kinase/CaM-kinase I/betaPIX signaling complex

Neuronal activity augments maturation of mushroom-shaped spines to form excitatory synapses, thereby strengthening synaptic transmission. We have delineated a Ca(2+)-signaling pathway downstream of the NMDA receptor that stimulates calmodulin-dependent kinase kinase (CaMKK) and CaMKI to promote formation of spines and synapses in hippocampal neurons. CaMKK and CaMKI form a multiprotein signaling complex with the guanine nucleotide exchange factor (GEF) betaPIX and GIT1 that is localized in spines. CaMKI-mediated phosphorylation of Ser516 in betaPIX enhances its GEF activity, resulting in activation of Rac1, an established enhancer of spinogenesis. Suppression of CaMKK or CaMKI by pharmacological inhibitors, dominant-negative (dn) constructs and siRNAs, as well as expression of the betaPIX Ser516Ala mutant, decreases spine formation and mEPSC frequency. Constitutively-active Pak1, a downstream effector of Rac1, rescues spine inhibition by dnCaMKI or betaPIX S516A. This activity-dependent signaling pathway can promote synapse formation during neuronal development and in structural plasticity.

Membrane-associated stress proteins: more than simply chaperones

The protein- and/or lipid-mediated association of chaperone proteins to membranes is a widespread phenomenon and implicated in a number of physiological and pathological events that were earlier partially or completely overlooked. A temporary association of certain HSPs with membranes can re-establish the fluidity and bilayer stability and thereby restore the membrane functionality during stress conditions. The fluidity and microdomain organization of membranes are decisive factors in the perception and transduction of stresses into signals that trigger the activation of specific HS genes. Conversely, the membrane association of HSPs may result in the inactivation of membrane-perturbing signals, thereby switch off the heat shock response. Interactions between certain HSPs and specific lipid microdomains ("rafts") might be a previously unrecognized means for the compartmentalization of HSPs to specific signaling platforms, where key signaling proteins are known to be concentrated. Any modulations of the membranes, especially the raft-lipid composition of the cells can alter the extracellular release and thus the immuno-stimulatory activity of certain HSPs. Reliable techniques, allowing mapping of the composition and dynamics of lipid microdomains and simultaneously the spatio-temporal localization of HSPs in and near the plasma membrane can provide suitable means with which to address fundamental questions, such as how HSPs are transported to and translocated through the plasma membrane. The possession of such information is critical if we are to target the membrane association principles of HSPs for successful drug development in most various diseases.

The genomic response of the ipsilateral and contralateral cortex to stroke in aged rats

Aged rats recover poorly after unilateral stroke, whereas young rats recover readily possibly with the help from the contralateral, healthy hemisphere. In this study we asked whether anomalous, age-related changes in the transcriptional activity in the brains of aged rats could be one underlying factor contributing to reduced functional recovery. We analysed gene expression in the periinfarct and contralateral areas of 3-month- and 18-month-old Sprague Dawley rats. Our experimental end-points were cDNA arrays containing genes related to hypoxia signalling, DNA damage and apoptosis, cellular response to injury, axonal damage and re-growth, cell lineage differentiation, dendritogenesis and neurogenesis. The major transcriptional events observed were: (i) Early up-regulation of DNA damage and down-regulation of anti-apoptosis-related genes in the periinfarct region of aged rats after stroke; (ii) Impaired neurogenesis in the periinfarct area, especially in aged rats; (iii) Impaired neurogenesis in the contralateral (unlesioned) hemisphere of both young and aged rats at all times after stroke and (iv) Marked up-regulation, in aged rats, of genes associated with inflammation and scar formation. These results were confirmed with quantitative real-time PCR. We conclude that reduced transcriptional activity in the healthy, contralateral hemisphere of aged rats in conjunction with an early up-regulation of DNA damage-related genes and pro-apoptotic genes and down-regulation of axono- and neurogenesis in the periinfarct area are likely to account for poor neurorehabilitation after stroke in old rats.

Translating neuronal activity into dendrite elaboration: signaling to the nucleus

Growth and elaboration of neuronal processes is key to establishing neuronal connectivity critical for an optimally functioning nervous system. Neuronal activity clearly influences neuronal connectivity and does so via intracellular calcium signaling. A number of CaMKs and MAPKs convey the calcium signal initiated by neuronal activity. Several of these kinases interact with substrates in close proximity to the plasma membrane and alter dendrite structure locally via these local interactions. However, many calcium-activated kinases, such as Ras-MAPK and CaMKIV, target proteins in the nucleus, either by activating a downstream substrate that is a component of a signaling cascade or by directly acting within the nucleus. It is the activation of nuclear signaling and gene transcription that is thought to mediate global changes in dendrite complexity. The identification of calcium-sensitive transcription factors and transcriptional coactivators provides substantial evidence that gene transcription is a prevalent mechanism by which neuronal activity is translated into changes in dendrite complexity. The present review presents an overview of the role of neuronal activity in the development of neuronal dendrites, the signaling mechanisms that translate neuronal activity into gene transcription, and the transcribed effectors that regulate dendrite complexity.

Emerging aspects of membrane traffic in neuronal dendrite growth

Polarized growth of the neuron would logically require some form of membrane traffic to the tip of the growth cone, regulated in conjunction with other trafficking processes that are common to both neuronal and non-neuronal cells. Unlike axons, dendrites are endowed with membranous organelles of the exocytic pathway extending from the cell soma, including both rough and smooth endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC). Dendrites also have satellite Golgi-like cisternal stacks known as Golgi outposts that have no membranous connections with the somatic Golgi. Golgi outposts presumably serve both general and specific local trafficking needs, and could mediate membrane traffic required for polarized dendritic growth during neuronal differentiation. Recent findings suggest that dendritic growth, but apparently not axonal growth, relies very much on classical exocytic traffic, and is affected by defects in components of both the early and late secretory pathways. Within dendrites, localized processes of recycling endosome-based exocytosis regulate the growth of dendritic spines and postsynaptic compartments. Emerging membrane traffic processes and components that contribute specifically to dendritic growth are discussed.