TORC1 regulates activity-dependent CREB-target gene transcription and dendritic growth of developing cortical neurons

The HDAC inhibitor SAHA improves depressive-like behavior of CRTC1-deficient mice: Possible relevance for treatment-resistant depression

Major depression is a highly complex disabling psychiatric disorder affecting millions of people worldwide. Despite the availability of several classes of antidepressants, a substantial percentage of patients are unresponsive to these medications. A better understanding of the neurobiology of depression and the mechanisms underlying antidepressant response is thus critically needed. We previously reported that mice lacking CREB-regulated transcription coactivator 1 (CRTC1) exhibit a depressive-like phenotype and a blunted antidepressant response to the selective serotonin reuptake inhibitor fluoxetine. In this study, we similarly show that Crtc1(-/-) mice are resistant to the antidepressant effect of chronic desipramine in a behavioral despair paradigm. Supporting the blunted response to this tricyclic antidepressant, we found that desipramine does not significantly increase the expression of Bdnf and Nr4a1-3 in the hippocampus and prefrontal cortex of Crtc1(-/-) mice. Epigenetic regulation of neuroplasticity gene expression has been associated with depression and antidepressant response, and histone deacetylase (HDAC) inhibitors have been shown to have antidepressant-like properties. Here, we show that unlike conventional antidepressants, chronic systemic administration of the HDAC inhibitor SAHA partially rescues the depressive-like behavior of Crtc1(-/-) mice. This behavioral effect is accompanied by an increased expression of Bdnf, but not Nr4a1-3, in the prefrontal cortex of these mice, suggesting that this epigenetic intervention restores the expression of a subset of genes by acting downstream of CRTC1. These findings suggest that CRTC1 alterations may be associated with treatment-resistant depression, and support the interesting possibility that targeting HDACs may be a useful therapeutic strategy in antidepressant development.

Methyl-Arginine Profile of Brain from Aged PINK1-KO+A53T-SNCA Mice Suggests Altered Mitochondrial Biogenesis

Hereditary Parkinson's disease can be triggered by an autosomal dominant overdose of alpha-Synuclein (SNCA) or the autosomal recessive deficiency of PINK1. We recently showed that the combination of PINK1-knockout with overexpression of A53T-SNCA in double mutant (DM) mice potentiates phenotypes and reduces survival. Now we studied brain hemispheres of DM mice at age of 18 months in a hypothesis-free approach, employing a quantitative label-free global proteomic mass spectrometry scan of posttranslational modifications focusing on methyl-arginine. The strongest effects were documented for the adhesion modulator CMAS, the mRNA decapping/deadenylation factor PATL1, and the synaptic plasticity mediator CRTC1/TORC1. In addition, an intriguing effect was observed for the splicing factor PSF/SFPQ, known to interact with the dopaminergic differentiation factor NURR1 as well as with DJ-1, the protein responsible for the autosomal recessive PARK7 variant of PD. CRTC1, PSF, and DJ-1 are modulators of PGC1alpha and of mitochondrial biogenesis. This pathway was further stressed by dysregulations of oxygen sensor EGLN3 and of nuclear TMPO. PSF and TMPO cooperate with dopaminergic differentiation factors LMX1B and NURR1. Further dysregulations concerned PRR18, TRIO, HNRNPA1, DMWD, WAVE1, ILDR2, DBNDD1, and NFM. Thus, we report selective novel endogenous stress responses in brain, which highlight early dysregulations of mitochondrial homeostasis and midbrain vulnerability.

Functional Impact of Corticotropin-Releasing Factor Exposure on Tau Phosphorylation and Axon Transport

Stress exposure or increased levels of corticotropin-releasing factor (CRF) induce hippocampal tau phosphorylation (tau-P) in rodent models, a process that is dependent on the type-1 CRF receptor (CRFR1). Although these preclinical studies on stress-induced tau-P provide mechanistic insight for epidemiological work that identifies stress as a risk factor for Alzheimer’s disease (AD), the actual impact of stress-induced tau-P on neuronal function remains unclear. To determine the functional consequences of stress-induced tau-P, we developed a novel mouse neuronal cell culture system to explore the impact of acute (0.5hr) and chronic (2hr) CRF treatment on tau-P and integral cell processes such as axon transport. Consistent with in vivo reports, we found that chronic CRF treatment increased tau-P levels and caused globular accumulations of phosphorylated tau in dendritic and axonal processes. Furthermore, while both acute and chronic CRF treatment led to significant reduction in CREB activation and axon transport of brain-derived neurotrophic factor (BDNF), this was not the case with mitochondrial transport. Acute CRF treatment caused increased mitochondrial velocity and distance traveled in neurons, while chronic CRF treatment modestly decreased mitochondrial velocity and greatly increased distance traveled. These results suggest that transport of cellular energetics may take priority over growth factors during stress. Tau-P was required for these changes, as co-treatment of CRF with a GSK kinase inhibitor prevented CRF-induced tau-P and all axon transport changes. Collectively, our results provide mechanistic insight into the consequences of stress peptide-induced tau-P and provide an explanation for how chronic stress via CRF may lead to neuronal vulnerability in AD.

Memory acquisition and retrieval impact different epigenetic processes that regulate gene expression

Background

A fundamental question in neuroscience is how memories are stored and retrieved in the brain. Long-term memory formation requires transcription, translation and epigenetic processes that control gene expression. Thus, characterizing genome-wide the transcriptional changes that occur after memory acquisition and retrieval is of broad interest and importance. Genome-wide technologies are commonly used to interrogate transcriptional changes in discovery-based approaches. Their ability to increase scientific insight beyond traditional candidate gene approaches, however, is usually hindered by batch effects and other sources of unwanted variation, which are particularly hard to control in the study of brain and behavior.

Results

We examined genome-wide gene expression after contextual conditioning in the mouse hippocampus, a brain region essential for learning and memory, at all the time-points in which inhibiting transcription has been shown to impair memory formation. We show that most of the variance in gene expression is not due to conditioning and that by removing unwanted variance through additional normalization we are able provide novel biological insights. In particular, we show that genes downregulated by memory acquisition and retrieval impact different functions: chromatin assembly and RNA processing, respectively. Levels of histone 2A variant H2AB are reduced only following acquisition, a finding we confirmed using quantitative proteomics. On the other hand, splicing factor Rbfox1 and NMDA receptor-dependent microRNA miR-219 are only downregulated after retrieval, accompanied by an increase in protein levels of miR-219 target CAMKIIγ.

Conclusions

We provide a thorough characterization of coding and non-coding gene expression during long-term memory formation. We demonstrate that unwanted variance dominates the signal in transcriptional studies of learning and memory and introduce the removal of unwanted variance through normalization as a necessary step for the analysis of genome-wide transcriptional studies in the context of brain and behavior. We show for the first time that histone variants are downregulated after memory acquisition, and splicing factors and microRNAs after memory retrieval. Our results provide mechanistic insights into the molecular basis of cognition by highlighting the differential involvement of epigenetic mechanisms, such as histone variants and post-transcriptional RNA regulation, after acquisition and retrieval of memory.

Rcan1 deficiency impairs neuronal migration and causes periventricular heterotopia

Periventricular heterotopia (PH) is a cortical malformation characterized by aggregation of neurons lining the lateral ventricles due to abnormal neuronal migration. The molecular mechanism underlying the pathogenesis of PH is unclear. Here we show that Regulators of calcineurin 1 (Rcan1), a Down syndrome-related gene, plays an important role in radial migration of rat cortical neurons. Downregulation of Rcan1 by expressing shRNA impaired neural progenitor proliferation and led to defects in radial migration and PH. Two isoforms of Rcan1 (Rcan1-1 and Rcan1-4) are expressed in the rat brain. Migration defects due to downregulation of Rcan1 could be prevented by shRNA-resistant expression of Rcan1-1 but not Rcan1-4. Furthermore, we found that Rcan1 knockdown significantly decreased the expression level of Flna, an F-actin cross-linking protein essential for cytoskeleton rearrangement and cell migration, mutation of which causes the most common form of bilateral PH in humans. Finally, overexpression of FLNA in Rcan1 knockdown neurons prevented migration abnormalities. Together, these findings demonstrate that Rcan1 acts upstream from Flna in regulating radial migration and suggest that impairment of Rcan1-Flna pathway may underlie PH pathogenesis.

De novo mutations in SIK1 cause a spectrum of developmental epilepsies

Developmental epilepsies are age-dependent seizure disorders for which genetic causes have been increasingly identified. Here we report six unrelated individuals with mutations in salt-inducible kinase 1 (SIK1) in a series of 101 persons with early myoclonic encephalopathy, Ohtahara syndrome, and infantile spasms. Individuals with SIK1 mutations had short survival in cases with neonatal epilepsy onset, and an autism plus developmental syndrome after infantile spasms in others. All six mutations occurred outside the kinase domain of SIK1 and each of the mutants displayed autophosphorylation and kinase activity toward HDAC5. Three mutations generated truncated forms of SIK1 that were resistant to degradation and also showed changes in sub-cellular localization compared to wild-type SIK1. We also report the human neuropathologic examination of SIK1-related developmental epilepsy, with normal neuronal morphology and lamination but abnormal SIK1 protein cellular localization. Therefore, these results expand the genetic etiologies of developmental epilepsies by demonstrating SIK1 mutations as a cause of severe developmental epilepsy.

Salt-inducible kinase 1 (SIK1) is induced by gastrin and inhibits migration of gastric adenocarcinoma cells

Salt-inducible kinase 1 (SIK1/Snf1lk) belongs to the AMP-activated protein kinase (AMPK) family of kinases, all of which play major roles in regulating metabolism and cell growth. Recent studies have shown that reduced levels of SIK1 are associated with poor outcome in cancers, and that this involves an invasive cellular phenotype with increased metastatic potential. However, the molecular mechanism(s) regulated by SIK1 in cancer cells is not well explored. The peptide hormone gastrin regulates cellular processes involved in oncogenesis, including proliferation, apoptosis, migration and invasion. The aim of this study was to examine the role of SIK1 in gastrin responsive adenocarcinoma cell lines AR42J, AGS-GR and MKN45. We show that gastrin, known to signal through the Gq/G11-coupled CCK2 receptor, induces SIK1 expression in adenocarcinoma cells, and that transcriptional activation of SIK1 is negatively regulated by the Inducible cAMP early repressor (ICER). We demonstrate that gastrin-mediated signalling induces phosphorylation of Liver Kinase 1B (LKB1) Ser-428 and SIK1 Thr-182. Ectopic expression of SIK1 increases gastrin-induced phosphorylation of histone deacetylase 4 (HDAC4) and enhances gastrin-induced transcription of c-fos and CRE-, SRE-, AP1- and NF-κB-driven luciferase reporter plasmids. We also show that gastrin induces phosphorylation and nuclear export of HDACs. Next we find that siRNA mediated knockdown of SIK1 increases migration of the gastric adenocarcinoma cell line AGS-GR. Evidence provided here demonstrates that SIK1 is regulated by gastrin and influences gastrin elicited signalling in gastric adenocarcinoma cells. The results from the present study are relevant for the understanding of molecular mechanisms involved in gastric adenocarcinomas.

Towards a better understanding of cognitive behaviors regulated by gene expression downstream of activity-dependent transcription factors

In the field of molecular and cellular neuroscience, it is not a trivial task to see the forest for the trees, where numerous, and seemingly independent, molecules often work in concert to control critical steps of synaptic plasticity and signalling. Here, we will first summarize our current knowledge on essential activity-dependent transcription factors (TFs) such as CREB, MEF2, Npas4 and SRF, then examine how various transcription cofactors (TcoFs) also contribute to defining the transcriptional outputs during learning and memory. This review finally attempts a provisory synthesis that sheds new light on some of the emerging principles of neuronal circuit dynamics driven by activity-regulated gene transcription to help better understand the intricate relationship between activity-dependent gene expression and cognitive behavior.

Excitatory GABA induces BDNF transcription via CRTC1 and phosphorylated CREB-related pathways in immature cortical cells

Although the excitatory action of GABA has been shown to activate the expression of brain-derived neurotrophic factor (BDNF), its molecular mechanisms remain unclear. Using cultured rat cortical cells, we here demonstrated that GABA induced Bdnf mRNA expression mainly via L-type voltage-dependent Ca(2+) channels (L-VDCC) at the early stage and inhibited it at the late stage of the culture, which corresponded to the excitatory and inhibitory states of cortical cells. The excitatory GABA-induced Bdnf mRNA expression was controlled by multiple Ca(2+) signaling pathways including Ca(2+) /calmodulin-dependent protein kinase (CaMK), mitogen-activated protein kinase (MAPK) and calcineurin (CN). The Bdnf-promoter IV (Bdnf-pIV) was activated by GABA, mainly via cAMP-response element (CRE)/CREB, and this was prevented by the over-expression of a dominant negative CREB. The nuclear translocation of CREB-regulated transcriptional coactivator 1 (CRTC1) was selectively induced by the GABA-induced CN pathway to activate Bdnf-pIV. On the other hand, GABA-induced Gal4-CREB-dependent transcription, which was controlled by multiple Ca(2+) signaling pathways, was prevented when the serine at position 133 of Gal4-CREB was mutated to alanine. Taken together, the excitatory action of GABA transcriptionally activated Bdnf expression through the combination of nuclear-localized CRTC1 and phosphorylated CREB in immature cortical cells, and may be the molecular mechanisms underlying Bdnf expression to control neuronal development. We demonstrated that GABA induced Bdnf expression at the early stage of the culture, in which GABA exerted its excitatory action. The excitatory GABA-induced Bdnf expression was controlled by multiple Ca(2+) signaling pathways evoked via L-VDCC. Both the CREB coactivator, CRTC1 and CREB phosphorylation participated in excitatory GABA-induced Bdnf transcription. Our present study indicates the mechanism underlying the excitatory GABA-induced Bdnf expression in immature neurons and provide new insights into molecular mechanisms underlying Bdnf expression to control neuronal development.

The effects of early-life seizures on hippocampal dendrite development and later-life learning and memory

Severe childhood epilepsy is commonly associated with intellectual developmental disabilities. The reasons for these cognitive deficits are likely multifactorial and will vary between epilepsy syndromes and even among children with the same syndrome. However, one factor these children have in common is the recurring seizures they experience - sometimes on a daily basis. Supporting the idea that the seizures themselves can contribute to intellectual disabilities are laboratory results demonstrating spatial learning and memory deficits in normal mice and rats that have experienced recurrent seizures in infancy. Studies reviewed here have shown that seizures in vivo and electrographic seizure activity in vitro both suppress the growth of hippocampal pyramidal cell dendrites. A simplification of dendritic arborization and a resulting decrease in the number and/or properties of the excitatory synapses on them could help explain the observed cognitive disabilities. There are a wide variety of candidate mechanisms that could be involved in seizure-induced growth suppression. The challenge is designing experiments that will help focus research on a limited number of potential molecular events. Thus far, results suggest that growth suppression is NMDA receptor-dependent and associated with a decrease in activation of the transcription factor CREB. The latter result is intriguing since CREB is known to play an important role in dendrite growth. Seizure-induced dendrite growth suppression may not occur as a single process in which pyramidal cells dendrites simply stop growing or grow slower compared to normal neurons. Instead, recent results suggest that after only a few hours of synchronized epileptiform activity in vitro dendrites appear to partially retract. This acute response is also NMDA receptor dependent and appears to be mediated by the Ca(+2)/calmodulin-dependent phosphatase, calcineurin. An understanding of the staging of seizure-induced growth suppression and the underlying molecular mechanisms will likely prove crucial for developing therapeutic strategies aimed at ameliorating the intellectual developmental disabilities associated with intractable childhood epilepsy.

The Drosophila transcription factor Adf-1 (nalyot) regulates dendrite growth by controlling FasII and Staufen expression downstream of CaMKII and neural activity

Memory deficits in Drosophila nalyot mutants suggest that the Myb family transcription factor Adf-1 is an important regulator of developmental plasticity in the brain. However, the cellular functions for this transcription factor in neurons or molecular mechanisms by which it regulates plasticity remain unknown. Here, we use in vivo 3D reconstruction of identifiable larval motor neuron dendrites to show that Adf-1 is required cell autonomously for dendritic development and activity-dependent plasticity of motor neurons downstream of CaMKII. Adf-1 inhibition reduces dendrite growth and neuronal excitability, and results in motor deficits and altered transcriptional profiles. Surprisingly, analysis by comparative chromatin immunoprecipitation followed by sequencing (ChIP-Seq) of Adf-1, RNA Polymerase II (Pol II), and histone modifications in Kc cells shows that Adf-1 binding correlates positively with high Pol II-pausing indices and negatively with active chromatin marks such as H3K4me3 and H3K27ac. Consistently, the expression of Adf-1 targets Staufen and Fasciclin II (FasII), identified through larval brain ChIP-Seq for Adf-1, is negatively regulated by Adf-1, and manipulations of these genes predictably modify dendrite growth. Our results imply mechanistic interactions between transcriptional and local translational machinery in neurons as well as conserved neuronal growth mechanisms mediated by cell adhesion molecules, and suggest that CaMKII, Adf-1, FasII, and Staufen influence crucial aspects of dendrite development and plasticity with potential implications for memory formation. Further, our experiments reveal molecular details underlying transcriptional regulation by Adf-1, and indicate active interaction between Adf-1 and epigenetic regulators of gene expression during activity-dependent neuronal plasticity.

Clock and light regulation of the CREB coactivator CRTC1 in the suprachiasmatic circadian clock

The CREB/CRE transcriptional pathway has been implicated in circadian clock timing and light-evoked clock resetting. To date, much of the work on CREB in circadian physiology has focused on how changes in the phosphorylation state of CREB regulate the timing processes. However, beyond changes in phosphorylation, CREB-dependent transcription can also be regulated by the CREB coactivator CRTC (CREB-regulated transcription coactivator), also known as TORC (transducer of regulated CREB). Here we profiled both the rhythmic and light-evoked regulation of CRTC1 and CRTC2 in the murine suprachiasmatic nucleus (SCN), the locus of the master mammalian clock. Immunohistochemical analysis revealed rhythmic expression of CRTC1 in the SCN. CRTC1 expression was detected throughout the dorsoventral extent of the SCN in the middle of the subjective day, with limited expression during early night, and late night expression levels intermediate between mid-day and early night levels. In contrast to CRTC1, robust expression of CRTC2 was detected during both the subjective day and night. During early and late subjective night, a brief light pulse induced strong nuclear accumulation of CRTC1 in the SCN. In contrast with CRTC1, photic stimulation did not affect the subcellular localization of CRTC2 in the SCN. Additionally, reporter gene profiling and chromatin immunoprecipitation analysis indicated that CRTC1 was associated with CREB in the 5' regulatory region of the period1 gene, and that overexpression of CRTC1 leads to a marked upregulation in period1 transcription. Together, these data raise the prospect that CRTC1 plays a role in fundamental aspects of SCN clock timing and entrainment.

Calcineurin signaling mediates activity-dependent relocation of the axon initial segment

The axon initial segment (AIS) is a specialized neuronal subcompartment located at the beginning of the axon that is crucially involved in both the generation of action potentials and the regulation of neuronal polarity. We recently showed that prolonged neuronal depolarization produces a distal shift of the entire AIS structure away from the cell body, a change associated with a decrease in neuronal excitability. Here, we used dissociated rat hippocampal cultures, with a major focus on the dentate granule cell (DGC) population, to explore the signaling pathways underlying activity-dependent relocation of the AIS. First, a pharmacological screen of voltage-gated calcium channels (VGCCs) showed that AIS relocation is triggered by activation of L-type Cav1 VGCCs with negligible contribution from any other VGCC subtypes. Additional pharmacological analysis revealed that downstream signaling events are mediated by the calcium-sensitive phosphatase calcineurin; inhibition of calcineurin with either FK506 or cyclosporin A totally abolished both depolarization- and optogenetically-induced activity-dependent AIS relocation. Furthermore, calcineurin activation is sufficient for AIS plasticity, because expression of a constitutively active form of the phosphatase resulted in relocation of the AIS of DGCs without a depolarizing stimulus. Finally, we assessed the role of calcineurin in other forms of depolarization-induced plasticity. Neither membrane resistance changes nor spine density changes were affected by FK506 treatment, suggesting that calcineurin acts via a separate pathway to modulate AIS plasticity. Together, these results emphasize calcineurin as a vital player in the regulation of intrinsic plasticity as governed by the AIS.

PDE10 inhibition increases GluA1 and CREB phosphorylation and improves spatial and recognition memories in a Huntington's disease mouse model

Huntington's disease (HD) causes motor disturbances, preceded by cognitive impairment, in patients and mouse models. We showed that increased hippocampal cAMP-dependent protein kinase (PKA) signaling disrupts recognition and spatial memories in R6 HD mouse models. However, unchanged levels of hippocampal phosphorylated (p) cAMP-responsive element-binding protein (CREB) suggested unaltered nuclear PKA activity in R6 mice. Here, we extend this finding by showing that nuclear pPKA catalytic subunit (Thr197) and pPKA substrate levels were unaltered in the hippocampus of R6/1 mice. Phosphodiesterases (PDEs) play an important role in the regulation of PKA activity. PDE10A, a cAMP/cGMP dual-substrate PDE, was reported to be restricted to the nuclear region in nonstriatal neurons. Using cell fractionation we confirmed that PDE10A was enriched in nuclear fractions, both in wild-type and R6/1 mice hippocampus, without differences in its levels or intracellular distribution between genotypes. We next investigated whether inhibition of PDE10 with papaverine could improve cognitive function in HD mice. Papaverine treatment improved spatial and object recognition memories in R6/1 mice, and significantly increased pGluA1 and pCREB levels in R6/1 mice hippocampus. Papaverine likely acted through the activation of the PKA pathway as the phosphorylation level of distinct cGMP-dependent kinase (cGK) substrates was not modified in either genotype. Moreover, hippocampal cAMP, but not cGMP, levels were increased after acute papaverine injection. Our results show that inhibition of PDE10 improves cognition in R6 mice, at least in part through increased GluA1 and CREB phosphorylation. Thus, PDE10 might be a good therapeutic target to improve cognitive impairment in HD.

Increasing CRTC1 function in the dentate gyrus during memory formation or reactivation increases memory strength without compromising memory quality

Memory stabilization following encoding (synaptic consolidation) or memory reactivation (reconsolidation) requires gene expression and protein synthesis (Dudai and Eisenberg, 2004; Tronson and Taylor, 2007; Nader and Einarsson, 2010; Alberini, 2011). Although consolidation and reconsolidation may be mediated by distinct molecular mechanisms (Lee et al., 2004), disrupting the function of the transcription factor CREB impairs both processes (Kida et al., 2002; Mamiya et al., 2009). Phosphorylation of CREB at Ser133 recruits CREB binding protein (CBP)/p300 coactivators to activate transcription (Chrivia et al., 1993; Parker et al., 1996). In addition to this well known mechanism, CREB regulated transcription coactivators (CRTCs), previously called transducers of regulated CREB (TORC) activity, stimulate CREB-mediated transcription, even in the absence of CREB phosphorylation. Recently, CRTC1 has been shown to undergo activity-dependent trafficking from synapses and dendrites to the nucleus in excitatory hippocampal neurons (Ch'ng et al., 2012). Despite being a powerful and specific coactivator of CREB, the role of CRTC in memory is virtually unexplored. To examine the effects of increasing CRTC levels, we used viral vectors to locally and acutely increase CRTC1 in the dorsal hippocampus dentate gyrus region of mice before training or memory reactivation in context fear conditioning. Overexpressing CRTC1 enhanced both memory consolidation and reconsolidation; CRTC1-mediated memory facilitation was context specific (did not generalize to nontrained context) and long lasting (observed after virally expressed CRTC1 dissipated). CREB overexpression produced strikingly similar effects. Therefore, increasing CRTC1 or CREB function is sufficient to enhance the strength of new, as well as established reactivated, memories without compromising memory quality.

Role of salt-inducible kinase 1 in the activation of MEF2-dependent transcription by BDNF

Substantial evidence supports a role for myocyte enhancer factor 2 (MEF2)-mediated transcription in neuronal survival, differentiation and synaptic function. In developing neurons, it has been shown that MEF2-dependent transcription is regulated by neurotrophins. Despite these observations, little is known about the cellular mechanisms by which neurotrophins activate MEF2 transcriptional activity. In this study, we examined the role of salt-inducible kinase 1 (SIK1), a member of the AMP-activated protein kinase (AMPK) family, in the regulation of MEF2-mediated transcription by the neurotrophin brain-derived neurotrophic factor (BDNF). We show that BDNF increases the expression of SIK1 in primary cultures of rat cortical neurons through the extracellular signal-regulated kinase 1/2 (ERK1/2)-signaling pathway. In addition to inducing SIK1 expression, BDNF triggers the phosphorylation of SIK1 at Thr182 and its translocation from the cytoplasm to the nucleus of cortical neurons. The effects of BDNF on the expression, phosphorylation and, translocation of SIK1 are followed by the phosphorylation and nuclear export of histone deacetylase 5 (HDAC5). Blockade of SIK activity with a low concentration of staurosporine abolished BDNF-induced phosphorylation and nuclear export of HDAC5 in cortical neurons. Importantly, stimulation of HDAC5 phosphorylation and nuclear export by BDNF is accompanied by the activation of MEF2-mediated transcription, an effect that is suppressed by staurosporine. Consistent with these data, BDNF induces the expression of the MEF2 target genes Arc and Nur77, in a staurosporine-sensitive manner. In further support of the role of SIK1 in the regulation of MEF2-dependent transcription by BDNF, we found that expression of wild-type SIK1 or S577A SIK1, a mutated form of SIK1 which is retained in the nucleus of transfected cells, is sufficient to enhance MEF2 transcriptional activity in cortical neurons. Together, these data identify a previously unrecognized mechanism by which SIK1 mediates the activation of MEF2-dependent transcription by BDNF.

Molecular, cellular, and structural mechanisms of cocaine addiction: a key role for microRNAs

The rewarding properties of cocaine play a key role in establishing and maintaining the drug-taking habit. However, as exposure to cocaine increases, drug use can transition from controlled to compulsive. Importantly, very little is known about the neurobiological mechanisms that control this switch in drug use that defines addiction. MicroRNAs (miRNAs) are small non-protein coding RNA transcripts that can regulate the expression of messenger RNAs that code for proteins. Because of their highly pleiotropic nature, each miRNA has the potential to regulate hundreds or even thousands of protein-coding RNA transcripts. This property of miRNAs has generated considerable interest in their potential involvement in complex psychiatric disorders such as addiction, as each miRNA could potentially influence the many different molecular and cellular adaptations that arise in response to drug use that are hypothesized to drive the emergence of addiction. Here, we review recent evidence supporting a key role for miRNAs in the ventral striatum in regulating the rewarding and reinforcing properties of cocaine in animals with limited exposure to the drug. Moreover, we discuss evidence suggesting that miRNAs in the dorsal striatum control the escalation of drug intake in rats with extended cocaine access. These findings highlight the central role for miRNAs in drug-induced neuroplasticity in brain reward systems that drive the emergence of compulsive-like drug use in animals, and suggest that a better understanding of how miRNAs control drug intake will provide new insights into the neurobiology of drug addiction.

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.

Deletion of CREB-regulated transcription coactivator 1 induces pathological aggression, depression-related behaviors, and neuroplasticity genes dysregulation in mice

BACKGROUND

Mood disorders are polygenic disorders in which the alteration of several susceptibility genes results in dysfunctional mood regulation. However, the molecular mechanisms underlying their transcriptional dysregulation are still unclear. The transcription factor cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) and the neurotrophin brain-derived neurotrophic factor (BDNF) have been implicated in rodent models of depression. We previously provided evidence that Bdnf expression critically rely on a potent CREB coactivator called CREB-regulated transcription coactivator 1 (CRTC1).

METHODS

To further evaluate the role of CRTC1 in the brain, we generated a knockout mouse line and analyzed its behavioral and molecular phenotype.

RESULTS

We found that mice lacking CRTC1 associate neurobehavioral endophenotypes related to mood disorders. Crtc1(-/-) mice exhibit impulsive aggressiveness, social withdrawal, and decreased sexual motivation, together with increased behavioral despair, anhedonia, and anxiety-related behavior in the novelty-induced hypophagia test. They also present psychomotor retardation as well as increased emotional response to stressful events. Crtc1(-/-) mice have a blunted response to the antidepressant fluoxetine in behavioral despair paradigms, whereas fluoxetine normalizes their aggressiveness and their behavioral response in the novelty-induced hypophagia test. Crtc1(-/-) mice strikingly show, in addition to a reduced dopamine and serotonin turnover in the prefrontal cortex, a concomitant decreased expression of several susceptibility genes involved in neuroplasticity, including Bdnf, its receptor TrkB, the nuclear receptors Nr4a1-3, and several other CREB-regulated genes.

CONCLUSIONS

Collectively, these findings support a role for the CRTC1-CREB pathway in mood disorders etiology and behavioral response to antidepressants and identify CRTC1 as an essential coactivator of genes involved in mood regulation.

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.

ATP and noradrenaline activate CREB in astrocytes via noncanonical Ca(2+) and cyclic AMP independent pathways

In neurons, it is well established that CREB contributes to learning and memory by orchestrating the translation of experience into the activity-dependent (i.e., driven by neurotransmitters) transcription of plasticity-related genes. The activity-dependent CREB-triggered transcription requires the concerted action of cyclic AMP/protein kinase A and Ca(2+) /calcineurin via the CREB-regulated transcription co-activator (CRTC). It is not known, however, whether a comparable molecular sequence occurs in astrocytes, despite the unquestionable contribution of these cells to brain plasticity. Here we sought to determine whether and how ATP and noradrenaline cause CREB-dependent transcription in rat cortical astrocyte cultures. Both transmitters induced CREB phosphorylation (Western Blots), CREB-dependent transcription (CRE-luciferase reporter assays), and the transcription of Bdnf, a canonical regulator of synaptic plasticity (quantitative RT-PCR). We indentified a Ca(2+) and diacylglycerol-independent protein kinase C at the uppermost position of the cascade leading to CREB-dependent transcription. Notably, CREB-dependent transcription was partially dependent on ERK1/2 and CRTC, but independent of cyclic AMP/protein kinase A or Ca(2+) /calcineurin. We conclude that ATP and noradrenaline activate CREB-dependent transcription in cortical astrocytes via an atypical protein kinase C. It is of relevance that the signaling involved be starkly different to the one described in neurons since there is no convergence of Ca(2+) and cyclic AMP-dependent pathways on CRTC, which, moreover, exerts a modulatory rather than a central role. Our data thus point to the existence of an alternative, non-neuronal, glia-based role of CREB in plasticity.

Transducer of regulated CREB-binding proteins (TORCs) transcription and function is impaired in Huntington's disease

Huntington's disease (HD) is an incurable neurological disorder caused by an abnormal glutamine repeat expansion in the huntingtin (Htt) protein. In the present studies, we investigated the role of Transducers of Regulated cAMP response element-binding (CREB) protein activity (TORCs) in HD, since TORCs play an important role in the expression of the transcriptional co-regulator peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α), whose expression is impaired in HD. We found significantly decreased TORC1 expression levels in STHdhQ111 cells expressing mutant Htt, in the striatum of NLS-N171-82Q, R6/2 and HdhQ111 HD transgenic mice and in postmortem striatal tissue from HD patients. TORC1 overexpression in wild-type (WT) and Htt striatal cells increased CREB mRNA and protein levels, PGC-1α promoter activity, mRNA expression of the PGC-1α, NRF-1, Tfam and CytC genes, mitochondrial DNA content, mitochondrial activity and mitochondrial membrane potential. TORC1 overexpression also increased the resistance of striatal cells to 3-nitropropionic (3-NP) acid-mediated toxicity. In cultured WT and mutant Htt striatal cells, small hairpin RNA-mediated TORC1 knockdown resulted in decreased PGC-1α expression and increased susceptibility to 3-NP-induced toxicity. Overexpression of PGC-1α partially prevented TORC1 knockdown-mediated increased susceptibility of Htt striatal cells to 3-NP. Specific knockdown of TORC1 in the striatum of NLS-N171-82Q HD transgenic mice induced neurodegeneration. Lastly, knockdown of Htt prevents transcriptional repression of TORC1 and CREB in Htt striatal cells. These findings show that impaired expression and function of TORC1, which results in a reduction in PGC-1α, plays an important role in mitochondrial dysfunction in HD.

Sirt1 mediates neuroprotection from mutant huntingtin by activation of the TORC1 and CREB transcriptional pathway

Sirt1, a NAD-dependent protein deacetylase, has emerged as a key regulator of mammalian transcription in response to cellular metabolic status and stress. Here we show that Sirt1 has a neuroprotective role in models of Huntington's disease, an inherited neurodegenerative disorder caused by a glutamine repeat expansion in huntingtin protein (HTT). Brain-specific knockout of Sirt1 results in exacerbation of brain pathology in a mouse model of Huntington's disease, whereas overexpression of Sirt1 improves survival, neuropathology and the expression of brain-derived neurotrophic factor (BDNF) in Huntington's disease mice. We show that Sirt1 deacetylase activity directly targets neurons to mediate neuroprotection from mutant HTT. The neuroprotective effect of Sirt1 requires the presence of CREB-regulated transcription coactivator 1 (TORC1), a brain-specific modulator of CREB activity. We show that under normal conditions, Sirt1 deacetylates and activates TORC1 by promoting its dephosphorylation and its interaction with CREB. We identified BDNF as a key target of Sirt1 and TORC1 transcriptional activity in both normal and Huntington's disease neurons. Mutant HTT interferes with the TORC1-CREB interaction to repress BDNF transcription, and Sirt1 rescues this defect in vitro and in vivo. These studies suggest a key role for Sirt1 in transcriptional networks in both the normal and Huntington's disease brain and offer an opportunity for therapeutic development.

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.

Cocaine induces the expression of MEF2C transcription factor in rat striatum through activation of SIK1 and phosphorylation of the histone deacetylase HDAC5

Distinct forms of MEF2 transcription factor act as positive or negative regulators of dendritic spine formation, with MEF2C playing a key regulator role in synapse plasticity. We report here that acute cocaine treatment of rats induced the expression of MEF2C in the striatum through a recently discovered transduction pathway. Repeated injections were found to induce MEF2C to a lesser extent. The mechanism by which MEF2C was induced involves the subsequent activation of the salt-inducible kinase SIK1 and the phosphorylation of HDAC5, a member of the class IIa of HDACs. Cocaine activated SIK1 by phosphorylation on Thr-182 residue, which was accompanied by the nuclear import of the kinase. In the nuclear compartment, SIK1 then phosphorylated HDAC5 causing the shuttling of its phospho-form from the nucleus to the cytoplasm of striatal cells. Activation of SIK1 by cocaine was further validated by the phosphorylation of TORC1/3, which was followed by the shuttling of TORC proteins from the nucleus to the cytoplasm. Activation of MEF2C was assessed by measuring the expression of the MEF2C gene itself, since the gene is known to be under the control of its own product. Since MEF2C plays a key role in memory/learning processes, activation of this pathway by cocaine is probably involved in plasticity mechanisms whereby the drug establishes its long-term effects such as drug dependence.

A potent inhibitor of SIK2, 3, 3', 7-trihydroxy-4'-methoxyflavon (4'-O-methylfisetin), promotes melanogenesis in B16F10 melanoma cells

Flavonoids, which are plant polyphenols, are now widely used in supplements and cosmetics. Here, we report that 4′-methylflavonoids are potent inducers of melanogenesis in B16F10 melanoma cells and in mice. We recently identified salt inducible kinase 2 (SIK2) as an inhibitor of melanogenesis via the suppression of the cAMP-response element binding protein (CREB)-specific coactivator 1 (TORC1). Using an in vitro kinase assay targeting SIK2, we identified fisetin as a candidate inhibitor, possibly being capable of promoting melanogenesis. However, fisetin neither inhibited the CREB-inhibitory activity of SIK2 nor promoted melanogenesis in B16F10 melanoma cells. Conversely, mono-methyl-flavonoids, such as diosmetin (4′-O-metlylluteolin), efficiently inhibited SIK2 and promoted melanogenesis in this cell line. The cAMP-CREB system is impaired in A^y/a mice and these mice have yellow hair as a result of pheomelanogenesis, while Sik2^+/−; A^y/a mice also have yellow hair, but activate eumelanogenesis when they are exposed to CREB stimulators. Feeding Sik2^+/−; A^y/a mice with diets supplemented with fisetin resulted in their hair color changing to brown, and metabolite analysis suggested the presence of mono-methylfisetin in their feces. Thus, we decided to synthesize 4′-O-methylfisetin (4′MF) and found that 4′MF strongly induced melanogenesis in B16F10 melanoma cells, which was accompanied by the nuclear translocation of TORC1, and the 4′-O-methylfisetin-induced melanogenic programs were inhibited by the overexpression of dominant negative TORC1. In conclusion, compounds that modulate SIK2 cascades are helpful to regulate melanogenesis via TORC1 without affecting cAMP levels, and the combined analysis of Sik2^+/− mice and metabolites from these mice is an effective strategy to identify beneficial compounds to regulate CREB activity in vivo.

Cellular mechanisms underlying the regulation of dendritic development by hepatocyte growth factor

Acquisition of a mature dendritic morphology is critical for neural information processing. In particular, hepatocyte growth factor (HGF) controls dendritic arborization during brain development. However, the cellular mechanisms underlying the effects of HGF on dendritic growth remain elusive. Here, we show that HGF increases dendritic length and branching of rat cortical neurons through activation of the mitogen-activated protein kinase (MAPK) signaling pathway. Activation of MAPK by HGF leads to the rapid and transient phosphorylation of cAMP response element-binding protein (CREB), a key step necessary for the control of dendritic development by HGF. In addition to CREB phosphorylation, regulation of dendritic growth by HGF requires the interaction between CREB and CREB-regulated transcription coactivator 1 (CRTC1), as expression of a mutated form of CREB unable to bind CRTC1 completely abolished the effects of HGF on dendritic morphology. Treatment of cortical neurons with HGF in combination with brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family that regulates dendritic development via similar mechanisms, showed additive effects on MAPK activation, CREB phosphorylation and dendritic growth. Collectively, these results support the conclusion that regulation of cortical dendritic morphology by HGF is mediated by activation of the MAPK pathway, phosphorylation of CREB and interaction of CREB with CRTC1.

The distribution of messenger RNAs encoding the three isoforms of the transducer of regulated cAMP responsive element binding protein activity in the rat forebrain

Increasing evidence indicates that the cAMP responsive element binding protein (CREB)-dependent transcriptional activation of a number of genes requires the CREB co-activator: transducer of regulated CREB activity (TORC). Because of the central importance of CREB in many brain functions, we examined the topographic distribution of TORC1, 2, and 3 mRNAs in specific regions of the rat forebrain. In situ hybridisation analysis revealed that TORC1 is the most abundant isoform in most forebrain structures, followed by TORC2 and TORC3. All three TORC isoforms were found in a number of brain nuclei, the ventricular ependyma and pia mater. Although high levels of TORC1 were widely distributed in the forebrain, TORC2 was found in discrete nuclei and TORC3 mostly in the ependyma, and pia mater. The relative expression of TORC isoforms was confirmed by quantitative reverse transcriptase-polymerase chain reaction analysis in the hippocampus and hypothalamus. In the paraventricular nucleus of the hypothalamus, TORC1 and 2 mRNAs were abundant in the parvicellular and magnocellular neuroendocrine compartments, whereas TORC3 expression was low. All three isoform mRNAs were found elsewhere in the hypothalamus, with the most prominent expression of TORC1 in the ventromedial nucleus, TORC2 in the dorsomedial and arcuate nuclei, TORCs 1 and 2 in the supraoptic nucleus, and TORC2 in the suprachiasmatic nucleus. These differential distribution patterns are consistent with complex roles for all three TORC isoforms in diverse brain structures, and provide a foundation for further studies on the mechanisms of CREB/TORC signalling on brain function.

Serum inducible kinase is a positive regulator of cortical dendrite development and is required for BDNF-promoted dendritic arborization

Serum inducible kinase (SNK), also known as polo-like kinase 2 (PLK2), is a known regulator of mitosis, synaptogenesis and synaptic homeostasis. However, its role in early cortical development is unknown. Herein, we show that snk is expressed in the cortical plate from embryonic day 14, but not in the ventricular/subventricular zones (VZ/SVZ), and SNK protein localizes to the soma and dendrites of cultured immature cortical neurons. Loss of SNK impaired dendritic but not axonal arborization in a dose-dependent manner and overexpression had opposite effects, both in vitro and in vivo. Overexpression of SNK also caused abnormal branching of the leading process of migrating cortical neurons in electroporated cortices. The kinase activity was necessary for these effects. Extracellular signal-regulated kinase (ERK) pathway activity downstream of brain-derived neurotrophic factor (BDNF) stimulation led to increases in SNK protein expression via transcriptional regulation, and this upregulation was necessary for the growth-promoting effect of BDNF on dendritic arborization. Taken together, our results indicate that SNK is essential for dendrite morphogenesis in cortical neurons.

Pituitary adenylate cyclase-activating peptide induces long-lasting neuroprotection through the induction of activity-dependent signaling via the cyclic AMP response element-binding protein-regulated transcription co-activator 1

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuroprotective peptide which exerts its effects mainly through the cAMP-protein kinase A (PKA) pathway. Here, we show that in cortical neurons, PACAP-induced PKA signaling exerts a major part of its neuroprotective effects indirectly, by triggering action potential (AP) firing. Treatment of cortical neurons with PACAP induces a rapid and sustained PKA-dependent increase in AP firing and associated intracellular Ca²⁺ transients, which are essential for the anti-apoptotic actions of PACAP. Transient exposure to PACAP induces long-lasting neuroprotection in the face of apoptotic insults which is reliant on AP firing and the activation of cAMP response element (CRE) binding protein (CREB)-mediated gene expression. Although direct, activity-independent PKA signaling is sufficient to trigger phosphorylation on CREB’s activating serine-133 site, this is insufficient for activation of CREB-mediated gene expression. Full activation is dependent on CREB-regulated transcription co-activator 1 (CRTC1), whose PACAP-induced nuclear import is dependent on firing activity-dependent calcineurin signaling. Over-expression of CRTC1 is sufficient to rescue PACAP-induced CRE-mediated gene expression in the face of activity-blockade, while dominant negative CRTC1 interferes with PACAP-induced, CREB-mediated neuroprotection. Thus, the enhancement of AP firing may play a significant role in the neuroprotective actions of PACAP and other adenylate cyclase-coupled ligands.

Salt-inducible kinase 1 is present in lung alveolar epithelial cells and regulates active sodium transport

Salt-inducible kinase 1 (SIK1) in epithelial cells mediates the increases in active sodium transport (Na(+), K(+)-ATPase-mediated) in response to elevations in the intracellular concentration of sodium. In lung alveolar epithelial cells increases in active sodium transport in response to β-adrenergic stimulation increases pulmonary edema clearance. Therefore, we sought to determine whether SIK1 is present in lung epithelial cells and to examine whether isoproterenol-dependent stimulation of Na(+), K(+)-ATPase is mediated via SIK1 activity. All three SIK isoforms were present in airway epithelial cells, and in alveolar epithelial cells type 1 and type 2 from rat and mouse lungs, as well as from human and mouse cell lines representative of lung alveolar epithelium. In mouse lung epithelial cells, SIK1 associated with the Na(+), K(+)-ATPase α-subunit, and isoproterenol increased SIK1 activity. Isoproterenol increased Na(+), K(+)-ATPase activity and the incorporation of Na(+), K(+)-ATPase molecules at the plasma membrane. Furthermore, those effects were abolished in cells depleted of SIK1 using shRNA, or in cells overexpressing a SIK1 kinase-deficient mutant. These results provide evidence that SIK1 is present in lung epithelial cells and that its function is relevant for the action of isoproterenol during regulation of active sodium transport. As such, SIK1 may constitute an important target for drug discovery aimed at improving the clearance of pulmonary edema.

Stress induces parallel changes in corticotrophin-releasing hormone (CRH) Transcription and nuclear translocation of transducer of regulated cAMP response element-binding activity 2 in hypothalamic CRH neurones

Recent studies in vitro have shown that the cAMP response element-binding (CREB) co-activator, transducer of regulated CREB activity (TORC), is required for transcriptional activation of the corticotrophin-releasing hormone (CRH) gene. To determine the physiological importance of TORC2 regulating CRH transcription during stress, we examined the localisation of TORC2 in CRH neurones, as well as the relationship between changes in CRH heterogeneous nuclear (hn)RNA, nuclear translocation of TORC2 and binding of TORC2 to the CRH promoter. Immunohistochemistry revealed TORC2 immunoreactivity (irTORC2) in the dorsolateral (magnocellular) and dorsomedial (parvocellular) regions of the hypothalamic paraventricular nucleus (PVN). Although staining was mostly cytosolic under basal conditions, there was a marked increase in nuclear irTORC2 in the dorsomedial region after 30 min of restraint, concomitant with increases in CRH hnRNA levels. Levels of nuclear irTORC2 and CRH hnRNA had returned to basal 4 h after stress. Double-staining immunohistochemistry showed TORC2 co-staining in 100% of detected CRH neurones, and nuclear translocation after 30 min of restraint in 61%. Cellular distribution of TORC2 in the dorsolateral PVN was unaffected by restraint. Chromatin immunoprecipitation experiments showed recruitment of TORC2 and phosphorylated CREB (pCREB) by the CRH promoter after 30 min of restraint, but 4 h after stress only pCREB was associated with the CRH promoter. The demonstration that TORC2 translocates to the nucleus of hypothalamic CRH neurones and interacts with the CRH promoter in conjunction with the activation of CRH transcription during restraint stress, provides strong evidence for the involvement of TORC2 in the physiological regulation of CRH transcription.

SIK2 is a key regulator for neuronal survival after ischemia via TORC1-CREB

The cAMP responsive element-binding protein (CREB) functions in a broad array of biological and pathophysiological processes. We found that salt-inducible kinase 2 (SIK2) was abundantly expressed in neurons and suppressed CREB-mediated gene expression after oxygen-glucose deprivation (OGD). OGD induced the degradation of SIK2 protein concomitantly with the dephosphorylation of the CREB-specific coactivator transducer of regulated CREB activity 1 (TORC1), resulting in the activation of CREB and its downstream gene targets. Ca(2+)/calmodulin-dependent protein kinase I/IV are capable of phosphorylating SIK2 at Thr484, resulting in SIK2 degradation in cortical neurons. Neuronal survival after OGD was significantly increased in neurons isolated from sik2(-/-) mice, and ischemic neuronal injury was significantly reduced in the brains of sik2(-)(/-) mice subjected to transient focal ischemia. These findings suggest that SIK2 plays critical roles in neuronal survival, is modulated by CaMK I/IV, and regulates CREB via TORC1.

Role of salt-induced kinase 1 in androgen neuroprotection against cerebral ischemia

Androgens within physiological ranges protect castrated male mice from cerebral ischemic injury. Yet, underlying mechanisms are unclear. Here, we report that, after middle cerebral artery occlusion (MCAO), salt-induced kinase 1 (SIK1) was induced by a potent androgen-dihydrotestosterone (DHT) at protective doses. To investigate whether SIK1 contributes to DHT neuroprotection after cerebral ischemia, we constructed lentivirus-expressing small interference RNA (siRNA) against SIK1. The SIK1 knockdown by siRNA exacerbated oxygen-glucose deprivation (OGD)-induced cell death in primary cortical neurons, suggesting that SIK1 is an endogenous neuroprotective gene against cerebral ischemia. Furthermore, lentivirus-mediated SIK1 knockdown increased both cortical and striatal infarct sizes in castrated mice treated with a protective dose of DHT. Earlier studies show that SIK1 inhibits histone deacetylase (HDAC) activities by acting as a class IIa HDAC kinase. We observed that SIK1 knockdown decreased histone H3 acetylation in primary neurons. The SIK1 siRNA also exacerbated OGD-induced neuronal death in the presence of trichostatin A (TSA), an HDAC inhibitor, and decreased histone H3 acetylation at 4 hours reoxygenation in TSA-treated neurons. Finally, we showed that DHT at protective doses prevented ischemia-induced histone deacetylation after MCAO. Our finding suggests that SIK1 contributes to neuroprotection by androgens within physiological ranges by inhibiting histone deacetylation.

RNA polymerase 1-driven transcription as a mediator of BDNF-induced neurite outgrowth

Neurite outgrowth is essential for development of the nervous system. Neurotrophins including BDNF are among extracellular signals that regulate neurite outgrowth. The ERK1/2 pathway contributes to intracellular signaling networks transducing the pro-neuritic effects of BDNF. In the nucleolus, RNA polymerase-1 (Pol1)-mediated transcription regulates ribosomal biogenesis, enabling cellular protein synthesis and growth. Hence, we tested the possibility that Pol1 is an effector for pro-neuritic signals such as BDNF. We report that Pol1-mediated nucleolar transcription was increased by BDNF in an ERK1/2-dependent manner in rat forebrain neurons. Conversely, in cultured hippocampal neurons, knockdown of a Pol1 coactivator, transcription initiation factor 1A (TIF1A), attenuated BDNF- or ERK1/2-induced neurite outgrowth. Also, upon overexpression, a constitutively active mutant of TIF1A strongly promoted neurite outgrowth, including increases in total neurite length and branching. Finally, overexpression of wild-type TIF1A enhanced the pro-neuritic effects of ERK1/2 activation. These observations indicate that the Pol1-mediated nucleolar transcription regulates neurite outgrowth and serves as a major pro-neuritic effector of the BDNF-activated ERK1/2 pathway. Thus, development of the nervous system appears critically dependent on the nucleolus.

beta-Amyloid disrupts activity-dependent gene transcription required for memory through the CREB coactivator CRTC1

Activity-dependent gene expression mediating changes of synaptic efficacy is important for memory storage, but the mechanisms underlying gene transcriptional changes in age-related memory disorders are poorly understood. In this study, we report that gene transcription mediated by the cAMP-response element binding protein (CREB)-regulated transcription coactivator CRTC1 is impaired in neurons and brain from an Alzheimer's disease (AD) transgenic mouse expressing the human beta-amyloid precursor protein (APP(Sw,Ind)). Suppression of CRTC1-dependent gene transcription by beta-amyloid (Abeta) in response to cAMP and Ca(2+) signals is mediated by reduced calcium influx and disruption of PP2B/calcineurin-dependent CRTC1 dephosphorylation at Ser151. Consistently, expression of CRTC1 or active CRTC1 S151A and calcineurin mutants reverse the deficits on CRTC1 transcriptional activity in APP(Sw,Ind) neurons. Inhibition of calcium influx by pharmacological blockade of L-type voltage-gated calcium channels (VGCCs), but not by blocking NMDA or AMPA receptors, mimics the decrease on CRTC1 transcriptional activity observed in APP(Sw,Ind) neurons, whereas agonists of L-type VGCCs reverse efficiently these deficits. Consistent with a role of CRTC1 on Abeta-induced synaptic and memory dysfunction, we demonstrate a selective reduction of CRTC1-dependent genes related to memory (Bdnf, c-fos, and Nr4a2) coinciding with hippocampal-dependent spatial memory deficits in APP(Sw,Ind) mice. These findings suggest that CRTC1 plays a key role in coupling synaptic activity to gene transcription required for hippocampal-dependent memory, and that Abeta could disrupt cognition by affecting CRTC1 function.

Regulation of dendritic development by BDNF requires activation of CRTC1 by glutamate

Dendritic growth is essential for the establishment of a functional nervous system. Among extrinsic signals that control dendritic development, substantial evidence indicates that BDNF regulates dendritic morphology. However, little is known about the underlying mechanisms by which BDNF controls dendritic growth. In this study, we show that the MAPK signaling pathway and the transcription factor cAMP response element-binding protein (CREB) mediate the effects of BDNF on dendritic length and complexity. However, phosphorylation of CREB alone is not sufficient for the stimulation of dendritic growth by BDNF. Thus, using a mutant form of CREB unable to bind CREB-regulated transcription coactivator (CRTC1), we demonstrate that this effect also requires a functional interaction between CREB and CRTC1. Moreover, inhibition of CRTC1 expression by shRNA-mediated knockdown abolished BDNF-induced dendritic growth of cortical neurons. Interestingly, we found that nuclear translocation of CRTC1 results from activation of NMDA receptors by glutamate, a process that is essential for the effects of BDNF on dendritic development. Together, these data identify a previously unrecognized mechanism by which CREB and the coactivator CRTC1 mediate the effects of BDNF on dendritic growth.

Striatal microRNA controls cocaine intake through CREB signalling

Cocaine addiction is characterized by a gradual loss of control over drug use, but the molecular mechanisms regulating vulnerability to this process remain unclear. Here we report that microRNA-212 (miR-212) is upregulated in the dorsal striatum of rats with a history of extended access to cocaine. Striatal miR-212 decreases responsiveness to the motivational properties of cocaine by markedly amplifying the stimulatory effects of the drug on cAMP response element binding protein (CREB) signalling. This action occurs through miR-212-enhanced Raf1 activity, resulting in adenylyl cyclase sensitization and increased expression of the essential CREB co-activator TORC (transducer of regulated CREB; also known as CRTC). Our findings indicate that striatal miR-212 signalling has a key role in determining vulnerability to cocaine addiction, reveal new molecular regulators that control the complex actions of cocaine in brain reward circuitries and provide an entirely new direction for the development of anti-addiction therapeutics based on the modulation of noncoding RNAs.

Synapse formation and remodeling

Synapses are specialized structures that mediate information flow between neurons and target cells, and thus are the basis for neuronal system to execute various functions, including learning and memory. There are around 10(11) neurons in the human brain, with each neuron receiving thousands of synaptic inputs, either excitatory or inhibitory. A synapse is an asymmetric structure that is composed of pre-synaptic axon terminals, synaptic cleft, and postsynaptic compartments. Synapse formation involves a number of cell adhesion molecules, extracellular factors, and intracellular signaling or structural proteins. After the establishment of synaptic connections, synapses undergo structural or functional changes, known as synaptic plasticity which is believed to be regulated by neuronal activity and a variety of secreted factors. This review summarizes recent progress in the field of synapse development, with particular emphasis on the work carried out in China during the past 10 years (1999-2009).

The neuroprotective effects of Tanshinone IIA are associated with induced nuclear translocation of TORC1 and upregulated expression of TORC1, pCREB and BDNF in the acute stage of ischemic stroke

The activation of cAMP response element binding protein (CREB) and transcription of CRE-targeted genes play critical roles in various physiological and pathological conditions. Transducers of regulated CREB activity (TORCs) represent a new family of conserved CREB coactivators that promote the activation of CRE-targeted genes. Tanshinone IIA (TSA) has been proven to protect the brain against focal ischemia injury. However, little is known regarding the underlying mechanisms. Herein, we examined the activity-dependent nuclear translocation of TORC1 (transducer of regulated CREB activity 1) and the expression of TORC1, phosphorylated CREB (pCREB) and BDNF (brain-derived neurotrophic factor) at the early time of ischemic stroke as well as after the treatment with TSA. We observed a bimodal increase in pCREB, TORC1 and BDNF protein levels and transient nuclear accumulation of TORC1 in the acute stage of ischemia. Compared with vehicle group, TSA (20mg/kg) dramatically lessened neurological deficits scores, brain water contents and infarct sizes, significantly enhanced nuclear accumulation of TORC1 and upregulated the expression of TORC1, pCREB and BDNF (P<0.05). Collectively, the present results suggest that TSA protects rat brain from pristine ischemic damage in cerebral cortex, which might be correlated with induced nuclear translocation of TORC1 and upregulated expression of TORC1, pCREB and BDNF.

Regulation of Kv channel expression and neuronal excitability in rat medial nucleus of the trapezoid body maintained in organotypic culture

Principal neurons of the medial nucleus of the trapezoid body (MNTB) express a spectrum of voltage-dependent K(+) conductances mediated by Kv1-Kv4 channels, which shape action potential (AP) firing and regulate intrinsic excitability. Postsynaptic factors influencing expression of Kv channels were explored using organotypic cultures of brainstem prepared from P9-P12 rats and maintained in either low (5 mm, low-K) or high (25 mm, high-K) [K(+)](o) medium. Whole cell patch-clamp recordings were made after 7-28 days in vitro. MNTB neurons cultured in high-K medium maintained a single AP firing phenotype, while low-K cultures had smaller K(+) currents, enhanced excitability and fired multiple APs. The calyx of Held inputs degenerated within 3 days in culture, having lost their major afferent input; this preparation of calyx-free MNTB neurons allowed the effects of postsynaptic depolarisation to be studied with minimal synaptic activity. The depolarization caused by the high-K aCSF only transiently increased spontaneous AP firing (<2 min) and did not measurably increase synaptic activity. Chronic depolarization in high-K cultures raised basal levels of [Ca(2+)](i), increased Kv3 currents and shortened AP half-widths. These events relied on raised [Ca(2+)](i), mediated by influx through voltage-gated calcium channels (VGCCs) and release from intracellular stores, causing an increase in cAMP-response element binding protein (CREB) phosphorylation. Block of VGCCs or of CREB function suppressed Kv3 currents, increased AP duration, and reduced Kv3.3 and c-fos expression. Real-time PCR revealed higher Kv3.3 and Kv1.1 mRNA in high-K compared to low-K cultures, although the increased Kv1.1 mRNA was mediated by a CREB-independent mechanism. We conclude that Kv channel expression and hence the intrinsic membrane properties of MNTB neurons are homeostatically regulated by [Ca(2+)](i)-dependent mechanisms and influenced by sustained depolarization of the resting membrane potential.