Cell biological mechanisms of activity-dependent synapse to nucleus translocation of CRTC1 in neurons

A rapid and bidirectional reporter of neural activity reveals neural correlates of social behaviors in Drosophila

Neural activity is modulated over different timescales encompassing subseconds to hours, reflecting changes in external environment, internal state and behavior. Using Drosophila as a model, we developed a rapid and bidirectional reporter that provides a cellular readout of recent neural activity. This reporter uses nuclear versus cytoplasmic distribution of CREB-regulated transcriptional co-activator (CRTC). Subcellular distribution of GFP-tagged CRTC (CRTC::GFP) bidirectionally changes on the order of minutes and reflects both increases and decreases in neural activity. We established an automated machine-learning-based routine for efficient quantification of reporter signal. Using this reporter, we demonstrate mating-evoked activation and inactivation of modulatory neurons. We further investigated the functional role of the master courtship regulator gene fruitless (fru) and show that fru is necessary to ensure activation of male arousal neurons by female cues. Together, our results establish CRTC::GFP as a bidirectional reporter of recent neural activity suitable for examining neural correlates in behavioral contexts.

Protein transport from pre- and postsynapse to the nucleus: Mechanisms and functional implications

The extreme length of neuronal processes poses a challenge for synapse-to-nucleus communication. In response to this challenge several different mechanisms have evolved in neurons to couple synaptic activity to the regulation of gene expression. One of these mechanisms concerns the long-distance transport of proteins from pre- and postsynaptic sites to the nucleus. In this review we summarize current evidence on mechanisms of transport and consequences of nuclear import of these proteins for gene transcription. In addition, we discuss how information from pre- and postsynaptic sites might be relayed to the nucleus by this type of long-distance signaling. When applicable, we highlight how long-distance protein transport from synapse-to-nucleus can provide insight into the pathophysiology of disease or reveal new opportunities for therapeutic intervention.

Combining multiple fluorescence imaging techniques in biology: when one microscope is not enough

While fluorescence microscopy has proven to be an exceedingly useful tool in bioscience, it is difficult to offer simultaneous high resolution, fast speed, large volume, and good biocompatibility in a single imaging technique. Thus, when determining the image data required to quantitatively test a complex biological hypothesis, it often becomes evident that multiple imaging techniques are necessary. Recent years have seen an explosion in development of novel fluorescence microscopy techniques, each of which features a unique suite of capabilities. In this Technical Perspective, we highlight recent studies to illustrate the benefits, and often the necessity, of combining multiple fluorescence microscopy modalities. We provide guidance in choosing optimal technique combinations to effectively address a biological question. Ultimately, we aim to promote a more well-rounded approach in designing fluorescence microscopy experiments, leading to more robust quantitative insight.

Experience-Regulated Neuronal Signaling in Maternal Behavior

Maternal behavior is shaped and challenged by the changing developmental needs of offspring and a broad range of environmental factors, with evidence indicating that the maternal brain exhibits a high degree of plasticity. This plasticity is displayed within cellular and molecular systems, including both intra- and intercellular signaling processes as well as transcriptional profiles. This experience-associated plasticity may have significant overlap with the mechanisms controlling memory processes, in particular those that are activity-dependent. While a significant body of work has identified various molecules and intracellular processes regulating maternal care, the role of activity- and experience-dependent processes remains unclear. We discuss recent progress in studying activity-dependent changes occurring at the synapse, in the nucleus, and during the transport between these two structures in relation to maternal behavior. Several pre- and postsynaptic molecules as well as transcription factors have been found to be critical in these processes. This role reflects the principal importance of the molecular and cellular mechanisms of memory formation to maternal and other behavioral adaptations.

New Insights Into the Pivotal Role of CREB-Regulated Transcription Coactivator 1 in Depression and Comorbid Obesity

Depression and obesity are major public health concerns, and there is mounting evidence that they share etiopathophysiological mechanisms. The neurobiological pathways involved in both mood and energy balance regulation are complex, multifactorial and still incompletely understood. As a coactivator of the pleiotropic transcription factor cAMP response element-binding protein (CREB), CREB-regulated transcription coactivator 1 (CRTC1) has recently emerged as a novel regulator of neuronal plasticity and brain functions, while CRTC1 dysfunction has been associated with neurodegenerative and psychiatric diseases. This review focuses on recent evidence emphasizing the critical role of CRTC1 in the neurobiology of depression and comorbid obesity. We discuss the role of CRTC1 downregulation in mediating chronic stress-induced depressive-like behaviors, and antidepressant response in the light of the previously characterized Crtc1 knockout mouse model of depression. The putative role of CRTC1 in the alteration of brain energy homeostasis observed in depression is also discussed. Finally, we highlight rodent and human studies supporting the critical involvement of CRTC1 in depression-associated obesity.

Neuronal activity regulates the nuclear proteome to promote activity-dependent transcription

The formation and plasticity of neuronal circuits relies on dynamic activity-dependent gene expression. Although recent work has revealed the identity of important transcriptional regulators and of genes that are transcribed and translated in response to activity, relatively little is known about the cell biological mechanisms by which activity alters the nuclear proteome of neurons to link neuronal stimulation to transcription. Using nucleus-specific proteomic mapping in silenced and stimulated neurons, we uncovered an understudied mechanism of nuclear proteome regulation: activity-dependent proteasome-mediated degradation. We found that the tumor suppressor protein PDCD4 undergoes rapid stimulus-induced degradation in the nucleus of neurons. We demonstrate that degradation of PDCD4 is required for normal activity-dependent transcription and that PDCD4 target genes include those encoding proteins critical for synapse formation, remodeling, and transmission. Our findings highlight the importance of the nuclear proteasome in regulating the activity-dependent nuclear proteome and point to a specific role for PDCD4 as a regulator of activity-dependent transcription in neurons.

Ca2+/CaM binding to CaMKI promotes IMA-3 importin binding and nuclear translocation in sensory neurons to control behavioral adaptation

Sensory and behavioral plasticity are essential for animals to thrive in changing environments. As key effectors of intracellular calcium signaling, Ca²⁺/calmodulin-dependent protein kinases (CaMKs) can bridge neural activation with the many regulatory processes needed to orchestrate sensory adaptation, including by relaying signals to the nucleus. Here, we elucidate the molecular mechanism controlling the cell activation-dependent nuclear translocation of CMK-1, the Caenorhabditis elegans ortholog of mammalian CaMKI/IV, in thermosensory neurons in vivo. We show that an intracellular Ca²⁺ concentration elevation is necessary and sufficient to favor CMK-1 nuclear import. The binding of Ca²⁺/CaM to CMK-1 increases its affinity for IMA-3 importin, causing a redistribution with a relatively slow kinetics, matching the timescale of sensory adaptation. Furthermore, we show that this mechanism enables the encoding of opposite nuclear signals in neuron types with opposite calcium-responses and that it is essential for experience-dependent behavioral plasticity and gene transcription control in vivo. Since CaMKI/IV are conserved regulators of adaptable behaviors, similar mechanisms could exist in other organisms and for other sensory modalities.

Sediment quality assessment combining chemical and biological (non)target analysis

Aquatic sediments act as a storage for diverse mixtures of organic and inorganic contaminants. Nevertheless, most evaluations of contaminated sediments have been limited to the assessment of concentrations of target compounds and lethal effects on some test species. To identify the organic contaminants causing sub-lethal effects of contaminated sediment, this study combined chemical and biological (non)target analysis involving comprehensive two-dimensional gas chromatography coupled with a time-of-flight Mass Spectrometer (GCxGC/ToF-MS) analysis, embryonic malformation and high-throughput sequencing (RNA-seq) analysis on developing flounder. Polycyclic aromatic hydrocarbons were more abundant in the sediment extract of Yeongil Bay (SEY), while Jinhae Bay (SEJ) was contaminated with a large amount of unidentified chemicals. The unidentified chemicals of SEJ included branched alkanes, oxygenated cycloalkanes, heterogeneous hydrocarbons, and other unknown compounds. Percentage of pericardial edema was the highest in embryonic flounder exposed to SEY. Consistent with the morphogenesis results, the expression level of genes related to heart formation including the nkx2.5 and robo1 was greater in embryonic flounder exposed to SEY. In the analyses of differential gene expression profiles (cutoff P < 0.05), by RNA-seq, embryos exposed to SEJ showed changes related to cell differentiation, cell part morphogenesis, neurogenesis, and neuron development. Genes related to neurogenesis and positive regulation of molecular functions variated significantly in embryos exposed to SEY. These results demonstrated the advantages of combining target and non-target analysis to accurately evaluate the major chemical groups causing sediment toxicity. Therefore, this work provided a useful approach to tracking and revealing the causes of toxic effects and identifying potential toxic mechanisms.

Importin-α2 mediates brain development, learning and memory consolidation in Drosophila

Neuronal development and memory consolidation are conserved processes that rely on nuclear-cytoplasmic transport of signaling molecules to regulate gene activity and initiate cascades of downstream cellular events. Surprisingly, few reports address and validate this widely accepted perspective. Here we show that Importin-α2 (Imp-α2), a soluble nuclear transporter that shuttles cargoes between the cytoplasm and nucleus, is vital for brain development, learning and persistent memory in Drosophila melanogaster. Mutations in importin-α2 (imp-α2, known as Pendulin or Pen and homologous with human KPNA2) are alleles of mushroom body miniature B (mbmB), a gene known to regulate aspects of brain development and influence adult behavior in flies. Mushroom bodies (MBs), paired associative centers in the brain, are smaller than normal due to defective proliferation of specific intrinsic Kenyon cell (KC) neurons in mbmB mutants. Extant KCs projecting to the MB β-lobe terminate abnormally on the contralateral side of the brain. mbmB adults have impaired olfactory learning but normal memory decay in most respects, except that protein synthesis-dependent long-term memory (LTM) is abolished. This observation supports an alternative mechanism of persistent memory in which mutually exclusive protein-synthesis-dependent and -independent forms rely on opposing cellular mechanisms or circuits. We propose a testable model of Imp-α2 and nuclear transport roles in brain development and conditioned behavior. Based on our molecular characterization, we suggest that mbmB is hereafter referred to as imp-α2^mbmB.

Genomic Decoding of Neuronal Depolarization by Stimulus-Specific NPAS4 Heterodimers

Cells regulate gene expression in response to salient external stimuli. In neurons, depolarization leads to the expression of inducible transcription factors (ITFs) that direct subsequent gene regulation. Depolarization encodes both a neuron's action potential (AP) output and synaptic inputs, via excitatory postsynaptic potentials (EPSPs). However, it is unclear if distinct types of electrical activity can be transformed by an ITF into distinct modes of genomic regulation. Here, we show that APs and EPSPs in mouse hippocampal neurons trigger two spatially segregated and molecularly distinct induction mechanisms that lead to the expression of the ITF NPAS4. These two pathways culminate in the formation of stimulus-specific NPAS4 heterodimers that exhibit distinct DNA binding patterns. Thus, NPAS4 differentially communicates increases in a neuron's spiking output and synaptic inputs to the nucleus, enabling gene regulation to be tailored to the type of depolarizing activity along the somato-dendritic axis of a neuron.

(Epi)genetic regulation of CRTC1 in human eating behaviour and fat distribution

BACKGROUND

In brain, CREB-regulated transcription co-activator 1 (CRTC1) is involved in metabolic dysregulation. In humans a SNP in CRTC1 was associated to body fat percentage and two SNPs affected RNA Pol II binding and chromatin structure, implying epigenetic regulation of CRTC1. We sought to understand the relevance of CRTC1 SNPs, DNA methylation and expression in human eating behaviour and its relationship to clinical variables of obesity in blood and adipose tissue.

METHODS

13 CRTC1 SNPs were included to analyze eating behaviour. For rs7256986, follow up association analyses were applied on DNA methylation, CRTC1 expression and clinical parameters. Linear regression was used throughout the study adjusted for age, sex and BMI. Besides data extraction from previous work, rs7256986 was de-novo genotyped and DNA methylation was evaluated by using pyrosequencing.

FINDINGS

We found several SNPs in the CRTC1 locus nominally associated with human eating behaviour or 2hr postprandial insulin levels and observed a correlation with alcohol and coffee intake (all P < 0.05). G-allele carriers of rs7256986 showed slightly increased hip circumference. We showed that rs7256986 represents a methylation quantitative trait locus (meQTL) in whole blood and adipose tissue. The presence of the SNP and/or DNA methylation correlated with CRTC1 gene expression which in turn, related to BMI and fat distribution.

INTERPRETATION

Our data support the known role of CRCT1 regulating energy metabolism in brain. Here, we highlight relevance of CRTC1 regulation in blood and adipose tissue. FUND: IFB AdiposityDiseases (BMBF); n609020-Scientia Fellows; Helse-SørØst; DFG: CRC 1052/1 and/2; Kompetenznetz Adipositas, German Diabetes Association.

Nuclear Entry of CRTC1 as Druggable Target of Acquired Pigmentary Disorder

Rationale: SOX10 (SRY-related HMG-box 10) and MITF-M (microphthalmia-associated transcription factor M) restrict the expression of melanogenic genes, such as TYR (tyrosinase), in melanocytes. DACE (diacetylcaffeic acid cyclohexyl ester) inhibits melanin production in α-MSH (α-melanocyte stimulating hormone)-activated B16-F0 melanoma cells. In this study, we evaluated the antimelanogenic activity of DACE in vivo and elucidated the molecular basis of its action.

Methods: We employed melanocyte cultures and hyperpigmented skin samples for pigmentation assays, and applied chromatin immunoprecipitation, immunoblotting, RT-PCR or siRNA-based knockdown for mechanistic analyses.

Results: Topical treatment with DACE mitigated UV-B-induced hyperpigmentation in the skin with attenuated expression of MITF-M and TYR. DACE also inhibited melanin production in α-MSH- or ET-1 (endothelin 1)-activated melanocyte cultures. As a mechanism, DACE blocked the nuclear import of CRTC1 (CREB-regulated co-activator 1) in melanocytes. DACE resultantly inhibited SOX10 induction, and suppressed the transcriptional abilities of CREB/CRTC1 heterodimer and SOX10 at MITF-M promoter, thereby ameliorating facultative melanogenesis. Furthermore, this study unveiled new issues in melanocyte biology that i) KPNA1 (Impα5) escorted CRTC1 as a cargo across the nuclear envelope, ii) SOX10 was inducible in the melanogenic process, and iii) CRTC1 could direct SOX10 induction at the transcription level.

Conclusion: We propose the targeting of CRTC1 as a unique strategy in the treatment of acquired pigmentary disorders.

TorsinA dysfunction causes persistent neuronal nuclear pore defects

A critical challenge to deciphering the pathophysiology of neurodevelopmental disease is identifying which of the myriad abnormalities that emerge during CNS maturation persist to contribute to long-term brain dysfunction. Childhood-onset dystonia caused by a loss-of-function mutation in the AAA+ protein torsinA exemplifies this challenge. Neurons lacking torsinA develop transient nuclear envelope (NE) malformations during CNS maturation, but no NE defects are described in mature torsinA null neurons. We find that during postnatal CNS maturation torsinA null neurons develop mislocalized and dysfunctional nuclear pore complexes (NPC) that lack NUP358, normally added late in NPC biogenesis. SUN1, a torsinA-related molecule implicated in interphase NPC biogenesis, also exhibits localization abnormalities. Whereas SUN1 and associated nuclear membrane abnormalities resolve in juvenile mice, NPC defects persist into adulthood. These findings support a role for torsinA function in NPC biogenesis during neuronal maturation and implicate altered NPC function in dystonia pathophysiology.

The C-terminus of NMDAR GluN1-1a Subunit Translocates to Nucleus and Regulates Synaptic Function

NMDARs, the Ca²⁺ permeable channels, play central roles in synaptic plasticity, brain development, learning, and memory. NMDAR binding partners and associated signaling has been extensively studied in synapse-to-nucleus communications. However, whether NMDARs could directly regulate synapse-to-nucleus communications is largely unknown. Here, we analyze the four alternative splicing of the C-terminus isoforms of GluN1 (1a, 2a, 3a, and 4a), and find that C1 domain of GluN1 is necessary for nuclear localization. Besides, we find that the 10 basic amino acids in C1 domain determine the nuclear localization of GluN1 C-terminus. Further investigating the expression patterns of the full length of GluN1 four isoforms shows that only GluN-1a exhibits the cytoplasmic and nucleus distribution in primary hippocampal neurons. Electrophysiological analyses also show that over-expression of GluN1 C-terminus without C1 domain doesn't affect synaptic transmission, whereas GluN1 C-terminus containing C1 domain potentiates NMDAR-mediated synaptic transmission. Our data suggested that the 10 basic amino acids in C1 domain determine translocation of GluN1 C-terminus into nucleus and regulate synaptic transmission.

Landscape of nuclear transport receptor cargo specificity

Nuclear transport receptors (NTRs) recognize localization signals of cargos to facilitate their passage across the central channel of nuclear pore complexes (NPCs). About 30 different NTRs constitute different transport pathways in humans and bind to a multitude of different cargos. The exact cargo spectrum of the majority of NTRs, their specificity and even the extent to which active nucleocytoplasmic transport contributes to protein localization remains understudied because of the transient nature of these interactions and the wide dynamic range of cargo concentrations. To systematically map cargo-NTR relationships in situ, we used proximity ligation coupled to mass spectrometry (BioID). We systematically fused the engineered biotin ligase BirA* to 16 NTRs. We estimate that a considerable fraction of the human proteome is subject to active nuclear transport. We quantified the specificity and redundancy in NTR interactions and identified transport pathways for cargos. We extended the BioID method by the direct identification of biotinylation sites. This approach enabled us to identify interaction interfaces and to discriminate direct versus piggyback transport mechanisms. Data are available via ProteomeXchange with identifier PXD007976.

The malleable brain: plasticity of neural circuits and behavior - a review from students to students

One of the most intriguing features of the brain is its ability to be malleable, allowing it to adapt continually to changes in the environment. Specific neuronal activity patterns drive long-lasting increases or decreases in the strength of synaptic connections, referred to as long-term potentiation and long-term depression, respectively. Such phenomena have been described in a variety of model organisms, which are used to study molecular, structural, and functional aspects of synaptic plasticity. This review originated from the first International Society for Neurochemistry (ISN) and Journal of Neurochemistry (JNC) Flagship School held in Alpbach, Austria (Sep 2016), and will use its curriculum and discussions as a framework to review some of the current knowledge in the field of synaptic plasticity. First, we describe the role of plasticity during development and the persistent changes of neural circuitry occurring when sensory input is altered during critical developmental stages. We then outline the signaling cascades resulting in the synthesis of new plasticity-related proteins, which ultimately enable sustained changes in synaptic strength. Going beyond the traditional understanding of synaptic plasticity conceptualized by long-term potentiation and long-term depression, we discuss system-wide modifications and recently unveiled homeostatic mechanisms, such as synaptic scaling. Finally, we describe the neural circuits and synaptic plasticity mechanisms driving associative memory and motor learning. Evidence summarized in this review provides a current view of synaptic plasticity in its various forms, offers new insights into the underlying mechanisms and behavioral relevance, and provides directions for future research in the field of synaptic plasticity. Read the Editorial Highlight for this article on page 788. Cover Image for this issue: doi: 10.1111/jnc.13815.

Regulated transport of signaling proteins from synapse to nucleus

Synapse-to-nucleus communication is essential for neural development, plasticity, and repair. In addition to fast electrochemical signaling, neurons employ a slower mechanism of protein transport from synapse-to-nucleus. This mechanism provides potential advantages, including the encoding of spatial information. Many synaptonuclear signaling proteins are transported from the postsynaptic compartment to the nucleus in an activity-dependent manner. The phosphorylation state of two such proteins, CRTC1 and Jacob, is dependent on the stimulus type. While most studies have focused on postsynaptic synaptonuclear communication, a transcriptional co-repressor, CtBP1, was recently discovered to undergo activity-dependent translocation from the presynaptic compartment to the nucleus. Recent evidence indicates that synapse-to-nucleus communication could be cell type-specific, including the identification of a distinct mechanism of excitation-transcription coupling in inhibitory neurons.

Deregulation of CRTCs in Aging and Age-Related Disease Risk

Advances in public health in the past century have seen a sharp increase in human life expectancy. With these changes have come an increased prevalence of age-related pathologies and health burdens in the elderly. Patient age is the biggest risk factor for multiple chronic conditions that often occur simultaneously within a single individual. An alternative to disease-centric therapeutic approaches is that of 'geroscience', which aims to define molecular mechanisms that link age to overall disease risk. One such mechanism is deregulation of CREB-regulated transcriptional coactivators (CRTCs). Initially identified for their role in modulating CREB transcription, the past 5 years has seen an expansion in knowledge of new cellular regulators and roles of CRTCs beyond CREB. CRTCs have been shown to modulate organismal aging in Caenorhabditis elegans and to impact on age-related diseases in humans. We discuss CRTC deregulation as a new driver of aging that integrates the link between age and disease risk.

Screening the Molecular Framework Underlying Local Dendritic mRNA Translation

In the last decade, bioinformatic analyses of high-throughput proteomics and transcriptomics data have enabled researchers to gain insight into the molecular networks that may underlie lasting changes in synaptic efficacy. Development and utilization of these techniques have advanced the field of learning and memory significantly. It is now possible to move from the study of activity-dependent changes of a single protein to modeling entire network changes that require local protein synthesis. This data revolution has necessitated the development of alternative computational and statistical techniques to analyze and understand the patterns contained within. Thus, the focus of this review is to provide a synopsis of the journey and evolution toward big data techniques to address still unanswered questions regarding how synapses are modified to strengthen neuronal circuits. We first review the seminal studies that demonstrated the pivotal role played by local mRNA translation as the mechanism underlying the enhancement of enduring synaptic activity. In the interest of those who are new to the field, we provide a brief overview of molecular biology and biochemical techniques utilized for sample preparation to identify locally translated proteins using RNA sequencing and proteomics, as well as the computational approaches used to analyze these data. While many mRNAs have been identified, few have been shown to be locally synthesized. To this end, we review techniques currently being utilized to visualize new protein synthesis, a task that has proven to be the most difficult aspect of the field. Finally, we provide examples of future applications to test the physiological relevance of locally synthesized proteins identified by big data approaches.

CRTC1 nuclear localization in the hippocampus of the pilocarpine-induced status epilepticus model of temporal lobe epilepsy

cAMP response-element binding protein (CREB)-dependent genes are differentially expressed in brains of temporal lobe epilepsy (TLE) patients and also in animal models of TLE. Previous studies have demonstrated the importance of CREB regulated transcription in TLE. However, the role of the key regulator of CREB activity, CREB-regulated transcription coactivator 1 (CRTC1), has not been explored in epilepsy. In the present study the pilocarpine-induced status epilepticus (SE) model of TLE was used to study the regulation of CRTC1 during and following SE. Nuclear translocation of CRTC1 is critical for its transcriptional activity, and dephosphorylation at serine 151 residue via calcineurin phosphatase regulates cytoplasmic to nuclear transit of CRTC1. Here, we examined the localization and phosphorylation (Ser151) of CRTC1 in SE-induced rat hippocampus at two different time points after SE onset. One hour after SE onset, we found that CRTC1 translocates to the nucleus of CA1 neurons but not CA3 or dentate granule neurons. We further found that this CRTC1 nuclear localization is independent of Ser151 dephosphorylation since we did not detect any difference in dephosphorylation of Ser151 between control and SE animals at this time point. In contrast, 48 h after SE CRTC1 shows increased nuclear localization in the dentate gyrus (DG) of the SE-induced rats. At 48 h after SE, FK506 treatment blocked CRTC1 nuclear localization and dephosphorylation of Ser151. Our results provide evidence that CREB cofactor CRTC1 translocates into the nucleus of a distinct subset of hippocampal neurons during and following SE and this translocalization is regulated by calcineurin at a later time point following SE. Nuclear CRTC1 can bind to CREB possibly altering transcription during epileptogenesis.