Importin-mediated nuclear transport in neurons

Nucleic acid therapies for CNS diseases: Pathophysiology, targets, barriers, and delivery strategies

Nucleic acid therapeutics have emerged as one of the very advanced and efficacious treatment approaches for debilitating health conditions, including those diseases affecting the central nervous system (CNS). Precise targeting with an optimal control over gene regulation confers long-lasting benefits through the administration of nucleic acid payloads via viral, non-viral, and engineered vectors. The current review majorly focuses on the development and clinical translational potential of non-viral vectors for treating CNS diseases with a focus on their specific design and targeting approaches. These carriers must be able to surmount the various intracellular and extracellular barriers, to ensure successful neuronal transfection and ultimately attain higher therapeutic efficacies. Additionally, the specific challenges associated with CNS administration also include the presence of blood-brain barrier (BBB), the complex pathophysiological and biochemical changes associated with different disease conditions and the existence of non-dividing cells. The advantages offered by lipid-based or polymeric systems, engineered proteins, particle-based systems coupled with various approaches of neuronal targeting have been discussed in the context of a variety of CNS diseases. The possibilities of rapid yet highly efficient gene modifications rendered by the breakthrough methodologies for gene editing and gene manipulation have also opened vast avenues of research in neuroscience and CNS disease therapy. The current review also underscores the extensive scientific efforts to optimize specialized, efficacious yet non-invasive and safer administration approaches to overcome the therapeutic delivery challenges specifically posed by the CNS transport barriers and the overall obstacles to clinical translation.

The Nrf2-Keap1 pathway is activated by steroid hormone signaling to govern neuronal remodeling

The evolutionarily conserved Nrf2-Keap1 pathway is a key antioxidant response pathway that protects cells/organisms against detrimental effects of oxidative stress. Impaired Nrf2 function is associated with cancer and neurodegenerative diseases in humans. However, the function of the Nrf2-Keap1 pathway in the developing nervous systems has not been established. Here we demonstrate a cell-autonomous role of the Nrf2-Keap1 pathway, composed of CncC/Nrf2, Keap1, and MafS, in governing neuronal remodeling during Drosophila metamorphosis. Nrf2-Keap1 signaling is activated downstream of the steroid hormone ecdysone. Mechanistically, the Nrf2-Keap1 pathway is activated via cytoplasmic-to-nuclear translocation of CncC in an importin- and ecdysone-signaling-dependent manner. Moreover, Nrf2-Keap1 signaling regulates dendrite pruning independent of its canonical antioxidant response pathway, acting instead through proteasomal degradation. This study reveals an epistatic link between the Nrf2-Keap1 pathway and steroid hormone signaling and demonstrates an antioxidant-independent but proteasome-dependent role of the Nrf2-Keap1 pathway in neuronal remodeling.

Flotillin-1 interacts with the serotonin transporter and modulates chronic corticosterone response

Aberrant serotonergic neurotransmission in the brain is considered at the core of the pathophysiological mechanisms involved in neuropsychiatric disorders. Gene by environment interactions contribute to the development of depression and involve modulation of the availability and functional activity of the serotonin transporter (SERT). Using behavioral and in vivo electrophysiological approaches together with biochemical, molecular-biological and molecular imaging tools we establish Flotillin-1 (Flot1) as a novel protein interacting with SERT and demonstrate its involvement in the response to chronic corticosterone (CORT) treatment. We show that genetic Flot1 depletion augments chronic CORT-induced behavioral despair and describe concomitant alterations in the expression of SERT, activity of serotonergic neurons and alterations of the glucocorticoid receptor transport machinery. Hence, we propose a role for Flot1 as modulatory factor for the depressogenic consequences of chronic CORT exposure and suggest Flotillin-1-dependent regulation of SERT expression and activity of serotonergic neurotransmission at the core of the molecular mechanisms involved.

Near-Infrared Light-Controlled Gene Expression and Protein Targeting in Neurons and Non-neuronal Cells

Near-infrared (NIR) light-inducible binding of bacterial phytochrome BphP1 to its engineered partner, QPAS1, is used for optical protein regulation in mammalian cells. However, there are no data on the application of the BphP1-QPAS1 pair in cells derived from various mammalian tissues. Here, we tested the functionality of two BphP1-QPAS1-based optogenetic tools-an NIR- and blue-light-sensing system for control of protein localization (iRIS) and an NIR light-sensing system for transcription activation (TA)-in several cell types, including cortical neurons. We found that the performance of these optogenetic tools often relied on physiological properties of a specific cell type, such as nuclear transport, which could limit the applicability of the blue-light-sensitive component of iRIS. In contrast, the NIR-light-sensing component of iRIS performed well in all tested cell types. The TA system showed the best performance in cervical cancer (HeLa), bone cancer (U-2 OS), and human embryonic kidney (HEK-293) cells. The small size of the QPAS1 component allowed the design of adeno-associated virus (AAV) particles, which were applied to deliver the TA system to neurons.

Deciphering glycomics and neuroproteomic alterations in experimental traumatic brain injury: Comparative analysis of aspirin and clopidogrel treatment

As populations age, the number of patients sustaining traumatic brain injury (TBI) and concomitantly receiving preinjury antiplatelet therapy such as aspirin (ASA) and clopidogrel (CLOP) is rising. These drugs have been linked with unfavorable clinical outcomes following TBI, where the exact mechanism(s) involved are still unknown. In this novel work, we aimed to identify and compare the altered proteome profile imposed by ASA and CLOP when administered alone or in combination, prior to experimental TBI. Furthermore, we assessed differential glycosylation PTM patterns following experimental controlled cortical impact model of TBI, ASA, CLOP, and ASA + CLOP. Ipsilateral cortical brain tissues were harvested 48 h postinjury and were analyzed using an advanced neuroproteomics LC-MS/MS platform to assess proteomic and glycoproteins alterations. Of interest, differential proteins pertaining to each group (22 in TBI, 41 in TBI + ASA, 44 in TBI + CLOP, and 34 in TBI + ASA + CLOP) were revealed. Advanced bioinformatics/systems biology and clustering analyses were performed to evaluate biological networks and protein interaction maps illustrating molecular pathways involved in the experimental conditions. Results have indicated that proteins involved in neuroprotective cellular pathways were upregulated in the ASA and CLOP groups when given separately. However, ASA + CLOP administration revealed enrichment in biological pathways relevant to inflammation and proinjury mechanisms. Moreover, results showed differential upregulation of glycoproteins levels in the sialylated N-glycans PTMs that can be implicated in pathological changes. Omics data obtained have provided molecular insights of the underlying mechanisms that can be translated into clinical bedside settings.

Calcium, Reactive Oxygen Species, and Synaptic Plasticity

In this review article, we address how activity-dependent Ca2+ signaling is crucial for hippocampal synaptic/structural plasticity and discuss how changes in neuronal oxidative state affect Ca2+ signaling and synaptic plasticity. We also analyze current evidence indicating that oxidative stress and abnormal Ca2+ signaling contribute to age-related synaptic plasticity deterioration.

Importin-7 mediates memory consolidation through regulation of nuclear translocation of training-activated MAPK in Drosophila

Translocation of signaling molecules, MAPK in particular, from the cytosol to nucleus represents a universal key element in initiating the gene program that determines memory consolidation. Translocation mechanisms and their behavioral impact, however, remain to be determined. Here, we report that a highly conserved nuclear transporter, Drosophila importin-7 (DIM-7), regulates import of training-activated MAPK for consolidation of long-term memory (LTM). We show that silencing DIM-7 functions results in impaired LTM, whereas overexpression of DIM-7 enhances LTM. This DIM-7-dependent regulation of LTM is confined to a consolidation time window and in mushroom body neurons. Image data show that bidirectional alteration in DIM-7 expression results in proportional changes in the intensity of training-activated MAPK accumulated within the nuclei of mushroom body neurons during LTM consolidation. Such DIM-7-regulated nuclear accumulation of activated MAPK is observed only in the training specified for LTM induction and determines the amplitude, but not the time course, of memory consolidation.

Protein trafficking from synapse to nucleus in control of activity-dependent gene expression

Long-lasting changes in neuronal excitability require activity-dependent gene expression and therefore the transduction of synaptic signals to the nucleus. Synaptic activity is rapidly relayed to the nucleus by membrane depolarization and the propagation of Ca(2+)-waves. However, it is unlikely that Ca(2+)-transients alone can explain the specific genomic response to the plethora of extracellular stimuli that control gene expression. In recent years a steadily growing number of studies report the transport of proteins from synapse to nucleus. Potential mechanisms for active retrograde transport and nuclear targets for these proteins have been identified and recent reports assigned first functions to this type of long-distance signaling. In this review we will discuss how the dissociation of synapto-nuclear protein messenger from synaptic and extrasynaptic sites, their transport, nuclear import and the subsequent genomic response relate to the prevailing concept behind this signaling mechanism, the encoding of signals at their site of origin and their decoding in the nucleus.

Axonal transcription factors: novel regulators of growth cone-to-nucleus signaling

Developing axons contain transcripts that are locally translated to influence the axonal proteome. Recent studies have shown that axonal transcripts include mRNAs encoding transcription factors. These mRNAs are translated to produce transcription factors that can be retrogradely trafficked back to the nucleus, where they regulate gene expression programs. These findings point to a novel mechanism of growth cone-to-nucleus signaling that occurs when growth cones encounter extracellular signaling molecules that stimulate local translation of these transcription factors, thereby influencing gene transcription. Here we summarize recent findings on local translation of transcription factors in axons and their roles in different neuronal processes such as neuronal specification, survival, and axon regeneration. Comprehensive axonal transcriptome studies have revealed transcripts that encode many more transcription factors and cofactors, suggesting a potentially broad role for this type of signaling. We review the progress on the approaches and tools that have been developed to study local translation and retrograde trafficking of transcription factors. We also highlight the challenges in the field and discuss the potential routes to resolving them.

Fasting induced cytoplasmic Fto expression in some neurons of rat hypothalamus

Fat mass and obesity associated protein (Fto) is a nucleic acid demethylase, with a preference for thymine or uracil, according to the recent structural data. This fact suggests that methylated single-stranded RNA, rather than DNA, may be the primary Fto substrate. Fto is abundantly expressed in all hypothalamic sites governing feeding behavior. Considering that selective modulation of Fto levels in the hypothalamus can influence food intake, we set out to investigate the effect of 48 h fasting on the Fto expression in lateral hypothalamic area, paraventricular, ventromedial and arcuate nucleus, the regulatory centres of energy homeostasis. We have demonstrated that 48 h fasting causes not only an increase in the overall hypothalamic levels of both Fto mRNA and protein, but also alters Fto intracellular distribution. This switch happens in some neurons of paraventricular and ventromedial nucleus, as well as lateral hypothalamic area, resulting in the majority of the enzyme being localized outside the cell nuclei. Interestingly, the change in the Fto intracellular localization was not observed in neurons of arcuate nucleus, suggesting that fasting did not universally affect Fto in all of the hypothalmic sites involved in energy homeostasis regulation. Both Fto mRNA and catechol-O-methyltransferaze mRNA were upregulated in the identical time-dependent manner in fasting animals. This fact, combined with the knowledge of the Fto substrate preference, may provide further insight into monoamine metabolism in the state of disturbed energy homeostasis.

Neuronal activity-regulated gene transcription: how are distant synaptic signals conveyed to the nucleus?

Synaptic activity can trigger gene expression programs that are required for the stable change of neuronal properties, a process that is essential for learning and memory. Currently, it is still unclear how the stimulation of dendritic synapses can be coupled to transcription in the nucleus in a timely way given that large distances can separate these two cellular compartments. Although several mechanisms have been proposed to explain long distance communication between synapses and the nucleus, the possible co-existence of these models and their relevance in physiological conditions remain elusive. One model suggests that synaptic activation triggers the translocation to the nucleus of certain transcription regulators localised at postsynaptic sites that function as synapto-nuclear messengers. Alternatively, it has been hypothesised that synaptic activity initiates propagating regenerative intracellular calcium waves that spread through dendrites into the nucleus where nuclear transcription machinery is thereby regulated. It has also been postulated that membrane depolarisation of voltage-gated calcium channels on the somatic membrane is sufficient to increase intracellular calcium concentration and activate transcription without the need for transported signals from distant synapses. Here I provide a critical overview of the suggested mechanisms for coupling synaptic stimulation to transcription, the underlying assumptions behind them and their plausible physiological significance.

NMDAR signaling facilitates the IPO5-mediated nuclear import of CPEB3

Cytoplasmic polyadenylation element-binding protein (CPEB)3 is a nucleocytoplasm-shuttling RNA-binding protein and predominantly resides in the cytoplasm where it represses target RNA translation. When translocated into the nucleus, CPEB3 binds to Stat5b and downregulates Stat5b-dependent transcription. In neurons, the activation of N-methyl-d-aspartate receptors (NMDARs) accumulates CPEB3 in the nucleus and redistributes CPEB3 in the nucleocytoplasmic compartments to control gene expression. Nonetheless, it is unclear which karyopherin drives the nuclear import of CPEB3 and which transport direction is most affected by NMDA stimulation to increase the nuclear pool of CPEB3. Here, we have identified that the karyopherins, IPO5 and CRM1, facilitate CPEB3 translocation by binding to RRM1 and a leucine-containing motif of CPEB3, respectively. NMDAR signaling increases RanBP1 expression and reduces the level of cytoplasmic GTP-bound Ran. These changes enhance CPEB3-IPO5 interaction, which consequently accelerates the nuclear import of CPEB3. This study uncovers a novel NMDA-regulated import pathway to facilitate the nuclear translocation of CPEB3.

Stimulation-dependent intraspinal microtubules and synaptic failure in Alzheimer's disease: a review

There are many microtubules in axons and dendritic shafts, but it has been thought that there were fewer microtubules in spines. Recently, there have been four reports that observed the intraspinal microtubules. Because microtubules originate from the centrosome, these four reports strongly suggest a stimulation-dependent connection between the nucleus and the stimulated postsynaptic membrane by microtubules. In contrast, several pieces of evidence suggest that spine elongation may be caused by the polymerization of intraspinal microtubules. This structural mechanism for spine elongation suggests, conversely, that the synapse loss or spine loss observed in Alzheimer's disease may be caused by the depolymerization of intraspinal microtubules. Based on this evidence, it is suggested that the impairment of intraspinal microtubules may cause spinal structural change and block the translocation of plasticity-related molecules between the stimulated postsynaptic membranes and the nucleus, resulting in the cognitive deficits of Alzheimer's disease.

Functional complexity of the axonal growth cone: a proteomic analysis

The growth cone, the tip of the emerging neurite, plays a crucial role in establishing the wiring of the developing nervous system. We performed an extensive proteomic analysis of axonal growth cones isolated from the brains of fetal Sprague-Dawley rats. Approximately 2000 proteins were identified at ≥ 99% confidence level. Using informatics, including functional annotation cluster and KEGG pathway analysis, we found great diversity of proteins involved in axonal pathfinding, cytoskeletal remodeling, vesicular traffic and carbohydrate metabolism, as expected. We also found a large and complex array of proteins involved in translation, protein folding, posttranslational processing, and proteasome/ubiquitination-dependent degradation. Immunofluorescence studies performed on hippocampal neurons in culture confirmed the presence in the axonal growth cone of proteins representative of these processes. These analyses also provide evidence for rough endoplasmic reticulum and reveal a reticular structure equipped with Golgi-like functions in the axonal growth cone. Furthermore, Western blot revealed the growth cone enrichment, relative to fetal brain homogenate, of some of the proteins involved in protein synthesis, folding and catabolism. Our study provides a resource for further research and amplifies the relatively recently developed concept that the axonal growth cone is equipped with proteins capable of performing a highly diverse range of functions.

Long-distance signaling from synapse to nucleus via protein messengers

The communication between synapses and the cell nucleus has attracted considerable interest for many years. This interest is largely fueled by the idea that synapse-to-nucleus signaling might specifically induce the expression of genes that make long-term memory "stick." However, despite many years of research, it is still essentially unclear how synaptic signals are conveyed to the nucleus, and it remains to a large degree enigmatic how activity-induced gene expression feeds back to synaptic function. In this chapter, we will focus on the activity-dependent synapto-nuclear trafficking of protein messengers and discuss the underlying mechanisms of their retrograde transport and their supposed functional role in neuronal plasticity.

Calcium signaling in synapse-to-nucleus communication

Changes in the intracellular concentration of calcium ions in neurons are involved in neurite growth, development, and remodeling, regulation of neuronal excitability, increases and decreases in the strength of synaptic connections, and the activation of survival and programmed cell death pathways. An important aspect of the signals that trigger these processes is that they are frequently initiated in the form of glutamatergic neurotransmission within dendritic trees, while their completion involves specific changes in the patterns of genes expressed within neuronal nuclei. Accordingly, two prominent aims of research concerned with calcium signaling in neurons are determination of the mechanisms governing information conveyance between synapse and nucleus, and discovery of the rules dictating translation of specific patterns of inputs into appropriate and specific transcriptional responses. In this article, we present an overview of the avenues by which glutamatergic excitation of dendrites may be communicated to the neuronal nucleus and the primary calcium-dependent signaling pathways by which synaptic activity can invoke changes in neuronal gene expression programs.

Mechanisms of specificity in neuronal activity-regulated gene transcription

The brain is a highly adaptable organ that is capable of converting sensory information into changes in neuronal function. This plasticity allows behavior to be accommodated to the environment, providing an important evolutionary advantage. Neurons convert environmental stimuli into long-lasting changes in their physiology in part through the synaptic activity-regulated transcription of new gene products. Since the neurotransmitter-dependent regulation of Fos transcription was first discovered nearly 25 years ago, a wealth of studies have enriched our understanding of the molecular pathways that mediate activity-regulated changes in gene transcription. These findings show that a broad range of signaling pathways and transcriptional regulators can be engaged by neuronal activity to sculpt complex programs of stimulus-regulated gene transcription. However, the shear scope of the transcriptional pathways engaged by neuronal activity raises the question of how specificity in the nature of the transcriptional response is achieved in order to encode physiologically relevant responses to divergent stimuli. Here we summarize the general paradigms by which neuronal activity regulates transcription while focusing on the molecular mechanisms that confer differential stimulus-, cell-type-, and developmental-specificity upon activity-regulated programs of neuronal gene transcription. In addition, we preview some of the new technologies that will advance our future understanding of the mechanisms and consequences of activity-regulated gene transcription in the brain.

Drosophila Importin-α2 is involved in synapse, axon and muscle development

Nuclear import is required for communication between the cytoplasm and the nucleus and to enact lasting changes in gene transcription following stimuli. Binding to an Importin-α molecule in the cytoplasm is often required to mediate nuclear entry of a signaling protein. As multiple isoforms of Importin-α exist, some may be responsible for the entry of distinct cargoes rather than general nuclear import. Indeed, in neuronal systems, Importin-α isoforms can mediate very specific processes such as axonal tiling and communication of an injury signal. To study nuclear import during development, we examined the expression and function of Importin-α2 in Drosophila melanogaster. We found that Importin-α2 was expressed in the nervous system where it was required for normal active zone density at the NMJ and axonal commissure formation in the central nervous system. Other aspects of synaptic morphology at the NMJ and the localization of other synaptic markers appeared normal in importin-α2 mutants. Importin-α2 also functioned in development of the body wall musculature. Mutants in importin-α2 exhibited errors in muscle patterning and organization that could be alleviated by restoring muscle expression of Importin-α2. Thus, Importin-α2 is needed for some processes in the development of both the nervous system and the larval musculature.

The Huntington's disease mutation impairs Huntingtin's role in the transport of NF-κB from the synapse to the nucleus

Expansion of a polyglutamine (polyQ) tract in the Huntingtin (Htt) protein causes Huntington's disease (HD), a fatal inherited neurodegenerative disorder. Loss of the normal function of Htt is thought to be an important pathogenetic component of HD. However, the function of wild-type Htt is not well defined. Htt is thought to be a multifunctional protein that plays distinct roles in several biological processes, including synaptic transmission, intracellular transport and neuronal transcription. Here, we show with biochemical and live cell imaging studies that wild-type Htt stimulates the transport of nuclear factor κ light-chain-enhancer of activated B cells (NF-κB) out of dendritic spines (where NF-κB is activated by excitatory synaptic input) and supports a high level of active NF-κB in neuronal nuclei (where NF-κB stimulates the transcription of target genes). We show that this novel function of Htt is impaired by the polyQ expansion and thus may contribute to the etiology of HD.

Expression of the nuclear transport protein importin ß-1 and its association with the neurokinin 3 receptor in the rat hypothalamus following acute hyperosmotic challenge

The tachykinin NK3 receptor (NK3R) is a G-protein coupled receptor that is activated, internalized, and trafficked to the nuclei of magnocellular neurons in the paraventricular nucleus of the hypothalamus (PVN) in response to acute hyperosmolarity. The lack of information on the nuclear import pathway raises concerns about the physiological role of nuclear NK3R. NK3R contains a nuclear localizing sequence (NLS) and this raises the possibility that importins are involved in transport of NK3R through the nuclear pore complex. The following experiments utilized: (1) co-immunoprecipitation to determine if NK3R is associated with importin ß-1 following activation in response to acute hyperosmolarity in vivo, and (2) immuno-neutralization of importin ß-1 in vitro to determine if nuclear transport of NK3R was blocked. Rats were given an i.v. injection of hypertonic saline (2 M) and 10 min after the infusion, the PVN was removed and homogenized. Importin ß-1 co-immunoprecipitated with the NK3R following treatment with 2 M NaCl, but not following isotonic saline treatment. Immuno-neutralization of importin ß-1 decreased the transport of NK3R into the nuclei in a time dependent fashion. The results indicate that in response to acute hyperosmotic challenge, NK3R associates with importin ß-1 which enables the nuclear transport of NK3R. This is the first in vivo study linking importin ß-1 and the nuclear transport of a G protein coupled receptor, the NK3R, in brain.

The nuclear import of Frizzled2-C by Importins-beta11 and alpha2 promotes postsynaptic development

Synapse-to-nucleus signaling is critical for synaptic development and plasticity. In Drosophila, the ligand Wingless causes the C-terminus of its Frizzled2 receptor (Fz2-C) to be cleaved and translocated from the postsynaptic density to nuclei. The mechanism of nuclear import is unknown and the developmental consequences of this translocation are uncertain. Here, we show that Fz2-C localization to muscle nuclei requires the nuclear import factors, Importin-β11 and Importin-α2 and that this pathway promotes the postsynaptic development of the subsynaptic reticulum (SSR), an elaboration of the postsynaptic plasma membrane. importin-β11 and dfz2 mutants have less SSR and some boutons lacking the postsynaptic marker Discs Large. These developmental defects in importin-β11 can be overcome by expression of Fz2-C fused to a nuclear localization sequence that can bypass Importin-β11. Thus, Wnt-activated growth of the postsynaptic membrane is mediated by the synapse-to-nucleus translocation and active nuclear import of Fz2-C via a selective Importin-β11/α2 pathway.

Importin-beta11 regulates synaptic phosphorylated mothers against decapentaplegic, and thereby influences synaptic development and function at the Drosophila neuromuscular junction

Importin proteins act both at the nuclear pore to promote substrate entry and in the cytosol during signal trafficking. Here, we describe mutations in the Drosophila gene importin-beta11, which has not previously been analyzed genetically. Mutants of importin-beta11 died as late pupae from neuronal defects, and neuronal importin-beta11 was present not only at nuclear pores but also in the cytosol and at synapses. Neurons lacking importin-beta11 were viable and properly differentiated but exhibited discrete defects. Synaptic transmission was defective in adult photoreceptors and at larval neuromuscular junctions (NMJs). Mutant photoreceptor axons formed grossly normal projections and synaptic terminals in the brain, but synaptic arbors on larval muscles were smaller while still containing appropriate synaptic components. Bone morphogenic protein (BMP) signaling was the apparent cause of the observed NMJ defects. Importin-beta11 interacted genetically with the BMP pathway, and at mutant synaptic boutons, a key component of this pathway, phosphorylated mothers against decapentaplegic (pMAD), was reduced. Neuronal expression of an importin-beta11 transgene rescued this phenotype as well as the other observed neuromuscular phenotypes. Despite the loss of synaptic pMAD, pMAD persisted in motor neuron nuclei, suggesting a specific impairment in the local function of pMAD. Restoring levels of pMAD to mutant terminals via expression of constitutively active type I BMP receptors or by reducing retrograde transport in motor neurons also restored synaptic strength and morphology. Thus, importin-beta11 function interacts with the BMP pathway to regulate a pool of pMAD that must be present at the presynapse for its proper development and function.

Activity-dependent anchoring of importin alpha at the synapse involves regulated binding to the cytoplasmic tail of the NR1-1a subunit of the NMDA receptor

Synaptic plasticity, the capacity of neurons to change the strength of their connections with experience, provides a mechanism for learning and memory in the brain. Long-term plasticity requires new transcription, indicating that synaptically generated signals must be transported to the nucleus. Previous studies have described a role for importin nuclear transport adaptors in mediating the retrograde transport of signals from synapse to nucleus during plasticity. Here, we investigated the possibility that stimulus-induced translocation of importins from synapse to nucleus involves activity-dependent anchoring of importins at the synapse. We show that importin alpha binds to a nuclear localization signal (NLS) present in the cytoplasmic tail of NR1-1a. This interaction is disrupted by activation of NMDA receptors in cultured neurons and by stimuli that trigger late-phase, but not early-phase, long-term potentiation of CA3-CA1 synapses in acute hippocampal slices. In vitro PKC phosphorylation of GST-NR1-1a abolishes its ability to bind importin alpha in brain lysates, and the interaction of importin alpha and NR1 in neurons is modulated by PKC activity. Together, our results indicate that importin alpha is tethered at the postsynaptic density by binding to the NLS present in NR1-1a. This interaction is activity dependent, with importin alpha being released following NMDA receptor activation and phosphorylation rendering it available to bind soluble cargoes and transport them to the nucleus during transcription-dependent forms of neuronal plasticity.

Molluscan memory of injury: evolutionary insights into chronic pain and neurological disorders

Molluscan preparations have yielded seminal discoveries in neuroscience, but the experimental advantages of this group have not, until now, been complemented by adequate molecular or genomic information for comparisons to genetically defined model organisms in other phyla. The recent sequencing of the transcriptome and genome of Aplysia californica, however, will enable extensive comparative studies at the molecular level. Among other benefits, this will bring the power of individually identifiable and manipulable neurons to bear upon questions of cellular function for evolutionarily conserved genes associated with clinically important neural dysfunction. Because of the slower rate of gene evolution in this molluscan lineage, more homologs of genes associated with human disease are present in Aplysia than in leading model organisms from Arthropoda (Drosophila) or Nematoda (Caenorhabditis elegans). Research has hardly begun in molluscs on the cellular functions of gene products that in humans are associated with neurological diseases. On the other hand, much is known about molecular and cellular mechanisms of long-term neuronal plasticity. Persistent nociceptive sensitization of nociceptors in Aplysia displays many functional similarities to alterations in mammalian nociceptors associated with the clinical problem of chronic pain. Moreover, in Aplysia and mammals the same cell signaling pathways trigger persistent enhancement of excitability and synaptic transmission following noxious stimulation, and these highly conserved pathways are also used to induce memory traces in neural circuits of diverse species. This functional and molecular overlap in distantly related lineages and neuronal types supports the proposal that fundamental plasticity mechanisms important for memory, chronic pain, and other lasting alterations evolved from adaptive responses to peripheral injury in the earliest neurons. Molluscan preparations should become increasingly useful for comparative studies across phyla that can provide insight into cellular functions of clinically important genes.

ARHI (DIRAS3), an imprinted tumour suppressor gene, binds to importins and blocks nuclear import of cargo proteins

ARHI (aplasia Ras homologue member I; also known as DIRAS3) is an imprinted tumour suppressor gene, the expression of which is lost in the majority of breast and ovarian cancers. Unlike its homologues Ras and Rap, ARHI functions as a tumour suppressor. Our previous study showed that ARHI can interact with the transcriptional activator STAT3 (signal transducer and activator of transcription 3) and inhibit its nuclear translocation in human breast- and ovarian-cancer cells. To identify proteins that interact with ARHI in nuclear translocation, in the present study, we performed proteomic analysis and identified several importins that can associate with ARHI. To further explore this novel finding, we purified 10 GST (glutathione transferase)-importin fusion proteins (importins 7, 8, 13, beta1, alpha1, alpha3, alpha5, alpha6, alpha7 and mutant alpha1). Using a GST-pulldown assay, we found that ARHI can bind strongly to most importins; however, its binding is markedly reduced with an importin alpha1 mutant that contains an altered NLS (nuclear-localization signal) domain. In addition, an ARHI N-terminal deletion mutant exhibits greatly reduced binding to all importins compared with wild-type ARHI. In nuclear-import assays, the addition of ARHI blocked nuclear localization of phosphorylated STAT3. ARHI also inhibits the interaction of Ran-importin complexes with GFP (green fluorescent protein) fusion proteins that contain an NLS domain and a beta-like import receptor-binding domain, thereby blocking their nuclear localization. By conducting GST-pulldown assays, we found that ARHI could compete for Ran-importin binding. Thus ARHI-induced disruption of importin-binding to cargo proteins, including STAT3, could serve as an important regulatory mechanism that contributes to the tumour-suppressor function of ARHI.

Multifunctional basic motif in the glycine receptor intracellular domain induces subunit-specific sorting

The strychnine-sensitive glycine receptor (GlyR) is a ligand-gated ion channel that mediates fast synaptic inhibition in the vertebrate central nervous system. As a member of the family of Cys-loop receptors, it assembles from five homologous subunits (GlyRalpha1-4 and -beta). Each subunit contains an extracellular ligand binding domain, four transmembrane domains (TM), and an intracellular domain, formed by the loop connecting TM3 and TM4 (TM3-4 loop). The TM3-4 loops of the subunits GlyRalpha1 and -alpha3 harbor a conserved basic motif, which is part of a potential nuclear localization signal. When tested for functionality by live cell imaging of green fluorescent protein and beta-galactosidase-tagged domain constructs, the TM3-4 loops of GlyRalpha1 and -alpha3, but not of GlyRalpha2 and -beta, exhibited nuclear sorting activity. Subunit specificity may be attributed to slight amino acid alterations in the basic motif. In yeast two-hybrid screening and GST pulldown assays, karyopherin alpha3 and alpha4 were found to interact with the TM3-4 loop, providing a molecular mechanism for the observed intracellular trafficking. These results indicate that the multifunctional basic motif of the TM3-4 loop is capable of mediating a karyopherin-dependent intracellular sorting of full-length GlyRs.

Axonal ligation induces transient redistribution of TDP-43 in brainstem motor neurons

Nuclear exclusion of TAR DNA binding protein 43 (TDP-43) and formation of cytosolic aggregates are a pathological characteristic of amyotrophic lateral sclerosis (ALS). However, the molecular basis of the aberrant distribution of TDP-43 remains elusive. Here, we show evidence that axonal ligation induced transient nuclear exclusion and peripheral accumulation of TDP-43, without apparent cytosolic aggregates in hypoglossal neurons in mice. Immunohistochemistry showed marked loss of nuclear TDP-43 7-14 days after ligation, which was accompanied by reduction of choline acetyltransferase (ChAT). TDP-43 staining was restored in the nucleus on day 28 exclusively in the neurons with normalized ChAT expression. We also showed that importin beta, which was shown to mediate nuclear transport of TDP-43 was downregulated transiently by nerve ligation. The analysis of the peripheral nerves proximal to the ligation revealed that TDP-43 markedly accumulated with a concomitant decrease in active autophagosome. Moreover, we showed that TDP-43 was present in the microsome fraction containing endoplasmic reticulum (ER) or autophagosomes in the brainstem section, indicating that TDP-43 is axonally transported with vesicles. These results indicate that axonal damage is associated with redistribution of TDP-43 through the combination of defective axonal autophagy periphery and the impaired nuclear transport system in the soma. Moreover, it was also shown that transient redistribution of TDP-43 does not prevent motor neurons from axonal regeneration. Therefore, our data suggest that the subcellular distribution of TDP-43 correlates to the innervation status of motor neurons, which may be governed by unidentified cause of ALS.

Nuclear transport factors in neuronal function

Active nucleocytoplasmic transport of macromolecules requires soluble transport carriers of the importin/karyopherin superfamily. Although the nuclear transport machinery is essential in all eukaryotic cells, neurons must also mobilise importins and associated proteins to overcome unique spatiotemporal challenges. These include switches in importin alpha subtype expression during neuronal differentiation, localized axonal synthesis of importin beta1 to coordinate a retrograde injury signaling complex on axonal dynein, and trafficking of regulatory and signaling molecules from synaptic terminals to cell bodies. Targeting of RNAs encoding critical components of the importins complex and the Ran system to axons allows sophisticated local regulation of the system for mobilization upon need. Finally, a number of importin family members have been associated with mental or neurodegenerative diseases. The extended roles recently discovered for importins in the nervous system might also be relevant in non-neuronal cells, and the localized modes of importin regulation in neurons offer new avenues to interrogate their cytoplasmic functions.

Importin-mediated retrograde transport of CREB2 from distal processes to the nucleus in neurons

Signals received at distal synapses of neurons must be conveyed to the nucleus to initiate the changes in transcription that underlie long-lasting synaptic plasticity. The presence of importin nuclear transporters and of select transcription factors at synapses raises the possibility that importins directly transport transcription factors from synapse to nucleus to modulate gene expression. Here, we show that cyclic AMP response element binding protein 2 (CREB2)/activating transcription factor 4 (ATF4), a transcriptional repressor that modulates long-term synaptic plasticity and memory, localizes to distal dendrites of rodent hippocampal neurons and neurites of Aplysia sensory neurons (SNs) and binds to specific importin alpha isoforms. Binding of CREB2 to importin alpha is required for its transport from distal dendrites to the soma and for its translocation into the nucleus. CREB2 accumulates in the nucleus during long-term depression (LTD) but not long-term potentiation of rodent hippocampal synapses, and during LTD but not long-term facilitation (LTF) of Aplysia sensory-motor synapses. Time-lapse microscopy of CREB2 tagged with a photoconvertible fluorescent protein further reveals retrograde transport of CREB2 from distal neurites to the nucleus of Aplysia SN during phenylalanine-methionine-arginine-phenylalanine-amide (FMRFamide)-induced LTD. Together, our findings indicate that CREB2 is a novel cargo of importin alpha that translocates from distal synaptic sites to the nucleus after stimuli that induce LTD of neuronal synapses.

Regional distribution of importin subtype mRNA expression in the nervous system: study of early postnatal and adult mouse

Importin-alpha and beta1 mediate the translocation of macromolecules bearing nuclear localization signals across the nuclear pore complex. Five importin-alpha isoforms have been identified in mice and six in human. Some of these importins play an important role in neural activity such as long term potentiation, but the functional differences of each isoform in the CNS are still unclear. We performed in situ hybridization (ISH) using non-isotopic probes to clarify the expression patterns of importin-alpha subtypes (alpha5, alpha7, alpha1, alpha4, alpha3) and importin-beta1 in the mouse CNS of adult and early postnatal stages. The mRNAs of the importin-alpha subtypes and importin beta1 were expressed throughout the CNS with specific patterns; importin-alpha5, alpha7, alpha3, and beta1 showed moderate to high expression levels throughout the brain and spinal cord; importin-alpha4 showed a lack of expression in limited regions; and importin-alpha1 showed a low expression level throughout the brain and spinal cord but with a moderate expression level in the olfactory bulb and reticular system. We also demonstrated that importin-alphas and beta1 mRNAs were predominantly expressed in neurons in the adult mouse brain by using double-labeling fluorescence ISH and immunohistochemistry. Moreover, importin-alphas and beta1 mRNAs were detected throughout the CNS of postnatal mice and were highly expressed in the external granule layer of the cerebellar cortex on postnatal days 0, 4, and 10. This is the first report of importin-alphas and beta1 expression throughout the CNS of adult mice, as well as in the developing brain, including cell type specific localization.

RNA editing, DNA recoding and the evolution of human cognition

RNA editing appears to be the major mechanism by which environmental signals overwrite encoded genetic information to modify gene function and regulation, particularly in the brain. We suggest that the predominance of Alu elements in the human genome is the result of their evolutionary co-adaptation as a modular substrate for RNA editing, driven by selection for higher-order cognitive function. We show that RNA editing alters transcripts from loci encoding proteins involved in neural cell identity, maturation and function, as well as in DNA repair, implying a role for RNA editing not only in neural transmission and network plasticity but also in brain development, and suggesting that communication of productive changes back to the genome might constitute the molecular basis of long-term memory and higher-order cognition.

High-fat diet-induced changes in body mass and hypothalamic gene expression in wild-type and leptin-deficient mice

We tested whether diet-induced obesity results from increased energy consumption, is associated with changes in expression of genes involved in leptin signal transduction, and is altered by hyperleptinemia. C57BL/6 mice were fed a low-fat diet (LFD) or high-fat diet (HFD) for up to 15 weeks. HFD mice weighed significantly more than LFD controls by 3 weeks, despite consuming less energy. HFD mice had significantly greater leptin, insulin, and glucose levels than LFD mice, suggesting leptin and insulin resistance. Adiponectin levels declined with age but were unaffected by diet. HFD was associated with altered hypothalamic expression of genes whose products regulate the activity or nuclear translocation of STAT3, an important mediator of leptin actions. Expression of two isoforms of the leptin receptor decreased at 15 weeks in hypothalami of HFD mice in a tissue-specific manner. The type of fat (saturated versus unsaturated) did not influence weight gain on an HFD, but animals on LFD gained significantly more weight and adiposity if the dietary fat consisted mostly of saturated fats; this occurred despite no difference in energy consumption or absorption. Replacement of leptin to leptin-deficient ob/ob mice decreased hypothalamic leptin receptor expression and did not prevent HFD-induced weight gain. It is concluded that (1) increased energy consumption is not required for HFD-induced obesity in C57BL/6 mice, (2) HFD results in weight gain partly by modulating hypothalamic leptin-signaling pathways, (3) saturated fats induce weight gain even when total fat content of the diet is low, and (4) the effects of HFD are manifest in the presence or absence of circulating leptin.

Sexual and social stimuli elicit rapid and contrasting genomic responses

Sensory physiology has been shown to influence female mate choice, yet little is known about the mechanisms within the brain that regulate this critical behaviour. Here we examine preference behaviour of 58 female swordtails, Xiphophorus nigrensis, in four different social environments (attractive and unattractive males, females only, non-attractive males only and asocial conditions) followed by neural gene expression profiling. We used a brain-specific cDNA microarray to identify patterns of genomic response and candidate genes, followed by quantitative PCR (qPCR) examination of gene expression with variation in behaviour. Our microarray results revealed patterns of genomic response differing more between classes of social stimuli than between presence versus absence of stimuli. We identified suites of genes showing diametrically opposed patterns of expression: genes that are turned ‘on’ while females interact with attractive males are turned ‘off’ when interacting with other females, and vice versa. Our qPCR results identified significant predictive relationships between five candidate genes and specific mate choice behaviours (preference and receptivity) across females exposed to males, with no significant patterns identified in female or asocial conditions or with overall locomotor activity. The identification of stimulus- and behaviour-specific responses opens an exciting window into the molecular pathways associated with social behaviour and mechanisms that underlie sexual selection.

Stability of the fittest: organizing learning through retroaxonal signals

Classically, neurons communicate by anterograde conduction of action potentials. However, information can also pass backward along axons, a process that is essential during the development of the nervous system. Here we propose a role for such 'retroaxonal' signals in adult learning. We hypothesize that strengthening of a neuron's output synapses stabilizes recent changes in the same neuron's inputs. During learning, the input synapses of many neurons undergo transient changes, resulting in altered spiking activity. If this in turn promotes strengthening of output synapses, the recent synaptic changes will be stabilized; otherwise they will decay. A representation of sensory stimuli therefore evolves that is tailored to the demands of behavioral tasks. We describe a candidate molecular mechanism for this process involving the activation of CREB by retrograde neurotrophin signals.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

The Endoplasmic Reticulum as an Integrator of Multiple Dendritic Events

Dendrites are integrating elements that receive numerous subsets of heterogeneous synaptic inputs, which generate temporally and spatially distinct changes in membrane potential and intracellular Ca2+ levels in local domains. The ubiquitously distributed endoplasmic reticulum (ER) in dendrites is luminally connected to the bulk ER in the soma, constituting a huge interconnected intracellular network that allows rapid Ca2+ diffusion and equilibration. The ER is an excitable organelle that can elicit or terminate cytosolic Ca2+ signals in local or global domains. The absolute level or changes in the Ca2+ concentration in the ER lumen are also very important for the synthesis and maturation of proteins, regulation of gene expression, mitochondrial functions, neuronal excitability, and synaptic plasticity. Through the connected lumen of the ER, information from multiple dendritic events in neurons appears to be delivered into the bulk ER in the soma. Therefore, the ER network in neurons is emerging as a conveyor and integrator of signals. In this article, we will discuss the various roles of the ER and the functional and structural organization of the ER network in neurons. NEUROSCIENTIST 14(1):68—77, 2008.

Location, location, location: altered transcription factor trafficking in neurodegeneration

Neurons may be particularly sensitive to disruptions in transcription factor trafficking. Survival and injury signals must traverse dendrites or axons, in addition to soma, to affect nuclear transcriptional responses. Transcription factors exhibit continued nucleocytoplasmic shuttling; the predominant localization is regulated by binding to anchoring proteins that mask nuclear localization/export signals and/or target the factor for degradation. Two functional groups of karyopherins, importins and exportins, mediate RanGTPase-dependent transport through the nuclear pore. A growing number of recent studies, in Alzheimer, Parkinson, and Lewy body diseases, amyotrophic lateral sclerosis, and human immunodeficiency virus encephalitis, implicate aberrant cytoplasmic localization of transcription factors and their regulatory kinases in degenerating neurons. Potential mechanisms include impaired nuclear import, enhanced export, suppression of degradation, and sequestration in protein aggregates or organelles and may reflect unmasking of alternative cytoplasmic functions, both physiologic and pathologic. Some "nuclear" factors also function in mitochondria, and importins are also involved in axonal protein trafficking. Detrimental consequences of a decreased nuclear to cytoplasmic balance include suppression of neuroprotective transcription mediated by cAMP- and electrophile/antioxidant-response elements and gain of toxic cytoplasmic effects. Studying the pathophysiologic mechanisms regulating transcription factor localization should facilitate strategies to bypass deficits and restore adaptive neuroprotective transcriptional responses.

Visual stimulation activates ERK in synaptic and somatic compartments of rat cortical neurons with parallel kinetics

BACKGROUND

Extracellular signal-regulated kinase (ERK) signalling pathway plays a crucial role in regulating diverse neuronal processes, such as cell proliferation and differentiation, and long-term synaptic plasticity. However, a detailed understanding of the action of ERK in neurons is made difficult by the lack of knowledge about its subcellular localization in response to physiological stimuli. To address this issue, we have studied the effect of visual stimulation in vivo of dark-reared rats on the spatial-temporal dynamics of ERK activation in pyramidal neurons of the visual cortex.

METHODOLOGY/PRINCIPAL FINDINGS

Using immunogold electron microscopy, we show that phosphorylated ERK (pERK) is present in dendritic spines, both at synaptic and non-synaptic plasma membrane domains. Moreover, pERK is also detected in presynaptic axonal boutons forming connections with dendritic spines. Visual stimulation after dark rearing during the critical period causes a rapid increase in the number of pERK-labelled synapses in cortical layers I-II/III. This visually-induced activation of ERK at synaptic sites occurs in pre- and post-synaptic compartments and its temporal profile is identical to that of ERK activation in neuronal cell bodies.

CONCLUSIONS/SIGNIFICANCE

Visual stimulation in vivo increases pERK expression at pre- and post-synaptic sites of axo-spinous junctions, suggesting that ERK plays an important role in the local modulation of synaptic function. The data presented here support a model in which pERK can have early and late actions both centrally in the cell nucleus and peripherally at synaptic contacts.

Dynamics of glucocorticoid receptor and mineralocorticoid receptor: implications from live cell imaging studies

Adrenal corticosteroids (cortisol in humans/corticosterone in rodents) readily enter the brain and exert markedly diverse effects, such as the stress response of target neural cells. These effects are regulated via two receptor systems, the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR), both of which are ligand-inducible transcription factors. It is generally accepted that GR and MR predominantly reside in the cytoplasm in the absence of corticosterone (CORT), and are quickly translocated into the nucleus upon binding CORT. Then these receptors form dimers to bind hormone-responsive elements and regulate the expression of target genes. Given the different actions of MR and GR in the central nervous system, it is important to elucidate how the trafficking of these receptors between the cytoplasm and nucleus and their interaction are regulated by ligands or other molecules to exert transcriptional activity. However, the precise mechanisms of these processes are still not completely clarified. To address these issues, we have tried to observe more dynamic subcellular trafficking processes in living cells by employing a green fluorescent protein. In this review, we describe our recent studies of corticosteroid receptor dynamics in living cells focusing on three points: (1) the effects of a ligand, corticosteroid; (2) the carrier molecules involved in active nuclear transport, importins, and (3) the possibility of heterodimer formation. These studies demonstrate that GR and MR were quickly translocated from the cytoplasm to the nucleus after CORT treatment by associating with importin molecules. GR and MR differed in their response to the concentration of CORT in neural cells and non-neural cells. In the nuclear region, we detected GR-MR heterodimers, which were affected by changes in CORT concentrations in response to various hormonal milieus such as circadian rhythm and stress.

Retrograde signaling in injured nerve--the axon reaction revisited

Injury to axons elicits changes in macromolecule synthesis in the corresponding cell bodies that are critical for an effective regenerative response. For decades the most easily studied aspect of this phenomenon was the onset of chromatolysis, a suite of structural changes in the cell body characterized by swelling, shifting of the nucleus and dispersal of Nissl bodies. The question: 'what is the signal for chromatolysis?' received no less than 10 possible answers in a comprehensive review article published more than three decades ago. Here we come back to this 36 years old question, and review progress on understanding the mechanism of retrograde injury signaling in lesioned peripheral nerves. Recent work suggests that this is based on local axonal synthesis of critical carrier proteins, including importins and vimentin that link diverse signaling molecules to the dynein retrograde motor. A multiplicity of binding sites and of potential signaling molecules, including transcription factors and MAP kinases (Erk, Jnk), may allow diverse options for information-rich encoding of the injury status of the axon for transmission to the cell body.