Importin alpha transports CaMKIV to the nucleus without utilizing importin beta

Analysis of the effects of importin α1 on the nuclear translocation of IL-1α in HeLa cells

Interleukin-1α (IL-1α), a cytokine released by necrotic cells, causes sterile inflammation. On the other hand, IL-1α is present in the nucleus and also regulates the expression of many proteins. A protein substrate containing a classical nuclear localization signal (cNLS) typically forms a substrate/importin α/β complex, which is subsequently transported to the nucleus. To the best of our knowledge, no study has directly investigated whether IL-1α-which includes cNLS-is imported into the nucleus in an importin α/β-dependent manner. In this study, we noted that all detected importin α subtypes interacted with IL-1α. In HeLa cells, importin α1-mediated nuclear translocation of IL-1α occurred at steady state and was independent of importin β1. Importin α1 not only was engaged in IL-1α nuclear transport but also concurrently functioned as a molecule that regulated IL-1α protein level in the cell. Furthermore, we discussed the underlying mechanism of IL-1α nuclear translocation by importin α1 based on our findings.

Nuclear Import and Export of YAP and TAZ

Yes-associated Protein (YAP) and its paralog Transcriptional Coactivator with PDZ-binding Motif (TAZ) are major regulators of gene transcription/expression, primarily controlled by the Hippo pathway and the cytoskeleton. Integrating an array of chemical and mechanical signals, they impact growth, differentiation, and regeneration. Accordingly, they also play key roles in tumorigenesis and metastasis formation. Their activity is primarily regulated by their localization, that is, Hippo pathway- and/or cytoskeleton-controlled cytosolic or nuclear sequestration. While many details of such prevailing retention models have been elucidated, much less is known about their actual nuclear traffic: import and export. Although their size is not far from the cutoff for passive diffusion through the nuclear pore complex (NPC), and they do not contain any classic nuclear localization (NLS) or nuclear export signal (NES), evidence has been accumulating that their shuttling involves mediated and thus regulatable/targetable processes. The aim of this review is to summarize emerging information/concepts about their nucleocytoplasmic shuttling, encompassing the relevant structural requirements (NLS, NES), nuclear transport receptors (NTRs, karyophererins), and NPC components, along with the potential transport mechanisms and their regulation. While dissecting retention vs. transport is often challenging, the emerging picture suggests that YAP/TAZ shuttles across the NPC via multiple, non-exclusive, mediated mechanisms, constituting a novel and intriguing facet of YAP/TAZ biology.

Anti-phospholipase A2 receptor antibodies directly induced podocyte damage in vitro

Background

The pathogenesis of primary membranous nephropathy (MN) involves the antibodies against antigens on the cell surface of podocytes, with the majority of M-type phospholipase A2 receptor (PLA2R), and a profound podocyte dysfunction. The effects of anti-PLA2R antibodies directly to the podocytes remain unclear.

Methods

Anti-PLA2R antibodies from patients with PLA2R-associated MN were affinity-purified using a column coupled with recombinant human PLA2R protein. Their effects on conditionally immortalized human podocytes were assessed by apoptosis assays, cellular calcium detection, wound healing assay, and immunofluorescent staining. Proteomics analysis was performed by LC-MS/MS and on PANTHER database.

Results

The stimulation by anti-PLA2R antibodies could induce early-stage apoptosis of podocytes (MFI of Annexin V = 104.3 ± 19.2 vs. 36.7 ± 7.6, p = 0.004). The increase of calcium concentration in podocytes (MFI = 3309.3 ± 363.6 vs. 1776.3 ± 212.7, p = 0.015) might attribute to the endoplasmic reticulum calcium efflux. The expression of calcium/calmodulin-dependent protein kinase IV (CaMK4) was also increased (MFI = 134.4 ± 9.8 vs. 105.3 ± 10.1, p = 0.011). Proteomics results suggested that anti-PLA2R antibody treatment led to damage on cellular structure, and produced functional disorders on protein binding, actin filament binding, and microtubule motor activity. The staining of F-actin on foot process was reduced (MFI = 27.3 ± 2.8 vs. 47.5 ± 1.0, p = 0.001) and the motility and adherence capacity of podocytes were reduced (number of migrated cells = 44.7 ± 3.1 vs. 53.3 ± 4.9, p = 0.001) after incubation with anti-PLA2R antibodies.

Conclusion

These data indicate that anti-PLA2R antibodies may directly induce podocyte damage independent of the complement system, which expands the mechanism of anti-PLA2R antibodies on MN.

Nrf2 improves hippocampal synaptic plasticity, learning and memory through the circ-Vps41/miR-26a-5p/CaMKIV regulatory network

Antioxidant response transcription factor nuclear factor erythroid-2-related factor 2 (Nrf2/Nfe2l2) is a neuroprotective agent in learning and memory impairment. This study provides a new perspective to explore the regulatory mechanisms of Nrf2. Here, we found that Nrf2 regulated circular RNA circ-Vps41 to increase hippocampal synaptic plasticity; Nrf2 bound the Vps41 promoter to activate transcription of the Vps41 gene and promote expression of circ-Vps41; circ-Vps41 positively correlated with Nrf2, synaptic plasticity, and learning and memory but negatively correlated with reactive oxygen species; and Nrf2 promoted CaMKIV expression by increasing levels of circ-Vps41, which can absorb miR-26a-5p that targets CaMKIV. Our findings revealed a new circRNA-based regulatory network regulated by Nrf2 and provided novel insights into the potential mechanism involved in the improvement of learning and memory impairment.

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.

Aberrantly glycosylated IgG elicits pathogenic signaling in podocytes and signifies lupus nephritis

Lupus nephritis (LN) is a serious complication occurring in 50% of patients with systemic lupus erythematosus (SLE) for which there is a lack of biomarkers, a lack of specific medications, and a lack of a clear understanding of its pathogenesis. The expression of calcium/calmodulin kinase IV (CaMK4) is increased in podocytes of patients with LN and lupus-prone mice, and its podocyte-targeted inhibition averts the development of nephritis in mice. Nephrin is a key podocyte molecule essential for the maintenance of the glomerular slit diaphragm. Here, we show that the presence of fucose on N-glycans of IgG induces, whereas the presence of galactose ameliorates, podocyte injury through CaMK4 expression. Mechanistically, CaMK4 phosphorylates NF-κB, upregulates the transcriptional repressor SNAIL, and limits the expression of nephrin. In addition, we demonstrate that increased expression of CaMK4 in biopsy specimens and in urine podocytes from people with LN is linked to active kidney disease. Our data shed light on the role of IgG glycosylation in the development of podocyte injury and propose the development of “liquid kidney biopsy” approaches to diagnose LN.

Attenuated β-adrenergic response in calcium/calmodulin-dependent protein kinase IV-knockout mice

In the present study, we examined the importance of Ca2+/calmodulin-dependent protein kinase IV (CaMKIV) in the regulation of cardiac function using genetically modified CaMKIV-null mice. RT-PCR analysis revealed decreased expression of voltage-dependent calcium channels in the cardiac myocytes of CaMKIV-null mice compared with wild-type mice. CaMKIV-null mice showed shortened QT time on electrocardiograms. Pharmacological analysis revealed decreased responsiveness to the β-adrenergic blocker propranolol in CaMKIV-null mice, whereas the plasma norepinephrine level was not affected. CaMKIV-null mice showed decreased baroreflex on electrocardiograms. Heart rate variability analysis showed unstable R-R intervals, a decreased low frequency power/high frequency power (LF/HF) ratio, and increased standard deviation of the normal to normal R-R intervals (SDNN) in CaMKIV-null mice, suggesting decreased responsiveness to β-adrenergic stimulation in CaMKIV-null mice. Atrial contraction analysis and cardiac action potential recording showed a decreased response to the β-adrenoceptor agonist isoproterenol in CaMKIV-null mice. Furthermore, fluorescence imaging in a CRE-hrGFP assay revealed a decreased response to isoproterenol in CaMKIV-null cardiac myocytes. Taken together, our data strongly suggest a significant effect of CaMKIV gene ablation on cardiac β-adrenergic signal transduction.

Novel Ran-RCC1 Inhibitory Peptide-Loaded Nanoparticles Have Anti-Cancer Efficacy In Vitro and In Vivo

The delivery of anticancer agents to their subcellular sites of action is a significant challenge for effective cancer therapy. Peptides, which are integral to several oncogenic pathways, have significant potential to be utilised as cancer therapeutics due to their selectivity, high potency and lack of normal cell toxicity. Novel Ras protein-Regulator of chromosome condensation 1 (Ran-RCC1) inhibitory peptides designed to interact with Ran, a novel therapeutic target in breast cancer, were delivered by entrapment into polyethylene glycol-poly (lactic-co-glycolic acid) PEG-PLGA polymeric nanoparticles (NPs). A modified double emulsion solvent evaporation technique was used to optimise the physicochemical properties of these peptide-loaded biodegradable NPs. The anti-cancer activity of peptide-loaded NPs was studied in vitro using Ran-expressing metastatic breast (MDA-MB-231) and lung cancer (A549) cell lines, and in vivo using Solid Ehrlich Carcinoma-bearing mice. The anti-metastatic activity of peptide-loaded NPs was investigated using migration, invasion and colony formation assays in vitro. A PEG-PLGA-nanoparticle encapsulating N-terminal peptide showed a pronounced antitumor and anti-metastatic action in lung and breast cancer cells in vitro and caused a significant reduction of tumor volume and associated tumor growth inhibition of breast cancer model in vivo. These findings suggest that the novel inhibitory peptides encapsulated into PEGylated PLGA NPs are delivered effectively to interact and deactivate Ran. This novel Ran-targeting peptide construct shows significant potential for therapy of breast cancer and other cancers mediated by Ran overexpression.

High throughput screening, docking, and molecular dynamics studies to identify potential inhibitors of human calcium/calmodulin-dependent protein kinase IV

Calcium/calmodulin-dependent protein kinase IV (CAMKIV) is associated with many diseases including cancer and neurodegenerative disorders and thus being considered as a potential drug target. Here, we have employed the knowledge of three-dimensional structure of CAMKIV to identify new inhibitors for possible therapeutic intervention. We have employed virtual high throughput screening of 12,500 natural compounds of Zinc database to screen the best possible inhibitors of CAMKIV. Subsequently, 40 compounds which showed significant docking scores (-11.6 to -10.0 kcal/mol) were selected and further filtered through Lipinski rule and drug likeness parameter to get best inhibitors of CAMKIV. Docking results are indicating that ligands are binding to the hydrophobic cavity of the kinase domain of CAMKIV and forming a significant number of non-covalent interactions. Four compounds, ZINC02098378, ZINC12866674, ZINC04293413, and ZINC13403020, showing excellent binding affinity and drug likeness were subjected to molecular dynamics simulation to evaluate their mechanism of interaction and stability of protein-ligand complex. Our observations clearly suggesting that these selected ligands may be further employed for therapeutic intervention to address CAMKIV associated diseases. Communicated by Ramaswamy H. Sarma.

Importin α: functions as a nuclear transport factor and beyond

Nucleocytoplasmic transport is an essential process in eukaryotes. The molecular mechanisms underlying nuclear transport that involve the nuclear transport receptor, small GTPase Ran, and the nuclear pore complex are highly conserved from yeast to humans. On the other hand, it has become clear that the nuclear transport system diverged during evolution to achieve various physiological functions in multicellular eukaryotes. In this review, we first summarize the molecular mechanisms of nuclear transport and how these were elucidated. Then, we focus on the diverse functions of importin α, which acts not merely an import factor but also as a multi-functional protein contributing to a variety of cellular functions in higher eukaryotes.

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.

Calmodulin kinases: essential regulators in health and disease

Neuronal activity induces intracellular Ca²⁺ increase, which triggers activation of a series of Ca²⁺ -dependent signaling cascades. Among these, the multifunctional Ca²⁺ /calmodulin-dependent protein kinases (CaMKs, or calmodulin kinases) play key roles in neuronal transmission, synaptic plasticity, circuit development and cognition. The most investigated CaMKs for these roles in neuronal functions are CaMKI, CaMKII, CaMKIV and we will shed light on these neuronal CaMKs' functions in this review. Catalytically active members of CaMKs currently are CaMKI, CaMKII, CaMKIV and CaMKK. Although they all necessitate the binding of Ca²⁺ and calmodulin complex (Ca²⁺ /CaM) for releasing autoinhibition, each member of CaMK has distinct activation mechanisms-autophosphorylation mediated autonomy of multimeric CaMKII and CaMKK-dependent phosphoswitch-induced activation of CaMKI or CaMKIV. Furthermore, each CaMK shows distinct subcellular localization that underlies specific compartmentalized function in each activated neuron. In this review, we first summarize these molecular characteristics of each CaMK as to regulation and subcellular localization, and then describe each biological function. In the last section, we also focus on the emerging role of CaMKs in pathophysiological conditions by introducing the recent studies, especially focusing on drug addiction and depression, and discuss how dysfunctional CaMKs may contribute to the pathology of the neuropsychological disorders. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

Karyopherin alpha 2 expression is a novel diagnostic and prognostic factor for colorectal cancer

Colorectal cancer (CRC) is the fourth most common cancer and the second leading cause of cancer-associated mortality in Western countries. CRC treatment is dependent on the preoperative and postoperative condition of patients. At present, the prognostic value of conventional parameters for the estimation of patient prognosis is limited. The aim of the present study was to investigate the expression of karyopherin α2 (KPNA2) in cancerous and healthy colon tissues and to evaluate the prognostic factors for patients with primary CRC. KPNA2 expression in CRC and paired normal tissues was analyzed by immunohistochemistry and reverse transcription-quantitative polymerase chain reaction (RT-qPCR). In addition, serum KPNA2 expression was evaluated by enzyme-linked immunosorbent assay. Subsequently, the association between KPNA2 expression in CRC tissues and patient clinicopathological features was analyzed. Kaplan-Meier analysis was utilized to investigate the prognostic value of KPNA2 expression on overall survival rates following radical surgery for the treatment of CRC. Immunohistochemistry and RT-qPCR revealed that KPNA2 expression was significantly increased in CRC tissues compared with paired normal tissues. Serum KPNA2 expression was significantly increased in CRC patients compared with healthy individuals. Furthermore, KPNA2 expression was observed to positively correlate with Tumor-Node-Metastasis stage, lymph node involvement, tumor differentiation, infiltration depth, lymphovascular invasion and perineural invasion, which are factors known to affect the prognosis of CRC patients following surgery. In addition, increased KPNA2 expression was associated with decreased overall survival and disease-free survival rates. Patients not suited for surveillance regimens may be identified at initial biopsy test with a positive KPNA2 immunohistochemistry. Increased serum expression of KPNA2 may be utilized as a diagnostic factor for patients with CRC. High nuclear KPNA2 expression may serve as a novel predictor of survival following radical colorectal surgery in CRC patients. The results of the present study may improve individualized risk stratification, leading to the optimization of therapies for CRC patients.

Tau accumulation induces synaptic impairment and memory deficit by calcineurin-mediated inactivation of nuclear CaMKIV/CREB signaling

Intracellular accumulation of wild-type tau is a hallmark of sporadic Alzheimer's disease (AD), but the molecular mechanisms underlying tau-induced synapse impairment and memory deficit are poorly understood. Here we found that overexpression of human wild-type full-length tau (termed hTau) induced memory deficits with impairments of synaptic plasticity. Both in vivo and in vitro data demonstrated that hTau accumulation caused remarkable dephosphorylation of cAMP response element binding protein (CREB) in the nuclear fraction. Simultaneously, the calcium-dependent protein phosphatase calcineurin (CaN) was up-regulated, whereas the calcium/calmodulin-dependent protein kinase IV (CaMKIV) was suppressed. Further studies revealed that CaN activation could dephosphorylate CREB and CaMKIV, and the effect of CaN on CREB dephosphorylation was independent of CaMKIV inhibition. Finally, inhibition of CaN attenuated the hTau-induced CREB dephosphorylation with improved synapse and memory functions. Together, these data indicate that the hTau accumulation impairs synapse and memory by CaN-mediated suppression of nuclear CaMKIV/CREB signaling. Our findings not only reveal new mechanisms underlying the hTau-induced synaptic toxicity, but also provide potential targets for rescuing tauopathies.

LMTK3 Represses Tumor Suppressor-like Genes through Chromatin Remodeling in Breast Cancer

LMTK3 is an oncogenic receptor tyrosine kinase (RTK) implicated in various types of cancer, including breast, lung, gastric, and colorectal cancer. It is localized in different cellular compartments, but its nuclear function has not been investigated so far. We mapped LMTK3 binding across the genome using ChIP-seq and found that LMTK3 binding events are correlated with repressive chromatin markers. We further identified KRAB-associated protein 1 (KAP1) as a binding partner of LMTK3. The LMTK3/KAP1 interaction is stabilized by PP1α, which suppresses KAP1 phosphorylation specifically at LMTK3-associated chromatin regions, inducing chromatin condensation and resulting in transcriptional repression of LMTK3-bound tumor suppressor-like genes. Furthermore, LMTK3 functions at distal regions in tethering the chromatin to the nuclear periphery, resulting in H3K9me3 modification and gene silencing. In summary, we propose a model where a scaffolding function of nuclear LMTK3 promotes cancer progression through chromatin remodeling.

Ca(2+)/Calmodulin-Dependent Protein Kinase IV Promotes Interplay of Proteins in Chromatoid Body of Male Germ Cells

The chromatoid body is a granule-like structure of male germ cells, containing many proteins and RNAs, and is important for spermatogenesis. However, the molecular mechanisms for the formation and function of the chromatoid body are still elusive. Here, we report that Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) accumulates in the chromatoid body by immunofluorescence staining, indicating that CaMKIV is a new component of the chromatoid body. Furthermore, we find that CaMKIV can interplay with the other components of the chromatoid body by immunoprecipitation: mouse VASA homologue (MVH), mouse homologue of PIWI, PIWIL1 (MIWI), and kinesin KIF17b. Importantly, interplay between KIF17b and MVH or MIWI can be potentially regulated by CaMKIV. These results imply that CaMKIV plays a role in maintenance the structure of chromatoid body by regulating the associations of proteins in it.

Drosophila TIM binds importin α1, and acts as an adapter to transport PER to the nucleus

Regulated nuclear entry of clock proteins is a conserved feature of eukaryotic circadian clocks and serves to separate the phase of mRNA activation from mRNA repression in the molecular feedback loop. In Drosophila, nuclear entry of the clock proteins, PERIOD (PER) and TIMELESS (TIM), is tightly controlled, and impairments of this process produce profound behavioral phenotypes. We report here that nuclear entry of PER-TIM in clock cells, and consequently behavioral rhythms, require a specific member of a classic nuclear import pathway, Importin α1 (IMPα1). In addition to IMPα1, rhythmic behavior and nuclear expression of PER-TIM require a specific nuclear pore protein, Nup153, and Ran-GTPase. IMPα1 can also drive rapid and efficient nuclear expression of TIM and PER in cultured cells, although the effect on PER is mediated by TIM. Mapping of interaction domains between IMPα1 and TIM/PER suggests that TIM is the primary cargo for the importin machinery. This is supported by attenuated interaction of IMPα1 with TIM carrying a mutation previously shown to prevent nuclear entry of TIM and PER. TIM is detected at the nuclear envelope, and computational modeling suggests that it contains HEAT-ARM repeats typically found in karyopherins, consistent with its role as a co-transporter for PER. These findings suggest that although PER is the major timekeeper of the clock, TIM is the primary target of nuclear import mechanisms. Thus, the circadian clock uses specific components of the importin pathway with a novel twist in that TIM serves a karyopherin-like role for PER.

In Drosophila, circadian rhythms are driven by a negative feedback loop that includes the key regulators, period (per) and timeless (tim). To generate this feedback loop, PER and TIM proteins first accumulate in the cytoplasm and then translocate to the nucleus where PER represses transcription. Thus, the nuclear import of PER-TIM proteins is a critical step to separate the phases of activation and repression of mRNA synthesis. In this study, we discovered that a member of the nuclear import machinery, importin α1 is an essential component of this feedback loop. Flies lacking importin α1 (IMPα1) display arrhythmic behavior and cytoplasmic expression of both PER and TIM at all times. In cultured S2 cells, IMPα1 expression directly facilitates nuclear import of TIM, but the effect on PER appears to be indirect. TIM expression is detected at the nuclear envelope and it interacts with other components of the nuclear transport machinery, which we show are also required for nuclear expression of TIM-PER and for behavioral rhythms. Our results thus suggest that TIM functions to link PER to the nuclear import machinery through IMPα1. Altogether, this study provides the mechanistic basis of a crucial step in the circadian clock mechanism.

The α-importome of mammalian germ cell maturation provides novel insights for importin biology

Importin α proteins function as adaptors to connect a cargo protein and importin β1 in the classical nuclear import pathway. Here we measure for the first time the stoichiometry of importins α2, α3, α4, and β1 in primary cells corresponding to 2 successive stages of rat spermatogenesis: meiotic spermatocytes and haploid round spermatids. Importin α2 levels were more than 2-fold higher in spermatocytes than in spermatids, while importins α4 and β1 levels did not differ significantly. We performed a comprehensive proteomics analysis to identify binding proteins in spermatocytes and spermatids using recombinant importin α2 and α4 proteins. Among the 100 candidate partners, 42 contained a strong classical nuclear localization signal (cNLS; score of>6 by cNLS Mapper), while 8 nuclear proteins lacked any cNLS. In addition, we developed a new strategy to predict which cargoes bind to importin α through the conserved C-terminal acidic domain (ARM repeats 9-10), and provided functional validation of a predicted importin α C-terminal binding segment in Senataxin and Smarca4. Evaluation of this set of candidate binding partners from spermatogenic cells using several bioinformatics approaches provides new evidence that individual importin αs may serve unique and nonredundant roles in mediating cellular differentiation.

IB4-binding sensory neurons in the adult rat express a novel 3' UTR-extended isoform of CaMK4 that is associated with its localization to axons

Calcium/calmodulin-dependent protein kinase 4 (gene and transcript: CaMK4; protein: CaMKIV) is the nuclear effector of the Ca(2+) /calmodulin kinase (CaMK) pathway where it coordinates transcriptional responses. However, CaMKIV is present in the cytoplasm and axons of subpopulations of neurons, including some sensory neurons of the dorsal root ganglia (DRG), suggesting an extranuclear role for this protein. We observed that CaMKIV was expressed strongly in the cytoplasm and axons of a subpopulation of small-diameter DRG neurons, most likely cutaneous nociceptors by virtue of their binding the isolectin IB4. In IB4+ spinal nerve axons, 20% of CaMKIV was colocalized with the endocytic marker Rab7 in axons that highly expressed CAM-kinase-kinase (CAMKK), an upstream activator of CaMKIV, suggesting a role for CaMKIV in signaling though signaling endosomes. Using fluorescent in situ hybridization (FISH) with riboprobes, we also observed that small-diameter neurons expressed high levels of a novel 3' untranslated region (UTR) variant of CaMK4 mRNA. Using rapid amplification of cDNA ends (RACE), reverse-transcription polymerase chain reaction (RT-PCR) with gene-specific primers, and cDNA sequencing analyses we determined that the novel transcript contains an additional 10 kb beyond the annotated gene terminus to a highly conserved alternate polyadenylation site. Quantitative PCR (qPCR) analyses of fluorescent-activated cell sorted (FACS) DRG neurons confirmed that this 3'-UTR-extended variant was preferentially expressed in IB4-binding neurons. Computational analyses of the 3'-UTR sequence predict that UTR-extension introduces consensus sites for RNA-binding proteins (RBPs) including the embryonic lethal abnormal vision (ELAV)/Hu family proteins. We consider the possible implications of axonal CaMKIV in the context of the unique properties of IB4-binding DRG neurons.

Nuclear calcium signaling in spinal neurons drives a genomic program required for persistent inflammatory pain

Persistent pain induced by noxious stimuli is characterized by the transition from normosensitivity to hypersensitivity. Underlying mechanisms are not well understood, although gene expression is considered important. Here, we show that persistent nociceptive-like activity triggers calcium transients in neuronal nuclei within the superficial spinal dorsal horn, and that nuclear calcium is necessary for the development of long-term inflammatory hypersensitivity. Using a nucleus-specific calcium signal perturbation strategy in vivo complemented by gene profiling, bioinformatics, and functional analyses, we discovered a pain-associated, nuclear calcium-regulated gene program in spinal excitatory neurons. This includes C1q, a modulator of synaptic spine morphogenesis, which we found to contribute to activity-dependent spine remodelling on spinal neurons in a manner functionally associated with inflammatory hypersensitivity. Thus, nuclear calcium integrates synapse-to-nucleus communication following noxious stimulation and controls a spinal genomic response that mediates the transition between acute and long-term nociceptive sensitization by modulating functional and structural plasticity.

Overexpression of KPNA2 correlates with poor prognosis in patients with gastric adenocarcinoma

This study aims to investigate the expression and significance of KPNA2 in human gastric adenocarcinoma progression and prognosis. Using immunohistochemistry and real-time reverse transcriptase polymerase chain reaction assay, we identified abnormally elevated expression of KPNA2 in gastric adenocarcinoma tissues compared to paired normal stomach mucosa tissues in 30 patients (p < 0.05). In order to investigate the correlations between KPNA2 and the clinicopathological features of gastric adenocarcinoma, the expression of KPNA2 in 142 patients with gastric adenocarcinoma was detected by immunohistochemistry, and the results showed that overexpression of KPNA2 was associated with the size of tumor (p < 0.001), histological grade (p < 0.001), lymph node involvement (p = 0.001), and tumor node metastasis stage (p < 0.001). Kaplan-Meier survival analysis showed that patients with high KPNA2 expression showed a significantly shorter overall survival time compared with patients with low KPNA2 expression. Multivariate analysis suggested that KPNA2 expression might be an independent prognostic indicator (p < 0.001) for the survival of patients with gastric adenocarcinoma. In conclusion, overexpression of KPNA2 is closely related to progression of gastric adenocarcinoma and might be regarded as an independent predictor of poor prognosis for gastric adenocarcinoma.

Quantitative proteomics reveals regulation of karyopherin subunit alpha-2 (KPNA2) and its potential novel cargo proteins in nonsmall cell lung cancer

The process of nucleocytoplasmic shuttling is mediated by karyopherins. Dysregulated expression of karyopherins may trigger oncogenesis through aberrant distribution of cargo proteins. Karyopherin subunit alpha-2 (KPNA2) was previously identified as a potential biomarker for nonsmall cell lung cancer by integration of the cancer cell secretome and tissue transcriptome data sets. Knockdown of KPNA2 suppressed the proliferation and migration abilities of lung cancer cells. However, the precise molecular mechanisms underlying KPNA2 activity in cancer remain to be established. In the current study, we applied gene knockdown, subcellular fractionation, and stable isotope labeling by amino acids in cell culture-based quantitative proteomic strategies to systematically analyze the KPNA2-regulating protein profiles in an adenocarcinoma cell line. Interaction network analysis revealed that several KPNA2-regulating proteins are involved in the cell cycle, DNA metabolic process, cellular component movements and cell migration. Importantly, E2F1 was identified as a potential novel cargo of KPNA2 in the nuclear proteome. The mRNA levels of potential effectors of E2F1 measured using quantitative PCR indicated that E2F1 is one of the "master molecule" responses to KPNA2 knockdown. Immunofluorescence staining and immunoprecipitation assays disclosed co-localization and association between E2F1 and KPNA2. An in vitro protein binding assay further demonstrated that E2F1 interacts directly with KPNA2. Moreover, knockdown of KPNA2 led to subcellular redistribution of E2F1 in lung cancer cells. Our results collectively demonstrate the utility of quantitative proteomic approaches and provide a fundamental platform to further explore the biological roles of KPNA2 in nonsmall cell lung cancer.

Intrinsic and extrinsic negative regulators of nuclear protein transport processes

The nuclear-cytoplasmic protein transport is a critical process in cellular events. The identification of transport signals (nuclear localization signal and nuclear export signal) and their receptors has facilitated our understanding of this expanding field. Nuclear transport must be appropriately regulated to deliver proteins through the nuclear pore when their functions are required in the nucleus, and to export them into the cytoplasm when they are not needed in the nucleus. Altered nuclear transport processes have been observed in stressed cells, which would change gene expressions. Some viruses interfere with nuclear transport in host cells to evade immune defense. Moreover, certain transport factors negatively regulate nuclear protein transport in cells. Understanding the regulatory mechanisms of nuclear-cytoplasmic trafficking not only provides important information about cellular processes, but also is of use for developing specific inhibitors for transport pathways.

The bovine immunodeficiency virus Rev protein: identification of a novel nuclear import pathway and nuclear export signal among retroviral Rev/Rev-like proteins

The Rev protein is essential for the replication of lentiviruses. Rev is a shuttling protein that transports unspliced and partially spliced lentiviral RNAs from the nucleus to the cytoplasm via the nucleopore. To transport these RNAs, the human immunodeficiency virus type 1 (HIV-1) Rev uses the karyopherin β family importin β and CRM1 proteins that interact with the Rev nuclear localization signal (NLS) and nuclear exportation signal (NES), respectively. Recently, we reported the presence of new types of bipartite NLS and nucleolar localization signal (NoLS) in the bovine immunodeficiency virus (BIV) Rev protein. Here we report the characterization of the nuclear import and export pathways of BIV Rev. By using an in vitro nuclear import assay, we showed that BIV Rev is transported into the nucleus by a cytosolic and energy-dependent importin α/β classical pathway. Results from glutathione S-transferase (GST) pulldown assays that showed the binding of BIV Rev with importins α3 and α5 were in agreement with those from the nuclear import assay. We also identified a leptomycin B-sensitive NES in BIV Rev, which indicates that the protein is exported via CRM1 like HIV-1 Rev. Mutagenesis experiments showed that the BIV Rev NES maps between amino acids 109 to 121 of the protein. Remarkably, the BIV Rev NES was found to be of the cyclic AMP (cAMP)-dependent protein kinase inhibitor (PKI) type instead of the HIV-1 Rev type. In summary, our data showed that the nuclear import mechanism of BIV Rev is novel among Rev proteins characterized so far in lentiviruses.

Involvement of classical bipartite/karyopherin nuclear import pathway components in acrosomal trafficking and assembly during bovine and murid spermiogenesis

This study arose from our finding that SubH2Bv, a histone H2B variant residing in the subacrosomal compartment of mammalian spermatozoa, contains a bipartite nuclear localization signal (bNLS) but in spite of this did not enter the spermatid nucleus. Instead, it associated with proacrosomic and acrosomic vesicles, which were targeted to the nuclear surface to form the acrosome. On this basis we proposed that SubH2Bv targets proacrosomic/acrosomic vesicles from the Golgi apparatus to the nuclear envelope by utilizing the classical bipartite/karyopherin alpha (KPNA) nuclear import pathway. To test the protein's nuclear targeting ability, SubH2Bv, with and without targeted mutations of the basic residues of bNLS, as well as bNLS alone, were transfected into mammalian cells as GFP-fusion proteins. Only the intact bNLS conferred nuclear entry. Subsequently, we showed that a KPNA, most likely KPNA6, occupies the same sperm head compartment and follows the same pattern of acrosomal association during spermiogenesis as SubH2Bv. Sperm head fractionation combined with Western blotting located this KPNA to the subacrosomal layer of the perinuclear theca, while immunocytochemistry of testicular sections showed that it associates with the surface of proacrosomic/acrosomic vesicles during acrosomal biogenesis. The identical sperm-localization and testicular-expression patterns between KPNA and SubH2Bv suggested a potential binding interaction between these proteins. This was supported by recombinant SubH2Bv affinity pull-down assays on germ cell extracts. The results of this study provide a compelling argument that these two nuclear homing proteins work in concert to direct the acrosomic vesicle to the nucleus. Their final residence in the subacrosomal layer of the perinuclear theca of spermatozoa indicates a role for SubH2Bv and KPNA in acrosomal-nuclear docking.

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.

Nuclear exportin receptor CAS regulates the NPI-1-mediated nuclear import of HIV-1 Vpr

Vpr, an accessory protein of human immunodeficiency virus type 1, is a multifunctional protein that plays an important role in viral replication. We have previously shown that the region between residues 17 and 74 of Vpr (Vpr_N17C74) contained a bona fide nuclear localization signal and it is targeted Vpr_N17C74 to the nuclear envelope and then imported into the nucleus by importin α (Impα) alone. The interaction between Impα and Vpr is important not only for the nuclear import of Vpr but also for HIV-1 replication in macrophages; however, it was unclear whether full-length Vpr enters the nucleus in a manner similar to Vpr_N17C74. This study investigated the nuclear import of full-length Vpr using the three typical Impα isoforms, Rch1, Qip1 and NPI-1, and revealed that full-length Vpr is selectively imported by NPI-1, but not Rch1 and Qip1, after it makes contact with the perinuclear region in digitonin-permeabilized cells. A binding assay using the three Impα isoforms showed that Vpr bound preferentially to the ninth armadillo repeat (ARM) region (which is also essential for the binding of CAS, the export receptor for Impα) in all three isoforms. Comparison of biochemical binding affinities between Vpr and the Impα isoforms using surface plasmon resonance analysis demonstrated almost identical values for the binding of Vpr to the full-length isoforms and to their C-terminal domains. By contrast, the data showed that, in the presence of CAS, Vpr was released from the Vpr/NPI-1 complex but was not released from Rch1 or Qip1. Finally, the NPI-1–mediated nuclear import of Vpr was greatly reduced in semi-intact CAS knocked-down cells and was recovered by the addition of exogenous CAS. This report is the first to show the requirement for and the regulation of CAS in the functioning of the Vpr-Impα complex.

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.

Nuclear retention of importin α coordinates cell fate through changes in gene expression

Various cellular stresses including oxidative stress induce a collapse of the Ran gradient, which causes accumulation of importin α in the nucleus and a subsequent block of nuclear protein import. However, it is unknown whether accumulated importin α performs roles in the nucleus after its migration in response to stress. In this study, we found that nuclear-retained importin α2 binds with DNase I-sensitive nuclear component(s) and exhibits selective upregulation of mRNA encoding Serine/threonine kinase 35 (STK35) by microarray analysis. Chromatin immunoprecipitation and promoter analysis demonstrated that importin α2 can access to the promoter region of STK35 and accelerate its transcription in response to hydrogen peroxide exposure. Furthermore, constitutive overexpression of STK35 proteins enhances caspase-independent cell death under oxidative stress conditions. These results collectively reveal that nuclear-localized importin α2 influences gene expression and contributes directly to cell fate outcomes including non-apoptotic cell death.

Cross-talk between distinct nuclear import pathways enables efficient nuclear import of E47 in conjunction with its partner transcription factors

Study of the nuclear import behavior of E47 in conjunction with its partner transcription factors shows that although the nuclear import of E47 is importin α dependent, it is capable of accumulating in the nucleus under importin α–blocked conditions by virtue of its interaction with its binding partners NeuroD1 and MyoD.

Nuclear import of karyophilic proteins is carried out by a variety of mechanisms. We previously showed that two basic helix-loop-helix proteins, NeuroD1 and E47, synergistically affect each other's nuclear import. In this study, we dissected the molecular pathways underlying nuclear import of the NeuroD1/E47 heterodimer. In vitro nuclear import assays indicated that importin α family members are the major nuclear import receptors for E47. However, inhibition of importin α resulted in cytoplasmic retention of E47 that could be rescued by its binding partner, NeuroD1, through heterodimerization. In addition, nuclear import of NeuroD1 was importin α independent but importin β1 dependent. In primary neurons, localization of endogenous E47 was not affected by importin α inhibition, suggesting that neuronal E47 could be imported into the nucleus as a heterodimer with NeuroD1 by using importin β1 alone. We also found that E47 had similar nuclear import characteristics in C2C12 cells, where E47 heterodimerized with MyoD, another helix-loop-helix protein, suggesting functional conservation within the same family of transcription factors. Collectively, our data reveal that E47 is imported into the nucleus via multiple pathways, depending on the molecular binding mode, establishing a previously uncharacterized cross-talk between two distinct nuclear import pathways.

Generation and characterization of a monoclonal antibody against importin α7/NPI-2

Many nuclear proteins are transported into the nucleus via the importin α/β-mediated pathway. Importin α comprises a multigene family. In this study, we generated and characterized a rat monoclonal antibody (MAb) 3F8 to importin α7. The antibody was generated by the hybridization of mouse myeloma cells with lymph node cells from an immunized rat. The MAb 3F8 specifically recognized importin α7 among importin α isoforms as evidenced by immunoblotting analysis. Furthermore, MAb 3F8 detected exogenous importin α7 in COS-7 cells by immunofluorescence. This MAb will be useful in the analysis of the isoform-specific function of importin α7.

Cardiac pressure overload hypertrophy is differentially regulated by β-adrenergic receptor subtypes

In isolated myocytes, hypertrophy induced by norepinephrine is mediated via α(1)-adrenergic receptors (ARs) and not β-ARs. However, mice with deletions of both major cardiac α(1)-ARs still develop hypertrophy in response to pressure overload. Our purpose was to better define the role of β-AR subtypes in regulating cardiac hypertrophy in vivo, important given the widespread clinical use of β-AR antagonists and the likelihood that patients treated with these agents could develop conditions of further afterload stress. Mice with deletions of β(1), β(2), or both β(1)- and β(2)-ARs were subjected to transverse aortic constriction (TAC). After 3 wk, β(1)(-/-) showed a 21% increase in heart to body weight vs. sham controls, similar to wild type, whereas β(2)(-/-) developed exaggerated (49% increase) hypertrophy. Only when both β-ARs were ablated (β(1)β(2)(-/-)) was hypertrophy totally abolished. Cardiac function was preserved in all genotypes. Several known inhibitors of cardiac hypertrophy (FK506 binding protein 5, thioredoxin interacting protein, and S100A9) were upregulated in β(1)β(2)(-/-) compared with the other genotypes, whereas transforming growth factor-β(2), a positive mediator of hypertrophy was upregulated in all genotypes except the β(1)β(2)(-/-). In contrast to recent reports suggesting that angiogenesis plays a critical role in regulating cardiac hypertrophy-induced heart failure, we found no evidence that angiogenesis or its regulators (VEGF, Hif1α, and p53) play a role in compensated cardiac hypertrophy. Pressure overload hypertrophy in vivo is dependent on a coordination of signaling through both β(1)- and β(2)-ARs, mediated through several key cardiac remodeling pathways. Angiogenesis is not a prerequisite for compensated cardiac hypertrophy.

Signals and pathways regulating nucleolar retention of novel putative nucleolar GTPase NGP-1(GNL-2)

NGP-1(GNL-2) is a putative GTPase, overexpressed in breast carcinoma and localized in the nucleolus. NGP-1 belongs to the MMR1-HSR1 family of large GTPases that are emerging as crucial coordinators of signaling cascades in different cellular compartments. The members of this family share very closely related G-domains, but the signals and pathways regulating their subcellular localization and their functional relevance remain unknown. To improve our understanding of the nuclear transport mechanism of NGP-1, we have identified two nucleolar localization signals (NoLS) that are independently shown to translocate NGP-1 as well the heterologous protein to the nucleolus. Site-specific mutagenesis and immunofluorescence studies suggest that the tandem repeats of positively charged amino acids are critical for NGP-1 NoLS function. Interestingly, amino-terminal (NGP-1(1-100)) and carboxyl-terminal (NGP-1(661-731)) signals independently interact with receptors importin-β and importin-α, respectively. This investigation, for the first time, provides evidence that the interaction of importin-α with C-terminal NoLS (NGP-1(661-731)) was able to target the heterologous protein to the nucleolar compartment. Structural modeling analysis and alanine scanning mutagenesis of conserved G-domains suggest that G4 and G5 motifs are critical for GTP binding of NGP-1 and further show that the nucleolar localization of NGP-1 is regulated by a GTP gating-mediated mechanism. In addition, our data suggest that an ongoing transcription is essential for efficient localization of NGP-1 to the nucleolus. We have observed a high level of NGP-1 expression in the mitogen-activated primary human peripheral blood mononuclear cells (hPBMC) as well as in human fetal brain-derived neural precursor cells (hNPCs) in comparison to cells undergoing differentiation. Overall, the results suggest that multiple mechanisms are involved in the localization of NGP-1 to the nucleolus for the regulation of nucleolar function in cell growth and proliferation.

HIV-1 accessory protein Vpr: relevance in the pathogenesis of HIV and potential for therapeutic intervention

The HIV protein, Vpr, is a multifunctional accessory protein critical for efficient viral infection of target CD4⁺ T cells and macrophages. Vpr is incorporated into virions and functions to transport the preintegration complex into the nucleus where the process of viral integration into the host genome is completed. This action is particularly important in macrophages, which as a result of their terminal differentiation and non-proliferative status, would be otherwise more refractory to HIV infection. Vpr has several other critical functions including activation of HIV-1 LTR transcription, cell-cycle arrest due to DCAF-1 binding, and both direct and indirect contributions to T-cell dysfunction. The interactions of Vpr with molecular pathways in the context of macrophages, on the other hand, support accumulation of a persistent reservoir of HIV infection in cells of the myeloid lineage. The role of Vpr in the virus life cycle, as well as its effects on immune cells, appears to play an important role in the immune pathogenesis of AIDS and the development of HIV induced end-organ disease. In view of the pivotal functions of Vpr in virus infection, replication, and persistence of infection, this protein represents an attractive target for therapeutic intervention.

Importin alpha protein acts as a negative regulator for Snail protein nuclear import

Snail, a zinc finger-containing transcriptional regulator, migrates into the nucleus where it controls gene expression. We demonstrated previously that importin β1 directly recognizes the zinc finger domain of Snail and transports it into the nucleus. Here, using in vitro and in vivo assays, we show that importin α, an adaptor protein for importin β1, negatively regulates the nuclear import of Snail mediated by importin β1. In vitro binding assays indicated that importin α interacted with the zinc finger domain of Snail to compete with the binding of importin β1 and that Snail did not form a ternary complex with importin α/importin β1. Overexpression of importin α in A549 cells reduced the endogenous Snail protein level, which was restored by inhibitors of the proteasome and glycogen synthase kinase 3β. Furthermore, knockdown of importin α by siRNA treatment increased the endogenous Snail protein level in several cancer cell lines. This study provides a novel regulatory mechanism of the nuclear protein import process by importin α and gives an implication to control Snail activity by inhibiting its nuclear localization.

Proteomic analysis of importin α-interacting proteins in adult mouse brain

Many transport factors, such as importins and exportins, have been identified, and the molecular mechanisms underlying nucleocytoplasmic transport have been characterized. The specific molecules that are carried by each transport factor and the temporal profiles that characterize the movements of various proteins into or out of the nucleus, however, have yet to be elucidated. Here, we used a proteomic approach to identify molecules that are transported into the nuclei of adult mouse brain cells via importin α5. We identified 48 proteins in total, among which we chose seven to characterize more extensively: acidic (leucine-rich) nuclear phosphoprotein 32 family member A (Anp32a), far upstream element binding protein 1 (FUBP1), thyroid hormone receptor β1 (TRβ1), transaldolase 1, CDC42 effector protein 4 (CDC42-ep4), Coronin 1B, and brain-specific creatine kinase (CK-B). Analyses using green fluorescent protein (GFP)-fused proteins showed that Anp32a, FUBP1, and TRβ1 were localized in the nucleus, whereas transaldolase 1, CDC42-ep4, CK-B, and Coronin 1B were distributed in both the cytoplasm and nucleus. Using a digitonin-permeabilized in vitro transport assay, we demonstrated that, with the exception of CK-B, these proteins were transported into the nucleus by importin α5 together with importin β and Ran. Further, we found that leptomycin B (LMB) treatment increased nuclear CK-B-GFP signals, suggesting that CK-B enters the nucleus and is then exported in a CRM1-dependent manner. Thus, we identified a comprehensive set of candidate proteins that are transported into the nucleus in a manner dependent on importin α5, which enhances our understanding of nucleocytoplasmic signaling in neural cells.

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

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

Identification of a nuclear localization signal in the polo box domain of Plk1

Polo-like kinase 1 plays an essential role in mitosis and cytokinesis. Expression and nuclear localization of Plk1 during the S phase are necessary for its functions. Although it was reported that a bipartite nuclear localization signal located at the N-terminal kinase domain is required for nuclear import of Plk1, Plk1 carrying mutations in the polo box I of the polo box domain exhibited increased cytoplasmic accumulation. We further showed that the polo box domain was able to confer nuclear import of beta-galactosidase in vivo and GST-EGFP in vitro. The import carriers transportin and importin alpha were found to interact with the polo box domain directly in a Ran-GTP sensitive manner. These results indicate the presence of a nuclear localization signal in the polo box domain. A 38 amino acid sequence with the function of nuclear localization signal was identified to interact with transportin. Our findings demonstrated that a transportin-dependent nuclear localization signal is present in the polo box domain of Plk1, possibly required for efficient nuclear import. Showing little similarity to the M9 sequence, the 38 amino acid sequence identified here likely represents a novel nuclear localization signal.

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

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

Repression of Ca2+/calmodulin-dependent protein kinase IV signaling accelerates retinoic acid-induced differentiation of human neuroblastoma cells

Neuroblastoma cells having stem cell-like qualities are widely employed models for the study of neural stem/progenitor cell proliferation and differentiation. We find that human BE(2)C neuroblastoma cells possess a signaling cascade initiated by Ca(2+) influx via voltage-dependent calcium channels and the N-methyl-D-aspartate (NMDA) receptor and culminating in nuclear calmodulin-dependent protein kinase IV (CaMKIV)-mediated phosphorylation and activation of the transcription factors Ca(2+)/cyclic AMP-response element-binding protein (CREB) and ATF1 (activating transcription factor-1). This pathway functions to maintain BE(2)C cells in an undifferentiated, proliferative state. Parallel to this Ca(2+)-dependent pathway is a hormone-responsive program by which retinoic acid (RA) initiates the differentiation of BE(2)C cells toward a neuronal lineage. This is evidenced by RA-dependent induction of the cell cycle inhibitor p21/Cip1 (Cdk-interacting protein 1) and cell cycle arrest, induction of the neuroblastic marker doublecortin and of the neuron-specific intermediate filament protein, peripherin, and by RA-stimulated extension of neuritic processes. During neuronal differentiation there is a complex antagonistic interplay between these two major signaling pathways. RA down-regulates expression of CaMKIV and one of its upstream activators, CaMKK1 (calmodulin-dependent protein kinase kinase 1). This is accompanied by RA-induced suppression of activating phosphorylation of CREB with a time course paralleling that of CaMKIV down-regulation. RA-induced repression of the Ca(2+)/calmodulin-dependent protein kinase kinase/CaMKIV/CREB pathway appears to be involved in regulating the timing of neuronal differentiation, as shown by the effect of RNA interference of CaMKIV to markedly accelerate RA-dependent up-regulation of p21/Cip1 and doublecortin expression and RA-promoted neurite outgrowth. RA-induced repression of the CaMKIV signaling pathway may represent an early event in retinoid-dependent neuronal differentiation.

The role of the nuclear transport system in cell differentiation

The eukaryotic cell nuclear transport system selectively mediates molecular trafficking to facilitate the regulation of cellular processes. The components of this system include diverse transport factors such as importins and nuclear pore components that are precisely organized to coordinate cellular events. A number of studies have demonstrated that the nuclear transport system is indispensible in many types of cellular responses. In particular, the nuclear transport machinery has been shown to be an important regulator of development, organogenesis, and tissue formation, wherein altered nuclear transport of key transcription factors can lead to disease. Importantly, precise switching between distinct forms of importin alpha is central to neural lineage specification, consistent with the hypothesis that importin expression can be a key mediator of cell differentiation.

Nucleocytoplasmic transport: a thermodynamic mechanism

The nuclear pore supports molecular communication between cytoplasm and nucleus in eukaryotic cells. Selective transport of proteins is mediated by soluble receptors, whose regulation by the small GTPase Ran leads to cargo accumulation in, or depletion from, the nucleus, i.e., nuclear import or nuclear export. We consider the operation of this transport system by a combined analytical and experimental approach. Provocative predictions of a simple model were tested using cell-free nuclei reconstituted in Xenopus egg extract, a system well suited to quantitative studies. We found that accumulation capacity is limited, so that introduction of one import cargo leads to egress of another. Clearly, the pore per se does not determine transport directionality. Moreover, different cargo reach a similar ratio of nuclear to cytoplasmic concentration in steady-state. The model shows that this ratio should in fact be independent of the receptor-cargo affinity, though kinetics may be strongly influenced. Numerical conservation of the system components highlights a conflict between the observations and the popular concept of transport cycles. We suggest that chemical partitioning provides a framework to understand the capacity to generate concentration gradients by equilibration of the receptor-cargo intermediary.