Involvement of p62 nucleoporin in angiotensin II-induced nuclear translocation of STAT3 in brain neurons

Protein phosphatases at the nuclear envelope

The nuclear envelope (NE) is a unique topological structure formed by lipid membranes (Inner and Outer Membrane: IM and OM) interrupted by open channels (Nuclear Pore complexes). Besides its well-established structural role in providing a physical separation between the genome and the cytoplasm and regulating the exchanges between the two cellular compartments, it has become quite evident in recent years that the NE also represents a hub for localized signal transduction. Mechanical, stress, or mitogen signals reach the nucleus and trigger the activation of several pathways, many effectors of which are processed at the NE. Therefore, the concept of the NE acting just as a barrier needs to be expanded to embrace all the dynamic processes that are indeed associated with it. In this context, dynamic protein association and turnover coupled to reversible post-translational modifications of NE components can provide important clues on how this integrated cellular machinery functions as a whole. Reversible protein phosphorylation is the most used mechanism to control protein dynamics and association in cells. Keys to the reversibility of the system are protein phosphatases and the regulation of their activity in space and time. As the NE is clearly becoming an interesting compartment for the control and transduction of several signalling pathways, in this review we will focus on the role of Protein Phosphatases at the NE since the significance of this class of proteins in this context has been little explored.

Nucleoporin phosphorylation triggered by the encephalomyocarditis virus leader protein is mediated by mitogen-activated protein kinases

Cardioviruses disrupt nucleocytoplasmic transport through the activity of their leader (L) protein. We have shown that hyperphosphorylation of nuclear pore proteins (nucleoporins or Nups), including Nup62, Nup153, and Nup214, is central to this L protein function and requires one or more cytosolic kinases. In this study, potential cellular enzymes involved in encephalomyocarditis virus (EMCV) L-directed Nup phosphorylation were screened with a panel of specific, cell-permeating kinase inhibitors. Extracellular signal-regulated receptor kinase (ERK) and p38 mitogen-activated protein kinase inhibitors (U0126 and SB203580) were sufficient to block Nup hyperphosphorylation in EMCV-infected or L-expressing cells. Recombinant L alone, in the absence of infection, triggered activation of ERK and p38, independent of their upstream signaling cascades. Conserved residues within the L zinc finger (Cys(19)) and acidic domain (Asp(48),(51),(52),(55)) were essential for this activation and for the phosphorylation of Nups, suggesting that the phenomena are linked. Analysis of the hyperphosphorylated Nup species revealed only phosphoserine and phosphothreonine residues. The sizes of the tryptic phosphopeptides derived from Nup62 were compatible with sites in the Phe/Gly repeat domain which display common consensus sequences for ERK and p38 substrates. The results provide strong evidence that ERK and p38 are the probable effector kinases required for L-dependent inhibition of nuclear trafficking.

Mengovirus-induced rearrangement of the nuclear pore complex: hijacking cellular phosphorylation machinery

Representatives of several picornavirus genera have been shown previously to significantly enhance non-controllable bidirectional exchange of proteins between nuclei and cytoplasm. In enteroviruses and rhinoviruses, enhanced permeabilization of the nuclear pores appears to be primarily due to proteolytic degradation of some nucleoporins (protein components of the pore), whereas this effect in cardiovirus-infected cells is triggered by the leader (L) protein, devoid of any enzymatic activities. Here, we present evidence that expression of L alone was sufficient to cause permeabilization of the nuclear envelope in HeLa cells. In contrast to poliovirus, mengovirus infection of these cells did not elicit loss of nucleoporins Nup62 and Nup153 from the nuclear pore complex. Instead, nuclear envelope permeabilization was accompanied by hyperphosphorylation of Nup62 in cells infected with wild-type mengovirus, whereas both of these alterations were suppressed in L-deficient virus mutants. Since phosphorylation of Nup62 (although less prominent) did accompany permeabilization of the nuclear envelope prior to its mitotic disassembly in uninfected cells, we hypothesize that cardiovirus L alters the nucleocytoplasmic traffic by hijacking some components of the normal cell division machinery. The variability and biological significance of picornaviral interactions with the nucleocytoplasmic transport in the infected cells are discussed.

Ethanol inhibition of angiotensin II-stimulated Tyr705 and Ser727 STAT3 phosphorylation in cultured rat hepatocytes: relevance to activation of p42/44 mitogen-activated protein kinase

Angiotensin (Ang) II-stimulated phosphorylation of signal transducer and activator transcription (STAT) 3 in rat hepatocytes and the effects of ethanol on this activation were investigated. Angiotensin II (100 nM) stimulated Tyr705 and Ser727 phosphorylation of STAT3 and formation of sis-inducing factor complexes. In the presence of U-0126 (10microM), a p42/44 mitogen-activated protein kinase (MAPK) kinase inhibitor, Ang II further increased Tyr705 phosphorylation of STAT3 but completely abrogated Ser727 phosphorylation of STAT3. Inhibition of p42/44MAPK also increased STAT3 DNA-binding activity. Pretreatment with ethanol (100mM) for 24h resulted in decrease in Tyr705 phosphorylation of STAT3 by ethanol alone and inhibition of Tyr705 phosphorylation of STAT3 stimulated by Ang II. Although ethanol potentiates Ang II stimulated p42/44 MAPK activation in hepatocytes, ethanol inhibited Ser727 phosphorylation of STAT3 stimulated by Ang II. Angiotensin II-stimulated STAT3-binding activity was not significantly affected by ethanol treatment. These results suggest a negative regulation of Ang II-stimulated STAT3 tyrosine phosphorylation and STAT3-binding activity through p42/44 MAPK activation in hepatocytes. However, ethanol modulation of Ang II-stimulated STAT3 phosphorylation occurs by MAPK independent mechanisms. Ethanol potentiation of MAPK signaling without suppression of STAT3 function may modulate the course of alcoholic liver injury.

Functional Analysis of Nuclear Pore Complex Protein Nup62/p62 Using Monoclonal Antibodies

The nuclear pore complex (NPC) is an enormous structure embedded in the double membrane of the nuclear envelope that acts as a passageway for nucleocytoplasmic transport. The vertebrate NPC is comprised of about 30 unique proteins. Nup62/p62, a major component of the NPC, has been reported to interact directly with several nuclear transport factors, including importin-beta and NTF2. However, it has not been shown how the interaction of Nup62/p62 with transport factors is involved in nucleocytoplasmic transport. The present study reports on the preparation of monoclonal antibodies (MAbs) directed against human Nup62/p62 and a functional analysis of Nup62/p62 using antibodies in living cells. Hybridomas producing the antibodies were produced by the hybridization of mouse myeloma cells with medial iliac lymph node cells from an immunized rat. These MAbs specifically recognized Nup62/p62 as evidenced by immunoblotting analysis using a nuclear membrane fraction. In the immunostaining using MAbs, a punctuate nuclear rim staining pattern was observed. Moreover, cytoplasmic injected-anti-Nup62/p62 MAbs were rapidly targeted to the nuclear pore of cultured cells and some of them inhibited normal cell division, causing the formation of abnormal nuclei. The antibodies described in this study provide the means for immunochemical analyses of the NPC protein Nup62/p62 in mammalian cells, and represent useful molecular tools that should permit a better understanding of the biological roles and cellular dynamics of this protein in nucleocytoplasmic transport, cell division, and nuclear organization.

Differential expression of nuclear AT1 receptors and angiotensin II within the kidney of the male congenic mRen2. Lewis rat

We established a new congenic model of hypertension, the mRen(2). Lewis rat and assessed the intracellular expression of angiotensin peptides and receptors in the kidney. The congenic strain was established from the backcross of the (mRen2)27 transgenic rat that expresses the mouse renin 2 gene onto the Lewis strain. The 20-wk-old male congenic rats were markedly hypertensive compared with the Lewis controls (systolic blood pressure: 195 +/- 2 vs. 107 +/- 2 mmHg, P < 0.01). Although plasma ANG II levels were not different between strains, circulating levels of ANG-(1-7) were 270% higher and ANG I concentrations were 40% lower in the mRen2. Lewis rats. In contrast, both cortical (CORT) and medullary (MED) ANG II concentrations were 60% higher in the mRen2. Lewis rats, whereas tissue ANG I was 66 and 84% lower in CORT and MED. For both strains, MED ANG II, ANG I, and ANG-(1-7) were significantly higher than CORT levels. Intracellular ANG II binding distinguished nuclear (NUC) and plasma membrane (PM) receptor using the ANG II radioligand 125I-sarthran. Isolated CORT nuclei exhibited a high density (Bmax >200 fmol/mg protein) and affinity for the sarthran ligand (KD<0.5 nM); the majority of these sites (>95%) were the AT1 receptor subtype. CORT ANG II receptor Bmax and KD values in nuclei were 75 and 50% lower, respectively, for the mRen2. Lewis vs. the Lewis rats. In the MED, the PM receptor density (Lewis: 50 +/- 4 vs. mRen2. Lewis: 21 +/- 5 fmol/mg protein) and affinity (Lewis: 0.31 +/- 0.1 vs. 0.69 +/- 0.1 nM) were lower in the mRen2. Lewis rats. In summary, the hypertensive mRen2. Lewis rats exhibit higher ANG II in both CORT and MED regions of the kidney. Evaluation of intracellular ANG II receptors revealed lower CORT NUC and MED PM AT1 sites in the mRen2. Lewis. The downregulation of AT1 sites in the mRen2. Lewis rats may reflect a compensatory response to dampen the elevated levels of intrarenal ANG II.

NF-kappa B functions in synaptic signaling and behavior

Ca(2+)-regulated gene transcription is essential to diverse physiological processes, including the adaptive plasticity associated with learning. We found that basal synaptic input activates the NF-kappa B transcription factor by a pathway requiring the Ca(2+)/calmodulin-dependent kinase CaMKII and local submembranous Ca(2+) elevation. The p65:p50 NF-kappa B form is selectively localized at synapses; p65-deficient mice have no detectable synaptic NF-kappa B. Activated NF-kappa B moves to the nucleus and could directly transmute synaptic signals into altered gene expression. Mice lacking p65 show a selective learning deficit in the spatial version of the radial arm maze. These observations suggest that long-term changes to adult neuronal function caused by synaptic stimulation can be regulated by NF-kappa B nuclear translocation and gene activation.

Identification of differential gene expression profiles in rat cortical cells exposed to the neuroactive agents trimethylolpropane phosphate and bicuculline

Recent technological advancements in microfabrication combined with the rapid acquisition of full genome sequence data have led to the development of DNA arrays that have the capacity to monitor the expression levels of thousands of genes simultaneously. The development of this technology enables the use of functional genomics approaches to identify molecular markers associated with cellular responsiveness to cytotoxic exposures. Databases containing unique cell-response profiles associated with specific toxicants or classes of toxicants can then be used in conjunction with cell-based biosensor platforms for environmental surveillance and toxicological assessment. An important issue that must be addressed, however, is whether DNA arrays can be used to identify transient gene modulation events in a reproducible manner. To address this issue, we utilized a primary embryonic rat (day 18) cortical cell model system and examined the RNA of both chemically treated and untreated cells using radioisotope-labeled cDNA probes and commercially available nylon membrane arrays. Using this approach, we examined experimental variability, basal gene expression variability, the occurrence of false positives, and the reproducibility of gene expression profiles obtained after chemical exposure. Minimal differences in gene modulation were observed between RNA samples from independently cultured cortical cells when array experiments were conducted in parallel (Pearson correlation coefficient for gene intensities =0.98). In contrast, significant differences in gene expression were observed between array experiments conducted at different times with an identical RNA source (Pearson correlation coefficient for gene intensities=0.91). Our results suggest the effect of basal gene activity differences in independently isolated cell cultures is negligible and that experimental variability possibly associated with the handling of RNA samples, differences in reverse transcription efficiency, hybridization, and/or signal acquisition are the primary contributors to variability in measurements. Using cDNA array analysis of unexposed cells from three independent cell culture preparations, we calculated false positive gene modulation events as a function of the threshold absolute value of log(2) >1.0. The number of false positives using this criteria was 1-10 gene/ESTs/5109 actively transcribed gene/ESTs represented on the array. Using three independent replicate experiments of untreated cortical cell cultures, we determined that a threshold criterion of absolute value of log(2) >0.63 for triplicate experiments would reduce the expected number of false positives in our experiments to less than one. Using this criterion, reproducible gene expression profiles were identified in cortical cells exposed to the neuroactive agents trimethylolpropane phosphate and bicuculline.

Application of a rapid method (targeted display) for the identification of differentially expressed mRNAs following NGF-induced neuronal differentiation in PC12 cells

Nerve growth factor (NGF)-induced differentiation of the rat pheochromocytoma, PC12, cell line presents a model system for the study of early gene expression changes involved in neuronal differentiation. Rapid alterations in mRNA expression patterns were investigated in PC12 cells following exposure to NGF using a set of statistically designed primers that exhibit coding-strand bias, and the products were analyzed on agarose gels. This simple and rapid method (targeted display) generated reproducible expression profiles, indicating a complex pattern of gene regulation, and resulted in the identification of a number of NGF-regulated transcripts. Thirty-two of these were selected at random and sequenced, revealing 19 known and 13 novel genes (or ESTs). Northern blot analysis and RT-PCR confirmed the differential regulation of 22 genes (16 known, 6 novel) and demonstrated 1 false positive result. Antisense application of one isolated gene product, the serine/threonine kinase MARK1, prevented neuronal differentiation in transiently transfected PC12 cells.