Abstract P-18: Phosphorylation of RNA Polymerase II C-Terminal Domain Affects Transcript Elongation Through Chromatin

Background: Transcription is the central point of gene regulation where the efficient maintenance of chromatin structure during the passage of RNA polymerase (Pol II) is critical for cell survival and functioning. The phosphorylation of carboxy-terminal domain (CTD) of the large subunit (Rpb1) of Pol II plays a key role in transcription through chromatin providing the binding and dissociation of factors essential for the mRNA biogenesis. Although the regulatory effect of chromatin structure on multiple stages of transcription has been well established, the role of CTD phosphorylation itself has not been systematically addressed. Methods: The effect of differentially phosphorylated Pol II-CTD on transcript elongation through chromatin was studied using in vitro transcription system based on mononucleosomes precisely positioned on DNA. The unphosphorylated and hyperphosphorylated Pol II-CTD were obtained using yeast genetics as well as in vitro kinase or phosphatases. Transcription rate and positions of pausing were measured using authentic elongation complexes comprising Pol II having different CTD phosphorylation states. The quantitative analysis of the transcripts was conducted using denaturing PAGE. Results: We observed a significant difference in the transcription through chromatin depending on CTD phosphorylation level. Thus, experiments on transcription of nucleosomes with Pol II isoforms have shown that the hyperphosphorylated form more efficiently transcribes the nucleosome and leads to a faster accumulation of the full-length RNA product than the non-phosphorylated isoform of Pol II. The non-phosphorylated isoform of the enzyme is characterized by a stronger pause in the early nucleosomal region and a slower accumulation of the full-length RNA product. Conclusion: Hyperphosphorylated form more efficiently transcribes the nucleosome and leads to a faster accumulation of the full-length RNA product as compared with the non-phosphorylated isoform of Pol II. A preliminary model of the effect of Pol II hyperphosphorylation on nucleosomal DNA transcription is proposed.

Phosphorylation and Dephosphorylation of Tau Protein by the Catalytic Subunit of PKA, as Probed by Electrophoretic Mobility Retard

Background: Tau is a microtubule associated protein that regulates the stability of microtubules and the microtubule-dependent axonal transport. Its hyperphosphorylated form is one of the hallmarks of Alzheimer’s disease and other tauopathies and the major component of the paired helical filaments that form the abnormal proteinaceous tangles found in these neurodegenerative diseases. It is generally accepted that the phosphorylation extent of tau is the result of an equilibrium in the activity of protein kinases and phosphatases. Disruption of the balance between both types of enzyme activities has been assumed to be at the origin of tau hyperphosphorylation and the subsequent toxicity and progress of the disease. Objective: We explore the possibility that, beside the phosphatase action on phosphorylated tau, the catalytic subunit of PKA catalyzes both tau phosphorylation and also tau dephosphorylation, depending on the ATP/ADP ratio. Methods: We use the shift in the relative electrophoretic mobility suffered by different phosphorylated forms of tau, as a sensor of the catalytic action of the enzyme. Results: The results are in agreement with the long-known thermodynamic reversibility of the phosphorylation reaction (ATP + Protein = ADP+Phospho-Protein) catalyzed by PKA and many other protein kinases. Conclusion: The results contribute to put the compartmentalized energy state of the neuron and the mitochondrial-functions disruption upstream of tau-related pathologies.

CK2-Dependent Phosphorylation of the Brg1 Chromatin Remodeling Enzyme Occurs during Mitosis

Brg1 (Brahma-related gene 1) is one of two mutually exclusive ATPases that can act as the catalytic subunit of mammalian SWI/SNF (mSWI/SfigureNF) chromatin remodeling enzymes that facilitate utilization of the DNA in eukaryotic cells. Brg1 is a phospho-protein, and its activity is regulated by specific kinases and phosphatases. Previously, we showed that Brg1 interacts with and is phosphorylated by casein kinase 2 (CK2) in a manner that regulates myoblast proliferation. Here, we use biochemical and cell and molecular biology approaches to demonstrate that the Brg1-CK2 interaction occurred during mitosis in embryonic mouse somites and in primary myoblasts derived from satellite cells isolated from mouse skeletal muscle tissue. The interaction of CK2 with Brg1 and the incorporation of a number of other subunits into the mSWI/SNF enzyme complex were independent of CK2 enzymatic activity. CK2-mediated hyperphosphorylation of Brg1 was observed in mitotic cells derived from multiple cell types and organisms, suggesting functional conservation across tissues and species. The mitotically hyperphosphorylated form of Brg1 was localized with soluble chromatin, demonstrating that CK2-mediated phosphorylation of Brg1 is associated with specific partitioning of Brg1 within subcellular compartments. Thus, CK2 acts as a mitotic kinase that regulates Brg1 phosphorylation and subcellular localization.

CK2-dependent phosphorylation of the Brg1 chromatin remodeling enzyme occurs during mitosis

ABSTRACTBrg1 (Brahma related gene 1) is one of two mutually exclusive ATPases that can act as the catalytic subunit of mammalian SWI/SNF chromatin remodeling enzymes that facilitate utilization of the DNA in eukaryotic cells. Brg1 is a phospho-protein and its activity is regulated by specific kinases and phosphatases. Previously, we showed that Brg1 interacts with and is phosphorylated by casein kinase 2 (CK2) in a manner that regulates myoblast proliferation. Here we demonstrate that the Brg1-CK2 interaction occurred during mitosis in embryonic somites and in primary myoblasts derived from satellite cells isolated from muscle tissue. The interaction of CK2 activity with Brg1 and the incorporation of a number of other subunits into the mSWI/SNF enzyme complex were independent of CK2 enzymatic activity. CK2-mediated hyperphosphorylation of Brg1 was observed in mitotic cells derived from multiple cell types and organisms, suggesting functional conservation across tissues and species. The mitotically hyperphosphorylated form of Brg1 was localized with soluble chromatin, demonstrating that CK2-mediated phosphorylation of Brg1 is associated with specific partitioning of Brg1 within sub-cellular compartments. Thus CK2 acts a mitotic kinase that regulates Brg1 phosphorylation and sub-cellular localization.HIGHLIGHTSInteractions between CK2 and the Brg1 chromatin remodeling enzyme occur during mitosisCK2-Brg1 interactions are independent of CK2 catalytic activityCK2-mediated phosphorylation of Brg1 is a mitotic eventCK2-mediated phosphorylation of Brg1 is conserved across mammalian cell typesThe mitotically hyperphosphorylated form of Brg1 is localized with soluble chromatin

Decitabine-Induced Changes in Human Myelodysplastic Syndrome Cell Line SKM-1 Are Mediated by FOXO3A Activation

The epigenetic silencing of tumor suppressor genes in myelodysplastic syndromes (MDS) can potentially confer a growth advantage to individual cellular clones. Currently, the recommended treatment for patients with high-risk MDS is the methylation agent decitabine (DAC), a drug that can induce the reexpression of silenced tumor suppressor genes. We investigated the effects of DAC treatment on the myeloid MDS cell line SKM-1 and investigated the role of FOXO3A, a potentially tumor-suppressive transcription factor, by silencing its expression prior to DAC treatment. We found that FOXO3A exists in an inactive, hyperphosphorylated form in SKM-1 cells, but that DAC both induces FOXO3A expression and reactivates the protein by reducing its phosphorylation level. Furthermore, we show that this FOXO3A activation is responsible for the DAC-induced differentiation of SKM-1 cells into monocytes, as well as for SKM-1 cell cycle arrest, apoptosis, and autophagy. Collectively, these results suggest that FOXO3A reactivation may contribute to the therapeutic effects of DAC in MDS.

Mitochondrial Dysfunction Contributes to the Pathogenesis of Alzheimer’s Disease

Alzheimer’s disease (AD) is a neurodegenerative disease that affects millions of people worldwide. Currently, there is no effective treatment for AD, which indicates the necessity to understand the pathogenic mechanism of this disorder. Extracellular aggregates of amyloid precursor protein (APP), called Aβpeptide and neurofibrillary tangles (NFTs), formed by tau protein in the hyperphosphorylated form are considered the hallmarks of AD. Accumulative evidence suggests that tau pathology and Aβaffect neuronal cells compromising energy supply, antioxidant response, and synaptic activity. In this context, it has been showed that mitochondrial function could be affected by the presence of tau pathology and Aβin AD. Mitochondria are essential for brain cells function and the improvement of mitochondrial activity contributes to preventing neurodegeneration. Several reports have suggested that mitochondria could be affected in terms of morphology, bioenergetics, and transport in AD. These defects affect mitochondrial health, which later will contribute to the pathogenesis of AD. In this review, we will discuss evidence that supports the importance of mitochondrial injury in the pathogenesis of AD and how studying these mechanisms could lead us to suggest new targets for diagnostic and therapeutic intervention against neurodegeneration.

Tau phosphorylation in hippocampus results in toxic gain-of-function

The MAP (microtubule-associated protein) tau binds to tubulin, the main component of MTs (microtubules), which results in the stabilization of MT polymers. Tau binds to the C-terminal of tubulin, like other MAPs (including motor proteins such as kinesin) and it therefore may compete with these proteins for the same binding site in the tubulin molecule. In pathological conditions, tau is the main component of aberrant protein aggregates found in neurodegenerative disorders known as tauopathies where tau is present in its hyperphosphorylated form. GSK3 (glycogen synthase kinase 3, also known as tau kinase I) has been described as one of the main kinases involved in tau modifications. We have analysed the role of phospho-tau as a neurotoxic agent. We have analysed a transgenic mouse model which overexpresses GSK3β. In this transgenic mouse, a clear degeneration of the dentate gyrus, which increases with age, was found. In a double transgenic mouse, which overexpresses GSK3 and tau at the same time, dentate gyrus degeneration was dramatically increased. This result may suggest that phospho-tau may be toxic inside neurons of the dentate gyrus. Once neuronal degeneration takes place, intracellular tau is secreted to the extracellular space. The present review discusses the toxicity of this extracellular tau for surrounding neurons.

Abstract 1511: Gap Junction Internalization and Autophagic Degradation in the Failing Heart

INTRODUCTION Gap junctions (GJs) mediate electrical coupling between cardiomyocytes, and are required for coordinated electrical activation and contraction of the heart. GJs are remodeled in diseased myocardium, resulting in impaired intercellular coupling and a predisposition to life threatening arrhythmias. To understand the mechanisms of GJ remodeling in heart failure (HF), we examined the ultrastructure of GJs in failing canine myocardium. METHODS and RESULTS Using transmission electron microscopy, we observed the formation of GJs between lateral membranes of ventricular myocytes in rapid pacing induced heart failure (HF) in dogs. Lateral GJs exhibited complex membrane bending and extensive internalization. Lateral GJs were frequently adjacent to mitochondria and internalized GJs often formed concentric ring structures surrounding cellular debris. Internalized GJ membranes were associated with multi-lamellar membrane structures, with features characteristic of autophagosomes. Connexin43 (Cx43) and the autophagosome marker GFP-LC3 were extensively co-localized when expressed in HeLa cells. A hyperphosphorylated form of Cx43 previously associated with GJ degradation co-fractionated with LC3-II (autophagosome targeted form) in a Triton-insoluble and buoyant fraction of ventricular tissue. The hyperphosphorylated/buoyant population of Cx43 as well as LC3-II were both significantly increased (n=5 each, p=0.011 and p=0.016 respectively), and whole tissue levels of Cx43 were significantly decreased (n=5, p=0.01), in pacing-induced HF. CONCLUSION We show that structurally heterogeneous GJs are present at the lateral membranes of cardiomyocytes in HF, suggesting aberrant targeting of GJs. Internalized GJs are degraded via autophagy, which involves a hyperphosphorylated form of Cx43, and is enhanced in HF. These findings reveal a novel pathway of GJ targeting and degradation in HF, and suggest potential therapeutic targets for HF-related conduction slowing and arrhythmias.

The α Isoform of Protein Kinase CKI Is Responsible for Hepatitis C Virus NS5A Hyperphosphorylation

ABSTRACT Hepatitis C virus (HCV) has been the subject of intensive studies for nearly two decades. Nevertheless, some aspects of the virus life cycle are still a mystery. The HCV nonstructural protein 5A (NS5A) has been shown to be a modulator of cellular processes possibly required for the establishment of viral persistence. NS5A is heavily phosphorylated, and a switch between a basally phosphorylated form of NS5A (p56) and a hyperphosphorylated form of NS5A (p58) seems to play a pivotal role in regulating HCV replication. Using kinase inhibitors that specifically inhibit the formation of NS5A-p58 in cells, we identified the CKI kinase family as a target. NS5A-p58 increased upon overexpression of CKI-α, CKI-δ, and CKI-ε, whereas the RNA interference of only CKI-α reduced NS5A hyperphosphorylation. Rescue of inhibition of NS5A-p58 was achieved by CKI-α overexpression, and we demonstrated that the CKI-α isoform is targeted by NS5A hyperphosphorylation inhibitors in living cells. Finally, we showed that down-regulation of CKI-α attenuates HCV RNA replication.

RelA/p65 Regulation of IκBβ

ABSTRACT IκB inhibitor proteins are the primary regulators of NF-κB. In contrast to the defined regulatory interplay between NF-κB and IκBα, much less is known regarding the regulation of IκBβ by NF-κB. Here, we describe in detail the regulation of IκBβ by RelA/p65. Using p65 −/− fibroblasts, we show that IκBβ is profoundly reduced in these cells, but not in other NF-κB subunit knockouts. This regulation prevails during embryonic and postnatal development in a tissue-specific manner. Significantly, in both p65 −/− cells and tissues, IκBα is also reduced, but not nearly to the same extent as IκBβ, thus highlighting the degree to which IκBβ is dependent on p65. This dependence is based on the ability of p65 to stabilize IκBβ protein from the 26S proteasome, a process mediated in large part through the p65 carboxyl terminus. Furthermore, IκBβ was found to exist in both a basally phosphorylated and a hyperphosphorylated form. While the hyperphosphorylated form is less abundant, it is also more stable and less dependent on p65 and its carboxyl domain. Finally, we show that in p65 −/− fibroblasts, expression of a proteolysis-resistant form of IκBβ, but not IκBα, causes a severe growth defect associated with apoptosis. Based on these findings, we propose that tight control of IκBβ protein by p65 is necessary for the maintenance of cellular homeostasis.