Nune Darbinian

Purα Is Essential for Postnatal Brain Development and Developmentally Coupled Cellular Proliferation As Revealed by Genetic Inactivation in the Mouse

ABSTRACT The single-stranded DNA- and RNA-binding protein, Purα, has been implicated in many biological processes, including control of transcription of multiple genes, initiation of DNA replication, and RNA transport and translation. Deletions of the PURA gene are frequent in acute myeloid leukemia. Mice with targeted disruption of the PURA gene in both alleles appear normal at birth, but at 2 weeks of age, they develop neurological problems manifest by severe tremor and spontaneous seizures and they die by 4 weeks. There are severely lower numbers of neurons in regions of the hippocampus and cerebellum of PURA−/− mice versus those of age-matched +/+ littermates, and lamination of these regions is aberrant at time of death. Immunohistochemical analysis of MCM7, a protein marker for DNA replication, reveals a lack of proliferation of precursor cells in these regions in the PURA−/− mice. Levels of proliferation were also absent or low in several other tissues of the PURA−/− mice, including those of myeloid lineage, whereas those of PURA+/− mice were intermediate. Evaluation of brain sections indicates a reduction in myelin and glial fibrillary acidic protein labeling in oligodendrocytes and astrocytes, respectively, indicating pathological development of these cells. At postnatal day 5, a critical time for cerebellar development, Purα and Cdk5 were both at peak levels in bodies and dendrites of Purkinje cells of PURA+/+ mice, but both were absent in dendrites of PURA−/− mice. Purα and Cdk5 can be coimmunoprecipitated from brain lysates of PURA+/+ mice. Immunohistochemical studies reveal a dramatic reduction in the level of both phosphorylated and nonphosphorylated neurofilaments in dendrites of the Purkinje cell layer and of synapse formation in the hippocampus. Overall results are consistent with a role for Purα in developmentally timed DNA replication in specific cell types and also point to a newly emerging role in compartmentalized RNA transport and translation in neuronal dendrites.

Beta-amyloid deposition and Alzheimer's type changes induced by Borrelia spirochetes

The pathological hallmarks of Alzheimers disease (AD) consist of beta-amyloid plaques and neurofibrillary tangles in affected brain areas. The processes, which drive this host reaction are unknown. To determine whether an analogous host reaction to that occurring in AD could be induced by infectious agents, we exposed mammalian glial and neuronal cells in vitro to Borrelia burgdorferi spirochetes and to the inflammatory bacterial lipopolysaccharide (LPS). Morphological changes analogous to the amyloid deposits of AD brain were observed following 2-8 weeks of exposure to the spirochetes. Increased levels of beta-amyloid precursor protein (AbetaPP) and hyperphosphorylated tau were also detected by Western blots of extracts of cultured cells that had been treated with spirochetes or LPS. These observations indicate that, by exposure to bacteria or to their toxic products, host responses similar in nature to those observed in AD may be induced.

Beta amyloid and hyperphosphorylated tau deposits in the pancreas in type 2 diabetes

Strong epidemiologic evidence suggests an association between Alzheimer disease (AD) and type 2 diabetes. To determine if amyloid beta (Abeta) and hyperphosphorylated tau occurs in type 2 diabetes, pancreas tissues from 21 autopsy cases (10 type 2 diabetes and 11 controls) were analyzed. APP and tau mRNAs were identified in human pancreas and in cultured insulinoma beta cells (INS-1) by RT-PCR. Prominent APP and tau bands were detected by Western blotting in pancreatic extracts. Aggregated Abeta, hyperphosphorylated tau, ubiquitin, apolipoprotein E, apolipoprotein(a), IB1/JIP-1 and JNK1 were detected in Langerhans islets in type 2 diabetic patients. Abeta was co-localized with amylin in islet amyloid deposits. In situ beta sheet formation of islet amyloid deposits was shown by infrared microspectroscopy (SIRMS). LPS increased APP in non-neuronal cells as well. We conclude that Abeta deposits and hyperphosphorylated tau are also associated with type 2 diabetes, highlighting common pathogenetic features in neurodegenerative disorders, including AD and type 2 diabetes and suggesting that Abeta deposits and hyperphosphorylated tau may also occur in other organs than the brain.

Evidence for dysregulation of cell cycle by human polyomavirus, JCV, late auxiliary protein

The late region of the human neurotropic JC virus encodes a 71 amino acid protein, named Agnoprotein, whose biological function remains elusive. Here we demonstrate that in the absence of other viral proteins, expression of Agnoprotein can inhibit cell growth by deregulating cell progression through the cell cycle stages. Cells with constitutive expression of Agnoprotein were largely accumulated at the G2/M stage and that decline in the activity of cyclins A and B is observed in these cells. Agnoprotein showed the ability to augment p21 promoter activity in transient transfection assay and a noticeable increase in the level of p21 is detected in cells continuously expressing Agnoprotein. Results from binding studies revealed the interaction of Agnoprotein with p53 through the N-terminal of the Agnoprotein spanning residues 1–36. Co-expression of p53 and Agnoprotein further stimulated transcription of the p21 promoter. Thus, the interaction of p53 and Agnoprotein can lead to a higher level of p21 expression and suppression of cell cycle progression during the cell cycle.

Role of JC Virus Agnoprotein in DNA Repair

ABSTRACT The late region of human neurotropic JC virus encodes a small 71-amino-acid agnoprotein that is also found in the polyomaviruses simian virus 40 and BK virus. Several functions of agnoprotein have been identified, including roles in regulating viral transcription and virion maturation. Earlier studies showed that agnoprotein expressed alone induced p21/WAF-1 expression and caused cells to accumulate in the G2/M stage of the cell cycle. Here we report that agnoprotein expression sensitized cells to the cytotoxic effects of the DNA-damaging agent cisplatin. Agnoprotein reduced the viability of cisplatin-treated cells and increased chromosome fragmentation and micronucleus formation. Whereas cisplatin-treated control cells accumulated in S phase, cells expressing agnoprotein did not, instead becoming aneuploid. Agnoprotein expression correlated with impaired double-strand-break repair activity in cellular extracts and reduced expression of the Ku70 and Ku80 DNA repair proteins. After agnoprotein expression, much of the Ku70 protein was located in the perinuclear space, where agnoprotein was also found. Results from binding studies showed an interaction of agnoprotein with Ku70 which was mediated by the N terminus. The ability of agnoprotein to inhibit double-strand break repair activity when it was added to cellular extracts was also mediated by the N terminus. We conclude that agnoprotein inhibits DNA repair after DNA damage and interferes with DNA damage-induced cell cycle regulation. Since Ku70 is a subunit of the DNA-dependent protein kinase that is responsible both for double-strand break repair and for signaling damage-induced cell cycle arrest, the modulation of Ku70 and/or Ku80 by agnoprotein may represent an important event in the polyomavirus life cycle and in cell transformation.

Growth inhibition of glioblastoma cells by human Purα

Purα is a multifunctional DNA‐ and RNA‐binding protein implicated in a variety of biological events including transcription and replication. Further, this protein has the ability to form a complex with several cellular proteins which are important for cell proliferation including the transcription factor, E2F‐1. Purα has a modular structure highlighted by alternating three basic aromatic class I and two acidic leucine‐rich class II repeats in the central region of the protein. Here, we demonstrate that ectopic overexpression of Purα suppresses proliferation of a variety of transformed and tumor cells including human glioblastoma. By utilizing various deletion mutants of Purα in colony formation assay, we identified the region spanning the first class II repeat (residues 107–131) and the second class I repeat (residues 148–170) of Purα which participate in growth inhibitory action of Purα. Results from protein transduction experiments using the synthetic peptides representing residues 109–131 and 123–154 of Purα in fusion with the arginine rich domain of HIV‐1 Tat revealed cellular internalization and nuclear appearance of the Tat–Purα fusion peptide after 2 h and its detection in nuclei up to 24 h after treatment. Glioblastoma cells treated with Tat–Purα (109–131) and Tat–Purα (123–154) exhibited 41 and 47% decrease, respectively, in proliferation. Altogether these results illustrate the efficacy of Purα in suppressing glioblastoma cell growth and provide evidence for the potential use of this protein and its derivative(s) in blocking proliferation of tumor cells.

Helix‐destabilizing properties of the human single‐stranded DNA‐ and RNA‐binding protein Purα

Purα is a ubiquitous nucleic acid‐binding protein which has been implicated in the control of eukaryotic gene transcription. Further, Purα associates with DNA sequences positioned in close proximity to viral and cellular origins of replication suggesting a role for this protein in DNA replication. As initiation of transcription and replication require alteration in the structure of duplex DNA, we investigated the DNA unwinding activity of this single‐stranded nucleic acid‐binding protein. Here we demonstrate that Purα has the ability to displace an oligonucleotide annealed to single‐stranded M13 DNA. The helix unwinding activity of Purα was dose‐, time‐ and temperature‐dependent and ATP‐independent. Results from mapping studies revealed that the central region of Purα, spanning amino acids 72–274, was important for the helix‐destabilizing activity of this protein. The region of Purα which was involved in the helix‐destabilizing activity mapped to the DNA‐binding domain of this protein. Results from heat inactivation experiments demonstrated that the helix‐destabilizing activity of Purα correlates with its capacity to interact with DNA containing the PUR element. Taken together, these studies demonstrate that Purα possesses helix‐destabilizing activity and that this activity maps to and correlates with its ability to interact with DNA. J. Cell. Biochem. 80:589–595, 2001.

Functional interaction between cyclin T1/cdk9 and Purα determines the level of TNFα promoter activation by Tat in glial cells

Abstract In addition to its stimulatory effect on transcription of the HIV-1 LTR, the early protein of HIV-1, Tat, exhibits detrimental effects on the CNS by deregulating the expression of several cytokines and immunomodulators including TNFα. Activation of the viral promoter by Tat requires several cellular proteins including cyclin T1 and its partner, cdk9, which upon association with the TAR sequence of the LTR, forms a complex that enhances the activity of RNA polymerase II. Here, we examined the involvement of cyclin T1/cdk9 in Tat-mediated transcriptional activation of the TNFα promoter which has no TAR sequence. Results from transfection of human astrocytic cells revealed that both cyclin T1 and cdk9 stimulate the basal promoter activity of TNFα, although the level of such activation is decreased in the presence of Tat. Ectopic expression of Purα, a brain-derived regulatory protein which binds to Tat, enhanced the basal level of TNFα transcription, yet exerted a negative effect on the level of Tat activation of the TNFα promoter. The antagonistic effect of Purα and Tat upon the TNFα promoter was diminished in the presence of cyclin T1 and cdk9, suggesting cooperativity of Purα with cyclin T1 and cdk9 in Tat activation of the TNFα promoter. Results from protein–protein binding studies showed the interaction of Purα with both cyclin T1 and cdk9 through distinct domains of Purα which are in juxtaposition with each other. Interestingly, the site for cyclin T1 binding within Purα is adjacent to the region which is important for Tat/Purα association. In light of these observations, we propose a model which ascribes a bridging role for Purα in assembling Tat, cyclin T1, and cdk9 around the promoter region of TAR-negative genes such as TNFα, which is responsive to Tat activation.

Evidence for Regulation of Long Terminal Repeat Transcription by Wnt Transcription Factor TCF-4 in Human Astrocytic Cells

ABSTRACT The Wnt signaling pathway plays an important role in neural cell development and function. The key components of this pathway, β-catenin and its partner TCF-4/LEF-1, exert their effects on transcription by entering the nuclei, where they associate with the TCF-4/LEF-1 DNA motif positioned in the promoters of several important genes. Here we examined the role of TCF-4 upon transcription of the human immunodeficiency virus type 1 (HIV-1) promoter in human astrocytic cells. Our results showed that expression of TCF-4 in human astrocytic cells (U-87MG cells) decreased the basal and Tat-mediated transcription of the HIV-1 long terminal repeat (LTR). Results from promoter deletion studies revealed that the promoter sequence of the LTR with no classical binding motif for TCF-4/LEF-1, which spans positions −80 to +80 of the LTR, remained responsive to down-regulation by TCF-4. Noticeably, removal of the sequences between positions −80 and −68 decreased the negative effect of TCF-4 on viral gene transcription. A mutant variant of TCF-4 with no binding site for β-catenin was able to down-regulate LTR transcription, suggesting that β-catenin may not be directly involved in the observed regulatory events. Results from the glutathione S-transferase pull-down assay as well as the combined immunoprecipitation and Western blot analysis of protein extract from U-87MG cells revealed an interaction of Tat with TCF-4. Subcellular examination of TCF-4 and Tat in cells expressing either protein alone showed a predominantly nuclear accumulation of these proteins. However, in cells which coexpressed both TCF-4 and Tat, significant levels of these proteins were found in the cytoplasm. All together, these observations provide evidence for the cooperative interaction of TCF-4, the important transcription factor of the Wnt pathway, with Tat; this interaction may determine the level of viral gene transcription in human astrocytic cells.

Tat-induced deregulation of neuronal differentiation and survival by nerve growth factor pathway.

HIV-1 enters the brain at the early stage of infection and resides primarily in a limited number of macrophages/microglia and astrocytes. Infection of these cells, however, may not explain the massive neuronal pathology which is seen in AIDS-associated dementia, suggesting a role for factors released from HIV-1 infected cells that trigger a cascade of events leading to neurodegeneration. Our results indicate that Tat, the potent regulatory protein of HIV-1 which is secreted by infected cells and can affect neighboring uninfected cells by transcellular means, can influence multiple biological events that lead to neuronal injury. These findings demonstrate that treatment of neuronal cells with Tat affects MAPK/ERK1/2 activity, the downstream central component of the nerve growth factor (NGF) signaling pathway. Furthermore, our data indicate that treatment of cells with Tat severely decreases expression of p35, a neuron-specific activator of cdk5, a cyclin dependent kinase that phosphorylates several neuronal proteins including neurofilament, and plays an important role in neuronal differentiation and survival. In parallel, Tat can bind to the cellular protein, Puralpha, which associates with cdk5. Further, results from Puralpha knockout animals revealed a decrease in p35 activity, pointing to the importance of Puralpha association with cdk5 in the activity of cdk5:p35 complex. These data demonstrate the cooperativity between HIV-1 Tat and the Puralpha in deregulation of the NGF signal transduction pathway in neuronal cells.