APP RNA splicing is not affected by differentiation of neurons and glia in culture

Amyloid precursor protein (APP) gene expression was investigated in primary cultures of neurons, astrocytes, microglial cells and oligodendrocytes. Neurons from various rat brain regions, as well as oligodendrocytes, contained RNA encoding APP695, while astrocytes and microglial cells expressed high levels of RNAs for APP770 and APP751. It was studied whether the cell type-specific regulation of APP gene expression could be modified by induction of cellular differentiation in vitro. While neuronal differentiation of PC12 cells has been shown to correspond with an altered pattern of APP splicing, in the primary cultures neither the time in culture nor a treatment of the cells with appropriate differentiation factors affected this pattern.     

p250GAP,a neural RhoGAP protein,is associated with and phosphorylated by Fyn

Fyn is a member of the Src-family protein tyrosine kinases and plays important roles in both neurons and oligodendrocytes. Here we report association of Fyn with p250GAP, a RhoGAP protein that is expressed predominantly in brain. p250GAP interacts with Fyn both in vitro and in vivo. p250GAP is tyrosine phosphorylated by Fyn when co-expressed in HEK293T cells. This phosphorylation appears to enhance the interaction between p250GAP and Fyn. Furthermore, the level of tyrosine phosphorylation of p250GAP increases upon differentiation of the oligodendrocyte cell line CG4. Given that Fyn activity is up-regulated during oligodendrocyte maturation, the results argue that p250GAP is phosphorylated by Fyn in oligodendrocytes. Tyrosine phosphorylation of p250GAP by Fyn would regulate its RhoGAP activity, subcellular localization, or interactions with other proteins, leading to morphological and phenotypic changes of oligodendrocytes.     

The p53-activated gene,PAG608, requires a zinc finger domain for nuclear localization and oxidative stress-induced apoptosis

The p53-activated gene PAG608, which encodes a nuclear zinc finger protein, is a p53-inducible gene that contributes to p53-mediated apoptosis. However, the mechanisms by which PAG608 is involved in the apoptosis of neuronal cells are still obscure. In this study, we demonstrated that expression of p53 was induced by 100 microm 6-hydroxydopamine (6-OHDA), accompanied by increased PAG608 expression in PC12 cells. On the other hand, transient or permanent transfection of antisense PAG608 cDNA into PC12 cells significantly prevented apoptotic cell death induced by 100 microm 6-OHDA or 200 microm hydrogen peroxide but not by 250 microm 1-methyl-4-phenylpyridinium ion. The 6-OHDA-induced activation of caspase-3, DNA fragmentation, loss of mitochondrial membrane potential, and induction of p53 and Bax were also prevented in PC12 cells that stably expressed antisense PAG608 cDNA. These results suggest that PAG608 is associated with the apoptotic pathway induced by these oxidative stress-generating reagents, upstream of the collapse in the mitochondrial membrane potential in PC12 cells. Interestingly, transient transfection with PAG608 cDNA increased p53 expression in both PC12 cells and B65 cells, indicating that PAG608 induced by p53 is able to induce p53 expression in these cells inversely. Furthermore, transient transfection of a truncated mutant PAG608 cDNA, lacking the first zinc finger domain, inhibited 6-OHDA-induced cell death and altered the nuclear and nucleolar localization of wild-type PAG608 in PC12 cells. These results suggest that PAG608 may induce or regulate p53 expression and translocate to the nucleus and nucleolus using its first zinc finger domain during oxidative stress-induced apoptosis of catecholamine-containing cells.     

NBS1, the Nijmegen breakage syndrome gene product, regulates neuronal proliferation and differentiation

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder, characterized by progressive microcephaly, growth retardation, immunodeficiency, and pre-disposition to tumor formation. To investigate the functions of the NBS gene product, NBS1, on neurons, PC12 cells overexpressing NBS1 and related mutants and primary cortical neuronal culture were used in the present study. Small interfering RNA (siRNA) was applied to repress the expression of endogenous Nbs1 in PC12 cells and primary cortical neurons. We demonstrated that overexpression of NBS1 increases cellular proliferation and decreases the apoptosis of PC12 cells in serum withdrawal and ionizing irradiation, through the activation of phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathway. Overexpression of NBS1 also decreases neurite elongation on PC12 cells under nerve growth factor stimulation. Transfection of NBS1-overexpressing PC12 cells with a dominant negative Akt mutant attenuates the neuroprotection and cellular proliferation effects of NBS1 while having no effect on neurite elongation. PC12 cells overexpressing NBS657del5 and NBS653 mutants, in which the major NBS1 protein in cells are truncated proteins, have decreased cellular proliferation, increased cell death, and decreased neurite elongation compared with those of control PC12 cells. Repression of Nbs1 by siRNA decreases the PI 3-kinase activity and Akt phosphorylation levels, and induces neurite elongation in PC12 cells even without nerve growth factor stimulation. Repression of Nbs1 by siRNA in primary cortical neurons also increased neurite elongation, but increased neuronal death. We conclude that NBS1 can regulate neuronal proliferation and neuroprotection via PI 3-kinase/Akt pathway while regulating neuronal differentiation in a different pathway. Excessive accumulation of truncated protein secondary to 657del5 mutation may be detrimental to neurons, leading to defective neuronal proliferation and differentiation.     

SNT, a differentiation-specific target of neurotrophic factor-induced tyrosine kinase activity in neurons and PC12 cells.

To elucidate the signal transduction mechanisms used by ligands that induce differentiation and the cessation of cell division, we utilized p13suc1-agarose, a reagent that binds p34cdc2/cdk2. By using this reagent, we identified a 78- to 90-kDa species in PC12 pheochromocytoma cells that is rapidly phosphorylated on tyrosine following treatment with the differentiation factors nerve growth factor (NGF) and fibroblast growth factor but not by the mitogens epidermal growth factor or insulin. This species, called SNT (suc-associated neurotrophic factor-induced tyrosine-phosphorylated target), was also phosphorylated on tyrosine in primary rat cortical neurons treated with the neurotrophic factors neurotrophin-3, brain-derived neurotrophic factor, and fibroblast growth factor but not in those treated with epidermal growth factor. In neuronal and fibroblast cells, where NGF can also act as a mitogen, SNT was tyrosine phosphorylated to a much greater extent during NGF-induced differentiation than during NGF-induced proliferation. SNT was phosphorylated in vitro on serine, threonine, and tyrosine in p13suc1-agarose precipitates from NGF-treated PC12 cells, indicating that this protein may be a substrate of kinase activities associated with p13suc1-p34cdc2/cdk2 complexes. In addition, SNT was associated predominantly with nuclear fractions following subcellular fractionation of NGF-treated PC12 cells. Finally, in PC12 cells, NGF-stimulated tyrosine phosphorylation of SNT was dependent on the levels of Trk tyrosine kinase activity and was constitutively induced by expression of pp60v-src. However, Ras was not required for constitutive SNT tyrosine phosphorylation, suggesting that this protein functions distally to Trk and pp60v-src but in a pathway parallel to that of Ras. SNT is the first identified specific target of differentiation factor-induced tyrosine kinase activity in neuronal cells.     

Nerve growth factor withdrawal-mediated apoptosis in naive and differentiated PC12 cells through p53/caspase-3-dependent and -independent pathways

Programmed cell death is regulated in response to a variety of stimuli, including the tumor suppressor protein p53, that can mediate cell cycle arrest through p21/Waf1 and apoptosis through the Bcl-2/Bax equilibrium and caspases. Neuronal cell apoptosis has been reported to require p53, whereas other data suggest that neuronal cell death may be independent of p53. Comparison of wild type PC12 to a temperature-sensitive PC12 cell line that depresses the normal function of p53 has permitted investigation of the importance of p53 in a variety of cell functions. This study examined the role of p53 in trophic factor withdrawal-mediated apoptosis in both na?ve and differentiated PC12 cells. Our data show that as PC12 cells differentiate they are more poised to undergo apoptosis than their undifferentiated counterparts. Survival assays with XTT (sodium 3'-1-(phenylaminocarbonyl)-3,4-tetrazolium-bis(4-methoxy-6-nitro)benzene sulfonic acid) and TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) demonstrated that lack of p53 is initially protective against apoptosis. The window of protection is about 20 h for na?ve and 36 h for differentiated cells. Apoptosis involved caspases 3, 6, and 9. However, caspase 3 activation was absent in cells lacking p53, concomitant with the delayed apoptosis. When the expression of caspase 3 was silenced with interference RNA, wild type PC12 cells revealed a morphology and biochemistry similar to PC12[p53ts] cells, indicating that caspase 3 accounts for the observed delay in apoptosis in p53 dysfunction. These results suggest that p53 is important, but not essential, in factor withdrawal-mediated apoptosis. Parallel pathways of caspase-mediated apoptosis are activated later in the absence of functional p53.     

p53-independent endoplasmic reticulum stress-mediated cytotoxicity of a Newcastle disease virus strain in tumor cell lines

While Newcastle disease virus (NDV) causes serious infections in birds, it is apparently nonpathogenic in mammalian species, including humans. Previous observations and small-scale clinical trials indicated that NDV exerts oncolytic effects. Isolates of NDV were found to have selective affinity to transformed cells. We previously showed that the attenuated NDV strain MTH-68/H causes apoptotic cell death in cultures of PC12 rat pheochromocytoma cells. The aim of the present study was to extend MTH-68/H cytotoxicity testing with human tumor cell lines and to analyze certain biochemical aspects of its oncolytic effect. MTH-68/H was found to be able to kill a wide range of transformed cells by apoptosis. While caspase-8 and caspase-9 are not involved in MTH-68/H-induced apoptosis, activation of caspase-3 and caspase-12 was detected in virus-infected PC12 cells. A human glioblastoma cell line with repressible expression of the p53 protein did not show any difference in MTH-68/H sensitivity in its p53-expressing and p53-depleted states, indicating that the apoptotic process induced by MTH-68/H does not depend on p53. Apoptosis was accompanied by virus replication in two tumor cell lines tested (PC12 cells and HeLa human cervical cells), and signs of endoplasmic reticulum stress (phosphorylation of protein kinase R-like endoplasmic reticulum kinase and eIF2alpha) were also detected in transformed cells. In contrast, proliferation of nontransformed mouse and rat fibroblast cell lines and human primary fibroblasts was not affected by MTH-68/H treatment. MTH-68/H thus selectively kills tumor cell cultures by inducing endoplasmic reticulum stress leading to p53-independent apoptotic cell death.     

P53 transformation-related protein accumulates in the nucleus of transformed fibroblasts in association with the chromatin and is found in the cytoplasm of non-transformed fibroblasts.

The subcellular localization of the p53 molecule was studied in transformed and non-transformed fibroblasts. A newly established transformed cell line obtained by treating primary embryonic mouse cells in vitro with the chemical carcinogen methylcholanthrene was compared with the embryonic parent fibroblasts. The transformed cells lost the spindle shape characteristic of the parent fibroblasts, acquired an accelerated growth rate, developed into tumors when injected into syngeneic mice and expressed high levels of p53 synthesis estimated by immunoprecipitation of [35S]methionine-labeled cell extracts. The cellular localization of the p53 molecule was studied by immunofluorescent staining of fixed cells with monoclonal antibodies and by immunoprecipitation of [35S]-methionine-labeled p53 from various subcellular fractions. p53 was mainly found in the nucleus of the transformed fibroblast, while in the parent non-transformed primary embryonic cells, p53 was detected in the cytoplasm in a Triton X-100 soluble fraction, and associated with the cytoskeleton. The modulated distribution of p53 was also confirmed by analyzing a wide range of independently established transformed and non-transformed fibroblastic cell lines growing in vitro. The switch from the cytoplasmic localization of p53 in the non-transformed fibroblasts to a chromatin-associated accumulation in the transformed cells suggests a possible mechanism by which this protein may function in the transformed fibroblasts.     

MKK7γ1 reverses nerve growth factor signals: proliferation and cell death instead of neuritogenesis and protection

c-Jun N-terminal kinases (JNKs) are the exclusive downstream substrates of mitogen-activated protein kinase kinase 7 (MKK7). Recently, we have shown that a single MKK7 splice variant, MKK7γ1, substantially changes the functions of JNKs in naïve PC12 cells. Here we provide evidence that MKK7γ1 blocks NGF-mediated differentiation and sustains proliferation by interfering with the NGF-triggered differentiation programme at several levels: (i) down-regulation of the NGF receptors TrkA and p75; (ii) attenuation of the differentiation-promoting pathways ERK1/2 and AKT; (iii) increase of JNK1 and JNK2, especially the JNK2 54 kDa splice variants; (iv) repression of the cyclin-dependent kinase inhibitor p21^WAF1/CIP1, which normally supports NGF-mediated cell cycle arrest; (v) strong induction of the cell cycle promoter CyclinD1, and (vi) profound changes of p53 functions. Moreover, MKK7γ1 substantially changes the responsiveness to stress. Whereas NGF differentiation protects PC12 cells against taxol-induced apoptosis, MKK7γ1 triggers an escape from cell cycle arrest and renders transfected cells sensitive to taxol-induced death. This stress response completely differs from naïve PC12 cells, where MKK7γ1 protects against taxol-induced cell death. These novel aspects on the regulation of JNK signalling emphasise the importance of MKK7γ1 in its ability to reverse basic cellular programmes by simply using JNKs as effectors. Furthermore, our results highlight the necessity for the cells to balance the expression of JNK activators to ensure precise intracellular processes.     

Nuclear accumulation of p53 protein is mediated by several nuclear localization signals and plays a role in tumorigenesis.

The basic carboxy terminus of p53 plays an important role in directing the protein into the nuclear compartment. The C terminus of the p53 molecule contains a cluster of several nuclear localization signals (NLSs) that mediate the migration of the protein into the cell nucleus. NLSI, the most active domain, is highly conserved in genetically diverged species and shares perfect homology with consensus NLS sequences found in other nuclear proteins. The other two NLSs, II and III, appear to be less effective and less conserved. Although nuclear localization is dictated primarily by the NLSs inherent in the primary amino acid sequence, the actual nuclear homing can be modified by interactions with other proteins expressed in the cell. Comparison between wild-type p53 and naturally occurring mutant p53 showed that both protein categories could migrate into the nucleus of rat primary embryonic fibroblasts by essentially similar mechanisms. Nuclear localization of both proteins was totally dependent on the existence of functional NLS domains. In COS cells, however, we found that NLS-deprived wild-type p53 molecules could migrate into the nucleus by complexing with another nuclear protein, simian virus 40 large-T antigen. Wild-type and mutant p53 proteins differentially complexed with viral or cellular proteins, which may significantly affect the ultimate compartmentalization of p53 in the cell; this finding suggests that the actual subcellular compartmentalization of proteins may differ in various cell type milieux and may largely be affected by the ability of these proteins to complex with other proteins expressed in the cell. Experiments designed to test the physiological significance of p53 subcellular localization indicated that nuclear localization of mutant p53 is essential for this protein to enhance the process of malignant transformation of partially transformed cells, suggesting that p53 functions within the cell nucleus.     

The role of the p53 protein in nitrosative stress-induced apoptosis of PC12 rat pheochromocytoma cells

PC12 rat pheochromocytoma cells are widely used to investigate signaling pathways. The p143p53PC12 cell line expresses a Val143Ala mutant p53 protein that is less capable of binding to the p53 consensus site in DNA than its wild-type counterpart. Nitric oxide (NO), depending on its concentration, is able to activate several signal transduction pathways. We used sodium nitroprusside (SNP), an NO donor compound, to analyze NO-induced cellular stress in order to clarify the mechanism and role of nitrosative stress in pathological processes, including inflammation and cancer. SNP caused cell death when applied at a concentration of 400 μM, p143p53PC12 cells showing higher sensitivity than wild-type PC12 cells. The mechanisms leading to the increased SNP-sensitivity of p143p53PC12 cells were then investigated. The 400-μM SNP treatment caused stress kinase activation, phosphorylation of the eukaryotic initiation factor eIF2α and p53 protein, proteolytic activation of protein kinase R, caspase-9, and caspase-3, p53 stabilization, CHOP induction, cytochrome c release from mitochondria, and a decline in the level of the Bcl-2 protein in both cell lines. All these SNP-induced changes were more robust and/or permanent in cells with the mutant p53 protein. We thus conclude that (1) the main cause of the SNP-induced apoptosis of PC12 cells is the repression of the bcl-2 gene, evoked through p53 stabilization, stress kinase activation, and CHOP induction; (2) the higher SNP sensitivity of p143p53PC12 cells is the consequence of the stronger and earlier activation of the intrinsic apoptotic pathway.     

A novel mechanism of non-Aβ component of Alzheimer's disease amyloid (NAC) neurotoxicity. Interplay between p53 protein and cyclin-dependent kinase 5 (Cdk5)

The non-Aβ component of Alzheimer's disease (AD) amyloid (NAC) is produced from the precursor protein NACP/α-synuclein (ASN) by till now unknown mechanism. Previous study showed that like ASN, NAC peptide induced oxidative/nitrosative stress and apoptosis. Our present study focused on the mechanisms of PC12 cells death evoked by NAC peptide, with particular consideration on the role of p53 protein. On the basis of molecular and transmission electron microscopic (TEM) analysis it was found that exogenous NAC peptide (10 μM) caused mitochondria dysfunction, enhanced free radical generation, and induced both apoptotic and autophagic cell death. Morphological and immunocytochemical evidence from TEM showed marked changes in expression and in translocation of proapoptotic protein Bax. We also observed time-dependent enhancement of Tp53 gene expression after NAC treatment. Free radicals scavenger N-tert-butyl-alpha-phenylnitrone (PBN, 1 mM) and p53 inhibitor (α-Pifithrin, 20 μM) significantly protected PC12 cells against NAC peptide-evoked cell death. In addition, exposure to NAC peptide resulted in higher expression of cyclin-dependent kinase 5 (Cdk5), one of the enzymes responsible for p53 phosphorylation and activation. Concomitantly, we observed the increase of expression of Cdk5r1 and Cdk5r2 genes, coding p35 and p39 peptides that are essential regulators of Cdk5 activity. Moreover, the specific Cdk5 inhibitor (BML-259, 10 μM) protected large population of cells against NAC-evoked cell death. Our findings indicate that NAC peptide exerts its toxic effect by activation of p53/Cdk5 and Bax-dependent apoptotic signaling pathway.     

Activation of Cell Cycle Regulatory Proteins in the Apoptosis of Terminally Differentiated Oligodendrocytes

There is increasing evidence that proteins normally involved in the cell cycle play a role in the regulation of neuronal apoptotic death following various insults. However, it is not clear if the same mechanisms regulate cell death of oligodendrocytes as well. In this study, we investigated the mechanism of ceramide-induced apoptosis in primary rat oligodendrocytes. We show that ceramide treatment initiates a cascade of biochemical events involving cell cycle regulatory proteins. Although at the time of induction of cell death the oligodendrocytes are postmitotic, activation of c-myc and translocation of Cdc25A into the nucleus can be demonstrated. Of particular interest are the findings of the up-regulation of PCNA and down-regulation of p21WAF1/CIP1 protein, an inhibitor of cell-cycle progression. The current results show that activation of regulatory cell-cycle proteins at the oligodendrocytes G1-S checkpoint may constitute a crucial step of the death pathway of oligodendrocytes.     

The A2A adenosine receptor rescues neuritogenesis impaired by p53 blockage via KIF2A,a kinesin family member

The A_2A adenosine receptor (A_2AR) is a G‐protein–coupled receptor. We previously reported that the C terminus of the A_2AR binds to translin‐associated protein X (TRAX) and modulates nerve growth factor (NGF)‐evoked neurite outgrowth in PC12 cells. Herein, we show that neuritogenesis of primary hippocampal neurons requires p53 because blockage of p53 suppressed neurite outgrowth. The impaired neuritogenesis caused by p53 blockage was rescued by activation of the A_2AR (designated the A_2A rescue effect) in a TRAX‐dependent manner. Importantly, suppression of a TRAX‐interacting protein (kinesin heavy chain member 2A, KIF2A) inhibited the A_2A rescue effect, whereas overexpression of KIF2A caused a rescue effect. Expression of a KIF2A fragment (KIF2A₅₁₄), which disturbed the interaction between KIF2A and TRAX, blocked the rescue effect. Transient colocalization of TRAX and KIF2A was detected in the nucleus of PC12 cells upon NGF treatment. These data suggest that functional interaction between KIF2A and TRAX is critical for the A_2A rescue effect. Moreover, p53 blockage during NGF treatment prevented the redistribution of KIF2A from the nucleus to the cytoplasmic region. Expression of a nuclear‐retained KIF2A variant (NLS‐KIF2A) did not rescue the impaired neurite outgrowth as did the wild‐type KIF2A. Therefore, redistribution of KIF2A to the cytoplasmic fraction is a prerequisite for neurite outgrowth. Collectively, we demonstrate that KIF2A functions downstream of p53 to mediate neuritogenesis of primary hippocampal neurons and PC12 cells. Stimulation of the A_2AR rescued neuritogenesis impaired by p53 blockage via an interaction between TRAX and KIF2A. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 604–621, 2010