Ubiquitin binds the amyloid β peptide and interferes with its clearance pathways

Several lines of evidence point to a compromised proteostasis associated with a reduction of the Ubiquitin Proteasome System (UPS) activity in patients affected by Alzheimer's Disease (AD) and suggest that the amyloid β peptide (Aβ) is an important player in the game. Inspired also by many reports, underlining the presence of ubiquitin (Ub) in the amyloid plaques of AD brains, here we set out to test whether Ub may bind the Aβ peptide and have any effect on its clearance pathways. By using an integrated array of MALDI-TOF/UPLC-HRMS, fluorescence, NMR, SPR, Microscale Thermophoresis (MST) and molecular dynamics studies, we consistently demonstrated that Aβ40 binds Ub with a 1 : 1 stoichiometry and K d in the high micromolar range. In particular, we show that the N-terminal domain of the Aβ peptide (through residues D1, E3 and R5) interacts with the C-terminal tail of Ub (involving residues K63 and E64), inducing the central region of Aβ (14HQKLVFFAEDVGSNK28) to adopt a mixed α-helix/β-turn structure. ELISA assays, carried out in neuroblastoma cell lysates, suggest that Aβ competitively binds Ub also in the presence of the entire pool of cytosolic Ub binding proteins. Ub-bound Aβ has a lower tendency to aggregate into amyloid-like fibrils and is more slowly degraded by the Insulin Degrading Enzyme (IDE). Finally, we observe that the water soluble fragment Aβ1-16 significantly inhibits Ub chain growth reactions. These results evidence how the non-covalent interaction between Aβ peptides and Ub may have relevant effects on the regulation of the upstream events of the UPS and pave the way to future in vivo studies addressing the role played by Aβ peptide in the malfunction of proteome maintenance occurring in AD.

Effect of antioxidant diets on mitochondrial gene expression in rat brain during aging

Age-related increase of reactive oxygen species (ROS) is particularly detrimental in postmitotic tissues. Calorie restriction (CR) has been shown to exert beneficial effects, consistent with reduced ROS generation by mitochondria. Many antioxidant compounds also mimic such effects. N-acetyl cysteine (NAC) provides thiol groups to glutathione and to mitochondrial respiratory chain proteins; thus, it may counteract both ROS generation and effects. In the present study we investigated, in different rat brain areas during aging (6, 12, and 28 months), the effect of 1-year treatment with CR and dietary supplementation with NAC on the expression of subunit 39 kDa and ND-1 (mitochondrial respiratory complex I), subunit IV (complex IV), subunit alpha of F0F1-ATP synthase (complex V) and of adenine nucleotide translocator, isoform 1 (ANT-1). The observed age-related changes of expression were prevented by the dietary treatments. The present study provides further evidence for the critical role of mitochondria in the aging process.

Diabetic patients and retinal proliferation: an evaluation of the role of vascular endothelial growth factor (VEGF)

Vascular endothelial growth factor (VEGF) has been shown to play a major role in intraocular neovascularisation in ischaemic retinal diseases. The aim of this study was to evaluate the concentration of VEGF in vitreous, aqueous and epiretinal membranes of diabetic and non-diabetic patients, with other pathological conditions requiring surgical intervention. Higher VEGF concentration were found in samples from the eyes of diabetic patients versus other pathologies as well as in epiretinal membranes versus the other eye compartments in diabetic patients. However, high VEGF levels were also found in retinal detachment and proliferative vitreoretinopathy of non-diabetic patients. We concluded that VEGF is produced locally and plays a fundamental, but not specific, role in diabetic retinal neovascularisation and proliferation.

Role of the JAK/STAT signal transduction pathway in the regulation of gene expression in CNS

Over the last 20 years the JAK/STAT signal transduction pathway has been extensively studied. An enormous amount of data on different cell signal transduction pathways is now available. The JAK/STAT signal transduction pathway is one of the intracellular signaling pathways activated by cytokines and growth factors that was first studied in the hematopoietic system, but recent data demonstrate that this signal transduction is also greatly utilized by other systems. The JAK/STAT pathway is a signaling cascade that links the activation of specific cell membrane receptors to nuclear gene expression. This review is focused on the role of JAK/STAT signal transduction pathway activation in the central nervous system (CNS).

JAK/STAT signaling pathway mediates cytokine-induced iNOS expression in primary astroglial cell cultures

The production of nitric oxide by the calcium-independent inducible nitric oxide synthase (iNOS) in glial cells has been implicated in the neuropathogenesis of various diseases. It is well known that in response to lipopolysaccharide (LPS) and cytokines, such as IFN-gamma, glial cells are induced to synthesize large amount of nitric oxide (NO) (Bolaños et al., 1996; Nicoletti et al., 1998). The signaling transduction pathways for iNOS transcription in astroglial cells have however not yet been established. Because IFN-gamma receptor chains are associated with Janus tyrosine kinases (JAK1 and JAK2) (Darnell et al., 1994), we analyzed the involvement of the JAK/STAT signal transduction pathway in iNOS expression. Our study shows increased JAK2 and STAT1 alpha/beta tyrosine phosphorylation in primary astroglial cell culture after treatment with IFN-gamma and LPS. A temporal correlation was observed between JAK2 and STAT1 alpha/beta tyrosine phosphorylation, the appearance of interferon-regulatory factor-1 (IRF-1) mRNA and the iNOS expression. Inhibition experiments showed that JAK2 and STAT1 alpha/beta tyrosine phosphorylation were necessary for IFN gamma-mediated iNOS induction in astroglial cells. We conclude that JAK2 and STAT1 alpha/beta tyrosine phosphorylation is an early event involved in the expression of iNOS in astroglial cells.

Nitric oxide synthase induction in astroglial cell cultures: effect on heat shock protein 70 synthesis and oxidant/antioxidant balance

Glial cells in the nervous system can produce nitric oxide in response to cytokines. This production is mediated by the inducible isoform of nitric oxide synthase. Radical oxygen species (ROS) and nitric oxide (NO) derivatives have been claimed to play a crucial role in many different processes, both physiological such as neuromodulation, synaptic plasticity, response to glutamate, and pathological such as ischemia and various neurodegenerative disorders. In the present study we investigated the effects of NO synthase (iNOS) induction in astrocyte cultures on the synthesis of heat shock proteins, the activity of respiratory chain complexes and the oxidant/antioxidant balance. Treatment of astrocyte cultures for 18 hr with LPS and INFgamma produced a dose dependent increase of iNOS associated with an increased synthesis of hsp70 stress proteins. This effect was abolished by the NO synthase inhibitor L-NMMA and significantly decreased by addition of SOD/CAT in the medium. Time course experiments showed that iNOS induced protein expression increased significantly by 2 hr after treatment with LPS and INFgamma and reached a plateau at 18 hr; hsp70 protein synthesis peaked around 18 and 36 hr after the same treatment. Addition to astrocytes of the NO donor sodium nitroprusside resulted in a dose dependent increase in hsp70 protein that was comparable to that found after a mild heat shock. Additionally, a decrease in cytochrome oxidase activity, a marked decrease in ATP and protein sulfhydryl contents, an increase in the activity of the antioxidant enzymes mt-SOD and catalase were found which were abolished by L-NMMA. These findings suggest the importance of mitochondrial energy impairment as a critical determinant of the susceptibility of astrocytes to neurotoxic processes and point to a possible pivotal role of hsp70 in the signalling pathways of stress tolerance.

Structural features of the rat GFAP gene and identification of a novel alternative transcript

The glial fibrillary acidic protein (GFAP) is expressed in a cell-specific manner and represents the major subunit of intermediate filaments of astroglial cells. The knowledge of the gene structure is an important step for further understanding the mechanisms of cell-specific expression. In the present study, we report the complete sequence of the rat GFAP gene and provide evidence for the existence, in the rat brain, of a novel alternative transcript. Since three different transcripts, indicated as GFAPalpha, beta, and gamma, have been previously reported (Feinstein et al. [1992] J. Neurosci. Res. 32:1-14; Zelenika et al. [1995] Mol. Brain Res. 30:251-258), we called this novel mRNA isoform GFAPdelta. It is generated by the alternative splicing of a novel exon located in the classic seventh intron. This alternative exon (called VII+) contains a 101-bp coding sequence in frame with exon VII and interrupted by a stop codon TAA at position +5451. Therefore, the novel GFAPdelta transcript encodes for an hypothetical GFAP where the forty-two carboxy-terminal amino acids encoded by exon VIII and IX are replaced by thirty-three amino acids encoded by exon VII+. Northern blot analysis with a specific probe for exon VII+ revealed a 4.2-kb mRNA, expressed in several brain areas, but absent in extracerebral tissues (lung, heart, kidney, liver, spleen). The previously discovered GFAP isoforms (alpha, beta, and gamma) produce hypothetical translation products differing in the amino-terminal Head domain. The present data suggest, for the first time, the possible existence of GFAP isoforms differing in the carboxy-terminal Tail domain.

GFAPbeta mRNA expression in the normal rat brain and after neuronal injury

GFAPbeta mRNA is an alternative transcript of the glial fibrillary acidic protein (GFAP) gene, whose transcriptional start site is located 169 nucleotides upstream to the classical GFAPalpha mRNA. By an RT-PCR method with primers on separate exons, we were able to confirm the presence of GFAP transcripts with a longer 5' untranslated region in all the examined areas of rat brain and in primary cultures of astroglial cells. Northern blot analysis, using an oligoprobe specific for the 5' region of GFAPbeta, revealed a single hybridization band of 2.9 kb in all the brain regions examined and in primary cultures of astroglial cells. The availability of the quantitative Northern blot assay allowed further studies on the regulation of GFAPbeta expression in vivo. Since it is well-known that neuronal brain injury is one of the most powerful inducers of GFAP, we examined the expression of GFAPalpha and beta after a neurotoxic lesion in the rat hippocampus. Results obtained show a parallel increase in both GFAP transcripts with an identical time-course, suggesting that regulatory regions of the gene influence in similar way the rate of transcription at the two different start sites (alpha and beta) or that a similar post-transcriptional mechanism is involved in regulating both mRNA isoforms.

Effect of nitric oxide synthase induction on the expression of mitochondrial respiratory chain enzyme subunits in mixed cortical and astroglial cell cultures

In the present study we evaluated the effects of NO synthase (NOS) induction on the regulation of cytochrome c oxidase (CO) and F0F1-ATPase subunit expression in astroglial and mixed cortical cell cultures. In mixed cortical cell cultures, 18 h of treatment with lipopolysaccharide (LPS, 0.1 microgram/mL) plus interferon-gamma (INF-gamma, 10 U/mL) caused an increase of mRNAs for CO-I, F0F1-ATPase 6 and also for iNOS at 20 DIV. The induction of both CO-I and F0F1-ATPase 6 was abolished by the NOS inhibitor N-monomethyl-L-arginine (NMMA) or by the enzymatic scavenger superoxide dismutase/catalase (SOD/CAT). In primary astroglial cell cultures, treatment for 18 h with increasing concentrations of LPS and INF gamma, produced an increase in the amount of mitochondrial encoded CO-I and -II subunits, with no significant modifications of nuclear encoded subunit IV. An increase was also observed at level of transcription for CO-I and -II, and F0F1-ATPase 6 mRNAs. These effects were abolished by addition of NMMA or SOD/CAT. mRNA induction of CO-I was higher in mixed cortical than in astroglial cell cultures while that of F0F1-ATPase 6 was similar in both cell types. These results suggest that the expression of mitochondrial encoded subunits (CO-I, CO-II and F0F1-ATPase 6) is up-regulated in response to oxygen and NO reactive species. The activity of cytochrome c oxidase decreased after LPS/INF gamma treatment in both astroglial and mixed cortical cultures. The activity of ATP synthase was unmodified, while ATP content drastically decreased after LPS/INF gamma treatment, in both astroglial and mixed cortical cultures. The enzymatic activities of catalase and Mn-SOD (mitochondrial) showed a significant increase after LPS/INF gamma treatment, which was abolished by NMMA.

Regulation of cytochrome c oxidase and FoF1-ATPase subunits expression in rat brain during aging

In the present study we analyzed the age-dependent changes of mRNA levels for cytochrome c oxidase and FoF1-ATP synthase subunits in rat cerebral cortex and cerebellum. To establish whether the regulation of expression is transcriptional or post-transcriptional, the results were compared to those related to protein subunits levels, of the same enzymatic complexes, previously observed. The different patterns of age-related changes of mRNA subunits, in particular the lower increments, compared with those related to protein subunits, indicate that post-transcriptional mechanisms of regulation might be involved in the coordinated expression of the various subunits of each complex. Northern blotting analyses of RNA from the cerebellum of rats at the various ages, showed also differences in age-dependent patterns of transcription between cerebral cortex and cerebellum. Moreover, the major age-dependent changes of mitochondrial-encoded subunits, compared with the nuclear-encoded ones, previously observed at proteins level, occur also during transcription.

A neural-specific hypomethylated domain in the 5' flanking region of the glial fibrillary acidic protein gene

In the present study we examined the methylation status of the glial fibrillary acidic protein (GFAP) gene promoter, analyzing various CG sites in both the human and rat gene in GFAP-expressing and nonexpressing tissues. Moreover, we studied the methylation of specific CG sites in different rat brain areas during postnatal development, in cell cultures highly enriched in specific neural- or non-neural-cell types (fibroblasts), and in human gliomas. The obtained results do not support a simple correlation between demethylation and expression of the GFAP gene but help to identify a cluster of CG sites in the 5'flanking region (from -1176 to -1471 in the rat) that are hypomethylated in neural cell types and localized in a region highly conserved between rat, mouse and human GFAP promoters. Neural-specific hypomethylation of this conserved zone can be observed also in the human GFAP gene both in normal brain tissue and neoplastic glial cells. A higher demethylation of the -1176 site at early stage of postnatal life was observed in specific rat brain areas, such as hippocampus and cerebellum. The most dramatic differences were observed in the cerebellum where a peak of demethylation of the -1176 site was detected at 15 days of postnatal life, followed by an intense remethylation of this site. Results of experiments in the CG4 glial progenitor cell line showed that demethylation of the -1176 site is already established before transcriptional activation of the GFAP gene. Moreover, results of experiments in primary cell cultures show that in neuronal cell types, such as cerebellar granule cells and embryonic cerebral hemisphere neurons, the level of demethylation of the -1176 site is comparable to that observed in cultured astrocytes. In contrast a high level of methylation can be observed in cultured non-neural cell types (fibroblasts). Such neural-specific hypomethylation could be established in a very early stage in the progression along the neural cell lineage and could play a role in maintaining a local open chromatin conformation which is then necessary to allow the interaction with specific regulatory factors present in astroglial cells.

Changes of mitochondrial cytochrome c oxidase and FoF1 ATP synthase subunits in rat cerebral cortex during aging

The contents of subunits I, II/III, and IV of cytochrome c oxidase and of subunits alpha, beta and gamma of FoF1 ATP synthase in inner mitochondrial membrane proteins purified from cerebral cortex of rat at 2, 6, 12, 18, 24, and 26 months of age were analyzed by western blot. Age-related changes in the content of subunits, either of mitochondrial or nuclear origin, were observed. All the cytochrome c oxidase (COX) subunits examined showed an age-related increase from 2-month-old rats up to 24 months with a decrease at the oldest age (26 months). The same pattern of age-dependent changes was observed for gamma ATP synthase, while the alpha and beta subunits increased progressively up to 26 months.

AMPA-selective glutamate receptor subunits in the rat hippocampus during aging

The levels of mRNAs for the subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-selective glutamate receptors (GluR-1, -2, -3, -4) in the rat hippocampus during aging were measured by Northern blotting. The distribution of these receptors was also examined at the protein level by immunoblotting using antibodies to GluR-1 and to an epitope common to GluR-2 and GluR-3 (denoted GluR-2/3). During aging a significant decrease of GluR-1 protein, but no change in the corresponding mRNA level, was observed. No differences in the level of GluR-2/3 protein and GluR-2, -3, and -4 mRNAs at the various ages examined (4, 12, and 24 months) were detected. Our results show that AMPA receptors are only slightly influenced by the aging process in the rat hippocampus. The slight decrease in GluR-1 protein content, not accompanied by a parallel decrease in the GluR-1 mRNA level, might be explained by a decreased translational efficiency or an increased protein degradation of the GluR-1 subunit.

Tissue-specific DNA methylation patterns of the rat glial fibrillary acidic protein gene

The glial fibrillary acidic protein (GFAP) is an intermediate filament protein, specific of the cytoskeleton of astrocytes in the central nervous system. In the present work, as a preliminary step to the study of glial-specific gene expression, we cloned the rat GFAP gene, and we report the sequence of 1.9 kb of the 5' flanking region, exon 1, and the majority of the first intron. By digestion with methylation-sensitive restriction enzymes followed by Southern blot analysis, the methylation status of various CpG sites was examined in this genomic segment. We tested whether structural modification of the GFAP gene, such as DNA methylation, could be related to its tissue-specific transcriptional activity. Therefore, we compared a GFAP-expressing cell population (primary culture of astroglial cells), a mixed population of GFAP-expressing and -nonexpressing cells (adult rat cerebral hemispheres), and a GFAP-nonexpressing tissue (liver). In the 5' flanking region we identified a CpG site at position -1176 whose level of methylation is inversely correlated to GFAP expression. In primary cultured astrocytes, 75% of the GFAP gene alleles were demethylated at this site, while the corresponding value obtained for the cerebral hemispheres was 45%, and for liver only 9%. On the basis of the sequence data, a CpG-rich region (putative CpG island) was identified extending from -38 to +347 and overlapping 80% of the first exon. HhaI and HpaII sites located in the putative CpG island showed a relatively high level of methylation in all the cell populations examined, and did not show any clear correlation with the level of GFAP gene expression or with the methylation status of the -1176 site. Further in vivo developmental studies and in vitro differentiation studies are necessary to better understand the functional differences of the various methylatable CpG sites in the 5' end of the GFAP gene.

ADP-ribosylation of proteins in brain regions of rats during postnatal development

Post-translational modifications of chromatin-bound proteins play an important role in the regulation of eukaryotic gene expression. Processes such as acetylation, methylation, phosphorylation and ADP-ribosylation may alter the interaction of these proteins with DNA and consequently affect chromatin conformation and the binding of enzymes and other molecules involved in the regulation of gene expression. In the present study the process of ADP-ribosylation of chromosomal proteins (histone and non-histone proteins) in some rat brain regions during postnatal development was investigated; also the effect of epidermal growth factor (EGF) on this process in fetal brain slices was studied. It has been found that the process of ADP-ribosylation of total histones extracted from rat cerebral cortex and cerebellum at 1, 10 and 30 days of age, increases from 1 to 10 days of age (i.e. the period of maximal cell proliferation) and decreases thereafter, while the process of ADP-ribosylation of non-histone proteins (NHPs) sharply decreases during the same developmental period. The addition of EGF to fetal brain slices causes a significant increase of ADP-ribosylation of total histones (particularly of the histone H1 fraction) and also of NHPs and microsomal proteins. This result is in agreement with the effect of EGF as a mitogen factor, previously shown in astroglial cell cultures.