Comparative studies of the effects of galactosamine and actinomycin D on nuclear ribonucleoprotein particles from rat liver

Acute and chronic excitotoxicity in ischemic stroke and late-onset Alzheimer's disease

Stroke and Alzheimer's disease are common neurological disorders and often occur in the same individuals. The comorbidity of the two neurological disorders represents a grave health threat to older populations. This review presents a brief background of the development of novel concepts and their clinical potentials. The activity of glutamatergic N-methyl-D-aspartate receptors and N-methyl-D-aspartate receptor-mediated Ca 2+ influx is critical for neuronal function. An ischemic insult induces prompt and excessive glutamate release and drastic increases of intracellular Ca 2+ mainly via N-methyl-D-aspartate receptors, particularly of those at the extrasynaptic site. This Ca 2+ -evoked neuronal cell death in the ischemic core is dominated by necrosis within a few hours and days known as acute excitotoxicity. Furthermore, mild but sustained Ca 2+ increases under neurodegenerative conditions such as in the distant penumbra of the ischemic brain and early stages of Alzheimer's disease are not immediately toxic, but gradually set off deteriorating Ca 2+ -dependent signals and neuronal cell loss mostly because of activation of programmed cell death pathways. Based on the Ca 2+ hypothesis of Alzheimer's disease and recent advances, this Ca 2+ -activated "silent" degenerative excitotoxicity evolves from years to decades and is recognized as a unique slow and chronic neuropathogenesis. The N-methyl-D-aspartate receptor subunit GluN3A, primarily at the extrasynaptic site, serves as a gatekeeper for the N-methyl-D-aspartate receptor activity and is neuroprotective against both acute and chronic excitotoxicity. Ischemic stroke and Alzheimer's disease, therefore, share an N-methyl-D-aspartate receptor- and Ca 2+ -mediated mechanism, although with much different time courses. It is thus proposed that early interventions to control Ca 2+ homeostasis at the preclinical stage are pivotal for individuals who are susceptible to sporadic late-onset Alzheimer's disease and Alzheimer's disease-related dementia. This early treatment simultaneously serves as a preconditioning therapy against ischemic stroke that often attacks the same individuals during abnormal aging.

Extrasynaptic NMDA receptors in acute and chronic excitotoxicity: implications for preventive treatments of ischemic stroke and late-onset Alzheimer's disease

Stroke and late-onset Alzheimer's disease (AD) are risk factors for each other; the comorbidity of these brain disorders in aging individuals represents a significant challenge in basic research and clinical practice. The similarities and differences between stroke and AD in terms of pathogenesis and pathophysiology, however, have rarely been comparably reviewed. Here, we discuss the research background and recent progresses that are important and informative for the comorbidity of stroke and late-onset AD and related dementia (ADRD). Glutamatergic NMDA receptor (NMDAR) activity and NMDAR-mediated Ca²⁺ influx are essential for neuronal function and cell survival. An ischemic insult, however, can cause rapid increases in glutamate concentration and excessive activation of NMDARs, leading to swift Ca²⁺ overload in neuronal cells and acute excitotoxicity within hours and days. On the other hand, mild upregulation of NMDAR activity, commonly seen in AD animal models and patients, is not immediately cytotoxic. Sustained NMDAR hyperactivity and Ca²⁺ dysregulation lasting from months to years, nevertheless, can be pathogenic for slowly evolving events, i.e. degenerative excitotoxicity, in the development of AD/ADRD. Specifically, Ca²⁺ influx mediated by extrasynaptic NMDARs (eNMDARs) and a downstream pathway mediated by transient receptor potential cation channel subfamily M member (TRPM) are primarily responsible for excitotoxicity. On the other hand, the NMDAR subunit GluN3A plays a "gatekeeper" role in NMDAR activity and a neuroprotective role against both acute and chronic excitotoxicity. Thus, ischemic stroke and AD share an NMDAR- and Ca²⁺-mediated pathogenic mechanism that provides a common receptor target for preventive and possibly disease-modifying therapies. Memantine (MEM) preferentially blocks eNMDARs and was approved by the Federal Drug Administration (FDA) for symptomatic treatment of moderate-to-severe AD with variable efficacy. According to the pathogenic role of eNMDARs, it is conceivable that MEM and other eNMDAR antagonists should be administered much earlier, preferably during the presymptomatic phases of AD/ADRD. This anti-AD treatment could simultaneously serve as a preconditioning strategy against stroke that attacks ≥ 50% of AD patients. Future research on the regulation of NMDARs, enduring control of eNMDARs, Ca²⁺ homeostasis, and downstream events will provide a promising opportunity to understand and treat the comorbidity of AD/ADRD and stroke.

An assessment of the evidence for the role of ribonucleoprotein particles in the maturation of eukaryote mRNA

This article has sought to draw together, on the one hand, what is known of mRNA processing and its control and, on the other hand, what is known of the structure and validity of hnRNP and snRNP particles. At the same time, it has attempted to synthesize these two themes into a critical assessment of the evidence which suggests that the particles are intimately involved in processing. It cannot be said that the case is proven. The evidence is compelling but circumstantial. The last few years have seen the development of the first in vitro splicing systems (Weingartner and Keller, 1981; Goldenberg and Raskus, 1981; Kole and Weissman, 1982), the isolation of monoclonal antibodies to defined snRNP (Lerner et al., 1981a; Billings et al., 1982) and hnRNP proteins (Hugle et al., 1982), and the ability to use artificial lipid vesicles to transfer antisera (Lenk et al., 1982) and radioactive snRNA (Gross and Cetron, 1982) into cells. It is to be hoped that further refinements of these and other techniques will allow us to solve this, one of the major outstanding problems of molecular biology.

The effect of inhibition of RNA synthesis by actinomycin D on the population of basic polypeptides of the 30S unclear ribonucleoprotein particles

In rats injected intraperitoneally with actinomycin D (2 mg/kg body weight) 12 h earlier, the yield of the 30S ribonucleoprotein particles isolated from liver nuclei by extraction with 0.1 M NaCl at pH 8.0 decreased by 60 per cent. The protein-to-RNA ratio of these particles increased to 32:1 from the ratio 4.4:1 found in the same particles isolated from the nuclei of liver of control rats. The particles isolated from the liver nuclei of rats injected with actinomycin D were depleted of all charge isomers of the two most prominent polypeptides (33,000 and 39,000 daltons) present in the particles of liver of control animals. The most abundant protein in these particles was a 43,000 dalton polypeptide. This polypeptide is the least prominent of the 3 major polypeptides present in the control particles. The same charge isomers of the 43,000 dalton polypeptide were present in the nuclear ribonucleoprotein particles isolated from the liver of control animals and from the liver of animals treated with actinomycin D 12 h earlier. In control animals the nuclear ribonucleoprotein monoparticles isolated from kidney contained 3 major polypeptides of the same molecular weight with the same distribution of their charge isomers as were present in the particles isolated from liver nuclei. The injection of actinomycin D 12 h earlier was without any effect on the protein composition of the 30S nuclear ribonucleoprotein particles of rat kidney.

Comparison of the nuclear ribonucleoproteins containing the transcripts of adenovirus-2 and HeLa cell dna

A method as devised allowing the preparation of hnRNA-containing ribonucleproteins (hnRNP) frm HeLa cells infected with adenovirus type 2 under conditions where the extraction of viral replication complexes as minimal. Approximately 60% of the RNA from such hnRNP hybridized with adenovirus DNA. The hnRNP from infected cells had the same general characteristics as those from uninfected cells. Their size was heterogeneous (30-260 S) and depended upon that of their RNA. Their CsCl densities were identical (1.39-1.40 g/ml), indicating the same protein:RNA ratio. Their proteins were found in the same molecular weight range, between 25 000 and 200 000. The major proteins of hnRNP from HeLa cells were present in hnRNP from adenovirus-infected cells. As 60% of the cellular RNA was replaced by adenovirus RNA in hnRNP, this in dicated that there was not stringent specificity in the RNA-protein interactions. The relative proportions of the proteins were identifical in both cases, suggesting that the hnRNP assembly was independent of nucleotide sequences at least for the major proteins. The hnRNP from infected and uninfected cells differed by the size of their RNA, which was larger after infection, and by the presence of six additional minor polypeptides after infection. However, it cannot be excluded that the presence of these polypeptides in hnRNP resulted from non-specific adsorption.

Composition of hnRNA-associated proteins in rat liver is specifically altered after cycloheximide treatment

Rats were treated with non-lethal doses of alpha-amanitin or cycloheximide. Nuclei were prepared and the particles carrying heterogeneous nuclear RNA (30--40 S-particles) isolated by density gradient centrifugation. In the case of alpha-amanitin the yield of particles was reduced to about 45%. Cycloheximide affected the composition of proteins associated with the nuclear RNA. In particular, the concentration of a 110 000 molecular weight protein as determined by sodium dodecylsulfate gel electrophoresis was reduced to 20--30% after 2 h and then increased to 150--180% of the control before it approached the normal level after 30 h. Possible mechanisms underlying these changes are discussed.

Maturation and nucleo-cytoplasmic transport of rat-liver ribosomal RNA upon D-galactosamine inhibition of transcription

D-Galactosamine (250 mg/kg body weight) causes 90--95% inhibition of [14C]orotate or inorganic [32P]phosphate incorporation in vivo into rat liver nuclear RNA within 30 min. The transcription of both nucleolar and nucleoplasmic genes is inhibited to the same extent. Under these conditions, prelabbeled 45-S pre-rRNA is processed quantitatively to nuclear 28-S and 18-S rRNA. The nucleocytoplasmic transport of both 28-S and 18-S rRNA remains unaltered for about 60 min after blockage of transcription. At this stage the nucleo-cytoplasmic transport of 18-S rRNA is almost completed. It is concluded that formation and nucleo-cytoplasmic transport of ribosomes is independent of concurrent transcription of rRNA or nucleoplasmic genes. At later stages, the nucleocytoplasmic transport of 28-S rRNA is delayed and its partial degaradation in the nucleus may take place. This effect is correlated with a decreased (up to 40% of controls) labelling of nuclear proteins. However, the labelling of total cellular or microsomal proteins remains unchanged up to 3 h after D-galactosamine administration. It is suggested that the last nuclear steps of ribosome formation are dependent on the continuous supply of rapidly-labelled nuclear proteins.

Low molecular weight RNAs as components of nuclear ribonucleoprotein particles containing heterogeneous nuclear RNA

70-130 S polyparticles as well as 38 S monoparticles were isolated from rat liver nuclei and analyzed in respect to their RNA components by microgel polyacrylamide electrophoresis in formamide. In addition to the high molecular weight polydisperse hnRNA of polyparticles several low molecular weight RNAs (snRNA) were detected. There are at least six distinct snRNA species in polyparticles. Except for one species, which is missing, 38 S monoparticles showed a similar snRNA pattern. From densitometer tracings of microgels the snRNAs were estimated to represent about 11% of the total polyparticle RNA. The number of nucleotides for the various snRNAs were determined from a plot of relative electrophoretic mobility versus log number of nucleotides. The possibility that the snRNAs are degradation products of the hnRNA was excluded on the basis of the following findings. (1) The snRNA pattern was similar in mono- and polyparticles. (2) Whereas the hnRNA of polyparticles incubated at 37 degrees C was extensively degraded, the snRNA did not show a corresponding increase. (3) After a 30 min pulse with [3H]orotate the hn RNA was readily labeled; none of the snRNAs, however, incorporated radioactivity. The snRNAs were still found after treatment of polyparticles with 2 M NaCl excluding contamination by nucleoplasm.

The effect of sodium chloride on the structure of ribonucleoprotein particles from rat liver nuclei

38S (monoparticles) and greater than 50--200S ribonucleoprotein particles (polyparticles) from rat liver nuclei were treated with increasing concentrations of sodium chloride. Treatment of 38S or greater than 50--200S particles, with 0.14, 0.25, 0.5, 1.0, and 2.0M NaCl resulted in a decrease of protein to RNA ratios from 8 to 3.1 for 38S particles and from 4.0 to 1.5 for greater than 20--200S particles. Correspondingly the densities in CsCl increased. Whereas the maximum of the sedimentation profile of polyparticles decreased from 90S to 50S after treatment with increasing NaCl concentrations, a discontinuous change was found in the case of monoparticles. It was shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis that the proteins which were dissociated by NaCl were in the molecular weight range of 30--45 000. Four of the 5 small molecular weight RNAs in the range of 4.5 to 8S remained tightly associated even after treatment of polyparticles with 2.0M NaCl. When 38S or 70--200S nRNP particles were exposed to increasing concentrations of NaCl (0.25, 0.5, 1.0, 2.0M), the molar ellipticity at 264 nm increased progressively to about 40%. Upon NaCl treatment of polyparticles and successive removal of the dissociated proteins by centrifugation the increase in the positive CD band at 264 nm was only 15%.

Studies on the preparation and properties of ribonucleoprotein particles from rat liver nuclei

Ribonucleoprotein particles of 38 S were extracted from rat liver nuclei with isotonic salt buffer under concomitant sonication. The fate of the endogeneous nuclear RNAases assayed with poly(A), high molecular weight yeast RNA and rapidly labeled hnRNA was followed during the preparation of 38-S nuclear ribonucleoprotein (nRNP) particles. Essentially all the RNAase activity could be removed from the particle preparation. The effect of synthetic RNAase inhibitors on the nRNP particles was studied. Upon extraction of nuclei with 0.14 M NaCl, approximately 38% of the total nuclear radioactivity was found in the 38-S nRNP particles. By two successive extractions of the remaining chromatin with either isotonic or 0.22 and 0.3 M NaCl, an additional 25 and 9% of rapidly labeled hnRNA of 38 S particle were dissociated from chromatin, respectively. The chromatin components, DNA, nonhistone proteins, histones and RNA were determined after successive salt extractions. Particularly alterations in the nonhistone proteins and RNA were found. The protein patterns upon SDS-acrylamide gel electrophoresis of the salt-extracted chromatin preparations were compared with those of the 38-S nRNP particles. Particularly proteins in the molecular weight range of 32 000-43 000 were dissociated from chromatin after treatment with 0.22 or 0.3 M NaCl.