Ribosomal RNA in Alzheimer's disease and aging

MicroRNAs and long non-coding RNAs as novel regulators of ribosome biogenesis

Ribosome biogenesis is the fine-tuned, essential process that generates mature ribosomal subunits and ultimately enables all protein synthesis within a cell. Novel regulators of ribosome biogenesis continue to be discovered in higher eukaryotes. While many known regulatory factors are proteins or small nucleolar ribonucleoproteins, microRNAs (miRNAs), and long non-coding RNAs (lncRNAs) are emerging as a novel modulatory layer controlling ribosome production. Here, we summarize work uncovering non-coding RNAs (ncRNAs) as novel regulators of ribosome biogenesis and highlight their links to diseases of defective ribosome biogenesis. It is still unclear how many miRNAs or lncRNAs are involved in phenotypic or pathological disease outcomes caused by impaired ribosome production, as in the ribosomopathies, or by increased ribosome production, as in cancer. In time, we hypothesize that many more ncRNA regulators of ribosome biogenesis will be discovered, which will be followed by an effort to establish connections between disease pathologies and the molecular mechanisms of this additional layer of ribosome biogenesis control.

Characterization of Cerebellum-Specific Ribosomal DNA Epigenetic Modifications in Alzheimer's Disease: Should the Cerebellum Serve as a Control Tissue After All?

Alzheimer's disease (AD) is a neurodegenerative disease, known as the most common form of dementia. In AD onset, abnormal rRNA expression has been reported to be linked in pathogenesis. Although region-specific expression patterns have previously been reported in AD, it is not until recently that the cerebellum has come under the spotlight. Specifically, it is unclear whether DNA methylation is the mechanism involved in rRNA expression regulation in AD. Hence, we sought to explore the rDNA methylation pattern of two different brain regions - auditory cortex and cerebellum - from AD and age-/sex-matched controls. Our results showed differential hypermethylation at an upstream CpG region to the rDNA promoter when comparing cerebellum controls to auditory cortex controls. This suggests a possible regulatory region from rDNA expression regulation. Moreover, when comparing between AD and control cerebellum samples, we observed hypermethylation of the rDNA promoter region as well as an increase in rDNA content. In addition, we also observed increased rRNA levels in AD compared to control cerebellum. Although still considered a pathology-free brain region, there are growing findings that continue to suggest otherwise. Indeed, cerebellum from AD has been recently described as affected by the disease, presenting a unique pattern of molecular alterations. Given that we observed that increased rDNA promoter methylation did not silence rDNA gene expression, we suggest that rDNA promoter hypermethylation is playing a protective role in rDNA genomic stability and, therefore, increasing rRNA levels in AD cerebellum.

Emerging Role of the Nucleolar Stress Response in Autophagy

Autophagy represents a conserved self-digestion program, which allows regulated degradation of cellular material. Autophagy is activated by cellular stress, serum starvation and nutrient deprivation. Several autophagic pathways have been uncovered, which either non-selectively or selectively target the cellular cargo for lysosomal degradation. Autophagy engages the coordinated action of various key regulators involved in the steps of autophagosome formation, cargo targeting and lysosomal fusion. While non-selective (macro)autophagy is required for removal of bulk material or recycling of nutrients, selective autophagy mediates specific targeting of damaged organelles or protein aggregates. By proper action of the autophagic machinery, cellular homeostasis is maintained. In contrast, failure of this fundamental process is accompanied by severe pathophysiological conditions. Hallmarks of neuropathological disorders are for instance accumulated, mis-folded protein aggregates and damaged mitochondria. The nucleolus has been recognized as central hub in the cellular stress response. It represents a sub-nuclear organelle essential for ribosome biogenesis and also functions as stress sensor by mediating cell cycle arrest or apoptosis. Thus, proper nucleolar function is mandatory for cell growth and survival. Here, I highlight the emerging role of nucleolar factors in the regulation of autophagy. Moreover, I discuss the nucleolar stress response as a novel signaling pathway in the context of autophagy, health and disease.

Evidence for nucleolar dysfunction in Alzheimer's disease

The nucleolus is a dynamically changing organelle that is central to a number of important cellular functions. Not only is it important for ribosome biogenesis, but it also reacts to stress by instigating a nucleolar stress response and is further involved in regulating the cell cycle. Several studies report nucleolar dysfunction in Alzheimer's disease (AD). Studies have reported a decrease in both total nucleolar volume and transcriptional activity of the nucleolar organizing regions. Ribosomes appear to be targeted by oxidation and reduced protein translation has been reported. In addition, several nucleolar proteins are dysregulated and some of these appear to be implicated in classical AD pathology. Some studies also suggest that the nucleolar stress response may be activated in AD, albeit this latter research is rather limited and requires further investigation. The purpose of this review is to draw the connections of all these studies together and signify that there are clear changes in the nucleolus and the ribosomes in AD. The nucleolus is therefore an organelle that requires more attention than previously given in relation to understanding the biological mechanisms underlying the disease.

Activity dependent LoNA regulates translation by coordinating rRNA transcription and methylation

The ribosome is indispensable for precisely controlling the capacity of protein synthesis. However, how translational machinery is coordinated to meet the translational demands remains elusive. Here, we identify a nucleolar-specific lncRNA (LoNA), its 5' portion binds and sequesters nucleolin to suppress rRNA transcription, and its snoRNA like 3' end recruits and diminishes fibrillarin activity to reduce rRNA methylation. Activity-dependent decrease of LoNA leads to elevated rRNA and ribosome levels, an increased proportion of polysomes, mRNA polysome loading, and protein translation. In addition, transport of ribosomes to synapses is particularly promoted, resulting in increased levels of AMPA/NMDA receptor, enhanced synaptic plasticity, long-term potentiation and consolidated memory. Strikingly, hippocampal LoNA deficiency not only enhances long-term memory in WT mice, but also restores impaired memory function in APP/PS1 transgenic mice. Together, these findings reveal the multifaceted role of LoNA in modulating ribosome biogenesis to meet the translational demands of long-term memory.

Determining the Molecular Pathways Underlying the Protective Effect of Non-Steroidal Anti-Inflammatory Drugs for Alzheimer's Disease: A Bioinformatics Approach

Alzheimer's disease (AD) represents a substantial unmet need, due to increasing prevalence in an ageing society and the absence of a disease modifying therapy. Epidemiological evidence shows a protective effect of non steroidal anti inflammatory (NSAID) drugs, and genome wide association studies (GWAS) show consistent linkage to inflammatory pathways; both observations suggesting anti-inflammatory compounds might be effective in AD therapy although clinical trials to date have not been positive. In this study, we use pathway enrichment and fuzzy logic to identify pathways (KEGG database) simultaneously affected in both AD and by NSAIDs (Sulindac, Piroxicam, Paracetamol, Naproxen, Nabumetone, Ketoprofen, Diclofenac and Aspirin). Gene expression signatures were derived for disease from both blood (n = 344) and post-mortem brain (n = 690), and for drugs from immortalised human cell lines exposed to drugs of interest as part of the Connectivity Map platform. Using this novel approach to combine datasets we find striking overlap between AD gene expression in blood and NSAID induced changes in KEGG pathways of Ribosome and Oxidative Phosphorylation. No overlap was found in non NSAID comparison drugs. In brain we find little such overlap, although Oxidative Phosphorylation approaches our pre-specified significance level. These findings suggest that NSAIDs might have a mode of action beyond inflammation and moreover that their therapeutic effects might be mediated in particular by alteration of Oxidative Phosphorylation and possibly the Ribosome pathway. Mining of such datasets might prove increasingly productive as they increase in size and richness.

Increased Protein Stability and Decreased Protein Turnover in the Caenorhabditis elegans Ins/IGF-1 daf-2 Mutant

In Caenorhabditis elegans, cellular proteostasis is likely essential for longevity. Autophagy has been shown to be essential for lifespan extension of daf-2 insulin/IGF mutants. Therefore, it can be hypothesized that daf-2 mutants achieve this phenotype by increasing protein turnover. However, such a mechanism would exert a substantial energy cost. By using classical ³⁵S pulse-chase labeling, we observed that protein synthesis and degradation rates are decreased in young adults of the daf-2 insulin/IGF mutants. Although reduction of protein turnover may be energetically favorable, it may lead to accumulation and aggregation of damaged proteins. As this has been shown not to be the case in daf-2 mutants, another mechanism must exist to maintain proteostasis in this strain. We observed that proteins isolated from daf-2 mutants are more soluble in acidic conditions due to increased levels of trehalose. This suggests that trehalose may decrease the potential for protein aggregation and increases proteostasis in the daf-2 mutants. We postulate that daf-2 mutants save energy by decreasing protein turnover rates and instead stabilize their proteome by trehalose.

Impaired APP activity and altered Tau splicing in embryonic stem cell-derived astrocytes obtained from an APPsw transgenic minipig

Animal models of familial juvenile onset of Alzheimer's disease (AD) often fail to produce diverse pathological features of the disease by modification of single gene mutations that are responsible for the disease. They can hence be poor models for testing and development of novel drugs. Here, we analyze in vitro-produced stem cells and their derivatives from a large mammalian model of the disease created by overexpression of a single mutant human gene (APPsw). We produced hemizygous and homozygous radial glial-like cells following culture and differentiation of embryonic stem cells (ESCs) isolated from embryos obtained from mated hemizygous minipigs. These cells were confirmed to co-express varying neural markers, including NES, GFAP and BLBP, typical of type one radial glial cells (RGs) from the subgranular zone. These cells had altered expression of CCND1 and NOTCH1 and decreased expression of several ribosomal RNA genes. We found that these cells were able to differentiate into astrocytes upon directed differentiation. The astrocytes produced had decreased α- and β-secretase activity, increased γ-secretase activity and altered splicing of tau. This indicates novel aspects of early onset mechanisms related to cell renewal and function in familial AD astrocytes. These outcomes also highlight that radial glia could be a potentially useful population of cells for drug discovery, and that altered APP expression and altered tau phosphorylation can be detected in an in vitro model of the disease. Finally, it might be possible to use large mammal models to model familial AD by insertion of only a single mutation.

Summary: Insight into astrocyte and radial glia pathology in an in vitro culture system derived from the APPsw pig.

In vitro caloric restriction induces protective genes and functional rejuvenation in senescent SAMP8 astrocytes

Astrocytes are key cells in brain aging, helping neurons to undertake healthy aging or otherwise letting them enter into a spiral of neurodegeneration. We aimed to characterize astrocytes cultured from senescence-accelerated prone 8 (SAMP8) mice, a mouse model of brain pathological aging, along with the effects of caloric restriction, the most effective rejuvenating treatment known so far. Analysis of the transcriptomic profiles of SAMP8 astrocytes cultured in control conditions and treated with caloric restriction serum was performed using mRNA microarrays. A decrease in mitochondrial and ribosome mRNA, which was restored by caloric restriction, confirmed the age-related profile of SAMP8 astrocytes and the benefits of caloric restriction. An amelioration of antioxidant and neurodegeneration-related pathways confirmed the brain benefits of caloric restriction. Studies of oxidative stress and mitochondrial function demonstrated a reduction of oxidative damage and partial improvement of mitochondria after caloric restriction. In summary, caloric restriction showed a significant tendency to normalize pathologically aged astrocytes through the activation of pathways that are protective against the age-related deterioration of brain physiology.

Identification of radiation-induced expression changes in nonimmortalized human T cells

In the event of a radiation accident or attack, it will be imperative to quickly assess the amount of radiation exposure to accurately triage victims for appropriate care. RNA-based radiation dosimetry assays offer the potential to rapidly screen thousands of individuals in an efficient and cost-effective manner. However, prior to the development of these assays, it will be critical to identify those genes that will be most useful to delineate different radiation doses. Using global expression profiling, we examined expression changes in nonimmortalized T cells across a wide range of doses (0.15-12 Gy). Because many radiation responses are highly dependent on time, expression changes were examined at three different times (3, 8, and 24 h). Analyses identified 61, 512 and 1310 genes with significant linear dose-dependent expression changes at 3, 8 and 24 h, respectively. Using a stepwise regression procedure, a model was developed to estimate in vitro radiation exposures using the expression of three genes (CDKN1A, PSRC1 and TNFSF4) and validated in an independent test set with 86% accuracy. These findings suggest that RNA-based expression assays for a small subset of genes can be employed to develop clinical biodosimetry assays to be used in assessments of radiation exposure and toxicity.

Insulin and insulin-like growth factor resistance in alcoholic neurodegeneration

BACKGROUND

Chronic alcohol feeding of adult Long Evans rats causes major central nervous system abnormalities that link neuronal loss and impaired acetylcholine homeostasis to ethanol inhibition of insulin and insulin-like growth factor (IGF) signaling and increased oxidative stress.

OBJECTIVES

We now characterize the integrity of insulin and IGF signaling mechanisms and assess molecular indices of neurodegeneration in the cerebellar vermis and anterior cingulate gyrus of human alcoholics.

RESULTS

Alcoholic cerebella had increased neuronal loss, gliosis, lipid peroxidation, and DNA damage relative to control. Quantitative RT-PCR studies demonstrated reduced expression of insulin, insulin receptor and IGF-II receptor in the anterior cingulate, and reduced expression of insulin, IGF-I, and their corresponding receptors in the vermis. Competitive equilibrium binding assays revealed significantly reduced specific binding to the insulin, IGF-I, and IGF-II receptors in both the anterior cingulate and vermis of alcoholic brains. These effects of chronic alcohol abuse were associated with significantly reduced expression of choline acetyltransferase, which is needed for acetylcholine biosynthesis.

CONCLUSIONS

The results suggest that alcoholic neurodegeneration in humans is associated with insulin and IGF resistance with attendant impairment of neuronal survival mechanisms and acetylcholine homeostasis.

Nuclear bodies in neurodegenerative disease

Neurodegenerative diseases are characterized by a relentlessly progressive loss of the functional and structural integrity of the central nervous system. In many cases, these diseases arise sporadically and the causes are unknown. The abnormal aggregation of protein within the cytoplasm or the nucleus of brain cells represents a unifying pathological feature of these diseases. There is increasing evidence for nuclear dysfunction in neurodegenerative diseases. How this relates to protein aggregation in the context of "cause and effect" remains to be determined in most cases. Co-ordinated nuclear function is predicated on the activity of distinct nuclear subdomains, or nuclear bodies, each responsible for a specific function. If nuclear dysfunction represents an important etiopathological feature in neurodegenerative disease, then this should be reflected by functional and/or morphological alterations in this nuclear compartmentalization. For most neurodegenerative diseases, evidence for nuclear dysfunction, with attendant consequences for nuclear architecture, is only beginning to emerge. In this review, I will discuss neurodegenerative diseases in the context of nuclear dysfunction and, more specifically, alterations in nuclear bodies. Although research in this field is in its infancy, identifying alterations in the nucleus in neurodegenerative disease has potentially profound implications for elucidating the pathogenesis of these disorders.

Quality control for microarray analysis of human brain samples: The impact of postmortem factors, RNA characteristics, and histopathology

The quality of results from microarray studies depends on RNA quality, which can be significantly influenced by postmortem factors. The aim of this study was to determine which postmortem factors and/or RNA electropherogram characteristics best correspond to microarray output and can be used to prospectively screen RNA prior to microarray analysis. Total RNA was extracted (N=125) from gray and white matter of postmortem frontal and occipital lobe tissue, acquired from normal controls, and patients with schizophrenia, bipolar disorder or major depression. Electropherograms were generated by the Agilent BioAnalyzer 2100, allowing calculation of the 28S/18S ratio, the 18S/baseline peak ratio and the RNA Integrity Number (RIN). These values were compared to post-hybridization image analysis of Affymetrix microarrays. The postmortem variables correlated with some quality measures but could not be used as effective screening tools. Logistic regression demonstrated that all three electropherogram measures were predictive for microarray quality, and that the RIN threshold predictive of "good quality" (>35% present calls) was most consistent with that of prior studies. The optimal RIN must be determined by the investigator's specifications for false inclusion and false exclusion. In contrast to RIN, the quality threshold for the 28S/18S ratio has proven unacceptably variable, due to sensitivity to slight differences in protocol and/or tissue source. In conclusion, the measures we found useful as screening criteria do not replace the need to exclude samples after a microarray analysis is performed, as an acceptable percent call rate and other measures of microarray quality represent the desired endpoint.

Evaluation of the nucleolar organizer regions in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder in middle and late age. Ribosomal RNA (rRNA) genes are located in the nucleolus (nucleolar organizer regions = NORs). There are increased deposits of beta-amyloid protein in the brains of the patients with AD and aged individuals with Down's syndrome (DS). The beta-amyloid gene is located in the acrocentric chromosome 21 that is responsible for rRNA synthesis. Therefore, it is possible that there is a relationship between ribosomal genes and AD.

OBJECTIVE

To investigate the activities of ribosomal genes of AD patients by comparing the activities of NORs in AD patients and healthy controls with the silver-staining method.

METHODS

NOR surface/the total nucleus surface proportions in interphase nuclei, and silver stainability and satellite association (SA) of acrocentric chromosomes in the metaphases of cultivated lymphocytes of 20 AD patients and 20 healthy controls (10 elderly and 10 young) were evaluated.

RESULTS

A decrease in NOR surface/total nucleus surface proportions has been observed in the interphase nucleus of AD patients when compared with elderly controls (p = 0.035). When compared with the sizes of Ag+ segments of acrocentric chromosomes of AD patients and control groups, the Ag-staining size 1 of the chromosome 22 of AD patients was found to be more increased than that of the young controls (p = 0.018). There was no statistically significant difference between AD patients and control groups regarding the number of Ag+ acrocentric chromosomes, Ag+ chromosome 21 and SA frequency (p > 0.05). It has been found that there is only a slight increase in the total number of chromosomes in SA in AD patients when compared with elderly controls (p = 0.05).

CONCLUSION

The decrease in NOR surface/total nucleus surface proportions of AD patients may indicate a reduction in the activity of the ribosomal genes of these patients.

Towards standardization of RNA quality assessment using user-independent classifiers of microcapillary electrophoresis traces

While it is universally accepted that intact RNA constitutes the best representation of the steady-state of transcription, there is no gold standard to define RNA quality prior to gene expression analysis. In this report, we evaluated the reliability of conventional methods for RNA quality assessment including UV spectroscopy and 28S:18S area ratios, and demonstrated their inconsistency. We then used two new freely available classifiers, the Degradometer and RIN systems, to produce user-independent RNA quality metrics, based on analysis of microcapillary electrophoresis traces. Both provided highly informative and valuable data and the results were found highly correlated, while the RIN system gave more reliable data. The relevance of the RNA quality metrics for assessment of gene expression differences was tested by Q-PCR, revealing a significant decline of the relative expression of genes in RNA samples of disparate quality, while samples of similar, even poor integrity were found highly comparable. We discuss the consequences of these observations to minimize artifactual detection of false positive and negative differential expression due to RNA integrity differences, and propose a scheme for the development of a standard operational procedure, with optional registration of RNA integrity metrics in public repositories of gene expression data.

Evaluating RNA status for RT-PCR in extracts of postmortem human brain tissue

Total RNA was extracted from 105 individual postmortem human brain samples representing a range of postmortem conditions. To improve upon parameters currently used to screen for RNA quality, electropherogram patterns generated by the Agilent Bioanalyzer 2100 were compared to the average score in random hexamer-primed reverse transcription real-time PCR for four housekeeping genes in each RNA sample. The ribosomal ratio (28S to 18S) was found to be unrelated to the housekeeping gene score (r = -0.06; P = 0.50), and there was no threshold value in the ratio that could be applied to effectively categorize the RNA degradation. Although the housekeeping gene score correlated significantly with the percentage of area in the electropherogram corresponding to moderate to high molecular weight intact mRNA (r = 0.41; P = 0.0001), the best discriminator was determined to be the ratio of the 18S peak height to the highest peak in the tRNA to 18S rRNA baseline. Applying a lower boundary of 2.12 for the ratio allowed for the screening out of samples with the lowest housekeeping gene scores without excluding better-quality samples. This measure represents a marked improvement over the 28S to 18S ratio, which proved to be a misleading indicator of the state of the mRNA for use in random hexamer-primed reverse transcription PCR.

Selective labeling and detection of specific RNAs in an RNA mixture

We report here a unique approach to selectively label and detect specific RNA in an RNA mixture (without separation or purification) using DNA polymerase, dNTP labels, and a short synthetic DNA template complementary to the 3(')-terminus of the RNA. The detection sensitivity is high, at attomole level (10-18 mole). The selective principle was demonstrated by individually labeling and detecting RNAs in a RNA mixture when different templates were provided. By taking advantage of the template-directed selectivity, poly(A) tail-containing mRNA in total RNA was detected and labeled at the 3(')-terminal on a poly(T) template. Nonradioactive labels, such as fluorophore and antigen labels, may also be used; this method can be applied in methodology for direct detection and quantification of viral RNAs.

Alterations of chaperone protein expression in presenilin mutant neurons in response to glutamate excitotoxicity

Mutations in the presenilin-1 (PS1) gene underlie the most common form of familial dementia of the Alzheimer type (DAT). We demonstrated previously that the expression of PS1 with a M146V mutation in transgenic mice potentiates glutamate toxicity to neurons, due to an altered calcium homeostasis. Here, using a subtractive cDNA library approach, we report the identification of several genes, the altered expression of which may be associated with this unique PS1-related vulnerability to glutamate. The identified genes, including chaperonin subunit 2 and nucleophosmin 1/B23, are involved in the intracellular trafficking of proteins and ions. Northern blot analysis revealed that the effect of glutamate on calcium-binding proteins was augmented in neurons from PS1 mutation mice, compared with neurons from mice lacking other genes relevant to the pathogenesis of DAT (FE65 and APOE) or neurons from control wild-type mice. Interestingly, mRNA for two chaperone proteins were expressed at lower levels specifically in neurons from PS1 mutant mice. These findings suggest that PS1 mutations may, in part, contribute to the development of DAT via altered expression of chaperone proteins.

Quantitative evaluation of the rRNA in Alzheimer's disease

The ribosomal RNA (rRNA) genes are located in nucleolus during active transcription and are transcribed by RNA polymerase I. This group of genes is involved in transcription and translation processes which can modulate gene expression. The association between rRNA levels and aging has been reported. In the present study, we investigated the ratio of mature rRNA 28S and 18S in peripheral blood of 15 Alzheimer's disease (AD) patients, 15 elderly healthy controls and 15 healthy young controls. Our results showed a statistically significant decrease of the mature rRNA 28S/18S ratio in AD patients when compared with the elderly and young control groups. Thus we can suggest that there is a possible change in the transcriptional or maturation process or a preferential degradation of the 28S subunit in AD.

Age-associated DNA damage is accelerated in the senescence-accelerated mice

We investigated how the DNA status correlates with the aging process in organisms, in different organs and in tissues using two inbred strains of mice, which are genetically related but have different senescence patterns. The SAMP1 mice belong to an accelerated senescence-prone and short lived strain, the other, SAMR1 mice are from an accelerated senescence-resistant and long lived strain. Using the alkaline filter elution technique, pieces of tissues from six organs: lung, intestine, liver, brain, muscle, and heart have been examined for DNA damage, mainly DNA single strand breaks. It was shown that in newborns the DNA damage is minimal, and it was increased significantly with calendric age in all organs in both strains. Although the correlation of DNA damage with aging differed in the different six organs, damage was significantly higher in SAMP1 mice than SAMR1 mice at later life in all organs. This is another remarkable example for the strong correlation of DNA damage and aging process, especially with senescence acceleration.

Doença de Alzheimer

O artigo apresenta uma revisão sucinta dos aspectos genéticos da doença de Alzheimer e da metodologia empregada. Três genes distintos foram responsabilizados pela afecção até o momento: o da APP<FONT FACE="Symbol">-</FONT> responsável pela substância precursora da b-amilóide, a qual se deposita intensamente no cérebro dos afetados e está associada ao quadro demencial <FONT FACE="Symbol">-</FONT>, o gene da presenilina 1 (PS1) e o da presenilina 2 (PS2), proteínas de membrana celular. O gene da PS1 é responsável por cerca de 40% dos casos familiares e de acometimento precoce da DA . Os genes da ApoE4, da a-2-macroglobulina e da catepsina D, envolvidos no metabolismo da b-amilóide, foram caracterizados como fatores de risco para a DA. O gene da ApoE4 é fator de risco em cerca de 50% dos casos de DA esporádicos e de acometimento tardio. Muitos outros genes foram ainda associados à DA e são apresentados brevemente. São discutidas a conduta <FONT FACE="Symbol">-</FONT> quanto ao aconselhamento genético para familiares de afetados <FONT FACE="Symbol">-</FONT> e a utilização de diagnóstico molecular na predisposição genética à afecção. É apresentado como mecanismo comum às síndromes progeróides genéticas, como a DA, a alteração da atividade dos genes ribossômicos.