[The nucleolus: structure, functions, amd associated diseases]

Inheriting nuclear organization: can nuclear lamins impart spatial memory during post-mitotic nuclear assembly?

Cell type and tissue architecture correlate with genome organization in higher eukaryotes, and structural nuclear landmarks are faithfully transmitted from one cell generation to the next. However, how nuclear components find their place in the nucleus after mitosis is still a matter of debate. As the major structural proteins within nuclei, the nuclear lamins are good candidates to re-establish nuclear compartments following mitosis. Human cells with reduced expression of the major B-type lamin protein, lamin B1, were generated using RNA interference. Mitotic and nuclear assembly phenotypes were then visualized in both fixed and living cells. Mitotic defects in lamin B1-depleted cells correlated with a general deterioration in nuclear compartmentalization and chromatin structure, frequent failure of chromosome segregation, and profound disorganization of centromeres. Examination of cells with normal lamin B1 expression indicated that small lamin B1 foci remain associated with major nuclear compartments--chromatin, nucleoli, and nuclear speckles--during an unperturbed mitosis. Our experiments show that normal lamin B1 expression is required for successful cell division and provide preliminary evidence that lamin B1-containing remnants of the interphase nucleoskeleton persist throughout mitosis. We suggest that these residual structures provide landmarks that are targeted during nuclear reassembly to allow key features of nuclear organization to be inherited from one cell cycle to the next.

[<< Catch me if you can >>: how the structural and functional integrity of eukaryotic RNA molecules is monitored by surveillance mechanisms]

Cellular RNAs are invariably organized in ribonucleoprotein particles, or RNPs, regardless of their size, structure or function. RNPs are monitored by active surveillance mechanisms for their structural and functional integrity at every single step of their "life". A limited number of key endoRNase and/or exoRNase activities are recruited to multiple metabolic pathways by specific adaptors. These trans-acting factors are often endowed with synthesis activities in the formation of mature RNA termini, as well as degradation and surveillance activities. Quality control mechanisms are robust because they are partially redundant. The actual mechanisms that discriminate aberrant RNAs from normal RNAs are still loosely defined. Surveillance is essential to cellular homeostasis and has been linked to several human diseases.

Lamin B1 maintains the functional plasticity of nucleoli

The dynamic ability of genomes to interact with discrete nuclear compartments appears to be essential for chromatin function. However, the extent to which structural nuclear proteins contribute to this level of organization is largely unresolved. To test the links between structure and function, we evaluated how nuclear lamins contribute to the organization of a major functional compartment, the nucleolus. HeLa cells with compromised expression of the genes encoding lamins were analyzed using high-resolution imaging and pull-down assays. When lamin B1 expression was depleted, inhibition of RNA synthesis correlated with complex structural changes within the nucleolar active centers until, eventually, the nucleoli were dispersed completely. With normal lamin expression, the nucleoli were highly plastic, with dramatic and freely reversible structural changes correlating with the demand for ribosome biogenesis. Preservation of the nucleolar compartment throughout these structural transitions is shown to be linked to lamin B1 expression, with the lamin B1 protein interacting with the major nucleolar protein nucleophosmin/B23.

[Diamond-Blackfan anemia reveals the dark side of ribosome biogenesis]

Diamond-Blackfan anemia (DBA), a rare congenital erythroblastopenia, has recently become a paradigm for a growing set of genetic diseases linked to mutations in genes encoding ribosomal proteins or factors involved in ribosome biogenesis. Recent studies of the structure and the function of ribosomal proteins affected in DBA indicate that their mutation in DBA primarily impacts ribosome biogenesis. Accordingly, cells from DBA patients display anomalies in the maturation of ribosomal RNAs. The explanation of this unexpected link between ribosome biogenesis, a ubiquitous process, and a disease mostly affecting erythroid differentiation may stem in part from the emerging concept of ribosomal stress response, a signaling pathway triggering cell cycle arrest in response to a defect in ribosome synthesis. Future studies of DBA and other diseases related to defects in ribosome biogenesis are likely to rapidly provide important insights into the regulatory mechanisms linking cell cycle progression to this major metabolic pathway.

AgNOR status in Down's syndrome infants and a plausible phenotype formation hypothesis

Down's syndrome (DS) or trisomy 21 is the most frequent genetic birth defect associated with mental retardation. Although DS has been known for more than a 100 years and its chromosomal basis recognized for half a century (1959), the underlying patho-mechanisms for the phenotype formation remain elusive and cannot be fully explained by simple gene dosage effect. The general consensus is that the extra chromosome 21 genes perturb the global metabolism of the body cells. Our experiments show that the most prominent metabolic perturbation occurs during ribosome biogenesis in the cells of DS babies/infants. In humans, ribosomal RNA (rRNA) gene families or nucleolar organizer regions (NORs) are localized at the secondary constriction (on the satellite stalks) of five pairs of acrocentric chromosomes (13, 14, 15, 21 and 22) and their activities are evaluated specifically either in metaphase or interphase through a procedure known as AgNOR or silver staining. Our successive AgNOR studies, supported by RNA and nuclear protein measurement, show that cells from DS infants produce more ribosomes than expected, accounting for the extra set of active rRNA gene family (1/6-1/11) situated on the extra chromosome 21. Thus, the presence of an extra chromosome 21 stimulates a global increase in ribosome biogenesis in cooperation with other NOR-bearing chromosomes, causing unnecessary rRNA and ribosomal proteins synthesis compared to controls. Following the description of NORs, AgNOR, AgNOR-proteins, AgNOR measurement and our experimental results, we propose that the extra RNA and protein synthesis can cause a fundamental handicap to DS infants, contributing to the formation of DS phenotypes, due to the wasted energy in producing unnecessary macromolecules, including energy (GTP)-dependent transport of the excessive ribosomes from the nucleus to the cytoplasm.

The KH-Tudor Domain of A-Kinase Anchoring Protein 149 Mediates RNA-Dependent Self-Association

A-Kinase anchoring proteins (AKAPs) control the subcellular localization and temporal specificity of protein phosphorylation mediated by cAMP-dependent protein kinase. AKAP149 (AKAP1) is found in mitochondria and in the endoplasmic reticulum-nuclear envelope network where it anchors protein kinases, phosphatases, and a phosphodiesterase. AKAP149 harbors in its COOH-terminal part one KH and one Tudor domain, both known to be involved in RNA binding. We investigated the properties of the COOH-terminal domain of AKAP149. We show here that AKAP149 is a self-associating protein with RNA binding features. The KH domain of AKAP149 is sufficient for self-association in a RNA-dependent manner. The Tudor domain is not necessary for self-association, but it is required together with the KH domain for targeting to well-defined nuclear foci. These foci are spatially closely related to nucleolar subcompartments. We also show that the KH-Tudor-containing domain of AKAP149 binds RNA in vitro and in RNA coprecipitation experiments. AKAP149 emerges as a scaffolding protein involved in the integration of intracellular signals and possibly in RNA metabolism.

NOR Expression increases in interphase lymphocytes of Down syndrome babies/children as AgNORs surface, according to the mitogen concentration in the culture medium

The extra chromosome 21 of Down syndrome (DS) or trisomy 21 patients contains an average of 40 extra copies of rRNA genes and the in vivo/in vitro regulation of the activity of these genes is not fully understood. The objective of this work was to compare the NORs expression pattern in interphase lymphocytes of DS patients with regular trisomy 21 and control individuals according to phytohemagglutinin (PHA) concentration (0.37, 0.75, 1.48 and 2.21 ml) per 100 ml of medium. Because the AgNOR staining is an indicator of the active rRNA genes, comparison of the image analysis values of the AgNOR area in 72 h cultivated lymphocytes for each concentration of PHA between DS patients (N=30) and controls (N=24) provided a plausible conclusion on the regulation of the extra rRNA genes in DS lymphocytes. The nucleolus organizer regions area/total nuclear area (NORa/TNa) was calculated using an in-house computer program. Fifty consecutive interphases per PHA concentration were analysed for each individual, for determination of the NORa/TNa. In contrast to healthy controls, NORa/TNa of lymphocytes from DS patient babies/children (0-8 years old) increased gradually in parallel with the PHA concentration in the culture medium: 10.44+/-1.72% for 0.37 ml of PHA, 11.74+/-1.93% for 0.75 ml of PHA, 13.25+/-2.03% for 1.48 ml of PHA and 13.43+/-2.08% for 2.21 ml of PHA per 100 ml of medium. Contrary to control cells (in which the NORa/TNa ratio according to PHA concentration in the culture medium remains constant), DS interphase lymphocytes in culture do not down-regulate their NOR expression. These results obtained from interphase NORs are consistent with the previous results obtained by evaluating the mean of AgNOR+ chromosome number in metaphase cells, also in relation to the mitogen concentration in the culture medium.

Age-dependent decreases in mitogen-stimulation level and RNA content in peripheral blood mononuclear cells of down syndrome patients

OBJECTIVE

The aim of the present study was to determine whether or not the phytohemagglutinin (PHA)-activated proliferation and average RNA content in peripheral blood mononuclear cells (PBMC) of Down syndrome (DS) patients change with age.

METHOD

Stimulated portion of PBMC and total RNA levels in these cells after 72 h of PHA stimulation from 38 DS patients were compared with 28 age-matched healthy controls using flow cytometric measurement.

RESULTS

Decreased ratio of PBMC from DS patients undergoes mitogenic stimulation with age (r = -0.84, P = 0.000). This decrease is not observed in the cells of control individuals (r = 0.03, P = 0.869). Stimulated PBMC in infants with DS have higher level of RNA contents compared to controls (Z = 2.227, P = 0.026). While RNA content in mitogen-stimulated PBMC of DS decreased progressively and significantly with age (r = -0.70, P = 0.000), no significant age-related change in RNA content was found among the cells of healthy individuals in the range of 0-27 year old (r = 0.275, P = 0.157, P > 0.05).

CONCLUSION

Age-dependent decreases in mitogen-activated proliferation ratio and average RNA content of PBMC from DS patients appear as regular events. These results may contribute to the explanation of the immune deficiency seen in DS patients since the PHA-stimulated cells are principally T-lymphocytes. This is the first report on the decrease in PHA-stimulated proliferation ratio (stimulability) and RNA level in PBMC of DS patients in relation to age.

Effects of Feed Restriction on the Nucleolar Structure and Function in Lymphocytes

Since malnutrition leads to adverse effects of several drugs on haemopoiesis and blood morphology, we tested if malnutrition itself could affect the nucleolar structure and function in rat lymphocytes. We report changes in the proportion of different nucleolar types that were dependent on the severity of feed restriction. The observed changes resemble those seen with cytostatics suggesting a possible link between feed restriction and haemotoxicity.

Higher NORs-expression in lymphocyte of trisomy 21 babies/children: In vivo evaluation

Extra chromosome 21 of Down syndrome (DS) or trisomy 21 patients contains an average of 40 extra copies of rRNA genes and the in vivo regulation of these genes is not known. The objective of this work is to compare the NORs expression in interphase nuclei in non-stimulated lymphocytes of DS patients and healthy controls. Because the AgNOR staining is the indicator of the active rRNA genes, comparison of the image analysis values of AgNORs area between DS's and healthy controls' interphase lymphocytes is considered to be sufficient to evaluate the level of rDNA activities in the two groups. The Nucleolus Organizer Regions area/Total Nuclear area (NORa/TNa) was calculated using a computer program designed by us. 100 consecutive NORa/TNa per individual were evaluated. We report that 24 DS children's peripheral lymphocytes show significantly higher NORa/TNa mean value (6.32 +/- 1.77%) than that of the 20 healthy controls' cells (5.31 +/- 1.34%) (2-tailed Mann-Whitney U test, z = 19.4, P = 0.000). The same is true for the nucleolus (AgNOR spot) number per nucleus. The mean value of nucleoli number per nucleus in DS lymphocytes was significantly higher than that of the controls: z = 14.6, P = 0.000. In conclusion, extra rRNA genes on the chromosome 21 are not down-regulated in DS patients' lymphocytes. Rather, extra NORs expressions in 'in vivo' condition contribute to the increase of AgNORs area and AgNOR spots number per nucleus. This is the first work on the comparison of NORs activities in resting (non-stimulated) interphase lymphocytes between DS and healthy controls.

NOR expression increases on metaphase chromosomes of down syndrome lymphocytes in concordance with mitogen concentration in culture medium

BACKGROUND

Regulation of nucleolus organizer region (NOR) expression in trisomy 21 (Down syndrome [DS]) cells is not fully explained. This work compared NOR expression on metaphase chromosomes in gradiently stimulated lymphocytes from DS patients with those from healthy controls.

METHOD

Conventional peripheral blood culture (72 h) and chromosomal preparation procedures were used except that blood samples from each individual were cultivated in the same but gradiently increasing concentrations (0.37, 0.75, 1.48, and 2.21 ml) of phytohemagglutinin (PHA) per 100 ml of medium. One hundred consecutive metaphases per concentration were analyzed for scoring the means of the active NORs bearing chromosomes (AgNOR+ chromosome) per individual and per concentration.

RESULTS

In contrast to healthy controls (n=24), AgNOR+ chromosomal number in lymphocytes from 30 DS patients increased in concordance to the gradient of PHA concentration in the culture medium.

CONCLUSION

DS lymphocytes do not downregulate their NOR expression in the limit of control cells. This in vitro result may serve as a clue for the explanation of the DS phenotype due to the wasted energy in producing unnecessary rRNA transcripts and AgNOR proteins in utero during organogenesis. These results also indicate that precautions must be used in routine work of NOR evaluation/interpretation in DS lymphocytes.

Flow cytometric comparison of RNA content in peripheral blood mononuclear cells of down syndrome patients and control individuals

OBJECTIVE

Trisomy 21 or Down syndrome (DS) is the most common genetic cause of mental retardation associated with the immunologic and other known defects. Extra chromosome 21 of DS patients contains an average of 40 extra copies of rRNA genes and the in vivo regulation of these genes' activity is not known. Because over 80% of total cellular RNA is rRNA, the measurement of total cellular RNA provides information on rRNA content. The aim of this work was to determine whether or not the additional chromosome 21 causes any increase in total cellular RNA content in mononuclear cells from peripheral blood (PBMNCs) of these patients and whether or not this content is modified with age.

METHOD

PBMNCs of 48 patients with DS and 48 healthy controls were studied. RNA content of isolated PBMNCs was evaluated by flow cytometric measurements.

RESULTS

Average RNA content of younger DS patients' cells was significantly higher than that of healthy controls (P=0.003). Furthermore, the RNA content decreased significantly with increasing age of DS patients (r=-0.377, P=0.008) in the range of 0-26 year old, whereas no significant relationship was found between age and PBMNCs' RNA content of healthy controls in the same range of ages.

CONCLUSION

RNA content of PBMNCs from DS patients decreases rapidly with age. This is the first work on the age-dependent decrease of the RNA content in PBMNCs of DS patients.