Stability of the small gamma-tubulin complex requires HCA66, a protein of the centrosome and the nucleolus

Validating RRP12 Expression and Its Prognostic Significance in HCC Based on Data Mining and Bioinformatics Methods

RRP12 (ribosomal RNA processing 12 homolog) is a nucleolar protein involved in the maturation and transport of eukaryotic ribosomal subunits and is a type of RNA binding protein. In recent years, considerable research has indicated that RRP12 is associated with the occurrence and development of multiple cancers. However, there is no research on RRP12 in hepatocellular carcinoma. Herein, we aimed to explore the role and significance of RRP12 in hepatocellular carcinoma.We used the TIMER and GEPIA databases to perform pan-cancer analyses of RRP12. The impact of RRP12 on the prognosis was analyzed through the GEPIA database. The relationship between RRP12 and immune cell infiltration was investigated by TIMER and GEPIA databases. Moreover, the expression of RRP12 in various liver cancer cells was evaluated by Western Blot to determine the cell line for the next experiment. Scratch test, Transwell test, and Edu tests were applied to validate the effects of RRP12 on the function of liver cancer cells. And the data were statistically analyzed.Pan-cancer analysis found that RPP12 was significantly upregulated in many cancers. Moreover, the prognostic analysis revealed that the difference in the expression of RRP12 has statistical significance for the overall survival rate and disease-free survival rate of liver cancer patients. In order to analyze the correlation between the expression level of RRP12 and clinical parameters, it was found that there was a significant negative correlation with tumor stage, tumor grade and tumor size. Univariate and multivariate analysis showed that RRP12 could be used as an independent prognostic factor for patients with hepatocellular carcinoma. Cellular experiments have proved that knocking down RRP12 can inhibit the proliferation, invasion, and metastasis of liver cancer cells.Therefore, RRP12 significantly affects the occurrence and development of HCC. Hence, RRP12 can become a potential target and prognostic biomarker for the treatment of hepatocellular carcinoma.

Cyanidin 3-glucoside modulated cell cycle progression in liver precancerous lesion, in vivo study

BACKGROUND

Cyanidin-3-O-glucoside (cyan) exhibits antioxidant and anticancer properties. The cell cycle proteins and antimitotic drugs might be promising therapeutic targets in hepatocellular carcinoma.

AIM

To investigate the effect of cyan administration on cell cycle in hepatic precancerous lesion (PCL) induced by diethylnitrosamine/2-acetylaminofluorene (DEN/2-AAF) in Wistar rats.

METHODS

In vivo, DEN/2-AAF-induced hepatic PCL, rats were treated with three doses of cyan (10, 15, and 20 mg/kg/d, for four consecutive days per week for 16 wk). Blood and liver tissue samples were collected for measurement of the followings; alpha fetoprotein (AFP) liver function and RNA panel differential expression was evaluated via real time polymerase chain reaction. Histopathological examination of liver sections stained with H&E and immunohistochemical study using glutathione S-transferase placental (GSTP) and proliferating cell nuclear antigen (PCNA) antibodies were assessed.

RESULTS

Cyan administration mitigated the effect of DEN/2-AFF induced PCL, decreased AFP levels, and improved liver function. Remarkably, treatment with cyan dose dependently decreased the long non-coding RNA MALAT1 and tubulin gamma 1 mRNA expressions and increased the levels of miR-125b, all of which are involved in cell cycle and mitotic spindle assembly. Of note, cyan decreased GSTP foci percent area and PCNA positively stained nuclei.

CONCLUSION

Our results indicated that cyan could be used as a potential therapeutic agent to inhibit liver carcinogenesis in rat model via modulation of cell cycle.

Neurymenolide A, a Novel Mitotic Spindle Poison from the New Caledonian Rhodophyta Phacelocarpus neurymenioides

The marine α-pyrone macrolide neurymenolide A was previously isolated from the Fijian red macroalga, Neurymenia fraxinifolia, and characterized as an antibacterial agent against antibiotic-resistant strains that also exhibited moderate cytotoxicity in vitro against cancer cell lines. This compound was also shown to exhibit allelopathic effects on Scleractinian corals. However, to date no mechanism of action has been described in the literature. The present study showed, for the first time, the isolation of neurymenolide A from the New Caledonian Rhodophyta, Phacelocarpus neurymenioides. We confirmed the compound’s moderate cytotoxicity in vitro against several human cell lines, including solid and hematological malignancies. Furthermore, we combined fluorescence microscopy and flow cytometry to demonstrate that treatment of U-2 OS osteosarcoma human cells with neurymenolide A could block cell division in prometaphase by inhibiting the correct formation of the mitotic spindle, which induced a mitotic catastrophe that led to necrosis and apoptosis. Absolute configuration of the stereogenic center C-17 of neurymenolide A was deduced by comparison of the experimental and theoretical circular dichroism spectra. Since the total synthesis of this compound has already been described, our findings open new avenues in cancer treatment for this class of marine molecules, including a new source for the natural product.

Regulation of microtubule nucleation mediated by γ-tubulin complexes

The microtubule cytoskeleton is critically important for spatio-temporal organization of eukaryotic cells. The nucleation of new microtubules is typically restricted to microtubule organizing centers (MTOCs) and requires γ-tubulin that assembles into multisubunit complexes of various sizes. γ-Tubulin ring complexes (TuRCs) are efficient microtubule nucleators and are associated with large number of targeting, activating and modulating proteins. γ-Tubulin-dependent nucleation of microtubules occurs both from canonical MTOCs, such as spindle pole bodies and centrosomes, and additional sites such as Golgi apparatus, nuclear envelope, plasma membrane-associated sites, chromatin and surface of pre-existing microtubules. Despite many advances in structure of γ-tubulin complexes and characterization of γTuRC interacting factors, regulatory mechanisms of microtubule nucleation are not fully understood. Here, we review recent work on the factors and regulatory mechanisms that are involved in centrosomal and non-centrosomal microtubule nucleation.

Emerging genotype-phenotype relationships in patients with large NF1 deletions

The most frequent recurring mutations in neurofibromatosis type 1 (NF1) are large deletions encompassing the NF1 gene and its flanking regions (NF1 microdeletions). The majority of these deletions encompass 1.4-Mb and are associated with the loss of 14 protein-coding genes and four microRNA genes. Patients with germline type-1 NF1 microdeletions frequently exhibit dysmorphic facial features, overgrowth/tall-for-age stature, significant delay in cognitive development, large hands and feet, hyperflexibility of joints and muscular hypotonia. Such patients also display significantly more cardiovascular anomalies as compared with patients without large deletions and often exhibit increased numbers of subcutaneous, plexiform and spinal neurofibromas as compared with the general NF1 population. Further, an extremely high burden of internal neurofibromas, characterised by >3000 ml tumour volume, is encountered significantly, more frequently, in non-mosaic NF1 microdeletion patients than in NF1 patients lacking such deletions. NF1 microdeletion patients also have an increased risk of malignant peripheral nerve sheath tumours (MPNSTs); their lifetime MPNST risk is 16–26%, rather higher than that of NF1 patients with intragenic NF1 mutations (8–13%). NF1 microdeletion patients, therefore, represent a high-risk group for the development of MPNSTs, tumours which are very aggressive and difficult to treat. Co-deletion of the SUZ12 gene in addition to NF1 further increases the MPNST risk in NF1 microdeletion patients. Here, we summarise current knowledge about genotype–phenotype relationships in NF1 microdeletion patients and discuss the potential role of the genes located within the NF1 microdeletion interval whose haploinsufficiency may contribute to the more severe clinical phenotype.

Apaf-1: Regulation and function in cell death

Apoptosis, a form of programmed cell death, is responsible for eliminating damaged or unnecessary cells in multicellular organisms. Various types of intracellular stress trigger apoptosis by induction of cytochrome c release from mitochondria into the cytosol. Apoptotic protease activating factor-1 (Apaf-1) is a key molecule in the intrinsic or mitochondrial pathway of apoptosis, which oligomerizes in response to cytochrome c release and forms a large complex known as apoptosome. Procaspase-9, an initiator caspase in the mitochondrial pathway, is recruited and activated by the apoptosome leading to downstream caspase-3 processing. Various cellular proteins and small molecules can modulate apoptosome formation and function directly or indirectly. Despite recent progress in understanding the mitochondrial pathway of apoptosis, numerous questions such as the molecular mechanism of Apaf-1 oligomerization and caspase-9 activation remain poorly understood. In addition, reports have emerged showing non-apoptotic functions for Apaf-1. The current review summarizes the latest findings regarding structure-function relationship of Apaf-1 as well as its modifiers.

Apaf1-deficient cortical neurons exhibit defects in axonal outgrowth

The establishment of neuronal polarity and axonal outgrowth are key processes affecting neuronal migration and synapse formation, their impairment likely leading to cognitive deficits. Here we have found that the apoptotic protease activating factor 1 (Apaf1), apart from its canonical role in apoptosis, plays an additional function in cortical neurons, where its deficiency specifically impairs axonal growth. Given the central role played by centrosomes and microtubules in the polarized extension of the axon, our data suggest that Apaf1-deletion affects axonal outgrowth through an impairment of centrosome organization. In line with this, centrosomal protein expression, as well as their centrosomal localization proved to be altered upon Apaf1-deletion. Strikingly, we also found that Apaf1-loss affects trans-Golgi components and leads to a robust activation of AMP-dependent protein kinase (AMPK), this confirming the stressful conditions induced by Apaf1-deficiency. Since AMPK hyper-phosphorylation is known to impair a proper axon elongation, our finding contributes to explain the effect of Apaf1-deficiency on axogenesis. We also discovered that the signaling pathways mediating axonal growth and involving glycogen synthase kinase-3β, liver kinase B1, and collapsing-response mediator protein-2 are altered in Apaf1-KO neurons. Overall, our results reveal a novel non-apoptotic role for Apaf1 in axonal outgrowth, suggesting that the neuronal phenotype due to Apaf1-deletion could not only be fully ascribed to apoptosis inhibition, but might also be the result of defects in axogenesis. The discovery of new molecules involved in axonal elongation has a clinical relevance since it might help to explain neurological abnormalities occurring during early brain development.

Centrosomal nucleolin is required for microtubule network organization

Nucleolin is a pleiotropic protein involved in a variety of cellular processes. Although multipolar spindle formation has been observed after nucleolin depletion, the roles of nucleolin in centrosome regulation and functions have not been addressed. Here we report using immunofluorescence and biochemically purified centrosomes that nucleolin co-localized only with one of the centrioles during interphase which was further identified as the mature centriole. Upon nucleolin depletion, cells exhibited an amplification of immature centriole markers surrounded by irregular pericentrin staining; these structures were exempt from maturation markers and unable to nucleate microtubules. Furthermore, the microtubule network was disorganized in these cells, exhibiting frequent non-centrosomal microtubules. At the mature centriole a reduced kinetics in the centrosomal microtubule nucleation phase was observed in live silenced cells, as well as a perturbation of microtubule anchoring. Immunoprecipitation experiments showed that nucleolin belongs to protein complexes containing 2 key centrosomal proteins, γ-tubulin and ninein, involved in microtubule nucleation and anchoring steps. Altogether, our study uncovered a new role for nucleolin in restricting microtubule nucleation and anchoring at centrosomes in interphase cells.

hCLE/C14orf166 associates with DDX1-HSPC117-FAM98B in a novel transcription-dependent shuttling RNA-transporting complex

hCLE/C14orf166 is a nuclear and cytoplasmic protein that interacts with the RNAP II, modulates nuclear RNA metabolism and is present in cytoplasmic RNA granules involved in localized translation. Here we have studied whether hCLE shares common interactors in the nucleus and the cytosol, which could shed light on its participation in the sequential phases of RNA metabolism. Nuclear and cytoplasmic purified hCLE-associated factors were identified and proteins involved in mRNA metabolism, motor-related proteins, cytoskeletal and translation-related factors were found. Purified hCLE complexes also contain RNAs and as expected some hCLE-interacting proteins (DDX1, HSPC117, FAM98B) were found both in the nucleus and the cytoplasm. Moreover, endogenous hCLE fractionates in protein complexes together with DDX1, HSPC117 and FAM98B and silencing of hCLE down-regulates their nuclear and cytosolic accumulation levels. Using a photoactivatable hCLE-GFP protein, nuclear import and export of hCLE was observed indicating that hCLE is a shuttling protein. Interestingly, hCLE nuclear import required active transcription, as did the import of DDX1, HSPC117 and FAM98B proteins. The data indicate that hCLE probably as a complex with DDX1, HSPC117 and FAM98B shuttles between the nucleus and the cytoplasm transporting RNAs suggesting that this complex has a prominent role on nuclear and cytoplasmic RNA fate.

Par6γ is at the mother centriole and controls centrosomal protein composition through a Par6α-dependent pathway

The centrosome contains two centrioles that differ in age, protein composition and function. This non-membrane bound organelle is known to regulate microtubule organization in dividing cells and ciliogenesis in quiescent cells. These specific roles depend on protein appendages at the older, or mother, centriole. In this study, we identified the polarity protein partitioning defective 6 homolog gamma (Par6γ) as a novel component of the mother centriole. This specific localization required the Par6γ C-terminus, but was independent of intact microtubules, the dynein/dynactin complex and the components of the PAR polarity complex. Par6γ depletion resulted in altered centrosomal protein composition, with the loss of a large number of proteins, including Par6α and p150(Glued), from the centrosome. As a consequence, there were defects in ciliogenesis, microtubule organization and centrosome reorientation during migration. Par6γ interacted with Par3 and aPKC, but these proteins were not required for the regulation of centrosomal protein composition. Par6γ also associated with Par6α, which controls protein recruitment to the centrosome through p150(Glued). Our study is the first to identify Par6γ as a component of the mother centriole and to report a role of a mother centriole protein in the regulation of centrosomal protein composition.

Mammalian HCA66 protein is required for both ribosome synthesis and centriole duplication

Ribosome production, one of the most energy-consuming biosynthetic activities in living cells, is adjusted to growth conditions and coordinated with the cell cycle. Connections between ribosome synthesis and cell cycle progression have been described, but the underlying mechanisms remain only partially understood. The human HCA66 protein was recently characterized as a component of the centrosome, the major microtubule-organizing center (MTOC) in mammalian cells, and was shown to be required for centriole duplication and assembly of the mitotic spindle. We show here that HCA66 is also required for nucleolar steps of the maturation of the 40S ribosomal subunit and therefore displays a dual function. Overexpression of a dominant negative version of HCA66, accumulating at the centrosome but absent from the nucleoli, alters centrosome function but has no effect on pre-rRNA processing, suggesting that HCA66 acts independently in each process. In yeast and HeLa cells, depletion of MTOC components does not impair ribosome synthesis. Hence our results suggest that both in yeast and human cells, assembly of a functional MTOC and ribosome synthesis are not closely connected processes.

The where, when and how of microtubule nucleation – one ring to rule them all

The function of microtubules depends on their arrangement into highly ordered arrays. Spatio-temporal control over the formation of new microtubules and regulation of their properties are central to the organization of these arrays. The nucleation of new microtubules requires γ-tubulin, an essential protein that assembles into multi-subunit complexes and is found in all eukaryotic organisms. However, the way in which γ-tubulin complexes are regulated and how this affects nucleation and, potentially, microtubule behavior, is poorly understood. γ-tubulin has been found in complexes of various sizes but several lines of evidence suggest that only large, ring-shaped complexes function as efficient microtubule nucleators. Human γ-tubulin ring complexes (γTuRCs) are composed of γ-tubulin and the γ-tubulin complex components (GCPs) 2, 3, 4, 5 and 6, which are members of a conserved protein family. Recent work has identified additional unrelated γTuRC subunits, as well as a large number of more transient γTuRC interactors. In this Commentary, we discuss the regulation of γTuRC-dependent microtubule nucleation as a key mechanism of microtubule organization. Specifically, we focus on the regulatory roles of the γTuRC subunits and interactors and present an overview of other mechanisms that regulate γTuRC-dependent microtubule nucleation and organization.

Nuclear γ-tubulin associates with nucleoli and interacts with tumor suppressor protein C53

γ-Tubulin is assumed to be a typical cytosolic protein necessary for nucleation of microtubules from microtubule organizing centers. Using immunolocalization and cell fractionation techniques in combination with siRNAi and expression of FLAG-tagged constructs, we have obtained evidence that γ-tubulin is also present in nucleoli of mammalian interphase cells of diverse cellular origins. Immunoelectron microscopy has revealed γ-tubulin localization outside fibrillar centers where transcription of ribosomal DNA takes place. γ-Tubulin was associated with nucleolar remnants after nuclear envelope breakdown and could be translocated to nucleoli during mitosis. Pretreatment of cells with leptomycin B did not affect the distribution of nuclear γ-tubulin, making it unlikely that rapid active transport via nuclear pores participates in the transport of γ-tubulin into the nucleus. This finding was confirmed by heterokaryon assay and time-lapse imaging of photoconvertible protein Dendra2 tagged to γ-tubulin. Immunoprecipitation from nuclear extracts combined with mass spectrometry revealed an association of γ-tubulin with tumor suppressor protein C53 located at multiple subcellular compartments including nucleoli. The notion of an interaction between γ-tubulin and C53 was corroborated by pull-down and co-immunoprecipitation experiments. Overexpression of γ-tubulin antagonized the inhibitory effect of C53 on DNA damage G(2) /M checkpoint activation. The combined results indicate that aside from its known role in microtubule nucleation, γ-tubulin may also have nuclear-specific function(s).

Apaf1 plays a pro-survival role by regulating centrosome morphology and function

The apoptotic protease activating factor 1 (Apaf1) is the main component of the apoptosome, and a crucial factor in the mitochondria-dependent death pathway. Here we show that Apaf1 plays a role in regulating centrosome maturation. By analyzing Apaf1-depleted cells, we have found that Apaf1 loss induces centrosome defects that impair centrosomal microtubule nucleation and cytoskeleton organization. This, in turn, affects several cellular processes such as mitotic spindle formation, cell migration and mitochondrial network regulation. As a consequence, Apaf1-depleted cells are more fragile and have a lower threshold to stress than wild-type cells. In fact, we found that they exhibit low Bcl-2 and Bcl-XL expression and, under apoptotic treatment, rapidly release cytochrome c. We also show that Apaf1 acts by regulating the recruitment of HCA66, with which it interacts, to the centrosome. This function of Apaf1 is carried out during the cell life and is not related to its apoptotic role. Therefore, Apaf1 might also be considered a pro-survival molecule, whose absence impairs cell performance and causes a higher responsiveness to stressful conditions.

SAP-like domain in nucleolar spindle associated protein mediates mitotic chromosome loading as well as interphase chromatin interaction

Nucleolar spindle associated protein (NuSAP) is a microtubule-stabilizing protein that localizes to chromosome arms and chromosome-proximal microtubules during mitosis and to the nucleus, with enrichment in the nucleoli, during interphase. The critical function of NuSAP is underscored by the finding that its depletion in HeLa cells results in various mitotic defects. Moreover, NuSAP is found overexpressed in multiple cancers and its expression levels often correlate with the aggressiveness of cancer. Due to its localization on chromosome arms and combination of microtubule-stabilizing and DNA-binding properties, NuSAP takes a special place within the extensive group of spindle assembly factors. In this study, we identify a SAP-like domain that shows DNA binding in vitro with a preference for dsDNA. Deletion of the SAP-like domain abolishes chromosome arm binding of NuSAP during mitosis, but is not sufficient to abrogate its chromosome-proximal localization after anaphase onset. Fluorescence recovery after photobleaching experiments revealed the highly dynamic nature of this NuSAP-chromatin interaction during mitosis. In interphase cells, NuSAP also interacts with chromatin through its SAP-like domain, as evident from its enrichment on dense chromatin regions and intranuclear mobility, measured by fluorescence correlation spectroscopy. The obtained results are in agreement with a model where NuSAP dynamically stabilizes newly formed microtubules on mitotic chromosomes to enhance chromosome positioning without immobilizing these microtubules. Interphase NuSAP-chromatin interaction suggests additional functions for NuSAP, as recently identified for other nuclear spindle assembly factors with a role in gene expression or DNA damage response.

Ribosome synthesis-unrelated functions of the preribosomal factor Rrp12 in cell cycle progression and the DNA damage response

Given the high metabolic cost required to generate ribosomes, it has been assumed that proteins involved in ribosome synthesis might establish functional cross talk with other intracellular processes to efficiently couple ribosome production and cell growth. However, such interconnections have remained elusive due to the difficulty in separating the intra- and extraribosomal roles of ribosome biogenesis factors. Using a yeast functional screen, I have discovered that Rrp12, a conserved protein involved in ribosome maturation and export, plays roles in the cell cycle and the DNA damage response. These results indicate that Rrp12 participates in a karyopherin Kap121-dependent import route that is crucial for nuclear sequestration of ribonucleotide reductase subunits and, thereby, ensures the proper kinetics of deoxyribonucleotide production during the cell cycle. Within this route, Rrp12 acts as a cofactor important for the full functionality of Kap121. This activity is mechanistically different from the known roles of Rrp12 in ribosome biogenesis. I propose that the functional duality of Rrp12 may couple the control of ribosome production to the regulation of other cellular processes during cell cycle progression.

NEDD1 is crucial for meiotic spindle stability and accurate chromosome segregation in mammalian oocytes

Defects in meiotic spindle structure contribute to chromosome segregation errors leading to genomic instability in oocytes and embryos upon fertilization. In this study, we analyzed the mechanisms that control spindle microtubule nucleation and stability in mammalian oocytes, and identified NEDD1/GCP-WD as a key regulator. NEDD1 specifically co-localizes with gamma-tubulin and pericentrin at microtubule-organizing centers (MTOCs) in mouse oocytes arrested at prophase-I. During metaphase-I and metaphase-II, the protein remains associated with MTOCs, in a pericentrin dependent manner. Notably, knockdown of Nedd1 transcripts using specific siRNAs resulted in a high incidence (65-70%) of metaphase-I arrest. The arrested oocytes were characterized by disrupted meiotic spindle structure, reduced microtubule density and significant chromosome misalignment. Detection of MAD2 at kinetochores indicated an absence of stable chromosome-microtubule attachment as well as activation of the spindle assembly checkpoint (SAC). Importantly, the disruption of meiotic spindle stability was associated with decreased gamma-tubulin at MTOCs in NEDD1-depleted oocytes, as well as a high frequency of chromosome non-disjunction errors leading to aneuploidy (50%) in the oocytes that did progress to metaphase-II. This study demonstrates that NEDD1 is an essential component of acentriolar oocyte MTOCs, which functions in the regulation of meiotic spindle stability. Moreover, it underscores that disruption of spindle stability in oocytes can lead to chromosomes segregation errors that are not fully resolved by SAC.

Double duty for nuclear proteins--the price of more open forms of mitosis

During cell division, eukaryotic cells pass on their genetic material to the next generation by undergoing mitosis, which segregates their chromosomes. During mitosis, the nuclear envelope, nuclear pore complexes and nucleolus must also be segregated. Cells achieve this in a range of different forms of mitosis, from closed, in which these nuclear structures remain intact, to open, in which these nuclear structures are disassembled. In between lies a smorgasbord of intermediate forms of mitosis, displaying varying degrees of nuclear disassembly. Gathering evidence is revealing links between the extent of nuclear disassembly and the evolution of new roles for nuclear proteins during mitosis. We propose that proteins with such double duties help coordinate reassembly of the nucleus with chromosomal segregation.

Arabidopsis cortical microtubules are initiated along, as well as branching from, existing microtubules

The principles by which cortical microtubules self-organize into a global template hold important implications for cell wall patterning. Microtubules move along bundles of microtubules, and neighboring bundles tend to form mobile domains that flow in a common direction. The bundles themselves move slowly and for longer than the individual microtubules, with domains describing slow rotary patterns. Despite this tendency for colinearity, microtubules have been seen to branch off extant microtubules at approximately 45 degrees . To examine this paradoxical behavior, we investigated whether some microtubules may be born on and grow along extant microtubule(s). The plus-end markers Arabidopsis thaliana end binding protein 1a, AtEB1a-GFP, and Arabidopsis SPIRAL1, SPR1-GFP, allowed microtubules of known polarity to be distinguished from underlying microtubules. This showed that the majority of microtubules do branch but in a direction heavily biased toward the plus end of the mother microtubule: few grow backward, consistent with the common polarity of domains. However, we also found that a significant proportion of emergent comets do follow the axes of extant microtubules, both at sites of apparent microtubule nucleation and at cross-over points. These phenomena help explain the persistence of bundles and counterbalance the tendency to branch.