Ribosomal protein S19 deficiency in zebrafish leads to developmental abnormalities and defective erythropoiesis through activation of p53 protein family

Rare Genetic Blood Disease Modeling in Zebrafish

Hematopoiesis results in the correct formation of all the different blood cell types. In mammals, it starts from specific hematopoietic stem and precursor cells residing in the bone marrow. Mature blood cells are responsible for supplying oxygen to every cell of the organism and for the protection against pathogens. Therefore, inherited or de novo genetic mutations affecting blood cell formation or the regulation of their activity are responsible for numerous diseases including anemia, immunodeficiency, autoimmunity, hyper- or hypo-inflammation, and cancer. By definition, an animal disease model is an analogous version of a specific clinical condition developed by researchers to gain information about its pathophysiology. Among all the model species used in comparative medicine, mice continue to be the most common and accepted model for biomedical research. However, because of the complexity of human diseases and the intrinsic differences between humans and other species, the use of several models (possibly in distinct species) can often be more helpful and informative than the use of a single model. In recent decades, the zebrafish (Danio rerio) has become increasingly popular among researchers, because it represents an inexpensive alternative compared to mammalian models, such as mice. Numerous advantages make it an excellent animal model to be used in genetic studies and in particular in modeling human blood diseases. Comparing zebrafish hematopoiesis to mammals, it is highly conserved with few, significant differences. In addition, the zebrafish model has a high-quality, complete genomic sequence available that shows a high level of evolutionary conservation with the human genome, empowering genetic and genomic approaches. Moreover, the external fertilization, the high fecundity and the transparency of their embryos facilitate rapid, in vivo analysis of phenotypes. In addition, the ability to manipulate its genome using the last genome editing technologies, provides powerful tools for developing new disease models and understanding the pathophysiology of human disorders. This review provides an overview of the different approaches and techniques that can be used to model genetic diseases in zebrafish, discussing how this animal model has contributed to the understanding of genetic diseases, with a specific focus on the blood disorders.

Significance of Clonal Mutations in Bone Marrow Failure and Inherited Myelodysplastic Syndrome/Acute Myeloid Leukemia Predisposition Syndromes

Clonal hematopoiesis as a hallmark of myelodysplastic syndrome (MDS) is mediated by the selective advantage of clonal hematopoietic stem cells in a context-specific manner. Although primary MDS emerges without known predisposing cause and is associated with advanced age, secondary MDS may develop in younger patients with bone marrow failure syndromes or after exposure to chemotherapy, respectively. This article discusses recent advances in the understanding of context-dependent clonal hematopoiesis in MDS with focus on clonal evolution in inherited and acquired bone marrow failure syndromes.

A small molecule p53 activator attenuates Fanconi anemia leukemic stem cell proliferation

Although p53 mutations are common in solid tumors, such mutations are found at a lower frequency in hematologic malignancies. In the genetic disorder Fanconi anemia (FA), p53 has been proposed as an important pathophysiological factor for two important hematologic hallmarks of the disease: bone marrow failure and leukemogenesis. Here we show that low levels of the p53 protein enhance the capacity of leukemic stem cells from FA patients to repopulate immunodeficient mice. Furthermore, boosting p53 protein levels with the use of the small molecule Nutlin-3 reduced leukemia burden in recipient mice. These results demonstrate that the level of p53 protein plays a crucial role in FA leukemogenesis.

Innate immune system activation in zebrafish and cellular models of Diamond Blackfan Anemia

Deficiency of ribosomal proteins (RPs) leads to Diamond Blackfan Anemia (DBA) associated with anemia, congenital defects, and cancer. While p53 activation is responsible for many features of DBA, the role of immune system is less defined. The Innate immune system can be activated by endogenous nucleic acids from non-processed pre-rRNAs, DNA damage, and apoptosis that occurs in DBA. Recognition by toll like receptors (TLRs) and Mda5-like sensors induces interferons (IFNs) and inflammation. Dying cells can also activate complement system. Therefore we analyzed the status of these pathways in RP-deficient zebrafish and found upregulation of interferon, inflammatory cytokines and mediators, and complement. We also found upregulation of receptors signaling to IFNs including Mda5, Tlr3, and Tlr9. TGFb family member activin was also upregulated in RP-deficient zebrafish and in RPS19-deficient human cells, which include a lymphoid cell line from a DBA patient, and fetal liver cells and K562 cells transduced with RPS19 shRNA. Treatment of RP-deficient zebrafish with a TLR3 inhibitor decreased IFNs activation, acute phase response, and apoptosis and improved their hematopoiesis and morphology. Inhibitors of complement and activin also had beneficial effects. Our studies suggest that innate immune system contributes to the phenotype of RPS19-deficient zebrafish and human cells.

RPL27A is a target of miR-595 and may contribute to the myelodysplastic phenotype through ribosomal dysgenesis

We investigated the functional consequences following deletion of a microRNA (miR) termed miR-595 which resides on chromosome 7q and is localised within one of the commonly deleted regions identified for Myelodysplasia (MDS) with monosomy 7 (−7)/isolated loss of 7q (7q-). We identified several targets for miR-595, including a large ribosomal subunit protein RPL27A. RPL27A downregulation induced p53 activation, apoptosis and inhibited proliferation. Moreover, p53-independent effects were additionally identified secondary to a reduction in the ribosome subunit 60s. We confirmed that RPL27A plays a pivotal role in the maintenance of nucleolar integrity and ribosomal synthesis/maturation. Of note, RPL27A overexpression, despite showing no significant effects on p53 mRNA levels, did in fact enhance cellular proliferation. In normal CD34+ cells, RPL27A knockdown preferentially blocked erythroid proliferation and differentiation. Lastly, we show that miR-595 expression appears significantly downregulated in the majority of primary samples derived from MDS patients with (−7)/(7q-), in association with RPL27A upregulation. This significant downregulation of miR-595 is also apparent when higher risk MDS cases are compared to lower risk cases. The potential clinical importance of these findings requires further validation.

PIM1 destabilization activates a p53-dependent response to ribosomal stress in cancer cells

Defects in ribosome biogenesis triggers a stress response (ribosomal stress) that can lead to growth arrest and apoptosis. Signaling pathways activated by ribosomal stress are specifically involved in the pathological mechanism of a group of disorders defined as ribosomopathies. However, more generally, the quality control of ribosome synthesis is part of the regulatory circuits that control cell metabolism. A number of studies identified tumor suppressor p53 as a central player in ribosomal stress. We have previously reported that the kinase PIM1 plays a role as a sensor for ribosome deficiency. In this report we address the relationship between PIM1 and p53 in cancer cell lines after depletion of a ribosomal protein. We identified a novel signaling pathway that includes the kinase AKT and the ubiquitin ligase MDM2. In fact, our results indicate that the lower level of PIM1, induced by ribosomal stress, causes inactivation of AKT, inhibition of MDM2 and a consequent p53 stabilization. Therefore, we propose that activation of p53 in response to ribosomal stress, is dependent on the pathway PIM1-AKT-MDM2. In addition, we report evidence that PIM1 level may be relevant to assess the sensitivity of cancer cells to chemotherapeutic drugs that induce ribosomal stress.

Ribosome biogenesis protein Urb2 regulates hematopoietic stem cells development via P53 pathway in zebrafish

Ribosome biogenesis is a significant process in cells. Dysfunction in this process will result in the defects of protein synthesis and consequently cause the development of specific diseases called ribosomopathies. Mutations in ribosome biogenesis protein Rps19, Rpl5, or Rpl11 can lead to hematopoietic defects in human, thus triggering the disease Diamond Blackfan anemia. However, the regulatory mechanisms of ribosome biogenesis in hematopoiesis remain incompletely understood. In this study, we describe a zebrafish mutant cq42, which carries a nonsense mutation in the gene that encodes ribosome biogenesis 2 homolog (Urb2). Urb2 is strongly expressed in the caudal hematopoietic tissue (CHT) during hematopoietic stem cells (HSCs) expanding. Molecular characterization of urb2^cq42 larvae suggest that urb2 deficiency notably decrease the population of HSCs in CHT and early T cells in thymus. Further analysis shows that compromised cell proliferation and superfluous apoptosis are observed in the CHT of urb2^cq42 mutant. P53 pathway is upregulated in the urb2^cq42 larvae and loss-of-function of P53 can fully rescue the hematopoietic defects in urb2^cq42 mutant. These data demonstrate that urb2 is essential for HSCs development through the regulation of P53 pathway.

SF3B1 deficiency impairs human erythropoiesis via activation of p53 pathway: implications for understanding of ineffective erythropoiesis in MDS

BACKGROUND

SF3B1 is a core component of splicing machinery. Mutations in SF3B1 are frequently found in myelodysplastic syndromes (MDS), particularly in patients with refractory anemia with ringed sideroblasts (RARS), characterized by isolated anemia. SF3B1 mutations have been implicated in the pathophysiology of RARS; however, the physiological function of SF3B1 in erythropoiesis remains unknown.

METHODS

shRNA-mediated approach was used to knockdown SF3B1 in human CD34+ cells. The effects of SF3B1 knockdown on human erythroid cell differentiation, cell cycle, and apoptosis were assessed by flow cytometry. RNA-seq, qRT-PCR, and western blot analyses were used to define the mechanisms of phenotypes following knockdown of SF3B1.

RESULTS

We document that SF3B1 knockdown in human CD34+ cells leads to increased apoptosis and cell cycle arrest of early-stage erythroid cells and generation of abnormally nucleated late-stage erythroblasts. RNA-seq analysis of SF3B1-knockdown erythroid progenitor CFU-E cells revealed altered splicing of an E3 ligase Makorin Ring Finger Protein 1 (MKRN1) and subsequent activation of p53 pathway. Importantly, ectopic expression of MKRN1 rescued SF3B1-knockdown-induced alterations. Decreased expression of genes involved in mitosis/cytokinesis pathway including polo-like kinase 1 (PLK1) was noted in SF3B1-knockdown polychromatic and orthochromatic erythroblasts comparing to control cells. Pharmacologic inhibition of PLK1 also led to generation of abnormally nucleated erythroblasts.

CONCLUSIONS

These findings enabled us to identify novel roles for SF3B1 in human erythropoiesis and provided new insights into its role in regulating normal erythropoiesis. Furthermore, these findings have implications for improved understanding of ineffective erythropoiesis in MDS patients with SF3B1 mutations.

Somatic mutations and clonal hematopoiesis in congenital neutropenia

Severe congenital neutropenia (SCN) and Shwachman-Diamond syndrome (SDS) are congenital neutropenia syndromes with a high rate of leukemic transformation. Hematopoietic stressors may contribute to leukemic transformation by increasing the mutation rate in hematopoietic stem/progenitor cells (HSPCs) and/or by promoting clonal hematopoiesis. We sequenced the exome of individual hematopoietic colonies derived from 13 patients with congenital neutropenia to measure total mutation burden and performed error-corrected sequencing on a panel of 46 genes on 80 patients with congenital neutropenia to assess for clonal hematopoiesis. An average of 3.6 ± 1.2 somatic mutations per exome was identified in HSPCs from patients with SCN compared with 3.9 ± 0.4 for healthy controls (P = NS). Clonal hematopoiesis due to mutations in TP53 was present in 48% (13/27) of patients with SDS but was not seen in healthy controls (0/17, P < .001) or patients with SCN (0/40, P < .001). Our SDS cohort was young (median age 6.3 years), and many of the patients had multiple TP53 mutations. Conversely, clonal hematopoiesis due to mutations of CSF3R was present in patients with SCN but was not detected in healthy controls or patients with SDS. These data show that hematopoietic stress, including granulocyte colony-stimulating factor, do not increase the mutation burden in HSPCs in congenital neutropenia. Rather, distinct hematopoietic stressors result in the selective expansion of HSPCs carrying specific gene mutations. In particular, in SDS there is enormous selective pressure to expand TP53-mutated HSPCs, suggesting that acquisition of TP53 mutations is an early, likely initiating event, in the transformation to myelodysplastic syndrome/acute myeloid leukemia in patients with SDS.

Erythroblast differentiation at spleen in Q137E mutant ribosomal protein S19 gene knock-in C57BL/6J mice

We recently found that erythroblast-like cells derived from human leukaemia K562 cells express C5a receptor (C5aR) and produce its antagonistic and agonistic ligand ribosomal protein S19 (RP S19) polymer, which is cross-linked between K122 and Q137 by tissue transglutaminases. RP S19 polymer binds to the reciprocal C5aRs on erythroblast-like cells and macrophage-like cells derived from human monocytic THP-1 cells and promotes differentiation into reticulocyte-like cells through enucleation in vitro. To examine the roles of RP S19 polymer in mouse erythropoiesis, we prepared Q137E mutant RP S19 gene knock-in C57BL/6J mice. In contrast to wild-type mice, erythroblast numbers at the preliminary stage (CD71^high/TER119^low) in spleen based on transferrin receptor (CD71) and glycophorin A (TER119) values and erythrocyte numbers in orbital artery bloods were not largely changed in knock-in mice. Conversely, erythroblast numbers at the early stage (CD71^high/TER119^high) were significantly decreased in spleen by knock-in mice. The reduction of early erythroblast numbers in spleen was enhanced by the phenylhydrazine-induced pernicious anemia model knock-in mice and was rescued by a functional analogue of RP S19 dimer S-tagged C5a/RP S19. These data indicated that RP S19 polymer plays the roles in the early erythroblast differentiation of C57BL/6J mouse spleen.

How Ribosomes Translate Cancer

A wealth of novel findings, including congenital ribosomal mutations in ribosomopathies and somatic ribosomal mutations in various cancers, have significantly increased our understanding of the relevance of ribosomes in oncogenesis. Here, we explore the growing list of mechanisms by which the ribosome is involved in carcinogenesis-from the hijacking of ribosomes by oncogenic factors and dysregulated translational control, to the effects of mutations in ribosomal components on cellular metabolism. Of clinical importance, the recent success of RNA polymerase inhibitors highlights the dependence on "onco-ribosomes" as an Achilles' heel of cancer cells and a promising target for further therapeutic intervention.Significance: The recent discovery of somatic mutations in ribosomal proteins in several cancers has strengthened the link between ribosome defects and cancer progression, while also raising the question of which cellular mechanisms such defects exploit. Here, we discuss the emerging molecular mechanisms by which ribosomes support oncogenesis, and how this understanding is driving the design of novel therapeutic strategies. Cancer Discov; 7(10); 1069-87. ©2017 AACR.

Lymphoblastoid cell lines from Diamond Blackfan anaemia patients exhibit a full ribosomal stress phenotype that is rescued by gene therapy

Diamond Blackfan anaemia (DBA) is a congenital bone marrow failure syndrome characterised by selective red cell hypoplasia. DBA is most often due to heterozygous mutations in ribosomal protein (RP) genes that lead to defects in ribosome biogenesis and function and result in ribosomal stress and p53 activation. The molecular mechanisms underlying this pathology are still poorly understood and studies on patient erythroid cells are hampered by their paucity. Here we report that RP-mutated lymphoblastoid cell lines (LCLs) established from DBA patients show defective rRNA processing and ribosomal stress features such as reduced proliferation, decreased protein synthesis, and activation of p53 and its target p21. These phenotypic alterations were corrected by gene complementation. Our data indicate that DBA LCLs could be a useful model for molecular and pharmacological investigations.

Genome-Wide Identification and Comprehensive Expression Profiling of Ribosomal Protein Small Subunit (RPS) Genes and their Comparative Analysis with the Large Subunit (RPL) Genes in Rice

Ribosomal proteins (RPs) are indispensable in ribosome biogenesis and protein synthesis, and play a crucial role in diverse developmental processes. Our previous studies on Ribosomal Protein Large subunit (RPL) genes provided insights into their stress responsive roles in rice. In the present study, we have explored the developmental and stress regulated expression patterns of Ribosomal Protein Small (RPS) subunit genes for their differential expression in a spatiotemporal and stress dependent manner. We have also performed an in silico analysis of gene structure, cis-elements in upstream regulatory regions, protein properties and phylogeny. Expression studies of the 34 RPS genes in 13 different tissues of rice covering major growth and developmental stages revealed that their expression was substantially elevated, mostly in shoots and leaves indicating their possible involvement in the development of vegetative organs. The majority of the RPS genes have manifested significant expression under all abiotic stress treatments with ABA, PEG, NaCl, and H2O2. Infection with important rice pathogens, Xanthomonas oryzae pv. oryzae (Xoo) and Rhizoctonia solani also induced the up-regulation of several of the RPS genes. RPS4, 13a, 18a, and 4a have shown higher transcript levels under all the abiotic stresses, whereas, RPS4 is up-regulated in both the biotic stress treatments. The information obtained from the present investigation would be useful in appreciating the possible stress-regulatory attributes of the genes coding for rice ribosomal small subunit proteins apart from their functions as house-keeping proteins. A detailed functional analysis of independent genes is required to study their roles in stress tolerance and generating stress- tolerant crops.

Osteosarcoma: Molecular Pathogenesis and iPSC Modeling

Rare hereditary disorders provide unequivocal evidence of the importance of genes in human disease pathogenesis. Familial syndromes that predispose to osteosarcomagenesis are invaluable in understanding the underlying genetics of this malignancy. Recently, patient-derived induced pluripotent stem cells (iPSCs) have been successfully utilized to model Li-Fraumeni syndrome (LFS)-associated bone malignancy, demonstrating that iPSCs can serve as an in vitro disease model to elucidate osteosarcoma etiology. We provide here an overview of osteosarcoma predisposition syndromes and review recently established iPSC disease models for these familial syndromes. Merging molecular information gathered from these models with the current knowledge of osteosarcoma biology will help us to gain a deeper understanding of the pathological mechanisms underlying osteosarcomagenesis and will potentially aid in the development of future patient therapies.

Probing the mechanisms underlying human diseases in making ribosomes

Ribosomes are essential, highly complex machines responsible for protein synthesis in all growing cells. Because of their importance, the process of building these machines is intricately regulated. Although the proteins involved in regulating ribosome biogenesis are just beginning to be understood, especially in human cells, the consequences for dysregulating this process have been even less studied. Such interruptions in ribosome synthesis result in a collection of human disorders known as ribosomopathies. Ribosomopathies, which occur due to mutations in proteins involved in the global process of ribosome biogenesis, result in tissue-specific defects. The questions posed by this dichotomy and the steps taken to address these questions are therefore the focus of this review: How can tissue-specific disorders result from alterations in global processes? Could ribosome specialization account for this difference?

The Pu.1 target gene Zbtb11 regulates neutrophil development through its integrase-like HHCC zinc finger

In response to infection and injury, the neutrophil population rapidly expands and then quickly re-establishes the basal state when inflammation resolves. The exact pathways governing neutrophil/macrophage lineage outputs from a common granulocyte-macrophage progenitor are still not completely understood. From a forward genetic screen in zebrafish, we identify the transcriptional repressor, ZBTB11, as critical for basal and emergency granulopoiesis. ZBTB11 sits in a pathway directly downstream of master myeloid regulators including PU.1, and TP53 is one direct ZBTB11 transcriptional target. TP53 repression is dependent on ZBTB11 cys116, which is a functionally critical, metal ion-coordinating residue within a novel viral integrase-like zinc finger domain. To our knowledge, this is the first description of a function for this domain in a cellular protein. We demonstrate that the PU.1–ZBTB11–TP53 pathway is conserved from fish to mammals. Finally, Zbtb11 mutant rescue experiments point to a ZBTB11-regulated TP53 requirement in development of other organs.

Neutrophils are increased in response to injury and infection but how they form from a common granulocyte-macrophage progenitor is unclear. Here, the authors identify a role for the transcriptional repressor ZBTB11 in zebrafish, which is regulated by master myeloid regulators and represses TP53.

Molecular convergence in ex vivo models of Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a congenital bone marrow failure syndrome characterized by erythroid hypoplasia, usually without perturbation of other hematopoietic lineages. Approximately 65% of DBA patients with autosomal dominant inheritance have heterozygous mutations or deletions in ribosomal protein (RP) genes while <1% of patients with X-linked inheritance have been identified with mutations in the transcription factor GATA1 Erythroid cells from patients with DBA have not been well characterized, and the mechanisms underlying the erythroid specific effects of either RP or GATA1 associated DBA remain unclear. We have developed an ex vivo culture system to expand peripheral blood CD34⁺ progenitor cells from patients with DBA and differentiate them into erythroid cells. Cells from patients with RP or GATA1 mutations showed decreased proliferation and delayed erythroid differentiation in comparison with controls. RNA transcript analyses of erythroid cells from controls and patients with RP or GATA1 mutations showed distinctive differences, with upregulation of heme biosynthesis genes prominently in RP-mediated DBA and failure to upregulate components of the translational apparatus in GATA1-mediated DBA. Our data show that dysregulation of translation is a common feature of DBA caused by both RP and GATA1 mutations. This trial was registered at www.clinicaltrials.gov as #NCT00106015.

SBDS-Deficient Cells Have an Altered Homeostatic Equilibrium due to Translational Inefficiency Which Explains their Reduced Fitness and Provides a Logical Framework for Intervention

Ribosomopathies are a family of inherited disorders caused by mutations in genes necessary for ribosomal function. Shwachman-Diamond Bodian Syndrome (SDS) is an autosomal recessive disease caused, in most patients, by mutations of the SBDS gene. SBDS is a protein required for the maturation of 60S ribosomes. SDS patients present exocrine pancreatic insufficiency, neutropenia, chronic infections, and skeletal abnormalities. Later in life, patients are prone to myelodisplastic syndrome and acute myeloid leukemia (AML). It is unknown why patients develop AML and which cellular alterations are directly due to the loss of the SBDS protein. Here we derived mouse embryonic fibroblast lines from an Sbds^R126T/R126T mouse model. After their immortalization, we reconstituted them by adding wild type Sbds. We then performed a comprehensive analysis of cellular functions including colony formation, translational and transcriptional RNA-seq, stress and drug sensitivity. We show that: 1. Mutant Sbds causes a reduction in cellular clonogenic capability and oncogene-induced transformation. 2. Mutant Sbds causes a marked increase in immature 60S subunits, limited impact on mRNA specific initiation of translation, but reduced global protein synthesis capability. 3. Chronic loss of SBDS activity leads to a rewiring of gene expression with reduced ribosomal capability, but increased lysosomal and catabolic activity. 4. Consistently with the gene signature, we found that SBDS loss causes a reduction in ATP and lactate levels, and increased susceptibility to DNA damage. Combining our data, we conclude that a cell-specific fragile phenotype occurs when SBDS protein drops below a threshold level, and propose a new interpretation of the disease.

Shwachman Diamond syndrome (SDS) is an inherited disease. SDS presents, as hallmarks, exocrine pancreatic insufficiency, increased rate of infections, and higher incidence of leukemia. Most cases are due to mutations in the SBDS gene. SBDS encodes for a ribosome maturation factor. In this study, we immortalized mouse fibroblasts carrying one of the most common mutation of SDS patients and performed a thorough analysis of their properties. We show that the loss of SBDS activity causes a rewiring of gene expression and cellular metabolism. Overall we find a reduction of protein synthesis capability, a lower energy status, and increased lysosomal capability. SBDS mutant cells have an increased susceptibility to various forms of stress, but are strikingly resistant to oncogene-induced transformation. We propose a model that explains the complex phenotype of SDS patients and suggests roads for a rationale treatment.

Transcriptome Profiling Identifies Ribosome Biogenesis as a Target of Alcohol Teratogenicity and Vulnerability during Early Embryogenesis

Fetal alcohol spectrum disorder (FASD) is a leading cause of neurodevelopmental disability. Individuals with FASD may exhibit a characteristic facial appearance that has diagnostic utility. The mechanism by which alcohol disrupts craniofacial development is incompletely understood, as are the genetic factors that can modify individual alcohol vulnerability. Using an established avian model, we characterized the cranial transcriptome in response to alcohol to inform the mechanism underlying these cells’ vulnerability. Gallus gallus embryos having 3–6 somites were exposed to 52 mM alcohol and the cranial transcriptomes were sequenced thereafter. A total of 3422 genes had significantly differential expression. The KEGG pathways with the greatest enrichment of differentially expressed gene clusters were Ribosome (P = 1.2 x 10⁻¹⁷, 67 genes), Oxidative Phosphorylation (P = 4.8 x 10⁻¹², 60 genes), RNA Polymerase (P = 2.2 x 10⁻³, 15 genes) and Spliceosome (P = 2.6 x 10⁻², 39 genes). The preponderance of transcripts in these pathways were repressed in response to alcohol. These same gene clusters also had the greatest altered representation in our previous comparison of neural crest populations having differential vulnerability to alcohol-induced apoptosis. Comparison of differentially expressed genes in alcohol-exposed (3422) and untreated, alcohol-vulnerable (1201) transcriptomes identified 525 overlapping genes of which 257 have the same direction of transcriptional change. These included 36 ribosomal, 25 oxidative phosphorylation and 7 spliceosome genes. Using a functional approach in zebrafish, partial knockdown of ribosomal proteins zrpl11, zrpl5a, and zrps3a individually heightened vulnerability to alcohol-induced craniofacial deficits and increased apoptosis. In humans, haploinsufficiency of several of the identified ribosomal proteins are causative in craniofacial dysmorphologies such as Treacher Collins Syndrome and Diamond-Blackfan Anemia. This work suggests ribosome biogenesis may be a novel target mediating alcohol’s damage to developing neural crest. Our findings are consistent with observations that gene-environment interactions contribute to vulnerability in FASD.

On the Molecular Evolution of Leptin, Leptin Receptor, and Endospanin

Over a decade passed between Friedman's discovery of the mammalian leptin gene (1) and its cloning in fish (2) and amphibians (3). Since 2005, the concept of gene synteny conservation (vs. gene sequence homology) was instrumental in identifying leptin genes in dozens of species, and we now have leptin genes from all major classes of vertebrates. This database of LEP (leptin), LEPR (leptin receptor), and LEPROT (endospanin) genes has allowed protein structure modeling, stoichiometry predictions, and even functional predictions of leptin function for most vertebrate classes. Here, we apply functional genomics to model hundreds of LEP, LEPR, and LEPROT proteins from both vertebrates and invertebrates. We identify conserved structural motifs in each of the three leptin signaling proteins and demonstrate Drosophila Dome protein's conservation with vertebrate leptin receptors. We model endospanin structure for the first time and identify endospanin paralogs in invertebrate genomes. Finally, we argue that leptin is not an adipostat in fishes and discuss emerging knockout models in fishes.

Fish to Learn: Insights into Blood Development and Blood Disorders from Zebrafish Hematopoiesis

Since its introduction in early 1980s, the zebrafish (Danio rerio) has become an invaluable vertebrate animal model system to study many human disorders in almost all systems, from hepatic and brain pathology, to autoimmune and psychiatric disorders. Hematopoiesis between zebrafish and mammals is highly conserved, making the zebrafish an attractive model to study hematopoietic development and blood disorders. Unique attributes of the zebrafish include the ability to perform large-scale genetic and chemical screens in vivo, study development at the cellular level, and use transgenic fish to dissect mechanisms of disease or drug effects. This review summarizes major discoveries that helped define molecular control of hematopoiesis in vertebrates and specific contributions from studies in zebrafish.

Regulatory Roles of Rpl22 in Hematopoiesis: An Old Dog with New Tricks

Ribosomal proteins have long been known to serve critical roles in facilitating the biogenesis of the ribosome and its ability to synthesize proteins. However, evidence is emerging that suggests ribosomal proteins are also capable of performing tissue-restricted, regulatory functions that impact normal development and pathological conditions, including cancer. The challenge in studying such regulatory functions is that elimination of many ribosomal proteins also disrupts ribosome biogenesis and/or function. Thus, it is difficult to determine whether developmental abnormalities resulting from ablation of a ribosomal protein result from loss of core ribosome functions or from loss of the regulatory function of the ribosomal protein. Rpl22, a ribosomal protein component of the large 60S subunit, provides insight into this conundrum; Rpl22 is dispensable for both ribosome biogenesis and protein synthesis yet its ablation causes tissue-restricted disruptions in development. Here we review evidence supporting the regulatory functions of Rpl22 and other ribosomal proteins.

Neuronal GPCR OCTR-1 regulates innate immunity by controlling protein synthesis in Caenorhabditis elegans

Upon pathogen infection, microbial killing pathways and cellular stress pathways are rapidly activated by the host innate immune system. These pathways must be tightly regulated because insufficient or excessive immune responses have deleterious consequences. Increasing evidence indicates that the nervous system regulates the immune system to confer coordinated protection to the host. However, the precise mechanisms of neural-immune communication remain unclear. Previously we have demonstrated that OCTR-1, a neuronal G protein-coupled receptor, functions in the sensory neurons ASH and ASI to suppress innate immune responses in non-neural tissues against Pseudomonas aeruginosa in Caenorhabditis elegans. In the current study, by using a mass spectrometry-based quantitative proteomics approach, we discovered that OCTR-1 regulates innate immunity by suppressing translation and the unfolded protein response (UPR) pathways at the protein level. Functional assays revealed that OCTR-1 inhibits specific protein synthesis factors such as ribosomal protein RPS-1 and translation initiation factor EIF-3.J to reduce infection-triggered protein synthesis and UPR. Translational inhibition by chemicals abolishes the OCTR-1-controlled innate immune responses, indicating that activation of the OCTR-1 pathway is dependent on translation upregulation such as that induced by pathogen infection. Because OCTR-1 downregulates protein translation activities, the OCTR-1 pathway could function to suppress excessive responses to infection or to restore protein homeostasis after infection.

Selenoprotein H is an essential regulator of redox homeostasis that cooperates with p53 in development and tumorigenesis

Selenium, an essential micronutrient known for its cancer prevention properties, is incorporated into a class of selenocysteine-containing proteins (selenoproteins). Selenoprotein H (SepH) is a recently identified nucleolar oxidoreductase whose function is not well understood. Here we report that seph is an essential gene regulating organ development in zebrafish. Metabolite profiling by targeted LC-MS/MS demonstrated that SepH deficiency impairs redox balance by reducing the levels of ascorbate and methionine, while increasing methionine sulfoxide. Transcriptome analysis revealed that SepH deficiency induces an inflammatory response and activates the p53 pathway. Consequently, loss of seph renders larvae susceptible to oxidative stress and DNA damage. Finally, we demonstrate that seph interacts with p53 deficiency in adulthood to accelerate gastrointestinal tumor development. Overall, our findings establish that seph regulates redox homeostasis and suppresses DNA damage. We hypothesize that SepH deficiency may contribute to the increased cancer risk observed in cohorts with low selenium levels.

Toward RNA Repair of Diamond Blackfan Anemia Hematopoietic Stem Cells

Diamond blackfan anemia (DBA) is a well-known inherited bone marrow failure syndrome mostly caused by mutations in ribosomal protein (RP) genes but also rarely in the hematopoietic transcription factor gene, GATA1, or TSR2, a ribosomal protein (Rps26) chaperone gene. About 25% of patients have heterozygous mutations in the RPS19 gene, which leads to haploinsufficiency of Rps19 protein in most cases. However, some RPS19 missense mutations appear to act in a dominant negative fashion. DBA typically leads to a hypoplastic anemia that becomes apparent during the first year of life, and standard treatment includes steroids or red blood cell transfusions, each modality having attendant side effects. The only curative therapy is allogeneic stem-cell transplantation, but this option is limited to patients with a histocompatible donor. DBA-mutant embryonic, induced pluripotent, and hematopoietic stem cells all exhibit growth abnormalities that can be corrected by DNA gene transfer, suggesting the possibility of ex vivo autologous gene therapy. The authors have been interested in the application of spliceosome-mediated mRNA trans-splicing (SMaRT) technology to RNA repair of DBA stem cells. Compared with gene replacement or other RNA re-programming approaches, SMaRT has several potential advantages. First, delivery of the entire normal cDNA is unnecessary, thus minimizing the overall size of the construct for packaging into a viral delivery vector. Second, RNA transcription of the corrected gene relies on the cell's endogenous transcriptional, processing, and regulatory machinery, thereby ensuring faithful and contextual expression. Third, RNA trans-splicing employs the endogenous spliceosome enzymatic machinery present in nearly all cells. Fourth, RNA trans-splicing converts mutant transcripts into therapeutically useful mRNA, and thus may be capable of treating disorders caused by dominant negative mutations. This review critically assesses prospects for both gene and RNA repair in DBA stem cells.

Ribosomopathies: how a common root can cause a tree of pathologies

Defects in ribosome biogenesis are associated with a group of diseases called the ribosomopathies, of which Diamond-Blackfan anemia (DBA) is the most studied. Ribosomes are composed of ribosomal proteins (RPs) and ribosomal RNA (rRNA). RPs and multiple other factors are necessary for the processing of pre-rRNA, the assembly of ribosomal subunits, their export to the cytoplasm and for the final assembly of subunits into a ribosome. Haploinsufficiency of certain RPs causes DBA, whereas mutations in other factors cause various other ribosomopathies. Despite the general nature of their underlying defects, the clinical manifestations of ribosomopathies differ. In DBA, for example, red blood cell pathology is especially evident. In addition, individuals with DBA often have malformations of limbs, the face and various organs, and also have an increased risk of cancer. Common features shared among human DBA and animal models have emerged, such as small body size, eye defects, duplication or overgrowth of ectoderm-derived structures, and hematopoietic defects. Phenotypes of ribosomopathies are mediated both by p53-dependent and -independent pathways. The current challenge is to identify differences in response to ribosomal stress that lead to specific tissue defects in various ribosomopathies. Here, we review recent findings in this field, with a particular focus on animal models, and discuss how, in some cases, the different phenotypes of ribosomopathies might arise from differences in the spatiotemporal expression of the affected genes.

Summary: This paper reviews recent data on Diamond Blackfan anemia and discusses them in connection with other ribosomopathies.

The Roles of RNA Polymerase I and III Subunits Polr1c and Polr1d in Craniofacial Development and in Zebrafish Models of Treacher Collins Syndrome

Ribosome biogenesis is a global process required for growth and proliferation of all cells, yet perturbation of ribosome biogenesis during human development often leads to tissue-specific defects termed ribosomopathies. Transcription of the ribosomal RNAs (rRNAs) by RNA polymerases (Pol) I and III, is considered a rate limiting step of ribosome biogenesis and mutations in the genes coding for RNA Pol I and III subunits, POLR1C and POLR1D cause Treacher Collins syndrome, a rare congenital craniofacial disorder. Our understanding of the functions of individual RNA polymerase subunits, however, remains poor. We discovered that polr1c and polr1d are dynamically expressed during zebrafish embryonic development, particularly in craniofacial tissues. Consistent with this pattern of activity, polr1c and polr1d homozygous mutant zebrafish exhibit cartilage hypoplasia and cranioskeletal anomalies characteristic of humans with Treacher Collins syndrome. Mechanistically, we discovered that polr1c and polr1d loss-of-function results in deficient ribosome biogenesis, Tp53-dependent neuroepithelial cell death and a deficiency of migrating neural crest cells, which are the primary progenitors of the craniofacial skeleton. More importantly, we show that genetic inhibition of tp53 can suppress neuroepithelial cell death and ameliorate the skeletal anomalies in polr1c and polr1d mutants, providing a potential avenue to prevent the pathogenesis of Treacher Collins syndrome. Our work therefore has uncovered tissue-specific roles for polr1c and polr1d in rRNA transcription, ribosome biogenesis, and neural crest and craniofacial development during embryogenesis. Furthermore, we have established polr1c and polr1d mutant zebrafish as models of Treacher Collins syndrome together with a unifying mechanism underlying its pathogenesis and possible prevention.

Ribosomes synthesize all proteins, and are therefore critical for cell growth and proliferation. Ribosome biogenesis, or the process of making ribosomes, is one of the most energy consuming processes within a cell, and disruptions in ribosome biogenesis can lead to congenital disorders termed ribosomopathies. Interestingly, individual ribosomopathies are characterized by tissue-specific phenotypes, which is surprising given the universal importance of ribosomes. Treacher Collins syndrome (TCS) for example, is a ribosomopathy characterized by anomalies of facial bones, palate, eyes and ears. Mutations in TCOF1, POLR1C, and POLR1D are associated with the underlying etiology of TCS. TCOF1 plays an important role in the synthesis of ribosomal RNA, one of the rate-limiting steps of ribosome biogenesis. Consequently, TCOF1 is essential for the survival and proliferation of neural crest cell progenitors, which are the precursors of craniofacial bone, cartilage and connective tissue. In contrast, the functions of POLR1C and POLR1D, which are subunits of RNA Polymerases I and III remain unknown. Here we examined the function of polr1c and polr1d during zebrafish development and discovered that these genes display dynamic spatiotemporal activity during embryogenesis with enriched expression in craniofacial tissues. Furthermore, we observed that polr1c and polr1d loss-of-function zebrafish exhibit anomalies in craniofacial cartilage development, which reflects the characteristic features of TCS. An examination of polr1c^-/- and polr1d^-/- mutants revealed that diminished ribosome biogenesis results in neuroepithelial cell death and a deficiency of migrating neural crest cells, which are the progenitors of the craniofacial skeleton. Moreover, the cell death observed in polr1c^-/- and polr1d^-/- mutants is Tp53-dependent, and inhibition of tp53 is sufficient to repress cell death and rescue cranioskeletal cartilage formation in polr1c^-/- and polr1d^-/- mutant embryos. These studies provide evidence for tissue-specific functions of polr1c and polr1d during embryonic development, while also establishing polr1c and polr1d loss-of-function zebrafish mutants as models of Treacher Collins syndrome.

Combined Transcriptomic and Proteomic Approach to Identify Toxicity Pathways in Early Life Stages of Japanese Medaka (Oryzias latipes) Exposed to 1,2,5,6-Tetrabromocyclooctane (TBCO)

Currently, the novel brominated flame retardant 1,2,5,6-tetrabromocyclooctane (TBCO) is considered a potential replacement for hexabromocyclododecane (HBCD). Therefore, use of TBCO could increase in the near future. To assess potential toxicological risks to aquatic organisms, embryos of Japanese medaka (Oryzias latipes) were exposed to 10, 100, or 1000 μg/L TBCO from 2 h postfertilization until 1 day post-hatch. TBCO accumulated in embryos in the order of 0.43-1.3 × 10(4)-fold, and the rate constant of accumulation was 1.7-1.8 per day. The number of days to hatch and the hatching success of embryos exposed to the medium and the greatest concentrations of TBCO were impaired. Responses of the transcriptome (RNA-seq) and proteome were characterized in embryos exposed to 100 μg/L TBCO because this was the least concentration of TBCO that caused an effect on hatching. Consistent with effects on hatching, proteins whose abundances were reduced by exposure to TBCO were enriched in embryo development and hatching pathways. Also, on the basis of the responses of transcriptome and proteome, it was predicted that TBCO might impair vision and contraction of cardiac muscle, respectively, and these effects were confirmed by targeted bioassays. This study provided a comprehensive understanding of effects of TBCO on medaka at early life stages and illustrated the power of "omics" to explain and predict phenotypic responses to chemicals.

Erythrocyte glutathione is a novel biomarker of Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a congenital red cell aplasia with mutations in ribosomal protein (RP) genes. Elevated activity of erythrocyte adenosine deaminase (eADA) has been utilized as a biomarker of DBA. We examined erythrocyte reduced glutathione (GSH) as well as eADA in 22 patients in 18 DBA families, in whom RP gene mutations had been identified. Simultaneous evaluation of both eADA and GSH demonstrated that all examined DBA patients showed elevated values of either eADA or GSH, whereas presence of both eADA and GSH elevation was able to distinguish DBA patients from 34 normal controls and 14 unaffected members of the DBA families. Furthermore, a support vector machines analysis using both eADA and GSH levels yielded a formula to differentiate DBA from both normal controls and non-DBA family members. To confirm the usefulness of the formula, we analyzed additional 7 patients diagnosed by the clinical criteria. Although eADA showed within normal values in 3 patients, all of these patients were diagnosed as 'DBA' by use of the formula. Because extensive analysis of the RP genes failed to detect no causative mutation in approximately 40% of clinically diagnosed DBA patients, GSH may be useful an additional biomarker for diagnosis of DBA.

Mice with a Mutation in the Mdm2 Gene That Interferes with MDM2/Ribosomal Protein Binding Develop a Defect in Erythropoiesis

MDM2, an E3 ubiquitin ligase, is an important negative regulator of tumor suppressor p53. In turn the Mdm2 gene is a transcriptional target of p53, forming a negative feedback loop that is important in cell cycle control. It has recently become apparent that the ubiquitination of p53 by MDM2 can be inhibited when certain ribosomal proteins, including RPL5 and RPL11, bind to MDM2. This inhibition, and the resulting increase in p53 levels has been proposed to be responsible for the red cell aplasia seen in Diamond-Blackfan anemia (DBA) and in 5q- myelodysplastic syndrome (MDS). DBA and 5q- MDS are associated with inherited (DBA) or acquired (5q- MDS) haploinsufficiency of ribosomal proteins. A mutation in Mdm2 causing a C305F amino acid substitution blocks the binding of ribosomal proteins. Mice harboring this mutation (Mdm2^C305F), retain a normal p53 response to DNA damage, but lack the p53 response to perturbations in ribosome biogenesis. While studying the interaction between RP haploinsufficiency and the Mdm2^C305F mutation we noticed that Mdm2^C305F homozygous mice had altered hematopoiesis. These mice developed a mild macrocytic anemia with reticulocytosis. In the bone marrow (BM), these mice showed a significant decrease in Ter119^hi cells compared to wild type (WT) littermates, while no decrease in the number of mature erythroid cells (Ter119^hiCD71^low) was found in the spleen, which showed compensated bone marrow hematopoiesis. In methylcellulose cultures, BFU-E colonies from the mutant mice were slightly reduced in number and there was a significant reduction in CFU-E colony numbers in mutant mice compared with WT controls (p < 0.01). This erythropoietic defect was abrogated by concomitant p53 deficiency (Trp53^ko/ko). Further investigation revealed that in Mdm2^C305F animals, there was a decrease in Lin^-Sca-1⁺c-Kit⁺ (LSK) cells, accompanied by significant decreases in multipotent progenitor (MPP) cells (p < 0.01). Competitive BM repopulation experiments showed that donor BM harboring the Mdm2^C305F mutation possessed decreased repopulation capacity compared to WT BM, suggesting a functional stem cell deficit. These results suggest that there is a fine tuned balance in the interaction of ribosomal proteins with the MDM2/p53 axis which is important in normal hematopoiesis.

Rpl22 Loss Impairs the Development of B Lymphocytes by Activating a p53-Dependent Checkpoint

Although ribosomal proteins facilitate the ribosome’s core function of translation, emerging evidence suggests that some ribosomal proteins are also capable of performing tissue-restricted functions either from within specialized ribosomes or from outside of the ribosome. In particular, we have previously demonstrated that germline ablation of the gene encoding ribosomal protein Rpl22 causes a selective and p53-dependent arrest of ab T cell progenitors at the b-selection checkpoint. We have now identified a crucial role for Rpl22 during early B cell development. Germline ablation of Rpl22 results in a reduction in the absolute number of B-lineage progenitors in the bone marrow beginning at the pro–B cell stage. Although Rpl22-deficient pro–B cells are hyporesponsive to IL-7, a key cytokine required for early B cell development, the arrest of B cell development does not result from disrupted IL-7 signaling. Instead, p53 induction appears to be responsible for the developmental defects, as Rpl22 deficiency causes increased expression of p53 and activation of downstream p53 target genes, and p53 deficiency rescues the defect in B cell development in Rpl22-deficient mice. Interestingly, the requirement for Rpl22 in the B cell lineage appears to be developmentally restricted, because Rpl22-deficient splenic B cells proliferate normally in response to Ag receptor and Toll receptor stimuli and undergo normal class-switch recombination. These results indicate that Rpl22 performs a critical, developmentally restricted role in supporting early B cell development by preventing p53 induction.

Role of ribosomal protein mutations in tumor development (Review)

Ribosomes are cellular machines essential for protein synthesis. The biogenesis of ribosomes is a highly complex and energy consuming process that initiates in the nucleolus. Recently, a series of studies applying whole-exome or whole-genome sequencing techniques have led to the discovery of ribosomal protein gene mutations in different cancer types. Mutations in ribosomal protein genes have for example been found in endometrial cancer (RPL22), T-cell acute lymphoblastic leukemia (RPL10, RPL5 and RPL11), chronic lymphocytic leukemia (RPS15), colorectal cancer (RPS20), and glioma (RPL5). Moreover, patients suffering from Diamond-Blackfan anemia, a bone marrow failure syndrome caused by mutant ribosomal proteins are also at higher risk for developing leukemia, or solid tumors. Different experimental models indicate potential mechanisms whereby ribosomal proteins may initiate cancer development. In particular, deregulation of the p53 tumor suppressor network and altered mRNA translation are mechanisms likely to be involved. We envisage that changes in expression and the occurrence of ribosomal protein gene mutations play important roles in cancer development. Ribosome biology constitutes a re-emerging vital area of basic and translational cancer research.

The Ribosome Biogenesis Protein Nol9 Is Essential for Definitive Hematopoiesis and Pancreas Morphogenesis in Zebrafish

Ribosome biogenesis is a ubiquitous and essential process in cells. Defects in ribosome biogenesis and function result in a group of human disorders, collectively known as ribosomopathies. In this study, we describe a zebrafish mutant with a loss-of-function mutation in nol9, a gene that encodes a non-ribosomal protein involved in rRNA processing. nol9sa1022/sa1022 mutants have a defect in 28S rRNA processing. The nol9sa1022/sa1022 larvae display hypoplastic pancreas, liver and intestine and have decreased numbers of hematopoietic stem and progenitor cells (HSPCs), as well as definitive erythrocytes and lymphocytes. In addition, ultrastructural analysis revealed signs of pathological processes occurring in endothelial cells of the caudal vein, emphasizing the complexity of the phenotype observed in nol9sa1022/sa1022 larvae. We further show that both the pancreatic and hematopoietic deficiencies in nol9sa1022/sa1022 embryos were due to impaired cell proliferation of respective progenitor cells. Interestingly, genetic loss of Tp53 rescued the HSPCs but not the pancreatic defects. In contrast, activation of mRNA translation via the mTOR pathway by L-Leucine treatment did not revert the erythroid or pancreatic defects. Together, we present the nol9sa1022/sa1022 mutant, a novel zebrafish ribosomopathy model, which recapitulates key human disease characteristics. The use of this genetically tractable model will enhance our understanding of the tissue-specific mechanisms following impaired ribosome biogenesis in the context of an intact vertebrate.

Guilty as CHARGED: p53's expanding role in disease

Unrestrained p53 activity during development, as occurs upon loss of the p53 negative regulators Mdm2 or Mdmx, causes early embryonic lethality. Surprisingly, co-expression of wild-type p53 and a transcriptionally-dead variant of p53, with mutations in both transactivation domains (p53(L25Q,W26S,F53Q,F54S)), also causes lethality, but later in gestation and in association with a host of very specific phenotypes reminiscent of a syndrome known as CHARGE. Molecular analyses revealed that wild-type p53 is inappropriately activated in p53(5,26,53,54/)(+) embryos, triggering cell-cycle arrest or apoptosis during development to cause CHARGE phenotypes. In addition, CHARGE syndrome is typically caused by mutations in the CHD7 chromatin remodeler, and we have shown that activated p53 contributes to phenotypes caused by CHD7-deficiency. Together, these studies provide new insight into CHARGE syndrome and expand our understanding of the role of p53 in diseases other than cancer.

Glucocorticoids improve erythroid progenitor maintenance and dampen Trp53 response in a mouse model of Diamond-Blackfan anaemia

Diamond-Blackfan anaemia (DBA) is a rare congenital disease causing severe anaemia and progressive bone marrow failure. The majority of patients carry mutations in ribosomal proteins, which leads to depletion of erythroid progenitors in the bone marrow. As many as 40% of all DBA patients receive glucocorticoids to alleviate their anaemia. However, despite their use in DBA treatment for more than half a century, the therapeutic mechanisms of glucocorticoids remain largely unknown. Therefore we sought to study disease specific effects of glucocorticoid treatment using a ribosomal protein s19 (Rps19) deficient mouse model of DBA. This study determines for the first time that a mouse model of DBA can respond to glucocorticoid treatment, similar to DBA patients. Our results demonstrate that glucocorticoid treatment reduces apoptosis, rescues erythroid progenitor depletion and premature differentiation of erythroid cells. Furthermore, glucocorticoids prevent Trp53 activation in Rps19-deficient cells- in a disease-specific manner. Dissecting the therapeutic mechanisms behind glucocorticoid treatment of DBA provides indispensible insight into DBA pathogenesis. Identifying mechanisms important for DBA treatment also enables development of more disease-specific treatments of DBA.

RAP-011 improves erythropoiesis in zebrafish model of Diamond-Blackfan anemia through antagonizing lefty1

Ribosome deficiency in zebrafish leads to defects in erythroid maturation and is reversed by RAP-011 treatment. Identification of lefty1 as a key mediator of erythropoiesis.

Cytokinesis failure in RhoA-deficient mouse erythroblasts involves actomyosin and midbody dysregulation and triggers p53 activation

RhoA GTPase has been shown in vitro in cell lines and in vivo in nonmammalian organisms to regulate cell division, particularly during cytokinesis and abscission, when 2 daughter cells partition through coordinated actomyosin and microtubule machineries. To investigate the role of this GTPase in the rapidly proliferating mammalian erythroid lineage, we developed a mouse model with erythroid-specific deletion of RhoA. This model was proved embryonic lethal as a result of severe anemia by embryonic day 16.5 (E16.5). The primitive red blood cells were enlarged, poikilocytic, and frequently multinucleated, but were able to sustain life despite experiencing cytokinesis failure. In contrast, definitive erythropoiesis failed and the mice died by E16.5, with profound reduction of maturing erythroblast populations within the fetal liver. RhoA was required to activate myosin-regulatory light chain and localized at the site of the midbody formation in dividing wild-type erythroblasts. Cytokinesis failure caused by RhoA deficiency resulted in p53 activation and p21-transcriptional upregulation with associated cell-cycle arrest, increased DNA damage, and cell death. Our findings demonstrate the role of RhoA as a critical regulator for efficient erythroblast proliferation and the p53 pathway as a powerful quality control mechanism in erythropoiesis.

Mutation of kri1l causes definitive hematopoiesis failure via PERK-dependent excessive autophagy induction

Dysregulation of ribosome biogenesis causes human diseases, such as Diamond-Blackfan anemia, del (5q-) syndrome and bone marrow failure. However, the mechanisms of blood disorders in these diseases remain elusive. Through genetic mapping, molecular cloning and mechanism characterization of the zebrafish mutant cas002, we reveal a novel connection between ribosomal dysfunction and excessive autophagy in the regulation of hematopoietic stem/progenitor cells (HSPCs). cas002 carries a recessive lethal mutation in kri1l gene that encodes an essential component of rRNA small subunit processome. We show that Kri1l is required for normal ribosome biogenesis, expansion of definitive HSPCs and subsequent lineage differentiation. Through live imaging and biochemical studies, we find that loss of Kri1l causes the accumulation of misfolded proteins and excessive PERK activation-dependent autophagy in HSPCs. Blocking autophagy but not inhibiting apoptosis by Bcl2 overexpression can fully rescue hematopoietic defects, but not the lethality of kri1l^cas002 embryos. Treatment with autophagy inhibitors (3-MA and Baf A1) or PERK inhibitor (GSK2656157), or knockdown of beclin1 or perk can markedly restore HSPC proliferation and definitive hematopoietic cell differentiation. These results may provide leads for effective therapeutics that benefit patients with anemia or bone marrow failure caused by ribosome disorders.

Ribosomal Protein Mutations Result in Constitutive p53 Protein Degradation through Impairment of the AKT Pathway

Mutations in ribosomal protein (RP) genes can result in the loss of erythrocyte progenitor cells and cause severe anemia. This is seen in patients with Diamond-Blackfan anemia (DBA), a pure red cell aplasia and bone marrow failure syndrome that is almost exclusively linked to RP gene haploinsufficiency. While the mechanisms underlying the cytopenia phenotype of patients with these mutations are not completely understood, it is believed that stabilization of the p53 tumor suppressor protein may induce apoptosis in the progenitor cells. In stark contrast, tumor cells from zebrafish with RP gene haploinsufficiency are unable to stabilize p53 even when exposed to acute DNA damage despite transcribing wild type p53 normally. In this work we demonstrate that p53 has a limited role in eliciting the anemia phenotype of zebrafish models of DBA. In fact, we find that RP-deficient embryos exhibit the same normal p53 transcription, absence of p53 protein, and impaired p53 response to DNA damage as RP haploinsufficient tumor cells. Recently we reported that RP mutations suppress activity of the AKT pathway, and we show here that this suppression results in proteasomal degradation of p53. By re-activating the AKT pathway or by inhibiting GSK-3, a downstream modifier that normally represses AKT signaling, we are able to restore the stabilization of p53. Our work indicates that the anemia phenotype of zebrafish models of DBA is dependent on factors other than p53, and may hold clinical significance for both DBA and the increasing number of cancers revealing spontaneous mutations in RP genes.

The p53 tumor suppressor is the most commonly mutated gene in human cancers. However, cancer cells exploit multiple mechanisms to silence the p53 pathway in addition to inactivation of the p53 gene. We previously reported that one of these mechanisms is found in tumor cells with ribosomal protein (RP) gene mutations. These cells transcribe wild type p53 mRNA yet do not stabilize p53 protein when exposed to DNA damaging agents. In this work we demonstrate that this loss of p53 protein is due to its constitutive degradation. This degradation is due to impairment of the AKT pathway, which normal signals for p53 to stabilize when the DNA is damaged. By re-activating the AKT pathway in RP-mutant cells we are able to restore p53 stabilization and activity, which may hold clinical significance for cancer treatment.

Alteration of heme metabolism in a cellular model of Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a congenital pure red cell aplasia often associated with skeletal malformations. Mutations in ribosomal protein coding genes, mainly in RPS19, account for the majority of DBA cases. The molecular mechanisms underlying DBA pathogenesis are still not completely understood. Alternative spliced isoforms of FLVCR1 (feline leukemia virus subgroup C receptor 1) transcript coding for non-functional proteins have been reported in some DBA patients. Consistently, a phenotype very close to DBA has been described in animal models of FLVCR1 deficiency. FLVCR1 gene codes for two proteins: the plasma membrane heme exporter FLVCR1a and the mitochondrial heme exporter FLVCR1b. The coordinated expression of both FLVCR1 isoforms regulates an intracellular heme pool, necessary for proper expansion and differentiation of erythroid precursors. Here, we investigate the role of FLVCR1 isoforms in a cellular model of DBA. RPS19-downregulated TF1 cells show reduced FLVCR1a and FLVCR1b mRNA levels associated with heme overload. The downregulation of FLVCR1 isoforms affects cell cycle progression and apoptosis in differentiating K562 cells, a phenotype similar to DBA. Taken together, these data suggest that alteration of heme metabolism could play a role in the pathogenesis of DBA.

Ribosomal proteins as novel players in tumorigenesis

Ribosome biogenesis is the most demanding energetic and metabolic expenditure of the cell. The nucleolus, a nuclear compartment, coordinates rRNA transcription, maturation, and assembly into ribosome subunits. The transcription process is highly coordinated with ribosome biogenesis. In this context, ribosomal proteins (RPs) play a crucial role. In the last decade, an increasing number of studies have associated RPs with extraribosomal functions related to proliferation. Importantly, the expression of RPs appears to be deregulated in several human disorders due, at least in part, to genetic mutations. Although the deregulation of RPs in human malignancies is commonly observed, a more complex mechanism is believed to be involved, favoring the tumorigenic process, its progression and metastasis. This review explores the roles of the most frequently mutated oncogenes and tumor suppressor genes in human cancer that modulate ribosome biogenesis, including their interaction with RPs. In this regard, we propose a new focus for novel therapies.

Potential extra-ribosomal functions of ribosomal proteins in Saccharomyces cerevisiae

Ribosomal proteins (RPs), are essential components of the ribosomes, the molecular machines that turn mRNA blueprints into proteins, as they serve to stabilize the structure of the rRNA, thus improving protein biosynthesis. In addition, growing evidence suggests that RPs can function in other cellular roles. In the present review, we summarize several potential extra-ribosomal functions of RPs in ribosomal biogenesis, transcription activity, translation process, DNA repair, replicative life span, adhesive growth, and morphological transformation in Saccharomyces cerevisiae. However, the future in-depth studies are needed to identify these novel secondary functions of RPs in S. cerevisiae.

Disruption of the 5S RNP-Mdm2 interaction significantly improves the erythroid defect in a mouse model for Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a congenital erythroid hypoplasia caused by haploinsufficiency of genes encoding ribosomal proteins (RPs). Perturbed ribosome biogenesis in DBA has been shown to induce a p53-mediated ribosomal stress response. However, the mechanisms of p53 activation and its relevance for the erythroid defect remain elusive. Previous studies have indicated that activation of p53 is caused by the inhibition of mouse double minute 2 (Mdm2), the main negative regulator of p53, by the 5S ribonucleoprotein particle (RNP). Meanwhile, it is not clear whether this mechanism solely mediates the p53-dependent component found in DBA. To approach this question, we crossed our mouse model for RPS19-deficient DBA with Mdm2(C305F) knock-in mice that have a disrupted 5S RNP-Mdm2 interaction. Upon induction of the Rps19 deficiency, Mdm2(C305F) reversed the p53 response and improved expansion of hematopoietic progenitors in vitro, and ameliorated the anemia in vivo. Unexpectedly, disruption of the 5S RNP-Mdm2 interaction also led to selective defect in erythropoiesis. Our findings highlight the sensitivity of erythroid progenitor cells to aberrations in p53 homeostasis mediated by the 5S RNP-Mdm2 interaction. Finally, we provide evidence indicating that physiological activation of the 5S RNP-Mdm2-p53 pathway may contribute to functional decline of the hematopoietic system in a cell-autonomous manner over time.

Cellular differences in protein synthesis regulate tissue homeostasis

Although sometimes considered a "house-keeping" function, multiple aspects of protein synthesis are regulated differently among somatic cells, including stem cells, and can be modulated in a cell-type-specific manner. These differences are required to establish and maintain differences in cell identity, cell function, tissue homeostasis, and tumor suppression.

Ribosomal proteins as unrevealed caretakers for cellular stress and genomic instability

Ribosomal proteins (RPs) have gained much attention for their extraribosomal functions particularly with respect to p53 regulation. To date, about fourteen RPs have shown to bind to MDM2 and regulate p53. Upon binding to MDM2, the RPs suppress MDM2 E3 ubiquitin ligase activity resulting in the stabilization and activation of p53. Of the RPs that bind to MDM2, RPL5 and RPL11 are the most studied and RPL11 appears to have the most significant role in p53 regulation. Considering that more than 17% of RP species have been shown to interact with MDM2, one of the questions remains unresolved is why so many RPs bind MDM2 and modulate p53. Genes encoding RPs are widely dispersed on different chromosomes in both mice and humans. As components of ribosome, RP expression is tightly regulated to meet the appropriate stoichiometric ratio between RPs and rRNAs. Once genomic instability (e.g. aneuploidy) occurs, transcriptional and translational changes due to change of DNA copy number can result in an imbalance in the expression of RPs including those that bind to MDM2. Such an imbalance in RP expression could lead to failure to assemble functional ribosomes resulting in ribosomal stress. We propose that RPs have evolved ability to regulate MDM2 in response to genomic instability as an additional layer of p53 regulation. Full understanding of the biological roles of RPs could potentially establish RPs as a novel class of therapeutic targets in human diseases such as cancer.