Sbds is required for Rac2-mediated monocyte migration and signaling downstream of RANK during osteoclastogenesis

Knockdown of the Shwachman-Diamond syndrome gene, SBDS, induces galectin-1 expression and impairs cell growth

Shwachman-Diamond syndrome (SDS) is an autosomal recessive disorder characterized by exocrine pancreatic insufficiency and bone marrow failure. The depletion of SBDS protein by RNA interference has been shown to cause inhibition of cell proliferation in several cell lines. However, the precise mechanism by which the loss of SBDS leads to inhibition of cell growth remains unknown. To evaluate the impaired growth of SBDS-knockdown cells, we analyzed Epstein-Barr virus-transformed lymphoblast cells (LCLs) derived from two patients with SDS (c. 183_184TA > CT and c. 258 + 2 T > C). After 3 days of culture, the growth of LCL-SDS cell lines was considerably less than that of control donor cells. By annealing control primer-based GeneFishing PCR screening, we found that galectin-1 (Gal-1) mRNA expression was elevated in LCL-SDS cells. Western blot analysis showed that the level of Gal-1 protein expression was also increased in LCL-SDS cells as well as in SBDS-knockdown 32Dcl3 murine myeloid cells. We confirmed that recombinant Gal-1 inhibited the proliferation of both LCL-control and LCL-SDS cells and induced apoptosis (as determined by annexin V-positive staining). These results suggest that the overexpression of Gal-1 contributes to abnormal cell growth in SBDS-deficient cells.

Fanca deficiency is associated with alterations in osteoclastogenesis that are rescued by TNFα

BACKGROUND

Hematopoietic stem cells (HSCs) reside in the bone marrow (BM) niche, which includes bone-forming and bone-resorbing cells, i.e., osteoblasts (OBs) and osteoclasts (OCs). OBs originate from mesenchymal progenitors, while OCs are derived from HSCs. Self-renewal, proliferation and differentiation of HSCs are under the control of regulatory signals generated by OBs and OCs within the BM niche. Consequently, OBs and OCs control both bone physiology and hematopoiesis. Since the human developmental and bone marrow failure genetic syndrome fanconi anemia (FA) presents with skeletal abnormalities, osteoporosis and HSC impairment, we wanted to test the hypothesis that the main pathological abnormalities of FA could be related to a defect in OC physiology and/or in bone homeostasis.

RESULTS

We revealed here that the intrinsic differentiation of OCs from a Fanca^-/- mouse is impaired in vitro due to overactivation of the p53-p21 axis and defects in NF-kB signaling. The OC differentiation abnormalities observed in vitro were rescued by treating Fanca^-/- cells with the p53 inhibitor pifithrin-α, by treatment with the proinflammatory cytokine TNFα or by coculturing them with Fanca-proficient or Fanca-deficient osteoblastic cells.

CONCLUSIONS

Overall, our results highlight an unappreciated role of Fanca in OC differentiation that is potentially circumvented in vivo by the presence of OBs and TNFα in the BM niche.

Immunoporosis: Role of Innate Immune Cells in Osteoporosis

Osteoporosis or porous bone disorder is the result of an imbalance in an otherwise highly balanced physiological process known as 'bone remodeling'. The immune system is intricately involved in bone physiology as well as pathologies. Inflammatory diseases are often correlated with osteoporosis. Inflammatory mediators such as reactive oxygen species (ROS), and pro-inflammatory cytokines and chemokines directly or indirectly act on the bone cells and play a role in the pathogenesis of osteoporosis. Recently, Srivastava et al. (Srivastava RK, Dar HY, Mishra PK. Immunoporosis: Immunology of Osteoporosis-Role of T Cells. Frontiers in immunology. 2018;9:657) have coined the term "immunoporosis" to emphasize the role of immune cells in the pathology of osteoporosis. Accumulated pieces of evidence suggest both innate and adaptive immune cells contribute to osteoporosis. However, innate cells are the major effectors of inflammation. They sense various triggers to inflammation such as pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), cellular stress, etc., thus producing pro-inflammatory mediators that play a critical role in the pathogenesis of osteoporosis. In this review, we have discussed the role of the innate immune cells in great detail and divided these cells into different sections in a systemic manner. In the beginning, we talked about cells of the myeloid lineage, including macrophages, monocytes, and dendritic cells. This group of cells explicitly influences the skeletal system by the action of production of pro-inflammatory cytokines and can transdifferentiate into osteoclast. Other cells of the myeloid lineage, such as neutrophils, eosinophils, and mast cells, largely impact osteoporosis via the production of pro-inflammatory cytokines. Further, we talked about the cells of the lymphoid lineage, including natural killer cells and innate lymphoid cells, which share innate-like properties and play a role in osteoporosis. In addition to various innate immune cells, we also discussed the impact of classical pro-inflammatory cytokines on osteoporosis. We also highlighted the studies regarding the impact of physiological and metabolic changes in the body, which results in chronic inflammatory conditions such as ageing, ultimately triggering osteoporosis.

Repolarization of HSC attenuates HSCs failure in Shwachman-Diamond syndrome

Shwachman-Diamond syndrome (SDS) is a bone marrow failure (BMF) syndrome associated with an increased risk of myelodysplasia and leukemia. The molecular mechanisms of SDS are not fully understood. We report that primitive hematopoietic cells from SDS patients present with a reduced activity of the small RhoGTPase Cdc42 and concomitantly a reduced frequency of HSCs polar for polarity proteins. The level of apolarity of SDS HSCs correlated with the magnitude of HSC depletion in SDS patients. Importantly, exogenously provided Wnt5a or GDF11 that elevates the activity of Cdc42 restored polarity in SDS HSCs and increased the number of HSCs in SDS patient samples in surrogate ex vivo assays. Single cell level RNA-Seq analyses of SDS HSCs and daughter cells demonstrated that SDS HSC treated with GDF11 are transcriptionally more similar to control than to SDS HSCs. Treatment with GDF11 reverted pathways in SDS HSCs associated with rRNA processing and ribosome function, but also viral infection and immune function, p53-dependent DNA damage, spindle checkpoints, and metabolism, further implying a role of these pathways in HSC failure in SDS. Our data suggest that HSC failure in SDS is driven at least in part by low Cdc42 activity in SDS HSCs. Our data thus identify novel rationale approaches to attenuate HSCs failure in SDS.

Genetic Basis of Response of Ghanaian Local Chickens to Infection With a Lentogenic Newcastle Disease Virus

Newcastle disease (ND) is a global threat to domestic poultry, especially in rural areas of Africa and Asia, where the loss of entire backyard local chicken flocks often threatens household food security and income. To investigate the genetics of Ghanaian local chicken ecotypes to Newcastle disease virus (NDV), in this study, three popular Ghanaian chicken ecotypes (regional populations) were challenged with a lentogenic NDV strain at 28 days of age. This study was conducted in parallel with a similar study that used three popular Tanzanian local chicken ecotypes and after two companion studies in the United States, using Hy-line Brown commercial laying birds. In addition to growth rate, NDV response traits were measured following infection, including anti-NDV antibody levels [pre-infection and 10 days post-infection (dpi)], and viral load (2 and 6 dpi). Genetic parameters were estimated, and two genome-wide association study analysis methods were used on data from 1,440 Ghanaian chickens that were genotyped on a chicken 600K Single Nucleotide Polymorphism (SNP) chip. Both Ghana and Tanzania NDV challenge studies revealed moderate to high (0.18 – 0.55) estimates of heritability for all traits, except viral clearance where the heritability estimate was not different from zero for the Tanzanian ecotypes. For the Ghana study, 12 quantitative trait loci (QTL) for growth and/or response to NDV from single-SNP analyses and 20 genomic regions that explained more than 1% of genetic variance using the Bayes B method were identified. Seven of these windows were also identified as having at least one significant SNP in the single SNP analyses for growth rate, anti-NDV antibody levels, and viral load at 2 and 6 dpi. An important gene for growth during stress, CHORDC1 associated with post-infection growth rate was identified as a positional candidate gene, as well as other immune related genes, including VAV2, IL12B, DUSP1, and IL17B. The QTL identified in the Ghana study did not overlap with those identified in the Tanzania study. However, both studies revealed QTL with genes vital for growth and immune response during NDV challenge. The Tanzania parallel study revealed an overlapping QTL on chromosome 24 for viral load at 6 dpi with the US NDV study in which birds were challenged with NDV under heat stress. This QTL region includes genes related to immune response, including TIRAP, ETS1, and KIRREL3. The moderate to high estimates of heritability and the identified QTL suggest that host response to NDV of local African chicken ecotypes can be improved through selective breeding to enhance increased NDV resistance and vaccine efficacy.

Opposing roles of hematopoietic-specific small GTPase Rac2 and the guanine nucleotide exchange factor Vav1 in osteoclast differentiation

Vav1 regulates Rac activation as a hematopoietic-specific Rho/Rac-family guanine nucleotide exchange factor. Rac is a subfamily of Rho GTPases that regulates the bone-resorbing capacity of osteoclasts (OCs). In this study, we show that hematopoietic-specific Rac2 and Vav1 play opposing roles by enhancing or attenuating OC differentiation, respectively. This was demonstrated by higher and lower bone density in the femurs from Rac2-deficient (Rac2-/-) and Vav1-deficient (Vav1-/-) mice, respectively, compared to the wild-type (WT) mice. Accordingly, Rac2-/- cells displayed low numbers of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells (41%) compared to WT cells, whereas, Vav1-/- cells showed high TRAP-positive cell numbers (150%), and the double-knockout Rac2-/-Vav1-/- mice nullified the effects on OC numbers achieved by the individual knockouts. These reciprocal roles of Rac2 and Vav1 in OC differentiation were confirmed by reduced and increased levels of OC-specific markers, such as TRAP, calcitonin receptor, cathepsin K, and DC-STAMP in the Rac2-/- and Vav1-/- OCs, respectively. Our findings of decrease and increase in actin ring formation and αvβ3 integrin-mediated adhesion in Rac2-/- and Vav1-/- mice, respectively, suggest that Vav1 and its downstream GTPase, Rac2, may counteract to fine-tune OC differentiation and bone resorption.

Dual regulation of p53 by the ribosome maturation factor SBDS

The Shwachman-Bodian Diamond syndrome (SBDS)-associated gene, SBDS, is involved in rRNA synthesis and ribosome maturation, but the role of SBDS in cancer is largely elusive. In this study, we found that SBDS is often overexpressed or amplified in human cancers, and high level of endogenous SBDS is significantly associated with unfavorable prognosis. Conversely, knockdown of SBDS leads to p53 stabilization and activation through the ribosomal stress-RPL5/RPL11-MDM2 pathway, resulting in the repression of cancer cell proliferation and invasion. Interestingly, ectopic SBDS in the nucleoplasm also suppresses tumor cell growth and proliferation in vitro and in vivo. Mechanistically, ectopically expressed SBDS triggered by, for example, ribosomal stress binds to the transactivation domain of p53 and perturbs the MDM2-p53 interaction, consequently leading to impaired p53 ubiquitination and proteasomal degradation. Altogether, our finding for the first time demonstrates the dual functions of SBDS in cancer development by coordinating ribosome biogenesis and p53 activity.

Multi-omics Data Integration for Identifying Osteoporosis Biomarkers and Their Biological Interaction and Causal Mechanisms

Osteoporosis is characterized by low bone mineral density (BMD). The advancement of high-throughput technologies and integrative approaches provided an opportunity for deciphering the mechanisms underlying osteoporosis. Here, we generated genomic, transcriptomic, methylomic, and metabolomic datasets from 119 subjects with high (n = 61) and low (n = 58) BMDs. By adopting sparse multiple discriminative canonical correlation analysis, we identified an optimal multi-omics biomarker panel with 74 differentially expressed genes (DEGs), 75 differentially methylated CpG sites (DMCs), and 23 differential metabolic products (DMPs). By linking genetic data, we identified 199 targeted BMD-associated expression/methylation/metabolite quantitative trait loci (eQTLs/meQTLs/metaQTLs). The reconstructed networks/pathways showed extensive biomarker interactions, and a substantial proportion of these biomarkers were enriched in RANK/RANKL, MAPK/TGF-β, and WNT/β-catenin pathways and G-protein-coupled receptor, GTP-binding/GTPase, telomere/mitochondrial activities that are essential for bone metabolism. Five biomarkers (FADS2, ADRA2A, FMN1, RABL2A, SPRY1) revealed causal effects on BMD variation. Our study provided an innovative framework and insights into the pathogenesis of osteoporosis.

Genetic Analyses of Tanzanian Local Chicken Ecotypes Challenged with Newcastle Disease Virus

Newcastle Disease (ND) is a continuing global threat to domestic poultry, especially in developing countries, where severe outbreaks of velogenic ND virus (NDV) often cause major economic losses to households. Local chickens are of great importance to rural family livelihoods through provision of high-quality protein. To investigate the genetic basis of host response to NDV, three popular Tanzanian chicken ecotypes (regional populations) were challenged with a lentogenic (vaccine) strain of NDV at 28 days of age. Various host response phenotypes, including anti-NDV antibody levels (pre-infection and 10 days post-infection, dpi), and viral load (2 and 6 dpi) were measured, in addition to growth rate. We estimated genetic parameters and conducted genome-wide association study analyses by genotyping 1399 chickens using the Affymetrix 600K chicken SNP chip. Estimates of heritability of the evaluated traits were moderate (0.18–0.35). Five quantitative trait loci (QTL) associated with growth and/or response to NDV were identified by single-SNP analyses, with some regions explaining ≥1% of genetic variance based on the Bayes-B method. Immune related genes, such as ETS1, TIRAP, and KIRREL3, were located in regions associated with viral load at 6 dpi. The moderate estimates of heritability and identified QTL indicate that NDV response traits may be improved through selective breeding of chickens to enhance increased NDV resistance and vaccine efficacy in Tanzanian local ecotypes.

Integrative genomic analysis predicts novel functional enhancer-SNPs for bone mineral density

Osteoporosis is a skeletal disorder characterized by low bone mineral density (BMD) and deterioration of bone microarchitecture. To identify novel genetic loci underlying osteoporosis, an effective strategy is to focus on scanning of variants with high potential functional impacts. Enhancers play a crucial role in regulating cell-type-specific transcription. Therefore, single-nucleotide polymorphisms (SNPs) located in enhancers (enhancer-SNPs) may represent strong candidate functional variants. Here, we performed a targeted analysis for potential functional enhancer-SNPs that may affect gene expression and biological processes in bone-related cells, specifically, osteoblasts, and peripheral blood monocytes (PBMs), using five independent cohorts (n = 5905) and the genetics factors for osteoporosis summary statistics, followed by comprehensive integrative genomic analyses of chromatin states, transcription, and metabolites. We identified 15 novel enhancer-SNPs associated with femoral neck and lumbar spine BMD, including 5 SNPs mapped to novel genes (e.g., rs10840343 and rs10770081 in IGF2 gene) and 10 novel SNPs mapped to known BMD-associated genes (e.g., rs2941742 in ESR1 gene, and rs10249092 and rs4342522 in SHFM1 gene). Interestingly, enhancer-SNPs rs10249092 and rs4342522 in SHFM1 were tightly linked, but annotated to different enhancers in PBMs and osteoblasts, respectively, suggesting that even tightly linked SNPs may regulate the same target gene and contribute to the phenotype variation in cell-type-specific manners. Importantly, ten enhancer-SNPs may also regulate BMD variation by affecting the serum metabolite levels. Our findings revealed novel susceptibility loci that may regulate BMD variation and provided intriguing insights into the genetic mechanisms of osteoporosis.

Meta-Analysis of Genome-Wide Association Studies Identifies Novel Functional CpG-SNPs Associated with Bone Mineral Density at Lumbar Spine

Osteoporosis is a serious public health issue, which is mostly characterized by low bone mineral density (BMD). To search for additional genetic susceptibility loci underlying BMD variation, an effective strategy is to focus on testing of specific variants with high potential of functional effects. Single nucleotide polymorphisms (SNPs) that introduce or disrupt CpG dinucleotides (CpG-SNPs) may alter DNA methylation levels and thus represent strong candidate functional variants. Here, we performed a targeted GWAS for 63,627 potential functional CpG-SNPs that may affect DNA methylation in bone-related cells, in five independent cohorts (n = 5905). By meta-analysis, 9 CpG-SNPs achieved a genome-wide significance level (p < 7.86 × 10-7) for association with lumbar spine BMD and additional 15 CpG-SNPs showed suggestive significant (p < 5.00 × 10-5) association, of which 2 novel SNPs rs7231498 (NFATC1) and rs7455028 (ESR1) also reached a genome-wide significance level in the joint analysis. Several identified CpG-SNPs were mapped to genes that have not been reported for association with BMD in previous GWAS, such as NEK3 and NFATC1 genes, highlighting the enhanced power of targeted association analysis for identification of novel associations that were missed by traditional GWAS. Interestingly, several genomic regions, such as NEK3 and LRP5 regions, contained multiple significant/suggestive CpG-SNPs for lumbar spine BMD, suggesting that multiple neighboring CpG-SNPs may synergistically mediate the DNA methylation level and gene expression pattern of target genes. Furthermore, functional annotation analyses suggested a strong regulatory potential of the identified BMD-associated CpG-SNPs and a significant enrichment in biological processes associated with protein localization and protein signal transduction. Our results provided novel insights into the genetic basis of BMD variation and highlighted the close connections between genetic and epigenetic mechanisms of complex disease.

Methods to Investigate the Role of Rho GTPases in Osteoclast Function

The actin cytoskeleton is essential for the biology of osteoclasts, in particular during bone resorption. As key regulators of actin dynamics, the small GTPases of the Rho family are very important in the control of osteoclast activity. The study of Rho GTPase signaling pathways is essential to uncover the mechanisms of bone resorption and can have interesting applications for the treatment of osteolytic diseases. In this chapter, we describe various techniques to obtain primary osteoclasts from murine bone marrow cells, to measure Rho GTPase activation levels, to monitor bone resorption activity of osteoclasts and to introduce the expression of proteins of interest using a retroviral approach. We illustrate the different methods with experimental examples of the effect of Rac1 activation by the exchange factor Dock5 on bone resorption by osteoclasts.

Molecular basis of the human ribosomopathy Shwachman-Diamond syndrome

Mutations that target the ubiquitous process of ribosome assembly paradoxically cause diverse tissue-specific disorders (ribosomopathies) that are often associated with an increased risk of cancer. Ribosomes are the essential macromolecular machines that read the genetic code in all cells in all kingdoms of life. Following pre-assembly in the nucleus, precursors of the large 60S and small 40S ribosomal subunits are exported to the cytoplasm where the final steps in maturation are completed. Here, I review the recent insights into the conserved mechanisms of ribosome assembly that have come from functional characterisation of the genes mutated in human ribosomopathies. In particular, recent advances in cryo-electron microscopy, coupled with genetic, biochemical and prior structural data, have revealed that the SBDS protein that is deficient in the inherited leukaemia predisposition disorder Shwachman-Diamond syndrome couples the final step in cytoplasmic 60S ribosomal subunit maturation to a quality control assessment of the structural and functional integrity of the nascent particle. Thus, study of this fascinating disorder is providing remarkable insights into how the large ribosomal subunit is functionally activated in the cytoplasm to enter the actively translating pool of ribosomes.

Functional relevance for associations between osteoporosis and genetic variants

Osteoporosis is characterized by increased bone loss and deterioration of bone microarchitecture, which will lead to reduced bone strength and increased risk of fragility fractures. Previous studies have identified many genetic loci associated with osteoporosis, but functional mechanisms underlying the associations have rarely been explored. In order to explore the potential molecular functional mechanisms underlying the associations for osteoporosis, we performed integrative analyses by using the publically available datasets and resources. We searched 128 identified osteoporosis associated SNPs (P<10-6), and 8 SNPs exert cis-regulation effects on 11 eQTL target genes. Among the 8 SNPs, 2 SNPs (RPL31 rs2278729 and LRP5 rs3736228) were confirmed to impact the expression of 3 genes (RPL31, CPT1A and MTL5) that were differentially expressed between human subjects of high BMD group and low BMD group. All of the functional evidence suggested the important functional mechanisms underlying the associations of the 2 SNPs (rs2278729 and rs3736228) and 3 genes (RPL31, CPT1A and MTL5) with osteoporosis. This study may provide novel insights into the functional mechanisms underlying the osteoporosis associated genetic variants, which will help us to comprehend the potential mechanisms underlying the genetic association for osteoporosis.

Shwachman-Bodian-Diamond syndrome (SBDS) protein deficiency impairs translation re-initiation from C/EBPα and C/EBPβ mRNAs

Mutations in the Shwachman–Bodian–Diamond Syndrome (SBDS) gene cause Shwachman–Diamond Syndrome (SDS), a rare congenital disease characterized by bone marrow failure with neutropenia, exocrine pancreatic dysfunction and skeletal abnormalities. The SBDS protein is important for ribosome maturation and therefore SDS belongs to the ribosomopathies. It is unknown, however, if loss of SBDS functionality affects the translation of specific mRNAs and whether this could play a role in the development of the clinical features of SDS. Here, we report that translation of the C/EBPα and -β mRNAs, that are indispensible regulators of granulocytic differentiation, is altered by SBDS mutations or knockdown. We show that SBDS function is specifically required for efficient translation re-initiation into the protein isoforms C/EBPα-p30 and C/EBPβ-LIP, which is controlled by a single cis-regulatory upstream open reading frame (uORF) in the 5′ untranslated regions (5′ UTRs) of both mRNAs. Furthermore, we show that as a consequence of the C/EBPα and -β deregulation the expression of MYC is decreased with associated reduction in proliferation, suggesting that failure of progenitor proliferation contributes to the haematological phenotype of SDS. Therefore, our study provides the first indication that disturbance of specific translation by loss of SBDS function may contribute to the development of the SDS phenotype.

Integrative Analysis of Transcriptomic and Epigenomic Data to Reveal Regulation Patterns for BMD Variation

Integration of multiple profiling data and construction of functional gene networks may provide additional insights into the molecular mechanisms of complex diseases. Osteoporosis is a worldwide public health problem, but the complex gene-gene interactions, post-transcriptional modifications and regulation of functional networks are still unclear. To gain a comprehensive understanding of osteoporosis etiology, transcriptome gene expression microarray, epigenomic miRNA microarray and methylome sequencing were performed simultaneously in 5 high hip BMD (Bone Mineral Density) subjects and 5 low hip BMD subjects. SPIA (Signaling Pathway Impact Analysis) and PCST (Prize Collecting Steiner Tree) algorithm were used to perform pathway-enrichment analysis and construct the interaction networks. Through integrating the transcriptomic and epigenomic data, firstly we identified 3 genes (FAM50A, ZNF473 and TMEM55B) and one miRNA (hsa-mir-4291) which showed the consistent association evidence from both gene expression and methylation data; secondly in network analysis we identified an interaction network module with 12 genes and 11 miRNAs including AKT1, STAT3, STAT5A, FLT3, hsa-mir-141 and hsa-mir-34a which have been associated with BMD in previous studies. This module revealed the crosstalk among miRNAs, mRNAs and DNA methylation and showed four potential regulatory patterns of gene expression to influence the BMD status. In conclusion, the integration of multiple layers of omics can yield in-depth results than analysis of individual omics data respectively. Integrative analysis from transcriptomics and epigenomic data improves our ability to identify causal genetic factors, and more importantly uncover functional regulation pattern of multi-omics for osteoporosis etiology.

Blockade of CD26 signaling inhibits human osteoclast development

Bone remodeling is maintained by the delicate balance between osteoblasts (OBs) and osteoclasts (OCs). However, the role of CD26 in regulating bone remodeling has not yet been characterized. We herein show that CD26 is preferentially expressed on normal human OCs and is intensely expressed on activated human OCs in osteolytic bone alterations. Macrophage-colony stimulating factor (M-CSF) and soluble receptor activator of NF-κB ligand (sRANKL) induced human OC differentiation, in association with CD26 expression on monocyte-macrophage lineage cells. CD26 expression was accompanied by increased phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), which is crucial for early human OC differentiation. The humanized anti-CD26 monoclonal antibody, huCD26mAb, impaired the formation and function of tartrate-resistant acid phosphatase (TRAP)/CD26 positive multi-nucleated (nuclei > 3) OCs with maturation in the manner of dose-dependency. It was revealed that huCD26mAb inhibits early OC differentiation via the inactivation of MKK3/6, p38 MAPK and subsequent dephosphorylation of microphthalmia-associated transcription factor (mi/Mitf). These inhibitions occur immediately after RANKL binds to RANK on the human OC precursor cells and were demonstrated using the OC functional assays. huCD26mAb subsequently impaired OC maturation and bone resorption by suppressing the expression of TRAP and OC fusion proteins. In addition, p38 MAPK inhibitor also strongly inhibited OC formation and function. Our results suggest that the blockade of CD26 signaling impairs the development of human functional OCs by inhibiting p38 MAPK-mi/Mitf phosphorylation pathway and that targeting human OCs with huCD26mAb may have therapeutic potential for the treatment of osteolytic lesions following metastasis to alleviate bone destruction and reduce total skeletal-related events (SREs).

Marrow failure: a window into ribosome biology

Diamond-Blackfan anemia, Shwachman-Diamond syndrome, and dyskeratosis congenita are inherited syndromes characterized by marrow failure, congenital anomalies, and cancer predisposition. Genetic and molecular studies have uncovered distinct abnormalities in ribosome biogenesis underlying each of these 3 disorders. How defects in ribosomes, the essential organelles required for protein biosynthesis in all cells, cause tissue-specific abnormalities in human disease remains a question of fundamental scientific and medical importance. Here we review the overlapping and distinct clinical features of these 3 syndromes and discuss current knowledge regarding the ribosomal pathways disrupted in each of these disorders. We also explore the increasing complexity of ribosome biology and how this informs our understanding of developmental biology and human disease.

F-spondin inhibits differentiation of clastic precursors via lipoprotein receptor-related protein 8 (LRP8)

BACKGROUND

F-spondin, known to be a secreted neuronal glycoprotein, is highly expressed on the tooth root surface. The authors previously reported that F-spondin is one of the specific markers of cementoblasts in periodontal tissue. In chronic periodontitis, significant cemental resorption rarely occurs on the root side, although alveolar bone resorption by osteoclasts is one of the major pathologic changes. Thus, it was hypothesized that secretory F-spondin from cementoblasts might be involved in differentiation of clastic cells on the root surface. The authors studied effects of secretory F-spondin from F-spondin-expressing cells and its pathway on receptor activator of nuclear factor-κB ligand (RANKL)-mediated differentiation of clastic cells.

METHODS

Osteoclast precursors were used in this study. With a chamber assay, the authors examined effects of secretory molecules from F-spondin-expressing cells of transgenic mice on RANKL-induced clastic cell differentiation.

RESULTS

Secretory molecules from F-spondin-overexpressing cells significantly inhibited the RANKL-mediated tartrate-resistant acid phosphatase (TRAP)-positive cells from primary progenitor cells with the chamber system. F-spondin suppressed RANKL-mediated nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1); TRAP; cathepsin K; and dendritic cell-specific transmembrane protein (DC-STAMP) expression in the cells. The suppressive effect of F-spondin on RANKL-induced differentiation of clastic cells was partially blocked by knockdown of low-density lipoprotein receptor-related protein 8 (LRP8).

CONCLUSIONS

These findings indicate that secretory factors from F-spondin-expressing cells, including F-spondin, downregulate differentiation of clastic precursors. Moreover, F-spondin inhibits RANKL-mediated differentiation of clastic cells partially via LRP8. It is suggested that secretory F-spondin may act protectively from cemental resorption partially via LRP8 in periodontal tissue.

Inherited bone marrow failure syndromes in adolescents and young adults

The inherited bone marrow failure syndromes are a diverse group of genetic diseases associated with inadequate production of one or more blood cell lineages. Examples include Fanconi anemia, dyskeratosis congenita, Diamond-Blackfan anemia, thrombocytopenia absent radii syndrome, severe congenital neutropenia, and Shwachman-Diamond syndrome. The management of these disorders was once the exclusive domain of pediatric subspecialists, but increasingly physicians who care for adults are being called upon to diagnose or treat these conditions. Through a series of patient vignettes, we highlight the clinical manifestations of inherited bone marrow failure syndromes in adolescents and young adults. The diagnostic and therapeutic challenges posed by these diseases are discussed.

Ribosomopathies: mechanisms of disease

Ribosomopathies are diseases caused by alterations in the structure or function of ribosomal components. Progress in our understanding of the role of the ribosome in translational and transcriptional regulation has clarified the mechanisms of the ribosomopathies and the relationship between ribosomal dysfunction and other diseases, especially cancer. This review aims to discuss these topics with updated information.

Key genes associated with osteoporosis revealed by genome wide gene expression analysis

Gene expression profiles of circulating monocytes were analyzed to identify key genes associated with osteoporosis. Raw microarray data were downloaded from gene expression omnibus under accession number GSE7158, including 8 microarray dataset for patients with high peak bone mass (PBM) and 8 for low PBM. Package linear models for microarray data of R was adopted to screen out differentially expressed genes (DEGs). Gene ontology enrichment analysis and Kyoto encyclopedia of genes and genomes pathway analysis were performed with plug-ins of cytoscape. Protein-protein interaction network was constructed using FunCoup. A total of 283 DEGs were identified in low-PBM group, including 135 up- and 148 down-regulated genes. A considerable part of DEGs were localized in plasma membrane. Several ion transport-related pathways were revealed, such as mineral absorption and carbohydrate digestion and absorption. A range of DEGs were identified and some of them were related to calcium transport as well as osteoporosis. These findings are helpful in disclosing the pathogenetic mechanisms of osteoporosis.

The ever-expanding conundrum of primary osteoporosis: aetiopathogenesis, diagnosis, and treatment

In recent years, as knowledge regarding the etiopathogenetic mechanisms of bone involvement characterizing many diseases has increased and diagnostic techniques evaluating bone health have progressively improved, the problem of low bone mass/quality in children and adolescents has attracted more and more attention, and the body evidence that there are groups of children who may be at risk of osteoporosis has grown. This interest is linked to an increased understanding that a higher peak bone mass (PBM) may be one of the most important determinants affecting the age of onset of osteoporosis in adulthood. This review provides an updated picture of bone pathophysiology and characteristics in children and adolescents with paediatric osteoporosis, taking into account the major causes of primary osteoporosis (PO) and evaluating the major aspects of bone densitometry in these patients. Finally, some options for the treatment of PO will be briefly discussed.

Modulation of osteoclast differentiation and bone resorption by Rho GTPases

Bone is a dynamic tissue constantly renewed through a regulated balance between bone formation and resorption. Excessive bone degradation by osteoclasts leads to pathological decreased bone density characteristic of osteolytic diseases such as post-menopausal osteoporosis or bone metastasis. Osteoclasts are multinucleated cells derived from hematopoietic stem cells via a complex differentiation process. Their unique ability to resorb bone is dependent on the formation of the actin-rich sealing zone. Within this adhesion structure, the plasma membrane differentiates into the ruffled border where protons and proteases are secreted to demineralize and degrade bone, respectively. On the bone surface, mature osteoclasts alternate between stationary resorptive and migratory phases. These are associated with profound actin cytoskeleton reorganization, until osteoclasts die of apoptosis. In this review, we highlight the role of Rho GTPases in all the steps of osteoclasts differentiation, function, and death and conclude on their interest as targets for treatment of osteolytic pathologies.

Ribosome biogenesis in skeletal development and the pathogenesis of skeletal disorders

The skeleton affords a framework and structural support for vertebrates, while also facilitating movement, protecting vital organs, and providing a reservoir of minerals and cells for immune system and vascular homeostasis. The mechanical and biological functions of the skeleton are inextricably linked to the size and shape of individual bones, the diversity of which is dependent in part upon differential growth and proliferation. Perturbation of bone development, growth and proliferation, can result in congenital skeletal anomalies, which affect approximately 1 in 3000 live births [1]. Ribosome biogenesis is integral to all cell growth and proliferation through its roles in translating mRNAs and building proteins. Disruption of any steps in the process of ribosome biogenesis can lead to congenital disorders termed ribosomopathies. In this review, we discuss the role of ribosome biogenesis in skeletal development and in the pathogenesis of congenital skeletal anomalies. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

Overexpression of developmentally regulated GTP-binding protein-2 increases bone loss

The developmentally regulated GTP-binding protein-2 (DRG2) is a novel subclass of GTP-binding proteins. Many functional characteristics of osteoclasts (OC) are associated with small GTPases. We hypothesized that DRG2 affects bone mass via modulating OC activity. Using DRG2 transgenic mice, we investigated the role of DRG2 in bone remodeling. DRG2 overexpression caused a decrease in bone mass and an increase in the number and activity of OC in vivo. DRG2 overexpression increased fusion, spreading, survival, and resorption activity of OC in vitro. Downregulation of DRG2 by siRNA decreased fusion, spreading, and survival of OC, supporting the observations found in DRG2 transgenic OC. Transgenic mature OCs were larger, with actin rings and higher ERK, Akt, Rac1 and Rho activities than wild-type OCs. Inhibition of these proteins abolished the effects of DRG2 on formation of large OCs with actin rings, implying that DRG2 affects cytoskeleton reorganization in a Rac1/Rho/ERK/Akt-dependent manner. In summary, DRG2 is associated with survival and cytoskeleton organization of OC under influence of macrophage colony-stimulating factor, and its overexpression leads to elevated bone resorptive activity of OC, resulting in bone loss.

Impaired growth, hematopoietic colony formation, and ribosome maturation in human cells depleted of Shwachman-Diamond syndrome protein SBDS

BACKGROUND

Shwachman-Diamond syndrome (SDS), associated with SBDS mutations, is characterized by pancreatic exocrine dysfunction and marrow failure. Sdo1, the yeast ortholog of SBDS, is implicated in maturation of the 60S ribosomal subunit, with delayed export of 60S-like particles from the nucleoplasm when depleted. Sdo1 is needed for release of the anti-subunit association factor Tif6 from 60S subunits, and Tif6 may not be recycled to the nucleus when Sdo1 is absent.

METHODS

To clarify the role of SBDS in human ribosome function, TF-1 erythroleukemia and A549 lung carcinoma cells were transfected with vectors expressing RNAi against SBDS.

RESULTS

Growth and hematopoietic colony forming potential of TF-1 knockdown cells were markedly hindered when compared to controls. To analyze the effect of SBDS on 60S subunit maturation in A549 cells, subunit localization was assessed by transfection with a vector expressing a fusion between human RPL29 and GFP: we found a higher percentage of SBDS-depleted cells with nuclear localization of 60S subunits. Polysome analysis of TF-1 knockdown cells showed a decrease in free 60S and 80S subunits. We also analyzed the levels of eIF6 (human ortholog of Tif6) following near-complete knockdown of SBDS in TF-1 cells and found an approximately 20% increase in the amount of eIF6 associated with the 60S subunit.

CONCLUSIONS

We conclude that knockdown of SBDS leads to growth inhibition and defects in ribosome maturation, suggesting a role for wild-type SBDS in nuclear export of pre-60S subunits. Furthermore, knockdown of SBDS may interfere with eIF6 recycling.

Rac regulates PtdInsP₃ signaling and the chemotactic compass through a redox-mediated feedback loop

Directional cell migration is an essential requirement for efficient neutrophil translocation to sites of infection and requires the establishment of a polarized cell characterized by an actin-rich leading edge facing the chemoattractant gradient. The asymmetrical accumulation of phosphatidylinositol(3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] in the up-gradient leading edge is a hallmark of polarization and regulates the recruitment and localization of various effector proteins at the leading-edge plasma membrane. How shallow gradients of chemoattractants trigger and maintain a much steeper intracellular gradient of PtdIns(3,4,5)P(3) is a critical question in the study of leukocyte chemotaxis. Our data demonstrate that the migration of neutrophils toward the chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenylalanine depends on the generation of reactive oxygen species by the phagocytic NADPH oxidase (NOX2) and subsequent oxidation and inhibition of phosphatase and tensin homolog. Moreover, we show that events downstream of PtdIns(3,4,5)P(3), including phosphorylation of AKT, Rac activation, uncapping of actin filaments, and directional migration, can be attenuated by ROS scavengers or genetic ablation of NOX2. Using Rac mutants that are defective in their ability to activate NOX2, we show that Rac regulates a redox-mediated feedback loop that mediates directional migration of neutrophils.

Of blood, bones, and ribosomes: is Swachman-Diamond syndrome a ribosomopathy?

Mutations in the human SBDS (Shwachman-Bodian-Diamond syndrome) gene are the most common cause of Shwachman-Diamond syndrome, an inherited bone marrow failure syndrome. In this issue of Genes & Development, Finch and colleagues (pp. 917-929) establish that SBDS functions in ribosome synthesis by promoting the recycling of eukaryotic initiation factor 6 (eIF6) in a GTP-dependent manner. This work supports the idea that a ribosomopathy may underlie this syndrome.

Uncoupling of GTP hydrolysis from eIF6 release on the ribosome causes Shwachman-Diamond syndrome

Removal of the assembly factor eukaryotic initiation factor 6 (eIF6) is critical for late cytoplasmic maturation of 60S ribosomal subunits. In mammalian cells, the current model posits that eIF6 release is triggered following phosphorylation of Ser 235 by activated protein kinase C. In contrast, genetic studies in yeast indicate a requirement for the ortholog of the SBDS (Shwachman-Bodian-Diamond syndrome) gene that is mutated in the inherited leukemia predisposition disorder Shwachman-Diamond syndrome (SDS). Here, by isolating late cytoplasmic 60S ribosomal subunits from Sbds-deleted mice, we show that SBDS and the GTPase elongation factor-like 1 (EFL1) directly catalyze eIF6 removal in mammalian cells by a mechanism that requires GTP binding and hydrolysis by EFL1 but not phosphorylation of eIF6 Ser 235. Functional analysis of disease-associated missense variants reveals that the essential role of SBDS is to tightly couple GTP hydrolysis by EFL1 on the ribosome to eIF6 release. Furthermore, complementary NMR spectroscopic studies suggest unanticipated mechanistic parallels between this late step in 60S maturation and aspects of bacterial ribosome disassembly. Our findings establish a direct role for SBDS and EFL1 in catalyzing the translational activation of ribosomes in all eukaryotes, and define SDS as a ribosomopathy caused by uncoupling GTP hydrolysis from eIF6 release.