Coordinated transcriptional regulation of bone homeostasis by Ebf1 and Zfp521 in both mesenchymal and hematopoietic lineages

Deletion of the transcription factor EBF1 in perivascular stroma disrupts skeletal homeostasis and precipitates premature aging of the marrow microenvironment

Early B cell factor 1 (EBF1) is a transcription factor expressed by multiple lineages of stromal cells within the bone marrow. While cultures of Ebf1-deficient cells have been demonstrated to have impaired differentiation into either the osteoblast or adipogenic lineage in vitro by several groups, in vivo there has been a nominal consequence of the loss of EBF1 on skeletal development. In this study we used Prx-cre driven deletion of Ebf1 to eliminate EBF1 from the entire mesenchymal lineage of the skeleton and resolve this discrepancy. We report here that EBF1 is expressed primarily in the Mesenchymal Stem and Progenitor Cell (MSPC)-Adipo, MSPC-Osteo, and the Early Mesenchymal Progenitors, and that loss of EBF1 has a plethora of consequences to maintenance of the skeleton throughout adulthood. Stroma from the Prx-cre;Ebf1fl/fl bones had impaired osteogenic differentiation, an age-dependent loss of CFU-F, and elevated senescence accompanying Ebf1-deletion. New bone formation was reduced after 3 months, and resulted in a quiescent bone environment with fewer osteoblasts and an accompanied reduction in osteoclast-mediated remodeling. Consequently, bones were less ductile at a younger age, and deletion of EBF1 dramatically impaired fracture repair. Disruption of EBF1 in perivascular populations also rearranged the vascular network within these bones and disrupted cytokine signaling from key hematopoietic niches resulting in anemia, reductions in B cells, and myeloid skewing of marrow hematopoietic lineages. Mechanistically we observed disrupted BMP signaling within Ebf1-deficient progenitors with reduced SMAD1-phosphorylation, and elevated secretion of the soluble BMP-inhibitor Gremlin from the MSPC-Adipo cells. Ebf1-deficient progenitors also exhibited posttranslational suppression of glucocorticoid receptor expression. Together, these results suggest that EBF1 signaling is required for mesenchymal progenitor mobilization to maintain the adult skeleton, and that the primary action of EBF1 in the early mesenchymal lineage is to promote proliferation, and differentiation of these perivascular cells to sustain a healthy tissue.

Fine-tuning osteoclastogenesis: An insight into the cellular and molecular regulation of osteoclastogenesis

Osteoclasts, the bone-resorbing cells, are essential for the bone remodeling process and are involved in the pathophysiology of several bone-related diseases. The extensive corpus of in vitro research and crucial mouse model studies in the 1990s demonstrated the key roles of monocyte/macrophage colony-stimulating factor, receptor activator of nuclear factor kappa B ligand (RANKL) and integrin αvβ3 in osteoclast biology. Our knowledge of the molecular mechanisms by which these variables control osteoclast differentiation and function has significantly advanced in the first decade of this century. Recent developments have revealed a number of novel insights into the fundamental mechanisms governing the differentiation and functional activity of osteoclasts; however, these mechanisms have not yet been adequately documented. Thus, in the present review, we discuss various regulatory factors including local and hormonal factors, innate as well as adaptive immune cells, noncoding RNAs (ncRNAs), etc., in the molecular regulation of the intricate and tightly regulated process of osteoclastogenesis. ncRNAs have a critical role as epigenetic controllers of osteoclast physiologic activities, including differentiation and bone resorption. The primary ncRNAs, which include micro-RNAs, circular RNAs, and long noncoding RNAs, form a complex network that affects gene transcription activities associated with osteoclast biological activity. Greater knowledge of the involvement of ncRNAs in osteoclast biological activities will contribute to the treatment and management of several skeletal diseases such as osteoporosis, osteoarthritis, rheumatoid arthritis, etc. Moreover, we further outline potential therapies targeting these regulatory pathways of osteoclastogenesis in distinct bone pathologies.

PTH regulates osteogenesis and suppresses adipogenesis through Zfp467 in a feed-forward, PTH1R-cyclic AMP-dependent manner

Conditional deletion of the PTH1R in mesenchymal progenitors reduces osteoblast differentiation, enhances marrow adipogenesis, and increases zinc finger protein 467 (Zfp467) expression. In contrast, genetic loss of Zfp467 increased Pth1r expression and shifts mesenchymal progenitor cell fate toward osteogenesis and higher bone mass. PTH1R and ZFP467 could constitute a feedback loop that facilitates PTH-induced osteogenesis and that conditional deletion of Zfp467 in osteogenic precursors would lead to high bone mass in mice. Prrx1Cre; Zfp467fl/fl but not AdipoqCre; Zfp467fl/fl mice exhibit high bone mass and greater osteogenic differentiation similar to the Zfp467-/- mice. qPCR results revealed that PTH suppressed Zfp467 expression primarily via the cyclic AMP/PKA pathway. Not surprisingly, PKA activation inhibited the expression of Zfp467 and gene silencing of Pth1r caused an increase in Zfp467 mRNA transcription. Dual fluorescence reporter assays and confocal immunofluorescence demonstrated that genetic deletion of Zfp467 resulted in higher nuclear translocation of NFκB1 that binds to the P2 promoter of the Pth1r and increased its transcription. As expected, Zfp467-/- cells had enhanced production of cyclic AMP and increased glycolysis in response to exogenous PTH. Additionally, the osteogenic response to PTH was also enhanced in Zfp467-/- COBs, and the pro-osteogenic effect of Zfp467 deletion was blocked by gene silencing of Pth1r or a PKA inhibitor. In conclusion, our findings suggest that loss or PTH1R-mediated repression of Zfp467 results in a pathway that increases Pth1r transcription via NFκB1 and thus cellular responsiveness to PTH/PTHrP, ultimately leading to enhanced bone formation.

Expression of the preadipocyte marker ZFP423 is dysregulated between well-differentiated and dedifferentiated liposarcoma

Background

Well-differentiated and dedifferentiated liposarcomas are rare soft tissue tumors originating in adipose tissue that share genetic abnormalities but have significantly different metastatic potential. Dedifferentiated liposarcoma (DDLPS) is highly aggressive and has an overall 5-year survival rate of 30% as compared to 90% for well-differentiated liposarcoma (WDLPS). This discrepancy may be connected to their potential to form adipocytes, where WDLPS is adipogenic but DDLPS is adipogenic-impaired. Normal adipogenesis requires Zinc Finger Protein 423 (ZFP423), a transcriptional coregulator of Perixosome Proliferator Activated Receptor gamma (PPARG2) mRNA expression that defines committed preadipocytes. Expression of ZFP423 in preadipocytes is promoted by Seven-In-Absentia Homolog 2 (SIAH2)-mediated degradation of Zinc Finger Protein 521 (ZFP521). This study investigated the potential role of ZFP423, SIAH2 and ZFP521 in the adipogenic potential of WDLPS and DDLPS.

Methods

Human WDLPS and DDLPS fresh and paraffin-embedded tissues were used to assess the gene and protein expression of proadipogenic regulators. In parallel, normal adipose tissue stromal cells along with WDLPS and DDLPS cell lines were cultured, genetically modified, and induced to undergo adipogenesis in vitro.

Results

Impaired adipogenic potential in DDLPS was associated with reduced ZFP423 protein levels in parallel with reduced PPARG2 expression, potentially involving regulation of ZFP521. SIAH2 protein levels did not define a clear distinction related to adipogenesis in these liposarcomas. However, in primary tumor specimens, SIAH2 mRNA was consistently upregulated in DDLPS compared to WDLPS when assayed by fluorescence in situ hybridization or real-time PCR.

Conclusions

These data provide novel insights into ZFP423 expression in adipogenic regulation between WDLPS and DDLPS adipocytic tumor development. The data also introduces SIAH2 mRNA levels as a possible molecular marker to distinguish between WDLPS and DDLPS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09379-6.

Expression and function of Ebf1 gene during chondrogenesis in chick embryo limb buds

The expression profile of early B-cell factor (Ebf) genes and loss of function experiments denote a crucial role for these genes during the late stage of skeletogenesis. However, little is known regarding the expression and function of these genes during the early stage of skeletogenesis. Therefore, this study aimed to detail the spatiotemporal expression pattern of cEbf1, in comparison to cEbf2 and cEbf3, in chick limb buds and investigate its function during chondrogenesis. cEbf1-3 were co-expressed in the distal mesenchyme from a very early stage and later in the outer perichondrium and the surrounding noncartilaginous mesenchymal cells. Ebf1 loss of function through injection of RCASBP virus-carrying Ebf1 dominant-negative form (ΔEbf1) into the wing buds resulted in shortened skeletal elements with a clear defect in the chondrocyte differentiation program. In RCASBP-ΔEbf1 injected wing, the chondrogenesis was initiated normally but hindered at the maturation stage. Subsequently, the chondrocytes failed to become mature or hypertrophic and the long bone diaphysis was not properly developed. The final phenotype included shorter, thicker, and fused long bones. These phenotypic changes were associated with downregulation of the early [Sox9 and collagen type II (Col2a1)] and the late [alkaline phosphatase (AP)] chondrocytes differentiation markers in the limb buds. These results conclude that cEbf1 could be involved in a molecular cascade that promotes the terminal stages of chondrogenesis in the long bone anlagen.

EBF1 is expressed in pericytes and contributes to pericyte cell commitment

Early B-cell factor-1 (EBF1) is a transcription factor with an important role in cell lineage specification and commitment during the early stage of cell maturation. Originally described during B-cell maturation, EBF1 was subsequently identified as a crucial molecule for proper cell fate commitment of mesenchymal stem cells into adipocytes, osteoblasts and muscle cells. In vessels, EBF1 expression and function have never been documented. Our data indicate that EBF1 is highly expressed in peri-endothelial cells in both tumor vessels and in physiological conditions. Immunohistochemistry, quantitative reverse transcription polymerase chain reaction (RT-qPCR) and fluorescence-activated cell sorting (FACS) analysis suggest that EBF1-expressing peri-endothelial cells represent bona fide pericytes and selectively express well-recognized markers employed in the identification of the pericyte phenotype (SMA, PDGFRβ, CD146, NG2). This observation was also confirmed in vitro in human placenta-derived pericytes and in human brain vascular pericytes (HBVP). Of note, in accord with the key role of EBF1 in the cell lineage commitment of mesenchymal stem cells, EBF1-silenced HBVP cells showed a significant reduction in PDGFRβ and CD146, but not CD90, a marker mostly associated with a prominent mesenchymal phenotype. Moreover, the expression levels of VEGF, angiopoietin-1, NG2 and TGF-β, cytokines produced by pericytes during angiogenesis and linked to their differentiation and activation, were also significantly reduced. Overall, the data suggest a functional role of EBF1 in the cell fate commitment toward the pericyte phenotype.

Early B-cell Factor1 (Ebf1) promotes early osteoblast differentiation but suppresses osteoblast function

Early B cell factor 1 (Ebf1) is a transcription factor that regulates B cell, neuronal cell and adipocyte differentiation. We and others have shown that Ebf1 is expressed in osteoblasts and that global deletion of Ebf1 results in increased bone formation in vivo. However, as Ebf1 is expressed in multiple tissues and cell types, it has remained unclear, which of the phenotypic changes in bone are derived from bone cells. The aim of this study was to determine the cell-autonomous and differentiation stage-specific roles of Ebf1 in osteoblasts. In vitro, haploinsufficient Ebf1^+/- calvarial cells showed impaired osteoblastic differentiation indicated by lower alkaline phosphatase (ALP) activity and reduced mRNA expression of osteoblastic genes, while overexpression of Ebf1 in wild type mouse calvarial cells led to enhanced osteoblast differentiation with increased expression of Osterix (Osx). We identified a putative Ebf1 binding site in the Osterix promoter by ChIP assay in MC3T3-E1 osteoblasts and showed that Ebf1 was able to activate Osx-luc reporter construct that included this Ebf1 binding site, suggesting that Ebf1 indeed regulates osteoblast differentiation by inducing Osterix expression. To reconcile our previous data and that of others with our novel findings, we hypothesized that Ebf1 could have a dual role in osteoblast differentiation promoting early but inhibiting late stages of differentiation and osteoblast function. To test this hypothesis in vivo, we generated conditional Ebf1 knockout mice, in which Ebf1 deletion was targeted to early or late osteoblasts by crossing Ebf1^fl/fl mice with Osx- or Osteocalcin (hOC)-Cre mouse lines, respectively. Deletion of Ebf1 in early Ebf1_Osx^-/- osteoblasts resulted in significantly increased bone volume and trabecular number at 12 weeks by μCT analysis, while Ebf1_hOC^-/- mice did not have a bone phenotype. To conclude, our data demonstrate that Ebf1 promotes early osteoblast differentiation by regulating Osterix expression. However, Ebf1 inhibits bone accrual in the Osterix expressing osteoblasts in vivo but it is redundant in the maintenance of mature osteoblast function.

The role of Zfp467 in mediating the pro-osteogenic and anti-adipogenic effects on bone and bone marrow niche

Conditional deletion of the PTH receptor (Pth1r) in mesenchymal progenitors reduces osteoblast differentiation and bone mass while enhancing adipogenesis and bone marrow adipose tissue. Mechanistically, PTH suppresses the expression of Zfp467, a pro-adipogenic zinc finger transcription factor. Consequently, Pth1r deficiency in mesenchymal progenitors leads to increased Zfp467 expression. Based on these observations, we hypothesized that genetic loss of Zfp467 would lead to a shift in marrow progenitor cell fate towards osteogenesis and increased bone mass. To test this hypothesis, we generated Zfp467-/- mice. Zfp467-/- mice (-/-) were significantly smaller than Zfp467+/+ mice (+/+). μCT showed significantly higher trabecular bone and cortical bone area in -/- vs. +/+, and histomorphometry showed higher structural and dynamic formation parameters in -/- mice vs. +/+. Femoral gene expression including Alpl, Sp7, and Acp5 were increased in -/-mice, whereas Adiponectin, Cebpa, Lepr, and Ppraγ mRNA were lower in -/- mice. Similarly, Fabp4 and Lep in the inguinal depot were also decreased in -/- mice. Moreover, marrow adipocyte numbers were reduced in -/- vs +/+ mice (p<0.007). In vitro, COBs and BMSCs-/- showed more positive ALP and Alizarin Red staining and a decrease in ORO droplets. Pth1r mRNA and protein levels were increased in COBs and BMSCs from -/- mice vs +/+ (p<0.02 for each parameter, -/- vs. +/+). -/- cells also exhibited enhanced endogenous levels of cAMP vs. control cells. Moreover, in an ovariectomy (OVX) mouse model, Zfp467-/- mice had significantly lower fat mass but similar bone mass compared to OVX +/+ mice. In contrast, in a high fat diet (HFD) mouse model, in addition to reduced adipocyte volume and adipogenesis related gene expression in both peripheral and bone marrow fat tissue, greater osteoblast number and higher osteogenesis related gene expression were also observed in -/- HFD mice vs. +/+ HFD mice. Taken together, these results demonstrate that ZFP467 negatively influences skeletal homeostasis and favors adipogenesis. Global deletion of Zfp467 increases PTHR1, cAMP and bone turnover, hence its repression is a component of PTH signaling and its regulation. These data support a critical role for Zfp467 in early lineage allocation and provide a novel potential mechanism by which PTH acts in an anabolic manner on the bone remodeling unit.

Zfp521 is essential for the quiescence and maintenance of adult hematopoietic stem cells under stress

Zinc finger protein 521 (Zfp521), a quiescent hematopoietic stem cell (HSC)-enriched transcription factor, is involved in the self-renewal and differentiation of fetal liver HSC. However, its role in adult hematopoiesis remains elusive. Here, we found that Zfp521 deletion did not inhibit adult hematopoiesis under homeostatic conditions. In contrast, Zfp521-null chimeric mice showed significantly reduced pool size of HSC and hematopoietic progenitor cells associated with increased apoptosis and loss of quiescence. Competitive serial transplantation assays revealed that Zfp521 regulates HSC self-renewal and differentiation under regenerative stress. Mechanistically, Zfp521 transcriptionally repressed Rela expression by increasing H3K9ac and decreasing H3K9me3 levels in its promoter. Knockdown of Rela inhibited the hyper-activated NF-κB pathway and reversed the loss of quiescence in Zfp521-null HSC under stress. Thus, our results reveal a previously unrecognized role for Zfp521 as critical regulator of quiescence and self-renewal of HSC in adult hematopoiesis mediated at least partly by controlling Rela expression.

SIAH2 is Expressed in Adipocyte Precursor Cells and Interacts with EBF1 and ZFP521 to Promote Adipogenesis

OBJECTIVE

Expression of zinc finger protein 423 (ZFP423), a key proadipogenic transcription factor in adipocyte precursor cells, is regulated by interaction of the proadipogenic early B-cell factor 1 (EBF1) and antiadipogenic ZFP521. The ubiquitin ligase seven-in-absentia homolog 2 (SIAH2) targets ZFP521 for degradation. This study asked whether SIAH2 is expressed in adipocyte precursor cells and whether SIAH2 interacts with ZFP521 and EBF1 to regulate ZFP521 protein levels during adipogenesis.

METHODS

SIAH2 expression in precursor cells was assessed in primary cells and tissues from wild-type and SIAH2 null mice fed a control or high-fat diet. Primary cells, 3T3-L1 preadipocytes, and HEK293T cells were used to analyze Siah2, Ebf1, and Zfp521 expression and SIAH2-mediated changes in ZFP521 and EBF1 protein levels.

RESULTS

Siah2 is expressed in platelet-derived growth factor receptor α (PDGFRα)⁺ and stem cell antigen-1 (SCA1)⁺ adipocyte precursor cells. SIAH2 depletion reduces Ebf1 gene expression and increases EBF1 protein levels in early but not late adipogenesis. In early adipogenesis, SIAH2 forms a protein complex with EBF1 and ZFP521 to enhance SIAH2-mediated ubiquitylation and degradation of ZFP521 while increasing EBF1 protein levels.

CONCLUSIONS

Siah2 is expressed in PDGFRα+ adipocyte precursor cells and is linked to precursor cell commitment to adipogenesis by interacting with EBF1 and ZFP521 proteins to target the antiadipogenic ZFP521 for degradation.

The Classic Lobe Eye Phenotype of Drosophila Is Caused by Transposon Insertion-Induced Misexpression of a Zinc-Finger Transcription Factor

Drosophila Lobe (L) alleles were first discovered ∼100 years ago as spontaneous dominant mutants with characteristic developmental eye defects. However, the molecular basis for L dominant eye phenotypes has not been clearly understood. A previous work reported identification of CG10109/PRAS40 as the L gene, but subsequent analyses suggested that PRAS40 may not be related to L Here, we revisited the L gene to clarify this discrepancy and understand the basis for the dominance of L mutations. Genetic analysis localized the L gene to Oaz, which encodes a homolog of the vertebrate zinc finger protein 423 (Zfp423) family transcriptional regulators. We demonstrate that RNAi knockdown of Oaz almost completely restores all L dominant alleles tested. Lrev6-3 , a revertant allele of the L2 dominant eye phenotype, has an inframe deletion in the Oaz coding sequence. Molecular analysis of L dominant mutants identified allele-specific insertions of natural transposons (roo[ ]L1 , hopper[ ]L5 , and roo[ ]Lr ) or alterations of a preexisting transposon (L2 -specific mutations in roo[ ]Mohr) in the Oaz region. In addition, we generated additional L2 -reversion alleles by CRISPR targeting at Oaz These new loss-of-function Oaz mutations suppress the dominant L eye phenotype. Oaz protein is not expressed in wild-type eye disc but is expressed ectopically in L2/+ mutant eye disc. We induced male recombination between Oaz-GAL4 insertions and the L2 mutation through homologous recombination. By using the L2 -recombined GAL4 reporters, we show that Oaz-GAL4 is expressed ectopically in L2 eye imaginal disc. Taken together, our data suggest that neomorphic L eye phenotypes are likely due to misregulation of Oaz by spontaneous transposon insertions.

Patient genetics is linked to chronic wound microbiome composition and healing

The clinical importance of microbiomes to the chronicity of wounds is widely appreciated, yet little is understood about patient-specific processes shaping wound microbiome composition. Here, a two-cohort microbiome-genome wide association study is presented through which patient genomic loci associated with chronic wound microbiome diversity were identified. Further investigation revealed that alternative TLN2 and ZNF521 genotypes explained significant inter-patient variation in relative abundance of two key pathogens, Pseudomonas aeruginosa and Staphylococcus epidermidis. Wound diversity was lowest in Pseudomonas aeruginosa infected wounds, and decreasing wound diversity had a significant negative linear relationship with healing rate. In addition to microbiome characteristics, age, diabetic status, and genetic ancestry all significantly influenced healing. Using structural equation modeling to identify common variance among SNPs, six loci were sufficient to explain 53% of variation in wound microbiome diversity, which was a 10% increase over traditional multiple regression. Focusing on TLN2, genotype at rs8031916 explained expression differences of alternative transcripts that differ in inclusion of important focal adhesion binding domains. Such differences are hypothesized to relate to wound microbiomes and healing through effects on bacterial exploitation of focal adhesions and/or cellular migration. Related, other associated loci were functionally enriched, often with roles in cytoskeletal dynamics. This study, being the first to identify patient genetic determinants for wound microbiomes and healing, implicates genetic variation determining cellular adhesion phenotypes as important drivers of infection type. The identification of predictive biomarkers for chronic wound microbiomes may serve as risk factors and guide treatment by informing patient-specific tendencies of infection.

Chronic, or non-healing, wounds represent a costly burden to patients, and bacterial infection of wounds is an important driver of chronicity. A variety of bacterial species often occur in chronic wounds, but it is unknown why certain species are observed in some wound infections and not others. In this study, genetic variation of wound clinic patients was compared to the bacteria observed in their infected wounds. Through these comparisons, genetic variation in the TLN2 and ZNF521 genes was found to be associated with both the number of bacteria observed in wounds and the abundance of common pathogens (primarily Pseudomonas aeruginosa and Staphylococcus epidermidis). Moreover, Pseudomonas infected wounds were found to have fewer species present and wounds with fewer species were slower to heal. Furthermore, patient genes associated with microbiomes commonly encode proteins known to be important for cellular structures important to healing and to which bacteria directly interact. Experimental investigation of one such gene, TLN2, identified genotype-dependent differences in the expression of functionally different versions of TLN2 that is hypothesized to shape differences in cellular adhesion structures. Finally, a new statistical approach is presented in which patient biomarkers are used to predict the number of species observed during infection. Overall, our results describe how patient genetic variation influence the types of bacteria likely to infect an individual as well as influence healing.

Zfp521 SUMOylation facilities erythroid hematopoietic reconstitution under stress

Zinc finger protein 521 (Zfp521) is a key transcriptional factor in regulation of hematopoiesis. SUMOylation, a protein post-translational modification process, plays important roles in various biological process including hematopoiesis. However, whether Zfp521 can be SUMOylated and how it affects hematopoiesis is unknown. In this study, we confirmed that Zfp521 can be modified by SUMO1 and lysine 1146 was the primary SUMOylation site. Under homeostatic condition, Zfp521 SUMOylation-deficient mice had normal mature blood cells and primitive cells. However, in bone marrow (BM) transplantation assay, recipient mice transplanted with BM cells from Zfp521 SUMOylation-deficient mice had a significantly decreased R2 population of erythroid lineage in BM and spleen compared with those transplanted with BM cells from wild-type mice. Our results found a novel function of Zfp521 SUMOylation in erythroid reconstitution under stress, which might be a new therapeutic target in future.

Osteoclastogenesis inhibition by mutated IGSF23 results in human osteopetrosis

OBJECTIVES

Osteopetrosis is a rare inherited skeletal disease characterized by increased bone mineral density due to the loss of osteoclast function or differentiation potential.

MATERIALS AND METHODS

The study involved a Chinese patient with osteopetrosis (the proband) and her immediate family members and 180 controls without osteopetrosis. Bone density of the femoral neck, lumbar spine and total body was measured using dual-energy x-ray absorptiometry. Osteoclast differentiation by the participants' peripheral blood mononuclear cells (PBMCs) was investigated using tartrate-resistant acid phosphatase (TRAP) staining. Osteoblast differentiation was examined with Alizarin Red S staining. Reverse transcription-quantitative PCR was used to amplify immunoglobulin superfamily member 23 (IGSF23), c-FOS and nuclear factor of activated T cells 1 (NFATC1).

RESULTS

We found a homozygous mutation (c.295C>T) in the IGSF23 gene in two osteopetrosis samples. The mutation led to the formation of a stop codon, causing loss of the immunoglobulin-like domain and the whole transmembrane domain. PBMCs from the proband (IGSF23^-/- ) exhibited poor ability for differentiating into mature osteoclasts in vitro. Overexpression of IGSF23 rescued the ability of IGSF23^-/- PBMCs to differentiate into osteoclasts. Moreover, knockdown of IGSF23 reversed the bone loss in OVX mice by injecting AAV-shIGSF23 into mice femoral bone marrow cavity. Furthermore, we also found that the IGSF23 mutation led to decreased c-Fos and NFATC1 expression levels by inhibiting the mitogen-activated protein kinase signalling pathways.

CONCLUSIONS

IGSF23-mediated osteoclast differentiation of PBMCs may serve as a potential target in osteoporosis therapy.

ZNF521 Has an Inhibitory Effect on the Adipogenic Differentiation of Human Adipose-Derived Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are multipotent progenitors present in the bone marrow stroma and in subcutaneous abdominal fat, an abundant and easily accessible source of MSCs with the ability to differentiate along multiple lineage pathways. The stem cell-associated transcription co-factor Zinc Finger Protein 521 (ZNF521/zfp521) has been implicated in the control of the homeostasis of hematopoietic, neural and osteo-adipogenic progenitors. Here we document through the analysis of a panel of human adipose-derived stem cells (hADSCs), that ZNF521 strongly inhibits the generation of mature adipocytes. Enforced overexpression of ZNF521 in these cells resulted in a significant delay and reduction in adipocyte differentiation upon exposure to inducers of adipogenesis. Of particular relevance, ZNF521 was able to inhibit the expression of ZNF423, recently identified as an essential commitment factor necessary for the generation of pre-adipocytes. Conversely, silencing of ZNF521 was found to significantly enhance the adipogenic differentiation of hADSCs. Inhibition of adipogenesis by ZNF521 was at least in part due to inhibition of EBF1. Taken together, these results confirm a role for ZNF521 as a key negative regulator of adipocyte differentiation of hADSCs.

Zfp423 Regulates Skeletal Muscle Regeneration and Proliferation

Satellite cells (SCs) are skeletal muscle stem cells that proliferate in response to injury and provide myogenic precursors for growth and repair. Zfp423 is a transcriptional cofactor expressed in multiple immature cell populations, such as neuronal precursors, mesenchymal stem cells, and preadipocytes, where it regulates lineage allocation, proliferation, and differentiation. Here, we show that Zfp423 regulates myogenic progression during muscle regeneration. Zfp423 is undetectable in quiescent SCs but becomes expressed during SC activation. After expansion, Zfp423 is gradually downregulated as committed SCs terminally differentiate. Mice with satellite-cell-specific Zfp423 deletion exhibit severely impaired muscle regeneration following injury, with aberrant SC expansion, defective cell cycle exit, and failure to transition efficiently from the proliferative stage toward commitment. Consistent with a cell-autonomous role of Zfp423, shRNA-mediated knockdown of Zfp423 in myoblasts inhibits differentiation. Surprisingly, forced expression of Zfp423 in myoblasts induces differentiation into adipocytes and arrests myogenesis. Affinity purification of Zfp423 in myoblasts identified Satb2 as a nuclear partner of Zfp423 that cooperatively enhances Zfp423 transcriptional activity, which in turn affects myoblast differentiation. In conclusion, by controlling SC expansion and proliferation, Zfp423 is essential for muscle regeneration. Tight regulation of Zfp423 expression is essential for normal progression of muscle progenitors from proliferation to differentiation.

ZNF521 Represses Osteoblastic Differentiation in Human Adipose-Derived Stem Cells

Human adipose-derived stem cells (hADSCs) are multipotent mesenchymal cells that can differentiate into adipocytes, chondrocytes, and osteocytes. During osteoblastogenesis, the osteoprogenitor cells differentiate into mature osteoblasts and synthesize bone matrix components. Zinc finger protein 521 (ZNF521/Zfp521) is a transcription co-factor implicated in the regulation of hematopoietic, neural, and mesenchymal stem cells, where it has been shown to inhibit adipogenic differentiation. The present study is aimed at determining the effects of ZNF521 on the osteoblastic differentiation of hADSCs to clarify whether it can influence their osteogenic commitment. The enforced expression or silencing of ZNF521 in hADSCs was achieved by lentiviral vector transduction. Cells were cultured in a commercial osteogenic medium for up to 20 days. The ZNF521 enforced expression significantly reduced osteoblast development as assessed by the morphological and molecular criteria, resulting in reduced levels of collagen I, alkaline phosphatase, osterix, osteopontin, and calcium deposits. Conversely, ZNF521 silencing, in response to osteoblastic stimuli, induced a significant increase in early molecular markers of osteogenesis and, at later stages, a remarkable enhancement of matrix mineralization. Together with our previous findings, these results show that ZNF521 inhibits both adipocytic and osteoblastic maturation in hADSCs and suggest that its expression may contribute to maintaining the immature properties of hADSCs.

Zinc Finger Protein 521 Regulates Early Hematopoiesis through Cell-Extrinsic Mechanisms in the Bone Marrow Microenvironment

Zinc finger protein 521 (ZFP521), a DNA-binding protein containing 30 Krüppel-like zinc fingers, has been implicated in the differentiation of multiple cell types, including hematopoietic stem and progenitor cells (HSPC) and B lymphocytes. Here, we report a novel role for ZFP521 in regulating the earliest stages of hematopoiesis and lymphoid cell development via a cell-extrinsic mechanism. Mice with inactivated Zfp521 genes (Zfp521^-/-) possess reduced frequencies and numbers of hematopoietic stem and progenitor cells, common lymphoid progenitors, and B and T cell precursors. Notably, ZFP521 deficiency changes bone marrow microenvironment cytokine levels and gene expression within resident HSPC, consistent with a skewing of hematopoiesis away from lymphopoiesis. These results advance our understanding of ZFP521's role in normal hematopoiesis, justifying further research to assess its potential as a target for cancer therapies.

ZNF423: A New Player in Estrogen Receptor-Positive Breast Cancer

Preventive therapy can target hormone-responsive breast cancer (BC) by treatment with selective estrogen receptor modulators (SERMs) and reduce the incidence of BC. Genome-wide association studies have identified single nucleotide polymorphisms (SNPs) with relevant predictive values, SNPs in the ZNF423 gene were associated with decreased risk of BC during SERM therapy, and SNPs in the Cathepsin O gene with an increased risk. ZNF423, which was not previously associated with BC is a multifunctional transcription factor known to have a role in development, neurogenesis, and adipogenesis and is implicated in other types of cancer. ZNF423 is transcriptionally controlled by the homolog ZNF521, early B cell factor transcription factor, epigenetic silencing of the promoter by CpG island hyper-methylation, and also by ZNF423 itself in an auto-regulatory loop. In BC cells, ZNF423 expression is found to be induced by estrogen, dependent on the binding of the estrogen receptor and calmodulin-like 3 to SNPs in ZNP423 intronic sites in proximity to consensus estrogen response elements. ZNF423 has also been shown to play a mechanistic role by trans-activating the tumor suppressor BRCA1 and thus modulating the DNA damage response. Even though recent extensive trial studies did not classify these SNPs with the highest predictive values, for inclusion in polygenic SNP analysis, the mechanism unveiled in these studies has introduced ZNF423 as a factor important in the control of the estrogen response. Here, we aim at providing an overview of ZNF423 expression and functional role in human malignancies, with a specific focus on its implication in hormone-responsive BC.

ZFP521 regulates murine hematopoietic stem cell function and facilitates MLL-AF9 leukemogenesis in mouse and human cells

The concept that tumor-initiating cells can co-opt the self-renewal program of endogenous stem cells as a means of enforcing their unlimited proliferative potential is widely accepted, yet identification of specific factors that regulate self-renewal of normal and cancer stem cells remains limited. Using a comparative transcriptomic approach, we identify ZNF521/Zfp521 as a conserved hematopoietic stem cell (HSC)-enriched transcription factor in human and murine hematopoiesis whose function in HSC biology remains elusive. Competitive serial transplantation assays using Zfp521-deficient mice revealed that ZFP521 regulates HSC self-renewal and differentiation. In contrast, ectopic expression of ZFP521 in HSCs led to a robust maintenance of progenitor activity in vitro. Transcriptional analysis of human acute myeloid leukemia (AML) patient samples revealed that ZNF521 is highly and specifically upregulated in AMLs with MLL translocations. Using an MLL-AF9 murine leukemia model and serial transplantation studies, we show that ZFP521 is not required for leukemogenesis, although its absence leads to a significant delay in leukemia onset. Furthermore, knockdown of ZNF521 reduced proliferation in human leukemia cell lines possessing MLL-AF9 translocations. Taken together, these results identify ZNF521/ZFP521 as a critical regulator of HSC function, which facilitates MLL-AF9-mediated leukemic disease in mice.

Zinc finger protein 521 overexpression increased transcript levels of Fndc5 in mouse embryonic stem cells

Zinc finger protein 521 is highly expressed in brain, neural stem cells and early progenitors of the human hematopoietic cells. Zfp521 triggers the cascade of neurogenesis in mouse embryonic stem cells through inducing expression of the early neuroectodermal genes Sox1, Sox3 and Pax6. Fndc5, a precursor of Irisin has inducing effects on the expression level of brain derived neurotrophic factor in hippocampus. Therefore, it is most likely that Fndc5 may play an important role in neural differentiation. To exhibit whether the expression of this protein is under regulation with Zfp521, we overexpressed Zfp521 in a stable transformants of mESCs expressing EGFP under control of Fndc5 promoter. Increased expression of Zfp521 enhanced transcription levels of both EGFP and endogenous Fndc5. This result was confirmed by overexpression the aforementioned vectors in HEK cells and indicated that Zfp521 functions upstream of Fndc5 expression. It is most likely that Zfp521 may act through the binding to its response element on Fndc5 core promoter. Therefore it is concluding that an enhanced expression of Fndc5 in neural progenitor cells is stimulated by Zfp521 overexpression in these cells.

Interaction of CCR4-NOT with EBF1 regulates gene-specific transcription and mRNA stability in B lymphopoiesis

Yang et al. used SILAC-based mass spectrometry of proteins associated with endogenous EBF1 in pro-B cells and identified most components of the multifunctional CCR4–NOT complex, which regulates transcription and mRNA degradation. The interaction of the CCR4–NOT complex with EBF1 diversifies the function of EBF1 in a context-dependent manner and may coordinate transcriptional and post-transcriptional gene regulation.

Transcription factor EBF1 (early B-cell factor 1) regulates early B-cell differentiation by poising or activating lineage-specific genes and repressing genes associated with alternative cell fates. To identify proteins that regulate the diverse functions of EBF1, we used SILAC (stable isotope labeling by amino acids in cell culture)-based mass spectrometry of proteins associated with endogenous EBF1 in pro-B cells. This analysis identified most components of the multifunctional CCR4–NOT complex, which regulates transcription and mRNA degradation. CNOT3 interacts with EBF1, and we identified histidine 240 in EBF1 as a critical residue for this interaction. Complementation of Ebf1^−/− progenitors with EBF1H240A revealed a partial block of pro-B-cell differentiation and altered expression of specific EBF1 target genes that show either reduced transcription or increased mRNA stability. Most deregulated EBF1 target genes show normal occupancy by EBF1H240A, but we also detected genes with altered occupancy, suggesting that the CCR4–NOT complex affects multiple activities of EBF1. Mice with conditional Cnot3 inactivation recapitulate the block of early B-cell differentiation, which we found to be associated with an impaired autoregulation of Ebf1 and reduced expression of pre-B-cell receptor components. Thus, the interaction of the CCR4–NOT complex with EBF1 diversifies the function of EBF1 in a context-dependent manner and may coordinate transcriptional and post-transcriptional gene regulation.

Transcriptional regulation of the proto-oncogene Zfp521 by SPI1 (PU.1) and HOXC13

The mouse zinc‐finger gene Zfp521 (also known as ecotropic viral insertion site 3; Evi3; and ZNF521 in humans) has been identified as a B‐cell proto‐oncogene, causing leukemia in mice following retroviral insertions in its promoter region that drive Zfp521 over‐expression. Furthermore, ZNF521 is expressed in human hematopoietic cells, and translocations between ZNF521 and PAX5 are associated with pediatric acute lymphoblastic leukemia. However, the regulatory factors that control Zfp521 expression directly have not been characterized. Here we demonstrate that the transcription factors SPI1 (PU.1) and HOXC13 synergistically regulate Zfp521 expression, and identify the regions of the Zfp521 promoter required for this transcriptional activity. We also show that SPI1 and HOXC13 activate Zfp521 in a dose‐dependent manner. Our data support a role for this regulatory mechanism in vivo, as transgenic mice over‐expressing Hoxc13 in the fetal liver show a strong correlation between Hoxc13 expression levels and Zfp521 expression. Overall these experiments provide insights into the regulation of Zfp521 expression in a nononcogenic context. The identification of transcription factors capable of activating Zfp521 provides a foundation for further investigation of the regulatory mechanisms involved in ZFP521‐driven cell differentiation processes and diseases linked to Zfp521 mis‐expression.

Zfp521 promotes B-cell viability and cyclin D1 gene expression in a B cell culture system

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Knockdown of Zfp521 in BCL1 cell culture reduces viability and promotes apoptosis.

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Genes expressed in B cells are down-regulated in cells with Zfp521 knockdown.

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Cyclin D1 expression is increased in mouse tumors with Zfp521 over-expression.

Leukemia arises due to the dysregulated proliferation of hematopoietic progenitor cells. Errors in the multi-step commitment process result in excessive numbers of immature lymphocytes, causing malignant disease. Genes involved in the differentiation of lymphocytes are often associated with leukemia. One such gene, Zfp521, has been found to cause B-cell leukemia in mice when over-expressed. The role of Zfp521 in B-cell differentiation, and the mechanisms by which it leads to leukemic transformation, are unclear. In this study we report that Zfp521 knockdown causes apoptosis in a B-cell culture system and promotes down-regulation of genes acting at late stages of B-cell differentiation. We identify Pax5 and cyclin D1 as Zfp521 target genes, and suggest that excessive B-cell proliferation observed in mice with retroviral insertions near the Zfp521 gene is due to an up-regulation of cyclin D1 in B-cells. Overall, these results suggest links between dysregulated Zfp521 and B-cell survival.

ZNF423 and ZNF521: EBF1 Antagonists of Potential Relevance in B-Lymphoid Malignancies

The development of the B-lymphoid cell lineage is tightly controlled by the concerted action of a network of transcriptional and epigenetic regulators. EBF1, a central component of this network, is essential for B-lymphoid specification and commitment as well as for the maintenance of the B-cell identity. Genetic alterations causing loss of function of these B-lymphopoiesis regulators have been implicated in the pathogenesis of B-lymphoid malignancies, with particular regard to B-cell acute lymphoblastic leukaemias (B-ALLs), where their presence is frequently detected. The activity of the B-cell regulatory network may also be disrupted by the aberrant expression of inhibitory molecules. In particular, two multi-zinc finger transcription cofactors named ZNF423 and ZNF521 have been characterised as potent inhibitors of EBF1 and are emerging as potentially relevant contributors to the development of B-cell leukaemias. Here we will briefly review the current knowledge of these factors and discuss the importance of their functional cross talk with EBF1 in the development of B-cell malignancies.

ZFP521 contributes to pre-B-cell lymphomagenesis through modulation of the pre-B-cell receptor signaling pathway

ZFP521 was previously identified as a putative gene involved in induction of B-cell lymphomagenesis. However, the contribution of ZFP521 to lymphomagenesis has not been confirmed. In this study, we sought to elucidate the role of ZFP521 in B-cell lymphomagenesis. To this end, we used a retroviral insertion method to show that ZFP521 was a target of mutagenesis in pre-B-lymphoblastic lymphoma cells. The pre-B-cell receptor (pre-BCR) signaling molecules BLNK, BTK and BANK1 were positively regulated by the ZFP521 gene, leading to enhancement of the pre-BCR signaling pathway. In addition, c-myc and c-jun were upregulated following activation of ZFP521. Stimulation of pre-BCR signaling using anti-Vpreb antibodies caused aberrant upregulation of c-myc and c-jun and of Ccnd3, which encodes cyclin D3, thereby inducing the growth of pre-B cells. Stimulation with Vpreb affected the growth of pre-B cells, and addition of interleukin (IL)-7 receptor exerted competitive effects on pre-B-cell growth. Knockdown of BTK and BANK1, targets of ZFP521, suppressed the effects of Vpreb stimulation on cell growth. Furthermore, in human lymphoblastic lymphoma, analogous to pre-B-cell lymphoma in mice, the expression of ZNF521, the homolog of ZFP521 in humans, was upregulated. In conclusion, our data showed that the ZFP521 gene comprehensively induced pre-B-cell lymphomagenesis by modulating the pre-B-cell receptor signaling pathway.

Computational Modeling of a Transcriptional Switch Underlying B-Lymphocyte Lineage Commitment of Hematopoietic Multipotent Cells

Despite progresses in identifying the cellular mechanisms at the basis of the differentiation of hematopoietic stem/progenitor cells, little is known about the regulatory circuitry at the basis of lineage commitment of hematopoietic multipotent progenitors. To address this issue, we propose a computational approach to give further insights in the comprehension of this genetic mechanism. Differently from T lymphopoiesis, however, there is at present no mathematical model describing lineage restriction of multipotent progenitors to early B-cell precursors. Here, we provide a first model-constructed on the basis of current experimental evidence from literature and of publicly available microarray datasets-of the genetic regulatory network driving the cellular fate determination at the stage of lymphoid lineage commitment, with particular regard to the multipotent-B-cell progenitor transition. By applying multistability analysis methods, we are able to assess the capability of the model to capture the experimentally observed switch-like commitment behavior. These methods allow us to confirm the central role of zinc finger protein 521 (ZNF521) in this process, that we had previously reported, and to identify a novel putative functional interaction for ZNF521, which is essential to realize such characteristic behavior. Moreover, using the devised model, we are able to rigorously analyze the mechanisms underpinning irreversibility of the physiological commitment step and to devise a possible reprogramming strategy, based on the combined modification of the expression of ZNF521 and EBF1.

The Genetic Landscape of Hematopoietic Stem Cell Frequency in Mice

Prior efforts to identify regulators of hematopoietic stem cell physiology have relied mainly on candidate gene approaches with genetically modified mice. Here we used a genome-wide association study (GWAS) strategy with the hybrid mouse diversity panel to identify the genetic determinants of hematopoietic stem/progenitor cell (HSPC) frequency. Among 108 strains, we observed ∼120- to 300-fold variation in three HSPC populations. A GWAS analysis identified several loci that were significantly associated with HSPC frequency, including a locus on chromosome 5 harboring the homeodomain-only protein gene (Hopx). Hopx previously had been implicated in cardiac development but was not known to influence HSPC biology. Analysis of the HSPC pool in Hopx^−/− mice demonstrated significantly reduced cell frequencies and impaired engraftment in competitive repopulation assays, thus providing functional validation of this positional candidate gene. These results demonstrate the power of GWAS in mice to identify genetic determinants of the hematopoietic system.

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Genetic variation across mouse strains influences hematopoietic stem cell frequency

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This variation can be exploited for genome-wide association studies

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Hopx is a regulator of hematopoietic stem/progenitor cell function

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This approach can be used to identify genetic determinants of other stem cell systems

Validation of a novel shotgun proteomic workflow for the discovery of protein-protein interactions: focus on ZNF521

The study of protein-protein interactions is increasingly relying on mass spectrometry (MS). The classical approach of separating immunoprecipitated proteins by SDS-PAGE followed by in-gel digestion is long and labor-intensive. Besides, it is difficult to integrate it with most quantitative MS-based workflows, except for stable isotopic labeling of amino acids in cell culture (SILAC). This work describes a fast, flexible and quantitative workflow for the discovery of novel protein-protein interactions. A cleavable cross-linker, dithiobis[succinimidyl propionate] (DSP), is utilized to stabilize protein complexes before immunoprecipitation. Protein complex detachment from the antibody is achieved by limited proteolysis. Finally, protein quantitation is performed via (18)O labeling. The workflow has been optimized concerning (i) DSP concentration and (ii) incubation times for limited proteolysis, using the stem cell-associated transcription cofactor ZNF521 as a model target. The interaction of ZNF521 with the core components of the nuclear remodelling and histone deacetylase (NuRD) complex, already reported in the literature, was confirmed. Additionally, interactions with newly discovered molecular partners of potentially relevant functional role, such as ZNF423, Spt16, Spt5, were discovered and validated by Western blotting.

UMG Lenti: novel lentiviral vectors for efficient transgene- and reporter gene expression in human early hematopoietic progenitors

Lentiviral vectors are widely used to investigate the biological properties of regulatory proteins and/or of leukaemia-associated oncogenes by stably enforcing their expression in hematopoietic stem and progenitor cells. In these studies it is critical to be able to monitor and/or sort the infected cells, typically via fluorescent proteins encoded by the modified viral genome. The most popular strategy to ensure co-expression of transgene and reporter gene is to insert between these cDNAs an IRES element, thus generating bi-cistronic mRNAs whose transcription is driven by a single promoter. However, while the product of the gene located upstream of the IRES is generally abundantly expressed, the translation of the downstream cDNA (typically encoding the reporter protein) is often inconsistent, which hinders the detection and the isolation of transduced cells. To overcome these limitations, we developed novel lentiviral dual-promoter vectors (named UMG-LV5 and –LV6) where transgene expression is driven by the potent UBC promoter and that of the reporter protein, EGFP, by the minimal regulatory element of the WASP gene. These vectors, harboring two distinct transgenes, were tested in a variety of human haematopoietic cell lines as well as in primary human CD34⁺ cells in comparison with the FUIGW vector that contains the expression cassette UBC-transgene-IRES-EGFP. In these experiments both UMG-LV5 and UMG–LV6 yielded moderately lower transgene expression than FUIGW, but dramatically higher levels of EGFP, thereby allowing the easy distinction between transduced and non-transduced cells. An additional construct was produced, in which the cDNA encoding the reporter protein is upstream, and the transgene downstream of the IRES sequence. This vector, named UMG-LV11, proved able to promote abundant expression of both transgene product and EGFP in all cells tested. The UMG-LVs represent therefore useful vectors for gene transfer-based studies in hematopoietic stem and progenitor cells, as well as in non-hematopoietic cells.

Gene expression profile induced by ovariectomy in bone marrow of mice: a functional approach to identify new candidate genes associated to osteoporosis risk in women

Osteoporosis is a multifactorial skeletal pathology with a main genetic component. To date, however, the majority of genes associated with this pathology remain unknown since genes cataloged to date only explain a part of the heritability of bone phenotypes. In the present study, we have used a genome-wide gene expression approach by means of microarrays to identify new candidate genes involved in the physiopathology of osteoporosis, using as a model the ovariectomized (OVX) mice by comparing global bone marrow gene expression of the OVX mice with those of SHAM operated mice. One hundred and eighty transcripts were found to be differentially expressed between groups. The analysis showed 23 significant regulatory networks, of which the top five canonical pathways included B-cell development, primary immunodeficiency signaling, PI3K signaling in B-cells, phospholipase C signaling, and FcgRIIB signaling in B-cells. Twelve differentially expressed genes were validated by MALDI-TOF mass spectrometry with good reproducibility. Finally, the association to bone phenotypes of SNPs in genes whose expression was increased (IL7R and CD79A) or decreased (GPX3 and IRAK3) by OVX in mice was analyzed in a cohort of 706 postmenopausal women. We detected an association of a SNP in a gene involved in the detoxification of free radicals like glutathione peroxidase 3 (GPX3) with femoral neck BMD (rs8177447, P=0.043) and two SNPs in the Ig-alpha protein of the B-cell antigen component gene (CD79A) with lumbar spine BMD (rs3810153 and rs1428922, P=0.016 and P=0.001, respectively). These results reinforce the role of antioxidant pathways and of B-cells in bone metabolism. Furthermore, it shows that a genome-wide gene expression approach in animal models is a useful method for detecting genes associated to BMD and osteoporosis risk in humans.

Critical role of zinc finger protein 521 in the control of growth, clonogenicity and tumorigenic potential of medulloblastoma cells

The stem cell-associated transcription co-factor ZNF521 has been implicated in the control of hematopoietic, osteo-adipogenic and neural progenitor cells. ZNF521 is highly expressed in cerebellum and in particular in the neonatal external granule layer that contains candidate medulloblastoma cells-of-origin, and in the majority of human medulloblastomas. Here we have explored its involvement in the control of human and murine medulloblastoma cells.

The effect of ZNF521 on growth and tumorigenic potential of human medulloblastoma cell lines as well as primary Ptc1^−/+ mouse medulloblastoma cells was investigated in a variety of in vitro and in vivo assays, by modulating its expression using lentiviral vectors carrying the ZNF521 cDNA, or shRNAs that silence its expression.

Enforced overexpression of ZNF521 in DAOY medulloblastoma cells significantly increased their proliferation, growth as spheroids and ability to generate clones in single-cell cultures and semisolid media, and enhanced their migratory ability in wound-healing assays. Importantly, ZNF521-expressing cells displayed a greatly enhanced tumorigenic potential in nude mice. All these activities required the ZNF521 N-terminal motif that recruits the nucleosome remodeling and histone deacetylase complex, which might therefore represent an appealing therapeutic target. Conversely, silencing of ZNF521 in human UW228 medulloblastoma cells that display high baseline expression decreased their proliferation, clonogenicity, sphere formation and wound-healing ability. Similarly, Zfp521 silencing in mouse Ptc1^−/+ medulloblastoma cells drastically reduced their growth and tumorigenic potential.

Our data strongly support the notion that ZNF521, through the recruitment of the NuRD complex, contributes to the clonogenic growth, migration and tumorigenicity of medulloblastoma cells.

Direct transcriptional repression of Zfp423 by Zfp521 mediates a bone morphogenic protein-dependent osteoblast versus adipocyte lineage commitment switch

Osteoblasts and adipocytes arise from a common mesenchymal precursor cell. The cell fate decision of a mesenchymal precursor cell is under the influence of molecular cues and signaling pathways that lead to the activation or repression of lineage-specific transcription factors. The molecular mechanisms determining osteoblast versus adipocyte lineage specificity in response to bone morphogenic protein (BMP) remain unclear. In this study, we describe the mechanism through which Zfp521 (ZNF521), a regulator of lineage progression in multiple immature cell populations, regulates lineage specification of mesenchymal progenitor cells during BMP-induced differentiation events. In vivo deletion or in vitro knockdown of Zfp521 in mesenchymal precursors resulted in increased expression of the adipocyte determinant factor Zfp423 (ZNF423). This was concurrent with the loss of histone H3K9 methylation and an increase in histone H3K9 acetylation at the Zfp423 promoter, which together are indicative of decreased gene repression. Indeed, we found that Zfp521 occupies and represses the promoter and intronic enhancer regions of Zfp423. Accordingly, conditional deletion of Zfp521 inhibited heterotopic bone formation in response to local injection of BMP2. In contrast, marrow adiposity within BMP2-induced bone was markedly enhanced in Zfp521-deficient mice, suggesting that precursor cells lacking Zfp521 differentiate preferentially into adipocytes instead of osteoblasts in response to BMP2. Consistent with a cell-autonomous role of Zfp521 in mesenchymal precursors, knockdown of Zfp521 in stromal cells prevented BMP2-induced osteoblast marker expression and simultaneously enhanced lipid accumulation and expression of adipocyte-related genes. Taken together, the data suggest that Zfp521 is a cell fate switch critical for BMP-induced osteoblast commitment and identify Zfp521 as the intrinsic repressor of Zfp423 and hence of adipocyte commitment during BMP-induced mesenchymal precursor differentiation.

Expression profiling and functional implications of a set of zinc finger proteins, ZNF423, ZNF470, ZNF521, and ZNF780B, in primary osteoarthritic articular chondrocytes

Articular chondrocytes are responsible for the maintenance of healthy articulations; indeed, dysregulation of their functions, including the production of matrix proteins and matrix-remodeling proteases, may result in fraying of the tissue and development of osteoarthritis (OA). To explore transcriptional mechanisms that contribute to the regulation of chondrocyte homeostasis and may be implicated in OA development, we compared the gene expression profile of a set of zinc finger proteins potentially linked to the control of chondrocyte differentiation and/or functions (ZNF423, ZNF470, ZNF521, and ZNF780B) in chondrocytes from patients affected by OA and from subjects not affected by OA. This analysis highlighted a significantly lower expression of the transcript encoding ZNF423 in chondrocytes from OA, particularly in elderly patients. Interestingly, this decrease was mirrored by the similarly reduced expression of PPARγ, a known target of ZNF423 with anti-inflammatory and chondroprotective properties. The ZNF521 mRNA instead was abundant in all primary chondrocytes studied; the RNAi-mediated silencing of this gene significantly altered the COL2A/COL1 expression ratio, associated with the maintenance of the differentiated phenotype, in chondrocytes cultivated in alginate beads. These results suggest a role for ZNF423 and ZNF521 in the regulation of chondrocyte homeostasis and warrant further investigations to elucidate their mechanism of action.

Advances in the regulation of osteoclasts and osteoclast functions

Osteoclasts are derived from mononuclear hematopoietic myeloid lineage cells, which are formed in the bone marrow and are attracted to the bloodstream by factors, including sphingsine-1 phosphate. These circulating precursors are attracted to bone surfaces undergoing resorption by chemokines and other factors expressed at these sites, where they fuse to form multinucleated bone-resorbing cells. All aspects of osteoclast formation and functions are regulated by macrophage-colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL), cytokines essential for osteoclast formation and expressed by a variety of cell types, including osteoblast lineage cells. Since the discovery of RANKL in the mid-1990s, mouse genetic and molecular studies have revealed numerous signaling pathways activated by RANKL and M-CSF. More recent studies indicate that osteoclasts and their precursors regulate immune responses and osteoblast formation and functions by means of direct cell-cell contact through ligands and receptors, such as ephrins and Ephs, and semaphorins and plexins, and through expression of clastokines. There is also growing recognition that osteoclasts are immune cells with roles in immune responses beyond mediating the bone destruction that can accompany them. This article reviews recent advances in the understanding of the molecular mechanisms regulating osteoclast formation and functions and their interactions with other cells in normal and pathologic states.