Differential biological activities of mammalian Id proteins in muscle cells

Integrated transcriptomics and metabolomics study of embryonic breast muscle of Jiaji ducks

Because number of matured muscle fibers in poultry does not increase after birth, the meat yield is mainly determined during embryogenesis. We previously indicated breast muscle grew rapidly from 18th day after hatching (E18) to E27, and almost stopped from E27 to E34 of Jiaji ducks, while the mechanism is unclear. This study utilized RNA-seq to explore the related genes of muscle development and their relationship with small molecule metabolites at E18, E27 and E34 of Jiaji ducks. Several thousand differentially expressed genes (DEGs) were detected among E18, E27 and E34. DEGs expression profiles included 8 trend maps, among which trend 1 was opposite to and trend 6 was consistent with breast muscle development trend of Jiaji ducks. Through joint analysis between trend 1 of DEGs and trend 1 of differential metabolites (DEMs), protein digestion and absorption pathway stood out. The decrease of COL8A2 gene expression will lead to the decrease of arginine content, which will inhibit the development of breast muscle in embryonic Jiaji duck. Similarly, joint analysis between trend 6 of DEGs and trend 6 of DEMs indicated the increase of GAMT gene expression will cause the increase of proline content, and then promote the development of breast muscle of Jiaji duck in embryonic period. These results will be helpful for further understanding the mechanism of muscle yields of Jiaji ducks.

Dephosphorylation of HDAC4 by PP2A-Bδ unravels a new role for the HDAC4/MEF2 axis in myoblast fusion

Muscle formation is controlled by a number of key myogenic transcriptional regulators that govern stage-specific gene expression programs and act as terminal effectors of intracellular signaling pathways. To date, the role of phosphatases in the signaling cascades instructing muscle development remains poorly understood. Here, we show that a specific PP2A-B55δ holoenzyme is necessary for skeletal myogenesis. The primary role of PP2A-B55δ is to dephosphorylate histone deacetylase 4 (HDAC4) following myocyte differentiation and ensure repression of Myocyte enhancer factor 2D (MEF2D)-dependent gene expression programs during myogenic fusion. As a crucial HDAC4/MEF2D target gene that governs myocyte fusion, we identify ArgBP2, an upstream inhibitor of Abl, which itself is a repressor of CrkII signaling. Consequently, cells lacking PP2A-B55δ show upregulation of ArgBP2 and hyperactivation of CrkII downstream effectors, including Rac1 and FAK, precluding cytoskeletal and membrane rearrangements associated with myoblast fusion. Both in vitro and in zebrafish, loss-of-function of PP2A-B55δ severely impairs fusion of myocytes and formation of multinucleated muscle fibers, without affecting myoblast differentiation. Taken together, our results establish PP2A-B55δ as the first protein phosphatase to be involved in myoblast fusion and suggest that reversible phosphorylation of HDAC4 may coordinate differentiation and fusion events during myogenesis.

CCAAT/Enhancer Binding Protein β inhibits myogenic differentiation via ID3

Myogenesis is regulated by the coordinated expression of muscle regulatory factors, a family of transcription factors that includes MYOD, MYF5, myogenin and MRF4. Muscle regulatory factors are basic helix-loop-helix transcription factors that heterodimerize with E proteins to bind the regulatory regions of target genes. Their activity can be inhibited by members of the Inhibitor of DNA binding and differentiation (ID) family, which bind E-proteins with high affinity, thereby preventing muscle regulatory factor-dependent transcriptional responses. CCAAT/Enhancer Binding protein beta (C/EBPβ) is a transcription factor expressed in myogenic precursor cells that acts to inhibit myogenic differentiation, though the mechanism remains poorly understood. We identify Id3 as a novel C/EBPβ target gene that inhibits myogenic differentiation. Overexpression of C/EBPβ stimulates Id3 mRNA and protein expression, and is required for C/EBPβ-mediated inhibition of myogenic differentiation. Misexpression of C/EBPβ in myogenic precursors, such as in models of cancer cachexia, prevents the differentiation of myogenic precursors and we show that loss of Id3 rescues differentiation under these conditions, suggesting that the stimulation of Id3 expression by C/EBPβ is an important mechanism by which C/EBPβ inhibits myogenic differentiation.

The transcription factor Lef1 switches partners from β-catenin to Smad3 during muscle stem cell quiescence

Skeletal muscle stem cells (MuSCs), also known as satellite cells, persist in adult mammals by entering a state of quiescence (G₀) during the early postnatal period. Quiescence is reversed during damage-induced regeneration and re-established after regeneration. Entry of cultured myoblasts into G₀ is associated with a specific, reversible induction of Wnt target genes, thus implicating members of the Tcf and Lef1 (Tcf/Lef) transcription factor family, which mediate transcriptional responses to Wnt signaling, in the initiation of quiescence. We found that the canonical Wnt effector β-catenin, which cooperates with Tcf/Lef, was dispensable for myoblasts to enter quiescence. Using pharmacological and genetic approaches in cultured C2C12 myoblasts and in MuSCs, we demonstrated that Tcf/Lef activity during quiescence depended not on β-catenin but on the transforming growth factor-β (TGF-β) effector and transcriptional coactivator Smad3, which colocalized with Lef1 at canonical Wnt-responsive elements and directly interacted with Lef1 specifically in G₀ Depletion of Smad3, but not β-catenin, reduced Lef1 occupancy at target promoters, Tcf/Lef target gene expression, and self-renewal of myoblasts. In vivo, MuSCs underwent a switch from β-catenin-Lef1 to Smad3-Lef1 interactions during the postnatal switch from proliferation to quiescence, with β-catenin-Lef1 interactions recurring during damage-induced reactivation. Our findings suggest that the interplay of Wnt-Tcf/Lef and TGF-β-Smad3 signaling activates canonical Wnt target promoters in a manner that depends on β-catenin during myoblast proliferation but is independent of β-catenin during MuSC quiescence.

Introduction of ID2 Enhances Invasiveness in ID2-null Oral Squamous Cell Carcinoma Cells via the SNAIL Axis

AIM

Inhibitor of DNA-binding (ID) proteins are negative regulators of basic helix-loop-helix transcription factors that generally stimulate cell proliferation and inhibit differentiation. However, the role of ID2 in cancer progression remains ambiguous. Here, we investigated the function of ID2 in ID2-null oral squamous cell carcinoma (OSCC) cells.

MATERIALS AND METHODS

We introduced an ID2 cDNA construct into ID2-null OSCC cells and compared them with empty-vector-transfected cells in terms of cell proliferation, invasion, and activity and expression of matrix metalloproteinase (MMP).

RESULTS

ID2 introduction resulted in enhanced malignant phenotypes. The ID2-expressing cells showed increased N-cadherin, vimentin, and E-cadherin expression and epithelial-mesenchymal transition. In addition, cell invasion drastically increased with increased expression and activity of MMP2. Immunoprecipitation revealed a direct interaction between ID2 and zinc finger transcription factor, snail family transcriptional repressor 1 (SNAIL1).

CONCLUSION

ID2 expression triggered a malignant phenotype, especially of invasive properties, through the ID2-SNAIL axis. Thus, ID2 represents a potential therapeutic target for OSCC.

Targeting ID2 expression triggers a more differentiated phenotype and reduces aggressiveness in human salivary gland cancer cells

Inhibitors of DNA-binding (ID) proteins are negative regulators of basic helix-loop-helix transcription factors and generally stimulate cell proliferation and inhibit differentiation. We previously determined that ID1 was highly expressed in aggressive salivary gland cancer (SGC) cells in culture. Here, we show that ID2 is also expressed in aggressive SGC cells. ID2 knockdown triggers important changes in cell behavior, that is, it significantly reduces the expression of N-cadherin, vimentin and Snail, induces E-cadherin expression and leads to a more differentiated phenotype exemplified by changes in cell shape. Moreover, ID2 knockdown almost completely suppresses invasion and the expression of matrix metalloproteinase 9. In conclusion, ID2 expression maintains an aggressive phenotype in SGC cells, and ID2 repression triggers a reduction in cell aggressiveness. ID2 therefore represents a potential therapeutic target during SGC progression. ID proteins are negative regulators of basic helix-loop-helix transcription factors and generally stimulate cell proliferation and inhibit differentiation. ID2 knockdown triggers important changes in cell behavior, that is, it significantly reduces the expression of N-cadherin, vimentin and Snail, induces E-cadherin expression and leads to a more differentiated phenotype exemplified by changes in cell shape. ID2 therefore represents a potential therapeutic target during SGC progression.

Scaffold protein enigma homolog 1 overcomes the repression of myogenesis activation by inhibitor of DNA binding 2

Enigma Homolog 1 (ENH1) is a scaffold protein for signaling proteins and transcription factors. Previously, we reported that ENH1 overexpression promotes the differentiation of C2C12 myoblasts. However, the molecular mechanism underlying the role of ENH1 in the C2C12 cells differentiation remains elusive. ENH1 was shown to inhibit the proliferation of neuroblastoma cells by sequestering Inhibitor of DNA binding protein 2 (Id2) in the cytosol. Id2 is a repressor of basic Helix-Loop-Helix transcription factors activity and prevents myogenesis. Here, we found that ENH1 overcome the Id2 repression of C2C12 cells myogenic differentiation and that ENH1 overexpression promotes mice satellite cells activation, the first step toward myogenic differentiation. In addition, we show that ENH1 interacted with Id2 in C2C12 cells and mice satellite cells. Collectively, our results suggest that ENH1 plays an important role in the activation of myogenesis through the repression of Id2 activity.

miR-98 delays skeletal muscle differentiation by down-regulating E2F5

A genome-wide screen had previously shown that knocking down miR-98 and let-7g, two miRNAs of the let-7 family, leads to a dramatic increase in terminal myogenic differentiation. In the present paper, we report that a transcriptomic analysis of human myoblasts, where miR-98 was knocked down, revealed that approximately 240 genes were sensitive to miR-98 depletion. Among these potential targets of miR-98, we identified the transcriptional repressor E2F5 and showed that it is a direct target of miR-98. Knocking down simultaneously E2F5 and miR-98 almost fully restored normal differentiation, indicating that E2F5 is involved in the regulation of skeletal muscle differentiation. We subsequently show that E2F5 can bind to the promoters of two inhibitors of terminal muscle differentiation, ID1 (inhibitor of DNA binding 1) and HMOX1 (heme oxygenase 1), which decreases their expression in skeletal myoblasts. We conclude that miR-98 regulates muscle differentiation by altering the expression of the transcription factor E2F5 and, in turn, of multiple E2F5 targets.

A systems approach and skeletal myogenesis

Skeletal myogenesis depends on the strict regulation of the expression of various gene subsets. Therefore, the understanding of genome wide gene regulation is imperative for elucidation of skeletal myogenesis. In recent years, systems approach has contributed to the understanding of various biological processes. Our group recently revealed the critical genome network of skeletal myogenesis by using a novel systems approach combined with whole-mount in situ hybridization (WISH) database, high-throughput screening, and microarray analysis. In this paper, we introduce our systems approach for understanding the myogenesis regulatory network and describe the advantages of systems approach.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

Olig1 and ID4 interactions in living cells visualized by bimolecular fluorescence complementation technique

Olig1, a member of class B basic-helix-loop-helix (bHLH), plays key roles in early oligodendrocyte specification. Inhibitors of DNA binding (Id) is another sub-class of HLH proteins, act as dominant-negative regulators of bHLH proteins, which can form heterodimers with class A or B bHLH proteins, but lack the critical basic DNA binding domain. Id4 was recently found to interact with olig1 and inhibit oligodendrocyte differentiation. However, there still no direct evidence to reveal the spatial and temporal interaction of olig1 and ID4 in living cells. In this study, we performed bimolecular fluorescence complementation (BiFC) analysis to further characterize the distinct subcellular localization of olig1, ID4 and their dimer in living SW1116 cells. To examine the subcellular localization of olig1 and ID4 by themselves, the olig1-EGFP or ID4-DsRed2 fusion proteins were also expressed in SW1116 cells, respectively. As predicted, the olig1-EGFP fusion proteins were located in the nucleus, and ID4-DsRed2 fusion proteins were located in the cytoplasm. When olig1-EGFP and ID4-DsRed2 fusion proteins were co-expressed, the green and red signals were co-located in the cytoplasm. Using BiFC, the strong BiFC signals could be detected in pBiFC-olig1VN173 and pBiFC-ID4VC155 co-transfected cells and the fluorescence signal was located in the cytoplasm. These results collectively confirmed that olig1 and ID4 could interact and form dimer in living cells, and ID4 could block the transport of olig1 from cytoplasm to nucleus.

NF-kappaB signaling: a tale of two pathways in skeletal myogenesis

NF-kappaB is a ubiquitiously expressed transcription factor that plays vital roles in innate immunity and other processes involving cellular survival, proliferation, and differentiation. Activation of NF-kappaB is controlled by an IkappaB kinase (IKK) complex that can direct either canonical (classical) NF-kappaB signaling by degrading the IkappaB inhibitor and releasing p65/p50 dimers to the nucleus, or causes p100 processing and nuclear translocation of RelB/p52 via a noncanonical (alternative) pathway. Under physiological conditions, NF-kappaB activity is transiently regulated, whereas constitutive activation of this transcription factor typically in the classical pathway is associated with a multitude of disease conditions, including those related to skeletal muscle. How NF-kappaB functions in muscle diseases is currently under intense investigation. Insight into this role of NF-kappaB may be gained by understanding at a more basic level how this transcription factor contributes to skeletal muscle cell differentiation. Recent data from knockout mice support that the classical NF-kappaB pathway functions as an inhibitor of skeletal myogenesis and muscle regeneration acting through multiple mechanisms. In contrast, alternative NF-kappaB signaling does not appear to be required for myofiber conversion, but instead functions in myotube homeostasis by regulating mitochondrial biogenesis. Additional knowledge of these signaling pathways in skeletal myogenesis should aid in the development of specific inhibitors that may be useful in treatments of muscle disorders.

A systems approach reveals that the myogenesis genome network is regulated by the transcriptional repressor RP58

We created a whole-mount in situ hybridization (WISH) database, termed EMBRYS, containing expression data of 1520 transcription factors and cofactors expressed in E9.5, E10.5, and E11.5 mouse embryos--a highly dynamic stage of skeletal myogenesis. This approach implicated 43 genes in regulation of embryonic myogenesis, including a transcriptional repressor, the zinc-finger protein RP58 (also known as Zfp238). Knockout and knockdown approaches confirmed an essential role for RP58 in skeletal myogenesis. Cell-based high-throughput transfection screening revealed that RP58 is a direct MyoD target. Microarray analysis identified two inhibitors of skeletal myogenesis, Id2 and Id3, as targets for RP58-mediated repression. Consistently, MyoD-dependent activation of the myogenic program is impaired in RP58 null fibroblasts and downregulation of Id2 and Id3 rescues MyoD's ability to promote myogenesis in these cells. Our combined, multi-system approach reveals a MyoD-activated regulatory loop relying on RP58-mediated repression of muscle regulatory factor (MRF) inhibitors.

The fibrodysplasia ossificans progressiva R206H ACVR1 mutation activates BMP-independent chondrogenesis and zebrafish embryo ventralization

Patients with classic fibrodysplasia ossificans progressiva, a disorder characterized by extensive extraskeletal endochondral bone formation, share a recurrent mutation (R206H) within the glycine/serine-rich domain of ACVR1/ALK2, a bone morphogenetic protein type I receptor. Through a series of in vitro assays using several mammalian cell lines and chick limb bud micromass cultures, we determined that mutant R206H ACVR1 activated BMP signaling in the absence of BMP ligand and mediated BMP-independent chondrogenesis that was enhanced by BMP. We further investigated the interaction of mutant R206H ACVR1 with FKBP1A, a glycine/serine domain-binding protein that prevents leaky BMP type I receptor activation in the absence of ligand. The mutant protein exhibited reduced binding to FKBP1A in COS-7 simian kidney cell line assays, suggesting that increased BMP pathway activity in COS-7 cells with R206H ACVR1 is due, at least in part, to decreased binding of this inhibitory factor. Consistent with these findings, in vivo analyses of zebrafish embryos showed BMP-independent hyperactivation of BMP signaling in response to the R206H mutant, resulting in increased embryonic ventralization. These data support the conclusion that the mutant R206H ACVR1 receptor in FOP patients is an activating mutation that induces BMP signaling in a BMP-independent and BMP-responsive manner to promote chondrogenesis, consistent with the ectopic endochondral bone formation in these patients.

Expression and prognostic values of Id-1 and Id-3 in gastric adenocarcinoma

BACKGROUND

Id (inhibitor of differentiation/DNA binding)-1 and -3 are involved in neoangiogenesis; they antagonize basic helix-loop-helix proteins, inhibit differentiation, and enhance cell proliferation. The aim of this study was to investigate Id-1 and -3 expression in gastric tumors and their clinical relevance in gastric cancer.

MATERIALS AND METHODS

We investigated Id-1 and Id-3 expression in gastric cancer samples by immunohistochemistry and Western blotting, and further analyzed the relationship between expression of Id-1 and Id-3 and clinicopathologic characteristics.

RESULTS

Expression of Id-1 and -3 was found significantly more often in gastric cancers than in matched adjacent nonmalignant tissues. Cancer samples with poor or moderate histologic differentiation showed significantly stronger Id-1 and -3 expression than cancer samples with high differentiation. In cancer samples, strong or moderate expression of Id-3, but not Id-1, was a strong independent predictor for shorter overall survival in multivariate analysis.

CONCLUSIONS

The level of Id-1 and -3 protein expression was associated with the malignant potential of gastric tumors. In cancer samples, stronger Id-1 and -3 expression is associated with poor differentiation and more aggressive behavior of tumor cells, resulting in poor clinical outcome. Consequently, Id-3 might be used to independently predict survival of patients with gastric cancer.

The intracellular localization of ID2 expression has a predictive value in non small cell lung cancer

BACKGROUND

ID2 is a member of a subclass of transcription regulators belonging to the general bHLH (basic-helix-loop-helix) family of transcription factors. In normal cells, ID2 is responsible for regulating the balance between proliferation and differentiation. More recent studies have demonstrated that ID2 is involved in tumor progression in several cancer types such as prostate or breast.

METHODOLOGY/PRINCIPAL FINDINGS

In this work, we investigated, for the first time, the relationship between the expression of ID2 in non-small cell lung cancer (NSCLC) patients and the clinicopathological features and prognosis of these patients. Immunohistochemistry was performed on tissue microarrays, which included 62 NSCLC tumors. In malignant tissues, ID2 expression has been detected in both the nuclear and cytoplasmic compartments, but we have demonstrated that only nuclear expression of ID2 is inversely correlated with the differentiation grade of the tumor (p = 0.007). Interestingly, among patients with poorly differentiated tumors, high nuclear expression of ID2 was an independent and unfavorable prognostic factor for survival (p = 0.036).

CONCLUSIONS

These results suggest that ID2 could be involved in tumor dedifferentiation processes of NSCLC, and could be used as prognostic marker for patients with poorly differentiated tumors.

Rho/Rho-associated kinase signal regulates myogenic differentiation via myocardin-related transcription factor-A/Smad-dependent transcription of the Id3 gene

RhoA is known to be involved in myogenic differentiation, but whether it acts as a positive or negative regulator is controversial. To resolve this issue, we investigated the differentiation stage-specific roles of RhoA and its effector, Rho-associated kinase, using C2C12 myoblasts. We found that proliferating myoblasts show high levels of RhoA and serum-response factor activities and strong expression of the downstream target of RhoA, myocardin-related transcription factor-A (MRTF-A or MAL); these activities and expression are markedly lower in differentiating myocytes. We further demonstrated that, in proliferating myoblasts, an increase in MRTF-A, which forms a complex with Smad1/4, strikingly activates the expression level of the Id3 gene; the Id3 gene product is a potent inhibitor of myogenic differentiation. Finally, we found that during differentiation, one of the forkhead transcription factors translocates into the nucleus and suppresses Id3 expression by preventing the association of the MRTF-A-Smad complex with the Id3 promoter, which leads to the enhancement of myogenic differentiation. We conclude that RhoA/Rho-associated kinase signaling plays positive and negative roles in myogenic differentiation, mediated by MRTF-A/Smad-dependent transcription of the Id3 gene in a differentiation stage-specific manner.

Glucose induces increases in levels of the transcriptional repressor Id2 via the hexosamine pathway

Changes in glucose levels are known to directly alter gene expression. A number of previous studies have found that these effects are in part mediated by modulating the levels and the activity of transcription factors. We have investigated an alternative mechanism by which glucose might regulate gene expression by modulating levels of a transcriptional repressor. We have focused on Id2, which is a protein that indirectly regulates gene expression by sequestering certain transcription factors and preventing them from forming functional dimers. Id2 targets include the class A basic helix-loop-helix transcription factors and the sterol regulatory element-binding protein (SREBP)-1. We demonstrate that increases in glucose levels cause a rapid increase in levels of Id2 in J774.2 macrophages, and a number of lines of evidence indicate that this is via the hexosamine pathway because 1) the effect of glucose requires glutamine; 2) the effect of glucose is mimicked by low levels of glucosamine; 3) the effect of glucose is inhibited by azaserine, an inhibitor of glutamine:fructose-6-phosphate amidotransferase (GFAT); and 4) adenoviral mediated overexpression of GFAT increases levels of Id2. We go on to show that increases in Id2 can have functional effects on metabolic genes, because Id2 blocked the SREBP-1-induced induction of hormone-sensitive lipase (HSL) promoter activity, whereas Id2 alone does not modulate activity of the HSL promoter. In summary, these studies define a new mechanism by which glucose uses the hexosamine pathway to regulate gene expression by increasing levels of a transcriptional repressor.

Dynamic Id2 expression in the medial and lateral domains of avian dermamyotome

Id2 cDNA was isolated from a subtractive screen of stage-12 quail caudal somites. In situ hybridisation analysis identified the previously un-described expression of Id2 mRNA in distinct medial and lateral domains of the somitic dermamyotome in both quail and chick embryos. Id2 expression in somites was highly dynamic being first initiated in the lateral domain of the dermamyotome of stage-8-10 embryos, followed by expression in a separate medial domain. Id2 mRNA during subsequent embryonic development could be detected in both medial and lateral domains in the anterior to mid regions while the posterior, recently segmented somites, showed expression only in the lateral domain, which was eventually down regulated in the anterior-most somites. Tissue manipulation studies revealed that Id2 expression in somites required positive signalling from not only axial structures and lateral plate mesoderm but also surface ectoderm. In addition, Id2 expression was also observed in anterior and posterior domains of developing avian limb buds and interdigital tissue.

Transcriptional program of bone morphogenetic protein-2-induced epithelial and smooth muscle differentiation of pluripotent human embryonal carcinoma cells

Pluripotent human embryonal carcinoma NTera2/cloneD1 (NT2/D1) cells respond to multiple vertebrate patterning factors and offer a unique model system to investigate the signaling events associated with lineage determination and cell differentiation. Here, we define the temporal changes in global gene expression patterns in NT2/D1 cells upon treatment with bone morphogenetic protein-2 (BMP-2). Exposure to BMP-2 rapidly induced the expression of several transcription factors involved in establishing non-neural ectodermal fate followed by the appearance of epithelial-specific markers. Subsequent loss of stem cell markers was coupled to gene expression changes associated with decreased proliferative activity. Temporal clustering of gene expression patterns revealed a concurrent down-regulation of multiple transcripts involved in neurogenesis, neurite outgrowth, and axonal guidance, suggesting that the BMP-mediated differentiation process involves pro-epithelial as well as anti-neurogenic mechanisms. In addition, increased expression of smooth muscle markers both by gene expression and immunohistochemistry was detected. Several neural crest markers were induced preceding such a differentiation, compatible with a neural crest origin of NT2/D1-derived smooth muscle cells. Comparison of changes in transcript expression between BMP-2-induced epithelial versus all-trans-retinoic acid (ATRA)-induced neural differentiation revealed potential candidates for regulation of BMP-2 signaling and suppression of neural fate by BMP-2. This study suggests that BMP-2-induced differentiation of NT2/D1 cells provides a powerful assay to study early human epithelial and smooth muscle development.

Mechanisms regulating lineage diversity during mammalian cerebral cortical neurogenesis and gliogenesis

During mammalian cerebral cortical development, neural stem cells (NSCs) present within periventricular generative zones give rise to successive waves of neurons and radial glia, followed by oligodendrocytes and astrocytes. The molecular and cellular mechanisms that orchestrate these precisely timed and progressive maturational events are still largely undefined. These developmental processes are likely to involve the dynamic interplay of environmental signals, cell-cell interactions and transcriptional regulatory events. The bone morphogenetic proteins (BMPs), an expanding subclass of the transforming growth factor beta cytokine superfamily, may represent an important set of environmental cues for these progressive maturational events because of the broad profiles of developmental expression of the requisite BMP ligands, receptor subunits and intracellular transduction elements, and because of their versatile roles in promoting a spectrum of cellular processes intimately involved in progressive neural fate decisions. The BMPs also interact with complementary regional environmental signals such as the basic fibroblast growth factor (bFGF) and sonic hedgehog (Shh) that promote earlier stages of NSC expansion, self-renewal, lineage restriction and incipient lineage commitment. The ability of these cytokines and trophic signals to act within specific neurodevelopmental contexts may, in turn, depend on the composite actions of cell-cell contact-associated signals, such as Notch-Hes-mediated lateral inhibitory pathways, and additional transcriptional modulatory events, such as those mediated by members of the inhibitor of differentiation (ID) gene family that encode a novel set of negative basic helix-loop-helix (bHLH) transcription factors. In this chapter, we will examine the distinct roles of these different classes of developmental cues in defining the biological properties of an integrated cerebral cortical developmental signaling network. Ongoing studies in this exciting area of mammalian central nervous system (CNS) development will help to identify important molecular and cellular targets for evolving pharmacological, gene and stem cell therapeutic interventions to combat the pathological sequelae of a spectrum of acquired and genetic disorders of the central nervous system.

Id2 expression during apoptosis and satellite cell activation in unloaded and loaded quail skeletal muscles

Inhibitor of differentiation-2 (Id2) is a basic helix-loop-helix protein that acts as a negative regulator of the myogenic regulatory transcription factor family, but Id2 has also been implicated in apoptosis in several cell lines. In this study, we tested the hypothesis that Id2 has a role in both apoptosis-associated muscle atrophy and muscle hypertrophy. A weight corresponding to 12% of the body weight was attached to one wing of Japanese quail to induce hypertrophy in the patagialis (PAT) muscle. Birds in group 1 were killed after 5 (n = 8), 7 (n = 10), or 14 days (n = 10) of loading. The left wing was loaded for 14 days in group 2 birds, and then the weight was removed and the PAT was examined after 7 (n = 10), 14 (n = 10), or 21 (n = 5) days of unloading. A time-released bromodeoxyuridine (BrdU) pellet was implanted subcutaneously with wing weighting to identify activated satellite cells during loading. The left wing was loaded for 14 days, unloaded for 14 days, and then the weight was reattached for a subsequent 7 (n = 10) or 14 days (n = 10) in group 3 birds. BrdU was implanted on the second loading phase in this group. Id2 mRNA as measured by kinetic PCR increased by 3.9-, 2.7-, and 1.6-fold, relative to control levels after 7, 14, and 21 days of unloading (group 2). Id2 protein as estimated by Western blots increased by 1.5-, 1.4-, and 0.75-fold after 7, 14, and 21 days of unloading (group 2). Muscle unloading induced apoptosis, because poly(ADP-ribose) polymerase-(PARP)-positive nuclei increased and caspase 8 levels increased by 2.6- and 1.7-fold after 7 or 14 days of unloading, respectively (group 2). Although BrdU-positive nuclei increased during loading (groups 1 and 3), 50% failed to survive during unloading (group 2). Id2 mRNA increased by 2.2- and 1.8-fold after 5 and 7 days of loading, respectively, but decreased to control levels by 14 days of loading in group 1. Id2 protein levels increased 2.1-fold after 5 days of loading (group 1). In contrast, Id2 did not increase in reloaded muscles of group 3 birds. These data suggest that Id2 may have a role in apoptosis-associated atrophy of skeletal muscles, but its role in muscle hypertrophy is less clear.

Characterization of heterokaryons between skeletal myoblasts and preadipocytes: myogenic potential of 3T3-L1 preadipocytes

It has been shown previously that heterokaryons between myoblasts and non-myogenic cells disturb myogenic differentiation (Hirayama et al. (2001); Cell Struct. Funct. 26, 37-47), suggesting that some myogenesis inhibitory factors exist in non-myogenic cells. Skeletal myoblasts and adipose cells are derived from a common mesodermal stem cell, indicating that both cells have a closer relationship in the developmental lineage than the other somatic cells. To investigate the functional relationship between myoblasts and adipose cells, heterokaryons between quail myoblasts and 3T3-L1 cells, a mouse preadipocyte cell line, were prepared and examined for characteristics of myogenic differentiation. Myogenic differentiation was inhibited in the heterokaryons between quail myoblasts and well-differentiated (adipocytes) 3T3-L1 cells. On the contrary, normal myogenic differentiation proceeded in the heterokaryons between quail myoblasts and undifferentiated (preadipocytes) 3T3-L1 cells. Further investigation showed that the mouse myogenin gene from 3T3-L1 cells was transactivated in the heterokaryons between quail myoblasts and undifferentiated 3T3-L1 cells. The results demonstrated that undifferentiated 3T3-L1 cells have no myogenesis inhibitory factors but acquire these during terminal differentiation into adipocytes.

Genes that are differentially expressed in rat vibrissa follicle germinative epithelium in vivo show altered expression patterns after extended organ culture

Hair growth depends on maintenance of signalling between the dermal papilla and the germinative epithelium (GE), from which the differentiated layers of the hair fibre originate. Because no molecular studies have been reported which concentrate specifically on GE cells either in vivo or in vitro, we prepared a cDNA library enriched for messages which were highly expressed in GE cells to identify genes that may be involved in hair growth control. Of 35 subtracted library clones sequenced, 23 shared extensive homology with previously determined cDNA sequences, including LEF-1 and id4. Hair follicle organ culture models are often used to investigate the molecular basis of hair growth, although hair growth arrest occurs relatively rapidly in vitro. As an indicator of their role in follicle activities, we compared the expression of GE-specific clones in different regions of freshly isolated vibrissa follicles, with the corresponding regions of growth arrested, cultured follicles. Changes in the expression of some of these clones indicates that they could be related to fundamental cellular activities in the follicle. A library enriched for GE-specific clones therefore provides a useful source of candidate molecules for studies of follicular epithelial cell behaviour, both in vivo and in vitro.

The MyoD-inducible p204 protein overcomes the inhibition of myoblast differentiation by Id proteins

The murine p204 protein level is highest in heart and skeletal muscle. During the fusion of cultured myoblasts to myotubes, the p204 level increases due to transcription dependent on the muscle-specific MyoD protein, and p204 is phosphorylated and translocated from the nucleus to the cytoplasm. p204 overexpression accelerates myoblast fusion in differentiation medium and triggers this process even in growth medium. Here we report that p204 is required for the differentiation of C2C12 myoblasts. We propose that it enables the differentiation, at least in part, by overcoming the inhibition of the activities of the MyoD and E47 proteins by the Id proteins: Id1, Id2, and Id3. These are known to inhibit skeletal muscle differentiation by binding and blocking the activity of MyoD, E12/E47, and other myogenic basic helix-loop-helix (bHLH) proteins. Our hypothesis is based on the following findings. (i) A decrease in the p204 level in C2C12 myoblasts by antisense RNA (a) increased the level of the Id2; (b) inhibited the MyoD-, E12/E47-, and other bHLH protein-dependent accumulation of the muscle-specific myosin heavy-chain protein; and (c) inhibited the fusion of myoblasts to myotubes in differentiation medium. (ii) p204 bound to the Id proteins in vitro and in vivo. (iii) In the binding of p204 to Id2, the b segment of p204 and the HLH segment of Id2 were involved. (iv) Addition of p204 overcame the inhibition by the Id proteins of the binding of MyoD and E47 to DNA in vitro. (v) Overexpression of p204 in myoblasts (a) decreased the level of the Id proteins, even in a culture in growth medium, and (b) overcame the inhibition by the Id proteins of MyoD- and E47 dependent transcription and also overcame the inhibition by Id2 of the fusion of myoblasts to myotubes.

Direct binding of Smad1 and Smad4 to two distinct motifs mediates bone morphogenetic protein-specific transcriptional activation of Id1 gene

Bone morphogenetic proteins (BMPs) are potent inhibitors of myoblast differentiation and inducers of bone formation both in vivo and in vitro. Expression of Id1, a negative regulator of basic helix-loop-helix transcription factors, is up-regulated by BMPs and contributes to the antimyogenic effects of this family of cytokines. In this report, we have identified a specific BMP-2 immediate early response enhancer in the human Id1 gene. Transcriptional activation of the enhancer was increased by overexpression of BMP-responsive Smads, and Smad4 and was completely abrogated in Smad4-deficient cells. Deletion analysis demonstrates that the responsive region is composed of two separate DNA binding elements, a set of overlapping GC boxes, which bind BMP-regulated Smads upon BMP stimulation, and three repeats of CAGAC boxes. Gel shift and oligonucleotide pull-down assays demonstrated that these two types of motifs were capable of binding their corresponding Smads. However, deletion or mutation of either DNA binding element was nonadditive, since disruption of either GC or CAGAC boxes resulted in complete or severe loss of BMP-2 responsiveness. These data suggest the simultaneous requirement of two independent DNA binding elements to allow functional cooperativity of BMP-regulated Smads and Smad4 in BMP-activated gene promoters.

Assessment of gene regulation by bone morphogenetic protein 2 in human marrow stromal cells using gene array technology

Marrow stromal cells can differentiate into osteoblasts, adipocytes, myoblasts, and chondrocytes. Bone morphogenetic protein 2 (BMP-2) is a potent stimulator of osteoblastic differentiation, and identification of the genes regulated by BMP-2 in these cells should provide insight into the mechanism(s) of osteoblastic differentiation. Thus, we used a conditionally immortalized human marrow stromal cell line (hMS) and a gene expression microarray containing probes for a total of 6800 genes to compare gene expression in control and BMP-2-treated cultures. A total of 51 genes showed a consistent change in messenger RNA (mRNA) frequency between two repeat experiments. Seventeen of these genes showed a change in expression of at least 3-fold in BMP-2-treated cultures over control cultures. These included nuclear binding factors (10 genes), signal transduction pathway genes (2 genes), molecular transport (1 gene), cell surface proteins (2 genes) and growth factors (2 genes). Of particular interest were four of the nuclear binding factor genes ID-1, ID-2, ID-3, and ID-4. These encode dominant negative helix-loop-helix (dnHLH) proteins that lack the nuclear binding domain of the basic HLH proteins and thus have no transcriptional activity. They have been implicated in blocking both myogenesis and adipogenesis. Other transcription factors up-regulated at least 3-fold by BMP-2 included Dlx-2, HES-1, STAT1, and JunB. The changes in these nuclear binding factor mRNA levels were confirmed by real-time reverse-transcriptase-polymerase chain reaction (RT-PCR). A further three transcription factors, core binding factor beta (CBFbeta), AREB6, and SOX4, showed changes in expression of between 2- and 3-fold with BMP-2 treatment. In summary, we have used a gene chip microarray to identify a number of BMP-2 responsive genes in hMS cells. Thus, these studies provide potential candidate genes that may induce osteoblastic differentiation or, in the case of the ID proteins, block differentiation along alternate pathways.

Id-1, ITF-2, and Id-2 comprise a network of helix-loop-helix proteins that regulate mammary epithelial cell proliferation, differentiation, and apoptosis

Mammary epithelial cells proliferate, invade the stroma, differentiate, and die in adult mammals by mechanisms that are poorly understood. We found that Id-1, an inhibitor of basic helix-loop-helix transcription factors, regulates mammary epithelial cell growth, differentiation, and invasion in culture. Here, we show that Id-1 is expressed highly during mammary development in virgin mice and during early pregnancy, when proliferation and invasion are high. During mid-pregnancy, Id-1 expression declined to undetectable levels as the epithelium differentiated fully. Surprisingly, Id-1 increased during involution, when the epithelium undergoes extensive apoptosis. To determine whether Id-1 regulates both proliferation and apoptosis, we constitutively expressed Id-1 in mammary epithelial cell cultures. Id-1 stimulated proliferation in sparse cultures but induced apoptosis in dense cultures, which reflect epithelial cell density during early pregnancy and involution, respectively. To understand how Id-1 acts, we screened a yeast two-hybrid library from differentiating mammary epithelial cells and identified ITF-2, a basic helix-loop-helix transcription factor, as an Id-1-interacting protein. Overexpression of ITF-2 significantly reduced Id-1-stimulated proliferation and apoptosis. We show further that, in contrast to Id-1, Id-2 was expressed highly in differentiated mammary epithelial cells in vivo and in culture. In culture, Id-2 antisense transcripts blocked differentiation. Our results suggest that Id-1, ITF-2, and Id-2 comprise a network of interacting molecular switches that govern mammary epithelial cell phenotypes.

A role for the helix-loop-helix protein Id2 in the control of oligodendrocyte development

Compared to neurons, the intracellular mechanisms that control glial differentiation are still poorly understood. We show here that oligodendrocyte lineage cells express the helix-loop-helix proteins Mash1 and Id2. Although Mash1 has been found to regulate neuronal development, we found that in the absence of Mash1 oligodendrocyte differentiation occurs normally. In contrast, we found that overexpression of Id2 powerfully inhibits oligodendrocyte differentiation, that Id2 normally translocates out of the nucleus at the onset of differentiation, and that absence of Id2 induces premature oligodendrocyte differentiation in vitro. These findings demonstrate that Id2 is a component of the intracellular mechanism that times oligodendrocyte differentiation and point to the existence of an as yet unidentified MyoD-like bHLH protein necessary for oligodendrocyte differentiation.

ID helix-loop-helix proteins in cell growth, differentiation and tumorigenesis

The ubiquitously expressed family of ID helix-loop-helix (HLH) proteins function as dominant negative regulators of basic HLH (bHLH) transcriptional regulators that drive cell lineage commitment and differentiation in metazoa. Recent data from cell line and in vivo studies have implicated the functions of ID proteins in other cellular processes besides negative regulation of cell differentiation. ID proteins play key roles in the regulation of lineage commitment, cell fate decisions and in the timing of differentiation during neurogenesis, lymphopoiesis and neovascularisation (angiogenesis). They are essential for embryogenesis and for cell cycle progression, and they function as positive regulators of cell proliferation. ID proteins also possess pro-apoptotic properties in a variety of cell types and function as cooperating or dominant oncoproteins in immortalisation of rodent and human cells and in tumour induction in Id-transgenic mice. In several human tumour types, the expression of ID proteins is deregulated, and loss- and gain-of-function studies implicate ID functions in the regulation of tumour growth, vascularisation, invasiveness and metastasis. More recent biochemical studies have also revealed an emerging ‘molecular promiscuity’ of mammalian ID proteins: they directly interact with and modulate the activities of several other families of transcriptional regulator, besides bHLH proteins.

MyoD-dependent induction during myoblast differentiation of p204, a protein also inducible by interferon

p204, an interferon-inducible p200 family protein, inhibits rRNA synthesis in fibroblasts by blocking the binding of the upstream binding factor transcription factor to DNA. Here we report that among 10 adult mouse tissues tested, the level of p204 was highest in heart and skeletal muscles. In cultured C2C12 skeletal muscle myoblasts, p204 was nucleoplasmic and its level was low. During myoblast fusion this level strongly increased, p204 became phosphorylated, and the bulk of p204 appeared in the cytoplasm of the myotubes. Leptomycin B, an inhibitor of nuclear export that blocked myoblast fusion, inhibited the nuclear export signal-dependent translocation of p204 to the cytoplasm. The increase in the p204 level during myoblast fusion was a consequence of MyoD transcription factor binding to several MyoD-specific sequences in the gene encoding p204, followed by transcription. Overexpression of p204 (in C2C12 myoblasts carrying an inducible p204 expression plasmid) accelerated the fusion of myoblasts to myotubes in differentiation medium and induced the fusion even in growth medium. The level of p204 in mouse heart muscle strongly increased during differentiation; it was barely detectable in 10. 5-day-old embryos, reached the peak level in 16.5-day-old embryos, and remained high thereafter. p204 is the second p200 family protein (after p202a) found to be involved in muscle differentiation. (p202a was formerly designated p202. The new designation is due to the identification of a highly similar protein-p202b [H. Wang, G. Chatterjee, J. J. Meyer, C. J. Liu, N. A. Manjunath, P. Bray-Ward, and P. Lengyel, Genomics 60:281-294, 1999].) These results reveal that p204 and p202a function in both muscle differentiation and interferon action.

Protein binding by the 3' untranslated region of alpha-striated tropomyosin

Tropomyosin is a component protein of the thin filament system in striated muscle, regulating the interaction between actin and myosin. The 3' untranslated region of the alpha-striated tropomyosin gene (TM UTR) induces muscle differentiation when expressed in primary fibroblasts, but the mechanism has not been defined. We hypothesize that fibroblasts utilize resident proteins to effect this response, perhaps by TM UTR binding to protein(s). In order to facilitate identification of protein(s) involved in mediating this differentiation response, we investigated the potential for this sequence to bind to cellular protein utilizing electrophoretic mobility gel shifting analysis (EMSA) with and without UV cross-linking. Under very specific conditions (including pH, KCl, and Mg concentration and extent of phosphorylation of protein), the TM UTR is able to bind protein in cells that differentiate upon TM UTR expression. Protein binding is significantly more extensive in cytoplasmic than nuclear protein preparations. Secondary structure of the RNA probe facilitates protein binding. The molecular masses of bound proteins are approximately 42 and 115 kDa under basal conditions. EMSA analysis of extract from cultured skeletal muscle confirms that protein binding by the TM UTR occurs in this cell type, and is more extensive in less differentiated cells. The demonstration of highly regulated protein binding by the TM UTR raises the possibility that this sequence may cause differentiation by binding to endogenous proteins, and further that this sequence may play a role in normal differentiation. Identification of proteins bound by the TM UTR will be necessary to completely define the mechanism by which it causes differentiation.

Reciprocal Id expression and myelin gene regulation in Schwann cells

Id proteins are thought to act as dominant negative antagonists of basic helix-loop-helix (bHLH) transcription factors that direct differentiation in various cell types. We found that Schwann cells express all four Id-family genes and that their transcript levels were reciprocally regulated in pairs during nerve maturation in vivo and cAMP-mediated differentiation in vitro. The rapid induction as part of the early response to axonal membranes and cytokines suggested that Id3 is involved in myelin gene repression. An inverse relationship between Id1/3 and myelin P0 expression was consistent with a role for these two Id proteins as inhibitors of differentiation, and Id1/3 proteins strongly repressed myelin gene promoter activity. Nuclear factors isolated from Schwann cells and intact sciatic nerves were found to bind three different HLH recognition sequences (E boxes) in the proximal region of the P0 promoter, and production of these DNA binding complexes was altered during differentiation. These data support the concept that Id proteins regulate myelin gene expression by controlling the formation of specific bHLH DNA binding complexes with different E-box preferences.