Regulation of Id3 cell cycle function by Cdk-2-dependent phosphorylation

Id proteins: emerging roles in CNS disease and targets for modifying neural stemcell behavior

Neural stem/progenitor cells (NSPCs) are found in the adult brain and spinal cord, and endogenous or transplanted NSPCs contribute to repair processes and regulate immune responses in the CNS. However, the molecular mechanisms of NSPC survival and integration as well as their fate determination and functionality are still poorly understood. Inhibitor of DNA binding (Id) proteins are increasingly recognized as key determinants of NSPC fate specification. Id proteins act by antagonizing the DNA-binding activity of basic helix-loop-helix (bHLH) transcription factors, and the balance of Id and bHLH proteins determines cell fate decisions in numerous cell types and developmental stages. Id proteins are central in responses to environmental changes, as they occur in CNS injury and disease, and cellular responses in adult NSPCs implicate Id proteins as prime candidates for manipulating stemcell behavior. Here, we outline recent advances in understanding Id protein pleiotropic functions in CNS diseases and propose an integrated view of Id proteins and their promise as potential targets in modifying stemcell behavior to ameliorate CNS disease.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

The Id-protein family in developmental and cancer-associated pathways

Inhibitors of DNA binding and cell differentiation (Id) proteins are members of the large family of the helix-loop-helix (HLH) transcription factors, but they lack any DNA-binding motif. During development, the Id proteins play a key role in the regulation of cell-cycle progression and cell differentiation by modulating different cell-cycle regulators both by direct and indirect mechanisms. Several Id-protein interacting partners have been identified thus far, which belong to structurally and functionally unrelated families, including, among others, the class I and II bHLH transcription factors, the retinoblastoma protein and related pocket proteins, the paired-box transcription factors, and the S5a subunit of the 26 S proteasome. Although the HLH domain of the Id proteins is involved in most of their protein-protein interaction events, additional motifs located in their N-terminal and C-terminal regions are required for the recognition of diverse protein partners. The ability of the Id proteins to interact with structurally different proteins is likely to arise from their conformational flexibility: indeed, these proteins contain intrinsically disordered regions that, in the case of the HLH region, undergo folding upon self- or heteroassociation. Besides their crucial role for cell-fate determination and cell-cycle progression during development, other important cellular events have been related to the Id-protein expression in a number of pathologies. Dysregulated Id-protein expression has been associated with tumor growth, vascularization, invasiveness, metastasis, chemoresistance and stemness, as well as with various developmental defects and diseases. Herein we provide an overview on the structural properties, mode of action, biological function and therapeutic potential of these regulatory proteins.

The Id-protein family in developmental and cancer-associated pathways

Inhibitors of DNA binding and cell differentiation (Id) proteins are members of the large family of the helix-loop-helix (HLH) transcription factors, but they lack any DNA-binding motif. During development, the Id proteins play a key role in the regulation of cell-cycle progression and cell differentiation by modulating different cell-cycle regulators both by direct and indirect mechanisms. Several Id-protein interacting partners have been identified thus far, which belong to structurally and functionally unrelated families, including, among others, the class I and II bHLH transcription factors, the retinoblastoma protein and related pocket proteins, the paired-box transcription factors, and the S5a subunit of the 26 S proteasome. Although the HLH domain of the Id proteins is involved in most of their protein-protein interaction events, additional motifs located in their N-terminal and C-terminal regions are required for the recognition of diverse protein partners. The ability of the Id proteins to interact with structurally different proteins is likely to arise from their conformational flexibility: indeed, these proteins contain intrinsically disordered regions that, in the case of the HLH region, undergo folding upon self- or heteroassociation. Besides their crucial role for cell-fate determination and cell-cycle progression during development, other important cellular events have been related to the Id-protein expression in a number of pathologies. Dysregulated Id-protein expression has been associated with tumor growth, vascularization, invasiveness, metastasis, chemoresistance and stemness, as well as with various developmental defects and diseases. Herein we provide an overview on the structural properties, mode of action, biological function and therapeutic potential of these regulatory proteins.

Structural prediction of the interaction of the tumor suppressor p27KIP1 with cyclin A/CDK2 identifies a novel catalytically relevant determinant

BACKGROUND

The cyclin-dependent kinase 2 (CDK2) together with its cyclin E and A partners is a central regulator of cell growth and division. Deregulation of CDK2 activity is associated with diseases such as cancer. The analysis of substrates identified S/T-P-X-R/K/H as the CDK2 consensus sequence. The crystal structure of cyclin A/CDK2 with a short model peptide supports this sequence and identifies key interactions. However, CDKs use additional determinants to recognize substrates, including the RXL motif that is read by the cyclin subunits. We were interested to determine whether additional amino acids beyond the minimal consensus sequence of the well-studied substrate and tumor suppressor p27^KIP1 were relevant for catalysis.

RESULTS

To address whether additional amino acids, close to the minimal consensus sequence, play a role in binding, we investigate the interaction of cyclin A/CDK2 with an in vivo cellular partner and CDK inhibitor p27^KIP1. This protein is an intrinsically unfolded protein and, in particular, the C-terminal half of the protein has not been accessible to structural analysis. This part harbors the CDK2 phosphorylation site. We used bioinformatics tools, including MODELLER, iTASSER and HADDOCK, along with partial structural information to build a model of the C-terminal region of p27^KIP1 with cyclin A/CDK2. This revealed novel interactions beyond the consensus sequence with a proline and a basic amino acid at the P + 1 and the P + 3 sites, respectively. We suggest that the lysine at P + 2 might regulate the reversible association of the second counter ion in the active site of CDK2. The arginine at P + 7 interacts with both cyclin A and CDK2 and is important for the catalytic turnover rate.

CONCLUSION

Our modeling identifies additional amino acids in p27^KIP1 beyond the consensus sequence that contribute to the efficiency of substrate phosphorylation.

Diversity, expansion, and evolutionary novelty of plant DNA-binding transcription factor families

Plant transcription factors (TFs) that interact with specific sequences via DNA-binding domains are crucial for regulating transcriptional initiation and are fundamental to plant development and environmental response. In addition, expansion of TF families has allowed functional divergence of duplicate copies, which has contributed to novel, and in some cases adaptive, traits in plants. Thus, TFs are central to the generation of the diverse plant species that we see today. Major plant agronomic traits, including those relevant to domestication, have also frequently arisen through changes in TF coding sequence or expression patterns. Here our goal is to provide an overview of plant TF evolution by first comparing the diversity of DNA-binding domains and the sizes of these domain families in plants and other eukaryotes. Because TFs are among the most highly expanded gene families in plants, the birth and death process of TFs as well as the mechanisms contributing to their retention are discussed. We also provide recent examples of how TFs have contributed to novel traits that are important in plant evolution and in agriculture.This article is part of a Special Issue entitled: Plant Gene Regulatory Mechanisms and Networks, edited by Dr. Erich Grotewold and Dr. Nathan Springer.

Interacting mechanism of ID3 HLH domain towards E2A/E12 transcription factor - An Insight through molecular dynamics and docking approach

Inhibitor of DNA binding protein 3 (ID3) has long been characterized as an oncogene that implicates its functional role through its Helix-Loop-Helix (HLH) domain upon protein-protein interaction. An insight into the dimerization brought by this domain helps in identifying the key residues that favor the mechanism behind it. Molecular dynamics (MD) simulations were performed for the HLH proteins ID3 and Transcription factor E2-alpha (E2A/E12) and their ensemble complex (ID3-E2A/E12) to gather information about the HLH domain region and its role in the interaction process. Further evaluation of the results by Principal Component Analysis (PCA) and Free Energy Landscape (FEL) helped in revealing residues of E2A/E12: Lys570, Ala595, Val598, and Ile599 and ID3: Glu53, Gln63, and Gln66 buried in their HLH motifs imparting key roles in dimerization process. Furthermore the T-pad analysis results helped in identifying the key fluctuations and conformational transitions using the intrinsic properties of the residues present in the domain region of the proteins thus specifying their crucial role towards molecular recognition. The study provides an insight into the interacting mechanism of the ID3-E2A/E12 complex and maps the structural transitions arising in the essential conformational space indicating the key structural changes within the helical regions of the motif. It thereby describes how the internal dynamics of the proteins might regulate their intrinsic structural features and its subsequent functionality.

Id2a is required for hepatic outgrowth during liver development in zebrafish

During development, inhibitor of DNA binding (Id) proteins, a subclass of the helix-loop-helix family of proteins, regulate cellular proliferation, differentiation, and apoptosis in various organs. However, a functional role of Id2a in liver development has not yet been reported. Here, using zebrafish as a model organism, we provide in vivo evidence that Id2a regulates hepatoblast proliferation and cell death during liver development. Initially, in the liver, id2a is expressed in hepatoblasts and after their differentiation, id2a expression is restricted to biliary epithelial cells. id2a knockdown in zebrafish embryos had no effect on hepatoblast specification or hepatocyte differentiation. However, liver size was greatly reduced in id2a morpholino-injected embryos, indicative of a hepatic outgrowth defect attributable to the significant decrease in proliferating hepatoblasts concomitant with the significant increase in hepatoblast cell death. Altogether, these data support the role of Id2a as an important regulator of hepatic outgrowth via modulation of hepatoblast proliferation and survival during liver development in zebrafish.

Id proteins in the vasculature: from molecular biology to cardiopulmonary medicine

The inhibitors of differentiation (Id) proteins belong to the helix-loop-helix group of transcription factors and regulate cell differentiation and proliferation. Recent studies have reported that Id proteins play important roles in cardiogenesis and formation of the vasculature. We have also demonstrated that heritable pulmonary arterial hypertension (HPAH) patients have dysregulated Id gene expression in pulmonary artery smooth muscle cells. The interaction between bone morphogenetic proteins and other growth factors or cytokines regulates Id gene expression, which impacts on pulmonary vascular cell differentiation and proliferation. Exploration of the roles of Id proteins in vascular remodelling that occurs in PAH and atherosclerosis might provide new insights into the molecular basis of these diseases. In addition, current progress in identification of the interactors of Id proteins will further the understanding of the function of Ids in vascular cells and enable the identification of novel targets for therapy in PAH and other cardiovascular diseases.

Self-recognition behavior of a helix-loop-helix domain by a fragment scan

The inhibitors of DNA binding Id1-4 are helix-loop-helix (HLH) proteins that exert their biological function by interacting with members of the basic-HLH (bHLH) transcription-factor family. The HLH domains of the Id and bHLH proteins allow both self- and hetero-association. Due to their abnormal expression in cancer cells, the Id proteins are potential protein targets for cancer treatment. Suitable Id-protein inactivators should promote self-association and/or prevent hetero-association. In this work we evaluated the ability of the Id-protein HLH domain to recognize itself in form of short sequences extracted from the helical and loop regions. We performed a peptide scan of the Id1 HLH domain 64-106 based on three-residue overlapping octapeptides. Interaction of each octapeptide with the natively folded Id1 HLH domain was investigated by CD and fluorescence spectroscopy. The results from both techniques showed that the helix-based but not the loop-based octapeptides interacted with the Id1 HLH domain in the low-micromolar range. In contrast, a nitrotyrosine-containing analog of the Id1 HLH region, which was unable to reproduce the native-like conformation, quenched only the 2-amino-benzoyl-(Abz)-labeled loop-based octapeptides. This opposite self-recognition pattern suggests that the short helix-based and loop-based sequences should be able to distinguish different folding states of the Id1 HLH domain. This feature may be biologically relevant, as the Id proteins are predicted to behave as intrinsically disordered proteins, being in equilibrium between rapidly exchanging monomeric conformations and structurally better-defined homo-/heterodimers displaying the parallel four-helix bundle.

The ID proteins: master regulators of cancer stem cells and tumour aggressiveness

Inhibitor of DNA binding (ID) proteins are transcriptional regulators that control the timing of cell fate determination and differentiation in stem and progenitor cells during normal development and adult life. ID genes are frequently deregulated in many types of human neoplasms, and they endow cancer cells with biological features that are hijacked from normal stem cells. The ability of ID proteins to function as central 'hubs' for the coordination of multiple cancer hallmarks has established these transcriptional regulators as therapeutic targets and biomarkers in specific types of human tumours.

Id proteins: small molecules, mighty regulators

The family of inhibitor of differentiation (Id) proteins is a group of evolutionarily conserved molecules, which play important regulatory roles in organisms ranging from Drosophila to humans. Id proteins are small polypeptides harboring a helix-loop-helix (HLH) motif, which are best known to mediate dimerization with other basic HLH proteins, primarily E proteins. Because Id proteins do not possess the basic amino acids adjacent to the HLH motif necessary for DNA binding, Id proteins inhibit the function of E protein homodimers, as well as heterodimers between E proteins and tissue-specific bHLH proteins. However, Id proteins have also been shown to have E protein-independent functions. The Id genes are broadly but differentially expressed in a variety of cell types. Transcription of the Id genes is controlled by transcription factors such as C/EBPβ and Egr as well as by signaling pathways triggered by different stimuli, which include bone morphogenic proteins, cytokines, and ligands of T cell receptors. In general, Id proteins are capable of inhibiting the differentiation of progenitors of different cell types, promoting cell-cycle progression, delaying cellular senescence, and facilitating cell migration. These properties of Id proteins enable them to play significant roles in stem cell maintenance, vasculogenesis, tumorigenesis and metastasis, the development of the immune system, and energy metabolism. In this review, we intend to highlight the current understanding of the function of Id proteins and discuss gaps in our knowledge about the mechanisms whereby Id proteins exert their diverse effects in multiple cellular processes.

E proteins in lymphocyte development and lymphoid diseases

As members of the basic helix-loop-helix (bHLH) family of transcription factors, E proteins function in the immune system by directing and maintaining a vast transcriptional network that regulates cell survival, proliferation, differentiation, and function. Proper activity of this network is essential to the functionality of the immune system. Aberrations in E protein expression or function can cause numerous defects, ranging from impaired lymphocyte development and immunodeficiency to aberrant function, cancer, and autoimmunity. Additionally, disruption of inhibitor of DNA-binding (Id) proteins, natural inhibitors of E proteins, can induce additional defects in development and function. Although E proteins have been investigated for several decades, their study continues to yield novel and exciting insights into the workings of the immune system. The goal of this chapter is to discuss the various classical roles of E proteins in lymphocyte development and highlight new and ongoing research into how these roles, if compromised, can lead to disease.

Expression pattern of id proteins in medulloblastoma

Inhibitor of DNA binding or inhibitor of differentiation (Id) proteins are up regulated in a variety of neoplasms, particularly in association with high-grade, poorly differentiated tumors, while differentiated tissues show little or no Id expression. The four Id genes are members of the helix-loop-helix (HLH) family of transcription factors and act as negative regulators of transcription by binding to and sequestering HLH complexes. We tested the hypothesis that Id proteins are overexpressed in medulloblastoma by performing immunohistochemistry using a medulloblastoma tissue microarray with 45 unique medulloblastoma and 11 normal control cerebella, and antibodies specific for Id1, Id2, Id3, and Id4. A semi-quantitative staining score that took staining intensity and the proportion of immunoreactive cells into account was used. Id1 was not detected in normal cerebella or in medulloblastoma cells, but 78 % of tumors showed strong Id1 expression in endothelial nuclei of tumor vessels. Id2 expression was scant in normal cerebella and increased in medulloblastoma (median staining score: 4). Id3 expression was noted in some neurons of the developing cerebellar cortex, but it was markedly up regulated in medulloblastoma (median staining score: 12) and in tumor endothelial cells. Id4 was not expressed in normal cerebella or in tumor cells. Id2 or Id3 overexpression drove proliferation in medulloblastoma cell lines by altering the expression of critical cell cycle regulatory proteins in favor of cell proliferation. This study shows that Id1 expression in endothelial cells may contribute to angiogenic processes and that increased expression of Id2 and Id3 in medulloblastoma is potentially involved in tumor cell proliferation and survival.

Forced expression of cyclin-dependent kinase 6 confers resistance of pro-B acute lymphocytic leukemia to Gleevec treatment

The gene encoding c-ABL, a nonreceptor protein tyrosine kinase, is involved in a chromosomal translocation resulting in expression of a BCR-Abl fusion protein that causes most chronic myelogenous and some acute lymphocytic leukemias (CML and ALL) in humans. The Abelson murine leukemia virus (A-MuLV) expresses an alternative form of c-Abl, v-Abl, that transforms murine pro-B cells, resulting in acute leukemia and providing an experimental model for human disease. Gleevec (STI571) inhibits the Abl kinase and has shown great utility against CML and ALL in humans, although its usefulness is limited by acquired resistance. Since STI571 is active against A-MuLV-transformed cells in vitro, we performed a retroviral cDNA library screen for genes that confer resistance to apoptosis induced by STI571. We found that forced expression of Cdk6 promotes continued cell division and decreased apoptosis of leukemic cells. We then determined that the transcription factor E2A negatively regulates Cdk6 transcription in leukemic pro-B cells and that the v-Abl kinase stimulates Cdk6 expression via an extracellular signal-regulated kinase 1-dependent pathway. Finally, we show that the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor PD0332991 can act synergistically with STI571 to enhance leukemic cell death, suggesting a potential role for CDK6 inhibitors in the treatment of STI571-resistant CML or ALL.

The role of cell cycle in retinal development: cyclin-dependent kinase inhibitors co-ordinate cell-cycle inhibition, cell-fate determination and differentiation in the developing retina

The mature retina is formed through multi-step developmental processes, including eye field specification, optic vesicle evagination, and cell-fate determination. Co-ordination of these developmental events with cell-proliferative activity is essential to achieve formation of proper retinal structure and function. In particular, the molecular and cellular dynamics of the final cell cycle significantly influence the identity that a cell acquires, since cell fate is largely determined at the final cell cycle for the production of postmitotic cells. This review summarizes our current understanding of the cellular mechanisms that underlie the co-ordination of cell-cycle and cell-fate determination, and also describes a molecular role of cyclin-dependent kinase inhibitors (CDKIs) as co-ordinators of cell-cycle arrest, cell-fate determination and differentiation.

gammadelta and alphabeta T cell lineage choice: resolution by a stronger sense of being

A common bipotent thymocyte precursor gives rise to both lineages of T cells, alphabeta and gammadelta. However, the cell intrinsic and extrinsic factors that influence alphabeta- versus gammadelta-lineage bifurcation remain controversial. gammadelta T cells play a unique and vital role in host defense, from maintaining integrity at epithelial and mucosal barriers to their newly defined role as an important innate source of interleukin-17. Although a T cell receptor (TCR)-independent fate choice may take place, emerging data supports a model in which the differential signaling capacity of alphabeta and gammadeltaTCRs play an instructional role in specifying lineage fate, with strength of signal measured by the amount of ERK/MAPK pathway activation. Here we discuss how the interplay between intrinsic TCR signals and cell extrinsic signals provided by Notch and TCR ligands help to assign and support a final lineage fate decision.

TWIST family of basic helix-loop-helix transcription factors mediate human mesenchymal stem cell growth and commitment

The TWIST family of basic helix-loop-helix transcription factors, Twist-1 and Dermo-1 are known mediators of mesodermal tissue development and contribute to correct patterning of the skeleton. In this study, we demonstrate that freshly purified human bone marrow-derived mesenchymal stromal/stem cells (MSC) express high levels of Twist-1 and Dermo-1 which are downregulated following ex vivo expansion. Enforced expression of Twist-1 or Dermo-1 in human MSC cultures increased expression of the MSC marker, STRO-1, and the early osteogenic transcription factors, Runx2 and Msx2. Conversely, overexpression of Twist-1 and Dermo-1 was associated with a decrease in the gene expression of osteoblast-associated markers, bone morphogenic protein-2, bone sialoprotein, osteopontin, alkaline phosphatase and osteocalcin. High expressing Twist-1 or Dermo-1 MSC lines exhibited an enhanced proliferative potential of approximately 2.5-fold compared with control MSC populations that were associated with elevated levels of Id-1 and Id-2 gene expression. Functional studies demonstrated that high expressing Twist-1 and Dermo-1 MSC displayed a decreased capacity for osteo/chondrogenic differentiation and an enhanced capacity to undergo adipogenesis. These findings implicate the TWIST gene family members as potential mediators of MSC self-renewal and lineage commitment in postnatal skeletal tissues by exerting their effects on genes involved in the early stages of bone development.

Elevated endogenous expression of the dominant negative basic helix-loop-helix protein ID1 correlates with significant centrosome abnormalities in human tumor cells

Background

ID proteins are dominant negative inhibitors of basic helix-loop-helix transcription factors that have multiple functions during development and cellular differentiation. Ectopic (over-)expression of ID1 extends the lifespan of primary human epithelial cells. High expression levels of ID1 have been detected in multiple human malignancies, and in some have been correlated with unfavorable clinical prognosis. ID1 protein is localized at the centrosomes and forced (over-)expression of ID1 results in errors during centrosome duplication.

Results

Here we analyzed the steady state expression levels of the four ID-proteins in 18 tumor cell lines and assessed the number of centrosome abnormalities. While expression of ID1, ID2, and ID3 was detected, we failed to detect protein expression of ID4. Expression of ID1 correlated with increased supernumerary centrosomes in most cell lines analyzed.

Conclusions

This is the first report that shows that not only ectopic expression in tissue culture but endogenous levels of ID1 modulate centrosome numbers. Thus, our findings support the hypothesis that ID1 interferes with centrosome homeostasis, most likely contributing to genomic instability and associated tumor aggressiveness.

Marked induction of the helix-loop-helix protein Id3 promotes the gammadelta T cell fate and renders their functional maturation Notch independent

alphabeta and gammadelta T cells arise from a common thymocyte progenitor during development in the thymus. Emerging evidence suggests that the pre-T cell receptor (pre-TCR) and gammadelta T cell receptor (gammadeltaTCR) play instructional roles in specifying the alphabeta and gammadelta T-lineage fates, respectively. Nevertheless, the signaling pathways differentially engaged to specify fate and promote the development of these lineages remain poorly understood. Here, we show that differential activation of the extracellular signal-related kinase (ERK)-early growth response gene (Egr)-inhibitor of DNA binding 3 (Id3) pathway plays a defining role in this process. In particular, Id3 expression served to regulate adoption of the gammadelta fate. Moreover, Id3 was both necessary and sufficient to enable gammadelta-lineage cells to differentiate independently of Notch signaling and become competent IFNgamma-producing effectors. Taken together, these findings identify Id3 as a central player that controls both adoption of the gammadelta fate and its maturation in the thymus.

Cyclin-dependent kinase antagonizes promyelocytic leukemia zinc-finger through phosphorylation

Acute promyelocytic leukemia is associated with chromosomal translocations that involve the RARalpha gene and several distinct loci producing a variety of fusion proteins. One such fusion partner is promyelocytic leukemia zinc-finger gene (PLZF), a member of the POK (POZ and Krüppel) family of transcriptional repressors that is a key developmental regulator, stem cell maintenance factor and tumor suppressor. Overexpression of PLZF has been shown to induce cell cycle arrest at the G(1) to S transition and repress the expression of key pro-proliferative genes such as CCNA2 and MYC. However, given this data suggesting an important growth inhibitory role for PLZF, relatively little is known regarding regulation of its activity. Here we show that the main cyclin-dependent kinase involved at the G(1) to S transition (CDK2) phosphorylates PLZF at two consensus sites found within PEST domains present in the hinge region of the protein. This phosphorylation triggers the ubiquitination and subsequent degradation of PLZF, which impairs PLZF transcriptional repression ability and antagonizes its growth inhibitory effects. This critical mechanism of PLZF regulation may thus be relevant for cell cycle progression during the development and the pathogenesis of human cancer.

Interference of the dominant negative helix-loop-helix protein ID1 with the proteasomal subunit S5A causes centrosomal abnormalities

The inhibitor of DNA-binding (ID) proteins are dominant-negative inhibitors of basic helix-loop-helix transcription factors that have multiple functions during development and cellular differentiation. High-level expression of some ID family members has been observed in human malignancies, and in some cases was correlated with poor clinical prognosis. Ectopic ID1 expression extends the life span of primary human epithelial cells, inhibits cellular differentiation and induces centrosome duplication errors, thus suggesting that ID1 may have oncogenic activities. ID1 can bind to the proteasomal subunit S5A/Rpn10, but the biological consequences of the interaction have not been studied in detail. Here, we show that ID1's ability to induce supernumerary centrosomes correlates with S5A binding. Similar to ID1, a fraction of the S5A protein localizes to centrosomal structures. Furthermore, partial depletion of S5A by RNA interference causes accumulation of cells with supernumerary centrosomes. These results are consistent with the model that ID1 dysregulates centrosome homeostasis at least in part by interfering with S5A activities at the centrosome.

Protein kinase A-regulated nucleocytoplasmic shuttling of Id1 during angiogenesis

Id1, an inhibitory partner of basic-helix-loop-helix transcriptional factors, has recently been recognized as a potent contributor to angiogenesis. However, the molecular mechanism underlying its role in angiogenesis remains essentially unknown. Herein we demonstrate the subcellular localization of Id1 to be altered depending on the cellular context of vascular endothelial cells. Id1 was localized in the nuclei of human umbilical vein endothelial cells (HUVECs) cultured on uncoated plates, whereas it was translocated to the cytoplasm in HUVECs on Matrigel along with the formation of capillary-like structures. Treatment with the nuclear export inhibitor leptomycin B and mutagenesis analysis using green fluorescent protein-fused Id1 revealed CRM1/exportin-dependent nuclear export of Id1 in HUVECs on Matrigel. This nuclear export of Id1 was inhibited by protein kinase A (PKA) activation by dibutyryl cyclic AMP and forskolin but was promoted by PKA inactivation by H-89 and MDL-12,330A. Mutagenesis analysis of Id1 showed the phosphorylation of Ser-5 to possibly mediate the effect of PKA. These results suggest the function of Id1 as a transcriptional factor to be controlled by nucleocytoplasmic shuttling during angiogenesis and that PKA might be involved in this process. This may serve as a novel mechanism regulating angiogenesis and as a possible target for therapeutic vascular regeneration.

Id3 is a novel regulator of p27kip1 mRNA in early G1 phase and is required for cell-cycle progression

P27kip is a key inhibitory protein of the cell-cycle progression, which is rapidly downregulated in early G1 phase by a post-translational mechanism involving the proteosomal degradation. In this study, using a wounding model that induces cell-cycle entry of human dermal fibroblasts, we demonstrate that p27mRNA is downregulated when cells progress into the G1 phase, and then it returns to its basal level when cells approach the S phase. By using a quantitative polymerase chain reaction screening we identified inhibitors of differentiation (Id3), a bHLH transcriptional repressor, as a candidate mediator accounting for p27 mRNA decrease. Id3 silencing, using an small interfering RNA approach, reversed the injury mediated p27 downregulation demonstrating that Id3 is involved in the transcriptional repression of p27. Reporter gene experiments and a chromatin immunoprecipitation assay showed that Id3 likely exerts its repressive action through ELK1 inhibition. By inhibiting early p27 downregulation, Id3 depletion blocked (i) the G1-phase progression as assessed by the inhibition of pRb phosphorylation and p130 degradation and (ii) the G1/S transition as observed by the inhibition of cyclin A induction, demonstrating that p27 mRNA decrease is required for cell proliferation. Apart from its effect on the early p27 diminution, Id3 appears also involved in the control of the steady-state level of p27 at the G1/S boundary. In conclusion, this study identifies a novel mechanism of p27 regulation which besides p27 protein degradation also implicates a transcriptional mechanism mediated by Id3.

Generalized, switch-like competitive heterodimerization networks

High-dimensional switches have been proposed as a way to model cellular differentiation, particularly in the context of basic Helix-Loop-Helix (bHLH) competitive heterodimerization networks. A previous study derived a simple rule showing how many elements can be co-expressed, depending on the rate of competition within the network. A limitation to that rule, however, is that many biochemical parameters were considered to be identical. Here, we derive a generalized rule. This in turns allows one to study more ways in which these networks could be regulated, linking intrinsic cellular differentiation determinants to extracellular cues.

Synthesis and conformational analysis of Id2 protein fragments: impact of chain length and point mutations on the structural HLH motif

The Id proteins are negative regulators of several basic-helix-loop-helix (HLH) transcription factors, including the ubiquitous E factors and the tissue-specific myogenin-regulating factors. Id1 through Id4 contain highly identical HLH domains but different N- and C-terminal extensions. Beside the heterodimerization with the parent HLH factors, Id2 was shown to additionally interact with the retinoblastoma protein and to be overexpressed in neuroblastoma. Thus, Id2 represents an interesting target for cancer therapy based on the inhibition of protein-protein interactions. Here we present the synthesis and circular dichroism (CD) analysis of peptides derived from point mutations and N-/C-terminal truncations of Id2. The helix character of the HLH domain (residues 36-76) was reduced upon substitution of Met39/-62 and Cys42 with Nle and Ser, respectively, suggesting a structural role of these side chains. The largest sequence that could be obtained by stepwise solid-phase peptide synthesis (SPPS) with Fmoc strategy spanned the entire HLH motif (with Cys42 replaced by Ser) and part of the C-terminus (residues 77-110). This 75-residue long fragment was less helical than the isolated HLH domain and had propensity to aggregate, which was correlated with the presence of the flanking residues C-terminal to helix-2. By CD analysis of an equimolar mixture of the sequence 36-110 with the N-terminus 1-35, noncovalent interactions between the two peptides were detected, which, however, changed upon aging. In contrast, the mixture of the HLH sequence 36-76 with the N-terminus was characterized by a stabilized helix structure that was maintained also upon aging. Presumably, the N-terminal region interacted with the folded HLH motif in a specific manner, whereas only unspecific, weak contacts occurred with the partly unfolded HLH domain and/or the immediate flanking residues 77-110.

The basic helix-loop-helix transcription factor HEBAlt is expressed in pro-T cells and enhances the generation of T cell precursors

The basic helix-loop-helix (bHLH) transcription factors HEB and E2A are critical mediators of gene regulation during lymphocyte development. We have cloned a new transcription factor, called HEBAlt, from a pro-T cell cDNA library. HEBAlt is generated by alternative transcriptional initiation and splicing from the HEB gene locus, which also encodes the previously characterized E box protein HEBCan. HEBAlt contains a unique N-terminal coding exon (the Alt domain) that replaces the first transactivation domain of HEBCan. Downstream of the Alt domain, HEBAlt is identical to HEBCan, including the DNA binding domain. HEBAlt is induced in early thymocyte precursors and down-regulated permanently at the double negative to double positive (DP) transition, whereas HEBCan mRNA expression peaks at the DP stage of thymocyte development. HEBAlt mRNA is up-regulated synergistically by a combination of HEBCan activity and Delta-Notch signaling. Retroviral transduction of HEBAlt or HEBCan into hemopoietic stem cells followed by OP9-DL1 coculture revealed that HEBAlt-transduced precursors generated more early T lineage precursors and more DP pre-T cells than control transduced cells. By contrast, HEBCan-transduced cells that maintained high level expression of the HEBCan transgene were inhibited in expansion and progression through T cell development. HEB(-/-) fetal liver precursors transduced with HEBAlt were rescued from delayed T cell specification, but HEBCan-transduced HEB(-/-) precursors were not. Therefore, HEBAlt and HEBCan are functionally distinct transcription factors, and HEBAlt is specifically required for the efficient generation of early T cell precursors.

A short Id2 protein fragment containing the nuclear export signal forms amyloid-like fibrils

The negative regulator of DNA-binding/cell-differentiation Id2 is a small protein containing a central helix-loop-helix (HLH) motif and a C-terminal nuclear export signal (NES). Whereas the former is essential for Id2 dimerization and nuclear localization, the latter is responsible for the transport of Id2 from the nucleus to the cytoplasm. Whereas the isolated Id2 HLH motif is highly helical, large C-terminal Id2 fragments including the NES sequence are either unordered or aggregation-prone. To study the conformational properties of the isolated NES region, we synthesized the Id2 segment 103-124. The latter was insoluble in water and only temporarily soluble in water/alcohol mixtures, where it formed quickly precipitating beta-sheets. Introduction of a positively charged N-terminal tail prevented aggressive precipitation and led to aggregates consisting of long fibrils that bound thioflavin T. These results show an interesting structural aspect of the Id2 NES region, which might be of significance for both protein folding and function.

Non-redundant inhibitor of differentiation (Id) gene expression and function in human prostate epithelial cells

BACKGROUND

The four Id (inhibitor of differentiation) proteins (Id1, Id2, Id3, and Id4) dimerize and neutralize the transcriptional activity of basic helix-loop-helix (bHLH) proteins. The Id proteins negatively regulate differentiation and promote proliferation hence the expression of specific subsets of Id proteins is high in many different types of cancers. However, the expression of all the Id isoforms and their potential function in specific cancer cell types is not known. In this study, the expression and function of all four Id isoforms in prostate cancer cell lines was investigated to gain a better understanding of the role of each Id isoform in normal prostate epithelial and prostate cancer cells.

METHODS

Id gene and protein expression was evaluated in the context of androgen response. The cellular function of Id isoforms was evaluated by targeted loss of function of Id genes.

RESULTS

The four Id isoforms are differentially expressed and regulated in normal human prostate epithelial cells versus prostate cancer cell lines DU145 and LNCaP. Id4 is present only in AR positive cells (normal and LNCaP) and its expression regulated by androgens. Loss of Id1 and Id3 expression by siRNA results in loss of proliferation. Loss of Id2 had no effect on proliferation but increased apoptosis.

CONCLUSIONS

A complex equilibrium between Id isoforms determines the cell fate. Id1 and Id3 target cellular proliferation, Id2 targets apoptosis, and Id4 may act as a potential tumor suppressor in prostate epithelial cells.

At the crossroads: diverse roles of early thymocyte transcriptional regulators

Transcriptional regulation of T-cell development involves successive interactions between complexes of transcriptional regulators and their binding sites within the regulatory regions of each gene. The regulatory modules that control expression of T-lineage genes frequently include binding sites for a core set of regulators that set the T-cell-specific background for signal-dependent control, including GATA-3, Notch/CSL, c-myb, TCF-1, Ikaros, HEB/E2A, Ets, and Runx factors. Additional regulators in early thymocytes include PU.1, Id-2, SCL, Spi-B, Erg, Gfi-1, and Gli. Many of these factors are involved in simultaneous regulation of non-T-lineage genes, T-lineage genes, and genes involved in cell cycle control, apoptosis, or survival. Potential and known interactions between early thymic transcription factors such as GATA-3, SCL, PU.1, Erg, and Spi-B are explored. Regulatory modules involved in the expression of several critical T-lineage genes are described, and models are presented for shifting occupancy of the DNA-binding sites in the regulatory modules of pre-Talpha, T-cell receptor beta (TCRbeta), recombinase activating genes 1 and 2 (Rag-1/2), and CD4 during T-cell development. Finally, evidence is presented that c-kit, Erg, Hes-1, and HEBAlt are expressed differently in Rag-2(-/-) thymocytes versus normal early thymocytes, which provide insight into potential regulatory interactions that occur during normal T-cell development.

Molecular mechanisms regulating expression and function of transcription regulator inhibitor of differentiation 3

The transcription factor antagonist inhibitor of differentiation 3 (Id3) has been implicated in many diverse developmental, physiological and pathophysiological processes. Its expression and function is subjected to many levels of complex regulation. This review summarizes the current understanding of these mechanisms and describes how they might be related to the diverse functions that have been attributed to the Id3 protein. Detailed understanding of these mechanisms should provide insights towards the development of therapeutic approaches to various diseases, including cancer and atherogenesis.

Regulation of inhibitor of differentiation gene 3 (Id3) expression by Sp2-motif binding factor in myogenic C2C12 cells: Downregulation of DNA binding activity following skeletal muscle differentiation

Id3 is a member of the Id family of transcriptional regulators that have been implicated in the development of multiple tissues. Altered expression of the Id genes and proteins contribute to carcinogenesis and atherosclerosis. Id3 is highly expressed in proliferating skeletal muscle cells but becomes downregulated upon terminal differentiation. We have identified several DNase I protected footprints within a proximal region of the mouse Id3 promoter that has been shown previously to support high levels of transcriptional activity in proliferating skeletal muscle cells. Two of these sites interacted, respectively, in vitro with Sp2 and Egr-1 proteins present in muscle cell nuclear extracts. Mutation analysis revealed that the Sp2 site accounted for a major part of the Id3 promoter activity in proliferating muscle cells whereas the Egr-1 site was dispensable. Consistent with the previously observed downregulation of the endogenous Id3 gene, protein binding to the Sp2 site was substantially reduced with extracts from differentiated muscle cells. Our results reveal Id3 as a potential target for Sp2 and raise the possibility that acute activation and the chronic and maintained expression of Id3 gene might be regulated by different mechanisms.

Identification and expression profile of the ID gene family in the rainbow trout (Oncorhynchus mykiss)

ID proteins are negative regulators of basic helix-loop-helix transcription factors governing growth and development in mammals. However, little is known about the ID gene function and expression in fish. We report the identification and characterization of two new rainbow trout ID genes (ID1D and ID2B) and extend our expression analyses of two previously identified ID genes (ID1A and ID2A). Phylogenetic analyses indicate an evolutionary relationship between ID1A and ID1D and between ID1B and ID1C, suggesting a mechanism of divergence throughout salmonid evolution. To access the expression of these genes in adult and developing fish, we measured the relative transcript abundance of four ID1 and two ID2 genes by real-time PCR. ID1 transcripts were expressed in a variety of tissues and the ID1 paralogues showed similar patterns of expression, whereas the ID2 paralogues were differentially expressed. To access the role of the ID genes during embryonic development, gene expression was measured at early (day 0 and day 2), mid (day 9 and day 18) and late (day 30 and day 50) embryonic development. ID1A and ID1D expression remained unchanged throughout embryonic development, while ID1B and ID1C were lowest during early, highest at mid, and decreased during late embryonic development. The ID2 transcripts revealed the highest expression in unfertilized eggs and day 2 embryos, and remained low throughout the remainder of embryonic development. The sequence analyses and gene expression patterns implicate gene and genome duplication in rainbow trout ID gene evolution and suggest an extensive role for the IDs in rainbow trout growth and development.

Cyclin E

E-type cyclins (cyclin E1 and cyclin E2) are expressed during the late G1 phase of the cell cycle until the end of the S-phase. The activity of cyclin E is limiting for the passage of cells through the restriction point "R" which marks a "point of no return" for cells entering the division cycle from a resting state or passing from G1 into S-phase. Expression of cyclin E is regulated on the level of gene transcription mainly by members of the E2F trrnscription factor family and by its degradation via the proteasome pathway. Cyclin E binds and activates the kinase Cdk2 and by phosphorylating its substrates, the so-called "pocket proteins", the cyclic/Cdk2 complexes initiate a cascade of events that leads to the expression of S-phase specific genes. Aside from this specific function as a regulator of S-phase-entry, cyclin E plays a direct role in the initiation of DNA replication, the control of genomic stability, and the centrosome cycle. Surprisingly, recent studies have shown that the once thought essential cyclin E is dispensable for the development of higher eukaryotes and for the mitotic division of eukaryotic cells. Nevertheless, high level cyclin E expression has been associated with the initiation or progression of different human cancers, in particular breast cancer but also leukemia, lymphoma and others. Transgenic mouse models in which cyclin E is constitutively expressed develop malignant diseases, supporting the notion of cyclin E as a dominant onco-protein.

Differential regulation of granulopoiesis by the basic helix-loop-helix transcriptional inhibitors Id1 and Id2

Inhibitor of DNA binding (Id) proteins function as inhibitors of members of the basic helix-loop-helix family of transcription factors and have been demonstrated to play an important role in regulating lymphopoiesis. However, the role of these proteins in regulation of myelopoiesis is currently unclear. In this study, we have investigated the role of Id1 and Id2 in the regulation of granulopoiesis. Id1 expression was initially up-regulated during early granulopoiesis, which was then followed by a decrease in expression during final maturation. In contrast, Id2 expression was up-regulated in terminally differentiated granulocytes. In order to determine whether Id expression plays a critical role in regulating granulopoiesis, Id1 and Id2 were ectopically expressed in CD34(+) cells by retroviral transduction. Our experiments demonstrate that constitutive expression of Id1 inhibits eosinophil development, whereas in contrast neutrophil differentiation was modestly enhanced. Constitutive Id2 expression accelerates final maturation of both eosinophils and neutrophils, whereas inhibition of Id2 expression blocks differentiation of both lineages. Transplantation of beta2-microglobulin(-/-) nonobese diabetic severe combined immunodeficient (NOD/SCID) mice with CD34(+) cells ectopically expressing Id1 resulted in enhanced neutrophil development, whereas ectopic expression of Id2 induced both eosinophil and neutrophil development. These data demonstrate that both Id1 and Id2 play a critical, although differential role in granulopoiesis.

Nucleo-cytoplasmic shuttling of Id2, a negative regulator of basic helix-loop-helix transcription factors

Id proteins function as negative regulators for basic helix-loop-helix transcriptional factors that play important roles in cell fate determination. They preferentially associate with ubiquitously expressed E proteins of the basic helix-loop-helix family and prevent them from binding to DNA and activating transcription. Although their small size suggests that Id proteins enter and exit the nucleus by passive diffusion, several studies have indicated that other pathways may regulate their subcellular localization. In this study, we obtained evidence that Id2 has the ability to shuttle between the nucleus and the cytoplasm. When passive diffusion was prevented by fusion with green fluorescent protein (GFP), Id2 was predominantly localized in the cytoplasm. Using GFP fusion constructs, we demonstrated that the C-terminal region is required for cytoplasmic localization. Nuclear accumulation of GFP-Id2 in cells treated with the nuclear export inhibitor leptomycin B suggests that the nuclear export receptor chromosome region maintenance protein 1 mediates the cytoplasmic localization of Id2. Id2 contains two putative leucine-rich nuclear export signals, and the nuclear export signal in the C-terminal region is essential for nuclear export. On the other hand, the helix-loop-helix domain is important for nuclear localization. Finally, experiments using reporter assays revealed an inverse correlation between nuclear export and transcriptional repression via the E-box sequence. Based on all these findings, we propose that nucleo-cytoplasmic shuttling is a novel mechanism for the regulation of Id2 function.

Id family of transcription factors and vascular lesion formation

Vascular smooth muscle cell (VSMC) modulation to a de-differentiated phenotype and proliferation are key components of vascular lesion formation. Understanding how these processes are regulated is essential to understanding the progression of vascular diseases such as atherosclerosis and in-stent restenosis. The Id family of helix-loop-helix (HLH) transcription factors has emerged as important regulators of cellular growth and differentiation. Recent published findings have implicated the Id proteins as important regulators of growth and phenotypic modulation in VSMC and in the vascular response to injury. In this review, we summarize what is known regarding how the Id proteins function to control cellular growth and differentiation and their role in vascular lesion formation.

Phosphorylation regulates Id3 function in vascular smooth muscle cells

Understanding the mechanisms that regulate cell cycle progression in vascular smooth muscle cells (VSMCs) is key to understanding and modulating vascular lesion formation. Results of the present study provide the first evidence that phosphorylation of the helix-loop-helix factor Id3 in VSMCs occurs in vitro and in vivo and provides a regulatory switch controlling Id3-induced regulation of p21Cip1 and VSMC growth.

The helix-loop-helix protein ID1 localizes to centrosomes and rapidly induces abnormal centrosome numbers

ID1 is a member of the inhibitor of DNA binding/differentiation (ID) family of dominant negative helix-loop-helix transcription factors. ID-proteins have been implicated in the control of differentiation and transcriptional modulation of various cell cycle regulators and high levels of ID1 expression are frequently detected in various cancer types. However, it is unclear whether ID1 is a marker of highly proliferative cancer cells or whether it directly contributes to the tumor phenotype. A detailed analysis of ID1-expressing human cells revealed that a fraction of ID1 localizes to centrosomes. Ectopic expression of ID1 in primary cells and tumor cell lines resulted in accumulation of cells with abnormal centrosome numbers. There was no evidence for centrosomal localization or induction of centrosome abnormalities by the other ID family members. Hence, ID1 may contribute to oncogenesis not only by inhibiting transcriptional activity of basic helix-loop-helix transcription factors and abrogate differentiation but also by subverting centrosome duplication.

Degradation of the Id2 developmental regulator: targeting via N-terminal ubiquitination

Degradation of cellular proteins via the ubiquitin-proteasome system (UPS) involves: (i) generation of a substrate-anchored polyubiquitin degradation signal and (ii) destruction of the tagged protein by the 26S proteasome with release of free and reusable ubiquitin. For most substrates, it is believed that the first ubiquitin moiety is conjugated to a epsilon-NH(2) group of an internal Lys residue. Recent findings indicate that for several proteins, the first ubiquitin moiety is fused, in a linear manner, to the free alpha-NH(2) group of the protein. Here, we demonstrate that the inhibitor of differentiation (or inhibitor of DNA binding) 2, Id2, that downregulates gene expression in undifferentiated and self-renewing cells, is degraded by the UPS following ubiquitination at its N-terminal residue. Lysine-less (LL) Id2 is degraded efficiently by the proteasome following ubiquitination. Fusion of a Myc tag to the N-terminal but not to the C-terminal residue of Id2 stabilizes the protein. Furthermore, deletion of the first 15 N-terminal residues of Id2 stabilizes the protein, suggesting that this domain serves as a recognition element, possibly for the ubiquitin ligase, E3. The mechanisms and structural motives that govern Id2 stability may have important implications to the regulation of the protein during normal differentiation and malignant transformation.

A Dual Role of Cyclin E in Cell Proliferation and Apotosis May Provide a Target for Cancer Therapy

Cyclin E is essential for progression through the G1-phase of the cell cycle and initiation of DNA replication by interacting with and activating its catalytic partner, the cyclin dependent kinase 2 (Cdk2). Rb, as well as Cdc6, NPAT, and nucleophosmin, critical components of cell proliferation and DNA replication, respectively, are targets of Cyclin E/Cdk2 phosphorylation. There are a number of putative binding sites for E2F in the cyclin E promoter region, suggesting an E2F-dependent regulation. Skp2 and Fbw7 are novel proteins, responsible for ubiquitin-dependent proteolysis of Cyclin E. The tight regulation of cyclin E expression, both at the transcriptional level and by ubiquitin-mediated proteolysis, indicates that it has a major role in the control of the G1- and S-phase transitions. Cyclin E is also transcriptionally regulated during radiation-induced apoptosis of hematopoietic cells. In addition to its biological roles, deregulated cyclin E expression has an established role in tumorigenesis. Cell cycle regulatory molecules, such as cyclin E, are frequently deregulated in different types of cancers, where overexpressed native or low molecular weight forms of Cyclin E have a significant role in oncogenesis. During apoptosis of hematopoietic cells, caspase-dependent proteolysis of Cyclin E generates a p18-Cyclin E variant. Understanding the role of Cyclin E in apoptosis may provide a novel target, which may be effective in cancer therapy. This review summarizes what is known about the biological role of cyclin E, its deregulation in cancer, and the opportunities it may provide as a target in clinical therapy.

Transcriptional control of the cell cycle in mammary gland development and tumorigenesis

Over the past several years it has become increasingly evident that normal development and cancer share many properties. Both processes involve alterations in cell proliferation and differentiation, cell death, neovascularization, and cell motility and invasion. Thus, genes involved in normal development are frequently utilized in neoplasia. During development, numerous transcriptional regulatory mechanisms are used to ensure tight control over cellular proliferation. In this review we focus on a number of transcription factor families (homeobox, STAT, and Ets), and on inhibitors of transcription factors (Id), which have been implicated in controlling the cell cycle not only in normal mammary gland development but also in breast tumorigenesis.

Regulation of TCF ETS-domain transcription factors by helix-loop-helix motifs

DNA binding by the ternary complex factor (TCF) subfamily of ETS-domain transcription factors is tightly regulated by intramolecular and intermolecular interactions. The helix-loop-helix (HLH)-containing Id proteins are trans-acting negative regulators of DNA binding by the TCFs. In the TCF, SAP-2/Net/ERP, intramolecular inhibition of DNA binding is promoted by the cis-acting NID region that also contains an HLH-like motif. The NID also acts as a transcriptional repression domain. Here, we have studied the role of HLH motifs in regulating DNA binding and transcription by the TCF protein SAP-1 and how Cdk-mediated phosphorylation affects the inhibitory activity of the Id proteins towards the TCFs. We demonstrate that the NID region of SAP-1 is an autoinhibitory motif that acts to inhibit DNA binding and also functions as a transcription repression domain. This region can be functionally replaced by fusion of Id proteins to SAP-1, whereby the Id moiety then acts to repress DNA binding in cis. Phosphorylation of the Ids by cyclin-Cdk complexes results in reduction in protein-protein interactions between the Ids and TCFs and relief of their DNA-binding inhibitory activity. In revealing distinct mechanisms through which HLH motifs modulate the activity of TCFs, our results therefore provide further insight into the role of HLH motifs in regulating TCF function and how the inhibitory properties of the trans-acting Id HLH proteins are themselves regulated by phosphorylation.

Id proteins in development, cell cycle and cancer

Id proteins are important parts of signaling pathways involved in development, cell cycle and tumorigenesis. They were first shown to act as dominant negative antagonists of the basic helix-loop-helix family of transcription factors, which positively regulate differentiation in many cell lineages. The Id proteins do this by associating with the ubiquitous E proteins and preventing them from binding DNA or other transcription factors. Id proteins also associate with Ets transcription factors and the Rb family of tumor suppressor proteins, and are downstream targets of transforming growth factor beta and bone morphogenic protein signaling. Thus, the Id proteins have become important molecules for understanding basic biological processes as well as targets for potential therapeutic intervention in human disease.

Basic helix-loop-helix factors in cortical development

Transcription factors with bHLH motifs modulate critical events in the development of the mammalian neocortex. Multipotent cortical progenitors are maintained in a proliferative state by bHLH factors from the Id and Hes families. The transition from proliferation to neurogenesis involves a coordinate increase in the activity of proneural bHLH factors (Mash1, Neurogenin1, and Neurogenin2) and a decrease in the activity of Hes and Id factors. As development proceeds, inhibition of proneural bHLH factors in cortical progenitors promotes the formation of astrocytes. Finally, the formation of oligodendrocytes is triggered by an increase in the activity of bHLH factors Olig1 and Olig2 that may be coupled with a decrease in Id activity. Thus, bHLH factors have key roles in corticogenesis, affecting the timing of differentiation and the specification of cell fate.