The Cdk1 Complex Plays a Prime Role in Regulating N-Myc Phosphorylation and Turnover in Neural Precursors

Huwe1 ubiquitin ligase is essential to synchronize neuronal and glial differentiation in the developing cerebellum

We have generated a knockout mouse strain in which the gene coding for the ubiquitin ligase Huwe1 has been inactivated in cerebellar granule neuron precursors (CGNPs) and radial glia. These mice have a high rate of postnatal lethality and profound cerebellar abnormalities. The external granule layer of the cerebellum, which contains CGNPs, is expanded and displays aberrant proliferation and impaired differentiation of the progenitor cell population. The uncontrolled proliferation of the CGNPs is associated with accumulation of the N-Myc oncoprotein, a substrate of Huwe1, and con-sequent activation of the signaling events downstream to N-Myc. Furthermore, loss of Huwe1 in Bergmann glia leads to extensive disorganization of this cell population with layering aberrations, severe granule neuron migration defects, and persistence of ectopic clusters of granule neurons in the external granule layer. Our findings uncover an unexpected role for Huwe1 in regulating Berg-mann glia differentiation and indicate that this ubiquitin ligase orchestrates the programming of the neural progenitors that give rise to neurons and glia in the cerebellum.

Myc proteins as therapeutic targets

Myc proteins (c-myc, Mycn and Mycl) target proliferative and apoptotic pathways vital for progression in cancer. Amplification of the MYCN gene has emerged as one of the clearest indicators of aggressive and chemotherapy-refractory disease in children with neuroblastoma, the most common extracranial solid tumor of childhood. Phosphorylation and ubiquitin-mediated modulation of Myc protein influence stability and represent potential targets for therapeutic intervention. Phosphorylation of Myc proteins is controlled in-part by the receptor tyrosine kinase/phosphatidylinositol 3-kinase/Akt/mTOR signaling, with additional contributions from Aurora A kinase. Myc proteins regulate apoptosis in part through interactions with the p53/Mdm2/Arf signaling pathway. Mutation in p53 is commonly observed in patients with relapsed neuroblastoma, contributing to both biology and therapeutic resistance. This review examines Myc function and regulation in neuroblastoma, and discusses emerging therapies that target Mycn.

Phosphorylation by Cdk2 is required for Myc to repress Ras-induced senescence in cotransformation

The MYC and RAS oncogenes are frequently activated in cancer and, together, are sufficient to transform rodent cells. The basis for this cooperativity remains unclear. We found that although Ras interfered with Myc-induced apoptosis, Myc repressed Ras-induced senescence, together abrogating two main barriers of tumorigenesis. Inhibition of cellular senescence required phosphorylation of Myc at Ser-62 by cyclin E/cyclin-dependent kinase (Cdk) 2. Cdk2 interacted with Myc at promoters, where it affected Myc-dependent regulation of genes, including Bmi-1, p16, p21, and hTERT, which encode proteins known to control senescence. Repression of senescence by Myc was abrogated by the Cdk inhibitor p27Kip1, which is induced by antiproliferative signals like IFN-gamma or by pharmacological inhibitors of Cdk2 but not by inhibitors of other Cdks. In contrast, a phospho-mimicking Myc-S62D mutant was resistant to these manipulations. Inhibition of cyclin E/Cdk2 reversed the senescence-associated gene expression pattern imposed by Myc/cyclin E/Cdk2. This indicates a role of Cdk2 as a transcriptional cofactor and activator of the antisenescence function of Myc and provides mechanistic insight into the Myc-p27Kip1 antagonism. Finally, our findings highlight that pharmacological inhibition of Cdk2 activity is a potential therapeutical principle for cancer therapy, in particular for tumors with activated Myc or Ras.

Myc's other life: stem cells and beyond

Over the last three decades genetic and biochemical studies have revealed the pleiotropic effects of the Myc oncoprotein. While cell line studies have defined the intracellular processes regulated by Myc such as proliferation, differentiation, and metabolic growth, in vivo studies have confirmed these functions, and revealed roles in acquisition and maintenance of stem cell properties. These roles may be partially mediated by Myc's capacity to modify the chromatin landscape on a global scale. Myc also regulates numerous protein-coding transcripts, and many noncoding RNAs (rRNAs, tRNAs, and miRNAs). As Myc activity directly correlates with protein expression, further complexity is provided by post-translational modifications that regulate Myc in normal stem cells or deregulate it in malignant stem cells.

Patched1 deletion increases N-Myc protein stability as a mechanism of medulloblastoma initiation and progression

Medulloblastoma tumorigenesis caused by inactivating mutations in the PATCHED1 (PTCH1) gene is initiated by persistently activated Sonic Hedgehog (Shh) signaling in granule neuron precursors (GNPs) during the late stages of cerebellar development. Both normal cerebellar development and Shh-driven medulloblastoma tumorigenesis require N-Myc expression. However, the mechanisms by which N-Myc affects the stages of medulloblastoma initiation and progression are unknown. Here we used a mouse model of Ptch1 heterozygosity and medulloblastoma to show that increased N-Myc expression characterized the earliest selection of focal GNP hyperplasia destined for later tumor progression. Step-wise loss of Ptch1 expression, from tumor initiation to progression, led to incremental increases in N-Myc protein, rather than mRNA, expression. Increased N-Myc resulted in enhanced proliferation and death resistance of perinatal GNPs at tumor initiation. Sequential N-Myc protein phosphorylation at serine-62 and serine-62/threonine-58 characterized the early and late stages of medulloblastoma tumorigenesis, respectively. Shh pathway activation led to increased Myc protein stability and reduced expression of key regulatory factors. Taken together our data identify N-Myc protein stability as the result of loss of Ptch1, which distinguishes normal cerebellar development from medulloblastoma tumorigenesis.

SHH pathway and cerebellar development

The morphogenetic factor Sonic hedgehog (SHH) has been discovered as one of the masterplayers in cerebellar patterning and was subjected to intensive investigation during the last decade. During early postnatal development, this continuously secreted cholesterol-modified protein drives the expansion of the largest neuronal population of the brain, the granular cells. Moreover, it acts on Bergmann glia differentiation and would potentially affect Purkinje cells homeostasis at adult age. The cerebellar cortex constituted an ideal developmental model to dissect out the upstream mechanisms and downstream targets of this complex pathway. Its deep understanding discloses some of the mechanistic disorders underlying pediatric tumorigenesis, congenital ataxia, and mental retardation. Therapeutical use of its regulators has been consolidated on murine transgenic models and is now considered as a realistic human clinical application. Here, we will review the most recent advances made in the comprehensive understanding of SHH involvement in cerebellar development and pathology.

Stabilization of N-Myc is a critical function of Aurora A in human neuroblastoma

In human neuroblastoma, amplification of the MYCN gene predicts poor prognosis and resistance to therapy. In a shRNA screen of genes that are highly expressed in MYCN-amplified tumors, we have identified AURKA as a gene that is required for the growth of MYCN-amplified neuroblastoma cells but largely dispensable for cells lacking amplified MYCN. Aurora A has a critical function in regulating turnover of the N-Myc protein. Degradation of N-Myc requires sequential phosphorylation by cyclin B/Cdk1 and Gsk3. N-Myc is therefore degraded during mitosis in response to low levels of PI3-kinase activity. Aurora A interacts with both N-Myc and the SCF(Fbxw7) ubiquitin ligase that ubiquitinates N-Myc and counteracts degradation of N-Myc, thereby uncoupling N-Myc stability from growth factor-dependent signals.

Insulin receptor substrate 1 is an effector of sonic hedgehog mitogenic signaling in cerebellar neural precursors

Sonic hedgehog (SHH) and insulin-like growth factor (IGF) signaling are essential for development of many tissues and are implicated in medulloblastoma, the most common solid pediatric malignancy. Cerebellar granule neuron precursors (CGNPs), proposed cells-of-origin for specific classes of medulloblastomas, require SHH and IGF signaling for proliferation and survival during development of the cerebellum. We asked whether SHH regulates IGF pathway components in proliferating CGNPs. We report that SHH-treated CGNPs showed increased levels of insulin receptor substrate 1 (IRS1) protein, which was also present in the germinal layer of the developing mouse cerebellum and in mouse SHH-induced medulloblastomas. Previous roles for IRS1, an oncogenic protein that is essential for IGF-mediated proliferation in other cell types, have not been described in SHH-mediated CGNP proliferation. We found that IRS1 overexpression can maintain CGNP proliferation in the absence of SHH. Furthermore, lentivirus-mediated knock down experiments have shown that IRS1 activity is required for CGNP proliferation in slice explants and dissociated cultures. Contrary to traditional models for SHH signaling that focus on gene transcription, SHH stimulation does not regulate Irs1 transcription but rather stabilizes IRS1 protein by interfering with mTOR-dependent IRS1 turnover and possibly affects Irs1 mRNA translation. Thus, we have identified IRS1 as a novel effector of SHH mitogenic signaling that may serve as a future target for medulloblastoma therapies. Our findings also indicate a previously unreported interaction between the SHH and mTOR pathways, and provide an example of a non-classical means for SHH-mediated protein regulation during development.

The Fbxw7/hCdc4 tumor suppressor in human cancer

Fbxw7/hCdc4 is a member of the F-box family of proteins, which function as interchangeable substrate recognition components of the SCF ubiquitin ligases. SCF(Fbxw7/hCdc4) targets several important oncoproteins including c-Myc, c-Jun, cyclin E1, and Notch, for ubiquitin-dependent proteolysis. Recent studies have shown that FBXW7/hCDC4 is mutated in a variety of human tumor types, suggesting that it is a general tumor suppressor in human cancer. Alteration of Fbxw7/hCdc4 function is linked to defects in differentiation, cellular proliferation, and genetic instability. In this review, we summarize what is known about Fbxw7/hCdc4-mediated degradation in the regulation of cellular proliferation and discuss how alteration of its function contributes to human tumorigenesis.

The origins of medulloblastoma subtypes

Childhood tumors containing cells that are morphologically and functionally similar to normal progenitor cells provide fertile ground for investigating the links between development and cancer. In this respect, integrated studies of normal cerebellar development and the medulloblastoma, a malignant embryonal tumor of the cerebellum, have proven especially fruitful. Emerging evidence indicates that the different precursor cell populations that form the cerebellum and the cell signaling pathways that regulate its development likely represent distinct compartments from which the various subtypes of medulloblastoma arise. Definitive characterization of each medulloblastoma subtype will undoubtedly improve treatment of this disease and provide important insights to the origins of cancer.

Ubiquitination of Myc: proteasomal degradation and beyond

The level of Myc proteins is a critical determinant of cellular proliferation and apoptosis. Ubiquitination of Myc plays a key role in controlling protein levels by stimulating proteasomal degradation of the protein. Some experiments suggest that ubiquitination may also regulate Myc function in addition to turnover. This review attempts to summarize current knowledge about this field.

Forkhead transcription factor FoxM1 regulates mitotic entry and prevents spindle defects in cerebellar granule neuron precursors

The forkhead transcription factor FoxM1 has been reported to regulate, variously, proliferation and/or spindle formation during the G2/M transition of the cell cycle. Here we define specific functions of FoxM1 during brain development by the investigation of FoxM1 loss-of-function mutations in the context of Sonic hedgehog (Shh)-induced neuroproliferation in cerebellar granule neuron precursors (CGNP). We show that FoxM1 is expressed in the cerebellar anlagen as well as in postnatal proliferating CGNP and that it is upregulated in response to activated Shh signaling. To determine the requirements for FoxM1 function, we used transgenic mice carrying conventional null alleles or conditionally targeted alleles in conjunction with specific Cre recombinase expression in CGNP or early neural precursors driven by Math1 or Nestin enhancers. Although the overall cerebellar morphology was grossly normal, we observed that the entry into mitosis was postponed both in vivo and in Shh-treated CGNP cultures. Cell cycle analysis and immunohistochemistry with antibodies against phosphorylated histone H3 indicated a significant delay in the G2/M transition. Consistent with this, FoxM1-deficient CGNP showed decreased levels of the cyclin B1 and Cdc25b proteins. Furthermore, the loss of FoxM1 resulted in spindle defects and centrosome amplification. These findings indicate that the functions of FoxM1 in Shh-induced neuroproliferation are restricted to the regulation of the G2/M transition in CGNP, most probably through transcriptional effects on target genes such as those coding for B-type cyclins.

Shifting paradigms in Hedgehog signaling

Hedgehog (Hh) signaling proteins regulate multiple developmental and adult homeostatic processes. A defining feature of Hh signaling is that relatively small changes in the concentration of Hh ligand elicit dramatically different cellular responses. As a result, the processing, release and trafficking of Hh ligands must be tightly regulated to ensure proper signaling. In addition, sensitive and specific intracellular signaling cascades are needed to interpret subtle differences in the level of Hh signal to execute an appropriate response. A detailed understanding of the mechanisms that regulate these responses is critical to shaping our view of this key regulatory system.

Systems biology by the rules: hybrid intelligent systems for pathway modeling and discovery

BACKGROUND

Expert knowledge in journal articles is an important source of data for reconstructing biological pathways and creating new hypotheses. An important need for medical research is to integrate this data with high throughput sources to build useful models that span several scales. Researchers traditionally use mental models of pathways to integrate information and development new hypotheses. Unfortunately, the amount of information is often overwhelming and these are inadequate for predicting the dynamic response of complex pathways. Hierarchical computational models that allow exploration of semi-quantitative dynamics are useful systems biology tools for theoreticians, experimentalists and clinicians and may provide a means for cross-communication.

RESULTS

A novel approach for biological pathway modeling based on hybrid intelligent systems or soft computing technologies is presented here. Intelligent hybrid systems, which refers to several related computing methods such as fuzzy logic, neural nets, genetic algorithms, and statistical analysis, has become ubiquitous in engineering applications for complex control system modeling and design. Biological pathways may be considered to be complex control systems, which medicine tries to manipulate to achieve desired results. Thus, hybrid intelligent systems may provide a useful tool for modeling biological system dynamics and computational exploration of new drug targets. A new modeling approach based on these methods is presented in the context of hedgehog regulation of the cell cycle in granule cells. Code and input files can be found at the Bionet website: www.chip.ord/~wbosl/Software/Bionet.

CONCLUSION

This paper presents the algorithmic methods needed for modeling complicated biochemical dynamics using rule-based models to represent expert knowledge in the context of cell cycle regulation and tumor growth. A notable feature of this modeling approach is that it allows biologists to build complex models from their knowledge base without the need to translate that knowledge into mathematical form. Dynamics on several levels, from molecular pathways to tissue growth, are seamlessly integrated. A number of common network motifs are examined and used to build a model of hedgehog regulation of the cell cycle in cerebellar neurons, which is believed to play a key role in the etiology of medulloblastoma, a devastating childhood brain cancer.

Morphogen to mitogen: the multiple roles of hedgehog signalling in vertebrate neural development

Sonic hedgehog has received an enormous amount of attention since its role as a morphogen that directs ventral patterning in the spinal cord was discovered a decade ago. Since that time, a bewildering array of information has been generated concerning both the components of the hedgehog signalling pathway and the remarkable number of contexts in which it functions. Nowhere is this more evident than in the nervous system, where hedgehog signalling has been implicated in events as disparate as axonal guidance and stem cell maintenance. Here we review our present knowledge of the hedgehog signalling pathway and speculate about areas in which further insights into this versatile pathway might be forthcoming.

The Ins and Outs of MYC Regulation by Posttranslational Mechanisms

The proteins of the MYC family are key regulators of cell behavior. MYC, originally identified as an oncoprotein, affects growth, proliferation, differentiation, and apoptosis of cells through its ability to regulate a significant number of genes. In addition MYC governs events associated with tumor progression, including genetic stability, migration, and angiogenesis. The pleiotropic activities attributed to MYC and their balanced control requires that the expression and function of MYC is tightly controlled. Indeed many different pathways and factors have been identified that impinge on MYC gene expression and protein function. In particular the protein is subject to different posttranslational modifications, including phosphorylation, ubiquitinylation, and acetylation. Here we discuss the latest developments regarding these modifications that control various aspects of MYC function, including its stability, the interaction with partner proteins, and the transcriptional potential.

Inhibition of IGF-I receptor in anchorage-independence attenuates GSK-3beta constitutive phosphorylation and compromises growth and survival of medulloblastoma cell lines

We have previously reported that insulin-like growth factor-I (IGF-I) supports growth and survival of mouse and human medulloblastoma cell lines, and that IGF-I receptor (IGF-IR) is constitutively phosphorylated in human medulloblastoma clinical samples. Here, we demonstrate that a specific inhibitor of insulin-like growth factor-I receptor (IGF-IR), NVP-AEW541, attenuated growth and survival of mouse (BsB8) and human (D384, Daoy) medulloblastoma cell lines. Cell cycle analysis demonstrated that G1 arrest and apoptosis contributed to the action of NVP-AEW54. Interestingly, very aggressive BsB8 cells, which derive from cerebellar tumors of transgenic mice expressing viral oncoprotein (large T-antigen from human polyomavirus JC) became much more sensitive to NVP-AEW541 when exposed to anchorage-independent culture conditions. This high sensitivity to NVP-AEW54 in suspension was accompanied by the loss of GSK-3beta constitutive phosphorylation and was independent from T-antigen-mediated cellular events (Supplementary Materials). BsB8 cells were partially rescued from NVP-AEW541 by GSK3beta inhibitor, lithium chloride and were sensitized by GSK3beta activator, sodium nitroprusside (SNP). Importantly, human medulloblastoma cells, D384, which demonstrated partial resistance to NVP-AEW541 in suspension cultures, become much more sensitive following SNP-mediated GSK3beta dephosphorylation (activation). Our results indicate that hypersensitivity of medulloblastoma cells in anchorage-independence is linked to GSK-3beta activity and suggest that pharmacological intervention against IGF-IR with simultaneous activation of GSK3beta could be highly effective against medulloblastomas, which have intrinsic ability of disseminating the CNS via cerebrospinal fluid.

Role of post-translational modifications in regulating c-Myc proteolysis, transcriptional activity and biological function

The Myc proteins play a central role in cellular proliferation, differentiation, apoptosis and tumorigenesis. Although it is clear that multiple molecular mechanisms mediate these functions, it is unclear how individual mechanisms contribute and if different mechanisms work in concert or separately in mediating the diverse biological functions of c-Myc. Similarly, the role of post-translational modifications in regulating c-Myc molecular and biological properties has remained uncertain, despite over 20 years of research. In particular, phosphorylation of the N-terminal transcriptional regulatory domain has been shown to have a variety of consequences ranging from dramatic effects on apoptosis, tumorigenesis and c-Myc proteolysis to negligible effects on cellular transformation and transcriptional activity. This review attempts to provide a comprehensive and critical evaluation of the accumulated evidence to address the complex and controversial issues surrounding the role of post-translational modifications in c-Myc function, focusing on phosphorylation and ubiquitination of the N-terminal transcriptional regulatory domain. An overall model emerges that suggests phosphorylation and ubiquitination play critical roles in cell cycle progression, cell growth, apoptosis and tumorigenesis that are mediated by phosphorylation-dependent transcriptional activation of distinct sets of target genes and synchronized proteolysis.

Identification of an unexpected link between the Shh pathway and a G2/M regulator, the phosphatase CDC25B

Sonic hedgehog (Shh) signaling controls numerous aspects of vertebrate development, including proliferation of precursors in different organs. Identification of molecules that link the Shh pathway to cell cycle machinery is therefore of major importance for an understanding of the mechanisms underlying Shh-dependent proliferation. Here, we show that an actor in the control of entry into mitosis, the phosphatase CDC25B, is transcriptionally upregulated by the Shh/Gli pathway. Unlike other G2/M regulators, CDC25B is highly expressed in domains of Shh activity, including the ventral neural tube and the posterior limb bud. Loss- and gain-of-function experiments reveal that Shh contributes to CDC25B transcriptional activation in the neural tube both of chick and mouse embryos. Moreover, CDC25B transcripts are absent from the posterior limb bud of Shh-/- mice, while anterior grafts of Shh-expressing cells in the chicken limb bud induce ectopic CDC25B expression. Arresting the cell cycle does not reduce the level of CDC25B expression in the neural tube strongly suggesting that the upregulation of CDC25B is not an indirect consequence of the Shh-dependent proliferation. These data reveal an unexpected developmental link between the Shh pathway and a participant in G2/M control.

Neural Stem Cells and Their Manipulation

Extracellular signals dictate the biological processes of neural stem cells (NSCs) both in vivo and in vitro. The intracellular response elicited by these signals is dependent on the context in which the signal is received, which in turn is decided by previous and concurrent signals impinging on the cell. A synthesis of signaling pathways that control proliferation, survival, and differentiation of NSCs in vivo and in vitro will lead to a better understanding of their biology, and will also permit more precise and reproducible manipulation of these cells to particular end points. In this review we summarize the known signals that cause proliferation, survival, and differentiation in mammalian NSCs.