N-myc is essential during neurogenesis for the rapid expansion of progenitor cell populations and the inhibition of neuronal differentiation

Development and malformations of the cerebellum in mice

The cerebellum is the primary motor coordination center of the CNS and is also involved in cognitive processing and sensory discrimination. Multiple cerebellar malformations have been described in humans, however, their developmental and genetic etiologies currently remain largely unknown. In contrast, there is extensive literature describing cerebellar malformations in the mouse. During the past decade, analysis of both spontaneous and gene-targeted neurological mutant mice has provided significant insight into the molecular and cellular mechanisms that regulate cerebellar development. Cerebellar development occurs in several distinct but interconnected steps. These include the establishment of the cerebellar territory along anterior-posterior and dorsal-ventral axes of the embryo, initial specification of the cerebellar cell types, their subsequent proliferation, differentiation and migration, and, finally, the interconnection of the cerebellar circuitry. Our understanding of the basis of these developmental processes is certain to provide insight into the nature of human cerebellar malformations.

Gene and protein expression profiling during differentiation of neuroblastoma cells triggered by 13-cis retinoic acid

PURPOSE

The precise changes in RNA and protein expression that accompany neuroblastoma differentiation remain unknown. The authors used microarray technologies to screen molecules associated with the differentiation of neuroblastoma (NB) cells induced by 13-cis retinoic acid.

METHODS

The authors quantified the expression of 2,061 RNA transcripts related to oncogenesis and of 380 proteins expressed in SK-N-SH and CHP-134 NB cell lines in the presence or absence of 13-cis retinoic acid.

RESULTS

Hierarchical clustering captured gene expression altered during neuroblastoma differentiation induced by 13-cis retinoic acid. Several genes were further abstracted based on P values below 0.0017 or protein chips observed in both NB cell lines. The altered expressions of gene products revealed by both DNA and protein chips were in agreement. The expressions of N-myc, cyclin D3, and Wnt10B were downregulated, whereas those of retinoblastoma (RB) and related genes (p107, RB2/p130, p300/CBP, E2F-1, DP-1) as well as others were upregulated.

CONCLUSIONS

These results suggest that microarray technology can screen for genes that are important in neuroblastoma differentiation.

Cell cycle regulation and neural differentiation

The general mechanisms that control the cell cycle in mammalian cells have been studied in depth and several proteins that are involved in the tight regulation of cell cycle progression have been identified. However, the analysis of which molecules participate in cell cycle exit of specific cell lineages is not exhaustive yet. Moreover, the strict relation between cell cycle exit and induction of differentiation has not been fully understood and seems to depend on the cell type. Several in vivo and in vitro studies have been performed in the last few years to address these issues in cells of the nervous system. In this review, we focus our attention on cyclin-cyclin-dependent kinase complexes, cyclin kinase inhibitors, genes of the retinoblastoma family, p53 and N-Myc, and we aim to summarize the latest evidence indicating their involvement in the control of the cell cycle and induction of differentiation in different cell types of the peripheral and central nervous systems. Studies on nervous system tumors and a possible contributory role in tumorigenesis of polyomavirus T antigen are reported to point out the critical contribution of some cell cycle regulators to normal neural and glial development.

Transcriptional profiling of the Sonic hedgehog response: a critical role for N-myc in proliferation of neuronal precursors

Cerebellar granule cells are the most abundant neurons in the brain, and granule cell precursors (GCPs) are a common target of transformation in the pediatric brain tumor medulloblastoma. Proliferation of GCPs is regulated by the secreted signaling molecule Sonic hedgehog (Shh), but the mechanisms by which Shh controls proliferation of GCPs remain inadequately understood. We used DNA microarrays to identify targets of Shh in these cells and found that Shh activates a program of transcription that promotes cell cycle entry and DNA replication. Among the genes most robustly induced by Shh are cyclin D1 and N-myc. N-myc transcription is induced in the presence of the protein synthesis inhibitor cycloheximide, so it appears to be a direct target of Shh. Retroviral transduction of N-myc into GCPs induces expression of cyclin D1, E2F1, and E2F2, and promotes proliferation. Moreover, dominant-negative N-myc substantially reduces Shh-induced proliferation, indicating that N-myc is required for the Shh response. Finally, cyclin D1 and N-myc are overexpressed in murine medulloblastoma. These findings suggest that cyclin D1 and N-myc are important mediators of Shh-induced proliferation and tumorigenesis.

Genomic binding by the Drosophila Myc, Max, Mad/Mnt transcription factor network

The Myc/Max/Mad transcription factor network is critically involved in cell behavior; however, there is relatively little information on its genomic binding sites. We have employed the DamID method to carry out global genomic mapping of the Drosophila Myc, Max, and Mad/Mnt proteins. Each protein was tethered to Escherichia coli DNA adenine-methyltransferase (Dam) permitting methylation proximal to in vivo binding sites in Kc cells. Microarray analyses of methylated DNA fragments reveals binding to multiple loci on all major Drosophila chromosomes. This approach also reveals dynamic interactions among network members as we find that increased levels of dMax influence the extent of dMyc, but not dMnt, binding. Computer analysis using the REDUCE algorithm demonstrates that binding regions correlate with the presence of E-boxes, CG repeats, and other sequence motifs. The surprisingly large number of directly bound loci ( approximately 15% of coding regions) suggests that the network interacts widely with the genome. Furthermore, we employ microarray expression analysis to demonstrate that hundreds of DamID-binding loci correspond to genes whose expression is directly regulated by dMyc in larvae. These results suggest that a fundamental aspect of Max network function involves widespread binding and regulation of gene expression.

Nmyc upregulation by sonic hedgehog signaling promotes proliferation in developing cerebellar granule neuron precursors

Hedgehog pathway activation is required for expansion of specific neuronal precursor populations during development and is etiologic in the human cerebellar tumor, medulloblastoma. We report that sonic hedgehog (Shh) signaling upregulates expression of the proto-oncogene Nmyc in cultured cerebellar granule neuron precursors (CGNPs) in the absence of new protein synthesis. The temporal-spatial expression pattern of Nmyc, but not other Myc family members, precisely coincides with regions of hedgehog proliferative activity in the developing cerebellum and is observed in medulloblastomas of Patched (Ptch) heterozygous mice. Overexpression of Nmyc promotes cell-autonomous G(1) cyclin upregulation and CGNP proliferation independent of Shh signaling. Furthermore, Myc antagonism in vitro significantly decreases proliferative effects of Shh in cultured CGNPs. Together, these findings identify Nmyc as a direct target of the Shh pathway that functions to regulate cell cycle progression in cerebellar granule neuron precursors.

Development of mice expressing a single D-type cyclin

D-cyclins (cyclins D1, D2, and D3) are components of the core cell cycle machinery. To directly test the ability of each D-cyclin to drive development of various lineages, we generated mice expressing only cyclin D1, or only cyclin D2, or only cyclin D3. We found that these "single-cyclin" embryos develop normally until late gestation. Our analyses revealed that in single-cyclin embryos, the tissue-specific expression pattern of D-cyclins was lost. Instead, mutant embryos ubiquitously expressed the remaining D-cyclin. These findings suggest that the functions of the three D-cyclins are largely exchangeable at this stage. Later in life, single-cyclin mice displayed focused abnormalities, resulting in premature mortality. "Cyclin D1-only" mice developed severe megaloblastic anemia, "cyclin D2-only" mice presented neurological abnormalities, and "cyclin D3-only" mice lacked normal cerebella. Analyses of the affected tissues revealed that these compartments failed to sufficiently up-regulate the remaining, intact D-cyclin. In particular, we found that in cerebellar granule neuron precursors, the N-myc transcription factor communicates with the cell cycle machinery via cyclins D1 and D2, but not D3, explaining the inability of D3-only mice to up-regulate cyclin D3 in this compartment. Hence, the requirement for a particular cyclin in a given tissue is likely caused by specific transcription factors, rather than by unique properties of cyclins.