Pei Cheng

To address the role of N- myc in neurogenesis and in nervous system tumors, it was conditionally disrupted in neuronal progenitor cells (NPCs) with a nestin-Cre transgene. Null mice display ataxia, behavioral abnormalities, and tremors that correlate with a twofold decrease in brain mass that disproportionately affects the cerebellum (sixfold reduced in mass) and the cerebral cortex, both of which show signs of disorganization. In control mice at E12.5, we observe a domain of high N-Myc protein expression in the rapidly proliferating cerebellar primordium. Targeted deletion of N- myc results in severely compromised proliferation as shown by a striking decrease in S phase and mitotic cells as well as in cells expressing the Myc target gene cyclin D2, whereas apoptosis is unaffected. Null progenitor cells also have comparatively high levels of the cdk inhibitors p27 Kip1 and p18 Ink4c , whereas p15 Ink4b , p21 Cip1 , and p19 Ink4d levels are unaffected. Many null progenitors also exhibit altered nuclear morphology and size. In addition, loss of N- myc disrupts neuronal differentiation as evidenced by ectopic staining of the neuron specific marker βTUBIII in the cerebrum. Furthermore, in progenitor cell cultures derived from null embryonic brain, we observe a dramatic increase in neuronal differentiation compared with controls. Thus, N- myc is essential for normal neurogenesis, regulating NPC proliferation, differentiation, and nuclear size. Its effects on proliferation and differentiation appear due, at least in part, to down-regulation of a specific subset of cyclin-dependent kinase inhibitors.

The proteins encoded by the myc proto-oncogene family are involved in cell proliferation, apoptosis, differentiation, and neoplasia. Myc acts through dimerization with Max to bind DNA and activate transcription. Homologs of the myc and max genes were cloned from the fruit fly Drosophila melanogaster and their protein products (dMyc and dMax) were shown to heterodimerize, recognize the same DNA sequence as their vertebrate homologs, and activate transcription. The dMyc protein is likely encoded by the Drosophila gene diminutive (dm), a mutation in which results in small body size and female sterility caused by degeneration of the ovaries. These findings indicate a potential role for Myc in germ cell development and set the stage for genetic analysis of Myc and Max.

The corepressor mSin3A is the core component of a chromatin-modifying complex that is recruited by multiple gene-specific transcriptional repressors. In order to understand the role of mSin3A during development, we generated constitutive germ line as well as conditional msin3A deletions. msin3A deletion in the developing mouse embryo results in lethality at the postimplantation stage, demonstrating that it is an essential gene. Blastocysts derived from preimplantation msin3A null embryos and mouse embryo fibroblasts (MEFs) lacking msin3A display a significant reduction in cell division. msin3A null MEFs also show mislocalization of the heterochromatin protein, HP1alpha, without alterations in global histone acetylation. Heterozygous msin3A(+/-) mice with a systemic twofold decrease in mSin3A protein develop splenomegaly as well as kidney disease indicative of a disruption of lymphocyte homeostasis. Conditional deletion of msin3A from developing T cells results in reduced thymic cellularity and a fivefold decrease in the number of cytotoxic (CD8) T cells, while helper (CD4) T cells are unaffected. We show that CD8 development is dependent on mSin3A at a step downstream of T-cell receptor signaling and that loss of mSin3A specifically decreases survival of double-positive and CD8 T cells. Thus, msin3A is a pleiotropic gene which, in addition to its role in cell cycle progression, is required for the development and homeostasis of cells in the lymphoid lineage.