Biomedical subjects

Paul S Knoepfler

Publications and source records attributed to Paul S Knoepfler.

8 recordsLinked to original sources

PubMed2006

N-myc is an essential downstream effector of Shh signaling during both normal and neoplastic cerebellar growth.

We examined the genetic requirements for the Myc family of oncogenes in normal Sonic hedgehog (Shh)-mediated cerebellar granule neuronal precursor (GNP) expansion and in Shh pathway-induced medulloblastoma formation. In GNP-enriched cultures derived from N-myc(Fl/Fl) and c-myc(Fl/Fl) mice, disruption of N-myc, but not c-myc, inhibited the proliferative response to Shh. Conditional deletion of c-myc revealed that, although it is necessary for the general regulation of brain growth, it is less important for cerebellar development and GNP expansion than N-myc. In vivo analysis of compound mutants carrying the conditional N-myc null and the activated Smoothened (ND2:SmoA1) alleles showed, that although granule cells expressing the ND2:SmoA1 transgene are present in the N-myc null cerebellum, no hyperproliferation or tumor formation was detected. Taken together, these findings provide in vivo evidence that N-myc acts downstream of Shh/Smo signaling during GNP proliferation and that N-myc is required for medulloblastoma genesis even in the presence of constitutively active signaling from the Shh pathway.

Animals↗

PubMed2006

N-Myc and the cyclin-dependent kinase inhibitors p18Ink4c and p27Kip1 coordinately regulate cerebellar development.

Conditional N-Myc deletion limits the proliferation of granule neuron progenitors (GNPs), perturbs foliation, and leads to reduced cerebellar mass. We show that c-Myc mRNA levels increase in N-Myc-null GNPs and that simultaneous deletion of both c- and N-Myc exacerbates defective cerebellar development. Moreover, N-Myc loss has been shown to trigger the precocious expression of two cyclin-dependent kinase inhibitors, Kip1 and Ink4c, in the cerebellar primordium. We now further demonstrate that the engineered disruption of the Kip1 and Ink4c genes in N-Myc-null cerebella partially rescues GNP cell proliferation and cerebellar foliation. These results provide definitive genetic evidence that expression of N-Myc and concomitant down-regulation of Ink4c and Kip1 contribute to the proper development of the cerebellum.

Animals↗

PubMed2006

Myc influences global chromatin structure.

The family of myc proto-oncogenes encodes transcription factors (c-, N-, and L-Myc) that regulate cell growth and proliferation and are involved in the etiology of diverse cancers. Myc proteins are thought to function by binding and regulating specific target genes. Here we report that Myc proteins are required for the widespread maintenance of active chromatin. Disruption of N-myc in neuronal progenitors and other cell types leads to nuclear condensation accompanied by large-scale changes in histone modifications associated with chromatin inactivation, including hypoacetylation and altered methylation. These effects are largely reversed by exogenous Myc as well as by differentiation and are mimicked by the Myc antagonist Mad1. The first chromatin changes are evident within 6 h of Myc loss and lead to changes in chromatin structure. Myc widely influences chromatin in part through upregulation of the histone acetyltransferase GCN5. This study provides the first evidence for regulation of global chromatin structure by an oncoprotein and may explain the broad effects of Myc on cell behavior and tumorigenesis.

Acetylation↗

PubMed2006

Neural precursor cycling at sonic speed: N-Myc pedals, GSK-3 brakes.

Signaling by the sonic hedgehog (Shh) pathway is essential for neural precursor population expansion during normal central nervous system (CNS) development, and is implicated in the childhood brain tumor, medulloblastoma. The proto-oncogene N-myc plays essential roles as a downstream effector of Shh proliferative effects in neural precursors of the cerebellum, where medulloblastomas arise. It is likely that N-Myc has analogous functions in medulloblastomas and other CNS tumors where it is highly expressed due to altered regulation or gene amplification. Myc destabilization occurs in response to phosphorylation by GSK-3beta. N-Myc degradation is required for cerebellar neural precursors to exit the cell cycle. During mitosis in cerebellar neural precursors, levels of N-Myc primed for phosphorylation by GSK-3beta increase, due to cdk1 complex activity towards N-Myc. GSK-3beta is kept in check by insulin-like growth factor signaling, which also plays critical roles in brain development and cancer. These findings indicate that therapeutic strategies targeting N-myc and the IGF pathway might be effective against medulloblastoma.

Animals↗

PubMed2005

Nmyc plays an essential role during lung development as a dosage-sensitive regulator of progenitor cell proliferation and differentiation.

Understanding how lung progenitor cells balance proliferation against differentiation is relevant to clinical disorders such as bronchopulmonary dysplasia of premature babies and lung cancer. Previous studies have established that lung development is severely disrupted in mouse mutants with reduced levels of the proto-oncogene Nmyc, but the precise mechanisms involved have not been explored. We show here that Nmyc expression in the embryonic lung is normally restricted to a distal population of undifferentiated epithelial cells, a high proportion of which are in the S phase of the cell cycle. Overexpression of NmycEGFP in the epithelium under the control of surfactant protein C (Sftpc) regulatory elements expands the domain of S phase cells and upregulates numerous genes associated with growth and metabolism, as shown by transcriptional microarray. In addition, there is marked inhibition of differentiation, coupled with an expanded domain of expression of Sox9 protein, which is also normally restricted to the distal epithelial compartment. By contrast, conditional deletion of Nmyc leads to reduced proliferation, epithelial differentiation and high levels of apoptosis in both epithelium and mesenchyme. Unexpectedly, about 50% of embryos in which only one copy of Nmyc is deleted die perinatally, with similarly abnormal lungs. We propose a model in which Nmyc is essential in the developing lung for maintaining a distal population of undifferentiated, proliferating progenitor cells.

Animals↗

PubMed2004

HBP1 and Mad1 repressors bind the Sin3 corepressor PAH2 domain with opposite helical orientations.

Recruitment of the histone deacetylase (HDAC)-associated Sin3 corepressor is an obligatory step in many eukaryotic gene silencing pathways. Here we show that HBP1, a cell cycle inhibitor and regulator of differentiation, represses transcription in a HDAC/Sin3-dependent manner by targeting the mammalian Sin3A (mSin3A) PAH2 domain. HBP1 is unrelated to the Mad1 repressor for which high-resolution structures in complex with PAH2 have been described. We show that like Mad1, the HBP1 transrepression domain binds through a helical structure to the hydrophobic cleft of mSin3A PAH2. Notably, the HBP1 helix binds PAH2 in a reversed orientation relative to Mad1 and, equally unexpectedly, this is correlated with a chain reversal of the minimal Sin3 interaction motifs. These results not only provide insights into how multiple, unrelated transcription factors recruit the same coregulator, but also have implications for how sequence similarity searches are conducted.

Amino Acid Motifs↗

PubMed2004

Pbx marks genes for activation by MyoD indicating a role for a homeodomain protein in establishing myogenic potential.

Skeletal muscle differentiation is initiated by the transcription factor MyoD, which binds directly to the regulatory regions of genes expressed during skeletal muscle differentiation and initiates chromatin remodeling at specific promoters. It is not known, however, how MyoD initially recognizes its binding site in a chromatin context. Here we show that the H/C and helix III domains, two domains of MyoD that are necessary for the initiation of chromatin remodeling at the myogenin locus, together regulate a restricted subset of genes, including myogenin. These domains are necessary for the stable binding of MyoD to the myogenin promoter through an interaction with an adjacent protein complex containing the homeodomain protein Pbx, which appears to be constitutively bound at this site. This demonstrates a specific mechanism of targeting MyoD to loci in inactive chromatin and reveals a critical role of homeodomain proteins in marking specific genes for activation in the muscle lineage.

Animals↗

PubMed2002

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

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 betaTUBIII 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.

Animals↗