Biomedical subjects

Z Dave Sharp

Publications and source records attributed to Z Dave Sharp.

4 recordsLinked to original sources

PubMed2006

Estrogen-receptor-alpha exchange and chromatin dynamics are ligand- and domain-dependent.

We report a mammalian-based promoter chromosomal array system developed for single-cell studies of transcription-factor function. Designed after the prolactin promoter-enhancer, it allows for the direct visualization of estrogen receptor alpha (ERalpha) and/or Pit-1 interactions at a physiologically regulated transcription locus. ERalpha- and ligand-dependent cofactor recruitment, large-scale chromatin modifications and transcriptional activity identified a distinct fingerprint of responses for each condition. Ligand-dependent transcription (more than threefold activation compared with vehicle, or complete repression by mRNA fluorescent in situ hybridization) at the array correlated with its state of condensation, which was assayed using a novel high throughput microscopy approach. In support of the nuclear receptor hit-and-run model, photobleaching studies provided direct evidence of very transient ER-array interactions, and revealed ligand-dependent changes in k(off). ERalpha-truncation mutants indicated that helix-12 and interactions with co-regulators influenced both large-scale chromatin modeling and photobleaching recovery times. These data also showed that the ERalpha DNA-binding domain was insufficient for array targeting. Collectively, quantitative observations from this physiologically relevant biosensor suggest stochastic-based dynamics influence gene regulation at the promoter level.

Active Transport, Cell Nucleus↗

PubMed2004

Inactivating Pit-1 mutations alter subnuclear dynamics suggesting a protein misfolding and nuclear stress response.

Pit-1, a POU-class nuclear DNA-binding transcription factor, specifies three of the parenchymal cell types in anterior pituitary ontogeny. Using fluorescent fusions and live cell imaging, we have compared the dynamic behavior of wild-type and inactivating Pit-1 point mutations. Fluorescence recovery after photobleaching (FRAP) and real-time extraction data indicate that wild-type Pit-1 has a dynamic mobility profile, with t(1/2s) approximately 5-7 s when expressed from low to high amounts, respectively. Biochemically, Pit-1 is approximately 50% retained according to direct observation during extraction, indicating a dynamic interaction with nuclear structure. An analysis of transiently expressed Pit-1 carrying two different debilitating mutations reveals that they translocate normally to the nucleus, but exhibit two different levels of mobility, both clearly distinguishable from wild-type Pit-1. At low expression levels, the t(1/2s) of Pit(W261C) and Pit(A158P) are extremely rapid (0.3 and 0.6 s t(1/2s), respectively). At higher expression levels, unlike wild-type Pit-1, both mutant proteins become immobilized and insoluble, and fractionate completely with the insoluble nuclear matrix. Relative to wild-type, over expression of mutated Pit-1 elicits a nuclear stress response indicated by increased levels of heat shock inducible heat shock protein 70 (Hsp70), and reorganization of heat shock factor-1. The decreased mobility of Pit(A158P) relative to Pit(W261C) at low expression levels correlates with its ability to partially activate when expressed at low levels and its ability to bind cognate DNA. At high expression levels, lower Pit(A158P) activation correlates with its immobilization and insolubility. These data suggest a link between specific rates of intranuclear mobility and Pit-1 transcription function, perhaps to insure sufficient interactions with chromatin, or in the case of non-DNA binding Pit-1, interaction as a repressor. These data imply inactivating mutations can lead to an intranuclear sorting away from transcription related pathways, and at least in part to a misfolded protein pathway. Taken together, caution is suggested when interpreting point (or other) mutational analyses of transactivator function, as new compartmentation, especially in the context of expression levels, may cloud the distinction between defining functional molecular domains and intranuclear processing of misfolded proteins.

Cell Nucleus↗

PubMed2003

Minimal effects of dietary restriction on neuroendocrine carcinogenesis in Rb+/- mice.

The efficacy of dietary restriction in retarding tumor growth is well established in rodents. However, gene and cell lineage specificity of dietary restriction effects is far less defined. Mice with a single copy of the retinoblastoma susceptibility gene (Rb) develop a well-established syndrome of mouse neuroendocrine neoplasia associated with Rb deficiency. Thus, if DR represses tumor growth in this model, it should be unambiguously attributed to the Rb defect in neuroendocrine cell lineages. To address this possibility, Rb(+/-) mice were entered into a diet restriction study. Surprisingly, 40-50% reductions in dietary intake, relative to an ad libitum group, started on either postnatal day 28 or 42 had little to no effect on either the frequency or growth of pituitary tumors either during the latency period (postnatal day 224) or at the time of their natural death. Consistent with cross-section data, survival of 65 diet restricted Rb(+/-) mice was almost identical to that of 67 Rb(+/-) mice fed ad libitum (AL); median life span was 414 and 436 days for AL and DR groups, respectively. These findings indicate that diet restriction provides no significant benefit in delaying growth and progression of neuroendocrine tumors exhibiting loss of RB function. They also introduce the possibility that RB is required for the tumor-repressive effects of DR.

Animals↗

PubMedDate not supplied

Molecular dynamics and nuclear receptor function.

The development of live cell and biochemical analysis methods has led to an increase in our understanding of the dynamic regulation of transcription. Live single cell studies using photobleaching techniques indicate that many proteins have a high nuclear mobility. Pioneering work using promoter array systems based on the lac operon or the mouse mammary tumor virus promoter enabled the study of chromatin structure, promoter occupancy and protein-chromatin interaction dynamics in relation to transcription. Chromatin immunoprecipitation (ChIP)-based assays allow an exhaustive analysis of the temporal recruitment of proteins to an endogenous promoter and provide evidence of cyclic protein-protein and protein-promoter interactions. Although reflecting different timescales, both ChIP and live cell studies indicate a highly dynamic control of transcription that until now has gone undetected and unappreciated.

Animals↗