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This issue's Molecular Biology Select highlights recent studies that further the understanding of how chromatin structure is regulated and maintained in a variety of different biological contexts to ensure the precise regulation of gene expression. Two of the studies report noncoding RNAs, which are cofactors that direct proteins to modulate chromatin structure. The other three studies delve into...
The yeast sirtuin (Sir2) is a histone deacetylase that modulates yeast replicative life span by suppressing genome instability through chromatin modification. In this issue, Oberdoerffer et al. (2008) report that SIRT1, the mammalian ortholog of Sir2, is involved in DNA damage-induced chromatin reorganization, which promotes genome stability in mammalian cells.
Loss of cell polarity proteins such as Scribble induces neoplasia in Drosophila by promoting uncontrolled proliferation. In mammals, the role that polarity proteins play during tumorigenesis is not well understood. Here, we demonstrate that depletion of Scribble in mammary epithelia disrupts cell polarity, blocks three-dimensional morphogenesis, inhibits apoptosis, and induces dysplasia in vivo that...
When and why did cell polarization arise? Recent work in bacteria and yeast suggests that polarization may have evolved to restrict senescence to one daughter during division by enabling the differential segregation of damaged material. In more complex organisms, polarity functions have diversified to permit the differential inheritance of centrosomes, RNAs, proteins, and membranes, which is essential...
MALAT1 is a long noncoding RNA known to be misregulated in many human cancers. We have identified a highly conserved small RNA of 61 nucleotides originating from the MALAT1 locus that is broadly expressed in human tissues. Although the long MALAT1 transcript localizes to nuclear speckles, the small RNA is found exclusively in the cytoplasm. RNase P cleaves the nascent MALAT1 transcript downstream...
Mutations in LRP5, a coreceptor for Wnt proteins, cause the disease osteoporosis pseudoglioma. A new study by Yadav et al. (2008) now challenges the view that LRP5 controls bone mass through Wnt signaling in bone and argues instead that LRP5 regulates bone mass indirectly through its effects on serotonin synthesis in the gut.
Disruption of the Transient Receptor Potential (TRP) mucolipin 1 (TRPML1) channel results in the neurodegenerative disorder mucolipidosis type IV (MLIV), a lysosomal storage disease with severe motor impairments. The mechanisms underlying MLIV are poorly understood and there is no treatment. Here, we report a Drosophila MLIV model, which recapitulates the key disease features, including abnormal intracellular...
The zonula adherens (ZA) is a specialized cadherin-based structure found at the contacts between epithelial cells. Meng et al. (2008) now identify a protein complex containing the microtubule minus-end-binding protein Nezha, which provides a critical link between microtubules and cadherins in ZA biogenesis and maintenance.
Loss- and gain-of-function mutations in the broadly expressed gene Lrp5 affect bone formation, causing osteoporosis and high bone mass, respectively. Although Lrp5 is viewed as a Wnt coreceptor, osteoblast-specific disruption of β-Catenin does not affect bone formation. Instead, we show here that Lrp5 inhibits expression of Tph1, the rate-limiting biosynthetic enzyme for serotonin in enterochromaffin...
N-acylphosphatidylethanolamines (NAPEs) are a relatively abundant group of plasma lipids of unknown physiological significance. Here, we show that NAPEs are secreted into circulation from the small intestine in response to ingested fat and that systemic administration of the most abundant circulating NAPE, at physiologic doses, decreases food intake in rats without causing conditioned taste aversion...
Disruption of protein folding in the endoplasmic reticulum (ER) causes unfolded proteins to accumulate, triggering the unfolded protein response (UPR). UPR outputs in turn decrease ER unfolded proteins to close a negative feedback loop. However, because it is infeasible to directly measure the concentration of unfolded proteins in vivo, cells are generically described as experiencing “ER stress” whenever...
Genomic instability and alterations in gene expression are hallmarks of eukaryotic aging. The yeast histone deacetylase Sir2 silences transcription and stabilizes repetitive DNA, but during aging or in response to a DNA break, the Sir complex relocalizes to sites of genomic instability, resulting in the desilencing of genes that cause sterility, a characteristic of yeast aging. Using embryonic stem...
The ability to evolve is a fundamental feature of biological systems, but the mechanisms underlying this capacity and the evolutionary dynamics of conserved core processes remain elusive. We show that yeast cells deleted of MYO1, encoding the only myosin II normally required for cytokinesis, rapidly evolved divergent pathways to restore growth and cytokinesis. The evolved cytokinesis phenotypes correlated...
Of the myriad alterations in gene copy number found in cancer cells, which alterations are critical for the cancer phenotype? In this issue of Cell, Zender et al. (2008) describe an integrative genomics approach to identify new tumor suppressor genes involved in hepatocellular carcinoma.
Balancing the capacity for protein maturation with changes in protein flux through the endoplasmic reticulum (ER) is crucial for maintaining ER homeostasis. In this issue, Merksamer et al. (2008) exploit a redox-sensitive fluorescent protein to monitor the environment inside the ER of living yeast, illuminating how this organelle responds to different perturbations.
During mitosis, sister chromatids congress to the spindle equator and are subsequently segregated via attachment to dynamic kinetochore microtubule (kMT) plus ends. A major question is how kMT plus-end assembly is spatially regulated to achieve chromosome congression. Here we find in budding yeast that the widely conserved kinesin-5 sliding motor proteins, Cin8p and Kip1p, mediate chromosome congression...
Epithelial cells contain noncentrosomal microtubules (MTs), whose minus ends are oriented apically. In contrast with the well-known interactions of the minus ends with the centrosome, little is known about the termination site of the noncentrosomal minus ends. Here we show that a population of MT minus ends is anchored at the zonula adherens (ZA), the apical-most part of the cadherin-based adherens...
Cancers are highly heterogeneous and contain many passenger and driver mutations. To functionally identify tumor suppressor genes relevant to human cancer, we compiled pools of short hairpin RNAs (shRNAs) targeting the mouse orthologs of genes recurrently deleted in a series of human hepatocellular carcinomas and tested their ability to promote tumorigenesis in a mosaic mouse model. In contrast to...
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