Impaired Expression of Rearranged Immunoglobulin Genes and Premature p53 Activation Block B Cell Development in BMI1 Null Mice.

B cell development is a highly regulated process that requires stepwise rearrangement of immunoglobulin genes to generate a functional B cell receptor (BCR). The polycomb group protein BMI1 is required for B cell development, but its function in developing B cells remains poorly defined. We demonstrate that BMI1 functions in a cell-autonomous manner at two stages during early B cell development. First, loss of BMI1 results in a differentiation block at the pro-B cell to pre-B cell transition due to the inability of BMI1-deficient cells to transcribe newly rearranged Igh genes. Accordingly, introduction of a pre-rearranged Igh allele partially restored B cell development in Bmi1-/- mice. In addition, BMI1 is required to prevent premature p53 signaling, and as a consequence, Bmi1-/- large pre-B cells fail to properly proliferate. Altogether, our results clarify the role of BMI1 in early B cell development and uncover an unexpected function of BMI1 during VDJ recombination.

Transcriptional repression of Sin3B by Bmi-1 prevents cellular senescence and is relieved by oncogene activation.

The Polycomb group protein Bmi-1 is an essential regulator of cellular senescence and is believed to function largely through the direct repression of the Ink4a/Arf locus. However, concurrent deletion of Ink4a/Arf does not fully rescue the defects detected in Bmi-1(-/-) mice, indicating that additional Bmi-1 targets remain to be identified. The expression of the chromatin-associated Sin3B protein is stimulated by oncogenic stress, and is required for oncogene-induced senescence. Here we demonstrate that oncogenic stress leads to the dissociation of Bmi-1 from the Sin3B locus, resulting in increased Sin3B expression and subsequent entry into cellular senescence. Furthermore, Sin3B is required for the senescent phenotype and elevated levels of reactive oxygen species elicited upon Bmi-1 depletion. Altogether, these results identify Sin3B as a novel direct target of Bmi-1, and establish Bmi-1-driven repression of Sin3B as an essential regulator of cellular senescence.

Ras-induced senescence and its physiological relevance in cancer.

Activated oncogenes like Ras have traditionally been thought as promoting unrestrained proliferation; therefore, the concept of oncogene-induced senescence has been, and still is, controversial. The counter-intuitive notion that activation of oncogenes leads to the prevention of cellular proliferation has initially been fueled by in vitro studies using ectopic expression of activated Ras in primary fibroblasts. While these initial studies demonstrated unambiguously the existence of a new type of cellular senescence, induced by oncogenes in an ex-vivo system, questions were raised about the physiological relevance of this process. Indeed, recent technical advances in mouse modeling for cancer have suggested that the occurrence of Ras-induced senescence is highly dependent on the cellular context, as well as the level of expression of activated Ras, and may not be pertinent to the study of human cancer initiation and/or progression. However, our increased knowledge of the molecular basis for cellular senescence has led to a better understanding of the molecular events modulating cancer progression in vivo. Recent studies have not only clearly established the incidence of cellular senescence in pre-neoplasic lesions, but also its role as a potential tumor-suppressor mechanism in vivo. Here, we review the recent and exciting new findings regarding the physiological relevance of Ras-induced senescence, and discuss their implications in terms of cancer therapy.

Sin3B expression is required for cellular senescence and is up-regulated upon oncogenic stress.

Serial passage of primary mammalian cells or strong mitogenic signals induce a permanent exit from the cell cycle called senescence. A characteristic of senescent cells is the heterochromatinization of loci encoding pro-proliferative genes, leading to their transcriptional silencing. Senescence is thought to represent a defense mechanism against uncontrolled proliferation and cancer. Consequently, genetic alterations that allow senescence bypass are associated with susceptibility to oncogenic transformation. We show that fibroblasts genetically inactivated for the chromatin-associated Sin3B protein are refractory to replicative and oncogene-induced senescence. Conversely, overexpression of Sin3B triggers senescence and the formation of senescence-associated heterochromatic foci. Although Sin3B is strongly up-regulated upon oncogenic stress, decrease in expression of Sin3B is associated with tumor progression in vivo, suggesting that expression of Sin3B may represent a barrier against transformation. Together, these results underscore the contribution of senescence in tumor suppression and suggest that expression of chromatin modifiers is modulated at specific stages of cellular transformation. Consequently, these findings suggest that modulation of Sin3B-associated activities may represent new therapeutic opportunities for treatment of cancers.

Chromatin modifications: the driving force of senescence and aging?

An emerging field of investigation in the search for treatment of human disease is the modulation of chromatin modifications. Chromatin modifications impart virtually all processes occurring in the mammalian nucleus, from regulation of transcription to genomic stability and nuclear high order organization. It has been well recognized that, as the mammalian cell ages, its chromatin structure evolves, both at a global level and at specific loci. While these observations are mostly correlative, recent technical developments allowing loss-of-function experiments and genome-wide approaches have permitted the identification of a causal relationship between specific changes in chromatin structure and the aging phenotype. Here we review the evidence pointing to the modulation of chromatin structure as a potential driving force of cellular aging in mammals.

Expansion and intragenic homogenization of spider silk genes since the Triassic: evidence from Mygalomorphae (tarantulas and their kin) spidroins.

Spiders spin a diverse array of silk fibers that are predominately composed of repetitive proteins (spidroins) encoded by a gene family. Characterization of this gene family has focused on spidroins synthesized by the Araneomorphae (true spiders), whereas only a single sequence is known from the Mygalomorphae (tarantulas and their kin). To better understand the diversity and evolution of the spidroin gene family, we surveyed the silk gland transcriptomes of 4 divergent mygalomorph species. Through expressed sequence tag screening and probing of silk gland cDNA libraries, we discovered 6 novel mygalomorph spidroins and an approximately 8-kb cDNA of the previously reported Euagrus chisoseus fibroin 1. Mygalomorph spidroin cDNAs encode tandem iterations of sequence repeats, followed by a nonrepetitive carboxy-terminal domain. Though highly homogenized at the nucleotide level within a cDNA (89-100% identical), these repeats exhibit extensive variation across spidroins, consistent with intragenic repeats evolving in concert. Extreme homogeneity of intragenic repeats is also characteristic of araneomorph spidroins, suggesting that modular architecture and its maintenance through concerted evolution have persisted since the mygalomorph/araneomorph split (> or =240 MYA). Phylogenetic analyses of C-terminal sequences grouped all mygalomorph spidroins, except Aliatypus fibroin 1, in a clade. Aliatypus fibroin 1 was instead more closely related to a subset of araneomorph spidroins, including those used in prey wrapping. Our results suggest that spidroin paralogs existed prior to the divergence of mygalomorphs and araneomorphs, followed by a far greater expansion of this gene family in araneomorphs, paralleling the dramatic functional diversification of their silk gland anatomy.

Silk genes support the single origin of orb webs.

The orb web is a spectacular evolutionary innovation that enables spiders to catch flying prey. This elegant, geometric structure is woven with silk fibers that are renowned for their superior mechanical properties. We used silk gland expression libraries to address a long-standing controversy concerning the evolution of the orb-web architecture. Contrary to the view that the orb-web design evolved multiple times, we found that the distribution and phylogeny of silk proteins support a single, ancient origin of the orb web at least 136 million years ago. Furthermore, we substantially expanded the repository of silk sequences that can be used for the synthesis of high-performance biomaterials.