Bright/Arid3a has been characterized both as an activator of immunoglobulin heavy-chain transcription and as a proto-oncogene. Although Bright expression is highly B lineage stage restricted in adult mice, its expression in the earliest identifiable hematopoietic stem cell (HSC) population suggests that Bright might have additional functions. We showed that >99% of Bright ؊/؊ embryos die at midgestation from failed hematopoiesis. Bright ؊/؊ embryonic day 12.5 (E12.5) fetal livers showed an increase in the expression of immature markers. Colony-forming assays indicated that the hematopoietic potential of Bright ؊/؊ mice is markedly reduced. Rare survivors of lethality, which were not compensated by the closely related paralogue Bright-derived protein (Bdp)/Arid3b, suffered HSC deficits in their bone marrow as well as B lineage-intrinsic developmental and functional deficiencies in their peripheries. These include a reduction in a natural antibody, B-1 responses to phosphocholine, and selective T-dependent impairment of IgG1 class switching. Our results place Bright/Arid3a on a select list of transcriptional regulators required to program both HSC and lineage-specific differentiation.The formation and maintenance of blood throughout fetal and adult life rely on the self-renewal of hematopoietic stem cells (HSCs). Rare HSCs arise in the embryonic yolk sac and aorta-gonad mesonephros AGM, seed the fetal liver, and then circulate in the bone marrow of adult mammals. Fetal and adult HSC progenitors become progressively dedicated to differentiation into erythrocytes, myeloid cells, and lymphocytes. Transcription factors critical for the specification and formation of HSCs cover a wide range of DNA binding protein families. An emerging theme is that many of these same regulators are required later for the differentiation of individual blood lineages, which explains why a number of HSC transcription factors were discovered and originally characterized because of their deregulation in hematopoietic malignancies.Bright/Arid3a/Dril1 is the founder of the AT-rich interaction domain (ARID) superfamily of DNA binding proteins (18,60). Bright, in a complex with Bruton's tyrosine kinase (Btk) and TFII-I, binds to specific AT-rich motifs within the nuclearmatrix attachment regions (MARs) of the immunoglobulin heavy-chain (IgH) intronic enhancer (E) and selected IgH promoters to activate IgH transcription (18,25,30,43,44,55,57,58). B cell-specific, transgenic overexpression of Bright leads to partial blocks at both the late-pre-B and T1 immature stages, skewed marginal-zone (MZ) B cell development, increased natural IgM antibody production, and intrinsic autoimmunity (49). Transgenic dominant negative (DN) inhibition of Bright DNA binding results in reduced levels of IgM in serum and functional perturbation of IgM secretion by B-1 cells (39,48). A small pool of Bright cycles from the nucleus into plasma membrane lipid rafts, where it associates with Btk to dampen antigen receptor signaling (48).While highly B lineage restricted in...

Mutations in the Huntington locus (htt) have devastating consequences. Gain-of-poly-Q repeats in Htt protein causes Huntington's disease (HD), while htt-/- mutants display early embryonic lethality. Despite its importance, the function of Htt remains elusive. To address this, we compared more than 3,700 compounds in three syngeneic mouse embryonic stem cell (mESC) lines: htt-/-, extended poly-Q (Htt-Q140/7), and wildtype mESCs (Htt-Q7/7) using untargeted metabolite profiling. While Htt-Q140/7 cells, did not show major differences in cellular bioenergetics, we find extensive metabolic aberrations in htt-/- mESCs, including: (i) complete failure of ATP production despite preservation of the mitochondrial membrane potential; (ii) near-maximal glycolysis, with little or no glycolytic reserve; (iii) marked ketogenesis; (iv) depletion of intracellular NTPs; (v) accelerated purine biosynthesis and salvage; and (vi) loss of mitochondrial structural integrity. Together, our findings reveal that Htt is necessary for mitochondrial structure and function from the earliest stages of embryogenesis, providing a molecular explanation for htt-/- early embryonic lethality.

Despite their origin from the inner cell mass, embryonic stem (ES) cells undergo differentiation to the trophectoderm (TE) lineage by repression of the ES cell master regulator Oct4 or activation of the TE master regulator Caudal-type homeobox 2 (Cdx2). In contrast to the in-depth studies of ES cell self-renewal and pluripotency, few TE-specific regulators have been identified, thereby limiting our understanding of mechanisms underlying the first cell fate decision. Here we show that up-regulation and nuclear entry of AT-rich interactive domain 3a (Arid3a) drives TE-like transcriptional programs in ES cells, maintains trophoblast stem (TS) cell selfrenewal, and promotes further trophoblastic differentiation both upstream and independent of Cdx2. Accordingly, Arid3a-/-mouse post-implantation placental development is severely impaired, resulting in early embryonic death. We provide evidence that Arid3a directly activates TE-specific and trophoblast lineage-specific genes while directly repressing pluripotency genes via differential regulation of epigenetic acetylation or deacetylation. Our results identify Arid3a as a critical regulator of TE and placental development through execution of the commitment and differentiation phases of the first cell fate decision.

SummaryWe show here that singular loss of the Bright/Arid3A transcription factor leads to reprograming of mouse embryonic fibroblasts (MEFs) and enhancement of standard four-factor (4F) reprogramming. Bright-deficient MEFs bypass senescence and, under standard embryonic stem cell (ESC) culture conditions, spontaneously form clones that in vitro express pluripotency markers, differentiate to all germ lineages, and in vivo form teratomas and chimeric mice. We demonstrate that BRIGHT binds directly to the promoter/enhancer regions of Oct4, Sox2, and Nanog to contribute to their repression in both MEFs and ESCs. Thus, elimination of the BRIGHT barrier may provide an approach for somatic cell reprogramming.

Huntington’s disease (HD) is a devastating neurological disorder that is caused by an expansion of the poly-Q tract in exon 1 of the Huntingtin gene (HTT). HTT is an evolutionarily conserved and ubiquitously expressed protein that has been linked to a variety of functions including transcriptional regulation, mitochondrial function, and vesicle transport. This large protein has numerous caspase and calpain cleavage sites and can be decorated with several post-translational modifications such as phosphorylations, acetylations, sumoylations, and palmitoylations. However, the exact function of HTT and the role played by its modifications in the cell are still not well understood. Scrutiny of HTT function has been focused on a single, full length mRNA. In this study, we report the discovery of 5 novel HTT mRNA splice isoforms that are expressed in normal and HTT-expanded human embryonic stem cell (hESC) lines as well as in cortical neurons differentiated from hESCs. Interestingly, none of the novel isoforms generates a truncated protein. Instead, 4 of the 5 new isoforms specifically eliminate domains and modifications to generate smaller HTT proteins. The fifth novel isoform incorporates a previously unreported additional exon, dubbed 41b, which is hominid-specific and introduces a potential phosphorylation site in the protein. The discovery of this hominid-specific isoform may shed light on human-specific pathogenic mechanisms of HTT, which could not be investigated with current mouse models of the disease.