Getting off to a good start.

RNA polymerase II transcription can efficiently occur when mouse embryonic stem cells lack TBP and TBP-like proteins, confirming that this initiation factor may not be as essential as once thought.

Single cell transcriptomics reveal trans-differentiation of pancreatic beta cells following inactivation of the TFIID subunit Taf4.

Regulation of gene expression involves a complex and dynamic dialogue between transcription factors, chromatin remodelling and modification complexes and the basal transcription machinery. To address the function of the Taf4 subunit of general transcription factor TFIID in the regulation of insulin signalling, it was inactivated in adult murine pancreatic beta cells. Taf4 inactivation impacted the expression of critical genes involved in beta-cell function leading to increased glycaemia, lowered plasma insulin levels and defective glucose-stimulated insulin secretion. One week after Taf4-loss, single-cell RNA-seq revealed cells with mixed beta cell, alpha and/or delta cell identities as well as a beta cell population trans-differentiating into alpha-like cells. Computational analysis of single-cell RNA-seq defines how known critical beta cell and alpha cell determinants may act in combination with additional transcription factors and the NuRF chromatin remodelling complex to promote beta cell trans-differentiation.

Hypomorphic Pathogenic Variants in TAF13 Are Associated with Autosomal-Recessive Intellectual Disability and Microcephaly.

In two independent consanguineous families each with two children affected by mild intellectual disability and microcephaly, we identified two homozygous missense variants (c.119T>A [p.Met40Lys] and c.92T>A [p.Leu31His]) in TATA-box-binding-protein-associated factor 13 (TAF13). Molecular modeling suggested a pathogenic effect of both variants through disruption of the interaction between TAF13 and TAF11. These two proteins form a histone-like heterodimer that is essential for their recruitment into the general RNA polymerase II transcription factor IID (TFIID) complex. Co-immunoprecipitation in HeLa cells transfected with plasmids encoding TAF11 and TAF13 revealed that both variants indeed impaired formation of the TAF13-TAF11 heterodimer, thus confirming the protein modeling analysis. To further understand the functional role of TAF13, we performed RNA sequencing of neuroblastoma cell lines upon TAF13 knockdown. The transcriptional profile showed significant deregulation of gene expression patterns with an emphasis on genes related to neuronal and skeletal functions and those containing E-box motives in their promoters. Here, we expand the spectrum of TAF-associated phenotypes and highlight the importance of TAF13 in neuronal functions.

TRF2 is recruited to the pre-initiation complex as a testis-specific subunit of TFIIA/ALF to promote haploid cell gene expression.

Mammalian genomes encode two genes related to the TATA-box binding protein (TBP), TBP-related factors 2 and 3 (TRF2 and TRF3). Male Trf2(-/-) mice are sterile and characterized by arrested spermatogenesis at the transition from late haploid spermatids to early elongating spermatids. Despite this characterization, the molecular function of murine Trf2 remains poorly characterized and no direct evidence exists to show that it acts as a bona fide chromatin-bound transcription factor. We show here that Trf2 forms a stable complex with TFIIA or the testis expressed paralogue ALF chaperoned in the cytoplasm by heat shock proteins. We demonstrate for the first time that Trf2 is recruited to active haploid cell promoters together with Tbp, Taf7l and RNA polymerase II. RNA-seq analysis identifies a set of genes activated in haploid spermatids during the first wave of spermatogenesis whose expression is down-regulated by Trf2 inactivation. We therefore propose that Trf2 is recruited to the preinitiation complex as a testis-specific subunit of TFIIA/ALF that cooperates with Tbp and Taf7l to promote haploid cell gene expression.

Essential role of the TFIID subunit TAF4 in murine embryogenesis and embryonic stem cell differentiation.

TAF4 (TATA-binding protein-associated factor 4) and its paralogue TAF4b are components of the TFIID core module. We inactivated the murine Taf4a gene to address Taf4 function during embryogenesis. Here we show that Taf4a(-/-) embryos survive until E9.5 where primary germ layers and many embryonic structures are identified showing Taf4 is dispensable for their specification. In contrast, Taf4 is required for correct patterning of the trunk and anterior structures, ventral morphogenesis and proper heart positioning. Overlapping expression of Taf4a and Taf4b during embryogenesis suggests their redundancy at early stages. In agreement with this, Taf4a(-/-) embryonic stem cells (ESCs) are viable and comprise Taf4b-containing TFIID. Nevertheless, Taf4a(-/-) ESCs do not complete differentiation into glutamatergic neurons and cardiomyocytes in vitro due to impaired preinitiation complex formation at the promoters of critical differentiation genes. We define an essential role of a core TFIID TAF in differentiation events during mammalian embryogenesis.

TAF4 inactivation reveals the 3 dimensional growth promoting activities of collagen 6A3.

Collagen 6A3 (Col6a3), a component of extracellular matrix, is often up-regulated in tumours and is believed to play a pro-oncogenic role. However the mechanisms of its tumorigenic activity are poorly understood. We show here that Col6a3 is highly expressed in densely growing mouse embryonic fibroblasts (MEFs). In MEFs where the TAF4 subunit of general transcription factor IID (TFIID) has been inactivated, elevated Col6a3 expression prevents contact inhibition promoting their 3 dimensional growth as foci and fibrospheres. Analyses of gene expression in densely growing Taf4(-/-) MEFs revealed repression of the Hippo pathway and activation of Wnt signalling. The Hippo activator Kibra/Wwc1 is repressed under dense conditions in Taf4(-/-) MEFs, leading to nuclear accumulation of the proliferation factor YAP1 in the cells forming 3D foci. At the same time, Wnt9a is activated and the Sfrp2 antagonist of Wnt signalling is repressed. Surprisingly, treatment of Taf4(-/-) MEFs with all-trans retinoic acid (ATRA) restores contact inhibition suppressing 3D growth. ATRA represses Col6a3 expression independently of TAF4 expression and Col6a3 silencing is sufficient to restore contact inhibition in Taf4(-/-) MEFs and to suppress 3D growth by reactivating Kibra expression to induce Hippo signalling and by inducing Sfrp2 expression to antagonize Wnt signalling. All together, these results reveal a critical role for Col6a3 in regulating both Hippo and Wnt signalling to promote 3D growth, and show that the TFIID subunit TAF4 is essential to restrain the growth promoting properties of Col6a3. Our data provide new insight into the role of extra cellular matrix components in regulating cell growth.

Trim24-repressed VL30 retrotransposons regulate gene expression by producing noncoding RNA.

Trim24 (Tif1α) and Trim33 (Tif1γ) interact to form a co-repressor complex that suppresses murine hepatocellular carcinoma. Here we show that Trim24 and Trim33 cooperatively repress retinoic acid receptor-dependent activity of VL30-class endogenous retroviruses (ERVs) in liver. In Trim24-knockout hepatocytes, VL30 derepression leads to accumulation of reverse-transcribed VL30 cDNA in the cytoplasm that correlates with activation of the viral-defense interferon responses mimicking the preneoplastic inflammatory state seen in human liver following exogenous viral infection. Furthermore, upon derepression, VL30 long terminal repeats (LTRs) act as promoter and enhancer elements deregulating expression of neighboring genes and generating enhancer RNAs that are required for LTR enhancer activity in hepatocytes in vivo. These data reinforce the role of the TRIM family of proteins in retroviral restriction and antiviral defense and provide an example of an ERV-derived oncogenic regulatory network.

Interconversion between active and inactive TATA-binding protein transcription complexes in the mouse genome.

The TATA binding protein (TBP) plays a pivotal role in RNA polymerase II (Pol II) transcription through incorporation into the TFIID and B-TFIID complexes. The role of mammalian B-TFIID composed of TBP and B-TAF1 is poorly understood. Using a complementation system in genetically modified mouse cells where endogenous TBP can be conditionally inactivated and replaced by exogenous mutant TBP coupled to tandem affinity purification and mass spectrometry, we identify two TBP mutations, R188E and K243E, that disrupt the TBP-BTAF1 interaction and B-TFIID complex formation. Transcriptome and ChIP-seq analyses show that loss of B-TFIID does not generally alter gene expression or genomic distribution of TBP, but positively or negatively affects TBP and/or Pol II recruitment to a subset of promoters. We identify promoters where wild-type TBP assembles a partial inactive preinitiation complex comprising B-TFIID, TFIIB and Mediator complex, but lacking TFIID, TFIIE and Pol II. Exchange of B-TFIID in wild-type cells for TFIID in R188E and K243E mutant cells at these primed promoters completes preinitiation complex formation and recruits Pol II to activate their expression. We propose a novel regulatory mechanism involving formation of a partial preinitiation complex comprising B-TFIID that primes the promoter for productive preinitiation complex formation in mammalian cells.

Cell-specific occupancy of an extended repertoire of CREM and CREB binding loci in male germ cells.

BACKGROUND: CREB and CREM are closely related factors that regulate transcription in response to various stress, metabolic and developmental signals. The CREMτ activator isoform is selectively expressed in haploid spermatids and plays an essential role in murine spermiogenesis. RESULTS: We have used chromatin immunoprecipitation coupled to sequencing (ChIP-seq) to map CREM and CREB target loci in round spermatids from adult mouse testis and spermatogonia derived GC1-spg cells respectively. We identify more than 9000 genomic loci most of which are cell-specifically occupied. Despite the fact that round spermatids correspond to a highly specialised differentiated state, our results show that they have a remarkably accessible chromatin environment as CREM occupies more than 6700 target loci corresponding not only to the promoters of genes selectively expressed in spermiogenesis, but also of genes involved in functions specific to other cell types. The expression of only a small subset of these target genes are affected in the round spermatids of CREM knockout animals. We also identify a set of intergenic binding loci some of which are associated with H3K4 trimethylation and elongating RNA polymerase II suggesting the existence of novel CREB and CREM regulated transcripts. CONCLUSIONS: We demonstrate that CREM and CREB occupy a large number of promoters in highly cell specific manner. This is the first study of CREM target promoters directly in a physiologically relevant tissue in vivo and represents the most comprehensive experimental analysis of CREB/CREM regulatory potential to date.

HMGB4, a novel member of the HMGB family, is preferentially expressed in the mouse testis and localizes to the basal pole of elongating spermatids.

We identified HMGB4, a novel member of the HMGB family lacking the acidic tail typically found in this family. HMGB4 is strongly and preferentially expressed in the adult mouse testis and weakly in the brain, but not in many other tissues. HMGB4 associates with chromatin, and in transfection assays, in contrast to HMGB1, it acts as a potent transcriptional repressor. During spermatogenesis, HMGB4 is present in the euchromatin of late pachytene spermatocytes and haploid round spermatids, whereas stronger expression is observed during the elongation phase, where it localizes to the basal pole of the nucleus in a manner mutually exclusive with H1FNT (H1T2) localized at the apical pole. HMGB4 basal localization is lost in H1FNT-mutant spermatids, showing that H1FNT provides a positional cue for organizing chromatin domains within the nucleus. These results show that HMGB4 and H1FNT specify distinct chromatin domains at the apical and basal poles of the elongating spermatid nucleus.

Specialization of the general transcriptional machinery in male germ cells.

In adult animals, spermatogenesis involves a continuous differentiation of the spermatogonial stem and progenitor cell population into mature sperm. A unique aspect of this developmental process is the germ cell-specific expression and function of paralogues of components of the general transcription machinery, notably subunits of TFIID. Genetic and biochemical studies show that these paralogues play critical, but mechanistically distinct roles in Drosophila and mouse spermatogenesis.

Repression of the human dihydrofolate reductase gene by a non-coding interfering transcript.

Alternative promoters within the same gene are a general phenomenon in gene expression. Mechanisms of their selective regulation vary from one gene to another and are still poorly understood. Here we show that in quiescent cells the mechanism of transcriptional repression of the major promoter of the gene encoding dihydrofolate reductase depends on a non-coding transcript initiated from the upstream minor promoter and involves both the direct interaction of the RNA and promoter-specific interference. The specificity and efficiency of repression is ensured by the formation of a stable complex between non-coding RNA and the major promoter, direct interaction of the non-coding RNA with the general transcription factor IIB and dissociation of the preinitiation complex from the major promoter. By using in vivo and in vitro assays such as inducible and reconstituted transcription, RNA bandshifts, RNA interference, chromatin immunoprecipitation and RNA immunoprecipitation, we show that the regulatory transcript produced from the minor promoter has a critical function in an epigenetic mechanism of promoter-specific transcriptional repression.

Proteolytic cleavage of ALF into alpha- and beta-subunits that form homologous and heterologous complexes with somatic TFIIA and TRF2 in male germ cells.

Male germ cells specifically express paralogues of components of the general transcription apparatus including ALF a paralogue of TFIIAalpha/beta. We show that endogenous ALF is proteolytically cleaved to give alpha- and beta-subunits and we map the proteolytic cleavage site by mass spectrometry. Immunoprecipitations show that ALFalpha- and beta-subunits form a series of homologous and heterologous complexes with somatic TFIIA which is coexpressed in male germ cells. In addition, we show that ALF is coexpressed in late pachytene spermatocytes and in haploid round spermatids with transcription factor TRF2, and that these proteins form stable complexes in testis extracts. Our observations highlight how cleavage of ALF and coexpression with TFIIA and TRF2 increases the combinatorial possibilities for gene regulation at different developmental stages of spermatogenesis.

Polar nuclear localization of H1T2, a histone H1 variant, required for spermatid elongation and DNA condensation during spermiogenesis.

Spermiogenesis entails a major biochemical and morphological restructuring of the germ cell involving replacement of the somatic histones by protamines packing the DNA into the condensed spermatid nucleus and elimination of the cytoplasm during the elongation phase. We describe H1T2, an histone H1 variant selectively and transiently expressed in male haploid germ cells during spermiogenesis. In round and elongating spermatids, H1T2 specifically localizes to a chromatin domain at the apical pole, revealing a polarity in the spermatid nucleus. Inactivation by homologous recombination shows that H1T2 is critical for spermiogenesis as male H1t2(-/-) mice have greatly reduced fertility. Analysis of spermiogenesis in H1t2 mutant mice shows delayed nuclear condensation and aberrant elongation. As a result, mutant spermatids are characterized by the presence of residual cytoplasm, acrosome detachment, and fragmented DNA. Hence, H1T2 is a protein required for proper cell restructuring and DNA condensation during the elongation phase of spermiogenesis.

Autocatalytic RNA cleavage in the human beta-globin pre-mRNA promotes transcription termination.

New evidence indicates that termination of transcription is an important regulatory step, closely related to transcriptional interference and even transcriptional initiation. However, how this occurs is poorly understood. Recently, in vivo analysis of transcriptional termination for the human beta-globin gene revealed a new phenomenon--co-transcriptional cleavage (CoTC). This primary cleavage event within beta-globin pre-messenger RNA, downstream of the poly(A) site, is critical for efficient transcriptional termination by RNA polymerase II. Here we show that the CoTC process in the human beta-globin gene involves an RNA self-cleaving activity. We characterize the autocatalytic core of the CoTC ribozyme and show its functional role in efficient termination in vivo. The identified core CoTC is highly conserved in the 3' flanking regions of other primate beta-globin genes. Functionally, it resembles the 3' processive, self-cleaving ribozymes described for the protein-encoding genes from the myxomycetes Didymium iridis and Physarum polycephalum, indicating evolutionary conservation of this molecular process. We predict that regulated autocatalytic cleavage elements within pre-mRNAs may be a general phenomenon and that functionally it may provide the entry point for exonucleases involved in mRNA maturation, turnover and, in particular, transcriptional termination.

Cell-specific nucleolar localization of TBP-related factor 2.

TATA-binding protein (TBP)-related factor 2 (TRF2) is one of four closely related RNA polymerase II transcription factors. We compared the intracellular localizations of TBP and TRF2 during the cell cycle and mitosis in HeLa cells. We show that during interphase, endogenous or exogenously expressed TRF2 is located almost exclusively in the nucleolus in HeLa or Cos cells. TRF2 localization is not affected by stress or mitotic stimuli, but TRF2 is rapidly released from the nucleolus upon inhibition of pol I transcription or treatment by RNase. These results suggest that localization of HeLa TRF2 requires a nucleolar-associated RNA species. In contrast, in 3T3 fibroblast cells, exogenously expressed TRF2 localizes to the nucleoplasm. Constitutive expression of ectopic TRF2 in 3T3 cells leads to a prolonged S phase of the cell cycle and reduced proliferation. Together with previous data, our results highlight the cell-specific localization and functions of TRF2. Furthermore, we show that during cell division, HeLa TRF2 and TBP are localized in the mitotic cytoplasm and TRF2 relocalizes into the nascent nucleoli immediately after mitosis, whereas TBP reassociates with the chromatin. Although partially contradictory results have been reported, our data are consistent with a model where only small proportion of the cellular TBP remains associated with specific promoter loci during mitosis.

[TBP, a universal transcription factor?]. / TBP, un facteur de transcription universel ?

The TATA binding protein (TBP) is a subunit of several macromolecular complexes required for transcription by the three nuclear RNA polymerases. This observation led to the idea that TBP is a "universal" transcription factor. The discovery of three TBP-related factors and a macromolecular complex which lacks TBP but can support RNA polymerase II transcription in vitro has led to a reappraisal of the universal character of TBP. Several in vivo studies have rather shown that TBP plays a specific role in the activation of a subset of cellular genes controlling the cell cycle. In mammals, the aminoterminal region of TBP plays a highly selective role in the maternal immunotolerance of pregnancy.

RNA polymerase II transcription in murine cells lacking the TATA binding protein.

Inactivation of the murine TATA binding protein (TBP) gene by homologous recombination leads to growth arrest and apoptosis at the embryonic blastocyst stage. However, after loss of TBP, RNA polymerase II (pol II) remains in a transcriptionally active phosphorylation state, and in situ run-on experiments showed high levels of pol II transcription comparable to those of wild-type cells. In contrast, pol I and pol III transcription was arrested. Our results show a differential dependency of the RNA polymerases on TBP and provide evidence for TBP-independent pol II transcriptional mechanisms that allow reinitiation and maintenance of gene transcription in vivo.

Distinct functions of TBP and TLF/TRF2 during spermatogenesis: requirement of TLF for heterochromatic chromocenter formation in haploid round spermatids.

TLF (TBP-like factor) is a protein commonly thought to belong to the general transcription initiation complex. TLF is evolutionarily conserved and has been shown to be essential for early development in C. elegans, zebrafish and Xenopus. In mammals however, TLF has a specialised function, as revealed by targeted mutation of the gene in the mouse germline. The TLF mutation elicits a complete arrest of late spermiogenesis and increased haploid cell apoptosis. We explored in more detail the molecular function that TLF plays in the differentiation program of male germ cells. A comparison of TBP and TLF reveals drastic differences, both in their temporal expression pattern and in their intracellular location. While TBP is ubiquitously expressed, TLF expression is strictly developmentally regulated, being very high in late pachytene spermatocytes, suggesting a function prior to the apoptosis of the haploid cells. A refined study of TLF-deficient mice reveals defective acrosome formation in early stage spermatids. Most importantly, our results uncover an unsuspected function of TLF in chromatin organisation. Indeed, early spermatids in TLF-deficient mice display a fragmentation of the chromocenter, a condensed structure formed by the association of centromeric heterochromatin and containing the HP1 proteins. This defect is likely to be the primary cause of spermatogenic failure in the TLF mutant mice.