Sequence and epigenetic determinants in the regulation of the Math6 gene by Neurogenin3

Atoh8 in Development and Disease

Atoh8 belongs to a large superfamily of transcriptional regulators called basic helix-loop-helix (bHLH) proteins. bHLH proteins have been identified in a wide range of organisms from yeast to humans. The members of this special group of transcription factors were found to be involved not only in embryonic development but also in disease initiation and its progression. Given their importance in several fundamental processes, the translation, subcellular location and turnover of bHLH proteins is tightly regulated. Alterations in the expression of bHLH proteins have been associated with multiple diseases also in context with Atoh8 which seems to unfold its functions as both transcriptional activator and repressor. Like many other bHLH transcription factors, so far, Atoh8 has also been observed to be involved in both embryonic development and carcinogenesis where it mainly acts as tumor suppressor. This review summarizes our current understanding of Atoh8 structure, function and regulation and its complex and partially controversial involvement in development and disease.

DLX3 regulates osteogenic differentiation of bone marrow mesenchymal stem cells via Wnt/β-catenin pathway mediated histone methylation of DKK4

OBJECTIVE

Distal-less homeobox 3 (DLX3) is an important transcription factor involved in the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). However, the underlying mechanism is not clear. This study investigated the underlying mechanism of DLX3 in osteogenic differentiation.

METHODS

DLX3 overexpression and knockdown in cells were achieved using lentiviruses. The osteogenic differentiation of BMSCs was detected using alkaline phosphatase expression, alizarin red staining, real-time quantitative polymerase chain reaction (RT-qPCR), Western blotting, and chromatin immunoprecipitation (ChIP) assays.

RESULTS

DLX3 overexpression promoted the osteogenic differentiation of BMSCs, whereas DLX3 knockdown reduced the osteogenic differentiation of BMSCs. RT-qPCR and Western blotting assays showed that DLX3 modulated osteogenic differentiation via the Wnt/β-catenin pathway. ChIP-qPCR showed that DLX3 knockdown promoted DKK4 expression by decreasing the enrichment of histone H3 lysine 27 trimethylation (H3K27me3) in the promotor region of DKK4.

CONCLUSION

Our data implied that DLX3 regulated Wnt/β-catenin pathway through histone modification of DKK4 during the osteogenic differentiation of BMSCs.

Protein Methyltransferase Inhibition Decreases Endocrine Specification Through the Upregulation of Aldh1b1 Expression

Understanding the mechanisms that promote the specification of pancreas progenitors and regulate their self-renewal and differentiation will help to maintain and expand pancreas progenitor cells derived from human pluripotent stem (hPS) cells. This will improve the efficiency of current differentiation protocols of hPS cells into β-cells and bring such cells closer to clinical applications for the therapy of diabetes. Aldehyde dehydrogenase 1b1 (Aldh1b1) is a mitochondrial enzyme expressed specifically in progenitor cells during mouse pancreas development, and we have shown that its functional inactivation leads to accelerated differentiation and deficient β-cells. In this report, we aimed to identify small molecule inducers of Aldh1b1 expression taking advantage of a mouse embryonic stem (mES) cell Aldh1b1 lacZ reporter line and a pancreas differentiation protocol directing mES cells into pancreatic progenitors. We identified AMI-5, a protein methyltransferase inhibitor, as an Aldh1b1 inducer and showed that it can maintain Aldh1b1 expression in embryonic pancreas explants. This led to a selective reduction in endocrine specification. This effect was due to a downregulation of Ngn3, and it was mediated through Aldh1b1 since the effect was abolished in Aldh1b1 null pancreata. The findings implicated methyltransferase activity in the regulation of endocrine differentiation and showed that methyltransferases can act through specific regulators during pancreas differentiation. Stem Cells 2019;37:640-651.

Modulation of the endocrine transcriptional program by targeting histone modifiers of the H3K27me3 mark

Posttranscriptional modifications of histones constitute an epigenetic mechanism that is closely linked to both gene silencing and activation events. Trimethylation of Histone3 at lysine 27 (H3K27me3) is a repressive mark that associates with developmental gene regulation during differentiation programs. In the developing pancreas, expression of the transcription factor Neurogenin3 in multipotent progenitors initiates endocrine differentiation that culminates in the generation of all pancreatic islet cell lineages, including insulin-producing beta cells. Previously, we showed that Neurogenin3 promoted the removal of H3K27me3 marks at target gene promoters in vitro, suggesting a functional connection between this factor and regulators of this chromatin mark. In the present study, we aimed to specifically evaluate whether targeting the activity of these histone modifiers can be used to modulate pancreatic endocrine differentiation. Our data show that chemical inhibition of the H3K27me3 demethylases Jmjd3/Utx blunts Neurogenin3-dependent gene activation in vitro. Conversely, inhibition of the H3K27me3 methyltransferase Ezh2 enhances both the transactivation ability of Neurogenin3 in cultured cells and the formation of insulin-producing cells during directed differentiation from pluripotent cells. These results can help improve current protocols aimed at generating insulin-producing cells for beta cell replacement therapy in diabetes.

Late-stage differentiation of embryonic pancreatic β-cells requires Jarid2

Jarid2 is a component of the Polycomb Repressor complex 2 (PRC2), which is responsible for genome-wide H3K27me3 deposition, in embryonic stem cells. However, Jarid2 has also been shown to exert pleiotropic PRC2-independent actions during embryogenesis. Here, we have investigated the role of Jarid2 during pancreas development. Conditional ablation of Jarid2 in pancreatic progenitors results in reduced endocrine cell area at birth due to impaired endocrine cell differentiation and reduced prenatal proliferation. Inactivation of Jarid2 in endocrine progenitors demonstrates that Jarid2 functions after endocrine specification. Furthermore, genome-wide expression analysis reveals that Jarid2 is required for the complete activation of the insulin-producing β-cell differentiation program. Jarid2-deficient pancreases exhibit impaired deposition of RNAPII-Ser5P, the initiating form of RNAPII, but no changes in H3K27me3, at the promoters of affected endocrine genes. Thus, our study identifies Jarid2 as a fine-tuner of gene expression during late stages of pancreatic endocrine cell development. These findings are relevant for generation of transplantable stem cell-derived β-cells.

A novel interaction between ATOH8 and PPP3CB

ATOH8 is a bHLH transcription factor playing roles in a variety of developmental processes such as neurogenesis, differentiation of pancreatic precursor cells, development of kidney and muscle, and differentiation of endothelial cells. PPP3CB belongs to the catalytic subunit of the serine/threonine phosphatase, calcineurin, which can dephosphorylate its substrate proteins to regulate their physiological activities. In our study, we demonstrated that ATOH8 interacts with PPP3CB in vitro with different approaches. We show that the conserved catalytic domain of PPP3CB interacts with both the N-terminus and the bHLH domain of ATOH8. Although the interaction domain of PPP3CB is conserved among all isoforms of calcineurin A, ATOH8 selectively interacts with PPP3CB instead of PPP3CA, probably due to the unique proline-rich region present in the N-terminus of PPP3CB, which controls the specificity of its interaction partners. Furthermore, we show that inhibition of the interaction with calcineurin inhibitor, cyclosporin A (CsA), leads to the retention of ATOH8 to the cytoplasm, suggesting that the interaction renders nuclear localization of ATOH8 which may be critical to control its activity as transcription factor.

Characterization of the transcriptional activity of the basic helix-loop-helix (bHLH) transcription factor Atoh8

The atonal-related Neurogenin/NeuroD family of basic helix-loop-helix (bHLH) transcription factors comprises potent inducers of neuronal and endocrine differentiation programs in the nervous and digestive system. Atonal homolog 8 (Atoh8) displays high similarity in the bHLH domain with NeuroD proteins. Yet, available evidences indicate that Atoh8 has distinctive features including a ubiquitous expression pattern in embryonic tissues and the ability to inhibit differentiation. To gain insights into Atoh8 function, we aimed at identifying Atoh8 targets and investigated the effects of Atoh8 on global gene expression patterns in pancreatic mPAC cells, a model of bHLH-dependent endocrine differentiation. Our data reveal that Atoh8 is a weak transcriptional activator and does not exhibit proendocrine activity. Conversely, it blocks the induction of a reduced group of gene targets of the atonal-related proendocrine factor Neurogenin3. We show that Atoh8 lacks a transactivation domain and possesses intrinsic repressor activity that depends on a conserved Proline-rich domain. Atoh8 binds the ubiquitous E protein E47 and its ability to repress transcription may partly result from its ability to inhibit E47/E47 and Neurogenin3/E47 dimer activities. These results reveal distinctive transcriptional properties of Atoh8 within the atonal-related bHLH family that may be associated with the acquisition of new biological functions.

Neurogenin3 cooperates with Foxa2 to autoactivate its own expression

The transcription factor Neurogenin3 functions as a master regulator of endocrine pancreas formation, and its deficiency leads to the development of diabetes in humans and mice. In the embryonic pancreas, Neurogenin3 is transiently expressed at high levels for a narrow time window to initiate endocrine differentiation in scattered progenitor cells. The mechanisms controlling these rapid and robust changes in Neurogenin3 expression are poorly understood. In this study, we characterize a Neurogenin3 positive autoregulatory loop whereby this factor may rapidly induce its own levels. We show that Neurogenin3 binds to a conserved upstream fragment of its own gene, inducing deposition of active chromatin marks and the activation of Neurog3 transcription. Additionally, we show that the broadly expressed endodermal forkhead factors Foxa1 and Foxa2 can cooperate synergistically to amplify Neurogenin3 autoregulation in vitro. However, only Foxa2 colocalizes with Neurogenin3 in pancreatic progenitors, thus indicating a primary role for this factor in regulating Neurogenin3 expression in vivo. Furthermore, in addition to decreasing Neurog3 autoregulation, inhibition of Foxa2 by RNA interference attenuates Neurogenin3-dependent activation of the endocrine developmental program in cultured duct mPAC cells. Hence, these data uncover the potential functional cooperation between the endocrine lineage-determining factor Neurogenin3 and the widespread endoderm progenitor factor Foxa2 in the implementation of the endocrine developmental program in the pancreas.

Replication-coupled chromatin assembly generates a neuronal bilateral asymmetry in C. elegans

Although replication-coupled chromatin assembly is known to be important for the maintenance of patterns of gene expression through sequential cell divisions, the role of replication-coupled chromatin assembly in controlling cell differentiation during animal development remains largely unexplored. Here we report that the CAF-1 protein complex, an evolutionarily conserved histone chaperone that deposits histone H3-H4 proteins onto replicating DNA, is required to generate a bilateral asymmetry in the C. elegans nervous system. A mutation in 1 of 24 C. elegans histone H3 genes specifically eliminates this aspect of neuronal asymmetry by causing a defect in the formation of a histone H3-H4 tetramer and the consequent inhibition of CAF-1-mediated nucleosome formation. Our results reveal that replication-coupled nucleosome assembly is necessary to generate a bilateral asymmetry in C. elegans neuroanatomy and suggest that left-right asymmetric epigenetic regulation can establish bilateral asymmetry in the nervous system.