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The proliferation and differentiation of muscle precursor cells require myogenic regulatory factors and chromatin modifiers whose concerted action dynamically regulates access to DNA and allows reprogramming of cells towards terminal differentiation. Type 2 deiodinase (D2), the thyroid hormone (TH)-activating enzyme, is sharply upregulated during myoblast differentiation, whereas type 3 deiodinase (D3), the TH-inactivating enzyme, is downregulated. The molecular determinants controlling synchronized D2 and D3 expression in muscle differentiation are completely unknown. Here, we report that the histone H3 demethylating enzyme (LSD-1) is essential for transcriptional induction of D2 and repression of D3. LSD-1 relieves the repressive marks (H3-K9me2-3) on the Dio2 promoter and the activation marks (H3-K4me2-3) on the Dio3 promoter. LSD-1 silencing impairs the D2 surge in skeletal muscle differentiation while inducing D3 expression thereby leading to a global decrease in intracellular TH production. Furthermore, endogenous LSD-1 interacts with FoxO3a, and abrogation of FoxO3-DNA binding compromises the ability of LSD-1 to induce D2. Our data reveal a novel epigenetic control of reciprocal deiodinases expression and provide a molecular mechanism by which LSD-1, through the opposite regulation of D2 and D3 expression, acts as a molecular switch that dynamically finely tunes the cellular needs of active TH during myogenesis.

Although it is well known that RET gene is strongly activated by retinoic acid (RA) in neuroblastoma cells, the mechanisms underlying such activation are still poorly understood. Here we show that a complex series of molecular events, that include modifications of both chromatin and DNA methylation state, accompany RA-mediated RET activation. Our results indicate that the primary epigenetic determinants of RA-induced RET activation differ between enhancer and promoter regions. At promoter region, the main mark of RET activation was the increase of H3K4me3 levels while no significant changes of the methylation state of H3K27 and H3K9 were observed. At RET enhancer region a bipartite chromatin domain was detected in unstimulated cells and a prompt demethylation of H3K27me3 marked RET gene activation upon RA exposure. Moreover, ChIP experiments demonstrated that EZH2 and MeCP2 repressor complexes were associated to the heavily methylated enhancer region in the absence of RA while both complexes were displaced during RA stimulation. Finally, our data show that a demethylation of a specific CpG site at the enhancer region could favor the displacement of MeCP2 from the heavily methylated RET enhancer region providing a novel potential mechanism for transcriptional regulation of methylated RA-regulated loci.

We have recently reported that a novel MBD2 interactor (MBDin) has the capacity to reactivate transcription from MBD2-repressed methylated promoters even in the absence of demethylation events. Here we show that another unrelated protein, TACC3, displays a similar activity on methylated genes. In addition the data reported here provide possible molecular mechanisms for the observed phenomenon. Immunoprecipitation experiments showed that MBD2/TACC3 form a complex in vivo with the histone acetyltransferase pCAF. MBD2 could also associate with HDAC2, a component of MeCP1 repression complex. However, we found that the complexes formed by MBD2 with TACC3/pCAF and with HDAC2 were mutually exclusive. Moreover, HAT enzymatic assays demonstrated that HAT activity associates with MBD2 in vivo and that such association significantly increased when TACC3 was over-expressed. Overall our findings suggest that TACC3 can be recruited by MBD2 on methylated promoters and is able to reactivate transcription possibly by favoring the formation of an HAT-containing MBD2 complex and, thus, switching the repression potential of MBD2 in activation even prior to eventual demethylation.

The sequence organization of porcine DNA isolated from thyroid has been analyzed by hydroxylapatite (HAP) chromatography. The reassociation of 0.4 kilobase (Kb) DNA fragments shows, besides the presence of 5% inverted repeat sequences (foldback DNA), that 45% of the genome is represented by high (10%) and intermediate (35%) repetitive components, whereas the remaining 50% is unique sequences. 30% of the unique sequences consists of 1,000 nucleotide fragments interspersed with repetitive elements 400 nucleotides in length. The remaining 20% is longer unique sequences (10,000 nucleotides) apparently not linked to repetitive elements.

We report the structural organization of an 80 Kb segment of rat DNA, which encodes for about 40% of Thyroglobulin mRNA at the 3' end. The codogenic information included in this segment is splitted in 17 exons of homogeneous size (about 200 bp). The seven exons at the extreme 3' end have been precisely defined by DNA sequence analysis. No clear sequence homology is found among the exons, even though their coding capacity is quite similar, from 55 to 63 aminoacids residues. We located 2 hormonogenic (T4 forming) sites on the extreme 3' end of the gene in different exons. The DNA sequence coding for these functional sites shows a 70% homology in a 50 nucleotides segment. In addition we found a remnant of this sequence in other exons of the gene. Two large introns have been found on the 3' end of the gene: one is 17 Kb and the other one is more than 30 Kb long. On the basis of these findings and of preliminary studies on the remaining 5' end of the gene, we can predict that the minimum length of the rat TGB gene will be 150 Kb, which makes this gene the largest so far identified eukaryotic gene. We propose in addition that the 3' end exons arose by duplication of a common ancestor.