Kazuhiko Igarashi

The establishment of central tolerance in the thymus is governed by precise gene regulatory networks, but how disrupted chromatin architecture leads to autoimmunity is unclear. The genome organizer SATB1 is essential for T cell development and its loss triggers a severe autoimmune phenotype. Here, we identify a pathogenic transcriptional axis, involving BACH1 and cMAF, that is unleashed upon SATB1 deletion. Using integrated multi-omics in T cell-specific Satb1 -knockout mice, we demonstrate that SATB1 constrains BACH1 chromatin occupancy. In its absence, BACH1 redistributes to promoter-proximal regions and SATB1-bound immune loci, where it facilitates the recruitment of the transcription factor cMAF. This BACH1-cMAF complex drives a pro-inflammatory transcriptional program in thymocytes, which seeds the periphery and results in a systemic autoimmune disease. Strikingly, genetic ablation of Bach1 in Satb1 -deficient mice rescues the pathology, normalizes immunity and prevents mortality. Furthermore, genetic or pharmacological inhibition of cMAF ameliorates the disease. The pro-inflammatory signature in mutant T cells overlaps with T cells from Systemic Lupus Erythematosus (SLE) patients. Our findings reveal a BACH1-cMAF axis that is derepressed upon SATB1 loss and bridges disrupted thymic chromatin organization to peripheral autoimmunity, nominating new therapeutic targets.

Methionine adenosyltransferase synthesizes S-adenosylmethionine (SAM), the universal methyl donor. Its ubiquitous isozyme MAT2 is present in the nucleus and regulates gene expression through histone methylation. However, the physiological significance of its nuclear enzymatic activity remains unclear. Here, we investigate the role of nuclear MAT2 in antibody class switch recombination (CSR), a crucial process in adaptive immunity. We established murine B-cell lines expressing either wild-type (WT) or catalytically-inactive (D134A) MAT2A subunits, targeted to the nucleus with nuclear localization signal (NLS). Inhibition of nuclear MAT2 enzymatic activity by expressing NLS-MAT2A(D134A) significantly suppressed CSR from IgM to IgA and reduced intracellular SAM levels. Chromatin immunoprecipitation revealed that MAT2A localized to the IgA switch region. Interestingly, while the expression of germline transcripts was enhanced, the expression of activation-induced cytidine deaminase (AID), an essential enzyme for CSR, was downregulated in NLS-MAT2A(D134A) cells. Our findings demonstrate that the enzymatic activity of MAT2 in the nucleus is indispensable for CSR, likely by ensuring proper AID expression subsequent to germline transcription, thus revealing a critical link between nuclear SAM metabolism and adaptive immunity.

The nucleolus is the site of rDNA transcription and ribosome biogenesis. Alterations in nucleolar function and architecture correlate with drastic heterochromatin rearrangement and global changes in gene expression. However, the precise mechanism that connects nucleolar function to heterochromatin organization and transcription is yet unknown. Here, we report that the RNA polymerase II (RNA pol II) transactivator and chromatin condenser, Positive Coactivator 4 (PC4), is a bona fide nucleolar protein. PC4 showed dynamic nucleolar accumulation, which is critical for rDNA transcription. The lysine acetyltransferase, KAT5 (Tip60) acetylates PC4 at K35, which facilitates nucleolar release of PC4 and concomitated inhibition of rDNA transcription. By employing PC4 mutant, which is defective in nucleolar accumulation, we found that nucleolar PC4 is crucial for RNA pol I-mediated rDNA transcription. To validate this significant novel role of PC4, in the context of nucleolus organization and function, at the organismal level, we looked into B cell-specific conditional knockout of Sub1 encoding PC4 in mice, which revealed that indeed the rDNA transcription and protein synthesis in B cells are severely repressed in the absence of PC4. Furthermore, PC4 CKO B cells were associated with the loss of H3K9me3-marked heterochromatin foci but not global H3K9me3 levels. LC–MS/MS analysis of the H3K9me3 chromatin complexes revealed that most non-histone heterochromatin proteins were reduced or absent in the constitutive heterochromatin of PC4 CKO B cells. These findings establish PC4 as a critical functional component of nucleolus for rDNA transcription.

Head and neck squamous cell carcinoma (HNSCC) remains difficult to treat due to the lack of molecularly targeted drugs with high anti-tumor efficacy and safety. To investigate the involvement of the transcription factor BACH1, which is known to promote the metastasis of various cancers, in the malignant properties of HNSCC cells, we examined the effects of BACH1 depletion with short interfering RNAs in human HNSCC cell lines. We found that knockdown of BACH1 induced cell death due to depletion of intracellular labile iron which is not tightly bound to protein. Mitochodrial electron transfer chain activity was severely reduced but efficiently resuced with supplementation of iron in the medium. By combining chromatin immunoprecipitation-sequence and RNA-sequence analyses, we found that BACH1 represses ferritin genes in HNSCC cells. BACH1 knockdown was found to enhance the effect of tipifarnib, a selective inhibitor of farnesyltransferase required for HRAS activation, resulting in efficient inhibition of proliferation of HNSCC cell lines in vitro. These results indicate that BACH1 is essential for maintaining the amount of labile iron in HNSCC cells to evade metabolic cell death of iron deficiency and to proliferate. BACH1 can be a target for conferring Tipifarnib sensitivity to HNSCC cells.

Target of rapamycin (TOR) signaling pathways are major regulators of starch accumulation in various eukaryotes. However, the underlying molecular mechanisms of this regulation remain elusive. Here, we report the role of TOR signaling in starch degradation in the unicellular red alga Cyanidioschyzon merolae. Reanalysis of our previously published phosphoproteome data showed that phosphorylation of the serine residue at position 264 of a protein similar to α-glucan water dikinase (CmGWD), a key regulator of starch degradation, was not increased by rapamycin treatment. In the CmGWD knockout strain, starch content increased and starch phosphorylation decreased, indicating that CmGWD is a functional GWD. CmGWD-dependent starch degradation under dark conditions was alleviated by rapamycin treatment. The overexpression of a phosphomimic CmGWD variant, in which Ser264 was replaced by aspartic acid, or a dephosphomimic CmGWD variant, in which Ser264 was replaced by alanine, resulted in 0.6-fold lower and 1.6-fold higher starch accumulation compared to the wild-type CmGWD-overexpressing strain, respectively. The starch levels corresponded with starch phosphorylation status. Furthermore, the dephosphomimic CmGWD-overexpressing strain accumulated nearly the same amount of starch with or without rapamycin treatment as the rapamycin-treated wild-type CmGWD-overexpressing strain. In contrast, rapamycin treatment did not trigger an increase in starch accumulation in the phosphomimic CmGWD-overexpressing strain. These results indicate that TOR signaling regulates starch degradation in C. merolae by altering the phosphorylation state of Ser264 in CmGWD.

Ferroptosis is a cell death mechanism mediated by iron-dependent lipid peroxidation. Although ferroptosis has garnered attention as a cancer-suppressing mechanism, there are still limited markers available for identifying ferroptotic cells or assessing their sensitivity to ferroptosis. The study focused on biliverdin, an endogenous reducing substance in cells, and examined the dynamics of intracellular biliverdin during ferroptosis using a biliverdin-binding cyanobacteriochrome. It was found that intracellular biliverdin decreases during ferroptosis and that this decrease is specific to ferroptosis amongst different forms of cell death. Furthermore, the feasibility of predicting sensitivity to ferroptosis by measuring intracellular biliverdin was demonstrated using a ferroptosis model induced by the re-expression of the transcription factor BACH1. These findings provide further insight into ferroptosis research and are expected to contribute to the development of cancer therapies that exploit ferroptosis.