Simone Sidoli

Histone proteins package DNA into nucleosomes, forming chromatin and thereby safeguarding genome integrity. Proper histone expression is essential for cell proliferation and chromatin organization, yet the upstream regulators of histone supply remain incompletely understood. PRMT5, a protein arginine methyltransferase that catalyzes symmetric dimethylation of arginine residues, is frequently overexpressed in cancer and targeted in multiple clinical trials, but its cell-essential function remains unclear. Using time-resolved nascent transcriptional profiling, quantitative proteomics, and imaging, we show that PRMT5 activity is required to sustain histone transcription and histone protein synthesis during S phase. PRMT5 inhibition or knockdown leads to rapid, p53-independent depletion of histone mRNA, leading to loss of histone proteins and accumulation of replication-associated nuclear abnormalities. Parallel depletion of the histone gene transcription factor HINFP produces similar genomic instability, linking defective histone production to loss of genome integrity. We further show that histone H4 accumulates at both histone genes and histone locus bodies (HLBs) and that H4R3me2s is enriched at HLBs. These findings support a model in which PRMT5-mediated methylation of histone H4 regulates histone transcription. Our findings establish PRMT5 as a central coordinator of histone homeostasis and provide a mechanistic rationale for its essential role in proliferating cells.

Summary: Ubiquitously expressed chromatin regulators orchestrate cell-type-specific gene expression programs, yet how they acquire distinct regulatory functions during development remains fundamentally unresolved ^1,2 . Using temporally controlled protein depletion during human cortical neurogenesis in vitro , we show that the autism-associated histone methyltransferase EHMT1 ^3,4 is developmentally repurposed from epiblast to cortical neurons. Broadly expressed zinc finger proteins recruit EHMT1 to distinct cis-regulatory elements within changing chromatin landscapes associated with lineage-specific transcription factor activity, with extensive remodeling of EHMT1 enhancer occupancy during neural induction dependent on ZNF462. This genomic rewiring is accompanied by a fundamental shift from restraining premature developmental gene expression in pluripotent cells to supporting neuronal programs, with distinct transcriptional consequences of EHMT1 loss across neural progenitors and excitatory and inhibitory neuronal subtypes. Timed depletion and recovery experiments identify the neural progenitor stage as the onset of a developmental period during which EHMT1 is required for faithful neuronal maturation. Remarkably, EHMT1 can re-engage stage-specific targets after prolonged deficiency, allowing extensive transcriptional recovery with only limited molecular scarring. These findings reveal how ubiquitous epigenetic machinery acquires developmental specificity while demonstrating the resilience of developmental gene regulatory programs to prolonged disruption, with implications for neurodevelopmental disorders.

Background: Succinate dehydrogenase (SDH) is a heterotetrameric enzyme in both the tricarboxylic acid (TCA) cycle and the electron transport chain. SDH-deficiency has been shown to drive the oncogenesis of cancer types including gastrointestinal stromal tumors (GIST), paraganglioma/pheochromocytoma, and clear cell renal cell carcinoma. Previous work on SDH loss has shown increases in succinate, decreases in fumarate, as well as increase in the succinate to α-ketoglutarate ratio, and correlated these changes to metabolite signaling and epigenetic dysregulation. Alterations to other TCA intermediates, including succinyl-CoA, the acyl-donor for succinylation, and the resulting cellular effects has not been fully explored. Methods: We used liquid chromatography high resolution mass spectrometry to quantify TCA cycle metabolites across the NCI-60 panel of cell lines, three SDH-competent and SDH-deficient model systems, and human tumor samples. We measured histone post-translational modifications in SHD-deficient and SDH-competent models and examined the correlation and potential pharmacological targeting of described desuccinylase enzymes by protein and cell assays. Results: We found an increase in succinyl-CoA in multiple SDH-deficient models, but no correlation between succinate to fumarate ratio and succinyl-CoA concentrations in the NCI-60. Interestingly, we found that while there was an increase in succinyl-CoA in the SDH-deficient cells, there was no corresponding increase in histone succinylation. We also observed a decrease in lysine malonylation and glutarylation, two other anionic lysine modifications. Cell viability and clonogenic assays revealed very weak or no selective effect of desuccinylase targeting in SDH-deficient models. Similarly, using SDH-deficient versus SDH-competent GIST patient samples, we examined both protein expression and metabolite abundance at the bulk tumor level and found no significant increase in succinyl-CoA or desuccinylase expression, despite the increase in succinate to fumarate ratio. Conclusions: We found that SDH-deficiency led to altered central carbon metabolism including increases to succinate and succinyl-CoA. Despite data that suggested the upregulation of desuccinylases may mediate a disconnect between succinyl-CoA concentration and histone succinylation, we found weak or no reductions in cell viability in response to pharmacological inhibition of desuccinylases. Additionally, no correlation of SDH loss with desuccinylase expression and succinyl-CoA in bulk tumor samples was observed.

Tissue microenvironments differ widely in nutrient composition; yet how local nutrient conditions shape epigenetic programs that direct immune cell fate is poorly understood. We identify histone lysine propionylation (Kpr) as a chromatin mechanism linking nutrient availability to T cell fate decisions. Proteomic analysis reveals that Kpr is broadly reduced in exhausted T cells. H3K18pr is enriched at stemness-associated loci in effector T cells and depleted upon exhaustion. Isoleucine and propionate supply propionyl-CoA in T cells, and restricted nutrient availability within the tumor microenvironment limits histone Kpr, promoting T cell exhaustion. Disrupting isoleucine catabolism in T cells further suppresses Kpr, without altering histone acetylation, and impairs T cell function and antitumor immunity in a manner rescued by propionate. Dietary supplementation of isoleucine and propionate increases Kpr in tumor infiltrating lymphocytes, limits exhaustion, and improves tumor control. These findings establish a microenvironment-chromatin axis crucial for antitumor immunity and amenable to nutritional intervention.

SETD2 is a critical tumor suppressor in lung cancer that catalyzes histone H3 lysine 36 (H3K36) trimethylation. Oncogenic histone H3 alleles, including H3 ^K36M , causally drive cancer formation by dominantly inhibiting SETD2 and other H3K36 methyltransferases. Here we show that expression of the H3 ^K36M oncogene paradoxically suppresses tumor growth in KRAS-driven lung cancer by promoting transcription of repetitive elements and accumulation of cytoplasmic dsRNA, activating antiviral-like immune clearance. Unexpectedly, all attendant effects of H3 ^K36M expression require SETD2 activity despite widespread loss of H3K36 trimethylation. Reducing H3K36 dimethylation through genetic or pharmacologic attenuation of NSD2 phenocopies H3 ^K36M expression, revealing NSD2 as the relevant target through which H3 ^K36M triggers immune activation. Mechanistically, H3 ^K36M expression or NSD2 attenuation reveals a SETD2-dependent mechanism that limits EZH2-mediated repression of repetitive elements, enabling dsRNA accumulation and immune activation. These findings establish opposing roles for NSD2- and SETD2-dependent H3K36 methylation in lung tumor immunity and reveal an NSD2–SETD2–EZH2 axis that can be targeted to modulate viral mimicry and immune surveillance in cancer.

The cell nucleus is an active metabolic site. Numerous enzymes best known for their roles in cytosolic or mitochondrial pathways also function in the nucleus, where they contribute to gene regulation and DNA replication and repair. Although metabolites can diffuse through nuclear pores, it remains unclear the extent to which the nucleus and cytosol operate as continuous versus distinct metabolic spaces. Both compartments require acetyl-CoA—for example, for histone acetylation and lipid synthesis—and the acetyl-CoA generating enzyme ATP-citrate lyase (ACLY) resides in both locations, but the significance of its dual localization is incompletely understood. Using cell lines in which ACLY is localized to either compartment, we find that ACLY in either location supports fatty acid synthesis and histone acetylation, yet compartment-localized ACLY enables finer control. Nuclear ACLY preserves histone H3K23 acetylation under glucose limitation and modulates specific transcriptional programs, whereas cytosolic ACLY most efficiently supports lipid biosynthetic fluxes. Thus, local synthesis defines a preferential metabolic fate, providing more precise regulation.

Maintenance of cellular homeostasis requires tight coordination between protein synthesis and degradation, particularly at old age and under conditions of stress including starvation. Autophagy contributes to sustain this balance by degrading cytoplasmic proteins and organelles. It thus is essential to prevent the accumulation of damaged proteins and organelles and to recycle nutrients. Of the three forms of autophagy, macroautophagy, chaperone mediated autophagy, and (endosomal) microautophagy (e-MI), the latter remains the least well understood. During e-MI, cytosolic substrate proteins are captured into late endosomes via ESCRT-dependent multivesicular body formation and then degraded in late endosomes or lysosomes. e-MI is thought to contribute to protein quality control under basal conditions and under stress. Importantly, very little is known about the endogenous substrates of e-MI in flies and thus about its physiological role. Performing integrative multi-omic analyses in Drosophila larval fat body that has functions similar to mammalian liver and adipose tissue, we identified 153 high-confidence endogenous e-MI substrates with the degradation of ribosomal proteins by e-MI being the most strongly affected functional category. Generally, we found that starvation caused the depletion of proteins involved in translation, aminoacyl-tRNA synthesis, and ribosomal biogenesis, without affecting their level of transcripts. Importantly, we observe a striking specificity between e-MI and macroautophagy, as the two pathways largely target distinct protein sets including different subsets of ribosomal proteins. Our metabolomic analysis further shows that genetic inhibition of e-MI reverses the reduced levels of amino acid caused by starvation. Together, our findings reveal ribosome turnover as a central physiological function of Drosophila e-MI and establish e-MI as a pathway driving metabolic adaptation during starvation. HIGHLIGHTS: Proteomic and metabolomic analysis of Drosophila fat body under starvation stress Systematic identification of endosomal microautophagy (e-MI) substrates. Ribosomal proteins are a major target of e-MI. e-MI and macroautophagy degrade distinct proteins incl. different ribosomal ones. Prolonged starvation alters amino acid levels.

Summary: Sex differences strongly influence susceptibility to metabolic dysfunction-associated steatotic liver disease (MASLD), yet the regulatory mechanisms underlying these differences remain incompletely understood. To examine sex-specific hepatic adaptation to a high-fat (HF) diet mouse model of MASLD, we integrated proteomics, transcriptomics, and Oxford Nanopore direct RNA sequencing for transcriptome-wide m6A profiling in male and female mouse livers. Female mice were relatively protected from HF diet–induced hepatic steatosis and exhibited distinct proteome remodeling enriched for peroxisomal pathways. In contrast, transcriptomic responses in females were dominated by inflammatory signatures and did not recapitulate the metabolic adaptations observed at the protein level, revealing extensive RNA–protein discordance and post-transcriptional remodeling. Integrated RNA–protein analyses identified female-specific amplification of peroxisomal proteins despite modest transcript-level changes. HF diet also induced sex-specific remodeling of m6A RNA methylation and altered regulation of the m6A methylation system. Notably, reduced 3′ UTR m6A methylation of peroxisomal transcripts inversely correlated with increased protein abundance relative to RNA expression in female mice. Together, these findings implicate m6A-associated post-transcriptional regulation in sex-specific hepatic adaptation to HF diet exposure and the basis for discordance between many of the mRNAs and proteins in the liver.

Early transcriptional regulation is a major kinetic bottleneck in gene expression, yet the dynamics of RNA polymerase II in living cells remain poorly understood. Here, we use single-molecule imaging to quantify RNA polymerase II chromatin engagement in live human cells. We find that RNA polymerase II infrequently enters a long-lived binding state associated with productive elongation, with ~94% of chromatin-associated events dissociating within tens of seconds. Single-cell analysis reveals substantial heterogeneity in the fraction of RNA polymerase II reaching productive elongation. Spatial heatmap analysis identifies RNA polymerase II clusters enriched for repeated long-lived binding events whose abundance is selectively perturbed by transcriptional inhibitors. Acute degradation of the TFIID subunit TAF1 reduces RNA polymerase II clustering and increases the global pausing index, supporting a model in which TAF1 promotes pause release and entry into productive elongation. Together, these findings provide a single-molecule framework for quantifying transcription kinetics in individual living cells.