Mitochondrial Enzymes Moonlight in the Nucleus to Drive Gene Activation

A central mystery in cell biology is how metabolic state is translated into gene expression changes in real time. Acetyl-CoA — the essential currency for histone acetylation — cannot cross membranes freely, meaning nuclei must either import metabolite precursors or synthesize acetyl-CoA locally. This study reveals an elegant solution: mitochondrial TCA cycle enzymes physically relocate to the nucleus to run a dedicated metabolic pathway on-site.

Using subcellular fractionation across multiple human and mouse cell lines, the authors showed that citrate synthase (CS), aconitase 2 (ACO2), and isocitrate dehydrogenase 2 (IDH2) are consistently detected in nuclear fractions, while other TCA enzymes (SDHA, FH, MDH2) remain mitochondria-restricted. Imaging flow cytometry of 10,000 individual cells per line and high-resolution z-stack confocal microscopy confirmed genuine nuclear residence, with the enzymes forming puncta-like condensates suggestive of association with transcriptional hotspots. Critically, intact isolated nuclei — rigorously stripped of mitochondrial contamination — retained ACO2 and IDH2 signal, ruling out artifactual co-isolation.

Isotope tracing with ¹³C-labeled alpha-ketoglutarate in isolated intact nuclei demonstrated that IDH2 and ACO2 catalyze reductive carboxylation of α-KG to generate citrate within the nucleus, independent of any mitochondrial contribution. ACLY then converts this citrate to acetyl-CoA locally. Deep mass spectrometric profiling of histone modifications confirmed that acetyl carbons originating from α-KG are incorporated into histone H3 acetylation marks (H3Ac), directly linking the nuclear metabolic reaction to epigenetic writing.

Immunoaffinity proteomics showed that ACO2 and IDH2 (but not CS) physically associate with histone H3 and form a complex with the histone acetyltransferase KAT2A/GCN5. Manipulating nuclear-specific levels of IDH2 and ACO2 altered global H3Ac marks and chromatin accessibility (assessed by ATAC-seq), and high-throughput sequencing identified enrichment of the pioneering transcription factor FOXA1 at promoters of proliferative genes. Genetic loss- and gain-of-function experiments in multiple mouse tumor models confirmed that the IDH2-ACO2-KAT2A axis drives hyperproliferative phenotypes in vivo, and high nuclear expression of both enzymes correlates with aggressive tumor characteristics in patient datasets.

Taken together, this work establishes a paradigm in which a dedicated nuclear metabolic pathway — reductive carboxylation of α-KG by IDH2 and ACO2 — generates acetyl-CoA at chromatin to directly fuel KAT2A-mediated histone acetylation, chromatin opening, and activation of proliferative transcriptional programs. The axis may be broadly relevant to any context requiring rapid epigenetic reprogramming.