2-oxoglutarate (2-OG or α-ketoglutarate) relates mitochondrial metabolism to cell function by modulating the activity of 2-OG dependent dioxygenases involved in the hypoxia response and DNA/histone modifications. However, metabolic pathways that regulate these oxygen and 2-OG sensitive enzymes remain poorly understood. Here, using CRISPR Cas9 genome-wide mutagenesis to screen for genetic determinants of 2-OG levels, we uncover a redox sensitive mitochondrial lipoylation pathway, dependent on the mitochondrial hydrolase ABHD11, that signals changes in mitochondrial 2-OG metabolism to 2-OG dependent dioxygenase function. ABHD11 loss or inhibition drives a rapid increase in 2-OG levels by impairing lipoylation of the 2-OG dehydrogenase complex (OGDHc)—the rate limiting step for mitochondrial 2-OG metabolism. Rather than facilitating lipoate conjugation, ABHD11 associates with the OGDHc and maintains catalytic activity of lipoyl domain by preventing the formation of lipoyl adducts, highlighting ABHD11 as a regulator of functional lipoylation and 2-OG metabolism.

Hypoxia-inducible transcription factors (HIFs) facilitate cellular adaptations to low-oxygen environments. However, it is increasingly recognised that HIFs may be activated in response to metabolic stimuli, even when oxygen is present. Understanding the mechanisms for the crosstalk that exists between HIF signalling and metabolic pathways is therefore important. This review focuses on the metabolic regulation of HIFs by small molecule metabolites and iron, highlighting the latest studies that explore how tricarboxylic acid (TCA) cycle intermediates, 2-hydroxyglutarate (2-HG) and intracellular iron levels influence the HIF response through modulating the activity of prolyl hydroxylases (PHDs). We also discuss the relevance of these metabolic pathways in physiological and disease contexts. Lastly, as PHDs are members of a large family of 2-oxoglutarate (2-OG) dependent dioxygenases that can all respond to metabolic stimuli, we explore the broader role of TCA cycle metabolites and 2-HG in the regulation of 2-OG dependent dioxygenases, focusing on the enzymes involved in chromatin remodelling.

Cells produce considerable genotoxic formaldehyde from an unknown source. We carry out a genome-wide CRISPR-Cas9 genetic screen in metabolically engineered HAP1 cells that are auxotrophic for formaldehyde to find this cellular source. We identify histone deacetylase 3 (HDAC3) as a regulator of cellular formaldehyde production. HDAC3 regulation requires deacetylase activity, and a secondary genetic screen identifies several components of mitochondrial complex I as mediators of this regulation. Metabolic profiling indicates that this unexpected mitochondrial requirement for formaldehyde detoxification is separate from energy generation. HDAC3 and complex I therefore control the abundance of a ubiquitous genotoxic metabolite.

The combined effects of intracellular lactate and proton accumulation on cell volume, V_c, were investigated in resting Rana temporaria striated muscle fibres. Intracellular lactate and H⁺ concentrations were simultaneously increased by exposing resting muscle fibres to extracellular solutions that contained 20–80 mm sodium lactate. Cellular H⁺ and lactate entry was confirmed using pH‐sensitive electrodes and ¹H‐NMR, respectively, and effects on V_c were measured using confocal microscope xz‐scanning. Exposure to extracellular lactate up to 80 mm produced significant changes in pH and intracellular lactate (from a pH of 7.24 ± 0.03, n= 8, and 4.65 ± 1.07 mm, n= 6, respectively, in control fibres, to 6.59 ± 0.03, n= 4, and 26.41 ± 0.92 mm, n= 3, respectively) that were comparable to those observed following fatiguing stimulation (6.30–6.70 and 18.04 ± 1.78 mm, n= 6, respectively). Yet, the increase in intracellular osmolarity expected from such an increase in intracellular lactate did not significantly alter V_c. Simulation of these experimental results, modified from the charge difference model of Fraser & Huang, demonstrated that such experimental manoeuvres produced changes in intracellular [H⁺] and [lactate] comparable to those observed during muscle fatigue, and accounted for this paradoxical conservation of V_c through balancing negative osmotic effects resulting from the net cation efflux that would follow a titration of intracellular membrane‐impermeant anions by the intracellular accumulation of protons. It demonstrated that with established physiological values for intracellular buffering capacity and the permeability ratio of lactic acid and anionic lactate, P_LacH: P_Lac−, this would provide a mechanism that precisely balanced any effect on cell volume resulting from lactate accumulation during exercise.

Paredes et al. (1) describe polymerase-δ interacting protein 2 (Poldip2) as a novel regulator of mitochondrial lipoylation through stabilization of Ac-CoA synthetase medium-chain family member 1 (ACSM1). We have several concerns with their proposed model based on the following reasons. Prior mammalian and yeast biochemical studies are not consistent with a significant physiological role for lipoate scavenging in eukaryotes (2, 3). Genetic depletion or germline mutations in de novo lipoic acid synthesis enzymes (LIAS, LIPT1, and LIPT2) result in loss of mitochondrial lipoylation, respiration, and developmental defects in mammals, which are not reversed with exogenous lipoic acid (2 … [↵][1]1To whom correspondence may be addressed. Email: dieckman{at}email.arizona.edu, alexander.kastaniotis{at}oulu.fi, or jan33{at}cam.ac.uk. [1]: #xref-corresp-1-1