Mitochondrial complex III is essential for suppressive function of regulatory T cells.

Regulatory T cells (Treg cells), a distinct subset of CD4+ T cells, are necessary for the maintenance of immune self-tolerance and homeostasis1,2. Recent studies have demonstrated that Treg cells exhibit a unique metabolic profile, characterized by an increase in mitochondrial metabolism relative to other CD4+ effector subsets3,4. Furthermore, the Treg cell lineage-defining transcription factor, Foxp3, has been shown to promote respiration5,6; however, it remains unknown whether the mitochondrial respiratory chain is required for the T cell-suppression capacity, stability and survival of Treg cells. Here we report that Treg cell-specific ablation of mitochondrial respiratory chain complex III in mice results in the development of fatal inflammatory disease early in life, without affecting Treg cell number. Mice that lack mitochondrial complex III specifically in Treg cells displayed a loss of T cell-suppression capacity without altering Treg cell proliferation and survival. Treg cells deficient in complex III showed decreased expression of genes associated with Treg function, whereas Foxp3 expression remained stable. Loss of complex III in Treg cells increased DNA methylation as well as the metabolites 2-hydroxyglutarate (2-HG) and succinate that inhibit the ten-eleven translocation (TET) family of DNA demethylases7. Thus, Treg cells require mitochondrial complex III to maintain immune regulatory gene expression and suppressive function.

TCA Cycle and Mitochondrial Membrane Potential Are Necessary for Diverse Biological Functions.

Mitochondrial metabolism is necessary for the maintenance of oxidative TCA cycle function and mitochondrial membrane potential. Previous attempts to decipher whether mitochondria are necessary for biological outcomes have been hampered by genetic and pharmacologic methods that simultaneously disrupt multiple functions linked to mitochondrial metabolism. Here, we report that inducible depletion of mitochondrial DNA (ρ(ο) cells) diminished respiration, oxidative TCA cycle function, and the mitochondrial membrane potential, resulting in diminished cell proliferation, hypoxic activation of HIF-1, and specific histone acetylation marks. Genetic reconstitution only of the oxidative TCA cycle function specifically in these inducible ρ(ο) cells restored metabolites, resulting in re-establishment of histone acetylation. In contrast, genetic reconstitution of the mitochondrial membrane potential restored ROS, which were necessary for hypoxic activation of HIF-1 and cell proliferation. These results indicate that distinct mitochondrial functions associated with respiration are necessary for cell proliferation, epigenetics, and HIF-1 activation.

A transposase strategy for creating libraries of circularly permuted proteins.

A simple approach for creating libraries of circularly permuted proteins is described that is called PERMutation Using Transposase Engineering (PERMUTE). In PERMUTE, the transposase MuA is used to randomly insert a minitransposon that can function as a protein expression vector into a plasmid that contains the open reading frame (ORF) being permuted. A library of vectors that express different permuted variants of the ORF-encoded protein is created by: (i) using bacteria to select for target vectors that acquire an integrated minitransposon; (ii) excising the ensemble of ORFs that contain an integrated minitransposon from the selected vectors; and (iii) circularizing the ensemble of ORFs containing integrated minitransposons using intramolecular ligation. Construction of a Thermotoga neapolitana adenylate kinase (AK) library using PERMUTE revealed that this approach produces vectors that express circularly permuted proteins with distinct sequence diversity from existing methods. In addition, selection of this library for variants that complement the growth of Escherichia coli with a temperature-sensitive AK identified functional proteins with novel architectures, suggesting that PERMUTE will be useful for the directed evolution of proteins with new functions.

Hexokinase 2 is dispensable for T cell-dependent immunity.

BACKGROUND: T cells and cancer cells utilize glycolysis for proliferation. The hexokinase (1-4) family of enzymes catalyze the first step of glycolysis. Hexokinase 2 (HK2) is one of the most highly upregulated metabolic enzymes in both cancer and activated T cells. HK2 is required for the development and/or growth of cancer in several cancer models, but the necessity of HK2 in T cells is not fully understood. The clinical applicability of HK2 inhibition in cancer may be significantly limited by any potential negative effects of HK2 inhibition on T cells. Therefore, we investigated the necessity of HK2 for T cell function. In order to identify additional therapeutic cancer targets, we performed RNA-seq to compare in vivo proliferating T cells to T cell leukemia. METHODS: HK2 was genetically ablated in mouse T cells using a floxed Hk2 allele crossed to CD4-Cre. CD4+ and CD8+ cells from mice were characterized metabolically and tested in vitro. T cell function in vivo was tested in a mouse model of colitis, Th2-mediated lung inflammation, and viral infection. Treg function was tested by crossing Hk2-floxed mice to FoxP3-Cre mice. Hematopoietic function was tested by deleting HK2 from bone marrow with Vav1-iCre. RNA-seq was used to compare T cells proliferating in response to virus with primary T-ALL leukemia induced with mutant Notch1 expression. RESULTS: We unexpectedly report that HK2 is largely dispensable for in vitro T cell activation, proliferation, and differentiation. Loss of HK2 does not impair in vivo viral immunity and causes only a small impairment in the development of pathological inflammation. HK2 is not required for Treg function or hematopoiesis in vivo. One hundred sixty-seven metabolic genes were identified as being differentially expressed between T cells and leukemia. CONCLUSIONS: HK2 is a highly upregulated enzyme in cancer and in T cells. The requirement for HK2 in various cancer models has been described previously. Our finding that T cells are able to withstand the loss of HK2 indicates that HK2 may be a promising candidate for cancer therapy. Furthermore, we identify several other potential metabolic targets in T-ALL leukemia that could spare T cell function.

Mitochondrial control of immunity: beyond ATP.

Mitochondria are important signalling organelles, and they dictate immunological fate. From T cells to macrophages, mitochondria form the nexus of the various metabolic pathways that define each immune cell subset. In this central position, mitochondria help to control the various metabolic decision points that determine immune cell function. In this Review, we discuss how mitochondrial metabolism varies across different immune cell subsets, how metabolic signalling dictates cell fate and how this signalling could potentially be targeted therapeutically.

The Structure of a Thermophilic Kinase Shapes Fitness upon Random Circular Permutation.

Proteins can be engineered for synthetic biology through circular permutation, a sequence rearrangement in which native protein termini become linked and new termini are created elsewhere through backbone fission. However, it remains challenging to anticipate a protein's functional tolerance to circular permutation. Here, we describe new transposons for creating libraries of randomly circularly permuted proteins that minimize peptide additions at their termini, and we use transposase mutagenesis to study the tolerance of a thermophilic adenylate kinase (AK) to circular permutation. We find that libraries expressing permuted AKs with either short or long peptides amended to their N-terminus yield distinct sets of active variants and present evidence that this trend arises because permuted protein expression varies across libraries. Mapping all sites that tolerate backbone cleavage onto AK structure reveals that the largest contiguous regions of sequence that lack cleavage sites are proximal to the phosphotransfer site. A comparison of our results with a range of structure-derived parameters further showed that retention of function correlates to the strongest extent with the distance to the phosphotransfer site, amino acid variability in an AK family sequence alignment, and residue-level deviations in superimposed AK structures. Our work illustrates how permuted protein libraries can be created with minimal peptide additions using transposase mutagenesis, and it reveals a challenge of maintaining consistent expression across permuted variants in a library that minimizes peptide additions. Furthermore, these findings provide a basis for interpreting responses of thermophilic phosphotransferases to circular permutation by calibrating how different structure-derived parameters relate to retention of function in a cellular selection.

An unusual cause of headache in a medical tourist.

With booming medical tourism, Indian doctors are seeing a lot of patients from other countries for varied medical conditions. A citizen of Nairobi presented for treatment of his complaints consisting of severe episodic headache, with abdominal pain, spermatorrhea, decreased libido, constipation, and impotence. On detailed evaluation, he was found to have developed dependence to khat chewing; a social habit in his native country and his symptoms were attributed to effects of khat withdrawal. He improved after treatment with topiramate and escitalopram and lifestyle modification. Physicians need to be aware about various cultural habits of addiction in different parts of the world and their common presentations in view of globalization of health care.

Targeting metabolism for lupus therapy.

Drugs that correct defects in T cell metabolism may prove to be promising treatments for lupus.