SUMMARY It is widely appreciated that T cells increase glycolytic flux during activation, however the role of mitochondrial flux is unclear. Here we have shown that mitochondrial metabolism, in the absence of glucose metabolism, was sufficient to support interleukin-2 (IL-2) induction. Furthermore, we used mice with reduced mitochondrial reactive oxygen species (mROS) production in T cells (T-Uqcrfs−/− mice) to show that mitochondria are required for T cell activation to produce mROS for activation of nuclear factor of activated T cells (NFAT) and subsequent IL-2 induction. These mice could not induce antigen-specific expansion of T cells in vivo, however Uqcrfs1−/− T cells retained the ability to proliferate in vivo under lymphopenic conditions. This suggests that Uqcrfs1−/− T cells were not lacking bioenergetically, but rather lacked specific ROS-dependent signaling events needed for antigen-specific expansion. Thus, mitochondrial metabolism is a critical component of T cell activation through production of complex III ROS.

AMP-activated protein kinase (AMPK) is an energy sensor activated by increases in [AMP] or by oxidant stress (reactive oxygen species [ROS]). Hypoxia increases cellular ROSEnergy-dependent cellular processes require ATP, which is derived from mitochondrial oxidative phosphorylation and/or from glycolysis. Conditions that limit the cellular oxygen supply to the point that oxidative metabolism suffers can threaten cell survival because they undermine the ability of the cell to sustain essential ATP-dependent processes. Given that ATP supply is critical for survival, multiple systems have evolved to protect cells from the consequences of oxygen supply limitation and metabolic substrate deprivation. In this regard, defense of cellular energy substrates is mediated by the AMPactivated protein kinase (AMPK) system, which has been described as a "fuel gauge of the cell" (17). Activation of AMPK leads to increased glucose uptake via translocation of the transporter GLUT4 to the plasma membrane (42) and activation of glycolytic enzymes such as 6-phosphofructo-2-kinase, leading to enhanced glycolytic capacity via increases in fructose-2,6-bisphosphate (36). These and other responses triggered by AMPK activation confer protection against hypoxic injury in tissues such as the heart by preserving energy supply, mitochondrial metabolism, and glycolytic flux (43).AMPK activation requires phosphorylation of Thr-172 in the activation loop of its ␣ subunit (21) and is regulated by increases in cellular AMP and decreases in ATP via allosteric mechanisms and by the activity of upstream kinases and phosphatases that control AMPK phosphorylation and dephosphorylation at the critical threonine residue (8,23). Perhaps the best-described AMPK kinase is the tumor suppressor LKB1, which phosphorylates the catalytic ␣ subunit of AMPK in an AMP-dependent manner (20,53). A second activation pathway occurs independently of cellular AMP levels and involves Ca 2ϩ /calmodulin-dependent protein kinase kinase (CaMKK␤) (25,52). In that pathway, activation is initiated by a rise in [Ca 2ϩ ] i which increases the activity of CaMKK␤, which then phosphorylates AMPK. In vitro studies using both endogenous and recombinant proteins show that LKB1 and CaMKK␤ phosphorylate AMPK at 20,52). Activation of AMPK can also occur in response to oxidant stress (26,47), as demonstrated by the observation that H 2 O 2 administration triggers phosphorylation of AMPK and its downstream target, acyl coenzyme A carboxylase (ACC) (7, 28). However, the mechanism underlying this reactive oxygen species (ROS)-dependent activation of AMPK is not known.During severe oxygen deprivation, limitations in mitochondrial respiration lead to increases in the AMP/ATP ratio, which then trigger AMPK activation. However, AMPK-mediated protection might be greater if the kinase was activated before the cell reached the point where the AMP/ATP ratio has increased. Some beneficial effects of AMPK signaling, including the activation of gene expression or mitochondrial biogenesis (39), are not...

Mammalian homologues of Drosophila melanogaster transient receptor potential (TRP) are a large family of multimeric cation channels that act, or putatively act, as sensors of one or more chemical factor1,2. Major research objectives are the identification of endogenous activators and the determination of cellular and tissue functions of these novel channels. Here we show activation of TRPC5 homomultimeric and TRPC5-TRPC1 heteromultimeric channels3-5 by extracellular reduced thioredoxin acting by breaking a disulphide bridge in the predicted extracellular loop adjacent to the ion-selectivity filter of TRPC5. Thioredoxin is an endogenous redox protein with established intracellular functions, but it is also secreted and its extracellular targets are largely unknown6-9. Particularly high extracellular concentrations of thioredoxin are apparent in rheumatoid arthritis8,10-12, an inflammatory joint disease disabling millions of people worldwide13. We show that TRPC5 and TRPC1 are expressed in secretory fibroblast-like synoviocytes from patients with rheumatoid arthritis, endogenous TRPC5-TRPC1 channels of the cells are activated by reduced thioredoxin, and blockade of the channels enhances secretory activity and prevents suppression of secretion by thioredoxin. The data suggest a novel ion channel activation mechanism that couples extracellular thioredoxin to cell function.Striking activators of TRPC5 are extracellular lanthanide ions4,14,15. Effects of these ions depend on a glutamic acid residue at position 54314 in the predicted extracellular loop adjacent to the ion pore (Supplementary Fig. 1-2). This structural feature may, therefore, have functional importance in enabling extracellular factors to activate the channels. Because lanthanides are unlikely physiological activators we were interested in alternatives and developed a hypothesis based on amino acid sequence alignment which showed two cysteine residues near glutamic acid 543 that are conserved in TRPC5, TRPC4 and TRPC1 ( Supplementary Fig. 2), a subset of the seven TRPC channels1-5. TRPC5 and TRPC4 have similar functional properties4 and both form heteromultimers with TRPC13-5, a subunit that has weak targeting to the plasma membrane when expressed in isolation3,16. Pairs of cysteine residues may be covalently linked by a disulphide bridge that can be cleaved by reduction. We therefore applied the chemical reducing agent dithiothreitol (DTT) to HEK 293 cells expressing TRPC515,16. There was channel activation with the characteristic current-voltage relationship (I-V) of TRPC5 and block by 2-APB, an inhibitor of TRPC55 (Fig. 1a, b, d). Current recovered on wash-out of DTT (data not shown). Similarly, the membrane-impermeable disulphide reducing agent TCEP (Fig. 1c, d) activated TRPC5, whereas the thiol reagent MTSET had no effect (Fig. 1d). TRPC5 was inhibited by cadmium ions only after pre-treatment with DTT ( Fig. 1e, f), consistent with the metal ion acting by re-engaging cysteines17. Other TRP channels lacking the cysteine pair in a similar po...

ORAI1 is the pore-forming subunit of the Ca2+ release-activated Ca2+ (CRAC) channel, which is responsible for store-operated Ca2+ entry in lymphocytes. A role for ORAI1 in T cell function in vivo has been inferred from in vitro studies of T cells from human immunodeficient patients with mutations in ORAI1 and Orai1−/− mice, but a detailed analysis of T cell-mediated immune responses in vivo in mice lacking functional ORAI1 has been missing. We therefore generated Orai1 knock-in mice (Orai1KI/KI) expressing a nonfunctional ORAI1-R93W protein. Homozygosity for the equivalent ORAI1-R91W mutation abolishes CRAC channel function in human T cells resulting in severe immunodeficiency. Homozygous Orai1KI/KI mice die neonatally, but Orai1KI/KI fetal liver chimeric mice are viable and show normal lymphocyte development. T and B cells from Orai1KI/KI mice display severely impaired store-operated Ca2+ entry and CRAC channel function resulting in a strongly reduced expression of several key cytokines including IL-2, IL-4, IL-17, IFN-γ, and TNF-α in CD4+ and CD8+ T cells. Cell-mediated immune responses in vivo that depend on Th1, Th2, and Th17 cell function were severely attenuated in ORAI1-deficient mice. Orai1KI/KI mice lacked detectable contact hypersensitivity responses and tolerated skin allografts significantly longer than wild-type mice. In addition, T cells from Orai1KI/KI mice failed to induce colitis in an adoptive transfer model of inflammatory bowel disease. These findings reaffirm the critical role of ORAI1 for T cell function and provide important insights into the in vivo functions of CRAC channels for T cell-mediated immunity.

The discovery of TREM2 as a myeloid-specific Alzheimer’s disease (AD) risk gene has accelerated research into the role of microglia in AD. While TREM2 mouse models have provided critical insight, the normal and disease-associated functions of TREM2 in human microglia remain unclear. To examine this question, we profile microglia differentiated from isogenic, CRISPR-modified TREM2-knockout induced pluripotent stem cell (iPSC) lines. By combining transcriptomic and functional analyses with a chimeric AD mouse model, we find that TREM2 deletion reduces microglial survival, impairs phagocytosis of key substrates including APOE, and inhibits SDF-1α/CXCR4-mediated chemotaxis, culminating in an impaired response to beta-amyloid plaques in vivo. Single-cell sequencing of xenotransplanted human microglia further highlights a loss of disease-associated microglial (DAM) responses in human TREM2 knockout microglia that we validate by flow cytometry and immunohistochemistry. Taken together, these studies reveal both conserved and novel aspects of human TREM2 biology that likely play critical roles in the development and progression of AD.