SUMMARY Activated effector T (TE) cells augment anabolic pathways of metabolism, such as aerobic glycolysis, while memory T (TM) cells engage catabolic pathways, like fatty acid oxidation (FAO). However, signals that drive these differences remain unclear. Mitochondria are metabolic organelles that actively transform their ultrastructure. Therefore, we questioned whether mitochondrial dynamics controls T cell metabolism. We show that TE cells have punctate mitochondria, while TM cells maintain fused networks. The fusion protein Opa1 is required for TM, but not TE cells after infection and enforcing fusion in TE cells imposes TM cell characteristics and enhances antitumor function. Our data suggest that, by altering cristae morphology, fusion in TM cells configures electron transport chain (ETC) complex associations favoring oxidative phosphorylation (OXPHOS) and FAO, while fission in TE cells leads to cristae expansion, reducing ETC efficiency and promoting aerobic glycolysis. Thus, mitochondrial remodeling is a signaling mechanism that instructs T cell metabolic programming.

Starvation-induced autophagosomes engulf cytosol and/or organelles and deliver them to lysosomes for degradation, thereby re-supplying depleted nutrients. Despite advances in understanding the molecular basis of this process, the membrane origin of autophagosomes remains unclear. Here, we demonstrate that, in starved cells, autophagosomes are derived from the outer membranes of mitochondria. In time-lapse movies, the early autophagosomal marker, mApg5, transiently localizes to punctae on the surface of mitochondria, followed by the late autophagosomal marker, LC3. A unique tail-anchored outer mitochondrial membrane protein, but not other outer nor inner mitochondrial membrane proteins, labels autophagosomes and diffuses into newly forming autophagosomes from mitochondria. The fluorescent lipid, NBD-PS (which converts to PE in mitochondria) transfers from mitochondria to autophagosomes in starved cells. In addition, when mitochondria/ER connections are perturbed by loss of mitofusin2, starvation-induced autophagosomes do not form. Mitochondria thus play a central role in starvation-induced autophagy, serving as membrane source of autophagosomes.

Mitochondria are highly dynamic organelles that mediate essential cell functions such as apoptosis and cell-cycle control in addition to their role as efficient ATP generators. Mitochondrial morphology changes are tightly regulated, and their shape can shift between small, fragmented units and larger networks of elongated mitochondria. We demonstrate that mitochondrial elements become significantly elongated and interconnected shortly after nutrient depletion. This mitochondrial morphological shift depends on the type of starvation, with an additive effect observed when multiple nutrients are depleted simultaneously. We further show that starvation-induced mitochondrial elongation is mediated by downregulation of dynamin-related protein 1 (Drp1) through modulation of two Drp1 phosphorylation sites, leading to unopposed mitochondrial fusion. Finally, we establish that mitochondrial tubulation upon nutrient deprivation protects mitochondria from autophagosomal degradation, which could permit mitochondria to maximize energy production and supply autophagosomal membranes during starvation.autophagy | mitofusin M itochondria are dynamic organelles that mediate many essential cell functions. Depending on the cellular context, mitochondria shift between fragmented and tubular network-like morphologies by means of coordinated fission and fusion (1, 2). Proteins responsible for mitochondrial fusion include the outer mitochondrial membrane proteins mitofusin 1 (Mfn1) and mitofusin 2 (Mfn2) (3-5) and the inner membrane protein optic atrophy 1 (Opa1) (6). Fission is mediated by dynamin-related protein 1 (Drp1) (7,8) and its interaction with binding partners Fis1 and/or mitochondrial fission factor (Mff) (9, 10). Multiple mechanisms, including phosphorylation, sumoylation, and ubiquitination, coordinate Drp1 fission capacity (11)(12)(13)(14). Phosphorylation of Drp1 at S616 by Cdk1/cyclin B results in increased Drp1 fission activity (13). Conversely, phosphorylation of Drp1 at S637 by PKA decreases fission by causing Drp1 retention in the cytosol, whereas dephosphorylation of S637 by calcineurin causes Drp1 translocation to the mitochondria and increased mitochondrial fission (11, 15).Mitochondrial morphological dynamics are linked to regulation of many specific cell functions. Changes in mitochondrial cristae and mitochondrial fragmentation, for example, play a vital role in apoptosis (16,17). Ca 2+ transfer (18), cell-cycle regulation (13,19), and mitochondrial quality control (20, 21) are all closely tied to changes in mitochondrial morphology. Furthermore, stress conditions and changes in energy source can induce significant mitochondrial morphological changes (22, 23). Very recently, nutrient starvation was shown to induce mitochondrial elongation and to protect mitochondria from autophagic degradation (24). In addition to the above functions, mitochondria have recently been linked to autophagosome biogenesis during starvation conditions (25), and it is possible that mitochondrial morphological changes play a role in th...