Adrenergic stimulation of brown adipocytes alters mitochondrial dynamics, including the mitochondrial fusion protein optic atrophy 1 (OPA1). However, direct mechanisms linking OPA1 to brown adipose tissue (BAT) physiology are incompletely understood. We utilized a mouse model of selective OPA1 deletion in BAT (OPA1 BAT KO) to investigate the role of OPA1 in thermogenesis. OPA1 is required for cold-induced activation of thermogenic genes in BAT. Unexpectedly, OPA1 deficiency induced fibroblast growth factor 21 (FGF21) as a BATokine in an activating transcription factor 4 (ATF4)-dependent manner. BAT-derived FGF21 mediates an adaptive response, by inducing browning of white adipose tissue, increasing resting metabolic rates, and improving thermoregulation. However, mechanisms independent of FGF21, but dependent on ATF4 induction, promote resistance to diet-induced obesity in OPA1 BAT KO mice. These findings uncover a homeostatic mechanism of BAT-mediated metabolic protection governed in part by an ATF4-FGF21 axis, that is activated independently of BAT thermogenic function.

Patients with type 1 diabetes mellitus (T1DM) have increased thrombosis and platelet activation. The mechanisms for platelet hyperactivation in diabetes are incompletely understood. T1DM is accompanied by hyperglycemia, dyslipidemia, and increased inflammation in addition to an altered hormonal milieu. In vitro analysis of platelets revealed that normal glucose reduces platelet activation whereas hyperglycemic conditions increase platelet activation. We therefore hypothesized that hyperglycemia increases platelet glucose utilization, which increases platelet activation to promote thrombosis. Glucose uptake and glycolysis were increased in platelets isolated from mice given streptozotocin (STZ) to induce T1DM in concert with induction of GLUT3. Platelets from STZ-induced diabetic mice exhibited increased activation after administration of proteaseactivated receptor 4 peptide and convulxin. In contrast, platelets isolated from GLUT1 and GLUT3 double-knockout (DKO) mice, which lack the ability to use glucose, failed to increase activation in hyperglycemic mice. Diabetic mice displayed decreased survival in a collagen/epinephrine-induced pulmonary embolism model of in vivo platelet activation relative to nondiabetic controls. Survival after pulmonary embolism was increased in diabetic DKO mice relative to nondiabetic controls. These data reveal that increased platelet glucose metabolism in vivo contributes to increased platelet activation and thrombosis in a model of T1DM.

Summary Increased intracellular reactive oxygen species (ROS) promote platelet activation. The sources of platelet-derived ROS are diverse and whether or not mitochondrial derived ROS, modulates platelet function is incompletely understood. Studies of platelets from patients with sickle cell disease, and diabetes suggest a correlation between mitochondrial ROS and platelet dysfunction. Therefore, we generated mice with a platelet specific knockout of superoxide dismutase 2 (SOD2-KO) to determine if increased mitochondrial ROS increases platelet activation. SOD2-KO platelets demonstrated decreased SOD2 activity and increased mitochondrial ROS, however total platelet ROS was unchanged. Mitochondrial function and content were maintained in non-stimulated platelets. However SOD2-KO platelets demonstrated decreased mitochondrial function following thrombin stimulation. In vitro platelet activation and spreading was normal and in vivo, deletion of SOD2 did not change tail-bleeding or arterial thrombosis indices. In pathophysiological models mediated by platelet-dependent immune mechanisms such as sepsis and autoimmune inflammatory arthritis, SOD2-KO mice were phenotypically identical to wild-type controls. These data demonstrate that increased mitochondrial ROS does not result in platelet dysfunction.