Mitochondrial matrix protein Letmd1 maintains thermogenic capacity of brown adipose tissue

Brown adipose tissue (BAT) has abundant mitochondria with the unique capability of generating heat via uncoupled respiration. Mitochondrial uncoupling protein 1 (Ucp1) is activated in BAT during cold stress and dissipates mitochondrial proton motive force generated by the electron transport chain to generate heat. However, other mitochondrial factors required for brown adipocyte respiration and thermogenesis under cold stress are largely unknown. Here we identify LETM1 domain-containing protein 1 (Letmd1) is a BAT-enriched, cold-induced protein that is required for cold-stimulated respiration and thermogenesis of BAT. Proximity labeling studies reveal that Letmd1 is a mitochondrial matrix protein. Letmd1 knockout mice display aberrant BAT mitochondria and fail to carry out adaptive thermogenesis under cold stress. Letmd1 knockout BAT is deficient in oxidative phosphorylation (OXPHOS) complex proteins and has impaired mitochondrial respiration. Taken together, we identify that the BAT-enriched mitochondrial matrix protein Letmd1 is required for cold-stimulated respiration and thermogenic function of BAT.

Thermogenic Activation Downregulates High Mitophagy Rate in Human Masked and Mature Beige Adipocytes

Thermogenic brown and beige adipocytes oxidize metabolic substrates producing heat, mainly by the mitochondrial uncoupling protein UCP1, and can thus counteract obesity. Masked beige adipocytes possess white adipocyte-like morphology, but can be made thermogenic by adrenergic stimuli. We investigated the regulation of mitophagy upon thermogenic activation of human masked and mature beige adipocytes. Human primary abdominal subcutaneous adipose-derived stromal cells (hASCs) and Simpson–Golabi–Behmel syndrome (SGBS) preadipocytes were differentiated to white and beige adipocytes, then their cAMP-induced thermogenic potential was assessed by detecting increased expressions of UCP1, mitochondrial DNA content and respiratory chain complex subunits. cAMP increased the thermogenic potential of white adipocytes similarly to beige ones, indicating the presence of a masked beige population. In unstimulated conditions, a high autophagic flux and mitophagy rates (demonstrated by LC3 punctae and TOM20 co-immunostaining) were observed in white adipocytes, while these were lower in beige adipocytes. Silencing and gene expression experiments showed that the ongoing mitophagy was Parkin-independent. cAMP treatment led to the downregulation of mitophagy through PKA in both types of adipocytes, resulting in more fragmented mitochondria and increased UCP1 levels. Our data indicates that mitophagy is repressed upon encountering a short-term adrenergic stimulus, as a fast regulatory mechanism to provide high mitochondrial content for thermogenesis.

Mitochondrial uncoupling protein 2 (UCP2), but not UCP3, is sensitive to oxygen concentration in cells

One of the important hallmarks of cardiovascular disease is mitochondrial dysfunction, which results in abnormal energy metabolism and increased ROS production in cardiomyocytes. Members of the mitochondrial uncoupling protein family, UCP2 and UCP3, are thought to be beneficial by reducing ROS due to mild uncoupling. More recent hypotheses suggest the involvement of both proteins in cell metabolism by the transport of yet unknown substrates. The protein expression pattern under physiological and pathological conditions is an important clue for the evaluation of UCP2/UCP3 function, however, there is still no consensus about it. Previously, we demonstrated that only UCP3 is present in the adult murine heart under physiological conditions and correlated it with the predominant use of fatty acids for oxidation. In contrast, UCP2 was found only in very young (stem cell – like) cardiomyocytes, that rely mostly on glycolysis. Here, we employed three different models (ex vivo heart ischemia-reperfusion model, myocardial infarction model, and embryonic stem cell differentiation into cardiomyocytes under hypoxic conditions) to evaluate the abundance of both proteins under ischemia and hypoxia conditions. We found that (i) oxygen shortage or bursts did not influence UCP3 levels in the heart and ii) UCP2 was not present in healthy, ischemic, or re-perfused hearts. However, (iii) UCP2 was sensitive to the oxygen concentration in stem cells, in which UCP2 is normally expressed. These results further support the idea, that two highly homologous proteins – UCP2 and UCP3 – are abundant in different cells and tissues, and differently regulated under physiological and pathological conditions.