Mahdaviani 2015 Abstract MiPschool Greenville 2015

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Lipid droplets interact with an exclusive sub-population of mitochondria in brown adipocyte.

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Mahdaviani K, Twig G, Wikstrom JD, Benador I, Chess D, Trudeau K, Liesa M, Shirihai OS (2015)

Event: MiPschool Greenville 2015

We have recently shown that mitochondrial dynamics is a physiological regulator of adrenergically‐induced changes in energy expenditure and lipid metabolism in brown adipocyte (BA)[1]. In this study we addressed the hypothesis that local changes to mitochondrial dynamics at subcellular levels play a role in generating sub-populations of mitochondria with specialized functions.

Mitochondrial membrane potential, motility and morphology were measured using TMRE and PAGFP respectively with Zeiss LSM 710 confocal microscope. NADH autofluorescent was used to measure the NADH content of the mitochondria. Protein import and turnover were measured using MitoTimer probe.

Confocal microscopy of brown adipocytes harvested from wild-type C57BL6/J mice identifies the mitochondria surrounding the lipid droplets as an exclusive sub-population of mitochondria. Peri-droplret mitochondria (PDM) in BA are more elongated as compared to cytosolic-mitochondria (CM) (average length of PDM and CM are 4.9 and 3.2 µm respectively). This form of mitochondria is more commonly associated with coupled respiration. Indeed, measurements of membrane potential and NADH show that PDM have higher mitochondrial membrane potential and higher NADH content. Interestingly the heterogeneity in membrane potential can be manipulated by mitochondrial dynamic protein expression and by hormonal stimulation. As the level of coupling and ATP synthesis in BA may be influenced by the levels of ATP synthase, which is expressed at very low levels in this tissue, we tested the ATP synthase expression in this specific set of mitochondria. Immune fluorescence and super-resolution microscopy indicated that ATP synthase in the brown adipocyte is concentrated in PDM. Since mitochondrial composition has been shown in other cells to be equilibrated and homogenized by continuous mitochondrial fusion and fission, we next measured the levels of fusion and equilibration across the mitochondrial population in the BA. Using fusion assay and matrix-targeted PAGFP we found that following photo-conversion the fluorescent intensity of PAGFP in the matrix of PDM stays ~10% higher, 65 min after photo activation in PDM, indicating lower fusion activity. Ratiometric analysis of MitoTimer probe showed that PDM have similar rate of protein turnover, but higher protein imports and protein content possibly to compensate for the lack of fusion with the rest of the mitochondrial population. This finding is also supported by their higher Tomm20 content.

In conclusion, PDM represent a sub-population of mitochondria with unique protein composition, architecture and activity that is likely maintained due to their interaction with lipid droplets as well as their reduced level of interaction with the rest of the mitochondrial population.


O2k-Network Lab: SE Stockholm Wikstroem J


Labels: MiParea: mt-Structure;fission;fusion, mt-Membrane, Comparative MiP;environmental MiP 


Organism: Mouse  Tissue;cell: Fat 


Regulation: mt-Membrane potential 


Event: Poster 


Affiliations

1-Dept Med, Boston Univ School Med, MA, USA; 2-Dept Mol Biosc, Wenner-Gren Inst, Stockholm Univ, Sweden; 3-Dept Med and Dr Pinchas Bornstein Talpiot Med Leadership Program, Chaim Sheba Med Center, Tel-Hashomer, Israel. - kiana.mahdaviani@gmail.com

References and acknowledgements

Wikstrom JD, Mahdaviani K, Liesa M, Sereda SB, Si Y, Las G, Twig G, Petrovic N, Zingaretti C, Graham A, Cinti S, Corkey BE, Cannon B, Nedergaard J, Shirihai OS (2014) Hormone-induced mitochondrial fission is utilized by brown adipocytes as an amplification pathway for energy expenditure. EMBO J 33:418-36.