Contents

📒 Hatano 2015

Distinct Functional Roles of Cardiac Mitochondrial Subpopulations Revealed by a 3D Simulation Model1

Sciwheel.

Introduction

  • Mitochondria located in specific cell regions are reported to have different morphological and biochemical properties => subsarcolemmal mitochondria (SSM) and interfibrillar mitochondria (IFM)
  • IFM and SSM are differentially affected under pathological conditions

Materials and Methods

3D cardiomyocyte model

Previous model2 https://els-jbs-prod-cdn.literatumonline.com/cms/attachment/1af0ec73-23a6-40f9-b4b8-fd72dff6e129/gr1_lrg.jpg

  • Added oxygen and myoglobin buffering
  • compared simulated oxygen and NADH distributions with the experimental results of Takahashi et al

Modeling SSM and IFM

  • Mitochondria facing the surface sarcolemma were taken to be SSM and others were taken to be IFM
  • 10% of the mitochondrial node volume was treated as cytosolic space (corresponding to intermembrane space). Ions and metabolites freely diffusable between through OMM.
  • IFM have ∼50% higher oxidative capacity and TCA activity than SSM in reports.
    • equal model: same capacity
    • hetero model: 50% higher enzymatic activity in IFM

https://user-images.githubusercontent.com/40054455/86618472-8d79a200-bfeb-11ea-8d33-41d97829b630.png

Oxygen diffusion and oxidative phosphorylation

  • myoglobin as an immobile oxygen buffer and adopted a fast equilibrium assumption https://user-images.githubusercontent.com/40054455/86618462-8b174800-bfeb-11ea-840a-39791951b1ea.png https://user-images.githubusercontent.com/40054455/86618467-8c487500-bfeb-11ea-8733-f9b5d17dd101.png, α = 1.5 μM / mmHg
  • Rates of oxygen consumption and proton pumping (complex IV activity) multiplied by the MM factor of oxygen.
  • The governing equations are shown in the Appendix with rate constants obtained from the literature

Simulation protocols

  • simulation with the published experiment by Takahashi et al. measured intracellular oxygen and intramitochondrial NADH gradients. rat cardiomyocytes
  • simulate electrical pacing, we applied a current pulse (100 μA/cm², duration 0.5 ms)

Results

Validation of the oxygen diffusion model

https://els-jbs-prod-cdn.literatumonline.com/cms/attachment/c5a62901-6725-44f2-b70d-eb5ab6bf9210/gr2_lrg.jpg

  • The simulated rate of oxygen consumption was 0.23 μM/ms, which is comparable to the rates observed experimentally (0.15 μM/ms in Fig. 2 C and 0.24 μM/ms in Fig. 2 D

SSM and IFM responses to changes in contraction frequency

https://els-jbs-prod-cdn.literatumonline.com/cms/attachment/2cd7fdd1-09d2-4d8f-b4fc-b759e90ee888/gr3_lrg.jpg

  • Despite the higher NADH and inner membrane potential levels, IFM synthesized ATP at nearly the same rate as SSM. This strongly indicate a dominant effect of the local environment on the apparent functional differences between IFM and SSM

Response to hypoxia

https://els-jbs-prod-cdn.literatumonline.com/cms/attachment/4245ae5a-63ce-411c-8b57-b819e1a31c61/gr4_lrg.jpg

  • Immediately below the sarcolemma, PO2 decreased to 0.042 mmHg because of the transportation barrier created by the sarcolemma, whereas in the core region, PO2 dropped to <10−5 mmHg
  • proton pump activity in core IFM dropped to nearly zero, resulting in mitochondrial membrane potential deprivation, whereas SSM continued to function. ATPase activity dropped to minus values in core IFM
  • creatine phosphate (CrP) decreased fairly rapidly, and after depletion ATP started to drop
  • peak Ca2+ transient levels decreased after a latent peroid from depletion of SR Ca2+ content caused by depressed SR Ca2+-ATPase (SERCA) activity

Comparing the hypoxic response of the hetero and equal models

https://els-jbs-prod-cdn.literatumonline.com/cms/attachment/238e4166-a176-4e64-8b3f-e2f2e0188f26/gr5_lrg.jpg

  • hetero (dotted line) and equal (solid line) models
  • Under normoxic conditions, the contribution of SSM to ATP production was relatively small, only ∼9.5% in the equal model and 5.8% in the hetero model
  • under hypoxic conditions, the role of SSM dramatically increased in both models, with >70% of ATP being produced in SSM in the later stage of hypoxia
  • Regarding ATPase activity, IFM (especially outer IFM) showed less negative activity in the hetero model

Discussion

Validation of the model

  • EC coupling validated previously2
  • Oxygen and NADH gradients

Effect of subcellular location on mitochondrial Ca2+ handling

  • SSM showed lower Ca2+ levels, weaker NADH recovery, and higher ADP levels, indicating a dominant effect of the local environment
  • IFM show higher Ca2+ accumulation, but species dependent
  • But sarcolemmal Na+-Ca2+ exchanger distribution between the surface and t-tubules would change the mitochondrial Ca2+ environment

Response to hypoxia

  • steep oxygen gradients, core IFM stopped functioning and even started to consume ATP reserves, whereas in SSM function was upregulated
  • ATP and CrP diffusion is fast enough to keep [ATP] constant throughout the cell under normoxic conditions
  • ATP produced in SSM may preferentially serve to maintain the reactions of neighboring organelles and molecules, and sustain them for longer periods of time; contractile activity supported by IFM deteriorates quickly

Possible differences in intrinsic properties between IFM and SSM

  • IFM showed a higher NADH, inner membrane potential, and faster recovery after a change in the pacing frequency in the hetero model compared with the equal model
  • the role of SSM in ATP production increased significantly under hypoxic conditions (upregulated by 5x)

Study limitations

  • Not including some components working in low-[ATP] conditions, i.e., glycolysis, adenylate kinase, acidosis, and ADP- and AMP-dependent reactions, including KATP channels and myosin ATPase

Appendix

https://user-images.githubusercontent.com/40054455/86618469-8ce10b80-bfeb-11ea-8e27-962915f9f10d.png https://user-images.githubusercontent.com/40054455/86618474-8d79a200-bfeb-11ea-8fad-00819614d18f.png

Reference


  1. Hatano A, Okada J, Washio T, Hisada T, Sugiura S. Distinct functional roles of cardiac mitochondrial subpopulations revealed by a 3D simulation model. Biophys J. 2015;108(11):2732-9. PMC4457478↩︎

  2. Hatano A, Okada J, Hisada T, Sugiura S. Critical role of cardiac t-tubule system for the maintenance of contractile function revealed by a 3D integrated model of cardiomyocytes. J. Biomech. 2012;45(5):815-823. doi:10.1016/j.jbiomech.2011.11.022. ↩︎