Contents

📒 Stiles 2012

Mitochondrial dynamics and morphology in beta-cells1

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Mitochondrial dynamics

Mitochondria are dynamic organelles that function as heterogeneous networks, undergoing fusion and fission events, together termed mitochondrial dynamics.

It has been established that mitochondrial dynamics can influence various aspects of mitochondrial biology including mitochondrial biogenesis, bioenergetics, heterogeneity and elimination.3

Fusion

Fusion is regulated by at least three mitochondrially localized GTPases: mitofusin 1 (Mfn1 @ OMM), mitofusin 2 (Mfn2 @ OMM), and optic atrophy protein 1 (Opa1 @ IMM).

Fission

Fission is mediated by the transmembrane protein Fis1, the cytosolic GTPase dynamin related protein 1 (Drp1/DNM1L),and mitochondrial fission factor (Mff).

Occurs at mitochondria-ER contacts.

Relation to bioenergetics

Mitochondrial dynamics facilitate the maintenance of a metabolically efficient mitochondrial population and disruption of either fusion or fission alters mitochondrial morphology and functionality

Dysfunctional glucose-stimulated insulin secretion (GSIS) in models of type 2 diabetes with mitochondrial dysfunction, including changes in respiratory chain activity.

Hypothesis that an impaired balance between fusion and fission contribute to the deterioration of beta-cell function in the progression of diabetes.

Disruptions in mitochondrial fusion and fission alter beta-cell function

Overexpression or knockdown of mitochondrial dynamics: OPA1, Mfn1, Drp1, Fis1.

Mitochondrial dynamics and autophagy

Alterations in beta-cell autophagy contribute to the development of diabetes, with decreased basal and glucose-stimulated insulin secretion as well as decreased glucose-induce changes to cytosolic Ca2+ concentrations.

Twig et al. demonstrated that mitochondrial fusion and fission play a prominent role in regulating mitochondrial autophagy, termed mitophagy, which is the process by which dysfunctional mitochondria are degraded by autophagy.

https://ars.els-cdn.com/content/image/1-s2.0-S1521690X12000656-gr2_lrg.jpg
The quality control cycle of a mitochondrion. Mitochondrial fusion,1 fission,2 and autophagy

Characteristics of beta-cell mitochondria, exposure to high concentrations of nutrients

GSIS: Glucose-stimulated insulin secretion.

Mitochodria as part of the nutrient(glucose, amino acids, lipids) sensor.3

Nutrient -> OXPHOS (@Mitochondria) -> ATP -> closure of K-ATP channel -> depolarize of plasma membrane -> Ca influx -> insulin secretion.

GSIS occurs in two distinct phases, the first a triggering phase, which is followed by a secondary amplifying phase. Mitochondria metabolism participates in both phases.

Mitochondria in beta-cells (human & INS-1) formed tubular networks throughout the cytosol.

https://ars.els-cdn.com/content/image/1-s2.0-S1521690X12000656-gr1_lrg.jpg
Mitochondrial morphology and dynamics in beta-cells

  • (A) Three-dimensional reconstruction of the mitochondria within an entire primary mouse beta-cell. Mitochondria are densely packed within the beta-cell.
  • (B) Mitochondrial fusion assays in INS-1 cells demonstrate normal mitochondrial fusion in control cells and inhibition of mitochondrial fusion in cells treated with HFG2 for 24 h. Decreased mitochondrial fusion capacity in HFG.
  • (C) Mitochondrial fragmentation in INS-1 cells exposed to HFG2 for 24 h. Marked mitochodrial network fragmentation within 4 hours.

Evidence for Changes in Beta-Cell Mitochondrial Dynamics in Diabetes

Diabetic beta-cells have reduced ATP levels, a lower ATP/ADP ratio and impaired hyperpolarization of the mitochondrial membrane. Increased protein expression of UCP-2, complex I and the ATP synthase of the respiratory chain, and a higher level of reactive nitrogen species were also found in type 2 diabetic islets.

Mitochondrial morphology is changed to a more fragmented state in T2DM.

Mitochondrial Dynamics May Influence Other Regulators of Beta-Cell Function

ROS singaling (electron leak)

Inhibition of mitochondrial fission with Fis1 RNAi in INS-1 cells reduces mitophagy and leads to the accumulation of oxidized protein.

Mitochondrial fragmentation was necessary for the high glucose-induced respiration responsible for the increased ROS production and that inhibition of fission with Drp1-DN prevented this ROS production.

The use of rotenone to inhibit ETC activity at complex I results in decreased ∆ψmt, increased ROS production, and fragmented mitochondrial morphology.

Calcium signaling

Proxomity of ER and mitochondria facilitates calcium buffering.

Mitochondrial membrane potential is a driving force for Ca2+ uptake into the mitochondrial matrix where it regulates the activity of Kreb Cycle (TCA cycle, citric acid cycle) enzymes, which can influence oxidative phosphorylation.

Ca2+ overload can induce the mitochondrial permeability transition pore (mPTP), ΔΨ depolarization, and apoptosis.

Increased intracellular calcium concentrations is the primary trigger for insulin granule exocytosis.

Coupling efficiency (proton leak)

Changes in proton leak and ΔΨ are often associated with mitochondrial fragmentation in cells such as skeletal muscle and adipocytes.

75% of the respiration measured from INS1 cells was reported to be uncoupled in comparison to 20% in myoblasts.

The role of proton leak

  • TCA cycle would run in high flux even in high concentrations of nutrients
  • Protect beta-cell from nutrient-induced ROS production
  • Determine the dynamic range for nutrient sensing

  1. Stiles L, Shirihai OS. Mitochondrial dynamics and morphology in beta-cells. Best Pract Res Clin Endocrinol Metab. 2012 Dec;26(6):725–38. PMC5967392 ↩︎

  2. HFG: high glucose concentrations to the free fatty acids ↩︎