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Autophagy capacity and sub-mitochondrial heterogeneity shape Bnip3-induced mitophagy regulation of apoptosis.

Choe SC, Hamacher-Brady A, Brady NR - Cell Commun. Signal (2015)

Bottom Line: Previously, we have shown that Bnip3-activated mitophagy prior to apoptosis induction can reduce mitochondrial activation of caspases, suggesting that a reduction to mitochondrial levels may be pro-survival.Our model, supported by experimental evidence, identified an emergent regulatory structure within canonical apoptosis signaling.We show that the extent of mitophagy is determined by levels and spatial localization of autophagy capacity, and subcellular mitochondrial protein heterogeneities.

View Article: PubMed Central - PubMed

Affiliation: Systems Biology of Cell Death Mechanisms, German Cancer Research Center (DKFZ), Heidelberg, Germany.

ABSTRACT

Background: Mitochondria are key regulators of apoptosis. In response to stress, BH3-only proteins activate pro-apoptotic Bcl2 family proteins Bax and Bak, which induce mitochondrial outer membrane permeabilization (MOMP). While the large-scale mitochondrial release of pro-apoptotic proteins activates caspase-dependent cell death, a limited release results in sub-lethal caspase activation which promotes tumorigenesis. Mitochondrial autophagy (mitophagy) targets dysfunctional mitochondria for degradation by lysosomes, and undergoes extensive crosstalk with apoptosis signaling, but its influence on apoptosis remains undetermined. The BH3-only protein Bnip3 integrates apoptosis and mitophagy signaling at different signaling domains. Bnip3 inhibits pro-survival Bcl2 members via its BH3 domain and activates mitophagy through its LC3 Interacting Region (LIR), which is responsible for binding to autophagosomes. Previously, we have shown that Bnip3-activated mitophagy prior to apoptosis induction can reduce mitochondrial activation of caspases, suggesting that a reduction to mitochondrial levels may be pro-survival. An outstanding question is whether organelle dynamics and/or recently discovered subcellular variations of protein levels responsible for both MOMP sensitivity and crosstalk between apoptosis and mitophagy can influence the cellular apoptosis decision event. To that end, here we undertook a systems biology analysis of mitophagy-apoptosis crosstalk at the level of cellular mitochondrial populations.

Results: Based on experimental findings, we developed a multi-scale, hybrid model with an individually adaptive mitochondrial population, whose actions are determined by protein levels, embedded in an agent-based model (ABM) for simulating subcellular dynamics and local feedback via reactive oxygen species signaling. Our model, supported by experimental evidence, identified an emergent regulatory structure within canonical apoptosis signaling. We show that the extent of mitophagy is determined by levels and spatial localization of autophagy capacity, and subcellular mitochondrial protein heterogeneities. Our model identifies mechanisms and conditions that alter the mitophagy decision within mitochondrial subpopulations to an extent sufficient to shape cellular outcome to apoptotic stimuli.

Conclusion: Overall, our modeling approach provides means to suggest new experiments and implement findings at multiple scales in order to understand how network topologies and subcellular heterogeneities can influence signaling events at individual organelle level, and hence, determine the emergence of heterogeneity in cellular decisions due the actions of the collective intra-cellular population.

No MeSH data available.


Related in: MedlinePlus

Single mitochondrion dynamics. a Illustration of Bnip3 dual-functionality due to LIR and BH3 domains. ROS and Bcl2/xL (gray boxes) are points of crosstalk between two distinct branches: LIR-induced mitochondrial autophagy (mitophagy) pathway (blue) and apoptosis signaling by activator BH3 proteins (e.g. tBid), which induces Bax-mediated MOMP and cytochrome c release to induce caspase cascade (red). b Level values of the ODE species represent the mitophagy (blue) versus apoptosis (red) activity capacity for a mitochondrion. The shaded areas indicate range of activity as a function of increasing tBid activation (direction of arrows) and 20 % Bnip3 pre-activation. The overlap shows competition between both pathways via Bnip3. c Illustration of Bnip3 mutants with constitutively-active (2SE) and constitutively-inactive (2SA) LIR domain. d Scenarios of increasingly delayed timing of tBid activation (t = 0, 10, 50) for all mutants of Bnip3 with increasing tBid (direction of arrow) activation. e ROS production as a function of different tBid and autophagic vesicles (AV) level combinations for all three Bnip3 mutants
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Fig1: Single mitochondrion dynamics. a Illustration of Bnip3 dual-functionality due to LIR and BH3 domains. ROS and Bcl2/xL (gray boxes) are points of crosstalk between two distinct branches: LIR-induced mitochondrial autophagy (mitophagy) pathway (blue) and apoptosis signaling by activator BH3 proteins (e.g. tBid), which induces Bax-mediated MOMP and cytochrome c release to induce caspase cascade (red). b Level values of the ODE species represent the mitophagy (blue) versus apoptosis (red) activity capacity for a mitochondrion. The shaded areas indicate range of activity as a function of increasing tBid activation (direction of arrows) and 20 % Bnip3 pre-activation. The overlap shows competition between both pathways via Bnip3. c Illustration of Bnip3 mutants with constitutively-active (2SE) and constitutively-inactive (2SA) LIR domain. d Scenarios of increasingly delayed timing of tBid activation (t = 0, 10, 50) for all mutants of Bnip3 with increasing tBid (direction of arrow) activation. e ROS production as a function of different tBid and autophagic vesicles (AV) level combinations for all three Bnip3 mutants

Mentions: We first developed an ODE model for a single mitochondrion based on experimental findings in order to evaluate dynamic behavior stemming from Bnip3-mediated mitophagy and apoptosis signaling (Fig. 1a, Additional file 1: Figure S1). We concentrated on qualitative analysis by parameterizing our model with relative levels and constants (Additional file 2: Figure S2). In the apoptotic pathway (Fig. 1a, red), the BH3 domain of Bnip3 suppresses anti-apoptotic Bcl2 member function [40], acting as a sensitizer BH3 protein to tBid-mediated activation of Bax [41], and leads to ROS generation and caspase activation [21, 42]. In parallel, the mitophagy pathway (Fig. 1a, blue) is active when the phosphorylated LIR domain of Bnip3 binds autophagosomes, and Bcl-xL positively regulates mitophagy [14]. Note, as ROS are central signaling messengers in apoptosis, autophagy and mitophagy [30–32], ROS was included as a prerequisite for activation of Bnip3 signaling [43].Fig. 1


Autophagy capacity and sub-mitochondrial heterogeneity shape Bnip3-induced mitophagy regulation of apoptosis.

Choe SC, Hamacher-Brady A, Brady NR - Cell Commun. Signal (2015)

Single mitochondrion dynamics. a Illustration of Bnip3 dual-functionality due to LIR and BH3 domains. ROS and Bcl2/xL (gray boxes) are points of crosstalk between two distinct branches: LIR-induced mitochondrial autophagy (mitophagy) pathway (blue) and apoptosis signaling by activator BH3 proteins (e.g. tBid), which induces Bax-mediated MOMP and cytochrome c release to induce caspase cascade (red). b Level values of the ODE species represent the mitophagy (blue) versus apoptosis (red) activity capacity for a mitochondrion. The shaded areas indicate range of activity as a function of increasing tBid activation (direction of arrows) and 20 % Bnip3 pre-activation. The overlap shows competition between both pathways via Bnip3. c Illustration of Bnip3 mutants with constitutively-active (2SE) and constitutively-inactive (2SA) LIR domain. d Scenarios of increasingly delayed timing of tBid activation (t = 0, 10, 50) for all mutants of Bnip3 with increasing tBid (direction of arrow) activation. e ROS production as a function of different tBid and autophagic vesicles (AV) level combinations for all three Bnip3 mutants
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4528699&req=5

Fig1: Single mitochondrion dynamics. a Illustration of Bnip3 dual-functionality due to LIR and BH3 domains. ROS and Bcl2/xL (gray boxes) are points of crosstalk between two distinct branches: LIR-induced mitochondrial autophagy (mitophagy) pathway (blue) and apoptosis signaling by activator BH3 proteins (e.g. tBid), which induces Bax-mediated MOMP and cytochrome c release to induce caspase cascade (red). b Level values of the ODE species represent the mitophagy (blue) versus apoptosis (red) activity capacity for a mitochondrion. The shaded areas indicate range of activity as a function of increasing tBid activation (direction of arrows) and 20 % Bnip3 pre-activation. The overlap shows competition between both pathways via Bnip3. c Illustration of Bnip3 mutants with constitutively-active (2SE) and constitutively-inactive (2SA) LIR domain. d Scenarios of increasingly delayed timing of tBid activation (t = 0, 10, 50) for all mutants of Bnip3 with increasing tBid (direction of arrow) activation. e ROS production as a function of different tBid and autophagic vesicles (AV) level combinations for all three Bnip3 mutants
Mentions: We first developed an ODE model for a single mitochondrion based on experimental findings in order to evaluate dynamic behavior stemming from Bnip3-mediated mitophagy and apoptosis signaling (Fig. 1a, Additional file 1: Figure S1). We concentrated on qualitative analysis by parameterizing our model with relative levels and constants (Additional file 2: Figure S2). In the apoptotic pathway (Fig. 1a, red), the BH3 domain of Bnip3 suppresses anti-apoptotic Bcl2 member function [40], acting as a sensitizer BH3 protein to tBid-mediated activation of Bax [41], and leads to ROS generation and caspase activation [21, 42]. In parallel, the mitophagy pathway (Fig. 1a, blue) is active when the phosphorylated LIR domain of Bnip3 binds autophagosomes, and Bcl-xL positively regulates mitophagy [14]. Note, as ROS are central signaling messengers in apoptosis, autophagy and mitophagy [30–32], ROS was included as a prerequisite for activation of Bnip3 signaling [43].Fig. 1

Bottom Line: Previously, we have shown that Bnip3-activated mitophagy prior to apoptosis induction can reduce mitochondrial activation of caspases, suggesting that a reduction to mitochondrial levels may be pro-survival.Our model, supported by experimental evidence, identified an emergent regulatory structure within canonical apoptosis signaling.We show that the extent of mitophagy is determined by levels and spatial localization of autophagy capacity, and subcellular mitochondrial protein heterogeneities.

View Article: PubMed Central - PubMed

Affiliation: Systems Biology of Cell Death Mechanisms, German Cancer Research Center (DKFZ), Heidelberg, Germany.

ABSTRACT

Background: Mitochondria are key regulators of apoptosis. In response to stress, BH3-only proteins activate pro-apoptotic Bcl2 family proteins Bax and Bak, which induce mitochondrial outer membrane permeabilization (MOMP). While the large-scale mitochondrial release of pro-apoptotic proteins activates caspase-dependent cell death, a limited release results in sub-lethal caspase activation which promotes tumorigenesis. Mitochondrial autophagy (mitophagy) targets dysfunctional mitochondria for degradation by lysosomes, and undergoes extensive crosstalk with apoptosis signaling, but its influence on apoptosis remains undetermined. The BH3-only protein Bnip3 integrates apoptosis and mitophagy signaling at different signaling domains. Bnip3 inhibits pro-survival Bcl2 members via its BH3 domain and activates mitophagy through its LC3 Interacting Region (LIR), which is responsible for binding to autophagosomes. Previously, we have shown that Bnip3-activated mitophagy prior to apoptosis induction can reduce mitochondrial activation of caspases, suggesting that a reduction to mitochondrial levels may be pro-survival. An outstanding question is whether organelle dynamics and/or recently discovered subcellular variations of protein levels responsible for both MOMP sensitivity and crosstalk between apoptosis and mitophagy can influence the cellular apoptosis decision event. To that end, here we undertook a systems biology analysis of mitophagy-apoptosis crosstalk at the level of cellular mitochondrial populations.

Results: Based on experimental findings, we developed a multi-scale, hybrid model with an individually adaptive mitochondrial population, whose actions are determined by protein levels, embedded in an agent-based model (ABM) for simulating subcellular dynamics and local feedback via reactive oxygen species signaling. Our model, supported by experimental evidence, identified an emergent regulatory structure within canonical apoptosis signaling. We show that the extent of mitophagy is determined by levels and spatial localization of autophagy capacity, and subcellular mitochondrial protein heterogeneities. Our model identifies mechanisms and conditions that alter the mitophagy decision within mitochondrial subpopulations to an extent sufficient to shape cellular outcome to apoptotic stimuli.

Conclusion: Overall, our modeling approach provides means to suggest new experiments and implement findings at multiple scales in order to understand how network topologies and subcellular heterogeneities can influence signaling events at individual organelle level, and hence, determine the emergence of heterogeneity in cellular decisions due the actions of the collective intra-cellular population.

No MeSH data available.


Related in: MedlinePlus