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Caspase cleavage product of BAP31 induces mitochondrial fission through endoplasmic reticulum calcium signals, enhancing cytochrome c release to the cytosol.

Breckenridge DG, Stojanovic M, Marcellus RC, Shore GC - J. Cell Biol. (2003)

Bottom Line: Cell.Inhibition of Drp1 or ER-mitochondrial Ca2+ signaling prevented p20-induced fission of mitochondria. p20 strongly sensitized mitochondria to caspase-8-induced cyt.c release, whereas prolonged expression of p20 on its own ultimately induced caspase activation and apoptosis through the mitochondrial apoptosome stress pathway.Therefore, caspase-8 cleavage of BAP31 at the ER stimulates Ca2+-dependent mitochondrial fission, enhancing the release of cyt.c in response to this initiator caspase.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, McGill University, Montreal, Quebec, Canada H3G 1Y6.

ABSTRACT
Stimulation of cell surface death receptors activates caspase-8, which targets a limited number of substrates including BAP31, an integral membrane protein of the endoplasmic reticulum (ER). Recently, we reported that a caspase-resistant BAP31 mutant inhibited several features of Fas-induced apoptosis, including the release of cytochrome c (cyt.c) from mitochondria (Nguyen, M., D.G. Breckenridge, A. Ducret, and G.C. Shore. 2000. Mol. Cell. Biol. 20:6731-6740), implicating ER-mitochondria crosstalk in this pathway. Here, we report that the p20 caspase cleavage fragment of BAP31 can direct pro-apoptotic signals between the ER and mitochondria. Adenoviral expression of p20 caused an early release of Ca2+ from the ER, concomitant uptake of Ca2+ into mitochondria, and mitochondrial recruitment of Drp1, a dynamin-related protein that mediates scission of the outer mitochondrial membrane, resulting in dramatic fragmentation and fission of the mitochondrial network. Inhibition of Drp1 or ER-mitochondrial Ca2+ signaling prevented p20-induced fission of mitochondria. p20 strongly sensitized mitochondria to caspase-8-induced cyt.c release, whereas prolonged expression of p20 on its own ultimately induced caspase activation and apoptosis through the mitochondrial apoptosome stress pathway. Therefore, caspase-8 cleavage of BAP31 at the ER stimulates Ca2+-dependent mitochondrial fission, enhancing the release of cyt.c in response to this initiator caspase.

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A proposed mechanism of p20-induced mitochondrial fission. (A) p20 triggers a specific Ca2+ signal from the ER that is decoded by mitochondria. Mitochondria, in turn, recruit Drp1, which initiates organelle fission. Lowering ER Ca2+ stores by pretreatment with TG or expression of b5-BCL-2, chelating the Ca2+ released to the cytosol with BAPTA, blocking mitochondrial uptake of Ca2+ with Ru360, or inhibition of Drp1 by expression of Drp1K38E all prevent p20-induced mitochondrial fission. (B) A model depicting how in intact cells, cleavage of BAP31 at the ER sensitizes mitochondria to caspase-8–driven cyt.c release. Stimulation of Fas leads to caspase-8–dependent processing of BAP31 and BID, generating p20 and tBID. tBID translocates to mitochondria, where it induces the oligomerization of BAX/BAK into pores in the OMM. Simultaneously, p20 triggers ER Ca2+ release, causing Drp-1 translocation to mitochondria and subsequent organelle fission, enhancing the release of cyt.c to the cytosol.
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fig8: A proposed mechanism of p20-induced mitochondrial fission. (A) p20 triggers a specific Ca2+ signal from the ER that is decoded by mitochondria. Mitochondria, in turn, recruit Drp1, which initiates organelle fission. Lowering ER Ca2+ stores by pretreatment with TG or expression of b5-BCL-2, chelating the Ca2+ released to the cytosol with BAPTA, blocking mitochondrial uptake of Ca2+ with Ru360, or inhibition of Drp1 by expression of Drp1K38E all prevent p20-induced mitochondrial fission. (B) A model depicting how in intact cells, cleavage of BAP31 at the ER sensitizes mitochondria to caspase-8–driven cyt.c release. Stimulation of Fas leads to caspase-8–dependent processing of BAP31 and BID, generating p20 and tBID. tBID translocates to mitochondria, where it induces the oligomerization of BAX/BAK into pores in the OMM. Simultaneously, p20 triggers ER Ca2+ release, causing Drp-1 translocation to mitochondria and subsequent organelle fission, enhancing the release of cyt.c to the cytosol.

Mentions: Based on studies using pharmacological modulators of Ca2+ signaling and inhibitors of apoptosis and mitochondrial fission, our results suggest that p20 induces an apoptotic pathway between the ER and mitochondria (Fig. 8 A). This is initiated by ER Ca2+ release coupled to mitochondrial Ca2+ uptake. An important caveat, of course, is that such conclusions rely on the specificity of the inhibitors that are widely used to interfere with Ca2+ signaling. Moreover, it cannot be ruled out that additional mechanisms are also involved. Importantly, however, it has been demonstrated that Drp1 recruitment to mitochondria initiates fission (Labrousse et al., 1999; Smirnova et al., 2001). Because either the lowering of ER Ca2+ stores, or chelating cytosolic Ca2+, or preventing mitochondrial Ca2+ uptake all prevented p20-induced fission of mitochondria, it is likely that ER-mitochondrial Ca2+ transmission acts upstream of Drp1 translocation in this context. Drp1 recruitment is likely mediated by an OMM receptor protein(s), and this complex likely cooperates with an inner mitochondrial membrane reorganizing enzyme(s) to mediate organelle fission (Shaw and Nunnari, 2002). Mitochondrial membranes are often in close proximity and privileged Ca2+ exchange between the two organelles has previously been implicated during apoptosis. For example, IP3 receptor– and ryanodine receptor–mediated Ca2+ spikes that modulate mitochondrial metabolism in healthy cells also sensitize mitochondria to pro-apoptotic stimuli during cell death (Szalai et al., 1999; Hajnoczky et al., 2000). Moreover, manipulations that increase [Ca2+]ER also increase agonist-induced Ca2+ spikes and enhance mitochondrial cyt.c release and apoptosis, whereas a lowering of ER Ca2+ stores has the opposite effect (Nakamura et al., 2000; Pinton et al., 2001). Modulation of the frequency, amplitude and spatio-temporal pattern of ER Ca2+ release during apoptosis may determine how mitochondria respond to Ca2+ signals (Berridge et al., 2000; Pacher and Hajnoczky, 2001). Our results suggest that caspase cleavage of BAP31 may be one mechanism to generate such pro-apoptotic ER-mitochondrial Ca2+-dependent crosstalk in the Fas pathway.


Caspase cleavage product of BAP31 induces mitochondrial fission through endoplasmic reticulum calcium signals, enhancing cytochrome c release to the cytosol.

Breckenridge DG, Stojanovic M, Marcellus RC, Shore GC - J. Cell Biol. (2003)

A proposed mechanism of p20-induced mitochondrial fission. (A) p20 triggers a specific Ca2+ signal from the ER that is decoded by mitochondria. Mitochondria, in turn, recruit Drp1, which initiates organelle fission. Lowering ER Ca2+ stores by pretreatment with TG or expression of b5-BCL-2, chelating the Ca2+ released to the cytosol with BAPTA, blocking mitochondrial uptake of Ca2+ with Ru360, or inhibition of Drp1 by expression of Drp1K38E all prevent p20-induced mitochondrial fission. (B) A model depicting how in intact cells, cleavage of BAP31 at the ER sensitizes mitochondria to caspase-8–driven cyt.c release. Stimulation of Fas leads to caspase-8–dependent processing of BAP31 and BID, generating p20 and tBID. tBID translocates to mitochondria, where it induces the oligomerization of BAX/BAK into pores in the OMM. Simultaneously, p20 triggers ER Ca2+ release, causing Drp-1 translocation to mitochondria and subsequent organelle fission, enhancing the release of cyt.c to the cytosol.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2172754&req=5

fig8: A proposed mechanism of p20-induced mitochondrial fission. (A) p20 triggers a specific Ca2+ signal from the ER that is decoded by mitochondria. Mitochondria, in turn, recruit Drp1, which initiates organelle fission. Lowering ER Ca2+ stores by pretreatment with TG or expression of b5-BCL-2, chelating the Ca2+ released to the cytosol with BAPTA, blocking mitochondrial uptake of Ca2+ with Ru360, or inhibition of Drp1 by expression of Drp1K38E all prevent p20-induced mitochondrial fission. (B) A model depicting how in intact cells, cleavage of BAP31 at the ER sensitizes mitochondria to caspase-8–driven cyt.c release. Stimulation of Fas leads to caspase-8–dependent processing of BAP31 and BID, generating p20 and tBID. tBID translocates to mitochondria, where it induces the oligomerization of BAX/BAK into pores in the OMM. Simultaneously, p20 triggers ER Ca2+ release, causing Drp-1 translocation to mitochondria and subsequent organelle fission, enhancing the release of cyt.c to the cytosol.
Mentions: Based on studies using pharmacological modulators of Ca2+ signaling and inhibitors of apoptosis and mitochondrial fission, our results suggest that p20 induces an apoptotic pathway between the ER and mitochondria (Fig. 8 A). This is initiated by ER Ca2+ release coupled to mitochondrial Ca2+ uptake. An important caveat, of course, is that such conclusions rely on the specificity of the inhibitors that are widely used to interfere with Ca2+ signaling. Moreover, it cannot be ruled out that additional mechanisms are also involved. Importantly, however, it has been demonstrated that Drp1 recruitment to mitochondria initiates fission (Labrousse et al., 1999; Smirnova et al., 2001). Because either the lowering of ER Ca2+ stores, or chelating cytosolic Ca2+, or preventing mitochondrial Ca2+ uptake all prevented p20-induced fission of mitochondria, it is likely that ER-mitochondrial Ca2+ transmission acts upstream of Drp1 translocation in this context. Drp1 recruitment is likely mediated by an OMM receptor protein(s), and this complex likely cooperates with an inner mitochondrial membrane reorganizing enzyme(s) to mediate organelle fission (Shaw and Nunnari, 2002). Mitochondrial membranes are often in close proximity and privileged Ca2+ exchange between the two organelles has previously been implicated during apoptosis. For example, IP3 receptor– and ryanodine receptor–mediated Ca2+ spikes that modulate mitochondrial metabolism in healthy cells also sensitize mitochondria to pro-apoptotic stimuli during cell death (Szalai et al., 1999; Hajnoczky et al., 2000). Moreover, manipulations that increase [Ca2+]ER also increase agonist-induced Ca2+ spikes and enhance mitochondrial cyt.c release and apoptosis, whereas a lowering of ER Ca2+ stores has the opposite effect (Nakamura et al., 2000; Pinton et al., 2001). Modulation of the frequency, amplitude and spatio-temporal pattern of ER Ca2+ release during apoptosis may determine how mitochondria respond to Ca2+ signals (Berridge et al., 2000; Pacher and Hajnoczky, 2001). Our results suggest that caspase cleavage of BAP31 may be one mechanism to generate such pro-apoptotic ER-mitochondrial Ca2+-dependent crosstalk in the Fas pathway.

Bottom Line: Cell.Inhibition of Drp1 or ER-mitochondrial Ca2+ signaling prevented p20-induced fission of mitochondria. p20 strongly sensitized mitochondria to caspase-8-induced cyt.c release, whereas prolonged expression of p20 on its own ultimately induced caspase activation and apoptosis through the mitochondrial apoptosome stress pathway.Therefore, caspase-8 cleavage of BAP31 at the ER stimulates Ca2+-dependent mitochondrial fission, enhancing the release of cyt.c in response to this initiator caspase.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, McGill University, Montreal, Quebec, Canada H3G 1Y6.

ABSTRACT
Stimulation of cell surface death receptors activates caspase-8, which targets a limited number of substrates including BAP31, an integral membrane protein of the endoplasmic reticulum (ER). Recently, we reported that a caspase-resistant BAP31 mutant inhibited several features of Fas-induced apoptosis, including the release of cytochrome c (cyt.c) from mitochondria (Nguyen, M., D.G. Breckenridge, A. Ducret, and G.C. Shore. 2000. Mol. Cell. Biol. 20:6731-6740), implicating ER-mitochondria crosstalk in this pathway. Here, we report that the p20 caspase cleavage fragment of BAP31 can direct pro-apoptotic signals between the ER and mitochondria. Adenoviral expression of p20 caused an early release of Ca2+ from the ER, concomitant uptake of Ca2+ into mitochondria, and mitochondrial recruitment of Drp1, a dynamin-related protein that mediates scission of the outer mitochondrial membrane, resulting in dramatic fragmentation and fission of the mitochondrial network. Inhibition of Drp1 or ER-mitochondrial Ca2+ signaling prevented p20-induced fission of mitochondria. p20 strongly sensitized mitochondria to caspase-8-induced cyt.c release, whereas prolonged expression of p20 on its own ultimately induced caspase activation and apoptosis through the mitochondrial apoptosome stress pathway. Therefore, caspase-8 cleavage of BAP31 at the ER stimulates Ca2+-dependent mitochondrial fission, enhancing the release of cyt.c in response to this initiator caspase.

Show MeSH
Related in: MedlinePlus