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

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.

Mitochondria and cancer: is there a morphological connection?

“…Fis1 overexpression not only induces fragmentation of the mitochondrial network, but also leads to the release of cytochrome c from the mitochondria caspase activation and cell death . Silencing of either Drp1 or Fis1 confers resistance to several apoptotic stimuli and impairs cytochrome c release suggesting that fission molecules are required for MMP to occur (Frank et al, 2001;Karbowski et al, 2002;Breckenridge et al, 2003;Lee et al, 2004;Germain et al, 2005). However, translocation of Bax to the mitochondria remains unaffected by fission inhibition, suggesting that fission molecules act downstream of Bax translocation.…”

Mitochondrial dynamics in the regulation of neuronal cell death

“…Moreover, inhibition of Drp1 activity by expression of the dominant negative mutant could prevent mitochondrial fission, cytochrome c release, and the subsequent cell death [28]. Another study examining an ER initiated apoptotic pathway revealed that inhibition of Drp1 activity could also prevent mitochondrial fission and cell death [63,64]. More recently, downregulation of the mitochondrial membrane bound fission protein, Fis1, could prevent mitochondrial fission and cell death [57].…”

Mitochondrial factors with dual roles in death and survival

“…Interestingly, activation of programmed cell death leads to excessive fission of tubular mitochondria into short punctate structures, also referred to as mitochondrial fragmentation. This shortening of mitochondria may be the earliest morphological change detected following a death stimulus (Breckenridge et al, 2003;Karbowski and Youle, 2003). Although fission machinery appears to be a critical component of cell death-associated mitochondrial fragmentation, reshaping of thin tubular mitochondria into spherical shapes could also give the appearance of fission even though none has occurred.…”

“…Indeed, human Drp1 was shown to be responsible for the excessive mitochondrial fission/fragmentation that occurs during programmed cell death in mammalian cells (Frank et al, 2001;Breckenridge et al, 2003;James et al, 2003). Furthermore, Drp1 plays an important role in cytochrome c release.…”