Apoptosis caused by cathepsins does not require Bid signaling in an in vivo model of progressive myoclonus epilepsy (EPM1)

Abstract

Apoptosis can be mediated by mechanisms other than the traditional caspase-mediated cleavage cascade. There is growing recognition that alternative proteolytic enzymes such as the lysosomal cathepsin proteases can initiate or propagate proapoptotic signals, but it is currently unclear how cathepsins achieve these actions. Recent in vitro evidence suggests that cathepsins cleave the proapoptotic Bcl-2 family member Bid, thereby activating it and allowing it to induce the mitochondrial release of cytochrome c and subsequent apoptosis. We have tested this hypothesis in vivo by breeding mice that lack cathepsin inhibition (cystatin B-deficient mice) to Bid-deficient mice, to determine whether the apoptosis caused by cathepsins is dependent on Bid signaling. We found that cathepsins are still able to promote apoptosis even in the absence of Bid, indicating that these proteases mediate apoptosis via a different pathway, or that some other molecule can functionally substitute for Bid in this system.

Cysteine Cathepsins Trigger Caspase-dependent Cell Death through Cleavage of Bid and Antiapoptotic Bcl-2 Homologues

“…This was later confirmed in several cellular models such as LeuLeuOMe-induced apoptosis in HeLa cells, with multiple cysteine cathepsins suggested to be responsible for the cleavage of Bid (32,39). However, apoptosis was found to be induced also in the absence of Bid cleavage in cellular models (35) and in an in vivo model of progressive myoclonus epilepsy (40), suggesting that cathepsins can induce apoptosis through cleavage of other cytosolic proteins, which however, remained unidentified.…”

Lysosomes in cell death

“…The data are consistent with a report showing that pharmacological inhibition of cathepsin B, L, and D activities does not suppress Bid cleavage, or procaspase-9 and -3 activation following lysosomal photodamage in murine hepatoma 1c1c7 cells, suggesting that other lysosomal proteases might be responsible for Bid cleavage (Reiners et al, 2002). Moreover, a study employing a model of cystatin B/Bid double knockout mice showed that the absence of Bid did not rescue the neurological phenotype caused by constitutional activation of cathepsins (due to the lack of the endogenous inhibitor cystatin B), suggesting that Bid is not required for cathepsin-mediated apoptosis in this in vivo model of cell death (Houseweart et al, 2003b). Finally, Bid does not contribute to apoptosis induced by the lysosomotropic photosensitizers ciprofloxacin or norfloxacin, as Bid-deficient mouse embryonic fibroblasts are not protected against mitochondrial membrane permeabilization (Boya et al, 2003a).…”

Transcriptional profiles of unirradiated or UV-irradiated human cells expressing either the cancer-prone XPB/CS allele or the noncancer-prone XPB/TTD allele

“…The cathepsin V belongs to the papain-like lysosomal cysteine protease family. Recently, it has been shown that lysosomal cathepsin proteases can initiate or propagate proapoptotic signals; however, it is unclear how cathepsins achieve these actions (Houseweart et al, 2003).…”