Disruption, Lysosome leads to N/A, Mitochondrial dysfunction 1

Disrupted lysosomal membrane release the content of lysosomes including cathepsins. Cathepsins take part in activation of BH3-only proteins, which directly or indirectly activate pro-apoptic Bax and Bak proteins. Once activated Bax and Bak form dimers and higher order oligomers, in order to form pores in outer mitochondrial membrane and cause mitochondrial injury.

Many evidences suggest that lysosomal disruption usually precedes mitochondrial injury (Guicciardi et al., 2000; Brunk et al., 2001; Zhao et al. 2003; Droga-Mazovec et al., 2008), with lysosomal proteases inducing mitochondrial dysfunction.

Zhao and colleagues have also proposed the existence of a positive feed-back mechanism between lysosomal damage and mitochondrial damage, in which early lysosomal rupture causes mitochondrial rupture and leakage of mitochondrial proteins that increase lysosomal damage and consequent apoptosis (Zhao et al., 2000).

The pathway between lysosomal membrane permeabilization (LMP) and mitochondrial membrane permeabilization (MMP) is regulated principally by Bcl-2 family of proteins. The family is subdivided into anti-apoptotic multidomain proteins (such as Bcl-2, Bcl-xl, Bcl-W, Mcl-1 and A1), pro-apoptotic multidomain proteins (Bax and Bak) and pro-apoptotic BH3-only proteins (such as Bid, Puma, Noxa, Bim, Bad, and Bik) (Fletcher and Huang, 2006; Youle and Strasser, 2008).

Cathepsins, released after lysosomal damage, have a role in the cell death through the cleavage of BH3-only proteins, such as Bid, to generate active tBid (truncated Bid) (Blomgran et al., 2007; Cirman et al., 2004; Droga-Mazovec et al., 2008; Houseweart et al., 2003; Stoka et al., 2001) and by degradation of the anti-apoptotic Bcl-2 molecules Bcl-2, Bcl-xl and Mcl-1 (Blomgran et al., 2007; Droga-Mazovec et al., 2008). However it was shown that though Bid is not the only substrate of lysosomal enzymes that induce cytochrome c release, it is the major one (Stoka et al., 2001). Droga-Mazovec and colleagues showed that Bid is cleaved by cathepsins in human liver carcinoma cells (HepG2) (Droga-Mazovec et al., 2008), while other study showed that particularly cathepsin B is active in hepatocytes (Guicciardi et al., 2000).

Bid is also cleaved by caspase 8, which represents a link between extrinsic and intrinsic (mitochondrial) pathway (Li et al., 1998).

Activated BH3-only proteins continue to activate pro-apoptic proteins Bax and Bak. Sarosiek et al. observed that Bid preferentially activates Bak, while Bim activates Bax (Sarosiek et al., 2013). The activation of Bax and Bak occurs after LMP, but before mitochondrial release of cytochrome c and caspase-3 activation (Boya et al., 2003). Currently there are two models describing activation of Bax and Bak proteins and the role of anti-apoptic and pro-apoptic multidomain proteins in it. In the indirect model, Bax and Bak are sequestered and inactivated by anti-apoptotic Bcl-2 proteins. The binding of pro-apoptotic BH3-only proteins to these Bcl-2 proteins triggers the release of Bax and Bak. The direct model proposes that Bax and Bak are activated by direct binding of pro-apoptotic BH3-only proteins, called the activators (Bid, Bim or Puma). However, these activators are normally sequestered by anti-apoptotic Bcl-2 proteins. In order to release the activators, other BH3-only proteins, called senzitizers, neutralize anti-apoptotic Bcl-2 proteins (Brenner and Mak, 2009; Willis et al., 2007).

Bak and Bax go under major conformational changes after binding of BH3 only proteins (as reviewed by Westphal et al., 2014). Once activated Bak or Bax molecules bind reciprocally to form symmetric homodimers. It is thought that homodimers of Bak or Bax must then associate to higher order oligomers to porate the mitochondrial outer membrane (Uren et al., 2017). Heterodimers form only a minor population compared with homodimers (Dewson et al., 2012; Mikhailov et al., 2003).

Bak and Bax shallow insertion into the outer leaflet of mitochondrial membrane (Westphal et al., 2014; Oh KJ et al., 2010) may destabilize the lamellar structure of the bilayer to induce lipidic pores in mitochondrial membrane. This induces release of proteins from the space between inner and outer mitochondrial membrane (Newmeyer et al., 2003).

Murine (Stoka et al., 2001; Zhang et al., 2009; Lindsten et al., 2000)

Human (Boya et al., 2003; Cirman et al., 2004)