5. Necrosis
 

5. Cell Death Through Necrosis

 

Necrosis, which is usually referred to as a passive process and is not dependent on de novo macromolecular synthesis, is a non-programmed cell death that can occur as a result of a few external reasons such as infection, toxins and physical injuries (Yan G E et al., 2020). This, in turn, will cause changes in morphology like cytoplasmic augmenting, which takes place as a result of interruption in of ionic pumps such as calcium influx, plasma membrane break and consequent loss and damage of intracellular organelles without any acute attenuation of chromatin; however, arbitrary dilapidation of DNA is observed (Syntichaki P and Tavernarakis N 2002) (Won S J et al., 2002 as cited in Yan G E et al., 2020) (Weerasinghe P and Buja L M., 2012 as cited in Yan G E et al., 2020 ) (Yan G E et al., 2020).

 

Cell Death Through Necrosis References

 

1.        Syntichaki, P. & Tavernarakis, N. Death by necrosis. EMBO Rep. 3, 604–609 (2002).

2.        Weerasinghe, P. & Buja, L. M. Oncosis: An important non-apoptotic mode of cell death. Exp. Mol. Pathol. 93, 302–308 (2012).

3.        Won, S.-J., Kim, D.-Y. & Gwag, B.-J. Cellular and Molecular Pathways of Ischemic Neuronal Death. BMB Rep. 35, 67–86 (2002).

4.        Yan, G., Elbadawi, M. & Efferth, T. Multiple cell death modalities and their key features (Review). World Acad. Sci. J. (2020). doi:10.3892/wasj.2020.40

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5.1. Necroptosis Molecular Mechanism

 

The molecular mechanism of necroptosis can be explored through the study of TNFα, RIPK3 and caspase 8 (Dhuriya Y K and Shama D., 2018).

 

Toll-like receptors (TLR3 and TLR4), which are members of TNF superfamily receptors, along with interferon receptors, are responsible for the induction of necroptosis, although TNFR1-facilitated necroptosis is thoroughly categorised (Dhuriya Y K and Shama D., 2018).

 

According to solid features, there are three categories of necroptosis:

 

• (i) TNFα stimulated extrinsic necroptosis

• (ii) Reactive oxygen species (Ros), which triggers intrinsic necroptosis

• (iii) Intrinsic necroptosis, which is facilitated by ischemia 

 

Classical necroptosis, which is facilitated by TNFα, attaches with the complementary receptor, resulting in the configuration of complex I (a membrane signalling complex), consisting of TRADD (an adaptor molecule, responsible for conscription of RIPK1 to TNFR1), FADD, RIPK1, TRAF2/TRAF5 and cIAP1/cIAP2 (Micheau O and Tschopp J.,2003) (Han J et al., 2011) (Dhuriya Y K and Shama D., 2018). As TRADD recruits RIPK1 to TNFR1, then cIAPs and TRAF2/3/5 are conscripted to complex 1 (Vandenabeele P et al., 2010 as cited in Dhuriya Y K and Shama D., 2018) (Dhuriya Y K and Shama D., 2018).

 

In order to establish a stable complex 1 and induce another pathway that ends up to cell survival pathways such as NFκB and MAPK mediated pathway, then there is a need for RIPK1 to be ubiquitinated through induction of cIAP1/2 and TRAF2/5 (Newton K et al., 2014 as cited in Dhuriya Y K and Shama D., 2018) (Dhuriya Y K and Shama D., 2018).

 

Prosurvival efficacy of NFκB, which is facilitated through cIAP1/2 and cellular FLICE-like inhibitory protein (cFLIP1), plays a role in offsetting the cytotoxic effect of TNFα (Kreuz S et al., 2001) (Papa S et al., 2004 as cited in Dhuriya Y K and Shama D., 2018) (Dhuriya Y K and Shama D., 2018). 

 

Consequently, it is concluded that complex 1 plays a critical role as a checkpoint for cell survival and necroptosis (Oberst A et al., 2011) (Dhuriya Y K and Shama D., 2018). 

 

It is a common process for caspase 8 and cFLIP1 to arrange in a heterodimer and inactivate apoptosis, which eventually causes inhibition of caspase 8 (Dhuriya Y K and Shama D., 2018). Caspase 8 inactivates RIPK3 and RIPK1, which results in switching off necroptosis caused by activation of exogenous apoptosis through caspase 8 itself (Dhuriya Y K and Shama D., 2018). Furthermore, dissociation of complex 1 and the inhibition of ubiquitination, which is facilitated by cylindromatosis (CYLD), results in induction of RIPK1, caused by eradication or inactivation of caspase 8 (ó Donnell M A et al., 2011) (Pop C et al., 2011) (Dhuriya Y K and Shama D., 2018). In turn, complex II is developed as a result of the connection between FADD, TRADD, RIPK3 and caspase 8, which occurs through the elimination of the ubiquitin sequence from RIPK1 (Dhuriya Y K and Shama D., 2018).

 

Necrosome is constructed through communication between RIPK1 and RIPK3 via Receptor Homology Domain (RHD) and both essential for the activation of necroptosis, however, the overexpression of only RIPK3 in cells is sufficient for necroptosis to take place. Formation of necrosome, in turn, prompts downstream signaling and consequently causes the occurrence of necroptosis (Xie T et al., 2013) (Dondelinger Y et al., 2013) (Dhuriya Y K and Shama D., 2018). 

 

Finally, Mixed Lineage Kinase domain Like protein (MLKL) can be induced through phosphorylation by RIPK3, and is known as a pseudokinase, which is responsible for activation of necroptosis in two ways (Sun X et al., 1999) (Cai Z et al., 2014) after being phosphorylated by RIPK3 (Dhuriya Y K and Shama D., 2018): 

 

• (i) functioning as a stage for recruiting sodium ion or calcium channels in the plasma membrane  (Sun X et al., 1999) (Cai Z et al., 2014) (Dhuriya Y K and Shama D., 2018).

• (ii) Networking with phosphatidyl inositol phosphate amino terminal, resulting in induction of pore foundation in the plasma membrane (Sun X et al., 1999) (Cai Z et al., 2014) (Dhuriya Y K and Shama D., 2018).

 

Necroptosis Molecular Mechanism References

 

1.        Cai, Z. et al. Plasma membrane translocation of trimerized MLKL protein is required for TNF-induced necroptosis. Nat. Cell Biol. 16, 55–65 (2014).

2.        Dhuriya, Y. K. & Sharma, D. Necroptosis: a regulated inflammatory mode of cell death. J. Neuroinflammation15, 199 (2018).

3.        Dondelinger, Y. et al. RIPK3 contributes to TNFR1-mediated RIPK1 kinase-dependent apoptosis in conditions of cIAP1/2 depletion or TAK1 kinase inhibition. Cell Death Differ. 20, 1381–1392 (2013).

4.        Kreuz, S., Siegmund, D., Scheurich, P. & Wajant, H. NF-κB Inducers Upregulate cFLIP, a Cycloheximide-Sensitive Inhibitor of Death Receptor Signaling. Mol. Cell. Biol. 21, 3964–3973 (2001).

5.        Micheau, O. & Tschopp, J. Induction of TNF Receptor I-Mediated Apoptosis via Two Sequential Signaling Complexes. Cell 114, 181–190 (2003).

6.        Newton, K. et al. Activity of Protein Kinase RIPK3 Determines Whether Cells Die by Necroptosis or Apoptosis. Science (80-. ). 343, 1357–1360 (2014).

7.        O’Donnell, M. A. et al. Caspase 8 inhibits programmed necrosis by processing CYLD. Nat. Cell Biol. 13, 1437–1442 (2011).

8.        Oberst, A. et al. Catalytic activity of the caspase-8–FLIPL complex inhibits RIPK3-dependent necrosis. Nature471, 363–367 (2011).

9.        Papa, S. et al. Gadd45β mediates the NF-κB suppression of JNK signalling by targeting MKK7/JNKK2. Nat. Cell Biol. 6, 146–153 (2004).

10.     Pop, C. et al. FLIPL induces caspase 8 activity in the absence of interdomain caspase 8 cleavage and alters substrate specificity. Biochem. J. 433, 447–457 (2011).

11.     Sun, X. et al. RIP3, a Novel Apoptosis-inducing Kinase. J. Biol. Chem. 274, 16871–16875 (1999).

12.     Vandenabeele, P., Galluzzi, L., Vanden Berghe, T. & Kroemer, G. Molecular mechanisms of necroptosis: an ordered cellular explosion. Nat. Rev. Mol. Cell Biol. 11, 700–714 (2010).

13.     Xie, T. et al. Structural Insights into RIP3-Mediated Necroptotic Signaling. Cell Rep. 5, 70–78 (2013).

 

5.2. Pyroptosis

 

According to Ros U et al., 2020 "An extremely inflammatory form of regulated cell death, which is related to the development of the membrane pores by Gasdermins (GSDMs) is known as pyroptosis" (Ros U et al., 2020). Within the inflammasome complex , pyroptosis is usually induced by inflammatory caspases 1 or 11 (Galluzzi L et al., 2018) (Ros U et al., 2020). Two various inflammasome pathways: canonical and non-canonical, are known to result in  pyroptosis (Ros U et al., 2020).

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In the canonical pathway, inflammasome via pattern recognition receptors picks up danger signals such as Pathogen- or Damage-Associated Molecular patters (PAMPs and DAMPS) (Ros U et al., 2020). In addition, splitting and stimulating GSDMD and activating inflammatory cytokines IL-1β and IL-18 occurs through induction of caspase 1 within the inflammasome (Ros U et al., 2020).

 

In the non-canonical pathway, oligomerisation and induction of caspase 4 and 5 (in human) and 11 (in mouse) takes place as intracellular lipopolysaccharide (LPS) attaches to these caspases (Ros U et al., 2020). This, in turn, results in cleavage of a conserved region in GSDMD, induced by the same above inflammatory caspases (Ros U et al., 2020). 

 

Pyroptosis References 

 

1.        Galluzzi, L. et al. Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018. Cell Death Differ. 25, 486–541 (2018).

2.        Ros, U., Pedrera, L. & Garcia-Saez, A. J. Partners in Crime: The Interplay of Proteins and Membranes in Regulated Necrosis. Int. J. Mol. Sci. 21, 2412 (2020).