Cell death pathways: intricate connections and disease implications

Abstract

Various forms of cell death have been identified over the last decades with each relying on a different subset of proteins for the activation and execution of their respective pathway(s). In addition to the three best characterized pathways-apoptosis, necroptosis, and pyroptosis-other forms of regulated cell death including autophagy-dependent cell death (ADCD), mitochondrial permeability transition pore (MPTP)-mediated necrosis, parthanatos, NETosis and ferroptosis, and their relevance for organismal homeostasis are becoming better understood. Importantly, it is increasingly clear that none of these pathways operate alone. Instead, a more complex picture is emerging with many pathways sharing components and signaling principles. Finally, a number of cell death regulators are implicated in human diseases and represent attractive therapeutic targets. Therefore, better understanding of physiological and mechanistic aspects of cell death signaling should yield improved reagents for addressing unmet medical needs.

Keywords: RIPK1; apoptosis; caspase; necroptosis; pyroptosis.

Figure 1. Intrinsic and extrinsic apoptosis

Intrinsic apoptosis can be induced by various stimuli (e.g., GF (growth factor) withdrawal) by shifting the equilibrium of pro‐survival Bcl‐2 and BH3‐only proteins. Sequestering of pro‐survival Bcl‐2 proteins or directed binding of BH3‐only proteins to Bax and Bak induces oligomerization of Bax and Bak leading to MOMP and cytochrome c and Smac release. Bok can lead to MOMP independent of Bcl‐2 family members. Released cytochrome c binds Apaf‐1 and induces apoptosome formation which recruits caspase‐9. Activated caspase‐9 induces caspase‐3/7 cleavage and activation (cCasp3/7) leading to apoptosis. Extrinsic apoptosis can be induced by binding of select group of TNF family ligands to, their receptors leading to DISC formation by recruitment of adapter FADD/TRADD and caspase‐8. Caspase‐8 autoprocesses itself (cCasp8—cleaved/activated caspase‐8) and can directly activate caspase‐3 or cleave Bid to generated tBid and triggers intrinsic apoptosis. Caspase‐3/7/9 activity can be inhibited by XIAP, which itself can be antagonized by Smac. Lastly, apoptosis can also be induced by granzyme B and Perforin released form immune cells. Once taken up by the target cell, granzyme B can induces cell death by caspase‐3 or by directly activating apoptosis effectors (e.g., CAD).

Figure 2. Necroptosis

Necroptosis can be induced by different stimuli. Upon binding to its receptor, TNF induces complex formation leading to NF‐κB and MAPK activation. Prolonged signaling and inhibition of caspases leads to RIP1 translocation to the cytosol forming complex II. RIP1 autophosphorylates recruiting RIP3. RIP3 phosphorylates itself as well as MLKL leading to MLKL oligomerization which induces membrane perturbation and cell lysis. LPS or Poly(I:C)‐induced TLR3/4 signaling also can stimulate necroptosis through adaptor TRIF, which engages RIP1 or RIP3. Sensing of Z‐DNA by ZBP1 leads to binding to RIP3 and cell death, which can be inhibited by RIP1.

Figure 3. Pyroptosis

Pyroptotic cell death can be induced by various stimuli that activate inflammasome. The activation of NLRP3 prompts its binding to ASC and caspase‐1 forming the inflammasome. Caspase‐1 processes pro‐IL‐1β and pro‐IL‐18 to their active forms. In parallel, caspase‐1 cleaves GSDMD separating the inhibitory C‐ and active N‐terminal domains. GSDMD‐N then translocates to the membrane inducing cell lysis and cytokine release. Non‐canonical inflammasome activation consists of priming which induces expression of several pathway genes (caspase‐11). Intracellular LPS can be sensed by caspase‐11 leading to its activation and processing of GSDMD and caspase‐1. Cell lysis can then also activate canonical inflammasome signaling. Green arrows indicate upregulation by gene expression, while black arrows indicate interactions/cleavage events or inhibition.

Figure 4. Key features of different forms of regulated cell death

Overview of regulated cell death pathways highlighting the stimuli, key features as well as positive and negative regulators of the pathways. Gray boxes indicate cell death pathway, green boxes show negative regulators/inhibitors, while red boxes indicate activators.

Figure 5. Crosstalk between different forms of cell death

A more complex network between the different cell death pathways has been established over the years. Arrows indicate established interconnectivity between apoptosis, necroptosis, and pyroptosis. For details, please refer to the main text. Black arrows indicate crosstalk events, while red arrows indicate inhibition.