Diagram of PANoptosome structure showing key proteins involved in PANoptosis, including RIPK1, RIPK3, ASC, AIM2, FADD, NLPR3, and Caspase-8.What is PANoptosis?

PANoptosis is a distinctive form of cell death that combines features of innate immunity, inflammation, and cell lysis, setting it apart from other cell death pathways such as pyroptosis, apoptosis, and necroptosis. It is triggered by innate immune sensors and orchestrated by caspases and receptor-interacting protein kinases (RIPKs) through the formation of multiprotein PANoptosome complexes (1-2).

Key Features of PANoptosis

  • Innate Immune Response: PANoptosis is initiated by innate immune sensors that detect pathogens, damage-associated molecular patterns (DAMPs), or cytokines (3,4).
  • Multiprotein Complex: This pathway involves the assembly of the PANoptosome, a multiprotein complex that includes key molecules from pyroptosis, apoptosis, and necroptosis pathways (4-6).
  • Crosstalk with Other Cell Death Pathways: While PANoptosis shares components and interacts with apoptosis, pyroptosis, and necroptosis, it remains a distinct pathway with unique characteristics (6-8).
  • Inflammatory Response: PANoptosis induces a robust inflammatory response, resulting in the release of DAMPs and pro-inflammatory cytokines (1, 4, 9).

Mechanisms of PANoptosis

The molecular machinery of PANoptosis involves the coordinated activation of several key proteins and complexes from apoptosis, pyroptosis, and necroptosis (10, 16) as shown in Fig. 1.

Illustration showing the mechanism of PANoptosis, highlighting the formation of the PANoptosome complex and its interaction with pyroptosis, apoptosis, and necroptosis pathways. Key proteins such as RIPK3, MLKL, Caspase-8, and GSDMD are involved in the process, leading to the release of DAMPs and proinflammatory cytokines.

1. Apoptosis

This pathway involves the activation of caspases, particularly caspase-3 and caspase-7, which lead to controlled cell dismantling. It can be triggered via intrinsic (mitochondrial) or extrinsic (death receptor-mediated) pathways.

2. Pyroptosis

Mediated by inflammasomes such as NLRP3, pyroptosis involves the activation of caspase-1, which cleaves gasdermin D (GSDMD), forming pores in the cell membrane and leading to cell lysis and the release of pro-inflammatory cytokines like IL-1β and IL-18.

3. Necroptosis

This pathway is regulated by receptor-interacting protein kinases RIPK1 and RIPK3, which phosphorylate mixed lineage kinase domain-like pseudokinase (MLKL). MLKL translocates to the plasma membrane, disrupting membrane integrity and causing cell death.

The convergence of these pathways is facilitated by a multiprotein complex known as the PANoptosome. This complex acts as a signaling hub, integrating inputs from various stress and damage signals to coordinate the activation of apoptosis, pyroptosis, and necroptosis.

PANoptosomes: The Cellular Orchestrators of PANoptosis

PANoptosomes are specialized cellular structures that facilitate the execution of PANoptosis, a complex form of cell death that integrates mechanisms from pyroptosis, apoptosis, and necroptosis (1-2). These multiprotein complexes play a crucial role in the regulation and coordination of the signals and effector molecules involved in these distinct but interconnected cell death pathways.

Composition and Structure of PANoptosomes

PANoptosomes are composed of various proteins and enzymes that are key players in pyroptosis, apoptosis, and necroptosis (1,2,8). These include:

  1. Inflammasome Components: Proteins such as NLRP3, ASC, and caspase-1, which are central to the activation of pyroptosis.
  2. Apoptotic Proteins: Caspases such as caspase-8 and caspase-3, as well as other apoptosis-regulating proteins like BCL-2 family members.
  3. Necroptotic Proteins: RIPK1, RIPK3, and MLKL, which are essential for the execution of necroptosis.

Mechanism of PANoptosome Action

PANoptosomes act as signaling hubs that integrate inputs from various cellular sensors and transduce these signals to execute cell death (1,2,8). The mechanism involves:

  1. Signal Integration: PANoptosomes integrate signals from pattern recognition receptors (PRRs) that detect pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). This integration ensures a coordinated response to cellular stress or infection.
  2. Protein Complex Assembly: Upon activation, key proteins from pyroptosis, apoptosis, and necroptosis pathways are recruited to the PANoptosome. This assembly is crucial for the simultaneous activation of multiple cell death pathways.
  3. Caspase Activation: PANoptosomes facilitate the activation of initiator caspases (e.g., caspase-8), which in turn activate executioner caspases (e.g., caspase-3) and other downstream effectors.
  4. Effector Molecule Activation: The activation of effector molecules such as GSDMD in pyroptosis, executioner caspases in apoptosis, and MLKL in necroptosis leads to the dismantling of the cell through multiple mechanisms.

Biological Significance of PANoptosis

PANoptosis represents an evolutionary adaptation for effective pathogen defense and tissue homeostasis (1, 7,11,12, 13). The interplay of different cell death pathways allows for:

  • Robust Defense: By combining pyroptosis, apoptosis, and necroptosis, PANoptosis ensures the efficient removal of infected or damaged cells, even if one pathway is inhibited by pathogens.
  • Inflammatory Regulation: The inflammatory nature of pyroptosis and necroptosis is balanced by the more controlled apoptotic process, modulating immune responses to avoid excessive tissue damage.
  • Adaptability: PANoptosis can be tailored to different cellular contexts, ensuring appropriate responses to diverse stress signals.

Clinical Relevance

The clinical implications of PANoptosis are vast, particularly in the context of infectious diseases, cancer, and inflammatory conditions (1, 7,11,12, 13,14).

  1. Infectious Diseases: Many pathogens evolve mechanisms to inhibit individual cell death pathways. PANoptosis, by integrating multiple pathways, can overcome such evasion strategies, making it a critical component of host defense. Targeting the regulatory nodes of PANoptosis could enhance pathogen clearance and improve outcomes in infections.
  2. Cancer: Tumors often develop resistance to apoptosis. PANoptosis, through its multi-pathway approach, could bypass such resistance, offering new therapeutic avenues. Drugs that can simultaneously activate pyroptosis, apoptosis, and necroptosis may prove effective against resistant cancers.
  3. Inflammatory Diseases: Dysregulated cell death contributes to chronic inflammation and tissue damage. Understanding PANoptosis provides insights into the balance between cell death and inflammation, potentially leading to novel treatments for conditions like autoimmune diseases and chronic inflammatory disorders.
  4. Therapeutic Targeting: Modulating PANoptosis could offer precise control over cell death in various diseases. For instance, enhancing PANoptosis could be beneficial in infections and cancer, while inhibiting it might be therapeutic in conditions characterized by excessive cell death and inflammation.

Therapeutic Strategies

Research into PANoptosis has revealed several potential therapeutic strategies:

  • Targeting PANoptosome Components: Drugs that activate or inhibit key proteins in the PANoptosome can modulate the PANoptosis response. For instance, targeting caspase-8, RIPK1, or NLRP3 could selectively induce cell death in cancer cells (1).
  • Combining Therapies: PANoptosis-targeted therapies can be combined with existing treatments like chemotherapy, radiotherapy, and immunotherapy to enhance their effectiveness. For example, combining chemotherapeutic agents with PANoptosis inducers may improve cancer cell killing (14).
  • Nanoparticle-based Therapies: Novel delivery systems, such as nanoparticles, can be used to target PANoptosis pathways specifically within tumors, reducing side effects and increasing therapeutic efficacy (15).

In Vitro and Cell-Based Assays for Validating Mechanism of Action (MOA) and Potency of Therapeutic Drugs Targeting PANoptosis/PANoptosomes

In Vitro Assays

Cell-Based Assays

  • Immunofluorescence staining:
    • To visualize key markers of panoptosis, such as PANoptosome components, cleaved caspases link, and RIPK activation.
    • Co-localization studies with markers of other cell death pathways can help differentiate panoptosis.
  • Cytotoxicity, Cell Viability and Death Assays:
    • Luminescence and Fluorescence Detection: Used to monitor cell viability, apoptosis, and necrosis.
  • Cytokine Release Assays:
    • Measure the release of cytokines like IL-1β, which is indicative of pyroptosis.
  • Reporter gene assays:
    • To monitor the activation of specific signaling pathways involved in panoptosis.

Additional Considerations

  • Combination of assays: It’s often necessary to use multiple assays to confirm panoptosis and distinguish it from other cell death types.
  • Specific markers: Identifying unique markers for panoptosis can help in its accurate detection and characterization.
  • Inhibition studies: Using specific inhibitors of panoptosis components can provide further validation of the pathway’s involvement.

It’s important to note that the field of panoptosis research is rapidly evolving, and new assays and techniques are being developed. Therefore, staying updated on the latest advancements is crucial for accurate and reliable detection and validation of this complex cell death process.

Conclusion

PANoptosis represents a significant advancement in our understanding of cell death, offering new insights into the regulation of immune responses and tissue homeostasis. Its integration of apoptosis, pyroptosis, and necroptosis provides a robust mechanism for cell death, with substantial implications for cancer therapy and inflammatory diseases. Continued research into the regulation and modulation of PANoptosis will likely reveal new therapeutic opportunities, paving the way for innovative treatments that improve patient outcomes.

References

  1. Wang, L., Zhu, Y., Zhang, L. et al. Mechanisms of PANoptosis and relevant small-molecule compounds for fighting diseases. Cell Death Dis 14, 851 (2023). 
  2. Samir, P., Malireddi, R.K.S., Kanneganti, T-D. The PANoptosome: A Deadly Protein Complex Driving Pyroptosis, Apoptosis, and Necroptosis (PANoptosis). Front. Cell. Infect. Microbiol.. 2020. 10:2020. 
  3. Chen, W., Gullett, J.M., Tweedell, R.E. Innate immune inflammatory cell death: PANoptosis and PANoptosomes in host defense and disease. 2023. Eur. J. Immunol. 53: 2250235.
  4. Pandian, N., Kanneganti, T-D. PANoptosis: A Unique Innate Immune Inflammatory Cell Death Modality. J Immunol 2022; 209: 1625–1633. 
  5. Liu, X., Miao, M., Sun, J. PANoptosis: a potential new target for programmed cell death in breast cancer treatment and prognosis. Apoptosis. 2024 ;29:277-288. 
  6. Gullett, J.M., Tweedell, R.E., Kanneganti, T.D. It’s All in the PAN: Crosstalk, Plasticity, Redundancies, Switches, and Interconnectedness Encompassed by PANoptosis Underlying the Totality of Cell Death-Associated Biological Effects. Cells. 2022 11:1495. 
  7. Gao, J., Xiong, A., Liu, J. et al. PANoptosis: bridging apoptosis, pyroptosis, and necroptosis in cancer progression and treatment. Cancer Gene Ther 31, 970–983 (2024). 
  8. Pandeya, A., Kannegant, T-D. Therapeutic potential of PANoptosis: innate sensors, inflammasomes, and RIPKs in PANoptosomes. Trends Mol. Med. 2024. 30:74-88. 
  9. Ocansey, D.K.W., Qian, F., Cai, P. Current evidence and therapeutic implication of PANoptosis in cancer. Theranostics. 2024. 14: 640-661. 
  10. Malireddi, R.K.S., Kesavardhana, S., & Kanneganti, T.D. (2019). ZBP1 and TAK1: Master Regulators of NLRP3 Inflammasome/Pyroptosis, Apoptosis, and Necroptosis (PAN-optosis). Front. Cell. Infect. Microbiol. 9:406. 
  11. Zhu, P., Ke, Z-R., Chen, J-X. Advances in mechanism and regulation of PANoptosis: Prospects in disease treatment. Front. Immunol. 2023. 14: 2023 
  12. Sun, X., Yang, Y., Meng, X. PANoptosis: Mechanisms, biology, and role in disease. Immunol. Rev. 321:246-262. 
  13. Place, D.E., Lee, S., Kanneganti, T.D. PANoptosis in microbial infection. Curr Opin Microbiol. 2021. 59:42-49. 
  14. Fontana, F., Raimondi, M., Marzagalli, M. The emerging role of paraptosis in tumor cell biology: Perspectives for cancer prevention and therapy with natural compounds, Biochimica et Biophysica Acta (BBA) – Reviews on Cancer. 2020. 1873:188338. 
  15. Chang, J., Yu, B., Saltzman, W.M. Nanoparticles as a Therapeutic Delivery System for Skin Cancer Prevention and Treatment. JID Innov. 2023. 3:100197. 
  16. Christgen, S., Zheng, M., Kesavardhana, S., Karki R, Identification of the PANoptosome: A Molecular Platform Triggering Pyroptosis, Apoptosis, and Necroptosis (PANoptosis). Front Cell Infect Microbiol. 2020. 10:237. 

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