Inflammasome pathway

The inflammasome pathway is crucial to the innate immune system, detecting pathogenic microorganisms and cellular stress to initiate inflammatory responses. Inflammasomes are cytosolic multiprotein complexes that form in response to various stimuli. They serve as platforms for activating inflammatory caspases, particularly caspase-1, which processes pro-inflammatory cytokines such as IL-1β and IL-18 into their active forms.

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Different inflammasome complexes have been discovered, each characterized by a distinct pattern recognition receptor (PRR) and specific activation triggers. The most well-studied inflammasome is NLRP3 (NOD-like receptor family pyrin domain containing 3), which includes three main components: the sensor protein NLRP3, the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD), and the effector enzyme caspase-1. NLRP3 acts as a pattern recognition receptor (PRR) that detects a wide range of pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs).

The NLRP3 inflammasome is activated in a 2-step process. First, the activation of TLR4 by lipopolysaccharides (LPS) from Gram-negative bacteria (signal 1) triggers a signaling cascade that stimulates the NF-κB pathway. NF-κB is a transcription factor that regulates the expression of various pro-inflammatory genes, including those encoding for NLRP3, pro-IL-1β and pro-IL-18. This process, known as priming, is critical for the subsequent inflammasome activation³.

Second, upon sensing specific stimuli (signal 2), NLRP3 undergoes conformational changes and oligomerizes to form a scaffold, recruiting the ASC adaptor protein and pro-caspase 1. ASC/TMS1 is an adaptor protein crucial in assembling the inflammasome complex containing two domains: a pyrin domain (PYD) and a caspase activation and recruitment domain (CARD). The PYD of ASC interacts with the PYD of NLRP3, while its CARD interacts with the CARD of pro-caspase-1, enabling the formation of the inflammasome complex¹.

Caspase-1, a cysteine protease, is initially synthesized as an inactive zymogen known as pro-caspase-1. Pro-caspase-1 is cleaved into its active form upon recruitment to the inflammasome complex. The active caspase-1 then processes pro-IL-1β and pro-IL-18 into their mature, bioactive forms, IL-1β and IL-18, which are released from the cell to drive inflammatory responses².

The inflammasome pathway is strictly controlled to prevent excessive inflammation, which can contribute to tissue damage and disease. Numerous mechanisms exist to regulate the activation of the NLRP3 inflammasome. For example, post-translational modifications such as ubiquitination and phosphorylation can modulate the activity of NLRP3 and other inflammasome components. In addition, autophagy, a cellular process that degrades and recycles damaged organelles and proteins, can negatively regulate inflammasome activation by removing damaged mitochondria and other sources of ROS. Furthermore, the dysregulation of the inflammasome pathway has been implicated in various inflammatory and autoimmune diseases, including gout, type 2 diabetes, and Alzheimer's disease. Excessive or inappropriate inflammasome activation leads to chronic inflammation and tissue damage in these conditions. Therefore, understanding the molecular mechanisms that regulate inflammasome activation is imperative to developing therapeutic strategies to treat these diseases⁴.

The inflammasome pathway is a complex and tightly regulated process essential to the innate immune response. Our poster showcases key components such as NLRP3, ASC, caspase-1, NF-κB, lysosomes, and TLR4 that work together to detect and respond to pathogenic and stress signals, activating pro-inflammatory cytokines. Proper regulation of this pathway is pivotal for maintaining immune homeostasis and preventing inflammatory diseases. Active research continues to uncover new insights into the molecular mechanisms governing inflammasome activation and regulation, offering potential avenues for therapeutic intervention.

References

1. Zheng, D., Liwinski, T. & Elinav, E. Inflammasome activation and regulation: toward a better understanding of complex mechanisms.  Cell Discov  6, 36 (2020).

2. Franchi, L., Eigenbrod, T., Muñoz-Planillo, R. et al. The inflammasome: a caspase-1-activation platform that regulates immune responses and disease pathogenesis.  Nat. Immunol.  10, 241–247 (2009).

3. Su, Q. et al. Effects of the TLR4/Myd88/NF-κB Signaling Pathway on NLRP3 Inflammasome in Coronary Microembolization-Induced Myocardial Injury.  Cell Physiol. Biochem.  47, 1497–1508 (2018).

4. Li, Y. et al. Inflammasomes as therapeutic targets in human diseases.  Sig. Transduct. Target Ther.  6, 247 (2021).