Activation

CARD-CARD Interaction Mediated Ring Formation

Caspase-1, normally in its physiologically inactive zymogen form, autoactivates when it is assembled into the filamentous inflammasome complex by autoproteolysis into the p10 and p20 subunits.^[14][15] The inflammasome complex is a ring complex composed of trimers of a signal specific sensor protein such as those of the NLR family and the AIM-1 (Absent in Melanoma) like receptors, an adaptor protein such as ASC, and a caspase, in this case Caspase-1. In some cases, where the signaling proteins contain their own CARDs, like in NLRP1 and NLRC4, the CARD –CARD interaction is direct, meaning there is no adaptor protein in the complex. There are a variety of sensor and adaptor proteins, the various combinations of which confer the inflammasomes' responses to specific signals. This allows the cell to have varying degrees of inflammatory responses based on the severity of the danger signal received.^[16][17]

Inhibition

CARD only proteins (COPs) as their name implies, are proteins that only contain the non-catalytic CARDs. Owing to the importance of CARD-CARD interactions in inflammasome formation, many COPs are known inhibitors of Caspase activation. For Caspase-1, genes for specific COPs—ICEBERG, COP1 (ICE/Pseudo-ICE), and INCA (Inhibitory Card)—are all found near its locus, and are thus thought to have emerged from gene duplication events and subsequent deletions of the catalytic domains. Though they all interact with the inflammasomes using CARD–CARD interactions, they differ in the way they carry out their inhibitory functions as well as in their effectiveness at doing so.^[15][18][19]

For example, ICEBERG nucleates the formation of Caspase-1 filaments and is thus incorporated into the filaments, but lacks the ability to inhibit the activation of inflammasomes. Instead, it is thought to inhibit Caspase-1 activation by interfering with the interaction of Caspase-1 with other important CARD containing proteins.^[15][18][19]

INCA, on the other hand, directly blocks inflammasome assembly by capping Caspase-1 CARD oligomers, thus blocking further polymerization into the inflammasome filaments.^{[18][19][20][13]}

Similarly, some POPs (Pyrin only proteins) are also known to regulate Caspase-1 activation through inhibition of inflammasome activation by binding to and blocking PYD interactions, which also play a role in the formation of the inflammasomes, though the exact mechanisms are not yet well established.^[19][21]

Inhibitors

• Belnacasan (VX-765) ^[22]
• Pralnacasan (VX-740) ^[23]

Proteolytic cleavage

Activated Caspase 1 proteolytically cleaves pro IL-1β and pro IL-18 into their active forms, IL-1β and IL-18. The active cytokines lead to a downstream inflammatory response. It also cleaves Gasdermin D into its active form, which leads to pyroptosis.^[13]

Inflammatory response

Once matured, the cytokines initiate downstream signaling events to induce a proinflammatory response as well as to activate the expression of antiviral genes. The speed, specificity and types of response are dependent on the signal received as well as the sensor protein that received it. Signals that can be received by the inflammasomes include viral double stranded RNA, urea, free radicals, and other signals associated with cellular danger, even byproducts of other immune response pathways.^[24]

The mature cytokines themselves do not contain the necessary sorting sequences to enter the ER-Golgi secretory pathway, and thus are not excreted from the cell by conventional methods. However, it is theorized that the release of these proinflammatory cytokines is not reliant on cellular rupture via pyroptosis, and is in fact, an active process. There exists evidence both for and against this hypothesis. The fact that for many cell types, the cytokines are secreted despite them showing absolutely no signs of pyroptosis, supports this hypothesis.^[17][25] However, some experiments show that Gasdermin D nonfunctional mutants still had normal cleavage of the cytokines but lacked the ability to secrete them, indicating pyroptosis may in fact be necessary for secretion in some way.^[26]

Pyroptosis response

Following the inflammatory response, an activated Caspase-1 can induce pyroptosis, a lytic form of cell death, depending on the signal received as well as the specific inflammasome sensor domain protein that received it. Though pyroptosis may or may not be required for the full inflammatory response, the inflammatory response is fully required before pyroptosis can occur.^[17] In order to induce pyroptosis, Caspase-1 cleaves Gasdermin D into fragments that form pores in the plasma membrane. As a result of osmotic pressure, these pores promote the influx of fluid, resulting in cell lysis and death.^[27]

Other roles

Caspase-1 has also been shown to induce necrosis and may also function in various developmental stages. Studies of a similar protein in mice suggest a role in the pathogenesis of Huntington's disease. Alternative splicing of the gene results in five transcript variants encoding distinct isoforms.^[28] Recent studies implicated caspase-1 in promoting CD4 T-cell death and inflammation by HIV, two signature events that fuel HIV disease progression to AIDS.^[29][30][31] Caspase-1 activity has also been implicated in lysosome acidification following phagocytosis of bacteria^[32] and immune complexes.^[33]