NLRP3: More than an Inflammasome?

Inflammasome-Independent Mechanism of NLRP3 is Critical for Platelet GPIb-IX Function and Thrombosis

NLR family pyrin domain containing protein (NLRP) 3 is part of the NLRP3 inflammasome complex which is formed between NLRP3, the adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC), and pro-caspase 1 and is involved in the production and activation/processing of interleukin (IL)-1β by inflammatory cells such as monocytes, macrophages, and neutrophils in response to danger signals such as pathogen-associated molecular patterns and damage-associated molecular patterns. The discovery of an inflammasome complex being involved in the processing of IL-1β was identified in 2002 by several groups,[1] [2] [3] who identified two of the known inflammasomes—NLRP1 and NLRP3. The importance of the NLRP3 inflammasome has been demonstrated in various diseases, including atherosclerosis and cardiovascular disease (reviewed in Grebe et al[4] and Toldo and Abbate[5]), Alzheimer's disease (reviewed in McManus and Latz [6]), and other age-related diseases (reviewed in Liang et al[7]). While predominantly investigated within the context of monocytes, macrophages, and neutrophils, recent work has investigated the NRLP3 inflammasome and IL-1β in the context of nontraditional cells such as platelets.[8] [9] [10] [11] [12] [13] [14] [15] [16]

Since the discovery of the inflammasome, the focus of the literature regarding NLRP3 has predominantly been on NLRP3 in the context of the inflammasome complex rather than the actions of the NLRP3 protein itself. In this issue, Chen et al[9] have demonstrated that the NLRP3 protein has an important role within platelets that is independent of the NLRP3 inflammasome complex ([Fig. 1]). Consistent with previous findings by Qiao et al,[13] NLRP3 deficiency, both systemic and platelet-specific, impairs hemostatic function. Additionally, the difference in bleeding time and clot formation was NLRP3 protein-dependent and NLRP3 inflammasome-independent as demonstrated by the lack of change in the ASC^−/− and caspase-1^−/− knockout models.

The NLRP3 protein, not NLRP3 inflammasome, promoted adhesion of platelets to both collagen and von Willebrand factor (vWF) under conditions of low and high shear. Absence of the NLRP3 protein led to a reduction in their binding affinity to vWF and additionally altered downstream signaling pathways of glycoprotein (GP) Ib-IX by reducing phosphorylation of key molecules. As Qiao et al[13] also reported, a reduction in aggregation in response to low concentrations of adenosine diphosphate, collagen, and thrombin was observed in the absence of NLRP3 (either NLRP3^−/− or with NLRP3 inhibitor, CY-09) that was not maintained in the presence of stronger agonist concentrations.

Chen et al[9] have identified that the NLRP3 protein, in platelets, is an important link between the cAMP/PKA (cyclic adenosine monophosphate/protein kinase A) axis and the binding of GPIbα to either filamin A or 14-3-3ζ during activation of GPIb-IX. cAMP/PKA-dependent phosphorylation of NLRP3 occurs via the NACHT domain; however, when cAMP/PKA levels are reduced or absent, dephosphorylation/activation of NLRP3 occurs, leading to increased binding to filamin A (via the PYD domain) and 14-3-3ζ and a subsequent increase in binding affinity to vWF. They further demonstrated the importance of the NLRP3 protein independent of inflammasome assembly both in the systemic knockout and platelet specifically by demonstrating reduction in infarct volume and improved neurological assessment in a transient middle cerebral artery occlusion model and reduced thrombus weight and length in a deep vein thrombosis model, with these effects not occurring in the ASC^−/− or caspase^−/− knockout mouse models.

The potential for NLRP3 as a therapeutic target[17] [18] is currently of considerable interest. MCC950, an NLRP3 inhibitor being widely investigated, targets the NACHT domain and prevents conformational change and activation of NLRP3,[19] thus inhibiting both the NLRP3 protein and inflammasome. Based on the findings of Chen et al,[9] inhibition of NLRP3 may assist in the treatment of thrombotic disorders as well as the expected anti-inflammatory effects of NLRP3 inhibition. While inhibition of the IL-1β pathways is attractive, excessive inhibition could lead to other complications such as severe infection/sepsis as observed in the CANTOS trial,[20] therefore, achieving a balance is important. Based on the present study, NLRP3 may not only be as effective as IL-1β suppression in dampening inflammatory atherosclerosis,[21] and it may also deserve further investigation as an antithrombotic target and may be beneficial in diseases such as deep vein thrombosis and stroke.

Fig. 1 NLRP3 within the inflammasome complex is involved in the production of interleukin-1β and other caspase-1-dependent cytokines (simplified pathway, left). In this issue, Chen et al[9] have proposed an alternative role for the NLRP3 protein in altering the binding affinity of GPIbα to vWF (right), which is independent of inflammasome activity, and which has implications for thrombosis. This is regulated by the cAMP/PKA pathway in which dephosphorylation of NLRP3 promotes binding to filamin A. Created with BioRender.com. cAMP, cyclic adenosine monophosphate; PKA, protein kinase A; vWF, von Willebrand factor.

Publikationsverlauf

Eingereicht: 09. September 2024

Angenommen: 10. September 2024

Accepted Manuscript online:
11. September 2024

Artikel online veröffentlicht:
01. Oktober 2024

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