Enigmatic inflammasomes – Sequel (Part 2)

Abstract

In this issue, we introduce the second part of our review series focusing on lesser‐known enigmatic inflammasomes. This part of the collection introduces one more under‐studied NLR, NLRP7, and not only its role as a regulator of inflammation in response to bacterial infections but also its non‐inflammasome role in early pregnancy. In addition, the enigmatic function of extracellular ASC specks is also introduced, where extracellular ASC specks are presented as ‘danger signals’ to propagate inflammation. The series is concluded with an article that reviews the immunometabolic regulation of all of these lesser‐known NLRs, demonstrating that metabolic regulation of inflammasome activation is central for the whole NLR family. These three reviews, together with the four articles that were published in the first part of the series in December 2020, offer new insights into the complex functions of NLRs, well beyond the well‐known NLRP3. The review series as a whole provides a thought‐provoking platform with some of the latest findings in the NLR field and sparks our imagination into what might be discovered in this space in the future.

The term ‘inflammasome’ was first coined almost 20 years ago by Martinon & Tschopp ¹ in order to describe a stimulus‐induced cytoplasmic multimeric protein complex that promotes the maturation of pro‐inflammatory interleukin‐1β (IL‐1β) and interleukin‐18 (IL‐18) and the induction of an inflammatory cell death termed pyroptosis. Over the past 20 years, intense research in this area has unravelled key aspects of inflammasome activation and its role in disease processes. As inflammasomes represent important sensors of the innate immune response, it is logical that their dysregulation is associated with augmented inflammation, at different clinical contexts, which can be detrimental to the host.

Two types of inflammasome activation have been described: canonical and non‐canonical inflammasome activation. In the case of canonical inflammasome activation, two sequential stimuli are required for activation. There is a priming signal (for the transcription of pro‐IL‐1β and pro‐IL‐18), and upon the second signal, which triggers the formation of the inflammasome complex, the inflammasome sensors (nucleotide oligomerization and binding domain (NOD)‐like receptors (NLRs)) initiate inflammasome assembly by oligomerizing and recruiting pro‐caspase‐1, with or without the ASC adapter. This leads to the autocatalytic cleavage of caspase‐1 and processing of pro‐IL‐1β and pro‐IL‐18 to their mature and bioactive forms and cleavage of gasdermin D (GSDMD), ² which is the executioner of pyroptosis. In inflammasomes where oligomerization of the NLR occurs with ASC, they form what are called ‘ASC specks’ and these are believed to be also released in the extracellular space during pyroptosis propagating inflammation.

In the case of the newly discovered ‘non‐canonical inflammasome’, it signals in a caspase‐1‐independent manner through direct recognition of cytosolic LPS by the CARDs of caspase‐4 and caspase‐5 (in humans) and caspase‐11 (in mice). This elicits caspase dimerization and activation, resulting in cleavage of GSDMD. ³

The NLR family of sensors is a large receptor family of more than 20 members. To date, only about half of the NLRs have been characterized in any detail, with four key inflammasomes, namely NLRP1, NLRP3, NLRC4, and AIM2 receiving most of the focus. The remaining, lesser‐known enigmatic NLRs whose functions seem to range from non‐canonical inflammasome activation, pathogen/damage sensing, suppression or modulation of inflammation and even control of embryonic development are the focus of this review series. Our review series illustrates an exciting range of current advances in our understanding of enigmatic inflammasomes and is being published in two parts. The first four reviews were included in the December 2020 issue of this journal, and they focused on enigmatic inflammasomes such as NLRC5, which has a unique function that bridges both innate and adaptive immune responses by regulating MHC class I expression and antigen presentation ⁴ ; NLRX1, which the authors discuss the structure of NLRX1 and the importance of its mitochondrial localization, and its role as a central homeostatic gatekeeper between mitochondrial biology and immunological response ⁵ ; NLRP6, which can not only activate both a canonical and a non‐canonical inflammasome but also exert ‘inflammasome‐independent’ functions regulating gut microbiome composition, goblet cell function and related susceptibility to gastrointestinal inflammatory, infectious and neoplastic diseases ⁶ ; and NLRP9, which is unique in the fact that it is proposed to be expressed and functioned solely in the reproduction system. ⁷ This second part of the series provides current advances on our understanding of NLRP7, unknown functions of extracellular ASC specks but also provide specific examples of immunometabolism linked with these lesser‐known NLRs.

In the first article of this issue, Carriere et al ⁸ introduce us to NLRP7. NLRP7 is an NLR that was originally described as a negative regulator of inflammation, but contrary to the original findings of its involvement in anti‐inflammatory responses, recent evidence supports the activation of an NLRP7 inflammasome upon bacterial infection, suggesting that NLRP7 specifically senses bacterial di‐ and triacylated lipopeptides. In addition to its role in innate immune responses against bacteria, the authors also discuss other functions of NLRP7, which are inflammasome‐independent. They present growing evidence that NLRP7 also plays an important role during early pregnancy by affecting oocyte maturation, endometrial remodelling and placental development. In addition, they discuss accumulating evidence that links NLRP7 with molar pregnancies as numerous NLRP7 gene variants seem to be clearly associated with reproduction and imprinting defects. Carriere et al ⁸ excitingly present the role of NLRP7, not only in inflammasome signalling during inflammatory responses, but also during reproduction, and discuss possible consequences for NLRP7 variants.

All the articles so far have been presenting recent advances relating to lesser‐known NLRs, the review article by de Souza and colleagues introduce us to the lesser‐known functions of apoptosis‐associated speck‐like protein containing a caspase recruit domain (ASC) and its enigmatic extracellular function. ⁹ As already discussed, during inflammasome activation ASC oligomerizes with NLRs within the cytosol to form ‘ASC specks’. A lesser‐known function of ASC specks is that during pyroptosis, they seem to be released in the extracellular space and act as ‘danger signals’ propagating inflammation in nearby cells. The authors discuss mechanisms of ASC speck formation and release to the extracellular space and report recent findings were extracellular ASC specks (eASC) that have been detected in the circulation of individuals with inflammatory and autoimmune conditions. Most importantly, the authors describe the role of eASC in amyloid diseases, such as Alzheimer's disease, where it has been shown that aggregation of ASC into prion‐like ASC specks has the potential to aggravate amyloid pathology by cross‐seeding with several amyloidogenic peptides in different tissues. de Souza et al ⁹  have focused their review on two main special features of eASCs, which could underlie most of their functions: 1) on the prion‐like propensity of ASC to polymerize and 2) on the finding that eASC specks are biologically active and can reach the circulation augmenting inflammation in nearby cells. These abilities give eASCs diverse functions from playing a pivotal role in the innate immune defences to propagating inflammation and being implicated in a wide spectrum of pathologies.

The final review of the series by Jimenez‐Duran and Triantafilou ¹⁰ nicely concludes the review series by considering the metabolic regulation of lesser‐known inflammasomes. There is growing evidence of metabolic regulation of NLRP3 inflammasome activation, but little is known regarding other NLRs. The authors introduce us to NLRP1, which has been reported to have both protective and worsening effects in different immunometabolic diseases. It seems to be able to act as a sensor of cellular stress, glucose and lipid metabolism and has been linked to microbiome dysbiosis and inflammatory bowel disease.

In addition, the authors introduce us to a truly enigmatic inflammasome, NLRC3, whose functions are mostly undiscovered. However, NLRC3 has been shown to play a major role in T‐cell metabolism, where it seems to act as a brake for aerobic glycolysis, which is the hallmark of activated T cells and Th1 cell differentiation. Although in the first part of the series, Pickering and Booty ⁵ introduced us to NLRX1, in the second part of the review series, Jimenez‐Duran and Triantafilou ¹⁰ introduce us to the metabolic regulation of NLRX1. With its location in the outer mitochondrial membrane, NLRX1 is key in modulating cellular metabolism and has been linked to a variety of immunometabolic diseases, such as cancer, colitis, inflammatory bowel disease and CNS inflammation. They also introduce NLRP6 and metabolism and its effect on host–microbiota communications, as well as NLRP12 and its protective role in inflammatory bowel disease and its function as a regulator of the colonic microbiota and suppressor of inflammation to microbial ligands. Finally, the crosstalk of lesser‐known inflammasomes with other signalling pathways, such as the complement system, is discussed with a focus on the complement–inflammasome–immunometabolism axis. Jimenez‐Duran and Triantafilou conclude the review series with a thought‐provoking discussion regarding the metabolic control of these lesser‐known inflammasomes, how these implicate a variety of perturbations in the glycolytic pathway, and mitochondrial and lipid metabolism and how it is linked to several autoimmune and metabolic diseases.

The three articles in this final instalment of the review series not only provide us with recent advances to our knowledge of enigmatic inflammasomes, such as NLRP7, but also introduce us to novel concepts of inflammasome biology such as the existence of extracellular NLR‐ASC specks propagating inflammasome activation in the nearby environment, as well as how immunometabolism regulates these lesser‐known inflammasomes. Together with the first part of the series, we provide an exciting review series that presents advances in the field of inflammasome activation beyond the well‐known NLRP3 and provide us with some thought‐provoking discussions on which are the areas that need to be addressed in the field of inflammasome research. As demonstrated by this review series, almost twenty years since the discovery of NLRs, we still have several under‐studied NLRs. Due to their diversity of function, it would be exciting to watch this space in the future. We hope you enjoy the review series as much as we have and inspire you to look a little closer at these enigmatic NLRs!

Triantafilou K. Enigmatic inflammasomes – Sequel (Part 2). Immunology. 2021;163:345–347. 10.1111/imm.13387