NLR-mediated control of inflammasome assembly in the host response against bacterial pathogens

Abstract

The host response against diverse bacterial pathogens involves activation of specialized immune cells and elaboration of pro-inflammatory cytokines that help to coordinate appropriate host defense. Members of the interleukin-1 (IL-1) cytokine family, IL-1β and IL-18, are central players in this process. Extracellular release of the mature, active form of these cytokines requires their processing by the cysteine protease caspase-1, which therefore serves as a key regulator of the inflammatory response. In addition to its role in secretion of pro-inflammatory cytokines, caspase-1 is also required for a form of cell death, recently termed pyroptosis, that occurs in macrophages infected by certain bacterial pathogens. Caspase-1 itself is synthesized as a pro-enzyme, which must first be activated by autocatalytic cleavage. This activation requires recruitment of caspase-1 into multiprotein complexes known as inflammasomes. The Nod-like receptor (NLR) family of cytosolic proteins play an important role in detecting inflammatory stimuli and subsequently mediate inflammasome assembly. A common feature of NLR proteins that trigger inflammasome assembly in response to bacterial infection is that they appear to sense membrane perturbation or delivery of bacterial components into the cytosol through bacterial pore-forming toxins or bacterial secretion systems. This review will discuss the recent developments regarding caspase-1 activation in response to bacterial infection, cross-talk between caspase-1 and other pathways involved in regulating cell death, and recent findings that a number of bacterial pathogens possess mechanisms to inhibit caspase-1 activation.

Introduction

Appropriate responses of multicellular eukaryotes against diverse microorganisms requires recruitment of inflammatory cells and release of pro-inflammatory and antimicrobial mediators. The Toll-like receptors (TLRs) and NOD-like receptors (NLRs) are two major families of germ-line encoded receptors that play crucial roles in the host response to bacterial infection by detecting conserved microbial structural components such as lipopolysaccharide (LPS), peptidoglycan, or lipoteichoic acid, and initiating transcriptional programs that mediate activation of the host immune response. Throughout this review, we will use the newly established nomenclature in referring to members of the NLR family [1].

TLRs are membrane-bound receptors located on the plasma membrane and on vesicles of the endocytic pathway, while NLRs are present within the cytosol. Although TLRs and their downstream signaling pathways are essential for recognition of microbial components that were originally termed ‘pathogen associated molecular patterns’ (PAMPs), they are insufficient to distinguish between pathogenic and non-pathogenic bacteria because they recognize molecules present in both classes of bacteria. That role appears to be played at least partially by certain members of the NLR family, which detect the presence of microbial components within the host cell cytosol and respond by triggering caspase-1 activation. For example, the microbial components that have been identified as being detected by NLRs, such as muramyl di-peptide (MDP) [2], [3] and flagellin [4], [5], are not unique to pathogenic bacteria. However, the presence of these microbial components within the cytosol of the host cell appears to depend upon their delivery via specialized secretion systems or pore-forming toxins. The ability of NLRs to respond specifically to pathogenic microbes is thus linked to an activity – the formation of pores in cellular membranes – that appears to be preferentially associated with microbial pathogens.

NLRs possess a characteristic domain architecture, consisting of an N-terminal Pyrin domain (PYD) or caspase recruitment domain (CARD), a central nucleotide binding/oligomerization domain (NOD, also known as NACHT), and C-terminal leucine-rich repeats (LRRs) (Fig. 1). The CARD or PYD mediate homophilic protein-protein interactions with other CARD or PYD containing proteins. NLRs bear striking similarity to the resistance, or R, proteins of plants, which also contain a variable N-terminus, central NOD domain, and C-terminal LRR [6]. Like mammalian NLRs, plant R proteins play a critical role in immunity of plants to infection with microbial pathogens. Unlike mammalian NLRs that have been characterized thus far however, plant R proteins detect the presence of virulence factors, either via a direct interaction, or through an intermediary protein that is modified by the virulence factor. Based on analogy with R proteins as well as mammalian Apaf-1, which also contains a similar domain architecture to NLRs, the LRRs are believed to maintain the NLRs in an auto-inhibited state, and to mediate recognition of a specific signal leading to a conformational change that promotes oligomerization of the NLR into a multiprotein complex, termed the inflammasome, within which caspase-1 is activated [7].

Caspase-1 is the founding member of a family of cysteine proteases that cleave proteins at specific sequences following aspartyl residues [8]. Originally termed Interleukin Converting Enzyme (ICE), caspase-1 was identified based on its ability to cleave the pro-form of IL-1β to mature, active IL-1β [9]. Since then, a great deal of research has focused on the mechanisms by which caspase-1 is activated. Discovery in the late 1990s of a large family of cytosolic proteins containing C-terminal LRRs, a central nucleotide binding domain, and N-terminal homotypic protein–protein interaction domains of the death domain superfamily led to the emerging idea that these proteins were regulators of cell death and pro-inflammatory signaling [10], [11]. The apoptosis associated speck-like protein containing a CARD (ASC), which contains both a CARD and PYD, was also identified as playing a role in regulating caspase-1 activation, although it was initially unclear whether ASC was a positive or negative regulator of caspase-1 [12].

The fields of caspase-1 biology and bacterial pathogenesis converged with a series of studies demonstrating that infection of macrophages by different bacterial pathogens resulted in a caspase-1-dependent cell death [13], [14]. Due to the release of the pro-inflammatory cytokines, IL-1β and IL-18, upon caspase-1 activation, this form of cell death was termed ‘pyroptosis’ to reflect its pro-inflammatory nature and distinguish it from classical apoptosis [15], [16]. The caspase-1-dependent death of macrophages in response to bacterial infection was shown to depend on the presence of bacterial virulence determinants, notably bacterial secretion systems [13], [14], [17], [18] or pore-forming toxins [19]. Generation of mice deficient in ASC or different NLRs led to the discovery that different NLRs were important for activation of caspase-1 in response to different signals [20]. Furthermore, ASC was defined as an adaptor necessary for bridging the CARD of caspase-1 and the PYD of the NLRP proteins, thus playing a positive role in caspase-1 activation [7], [21].

Much of this literature has recently been reviewed elsewhere [22], [23], [24], [25]. In this review, we discuss a number of outstanding questions that remain concerning the nature of the signals that are sensed by different inflammasomes, cross-talk between inflammasomes and other signaling pathways, and the recent observations that some bacterial pathogens can interfere with caspase-1 activation. We apologize to those whose work could not be cited due to space constraints.