The Emerging Role of Inflammasomes as Central Mediators in Inflammatory Bladder Pathology

Abstract

Irritative voiding symptoms (e.g. increased frequency and urgency) occur in many common pathologic conditions such as urinary tract infections and bladder outlet obstruction, and these conditions are well-established to have underlying inflammation that directly triggers these symptoms. However, it remains unclear as to how such diverse stimuli individually generate a common inflammatory process. Jürg Tschopp provided substantial insight into this conundrum when, working with extracts from THP-1 cells, he reported the existence of the inflammasome. He described it as a structure that senses multiple diverse signals from intracellular/extracellular sources and pathogens and triggers inflammation by the maturation and release of the pro-inflammatory cytokines interleukin-1β and interleukin-18. Recently, many of these sensors were found in the bladder and the nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3, has been shown to be a central mediator of inflammation in several urological diseases. In this review, we introduce the nucleotide-binding domain, leucine-rich-containing family, pyrin domaincontaining-3 inflammasome, highlight its emerging role in several common urologic conditions, and speculate on the potential involvement of other inflammasomes in bladder pathology.

Keywords: Bladder cancer; Bladder outlet obstruction; Inflammasomes; NLRP3; Urinary tract infection; Urothelial carcinoma.

Fig. 1

Domain structure of the components of the NLRP3 inflammasome. The NLRP3 inflammasome contains 3 components: 1) NLRP3 (the nod-like receptor), 2) ASC, and 3) procaspase-1. NLRP3 consists of 3 separate domains: a C-terminal LRR domain, a NACHT domain and an N-terminal PYD. ASC is composed of a PYD and a CARD, and is the adaptor protein that binds pro-caspase-1. Pro-caspase-1 has a CARD and 2 subunits (p20 and p10) which are cleaved to form active caspase-1 by the inflammasome.

Fig. 2

Priming of the NLRP3 inflammasome in the canonical pathway. The canonical pathway of NLRP3 inflammasome activation requires a priming step. This occurs when a DAMP or PAMP (such as LPS) binds to a non-NLR receptor (such as TLR) or there is signaling though IL-1R or tumor necrosis factor receptor. The effect of this binding leads to further signaling through either NF-κB and/or BRCC36. Signaling through NF-κB leads to production of de novo pro-IL-1β and other components of the NLRP3 inflammasome while signaling through BRCC36 leads to deubiquination of existing NLRP3. Deubiquination then frees NLRP3 to be activated, i.e. “licenses” it for activation.

Fig. 3

Activation of the NLRP3 inflammasome in the canonical pathway. The second step of the canonical pathway is activation. DAMPs and PAMPs trigger this step and lead to activation of the NLRP3 inflammasome. Proposed cellular events involved include ROS, Ca²⁺ signaling, and, most likely, K⁺ efflux. Regardless, NLRP3 inflammasome assembly is initiated and the catalytic domain of the never in mitosis gene a-related kinase 7 interacts with the LRR domain of NLRP3 causing oligomerization through binding of NACHT domains (the figure depicts only a dimer for simplicity). This leads to NLRP3 binding to ASC and ASC in turn binding to pro-caspase-1. Pro-caspase-1 molecules in close proximity lead autocatalytic cleavage of pro-caspase-1 to caspase-1 (induced proximity). Activated caspase-1 then cleaves pro-IL-1β and pro-IL-18 into their active forms (IL-1β and IL-18). Caspase-1 also cleaves gasdermin D whose N-terminus is responsible for forming a pore that results in pyroptosis and the release of IL-1β and IL-18.

Fig. 4

Non-canonical pathway of NLRP3 inflammasome activation. NLRP3 activation in the noncanonical pathway requires activation of caspase-11 through direct binding to a cytosolic PAMP, like LPS, which then facilitates the oligomerization of the caspase. Caspase-11 can directly cleave gasdermin D and cause pyroptosis or can lead to the release of ATP that acts as a DAMP, stimulating K⁺ loss and the assembly of the NRP3 inflammasome with subsequent maturation of IL-1β and IL-18.

Fig. 5

Alternative pathway of NLRP3 inflammasome activation. The alternative pathway is the third mechanism of NLRP3 inflammasome activation. Similar to the canonical pathway's priming step, LPS binds to TLR4 that leads to upregulation of NLRP3 and pro-IL-1β through the NF-κB pathway. However, instead of the K⁺ efflux, NLRP3 inflammasome activation relies on toll/interleukin-1 receptor-domain-containing adapter-inducing interferon-β, receptor-interacting protein kinase 1, Fas-associated protein with death domain, and caspase-8 that are upregulated by the TLR4 signaling.

Fig. 6

NLRP3 inflammasome activation in BOO. The NLRP3 inflammasomes is thought to be activated by at least 3 repetitive insults during BOO: hypoxia/reperfusion, high pressure and increased stretching. Hypoxia/reperfusion leads to production of ROS, a known trigger for the inflammasome. Increased pressure and repetitive stretch both release ATP, a known NLRP3 DAMP. Furthermore, bladder stretching releases acetylcholine that can act back on cholinergic receptors to trigger release of more ATP. The increase in extracellular ATP caused by pressure, stretch, and acetylcholine can drive yet another positive feedback loop known as ATP-dependent ATP release. The result is a tremendous amount of extracellular ATP, which then serves as a trigger for NLRP3 activation.

Fig. 7

Immunohistochemistry localizing inflammasomes to the urothelium. Antibody staining of rat bladders for NLRP1, NLRP3, NLRP6, NLRP7, NLRP12, NLRC4, and AIM2 and photographed at high magnification. Differential staining demonstrates that each inflammasome preferentially localizes to the urothelium. Reprinted with permission from [19].