Structure of the NLRP3 decamer bound to the cytokine release inhibitor CRID3.

NLRP3 is an intracellular sensor protein that when activated by a broad spectrum of exogenous and endogenous stimuli leads to inflammasome formation and pyroptosis1,2. The conformational states of NLRP3 and the way antagonistic small molecules act at the molecular level remain poorly understood2,3. Here we report the cryo-electron microscopy structures of full-length human NLRP3 in its native form and complexed with the inhibitor CRID3 (also named MCC950)4. Inactive, ADP-bound NLRP3 is a decamer composed of homodimers of intertwined leucine-rich repeat (LRR) domains that assemble back-to-back as pentamers. The NACHT domain is located at the apical axis of this spherical structure. One pyrin domain dimer is in addition formed inside the LRR cage. Molecular contacts between the concave sites of two opposing LRR domains are mediated by an acidic loop that extends from an LRR transition segment. Binding of CRID3 considerably stabilizes the NACHT and LRR domains relative to each other. CRID3 binds into a cleft, connecting four subdomains of the NACHT with the transition LRR. Its central sulfonylurea group interacts with the Walker A motif of the NLRP3 nucleotide-binding domain and is sandwiched between two arginine residues, which explains the specificity of NLRP3 for this chemical entity. With the determination of the binding site of this key therapeutic agent, specific targeting of NLRP3 for the treatment of autoinflammatory and autoimmune diseases and rational drug optimization is within reach.

Microglia-derived ASC specks cross-seed amyloid-ß in Alzheimer's disease.

The spreading of pathology within and between brain areas is a hallmark of neurodegenerative disorders. In patients with Alzheimer's disease, deposition of amyloid-ß is accompanied by activation of the innate immune system and involves inflammasome-dependent formation of ASC specks in microglia. ASC specks released by microglia bind rapidly to amyloid-ß and increase the formation of amyloid-ß oligomers and aggregates, acting as an inflammation-driven cross-seed for amyloid-ß pathology. Here we show that intrahippocampal injection of ASC specks resulted in spreading of amyloid-ß pathology in transgenic double-mutant APPSwePSEN1dE9 mice. By contrast, homogenates from brains of APPSwePSEN1dE9 mice failed to induce seeding and spreading of amyloid-ß pathology in ASC-deficient APPSwePSEN1dE9 mice. Moreover, co-application of an anti-ASC antibody blocked the increase in amyloid-ß pathology in APPSwePSEN1dE9 mice. These findings support the concept that inflammasome activation is connected to seeding and spreading of amyloid-ß pathology in patients with Alzheimer's disease.

ATP-binding and hydrolysis of human NLRP3.

The innate immune system uses inflammasomal proteins to recognize danger signals and fight invading pathogens. NLRP3, a multidomain protein belonging to the family of STAND ATPases, is characterized by its central nucleotide-binding NACHT domain. The incorporation of ATP is thought to correlate with large conformational changes in NLRP3, leading to an active state of the sensory protein. Here we analyze the intrinsic ATP hydrolysis activity of recombinant NLRP3 by reverse phase HPLC. Wild-type NLRP3 appears in two different conformational states that exhibit an approximately fourteen-fold different hydrolysis activity in accordance with an inactive, autoinhibited state and an open, active state. The impact of canonical residues in the nucleotide binding site as the Walker A and B motifs and sensor 1 and 2 is analyzed by site directed mutagenesis. Cellular experiments show that reduced NLRP3 hydrolysis activity correlates with higher ASC specking after inflammation stimulation. Addition of the kinase NEK7 does not change the hydrolysis activity of NLRP3. Our data provide a comprehensive view on the function of conserved residues in the nucleotide-binding site of NLRP3 and the correlation of ATP hydrolysis with inflammasome activity.

ß-Amyloid Clustering around ASC Fibrils Boosts Its Toxicity in Microglia.

Alzheimer's disease is the world's most common neurodegenerative disorder. It is associated with neuroinflammation involving activation of microglia by ß-amyloid (Aß) deposits. Based on previous studies showing apoptosis-associated speck-like protein containing a CARD (ASC) binding and cross-seeding extracellular Aß, we investigate the propagation of ASC between primary microglia and the effects of ASC-Aß composites on microglial inflammasomes and function. Indeed, ASC released by a pyroptotic cell can be functionally built into the neighboring microglia NOD-like receptor protein (NLRP3) inflammasome. Compared with protein-only application, exposure to ASC-Aß composites amplifies the proinflammatory response, resulting in pyroptotic cell death, setting free functional ASC and inducing a feedforward stimulating vicious cycle. Clustering around ASC fibrils also compromises clearance of Aß by microglia. Together, these data enable a closer look at the turning point from acute to chronic Aß-related neuroinflammation through formation of ASC-Aß composites.

Alternative splicing regulates stochastic NLRP3 activity.

Leucine-rich repeat (LRR) domains are evolutionarily conserved in proteins that function in development and immunity. Here we report strict exonic modularity of LRR domains of several human gene families, which is a precondition for alternative splicing (AS). We provide evidence for AS of LRR domain within several Nod-like receptors, most prominently the inflammasome sensor NLRP3. Human NLRP3, but not mouse NLRP3, is expressed as two major isoforms, the full-length variant and a variant lacking exon 5. Moreover, NLRP3 AS is stochastically regulated, with NLRP3 ∆ exon 5 lacking the interaction surface for NEK7 and hence loss of activity. Our data thus reveals unexpected regulatory roles of AS through differential utilization of LRRs modules in vertebrate innate immunity.

NLRP3 inflammasome assembly is regulated by phosphorylation of the pyrin domain.

NLRP3 is a cytosolic pattern recognition receptor that senses microbes and endogenous danger signals. Upon activation, NLRP3 forms an inflammasome with the adapter ASC, resulting in caspase-1 activation, release of proinflammatory cytokines and cell death. How NLRP3 activation is regulated by transcriptional and posttranslational mechanisms to prevent aberrant activation remains incompletely understood. Here, we identify three conserved phosphorylation sites in NLRP3 and demonstrate that NLRP3 activation is controlled by phosphorylation of its pyrin domain (PYD). Phosphomimetic residues in NLRP3 PYD abrogate inflammasome activation and structural modeling indicates that phosphorylation of the PYD regulates charge-charge interaction between two PYDs that are essential for NLRP3 activation. Phosphatase 2A (PP2A) inhibition or knock-down drastically reduces NLRP3 activation, showing that PP2A can license inflammasome assembly via dephosphorylating NLRP3 PYD. These results propose that the balance between kinases and phosphatases acting on the NLRP3 PYD is critical for NLRP3 activation.