Mitochondrial reactive oxygen species induces NLRP3-dependent lysosomal damage and inflammasome activation.

The nucleotide-binding oligomerization domain-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome drives many inflammatory processes and mediates IL-1 family cytokine release. Inflammasome activators typically damage cells and may release lysosomal and mitochondrial products into the cytosol. Macrophages triggered by the NLRP3 inflammasome activator nigericin show reduced mitochondrial function and decreased cellular ATP. Release of mitochondrial reactive oxygen species (ROS) leads to subsequent lysosomal membrane permeabilization (LMP). NLRP3-deficient macrophages show comparable reduced mitochondrial function and ATP loss, but maintain lysosomal acidity, demonstrating that LMP is NLRP3 dependent. A subset of wild-type macrophages undergo subsequent mitochondrial membrane permeabilization and die. Both LMP and mitochondrial membrane permeabilization are inhibited by potassium, scavenging mitochondrial ROS, or NLRP3 deficiency, but are unaffected by cathepsin B or caspase-1 inhibitors. In contrast, IL-1ß secretion is ablated by potassium, scavenging mitochondrial ROS, and both cathepsin B and caspase-1 inhibition. These results demonstrate interplay between lysosomes and mitochondria that sustain NLRP3 activation and distinguish cell death from IL-1ß release.

The second transmembrane domain of P2X7 contributes to dilated pore formation.

Activation of the purinergic receptor P2X7 leads to the cellular permeability of low molecular weight cations. To determine which domains of P2X7 are necessary for this permeability, we exchanged either the C-terminus or portions of the second transmembrane domain (TM2) with those in P2X1 or P2X4. Replacement of the C-terminus of P2X7 with either P2X1 or P2X4 prevented surface expression of the chimeric receptor. Similarly, chimeric P2X7 containing TM2 from P2X1 or P2X4 had reduced surface expression and no permeability to cationic dyes. Exchanging the N-terminal 10 residues or C-terminal 14 residues of the P2X7 TM2 with the corresponding region of P2X1 TM2 partially restored surface expression and limited pore permeability. To further probe TM2 structure, we replaced single residues in P2X7 TM2 with those in P2X1 or P2X4. We identified multiple substitutions that drastically changed pore permeability without altering surface expression. Three substitutions (Q332P, Y336T, and Y343L) individually reduced pore formation as indicated by decreased dye uptake and also reduced membrane blebbing in response to ATP exposure. Three others substitutions, V335T, S342G, and S342A each enhanced dye uptake, membrane blebbing and cell death. Our results demonstrate a critical role for the TM2 domain of P2X7 in receptor function, and provide a structural basis for differences between purinergic receptors.

Visualization of bacterial toxin induced responses using live cell fluorescence microscopy.

Bacterial toxins bind to cholesterol in membranes, forming pores that allow for leakage of cellular contents and influx of materials from the external environment. The cell can either recover from this insult, which requires active membrane repair processes, or else die depending on the amount of toxin exposure and cell type(1). In addition, these toxins induce strong inflammatory responses in infected hosts through activation of immune cells, including macrophages, which produce an array of pro-inflammatory cytokines(2). Many Gram positive bacteria produce cholesterol binding toxins which have been shown to contribute to their virulence through largely uncharacterized mechanisms. Morphologic changes in the plasma membrane of cells exposed to these toxins include their sequestration into cholesterol-enriched surface protrusions, which can be shed into the extracellular space, suggesting an intrinsic cellular defense mechanism(3,4). This process occurs on all cells in the absence of metabolic activity, and can be visualized using EM after chemical fixation(4). In immune cells such as macrophages that mediate inflammation in response to toxin exposure, induced membrane vesicles are suggested to contain cytokines of the IL-1 family and may be responsible both for shedding toxin and disseminating these pro-inflammatory cytokines(5,6,7). A link between IL-1ß release and a specific type of cell death, termed pyroptosis has been suggested, as both are caspase-1 dependent processes(8). To sort out the complexities of this macrophage response, which includes toxin binding, shedding of membrane vesicles, cytokine release, and potentially cell death, we have developed labeling techniques and fluorescence microscopy methods that allow for real time visualization of toxin-cell interactions, including measurements of dysfunction and death (Figure 1). Use of live cell imaging is necessary due to limitations in other techniques. Biochemical approaches cannot resolve effects occurring in individual cells, while flow cytometry does not offer high resolution, real-time visualization of individual cells. The methods described here can be applied to kinetic analysis of responses induced by other stimuli involving complex phenotypic changes in cells.

Macrophage responses to bacterial toxins: a balance between activation and suppression.

Toxins secreted by bacteria can impact the host in a number of different ways. In some infections, toxins play a crucial and central role in pathogenesis (i.e., anthrax), while in other bacterial infections, the role of toxins is less understood. The cholesterol-dependent cytolysins (CDCs), of which streptolysin O is a prototype, are a class of pore-forming toxins produced by many gram-positive bacteria and have only been studied in a few experimental infection models. Our laboratory has demonstrated that CDCs have effects on macrophages that are both pro- and anti-inflammatory. Here, we review evidence that CDCs promote inflammation by driving secretion of IL-1ß and HMGB-1 from macrophages in a NLRP3-dependent manner, while also causing shedding of membrane microvesicles from cells that can interact with macrophages and inhibit TNF-α release. CDCs thus impact macrophage function in ways that may be both beneficial and detrimental to the host.