Over the past several decades, much has been revealed about the nature of the host innate immune response to microorganisms, with the identification of pattern recognition receptors (PRRs) and pathogen-associated molecular patterns, which are the conserved microbial motifs sensed by these receptors. It is now apparent that these same PRRs can also be activated by non-microbial signals, many of which are considered as damage-associated molecular patterns. The sterile inflammation that ensues either resolves the initial insult or leads to disease. Here, we review the triggers and receptor pathways that result in sterile inflammation and its impact on human health.

Sterile inflammation: sensing and reacting to damage

Inflammasomes are key signalling platforms that detect pathogenic microorganisms and sterile stressors, and that activate the highly pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18. In this Review, we discuss the complex regulatory mechanisms that facilitate a balanced but effective inflammasome-mediated immune response, and we highlight the similarities to another molecular signalling platform - the apoptosome - that monitors cellular health. Extracellular regulatory mechanisms are discussed, as well as the intracellular control of inflammasome assembly, for example, via ion fluxes, free radicals and autophagy.

/pdf/activation-and-regulation-of-the-inflammasomes-3jj26fqvmx.pdf

Activation and regulation of the inflammasomes

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

https://www.um.es/biomybiotec/web/Seminarios/2009/papers/Pablo_Pelegrín_Annu%20Rev%20Immunol%202009%20Martinon-1.pdf

The Inflammasomes: Guardians of the Body

The shapes of eukaryotic cells and ultimately the organisms that they form are defined by cycles of mechanosensing, mechanotransduction and mechanoresponse Local sensing of force or geometry is transduced into biochemical signals that result in cell responses even for complex mechanical parameters such as substrate rigidity and cell-level form These responses regulate cell growth, differentiation, shape changes and cell death Recent tissue scaffolds that have been engineered at the micro- and nanoscale level now enable better dissection of the mechanosensing, transduction and response mechanisms

https://faculty.washington.edu/nsniadec/ME599/S09/private/25Vogel.pdf

Local force and geometry sensing regulate cell functions.

Calcitonin-gene-related peptide (CGRP) and adrenomedullin are related peptides with distinct pharmacological profiles. Here we show that a receptor with seven transmembrane domains, the calcitonin-receptor-like receptor (CRLR), can function as either a CGRP receptor or an adrenomedullin receptor, depending on which members of a new family of single-transmembrane-domain proteins, which we have called receptor-activity-modifying proteins or RAMPs, are expressed. RAMPs are required to transport CRLR to the plasma membrane. RAMP1 presents the receptor at the cell surface as a mature glycoprotein and a CGRP receptor. RAMP2-transported receptors are core-glycosylated and are adrenomedullin receptors.

/pdf/ramps-regulate-the-transport-and-ligand-specificity-of-the-3dt9v953y2.pdf

RAMPs regulate the transport and ligand specificity of the calcitonin-receptor-like receptor.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/j.febslet.2004.07.009

Pannexin membrane channels are mechanosensitive conduits for ATP

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/j.febslet.2005.12.004

Activation of pannexin 1 channels by ATP through P2Y receptors and by cytoplasmic calcium

ATP is a widely used extracellular signaling molecule. The mechanism of ATP release from cells is presently unresolved and may be either vesicular or channel-mediated. Erythrocytes release ATP in response to low oxygen or to shear stress. In the absence of vesicles, the release has to be through channels. Erythrocytes do not form gap junctions. Yet, here we show with immunohistochemical and electrophysiological data that erythrocytes express the gap junction protein pannexin 1. This protein, in addition to forming gap junction channels in paired oocytes, can also form a mechanosensitive and ATP-permeable channel in the nonjunctional plasma membrane. Consistent with a role of pannexin 1 as an ATP release channel, ATP release by erythrocytes was attenuated by the gap junction blocker carbenoxolone. Furthermore, under conditions of ATP release, erythrocytes took up fluorescent tracer molecules permeant to gap junction channels.

http://www.pnas.org/content/103/20/7655.full.pdf

Pannexin 1 in erythrocytes: function without a gap.

The pannexin 1 channel activates the inflammasome in neurons and astrocytes.

Transcriptional activity of heat shock (hsp) genes is controlled by a heat-activated, group-specific transcription factor(s) recognizing arrays of inverted repeats of the element NGAAN. To date genes for two human factors, HSF1 and HSF2, have been isolated. To define their properties as well as the changes they undergo during heat stress activation, we prepared polyclonal antibodies to these factors. Using these tools, we have shown that human HeLa cells constitutively synthesize HSF1, but we were unable to detect HSF2. In unstressed cells HSF1 is present mainly in complexes with an apparent molecular mass of about 200 kDa, unable to bind to DNA. Heat treatment induces a shift in the apparent molecular mass of HSF1 to about 700 kDa, concomitant with the acquisition of DNA-binding ability. Cross-linking experiments suggest that this change in complex size may reflect the trimerization of monomeric HSF1. Human HSF1 expressed in Xenopus oocytes does not bind DNA, but derepression of DNA-binding activity, as well as oligomerization of HSF1, occurs during heat treatment at the same temperature at which hsp gene expression is induced in this organism, suggesting that a conserved Xenopus protein(s) plays a role in this regulation. Inactive HSF1 resides in the cytoplasm of human cells; on activation it rapidly translocates to a soluble nuclear fraction, and shortly thereafter it becomes associated with the nuclear pellet. On heat shock, activatable HSF1, which might already have been posttranslationally modified in the unstressed cell, undergoes further modification. These different process provide multiple points of regulation of hsp gene expression.

Gerhard Dahl

Papers

Pannexin membrane channels are mechanosensitive conduits for ATP

Activation of pannexin 1 channels by ATP through P2Y receptors and by cytoplasmic calcium

Pannexin 1 in erythrocytes: function without a gap.

The pannexin 1 channel activates the inflammasome in neurons and astrocytes.

Activation of human heat shock genes is accompanied by oligomerization, modification, and rapid translocation of heat shock transcription factor HSF1.