Editors' ChoiceImmunology

Dangerous sugar signals

Sci. Signal.  09 Aug 2016:
Vol. 9, Issue 440, pp. ec179
DOI: 10.1126/scisignal.aai7399

The NRLP3 inflammasome produces the proinflammatory cytokines interleukin (IL)–1β and IL-18 in response to activation of pattern recognition receptors on macrophages and dendritic cells by microbial components, such as peptidoglycan, and small molecules that act as danger signals. Peptidoglycan is a polysaccharide that is part of the cell wall in Gram-positive bacteria, and its backbone is composed of the sugars N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG). Wolf et al. found that production of IL-1β was increased by NAG, but not by NAM, when these sugars were delivered into the cytosol of macrophages. The authors confirmed previous reports that NAG inhibits the glycolytic enzyme hexokinase. Both the increase in IL-1β production and inhibition of hexokinase required the acetylation of NAG. The interaction of hexokinase with the voltage-dependent anion channel (VDAC) tethers the enzyme to the mitochondrial outer membrane, an interaction that is affected by signaling pathways or feedback inhibition by its end product, glucose-6-phosphate. The cytosolic pool of either endogenous or GFP-tagged hexokinase increased in macrophages treated with peptidoglycan or NAG but not in those treated with NAM. These results suggested that NAG disrupted the interaction of hexokinase with VDAC and that disruption of this interaction could activate the NLRP3 inflammasome. Indeed, delivery into macrophages of a peptide that disrupts the association of hexokinase with VDAC activated the NLRP3 inflammasome as assessed by various measures, including an increase in IL-1β and IL-18 secretion. Intraperitoneal injection of this peptide into mice triggered neutrophil infiltration into the peritoneal cavity. Furthermore, delivery of glucose-6-phosphate into the cytosol of macrophages increased IL-1β secretion and dissociated hexokinase from mitochondria, and the increase in IL-1β secretion was mimicked by manipulations that result in the buildup of glucose-6-phosphate. Thus, hexokinase acts as a pattern recognition receptor for both peptidoglycan and high amounts of glucose-6-phosphate, suggesting that it may mediate inflammatory immune responses to metabolic perturbations.

A. J. Wolf, C. N. Reyes, W. Liang, C. Becker, K. Shimada, M. L. Wheeler, H. C. Cho, N. I. Popescu, K. M. Coggeshall, M. Arditi, D. M. Underhill, Hexokinase is an innate immune receptor for the detection of bacterial peptidoglycan. Cell 166, 624–636 (2016). [PubMed]