NADase

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Introduction

Streptococcus pyogenes, group A Streptococcus (GAS) is an exclusively human pathogen that can cause pharyngitis, skin and underlying soft tissue infections, and even produce life-threatening invasive infection syndromes. In the past few decades, the pathogen's comeback in North America, Europe, and some developing countries, which has stimulated scientists to pay close attention to the pathogenesis of GAS.

Among the many virulence factors that GAS can produce, two secreted proteins interact to form a complex toxin, which has harmful effects on human epithelial cells in vitro. Streptolysin O (SLO) is a typical member of the cholesterol-dependent cytolysin family of toxins, representatives of which are produced by diverse gram-positive bacteria. Like other members of this family, SLO binds to cholesterol, oligomerizes, and inserts into the plasma membrane to create large pores in the membrane of eukaryotic cells. Studies have shown that SLO can be used as a vehicle for the second GAS protein and NAD⁺ - glycohydrolase (NADase) to translocate into the cytoplasm of host epithelial cells.

In addition to cleaving NAD to produce nicotinamide and ADP-ribose, GAS NADase can also produce cyclic ADP-ribose (cADPR) in a side reaction. cADPR can act as an effector of the intracellular Ca²⁺ signaling pathway, and may contribute to the cellular effects related to SLO and NADase, such as reducing bacterial uptake, enhancing cytotoxicity, and inducing cell apoptosis.

Studies on thein vitroeffects of SLO and NADase have shown that the combined effects of proteins contribute to the virulence of GAS. However, Stevens et al. reached conflicting conclusions in epidemiological studies. They noted that the temporal appearance of NADase activity in M1 strain of GAS is related to the appearance of this serotype in invasive GAS infection syndromes. On the other hand, a study on GAS infections among aboriginal people in Australia found that there was no relationship between the production of NADase and the severity or outcome of GAS infections.

Figure 1. Fungal NAD⁺ metabolism gets extracellular

Discussion

At present, people have not yet fully understood the potential importance and specific mode of action of NADase in the pathogenesis of GAS. Although all GAS strains have the ngagene, only 56% to 92% of clinical isolates produced clear NADase activity during growth in vitro. These results indicate that NADase is not necessary for GAS infection, and it is also possible that the regulation of enzyme expression during infection is different from that during growth in vitro. All these findings are limited by the potential biases in how strains were selected and the possible differences between NADase production in vitro versus in vivo.

Although mice are not usually the natural host of GAS, murine models have been shown to be useful for assessing the role of determinants of GAS virulence in pathogenesis. A series of research results have proved the role of NADase in virulence, and showed that in the absence of NADase, SLO expression may mitigate the virulence of GAS.  

The results of this study extend previous in vitroobservations on the pathogenesis of NADase. We must treat animal models of GAS infection with caution, because human beings are the only natural hosts of this organism. Nevertheless, the mouse infection model used in these studies has been used as efficient test systems to study the contribution to the pathogenesis of several putative virulence determinants, including M protein, cysteine protease, hyaluronic acid capsular polysaccharide, and streptolysin S.

At present, the mechanism by which NADase intoxicates host cells is still uncertain. It is possible that it may disrupt intracellular signaling pathways by depleting intracellular NAD⁺, ADP ribosylation of cellular proteins, and so on. The discovery that NADase contributes to virulence in vivoreinforces the view that SLO and NADase play an important role in GAS infection, and emphasizes the importance of further research to determine the molecular basis of NADase toxicity.