Functional and structural analyses reveal that a dual domain sialidase protects bacteria from complement killing through desialylation of complement factors

The complement system: A key player in the host response to infections

Infections are one of the most significant healthcare and economic burdens across the world as underscored by the recent coronavirus pandemic. Moreover, with the increasing incidence of antimicrobial resistance, there is an urgent need to better understand host-pathogen interactions to design effective treatment strategies. The complement system is a key arsenal of the host defense response to pathogens and bridges both innate and adaptive immunity. However, in the contest between pathogens and host defense mechanisms, the host is not always victorious. Pathogens have evolved several approaches, including co-opting the host complement regulators to evade complement-mediated killing. Furthermore, deficiencies in the complement proteins, both genetic and therapeutic, can lead to an inefficient complement-mediated pathogen eradication, rendering the host more susceptible to certain infections. On the other hand, overwhelming infection can provoke fulminant complement activation with uncontrolled inflammation and potentially fatal tissue and organ damage. This review presents an overview of critical aspects of the complement-pathogen interactions during infection and discusses perspectives on designing therapies to mitigate complement dysfunction and limit tissue injury.

A "terminal" case of glycan catabolism: Structural and enzymatic characterization of the sialidases of Clostridium perfringens

Sialic acids are commonly found on the terminal ends of biologically important carbohydrates, including intestinal mucin O-linked glycans. Pathogens such as Clostridium perfringens, the causative agent of necrotic enteritis in poultry and humans, have the ability to degrade host mucins and colonize the mucus layer, which involves removal of the terminal sialic acid by carbohydrate-active enzymes (CAZymes). Here, we present the structural and biochemical characterization of the GH33 catalytic domains of the three sialidases of C. perfringens and probe their substrate specificity. The catalytically active domains, which we refer to as NanHGH33, NanJGH33, and NanIGH33, displayed differential activity on various naturally occurring forms of sialic acid. We report the X-ray crystal structures of these domains in complex with relevant sialic acid variants revealing the molecular basis of how each catalytic domain accommodates different sialic acids. NanHGH33 displays a distinct preference for α-2,3-linked sialic acid, but can process α-2,6-linked sialic acid. NanJGH33 and NanIGH33 both exhibit the ability to process α-2,3- and α-2,6-linked sialic acid without any significant apparent preference. All three enzymes were sensitive to generic and commercially available sialidase inhibitors, which impeded sialidase activity in cultures as well as the growth of C. perfringens on sialylated glycans. The knowledge gained in these studies can be applied to in vivo models for C. perfringens growth and metabolism of mucin O-glycans, with a view toward future mitigation of bacterial colonization and infection of intestinal tissues.

A pleiotropic role of sialidase in the pathogenicity of Porphyromonas gingivalis

As one of the keystone pathogens of periodontitis, the oral bacterium Porphyromonas gingivalis produces an array of virulence factors, including a recently identified sialidase (PG0352). Our previous report involving loss-of-function studies indicated that PG0352 plays an important role in the pathophysiology of P. gingivalis. However, this report had not been corroborated by gain-of-function studies or substantiated in different P. gingivalis strains. To fill these gaps, herein we first confirm the role of PG0352 in cell surface structures (e.g., capsule) and serum resistance using P. gingivalis W83 strain through genetic complementation and then recapitulate these studies using P. gingivalis ATCC33277 strain. We further investigate the role of PG0352 and its counterpart (PGN1608) in ATCC33277 in cell growth, biofilm formation, neutrophil killing, cell invasion, and P. gingivalis-induced inflammation. Our results indicate that PG0352 and PGN1608 are implicated in P. gingivalis cell surface structures, hydrophobicity, biofilm formation, resistance to complement and neutrophil killing, and host immune responses. Possible molecular mechanisms involved are also discussed. In summary, this report underscores the importance of sialidases in the pathophysiology of P. gingivalis and opens an avenue to elucidate their underlying molecular mechanisms.