Abstract
Porphyromonas gingivalis is the only known human-associated prokaryote that produces a peptidylarginine deiminase (PPAD), a protein-modifying enzyme that is secreted along with a number of virulence factors via a type IX secretion system (T9SS). While the function of PPAD in P. gingivalis physiology is not clear, human peptidylarginine deiminases are known to convert positively charged arginine residues within proteins to neutral citrulline and, thereby, impact protein conformation and function. Here, we report that the lack of citrullination in a PPAD deletion mutant (Δ8820) enhances biofilm formation. More Δ8820 cells attached to the surface than the parent strain during the early stages of biofilm development and, ultimately, mature Δ8820 biofilms were comprised of significantly more cell–cell aggregates and extracellular matrix. Imaging by electron microscopy discovered that Δ8820 biofilm cells secrete copious amounts of protein aggregates. Furthermore, gingipain-derived adhesin proteins, which are also secreted by the T9SS were predicted by mass spectrometry to be citrullinated and citrullination of these targets by wild-type strain 381 in vitro was confirmed. Lastly, Δ8820 biofilms contained more gingipain-derived adhesin proteins and more gingipain activity than 381 biofilms. Overall, our findings support the model that citrullination of T9SS cargo proteins known to play a key role in colonization, such as gingipain-derived adhesin proteins, is an underlying mechanism that modulates P. gingivalis biofilm development.
Highlights
Porphyromonas gingivalis, an oral bacterium primarily known for its etiological role in periodontal disease, is implicated in rheumatoid arthritis, which is a common comorbidity in periodontal disease.[1]
Previous findings show that removal of free L-arginine from the environment by arginine deiminases inhibits biofilm formation of P. gingivalis, while addition of L-arginine can enhance attachment.[27,28,29]
These findings are specific to free L-arginine, they demonstrate the importance of L-arginine in and the impact of arginine deiminases on P. gingivalis biofilm formation
Summary
Porphyromonas gingivalis, an oral bacterium primarily known for its etiological role in periodontal disease, is implicated in rheumatoid arthritis, which is a common comorbidity in periodontal disease.[1]. While humans express five different isotypes (PAD1–4 and PAD 6) that play roles in both health and disease, PPAD is the only known prokaryotic PPAD.[4,5,6,7,8,9,10,11,12] Currently, data support the model that citrullination of peptides or proteins by PPAD in the periodontium can lead to a breakdown in tolerance and, thereby, the production of ACPAs, and the development or progression of rheumatoid arthritis.[2,3,5,13,14,15,16,17] the link to rheumatoid arthritis has driven an extensive amount of PPAD research, the role of PPAD in the basic physiology of P. gingivalis, in particular its impact during biofilm growth, is still unclear
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