Proteomics of Bacterial and Mouse Extracellular Vesicles Released in the Gastrointestinal Tract of Nutrient Stressed Animals Reveals an Interplay Between Microbial Serine Proteases and Mammalian Serine Protease Inhibitors

Bacterial extracellular vesicles (BEVs) produced by members of the intestinal microbiota can contribute not only to digestion but also mediate microbe-host cell communication via the transfer of functional biomolecules to mammalian host cells. An unresolved question is what host factors and conditions influence BEV cargoes and how do they impact on host cell function? To address this question, we analysed and compared the proteome of BEVs released by the major human gastrointestinal tract (GIT) symbiont Bacteroides thetaiotaomicron (Bt) in vivo in fed versus fasted animals using nano-liquid chromatography with tandem mass spectrometry (LC-MSMS). Among the proteins whose abundance was negatively affected by fasting, nine of ten proteins of the serine protease family, including the regulatory protein dipeptidyl peptidase-4 (DPP-4), were significantly decreased in BEVs produced in the GIT of fasted animals. Strikingly, in extracellular vesicles produced by the intestinal epithelium of the same fasted mice, the proteins with the most increased abundance were serine protease inhibitors (serpins). Together, these findings suggest a dynamic interaction between GI bacteria and the host. Additionally, they indicate a regulatory role for the host in determining the balance between bacterial serine proteases and host serpins exported in bacterial and host extracellular vesicles.