SypC, a symbiont outer membrane vesicle protein, impacts the development of the squid–vibrio partnership

Bacterial outer membrane vesicles (OMVs) and the cargo they carry are increasingly recognized as a means of communication between microbial symbionts and the cells of their host. However, few studies have focused on the biochemical and molecular mechanisms underlying OMV signaling during symbiosis onset and development. We show here that SypC, an OMV protein of the bioluminescent symbiont Vibrio fischeri, is taken up by cells of the squid host Euprymna scolopes where it assumes a new function, i.e., the facilitation of symbiont-induced light-organ morphogenesis. SypC is a Wza-like outer membrane protein found in host-associated Vibrionaceae and is essential for V. fischeri biofilm formation. Colonization or direct treatment with V. fischeri OMVs triggers host development, which was reduced or delayed if the host is instead exposed to a ∆sypC mutant or ∆sypC OMVs. RNA-seq analyses comparing light organs colonized by either the mutant or its parent revealed differential expression of host genes associated with immune responses and tissue morphogenesis. In immunocytochemical imaging, SypC-bearing OMVs were taken up by the host's macrophage-like cells near the light-organ crypts, revealing the mechanism by which SypC travels through tissue to trigger morphogenesis. Taken together, the data provide evidence that in addition to its role in biofilm formation and colonization, SypC has a second function promoting the induction of symbiotic-tissue development. These findings provide a critical piece of a puzzle whereby a rich array of host and symbiont molecules work in concert to orchestrate normal symbiont colonization and host development within the first hours to days of symbiosis.