Borve, A. | bioRxiv Science

bioRxiv2022

The Toll and Imd pathway, the complement system and lectins during immune response of the nemertean Lineus ruber

Innate immunity is the first line of defense against pathogens. In animals, the Toll pathway, the Imd pathway, the complement system, and lectins are well-known mechanisms involved in innate immunity. Although these pathways and systems are well understood in vertebrates and arthropods, they are understudied in other invertebrates. In order to shed light on immunity in the nemertean Lineus ruber, we performed a transcriptomic survey and identified the main components of the Toll pathway (e.g. myD88, dorsal/dif/NF{kappa}B-p65), the Imd pathway (e.g. imd, relish/NF{kappa}B-p105/100), the complement system (e.g. C3, cfb) and some lectins (FreD-Cs and C-lectins). In situ hybridization showed that TLR{beta}1, TLR{beta}2 and imd and are expressed in the nervous system, the complement gene C3-1 is expressed in the gut and the lectins in the nervous system, the blood, and the gut. To reveal their potential role in defense mechanisms, we performed immune challenge experiments, in which Lineus ruber specimens were exposed to the gram-negative bacteria Vibrio diazotrophicus. Our results show the upregulation of specific components of the Toll pathway (TLR3, TLR{beta}1, and TLR{beta}2), the complement system (C3-1), and lectins (c-lectin2 and fred-c5). Therefore, similarly to what occurs in other invertebrates, our study shows that components of the Toll pathway, the complement system and lectins are involved in the immune response in the nemertean Lineus ruber. The presence of these pathways and systems in Lineus ruber, but also in other spiralians, in protostomes and in deuterostomes suggest that these pathways and systems were involved in the immune response in the stem species of Bilateria.

immunology↗

bioRxiv2022

Peripheral and central employment of acid-sensing ion channels during early bilaterian evolution

Nervous systems are endowed with rapid chemosensation and intercellular signaling by ligand-gated ion channels (LGICs). While a complex, bilaterally symmetrical nervous system is a major innovation of bilaterian animals, the employment of specific LGICs during early bilaterian evolution is poorly understood. We therefore questioned bilaterian animals employment of acid-sensing ion channels (ASICs), LGICs that mediate fast excitatory responses to decreases in extracellular pH in vertebrate neurons. Our phylogenetic analysis identified an earlier emergence of ASICs from the overarching DEG/ENaC superfamily than previously thought and suggests that ASICs were a bilaterian innovation. Our broad examination of ASIC gene expression and biophysical function in each major bilaterian lineage of Xenacoelomorpha, Protostomia, and Deuterostomia, suggests that the earliest bilaterian ASICs were probably expressed in the periphery, before being incorporated into the brain as it emerged independently in certain deuterostomes and xenacoelomorphs. The loss of certain peripheral cells from Ecdysozoa when they split from other protostomes likely explains their loss of ASICs, and thus the absence of ASICs in model organisms Drosophila and C. elegans. Thus, our use of diverse bilaterians in the investigation of LGIC expression and function offers a unique hypothesis on the employment of LGICs in early bilaterian evolution.

evolutionary biology↗