Beyond TLR4 and Its Alternative Lipopolysaccharide (LPS) Sensing Pathways in Zebrafish

Due to their evolutionary divergence from mammals, zebrafish (Zf, Danio rerio), which are frequently employed in biomedical research, provide a distinctive viewpoint on innate immune systems. The Toll-like receptor 4/myeloid differentiation factor 2/cluster of differentiation 14 (TLR4/MD-2/CD14) complex in mammals detects lipopolysaccharide (LPS), a crucial component of Gram-negative bacteria, and it causes potent inflammatory reactions through a Toll/interleukin-1 receptor domain-containing adapter-inducing interferon-β (TRIF)-dependent and myeloid differentiation primary response 88 (MyD88)-dependent pathways. However, key components of this system, such as a responsive TLR4 axis and a functional CD14 ortholog, are absent in Zf. The Zf species nevertheless reacts to LPS, which leads to research into other recognition systems. This review looks at a number of TLR4-independent processes in Zf, such as scavenger receptors (SRs) including scavenger receptor class B type 1 (SR-BI) and cluster of differentiation 36 (CD36), nucleotide-binding oligomerization domain-containing protein 1 (NOD1)-dependent cytosolic sensing, peptidoglycan recognition proteins (PGRPs), Complement Component 3 (C3), and caspase-1-like protein 2 (Caspy2)-mediated inflammasome activation. An alternative and flexible immune system that makes up for the lack of canonical TLR4 signaling is revealed by these mechanisms. Additionally, the discovery of lymphocyte antigen 96 (ly96), an ortholog of MD-2 found in Zf, suggests evolutionary similarity; however, as it is only functional in artificial systems, it demonstrates minimal overlap with mammalian MD-2 activity. Knowing these pathways provides important information for studying inflammation, infection, and immunological modulation in vertebrates using Zf as a model. It also clarifies the evolutionary flexibility of innate immune recognition.