Toll-like receptor 4 pathway evolutionary trajectory and functional emergence.

Area of Science:

• Molecular Biology
• Immunology
• Evolutionary Biology

Background:

• Toll-like receptors 4 (TLR4) recognize bacterial lipopolysaccharides (LPS), initiating cytokine production via TIRAP-MyD88 or TRAM-TRIF signaling pathways.
• The TRAM-TRIF pathway is largely specific to TLR4, while the TIRAP-MyD88 pathway is common to other TLRs.
• Previous research identified Callorhincus milli as an ancestral organism possessing both TRAM and TRIF proteins.

Purpose of the Study:

• To investigate the evolutionary conservation and functional significance of TRAM and TRIF TIR domains in TLR4 signaling.
• To compare the TRAM-TRIF complex structure and energetics between the ancestral Callorhincus milli and Homo sapiens.

Main Methods:

• Molecular modeling of TRAM-TRIF complexes from representative organisms across taxa.
• Calculation of interaction surfaces, energetics, and electrostatic potential.
• Normal Mode Analysis (NMA) and molecular dynamics simulations to identify functionally important residues and network parameters.

Main Results:

• Tetrameric TRAM-TRIF complexes (TRAM dimer and TRIF dimer) demonstrated greater stability than trimeric models.
• Critical residues for TIRAP, TRIF, and MyD88 signaling were conserved.
• Key residues essential for TRAM signaling were found to be non-conserved in Callorhincus milli.

Conclusions:

• The findings suggest the presence of functional TIRAP-MyD88-mediated TLR4 and TRIF-mediated TLR3 signaling in the ancestral Callorhincus milli.
• The evolution of TLR4 signaling appears to involve the gradual acquisition of function through the orchestration of multiple motifs over evolutionary time.