A recent study conducted by researchers at Kanazawa University has shed light on the functioning of Transient receptor potential vanilloid member 1 (TRPV1), which is a protein responsible for sensing heat and chili molecules. The team employed high-speed atomic force microscopy to investigate the structural variations of the protein when bound to ligands that either stimulate or suppress its activation.
The TRPV1 protein plays a crucial role in the skin's ability to detect heat, whether it originates from elevated temperatures or compounds like capsaicin found in chili peppers. However, the underlying mechanisms governing TRPV1's function have remained elusive. This study, led by Ayumi Sumino from Kanazawa University and Motoyuki Hattori from Fudan University, offers significant insights into this mechanism.
By utilizing high-speed atomic force microscopy, the researchers compared the TRPV1 protein in its unbound state to its state when bound to stimulating or suppressing ligands, known as agonists and antagonists, respectively. This allowed them to obtain “the first experimental evidence showing the correlation between molecular fluctuation and the gating state (ligand binding).”
Once activated, the TRPV1 channel opens, enabling the passage of ions and signaling the nervous system about the presence of a harmful stimulus. In 2011, researchers at the Howard Hughes Medical Institute proposed a theoretical framework for receptor activation based on thermodynamics, which has since been supported by experimental data.
According to the theoretical basis, the protein responds to heat by undergoing changes in heat capacity, which are associated with fluctuations in its conformation. While previous cryo electron microscopy studies provided structural information about the TRPV1 protein, they did not clarify how conformational fluctuations relate to stimulating or suppressing ligands. Moreover, it remained uncertain whether temperature and chili sensing shared the same molecular mechanism.
Atomic force microscopy (AFM) is a technique that detects surface topologies by measuring the forces acting on a nanoscale tip positioned directly above the surface. Although the microscope was initially invented in 1986, recent advancements at Kanazawa University have facilitated high-speed imaging, offering insights into structural dynamics.
Sumino, Hattori, and their team employed high-speed AFM to visualize the TRPV1 receptor in both its unbound state and when bound to agonist or antagonist ligands. They used resiniferatoxin as the agonist, which is 1,000 times hotter than capsaicin, and capsazepine as the antagonist, which blocks capsaicin-induced pain.
Through their observations of the captured structures, the researchers noted fluctuations in the conformation of both the bound and unbound states of TRPV1. They discovered that resiniferatoxin increased conformational fluctuations, whereas capsazepine suppressed them.
Although the conformational fluctuations observed were minute, measuring at approximately an Angstrom, the researchers highlighted existing evidence in scientific literature suggesting that such small changes can significantly impact the ion permeability of a channel. In their report, the researchers concluded, “Overall, this study suggests the importance of structural fluctuation, which would be a key factor for the heat-sensing of TRPV1.”
Source: Kanazawa University