Research　Project

(1) Somatic sensation enables animal body to feel, to ache, to chill, and to know what its part are doing. It is sensitive to many kinds of stimuli. The sensing transduction occurs in nerve endings of sensory neurons. Various noxious stimuli are sensed by membrane proteins mainly including ion channels belonging to the transient receptor potential (TRP) channel family. A main member of the rodent TRP family in sensory neurons, TRPA1, functions as a sensor for cold temperature and also for chemicals such as allyl isothiocyanate, caffeine, oxidized epigallocatechin gallate, acrolein present in wasabi, coffee, green tea, and vehicle exhaust. In recent years, the species-specific differences of TRPA1 functions, however, have been reported. The human and rhesus monkey TRPA1s are not activated by cold and TRPA1s from several tetrapod vertebrates are activated by high temperature. In addition, it has been shown that rodent and human TRPA1s respond differently to chemical ligands. These observations suggested the possibility that nociceptive receptors such as TRPA1 are likely to play important roles in the survival strategy during the course of evolution and environmental adaptation of taxa.

Therefore, to understand the functional evolution of TRPA1 in vertebrates, we first focused on TRPA1s from several fish species. We investigated the functional properties of zebrafish TRPA1 paralogs (zTRPA1a and b) and Takifugu TRPA1. We found that zTRPA1b and Takifugu TRPA1 show statistically significant cold and heat-induced responses. This was the first demonstrating the function of TRPA1 as a dual thermal sensor. Further, we observed that fish TRPA1s were gradually heat-activated over about 25 °C with no clear threshold. It has been reported that TRPA1s from several tetrapod vertebrates are activated by high temperature with a clear temperature threshold. Because water has a high heat capacity, water temperature responds less to the addition and extraction of heat energy and is relatively stable compared with temperature on land.  Therefore, the gradual heat activation must be one of the specific features of TRPA1 of vertebrates living in water environment. 

To further approach the functional evolution of nociceptor, TRPA1 during the land advance of animals, our research project is to characterize the function of TRPA1s from other teleost fishes, lobe-finned fishes which are evolutionarily closer to tetrapod vertebrates, and tailed amphibians.

(2) Shiga prefecture has several characteristic land features. Lake Biwa, Japan’s largest lake, is located in the center of this prefecture and divides the prefecture into four different plain areas.  This prefecture is surrounded by mountain chains. These geographical features may have influenced the distribution and local diversity of animals. We focused on aquatic animals, especially, salamanders, frogs, and the rare fish in Shiga prefecture, and investigate their distribution situation and the molecular phylogeny based on DNA analysis.