Shawn Xu, PhD
Bernard W. Agranoff Collegiate Professor of the Life Sciences
Department of Molecular & Integrative Physiology
University of Michigan
(March 20, 2018)
Eyeless But Not Blind: Photosensation In C. Elegans
Mammals make use of their senses (sight, hearing, taste, touch, and smell, as well as proprioception) to interact and navigate with their environment. Sensory input, such as sound, is transduced (or changed) into electrical signals. The worm C. elegans is a useful model system for studies on the molecular basis of how sensory inputs are transduced. Doctor Xu discussed his work examining how worms sense their world. His work has demonstrated that worms can sense and avoid light, mediated by a particular photoreceptor protein, LITE-1.
The Xu lab is a sensory neuroscience lab studying how animals interact and communicate with both the external and internal environments through the sensory systems, which are essential for survival and the quality of life. The questions they study in the lab include: i) Sensory transduction: how do sensory neurons/cells detect and distinguish different sensory cues and then transduce them into electric outputs? ii) Sensory processing: how do neural circuits process sensory information from sensory neurons to turn it into a behavioral output? iii) In addition to behavior, sensory stimuli also modulate other physiological processes such as aging and longevity, so they are also interested in characterizing how the sensory environment modulates aging. In his lecture, he covered only work on sensory transduction.
There are five common senses in mammals: vision, hearing, smell, taste, and touch or somatosensation that includes temperature sensation, and a sixth sense: propriopception, which is important for controlling body posture and balance during movement. Work from many labs have demonstrated that worms sense smell, tastants, touch, and temperature. The Xu lab has discovered a new sense in worms, proprioception, and found it requires a TRP channel. Instead of giving an overview of work on all these different kinds of sensory modalities, he focused on the sensing of light.
C. elegans were thought be insensitive to light because they live in the dark soil. Interestingly, they found that worms are light sensitive as they avoid light, and engage in phototaxis behavior. This photoavoidance behavior is important for their survival and also provide a potential mechanism for keeping them in the dark. Worms are most sensitive to UV light, and they estimate that the amount of UV in the sunlight would be sufficient to drive this behavior. Through a combination of behavioral, genetic, functional imaging and electrophysiological studies, they have made a number of discoveries, which are summarized as follows: 1) worms sense light through a subset of ciliary photoreceptor neurons and engage in phototaxis behavior mediated by LITE-1. 2) LITE-1 is a new type of photoreceptor protein with an exceptionally high photon-capturing ability. 3) LITE-1 may not have a prosthetic chromophore and requires two tryptophan residues for photosensitivity. 4) Doctor Xu proposed that the two tryptophan residues participate the formation of the chromophore for LITE-1.