The mechanism of extra-visual circadian photoentrainment in mammals

Project Summary. Direct exposure to sunlight subjects cells to potentially damaging
solar radiation. For superficial tissues such as the cornea, retina, and skin this is
unavoidable. Biological circadian rhythms have evolved as a way to anticipate sunlight:
by synchronizing cellular and behavioral rhythms to the 24-hour solar cycle, or
“photoentrainment”. Most tissues in mammals contain an autonomous molecular
circadian clock, and many of these are synchronized by the brain's central clock, the
suprachiasmatic nucleus (SCN). However, we have found that some tissues located
near the body's surface, such as skin and retina, have an ability to directly synchronize
their local circadian clocks to short-wavelength light using the opsin, Opn5. In the case
of retina and skin clocks, rhythmic environmental light dominates systemic circadian
synchronization cues. The central hypothesis of this work is that opsins in cells typically
exposed to sunlight respond to short-wavelength light, causing the release of diffusible
signals which synchronize adjacent circadian clocks. This allows tissues to maintain a
phase relationship with environmental light regardless of the phase of the animal's
behavior. The specific aims of this proposal are to identify the intracellular molecular
mechanism Opn5 uses to signal the presence of light using skin and retina as model
circadian systems (Aim 1), identify the nature of diffusible signaling by which Opn5-cells
influence adjacent cells and tissues (Aim 2), and determine the physiological
significance of Opn5-mediated circadian clock synchronization in vivo (Aim 3). Our
approach will be to focus on skin as a model circadian tissue due to its physiologic
significance, its natural exposure to light, and the relative ease of distinguishing Opn5-
specific photoactivation. However, studies will be repeated in retina for analyses on the
similarity of signaling mechanisms in another photoentrainable tissue. The studies will
employ a mixture of in vivo, ex vivo (organotypic tissue explants), and cell culture
methods. The innovation of this project lies in the measurement of direct responses of
clocks within mammalian tissues to environmental light and the mechanisms opsins
employ outside of the visual system. The improvements of this proposal over past work
are the 1) assessment of Opn5 signaling cascades for which there is still a gap in
published literature, 2) assessment of tissue-level signaling dynamics and 3) a focus on
the physiologic impacts of lighting environment on tissue health.

Grant Number: 5R01GM152641-02
NIH Institute/Center: NIH
Principal Investigator: Ethan Buhr