Regulation of Circadian Rhythms in and by Glia

Washington University, Saint Louis MO

Investigators

Abstract

The discoveries of specific molecules, cells, and brain areas that mediate daily behaviors have made circadian biology one of the fastest growing and most integrative fields in neuroscience. In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus regulate rhythms in physiology and behavior. Recently, our lab and others have also found other circadian oscillators in the brain and body. We are now poised to identify the cells capable of circadian pacemaking, the molecular mechanisms underlying their rhythmicity, and the signaling pathways that synchronize them to coordinate behavior. This proposal focuses on glia as a system to test the role of specific genes and molecules in circadian rhythm generation and communication. Although glia far outnumber the approximately 10,500 neurons in the SCN, they are seriously understudied. Recent studies on glia have challenged traditional neurobiology. Glia communicate with one another and with neurons through conventional and novel chemical signals. Glia can regulate synapse formation, control synaptic strength and, under specific conditions, differentiate into neurons. Conversely, neural impulse activity regulates a wide range of glial activities, including their proliferation and differentiation. It is not known whether glia are circadian pacemakers, precursor pacemaker cells, targets for pacemakers, or support cells to pacemakers. These features emphasize the importance of studying neural-glial interactions in the circadian system. Glia will be studied as model circadian oscillators, first as targets for SCN signaling and then as potential modulators of circadian timing. Transgenic mice expressing firefly luciferase under the control of either the Period1 or Period2 promoter will allow real-time observations of core clock mechanisms of different cell types. Co-cultures of SCN explants with pure glia will test when and which SCN cells secrete timing signals that are sufficient to synchronize rhythms in target cells. Replacing the SCN with other brain regions in these co-cultures will test whether the SCN are uniquely capable of imposing periodicity on other cells. Pharmacological blockade of candidate transmitter pathways will test their necessity for this neural-to-glia coordination. The capacity of glia to modulate neural function will be examined in three ways. Recording bioluminescence from transgenic SCN explants co-cultured with glia of different circadian phenotypes will test whether glia secrete factors that influence SCN periodicity. Long-term, multisite electrical recording from SCN neurons will specifically assess the role of glia in neural rhythmicity. Finally, locomotor recording from SCN-lesioned and -chimeric animals will test the possibility that transplanted glia regulate behavioral rhythmicity. The proposed research will reveal mechanisms that may coordinate activity across the brain. By examining circadian timing in glia and the modulation of circadian timing by glia, this work makes testable predictions about neuron-glia interactions. The findings will be relevant to daily physiology and intercellular signaling. The impact of this work will be broadened with the generation and distribution of transgenic, circadian mutant mice and the recruitment of new scientists, including those traditionally underrepresented. Students will become involved in the research through hands-on research internships, outreach programs, two undergraduate courses and two graduate courses.