Date of Award
Doctor of Philosophy (PhD)
The circadian clock regulates gene transcription to control daily rhythms in cellular function, behavior, and disease. Circadian dysfunction is a symptom of aging and neurodegenerative diseases, and recent studies suggest reciprocal regulation between impaired clock function and neurodegeneration. In addition to the clock regulating neuronal activity, non-neuronal glia cells of the brain also possess functional circadian clocks which control their responses to daily oscillations in brain activity, cellular stress, and metabolism. Astrocytes directly support brain function through synaptic interactions, neuronal metabolic support, neuroinflammatory regulation, and control of neurovascular coupling at blood and cerebrospinal fluid (CSF) barriers. Emerging evidence suggests that the astrocyte circadian clock may be involved in many of these processes, and that clock disruption could influence neurodegeneration by disrupting several aspects of astrocyte function.The circadian clock is coordinated by positive and negative mediators that generate transcriptional-translational feedback loops and rhythmic gene expression across time-of-day. At the center of these feedback loops is BMAL1, the only circadian clock gene for which deletion abrogates cellular and behavioral rhythms. Depletion of Bmal1 in astrocytes leads to their cell-type specific activation. Activated glia influence the progression of chronic neurodegenerative conditions such as Alzheimer's disease (AD) through functions that alter the course of pathogenesis and neuroinflammation. Activated glia may be particularly important for limiting or exacerbating the accumulation of toxic protein aggregates common to many neurodegenerative diseases, but studies of the impact of astrocyte activation on protein aggregates have produced conflicting results. Thus, we first investigated how the circadian clock functions in astrocytes to regulate amyloid beta (Aβ) deposition and degradation in models of AD. To address these questions, we generated mice with astrocyte-specific knockout of the master circadian clock gene Bmal1 (BMAL1 aKO mice), which renders astrocytes transcriptionally arrhythmic. We observed that astrocytes not only enter an activated state exhibited by transcriptional markers, but they also differentially express disease-modifying genes. We crossed BMAL1 aKO mice to the APP/PS1 and the APP-NL-G-F/wt models of Aβ accumulation. Similar to the wildtype BMAL1 aKO mice, astrocyte-specific Bmal1 deficiency in both the APP/PS1 and APP-NL-G-F/wt models also strongly increases activation of astrocytes around Aβ plaques but does not affect plaque accumulation or neuronal dystrophy. These results show that while BMAL1 regulates astrocyte activation and transcription, this does not alter Aβ plaque accumulation. More generally, our results demonstrate that the effect of astrocyte activation on Aβ plaque accumulation is likely dependent on the specific transcriptional activation state of the astrocyte. We then investigated how deletion of Bmal1 influences endolysosome function, autophagy, and degradation dynamics of extracellular proteins. In vitro, Bmal1-deficient astrocytes exhibit more endocytosis and degradation of extracellular proteins, lysosome-dependent protein cleavage, and accumulation of LAMP1, RAB7, and acidified autophagosomes. In vivo, BMAL1 aKO brains show accumulation of autophagosome-like structures by electron microscopy. RNA sequencing of isolated astrocytes from young and aged BMAL1 aKO mice indicates broad dysregulation of endolysosome pathways involved in receptor-mediated endocytosis and lysosome function. Since a clear link has been established between neurodegeneration and endolysosome dysfunction over the course of aging, this work provides insight into how the circadian clock may be a master regulator of these crucial astrocyte functions in health and disease. Understanding how the clock regulates astrocyte autophagy and endolysosome function may give insights into methods by which to control not only protein aggregation in neurodegenerative conditions, but likely other disease processes as well.
Chair and Committee
Erik S. Musiek
John Cirrito, Abhinav Diwan, Jin-Moo Lee, Marco Colonna,
Mckee, Celia Augusta, "Circadian Protein BMAL1 Regulates Astrocyte Activation and Protein Degradation in Health and Disease" (2023). Arts & Sciences Electronic Theses and Dissertations. 2890.