The Role of the Circadian Clock as a Switch for Reactive Astrocyte Status in Neurodegenerative Diseases
Student thesis: Doctoral Thesis
Related Research Unit(s)
Neurodegenerative diseases (ND) in the central nervous system, such as Alzheimer’s and Parkinson’s disease, are accompanied by reactive astrogliosis. However, whether reactive astrocytes are neurotoxic, neuroprotective, or both, and how their functions are controlled remains to be elucidated. Recent studies have reported the roles of circadian clocks in astrocytes. The circadian clock, which is intrinsic in most cells, is an endogenous oscillator generating daily biological rhythms from the molecular to the behavioral level through the transcriptional regulation of thousands of clock genes. To study whether circadian clocks control reactive astrocytes status and functions, circadian clock gene expression was examined in reactive astrocytes cultured with lipopolysaccharide-activated microglia, mimicking neuroinflammation presented in all ND. The circadian clock gene expression was reduced in reactive astrocytes compared to non-reactive control, indicating the presence of the altered circadian clock. This reduction is due to the diminished DNA binding of the circadian transcription factor BMAL1 to the promoter of the clock genes. This circadian clock change is induced by activated microglia, partially via secreted cytokines. We next studied the functional changes in astrocytes from mice with the circadian clock disrupted globally, Bmal1-/-, or specifically in astrocytes, Gfap-cre;Bmal1f/f. Disruption of the circadian clock specifically in astrocytes causes reactive astrogliosis. RNA sequencing analysis using astrocytes from Bmal1-/- and wild-type mice identified cell migration and immune responses as potential clock-controlled functions. This result is supported by the result of RNA sequencing analysis using reactive glia of the neuroinflammatory model. To verify cell migration is regulated by the circadian clock, stereotaxic injections of reactive astrocytes and Bmal1-/- astrocytes to the brain were performed and they both have enhanced migration compared to control. To show the circadian regulation of migration is not limited to astrocytes in neuroinflammation, expression of the circadian clock genes in astrocytes treated with amyloid-β, which aggregates in Alzheimer’s disease, was measured. Amyloid-β activates astrocytes and downregulates Bmal1, indicating the alteration of the circadian clock. RNA sequencing analysis using the same samples identified migration-related pathways changed. The result suggested that migration is regulated by the circadian clock in neuroinflammation and Alzheimer’s disease. Our findings provide insights into the function of the altered circadian clocks in reactive astrocytes in ND.