Microglial Clock Dysfunction in Neuroinflammation Compromises Glial Interaction and Disrupts Neuroimmune Homeostasis

Abstract

Circadian clocks are endogenous oscillators built on a transcriptional-translational negative feedback loop and are intrinsic to nearly all cell types, including microglia. They generate daily physiological rhythms and regulate immune processes, including cytokine production and immune response severity in macrophages. Microglia are phagocytes in the central nervous system (CNS) and are critical in maintaining immune responses and homeostasis. Although accumulated studies have reported time-of-day-dependent variations in microglial immune responses, how intrinsic microglial clocks respond to inflammatory stimuli and influence microglial function remains unclear.

To address these, we first investigated whether microglial physiological activity was governed by their circadian clocks. Activation markers and morphological analysis across multiple time points revealed circadian oscillations in microglial activity within healthy brains. Therefore, we next examined whether this rhythmic microglial activation was disturbed under inflammation conditions. Lipopolysaccharide (LPS)-induced neuroinflammation caused phase-shifting of the circadian transcription factor Bmal1, with dampened or abolished circadian rhythms significance in its targets including circadian gene Per1, microglial activation marker Iba1 and abolished in inflammatory cytokine Ccl5, leading to sustained immune activation with disrupted rhythmicity. Nevertheless, in BMAL1 deficient microglia, cytokines expression levels were stable, which led to a new question: if the pro-inflammatory functions were not involved, what the non-immune related functions of microglial clocks were.

To directly address the physiological role of circadian clocks in microglia, we conducted microglial transplantation into the corpus callosum (CC). Transplanting wild-type or BMAL1 knockout (BKO) microglia into CC revealed that circadian clocks are essential for the effective recruitment of oligodendrocyte progenitor cells (OPCs), similar with neuroinflammatory activated microglia with disrupted clocks. On the other hand, as another critical subset of glia in the CNS, astrocytes density was also promoted by transplanted microglia, while this effect blunted when transplanted with BKO or neuroinflammatory activated microglia. The similar effects between astrocytes and OPCs dynamics identified that both genetic clock ablation and neuroinflammation activation-induced clock disruption significantly attenuated the ability of microglia with regulating OPC and astrocyte dynamics, converge on impairing the microglial functions in glial cell interactions.

Our findings establish the role of microglial circadian clocks in regulating non-immune related functions in microglia, including intercellular communications, and these effects are impaired after neuroinflammatory activations in pathological conditions. Preserving circadian function in microglia offer a potential therapeutic avenue to mitigate neuroinflammation and support brain homeostasis.

Date of Award	14 Oct 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jin Young KIM (Supervisor)