High-fat diet-induced MAFLD disrupts sleep architecture through hippocampal neuroinflammation and bifurcated IRS-1/Akt/mTOR signaling in mice

Technical Summary: High-fat diet-induced MAFLD disrupts sleep architecture through hippocampal neuroinflammation and bifurcated IRS-1/Akt/mTOR signaling in mice

Problem Statement
Metabolic dysfunction-associated fatty liver disease (MAFLD) is epidemiologically linked to sleep disturbances, including reduced sleep duration, poor quality, and fragmentation. While the bidirectional relationship between hepatic steatosis and sleep disruption is established, the central neural pathways and molecular mechanisms through which a fatty liver communicates with sleep-regulatory circuits remain unexplored. Specifically, the role of the hippocampus—a brain region critical for both memory and sleep-wake regulation—and its susceptibility to metabolic and inflammatory insults in the context of MAFLD-induced sleep disruption has not been characterized.

Methodology
The study utilized a murine model to investigate the causal link between diet-induced MAFLD and sleep architecture.

• Animal Model: Male BALB/c mice (n=6 per group) were fed either a control diet (CD) or a 60% high-fat diet (HFD) for 16 weeks to induce MAFLD.
• Sleep Recording: Following a 14-day recovery period post-surgery, continuous 24-hour video-EEG/EMG recordings were performed. Sleep-wake states (Wakefulness, NREM, REM) were scored manually in 10-second epochs.
• Spectral and Circadian Analysis: NREM sleep delta power (1–4 Hz) was analyzed using Fast Fourier Transform (FFT). Circadian rhythms of NREM delta power were assessed using Cosinor analysis to determine mesor, amplitude, and acrophase.
• Molecular and Histological Assessment:
  • Systemic/Peripheral: Serum levels of liver enzymes (ALT, AST), lipids (TG, TC), and pro-inflammatory cytokines (TNF-α, IL-1β) were measured. Liver histology (H&E and Oil Red O staining) confirmed hepatic steatosis.
  • Central (Hippocampus): Hippocampal tissue was analyzed for neuronal morphology (Nissl staining), local cytokine levels (TNF-α, IL-1β via ELISA), and the phosphorylation status of the insulin signaling axis.
  • Signaling Pathway: Western blot analysis quantified the expression and phosphorylation of IRS-1 (Ser307), Akt (Ser473), and mTOR (Ser2448).

Key Results

1. Induction of MAFLD: HFD feeding successfully induced hepatic steatosis, elevated liver enzymes (ALT, AST), and a systemic pro-inflammatory state characterized by increased serum TNF-α and IL-1β.
2. Sleep Architecture Disruption: HFD-fed mice exhibited profound sleep disturbances compared to controls:
   • Reduced Duration: Significant decreases in total 24-hour NREM and REM sleep duration, primarily driven by deficits during the light (rest) phase.
   • Severe Fragmentation: A marked increase in the number of sleep bouts and a drastic reduction in mean bout duration for both NREM and REM sleep.
   • Attenuated Slow-Wave Activity: NREM sleep delta power (1–4 Hz) was significantly suppressed across the frequency band.
   • Circadian Dampening: Cosinor analysis revealed a blunted circadian rhythm of NREM delta power, characterized by a reduced mesor, diminished amplitude, and a phase advance.
3. Hippocampal Pathology: The HFD group displayed distinct hippocampal injury, including neuronal disorganization and cytoplasmic shrinkage in the CA1 region, alongside elevated local levels of TNF-α and IL-1β.
4. Bifurcated Signaling Dysregulation: Western blot analysis revealed a specific, paradoxical signaling profile in the hippocampus of HFD mice:
   • Insulin Resistance: Increased inhibitory phosphorylation of IRS-1 at Ser307 and decreased activation of Akt (reduced p-Akt Ser473).
   • Paradoxical mTOR Activation: Despite the suppression of Akt, mTOR phosphorylation at Ser2448 was significantly elevated.

Key Contributions

• Establishment of a Liver–Hippocampus–Sleep Axis: This study provides the first functional evidence linking diet-induced hepatic steatosis to disrupted sleep architecture via hippocampal pathology.
• Identification of a Specific Molecular Mechanism: The research delineates a "bifurcated" dysregulation of the IRS-1/Akt/mTOR pathway in the hippocampus, where insulin resistance (p-IRS-1 up, p-Akt down) coexists with mTOR hyperactivation (p-mTOR up).
• Characterization of Sleep Phenotype: The study details the specific nature of sleep disruption in MAFLD, highlighting not just reduced sleep quantity but also severe fragmentation and the loss of restorative slow-wave activity.

Significance and Claims
The authors claim that their findings establish a novel mechanistic framework explaining the epidemiological association between metabolic liver disease and sleep disorders. They posit that MAFLD-driven systemic inflammation compromises blood-brain barrier integrity, triggering hippocampal neuroinflammation and insulin resistance. This central pathology, specifically the dysregulation of the IRS-1/Akt/mTOR axis, disrupts the oscillatory networks required for normal sleep architecture.

The study concludes that targeting this liver–hippocampus–sleep axis, particularly the dysregulated signaling nodes, may offer new therapeutic strategies for treating sleep disorders in patients with MAFLD. The authors explicitly state that this is the first study to connect diet-induced hepatic steatosis to sleep disruption via hippocampal insulin resistance and neuroinflammation, addressing a critical gap in understanding how metabolic liver disease alters brain function.

Limitations Noted by Authors
The paper acknowledges several limitations, including the use of a modest sample size of only male mice, the correlative nature of the design (lacking region-specific genetic or pharmacological manipulation to prove causality), and the inability to perform within-subject pre-diet baseline sleep recordings due to technical constraints of long-term electrode implantation. Additionally, the specific upstream activator of the paradoxical mTOR hyperphosphorylation remains to be identified.