Hyperglycemia Enhances Brain Susceptibility to Lipopolysaccharide-Induced Neuroinflammation via Astrocyte Reprogramming

Academic Background Introduction

Diabetes is a chronic metabolic disease affecting millions of people worldwide, characterized by hyperglycemia, or elevated blood glucose levels. For a long time, research has primarily focused on the adverse effects of hyperglycemia on peripheral organs. However, in recent years, scientists have begun to realize that hyperglycemia also has profound effects on the central nervous system (CNS). Hyperglycemia plays an important role in the pathologies of brain inflammation and neurodegenerative diseases. Diabetic patients are more prone to cognitive dysfunction and more severe brain damage compared to healthy individuals. However, the specific impact of hyperglycemia on brain-resident cells, particularly astrocytes, remains poorly understood. This study aims to explore the role of hyperglycemia in driving brain inflammation.

Introduction to the Paper

The paper titled “Hyperglycemia enhances brain susceptibility to lipopolysaccharide-induced neuroinflammation via astrocyte reprogramming” was published in the journal “Journal of Neuroinflammation.” The authors include Kyung-Seo Lee, Sung-Hyun Yoon, Inhwa Hwang, Jeong-Hwa Ma, Euimo Yang, Rebekah Hyeyoon Kim, Eosu Kim, and Je-Wook Yu (corresponding author). The authors are from various research departments at the Yonsei University College of Medicine in Seoul, South Korea. The paper was published in 2024 and was supported by the National Research Foundation funded by the Korean government.

Introduction to Research Details

Research Process

The study first induced hyperglycemia in mice using streptozotocin (STZ) and then performed relevant tests and analyses.

• Experimental Mice: C57BL/6 mice, Aldh1l1CreERT2 mice, Rosa26-tdTomatofl/fl mice, and Tmem119-EGFP mice.
• Hyperglycemia Induction: Eight-week-old male C57BL/6 mice were administered STZ (150mg/kg). Five days later, fasting blood glucose was measured via tail vein blood sample. Mice with blood glucose levels exceeding 250 mg/dL were selected for this study.
• Peripheral Inflammation Induction: Mice were intraperitoneally injected with lipopolysaccharide (LPS, 0.5mg/kg).

The research process included: 1. Flow Cytometry Analysis: Detection of changes in brain cell populations. - Mice were placed under hyperglycemic and control conditions. - Analysis of the number of immune cells (CD45+) and non-immune cells (CD45-), detecting changes in microglia (CD45medCD11bmed) and infiltrated myeloid cells (CD45hiCD11bhi). - Increase in astrocytes detected using GFAP and Acsa2 antibodies.

1. Immunohistochemistry Analysis: Staining astrocytes and microglia using GFAP and Iba1 antibodies respectively.

2. RNA Sequencing Analysis: Isolation and analysis of transcriptome changes in astrocytes.

   • Separation of tdTomato-labeled astrocytes via cell sorter.
   • Differential gene expression analysis and Gene Ontology (GO) enrichment analysis.

3. Behavioral Testing:

   • Novel Object Recognition (NOR) task and Y-maze task to assess cognitive function.
   • Behavioral tests conducted post-LPS injection in mice, calculating exploration time and spontaneous alternation.

4. Blood-Brain Barrier (BBB) Permeability Testing: Evaluation of BBB integrity using Evans blue dye leakage.

Main Results

1. Astrocyte Proliferation: The study found a significant increase in the number of astrocytes in the brains of STZ-induced hyperglycemic mice, with little impact on microglia or infiltrated myeloid cells. Findings indicate hyperglycemia induces astrocyte proliferation.

2. Astrocyte Reprogramming:

   • Under hyperglycemic conditions, astrocytes showed significant transcriptional changes, particularly genes related to cell proliferation and inflammatory responses.
   • Hyperglycemia-induced astrocytes exhibited specific A1 reactive gene upregulation.

3. Enhanced Peripheral LPS-Induced Neuroinflammation:

   • Hyperglycemic states enhanced LPS-induced myeloid cell infiltration into the brain.
   • Increased BBB permeability, with reduced astrocytic expression of Aquaporin 4 (AQP4) further confirming BBB damage.
   • Evaluation of brain inflammation states indicated significantly increased expression of cytokines (such as TNF-α) induced by LPS under hyperglycemic conditions.

4. No Significant Impact of Hyperglycemia on Peripheral Inflammation:

   • In peripheral organs, myeloid cells and their reactivity did not exhibit significant changes under hyperglycemic conditions.
   • Under high glucose conditions, peripheral macrophages showed no significant changes in IL-6 production in response to LPS or poly(I:C).

5. Glucose-Driven Reprogramming of Astrocytes:

   • In high glucose environments, astrocytic expression of glycolytic enzymes and reactivity to LPS were significantly enhanced.

6. Cognitive Impairment:

   • Behavioral tests showed that mild peripheral LPS injection significantly impaired cognitive function in hyperglycemic mice, while normoglycemic mice did not exhibit such effects.

Conclusion and Significance

This study indicates that hyperglycemia directly induces astrocyte reprogramming to a proliferative and pro-inflammatory phenotype, enhancing brain sensitivity to mild peripheral inflammation. This discovery underscores the critical role of astrocytes in hyperglycemia-mediated brain pathological states and may provide new avenues for treating diabetes-related brain pathologies. For instance, interventions targeting hyperglycemia-induced astrocyte reprogramming could become a new therapeutic target to mitigate the brain pathologies caused by diabetes complications.

Research Highlights

• Innovation: First demonstration that hyperglycemia directly causes proliferation and reprogramming of astrocytes, shifting to a pro-inflammatory phenotype.
• Important Findings: Hyperglycemia significantly enhances brain sensitivity to mild peripheral inflammatory stimuli, increasing myeloid cell brain infiltration and risk of cognitive impairment.
• Practical Value: Provides new research directions and potential therapeutic targets for treating diabetes-related brain pathologies.

This study further deepens the understanding of the specific impacts of hyperglycemia on astrocytes and brain inflammation and validates the critical role of astrocytes in maintaining CNS homeostasis. It also offers potential intervention strategies for preventing and treating brain pathologies in diabetic patients.