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Contributor Information

  • Authors: Matthew J. Moulton, Scott Barish, Isha Ralhan, Jinlan Chang, Lindsey D. Goodman, Jake G. Harland, Paul C. Marcogliese, Jan O. Johansson, Maria S. Ioannou, Hugo J. Bellen

  • Affiliations: Various departments at Baylor College of Medicine, Texas Children’s Hospital, University of Alberta, Artery Therapeutics, Inc.

  • Corresponding Author: Hugo J. Bellen (hbellen@bcm.edu)

  • Publication Details: Inaugural Articles by members of the National Academy of Sciences, accepted on November 11, 2021

Introduction to Alzheimer's Disease (AD)

  • Prevalence: Affects ~2% of the U.S. population, defines ~70% of dementia cases.

  • Pathological Characteristics: Defined by the accumulation of amyloid-β (Aβ) peptides in plaques and phosphorylated tau in neurofibrillary tangles.

  • Treatment Challenges: Therapeutic strategies focused on Aβ reduction largely unsuccessful in slowing disease progression.

  • Concept of Multiple Insults: Aβ may not always correlate with cognitive decline; suggests other factors contribute to disease onset.

Genetic Risk Factors for AD

  • GWAS (Genome-Wide Association Studies): Over 40 risk variants identified.

  • Key Genes: Genes linked to lipid metabolism (e.g., TREM2, ABCA1, ABCA7) and endocytosis (e.g., BIN1, CD2AP, AP2A2).

  • Role of APOE4: Present in ~40-60% of AD patients, strong association with earlier onset and impaired fatty acid metabolism.

  • Implications of Genetic Variants: Variants believed to contribute to neuronal demise through dysfunctional lipid transfer mechanisms.

Reactive Oxygen Species (ROS) in AD

  • ROS Dynamics: Can be damaging at high levels; involved in lipid peroxidation leading to neurodegeneration.

  • Evidence of Elevated ROS: Observed in postmortem tissues of individuals with preclinical AD and mild cognitive impairment.

  • Hypothesis: A vicious cycle involving ROS and Aβ production may accelerate AD progression.

Study Objective

  • Investigate how AD-associated genes influence ROS-induced lipid droplet (LD) formation in glia and their neuroprotective effects against neurotoxicity.

Key Findings

Role of ROS and LD Formation

  • Mechanism of Neuroprotection: Elevated ROS in neurons triggers lipid synthesis which helps sequester toxic peroxidated lipids in glial cells.

  • APOD/E-dependent Lipid Transfer: The transfer of lipids from neurons to glia relies on apolipoproteins such as GLaz and APOE.

Genetic Factors Impacting LD Formation

  • Identified Genes: Key genes affecting LD formation under ROS conditions include human homologs: ABCA1, ABCA7, VLDLR, VPS26, VPS35, AP2A, PICALM, CD2AP.

  • Gene Impacts: Loss of function of these genes leads to impaired LD formation and increased neurotoxicity.

Experimental Insights

  • Utilized Drosophila models for testing impacts of genetic knockdown via RNAi on LD formation and neurodegeneration.

  • Results indicated that ROS synergizes with Aβ42 to accelerate neuronal death.

Therapeutic Implications

  • ABCA1 Agonist Peptide: A peptide agonist of ABCA1 was found to restore LD formation in a humanized APOE4 fly model, showing promise for future therapies.

  • Neuronal and Glial Interactions: Highlights the importance of neuron-glia interactions for maintaining cellular health in the context of AD.

Detailed Mechanistic Insights

ROS-Induced Neuronal Lipogenesis

  • Mitochondrial Dysfunction: Defective mitochondria produce increased ROS, activating JNK and SREBP transcription factors, leading to increased lipid synthesis.

  • Sensitivity of LD Formation: Loss of one copy of genes manipulating LD formation significantly impairs the protective process against ROS toxicity.

Gene Functions and Endocytosis in Glia

  • Glial Apolipoprotein Roles: GLaz (APOD homolog) is crucial for lipid transfer from neurons to glia, with LRP1 being essential for glial uptake.

  • Retromer Role: VPS26 and VPS35: Knockdown leads to impaired LD formation, further indicating the importance of proper endocytic recycling for glial health.

AD-Risk Genes and LD Dynamics

  • Critical Role of CD2AP and PICALM: Both are implicated in glial LD formation and protecting against degeneration by sequestering toxic products from neurons.

  • Synergistic Effects: Evidence of a feed-forward mechanism where interactions between ROS and Aβ exacerbate neurotoxicity.

Clinical Implications and Future Directions

  • Antioxidants in AD Treatment: Reduced antioxidant capacity is noted in AD; studying compounds that enhance this pathway may benefit treatment approaches.

  • Focus on Glial Health: Understanding how glia manage lipid handling and ROS could reveal crucial targets for therapeutic intervention.

  • Potential for Drug Development: Compounds that stimulate lipid transfer or mitigate ROS damage could emerge as major therapeutic options in managing AD.

Conclusion

  • Multifactorial nature of AD emphasizes the need for integrated approaches focusing on genetic, environmental, and cellular factors influencing disease onset and progression. This research underscores the intricate interplay between neuroprotective mechanisms and potential targets for pharmacological intervention.