Hussein Yassine, M.D.: Deep dive into the “Alzheimer’s Gene” (APOE), Brain Health, and Omega-3s

Brain Architecture and Function

• Brain is a lipid organ containing cholesterol and various fatty acids
• Composed of three main cell types: 
  • Neurons: responsible for firing and forming synapses, regulating brain functions
  • Glial cells (astrocytes): support neurons by regulating energy storage and production
  • Microglial cells: immune cells responsible for cleaning up toxic proteins and infections
• Blood-​​brain barrier: separates blood vessels from the brain, protecting it from toxic substances and maintaining a stable environment 
  • Composed of endothelial cells, pericytes, and mural cells
• Cerebrospinal fluid (CSF): acts as the brain’s sewage system, clearing metabolites and byproducts from brain cells 
  • Choroid plexus pumps clear fluid into the CSF, which then washes the brain and clears metabolites into the blood for excretion

APOE Genotype and Alzheimer’s Disease

• APOE4 genotype increases the risk of Alzheimer’s disease
• More people are becoming aware of their APOE genotype through genetic testing (e.g., 23andMe)

• Understanding the brain’s architecture and function is crucial for studying the impact of APOE genotypes on Alzheimer’s disease and other neurodegenerative conditions
  Brain Structure and Energy Utilization

• Brain is distinct in appearance and function 
  • Lacks the redness of other organs, looks sterile
  • Disproportionately lipid-​​laden compared to other organs 
    • Every cell has a bilayer of cholesterol for fluidity and transporters
    • Brain cells require more lipids for their function
• Brain operates by firing electrical impulses 
  • Requires specific “cables” for instantaneous communication
  • Lipids facilitate this process (e.g., myelin in neurons, synaptosomes in synapses)
• Brain is energy-demanding 
  • Weighs about 2% of body weight, responsible for ~20% of energy consumption
  • Entire function is based on electrical potentials
  • Optimized for signal transduction and rapid potential recurrence

Brain Energy Extraction and Utilization

• Brain prefers glucose as the predominant source of energy 
  • Glucose regulation in the brain differs from the rest of the body
  • Blood-​​brain barrier has a predominance of GLUT1 transporters 
    • Not controlled by insulin, exercise, or diet
    • Regulated to protect the brain from systemic hyperglycemia or hypoglycemia
• Brain is not efficient in utilizing fat as a source of ATP 
  • No significant fat storage sites in the brain
  • Prefers to use fat for myelin sheath, synaptisome, membrane fluidity, and depolarization
  • Using fat for ATP production results in oxidative stress
• Neurons prefer lactate over glucose 
  • Astrocytes (helper cells) take up glucose, process it into lactate, and shuttle it to neurons
  • Neurons can produce ATP from lactate with less damage
• In prolonged fasting and glycogen depletion, the brain becomes efficient in extracting ketone bodies to maintain function

Overall, the brain is a unique organ with distinct structure and energy utilization. It relies heavily on glucose and lactate for energy, with a complex system of astrocytes and neurons working together to maintain optimal function. In times of glucose scarcity, the brain can adapt to utilize ketone bodies for energy.
Microglial Cells and Energy Consumption

• Microglial cells: unsure of their energy source (glucose or residual lactate)
• Distribution of ATP consumption between glial cells and neurons is unclear

APOE and APOC3 in Lipoprotein Orchestra

• APOE and APOC3 act as conductors in the lipoprotein orchestra
• APOE and APOC3 regulate the speed of lipoprotein processes
• APOE and APOC3 are exchangeable lipoproteins, can jump between different lipoprotein populations
• APOE helps clear lipoproteins, APOC3 maintains lipoproteins in circulation

APOE in the Blood vs. the Brain

• APOE in the blood has multiple roles, appears on HDL, LDL, IDL, and VLDL
• APOE concentration in the blood is elusive, not as informative as APOB or APOA1 concentrations
• APOE in the brain supports astrocytes, helps with lipid exchange and inflammation regulation
• APOE levels increase drastically after neuronal damage to aid in repair
  APOE and Its Role in Inflammation and Alzheimer’s Disease
• APOE is a protein that exists in different forms (isoforms)
  • APOE4 is the ancestral variant, which has a greater capacity to aggregate and produce a strong inflammatory response
  • APOE3 and APOE2 are more recent variants that emerged due to evolutionary pressures and changes in diet
• APOE4 has been linked to a higher risk of Alzheimer’s disease in modern humans 
  • APOE4 makes Glute One less successful at the blood-​​brain barrier, which affects glucose utilization in the brain
  • APOE3 and APOE2 favor a more robust Glute One expression, allowing for better glucose utilization
• APOE4 carriers may have had better survival rates in the past due to its protective effects against infections and parasites 
  • However, as human lifespan increased and environments became more aseptic, the disadvantages of APOE4 (such as Alzheimer’s risk) became more apparent
• The link between APOE isoforms and Alzheimer’s risk was first discovered in the late 1980s by researcher Alan Roses at Duke University
  APOE4 and Alzheimer’s Disease
• APOE4 is associated with a higher risk of Alzheimer’s disease 
  • Discovered by Alan Roses in the 1990s
  • APOE4 carriers have a 2–4 times higher risk of Alzheimer’s if they have one copy, and a 12 times higher risk if they have two copies
• APOE4 risk varies by ethnicity 
  • Lower risk in Nigerian and Hispanic populations compared to Caucasian and Asian populations
  • Linkage disequilibrium (co-​​inheritance of gene variants) differs between ethnicities, which may explain the varying risk
• APOE4 interacts with other factors, such as diabetes 
  • Colombian study found that APOE4 carriers with diabetes had a higher risk of Alzheimer’s than APOE4 carriers without diabetes or diabetics without APOE4
  • Suggests a gene-​​environment interaction

APOE4, Glucose, and Diet

• APOE4 affects the function of glucose transporter 1 (GLUT1) at the blood-​​brain barrier 
  • In an insulin-​​resistant environment, APOE4 carriers may be less capable of regulating glucose fluctuations
• APOE4 also affects the transport of omega‑3 fatty acids into the brain
• Type 2 diabetes may be a factor in the development of Alzheimer’s in APOE4 carriers 
  • APOE4 itself can create an insulin-​​resistant environment with aging
  • The relationship between APOE4, type 2 diabetes, and Alzheimer’s is complex and may depend on individual factors

Matching Diet to Genotype

• Current dietary patterns in Western countries do not differ by APOE4 genotype
• Research is ongoing to determine how APOE4 interacts with diet and how that interaction can change disease processes
• There may be a rationale for changing dietary patterns to produce better outcomes for APOE4 carriers
  APOE4 and Aging
• APOE4 is a disease of aging 
  • Alzheimer’s disease is also a disease of aging
  • APOE4 carriers do not have frank presentations of disease at younger ages
  • APOE4 carriers may develop problems as they age, but not all of them
• Factors that may contribute to APOE4 carriers developing problems: 
  • Traumatic brain injury
  • Type 2 diabetes
  • Inheriting another protein that increases Alzheimer’s risk
• Aging APOE4 individuals with second and third hits may strain the brain and create an environment where diet makes a difference

Omega‑3 Fatty Acids

• Significant portion of the brain is composed of polyunsaturated fatty acids (DHA, EPA, and arachidonic acid) 
  • DHA and EPA are omega-​​3s, while arachidonic acid is an omega‑6
  • Omega-​​3s and omega-​​6s are important for membrane fluidity and neuronal firing
• DHA is the most common omega‑3 in the brain, while EPA is less abundant but has stronger anti-​​inflammatory effects
• Limited interconversion between DHA and EPA
• Human body cannot efficiently make omega-​​3s from scratch 
  • Requires precursor alpha-​​linoleic acid (ALA)
  • Only 0.5% to 5% of DHA and EPA can come from ALA conversion
• US diet is not enriched in DHA or EPA 
  • Low consumption may not provide enough omega-​​3s to the brain, potentially resulting in disease
• Supplementing with omega-​​3s may not make a difference due to conflicting research and varying supplement quality

Omega‑3 Sources

• Omega-​​3s can come from various sources: 
  • Microalgae (triglyceride-​​based)
  • Fish oils (ethyl esters)
  • Krill oils (phospholipid-​​based)
• Marine origin is necessary for EPA and DHA due to the complex carbon fixation that only algae can do

Omega‑3 Consumption and Health Outcomes

• People who do not eat fish or seafood may be at a higher risk of disease
• Meeting a certain threshold of omega‑3 consumption may provide benefits, but exceeding that threshold may not provide additional benefits
  Vitamin C and Health Protection
• Insufficient vitamin C can cause scurvy 
• Little evidence suggests that supplementing vitamin C offers health protection beyond the RDA

Reduce-​​It Study

• Background: Japanese studies suggested a certain EPA to arachidonic acid ratio was associated with less cardiovascular disease
• Reduce-​​It study: Large multicenter trial with 4 grams of pure EPA given to people with diabetes and high triglycerides
• Results: Those given 4 grams of EPA had substantially better outcomes compared to the placebo group
• Criticism: The placebo used was mineral oil, which may have increased cardiovascular events

Strength Trial

• Predominantly high dose EPA supplement (4 grams omega‑3)
• Compared to corn oil, the study showed no difference in outcomes
• Questions raised about the mineral oil in the Reduce-​​It study, the dose or ratio of EPA to DHA, and the relevance of supplementing to excess

EPA as a Drug or Supplement

• If EPA works through a specific cascade like statins, it could be considered a drug
• If the mechanism is unclear, it might be considered a supplement
• One possibility is that EPA has potent anti-​​inflammatory effects on plaques, leading to less plaque rupture and fewer events

Omega-​​3s and the Brain

• The lab is more interested in figuring out a link between omega-​​3s and the brain, rather than cardiovascular disease
• The Reduce-​​It study and high dose EPA in cardiovascular disease serve as a contrasting example to different systems
  DHA and Alzheimer’s Disease
• Alzheimer’s dementia is a chronic, slow process happening over years 
• DHA deficiency is detrimental to brain function 
  • Studies show that not having enough polyunsaturated fatty acids during development is associated with poorer outcomes in school
  • FDA recommends supplementing infant formula with DHA and arachidonic acid (AA)
• The role of omega‑3 in the diet between ages 6 and 60 is difficult to understand 
  • Studies show discrepant results, some suggesting cognitive impairments, anxiety disorders, mood disorders, and depression, while others do not find these associations
• The half-​​life of lipids in the brain is different than in other compartments of the body 
  • DHA in neuronal membranes lasts longer than DHA on the surface of phospholipids on HDL or with an albumin carrier in blood
  • The brain needs a stable depot of lipids that do not dramatically fluctuate

APOE and DHA

• APOE4 carriers have a greater uptake of DHA in their brain compared to non-carriers 
  • Younger APOE4 carriers have a higher consumption of DHA, which may be important for maintaining cognitive function
• Epidemiology studies suggest that APOE4 carriers who consume high amounts of omega-​​3s from fish and other sources have significantly less disease later in life
• After a certain age (around 55–70), the ability of the APOE4 brain to capture DHA from blood gets compromised 
  • This happens around the same time that the blood-​​brain barrier and glucose transporters are compromised
• A clinical trial found that 2 grams of omega-​​3s had no effect on Alzheimer’s patients, but APOE4 carriers were less likely to respond than non-carriers 
  • The trial focused on DHA, which is the main omega‑3 that makes up the building blocks of synapses in the brain
  • EPA may have a different application in the brain, potentially focusing on vascular disease and small atherosclerotic strokes
    Prevent E Four Trial
• Large trial to study the effect of DHA on younger APOE Four carriers before they have dementia or clinical symptoms 
• Decision made in 2017 to focus on 55 to 70-year-olds
• Trial started in 2019, paused due to COVID, expected to wrap up in 2023 and be published between 2024–2025
• Aim to see if high dose DHA supplementation can slow down the progression of disease in this population

Challenges in Dementia Research

• Dementia is more subtle and complicated than atherosclerosis
• Requires new set of tools, such as brain imaging studies and biomarkers
• Goal is to find a biomarker that can predict protection against dementia or cognitive decline decades later

APOE Four and Brain Degradation

• Study found that older APOE Four brains with dementia have upregulated enzymes that facilitate auto-digestion
• This is due to the failure in utilizing glucose as a source of energy
• Recommendations for younger APOE Four carriers may be different than for older APOE Four carriers in a state of energy deprivation and failure

Ketone Supplementation

• Some studies suggest APOE Four carriers with dementia are less likely to benefit from ketone supplementation compared to non-carriers
• Possible reason: degenerating blood-​​brain barrier compromises the transport of ketone bodies, glucose, and omega‑3 fatty acids
• Dietary interventions at late stages may not be effective; importance of early intervention

Recommendations for APOE Four Carriers

• No strong evidence to support omega‑3 supplementation yet
• Recommend consuming at least one serving of fatty fish per week for APOE Four carriers
• Exercise may have benefits for Alzheimer’s prevention, but no high-​​quality randomized clinical trials specifically for APOE Four carriers
  APOE4 and Alzheimer’s Disease
• APOE4 carriers who exercise have less amyloid plaque buildup compared to non-​​exercising carriers 
• High level of education and hypertension control protect APOE4 carriers from dementia
• Advice for APOE4 carriers:
• Maintain adequate omega‑3 consumption from fatty fish (not supplements)
• Ensure blood pressure is controlled
• Engage in regular exercise throughout life
• Modifying APOE4 risk refers to modifying the vascular effects of APOE4
• Provides both cardiovascular and brain benefits
• Need for biomarkers and functional imaging studies to speed up clinical trials and improve understanding of interventions

Challenges in Alzheimer’s Research

• Logistically complicated and expensive to study younger APOE4 carriers in lifestyle interventions
• Relying on epidemiology makes it difficult to disentangle causal relationships
• Faster clinical trials and better understanding of interventions needed to prevent Alzheimer’s disease