ApoE in AD and Related Dementias
The Neurobiology of Alzheimer's Disease Lab is investigating the roles of apolipoprotein E (apoE) in Alzheimer's disease (AD) and related dementias. The pathological hallmarks of AD include the deposition of extracellular amyloid-beta (Aβ) aggregates as senile plaques and intracellular hyperphosphorylated tau aggregates as neurofibrillary tangles, along with neuronal loss and glial activation.
Pathobiology of APOE
Polymorphism in the APOE gene is a major genetic risk determinant for late-onset AD with the ɛ4 allele (APOE4) increasing and ɛ2 allele (APOE2) decreasing risk relative to the common ɛ3 allele (APOE3). Mounting evidence demonstrates that a major pathway by which apoE4 increases AD risk is by driving earlier and more-abundant amyloid pathology in the brains of APOE4 carriers. In addition, recent studies indicate that apoE influences tau pathology, tau-mediated neurodegeneration and microglial responses to AD-related pathologies. Furthermore, apoE4 is either pathogenic or less efficient in multiple brain homeostatic pathways, including lipid transport, synaptic integrity and plasticity, glucose metabolism, and cerebrovascular function. These findings indicate that apoE isoforms differentially modulate Aβ-dependent and Aβ-independent pathways; thus, understanding the pathobiology of apoE holds promise for the development of strategies for AD therapy.
In peripheral tissues, apoE is primarily produced by the liver and macrophages and mediates cholesterol metabolism in an isoform-dependent manner. In the central nervous system (CNS), apoE is most abundantly expressed in astrocytes and also in microglia, vascular mural cells, and to a lesser extent, stressed neurons.
Using novel apoE inducible models, Dr. Bu's lab is investigating how apoE produced by different cell types in the CNS and periphery affects brain functions and AD pathogenesis. A major function of apoE is to transport cholesterol to neurons via two classes of receptors, the low-density lipoprotein receptor (LDLR) family members and cell surface heparan sulfate proteoglycans (HSPGs). Using cellular and animal models combined with human studies, the lab investigates the complex mechanisms underlying isoform-dependent effects of apoE and apoE receptors in normal CNS functions, Aβ clearance, and amyloid and tau pathologies as well as cognition during aging and AD.
The role of apolipoprotein E4 in Alzheimer's disease pathogenesis. Sources: Nature Reviews Neurology. 2013;9:106.
ApoE in Aβ seeding and aggregation
Aβ aggregation occurs when Aβ is overproduced or less efficiently cleared or both. The formation of Aβ fibrils, a major component of amyloid plaques, follows three kinetic stages: a lag phase, a growth phase and a plateau phase. Although there is strong evidence showing that apoE4 enhances amyloid pathology, the critical stage that apoE4 has the strongest impact in amyloid development is not clear. Using apoE inducible mouse models, Dr. Bu's lab showed that astrocytic overexpression of apoE4, not apoE3; during the initial Aβ seeding stage enhances amyloid pathology in an amyloid mouse model. However, expression of apoE4 or apoE3 during the plaque rapid growing period has no effect on Aβ deposition. These findings demonstrated that apoE plays a critical role in the initial seeding stage of Aβ deposition, suggesting that strategies targeting apoE4 to reduce Aβ pathology should focus on early prevention.
ApoE4 promotes amyloid seeding during the Aβ nucleation stage (0-6 months of age), but not during the rapid growing stage (6-9 months of age), leading to enhanced amyloid deposition. Source: Neuron. 2017, 96:1024.
ApoE and apoE receptors in Aβ clearance
As Aβ is continuously generated in the brain as a result of amyloid precursor protein (APP) processing, efficient clearance is critical for preventing Aβ accumulation and subsequent aggregation. Using an in vivo microdialysis technique that allowed for examination of the Aβ metabolism in living animals, Dr. Bu's lab demonstrated that apoE mediates the Aβ clearance of in an apoE isoform-dependent manner with apoE4 impairing Aβ clearance. In addition, the low-density lipoprotein receptor-related protein 1 (LRP1), a major receptor for both apoE and Aβ, plays essential roles in brain Aβ clearance.
Using unique mouse models, lab researchers also showed that conditional knockout of LRP1 in mouse forebrain neurons, astrocytes and vascular cells impairs Aβ clearance, leading to increased brain Aβ and exacerbated amyloid plaque deposition in the amyloid model mice without affecting Aβ production. These studies highlighted the importance of various brain cell types in the process of brain Aβ clearance. Heparan sulfate proteoglycans (HSPGs) are abundant cell surface receptors that co-localize with amyloid plaques. Dr. Bu's lab has shown that genetically engineered mice lacking heparan sulfates in forebrain neurons were protected from amyloid deposition due to a faster clearance of abeta and reduction in Aβ aggregation. Importantly, it was shown that several HSPG species are more prevalent in human AD postmortem brain tissue. This finding suggests that targeting Aβ-HSPG interactions might be an effective strategy for AD prevention and treatment.
APOE and other neurodegenerative diseases
Brains affected by Alzheimer's disease have extensive tau pathology in addition to amyloids. As such, AD is considered a "secondary tauopathy," with primary tauopathies referring to disorders in which tau pathology is not accompanied by amyloid, such as progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). To examine the association of the APOE genotype in tau pathology, Dr. Bu's research team established an adeno-associated virus (AAV)-mediated gene delivery approach to express TauP301L in human apoE-targeted replacement (TR) mice with different isoforms.
The lab found that apoE2-TR mice expressing human tau exhibited significant increases in hyperphosphorylated tau species, thioflavin-S-positive tau aggregates, astrocytosis and behavioral abnormalities. Importantly, the APOE2 allele is associated with increased tau pathology in the brains of people with PSP. Together, these findings support a pathogenic risk of the APOE2 gene allele with primary tauopathies. In addition, APOE4 has recently been discovered as a strong risk gene for Lewy body dementia. However, it is not clear how apoE4 drives DLB risk and whether it directly affects α-synuclein pathology. Using the AAV gene delivery system, Dr. Bu's lab demonstrates the pathogenic role of apoE4 in driving α-synuclein pathology and provides pathogenic insights on how APOE4 increases the risk of DLB.
Increased hyperphosphorylated tau species and thioflavin S-positive tau aggregates in TauP301L-APOE2 mice. Source: Nature Communications. 2018;9:4388.