Apolipoprotein E (apoE) isoforms differ by single amino acids at residues 112 and 158 (apoE3: Cys112,
Cys158; apoE3: Cys112, Arg158; apoE4: Arg112, Arg158). These small differences cause a profound change
in the apoE’s structure and function. ApoE4 is associated with the highest risk of onset of Alzheimer’s disease
(AD), whereas apoE2 is “protective” or “less risky” than apoE3. Defining the unique biological properties of
apoE2 will provide insights into its protective roles and may suggest novel therapeutic approaches.
This proposal builds upon our model of apoE involvement in AD. We posit that apoE is induced in
injured/stressed neurons and undergoes neuron-specific proteolysis (apoE4>apoE3>apoE2) to generate a
series of neurotoxic apoE fragments (12–29 kDa). The fragments escape the secretory pathway and enter the
cytosol where they cause mitochondrial dysfunction and stimulate phosphorylation of tau. Susceptibility to
proteolysis and fragment formation occurs secondary to domain interaction (Arg61 ionic interaction with
Glu255) and displays an isoform hierarchy (apoE4>apoE3>apoE2).
We hypothesize that unique features of apoE2 make it less of a risk for AD. We will test this hypothesis by
comparing the effects of apoE2 expression in neurons in transgenic mice with those of our apoE3 and apoE4
mice and cultured neurons to determine how apoE2 modulates apoE-associated neuropathology and
neurodegeneration. We will investigate the biochemical, histopathological and behavioral consequences of
apoE2 expression. Quantitation and characterization of the apoE2 fragments in the brain and plasma will allow
correlation of the fragments with the “protective” or “less risky” role of this isoform in neuropathology. Our data
indicate that apoE2 may be a risk, but just much less. The structural features of apoE2, such as reduced
susceptibility to domain interaction and the unique presence of Cys158, a residue known to significantly alter
the conformation of the molecule, likely moderates the effects on neurons. Understanding the effects of these
structural elements will provide insights into how apoE2 is protective.
The impact of apoE2 on mitochondrial function will be studied. Numerous studies indicate that
mitochondrial dysfunction is an early pathological event in AD. In cultured Neuro-2a cells and primary neurons
from apoE transgenic mice we will determine the mechanisms of action of apoE2 on cellular mitochondrial
activity and energetics and how apoE2 may alter/modulate mitochondrial function, as compared to apoE3 and
apoE4. ApoE4 fragments interact with mitochondria and impair mitochondrial respiratory enzyme levels and
activity. We will extend our studies to apoE2 and apoE2 fragments with mitochondrial activity. Alteration of
mitochondrial proteins will be coupled with the latest proteomic techniques and elucidation of apoE2
interactomes to identify unique pathways involved in apoE2 modulation of metabolism.