The Function of Apolipoprotein E in Neurobiology

Areas of Investigation
The major focus of our laboratory is to elucidate the function of apolipoprotein (apo) E in neurobiology. We are especially interested in determining the basis of the differential effects of the apoE isoforms in neuronal repair/remodeling and in neuroprotection.

Significance
ApoE is produced in abundance in the brain and serves as the principal lipid transport vehicle in cerebrospinal fluid. It is induced at high concentrations after peripheral nerve injury and plays a key role in neuronal repair. ApoE4 is a major susceptibility gene associated with 40–65% of cases of sporadic and familial Alzheimer's disease. ApoE4 increases the occurrence and lowers the age of onset of Alzheimer's disease and is also associated with poor clinical outcome in patients with acute head trauma.  Defining the effects of the three apoE isoforms on the structure and function of the brain should provide crucial insights into the contribution of apoE4 to neurological disease.

Approaches
We use biophysical (X-ray diffraction and crystallography) techniques in combination with cell and molecular biology, and transgenic and gene knockout mice to study the molecular mechanisms apoE and lipids in normal development and disease pathology of Alzheimer's disease and other neurodegenerative diseases.

Contributions
My laboratory described apoE's ligand function, determined its protein and gene sequences, mapped the amino acid residues involved in receptor binding, and defined the three-dimensional structure of the ligand-binding domain. Understanding the structure and function of apoE laid the groundwork for the recent explosion of studiesconcerning apoE4 and Alzheimer's disease. My laboratory was involved in describing the role of apoE in peripheral nerve regeneration, lipid transport in the nervous system, and cytoskeletal stability and neurite extension and remodeling. By establishing brain-specific apoE transgenic mice, we demonstrated detrimental effects on apoE4 on the central nervous system in vivo.

Research Summary
We are pursuing several intriguing potential mechanisms to explain the differential effects of apoE4 versus apoE3. For example, carboxyl-terminal-truncated forms of apoE4 may alter the cytoskeleton and induce the formation of inclusions resembling the neurofibrillary tangles seen in postmortem Alzheimer's disease brains. There may be differences in their intracellular accumulation and distribution, possibly reflecting differences in their intracellular handling or processing. ApoE4 may interact with the amyloid b peptide (Ab), which has neurotoxic effects and appears to be an important factor in the pathogenesis of Alzheimer's disease. ApoE4 and the 42amino acid form of Ab stimulate lysosomal leakage and potentiate cell death and apoptosis in neurons. Our structure/function analyses, in collaboration with Dr. Karl Weisgraber, suggest that many detrimental effects of apoE4 are mediated by a reactive intermediate or molten globule. Thermal and chemical denaturation studies have shown that apoE4 is less stable than apoE3 and is more prone to form a molten globule.