Strickland MR, Rau MJ, Summers B, Basore K, Wulf J 2nd, Jiang H, Chen Y, Ulrich JD, Randolph GJ, Zhang R, Fitzpatrick JA, Cashikar AG, Holtzman DM. Apolipoprotein E secreted by astrocytes forms antiparallel dimers in discoidal lipoproteins. Neuron. 2024 Apr 3;112(7):1100-1109.e5. Epub 2024 Jan 23 PubMed.
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CAMed, Boston University
This study uses antibody labeling and cryogenic electron microscopy to propose a model of lipidated apoE, the major functional form of this important apolipoprotein. Currently we have atomic structures of lipid-free, full-length modified human apoE and its globular N-terminal domain, which are dramatically different from the extended conformation of apoE on the lipid. Prior studies have provided extensive (albeit indirect) low-resolution information on lipidated apoE, which led to the development of the antiparallel double-belt models. In these models, two protein copies in a largely extended highly α-helical conformation run antiparallel to form a double belt around the lipoprotein circumference. These models resemble the general features of lipidated apoA-I on HDL, for which much more direct structural information is now available. Similar details for apoE are still lacking, such as the helix registry in the double belt and the N-terminal conformation on lipid particles of various sizes. Such details are important for understanding apoE receptor binding (which critically depends on the N-terminal domain conformation) and the isoform-specific effects of apoE in cardiovascular and Alzheimer’s diseases.
The results of the current study, in particular binding of dimeric antibody fragments, are consistent with the antiparallel double-belt model of apoE. These results provide strong supportive evidence for this model but do not directly prove it due to limited resolution. The resolution of the cryoEM data and models needs to be improved before any new structural details and their biological ramifications become apparent. This study represents an interesting development in this direction.
The authors cite the resolution of the cryoEM data in the 8À range, with the best data at 7.7À resolution. At this resolution, one should be able to resolve individual α-helices in the double belt (since the helical diameter is approximately 10À). It is therefore surprising that the helices cannot be resolved. Moreover, the protein position on the lipid is not clearly discernable.
The conformation of the N-terminal domain of apoE on the particle is also unclear from the current study. On small 12.4 nm particles used for cryoEM analysis, this domain perhaps is not entirely extended in the double belt; if so, it should stick out from the surface. Whether the small pointed end seen in cryoEM maps represents N-terminal domain remains unclear. Hopefully future higher-resolution studies can answer these central questions in apoE biology.
Larger macromolecular complexes are generally more conducive to structural cryoEM studies. Perhaps if larger-size apoE2-containing particles were used for cryoEM (e.g., 20 nm in diameter) rather than smaller apoE4-containing particles used here (12.4 nm), better resolution could be obtained. Another potential approach to improve the resolution is following the protocols of Weisgraber’s team, who managed to crystallize lipidated apoE. Typically, samples that can be crystallized are suitable for high-resolution cryoEM analysis. Obtaining such lipoprotein samples remains a major technical challenge.
View all comments by Olga GurskyHong Kong University of Science & Technology
As we explore the pathobiology of ApoE in AD and related dementias, there is an urgent need to understand the biochemical and biophysical properties of ApoE with the three main isoforms, ApoE2, ApoE3, and ApoE4 having differential effects on amyloids, cerebrovasculature, immune response, and tau-mediated neurodegeneration.
Due to the aggregative nature of recombinant ApoE, there has not been a complete structure of ApoE, although helpful information has been generated using ApoE fragments or mutated ApoE. Importantly, because the physiological form of ApoE is lipidated, both peripheral and CNS, it is perhaps more relevant to understand the structures of lipidated ApoE, in particular how they differ when lipoprotein particles contain different ApoE isoforms.
Using astrocyte-secreted ApoE and recombinant ApoE/lipoprotein, taking advantage of the negative stain transmission and cryogenic electron microscopy (TEM and cryo-EM) technology, the authors of this exciting paper found that each astrocyte-secreted ApoE-containing lipoprotein has two ApoE molecules that are in an antiparallel conformation.
One key approach used in this study is the application of two ApoE antibodies recognizing two different regions of ApoE. By assessing their binding, the authors were able to conclude there are two anti-parallel molecules of ApoE on each lipoprotein particle. Although the resolution of the structure did not allow for a visualization on the details of ApoE structure, this study clearly paves the way for obtaining high-resolution structural information of lipidated ApoE.
In the brain, ApoE's primary function is to transport lipids from glial cells, primarily astrocytes, to neurons and other brain cell types which need lipids for synaptic and brain functions. ApoE also plays a key role in injury repair, where it is critical for “organizing” lipids for cellular uptake leading to storage and/or re-utilization.
In the setting of AD, ApoE is co-deposited with Ab as amyloid plaques, with little or no lipids. It is unclear whether the amyloid-associated ApoE represents a pool of non-lipidated ApoE, or lipidated ApoE that has undergone a de-lipidation process, e.g., in an intracellular environment prior to forming aggregates with Ab.
More importantly, the three major ApoE isoforms have differential effects on amyloid formation, as well as several other AD-related pathways. How ApoE/lipoprotein particles differ in an ApoE isoform-dependent manner is not entirely clear, but this and other studies have shown that ApoE2 particles are larger than ApoE3, which is larger than ApoE4. The poorer lipidation status of ApoE4 likely contributes to its many pathogenic effects. As such, we hope there will be new structural information, along with biochemical and biophysical characterization of ApoE isoforms, to aid our understanding of ApoE in AD pathogenesis.
View all comments by Guojun BuMake a Comment
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