Identification and Quantitation of Novel ABI3 Isoforms Relative to Alzheimer's Disease Genetics and Neuropathology

Elucidating the actions of genetic polymorphisms associated with the risk of Alzheimer's disease (AD) may provide novel insights into underlying mechanisms. Two polymorphisms have implicated ABI3 as a modulator of AD risk. Here, we sought to identify ABI3 isoforms expressed in human AD and non-AD brain, quantify the more abundant isoforms as a function of AD genetics and neuropathology, and provide an initial in vitro characterization of the proteins produced by these novel isoforms. We report that ABI3 expression is increased with AD neuropathology but not associated with AD genetics. Single-cell RNAseq of APP/PS1 mice showed that Abi3 is primarily expressed by microglia, including disease-associated microglia. In human brain, several novel ABI3 isoforms were identified, including isoforms with partial or complete loss of exon 6. Expression of these isoforms correlated tightly with total ABI3 expression but were not influenced by AD genetics. Lastly, we performed an initial characterization of these isoforms in transfected cells and found that, while full-length ABI3 was expressed in a dispersed punctate fashion within the cytosol, isoforms lacking most or all of exon six tended to form extensive protein aggregates. In summary, ABI3 expression is restricted to microglia, is increased with Alzheimer's neuropathology, and includes several isoforms that display a variable tendency to aggregate when expressed in vitro.