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

Abstract

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.

Expression of INPP5D Isoforms in Human Brain: Impact of Alzheimer’s Disease Neuropathology and Genetics

“…The cDNA copy numbers for each sample were determined by quantitative polymerase chain reaction (qPCR) relative to standard curves that were amplified in parallel and were based upon previously purified and quantified PCR products, as described previously [ 27 , 31 , 32 , 33 ]. INPP5D isoforms were identified using ENSEMBL (version GRCh38:CM000664.2) and are referred to with ENSEMBL nomenclature.…”

“…Cell suspensions were centrifuged at 300× g for 5 min, and RNA was extracted from cell pellets using RNeasy kits according to the manufacturer’s instructions (Life Technologies, Carlsbad, CA USA). RNA was converted to cDNA by using random hexamers and SuperScript IV (Thermo, Waltham, MA USA) and subjected to PCR as described previously for human brain cDNA [ 27 , 31 , 32 , 33 ].…”

Orchestration of synaptic functions by WAVE regulatory complex-mediated actin reorganization

“…ABI3 was also recently proposed as an early biomarker for AD 148 . Overall, a series of these studies have strongly implicated ABI3 in AD pathogenesis, likely through its involvement in microglial motility and/or phagocytosis, in relation to microglial migration into amyloid plaques 149 , 150 . Whether ABI3 is also involved in other neurodegenerative disorders remains to be determined.…”

The effect of Abi3 locus deletion on the progression of Alzheimer’s disease-related pathologies

“…Recent human genetics studies of AD identified a risk variant in the Abelson interactor family member 3 ( ABI3 ) locus in late-onset AD (LOAD) patients ( 2 , 15 – 17 ). ABI3 is a microglia-enriched gene ( 16 , 18 , 19 ). It participates in the WASP-family verprolin homologous protein (WAVE) regulatory complex, which is involved in actin cytoskeleton organization ( 20 , 21 ).…”