Summary: Amyloid plaque formation in the brain leads to the development of spheroid-shaped swellings along axons near plaque deposits. These swellings result from accumulating lysosomes—organelles that digest cellular waste. As the spheroids enlarge, they can disrupt electrical signal transmission between brain regions.
Source: Yale
Amyloid plaque accumulation in the brain is a defining feature of Alzheimer’s disease, yet therapies aimed solely at removing plaques have produced mixed clinical results.
Researchers at Yale report that the swelling produced by a plaque-related byproduct may be the primary driver of the cognitive symptoms associated with the disease. Their findings, published Nov. 30, identify both a cellular mechanism that impairs neural communication and a potential biomarker that could guide diagnosis and new treatments.
The team found that each amyloid plaque can trigger the formation of numerous spheroid-shaped swellings along the axons—the thin, cable-like projections that transmit electrical signals between neurons—around the plaque deposits.
These spheroids form as lysosomes, the cell’s waste-processing organelles, progressively accumulate within axons. As lysosomal material builds up, the swellings grow larger and begin to interfere with the normal propagation of action potentials, effectively blunting electrical signals traveling between distant brain regions.
This accumulation of lysosomes and the resulting axonal swelling appear to be a key factor in the breakdown of circuit function that underlies dementia symptoms, according to the researchers.
“We have identified a potential signature of Alzheimer’s which has functional repercussions on brain circuitry, with each spheroid having the potential to disrupt activity in hundreds of neuronal axons and thousands of interconnected neurons,” said Dr. Jaime Grutzendler, the Dr. Harry M. Zimmerman and Dr. Nicholas and Viola Spinelli Professor of Neurology and Neuroscience at Yale School of Medicine and senior author of the study.
The investigators linked this lysosomal buildup to a lysosomal protein called PLD3. PLD3 becomes enriched in axonal spheroids and appears to drive the growth and clumping of endolysosomal vesicles along axons, contributing to spheroid enlargement and the eventual breakdown of electrical conduction.
In mouse models that mimic key features of Alzheimer’s disease, the researchers used gene therapy to remove PLD3 from neurons. This intervention sharply reduced axonal swelling, restored more normal electrical conduction along affected axons, and improved the functional connectivity of the brain regions joined by those axons.
Based on these results, PLD3 has potential utility in two important ways: as a biomarker to help assess Alzheimer’s disease risk or progression, and as a therapeutic target. Targeting PLD3 or other regulators of lysosomal biogenesis could prevent or reverse spheroid formation and restore circuit function, possibly independently of amyloid plaque removal.
“It may be possible to eliminate this breakdown of the electrical signals in axons by targeting PLD3 or other molecules that regulate lysosomes, independent of the presence of plaques,” Grutzendler said. This suggests a therapeutic strategy focused on preserving axonal conductivity and neural network integrity rather than solely eliminating plaques.
About this Alzheimer’s disease research news
Author: Bill Hathaway
Source: Yale
Contact: Bill Hathaway – Yale
Image: The image is in the public domain
Original Research: Open access.
Title: PLD3 affects axonal spheroids and network defects in Alzheimer’s disease by Peng Yuan et al. Nature Communications
Abstract
PLD3 affects axonal spheroids and network defects in Alzheimer’s disease
The precise mechanisms that lead to cognitive decline in Alzheimer’s disease remain incompletely understood. In this study, the authors identify amyloid-plaque-associated axonal spheroids as major contributors to neural network dysfunction.
Using live calcium and voltage imaging in a mouse model, the researchers observed severe disruption of long-range axonal connectivity. They demonstrate that expanding spheroids act as electrical sinks: as spheroids grow, they introduce action-potential conduction blockades in a size-dependent manner.
Spheroid enlargement correlated with an age-dependent accumulation of large endolysosomal vesicles and was mechanistically tied to Pld3, a gene encoding a lysosomal protein that is highly enriched in these spheroids. Neuronal overexpression of Pld3 increased endolysosomal vesicle accumulation and worsened spheroid growth and conduction blockades. Conversely, deletion of Pld3 reduced vesicle and spheroid size and improved electrical conduction and network function.
These findings indicate that selectively modulating endolysosomal biogenesis in neurons could reverse axonal spheroid–induced circuit abnormalities in Alzheimer’s disease, offering a complementary therapeutic approach that does not rely solely on amyloid removal.