Axonal Spheroids in Alzheimer's Disease, Connected to Amyloid and Autophagy

Axonal spheroids are a feature of neurodegenerative conditions, bubbles that form on axons and can contain entire cell organelles, in addition to molecular debris and other cell components. These spheroids are comparatively poorly understood, but are thought to be connected to the processes of autophagy and other modes of clearance of waste. Spheroids disrupt axonal function, and may rupture to spill their contents outside the cell, causing further issues. Today's open access paper provides more evidence for the connection to portions of autophagy, specifically the formation of endolysosomes, when an endosome carrying materials ingested by the cell merges with a lysosome in order for those materials to be broken down by enzymes. Connections are also made to the presence of amyloid-β in the aging brain, in that axons close to plaque are those that form axonal spheroids.

Whether the mechanisms underlying these correlations are direct or indirect is a matter for speculation. It has the look of a garbage catastrophe: cellular recycling systems, such as autophagy, that can manage the load in a youthful brain become overloaded in an aged brain. That may be due to the presence of amyloid-β plaques and other problematic materials outside cells that are taken up into endosomes, rising levels of damaged molecular machinery inside cells that requires recycling, or loss of efficiency in autophagy due to that damage, but the end result is a series of maladaptive, possibly compensatory phenomena such as the creation of axonal spheroids. These structures are potentially useful to eject material from the cell, but also harmful to neural function and the surrounding environment.

PLD3 affects axonal spheroids and network defects in Alzheimer's disease

Here we show that hundreds of axons around each amyloid plaque develop spheroids and, rather than being retraction bulbs from degenerating axons, these structures are stable for extended periods of time and could therefore have an ongoing detrimental effect on neuronal connectivity. Given the similarity in the morphology, organelle and biochemical content of plaque-associated axonal spheroids (PAASs) in mice and humans, it is probable that, in humans, these are also stable structures that could disrupt neural circuits for extended intervals.

To better understand the effect of PAASs on axonal function, we implemented in vivo Ca2+ and voltage imaging in individual cortical axons and cell bodies. Both Ca2+ and voltage imaging revealed that a substantial proportion of axons in a mouse model of Alzheimer's disease (AD) had disrupted AP conduction and an overall increase in the threshold for action potential propagation manifested by conduction blockades. This was due to the presence of axonal spheroids and was shown to be correlated with their size. The finding that larger PAASs caused more severe conduction blocks was consistent with computational modelling showing that PAASs resemble electrical capacitors that function as current sinks, and that PAAS size is a major determinant of the degree of conduction defects. Together, our data suggest that the large number of amyloid deposits present in the AD brain have the potential to substantially affect neural networks by widespread disruption of axonal connectivity.

Mechanistically, we found that enlarged LAMP1-positive vesicles (ELPVs) - which probably include multivesicular bodies (MVBs), endolysosomes, and autolysosomes - accumulate within axonal spheroids and that their presence is correlated with spheroid size. Moreover, we found an increased presence of ELPVs within spheroids in older Alzheimer's model mice and in more severely impaired human patients with AD, indicating that ELPV accumulation may be a key feature of disease progression. MVBs are crucial intermediate organelles that evolve through the maturation of endosomes and fuse with autophagosomes and lysosomes. Thus, dysregulation in MVB biogenesis has the potential to affect the normal generation of fusion vesicles.

Spheroid growth was also mechanistically linked with Pld3 - a potential Alzheimer's-disease-associated risk gene that encodes a lysosomal protein that is highly enriched in axonal spheroids. Neuronal overexpression of Pld3 led to endolysosomal vesicle accumulation and spheroid enlargement, which worsened axonal conduction blockades. By contrast, Pld3 deletion reduced endolysosomal vesicle and spheroid size, leading to improved electrical conduction and neural network function. Thus, targeted modulation of endolysosomal biogenesis in neurons could potentially reverse axonal spheroid-induced neural circuit abnormalities in Alzheimer's disease, independent of amyloid removal.