Aging linked to impaired garbage collection in the brain
A mouse study has shown that, as cells age, their ability to remove damaged proteins and structures declines.
The process of waste management, called autophagy, involves a component within the cell (an autophagosome) engulfing misfolded proteins or damaged structures (putting them in a garbage bag, essentially). The autophagosome then fuses with a second cellular structure, called a lysosome, that contains the enzymes needed to breakdown the garbage, allowing the components to be recycled and reused.
It’s thought that this decline in autophagy makes neurons more vulnerable to genetic or environmental risks.
The mouse study found that aging brought a significant decrease in the number of autophagosomes produced, along with pronounced defects in their structure.
However, activating the protein WIPI2B restored autophagosome formation.
Breakdown in cleaning process in mitochondria linked to Alzheimer's
A cleaning process in brain cells called mitophagy breaks down defective mitochondria and reuses the proteins that they consist of. When the process breaks down, defective mitochondria accumulate in brain cells.
Research has now found that this is markedly present in cells from both humans and animals with Alzheimer's. Moreover, when active substances targeted at the cleaning process were tried in live animals, their Alzheimer's symptoms almost disappeared.
Microglia may spread toxic tau during early Alzheimer's
A 2015 study found how toxic tau fibrils spread during the early stages of Alzheimer's disease. Apparently the fibrils (accumulations of tau proteins) can be carried from one neuron to another by microglia.
Microglia act as the brain's immune cells, in which role they identify and clear damage and infection. They clear damage by first engulfing dead cells, debris, inactive synapses or even unhealthy neurons, then releasing nano-scale particles called exosomes, which can be absorbed by other cells.
It used to be thought that exosomes simply help the cell to get rid of waste products. It now appears that cells throughout the body use exosomes to transmit information. This requires them to contain both proteins and genetic material, which other cells can absorb. Hence their ability to spread tau protein, and hence, it now seems, their ability to also transport amyloid-beta.
Microglia link Alzheimer’s amyloid and tau
Amyloid plaques and tau tangles are key biomarkers for Alzheimer’s, but research indicates that it is the tau tangles that are the real problem — the main problem with amyloid plaques is that they lead to tau tangles. A new study indicates how that happens.
A mouse study modified the TREM2 genes, which affect the health of microglia. So some mice carried the common variant of the gene, meaning that their microglia were fully functional, and some carried the risky variant, or no gene at all.
When seeded with tau protein from Alzheimer’s patients, those brains with weakened microglia produced more tau tangle-like structures near the amyloid plaques than in mice with functional microglia.
It was also revealed that microglia normally form a cap over amyloid plaques that limits their toxicity to nearby neurons. When the microglia failed to do that, neurons suffered more damage, creating an environment that fostered the formation of tau tangle-like lesions.
The findings were supported by the finding that humans with TREM2 mutations who died with Alzheimer’s had more tau tangle-like structures near their amyloid plaques than people who died with Alzheimer’s but didn’t have the risky gene.
However, it should be noted that in more advanced stages of Alzheimer’s, mice with the common TREM2 variant showed faster plaque growth. This appears to be linked to the gene inducing microglia to produce ApoE, which enhances aggregate formation.
The finding adds to evidence that Alzheimer's treatment has to take into account the stage at which the disease is at.
Another study that modified the TREM2 gene in mice found that the difference between those with the gene and those without was not in the amount of tau tangles, but rather in the way their immune cells responded to the tau tangles. The microglia in mice with TREM2 were active, releasing compounds that in some circumstances help fight disease, but in this case primarily injured and killed nearby neurons. The microglia in mice without TREM2 were much less active, and their neurons were relatively spared.
Overactive microglia have multiple effects
A study found that, if the gene for the TDP-43 protein was turned off in microglia, its activity increased, and amyloid-beta was removed very efficiently. However, when TDP-43 was switched off in microglia in mice, it didn’t just get better at removing amyloid-beta, but also led to a significant loss of synapses.
Clearly, dysfunction of microglia is a complicated business, and it’s suggested that such dysfunction may be one reason why many Alzheimer's medications reduce amyloid plaques but fail to improve cognition.
Classifying brain microglia
Microglia come in many forms. A survey of brain microglia has classified microglia into at least nine distinct groups, including some types never detected in the past. Some types appeared almost exclusively in the embryonic or newborn stages, others only after injury.
One group tended to cluster near the brain's developing white matter. Another appears to be very inflammatory compared with other microglia, and has been found in people with MS.
Microglia were most diverse early in brain development, in the aged brain and in disease.
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