A research team has identified a key cellular pathway common to a rare childhood disorder and the more common Alzheimer's disease, revealing a unified mechanism of brain deterioration in both conditions.
The study, led by a team from the University of California, San Diego and published in the journal Immunity, showed how immune cells in the brain, known as microglia, respond to the accumulation of cellular waste, and how this response can turn from defensive to destructive, opening new horizons for understanding and treating neurodegenerative diseases.
This discovery is related to Sanfilippo syndrome type A, a rare genetic disorder caused by a mutation in a single gene that prevents the production of the sulfamidase enzyme, which lysosomal enzymes, small cellular structures, depend on to break down nutrients and recycle damaged parts. Without this enzyme, waste accumulates inside cells, leading to seizures, dementia, and premature death in affected children.
The team observed, through studying an animal model, that this waste accumulates in several types of cells, but microglia in the brain are the most affected, as they enlarge and become filled with fats and proteins, losing their ability to protect nerve cells.
Researchers identified a protein family known as MITF/TFE that acts as a master genetic switch. These switches are activated from an "off" to a "on" state when lysosomal enzymes in glial cells become overburdened with stress. This triggers a change in their genetic program in an attempt to protect the brain. However, over time, this response becomes ineffective and transforms into a driver of inflammation and neuronal death. Even more surprisingly, the researchers found that these same genetic switches are active in the microglia of Alzheimer's patients. This suggests that the stress process resulting from lysosomal failure in Sanfilippo is the same as that occurring in the brains of Alzheimer's patients. Unlike complex neurodegenerative diseases associated with aging, Sanfilippo has a clear and direct genetic cause, making it an ideal model for studying these mechanisms.
The study's lead author, Christopher Balak, a postdoctoral researcher in Christopher Glass's lab at the University of California, explained that this discovery provided a clear framework for studying common degenerative diseases and understanding their mechanisms. He noted that many researchers believe that amyloid plaques, which form outside glial cells, cause lysosome failure from the outside, but their study proved that damage can come directly from within the cell itself, and that lysosomes alone are sufficient to cause neurodegeneration, which may also apply to major diseases such as Alzheimer's.
By identifying the MITF/TFE family as a key driver of this process, the study opens the door to the development of new drugs that target these genetic switches, rather than traditional approaches that focus on cell surface receptors, which may allow microglia to be kept in a protective state and prevented from causing brain damage.
The team also found that these cells try to reduce damage in the early stages of the disease before it becomes exhausted, suggesting that early intervention using enzyme replacement therapy or cell therapies may be more effective if done before immune cells turn into the harmful state, which represents new hope in treating these intractable diseases.
