Shared cellular pathway drives neurodegeneration in rare childhood dementia and Alzheimer’s

Researchers at University of California San Diego and their colleagues have identified a key cellular pathway that drives brain degeneration in both a rare childhood disorder and the far more common Alzheimer's disease. The study, published in Immunity, reveals how the brain's immune cells respond to waste buildup and provides a novel roadmap for understanding and treating neurodegenerative conditions.

Children with Sanfilippo syndrome type A, also known as Mucopolysaccharidosis Type IIIA (MPS IIIA), experience seizures and dementia among other symptoms, leading to early death. The condition is caused by a single gene variant that blocks the production of the enzyme sulfamidase. Normally, tiny structures within cells called lysosomes use sulfamidase to break down nutrients into usable energy, destroy harmful invaders like bacteria, and recycle old cell parts for reuse. In the absence of the enzyme, debris accumulates.

The researchers studied a mouse model of MPS IIIA, finding that while this waste builds up in many cell types, microglia - the brain's dedicated immune cells - are impacted the most. These cells expand as they become clogged with fats and proteins, losing their ability to protect neurons. 

The researchers identified a family of proteins, known as MITF/TFE, that act as master genetic switches. When lysosomes in microglia become overburdened and stressed, these switches are flipped from the "off" to the "on" position, triggering a massive change in the microglia's genetic program in order to protect the brain. But this response eventually becomes maladaptive, fueling inflammation and contributing to the death of neurons.

Surprisingly, the researchers found that the same MITF/TFE switches are turned on in response to waste accumulation in the microglia of human Alzheimer's patients as well. This suggests that the stress response triggered by lysosomal failure in MPS IIIA is the same process occurring in the aging brains of Alzheimer's patients. But unlike complex neurodegenerative diseases of aging, MPS IIIA has a clear-cut cause.

"It gave us a really clear framework to study what we see in common neurodegenerative diseases and try to figure out mechanisms that are causing them," said first author Christopher Balak, PhD, a post-doctoral researcher in the lab of corresponding author, Christopher Glass, PhD, professor of cellular and molecular medicine at UC San Diego School of Medicine.

Many researchers believe that amyloid plaques, which are external to microglia, cause lysosomes to fail from the "outside in." 

However, we show in this paper that the damage can come directly from inside the cell. We know lysosomes alone are sufficient to cause neurodegeneration from rare disorders like MPS IIIA. The same thing could be happening in, or at least contributing to, major diseases like Alzheimer's disease."

Christopher Balak, PhD, first author

By identifying the MITF/TFE protein family as the primary drivers of this process, the research points to a new target for drug development. By modulating these genetic switches, scientists may one day be able to maintain microglia in a protective state and prevent them from further damaging the brain.

"Most microglia-targeted drugs go after receptors on the cell surface," said Balak. "I think this work points to a little bit of a different strategy, instead going after the lysosomal program inside the cell."

The team found that microglia attempt to minimize damage early in the disease process before becoming overwhelmed. This suggests that early intervention in neurodegenerative diseases using enzyme replacement or cell therapies could be most effective when administered before the "genetic switch" flips the immune cells into a harmful state.