A recent study published in Nature investigated how aging alters neuronal protein degradation, aggregation, and transfer to microglia, focusing on synaptic proteins in mouse models of brain aging.
Age and Neurodegenerative Disease
Age is a significant risk factor for neurodegenerative diseases, affecting over one in twelve people globally. As the brain ages, neurons become less efficient at maintaining proteostasis (protein synthesis, folding, transport, and degradation). Disruptions in this balance can lead to protein misfolding, deposition, and aggregation, which impairs brain function and is associated with memory loss, cognitive decline, and dementia.
Experimental Design
Researchers used genetically engineered mouse models to selectively label newly synthesized neuronal proteins in living brains through Bioorthogonal Non-Canonical Amino Acid Tagging (BONCAT). Young, middle-aged, and aged mice were fed non-canonical amino acids and subjected to pulse-chase experiments to evaluate protein degradation over specific periods. Adeno-associated virus (AAV) vectors delivered neuronal labeling machinery in other experiments. Various brain regions, including the cortex, hippocampus, striatum, and hypothalamus, were dissected and analyzed.
Data Analysis
Labeled proteins were enriched and quantified using liquid chromatography–mass spectrometry (LC-MS) combined with tandem mass tag (TMT) multiplexing to enable precise comparisons across ages and brain regions. Protein half-lives were estimated using established kinetic models. Protein aggregates were isolated via detergent-based fractionation. To examine how immune cells process neuronal proteins, fluorescence-activated cell sorting (FACS) was used to isolate microglia for analysis of neuron-derived proteins.
Key Findings
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Slowed Neuronal Protein Degradation: Neuronal protein degradation significantly slowed with age across all examined brain regions.
On average, protein half-lives nearly doubled between young and aged mice, indicating a widespread age-related decline in protein turnover, primarily emerging after middle age.
The hippocampus and sensory cortex showed particularly strong effects.
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Affected Proteins: Proteins most impacted by age were enriched in synaptic structures, mitochondria, and cell junctions—components critical for neuronal communication and metabolic function. Many of these proteins are encoded by genes previously linked to neurodegenerative and neurodevelopmental disorders.
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Protein Aggregation: Aging neurons accumulated a large number of aggregated proteins. Analysis identified over 1,700 neuronal proteins within insoluble aggregates in aged brains. Nearly half of these aggregated proteins also exhibited reduced degradation rates, indicating a connection between impaired turnover and aggregation, with synaptic proteins being highly represented.
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Microglial Uptake: Microglia in aged brains contained significantly higher levels of slowly degrading, neuron-derived proteins compared to young brains. These proteins were enriched for synaptic markers and localized within microglial lysosomes, suggesting active uptake and processing. Over 50% of these accumulated proteins in aged microglia showed prior evidence of defective degradation or aggregation within neurons.
Implications
The findings indicate that aging significantly impairs the brain's ability to maintain protein balance within neurons, leading to slower degradation, widespread aggregation, and accumulation of synaptic proteins. Microglia appear to play a compensatory role by selectively engulfing these aging-related neuronal proteins, especially those associated with synapses. While this mechanism may initially support neuronal homeostasis, increased demand with age could contribute to microglial stress and broader age-related neuropathological vulnerability.
These results, derived from mouse models, highlight neuronal proteostasis as a critical area for preserving brain function during aging.