Brain Protein HAPLN2 Aggregates With Age and Fuels Neuroinflammation
A newly identified extracellular matrix protein accumulates in aging mouse and human brains, triggering microglial inflammation that may accelerate brain decline.
Summary
Researchers identified HAPLN2, a hyaluronic acid-binding extracellular matrix protein found at nodes of Ranvier, as a novel age-dependent aggregating protein in mouse brains. Using sarkosyl-insolubility fractionation and mass spectrometry, the team showed HAPLN2 accumulates progressively with age, driven by rising hyaluronic acid levels and declining microglial clearance capacity. HAPLN2 oligomers directly triggered inflammatory responses in microglia both in cell culture and in living animals. Critically, age-associated HAPLN2 aggregation was also confirmed in human cerebellar tissue, suggesting translational relevance. The findings position HAPLN2 as a novel contributor to neuroinflammatory brain aging, even in the absence of classical neurodegenerative disease.
Detailed Summary
Protein aggregation is well established as a driver of neurodegenerative diseases like Alzheimer's and Parkinson's, but aggregates also accumulate in cognitively normal aging brains. The identity and functional consequences of these age-associated aggregates—especially outside of disease contexts—remain poorly understood. This study set out to systematically map proteins that become insoluble with age in healthy mouse brains and characterize one novel candidate in depth.
Using sarkosyl-based fractionation to isolate insoluble proteins, followed by quantitative mass spectrometry, the researchers screened brain tissue from young and old mice and identified hyaluronan and proteoglycan link protein 2 (HAPLN2) as a top hit. HAPLN2 is a component of the extracellular matrix (ECM) concentrated at nodes of Ranvier, where it stabilizes perineuronal nets by linking hyaluronic acid to chondroitin sulfate proteoglycans. Its aggregation had not previously been linked to aging.
The study identified two key mechanisms driving HAPLN2 accumulation with age. First, hyaluronic acid levels rise in the aging brain, and this excess HA promotes HAPLN2 aggregate formation—likely by altering the ECM environment in which HAPLN2 normally resides. Second, microglial phagocytic function declines with age, impairing the clearance of extracellular HAPLN2. Together, these factors create a permissive environment for aggregate buildup. Using recombinant HAPLN2 oligomers applied to cultured microglia and injected into mouse brains in vivo, the team demonstrated that HAPLN2 aggregates directly stimulate microglial inflammatory responses, including upregulation of pro-inflammatory cytokines. This creates a potential feed-forward loop where aggregation and neuroinflammation reinforce each other.
Importantly, the researchers validated their findings in human tissue, detecting age-associated HAPLN2 aggregation in human cerebellar samples. The cerebellum, which is particularly rich in perineuronal nets and HAPLN2 expression, emerged as a primary site of accumulation. This cross-species confirmation strengthens the translational relevance of the mouse findings and raises the possibility that HAPLN2 plays a role in human cerebellar aging.
The study is notable for situating HAPLN2 aggregation within the broader context of ECM dysregulation in aging—a relatively underexplored dimension of brain aging compared to intracellular proteostasis. The findings suggest that targeting HAPLN2 aggregation, reducing excess hyaluronic acid, or restoring microglial clearance capacity could represent novel strategies for mitigating age-associated neuroinflammation. Several caveats remain, including the need to understand the precise molecular triggers for HAPLN2 oligomerization and to determine whether HAPLN2 aggregation causally contributes to cognitive or functional decline, or is primarily a biomarker of aging.
Key Findings
- HAPLN2, an ECM protein at nodes of Ranvier, progressively forms sarkosyl-insoluble aggregates in aging mouse brains.
- Rising hyaluronic acid levels and declining microglial phagocytosis together drive age-dependent HAPLN2 accumulation.
- HAPLN2 oligomers directly trigger microglial inflammatory responses in vitro and in vivo.
- Age-associated HAPLN2 aggregation was confirmed in human cerebellar tissue, suggesting translational relevance.
- The cerebellum is identified as a primary site of HAPLN2 aggregation, linked to its high perineuronal net density.
Methodology
Sarkosyl-insoluble fractionation and quantitative mass spectrometry were used to identify age-dependent aggregating proteins in young versus old mouse brains. Functional studies employed recombinant HAPLN2 oligomers applied to cultured microglia and injected in vivo, with validation in human cerebellar post-mortem tissue.
Study Limitations
The study does not directly demonstrate that HAPLN2 aggregation causally impairs cognitive or motor function, leaving its functional consequences incompletely defined. The precise molecular mechanism by which hyaluronic acid drives HAPLN2 oligomerization requires further elucidation. Human tissue analysis was limited to the cerebellum and was correlational rather than mechanistic.
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