Perineuronal net deglycosylation associates with tauopathy-induced gliosis and neurodegeneration

Abstract

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by clinical symptoms of memory and cognitive deficiencies. Postmortem evaluation of AD brain tissue shows proteinopathy that closely associate with the progression of this dementing disorder, including the accumulation of extracellular beta amyloid (Aβ) and intracellular hyperphosphorylated tau (pTau) with neurofibrillary tangles (NFTs). Current therapies targeting Aβ have limited clinical efficacy and life-threatening side effects and highlight the need for alternative treatments targeting pTau and other pathophysiologic mechanisms driving AD pathogenesis. The brain's extracellular matrices (ECM), particularly perineuronal nets (PNNs), play a crucial role in brain functioning and neurocircuit stability, and reorganization of these unique PNN matrices has been associated with the progression of AD and accumulation of pTau in humans. We hypothesize that AD-associated changes in PNNs may in part be driven by the accumulation of pTau within the brain. In this work, we investigated whether the presence of pTau influenced PNN structural integrity and PNN chondroitin sulfate-glycosaminoglycan (CS-GAG) compositional changes in two transgenic mouse models expressing tauopathy-related AD pathology, PS19 (P301S) and Tau4RTg2652 mice. We show that PS19 mice exhibit an age-dependent loss of hippocampal PNN CS-GAGs, but not the underlying aggrecan core protein structures, in association with pTau accumulation, gliosis, and neurodegeneration. The loss of PNN CS-GAGs were linked to shifts in CS-GAG sulfation patterns to favor the neuroregenerative isomer, 2S6S-CS. Conversely, Tau4RTg2652 mice exhibit stable PNN structures and normal CS-GAG isomer composition despite robust pTau accumulation, suggesting a critical interaction between neuronal PNN glycan integrity and neighboring glial cell activation. Overall, our findings provide insights into the complex relationship between PNN CS-GAGs, pTau pathology, gliosis, and neurodegeneration in mouse models of tauopathy, and offer new therapeutic insights and targets for AD treatment.

Keywords: Alzheimer's disease; chondroitin sulfates; extracellular matrix; glycosaminoglycans; perineuronal nets.

Figure 1.. 9m-PS19 mice exhibit loss of WFA⁺ PNNs in the presence of pTau accumulation and gliosis.

9-month-old PS19 mice exhibit A) striking loss of hippocampal WFA⁺ PNNs in association with regional pTau accumulation. 5-region stereology analysis (Bregma −1.4 to −1.8 mm) of B) AT8 (pTau), C) GFAP (astrogliosis), D) WFA (PNN CS-GAGs), E) ACAN (PNN CSPG), and F) Iba1 (microgliosis) were performed and then G) averaged and quantified. Solid arrows: dorsal hippocampus; Open arrow: retrosplenial cortex. Scale bar: A) 1 mm, B-F) 0.5 mm, representative images from male mice, dapi included in all images. Statistics reported in Supplemental Table 1.

Figure 2.. 6m-PS19 mice exhibit stable PNNs in the presence of pTau accumulation without gliosis.

5-region stereology analysis (Bregma −1.4 to −1.8 mm) of 6-month-old PS19 mice exhibit regional accumulation of A) pTau (AT8) in the CA3 (white arrow) without changes in B) astrogliosis (GFAP), C) WFA (PNN CS-GAGs), D) ACAN (PNN CSPG), or E) microgliosis (Iba1). Scale bar: 0.5 mm, representative images from male mice, dapi included in all images. Statistics reported in Supplemental Table 3.

Figure 3.. 4m-Tg2652 mice exhibit stable PNNs in the presence of pTau accumulation without gliosis.

5-region stereology analysis (Bregma −1.4 to −1.8 mm) of 4-month-old Tg2652 mice exhibit robust accumulation of A) pTau (AT8) without B) gliosis (GFAP, astrogliosis) in association with increased C) WFA⁺ PNNs (PNN CS-GAGs) and D) ACAN (PNN CSPG). Scale bar: 0.5 mm, representative images from male mice, dapi included in all images. Statistics reported in Supplemental Table 4.

Figure 4.. 9m-PS19 mice exhibit altered hippocampal CS isomer composition in association with PNN glycan loss, pTau accumulation, and neuroinflammation.

PNN matrices are comprised of A) non-sulfated (0S-CS), mono-sulfated (4S-CS, 6S-CS) and di-sulfated (2S6S-, 4S6S-CS) isomers. 9-month-old PS19 mice exhibit B) decreased mono-sulfated 6S-CS isomer and increased 2S6S-CS isomer compared to controls, which is visualized in C) the illustration representing flux through the 2S6S-CS isomer biosynthesis branch. These isomer changes occur in association with D) loss of WFA (PNN CS-GAGs) and gain of E) AT8 (pTau), F) GFAP (astrogliosis), and G) Iba1 (microgliosis). Statistics reported in Supplemental Tables 5 and 6.

Figure 5.. 9m-PS19 mice exhibit increased neurodegeneration in association with pTau accumulation, gliosis, and altered CS-GAG sulfation patterning.

TUNEL staining in 9-month-old A) wt and age-matched PS19 mice showed selective increase in apoptosis within the B) DG of the dorsal hippocampus that positively colocalized with C) neurons labeled with NeuN. The increase in DG-TUNEL associated with D) pTau (AT8), E) astrogliosis (GFAP), and F) microgliosis (Iba1), but not G) WFA (PNN CS-GAG) loss. DG-TUNEL staining also associated with H) decreased 6S-CS isomer and I) increased 2S6S-CS isomer hippocampal content. Scale bar: A,B) 0.5 mm, a,b) 0.250 mm, representative images from male mice, dapi included in all images. Statistics reported in Supplemental Tables 7–9.