Tau aggregation drives tauopathies, but what orchestrates the aggregation? The synaptic protein Bassoon plays its part, according to scientists led by Cristian Lasagna-Reeves at the Indiana University School of Medicine, Indianapolis. In the November 7 Nature Neuroscience, they reported that complexes of Bassoon and tau oligomers seed new tangles. In a mouse model of tauopathy, knocking down the synaptic protein halted tau spreading and prevented gliosis and neurodegeneration. These findings suggest that quieting Bassoon-tau interactions may quell pathological tau seeding.
- Bassoon latches onto tau, encouraging its propagation.
- Without the synaptic protein, mice developed fewer tangles and no gliosis or neurodegeneration.
- Tau seeds from AD and PSP brain tissue imbued with Bassoon.
“This interesting paper provides compelling evidence that Bassoon interacts with tau seeds and influences tau pathology and neurotoxicity,” wrote Kaoru Yamada and Takeshi Iwatsubo, University of Tokyo.
Nicholas Seyfried of Emory University in Atlanta expressed similar sentiments. “A main takeaway is that Bassoon, and potentially other presynaptic scaffolding proteins, enhances tau seeding and toxicity in tauopathies,” he wrote (comments below).
To search for proteins bound to pathological tau, first author Pablo Martinez isolated tau seeds from 3-month-old mice expressing a human tau isoform with the P301S mutation. In these animals, tau seeding begins as early as 6 weeks, yet neurofibrillary tangles don’t bloom until 6 months. Martinez centrifuged brain tissue homogenates, then separated large complexes of tau from the supernatant using size exclusion chromatography, testing fractions in a cell-based assay for tau seeding activity. Seeds accounted for about 5 percent of the total tau.
From the fraction with the most seeds, the scientists immunoprecipitated tau and identified other proteins that came along for the ride using mass spectrometry. About 800 proteins clung to tau seeds. These were not found in fractions that only contained tau monomers. Many were synaptic proteins, echoing previous data suggesting that synapses act as routes for the spread of pathological tau (Feb 2012 news; Jun 2016 news; Jan 2020 news).
To prioritize these hits, Martinez searched to see if any were documented among the 600 potential therapeutic targets for Alzheimer’s disease listed on the Accelerating Medicines Partnership in Alzheimer's Disease (AMP-AD) consortium’s knowledge portal. Bassoon caught the researchers’ attention. This presynaptic scaffolding protein has ties to tauopathies. Missense mutations in the BSN gene occur in people with a progressive supranuclear palsy-like disease, and expression of the gene, also known as ZNF231, increases in multiple-system atrophy, a glial synucleinopathy that can have concomitant tau inclusions (Yabe et al., 2018; Hashida et al., 1998; Nagaishi et al., 2011). Bassoon also accumulates within neurons during multiple sclerosis, a demyelinating disorder in which intraneuronal phospho-tau aggregates have also been spotted (Schattling et al., 2019; LaCroix et al., 2022).
Does Bassoon bind tau in the mouse brain? In cortical slices from the P301S mice, Bassoon and PHF1-positive phospho-tau co-localized within presynapses, suggesting close proximity. To confirm tau and Bassoon’s interaction in cells, the scientists used a fluorescence-based proximity ligation assay. In cultured kidney cells, Bassoon and P301S tau, but not wild-type tau, bound each other (see image below).
Extracts from kidney cells expressing Bassoon and P301S tau also seeded more tau tangles in a cell-based assay than those without the protein. Moreover, the eyes of fruit flies overexpressing P301L tau and Bassoon degenerated, a sign of neurodegeneration in these animals. The damage worsened if the flies expressed one of the BSN variants linked to PSP.
What about the human brain? The scientists saw Bassoon entangled with tau in brain tissue from six people who had had AD, and six who had had PSP. All samples were from the Brain Research Center at Johns Hopkins University. Bassoon and tau co-immunoprecipitated from brain homogenate fractions that seeded the most tangles, and the proteins co-localized in tissue slices.
All told, the data suggest that bassoon might hasten the spread of tau toxicity by ushering the protein through synapses. Would downregulating Bassoon slow down the spread of tau then? To find out, the researchers knocked down the BSN gene in the brains of newborn P301S mice. By 4 months old, Bassoon expression was down 80 percent. Compared to mock knockdowns, BSN knockdowns had 50 to 75 percent fewer p-tau aggregates, as measured by PHF-1 and pThr231, and half as many MC1-positive tau tangles (see image below).
Better Without Bassoon. Compared to P301S mice (top), those with Bassoon knocked down (bottom) had fewer tangles (green, left), sparse reactive microglia (green, middle), and fewer reactive astrocytes (red) in the hippocampus. [Courtesy of Martinez et al., Nature Neuroscience, 2022.]
Knocking down Bassoon alleviated other pathologies too. P301S mice develop gliosis and, as their neurons degenerate, they lose grip strength. However, BSN knockdowns showed no signs of hippocampal atrophy, held onto a rod as well as did wild-type mice, and had no more reactive Iba1-positive microglia and GFAP-positive astrocytes than controls.
All told, downregulating Bassoon expression reduced tauopathy, gliosis, and tau-driven neurodegeneration. Lasagna-Reeves is working with other Indiana U researchers through the TREAT-AD consortium to find small interfering RNA drugs to knock down Bassoon.
Do any of the other 799 proteins that immunoprecipitated with tau seeds exacerbate tauopathies also? Some researchers were concerned about non-specific binding since tau itself was not among the 100 most abundant proteins in the seed fractions. Still, Lasagna-Reeves will characterize some of these other proteins in tauopathy models as he did with Bassoon. He is also identifying the interactomes of the tau seeds isolated from the AD and PSP tissue.—Chelsea Weidman Burke
Research Models Citations
- Mice Tell Tale of Tau Transmission, Alzheimer’s Progression
- Excited Neurons Release More Aberrant Tau
- Connectivity, Not Proximity, Predicts Tau Spread
- Yabe I, Yaguchi H, Kato Y, Miki Y, Takahashi H, Tanikawa S, Shirai S, Takahashi I, Kimura M, Hama Y, Matsushima M, Fujioka S, Kano T, Watanabe M, Nakagawa S, Kunieda Y, Ikeda Y, Hasegawa M, Nishihara H, Ohtsuka T, Tanaka S, Tsuboi Y, Hatakeyama S, Wakabayashi K, Sasaki H. Mutations in bassoon in individuals with familial and sporadic progressive supranuclear palsy-like syndrome. Sci Rep. 2018 Jan 16;8(1):819. PubMed.
- Hashida H, Goto J, Zhao N, Takahashi N, Hirai M, Kanazawa I, Sakaki Y. Cloning and mapping of ZNF231, a novel brain-specific gene encoding neuronal double zinc finger protein whose expression is enhanced in a neurodegenerative disorder, multiple system atrophy (MSA). Genomics. 1998 Nov 15;54(1):50-8. PubMed.
- Nagaishi M, Yokoo H, Nakazato Y. Tau-positive glial cytoplasmic granules in multiple system atrophy. Neuropathology. 2011 Jun;31(3):299-305. PubMed.
- Schattling B, Engler JB, Volkmann C, Rothammer N, Woo MS, Petersen M, Winkler I, Kaufmann M, Rosenkranz SC, Fejtova A, Thomas U, Bose A, Bauer S, Träger S, Miller KK, Brück W, Duncan KE, Salinas G, Soba P, Gundelfinger ED, Merkler D, Friese MA. Bassoon proteinopathy drives neurodegeneration in multiple sclerosis. Nat Neurosci. 2019 Jun;22(6):887-896. Epub 2019 Apr 22 PubMed.
- LaCroix MS, Mirbaha H, Shang P, Zandee S, Foong C, Prat A, White CL 3rd, Stuve O, Diamond MI. Tau seeding in cases of multiple sclerosis. Acta Neuropathol Commun. 2022 Oct 11;10(1):146. PubMed.
- Martinez P, Patel H, You Y, Jury N, Perkins A, Lee-Gosselin A, Taylor X, You Y, Viana Di Prisco G, Huang X, Dutta S, Wijeratne AB, Redding-Ochoa J, Shahid SS, Codocedo JF, Min S, Landreth GE, Mosley AL, Wu YC, McKinzie DL, Rochet JC, Zhang J, Atwood BK, Troncoso J, Lasagna-Reeves CA. Bassoon contributes to tau-seed propagation and neurotoxicity. Nat Neurosci. 2022 Dec;25(12):1597-1607. Epub 2022 Nov 7 PubMed.