Might SORL1 Bind Tau in Glia, Fuel Its Aggregation?

Over the last few decades, evidence has emerged to peg the sortilin-related receptor, SORL1, as a risk factor for Alzheimer’s disease and even as the fourth gene for familial AD. SORL1 helps traffic amyloid precursor protein through the endolysosomal system, where delays can increase the production of Aβ. Could the receptor be involved in tau pathology, too? Yes, say scientists led by Bradley Hyman at Massachusetts General Hospital in Charlestown and Dudley Strickland of the University of Maryland, Baltimore. In the April 22 Journal of Biological Chemistry, they reported that, in vitro, human recombinant SORL1 bound tau and that the two co-localized in neuroglioma cells. What’s more, SORL1 seems to have a hand in tau seeding because knocking the receptor down reduced tau aggregation, while expressing the N1358S variant increased it. This might explain why this mutation would increase the risk for AD. Scientists have been unsure how, or if, the variant affects SORL1 function.

  • In surface plasmon resonance experiments, tau binds SORL1.
  • In neuroglioma cells, the two co-localize.
  • Knocking down SORL1 limits tau seeding; overexpressing SORL1 exacerbates it.
  • So does the SORL1 N1358S risk variant.

Kenneth Kosik at the University of California, Santa Barbara, called this a well-executed study. “In essence, the problem we face is that, internalized via LRP1, through which it enters endosomes, tau must then escape the endosome before it can assemble into cytoplasmic neurofibrillary tangles. This new discovery offers evidence that tau is handed off to SORL1 inside the cell … and most importantly, that this interaction promotes seeding,” he wrote (comment below).

A transmembrane protein, SORL1 primarily resides on endosomes. It shuttles cargo, such as amyloid precursor protein, from the cell surface, though only 10 percent of total SORL1 associates with the cell membrane at any one time (Jul 2023 news; Jacobsen et al., 2001). Since the receptor contains LDL ligand-binding domains similar to those in LDL receptor-related protein 1 (LRP1), a proposed cell-surface tau receptor, the authors wondered if SORL1 binds tau, too (Mar 2020 news; Cooper et al., 2021)

To find out, co-first authors Joanna Cooper at U Maryland and Aurelien Lathuiliere of Mass General turned to surface plasmon resonance spectroscopy, which measures the interaction between one immobilized biomolecule on a thin surface and another by detecting changes in light bouncing off that surface. They found recombinant human SORL1 latched tightly to recombinant human tau with a dissociation constant of 59 nM.

To see if the two bound in cells, Cooper and Lathuiliere tracked radiolabeled tau in H4 neuroglioma cells, which express both LRP1 and SORL1. Adding an anti-LRP1 antibody to the medium blocked most tau uptake into the cells, whereas knocking down SORL1 had no effect. Still, 17 percent of fluorescently labeled tau co-localized with SORL1, as seen using confocal microscopy, indicating that the two bind once tau is inside the cell (image below).

Curious if this binding happens within endosomes, the scientists tested it at pH 5.5, the acidity of those organelles. Indeed, in plasmon resonance experiments, they bound as tightly at pH 5.5 as at pH 7.4. “We think that tau dissociates from LRP1 at endosomal pH (~pH 5.5) and is picked up by SORL1,” Strickland told Alzforum. Strickland said they are now mapping the co-localization using markers for various organelles.

Gregory Petsko of Brigham and Women’s Hospital in Boston had a different interpretation. Since tau is not an integral membrane protein, he thinks that SORL1 must meet tau at the cell surface where the pH is more neutral. “It is that SORL1 binds tau at pH 7.4 that is the important finding,” he wrote (comment below).

SORL1 Meets Tau. In a neuroglioma cell, SORL1 (turquoise) co-localized (white) with tau (purple). [Courtesy of Cooper et al., Journal of Biological Chemistry, 2024.]

For his part, Marc Diamond, UT Southwestern, Dallas, also wondered if the binding takes place in the endosome, questioning the specificity of the co-localization data. “If you co-stain for LDL receptors, or a dozen related receptors, how often would they co-localize with tau that has been taken up?” he asked. “I would want to see other transmembrane receptors that could potentially bind tau as negative controls,” he added (comment below).

How would binding to SORL1 fit with tau biology? To see if the liaison affected tau aggregation, Cooper and Lathuiliere studied seeding in reporter cell lines expressing fluorescent P301S tau (Oct 2014 news). In cells overexpressing SORL1, tau extracted from AD brain tissue seeded 13 times more tau aggregates than in control cells. In contrast, silencing SORL1 in neuroglioma cells expressing the same fluorescent P301S tau biosensor halved tau aggregation. The authors concluded that SORL1 encourages tau seeding.

Next, the scientists turned their attention to the SORL1 mutant N1358S, which was found in a French family with a history of autosomal dominant AD but with no mutations in APP or presenilins 1 or 2.

HEK cells overexpressing N1358S SORL1 did not take in any more tau than cells expressing the wild-type receptor. However, they seeded 1.5-fold more tau fibrils. The point mutation lies in the LDL ligand binding region, one that is also found in LRP1, suggesting that the domain is a binding site for tau. The authors believe that N1358S might weaken that binding—though they did not test it directly with surface plasmon resonance—possibly allowing the protein to escape the endosome and aggregate (image below). To the authors, this suggests that N1358S might be pathogenic.

Petsko was not convinced. “The clinical pathogenicity of this variant is not well-established,” he wrote, noting that the asparagine to serine switches are generally benign. Indeed, only one of three prediction algorithms pegged N1358S as pathogenic (Apr 2012 news; Nov 2022 news).

Sordid Model. Tau (red, left), taken into cells via LRP1, is shunted to SORL1 in endosomes or degraded by lysosomes. In AD, pathogenic forms of tau (right) also enter cells through LRP1 but escape degradation and slip from SORL1’s grip into the cytosol, where they aggregate. The N1358S variant exacerbates this process. [Courtesy of Cooper et al., Journal of Biological Chemistry, 2024.]

All told, dysfunctional SORL1 might be a double-edged sword, releasing tau into the cytoplasm where it might aggregate, while retaining APP in endosomes where it can be processed to release Aβ, as reported previously. “This increases the attractiveness of SORL1 as a potential drug target,” wrote Sam Gandy of the Icahn School of Medicine, New York (comment below). “It will be interesting to see whether the pharmacological chaperones that stabilize SORL1 and modulate amyloidogenesis have a beneficial, neutral, or detrimental effect on SORL1 promotion of tau seeding,” he added.

One such chaperone, called R55, purportedly binds to and stabilizes the retromer complex, which maintains SORL1 trafficking of APP (Apr 2014 news). Petsko and Scott Small of Columbia University in New York, who co-developed R55, told Alzforum that it has not been tested in the clinic because it does not bind tightly enough to the retromer and because it is unstable in vivo. “I think that it would be interesting to test to see whether R55 modulates tau seeding even if it is not the world’s best lead compound,” said Gandy.—Chelsea Weidman Burke

Comments

    • User Profile Image Kenneth Kosik
      University of California, Santa Barbara
    • Posted:

    This paper by J.M. Cooper et al. is a well-executed study that advances our thinking about the route tau travels once internalized. In essence, the problem we face is that, internalized via LRP1 through which it enters endosomes, tau must escape the endosome before it assembles into cytoplasmic neurofibrillary tangles. This new discovery offers evidence that tau is handed off to SORL1 inside the cell, mediated by the reduced pH of endosomes and, most importantly, this interaction promotes seeding in relation to this second, endosomal compartment. Interestingly, unlike the interaction between LRP1 and tau, which is lysine-dependent, the SORL1 interaction is not.

    The future direction inferred from this work is a detailed in silico analysis of the physico-chemical interaction between tau and SORL1 that addresses the likely possibility that the interaction lowers the energy barrier to tau fibrillization. With multiple tau-binding sites on SORL1, molecular dynamic studies will be challenging, but the many advances in modeling that utilize generative models, such as RFdiffusion (Watson et al., 2023) and related simulations for binding studies, will be highly informative.

    References:

    Watson JL, Juergens D, Bennett NR, Trippe BL, Yim J, Eisenach HE, Ahern W, Borst AJ, Ragotte RJ, Milles LF, Wicky BI, Hanikel N, Pellock SJ, Courbet A, Sheffler W, Wang J, Venkatesh P, Sappington I, Torres SV, Lauko A, De Bortoli V, Mathieu E, Ovchinnikov S, Barzilay R, Jaakkola TS, DiMaio F, Baek M, Baker D. De novo design of protein structure and function with RFdiffusion. Nature. 2023 Jul 11; PubMed.

    View all comments by Kenneth Kosik
    • User Profile Image Gregory Petsko
      Harvard Medical School and Brigham&Women's Hospital
    • Posted:

    The paper is interesting and potentially important. That retromer-regulated trafficking is important in tauopathies, including Alzheimer’s disease, is well-established (e.g., Mishra et al., 2023). However, the direct binding of tau to SORL1 is a new finding, though I wish they had included primary cortical neurons as well as—or instead of—H4 neuroglioma and CHO cells. Nevertheless, this adds to our understanding of the role that SORL1-retromers may play in neurodegenerative diseases, where pathological aggregation of tau is a phenotype.

    I am less certain of the relevance of these findings to the mechanism of pathogenesis of SORL1 variants. The authors chose to look at two variants: G511R and N1358S. They found no differences in tau uptake or seeding with expression of G511R mutant SORL1, despite the fact that this mutation is known to disrupt peptide cargo binding to the VPS10 domain of the protein. And their choice of N1358S is a curious one. I disagree with their statement that “In silico predictions suggest that the N1358S variant is likely to have a pathogenic effect.” On structural considerations, mutation of an asparagine to serine is generally considered a relatively benign change. And the clinical pathogenicity of this variant is not well-established. There is only a single report of this mutation in one French family, and though three members of the family are stated to have dementia, only two are genotyped. Alzforum’s SORL1 mutation database reports that, in a study that included 15,808 Alzheimer’s cases and 16,097 control subjects from multiple European and American cohorts, N1358S was observed only once among the AD cases (Holstege et al., 2022), and in another study investigating the effects of SORL1 missense mutations on protein processing, the N1358S variant did not affect the maturation (glycosylation) of SORL1 overexpressed in HEK293 cells (Rovelet-Lecrux et al., 2021). I worry that this variant may not be pathogenic.

    There also might be a problem with the logic here: Most pathogenic SORL1 missense variants reduce the expression of SORL1 on the cell surface (e.g., Fazeli et al., 2024). This should lead to less internalization of tau and reduced seeding, not more, in contrast to what they observe for N1358S. Multiple, more clearly pathogenic variants—with established effects on SORL1’s biochemical properties and secretion—need to be examined for their effects on tau internalization and seeding before the clinical relevance of the author’s findings can be determined.

    If anything, I think the authors underemphasized the importance of their observation that SORL1 bound tau avidly at both pH 7.4 and 5.5. They stressed that, because of this, binding can occur in the endosome, unlike the case for LRP receptors. But that is not what’s most interesting here. That SORL1 binds tau at pH 5.5 is completely expected. SORL1 is different from the LRPs: it binds its cargo proteins, such as APP, much tighter at the low pH of the endosome than at the neutral pH of the extracellular environment because it needs to carry them back to the cell surface for recycling, where they will be released (Mehmedbasic et al., 2015). That works for the type 1 integral membrane proteins that form its usual cargo, because such proteins, as with SORL1, will be endocytosed and the two will therefore encounter each other in the endosome. But tau is not an integral membrane protein. For it to be a cargo for SORL1, tau must encounter the SORL1 ectodomain at the cell surface, so that SORL1 can facilitate its endocytosis. That SORL1 binds tau at pH 7.4 is the important finding.

    References:

    Mishra S, Knupp A, Kinoshita C, Williams CA, Rose SE, Martinez R, Theofilas P, Young JE. Pharmacologic enhancement of retromer rescues endosomal pathology induced by defects in the Alzheimer's gene SORL1. Stem Cell Reports. 2023 Dec 12;18(12):2434-2450. Epub 2023 Nov 9 PubMed.

    Holstege H, Hulsman M, Charbonnier C, Grenier-Boley B, Quenez O, Grozeva D, van Rooij JG, Sims R, Ahmad S, Amin N, Norsworthy PJ, Dols-Icardo O, Hummerich H, Kawalia A, Amouyel P, Beecham GW, Berr C, Bis JC, Boland A, Bossù P, Bouwman F, Bras J, Campion D, Cochran JN, Daniele A, Dartigues JF, Debette S, Deleuze JF, Denning N, DeStefano AL, Farrer LA, Fernández MV, Fox NC, Galimberti D, Genin E, Gille JJ, Le Guen Y, Guerreiro R, Haines JL, Holmes C, Ikram MA, Ikram MK, Jansen IE, Kraaij R, Lathrop M, Lemstra AW, Lleó A, Luckcuck L, Mannens MM, Marshall R, Martin ER, Masullo C, Mayeux R, Mecocci P, Meggy A, Mol MO, Morgan K, Myers RM, Nacmias B, Naj AC, Napolioni V, Pasquier F, Pastor P, Pericak-Vance MA, Raybould R, Redon R, Reinders MJ, Richard AC, Riedel-Heller SG, Rivadeneira F, Rousseau S, Ryan NS, Saad S, Sanchez-Juan P, Schellenberg GD, Scheltens P, Schott JM, Seripa D, Seshadri S, Sie D, Sistermans EA, Sorbi S, van Spaendonk R, Spalletta G, Tesi N, Tijms B, Uitterlinden AG, van der Lee SJ, Visser PJ, Wagner M, Wallon D, Wang LS, Zarea A, Clarimon J, van Swieten JC, Greicius MD, Yokoyama JS, Cruchaga C, Hardy J, Ramirez A, Mead S, van der Flier WM, van Duijn CM, Williams J, Nicolas G, Bellenguez C, Lambert JC. Exome sequencing identifies rare damaging variants in ATP8B4 and ABCA1 as risk factors for Alzheimer's disease. Nat Genet. 2022 Dec;54(12):1786-1794. Epub 2022 Nov 21 PubMed.

    Rovelet-Lecrux A, Feuillette S, Miguel L, Schramm C, Pernet S, Quenez O, Ségalas-Milazzo I, Guilhaudis L, Rousseau S, Riou G, Frébourg T, Campion D, Nicolas G, Lecourtois M. Impaired SorLA maturation and trafficking as a new mechanism for SORL1 missense variants in Alzheimer disease. Acta Neuropathol Commun. 2021 Dec 18;9(1):196. PubMed.

    Fazeli E, Child DD, Bucks SA, Stovarsky M, Edwards G, Rose SE, Yu CE, Latimer C, Kitago Y, Bird T, Jayadev S, Andersen OM, Young JE. A familial missense variant in the Alzheimer's disease gene SORL1 impairs its maturation and endosomal sorting. Acta Neuropathol. 2024 Jan 20;147(1):20. PubMed.

    Mehmedbasic A, Christensen SK, Nilsson J, Rüetschi U, Gustafsen C, Poulsen AS, Rasmussen RW, Fjorback AN, Larson G, Andersen OM. SorLA complement-type repeat domains protect the amyloid precursor protein against processing. J Biol Chem. 2015 Feb 6;290(6):3359-76. Epub 2014 Dec 18 PubMed.

    View all comments by Gregory Petsko
    • User Profile Image Marc Diamond
      University of Texas, Southwestern Medical Center
    • Posted:

    The take-home message for me is that any assertion of a single protein “receptor” for tau at the surface should be taken with a grain of salt. This paper purports to show that SORL1 is a “receptor” for tau. However, one of the final figures indicates that only about ~18 percent of SORL1 positive endosomes are positive for tau, in other words, the vast majority do not show co-localization with tau that has been taken up. I would like to see other transmembrane receptors that could potentially bind tau used as negative controls. For example, if you co-stain for LDL receptor, or a dozen related receptors, how often would they co-localize with tau that has been taken up? A lot of negative controls are needed here and in prior studies.

    Here the authors also use an inhibitory peptide, RAP, for SORL1 that was supposedly a “specific” inhibitor of LRP1. I am probably missing something here, but doesn’t this data contradict prior work on LRP1? I would say proper controls are missing in order to interpret specificity. This also calls into question the prior work by Ken Kosik and Jennifer Rauch that LRP1 is a specific tau receptor.

    In summary, I would say that we still don’t know if there is a specific protein on the cell surface that is a tau “receptor,” in a meaningful sense of the word, i.e., that there is a specific interacting protein as there is for, say, transferrin.

    View all comments by Marc Diamond
    • User Profile Image Sam Gandy
      Icahn School of Medicine at Mount Sinai
    • Posted:

    SORL1 is best known as a modulator of APP sorting, metabolism, and amyloidogenesis. This new data extends the action of SORL1 to tauopathy, providing an unexpected convergence of two key pathways in Alzheimer pathogenesis. Tau aggregates form in neurons and synapses and transmit tauopathy to neighboring cells by a microglial microvesicle system. In this way, tauopathy spreads to anatomically connected brain regions by prion-like mechanisms. Soluble tau aggregates (tau oligomers) are the most toxic species that initiate neurodegeneration in tauopathies. Tau oligomers are internalized by brain cells, but the precise cellular and molecular mechanisms that underlie the internalization of tau oligomers have remained elusive.

    Various pathways for neuronal internalization of tau oligomers have been described, including a heparan sulfate proteoglycan-mediated pathway, a clathrin-mediated pathway, and a caveolae-mediated pathway. Yet, a comprehensive understanding of the cell-surface receptor(s) for tau oligomers remained unknown until now. Using surface plasmon resonance measurements, Cooper et al. observed high-affinity binding of tau to the vacuolar protein sorting 10 (VPS10) domain of SORL1. This interaction was exacerbated by a pathogenic mutation in SORL1. The N1358S mutant significantly increased tau seeding when compared to WT SORL1, identifying for the first time a potential mechanism that connects this specific SORL1 mutation to Alzheimer’s disease. This connection is not via the usual SORL1 modulation of amyloidogenesis, but through the interaction of SORL1 with tau.

    Together, SORL1 studies identify it as a receptor that contributes to both the trafficking of APP and the seeding of pathogenic tau. This increases the attractiveness of SORL1 as a potential drug target, a concept pioneered by the pharmacological chaperones developed by Scott Small and Gregory Petsko. It will be interesting to see whether the pharmacological chaperones that stabilize SORL1 and modulate amyloidogenesis have a beneficial, neutral, or detrimental effect on SORL1 promotion of tau seeding.

    View all comments by Sam Gandy

  5. This is an interesting mechanistic study by Cooper et al., looking at the interactions between tau and SORL1, a protein with known associations to AD. This new study expands on the authors’ prior biochemical work looking at the tau-LRP1 interaction and it helps to build on our picture of tau protein regulation in the endolysosomal system.

    One of the interesting findings is the lack of pH regulation of tau-SORL1 binding, which contrasts with binding to the endocytic receptor LRP1. The work provides a mechanistic picture of how these proteins could contribute to tau processing, and it gives us further evidence that there are likely many proteins that are critical for tau handling in the cell.

    Another significant highlight from this work is the importance of cell type when studying tau neurobiology. The authors see a difference in SORL1 function in CHO versus H4 neuroblastoma cells. We should take care not to overinterpret results that come from only one model system and, ideally, test hypotheses in different cellular conditions. The inherent complexity of tau neurobiology, including isoform composition, different forms of aggregates, post-translational modifications, etc., suggests that there are likely many mechanisms that can contribute to disease pathology. Hopefully, by leveraging our understanding of human patient genetics, coupled with more careful mechanistic work, such as in this paper, we can begin to define these pathways more clearly.  

    View all comments by Jennifer Rauch

  6. Cooper et al. report interesting findings consistent with the notion that SORL1 may operate as an intracellular receptor for internalized tau, in a process that modulates tau seeding and may thus be relevant for tau spreading in AD. While there is mounting evidence indicating that tau is primarily internalized via LRP1, tau binding appears to be pH-dependent in this case and thus the acidic environment of endosomes may allow it to detach from LRP1, reminiscent of how low-density lipoprotein (LDL) detaches from its receptor in the endocytic compartment. However, the SORL1-tau interaction is largely pH-independent based on surface plasmon resonance analyses presented in this study and thus it is conceivable that tau released by LRP1 may interact with SORL1, but only in the mildly acidic endosomal compartment.

    Interestingly, both loss of SORL1 and expression of a disease-linked SORL1 mutant, N1358S, promote tau seeding in non-neuronal cell lines, suggesting potential disease relevance of this interaction, perhaps via modulation of endosomal or lysosomal escape. However, the specific mechanisms mediating this seeding remain to be discovered.

    Overall, this study proposes a new hypothesis on the disease mechanisms driven by SORL1 and one that does not appear to be directly related to amyloid. It will be important to replicate these interesting findings in relevant CNS models, such as iPSC-derived human neurons, and validate them in preclinical models of tau spreading. For this purpose, utilizing genetic backgrounds with SORL1 loss of function and expression of disease mutations, such as N1358S, will be critical. Finally, since SORL1 trafficking and processing has been established to be largely retromer-dependent, it will be important to understand whether retromer deficiency affects tau seeding via mechanisms that involve SORL1 or alternate mechanisms. The recent study from Li Gan and colleagues identifying retromer components as modifiers of tau propagation in iPSC-derived neurons expressing the FTD-linked P301S MAPT mutation in a 4R tau background certainly provide strong incentive to pursue this line of investigation (Parra Bravo et al., 2024).

    References:

    Parra Bravo C, Giani AM, Madero-Perez J, Zhao Z, Wan Y, Samelson AJ, Wong MY, Evangelisti A, Cordes E, Fan L, Ye P, Zhu D, Pozner T, Mercedes M, Patel T, Yarahmady A, Carling GK, Sterky FH, Lee VM, Lee EB, DeTure M, Dickson DW, Sharma M, Mok SA, Luo W, Zhao M, Kampmann M, Gong S, Gan L. Human iPSC 4R tauopathy model uncovers modifiers of tau propagation. Cell. 2024 May 9;187(10):2446-2464.e22. Epub 2024 Apr 5 PubMed.

    View all comments by Gilbert Di Paolo

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References

Mutation Interactive Images Citations

  1. SORL1

News Citations

  1. When Missense Variants Derail SORL1 Traffic, Destination Is Dementia
  2. Tau Receptor Identified on Cell Surface
  3. Cellular Biosensor Detects Tau Seeds Long Before They Sprout Pathology
  4. New Genetic Insights Into AD: SORL1 and Natural Selection
  5. Rare Variants in Lipid Transporter Genes Increase Risk for Alzheimer’s Disease
  6. Could Bolstering the Retromer Thwart Alzheimer’s?

Mutations Citations

  1. SORL1 N1358S

Paper Citations

  1. Jacobsen L, Madsen P, Jacobsen C, Nielsen MS, Gliemann J, Petersen CM. Activation and functional characterization of the mosaic receptor SorLA/LR11. J Biol Chem. 2001 Jun 22;276(25):22788-96. Epub 2001 Apr 9 PubMed.
  2. Cooper JM, Lathuiliere A, Migliorini M, Arai AL, Wani MM, Dujardin S, Muratoglu SC, Hyman BT, Strickland DK. Regulation of tau internalization, degradation, and seeding by LRP1 reveals multiple pathways for tau catabolism. J Biol Chem. 2021 Jan-Jun;296:100715. Epub 2021 Apr 28 PubMed.

Further Reading

Primary Papers

  1. Cooper JM, Lathuiliere A, Su EJ, Song Y, Torrente D, Jo Y, Weinrich N, Sales JD, Migliorini M, Sisson TH, Lawrence DA, Hyman BT, Strickland DK. SORL1 is a receptor for tau that promotes tau seeding. J Biol Chem. 2024 Jun;300(6):107313. Epub 2024 Apr 23 PubMed.

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