Tracy TE, Madero-Pérez J, Swaney DL, Chang TS, Moritz M, Konrad C, Ward ME, Stevenson E, Hüttenhain R, Kauwe G, Mercedes M, Sweetland-Martin L, Chen X, Mok SA, Wong MY, Telpoukhovskaia M, Min SW, Wang C, Sohn PD, Martin J, Zhou Y, Luo W, Trojanowski JQ, Lee VM, Gong S, Manfredi G, Coppola G, Krogan NJ, Geschwind DH, Gan L. Tau interactome maps synaptic and mitochondrial processes associated with neurodegeneration. Cell. 2022 Feb 17;185(4):712-728.e14. Epub 2022 Jan 20 PubMed.

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  1. The authors are to be commended for a beautifully presented study that should serve as a useful resource in the years to come. The study showcases an elegant application of the APEX method to map binding partners of the 2N4R tau isoform in an in vitro human neuron paradigm, in the presence or absence of KCl stimulation. It nicely demonstrates both the power and limitations of this method, and how it can be paired with follow-on orthogonal techniques to reveal meaningful insights.

    Naturally, with this approach, its strength in mapping the spatial relationship of proteins also is its predominant weakness, because proteins that reside in proximity do not have to work together, or even influence each other. It therefore will still require a major effort to understand which of the proteins that reside in proximity to tau can truly be considered interactors before we can make biological sense of these data.

    The authors have begun this process by complementing their APEX data with affinity capture mass spectrometry analyses for a subset of their analyses, exploiting the inclusion of terminal FLAG-tags on their APEX-tau fusion constructs, and by showing that a subset of shortlisted candidate tau interactors have altered levels in human postmortem brains from individuals who succumbed to tauopathies.

    I was excited to find that the study corroborated several earlier findings, including, from our own lab, the prominent association of tau with the ribonucleo-proteome. I look forward to digging deeper into the data files to see what else they may tell us about tau candidate interactors.

    I would be remiss if I was not to leave a suggestion here for the authors that, in my mind, would greatly facilitate the task of integrating and comparing results from this and prior studies, i.e., to add to the Supplemental Data files the relative enrichment levels of the proteins that were observed in proximity to, or were co-purifying with, tau (in addition to the binary yes/no labels currently shown). An obvious next step would be to apply these cutting-edge methods to an in vivo paradigm, preferably based on a model that recapitulates more closely the tau isoform balance observed in human brains.

    View all comments by Gerold Schmitt-Ulms

  2. This is an interesting and comprehensive analysis of the tau protein interaction network. The study identifies many cytoskeletal protein interactors, which have been previously noted to interact with tau.

    The group also identified a large network of proteins associated with vesicles and the presynaptic SNARE complex, which are part of the tau protein interaction network (PIN). This observation is interesting because tau is well-known to be secreted, which enables propagation of tau in disease. The identification of this tau synaptic/vesicular PIN potentially provides some targets for therapeutic intervention to reduce such propagation.

    This group includes multiple proteins involved in active zone docking, including DNM1, RABGAP3, Syntaxin 3, MINT1, etc. It’s interesting to note that N- and C-terminal APEX tags had about 55 percent overlap, which fits with prior results showing effects of chimeric tags producing varied results depending on whether they are on the N- or C-terminus.

    The authors also examined a large component of mitochondrial proteins as part of the tau PIN; this group includes multiple members of the inner mitochondrial membrane. I find the presence of proteins of the inner mitochondrial membrane surprising, because tau is not a mitochondrial protein; this suggests to me that tau is involved in trafficking these proteins from the nucleus to the mitochondria. The association of TOMM40 in this group is interesting because it is produced from a gene in the tau locus. Thus, the co-localization of these two genes might be functional. The authors find that many of these interactions decrease with pathology in human brain, which is consistent with a loss of the basal functioning of tau as it fibrillizes.

    A weakness of the study is that it omits any discussion of the actions of tau under conditions of stress or fibrillization. The iPSCs used in this study were not subject to any type of stress, and only stimulated with KCl. It is known that stress causes tau to become phosphorylated by proline-directed kinases, which causes dissociation of tau from microtubules and its oligomerization. The actions of pathologically phosphorylated or oligomeric tau are thought to be essential to its pathophysiology. This manuscript distinctly omits any discussion of these interactions.

    Interactions that involve RNA and DNA metabolism as well as the ribosome are presented in passing (e.g., Fig. 2 and 4) but are not discussed, even though these appear increased in the mutant tau (V337M and P301L) paradigm and are known to dramatically change with disease and contribute strongly to the pathophysiology of tauopathies (Jiang et al., 2021; Koren et al., 2019). 

    Despite the lack of evaluation of tau biology during the stress response, this study provides insight into basal functioning of tau protein and uses a powerful proteomic tool that strongly validates much of the work done by other groups previously. It is a solid manuscript that certainly represents an important addition to the literature on tau PINs.

    References:

    Jiang L, Lin W, Zhang C, Ash PE, Verma M, Kwan J, van Vliet E, Yang Z, Cruz AL, Boudeau S, Maziuk BF, Lei S, Song J, Alvarez VE, Hovde S, Abisambra JF, Kuo MH, Kanaan N, Murray ME, Crary JF, Zhao J, Cheng JX, Petrucelli L, Li H, Emili A, Wolozin B. Interaction of tau with HNRNPA2B1 and N6-methyladenosine RNA mediates the progression of tauopathy. Mol Cell. 2021 Oct 21;81(20):4209-4227.e12. Epub 2021 Aug 27 PubMed.

    Koren SA, Hamm MJ, Meier SE, Weiss BE, Nation GK, Chishti EA, Arango JP, Chen J, Zhu H, Blalock EM, Abisambra JF. Tau drives translational selectivity by interacting with ribosomal proteins. Acta Neuropathol. 2019 Apr;137(4):571-583. Epub 2019 Feb 13 PubMed.

    View all comments by Benjamin Wolozin

  3. This very interesting work led by Dr. Li Gan and a team of collaborators creatively used proximity labeling and pull-down experiments in human iPSC-derived neurons to map tau interactomes with high spatiotemporal resolution under different conditions. The Gan lab has done pioneering work on the role of tau acetylation in disease, and the current work identifies additional potential disease mechanisms and targets for future therapy development.

    The first part of this study used peroxidase-based proximity labeling and interactome mapping via mass spectrometry to identify neuronal activity-dependent changes in the tau interactome. This method allows for a high degree of spatiotemporal resolution. Tagging tau on either the N- or C-terminus identified a surprisingly large percentage of unique interactors. A wealth of additional spatial information comes from the enrichment for biotin-modified peptides, which made it possible to not only identify proximal proteins but also map proximal biotinylated tyrosine residues at high resolution. One can envision how this discovery platform can be used to map and compare interactomes under a variety of different conditions in neuronal health and disease.

    In the present study, the precise mapping of biotinylation sites led to the discovery that neuronal activity enhanced interactions with specific synaptic vesicle-associated proteins, providing a potential mechanism for activity-dependent tau release from neurons via the presynaptic vesicle fusion machinery. It will be interesting to see how this mechanism vs. exosome-mediated tau release may contribute to the observed spread of tau pathology across brain regions.

    A more conventional co-immunoprecipitation approach was used in the second part of this study to compare the tau-associated proteome between wild-type (WT) and FTD-associated mutant tau. It would have been interesting to see how this alternative approach compares to the proximity labeling and interactome mapping method used before.

    The main finding here was a weakened interaction of tau carrying FTD-causing mutations with mitochondrial proteins, potentially impairing mitochondrial bioenergetics as shown in isogenic iPSC-derived neurons carrying the V337M mutation. While this finding suggests a potential disease mechanism related to compromised energy metabolism, it remains to be seen how this loss-of-function phenotype of mutant tau in vitro relates to an autosomal-dominant mutation causing early onset dementia in human patients. 

    Finally, the analysis of existing proteomic datasets revealed a significant reduction in the levels of WT tau interactors modified by FTD-causing mutation in AD, FTD, and PSP brain tissue compared with those in control cases. This highlights a major strength of this study in combining different approaches to probe tau interactomes and to explore potential connections to the disease process.

    While the mechanism behind the reduced abundance of WT tau interactors is unclear at this point, taken together these findings suggest that defects in mitochondrial bioenergetics may represent a converging disease mechanism across primary tauopathies and AD.

    View all comments by Wilfried Rossoll

  4. In this elegant study, the authors generated an atlas of tau-interacting proteins using proximity labeling and immunoprecipitation approaches to capture both wild-type (WT) and mutant tau interacting partners.

    One novel aspect of this paper was that the authors engineered human iPSC-derived neurons to express APEX2 either N- or C-terminal to full-length tau for proximity labeling of tau-interacting proteins with biotin and identification by mass spectrometry. Remarkably, nearly half of all tau interactions were uniquely detected in either N-APEX or C-APEX tau neurons, revealing unique domain-specific functions of tau and corresponding proteolytic fragments that are produced in disease. Mapping sites of biotinylation also allowed the authors to resolve contact regions between tau and the interacting proteins, highlighting both unique cytosolic and nuclear partners. The authors further leveraged these engineered human neurons to capture activity-dependent changes in tau-interacting partners following KCl treatment, which revealed enrichment of proteins that regulate synaptic vesicle exocytosis, and may provide new mechanisms and drug targets that underlie tau secretion.

    Additional studies highlighted differences between tau interacting partners of WT and mutant tau (TauP301L or TauV337M). Here, they observed a loss of interaction between mutated tau and mitochondrial proteins compared to WT tau, suggesting that energy defects in disease are linked to tau dysfunction.

    Finally, a strength of the paper is that the authors used proteomic data from human brain tissue to show that in AD, and other tauopathies, levels of tau interactors correlate with disease severity. Overall, this paper not only confirms many of the known components of the tau interactome, but also provides a strong resource of novel tau-protein interactions that can be followed up for future mechanistic studies.

    View all comments by Nicholas Seyfried

  5. This is a wonderfully comprehensive analysis of interactions between neuronal proteins and tau. We’ve known for a long time that tau is distributed among many different compartments in neurons, but there’s still a lot that’s unknown about how this process is regulated, particularly in the context of disease-causing tau mutations.

    I find the new interactions with mitochondrial proteins particularly exciting. Mitochondrial dysfunction, including reduced bioenergetics, is well described in the early stage of tauopathies, and abnormal tau may disrupt coupling between mitochondria and the ER. I look forward to further studies on how these novel interactions may play a role in these processes.  

    One limitation of this study is the focus on iPSC-derived neurons. In future it will be important to understand whether these interactions vary between the many different neuron subpopulations, which may shed light on regional vulnerabilities to tau toxicity in the brain.

    View all comments by Amy Pooler

  6. Aβ has been reported to enter mitochondria, and it is certainly worth considering if tau does the same, especially in the setting of dysregulated mitophagy.

    Given the gradual failure of cellular energetics across numerous cell types in Alzheimer's, it is important to find out more about this phenomenon.

    I haven't read the paper yet, but I wonder if the majority of mitochondrial proteins in this experiment incidentally contacted tau in the cytoplasm before they were imported to mitochondria, as opposed to contacting tau within the inner membrane? For example, TOMM40 on the outer membrane is a nuclear gene on chromosome 19 which has been previously associated with Alzheimer's disease, and would be readily accessible to tau.

    My lab's unpublished RNA-Seq data from a number of non-neuronal cell types also implicates exocytosis of Aβ and tau as potentially causative mechanisms in AD, as well as implicating a number of mitochondrial pathways.

    Whatever the explanation for these mitochondrial contacts with tau, I think the authors have made an important observation, and this technique is quite elegant.

    View all comments by Christopher Janson

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