TEAD1 is crucial for developmental myelination, Remak bundles, and functional regeneration of peripheral nerves

  1. Matthew Grove
  2. Hyukmin Kim
  3. Shuhuan Pang
  4. Jose Paz Amaya
  5. Guoqing Hu
  6. Jiliang Zhou
  7. Michel Lemay
  8. Young-Jin Son

  • Curated by eLife

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    eLife assessment

    This important study demonstrates that the transcription factor TEAD1 is required for the function of Yap/Taz in Schwann cells, with conditional mouse mutants having very similar dysmyelinated phenotypes. Convincing histological evidence is shown for the role of TEAD1 itself, leaving open the function of other TEAD proteins in this system. This study will nevertheless be of great interest to researchers in the field of peripheral nerve development.

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Abstract

Previously we showed that the hippo pathway transcriptional effectors, YAP and TAZ, are essential for Schwann cells (SCs) to develop, maintain and regenerate myelin . Although TEAD1 has been implicated as a partner transcription factor, the mechanisms by which it mediates YAP/TAZ regulation of SC myelination are unclear. Here, using conditional and inducible knockout mice, we show that TEAD1 is crucial for SCs to develop and regenerate myelin. It promotes myelination by both positively and negatively regulating SC proliferation, enabling Krox20/Egr2 to upregulate myelin proteins, and upregulating the cholesterol biosynthetic enzymes FDPS and IDI1. We also show stage-dependent redundancy of TEAD1 and that non-myelinating SCs have a unique requirement for TEAD1 to enwrap nociceptive axons in Remak bundles. Our findings establish TEAD1 as a major partner of YAP/TAZ in developmental myelination and functional nerve regeneration and as a novel transcription factor regulating Remak bundle integrity.

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  1. Version published to 10.7554/elife.87394 on eLife
  2. eLife

    Author Response

    Reviewer #1 (Public Review):

    The manuscript by Grove and colleagues analyzes the role of TEAD1 transcription factors in all events regulating PNS myelin formation and maintenance and regeneration. Throughout the manuscript, the authors compare the results obtained to those they previously described in YAP/TAZ double knockout mice. Strengths of the manuscript are combined in vivo analyses by generating mutants constitutively lacking TEAD1 expression in myelinating Schwann cells (P0Cre//TEAD1f/f mice: cKO) and mutants in which TEAD1 expression can be ablated after tamoxifen-mediated recombination is myelinating Schwann cells (PlpCreER//TEAD1f/f mice: iKO). Using this approach the authors were able to assess the role of TEAD1 in all aspects related to PNS myelin: formation as well as maintenance and remyelination after injury. By exploiting these models, they were able to define the role of TEAD1 in regulating Schwann cell proliferation as well as in the cholesterol biosynthetic pathway. Collectively, their data indicate that TEAD 1 has a composite role in PNS myelination being required for developmental myelination, but dispensable for myelin maintenance. Further, they also describe a role for TEAD1 in promoting PNS remyelination after an injury event.

    Despite these strengths, there are some weaknesses that should be addressed by the authors:

    1. The manuscript would benefit from better and more detailed analysis of the role of the other TEAD transcription factors, as they are likely redundant in function to TEAD1. For example, since in cKO mice some fibers can escape the sorting defect and eventually myelinate, albeit at a lower level, could they determine whether TEAD2-4 transcription factors might compensate for TEAD1 absence in this setting?

    We speculate that other TEADs, most likely both TEAD2 and TEAD3, compensate TEAD1 in myelinating some developing axons. We also speculate that TEAD4 counteracts TEAD1, resulting in excessive proliferation of Schwann cells in Tead1 cKO. Unfortunately, because, unlike TEAD1, floxed/congenic alleles and IHC-compatible antibodies are not yet available for TEAD2-4, it is difficult to determine their roles. We attempted to knock down TEAD2-4 by injecting AAV-shRNAs into the sciatic nerves of WT and Tead1 iKO, but this intervention was not successful. Our future studies will determine compensatory and/or opposing roles of other TEADs during development and homeostasis and after nerve injury.

    1. A striking result of the study is the morphological defects observed in the process of axonal sorting and in the Remak fibers formation of TEAD1 cKO mice. To explain the sorting defect, the authors correctly analyze Schwann cell proliferation. However, since axonal sorting is mediated by the interaction between the extracellular matrix and intracellular cytoskeleton rearrangement, they should address also these two aspects. As per the Remak bundles and the poly-axonal myelination they observe, it is difficult to reconcile this "abnormal" myelination with the fact that TEAD1 cKO mice have a very severe myelinating phenotype, which is persistent in adulthood.

    It is noteworthy that we found radial sorting to be delayed, but not blocked, in Tead1 cKO, as we had previously reported for Yap/Taz cDKO mice in our earlier publication (Grove et al., eLIFE 2017). The primary reason that myelin development fails in Schwann cells lacking YAP/TAZ (or TEAD1 in the present report) is because they do not initiate myelination of sorted axons, not because of defective radial sorting. We showed that radial sorting was delayed in Schwann cells lacking YAP/TAZ because of their late S phase entry (Figure 4 in Grove et al., eLIFE 2017). In addition, our earlier report demonstrated that the key laminin receptor, integrin 6, is strongly downregulated but axons are nevertheless sorted out by Schwann cells in Yap/Taz cDKO (Figure 4-figure supplement 2 in Grove et al., eLIFE 2017). Our current view, therefore, is that extracellular matrix may contribute to reducing Schwann cell proliferation (Berti et al., 2011; Pellegatta et al., 2013; Yu, Feltri, Wrabetz, Strickland, & Chen, 2005), which helps to delay radial sorting, but that it is not required for Schwann cells lacking YAP/TAZ (or TEAD1) to sort axons (see the author response #2 in Grove et al., eLIFE 2017). Based on this information, we disagree with the reviewer that it is essential for us to address the role of extracellular matrix in delaying radial sorting in Tead1 cKO.

    Regarding Remak bundles, ‘thinly’ myelinated Remak bundles are only ‘occasionally’ observed in Tead1 cKO mice. Given that some large axons are still myelinated in Tead1 cKO mice, likely due to compensation by other TEADs, we speculate that Remak bundles are occasionally myelinated by other TEADs in Tead1 cKO. We have clarified our description and expanded our discussion of TEAD1 regulation of Remak bundles, including abnormal polyaxonal myelination.

    1. In the analyses of the cholesterol biosynthetic pathway, TEAD1 seems to be only partly involved. Again, which is the role of any of the other TEADs?

    Examining cholesterol biosynthesis pathways (SREBP1 and 2) and their target enzymes (SCD1, HMGCR, FDPS, IDI1) in Tead1 cKO and Yap/Taz cDKO, we showed that TEAD1 is required for upregulating FDPS and IDI1. These data suggest that TEAD1 plays a major role in mediating YAP/TAZ-driven cholesterol synthesis by upregulating FDPS and IDI1. It is also important to note that FDPS and IDI1 levels are reduced in TEAD1 cKO as ‘greatly’ as those in Yap/Taz cDKO (Figure 5). We therefore speculate that other TEADs compensate TEAD1 modestly, if at all, in upregulating FDPS and IDI1. We do not rule out the possibility, however, that other TEADs fully compensate TEAD1 in ‘maintaining’ cholesterol synthesis in adult Schwann cells. We will address these important questions in the future when the key resources mentioned above become available to study TEAD2-4.

    1. Why do cKO mice die before P60?

    In accordance with IACUC guidelines, we humanely euthanized Tead1 cKO mice before P60 because, like Yap/Taz cKO mice, they develop severe peripheral neuropathy.

  3. eLife

    eLife assessment

    This important study demonstrates that the transcription factor TEAD1 is required for the function of Yap/Taz in Schwann cells, with conditional mouse mutants having very similar dysmyelinated phenotypes. Convincing histological evidence is shown for the role of TEAD1 itself, leaving open the function of other TEAD proteins in this system. This study will nevertheless be of great interest to researchers in the field of peripheral nerve development.

  4. eLife

    Reviewer #1 (Public Review):

    The manuscript by Grove and colleagues analyzes the role of TEAD1 transcription factors in all events regulating PNS myelin formation and maintenance and regeneration. Throughout the manuscript, the authors compare the results obtained to those they previously described in YAP/TAZ double knockout mice. Strengths of the manuscript are combined in vivo analyses by generating mutants constitutively lacking TEAD1 expression in myelinating Schwann cells (P0Cre//TEAD1f/f mice: cKO) and mutants in which TEAD1 expression can be ablated after tamoxifen-mediated recombination is myelinating Schwann cells (PlpCreER//TEAD1f/f mice: iKO). Using this approach the authors were able to assess the role of TEAD1 in all aspects related to PNS myelin: formation as well as maintenance and remyelination after injury. By exploiting these models, they were able to define the role of TEAD1 in regulating Schwann cell proliferation as well as in the cholesterol biosynthetic pathway.

    Collectively, their data indicate that TEAD 1 has a composite role in PNS myelination being required for developmental myelination, but dispensable for myelin maintenance. Further, they also describe a role for TEAD1 in promoting PNS remyelination after an injury event.

    Despite these strengths, there are some weaknesses that should be addressed by the authors:

    1. The manuscript would benefit from better and more detailed analysis of the role of the other TEAD transcription factors, as they are likely redundant in function to TEAD1. For example, since in cKO mice some fibers can escape the sorting defect and eventually myelinate, albeit at a lower level, could they determine whether TEAD2-4 transcription factors might compensate for TEAD1 absence in this setting?

    2. A striking result of the study is the morphological defects observed in the process of axonal sorting and in the Remak fibers formation of TEAD1 cKO mice. To explain the sorting defect, the authors correctly analyze Schwann cell proliferation. However, since axonal sorting is mediated by the interaction between the extracellular matrix and intracellular cytoskeleton rearrangement they should address also these two aspects. As per the Remak bundles and the poly-axonal myelination they observe, it is difficult to reconcile this "abnormal" myelination with the fact that TEAD 1 cKO mice have a very severe myelinating phenotype, which is persistent in adulthood.

    3. In the analyses of the cholesterol biosynthetic pathway, TEAD1 seems to be only partly involved. Again, which is the role of any of the other TEADs?

    4. Why do cKO mice die before P60?

  5. eLife

    Reviewer #2 (Public Review):

    The manuscript addresses the role of TEAD1 in developmental myelination and nerve regeneration after nerve injury and establishes TEAD1 as a key component for YAP/TAZ-related Schwann cell biology. The authors use genetic and biochemical techniques, as well as immunostainings of tissues to address TEAD1's function in myelin biology. While the constitutive knockout of TEAD1 is convincing, the tamoxifen-induced variation requires some validation. Experimental procedures to study the effect of TEAD1 on myelin development and regeneration were properly performed. TEAD1 is believed to be the major driver of the TEAD family in regulating myelination in Schwann cells. However, the delineation of TEAD1 in myelin biology from the other TEAD family members TEAD2, 3, and 4 needs further verification. In particular, the biochemical techniques assessing the potentially competitive binding of TEAD1 versus TEAD2, 3, and 4 to YAP1 and TAZ (WWTR1) require a thorough functional validation. Overall, the identification of TEAD1 as the major driver of myelin in development and regeneration is a very important finding for Schwann cell biology.

  6. eLife

    Reviewer #3 (Public Review):

    The Hippo signalling pathway has been implicated in organ growth through the regulation of cell proliferation and apoptosis. The main transcriptional effectors of this pathway, the Yap and Taz proteins, associate with members of the TEAD family of transcription factors to drive diverse transcriptional programs of proliferation, growth, and differentiation. It has previously been shown that YAP/TAZ are essentially required in Schwann cells for developmental myelination, homeostasis, and regenerative myelination in the peripheral nervous system. All four members of the TEAD family are expressed in the Schwann cell lineage, raising the possibility that different aspects of YAP/TAZ role in the Schwann cell lineage are underpinned by differential associations with TEAD transcription factors. In this study, Grove and colleagues provide convincing evidence that TEAD1 is the main transcription factor through which YAP/TAZ affects myelination in development and following nerve injury. A careful comparison between Schwann cell-specific and inducible Yap/Taz and TEAD1 knock out animals reveal unique and redundant roles for TEAD1 in myelination by regulating Schwann cell proliferation, Krox20-dependent myelin gene expression, and cholesterol biosynthesis. Interestingly, their study appears to reveal a YAP/TAZ independent role for TEAD1 in non-myelinating Schwann cell ensheathment of low calibre axons and Remak bundle formation. The conclusions of this study are based on rigorous biochemical, immune-histochemical, electron microscopic, and functional analysis of mutant and wild-type nerves at different stages of postnatal development and following crush nerve injury.

    Perhaps the most surprising finding of this study is that TEAD1 can function independently from YAP/TAZ in one branch of the Schwann cell lineage (the authors had reported earlier that non-myelinating Schwann cells do not express YAP/TAZ).
    How TEAD1 transcriptional activity is modulated in these Remak Schwann cells is an interesting avenue of future research.

  7. Version published to 10.1101/2023.02.27.530298v3 on bioRxiv
  8. Version published to 10.1101/2023.02.27.530298v2 on bioRxiv
  9. Version published to 10.1101/2023.02.27.530298v1 on bioRxiv