The Axin scaffold protects the kinase GSK3β from cross-pathway inhibition

  1. Maire Gavagan
  2. Noel Jameson
  3. Jesse G Zalatan

  • Curated by eLife

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    This study presents a valuable and elegant kinetic analysis of the GSKbeta activity as a function of phosphorylation and Axin binding - providing insights into critical steps of Wnt pathway signaling. The results will be of big use to the broader signaling community, however, the incomplete dissection of the mechanism by which Axin binding inhibits GSKbeta inhibitory phosphorylation remains a weakness of this study. The work will be of broad interest to cell biologists and biochemists.

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Abstract

Multiple signaling pathways regulate the kinase GSK3β by inhibitory phosphorylation at Ser9, which then occupies the GSK3β priming pocket and blocks substrate binding. Since this mechanism should affect GSK3β activity toward all primed substrates, it is unclear why Ser9 phosphorylation does not affect other GSK3β-dependent pathways, such as Wnt signaling. We used biochemical reconstitution and cell culture assays to evaluate how Wnt-associated GSK3β is insulated from cross-activation by other signals. We found that the Wnt-specific scaffold protein Axin allosterically protects GSK3β from phosphorylation at Ser9 by upstream kinases, which prevents accumulation of pS9-GSK3β in the Axin•GSK3β complex. Scaffold proteins that protect bound proteins from alternative pathway reactions could provide a general mechanism to insulate signaling pathways from improper crosstalk.

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  1. Version published to 10.7554/elife.85444 on eLife
  2. eLife

    Author Response

    Reviewer #1 (Public Review):

    GSK3 is a multi-tasking kinase that recognises primed (i.e. phosphorylated) substrates. One of the mechanisms by which the activity of GSK3 can be regulated is through N-terminal (pSer9) phosphorylation. In this case, the phosphorylated N-terminus turns into a pseudo-substrate that occupies the substrate binding pocket and thus inhibits the activity of GSK3 towards its real substrates.

    One outstanding question is how this autoinhibitory mechanism can affect some, but not all signaling pathways that GSK3 is involved in. One example is WNT/CTNNB1 signaling. Here, GSK3 plays a central role in the turnover of CTNNB1 in the absence of WNT, but this pool of GSK3 is not affected by pSer9 phosphorylation.

    Gavagan et al. address this question using an in vitro approach with purified proteins. They identify a role for AXIN1 in protecting the "WNT signaling pool" of GSK3 from the auto- inhibition that occurs upon pSer9 phosphorylation.

    Specifically, they show that i) GSK3-pSer9 is less capable of binding and phosphorylating primed CTNNB1 - thus suggesting that GSK3-pSer9 does not contribute to WNT signaling, ii) in the presence of AXIN1, GSK3-pSer9 becomes more capable of binding and phosphorylating CTNNB1 - suggesting that Axin can promote binding of GSK3 and CTNNB1 even when the primed binding pocket on GSK3 is blocked initially, iii) AXIN1 specifically prevents the PKA mediated phosphorylation of GSK3B on pSer9 - while leaving the phosphorylation of other PKA substrates unaffected.

    Strengths:

    • The authors use an in vitro system in which they can reconstitute different interactions and reactions using purified proteins, thus allowing them to zoom in on specific biochemical events in isolation.
    • The authors measure the phosphorylation of primed substrates (pSer45-CTNNB1 or WNT- independent substrates) and quantify specific kinetic parameters (kcat, KM, and kcat/KM) - of wildtype non-phosphorylated GSK3B, pSer9GSK3B, or the non-phosphorylatable S9A-GSK3B, either in the presence or absence of AXIN1 (or an AXIN1 fragment).
    • The experiments appear to be well-controlled and the results appear to be interpreted correctly.

    Weaknesses:

    • Key experiments (e.g. Figures 2 and 3) are described as being performed as n=3 technical replicates rather than independent/biological replicates.

    We suggest that the replicates described in our work can properly be described as biological replicates, and we have updated the manuscript accordingly. We apologize for the confusion and elaborate on our reasoning below.

    Each replicate reported for our in vitro kinetic assays is an independent reaction prepared in a separate reaction vessel, and replicates were analyzed on separate gels. Thus, each reaction is a distinct biological sample and should have been described as a biological replicate. A technical replicate would have been repeat measurements of the same timepoint from a single reaction.

    Our original description as technical replicates was based on the notion that each replicate came from the same protein purification (biological sample). However, an analogy to cell culture experiments can illustrate why our initial reasoning was incorrect. In a cell culture experiment, cells from the same initial source are typically split into independent wells for biological replicates. Similarly, our proteins come from the same initial source but are split into independent reaction vessels for biological replicates.

    The critical point is that, regardless of the precise terminology, our replicates capture the variability between independent experiments.

    • The validation in a biologically relevant setting (i.e. a cellular context) is limited to Figure 4C, which shows that over-expression of AXIN1 reduces the total levels of pSer9-GSK3.

    The biochemical experiments presented in our work address a critical gap in the signaling field and, together with the in vivo validation in Figure 4C, establish a model that was previously speculative. We suggest that further in vivo experiments are beyond the scope of the current manuscript.

    The authors convincingly show that AXIN1 can play a role in shielding GSK3 from auto- inhibition. As it stands, the impact of this work on the field of WNT/CTNNB1 signaling is likely to remain limited. This is mainly due to the reason that the mechanism by which AXIN1 shields the WNT/CTNNB1 signaling pool of GSK3 from pSer9 inhibition remains unresolved. Based on the fact that a mini AXIN1 (i.e. an AXIN1 fragment) behaves the same as WT AXIN1, the authors conclude that AXIN1 likely causes allosteric changes on GSK3 but is less likely to block PKA from binding. They cannot conclusively show this, however, as they do not have evidence in favour of one or the other explanation.

    We thank the reviewer for this important comment which details the central concern raised in the review process. To address this point, we have collected additional biochemical data that conclusively shows that the Axin shielding effect is allosteric and not a steric block. We demonstrated that a minimal, 27 amino acid Axin peptide produces the same GSK3β shielding behavior as full length Axin and miniAxin. The minimal Axin peptide does not sterically occlude the GSK3β phosphorylation site. This data is included in a revised Fig 4A and described on lines 115-120 of the revised manuscript.

    However, this study does offer more insight into the compartmentalisation of GSK3 and the quantitative parameters may be used in computational models describing the different cellular activities of GSK3.

    This work also has conceptual significance: Scaffold proteins are known to promote signal transduction by bringing proteins together (often: kinases and substrates). Here, Gavagan et al. show that AXIN1 also plays a second role, namely in protecting one of its binding kinases (GSK3) from inhibitory signals. This could potentially hold for other scaffolding proteins as well.

    Reviewer #2 (Public Review):

    Gavagan et al. investigated the role of the scaffolding protein, Axin, in the cross-pathway inhibition of GSK3b. The authors utilize reconstituted Axin, b-catenin, GSK3b, and protein kinase A to test 2 models. In the first model, the formation of the complex consisting of Axin, b-catenin, and GSK3b overcomes inhibitory phosphorylation of serine 9 of GSK3b. In the second model, the binding of Axin to GSK3b inhibits serine 9 phosphorylation through allosteric effects. Previous literature has established that the phosphorylation of serine 9 of GSK3b inhibits its kinase activity. To provide a quantitative measure of inhibition, the authors determine the binding affinity and catalytic efficiency of GSK3b in comparison to GSK3b phosphoS9 towards b-catenin. Interestingly, the data demonstrate a 200-fold decrease in Kcat/Km and 7 fold increase in Km. It is unclear why serine 9 mutation to alanine increases the rate of B-catenin phosphorylation more than the GSK unphosphorylated protein in figure S10.

    We thank the reviewer for catching this inconsistency. In the Michaelis-Menten plots presented in the main text (Figure 2 & Figure 3D), rates for unphosphorylated GSK3β and GSK3β_S9A are indistinguishable. These plots were used to determine the kinetic parameters reported in Table S1 (now Supplementary file 1a). The purpose of Figure S10 (now Figure 2-figure supplement 8) was to confirm that these reactions were first order (linear) in enzyme concentration, but the reviewer is correct to flag the inconsistency in absolute rates. In Figure S10A (now Figure 2-figure supplement 8A), the rates for unphosphorylated GSK3β were ~2-3-fold lower than expected.

    We have reanalyzed the original frozen reaction timepoints on new western blots. The results were identical for unphosphorylated GSK3β and GSK3β_S9A, resolving the apparent discrepancy. Upon review of the original western blot images, we noted that they were relatively noisy, potentially indicating incomplete blot transfer or an antibody going bad. Because we were able to reanalyze the original samples and obtained internally consistent results, we suggest that the updated data should replace the original data. The updated data are included in a revised Figure S10A (now Figure 2-figure supplement 8A).

    Next, the authors tested if the addition of Axin could overcome this inhibition. Although the addition of Axin decreases the Km, thereby producing a 20-fold increase in catalytic efficiency, the addition of Axin does not rescue the catalytic turnover of the phosphorylated GSK3b. Hence, the authors propose that Axin does not rescue the kinase activity of GSK3b from the inhibitory effects of serine 9 phosphorylation.

    Next, the authors test if Axin protects GSK3b from phosphorylation by the upstream kinase PKA. Excitingly, the data show a decrease in binding affinity and catalytic efficiency of PKA with GSK3b phosphoS9 in comparison to GSK3b. The binding of Axin inhibits GSK3b serine 9 phosphorylation by PKA but does not inhibit the phosphorylation of other PKA substrates such as Creb. The authors demonstrate that a fragment of Axin, residues 384-518, behaves similarly to the full-length Axin to shield GSK3b from phosphorylation. However, it is unclear how this fragment may bind in the destruction complex and if Axin has allosteric effects on GSK3b.

  3. eLife

    eLife assessment

    This study presents a valuable and elegant kinetic analysis of the GSKbeta activity as a function of phosphorylation and Axin binding - providing insights into critical steps of Wnt pathway signaling. The results will be of big use to the broader signaling community, however, the incomplete dissection of the mechanism by which Axin binding inhibits GSKbeta inhibitory phosphorylation remains a weakness of this study. The work will be of broad interest to cell biologists and biochemists.

  4. eLife

    Reviewer #1 (Public Review):

    GSK3 is a multi-tasking kinase that recognises primed (i.e. phosphorylated) substrates. One of the mechanisms by which the activity of GSK3 can be regulated is through N-terminal (pSer9) phosphorylation. In this case, the phosphorylated N-terminus turns into a pseudo-substrate that occupies the substrate binding pocket and thus inhibits the activity of GSK3 towards its real substrates.

    One outstanding question is how this autoinhibitory mechanism can affect some, but not all signaling pathways that GSK3 is involved in. One example is WNT/CTNNB1 signaling. Here, GSK3 plays a central role in the turnover of CTNNB1 in the absence of WNT, but this pool of GSK3 is not affected by pSer9 phosphorylation.

    Gavagan et al. address this question using an in vitro approach with purified proteins. They identify a role for AXIN1 in protecting the "WNT signaling pool" of GSK3 from the auto-inhibition that occurs upon pSer9 phosphorylation.
    Specifically, they show that i) GSK3-pSer9 is less capable of binding and phosphorylating primed CTNNB1 - thus suggesting that GSK3-pSer9 does not contribute to WNT signaling, ii) in the presence of AXIN1, GSK3-pSer9 becomes more capable of binding and phosphorylating CTNNB1 - suggesting that Axin can promote binding of GSK3 and CTNNB1 even when the primed binding pocket on GSK3 is blocked initially, iii) AXIN1 specifically prevents the PKA mediated phosphorylation of GSK3B on pSer9 - while leaving the phosphorylation of other PKA substrates unaffected.

    Strengths:
    - The authors use an in vitro system in which they can reconstitute different interactions and reactions using purified proteins, thus allowing them to zoom in on specific biochemical events in isolation.
    - The authors measure the phosphorylation of primed substrates (pSer45-CTNNB1 or WNT-independent substrates) and quantify specific kinetic parameters (kcat, KM, and kcat/KM) - of wildtype non-phosphorylated GSK3B, pSer9GSK3B, or the non-phosphorylatable S9A-GSK3B, either in the presence or absence of AXIN1 (or an AXIN1 fragment).
    - The experiments appear to be well-controlled and the results appear to be interpreted correctly.

    Weaknesses:
    - Key experiments (e.g. Figures 2 and 3) are described as being performed as n=3 technical replicates rather than independent/biological replicates.
    - The validation in a biologically relevant setting (i.e. a cellular context) is limited to Figure 4C, which shows that over-expression of AXIN1 reduces the total levels of pSer9-GSK3.

    The authors convincingly show that AXIN1 can play a role in shielding GSK3 from auto-inhibition. As it stands, the impact of this work on the field of WNT/CTNNB1 signaling is likely to remain limited. This is mainly due to the reason that the mechanism by which AXIN1 shields the WNT/CTNNB1 signaling pool of GSK3 from pSer9 inhibition remains unresolved. Based on the fact that a mini AXIN1 (i.e. an AXIN1 fragment) behaves the same as WT AXIN1, the authors conclude that AXIN1 likely causes allosteric changes on GSK3 but is less likely to block PKA from binding. They cannot conclusively show this, however, as they do not have evidence in favour of one or the other explanation.

    However, this study does offer more insight into the compartmentalisation of GSK3 and the quantitative parameters may be used in computational models describing the different cellular activities of GSK3.

    This work also has conceptual significance: Scaffold proteins are known to promote signal transduction by bringing proteins together (often: kinases and substrates). Here, Gavagan et al. show that AXIN1 also plays a second role, namely in protecting one of its binding kinases (GSK3) from inhibitory signals. This could potentially hold for other scaffolding proteins as well.

  5. eLife

    Reviewer #2 (Public Review):

    Gavagan et al. investigated the role of the scaffolding protein, Axin, in the cross-pathway inhibition of GSK3b. The authors utilize reconstituted Axin, b-catenin, GSK3b, and protein kinase A to test 2 models. In the first model, the formation of the complex consisting of Axin, b-catenin, and GSK3b overcomes inhibitory phosphorylation of serine 9 of GSK3b. In the second model, the binding of Axin to GSK3b inhibits serine 9 phosphorylation through allosteric effects.

    Previous literature has established that the phosphorylation of serine 9 of GSK3b inhibits its kinase activity. To provide a quantitative measure of inhibition, the authors determine the binding affinity and catalytic efficiency of GSK3b in comparison to GSK3b phosphoS9 towards b-catenin. Interestingly, the data demonstrate a 200-fold decrease in Kcat/Km and 7 fold increase in Km. It is unclear why serine 9 mutation to alanine increases the rate of B-catenin phosphorylation more than the GSK unphosphorylated protein in figure S10. Next, the authors tested if the addition of Axin could overcome this inhibition. Although the addition of Axin decreases the Km, thereby producing a 20-fold increase in catalytic efficiency, the addition of Axin does not rescue the catalytic turnover of the phosphorylated GSK3b. Hence, the authors propose that Axin does not rescue the kinase activity of GSK3b from the inhibitory effects of serine 9 phosphorylation.

    Next, the authors test if Axin protects GSK3b from phosphorylation by the upstream kinase PKA. Excitingly, the data show a decrease in binding affinity and catalytic efficiency of PKA with GSK3b phosphoS9 in comparison to GSK3b. The binding of Axin inhibits GSK3b serine 9 phosphorylation by PKA but does not inhibit the phosphorylation of other PKA substrates such as Creb. The authors demonstrate that a fragment of Axin, residues 384-518, behaves similarly to the full-length Axin to shield GSK3b from phosphorylation. However, it is unclear how this fragment may bind in the destruction complex and if Axin has allosteric effects on GSK3b.

  6. Version published to 10.1101/2022.12.05.519208v1 on bioRxiv