ARHGAP18-ezrin functions as an autoregulatory module for RhoA in the assembly of distinct actin-based structures
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eLife assessment
This fundamental work substantially advances our understanding of how cells can tightly modulate small GTPase activity to build and maintain neighboring cytoskeletal structures, in this case microvilli. The evidence supporting these claims is compelling and is supported by both protein-protein interaction assays as well as cell biological studies. The work will be of interest to cell biologist studying the cytoskeleton as well as those interested in G-protein mediated regulation.
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Abstract
The location of different actin-based structures is largely regulated by Rho GTPases through specific effectors. We use the apical aspect of epithelial cells as a model system to investigate how RhoA is locally regulated to contribute to two distinct adjacent actin-based structures. Assembly of the non-muscle myosin-2 filaments in the terminal web is dependent on RhoA activity, and assembly of the microvilli also requires active RhoA for phosphorylation and activation of ezrin. We show that the RhoGAP, ARHGAP18, is localized by binding active microvillar ezrin, and this interaction enhances ARHGAP18’s RhoGAP activity. We present a model where ezrin-ARHGAP18 acts as a negative autoregulatory module to locally reduce RhoA activity in microvilli. Consistent with this model, loss of ARHGAP18 results in disruption of the distinction between microvilli and the terminal web including aberrant assembly of myosin-2 filaments forming inside microvilli. Thus, ARHGAP18, through its recruitment and activation by ezrin, fine-tunes the local level of RhoA to allow for the appropriate distribution of actin-based structures between the microvilli and terminal web. As RhoGAPs vastly outnumber Rho GTPases, this may represent a general mechanism whereby individual Rho effectors drive specific actin-based structures.
Article activity feed
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eLife assessment
This fundamental work substantially advances our understanding of how cells can tightly modulate small GTPase activity to build and maintain neighboring cytoskeletal structures, in this case microvilli. The evidence supporting these claims is compelling and is supported by both protein-protein interaction assays as well as cell biological studies. The work will be of interest to cell biologist studying the cytoskeleton as well as those interested in G-protein mediated regulation.
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Joint Public Review:
The assembly of the apical cytoskeleton of epithelial cells, i.e. the terminal web and microvilli (MV), requires precise control of actin dynamics and non-muscle myosin II (NM M2) contractility. Previous work from the Bretscher lab (Zaman et al, 2021) revealed a connection between ERM protein (ezrin) phosphorylation by LOK/SLK kinases and NM M2 activity and showed that ezrin negatively regulates RhoA. Here the authors now identify the missing link between ezrin and RhoA activity - the GAP ARHGAP18. Binding of ARHGAP18 to the ezrin FERM domain localizes its activity to the site of MV formation, maintaining optimal levels of active RhoA turn on the ezrin kinases LOK/SLK and prevents NM M2 activity (via reduced ROCK activity) within the growing MV. The results here establish that an ARHGAP18-ezrin interaction serves to …
Joint Public Review:
The assembly of the apical cytoskeleton of epithelial cells, i.e. the terminal web and microvilli (MV), requires precise control of actin dynamics and non-muscle myosin II (NM M2) contractility. Previous work from the Bretscher lab (Zaman et al, 2021) revealed a connection between ERM protein (ezrin) phosphorylation by LOK/SLK kinases and NM M2 activity and showed that ezrin negatively regulates RhoA. Here the authors now identify the missing link between ezrin and RhoA activity - the GAP ARHGAP18. Binding of ARHGAP18 to the ezrin FERM domain localizes its activity to the site of MV formation, maintaining optimal levels of active RhoA turn on the ezrin kinases LOK/SLK and prevents NM M2 activity (via reduced ROCK activity) within the growing MV. The results here establish that an ARHGAP18-ezrin interaction serves to tightly localize RhoA activity, promoting optimized signalling for MV formation.
The results from several complementary approaches strongly support the identification of ARHGAP18 as a critical component of a negative feedback loop that relies on interaction with ezrin for highly localized control of RhoA-GTP levels. The work is thoughtful and systematic. The results now bring into focus an elegant mechanism for controlling the formation of microvilli that relies on formation of a complex of key players - ezrin that is required for microvilli formation, LOK/SLK kinases that opens and activates ezrin at the membrane and ARHGAP18 that downregulates RhoA, the GTPase that activates LOK/SLK and NM M2.
The findings also suggest interesting possibilities for a similar mode of control in the building of related cellular protrusions, i.e. filopodia and stereocilia.There are a few questions remaining about the results. One concerns the strength of the ARHGAP18-ezrin FERM domain interaction. Also, the authors propose that activation of non-muscle Myo2 activation accounts for increased apical stiffness and that myosin filaments are present within microvilli in cells lacking ARHGAP. The distribution of the NM 2B heavy chain versus the pMLC seems at odds with the first proposition and the localization results don't quite seem to support the author's conclusion about the relocalization of NM 2B within MV. These are straightforward issues that the author should be able to clarify or address.
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