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1 Molecular Biology, University of Copenhagen, Copenhagen, Denmark
2 Hospital for Sick Children, University of Toronto, Toronto, Canada
3 St. Michael's Hospital Research Institute, Canada
* To whom correspondence should be addressed. E-mail: sfpedersen{at}aki.ku.dk.
Hyperosmotic shrinkage induces multiple cellular responses, including activation of volume-regulatory ion transport, cytoskeletal reorganization, and cell death. Here, we investigated the possible roles of ezrin/radixin/moesin (ERM) proteins in these events. Osmotic shrinkage of Ehrlich Lettre ascites (ELA) cells elicited the formation of long microvillus-like protrusions, rapid translocation of endogenous ERM proteins and GFP-tagged ezrin to the cortical region including these protrusions, and Thr567/564/558 (ezrin/radixin/moesin) phosphorylation of cortical ERM proteins. Reduced cell volume appeared to be the critical parameter in hypertonicity-induced ERM protein activation, whereas alterations in extracellular ionic strength or pHi were not involved. A shrinkage-induced increase in the level of membrane-associated PtdIns(4,5)P2 appeared to play an important role in ERM protein activation, which was prevented after PtdIns(4,5)P2 depletion by expression of the synaptojanin-2 phosphatase domain. While expression of constitutively active RhoA increased basal ERM phosphorylation, the Rho-Rho kinase pathway did not appear to be involved in shrinkage-induced ERM protein phosphorylation, which was also unaffected by the inhibition or absence of the Na+/H+ exchanger (NHE1). Ezrin knockdown by siRNA increased shrinkage-induced NHE1 activity, reduced basal and shrinkage-induced Rho activity, and attenuated the shrinkage-induced formation of microvillus-like protrusions. Hyperosmolarity-induced cell death was unaltered by ezrin knockdown or after phosphatidylinositol-3-kinase (PI3K) inhibition. In conclusion, ERM proteins are activated by osmotic shrinkage in a PtdIns(4,5)P2-dependent, NHE1-independent manner. This in turn mitigates the shrinkage-induced activation of NHE1, augments Rho activity, and may also contribute to F-actin rearrangement. In contrast, no evidence was found for the involvement of an NHE1-ezrin-PI3K-PKB pathway in counteracting shrinkage-induced death.
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