SUMMARY Group I metabotropic glutamate receptors (mGluR) induce long-term depression (LTD) that requires protein synthesis. Here, we demonstrate that Arc/Arg3.1 is translationally induced within 5 min of mGluR activation, and this response is essential for mGluR-dependent LTD. The increase in Arc/Arg3.1 translation requires eEF2K, a Ca2+/calmodulin-dependent kinase that binds mGluR and dissociates upon mGluR activation, whereupon it phosphorylates eEF2. Phospho-eEF2 acts to slow the elongation step of translation and inhibits general protein synthesis but simultaneously increases Arc/Arg3.1 translation. Genetic deletion of eEF2K results in a selective deficit of rapid mGluR-dependent Arc/Arg3.1 translation and mGluR-LTD. This rapid translational mechanism is disrupted in the fragile X disease mouse (Fmr1 KO) in which mGluR-LTD does not require de novo protein synthesis but does require Arc/Arg3.1. We propose a model in which eEF2K-eEF2 and FMRP coordinately control the dynamic translation of Arc/Arg3.1 mRNA in dendrites that is critical for synapse-specific LTD.

Actomyosin contractility regulates various cell biological processes including cytokinesis, adhesion and migration. While in lower eukaryotes, a-kinases control actomyosin relaxation, a similar role for mammalian a-kinases has yet to be established. Here, we examined whether TRPM7, a cation channel fused to an a-kinase, can affect actomyosin function. We demonstrate that activation of TRPM7 by bradykinin leads to a Ca 2 þ -and kinase-dependent interaction with the actomyosin cytoskeleton. Moreover, TRPM7 phosphorylates the myosin IIA heavy chain. Accordingly, low overexpression of TRPM7 increases intracellular Ca 2 þ levels accompanied by cell spreading, adhesion and the formation of focal adhesions. Activation of TRPM7 induces the transformation of these focal adhesions into podosomes by a kinase-dependent mechanism, an effect that can be mimicked by pharmacological inhibition of myosin II. Collectively, our results demonstrate that regulation of cell adhesion by TRPM7 is the combined effect of kinase-dependent and -independent pathways on actomyosin contractility.

A new Ca2+/calmodulin-dependent protein kinase has been recently discovered in mammalian cells. The major substrate of this kinase, a protein of relative molecular mass (Mr) approximately equal to 100,000 (100K), has been identified as elongation factor 2 (EF-2), which participates in protein synthesis. The in vivo activity of the EF-2 kinase depends upon growth factors and other agents affecting the level of Ca2+ and cAMP. Its effect on EF-2 activity, however, remained obscure. This work shows that the phosphorylation of EF-2 by the EF-2 kinase results in a drastic inhibition of polyphenylalanine synthesis in poly(U)-directed translation. Phosphorylated EF-2 is completely inactive in translation and, moreover, inhibits the activity of non-phosphorylated EF-2. Dephosphorylation of EF-2 by phosphatase restores its activity. Hence, the phosphorylation of EF-2 directly affects the elongation stage of translation and thus represents a novel mechanism of translational control.

Inactivation of APC is a strongly predisposing event in the development of colorectal cancer1,2, prompting us to search for vulnerabilities specific to cells that have lost APC function. Signalling through the mTOR pathway is known to be required for epithelial cell proliferation and tumour growth3-5 and the current paradigm suggests that a critical function of mTOR activity is to upregulate translational initiation through phosphorylation of 4EBP16,7. This model predicts that the mTOR inhibitor rapamycin, which does not efficiently inhibit 4EBP18, would be ineffective in limiting cancer progression in APC deficient lesions. Here we show that mTORC1 activity is absolutely required for the proliferation of APC deficient (but not wild type) enterocytes, revealing an unexpected opportunity for therapeutic intervention. Although APC deficient cells show the expected increases in protein synthesis, our studies reveals that it is translation elongation, and not initiation, which is the rate-limiting component. Mechanistically, mTORC1 mediated inhibition of eEF2 kinase is required for the proliferation of APC deficient cells. Importantly, treatment of established APC deficient adenomas with rapamycin (which can target eEF2 through the mTORC1 – S6K – eEF2K axis) causes tumour cells to undergo growth arrest and differentiation. Taken together our data suggest that inhibition of translation elongation using existing, clinically approved drugs such as the Rapalogs, would provide clear therapeutic benefit for patients at high-risk of developing colorectal cancer.