Fragile X syndrome is a common form of cognitive deficit caused by the functional absence of fragile X mental retardation protein (FMRP), a dendritic RNA-binding protein that represses translation of specific messages. Although FMRP is phosphorylated in a group I metabotropic glutamate receptor (mGluR) activity-dependent manner following brief protein phosphatase 2A (PP2A)-mediated dephosphorylation, the kinase regulating FMRP function in neuronal protein synthesis is unclear. Here we identify ribosomal protein S6 kinase (S6K1) as a major FMRP kinase in the mouse hippocampus, finding that activity-dependent phosphorylation of FMRP by S6K1 requires signaling inputs from mammalian target of rapamycin (mTOR), ERK1/2, and PP2A. Further, the loss of hippocampal S6K1 and the subsequent absence of phospho-FMRP mimic FMRP loss in the increased expression of SAPAP3, a synapse-associated FMRP target mRNA. Together these data reveal a S6K1-PP2A signaling module regulating FMRP function and place FMRP phosphorylation in the mGluR-triggered signaling cascade required for protein-synthesis-dependent synaptic plasticity.
"Akt is regulated by CHIP in a phosphorylation-dependent manner . S6K1 phosphorylates S499 of murine FMR1 (S500 in humans) . Therefore, we generated two FMR1 mutants: the FMR1 non-phospho mutant, in which S500 was mutated to Ala (HA-FMR1 S500A) and the FMR1 phospho-mimic mutant, in which S500 was mutated to Asp (HA- FMR1 S500D). "
[Show abstract][Hide abstract] ABSTRACT: The fragile X mental retardation 1 (FMR1) protein binds mRNA and acts as a negative regulator of translation. Lack of FMR1 causes the most common neurological disorder, fragile X syndrome, while its overexpression is associated with metastasis of breast cancer. Its activity has been well-studied in nervous tissue, but recent evidence as well as its role in cancer indicates that it also acts in other tissues. We have investigated the expression of FMR1 in brain and other tissues of mouse and examined its regulation. We detected expression of FMR1 in liver and heart tissues of mice as well as in brain tissue, supporting other contentions that it acts in non-nervous tissue. Expression of FMR1 inversely correlated with expression of the C-terminus of Hsc70-interacting protein (CHIP) and, based on the known activity of CHIP in protein homeostasis, we suggest that CHIP regulates expression of FMR1. CHIP ubiquitinated FMR1 for proteasomal degradation in a molecular chaperone-independent manner. FMR1 expression was reduced following treatment with okadaic acid, a phosphatase inhibitor, but not in CHIP-depleted cells. Also, a non-phospho FMR1 mutant was much less efficiently ubiquitinated by CHIP and had a longer half-life compared to either wild-type FMR or a phospho-mimic mutant. Taken together, our results demonstrate that CHIP regulates the levels of FMR1 as an E3 ubiquitin ligase in phosphorylation-dependent manner, suggesting that CHIP regulates FMR1-mediated translational repression by regulating the levels of FMR1.
Biochemical and Biophysical Research Communications 09/2014; 453(1). DOI:10.1016/j.bbrc.2014.09.099 · 2.30 Impact Factor
"We have reported alterations in S6K signaling in the ELS model ( Bernard et al . , 2013 ) , specifically S6K is overall activated or hyperphosphorylated following ELS selectively in synaptic fractions . S6K is implicated in the induction of mGluR - LTD ( Ceman et al . , 2003 ; Narayanan et al . , 2007 ; Bassell and Warren , 2008 ; Narayanan et al . , 2008 ) . In order to determine if this role is altered following ELS we induced mGluR - LTD in the presence of an S6K inhibitor ( PF - 4708671 , 25 mM ) ( Pearce et al . , 2010 ) . PF - 4708671 completely blocked mGluR - LTD in controls ( controls with PF - 4708671 : 106 . 14 . AE 9 . 71% , n ¼ 6 ; controls without PF - 4708671 : 80 . 32 AE "
[Show abstract][Hide abstract] ABSTRACT: Using the rat model of early life seizures (ELS), which has exaggerated mGluR mediated long-term depression of synaptic strength (mGluR-LTD) in adulthood, we probed the signaling cascades underlying mGluR-LTD induction. Several inhibitors completely blocked mGluR-LTD in control but not in ELS rats: the proteasome, the mammalian target of rapamycin (mTOR), S6 kinase (S6K), or L-type voltage-gated calcium channels (L-type VGCC). Inhibition of the Ca2+/calmodulin-dependent protein kinase II (CaMKII) resulted in a near complete block of mGluR-LTD in control rats and a slight reduction of mGluR-LTD in ELS rats. “Autonomous” CaMKII was found to be upregulated in ELS rats, while elevated S6K activity, which is stimulated by mTOR, was described previously. Thus, modulation of each of these factors was necessary for mGluR-LTD induction in control rats, but even their combined, permanent activation in the ELS rats was not sufficient to individually support mGluR-LTD induction following ELS. This implies that while these factors may act sequentially in controls to mediate mGluR-LTD, this is no longer the case after ELS. In contrast, activated ERK was found to be significantly down-regulated in ELS rats. Inhibition of MEK/ERK activation in control rats elevated mGluR-LTD to the exaggerated levels seen in ELS rats. Together, these results elucidate both the mechanisms that persistently enhance mGluR-LTD after ELS and the mechanisms underlying normal mGluR-LTD by providing evidence for multiple, convergent pathways that mediate mGluR-LTD induction. With our prior work, this ties these signaling cascades to the ELS behavioral phenotype that includes abnormal working memory, fear conditioning and socialization.
"The distinction is not trivial, given that pathophysiologic and pharmacologic pathways are being explored in order to improve neurodevelopmental outcome in monogenic disorders [Chang et al., 2008; Narayanan et al., 2008; Moss and Howlin, 2009; Gross et al., 2010]. In Fragile X syndrome (FXS), true social avoidance (anxiety) but not social indifference (an autism feature) appears to predict an ASD diagnosis, suggesting an atypical profile of autism in FXS [Budimirovic et al., 2006; Hall et al., 2010]. "
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