Excess Phosphoinositide 3-Kinase Subunit Synthesis and Activity as a Novel Therapeutic Target in Fragile X Syndrome

Abstract

Fragile X syndrome (FXS) is an inherited neurologic disease caused by loss of fragile X mental retardation protein (FMRP), which is hypothesized to mediate negative regulation of mRNA translation at synapses. A prominent feature of FXS animal models is exaggerated signaling through group 1 metabotropic glutamate receptors (gp1 mGluRs), and therapeutic strategies to treat FXS are targeted mainly at gp1 mGluRs. Recent studies, however, indicate that a variety of receptor-mediated signal transduction pathways are dysregulated in FXS, suggesting that FMRP acts on a common downstream signaling molecule. Here, we show that deficiency of FMRP results in excess activity of phosphoinositide 3-kinase (PI3K), a downstream signaling molecule of many cell surface receptors. In Fmr1 knock-out neurons, excess synaptic PI3K activity can be reduced by perturbation of gp1 mGluR-mediated signaling. Remarkably, increased PI3K activity was also observed in FMRP-deficient non-neuronal cells in the absence of gp1 mGluRs. Here, we show that FMRP regulates the synthesis and synaptic localization of p110␤, the catalytic subunit of PI3K. In wild type, gp1 mGluR activation induces p110␤ translation, p110␤ protein expression, and PI3K activity. In contrast, both p110␤ protein synthesis and PI3K activity are elevated and insensitive to gp1 mGluR stimulation in Fmr1 knock-out. This suggests that dysregulated PI3K signaling may underlie the synaptic impairments in FXS. In support of this hypothesis, we show that PI3K antagonists rescue three FXS-associated phenotypes: dysregulated synaptic protein synthesis, excess AMPA receptor internalization, and increased spine density. Targeting excessive PI3K activity might thus be a potent therapeutic strategy for FXS.

Therapeutic Strategies in Fragile X Syndrome: Dysregulated mGluR Signaling and Beyond

“…FMRP was shown to repress translation of individual target mRNAs as well as general protein synthesis (Zalfa et al, 2003;Lu et al, 2004;Dolen et al, 2007;Muddashetty et al, 2007;Gross et al, 2010;Osterweil et al, 2010). On the other hand, recent work suggests that FMRP can also act as a translational activator for specific target mRNAs (Bechara et al, 2009;Fahling et al, 2009;Gross et al, 2011).…”

“…They include in vivo and in vitro studies, as well as analyses of dendritic protrusion and filopodia density, dendritic spine classification, and dendritic arborization (see, eg, McKinney et al, 2005;de Vrij et al, 2008;Gross et al, 2010;Grossman et al, 2010). Some findings are contradictory; for example, several studies have reported significantly increased total dendritic spine density in cultured hippocampal Fmr1 KO neurons as well as in cortex and olfactorial bulb in vivo (Hayashi et al, 2007;Gross et al, 2010;Liu et al, 2011;Scotto-Lomassese et al, 2011), whereas other studies have detected increased numbers of filopodia, but no significant difference in total spine density in the cortex in vivo and in cultured hippocampal neurons (Irwin et al, 2002;de Vrij et al, 2008;Bilousova et al, 2009). Different experimental settings, such as age of the mice or neuronal cultures, might explain variations in the observed phenotypes.…”

“…Although the underlying molecular mechanisms are not fully understood yet, the dendritic spine phenotype in Fmr1 KO mouse models has been proven to be an important readout to evaluate novel therapeutic strategies in FXS (Dolen et al, 2007;de Vrij et al, 2008;Bilousova et al, 2009;Gross et al, 2010;Liu et al, 2011).…”

“…Over the past years, evidence is emerging that the PI3K/ mTOR signaling pathway is frequently linked to autism, via both inherited and exogenous mechanisms, suggesting that dysregulated PI3K/mTOR signaling might be a common mechanism in many ASDs (reviewed in Levitt and Campbell, 2009). The observation that PI3K/mTOR signaling in FXS is dysregulated Gross et al, 2010;Sharma et al, 2010) might therefore provide another interesting link between FXS and autism. Several single-gene mutations associated with autism affect components of the PI3K/mTOR signaling complex, for example, mutations or deletions in tuberous sclerosis complex 1/2 (TSC1/2) (Wiznitzer, 2004), and phosphatase and tensin homolog on chromosome ten (PTEN) (Butler et al, 2005;Redfern et al, 2010).…”

Dysregulation of mTOR Signaling in Neuropsychiatric Disorders: Therapeutic Implications

“…mGluR1/5 links via Homer to PIKE (PI3 kinase enhancer) at synapses, where it engages PI3K-mTOR signaling in response to synaptic stimulation. PIKE, an upstream activator of mTOR and identified target of FMRP (Darnell et al, 2011) is elevated at the synapses of Fmr1 KO mice (Gross et al, 2010;Sharma et al, 2010), providing a functional link between loss of FMRP and overactivated mTOR signaling (Figure 1). These findings identify dysregulation of mTOR signaling as a phenotypic feature common to FXS, TSC1 and 2, NF1, and PTEN-associated autism syndromes.…”