Elsevier

Neurobiology of Disease

Volume 45, Issue 3, March 2012, Pages 1145-1152
Neurobiology of Disease

Lithium reverses increased rates of cerebral protein synthesis in a mouse model of fragile X syndrome

https://doi.org/10.1016/j.nbd.2011.12.037Get rights and content

Abstract

Individuals with fragile X syndrome (FXS), an inherited form of cognitive disability, have a wide range of symptoms including hyperactivity, autistic behavior, seizures and learning deficits. FXS is caused by silencing of FMR1 and the consequent absence of fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein that associates with polyribosomes and negatively regulates translation. In a previous study of a mouse model of FXS (Fmr1 knockout (KO)) we demonstrated that in vivo rates of cerebral protein synthesis (rCPS) were elevated in selective brain regions suggesting that the absence of FMRP in FXS may result in dysregulation of cerebral protein synthesis. Lithium, a drug used clinically to treat bipolar disorder, has been used to improve mood dysregulation in individuals with FXS. We reported previously that in the Fmr1 KO mouse chronic dietary lithium treatment reversed or ameliorated both behavioral and morphological abnormalities. Herein we report that chronic dietary lithium treatment reversed the increased rCPS in Fmr1 KO mice with little effect on wild type mice. We also report our results of analyses of key signaling molecules involved in regulation of mRNA translation. Our analyses indicate that neither effects on the PI3K/Akt nor the MAPK/ERK 1/2 pathway fully account for the effects of lithium treatment on rCPS. Collectively our findings and those from other laboratories on the efficacy of lithium treatment in animal models support further studies in patients with FXS.

Highlights

► Chronic lithium reverses increased cerebral protein synthesis in Fmr1 KO mice ► Chronic lithium has little effect on cerebral protein synthesis in WT mice ► PI3K/Akt pathway signaling does not fully account for effects on hippocampal rCPS ► MAPK/ERK 1/2 pathway signaling does not fully account for effects on hippocampal rCPS

Introduction

Individuals with fragile X syndrome (FXS), an inherited form of intellectual disability, show a broad spectrum of morphologic, cognitive, behavioral, neurological, and psychological features (Chonchaiya et al., 2009). FXS is caused by the silencing of the FMR1 gene and the consequent absence of its protein product, fragile X mental retardation protein (FMRP) (Brown, 2002). FMRP is a RNA-binding protein that associates with polyribosomes and negatively regulates translation of certain mRNAs (Penagarikano et al., 2007). It is thought that absence of FMRP results in a dysregulation of protein synthesis and that it is this dysregulation that has such profound consequences for development and function of the nervous system. Protein synthesis in the nervous system is required for normal nervous system development and maintenance. It is also essential for implementation of long lasting changes in synaptic strength such as occur in long term potentiation (LTP) and long term depression (LTD). Some forms of both LTD and LTP are impaired in Fmr1 knockout (KO) mice (Huber et al., 2004, Larson et al., 2005, Li et al., 2002, Suvrathan et al., 2010, Wilson and Cox, 2007, Zhao et al., 2005), and it has been proposed that a dysregulation of protein synthesis may underlie these impairments.

In accord with the idea that a dysregulation of protein synthesis may be at the heart of the defect in FXS, we have shown that adult male Fmr1 KO mice have elevated regional rates of cerebral protein synthesis (rCPS) compared with wild type (WT) littermates (Qin et al., 2005). We measured rCPS in vivo with a quantitative autoradiographic method (Smith et al., 1988). Effects were regionally selective occurring mainly in hippocampus, thalamus and hypothalamus. Other investigators have monitored the incorporation of 35S-methionine/cysteine into new protein in vitro. Their findings also indicate that incorporation is higher in hippocampal slices from Fmr1 KO mice (Dölen et al., 2007, Osterweil et al., 2010).

Treatment for FXS is largely symptom-based and multidisciplinary. Current treatments include special education, behavioral interventions, physical therapy and symptom-directed pharmacotherapy. There are several classes of pharmacotherapeutic agents currently under investigation in FXS that are aimed at the biochemical processes thought to underlie the disease. Among them are mGluR antagonists, antibiotics, and GABA agonists (Levenga et al., 2010). Lithium, an effective mood stabilizer for the treatment of bipolar disorder, has been used successfully in individuals with FXS to stabilize mood dysregulation (Al-Semaan et al., 1999). Results of a pilot add-on trial demonstrated that lithium may improve behavior, adaptive skills, and verbal memory in patients with FXS (Berry-Kravis et al., 2008). We and other groups have demonstrated that chronic dietary lithium treatment ameliorates many of the behavioral deficits seen in Fmr1 KO mice (Liu et al., 2011, Mines et al., 2010, Yuskaitis et al., 2010). Moreover, lithium treatment partially normalized the immature dendritic spine morphology in medial prefrontal cortex (Liu et al., 2011). Lithium has also been shown to modify abnormal behavior and morphology in a Drosophila model of FXS (McBride et al., 2005). Taken together results from these studies suggest that lithium could provide significant therapeutic benefits in FXS.

How lithium treatment may effect these therapeutic outcomes is not understood. In the current study, we measure rCPS in WT and Fmr1 KO mice to investigate whether lithium may act through an effect on the control of translation. Our results indicate that, in addition to its effects on behavior and morphology, lithium also normalizes the elevated rCPS in Fmr1 KO mice. We also present in this paper effects of lithium treatment on some of the steps in signaling pathways that control cap-dependent translation.

Section snippets

Animals

Male WT and Fmr1 KO mice (n = 79), generated by FVB/NJ-fmr1tm1Cgr breeding pairs (heterozygous females and homozygous males), were used. The generation of Fmr1 KO mice and their genotyping by PCR amplification of tail DNA were described previously (Qin et al., 2005). All mice were group housed in a central facility and maintained under controlled conditions of normal humidity and temperature with standard alternating 12-h periods of light and darkness. All procedures were carried out in

Cerebral protein synthesis

We studied WT and Fmr1 KO mice under two conditions, control with normal rodent chow and experimental with lithium-supplemented chow. Physiological variables were measured at the time of determination of rCPS. The four groups were well matched with regard to age, body weight, and physiological status (Table 1). Arterial plasma leucine concentration was affected by the lithium treatment; leucine concentrations were lower in the lithium-treated mice regardless of genotype (F(1, 29) = 30.51, p < 

Discussion

The central finding of our study is that chronic treatment with lithium reversed the increased rCPS found in Fmr1 KO mice and had little, if any, effect on rCPS in WT mice. Increased rCPS in the untreated Fmr1 KO mice were found primarily in the limbic system and hypothalamus, whereas the effects of lithium occurred throughout the brain. We extended our studies to examine in hippocampus the effects of lithium treatment on some of the signaling pathways that influence translation. Our results

Acknowledgments

We thank Zengyan Xia for overseeing the breeding colony and Tom Burlin for analyzing plasma samples for amino acid concentrations. We also thank Dr. De-Maw Chuang for helpful discussions at the outset of these studies. The research was supported by the Intramural Research Program of the National Institute of Mental Health, National Institutes of Health.

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