Activation of Akt by lithium: Pro-survival pathways in aging

Abstract

The effects of lithium on senescence were investigated using the senescence-accelerated mouse prone 8 (SAMP8) mice and cultures of aging cerebellar granule cells. Our in vitro findings, using cerebellar granule neurons, demonstrate that lithium (1–10 mM) exerts neuroprotective effects in young cultures (7–8 days) against LY294002-induced Akt inhibition. Furthermore, lithium (10 mM) inhibits GSK-3β activity by upregulating p-GSK-3β (ser-9) and increases p-FOXO1 (Ser256) suggesting an effective anti-apoptotic effect. Our data also showed that lithium in aged cultures exerts anti-apoptotic effects via Akt activation and consequent inhibition of downstream targets regulated by this enzyme. Finally, the administration of lithium to senescence-accelerated mice (SAMP8) and senescence-accelerated resistant mice (SAMR1) at 3 months of age also caused increased Akt activity and p-FoxO-1. These results demonstrate the effectiveness of lithium in preventing age-related neural loss and the potential therapeutic applications of lithium in treatment/prevention of neurological disease.

Introduction

Lithium is a simple cation that has been used clinically since 1950 for the treatment of bipolar disorder (Manji et al., 1999). However, in the last decade numerous studies, either using animal models or human trials, suggest that this cation may delay progression of neurodegenerative diseases (Aghdam and Barger, 2007, Alvarez et al., 2002). Specifically, a clinical study performed with Alzheimer's patients demonstrated that lithium may be effective in treating neurological disease (Terao et al., 2006).

One of the hypothesized favorable actions of lithium on AD is via the blocking of tau hyper-phosphorylation. The mechanism or mechanisms responsible for widespread hyper-phosphorylation of proteins in AD is still unknown however the activation of signal transduction-related kinases are likely to play an important role. As such, the activation of C-Jun N-terminal kinase stress/activated protein kinase (Zhu et al., 2001), P38 (Zhu et al., 2000) and ERK (Perry et al., 1999), among others, have been extensively described in this process (Zhu et al., 2002, for review).

Another kinase that intimately associated with tau hyper-phosphorylation is glycogen synthase kinase 3 beta (GSK-3β) (Sugden et al., 2008), which is powerfully inhibited by (Aghdam and Barger, 2007). Lithium inhibits this enzyme by a direct competitive mechanism with Mg²⁺, suppressing the ATP-Mg-dependent catalytic activity of GSK-3β (Mora et al., 1999). In addition, lithium indirectly inhibits GSK-3β activity by triggering phosphorylation of GSK-3β at ser9 (Takashima, 2006). The inhibitory effect of lithium on GSK-3β may explain the anti-apoptotic and neuroprotective properties of this drug and support the potential application of this drug in neuropharmacology (Tajes et al., 2008, Caccamo et al., 2007, Wada et al., 2005, Yu et al., 2003).

In addition to its direct effects on GSK-3β, lithium has other targets. Lithium prevents the dephosphorylation of Akt through inhibition of PP2A and maintains Akt in an activated state (Mora et al., 2002). Interestingly, chronic treatment of rats with lithium increased the levels of anti-apoptotic proteins such as Bcl-2 and decreased the level of p53, a modulator of cell death proteins (Aghdam and Barger, 2007, Chalecka-Franaszek and Chuang, 1999). Likewise, chronic treatment of rats with lithium increases the levels of brain derived neurotrophic factor (BDNF) and thus activates TrkB receptors. Moreover, recent studies have demonstrated that chronic treatment of neuronal cell cultures with lithium increases the antioxidant capacity of the cells by increasing glutathione levels (Shao et al., 2008). In addition, lithium decreases the intracellular calcium increase mediated by the excitatory amino acid glutamate and may prevent cdk5/p35 breakdown and formation of the pro-apoptotic cdk5/p25 (Shao et al., 2005, Jordà et al., 2005). Therefore, the modulation of all these biochemical pathways could explain, in part, the neuroprotective properties of lithium in neuronal cell cultures against different insults such as glutamate excitotoxicity (Shao et al., 2005), ischemia, potassium withdrawal (Yeste-Velasco et al., 2007), colchicine (Jordà et al., 2005) and oxidative stress (Shao et al., 2008).

Importantly, with regards to neurodegeneration, lithium has been shown to be effective in treating experimental models of Alzheimer's disease (Alvarez et al., 2002), Parkinson's disease (Wada et al., 2005), and Huntington's disease (Wei et al., 2001). However, while the majority of lithium studies have established efficacy in models of neurodegeneration, few studies have examined the effects of lithium on the aging process, which precede and is intimately linked to neurodegenerative processes. Interestingly, a recent study performed in the worm Caenorhabditis elegans, a widely used model of aging, demonstrated that lithium delays the process of aging (McColl et al., 2008). Similarly, recent studies performed by our group, using the senescence-accelerated mouse model (SAMP8), demonstrated that this drug inhibited the enzyme GSK-3β and decreased tau phosphorylation (Gutierrez-Cuesta et al., 2007, Canudas et al., 2005). A clear and direct link between aging and the development of neurodegenerative diseases, as well as an understanding of the biological effects of lithium in vivo and in vitro is critical to evaluate the utility of this drug in the prevention strategy for neurodegeneration. Therefore, the aim of the present work was to study the neuroprotective effects of lithium in vitro, using aging cultures of cerebellar granule neurons (CGNs), which is an extensively used in vitro model of aging (Toescu and Verkhratsky, 2000; Mason et al., 1999) and in vivo using the well-characterized age-accelerated SAMP8 mouse model (Pallàs et al., 2008a, Pallàs et al., 2008b, Pelegrí et al., 2007, Sureda et al., 2006). A particular focus in this study was the activation of the pro-survival Akt pathway since preservation of this pathway in aging appears to be important in the treatment of neurological disorders.