There is strong evidence that expanded polyglutamine tracts cause the host protein to misfold [5], but polyglutamine pathogenesis is a complex, multifactorial process. In SCA1, the nuclear localization of the mutant protein [6], protein context, and protein modification are all factors in disease expression [7,8]. Emamian et al. [7] demonstrated that serine 776 (S776) in ataxin 1 (ATXN1) is essential for its pathogenicity in transgenic mice; Chen et al. [8] showed that phosphorylation of this residue by Akt enables 14-3-3 to interact with and stabilize ATXN1. Genetic studies revealed that overexpression of Hsp70 suppresses ATXN1-induced degeneration, possibly through its effect on the misfolded protein [9]. We sought to determine whether we could improve the neurological dysfunction of SCA1 mutant mice by using pharmacologic therapy that targets one of the potential early downstream consequences of misfolded ATXN1. An early event in the pathogenic cascade is the down-regulation of the mRNAs of several genes. Lin et al. demonstrated dysregulation of several neuronal genes in the Purkinje cells of SCA1 transgenic mice even before the onset of behavioral or neuropathological changes, and they observed similar changes in the tissue of humans with SCA1 [10]. Therefore, we rationalized that if we use a drug that enhances gene expression, we might slow SCA1 progression. Lithium was recently shown to exert neuroprotective effects, possibly by affecting gene transcription [11,12]. Lithium is widely known for its use in the treatment of bipolar disease. The mechanism of lithium's mood-stabilizing activity has not been elucidated, in part because lithium exerts its effects on a wide range of cellular functions: for example, it inhibits inositol production, affects the protein kinase C signaling pathway, and inhibits glycogen synthase kinase 3 (GSK3) [13–15]. Furthermore, several studies in cellular and animal models have shown that lithium can suppress neurodegeneration induced by various kinds of insults, such as Alzheimer disease amyloid-β peptide [16], glutamate [17], and ischemia [17]. Carmichael et al. reported that lithium treatment partially rescued cell death in cells overexpressing Huntington disease exon 1 fragment with 74 glutamines, possibly by inhibiting GSK3 and subsequently altering gene transcription [11].

Lithium has been reported to exert its neuroprotective effects predominantly by inhibiting apoptosis through inhibition of GSK3β [27]. Given the lack of evidence linking apoptosis with the pathogenesis of SCA1 [18,28,29], we think it is unlikely that the beneficial effects of lithium on neurological function arose from such antiapoptotic activities in Sca1^154Q/2Q mice. There are, however, several other possible modes of activity by which lithium may improve the course of the disease in Sca1^154Q/2Q mice. Lithium has diverse molecular targets (including GSK3β) that affect gene expression, and chronic lithium treatment has been reported to increase the DNA binding activity of the transcription factor AP-1 in rat brains [30]. Since alteration in gene expression is an early step in SCA1 pathogenesis, it is conceivable that lithium could exert some of its neuroprotective effect by favorably affecting gene expression in the mutant mice. Although it is unclear whether the enhanced levels of Icmt/Pccmt mRNA contributed to the improved behavioral profile of treated Sca1^154Q/2Q mice, the normalization of its levels suggest that lithium might help also correct some of the transcriptional dysregulation underlying SCA1.

Our Sholl analysis data suggest that the lithium treatment could improve cognitive dysfunction in Sca1^154Q/2Q mice, possibly by partially rescuing dendritic atrophy of the mutants' hippocampal pyramidal neurons. It is interesting that chronic lithium treatment has been reported to prevent the stress-induced decrease in dendritic length in CA3 hippocampal neurons [31]. Thus, it is plausible that the pathogenic mechanism of mutant ATXN1-induced dendritic atrophy shares common downstream pathways with that of stress-induced dendritic remodeling, which was shown to depend on excitatory amino acid activity [32]. Lithium could also affect neurotransmission or neurogenesis. Chronic lithium treatment up-regulates synaptosomal uptake of glutamate [33], and lithium treatment enhances long-term potentiation in the rat dentate gyrus [34]. It has also been reported that chronic administration of lithium enhances neurogenesis in the dentate gyrus of adult rodents [35].

Our trials in a faithful mouse model of SCA1, which expresses endogenous levels of mutant, full-length ATXN1 protein in a proper spatial and temporal pattern and reproduces many features of the human disease, strongly suggest that the therapeutic potential of lithium in this disease deserves serious consideration. Importantly, the lithium trial improved the motor impairment even when it was started after the onset of disease in Sca1^154Q/2Q mice. However, it remains unclear whether all the beneficial effects of lithium would last throughout the disease course. When we tested the mice on the rotarod at 20 weeks of age, we found signs of improved performance but the differences failed to reach statistical significance between the treated and untreated mutant mice (p = 0.102, by repeated measures ANOVA). Thus, lithium may not confer long-lasting benefits. Nonetheless, we cannot rule out whether using a less challenging paradigm might be required to delineate differences in the motor performance at this stage. We did not observe a significant improvement in life span (unpublished data), but life span measurement could also be confounded by various factors. For instance, it has been shown that long-term lithium treatment could disturb serum ionic balance in rodents [36] such that effects on normal functions of organs outside the central nervous system could have confounded life span measures. Further, because the Sca1^154Q/2Q mice express a CAG tract that is much longer than typically seen in human patients (in order to produce a phenotype during the short life span of the mouse [37]), their disease might be more aggressive than the human condition. Finally, the therapeutic and toxic potential of lithium have been well documented; tremor and lack of coordination are among the side effects reported, particularly when the dose is not carefully monitored [38]. Because such side effects could be of concern when treating patients with a disorder of cerebellar dysfunction, we evaluated tremor events in mice using a tremor monitoring system. We did not find any indication of increased tremor in chronically lithium-treated Sca1^154Q/2Q or wild-type mice (unpublished data), suggesting that the lithium trial at the doses used in our study did not induce tremors even in the context of an ATXN1 mutation.

It is interesting to note that lithium has produced beneficial effects in a cellular, Drosophila, and mouse model of Huntington disease toxicity in which fragments of polyglutamine expanded mutant huntingtin were expressed. Lithium chloride treatment reduced toxicity induced by overexpression of Huntington disease exon 1 fragment with 74 glutamines in neuronal and non-neuronal cell lines [11], and attenuated toxicity of the N-terminal part of mutant huntingtin with 120 glutamine repeats in Drosophila [39]. Wood and Morton recently showed that chronic lithium treatment improved motor function in a transgenic mouse model of Huntington disease (the R6/2 line) that expresses exon 1 of the human huntingtin gene with approximately 150 CAG repeats [40]. In this study, lithium improved the performance of the mice in one behavioral paradigm, the rotating rod, but only in animals treated postsymptomatically [40]. It remains to be seen if lithium therapy would be beneficial in mouse models of HD that express the full-length protein. This experiment is critical because protein context has increasingly been shown to be important in the pathogenesis of polyglutamine disorders, such that normal function of the full-length host protein may determine molecular pathways, pathogenic progression, and potential therapeutic targets [41,42]. Here, we show the beneficial effects of chronic lithium treatment on multiple measures in an SCA1 disease model—Sca1^154Q/2Q mice—that express the full-length mutant protein in the endogenous spatiotemporal pattern and reproduce most features of human SCA1. Future studies using other polyglutamine diseases, particularly the knock-in mouse models, will be necessary to determine whether lithium will prove effective for polyglutamine diseases in general or will have a more limited therapeutic role.