Molecular actions and therapeutic potential of lithium in preclinical and clinical studies of CNS disorders

Abstract

Lithium has been used clinically to treat bipolar disorder for over half a century, and remains a fundamental pharmacological therapy for patients with this illness. Although lithium's therapeutic mechanisms are not fully understood, substantial in vitro and in vivo evidence suggests that it has neuroprotective/neurotrophic properties against various insults, and considerable clinical potential for the treatment of several neurodegenerative conditions. Evidence from pharmacological and gene manipulation studies support the notion that glycogen synthase kinase-3 inhibition and induction of brain-derived neurotrophic factor-mediated signaling are lithium's main mechanisms of action, leading to enhanced cell survival pathways and alteration of a wide variety of downstream effectors. By inhibiting N-methyl-D-aspartate receptor-mediated calcium influx, lithium also contributes to calcium homeostasis and suppresses calcium-dependent activation of pro-apoptotic signaling pathways. In addition, lithium decreases inositol 1,4,5-trisphosphate by inhibiting phosphoinositol phosphatases, a process recently identified as a novel mechanism for inducing autophagy. Through these mechanisms, therapeutic doses of lithium have been demonstrated to defend neuronal cells against diverse forms of death insults and to improve behavioral as well as cognitive deficits in various animal models of neurodegenerative diseases, including stroke, amyotrophic lateral sclerosis, fragile X syndrome, as well as Huntington's, Alzheimer's, and Parkinson's diseases, among others. Several clinical trials are also underway to assess the therapeutic effects of lithium for treating these disorders. This article reviews the most recent findings regarding the potential targets involved in lithium's neuroprotective effects, and the implication of these findings for the treatment of a variety of diseases.

Figure 1. An overview of proposed signaling mechanisms underlying lithium’s neuroprotective effects

The neuroprotective effects of lithium against glutamate excitotoxicity are proposed to result from its interactions with cell survival and apoptotic machinery, as well as inhibition of receptor-mediated calcium entry. First, lithium can directly and indirectly reduce the activity of constitutively activated GSK-3 by multiple mechanisms, leading to disinhibition of several transcription factors, including CREB, HSF-1, and β-catenin, and subsequent induction of major cytoprotective proteins such as BDNF, VEGF, HSP70, and Bcl-2. GSK-3 is negatively regulated by Wnt-stimulated activation of the Frizzled receptor and decreased GSK-3 activity further reduces the activity of pro-apoptotic protein p53 and its downregulating effect on Bcl-2. Second, lithium-induced neurotrophic factors such as BDNF, in turn, activates its cell surface receptor and the downstream PI3K/Akt and MEK/ERK pathways. Both pathways are strongly associated with neuroprotective effects, which stimulate CREB and inhibit GSK-3. BDNF induction is an early and essential step for neuroprotection against glutamate excitotoxicity and may contribute to lithium-induced neurogenesis. Lithium also indirectly inhibits GSK-3 activity via PI3K-dependent activation of PKC and cAMP-dependent activation of PKA. Third, lithium inhibits NMDA receptor-mediated calcium influx, which in turn decreases subsequent activation of JNK, p38 kinase, and transcription factor AP-1. This NMDA receptor-mediated signaling plays a critical role in mediating glutamate-induced caspase activation and apoptosis. JNK activity is also inhibited by overexpression of HSP70. In addition, through inositol depletion, lithium reduces IP₃-mediated calcium release from the ER. Inhibition of intracellular calcium increase not only suppresses cellular stress, but also reduces the activity of calpain and calpain-mediated activation of pro-apoptotic Cdk5/p25 kinase. Lines with solid arrows represent stimulatory connections; lines with flattened ends represent inhibitory connections. Dashed lines represent pathways with reduced activity as a result of lithium treatment. Frizzled-R, Frizzled receptor; GPCR, G protein-coupled receptor; NMDA-R, NMDA receptor; RTK, receptor tyrosine kinase.

Figure 2. Lithium’s actions on inositol depletion and autophagy induction

Extracellular signal binding to its cell surface receptor, either GPCR or RTK, activates phospholipase C (PLC), which hydrolyzes the phospholipid PIP₂ to yield second messengers IP₃ and diacylglycerol (DAG). IP₃ is recycled by enzymes IPPase and IMPase and converted to inositol (mainly myo-inositol), which is required for PIP₂ re-synthesis. Lithium decreases intracellular inositol levels by directly inhibiting IPPase, IMPase, and inositol transporter (MIT) that uptakes extracellular inositol. Decreased intracellular inositol levels are expected to subsequently reduce PIP₂ and prevent the formation of IP₃ and DAG, thus blocking transmembrane signaling and trigging the induction of autophagy. Lines with solid arrows represent stimulatory connections; lines with flattened ends represent inhibitory connections. Dashed lines represent pathways with reduced activity as a result of lithium treatment. DAG, diacylglycerol; IP, inositol monophosphate; IP₂, inositol bisphosphate; MIT, inositol transporter; PLC, phospholipase C.

Figure 3. Inhibitory regulation of GSK-3 activity by lithium

Lithium regulates the activity of constitutively activated GSK-3 through multiple mechanisms. First, lithium is a competitive inhibitor of magnesium that directly inhibits ATP-magnesium-dependent catalytic activity of GSK-3. Second, lithium can indirectly increase serine phosphorylation of GSK-3 through PI3K-mediated phosphorylation/activation of Akt. The activity of GSK-3 is reduced by phosphorylation at this specific serine residue. Third, lithium can also disrupt the formation of the βArr2/PP2A/Akt complex that dephosphorylates/inactivates Akt, thereby increasing the serine phosphorylation of GSK-3. In addition, lithium can negatively regulate GSK-3 activity through other protein kinases including cAMP-dependent activation of PKA and PI3K-mediated activation of PKC (not shown), and through other mechanisms including downregulation of GSK-3 (not shown). Moreover, direct inhibition of GSK-3 by lithium interrupts the auto-regulation of GSK-3, by disinhibiting the inhibitory action of inhibitor-2 (I-2) on protein phosphatase-l (PP-1) that dephosphorylates GSK-3 at serine residues, and further decreases GSK-3 activity. Lines with solid arrows represent stimulatory connections; lines with flattened ends represent inhibitory connections. Dashed lines represent pathways with reduced activity as a result of lithium treatment. I-2, inhibitor-2; PP-1, protein phosphatase-l.

Figure 4. Negative regulation of Smad3/4-dependent transcription by lithium

Through stimulation of their cell surface receptors, TGF-β- and BDNF-triggered transcriptional activations are mediated by Smad3/4- and PI3K/Akt-dependent pathways, respectively. Lithium treatment reduces GSK-3β activity directly and indirectly via cAMP-dependent activation of PKA and BDNF-stimulated activation of PI3K/Akt pathways. These effects of lithium potentiate BDNF-induced phosphorylation/activation of CREB and increase cAMP response element (CRE)-mediated transactivation and expression of survival factors such as BDNF and Bcl-2. Increased BDNF-induced gene transcription causes sequestration of transcriptional co-activator p300, which suppresses Smad3/4-dependent transactivation and subsequently decreases the expression of TGF-β-responsive genes, PAI-1, and p21. Lines with solid arrows represent stimulatory connections; lines with flattened ends represent inhibitory connections. Dashed lines represent pathways with reduced activity as a result of lithium treatment. CRE, cAMP response element.