Synapse-specific accumulation of lithium in intracellular microdomains: a model for uncoupling coincidence detection in the brain

Lithium isotopes differentially modify mitochondrial amorphous calcium phosphate cluster size distribution and calcium capacity

Lithium is commonly prescribed as a mood stabilizer in a variety of mental health conditions, yet its molecular mode of action is incompletely understood. Many cellular events associated with lithium appear tied to mitochondrial function. Further, recent evidence suggests that lithium bioactivities are isotope specific. Here we focus on lithium effects related to mitochondrial calcium handling. Lithium protected against calcium-induced permeability transition and decreased the calcium capacity of liver mitochondria at a clinically relevant concentration. In contrast, brain mitochondrial calcium capacity was increased by lithium. Surprisingly, 7Li acted more potently than 6Li on calcium capacity, yet 6Li was more effective at delaying permeability transition. The size distribution of amorphous calcium phosphate colloids formed in vitro was differentially affected by lithium isotopes, providing a mechanistic basis for the observed isotope specific effects on mitochondrial calcium handling. This work highlights a need to better understand how mitochondrial calcium stores are structurally regulated and provides key considerations for future formulations of lithium-based therapeutics.

Dual effect of lithium on NFAT5 activity in kidney cells

Lithium salts are used widely for treatment of bipolar and other mental disorders. Lithium therapy is accompanied frequently by renal side effects, such as nephrogenic diabetes insipidus or chronic kidney disease (CKD), but the molecular mechanisms underlying these effects are still poorly understood. In the present study we examined the effect of lithium on the activity of the osmosensitive transcriptional activator nuclear factor of activated T cells 5 (NFAT5, also known as TonEBP), which plays a key role in renal cellular osmoprotection and urinary concentrating ability. Interestingly, we found different effects of lithium on NFAT5 activity, depending on medium osmolality and incubation time. When cells were exposed to lithium for a relative short period (24 h), NFAT5 activity was significantly increased, especially under isosmotic conditions, resulting in an enhanced expression of the NFAT5 target gene heat shock protein 70 (HSP70). Further analysis revealed that the increase of NFAT5 activity depended primarily on an enhanced activity of the c-terminal transactivation domain (TAD), while NFAT5 protein abundance was largely unaffected. Enhanced activity of the TAD is probably mediated by lithium-induced inhibitory phosphorylation of glycogen synthase kinase 3β (GSK-3β), which is in accordance with previous studies. When cells were exposed to lithium for a longer period (96 h), cellular NFAT5 activity and subsequently expression of HSP70 significantly decreased under hyperosmotic conditions, due to diminished NFAT5 protein abundance, also resulting from GSK-3β inhibition. Taken together, our results provide evidence that lithium has opposing effects on NFAT5 activity, depending on environmental osmolality and exposure duration. The potential impacts of these observations on the diverse effects of lithium on kidney function are discussed.

Lithium's role in neural plasticity and its implications for mood disorders

OBJECTIVE

Lithium (Li) is often an effective treatment for mood disorders, especially bipolar disorder (BPD), and can mitigate the effects of stress on the brain by modulating several pathways to facilitate neural plasticity. This review seeks to summarize what is known about the molecular mechanisms underlying Li's actions in the brain in response to stress, particularly how Li is able to facilitate plasticity through regulation of the glutamate system and cytoskeletal components.

METHOD

The authors conducted an extensive search of the published literature using several search terms, including Li, plasticity, and stress. Relevant articles were retrieved, and their bibliographies consulted to expand the number of articles reviewed. The most relevant articles from both the clinical and preclinical literature were examined in detail.

RESULTS

Chronic stress results in morphological and functional remodeling in specific brain regions where structural differences have been associated with mood disorders, such as BPD. Li has been shown to block stress-induced changes and facilitate neural plasticity. The onset of mood disorders may reflect an inability of the brain to properly respond after stress, where changes in certain regions may become 'locked in' when plasticity is lost. Li can enhance plasticity through several molecular mechanisms, which have been characterized in animal models. Further, the expanding number of clinical imaging studies has provided evidence that these mechanisms may be at work in the human brain.

CONCLUSION

This work supports the hypothesis that Li is able to improve clinical symptoms by facilitating neural plasticity and thereby helps to 'unlock' the brain from its maladaptive state in patients with mood disorders.

Lithium: Molecular interactions, clinical actions

Lithium is one of the most widely used drugs in neuropsychopharmacology. Preclinical scientists have made several advances in ascertaining the molecular mechanisms of action of this cation; such as its ability to stabilize monoamine levels, to interact with second messengers, and its neuroprotective effects, possibly over suicidal behaviors. Nevertheless, there remains a gap of knowledge between the pharmacological advances and the number of reliable clinical trials, creating a lack of evidence-based medicine to support medical prescriptions. In this review we examine lithium’s molecular mechanisms of action and evaluate their relevance in clinical applications.

Lithium-induced enhancement of mitochondrial oxidative phosphorylation in human brain tissue

OBJECTIVES

Extensive preclinical and clinical evidence suggests mitochondrial dysfunction in bipolar disorder. Studies of brain energy metabolism in bipolar disorder suggest an impairment of energy generation by mitochondrial oxidative phosphorylation. Lithium is an effective drug widely used in treating bipolar disorder, but its mechanism of action has remained uncertain. The aim of this study was to clarify the effect of lithium on mitochondrial oxidative phosphorylation.

METHODS

We spectrophotometrically determined the activities of the respiratory chain complexes I + III [antimycin A-sensitive nicotinamide adenine dinucleotide (NADH) cytochrome c oxidorductase], complexes II + III (succinate cytochrome c oxidoreductase), succinate dehydrogenase, and complex IV [cytochrome c oxidase (COX)], and of the mitochondrial matrix enzyme citrate synthase in postmortem human brain cortex homogenates following exposure to lithium (up to 10 mM).

RESULTS

Activities of complexes I + III and of complexes II + III were dose-dependently increased by lithium with maximum values at 1 mM (165%, p = 0.03, and 146%, p = 0.00002, of controls). Activity of succinate dehydrogenase remained unchanged up to 2 mM, but was raised at higher drug concentrations (maximum 220%, p = 0.01, of controls). In contrast, activity of COX was not significantly affected by the drug (decrease of 12% at 1 mM, p = 0.4).

CONCLUSIONS

Our study suggests that lithium stimulates mitochondrial respiratory chain enzyme activities at clinically relevant concentrations. Lithium's effect on the mitochondrial respiratory chain presents further evidence of the pathophysiological significance of mitochondrial dysfunction in bipolar disorder. The effect may be relevant to the therapeutic efficacy of the drug by potentially reversing a disease-related alteration.

Estradiol and lithium chloride specifically alter NMDA receptor subunit NR1 mRNA and excitotoxicity in primary cultures

Glutamate facilitates calcium influx via NMDAR, and excess calcium influx increases excitotoxicity--a pathological characteristic of neurological diseases. Both 17beta-estradiol (E2) and lithium influence NMDAR expression/signaling and excitotoxicity. This led us to hypothesize that combined E2 and lithium will alter NMDAR expression and excitotoxicity. We tested this hypothesis using primary cell cultures from the cortex and hippocampus of C57BL/6J fetal mice pretreated with E2, lithium chloride (LiCl) and combined E2/LiCl for 12, 24 or 48 h. We examined cultures for brain cell type and changes in cell type caused by experimental procedures using glia and neuron gene specific primers. These cultures expressed increased glial fibrillary acidic protein (GFAP) mRNA with low neurofilament-heavy chain (NF-H) mRNA expression. Subsequent analysis of cortical cell cultures indicated that combined E2/LiCl decreased NR1 mRNA expression after a 12 and 48 h treatment period. Combined E2/LiCl also reduced NR1 mRNA expression in hippocampal cultures but only after a 48 h treatment period. LiCl-treated hippocampal cultures also reduced NR1 mRNA expression after a 24 and 48 h treatment. We next examined the response of 48 h pretreated cultures to a toxic level of glutamate. Excitotoxicity was measured using fluorescein diacetate/propidium iodide (FDA/PI) cell viability assay. Results from FDA/PI assay revealed that LiCl pretreatment increased viability for cortical cultures while E2 and combined E2/LiCl reduced viability. All pretreatments for hippocampal cultures failed to increase viability. Our results showed combined E2/LiCl reduced NR1 mRNA and prevented protection against glutamate excitotoxicity in glial primary cultures.

Lithium: bipolar disorder and neurodegenerative diseases Possible cellular mechanisms of the therapeutic effects of lithium

Bipolar illness is a major psychiatric disorder that affects 1-3% of the worldwide population. Epidemiological studies have demonstrated that this illness is substantially heritable. However, the genetic characteristics remain unknown and a clear personality has not been identified for these patients. The clinical history of lithium began in mid-19th century when it was used to treat gout. In 1940, it was used as a substitute for sodium chloride in hypertensive patients. However, it was then banned, as it had major side effects. In 1949, Cade reported that lithium could be used as an effective treatment for bipolar disorder and subsequent studies confirmed this effect. Over the years, different authors have proposed many biochemical and biological effects of lithium in the brain. In this review, the main mechanisms of lithium action are summarised, including ion dysregulation; effects on neurotransmitter signalling; the interaction of lithium with the adenylyl cyclase system; inositol phosphate and protein kinase C signalling; and possible effects on arachidonic acid metabolism. However, none of the above mechanisms are definitive, and sometimes results have been contradictory. Recent advances in cellular and molecular biology have reported that lithium may represent an effective therapeutic strategy for treating neurodegenerative disorders like Alzheimer's disease, due to its effects on neuroprotective proteins like Bcl-2 and its actions on regulators of apoptosis and cellular resilience, such as GSK-3. However, results are contradictory and more specific studies into the use of lithium in therapeutic approaches for neurodegenerative diseases are required.

Inositol monophosphatase regulates localization of synaptic components and behavior in the mature nervous system of C. elegans

Although recent studies have provided significant molecular insights into the establishment of neuronal polarity in vitro, evidence is lacking on the corresponding phenomena in vivo, including correct localization of synaptic components and the importance of this process for function of the nervous system as a whole. RIA interneurons act as a pivotal component of the neural circuit for thermotaxis behavior in the nematode Caenorhabditis elegans and provide a suitable model to investigate these issues, having a neurite clearly divided into pre- and post-synaptic regions. In a screen for thermotaxis mutants, we identified the gene ttx-7, which encodes myo-inositol monophosphatase (IMPase), an inositol-producing enzyme regarded as a bipolar disorder-relevant molecule for its lithium sensitivity. Here we show that mutations in ttx-7 cause defects in thermotaxis behavior and localization of synaptic proteins in RIA neurons in vivo. Both behavioral and localization defects in ttx-7 mutants were rescued by expression of IMPase in adults and by inositol application, and the same defects were mimicked by lithium treatment in wild-type animals. These results suggest that IMPase is required in central interneurons of the mature nervous system for correct localization of synaptic components and thus for normal behavior.

Internalization of neuropeptide Y Y1 and Y5 and of pancreatic polypeptide Y4 receptors is inhibited by lithium in preference to sodium and potassium ions

The receptor-linked internalization of [125I] human neuropeptide Y (NPY) in Chinese hamster ovary (CHO) cells expressing the guinea-pig Y1 receptors or in human endometrial carcinoma-1B (Hec-1B) cells expressing the human Y5 receptor, as well as the receptor-linked internalization of human pancreatic polypeptide (hPP) receptor expressed in CHO cells, is selectively inhibited by low molarities of the Li+ cation. The Na+ and K+ cations decreased the receptor-linked internalization of agonist peptides only at high molar inputs, and largely in proportion to the reduction of cell surface binding of Y ligand peptides, dependent on ion concentration and the type of Y receptor examined. With particulates isolated from disrupted cells, there was no preferential inhibition by Li+ relative to Na+ in the binding of type-specific ligand peptides to Y receptors of any type. The observed difference could be connected to the known ability of Li+ to modify active conformations of signal transducers, which may also directly or indirectly affect the internalization motors. The decrease in the rate of Y receptor internalization by Li+ also points to a possible alteration of Y receptor signaling in vivo by lithium at acute therapeutically employed dose levels.

Li+ enhances GABAergic inputs to granule cells in the rat hippocampal dentate gyrus

Defects in GABAergic interneurons are thought to be involved in the pathophysiology of bipolar disorder, and Li+ has been used as a primary therapeutic agent in the treatment. We used the patch clamp technique to investigate whether Li+ affects on spontaneous GABAergic synaptic inputs to granule cells (GCs) in hippocampal dentate gyrus. Extracellularly applied Li+ (25 mM) markedly increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs), an effect completely blocked by picrotoxin or bicuculline. Li+ increased sIPSCs frequency in the presence of tetrodotoxin (TTX), but to a lesser extent than its absence. Li+ caused no change in the cumulative amplitude distribution of miniature IPSCs, indicating that a presynaptic mechanism is involved. When TTX was added in the presence of Li+, large-amplitude sIPSCs (>30 pA) were abolished specifically with no effect on small-amplitude sIPSCs (<20 pA). Intracellular Li+ (6 mM) applied via the patch pipette depolarized the resting membrane potential in fast-spiking interneurons, resulting in an increase in spontaneous action potential (AP) firing. This change, however, was not observed in GCs. These results suggest that Li(+)-induced spontaneous AP firing in GABAergic interneurons contributes to the increase in GABAergic synaptic inputs to GCs.

Opposite effects of lithium on proximal and distal caspases of immature and mature primary neurons correlate with earlier paradoxical actions on viability

To provide an explanation for earlier paradoxical findings of lithium on survival of mature and immature neurons, this study monitors changes in cytosolic caspases in rat cerebellar granule cells (CGC) grown 2-7 days in vitro (DIV), or in murine E-17 cortical neurons. Data show Li+ protects mature 7-DIV CGC parallel to a decrease in proximal and distal caspases but increases levels for immature 2-DIV-CGC or E-17 cortical neurons. Caspases mirror viability based on morphological analyses (dye uptake, phase-contrast, DNA fragmentation), and suggest protection occurs by suppressing activation of a cascade resulting in distal effectors that destroy proteins essential for neuronal survival. Protection was dose-dependent with EC50 3.0 mM and extended to 64 h in K+-serum deprived apoptotic media. Neuronal extracts contain a spectrum of proximal (-2, -8, -9) and distal (-3, -6) caspases sensitive to Li+ on assay with preferred peptide substrates and by immunoblotting. The lack of direct effect on activated cytosols indicates Li+ acts upstream only on intact cells, at sites for recruitment of pivotal procaspases. Alterations of procaspase-9 p46 and membrane-bound cytochrome c (Apaf-1) point to interaction with an intrinsic Mt-mediated pathway as one of the targets. The opposite effects on caspases and viability of immature or embryological neurons point to existence of alternative pathways that alter during neurite outgrowth suggesting the use of Li+ as a probe to unravel events relevant to neurogenesis.

Li+/Mg2+ competition at therapeutic intracellular Li+ levels in human neuroblastoma SH-SY5Y cells

OBJECTIVES

One proposed mechanism of lithium action in the treatment of bipolar disorder is that Li+ competes with Mg2+ for Mg2+ binding sites within the cell. In this study, we investigated this competition at therapeutic intracellular Li+ levels in human neuroblastoma SH-SY5Y cells.

METHODS

We used fluorescence spectroscopy and a Mg2+ indicator, furaptra, to investigate this competition in human neuroblastoma SH-SY5Y cells. Atomic absorption spectrophotometry was used for determination of the intracellular Li+ levels.

RESULTS

The neuroblastoma cells, incubated in 15 mM or 30 mM Li+-containing buffer, showed a significant increase in free intracellular Mg2+ levels [using a positive linear within-groups contrast t-test, the 15 mM condition produced t(2) = 5.0, one-tailed p < 0.02, and the 30 mM Li+-incubation conditions gave t(2) = 9.2, one-tailed p < 0.006] but did not significantly increase over time in the Li+-free condition [t(2) = 0.1, one-tailed p > 0.96]. At the earlier times during the incubation (1 or 10 min for the 15 mM or 30 mM Li+-containing buffers), the intracellular Li+ concentrations were 0.6-2.5 mM, values which are comparable to those reached in the brain of Li+-treated patients.

CONCLUSION

We demonstrated that competition between Li+ and Mg2+ can occur at therapeutic intracellular Li+ levels.

Lithium modulates desensitization of the glutamate receptor subtype gluR3 in Xenopus oocytes

Analysis of splice variants and site-directed mutants of the AMPA receptor GluR3 expressed in Xenopus oocytes has shown that lithium produces a large potentiation of the GluR3 flop splice variant and suggested that lithium might inhibit rapid desensitization, which is characteristic of this receptor (Karkanias, N. and Papke, R., Subtype-specific effects of lithium on glutamate receptor function. J. Neurophysiol., 81 (1999) 1506-1512). We now show that mutation of the 769R/ G desensitization site (Lomeli, H.M.J., Melcher, T., Hoger, T., Geiger, J.R., Kuner, T., Monyer, H., Higuchi, M.B.A. and Seeburg, P.H, Control of kinetic properties of AMPA receptor channels by nuclear RNA editing. Science, 9(266) (1994) 1709-1713) greatly attenuates the lithium-induced potentiation of GluR3. Additionally, experiments with the non-desensitizing site-directed mutant GluR3(L507Y) (Stern-Bach, Y., Russo, S., Neuman, M. and Rosenmund, C., A point mutation in the glutamate binding site blocks desensitization of AMPA receptors. Neuron, 21 (1998) 907-918) further confirms that lithium enhances GluR3 responses by reducing desensitization, since lithium's effects are reversed in this mutant. Lithium's effects on GluR3 desensitization are distinct from the effects of aniracetam on desensitization. Specifically, aniracetam, which potentiates wild-type AMPA receptors, is ineffective on the non-desensitizing GluR3(L507Y) mutant, but has synergistic effects with lithium on wild-type receptors.

Subtype-specific effects of lithium on glutamate receptor function

We report that substitution of sodium with lithium (Li+) in the extracellular solution causes subtype-specific changes in the inward and outward currents of glutamate receptors (GluRs), without a shift in reversal potential. Li+ produces an increase of inward and outward currents of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors and decreases in the currents of kainate (KA) and N-methyl-D-aspartate receptors. The greatest effect of Li+ was observed with GluR3. A concentration-response curve for GluR3 reveals that the potentiation caused by Li+ is greatest at saturating agonist concentrations. GluR1, which shows no potentiation by Li+ at 100 microM KA, shows a small but significant potentiation at saturating KA and glutamate concentrations. The effects of Li+ on outward current, where Li+ is not the primary charge carrier, and the lack of reversal potential shift argue for a mechanism of potentiation not associated with Li+ permeation. This potentiation of current is specific for Li+ because rubidium, although causing an increase of inward current, shifted the reversal potential and did not increase outward current. The effects of Li+ are different for KA, a weak desensitizing agonist, and glutamate, a strong desensitizing agonist, suggesting that Li+ might interact with a mechanism of desensitization. By using cyclothiazide (CTZ) to reduce desensitization of GluR3, we find that for low concentrations of KA and glutamate potentiation of the response by a combination of CTZ and Li+ is no greater than by CTZ or Li+ alone. However, at high concentrations of agonist, the potentiation of the response by a combination of CTZ and Li+ is significantly greater than by CTZ or Li+ alone. This potentiation was additive for glutamate but not for KA. At high agonist concentration in the presence of CTZ, the intrinsically lower desensitization produced with KA-evoked responses may preclude Li+ from potentiating the current to the same degree as it can potentiate glutamate-evoked responses. The additive effects of CTZ and Li+ were unique to the flop variant of GluR3.

Activation of KATP channels by Na/K pump in isolated cardiac myocytes and giant membrane patches

Strophanthidin inhibits KATP channels in 2,4-dinitrophenol-poisoned heart cells (). The current study shows that the Na/K pump interacts with KATP current (IK-ATP) via submembrane ATP depletion in isolated giant membrane patches and in nonpoisoned guinea pig cardiac cells in whole-cell configuration. IK-ATP was inhibited by ATP, glibenclamide, or intracellular Cs+. Na/K pump inactivation by substitution of cytoplasmic Na+ for Li+ or N-methylglucamine decreased both IK-ATP by 1/3 (1 mM ATP, zero calcium), and IC50 of ATP for IK-ATP (0.3 +/- 0.1 mM) by 2/5. The Na+/Li+ replacement had no effect on IK-ATP at low pump activity ([ATP] </= 0.1 mM or 100 microM ouabain) or when IK-ATP was completely inhibited by 10 mM ATP. In whole-cell configuration, ouabain inhibited up to 60% of inwardly rectifying IK-ATP at 1 mM ATP in the pipette but not at 10 mM ATP and 10 mM phosphocreatine when IK-ATP was always blocked. However, mathematical simulation of giant-patch experiments revealed that only 20% of ATP depletion may be attributed to the ATP concentration gradient in the bulk solution, and the remaining 80% probably occurs in the submembrane space.

Lithium homeostasis in Xenopus oocytes: implications for the study of signal transduction

The Xenopus oocyte has been shown to be a useful model for the study of signal transduction pathways. The present study investigated whether or not the oocyte could be used to study the effects of lithium on signal transduction mechanisms by comparing the dynamics of lithium homeostasis in the oocyte and a human immortalized hippocampal cell line using Flame Atomic Emission Spectroscopy (FAES). A biphasic pattern of lithium uptake was observed in the oocyte in the presence of 5 mM extracellular lithium. The late phase of lithium uptake, which started after 30 minutes of incubation time, was sensitive to phloretin, an inhibitor of Na+/Li+ counter-transport. Differences in lithium efflux kinetics further characterized the two observed phases of accumulation and also suggested that lithium might be distributed in different pools within the oocyte, including one sequestered in organelles or associated with cytosolic proteins. An analogous sequestered pool was not, however, observed in the hippocampal cell line indicating that lithium is distributed differently in these cell types. This suggests that the Xenopus oocyte might not be a suitable model for evaluating the effects of lithium on signal transduction pathways because of the unknown contribution of the sequestered pool on predicting relevant physiological effects.