Abstract
Although lithium has long been one of the most widely used pharmacological agents in psychiatry, its mechanisms of action at the cellular and molecular levels remain poorly understood. One of the targets of Li+ is the phosphoinositide pathway, but whereas the impact of Li+ on inositol lipid metabolism is well documented, information on physiological effects at the cellular level is lacking. We examined in two mammalian cell lines the effect of acute Li+ exposure on the mobilization of internal Ca2+ and phospholipase C (PLC)-dependent membrane conductances. We first corroborated by Western blots and immunofluorescence in HEK293 cells the presence of key signaling elements of a muscarinic PLC pathway (M1AchR, Gq, PLC-β1, and IP3Rs). Stimulation with carbachol evoked a dose-dependent mobilization of Ca, as determined with fluorescent indicators. This was due to release from internal stores and proved susceptible to the PLC antagonist U73122. Li+ exposure reproducibly potentiated the Ca response in a concentration-dependent manner extending to the low millimolar range. To broaden those observations to a neuronal context and probe potential Li modulation of electrical signaling, we next examined the cell line SHsy5y. We replicated the potentiating effects of Li on the mobilization of internal Ca, and, after characterizing the basic properties of the electrical response to cholinergic stimulation, we also demonstrated an equally robust upregulation of muscarinic membrane currents. Finally, by directly stimulating the signaling pathway at different links downstream of the receptor, the site of action of the observed Li effects could be narrowed down to the G protein and its interaction with PLC-β. These observations document a modulation of Gq/PLC/IP3-mediated signaling by acute exposure to lithium, reflected in distinct physiological changes in cellular responses.
Highlights
Lithium has been widely employed as a treatment of choice for bipolar disorders for well over six decades, but its exact mechanisms of action at the cellular and molecular levels have yet to be fully elucidated
The first goal was to determine whether the upregulation of the phospholipase C (PLC) pathway that had been observed in photoreceptors of the lancelet after iso-molar Na replacement with Li, can be reproduced in more standard preparations of mammalian cells, and obtains at much lower doses, which may be of pharmacological relevance
HEK-293 cells were initially selected as a convenient model system: microarray analysis (Atwood et al, 2011) had previously revealed significant mRNA levels for the core elements of the phosphoinositoid cascade: Gq and PLC-β; in addition, mRNA for diverse metabotropic receptors was found
Summary
Lithium has been widely employed as a treatment of choice for bipolar disorders for well over six decades, but its exact mechanisms of action at the cellular and molecular levels have yet to be fully elucidated. One of the earliest biochemical effects of lithium to be unveiled was the depletion of inositol and inositol-monophosphate (InsP1; Allison and Stewart, 1971; Allison et al, 1976), due to the inhibition of inositol monophosphatase (Hallcher and Sherman, 1980; Inhorn and Majerus, 1987) This spawned the celebrated inositol-depletion hypothesis of Li action (Berridge et al, 1982). The starting point of this work was the serendipitous observation that the photocurrent of light-sensitive neurons of the lancelet—which is mediated by melanopsin tapping into the phosphoinositide pathway—is augmented upon Na replacement with Li (Peinado et al, 2015) This was not due to a greater permeation of Li vs Na ions through the light-activated channels; the photo-induced mobilization of Ca from intracellular stores (Gomez and Nasi, 2009) was up-regulated by Li+. Establishing Li modulation of well-defined physiological processes that are important in neuronal communication can help understand its mode of action on the regulation of the activity of brain circuits
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