Lithium drives coordinated changes in the mouse synaptic phosphoproteome

Lithium is the gold standard mood stabiliser used to treat cycling mania and depression in bipolar disorder. Despite seven decades of clinical use, the mechanisms of its mood stabilisation are incompletely understood, fundamentally limiting development of improved alternatives. Two established lithium targets, glycogen synthase kinase 3β (GSK3β) and inositol monophosphatase, both modulate phosphorylation, suggesting lithium may exert broad effects on neuronal phosphorylation networks. We performed a discovery-phase in vitro screen of 140 kinases at 10mM LiCl and demonstrated that lithium inhibits 17 kinases beyond GSK3β. We therefore used untargeted quantitative phosphoproteomics to create a comprehensive map of lithium’s neural phosphorylation signature in lithium-treated mouse synaptoneurosomes. Samples were collected at dawn and dusk to match the peaks in phosphorylation that are induced by the sleep/wake cycle. Pathway analysis revealed convergence on synaptic plasticity, neurotransmitter release, and chemical transmission. Critically, lithium-sensitive phosphoproteins are significantly enriched in bipolar disorder genome-wide association study (GWAS) loci, providing independent genomic evidence that the phosphorylation networks we identified are relevant to bipolar pathophysiology. We identified novel kinase targets and phosphorylation sites not previously associated with lithium’s mechanism of action and tied them to bipolar pathology. We further refined existing models of lithium’s action by showing that GSK3β inhibition is temporally restricted to dawn, indicating cross talk with sleep/wake cycles of phosphorylation. Overall, our data demonstrate that lithium’s pleiotropic effects result from coordinated multi-kinase network reorganisation rather than single-target inhibition — a principle with direct implications for rational polypharmacology in mood stabiliser development.