Circadian Rhythms in Neuronal Models of Bipolar Disorder

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Bipolar disorder (BD) is a psychiatric disorder associated with heritable polygenic risk factors. While clinically heterogeneous, key clinical features of BD include disruptions in daily sleep and activity cycles indicating that circadian rhythm disruption is an important part of the disorder in many patients. Circadian rhythms are determined by molecular clocks comprised of “clock genes” whose expression is regulated by environmental factors such as light and temperature. In recent years, our group and others have made progress in developing cellular models of BD using induced pluripotent stem-cell (iPSC) derived neuronal progenitor cells (NPCs) and neurons. We have determined that chronotype (time of day preference, “morningness”) in BD patients and circadian rhythms in cells both predict the clinical response to lithium maintenance therapy. Using iPSC-based methods to grow human NPCs and neurons, we found that neurons from BD patients have a hyperexcitable phenotype that can be reversed by lithium selectively in neurons from lithium-responsive (Li-R) BD donors, but not in cells from lithium non-responders (Li-NR). Presently, we propose to build upon this work and further develop the iPSC-neuron model to further investigate circadian disruption in BD. In Aim 1, we will estimate the contributions of polygenic risk factors to circadian rhythm phenotypes in BD in excitatory neurons. We will study rhythms in iPSC-derived NPC and glutamatergic neurons grown from a well-characterized, extended family who share high genetic risk for BD, but are discordant for the BD diagnosis. Presumably, cells from these genetically similar individuals will contain BD risk genes across a gradient that can be quantified using polygenic risk scores (PRS). By evaluating the relationship between PRS and rhythm parameters, we will estimate the aggregate contributions of genetic risk for BD to the expression of circadian rhythm abnormalities. In Aim 2 we propose to assess the contribution of BD-associated gene sets to circadian rhythms in neurons. Genome-wide association studies (GWAS) of sleep and circadian phenotypes suggest the existence of a shared genetic basis for mood disorders and chronotype. In this aim, we will study synchronized cells over a 24 h period to identify rhythmically expressed genes in neural progenitor cells (NPCs) from BD patients and controls and describe rhythm differences between diagnostic groups. We expect to find that many genes change their rhythm or lose rhythms altogether in BD neurons. We will then use a molecular biology method called siRNA knockdown to reduce the expression of candidate genes linked to BD and the circadian clock, and assess their loss of function in circadian rhythm assays, under constant or temperature-entrained conditions. In Aim 3 we propose to measure circadian rhythms in iPSC-derived GABAergic medium-spiny neuron-like cells. Our previous cell-based studies of BD employed, excitatory glutamatergic neurons. Animal models of BD show important contributions of the clock genes on dopamine projections to inhibitory, GABAergic medium spiny neurons (MSN) in the striatum. In this aim, we will differentiate iPSCs from BD patients (Li-R and Li-NR) into inhibitory, GABAergic MSN-like neurons using an established, high efficiency protocol. We will use the Per2-luc circadian rhythm reporter to measure rhythms in live human GABAergic neurons for the first time and assess the effects of mood stabilizing drugs such as lithium and antipsychotic drugs upon them. At the conclusion of the study, we will have a more advanced understanding of which neurons lose rhythms in BD, which BD-risk genes are involved in the circadian disruption, and how circadian disruption affects the function of neurons. This information may help to better identify lithium- responsive BD patients and link them efficiently to effective treatments.