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While most of the efforts to uncover mechanisms contributing to bipolar disorder (BD) focused on phenotypes at the mature neuron stage, little research has considered events that may occur during earlier timepoints of neurodevelopment. Further, although aberrant calcium (Ca2+) signaling has been implicated in the etiology of this condition, the possible contribution of store-operated Ca2+entry (SOCE) is not well understood. Here, we report Ca2+and developmental dysregulations related to SOCE in BD patient induced pluripotent stem cell (iPSC)-derived neural progenitor cells (BD-NPCs) and cortical-like glutamatergic neurons. First, using a Ca2+re-addition assay we found that BD-NPCs and neurons had attenuated SOCE. Intrigued by this finding, we then performed RNA-sequencing and uncovered a unique transcriptome profile in BD-NPCs suggesting accelerated neurodifferentiation. Consistent with these results, we measured a slower rate of proliferation, increased neurite outgrowth, and decreased size in neurosphere formations with BD-NPCs. Also, we observed decreased subventricular areas in developing BD cerebral organoids. Finally, BD NPCs demonstrated high expression of the let-7family while BD neurons had increased miR-34a, both being microRNAs previously implicated in neurodevelopmental deviations and BD etiology. In summary, we present evidence supporting an accelerated transition towards the neuronal stage in BD-NPCs that may be indicative of early pathophysiological features of the disorder.

The AKT family of serine–threonine kinases functions downstream of phosphatidylinositol 3-kinase (PI3K) to transmit signals by direct phosphorylation of a number of targets, including the mammalian target of rapamycin (mTOR), glycogen synthase kinase 3β (GSK3β), and β-catenin. AKT binds to phosphatidylinositol (3,4,5)-triphosphate (PIP3) generated by PI3K activation, which results in its membrane localization and subsequent activation through phosphorylation by phosphoinositide-dependent protein kinase 1 (PDK1). Together, the PI3K–AKT signaling pathway plays pivotal roles in many cellular systems, including in the central nervous system where it governs both neurodevelopment and neuroplasticity. Recently, lysine residues (Lys14and Lys20) on AKT, located within its pleckstrin homology (PH) domain that binds to membrane-bound PIP3, have been found to be acetylated under certain cellular contexts in various cancer cell lines. These acetylation modifications are removed by the enzymatic action of the class III lysine deacetylases, SIRT1 and SIRT2, of the sirtuin family. The extent to which reversible acetylation regulates AKT function in other cell types remains poorly understood. We report here that AKT kinase activity is modulated by a class IIb lysine deacetylase, histone deacetylase 6 (HDAC6), in human neural progenitor cells (NPCs). We find that HDAC6 and AKT physically interact with each other in the neuronal cells, and in the presence of selective HDAC6 inhibition, AKT is acetylated at Lys163and Lys377located in the kinase domain, two novel sites distinct from the acetylation sites in the PH-domain modulated by the sirtuins. Measurement of the functional effect of HDAC6 inhibition on AKT revealed decreased binding to PIP3, a correlated decrease in AKT kinase activity, decreased phosphorylation of Ser552on β-catenin, and modulation of neuronal differentiation trajectories. Taken together, our studies implicate the deacetylase activity of HDAC6 as a novel regulator of AKT signaling and point to novel mechanisms for regulating AKT activity with small-molecule inhibitors of HDAC6 currently under clinical development.

Although circular RNAs (circRNAs) are enriched in the mammalian brain, very little is known about their potential involvement in brain function and psychiatric disease. Here, we show that circHomer1a, a neuronal-enriched circRNA abundantly expressed in the frontal cortex, derived from Homer protein homolog 1 (HOMER1), is significantly reduced in both the prefrontal cortex (PFC) and induced pluripotent stem cell-derived neuronal cultures from patients with schizophrenia (SCZ) and bipolar disorder (BD). Moreover, alterations in circHomer1awere positively associated with the age of onset of SCZ in both the dorsolateral prefrontal cortex (DLPFC) and orbitofrontal cortex (OFC). No correlations between the age of onset of SCZ and linear HOMER1mRNA were observed, whose expression was mostly unaltered in BD and SCZ postmortem brain. Using in vivo circRNA-specific knockdown of circHomer1ain mouse PFC, we show that it modulates the expression of numerous alternative mRNA transcripts from genes involved in synaptic plasticity and psychiatric disease. Intriguingly, in vivo circHomer1aknockdown in mouse OFC resulted in specific deficits in OFC-mediated cognitive flexibility. Lastly, we demonstrate that the neuronal RNA-binding protein HuD binds to circHomer1aand can influence its synaptic expression in the frontal cortex. Collectively, our data uncover a novel psychiatric disease-associated circRNA that regulates synaptic gene expression and cognitive flexibility.

Although circular RNAs (circRNAs) are enriched in the brain, their relevance for brain function and psychiatric disorders is poorly understood. Here, we show that circHomer1is inversely associated with relative HOMER1BmRNA isoform levels in both the orbitofrontal cortex (OFC) and stem-cell-derived neuronal cultures of subjects with psychiatric disorders. We further demonstrate that in vivo circHomer1knockdown (KD) within the OFC can inhibit the synaptic expression of Homer1bmRNA. Furthermore, we show that circHomer1directly binds to Homer1bmRNA and that Homer1b-specific KD increases synaptic circHomer1levels and improves OFC-mediated behavioral flexibility. Importantly, double circHomer1and Homer1b in vivoco-KD results in a complete rescue in circHomer1-associated alterations in both chance reversal learning and synaptic gene expression. Lastly, we uncover an RNA-binding protein that can directly bind to circHomer1and promote its biogenesis. Taken together, our data provide mechanistic insights into the importance of circRNAs in brain function and disease.

In this study, we examined changes in expression of calciumcalmodulin-dependent protein kinase IV (CaMKIV) in the mouse brain following chronic morphine treatment. Double immunohistochemical staining showed strong colocalization of CaMKIV with μ-opioid receptors. Chronic treatment with morphine produced an increase in expression of CaMKIV and phosphorylated cAMP response element-binding protein (pCREB) in the CA3 region of the hippocampus, whereas a decrease in CaMKIV and pCREB expression was observed in the caudate putamen. Interestingly, chronic morphine induced a decrease in protein expression of CaMKIV in the basolateral amygdale and the primary somatosensory cortex without any concomitant changes in pCREB. These findings suggest that CaMKIV-dependent signaling may play a role in chronic morphine-induced neuroplasticity in a brain region-specific manner.

Resting neurons use a specific calcium influx pathway to actively control transcription factor function.