Bipolar disorder (BD) is characterized by profound affective dysregulation. Periods of aversive symptoms (depression, anxiety, decreased appetitive drive), alternate with mania (a state of enhanced appetitive drive for reward and pleasure). The clinical manifestation is heterogeneous, with diverse patterns of predominant symptoms, severity and duration. Notably, there are no current robust neurocircuit models to account for these clinical manifestations. Imaging and postmortem studies point to the amygdala, a nucleus embedded in circuits involved in threat and reward responses. Recent breakthroughs from our group and others are beginning to characterize molecularly identifiable, functionally divergent sets of amygdala neurons, which separately encode and regulate aversive and appetitive behaviors. Specifically, distinct neuronal types within the mouse amygdala promote aversive/fear responses (`FEAR-ON' neurons), vs. appetitive/reward responses (`APPT-ON' neurons). Our preliminary data using single-cell RNA sequencing show that analogous molecularly defined neuronal populations are present in human amygdala. Our overarching hypothesis is that neuronal populations impacting valence encoding and motivated behavior (FEAR-ON vs. APPT-ON neurons), are disrupted in BD, contributing to depression, anxiety and mania. What factors may regulate the functions of FEAR-ON and APPT-ON circuitry in health and disease states? An answer may lie within the distinctive molecular signatures of these neurons, consistent with their opposing functions. First, FEAR-ON and APPT-ON neurons express distinct molecular factors known to regulate fear/threat and reward processing within the amygdala, including anxiogenic (e.g. corticotropic releasing hormone [CRH]) and anxiolytic (e.g. neurotensin receptor 2 [NTSR2]) factors, respectively. Second, a well- validated distinguishing feature of amygdala FEAR-ON and APPT-ON neurons is their distinct expression pattern of Wnt/? catenin signaling molecules. This feature indicates that Wnt/? catenin pathways differentially regulate FEAR-ON and APPT-ON neurons. Pilot data also show altered expression of key molecules, including Wnt7a and CRH in the amygdala of people with BD. Our specific hypothesis is that cell-specific FEAR-ON and APPT- ON molecular factors modulating stress/anxiety and reward/appetitive behaviors are altered in BD, and that disruption of Wnt/? catenin pathways contributes to distinct abnormalities FEAR-ON and APPT-ON neurons. Human postmortem studies combining single-cell RNAseq, multiplex mRNA/protein cell labeling and quantitative analyses of RDoC clinical domains will test the hypothesis that quantifiable clinical `fingerprints' in BD are predictive of distinct patterns of molecular changes in FEAR-ON and APPT-ON neurons (Aims 1 and 2). Causal manipulation in mouse genetic models will mechanistically test the hypothesis that a disruption of Wnt signaling causally alters expression of reward- and stress- related molecules in circuits linking deep amygdala nuclei to the CE and nucleus accumbens (Aim 3).
These studies will examine neural circuit-based mechanisms for understanding emotional instability across appetitive and aversive behaviors in BD. Focus on circuit- and cell-specific roles of Wnt/? catenin and stress/reward-related molecular pathways may be transformative in our understanding of BD and point to novel interventions and therapeutic approaches.
