Neuregulin 1 (NRG1) and ErbB4, critical neurodevelopmental genes, are implicated in schizophrenia, but the mediating mechanisms are unknown. Here we identify a genetically regulated, pharmacologically targetable, risk pathway associated with schizophrenia and with ErbB4 genetic variation involving increased expression of a PI3K-linked ErbB4 receptor (CYT-1) and the phosphoinositide 3-kinase subunit, p110δ (PIK3CD). In human lymphoblasts, 3,4,5 triphosphate [PI(3,4,5)P3] signaling is predicted by schizophrenia-associated ErbB4 genotype and PIK3CD levels and is impaired in patients with schizophrenia. In human brain, the same ErbB4 genotype again predicts increased PIK3CD expression. Pharmacological inhibition of p110δ using the small molecule inhibitor, IC87114, blocks the effects of amphetamine in a mouse pharmacological model of psychosis and reverses schizophrenia-related phenotypes in a rat neonatal ventral hippocampal lesion model. Consistent with these antipsychotic-like properties, IC87114 increases AKT phosphorylation in brains of treated mice, implicating a mechanism of action. Finally, in two family-based genetic studies, PIK3CD shows evidence of association with schizophrenia. Our data provide insight into a mechanism of ErbB4 association with schizophrenia; reveal a previously unidentified biological and disease link between NRG1-ErbB4, p110δ, and AKT; and suggest that p110δ is a previously undescribed therapeutic target for the treatment of psychiatric disorders.S chizophrenia is a severe neuropsychiatric disorder with a complex genetic etiology (1). Polymorphisms in the secreted growth factor neuregulin 1 (NRG1) have been associated with risk for schizophrenia (2-4) and recently, common genetic variation and structural microdeletions in ErbB4, a receptor tyrosine kinase for NRG1, have also been associated with the disorder (5-9). NRG1-ErbB4 signaling plays a critical role in neural development and synaptic plasticity (10-12) and NRG1 and ErbB4 mutant mice exhibit behavioral alterations (2, 12-14) consistent with other murine models of schizophrenia (15). Schizophrenia-associated genetic variation in these genes is implicated in human brain structure and function (6,16,17), but the biological mechanisms accounting for these diverse effects and how they translate into illness are unclear.Increased NRG1 and ErbB4 expression has been observed in schizophrenia postmortem brain tissue (5, 7, 18) and in neurons derived from induced pluripotent stem cells (IPSCs) of patients (19). Risk-associated polymorphisms in these genes affect expression of specific NRG1 and ErbB4 isoforms in human brain (7,18,20). In particular, risk polymorphisms in ErbB4 (rs7598440, rs839523, and rs707284) predict increased expression of the ErbB4 CYT-1 receptor (5, 7), one of two biologically occurring ErbB4 isoforms, the other being ErbB4 CYT-2 (21). Unlike ErbB4 CYT-2, ErbB4 CYT-1 includes a phosphoinositide 3-kinase (PI3K)-binding site and is capable of activating the PI3K pathway (21-24). Class IA PI3Ks are activated by recepto...