GABA-ERGIC DYSFUNCTION IN SCHIZOPHRENIA: FROM POSTMORTEM STUDIES TO ANIMAL MODELS

Abstract

Nine years have passed since the first postmortem DNA microarray studies of schizophrenia were published, and it appears that this technologyhas generated several important leads that continue to shape translational neuroscience studies investigating the cellular/molecular bases of schizophrenia. Specifically, independently replicated expression profiling studies of the prefrontal cortex in schizophrenia revealed abnormalities in expression of genes encoding 1) synaptic proteins, 2) proteins related to GABA signaling, 3) immune/chaperone system proteins, 4) metabolic pathway components and 5) oligodendrocyterelated proteins. The GABA-ergic systemic dysfunction is perhaps the most consistently observed deficit associated with schizophrenia. At a molecular level, the PFC of humans with schizophrenia is characterized by an interrelated transcript deficit that consists of downregulation of BDNF, TRKB, GAD67, SST, NPY, PARV, CCK and GABRAD genes, clearly implicating the cortical GABAergic interneuron as a central component of the pathophysiology underlying the disease. Of these, reduction of the transcript encoding glutamic acid decarboxylase 67 kDa (GAD67) is the most robust and consistently replicated finding across different cohorts. GAD67, the critical GABA synthesis enzyme in the brain, is downregulated in distinct interneuron populations of the human cortex. Each of these affected cell types mediate a different kind of inhibition: 1) a chandelier-cell subpopulationof parvalbumin (PARV)-GAD67neurons is responsible for regulating the output of projection neurons at the axon initial segment, 2) small basket andMartinotti cells containingGAD67+ CALB+SST(±NPY) are responsible for the inhibition of the distal dendritic tree of pyramidal cells, and 3) GAD67+CALR(±CCK) interneurons regulate both the dendritic inputs of pyramidal cells as well as provide input to other GABAergic neurons. To understand the GABAergic dysfunction in schizophreniawemust develop animalmodels that modulate gene expression in a phenotypic and regional fashion. Therefore, we hypothesized that transgenic mouse models directed to cortical downregulation of GAD67 in distinct interneuron subtypes should both mimic the molecular and cellular human postmortem findings in schizophrenia, and have distinct consequences on cortical functioning. To test this hypothesis, we developed a novel BAC-driven transgenic mice system that is capable of cell-type specific transcript downregulation using an endogenous miRNA processing cellular mechanism. We generated several transgenic mice lines with cell-type specific downregulation of GAD67 protein in theNPY+, CCK+and PV+ interneurons using exon-embedded miRNA. This transgenic approach allowed us rapid, cell type-specific in vivo downregulation of the transcripts of interest (reduction of GAD67 in specific interneuronal subpopulations), avoiding the labor-intensive and resource-demanding generation of conditional knockout animals. These animal models will allow us to gain a critical understanding of the mechanisms underlying cortical inhibition and the anatomical and behavioral consequences of disturbing this network. Furthermore, the mice generated in this proposal may be useful for testing current lead compounds with therapeutic indications for symptoms of schizophrenia, and aid in the knowledge-based development of drugs for this devastating disease.