Development of neuronal subtypes and local circuits in the hippocampus

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PROJECT SUMMARY/ABSTRACT During brain development, neurons must properly differentiate into distinct subtypes to assemble healthy circuits. Thus, disruption of this process can impact neural architecture and wiring, and contribute to disorders such as autism, schizophrenia, and epilepsy. The hippocampus is a brain structure crucial for learning and memory, and its function is compromised in these disorders. Excitatory pyramidal cells in area CA1 provide a major output of hippocampal computations to other brain regions. These cells can be parsed based on their physical position within CA1 as “deep” or “superficial.” Deep and superficial hippocampal pyramidal cells are distinct classes of neurons that exhibit differential molecular signatures, electrophysiological properties, sources of afferent input, and circuit connectivity with local inhibitory interneurons. Determining the mechanisms underlying their differentiation is crucial for understanding hippocampal development and function in both health and disease. Superficial pyramidal cells in CA1 preferentially express the transcriptional regulator Satb2, which controls gene expression by modifying chromatin structure. In humans, mutations of Satb2 cause developmental delay, intellectual disability, epilepsy, and autistic behaviors. Our preliminary data show that knocking out Satb2 during early development in mice disrupts the differentiation of superficial pyramidal cells in CA1. Furthermore, there are non-cell-autonomous changes to the migration and survival of distinct subtypes of interneurons in mutant mice relative to controls. In the present proposal, three specific aims will test the hypothesis that early expression of Satb2 is necessary for hippocampal pyramidal cell differentiation and circuit development in CA1, while later expression is necessary to promote experience- dependent synaptic plasticity. These experiments will use molecular genetic tools in mice to conditionally knock out Satb2 from pyramidal cells during both early and late developmental stages. Aim 1 will use electrophysiology and electrical stimulation to study the strength and plasticity of different sources of afferent input to deep and superficial CA1 pyramidal cells in acute slices. This aim will test the hypothesis that early Satb2 expression is necessary to establish differences in afferent input strength, while later expression is necessary for activity-driven synaptic plasticity of these inputs. Aim 2 will use paired whole-cell recordings between pyramidal cells (deep and superficial) and identified subtypes of interneurons to map circuits and study details of their synaptic physiology. This aim will test the hypothesis that early Satb2 expression is necessary to establish circuit motifs between local inhibitory interneurons and superficial pyramidal cells, while later expression is necessary to recruit new inhibitory synapses in response to environmental enrichment. Aim 3 will use single-cell RNA-seq and ATAC-seq to determine how Satb2 knockout alters gene expression and chromatin accessibility in CA1 at multiple developmental timepoints. This aim will provide molecular insight into how Satb2 controls gene expression in CA1 through development, and how its function may change over time.