Genetic dissection of auditory circuit assembly

Harvard Medical School, Boston MA

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Abstract

Project Summary Type I Spiral Ganglion Neurons (SGNs) encode everything we hear and send this information to the brain. To achieve rapid and reliable signal transmission, Type I SGNs exhibit a number of specialized properties, including the ability to respond to glutamate via large, AMPA-receptor rich post-synaptic densities opposite pre-synaptic ribbons in inner hair cells. Additionally, SGNs with different molecular identities make synapses with different morphologies and positions. Type Ia SGNs form larger post-synaptic densities and smaller pre-synaptic ribbons on the pillar side of the IHC, whereas Type Ib and Ic SGNs form smaller post-synaptic densities and larger pre- synaptic ribbons on the modiolar side. Ic synapses appear to be more vulnerable to acoustic trauma and aging, which may be why some people have trouble understanding what they hear despite normal auditory thresholds. The long term goal of this project is to understand how SGNs acquire the properties needed for the perception of sound. Here, we will define the molecular events that link SGN subtype identity to synaptic heterogeneity. We hypothesize that the transcription factors Gata3, c-Maf, and Mafb cooperate to deploy shared and subtype- specific programs of gene expression that are needed to make synapses with subtype-appropriate properties. In support of this idea, Gata3 is a pioneer transcription factor that plays a dynamic role in SGN development, with effects both on generic and SGN-specific programs of neuronal gene expression (Appler et al., 2013). Gata3 works in part by inducing expression of c-Maf and Mafb, which act redundantly to ensure generation of three SGN subtypes and independently to shape synaptic properties across subtypes (Yu et al., 2013, Bastille et al., 2023). We found that SGN subtypes express different levels and combinations of c-Maf and Mafb that orchestrate different programs of synaptic gene expression and thus ensure formation of synapses with subtype- appropriate properties (Bastille et al., 2023). Nothing is known about how this code is put in place or executed. Here, we will investigate how Gata3, c-Maf, and Mafb elicit different programs of gene expression across SGN subtypes. Using a combination of cutting-edge molecular techniques to assay chromatin structure and subtype- specific gene expression, we will define how Gata3 controls c-Maf and Mafb expression and activity through its effects on the epigenome. In parallel, we will map Gata3 and Maf factor binding sites in each SGN subtype and assay how various combinations of Gata3, c-Maf, and Mafb induce different transcriptional outcomes when they interact with regulatory elements for shared versus unique genes. Finally, we will use a genetic approach to test how specific changes in the Maf code influence SGN identity and synaptic heterogeneity in vivo. These studies will establish a molecular mechanism for a key feature of SGN diversity and thus reveal an entry point for modifying synaptic properties to restore normal hearing after acoustic trauma or with age. Further, heterozygous mutations in GATA3, c-MAF, and MAFB cause developmental syndromes in humans, including effects on hearing. Thus, this work also holds implications for understanding the origins of human disease.