In this project, Jeffrey Macklis will directly investigate interhemispheric callosal projection neuron (CPN) axonal growth cone (GC) molecular machinery, abnormalities of CPN connectivity and dose-dependent direct effects of aberrant cortical associative circuitry in mice mutant for autism spectrum disorder (ASD) risk gene Bcl11a/Ctip1. To do so, they will employ both a newly developed, powerful approach of subtype-specific GC purification and quantitative, high-depth proteomic and transcriptomic analysis, and a newly developed system for mosaic genetic analysis of interspersed neurons, including their GCs, all linked to ASD-related mouse behavior.

Macklis and colleagues have recently shown that the associative, interhemispheric CPN circuitry is disrupted in Bcl11a/Ctip1 mutant mice via both dysregulation of cortical sensori-motor areal allocation and aberrant subtype specification and balance¹,²; others have identified human BCL11A/CTIP1 mutation as a monogenic cause of autism and intellectual disability. Importantly, Bcl11a/Ctip1 mutant mice phenocopy brain development deficits (partial agenesis of the corpus callosum and microcephaly) and core social interaction behavioral impairments observed in individuals with ASD with BCL11A/CTIP1 mutations.

The Macklis lab has recently developed a powerful approach to identify directly from the brain subtype- and stage-specific axonal GC molecular networks (simultaneous with those of parent somata) that underlie the development of diverse synapses and precise connectivity, leading to normal and abnormal behavior. They will first apply this ‘subcellular RNA-proteome mapping’ approach, combined with their newly developed binary expression aleatory mosaic (BEAM) genetic mosaic analysis system, to CPN-specific GCs with Bcl11a/Ctip1 conditional gene deletion. In parallel, they will directly test linkage of affected circuitry in a dose-dependent manner with ASD-related mouse social interaction and cognitive behavior.

Macklis’ work on wild-type CPN GC proteomes and transcriptomes has already identified an entirely new biology indicating semi-autonomy of GCs with molecular mechanisms not detectable in their somata. This powerful and innovative approach can be combined with a newly developed system for dual-population mosaic analysis of spatially and temporally identical mutant versus wild-type GCs/somata from neurons of the same exact area of the brain. They will pursue the first ASD-directed application of these powerful new approaches to directly interrogate the GC molecular machinery of CPN circuit development and maintenance, and ASD-related behavior. This work will connect insights from an ASD risk gene to cortical circuits and behavior.