RUNX1T1 protein complexes in cortical excitatory neuron development

The cerebral cortex is responsible for higher-order cognitive functions like memory, learning and thought. These functions rely on a complex network composed of an array of excitatory and inhibitory neurons. Neuronal diversity is vital for the brain’s ability to perform complex tasks, as different neuron types perform specific tasks. Understanding how diversity is established offers insights into the construction of neural networks and how genetic mutations and neuronal deficiencies can impact behaviour. Moreover, knowledge of brain development and neuronal diversity is vital for advancing regenerative medicine. Cortical neurons are generated during embryogenesis from proliferative precursor cells in the embryonic brain. The acquisition of distinct identities is governed by genetic programs, driven by a specific class of proteins known as transcription factors. These nuclear proteins can turn genes ‘on’ or ‘off,’ thereby controlling the protein content and characteristics of cells. Transcription factors can function in two main ways: some bind to DNA, recognizing specific sequences to exert their effects. Others act as co-factors that attach to DNA-binding factors and recruit multiple other regulators, forming protein assemblies that either enhance or inhibit gene expression. Funded by a BBSRC project grant we recently discovered that the transcriptional co-factor RUNX1T1 plays a crucial role in specifying cortical inhibitory neurons during embryogenesis. This aligns with evolutionarily conserved roles for RUNX1T1 in flies and other species, where mutations in this gene lead to changes in cell fates. The importance of RUNX1T1 is underscored by human studies linking it to intellectual disability, mental retardation and autism spectrum disorder. The role of RUNX1T1 in the developing cerebral cortex, beyond the specification of cortical inhibitory neuron subtypes, remains unexplored. In addition to the cortical inhibitory interneuron lineage, we observe expression of Runx1t1 in the developing cerebral cortex, from the early stages of cortical excitatory neuron specification. Mouse embryos lacking RUNX1T1 show abnormalities in excitatory cell numbers and their projections, indicating deficiencies in the neuronal composition of the cortex and its ability to transmit information. Utilizing a robust method for identifying endogenous protein complexes in tissues, we discovered several putative interacting partners of RUNX1T1 in the embryonic brain, including DNA-binding proteins and other co-factors. Notably, 40% of these potential partners are candidate genes for neurodevelopmental disorders in humans. Our findings indicate that RUNX1T1 may serve as a critical co-regulator of disease-associated genes, potentially exerting pleiotropic effects on cortical excitatory neurons. Leveraging on our extensive toolbox and preliminary data we aim to discover the specific contributions of RUNX1T1 and its partner proteins to cortical excitatory neuron development. We will undertake a phenotypic characterisation of the developing cortex in RUNX1T1 mouse mutants. We will identify cortical neuronal types and genetic pathways impacted by the loss of RUNX1T1. We will investigate the mechanism of action of RUNX1T1 by identifying definitive interacting partners in the developing cerebral cortex. In line with the BBSRC’s priority area of Bioscience for Health, our findings will benefit not only academic researchers working in the field but also the wider public, as they will provide a better understanding of the mechanistic underpinnings of mutations in disease candidate genes paving the way for transformative interventional approaches.