A Transcriptional Switch Between KLF Activators and Repressors in Maturing Neocortex - Project Summary / Abstract The mammalian brain is not fully developed at birth. In humans, mice and other mammals, neocortical neurons largely complete neurogenesis, migration and the initial stages of axonal outgrowth in the embryo, but most refinement and myelination of axonal connections occurs after birth, and is profoundly influenced by neuronal activity and the hormonal milieu. These and other changes in the cellular and synaptic physiology of cortical neurons are achieved in part through massive changes in gene expression involving both upregulation of genes important for mature circuit function, and downregulation of transcriptional programs needed for proliferation, migration and axon outgrowth. Mechanisms controlling this transition are not well understood. Recent CRISPRi loss of function experiments identified a developmental switch between activators and repressors in the Kruppel-like (KLF) family of transcription factors that contribute to this transition. KLF activators are turned on before birth and activate a coordinated program of genes known to be necessary for axonal outgrowth. But unlike many genes with roles in early development, these genes are then turned off by a fall in KLF activators and a rise in redundant KLF repressors, a conserved event that occurs around the time of eye opening in rodents and around birth in humans. Here we address two important questions about this switch: 1) how is it initiated? And, 2) what developmental benefit might accrue from turning off a transcriptional program for axon outgrowth? We hypothesize that the benefit lies in permitting a period of extensive axonal refinement and pruning to proceed unopposed. We hypothesis that this transition is driven by a burst in thyroid hormone (TH) occurring at the same time. The more important repressor, Klf9 is known to be a TH target in multiple systems, and developmental loss of TH is known to impair pruning of cortical axons and if not treated, to produce profound and lasting Intellectual Disability. We will test these hypotheses by determining whether Klf9 expression depends on TH in cortical neurons and whether this is cell autonomous and reversible. We will also test whether repressors Klf9 and Klf13 are required for normal pruning of corticospinal and callosal axons, and whether increasing their expression is sufficient to rescue pruning after loss of TH. These experiments will empower a broader research program into mechanisms by which axonal refinement is coordinated with other features of circuit maturation under the control of neuronal activity and hormonal signaling.
