The Role of CNTNAP2 in the Development & Evolution of the Human Cerebral Cortex

The differing cognitive abilities of humans and other primates are accompanied by changes to forebrain neuronal circuit composition and function. This is underpinned by species-specific features of their development. Excitatory neurons in the human cerebral cortex have longer neurites and more elaborate neurite branching than chimpanzees or macaques, which is thought to contribute to differences in neural network performance. To study how differences in expression of single genes contributes to inter-species differences in neuronal form and function, my research focused on Contactin-Associated Protein-like 2 (CNTNAP2), a gene important for neuronal differentiation and synapse formation, for which there is accumulating evidence for differential use in the human cerebral cortex compared with other primates. Previous work in mice has found that loss of Cntnap2 function reduces neurite branching and dendritic spine density in vitro and in vivo. The potential evolutionary significance of this gene is highlighted by the presence of eight human accelerated regions (HARs), suggesting that there are human-specific aspects to its temporal and spatial expression during cortical development. The research presented in this thesis used human and non-human primate stem cell- derived forebrain neurons to study several aspects of CNTNAP2’s function in human cerebral cortex development. We present the first study of neurite outgrowth and neuronal activity in forebrain neurons generated from a human CNTNAP2 knockout (KO) pluripotent stem cell line. Differentiated human CNTNAP2 KO neurons have reduced neurite branching relative to wild type cells. Strikingly, the KO neurons were significantly more active - bursting more strongly and more frequently. We also applied a combination of bioinformatic and experimental approaches to show one or more of the CNTNAP2 HARs may be a gene enhancer. Our results indicate that loss of function mutations in CNTNAP2 contribute to human neurodevelopmental diseases through altering neuronal activity. This may be due to changes in neurite branching, and ultimately, to neuronal connectivity. These discoveries suggest a mechanism by which CNTNAP2 mutations causes disease in affected children, and may have contributed to the evolution of human-specialized brain function. This model will now be invaluable in deciphering the downstream molecular events caused by CNTNAP2 mutations, and may provide molecular targets for novel treatments in CNTNAP2 patients

Loss of CNTNAP2 Alters Human Cortical Excitatory Neuron Differentiation and Neural Network Development

BACKGROUND: Loss-of-function mutations in the contactin-associated protein-like 2 (CNTNAP2) gene are causal for neurodevelopmental disorders, including autism, schizophrenia, epilepsy and intellectual disability. CNTNAP2 encodes CASPR2, a single-pass transmembrane protein that belongs to the neurexin family of cell adhesion molecules. These proteins have a variety of functions in developing neurons, including connecting presynaptic and postsynaptic neurons, and mediating signalling across the synapse. METHODS: To study the effect of loss of CNTNAP2 function on human cerebral cortex development, and how this contributes to the pathogenesis of neurodevelopmental disorders, we generated human iPSCs from one neurotypical control donor null for full-length CNTNAP2, modelling cortical development from neurogenesis through to neural network formation in vitro. RESULTS: CNTNAP2 is particularly highly expressed in the first two populations of early-born excitatory cortical neurons, and loss of CNTNAP2 shifted the relative proportions of these two neuronal types. Live imaging of excitatory neuronal growth showed that loss of CNTNAP2 reduced neurite branching and overall neuronal complexity. At the network level, developing cortical excitatory networks null for CNTNAP2 had complex changes in activity compared to isogenic controls: an initial period of relatively reduced activity compared with isogenic controls, followed by a lengthy period of hyperexcitability, and then a further switch to reduced activity. CONCLUSIONS: Complete loss of CNTNAP2 contributes to the pathogenesis of neurodevelopmental disorders through complex changes in several aspects of human cerebral cortex excitatory neuron development that culminate in aberrant neural network formation and function