Pannexin 1 regulates dendritic spines in developing cortical neurons




Sanchez-Arias, Juan C.

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Sensory, cognitive, and emotional processing are rooted in the cerebral cortex. The cerebral cortex is comprised of six layers defined by the neurons within them that have distinctive connection, both within cortex itself and with other subcortical structures. Although still far from solving the mysteries of the mind, it is clear that networks form by neurons in the cerebral cortex provide the computational substrate for a remarkable range of behaviours. This neuron to neuron activation is mediated through dendritic spines, the postsynaptic target of most excitatory synapses in the cerebral cortex. Dendritic spines are small protrusions found along dendrites of neurons, and their number, as well as structural changes, accompany the development of synapses and establishment of neuronal networks. In fact, dendritic spines undergo rapid structural and functional changes guided by neuronal activity. This marriage between structural and functional plasticity, makes dendritic spines crucial in fine-tuning of networks in the brain; not surprisingly, dendritic spine aberrations are a hallmark of multiple neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. With this in mind, I considered Pannexin 1 (Panx1) an interesting novel candidate for a regulatory role on cortical neuronal network and dendritic spine development, for the following reasons. First, Panx1 transcripts are enriched in the brain and in the cortex are most abundant during the embryonic and early postnatal period. . This timing roughly corresponds to a period of synaptogenesis in the postnatal brain. Second, Panx1 localizes to postsynaptic compartments in neurons and its disruption leads to enhance excitability and potentiation of neuron-to neuron communication. Third, disruption of Panx1 (either knockdown or pharmacological blockade) leads to neurite outgrowth in neuron-like cells. Lastly, genetic variants in PANX1 have been associated with neurodevelopmental disorders. This dissertation explores the role of Panx1 in the development of dendritic spines and neuronal networks, furthering our understanding on cortical development and placing Panx1 as a novel regulator of structural and functional plasticity in the brain. Chapter 1 discusses core concepts on cortical development, with an emphasis on pyramidal neuron, the most abundant and only known projection neurons in the cerebral cortex. Here, I review the embryonic origin of pyramidal neurons, their postnasal development, and how cortical circuits are assembled. I finish this chapter with a brief review on Panx1 and its known expression and involvement in neuronal function. In Chapter 2 I explore the functional properties of neuronal networks and synaptic composition of cortical neurons in the absence of Panx1. Using live cell imaging and analysis of Ca2+ transients in dense primary cortical cultures, revealed that Panx1 knock-out (KO) cultures exhibit more and larger groups of significantly co-activated neurons, known as network ensembles. These network properties were not explained by differences in cell viability or cell-type composition. Examination of protein expression from cortical synaptosome preparations revealed that Panx1 is enriched in synaptic compartments, while also confirming a developmental downregulation. This analysis also revealed increased levels of the postsynaptic scaffolding protein PSD-95, along with the postsynaptic glutamate receptors GluA1 and GluN2A. Lastly, ex vivo tracing of dendritic spines on apical dendrites of Layer 5 pyramidal neurons in global and glutamatergic-specific Panx1 KO brain slices revealed higher spine densities in early and late postnatal development, with no differences in spine length. Analysis of dendritic spine densities in fixed cultured cortical neurons revealed an increase associated with Panx1 KO. Altogether, this work presents for the first time a connection between Panx1 and structural development of dendritic spines and suggest that Panx1 regulates cortical neuronal networks through changes in spine density. Chapter 3 examines the influence of Panx1 on spiny protrusions growth and movement. Spiny protrusion are long, thin, highly dynamic spine precursors. Taking advantage of a fluorescent signal localized to the plasma membrane, I visualized spiny protrusions and quantified their dynamics in wildtype (WT) and Panx1 KO developing cortical neurons, both under fixed and live conditions. I found that transient Panx1 expression is associated with decreased spiny protrusion density both in WT and Panx1 KO neurons. Using live cell imaging, I found that spiny protrusions are more stable and less motile in Panx1 KO neurons, while its transient expression reversed both of these phenotypes. These results suggest that Panx1 regulation of dendritic spines development is rooted partly in the regulation of spiny protrusion dynamics. Overall, this dissertation demonstrates that Panx1 negatively regulates dendritic spines in part by influencing spiny protrusion dynamics. It is reasonable to speculate that Panx1 regulation of dendritic spines underlies its newly discovered role in the formation network ensembles, providing a putative mechanism for previously described roles of Panx1 in synaptic plasticity. Likewise, this body of work furthers our understanding of Panx1 by filling some of the gaps attached to its developmental expression and association with neurodevelopmental disease.



pannexin, dendritic spine, network ensemble, neuronal development, synapse development, spiny protrusion