Final Report Summary - IONDYNDEV (Ionic dynamics and plasticity in developing neuronal networks)
Ion gradients are a fundamental feature of the nervous system and its development. They are established by the actions of ion pumps, transporters and channel proteins, which reside in the membrane of cells. Changes in intracellular ion composition have been implicated in a diverse array of neurobiological processes, including cell divisions during development, the establishment of synaptic connections and the regulation of neuronal excitability. And whilst a large body of work has examined the importance of intracellular calcium (Ca2+) dynamics during these processes, other ion species have received much less attention. This project focused upon two key ions: chloride ions (Cl-) and hydrogen ions (H+). To advance our understanding, we capitalised upon methods for directly measuring and manipulating ion concentrations in a spatially and temporally controlled manner. Working in the cortex of rodents and in human cortical cells derived from induced pluripotent stem cells, we used optical and electrophysiological methods to measure ion dynamics in defined populations of cells. Meanwhile, light-activated proteins were used to selectively control intracellular ion dynamics with unrivalled temporal resolution. The project addressed four experimental aims. The first series of experiments made the novel discovery that intracellular chloride levels change dynamically as progenitor cells divide and give rise to neurons. By optically controlling chloride levels, it was demonstrated that the timing of cell divisions are regulated via chloride-sensitive intracellular enzymes. The second series of experiments focused upon the development of cortical circuits and examined ion dynamics in a cell-type called astrocytes, which communicate closely with electrically-excitable neurons. Neuronal activity was found to be tightly-coupled to rapid hydrogen ion dynamics in neighbouring astrocytes, which was demonstrated for the first time in both rodent and human astrocytes. In the third series of experiments, intracellular chloride levels were shown to represent a key feature by which a developing neuron becomes functionally integrated into a neuronal circuit. This work revealed that elevated chloride levels enable immature neurons to strengthen their excitatory synaptic connections, through the integration of multiple neurotransmitter systems. The final series of experiments investigated how cells maintain their ion gradients during periods of elevated neuronal activity. Activity was found to modulate intracellular chloride levels on a range of timescales, including rapid increases associated with the strong activation of chloride-permeable GABA-A receptors, but also more sustained shifts that cause long-term changes in the strength of synaptic transmission. In conclusion therefore, the project has revealed new links between intracellular ion changes and the activity-dependent development and stability of neuronal networks. This advances our understanding of ion dynamics under normal conditions and in neurological disorders such as epilepsy.