Knowing When to Stop

Science Signaling  21 Apr 2009:
Vol. 2, Issue 67, pp. ec137
DOI: 10.1126/scisignal.267ec137

The response of cells to γ-aminobutyric acid (GABA) acting through ionotropic GABAA receptors depends on the relative internal and external concentrations of Cl, the ion that these channels conduct. Early in development, GABA depolarizes neurons because the Cl gradient is such that Cl preferentially leaves the cell, whereas once the potassium-chloride transporter KCC2 becomes abundant, the Cl gradient switches so that GABA hyperpolarizes cells. Bortone and Polleux show that this switch in the abundance of KCC2 allows GABA both to promote the migration of inhibitory interneurons to the cortex and to serve as the stop signal once they have reached their destination. The authors analyzed by time-lapse confocal imaging the migration of fluorescently labeled GABAergic interneurons in transgenic mice and found that migration gradually decreased from embryonic day 15 (E15) through postnatal day 7 (P7). Moreover, migration was predominantly controlled by the length of time in which movement paused rather than the rate of movement during mobile periods. In an in vitro system in which medial ganglionic eminence explants (source of interneurons) were cultured with dissociated pyramidal neurons, this cessation of interneuron movement through an increase in pause time was reproduced by 7 days in vitro. In vivo interneurons were initially negative for KCC2 (analyzed by immunofluorescence) and became positive for this transporter by P0. In the cultured explant system, KCC2 abundance was more variable and correlated with migratory behavior: Cells with abundant KCC2 responded to exogenous GABA with increased pause time, whereas cells with little KCC2 responded to antagonism of GABA (GABA plus bicuculline) with increased pause time. Thus, GABA appears to stimulate migration in low KCC2 cells and to terminate migration in high KCC2 cells. With short-hairpin RNA and rescue overexpression experiments, the authors demonstrated that KCC2 was necessary and sufficient for this switch in the response of the interneurons to GABA. Calcium imaging in the explant system showed that when KCC2 was knocked down, the interneurons displayed multiple spontaneous calcium transients per hour, whereas when KCC2 was overexpressed, no calcium transients were observed. Pharmacological analysis revealed that L-type voltage-gated calcium channels were most likely responsible for the calcium transients and blocking these channels prevented migration, whereas opening the channels decreased the proportion of sedentary interneurons. Finally, the authors showed that GABA and glutamate jointly control migration and that when KCC2 is overexpressed and glutamate receptors are blocked, a larger proportion of interneurons cease moving in response to a GABAA agonist than cease to move in response to GABAA activation when only KCC2 is overexpressed. Thus, GABA and glutamate provide complementary promigratory stimulation that is terminated by a switch in the cellular response to GABA at the appropriate time in development.

D. Bortone, F. Polleux, KCC2 expression promotes the termination of cortical interneuron migration in a voltage-sensitive calcium-dependent manner. Neuron 62, 53–71 (2009). [Online Journal]