Type 2K+Cl cotransporter is preferentially recruited to climbing fiber synapses during development and the stellate cell-targeting dendritic zone at adulthood in cerebellar Purkinje cells

Postnatal expression of the type 2K+Cl cotransporter (KCC2) in neurons lowers the Cl equilibrium potential to values that are more negative than the resting potential, thereby converting the action of Cl-permeable GABAA and glycine receptors from excitatory to inhibitory. In the present study, we investigated the spatiotemporal expression of KCC2 in mouse cerebella, particularly focusing on Purkinje cells (PCs). First, we confirmed the fundamental expression profiles of KCC2 in the cerebellum, i.e. neuron-specific expression, somatodendritic distribution, and postnatal upregulation. We also found preferential recruitment to climbing fiber (CF) synapses during the second and third postnatal weeks, when perisomatic innervation in PCs switches from CFs to basket cell axons (BAs) and also when single winner CFs translocate from somata to dendrites. In parallel with this synaptic recruitment, the intracellular distribution shifted from a diffuse cytoplasmic to a predominantly cell surface pattern. In adult PCs, CF synapse-associated accumulation was obscured. Instead, significantly high expression was noted on the surface of PC dendrites in the superficial two-thirds of the molecular layer, in which stellate cells reside and project axons to innervate PC dendrites. Thus, the somatodendritic distribution in PCs is regulated in relation to particular inputs or input zones. During development, timed recruitment of KCC2 to CF synapses will augment inhibitory GABAergic actions by incoming BAs, promoting the CF-to-BA switchover in perisomatic PC innervation. In adulthood, enriched KCC2 expression at the stellate cell-targeting territory of PC dendrites might help in maintaining intracellular Cl homeostasis and the polarity of GABAA receptor-mediated responses upon sustained activity of this interneuron.