Neuronal K+/Cl- co-transporter (KCC2) transgenes lacking neurone restrictive silencer element recapitulate CNS neurone-specific expression and developmental up-regulation of endogenous KCC2 gene

The K+/Cl- co-transporter KCC2 maintains the low intracellular chloride concentration required for fast synaptic inhibition and is exclusively expressed in neurones of the CNS. Here, we show that the KCC2 gene (alias SLC12a5) has multiple transcription start sites and characterize the activity of 6.8 kb of mouse KCC2 gene regulatory sequence (spanning 1.4 kb upstream from exon 1 to exon 2) using luciferase reporters. Overexpression of neurone-restrictive silencer factor repressed the reporter activity in vitro, apparently via a neurone restrictive silencer element (NRSEKCC2) within intron 1 of the mouse KCC2 gene. In transgenic mice, however, KCC2 reporters with or without deletion of the NRSEKCC2 were expressed exclusively in neurones and predominantly in the CNS with a similar pattern and developmental up-regulation as endogenous KCC2. Moreover, a third transgene with just a 1.4-kb KCC2 promoter region lacking the NRSEKCC2-bearing intron 1 was still expressed predominantly in neural tissues. Thus, developmental up-regulation of the KCC2 gene does not require NRSEKCC2 and the 1.4-kb KCC2 promoter is largely sufficient for neurone-specific expression of KCC2