Abstract
Ionotropic GABA transmission is mediated by anion (mainly Cl−)-permeable GABAA receptors (GABAARs). In immature neurons, GABA exerts depolarizing and sometimes functionally excitatory actions, based on active uptake of Cl− by the Na-K-2Cl cotransporter NKCC1. While functional evidence firmly shows NKCC1-mediated ion transport in immature and diseased neurons, molecular detection of NKCC1 in the brain has turned out to be extremely difficult. In this review, we describe the highly inconsistent data that are available on the cell type-specific expression patterns of the NKCC1 mRNA and protein in the CNS. We discuss the major technical caveats, including a lack of knock-out-controlled immunohistochemistry in the forebrain, possible effects of alternative splicing on the binding of antibodies and RNA probes, and the wide expression of NKCC1 in different cell types, which make whole-tissue analyses of NKCC1 useless for studying its neuronal expression. We also review novel single-cell RNAseq data showing that most of the NKCC1 in the adult CNS may, in fact, be expressed in non-neuronal cells, especially in glia. As future directions, we suggest single-cell NKCC1 mRNA and protein analyses and the use of genetically tagged endogenous proteins or systematically designed novel antibodies, together with proper knock-out controls, for the visualization of endogenous NKCC1 in distinct brain cell types and their subcellular compartments.
Highlights
NKCC1, encoded by Slc12a2, belongs to the SLC12 family of cation-chloride cotransporters (CCCs), which includes the Na-K-2Cl cotransporter 2 (NKCC2), the Na-Cl cotransporter (NCC), the K-Cl cotransporters 1–4 (KCC1-4), and two orphan members, CCC9 and CIP1, with largely unknown physiological roles
We address the question which cell types in the central nervous system (CNS) express NKCC1 and how this expression is regulated during development, by going, in a systematic manner, through the pertinent literature on NKCC1 protein and mRNA expression in the CNS
NKCC1 is usually localized in the basolateral membrane [37], but a notable exception is the choroid plexus epithelium, where NKCC1 is highly expressed in the apical membrane [38,39]
Summary
NKCC1, encoded by Slc12a2, belongs to the SLC12 family of cation-chloride cotransporters (CCCs), which includes the Na-K-2Cl cotransporter 2 (NKCC2), the Na-Cl cotransporter (NCC), the K-Cl cotransporters 1–4 (KCC1-4), and two orphan members, CCC9 and CIP1, with largely unknown physiological roles. For many of the CCC members, cell-specific expression patterns have been described (for review, see [1]). The role of KCC2 as the major neuronal Cl− extruder and its other functions have been described in several reviews [5,6,7]. In addition to the traditional immunohistochemical and in situ hybridization studies, we discuss new data that are available in single-cell RNAseq databases showing that NKCC1 expression in certain glial subtypes is very high—much higher than in neurons. We call for future studies employing novel single-cell methods for exploring cellular NKCC1 expression patterns in the developing and mature CNS
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