Beta-Amyloid (Aβ_1-42) Increases the Expression of NKCC1 in the Mouse Hippocampus

Alzheimer's disease (AD) is a neurodegenerative disorder with an increasing need for developing disease-modifying treatments as current therapies only provide marginal symptomatic relief. Recent evidence suggests the γ-aminobutyric acid (GABA) neurotransmitter system undergoes remodeling in AD, disrupting the excitatory/inhibitory (E/I) balance in the brain. Altered expression levels of K-Cl-2 (KCC2) and N-K-Cl-1 (NKCC1), which are cation-chloride cotransporters (CCCs), have been implicated in disrupting GABAergic activity by regulating GABA_A receptor signaling polarity in several neurological disorders, but these have not yet been explored in AD. NKCC1 and KCC2 regulate intracellular chloride [Cl^-]_i by accumulating and extruding Cl^-, respectively. Increased NKCC1 expression in mature neurons has been reported in these disease conditions, and bumetanide, an NKCC1 inhibitor, is suggested to show potential therapeutic benefits. This study used primary mouse hippocampal neurons to explore if KCC2 and NKCC1 expression levels are altered following beta-amyloid (Aβ_1-42) treatment and the potential neuroprotective effects of bumetanide. KCC2 and NKCC1 expression levels were also examined in 18-months-old male C57BL/6 mice following bilateral hippocampal Aβ_1-42 stereotaxic injection. No change in KCC2 and NKCC1 expression levels were observed in mouse hippocampal neurons treated with 1 nM Aβ_1-42, but NKCC1 expression increased 30-days post-Aβ_1-42-injection in the CA1 region of the mouse hippocampus. Primary mouse hippocampal cultures were treated with 1 nM Aβ_1-42 alone or with various concentrations of bumetanide (1 µM, 10 µM, 100 µM, 1 mM) to investigate the effect of the drug on cell viability. Aβ_1-42 produced 53.1 ± 1.4% cell death after 5 days, and the addition of bumetanide did not reduce this. However, the drug at all concentrations significantly reduced cell viability, suggesting bumetanide is highly neurotoxic. In summary, these results suggest that chronic exposure to Aβ_1-42 alters the balance of KCC2 and NKCC1 expression in a region-and layer-specific manner in mouse hippocampal tissue; therefore, this process most likely contributes to altered hippocampal E/I balance in this model. Furthermore, bumetanide induces hippocampal neurotoxicity, thus questioning its suitability for AD therapy. Further investigations are required to examine the effects of Aβ_1-42 on KCC2 and NKCC1 expression and whether targeting CCCs might offer a therapeutic approach for AD.