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Background: Neuropathological features of Alzheimer’s disease are characterized by the deposition of amyloid-β (Aβ) plaques and impairments in synaptic activity and memory. However, we know little about the physiological role of amyloid-β protein precursor (AβPP) from which Aβ derives. Objective: Evaluate APP deficiency induced alterations in neuronal electrical activity and mitochondrial protein expression. Methods: Utilizing electrophysiological, biochemical, pharmacological, and behavioral tests, we revealed aberrant local field potential (LFP), extracellular neuronal firing and levels of mitochondrial proteins. Result: We show that APP knockout (APP-/-) leads to increased gamma oscillations in the medial prefrontal cortex (mPFC) at 1-2 months old, which can be restored by baclofen (Bac), a γ-aminobutyric acid type B receptor (GABABR) agonist. A higher dose and longer exposure time is required for Bac to suppress neuronal firing in APP-/- mice than in wild type animals, indicating enhanced GABABR mediated activity in the mPFC of APP-/- mice. In line with increased GABABR function, the glutamine synthetase inhibitor, L-methionine sulfonate, significantly increases GABABR levels in the mPFC of APP-/- mice and this is associated with a significantly lower incidence of death. The results suggest that APP-/- mice developed stronger GABABR mediated inhibition. Using HEK 293 as an expression system, we uncover that AβPP functions to suppress GABABR expression. Furthermore, APP-/- mice show abnormal expression of several mitochondrial proteins. Conclusion: APP deficiency leads to both abnormal network activity involving defected GABABR and mitochondrial dysfunction, suggesting critical role of AβPP in synaptic and network function.

Amyloid precursor protein (APP) is enriched at the synapse, but its synaptic function is still poorly understood. We previously showed that GABAergic short-term plasticity is impaired in App knock-out (App-/-) animals, but the precise mechanism by which APP regulates GABAergic synaptic transmission has remained elusive. Using electrophysiological, biochemical, moleculobiological, and pharmacological analysis, here we show that APP can physically interact with KCC2, a neuron-specific K+-Cl- cotransporter that is essential for Cl- homeostasis and fast GABAergic inhibition. APP deficiency results in significant reductions in both total and membrane KCC2 levels, leading to a depolarizing shift in the GABA reversal potential (EGABA). Simultaneous measurement of presynaptic action potentials and inhibitory postsynaptic currents (IPSCs) in hippocampal neurons reveals impaired unitary IPSC amplitudes attributable to a reduction in α1 subunit levels of GABAAR. Importantly, restoration of normal KCC2 expression and function in App-/- mice rescues EGABA, GABAAR α1 levels and GABAAR mediated phasic inhibition. We show that APP functions to limit tyrosine-phosphorylation and ubiquitination and thus subsequent degradation of KCC2, providing a mechanism by which APP influences KCC2 abundance. Together, these experiments elucidate a novel molecular pathway in which APP regulates, via protein-protein interaction with KCC2, GABAAR mediated inhibition in the hippocampus. DOI: http://dx.doi.org/10.7554/eLife.20142.001, eLife digest Alzheimer’s disease is the most common form of dementia. One of the hallmarks of the disease is the formation of sticky protein clumps called amyloid plaques in the brain. These plaques are formed from specific fragments of a protein called APP. The intact form of APP is essential for synapses (the junctions across which neurons transmit signals) to form and work correctly. The hippocampus is one of the first brain regions to be affected in Alzheimer’s disease and is important for forming memories and emotions. In the hippocampus, GABAA receptors at synapses normally tightly regulate synaptic signaling by reducing the ability of the receiving neuron to respond, but this inhibition is disrupted in Alzheimer’s disease. Studies suggest that APP can affect how GABAA receptors transmit signals, but it is not known how it does so. One possibility is that APP regulates a protein called KCC2 that helps to maintain the inhibitory effect of GABAA receptors. To investigate this, Chen et al. genetically modified mice to lack the gene that produces APP. These mice had a lower level of KCC2 in their hippocampus than normal mice, and their GABAA receptors were less able to inhibit synaptic signaling. Further experiments demonstrated that APP physically interacts with KCC2 and maintains normal levels of the protein by preventing it from being chemically modified and degraded. Chen et al. also showed that treating mice that lack APP with specific compounds can restore KCC2 activity and return the behavior of synaptic GABAA receptors to normal. Future studies in mice (and eventually people) that exhibit symptoms of Alzheimer's disease will help to determine whether KCC2 is important in the development of the disease. If so, modifying the levels of the KCC2 protein in the brain could potentially help to slow down memory loss in Alzheimer’s disease. DOI: http://dx.doi.org/10.7554/eLife.20142.002