G-Protein-Coupled Inwardly Rectifying Potassium (GIRK) Channel Activation by the p75 Neurotrophin Receptor Is Required for Amyloid ß Toxicity.

Alzheimer's disease is characterized by cognitive decline, neuronal degeneration, and the accumulation of amyloid-beta (Aß). Although, the neurotoxic Aß peptide is widely believed to trigger neuronal dysfunction and degeneration in Alzheimer's disease, the mechanism by which this occurs is poorly defined. Here we describe a novel, Aß-triggered apoptotic pathway in which Aß treatment leads to the upregulation of G-protein activated inwardly rectifying potassium (GIRK/Kir3) channels, causing potassium efflux from neurons and Aß-mediated apoptosis. Although, GIRK channel activity is required for Aß-induced neuronal degeneration, we show that it is not sufficient, with coincident signaling by the p75 neurotrophin receptor (p75NTR) also required for potassium efflux and cell death. Our results identify a novel role for GIRK channels in mediating apoptosis, and provide a previously missing mechanistic link between the excitotoxicity of Aß and its ability to trigger cell death pathways, such as that mediated by p75NTR. We propose that this death-signaling pathway contributes to the dysfunction of neurons in Alzheimer's disease and is responsible for their eventual degeneration.

Inhibition of PIKfyve by YM-201636 dysregulates autophagy and leads to apoptosis-independent neuronal cell death.

The lipid phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P 2), synthesised by PIKfyve, regulates a number of intracellular membrane trafficking pathways. Genetic alteration of the PIKfyve complex, leading to even a mild reduction in PtdIns(3,5)P 2, results in marked neurodegeneration via an uncharacterised mechanism. In the present study we have shown that selectively inhibiting PIKfyve activity, using YM-201636, significantly reduces the survival of primary mouse hippocampal neurons in culture. YM-201636 treatment promoted vacuolation of endolysosomal membranes followed by apoptosis-independent cell death. Many vacuoles contained intravacuolar membranes and inclusions reminiscent of autolysosomes. Accordingly, YM-201636 treatment increased the level of the autophagosomal marker protein LC3-II, an effect that was potentiated by inhibition of lysosomal proteases, suggesting that alterations in autophagy could be a contributing factor to neuronal cell death.

p75 neurotrophin receptor mediates neuronal cell death by activating GIRK channels through phosphatidylinositol 4,5-bisphosphate.

The pan neurotrophin receptor p75(NTR) signals programmed cell death both during nervous system development and after neural trauma and disease in the adult. However, the molecular pathways by which death is mediated remain poorly understood. Here, we show that this cell death is initiated by activation of G-protein-coupled inwardly rectifying potassium (GIRK/Kir3) channels and a consequent potassium efflux. Death signals stimulated by neurotrophin-mediated cleavage of p75(NTR) activate GIRK channels through the generation and binding of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2/PIP2] to GIRK channels. Both GIRK channel activity and p75(NTR)-mediated neuronal death are inhibited by sequestration of PtdIns(4,5)P2 and application of GIRK channel inhibitors, whereas pertussis toxin treatment has no effect. Thus, p75(NTR) activates GIRK channels without the need for G(i/o)-proteins. Our results demonstrate a novel mode of activation of GIRK channels, representing an early step in the p75(NTR)-mediated cell death pathway and suggesting a function for these channels during nervous system development.

Palmitoylation of the C-terminal fragment of p75(NTR) regulates death signaling and is required for subsequent cleavage by gamma-secretase.

It has recently been shown that the p75 neurotrophin receptor (p75(NTR)), which is known to mediate neural cell death during development of the nervous system and in a range of adult neurodegenerative conditions, undergoes a regulated process of cell surface receptor cleavage, regulated intramembrane proteolysis (RIP). Here we show that neuronal death signaling occurs only following extracellular metalloprotease cleavage of p75(NTR) and palmitoylation of the resultant C-terminal fragment, causing its translocation to cholesterol-rich domains of the plasma membrane. Furthermore, death signaling is promoted by inhibition of intracellular gamma-secretase cleavage, a process which also occurs within the cholesterol-rich domains. In the presence of TrkA signaling, C-terminal fragment localization in these cholesterol-rich domains is prevented, thereby blocking neuronal death. Thus p75(NTR) activates neuronal death pathways in conditions where the balance of normal RIP is shifted toward extracellular domain cleavage due to increased metalloprotease activity, decreased TrkA activity or compromised gamma-secretase activity, all of which are features of neurodegenerative conditions such as Alzheimer's disease.