Endosomal hyper-acidification via proton-activated chloride channel deletion in neurons impairs AMPA receptor endocytosis and LTD

Abstract

Endosomal homeostasis is critical for neuronal function, including the post-synaptic trafficking of -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). Dynamic AMPAR trafficking is a major component of synaptic plasticity, such as hippocampal long-term potentiation (LTP) and long-term depression (LTD) and is thought to be required for learning and memory. Dramatic alteration of endosomal pH has been reported to negatively affect synaptic transmission and neural development, but the underlying mechanisms by which pH is involved in AMPAR trafficking are unclear. Here, we show that the proton-activated chloride (PAC) channel localizes to early and recycling endosomes along neuronal dendrites and prevents hyper-acidification of endosomes. To directly measure AMPAR endocytosis, we used a new method to assess LTD using HaloTag-GluA2 and found that the loss of PAC reduces AMPAR internalization during chemical LTD in primary neurons, while AMPAR trafficking in unstimulated cells or during chemical LTP is unaffected. Consistently, pyramidal neuron-specific PAC knockout mice had impaired hippocampal LTD, but not LTP, and performed poorly in the Morris water maze reversal test, exhibiting an inability to adapt to changing environments (also referred to as behavioral flexibility). We conclude that proper maintenance of pH by PAC is important during LTD to regulate AMPAR trafficking in a manner critical for animal physiology and behavior. SummaryThe ability to adapt to changing environments stems from the plasticity of neurons, which can modulate their synaptic strength in response to neural activity. We discovered a novel mechanism by which an endosomal proton-activated chloride channel (PAC) is involved in synaptic weakening, or long-term depression (LTD). To improve tools used to study LTD, we developed a live-cell imaging assay to directly observe -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) endocytosis. PAC-deficient neurons have hyper-acidified endosomes and fail to endocytose AMPARs during LTD. Neuron-specific PAC knockout mice have impaired hippocampal LTD and fail to adapt to changes in their environment. The role of endosomal pH in synaptic function is understudied, and our results provide a novel mechanism whereby PAC can affect synaptic LTD.