Neuronal Reprograming of Protein Homeostasis by Calcium-Dependent Regulation of the Heat Shock Response

Abstract

Protein quality control requires constant surveillance to prevent misfolding, aggregation, and loss of cellular function. There is increasing evidence in metazoans that communication between cells has an important role to ensure organismal health and to prevent stressed cells and tissues from compromising lifespan. Here, we show in C. elegans that a moderate increase in physiological cholinergic signaling at the neuromuscular junction (NMJ) induces the calcium (Ca ²⁺)-dependent activation of HSF-1 in post-synaptic muscle cells, resulting in suppression of protein misfolding. This protective effect on muscle cell protein homeostasis was identified in an unbiased genome-wide screening for modifiers of protein aggregation, and is triggered by downregulation of gei-11, a Myb-family factor and proposed regulator of the L-type acetylcholine receptor (AChR). This, in-turn, activates the voltage-gated Ca ²⁺ channel, EGL-19, and the sarcoplasmic reticulum ryanodine receptor in response to acetylcholine signaling. The release of calcium into the cytoplasm of muscle cells activates Ca ²⁺-dependent kinases and induces HSF-1-dependent expression of cytoplasmic chaperones, which suppress misfolding of metastable proteins and stabilize the folding environment of muscle cells. This demonstrates that the heat shock response (HSR) can be activated in muscle cells by neuronal signaling across the NMJ to protect proteome health.

Author Summary

The protein quality control machinery is responsible for preventing the accumulation of misfolded and damaged proteins and loss of cellular function. The capacity of cellular surveillance is limited however, leading to increased appearance of protein aggregates and risk for age-associated diseases. Here, we show that upregulation of acetylcholine receptors and moderate increased cholinergic activity leads to a calcium-dependent stress response that suppresses protein misfolding and restores homeostasis in C. elegans muscle cells. This involves gei-11 knockdown-dependent upregulation of acetylcholine receptors, and the release of calcium into the cytoplasm of muscle cells through cell membrane and sarcoplasmic reticulum specific channels. Subsequently, activation of the heat shock factor 1 (HSF-1) leads to the expression of cytoplasmic chaperones that suppress misfolding of metastable and aggregating proteins, restoring folding and muscle function. This reveals a new non-canonical mechanism for the cell non-autonomous regulation of the heat shock response to ensure balance between cells in a metazoan.