Mammalian copper homeostasis requires retromer-dependent recycling of the high-affinity copper transporter 1.

Rachel Curnock,Peter J Cullen

doi:10.1242/jcs.249201

Abstract

ABSTRACTThe concentration of essential micronutrients, such as copper (used here to describe both Cu+ and Cu2+), within the cell is tightly regulated to avoid their adverse deficiency and toxicity effects. Retromer-mediated sorting and recycling of nutrient transporters within the endo-lysosomal network is an essential process in regulating nutrient balance. Cellular copper homeostasis is regulated primarily by two transporters: the copper influx transporter copper transporter 1 (CTR1; also known as SLC31A1), which controls the uptake of copper, and the copper-extruding ATPase ATP7A, a recognised retromer cargo. Here, we show that in response to fluctuating extracellular copper, retromer controls the delivery of CTR1 to the cell surface. Following copper exposure, CTR1 is endocytosed to prevent excessive copper uptake. We reveal that internalised CTR1 localises on retromer-positive endosomes and, in response to decreased extracellular copper, retromer controls the recycling of CTR1 back to the cell surface to maintain copper homeostasis. In addition to copper, CTR1 plays a central role in the trafficking of platinum. The efficacy of platinum-based cancer drugs has been correlated with CTR1 expression. Consistent with this, we demonstrate that retromer-deficient cells show reduced sensitivity to the platinum-based drug cisplatin.

Highlights

Integral membrane proteins are delivered and removed from the cell surface by means of membrane trafficking pathways (Cullen and Steinberg, 2018)
In the present study, using RNAi-mediated suppression and CRISPR-Cas9-mediated knockout of VPS35, we have investigated whether the cell surface levels and trafficking of copper transporter 1 (CTR1) in response to changes in extracellular copper concentration are dependent on retromer expression
Retromer is required for the cell surface localization of CTR1 A multitude of metal ion transporters were previously identified as sorting nexin-27 (SNX27) and/or retromer cargo using a global unbiased approach (Steinberg et al, 2013)

Summary

Introduction

Integral membrane proteins are delivered and removed from the cell surface by means of membrane trafficking pathways (Cullen and Steinberg, 2018). The balance between delivery and internalisation defines the steady-state cell surface abundance of these proteins and provides the means to rapidly alter the abundance of individual proteins and larger groups of functionally related proteins in response to environmental cues. Comprising a number of intracellular membrane bound compartments, this network sorts internalised cell surface integral membrane proteins between two School of Biochemistry, Biomedical Sciences Building, University of Bristol, Bristol BS8 1TD, UK. Our understanding of the role of the endocytic retrieval and recycling pathways in adaptive responses has been hampered by a lack of basic understanding of the machinery that regulates these pathways (Bai et al, 2012; Grant and Donaldson, 2009; McNally and Cullen, 2018)

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