ABSTRACT Copper is one of the essential micronutrients utilized as a cofactor in a wide variety of biochemical reactions of metabolic pathways, including mitochondrial respiration and innate immune response. Cellular concentration and distribution of copper is regulated by copper‐specific transporters, chaperones, metallothionein proteins and amino acids. Transcription of a major copper metallothionein, CUP1 is epigenetically regulated in Saccharomyces cerevisiae . Mutations in histones dysregulate cellular copper homeostasis due to abnormal epigenetic changes and cause diseases in humans, such as cancerous growth and neurological disorders. Low or higher cellular concentration of copper is associated with disorders such as Menkes and Wilson's disease, respectively. Higher concentrations of copper cause caspase‐independent cell death known as cuproptosis and haemolytic anemia. We highlighted the existing knowledge regarding the significance of epigenetics and cellular factors in the regulation of copper metabolism and copper‐regulated protein trafficking. We have also proposed a few future directions to explore the role of cellular pH dynamics, stoichiometry among metals, amino acids and protein metabolism, histone modifications, autophagy and mitochondrial respiration in regulating cellular copper metabolism. Altogether, we provide a comprehensive summary of cellular factors targeting copper metabolism for dissecting the underlying complex mechanism of copper dynamics in normal physiology and diseases.

Neuroligins are synaptic cell adhesion proteins with a role in synaptic function, implicated in neurodevelopmental disorders. The autism spectrum disorder-associated substitution Arg451Cys (R451C) in NLGN3 promotes a partial misfolding of the extracellular domain of the protein leading to retention in the endoplasmic reticulum (ER) and the induction of the unfolded protein response (UPR). The reduced trafficking of R451C NLGN3 to the cell surface leads to altered synaptic function and social behavior. A screening in HEK-293 cells overexpressing NLGN3 of 2662 compounds (FDA-approved small molecule drug library), led to the identification of several glucocorticoids such as alclometasone dipropionate, desonide, prednisolone sodium phosphate, and dexamethasone (DEX), with the ability to favor the exit of full-length R451C NLGN3 from the ER. DEX improved the stability of R451C NLGN3 and trafficking to the cell surface, reduced the activation of the UPR, and increased the formation of artificial synapses between HEK-293 and hippocampal primary neurons. The effect of DEX was validated on a novel model system represented by neural stem progenitor cells and differentiated neurons derived from the R451C NLGN3 knock-in mouse, expressing the endogenous protein. This work shows a potential rescue strategy for an autism-linked mutation affecting cell surface trafficking of a synaptic protein.

Confocal microscopy images depict Wild-Type (row 1) and UGGT1 KO (row 2) mammalian cells co-expressing fluorescent fusions of a wild type glycoprotein which reaches its canonical localization at the cell membrane (column 2, green) and its rare disease associated mutant (column 3, red) confined to the Endoplasmic Reticulum (ER).Column 4: overlaps of the images in columns 2 and 3.The data suggest UGGT1-mediated ER retention of the disease-associated membrane glycoprotein mutant and constitute proof of principle of modulation of UGGT activity as rescue-of-secretion therapy for congenital rare disease caused by a responsive glycoprotein mutant.See Tax et al.Traffic 25(1):e12927.

In neurons, fast axonal transport (FAT) of vesicles occurs over long distances and requires constant and local energy supply for molecular motors in the form of adenosine triphosphate (ATP). FAT is independent of mitochondrial metabolism. Indeed, the glycolytic machinery is present on vesicles and locally produces ATP, as well as nicotinamide adenine dinucleotide bonded with hydrogen (NADH) and pyruvate, using glucose as a substrate. It remains unclear whether pyruvate is transferred to mitochondria from the vesicles as well as how NADH is recycled into NAD+ on vesicles for continuous glycolysis activity. The optimization of a glycolytic activity test for subcellular compartments allowed the evaluation of the kinetics of vesicular glycolysis in the brain. This revealed that glycolysis is more efficient on vesicles than in the cytosol. We also found that lactate dehydrogenase (LDH) enzymatic activity is required for effective vesicular ATP production. Indeed, inhibition of LDH or the forced degradation of pyruvate inhibited ATP production from axonal vesicles. We found LDHA rather than the B isoform to be enriched on axonal vesicles suggesting a preferential transformation of pyruvate to lactate and a concomitant recycling of NADH into NAD+ on vesicles. Finally, we found that LDHA inhibition dramatically reduces the FAT of both dense-core vesicles and synaptic vesicle precursors in a reconstituted cortico-striatal circuit on-a-chip. Together, this shows that aerobic glycolysis is required to supply energy for vesicular transport in neurons, similar to the Warburg effect.