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Rare genetic diseases preponderantly affect the nervous system causing neurodegeneration to neurodevelopmental disorders. This is the case for both Menkes and Wilson disease, arising from mutations in ATP7A and ATP7B, respectively. The ATP7A and ATP7B proteins localize to the Golgi and regulate copper homeostasis. We demonstrate genetic and biochemical interactions between ATP7 paralogs with the conserved oligomeric Golgi (COG) complex, a Golgi apparatus vesicular tether. Disruption ofDrosophilacopper homeostasis by ATP7 tissue-specific transgenic expression caused alterations in epidermis, aminergic, sensory, and motor neurons. Prominent among neuronal phenotypes was a decreased mitochondrial content at synapses, a phenotype that paralleled with alterations of synaptic morphology, transmission, and plasticity. These neuronal and synaptic phenotypes caused by transgenic expression of ATP7 were rescued by downregulation of COG complex subunits. We conclude that the integrity of Golgi-dependent copper homeostasis mechanisms, requiring ATP7 and COG, are necessary to maintain mitochondria functional integrity and localization to synapses.SIGNIFICANCE STATEMENTMenkes and Wilson disease affect copper homeostasis and characteristically afflict the nervous system. However, their molecular neuropathology mechanisms remain mostly unexplored. We demonstrate that copper homeostasis in neurons is maintained by two factors that localize to the Golgi apparatus, ATP7 and the conserved oligomeric Golgi (COG) complex. Disruption of these mechanisms affect mitochondrial function and localization to synapses as well as neurotransmission and synaptic plasticity. These findings suggest communication between the Golgi apparatus and mitochondria through homeostatically controlled cellular copper levels and copper-dependent enzymatic activities in both organelles.

BACKGROUND & AIMS: Wilson disease (WD) is an inherited disorder of copper metabolism that leads to copper accumulation and toxicity in the liver and brain. It is caused by mutations in the adenosine triphosphatase copper transporting b gene (ATP7B), which encodes a protein that transports copper from hepatocytes into the bile. We studied ATP7B-deficient cells and animals to identify strategies to decrease copper toxicity in patients with WD. METHODS: We used RNA-seq to compare gene expression patterns between wild-type and ATP7B-knockout HepG2 cells exposed to copper. We collected blood and liver tissues from Atp7b(-/-) and Atp7b(+/-) (control) rats (LPP) and mice; some mice were given 5 daily injections of an autophagy inhibitor (spautin-1) or vehicle. We obtained liver biopsies from 2 patients with WD in Italy and liver tissues from patients without WD (control). Liver tissues were analyzed by immunohistochemistry, immunofluorescence, cell viability, apoptosis assays, and electron and confocal microscopy. Proteins were knocked down in cell lines using small interfering RNAs. Levels of copper were measured in cell lysates, blood samples, liver homogenates, and subcellular fractions by spectroscopy. RESULTS: After exposure to copper, ATP7B-knockout cells had significant increases in the expression of 103 genes that regulate autophagy (including MAP1LC3A, known as LC3) compared with wild-type cells. Electron and confocal microscopy visualized more autophagic structures in the cytoplasm of ATP7B-knockout cells than wild-type cells after copper exposure. Hepatocytes in liver tissues from patients with WD and from Atp7b(-/-) mice and rats (but not controls) had multiple autophagosomes. In ATP7B-knockout cells, mammalian target of rapamycin (mTOR) had decreased activity and was dissociated from lysosomes; this resulted in translocation of the mTOR substrate transcription factor EB to the nucleus and activation of autophagy-related genes. In wild-type HepG2 cells (but not ATP7B-knockout cells), exposure to copper and amino acids induced recruitment of mTOR to lysosomes. Pharmacologic inhibitors of autophagy or knockdown of autophagy proteins ATG7 and ATG13 induced and accelerated the death of ATP7B-knockout HepG2 cells compared with wild-type cells. Autophagy protected ATP7B-knockout cells from copperinduced death. CONCLUSION: ATP7B-deficient hepatocytes, such as in those in patients with WD, activate autophagy in response to copper overload to prevent copper-induced apoptosis. Agents designed to activate this autophagic pathway might decrease copper toxicity in patients with WD.

Copper (Cu) is an important trace element required for the activity of essential enzymes. However, excess Cu compromises the redox balance in cells and tissues causing serious toxicity. The process of disposal of excess Cu from organisms relies on the activity of Cu-transporting ATPase ATP7B. ATP7B is mainly expressed in liver hepatocytes where it sequesters the potentially toxic metal and mediates its excretion into the bile. Mutations in the ATP7B gene cause Wilson disease (WD), which is characterized by the accumulation of toxic Cu in the liver due to the scarce expression of ATP7B as well as the failure of ATP7B mutants to pump Cu and/or traffic to the Cu-excretion sites. The most frequent ATP7B mutant, H1069Q, still presents a significant Cu-transporting activity, but undergoes retention within the endoplasmic reticulum (ER) where the mutant is rapidly degraded. Expression of this ATP7B mutant has been recently reported to activate the p38 and JNK stress kinase pathways, which, in turn, trigger quality control mechanisms leading to the arrest of ATP7B-H1069Q in the ER and to the acceleration of its degradation. However, the main molecular players operating in these p38/JNK-dependent ER quality control pathways remain to be discovered. By using a combination of RNAseq, bioinformatics and RNAi approaches, we found a cluster of ER quality control genes whose expression is controlled by p38 and JNK and is required for the efficient retention of the ATP7B-H1069Q mutant in the ER. Silencing these genes reduced the accumulation of the ATP7B mutant in the ER and facilitated the mutant sorting and export to the Golgi and post-Golgi copper excretion sites. In sum, our findings reveal the ER-associated genes that could be utilized for the correction of ATP7B mutants and, hence, for the normalization of Cu homeostasis in Wilson disease.

UNLABELLED Wilson disease (WD) is an autosomal recessive disorder that is caused by the toxic accumulation of copper (Cu) in the liver. The ATP7B gene, which is mutated in WD, encodes a multitransmembrane domain adenosine triphosphatase that traffics from the trans-Golgi network to the canalicular area of hepatocytes, where it facilitates excretion of excess Cu into the bile. Several ATP7B mutations, including H1069Q and R778L that are two of the most frequent variants, result in protein products, which, although still functional, remain in the endoplasmic reticulum. Thus, they fail to reach Cu excretion sites, resulting in the toxic buildup of Cu in the liver of WD patients. Therefore, correcting the location of these mutants by leading them to the appropriate functional sites in the cell should restore Cu excretion and would be beneficial to help large cohorts of WD patients. However, molecular targets for correction of endoplasmic reticulum-retained ATP7B mutants remain elusive. Here, we show that expression of the most frequent ATP7B mutant, H1069Q, activates p38 and c-Jun N-terminal kinase signaling pathways, which favor the rapid degradation of the mutant. Suppression of these pathways with RNA interference or specific chemical inhibitors results in the substantial rescue of ATP7B(H1069Q) (as well as that of several other WD-causing mutants) from the endoplasmic reticulum to the trans-Golgi network compartment, in recovery of its Cu-dependent trafficking, and in reduction of intracellular Cu levels. CONCLUSION Our findings indicate p38 and c-Jun N-terminal kinase as intriguing targets for correction of WD-causing mutants and, hence, as potential candidates, which could be evaluated for the development of novel therapeutic strategies to combat WD. (Hepatology 2016;63:1842-1859).