New insights into CNS requirements for the copper-ATPase, ATP7A. Focus on "Autonomous requirements of the Menkes disease protein in the nervous system".

Abstract

Editorial FocusNew insights into CNS requirements for the copper-ATPase, ATP7A. Focus on “Autonomous requirements of the Menkes disease protein in the nervous system”Sharon La FontaineSharon La FontaineCentre for Molecular and Medical Research and Centre for Cellular and Molecular Biology, School of Life and Environmental Sciences, Deakin University, Melbourne Campus, Burwood, Victoria, Australia; and The Florey Institute of Neuroscience and Mental Health, Parkville, Victoria, AustraliaPublished Online:01 Dec 2015https://doi.org/10.1152/ajpcell.00258.2015This is the final version - click for previous versionMoreSectionsPDF (548 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat copper is indispensable for development and function of the central nervous system (CNS). This is dramatically illustrated by the severe neuropathological deficits in Menkes disease, an X-linked copper deficiency disorder resulting from mutation of the gene that encodes an essential copper transporting P1B-type ATPase, ATP7A. Since its discovery over two decades ago, the role of ATP7A in copper transport and homeostasis has been inextricably linked to satisfying systemic and CNS requirements for copper. In a recent issue of American Journal of Physiology-Cell Physiology, Hodgkinson et al. (8) describe an important body of work, which for the first time distinguishes the CNS requirement for ATP7A from the CNS requirement for copper.The recognition of copper dysregulation as a key pathological feature in prominent neurodegenerative disorders such as Alzheimer's (AD), Parkinson's (PD), Huntington's (HD), and prion diseases has intensified research efforts to elucidate the mechanisms of copper regulation in the CNS. Copper is an essential structural or catalytic cofactor for numerous enzymes involved in vital CNS processes such as respiration, neurotransmitter synthesis, activation of neuropeptides and hormones, protection from oxidative damage, myelination, pigmentation, and iron metabolism, among others. In addition to functioning as cofactor, copper has specific roles in the CNS in neurodevelopment, synaptogenesis and axon extension (5), modulation of neurotransmitter receptor activity and synaptic transmission (2), and in a range of signaling cascades (7). CNS copper is precisely regulated to ensure appropriate levels and distribution for the maintenance of CNS function and to avoid damage from the redox activity of excess copper. ATP7A is a key player in CNS and systemic copper regulation, functioning at the brain barriers to regulate the entry into and exit of copper from the brain (10). Within cells, ATP7A at the trans-Golgi network delivers copper into the secretory pathway for metallation of copper-dependent enzymes and traffics towards the basolateral surface of polarized epithelial cells to expel excess copper.Loss of ATP7A from the CNS does not lead to neurodegeneration.The study by Hodgkinson et al. (8) provides new insight into the roles of ATP7A and copper in the CNS by deleting the mouse Atp7a gene from CNS cells, leaving systemic copper homeostasis, and hence brain copper levels, intact. This new mouse model (Atp7aNes), in which the Atp7a gene was specifically deleted in neural and glial cell precursors, has revealed potentially new roles for ATP7A in neural circuits and also will be pivotal in distinguishing and investigating the specific contributions of ATP7A and copper to neurodevelopment and neurodegeneration.Phenotypic, biochemical, histological, and behavioral parameters were compared across the Atp7aNes, wild-type (WT), and mottled brindled (Atp7aMo-br) mice, the latter a model of Menkes disease with Atp7a mutated in all cells. Table 1 summarizes the phenotypic and neurological differences between the Atp7aNes and Atp7aMo-br mice. Brain copper levels in the Atp7aNes mice were normal to elevated compared with WT mice, whereas copper levels in the Atp7aMo-br mice were approximately three-fold lower than WT, consistent with previous reports (Table 1). Some elevation of brain copper in the Atp7aNes mice would be expected, and perhaps it is surprising that a more significant increase in brain copper was not observed since ATP7A expression in the brain barriers [blood-brain barrier (BBB) and blood cerebrospinal fluid barrier (BCB)] was left intact, allowing normal copper entry into the brain. In the absence of ATP7A to export their copper, astrocytes, which are known to efficiently take up and store copper, and neurons should then accumulate copper, potentially explaining the observed elevated brain copper levels.Table 1. Comparison of phenotypic and neurological features between Atp7aMo-br and Atp7aNes micePhenotypic/Neurological AttributeAtp7aMo-brAtp7aNesPostnatal survival0% ≥ P1680% >4–5 moCoat colorHypopigmentedNormalGrowthRetardedRetarded (50% of WT at 1 yr)Brain copper levelsReduced (2–4 fold cf. WT)Normal to elevatedBrain ATP7A protein levelsApproximately normalMarked reductionSystemic ATP7A protein levelsApproximately normalNormalCuproenzyme activity TyrosinaseND/reduced based on hypopigmentationND/normal based on normal coat color Lysyl oxidaseReduced (50–60% of WT)ND Cytochrome c oxidaseReduced (20–30% of WT)Normal Cu,Zn-SOD1Reduced (70–80% of WT)Normal Peptidylglycine α-amidating monooxygenaseReducedND Dopamine-β-hydroxylaseReduced-undetectable at P10ReducedNeuropathology MyelinationMarked reductionNormal Purkinje neuron cytoskeletal pathology and degenerationSevereND Purkinje neuron ATP7A levelsMarked reductionND SynaptogenesisImpairedND Axonal outgrowthImpairedND Neuronal loss/cell death from cerebral cortex, hippocampus, and cerebellumEvidentNot evident Astrocytosis in neocortex, hippocampus, and cerebellumEvidentNDNMDA receptor-mediated excitotoxicitySusceptibleSusceptibleSeizure activitySpontaneousSusceptibleMotor functionNDImpairedHypohagiaNDPresentAnxietyNDPresentWT, wild-type; ND, not determined.It has been previously suggested and now supported by Hodgkinson et al. (8) that the loss of function of copper-dependent enzymes alone cannot account for the severe neurodegeneration that is characteristic of Menkes patients and mouse models. The Atp7aNes mice displayed defective copper transport into the secretory pathway but did not exhibit the typical neurodegenerative features and spontaneous seizures seen in the Atp7aMo-br mice (Table 1). These observations provide a clear demonstration that the loss of ATP7A-mediated copper delivery to cuproenzymes per se does not fully explain the neurodegeneration in Menkes disease.Copper plays a role at the synapse and in synaptic transmission (2) and in the past decade, a physiological role for copper in regulating neuronal excitability has emerged to account for the seizures and neuronal degeneration characteristic of Menkes disease. ATP7A-dependent release of synaptic copper from rat hippocampal neurons in response to NMDA receptor stimulation was neuroprotective, and in the absence of ATP7A, hippocampal neurons from Atp7aMo-br mice were sensitive to glutamate-mediated NMDA receptor-dependent excitotoxicity (see references 2, 10). The protective mechanism involved endogenous nitric oxide production and S-nitrosylation of the NMDA receptor as a means of modulating its excitotoxic responsiveness. Significantly, this copper-mediated neuroprotective S-nitrosylation of the NMDA receptor appears to also involve an association between copper and the prion protein (PrPC), an interaction that is blocked by Aβ oligomers, inducing neurotoxicity (2).For the first time, the Atp7aNes mice allow an assessment of the specific functional contribution of ATP7A to the copper-mediated neuroprotection against excitotoxic NMDA stimulation. Despite sensitivity to NMDA receptor activation, confirming a role for ATP7A in protection against NMDA receptor-mediated excitotoxicity, the absence of spontaneous seizure activity and severe neurodegeneration in the Atp7aNes mice suggests that other mechanisms also operate to promote neuroprotective synaptic copper release. Could exosomes play a role in copper-mediated neuroprotection? Based on the association of numerous metal homeostasis proteins with exosomes, and in particular metal-binding disease-associated proteins such as PrPC and Aβ, a case has been made for the role of exosomes in copper release from neurons and astrocytes (1).Niciu et al. (9) identified neuropathology in the Atp7aMo-br mice among subsets of neurons (for example, Purkinje cell degeneration) that was specifically related to mutant ATP7A in a role independent of copper transport. In addition, ATP7A plays a role in axonal outgrowth and synaptogenesis, and hence synaptic plasticity, and these phenomena are disrupted in Atp7aMo-br mice, also potentially contributing to neurodegeneration (5). The Atp7aNes mice now provide an opportunity to further dissect the specific role of ATP7A (or loss thereof) versus copper in cytoskeletal pathology and during development in axonal targeting and synaptogenesis.New insights into ATP7A function within the CNS.Observations in the Atp7aNes mice also raised the issue of the directionality of copper flow across the brain barriers, a topic that has generated some discussion in the literature. Documented evidence supports the long-held view that copper entry into the brain is mediated by ATP7A at the basolateral membrane of BBB endothelial cells, whereas the BCB, comprising epithelial cells of the choroid plexus (CP), functions in maintaining copper homeostasis of cerebrospinal fluid (CSF). Here, ATP7A at the basolateral membrane of CP epithelial cells functions to efflux excess copper back into the circulation, and the related but kinetically slower copper-ATPase, ATP7B, is concentrated at the apical membrane to transport copper into the CSF (10) (Fig. 1). However, based on ATP7A gene therapy at the CP (3) it was proposed that ATP7A at the apical membrane of CP cells plays a role in copper entry into the brain. Hodgkinson et al. (8) also detected increased abundance of ATP7A protein in CP epithelial cells of the Atp7aNes mice, which, combined with the elevated hippocampal copper levels, is consistent with a role for ATP7A-mediated copper entry via the BCB and may reflect compensatory changes arising from loss of ATP7A in the CNS. Exploiting the CP tropism of adeno-associated virus serotype 5 (AAV5) used for ATP7A gene therapy (3), targeted disruption of Atp7a expression, specifically in the CP epithelia of the Atp7aNes mice, may help to clarify its role in regulating the direction of copper transport across the BCB.Fig. 1.Schematic diagram showing the direction of copper flow across the blood-brain barrier (BBB) and blood cerebrospinal fluid (CSF) barrier (BCB). Red arrows denote ATP7A-mediated copper entry into the brain across the basolateral membrane of BBB endothelial cells and ATP7A-mediated copper copper exit from astrocytes and across the basolateral membrane of choroid plexus epithelial cells for export of excess copper back into the circulation. The multiple arrows reflect higher ATP7A expression in the choroid plexus than in the cerebral capillaries (10). The red dashed arrow represents potential ATP7A-mediated copper entry into the CSF across the apical membrane of choroid plexus epithelial cells. The blue arrow denotes ATP7B-mediated copper entry across the apical membrane of choroid plexus epithelial cells. Green arrows denote copper uptake into astrocytes, neurons, and choroid plexus epithelial cells, mostly likely via the major copper import protein CTR1.Download figureDownload PowerPointThe Atp7aNes mice have not only revealed potential new roles for ATP7A in neuronal circuits controlling normal growth and anxiety, but provide a tool to directly investigate these ATP7A functions, as well as copper handling within the amygdala, which is important for learned fear behaviors (6). Further, since copper dysregulation is a pathological feature of several neurodegenerative and cognitive diseases (e.g., AD, PD, prion disease), crossing the Atp7aNes mice with mouse models of these diseases will establish whether ATP7A depletion confers increased susceptibility to these diseases, hence revealing its contribution to learning and memory, and whether the CNS copper sufficiency of the Atp7aNes mice influences the neuropathology of the disease models.Concluding remarks.A significant outcome from the study by Hodgkinson et al. (8) was the demonstration that the severe neuropathology of Menkes disease stems from profound copper deficiency in the CNS, secondary to systemic copper deficiency, rather than from ATP7A deficiency within the CNS per se. This strengthens the argument for early copper delivery into the CNS. Lipophilic ionophoric copper compounds that can cross the brain barriers to rescue the copper deficiency would seem to hold most promise for treatment of Menkes disease. Several classes of such copper-coordinating compounds (hydroxyquinolines, dithiocarbamates, and thiosemicarbazones) are being investigated for their therapeutic potential in neurodegenerative diseases including AD, PD, and HD (4).In summary, this contribution by Hodgkinson et al. (8) helps to clarify the role of ATP7A and copper in CNS function and integrity, and it paves the way for new discoveries relating to the specific and unique roles of ATP7A in the CNS. The challenge is to exploit this new model to reveal the full extent of ATP7A function in the CNS, as well as novel therapeutic strategies based on rescue of critical copper-mediated processes.DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the author.AUTHOR CONTRIBUTIONSS.L. prepared figure; drafted manuscript; edited and revised manuscript; approved final version of manuscript.REFERENCES1. Bellingham SA, Guo B, Hill AF. The secret life of extracellular vesicles in metal homeostasis and neurodegeneration. Biol Cell. doi: 10.1111/boc.201500030. [Epub ahead of print].Crossref | ISI | Google Scholar2. D'Ambrosi N, Rossi L. Copper at synapse: release, binding and modulation of neurotransmission. Neurochem Int. doi: 10.1016/j.neuint.2015.07.006. [Epub ahead of print].Crossref | ISI | Google Scholar3. Donsante A, Yi L, Zerfas PM, Brinster LR, Sullivan P, Goldstein DS, Prohaska J, Centeno JA, Rushing E, Kaler SG. 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Hodgkinson VL, Zhu S, Wang Y, Ladomersky E, Nickelson K, Weisman GA, Lee J, Gitlin JD, Petris MJ. Autonomous requirements of the Menkes disease protein in the nervous system. Am J Physiol Cell Physiol 309: C660–C668, 2015.Link | ISI | Google Scholar9. Niciu MJ, Ma XM, El Meskini R, Pachter JS, Mains RE, Eipper BA. Altered ATP7A expression and other compensatory responses in a murine model of Menkes disease. Neurobiol Dis 27: 278–291, 2007.Crossref | PubMed | ISI | Google Scholar10. Telianidis J, Hung YH, Materia S, La Fontaine S. Role of the P-Type ATPases, ATP7A and ATP7B in brain copper homeostasis. Front Aging Neurosci 5: 44, 2013.Crossref | PubMed | ISI | Google ScholarAUTHOR NOTESAddress for reprint requests and other correspondence: S. La Fontaine, Deakin Univ., Melbourne Campus, School of Life and Environmental Sciences, Centre for Cellular and Molecular Biology, 221 Burwood Highway, Burwood, VIC, 3125, Australia (e-mail: sharon.[email protected]edu.au). Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation Cited ByXenon-inhibition of the MscL mechano-sensitive channel and the CopB copper ATPase under different conditions suggests direct effects on these proteins4 June 2018 | PLOS ONE, Vol. 13, No. 6Orchestration of dynamic copper navigation – new and missing pieces1 January 2017 | Metallomics, Vol. 9, No. 9Autonomous requirements of the Menkes disease protein in the nervous systemVictoria L. Hodgkinson, Sha Zhu, Yanfang Wang, Erik Ladomersky, Karen Nickelson, Gary A. Weisman, Jaekwon Lee, Jonathan D. Gitlin, and Michael J. Petris15 November 2015 | American Journal of Physiology-Cell Physiology, Vol. 309, No. 10 More from this issue > Volume 309Issue 11December 2015Pages C719-C721 Copyright & PermissionsCopyright © 2015 the American Physiological Societyhttps://doi.org/10.1152/ajpcell.00258.2015PubMed26468209History Published online 1 December 2015 Published in print 1 December 2015 Metrics