Biochemical study on the critical period for treatment of the mottled brindled mouse

Adaptive protein synthesis in genetic models of copper deficiency and childhood neurodegeneration

Rare inherited diseases caused by mutations in the copper transporters SLC31A1 (CTR1) or ATP7A induce copper deficiency in the brain and throughout the body, causing seizures and neurodegeneration in infancy. The mechanistic underpinnings of such neuropathology remains unclear. Here, we characterized the molecular mechanisms by which neuronal cells respond to copper depletion in multiple genetic model systems. Targeted deletion of CTR1 in neuroblastoma clonal cell lines produced copper deficiency that was associated with compromised copper-dependent Golgi and mitochondrial enzymes and a metabolic shift favoring glycolysis over oxidative phosphorylation. Proteomic and transcriptomic analysis revealed simultaneous upregulation of mTORC1 and S6K signaling, along with reduced PERK signaling in CTR1 KO cells. Patterns of gene and protein expression and pharmacogenomics show increased activation of the mTORC1-S6K pathway as a pro-survival mechanism, ultimately resulting in increased protein synthesis as measured by puromycin labeling. These effects of copper depletion were corroborated by spatial transcriptomic profiling of the cerebellum of Atp7a flx/Y :: Vil1 Cre/+ mice, in which copper-deficient Purkinje cells exhibited upregulated protein synthesis machinery and expression of mTORC1-S6K pathway genes. We tested whether increased activity of mTOR in copper-deficient neurons was adaptive or deleterious by genetic epistasis experiments in Drosophila . Copper deficiency dendritic phenotypes in class IV neurons are partially rescued by increased S6k expression or 4E-BP1 (Thor) RNAi, while epidermis phenotypes are exacerbated by Akt, S6k, or raptor RNAi. Overall, we demonstrate that increased mTORC1-S6K pathway activation and protein synthesis is an adaptive mechanism by which neuronal cells respond to copper depletion.

Significance Statement

Copper dysregulation is present in neurodevelopmental, neurodegenerative, and neurological conditions ranging from rare conditions such as Menkes disease and CTR1 deficiency to more common diseases like Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis. However, the precise mechanisms through which copper deficiency leads to pathology and the survival mechanisms by which the brain exhibits resilience remain unknown. We demonstrate that in a human cell line, Drosophila , and the mouse cerebellum, copper-depleted neuronal cells exhibit increased protein synthesis through two distinct signaling pathways: mTORC1 activation and decreased PERK (EIF2AK3) activity. This upregulation of protein synthesis facilitates resilience of neuronal cells to copper deficiency, including partial restoration of dendritic arborization. Our findings offer a new framework for understanding copper deficiency-related pathology in neurological disorders.

The Role of Copper Homeostasis in Brain Disease

In the human body, copper is an important trace element and is a cofactor for several important enzymes involved in energy production, iron metabolism, neuropeptide activation, connective tissue synthesis, and neurotransmitter synthesis. Copper is also necessary for cellular processes, such as the regulation of intracellular signal transduction, catecholamine balance, myelination of neurons, and efficient synaptic transmission in the central nervous system. Copper is naturally present in some foods and is available as a dietary supplement. Only small amounts of copper are typically stored in the body and a large amount of copper is excreted through bile and urine. Given the critical role of copper in a breadth of cellular processes, local concentrations of copper and the cellular distribution of copper transporter proteins in the brain are important to maintain the steady state of the internal environment. The dysfunction of copper metabolism or regulatory pathways results in an imbalance in copper homeostasis in the brain, which can lead to a myriad of acute and chronic pathological effects on neurological function. It suggests a unique mechanism linking copper homeostasis and neuronal activation within the central nervous system. This article explores the relationship between impaired copper homeostasis and neuropathophysiological progress in brain diseases.

Cerebrospinal Fluid-Directed rAAV9-rsATP7A Plus Subcutaneous Copper Histidinate Advance Survival and Outcomes in a Menkes Disease Mouse Model

Menkes disease is a lethal neurodegenerative disorder of copper metabolism caused by mutations in an evolutionarily conserved copper transporter, ATP7A. Based on our prior clinical and animal studies, we seek to develop a therapeutic approach suitable for application in affected human subjects, using the mottled-brindled (mo-br) mouse model that closely mimics the Menkes disease biochemical and clinical phenotypes. Here, we evaluate the efficacy of low-, intermediate-, and high-dose recombinant adeno-associated virus serotype 9 (rAAV9)-ATP7A delivered to the cerebrospinal fluid (CSF), in combination with subcutaneous administration of clinical-grade copper histidinate (sc CuHis, IND #34,166). Mutant mice that received high-dose (1.6 × 1010 vg) cerebrospinal fluid-directed rAAV9-rsATP7A plus sc copper histidinate showed 53.3% long-term (≥300-day) survival compared to 0% without treatment or with either treatment alone. The high-dose rAAV9-rsATP7A plus sc copper histidinate-treated mutant mice showed increased brain copper levels, normalized brain neurochemical levels, improvement of brain mitochondrial abnormalities, and normal growth and neurobehavioral outcomes. This synergistic treatment effect represents the most successful rescue to date of the mo-br mouse model. Based on these findings, and the absence of a large animal model, we propose cerebrospinal fluid-directed rAAV9-rsATP7A gene therapy plus subcutaneous copper histidinate as a potential therapeutic approach to cure or ameliorate Menkes disease.

Role of the P-Type ATPases, ATP7A and ATP7B in brain copper homeostasis

Over the past two decades there have been significant advances in our understanding of copper homeostasis and the pathological consequences of copper dysregulation. Cumulative evidence is revealing a complex regulatory network of proteins and pathways that maintain copper homeostasis. The recognition of copper dysregulation as a key pathological feature in prominent neurodegenerative disorders such as Alzheimer's, Parkinson's, and prion diseases has led to increased research focus on the mechanisms controlling copper homeostasis in the brain. The copper-transporting P-type ATPases (copper-ATPases), ATP7A and ATP7B, are critical components of the copper regulatory network. Our understanding of the biochemistry and cell biology of these complex proteins has grown significantly since their discovery in 1993. They are large polytopic transmembrane proteins with six copper-binding motifs within the cytoplasmic N-terminal domain, eight transmembrane domains, and highly conserved catalytic domains. These proteins catalyze ATP-dependent copper transport across cell membranes for the metallation of many essential cuproenzymes, as well as for the removal of excess cellular copper to prevent copper toxicity. A key functional aspect of these copper transporters is their copper-responsive trafficking between the trans-Golgi network and the cell periphery. ATP7A- and ATP7B-deficiency, due to genetic mutation, underlie the inherited copper transport disorders, Menkes and Wilson diseases, respectively. Their importance in maintaining brain copper homeostasis is underscored by the severe neuropathological deficits in these disorders. Herein we will review and update our current knowledge of these copper transporters in the brain and the central nervous system, their distribution and regulation, their role in normal brain copper homeostasis, and how their absence or dysfunction contributes to disturbances in copper homeostasis and neurodegeneration.

An overview and update of ATP7A mutations leading to Menkes disease and occipital horn syndrome

Menkes disease (MD) is a lethal multisystemic disorder of copper metabolism. Progressive neurodegeneration and connective tissue disturbances, together with the peculiar "kinky" hair, are the main manifestations. MD is inherited as an X-linked recessive trait, and as expected the vast majority of patients are males. MD occurs because of mutations in the ATP7A gene and the vast majority of ATP7A mutations are intragenic mutations or partial gene deletions. ATP7A is an energy-dependent transmembrane protein, which is involved in the delivery of copper to the secreted copper enzymes and in the export of surplus copper from cells. Severely affected MD patients die usually before the third year of life. A cure for the disease does not exist, but very early copper-histidine treatment may correct some of the neurological symptoms. This study reviews 274 published and 18 novel disease causing mutations identified in 370 unrelated MD patients, nonpathogenic variants of ATP7A, functional studies of the ATP7A mutations, and animal models of MD.

ATP7A-related copper transport diseases-emerging concepts and future trends

This Review summarizes recent advances in understanding copper-transporting ATPase 1 (ATP7A), and examines the neurological phenotypes associated with dysfunction of this protein. Involvement of ATP7A in axonal outgrowth, synapse integrity and neuronal activation underscores the fundamental importance of copper metabolism to neurological function. Defects in ATP7A cause Menkes disease, an infantile-onset, lethal condition. Neonatal diagnosis and early treatment with copper injections enhance survival in patients with this disease, and can normalize clinical outcomes if mutant ATP7A molecules retain small amounts of residual activity. Gene replacement rescues a mouse model of Menkes disease, suggesting a potential therapeutic approach for patients with complete loss-of-function ATP7A mutations. Remarkably, a newly discovered ATP7A disorder-isolated distal motor neuropathy-has none of the characteristic clinical or biochemical abnormalities of Menkes disease or its milder allelic variant occipital horn syndrome (OHS), instead resembling Charcot-Marie-Tooth disease type 2. These findings indicate that ATP7A has a crucial but previously unappreciated role in motor neuron maintenance, and that the mechanism underlying ATP7A-related distal motor neuropathy is distinct from Menkes disease and OHS pathophysiology. Collectively, these insights refine our knowledge of the neurology of ATP7A-related copper transport diseases and pave the way for further progress in understanding ATP7A function.

Menkes disease

Menkes disease (MD) is a lethal multisystemic disorder of copper metabolism. Progressive neurodegeneration and connective tissue disturbances, together with the peculiar 'kinky' hair are the main manifestations. MD is inherited as an X-linked recessive trait, and as expected the vast majority of patients are males. MD occurs due to mutations in the ATP7A gene and the vast majority of ATP7A mutations are intragenic mutations or partial gene deletions. ATP7A is an energy dependent transmembrane protein, which is involved in the delivery of copper to the secreted copper enzymes and in the export of surplus copper from cells. Severely affected MD patients die usually before the third year of life. A cure for the disease does not exist, but very early copper-histidine treatment may correct some of the neurological symptoms.

Altered ATP7A expression and other compensatory responses in a murine model of Menkes disease

Mutations in the copper-transporter ATP7A lead to severe neurodegeneration in the mottled brindled hemizygous male (MoBr/y) mouse and human patients with Menkes disease. Our earlier studies demonstrated cell-type- and -stage-specific changes in ATP7A protein expression during postnatal neurodevelopment. Here we examined copper and cuproenzyme levels in MoBr/y mice to search for compensatory responses. While all MoBr/y neocortical subcellular fractions had decreased copper levels, the greatest decrease (8-fold) was observed in cytosol. Immunostaining for ATP7A revealed decreased levels in MoBr/y hippocampal pyramidal and cerebellar Purkinje neurons. In contrast, an upregulation of ATP7A protein occurred in MoBr/y endothelial cells, perhaps to compensate for a lack of copper in the neuropil. MoBr/y astrocytes and microglia increased their physical association with the blood-brain barrier. No alterations in ATP7A levels were observed in ependymal cells, arguing for specificity in the alteration observed at the blood-brain barrier. The decreased expression of ATP7A protein in MoBr/y Purkinje cells was associated with impaired synaptogenesis and dramatic cytoskeletal dysfunction. Immunoblotting failed to reveal any compensatory increase in levels of ATP7B. While total levels of several cuproenzymes (peptide-amidating monooxygenase, SOD1, and SOD3) were unaltered in the MoBr/y brain, levels of amidated cholecystokinin (CCK8) and amidated pituitary adenylate cyclase-activating polypeptide (PACAP38) were reduced in a tissue-specific fashion. The compensatory changes observed in the neurovascular unit provide insight into the success of copper injections within a defined neurodevelopmental period.

Developmental changes in the expression of ATP7A during a critical period in postnatal neurodevelopment

ATP7A is a P-type ATPase that transports copper from cytosol into the secretory pathway for loading onto cuproproteins or efflux. Mutations in Atp7a cause Menkes disease, a copper-deficiency disorder fatal in the postnatal period due to severe neurodegeneration. Early postnatal copper injections are known to diminish degenerative changes in some human patients and mice bearing mutations in Atp7a. In situ hybridization studies previously demonstrated that ATP7A transcripts are expressed widely in the brain. ATP7A-specific antibody was used to study the neurodevelopmental expression and localization of ATP7A protein in the mouse brain. Based on immunoblot analyses, ATP7A expression is most abundant in the early postnatal period, reaching peak levels at P4 in neocortex and cerebellum. In the developing and adult brain, ATP7A levels are greatest in the choroid plexus/ependymal cells of the lateral and third ventricles. ATP7A expression decreases in most neuronal subpopulations from birth to adulthood. In contrast, ATP7A expression increases in CA2 hippocampal pyramidal and cerebellar Purkinje neurons. ATP7A is expressed in a subset of astrocytes, microglia, oligodendrocytes, tanycytes and endothelial cells. ATP7A is largely localized to the trans-Golgi network, adopting the cell-specific and developmentally-regulated morphology of this organelle. The presence of ATP7A in the axons of postnatal, but not adult, optic nerve suggests stage-specific roles for this enzyme. In sum, the precisely-regulated neurodevelopmental expression of ATP7A correlates well with the limited therapeutic window for effective treatment of Menkes disease.

Mitochondrial dysfunction in neurodegenerative diseases associated with copper imbalance

Copper is an essential transition metal ion for the function of key metabolic enzymes, but its uncontrolled redox reactivity is source of reactive oxygen species. Therefore a network of transporters strictly controls the trafficking of copper in living systems. Deficit, excess, or aberrant coordination of copper are conditions that may be detrimental, especially for neuronal cells, which are particularly sensitive to oxidative stress. Indeed, the genetic disturbances of copper homeostasis, Menkes' and Wilson's diseases, are associated with neurodegeneration. Furthermore, copper interacts with the proteins that are the hallmarks of neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, prion diseases, and familial amyotrophic lateral sclerosis. In all cases, copper-mediated oxidative stress is linked to mitochondrial dysfunction, which is a common feature of neurodegeneration. In particular we recently demonstrated that in copper deficiency, mitochondrial function is impaired due to decreased activity of cytochrome c oxidase, leading to production of reactive oxygen species, which in turn triggers mitochondria-mediated apoptotic neurodegeneration.

Menkes protein contributes to the function of peptidylglycine alpha-amidating monooxygenase

Menkes protein (ATP7A) is a P-type ATPase involved in copper uptake and homeostasis. Disturbed copper homeostasis occurs in patients with Menkes disease, an X-linked disorder characterized by mental retardation, neurodegeneration, connective tissue disorders, and early childhood death. Mutations in ATP7A result in malfunction of copper-requiring enzymes, such as tyrosinase and copper/zinc superoxide dismutase. The first step of the two-step amidation reaction carried out by peptidylglycine alpha-amidating monooxygenase (PAM) also requires copper. We used tissue from wild-type rats and mice and an ATP7A-specific antibody to determine that ATP7A is expressed at high levels in tissues expressing high levels of PAM. ATP7A is largely localized to the trans Golgi network in pituitary endocrine cells. The Atp7a mouse, bearing a mutation in the Atp7a gene, is an excellent model system for examining the consequences of ATP7A malfunction. Despite normal levels of PAM protein, levels of several amidated peptides were reduced in pituitary and brain extracts of Atp7a mice, demonstrating that PAM function is compromised when ATP7A is inactive. Based on these results, we conclude that a reduction in the ability of PAM to produce bioactive end-products involved in neuronal growth and development could contribute to many of the biological effects associated with Menkes disease.

Neurodegeneration in the animal model of Menkes' disease involves Bcl-2-linked apoptosis

Copper plays a key role in brain development, function and survival. Alteration of its homeostasis is suggested to be an aetiological factor in several neurodegenerative diseases. However, the molecular mechanisms relating copper to neurodegeneration are still unknown. In the present report, using morphological analyses of brain sections of mottled/brindled mutant (Mo(br/y)) mice, the animal model of the human genetic copper deficiency associated with neurodegeneration (Menkes' disease), we demonstrated that a high degree of apoptotic cells is present in the neocortex and in the hippocampus. Biochemical characterisation revealed decreased levels of copper content and of the activity of the mitochondrial copper-dependent enzyme cytochrome c oxidase. Copper, zinc-superoxide dismutase activity also shows a slight decrease, while no change was observed for glutathione content. Lower levels of ATP were also found, indicative of a copper-dependent impairment of energy metabolism. Changes appear to be specific for the brain, since no alterations in the activity of liver enzymes were found, although the level of copper was strongly decreased. We also tested biochemical factors involved in cell commitment to apoptosis. The expression of the anti-apoptotic protein Bcl-2, which plays a fundamental role in brain development and morphogenesis, was dramatically decreased and the levels of cytochrome c released from mitochondria into the cytosol were significantly increased. On the basis of these findings, we propose that down-regulation of Bcl-2 can cause neurodegeneration triggered by mitochondrial damage due to copper depletion during brain development in Mo(br/y) mice.

Increase of flux control of cytochrome c oxidase in copper-deficient mottled brindled mice

The brindled mottled mouse (Mobr), an animal model of the Menkes' copper deficiency syndrome, was used for the investigation of changes in respiratory flux control associated with cytochrome c oxidase deficiency in muscle. Enzymatic analysis of cardiac and skeletal muscles showed an approximately 2-fold decrease in cytochrome c oxidase activity of brindled mutants in both types of muscles as compared with controls. The activities of NADH-cytochrome c oxidoreductase (respiratory chain segment I-III) and succinate-cytochrome c oxidoreductase (segment II-III) were normal. Assessment of mitochondrial respiratory function was performed using chemically skinned musculus quadriceps or heart muscle fibers isolated from control and brindled mottled mice. In skeletal muscle, there was no difference found in maximal rates of respiration. In the Mobr hearts, this parameter was slightly lower than control. Alternately, the determination of flux control coefficients of cytochrome c oxidase performed by a step by step inhibition of respiration with increasing concentrations of azide or cyanide revealed significantly sharper inhibition curves for brindled mice than for control, indicating more than 2-fold elevated flux control coefficients of cytochrome c oxidase. This investigation proved essential in characterizing the metabolic effect of a cytochrome c oxidase deficiency. We conclude, therefore, that application of metabolic control analysis can be a valuable approach to study defects of mitochondrial oxidative phosphorylation.

Early copper therapy in classic Menkes disease patients with a novel splicing mutation

To correlate genotype with response to early copper histidine therapy in Menkes disease, an X-linked disorder of copper transport, we performed mutational analysis in 2 related males who began treatment at the age of 10 days and prenatally at 32 weeks' gestation, respectively. A G to T transversion at the -1 exonic position of a splice donor site was identified, predicting a glutamine to histidine substitution at codon 724 of the Menkes copper-transporting ATPase gene. The Q724H mutation disrupts proper splicing and generates five mutant transcripts that skip from one to four exons. None of these transcripts is predicted to encode a functional copper transport protein. Copper histidine treatment normalized circulating copper and ceruloplasmin levels but did not improve the baseline deficiency of dopamine-beta-hydroxylase, a copper-dependent enzyme. At the age of 36 months, the first patient was living and had neurodevelopmental abilities ranging from 10 to 15 months. The second patient also showed delayed neurodevelopment and died of pulmonary complications at the age of 5 1/2 months. We conclude that early copper histidine therapy does not normalize neurological outcome in patients with the Q724H splicing mutation, and suggest that preservation of some residual Menkes ATPase activity may be a general prerequisite for significant clinical efficacy from such treatment.

The mottled gene is the mouse homologue of the Menkes disease gene

The mottled mouse has been proposed as an animal model for Menkes disease, an X-linked disorder of copper transport. The recent isolation of a copper-transporting ATPase gene responsible for Menkes disease has allowed us to test this hypothesis. Here we report the isolation and sequence of the mouse homologue of this gene. We show that two mottled (Mo) alleles, dappled (Modp) and blotchy (Moblo), have abnormalities in the murine mRNA and that Modp has a partial gene deletion. These studies prove that the mottled mouse is the murine model for Menkes disease, providing the basis for future biochemical and therapeutic studies.

Deficiency of copper can cause neuronal degeneration

The aim of this article is to emphasize the important role that copper plays in the function of nerve cells. We are reporting preliminary data which suggest that the swelling of axons which we produce in rats by iminodipropionitrile, IDPN, is due to its chelating action on copper, and how conversely supplementation with copper abolishes both symptoms and lesions. The copper values we obtained by atomic absorption spectrophotometry of the spinal cord and brain from the animals fully support this contention. In comparing these results with the diseases that are known to be due to copper deficiency, namely Menkes disease in man, swayback in lambs and several neurological mutant mice, we find not only similar axonal swellings, but also amelioration of symptoms and lesions by early administration of copper. Considering the main forms in which copper is present, we discuss the cuproproteins, i.e. ceruloplasmin and metallothionein, and their role in transport and delivery of copper to various organs. Further, the many cuproenzymes i.e. superoxide dismutase, tryptophan-2,3-dioxygenase, lysine oxidase, cytochrome oxidase, monoamine oxidases, tyrosinase, dopamine-beta-hydroxylase and d-amino levulinate dehydratase are noted for their roles in the nervous system. Finally, we suggest that neuronal copper deficiency should be more fully investigated as a possible etiological factor in the more common neurodegenerative diseases, such as Alzheimer's disease and amyotrophic lateral sclerosis, ALS.