Selective ferroptosis vulnerability due to familial Alzheimer's disease presenilin mutations.

Mutations in presenilin 1 and 2 (PS1 and PS2) cause autosomal dominant familial Alzheimer's disease (FAD). Ferroptosis has been implicated as a mechanism of neurodegeneration in AD since neocortical iron burden predicts Alzheimer's disease (AD) progression. We found that loss of the presenilins dramatically sensitizes multiple cell types to ferroptosis, but not apoptosis. FAD causal mutations of presenilins similarly sensitizes cells to ferroptosis. The presenilins promote the expression of GPX4, the selenoprotein checkpoint enzyme that blocks ferroptosis by quenching the membrane propagation of lethal hydroperoxyl radicals. Presenilin γ-secretase activity cleaves Notch-1 to signal LRP8 expression, which then controls GPX4 expression by regulating the supply of selenium into the cell since LRP8 is the uptake receptor for selenoprotein P. Selenium uptake is thus disrupted by presenilin FAD mutations, suppressing GPX4 expression. Therefore, presenilin mutations may promote neurodegeneration by derepressing ferroptosis, which has implications for disease-modifying therapeutics.

Increased cortical expression of the zinc transporter SLC39A12 suggests a breakdown in zinc cellular homeostasis as part of the pathophysiology of schizophrenia.

Our expression microarray studies showed messenger RNA (mRNA) for solute carrier family 39 (zinc transporter), member 12 (SLC39A12) was higher in dorsolateral prefrontal cortex from subjects with schizophrenia (Sz) in comparison with controls. To better understand the significance of these data we ascertained whether SLC39A12 mRNA was altered in a number of cortical regions (Brodmann's area (BA) 8, 9, 44) from subjects with Sz, in BA 9 from subjects with mood disorders and in rats treated with antipsychotic drugs. In addition, we determined whether inducing the expression of SLC39A12 resulted in an increased cellular zinc uptake. SLC39A12 variant 1 and 2 mRNA was measured using quantitative PCR. Zinc uptake was measured in CHO cells transfected with human SLC39A12 variant 1 and 2. In Sz, compared with controls, SLC39A12 variant 1 and 2 mRNA was higher in all cortical regions studied. The were no differences in levels of mRNA for either variant of SLC39A12 in BA 9 from subjects with mood disorders and levels of mRNA for Slc39a12 was not different in the cortex of rats treated with antipsychotic drugs. Finally, expressing both variants in CHO-K1 cells was associated with an increase in radioactive zinc uptake. As increased levels of murine Slc39a12 mRNA has been shown to correlate with increasing cellular zinc uptake, our data would be consistent with the possibility of a dysregulated zinc homeostasis in the cortex of subjects with schizophrenia due to altered expression of SLC39A12.

Copper: from neurotransmission to neuroproteostasis.

Copper is critical for the Central Nervous System (CNS) development and function. In particular, different studies have shown the effect of copper at brain synapses, where it inhibits Long Term Potentation (LTP) and receptor pharmacology. Paradoxically, according to recent studies copper is required for a normal LTP response. Copper is released at the synaptic cleft, where it blocks glutamate receptors, which explain its blocking effects on excitatory neurotransmission. Our results indicate that copper also enhances neurotransmission through the accumulation of PSD95 protein, which increase the levels of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors located at the plasma membrane of the post-synaptic density. Thus, our findings represent a novel mechanism for the action of copper, which may have implications for the neurophysiology and neuropathology of the CNS. These data indicate that synaptic configuration is sensitive to transient changes in transition metal homeostasis. Our results suggest that copper increases GluA1 subunit levels of the AMPA receptor through the anchorage of AMPA receptors to the plasma membrane as a result of PSD-95 accumulation. Here, we will review the role of copper on neurotransmission of CNS neurons. In addition, we will discuss the potential mechanisms by which copper could modulate neuronal proteostasis ("neuroproteostasis") in the CNS with focus in the Ubiquitin Proteasome System (UPS), which is particularly relevant to neurological disorders such as Alzheimer's disease (AD) where copper and protein dyshomeostasis may contribute to neurodegeneration. An understanding of these mechanisms may ultimately lead to the development of novel therapeutic approaches to control metal and synaptic alterations observed in AD patients.

Presenilin promotes dietary copper uptake.

Dietary copper is essential for multicellular organisms. Copper is redox active and required as a cofactor for enzymes such as the antioxidant Superoxide Dismutase 1 (SOD1). Copper dyshomeostasis has been implicated in Alzheimer's disease. Mutations in the presenilin genes encoding PS1 and PS2 are major causes of early-onset familial Alzheimer's disease. PS1 and PS2 are required for efficient copper uptake in mammalian systems. Here we demonstrate a conserved role for presenilin in dietary copper uptake in the fly Drosophila melanogaster. Ubiquitous RNA interference-mediated knockdown of the single Drosophila presenilin (PSN) gene is lethal. However, PSN knockdown in the midgut produces viable flies. These flies have reduced copper levels and are more tolerant to excess dietary copper. Expression of a copper-responsive EYFP construct was also lower in the midgut of these larvae, indicative of reduced dietary copper uptake. SOD activity was reduced by midgut PSN knockdown, and these flies were sensitive to the superoxide-inducing chemical paraquat. These data support presenilin being needed for dietary copper uptake in the gut and so impacting on SOD activity and tolerance to oxidative stress. These results are consistent with previous studies of mammalian presenilins, supporting a conserved role for these proteins in mediating copper uptake.

Metal dyshomeostasis and oxidative stress in Alzheimer's disease.

Alzheimer's disease is the leading cause of dementia in the elderly and is defined by two pathological hallmarks; the accumulation of aggregated amyloid beta and excessively phosphorylated Tau proteins. The etiology of Alzheimer's disease progression is still debated, however, increased oxidative stress is an early and sustained event that underlies much of the neurotoxicity and consequent neuronal loss. Amyloid beta is a metal binding protein and copper, zinc and iron promote amyloid beta oligomer formation. Additionally, copper and iron are redox active and can generate reactive oxygen species via Fenton (and Fenton-like chemistry) and the Haber-Weiss reaction. Copper, zinc and iron are naturally abundant in the brain but Alzheimer's disease brain contains elevated concentrations of these metals in areas of amyloid plaque pathology. Amyloid beta can become pro-oxidant and when complexed to copper or iron it can generate hydrogen peroxide. Accumulating evidence suggests that copper, zinc, and iron homeostasis may become perturbed in Alzheimer's disease and could underlie an increased oxidative stress burden. In this review we discuss oxidative/nitrosative stress in Alzheimer's disease with a focus on the role that metals play in this process. Recent studies have started to elucidate molecular links with oxidative/nitrosative stress and Alzheimer's disease. Finally, we discuss metal binding compounds that are designed to cross the blood brain barrier and restore metal homeostasis as potential Alzheimer's disease therapeutics.

The Alzheimer's therapeutic PBT2 promotes amyloid-ß degradation and GSK3 phosphorylation via a metal chaperone activity.

Impaired metal ion homeostasis causes synaptic dysfunction and treatments for Alzheimer's disease (AD) that target metal ions have therefore been developed. The leading compound in this class of therapeutic, PBT2, improved cognition in a clinical trial with AD patients. The aim of the present study was to examine the cellular mechanism of action for PBT2. We show PBT2 induces inhibitory phosphorylation of the α- and ß-isoforms of glycogen synthase kinase 3 and that this activity is dependent on PBT2 translocating extracellular Zn and Cu into cells. This activity is supported when Aß:Zn aggregates are the source of extracellular Zn and adding PBT2 to Aß:Zn preparations promotes Aß degradation by matrix metalloprotease 2. PBT2-induced glycogen synthase kinase 3 phosphorylation appears to involve inhibition of the phosphatase calcineurin. Consistent with this, PBT2 increased phosphorylation of other calcineurin substrates, including cAMP response element binding protein and Ca²âº/calmodulin-dependent protein kinase. These data demonstrate PBT2 can decrease Aß levels by sequestering the Zn that promotes extracellular formation of protease resistant Aß:Zn aggregates, and that subsequent intracellular translocation of the Zn by PBT2 induces cellular responses with synapto-trophic potential. Intracellular translocation of Zn and Cu via the metal chaperone activity of PBT2 may be an important mechanism by which PBT2 improves cognitive function in people with AD.

Presenilins promote the cellular uptake of copper and zinc and maintain copper chaperone of SOD1-dependent copper/zinc superoxide dismutase activity.

Dyshomeostasis of extracellular zinc and copper has been implicated in ß-amyloid aggregation, the major pathology associated with Alzheimer disease. Presenilin mediates the proteolytic cleavage of the ß-amyloid precursor protein to release ß-amyloid, and mutations in presenilin can cause familial Alzheimer disease. We tested whether presenilin expression affects copper and zinc transport. Studying murine embryonic fibroblasts (MEFs) from presenilin knock-out mice or RNA interference of presenilin expression in HEK293T cells, we observed a marked decrease in saturable uptake of radiolabeled copper and zinc. Measurement of basal metal levels in 6-month-old presenilin 1 heterozygous knock-out (PS1(+/-)) mice revealed significant deficiencies of copper and zinc in several tissues, including brain. Copper/zinc superoxide dismutase (SOD1) activity was significantly decreased in both presenilin knock-out MEFs and brain tissue of presenilin 1 heterozygous knock-out mice. In the MEFs and PS1(+/-) brains, copper chaperone of SOD1 (CCS) levels were decreased. Zinc-dependent alkaline phosphatase activity was not decreased in the PS null MEFs. These data indicate that presenilins are important for cellular copper and zinc turnover, influencing SOD1 activity, and having the potential to indirectly impact ß-amyloid aggregation through metal ion clearance.

Iron-export ferroxidase activity of ß-amyloid precursor protein is inhibited by zinc in Alzheimer's disease.

Alzheimer's Disease (AD) is complicated by pro-oxidant intraneuronal Fe(2+) elevation as well as extracellular Zn(2+) accumulation within amyloid plaque. We found that the AD ß-amyloid protein precursor (APP) possesses ferroxidase activity mediated by a conserved H-ferritin-like active site, which is inhibited specifically by Zn(2+). Like ceruloplasmin, APP catalytically oxidizes Fe(2+), loads Fe(3+) into transferrin, and has a major interaction with ferroportin in HEK293T cells (that lack ceruloplasmin) and in human cortical tissue. Ablation of APP in HEK293T cells and primary neurons induces marked iron retention, whereas increasing APP695 promotes iron export. Unlike normal mice, APP(-/-) mice are vulnerable to dietary iron exposure, which causes Fe(2+) accumulation and oxidative stress in cortical neurons. Paralleling iron accumulation, APP ferroxidase activity in AD postmortem neocortex is inhibited by endogenous Zn(2+), which we demonstrate can originate from Zn(2+)-laden amyloid aggregates and correlates with Aß burden. Abnormal exchange of cortical zinc may link amyloid pathology with neuronal iron accumulation in AD.