Evidence for a novel heme-binding protein, HasAh, in Alzheimer disease

REVIEW: Pin1 in Alzheimer's disease

Proteolytic processing and phosphorylation of amyloid precursor protein (APP), and hyperphosphorylation of tau protein, have been shown to be increased in Alzheimer's disease (AD) brains, leading to increased production of beta-amyloid (Abeta) peptides and neurofibrillary tangles, respectively. These observations suggest that phosphorylation events are critical to the understanding of the pathogenesis and treatment of this devastating disease. Pin-1, one of the peptidyl-prolyl isomerases (PPIase), catalyzes the isomerization of the peptide bond between pSer/Thr-Pro in proteins, thereby regulating their biological functions which include protein assembly, folding, intracellular transport, intracellular signaling, transcription, cell cycle progression and apoptosis. A number of previous studies have shown that Pin1 is co-localized with phosphorylated tau in AD brain, and shows an inverse relationship to the expression of tau. Pin1 protects neurons under in vitro conditions. Moreover, recent studies demonstrate that APP is a target for Pin1 and thus, in Abeta production. Furthermore, Pin1 was found to be oxidatively modified and to have reduced activity in the hippocampus in mild cognitive impairment (MCI) and AD. Because of the diverse functions of Pin1, and the discovery that this protein is one of the oxidized proteins common to both MCI and AD brain, the question arises as to whether Pin1 is one of the driving forces for the initiation or progression of AD pathogenesis, finally leading to neurodegeneration and neuronal apoptosis. In the present review, we discuss the role of Pin1 with respect to Alzheimer's disease.

Tracing heme in a living cell: hemoglobin degradation and heme traffic in digest cells of the cattle tick Boophilus microplus

Heme is present in all cells, acting as a cofactor in essential metabolic pathways such as respiration and photosynthesis. Moreover, both heme and its degradation products, CO, iron and biliverdin, have been ascribed important signaling roles. However, limited knowledge is available on the intracellular pathways involved in the flux of heme between different cell compartments. The cattle tick Boophilus microplus ingests 100 times its own mass in blood. The digest cells of the midgut endocytose blood components and huge amounts of heme are released during hemoglobin digestion. Most of this heme is detoxified by accumulation into a specialized organelle, the hemosome. We followed the fate of hemoglobin and albumin in primary cultures of digest cells by incubation with hemoglobin and albumin labeled with rhodamine. Uptake of hemoglobin by digest cells was inhibited by unlabeled globin, suggesting the presence of receptor-mediated endocytosis. After endocytosis, hemoglobin was observed inside large digestive vesicles. Albumin was exclusively associated with a population of small acidic vesicles, and an excess of unlabeled albumin did not inhibit its uptake. The intracellular pathway of the heme moiety of hemoglobin was specifically monitored using Palladium-mesoporphyrin IX (Pd-mP) as a fluorescent heme analog. When pulse and chase experiments were performed using digest cells incubated with Pd-mP bound to globin (Pd-mP-globin), strong yellow fluorescence was found in large digestive vesicles 4 h after the pulse. By 8 h, the emission of Pd-mP was red-shifted and more evident in the cytoplasm, and at 12 h most of the fluorescence was concentrated inside the hemosomes and had turned green. After 48 h, the Pd-mP signal was exclusively found in hemosomes. In methanol, Pd-mP showed maximal emission at 550 nm, exhibiting a red-shift to 665 nm when bound to proteins in vitro. The red emission in the cytosol and at the boundary of hemosomes suggests the presence of heme-binding proteins, probably involved in transport of heme to the hemosome. The existence of an intracellular heme shuttle from the digestive vesicle to the hemosome acting as a detoxification mechanism should be regarded as a major adaptation of ticks to a blood-feeding way of life. To our knowledge, this is the first direct observation of intracellular transport of heme in a living eukaryotic cell. A similar approach, using Pd-mP fluorescence, could be applied to study heme intracellular metabolism in other cell types.

Redox proteomics identification of oxidized proteins in Alzheimer's disease hippocampus and cerebellum: an approach to understand pathological and biochemical alterations in AD

Alzheimer's disease (AD) is characterized by the presence of neurofibrillary tangles, senile plaques and loss of synapses. There is accumulating evidence that oxidative stress plays an important role in AD pathophysiology. Previous redox proteomics studies from our laboratory on AD inferior parietal lobule led to the identification of oxidatively modified proteins that were consistent with biochemical or pathological alterations in AD. The present study was focused on the identification of specific targets of protein oxidation in AD and control hippocampus and cerebellum using a redox proteomics approach. In AD hippocampus, peptidyl prolyl cis-trans isomerase, phosphoglycerate mutase 1, ubiquitin carboxyl terminal hydrolase 1, dihydropyrimidinase related protein-2 (DRP-2), carbonic anhydrase II, triose phosphate isomerase, alpha-enolase, and gamma-SNAP were identified as significantly oxidized protein with reduced enzyme activities relative to control hippocampus. In addition, no significant excessively oxidized protein spots were identified in cerebellum compared to control, consistent with the lack of pathology in this brain region in AD. The identification of oxidatively modified proteins in AD hippocampus was verified by immunochemical means. The identification of common oxidized proteins in different brain regions of AD brain suggests a potential role for these oxidized proteins and thereby oxidative stress in the pathogenesis of Alzheimer's disease.

Free heme toxicity and its detoxification systems in human

Severe hemolysis or myolysis occurring during pathological states, such as sickle cell disease, ischemia reperfusion, and malaria results in high levels of free heme, causing undesirable toxicity leading to organ, tissue, and cellular injury. Free heme catalyzes the oxidation, covalent cross-linking and aggregate formation of protein and its degradation to small peptides. It also catalyzes the formation of cytotoxic lipid peroxide via lipid peroxidation and damages DNA through oxidative stress. Heme being a lipophilic molecule intercalates in the membrane and impairs lipid bilayers and organelles, such as mitochondria and nuclei, and destabilizes the cytoskeleton. Heme is a potent hemolytic agent and alters the conformation of cytoskeletal protein in red cells. Free heme causes endothelial cell injury, leading to vascular inflammatory disorders and stimulates the expression of intracellular adhesion molecules. Heme acts as a pro-inflammatory molecule and heme-induced inflammation is involved in the pathology of diverse conditions; such as renal failure, arteriosclerosis, and complications after artificial blood transfusion, peritoneal endometriosis, and heart transplant failure. Heme offers severe toxic effects to kidney, liver, central nervous system and cardiac tissue. Although heme oxygenase is primarily responsible to detoxify free heme but other extra heme oxygenase systems also play a significant role to detoxify heme. A brief account of free heme toxicity and its detoxification systems along with mechanistic details are presented.

Heme catabolism and heme oxygenase in neurodegenerative disease

Heme oxygenase, the rate-limiting step in heme catabolism, appears to play an important role in a number of neurodegenerative disorders, such as Alzheimer disease. Interestingly, the spatial distribution of heme oxygenase-1 expression in diseased brain is essentially identical to that of the pathological expression of tau, suggesting a key role for both in disease progression. Like heme oxygenase, the expression, phosphorylation, and aggregation of tau are regulated through signal cascades, including the extracellular signal-regulated kinases, whose activities are modulated by oxidative stress. Therefore, the expression of tau and heme oxygenase-1 in a coordinated manner likely plays a pivotal role in the cytoprotection of neuronal cells. This places heme oxygenase at the center of disease pathogenesis and offers a novel therapeutic approach targeted at either the causes or consequences of enzyme induction.

Heme, iron, and the mitochondrial decay of ageing

Heme, the major functional form of iron, is synthesized in the mitochondria. Although disturbed heme metabolism causes mitochondrial decay, oxidative stress, and iron accumulation, all of which are hallmarks of ageing, heme has been little studied in nutritional deficiency, in ageing, or age-related disorders such as Alzheimer's disease (AD). Biosynthesis of heme requires Vitamin B(6), riboflavin, biotin, pantothenic acid, and lipoic acid and the minerals zinc, iron, and copper, micronutrients are essential for the production of succinyl-CoA, the precursor for porphyrins, by the TCA (Krebs) cycle. Only a small fraction of the porphyrins synthesized from succinyl-CoA are converted to heme, the rest are excreted out of the body together with the degradation products of heme (e.g. bilirubin). Therefore, the heme biosynthetic pathway causes a net loss of succinyl-CoA from the TCA cycle. The mitochondrial pool of succinyl-CoA may limit heme biosynthesis in deficiencies for micronutrients (e.g. iron or biotin deficiency). Ageing and AD are also associated with hypometabolism, increase in heme oxygenase-1, loss of complex IV, and iron accumulation. Heme is a common denominator for all these changes, suggesting that heme metabolism maybe altered in age-related disorders. Heme can also be a prooxidant: it converts less reactive oxidants to highly reactive free radicals. Free heme has high affinity for different cell structures (protein, membranes, and DNA), triggering site-directed oxidative damage. This review discusses heme metabolism as related to metabolic changes seen in ageing and age-related disorders and highlights the possible role in iron deficiency.

Homocysteine and Alzheimer's disease: a modifiable risk?

A hypothesis is proposed that reconciles the epidemiological observation of elevated homocysteine in Alzheimer's disease (AD) with clinical features of the disease, particularly evidence of increased oxidative stress. We propose homocysteine is involved in an iron dysregulation/oxidative stress cycle that has a central role in the pathogenesis of AD. The implications of the hypothesis and some strategies for testing it are discussed.

Different faces of the heme-heme oxygenase system in inflammation

The heme-heme oxygenase system has recently been recognized to possess important regulatory properties. It is tightly involved in both physiological as well as pathophysiological processes, such as cytoprotection, apoptosis, and inflammation. Heme functions as a double-edged sword. In moderate quantities and bound to protein, it forms an essential element for various biological processes, but when unleashed in large amounts, it can become toxic by mediating oxidative stress and inflammation. The effect of this free heme on the vascular system is determined by extracellular factors, such as hemoglobin/heme-binding proteins, haptoglobin, albumin, and hemopexin, and intracellular factors, including heme oxygenases and ferritin. Heme oxygenase (HO) enzyme activity results in the degradation of heme and the production of iron, carbon monoxide, and biliverdin. All these heme-degradation products are potentially toxic, but may also provide strong cytoprotection, depending on the generated amounts and the microenvironment. Pre-induction of HO activity has been demonstrated to ameliorate inflammation and mediate potent resistance to oxidative injury. A better understanding of the complex heme-heme

Heme and iron metabolism: role in cerebral hemorrhage

Heme and iron metabolism are of considerable interest and importance in normal brain function as well as in neurodegeneration and neuropathologically following traumatic injury and hemorrhagic stroke. After a cerebral hemorrhage, large numbers of hemoglobin-containing red blood cells are released into the brain's parenchyma and/or subarachnoid space. After hemolysis and the subsequent release of heme from hemoglobin, several pathways are employed to transport and metabolize this heme and its iron moiety to protect the brain from potential oxidative stress. Required for these processes are various extracellular and intracellular transporters and storage proteins, the heme oxygenase isozymes and metabolic proteins with differing localizations in the various brain-cell types. In the past several years, additional new genes and proteins have been discovered that are involved in the transport and metabolism of heme and iron in brain and other tissues. These discoveries may provide new insights into neurodegenerative diseases like Alzheimer's, Parkinson's, and Friedrich's ataxia that are associated with accumulation of iron in specific brain regions or in specific organelles. The present review will examine the uptake and metabolism of heme and iron in the brain and will relate these processes to blood removal and to the potential mechanisms underlying brain injury following cerebral hemorrhage.

Ferritin induction protects cortical astrocytes from heme-mediated oxidative injury

Hemin is released from hemoglobin after CNS hemorrhage and may contribute to its cytotoxic effect. In a prior study, we demonstrated that heme oxygenase-1 induction protected murine cortical astrocytes from hemoglobin toxicity. Since heme metabolism releases iron, this observation suggested that these cells are able to effectively sequester and detoxify free iron. In this study, we tested the hypotheses that astrocytes increased ferritin synthesis after exposure to heme-bound iron, and that this induction protected cells from subsequent exposure to toxic concentrations of hemin. Incubation with low micromolar concentrations of hemin, hemoglobin, or ferrous sulfate increased ferritin expression, as detected on immunoblots stained with a polyclonal antibody that was raised against horse spleen ferritin. Time course studies demonstrated an increase in ferritin levels within 2 h. Weak and scattered cellular staining was detected by immunohistochemistry in control, untreated cultures, while diffuse immunoreactivity was observed in cultures exposed to heme-bound iron. An enhanced ferritin band was detected on immunoblots from cultures that were treated with purified apoferritin, consistent with astrocytic ferritin uptake. Immunoreactivity after apoferritin treatment was not altered by concomitant treatment with cycloheximide. Pretreatment with apoferritin protected astrocytes from hemin toxicity in a concentration-dependent fashion between 1 and 4 mg/ml. At the highest concentration, cell death due to a 6-h exposure to 30 microM hemin was decreased by about 85%. A protective effect was also produced by induction of endogenous ferritin with nontoxic concentrations of ferrous sulfate, hemoglobin, or hemin. These results suggest that cortical astrocytes respond to exogenous heme-bound or free iron by rapidly increasing ferritin synthesis. The combined action of heme oxygenase-1 and ferritin may be a primary astrocytic defense against heme-mediated injury.

Heme deficiency may be a factor in the mitochondrial and neuronal decay of aging

Heme, a major functional form of iron in the cell, is synthesized in the mitochondria by ferrochelatase inserting ferrous iron into protoporphyrin IX. Heme deficiency was induced with N-methylprotoporphyrin IX, a selective inhibitor of ferrochelatase, in two human brain cell lines, SHSY5Y (neuroblastoma) and U373 (astrocytoma), as well as in rat primary hippocampal neurons. Heme deficiency in brain cells decreases mitochondrial complex IV, activates nitric oxide synthase, alters amyloid precursor protein, and corrupts iron and zinc homeostasis. The metabolic consequences resulting from heme deficiency seem similar to dysfunctional neurons in patients with Alzheimer's disease. Heme-deficient SHSY5Y or U373 cells die when induced to differentiate or to proliferate, respectively. The role of heme in these observations could result from its interaction with heme regulatory motifs in specific proteins or secondary to the compromised mitochondria. Common causes of heme deficiency include aging, deficiency of iron and vitamin B6, and exposure to toxic metals such as aluminum. Iron and B6 deficiencies are especially important because they are widespread, but they are also preventable with supplementation. Thus, heme deficiency or dysregulation may be an important and preventable component of the neurodegenerative process.

The role of iron and copper in the aetiology of neurodegenerative disorders: therapeutic implications

Abnormalities in the metabolism of the transition metals iron and copper have been demonstrated to play a crucial role in the pathogenesis of various neurodegenerative diseases. Metal homeostasis as it pertains to alterations in brain function in neurodegenerative diseases is reviewed in this article in depth. While there is documented evidence for alterations in the homeostasis, redox-activity and localisation of transition metals, it is also important to realise that alterations in specific copper- and iron-containing metalloenzymes appear to play a crucial role in the neurodegenerative process. These changes provide the opportunity to identify pathways where modification of the disease process can occur, potentially offering opportunities for clinical intervention. As understanding of disease aetiology evolves, so do the tools with which diseases are treated. In this article, we examine not only the possible mechanism of disease but also how pharmaceuticals may intervene, from direct and indirect antioxidant therapy to strategies involving gene therapy.

In situ oxidative catalysis by neurofibrillary tangles and senile plaques in Alzheimer's disease: a central role for bound transition metals

There is a great deal of evidence to support a pathogenic role of oxidative stress in Alzheimer's disease (AD), but the sources of reactive oxygen species have not been directly demonstrated. In this study, using a novel in situ detection system, we show that neurofibrillary tangles and senile plaques are major sites for catalytic redox reactivity. Pretreatment with deferoxamine or diethylenetriaminepentaacetic acid abolishes the ability of the lesions to catalyze the H2O2-dependent oxidation of 3,3'-diaminobenzidine (DAB), strongly suggesting the involvement of associated transition metal ions. Indeed, following chelated removal of metals, incubation with iron or copper salts reestablished lesion-dependent catalytic redox reactivity. Although DAB oxidation can also detect peroxidase activity, this was inactivated by H2O2 pretreatment before use of DAB, as shown by a specific peroxidase detection method. Model studies confirmed the ability of certain copper and iron coordination complexes to catalyze the H2O2-dependent oxidation of DAB. Also, the microtubule-associated protein tau, as an in vitro model for proteins relevant to AD pathology, was found capable of adventitious binding of copper and iron in a redox-competent manner. Our findings suggest that neurofibrillary tangles and senile plaques contain redox-active transition metals and may thereby exert prooxidant or possibly antioxidant activities, depending on the balance among cellular reductants and oxidants in the local microenvironment.