Egemen Savaskan

Amyloid-β and tau synergistically impair the oxidative phosphorylation system in triple transgenic Alzheimer's disease mice

Alzheimers disease (AD) is characterized by amyloid-beta (Aβ)-containing plaques, neurofibrillary tangles, and neuron and synapse loss. Tangle formation has been reproduced in P301L tau transgenic pR5 mice, whereas APPswPS2N141I double-transgenic APP152 mice develop Aβ plaques. Cross-breeding generates triple transgenic (tripleAD) mice that combine both pathologies in one model. To determine functional consequences of the combined Aβ and tau pathologies, we performed a proteomic analysis followed by functional validation. Specifically, we obtained vesicular preparations from tripleAD mice, the parental strains, and nontransgenic mice, followed by the quantitative mass-tag labeling proteomic technique iTRAQ and mass spectrometry. Within 1,275 quantified proteins, we found a massive deregulation of 24 proteins, of which one-third were mitochondrial proteins mainly related to complexes I and IV of the oxidative phosphorylation system (OXPHOS). Notably, deregulation of complex I was tau dependent, whereas deregulation of complex IV was Aβ dependent, both at the protein and activity levels. Synergistic effects of Aβ and tau were evident in 8-month-old tripleAD mice as only they showed a reduction of the mitochondrial membrane potential at this early age. At the age of 12 months, the strongest defects on OXPHOS, synthesis of ATP, and reactive oxygen species were exhibited in the tripleAD mice, again emphasizing synergistic, age-associated effects of Aβ and tau in perishing mitochondria. Our study establishes a molecular link between Aβ and tau protein in AD pathology in vivo, illustrating the potential of quantitative proteomics.

Post-learning intranasal oxytocin modulates human memory for facial identity

The nanopeptide oxytocin has physiological functions during labour and lactation. In addition, oxytocin is known to modulate aggression, anxiety, social behaviour and cognition. Little is known about its effects on memory for emotional stimuli. In the present single-blind, placebo-controlled, randomised study we have investigated the short- and long-term effects of a single post-learning dose (20 IU) of intranasal oxytocin on memory for facial identity and expression in 36 healthy young females and males using a face portrait recognition test. In the acquisition phase of the test, 60 different male faces with happy, angry or neutral expressions were presented to the volunteers. Thirty minutes and 24h after oxytocin administration, recognition memory tests were performed using portraits with neutral facial expressions, only. Oxytocin improved identity recognition memory independently of participants gender, for neutral and angry faces, whereas this effect was not present for happy faces. Oxytocin-treated subjects had a lower bias to judge not previously seen faces as being previously seen. Oxytocin had no effect on facial expression memory. In conclusion, oxytocin has distinct effects on memory performance for facial identity and may contribute to the modulation of social behaviour.

Reduced hippocampal MT2 melatonin receptor expression in Alzheimer's disease

Abstract:  The aim of the present study was to identify the distribution of the second melatonin receptor (MT2) in the human hippocampus of elderly controls and Alzheimers disease (AD) patients. This is the first report of immunohistochemical MT2 localization in the human hippocampus both in control and AD cases. The specificity of the MT2 antibody was ascertained by fluorescence microscopy using the anti‐MT2 antibody in HEK 293 cells expressing recombinant MT2, in immunoblot experiments on membranes from MT2 expressing cells, and, finally, by immunoprecipitation experiments of the native MT2. MT2 immunoreactivity was studied in the hippocampus of 16 elderly control and 16 AD cases. In controls, MT2 was localized in pyramidal neurons of the hippocampal subfields CA1‐4 and in some granular neurons of the stratum granulosum. The overall intensity of the MT2 staining was distinctly decreased in AD cases. The results indicate that MT2 may be involved in mediating the effects of melatonin in the human hippocampus, and this mechanism may be heavily impaired in AD.

Increased melatonin 1a-receptor immunoreactivity in the hippocampus of Alzheimer's disease patients.

The pineal secretory product melatonin has, in addition to regulating retinal, circadian and vascular functions, neuroprotective effects. Blood melatonin levels are often decreased in Alzheimers disease (AD), a progressively disabling neurodegenerative disorder. In this study we provide the first immunohistochemical evidence for the localization of melatonin 1a‐receptor (MT1) in aged human hippocampus and a comparison of AD cases. MT1 was localized to pyramidal neurons in the hippocampal cornu ammonis (CA)1‐4 subfields. There was a distinct increase in staining intensity in all AD cases indicating an up‐regulation of the receptor, possibly as a compensatory response to impaired melatonin levels in order to augment melatonins neuroprotective effects.

Distribution of Melatonin MT1 Receptor Immunoreactivity in Human Retina

Melatonin is synthesized in the pineal gland and retina during the night. Retinal melatonin is believed to be involved in local cellular modulation and in regulation of light-induced entrainment of circadian rhythms. The present study provides the first immunohistochemical evidence for the localization of melatonin 1a-receptor (MT1) in human retina of aged subjects. Ganglion, amacrine, and photoreceptor cells expressed MT1. In addition, MT1 immunoreactivity was localized to cell processes in the inner plexiform layer and to central vessels of the retina, as well as to retinal vessels but not to ciliary or choroidal vessels. These results support a variety of cellular and vascular effects of melatonin in the human retina. Preliminary evidence from patients with Alzheimers disease (AD) revealed increased MT1 immunoreactivity in ganglion and amacrine cells, as well as in vessels. In AD cases photoreceptor cells were degenerated and showed low MT1 expression.

Pineal and cortical melatonin receptors MT1 and MT2 are decreased in Alzheimer's disease

The pineal hormone melatonin is involved in physiological transduction of temporal information from the light dark cycle to circadian and seasonal behavioural rhythms, as well as possessing neuroprotective properties. Melatonin and its receptors MT1 and MT2, which belong to the family of G protein-coupled receptors, are impaired in Alzheimers disease (AD) with severe consequences to neuropathology and clinical symptoms. The present data provides the first immunohistochemical evidence for the cellular localization of the both melatonin receptors in the human pineal gland and occipital cortex, and demonstrates their alterations in AD. We localized MT1 and MT2 in the pineal gland and occipital cortex of 7 elderly controls and 11 AD patients using immunohistochemistry with peroxidase-staining. In the pineal gland both MT1 and MT2 were localized to pinealocytes, whereas in the cortex both receptors were expressed in some pyramidal and non-pyramidal cells. In patients with AD, parallel to degenerative tissue changes, there was an overall decrease in the intensity of receptors in both brain regions. In line with our previous findings, melatonin receptor expression in AD is impaired in two additional brain areas, and may contribute to disease pathology.

Melatonin protects SHSY5Y neuroblastoma cells from cobalt‐induced oxidative stress, neurotoxicity and increased β‐amyloid secretion

Heavy metals are increasingly being implicated as causative agents in neurodegenerative diseases such as Alzheimers disease (AD). Cobalt, a positively charged transition metal, has previously been shown to be in elevated levels in the brain of AD patients compared with age‐matched controls. In this study, we investigate the effects of cobalt as an inducer of oxidative stress/cell cytotoxicity and the resultant metabolic implications for neural cells. We show that cobalt is able to induce cell cytotoxicity (reduced MTT metabolism) and oxidative stress (reduced cellular glutathione). The pre‐treatment of cells with the pineal indoleamine melatonin, prevented cell cytotoxicity and the induction of oxidative stress. Cobalt treatment of SHSY5Y cells increased the release of β‐amyloid (Aβ) compared with untreated controls (ratio Aβ 40/42). Melatonin pre‐treatment reversed the deleterious effects of cobalt. These findings are significant as cobalt is an essential nutritional requirement, usually bound to cobalamin (vitamin B12), for all animals which in the unbound form could lead to neurotoxicity.

Peripheral Mitochondrial Dysfunction in Alzheimer’s Disease: Focus on Lymphocytes

Alzheimer’s disease (AD) is the most common progressive neurodegenerative disease. Today, AD affects millions of people worldwide and the number of AD cases will increase with increased life expectancy. The AD brain is marked by severe neurodegeneration like the loss of synapses and neurons, atrophy and depletion of neurotransmitter systems in the hippocampus and cerebral cortex. Recent findings suggest that these pathological changes are causally induced by mitochondrial dysfunction and increased oxidative stress. These changes are not only observed in the brain of AD patients but also in the periphery. In this review, we discuss the potential role of elevated apoptosis, increased oxidative stress and especially mitochondrial dysfunction as peripheral markers for the detection of AD in blood cells especially in lymphocytes. We discuss recent not otherwise published findings on the level of complex activities of the respiratory chain comprising mitochondrial respiration and the mitochondrial membrane potential (MMP). We obtained decreased basal MMP levels in lymphocytes from AD patients as well as enhanced sensitivity to different complex inhibitors of the respiratory chain. These changes are in line with mitochondrial defects obtained in AD cell and animal models, and in post-mortem AD tissue. Importantly, these mitochondrial alterations where not only found in AD patients but also in patients with mild cognitive impairment (MCI). These new findings point to a relevance of mitochondrial function as an early peripheral marker for the detection of AD and MCI.

The MT2 melatonin receptor subtype is present in human retina and decreases in Alzheimer's disease

The pineal and retinal melatonin regulates endogenous circadian rhythms, and has various physiological functions including neuromodulatory and vasoactive actions, antioxidative and neuroprotective properties. We have previously demonstrated that the melatonin 1a-receptor (MT(1)) is localized in human retinal cells and that the expression of MT(1) is increased in Alzheimers disease (AD) patients. We now present the first immunohistochemical evidence for the cellular distribution of the second melatonin receptor, MT(2), in the human retina and in AD patients. In elderly controls, MT(2) was localized to ganglion and bipolar cells in the inner nuclear layer, and to the inner segments of the photoreceptor cells. In addition, cellular processes in inner and outer plexiform layers were strongly positive for MT(2). In AD patients the overall intensity of MT(2)-staining was distinctly decreased in all observed cellular localizations. Our results indicate that MT(2) in the humans, similar to MT(1), may indeed be involved in transmitting melatonins effects in the retina, and AD pathology may impair MT(2) expression. Since our previous results showed an increase in MT(1) expression in AD retina, the two melatonin receptor subtypes appear to be differentially affected by the course of the neurodegenerative disorder.

Are G Protein‐Coupled Receptor Heterodimers of Physiological Relevance?—Focus on Melatonin Receptors

In mammals, the circadian hormone melatonin targets two seven‐transmembrane–spanning receptors, MT1 and MT2, of the G protein‐coupled receptor (GPCR) super‐family. Evidence accumulated over the last 15 yrs convincingly demonstrates that GPCRs, classically considered to function as monomers, are actually organized as homodimers and heterodimerize with other GPCR family members. These dimers are formed early in the biosynthetic pathway and remain stable throughout the entire life cycle. A growing number of observations demonstrate that GPCR oligomerization may occur in native tissues and may have important consequences on receptor function. The formation of MT1 and MT2 homodimers and MT1/MT2 heterodimers has been shown in heterologous expression systems at physiological expression levels. Formation of MT1/MT2 heterodimers remains to be shown in native tissues but is suggested by the documented co‐expression of MT1 and MT2 in many melatonin‐sensitive tissues, such as the hypothalamic suprachiasmatic nuclei, retina, arteries, and adipose tissue. Considering that multiple GPCRs are expressed simultaneously in most cells, the possible engagement into heterodimeric complexes has to be considered and taken into account for the interpretation of experimental data obtained from native tissues and knockout animals.