Alzheimer's Disease, Oestrogen and Mitochondria: an Ambiguous Relationship

Hormonal deficit in post-menopausal women has been proposed to be one risk factor in Alzheimer's disease (AD) since two thirds of AD patients are women. However, large treatment trials showed negative effects of long-term treatment with oestrogens in older women. Thus, oestrogen treatment after menopause is still under debate, and several hypotheses trying to explain the failure in outcome are under discussion. Concurrently, it was shown that amyloid-beta (Aβ) peptide, the main constituent of senile plaques, as well as abnormally hyperphosphorylated tau protein, the main component of neurofibrillary tangles, can modulate the level of neurosteroids which notably represent neuroactive steroids synthetized within the nervous system, independently of peripheral endocrine glands. In this review, we summarize the role of neurosteroids especially that of oestrogen in AD and discuss their potentially neuroprotective effects with specific regard to the role of oestrogens on the maintenance and function of mitochondria, important organelles which are highly vulnerable to Aβ- and tau-induced toxicity. We also discuss the role of Aβ-binding alcohol dehydrogenase (ABAD), a mitochondrial enzyme able to bind Aβ peptide thereby modifying mitochondrial function as well as oestradiol levels suggesting possible modes of interaction between the three, and the potential therapeutic implication of inhibiting Aβ-ABAD interactio

Genetically Engineered Triple MAPT-Mutant Human-Induced Pluripotent Stem Cells (N279K, P301L, and E10+16 Mutations) Exhibit Impairments in Mitochondrial Bioenergetics and Dynamics.

Pathological abnormalities in the tau protein give rise to a variety of neurodegenerative diseases, conjointly termed tauopathies. Several tau mutations have been identified in the tau-encoding gene MAPT, affecting either the physical properties of tau or resulting in altered tau splicing. At early disease stages, mitochondrial dysfunction was highlighted with mutant tau compromising almost every aspect of mitochondrial function. Additionally, mitochondria have emerged as fundamental regulators of stem cell function. Here, we show that compared to the isogenic wild-type triple MAPT-mutant human-induced pluripotent stem cells, bearing the pathogenic N279K, P301L, and E10+16 mutations, exhibit deficits in mitochondrial bioenergetics and present altered parameters linked to the metabolic regulation of mitochondria. Moreover, we demonstrate that the triple tau mutations disturb the cellular redox homeostasis and modify the mitochondrial network morphology and distribution. This study provides the first characterization of disease-associated tau-mediated mitochondrial impairments in an advanced human cellular tau pathology model at early disease stages, ranging from mitochondrial bioenergetics to dynamics. Consequently, comprehending better the influence of dysfunctional mitochondria on the development and differentiation of stem cells and their contribution to disease progression may thus assist in the potential prevention and treatment of tau-related neurodegeneration.Partial funding for open access charge: Universidad de Málag

ER-mitochondria contacts and cholesterol metabolism are disrupted by disease-associated tau protein

Abnormal tau protein impairs mitochondrial function, including transport, dynamics, and bioenergetics. Mitochondria interact with the endoplasmic reticulum (ER) via mitochondria-associated ER membranes (MAMs), which coordinate and modulate many cellular functions, including mitochondrial cholesterol metabolism. Here, we show that abnormal tau loosens the association between the ER and mitochondria in vivo and in vitro. Especially, ER-mitochondria interactions via vesicle-associated membrane protein-associated protein (VAPB)-protein tyrosine phosphatase-interacting protein 51 (PTPIP51) are decreased in the presence of abnormal tau. Disruption of MAMs in cells with abnormal tau alters the levels of mitochondrial cholesterol and pregnenolone, indicating that conversion of cholesterol into pregnenolone is impaired. Opposite effects are observed in the absence of tau. Besides, targeted metabolomics reveals overall alterations in cholesterol-related metabolites by tau. The inhibition of GSK3β decreases abnormal tau hyperphosphorylation and increases VAPB-PTPIP51 interactions, restoring mitochondrial cholesterol and pregnenolone levels. This study is the first to highlight a link between tau-induced impairments in the ER-mitochondria interaction and cholesterol metabolism

Insights into mitochondrial dysfunction: aging, amyloid-β, and tau-A deleterious trio

Significance: Alzheimer's disease (AD) is an age-related progressive neurodegenerative disorder mainly affecting elderly individuals. The pathology of AD is characterized by amyloid plaques (aggregates of amyloid-β [Aβ]) and neurofibrillary tangles (aggregates of tau), but the mechanisms underlying this dysfunction are still partially unclear. Recent Advances: A growing body of evidence supports mitochondrial dysfunction as a prominent and early, chronic oxidative stress-associated event that contributes to synaptic abnormalities and, ultimately, selective neuronal degeneration in AD. Critical Issues: In this review, we discuss on the one hand whether mitochondrial decline observed in brain aging is a determinant event in the onset of AD and on the other hand the close interrelationship of this organelle with Aβ and tau in the pathogenic process underlying AD. Moreover, we summarize evidence from aging and Alzheimer models showing that the harmful trio "aging, Aβ, and tau protein" triggers mitochondrial dysfunction through a number of pathways, such as impairment of oxidative phosphorylation (OXPHOS), elevation of reactive oxygen species production, and interaction with mitochondrial proteins, contributing to the development and progression of the disease. Future Directions: The aging process may weaken the mitochondrial OXPHOS system in a more general way over many years providing a basis for the specific and destructive effects of Aβ and tau. Establishing strategies involving efforts to protect cells at the mitochondrial level by stabilizing or restoring mitochondrial function and energy homeostasis appears to be challenging, but very promising route on the horizon

Transmission-selective muscle pathology induced by the active propagation of mutant huntingtin across the human neuromuscular synapse

Neuron-to-neuron transmission of aggregation-prone, misfolded proteins may potentially explain the spatiotemporal accumulation of pathological lesions in the brains of patients with neurodegenerative protein-misfolding diseases (PMDs). However, little is known about protein transmission from the central nervous system to the periphery, or how this propagation contributes to PMD pathology. To deepen our understanding of these processes, we established two functional neuromuscular systems derived from human iPSCs. One was suitable for long-term high-throughput live-cell imaging and the other was adapted to a microfluidic system assuring that connectivity between motor neurons and muscle cells was restricted to the neuromuscular junction. We show that the Huntington's disease (HD)-associated mutant HTT exon 1 protein (mHTTEx1) is transmitted from neurons to muscle cells across the human neuromuscular junction. We found that transmission is an active and dynamic process that starts before aggregate formation and is regulated by synaptic activity. We further found that transmitted mHTTEx1 causes HD-relevant pathology at both molecular and functional levels in human muscle cells, even in the presence of the ubiquitous expression of mHTTEx1. In conclusion, we have uncovered a causal link between mHTTEx1 synaptic transmission and HD pathology, highlighting the therapeutic potential of blocking toxic protein transmission in PMDs

Circadian control of DRP1 activity regulates mitochondrial dynamics and bioenergetics

Mitochondrial fission-fusion dynamics and mitochondrial bioenergetics, including oxidative phosphorylation and generation of ATP, are strongly clock controlled. Here we show that these circadian oscillations depend on circadian modification of dynamin-related protein 1 (DRP1), a key mediator of mitochondrial fission. We used a combination of in vitro and in vivo models, including human skin fibroblasts and DRP1-deficient or clock-deficient mice, to show that these dynamics are clock controlled via circadian regulation of DRP1. Genetic or pharmacological abrogation of DRP1 activity abolished circadian network dynamics and mitochondrial respiratory activity and eliminated circadian ATP production. Pharmacological silencing of pathways regulating circadian metabolism and mitochondrial function (e.g., sirtuins, AMPK) also altered DRP1 phosphorylation, and abrogation of DRP1 activity impaired circadian function. Our findings provide new insight into the crosstalk between the mitochondrial network and circadian cycles

Error-prone protein synthesis recapitulates early symptoms of Alzheimer disease in aging mice

Age-related neurodegenerative diseases (NDDs) are associated with the aggregation and propagation of specific pathogenic protein species (e.g., Aβ, α-synuclein). However, whether disruption of synaptic homeostasis results from protein misfolding per se rather than accumulation of a specific rogue protein is an unexplored question. Here, we show that error-prone translation, with its frequent outcome of random protein misfolding, is sufficient to recapitulate many early features of NDDs, including perturbed Ca2+ signaling, neuronal hyperexcitability, and mitochondrial dysfunction. Mice expressing the ribosomal ambiguity mutation Rps9 D95N exhibited disrupted synaptic homeostasis resulting in behavioral changes reminiscent of early Alzheimer disease (AD), such as learning and memory deficits, maladaptive emotional responses, epileptiform discharges, suppressed circadian rhythmicity, and sleep fragmentation, accompanied by hippocampal NPY expression and cerebral glucose hypometabolism. Collectively, our findings suggest that random protein misfolding may contribute to the pathogenesis of age-related NDDs, providing an alternative framework for understanding the initiation of AD. Keywords: Alzheimer; CP: Neuroscience; error-prone translation; neurodegenerative diseases; pathogenesis; protein misfolding; synaptic homeostasi

Inhibition of the Mitochondrial Enzyme ABAD Restores the Amyloid-β-Mediated Deregulation of Estradiol

Alzheimer's disease (AD) is a conformational disease that is characterized by amyloid-β (Aβ) deposition in the brain. Aβ exerts its toxicity in part by receptor-mediated interactions that cause down-stream protein misfolding and aggregation, as well as mitochondrial dysfunction. Recent reports indicate that Aβ may also interact directly with intracellular proteins such as the mitochondrial enzyme ABAD (Aβ binding alcohol dehydrogenase) in executing its toxic effects. Mitochondrial dysfunction occurs early in AD, and Aβ's toxicity is in part mediated by inhibition of ABAD as shown previously with an ABAD decoy peptide. Here, we employed AG18051, a novel small ABAD-specific compound inhibitor, to investigate the role of ABAD in Aβ toxicity. Using SH-SY5Y neuroblastoma cells, we found that AG18051 partially blocked the Aβ-ABAD interaction in a pull-down assay while it also prevented the Aβ42-induced down-regulation of ABAD activity, as measured by levels of estradiol, a known hormone and product of ABAD activity. Furthermore, AG18051 is protective against Aβ42 toxicity, as measured by LDH release and MTT absorbance. Specifically, AG18051 reduced Aβ42-induced impairment of mitochondrial respiration and oxidative stress as shown by reduced ROS (reactive oxygen species) levels. Guided by our previous finding of shared aspects of the toxicity of Aβ and human amylin (HA), with the latter forming aggregates in Type 2 diabetes mellitus (T2DM) pancreas, we determined whether AG18051 would also confer protection from HA toxicity. We found that the inhibitor conferred only partial protection from HA toxicity indicating distinct pathomechanisms of the two amyloidogenic agents. Taken together, our results present the inhibition of ABAD by compounds such as AG18051 as a promising therapeutic strategy for the prevention and treatment of AD, and suggest levels of estradiol as a suitable read-out

Mitochondria, neurosteroids and biological rhythms : implications in health and disease states

Mitochondria are considered as the “powerhouses” of cells because they synthesize the universal source of energy under the form of adenosine triphosphate (ATP) molecules via oxidative phosphorylation from nutritional sources. Thus, impaired mitochondrial function, especially in neurons that have high energy requirements, lead inevitably to disease, ranging from subtle alterations in neuronal function to cell death and neurodegenerative diseases, such as Alzheimer’s disease (AD). The purpose of this PhD thesis was therefore to deepen our understanding of the regulation of mitochondrial function and to identify key factors that are critical in the control of mitochondrial bioenergetics and dynamics. To achieve this goal, the thesis was divided into two main parts: 1) Since a growing body of evidence suggests that neurosteroids have a strong neuroprotective potential, the first part is based on the hypothesis that neurosteroids may exert a determinant action against neurodegeneration by improving mitochondrial bioenergetics, (A) under “healthy” conditions as well as (B) under pathological conditions (AD); 2) In the second part (C), we determined whether the biological clock, which coordinates a whole range of daily behaviors and physiological processes, is involved in the endogenous regulation of mitochondrial dynamics and bioenergetics. (A) In the first part of this thesis, we aimed to characterize the bioenergetic modulating profile of a panel of seven structurally diverse neurosteroids (progesterone, estradiol, estrone, testosterone, 3alpha-androstanediol, DHEA and allopregnanolone), known to be involved in brain function regulation. Our key findings were that: i) the majority of these steroids increased energy metabolism, mainly via an up-regulation of the mitochondrial activity and at least in part via receptor activation, and ii) neurosteroids regulated redox homeostasis by increasing the antioxidant activity as a compensatory mechanism to the reactive oxygen species (ROS) level enhancement which might result from the acceleration in oxygen consumption accompanied by a greater electron leakage from the electron transport chain. Additionally, each neurosteroid seems to have a specific bioenergetic profile. Together, these first data indicated that neurosteroids were indeed able to boost mitochondrial function in a delicate balance, possibly by regulating the expression of genes involved in glycolysis and oxidative phosphorylation, but also the content and activity of mitochondrial respiratory complexes. Further investigations are required to determine the underlying molecular mechanisms. (B) Based on these findings, we investigated in the next step whether neurosteroids were able to alleviate AD-related bioenergetic deficits. We distinguished the effects of several neurosteroids on ATP synthesis, mitochondrial membrane potential (MMP), mitochondrial respiration and glycolysis in two AD cellular models overexpressing either the amyloid precursor protein and amyloid-beta peptide (APP/Abeta) or the mutant form of tau producing abnormally hyperphosphorylated tau proteins, respectively. Key findings were that: i) APP/Abeta and mutant tau-overexpressing cells present distinct bioenergetic impairments, with APP/Abeta having the strongest deleterious effect on mitochondrial function; ii) the male steroid hormone, testosterone, was more efficient to alleviate mitochondrial deficits induced by APP/Abeta, whereas female steroid hormones, progesterone and estrogen, were more efficient to increase bioenergetic outcomes in our model of AD-related tauopathies. Together, our findings lend further evidence to the neuroprotective effects of neurosteroids in AD pathology and indicate that these molecules represent promising tools able to increase mitochondrial bioenergetics via enhanced mitochondrial respiration, in healthy and pathological conditions, respectively. Our results may open new avenues for drug development with regard to targeting mitochondria in neurodegeneration. (C) The aim of the second part of this thesis was to investigate more closely how mitochondrial function is endogenously regulated within the cells. Since a growing body of evidences shows that the circadian clock and metabolic homeostasis are connected in numerous ways via reciprocal regulation, we asked whether mitochondrial bioenergetics and dynamics may exhibit circadian oscillations and whether mitochondria themselves may be able to influence the circadian clock. We found that mitochondrial bioenergetics, including mitochondrial respiration and, consequently, generation of the byproducts ATP and ROS, is directly coupled to mitochondrial network which is, at least in part, regulated by clock-controlled phosphorylation of Drp1, the main factor involved in mitochondrial fission. The time-dependent reorganization of mitochondrial architecture in turn regulates the clock through circadian oscillation of mitochondrial ATP which can act as input signal through activation of AMP-activated protein kinase (AMPK). Our findings highlight new insights in the understanding of the reciprocal temporal crosstalk that governs the molecular interplay between the coupling of mitochondrial dynamics and metabolism and circadian rhythms. Overall, the studies performed in the present thesis importantly helped to deepen our knowledge about the modulation of mitochondrial function in health and disease states. Our findings could have multiple implications with regard to the regulation of metabolic homeostasis in health and disease states associated with mitochondrial impairments and / or circadian disruption