Caspase-cleaved tau expression induces mitochondrial dysfunction in immortalized cortical neurons: implications for the pathogenesis of Alzheimer disease

Influence of Tau on Neurotoxicity and Cerebral Vasculature Impairment Associated with Alzheimer's Disease

Alzheimer's disease is a fatal chronic neurodegenerative condition marked by a gradual decline in cognitive abilities and impaired vascular function within the central nervous system. This affliction initiates its insidious progression with the accumulation of two aberrant protein entities including Aβ plaques and neurofibrillary tangles. These chronic elements target distinct brain regions, steadily erasing the functionality of the hippocampus and triggering the erosion of memory and neuronal integrity. Several assumptions are anticipated for AD as genetic alterations, the occurrence of Aβ plaques, altered processing of amyloid precursor protein, mitochondrial damage, and discrepancy of neurotropic factors. In addition to Aβ oligomers, the deposition of tau hyper-phosphorylates also plays an indispensable part in AD etiology. The brain comprises a complex network of capillaries that is crucial for maintaining proper function. Tau is expressed in cerebral blood vessels, where it helps to regulate blood flow and sustain the blood-brain barrier's integrity. In AD, tau pathology can disrupt cerebral blood supply and deteriorate the BBB, leading to neuronal neurodegeneration. Neuroinflammation, deficits in the microvasculature and endothelial functions, and Aβ deposition are characteristically detected in the initial phases of AD. These variations trigger neuronal malfunction and cognitive impairment. Intracellular tau accumulation in microglia and astrocytes triggers deleterious effects on the integrity of endothelium and cerebral blood supply resulting in further advancement of the ailment and cerebral instability. In this review, we will discuss the impact of tau on neurovascular impairment, mitochondrial dysfunction, oxidative stress, and the role of hyperphosphorylated tau in neuron excitotoxicity and inflammation.

Tau phosphorylation suppresses oxidative stress-induced mitophagy via FKBP8 receptor modulation

Neurodegenerative diseases are often characterized by mitochondrial dysfunction. In Alzheimer's disease, abnormal tau phosphorylation disrupts mitophagy, a quality control process through which damaged organelles are selectively removed from the mitochondrial network. The precise mechanism through which this occurs remains unclear. Previously, we showed that tau which has been mutated at Thr-231 to glutamic acid to mimic an Alzheimer's-relevant phospho-epitope expressed early in disease selectively inhibits oxidative stress-induced mitophagy in C. elegans. Here, we use immortalized mouse hippocampal neuronal cell lines to extend that result into mammalian cells. Specifically, we show that phosphomimetic tau at Ser-396/404 (EC) or Thr-231/Ser-235 (EM) partly inhibits mitophagy induction by paraquat, a potent inducer of mitochondrial oxidative stress. Moreover, a combination of immunologic and biochemical approaches demonstrates that the levels of the mitophagy receptor FKBP8, significantly decrease in response to paraquat in cells expressing EC or EM tau mutants, but not in cells expressing wildtype tau. In contrast, paraquat treatment results in a decrease in the levels of the mitophagy receptors FUNDC1 and BNIP3 in the presence of both wildtype tau and the tau mutants. Interestingly, FKBP8 is normally trafficked to the endoplasmic reticulum during oxidative stress induced mitophagy, and our results support a model where this trafficking is impacted by disease-relevant tau, perhaps through a direct interaction. We provide new insights into the molecular mechanisms underlying tau pathology in Alzheimer's disease and highlight FKBP8 receptor as a potential target for mitigating mitochondrial dysfunction in neurodegenerative diseases.

Complicated Role of Post-translational Modification and Protease-Cleaved Fragments of Tau in Alzheimer's Disease and Other Tauopathies

Tau, a microtubule-associated protein predominantly localized in neuronal axons, plays a crucial role in promoting microtubule assembly, stabilizing their structure, and participating in axonal transport. Perturbations in tau's structure and function are implicated in the pathogenesis of neurodegenerative diseases collectively known as tauopathies, the most common disorder of which is Alzheimer's disease (AD). In tauopathies, it has been found that tau has a variety of post-translational modification (PTM) abnormalities and/or tau is cleaved into a variety of fragments by some specific proteolytic enzymes; however, the precise contributions of these abnormal modifications and fragments to disease onset and progression remain incompletely understood. Herein, we provide an overview about the involvement of distinctive abnormal tau PTMs and different tau fragments in the pathogenesis of AD and other tauopathies and discuss the involvement of proteolytic enzymes such as caspases, calpains, and asparagine endopeptidase in mediating tau cleavage while also addressing the intercellular transmission role played by tau. We anticipate that further exploration into PTMs and fragmented forms of tau will yield valuable insights for diagnostic approaches and therapeutic interventions targeting AD and other related disorders.

Caspases in Alzheimer's Disease: Mechanism of Activation, Role, and Potential Treatment

With the aging of the population, treatment of conditions emerging in old age, such as neurodegenerative disorders, has become a major medical challenge. Of these, Alzheimer's disease, leading to cognitive dysfunction, is of particular interest. Neuronal loss plays an important role in the pathophysiology of this condition, and over the years, a great effort has been made to determine the role of various factors in this process. Unfortunately, until now, the exact pathomechanism of this condition remains unknown. However, the most popular theories associate AD with abnormalities in the Tau and β-amyloid (Aβ) proteins, which lead to their deposition and result in neuronal death. Neurons, like all cells, die in a variety of ways, among which pyroptosis, apoptosis, and necroptosis are associated with the activation of various caspases. It is worth mentioning that Tau and Aβ proteins are considered to be one of the caspase activators, leading to cell death. Moreover, the protease activity of caspases influences both of the previously mentioned proteins, Tau and Aβ, converting them into more toxic derivatives. Due to the variety of ways caspases impact the development of AD, drugs targeting caspases could potentially be useful in the treatment of this condition. Therefore, there is a constant need to search for novel caspase inhibitors and evaluate them in preclinical and clinical trials.

Site-specific phosphorylation of tau impacts mitochondrial function and response to stressors

Phosphorylation of tau at sites associated with Alzheimer's disease (AD) likely plays a role in the disease progression. Mitochondrial impairment, correlating with increased presence of phosphorylated tau, has been identified as a contributing factor to neurodegenerative processes in AD. However, how tau phosphorylated at specific sites impacts mitochondrial function has not been fully defined. We examined how AD-relevant phosphomimetics of tau impact selected aspects of mitochondrial biology. To mimic phosphorylation at AD-associated sites, the serine/threonine (Ser/Thr) sites in wild-type green fluorescent protein (GFP)-tagged tau (T4) were converted to glutamic acid (E) to make pseudo-phosphorylated GFP-tagged Ser-396/404 (2EC) and GFP-tagged Thr-231/Ser-235 (2EM) constructs. These constructs were expressed in immortalized mouse hippocampal neuronal cell lines, and their impact on specific mitochondrial functions and responses to stressors were measured. Phosphomimetic tau altered mitochondrial distribution. Specifically, mitochondria accumulated in the soma of cells expressing either 2EC or 2EM and neurite-like extensions in 2EC cells were shorter. Additionally, adenosine triphosphate levels were reduced in both 2EC- and 2EM-expressing cells, and reactive oxygen species (ROS) production increased in 2EC cells during oxidation of succinate when compared to T4-expressing cells. Thapsigargin reduced mitochondrial membrane potential and increased ROS production in both 2EC and 2EM cells relative to T4 cells, with no significant difference in the effects of rotenone. These results show that tau phosphorylation at specific AD-relevant epitopes negatively affects mitochondria, with the extent of dysfunction and stress response varying according to the sites of phosphorylation. Altogether, these findings show that phosphorylated tau increases mitochondrial susceptibility to stressors and extend our understanding of potential mechanisms whereby phosphorylated tau promotes mitochondria dysfunction in tauopathies, including AD.

Mitochondria-tau association promotes cognitive decline and hippocampal bioenergetic deficits during the aging

Current studies indicate that pathological modifications of tau are associated with mitochondrial dysfunction, synaptic failure, and cognitive decline in neurological disorders and aging. We previously showed that caspase-3 cleaved tau, a relevant tau form in Alzheimer's disease (AD), affects mitochondrial bioenergetics, dynamics and synaptic plasticity by the opening of mitochondrial permeability transition pore (mPTP). Also, genetic ablation of tau promotes mitochondrial function boost and increased cognitive capacities in aging mice. However, the mechanisms and relevance of these alterations for the cognitive and mitochondrial abnormalities during aging, which is the primary risk factor for AD, has not been explored. Therefore, in this study we used aging C57BL/6 mice (2-15 and 28-month-old) to evaluate hippocampus-dependent cognitive performance and mitochondrial function. Behavioral tests revealed that aged mice (15 and 28-month-old) showed a reduced cognitive performance compared to young mice (2 month). Concomitantly, isolated hippocampal mitochondria of aged mice showed a significant decrease in bioenergetic-related functions including increases in reactive oxygen species (ROS), mitochondrial depolarization, ATP decreases, and calcium handling defects. Importantly, full-length and caspase-3 cleaved tau were preferentially present in mitochondrial fractions of 15 and 28-month-old mice. Also, aged mice (15 and 28-month-old) showed an increase in cyclophilin D (CypD), the principal regulator of mPTP opening, and a decrease in Opa-1 mitochondrial localization, indicating a possible defect in mitochondrial dynamics. Importantly, we corroborated these findings in immortalized cortical neurons expressing mitochondrial targeted full-length (GFP-T4-OMP25) and caspase-3 cleaved tau (GFP-T4C3-OMP25) which resulted in increased ROS levels and mitochondrial fragmentation, along with a decrease in Opa-1 protein expression. These results suggest that tau associates with mitochondria and this binding increases during aging. This connection may contribute to defects in mitochondrial bioenergetics and dynamics which later may conduce to cognitive decline present during aging.

Caspase-3 cleaved tau impairs mitochondrial function through the opening of the mitochondrial permeability transition pore

Mitochondrial dysfunction is a significant factor in the development of Alzheimer's disease (AD). Previous studies have demonstrated that the expression of tau cleaved at Asp421 by caspase-3 leads to mitochondrial abnormalities and bioenergetic impairment. However, the underlying mechanism behind these alterations and their impact on neuronal function remains unknown. To investigate the mechanism behind mitochondrial dysfunction caused by this tau form, we used transient transfection and pharmacological approaches in immortalized cortical neurons and mouse primary hippocampal neurons. We assessed mitochondrial morphology and bioenergetics function after expression of full-length tau and caspase-3-cleaved tau. We also evaluated the mitochondrial permeability transition pore (mPTP) opening and its conformation as a possible mechanism to explain mitochondrial impairment induced by caspase-3 cleaved tau. Our studies showed that pharmacological inhibition of mPTP by cyclosporine A (CsA) prevented all mitochondrial length and bioenergetics abnormalities in neuronal cells expressing caspase-3 cleaved tau. Neuronal cells expressing caspase-3-cleaved tau showed sustained mPTP opening which is mostly dependent on cyclophilin D (CypD) protein expression. Moreover, the impairment of mitochondrial length and bioenergetics induced by caspase-3-cleaved tau were prevented in hippocampal neurons obtained from CypD knock-out mice. Interestingly, previous studies using these mice showed a prevention of mPTP opening and a reduction of mitochondrial failure and neurodegeneration induced by AD. Therefore, our findings showed that caspase-3-cleaved tau negatively impacts mitochondrial bioenergetics through mPTP activation, highlighting the importance of this channel and its regulatory protein, CypD, in the neuronal damage induced by tau pathology in AD.

Pathological Impact of Tau Proteolytical Process on Neuronal and Mitochondrial Function: a Crucial Role in Alzheimer's Disease

Tau protein plays a pivotal role in the central nervous system (CNS), participating in microtubule stability, axonal transport, and synaptic communication. Research interest has focused on studying the role of post-translational tau modifications in mitochondrial failure, oxidative damage, and synaptic impairment in Alzheimer's disease (AD). Soluble tau forms produced by its pathological cleaved induced by caspases could lead to neuronal injury contributing to oxidative damage and cognitive decline in AD. For example, the presence of tau cleaved by caspase-3 has been suggested as a relevant factor in AD and is considered a previous event before neurofibrillary tangles (NFTs) formation.Interestingly, we and others have shown that caspase-cleaved tau in N- or C- terminal sites induce mitochondrial bioenergetics defects, axonal transport impairment, neuronal injury, and cognitive decline in neuronal cells and murine models. All these abnormalities are considered relevant in the early neurodegenerative manifestations such as memory and cognitive failure reported in AD. Therefore, in this review, we will discuss for the first time the importance of truncated tau by caspases activation in the pathogenesis of AD and how its negative actions could impact neuronal function.

Cogs in the autophagic machine-equipped to combat dementia-prone neurodegenerative diseases

Neurodegenerative diseases are often characterized by hydrophobic inclusion bodies, and it may be the case that the aggregate-prone proteins that comprise these inclusion bodies are in fact the cause of neurotoxicity. Indeed, the appearance of protein aggregates leads to a proteostatic imbalance that causes various interruptions in physiological cellular processes, including lysosomal and mitochondrial dysfunction, as well as break down in calcium homeostasis. Oftentimes the approach to counteract proteotoxicity is taken to merely upregulate autophagy, measured by an increase in autophagosomes, without a deeper assessment of contributors toward effective turnover through autophagy. There are various ways in which autophagy is regulated ranging from the mammalian target of rapamycin (mTOR) to acetylation status of proteins. Healthy mitochondria and the intracellular energetic charge they preserve are key for the acidification status of lysosomes and thus ensuring effective clearance of components through the autophagy pathway. Both mitochondria and lysosomes have been shown to bear functional protein complexes that aid in the regulation of autophagy. Indeed, it may be the case that minimizing the proteins associated with the respective neurodegenerative pathology may be of greater importance than addressing molecularly their resulting inclusion bodies. It is in this context that this review will dissect the autophagy signaling pathway, its control and the manner in which it is molecularly and functionally connected with the mitochondrial and lysosomal system, as well as provide a summary of the role of autophagy dysfunction in driving neurodegenerative disease as a means to better position the potential of rapamycin-mediated bioactivities to control autophagy favorably.

The Deficits of Insulin Signal in Alzheimer's Disease and the Mechanisms of Vanadium Compounds in Curing AD

Vanadium is a well-known essential trace element, which usually exists in oxidation states in the form of a vanadate cation intracellularly. The pharmacological study of vanadium began with the discovery of its unexpected inhibitory effect on ATPase. Thereafter, its protective effects on β cells and its ability in glucose metabolism regulation were observed from the vanadium compound, leading to the application of vanadium compounds in clinical trials for curing diabetes. Alzheimer's disease (AD) is the most common dementia disease in elderly people. However, there are still no efficient agents for treating AD safely to date. This is mainly because of the complexity of the pathology, which is characterized by senile plaques composed of the amyloid-beta (Aβ) protein in the parenchyma of the brain and the neurofibrillary tangles (NFTs), which are derived from the hyperphosphorylated tau protein in the neurocyte, along with mitochondrial damage, and eventually the central nervous system (CNS) atrophy. AD was also illustrated as type-3 diabetes because of the observations of insulin deficiency and the high level of glucose in cerebrospinal fluid (CSF), as well as the impaired insulin signaling in the brain. In this review, we summarize the advances in applicating the vanadium compound to AD treatment in experimental research and point out the limitations of the current study using vanadium compounds in AD treatment. We hope this will help future studies in this field.

Acetylated tau exacerbates learning and memory impairment by disturbing with mitochondrial homeostasis

Increased tau acetylation at K274 and K281 has been observed in the brains of Alzheimer's disease (AD) patients and animal models, and mitochondrial dysfunction are noticeable and early features of AD. However, the effect of acetylated tau on mitochondria has been unclear until now. Here, we constructed three type of tau forms, acetylated tau mutant by mutating its K274/K281 into Glutamine (TauKQ) to mimic disease-associated lysine acetylation, the non-acetylation tau mutant by mutating its K274/K281 into Arginine (TauKR) and the wild-type human full-length tau (TauWT). By overexpression of these tau forms in vivo and in vitro, we found that, TauKQ induced more severe cognitive deficits with neuronal loss, dendritic plasticity damage and mitochondrial dysfunctions than TauWT. Unlike TauWT induced mitochondria fusion, TauKQ not only induced mitochondria fission by decreasing mitofusion proteins, but also inhibited mitochondrial biogenesis via reduction of PGC-1a/Nrf1/Tfam levels. TauKR had no significant difference in the cognitive and mitochondrial abnormalities compared with TauWT. Treatment with BGP-15 rescued impaired learning and memory by attenuation of mitochondrial dysfunction, neuronal loss and dendritic complexity damage, which caused by TauKQ. Our data suggested that, acetylation at K274/281 was an important post translational modification site for tau neurotoxicity, and BGP-15 is a potential therapeutic drug for AD.

Cyclosporine A (CsA) prevents synaptic impairment caused by truncated tau by caspase-3

During Alzheimer's (AD), tau protein suffers from abnormal post-translational modifications, including cleaving by caspase-3. These tau forms affect synaptic plasticity contributing to the cognitive decline observed in the early stages of AD. In addition, caspase-3 cleaved tau (TauC3) impairs mitochondrial dynamics and organelles transport, which are both relevant processes for synapse. We recently showed that the absence of tau expression reverts age-associated cognitive and mitochondrial failure by blocking the mitochondrial permeability transition pore (mPTP). mPTP is a mitochondrial complex involved in calcium regulation and apoptosis. Therefore, we studied the effects of TauC3 against the dendritic spine and synaptic vesicle formation and the possible role of mPTP in these alterations. We used mature hippocampal mice neurons to express a reporter protein (GFP, mCherry), coupled to full-length human tau protein (GFP-T4, mCherry-T4), and coupled to human tau protein cleaved at D421 by caspase-3 (GFP-T4C3, mCherry-T4C3) and synaptic elements were evaluated. Treatment with cyclosporine A (CsA), an immunosuppressive drug with inhibitory activity on mPTP, prevented ROS increase and mitochondrial depolarization induced by TauC3 in hippocampal neurons. These results were corroborated with immortalized cortical neurons in which ROS increase and ATP loss induced by this tau form were prevented by CsA. Interestingly, TauC3 expression significantly reduced dendritic spine density (filopodia type) and synaptic vesicle number in hippocampal neurons. Also, neurons transfected with TauC3 showed a significant accumulation of synaptophysin protein in their soma. More importantly, all these synaptic alterations were prevented by CsA, suggesting an mPTP role in these negative changes derived from TauC3 expression.

Site-specific phosphorylation of tau impacts mitochondrial biology and response to stressors

Phosphorylation of tau at sites associated with Alzheimer's disease (AD) likely plays a role in the disease progression. Mitochondrial impairment, correlating with increased presence of phosphorylated tau, has been identified as a contributing factor to neurodegenerative processes in AD. However, how tau phosphorylated at specific sites impacts mitochondrial function has not been fully defined. We examined how AD-relevant phosphomimetics of tau impact selected aspects of mitochondrial biology. To mimic phosphorylation at AD-associated sites, the Ser/Thr sites in wild-type GFP tagged-tau (T4) were converted to glutamic acid (E) to make pseudophosphorylated GFP tagged-Ser-396/404 (2EC) and GFP tagged-Thr-231/Ser-235 (2EM) constructs. These constructs were expressed in neuronal HT22 cells and their impact on specific mitochondrial functions and responses to stressors were measured. Phosphomimetic tau altered mitochondrial distribution. Specifically, mitochondria accumulated in the soma of cells expressing either 2EC or 2EM, and neurite-like extensions in 2EC cells were shorter. Additionally, ATP levels were reduced in both 2EC and 2EM expressing cells, and ROS production increased in 2EC cells during oxidation of succinate when compared to T4 expressing cells. Thapsigargin reduced mitochondrial membrane potential (Ψ m ) and increased ROS production in both 2EC and 2EM cells relative to T4 cells, with no significant difference in the effects of rotenone. These results show that tau phosphorylation at specific AD-relevant epitopes negatively affects mitochondria, with the extent of dysfunction and stress response varying according to the sites of phosphorylation. Altogether, these findings extend our understanding of potential mechanisms whereby phosphorylated tau promotes mitochondria dysfunction in tauopathies, including AD.

Funding information

R01 AG067617.

All the Tau We Cannot See

Alzheimer's disease (AD) was described in 1906 as a dementing disease marked by the presence of two types of fibrillar aggregates in the brain: neurofibrillary tangles and senile plaques. The process of aggregation and formation of the aggregates has been a major focus of investigation ever since the discoveries that the tau protein is the predominant protein in tangles and amyloid β is the predominant protein in plaques. The idea that smaller, oligomeric species of amyloid may also be bioactive has now been clearly established. This review examines the possibility that soluble, nonfibrillar, bioactive forms of tau-the "tau we cannot see"-comprise a dominant driver of neurodegeneration in AD.

The Strategies for Treating "Alzheimer's Disease": Insulin Signaling May Be a Feasible Target

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by senile plaques formed by amyloid-beta (Aβ) extracellularly and neurofibrillary tangles (NFTs) formed by hyperphosphorylated tau protein intracellularly. Apart from these two features, insulin deficiency and insulin resistance have also been observed in AD brains. Thus, AD has also been referred to as type 3 diabetes by some of the scientists in this field. Insulin plays a pivotal role in learning and memory and is involved in regulating tau phosphorylation though the PI3KAkt-GSK3b signaling pathway. Interestingly, recent studies revealed that in AD brains the microglia transformed into a disease-associated microglia (DAM) status in a TREM2-dependent manner to restrain the toxicity of Aβ and propagation of tau. This also correlated with PI3K-Akt signaling through the adaptor of TREM2. Whether insulin has any effect on microglia activation in AD pathology is unclear so far. However, many studies demonstrated that diabetes increased the risk of AD. In this review, we summarize the main strategies for curing AD, including lowering the level of Aβ, suppressing the phosphorylation of tau, the ablation and/or repopulation of microglia, and especially the supply of insulin. We also propose that attention should be given to the influences of insulin on microglia in AD.

The use of fibroblasts as a valuable strategy for studying mitochondrial impairment in neurological disorders

Neurological disorders (NDs) are characterized by progressive neuronal dysfunction leading to synaptic failure, cognitive impairment, and motor injury. Among these diseases, Alzheimer's disease (AD), Parkinson's disease (PD), Huntington’s disease (HD), and amyotrophic lateral sclerosis (ALS) have raised a significant research interest. These disorders present common neuropathological signs, including neuronal dysfunction, protein accumulation, oxidative damage, and mitochondrial abnormalities. In this context, mitochondrial impairment is characterized by a deficiency in ATP production, excessive production of reactive oxygen species, calcium dysregulation, mitochondrial transport failure, and mitochondrial dynamics deficiencies. These defects in mitochondrial health could compromise the synaptic process, leading to early cognitive dysfunction observed in these NDs. Interestingly, skin fibroblasts from AD, PD, HD, and ALS patients have been suggested as a useful strategy to investigate and detect early mitochondrial abnormalities in these NDs. In this context, fibroblasts are considered a viable model for studying neurodegenerative changes due to their metabolic and biochemical relationships with neurons. Also, studies of our group and others have shown impairment of mitochondrial bioenergetics in fibroblasts from patients diagnosed with sporadic and genetic forms of AD, PD, HD, and ALS. Interestingly, these mitochondrial abnormalities have been observed in the brain tissues of patients suffering from the same pathologies. Therefore, fibroblasts represent a novel strategy to study the genesis and progression of mitochondrial dysfunction in AD, PD, HD, and ALS. This review discusses recent evidence that proposes fibroblasts as a potential target to study mitochondrial bioenergetics impairment in neurological disorders and consequently to search for new biomarkers of neurodegeneration.

Hyperphosphorylated Human Tau Accumulates at the Synapse, Localizing on Synaptic Mitochondrial Outer Membranes and Disrupting Respiration in a Mouse Model of Tauopathy

Neurogenerative disorders, such as Alzheimer’s disease (AD), represent a growing public health challenge in aging societies. Tauopathies, a subset of neurodegenerative disorders that includes AD, are characterized by accumulation of fibrillar and hyperphosphorylated forms of microtubule-associated protein tau with coincident mitochondrial abnormalities and neuronal dysfunction. Although, in vitro, tau impairs axonal transport altering mitochondrial distribution, clear in vivo mechanisms associating tau and mitochondrial dysfunction remain obscure. Herein, we investigated the effects of human tau on brain mitochondria in vivo using transgenic htau mice at ages preceding and coinciding with onset of tauopathy. Subcellular proteomics combined with bioenergetic assessment revealed pathologic forms of tau preferentially associate with synaptic over non-synaptic mitochondria coinciding with changes in bioenergetics, reminiscent of an aged synaptic mitochondrial phenotype in wild-type mice. While mitochondrial content was unaltered, mitochondrial maximal respiration was impaired in synaptosomes from htau mice. Further, mitochondria-associated tau was determined to be outer membrane-associated using the trypsin protection assay and carbonate extraction. These findings reveal non-mutant human tau accumulation at the synapse has deleterious effects on mitochondria, which likely contributes to synaptic dysfunction observed in the context of tauopathy.

Activation of the Nrf2 Pathway Prevents Mitochondrial Dysfunction Induced by Caspase-3 Cleaved Tau: Implications for Alzheimer’s Disease

Alzheimer’s disease (AD) is characterized by memory and cognitive impairment, accompanied by the accumulation of extracellular deposits of amyloid β-peptide (Aβ) and the presence of neurofibrillary tangles (NFTs) composed of pathological forms of tau protein. Mitochondrial dysfunction and oxidative stress are also critical elements for AD development. We previously showed that the presence of caspase-3 cleaved tau, a relevant pathological form of tau in AD, induced mitochondrial dysfunction and oxidative damage in different neuronal models. Recent studies demonstrated that the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) plays a significant role in the antioxidant response promoting neuroprotection. Here, we studied the effects of Nrf2 activation using sulforaphane (SFN) against mitochondrial injury induced by caspase-3 cleaved tau. We used immortalized cortical neurons to evaluate mitochondrial bioenergetics and ROS levels in control and SFN-treated cells. Expression of caspase-3 cleaved tau induced mitochondrial fragmentation, depolarization, ATP loss, and increased ROS levels. Treatment with SFN for 24 h significantly prevented these mitochondrial abnormalities, and reduced ROS levels. Analysis of Western blots and rt-PCR studies showed that SFN treatment increased the expression of several Nrf2-related antioxidants genes in caspase-3 cleaved tau cells. These results indicate a potential role of the Nrf2 pathway in preventing mitochondrial dysfunction induced by pathological forms of tau in AD.

2-Dodecyl-6-Methoxycyclohexa-2,5-Diene-1,4-Dione Ameliorates Diabetic Cognitive Impairment Through Inhibiting Hif3α and Apoptosis

Diabetes mellitus (DM) is an independent risk factor for cognitive impairment. Although the etiology of diabetic cognitive impairment is complex and multifactorial, the hippocampus neuronal apoptosis is recognized as a main cause of diabetes-induced cognitive impairment. 2-Dodecyl-6-methoxycyclohexa-2,5-diene-1,4-dione (DMDD) was purified from the roots of Averrhoa carambola L. Previous research demonstrated that DMDD was safe and effective in delaying some diabetic complications. However, the efficacy of DMDD to ameliorate diabetic cognitive impairment in type 2 diabetes mice has not been reported. In the present study, the behavioral evaluation was performed by Y maze and novel object recognition in db/db mice. Gene expression profiles were detected using mouse lncRNA microarray analysis in the hippocampi of db/db mice. Changes in the neurodegeneration-associated proteins and the apoptosis-related proteins were determined in both db/db mice and high glucose-treated HT22 cells by Western blotting. We observed that DMDD treatment significantly ameliorated the spatial working memory and object recognition memory impairment in db/db mice. Further study showed that neurodegeneration-associated protein tau was decreased after DMDD treatment in the hippocampi of db/db mice. Eleven lncRNAs and four mRNAs including pro-apoptotic gene Hif3a were significantly differently expressed after DMDD treatment in the hippocampi of db/db mice. The expression of Hif3a, cleaved parp, and caspase 3 proteins was significantly increased in the hippocampi of diabetic db/db mice compared with db/m control mice and then decreased after DMDD treatment. Similar beneficial effects of DMDD were observed in HG-treated HT22 cells. These data indicate that DMDD can alleviate cognitive impairment by inhibiting neuronal apoptosis through decreasing the expression of pro-apoptotic protein Hif3a. In conclusion, our study suggests that DMDD has great potential to be a new preventive and therapeutic compound for diabetic cognitive impairment.

Human Tau Isoforms and Proteolysis for Production of Toxic Tau Fragments in Neurodegeneration

The human tau protein is implicated in a wide range of neurodegenerative “tauopathy” diseases, consisting of Alzheimer’s disease (AD) and frontotemporal lobar degeneration which includes progressive supranuclear palsy, corticobasal degeneration, Pick’s disease, and FTLD-tau (frontotemporal dementia with parkinsonism caused by MAPT mutations). Tau gene transcripts in the human brain undergo alternative splicing to yield 6 different tau protein isoforms that are expressed in different ratios in neurodegeneration which result in tau pathology of paired-helical filaments, neurofibrillary tangles, and tau fibrillar aggregates with detrimental microtubule destabilization. Protease-mediated tau truncation is an important post-translational modification (PTM) which drives neurodegeneration in a tau fragment-dependent manner. While numerous tau fragments have been identified, knowledge of the proteolytic steps that convert each parent tau isoform into specific truncated tau fragments has not yet been fully defined. An improved understanding of the relationships between tau isoforms and their proteolytic processing to generate neurotoxic tau fragments is important to the field. This review evaluates tau isoform expression patterns including PTMs and mutations that influence proteolysis of tau to generate toxic fragments that drive cognitive deficits in AD and other tauopathy models. This assessment identifies the gap in the field on understanding the details of proteolytic steps used to convert each tau isoform into fragments. Knowledge of the processing mechanisms of tau isoforms can lead to new protease targeted drug strategies to prevent the formation of toxic tau fragments in tauopathy neurodegenerative diseases.

Alteration of mitochondrial homeostasis is an early event in a C. elegans model of human tauopathy

Tauopathies are a group of progressive neurodegenerative disorders characterized by the presence of insoluble intracellular tau filaments in the brain. Evidence suggests that there is a tight connection between mitochondrial dysfunction and tauopathies, including Alzheimer’s disease. However, whether mitochondrial dysfunction occurs prior to the detection of tau aggregates in tauopathies remains elusive. Here, we utilized transgenic nematodes expressing the full length of wild type tau in neuronal cells and monitored mitochondrial morphology alterations over time. Although tau-expressing nematodes did not accumulate detectable levels of tau aggregates during larval stages, they displayed increased mitochondrial damage and locomotion defects compared to the control worms. Chelating calcium restored mitochondrial activity and improved motility in the tau-expressing larvae suggesting a link between mitochondrial damage, calcium homeostasis and neuronal impairment in these animals. Our findings suggest that defective mitochondrial function is an early pathogenic event of tauopathies, taking place before tau aggregation and undermining neuronal homeostasis and organismal fitness. Understanding the molecular mechanisms causing mitochondrial dysfunction early in tauopathy will be of significant clinical and therapeutic value and merits further investigation.

Synaptic Mitochondria: An Early Target of Amyloid-β and Tau in Alzheimer's Disease

Alzheimer's disease (AD) is characterized by cognitive impairment and the presence of neurofibrillary tangles and senile plaques in the brain. Neurofibrillary tangles are composed of hyperphosphorylated tau, while senile plaques are formed by amyloid-β (Aβ) peptide. The amyloid hypothesis proposes that Aβ accumulation is primarily responsible for the neurotoxicity in AD. Multiple Aβ-mediated toxicity mechanisms have been proposed including mitochondrial dysfunction. However, it is unclear if it precedes Aβ accumulation or if is a consequence of it. Aβ promotes mitochondrial failure. However, amyloid β precursor protein (AβPP) could be cleaved in the mitochondria producing Aβ peptide. Mitochondrial-produced Aβ could interact with newly formed ones or with Aβ that enter the mitochondria, which may induce its oligomerization and contribute to further mitochondrial alterations, resulting in a vicious cycle. Another explanation for AD is the tau hypothesis, in which modified tau trigger toxic effects in neurons. Tau induces mitochondrial dysfunction by indirect and apparently by direct mechanisms. In neurons mitochondria are classified as non-synaptic or synaptic according to their localization, where synaptic mitochondrial function is fundamental supporting neurotransmission and hippocampal memory formation. Here, we focus on synaptic mitochondria as a primary target for Aβ toxicity and/or formation, generating toxicity at the synapse and contributing to synaptic and memory impairment in AD. We also hypothesize that phospho-tau accumulates in mitochondria and triggers dysfunction. Finally, we discuss that synaptic mitochondrial dysfunction occur in aging and correlates with age-related memory loss. Therefore, synaptic mitochondrial dysfunction could be a predisposing factor for AD or an early marker of its onset.

Mitochondrial DNA Manipulations Affect Tau Oligomerization

BACKGROUND

Mitochondrial dysfunction and tau aggregation occur in Alzheimer's disease (AD), and exposing cells or rodents to mitochondrial toxins alters their tau.

OBJECTIVE

To further explore how mitochondria influence tau, we measured tau oligomer levels in human neuronal SH-SY5Y cells with different mitochondrial DNA (mtDNA) manipulations.

METHODS

Specifically, we analyzed cells undergoing ethidium bromide-induced acute mtDNA depletion, ρ0 cells with chronic mtDNA depletion, and cytoplasmic hybrid (cybrid) cell lines containing mtDNA from AD subjects.

RESULTS

We found cytochrome oxidase activity was particularly sensitive to acute mtDNA depletion, evidence of metabolic re-programming in the ρ0 cells, and a relatively reduced mtDNA content in cybrids generated through AD subject mitochondrial transfer. In each case tau oligomer levels increased, and acutely depleted and AD cybrid cells also showed a monomer to oligomer shift.

CONCLUSION

We conclude a cell's mtDNA affects tau oligomerization. Overlapping tau changes across three mtDNA-manipulated models establishes the reproducibility of the phenomenon, and its presence in AD cybrids supports its AD-relevance.

Mitochondrial Calcium Signaling as a Therapeutic Target for Alzheimer's Disease

Mitochondria absorb calcium (Ca2+) at the expense of the electrochemical gradient generated during respiration. The influx of Ca2+ into the mitochondrial matrix helps maintain metabolic function and results in increased cytosolic Ca2+ during intracellular Ca2+ signaling. Mitochondrial Ca2+ homeostasis is tightly regulated by proteins located in the inner and outer mitochondrial membranes and by the cross-talk with endoplasmic reticulum Ca2+ signals. Increasing evidence indicates that mitochondrial Ca2+ overload is a pathological phenotype associated with Alzheimer's Disease (AD). As intracellular Ca2+ dysregulation can be observed before the appearance of typical pathological hallmarks of AD, it is believed that mitochondrial Ca2+ overload may also play an important role in AD etiology. The high mitochondrial Ca2+ uptake can easily compromise neuronal functions and exacerbate AD progression by impairing mitochondrial respiration, increasing reactive oxygen species formation and inducing apoptosis. Additionally, mitochondrial Ca2+ overload can damage mitochondrial recycling via mitophagy. This review will discuss the molecular players involved in mitochondrial Ca2+ dysregulation and the pharmacotherapies that target this dysregulation. As most of the current AD therapeutics are based on amyloidopathy, tauopathy, and the cholinergic hypothesis, they achieve only symptomatic relief. Thus, determining how to reestablish mitochondrial Ca2+ homeostasis may aid in the development of novel AD therapeutic interventions.

Single-Particle Tracking Method in Fluorescence Microscopy to Monitor Bioenergetic Responses of Individual Mitochondria

The spectroscopic methods commonly used to study mitochondria bioenergetics do not show the diversity of responses within a population of mitochondria (isolated or in a cell), and/or cannot measure individual dynamics. New methodological developments are necessary in order to improve quantitative and kinetic resolutions and eventually gain further insights on individual mitochondrial responses, such as studying activities of the mitochondrial permeability transition pore (mPTP ). The work reported herein is devoted to study responses of single mitochondria within a large population after isolation from cardiomyocytes. Mitochondria were preloaded with a commonly used membrane potential sensitive dye (TMRM), they are then deposited on a plasma-treated glass coverslip and subsequently energized or inhibited by additions of usual bioenergetics effectors. Responses were analyzed by fluorescence microscopy over few thousands of mitochondria simultaneously with a single organelle resolution. We report an automatic method to analyze each image of time-lapse stacks based on the TrackMate-ImageJ plug-in and specially made Python scripts. Images are processed to eliminate defects of illumination inhomogeneity, improving by at least two orders of magnitude the signal/noise ratio. This method enables us to follow the track of each mitochondrion within the observed field and monitor its fluorescence changes, with a time resolution of 400 ms, uninterrupted over the course of the experiment. Such methodological improvement is a prerequisite to further study the role of mPTP in single mitochondria during calcium transient loading.

Contribution of the Nrf2 Pathway on Oxidative Damage and Mitochondrial Failure in Parkinson and Alzheimer's Disease

The increase in human life expectancy has become a challenge to reduce the deleterious consequences of aging. Nowadays, an increasing number of the population suffer from age-associated neurodegenerative diseases including Parkinson's disease (PD) and Alzheimer's disease (AD). These disorders present different signs of neurodegeneration such as mitochondrial dysfunction, inflammation, and oxidative stress. Accumulative evidence suggests that the transcriptional factor nuclear factor (erythroid-derived 2)-like 2 (Nrf2) plays a vital defensive role orchestrating the antioxidant response in the brain. Nrf2 activation promotes the expression of several antioxidant enzymes that exert cytoprotective effects against oxidative damage and mitochondrial impairment. In this context, several studies have proposed a role of Nrf2 in the pathogenesis of PD and AD. Thus, we consider it important to summarize the ongoing literature related to the effects of the Nrf2 pathway in the context of these diseases. Therefore, in this review, we discuss the mechanisms involved in Nrf2 activity and its connection with mitochondria, energy supply, and antioxidant response in the brain. Furthermore, we will lead our discussion to identify the participation of the Nrf2 pathway in mitochondrial impairment and neurodegeneration present in PD and AD. Finally, we will discuss the therapeutic effects that the Nrf2 pathway activation could have on the cognitive impairment, neurodegeneration, and mitochondrial failure present in PD and AD.

Selective Neuron Vulnerability in Common and Rare Diseases-Mitochondria in the Focus

Mitochondrial dysfunction is a central feature of neurodegeneration within the central and peripheral nervous system, highlighting a strong dependence on proper mitochondrial function of neurons with especially high energy consumptions. The fitness of mitochondria critically depends on preservation of distinct processes, including the maintenance of their own genome, mitochondrial dynamics, quality control, and Ca²⁺ handling. These processes appear to be differently affected in common neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease, as well as in rare neurological disorders, including Huntington’s disease, Amyotrophic Lateral Sclerosis and peripheral neuropathies. Strikingly, particular neuron populations of different morphology and function perish in these diseases, suggesting that cell-type specific factors contribute to the vulnerability to distinct mitochondrial defects. Here we review the disruption of mitochondrial processes in common as well as in rare neurological disorders and its impact on selective neurodegeneration. Understanding discrepancies and commonalities regarding mitochondrial dysfunction as well as individual neuronal demands will help to design new targets and to make use of already established treatments in order to improve treatment of these diseases.

Interaction between Aβ and Tau in the Pathogenesis of Alzheimer's Disease

Extracellular neuritic plaques composed of amyloid‑β (Aβ) protein and intracellular neurofibrillary tangles containing phosphorylated tau protein are the two hallmark proteins of Alzheimer's disease (AD), and the separate neurotoxicity of these proteins in AD has been extensively studied. However, interventions that target Aβ or tau individually have not yielded substantial breakthroughs. The interest in the interactions between Aβ and tau in AD is increasing, but related drug investigations are in their infancy. This review discusses how Aβ accelerates tau phosphorylation and the possible mechanisms and pathways by which tau mediates Aβ toxicity. This review also describes the possible synergistic effects between Aβ and tau on microglial cells and astrocytes. Studies suggest that the coexistence of Aβ plaques and phosphorylated tau is related to the mechanism by which Aβ facilitates the propagation of tau aggregation in neuritic plaques. The interactions between Aβ and tau mediate cognitive dysfunction in patients with AD. In summary, this review summarizes recent data on the interplay between Aβ and tau to promote a better understanding of the roles of these proteins in the pathological process of AD and provide new insights into interventions against AD.

In vivo brain imaging of mitochondrial Ca2+ in neurodegenerative diseases with multiphoton microscopy

Mitochondria are involved in a large number of essential roles related to neuronal function. Ca2+ handling by mitochondria is critical for many of these functions, including energy production and cellular fate. Conversely, mitochondrial Ca2+ mishandling has been related to a variety of neurodegenerative diseases. Investigating mitochondrial Ca2+ dynamics is essential for advancing our understanding of the role of intracellular mitochondrial Ca2+ signals in physiology and pathology. Improved Ca2+ indicators, and the ability to target them to different cells and compartments, have emerged as useful tools for analysis of Ca2+ signals in living organisms. Combined with state-of-the-art techniques such as multiphoton microscopy, they allow for the study of mitochondrial Ca2+ dynamics in vivo in mouse models of the disease. Here, we provide an overview of the Ca2+ transporters/ion channels in mitochondrial membranes, and the involvement of mitochondrial Ca2+ in neurodegenerative diseases followed by a summary of the main tools available to evaluate mitochondrial Ca2+ dynamics in vivo using the aforementioned technique.

Mitochondrial Permeability Transition: A Pore Intertwines Brain Aging and Alzheimer's Disease

Advanced age is the greatest risk factor for aging-related brain disorders including Alzheimer's disease (AD). However, the detailed mechanisms that mechanistically link aging and AD remain elusive. In recent years, a mitochondrial hypothesis of brain aging and AD has been accentuated. Mitochondrial permeability transition pore (mPTP) is a mitochondrial response to intramitochondrial and intracellular stresses. mPTP overactivation has been implicated in mitochondrial dysfunction in aging and AD brains. This review summarizes the up-to-date progress in the study of mPTP in aging and AD and attempts to establish a link between brain aging and AD from a perspective of mPTP-mediated mitochondrial dysfunction.

Caspase-6-cleaved Tau fails to induce Tau hyperphosphorylation and aggregation, neurodegeneration, glial inflammation, and cognitive deficits

Active Caspase-6 (Casp6) and Tau cleaved by Casp6 at amino acids 402 (Tau∆D402) and 421 (Tau∆D421) are present in early Alzheimer disease intraneuronal neurofibrillary tangles, which are made primarily of filamentous Tau aggregates. To assess whether Casp6 cleavage of Tau contributes to Tau pathology and Casp6-mediated age-dependent cognitive impairment, we generated transgenic knock-in mouse models that conditionally express full-length human Tau (hTau) 0N4R only (CTO) or together with human Casp6 (hCasp6) (CTC). Region-specific hippocampal and cortical hCasp6 and hTau expression were confirmed with western blot and immunohistochemistry in 2-25-month-old brains. Casp6 activity was confirmed with Tau∆D421 and Tubulin cleaved by Casp6 immunopositivity in 3-25-month-old CTC, but not in CTO, brains. Immunoprecipitated Tau∆D402 was detected in both CTC and CTO brains, but was more abundant in CTC brains. Intraneuronal hippocampal Tau hyperphosphorylation at S202/T205, S422, and T231, and Tau conformational change were absent in both CTC and CTO brains. A slight accumulation of Tau phosphorylated at S396/404 and S202 was observed in Cornu Ammonis 1 (CA1) hippocampal neuron soma of CTC compared to CTO brains. Eighteen-month-old CTC brains showed rare argentophilic deposits that increased by 25 months, whereas CTO brains only displayed them sparsely at 25 months. Tau microtubule binding was equivalent in CTC and CTO hippocampi. Episodic and spatial memory measured with novel object recognition and Barnes maze, respectively, remained normal in 3-25-month-old CTC and CTO mice, in contrast to previously observed impairments in ACL mice expressing equivalent levels of hCasp6 only. Consistently, the CTC and CTO hippocampal CA1 region displayed equivalent dendritic spine density and no glial inflammation. Together, these results reveal that active hCasp6 co-expression with hTau generates Tau cleavage and rare age-dependent argentophilic deposits but fails to induce cognitive deficits, neuroinflammation, and Tau pathology.

Pathologically phosphorylated tau at S396/404 (PHF-1) is accumulated inside of hippocampal synaptic mitochondria of aged Wild-type mice

Brain aging is a natural process characterized by cognitive decline and memory loss. This impairment is related to mitochondrial dysfunction and has recently been linked to the accumulation of abnormal proteins in the hippocampus. Age-related mitochondrial dysfunction could be induced by modified forms of tau. Here, we demonstrated that phosphorylated tau at Ser 396/404 sites, epitope known as PHF-1, is increased in the hippocampus of aged mice at the same time that oxidative damage and mitochondrial dysfunction are observed. Most importantly, we showed that tau PHF-1 is located in hippocampal mitochondria and accumulates in the mitochondria of old mice. Finally, since two mitochondrial populations were found in neurons, we evaluated tau PHF-1 levels in both non-synaptic and synaptic mitochondria. Interestingly, our results revealed that tau PHF-1 accumulates primarily in synaptic mitochondria during aging, and immunogold electron microscopy and Proteinase K protection assays demonstrated that tau PHF-1 is located inside mitochondria. These results demonstrated the presence of phosphorylated tau at PHF-1 commonly related to tauopathy, inside the mitochondria from the hippocampus of healthy aged mice for the first time. Thus, this study strongly suggests that synaptic mitochondria could be damaged by tau PHF-1 accumulation inside this organelle, which in turn could result in synaptic mitochondrial dysfunction, contributing to synaptic failure and memory loss at an advanced age.

Tau Deletion Prevents Cognitive Impairment and Mitochondrial Dysfunction Age Associated by a Mechanism Dependent on Cyclophilin-D

Aging is an irreversible process and the primary risk factor for the development of neurodegenerative diseases, such as Alzheimer’s disease (AD). Mitochondrial impairment is a process that generates oxidative damage and ATP deficit; both factors are important in the memory decline showed during normal aging and AD. Tau is a microtubule-associated protein, with a strong influence on both the morphology and physiology of neurons. In AD, tau protein undergoes post-translational modifications, which could play a relevant role in the onset and progression of this disease. Also, these abnormal forms of tau could be present during the physiological aging that could be related to memory impairment present during this stage. We previously showed that tau ablation improves mitochondrial function and cognitive abilities in young wild-type mice. However, the possible contribution of tau during aging that could predispose to the development of AD is unclear. Here, we show that tau deletion prevents cognitive impairment and improves mitochondrial function during normal aging as indicated by a reduction in oxidative damage and increased ATP production. Notably, we observed a decrease in cyclophilin-D (CypD) levels in aged tau−/− mice, resulting in increased calcium buffering and reduced mitochondrial permeability transition pore (mPTP) opening. The mPTP is a mitochondrial structure, whose opening is dependent on CypD expression, and new evidence suggests that this could play an essential role in the neurodegenerative process showed during AD. In contrast, hippocampal CypD overexpression in aged tau−/− mice impairs mitochondrial function evidenced by an ATP deficit, increased mPTP opening, and memory loss; all effects were observed in the AD pathology. Our results indicate that the absence of tau prevents age-associated cognitive impairment by maintaining mitochondrial function and reducing mPTP opening through a CypD-dependent mechanism. These findings are novel and represent an important advance in the study of how tau contributes to the cognitive and mitochondrial failure present during aging and AD in the brain.

Tau Post-translational Modifications: Dynamic Transformers of Tau Function, Degradation, and Aggregation

Post-translational modifications (PTMs) on tau have long been recognized as affecting protein function and contributing to neurodegeneration. The explosion of information on potential and observed PTMs on tau provides an opportunity to better understand these modifications in the context of tau homeostasis, which becomes perturbed with aging and disease. Prevailing views regard tau as a protein that undergoes abnormal phosphorylation prior to its accumulation into the toxic aggregates implicated in Alzheimer's disease (AD) and other tauopathies. However, the phosphorylation of tau may, in fact, represent part of the normal but interrupted function and catabolism of the protein. In addition to phosphorylation, tau undergoes another forms of post-translational modification including (but not limited to), acetylation, ubiquitination, glycation, glycosylation, SUMOylation, methylation, oxidation, and nitration. A holistic appreciation of how these PTMs regulate tau during health and are potentially hijacked in disease remains elusive. Recent studies have reinforced the idea that PTMs play a critical role in tau localization, protein-protein interactions, maintenance of levels, and modifying aggregate structure. These studies also provide tantalizing clues into the possibility that neurons actively choose how tau is post-translationally modified, in potentially competitive and combinatorial ways, to achieve broad, cellular programs commensurate with the distinctive environmental conditions found during development, aging, stress, and disease. Here, we review tau PTMs and describe what is currently known about their functional impacts. In addition, we classify these PTMs from the perspectives of protein localization, electrostatics, and stability, which all contribute to normal tau function and homeostasis. Finally, we assess the potential impact of tau PTMs on tau solubility and aggregation. Tau occupies an undoubtedly important position in the biology of neurodegenerative diseases. This review aims to provide an integrated perspective of how post-translational modifications actively, purposefully, and dynamically remodel tau function, clearance, and aggregation. In doing so, we hope to enable a more comprehensive understanding of tau PTMs that will positively impact future studies.

Impact of Tau on Neurovascular Pathology in Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder and the most prevalent cause of dementia. The main cerebral histological hallmarks are represented by parenchymal insoluble deposits of amyloid beta (Aβ plaques) and neurofibrillary tangles (NFT), intracellular filamentous inclusions of tau, a microtubule-associated protein. It is well-established that cerebrovascular dysfunction is an early feature of AD pathology, but the detrimental mechanisms leading to blood vessel impairment and the associated neurovascular deregulation are not fully understood. In 90% of AD cases, Aβ deposition around the brain vasculature, known as cerebral amyloid angiopathy (CAA), alters blood brain barrier (BBB) essential functions. While the effects of vascular Aβ accumulation are better documented, the scientific community has only recently started to consider the impact of tau on neurovascular pathology in AD. Emerging compelling evidence points to transmission of neuronal tau to different brain cells, including astrocytes, as well as to the release of tau into brain interstitial fluids, which may lead to perivascular neurofibrillar tau accumulation and toxicity, affecting vessel architecture, cerebral blood flow (CBF), and vascular permeability. BBB integrity and functionality may therefore be impacted by pathological tau, consequentially accelerating the progression of the disease. Tau aggregates have also been shown to induce mitochondrial damage: it is known that tau impairs mitochondrial localization, distribution and dynamics, alters ATP and reactive oxygen species production, and compromises oxidative phosphorylation systems. In light of this previous knowledge, we postulate that tau can initiate neurovascular pathology in AD through mitochondrial dysregulation. In this review, we will explore the literature investigating tau pathology contribution to the malfunction of the brain vasculature and neurovascular unit, and its association with mitochondrial alterations and caspase activation, in cellular, animal, and human studies of AD and tauopathies.

Therapeutic Strategies to Target Calcium Dysregulation in Alzheimer's Disease

Alzheimer’s disease (AD) is the most common form of dementia, affecting millions of people worldwide. Unfortunately, none of the current treatments are effective at improving cognitive function in AD patients and, therefore, there is an urgent need for the development of new therapies that target the early cause(s) of AD. Intracellular calcium (Ca²⁺) regulation is critical for proper cellular and neuronal function. It has been suggested that Ca²⁺ dyshomeostasis is an upstream factor of many neurodegenerative diseases, including AD. For this reason, chemical agents or small molecules aimed at targeting or correcting this Ca²⁺ dysregulation might serve as therapeutic strategies to prevent the development of AD. Moreover, neurons are not alone in exhibiting Ca²⁺ dyshomeostasis, since Ca²⁺ disruption is observed in other cell types in the brain in AD. In this review, we examine the distinct Ca²⁺ channels and compartments involved in the disease mechanisms that could be potential targets in AD.

Mitochondria and Calcium in Alzheimer's Disease: From Cell Signaling to Neuronal Cell Death

Mitochondrial dysfunction has been implicated in the pathogenesis of almost all neurological diseases, including Alzheimer's disease (AD). Historically, a primary focus in this context has been the link between mitochondrial dynamics and amyloid β toxicity. Recent evidence suggests that dysregulation of mitochondrial calcium homeostasis is also related to tau and other risk factors in AD, although an ongoing challenge in the field is that data collected from different models or experimental settings have not always been consistent. We examine recent literature on mitochondrial dysregulation in AD, with special emphasis on mitochondrial calcium. We include data from in vitro systems, genetic animal models, and AD-derived human tissue, and discuss whether mitochondrial calcium transporters should be proposed as therapeutic candidates for the development of neuroprotective drugs against AD.

Hydroxytyrosol improves mitochondrial energetics of a cellular model of Alzheimer's disease

Mitochondrial energetic deficit is one of the hallmarks of neurodegenerative disorders, e.g. Alzheimer´s disease (AD). Adherence to a Mediterranean diet is associated with lower incidence of cognitive decline and AD and extra virgin olive oil's (poly)phenols such as oleuropein and hydroxytyrosol (HT) are being actively studied in this respect. In this study, we assessed the effects of HT on mitochondrial energetic dysfunction in the 7PA2 cells cellular model, i.e. one of the best cellular models of Aβ toxicity with a well-characterized mitochondrial dysfunction typically observed in AD. We report an increase of new mitochondria at 8 h post HT-treatment, which was followed by higher mitochondrial fusion. Further, ATP concentrations were significantly increased after 24 h of treatment with HT as compared with controls. Our data suggest that HT may revert the energetic deficit of a cellular model of AD by potentiating mitochondrial activity. Because HT is being proposed as dietary supplement or component of functional foods, future studies in appropriate animal models and - eventually - humans are warranted to further investigate its potential neuroprotective actions in AD.

Mitochondria in Alzheimer's disease and their potential role in Alzheimer's proteostasis

Alzheimer's disease (AD) is a progressive brain disorder characterized by memory loss and the accumulation of two insoluble protein aggregates, tau neurofibrillary tangles and beta-amyloid plaques. Widespread mitochondrial dysfunction also occurs and mitochondria from AD patients display changes in number, ultrastructure, and enzyme activities. Mitochondrial dysfunction in AD presumably links in some way to its other disease characteristics, either as a cause or consequence. This review characterizes AD-associated mitochondrial perturbations and considers their position in its pathologic hierarchy. It focuses on the crosstalk that occurs between mitochondria, nuclear gene expression, and cytosolic signaling pathways that serves to maintain cell homeostasis. To this point, recent evidence indicates mitochondria trigger retrograde responses that influence cell proteostasis in general and AD proteostasis specifically. Potentially pertinent retrograde responses include the mitochondrial unfolded protein response (mtUPR), integrated stress response (ISR), autophagy/mitophagy, and proteasome function. A fuller perspective of mitochondrial dysfunction in AD, and its relation to protein aggregation, could enhance our overall understanding of this disease.

Truncated Tau Induces Mitochondrial Transport Failure Through the Impairment of TRAK2 Protein and Bioenergetics Decline in Neuronal Cells

Mitochondria are highly specialized organelles essential for the synapse, and their impairment contributes to the neurodegeneration in Alzheimer's disease (AD). Previously, we studied the role of caspase-3-cleaved tau in mitochondrial dysfunction in AD. In neurons, the presence of this AD-relevant tau form induced mitochondrial fragmentation with a concomitant reduction in the expression of Opa1, a mitochondrial fission regulator. More importantly, we showed that caspase-cleaved tau affects mitochondrial transport, decreasing the number of moving mitochondria in the neuronal processes without affecting their velocity rate. However, the molecular mechanisms involved in these events are unknown. We studied the possible role of motor proteins (kinesin 1 and dynein) and mitochondrial protein adaptors (RhoT1/T2, syntaphilin, and TRAK2) in the mitochondrial transport failure induced by caspase-cleaved tau. We expressed green fluorescent protein (GFP), GFP-full-length, and GPF-caspase-3-cleaved tau proteins in rat hippocampal neurons and immortalized cortical neurons (CN 1.4) and analyzed the expression and localization of these proteins involved in mitochondrial transport regulation. We observed that hippocampal neurons expressing caspase-cleaved tau showed a significant accumulation of a mitochondrial population in the soma. These changes were accompanied by evident mitochondrial bioenergetic deficits, including depolarization, oxidative stress, and a significant reduction in ATP production. More critically, caspase-cleaved tau significantly decreased the expression of TRAK2 in immortalized and primary hippocampal neurons without affecting RhoT1/T2 and syntaphilin levels. Also, when we analyzed the expression of motor proteins-Kinesin 1 (KIF5) and Dynein-we did not detect changes in their expression, localization, and binding to the mitochondria. Interestingly, the expression of truncated tau significantly increases the association of TRAK2 with mitochondria compared with neuronal cells expressing full-length tau. Altogether these results indicate that caspase-cleaved tau may affect mitochondrial transport through the increase of TRAK2-mitochondria binding and reduction of ATP production available for the process of movement of these organelles. These observations are novel and represent a set of exciting findings whereby tau pathology could affect mitochondrial distribution in neurons, an event that may contribute to synaptic failure observed in AD.

Mechanisms of secretion and spreading of pathological tau protein

Accumulation of misfolded and aggregated forms of tau protein in the brain is a neuropathological hallmark of tauopathies, such as Alzheimer's disease and frontotemporal lobar degeneration. Tau aggregates have the ability to transfer from one cell to another and to induce templated misfolding and aggregation of healthy tau molecules in previously healthy cells, thereby propagating tau pathology across different brain areas in a prion-like manner. The molecular mechanisms involved in cell-to-cell transfer of tau aggregates are diverse, not mutually exclusive and only partially understood. Intracellular accumulation of misfolded tau induces several mechanisms that aim to reduce the cellular burden of aggregated proteins and also promote secretion of tau aggregates. However, tau may also be released from cells physiologically unrelated to protein aggregation. Tau secretion involves multiple vesicular and non-vesicle-mediated pathways, including secretion directly through the plasma membrane. Consequently, extracellular tau can be found in various forms, both as a free protein and in vesicles, such as exosomes and ectosomes. Once in the extracellular space, tau aggregates can be internalized by neighboring cells, both neurons and glial cells, via endocytic, pinocytic and phagocytic mechanisms. Importantly, accumulating evidence suggests that prion-like propagation of misfolding protein pathology could provide a general mechanism for disease progression in tauopathies and other related neurodegenerative diseases. Here, we review the recent literature on cellular mechanisms involved in cell-to-cell transfer of tau, with a particular focus in tau secretion.

NADPH oxidase contributes to oxidative damage and mitochondrial impairment induced by acute ethanol treatment in rat hippocampal neurons

Acute ethanol treatment induces neurodegeneration in cultured neurons and can lead to brain damage in animal models. Neuronal cells exposed to ethanol showed an increase in reactive oxygen species (ROS), oxidative damage and mitochondrial impairment contributing to synaptic failure. However, the underlying mechanisms of these events are not well understood. Here, we studied the contribution of NADPH oxidase, as a relevant source of ROS production in the brain, to mitochondrial impairment and oxidative stress induced by ethanol. We used primary hippocampal neurons subjected to an acute treatment of ethanol at increasing concentrations (25, 50, and 75 mM, 24 h), and we evaluated ROS production, mitochondrial function, and synaptic vesicle activity. Our studies showed that after ethanol administration, hippocampal neurons presented an increase in ROS levels, mitochondrial dysfunction, calcium handling defects, and synaptic impairment. Interestingly, treatment with the NADPH inhibitor, apocynin, significantly prevented oxidative stress, mitochondrial dysfunction, and the impairment of synaptic vesicle activity induced by ethanol treatment. These results indicate that NADPH oxidase could be a key participant in the molecular mechanism by which alcohol affects the brain.

The Role of Mitochondrial Impairment in Alzheimer´s Disease Neurodegeneration: The Tau Connection

Accumulative evidence has shown that mitochondrial dysfunction plays a pivotal role in the pathogenesis of Alzheimer's disease (AD). Mitochondrial impairment actively contributes to the synaptic and cognitive failure that characterizes AD. The presence of soluble pathological forms of tau like hyperphosphorylated at Ser396 and Ser404 and cleaved at Asp421 by caspase 3, negatively impacts mitochondrial bioenergetics, transport, and morphology in neurons. These adverse effects against mitochondria health will contribute to the synaptic impairment and cognitive decline in AD. Current studies suggest that mitochondrial failure induced by pathological tau forms is likely the result of the opening of the mitochondrial permeability transition pore (mPTP). mPTP is a mitochondrial mega-channel that is activated by increases in calcium and is associated with mitochondrial stress and apoptosis. This structure is composed of different proteins, where Ciclophilin D (CypD) is considered to be the primary mediator of mPTP activation. Also, new studies suggest that mPTP contributes to Aβ pathology and oxidative stress in AD. Further, inhibition of mPTP through the reduction of CypD expression prevents cognitive and synaptic impairment in AD mouse models. More importantly, tau protein contributes to the physiological regulation of mitochondria through the opening/interaction with mPTP in hippocampal neurons. Therefore, in this paper, we will discuss evidence that suggests an important role of pathological forms of tau against mitochondrial health. Also, we will discuss the possible role of mPTP in the mitochondrial impairment produced by the presence of tau pathology and its impact on synaptic function present in AD.

Mitochondrial dysfunction in Alzheimer's disease: Role in pathogenesis and novel therapeutic opportunities

Dysfunction of cell bioenergetics is a common feature of neurodegenerative diseases, the most common of which is Alzheimer's disease (AD). Disrupted energy utilization implicates mitochondria at its nexus. This review summarizes some of the evidence that points to faulty mitochondrial function in AD and highlights past and current therapeutic development efforts. Classical neuropathological hallmarks of disease (β-amyloid and τ) and sporadic AD risk genes (APOE) may trigger mitochondrial disturbance, yet mitochondrial dysfunction may incite pathology. Preclinical and clinical efforts have overwhelmingly centred on the amyloid pathway, but clinical trials have yet to reveal clear-cut benefits. AD therapies aimed at mitochondrial dysfunction are few and concentrate on reversing oxidative stress and cell death pathways. Novel research efforts aimed at boosting mitochondrial and bioenergetic function offer an alternative treatment strategy. Enhancing cell bioenergetics in preclinical models may yield widespread favourable effects that could benefit persons with AD. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

Fragmentation of the Golgi Apparatus in Neuroblastoma Cells Is Associated with Tau-Induced Ring-Shaped Microtubule Bundles

Abnormal fibrillary aggregation of tau protein is a pathological condition observed in Alzheimer's disease and other tauopathies; however, the presence and pathological significance of early non-fibrillary aggregates of tau remain under investigation. In cell and animal models expressing normal or modified tau, toxic effects altering the structure and function of several membranous organelles have also been reported in the absence of fibrillary structures; however, how these abnormalities are produced is an issue yet to be addressed. In order to obtain more insights into the mechanisms by which tau may disturb intracellular membranous elements, we transiently overexpressed human full-length tau and several truncated tau variants in cultured neuroblastoma cells. After 48 h of transfection, either full-length or truncated tau forms produced significant fragmentation of the Golgi apparatus (GA) with no changes in cell viability. Noteworthy is that in the majority of cells exhibiting dispersion of the GA, a ring-shaped array of cortical or perinuclear microtubule (Mt) bundles was also generated under the expression of either variant of tau. In contrast, Taxol treatment of non-transfected cells increased the amount of Mt bundles but not sufficiently to produce fragmentation of the GA. Tau-induced ring-shaped Mt bundles appeared to be well-organized and stable structures because they were resistant to Nocodazole post-treatment and displayed a high level of tubulin acetylation. These results further indicate that a mechanical force generated by tau-induced Mt-bundling may be responsible for Golgi fragmentation and that the repeated domain region of tau may be the main promoter of this effect.

Astrocytic Insulin-Like Growth Factor-1 Protects Neurons Against Excitotoxicity

Background

Exogenous insulin like growth factor-1 (IGF-1) is known to be neuroprotective in animal models with brain insults, while it can also cause hyperexcitability in rodents. In this regard, the role of endogenous IGF-1 in brain responses to brain insults like excitotoxicity, a common pathology in brain injuries, remains to be elucidated. Here, we investigated the potential role of cell-specific endogenous IGF-1 in the kainic acid (KA) -induced degeneration of the neurons.

Methods

Kainic acid was given to primary cultured cortical neurons and co-cultured astrocytes were added as a supportive system. We evaluated the cell proliferation rate, IGF-1 level in different groups and applied the PCR-Chip assay to explore the downstream of IGF-1. In addition, we applied the viral transfer of astrocytic IGF-1 to rodents treated with KA and assessed the associated molecular marker and behavioral outcomes in these rodents.

Results

We found KA induced increased cell death and hyperphosphorylated tau in neurons; co-cultured astrocytes could prevent these pathologies, and this rescuing effect was abrogated with blockade of the astrocytic IGF-1 with AG1024 (IGF-1R inhibitor). PCR-Chip assay identified that astrocytic IGF-1 could decrease the p-GSK-3 at Tyr 216 in neurons treated with KA and this effect was abrogated with AG1024 as well. In addition, in vivo study showed that gene transfer of astrocytic IGF-1 decreased p-tau and cognitive dysfunction in KA mice.

Conclusion

Our results show astrocytic IGF-1 exhibits neuroprotective properties in neurodegenerative processes in the CNS.

Mitochondrial Dysfunction and Stress Responses in Alzheimer's Disease

Alzheimer's disease (AD) patients display widespread mitochondrial defects. Brain hypometabolism occurs alongside mitochondrial defects, and correlates well with cognitive decline. Numerous theories attempt to explain AD mitochondrial dysfunction. Groups propose AD mitochondrial defects stem from: (1) mitochondrial-nuclear DNA interactions/variations; (2) amyloid and neurofibrillary tangle interactions with mitochondria, and (3) mitochondrial quality control defects and oxidative damage. Cells respond to mitochondrial dysfunction through numerous retrograde responses including the Integrated Stress Response (ISR) involving eukaryotic initiation factor 2α (eIF2α), activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP). AD brains activate the ISR and we hypothesize mitochondrial defects may contribute to ISR activation. Here we review current recognized contributions of the mitochondria to AD, with an emphasis on their potential contribution to brain stress responses.

Genetic ablation of tau improves mitochondrial function and cognitive abilities in the hippocampus

Tau is a key protein for microtubule stability; however, post-translationally modified tau contributes to neurodegenerative diseases by forming tau aggregates in the neurons. Previous reports from our group and others have shown that pathological forms of tau are toxic and impair mitochondrial function, whereas tau deletion is neuroprotective. However, the effects of tau ablation on brain structure and function in young mice have not been fully elucidated. Therefore, the aim of this study was to investigate the implications of tau ablation on the mitochondrial function and cognitive abilities of a litter of young mice (3 months old). Our results showed that tau deletion had positive effects on hippocampal cells by decreasing oxidative damage, favoring a mitochondrial pro-fusion state, and inhibiting mitochondrial permeability transition pore (mPTP) formation by reducing cyclophilin D (Cyp-D) protein. More importantly, tau deletion increased ATP production and improved the recognition memory and attentive capacity of juvenile mice. Therefore, the absence of tau enhanced brain function by improving mitochondrial health, which supplied more energy to the synapses. Thus, our work opens the possibility that preventing negative tau modifications could enhance brain function through the improvement of mitochondrial health.

Tau localises within mitochondrial sub-compartments and its caspase cleavage affects ER-mitochondria interactions and cellular Ca2+ handling

Intracellular neurofibrillary tangles (NFT) composed by tau and extracellular amyloid beta (Aβ) plaques accumulate in Alzheimer's disease (AD) and contribute to neuronal dysfunction. Mitochondrial dysfunction and neurodegeneration are increasingly considered two faces of the same coin and an early pathological event in AD. Compelling evidence indicates that tau and mitochondria are closely linked and suggests that tau-dependent modulation of mitochondrial functions might be a trigger for the neurodegeneration process; however, whether this occurs either directly or indirectly is not clear. Furthermore, whether tau influences cellular Ca²⁺ handling and ER-mitochondria cross-talk is yet to be explored. Here, by focusing on wt tau, either full-length (2N4R) or the caspase 3-cleaved form truncated at the C-terminus (2N4RΔC₂₀), we examined the above-mentioned aspects. Using new genetically encoded split-GFP-based tools and organelle-targeted aequorin probes, we assessed: i) tau distribution within the mitochondrial sub-compartments; ii) the effect of tau on the short- (8-10 nm) and the long- (40-50 nm) range ER-mitochondria interactions; and iii) the effect of tau on cytosolic, ER and mitochondrial Ca²⁺ homeostasis. Our results indicate that a fraction of tau is found at the outer mitochondrial membrane (OMM) and within the inner mitochondrial space (IMS), suggesting a potential tau-dependent regulation of mitochondrial functions. The ER Ca²⁺ content and the short-range ER-mitochondria interactions were selectively affected by the expression of the caspase 3-cleaved 2N4RΔC₂₀ tau, indicating that Ca²⁺ mis-handling and defects in the ER-mitochondria communications might be an important pathological event in tau-related dysfunction and thereby contributing to neurodegeneration. Finally, our data provide new insights into the molecular mechanisms underlying tauopathies.