Tau deficiency induces parkinsonism with dementia by impairing APP-mediated iron export

Loss of Tau results in defects in photoreceptor development and progressive neuronal degeneration in Drosophila

Accumulations of Tau, a microtubule-associated protein (MAP), into neurofibrillary tangles is a hallmark of Alzheimer's disease and other tauopathies. However, the mechanisms leading to this pathology are still unclear: the aggregates themselves could be toxic or the sequestration of Tau into tangles might prevent Tau from fulfilling its normal functions, thereby inducing a loss of function defect. Surprisingly, the consequences of losing normal Tau expression in vivo are still not well understood, in part due to the fact that Tau knockout mice show only subtle phenotypes, presumably due to the fact that mammals express several MAPs with partially overlapping functions. In contrast, flies express fewer MAP, with Tau being the only member of the Tau/MAP2/MAP4 family. Therefore, we used Drosophila to address the physiological consequences caused by the loss of Tau. Reducing the levels of fly Tau (dTau) ubiquitously resulted in developmental lethality, whereas deleting Tau specifically in neurons or the eye caused progressive neurodegeneration. Similarly, chromosomal mutations affecting dTau also caused progressive degeneration in both the eye and brain. Although photoreceptor cells initially developed normally in dTau knockdown animals, they subsequently degenerated during late pupal stages whereas weaker dTau alleles caused an age-dependent defect in rhabdomere structure. Expression of wild type human Tau partially rescued the neurodegenerative phenotype caused by the loss of endogenous dTau, suggesting that the functions of Tau proteins are functionally conserved from flies to humans.

Iron dysregulation in Huntington's disease

Huntington's disease (HD) is one of many neurodegenerative diseases with reported alterations in brain iron homeostasis that may contribute to neuropathogenesis. Iron accumulation in the specific brain areas of neurodegeneration in HD has been proposed based on observations in post-mortem tissue and magnetic resonance imaging studies. Altered magnetic resonance imaging signal within specific brain regions undergoing neurodegeneration has been consistently reported and interpreted as altered levels of brain iron. Biochemical studies using various techniques to measure iron species in human samples, mouse tissue, or in vitro has generated equivocal data to support such an association. Whether elevated brain iron occurs in HD, plays a significant contributing role in HD pathogenesis, or is a secondary effect remains currently unclear.

Caenorhabditis elegans: a model to investigate oxidative stress and metal dyshomeostasis in Parkinson's disease

Parkinson's disease (PD) is characterized by progressive motor impairment attributed to progressive loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta. Additional clinical manifestations include non-motor symptoms such as insomnia, depression, psychosis, and cognitive impairment. PD patients with mild cognitive impairment have an increased risk of developing dementia. The affected brain regions also show perturbed metal ion levels, primarily iron. These observations have led to speculation that metal ion dyshomeostasis plays a key role in the neuronal death of this disease. However, the mechanisms underlying this metal-associated neurodegeneration have yet to be completely elucidated. Mammalian models have traditionally been used to investigate PD pathogenesis. However, alternate animal models are also being adopted, bringing to bear their respective experimental advantage. The nematode, Caenorhabditis elegans, is one such system that has well-developed genetics, is amenable to transgenesis and has relatively low associated experimental costs. C. elegans has a well characterized neuronal network that includes a simple DAergic system. In this review we will discuss mechanisms thought to underlie PD and the use of C. elegans to investigate these processes.

Metals and cholesterol: two sides of the same coin in Alzheimer's disease pathology

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease. It begins years prior to the onset of clinical symptoms, such as memory loss and cognitive decline. Pathological hallmarks of AD include the accumulation of β-amyloid in plaques and hyperphosphorylated tau in neurofibrillary tangles. Copper, iron, and zinc are abnormally accumulated and distributed in the aging brain. These metal ions can adversely contribute to the progression of AD. Dysregulation of cholesterol metabolism has also been implicated in the development of AD pathology. To date, large bodies of research have been carried out independently to elucidate the role of metals or cholesterol on AD pathology. Interestingly, metals and cholesterol affect parallel molecular and biochemical pathways involved in AD pathology. The possible links between metal dyshomeostasis and altered brain cholesterol metabolism in AD are reviewed.

Seizure resistance without parkinsonism in aged mice after tau reduction

Tau is an emerging target for Alzheimer's disease (AD) and other conditions with epileptiform activity. Genetic tau reduction (in Tau(+/-) and Tau(-/-) mice) prevents deficits in AD models and has an excitoprotective effect, increasing resistance to seizures, without causing apparent neuronal dysfunction. However, most studies of tau reduction have been conducted in <1-year-old mice, and the effects of tau reduction in aged mice are less clear. Specifically, whether the excitoprotective effects of tau reduction persist with aging is unknown and whether tau reduction causes neuronal dysfunction, including parkinsonism, with aging is controversial. Here, we performed a comprehensive analysis of 2-year-old Tau(+/+), Tau(+/-), and Tau(-/-) mice. In aged mice, tau reduction still conferred resistance to pentylenetetrazole-induced seizures. Moreover, tau reduction did not cause parkinsonian abnormalities in dopamine levels or motor function and did not cause iron accumulation or impaired cognition, although Tau(-/-) mice had mild hyperactivity and decreased brain weight. Importantly, the excitoprotective effect in aged Tau(+/-) mice was not accompanied by detectable abnormalities, indicating that partially reducing tau or blocking its function may be a safe and effective therapeutic approach for AD and other conditions with increased excitability.

ADSC Therapy in Neurodegenerative Disorders

Neurodegenerative disorders, chronic diseases that can severely affect the patient's daily life, include amyotrophic lateral sclerosis, Parkinson's, Alzheimer's, and Huntington's diseases. However, these diseases all have the common characteristic that they are due to degenerative irreversibility, and thus no efficient drugs or therapy methods can mitigate symptoms completely. Stem cell therapy, such as adipose tissue-derived stem cells (ADSCs), is a promising treatment for incurable disorders. In this review, we summarized the previous studies using ADSCs to treat neurodegenerative disorders, as well as their therapeutic mechanisms. We also suggested possible expectations for future human clinical trials involving minimized intracerebroventricular combined with intravenous administration, using different cell lineages to finish complementary therapy as well as change the extracellular matrix to create a homing niche. Depending on successful experiments in relevant neurodegenerative disorders models, this could form the theoretical basis for future human clinical trials.

O-GlcNAc and neurodegeneration: biochemical mechanisms and potential roles in Alzheimer's disease and beyond

Alzheimer disease (AD) is a growing problem for aging populations worldwide. Despite significant efforts, no therapeutics are available that stop or slow progression of AD, which has driven interest in the basic causes of AD and the search for new therapeutic strategies. Longitudinal studies have clarified that defects in glucose metabolism occur in patients exhibiting Mild Cognitive Impairment (MCI) and glucose hypometabolism is an early pathological change within AD brain. Further, type 2 diabetes mellitus (T2DM) is a strong risk factor for the development of AD. These findings have stimulated interest in the possibility that disrupted glucose regulated signaling within the brain could contribute to the progression of AD. One such process of interest is the addition of O-linked N-acetylglucosamine (O-GlcNAc) residues onto nuclear and cytoplasmic proteins within mammals. O-GlcNAc is notably abundant within brain and is present on hundreds of proteins including several, such as tau and the amyloid precursor protein, which are involved in the pathophysiology AD. The cellular levels of O-GlcNAc are coupled to nutrient availability through the action of just two enzymes. O-GlcNAc transferase (OGT) is the glycosyltransferase that acts to install O-GlcNAc onto proteins and O-GlcNAcase (OGA) is the glycoside hydrolase that acts to remove O-GlcNAc from proteins. Uridine 5'-diphosphate-N-acetylglucosamine (UDP-GlcNAc) is the donor sugar substrate for OGT and its levels vary with cellular glucose availability because it is generated from glucose through the hexosamine biosynthetic pathway (HBSP). Within the brains of AD patients O-GlcNAc levels have been found to be decreased and aggregates of tau appear to lack O-GlcNAc entirely. Accordingly, glucose hypometabolism within the brain may result in disruption of the normal functions of O-GlcNAc within the brain and thereby contribute to downstream neurodegeneration. While this hypothesis remains largely speculative, recent studies using different mouse models of AD have demonstrated the protective benefit of pharmacologically increased brain O-GlcNAc levels. In this review we summarize the state of knowledge in the area of O-GlcNAc as it pertains to AD while also addressing some of the basic biochemical roles of O-GlcNAc and how these might contribute to protecting against AD and other neurodegenerative diseases.

The iron regulatory capability of the major protein participants in prevalent neurodegenerative disorders

As with most bioavailable transition metals, iron is essential for many metabolic processes required by the cell but when left unregulated is implicated as a potent source of reactive oxygen species. It is uncertain whether the brain’s evident vulnerability to reactive species-induced oxidative stress is caused by a reduced capability in cellular response or an increased metabolic activity. Either way, dys-regulated iron levels appear to be involved in oxidative stress provoked neurodegeneration. As in peripheral iron management, cells within the central nervous system tightly regulate iron homeostasis via responsive expression of select proteins required for iron flux, transport and storage. Recently proteins directly implicated in the most prevalent neurodegenerative diseases, such as amyloid-β precursor protein, tau, α-synuclein, prion protein and huntingtin, have been connected to neuronal iron homeostatic control. This suggests that disrupted expression, processing, or location of these proteins may result in a failure of their cellular iron homeostatic roles and augment the common underlying susceptibility to neuronal oxidative damage that is triggered in neurodegenerative disease.

A novel approach to rapidly prevent age-related cognitive decline

The loss of cognitive function is a pervasive and often debilitating feature of the aging process for which there are no effective therapeutics. We hypothesized that a novel metal chaperone (PBT2; Prana Biotechnology, Parkville, Victoria, Australia) would enhance cognition in aged rodents. We show here that PBT2 rapidly improves the performance of aged C57Bl/6 mice in the Morris water maze, concomitant with increases in dendritic spine density, hippocampal neuron number and markers of neurogenesis. There were also increased levels of specific glutamate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-d-aspartate), the glutamate transporter (VGLUT1) and glutamate itself. Markers of synaptic plasticity [calmodulin-dependent protein kinase II (CaMKII) and phosphorylated CaMKII, CREB, synaptophysin] were also increased following PBT2 treatment. We also demonstrate that PBT2 treatment results in a subregion-specific increase in hippocampal zinc, which is increasingly recognized as a potent neuromodulator. These data demonstrate that metal chaperones are a novel approach to the treatment of age-related cognitive decline.

Ceruloplasmin and β-amyloid precursor protein confer neuroprotection in traumatic brain injury and lower neuronal iron

Traumatic brain injury (TBI) is in part complicated by pro-oxidant iron elevation independent of brain hemorrhage. Ceruloplasmin (CP) and β-amyloid protein precursor (APP) are known neuroprotective proteins that reduce oxidative damage through iron regulation. We surveyed iron, CP, and APP in brain tissue from control and TBI-affected patients who were stratified according to time of death following injury. We observed CP and APP induction after TBI accompanying iron accumulation. Elevated APP and CP expression was also observed in a mouse model of focal cortical contusion injury concomitant with iron elevation. To determine if changes in APP or CP were neuroprotective we employed the same TBI model on APP(-/-) and CP(-/-) mice and found that both exhibited exaggerated infarct volume and iron accumulation postinjury. Evidence supports a regulatory role of both proteins in defence against iron-induced oxidative damage after TBI, which presents as a tractable therapeutic target.

Brain iron homeostasis: from molecular mechanisms to clinical significance and therapeutic opportunities

Iron has emerged as a significant cause of neurotoxicity in several neurodegenerative conditions, including Alzheimer's disease (AD), Parkinson's disease (PD), sporadic Creutzfeldt-Jakob disease (sCJD), and others. In some cases, the underlying cause of iron mis-metabolism is known, while in others, our understanding is, at best, incomplete. Recent evidence implicating key proteins involved in the pathogenesis of AD, PD, and sCJD in cellular iron metabolism suggests that imbalance of brain iron homeostasis associated with these disorders is a direct consequence of disease pathogenesis. A complete understanding of the molecular events leading to this phenotype is lacking partly because of the complex regulation of iron homeostasis within the brain. Since systemic organs and the brain share several iron regulatory mechanisms and iron-modulating proteins, dysfunction of a specific pathway or selective absence of iron-modulating protein(s) in systemic organs has provided important insights into the maintenance of iron homeostasis within the brain. Here, we review recent information on the regulation of iron uptake and utilization in systemic organs and within the complex environment of the brain, with particular emphasis on the underlying mechanisms leading to brain iron mis-metabolism in specific neurodegenerative conditions. Mouse models that have been instrumental in understanding systemic and brain disorders associated with iron mis-metabolism are also described, followed by current therapeutic strategies which are aimed at restoring brain iron homeostasis in different neurodegenerative conditions. We conclude by highlighting important gaps in our understanding of brain iron metabolism and mis-metabolism, particularly in the context of neurodegenerative disorders.

Glial cell ceruloplasmin and hepcidin differentially regulate iron efflux from brain microvascular endothelial cells

We have used an in vitro model system to probe the iron transport pathway across the brain microvascular endothelial cells (BMVEC) of the blood-brain barrier (BBB). This model consists of human BMVEC (hBMVEC) and C6 glioma cells (as an astrocytic cell line) grown in a transwell, a cell culture system commonly used to quantify metabolite flux across a cell-derived barrier. We found that iron efflux from hBMVEC through the ferrous iron permease ferroportin (Fpn) was stimulated by secretion of the soluble form of the multi-copper ferroxidase, ceruloplasmin (sCp) from the co-cultured C6 cells. Reciprocally, expression of sCp mRNA in the C6 cells was increased by neighboring hBMVEC. In addition, data indicate that C6 cell-secreted hepcidin stimulates internalization of hBMVEC Fpn but only when the end-feet projections characteristic of this glia-derived cell line are proximal to the endothelial cells. This hepcidin-dependent loss of Fpn correlated with knock-down of iron efflux from the hBMVEC; this result was consistent with the mechanism by which hepcidin regulates iron efflux in mammalian cells. In summary, the data support a model of iron trafficking across the BBB in which the capillary endothelium induce the underlying astrocytes to produce the ferroxidase activity needed to support Fpn-mediated iron efflux. Reciprocally, astrocyte proximity modulates the effective concentration of hepcidin at the endothelial cell membrane and thus the surface expression of hBMVEC Fpn. These results are independent of the source of hBMVEC iron (transferrin or non-transferrin bound) indicating that the model developed here is broadly applicable to brain iron homeostasis.

The role of iron and reactive oxygen species in cell death

The transition metal iron is essential for life, yet potentially toxic iron-catalyzed reactive oxygen species (ROS) are unavoidable in an oxygen-rich environment. Iron and ROS are increasingly recognized as important initiators and mediators of cell death in a variety of organisms and pathological situations. Here, we review recent discoveries regarding the mechanism by which iron and ROS participate in cell death. We describe the different roles of iron in triggering cell death, targets of iron-dependent ROS that mediate cell death and a new form of iron-dependent cell death termed ferroptosis. Recent advances in understanding the role of iron and ROS in cell death offer unexpected surprises and suggest new therapeutic avenues to treat cancer, organ damage and degenerative disease.

Iron chelation and multiple sclerosis

Histochemical and MRI studies have demonstrated that MS (multiple sclerosis) patients have abnormal deposition of iron in both gray and white matter structures. Data is emerging indicating that this iron could partake in pathogenesis by various mechanisms, e.g., promoting the production of reactive oxygen species and enhancing the production of proinflammatory cytokines. Iron chelation therapy could be a viable strategy to block iron-related pathological events or it can confer cellular protection by stabilizing hypoxia inducible factor 1α, a transcription factor that normally responds to hypoxic conditions. Iron chelation has been shown to protect against disease progression and/or limit iron accumulation in some neurological disorders or their experimental models. Data from studies that administered a chelator to animals with experimental autoimmune encephalomyelitis, a model of MS, support the rationale for examining this treatment approach in MS. Preliminary clinical studies have been performed in MS patients using deferoxamine. Although some side effects were observed, the large majority of patients were able to tolerate the arduous administration regimen, i.e., 6-8 h of subcutaneous infusion, and all side effects resolved upon discontinuation of treatment. Importantly, these preliminary studies did not identify a disqualifying event for this experimental approach. More recently developed chelators, deferasirox and deferiprone, are more desirable for possible use in MS given their oral administration, and importantly, deferiprone can cross the blood-brain barrier. However, experiences from other conditions indicate that the potential for adverse events during chelation therapy necessitates close patient monitoring and a carefully considered administration regimen.

Nigral iron elevation is an invariable feature of Parkinson's disease and is a sufficient cause of neurodegeneration

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor deficits accompanying degeneration of substantia nigra pars compactor (SNc) neurons. Although familial forms of the disease exist, the cause of sporadic PD is unknown. Symptomatic treatments are available for PD, but there are no disease modifying therapies. While the neurodegenerative processes in PD may be multifactorial, this paper will review the evidence that prooxidant iron elevation in the SNc is an invariable feature of sporadic and familial PD forms, participates in the disease mechanism, and presents as a tractable target for a disease modifying therapy.

Biological metals and metal-targeting compounds in major neurodegenerative diseases

Metals are functionally essential, but redistribute in neurodegenerative disease where they induce protein aggregates, catalyze radical formation, and lose bioavailability.

A role for tau at the synapse in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is characterized by brain deposition of amyloid plaques and tau neurofibrillary tangles along with steady cognitive decline. Although the mechanism by which AD pathogenesis occurs is unclear, accumulating evidence suggests that dysfunction and loss of synaptic connections may be an early event underlying disease progression. Profound synapse degeneration is observed in AD, and the density of these connections strongly correlates with cognitive ability. Initial investigations into AD-related synaptic changes focused on the toxic effects of amyloid. However, recent research suggests an emerging role for tau at the synapse. Even in the absence of tangles, mice overexpressing human tau display significant synaptic degeneration, suggesting that soluble, oligomeric tau is the synaptotoxic species. However, the localization of tau within synapses in both healthy and AD brains indicates that tau might play a role in normal synaptic function, which may be disrupted in disease. Tau is able to impact synaptic activity in several ways: studies show tau interacting directly with post-synaptic signaling complexes, regulating glutamatergic receptor content in dendritic spines, and influencing targeting and function of synaptic mitochondria. Early trials of tau-targeted immunotherapy reduce tau pathology and synapse loss, indicating that the toxic effects of tau may be reversible within a certain time frame. Understanding the role of tau in both normal and degenerating synapses is crucial for the development of therapeutic strategies designed to ameliorate synapse loss and prevent AD pathogenesis. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

An iron–dopamine index predicts risk of parkinsonian neurodegeneration in the substantia nigra pars compacta

Imaging of iron and dopamine by laser ablation-inductively coupled plasma-mass spectrometry reveals a risk index for parkinsonian neurodegeneration

Reducing iron in the brain: a novel pharmacologic mechanism of huperzine A in the treatment of Alzheimer's disease

Huperzine A (HupA), a natural inhibitor of acetylcholinesterase derived from a plant, is a licensed anti-Alzheimer's disease (AD) drug in China and a nutraceutical in the United States. In addition to acting as an acetylcholinesterase inhibitor, HupA possesses neuroprotective properties. However, the relevant mechanism is unknown. Here, we showed that the neuroprotective effect of HupA was derived from a novel action on brain iron regulation. HupA treatment reduced insoluble and soluble beta amyloid levels, ameliorated amyloid plaques formation, and hyperphosphorylated tau in the cortex and hippocampus of APPswe/PS1dE9 transgenic AD mice. Also, HupA decreased beta amyloid oligomers and amyloid precursor protein levels, and increased A Disintegrin And Metalloprotease Domain 10 (ADAM10) expression in these treated AD mice. However, these beneficial effects of HupA were largely abolished by feeding the animals with a high iron diet. In parallel, we found that HupA decreased iron content in the brain and demonstrated that HupA also has a role to reduce the expression of transferrin-receptor 1 as well as the transferrin-bound iron uptake in cultured neurons. The findings implied that reducing iron in the brain is a novel mechanism of HupA in the treatment of Alzheimer's disease.

Longitudinal assessment of tau and amyloid beta in cerebrospinal fluid of Parkinson disease

Tau gene has been consistently associated with the risk of Parkinson disease in recent genome wide association studies. In addition, alterations of the levels of total tau, phosphorylated tau [181P], and amyloid beta 1-42 in cerebrospinal fluid have been reported in patients with sporadic Parkinson disease and asymptomatic carriers of leucine-rich repeat kinase 2 mutations, in patterns that clearly differ from those typically described for patients with Alzheimer disease. To further determine the potential roles of these molecules in Parkinson disease pathogenesis and/or in tracking the disease progression, especially at early stages, the current study assessed all three proteins in 403 Parkinson disease patients enrolled in the DATATOP (Deprenyl and tocopherol antioxidative therapy of parkinsonism) placebo-controlled clinical trial, the largest cohort to date with cerebrospinal fluid samples collected longitudinally. These initially drug-naive patients at early disease stages were clinically evaluated, and cerebrospinal fluid was collected at baseline and then at endpoint, defined as the time at which symptomatic anti-Parkinson disease medications were determined to be required. General linear models were used to test for associations between baseline cerebrospinal fluid biomarker levels or their rates of change and changes in the Unified Parkinson Disease Rating Scale (total or part III motor score) over time. Robust associations among candidate markers are readily noted. Baseline levels of amyloid beta were weakly but negatively correlated with baseline Unified Parkinson Disease Rating Scale total scores. Baseline phosphorylated tau/total tau and phosphorylated tau/amyloid beta were significantly and negatively correlated with the rates of the Unified Parkinson Disease Rating Scale change. While medications (deprenyl and/or tocopherol) did not appear to alter biomarkers appreciably, a weak but significant positive correlation between the rate of change in total tau or total tau/amyloid beta levels and the change of the Unified Parkinson Disease Rating Scale was observed. Notably, these correlations did not appear to be influenced by APOE genotype. These results are one of the very first pieces of evidence suggesting that tau and amyloid beta are critically involved in early Parkinson disease progression, potentially by a different mechanism than that in Alzheimer disease, although their applications as Parkinson disease progression markers will likely require the addition of other proteins.

Antisense reduction of tau in adult mice protects against seizures

Tau, a microtubule-associated protein, is implicated in the pathogenesis of Alzheimer's Disease (AD) in regard to both neurofibrillary tangle formation and neuronal network hyperexcitability. The genetic ablation of tau substantially reduces hyperexcitability in AD mouse lines, induced seizure models, and genetic in vivo models of epilepsy. These data demonstrate that tau is an important regulator of network excitability. However, developmental compensation in the genetic tau knock-out line may account for the protective effect against seizures. To test the efficacy of a tau reducing therapy for disorders with a detrimental hyperexcitability profile in adult animals, we identified antisense oligonucleotides that selectively decrease endogenous tau expression throughout the entire mouse CNS--brain and spinal cord tissue, interstitial fluid, and CSF--while having no effect on baseline motor or cognitive behavior. In two chemically induced seizure models, mice with reduced tau protein had less severe seizures than control mice. Total tau protein levels and seizure severity were highly correlated, such that those mice with the most severe seizures also had the highest levels of tau. Our results demonstrate that endogenous tau is integral for regulating neuronal hyperexcitability in adult animals and suggest that an antisense oligonucleotide reduction of tau could benefit those with epilepsy and perhaps other disorders associated with tau-mediated neuronal hyperexcitability.

Iron accumulates in Huntington's disease neurons: protection by deferoxamine

Huntington’s disease (HD) is a progressive neurodegenerative disorder caused by a polyglutamine-encoding CAG expansion in the huntingtin gene. Iron accumulates in the brains of HD patients and mouse disease models. However, the cellular and subcellular sites of iron accumulation, as well as significance to disease progression are not well understood. We used independent approaches to investigate the location of brain iron accumulation. In R6/2 HD mouse brain, synchotron x-ray fluorescence analysis revealed iron accumulation as discrete puncta in the perinuclear cytoplasm of striatal neurons. Further, perfusion Turnbull’s staining for ferrous iron (II) combined with transmission electron microscope ultra-structural analysis revealed increased staining in membrane bound peri-nuclear vesicles in R6/2 HD striatal neurons. Analysis of iron homeostatic proteins in R6/2 HD mice revealed decreased levels of the iron response proteins (IRPs 1 and 2) and accordingly decreased expression of iron uptake transferrin receptor (TfR) and increased levels of neuronal iron export protein ferroportin (FPN). Finally, we show that intra-ventricular delivery of the iron chelator deferoxamine results in an improvement of the motor phenotype in R6/2 HD mice. Our data supports accumulation of redox-active ferrous iron in the endocytic / lysosomal compartment in mouse HD neurons. Expression changes of IRPs, TfR and FPN are consistent with a compensatory response to an increased intra-neuronal labile iron pool leading to increased susceptibility to iron-associated oxidative stress. These findings, together with protection by deferoxamine, support a potentiating role of neuronal iron accumulation in HD.

Amyloid-β oligomers induce synaptic damage via Tau-dependent microtubule severing by TTLL6 and spastin

Mislocalization and aggregation of Aβ and Tau combined with loss of synapses and microtubules (MTs) are hallmarks of Alzheimer disease. We exposed mature primary neurons to Aβ oligomers and analysed changes in the Tau/MT system. MT breakdown occurs in dendrites invaded by Tau (Tau missorting) and is mediated by spastin, an MT-severing enzyme. Spastin is recruited by MT polyglutamylation, induced by Tau missorting triggered translocalization of TTLL6 (Tubulin-Tyrosine-Ligase-Like-6) into dendrites. Consequences are spine loss and mitochondria and neurofilament mislocalization. Missorted Tau is not axonally derived, as shown by axonal retention of photoconvertible Dendra2-Tau, but newly synthesized. Recovery from Aβ insult occurs after Aβ oligomers lose their toxicity and requires the kinase MARK (Microtubule-Affinity-Regulating-Kinase). In neurons derived from Tau-knockout mice, MTs and synapses are resistant to Aβ toxicity because TTLL6 mislocalization and MT polyglutamylation are prevented; hence no spastin recruitment and no MT breakdown occur, enabling faster recovery. Reintroduction of Tau re-establishes Aβ-induced toxicity in TauKO neurons, which requires phosphorylation of Tau's KXGS motifs. Transgenic mice overexpressing Tau show TTLL6 translocalization into dendrites and decreased MT stability. The results provide a rationale for MT stabilization as a therapeutic approach.

Trasferrin receptor 2 gene regulation by microRNA 221 in SH-SY5Y cells treated with MPP⁺ as Parkinson's disease cellular model

Parkinson's disease (PD) is one of the most frequent human neurodegenerations. The neurodegeneration in PD is related to cellular iron increase but the mechanisms involved in iron accumulation remain unclear. Transferrin receptor type 2 (TFR2) is a protein expressed on cell membrane and involved in the cellular iron uptake. We hypothesized that microRNA 221 could regulate the expression of TfR2 in an in vitro model of Parkinson's disease, SH-SY5Y cells treated with MPP⁺. The miRNA 221 was selected by in silico analysis of several miRNAs predicted to target the TFR2 gene in SHSY5Y cells treated with MPP⁺. Taqman miRNA assay was used to evaluate the expression of the selected miRNAs. Using a luciferase assay we demonstrated the inhibition of TFR2 by miRNA 221. We show that in PD cellular model, TFR2 expression is regulated by miRNA 221. TFR2 and miR 221 are inversely correlated in SHSY5Y cells during the treatment with MPP⁺. Moreover, overexpression of miRNA 221 decreases the expression of TFR2, respectively, at the mRNA and protein levels. The inhibition of endogenous miRNA 221 also is able to regulate TFR2. These data suggest that miRNA 221 regulate TFR2 in PD model.

Mammalian iron transporters: families SLC11 and SLC40

This review is focused on the mammalian SLC11 and SLC40 families and their roles in iron homeostasis. The SLC11 family is composed of two members, SLC11A1 and SLC11A2. SLC11A1 is expressed in the lysosomal compartment of macrophages and in the tertiary granules of neutrophils, playing a key role in innate resistance against infection by intracellular microbes. SLC11A2 is a key player in iron metabolism and is ubiquitously expressed, most notably in the proximal duodenum, immature erythroid cells, brain, placenta and kidney. Intestinal iron absorption is mediated by SLC11A2 at the apical membrane of enterocytes, followed by basolateral exit via SLC40A1. To meet the daily requirement for iron, approximately 80% of the iron comes from the breakdown of hemoglobin following macrophage phagocytosis of senescent erythrocytes (iron recycling). Both SLC11A1 and SLC11A2 play an important role in macrophage iron recycling. SLC11A2 also transports iron into the cytosol across the membrane of endocytotic vesicles of the transferrin receptor-cycle. SLC40A1 is the sole member of the SLC40 family and is involved in the only cellular iron efflux mechanism described. SLC40A1 is highly expressed in several tissues and cells that play a critical role in body iron homeostasis. The signaling pathways that regulate SLC11A2 and SLC40A1 expression at transcriptional, post-transcriptional and post-translational levels are discussed. The roles of SLC11A2 and/or SLC40A1 in iron-associated disorders such as hemochromatosis, neurodegenerative diseases, and breast cancer are also summarized.

Allosteric heat shock protein 70 inhibitors rapidly rescue synaptic plasticity deficits by reducing aberrant tau

BACKGROUND

The microtubule-associated protein tau accumulates in neurodegenerative diseases known as tauopathies, the most common being Alzheimer's disease. One way to treat these disorders may be to reduce abnormal tau levels through chaperone manipulation, thus subverting synaptic plasticity defects caused by tau's toxic accretion.

METHODS

Tauopathy models were used to study the impact of YM-01 on tau. YM-01 is an allosteric promoter of triage functions of the most abundant variant of the heat shock protein 70 (Hsp70) family in the brain, heat shock cognate 70 protein (Hsc70). The mechanisms by which YM-01 modified Hsc70 activity and tau stability were evaluated with biochemical methods, cell cultures, and primary neuronal cultures from tau transgenic mice. YM-01 was also administered to acute brain slices of tau mice; changes in tau stability and electrophysiological correlates of learning and memory were measured.

RESULTS

Tau levels were rapidly and potently reduced in vitro and ex vivo upon treatment with nanomolar concentrations of YM-01. Consistent with Hsc70 having a key role in this process, overexpression of heat shock protein 40 (DNAJB2), an Hsp70 co-chaperone, suppressed YM-01 activity. In contrast to its effects in pathogenic tauopathy models, YM-01 had little activity in ex vivo brain slices from normal, wild-type mice unless microtubules were disrupted, suggesting that Hsc70 acts preferentially on abnormal pools of free tau. Finally, treatment with YM-01 increased long-term potentiation in tau transgenic brain slices.

CONCLUSIONS

Therapeutics that exploit the ability of chaperones to selectively target abnormal tau can rapidly and potently rescue the synaptic dysfunction that occurs in Alzheimer's disease and other tauopathies.

Metallostasis in Alzheimer's disease

2012 has been another year in which multiple large-scale clinical trials for Alzheimer's disease (AD) have failed to meet their clinical endpoints. With the social and financial burden of this disease increasing every year, the onus is now on the field of AD researchers to investigate alternative ideas to deliver outcomes for patients. Although several major clinical trials targeting Aβ have failed, three smaller clinical trials targeting metal interactions with Aβ have all shown benefit for patients. Here we review the genetic, pathological, biochemical, and pharmacological evidence that underlies the metal hypothesis of AD. The AD-affected brain suffers from metallostasis, or fatigue of metal trafficking, resulting in redistribution of metals into inappropriate compartments. The metal hypothesis is built upon a triad of transition elements: iron, copper, and zinc. The hypothesis has matured from early investigations showing amyloidogenic and oxidative stress consequences of these metals; recently, disease-related proteins, APP, tau, and presenilin, have been shown to have major roles in metal regulation, which provides insight into the pathway of neurodegeneration in AD and illuminates potential new therapeutic avenues.

Tau pathology and neurodegeneration

The pathway leading from soluble and monomeric to hyperphosphorylated, insoluble and filamentous tau protein is at the centre of many human neurodegenerative diseases, collectively referred to as tauopathies. Dominantly inherited mutations in MAPT, the gene that encodes tau, cause forms of frontotemporal dementia and parkinsonism, proving that dysfunction of tau is sufficient to cause neurodegeneration and dementia. However, most cases of tauopathy are not inherited in a dominant manner. The first tau aggregates form in a few nerve cells in discrete brain areas. These become self propagating and spread to distant brain regions in a prion-like manner. The prevention of tau aggregation and propagation is the focus of attempts to develop mechanism-based treatments for tauopathies.

A delicate balance: Iron metabolism and diseases of the brain

Iron is the most abundant transition metal within the brain, and is vital for a number of cellular processes including neurotransmitter synthesis, myelination of neurons, and mitochondrial function. Redox cycling between ferrous and ferric iron is utilized in biology for various electron transfer reactions essential to life, yet this same chemistry mediates deleterious reactions with oxygen that induce oxidative stress. Consequently, there is a precise and tightly controlled mechanism to regulate iron in the brain. When iron is dysregulated, both conditions of iron overload and iron deficiencies are harmful to the brain. This review focuses on how iron metabolism is maintained in the brain, and how an alteration to iron and iron metabolism adversely affects neurological function.

Melatonin in Alzheimer's disease

Alzheimer’s disease (AD), an age-related neurodegenerative disorder with progressive cognition deficit, is characterized by extracellular senile plaques (SP) of aggregated β-amyloid (Aβ) and intracellular neurofibrillary tangles, mainly containing the hyperphosphorylated microtubule-associated protein tau. Multiple factors contribute to the etiology of AD in terms of initiation and progression. Melatonin is an endogenously produced hormone in the brain and decreases during aging and in patients with AD. Data from clinical trials indicate that melatonin supplementation improves sleep, ameliorates sundowning and slows down the progression of cognitive impairment in AD patients. Melatonin efficiently protects neuronal cells from Aβ-mediated toxicity via antioxidant and anti-amyloid properties. It not only inhibits Aβ generation, but also arrests the formation of amyloid fibrils by a structure-dependent interaction with Aβ. Our studies have demonstrated that melatonin efficiently attenuates Alzheimer-like tau hyperphosphorylation. Although the exact mechanism is still not fully understood, a direct regulatory influence of melatonin on the activities of protein kinases and protein phosphatases is proposed. Additionally, melatonin also plays a role in protecting the cholinergic system and in anti-inflammation. The aim of this review is to stimulate interest in melatonin as a potentially useful agent in the prevention and treatment of AD.

Antisense oligonucleotides: treating neurodegeneration at the level of RNA

Adequate therapies are lacking for Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and other neurodegenerative diseases. The ability to use antisense oligonucleotides (ASOs) to target disease-associated genes by means of RNA may offer a potent approach for the treatment of these, and other, neurodegenerative disorders. In modifying the basic backbone chemistry, chemical groups, and target sequence, ASOs can act through numerous mechanisms to decrease or increase total protein levels, preferentially shift splicing patterns, and inhibit microRNAs, all at the level of the RNA molecule. Here, we discuss many of the more commonly used ASO chemistries, as well as the different mechanisms of action that can result from these specific chemical modifications. When applied to multiple neurodegenerative mouse models, ASOs that specifically target the detrimental transgenes have been shown to rescue disease associated phenotypes in vivo. These supporting mouse model data have moved the ASOs from the bench to the clinic, with two neuro-focused human clinical trials now underway and several more being proposed. Although still early in development, translating ASOs into human patients for neurodegeneration appears promising.

Therapeutic strategies for tau mediated neurodegeneration

Based on the amyloid hypothesis, controlling β-amyloid protein (Aβ) accumulation is supposed to suppress downstream pathological events, tau accumulation, neurodegeneration and cognitive decline. However, in recent clinical trials, Aβ removal or reducing Aβ production has shown limited efficacy. Moreover, while active immunisation with Aβ resulted in the clearance of Aβ, it did not prevent tau pathology or neurodegeneration. This prompts the concern that it might be too late to employ Aβ targeting therapies once tau mediated neurodegeneration has occurred. Therefore, it is timely and very important to develop tau directed therapies. The pathomechanisms of tau mediated neurodegeneration are unclear but hyperphosphorylation, oligomerisation, fibrillisation and propagation of tau pathology have been proposed as the likely pathological processes that induce loss of function or gain of toxic function of tau, causing neurodegeneration. Here we review the strategies for tau directed treatments based on recent progress in research on tau and our understanding of the pathomechanisms of tau mediated neurodegeneration.

NAP (davunetide) rescues neuronal dysfunction in a Drosophila model of tauopathy

Alzheimer's disease (AD) is a devastating neurodegenerative disease causing irreversible cognitive decline in the elderly. There is no disease-modifying therapy for this condition and the mechanisms underpinning neuronal dysfunction and neurodegeneration are unclear. Compromised cytoskeletal integrity within neurons is reported in AD. This is believed to result from loss-of-function of the microtubule-associated protein tau, which becomes hyper-phosphorylated and deposits into neurofibrillary tangles in AD. We have developed a Drosophila model of tauopathy in which abnormal human tau mediates neuronal dysfunction characterised by microtubule destabilisation, axonal transport disruption, synaptic defects and behavioural impairments. Here we show that a microtubule-stabilising drug, NAPVSIPQ (NAP), prevents as well as reverses these phenotypes even after they have become established. Moreover, it does not alter abnormal tau levels indicating that it by-passes toxic tau altogether. Thus, microtubule stabilisation is a disease-modifying therapeutic strategy protecting against tau-mediated neuronal dysfunction, which holds great promise for tauopathies like AD.

Progress and developments in tau aggregation inhibitors for Alzheimer disease

Pharmacological approaches directed toward Alzheimer disease are diversifying in parallel with a growing number of promising targets. Investigations on the microtubule-associated protein tau yielded innovative targets backed by recent findings about the central role of tau in numerous neurodegenerative diseases. In this review, we summarize the recent evolution in the development of nonpeptidic small molecules tau aggregation inhibitors (TAGIs) and their advancement toward clinical trials. The compounds are classified according to their chemical structures, providing correlative insights into their pharmacology. Overall, shared structure-activity traits are emerging, as well as specific binding modes related to their ability to engage in hydrogen bonding. Medicinal chemistry efforts on TAGIs together with encouraging in vivo data argue for successful translation to the clinic.

What Renders TAU Toxic

TAU is a microtubule-associated protein that under pathological conditions such as Alzheimer's disease (AD) forms insoluble, filamentous aggregates. When 20 years after TAU's discovery the first TAU transgenic mouse models were established, one declared goal that was achieved was the modeling of authentic TAU aggregate formation in the form of neurofibrillary tangles. However, as we review here, it has become increasingly clear that TAU causes damage much before these filamentous aggregates develop. In fact, because TAU is a scaffolding protein, increased levels and an altered subcellular localization (due to an increased insolubility and impaired clearance) result in the interaction of TAU with cellular proteins with which it would otherwise either not interact or do so to a lesser degree, thereby impairing their physiological functions. We specifically discuss the non-axonal localization of TAU, the role phosphorylation has in TAU toxicity and how TAU impairs mitochondrial functions. A major emphasis is on what we have learned from the four available TAU knock-out models in mice, and the knock-out of the TAU/MAP2 homolog PTL-1 in worms. It has been proposed that in human pathological conditions such as AD, a rare toxic TAU species exists which needs to be specifically removed to abrogate TAU's toxicity and restore neuronal functions. However, what is toxic in one context may not be in another, and simply reducing, but not fully abolishing TAU levels may be sufficient to abrogate TAU toxicity.

NAP (davunetide) modifies disease progression in a mouse model of severe neurodegeneration: protection against impairments in axonal transport

NAP (davunetide) is a novel neuroprotective compound with mechanism of action that appears to involve microtubule (MT) stabilization and repair. To evaluate, for the first time, the impact of NAP on axonal transport in vivo and to translate it to neuroprotection in a severe neurodegeneration, the SOD1-G93A mouse model for amyotrophic lateral sclerosis (ALS) was used. Manganese-enhanced magnetic resonance imaging (MRI), estimating axonal transport rates, revealed a significant reduction of the anterograde axonal transport in the ALS mice compared to healthy control mice. Acute NAP treatment normalized axonal transport rates in these ALS mice. Tau hyperphosphorylation, associated with MT dysfunction and defective axonal transport, was discovered in the brains of the ALS mice and was significantly reduced by chronic NAP treatment. Furthermore, in healthy wild type (WT) mice, NAP reversed axonal transport disruption by colchicine, suggesting drug-dependent protection against axonal transport impairment through stabilization of the neuronal MT network. Histochemical analysis showed that chronic NAP treatment significantly protected spinal cord motor neurons against ALS-like pathology. Sequential MRI measurements, correlating brain structure with ALS disease progression, revealed a significant damage to the ventral tegmental area (VTA), indicative of impairments to the dopaminergic pathways relative to healthy controls. Chronic daily NAP treatment of the SOD1-G93A mice, initiated close to disease onset, delayed degeneration of the trigeminal, facial and hypoglossal motor nuclei as was significantly apparent at days 90-100 and further protected the VTA throughout life. Importantly, protection of the VTA was significantly correlated with longevity and overall, NAP treatment significantly prolonged life span in the ALS mice.

Age-dependent effects of A53T alpha-synuclein on behavior and dopaminergic function

Expression of A53T mutant human alpha-synuclein under the mouse prion promoter is among the most successful transgenic models of Parkinson's disease. Accumulation of A53T alpha-synuclein causes adult mice to develop severe motor impairment resulting in early death at 8–12 months of age. In younger, pre-symptomatic animals, altered motor activity and anxiety-like behaviors have also been reported. These behavioral changes, which precede severe neuropathology, may stem from non-pathological functions of alpha-synuclein, including modulation of monoamine neurotransmission. Our analysis over the adult life-span of motor activity, anxiety-like, and depressive-like behaviors identifies perturbations both before and after the onset of disease. Young A53T mice had increased distribution of the dopamine transporter (DAT) to the membrane that was associated with increased striatal re-uptake function. DAT function decreased with aging, and was associated with neurochemical alterations that included increased expression of beta-synuclein and gamma synuclein. Prior to normalization of dopamine uptake, transient activation of Tau kinases and hyperphosphorylation of Tau in the striatum were also observed. Aged A53T mice had reduced neuron counts in the substantia nigra pars compacta, yet striatal medium spiny neuron dendritic spine density was largely maintained. These findings highlight the involvement of the synuclein family of proteins and phosphorylation of Tau in the response to dopaminergic dysfunction of the nigrostriatal pathway.

Direct modulation of microtubule stability contributes to anthracene general anesthesia

Recently, we identified 1-aminoanthracene as a fluorescent general anesthetic. To investigate the mechanism of action, a photoactive analogue, 1-azidoanthracene, was synthesized. Administration of 1-azidoanthracene to albino stage 40-47 tadpoles was found to immobilize animals upon near-UV irradiation of the forebrain region. The immobilization was often reversible, but it was characterized by a longer duration consistent with covalent attachment of the ligand to functionally important targets. IEF/SDS-PAGE examination of irradiated tadpole brain homogenate revealed labeled protein, identified by mass spectrometry as β-tubulin. In vitro assays with aminoanthracene-cross-linked tubulin indicated inhibition of microtubule polymerization, similar to colchicine. Tandem mass spectrometry confirmed anthracene binding near the colchicine site. Stage 40-47 tadpoles were also incubated 1 h with microtubule stabilizing agents, epothilone D or discodermolide, followed by dosing with 1-aminoanthracene. The effective concentration of 1-aminoanthracene required to immobilize the tadpoles was significantly increased in the presence of either microtubule stabilizing agent. Epothilone D similarly mitigated the effects of a clinical neurosteroid general anesthetic, allopregnanolone, believed to occupy the colchicine site in tubulin. We conclude that neuronal microtubules are "on-pathway" targets for anthracene general anesthetics and may also represent functional targets for some neurosteroid general anesthetics.

Tau loss attenuates neuronal network hyperexcitability in mouse and Drosophila genetic models of epilepsy

Neuronal network hyperexcitability underlies the pathogenesis of seizures and is a component of some degenerative neurological disorders such as Alzheimer's disease (AD). Recently, the microtubule-binding protein tau has been implicated in the regulation of network synchronization. Genetic removal of Mapt, the gene encoding tau, in AD models overexpressing amyloid-β (Aβ) decreases hyperexcitability and normalizes the excitation/inhibition imbalance. Whether this effect of tau removal is specific to Aβ mouse models remains to be determined. Here, we examined tau as an excitability modifier in the non-AD nervous system using genetic deletion of tau in mouse and Drosophila models of hyperexcitability. Kcna1(-/-) mice lack Kv1.1-delayed rectifier currents and exhibit severe spontaneous seizures, early lethality, and megencephaly. Young Kcna1(-/-) mice retained wild-type levels of Aβ, tau, and tau phospho-Thr(231). Decreasing tau in Kcna1(-/-) mice reduced hyperexcitability and alleviated seizure-related comorbidities. Tau reduction decreased Kcna1(-/-) video-EEG recorded seizure frequency and duration as well as normalized Kcna1(-/-) hippocampal network hyperexcitability in vitro. Additionally, tau reduction increased Kcna1(-/-) survival and prevented megencephaly and hippocampal hypertrophy, as determined by MRI. Bang-sensitive Drosophila mutants display paralysis and seizures in response to mechanical stimulation, providing a complementary excitability assay for epistatic interactions. We found that tau reduction significantly decreased seizure sensitivity in two independent bang-sensitive mutant models, kcc and eas. Our results indicate that tau plays a general role in regulating intrinsic neuronal network hyperexcitability independently of Aβ overexpression and suggest that reducing tau function could be a viable target for therapeutic intervention in seizure disorders and antiepileptogenesis.

Ceruloplasmin dysfunction and therapeutic potential for Parkinson disease

Ceruloplasmin is an iron-export ferroxidase that is abundant in plasma and also expressed in glia. We found a ∼80% loss of ceruloplasmin ferroxidase activity in the substantia nigra of idiopathic Parkinson disease (PD) cases, which could contribute to the pro-oxidant iron accumulation that characterizes the pathology. Consistent with a role for ceruloplasmin in PD etiopathogenesis, ceruloplasmin knockout mice developed parkinsonism that was rescued by iron chelation. Additionally, peripheral infusion of ceruloplasmin attenuated neurodegeneration and nigral iron elevation in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mouse model for PD. These findings show, in principle, that intravenous ceruloplasmin may have therapeutic potential in PD.

Age-appropriate cognition and subtle dopamine-independent motor deficits in aged tau knockout mice

The microtubule-associated protein tau is expressed throughout the nervous system, most highly in neurons but also in glial cells. Its functions in adult and aging mammals remain to be defined. Previous studies in mouse models found either protective or detrimental effects of genetic tau ablation. Though tau ablation prevented synaptic, network, and cognitive dysfunctions in several models of Alzheimer's disease and made mice more resistant to epileptic seizures, a recent study described a parkinsonian phenotype in aging Tau knockout mice. Here we tested cognition and motor functions in Tau(+/+), Tau(+/-), and Tau(-/-) mice at approximately 1 and 2 years of age. Tau ablation did not impair cognition and caused only minor motor deficits that were much more subtle than those associated with the aging process. Tau ablation caused a mild increase in body weight, which correlated with and might have contributed to some of the motor deficits. However, tau ablation did not cause significant dopaminergic impairments, and dopamine treatment did not improve the motor deficits, suggesting that they do not reflect extrapyramidal dysfunction.

Iron: effect of overload and deficiency

Iron is a redox active metal which is abundant in the Earth's crust. It has played a key role in the evolution of living systems and as such is an essential element in a wide range of biological phenomena, being critical for the function of an enormous array of enzymes, energy transduction mechanisms, and oxygen carriers. The redox nature of iron renders the metal toxic in excess and consequently all biological organisms carefully control iron levels. In this overview the mechanisms adopted by man to control body iron levels are described.Low body iron levels are related to anemia which can be treated by various forms of iron fortification and supplementation. Elevated iron levels can occur systemically or locally, each giving rise to specific symptoms. Systemic iron overload results from either the hyperabsorption of iron or regular blood transfusion and can be treated by the use of a selection of iron chelating molecules. The symptoms of many forms of neurodegeneration are associated with elevated levels of iron in certain regions of the brain and iron chelation therapy is beginning to find an application in the treatment of such diseases. Iron chelators have also been widely investigated for the treatment of cancer, tuberculosis, and malaria. In these latter studies, selective removal of iron from key enzymes or iron binding proteins is sought. Sufficient selectivity between the invading organism and the host has yet to be established for such chelators to find application in the clinic.Iron chelation for systemic iron overload and iron supplementation therapy for the treatment of various forms of anemia are now established procedures in clinical medicine. Chelation therapy may find an important role in the treatment of various neurodegenerative diseases in the near future.

Chaperone-dependent Neurodegeneration: A Molecular Perspective on Therapeutic Intervention

Maintenance of cellular homeostasis is regulated by the molecular chaperones. Under pathogenic conditions, aberrant proteins are triaged by the chaperone network. These aberrant proteins, known as "clients," have major roles in the pathogenesis of numerous neurological disorders, including tau in Alzheimer's disease, α-synuclein and LRRK2 in Parkinson's disease, SOD-1, TDP-43 and FUS in amyotrophic lateral sclerosis, and polyQ-expanded proteins such as huntingtin in Huntington's disease. Recent work has demonstrated that the use of chemical compounds which inhibit the activity of molecular chaperones subsequently alter the fate of aberrant clients. Inhibition of Hsp90 and Hsc70, two major molecular chaperones, has led to a greater understanding of how chaperone triage decisions are made and how perturbing the chaperone system can promote clearance of these pathogenic clients. Described here are major pathways and components of several prominent neurological disorders. Also discussed is how treatment with chaperone inhibitors, predominately Hsp90 inhibitors which are selective for a diseased state, can relieve the burden of aberrant client signaling in these neurological disorders.

Digital and analog chemical evolution

Living matter is the most elaborate, elegant, and complex hierarchical material known and is consequently the natural target for an ever-expanding scientific and technological effort to unlock and deconvolute its marvelous forms and functions. Our current understanding suggests that biological materials are derived from a bottom-up process, a spontaneous emergence of molecular networks in the course of chemical evolution. Polymer cooperation, so beautifully manifested in the ribosome, appeared in these dynamic networks, and the special physicochemical properties of the nucleic and amino acid polymers made possible the critical threshold for the emergence of extant cellular life. These properties include the precise and geometrically discrete hydrogen bonding patterns that dominate the complementary interactions of nucleic acid base-pairing that guide replication and ensure replication fidelity. In contrast, complex and highly context-dependent sets of intra- and intermolecular interactions guide protein folding. These diverse interactions allow the more analog environmental chemical potential fluctuations to dictate conformational template-directed propagation. When these two different strategies converged in the remarkable synergistic ribonucleoprotein that is the ribosome, this resulting molecular digital-to-analog converter achieved the capacity for both persistent information storage and adaptive responses to an ever-changing environment. The ancestral chemical networks that preceded the Central Dogma of Earth's biology must reflect the dynamic chemical evolutionary landscapes that allowed for selection, propagation, and diversification and ultimately the demarcation and specialization of function that modern biopolymers manifest. Not only should modern biopolymers contain molecular fossils of this earlier age, but it should be possible to use this information to reinvent these dynamic functional networks. In this Account, we review the first dynamic network created by modification of a nucleic acid backbone and show how it has exploited the digital-like base pairing for reversible polymer construction and information transfer. We further review how these lessons have been extended to the complex folding landscapes of templated peptide assembly. These insights have allowed for the construction of molecular hybrids of each biopolymer class and made possible the reimagining of chemical evolution. Such elaboration of biopolymer chimeras has already led to applications in therapeutics and diagnostics, to the construction of novel nanostructured materials, and toward orthogonal biochemical pathways that expand the evolution of existing biochemical systems. The ability to look beyond the primordial emergence of the ribosome may allow us to better define the origins of chemical evolution, to extend its horizons beyond the biology of today and ask whether evolution is an inherent property of matter unbounded by physical limitations imposed by our planet's diverse environments.