Remodelling of Ca2+ homeostasis in type I cortical astrocytes by hypoxia: evidence for association with Alzheimer's disease

HIF-1α Causes LCMT1/PP2A Deficiency and Mediates Tau Hyperphosphorylation and Cognitive Dysfunction during Chronic Hypoxia

Chronic hypoxia is a risk factor for Alzheimer's disease (AD), and the neurofibrillary tangle (NFT) formed by hyperphosphorylated tau is one of the two major pathological changes in AD. However, the effect of chronic hypoxia on tau phosphorylation and its mechanism remains unclear. In this study, we investigated the role of HIF-1α (the functional subunit of hypoxia-inducible factor 1) in tau pathology. It was found that in Sprague-Dawley (SD) rats, global hypoxia (10% O₂, 6 h per day) for one month induced cognitive impairments. Meanwhile it induced HIF-1α increase, tau hyperphosphorylation, and protein phosphatase 2A (PP2A) deficiency with leucine carboxyl methyltransferase 1(LCMT1, increasing PP2A activity) decrease in the rats' hippocampus. The results were replicated by hypoxic treatment in primary hippocampal neurons and C6/tau cells (rat C6 glioma cells stably expressing human full-length tau441). Conversely, HIF-1α silencing impeded the changes induced by hypoxia, both in primary neurons and SD rats. The result of dual luciferase assay proved that HIF-1α acted as a transcription factor of LCMT1. Unexpectedly, HIF-1α decreased the protein level of LCMT1. Further study uncovered that both overexpression of HIF-1α and hypoxia treatment resulted in a sizable degradation of LCMT1 via the autophagy--lysosomal pathway. Together, our data strongly indicated that chronic hypoxia upregulates HIF-1α, which obviously accelerated LCMT1 degradation, thus counteracting its transcriptional expression. The increase in HIF-1α decreases PP2A activity, finally resulting in tau hyperphosphorylation and cognitive dysfunction. Lowering HIF-1α in chronic hypoxia conditions may be useful in AD prevention.

Plasma hydrogen sulfide: A biomarker of Alzheimer's disease and related dementias

While heart disease remains a common cause of mortality, cerebrovascular disease also increases with age, and has been implicated in Alzheimer's disease and related dementias (ADRD). We have described hydrogen sulfide (H₂S), a signaling molecule important in vascular homeostasis, as a biomarker of cardiovascular disease. We hypothesize that plasma H₂S and its metabolites also relate to vascular and cognitive dysfunction in ADRD. We used analytical biochemical methods to measure plasma H₂S metabolites and MRI to evaluate indicators of microvascular disease in ADRD. Levels of total H₂S and specific metabolites were increased in ADRD versus controls. Cognition and microvascular disease indices were correlated with H₂S levels. Total plasma sulfide was the strongest indicator of ADRD, and partially drove the relationship between cognitive dysfunction and white matter lesion volume, an indicator of microvascular disease. Our findings show that H₂S is dysregulated in dementia, providing a potential biomarker for diagnosis and intervention.

Hypoxia-Induced Degenerative Protein Modifications Associated with Aging and Age-Associated Disorders

Aging is an inevitable time-dependent decline of various physiological functions that finally leads to death. Progressive protein damage and aggregation have been proposed as the root cause of imbalance in regulatory processes and risk factors for aging and neurodegenerative diseases. Oxygen is a modulator of aging. The oxygen-deprived conditions (hypoxia) leads to oxidative stress, cellular damage and protein modifications. Despite unambiguous evidence of the critical role of spontaneous non-enzymatic Degenerative Protein Modifications (DPMs) such as oxidation, glycation, carbonylation, carbamylation, and deamidation, that impart deleterious structural and functional protein alterations during aging and age-associated disorders, the mechanism that mediates these modifications is poorly understood. This review summarizes up-to-date information and recent developments that correlate DPMs, aging, hypoxia, and age-associated neurodegenerative diseases. Despite numerous advances in the study of the molecular hallmark of aging, hypoxia, and degenerative protein modifications during aging and age-associated pathologies, a major challenge remains there to dissect the relative contribution of different DPMs in aging (either natural or hypoxia-induced) and age-associated neurodegeneration.

Altered calcium signaling in cancer cells

It is the nature of the calcium signal, as determined by the coordinated activity of a suite of calcium channels, pumps, exchangers and binding proteins that ultimately guides a cell's fate. Deregulation of the calcium signal is often deleterious and has been linked to each of the 'cancer hallmarks'. Despite this, we do not yet have a full understanding of the remodeling of the calcium signal associated with cancer. Such an understanding could aid in guiding the development of therapies specifically targeting altered calcium signaling in cancer cells during tumorigenic progression. Findings from some of the studies that have assessed the remodeling of the calcium signal associated with tumorigenesis and/or processes important in invasion and metastasis are presented in this review. The potential of new methodologies is also discussed. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Presenilin/gamma-Secretase and Inflammation

Presenilins (PS) are the catalytic components of γ-secretase, an aspartyl protease that regulates through proteolytic processing the function of multiple signaling proteins. Specially relevant is the γ-secretase-dependent cleavage of the β-amyloid precursor protein (APP) since generates the β-amyloid (Aβ) peptides that aggregate and accumulate in the brain of Alzheimer's disease (AD) patients. Abnormal processing and/or accumulation of Aβ disrupt synaptic and metabolic processes leading to neuron dysfunction and neurodegeneration. Studies in presenilin conditional knockout mice have revealed that presenilin-1 is essential for age-dependent Aβ accumulation and inflammation. By contrast, mutations in the presenilin genes responsible for early onset familial AD cause rapid disease progression and accentuate clinical and pathological features including inflammation. In addition, a number of loss of function mutations in presenilin-1 have been recently associated to non-Alzheimer's dementias including frontotemporal dementia and dementia with Lewy bodies. In agreement, total loss of presenilin function in the brain results in striking neurodegeneration and inflammation, which includes activation of glial cells and induction of proinflammatory genes, besides altered inflammatory responses in the periphery. Interestingly, some non-steroidal anti-inflammatory drugs that slow cognitive decline and reduce the risk of AD, decrease amyloidogenic Aβ42 levels by modulating allosterically PS/γ-secretase. In this review, I present current evidence supporting a role of presenilin/γ-secretase signaling on gliogenesis and gliosis in normal and pathological conditions. Understanding the cellular mechanisms regulated by presenilin/γ-secretase during chronic inflammatory processes may provide new approaches for the development of effective therapeutic strategies for AD.

A review of the role of glial cells in understanding retinal disease

Retinal vascular diseases such as diabetic retinopathy and retinopathy of prematurity are major causes of visual loss. Although the focus of a great deal of research has been on the aetiology of vascular growth, it is now emerging that anomalies in other retinal cell types, especially glial cells, occur very early in the course of the disease. Glial cells have major roles in every stage of disease, from the earliest subtle variations in neural function, to the development of epi-retinal membranes and tractional detachment. Therefore, having a firm understanding of the function of retinal glia is important in our understanding of retinal disease and is crucial for the development of new treatment strategies.

Hypoxia and Alzheimer's disease

Numerous cardiorespiratory disorders result in persistent systemic hypoxia, or at worst (as a consequence of stroke) deprive the brain of oxygen completely for a period of time. Patients suffering from such conditions are much more susceptible to the development of dementias such as AD (Alzheimer's disease). Until recently, the cellular and molecular basis for the predisposition to AD by systemic hypoxia has been completely unknown. However, emerging evidence suggests that pathological cellular remodelling caused by chronic hypoxia shows striking similarities to those observed in the central nervous system as a consequence of AD. Furthermore, prolonged hypoxia can induce formation of Abetas (amyloid beta peptides), the primary neurotoxic elements of AD, which accumulate over years to form the extracellular plaques that are the hallmark feature of the disease. Hypoxia can lead to paradoxical increases in mitochondrial ROS (reactive oxygen species) generation upstream of Abeta formation. The downstream consequences of prolonged hypoxia include remodelling of functional expression of voltage-gated calcium channels and disturbance of intracellular calcium homoeostasis via disrupted calcium buffering and inhibition of calcium extrusion mechanisms. These effects can be mimicked by application of exogenous Abeta and, crucially, appear to depend on Abeta formation. Current knowledge supports the concept that prevention of the deleterious effects of hypoxia may prove beneficial in slowing or preventing the onset of AD.

Hypoxia suppresses astrocyte glutamate transport independently of amyloid formation

Sustained hypoxia alters the expression of numerous proteins and predisposes individuals to Alzheimer's disease (AD). We have previously shown that hypoxia in vitro alters Ca2+ homeostasis in astrocytes and promotes increased production of amyloid beta peptides (Abeta) of AD. Indeed, alteration of Ca2+ homeostasis requires amyloid formation. Here, we show that electrogenic glutamate uptake by astrocytes is suppressed by hypoxia (1% O2, 24h) in a manner that is independent of amyloid beta peptide formation. Thus, hypoxic suppression of glutamate uptake and expression levels of glutamate transporter proteins EAAT1 and EAAT2 were not mimicked by exogenous application of amyloid beta peptide, or by prevention of endogenous amyloid peptide formation (using inhibitors of either beta or gamma secretase). Thus, dysfunction in glutamate homeostasis in hypoxic conditions is independent of Abeta production, but will likely contribute to neuronal damage and death associated with AD following hypoxic events.

Chronic hypoxia inhibits Na+/Ca2+ exchanger expression in cortical astrocytes

Ca signalling is central to many diverse functions of astrocytes. Of the numerous proteins involved in Ca homeostasis, the Na(+)/Ca(2+) exchanger is of particular importance in signalling regulation. We have shown that Ca signaling is dramatically remodelled in astrocytes by periods of chronic hypoxia, in part by inhibition of Na(+)/Ca(2+) exchanger. Here, we demonstrate that bepridil-sensitive Ca extrusion (indicative of Na(+)/Ca(2+) exchanger activity) is suppressed following 24 h hypoxia (2.5 or 1% O2) owing to a loss of Na(+)/Ca(2+) exchanger expression, as determined using immunocytochemistry and Western blots. Hypoxic Na(+)/Ca(2+) exchanger 1 inhibition occurs at the level of transcription, as mRNA for Na(+)/Ca(2+) exchanger 1 was significantly suppressed by hypoxia. Our results show hypoxia perturbs Ca homeostasis in astrocytes via the suppression of Na(+)/Ca(2+) exchanger 1 expression.