Effect of Metal Chelators on γ-Secretase Indicates That Calcium and Magnesium Ions Facilitate Cleavage of Alzheimer Amyloid Precursor Substrate

Role of neurotoxicants in the pathogenesis of Alzheimer's disease: a mechanistic insight

Alzheimer's disease (AD) is the most conspicuous chronic neurodegenerative syndrome, which has become a significant challenge for the global healthcare system. Multiple studies have corroborated a clear association of neurotoxicants with AD pathogenicity, such as Amyloid beta (Aβ) proteins and neurofibrillary tangles (NFTs), signalling pathway modifications, cellular stress, cognitive dysfunctions, neuronal apoptosis, neuroinflammation, epigenetic modification, and so on. This review, therefore, aimed to address several essential mechanisms and signalling cascades, including Wnt (wingless and int.) signalling pathway, autophagy, mammalian target of rapamycin (mTOR), protein kinase C (PKC) signalling cascades, cellular redox status, energy metabolism, glutamatergic neurotransmissions, immune cell stimulations (e.g. microglia, astrocytes) as well as an amyloid precursor protein (APP), presenilin-1 (PSEN1), presenilin-2 (PSEN2) and other AD-related gene expressions that have been pretentious and modulated by the various neurotoxicants. This review concluded that neurotoxicants play a momentous role in developing AD through modulating various signalling cascades. Nevertheless, comprehension of this risk agent-induced neurotoxicity is far too little. More in-depth epidemiological and systematic investigations are needed to understand the potential mechanisms better to address these neurotoxicants and improve approaches to their risk exposure that aid in AD pathogenesis.Key messagesInevitable cascade mechanisms of how Alzheimer's Disease-related (AD-related) gene expressions are modulated by neurotoxicants have been discussed.Involvement of the neurotoxicants-induced pathways caused an extended risk of AD is explicited.Integration of cell culture, animals and population-based analysis on the clinical severity of AD is addressed.

Alterations of the Endoplasmic Reticulum (ER) Calcium Signaling Molecular Components in Alzheimer's Disease

Sustained imbalance in intracellular calcium (Ca²⁺) entry and clearance alters cellular integrity, ultimately leading to cellular homeostasis disequilibrium and cell death. Alzheimer’s disease (AD) is the most common cause of dementia. Beside the major pathological features associated with AD-linked toxic amyloid beta (Aβ) and hyperphosphorylated tau (p-tau), several studies suggested the contribution of altered Ca²⁺ handling in AD development. These studies documented physical or functional interactions of Aβ with several Ca²⁺ handling proteins located either at the plasma membrane or in intracellular organelles including the endoplasmic reticulum (ER), considered the major intracellular Ca²⁺ pool. In this review, we describe the cellular components of ER Ca²⁺ dysregulations likely responsible for AD. These include alterations of the inositol 1,4,5-trisphosphate receptors’ (IP₃Rs) and ryanodine receptors’ (RyRs) expression and function, dysfunction of the sarco-endoplasmic reticulum Ca²⁺ ATPase (SERCA) activity and upregulation of its truncated isoform (S1T), as well as presenilin (PS1, PS2)-mediated ER Ca²⁺ leak/ER Ca²⁺ release potentiation. Finally, we highlight the functional consequences of alterations of these ER Ca²⁺ components in AD pathology and unravel the potential benefit of targeting ER Ca²⁺ homeostasis as a tool to alleviate AD pathogenesis.

PRL-2 phosphatase is required for vascular morphogenesis and angiogenic signaling

Protein tyrosine phosphatases are essential modulators of angiogenesis and have been identified as novel therapeutic targets in cancer and anti-angiogenesis. The roles of atypical Phosphatase of Regenerative Liver (PRL) phosphatases in this context remain poorly understood. Here, we investigate the biological function of PRL phosphatases in developmental angiogenesis in the postnatal mouse retina and in cell culture. We show that endothelial cells in the retina express PRL-2 encoded by the Ptp4a2 gene, and that inducible endothelial and global Ptp4a2 mutant mice exhibit defective retinal vascular outgrowth, arteriovenous differentiation, and sprouting angiogenesis. Mechanistically, PTP4A2 deletion limits angiogenesis by inhibiting endothelial cell migration and the VEGF-A, DLL-4/NOTCH-1 signaling pathway. This study reveals the importance of PRL-2 as a modulator of vascular development.

Reversal of Calcium Dysregulation as Potential Approach for Treating Alzheimer's Disease

Despite decades of research and effort, there is still no effective disease-modifying treatment for Alzheimer's Disease (AD). Most of the recent AD clinical trials were targeting amyloid pathway, but all these trials failed. Although amyloid pathology is a hallmark and defining feature of AD, targeting the amyloid pathway has been very challenging due to low efficacy and serious side effects. Alternative approaches or mechanisms for our understanding of the major cause of memory loss in AD need to be considered as potential therapeutic targets. Increasing studies suggest that Ca2+ dysregulation in AD plays an important role in AD pathology and is associated with other AD abnormalities, such as excessive inflammation, increased ROS, impaired autophagy, neurodegeneration, synapse, and cognitive dysfunction. Ca2+ dysregulation in cytosolic space, Endoplasmic Reticulum (ER) and mitochondria have been reported in the context of various AD models. Drugs or strategies, to correct the Ca2+ dysregulation in AD, have been demonstrated to be promising as an approach for the treatment of AD in preclinical models. This review will discuss the mechanisms of Ca2+ dysregulation in AD and associated pathology and discuss potential approaches or strategies to develop novel drugs for the treatment of AD by targeting Ca2+ dysregulation.

Metal Ion Effects on Aβ and Tau Aggregation

Amyloid and tau aggregation are implicated in manifold neurodegenerative diseases and serve as two signature pathological hallmarks in Alzheimer's disease (AD). Though aging is considered as a prominent risk factor for AD pathogenesis, substantial evidence suggests that an imbalance of essential biometal ions in the body and exposure to certain metal ions in the environment can potentially induce alterations to AD pathology. Despite their physiological importance in various intracellular processes, biometal ions, when present in excessive or deficient amounts, can serve as a mediating factor for neurotoxicity. Recent studies have also demonstrated the contribution of metal ions found in the environment on mediating AD pathogenesis. In this regard, the neuropathological features associated with biometal ion dyshomeostasis and environmental metal ion exposure have prompted widespread interest by multiple research groups. In this review, we discuss and elaborate on findings from previous studies detailing the possible role of both endogenous and exogenous metal ions specifically on amyloid and tau pathology in AD.

Biometal Dyshomeostasis and Toxic Metal Accumulations in the Development of Alzheimer's Disease

Biometal dyshomeostasis and toxic metal accumulation are common features in many neurodegenerative disorders, including Alzheimer’s disease (AD), Parkinson’s disease, and Huntington’s disease. The neurotoxic effects of metal imbalance are generally associated with reduced enzymatic activities, elevated protein aggregation and oxidative stress in the central nervous system, in which a cascade of events lead to cell death and neurodegeneration. Although the links between biometal imbalance and neurodegenerative disorders remain elusive, a major class of endogenous proteins involved in metal transport has been receiving increasing attention over recent decades. The abnormal expression of these proteins has been linked to biometal imbalance and to the pathogenesis of AD. Here, we present a brief overview of the physiological roles of biometals including iron, zinc, copper, manganese, magnesium and calcium, and provide a detailed description of their transporters and their synergistic involvement in the development of AD. In addition, we also review the published data relating to neurotoxic metals in AD, including aluminum, lead, cadmium, and mercury.

Amyloid β production is regulated by β2-adrenergic signaling-mediated post-translational modifications of the ryanodine receptor

Alteration of ryanodine receptor (RyR)-mediated calcium (Ca²⁺) signaling has been reported in Alzheimer disease (AD) models. However, the molecular mechanisms underlying altered RyR-mediated intracellular Ca²⁺ release in AD remain to be fully elucidated. We report here that RyR2 undergoes post-translational modifications (phosphorylation, oxidation, and nitrosylation) in SH-SY5Y neuroblastoma cells expressing the β-amyloid precursor protein (βAPP) harboring the familial double Swedish mutations (APPswe). RyR2 macromolecular complex remodeling, characterized by depletion of the regulatory protein calstabin2, resulted in increased cytosolic Ca²⁺ levels and mitochondrial oxidative stress. We also report a functional interplay between amyloid β (Aβ), β-adrenergic signaling, and altered Ca²⁺ signaling via leaky RyR2 channels. Thus, post-translational modifications of RyR occur downstream of Aβ through a β2-adrenergic signaling cascade that activates PKA. RyR2 remodeling in turn enhances βAPP processing. Importantly, pharmacological stabilization of the binding of calstabin2 to RyR2 channels, which prevents Ca²⁺ leakage, or blocking the β2-adrenergic signaling cascade reduced βAPP processing and the production of Aβ in APPswe-expressing SH-SY5Y cells. We conclude that targeting RyR-mediated Ca²⁺ leakage may be a therapeutic approach to treat AD.

Metals and Neuronal Metal Binding Proteins Implicated in Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent age-related dementia affecting millions of people worldwide. Its main pathological hallmark feature is the formation of insoluble protein deposits of amyloid-β and hyperphosphorylated tau protein into extracellular plaques and intracellular neurofibrillary tangles, respectively. Many of the mechanistic details of this process remain unknown, but a well-established consequence of protein aggregation is synapse dysfunction and neuronal loss in the AD brain. Different pathways including mitochondrial dysfunction, oxidative stress, inflammation, and metal metabolism have been suggested to be implicated in this process. In particular, a body of evidence suggests that neuronal metal ions such as copper, zinc, and iron play important roles in brain function in health and disease states and altered homeostasis and distribution as a common feature across different neurodegenerative diseases and aging. In this focused review, we overview neuronal proteins that are involved in AD and whose metal binding properties may underlie important biochemical and regulatory processes occurring in the brain during the AD pathophysiological process.

Identification of tetrahydrocarbazoles as novel multifactorial drug candidates for treatment of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative brain disorder and the most frequent cause of dementia. To date, there are only a few approved drugs for AD, which show little or no effect on disease progression. Impaired intracellular calcium homeostasis is believed to occur early in the cascade of events leading to AD. Here, we examined the possibility of normalizing the disrupted calcium homeostasis in the endoplasmic reticulum (ER) store as an innovative approach for AD drug discovery. High-throughput screening of a small-molecule compound library led to the identification of tetrahydrocarbazoles, a novel multifactorial class of compounds that can normalize the impaired ER calcium homeostasis. We found that the tetrahydrocarbazole lead structure, first, dampens the enhanced calcium release from ER in HEK293 cells expressing familial Alzheimer's disease (FAD)-linked presenilin 1 mutations. Second, the lead structure also improves mitochondrial function, measured by increased mitochondrial membrane potential. Third, the same lead structure also attenuates the production of amyloid-beta (Aβ) peptides by decreasing the cleavage of amyloid precursor protein (APP) by β-secretase, without notably affecting α- and γ-secretase cleavage activities. Considering the beneficial effects of tetrahydrocarbazoles addressing three key pathological aspects of AD, these compounds hold promise for the development of potentially effective AD drug candidates.

Development and implementation of a high-throughput compound screening assay for targeting disrupted ER calcium homeostasis in Alzheimer's disease

Disrupted intracellular calcium homeostasis is believed to occur early in the cascade of events leading to Alzheimer's disease (AD) pathology. Particularly familial AD mutations linked to Presenilins result in exaggerated agonist-evoked calcium release from endoplasmic reticulum (ER). Here we report the development of a fully automated high-throughput calcium imaging assay utilizing a genetically-encoded FRET-based calcium indicator at single cell resolution for compound screening. The established high-throughput screening assay offers several advantages over conventional high-throughput calcium imaging technologies. We employed this assay for drug discovery in AD by screening compound libraries consisting of over 20,000 small molecules followed by structure-activity-relationship analysis. This led to the identification of Bepridil, a calcium channel antagonist drug in addition to four further lead structures capable of normalizing the potentiated FAD-PS1-induced calcium release from ER. Interestingly, it has recently been reported that Bepridil can reduce Aβ production by lowering BACE1 activity. Indeed, we also detected lowered Aβ, increased sAPPα and decreased sAPPβ fragment levels upon Bepridil treatment. The latter findings suggest that Bepridil may provide a multifactorial therapeutic modality for AD by simultaneously addressing multiple aspects of the disease.

Risk of new-onset atrial fibrillation in elderly patients with the overlap syndrome: a retrospective cohort study

OBJECTIVE

Co-existence of obstructive sleep apnea (OSA) and chronic obstructive pulmonary disease (COPD) is referred to as overlap syndrome. Overlap patients have greater degree of hypoxia and pulmonary hypertension than patients with OSA or COPD alone. Studies showed that elderly patients with OSA alone do not have increased risk of atrial fibrillation (AF) but it is not known if overlap patients have higher risk of AF. To determine whether elderly patients with overlap syndrome have an increased risk of AF.

METHODS

In this single center, community-based retrospective cohort analysis, data were collected on 2,873 patients > 65 years of age without AF, presenting in the year 2006. Patients were divided into OSA group (n = 60), COPD group (n = 416), overlap syndrome group (n = 28) and group with no OSA or COPD (n = 2369). The primary endpoint was incidence of new-onset AF over the following two years. Logistic regression was performed to adjust for heart failure (HF), coronary artery disease, hypertension (HTN), cerebrovascular disease, cardiac valve disorders, diabetes mellitus, hyperlipidemia, chronic kidney disease (CKD) and obesity.

RESULTS

The incidence of AF was 10% in COPD group, 6% in OSA group and 21% in overlap syndrome group (P < 0.05). After adjusting for age, sex, HF, CKD, and HTN, patients with overlap syndrome demonstrated a significant association with new-onset AF (OR = 3.66, P = 0.007). HF, CKD and HTN were also significantly associated with new-onset AF (P < 0.05).

CONCLUSION

Among elderly patients, the presence of overlap syndrome is associated with a marked increase in risk of new-onset AF as compared to the presence of OSA or COPD alone.

Ryanodine receptor blockade reduces amyloid-β load and memory impairments in Tg2576 mouse model of Alzheimer disease

In Alzheimer disease (AD), the perturbation of the endoplasmic reticulum (ER) calcium (Ca²⁺) homeostasis has been linked to presenilins, the catalytic core in γ-secretase complexes cleaving the amyloid precursor protein (APP), thereby generating amyloid-β (Aβ) peptides. Here we investigate whether APP contributes to ER Ca²⁺ homeostasis and whether ER Ca²⁺ could in turn influence Aβ production. We show that overexpression of wild-type human APP (APP(695)), or APP harboring the Swedish double mutation (APP(swe)) triggers increased ryanodine receptor (RyR) expression and enhances RyR-mediated ER Ca²⁺ release in SH-SY5Y neuroblastoma cells and in APP(swe)-expressing (Tg2576) mice. Interestingly, dantrolene-induced lowering of RyR-mediated Ca²⁺ release leads to the reduction of both intracellular and extracellular Aβ load in neuroblastoma cells as well as in primary cultured neurons derived from Tg2576 mice. This Aβ reduction can be accounted for by decreased Thr-668-dependent APP phosphorylation and β- and γ-secretases activities. Importantly, dantrolene diminishes Aβ load, reduces Aβ-related histological lesions, and slows down learning and memory deficits in Tg2576 mice. Overall, our data document a key role of RyR in Aβ production and learning and memory performances, and delineate RyR-mediated control of Ca²⁺ homeostasis as a physiological paradigm that could be targeted for innovative therapeutic approaches.

Augmented generation of protein fragments during wakefulness as the molecular cause of sleep: a hypothesis

Despite extensive understanding of sleep regulation, the molecular-level cause and function of sleep are unknown. I suggest that they originate in individual neurons and stem from increased production of protein fragments during wakefulness. These fragments are transient parts of protein complexes in which the fragments were generated. Neuronal Ca²⁺ fluxes are higher during wakefulness than during sleep. Subunits of transmembrane channels and other proteins are cleaved by Ca²⁺-activated calpains and by other nonprocessive proteases, including caspases and secretases. In the proposed concept, termed the fragment generation (FG) hypothesis, sleep is a state during which the production of fragments is decreased (owing to lower Ca²⁺ transients) while fragment-destroying pathways are upregulated. These changes facilitate the elimination of fragments and the remodeling of protein complexes in which the fragments resided. The FG hypothesis posits that a proteolytic cleavage, which produces two fragments, can have both deleterious effects and fitness-increasing functions. This (previously not considered) dichotomy can explain both the conservation of cleavage sites in proteins and the evolutionary persistence of sleep, because sleep would counteract deleterious aspects of protein fragments. The FG hypothesis leads to new explanations of sleep phenomena, including a longer sleep after sleep deprivation. Studies in the 1970s showed that ethanol-induced sleep in mice can be strikingly prolonged by intracerebroventricular injections of either Ca²⁺ alone or Ca²⁺ and its ionophore (Erickson et al., Science 1978;199:1219-1221; Harris, Pharmacol Biochem Behav 1979;10:527-534; Erickson et al., Pharmacol Biochem Behav 1980;12:651-656). These results, which were never interpreted in connection to protein fragments or the function of sleep, may be accounted for by the FG hypothesis about molecular causation of sleep.

New therapeutic targets in Alzheimer's disease: brain deregulation of calcium and zinc

The molecular determinants of Alzheimer's (AD) disease are still not completely known; however, in the past two decades, a large body of evidence has indicated that an important contributing factor for the disease is the development of an unbalanced homeostasis of two signaling cations: calcium (Ca²⁺) and zinc (Zn²⁺). Both ions serve a critical role in the physiological functioning of the central nervous system, but their brain deregulation promotes amyloid-β dysmetabolism as well as tau phosphorylation. AD is also characterized by an altered glutamatergic activation, and glutamate can promote both Ca²⁺ and Zn²⁺ dyshomeostasis. The two cations can operate synergistically to promote the generation of free radicals that further intracellular Ca²⁺ and Zn²⁺ rises and set the stage for a self-perpetuating harmful loop. These phenomena can be the initial steps in the pathogenic cascade leading to AD, therefore, therapeutic interventions aiming at preventing Ca²⁺ and Zn²⁺ dyshomeostasis may offer a great opportunity for disease-modifying strategies.