Glutamate induces autophagy via the two-pore channels in neural cells

Hspb1 and Lgals3 in spinal neurons are closely associated with autophagy following excitotoxicity based on machine learning algorithms

Excitotoxicity represents the primary cause of neuronal death following spinal cord injury (SCI). While autophagy plays a critical and intricate role in SCI, the specific mechanism underlying the relationship between excitotoxicity and autophagy in SCI has been largely overlooked. In this study, we isolated primary spinal cord neurons from neonatal rats and induced excitotoxic neuronal injury by high concentrations of glutamic acid, mimicking an excitotoxic injury model. Subsequently, we performed transcriptome sequencing. Leveraging machine learning algorithms, including weighted correlation network analysis (WGCNA), random forest analysis (RF), and least absolute shrinkage and selection operator analysis (LASSO), we conducted a comprehensive investigation into key genes associated with spinal cord neuron injury. We also utilized protein-protein interaction network (PPI) analysis to identify pivotal proteins regulating key gene expression and analyzed key genes from public datasets (GSE2599, GSE20907, GSE45006, and GSE174549). Our findings revealed that six genes-Anxa2, S100a10, Ccng1, Timp1, Hspb1, and Lgals3-were significantly upregulated not only in vitro in neurons subjected to excitotoxic injury but also in rats with subacute SCI. Furthermore, Hspb1 and Lgals3 were closely linked to neuronal autophagy induced by excitotoxicity. Our findings contribute to a better understanding of excitotoxicity and autophagy, offering potential targets and a theoretical foundation for SCI diagnosis and treatment.

The Key Role of Astrocytes in Amyotrophic Lateral Sclerosis and Their Commitment to Glutamate Excitotoxicity

In the last two decades, there has been increasing evidence supporting non-neuronal cells as active contributors to neurodegenerative disorders. Among glial cells, astrocytes play a pivotal role in driving amyotrophic lateral sclerosis (ALS) progression, leading the scientific community to focus on the "astrocytic signature" in ALS. Here, we summarized the main pathological mechanisms characterizing astrocyte contribution to MN damage and ALS progression, such as neuroinflammation, mitochondrial dysfunction, oxidative stress, energy metabolism impairment, miRNAs and extracellular vesicles contribution, autophagy dysfunction, protein misfolding, and altered neurotrophic factor release. Since glutamate excitotoxicity is one of the most relevant ALS features, we focused on the specific contribution of ALS astrocytes in this aspect, highlighting the known or potential molecular mechanisms by which astrocytes participate in increasing the extracellular glutamate level in ALS and, conversely, undergo the toxic effect of the excessive glutamate. In this scenario, astrocytes can behave as "producers" and "targets" of the high extracellular glutamate levels, going through changes that can affect themselves and, in turn, the neuronal and non-neuronal surrounding cells, thus actively impacting the ALS course. Moreover, this review aims to point out knowledge gaps that deserve further investigation.

NAADP-Evoked Ca2+ Signaling Leads to Mutant Huntingtin Aggregation and Autophagy Impairment in Murine Astrocytes

Huntington’s disease (HD) is a progressive neurodegenerative disease characterized by mutations in the huntingtin gene (mHtt), causing an unstable repeat of the CAG trinucleotide, leading to abnormal long repeats of polyglutamine (poly-Q) in the N-terminal region of the huntingtin, which form abnormal conformations and aggregates. Alterations in Ca2+ signaling are involved in HD models and the accumulation of mutated huntingtin interferes with Ca2+ homeostasis. Lysosomes are intracellular Ca2+ storages that participate in endocytic and lysosomal degradation processes, including autophagy. Nicotinic acid adenine dinucleotide phosphate (NAADP) is an intracellular second messenger that promotes Ca2+ release from the endo-lysosomal system via Two-Pore Channels (TPCs) activation. Herein, we show the impact of lysosomal Ca2+ signals on mHtt aggregation and autophagy blockade in murine astrocytes overexpressing mHtt-Q74. We observed that mHtt-Q74 overexpression causes an increase in NAADP-evoked Ca2+ signals and mHtt aggregation, which was inhibited in the presence of Ned-19, a TPC antagonist, or BAPTA-AM, a Ca2+ chelator. Additionally, TPC2 silencing revert the mHtt aggregation. Furthermore, mHtt has been shown co-localized with TPC2 which may contribute to its effects on lysosomal homeostasis. Moreover, NAADP-mediated autophagy was also blocked since its function is dependent on lysosomal functionality. Taken together, our data show that increased levels of cytosolic Ca2+ mediated by NAADP causes mHtt aggregation. Additionally, mHtt co-localizes with the lysosomes, where it possibly affects organelle functions and impairs autophagy.

Pharmacological inhibition of lysosomal two-pore channel 2 (TPC2) confers neuroprotection in stroke via autophagy regulation

Lysosomal function and organellar Ca²⁺ homeostasis become dysfunctional in Stroke causing disturbances in autophagy, the major process for the degradation of abnormal protein aggregates and dysfunctional organelles. However, the role of autophagy in Stroke is controversial since excessive or prolonged autophagy activation exacerbates ischemic brain injury. Of note, glutamate evokes NAADP-dependent Ca²⁺ release via lysosomal TPC2 channels thus controlling basal autophagy. Considering the massive release of excitotoxins in Stroke, autophagic flux becomes uncontrolled with abnormal formation of autophagosomes causing, in turn, disruption of excitotoxins clearance and neurodegeneration. Here, a fine regulation of autophagy via a proper pharmacological modulation of lysosomal TPC2 channel has been tested in preclinical Stroke models. Primary cortical neurons were subjected to oxygen and glucose deprivation+reoxygenation to reproduce in vitro brain ischemia. Focal brain ischemia was induced in rats by transient middle cerebral artery occlusion (tMCAO). Under these conditions, TPC2 protein expression as well as autophagy and endoplasmic reticulum (ER) stress markers were studied by Western blotting, while TPC2 localization and activity were measured by immunocytochemistry and single-cell video-imaging, respectively. TPC2 protein expression and immunosignal were highly modulated in primary cortical neurons exposed to extreme hypoxic conditions causing dysfunction in organellar Ca²⁺ homeostasis, ER stress and autophagy-induced cell death. TPC2 knocking down and pharmacological inhibition by Ned-19 during hypoxia induced neuroprotection. The effect of Ned-19 was reversed by the permeable form of TPC2 endogenous agonist, NAADP-AM. Of note, Ned-19 prevented ER stress, as measured by GRP78 (78 kDa glucose-regulated protein) protein reduction and caspase 9 downregulation. In this way Ned-19 restored organellar Ca²⁺ level. Interestingly, Ned-19 reduced the infarct volume and neurological deficits in rats subjected to tMCAO and prevented hypoxia-induced cell death by blocking autophagic flux. Collectively, the pharmacological inhibition of TPC2 lysosomal channel by Ned-19 protects from focal ischemia by hampering a hyperfunctional autophagy.

NAADP-Mediated Ca2+ Signalling

The discovery of NAADP-evoked Ca²⁺ release in sea urchin eggs and then as a ubiquitous Ca²⁺ mobilizing messenger has introduced several novel paradigms to our understanding of Ca²⁺ signalling, not least in providing a link between cell stimulation and Ca²⁺ release from lysosomes and other acidic Ca²⁺ storage organelles. In addition, the hallmark concentration-response relationship of NAADP-mediated Ca²⁺ release, shaped by striking activation/desensitization mechanisms, influences its actions as an intracellular messenger. There has been recent progress in our understanding of the molecular mechanisms underlying NAADP-evoked Ca²⁺ release, such as the identification of the endo-lysosomal two-pore channel family of cation channels (TPCs) as their principal target and the identity of NAADP-binding proteins that complex with them. The NAADP/TPC signalling axis has gained recent prominence in pathophysiology for their roles in such disease processes as neurodegeneration, tumorigenesis and cellular viral entry.

Targeting autophagy process in center nervous trauma

The central nervous system (CNS) is the primary regulator of physiological activity, and when CNS is compromised, its physical functions are affected. Spinal cord injury (SCI) and traumatic brain injury (TBI) are common trauma in CNS that are difficult to recover from, with a higher global disability and mortality rate. Autophagy is familiar to almost all researchers due to its role in regulating the degradation and recycling of cellular defective or incorrect proteins and toxic components, maintaining body balance and regulating cell health and function. Emerging evidence suggests it has a broad and long-lasting impact on pathophysiological process such as oxidative stress, inflammation, apoptosis, and angiogenesis, involving the alteration of autophagy marker expression and function recovery. Changes in autophagy level are considered a potential therapeutic strategy and have shown promising results in preclinical studies for neuroprotection following traumatic brain injury. However, the relationship between upward or downward autophagy and functional recovery following SCI or TBI is debatable. This article reviews the regulation and role of autophagy in repairing CNS trauma and the intervention effects of autophagy-targeted therapeutic agents to find more and better treatment options for SCI and TBI patients.

Two-pore channels: going with the flows

In recent years, our understanding of the structure, mechanisms and functions of the endo-lysosomal TPC (two-pore channel) family have grown apace. Gated by the second messengers, NAADP and PI(3,5)P₂, TPCs are an integral part of fundamental signal-transduction pathways, but their array and plasticity of cation conductances (Na⁺, Ca²⁺, H⁺) allow them to variously signal electrically, osmotically or chemically. Their relative tissue- and organelle-selective distribution, together with agonist-selective ion permeabilities provides a rich palette from which extracellular stimuli can choose. TPCs are emerging as mediators of immunity, cancer, metabolism, viral infectivity and neurodegeneration as this short review attests.

TPC2 rescues lysosomal storage in mucolipidosis type IV, Niemann-Pick type C1, and Batten disease

Lysosomes are cell organelles that degrade macromolecules to recycle their components. If lysosomal degradative function is impaired, e.g., due to mutations in lysosomal enzymes or membrane proteins, lysosomal storage diseases (LSDs) can develop. LSDs manifest often with neurodegenerative symptoms, typically starting in early childhood, and going along with a strongly reduced life expectancy and quality of life. We show here that small molecule activation of the Ca²⁺‐permeable endolysosomal two‐pore channel 2 (TPC2) results in an amelioration of cellular phenotypes associated with LSDs such as cholesterol or lipofuscin accumulation, or the formation of abnormal vacuoles seen by electron microscopy. Rescue effects by TPC2 activation, which promotes lysosomal exocytosis and autophagy, were assessed in mucolipidosis type IV (MLIV), Niemann–Pick type C1, and Batten disease patient fibroblasts, and in neurons derived from newly generated isogenic human iPSC models for MLIV and Batten disease. For in vivo proof of concept, we tested TPC2 activation in the MLIV mouse model. In sum, our data suggest that TPC2 is a promising target for the treatment of different types of LSDs, both in vitro and in‐vivo.

Extracellular Adenosine Triphosphate Binding to P2Y1 Receptors Prevents Glutamate-Induced Excitotoxicity: Involvement of Erk1/2 Signaling Pathway to Suppress Autophagy

Glutamate-induced neuroexcitotoxicity could be related to the pathophysiology of some neurodegenerative diseases including Parkinson’s disease and Alzheimer’s disease. Extracellular ATP exerts a wide variety of functions, such as attenuating Aβ-mediated toxicity, inhibiting N-Methyl-D-Aspartate (NMDA) receptor subunit combinations, and aggravating ischemic brain injury. However, the effect of extracellular ATP on glutamate-induced neuroexcitotoxicity remains largely unknown. Herein, we showed that extracellular ATP prevented the glutamate-induced excitotoxicity via binding to its P2Y1 receptors. We found that excessive glutamate triggered cellular reactive oxygen species (ROS) overproduction and mitochondrial membrane potential damage, which were significantly attenuated by extracellular ATP. Besides, glutamate activated autophagy, as illustrated by the increased protein level of autophagic marker LC3II and decreased level of p62, and glutamate-induced neuroexcitotoxicity could be completely abolished by autophagy inhibitor chloroquine. In addition, we revealed that extracellular ATP activated Erk1/2 signaling to suppress autophagy and to exert its neuroprotective effects, which was further reduced by autophagy agonist rapamycin and the selective Erk1/2 inhibitor PD0325901. Taken together, our findings suggest that extracellular ATP binding to P2Y1 receptors protected against glutamate-induced excitotoxicity via Erk1/2-mediated autophagy inhibition, implying the potential of ATP for the treatment of neurodegenerative disorders.

Regulation of Aging and Longevity by Ion Channels and Transporters

Despite significant advances in our understanding of the mechanisms that underlie age-related physiological decline, our ability to translate these insights into actionable strategies to extend human healthspan has been limited. One of the major reasons for the existence of this barrier is that with a few important exceptions, many of the proteins that mediate aging have proven to be undruggable. The argument put forth here is that the amenability of ion channels and transporters to pharmacological manipulation could be leveraged to develop novel therapeutic strategies to combat aging. This review delves into the established roles for ion channels and transporters in the regulation of aging and longevity via their influence on membrane excitability, Ca2+ homeostasis, mitochondrial and endolysosomal function, and the transduction of sensory stimuli. The goal is to provide the reader with an understanding of emergent themes, and prompt further investigation into how the activities of ion channels and transporters sculpt the trajectories of cellular and organismal aging.

Targeting the two-pore channel 2 in cancer progression and metastasis

The importance of Ca²⁺ signaling, and particularly Ca²⁺ channels, in key events of cancer cell function such as proliferation, metastasis, autophagy and angiogenesis, has recently begun to be appreciated. Of particular note are two-pore channels (TPCs), a group of recently identified Ca²⁺-channels, located within the endolysosomal system. TPC2 has recently emerged as an intracellular ion channel of significant pathophysiological relevance, specifically in cancer, and interest in its role as an anti-cancer drug target has begun to be explored. Herein, an overview of the cancer-related functions of TPC2 and a discussion of its potential as a target for therapeutic intervention, including a summary of clinical trials examining the TPC2 inhibitors, naringenin, tetrandrine, and verapamil for the treatment of various cancers is provided.

Autophagy in cancer: The cornerstone during glutamine deprivation

Over the past two decades, researchers have revealed the crucial functions of glutamine in supporting the hyperproliferation state of cancer cells. Glutamine acts on maintaining high energy production, supporting redox status and amino acid homeostasis. Therefore, cancer cells exhibit excessive uptake of the extracellular glutamine, synthesize it in some cases, and recycle intracellular and extracellular proteins to provide an additional source of glutamine to satisfy the increasing glutamine demand. On the other hand, autophagy's role is still debated regarding tumor initiation and progression. However, most cancer cells urgently need autophagy to overcome the existential threats during glutamine restriction stress. Downstream to various stress pathways induced during such a condition, autophagy is considered an indispensable cytoprotective tool to maintain cell integrity and survival. However, the overactivation of the autophagy process is related to lethal consequences. This review summarized glutamine pathways to control autophagy and highlighted autophagy's primary activation pathways, and discussed the roles during glutamine deprivation.

Autophagy: a multifaceted player in the fate of sperm

BACKGROUND

Autophagy is an intracellular catabolic process of degrading and recycling proteins and organelles to modulate various physiological and pathological events, including cell differentiation and development. Emerging data indicate that autophagy is closely associated with male reproduction, especially the biosynthetic and catabolic processes of sperm. Throughout the fate of sperm, a series of highly specialized cellular events occur, involving pre-testicular, testicular and post-testicular events. Nonetheless, the most fundamental question of whether autophagy plays a protective or harmful role in male reproduction, especially in sperm, remains unclear.

OBJECTIVE AND RATIONALE

We summarize the functional roles of autophagy in the pre-testicular (hypothalamic–pituitary–testis (HPG) axis), testicular (spermatocytogenesis, spermatidogenesis, spermiogenesis, spermiation) and post-testicular (sperm maturation and fertilization) processes according to the timeline of sperm fate. Additionally, critical mechanisms of the action and clinical impacts of autophagy on sperm are identified, laying the foundation for the treatment of male infertility.

SEARCH METHODS

In this narrative review, the PubMed database was used to search peer-reviewed publications for summarizing the functional roles of autophagy in the fate of sperm using the following terms: ‘autophagy’, ‘sperm’, ‘hypothalamic–pituitary–testis axis’, ‘spermatogenesis’, ‘spermatocytogenesis’, ‘spermatidogenesis’, ‘spermiogenesis’, ‘spermiation’, ‘sperm maturation’, ‘fertilization’, ‘capacitation’ and ‘acrosome’ in combination with autophagy-related proteins. We also performed a bibliographic search for the clinical impact of the autophagy process using the keywords of autophagy inhibitors such as ‘bafilomycin A1’, ‘chloroquine’, ‘hydroxychloroquine’, ‘3-Methyl Adenine (3-MA)’, ‘lucanthone’, ‘wortmannin’ and autophagy activators such as ‘rapamycin’, ‘perifosine’, ‘metformin’ in combination with ‘disease’, ‘treatment’, ‘therapy’, ‘male infertility’ and equivalent terms. In addition, reference lists of primary and review articles were reviewed for additional relevant publications. All relevant publications until August 2021 were critically evaluated and discussed on the basis of relevance, quality and timelines.

OUTCOMES

(i) In pre-testicular processes, autophagy-related genes are involved in the regulation of the HPG axis; and (ii) in testicular processes, mTORC1, the main gate to autophagy, is crucial for spermatogonia stem cell (SCCs) proliferation, differentiation, meiotic progression, inactivation of sex chromosomes and spermiogenesis. During spermatidogenesis, autophagy maintains haploid round spermatid chromatoid body homeostasis for differentiation. During spermiogenesis, autophagy participates in acrosome biogenesis, flagella assembly, head shaping and the removal of cytoplasm from elongating spermatid. After spermatogenesis, through PDLIM1, autophagy orchestrates apical ectoplasmic specialization and basal ectoplasmic specialization to handle cytoskeleton assembly, governing spermatid movement and release during spermiation. In post-testicular processes, there is no direct evidence that autophagy participates in the process of capacitation. However, autophagy modulates the acrosome reaction, paternal mitochondria elimination and clearance of membranous organelles during fertilization.

WIDER IMPLICATIONS

Deciphering the roles of autophagy in the entire fate of sperm will provide valuable insights into therapies for diseases, especially male infertility.

Ion Channels and Pumps in Autophagy: A Reciprocal Relationship

Autophagy, the process of cellular self-degradation, is intrinsically tied to the degradative function of the lysosome. Several diseases have been linked to lysosomal degradative defects, including rare lysosomal storage disorders and neurodegenerative diseases. Ion channels and pumps play a major regulatory role in autophagy. Importantly, calcium signaling produced by TRPML1 (transient receptor potential cation channel, mucolipin subfamily) has been shown to regulate autophagic progression through biogenesis of autophagic-lysosomal organelles, activation of mTORC1 (mechanistic target of rapamycin complex 1) and degradation of autophagic cargo. ER calcium channels such as IP₃Rs supply calcium for the lysosome, and lysosomal function is severely disrupted in the absence of lysosomal calcium replenishment by the ER. TRPML1 function is also regulated by LC3 (microtubule-associated protein light chain 3) and mTORC1, two critical components of the autophagic network. Here we provide an overview of the current knowledge about ion channels and pumps-including lysosomal V-ATPase (vacuolar proton-ATPase), which is required for acidification and hence proper enzymatic activity of lysosomal hydrolases-in the regulation of autophagy, and discuss how functional impairment of some of these leads to diseases.

NMDA receptors elicit flux-independent intracellular Ca2+ signals via metabotropic glutamate receptors and flux-dependent nitric oxide release in human brain microvascular endothelial cells

The excitatory neurotransmitter glutamate gates post-synaptic N-methyl-d-aspartate (NMDA) receptors (NMDARs) to mediate extracellular Ca²⁺ entry and stimulate neuronal nitric oxide (NO) synthase to release NO and trigger neurovascular coupling (NVC). Neuronal and glial NMDARs may also operate in a flux-independent manner, although it is unclear whether their non-ionotropic mode of action is involved in NVC. Recently, endothelial NMDARs were found to trigger Ca²⁺-dependent NO production and induce NVC, but the underlying mode of signaling remains elusive. Herein, we report that GluN1 protein, as well as GluN2C and GluN3B transcripts and proteins, were expressed and that NMDA did not elicit inward currents, but induced a dose-dependent increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the human brain microvascular endothelial cell line, hCMEC/D3. A multidisciplinary approach, including live cell imaging, whole-cell patch-clamp recordings, pharmacological manipulation and gene targeting, revealed that NMDARs increase the [Ca²⁺]_i in a flux-independent manner in hCMEC/D3 cells. The Ca²⁺ response to NMDA was triggered by endogenous Ca²⁺ release from the endoplasmic reticulum and the lysosomal Ca²⁺ stores and sustained by store-operated Ca²⁺ entry. Unexpectedly, pharmacological and genetic blockade of mGluR1 and mGluR5 dramatically impaired NMDARs-mediated Ca²⁺ signals. These findings indicate that NMDARs may increase the endothelial [Ca²⁺]_i in a flux-independent manner via group 1 mGluRs. However, imaging of DAF-FM fluorescence revealed that NMDARs may also induce Ca²⁺-dependent NO release by signaling in a flux-dependent manner. These findings, therefore, shed novel light on the mechanisms whereby brain microvascular endothelium decodes glutamatergic signaling and regulates NVC.

Bergamot rehabilitation AgaINst agitation in dementia (BRAINAID): Study protocol for a randomized, double-blind, placebo-controlled trial to assess the efficacy of furocoumarin-free bergamot loaded in a nanotechnology-based delivery system of the essential oil in the treatment of agitation in elderly affected by severe dementia

Pain is underdiagnosed and often not adequately treated, contributing to behavioral and psychological symptoms of dementia (BPSD). BPSD are treated with atypical antipsychotics that are associated with severe cerebrocardiovascular effects. Interestingly, treatment of pain may reduce agitation. Research is focusing on nonpharmacological treatment, such as aromatherapy, for pain and BPSD in dementia. This clinical study will assess the effect on agitation in severely demented elderly of BEO loaded in a nanotechnological odorless cream indistinguishable from placebo. This is a protocol for a randomized, double‐blind, placebo‐controlled trial (NCT04321889). A total of 134 patients aged ≥65 years with severe dementia (mini‐mental state examination <12) will be recruited and randomly allocated 1:1 to either BEO or placebo group. After baseline screening, BEO (80 mg) cream or placebo cream will be trans‐dermally applied on both arms twice a day for 4 weeks with a 4‐week follow‐up period. The effect on agitation will be the primary endpoint. Any adverse events will be reported. A double‐blind, clinical trial evaluating efficacy and safety of an essential oil endowed with strong analgesic properties has never been carried out before. This study could form the basis for a safer and more effective treatment of BPSD in severe dementia.

Mito-Tempo suppresses autophagic flux via the PI3K/Akt/mTOR signaling pathway in neuroblastoma SH-SY5Y cells

The generation of excessive mitochondrial reactive oxygen species (mtROS) is associated with glutamate-stimulated neurotoxicity and pathogenesis of Alzheimer's disease (AD). Impaired mitochondrial function is accompanied with oxidative stress that is a significant contributor to initiate autophagy, but the underlying mechanisms are not fully understood. The present study aimed to investigate the neuroprotective effects of Mito-Tempo on glutamate-induced neuroblastoma SH-SY5Y cell toxicity. SH-SY5Y cells were treated with 100 μM glutamate in the presence or absence of 50 and 100 μM Mito-Tempo for 24 h. Changes in cell viability were measured by MTT assay. Cytotoxicity and intracellular ROS accumulation were also evaluated using lactate dehydrogenase (LDH) activity assay and 2,7-dichlorofluorescein diacetate (DCFDA) Reactive Oxygen Species Assay kit, respectively. Mitochondrial membrane potential was analyzed by tetraethylbenzimidazoly-lcarbocyanine iodide (JC-1) staining. Expression of PI3K/AKT/mTOR pathway and autophagy markers, including LC3 (LC3-I/-II) and p62 (SQSTM1) were performed using Western blot analysis. Our results demonstrated that glutamate-exposed cells significantly increased cellular oxidative stress by enhancing ROS production. Glutamate treatment also increased LDH release follows the loss of mitochondrial membrane potential, caused cell viability loss. Treatment with Mito-Tempo not only attenuated the generation of ROS and improved mitochondrial membrane potential but also reduced the neurotoxicity of glutamate in a concentration-dependent manner, which leads to increased cell viability and decreased LDH release. Mito-Tempo has a greater protective effect by enhancing superoxide dismutase (SOD) activity and PI3K/AKT/mTOR phosphorylation. Moreover, Mito-Tempo treatment altered the autophagy process resulting in the decline in the ratio of the autophagy markers LC3-I/-II and p62 (SQSTM1). We propose that Mito-Tempo can improve neuronal properties against glutamate cytotoxicity through its direct free radical scavenging activity and inhibit excessive autophagy signaling pathway, therefore, allow for further studies to investigate the therapeutic potentials of Mito-Tempo in animal disease models and human.

Lysosomal calcium and autophagy

Lysosomal calcium is emerging as a modulator of autophagy and lysosomal compartment, an obligatory partner to complete the autophagic pathway. A variety of specific signals such as nutrient deprivation or oxidative stress can trigger lysosomal calcium-mediated nuclear translocation of the transcription factor EB (TFEB), a master regulator of global lysosomal function. Also, lysosomal calcium can promote the formation of autophagosome vesicles (AVs) by a mechanism that requires the production of the phosphoinositide PI3P by the VPS34 autophagic complex and the activation of the energy-sensing kinase AMPK. Additionally, lysosomal calcium plays a role in membrane fusion and fission events involved in cellular processes such as endocytic maturation, autophagosome-lysosome fusion, lysosomal exocytosis, and lysosomal reformation upon autophagy completion. Lysosomal calcium-dependent functions are defective in cellular and animal models of the non-selective cation channel TRPML1, whose mutations in humans cause the neurodegenerative lysosomal storage disease mucolipidosis type IV (MLIV). Lysosomal calcium is not only acting as a positive regulator of autophagy, but it is also responsible for turning-off this process through the reactivation of the mTOR kinase during prolonged starvation. More recently, it has been described the role of lysosomal calcium on an elegant sequence of intracellular signaling events such as membrane repair, lysophagy, and lysosomal biogenesis upon the induction of different grades of lysosomal membrane damage. Here, we will discuss these novel findings that re-define the importance of the lysosome and lysosomal calcium signaling at regulating cellular metabolism.

Pharmacological Modulators of Autophagy as a Potential Strategy for the Treatment of COVID-19

The family of coronaviruses (CoVs) uses the autophagy machinery of host cells to promote their growth and replication; thus, this process stands out as a potential target to combat COVID-19. Considering the different roles of autophagy during viral infection, including SARS-CoV-2 infection, in this review, we discuss several clinically used drugs that have effects at different stages of autophagy. Among them, we mention (1) lysosomotropic agents, which can prevent CoVs infection by alkalinizing the acid pH in the endolysosomal system, such as chloroquine and hydroxychloroquine, azithromycin, artemisinins, two-pore channel modulators and imatinib; (2) protease inhibitors that can inhibit the proteolytic cleavage of the spike CoVs protein, which is necessary for viral entry into host cells, such as camostat mesylate, lopinavir, umifenovir and teicoplanin and (3) modulators of PI3K/AKT/mTOR signaling pathways, such as rapamycin, heparin, glucocorticoids, angiotensin-converting enzyme inhibitors (IECAs) and cannabidiol. Thus, this review aims to highlight and discuss autophagy-related drugs for COVID-19, from in vitro to in vivo studies. We identified specific compounds that may modulate autophagy and exhibit antiviral properties. We hope that research initiatives and efforts will identify novel or "off-label" drugs that can be used to effectively treat patients infected with SARS-CoV-2, reducing the risk of mortality.

Cannabidiol induces autophagy via ERK1/2 activation in neural cells

Autophagy is a lysosomal catabolic process essential to cell homeostasis and is related to the neuroprotection of the central nervous system. Cannabidiol (CBD) is a non-psychotropic phytocannabinoid present in Cannabis sativa. Many therapeutic actions have been linked to this compound, including autophagy activation. However, the precise underlying molecular mechanisms remain unclear, and the downstream functional significance of these actions has yet to be determined. Here, we investigated CBD-evoked effects on autophagy in human neuroblastoma SH-SY5Y and murine astrocyte cell lines. We found that CBD-induced autophagy was substantially reduced in the presence of CB1, CB2 and TRPV1 receptor antagonists, AM 251, AM 630 and capsazepine, respectively. This result strongly indicates that the activation of these receptors mediates the autophagic flux. Additionally, we demonstrated that CBD activates autophagy through ERK1/2 activation and AKT suppression. Interestingly, CBD-mediated autophagy activation is dependent on the autophagy initiator ULK1, but mTORC1 independent. Thus, it is plausible that a non-canonical pathway is involved. Our findings collectively provide evidence that CBD stimulates autophagy signal transduction via crosstalk between the ERK1/2 and AKT kinases, which represent putative regulators of cell proliferation and survival. Furthermore, our study sheds light on potential therapeutic cannabinoid targets that could be developed for treating neurodegenerative disorders.

Lack of Autophagy Induction by Lithium Decreases Neuroprotective Effects in the Striatum of Aged Rats

The pharmacological modulation of autophagy is considered a promising neuroprotective strategy. While it has been postulated that lithium regulates this cellular process, the age-related effects have not been fully elucidated. Here, we evaluated lithium-mediated neuroprotective effects in young and aged striatum. After determining the optimal experimental conditions for inducing autophagy in loco with lithium carbonate (Li₂CO₃), we measured cell viability, reactive oxygen species (ROS) generation and oxygen consumption with rat brain striatal slices from young and aged animals. In the young striatum, Li₂CO₃ increased tissue viability and decreased ROS generation. These positive effects were accompanied by enhanced levels of LC3-II, LAMP 1, Ambra 1 and Beclin-1 expression. In the aged striatum, Li₂CO₃ reduced the autophagic flux and increased the basal oxygen consumption rate. Ultrastructural changes in the striatum of aged rats that consumed Li₂CO₃ for 30 days included electrondense mitochondria with disarranged cristae and reduced normal mitochondria and lysosomes area. Our data show that the striatum from younger animals benefits from lithium-mediated neuroprotection, while the striatum of older rats does not. These findings should be considered when developing neuroprotective strategies involving the induction of autophagy in aging.

Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)1

In 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field.

Two-pore and TRPML cation channels: Regulators of phagocytosis, autophagy and lysosomal exocytosis

The old Greek saying "Panta Rhei" ("everything flows") is true for all life and all living things in general. It also becomes nicely evident when looking closely into cells. There, material from the extracellular space is taken up by endocytic processes and transported to endosomes where it is sorted either for recycling or degradation. Cargo is also packaged for export through exocytosis involving the Golgi network, lysosomes and other organelles. Everything in this system is in constant motion and many proteins are necessary to coordinate transport along the different intracellular pathways to avoid chaos. Among these proteins are ion channels., in particular TRPML channels (mucolipins) and two-pore channels (TPCs) which reside on endosomal and lysosomal membranes to speed up movement between organelles, e.g. by regulating fusion and fission; they help readjust pH and osmolarity changes due to such processes, or they promote exocytosis of export material. Pathophysiologically, these channels are involved in neurodegenerative, metabolic, retinal and infectious diseases, cancer, pigmentation defects, and immune cell function, and thus have been proposed as novel pharmacological targets, e.g. for the treatment of lysosomal storage disorders, Duchenne muscular dystrophy, or different types of cancer. Here, we discuss the similarities but also differences of TPCs and TRPMLs in regulating phagocytosis, autophagy and lysosomal exocytosis, and we address the contradictions and open questions in the field relating to the roles TPCs and TRPMLs play in these different processes.

Roles of NAD+ and Its Metabolites Regulated Calcium Channels in Cancer

Nicotinamide adenine dinucleotide (NAD⁺) is an essential cofactor for redox enzymes, but also moonlights as a regulator for ion channels, the same as its metabolites. Ca²⁺ homeostasis is dysregulated in cancer cells and affects processes such as tumorigenesis, angiogenesis, autophagy, progression, and metastasis. Herein, we summarize the regulation of the most common calcium channels (TRPM2, TPCs, RyRs, and TRPML1) by NAD⁺ and its metabolites, with a particular focus on their roles in cancers. Although the mechanisms of NAD⁺ metabolites in these pathological processes are yet to be clearly elucidated, these ion channels are emerging as potential candidates of alternative targets for anticancer therapy.

Transcriptional block of AMPK-induced autophagy promotes glutamate excitotoxicity in nutrient-deprived SH-SY5Y neuroblastoma cells

We investigated the role of autophagy, a controlled lysosomal degradation of cellular macromolecules and organelles, in glutamate excitotoxicity during nutrient deprivation in vitro. The incubation in low-glucose serum/amino acid-free cell culture medium synergized with glutamate in increasing AMP/ATP ratio and causing excitotoxic necrosis in SH-SY5Y human neuroblastoma cells. Glutamate suppressed starvation-triggered autophagy, as confirmed by diminished intracellular acidification, lower LC3 punctuation and LC3-I conversion to autophagosome-associated LC3-II, reduced expression of proautophagic beclin-1 and ATG5, increase of the selective autophagic target NBR1, and decreased number of autophagic vesicles. Similar results were observed in PC12 rat pheochromocytoma cells. Both glutamate-mediated excitotoxicity and autophagy inhibition in starved SH-SY5Y cells were reverted by NMDA antagonist memantine and mimicked by NMDA agonists D-aspartate and ibotenate. Glutamate reduced starvation-triggered phosphorylation of the energy sensor AMP-activated protein kinase (AMPK) without affecting the activity of mammalian target of rapamycin complex 1, a major negative regulator of autophagy. This was associated with reduced mRNA levels of autophagy transcriptional activators (FOXO3, ATF4) and molecules involved in autophagy initiation (ULK1, ATG13, FIP200), autophagosome nucleation/elongation (ATG14, beclin-1, ATG5), and autophagic cargo delivery to autophagosomes (SQSTM1). Glutamate-mediated transcriptional repression of autophagy was alleviated by overexpression of constitutively active AMPK. Genetic or pharmacological AMPK activation by AMPK overexpression or metformin, as well as genetic or pharmacological autophagy induction by TFEB overexpression or lithium chloride, reduced the sensitivity of nutrient-deprived SH-SY5Y cells to glutamate excitotoxicity. These data indicate that transcriptional inhibition of AMPK-dependent cytoprotective autophagy is involved in glutamate-mediated excitotoxicity during nutrient deprivation in vitro.

Group 1 metabotropic glutamate receptors trigger glutamate-induced intracellular Ca2+ signals and nitric oxide release in human brain microvascular endothelial cells

Neurovascular coupling (NVC) is the mechanism whereby an increase in neuronal activity causes an increase in local cerebral blood flow (CBF) to ensure local supply of oxygen and nutrients to the activated areas. The excitatory neurotransmitter glutamate gates post-synaptic N-methyl-D-aspartate receptors to mediate extracellular Ca2+ entry and stimulate neuronal nitric oxide (NO) synthase to release NO, thereby triggering NVC. Recent work suggested that endothelial Ca2+ signals could underpin NVC by recruiting the endothelial NO synthase. For instance, acetylcholine induced intracellular Ca2+ signals followed by NO release by activating muscarinic 5 receptors in hCMEC/D3 cells, a widely employed model of human brain microvascular endothelial cells. Herein, we sought to assess whether also glutamate elicits metabotropic Ca2+ signals and NO release in hCMEC/D3 cells. Glutamate induced a dose-dependent increase in intracellular Ca2+ concentration ([Ca2+]i) that was blocked by α-methyl-4-carboxyphenylglycine and phenocopied by trans-1-amino-1,3-cyclopentanedicarboxylic acid, which, respectively, block and activate group 1 metabotropic glutamate receptors (mGluRs). Accordingly, hCMEC/D3 expressed both mGluR1 and mGluR5 and the Ca2+ response to glutamate was inhibited by their pharmacological blockade with, respectively, CPCCOEt and MTEP hydrochloride. The Ca2+ response to glutamate was initiated by endogenous Ca2+ release from the endoplasmic reticulum and endolysosomal Ca2+ store through inositol-1,4,5-trisphosphate receptors and two-pore channels, respectively, and sustained by store-operated Ca2+ entry. In addition, glutamate induced robust NO release that was suppressed by pharmacological blockade of the accompanying increase in [Ca2+]i. These data demonstrate for the first time that glutamate may induce metabotropic Ca2+ signals in human brain microvascular endothelial cells. The Ca2+ response to glutamate is likely to support NVC during neuronal activity, thereby reinforcing the emerging role of brain microvascular endothelial cells in the regulation of CBF.

Strategies for Neuroprotection in Multiple Sclerosis and the Role of Calcium

Calcium ions are vital for maintaining the physiological and biochemical processes inside cells. The central nervous system (CNS) is particularly dependent on calcium homeostasis and its dysregulation has been associated with several neurodegenerative disorders including Parkinson’s disease (PD), Alzheimer’s disease (AD) and Huntington’s disease (HD), as well as with multiple sclerosis (MS). Hence, the modulation of calcium influx into the cells and the targeting of calcium-mediated signaling pathways may present a promising therapeutic approach for these diseases. This review provides an overview on calcium channels in neurons and glial cells. Special emphasis is put on MS, a chronic autoimmune disease of the CNS. While the initial relapsing-remitting stage of MS can be treated effectively with immune modulatory and immunosuppressive drugs, the subsequent progressive stage has remained largely untreatable. Here we summarize several approaches that have been and are currently being tested for their neuroprotective capacities in MS and we discuss which role calcium could play in this regard.

The Interplay between Ca2+ Signaling Pathways and Neurodegeneration

Calcium (Ca²⁺) homeostasis is essential for cell maintenance since this ion participates in many physiological processes. For example, the spatial and temporal organization of Ca²⁺ signaling in the central nervous system is fundamental for neurotransmission, where local changes in cytosolic Ca²⁺ concentration are needed to transmit information from neuron to neuron, between neurons and glia, and even regulating local blood flow according to the required activity. However, under pathological conditions, Ca²⁺ homeostasis is altered, with increased cytoplasmic Ca²⁺ concentrations leading to the activation of proteases, lipases, and nucleases. This review aimed to highlight the role of Ca²⁺ signaling in neurodegenerative disease-related apoptosis, where the regulation of intracellular Ca²⁺ homeostasis depends on coordinated interactions between the endoplasmic reticulum, mitochondria, and lysosomes, as well as specific transport mechanisms. In neurodegenerative diseases, alterations-increased oxidative stress, energy metabolism alterations, and protein aggregation have been identified. The aggregation of α-synuclein, β-amyloid peptide (Aβ), and huntingtin all adversely affect Ca²⁺ homeostasis. Due to the mounting evidence for the relevance of Ca²⁺ signaling in neuroprotection, we would focus on the expression and function of Ca²⁺ signaling-related proteins, in terms of the effects on autophagy regulation and the onset and progression of neurodegenerative diseases.

NAADP Receptors

Of the established Ca²⁺-mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations, and its receptors undergo dramatic desensitization. Recent studies have identified a new class of calcium-release channel, the two-pore channels (TPCs), as the likely targets for NAADP regulation, even though the effect may be indirect. These channels localized at endolysosomes, where they mediate local Ca²⁺ release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca²⁺ mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca²⁺ release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca²⁺-induced Ca²⁺-release (CICR) channels at lysosomal-endoplasmic reticulum (ER) junctions. The third is to regulate plasma membrane excitability by the targeting of Ca²⁺ release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca²⁺-activated channels. In this review, I discuss the role of nicotinic acid adenine nucleotide diphosphate (NAADP)-mediated Ca²⁺ release from endolysosomal stores as a widespread trigger for intracellular calcium signaling mechanisms, and how studies of TPCs are beginning to enhance our understanding of the central role of lysosomes in Ca²⁺ signaling.

Neuropharmacology of the Neuropsychiatric Symptoms of Dementia and Role of Pain: Essential Oil of Bergamot as a Novel Therapeutic Approach

Aging of the population makes of dementia a challenge for health systems worldwide. The cognitive disturbance is a serious but not the only issue in dementia; behavioral and psychological syndromes known as neuropsychiatric symptoms of dementia remarkably reduce the quality of life. The cluster of symptoms includes anxiety, depression, wandering, delusions, hallucinations, misidentifications, agitation and aggression. The pathophysiology of these symptoms implicates all the neurotransmitter systems, with a pivotal role for the glutamatergic neurotransmission. Imbalanced glutamatergic and GABAergic neurotransmissions, over-activation of the extrasynaptic N-methyl-D-aspartate (NMDA) receptors and alterations of the latter have been linked to the development of neuropsychiatric symptoms experienced by almost the entire demented population. Drugs with efficacy and safety for prevention or long term treatment of these disorders are not available yet. Aromatherapy provides the best evidence for positive outcomes in the control of agitation, the most resistant symptom. Demented patients often cannot verbalize pain, resulting in unrelieved symptoms and contributing to agitation. Bergamot essential oil provides extensive preclinical evidence of analgesic properties. Incidentally, the essential oil of bergamot induces anxyolitic-like effects devoid of sedation, typical of benzodiazepines, with a noteworthy advantage for demented patients. These data, together with the reported safety profile, form the rational basis for bergamot as a neurotherapeutic to be trialed for the control of behavioral and psychological symptoms of dementia.

Neural stem cell-derived small extracellular vesicles attenuate apoptosis and neuroinflammation after traumatic spinal cord injury by activating autophagy

Spinal cord injury (SCI) can cause severe irreversible motor dysfunction and even death. Neural stem cell (NSC) transplantation can promote functional recovery after acute SCI in experimental animals, but numerous issues, including low-transplanted cell survival rate, cell de-differentiation, and tumor formation need to be resolved before routine clinical application is feasible. Recent studies have shown that transplanted stem cells facilitate regeneration through release of paracrine factors. Small extracellular vesicles (sEVs), the smallest known membrane-bound nanovesicles, are involved in complex intercellular communication systems and are an important vehicle for paracrine delivery of therapeutic agents. However, the application of NSC-derived small extracellular vesicles (NSC-sEVs) to SCI treatment has not been reported. We demonstrate that NSC-sEVs can significantly reduce the extent of SCI, improve functional recovery, and reduce neuronal apoptosis, microglia activation, and neuroinflammation in rats. Furthermore, our study suggests that NSC-sEVs can regulate apoptosis and inflammatory processes by inducing autophagy. In brief, NSC-sEVs increased the expression of the autophagy marker proteins LC3B and beclin-1, and promoted autophagosome formation. Following NSC-sEV infusion, the SCI area was significantly reduced, and the expression levels of the proapoptotic protein Bax, the apoptosis effector cleaved caspase-3, and the pro-inflammatory cytokines TNF-α, IL-1β, and IL-6 were significantly reduced, whereas the expression level of the anti-apoptotic protein Bcl-2 was upregulated. In the presence of the autophagy inhibitor 3MA, however, these inhibitory effects of NSC-sEVs on apoptosis and neuroinflammation were significantly reversed. Our results show for the first time that NSC-sEV treatment has the potential to reduce neuronal apoptosis, inhibit neuroinflammation, and promote functional recovery in SCI model rats at an early stage by promoting autophagy.

Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) Induces Intracellular Ca2+ Release through the Two-Pore Channel TPC1 in Metastatic Colorectal Cancer Cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) gates two-pore channels 1 and 2 (TPC1 and TPC2) to elicit endo-lysosomal (EL) Ca²⁺ release. NAADP-induced EL Ca²⁺ signals may be amplified by the endoplasmic reticulum (ER) through the Ca²⁺-induced Ca²⁺ release mechanism (CICR). Herein, we aimed at assessing for the first time the role of EL Ca²⁺ signaling in primary cultures of human metastatic colorectal carcinoma (mCRC) by exploiting Ca²⁺ imaging and molecular biology techniques. The lysosomotropic agent, Gly-Phe β-naphthylamide (GPN), and nigericin, which dissipates the ΔpH which drives Ca²⁺ refilling of acidic organelles, caused massive Ca²⁺ release in the presence of a functional inositol-1,4,5-trisphosphate (InsP₃)-sensitive ER Ca²⁺ store. Liposomal delivery of NAADP induced a transient Ca²⁺ release that was reduced by GPN and NED-19, a selective TPC antagonist. Pharmacological and genetic manipulations revealed that the Ca²⁺ response to NAADP was triggered by TPC1, the most expressed TPC isoform in mCRC cells, and required ER-embedded InsP₃ receptors. Finally, NED-19 and genetic silencing of TPC1 reduced fetal calf serum-induced Ca²⁺ signals, proliferation, and extracellular signal-regulated kinase and Akt phoshorylation in mCRC cells. These data demonstrate that NAADP-gated TPC1 could be regarded as a novel target for alternative therapies to treat mCRC.

FAM129A regulates autophagy in thyroid carcinomas in an oncogene-dependent manner

We previously proposed that high expression of FAM129A can be used as a thyroid carcinoma biomarker in preoperative diagnostic exams of thyroid nodules. Here, we identify that FAM129A expression is increased under nutrient and growth factor depletion in a normal thyroid cell line (PCCL3), overlapping with increased expression of autophagy-related protein and inhibition of AKT/mTOR/p70S6K. Supplementation of insulin, TSH and serum to the medium was able to reduce the expression of both FAM129A and autophagy-related protein and reestablish the AKT/mTOR/p70S6K axis. To determine the direct role of FAM129A on autophagy, FAM129A was transfected into PCCL3 cells. Its overexpression induced autophagic vesicles formation, evidenced by transmission electron microscopy. Co-expression of FAM129A and mCherry-EGFP-LC3B in PCCL3 showed an increased yellow puncta formation, suggesting that FAM129Ainduces autophagy. To further confirm its role on autophagy, we knockdown FAM129A in two thyroid carcinoma cell lines (TPC1 and FTC-236). Unexpectedly, FAM129A silencing increased autophagic flux, suggesting that FAM129A inhibits autophagy in these models. We next co-transfected PCCL3 cells with FAM129A and RET/PTC1 and tested autophagy in this context. Co-expression of FAM129A and RET/PTC1 oncogene in PCCL3 cells, inhibited RET/PTC1-induced autophagy. Together, our data suggest that, in normal cells FAM129A induces autophagy in order to maintain cell homeostasis and provide substrates under starvation conditions. Instead, in cancer cells, decreased autophagy may help the cells to overcome cell death. FAM129A regulates autophagy in a cell- and/or context-dependent manner. Our data reinforce the concept that autophagy can be used as a strategy for cancer treatment.

Endolysosomal Ca2+ Signalling and Cancer Hallmarks: Two-Pore Channels on the Move, TRPML1 Lags Behind!

The acidic vesicles of the endolysosomal (EL) system are emerging as an intracellular Ca²⁺ store implicated in the regulation of multiple cellular functions. The EL Ca²⁺ store releases Ca²⁺ through a variety of Ca²⁺-permeable channels, including Transient Receptor Potential (TRP) Mucolipin 1-3 (TRPML1-3) and two-pore channels 1-2 (TPC1-2), whereas EL Ca²⁺ refilling is sustained by the proton gradient across the EL membrane and/or by the endoplasmic reticulum (ER). EL Ca²⁺ signals may be either spatially restricted to control vesicle trafficking, autophagy and membrane repair or may be amplified into a global Ca²⁺ signal through the Ca²⁺-dependent recruitment of ER-embedded channels. Emerging evidence suggested that nicotinic acid adenine dinucleotide phosphate (NAADP)-gated TPCs sustain multiple cancer hallmarks, such as migration, invasiveness and angiogenesis. Herein, we first survey the EL Ca²⁺ refilling and release mechanisms and then focus on the oncogenic role of EL Ca²⁺ signaling. While the evidence in favor of TRPML1 involvement in neoplastic transformation is yet to be clearly provided, TPCs are emerging as an alternative target for anticancer therapies.

Hippocampal mGluR1-dependent long-term potentiation requires NAADP-mediated acidic store Ca2+ signaling

Acidic organelles, such as endosomes and lysosomes, store Ca²⁺ that is released in response to intracellular increases in the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). In neurons, NAADP and Ca²⁺ signaling contribute to synaptic plasticity, a process of activity-dependent long-term potentiation (LTP) [or, alternatively, long-term depression (LTD)] of synaptic strength and neuronal transmission that is critical for neuronal function and memory formation. We explored the function of and mechanisms regulating acidic Ca²⁺ store signaling in murine hippocampal neurons. We found that metabotropic glutamate receptor 1 (mGluR1) was coupled to NAADP signaling that elicited Ca²⁺ release from acidic stores. In turn, this released Ca²⁺-mediated mGluR1-dependent LTP by transiently inhibiting SK-type K⁺ channels, possibly through the activation of protein phosphatase 2A. Genetically removing two-pore channels (TPCs), which are endolysosomal-specific ion channels, switched the polarity of plasticity from LTP to LTD, indicating the importance of specific receptor store coupling and providing mechanistic insight into how mGluR1 can produce both synaptic potentiation and synaptic depression.

Glutamate triggers intracellular Ca2+ oscillations and nitric oxide release by inducing NAADP- and InsP3 -dependent Ca2+ release in mouse brain endothelial cells

The neurotransmitter glutamate increases cerebral blood flow by activating postsynaptic neurons and presynaptic glial cells within the neurovascular unit. Glutamate does so by causing an increase in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in the target cells, which activates the Ca²⁺ /Calmodulin-dependent nitric oxide (NO) synthase to release NO. It is unclear whether brain endothelial cells also sense glutamate through an elevation in [Ca²⁺ ]_i and NO production. The current study assessed whether and how glutamate drives Ca²⁺ -dependent NO release in bEND5 cells, an established model of brain endothelial cells. We found that glutamate induced a dose-dependent oscillatory increase in [Ca²⁺ ]_i , which was maximally activated at 200 μM and inhibited by α-methyl-4-carboxyphenylglycine, a selective blocker of Group 1 metabotropic glutamate receptors. Glutamate-induced intracellular Ca²⁺ oscillations were triggered by rhythmic endogenous Ca²⁺ mobilization and maintained over time by extracellular Ca²⁺ entry. Pharmacological manipulation revealed that glutamate-induced endogenous Ca²⁺ release was mediated by InsP₃ -sensitive receptors and nicotinic acid adenine dinucleotide phosphate (NAADP) gated two-pore channel 1. Constitutive store-operated Ca²⁺ entry mediated Ca²⁺ entry during ongoing Ca²⁺ oscillations. Finally, glutamate evoked a robust, although delayed increase in NO levels, which was blocked by pharmacologically inhibition of the accompanying intracellular Ca²⁺ signals. Of note, glutamate induced Ca²⁺ -dependent NO release also in hCMEC/D3 cells, an established model of human brain microvascular endothelial cells. This investigation demonstrates for the first time that metabotropic glutamate-induced intracellular Ca²⁺ oscillations and NO release have the potential to impact on neurovascular coupling in the brain.

Ion channels in the regulation of autophagy

Autophagy is a cellular process in which the cell degrades and recycles its own constituents. Given the crucial role of autophagy in physiology, deregulation of autophagic machinery is associated with various diseases. Hence, a thorough understanding of autophagy regulatory mechanisms is crucially important for the elaboration of efficient treatments for different diseases. Recently, ion channels, mediating ion fluxes across cellular membranes, have emerged as important regulators of both basal and induced autophagy. However, the mechanisms by which specific ion channels regulate autophagy are still poorly understood, thus underscoring the need for further research in this field. Here we discuss the involvement of major types of ion channels in autophagy regulation.

Importance of Autophagy in Mediating Human Immunodeficiency Virus (HIV) and Morphine-Induced Metabolic Dysfunction and Inflammation in Human Astrocytes

Under physiological conditions, the function of astrocytes in providing brain metabolic support is compromised under pathophysiological conditions caused by human immunodeficiency virus (HIV) and opioids. Herein, we examined the role of autophagy, a lysosomal degradation pathway important for cellular homeostasis and survival, as a potential regulatory mechanism during pathophysiological conditions in primary human astrocytes. Blocking autophagy with small interfering RNA (siRNA) targeting BECN1, but not the Autophagy-related 5 (ATG5) gene, caused a significant decrease in HIV and morphine-induced intracellular calcium release. On the contrary, inducing autophagy pharmacologically with rapamycin further enhanced calcium release and significantly reverted HIV and morphine-decreased glutamate uptake. Furthermore, siBeclin1 caused an increase in HIV-induced nitric oxide (NO) release, while viral-induced NO in astrocytes exposed to rapamycin was decreased. HIV replication was significantly attenuated in astrocytes transfected with siRNA while significantly induced in astrocytes exposed to rapamycin. Silencing with siBeclin1, but not siATG5, caused a significant decrease in HIV and morphine-induced interleukin (IL)-8 and tumor necrosis factor alpha (TNF-α) release, while secretion of IL-8 was significantly induced with rapamycin. Mechanistically, the effects of siBeclin1 in decreasing HIV-induced calcium release, viral replication, and viral-induced cytokine secretion were associated with a decrease in activation of the nuclear factor kappa B (NF-κB) pathway.

Cell Death-Autophagy Loop and Glutamate-Glutamine Cycle in Amyotrophic Lateral Sclerosis

Although we know that amyotrophic lateral sclerosis (ALS) is correlated with the glutamate-mediated corticomotor neuronal hyperexcitability, detailed ALS pathology remains largely unexplained. While a number of drugs have been developed, no cure exists so far. Here, we propose a hypothesis of neuronal cell death—incomplete autophagy positive-feedback loop—and summarize the role of the neuron-astrocyte glutamate-glutamine cycle in ALS. The disruption of these two cycles might ideally retard ALS progression. Cerebrovascular injuries (such as multiple embolization sessions and strokes) induce neuronal cell death and the subsequent autophagy. ALS impairs autophagosome-lysosome fusion and leads to magnified cell death. Trehalose rescues this impaired fusion step, significantly delaying the onset of the disease, although it does not affect the duration of the disease. Therefore, trehalose might be a prophylactic drug for ALS. Given that a major part of neuronal glutamate is converted from glutamine through neuronal glutaminase (GA), GA inhibitors may decrease the neuronal glutamate accumulation, and, therefore, might be therapeutic ALS drugs. Of these, Ebselen is the most promising one with strong antioxidant properties.

From mucolipidosis type IV to Ebola: TRPML and two-pore channels at the crossroads of endo-lysosomal trafficking and disease

What do lysosomal storage disorders such as mucolipidosis type IV have in common with Ebola, cancer cell migration, or LDL-cholesterol trafficking? LDL-cholesterol, certain bacterial toxins and viruses, growth factors, receptors, integrins, macromolecules destined for degradation or secretion are all sorted and transported via the endolysosomal system (ES). There are several pathways known in the ES, e.g. the degradation, the recycling, or the retrograde trafficking pathway. The ES comprises early and late endosomes, lysosomes and recycling endosomes as well as autophagosomes and lysosome related organelles. Contact sites between the ES and the endoplasmic reticulum or the Golgi apparatus may also be considered part of it. Dysfunction of this complex intracellular machinery can cause or contribute to the development of a number of diseases ranging from neurodegenerative, infectious, or metabolic diseases to retinal and pigmentation disorders as well as cancer and autophagy-related diseases. Endolysosomal ion channels such as mucolipins (TRPMLs) and two-pore channels (TPCs) play an important role in intracellular cation/calcium signaling and homeostasis and appear to critically contribute to the proper function of the endolysosomal trafficking network.