Inositol polyphosphate multikinase modulates redox signaling through nuclear factor erythroid 2-related factor 2 and glutathione metabolism

Maintenance of redox balance plays central roles in a plethora of signaling processes. Although physiological levels of reactive oxygen and nitrogen species are crucial for functioning of certain signaling pathways, excessive production of free radicals and oxidants can damage cell components. The nuclear factor erythroid 2-related factor 2 (Nrf2) signaling cascade is the key pathway that mediates cellular response to oxidative stress. It is controlled at multiple levels, which serve to maintain redox homeostasis within cells. We show here that inositol polyphosphate multikinase (IPMK) is a modulator of Nrf2 signaling. IPMK binds Nrf2 and attenuates activation and expression of Nrf2 target genes. Furthermore, depletion of IPMK leads to elevated glutathione and cysteine levels, resulting in increased resistance to oxidants. Accordingly, targeting IPMK may restore redox balance under conditions of cysteine and glutathione insufficiency.

Serine Racemase mediates subventricular zone neurogenesis via fatty acid metabolism

The adult subventricular zone (SVZ) is a neurogenic niche that continuously produces newborn neurons. Here we show that serine racemase (SR), an enzyme that catalyzes the racemization of L-serine to D-serine and vice versa, affects neurogenesis in the adult SVZ by controlling de novo fatty acid synthesis. Germline and conditional deletion of SR (nestin precursor cells) leads to diminished neurogenesis in the SVZ. Nestin-cre+ mice showed reduced expression of fatty acid synthase and its substrate malonyl-CoA, which are involved in de novo fatty acid synthesis. Global lipidomic analyses revealed significant alterations in different lipid subclasses in nestin-cre+ mice. Decrease in fatty acid synthesis was mediated by phospho Acetyl-CoA Carboxylase that was AMP-activated protein kinase independent. Both L- and D-serine supplementation rescued defects in SVZ neurogenesis, proliferation, and levels of malonyl-CoA in vitro. Our work shows that SR affects adult neurogenesis in the SVZ via lipid metabolism.

Loss of PI3k activity of inositol polyphosphate multikinase impairs PDK1-mediated AKT activation, cell migration, and intestinal homeostasis

Protein kinase B (AKT) is essential for cell survival, proliferation, and migration and has been associated with several diseases. Here, we demonstrate that inositol polyphosphate multikinase (IPMK's) lipid kinase property drives AKT activation via increasing membrane localization and activation of PDK1 (3-Phosphoinositide-dependent kinase 1), largely independent of class I PI3k (cPI3K). Deletion of IPMK impairs cell migration, which is partially associated with the abolition of PDK1-mediated ROCK1 disinhibition and subsequent myosin light chain (MLC) phosphorylation. IPMK is highly expressed in intestinal epithelial cells (IEC). Deleting IPMK in IEC reduced AKT phosphorylation and diminished the number of Paneth cells. Ablation of IPMK impaired IEC regeneration both basally and after chemotherapy-induced damage, suggesting a broad role for IPMK in activating AKT and intestinal tissue regeneration. In conclusion, the PI3k activity of IPMK is necessary for PDK1-mediated AKT activation and intestinal homeostasis.

A Novel Stereospecific Bioluminescent Assay for Detection of Endogenous d-Cysteine

The presence of endogenous d-stereoisomers of amino acids in mammals dispels a long-standing dogma about their existence. d-Serine and d-aspartate function as novel neurotransmitters in mammals. However, the stereoisomer with the fastest, spontaneous in vitro racemization rate, d-cysteine, has not been reported. We utilized a novel, stereospecific, bioluminescent assay to identify endogenous d-cysteine in substantial amounts in the eye, brain, and pancreas of mice. d-Cysteine is enriched in mice embryonic brains at day E9.5 (4.5 mM) and decreases progressively with development (μM levels). d-Cysteine is also present in significantly higher amounts in the human brain white matter compared with gray matter. In the luciferase assay, d-cysteine conjugates with cyano hydroxy benzothiazole in the presence of a base and reducing agent to form d-luciferin. d-Luciferin, subsequently, in the presence of firefly luciferase and ATP, emits bioluminescence proportional to the concentration of d-cysteine. The assay is stereospecific and allows the quantitative estimation of endogenous d-cysteine in tissues in addition to its specificity for d-cysteine. Future efforts aimed at bioluminescent in vivo imaging of d-cysteine may allow a more noninvasive means of its detection, thereby elucidating its function.

Identification of the NRF2 transcriptional network as a therapeutic target for trigeminal neuropathic pain

Trigeminal neuralgia, historically dubbed the "suicide disease," is an exceedingly painful neurologic condition characterized by sudden episodes of intense facial pain. Unfortunately, the only U.S. Food and Drug Administration (FDA)-approved medication for trigeminal neuralgia carries substantial side effects, with many patients requiring surgery. Here, we identify the NRF2 transcriptional network as a potential therapeutic target. We report that cerebrospinal fluid from patients with trigeminal neuralgia accumulates reactive oxygen species, several of which directly activate the pain-transducing channel TRPA1. Similar to our patient cohort, a mouse model of trigeminal neuropathic pain also exhibits notable oxidative stress. We discover that stimulating the NRF2 antioxidant transcriptional network is as analgesic as inhibiting TRPA1, in part by reversing the underlying oxidative stress. Using a transcriptome-guided drug discovery strategy, we identify two NRF2 network modulators as potential treatments. One of these candidates, exemestane, is already FDA-approved and may thus be a promising alternative treatment for trigeminal neuropathic pain.

Biliverdin reductase bridges focal adhesion kinase to Src to modulate synaptic signaling

Synapses connect discrete neurons into vast networks that send, receive, and encode diverse forms of information. Synaptic function and plasticity, the neuronal process of adapting to diverse and variable inputs, depend on the dynamic nature of synaptic molecular components, which is mediated in part by cell adhesion signaling pathways. Here, we found that the enzyme biliverdin reductase (BVR) physically links together key focal adhesion signaling molecules at the synapse. BVR-null (BVR^-/-) mice exhibited substantial deficits in learning and memory on neurocognitive tests, and hippocampal slices in which BVR was postsynaptically depleted showed deficits in electrophysiological responses to stimuli. RNA sequencing, biochemistry, and pathway analyses suggested that these deficits were mediated through the loss of focal adhesion signaling at both the transcriptional and biochemical level in the hippocampus. Independently of its catalytic function, BVR acted as a bridge between the primary focal adhesion signaling kinases FAK and Pyk2 and the effector kinase Src. Without BVR, FAK and Pyk2 did not bind to and stimulate Src, which then did not phosphorylate the N-methyl-d-aspartate (NMDA) receptor, a critical posttranslational modification for synaptic plasticity. Src itself is a molecular hub on which many signaling pathways converge to stimulate NMDAR-mediated neurotransmission, thus positioning BVR at a prominent intersection of synaptic signaling.

Mammalian D-cysteine: A novel regulator of neural progenitor cell proliferation: Endogenous D-cysteine, the stereoisomer with rapid spontaneous in vitro racemization rate, has major neural roles: Endogenous D-cysteine, the stereoisomer with rapid spontaneous in vitro racemization rate, has major neural roles

D-amino acids are being recognized as functionally important molecules in mammals. We recently identified endogenous D-cysteine in mammalian brain. D-cysteine is present in neonatal brain in substantial amounts (mM) and decreases with postnatal development. D-cysteine binds to MARCKS and a host of proteins implicated in cell division and neurodevelopmental disorders. D-cysteine decreases phosphorylation of MARCKS in neural progenitor cells (NPCs) affecting its translocation. D-cysteine controls NPC proliferation by inhibiting AKT signaling. Exogenous D-cysteine inhibits AKT phosphorylation at Thr 308 and Ser 473 in NPCs. D-cysteine treatment of NPCs led to 50% reduction in phosphorylation of Foxo1 at Ser 256 and Foxo3a at Ser 253. We hypothesize that in the developing brain endogenous D-cysteine is as a physiologic regulator of NPC proliferation by inhibiting AKT signaling mediated by Foxo1 and Foxo3a. Endogenous D-cysteine may regulate mammalian neurodevelopment with roles in schizophrenia and Alzheimer's disease (AD).

Inositol hexakisphosphate kinase-2 non-catalytically regulates mitophagy by attenuating PINK1 signaling

Inositol pyrophosphates, such as 5-diphosphoinositol pentakisphosphate (IP7), are generated by a family of inositol hexakisphosphate kinases (IP6Ks), of which IP6K2 has been implicated in various cellular functions including neuroprotection. Absence of IP6K2 causes impairment of oxidative phosphorylation regulated by creatine kinase-B. In the present study, we show that IP6K2 is involved in attenuation of PINK1-mediated mitochondrial autophagy (mitophagy) in the brain. Up-regulation of dynamin-related protein (Drp-1), as well as increased expression of mitochondrial biogenesis markers (PGC1-α and NRF-1) in the cerebella of IP6K2-deleted mice (IP6K2-knockout), point to the involvement of IP6K2 in the regulation of mitochondrial fission. Knockdown of IP6K2 also leads to augmented glycolysis, potentially as a compensatory mechanism for decreased mitochondrial respiration. Overexpressing IP6K2 as well as IP6K2-kinase dead mutant in IP6K2-knockdown N2A cells reverses the expression of mitophagy markers, demonstrating that IP6K2-induced mitoprotection is catalytically/kinase independent. IP6K2 supplementation in K2-PINK1 double-knockdown N2A cells fails to reverse the expression of the mitophagic marker, LC3-II, indicating that the mitoprotective effect of IP6K2 is dependent on PINK1. Overall, our study reveals a key neuroprotective role of IP6K2 in the prevention of PINK1-mediated mitophagy in the brain.

Quantitative measurement of reactive oxygen species in ex vivo mouse brain slices

Evaluating redox homeostasis involves gauging the levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS) directly in tissues and cells. The brain is especially metabolically active and is particularly vulnerable to excessive ROS and RNS. Here, we describe a methodology to quantitatively measure ROS in ex vivo mouse brain slices at baseline and after neural stimulation. Evaluating ROS in slices provides a more complete picture of neural redox signaling than when measured in isolated neurons or astrocytes. For complete details on the use and execution of this protocol, please refer to Vasavda et al. (2019).

Signaling by cGAS-STING in Neurodegeneration, Neuroinflammation, and Aging

Recognition of foreign or misplaced nucleic acids is one of the principal modes by which the immune system detects pathogenic entities. When cytosolic DNA is sensed, a signal is relayed via the cGAS-STING pathway: this involves the activation of cyclic GMP-AMP (cGMP-AMP) synthase (cGAS) and generation of the cyclic dinucleotide cGAMP, followed by the induction of stimulator of interferon genes (STING). The cGAS-STING pathway responds to viral, bacterial, and self-DNA. Whereas it generally mediates immune surveillance and is often neuroprotective, excessive engagement of the system can be deleterious. This is relevant in aging and age-related neurological diseases, where neuroinflammation contributes to disease progression. This review focuses on cGAS-STING signaling in aging, neurodegeneration, and neuroinflammation, and on therapeutic implications.

Cocaine-induced locomotor stimulation involves autophagic degradation of the dopamine transporter

Cocaine exerts its stimulant effect by inhibiting dopamine reuptake leading to increased dopamine signaling. This action is thought to reflect binding of cocaine to the dopamine transporter (DAT) to inhibit its function. However, cocaine is a relatively weak inhibitor of DAT, and many DAT inhibitors do not share the behavioral actions of cocaine. We previously showed that toxic levels of cocaine induce autophagic neuronal cell death. Here, we show that subnanomolar concentrations of cocaine elicit neural autophagy in vitro and in vivo. Autophagy inhibitors reduce the locomotor stimulant effect of cocaine in mice. Cocaine-induced autophagy degrades transporters for dopamine but not serotonin in the nucleus accumbens. Autophagy inhibition impairs cocaine conditioned place preference in mice. Our findings indicate that autophagic degradation of DAT modulates behavioral actions of cocaine.

Effects of hydrogen sulfide on mitochondrial function and cellular bioenergetics

Hydrogen sulfide (H₂S) was once considered to have only toxic properties, until it was discovered to be an endogenous signaling molecule. The effects of H₂S are dose dependent, with lower concentrations being beneficial and higher concentrations, cytotoxic. This scenario is especially true for the effects of H₂S on mitochondrial function, where higher concentrations of the gasotransmitter inhibit the electron transport chain, and lower concentrations stimulate bioenergetics in multiple ways. Here we review the role of H₂S in mitochondrial function and its effects on cellular physiology.

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Hydrogen sulfide (H₂S) plays central roles in mitochondrial homeostasis.

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Both excess H₂S and a paucity of H₂S have deleterious effects.

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One of the modes by which H₂S signals in mitochondria is by sulfhydrating target proteins.

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Administering H₂S (where scarcity of H₂S occurs) or inhibiting H₂S production (in case of excess H₂S) may be beneficial.

The inositol pyrophosphate 5-InsP7 drives sodium-potassium pump degradation by relieving an autoinhibitory domain of PI3K p85α

Sodium/potassium-transporting adenosine triphosphatase (Na⁺/K⁺-ATPase) is one of the most abundant cell membrane proteins and is essential for eukaryotes. Endogenous negative regulators have long been postulated to play an important role in regulating the activity and stability of Na⁺/K⁺-ATPase, but characterization of these regulators has been elusive. Mechanisms of regulating Na⁺/K⁺-ATPase homeostatic turnover are unknown. Here, we report that 5-diphosphoinositol 1,2,3,4,6-pentakisphosphate (5-InsP₇), generated by inositol hexakisphosphate kinase 1 (IP6K1), promotes physiological endocytosis and downstream degradation of Na⁺/K⁺-ATPase-α1. Deletion of IP6K1 elicits a twofold enrichment of Na⁺/K⁺-ATPase-α1 in plasma membranes of multiple tissues and cell types. Using a suite of synthetic chemical biology tools, we found that 5-InsP₇ binds the RhoGAP domain of phosphatidylinositol 3-kinase (PI3K) p85α to disinhibit its interaction with Na⁺/K⁺-ATPase-α1. This recruits adaptor protein 2 (AP2) and triggers the clathrin-mediated endocytosis of Na⁺/K⁺-ATPase-α1. Our study identifies 5-InsP₇ as an endogenous negative regulator of Na⁺/K⁺-ATPase-α1.

BVR-A Deficiency Leads to Autophagy Impairment through the Dysregulation of AMPK/mTOR Axis in the Brain-Implications for Neurodegeneration

Biliverdin reductase-A (BVR-A) impairment is associated with increased accumulation of oxidatively-damaged proteins along with the impairment of autophagy in the brain during neurodegenerative disorders. Reduced autophagy inhibits the clearance of misfolded proteins, which then form neurotoxic aggregates promoting neuronal death. The aim of our study was to clarify the role for BVR-A in the regulation of the mTOR/autophagy axis by evaluating age-associated changes (2, 6 and 11 months) in cerebral cortex samples collected from BVR-A knock-out (BVR-A^−/−) and wild-type (WT) mice. Our results show that BVR-A deficiency leads to the accumulation of oxidatively-damaged proteins along with mTOR hyper-activation in the cortex. This process starts in juvenile mice and persists with aging. mTOR hyper-activation is associated with the impairment of autophagy as highlighted by reduced levels of Beclin-1, LC3β, LC3II/I ratio, Atg5–Atg12 complex and Atg7 in the cortex of BVR-A^−/− mice. Furthermore, we have identified the dysregulation of AMP-activated protein kinase (AMPK) as a critical event driving mTOR hyper-activation in the absence of BVR-A. Overall, our results suggest that BVR-A is a new player in the regulation of autophagy, which may be targeted to arrive at novel therapeutics for diseases involving impaired autophagy.

The nonselective cation channel TRPV4 inhibits angiotensin II receptors

G-protein-coupled receptors (GPCRs) are a ubiquitously expressed family of receptor proteins that regulate many physiological functions and other proteins. They act through two dissociable signaling pathways: the exchange of GDP to GTP by linked G-proteins and the recruitment of β-arrestins. GPCRs modulate several members of the transient receptor potential (TRP) channel family of nonselective cation channels. How TRP channels reciprocally regulate GPCR signaling is less well-explored. Here, using an array of biochemical approaches, including immunoprecipitation and fluorescence, calcium imaging, phosphate radiolabeling, and a β-arrestin-dependent luciferase assay, we characterize a GPCR-TRP channel pair, angiotensin II receptor type 1 (AT1R), and transient receptor potential vanilloid 4 (TRPV4), in primary murine choroid plexus epithelial cells and immortalized cell lines. We found that AT1R and TRPV4 are binding partners and that activation of AT1R by angiotensin II (ANGII) elicits β-arrestin-dependent inhibition and internalization of TRPV4. Activating TRPV4 with endogenous and synthetic agonists inhibited angiotensin II-mediated G-protein-associated second messenger accumulation, AT1R receptor phosphorylation, and β-arrestin recruitment. We also noted that TRPV4 inhibits AT1R phosphorylation by activating the calcium-activated phosphatase calcineurin in a Ca2+/calmodulin-dependent manner, preventing β-arrestin recruitment and receptor internalization. These findings suggest that when TRP channels and GPCRs are co-expressed in the same tissues, many of these channels can inhibit GPCR desensitization.

IPMK Mediates Activation of ULK Signaling and Transcriptional Regulation of Autophagy Linked to Liver Inflammation and Regeneration

Autophagy plays a broad role in health and disease. Here, we show that inositol polyphosphate multikinase (IPMK) is a prominent physiological determinant of autophagy and is critical for liver inflammation and regeneration. Deletion of IPMK diminishes autophagy in cell lines and mouse liver. Regulation of autophagy by IPMK does not require catalytic activity. Two signaling axes, IPMK-AMPK-Sirt-1 and IPMK-AMPK-ULK1, appear to mediate the influence of IPMK on autophagy. IPMK enhances autophagy-related transcription by stimulating AMPK-depen-dent Sirt-1 activation, which mediates the deacetylation of histone 4 lysine 16. Furthermore, direct binding of IPMK to ULK and AMPK forms a ternary complex that facilitates AMPK-dependent ULK phosphorylation. Deletion of IPMK in cell lines and intact mice virtually abolishes lipophagy, promotes liver damage as well as inflammation, and impairs hepatocyte regeneration. Thus, targeting IPMK may afford therapeutic benefits in disabilities that depend on autophagy and lipophagy—specifically, in liver inflammation and regeneration.

IPMK is a physiological determinant of autophagy and is critical in liver inflammation. Two signaling axes, IPMK-AMPK-Sirt-1 and IPMK-AMPK-ULK1, appear to mediate the influence of IPMK on autophagy. Deletion of IPMK impairs lipophagy and hepatocyte regeneration.

Therapeutic Applications of Cysteamine and Cystamine in Neurodegenerative and Neuropsychiatric Diseases

Current medications for neurodegenerative and neuropsychiatric diseases such as Alzheimer's disease (AD), Huntington's disease (HD), Parkinson's disease (PD), and Schizophrenia mainly target disease symptoms. Thus, there is an urgent need to develop novel therapeutics that can delay, halt or reverse disease progression. AD, HD, PD, and schizophrenia are characterized by elevated oxidative and nitrosative stress, which play a central role in pathogenesis. Clinical trials utilizing antioxidants to counter disease progression have largely been unsuccessful. Most antioxidants are relatively non-specific and do not adequately target neuroprotective pathways. Accordingly, a search for agents that restore redox balance as well as halt or reverse neuronal loss is underway. The small molecules, cysteamine, the decarboxylated derivative of the amino acid cysteine, and cystamine, the oxidized form of cysteamine, respectively, mitigate oxidative stress and inflammation and upregulate neuroprotective pathways involving brain-derived neurotrophic factor (BDNF) and Nuclear factor erythroid 2-related factor 2 (Nrf2) signaling. Cysteamine can traverse the blood brain barrier, a desirable characteristic of drugs targeting neurodegeneration. This review addresses recent developments in the use of these aminothiols to counter neurodegeneration and neuropsychiatric deficits.

Selective Persulfide Detection Reveals Evolutionarily Conserved Antiaging Effects of S-Sulfhydration

Life on Earth emerged in a hydrogen sulfide (H₂S)-rich environment eons ago and with it protein persulfidation mediated by H₂S evolved as a signaling mechanism. Protein persulfidation (S-sulfhydration) is a post-translational modification of reactive cysteine residues, which modulate protein structure and/or function. Persulfides are difficult to label and study due to their reactivity and similarity with cysteine. Here, we report a facile strategy for chemoselective persulfide bioconjugation using dimedone-based probes, to achieve highly selective, rapid, and robust persulfide labeling in biological samples with broad utility. Using this method, we show persulfidation is an evolutionarily conserved modification and waves of persulfidation are employed by cells to resolve sulfenylation and prevent irreversible cysteine overoxidation preserving protein function. We report an age-associated decline in persulfidation that is conserved across evolutionary boundaries. Accordingly, dietary or pharmacological interventions to increase persulfidation associate with increased longevity and improved capacity to cope with stress stimuli.

Inhibition of IP6K1 suppresses neutrophil-mediated pulmonary damage in bacterial pneumonia

The significance of developing host-modulating personalized therapies to counteract the growing threat of antimicrobial resistance is well-recognized because such resistance cannot be overcome using microbe-centered strategies alone. Immune host defenses must be finely controlled during infection to balance pathogen clearance with unwanted inflammation-induced tissue damage. Thus, an ideal antimicrobial treatment would enhance bactericidal activity while preventing neutrophilic inflammation, which can induce tissue damage. We report that disrupting the inositol hexakisphosphate kinase 1 (Ip6k1) gene or pharmacologically inhibiting IP6K1 activity using the specific inhibitor TNP [N2-(m-(trifluoromethyl)benzyl) N6-(p-nitrobenzyl)purine] efficiently and effectively enhanced host bacterial killing but reduced pulmonary neutrophil accumulation, minimizing the lung damage caused by both Gram-positive and Gram-negative bacterial pneumonia. IP6K1-mediated inorganic polyphosphate (polyP) production by platelets was essential for infection-induced neutrophil-platelet aggregate (NPA) formation and facilitated neutrophil accumulation in alveolar spaces during bacterial pneumonia. IP6K1 inhibition reduced serum polyP levels, which regulated NPAs by triggering the bradykinin pathway and bradykinin-mediated neutrophil activation. Thus, we identified a mechanism that enhances host defenses while simultaneously suppressing neutrophil-mediated pulmonary damage in bacterial pneumonia. IP6K1 is, therefore, a legitimate therapeutic target for such disease.

Bilirubin Links Heme Metabolism to Neuroprotection by Scavenging Superoxide

Bilirubin is one of the most frequently measured metabolites in medicine, yet its physiologic roles remain unclear. Bilirubin can act as an antioxidant in vitro, but whether its redox activity is physiologically relevant is unclear because many other antioxidants are far more abundant in vivo. Here, we report that depleting endogenous bilirubin renders mice hypersensitive to oxidative stress. We find that mice lacking bilirubin are particularly vulnerable to superoxide (O₂^⋅-) over other tested reactive oxidants and electrophiles. Whereas major antioxidants such as glutathione and cysteine exhibit little to no reactivity toward O₂^⋅-, bilirubin readily scavenges O₂^⋅-. We find that bilirubin's redox activity is particularly important in the brain, where it prevents excitotoxicity and neuronal death by scavenging O₂^⋅- during NMDA neurotransmission. Bilirubin's unique redox activity toward O₂^⋅- may underlie a prominent physiologic role despite being significantly less abundant than other endogenous and exogenous antioxidants.

A Connective Tissue Mast-Cell-Specific Receptor Detects Bacterial Quorum-Sensing Molecules and Mediates Antibacterial Immunity

Quorum-sensing molecules (QSMs) are secreted by bacteria to signal population density. Upon reaching a critical concentration, QSMs induce transcriptional alterations in bacteria, which enable virulence factor expression and biofilm formation. It is unclear whether mammalian hosts can recognize QSMs to trigger responsive antibacterial immunity. We report that mouse mast-cell-specific G-protein-coupled receptor Mrgprb2 and its human homolog MRGPRX2 are receptors for Gram-positive QSMs, including competence-stimulating peptide (CSP)-1. CSP-1 activates Mrgprb2 and MRGPRX2, triggering mast cell degranulation, which inhibits bacterial growth and prevents biofilm formation. Such antibacterial functions are reduced in Mrgprb2-deficient mast cells, while wild-type mast cells fail to inhibit the growth of bacterial strains lacking CSP-1. Mrgprb2-knockout mice exhibit reduced bacterial clearance, while pharmacologically activating Mrgprb2 in vivo eliminates bacteria and improves disease score. These findings identify a host defense mechanism that uses QSMs as an "Achilles heel" and suggest MRGPRX2 as a potential therapeutic target for controlling bacterial infections.

Noncatalytic functions of IPMK are essential for activation of autophagy and liver regeneration

Macroautophagy/autophagy plays important roles in health and disease, but mechanisms of its activation are unclear. Recently we established IPMK (inositol polyphosphate multikinase) as a physiological determinant of autophagy independent of its catalytic activity. Two signaling axes, IPMK-AMPK-SIRT1 and IPMK-AMPK-ULK1, appear to mediate the influence of IPMK on autophagy. IPMK enhances autophagy-related transcription by stimulating AMPK-dependent SIRT1 activation, which mediates the deacetylation of histone 4 lysine 16. Furthermore, direct binding of IPMK to ULK and AMPK forms a ternary complex that facilitates AMPK-dependent ULK phosphorylation. Deletion of Ipmk virtually abolishes lipophagy, promotes liver damage and impairs hepatocyte regeneration. Our study establishes the importance of IPMK in regulation of autophagy and as a drug target for autophagy-related diseases.

MRGPRX4 is a G protein-coupled receptor activated by bile acids that may contribute to cholestatic pruritus

Patients suffering from cholestasis, the slowing or stoppage of bile flow, commonly report experiencing an intense, chronic itch. Numerous pruritogens are up-regulated in cholestatic patient sera, including bile acids (BAs). Acute injection of BAs results in itch in both mice and humans, and BA-modulating therapy is effective in controlling patient itch. Here, we present evidence that human sensory neuron-expressed Mas-related G protein-coupled receptor X4 (MRGPRX4), an orphan member of the Mrgpr family of GPCRs, is a BA receptor. Using Ca2+ imaging, we determined that pathophysiologically relevant levels of numerous BAs activated MRGPRX4. No mouse Mrgpr orthologs were activated by BAs. To assess the in vivo relevance of BA activation of MRGPRX4, we generated a humanized mouse with targeted expression of MRGPRX4 in itch-encoding sensory neurons. BAs activated MRGPRX4+ sensory neurons at higher levels compared with WT neurons. Compared with control animals, MRGPRX4+ mice scratched more upon acute injection of BAs and in a model of cholestatic itch. Overall, these data suggest that targeting MRGPRX4 is a promising strategy for alleviating cholestatic itch.

Redox Mechanisms in Neurodegeneration: From Disease Outcomes to Therapeutic Opportunities

SIGNIFICANCE

Once considered to be mere by-products of metabolism, reactive oxygen, nitrogen and sulfur species are now recognized to play important roles in diverse cellular processes such as response to pathogens and regulation of cellular differentiation. It is becoming increasingly evident that redox imbalance can impact several signaling pathways. For instance, disturbances of redox regulation in the brain mediate neurodegeneration and alter normal cytoprotective responses to stress. Very often small disturbances in redox signaling processes, which are reversible, precede damage in neurodegeneration. Recent Advances: The identification of redox-regulated processes, such as regulation of biochemical pathways involved in the maintenance of redox homeostasis in the brain has provided deeper insights into mechanisms of neuroprotection and neurodegeneration. Recent studies have also identified several post-translational modifications involving reactive cysteine residues, such as nitrosylation and sulfhydration, which fine-tune redox regulation. Thus, the study of mechanisms via which cell death occurs in several neurodegenerative disorders, reveal several similarities and dissimilarities. Here, we review redox regulated events that are disrupted in neurodegenerative disorders and whose modulation affords therapeutic opportunities.

CRITICAL ISSUES

Although accumulating evidence suggests that redox imbalance plays a significant role in progression of several neurodegenerative diseases, precise understanding of redox regulated events is lacking. Probes and methodologies that can precisely detect and quantify in vivo levels of reactive oxygen, nitrogen and sulfur species are not available.

FUTURE DIRECTIONS

Due to the importance of redox control in physiologic processes, organisms have evolved multiple pathways to counteract redox imbalance and maintain homeostasis. Cells and tissues address stress by harnessing an array of both endogenous and exogenous redox active substances. Targeting these pathways can help mitigate symptoms associated with neurodegeneration and may provide avenues for novel therapeutics. Antioxid. Redox Signal. 30, 1450-1499.

Impaired Redox Signaling in Huntington's Disease: Therapeutic Implications

Huntington's disease (HD) is a neurodegenerative disease triggered by expansion of polyglutamine repeats in the protein huntingtin. Mutant huntingtin (mHtt) aggregates and elicits toxicity by multiple mechanisms which range from dysregulated transcription to disturbances in several metabolic pathways in both the brain and peripheral tissues. Hallmarks of HD include elevated oxidative stress and imbalanced redox signaling. Disruption of antioxidant defense mechanisms, involving antioxidant molecules and enzymes involved in scavenging or reversing oxidative damage, have been linked to the pathophysiology of HD. In addition, mitochondrial function is compromised in HD leading to impaired bioenergetics and elevated production of free radicals in cells. However, the exact mechanisms linking redox imbalance to neurodegeneration are still elusive. This review will focus on the current understanding of aberrant redox homeostasis in HD and potential therapeutic interventions.

Identification of a bilirubin receptor that may mediate a component of cholestatic itch

Various pathologic conditions result in jaundice, a yellowing of the skin due to a buildup of bilirubin. Patients with jaundice commonly report experiencing an intense non-histaminergic itch. Despite this association, the pruritogenic capacity of bilirubin itself has not been described, and no bilirubin receptor has been identified. Here, we demonstrate that pathophysiologic levels of bilirubin excite peripheral itch sensory neurons and elicit pruritus through MRGPRs, a family of G-protein coupled receptors expressed in primary sensory neurons. Bilirubin binds and activates two MRGPRs, mouse MRGPRA1 and human MRGPRX4. In two mouse models of pathologic hyperbilirubinemia, we show that genetic deletion of either Mrgpra1 or Blvra, the gene that encodes the bilirubin-producing enzyme biliverdin reductase, attenuates itch. Similarly, plasma isolated from hyperbilirubinemic patients evoked itch in wild-type animals but not Mrgpra1^-/- animals. Removing bilirubin decreased the pruritogenic capacity of patient plasma. Based on these data, targeting MRGPRs is a promising strategy for alleviating jaundice-associated itch.

Jaundice causes the skin to yellow as a result of a build-up of a pigment called bilirubin. Normally, bilirubin is made in the liver and removed from the body in digestive fluid called bile, but people with liver or gallbladder problems may end up with too much bilirubin that accumulates in their blood and skin. One side effect of jaundice is intense and uncontrollable itching. Researchers are not sure what causes this itching, and there are few treatments that help to relieve it.

At the molecular level, itching sensations occur when compounds bind to particular receptors on the surface of nerve cells. One family of receptors that can trigger itch is called the Mas-related G-protein Coupled Receptor (MRGPR). Could one of these receptors trigger jaundice-related itching?

Now, Meixiong, Vasavda et al. show that bilirubin binds to and activates MRGPRs to cause itch in mice. Whereas injecting bilirubin into normal mice causes them to scratch, mice that have been genetically engineered to lack MRGPRs do not itch when their own bilirubin levels rise, or when they are injected with bilirubin or with plasma from patients who experience jaundice-related itching. Furthermore, removing bilirubin from the plasma of patients before it was injected into normal mice reduced the amount of itching that the mice felt.

Overall, the results reported by Meixiong, Vasavda et al. suggest that drugs that prevent bilirubin from attaching to MRGPRs might help to alleviate jaundice-related itching. However, researchers must first verify that bilirubin interacts with MRGPRs in people to cause itch. If bilirubin causes itch in people like in mice, scientists could then evaluate existing drugs or make new ones to prevent bilirubin from attaching to the MRGPRs.

Regulators of the transsulfuration pathway

The transsulfuration pathway is a metabolic pathway where transfer of sulfur from homocysteine to cysteine occurs. The pathway leads to the generation of several sulfur metabolites, which include cysteine, GSH and the gaseous signalling molecule hydrogen sulfide (H₂ S). Precise control of this pathway is critical for maintenance of optimal cellular function and, therefore, the key enzymes of the pathway, cystathionine β-synthase and cystathionine γ-lyase, are regulated at multiple levels. Disruption of the transsulfuration pathway contributes to the pathology of several conditions such as vascular dysfunction, Huntington's disease and during ageing. Treatment with donors of hydrogen sulfide and/or stimulation of this pathway have proved beneficial in several of these disorders. In this review, we focus on the regulation of the transsulfuration pathway pertaining to cysteine and H₂ S, which could be targeted to develop novel therapeutics. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Multiple aspects of male germ cell development and interactions with Sertoli cells require inositol hexakisphosphate kinase-1

Inositol hexakisphosphate kinase-1 (IP6K1) is required for male fertility, but the underlying mechanisms have been elusive. Here, we report that IP6K1 is required for multiple aspects of male germ cell development. This development requires selective interactions between germ cells and Sertoli cells, namely apical ectoplasmic specialization. Spermiation (sperm release) requires tubulobulbar complexes. We found that the apical ectoplasmic specialization and tubulobulbar complexes were poorly formed or disrupted in IP6K1 KOs. Deletion of IP6K1 elicited several aberrations, including: 1, sloughing off of round germ cells; 2, disorientation and malformation of elongating/elongated spermatids; 3, degeneration of acrosomes; 4, defects in germ-Sertoli cell interactions and 5, failure of spermiation. Eventually the sperm cells were not released but phagocytosed by Sertoli cells leading to an absence of sperm in the epididymis.

Developmental Alcohol Exposure Impairs Activity-Dependent S-Nitrosylation of NDEL1 for Neuronal Maturation

Neuronal nitric oxide synthase is involved in diverse signaling cascades that regulate neuronal development and functions via S-Nitrosylation-mediated mechanism or the soluble guanylate cyclase (sGC)/cyclic guanosine monophosphate (cGMP) pathway activated by nitric oxide. Although it has been studied extensively in vitro and in invertebrate animals, effects on mammalian brain development and underlying mechanisms remain poorly understood. Here we report that genetic deletion of "Nos1" disrupts dendritic development, whereas pharmacological inhibition of the sGC/cGMP pathway does not alter dendritic growth during cerebral cortex development. Instead, nuclear distribution element-like (NDEL1), a protein that regulates dendritic development, is specifically S-nitrosylated at cysteine 203, thereby accelerating dendritic arborization. This post-translational modification is enhanced by N-methyl-D-aspartate receptor-mediated neuronal activity, the main regulator of dendritic formation. Notably, we found that disruption of S-Nitrosylation of NDEL1 mediates impaired dendritic maturation caused by developmental alcohol exposure, a model of developmental brain abnormalities resulting from maternal alcohol use. These results highlight S-Nitrosylation as a key activity-dependent mechanism underlying neonatal brain maturation and suggest that reduction of S-Nitrosylation of NDEL1 acts as a pathological factor mediating neurodevelopmental abnormalities caused by maternal alcohol exposure.

Dimethyl fumarate targets GAPDH and aerobic glycolysis to modulate immunity

Activated immune cells undergo a metabolic switch to aerobic glycolysis akin to the Warburg effect, thereby presenting a potential therapeutic target in autoimmune disease. Dimethyl fumarate (DMF), a derivative of the Krebs cycle intermediate fumarate, is an immunomodulatory drug used to treat multiple sclerosis and psoriasis. Although its therapeutic mechanism remains uncertain, DMF covalently modifies cysteine residues in a process termed succination. We found that DMF succinates and inactivates the catalytic cysteine of the glycolytic enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in mice and humans, both in vitro and in vivo. It thereby down-regulates aerobic glycolysis in activated myeloid and lymphoid cells, which mediates its anti-inflammatory effects. Our results provide mechanistic insight into immune modulation by DMF and represent a proof of concept that aerobic glycolysis is a therapeutic target in autoimmunity.

Cysteine Metabolism in Neuronal Redox Homeostasis

Besides its essential role in protein synthesis, cysteine plays vital roles in redox homeostasis, being a component of the major antioxidant glutathione (GSH) and a potent antioxidant by itself. In addition, cysteine undergoes a variety of post-translational modifications that modulate several physiological processes. It is becoming increasingly clear that redox-modulated events play important roles not only in peripheral tissues but also in the brain where cysteine disposition is central to these pathways. Dysregulated cysteine metabolism is associated with several neurodegenerative disorders. Accordingly, restoration of cysteine balance has therapeutic benefits. This review discusses metabolic signaling pathways pertaining to cysteine disposition in the brain under normal and pathological conditions, highlighting recent findings on cysteine metabolism during aging and in neurodegenerative conditions such as Huntington's disease (HD) and molybdenum cofactor (MoCo) deficiency (MoCD) among others.

Gasotransmitter hydrogen sulfide signaling in neuronal health and disease

Hydrogen sulfide is a gaseous signaling molecule or gasotransmitter which plays important roles in a wide spectrum of physiologic processes in the brain and peripheral tissues. Unlike nitric oxide and carbon monoxide, the other major gasotransmitters, research on hydrogen sulfide is still in its infancy. One of the modes by which hydrogen sulfide signals is via a posttranslational modification termed sulfhydration/persulfidation, which occurs on reactive cysteine residues on target proteins, where the reactive SH group is converted to an SSH group. Sulfhydration is a substantially prevalent modification, which modulates the structure or function of proteins being modified. Thus, precise control of endogenous hydrogen sulfide production and metabolism is critical for maintenance of optimal cellular function, with excess generation and paucity, both contributing to pathology. Dysregulation of the reverse transsulfuration pathway which generates hydrogen sulfide occurs in several neurodegenerative diseases such as Parkinson's disease, Huntington's disease and Alzheimer's disease. Accordingly, treatment with donors of hydrogen sulfide or stimulation of the reverse transsulfuration have proved beneficial in several neurodegenerative states. In this review we focus on hydrogen sulfide mediated neuronal signaling processes that contribute to neuroprotection.

Inositol Polyphosphate Multikinase Inhibits Angiogenesis via Inositol Pentakisphosphate-Induced HIF-1α Degradation

RATIONALE

Inositol polyphosphate multikinase (IPMK) and its major product inositol pentakisphosphate (IP5) regulate a variety of cellular functions, but their role in vascular biology remains unexplored.

OBJECTIVE

We have investigated the role of IPMK in regulating angiogenesis.

METHODS AND RESULTS

Deletion of IPMK in fibroblasts induces angiogenesis in both in vitro and in vivo models. IPMK deletion elicits a substantial increase of VEGF (vascular endothelial growth factor), which mediates the regulation of angiogenesis by IPMK. The regulation of VEGF by IPMK requires its catalytic activity. IPMK is predominantly nuclear and regulates gene transcription. However, IPMK does not apparently serve as a transcription factor for VEGF. HIF (hypoxia-inducible factor)-1α is a major determinant of angiogenesis and induces VEGF transcription. IPMK deletion elicits a major enrichment of HIF-1α protein and thus VEGF. HIF-1α is constitutively ubiquitinated by pVHL (von Hippel-Lindau protein) followed by proteasomal degradation under normal conditions. However, HIF-1α is not recognized and ubiquitinated by pVHL in IPMK KO (knockout) cells. IP5 reinstates the interaction of HIF-1α and pVHL. HIF-1α prolyl hydroxylation, which is prerequisite for pVHL recognition, is interrupted in IPMK-deleted cells. IP5 promotes HIF-1α prolyl hydroxylation and thus pVHL-dependent degradation of HIF-1α. Deletion of IPMK in mouse brain increases HIF-1α/VEGF levels and vascularization. The increased VEGF in IPMK KO disrupts blood-brain barrier and enhances brain blood vessel permeability.

CONCLUSIONS

IPMK, via its product IP5, negatively regulates angiogenesis by inhibiting VEGF expression. IP5 acts by enhancing HIF-1α hydroxylation and thus pVHL-dependent degradation of HIF-1α.

DISC1 in Astrocytes Influences Adult Neurogenesis and Hippocampus-Dependent Behaviors in Mice

The functional role of genetic variants in glia in the pathogenesis of psychiatric disorders remains poorly studied. Disrupted-In-Schizophrenia 1 (DISC1), a genetic risk factor implicated in major mental disorders, has been implicated in regulation of astrocyte functions. As both astrocytes and DISC1 influence adult neurogenesis in the dentate gyrus (DG) of the hippocampus, we hypothesized that selective expression of dominant-negative C-terminus-truncated human DISC1 (mutant DISC1) in astrocytes would affect adult hippocampal neurogenesis and hippocampus-dependent behaviors. A series of behavioral tests were performed in mice with or without expression of mutant DISC1 in astrocytes during late postnatal development. In conjunction with behavioral tests, we evaluated adult neurogenesis, including neural progenitor proliferation and dendrite development of newborn neurons in the DG. The ameliorative effects of D-serine on mutant DISC1-associated behaviors and abnormal adult neurogenesis were also examined. Expression of mutant DISC1 in astrocytes decreased neural progenitor proliferation and dendrite growth of newborn neurons, and produced elevated anxiety, attenuated social behaviors, and impaired hippocampus-dependent learning and memory. Chronic treatment with D-serine ameliorated the behavioral alterations and rescued abnormal adult neurogenesis in mutant DISC1 mice. Our findings suggest that psychiatric genetic risk factors expressed in astrocytes could affect adult hippocampal neurogenesis and contribute to aspects of psychiatric disease through abnormal production of D-serine.

Antidepressant Actions of Ketamine Mediated by the Mechanistic Target of Rapamycin, Nitric Oxide, and Rheb

The weeks/months it takes for traditional antidepressants to act pose an obstacle in the management of depression. Ketamine's prompt and sustained antidepressant effects constitute a major advance. Multiple studies implicate glutamatergic signaling to protein synthesis machinery and synapse formation in ketamine's antidepressant effects. Here we review evidence linking ketamine to glutamate receptor subtypes and protein homeostasis. We describe a signaling cascade wherein nitric oxide drives the formation of a ternary protein complex comprised of glyceraldehyde 3-phosphate dehydrogenase, seven in absentia homolog 1, and Ras homolog enriched in brain downstream of the glutamate N-methyl-D-aspartate receptor. Seven in absentia homolog 1 ubiquitylates and degrades Ras homolog enriched in brain leading to inhibition of mechanistic target of rapamycin. Ketamine inhibits this molecular cascade leading to activation of mechanistic target of rapamycin and, in turn, to antidepressant actions.

Measuring G-protein-coupled Receptor Signaling via Radio-labeled GTP Binding

G-Protein-Coupled Receptors (GPCRs) are a large family of transmembrane receptors that play critical roles in normal cellular physiology and constitute a major pharmacological target for multiple indications, including analgesia, blood pressure regulation, and the treatment of psychiatric disease. Upon ligand binding, GPCRs catalyze the activation of intracellular G-proteins by stimulating the incorporation of guanosine triphosphate (GTP). Activated G-proteins then stimulate signaling pathways that elicit cellular responses. GPCR signaling can be monitored by measuring the incorporation of a radiolabeled and non-hydrolyzable form of GTP, [³⁵S]guanosine-5'-O-(3-thio)triphosphate ([³⁵S]GTPγS), into G-proteins. Unlike other methods that assess more downstream signaling processes, [³⁵S]GTPγS binding measures a proximal event in GPCR signaling and, importantly, can distinguish agonists, antagonists, and inverse agonists. The present protocol outlines a sensitive and specific method for studying GPCR signaling using crude membrane preparations of an archetypal GPCR, the µ-opioid receptor (MOR1). Although alternative approaches to fractionate cells and tissues exist, many are cost-prohibitive, tedious, and/or require non-standard laboratory equipment. The present method provides a simple procedure that enriches functional crude membranes. After isolating MOR1, various pharmacological properties of its agonist, [D-Ala, N-MePhe, Gly-ol]-enkephalin (DAMGO), and antagonist, naloxone, were determined.

Mutant Huntingtin Disrupts the Nuclear Pore Complex

Huntington's disease (HD) is caused by an expanded CAG repeat in the Huntingtin (HTT) gene. The mechanism(s) by which mutant HTT (mHTT) causes disease is unclear. Nucleocytoplasmic transport, the trafficking of macromolecules between the nucleus and cytoplasm, is tightly regulated by nuclear pore complexes (NPCs) made up of nucleoporins (NUPs). Previous studies offered clues that mHTT may disrupt nucleocytoplasmic transport and a mutation of an NUP can cause HD-like pathology. Therefore, we evaluated the NPC and nucleocytoplasmic transport in multiple models of HD, including mouse and fly models, neurons transfected with mHTT, HD iPSC-derived neurons, and human HD brain regions. These studies revealed severe mislocalization and aggregation of NUPs and defective nucleocytoplasmic transport. HD repeat-associated non-ATG (RAN) translation proteins also disrupted nucleocytoplasmic transport. Additionally, overexpression of NUPs and treatment with drugs that prevent aberrant NUP biology also mitigated this transport defect and neurotoxicity, providing future novel therapy targets.

Opportunities for the repurposing of PARP inhibitors for the therapy of non-oncological diseases

The recent clinical availability of the PARP inhibitor olaparib (Lynparza) opens the door for potential therapeutic repurposing for non-oncological indications. Considering (a) the preclinical efficacy data with PARP inhibitors in non-oncological diseases and (b) the risk-benefit ratio of treating patients with a compound that inhibits an enzyme that has physiological roles in the regulation of DNA repair, we have selected indications, where (a) the severity of the disease is high, (b) the available therapeutic options are limited, and (c) the duration of PARP inhibitor administration could be short, to provide first-line options for therapeutic repurposing. These indications are as follows: acute ischaemic stroke; traumatic brain injury; septic shock; acute pancreatitis; and severe asthma and severe acute lung injury. In addition, chronic, devastating diseases, where alternative therapeutic options cannot halt disease development (e.g. Parkinson's disease, progressive multiple sclerosis or severe fibrotic diseases), should also be considered. We present a preclinical and clinical action plan for the repurposing of PARP inhibitors.

LINKED ARTICLES

This article is part of a themed section on Inventing New Therapies Without Reinventing the Wheel: The Power of Drug Repurposing. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.2/issuetoc.

A Life of Neurotransmitters

Development of scientific creativity is often tied closely to mentorship. In my case, two years with Julius Axelrod, the sum total of my research training, was transformative. My mentoring generations of graduate students and postdoctoral fellows has been as nurturing for me as it has been for them. Work in our lab over fifty years has covered the breadth of neurotransmitters and related substances, focusing on the discovery and characterization of novel messenger molecules. I can't conceptualize a more rewarding professional life.

H2S production by reactive oxygen species in the carotid body triggers hypertension in a rodent model of sleep apnea

Sleep apnea is a prevalent respiratory disease in which episodic cessation of breathing causes intermittent hypoxia. Patients with sleep apnea and rodents exposed to intermittent hypoxia exhibit hypertension. The carotid body senses changes in blood O2 concentrations, and an enhanced carotid body chemosensory reflex contributes to hypertension in sleep apnea patients. A rodent model of intermittent hypoxia that mimics blood O2 saturation profiles of patients with sleep apnea has shown that increased generation of reactive oxygen species (ROS) in the carotid body enhances the chemosensory reflex and triggers hypertension. CO generated by heme oxygenase-2 (HO-2) induces a signaling pathway that inhibits hydrogen sulfide (H2S) production by cystathionine γ-lyase (CSE), leading to suppression of carotid body activity. We found that ROS inhibited CO generation by HO-2 in the carotid body and liver through a mechanism that required Cys(265) in the heme regulatory motif of heterologously expressed HO-2. We showed that ROS induced by intermittent hypoxia inhibited CO production and increased H2S concentrations in the carotid body, which stimulated its neural activity. In rodents, blockade of H2S synthesis by CSE, by either pharmacologic or genetic approaches, inhibited carotid body activation and hypertension induced by intermittent hypoxia. Thus, our results indicate that oxidant-induced inactivation of HO-2, which leads to increased CSE-dependent H2S production in the carotid body, is a critical trigger of hypertension in rodents exposed to intermittent hypoxia.