Trk receptors need neutral sphingomyelinase activity to promote cell viability

Membrane lipid raft homeostasis is directly linked to neurodegeneration

Age-associated neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD) are an unmet health need, with significant economic and societal implications, and an ever-increasing prevalence. Membrane lipid rafts (MLRs) are specialised plasma membrane microdomains that provide a platform for intracellular trafficking and signal transduction, particularly within neurons. Dysregulation of MLRs leads to disruption of neurotrophic signalling and excessive apoptosis which mirrors the final common pathway for neuronal death in ALS, PD and AD. Sphingomyelinase (SMase) and phospholipase (PL) enzymes process components of MLRs and therefore play central roles in MLR homeostasis and in neurotrophic signalling. We review the literature linking SMase and PL enzymes to ALS, AD and PD with particular attention to attractive therapeutic targets, where functional manipulation has been successful in preclinical studies. We propose that dysfunction of these enzymes is upstream in the pathogenesis of neurodegenerative diseases and to support this we provide new evidence that ALS risk genes are enriched with genes involved in ceramide metabolism (P=0.019, OR = 2.54, Fisher exact test). Ceramide is a product of SMase action upon sphingomyelin within MLRs, and it also has a role as a second messenger in intracellular signalling pathways important for neuronal survival. Genetic risk is necessarily upstream in a late age of onset disease such as ALS. We propose that manipulation of MLR structure and function should be a focus of future translational research seeking to ameliorate neurodegenerative disorders.

Interaction between a Novel Oligopeptide Fragment of the Human Neurotrophin Receptor TrkB Ectodomain D5 and the C-Terminal Fragment of Tetanus Neurotoxin

This article presents experimental evidence and computed molecular models of a potential interaction between receptor domain D5 of TrkB with the carboxyl-terminal domain of tetanus neurotoxin (Hc-TeNT). Computational simulations of a novel small cyclic oligopeptide are designed, synthesized, and tested for possible tetanus neurotoxin-D5 interaction. A hot spot of this protein-protein interaction is identified in analogy to the hitherto known crystal structures of the complex between neurotrophin and D5. Hc-TeNT activates the neurotrophin receptors, as well as its downstream signaling pathways, inducing neuroprotection in different stress cellular models. Based on these premises, we propose the Trk receptor family as potential proteic affinity receptors for TeNT. In vitro, Hc-TeNT binds to a synthetic TrkB-derived peptide and acts similar to an agonist ligand for TrkB, resulting in phosphorylation of the receptor. These properties are weakened by the mutagenesis of three residues of the predicted interaction region in Hc-TeNT. It also competes with Brain-derived neurotrophic factor, a native binder to human TrkB, for the binding to neural membranes, and for uptake in TrkB-positive vesicles. In addition, both molecules are located together in vivo at neuromuscular junctions and in motor neurons.

Alcohol Increases Exosome Release from Microglia to Promote Complement C1q-Induced Cellular Death of Proopiomelanocortin Neurons in the Hypothalamus in a Rat Model of Fetal Alcohol Spectrum Disorders

Microglia, a type of CNS immune cell, have been shown to contribute to ethanol-activated neuronal death of the stress regulatory proopiomelanocortin (POMC) neuron-producing β-endorphin peptides in the hypothalamus in a postnatal rat model of fetal alcohol spectrum disorders. We determined whether the microglial extracellular vesicle exosome is involved in the ethanol-induced neuronal death of the β-endorphin neuron. Extracellular vesicles were prepared from hypothalamic tissues collected from postnatal rats (both males and females) fed daily with 2.5 mg/kg ethanol or control milk formula for 5 d or from hypothalamic microglia cells obtained from postnatal rats, grown in cultures for several days, and then challenged with ethanol or vehicle for 24 h. Nanoparticle tracking analysis and transmission electron microscopy indicated that these vesicles had the size range and shape of exosomes. Ethanol treatments increased the number and the β-endorphin neuronal killing activity of microglial exosomes both in vivo and in vitro Proteomics analyses of exosomes of cultured microglial cells identified a large number of proteins, including various complements, which were elevated following ethanol treatment. Proteomics data involving complements were reconfirmed using quantitative protein assays. Ethanol treatments also increased deposition of the complement protein C1q in β-endorphin neuronal cells in both in vitro and in vivo systems. Recombinant C1q protein increased while C1q blockers reduced ethanol-induced C3a/b, C4, and membrane attack complex/C5b9 formations; ROS production; and ultimately cellular death of β-endorphin neurons. These data suggest that the complement system involving C1q-C3-C4-membrane attack complex and ROS regulates exosome-mediated, ethanol-induced β-endorphin neuronal death.SIGNIFICANCE STATEMENT Neurotoxic action of alcohol during the developmental period is recognized for its involvement in fetal alcohol spectrum disorders, but the lack of clear understanding of the mechanism of alcohol action has delayed the progress in therapeutic intervention of this disease. Proopiomelanocortin neurons known to regulate stress, energy homeostasis, and immune functions are reported to be killed by developmental alcohol exposure because of activation of microglial immune cells in the brain. While microglia are known to use extracellular vesicles to communicate with neurons for maintaining homeostasis, we show here that ethanol exposure during the developmental period hijacks this system to spread apoptotic factors, including complement protein C1q, to induce the membrane attack complex and reactive super-oxygen species for proopiomelanocortin neuronal killing.

Integrating Pathophysiology in Migraine: Role of the Gut Microbiome and Melatonin

BACKGROUND

The pathoetiology and pathophysiology of migraine are widely accepted as unknown.

METHODS

The current article reviews the wide array of data associated with the biological underpinnings of migraine and provides a framework that integrates previously disparate bodies of data.

RESULTS

The importance of alterations in stress- and pro-inflammatory cytokine- induced gut dysbiosis, especially butyrate production, are highlighted. This is linked to a decrease in the availability of melatonin, and a relative increase in the N-acetylserotonin/melatonin ratio, which has consequences for the heightened glutamatergic excitatory transmission in migraine. It is proposed that suboptimal mitochondria functioning and metabolic regulation drive alterations in astrocytes and satellite glial cells that underpin the vasoregulatory and nociceptive changes in migraine.

CONCLUSION

This provides a framework not only for classical migraine associated factors, such as calcitonin-gene related peptide and serotonin, but also for wider factors in the developmental pathoetiology of migraine. A number of future research and treatment implications arise, including the clinical utilization of sodium butyrate and melatonin in the management of migraine.

Effect of Sphingosine-1-Phosphate on Intracellular Free Ca²⁺ in Cat Esophageal Smooth Muscle Cells

A comprehensive collection of proteins senses local changes in intracellular Ca²⁺ concentrations ([Ca²⁺]_i) and transduces these signals into responses to agonists. In the present study, we examined the effect of sphingosine-1-phosphate (S1P) on modulation of intracellular Ca²⁺ concentrations in cat esophageal smooth muscle cells. To measure [Ca²⁺]_i levels in cat esophageal smooth muscle cells, we used a fluorescence microscopy with the Fura-2 loading method. S1P produced a concentration-dependent increase in [Ca²⁺]_i in the cells. Pretreatment with EGTA, an extracellular Ca²⁺ chelator, decreased the S1P-induced increase in [Ca²⁺]_i, and an L-type Ca²⁺-channel blocker, nimodipine, decreased the effect of S1P. This indicates that Ca²⁺ influx may be required for muscle contraction by S1P. When stimulated with thapsigargin, an intracellular calcium chelator, or 2-Aminoethoxydiphenyl borate (2-APB), an InsP₃ receptor blocker, the S1P-evoked increase in [Ca²⁺]_i was significantly decreased. Treatment with pertussis toxin (PTX), an inhibitor of G_i-protein, suppressed the increase in [Ca²⁺]_i evoked by S1P. These results suggest that the S1P-induced increase in [Ca²⁺]_i in cat esophageal smooth muscle cells occurs upon the activation of phospholipase C and subsequent release of Ca²⁺ from the InsP₃-sensitive Ca²⁺ pool in the sarcoplasmic reticulum. These results suggest that S1P utilized extracellular Ca²⁺ via the L type Ca²⁺ channel, which was dependent on activation of the S1P₄ receptor coupled to PTX-sensitive G_i protein, via phospholipase C-mediated Ca²⁺ release from the InsP₃-sensitive Ca²⁺ pool in cat esophageal smooth muscle cells.

Swedish Nerve Growth Factor Mutation (NGFR100W) Defines a Role for TrkA and p75NTR in Nociception

Nerve growth factor (NGF) exerts multiple functions on target neurons throughout development. The recent discovery of a point mutation leading to a change from arginine to tryptophan at residue 100 in the mature NGFβ sequence (NGFR100W) in patients with hereditary sensory and autonomic neuropathy type V (HSAN V) made it possible to distinguish the signaling mechanisms that lead to two functionally different outcomes of NGF: trophic versus nociceptive. We performed extensive biochemical, cellular, and live-imaging experiments to examine the binding and signaling properties of NGFR100W Our results show that, similar to the wild-type NGF (wtNGF), the naturally occurring NGFR100W mutant was capable of binding to and activating the TrkA receptor and its downstream signaling pathways to support neuronal survival and differentiation. However, NGFR100W failed to bind and stimulate the 75 kDa neurotrophic factor receptor (p75NTR)-mediated signaling cascades (i.e., the RhoA-Cofilin pathway). Intraplantar injection of NGFR100W into adult rats induced neither TrkA-mediated thermal nor mechanical acute hyperalgesia, but retained the ability to induce chronic hyperalgesia based on agonism for TrkA signaling. Together, our studies provide evidence that NGFR100W retains trophic support capability through TrkA and one aspect of its nociceptive signaling, but fails to engage p75NTR signaling pathways. Our findings suggest that wtNGF acts via TrkA to regulate the delayed priming of nociceptive responses. The integration of both TrkA and p75NTR signaling thus appears to regulate neuroplastic effects of NGF in peripheral nociception.SIGNIFICANCE STATEMENT In the present study, we characterized the naturally occurring nerve growth factor NGFR100W mutant that is associated with hereditary sensory and autonomic neuropathy type V. We have demonstrated for the first time that NGFR100W retains trophic support capability through TrkA, but fails to engage p75NTR signaling pathways. Furthermore, after intraplantar injection into adult rats, NGFR100W induced neither thermal nor mechanical acute hyperalgesia, but retained the ability to induce chronic hyperalgesia. We have also provided evidence that the integration of both TrkA- and p75NTR-mediated signaling appears to regulate neuroplastic effects of NGF in peripheral nociception. Our study with NGFR100W suggests that it is possible to uncouple trophic effect from nociceptive function, both induced by wild-type NGF.

Anti-GM1 ganglioside antibodies modulate membrane-associated sphingomyelin metabolism by altering neutral sphingomyelinase activity

Previous studies have shown that patients with Guillain-Barré syndrome express autoantibodies against ganglioside GM1 (GM1), although its pathogenic significance for the development of the disease remains to be elucidated. nSMase2 is the best characterized neutral sphingomyelinase (nSMase) found in neuronal cells. Activation of this enzyme leads to ceramide production, which is a known second messenger of the cell-death program in neuronal cells. We have explored the effects of anti-GM1 antibodies on sphingomyelin metabolism of PC12 cells stably transfected with human trk cDNA (PCtrk cells) by determining their effects on nSMase2 activity. The data we present here strongly suggest that anti-GM1 caused a significant change in sphingomyelin content of the membrane fraction in PCtrk cells. Both nSMase2 activity and the level of nSMase2 protein were significantly decreased by anti-GM1 treatment of PCtrk cells, while acidic SMase activities remained unchanged. Our results indicate, for the first time, that anti-GM1 may produce profound impacts on lipid metabolism in neuronal cell membranes.

Nogo-A interacts with TrkA to alter nerve growth factor signaling in Nogo-A-overexpressing PC12 cells

The Nogo-A protein, originally discovered as a potent myelin-associated inhibitor of neurite outgrowth, is also expressed by certain neurons, especially during development and after injury, but its role in neuronal function is not completely known. In this report, we overexpressed Nogo-A in PC12 cells to use as a model to identify potential neuronal signaling pathways affected by endogenously expressed Nogo-A. Unexpectedly, our results show that viability of Nogo-A-overexpressing cells was reduced progressively due to apoptotic cell death following NGF treatment, but only after 24 h. Inhibitors of neutral sphingomyelinase prevented this loss of viability, suggesting that NGF induced the activation of a ceramide-dependent cell death pathway. Nogo-A over-expression also changed NGF-induced phosphorylation of TrkA at tyrosines 490 and 674/675 from sustained to transient, and prevented the regulated intramembrane proteolysis of p75^NTR, indicating that Nogo-A was altering the function of the two neurotrophin receptors. Co-immunoprecipitation studies revealed that there was a physical association between TrkA and Nogo-A which appeared to be dependent on interactions in the Nogo-A-specific region of the protein. Taken together, our results indicate that Nogo-A influences NGF-mediated mechanisms involving the activation of TrkA and its interaction with p75^NTR.

Roles and regulation of neutral sphingomyelinase-2 in cellular and pathological processes

Our understanding of the functions of ceramide signaling has advanced tremendously over the past decade. In this review, we focus on the roles and regulation of neutral sphingomyelinase 2 (nSMase2), an enzyme that generates the bioactive lipid ceramide through the hydrolysis of the membrane lipid sphingomyelin. A large body of work has now implicated nSMase2 in a diverse set of cellular functions, physiological processes, and disease pathologies. We discuss different aspects of this enzyme's regulation from transcriptional, post-translational, and biochemical. Furthermore, we highlight nSMase2 involvement in cellular processes including inflammatory signaling, exosome generation, cell growth, and apoptosis, which in turn play important roles in pathologies such as cancer metastasis, Alzheimer's disease, and other organ systems disorders. Lastly, we examine avenues where targeted nSMase2-inhibition may be clinically beneficial in disease scenarios.

Identifying rare variants for genetic risk through a combined pedigree and phenotype approach: application to suicide and asthma

Suicidal behavior is a complex disorder, with evidence for genetic risk independent of other genetic risk factors including psychiatric disorders. Since 1996, over 3000 DNA samples from Utah suicide decedents have been collected and banked for research use through the Utah Medical Examiner. In addition, over 12,000 Utah suicides were identified through examination of death certificates back to 1904. By linking this data with the Utah Population Database, we have identified multiple extended pedigrees with increased risk for suicide completion. A number of medical conditions co-occur with suicide, including asthma, and this study was undertaken to identify genetic risk common to asthma and suicide. This study tests the hypothesis that a particular comorbid condition may identify a more homogeneous genetic subgroup, facilitating the identification of specific genetic risk factors in that group. From pedigrees at increased risk for suicide, we identified three pedigrees also at significantly increased familial risk for asthma. Five suicide decedents from each of these pedigrees, plus an additional three decedents not from these pedigrees with diagnosed asthma, and 10 decedents with close relatives with asthma were genotyped. Results were compared with 183 publicly available unaffected control exomes from 1000 Genomes and CEPH (Centre d'etude du polymorphisme humain) samples genotyped on the same platform. A further 432 suicide decedents were also genotyped as non-asthma suicide controls. Genotyping was done using the Infinium HumanExome BeadChip. For analysis, we used the pedigree extension of Variant Annotation, Analysis and Search Tool (pVAAST) to calculate the disease burden of each gene. The Phenotype Driven Variant Ontological Re-ranking tool (Phevor) then re-ranked our pVAAST results in context of the phenotype. Using asthma as a seed phenotype, Phevor traversed biomedical ontologies and identified genes with similar biological properties to those known to result in asthma. Our top associated genes included those related to neurodevelopment or neural signaling (brain-derived neurotrophic factor (BDNF), neutral sphingomyelinase 2 (SMPD2), homeobox b2 (HOXB2), neural cell adhesion molecule (NCAM2), heterogeneous nuclear ribonucleoprotein A0 (HNRNPA0)), inflammation (free fatty acid receptor 2 (FFAR2)) and inflammation with additional evidence of neuronal involvement (oxidized low density lipoprotein receptor 1 (OLR1), toll-like receptor 3 (TLR3)). Of particular interest, BDNF has been previously implicated in both psychiatric disorders and asthma. Our results demonstrate the utility of combining pedigree and co-occurring phenotypes to identify rare variants associated with suicide risk in conjunction with specific co-occurring conditions.