Browsing by Subject "Synapses"

Now showing 1 - 8 of 8
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    Dual regulation of voltage- and ligand-gated calcium channels by collapsin response mediator protein 2
    (2013-10-07) Brittain, Joel Matthew; Khanna, Rajesh; Cummins, Theodore R.; Oxford, G. S.; Quilliam, Lawrence; Thurmond, Debbie C.
    Synaptic transmission is coordinated by a litany of protein-protein interactions that rely on the proper localization and function of pre- and post-synaptic Ca2+ channels. The axonal guidance/specification collapsin response mediator protein-2 (CRMP-2) was identified as a potential partner of the pre-synaptic N-type voltage-gated Ca2+ channel (CaV2.2). CRMP-2 bound directly to CaV2.2 in two regions; the channel domain I-II intracellular loop and the distal C-terminus. Both proteins co-localized within presynaptic sites in hippocampal neurons. Overexpression in hippocampal neurons of a CRMP-2 protein fused to EGFP caused a significant increase in Ca2+ channel current density whereas lentivirus-mediated CRMP-2 knockdown abolished this effect. Cell surface biotinylation studies showed an increased number of CaV2.2 at the cell surface in CRMP-2–overexpressing neurons. Both activity- and CRMP-2-phosphoryation altered the interaction between CaV2.2 and CRMP-2. I identified a CRMP-2-derived peptide (called CBD3) that bound CaV2.2 and effectively disrupted the interaction between CaV2.2 and CRMP-2. CBD3 peptide fused to the HIV TAT protein (TAT-CBD3) decreased neuropeptide release from sensory neurons and excitatory synaptic transmission in dorsal horn neurons, and reversed neuropathic hypersensitivity produced by an antiretroviral drug. Unchecked Ca2+ influx via N-methyl-D-aspartate receptors (NMDARs) has been linked to activation of neurotoxic cascades culminating in cell death (i.e. excitotoxicity). CRMP-2 was suggested to affect NMDAR trafficking and possibly involved in neuronal survival following excitotoxicity. Based upon these studies, I hypothesized that a peptide from CRMP2 could preserve neurons in the face of excitotoxic challenges. Lentiviral–mediated CRMP2 knockdown or treatment with TAT-CBD3 blocked neuronal death following glutamate exposure likely via blunting toxicity from NMDAR-mediated delayed calcium deregulation. TAT-CBD3 induced internalization of the NMDAR subunit NR2B in dendritic spines without altering somal surface expression. TAT-CBD3 reduced NMDA-mediated Ca2+-influx and currents in cultured neurons. The presented work validates CRMP-2 as a novel modulator of pre- and post-synaptic Ca2+ channels and provides evidence that the TAT-CBD3 peptide could be useful as a potential therapeutic for both chronic neuropathic pain and excitotoxicity following stroke or other neuronal insults.
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    F-Actin regulation of SNARE-mediated insulin secretion
    (2013-10-07) Kalwat, Michael Andrew; Thurmond, Debbie C.; Atkinson, Simon; Hudmon, Andy; Mirmira, Raghavendra G.
    In response to glucose, pancreatic islet beta cells secrete insulin in a biphasic manner, and both phases are diminished in type 2 diabetes. In beta cells, cortical F-actin beneath the plasma membrane (PM) prevents insulin granule access to the PM and glucose stimulates remodeling of this cortical F-actin to allow trafficking of insulin granules to the PM. Glucose stimulation activates the small GTPase Cdc42, which then activates p21-activated kinase 1 (PAK1); both Cdc42 and PAK1 are required for insulin secretion. In conjunction with Cdc42-PAK1 signaling, the SNARE protein Syntaxin 4 dissociates from F-actin to allow SNARE complex formation and insulin exocytosis. My central hypothesis is that, in the pancreatic beta cell, glucose signals through a Cdc42-PAK1-mediated pathway to remodel the F-actin cytoskeleton to mobilize insulin granules to SNARE docking sites at the PM to evoke glucose stimulated second phase insulin secretion. To investigate this, PAK1 was inhibited in MIN6 beta cells with IPA3 followed by live-cell imaging of F-actin remodeling using the F-actin probe, Lifeact-GFP. PAK1 inhibition prevented normal glucose-induced F-actin remodeling. PAK1 inhibition also prevented insulin granule accumulation at the PM in response to glucose. The ERK pathway was implicated, as glucose-stimulated ERK activation was decreased under PAK1-depleted conditions. Further study showed that inhibition of ERK impaired insulin secretion and cortical F-actin remodeling. One of the final steps of insulin secretion is the fusion of insulin granules with the PM which is facilitated by the SNARE proteins Syntaxin 4 on the PM and VAMP2 on the insulin granule. PAK1 activation was also found to be critical for Syntaxin 4-F-actin complex dynamics in beta cells, linking the Cdc42-PAK1 signaling pathway to SNARE-mediated exocytosis. Syntaxin 4 interacts with the F-actin severing protein Gelsolin, and in response to glucose Gelsolin dissociates from Syntaxin 4 in a calcium-dependent manner to allow Syntaxin 4 activation. Disrupting the interaction between Syntaxin 4 and Gelsolin aberrantly activates endogenous Syntaxin 4, elevating basal insulin secretion. Taken together, these results illustrate that signaling to F-actin remodeling is important for insulin secretion and that F-actin and its binding proteins can impact the final steps of insulin secretion.
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    Involvement of Collapsin Response Mediator Protein 2 in Posttraumatic Sprouting in Acquired Epilepsy
    (2014) Wilson, Sarah Marie; Oxford, Gerry S.; Khanna, Rajesh; Jen, Joanna; Xu, Zao C.; Jin, Xiao-Ming
    Posttraumatic epilepsy, the development of temporal lobe epilepsy (TLE) following traumatic brain injury, accounts for 20% of symptomatic epilepsy. Reorganization of mossy fibers within the hippocampus is a common pathological finding of TLE. Normal mossy fibers project into the CA3 region of the hippocampus where they form synapses with pyramidal cells. During TLE, mossy fibers are observed to innervate the inner molecular layer where they synapse onto the dendrites of other dentate granule cells, leading to the formation of recurrent excitatory circuits. To date, the molecular mechanisms contributing to mossy fiber sprouting are relatively unknown. Recent focus has centered on the involvement of tropomycin-related kinase receptor B (TrkB), which culminates in glycogen synthase kinase 3β (GSK3β) inactivation. As the neurite outgrowth promoting collapsin response mediator protein 2 (CRMP2) is rendered inactive by GSK3β phosphorylation, events leading to inactivation of GSK3β should therefore increase CRMP2 activity. To determine the involvement of CRMP2 in mossy fiber sprouting, I developed a novel tool ((S)-LCM) for selectively targeting the ability of CRMP2 to enhance tubulin polymerization. Using (S)-LCM, it was demonstrated that increased neurite outgrowth following GSK3β inactivation is CRMP2 dependent. Importantly, TBI led to a decrease in GSK3β-phosphorylated CRMP2 within 24 hours which was secondary to the inactivation of GSK3β. The loss of GSK3β-phosphorylated CRMP2 was maintained even at 4 weeks post-injury, despite the transience of GSK3β-inactivation. Based on previous work, it was hypothesized that activity-dependent mechanisms may be responsible for the sustained loss of CRMP2 phosphorylation. Activity-dependent regulation of GSK3β-phosphorylated CRMP2 levels was observed that was attributed to a loss of priming by cyclin dependent kinase 5 (CDK5), which is required for subsequent phosphorylation by GSK3β. It was confirmed that the loss of GSK3β-phosphorylated CRMP2 at 4 weeks post-injury was likely due to decreased phosphorylation by CDK5. As TBI resulted in a sustained increase in CRMP2 activity, I attempted to prevent mossy fiber sprouting by targeting CRMP2 in vivo following TBI. While (S)-LCM treatment dramatically reduced mossy fiber sprouting following TBI, it did not differ significantly from vehicle-treated animals. Therefore, the necessity of CRMP2 in mossy fiber sprouting following TBI remains unknown.
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    miR‐133b as a potential regulator of a synaptic NPTX2 protein in Alzheimer's disease
    (Wiley, 2024) Han, Sang-Won; Park, Young Ho; Bice, Paula J.; Bennett, David A.; Kim, SangYun; Saykin, Andrew J.; Nho, Kwangsik; Radiology and Imaging Sciences, School of Medicine
    A synaptic protein, Neuronal Pentraxin 2 (NPTX2), has emerged as a pivotal biomarker for Alzheimer's dementia (AD). We identified candidate miRNAs targeting NPTX2 and performed association and mediation analyses using multi-omics data (N = 702). Among 44 candidate miRNAs, miR-133b was significantly associated with AD and Braak positivity. Higher miR-133b expression was also associated with higher NPTX2 gene expression and better cognition. Mediation analysis showed that miR-133b partially influences AD and cognition through the NPTX2 protein. Our integrated approach suggests a potential role of miR-133b in synaptic integrity and offers new insights into AD pathogenesis.
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    Oxidative metabolism in YAC128 mouse model of Huntington's disease
    (Oxford University Press, 2015-09-01) Hamilton, James; Pellman, Jessica J.; Brustovetsky, Tatiana; Harris, Robert A.; Brustovetsky, Nickolay; Department of Pharmacology and Toxicology, IU School of Medicine
    Alterations in oxidative metabolism are considered to be one of the major contributors to Huntington's disease (HD) pathogenesis. However, existing data about oxidative metabolism in HD are contradictory. Here, we investigated the effect of mutant huntingtin (mHtt) on oxidative metabolism in YAC128 mice. Both mHtt and wild-type huntingtin (Htt) were associated with mitochondria and the amount of bound Htt was four-times higher than the amount of bound mHtt. Percoll gradient-purified brain synaptic and non-synaptic mitochondria as well as unpurified brain, liver and heart mitochondria, isolated from 2- and 10-month-old YAC128 mice and age-matched WT littermates had similar respiratory rates. There was no difference in mitochondrial membrane potential or ADP and ATP levels. Expression of selected nuclear-encoded mitochondrial proteins in 2- and 10-month-old YAC128 and WT mice was similar. Cultured striatal and cortical neurons from YAC128 and WT mice had similar respiratory and glycolytic activities as measured with Seahorse XF24 analyzer in medium containing 10 mm glucose and 15 mm pyruvate. In the medium with 2.5 mm glucose, YAC128 striatal neurons had similar respiration, but slightly lower glycolytic activity. Striatal neurons had lower maximal respiration compared with cortical neurons. In vivo experiments with YAC128 and WT mice showed similar O2 consumption, CO2 release, physical activity, food consumption and fasted blood glucose. However, YAC128 mice were heavier and had more body fat compared with WT mice. Overall, our data argue against respiratory deficiency in YAC128 mice and, consequently, suggest that mitochondrial respiratory dysfunction is not essential for HD pathogenesis.
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    Syntaxin1A overexpression and pain insensitivity in individuals with 7q11.23 duplication syndrome
    (American Society for Clinical Investigation, 2024-02-22) Iadarola, Michael J.; Sapio, Matthew R.; Loydpierson, Amelia J.; Mervis, Carolyn B.; Fehrenbacher, Jill C.; Vasko, Michael R.; Maric, Dragan; Eisenberg, Daniel P.; Nash, Tiffany A.; Kippenhan, J. Shane; Garvey, Madeline H.; Mannes, Andrew J.; Gregory, Michael D.; Berman, Karen F.; Pharmacology and Toxicology, School of Medicine
    Genetic modifications leading to pain insensitivity phenotypes, while rare, provide invaluable insights into the molecular biology of pain and reveal targets for analgesic drugs. Pain insensitivity typically results from Mendelian loss-of-function mutations in genes expressed in nociceptive (pain-sensing) dorsal root ganglion (DRG) neurons that connect the body to the spinal cord. We document a pain insensitivity mechanism arising from gene overexpression in individuals with the rare 7q11.23 duplication syndrome (Dup7), who have 3 copies of the approximately 1.5-megabase Williams syndrome (WS) critical region. Based on parental accounts and pain ratings, people with Dup7, mainly children in this study, are pain insensitive following serious injury to skin, bones, teeth, or viscera. In contrast, diploid siblings (2 copies of the WS critical region) and individuals with WS (1 copy) show standard reactions to painful events. A converging series of human assessments and cross-species cell biological and transcriptomic studies identified 1 likely candidate in the WS critical region, STX1A, as underlying the pain insensitivity phenotype. STX1A codes for the synaptic vesicle fusion protein syntaxin1A. Excess syntaxin1A was demonstrated to compromise neuropeptide exocytosis from nociceptive DRG neurons. Taken together, these data indicate a mechanism for producing “genetic analgesia” in Dup7 and offer previously untargeted routes to pain control.