title: Astrocytic GAP43 Induced by the TLR4/NF-κB/STAT3 Axis Attenuates Astrogliosis-Mediated Microglial Activation and Neurotoxicity.
abstract: Growth-associated protein 43 (GAP43), a protein kinase C (PKC)-activated phosphoprotein, is often implicated in axonal plasticity and regeneration. In this study, we found that GAP43 can be induced by the endotoxin lipopolysaccharide (LPS) in rat brain astrocytes both in vivo and in vitro. The LPS-induced astrocytic GAP43 expression was mediated by Toll-like receptor 4 and nuclear factor-κB (NF-κB)- and interleukin-6/signal transducer and activator of transcription 3 (STAT3)-dependent transcriptional activation. The overexpression of the PKC phosphorylation-mimicking GAP43(S41D) (constitutive active GAP43) in astrocytes mimicked LPS-induced process arborization and elongation, while application of a NF-κB inhibitory peptide TAT-NBD or GAP43(S41A) (dominant-negative GAP43) or knockdown of GAP43 all inhibited astrogliosis responses. Moreover, GAP43 knockdown aggravated astrogliosis-induced microglial activation and expression of proinflammatory cytokines. We also show that astrogliosis-conditioned medium from GAP43 knock-down astrocytes inhibited GAP43 phosphorylation and axonal growth, and increased neuronal damage in cultured rat cortical neurons. These proneurotoxic effects of astrocytic GAP43 knockdown were accompanied by attenuated glutamate uptake and expression of the glutamate transporter excitatory amino acid transporter 2 (EAAT2) in LPS-treated astrocytes. The regulation of EAAT2 expression involves actin polymerization-dependent activation of the transcriptional coactivator megakaryoblastic leukemia 1 (MKL1), which targets the serum response elements in the promoter of rat Slc1a2 gene encoding EAAT2. In sum, the present study suggests that astrocytic GAP43 mediates glial plasticity during astrogliosis, and provides beneficial effects for neuronal plasticity and survival and attenuation of microglial activation.
  
  
  CONCLUSIONS
  Astrogliosis is a complex state in which injury-stimulated astrocytes exert both protective and harmful effects on neuronal survival and plasticity. In this study, we demonstrated for the first time that growth-associated protein 43 (GAP43), a well known growth cone protein that promotes axonal regeneration, can be induced in rat brain astrocytes by the proinflammatory endotoxin lipopolysaccharide via both nuclear factor-κB and signal transducer and activator of transcription 3-mediated transcriptional activation. Importantly, LPS-induced GAP43 mediates plastic changes of astrocytes while attenuating astrogliosis-induced microglial activation and neurotoxicity. Hence, astrocytic GAP43 upregulation may serve to indicate beneficial astrogliosis after CNS injury.
authors: Graduate Institute of Medical Sciences and Departments of Physiology and Department and Institute of Physiology, College of Medicine.
authors: Chia-Chi
authors: CC
authors: Hung
authors: Department of Nursing, Kang-Ning University, Taipei 114, Taiwan.
authors: Chun-Hua
authors: CH
authors: Lin
authors: Department and Institute of Physiology, College of Medicine.
authors: Hsuan
authors: H
authors: Chang
authors: Department and Institute of Physiology, College of Medicine, Brain Research Center, and.
authors: Chen-Yu
authors: http://orcid.org/0000-0002-5659-2207
authors: CY
authors: Wang
authors: Department and Institute of Physiology, College of Medicine.
authors: Shang-Hsuan
authors: SH
authors: Lin
authors: Department and Institute of Physiology, College of Medicine, Brain Research Center, and.
authors: Pei-Chien
authors: PC
authors: Hsu
authors: Department of Pediatrics, Division of Neurology, Emory University School of Medicine and Children's Healthcare of Atlanta, Atlanta, Georgia 30322.
authors: Yu-Yo
authors: YY
authors: Sun
authors: Institute of Biomedical Sciences, Academia Sinica, Taipei 115, Taiwan, and.
authors: Teng-Nan
authors: TN
authors: Lin
authors: Center for Neuropsychiatric Research, National Health Research Institutes, Miaoli County 350, Taiwan.
authors: Feng-Shiun
authors: FS
authors: Shie
authors: Brain Research Center, and Department of Life Sciences, Institute of Genomic Sciences, National Yang-Ming University, Taipei 112, Taiwan.
authors: Lung-Sen
authors: LS
authors: Kao
authors: Graduate Institute of Medical Sciences and Biochemistry and Molecular Cell Biology, College of Medicine, Taipei Medical University, Taipei 110, Taiwan.
authors: Chih-Ming
authors: CM
authors: Chou
authors: Graduate Institute of Medical Sciences and Departments of Physiology and Department and Institute of Physiology, College of Medicine, Brain Research Center, and yhlee3@ym.edu.tw.
authors: Yi-Hsuan
authors: http://orcid.org/0000-0002-3213-827X
authors: YH
authors: Lee
mesh_terms: D000818:Animals; D001253:Astrocytes / Q000187:drug effects*; D016207:Cytokines / Q000096:biosynthesis; D027342:Excitatory Amino Acid Transporter 2 / Q000096:biosynthesis / Q000235:genetics; D019922:GAP-43 Protein / Q000096:biosynthesis* / Q000235:genetics*; D005911:Gliosis / Q000235:genetics*; D008262:Macrophage Activation / Q000187:drug effects; D017628:Microglia / Q000187:drug effects*; D016328:NF-kappa B / Q000235:genetics*; D009474:Neurons; D020258:Neurotoxicity Syndromes / Q000235:genetics* / Q000473:pathology*; D010766:Phosphorylation; D051381:Rats; D017207:Rats, Sprague-Dawley; D050796:STAT3 Transcription Factor / Q000235:genetics*; D051197:Toll-Like Receptor 4 / Q000235:genetics*; D015534:Trans-Activators / Q000096:biosynthesis / Q000235:genetics; D014157:Transcription Factors
chemical_list: D016207:Cytokines; D027342:Excitatory Amino Acid Transporter 2; D019922:GAP-43 Protein; C000594702:Mrtfa protein, rat; D016328:NF-kappa B; D050796:STAT3 Transcription Factor; C493395:Slc1a2 protein, rat; C494088:Stat3 protein, rat; C495347:Tlr4 protein, rat; D051197:Toll-Like Receptor 4; D015534:Trans-Activators; D014157:Transcription Factors
keywords: EAAT2; GAP43; MKL1; astrogliosis; microglial activation; neurotoxicity