The Rheb-mTOR pathway is upregulated in reactive astrocytes of the injured spinal cord.
Astrocytes in the CNS respond to tissue damage by becoming reactive. They migrate, undergo hypertrophy, and form a glial scar that inhibits axon regeneration. Therefore, limiting astrocytic responses represents a potential therapeutic strategy to improve functional recovery. It was recently shown that the epidermal growth factor (EGF) receptor is upregulated in astrocytes after injury and promotes their transformation into reactive astrocytes. Furthermore, EGF receptor inhibitors were shown to enhance axon regeneration in the injured optic nerve and promote recovery after spinal cord injury. However, the signaling pathways involved were not elucidated. Here we show that in cultures of adult spinal cord astrocytes EGF activates the mTOR pathway, a key regulator of astrocyte physiology. This occurs through Akt-mediated phosphorylation of the GTPase-activating protein Tuberin, which inhibits Tuberin's ability to inactivate the small GTPase Rheb. Indeed, we found that Rheb is required for EGF-dependent mTOR activation in spinal cord astrocytes, whereas the Ras-MAP kinase pathway does not appear to be involved. Moreover, astrocyte growth and EGF-dependent chemoattraction were inhibited by the mTOR-selective drug rapamycin. We also detected elevated levels of activated EGF receptor and mTOR signaling in reactive astrocytes in vivo in an ischemic model of spinal cord injury. Furthermore, increased Rheb expression likely contributes to mTOR activation in the injured spinal cord. Interestingly, injured rats treated with rapamycin showed reduced signs of reactive gliosis, suggesting that rapamycin could be used to harness astrocytic responses in the damaged nervous system to promote an environment more permissive to axon regeneration.
Codeluppi
Simone
S
Burnham Institute for Medical Research, La Jolla, California 92037, USA.
Svensson
Camilla I
CI
Hefferan
Michael P
MP
Valencia
Fatima
F
Silldorff
Morgan D
MD
Oshiro
Masakatsu
M
Marsala
Martin
M
Pasquale
Elena B
EB
D000704:Analysis of Variance; D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D002867:Chromones / Q000494:pharmacology; D004195:Disease Models, Animal; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004815:Epidermal Growth Factor / Q000187:drug effects / Q000235:genetics; D066246:ErbB Receptors / Q000235:genetics / Q000378:metabolism; D027342:Excitatory Amino Acid Transporter 2 / Q000235:genetics / Q000378:metabolism; D005419:Flavonoids / Q000494:pharmacology; D005904:Glial Fibrillary Acidic Protein / Q000378:metabolism; D007166:Immunosuppressive Agents / Q000494:pharmacology / Q000627:therapeutic use; D008297:Male; D020559:Monomeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D009025:Morpholines / Q000494:pharmacology; D009479:Neuropeptides / Q000235:genetics / Q000378:metabolism*; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D000076205:Ras Homolog Enriched in Brain Protein; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000502:physiology*; D020123:Sirolimus / Q000494:pharmacology / Q000627:therapeutic use; D013119:Spinal Cord Injuries / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology*; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014162:Transfection / Q000379:methods; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology*; D014746:Vimentin / Q000235:genetics / Q000378:metabolism
D002867:Chromones; C519510:Crtc1 protein, rat; D004791:Enzyme Inhibitors; D027342:Excitatory Amino Acid Transporter 2; D005419:Flavonoids; D005904:Glial Fibrillary Acidic Protein; D007166:Immunosuppressive Agents; D009025:Morpholines; D009479:Neuropeptides; D012333:RNA, Messenger; D000076205:Ras Homolog Enriched in Brain Protein; C087693:Rheb protein, rat; D014157:Transcription Factors; D014746:Vimentin; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; D004815:Epidermal Growth Factor; D011494:Protein Kinases; C546845:mTOR protein, rat; D066246:ErbB Receptors; D058570:TOR Serine-Threonine Kinases; D020559:Monomeric GTP-Binding Proteins; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one; D020123:Sirolimus