Toll/interleukin-1 receptor domain-containing adapter inducing interferon-β mediates microglial phagocytosis of degenerating axons.
Following CNS injury, microglial phagocytosis of damaged endogenous tissue is thought to play an important role in recovery and regeneration. Previous work has focused on delineating mechanisms of clearance of neurons and myelin. Little, however, is known of the mechanisms underlying phagocytosis of axon debris. We have developed a novel microfluidic platform that enables coculture of microglia with bundles of CNS axons to investigate mechanisms of microglial phagocytosis of axons. Using this platform, we find that axon degeneration results in the induction of type-1 interferon genes within microglia. Pharmacologic and genetic disruption of Toll/interleukin-1 receptor domain-containing adapter inducing interferon-β (TRIF), a Toll-like receptor adapter protein, blocks induction of the interferon response and inhibits microglial phagocytosis of axon debris in vitro. In vivo, microglial phagocytosis of axons following dorsal root axotomy is impaired in mice in which TRIF has been genetically deleted. Furthermore, we identify the p38 mitogen-activated protein kinase (MAPK) cascade as a signaling pathway downstream of TRIF following axon degeneration and find that inhibition of p38 MAPK by SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole) also blocked clearance of axon debris. Finally, we find that TRIF-dependent microglial clearance of unmyelinated axon debris facilitates axon outgrowth. Overall, we provide evidence that TRIF-mediated signaling plays an unexpected role in axonal debris clearance by microglia, thereby facilitating a more permissive environment for axonal outgrowth. Our study has significant implications for the development of novel regenerative and restorative strategies for the many traumatic, neuroinflammatory, and neurodegenerative conditions characterized by CNS axon degeneration.
Departments of Biomedical Engineering, The Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA.
Suneil
S
Hosmane
Million Adane
MA
Tegenge
Labchan
L
Rajbhandari
Prech
P
Uapinyoying
Nishant
N
Ganesh Kumar
Nitish
N
Thakor
Arun
A
Venkatesan
D033942:Adaptor Proteins, Vesicular Transport / Q000737:chemistry / Q000172:deficiency / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000704:Analysis of Variance; D000818:Animals; D000831:Animals, Newborn; D001369:Axons / Q000473:pathology; D019771:Axotomy; D039481:CD11b Antigen / Q000378:metabolism; D002478:Cells, Cultured; D054357:Chemokine CXCL10 / Q000378:metabolism; D018920:Coculture Techniques; D004195:Disease Models, Animal; D004622:Embryo, Mammalian; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005727:Ganglia, Spinal / Q000293:injuries / Q000378:metabolism / Q000473:pathology; D005786:Gene Expression Regulation / Q000187:drug effects / Q000235:genetics; D006624:Hippocampus / Q000166:cytology; D006801:Humans; D007093:Imidazoles / Q000494:pharmacology; D016899:Interferon-beta / Q000378:metabolism*; D017737:Macrophages, Peritoneal / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D046228:Microarray Analysis; D046210:Microfluidic Analytical Techniques; D017628:Microglia / Q000502:physiology*; D009410:Nerve Degeneration / Q000139:chemically induced / Q000378:metabolism* / Q000473:pathology*; D009474:Neurons / Q000166:cytology / Q000187:drug effects; D009569:Nitric Oxide / Q000494:pharmacology; D010455:Peptides / Q000494:pharmacology; D010587:Phagocytosis / Q000235:genetics / Q000502:physiology*; D011725:Pyridines / Q000494:pharmacology; D000644:Quaternary Ammonium Compounds / Q000633:toxicity; D051381:Rats; D017207:Rats, Sprague-Dawley; D017472:Receptors, Interleukin-1 / Q000378:metabolism; D013997:Time Factors; D014162:Transfection
D033942:Adaptor Proteins, Vesicular Transport; D000255:Adenosine Triphosphate; D039481:CD11b Antigen; D054357:Chemokine CXCL10; D004791:Enzyme Inhibitors; D007093:Imidazoles; D016899:Interferon-beta; D009569:Nitric Oxide; D010455:Peptides; D011725:Pyridines; D000644:Quaternary Ammonium Compounds; D017472:Receptors, Interleukin-1; C485433:Cxcl10 protein, mouse; C476286:Ticam1 protein, mouse; C406802:diethylammonium-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate; C093642:SB 203580