title: Toll-like receptor 4 deficiency impairs microglial phagocytosis of degenerating axons.
abstract: Microglia are rapidly activated in the central nervous system (CNS) in response to a variety of injuries, including inflammation, trauma, and stroke. In addition to modulation of the innate immune response, a key function of microglia is the phagocytosis of dying cells and cellular debris, which can facilitate recovery. Despite emerging evidence that axonal debris can pose a barrier to regeneration of new axons in the CNS, little is known of the cellular and molecular mechanisms that underlie clearance of degenerating CNS axons. We utilize a custom micropatterned microfluidic system that enables robust microglial-axon co-culture to explore the role of Toll-like receptors (TLRs) in microglial phagocytosis of degenerating axons. We find that pharmacologic and genetic disruption of TLR4 blocks induction of the Type-1 interferon response and inhibits phagocytosis of axon debris in vitro. Moreover, TLR4-dependent microglial clearance of unmyelinated axon debris facilitates axon outgrowth. In vivo, microglial phagocytosis of CNS axons undergoing Wallerian degeneration in a dorsal root axotomy model is impaired in adult mice in which TLR4 has been deleted. Since purinergic receptors can influence TLR4-mediated signaling, we also explored a role for the microglia P2 receptors and found that the P2X7R contributes to microglial clearance of degenerating axons. Overall, we identify TLR4 as a key player in axonal debris clearance by microglia, thus creating a more permissive environment for axonal outgrowth. Our findings have significant implications for the development of protective and regenerative strategies for the many inflammatory, traumatic, and neurodegenerative conditions characterized by CNS axon degeneration.
authors.affiliation: Department of Neurology, Johns Hopkins University School of Medicine, Baltimore, Maryland.
authors.forename: Labchan
authors.initials: L
authors.lastname: Rajbhandari
authors.forename: Million Adane
authors.initials: MA
authors.lastname: Tegenge
authors.forename: Shiva
authors.initials: S
authors.lastname: Shrestha
authors.forename: Nishant
authors.initials: N
authors.lastname: Ganesh Kumar
authors.forename: Adeel
authors.initials: A
authors.lastname: Malik
authors.forename: Aditya
authors.initials: A
authors.lastname: Mithal
authors.forename: Suneil
authors.initials: S
authors.lastname: Hosmane
authors.forename: Arun
authors.initials: A
authors.lastname: Venkatesan
mesh_terms: D000818:Animals; D001369:Axons / Q000473:pathology*; D039481:CD11b Antigen / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000378:metabolism; D018920:Coculture Techniques; D016207:Cytokines / Q000378:metabolism; D004622:Embryo, Mammalian; D006624:Hippocampus / Q000166:cytology; D018345:Mice, Knockout; D008840:Microfilament Proteins / Q000378:metabolism; D046210:Microfluidic Analytical Techniques; D017628:Microglia / Q000378:metabolism*; D009410:Nerve Degeneration / Q000235:genetics / Q000473:pathology*; D009474:Neurons / Q000502:physiology; D010587:Phagocytosis / Q000235:genetics*; D051381:Rats; D017207:Rats, Sprague-Dawley; D058486:Receptors, Purinergic P2X7 / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D013449:Sulfonamides / Q000494:pharmacology; D051197:Toll-Like Receptor 4 / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism
chemical_list: C491049:Aif1 protein, mouse; D039481:CD11b Antigen; D002135:Calcium-Binding Proteins; D016207:Cytokines; D008840:Microfilament Proteins; D058486:Receptors, Purinergic P2X7; D013449:Sulfonamides; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4; C507035:ethyl 6-(N-(2-chloro-4-fluorophenyl)sulfamoyl)cyclohex-1-ene-1-carboxylate
keywords: axon degeneration; microglia; multiple sclerosis; phagocytosis; regeneration