title: TLR7 negatively regulates dendrite outgrowth through the Myd88-c-Fos-IL-6 pathway.
abstract: Toll-like receptors (TLRs) recognize both pathogen- and danger-associated molecular patterns and induce innate immune responses. Some TLRs are expressed in neurons and regulate neurodevelopment and neurodegeneration. However, the downstream signaling pathways and effectors for TLRs in neurons are still controversial. In this report, we provide evidence that TLR7 negatively regulates dendrite growth through the canonical myeloid differentiation primary response gene 88 (Myd88)-c-Fos-interleukin (IL)-6 pathway. Although both TLR7 and TLR8 recognize single-stranded RNA (ssRNA), the results of quantitative reverse transcription-PCR suggested that TLR7 is the major TLR recognizing ssRNA in brains. In both in vitro cultures and in utero electroporation experiments, manipulation of TLR7 expression levels was sufficient to alter neuronal morphology, indicating the presence of intrinsic TLR7 ligands. Besides, the RNase A treatment that removed ssRNA in cultures promoted dendrite growth. We also found that the addition of ssRNA and synthetic TLR7 agonists CL075 and loxoribine, but not R837 (imiquimod), to cultured neurons specifically restricted dendrite growth via TLR7. These results all suggest that TLR7 negatively regulates neuronal differentiation. In cultured neurons, TLR7 activation induced IL-6 and TNF-α expression through Myd88. Using Myd88-, IL-6-, and TNF-α-deficient neurons, we then demonstrated the essential roles of Myd88 and IL-6, but not TNF-α, in the TLR7 pathway to restrict dendrite growth. In addition to neuronal morphology, TLR7 knockout also affects mouse behaviors, because young mutant mice ∼2 weeks of age exhibited noticeably lower exploratory activity in an open field. In conclusion, our study suggests that TLR7 negatively regulates dendrite growth and influences cognition in mice.
authors.affiliation: Graduate Institute of Life Sciences, National Defense Medical Center, Taipei 114, Taiwan, Republic of China.
authors.forename: Hsin-Yu
authors.initials: HY
authors.lastname: Liu
authors.forename: Yun-Fen
authors.initials: YF
authors.lastname: Hong
authors.forename: Chiao-Ming
authors.initials: CM
authors.lastname: Huang
authors.forename: Chiung-Ya
authors.initials: CY
authors.lastname: Chen
authors.forename: Tzyy-Nan
authors.initials: TN
authors.lastname: Huang
authors.forename: Yi-Ping
authors.initials: YP
authors.lastname: Hsueh
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D003712:Dendrites / Q000502:physiology*; D015536:Down-Regulation / Q000502:physiology*; D005260:Female; D006131:Growth Inhibitors / Q000502:physiology*; D015850:Interleukin-6 / Q000502:physiology*; D008562:Membrane Glycoproteins / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D053594:Myeloid Differentiation Factor 88 / Q000502:physiology*; D011247:Pregnancy; D016760:Proto-Oncogene Proteins c-fos / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D051199:Toll-Like Receptor 7 / Q000502:physiology*
chemical_list: D006131:Growth Inhibitors; D015850:Interleukin-6; D008562:Membrane Glycoproteins; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D016760:Proto-Oncogene Proteins c-fos; C477038:Tlr7 protein, mouse; D051199:Toll-Like Receptor 7; C577686:interleukin-6, mouse