Abstract
BackgroundMicroglia reactivity is a hallmark of retinal degenerations and overwhelming microglial responses contribute to photoreceptor death. Minocycline, a semi-synthetic tetracycline analog, has potent anti-inflammatory and neuroprotective effects. Here, we investigated how minocycline affects microglia in vitro and studied its immuno-modulatory properties in a mouse model of acute retinal degeneration using bright white light exposure.MethodsLPS-treated BV-2 microglia were stimulated with 50 μg/ml minocycline for 6 or 24 h, respectively. Pro-inflammatory gene transcription was determined by real-time RT-PCR and nitric oxide (NO) secretion was assessed using the Griess reagent. Caspase 3/7 levels were determined in 661W photoreceptors cultured with microglia-conditioned medium in the absence or presence of minocycline supplementation. BALB/cJ mice received daily intraperitoneal injections of 45 mg/kg minocycline, starting 1 day before exposure to 15.000 lux white light for 1 hour. The effect of minocycline treatment on microglial reactivity was analyzed by immunohistochemical stainings of retinal sections and flat-mounts, and messenger RNA (mRNA) expression of microglia markers was determined using real-time RT-PCR and RNA-sequencing. Optical coherence tomography (OCT) and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) stainings were used to measure the extent of retinal degeneration and photoreceptor apoptosis.ResultsStimulation of LPS-activated BV-2 microglia with minocycline significantly diminished the transcription of the pro-inflammatory markers CCL2, IL6, and inducible nitric oxide synthase (iNOS). Minocycline also reduced the production of NO and dampened microglial neurotoxicity on 661W photoreceptors. Furthermore, minocycline had direct protective effects on 661W photoreceptors by decreasing caspase 3/7 activity. In mice challenged with white light, injections of minocycline strongly decreased the number of amoeboid alerted microglia in the outer retina and down-regulated the expression of the microglial activation marker translocator protein (18 kDa) (TSPO), CD68, and activated microglia/macrophage whey acidic protein (AMWAP) already 1 day after light exposure. Furthermore, RNA-seq analyses revealed the potential of minocycline to globally counter-regulate pro-inflammatory gene transcription in the light-damaged retina. The severe thinning of the outer retina and the strong induction of photoreceptor apoptosis induced by light challenge were nearly completely prevented by minocycline treatment as indicated by a preserved retinal structure and a low number of apoptotic cells.ConclusionsMinocycline potently counter-regulates microgliosis and light-induced retinal damage, indicating a promising concept for the treatment of retinal pathologies.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-015-0431-4) contains supplementary material, which is available to authorized users.
Highlights
Microglia reactivity is a hallmark of retinal degenerations and overwhelming microglial responses contribute to photoreceptor death
Gene expression of the chemoattractant protein CCL2, the pro-inflammatory cytokine IL6, and inducible nitric oxide synthase (iNOS), which catalyzes the generation of toxic nitric oxide (NO), were analyzed in LPS-activated BV-2 cells that were treated with minocycline for 6 and 24 h, respectively
(Fig. 1f, p < 0.0001 for LPS versus minocycline + LPS treatment). These findings indicate that minocycline action on microglia is a relatively rapid process and very likely interferes with immediate/early mechanisms of pro-inflammatory activation
Summary
Microglia reactivity is a hallmark of retinal degenerations and overwhelming microglial responses contribute to photoreceptor death. The expression of several receptors that are specific for the binding of chemokines, cytokines, complement factors, antibodies, or damage-associated molecular patterns enables these cells to recognize and immediately respond to pathological changes of their environment [7,8,9] Besides their supportive function in the healthy retina, microglia reactivity and age-related changes of microglia physiology contribute to degenerative pathologies of the retina and the entire CNS [3, 4, 8, 10,11,12,13,14,15,16,17]. Microglial activation cannot be just regarded as a bystander effect but rather actively contributes to photoreceptor cell death during retinal degeneration
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