Minocycline reduces inflammatory response and cell death in a S100B retina degeneration model

$\bf Background:$ Previous studies noted that intravitreal injection of S100B triggered a glaucoma-like degeneration of retina and optic nerve as well as microglia activation after 14 days. The precise role of microglia in our intravitreal S100B model is still unclear. Hence, microglia were inhibited through minocycline. The aim is to investigate whether microglia have a significant influence on the degeneration process or whether they are only a side effect in the model studied here. $\bf Methods:$ Minocycline was applied daily in rats by intraperitoneal injection using two different concentrations (13.5 mg/kg body weight, 25 mg/kg body weight). One day after treatment start, S100B or PBS was intravitreally injected in one eye per rat. The naïve groups received no injections. This resulted in a total of five groups (naïve $\it n$ = 14, PBS $\it n$ = 14, S100B $\it n$ = 13, 13.5 mg/kg mino $\it n$ = 15, 25 mg/kg mino $\it n$ = 15). At day 14, electroretinogram measurements were performed, followed by immunofluorescence and label-free quantitative proteomics analysis. The focus of these investigations was on the survival of RGCs as well as their axons, the response of the microglia, and the identification of further pathological modes of action of S100B. $\bf Results:$ The best signal transmission was detected via ERG in the 13.5 mg/kg mino group. The inhibition of the microglia protected optic nerve neurofilaments and decreased the negative impact of S100B on RGCs. However, the minocycline treatment could not trigger complete protection of RGCs. Furthermore, in retina and optic nerve, the minocycline treatment reduced the number and activity of S100B-triggered microglia in a concentration-dependent manner. Proteomics analysis showed that S100B application led to numerous metabolic functions and cellular stress, mainly an increased inflammatory response, glycolysis, and mitochondrial dysfunction, which caused oxidative stress in the retina. Importantly, the protective capability of lower dose of minocycline was unraveled by suppressing the apoptotic, inflammatory, and the altered metabolic processes caused by S100B insult in the retina. $\bf Conclusion:$ Intravitreally injected S100B not only led to a pro-inflammatory microglial reaction, but also a mitochondrial and metabolic dysfunction. Also, these results suggest that an excessive microglial response may be a significant degenerative factor, but not the only trigger for increased cell death

Retinal ganglion cell loss is accompanied by antibody depositions and increased levels of microglia after immunization with retinal antigens

$\textit {Background}$: Antibodies against retinal and optic nerve antigens are detectable in glaucoma patients. Recent studies using a model of experimental autoimmune glaucoma demonstrated that immunization with certain ocular antigens causes an immun-mediated retinal ganglion cell loss in rats. $\textit {Methodology/Principal Findings}$: Rats immunized with a retinal ganglion cell layer homogenate (RGA) had a reduced retinal ganglion cell density on retinal flatmounts (p = 0.007) and a lower number of $Brn3^{+}$retinal ganglion cells (p = 0.0001) after six weeks. The autoreactive antibody development against retina and optic nerve was examined throughout the study. The levels of autoreactive antibodies continuously increased up to 6 weeks (retina: p = 0.004; optic nerve: p = 0.000003). Additionally, antibody deposits were detected in the retina (p = 0.02). After 6 weeks a reactive gliosis (GFAP density: RGA: 174.7$\pm$41.9; CO: 137.6$\pm$36.8, p = 0.0006; $$ area: RGA: 8.5$\pm$3.4; CO: 5.9$\pm$3.6, p = 0.006) as well as elevated level of $Iba1^{+}$ microglia cells (p = 0.003) was observed in retinas of RGA animals. $\textit {Conclusions/Significance}$: Our findings suggest that these antibodies play a substantial role in mechanisms leading to retinal ganglion cell death. This seems to lead to glia cell activation as well as the invasion of microglia, which might be associated with debris clearance