Early pro-inflammatory cytokine elevations in the DBA/2J mouse model of glaucoma

Wilson et al. Journal of Neuroinflammation (2015) 12:176 DOI 10.1186/s12974-015-0399-0 JOURNAL OF NEUROINFLAMMATION RESEARCH Open Access Early pro-inflammatory cytokine elevations in the DBA/2J mouse model of glaucoma Gina N. Wilson1,2, Denise M. Inman1, Christine M. Dengler-Crish1, Matthew A. Smith1,3 and Samuel D. Crish1* Abstract Background: Neuroinflammation—astrogliosis, microglial activation, and changes in cytokine signaling—is a prominent feature of neurodegenerative disorders. Glaucoma is a group of chronic neurodegenerative conditions that make up the leading cause of irreversible blindness worldwide. Neuroinflammation has been postulated to play a significant role in the pathogenesis and progression of glaucomatous neurodegeneration. Though much is known regarding inflammation in the eye in glaucoma, little is known about cytokine activity outside of the retina where pathologies develop early. Methods: We traced the primary visual projection from the eye to the superior colliculus (SC) in DBA/2J and DBA/2J.Gpnmb+ (control) mice using the anterograde tracer cholera toxin-B (CTB) to assay axonal transport deficits. Forty-eight hours later, visual structures were microdissected from fresh tissue based on transport outcome. Using magnetic bead multiplexing assays, we measured levels of 20 cytokines in the retina, proximal and distal optic nerves, CTB-positive and negative SC subdivisions, cerebellum, and serum at different ages representing different stages of pathology. Results: Pro- and anti-inflammatory cytokine levels in mice often changed in the same direction based on strain, age, and tissue. Significant elevations in retinal pro-inflammatory cytokines were observed in young DBA/2J mice compared to controls, followed by an age-dependent decrease in the DBA/2J mice. Proximal optic nerve of young DBA/2J mice showed a 50 % or greater decrease in levels of certain cytokines compared to older DBA/2J cohorts and controls, while both proximal and distal optic nerve of DBA/2Js showed elevations in IL-1β at all ages compared to controls. Pro-inflammatory cytokine IL-6 levels varied in accordance with transport outcome in the SC: IL-6 was elevated 44–80 % in glaucomatous DBA/2J collicular regions deficient in anterograde transport from retinal ganglion cells (RGCs) compared to areas with intact transport. Conclusion: Dysregulation of cytokine signaling in the RGC projection of DBA/2J mice was evident early in distal retinal targets, well before intraocular pressure elevation or axonal degeneration begins. Keywords: Cytokines, Glaucoma, Inflammation, Intraocular pressure, Aging, Neurodegeneration Background Defects in axonal transport have been reported among the earliest pathologies in many neurodegenerative disorders [1, 2], including glaucoma [3–6]. Anterograde transport deficits have been found to precede retrograde deficits and overt structural degeneration of retinal ganglion cell (RGC) axons [7], indicating that an intact and semi-functional axon persists after initial onset of pathology. Furthermore, astrogliosis occurs in the superior * Correspondence: 1 Department of Pharmaceutical Sciences, Northeast Ohio Medical University, 4209 State Route 44, Rootstown, OH 44272, USA Full list of author information is available at the end of the article colliculus (SC) after transport deficits but before axon loss [8]. Given the early appearances of axon transport deficits and neuroinflammation, obvious questions concern the relationship between these two processes. While much has been reported on inflammation in the retina and optic nerve head, details of immune dysfunction such as changes in cytokine levels have not been examined further along the retinal projection—an area where we first see transport deficits and degeneration [4, 9, 10]. Glaucomatous neurodegeneration is predicted to afflict nearly 80 million people worldwide by the year 2020 [11]. Age and elevated intraocular pressure (IOP) are © 2015 Wilson et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Wilson et al. Journal of Neuroinflammation (2015) 12:176 major risk factors for glaucoma, with IOP currently comprising the only target of FDA-approved drug treatments. However, what actually blinds in the disease is the dysfunction and degeneration of RGCs [12, 13]. Also, given the common incidence of elevated IOP without glaucomatous vision loss as well as normal tension glaucoma, it is clear that many other factors play a role in the development and progression of this degenerative disease [14–16]. Abnormal activation of the immune system has been shown to produce glaucomatous pathology in the absence of elevated IOP [14]. Bosco and colleagues have shown microglial activation within young DBA/2J mouse retina, pre-laminar optic nerve, and nerve head [17, 18], which suggests a role for immunomodulatory molecules such as cytokines early in pathology in moderately elevated IOP contexts. At the other extreme, models of high IOP and nerve crush have also demonstrated some reliance on immunomodulation for either protection [19] or propagation of damage [20]. Cytokine activity is thought to play a substantial part in anterior chamber changes that can result in elevated IOP [21, 22]. Interestingly, increased IL-18 within anterior chamber structures has been evident even in young DBA/2J mice before they exhibit elevated IOP [21]. Additionally, increases in IL-6-type cytokines and members of the JAK-STAT (Janus kinase signal transducer and activator of transcription) signaling pathway were shown in early pressure-induced optic nerve head (ONH) injury [23]. Microglial release of IL-6 in mixed cultures of RGCs, microglia, and astrocytes in response to hydrostatic pressure illustrates an important link between cytokine levels and risk factors for developing glaucoma [24]. Upregulation of genes associated with immune responses (e.g., Edn2) have been localized to the retina and ONH of DBA/2J mice very early on in this disease model, and therapeutic attempts to block these responses have shown promise in ameliorating glaucomatous pathology [25]. Previous characterizations of cytokine levels in the context of glaucoma and related stressors, however, being primarily isolated to portions of the anterior chamber or retina, fail to tell the entire story regarding inflammatory signaling in glaucomatous neurodegeneration. Observations of microglial status within the proximal portion of (...truncated)