The Time Course of Gene Expression during Reactive Gliosis in the Optic Nerve

Abstract

Reactive gliosis is a complex process that involves changes in gene expression and morphological remodeling. The mouse optic nerve, where astrocytes, microglia and oligodendrocytes interact with retinal ganglion cell axons and each other, is a particularly suitable model for studying the molecular mechanisms of reactive gliosis. We triggered gliosis at the mouse optic nerve head by retro orbital nerve crush. We followed the expression profiles of 14,000 genes from 1 day to 3 months, as the optic nerve formed a glial scar. The transcriptome showed profound changes. These were greatest shortly after injury; the numbers of differentially regulated genes then dropped, returning nearly to resting levels by 3 months. Different genes were modulated with very different time courses, and functionally distinct groups of genes responded in partially overlapping waves. These correspond roughly to two quick waves of inflammation and cell proliferation, a slow wave of tissue remodeling and debris removal, and a final stationary phase that primarily reflects permanent structural changes in the axons. Responses from astrocytes, microglia and oligodendrocytes were distinctively different, both molecularly and morphologically. Comparisons to other models of brain injury and to glaucoma indicated that the glial responses depended on both the tissue and the injury. Attempts to modulate glial function after axonal injuries should consider different mechanistic targets at different times following the insult.

siRNA-Mediated Knockdown of the mTOR Inhibitor RTP801 Promotes Retinal Ganglion Cell Survival and Axon Elongation by Direct and Indirect Mechanisms

“…79 Time-course analyses of gene expression changes during reactive gliosis found more than 1300 genes differentially expressed within 1 day of ONC, with functionally distinct groups of temporally specific genes associated with inflammatory responses, tissue remodeling, and structural alterations. 80 Modulation of retinal gliosis to optimize RGC survival and axon regeneration must therefore consider multiple tissue-specific, injury-specific, and temporal factors.…”

The Complex Role of Neuroinflammation in Glaucoma

“…Changes in astrocyte morphology, such as enlargement of the cell body and processes, as well as up-regulation of cytoskeletal and extracellular matrix proteins, have been observed in the retina and ONH in human glaucoma (Wang et al 2002;Inman and Horner 2007;Hernandez et al 2008;Kompass et al 2008;Nikolskaya et al 2009). Several studies have demonstrated that this reactivity and remodeling of astrocytes, and the increased deposition of extracellular matrix occur in early stages of experimental glaucoma (Johnson and Morrison 2009;Howell et al 2011a;Johnson et al 2011;Lye-Barthel et al 2013;Qu and Jakobs 2013). In fact, just a short term elevation of IOP induces rapid and reversible morphological…”

“…Furthermore, changes in gene expression in astrocytes are observed at early stages of experimental glaucoma before RGC axon damage (Johnson et al 2000(Johnson et al , 2011Howell et al 2011a;Qu and Jakobs 2013). This early up-regulation of extracellular matrix in astrocytes of the ONH may function to repair or prevent damage to the blood -brain barrier in response to high IOP.…”

Differential Gene Expression in Glaucoma

“…In a mouse model of laser-induced ocular hypertension, TNFa levels were increased in the retina followed by oligodendrocyte loss in the optic nerve that preceded ganglion cell death (Nakazawa et al 2006). Although no study so far has compared gene expression in the optic nerve proper with that of the glial lamina in glaucoma, in some studies parts of the myelinated optic nerve were analyzed together with the glial lamina (Johnson et al 2007;Qu and Jakobs 2013). Not surprisingly, optic nerve damage is accompanied by down-regulation of oligodendrocyte-specific genes.…”