Glial coverage in the optic nerve expands in proportion to optic axon loss in chronic mouse glaucoma

Abstract

Within the white matter, axonal loss by neurodegeneration is coupled to glial cell changes in gene expression, structure and function commonly termed gliosis. Recently, we described the highly variable expansion of gliosis in degenerative optic nerves from the DBA/2J mouse model of chronic, age-related glaucoma. Here, to estimate and compare the levels of axonal loss with the expansion of glial coverage and axonal degeneration in DBA/2J nerves, we combined semiautomatic axon counts with threshold-based segmentation of total glial/scar areas and degenerative axonal profiles in plastic cross-sections. In nerves ranging from mild to severe degeneration, we found that the progression of axonal dropout is coupled to an increase of gliotic area. We detected a strong correlation between axon loss and the aggregate coverage by glial cells and scar, whereas axon loss did not correlate with the small fraction of degenerating profiles. Nerves with low to medium levels of axon loss displayed moderate glial reactivity, consisting of hypertrophic astrocytes, activated microglia and normal distribution of oligodendrocytes, with minimal reorganization of the tissue architecture. In contrast, nerves with extensive axonal loss showed prevalent rearrangement of the nerve, with loss of axon fascicle territories and enlarged or almost continuous gliotic and scar domains, containing reactive astrocytes, oligodendrocytes and activated microglia. These findings support the value of optic nerve gliotic expansion as a quantitative estimate of optic neuropathy that correlates with axon loss, applicable to grade the severity of optic nerve damage in mouse chronic glaucoma.

Loss of Fractalkine Signaling Exacerbates Axon Transport Dysfunction in a Chronic Model of Glaucoma

“…RGC degeneration in the DBA/2J model is well characterized within the optic nerve to include a progressive loss of RGC axons and a subsequent replacement by glia and extracellular matrix (John et al, 1998; Libby et al, 2005a; Schlamp et al, 2006; Lye-Barthel et al, 2013; Bosco et al, 2015a,b, 2016; Cooper et al, 2016). To determine if the potential impact on axonal transport by loss of Cx3cr1 affected optic nerve degeneration, we estimated the relative coverage of non-axonal area using segmentation of glia and extracellular matrix in nerve cross-sections (Bosco et al, 2015a,b, 2016).…”

“…To determine if the potential impact on axonal transport by loss of Cx3cr1 affected optic nerve degeneration, we estimated the relative coverage of non-axonal area using segmentation of glia and extracellular matrix in nerve cross-sections (Bosco et al, 2015a,b, 2016). For both the DBA/2J and Cx3cr1 gfp/gfp DBA/2J genotypes at 10–11 months, we observed a similar distribution of nerves with non-axonal area ranging from 10 to above 95% in individual nerves (Figures 4A–E).…”

“…The relative area devoid of axons or dystrophic axons in individual optic nerve cross-sections was measured by segmentation of glial cells and/or glial scar and extracellular matrix from 8-bit stitched RGB images of PPD stained nerves, as previously described (Bosco et al, 2015a,b, 2016). The green channel of the RGB image provided the greatest contrast to segment glial area from non-glial elements including axons and blood vessels.…”

Metabolic Vulnerability in the Neurodegenerative Disease Glaucoma

“…Analyses of optic nerve head and axonal glia in glaucoma to date have focused on morphological change (Lampert et al, 1968; Dai et al, 2012; Sun and Jakobs, 2012; Lye-Barthel et al, 2013; Bosco et al, 2016; Cooper et al, 2016), or cell loss. In the D2 optic nerve, astrocyte hypertrophy occurred early in the disease process and was maintained; oligodendrocytes were lost only after axon loss (Son et al, 2010).…”

“…Increasing glycogen stores, though, can maintain axon firing longer during periods of hypoglycemia in rat brain (Suh et al, 2007). Astrocytes form the glia lamina of the optic nerve head in rodents (Figure 6), and these cells undergo profound morphological change with ocular hypertension (Sun et al, 2009; Dai et al, 2012; Bosco et al, 2016; Cooper et al, 2016). Of note, the optic nerve head glia impacted by glaucoma no longer contact the basal lamina of the nerve (Dai et al, 2012), the ultrastructural location of glycogen granules (Brown, 2004).…”