Abstract
PURPOSE: Glaucoma is characterized by progressive loss of the retinal ganglion cells (RGCs) and their axons. Here we test an outstanding notion that microtubules (MTs) within RGC axons degrade before the loss of morphology ("MT hypothesis"). METHODS: The integrity of axonal MTs was interrogated by intrinsic second-harmonic generation (SHG) microscopy. Using DBA/2J mice as a model of glaucoma and DBA/2J-Gpnmb+ as a nonglaucomatous control, the relationship between MT disruption and morphology was quantitatively examined as a function of age and sex in the fresh retinal wholemounts. RESULTS: The mean SHG density (i.e., the mean SHG intensity per thickness) was significantly lower in DBA/2J than in DBA/2J-Gpnmb+ and also depended on sex and age. The loss of SHG density, indicating MT disruption within intact RGC axons, occurred in a sectorial manner near the loss of the retinal nerve fiber bundles. The decay rate of SHG density was approximately 97% higher than that of thickness. CONCLUSIONS: Collectively, the results indicate that the breakdown of MTs is pathology of glaucoma and likely a precursor of morphological atrophy. Based on a new finding that SHG density is highly variable and spatially discrete, a new model of RGC degeneration is proposed. This study validates SHG retinal imaging for elucidating the role and mechanism of MT deficiency in the course of glaucoma pathogenesis.