Impaired axonal transport at the optic nerve head contributes to neurodegeneration in a novel Cre-inducible mouse model of myocilin glaucoma

Abstract

Background: Degeneration of optic nerve (ON) axons and loss of retinal ganglion cells (RGCs) are the pathological hallmarks of Primary Open Angle Glaucoma (POAG). Elevation of intraocular pressure (IOP) due to dysfunction of trabecular meshwork (TM) is known to induce neurodegeneration. However, the early pathological events of glaucomatous neurodegeneration are poorly understood due to lack of robust and faithful mouse model that replicates all features of human POAG. Here, we report the generation and characterization of a novel Cre-inducible transgenic mouse model of myocilin (MYOC), the leading known genetic cause of POAG. Using this model, we further explore early pathological events of glaucomatous neurodegeneration due to chronic IOP elevation. Methods: We generated a Cre-inducible transgenic mouse model expressing DsRed-tagged Y437H mutant of human myocilin (Tg.CreMYOCY437H). A single intravitreal injection of helper adenovirus (HAd) 5 expressing empty cassette or Cre was performed in adult Tg.CreMYOCY437H mice, and glaucoma phenotypes including IOP, outflow facility, structural and functional loss of RGCs, ON degeneration, gliosis, and axonal transport deficits were examined at various stages of IOP elevation. Results: An intravitreal injection of HAd5-Cre led to selective MYOC expression in the TM at the level similar to endogenous Myoc. Expression of mutant MYOC induced biochemical and ultrastructural changes in TM leading to reduced outflow facility and significant IOP elevation. Notably, sustained IOP elevation led to significant functional and structural loss of RGCs and progressive ON degeneration. Glaucomatous neurodegeneration was associated with activation of astrocytes and neurodegenerative changes in the optic nerve head (ONH) region. Remarkably, chronic IOP elevation blocked anterograde axonal transport at the ONH prior to axonal degeneration and RGC loss. Interestingly, impaired axonal transport was associated with loss of cytoskeleton proteins including microtubules and neurofilaments resulting into accumulation of mitochondria in the ONH and neuronal dysfunction. Conclusions: Our studies indicate that Cre-inducible Tg.CreMYOCY437H mice recapitulates all glaucoma phenotypes observed in POAG patients. Importantly, sustained IOP elevation impairs axonal transport at ONH leading to glaucomatous neurodegeneration.