Differential protection by nicotinamide in a mouse model of glaucoma DBA/2J revealed by second-harmonic generation microscopy

Glaucoma is a blinding disease where the retinal ganglion cells and their axons degenerate. Degradation of axonal microtubules is thought to play a critical role in the pathogenesis, but the mechanism is unknown. Here we investigate whether microtubule disruption in glaucoma can be alleviated by metabolic rescue. The integrity of axonal microtubules and the morphology of the retinal nerve fibers were evaluated by second-harmonic generation microscopy in a mouse model of glaucoma, DBA/2J, which received a dietary supplement of nicotinamide (NAM) for reducing metabolic stress. It was compared with control DBA/2J, which did not receive NAM, and non-glaucomatous DBA/2J-Gpnmb+. We found that the morphology of the retinal nerve fibers, but not axonal microtubules, are significantly protected by NAM. The decoupling is analogous to microtubule deficit, a glaucoma pathology in which axonal microtubules exhibit rapid degradation compared to the morphology of the retinal nerve fibers. Understanding microtubule deficit could provide insights into the divergent responses to NAM. From co-registered images of second-harmonic generation and immunofluorescence, it was determined that microtubule deficit was not due to a shortage of tubulins. Furthermore, microtubule deficit colocalized with the sectors in which the retinal ganglion cells were disconnected from the brain, suggesting that microtubule disruption is associated with axonal transport deficit in glaucoma. Together, our data suggests significant role axonal microtubules play in glaucomatous degeneration, offering a new opportunity for neuroprotection.