Axonal Degeneration in Retinal Ganglion Cells Is Associated with a Membrane Polarity-Sensitive Redox Process

Abstract

Axonal degeneration is a pathophysiological mechanism common to several neurodegenerative diseases. The slow Wallerian degeneration (Wld S ) mutation, which results in reduced axonal degeneration in the central and peripheral nervous systems, has provided insight into a redox-dependent mechanism by which axons undergo self-destruction. We studied early molecular events in axonal degeneration with single-axon laser axotomy and time-lapse imaging, monitoring the initial changes in transected axons of purified retinal ganglion cells (RGCs) from wild-type and Wld S rat retinas using a polarity-sensitive annexin-based biosensor (annexin B12-Cys101,Cys260-N,NЈ-dimethyl-N-(iodoacetyl)-NЈ-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) ethylenediamine). Transected axons demonstrated a rapid and progressive change in membrane phospholipid polarity, manifested as phosphatidylserine externalization, which was significantly delayed and propagated more slowly in axotomized Wld S RGCs compared with wild-type axons. Delivery of bis(3-propionic acid methyl ester)phenylphosphine borane complex, a cell-permeable intracellular disulfide-reducing drug, slowed the onset and velocity of phosphatidylserine externalization in wild-type axons significantly, replicating the Wld S phenotype, whereas extracellular redox modulation reversed the Wld S phenotype. These findings are consistent with an intra-axonal redox mechanism for axonal degeneration associated with the initiation and propagation of phosphatidylserine externalization after axotomy.

Phosphatidylserine Externalization Results from and Causes Neurite Degeneration in Drosophila

“…Expression of Wld s , a chimeric protein containing nicotinamide mononucleotide adenyltransferase (NMNAT1), which synthesizes NAD + , in neurons prevents neurite degeneration AI ( Mack et al, 2001 ). Interestingly, transected axons of retinal ganglion cells isolated from transgenic Wld s rats showed a delay of PS exposure and halved the speed of PS exposure spread ( Almasieh et al, 2017 ). To test whether PS exposure on injured dendrites is also regulated by NAD + depletion in vivo , we examined Wld s -overexpressing C4 da neurons.…”

“…The onset of PS exposure coincides with early signs of dendrite degeneration such as membrane roughening and blebbing, but precedes dendrite fragmentation and engulfment. Moreover, PS exposure is blocked when degeneration of injured dendrites is inhibited by Wld s expression, suggesting that PS exposure is a downstream event of the neurite degeneration program triggered by NAD + depletion ( Almasieh et al, 2017 ; Gerdts et al, 2016 ). Interestingly, PS exposure in injured dendrites does not depend on caspase activity, consistent with previous findings that blocking caspase activity does not protect dendrites or axons from injury-induced degeneration ( Simon et al, 2012 ; Tao and Rolls, 2011 ).…”

Phosphatidylserine is a marker for axonal debris engulfment but its exposure can be decoupled from degeneration

“…Our genetic and pharmacological manipulations suggest that there is no single degenerative pathway that leads to the exposure of PS on the axons, and activation of both apoptosis-dependent and -independent degeneration results in PS exposure. Previous work showed reduced PS exposure on injured Wld s retinal gangilion neurons, indicating the involvement of the Wallerian degeneration pathway in exposure of PS 59 . We were able to pinpoint the involvement of Sarm1 in PS exposure of injured and vincristine-treated DRG neurons, as well as NAD + , allowing better understanding of the mechanism involved in PS exposure during Wallerian degeneration.…”

Change in phospholipid species of retinal layer in traumatic optic neuropathy model

“…During apoptosis, oxidized PS are produced (Matsura, ), that causes externalization of the membrane lipid bilayer (Tyurina et al, ). The initiation and propagation of PS externalization after transection were related to an intra‐axonal redox mechanism (Almasieh, Catrinescu, Binan, Costantino, & Levin, ). In our studies, TONI treatment tended to show an increase in PS (18:0/18:1) in the GCL and other retinal layers (Figure B, c, d), whereas PS (18:0/20:4) did not change in the GCL or other areas (Figure B, a, b).…”