Abstract
Purpose :
Sterile alpha and (Toll/interleukin receptor (TIR)) motif-containing 1 (SARM1) is a cell/axonal energy level sensor, which is activated by increased NMN/NAD(+) ratios (Figley, 2020). This enzyme is a key activator of Wallerian degeneration (WLD), and SARM1 inactivation results in a form of WLD-slow following axonal crush or section. While SARM1 knockout inhibits RGC loss in mouse models of pressure-related glaucoma, no data exists on the direct effects of SARM1 inactivation following acute axonal ischemia. We evaluated this using the rodent nonarteritic anterior ischemic optic neuropathy (rNAION) model and newly generated transgenic rats lacking active SARM1.
Methods :
Crispr-generated SARM1 SD knockout rats (SARM1(-)) were unilaterally rNAION induced with 11 seconds of 532nm laser light exposure/532nm wavelength/500um spot size/50mW measured at the cornea, following IV administration of 1ml/kg of 2.5mM rose Bengal solution. We induced a similar number of WT-SD animals with identical parameters. Optic nerve head (ONH) edema was evaluated 2d post-induction with SD-OCT, and RGCs were quantified by stereology following retinal flat-mount immunostaining with Brn3a(+) antibody. ON-axonal ultrastructure was evaluated at 5d post-induction with TEM, and ON histology evaluated at 30d post-induction with immunohistochemistry with SMI312 (axons) and MBP (myelin).
Results :
rNAION-induced SARM1(-) eyes (n=8) exhibited slightly less mean ONH edema than WT eyes (547.6±13.9um sem v 570.7±13.9um). Ultrastructurally, RGC axons in the center of the ON of SARM1(-) animals exhibited little detectable change 5d post-induction, although there was myelin fracturing, while there was structural dissolution of the axons of WT animals with loss of myelin structure. Immunohistochemically there was greater central axon and myelin preservation in SARM1(-) animals than in WTs. At 30d post-induction RGC loss was modestly reduced in the same SARM1(-) animals compared with WT (49.1±6.1% v 62.2±6.3% sem; p>0.05 (NS).
Conclusions :
Eliminating SARM1 activation by itself in ischemic axons seems to modestly inhibit RGC loss, although this is a non-significant effect with small group sizes. Despite ischemia, there is maintenance of the majority of ON axonal structural integrity for extended (5d) times. This ability may be useful in studies evaluating the combinatorial effects of neuroprotective treatments with independent mechanisms.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.