Fig. 5.
Changes in architecture of the motor unit induced by SOD1G93A are attenuated by ATF3 transgene coexpression. (A) Comparison of progressive changes in percentages of remaining motor neurons (black diamonds), large axons (blue squares), innervated NMJs (extrapolated from the actual denervation data in Fig. 3M) (green triangles), and weight (red circles) in (Left) SOD1G93A and (Right) ATF3/SOD1G93A mice. In SOD1G93A mice, all parameters rapidly decline over time. By contrast, in ATF3/SOD1G93A mice, all four parameters are indistinguishable from WT at 60 d, and only numbers of large motor neurons declined by 90 d. At 120 d, all parameters are improved in ATF3/SOD1G93A mice compared with SOD1G93A counterparts. Arrow highlights the 90-d time point at which ATF3/SOD1G93A mice clearly show a decline in numbers of surviving motor neurons without a decline of innervated NMJs or other parameters. (B and C) A schematic model of the disease course illustrates the dying back phenomenon in SOD1G93A mice and induction of collateral sprouts in ATF3/SOD1G93A mice as a mechanism to maintain maximal NMJ innervation. Intact motor neurons and axons are in purple/blue. Diseased motor neurons are indicated with white specks, and degenerating axons are indicated with white and black. New collateral axonal sprouts are yellow; intact muscle is red, and NMJs are green. (D) Terminal sprouts were determined when the nerve extended beyond the acetylcholine receptor (AChR) clusters in the innervated NMJ at any direction. Sprouting is presented as the percent of NMJs with sprouts relative to the total number of innervated NMJs in each group. Increased sprouting is detected at 120 d in ATF3/G93A compared with G93A mice. *P < 0.01 by ANOVA with Bonferroni postanalysis (n = 4–5 mice per time point). (E–G) Immunostaining with α-bungarotoxin (green) to label NMJs in the gastrocnemius muscle and antineurofilament (red) to mark innervating axons reveals sprouting at the NMJ in ATF3/G93A mice. The terminal sprout is indicated with an arrow.