The authors also showed that when fast fibers are converted to a slow phenotype, the MNs innervating those fibers express SV2A indicating a retrograde fiber type-specific signal that induces MN phenotype. In the SOD1 mouse slow muscle fibers may produce more MN survival promoting factors as compared with fast fibers. In support of this theory, slow fibers have been shown to express higher levels of Hsp70 as compared with more vulnerable fast fibers (Locke et al. 1991, 1994; Inhibitors,research,lifescience,medical Gifondorwa et al. 2012), and administration of recombinant Hsp70 can maintain

muscle innervation, delay symptom onset, and extend survival of SOD1G93A mice (Gifondorwa et al. 2007). Alternatively, slow muscles that contain Inhibitors,research,lifescience,medical more mitochondria may be better able to compensate for the JAK inhibitor increased oxidative stress shown to occur in the mutant SOD1 mice (e.g., Jang et al. 2010). Fast muscles may produce negative factors, including enhanced oxidative stress that promote NMJ dysfunction and denervation (e.g., Inhibitors,research,lifescience,medical Perlson et al. 2009). Empty cytoplasmic vacuoles The accumulations of small, empty vacuoles in mutant MN cytoplasm are observed by day 14. The vacuoles become more numerous by day 30 and at later stages the cytoplasm is full of these vacuoles. We are unable to definitely determine the

source of these vacuoles; however, it is unlikely that the vacuoles are an artifact of fixation as they were unique to SOD1G93A animals and not observed in WT animals. Similar vacuoles have been reported to originate from ER and may result from an unfolded protein response or ER stress (Ilieva et al. 2007; Nagata et al. 2007; Nishitoh et al. 2008; Kanekura et al. 2009). For example, vacuolization Inhibitors,research,lifescience,medical of the rER has been shown to occur in MNs following chronic excitotoxicity (Tarabal et al. 2005). Indeed, MNs that appear to Inhibitors,research,lifescience,medical be most susceptible in ALS, those that innervate fast, fatiguable muscle fibers, have been shown to initiate an unfolded

protein response at early as day 25 Mephenoxalone (Saxena et al. 2009), corresponding to the time when we begin to observe increased numbers of vacuoles. However, we did not observe vacuoles or other morphological changes selectively in MNs that innervate fast fibers, and we never observed structural perturbations of rER, even at late stages, although it is possible that the vacuoles originate from the smooth ER. We also observed small, empty vacuoles near the Golgi apparatus, suggesting either cis- or trans-Golgi elements as a possible source. These findings may indicate an early breakdown of cisternal maturation of Golgi membranes as previously suggested to occur in ALS mouse models (Gonatas et al. 1992, 2006; Mourelatos et al. 1996; Stieber et al. 1998; Martinez-Menárguez et al. 2001; Schaefer et al. 2007; Fan et al. 2008).