The role of Lynx1, an endogenous modulator of cholinergic transmission, in NMJ development, maintenance, and repair

The cholinergic system drives muscle contraction and plays a central role in the formation, maintenance, and repair of mammalian neuromuscular junctions (NMJs) and skeletal muscles. Because of these essential actions, much effort has been devoted to identifying primary and auxiliary modulatory components of the cholinergic system at NMJs and throughout skeletal muscles. Here, I asked if Lynx1, a GPI-anchored protein shown to modulate nAChRs in the brain, is present and affects the activity of nAChRs at NMJs. Molecular and cellular analysis revealed that Lynx1 levels increase in skeletal muscles, specifically at NMJs, during development. Its expression pattern also closely mirrors changes in cholinergic transmission in vivo and in vitro. As expected, I found by co-immunoprecipitation that Lynx1 interacts with muscle nAChRs and using electrophysiology, I show that Lynx1 desensitizes nAChRs to ACh at NMJs. These findings demonstrate that Lynx1 regulates the cholinergic system at NMJs, suggesting roles for this gene in developing and adult NMJs. To determine the role of Lynx1 at NMJs, I examined Lynx1 knockout mice at different ages. While deletion of Lynx1 has no discernable effect on developing NMJs, its absence increases the incidence of NMJs with age-related morphological features, such as fragmentation and denervation, in young adult and middle-aged mice. Loss of Lynx1 also increases the number of slow-type muscle fibers in young and middle-aged mice, another hallmark of aging. Along with these morphological changes, deletion of Lynx1 affects expression of genes associated with NMJ stability, myogenesis, and muscle atrophy in young adult and middle-aged mice. Not surprisingly, the loss of Lynx1 reduces the density and stability of nAChRs at NMJs. Because of these findings, I surmised that loss of Lynx1 would adversely affect NMJs under other physiological stressors. However, I found the opposite as the loss of Lynx1 augments the capacity of NMJs to repair damages during exercise, following injury to motor axons, and during the initial symptomatic stage of amyotrophic lateral sclerosis (ALS). Since Lynx1 modulates the activity of nAChRs, these contrasting findings likely represent the positive and negative effects of heightened cholinergic transmission on aging compared to injury and disease-afflicted NMJs.