Acetylcholinesterase and agrin: different functions, similar expression patterns, multiple roles

Increasing Agrin Function Antagonizes Muscle Atrophy and Motor Impairment in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a pediatric genetic disease, characterized by motor neuron (MN) death, leading to progressive muscle weakness, respiratory failure, and, in the most severe cases, to death. Abnormalities at the neuromuscular junction (NMJ) have been reported in SMA, including neurofilament (NF) accumulation at presynaptic terminals, immature and smaller than normal endplates, reduced transmitter release, and, finally, muscle denervation. Here we have studied the role of agrin in SMAΔ7 mice, the experimental model of SMAII. We observed a 50% reduction in agrin expression levels in quadriceps of P10 SMA mice compared to age-matched WT controls. To counteract such condition, we treated SMA mice from birth onwards with therapeutic agrin biological NT-1654, an active splice variant of agrin retaining synaptogenic properties, which is also resistant to proteolytic cleavage by neurotrypsin. Mice were analyzed for behavior, muscle and NMJ histology, and survival. Motor behavior was significantly improved and survival was extended by treatment of SMA mice with NT-1654. At P10, H/E-stained sections of the quadriceps, a proximal muscle early involved in SMA, showed that NT-1654 treatment strongly prevented the size decrease of muscle fibers. Studies of NMJ morphology on whole-mount diaphragm preparations revealed that NT-1654-treated SMA mice had more mature NMJs and reduced NF accumulation, compared to vehicle-treated SMA mice. We conclude that increasing agrin function in SMA has beneficial outcomes on muscle fibers and NMJs as the agrin biological NT-1654 restores the crosstalk between muscle and MNs, delaying muscular atrophy, improving motor performance and extending survival.

In Vitro Innervation as an Experimental Model to Study the Expression and Functions of Acetylcholinesterase and Agrin in Human Skeletal Muscle

Acetylcholinesterase (AChE) and agrin, a heparan-sulfate proteoglycan, reside in the basal lamina of the neuromuscular junction (NMJ) and play key roles in cholinergic transmission and synaptogenesis. Unlike most NMJ components, AChE and agrin are expressed in skeletal muscle and α-motor neurons. AChE and agrin are also expressed in various other types of cells, where they have important alternative functions that are not related to their classical roles in NMJ. In this review, we first focus on co-cultures of embryonic rat spinal cord explants with human skeletal muscle cells as an experimental model to study functional innervation in vitro. We describe how this heterologous rat-human model, which enables experimentation on highly developed contracting human myotubes, offers unique opportunities for AChE and agrin research. We then highlight innovative approaches that were used to address salient questions regarding expression and alternative functions of AChE and agrin in developing human skeletal muscle. Results obtained in co-cultures are compared with those obtained in other models in the context of general advances in the field of AChE and agrin neurobiology.

Effects of electroacupuncture on recovery of the electrophysiological properties of the rabbit gastrocnemius after contusion: an in vivo animal study

Background

Our preliminary studies indicated that electroacupuncture (EA) at the ST36 and Ashi acupoints could promote regeneration of the rabbit gastrocnemius (GM) by improving microcirculation perfusion, promoting the recovery of myofiber structures, and inhibiting excessive fibrosis. However, the effects of EA on recovery of the electrophysiological properties of the GM after contusion are not yet clear. Thus, the purpose of this study was to investigate the effects of EA at the Zusanli (ST36) and Ashi acupoints with regard to recovery of the electrophysiological properties of the rabbit GM after contusion.

Methods

Forty-five rabbits were randomly divided into three groups: normal, contusion, and EA. After an acute GM contusion was produced (in rabbits in the contusion and EA groups), rabbits in the EA group were treated with electrostimulation at the ST36 and Ashi acupoints with 0.4 mA (2 Hz) for 15 min. The contusion group received no EA treatment. At different time points (7, 14, and 28 days) after contusion, we performed surface electromyography (EMG) and measured the nerve conduction velocity (NCV) of the GM and the GM branch of the tibial nerve. We also examined acetylcholinesterase (AchE) and Agrin expression in the neuromuscular junction (NMJ) via immunohistochemistry.

Results

Compared with the contusion group, the EMG amplitude and NCV in rabbits in the EA group were significantly higher at all time points after contusion. AchE and Agrin expression in the EA group were significantly higher than those in the contusion group.

Conclusions

Our results showed that EA at the ST36 and Ashi acupoints effectively promoted recovery of the electrophysiological properties of the rabbit GM after contusion. The effects of EA were realized by promotion of the regeneration of myofibers and nerve fibers, as well as acceleration of NMJ reconstruction by upregulation of AchE and Agrin expression in the motor endplate area.

Non-synaptic roles of acetylcholinesterase and agrin

Proteins in living organisms have names that are usually derived from their function in the biochemical system their discoverer was investigating. Typical examples are acetylcholinesterase and agrin; however, for both of these, various other functions that are not related to the cholinergic system have been revealed. Our investigations have been focused on the alternative roles of acetylcholinesterase and agrin in the processes of muscle development and regeneration. Previously, we described a role for agrin in the development of excitability in muscle contraction. In this study, we report the effects of agrin on secretion of interleukin 6 in developing human muscle. At the myoblast stage, agrin increases interleukin 6 secretion. This effect seems to be general as it was observed in all of the cell models analysed (human, mouse, cell lines). After fusion of myoblasts into myotubes, the effects of agrin are no longer evident, although agrin has further effects at the innervation stage, at least in in vitro innervated human muscle. These effects of agrin are another demonstration of its non-synaptic roles that are apparently developmental-stage specific. Our data support the view that acetylcholinesterase and agrin participate in various processes during development of skeletal muscle.