The MuSK activator agrin has a separate role essential for postnatal maintenance of neuromuscular synapses

Abstract

The motoneural control of skeletal muscle contraction requires the neuromuscular junction (NMJ), a midmuscle synapse between the motor nerve and myotube. The formation and maintenance of NMJs are orchestrated by the muscle-specific receptor tyrosine kinase (MuSK). Motor neuron-derived agrin activates MuSK via binding to MuSK's coreceptor Lrp4, and genetic defects in agrin underlie a congenital myasthenic syndrome (an NMJ disorder). However, MuSK-dependent postsynaptic differentiation of NMJs occurs in the absence of a motor neuron, indicating a need for nerve/agrin-independent MuSK activation. We previously identified the muscle protein Dok-7 as an essential activator of MuSK. Although NMJ formation requires agrin under physiological conditions, it is dispensable for NMJ formation experimentally in the absence of the neurotransmitter acetylcholine, which inhibits postsynaptic specialization. Thus, it was hypothesized that MuSK needs agrin together with Lrp4 and Dok-7 to achieve sufficient activation to surmount inhibition by acetylcholine. Here, we show that forced expression of Dok-7 in muscle enhanced MuSK activation in mice lacking agrin or Lrp4 and restored midmuscle NMJ formation in agrin-deficient mice, but not in Lrp4-deficient mice, probably due to the loss of Lrp4-dependent presynaptic differentiation. However, these NMJs in agrin-deficient mice rapidly disappeared after birth, and postsynaptic specializations emerged ectopically throughout myotubes whereas exogenous Dok-7-mediated MuSK activation was maintained. These findings demonstrate that the MuSK activator agrin plays another role essential for the postnatal maintenance, but not for embryonic formation, of NMJs and also for the postnatal, but not prenatal, midmuscle localization of postsynaptic specializations, providing physiological and pathophysiological insight into NMJ homeostasis. neuromuscular junction | postsynaptic specialization | Dok-7 | Lrp4

Agrin and LRP4 antibodies as new biomarkers of myasthenia gravis

“…MG‐like symptoms were observed in rabbits immunized with LRP4, and, importantly, transfer of anti‐LRP4 IgG from immunized rabbits into naive mice caused similar MG‐like symptoms, reduced CMAP, and impaired transmission in recipients. Together, these observations demonstrate that LRP4 antibodies are causal to MG and support the notion that the agrin signaling is critical for NMJ maintenance in adulthood …”

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

“…Another rather unusual aspect of Dok-7 signaling is the finding that forced overexpression of Dok-7 is sufficient to form AChR clusters in cultured myotubes and to allow mice to survive for several weeks in the absence of agrin (334,422). Similarly, expression of Dok-7 via AAV vectors in skeletal muscles enlarges the NMJ and thereby ameliorates the disease in a mouse model for Emery-Dreifuss muscular dystrophy (11).…”

Postnatal knockdown of dok‐7 gene expression in mice causes structural defects in neuromuscular synapses and myasthenic pathology

“…; Tezuka et al . ). To examine whether MuSK‐mediated signaling is affected by postnatal knockdown of dok‐7 expression, we evaluated mRNA levels of several NMJ components that are known to be regulated by MuSK (Jones et al .…”

“…However, we previously showed that agrin has an essential role aside from MuSK activation in postnatal NMJ maintenance, but not in embryonic NMJ formation (Tezuka et al . ). In addition, collagen α3‐6 (IV) chains and biglycan selectively participate in the postnatal maintenance of NMJs (Fox et al .…”