Sarcoglycan Alpha Mitigates Neuromuscular Junction Decline in Aged Mice by Stabilizing LRP4

Abstract

During aging, acetylcholine receptor (AChR) clusters become fragmented and denervated at the neuromuscular junction (NMJ). Underpinning molecular mechanisms are not well understood. We showed that LRP4, a receptor for agrin and critical for NMJ formation and maintenance, was reduced at protein level in aged mice, which was associated with decreased MuSK tyrosine phosphorylation, suggesting compromised agrin-LRP4-MuSK signaling in aged muscles. Transgenic expression of LRP4 in muscles alleviated AChR fragmentation and denervation and improved neuromuscular transmission in aged mice. LRP4 ubiquitination was augmented in aged muscles, suggesting increased LRP4 degradation as a mechanism for reduced LRP4. We found that sarcoglycan α (SGα) interacted with LRP4 and delayed LRP4 degradation in cotransfected cells. AAV9-mediated expression of SGα in muscles mitigated AChR fragmentation and denervation and improved neuromuscular transmission in aged mice. These observations support a model where compromised agrin-LRP4-MuSK signaling serves as a pathological mechanism of age-related NMJ decline and identify a novel function of SGα in stabilizing LRP4 for NMJ stability in aged mice. This study provides evidence that LRP4, a receptor of agrin that is critical for NMJ formation and maintenance, is reduced at protein level in aged muscles. Transgenic expression of LRP4 in muscles ameliorates AChR fragmentation and denervation and improves neuromuscular transmission in aged mice, demonstrating a critical role of the agrin-LRP4-MuSK signaling. Our study also reveals a novel function of SGα to prevent LRP4 degradation in aged muscles. Finally, we show that NMJ decline in aged mice can be mitigated by AAV9-mediated expression of SGα in muscles. These observations provide insight into pathological mechanisms of age-related NMJ decline and suggest that improved agrin-LRP4-MuSK signaling may be a target for potential therapeutic intervention.

The neuromuscular junction is a focal point of mTORC1 signaling in sarcopenia

“…4a-d). In line with previous reports 37, 44 , we observed lower density of postsynaptic AChRs in cross sections from EDL muscles of 30mCON (trend) as well as 9mTSCmKO mice, (Fig. 4e).…”

DOK7 Gene Therapy Enhances Neuromuscular Junction Innervation and Motor Function in Aged Mice

“…A paper recently reported reduced levels of Lrp4 protein expression and MuSK activation in muscle of aged mice, suggesting impaired MuSK-mediated signaling due to loss of Lrp4 protein. Indeed, the authors demonstrated that transgenic expression of Lrp4 in skeletal muscle from the embryonic stage alleviates NMJ denervation and improves neuromuscular transmission and muscle strength in aged mice ( Zhao et al., 2018 ). Also, they found that Lrp4 interacts with sarcoglycan α (SGα) and demonstrated that intramuscular injection with an AAV serotype 9 vector expressing SGα fused with green fluorescent protein (GFP) (AAV9-SGα-GFP) at 22.5 months of age resulted in increased NMJ innervation, neuromuscular transmission, and muscle strength as compared with the AAV9-GFP-treated controls in 24-mo mice.…”

Impairment Mechanisms and Intervention Approaches for Aged Human Neuromuscular Junctions

“…However, CMS is not a degenerative disorder like aging but a developmental disorder, which has been reviewed previously ( Engel et al, 2010 , 2015 ; Webster, 2018 ). In aged mice, the agrin-LRP4-MuSK-AChR signaling pathway is implicated in aging-associated NMJ deficits ( Zhao et al, 2018 ). LRP4 protein levels are decreased in aged muscles and restoring the levels of LRP4 by transgenic expression or stabilization with sarcoglycan alpha maintained NMJ innervation, alleviated AChR fragmentation, and improved synaptic transmission ( Zhao et al, 2018 ).…”

“…In aged mice, the agrin-LRP4-MuSK-AChR signaling pathway is implicated in aging-associated NMJ deficits ( Zhao et al, 2018 ). LRP4 protein levels are decreased in aged muscles and restoring the levels of LRP4 by transgenic expression or stabilization with sarcoglycan alpha maintained NMJ innervation, alleviated AChR fragmentation, and improved synaptic transmission ( Zhao et al, 2018 ). Furthermore, viral-mediated upregulation of DOK7 in aged mouse muscle significantly enhanced motor function, muscle strength, NMJ innervation, and compound muscle action potential amplitudes ( Ueta et al, 2020 ).…”