The Sympathetic Nervous System is a Key Player in Skeletal Muscle Health During Exercise and Injury Restricted; Files Only
Klugherz, Tess (Spring 2024)
Abstract
The sympathetic nervous system (SNS) is crucial in regulating various physiological functions, yet its role in skeletal muscle is largely unknown. The neuromuscular junctions (NMJ), the sites where motor neurons connect with muscle fibers, are also innervated by sympathetic neurons through the Beta2 adrenergic receptor (β2AR). Here we examine the effects of exercise on the integrity of the NMJ following ablation of sympathetic signaling, in hopes of understanding the link between β2AR activation and muscle growth. Utilizing a surgical lumbar sympathectomy and a conditional knockout model, we explored how physical exercise might impact the activity of β2AR and acetylcholine receptor (AChR) signaling. Following aerobic exercise, we observed that sympathetic innervation is required for the upregulation of β2AR fluorescent intensity at the NMJ. Our data also suggests that the anatomy of the NMJ area is also dependent on sympathetic innervation, while its function of AChR signaling is not. Our study also investigated the downstream effects of β2AR activation, with a primary focus on protein degradation via Muscle RING Finger 1 (MuRF1). A closer examination with acute exercise of beta-less mice indicates that the sympathetic receptor is required to modulate MuRF1 expression, but not necessarily for AChR expression. From a clinical standpoint, we measured these markers in a spinal cord injury (SCI) model, and found evidence of sympathetic dysregulation from expression levels of MuRF1 and AChR subunits. Overall, our main findings illustrate that the SNS is required for exercise to impact β2AR activation and NMJ morphology. We hope this study provides insights on muscle health and recovery in the context of SCIs.
Table of Contents
Table of Contents
Introduction.................................................................................................................................... 1
Methods........................................................................................................................................... 6
Animals............................................................................................................................... 6
Surgical Lumbar Sympathectomy....................................................................................... 6
Aerobic Exercise.................................................................................................................. 6
Muscle Harvest.................................................................................................................... 7
Immunohistochemistry....................................................................................................... 7
NMJ Analysis........................................................................................................................ 8
AChR Fluorescence and Area....................................................................................... 8
B2AR Fluorescence..................................................................................................... 8
Acute Exercise...................................................................................................................... 9
Unilateral T2 Spinal Cord Injury.......................................................................................... 9
Quantitative Reverse Transcriptase-Polymerase Chain Reaction (PCR).......................... 10
Statistical Analysis............................................................................................................. 11
Results........................................................................................................................................... 12
Removing sympathetic innervation in skeletal muscle prevents β2AR localization and exercise does not recover activity. 12
Sympathetic innervation is required for NMJ morphology but not AChR localization following exercise. 13
Acute exercise of ADRB2 knockout mice fails to replicate changes in proteolysis and acetylcholine receptor expression. 15
SCI mice exhibit changes to the sympathetic and somatic nervous systems ................. 17
Discussion..................................................................................................................................... 20
Limitations...................................................................................................................... 24
Future Directions............................................................................................................. 25
Conclusion....................................................................................................................... 25
Works Cited................................................................................................................................... 27
List of Tables and Figures
Table 1. qRT-PCR Primer Sequences....................................................................................... 10
Figure 1. β2AR localization at NMJ following surgical sympathectomy and aerobic exercise. 13
Figure 2. NMJ Integrity is affected by sympathetic signaling and exercise.............................. 14
Figure 3. ADRB2 conditional knockout and acute exercise affect skeletal muscle physiology 16
Figure 4. SCI muscles experience elevated proteolysis and fetal AChR expression................. 19
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File download under embargo until 24 May 2026 | 2024-04-10 11:01:16 -0400 | File download under embargo until 24 May 2026 |
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