Hypothalamic Sirt1 protects terminal Schwann cells and neuromuscular junctions from age-related morphological changes.

Alison K Snyder‐Warwick,Akiko Satoh,Katherine B Santosa,Albina Jablonka‐Shariff,Shin‐Ichiro Imai

doi:10.1111/acel.12776

Abstract

SummaryNeuromuscular decline occurs with aging. The neuromuscular junction (NMJ), the interface between motor nerve and muscle, also undergoes age‐related changes. Aging effects on the NMJ components—motor nerve terminal, acetylcholine receptors (AChRs), and nonmyelinating terminal Schwann cells (tSCs)—have not been comprehensively evaluated. Sirtuins delay mammalian aging and increase longevity. Increased hypothalamic Sirt1 expression results in more youthful physiology, but the relationship between NMJ morphology and hypothalamic Sirt1 was previously unknown. In wild‐type mice, all NMJ components showed age‐associated morphological changes with ~80% of NMJs displaying abnormalities by 17 months of age. Aged mice with brain‐specific Sirt1 overexpression (BRASTO) had more youthful NMJ morphologic features compared to controls with increased tSC numbers, increased NMJ innervation, and increased numbers of normal AChRs. Sympathetic NMJ innervation was increased in BRASTO mice. In contrast, hypothalamic‐specific Sirt1 knockdown led to tSC abnormalities, decreased tSC numbers, and more denervated endplates compared to controls. Our data suggest that hypothalamic Sirt1 functions to protect NMJs in skeletal muscle from age‐related changes via sympathetic innervation.

Highlights

Muscle function is essential to overall health, stability, and safe mobility
We demonstrated that brain-specific Sirt1 overexpression (BRASTO) mice have a more youthful neuromuscular junction (NMJ) morphology, whereas dorsomedial hypothalamus (DMH)-specific Sirt1 knockdown mice display a more aged NMJ morphology
Because of the evidence supporting sympathetic innervation contributing to more youthful systemic physiology and NMJ morphology and function, we evaluated the presence of sympathetic innervation in our models via tyrosine hydroxylase (TH), b2-AR, and neuropeptide Y (NPY) staining

Summary

| RESULTS

Peak average body weight occurred at 14 months of age (41.34 Æ 4.9 g) and declined by 26% by 33 months of age (30.67 Æ 3.0 g) in WT mice. A variety of tSC morphological abnormalities (Figure 5) not observed in normal, healthy NMJs in control mice (13.6 Æ 0.74%, p < .05; Figure 6a) was seen in the NMJs from DMH-specific Sirt knockdown mice. A total of three b2-AR antibodies were tested (two monoclonal and one polyclonal), and all showed similar results This increased percentage of b2-AR-positive NMJs appears to be qualitatively consistent with higher mRNA expression levels of b2-AR and higher cAMP levels in aged BRASTO skeletal muscle, compared to those in age-matched control skeletal muscle (Satoh et al, 2013). Sirt knockdown in the DMH of young mice results in a more aged (approximately 9–10 months older) NMJ morphology These findings indicate that hypothalamic Sirt plays a critical role in protecting NMJ structures during the process of aging

| DISCUSSION

| CONCLUSIONS

Findings

| EXPERIMENTAL PROCEDURES

CONFLICT OF INTEREST