The effect of NAMPT deletion in projection neurons on the function and structure of neuromuscular junction (NMJ) in mice

Samuel Lundt,Shinghua Ding,Luis Polo-Parada,Xiaowan Wang,Nannan Zhang

doi:10.1038/s41598-019-57085-4

Samuel Lundt, Shinghua Ding + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-57085-4

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Journal: Scientific Reports	Publication Date: Jan 9, 2020
Citations: 16	License type: open-access

Affiliation: University of Missouri

Abstract

Nicotinamide adenine dinucleotide (NAD+) plays a critical role in energy metabolism and bioenergetic homeostasis. Most NAD+ in mammalian cells is synthesized via the NAD+ salvage pathway, where nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme, converting nicotinamide into nicotinamide mononucleotide (NMN). Using a Thy1-Nampt−/− projection neuron conditional knockout (cKO) mouse, we studied the impact of NAMPT on synaptic vesicle cycling in the neuromuscular junction (NMJ), end-plate structure of NMJs and muscle contractility of semitendinosus muscles. Loss of NAMPT impaired synaptic vesicle endocytosis/exocytosis in the NMJs. The cKO mice also had motor endplates with significantly reduced area and thickness. When the cKO mice were treated with NMN, vesicle endocytosis/exocytosis was improved and endplate morphology was restored. Electrical stimulation induced muscle contraction was significantly impacted in the cKO mice in a frequency dependent manner. The cKO mice were unresponsive to high frequency stimulation (100 Hz), while the NMN-treated cKO mice responded similarly to the control mice. Transmission electron microscopy (TEM) revealed sarcomere misalignment and changes to mitochondrial morphology in the cKO mice, with NMN treatment restoring sarcomere alignment but not mitochondrial morphology. This study demonstrates that neuronal NAMPT is important for pre-/post-synaptic NMJ function, and maintaining skeletal muscular function and structure.

Highlights

Nicotinamide adenine dinucleotide (NAD+) plays a critical role in energy metabolism and bioenergetic homeostasis
To further understand the phenomena induced by Nampt deletion, using FM1-43 imaging, we investigated if the endocytosis and exocytosis of synaptic vesicles, important for synaptic function, at the neuromuscular junction (NMJ) have been changed in Nampt−/− conditional knockout (cKO) mice
Our results show that Nampt−/− cKO mice expressed similar intracellular nicotinamide phosphoribosyltransferase (NAMPT) and extracellular NAMPT levels compared to the Namptf/f control mice, nicotinamide mononucleotide (NMN) administration did not affect iNAMPT and eNAMPT levels in the muscle (Fig. 3A–C)

Summary

Introduction

Nicotinamide adenine dinucleotide (NAD+) plays a critical role in energy metabolism and bioenergetic homeostasis. NAD+ and the NAD+ salvage pathway are vitally important to maintain bioenergetic homeostasis, the normal health and function of many different organs and tissues in the human body, with neurons and skeletal muscles being impacted greatly. Our recent study showed that the loss of NAMPT in projection neurons, using a Thy1-Nampt−/− conditional knockout (cKO) mouse model, produced neurodegeneration, muscle atrophy, abnormal NMJs and, eventually, death. These symptoms are similar to those observed in ALS. When these cKO mice were administered with NMN, these detrimental effects were ameliorated and lifespan was extended[22] These results imply a non-cell autonomous effect of neuronal NAMPT on skeletal muscle structure and function. Our study suggests that the disruption of bioenergetic homeostasis in projection neurons could significantly affect synaptic vesicle cycling at NMJs and muscular function and structure

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