Spinal TNF-α receptor 1 is differentially required for phrenic long-term facilitation (pLTF) over the course of motor neuron death in adult rats

Ryan D Lewis; Amy N Keilholz; Catherine L Smith; Ethan A Burd; Nicole L Nichols

doi:10.3389/fphys.2024.1488951

Spinal TNF-α receptor 1 is differentially required for phrenic long-term facilitation (pLTF) over the course of motor neuron death in adult rats

Front Physiol. 2024 Dec 5:15:1488951. doi: 10.3389/fphys.2024.1488951. eCollection 2024.

Authors

Ryan D Lewis^#¹, Amy N Keilholz^#^{2

3}, Catherine L Smith³, Ethan A Burd⁴, Nicole L Nichols^{3

5

6}

Affiliations

¹ Department of Biological Chemistry, Grinnell College, Grinnell, IA, United States.
² Department of Veterinary Pathobiology, College of Veterinary Medicine, University of Missouri, Columbia, MO, United States.
³ Department of Biomedical Sciences, College of Veterinary Medicine, University of Missouri, Columbia, MO, United States.
⁴ Department of Biology, Seton Hill University, Greensburg, PA, United States.
⁵ Department of Medical Pharmacology and Physiology, School of Medicine, University of Missouri, Columbia, MO, United States.
⁶ Dalton Cardiovascular Research Center, University of Missouri, Columbia, MO, United States.

^# Contributed equally.

Abstract

Introduction: Intrapleural injections of cholera toxin B conjugated to saporin (CTB-SAP) result in selective respiratory (e.g., phrenic) motor neuron death and mimics aspects of motor neuron disease [(e.g., amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA)], such as breathing deficits. This rodent model allows us to study the impact motor neuron death has on the output of surviving phrenic motor neurons as well as the compensatory mechanisms that are recruited. Microglial density in the phrenic motor nucleus as well as cervical gene expression of markers associated with inflammation (e.g., tumor necrosis factor α; TNF-α) are increased following CTB-SAP-induced phrenic motor neuron death, and ketoprofen (nonsteroidal anti-inflammatory drug) delivery attenuated phrenic long-term facilitation (pLTF) in 7 day (d) CTB-SAP rats but enhanced pLTF in 28d CTB-SAP rats.

Methods: Here, we worked to determine the impact of TNF-α in the phrenic motor nucleus by: 1) quantifying TNFR1 (a high affinity transmembrane receptor for TNF-α) expression; 2) investigating astrocytes (glial cells known to release TNF-α) by performing a morphological analysis in the phrenic motor nucleus; and 3) determining whether acute TNFR1 inhibition differentially affects phrenic plasticity over the course of CTB-SAP-induced motor neuron loss by delivering an inhibitor for TNF-α receptor 1 (sTNFR1i) in 7d and 28d male CTB-SAP and control rats.

Results: Results revealed that TNFR1 expression was increased on phrenic motor neurons of 28d CTB-SAP rats (p < 0.05), and that astrocytes were increased and exhibited reactive morphology (consistent with an activated phenotype; p < 0.05) in the phrenic motor nucleus of CTB-SAP rats. Additionally, we found that pLTF was attenuated in 7d CTB-SAP rats but enhanced in 28d CTB-SAP rats (p < 0.05) following intrathecal sTNFR1i delivery.

Conclusion: This work suggests that we could harness TNFR1 as a potential therapeutic agent in CTB-SAP rats and patients with respiratory motor neuron disease by increasing compensatory plasticity in surviving neurons to improve phrenic motor neuron function and breathing as well as quality of life. Future studies will focus on microglial and astrocytic cytokine release, the role they play in the differential mechanisms of pLTF utilized by 7d and 28d CTB-SAP rats, and potential therapies that target them.

Keywords: astrocyte; breathing; phrenic motor neuron death; plasticity; rat model; respiration.

Grants and funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This work was funded by a grant from the Spinal Cord Injury and Disease Research Program (NLN), and the University of Missouri College of Veterinary Medicine Committee on Research (NLN). RDL and EAB were supported by a Computational Neuroscience grant from the National Science Foundation, and ANK was supported by a National Institutes of Health Postdoctoral Training in Comparative Medicine T32 OD011126 grant.