C2 Spinal Cord Hemisection Reduces Mitochondrial Volume Density in Phrenic Motor Neurons

Abstract

The diaphragm muscle (DIAm) is the major inspiratory pump for breathing. Neural control of the DIAm during breathing involves the recruitment of fatigue resistant motor units comprising smaller phrenic motor neurons (PhMNs), which innervate type I and IIa fibers. More fatigable DIAm motor units comprise larger PhMNs that innervate type IIx/IIb fibers, which generate greater forces but are more fatigable. The fatigable motor units are only infrequently recruited for expulsive behaviors of the DIAm. Thus, there are marked differences in the activation history of DIAm motor units. Consistent with their higher activity levels, previous studies from our lab have shown that smaller PhMNs and type I and IIa DIAm fibers have greater mitochondrial volume density (MVD). PhMNs located in the cervical spinal cord (C3-C5) receive direct excitatory input from medullary premotor neurons, which is primarily ipsilateral. Thus, C2 spinal cord hemisection (C2SH) disrupts the neural pathway essential for breathing and significantly reduces DIAm activity on the same side. Therefore, we hypothesized that C2SH significantly reduces MVD of smaller PhMNs. In Sprague Dawley rats, PhMNs on both sides were retrogradely labeled by intrapleural injection of cholera toxin B (CTB) 3 days prior to C2SH (right side). In addition, DIAm EMG activity was recorded on both sides by inserting bipolar electrodes. Mitochondria were labeled by intraspinal injection of MitoTracker Red and imaged in identified PhMNs using 3D confocal microscopy. A series of optical slices (0.5 mm) of PhMNs were deconvolved to improve contrast, then thresholded for MitoTracker labeling followed by 3D reconstruction, providing detailed images of mitochondrial morphometry and distribution within PhMNs. In sham rats, the MVD of smaller (lower somal surface area) PhMNs was higher compared to larger PhMNs. C2SH reduced DIAm EMG activity on the ipsilateral side across a 14-day period, while activity on the contralateral side increased. After 14 days, C2SH reduced MVD by ~25% in smaller PhMNs on the ipsilateral side. We conclude that the reduced activity of smaller PhMNs induced by C2SH induced mitochondrial morphological changes, most likely due to reduced mitochondrial biogenesis. These results indicate that reduced DIAm activity associated with mechanical ventilation may impose mitochondrial remodeling that reduces mitochondrial oxidative capacity in PhMNs. These adaptations may disrupt mitochondrial function and impair weaning from mechanical ventilation. Research support by NIH grants HL166204, HL146114 and AG44615. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.