Citations

Neuromotor control of diaphragm muscle is dependent on the orderly recruitment of phrenic motor neurons (PhMNs) in a size‐dependent manner. Type S and FR motor units comprise smaller, more easily recruited PhMNs that innervate fatigue resistant, lower force‐producing type I and type IIa muscle fibers. Type FInt and FF motor units, comprise larger, rarely recruited PhMNs innervating the more fatigable, higher force‐producing type IIx and/or IIb muscle fibers. The activity and energy demand upon PhMNs of type S and FR motor units is greater than that of PhMNs from type FInt and FF units, due to the high duty cycle (~35%) of ventilation. Accordingly, activity of type FInt and FF motor units is required for higher force behaviors that are performed intermittently over much shorter durations. These include the expulsive/straining maneuvers during coughing, sneezing, defecation and parturition. The energy demand in both PhMNs and diaphragm muscle fibers is met by mitochondria. To preserve homeostasis, mitochondrial functions are regulated by dynamic, continuous cycles of fusion and fragmentation. We hypothesized that mitochondrial density and mitochondrial fusion will be greater in smaller PhMNs, due to their more frequent activation during ventilation. In six adult (6‐month old) Fischer 344 rats, PhMNs were retrogradely labeled by intrapleural injection of Alexa488‐conjugated CTB. Three days later, cervical transdural infusion of MitoTracker Red was performed every 10 min for 1.5 h under anesthesia. Spinal cord tissue was collected following transcardial perfusion with 4% paraformaldehyde, sectioned longitudinally at 70 μm, and mounted onto slides. Two‐channel sequential confocal z‐stack (0.50 μm step size) mosaics of PhMNs and mitochondria were acquired using a 60x oil immersion objective and analyzed using Elements software (Nikon). The surface areas of PhMNs were measured and divided into tertiles. The lower tertile of PhMNs (comprising type S and FR motor units) had greater mitochondrial volume densities, greater mitochondrial aspect ratios and form factors and greater mean individual mitochondrial volumes than upper tertile PhMNs (comprising type FInt and FF motor units). We further characterized the differences in mRNA expression of mitochondrial dynamics associated proteins (Mfn2 and Drp1) in large vs. small PhMNs using fluorescent in‐situ hybridization techniques. Our results suggest that PhMNs of type S and FR motor units have increased mitochondrial density, supporting their high energy demand to sustain ventilation. Their increased mitochondrial fusion may underlie their resilience to PhMN loss in aging and disease. Support or Funding Information Supported by a National Institutes of Health grants R01‐AG044615

Pro‐inflammatory cytokines, like TNFα, mediate a cellular stress response to inflammation that may target mitochondria. In the present study, we hypothesize that in motor neuron‐like NSC‐34 cells, TNFα induces mitochondrial fragmentation and biogenesis in order to maintain overall O 2 consumption of these cells. NSC‐34 cells were differentiated by serum depletion (~24 h) until neurite structures (>50 μm) were observed. Cells were then exposed to TNFα (20 ng/ml) for 24 h. Mitochondria were labeled using MitoTracker Green and imaged using confocal microscopy. Mitochondrial morphometry was assessed by measuring aspect ratio (ratio of long to short axis) and form factor (perimeter 2 /4p·Area). Expression of mitofusin 2 (Mfn2 – fusion protein) and dynamin related protein 1 (Drp1 – fission) was determined by Western blot. From 3D reconstruction of optical slices of NSC‐34 cells, cell and mitochondrial volumes were determined and mitochondrial volume density calculated. Mitochondrial biogenesis was assessed by measuring mitochondrial DNA, and expression of PGC1α was measured. An Agilent Seahorse XF Analyzer was used to measure cellular O 2 consumption. A quantitative histochemical method was used to measure the maximum velocity of the succinate dehydrogenase (SDH max ) reaction within NSC‐34 cells to determine the functionality of the tri‐carboxylic acid (TCA) cycle and the electron transport chain (ETC). Finally, reactive oxygen species (ROS) formation was measured using MitoSox fluorescence. After exposure to TNFα, aspect ratio and form factor of mitochondria decreased by ~15% compared to untreated controls with consistent changes in Mfn2 and Drp1 expression. After TNFα exposure, mitochondrial DNA expression increased as did mitochondrial volume density, together indicating increased mitochondrial biogenesis. After TNFα exposure, O 2 consumption per mitochondrion (normalized for mitochondrial volume density) decreased. This was consistent with a TNFα‐induced decrease in SDH max . Despite the decrease in O 2 consumption and SDH max per mitochondrion after TNFα exposure, ROS formation increased reflecting mitochondrial dysfunction. We conclude that mitochondrial fragmentation, biogenesis and increased mitochondrial volume density are a homeostatic mechanism to decrease O 2 consumption per mitochondrion thereby damping ROS formation, while maintaining overall O 2 consumption to meet ATP demand. Support or Funding Information Supported by NIH grants AG44615 and HL105355.

Neuromuscular dysfunction is common in old age. A hallmark of aging is the aggregation of damaged cytoplasmic structures, especially in post‐mitotic cells like motor neurons. Autophagy flux may be impaired as a result of deficits in the formation of double membrane autophagosomes, the fusion of autophagosomes with lysosomes, or lysosome‐based degradation. Our overarching hypothesis is that autophagy contributes to neuromuscular dysfunction in old age, and that this effect occurs in motor neurons. We evaluated changes in expression levels of core autophagy proteins necessary for the initiation (Beclin1) and elongation (LC3‐I, LC3‐II, ATG7 and ATG5‐ATG12 complex) phases of autophagsome formation, as well as lysosome degradation (assessed by p62 clearance), in adult male and female C57BL/6 mice at 6, 18, and 24 months of age (reflecting 100%, 90% and 75% survival, respectively). Protein expression was analyzed by Western blotting in the cervical spinal cord segments C2–C6. Expression of p62 was reduced in the cervical spinal cord of adult mice at 18 months compared to 24 months, suggesting increased clearance and autophagy flux at 18 months of age. There was no evidence of an effect of age on the expression of the other autophagy markers. In separate animals, immunohistochemical (IHC) analyses of the C2‐C6 segments was used to determine the relative expression of LC3 and p62 in gray and white matter regions. Across all age groups, LC3 and p62 immunoreactivity was evident in the gray matter with minimal expression in the white matter. The ratio of both LC3 and p62 in motor neurons vs. gray matter was compared using a mixed linear model with animal as a random effect. No effect of sex was evident. Increased expression of both LC3 and p62 was evident in choline acetyl transferase (ChAT)‐positive motor neurons (~2–3 fold) compared to the remainder of the gray matter. There was evidence of a difference in relative LC3 and p62 expression in motor neurons across age (F 349,2 =12.5, p<0.001), with a significant increase by 24 months of age (p<0.005). Taken together, these findings suggest that 1) there is an aging effect on autophagy markers in the cervical spinal cord of mice; 2) an increase in autophagy by 18 months of age reflects an increase in autophagosome clearance (reduced p62 expression) not restricted to motor neurons; and, 3) increased motor neuron expression of both LC3 and p62 by 24 months of age reflect autophagsome accumulation and autophagy impairment in motor neurons. These findings are also is in accordance with previous reports suggesting a period of vulnerability between 18 and 24 months of age in mice, where autophagy may play a role in the development of age‐related neuromuscular dysfunction. Support or Funding Information Supported by NIH R01 AG057052.

Inflammation triggers a homeostatic response that is mediated by pro‐inflammatory cytokines such as TNFα. We hypothesize that in a motor neuron‐like cell line, NSC‐34, TNFα decreases mitochondrial function, specifically succinate dehydrogenase (SDH) activity, which is a key enzyme in the tricarboxylic acid (TCA) cycle as well as complex II in the electron transport chain (ETC). Experiments were performed on cultured NSC‐34 cells that were differentiated by serum depletion until neurite structures (>50 μm) were observed. The differentiated cells were treated with TNFα (20 ng/ml) for 24 h. To measure the maximum velocity of the SDH reaction (SDH max ), cells were incubated in a solution containing 80 mM succinate and 1.5 mM nitro blue tetrazolium (NBT), the reaction indicator. As the SDH reaction proceeded, images were acquired every 15 s using a 40× objective on an Olympus IX71 inverted microscope. Linearity of the SDH reaction was confirmed across the 8‐min period, and SDH max was determined and expressed as mM fumarate/L tissue/min. To determine mitochondrial volume density, the cells were loaded with MitoTracker Green and imaged in 3D using a confocal microscope. The cell volume occupied by labeled mitochondria was determined from 3D reconstructions of 1 mm optical slices. In NSC‐34 cells exposed to TNFα, we found that mitochondrial volume density increased, and mitochondria were fragmented as compared to untreated controls. We also found that TNFα exposure decreased SDH max when normalized for mitochondrial volume density. In other cell types, we found that TNFα‐induces an increase in PGC1a and Drp1 expression and a decrease in Mfn2 expression, all of which affect mitochondrial structure. In related studies, we also found that TNFα exposure decreases O 2 consumption per mitochondrion in NSC‐34 cells. We conclude that by reducing the redox potential (SDH max ) of mitochondria, O 2 consumption per mitochondrion is reduced, thereby damping ROS formation. The impact of this homeostatic response is offset by mitochondrial biogenesis and an increase in mitochondrial volume density, thereby maintaining overall cellular O 2 consumption and ATP production but at reduced cost of ROS formation. Support or Funding Information Supported by NIH grants AG44615 and HL105355.

Neuroinflammation triggers a homeostatic response that is mediated by pro‐inflammatory cytokines such as TNFα. We hypothesize that in a motor neuron‐like cell line, NSC‐34, TNFα decreases mitochondrial O 2 consumption. NSC‐34 cells were differentiated by serum depletion (~24 h) until neurite structures (>50 μm) were observed. The cells were then exposed to TNFα (20 ng/ml) for 24 h. To determine mitochondrial volume density, the cells were loaded with MitoTracker Green and imaged in 3D using a Nikon A1R confocal microscope (60x oil immersion 1.4 NA). After treatment with TNFα, mitochondria appeared fragmented but mitochondrial volume density increased by ~50%, as compared to control untreated cells. A Agilent Seahorse XF Analyzer was used to determine the O 2 consumption of treated and untreated cells during a stress test that determines basal respiration, ATP production and maximum respiration. O 2 consumption was normalized to mitochondrial volume density in both treated and untreated groups. After TNFα exposure, basal respiration, ATP production and maximum respiration decreased per mitochondrion. However, overall O 2 consumption of the cells was unchanged after TNFα exposure due to an offsetting increase in mitochondrial volume density. Using MitoSox, we also measured reactive oxygen species (ROS) formation in treated and untreated NSC‐34 cells. ROS is a byproduct of mitochondrial O 2 consumption, and can cause cellular damage. After TNFα exposure, ROS formation increased compared to untreated cells. We conclude that mitochondrial biogenesis and an increase in mitochondrial volume density represent a homeostatic mechanism to reduce O 2 consumption the per mitochondrion, thereby damping ROS formation while maintaining overall cellular O 2 consumption and ATP production. Support or Funding Information Supported by NIH grant AG44615 and HL105355.

AbstractThe diaphragm muscle comprises various types of motor units that are recruited in an orderly fashion governed by the intrinsic electrophysiological properties (membrane capacitance as a function of somal surface area) of phrenic motor neurons (PhMNs). Glutamate is the main excitatory neurotransmitter at PhMNs and acts primarily via fast acting AMPA and N‐methyl‐D‐aspartic acid (NMDA) receptors. Differences in receptor expression may also contribute to motor unit recruitment order. We used single cell, multiplex fluorescence in situ hybridization to determine glutamatergic receptor mRNA expression across PhMNs based on their somal surface area. In adult male and female rats (n = 9) PhMNs were retrogradely labeled for analyses (n = 453 neurons). Differences in the total number and density of mRNA transcripts were evident across PhMNs grouped into tertiles according to somal surface area. A ~ 25% higher density of AMPA (Gria2) and NMDA (Grin1) mRNA expression was evident in PhMNs in the lower tertile compared to the upper tertile. These smaller PhMNs likely comprise type S motor units that are recruited first to accomplish lower force, ventilatory behaviors. In contrast, larger PhMNs with lower volume densities of AMPA and NMDA mRNA expression presumably comprise type FInt and FF motor units that are recruited during higher force, expulsive behaviors. Furthermore, there was a significantly higher cytosolic NMDA mRNA expression in small PhMNs suggesting a more important role for NMDA‐mediated glutamatergic neurotransmission at smaller PhMNs. These results are consistent with the observed order of motor unit recruitment and suggest a role for glutamatergic receptors in support of this orderly recruitment. image Cover Image for this issue: doi: 10.1111/jnc.14747.

Like all skeletal muscles, the diaphragm muscle accomplishes a range of motor behaviors by recruiting different motor unit types in an orderly fashion. Recruitment of phrenic motor neurons (PhMNs) is generally assumed to be based primarily on the intrinsic properties of PhMNs with an equal distribution of descending excitatory inputs to all PhMNs. However, differences in presynaptic excitatory input across PhMNs of varying sizes could also contribute to the orderly recruitment pattern. In the spinal cord of Sprague-Dawley rats, we retrogradely labeled PhMNs using cholera toxin B (CTB) and validated a robust confocal imaging-based technique that utilizes semiautomated processing to identify presynaptic glutamatergic (Glu) terminals within a defined distance around the somal membrane of PhMNs of varying size. Our results revealed an ~10% higher density of Glu terminals at PhMNs in the lower tertile of somal surface area. These smaller PhMNs are likely recruited first to accomplish lower force ventilatory behaviors of the diaphragm as compared with larger PhMNs in the upper tertile that are recruited to accomplish higher force expulsive behaviors. These results suggest that differences in excitatory synaptic input to PhMNs may also contribute to the orderly recruitment of diaphragm motor units.NEW & NOTEWORTHY The distribution of excitatory glutamatergic synaptic input to phrenic motor neurons differs across motor neurons of varying size. These findings support the size principle of motor unit recruitment that underlies graded force generation in a muscle, which is based on intrinsic electrophysiological properties of motor neurons resulting from differences in somal surface area. A higher density of glutamatergic inputs at smaller, more excitable motor neurons substantiates the earlier and more frequent recruitment of these units.

Midcervical spinal cord contusion injury results in tissue damage, disruption of spinal pathways, and motor neuron loss. Unilateral C4 contusion results in loss of 40%–50% of phrenic motor neurons ipsilateral to the injury (~25% of the total phrenic motor neuron pool). Over time after unilateral C4 contusion injury, diaphragm muscle (DIAm) electromyogram activity increases both contralateral and ipsilateral to the side of injury in rats, suggesting compensation because of increased activation of the surviving motor neurons. However, the impact of contusion injury on DIAm force generation is less clear. Transdiaphragmatic pressure (Pdi) was measured across motor behaviors over time after unilateral C4 contusion injury in adult male Sprague-Dawley rats. Maximum Pdi (Pdimax) was elicited by bilateral phrenic nerve stimulation at 7 days postinjury. We hypothesized that Pdimaxis reduced following unilateral C4 contusion injury, whereas ventilatory behaviors of the DIAm are unimpaired. In support of our hypothesis, Pdimaxwas reduced by ~25% after unilateral C4 contusion, consistent with the extent of phrenic motor neuron loss following contusion injury. One day after contusion injury, the Pdi amplitude during airway occlusion was reduced from ~30 to ~20 cmH2O, but this reduction was completely reversed by 7 days postinjury. Ventilatory behaviors (~10 cmH2O), DIAm-specific force, and muscle fiber cross-sectional area did not differ between the laminectomy and contusion groups. These results indicate that the large reserve capacity for DIAm force generation allows for higher-force motor behaviors to be accomplished despite motor neuron loss, likely reflecting changes in motor unit recruitment.NEW & NOTEWORTHY Respiratory muscles such as the diaphragm generate the pressures necessary to accomplish a variety of motor behaviors ranging from ventilation to near-maximal expulsive behaviors. However, the impact of contusion injury on diaphragm pressure generation across behaviors is not clear. The present study shows that contusion injury impairs maximal pressure generation while preserving the ability of the diaphragm to accomplish lower-force motor behaviors, likely reflecting changes in diaphragm motor unit recruitment.

Contusion-type injuries to the spinal cord are characterized by tissue loss and disruption of spinal pathways. Midcervical spinal cord injuries impair the function of respiratory muscles and may contribute to significant respiratory complications. This study systematically assessed the impact of a 100-kDy unilateral C4 contusion injury on diaphragm muscle activity across a range of motor behaviors in rats. Chronic diaphragm electromyography (EMG) was recorded before injury and at 1 and 7 days postinjury (DPI). Histological analyses assessed the extent of perineuronal net formation, white-matter sparing, and phrenic motoneuron loss. At 7 DPI, ∼45% of phrenic motoneurons were lost ipsilaterally. Relative diaphragm root mean square (RMS) EMG activity increased bilaterally across a range of motor behaviors by 7 DPI. The increase in diaphragm RMS EMG activity was associated with an increase in neural drive (RMS value at 75 ms after the onset of diaphragm activity) and was more pronounced during higher force, nonventilatory motor behaviors. Animals in the contusion group displayed a transient decrease in respiratory rate and an increase in burst duration at 1 DPI. By 7 days, following midcervical contusion, there was significant perineuronal net formation and white-matter loss that spanned 1 mm around the injury epicenter. Taken together, these findings are consistent with increased recruitment of remaining motor units, including more fatigable, high-threshold motor units, during higher force, nonventilatory behaviors. Changes in diaphragm EMG activity following midcervical contusion injury reflect complex adaptations in neuromotor control that may increase the risk of motor-unit fatigue and compromise the ability to sustain higher force diaphragm efforts.NEW & NOTEWORTHY The present study shows that unilateral contusion injury at C4 results in substantial loss of phrenic motoneurons but increased diaphragm muscle activity across a range of ventilatory and higher force, nonventilatory behaviors. Measures of neural drive indicate increased descending input to phrenic motoneurons that was more pronounced during higher force, nonventilatory behaviors. These findings reveal novel, complex adaptations in neuromotor control following injury, suggestive of increased recruitment of more fatigable, high-threshold motor units.