Citations

The diaphragm muscle (DIAm) must be able to generate sufficient forces to sustain ventilatory and non‐ventilatory behaviors throughout the lifespan. Aging‐related decreases in DIAm fiber cross‐sectional area and specific force, i.e., sarcopenia, may impair the ability to generate transdiaphragmatic pressures (Pdi) necessary to accomplish expulsive, non‐ventilatory behaviors essential for airway clearance. We examined the effects of aging on DIAm function in mice at 6, 18, and 24 months of age. At each age, DIAm‐phrenic nerve preparations were used to determine maximal force by ex vivo contractility of the DIAm as well as neuromuscular transmission failure by repetitive nerve and muscle stimulation. By 18 months, animals had reduced Pdi during maximal forces (e.g., during tracheal occlusion) and increased DIAm neuromuscular transmission failure compared to 6 months, but no change in DIAm maximum specific force. By 24 months, Pdi remained reduced during occlusion, DIAm neuromuscular transmission failure remained increased compared to 6 months, and DIAm maximum specific force was decreased compared to both 6 and 18 months. Using this global measure of neuromuscular transmission, it is evident that impaired transmission precedes muscle weakness. Support or Funding Information Supported by R01‐AG044615.

Neuromotor control of respiratory muscles such as the diaphragm muscle (DIAm), the main inspiratory muscle in mammals, is organized around motor units. Motor units comprise a motoneuron and the muscle fibers they innervate. Diversity in the contractile and fatigue properties of DIAm motor units permits generation of the wide range of behaviors necessary to sustain ventilation and maintain airways patent (e.g., sighs, which generate ~70% of maximal force in rats). Descending inputs to phrenic motoneurons innervating the DIAm is thought to be primarily unilateral in rats. After disrupting ipsilateral inputs by unilateral spinal cord hemisection at C2 (C2SH), latent, bilateral projections underlie recovery of ipsilateral DIAm activity. We hypothesized that DIAm EMG activity reflects bilateral descending premotor drive to motoneurons that varies across motor behaviors and conditions (anesthetized vs. awake). Using chronic DIAm EMG recordings in adult male Sprague‐Dawley rats, the bilateral vs. ipsilateral nature of premotor drive for each behavior and condition was evaluated by comparing the impact of C2SH. Three days prior to the C2SH surgery, a pair of DIAm electrodes was implanted bilaterally and DIAm EMG activity was assessed immediately prior to and after C2SH as well as at 3 days post‐C2SH in spontaneously‐breathing awake, restrained animals and following induction of ketamine‐xylazine anesthesia. DIAm EMG recordings comprised ~5 min of eupnea and exposure to hypoxia (10% O 2 )‐hypercapnia (5% CO 2 ). The completeness of C2SH surgery was confirmed by the absence of ipsilateral diaphragm EMG activity during eupnea at the time of C2SH surgery and at 3 days post‐C2SH in anesthetized animals, consistent with previous studies. In addition, histological assessment of disruption of anterolateral white matter tracts was conducted to verify the C2SH injury. Ipsilateral RMS EMG amplitude in anesthetized rats pre‐C2SH was ~30% and 45% of pre‐injury anesthetized sigh levels during eupnea and hypoxia‐hypercapnia, respectively. During awake conditions pre‐C2SH, ipsilateral RMS EMG amplitude was significantly higher (~50%, 70% and 200% of pre injury anesthetized sigh levels during eupnea, hypoxia‐hypercapnia and sighs, respectively). There were no differences across diaphragm muscle sides pre‐C2SH in RMS EMG amplitude. At 3 days post‐C2SH, ipsilateral RMS EMG activity was evident in anesthetized animals during hypoxia‐hypercapnia and sighs (~10% and 50% of pre‐injury anesthetized sigh levels) and in awake animals during eupnea, hypoxia‐hypercapnia and sighs (~25%, 50% and 125% of pre‐injury anesthetized sigh levels, respectively). These findings represent considerable differences in DIAm activity across motor behaviors and anesthetized vs. awake conditions and highlight physiologically‐relevant differences in the relative contribution of ipsilateral vs. bilateral inputs to phrenic motoneurons. Support or Funding Information NIH grant R01 HL96750.

Autophagy is a series of catabolic processes that involves degradation of cytoplasmic components through lysosomal pathways. There is an important balance that must be met, with impaired autophagy contributing to cell dysfunction and excessive autophagy leading to cell death likely via apoptosis. Indeed autophagy is a dynamic multi‐step process that is modulated positively and negatively at several steps. The role of autophagy in aging‐related motoneuron dysfunction or death is not known. Therefore, it is important to differentiate between the accumulation of autophagosomes due to increased induction of autophagy during conditions of cell stress common across aging‐related conditions, and the accumulation of autophagosomes due to inefficient clearance of damaged organelles and other sequestered cargoes, which reflects impairments in autophagy that would lead to cell dysfunction. We hypothesized that with aging, there is autophagy imbalance causing accumulation of autophagosomes and ultimately neuromuscular dysfunction. Male and female mice at 6 and 24 months of age were used to assess markers of autophagy in the spinal cord. Protein expression of autophagy‐related genes LC3A and LC3B increased in the cervical and lumbar spinal cord of old compared to young mice, whereas protein expression of Atg5, Atg7, and Bcn1 did not change with age. Quantitative real‐time RT‐PCR of microdissected phrenic motoneurons showed an increase in LC3 mRNA expression with age. Overall autophagic flux, which assesses inclusion (or exclusion) of cargo within autophagosomes, delivery of cargo to lysosomes, and subsequent breakdown and release of macromolecules back into the cytosol was evaluated using an mCherry‐GFP‐LC3 sensor. This genetic sensor is designed to monitor LC3 flux through autophagy pathways since the GFP signal is sensitive to the acidic/proteolytic conditions of the lysosome lumen, whereas mCherry fluorescence persists under these environments. We used motoneuron‐like NSC‐34 cells transfected with mCherry‐GFP‐LC3 plasmid to detect changes in the autophagic degradation rate in cells treated with tunicamycin (which leads to cell stress via induction of the unfolded protein response). We observed an increase in yellow vesicles indicating an accumulation of autophagosomes due to inefficient clearance of sequestered cargoes. This genetic sensor indicates impaired autophagy flux in motoneurons in conditions of elevated stress likely present in old age. Taken together, these results suggest impaired autophagy in motoneurons, including phrenic motoneurons, with an associated increase in markers of early autophagosome formation (LC3). Support or Funding Information Supported by R01‐AG044615.

Recent findings suggest that autophagy plays a role in the development of aging-related neuromuscular dysfunction; however, the contribution of autophagy impairment to the maintenance of diaphragm force generation in old age is unknown. This study shows that in old mice, chloroquine administration decreases maximal transdiaphragmatic pressure generation. These chloroquine effects suggest a susceptibility to impairments in autophagy in old age.

The diaphragm muscle is important in both sustaining ventilation and performing higher-force behaviors necessary for maintaining airway patency. Ventilatory behaviors (i.e., eupnea and response to hypoxia-hypercapnia) require only ~20% of maximal force generation, whereas coughing requires forces ~60% of maximal. Existing techniques to measure the full range of forces generated by respiratory muscles are limited by their invasiveness (e.g., balloon manometry for transdiaphragmatic pressure (Pdi) measurements), requirements for training, supervision and subjectivity (e.g., maximal inspiratory and expiratory pressures). Ultrasound-based shear-wave elastography (SWE) assesses muscle stiffness and has shown promising results in being used as a surrogate for Pdi to evaluate diaphragm muscle force. Previous literature has demonstrated a correlation between diaphragm SWE and Pdi at lower force behaviors and the goal of the present study was to evaluate the efficacy of SWE-based measurements of the diaphragm in correlating with Pdi measurements across the range of forces generated by the diaphragm. In this pilot study, we recruited 6 male and 5 female healthy volunteers. Subjects were positioned in a seated position with their torso unsupported. Pdi was measured during graded inspiratory threshold loading trials using a POWERbreathe Plus commercial inspiratory loading device, as well as during maximal inspiratory pressure (MIP). Simultaneously, SWE was performed at the zone of apposition in the right anterior axillary line, and diaphragm Young’s modulus and shear wave speed were subsequently measured. The average Pdi during MIP testing was 149 ± 66 cm H2O. Pdi during inspiratory loading at a low resistance was 39 ± 14 cm H2O and at a high resistance was 78 ± 29 cm H2O. Subjects were able to generate an average Pdi range of 39 ± 19 cm H2O across the graded resistance levels, and the average percent of MIP obtained was 54 ± 13%. Average Young’s modulus as measured with SWE during inspiratory loading at a low resistance was 46 ± 19 kPa and at a high resistance was 104 ± 19 kPa. From low to high resistance, Pdi, Young’s modulus, and shear wave speed increased by an average of 100%, 126%, and 59%, respectively. Taken together, SWE is a promising method of non-invasive assessment of diaphragm muscle forces across a range of voluntary behaviors. Support from NIH grant R21 DA055848 and the Mayo Clinic. This is the full abstract presented at the American Physiology Summit 2023 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.

Autophagy is a process through which damaged cytoplasmic structures are recycled and degraded within the cell. Aging can affect autophagy regulation in different steps and lead to the accumulation of altered proteins, leading to cell damage and death. Motor neuron (MN) loss is an important contributor to sarcopenia age-dependent motor dysfunction. Understanding the regulatory mechanisms of autophagy can help elucidate the motor dysfunction that occurs with aging and provide a target for developing future therapeutic agents. Previous immunofluorescence imaging studies in phrenic MNs showed an age-dependent increase in key autophagy proteins LC3 and p62, reflecting autophagosome accumulation and autophagy impairment. Accordingly, we hypothesized that aging in lumbar MNs leads to autophagy impairment reflected in regulatory protein expression. The current study assessed autophagy in choline acetyltransferase-positive MNs from spinal levels L1-L6, focused on crural muscles (extensor digitorum longus and soleus), using confocal immunofluorescence imaging. Expression of LC3 and p62 proteins in MNs were determined in male and female C57BL/6 mice at 6-, 18- and 24- months of age. Increased expression of LC3 in lumbar MNs was observed with aging across all three age groups and p62 expression in lumbar MNs was significantly increased by 24- and 18-months compared to 6-months (p<0.001). There was no sex effect observed in LC3, but p62 was significantly higher in lumbar MNs of female mice (p=0.007). Expression of both LC3 and p62 increased diffusely in ventral lumbar gray matter regions with age (p<0.001), an effect that was predominant in male and not female mice. Taken together, the results of this study provide evidence for that age-dependent changes in autophagy markers expression occurs in regions of the spinal cord (motor neurons and ventral horn gray matter). The neuronal mechanisms of aging effects include dysregulation of autophagy. Supported by NIH R01 AG057052. This is the full abstract presented at the American Physiology Summit 2023 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.

Transdiaphragmatic pressure (Pdi) measurements are used to determine diaphragm force generation across various motor behaviors. Previous studies in the diaphragm muscle found selective age-related changes at fast-twitch fatigable motor units including increased susceptibility to neuromuscular transmission failure and denervation. Accordingly, age-related changes at diaphragm motor units predominantly impair the ability to accomplish higher force behaviors, and these aging effects are evident prior to the onset of muscle atrophy. Recent findings suggest that autophagy plays a critical role in the development of age-related neuromuscular dysfunction; however, the possible contribution of autophagy impairment to the maintenance of diaphragm force generation in old age is unknown. We hypothesized that inhibition of autophagy (using chloroquine) decreases diaphragm force-generating capacity (Pdi) in mice susceptible to age-related neuromuscular dysfunction. Adult male and female C57BL/6 mice at 16-28 months of age were randomized to receive a single intraperitoneal injection of 50 mg/kg of chloroquine or vehicle (n= 9 per treatment group) four hours before Pdi measurements, with the experimenter blinded. Pdi was measured during eupnea (room air breathing), exposure to hypoxia/hypercapnia (10% O2/5% CO2), tracheal occlusion, spontaneous deep breaths (“sighs”) and during maximal activation elicited by bilateral phrenic nerve stimulation. Consistent with our hypothesis, there was an ~25% reduction in maximal Pdi in the chloroquine group (46 ± 12 cm H2O) compared to the vehicle control (61 ± 16 cm H2O; p < 0.05). There was no effect of chloroquine on Pdi during eupnea, hypoxia/hypercapnia, tracheal occlusion, or sigh behaviors. Acute inhibition of autophagy results in reduced maximal Pdi, consistent with that evident in old mice. Thus, we propose that autophagy impairment may be an important contributor to the reduced diaphragm force generation associated with aging. These findings help us further understand the mechanisms regulating age-related neuromuscular dysfunction and may serve as a foundation to develop novel treatments to mitigate aging effects. Support: NIH R01 AG057052. This is the full abstract presented at the American Physiology Summit 2023 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.

Neurotrophin brain-derived neurotrophic factor (BDNF) enhances neuromuscular transmission via activation of tropomyosin-related kinase receptor B (TrkB). Rat phrenic nerve-diaphragm muscle preparations were used to determine the rapid effect of BDNF on synaptic quantal release during repetitive stimulation. BDNF treatment significantly enhanced quantal release at all fiber types. BDNF increased synaptic vesicle cycling (measured using FM4-64 fluorescence uptake); conversely, inhibiting BDNF/TrkB signaling decreased FM4-64 uptake.

The diaphragm muscle is the primary inspiratory muscle in mammals and is highly active throughout life. The repetitive activation of the diaphragm muscle (and of other muscles driven by central pattern generator activity) presents an opportunity to analyze these physiological data on a per‐event basis. Here we highlight the development and implementation of the Automated Evaluation of Respiratory Signals (AERS) algorithm to detect individual respiratory events across a range of motor behaviors. Because different respiratory‐related signals provide different types of information, the AERS algorithm is designed to be robust regardless of the signal type—e.g., diaphragm electromyography (EMG) and transdiaphragmatic pressure (P di ). Individual respiratory event data obtained from adult rats were assessed using the AERS algorithm. This analysis revealed diaphragm root‐mean‐square (RMS) EMG and Pdi were lowest during eupnea, increased slightly in response to hypoxia‐hypercapnia (10% O 2 ‐ 5% CO 2 ), and increased approximately 3‐fold in response to airway occlusion compared to the eupneic baseline. The coefficient of variation (CV) within subjects was less than 10% across all behaviors for both RMS EMG and Pdi. By contrast, the CV was approximately 30% across subjects. Respiratory rate was increased by ~20 breaths/min during hypoxia‐hypercapnia compared to eupnea, as expected based on previous work from our lab and others. The respiratory rate CV within subjects was less than 10% during eupnea and hypoxia‐hypercapnia; during airway occlusion, the CV was ~17%, likely owing to the small number of respiratory events (2‐5) analyzed for this behavior per animal. For eupnea and hypoxia‐hypercapnia, the CV across subjects was ~20%, whereas during airway occlusion it was ~25%. Overall, our findings validate the mean values across various measures of respiratory motor output compared to standard subject means approach. However, per‐event analyses greatly enhance statistical power, allowing detection of physiologically relevant measures by taking within‐subject variability into account. Additionally, the AERS analyses show that various measures of respiratory variability as well as time‐frequency analyses can provide deeper insights into respiratory drive when accounting for individual respiratory events. These analyses can be effectively applied to a variety of models of respiratory motor unit loss or dysfunction (e.g., spinal cord injury and aging), allowing the extraction of large amounts of data from respiratory signals for more comprehensive assessments. Our findings suggest that the AERS approach may be particularly beneficial in reducing animal numbers in certain types of studies, for the detection of physiological changes reflected in respiratory signals with interventions causing gain or loss of function, and for analyses of respiratory variability, where breath‐to‐breath dynamics are important.

Sarcopenia is the aging‐related loss of force‐generating capacity and atrophy of skeletal muscles and is a major contributor to morbidity and mortality in the elderly. The molecular mechanisms underlying sarcopenia remain to be elucidated. Skeletal muscles exhibit different rates of sarcopenia, generally reflecting their specific muscle fiber type composition. Muscles primarily composed of type IIx and/or IIb muscle fibers (i.e., extensor digitorum longus; EDLm) develop sarcopenia at a faster rate than other muscles. In the diaphragm, muscle fiber cross‐sectional area (CSA) is reduced in old age, predominantly in type IIx and/or IIb fibers. Autophagy is a multistep, catabolic process that targets dysfunctional cytoplasmatic structures including proteins and organelles for degradation and recycling. Autophagy is impaired in old age across several tissues, including motor neurons and skeletal muscle. Expression of core autophagy proteins can indicate changes in autophagy, e.g., LC3 reflects initiation/elongation steps and p62 clearance reflects degradation. We hypothesize that impaired autophagy is associated with the development of sarcopenia in type‐identified muscle fibers of the EDLm. Indeed, at 24‐months of age, the CSA of type I and IIa fibers was not reduced, whereas the CSA of type IIx and/or IIb fibers was ~25% smaller (p<0.01 in both cases), compared to 6‐month‐old mice. These findings are consistent with fiber type selective sarcopenia in the EDLm. Expression of LC3 and p62 in the EDLm of mice at 6‐ and 24‐ months of age was evaluated via immunofluorescence by quantification of the number of LC3 and p62 puncta per muscle fiber type. In type I and IIa fibers, the number of LC3 puncta increased by ~30% and of p62 increased three‐fold in 24‐ compared to 6‐month‐old mice (p<0.01 in both cases). In type IIx and/or IIb fibers, the number of LC3 puncta decreased by ~60% and p62 puncta increased by ~50% in 24‐ compared to 6‐month‐old mice (p<0.01 in both cases). Accordingly, compared to 6 month‐old mice, autophagic degradation is impaired across muscle fiber types in 24‐month old mice, but likely to a greater extent in type I and IIa fibers of old mice given the increase in both LC3 and p62. Autophagic initiation is primarily reduced in type IIx and/or IIb fibers in the EDL as indicated by the reduced LC3 expression. Taken together these findings indicate that fiber type selective sarcopenia may reflect differences in autophagy across muscle fiber types.