Delivery of recombinant adeno-associated virus vectors to rat diaphragm muscle via direct intramuscular injection

Enhanced Diaphragm Muscle Function upon Satellite Cell Transplantation in Dystrophic Mice

The diaphragm muscle is essential for breathing, and its dysfunctions can be fatal. Many disorders affect the diaphragm, including muscular dystrophies. Despite the clinical relevance of targeting the diaphragm, there have been few studies evaluating diaphragm function following a given experimental treatment, with most of these involving anti-inflammatory drugs or gene therapy. Cell-based therapeutic approaches have shown success promoting muscle regeneration in several mouse models of muscular dystrophy, but these have focused mainly on limb muscles. Here we show that transplantation of as few as 5000 satellite cells directly into the diaphragm results in consistent and robust myofiber engraftment in dystrophin- and fukutin-related protein-mutant dystrophic mice. Transplanted cells also seed the stem cell reservoir, as shown by the presence of donor-derived satellite cells. Force measurements showed enhanced diaphragm strength in engrafted muscles. These findings demonstrate the feasibility of cell transplantation to target the diseased diaphragm and improve its contractility.

Targeting drug or gene delivery to the phrenic motoneuron pool

Phrenic motoneurons (PhrMNs) innervate diaphragm myofibers. Located in the ventral gray matter (lamina IX), PhrMNs form a column extending from approximately the third to sixth cervical spinal segment. Phrenic motor output and diaphragm activation are impaired in many neuromuscular diseases, and targeted delivery of drugs and/or genetic material to PhrMNs may have therapeutic application. Studies of phrenic motor control and/or neuroplasticity mechanisms also typically require targeting of PhrMNs with drugs, viral vectors, or tracers. The location of the phrenic motoneuron pool, however, poses a challenge. Selective PhrMN targeting is possible with molecules that move retrogradely upon uptake into phrenic axons subsequent to diaphragm or phrenic nerve delivery. However, nonspecific approaches that use intrathecal or intravenous delivery have considerably advanced the understanding of PhrMN control. New opportunities for targeted PhrMN gene expression may be possible with intersectional genetic methods. This article provides an overview of methods for targeting the phrenic motoneuron pool for studies of PhrMNs in health and disease.

Optogenetic activation of the diaphragm

Impaired diaphragm activation is common in many neuromuscular diseases. We hypothesized that expressing photoreceptors in diaphragm myofibers would enable light stimulation to evoke functional diaphragm activity, similar to endogenous bursts. In a mouse model, adeno-associated virus (AAV) encoding channelrhodopsin-2 (AAV9-CAG-ChR2-mVenus, 6.12 × 10¹¹ vg dose) was delivered to the diaphragm using a minimally invasive method of microinjection to the intrapleural space. At 8-18 weeks following AAV injection, mice were anesthetized and studied during spontaneous breathing. We first showed that diaphragm electromyographic (EMG) potentials could be evoked with brief presentations of light, using a 473 nm high intensity LED. Evoked potential amplitude increased with intensity or duration of the light pulse. We next showed that in a paralyzed diaphragm, trains of light pulses evoked diaphragm EMG activity which resembled endogenous bursting, and this was sufficient to generate respiratory airflow. Light-evoked diaphragm EMG bursts showed no diminution after up to one hour of stimulation. Histological evaluation confirmed transgene expression in diaphragm myofibers. We conclude that intrapleural delivery of AAV9 can drive expression of ChR2 in the diaphragm and subsequent photostimulation can evoke graded compound diaphragm EMG activity similar to endogenous inspiratory bursting.

Activation of Calpain Contributes to Mechanical Ventilation-Induced Depression of Protein Synthesis in Diaphragm Muscle

Mechanical ventilation (MV) is a clinical tool that provides respiratory support to patients unable to maintain adequate alveolar ventilation on their own. Although MV is often a life-saving intervention in critically ill patients, an undesired side-effect of prolonged MV is the rapid occurrence of diaphragmatic atrophy due to accelerated proteolysis and depressed protein synthesis. Investigations into the mechanism(s) responsible for MV-induced diaphragmatic atrophy reveal that activation of the calcium-activated protease, calpain, plays a key role in accelerating proteolysis in diaphragm muscle fibers. Moreover, active calpain has been reported to block signaling events that promote protein synthesis (i.e., inhibition of mammalian target of rapamycin (mTOR) activation). While this finding suggests that active calpain can depress muscle protein synthesis, this postulate has not been experimentally verified. Therefore, we tested the hypothesis that active calpain plays a key role in the MV-induced depression of both anabolic signaling events and protein synthesis in the diaphragm muscle. MV-induced activation of calpain in diaphragm muscle fibers was prevented by transgene overexpression of calpastatin, an endogenous inhibitor of calpain. Our findings indicate that overexpression of calpastatin averts MV-induced activation of calpain in diaphragm fibers and rescues the MV-induced depression of protein synthesis in the diaphragm muscle. Surprisingly, deterrence of calpain activation did not impede the MV-induced inhibition of key anabolic signaling events including mTOR activation. However, blockade of calpain activation prevented the calpain-induced cleavage of glutaminyl-tRNA synthetase in diaphragm fibers; this finding is potentially important because aminoacyl-tRNA synthetases play a central role in protein synthesis. Regardless of the mechanism(s) responsible for calpain’s depression of protein synthesis, these results provide the first evidence that active calpain plays an important role in promoting the MV-induced depression of protein synthesis within diaphragm fibers.

Calpains play an essential role in mechanical ventilation-induced diaphragmatic weakness and mitochondrial dysfunction

Mechanical ventilation (MV) is a life-saving intervention for many critically ill patients. Unfortunately, an unintended consequence of prolonged MV is the rapid development of diaphragmatic atrophy and contractile dysfunction, known as ventilator-induced diaphragm dysfunction (VIDD). Although the mechanism(s) responsible for VIDD are not fully understood, abundant evidence reveals that oxidative stress leading to the activation of the major proteolytic systems (i.e., autophagy, ubiquitin-proteasome, caspase, and calpain) plays a dominant role. Of the proteolytic systems involved in VIDD, calpain has received limited experimental attention due to the longstanding dogma that calpain plays a minor role in inactivity-induced muscle atrophy. Guided by preliminary experiments, we tested the hypothesis that activation of calpains play an essential role in MV-induced oxidative stress and the development of VIDD. This premise was rigorously tested by transgene overexpression of calpastatin, an endogenous inhibitor of calpains. Animals with/without transfection of the calpastatin gene in diaphragm muscle fibers were exposed to 12 h of MV. Results confirmed that overexpression of calpastatin barred MV-induced activation of calpain in diaphragm fibers. Importantly, deterrence of calpain activation protected the diaphragm against MV-induced oxidative stress, fiber atrophy, and contractile dysfunction. Moreover, prevention of calpain activation in the diaphragm forstalled MV-induced mitochondrial dysfunction and prevented MV-induced activation of caspase-3 along with the transcription of muscle specific E3 ligases. Collectively, these results support the hypothesis that calpain activation plays an essential role in the early development of VIDD. Further, these findings provide the first direct evidence that calpain plays an important function in inactivity-induced mitochondrial dysfunction and oxidative stress in skeletal muscle fibers.

•

Inhibiting calpains during mechanical ventilation protects the diaphragm.

•

Calpains play an important role in muscle atrophy and contractile dysfunction.

•

Calpain inhibition during mechanical ventilation prevents mitochondrial dysfunction.

•

Calpain-cleaved molecules may play important signaling roles.

•

Calpain activation cross-talks with other proteolytic systems.

Protective effect of exogenous hydrogen sulfide on diaphragm muscle fibrosis in streptozotocin-induced diabetic rats

IMPACT STATEMENT

Diabetes mellitus is a group of chronic metabolic disorders, which causes serious damage to a variety of organs, such as the retina, heart, and skeletal muscle. The diaphragm is an important skeletal muscle involved in respiration in mammals. Fibrosis of the diaphragm muscle affects its contractility, which in turn impairs respiratory function. Accumulating evidence suggests that exogenous hydrogen sulfide (H₂S) exhibits anti-fibrotic activity in diabetes mellitus, but whether and how H₂S exerts this anti-fibrotic effect in the diabetic diaphragm remains unclear. The current work for the first time reveals that exogenous H₂S attenuates hyperglycemia-induced fibrosis of the diaphragm muscle and strengthens diaphragmatic biomechanical properties in diabetes mellitus, and the mechanism may involve the alleviation of collagen deposition by suppression of the nucleotide-binding oligomerization domain-like receptor protein (NLRP) 3 inflammasome-mediated inflammatory reaction. Therefore, H₂S supplementation could be used as an efficient targeted therapy against the NLRP3 inflammasome in the diabetic diaphragm.

Effects of exercise preconditioning and HSP72 on diaphragm muscle function during mechanical ventilation

BACKGROUND

Mechanical ventilation (MV) is a life-saving measure for patients in respiratory failure. However, prolonged MV results in significant diaphragm atrophy and contractile dysfunction, a condition referred to as ventilator-induced diaphragm dysfunction (VIDD). While there are currently no clinically approved countermeasures to prevent VIDD, increased expression of heat shock protein 72 (HSP72) has been demonstrated to attenuate inactivity-induced muscle wasting. HSP72 elicits cytoprotection via inhibition of NF-κB and FoxO transcriptional activity, which contribute to VIDD. In addition, exercise-induced prevention of VIDD is characterized by an increase in the concentration of HSP72 in the diaphragm. Therefore, we tested the hypothesis that increased HSP72 expression is required for the exercise-induced prevention of VIDD. We also determined whether increasing the abundance of HSP72 in the diaphragm, independent of exercise, is sufficient to prevent VIDD.

METHODS

Cause and effect was determined by inhibiting the endurance exercise-induced increase in HSP72 in the diaphragm of exercise trained animals exposed to prolonged MV via administration of an antisense oligonucleotide targeting HSP72. Additional experiments were performed to determine if increasing HSP72 in the diaphragm via genetic (rAAV-HSP72) or pharmacological (BGP-15) overexpression is sufficient to prevent VIDD.

RESULTS

Our results demonstrate that the exercise-induced increase in HSP72 protein abundance is required for the protective effects of exercise against VIDD. Moreover, both rAAV-HSP72 and BGP-15-induced overexpression of HSP72 were sufficient to prevent VIDD. In addition, modification of HSP72 in the diaphragm is inversely related to the expression of NF-κB and FoxO target genes.

CONCLUSIONS

HSP72 overexpression in the diaphragm is an effective intervention to prevent MV-induced oxidative stress and the transcriptional activity of NF-κB and FoxO. Therefore, overexpression of HSP72 in the diaphragm is a potential therapeutic target to protect against VIDD.

Small-hairpin RNA and pharmacological targeting of neutral sphingomyelinase prevent diaphragm weakness in rats with heart failure and reduced ejection fraction

Heart failure with reduced ejection fraction (HFREF) increases neutral sphingomyelinase (NSMase) activity and mitochondrial reactive oxygen species (ROS) emission and causes diaphragm weakness. We tested whether a systemic pharmacological NSMase inhibitor or short-hairpin RNA (shRNA) targeting NSMase isoform 3 (NSMase3) would prevent diaphragm abnormalities induced by HFREF caused by myocardial infarction. In the pharmacological intervention, we used intraperitoneal injection of GW4869 or vehicle. In the genetic intervention, we injected adeno-associated virus serotype 9 (AAV9) containing shRNA targeting NSMase3 or a scrambled sequence directly into the diaphragm. We also studied acid sphingomyelinase-knockout mice. GW4869 prevented the increase in diaphragm ceramide content, weakness, and tachypnea caused by HFREF. For example, maximal specific forces (in N/cm²) were vehicle [sham 31 ± 2 and HFREF 26 ± 2 ( P < 0.05)] and GW4869 (sham 31 ± 2 and HFREF 31 ± 1). Respiratory rates were (in breaths/min) vehicle [sham 61 ± 3 and HFREF 84 ± 11 ( P < 0.05)] and GW4869 (sham 66 ± 2 and HFREF 72 ± 2). AAV9-NSMase3 shRNA prevented heightening of diaphragm mitochondrial ROS and weakness [in N/cm², AAV9-scrambled shRNA: sham 31 ± 2 and HFREF 27 ± 2 ( P < 0.05); AAV9-NSMase3 shRNA: sham 30 ± 1 and HFREF 30 ± 1] but displayed tachypnea. Both wild-type and ASMase-knockout mice with HFREF displayed diaphragm weakness. Our study suggests that activation of NSMase3 causes diaphragm weakness in HFREF, presumably through accumulation of ceramide and elevation in mitochondrial ROS. Our data also reveal a novel inhibitory effect of GW4869 on tachypnea in HFREF likely mediated by changes in neural control of breathing.

Increased SOD2 in the diaphragm contributes to exercise-induced protection against ventilator-induced diaphragm dysfunction

Mechanical ventilation (MV) is a life-saving intervention for many critically ill patients. Unfortunately, prolonged MV results in rapid diaphragmatic atrophy and contractile dysfunction, collectively termed ventilator-induced diaphragm dysfunction (VIDD). Recent evidence reveals that endurance exercise training, performed prior to MV, protects the diaphragm against VIDD. While the mechanism(s) responsible for this exercise-induced protection against VIDD remain unknown, increased diaphragm antioxidant expression may be required. To investigate the role that increased antioxidants play in this protection, we tested the hypothesis that elevated levels of the mitochondrial antioxidant enzyme superoxide dismutase 2 (SOD2) is required to achieve exercise-induced protection against VIDD. Cause and effect was investigated in two ways. First, we prevented the exercise-induced increase in diaphragmatic SOD2 via delivery of an antisense oligonucleotide targeted against SOD2 post-exercise. Second, using transgene overexpression of SOD2, we determined the effects of increased SOD2 in the diaphragm independent of exercise training. Results from these experiments revealed that prevention of the exercise-induced increases in diaphragmatic SOD2 results in a loss of exercise-mediated protection against MV-induced diaphragm atrophy and a partial loss of protection against MV-induced diaphragmatic contractile dysfunction. In contrast, transgenic overexpression of SOD2 in the diaphragm, independent of exercise, did not protect against MV-induced diaphragmatic atrophy and provided only partial protection against MV-induced diaphragmatic contractile dysfunction. Collectively, these results demonstrate that increased diaphragmatic levels of SOD2 are essential to achieve the full benefit of exercise-induced protection against VIDD.

•

Prolonged mechanical ventilation results in diaphragmatic weakness which is labeled as ventilator-induced diaphragm dysfunction (VIDD).

•

Endurance exercise training performed prior to mechanical ventilation protects the diaphragm against VIDD.

•

Preventing exercise-induced increases of superoxide dismutase 2 (SOD2) in the diaphragm partially abolishes exercise protection against VIDD.

•

Transgenic overexpression of SOD2 in the diaphragm provides only partial protection against VIDD.

•

We conclude that increases in SOD2 abundance in the diaphragm contributes to the exercise-induced protection against VIDD.

Crosstalk between autophagy and oxidative stress regulates proteolysis in the diaphragm during mechanical ventilation

Mechanical ventilation (MV) results in the rapid development of ventilator-induced diaphragm dysfunction (VIDD). While the mechanisms responsible for VIDD are not fully understood, recent data reveal that prolonged MV activates autophagy in the diaphragm, which may occur as a result of increased cellular reactive oxygen species (ROS) production. Therefore, we tested the hypothesis that (1) accelerated autophagy is a key contributor to VIDD; and that (2) oxidative stress is required to increase the expression of autophagy genes in the diaphragm. Our findings reveal that targeted inhibition of autophagy in the rat diaphragm prevented MV-induced muscle atrophy and contractile dysfunction. Attenuation of VIDD in these animals occurred as a result of increased diaphragm concentration of the antioxidant catalase and reduced mitochondrial ROS emission, which corresponded to reductions in the activity of calpain and caspase-3. To determine if increased ROS production is required for the upregulation of autophagy biomarkers in the diaphragm, rats that were administered the mitochondrial-targeted peptide SS-31 during MV. Results from this study demonstrated that mitochondrial ROS production in the diaphragm during MV is required for the increased expression of key autophagy genes (i.e. LC3, Atg7, Atg12, Beclin1 and p62), as well as for increased activity of cathepsin L. Together, these data reveal that autophagy is required for VIDD, and that autophagy inhibition reduces MV-induced diaphragm ROS production and prevents a positive feedback loop whereby increased autophagy is stimulated by oxidative stress, resulting in further increases in ROS and autophagy.

Gene therapy and respiratory neuroplasticity

Breathing is a life-sustaining behavior that in mammals is accomplished by activation of dedicated muscles responsible for inspiratory and expiratory forces acting on the lung and chest wall. Motor control is exerted by specialized pools of motoneurons in the medulla and spinal cord innervated by projections from multiple centers primarily in the brainstem that act in concert to generate both the rhythm and pattern of ventilation. Perturbations that prevent the accomplishment of the full range of motor behaviors by respiratory muscles commonly result in significant morbidity and increased mortality. Recent developments in gene therapy and novel targeting strategies have contributed to deeper understanding of the organization of respiratory motor systems. Gene therapy has received widespread attention and substantial progress has been made in recent years with the advent of improved tools for vector design. Genes can be delivered via a variety of plasmids, synthetic or viral vectors and cell therapies. In recent years, adeno-associated viruses (AAV) have become one of the most commonly used vector systems, primarily because of the extensive characterization conducted to date and the versatility in targeting strategies. Recent studies highlight the power of using AAV to selectively and effectively transduce respiratory motoneurons and muscle fibers with promising therapeutic effects. This brief review summarizes current evidence for the use of gene therapy in respiratory disorders with a primary focus on interventions that address motor control and neuroplasticity, including regeneration, in the respiratory system.

Cervical spinal cord injury exacerbates ventilator-induced diaphragm dysfunction

Cervical spinal cord injury (SCI) can dramatically impair diaphragm muscle function and often necessitates mechanical ventilation (MV) to maintain adequate pulmonary gas exchange. MV is a life-saving intervention. However, prolonged MV results in atrophy and impaired function of the diaphragm. Since cervical SCI can also trigger diaphragm atrophy, it may create preconditions that exacerbate ventilator-induced diaphragm dysfunction (VIDD). Currently, no drug therapy or clinical standard of care exists to prevent or minimize diaphragm dysfunction following SCI. Therefore, we first tested the hypothesis that initiating MV acutely after cervical SCI will exacerbate VIDD and enhance proteolytic activation in the diaphragm to a greater extent than either condition alone. Rats underwent controlled MV for 12 h following acute (∼24 h) cervical spinal hemisection injury at C2 (SCI). Diaphragm tissue was then harvested for comprehensive functional and molecular analyses. Second, we determined if antioxidant therapy could mitigate MV-induced diaphragm dysfunction after cervical SCI. In these experiments, SCI rats received antioxidant (Trolox, a vitamin E analog) or saline treatment prior to initiating MV. Our results demonstrate that compared with either condition alone, the combination of SCI and MV resulted in increased diaphragm atrophy, contractile dysfunction, and expression of atrophy-related genes, including MuRF1. Importantly, administration of the antioxidant Trolox attenuated proteolytic activation, fiber atrophy, and contractile dysfunction in the diaphragms of SCI + MV animals. These findings provide evidence that cervical SCI greatly exacerbates VIDD, but antioxidant therapy with Trolox can preserve diaphragm contractile function following acute SCI.

Inhibition of forkhead boxO-specific transcription prevents mechanical ventilation-induced diaphragm dysfunction

OBJECTIVES

Mechanical ventilation is a lifesaving measure for patients with respiratory failure. However, prolonged mechanical ventilation results in diaphragm weakness, which contributes to problems in weaning from the ventilator. Therefore, identifying the signaling pathways responsible for mechanical ventilation-induced diaphragm weakness is essential to developing effective countermeasures to combat this important problem. In this regard, the forkhead boxO family of transcription factors is activated in the diaphragm during mechanical ventilation, and forkhead boxO-specific transcription can lead to enhanced proteolysis and muscle protein breakdown. Currently, the role that forkhead boxO activation plays in the development of mechanical ventilation-induced diaphragm weakness remains unknown.

DESIGN

This study tested the hypothesis that mechanical ventilation-induced increases in forkhead boxO signaling contribute to ventilator-induced diaphragm weakness.

SETTING

University research laboratory.

SUBJECTS

Young adult female Sprague-Dawley rats.

INTERVENTIONS

Cause and effect was determined by inhibiting the activation of forkhead boxO in the rat diaphragm through the use of a dominant-negative forkhead boxO adeno-associated virus vector delivered directly to the diaphragm.

MEASUREMENTS AND MAIN RESULTS

Our results demonstrate that prolonged (12 hr) mechanical ventilation results in a significant decrease in both diaphragm muscle fiber size and diaphragm-specific force production. However, mechanically ventilated animals treated with dominant-negative forkhead boxO showed a significant attenuation of both diaphragm atrophy and contractile dysfunction. In addition, inhibiting forkhead boxO transcription attenuated the mechanical ventilation-induced activation of the ubiquitin-proteasome system, the autophagy/lysosomal system, and caspase-3.

CONCLUSIONS

Forkhead boxO is necessary for the activation of key proteolytic systems essential for mechanical ventilation-induced diaphragm atrophy and contractile dysfunction. Collectively, these results suggest that targeting forkhead boxO transcription could be a key therapeutic target to combat ventilator-induced diaphragm dysfunction.