Key aspects of phrenic motoneuron and diaphragm muscle development during the perinatal period

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Laryngeal and swallow dysregulation following acute cervical spinal cord injury

Laryngeal function is vital to airway protection. While swallow is mediated by the brainstem, mechanisms underlying increased risk of dysphagia after cervical spinal cord injury (SCI) are unknown. We hypothesized that loss of descending phrenic drive affects swallow and breathing differently, and loss of ascending spinal afferent information alters swallow regulation. We recorded electromyograms from upper airway and chest wall muscles in freely breathing pentobarbital-anesthetized cats and rats. Inspiratory laryngeal activity increased ~two-fold following C2 lateral-hemisection. Ipsilateral to the injury, crural diaphragm EMG amplitude was reduced during breathing (62 ± 25% change post-injury), but no animal had complete termination of activity; 75% of animals increased contralateral diaphragm recruitment, but this did not reach significance. During swallow, laryngeal adductor and pharyngeal constrictor muscles increased activity, and diaphragm activity was bilaterally suppressed. This was unexpected because of the ipsilateral-specific response during breathing. Swallow-breathing coordination was also disrupted and more swallows occurred during early expiration. Finally, to determine if the chest wall is a major source of feedback for laryngeal regulation, we performed T1 total transections in rats. As in the C2 lateral-hemisection, inspiratory laryngeal recruitment was the first feature noted. In contrast to the C2 lateral-hemisection, diaphragmatic drive increased after T1 transection. Overall, we found that SCI alters laryngeal drive during swallow and breathing, and reduced swallow-related diaphragm activity. Our results show behavior-specific effects, suggesting SCI affects swallow more than breathing, and emphasizes the need for additional studies on the effects of ascending afferents from the spinal cord on laryngeal function.

The phrenic neuromuscular system

The phrenic neuromuscular system consists of the phrenic motor nucleus in the mid-cervical spinal cord, the phrenic nerve, and the diaphragm muscle. This motor system helps sustain breathing throughout life, while also contributing to posture, coughing, swallowing, and speaking. The phrenic nerve contains primarily efferent phrenic axons and afferent axons from diaphragm sensory receptors but is also a conduit for autonomic fibers. On a breath-by-breath basis, rhythmic (inspiratory) depolarization of phrenic motoneurons occurs due to excitatory bulbospinal synaptic pathways. Further, a complex propriospinal network innervates phrenic motoneurons and may serve to coordinate postural, locomotor, and respiratory movements. The phrenic neuromuscular system is impacted in a wide range of neuromuscular diseases and injuries. Contemporary research is focused on understanding how neuromuscular plasticity occurs in the phrenic neuromuscular system and using this information to optimize treatments and rehabilitation strategies to improve breathing and related behaviors.

Diaphragm Pacing and a Model for Respiratory Rehabilitation After Spinal Cord Injury

BACKGROUND AND PURPOSE

Cervical spinal cord injury (CSCI) can cause severe respiratory impairment. Although mechanical ventilation (MV) is a lifesaving standard of care for these patients, it is associated with diaphragm atrophy and dysfunction. Diaphragm pacing (DP) is a strategy now used acutely to promote MV weaning and to combat the associated negative effects. Initial reports indicate that DP also may promote neuromuscular plasticity and lead to improvements in spontaneous diaphragm activation and respiratory function. These outcomes suggest the need for reevaluation of respiratory rehabilitation for patients with CSCI using DP and consideration of new rehabilitation models for these patients and their unique care needs.

SUMMARY OF KEY POINTS

This article discusses the rationale for consideration of DP as a rehabilitative strategy, particularly when used in combination with established respiratory interventions. In addition, a model of respiratory rehabilitation and recovery (RRR) is presented, providing a framework for rehabilitation and consideration of DP as an adjuvant rehabilitation approach. The model promotes goals such as respiratory recovery and independence, and lifelong respiratory health, via interdisciplinary care, respiratory training, quantitative measurement, and use of adjuvant strategies such as DP. Application of the model is demonstrated through a description of an inpatient rehabilitation program that applies model components to patients with CSCI who require DP.

RECOMMENDATIONS FOR CLINICAL PRACTICE

As DP use increases for patients with acute CSCI, so does the need and opportunity to advance rehabilitation approaches for these patients. This perspective article is a critical step in addressing this need and motivating the advancement of rehabilitation strategies for CSCI patients. (See Video Abstract, Supplemental Digital Content, available at: http://links.lww.com/JNPT/A348).

Sex-Specific Effects of Stress on Respiratory Control: Plasticity, Adaptation, and Dysfunction

As our understanding of respiratory control evolves, we appreciate how the basic neurobiological principles of plasticity discovered in other systems shape the development and function of the respiratory control system. While breathing is a robust homeostatic function, there is growing evidence that stress disrupts respiratory control in ways that predispose to disease. Neonatal stress (in the form of maternal separation) affects "classical" respiratory control structures such as the peripheral O₂ sensors (carotid bodies) and the medulla (e.g., nucleus of the solitary tract). Furthermore, early life stress disrupts the paraventricular nucleus of the hypothalamus (PVH), a structure that has emerged as a primary determinant of the intensity of the ventilatory response to hypoxia. Although underestimated, the PVH's influence on respiratory function is a logical extension of the hypothalamic control of metabolic demand and supply. In this article, we review the functional and anatomical links between the stress neuroendocrine axis and the medullary network regulating breathing. We then present the persistent and sex-specific effects of neonatal stress on respiratory control in adult rats. The similarities between the respiratory phenotype of stressed rats and clinical manifestations of respiratory control disorders such as sleep-disordered breathing and panic attacks are remarkable. These observations are in line with the scientific consensus that the origins of adult disease are often found among developmental and biological disruptions occurring during early life. These observations bring a different perspective on the structural hierarchy of respiratory homeostasis and point to new directions in our understanding of the etiology of respiratory control disorders. © 2021 American Physiological Society. Compr Physiol 11:1-38, 2021.

Is Innervation of the Neuromuscular Junction at the Diaphragm Modulated by sGC/cGMP Signaling?

We previously reported NO/sGC signaling in the upper respiratory pathway, receiving input from the respiratory neurons of the brainstem to phrenic motoneurons in the C3–C6 spinal cord. In order to assess whether innervation of the neuromuscular junction (NMJ) at the diaphragm is modulated by sGC/cGMP signaling, we performed unilateral 8-day continuous ligation of the phrenic nerve in rats. We examined sGCβ1 within the lower bulbospinal pathway (phrenic motoneurons, phrenic nerves and NMJs at the diaphragm) and the cGMP level in the contra- and ipsilateral hemidiaphragm. Additionally, we characterized the extent of phrenic nerve axonal degeneration and denervation at diaphragm NMJs. The results of our study show that continuous 8-day phrenic nerve ligation caused a marked increase in sGCβ1 (immunoreactivity and the protein level) in the ipsilateral phrenic motor pool. However, the protein sGCβ1 level in the phrenic nerve below its ligation and the cGMP level in the ipsilateral hemidiaphragm were evidently decreased. Using confocal analysis we discovered a reduction in sGCβ1-IR boutons/synaptic vesicles at the ipsilateral MNJs. These findings are consistent with the marked axonal loss (∼47%) and significant NMJs degeneration in the ipsilateral diaphragm muscle. The remarkable unilateral decrease in cGMP level in the diaphragm and the failure of EMG recordings in the ipsilateral hemidiaphragm muscle can be attributed to the fact that sGC is involved in transmitter release at the diaphragm NMJs via the sGC-cGMP pathway.

Phrenic motor neuron loss in an animal model of early onset hypertonia

Phrenic motor neuron (PhMN) development in early onset hypertonia is poorly understood. Respiratory disorders are one of the leading causes of morbidity and mortality in individuals with early onset hypertonia, such as cerebral palsy (CP), but they are largely overshadowed by a focus on physical function in this condition. Furthermore, while the brain is the focus of CP research, motor neurons, via the motor unit and neurotransmitter signaling, are the targets in clinical interventions for hypertonia. Furthermore, critical periods of spinal cord and motor unit development also coincide with the timing that the supposed brain injury occurs in CP. Using an animal model of early-onset spasticity (spa mouse [B6.Cg-Glrbspa/J] with a glycine receptor mutation), we hypothesized that removal of effective glycinergic neurotransmitter inputs to PhMNs during development will result in fewer PhMNs and reduced PhMN somal size at maturity. Adult spa (Glrb-/-), and wild-type (Glrb+/+) mice underwent unilateral retrograde labeling of PhMNs via phrenic nerve dip in tetramethylrhodamine. After three days, mice were euthanized, perfused with 4% paraformaldehyde, and the spinal cord excised and processed for confocal imaging. Spa mice had ~30% fewer PhMNs (P = 0.005), disproportionately affecting larger PhMNs. Additionally, a ~22% reduction in PhMN somal surface area (P = 0.019), an 18% increase in primary dendrites (P < 0.0001), and 24% decrease in dendritic surface area (P = 0.014) were observed. Thus, there are fewer larger PhMNs in spa mice. Fewer and smaller PhMNs may contribute to impaired diaphragm neuromotor control and contribute to respiratory morbidity and mortality in conditions of early onset hypertonia.NEW & NOTEWORTHY Phrenic motor neuron (PhMN) development in early-onset hypertonia is poorly understood. Yet, respiratory disorders are a common cause of morbidity and mortality. In spa mice, an animal model of early-onset hypertonia, we found ~30% fewer PhMNs, compared with controls. This PhMN loss disproportionately affected larger PhMNs. Thus, the number and heterogeneity of the PhMN pool are decreased in spa mice, likely contributing to the hypertonia, impaired neuromotor control, and respiratory disorders.

Inhibition of TrkB kinase activity impairs transdiaphragmatic pressure generation

Signaling via the tropomyosin-related kinase receptor subtype B (TrkB) regulates neuromuscular transmission, and inhibition of TrkB kinase activity by 1NMPP1 in TrkBF616A mice worsens neuromuscular transmission failure (NMTF). We hypothesized that acute inhibition of TrkB kinase activity will impair the ability of the diaphragm muscle to produce maximal transdiaphragmatic pressure (Pdi) without impacting the ability to generate forces associated with ventilation, consistent with the greater susceptibility to NMTF in motor units responsible for higher-force nonventilatory behaviors. Adult male and female TrkBF616A mice were injected with 1NMPP1 (n = 8) or vehicle (DMSO; n = 8) 1 h before Pdi measurements during eupneic breathing, hypoxia/hypercapnia (10% O2/5% CO2), tracheal occlusion, spontaneous deep breaths ("sighs") and during maximal activation elicited by bilateral phrenic nerve stimulation. In the vehicle-treated group, Pdi increased from ~10 cmH2O during eupnea and hypoxia/hypercapnia, to ~35 cmH2O during sighs and tracheal occlusion, and to ~65 cm H2O during maximal stimulation. There was no effect of acute 1NMPP1 treatment on Pdi generated during most behaviors, except during maximal stimulation (~30% reduction; P < 0.05). This reduction in maximal Pdi is generally similar to the worsening of NMTF previously reported with TrkB kinase inhibition in rodents. Accordingly, impaired TrkB signaling limits the range of motor behaviors accomplished by the diaphragm muscle and may contribute to neuromuscular dysfunction, primarily by impacting fatigable, higher force-generating motor units.NEW & NOTEWORTHY TrkB signaling plays an important role in maintaining neuromuscular function in the diaphragm muscle and may be necessary to accomplish the various motor behaviors ranging from ventilation to expulsive, behaviors requiring near-maximal forces. This study shows that inhibition of TrkB kinase activity impairs maximal pressure generation by the diaphragm muscle, but the ability to generate the lower pressures required for ventilatory behaviors is not impacted.

Impact of inflammation on developing respiratory control networks: rhythm generation, chemoreception and plasticity

The respiratory control network in the central nervous system undergoes critical developmental events early in life to ensure adequate breathing at birth. There are at least three "critical windows" in development of respiratory control networks: 1) in utero, 2) newborn (postnatal day 0-4 in rodents), and 3) neonatal (P10-13 in rodents, 2-4 months in humans). During these critical windows, developmental processes required for normal maturation of the respiratory control network occur, thereby increasing vulnerability of the network to insults, such as inflammation. Early life inflammation (induced by LPS, chronic intermittent hypoxia, sustained hypoxia, or neonatal maternal separation) acutely impairs respiratory rhythm generation, chemoreception and increases neonatal risk of mortality. These early life impairments are also greater in young males, suggesting sex-specific impairments in respiratory control. Further, neonatal inflammation has a lasting impact on respiratory control by impairing adult respiratory plasticity. This review focuses on how inflammation alters respiratory rhythm generation, chemoreception and plasticity during each of the three critical windows. We also highlight the need for additional mechanistic studies and increased investigation into how glia (such as microglia and astrocytes) play a role in impaired respiratory control after inflammation. Understanding how inflammation during critical windows of development disrupt respiratory control networks is essential for developing better treatments for vulnerable neonates and preventing adult ventilatory control disorders.

Why individuals with cerebral palsy are at higher risk for respiratory complications from COVID-19

Respiratory dysfunction is a leading cause of morbidity and mortality in individuals with cerebral palsy (CP). In children and adults with CP, movement and physical function is always affected. Yet, many clinicians overlook potential for impaired movement and function of the diaphragm muscle (DIAm) in individuals with CP. Since individuals with pre-existing respiratory disorders are at greater risk for respiratory complications if they contract COVID-19, understanding potential risks to individuals with CP is important. In this review we present research on respiratory function and DIAm force generation in children with CP. We compare this clinical work to basic science research investigating phrenic motor neuron and DIAm motor unit dysfunction in an animal model with CP symptoms, the spa mouse. Finally, we integrate the clinical and basic science work in respiratory function in CP, discussing potential for individuals with CP to have severe respiratory symptoms from COVID-19.

Age-dependent SMN expression in disease-relevant tissue and implications for SMA treatment

BACKGROUNDSpinal muscular atrophy (SMA) is caused by deficient expression of survival motor neuron (SMN) protein. New SMN-enhancing therapeutics are associated with variable clinical benefits. Limited knowledge of baseline and drug-induced SMN levels in disease-relevant tissues hinders efforts to optimize these treatments.METHODSSMN mRNA and protein levels were quantified in human tissues isolated during expedited autopsies.RESULTSSMN protein expression varied broadly among prenatal control spinal cord samples, but was restricted at relatively low levels in controls and SMA patients after 3 months of life. A 2.3-fold perinatal decrease in median SMN protein levels was not paralleled by comparable changes in SMN mRNA. In tissues isolated from nusinersen-treated SMA patients, antisense oligonucleotide (ASO) concentration and full-length (exon 7 including) SMN2 (SMN2-FL) mRNA level increases were highest in lumbar and thoracic spinal cord. An increased number of cells showed SMN immunolabeling in spinal cord of treated patients, but was not associated with an increase in whole-tissue SMN protein levels.CONCLUSIONSA normally occurring perinatal decrease in whole-tissue SMN protein levels supports efforts to initiate SMN-inducing therapies as soon after birth as possible. Limited ASO distribution to rostral spinal and brain regions in some patients likely limits clinical response of motor units in these regions for those patients. These results have important implications for optimizing treatment of SMA patients and warrant further investigations to enhance bioavailability of intrathecally administered ASOs.FUNDINGSMA Foundation, SMART, NIH (R01-NS096770, R01-NS062869), Ionis Pharmaceuticals, and PTC Therapeutics. Biogen provided support for absolute real-time RT-PCR.

One bout of neonatal inflammation impairs adult respiratory motor plasticity in male and female rats

Neonatal inflammation is common and has lasting consequences for adult health. We investigated the lasting effects of a single bout of neonatal inflammation on adult respiratory control in the form of respiratory motor plasticity induced by acute intermittent hypoxia, which likely compensates and stabilizes breathing during injury or disease and has significant therapeutic potential. Lipopolysaccharide-induced inflammation at postnatal day four induced lasting impairments in two distinct pathways to adult respiratory plasticity in male and female rats. Despite a lack of adult pro-inflammatory gene expression or alterations in glial morphology, one mechanistic pathway to plasticity was restored by acute, adult anti-inflammatory treatment, suggesting ongoing inflammatory signaling after neonatal inflammation. An alternative pathway to plasticity was not restored by anti-inflammatory treatment, but was evoked by exogenous adenosine receptor agonism, suggesting upstream impairment, likely astrocytic-dependent. Thus, the respiratory control network is vulnerable to early-life inflammation, limiting respiratory compensation to adult disease or injury.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Gestational intermittent hypoxia increases susceptibility to neuroinflammation and alters respiratory motor control in neonatal rats

Sleep disordered breathing (SDB) and obstructive sleep apnea (OSA) during pregnancy are growing health concerns because these conditions are associated with adverse outcomes for newborn infants. SDB/OSA during pregnancy exposes the mother and the fetus to intermittent hypoxia. Direct exposure of adults and neonates to IH causes neuroinflammation and neuronal apoptosis, and exposure to IH during gestation (GIH) causes long-term deficits in offspring respiratory function. However, the role of neuroinflammation in CNS respiratory control centers of GIH offspring has not been investigated. Thus, the goal of this hybrid review/research article is to comprehensively review the available literature both in humans and experimental rodent models of SDB in order to highlight key gaps in knowledge. To begin to address some of these gaps, we also include data demonstrating the consequences of GIH on respiratory rhythm generation and neuroinflammation in CNS respiratory control regions. Pregnant rats were exposed to daily intermittent hypoxia during gestation (G10-G21). Neuroinflammation in brainstem and cervical spinal cord was evaluated in P0-P3 pups that were injected with saline or lipopolysaccharide (LPS; 0.1mg/kg, 3h). In CNS respiratory control centers, we found that GIH attenuated the normal CNS immune response to LPS challenge in a gene-, sex-, and CNS region-specific manner. GIH also altered normal respiratory motor responses to LPS in newborn offspring brainstem-spinal cord preparations. These data underscore the need for further study of the long-term consequences of maternal SDB on the relationship between inflammation and the respiratory control system, in both neonatal and adult offspring.

Differences in lumbar motor neuron pruning in an animal model of early onset spasticity

Motor neuron (MN) development in early onset spasticity is poorly understood. For example, spastic cerebral palsy (sCP), the most common motor disability of childhood, is poorly predicted by brain imaging, yet research remains focused on the brain. By contrast, MNs, via the motor unit and neurotransmitter signaling, are the target of most therapeutic spasticity treatments and are the final common output of motor control. MN development in sCP is a critical knowledge gap, because the late embryonic and postnatal periods are not only when the supposed brain injury occurs but also are critical times for spinal cord neuromotor development. Using an animal model of early onset spasticity [ spa mouse (B6.Cg- Glrb^spa/J) with a glycine (Gly) receptor mutation], we hypothesized that removal of effective glycinergic neurotransmitter inputs to MNs during development will influence MN pruning (including primary dendrites) and MN size. Spa (Glrb-/-) and wild-type (Glrb+/+) mice, ages 4-9 wk, underwent unilateral retrograde labeling of the tibialis anterior muscle MNs via peroneal nerve dip in tetramethylrhodamine. After 3 days, mice were euthanized and perfused with 4% paraformaldehyde, and the spinal cord was excised and processed for confocal imaging. Spa mice had ~61% fewer lumbar tibialis anterior MNs ( P < 0.01), disproportionately affecting larger MNs. Additionally, a ~23% reduction in tibialis anterior MN somal surface area ( P < 0.01) and a 12% increase in primary dendrites ( P = 0.046) were observed. Thus MN pruning and MN somal surface area are abnormal in early onset spasticity. Fewer and smaller MNs may contribute to the spastic phenotype. NEW & NOTEWORTHY Motor neuron (MN) development in early onset spasticity is poorly understood. In an animal model of early onset spasticity, spa mice, we found ~61% fewer lumbar tibialis anterior MNs compared with controls. This MN loss disproportionately affected larger MNs. Thus number and heterogeneity of the MN pool are decreased in spa mice, likely contributing to the spastic phenotype.

Breathing: Motor Control of Diaphragm Muscle

Breathing occurs without thought but is controlled by a complex neural network with a final output of phrenic motor neurons activating diaphragm muscle fibers (i.e., motor units). This review considers diaphragm motor unit organization and how they are controlled during breathing as well as during expulsive behaviors.

Low back pain and gastroesophageal reflux in patients with COPD: the disease in the breath

COPD is a worsening condition that leads to a pathologic degeneration of the respiratory system. It represents one of the most important causes of mortality and morbidity in the world, and it is characterized by the presence of associated comorbidity. This article analyzes gastroesophageal reflux disease (GERD) and low back pain (LBP) in patients with COPD and tries to produce anatomo-clinical considerations on the reasons of the presence of these comorbidities. The considerations of the authors are based on the anatomic functions and characteristics of the respiratory diaphragm that are not always considered, from which elements useful to comprehend the symptomatic status of the patient can be deduced, finally improving the therapeutic approach. The information contained in the article can be of help to the clinician and for physiotherapy, and to all health professionals who gravitate around the patient’s care, improving the approach to the diaphragm muscle.

Diaphragm: A vital respiratory muscle in mammals

The diaphragm is a respiratory muscle that is primarily responsible for the respiratory function in normal individuals. In mammals, the diaphragm muscle has been studied from the early days of zoology, comparative and experimental anatomy, physiology, medicine, physics, and philosophy. However, even with these early advances in knowledge pertaining to the diaphragm, comprehensive morphological data on the diaphragm are still incomplete. In this review, we summarize the beginnings of the morphological description of the diaphragm, and we describe the current status of the known morphological and embryological features. In addition, we correlate how the impairment of the diaphragm muscle in Duchenne muscular dystrophy (DMD) can lead to patient deaths. DMD is the most common X-linked muscle degenerative disease and is caused by a lack of dystrophin protein. Dystrophin is an important muscle protein that links the cellular cytoskeleton with the extracellular matrix. In the absence of dystrophin, the muscle becomes susceptible to damage during muscle contraction. This review allows researchers to obtain an overview of the diaphragm, transcending the morphological data from animals described in conventional literature.

Mechanisms controlling neuromuscular junction stability

The neuromuscular junction (NMJ) is the synaptic connection between motor neurons and muscle fibers. It is involved in crucial processes such as body movements and breathing. Its proper development requires the guidance of motor axons toward their specific targets, the development of multi-innervated myofibers, and a selective synapse stabilization. It first consists of the removal of excessive motor axons on myofibers, going from multi-innervation to a single innervation of each myofiber. Whereas guidance cues of motor axons toward their specific muscular targets are well characterized, only few molecular and cellular cues have been reported as clues for selecting and stabilizing specific neuromuscular junctions. We will first provide a brief summary on NMJ development. We will then review molecular cues that are involved in NMJ stabilization, in both pre- and post-synaptic compartments, considering motor neurons and Schwann cells on the one hand, and muscle on the other hand. We will provide links with pathologies and highlight advances that can be brought both by basic research on NMJ development and clinical data resulting from the analyses of neurodegeneration of synaptic connections to obtain a better understanding of this process. The goal of this review is to highlight the findings toward understanding the roles of poly- or single-innervations and the underlying mechanisms of NMJ stabilization.

Bone morphogenetic protein signaling in vertebrate motor neurons and neuromuscular communication

An accurate communication between motor neurons and skeletal muscle fibers is required for the proper assembly, growth and maintenance of neuromuscular junctions (NMJs). Several signaling and extracellular matrix molecules play stimulatory and inhibitory roles on the assembly of functional synapses. Studies in Drosophila have revealed crucial functions for early morphogens, such as members of the Wnt and Bone Morphogenetic Proteins (BMP) signaling pathways, during the assembly and maturation of the NMJ. Here, we bring together recent findings that led us to propose that BMPs also work in vertebrate organisms as diffusible cues to communicate motor neurons and skeletal muscles.

Control of breathing activity in the fetus and newborn

Breathing movements have been demonstrated in the fetuses of every mammalian species investigated and are a critical component of normal fetal development. The classic sheep preparations instrumented for chronic fetal monitoring determined that fetal breathing movements (FBMs) occur in aggregates interspersed with long periods of quiescence that are strongly associated with neurophysiological state. The fetal sheep model also provided data regarding the neurochemical modulation of behavioral state and FBMs under a variety of in utero conditions. Subsequently, in vitro rodent models have been developed to advance our understanding of cellular, synaptic, network, and more detailed neuropharmacological aspects of perinatal respiratory neural control. This includes the ontogeny of the inspiratory rhythm generating center, the preBötzinger complex (preBötC), and the anatomical and functional development of phrenic motoneurons (PMNs) and diaphragm during the perinatal period. A variety of newborn animal models and studies of human infants have provided insights into age-dependent changes in state-dependent respiratory control, responses to hypoxia/hypercapnia and respiratory pathologies.

Genetic absence of the vesicular inhibitory amino acid transporter differentially regulates respiratory and locomotor motor neuron development

During mid to late embryonic development (E13 to birth in mice), the neuromotor system is refined by reducing motor neuron (MN) numbers and establishing nascent synaptic connections onto and by MNs. Concurrently, the response to GABAergic and glycinergic synaptic activity switches from postsynaptic excitation to inhibition. Our previous studies on mutant mice lacking glycinergic transmission or deficient in GABA suggests that altered MN activity levels during this developmental period differentially regulates MN survival and muscle innervation for respiratory and non-respiratory motor pools. To determine if combined loss of GABAergic and glycinergic transmission plays a similar or exaggerated role, we quantified MN number and muscle innervation in two respiratory (hypoglossal and phrenic) and two locomotor (brachial and lumbar) motor pools, in mice lacking vesicular inhibitory amino acid transporter, which display absent or severely impaired GABAergic and glycinergic neurotransmission. For respiratory MNs, we observed significant decreases in MN number (-20 % hypoglossal and -36 % phrenic) and diaphragm axonal branching (-60 %). By contrast, for non-respiratory brachial and lumbar MNs, we observed increases in MN number (+62 % brachial and +84 % lumbar) and axonal branching for innervated muscles (+123 % latissimus dorsi for brachial and +61 % gluteal for lumbar). These results show that combined absence of GABAergic and glycinergic neurotransmission causes distinct regional changes in MN number and muscle innervation, which are dependent upon the motor function of the specific motor pool.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Spinal TNF is necessary for inactivity-induced phrenic motor facilitation

A prolonged reduction in central neural respiratory activity elicits a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF), a 'rebound' increase in phrenic burst amplitude apparent once respiratory neural activity is restored. iPMF requires atypical protein kinase C (aPKC) activity within spinal segments containing the phrenic motor nucleus to stabilize an early transient increase in phrenic burst amplitude and to form long-lasting iPMF following reduced respiratory neural activity. Upstream signal(s) leading to spinal aPKC activation are unknown. We tested the hypothesis that spinal tumour necrosis factor-α (TNFα) is necessary for iPMF via an aPKC-dependent mechanism. Anaesthetized, ventilated rats were exposed to a 30 min neural apnoea; upon resumption of respiratory neural activity, a prolonged increase in phrenic burst amplitude (42 ± 9% baseline; P < 0.05) was apparent, indicating long-lasting iPMF. Pretreatment with recombinant human soluble TNF receptor 1 (sTNFR1) in the intrathecal space at the level of the phrenic motor nucleus prior to neural apnoea blocked long-lasting iPMF (2 ± 8% baseline; P > 0.05). Intrathecal TNFα without neural apnoea was sufficient to elicit long-lasting phrenic motor facilitation (pMF; 62 ± 7% baseline; P < 0.05). Similar to iPMF, TNFα-induced pMF required spinal aPKC activity, as intrathecal delivery of a ζ-pseudosubstrate inhibitory peptide (PKCζ-PS) 35 min following intrathecal TNFα arrested TNFα-induced pMF (28 ± 8% baseline; P < 0.05). These data demonstrate that: (1) spinal TNFα is necessary for iPMF; and (2) spinal TNFα is sufficient to elicit pMF via a similar aPKC-dependent mechanism. These data are consistent with the hypothesis that reduced respiratory neural activity elicits iPMF via a TNFα-dependent increase in spinal aPKC activity.

Sarcopenia: the gliogenic perspective

It has been approximately 25 years since Dr. Rosenberg first brought attention to sarcopenia. To date, this aging-associated condition is recognized as a chronic loss of muscle mass and is usually accompanied by dynapenia. Despite its poly-etiological factors, sarcopenia has a strong neurogenic component underlying this chrono-degeneration of muscle mass, as shown in recent studies. As it seems plausible to explain the origin of sarcopenia through a motor neuron degeneration model, the focus of sarcopenia research should combine neuroscience with the study of the original myocyte and satellite cells. Although a complete mechanism underlying the development of sarcopenia has yet to be elucidated, we propose that the primary trigger of sarcopenia could be gliogenic in origin based on the close relationship between the glia, neurons and non-neural cells, for example, the motor unit and its associated glia in both the central nervous system (CNS) and the peripheral nervous system (PNS). In addition to muscle cells, both of the neural cells are affected by aging.

Genetic deficiency of GABA differentially regulates respiratory and non-respiratory motor neuron development

Central nervous system GABAergic and glycinergic synaptic activity switches from postsynaptic excitation to inhibition during the stage when motor neuron numbers are being reduced, and when synaptic connections are being established onto and by motor neurons. In mice this occurs between embryonic (E) day 13 and birth (postnatal day 0). Our previous work on mice lacking glycinergic transmission suggested that altered motor neuron activity levels correspondingly regulated motor neuron survival and muscle innervation for all respiratory and non respiratory motor neuron pools, during this period of development [1]. To determine if GABAergic transmission plays a similar role, we quantified motor neuron number and the extent of muscle innervation in four distinct regions of the brain stem and spinal cord; hypoglossal, phrenic, brachial and lumbar motor pools, in mice lacking the enzyme GAD67. These mice display a 90% drop in CNS GABA levels ( [2]; this study). For respiratory-based motor neurons (hypoglossal and phrenic motor pools), we have observed significant drops in motor neuron number (17% decline for hypoglossal and 23% decline for phrenic) and muscle innervations (55% decrease). By contrast for non-respiratory motor neurons of the brachial lateral motor column, we have observed an increase in motor neuron number (43% increase) and muscle innervations (99% increase); however for more caudally located motor neurons within the lumbar lateral motor column, we observed no change in either neuron number or muscle innervation. These results show in mice lacking physiological levels of GABA, there are distinct regional changes in motor neuron number and muscle innervation, which appear to be linked to their physiological function and to their rostral-caudal position within the developing spinal cord. Our results also suggest that for more caudal (lumbar) regions of the spinal cord, the effect of GABA is less influential on motor neuron development compared to that of glycine.

Respiratory muscle plasticity

Muscle plasticity is defined as the ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it. As such, respiratory muscle is in a constant state of remodeling, and the basis of muscle's plasticity is its ability to change protein expression and resultant protein balance in response to varying environmental conditions. Here, we will describe the changes of respiratory muscle imposed by extrinsic changes in mechanical load, activity, and innervation. Although there is a large body of literature on the structural and functional plasticity of respiratory muscles, we are only beginning to understand the molecular-scale protein changes that contribute to protein balance. We will give an overview of key mechanisms regulating protein synthesis and protein degradation, as well as the complex interactions between them. We suggest future application of a systems biology approach that would develop a mathematical model of protein balance and greatly improve treatments in a variety of clinical settings related to maintaining both muscle mass and optimal contractile function of respiratory muscles.

Contribution of stretch to the change of activation properties of muscle fibers in the diaphragm at the transition from fetal to neonatal life

The transition from fetal to postnatal life involves clearance of liquid from the lung and airways, and rapid formation of a functional residual capacity. Despite the importance of the diaphragm in this process, the impact of birth on the mechanical and functional activity of its muscle fibers is not known. This study determined the contractile characteristics of individual “skinned” diaphragm fibers from 70 days (0.47) gestation to after birth in sheep. Based on differential sensitivity to the divalent ions calcium (Ca²⁺) and strontium (Sr²⁺), all fibers in the fetal diaphragm were classified as “fast,” whereas fibers from the adult sheep diaphragm exhibited a “hybrid” phenotype where both “fast” and “slow” characteristics were present within each single fiber. Transition to the hybrid phenotype occurred at birth, was evident after only 40 min of spontaneous breathing, and could be induced by simple mechanical stretch of diaphragm fibers from near-term fetuses (∼147 days gestation). Both physical stretch of isolated fibers, and mechanical ventilation of the fetal diaphragm in situ, significantly increased sensitivity to Ca²⁺ and Sr²⁺, maximum force generating capacity, and decreased passive tension in near-term and preterm fetuses; however, only fibers from near-term fetuses showed a complete transition to a “hybrid” activation profile. These findings suggest that stretch associated with the transition from a liquid to air-filled lung at birth induces physical changes of proteins determining the activation and elastic properties of the diaphragm. These changes may allow the diaphragm to meet the increased mechanical demands of breathing immediately after birth.

Reduced TrkB expression results in precocious age-like changes in neuromuscular structure, neurotransmission, and muscle function

Acute blockade of signaling through the tyrosine kinase receptor B (TrkB) attenuates neuromuscular transmission and fragments postsynaptic acetylcholine receptors (AChRs) in adult mice, suggesting that TrkB signaling is a key regulator of neuromuscular function. Using immunohistochemical, histological, and in vitro muscle contractile techniques, we tested the hypothesis that constitutively reduced TrkB expression would disrupt neuromuscular pre- and postsynaptic structure, neurotransmission, muscle fiber size, and muscle function in the soleus muscle of 6- to 8-mo-old TrkB⁺/⁻ mice compared with age-matched littermates. Age-like expansion of postsynaptic AChR area, AChR fragmentation, and denervation was observed in TrkB⁺/⁻ mice similar to that found in 24-mo-old wild-type mice. Neurotransmission failure was increased in TrkB⁺/⁻ mice, suggesting that these morphologic changes were sufficient to alter synaptic function. Reduced TrkB expression resulted in decreased muscle strength and fiber cross-sectional area. Immunohistochemical and muscle retrograde labeling experiments show that motor neuron number and size are unaffected in TrkB⁺/⁻ mice. These results suggest that TrkB- signaling at the neuromuscular junction plays a role in synaptic stabilization, neurotransmission, and muscle function and may impact the aging process of sarcopenia.

Phrenic motor unit recruitment during ventilatory and non-ventilatory behaviors

Phrenic motoneurons are located in the cervical spinal cord and innervate the diaphragm muscle, the main inspiratory muscle in mammals. Similar to other skeletal muscles, phrenic motoneurons and diaphragm muscle fibers form motor units which are the final element of neuromotor control. In addition to their role in sustaining ventilation, phrenic motor units are active in other non-ventilatory behaviors important for airway clearance such as coughing or sneezing. Diaphragm muscle fibers comprise all fiber types and are commonly classified based on expression of contractile proteins including myosin heavy chain isoforms. Although there are differences in contractile and fatigue properties across motor units, there is a matching of properties for the motor neuron and muscle fibers within a motor unit. Motor units are generally recruited in order such that fatigue-resistant motor units are recruited earlier and more often than more fatigable motor units. Thus, in sustaining ventilation, fatigue-resistant motor units are likely required. Based on a series of studies in cats, hamsters and rats, an orderly model of motor unit recruitment was proposed that takes into consideration the maximum forces generated by single type-identified diaphragm muscle fibers as well as the proportion of the different motor unit types. Using this model, eupnea can be accomplished by activation of only slow-twitch diaphragm motor units and only a subset of fast-twitch, fatigue-resistant units. Activation of fast-twitch fatigable motor units only becomes necessary when accomplishing tasks that require greater force generation by the diaphragm muscle, e.g., sneezing and coughing.

Axonal regeneration and neuronal function are preserved in motor neurons lacking ß-actin in vivo

The proper localization of ß-actin mRNA and protein is essential for growth cone guidance and axon elongation in cultured neurons. In addition, decreased levels of ß-actin mRNA and protein have been identified in the growth cones of motor neurons cultured from a mouse model of Spinal Muscular Atrophy (SMA), suggesting that ß-actin loss-of-function at growth cones or pre-synaptic nerve terminals could contribute to the pathogenesis of this disease. However, the role of ß-actin in motor neurons in vivo and its potential relevance to disease has yet to be examined. We therefore generated motor neuron specific ß-actin knock-out mice (Actb-MNsKO) to investigate the function of ß-actin in motor neurons in vivo. Surprisingly, ß-actin was not required for motor neuron viability or neuromuscular junction maintenance. Skeletal muscle from Actb-MNsKO mice showed no histological indication of denervation and did not significantly differ from controls in several measurements of physiologic function. Finally, motor axon regeneration was unimpaired in Actb-MNsKO mice, suggesting that ß-actin is not required for motor neuron function or regeneration in vivo.

Chronic assessment of diaphragm muscle EMG activity across motor behaviors

The diaphragm muscle is the main inspiratory muscle in mammals. Quantitative analyses documenting the reliability of chronic diaphragm EMG recordings are lacking. Assessment of ventilatory and non-ventilatory motor behaviors may facilitate evaluating diaphragm EMG activity over time. We hypothesized that normalization of diaphragm EMG amplitude across behaviors provides stable and reliable parameters for longitudinal assessments of diaphragm activity. We found that diaphragm EMG activity shows substantial intra-animal variability over 6 weeks, with coefficient of variation (CV) for different behaviors ∼ 29-42%. Normalization of diaphragm EMG activity to near maximal behaviors (e.g., deep breathing) reduced intra-animal variability over time (CV ∼ 22-29%). Plethysmographic measurements of eupneic ventilation were also stable over 6 weeks (CV ∼ 13% for minute ventilation). Thus, stable and reliable measurements of diaphragm EMG activity can be obtained longitudinally using chronically implanted electrodes by examining multiple motor behaviors. By quantitatively determining the reliability of longitudinal diaphragm EMG analyses, we provide an important tool for evaluating the progression of diseases or injuries that impair ventilation.

Tripartite purinergic modulation of central respiratory networks during perinatal development: the influence of ATP, ectonucleotidases, and ATP metabolites

ATP released during hypoxia from the ventrolateral medulla activates purinergic receptors (P2Rs) to attenuate the secondary hypoxic depression of breathing by a mechanism that likely involves a P2Y(1)R-mediated excitation of preBötzinger complex (preBötC) inspiratory rhythm-generating networks. In this study, we used rhythmically active in vitro preparations from embryonic and postnatal rats and ATP microinjection into the rostral ventral respiratory group (rVRG)/preBötC to reveal that these networks are sensitive to ATP when rhythm emerges at embryonic day 17 (E17). The peak frequency elicited by ATP at E19 and postnatally was the same ( approximately 45 bursts/min), but relative sensitivity was threefold greater at E19, reflecting a lower baseline frequency (5.6 +/- 0.9 vs 19.0 +/- 1.3 bursts/min). Combining microinjection techniques with ATP biosensors revealed that ATP concentration in the rVRG/preBötC falls rapidly as a result of active processes and closely correlates with inspiratory frequency. A phosphate assay established that preBötC-containing tissue punches degrade ATP at rates that increase perinatally. Thus, the agonist profile [ATP/ADP/adenosine (ADO)] produced after ATP release in the rVRG/preBötC will change perinatally. Electrophysiology further established that the ATP metabolite ADP is excitatory and that, in fetal but not postnatal animals, ADO at A(1) receptors exerts a tonic depressive action on rhythm, whereas A(1) antagonists extend the excitatory action of ATP on inspiratory rhythm. These data demonstrate that ATP is a potent excitatory modulator of the rVRG/preBötC inspiratory network from the time it becomes active and that ATP actions are determined by a dynamic interaction between the actions of ATP at P2 receptors, ectonucleotidases that degrade ATP, and ATP metabolites on P2Y and P1 receptors.

Neuromuscular adaptations to respiratory muscle inactivity

Cervical spinal cord injury results in significant functional impairment. It is important to understand the neuroplasticity in response to inactivity of respiratory muscles in order to prevent any associated effects that limit functional recovery. Recent studies have examined the mechanisms involved in inactivity-induced neuroplasticity of diaphragm motor units. Both spinal hemisection at C2 (C2HS) and tetrodotoxin (TTX)-induced phrenic nerve blockade result in diaphragm paralysis and inactivity of axon terminals. However, phrenic motoneurons are inactive with C2HS but remain active after TTX. Diaphragm muscle fibers ipsilateral to C2HS display minimal changes post-injury. Neuromuscular transmission is enhanced following C2HS but impaired following TTX. Synaptic vesicle pool size at diaphragm neuromuscular junctions increases after C2HS, but decreases after TTX. Thus, inactivity-induced neuromuscular plasticity reflects specific adaptations that depend on inactivity at the motoneuron rather than at axon terminals or muscle fibers. These results indicate that neuromuscular transmission and functional properties of diaphragm fibers can be maintained after spinal cord injury, providing a substrate for functional recovery and/or specific therapeutic approaches such as phrenic pacing.

Role of neurotrophins in recovery of phrenic motor function following spinal cord injury

Many individuals who sustain a cervical spinal cord injury are unable to maintain adequate ventilation due to diaphragm muscle paralysis. These patients become dependent on mechanical ventilators and this situation is associated with ongoing problems with pulmonary clearance, infections, and lung injury leading to significant morbidity and reduced life expectancy. Therefore, functional recovery of rhythmic phrenic activity and the ability to generate expulsive forces would dramatically affect the quality of life of patients with cervical spinal cord injury. Neurotrophins are very promising in that they have been shown to play an important role in modulating functional neuroplasticity. Specifically, brain-derived neurotrophic factor (BDNF) acting via the tropomyosin-related kinase receptor type B (TrkB) has been implicated in neuroplasticity following spinal cord injury. Our central hypothesis is that functional recovery of rhythmic phrenic activity after cervical spinal cord injury is enhanced by an increase in BDNF/TrkB signaling in phrenic motoneurons, providing a novel therapeutic target for patients.

Descending bulbospinal pathways and recovery of respiratory motor function following spinal cord injury

The rodent respiratory system is a relevant model for study of the intrinsic post-lesion mechanisms of neuronal plasticity and resulting recovery after high cervical spinal cord injury. An unilateral cervical injury (hemisection, lateral section or contusion) interrupts unilaterally bulbospinal respiratory pathways to phrenic motor neurons innervating the diaphragm and leads to important respiratory defects on the injured side. However, the ipsilateral phrenic nerve exhibits a spontaneous and progressive recovery with post-lesion time. Shortly after a lateral injury, this partial recovery depends on the activation of contralateral pathways that cross the spinal midline caudal to the injury. Activation of these crossed phrenic pathways after the injury depends on the integrity of phrenic sensory afferents. These pathways are located principally in the lateral part of the spinal cord and involve 30% of the medullary respiratory neurons. By contrast, in chronic post-lesion conditions, the medial part of the spinal cord becomes sufficient to trigger substantial ipsilateral respiratory drive. Thus, after unilateral cervical spinal cord injury, respiratory reactivation is associated with a time-dependent anatomo-functional reorganization of the bulbospinal respiratory descending pathways, which represents an adaptative strategy for functional compensation.

Trophic factor expression in phrenic motor neurons

The function of a motor neuron and the muscle fibers it innervates (i.e., a motor unit) determines neuromotor output. Unlike other skeletal muscles, respiratory muscles (e.g., the diaphragm, DIAm) must function from birth onwards in sustaining ventilation. DIAm motor units are capable of both ventilatory and non-ventilatory behaviors, including expulsive behaviors important for airway clearance. There is significant diversity in motor unit properties across different types of motor units in the DIAm. The mechanisms underlying the development and maintenance of motor unit diversity in respiratory muscles (including the DIAm) are not well understood. Recent studies suggest that trophic factor influences contribute to this diversity. Remarkably little is known about the expression of trophic factors and their receptors in phrenic motor neurons. This review will focus on the contribution of trophic factors to the establishment and maintenance of motor unit diversity in the DIAm, during development and in response to injury or disease.