Phrenic Motoneurons Research Articles

Multimodal Hox5 activity generates motor neuron diversity

Motor neurons (MNs) are the final output of circuits driving fundamental behaviors, such as respiration and locomotion. Hox proteins are essential in generating the MN diversity required for accomplishing these functions, but the transcriptional mechanisms that enable Hox paralogs to assign distinct MN subtype identities despite their promiscuous DNA binding motif are not well understood. Here we show that Hoxa5 modifies chromatin accessibility in all mouse spinal cervical MN subtypes and engages TALE co-factors to directly bind and regulate subtype-specific genes. We identify a paralog-specific interaction of Hoxa5 with the phrenic MN-specific transcription factor Scip and show that heterologous expression of Hoxa5 and Scip is sufficient to suppress limb-innervating MN identity. We also demonstrate that phrenic MN identity is stable after Hoxa5 downregulation and identify Klf proteins as potential regulators of phrenic MN maintenance. Our data identify multiple modes of Hoxa5 action that converge to induce and maintain MN identity.

Respiratory pathology in the TDP-43 transgenic mouse model of amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease that results in death within 2-5years of diagnosis. Respiratory failure is the most common cause of death in ALS. Mutations in the transactive response DNA binding protein 43 (TDP-43) encoded by the TARDBP gene are associated with abnormal cellular aggregates in neurons of patients with both familial and sporadic ALS. The role of these abnormal aggregates on breathing is unclear. Since respiratory failure is a major cause of death in ALS, we sought to determine the role of TDP-43 mutations on the respiratory motor unit in the Prp-hTDP-43A315T mouse model - a model that expresses human TDP-43 containing the A315T mutation. We assessed breathing using whole-body plethysmography, and investigated neuropathology in hypoglossal and phrenic respiratory motor units. Postmortem studies included quantification of hypoglossal and putative phrenic motor neurons, activated microglia and astrocytes in respiratory control centers, and assessment of hypoglossal and phrenic nerves of TDP43A315T mice. The male TDP43A315T mice display an early onset of rapid progression of disease, and premature death (less than 15weeks) compared to control mice and compared to female TDP43A315T mice who die between 20 and 35weeks of age. The TDP43A315T mice have progressive and profound breathing deficits at baseline and during a respiratory challenge. Histologically, hypoglossal and putative phrenic motor neurons of TDP43A315T mice are decreased and have increased microglial and astrocyte activation, indicating pronounced neurodegeneration and neuroinflammation. Further, there is axonopathy and demyelination in the hypoglossal and phrenic nerve of TDP43A315T mice. Thus, the TDP-43A315T mice have significant respiratory pathology and neuropathology, which makes them a useful translatable model for the study of novel therapies on breathing in ALS.

Hepatocyte Growth Factor Delivery to Injured Cervical Spinal Cord Using an Engineered Biomaterial Protects Respiratory Neural Circuitry and Preserves Functional Diaphragm Innervation.

A major portion of spinal cord injury (SCI) cases occur in the cervical region, where essential components of the respiratory neural circuitry are located. Phrenic motor neurons (PhMNs) housed at cervical spinal cord level C3-C5 directly innervate the diaphragm, and SCI-induced damage to these cells severely impairs respiratory function. In this study, we tested a biomaterial-based approach aimed at preserving this critical phrenic motor circuitry after cervical SCI by locally delivering hepatocyte growth factor (HGF). HGF is a potent mitogen that promotes survival, proliferation, migration, repair, and regeneration of a number of different cell and tissue types in response to injury. We developed a hydrogel-based HGF delivery system that can be injected into the intrathecal space for local delivery of high levels of HGF without damaging the spinal cord. Implantation of HGF hydrogel after unilateral C5 contusion-type SCI in rats preserved diaphragm function, as assessed by invivo recordings of both compound muscle action potentials and inspiratory electromyography amplitudes. HGF hydrogel also preserved PhMN innervation of the diaphragm, as assessed by both retrograde PhMN tracing and detailed neuromuscular junction morphological analysis. Furthermore, HGF hydrogel significantly decreased lesion size and degeneration of cervical motor neuron cell bodies, as well as reduced levels surrounding the injury site of scar-associated chondroitin sulfate proteoglycan molecules that limit axon growth capacity. Our findings demonstrate that local biomaterial-based delivery of HGF hydrogel to injured cervical spinal cord is an effective strategy for preserving respiratory circuitry and diaphragm function.

Orexin receptor 2 agonist activates diaphragm and genioglossus muscle through stimulating inspiratory neurons in the pre-Bötzinger complex, and phrenic and hypoglossal motoneurons in rodents.

Orexin-mediated stimulation of orexin receptors 1/2 (OX[1/2]R) may stimulate the diaphragm and genioglossus muscle via activation of inspiratory neurons in the pre-Bötzinger complex, which are critical for the generation of inspiratory rhythm, and phrenic and hypoglossal motoneurons. Herein, we assessed the effects of OX2R-selective agonists TAK-925 (danavorexton) and OX-201 on respiratory function. In in vitro electrophysiologic analyses using rat medullary slices, danavorexton and OX-201 showed tendency and significant effect, respectively, in increasing the frequency of inspiratory synaptic currents of inspiratory neurons in the pre-Bötzinger complex. In rat medullary slices, both danavorexton and OX-201 significantly increased the frequency of inspiratory synaptic currents of hypoglossal motoneurons. Danavorexton and OX-201 also showed significant effect and tendency, respectively, in increasing the frequency of burst activity recorded from the cervical (C3-C5) ventral root, which contains axons of phrenic motoneurons, in in vitro electrophysiologic analyses from rat isolated brainstem-spinal cord preparations. Electromyogram recordings revealed that intravenous administration of OX-201 increased burst frequency of the diaphragm and burst amplitude of the genioglossus muscle in isoflurane- and urethane-anesthetized rats, respectively. In whole-body plethysmography analyses, oral administration of OX-201 increased respiratory activity in free-moving mice. Overall, these results suggest that OX2R-selective agonists enhance respiratory function via activation of the diaphragm and genioglossus muscle through stimulation of inspiratory neurons in the pre-Bötzinger complex, and phrenic and hypoglossal motoneurons. OX2R-selective agonists could be promising drugs for various conditions with respiratory dysfunction.

Autophagy markers LC3 and p62 in aging lumbar motor neurons

Autophagy is a ubiquitous process through which damaged cytoplasmic structures are recycled and degraded within cells. Aging can affect autophagy regulation in different steps leading to the accumulation of damaged organelles and proteins, which can contribute to cell dysfunction and death. Motor neuron (MN) loss and sarcopenia are prominent features of neuromuscular aging. Previous studies on phrenic MNs showed increased levels of the autophagy proteins LC3 and p62 in 24 month compared to 6 month old mice, consistent with the onset of diaphragm muscle sarcopenia. In the present study, we hypothesized that aging leads to increased expression of the autophagy markers LC3 and p62 in single lumbar MNs. Expression of LC3 and p62 in lumbar MNs (spinal levels L1-L6) was assessed using immunofluorescence and confocal imaging of male and female mice at 6, 18 and 24 months of age, reflecting 100 %, 90 % and 75 % survival, respectively. A mixed linear model with animal as a random effect was used to compare relative LC3 and p62 expression in choline acetyl transferase-positive MNs across age groups. Expression of LC3 and p62 decreased in the white matter of the lumbar spinal cord with aging, with ~29 % decrease in LC3 and ~ 7 % decrease in p62 expression at 24 months of age compared to 6 months of age. There was no change in LC3 or p62 expression in the gray matter with age. LC3 expression in MNs relative to white matter increased significantly with age, with 150 % increase at 24 months of age compared to 6 months of age. Similarly, p62 expression in MNs relative to white matter increased significantly with age, with ~14 % increase at 24 months of age compared to 6 months of age. No effect of sex or MN pool was observed in LC3 and p62 expression in MNs. Overall, these data suggest autophagy impairment during elongation (increased LC3) and degradation (increased p62) phases with aging in lumbar MNs.

PTEN inhibition promotes robust growth of bulbospinal respiratory axons and partial recovery of diaphragm function in a chronic model of cervical contusion spinal cord injury

High spinal cord injury (SCI) leads to persistent and debilitating compromise in respiratory function. Cervical SCI not only causes the death of phrenic motor neurons (PhMNs) that innervate the diaphragm, but also damages descending respiratory pathways originating in the rostral ventral respiratory group (rVRG) located in the brainstem, resulting in denervation and consequent silencing of spared PhMNs located caudal to injury. It is imperative to determine whether interventions targeting rVRG axon growth and respiratory neural circuit reconnection are efficacious in chronic cervical contusion SCI, given that the vast majority of individuals are chronically-injured and most cases of SCI involve contusion-type damage to the cervical region. We therefore employed a rat model of chronic cervical hemicontusion to test therapeutic manipulations aimed at reconstructing damaged rVRG-PhMN-diaphragm circuitry to achieve recovery of respiratory function. At a chronic time point post-injury, we systemically administered: an antagonist peptide directed against phosphatase and tensin homolog (PTEN), a central inhibitor of neuron-intrinsic axon growth potential; an antagonist peptide directed against receptor-type protein tyrosine phosphatase sigma (PTPσ), another important negative regulator of axon growth capacity; or a combination of these two peptides. PTEN antagonist peptide (PAP4) promoted partial recovery of diaphragm motor activity out to nine months post-injury (though this effect depended on the anesthetic regimen used during recording), while PTPσ peptide did not impact diaphragm function after cervical SCI. Furthermore, PAP4 promoted robust growth of descending bulbospinal rVRG axons caudal to the injury within the denervated portion of the PhMN pool, while PTPσ peptide did not affect rVRG axon growth at this location that is critical to control of diaphragmatic respiratory function. In conclusion, we find that, when PTEN inhibition is targeted at a chronic time point following cervical contusion, our non-invasive PAP4 strategy can successfully promote significant regrowth of damaged respiratory neural circuitry and also partial recovery of diaphragm motor function.

Prolonged intermittent hypoxia differentially regulates phrenic motor neuron serotonin receptor expression in rats following chronic cervical spinal cord injury

Low-dose (< 2 h/day), acute intermittent hypoxia (AIH) elicits multiple forms of serotonin-dependent phrenic motor plasticity and is emerging as a promising therapeutic strategy to restore respiratory and non-respiratory motor function after spinal cord injury (SCI). In contrast, high-dose (> 8 h/day), chronic intermittent hypoxia (CIH) undermines some forms of serotonin-dependent phrenic motor plasticity and elicits pathology. CIH is a hallmark of sleep disordered breathing, which is highly prevalent in individuals with cervical SCI. Interestingly, AIH and CIH preconditioning differentially impact phrenic motor plasticity. Although mechanisms of AIH-induced plasticity in the phrenic motor system are well-described in naïve rats, we know little concerning how these mechanisms are affected by chronic SCI or intermittent hypoxia preconditioning. Thus, in a rat model of chronic, incomplete cervical SCI (lateral spinal hemisection at C2 (C2Hx), we assessed serotonin type 2A, 2B and 7 receptor expression in and near phrenic motor neurons and compared: 1) intact vs. chronically injured rats; and 2) the impact of preconditioning with varied “doses” of intermittent hypoxia (IH). While there were no effects of chronic injury or intermittent hypoxia alone, CIH affected multiple receptors in rats with chronic C2Hx. Specifically, CIH preconditioning (8 h/day; 28 days) increased serotonin 2A and 7 receptor expression exclusively in rats with chronic C2Hx. Understanding the complex, context-specific interactions between chronic SCI and CIH and how this ultimately impacts phrenic motor plasticity is important as we leverage AIH-induced motor plasticity to restore breathing and other non-respiratory motor functions in people with chronic SCI.

Inactivity-induced phrenic motor facilitation requires PKCζ activity within phrenic motor neurons.

Prolonged inhibition of respiratory neural activity elicits a long-lasting increase in phrenic nerve amplitude once respiratory neural activity is restored. Such long-lasting facilitation represents a form of respiratory motor plasticity known as inactivity-induced phrenic motor facilitation (iPMF). Although facilitation also occurs in inspiratory intercostal nerve activity after diminished respiratory neural activity (iIMF), it is of shorter duration. Atypical PKC activity in the cervical spinal cord is necessary for iPMF and iIMF, but the site and specific isoform of the relevant atypical PKC are unknown. Here, we used RNA interference to test the hypothesis that the zeta atypical PKC isoform (PKCζ) within phrenic motor neurons is necessary for iPMF but PKCζ within intercostal motor neurons is unnecessary for transient iIMF. Intrapleural injections of siRNAs targeting PKCζ (siPKCζ) to knock down PKCζ mRNA within phrenic and intercostal motor neurons were made in rats. Control rats received a nontargeting siRNA (NTsi) or an active siRNA pool targeting a novel PKC isoform, PKCθ (siPKCθ), which is required for other forms of respiratory motor plasticity. Phrenic nerve burst amplitude and external intercostal (T2) electromyographic (EMG) activity were measured in anesthetized and mechanically ventilated rats exposed to 30 min of respiratory neural inactivity (i.e., neural apnea) created by modest hypocapnia (20 min) or a similar recording duration without neural apnea (time control). Phrenic burst amplitude was increased in rats treated with NTsi (68 ± 10% baseline) and siPKCθ (57 ± 8% baseline) 60 min after neural apnea vs. time control rats (-3 ± 3% baseline), demonstrating iPMF. In contrast, intrapleural siPKCζ virtually abolished iPMF (5 ± 4% baseline). iIMF was transient in all groups exposed to neural apnea; however, intrapleural siPKCζ attenuated iIMF 5 min after neural apnea (50 ± 21% baseline) vs. NTsi (97 ± 22% baseline) and siPKCθ (103 ± 20% baseline). Neural inactivity elevated the phrenic, but not intercostal, responses to hypercapnia, an effect that was blocked by siPKCζ. We conclude that PKCζ within phrenic motor neurons is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for, transient iIMF.NEW & NOTEWORTHY We report important new findings concerning the mechanisms regulating a form of spinal neuroplasticity elicited by prolonged inhibition of respiratory neural activity, inactivity-induced phrenic motor facilitation (iPMF). We demonstrate that the atypical PKC isoform PKCζ within phrenic motor neurons is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for, transient inspiratory intercostal facilitation. Our findings are novel and advance our understanding of mechanisms contributing to phrenic motor plasticity.

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

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

Postnatal diaphragm muscle and phrenic motor neuron development is impaired in Spastic mice

In rats, there is a strong association between the emergence of mature diaphragm muscle (DIAm) myosin heavy chain isoform expression and the expansion of phrenic motor neuron (PhMN) size (from ~21-28 days postnatal (P)), alongside the manifestation of mature morphological and functional properties. In the Spastic ( Spa) mutant mouse, mutated glycine receptors lead to a spasticity phenotype emerging between P14-28, concomitant with DIAm and PhMN development. In adult Spa mice, DIAm weakness, fiber atrophy and loss of larger PhMNs is evident. We aim to characterize whether these DIAm and PhMN deficits occur contemporaneously to spasticity. Using wildtype (WT) and Spa mice at P14, P21, P28, and P90 (maturity) we evaluated DIAm and PhMN physiologic properties. Ex vivo DIAm strips were used for functional assessments of maximum specific force (Po) and fatigue. DIAm strips were also fresh-frozen, and 8 μm cross-sections were cut and stained with H&amp;E to evaluate cross-sectional area (CSA) of individual DIAm fibers. PhMNs were labelled via rhodamine phrenic nerve dip, processed for longitudinal cryosectioning (70 μm) and prepared for confocal imaging and PhMN assessment. In WT mice, DIAm specific force increased at each age (P14, P21, P28 and P90), whilst in Spa mice, DIAm specific force was reduced from P21 compared to WT. Similarly, DIAm fiber CSA was reduced in Spa compared to WT mice at P21, P28 and P90. The number of PhMNs was similar between genotypes at P21, with a reduction in the number of PhMNs in Spa mice compared to WT evident at P28 continuing to P90. In Spa mice, PhMN size was smaller compared to WT at P21, unchanged at P28 and reduced at P90. The changes in Spa DIAm and PhMNs during the first postnatal month are consistent with the emergence of spasticity from P14 to P28. As most changes occur at ~P21, we are optimistic that interventions prior to weaning may promote the normal growth of DIAm and survival of PhMNs and may stave off later life DIAm behavioral deficits. HL166204 and HL146114. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Chemogenetic phrenic motoneuron activation enables increased tidal volume

Reduced output of the phrenic neuromuscular system and the resultant respiratory insuffciency occurs in many neuromuscular disorders. We hypothesized that expressing an excitatory DREADD (designer receptors exclusively activated by designer drugs) in phrenic motoneurons would enable DREADD-ligand induced increases in inspiratory tidal volume in awake, freely behaving rats. ChAT-Cre rats (n= 9; female n= 3) underwent bilateral injections of a Cre-dependent viral construct, AAV9-hSyn-DIO-hM3D(Gq)-mCherry (titer: 2.07x1012 vg/mL) into the mid-cervical (C4) ventral horn (1 μl injected per side). The combination of focal AAV injection and Cre-dependent expression was expected to drive hM3D(Gq) expression primarily in phrenic motoneurons, which are ChAT-positive and located in the C3-C5 ventral horn. Whole-body plethysmography was used to measure breathing frequency, tidal volume, and minute ventilation before and after intravenous delivery of a DREADD ligand, JHU37160 (J60), or saline (sham). Delivery of the ligand produced an increase in inspiratory tidal volume compared to saline infusion (two-way RM ANOVA, p= 0.037). Respiratory rate was similar between the two conditions (p= 0.582). Two weeks after plethysmography recordings, rats were urethane anesthetized and phrenic nerve electrical activity was directly recorded using suction electrodes. This procedure was done to directly assess the impact of intravenous delivery of the DREADD ligand on phrenic motoneuron output. The J60 ligand caused a rapid, sustained, and bilateral increase in phrenic nerve efferent burst amplitude (one-way RM ANOVA, p&lt; 0.001), whereas saline injection had no impact. The increase in phrenic burst amplitude lasted up to 100 minutes, at which point the experiment was stopped. We conclude that expression of an excitatory DREADD in phrenic motoneurons enables DREADD ligand induced increases in tidal volume in the awake, unrestrained rat. The phrenic nerve recordings verify that this response very likely reflects activation of phrenic motoneurons. Funding: R01HD052682-14 (DDF), R01HL153140-04 (DDF), University of Florida Graduate Fellowship (ESB). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Mitochondrial Fragmentation Increases with Aging in Rats

The diaphragm muscle (DIAm) is the major pump muscle for breathing. Neural control of the DIAm involves recruitment of phrenic motor neurons (PhMNs) comprising motor units that vary in their mechanical and fatigue properties. Smaller PhMNs innervating fatigue resistant DIAm fibers are highly active being recruited during breathing, while larger PhMNs are infrequently recruited during more forceful expulsive behaviors. Previously, we found that to support their higher activity levels, smaller PhMNs have higher mitochondrial volume density and are less fragmented compared to larger PhMNs. With aging, larger PhMNs are more vulnerable and ~30% are lost, and those surviving display greater mitochondrial fragmentation. We also found that older rats exhibit elevated levels of the proinflammatory cytokine tumor necrosis factor alpha (TNFa), which in other tissue has been linked to mitochondrial fragmentation via activation of the IRE1a S724-mediated endoplasmic reticulum (ER) stress pathway. Accordingly, we hypothesized that in 24-month-old rats, increased TNFa levels activate the pIRE1aS724-mediated ER stress pathway, leading to XBP1s splicing, which transcriptionally increases expression of cyclin-dependent kinases (CDK1 and CDK5) that mediate phosphorylation of DRP1 at Serine 616 (pDRP1S616), and ultimately mitochondrial fragmentation. We found that serum TNFa levels increased ~3.5 fold in 24-month rats compared to 6-month rats. Consistent with elevated TNFa, we found increased pIRE1aS724 and XBP1s splicing in the cervical ventral horn of 24-month rats. In retrogradely labeled PhMNs, we found XBP1s was localized to the nucleus, consistent with transcriptional activation of gene expression. Bioinformatic analysis and ChIP assay confirmed XBP1s binding to the CDK1 and CDK5 promoter regions in rat cervical ventral horn. Both CDK1 and CDK5 mRNA and protein were elevated in the cervical ventral of 24-month rats. In aged rats, pDRP1S616 phosphorylation increased with a corresponding increase in mitochondrial fragmentation. Our results support our hypothesis that chronic inflammation (elevated TNFa levels) in older rats activate the pIRE1aS724-mediated ER stress pathway, leading to XBP1s splicing, which transcriptionally targets CDK1 and CDK5 expression mediating pDRP1S616 phosphorylation and mitochondrial fragmentation. Research Supported by NIH R01 grant AG44615. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Longitudinal Assessment of Outcome Measures of Diaphragm Motor Unit Connectivity As Biomarkers of Phrenic Motor Unit Degeneration and Compensation

Phrenic motor neurons (MNs) form the final link from the central nervous system (CNS) to the diaphragm muscle. Ventilatory function is modulated by the CNS via groups of muscle fibers each controlled by a single MN (motor units). Processes such as injury, aging, and neurodegeneration can result in losses of MNs, and their motor unit (MU) counterparts, but MNs and MUs have some capacity for compensation through mechanisms such as axonal sprouting and collateral sprouting to regain connectivity with denervated muscle. Motor unit number (motor unit number estimation, MUNE), MU size (single motor unit potential amplitude, SMUP), and summated muscle excitation (compound muscle action potential amplitude, CMAP) have been used clinically and preclinically for decades to track MU connectivity but efforts have largely focused on limb muscles. We aimed to apply these measures to a model of phrenic MN degeneration (intrapleural injection of cholera toxin B fragment conjugated to saporin, CTB-SAP).We hypothesized that CTB-SAP-induced phrenic motor neuron death would result in loss of diaphragm MU number and compensatory increases in MU size. Under isoflurane anesthesia, rats were bilaterally, intrapleurally injected with CTB-SAP (25 μg) to target phrenic MNs and extra CTB (25 μg) to label surviving phrenic MNs, and control rats received unconjugated CTB (25 μg) and SAP (25 μg). Diaphragm CMAP, SMUP, and MUNE were acquired bilaterally and averaged at baseline/pre-injection (n=14), as well as 7 (n=14), 14 (n=14), 21 (n=6), and 28 (n=14) days post-injection. No baseline differences between groups were found. MUNE showed a significant change for time (p&lt;0.05), treatment (p&lt;0.001), and interaction time x treatment (p&lt;0.01) with ~40% reduction of MUNE in CTB-SAP rats. Average SMUP of CTB-SAP rats demonstrated a significant change with time (p&lt;0.05) and treatment (p&lt;0.01), but no significant interaction with ~50-60% increase in SMUP amplitude. CMAP showed no significant change. Our results show a significant ~40% loss of MUs in CTB-SAP rats, similar to previous studies on morphological MN counts. In parallel with MU loss, MUs enlarged (increased SMUP), consistent with compensatory increases of MU size likely through MU collateral sprouting. In conclusion, our findings suggest that the use of diaphragm MU electrophysiological measures can provide insights into the onset and progression of CTB-SAP-induced pathology concerning the loss of phrenic MU integrity. Hence, these biomarkers can be used to investigate mechanisms of phrenic motor neuron degeneration and compensation, and to expedite the translation of experimental findings to clinical testing. Supported by a Spinal Cord Injury and Disease Research grant. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Differential Responses of Spinal Microglia to Low-Dose LPS Delivered in the Daily Rest vs Active Phase

Virtually all lung and neural disorders are associated with systemic and/or neuro-inflammation. Neuro-inflammation impairs important forms of neuroplasticity associated with cognitive and spinal motor learning ( e.g., respiratory motor plasticity). Microglia are resident CNS innate immune cells that regulate diverse processes including neuroinflammation and multiple forms of neuroplasticity via neuron-microglia interactions. Brain microglia exhibit profound diurnal rhythms in their response to pro-inflammatory stimuli, such as toll-like receptor 4 (TLR4) activation. Although hippocampal microglia develop a pro-inflammatory phenotype in response to TLR4 activation during the daily rest phase, little is known concerning spinal microglial responses to inflammatory stimuli across the daily rest/active cycle. Thus, we tested the hypothesis that ventral spinal microglia near the phrenic motor nucleus differentially respond to low-dose systemic lipopolysaccharide (LPS) delivered in the rest (12PM) vs active (12AM) phase. Specifically, we investigated microglial responses to systemic inflammation induced by low-dose LPS (0.1mg/kg, i.p.) in 2 populations of adult Sprague-Dawley rats (both 3-6 months old): 1) SD wildtype and 2) SD rats with humanized neuro-neutral apolipoprotein 3 (h APOE3) knock-in. All rats received intrapleural Cholera toxin B (CtB) injections 2 weeks prior to harvest to identify phrenic motor neurons. Twenty-four hours post-LPS or vehicle (0.9% saline), rats were perfused with 1X PBS and fixed with 4% PFA. Spinal tissues containing the phrenic motor nucleus (cervical 3-6) were harvested and dehydrated in 30% sucrose for immunohistochemistry. Activated microglia near phrenic motor neurons were visualized with an Iba-1 antibody and quantified and phenotyped based on morphology. Since no differences in microglia were found between wildtype and h APOE3 rats, data were combined into a single group for analyses. In vehicle-treated rats, robust Iba-1 staining near phrenic motor neurons was observed in the active, but not rest phase. In contrast, Iba-1 staining in LPS treated rats was elevated only when LPS was administered in the rest (not active) phase. After rest-phase LPS injections, microglial cell bodies were larger and had more processes compared to rest-phase controls. In the active phase, spinal microglia were similar between vehicle and LPS injected rats. Thus, spinal microglia exhibit a profound diurnal rhythm in (morphological) responses to systemic inflammation ( i.e. LPS). Further studies are needed to identify specific molecules driving time-of-day differences in spinal microglial responses to pro-inflammatory stimuli. Nevertheless, these findings increase our understanding of how diurnal rhythms impact cellular responses to physiological challenges, such as inflammation. Further, our results yield clues concerning diurnal variations in phrenic motor plasticity, since reciprocal phrenic motor neuron/microglial communication is an important regulator of phrenic motor plasticity. NIH R01HL148030 (GSM), NIH R01HL149800 (GSM), NIH T32HL134621-5 (ABM). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Spinal TNF-α receptor expression in a rodent model of respiratory motor neuron death

Intrapleural injection of cholera toxin B fragment conjugated to saporin (CTB-SAP) selectively kills respiratory motor neurons, and mimics aspects of neurodegenerative diseases including breathing deficits. In this model, phrenic long-term facilitation (pLTF) induced by acute intermittent hypoxia is enhanced following 7 days (d) of CTB-SAP vs. control rats, whereas pLTF in 28d CTB-SAP treated rats is comparable to pLTF in controls suggesting that the underlying mechanisms of plasticity at both time points differ. Inflammation is a hallmark of neurodegenerative diseases and is known to prevent pLTF. Furthermore, ketoprofen (NSAID) delivery attenuates pLTF in 7d CTB-SAP-treated rats, but enhances it in 28d CTB-SAP-treated rats; this suggests that inflammation contributes to pLTF in 7d CTB-SAP-treated rats, but undermines pLTF in 28d CTB-SAP-treated rats. In addition, cervical spinal inflammatory marker gene expression ( e.g., TNF-a) is increased in CTB-SAP rats, and pilot data suggest that soluble TNF receptor 1 (sTNFR1) inhibition differentially affects pLTF (Nichols and Smith , ibid). Thus, the goal of the current study is to investigate TNFR1 as a pharmacological target that can modulate inflammation to maximize pLTF and breathing in CTB-SAP rats. We hypothesized that C4 TNFR1 expression would be increased on phrenic motor neurons and glial cells of CTB-SAP rats vs. controls. To address this, we studied TNFR1 expression on phrenic motor neurons and glial cells using immunohistochemistry, confocal microscopy, and ImageJ analysis in male Sprague Dawley rats 7 and 28 days after intrapleural injection of: CTB-SAP or un-conjugated CTB and SAP (control); n=8/group. Our preliminary data (n=4/group) suggest that phrenic TNFR1 expression is increased in 7d CTB-SAP rats vs. controls (p&lt;0.05), but is unaffected in 28d CTB-SAP rats vs. controls (p&gt;0.05). Further data analysis is underway for phrenic as well as glial TNFR1 expression in CTB-SAP rats vs. controls. If C4 TNFR1 expression is increased, this would suggest that TNFR1 is a viable target for therapeutic intervention in CTB-SAP rats, and potentially in patients with breathing deficits due to respiratory motor neuron death. NSF Computational Neuroscience grant, MU CVM COR grant, and Spinal Cord Injury &amp; Disease Research Program grant. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Chloroquine Affects Frequency-Dependent Endocytosis in Diaphragm Muscle of Old Mice

Chloroquine is an antimalarial agent commonly used as an autophagy inhibitor by preventing autolysosomal fusion. Autophagy is an important cellular process through which defective cytoplasmic structures are recycled and degraded that has been implicated in aging neuromuscular effects. We recently showed that acute chloroquine treatment blunts the maximal forces generated by the diaphragm muscle, likely by impairing neuromuscular transmission generally consistent with aging effects in the diaphragm muscle. In addition, old mice show increased accumulation of autophagy proteins in phrenic motor neurons, consistent with impaired autophagy flux. In the present study, we hypothesized that chloroquine treatment results in accumulation of bulk endosomes pre-synaptically. Each hemidiaphragm with attached phrenic nerve was excised from 24-month-old male and female C57BL/6 mice to evaluate frequency dependent (0 vs 80 Hz stimulation) uptake of the styryl dye FM4-64, given that 80-Hz stimulation frequency induces bulk endocytosis after synaptic vesicle retrieval. Diaphragm-phrenic nerve preparations were incubated in 5 μM FM4-64 for 1 h prior to 6-min nerve stimulation (0.5 ms supramaximal pulses in 80 Hz, 200 ms trains delivered at 40% duty cycle) followed by a 5-min post-stimulation incubation. FM4-64 uptake at diaphragm neuromuscular junctions (NMJs) was measured using confocal microscopy. Preparations were randomly assigned to co-incubation with chloroquine (50 μM) or vehicle (Rees-Simpson solution). There was minimal FM4-64 uptake in the absence of phrenic nerve stimulation, consistent with the activity-dependent uptake of the styryl dye. With 80 Hz stimulation, high levels of FM4-64 uptake were evident in both the chloroquine and vehicle-treated groups without significant differences across groups. FM4-64 fluorescence was evident throughout the presynpatic terminal with few areas of dense accumulation, consistent with retention of endosomes. There was no evidence of greater endosome accumulation with chloroquine treatment. Of note, a slight increase in FM4-64 uptake was evident in the chloroquine treated groups compared to vehicle in the non-stimulated NMJs. These results are consistent with FM4-64 uptake reflecting bulk endocytosis that mainly occurs during high frequency stimulation. The impairment in autophagy flux following chloroquine treatment is expected to result in endosome accumulation but it is possible that the stimulation protocol was insuffcient to elucidate this possibility. Collectively, the findings of this study are consistent with impaired autophagy at diaphragm presynaptic terminals which may underlie neuromuscular deficits observed in old age. Supported by NIH grant R01 AG057052 and the Mayo Clinic. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Phrenic Motor Neuron Fractalkine Signaling to Nearby Microglia Regulates Phrenic Long-Term Facilitation

Although microglia are traditionally regarded as innate CNS immune cells, recent studies demonstrate that microglia also play key roles in normal CNS functions. For example, microglia engage in bidirectional communication with other cell types ( e.g., neurons) to regulate diverse processes including neuronal excitability, architecture and synaptic plasticity. Although neuron-microglia interactions regulate hippocampal synaptic plasticity, little is known concerning the role of microglia in regulating respiratory motor plasticity. One well-studied form of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a prolonged increase in phrenic motor output following acute intermittent hypoxia (AIH). Whereas AIH consisting of moderate hypoxic episodes (mAIH) elicits pLTF by a serotonin-dominant, adenosine-constrained mechanism, severe AIH (sAIH) elicits pLTF by an adenosine-dominant, serotonin-constrained mechanism. Neurons interact with microglia via release of fractalkine (CX3CL1), activating responses from nearby microglia, the only CNS cell-type known to express fractalkine receptors (CX3CR1). Cervical spinal fractalkine protein injections elicit phrenic motor facilitation similar to pLTF and trigger glial ATP/adenosine release, consistent with the hypothesis that neuronal fractalkine release during hypoxia elicits extracellular adenosine accumulation, thereby modulating AIH-induced pLTF. Here, we tested the hypothesis that phrenic motor neuron fractalkine protein underlies the adenosine-constraint to mAIH-induced pLTF, and drives sAIH-induced pLTF. In adult male Sprague Dawley rats, siRNA knockdown of phrenic motor neuron fractalkine mRNA and protein was achieved via intrapleural injections of siRNAs targeting fractalkine mRNA for 3 successive days. Fractalkine knock-down: 1) enhanced serotonin-dependent mAIH-induced pLTF (100±7%; n=6; vs controls: 50±5%; n=7; p&lt;0.001), but 2) attenuated adenosine-dependent, sAIH-induced pLTF (23±12%; n=5; vs controls: 88±7%; n=7; p=0.001). These findings suggest that hypoxia triggers dose-dependent phrenic motor neuron fractalkine release, activating CX3CR1 on nearby microglia. Subsequent microglial ATP release and conversion to adenosine activates phrenic motor neuron adenosine 2A receptors which: 1) undermines mAIH-induced (serotonin-dominant) pLTF; or 2) reaches levels suffcient to drive sAIH-induced (adenosine-dominant) pLTF. These findings increase our understanding of how spinal intercellular interactions regulate AIH-induced phrenic motor plasticity. NIH R01HL148030 (GSM), NIH R01HL149800 (GSM), NIH T32HL134621-5 (ABM). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

BDNF/TrkB Induces pCREBS133 Phosphorylation and Activates Mitochondrial Biogenesis in Rat Phrenic Motor Neurons

Neural control of the diaphragm muscle (DIAm) is essential to sustain breathing. During breathing, fatigue resistant DIAm motor units are recruited comprising smaller phrenic motor neurons (PhMNs) and type I and IIa muscle fibers. Larger PhMNs comprise more fatigable DIAm motor units that are infrequently active during expulsive behaviors. The difference in activation history is reflected in the higher mitochondrial volume density in smaller PhMNs. In PhMNs, brain-derived neurotrophic factor (BDNF) signals through its high-affnity receptor, tropomyosin receptor kinase B (TrkB.FL). Previously, we found that intrathecal BDNF treatment induces CREB phosphorylation at Serine 133 (pCREBs133). We hypothesized that pCREBs133 binds to the PGC1a promoter, transcriptionally targeting PGC1a expression, mediating downstream increased expression of NRF1, NRF2, and TFAM and mitochondrial biogenesis. To test this hypothesis, we used a chemosensitive TrkBF616 rat model with a knock-in allele sensitive to 1NMPP1 in that TrkB kinase is inhibited following 1NMPP1, a small molecule derivative of the protein kinase inhibitor protein phosphatase 1 that plays a significant role inhibiting the TrkB phosphorylation and activation. Systemic treatment of 1NMPP1 in TrkBF616 rats inhibits TrkB kinase activity. A bioinformatic analysis was performed to identify a putative binding site for pCREBs133 in the PGC1a promoter sequence in rats. Using a chromatin immunoprecipitation (ChIP) assay we found that pCREBS133 binds to the PGC1a promoter in rat cervical ventral horn. We showed that intrathecal BDNF treatment increased pCREBs133 phosphorylation, which was blocked by 1NMPP1 treatment in TrkBF616 rats. PGC1a, NRF1/2, and TFAM protein was compared in cervical ventral horn lysates from BDNF- and BDNF+1NMPP1-treated TrkBF616 rats by qRT-PCR and Western Blot. Relative mtDNA copy number (normalized to nuclear DNA) was quantified by qPCR to assess the mitochondrial biogenesis. All results were analyzed by paired t-test. Following 1NMPP1-induced inhibition of TrkB kinase, PGC1a, NRF1/2, and TFAM mRNA and protein expression was reduced. Consistent with these findings, mtDNA copy number was decreased following TrkB kinase inhibition by 1NMPP1. We conclude that inhibition of TrkB kinase by 1NMPP1 reduces PGC1a-mediated mitochondrial biogenesis in rat cervical ventral horn. Supported by NIH grants AG44615 and HL146114. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Respiration and Cough in Pompe Disease: Results from a neuromechanical computational model of the respiratory control system

Late Onset Pompe Disease (LOPD) is an incurable, rare progressive genetic disease. It is characterized by glycogen build-up, due to mutations in the gene encoding the lysosomal enzyme acid alpha glucosidase, throughout muscle and neural regions. This accumulation is associated with cell death and muscle fiber swelling, eventually leading to respiratory insuffciency and airway clearance (i.e. cough) deficiency. Impairments in effective airway clearance result from the inability to clear normal pulmonary volume. Emerging evidence indicates significant respiratory neuron dysfunction, altering pulmonary function in humans and animal models. It has been suggested that respiratory and airway clearance insuffciency observed in LOPD may be attributed to medullary motoneurons potentially associated with the respiratory and cough pattern generators (Fuller et al., 2013; DeRuisseau et al., 2009). However, the precise neural mechanisms that suppress breathing and cough are not fully understood. One possible target for LOPD respiratory insuffciencies may be reduced medullary inspiratory neuron activity within this network, which could result in decreased phrenic motoneuron output. Using a stochastic neural network simulator that consisted of discrete “integrate and fire” populations, we simulated a systematic decrease in three medullary inspiratory neuron populations: inspiratory driver (I-Driver), inspiratory decrementing (I-Dec), or inspiratory augmenting (I-Aug) neurons. We simulated breathing and three cough trials. By targeting these populations, we expected global decreased neural bursts, suppressed inspiratory motor drive, and/or suppressed expiratory motoneuron output. Simulations indicated that when the I-Dec motoneuron population decreased by 25%, it resulted in apnea and abolished cough. Reducing the I-Aug motoneuron population by 20% resulted in longer inspiratory phase durations (TI) and a decreased respiratory rate. During cough, there was a decrease in the duration and amplitudes of the expiratory motoneuronal activity associated with decreased cough receptor and pulmonary stretch receptor afferent feedback. When the I-Driver motoneuron population was reduced by 25%, TI decreased and I amplitude increased during respiration. During cough, lumbar and phrenic motoneuron discharge duration and amplitude decreased. These results are consistent with previously reported in vivo data. Overall, our simulation data support the plausibility of glycogen storage dysfunction impairing medullary neuronal and premotoneuronal excitability within the inspiratory network. Our results also suggest that some neurons may be more sensitive to glycogen storage than others within the brainstem inspiratory network. Supported by NHLBI L30HL165496, NIH T32 HL 134621 and 3OT2OD023854. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Brain Derived Neurotrophic Factor (BDNF) Immunolabeling in C4-C5 Spinal Cord of Adult Female Rats Following Acute Intermittent Hypoxia (AIH) Exposure Composed of 5,3-min Hypoxic Episodes (AIH5x3min) at 4-weeks After mid-thoracic SCI

Acute intermittent hypoxia (AIH) induces spinal neural plasticity in both respiratory and non-respiratory somatic motor systems. AIH-induced neural plasticity has been best studied in the respiratory neural control system, where exposure to moderate AIH (9-15% inspired O2), typically composed of 3,5-min hypoxic episodes (AIH3x5min), has been shown to consistently elicit a robust increase in phrenic motor activity due to a serotonin-dependent mechanism that promotes de novo synthesis of BDNF and activation of high affnity BDNF-TrkB receptors to initiate ERK MAP kinase signaling in phrenic motoneurons. Whether other protocols that use a total duration of 15 min of hypoxic exposure, but with an increased number of shorter duration hypoxic episodes ( e.g, AIH5x3min) similarly elicit serotonin-dependent BDNF pathway-mediated neural plasticity mechanisms in this region remains to be determined. To begin to assess this possibility, we evaluated BDNF expression in C4-C5 spinal cord segments containing the phrenic motor nucleus following single exposure to AIH5x3min (12% O2) in urethane-anesthetized spontaneously breathing adult female rats at 4-weeks after mid-thoracic SCI. Experiments using AIH3x5min (12% O2) exposure and intermittent normoxia (Nx, 21% O2) exposure served as controls. At ~60-90 min after the AIH or Nx exposure protocol, the rats were transcardially perfused with saline followed by 4% PFA, and various regions of the CNS, including the C3-C5 spinal cord were removed, post-fixed, and cryoprotected. 20 μm thick transverse sections were then cut to obtain sets of slides for single- or double-label fluorescence IHC staining to localize BDNF protein expression in the C4-C5 spinal cord, including the region containing the phrenic motor nucleus. Preliminary analyses show that both AIH5x3min and AIH3x5min exposures elicit increased BDNF expression when compared to Nx exposure, with both diffuse cytosolic and punctate immunofluorescence labeling in and around neurons within the phrenic motor nucleus region, including labeling of presumptive phrenic motoneurons. No labeling was seen in adjacent tissue sections that were incubated without the primary antibody. Immunofluorescence labeling was also seen in the more medial regions of lamina VIII in the ventral horn, the medial regions of laminae VI and VII, and throughout lamina X; these regions have been reported to contain the V0-V3 classes of spinal interneurons (SpINs) albeit we did not verify BDNF labeling of specific classes of presumptive SpINs. These preliminary observations suggest that AIH5x3min exposure likely recruits a serotonin-dependent BDNF pathway-mediated mechanism for neural plasticity in a manner similar to that seen for AIH3x5min exposure. APS Summer Undergraduate Research Fellowship; DOD CDMRP W81XWH-17-1-0260; NYS DOH SCIRB C37711GG. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.