Citations

In previous studies, we found that after cervical spinal cord hemisection at C2 (SH), neuromuscular transmission improved in the rat diaphragm muscle (DIAm). We also found that exogenous brain‐derived neurotrophic factor (BDNF) treatment enhanced neuromuscular transmission, whereas inhibiting activation of its receptor (TrkB) worsened neuromuscular transmission. We hypothesized that expression of BDNF and/or its receptor TrkB at phrenic motoneurons would increase following SH. In the present study, adult male rats underwent SH or sham surgery. Phrenic motoneurons were labeled by injection of cholera toxin B into the DIAm and retrogradely‐labeled phrenic motoneurons were sampled by laser capture microdissection (LCM). BDNF and TrkB receptor (truncated – T1 and full length – FL) mRNA were measured in LCM‐microdissected motoneurons using quantitative real‐time RT‐PCR and expressed as a ratio to ribosomal S16 RNA. We found that BDNF mRNA expression in phrenic motoneurons increased by 3 days after SH (~2‐fold). Similarly, both TrkB.T1 and TrkB.FL mRNA increased by 3 days after SH (~17‐ and ~11‐fold, respectively). It is possible that the relative expression of neurotrophins and their receptors (truncated or full‐length) may determine the time‐course of restoration of phrenic activity following SH. Supported by NIH grants AR51173 and HL37680 and the Mayo Foundation.

Reactive airway diseases are significant sources of pulmonary morbidity in neonatal and pediatric patients. Supplemental oxygen exposure in premature infants contributes to airway diseases such as asthma and promotes development of airway remodeling, characterized by increased airway smooth muscle (ASM) mass and extracellular matrix (ECM) deposition. Decreased plasma membrane caveolin-1 (CAV1) expression has been implicated in airway disease and may contribute to airway remodeling and hyperreactivity. Here, we investigated the impact of clinically relevant moderate hyperoxia (50% O2) on airway remodeling and caveolar protein expression in a neonatal mouse model. Within 12 h of birth, litters of B6129SF2J mice were randomized to room air (RA) or 50% hyperoxia exposure for 7 days with or without caveolin-1 scaffolding domain peptide (CSD; caveolin-1 mimic; 10 µl, 0.25 mM daily via intraperitoneal injection) followed by 14 days of recovery in normoxia. Moderate hyperoxia significantly increased airway reactivity and decreased pulmonary compliance at 3 wk. Histologic assessment demonstrated airway wall thickening and increased ASM mass following hyperoxia. RNA from isolated ASM demonstrated significant decreases in CAV1 and cavin-1 in hyperoxia-exposed animals while cavin-3 was increased. Supplementation with intraperitoneal CSD mitigated both the physiologic and histologic changes observed with hyperoxia. Overall, these data show that moderate hyperoxia is detrimental to developing airway and may predispose to airway reactivity and remodeling. Loss of CAV1 is one mechanism through which hyperoxia produces these deleterious effects. Supplementation of CAV1 using CSD or similar analogs may represent a new therapeutic avenue for blunting hyperoxia-induced pulmonary damage in neonates.

ABSTRACT Recent studies have demonstrated an effect of neurotrophins, particularly brain‐derived neurotrophic factor (BDNF), on airway contractility [ via increased airway smooth muscle (ASM) intracellular calcium [Ca 2+ ] i ] and remodeling (ASM proliferation and extracellular matrix formation) in the context of airway disease. In the present study, we examined the role of BDNF in allergen‐induced airway inflammation using 2 transgenic models: 1 ) tropomyosin‐related kinase B (TrkB) conditional knockin (TrkBKI) mice allowing for inducible, reversible disruption of BDNF receptor kinase activity by administration of 1NMPP1, a PP1 derivative, and 2) smooth muscle‐specific BDNF knockout (BDNF fl/fl /SMMHCll Cre/0 ) mice. Adult mice were intranasally challenged with PBS or mixed allergen ( Altemaria alternata, Aspergillus fumigatus , house dust mite, and ovalbumin) for 4 wk. Our data show that administration of 1NMPP1 in TrkBKI mice during the 4‐wk allergen challenge blunted airway hyper‐responsiveness (AHR) and reduced fibronectin mRNA expression in ASM layers but did not reduce inflammation per se . Smooth muscle‐specific deletion of BDNF reduced AHR and blunted airway fibrosis but did not significantly alter airway inflammation. Together, our novel data indicate that TrkB signaling is a key modulator of AHR and that smooth muscle‐derived BDNF mediates these effects during allergic airway inflammation.—Britt, R. D., Jr., Thompson, M. A., Wicher, S. A., Manlove, L. J., Roesler, A., Fang, Y.‐H., Roos, C., Smith, L., Miller, J. D., Pabelick, C. M., Prakash, Y. S. Smooth muscle brain‐derived neurotrophic factor contributes to airway hyperreactivity in a mouse model of allergic asthma. FASEB J. 33, 3024–3034 (2019). www.fasebj.org

The effect of chronic exogenous testosterone (T) treatment on neuromuscular transmission in the diaphragm (Dia) muscle of adult male rats was determined. The contribution of neuromuscular transmission failure (NTF) to Dia fatigue was evaluated by superimposing intermittent direct muscle stimulation on repetitive nerve stimulation of isometric contraction in vitro. T treatment significantly reduced the contribution of NTF to Dia fatigue by ∼20% ( P < 0.001). Fiber type-specific effects on NTF were determined by measuring Dia fiber glycogen levels subsequent to repetitive nerve or muscle stimulation. T treatment had no effect on glycogen depletion in Dia type I and IIa fibers regardless of stimulation route. In the control group, type IIx fibers demonstrated significantly less glycogen depletion after nerve stimulation compared with direct muscle stimulation ( P < 0.05), suggesting the presence of NTF. In contrast, T treatment increased glycogen depletion of type IIx fibers during nerve stimulation to levels similar to those after direct muscle stimulation. These data indicate that testosterone treatment substantially improves neuromuscular transmission in the Dia.

It has been found that maximum specific force (Fmax; force per cross-sectional area) of rat diaphragm muscle doubles from birth to 84 days (adult). We hypothesize that this developmental change in Fmax reflects an increase in myosin heavy chain (MHC) content per half-sarcomere (an estimate of the number of cross bridges in parallel) and/or a greater force per cross bridge in fibers expressing fast MHC isoforms compared with slow and neonatal MHC isoforms (MHCslow and MHCneo, respectively). Single Triton 100-X-permeabilized fibers were activated at a pCa of 4.0. MHC isoform expression was determined by SDS-PAGE. MHC content per half-sarcomere was determined by densitometric analysis and comparison to a standard curve of known MHC concentrations. MHC content per half-sarcomere progressively increased during early postnatal development. When normalized for MHC content per half-sarcomere, fibers expressing MHCslow and coexpressing MHCneoproduced less force than fibers expressing fast MHC isoforms. We conclude that lower force per cross bridge in fibers expressing MHCslow and MHCneo contributes to the lower Fmax seen in early postnatal development.