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

Vitamin A Protects the Preterm Lamb Diaphragm Against Adverse Effects of Mechanical Ventilation

Background: Preterm infants are deficient in vitamin A, which is essential for growth and development of the diaphragm. Preterm infants often require mechanical ventilation (MV) for respiratory distress. In adults, MV is associated with the development of ventilation-induced diaphragm dysfunction and difficulty weaning from the ventilator. We assessed the impact of MV on the preterm diaphragm and the protective effect of vitamin A during MV.

Methods: Preterm lambs delivered operatively at ∼131 days gestation (full gestation: 150 days) received respiratory support by synchronized intermittent mandatory ventilation for 3 days. Lambs in the treated group received daily (24 h) enteral doses of 2500 IU/kg/day vitamin A combined with 250 IU/kg/day retinoic acid (VARA) during MV, while MV control lambs received saline. Unventilated fetal reference lambs were euthanized at birth, without being allowed to breathe. The fetal diaphragm was collected to quantify mRNA levels of myosin heavy chain (MHC) isoforms, atrophy genes, antioxidant genes, and pro-inflammatory genes; to determine ubiquitin proteasome pathway activity; to measure the abundance of protein carbonyl, and to investigate metabolic signaling.

Results: Postnatal MV significantly decreased expression level of the neonatal MHC gene but increased expression level of MHC IIx mRNA level (p < 0.05). Proteasome activity increased after 3 days MV, accompanied by increased MuRF1 mRNA level and accumulated protein carbonyl abundance. VARA supplementation decreased proteasome activity and FOXO1 signaling, down-regulated MuRF1 expression, and reduced reactive oxidant production.

Conclusion: These findings suggest that 3 days of MV results in abnormal myofibrillar composition, activation of the proteolytic pathway, and oxidative injury of diaphragms in mechanically ventilated preterm lambs. Daily enteral VARA protects the preterm diaphragm from these adverse effects.

Gestational age at time of in utero lipopolysaccharide exposure influences the severity of inflammation-induced diaphragm weakness in lambs

The preterm diaphragm is functionally immature compared with its term counterpart. In utero inflammation further exacerbates preterm diaphragm dysfunction. We hypothesized that preterm lambs are more vulnerable to in utero inflammation-induced diaphragm dysfunction compared with term lambs. Pregnant ewes received intra-amniotic (IA) injections of saline or 10 mg lipopolysaccharide (LPS) 2 or 7 days before delivery at 121 days (preterm) or ∼145 days (term) of gestation. Diaphragm contractile function was assessed in vitro. Plasma cytokines, diaphragm myosin heavy chain (MHC) isoforms, and oxidative stress were evaluated. Maximum diaphragm force in preterm control lambs was significantly lower (22%) than in term control lambs ( P < 0.001). Despite similar inflammatory cytokine responses to in utero LPS exposure, diaphragm function in preterm and term lambs was affected differentially. In term lambs, maximum force after a 2-day LPS exposure was significantly lower than in controls (by ~20%, P < 0.05). In preterm lambs, maximum forces after 2-day and 7-day LPS exposures were significantly lower than in controls (by ~30%, P < 0.05). Peak twitch force after LPS exposure was significantly lower in preterm than in controls, but not in term lambs. In term lambs, LPS exposure increased the proportion of MHC-I fibers, increased twitch contraction times, and increased fatigue resistance relative to controls. In preterm diaphragm, the cross-sectional area of embryonic MHC fibers was significantly lower after 7-day versus 2-day LPS exposures. We conclude that preterm lambs are more vulnerable to IA LPS-induced diaphragm dysfunction than term lambs. In utero inflammation exacerbates diaphragm dysfunction and may increase susceptibility to postnatal respiratory failure.

Understanding the Full Spectrum of Organ Injury Following Intrapartum Asphyxia

Birth asphyxia is a significant global health problem, responsible for ~1.2 million neonatal deaths each year worldwide. Those who survive often suffer from a range of health issues including brain damage-manifesting as cerebral palsy (CP)-respiratory insufficiency, cardiovascular collapse, and renal dysfunction, to name a few. Although the majority of research is directed toward reducing the brain injury that results from intrapartum birth asphyxia, the multi-organ injury observed in surviving neonates is of equal importance. Despite the advent of hypothermia therapy for the treatment of hypoxic-ischemic encephalopathy (HIE), treatment options following asphyxia at birth remain limited, particularly in low-resource settings where the incidence of birth asphyxia is highest. Furthermore, although cooling of the neonate results in improved neurological outcomes for a small proportion of treated infants, it does not provide any benefit to the other organ systems affected by asphyxia at birth. The aim of this review is to summarize the current knowledge of the multi-organ effects of intrapartum asphyxia, with particular reference to the findings from our laboratory using the precocial spiny mouse to model birth asphyxia. Furthermore, we reviewed the current treatments available for neonates who have undergone intrapartum asphyxia, and highlight the emergence of maternal dietary creatine supplementation as a preventative therapy, which has been shown to provide multi-organ protection from birth asphyxia-induced injury in our preclinical studies. This cheap and effective nutritional supplement may be the key to reducing birth asphyxia-induced death and disability, particularly in low-resource settings where current treatments are unavailable.

Maternal Creatine Supplementation during Pregnancy Prevents Long-Term Changes in Diaphragm Muscle Structure and Function after Birth Asphyxia

Using a model of birth asphyxia, we previously reported significant structural and functional deficits in the diaphragm muscle in spiny mice, deficits that are prevented by supplementing the maternal diet with 5% creatine from mid-pregnancy. The long-term effects of this exposure are unknown. Pregnant spiny mice were fed control or 5% creatine-supplemented diet for the second half of pregnancy, and fetuses were delivered by caesarean section with or without 7.5 min of in-utero asphyxia. Surviving pups were raised by a cross-foster dam until 33±2 days of age when they were euthanized to obtain the diaphragm muscle for ex-vivo study of twitch tension and muscle fatigue, and for structural and enzymatic analyses. Functional analysis of the diaphragm revealed no differences in single twitch contractile parameters between any groups. However, muscle fatigue, induced by stimulation of diaphragm strips with a train of pulses (330 ms train/sec, 40 Hz) for 300 sec, was significantly greater for asphyxia pups compared with controls (p<0.05), and this did not occur in diaphragms of creatine + asphyxia pups. Birth asphyxia resulted in a significant increase in the proportion of glycolytic, fast-twitch fibres and a reduction in oxidative capacity of Type I and IIb fibres in male offspring, as well as reduced cross-sectional area of all muscle fibre types (Type I, IIa, IIb/d) in both males and females at 33 days of age. None of these changes were observed in creatine + asphyxia animals. Thus, the changes in diaphragm fatigue and structure induced by birth asphyxia persist long-term but are prevented by maternal creatine supplementation.

In utero LPS exposure impairs preterm diaphragm contractility

Preterm birth is associated with inflammation of the fetal membranes (chorioamnionitis). We aimed to establish how chorioamnionitis affects the contractile function and phenotype of the preterm diaphragm. Pregnant ewes received intra-amniotic injections of saline or 10 mg LPS, 2 days or 7 days before delivery at 121 days of gestation (term = 150 d). Diaphragm strips were dissected for the assessment of contractile function after terminal anesthesia. The inflammatory cytokine response, myosin heavy chain (MHC) fibers, proteolytic pathways, and intracellular molecular signaling were analyzed using quantitative PCR, ELISA, immunofluorescence staining, biochemical assays, and Western blotting. Diaphragm peak twitch force and maximal tetanic force were approximately 30% lower than control values in the 2-day and 7-day LPS groups. Activation of the NF-κB pathway, an inflammatory response, and increased proteasome activity were observed in the 2-day LPS group relative to the control or 7-day LPS group. No inflammatory response was evident after a 7-day LPS exposure. Seven-day LPS exposure markedly decreased p70S6K phosphorylation, but no effect on other signaling pathways was evident. The proportion of MHC IIa fibers was lower than that for control samples in the 7-day LPS group. MHC I fiber proportions did not differ between groups. These results demonstrate that intrauterine LPS impairs preterm diaphragmatic contractility after 2-day and 7-day exposures. Diaphragm dysfunction, resulting from 2-day LPS exposure, was associated with a transient activation of proinflammatory signaling, with subsequent increased atrophic gene expression and enhanced proteasome activity. Persistently impaired contractility for the 7-day LPS exposure was associated with the down-regulation of a key component of the protein synthetic signaling pathway and a reduction in the proportions of MHC IIa fibers.

Changes in muscle composition during the development of diving ability in the Australian fur seal

During development the Australian fur seal transitions from a terrestrial, maternally dependent pup to an adult marine predator. Adult seals have adaptations that allow them to voluntarily dive at depth for long periods, including increased bradycardic control, increased myoglobin levels and haematocrit. To establish whether the profile of skeletal muscle also changes in line with the development of diving ability, biopsy samples were collected from the trapezius muscle of pups, juveniles and adults. The proportions of different fibre types and their oxidative capacity were determined. Only oxidative fibre types (Type I and IIa) were identified, with a significant change in proportions from pup to adult. There was no change in oxidative capacity of Type I and IIa fibres between pups and juveniles but there was a two-fold increase between juveniles and adults. Myoglobin expression increased between pups and juveniles, suggesting improved oxygen delivery, but with no increase in oxidative capacity, oxygen utilisation within the muscle may still be limited. Adult muscle had the highest oxidative capacity, suggesting that fibres are able to effectively utilise available oxygen during prolonged dives. Elevated levels of total creatine in the muscles of juveniles may act as an energy buffer when fibres are transitioning from a fast to slow fibre type.