Neonatal Muscle Growth Research Articles

The role of sympathetic innervation in neonatal muscle growth and neuromuscular contractures.

Neonatal brachial plexus injury (NBPI), a leading cause of pediatric upper limb paralysis, results in disabling and incurable muscle contractures that are driven by impaired longitudinal growth of denervated muscles. A rare form of NBPI, which maintains both afferent and sympathetic muscle innervation despite motor denervation, protects against contractures. We have previously ruled out a role for NRG/ErbB signaling, the predominant pathway governing antegrade afferent neuromuscular transmission, in modulating the formation of contractures. Our current study therefore investigated the contributions of sympathetic innervation of skeletal muscle in modulating NBPI-induced contractures. Through chemical sympathectomy and pharmacologic modification with a β2 -adrenergic agonist, we discovered that sympathetic innervation alone is neither required nor sufficient to modulate contracture formation in neonatal mice. Despite this, sympathetic innervation plays an intriguing sex-specific role in mediating neonatal muscle growth, as the cross-sectional area (CSA) and volume of normally innervated male muscles were diminished by ablation of sympathetic neurons and increased by β-adrenergic stimulation. Intriguingly, the robust alterations in CSA occurred with minimal changes to normal longitudinal muscle growth as determined by sarcomere length. Instead, β-adrenergic stimulation exacerbated sarcomere overstretch in denervated male muscles, indicating potentially discrete regulation of muscle width and length. Future investigations into the mechanistic underpinnings of these distinct aspects of muscle growth are thus essential for improving clinical outcomes in patients affected by muscle disorders in which both length and width are affected.

Sex-specific role of myostatin signaling in neonatal muscle growth, denervation atrophy, and neuromuscular contractures.

Neonatal brachial plexus injury (NBPI) causes disabling and incurable muscle contractures that result from impaired longitudinal growth of denervated muscles. This deficit in muscle growth is driven by increased proteasome-mediated protein degradation, suggesting a dysregulation of muscle proteostasis. The myostatin (MSTN) pathway, a prominent muscle-specific regulator of proteostasis, is a putative signaling mechanism by which neonatal denervation could impair longitudinal muscle growth, and thus a potential target to prevent NBPI-induced contractures. Through a mouse model of NBPI, our present study revealed that pharmacologic inhibition of MSTN signaling induces hypertrophy, restores longitudinal growth, and prevents contractures in denervated muscles of female but not male mice, despite inducing hypertrophy of normally innervated muscles in both sexes. Additionally, the MSTN-dependent impairment of longitudinal muscle growth after NBPI in female mice is associated with perturbation of 20S proteasome activity, but not through alterations in canonical MSTN signaling pathways. These findings reveal a sex dimorphism in the regulation of neonatal longitudinal muscle growth and contractures, thereby providing insights into contracture pathophysiology, identifying a potential muscle-specific therapeutic target for contracture prevention, and underscoring the importance of sex as a biological variable in the pathophysiology of neuromuscular disorders.

NRG/ErbB signaling regulates neonatal muscle growth but not neuromuscular contractures in neonatal brachial plexus injury.

Neonatal brachial plexus injury (NBPI) causes disabling and incurable muscle contractures that are driven by impaired growth of denervated muscles. A rare form of NBPI, which maintains afferent muscle innervation despite motor denervation, does not cause contractures. As afferent innervation regulates various aspects of skeletal muscle homeostasis through NRG/ErbB signaling, our current study investigated the role of this pathway in modulating contracture development. Through pharmacologic modification with an ErbB antagonist and NRG1 isoforms, we discovered that NRG/ErbB signaling does not modulate the development of contractures in neonatal mice. Instead, ErbB inhibition impeded growth in nondenervated skeletal muscles, whereas increased ErbB activation exacerbated denervation‐induced skeletal muscle atrophy. This potential regulatory effect of NRG/ErbB signaling on neonatal muscle growth warrants deeper investigation.

258 Deficits in growth, muscle mass, and body composition following intrauterine growth restriction persisted in lambs at 60 d of age but were improved by daily clenbuterol supplementation

Abstract Intrauterine growth restriction (IUGR) reduces neonatal muscle growth and alters body composition in livestock. Our objective was to determine the effect of IUGR on juvenile growth and assess the benefits of treatment with clenbuterol (β2 adrenergic agonist) in IUGR offspring. Heat stress-induced IUGR lambs were born 28% lighter (P < 0.05) than controls. At 60 d of age, unsupplemented IUGR lambs had reduced (P < 0.05) bodyweight (BW), average daily gain, and crown-rump length compared to controls, but clenbuterol-supplemented IUGR lambs did not differ from controls. Crown circumference, body girth, and cannon bone length did not differ among groups. Bioelectrical impedance in live lambs and carcasses estimated that lean mass and mass of multiple muscle groups were reduced (P < 0.05) in unsupplemented IUGR lambs but not clenbuterol-supplemented IUGR lambs compared to controls. Estimated protein, fat, and protein/fat were likewise reduced (P < 0.05) in unsupplemented but not clenbuterol-supplemented IUGR lambs. Loin-eye area in chilled carcasses was 30% smaller (P < 0.05) in unsupplemented IUGR lambs but 19% larger (P < 0.05) in clenbuterol-supplemented IUGR lambs compared to controls. Proximate analysis revealed greater (P < 0.05) fat and reduced (P < 0.05) protein and protein/fat in loin muscles from unsupplemented but not clenbuterol-supplemented IUGR lambs compared to controls. At necropsy, hindlimbs, hearts, and flexor digitorum superficialis muscles tended to be lighter (P ≤ 0.09) and lungs and kidneys were lighter (P < 0.05) in IUGR lambs. Kidney weight was further reduced (P < 0.05) in clenbuterol-supplemented IUGR lambs. Brain/BW tended to be reduced (P ≤ 0.09) and lung/BW and kidney/BW were reduced (P < 0.05) in IUGR lambs, but lung weight and lung/BW were greater (P < 0.05) in clenbuterol-supplemented compared to unsupplemented IUGR lambs. We conclude that poor growth and asymmetric body composition previously observed in IUGR neonates persists in juveniles, but daily treatment with clenbuterol recovered growth and improved body composition in IUGR lambs.

Abundance of miRNA during muscle growth is not influenced by dietary protein inclusion levels in neonatal pigs

Postnatal muscle growth requires satellite cell (SC) activation and fusion with existing myofibers. Delay or absence of SC proliferation and differentiation leads to a reduction in muscle hypertrophy. Nutrition during neonatal life impacts developmental growth programing by altering SC activity. Growth as a percentage of body weight is highest during the neonatal period and restricting neonatal nutrition can lead to permanent muscle growth deficits. Reducing dietary intake of protein can improve gut function during weaning, however, this may limit amino acid availability for muscle growth. The feed‐induced protein synthesis response in muscle is highest in the neonatal period and subsequently progressively declines throughout life. The heightened responsiveness of protein synthesis during this period is mediated mainly by the activation of translation initiation factors. Post‐transcriptional regulation of RNA is mediated in part by micro‐RNAs (miRNA) by preventing translation and/or promoting degradation and are known to be important modulators of stem cell activity. Proliferation of SCs is promoted by miR‐133a, while differentiation is promoted by miR‐1, miR‐133b and miR‐206 through repression of Pax7. The objective of this study was to determine miRNA abundance during neonatal muscle growth in response to a protein deficient, adequate, and excess diets in pigs. Twenty‐eight piglets obtained at 1d of age received isocaloric dietary treatments of either an adequate protein diet (A), a 25% deficient protein diet (D), or a 25% excess protein diet (E) over a 17‐day trial. Piglet starting weights were 1.47kg±0.1 kg for D and 1.46±0.1 kg for E and A treatments. Final body weight and ADG were increased in E compared to D pigs (5.60±0.27kg vs 4.75±0.27kg and 0.27±0.01kg/d vs 0.21±0.01kg/d) despite an equal feed intake across treatments. Gene expression in the longissimus dorsi was evaluated by qPCR. There were no differences IGF1, or in myogenic genes MyoD and MyoG despite the increased growth rates of E treatment pigs. The SC marker Pax‐7 did not differ between treatments. Genes associated with lipid oxidation, PPARd and CPT1, were not different between treatments. There were no differences in degradation pathway genes muRF‐1, atrogin‐1, and FOXO despite the reduction in growth of D pigs. The differences in growth response without alterations in gene expression may be due to post‐transcriptional regulation, however, no differences were observed in miRNA expression. The differentiation promoting miRs‐1, 133b and 206 and proliferation promoting miR‐133a were not different between treatments. These data indicate that diets excess in protein increased growth rates which may lead a sustained increase in muscle mass throughout life. While miRNA expression was not different between treatments, miRNAs may still play a role in the differences observed in growth between D and E treatments through their activity and rate of binding to their target mRNAs.

Muscle Weakness and Fibrosis Due to Cell Autonomous and Non-cell Autonomous Events in Collagen VI Deficient Congenital Muscular Dystrophy

Congenital muscular dystrophies with collagen VI deficiency are inherited muscle disorders with a broad spectrum of clinical presentation and are caused by mutations in one of COL6A1–3 genes. Muscle pathology is characterized by fiber size variation and increased interstitial fibrosis and adipogenesis. In this study, we define critical events that contribute to muscle weakness and fibrosis in a mouse model with collagen VI deficiency. The Col6a1GT/GT mice develop non-progressive weakness from younger age, accompanied by stunted muscle growth due to reduced IGF-1 signaling activity. In addition, the Col6a1GT/GT mice have high numbers of interstitial skeletal muscle mesenchymal progenitor cells, which dramatically increase with repeated myofiber necrosis/regeneration. Our results suggest that impaired neonatal muscle growth and the activation of the mesenchymal cells in skeletal muscles contribute to the pathology of collagen VI deficient muscular dystrophy, and more importantly, provide the insights on the therapeutic strategies for collagen VI deficiency.

Intermittent Leucine Pulses during Continuous Feeding Alters Novel Components Involved in Skeletal Muscle Growth of Neonatal Pigs

Extra‐uterine growth restriction in low‐birth‐weight (LBW) infants is associated with the development of chronic metabolic derangements, and emphasizes the need for improved nutritional protocols to optimize growth. Supplementation with intermittent pulses of leucine in continuously fed neonatal pigs enhanced skeletal muscle protein synthesis through the upregulation of mammalian target of rapamycin signaling. Piglets (n = 23, 7‐d‐old) were continuously fed a milk formula via orogastric tube for 21 d, with an additional parenteral infusion of either leucine (800 μmol·kg−1·h−1, LEU) or alanine (CON) for 1 h every 4 h. To identify novel genes by which LEU may promote neonatal muscle growth, we performed RNA‐Seq analysis of longissimus dorsi muscle from 4 pigs per group, followed by gene ontology (GO) analysis of differentially expressed genes (DEG). In addition, protein abundance/activation of DEG products involved in both muscle protein anabolic or catabolic pathways was assessed by immunoblots. RNA‐Seq was performed on a HiSeq2000 sequencer analyzer. Reads were analyzed using CLCBio Genomics workbench software, assembled against the porcine reference genome, and normalized by a threshold of ‘reads per kilo base per million mapped reads’ (RPKM) ≥ 0.2. A total of 465 genes were differentially expressed between LEU and CON groups (RPKM P < 0.05). GO analyses showed developmental processes, cell junction assembly, regulation of cellular process and organ development as the terms most significantly enriched among the 465 genes. Applying more stringent parameters (RPKM P < 0.01 + >2 fold change), 18 differential expressed genes were identified between CON and LEU pigs, of which tumor necrosis factor receptor‐associated factor 6 (TRAF6) and mammalian Ste20‐like kinase 1 (MST1), both negative regulators of muscle growth, were decreased by > 2‐fold (P < 0.01) in LEU compared to CON. Western blot analysis also showed a downregulation of MST1 and TRAF6 abundance in LEU animals (P < 0.01) compared to CON. Together, these findings suggest that intermittent leucine pulses in continuously fed neonate pigs lead to downregulation in the gene and protein expression of signaling components that limit muscle growth.Support or Funding InformationNIH AR444474, NIHHD072891, USDA NIFA 2013‐67015‐20438, and USDA/ARS 6250‐51000‐055

An NF-κB - EphrinA5-Dependent Communication between NG2+ Interstitial Cells and Myoblasts Promotes Muscle Growth in Neonates

Skeletal muscle growth immediately following birth is critical for proper body posture and locomotion. However, compared with embryogenesis and adulthood, the processes regulating the maturation of neonatal muscles is considerably less clear. Studies in the 1960s predicted that neonatal muscle growth results from nuclear accretion of myoblasts preferentially at the tips of myofibers. Remarkably, little information has been added since then to resolve how myoblasts migrate to the ends of fibers. Here, we provide insight into this process by revealing a unique NF-κB-dependent communication between NG2(+) interstitial cells and myoblasts. NF-κB in NG2(+) cells promotes myoblast migration to the tips of myofibers through cell-cell contact. This occurs through expression of ephrinA5 from NG2(+) cells, which we further deduce is an NF-κB target gene. Together, these results suggest that NF-κB plays an important role in the development of newborn muscles to ensure proper myoblast migration for fiber growth.

Effects of level of nutrient intake and age on mammalian target of rapamycin, insulin, and insulin-like growth factor-1 gene network expression in skeletal muscle of young Holstein calves

The molecular mechanisms by which level of nutrient intake enhances skeletal muscle growth in young ruminants are not fully understood. We examined mammalian target of rapamycin (mTOR), insulin, and insulin-like growth factor-1 (IGF-1) gene network expression in semitendinosus muscle tissue of young male Holstein calves fed a conventional milk replacer plus conventional starter (CON) or an enhanced milk replacer plus high-protein starter (ENH) for 5 wk followed by a conventional starter or a high-protein starter until 10 wk of age. Feeding ENH led to greater concentration of plasma IGF-1 and leptin and greater carcass protein and fat mass throughout the study. Despite the greater plasma IGF-1 and protein mass at wk 5, calves fed ENH had lower expression of IGF1R, INSR, and RPS6KB1 but greater expression of IRS1 and PDPK1 in muscle tissue. Except for IGF1R expression, which did not differ at wk 10, these differences persisted at wk 10, suggesting a long-term effect of greater nutrient intake on physiological and molecular mechanisms. Components of mTOR complex (mTORC)1 and mTORC2 (RICTOR and RPTOR) and FOXO1 expression decreased by wk 10 regardless of diet. Overall, the present data revealed that greater nutrient intake throughout the milk-fed and early postweaning phase alters body mass composition partly by altering hormonal and molecular profiles of genes associated with glucose and amino acid signaling. Those networks may play a crucial role in coordinating neonatal muscle growth and metabolism in response to level of nutrient intake.

STIM1-Ca2+ Signaling Is Required for the Hypertrophic Growth of Skeletal Muscle in Mice

Immediately after birth, skeletal muscle must undergo an enormous period of growth and differentiation that is coordinated by several intertwined growth signaling pathways. How these pathways are integrated remains unclear but is likely to involve skeletal muscle contractile activity and calcium (Ca(2+)) signaling. Here, we show that Ca(2+) signaling governed by stromal interaction molecule 1 (STIM1) plays a central role in the integration of signaling and, therefore, muscle growth and differentiation. Conditional deletion of STIM1 from the skeletal muscle of mice (mSTIM1(-/-) mice) leads to profound growth delay, reduced myonuclear proliferation, and perinatal lethality. We show that muscle fibers of neonatal mSTIM1(-/-) mice cannot support the activity-dependent Ca(2+) transients evoked by tonic neurostimulation, even though excitation contraction coupling (ECC) remains unperturbed. In addition, disruption of tonic Ca(2+) signaling in muscle fibers attenuates downstream muscle growth signaling, such as that of calcineurin, mitogen-activated protein (MAP) kinases, extracellular signal-regulated kinase 1 and 2 (ERK1/2), and AKT. Based on our findings, we propose a model wherein STIM1-mediated store-operated calcium entry (SOCE) governs the Ca(2+) signaling required for cellular processes that are necessary for neonatal muscle growth and differentiation.

Nutritionally‐induced neonatal muscle growth retardation can be rescued by sustained muscle IGF‐I expression

Undernutrition prior to the attainment of maturation permanently compromises skeletal muscle mass. To determine if the loss in growth capacity can be overcome by precluding the developmental decline in local IGF expression, transgenic MLC/mIGF‐I mice with muscle‐specific IGF‐I overexpression were studied. From birth, nontransgenic (NTG) and transgenic (TG) littermates were suckled on NTG dams fed either a control 20 % (CP) or an 8% protein (LP) diet. At weaning all pups were fed the CP diet ad libitum and studied 0, 1, 3, and 17 wk later (n=7–9/age/genotype/diet). Hindlimb muscle masses at weaning were 20–25% smaller in all LP compared to CP pups (P<0.001); there was no protective effect of genotype. After 1 wk of refeeding, muscle mass of the LP TG and NTG pups were 6 and 20 % smaller, respectively than those of CP NTG pups. Over this week, food intake of TG and NTG LP pups were similar (2.98±0.08 g/d), but feed efficiency was higher in TG pups (0.39 and 0.33 g body weight/g food, respectively; pSD, 0.021; P<0.001). After 3 wk, muscle weights of LP TG pups were similar to those of CP NTG, and by 20 wk of age they were 20% heavier (P<0.001); LP NTG muscles remained 10% small than in CP NTG mice. The results indicate that the reduced muscle mass produced by undernutrition in early life is reversible provided appropriate anabolic stimuli are present during nutritional rehabilitation. (Supported by NIH AR46308 and USDA/ARS 6250‐51000‐043)

Regulation of muscle growth in neonates

This review reports recent findings on the multiple factors that regulate skeletal muscle growth in neonates. Skeletal muscle is the fastest growing protein mass in neonates. The high rate of neonatal muscle growth is due to accelerated rates of protein synthesis accompanied by the rapid accumulation of muscle nuclei. Feeding profoundly stimulates muscle protein synthesis in neonates and the response decreases with age. The feeding-induced stimulation of muscle protein synthesis is modulated by enhanced sensitivity to the postprandial rise in insulin and amino acids. Insulin and amino acid signaling components have been identified that are involved in the feeding-induced stimulation of protein synthesis in neonatal muscle. The enhanced activation of these signaling components in skeletal muscle of the neonate contributes to the high rate of muscle protein synthesis and rapid gain in muscle protein mass in neonates. Recent findings suggest that the immature muscle has a heightened capacity to activate signaling cascades that promote translation initiation in response to the postprandial rise in insulin and amino acids thereby enabling their efficient utilization for muscle growth. This capacity is further supported by enhanced satellite cell proliferation, but how these two processes are linked remains to be established.

Insulin and Amino Acids are Critical Regulators of Neonatal Muscle Growth

Nutrition Today: July 2008 - Volume 43 - Issue 4 - p 150-151 doi: 10.1097/01.NT.0000303338.45179.eb

Insulin and Amino Acids are Critical Regulators of Neonatal Muscle Growth

Newborn humans and animals grow at very rapid rates because they use the protein that they eat very efficiency to increase body protein mass. This high efficiency of protein deposition in neonates is largely due to their ability to markedly increase the amount of protein synthesized in their muscles when they eat. This enhanced stimulation of muscle protein synthesis after eating is mediated by the rise in the hormone, insulin, and the rise in amino acids, which are the building blocks of protein. Intracellular signaling components that respond to insulin and amino acids have been identified and these have been shown to be involved in the feeding-induced stimulation of protein synthesis in skeletal muscle of the neonate. The enhanced activation of these intracellular signaling components in neonatal muscle contributes to the high rate of muscle protein synthesis and rapid gain in skeletal muscle mass in newborns.

Neonatal muscle growth: a quantitative study.

AbstractVarious growth parameters of the rat extensor digitorum longus muscle were studied during the first week of neonatal growth, including changes in weight, length, girth and fiber number. These quantitative studies were supplemented by ultrastructural analysis of fiber diameter and morphology in an attempt to determine the source of “new” muscle fibers in the neonate. In the 2‐day‐old extensor digitorum longus many of the myofibers were enclosed with other filamented cells (a satellite myofiber and/or myotube) in a common basement membrane, forming a cluster. Clusters accounted for 40% of the total independent filamented cells and clusters in 2‐day‐old muscles, for 2% in the 5‐day‐old, and for less than 1% in the 8‐day‐old. The decrease in the percentage of clusters occurred concomitantly with an increase in the number of independent filamented cells. In the 2‐day‐old there were 2,932 ± 130 independent cells and clusters; this number increased by 90% in the 8‐day‐old muscle. Mean diameters of filamented cells (whether found independently or as part of a cluster) increased from 6.5 μm to 7.4 μm and to 9.5 μm on days 2, 5, and 8 respectively. Increased filamented cell diameter between days 2 and 5 was a result of growth of the smallest filamented cells into the medium‐size compartment. Between the fifth and eighth day postpartum, the increase was due to the growth of both small and medium‐size filamented cells. The largest fibers did not increase in diameter during the first week of postnatal development, maximal fiber diameter in the 2‐day‐old being 15.9 μm and in the 8‐day‐old, 16.1 μm. At two‐days the area through the widest girth of the muscle occupied by filamented cells was 35% of the total cross‐sectional area of the extensor digitorum longus. At eight days muscle fibers occupied 59% of the cross‐sectional area. To accommodate the myofibers the muscle belly increased its girth, and there was a decrease in the percentage of the area occupied by interstitial tissue, nerves, and blood vessels.