Insulin is required for amino acid stimulation of dual pathways for translational control in skeletal muscle in the late-gestation ovine fetus

Tissue-specific responses that constrain glucose oxidation and increase lactate production with the severity of hypoxemia in fetal sheep

Fetal hypoxemia decreases insulin and increases cortisol and norepinephrine concentrations and may restrict growth by decreasing glucose utilization and altering substrate oxidation. Specifically, we hypothesized that hypoxemia would decrease fetal glucose oxidation and increase lactate and pyruvate production. We tested this by measuring whole body glucose oxidation and lactate production, and molecular pathways in liver, muscle, adipose, and pancreas tissues of fetuses exposed to maternal hypoxemia for 9 days (HOX) compared with control fetal sheep (CON) in late gestation. Fetuses with more severe hypoxemia had lower whole body glucose oxidation rates, and HOX fetuses had increased lactate production from glucose. In muscle and adipose tissue, expression of the glucose transporter GLUT4 was decreased. In muscle, pyruvate kinase (PKM) and lactate dehydrogenase B (LDHB) expression was decreased. In adipose tissue, LDHA and lactate transporter (MCT1) expression was increased. In liver, there was decreased gene expression of PKLR and MPC2 and phosphorylation of PDH, and increased LDHA gene and LDH protein abundance. LDH activity, however, was decreased only in HOX skeletal muscle. There were no differences in basal insulin signaling across tissues, nor differences in pancreatic tissue insulin content, β-cell area, or genes regulating β-cell function. Collectively, these results demonstrate coordinated metabolic responses across tissues in the hypoxemic fetus that limit glucose oxidation and increase lactate and pyruvate production. These responses may be mediated by hypoxemia-induced endocrine responses including increased norepinephrine and cortisol, which inhibit pancreatic insulin secretion resulting in lower insulin concentrations and decreased stimulation of glucose utilization.NEW & NOTEWORTHY Hypoxemia lowered fetal glucose oxidation rates, based on severity of hypoxemia, and increased lactate production. This was supported by tissue-specific metabolic responses that may result from increased norepinephrine and cortisol concentrations, which decrease pancreatic insulin secretion and insulin concentrations and decrease glucose utilization. This highlights the vulnerability of metabolic pathways in the fetus and demonstrates that constrained glucose oxidation may represent an early event in response to sustained hypoxemia and fetal growth restriction.

Uteroplacental nutrient flux and evidence for metabolic reprogramming during sustained hypoxemia

Gestational hypoxemia is often associated with reduced birth weight, yet how hypoxemia controls uteroplacental nutrient metabolism and supply to the fetus is unclear. This study tested the effects of maternal hypoxemia (HOX) between 0.8 and 0.9 gestation on uteroplacental nutrient metabolism and flux to the fetus in pregnant sheep. Despite hypoxemia, uteroplacental and fetal oxygen utilization and net glucose and lactate uptake rates were similar in HOX (n = 11) compared to CON (n = 7) groups. HOX fetuses had increased lactate and pyruvate concentrations and increased net pyruvate output to the utero-placenta. In the HOX group, uteroplacental flux of alanine to the fetus was decreased, as was glutamate flux from the fetus. HOX fetuses had increased alanine and decreased aspartate, serine, and glutamate concentrations. In HOX placental tissue, we identified hypoxic responses that should increase mitochondrial efficiency (decreased SDHB, increased COX4I2) and increase lactate production from pyruvate (increased LDHA protein and LDH activity, decreased LDHB and MPC2), both resembling metabolic reprogramming, but with evidence for decreased (PFK1, PKM2), rather than increased, glycolysis and AMPK phosphorylation. This supports a fetal-uteroplacental shuttle during sustained hypoxemia whereby uteroplacental tissues produce lactate as fuel for the fetus using pyruvate released from the fetus, rather than pyruvate produced from glucose in the placenta, given the absence of increased uteroplacental glucose uptake and glycolytic gene activation. Together, these results provide new mechanisms for how hypoxemia, independent of AMPK activation, regulates uteroplacental metabolism and nutrient allocation to the fetus, which allow the fetus to defend its oxidative metabolism and growth.

Rates of myogenesis and myofiber numbers are reduced in late gestation IUGR fetal sheep

Intrauterine growth-restricted (IUGR) fetuses are born with reduced skeletal muscle mass. We hypothesized that reduced rates of myogenesis would contribute to fewer and smaller myofibers in IUGR fetal hindlimb muscle compared to the normally growing fetus. We tested this hypothesis in IUGR fetal sheep with progressive placental insufficiency produced by exposing pregnant ewes to elevated ambient temperatures from 38 to 116 days gestation (dGA; term = 147 dGA). Surgically catheterized control (CON, n = 8) and IUGR (n = 13) fetal sheep were injected with intravenous 5-bromo-2′-deoxyuridine (BrdU) prior to muscle collection (134 dGA). Rates of myogenesis, defined as the combined processes of myoblast proliferation, differentiation, and fusion into myofibers, were determined in biceps femoris (BF), tibialis anterior (TA), and flexor digitorum superficialis (FDS) muscles. Total myofiber number was determined for the entire cross-section of the FDS muscle. In IUGR fetuses, the number of BrdU+ myonuclei per myofiber cross-section was lower in BF, TA, and FDS (P < 0.05), total myonuclear number per myofiber cross-section was lower in BF and FDS (P < 0.05), and total myofiber number was lower in FDS (P < 0.005) compared to CON. mRNA expression levels of cyclins, cyclin-dependent protein kinases, and myogenic regulatory factors were lower (P < 0.05), and inhibitors of the cell cycle were higher (P < 0.05) in IUGR BF compared to CON. Markers of apoptosis were not different in IUGR BF muscle. These results show that in IUGR fetuses, reduced rates of myogenesis produce fewer numbers of myonuclei, which may limit hypertrophic myofiber growth. Fewer myofibers of smaller size contribute to smaller muscle mass in the IUGR fetus.

Skeletal muscle amino acid uptake is lower and alanine production is greater in late gestation intrauterine growth-restricted fetal sheep hindlimb

In a sheep model of intrauterine growth restriction (IUGR) produced from placental insufficiency, late gestation fetuses had smaller skeletal muscle mass, myofiber area, and slower muscle protein accretion rates compared with normally growing fetuses. We hypothesized that IUGR fetal muscle develops adaptations that divert amino acids (AAs) from protein accretion and activate pathways that conserve substrates for other organs. We placed hindlimb arterial and venous catheters into late gestation IUGR (n = 10) and control (CON, n = 8) fetal sheep and included an external iliac artery flow probe to measure hindlimb AA uptake rates. Arterial and venous plasma samples and biceps femoris muscle were analyzed by mass spectrometry-based metabolomics. IUGR fetuses had greater abundance of metabolites enriched within the alanine, aspartate, and glutamate metabolism pathway compared with CON. Net uptake rates of branched-chain AA (BCAA) were lower by 42%-73%, and muscle ammoniagenic AAs (alanine, glycine, and glutamine) were lower by 107%-158% in IUGR hindlimbs versus CON. AA uptake rates correlated with hindlimb weight; the smallest hindlimbs showed net release of ammoniagenic AAs. Gene expression levels indicated a decrease in BCAA catabolism in IUGR muscle. Plasma purines were lower and plasma uric acid was higher in IUGR versus CON, possibly a reflection of ATP conservation. We conclude that IUGR skeletal muscle has lower BCAA uptake and develops adaptations that divert AAs away from protein accretion into alternative pathways that sustain global energy production and nitrogen disposal in the form of ammoniagenic AAs for metabolism in other organs.

Prolonged amino acid infusion into intrauterine growth-restricted fetal sheep increases leucine oxidation rates

Overcoming impaired growth in an intrauterine growth-restricted (IUGR) fetus has potential to improve neonatal morbidity, long-term growth, and metabolic health outcomes. The extent to which fetal anabolic capacity persists as the IUGR condition progresses is not known. We subjected fetal sheep to chronic placental insufficiency and tested whether prolonged amino acid infusion would increase protein accretion in these IUGR fetuses. IUGR fetal sheep were infused for 10 days with either mixed amino acids providing ~2 g·kg^-1·day^-1 (IUGR-AA) or saline (IUGR-Sal) during late gestation. At the end of the infusion, fetal plasma leucine, isoleucine, lysine, methionine, and arginine concentrations were higher in the IUGR-AA than IUGR-Sal group ( P < 0.05). Fetal plasma glucose, oxygen, insulin, IGF-1, cortisol, and norepinephrine concentrations were similar between IUGR groups, but glucagon concentrations were fourfold higher in the IUGR-AA group ( P < 0.05). Net umbilical amino acid uptake rate did not differ between IUGR groups; thus the total amino acid delivery rate (net umbilical amino acid uptake + infusion rate) was higher in the IUGR-AA than IUGR-Sal group (30 ± 4 vs. 19 ± 1 μmol·kg^-1·min^-1, P < 0.05). Net umbilical glucose, lactate, and oxygen uptake rates were similar between IUGR groups. Fetal leucine oxidation rate, measured using a leucine tracer, was higher in the IUGR-AA than IUGR-Sal group (2.5 ± 0.3 vs. 1.7 ± 0.3 μmol·kg^-1·min^-1, P < 0.05). Fetal protein accretion rate was not statistically different between the IUGR groups (1.6 ± 0.4 and 0.8 ± 0.3 μmol·kg^-1·min^-1 in IUGR-AA and IUGR-Sal, respectively) due to variability in response to amino acids. Prolonged amino acid infusion into IUGR fetal sheep increased leucine oxidation rates with variable anabolic response.

The impact of IUGR on pancreatic islet development and β-cell function

Placental insufficiency is a primary cause of intrauterine growth restriction (IUGR). IUGR increases the risk of developing type 2 diabetes mellitus (T2DM) throughout life, which indicates that insults from placental insufficiency impair β-cell development during the perinatal period because β-cells have a central role in the regulation of glucose tolerance. The severely IUGR fetal pancreas is characterized by smaller islets, less β-cells, and lower insulin secretion. Because of the important associations among impaired islet growth, β-cell dysfunction, impaired fetal growth, and the propensity for T2DM, significant progress has been made in understanding the pathophysiology of IUGR and programing events in the fetal endocrine pancreas. Animal models of IUGR replicate many of the observations in severe cases of human IUGR and allow us to refine our understanding of the pathophysiology of developmental and functional defects in islet from IUGR fetuses. Almost all models demonstrate a phenotype of progressive loss of β-cell mass and impaired β-cell function. This review will first provide evidence of impaired human islet development and β-cell function associated with IUGR and the impact on glucose homeostasis including the development of glucose intolerance and diabetes in adulthood. We then discuss evidence for the mechanisms regulating β-cell mass and insulin secretion in the IUGR fetus, including the role of hypoxia, catecholamines, nutrients, growth factors, and pancreatic vascularity. We focus on recent evidence from experimental interventions in established models of IUGR to understand better the pathophysiological mechanisms linking placental insufficiency with impaired islet development and β-cell function.

PHYSIOLOGY AND ENDOCRINOLOGY SYMPOSIUM: Roles for insulin-supported skeletal muscle growth

Basic principles governing skeletal muscle growth and development, from a cellular point of view, have been realized for several decades. Skeletal muscle is marked by the capacity for rapid hypertrophy and increases in protein content. Ultimately, skeletal muscle growth is controlled by 2 basic means: 1) myonuclear accumulation stemming from satellite cell (myoblast) proliferation and 2) the balance of protein synthesis and degradation. Each process underlies the rapid changes in lean tissue accretion evident during fetal and neonatal growth and is particularly sensitive to nutritional manipulation. Although multiple signals converge to alter skeletal muscle mass, postprandial changes in the anabolic hormone insulin link feed intake with enhanced rates of protein synthesis in the neonate. Indeed, a consequence of insulin-deficient states such as malnutrition is reduced myoblast activity and a net loss of body protein. A well-characterized mechanism mediating the anabolic effect of insulin involves the phosphatidylinositol 3-kinase (PI3K)-mammalian target of rapamycin (mTOR) signaling pathway. Activation of mTOR leads to translation initiation control via the phosphorylation of downstream targets. Modulation of this pathway by insulin, as well as by other hormones and nutrients, accounts for enhanced protein synthesis leading to efficient lean tissue accretion and rapid skeletal muscle gain in the growing animal. Dysfunctional insulin activity during fetal and neonatal stages likely alters growth through cellular and protein synthetic capacities.

Exogenous amino acids suppress glucose oxidation and potentiate hepatic glucose production in late gestation fetal sheep

Acute amino acid (AA) infusion increases AA oxidation rates in normal late gestation fetal sheep. Because the fetal oxygen consumption rate does not change with increased AA oxidation, we hypothesized that AA infusion would suppress glucose oxidation pathways and that the additional carbon supply from AA would activate hepatic glucose production. To test this, late gestation fetal sheep were infused intravenously for 3 h with saline or exogenous AA (AA). Glucose tracer metabolic studies were performed and skeletal muscle and liver tissues samples were collected. AA infusion increased fetal arterial plasma branched chain AA, cortisol, and glucagon concentrations. Fetal glucose utilization rates were similar between basal and AA periods, yet the fraction of glucose oxidized and the glucose oxidation rate were decreased by 40% in the AA period. AA infusion increased expression of PDK4, an inhibitor of glucose oxidation, nearly twofold in muscle and liver. In liver, AA infusion tended to increase PCK1 gluconeogenic gene and PCK1 correlated with plasma cortisol concentrations. AA infusion also increased liver mRNA expression of the lactate transporter gene (MCT1), protein expression of GLUT2 and LDHA, and phosphorylation of AMPK, 4EBP1, and S6 proteins. In isolated fetal hepatocytes, AA supplementation increased glucose production and PCK1, LDHA, and MCT1 gene expression. These results demonstrate that AA infusion into fetal sheep competitively suppresses glucose oxidation and potentiates hepatic glucose production. These metabolic patterns support flexibility in fetal metabolism in response to increased nutrient substrate supply while maintaining a relatively stable rate of oxidative metabolism.

Impact of placental insufficiency on fetal skeletal muscle growth

Intrauterine growth restriction (IUGR) caused by placental insufficiency is one of the most common and complex problems in perinatology, with no known cure. In pregnancies affected by placental insufficiency, a poorly functioning placenta restricts nutrient supply to the fetus and prevents normal fetal growth. Among other significant deficits in organ development, the IUGR fetus characteristically has less lean body and skeletal muscle mass than their appropriately-grown counterparts. Reduced skeletal muscle growth is not fully compensated after birth, as individuals who were born small for gestational age (SGA) from IUGR have persistent reductions in muscle mass and strength into adulthood. The consequences of restricted muscle growth and accelerated postnatal "catch-up" growth in the form of adiposity may contribute to the increased later life risk for visceral adiposity, peripheral insulin resistance, diabetes, and cardiovascular disease in individuals who were formerly IUGR. This review will discuss how an insufficient placenta results in impaired fetal skeletal muscle growth and how lifelong reductions in muscle mass might contribute to increased metabolic disease risk in this vulnerable population.

Chronically Increased Amino Acids Improve Insulin Secretion, Pancreatic Vascularity, and Islet Size in Growth-Restricted Fetal Sheep

Placental insufficiency is associated with reduced supply of amino acids to the fetus and leads to intrauterine growth restriction (IUGR). IUGR fetuses are characterized by lower glucose-stimulated insulin secretion, smaller pancreatic islets with less β-cells, and impaired pancreatic vascularity. To test whether supplemental amino acids infused into the IUGR fetus could improve these complications of IUGR we used acute (hours) and chronic (11 d) direct fetal amino acid infusions into a sheep model of placental insufficiency and IUGR near the end of gestation. IUGR fetuses had attenuated acute amino acid-stimulated insulin secretion compared with control fetuses. These results were confirmed in isolated IUGR pancreatic islets. After the chronic fetal amino acid infusion, fetal glucose-stimulated insulin secretion and islet size were restored to control values. These changes were associated with normalization of fetal pancreatic vascularity and higher fetal pancreatic vascular endothelial growth factor A protein concentrations. These results demonstrate that decreased fetal amino acid supply contributes to the pathogenesis of pancreatic islet defects in IUGR. Moreover, the results show that pancreatic islets in IUGR fetuses retain their ability to respond to increased amino acids near the end of gestation after chronic fetal growth restriction.

Challenges in nourishing the intrauterine growth-restricted foetus - Lessons learned from studies in the intrauterine growth-restricted foetal sheep

Previous attempts to improve growth and development of the intrauterine growth-restricted (IUGR) foetus during pregnancy have not worked or caused harm. Our research identifies tissue-specific mechanisms underlying foetal growth restriction and then tests strategies to improve growth and ameliorate many of the metabolic problems before the infant is born. The goal of our studies is to reduce the impact of foetal growth restriction at critical stages of development on the lifelong complications of IUGR offspring.

CONCLUSION

Defining specific mechanisms that cause growth restriction in the foetus might identify specific nutrients and hormones that could be given to the mother to improve foetal growth and reduce metabolic complications, using strategies first tested in our IUGR animal model.

Chronic Hyperinsulinemia Increases Myoblast Proliferation in Fetal Sheep Skeletal Muscle

Insulin is an important fetal growth factor. However, chronic experimental hyperinsulinemia in the fetus fails to accelerate linear and lean mass growth beyond normal rates. Mechanisms preventing accelerated lean mass accretion during hyperinsulinemia are unknown. To address potential mechanisms, late-gestation fetal sheep were infused with iv insulin and glucose to produce euglycemic hyperinsulinemia (INS) or saline for 7-9 days. Fetal substrate uptake and protein metabolic rates were measured. INS fetuses had 1.5-fold higher insulin concentrations (P < .0001) and equivalent glucose concentrations. INS fetuses had 20% more Pax7(+) nuclei in the biceps femoris, which indicates the potential for hyperinsulinemia to increase the number of myoblasts within late-gestation fetal skeletal muscle. Additionally, the percentage of Pax7(+) myoblasts that expressed Ki-67 was 1.3-fold higher and expression of myogenic regulatory factors was 50% lower in INS fetuses (MYF5 and MYOG [myogenin], P < .005), which indicates a shift toward myoblast proliferation over differentiation. There were no differences for fetal body, organ, or muscle weights, although INS placentas weighed 28% less (P < .05). Protein synthesis and accretion rates did not change in INS fetuses, nor did fiber muscle size. Essential amino acid concentrations were lower in the INS group (P < .05) except for tryptophan. Umbilical blood flow, net total amino acids, and O2 uptakes rates did not differ between groups. Arterial O2 content was 33% lower (P < .005) and norepinephrine was 100% higher in the INS fetuses (P < .01), all of which are factors that may counteract fetal protein accretion during hyperinsulinemia despite an increase in myoblast proliferation.

Nutritional plane of twin-bearing ewes alters fetal mammary gland biochemical composition and mTOR/MAPK pathway signaling

Identifying the biochemical changes and molecular pathways that regulate fetal mammary development in response to maternal nutrition is important for understanding the link between fetal programming of mammary development and future lactation performance. Although there are published studies regarding biochemical changes in the developing mammary gland, there are currently no data on molecular pathway involvement in regulating ruminant fetal mammary development. This study investigated changes in fetal mammary biochemical indices and mechanistic target of rapamycin (mTOR)/mitogen activated protein kinase (MAPK) signaling at d 100 and 140 of gestation in an ovine model of restricted maternal nutrition. Ewes were randomly allocated to ad libitum (A) or maintenance (M) nutritional regimens, under New Zealand pastoral grazing conditions, from d 21 to 140 of pregnancy. At d 100 and 140 of pregnancy, a subgroup of twin-bearing dams was euthanized, and whole fetal mammary glands (fiber, skin, fat, and ducts) were collected. Mammary glands of fetuses carried by M-fed dams were heavier at d 100 than those of fetuses carried by A-fed dams ( = 0.03), with no difference in the abundance of mTOR/MAPK signaling proteins observed. At d 140, mammary glands of fetuses carried by M-fed dams were lighter ( = 0.07) than fetuses carried by A-fed dams because of decreased hyperplasia ( = 0.04) and hypertrophy ( = 0.09) but had increased protein synthetic capacity ( = 0.02). Increased protein synthetic capacity was associated with increased abundance of MAPK pathway signaling proteins eukaryotic intiation factor 4E (eIF4E)/eIF4E and mTOR pathway signaling proteins eukaryotic initiation factor 4E-binding protein 1 (4E-BP1)/4E-BP1 and ribosomal protein S6 (RPS6)/RPS6 ( ≤ 0.05). Increased abundance of MAPK/mTOR pathway proteins is proposed to mediate increased protein synthetic capacity via ribosome biogenesis and the availability of factors required to initiate protein translation. The primary regulator of 4E-BP1 phosphorylation at Ser65 and RPS6 at Ser235/236 is the activated form of mTOR: mTOR. To study potential tissue-specific mTOR, mTOR abundance mammary glands, separated into parenchyma and fat pad, were collected from d 140 fetuses carried by dams fed a lucerne-based pellet diet formulated to meet 100% of the NRC-recommended maintenance requirements. Results showed that the abundance of mTOR was primarily localized to the fat pad, indicating that the fat pad plays a potential role in regulating development of the fetal mammary gland.

Increased adrenergic signaling is responsible for decreased glucose-stimulated insulin secretion in the chronically hyperinsulinemic ovine fetus

Insulin may stimulate its own insulin secretion and is a potent growth factor for the pancreatic β-cell. Complications of pregnancy, such as diabetes and intrauterine growth restriction, are associated with changes in fetal insulin concentrations, secretion, and β-cell mass. However, glucose concentrations are also abnormal in these conditions. The direct effect of chronic fetal hyperinsulinemia with euglycemia on fetal insulin secretion and β-cell mass has not been tested. We hypothesized that chronic fetal hyperinsulinemia with euglycemia would increase glucose-stimulated insulin secretion (GSIS) and β-cell mass in the ovine fetus. Singleton ovine fetuses were infused with iv insulin to produce high physiological insulin concentrations, or saline for 7-10 days. The hyperinsulinemic animals also received a direct glucose infusion to maintain euglycemia. GSIS, measured at 133 ± 1 days of gestation, was significantly attenuated in the hyperinsulinemic fetuses (P < .05). There was no change in β-cell mass. The hyperinsulinemic fetuses also had decreased oxygen (P < .05) and higher norepinephrine (1160 ± 438 vs 522 ± 106 pg/mL; P < .005). Acute pharmacologic adrenergic blockade restored GSIS in the hyperinsulinemic-euglycemic fetuses, demonstrating that increased adrenergic signaling mediates decreased GSIS in these fetuses.

A physiological increase in insulin suppresses muscle-specific ubiquitin ligase gene activation in fetal sheep with sustained hypoglycemia

Decreased glucose transfer to the fetus is characteristic of pregnancies complicated by maternal under nutrition and placental insufficiency. Chronic experimental restriction of glucose transfer to the sheep fetus for the final 40% of gestation with a maternal insulin infusion (HG fetuses) results in fetal hypoglycemia, hypoinsulinemia, and decreased rates of fetal growth and protein accretion compared to controls (CON). Lower rates of fetal protein accretion are due to increased fetal protein breakdown and not decreased protein synthesis. However, the specific skeletal muscle pathways responsible for increased protein breakdown have not been determined. Nor has it been determined if low fetal glucose or insulin concentrations are more important for regulating these skeletal muscle protein breakdown pathways. We tested whether chronic restriction of glucose transfer to the fetus increased the ubiquitin-proteosome pathway or autophagy-lysosome pathway in fetal sheep skeletal muscle and found no evidence for an increase in the autophagy-lysosome pathway. However, HG fetuses had increase mRNA expression of MaFBx1 (twofold, P < 0.01) and a trend for increased mRNA expression of MuRF1 (P = 0.08) compared to CON. A subset of chronically hypoglycemic fetuses received an isoglycemic insulin infusion for the final 7 days of the maternal insulin infusion (HG + INS fetuses) and had MaFBx1 and MuRF1 mRNA concentrations similar to CON fetuses. These results demonstrate that fetuses exposed to sustained hypoglycemia have decreased protein accretion due to activation of the skeletal muscle ubiquitin-proteosome pathway and that a fetal hyperinsulinemic clamp can suppress this pathway even in the context of continued hypoglycemia.

Endocrine regulation of fetal skeletal muscle growth: impact on future metabolic health

Establishing sufficient skeletal muscle mass is essential for lifelong metabolic health. The intrauterine environment is a major determinant of the muscle mass that is present during the life course of an individual, because muscle fiber number is set at the time of birth. Thus, a compromised intrauterine environment from maternal nutrient restriction or placental insufficiency that restricts muscle fiber number can have permanent effects on the amount of muscle an individual will live with. Reduced muscle mass due to fewer muscle fibers persists even after compensatory or 'catch-up' postnatal growth occurs. Furthermore, muscle hypertrophy can only partially compensate for this limitation in fiber number. Compelling associations link low birth weight and decreased muscle mass to future insulin resistance, which can drive the development of the metabolic syndrome and type 2 diabetes, and the risk of cardiovascular events later in life. There are gaps in knowledge about the origins of reduced muscle growth at the cellular level and how these patterns are set during fetal development. By understanding the nutrient and endocrine regulation of fetal skeletal muscle growth and development, we can direct research efforts toward improving muscle growth early in life to prevent the development of chronic metabolic diseases later in life.

Muscle free amino acid profiles are related to differences in skeletal muscle growth between single and twin ovine fetuses near term

Twin sheep fetuses have reduced skeletal muscle weight near birth relative to singles as a result of restricted muscle hypertrophy. Intracellular free amino acids (FAA) are reported to regulate metabolic pathways which control muscle protein accretion, whereby reduced intracellular content of specific FAA may reduce their activation and therefore, muscle hypertrophy. The aim of this study was to determine whether differences in muscle weight between singleton and twin fetuses, under different maternal conditions is associated with reduced concentration of specific FAA. The FAA content in the semitendinosus muscle (ST) in singleton and twin fetuses (rank) at 140 days of gestation from heavy (H) or light (L) ewes fed ad libitum (A) or maintenance (M) level of nutrition was measured. Muscle weight was reduced in twin fetuses compared to singletons in all groups. Reduced concentrations of leucine, threonine and valine, but higher concentrations of methionine, ornithine, lysine and serine were found in twin fetuses compared to singletons. Maternal size and nutrition interaction with rank resulted in reduced glutamine in twins from HM-ewes (H-ewes under M nutrition) compared to their singleton counterparts. Maternal weight interaction with pregnancy rank reduced the concentration of arginine in twins, with a larger effect on H-ewes compared with L-ewes. Maternal size interaction with pregnancy rank resulted in twins from M-ewes to have lower alanine, while twins from A-ewes had lower aspartic acid concentration compared to singletons. The ST muscle weight was positively correlated only with arginine concentration after taking into account rank, size and nutrition. The present results indicate that reduced concentrations of specific intracellular FAA, such as arginine, leucine, valine, glutamine, which are known to play a role in muscle growth, could be acting as limiting factors for muscle hypertrophy in twin fetuses during late gestation. Ewe size and nutrition can influence the concentration of specific FAA in muscle and should be considered in any intervention plan to improve twin fetal muscle growth.

Hormonal and nutritional drivers of intrauterine growth

PURPOSE OF REVIEW

Size at birth is critical in determining life expectancy with both small and large neonates at risk of shortened life spans. This review examines the hormonal and nutritional drivers of intrauterine growth with emphasis on the role of foetal hormones as nutritional signals in utero.

RECENT FINDINGS

Nutrients drive intrauterine growth by providing substrate for tissue accretion, whereas hormones regulate nutrient distribution between foetal oxidative metabolism and mass accumulation. The main hormonal drivers of intrauterine growth are insulin, insulin-like growth factors and thyroid hormones. Together with leptin and cortisol, these hormones control cellular nutrient uptake and the balance between accretion and differentiation in regulating tissue growth. They also act indirectly via the placenta to alter the materno-foetal supply of nutrients and oxygen. By responding to nutrient and oxygen availability, foetal hormones optimize the survival and growth of the foetus with respect to its genetic potential, particularly during adverse conditions. However, changes in the intrauterine growth of individual tissues may alter their function permanently.

SUMMARY

In both normal and compromised pregnancies, intrauterine growth is determined by multiple hormonal and nutritional drivers which interact to produce a specific pattern of intrauterine development with potential lifelong consequences for health.

Increased amino acid supply potentiates glucose-stimulated insulin secretion but does not increase β-cell mass in fetal sheep

Amino acids and glucose acutely stimulate fetal insulin secretion. In isolated adult pancreatic islets, amino acids potentiate glucose-stimulated insulin secretion (GSIS), but whether amino acids have this same effect in the fetus is unknown. Therefore, we tested the effects of increased fetal amino acid supply on GSIS and morphology of the pancreas. We hypothesized that increasing fetal amino acid supply would potentiate GSIS. Singleton fetal sheep received a direct intravenous infusion of an amino acid mixture (AA) or saline (CON) for 10-14 days during late gestation to target a 25-50% increase in fetal branched-chain amino acids (BCAA). Early-phase GSIS increased 150% in the AA group (P < 0.01), and this difference was sustained for the duration of the hyperglycemic clamp (105 min) (P < 0.05). Glucose-potentiated arginine-stimulated insulin secretion (ASIS), pancreatic insulin content, and pancreatic glucagon content were similar between groups. β-Cell mass and area were unchanged between groups. Baseline and arginine-stimulated glucagon concentrations were increased in the AA group (P < 0.05). Pancreatic α-cell mass and area were unchanged. Fetal and pancreatic weights were similar. We conclude that a sustained increase of amino acid supply to the normally growing late-gestation fetus potentiated fetal GSIS but did not affect the morphology or insulin content of the pancreas. We speculate that increased β-cell responsiveness (insulin secretion) following increased amino acid supply may be due to increased generation of secondary messengers in the β-cell. This may be enhanced by the paracrine action of glucagon on the β-cell.

Effects of cortisol and dexamethasone on insulin signalling pathways in skeletal muscle of the ovine fetus during late gestation

Before birth, glucocorticoids retard growth, although the extent to which this is mediated by changes in insulin signalling pathways in the skeletal muscle of the fetus is unknown. The current study determined the effects of endogenous and synthetic glucocorticoid exposure on insulin signalling proteins in skeletal muscle of fetal sheep during late gestation. Experimental manipulation of fetal plasma glucocorticoid concentration was achieved by fetal cortisol infusion and maternal dexamethasone treatment. Cortisol infusion significantly increased muscle protein levels of Akt2 and phosphorylated Akt at Ser473, and decreased protein levels of phosphorylated forms of mTOR at Ser2448 and S6K at Thr389. Muscle GLUT4 protein expression was significantly higher in fetuses whose mothers were treated with dexamethasone compared to those treated with saline. There were no significant effects of glucocorticoid exposure on muscle protein abundance of IR-β, IGF-1R, PKCζ, Akt1, calpastatin or muscle glycogen content. The present study demonstrated that components of the insulin signalling pathway in skeletal muscle of the ovine fetus are influenced differentially by naturally occurring and synthetic glucocorticoids. These findings may provide a mechanism by which elevated concentrations of endogenous glucocorticoids retard fetal growth.

Acute supplementation of amino acids increases net protein accretion in IUGR fetal sheep

Placental insufficiency decreases fetal amino acid uptake from the placenta, plasma insulin concentrations, and protein accretion, thus compromising normal fetal growth trajectory. We tested whether acute supplementation of amino acids or insulin into the fetus with intrauterine growth restriction (IUGR) would increase net fetal protein accretion rates. Late-gestation IUGR and control (CON) fetal sheep received acute, 3-h infusions of amino acids (with euinsulinemia), insulin (with euglycemia and euaminoacidemia), or saline. Fetal leucine metabolism was measured under steady-state conditions followed by a fetal muscle biopsy to quantify insulin signaling. In CON, increasing amino acid delivery rates to the fetus by 100% increased leucine oxidation rates by 100%. In IUGR, amino acid infusion completely suppressed fetal protein breakdown rates but increased leucine oxidation rate by only 25%, resulting in increased protein accretion rates by 150%. Acute insulin infusion, however, had very little effect on amino acid delivery rates, fetal leucine disposal rates, or fetal protein accretion rates in CON or IUGR fetuses despite robust signaling of the fetal skeletal muscle insulin-signaling cascade. These results indicate that, when amino acids are given directly into the fetal circulation independently of changes in insulin concentrations, IUGR fetal sheep have suppressed protein breakdown rates, thus increasing net fetal protein accretion.

Developmental programming in response to intrauterine growth restriction impairs myoblast function and skeletal muscle metabolism

Fetal adaptations to placental insufficiency alter postnatal metabolic homeostasis in skeletal muscle by reducing glucose oxidation rates, impairing insulin action, and lowering the proportion of oxidative fibers. In animal models of intrauterine growth restriction (IUGR), skeletal muscle fibers have less myonuclei at birth. This means that myoblasts, the sole source for myonuclei accumulation in fibers, are compromised. Fetal hypoglycemia and hypoxemia are complications that result from placental insufficiency. Hypoxemia elevates circulating catecholamines, and chronic hypercatecholaminemia has been shown to reduce fetal muscle development and growth. We have found evidence for adaptations in adrenergic receptor expression profiles in myoblasts and skeletal muscle of IUGR sheep fetuses with placental insufficiency. The relationship of β-adrenergic receptors shifts in IUGR fetuses because Adrβ2 expression levels decline and Adrβ1 expression levels are unaffected in myofibers and increased in myoblasts. This adaptive response would suppress insulin signaling, myoblast incorporation, fiber hypertrophy, and glucose oxidation. Furthermore, this β-adrenergic receptor expression profile persists for at least the first month in IUGR lambs and lowers their fatty acid mobilization. Developmental programming of skeletal muscle adrenergic receptors partially explains metabolic and endocrine differences in IUGR offspring, and the impact on metabolism may result in differential nutrient utilization.

Prolonged infusion of amino acids increases leucine oxidation in fetal sheep

Maternal high-protein supplements designed to increase birth weight have not been successful. We recently showed that maternal amino acid infusion into pregnant sheep resulted in competitive inhibition of amino acid transport across the placenta and did not increase fetal protein accretion rates. To bypass placental transport, singleton fetal sheep were intravenously infused with an amino acid mixture (AA, n = 8) or saline [control (Con), n = 10] for ∼12 days during late gestation. Fetal leucine oxidation rate increased in the AA group (3.1 ± 0.5 vs. 1.4 ± 0.6 μmol·min(-1)·kg(-1), P < 0.05). Fetal protein accretion (2.6 ± 0.5 and 2.2 ± 0.6 μmol·min(-1)·kg(-1) in AA and Con, respectively), synthesis (6.2 ± 0.8 and 7.0 ± 0.9 μmol·min(-1)·kg(-1) in AA and Con, respectively), and degradation (3.6 ± 0.6 and 4.5 ± 1.0 μmol·min(-1)·kg(-1) in AA and Con, respectively) rates were similar between groups. Net fetal glucose uptake decreased in the AA group (2.8 ± 0.4 vs. 3.9 ± 0.1 mg·kg(-1)·min(-1), P < 0.05). The glucose-O(2) quotient also decreased over time in the AA group (P < 0.05). Fetal insulin and IGF-I concentrations did not change. Fetal glucagon increased in the AA group (119 ± 24 vs. 59 ± 9 pg/ml, P < 0.05), and norepinephrine (NE) also tended to increase in the AA group (785 ± 181 vs. 419 ± 76 pg/ml, P = 0.06). Net fetal glucose uptake rates were inversely proportional to fetal glucagon (r(2) = 0.38, P < 0.05), cortisol (r(2) = 0.31, P < 0.05), and NE (r(2) = 0.59, P < 0.05) concentrations. Expressions of components in the mammalian target of rapamycin signaling pathway in fetal skeletal muscle were similar between groups. In summary, prolonged infusion of amino acids directly into normally growing fetal sheep increased leucine oxidation. Amino acid-stimulated increases in fetal glucagon, cortisol, and NE may contribute to a shift in substrate oxidation by the fetus from glucose to amino acids.

Inhibition of mammalian target of rapamycin signaling by CCI-779 (temsirolimus) induces growth inhibition and cell cycle arrest in Cashmere goat fetal fibroblasts (Capra hircus)

The mammalian target of rapamycin (mTOR) is a Ser/Thr kinase. It plays an evolutionarily conserved role in regulating cell growth, proliferation, survival, and metabolism via different cellular processes. The purpose of this study was to explore the inhibitory effects of CCI-779 (temsirolimus), a specific mTOR inhibitor, on mTOR signaling, and examine the mechanism of cell growth suppression by CCI-779 in Cashmere goat fetal fibroblasts (GFb cells). GFb cells were sensitive to CCI-779 and the survival rate of cells treated with >3.0 μM of CCI-779 was significantly reduced compared with the control (p<0.01). CCI-779 inhibited the phosphorylation of mTOR (at Ser2448) and S6 (at Ser240/244), and the expression of mTOR, p70S6K, and S6. Thus, CCI-779 was toxic to GFb cells, and it induced a dose-dependent decrease in cell proliferation and caused G1/S cell cycle arrest. Taken together, these data show that CCI-779 can inhibit mTOR signaling and proliferation in GFb cells in vitro. Therefore, mTOR is an important regulator for GFb cell growth and proliferation.

The intrauterine growth restriction phenotype: fetal adaptations and potential implications for later life insulin resistance and diabetes

The intrauterine growth restricted (IUGR) fetus develops unique metabolic adaptations in response to exposure to reduced nutrient supply. These adaptations provide survival value for the fetus by enhancing the capacity of the fetus to take up and use nutrients, thereby reducing the need for nutrient supply. Each organ and tissue in the fetus adapts differently, with the brain showing the greatest capacity for maintaining nutrient supply and growth. Such adaptations, if persistent, also have the potential in later life to promote nutrient uptake and storage, which directly lead to complications of obesity, insulin resistance, reduced insulin production, and type 2 diabetes.

Maternal amino acid supplementation for intrauterine growth restriction

Maternal dietary protein supplementation to improve fetal growth has been considered as an option to prevent or treat intrauterine growth restriction. However, in contrast to balanced dietary supplementation, adverse perinatal outcomes in pregnant women who received high amounts of dietary protein supplementation have been observed. The responsible mechanisms for these adverse outcomes are unknown. This review will discuss relevant human and animal data to provide the background necessary for the development of explanatory hypotheses and ultimately for the development therapeutic interventions during pregnancy to improve fetal growth. Relevant aspects of fetal amino acid metabolism during normal pregnancy and those pregnancies affected by IUGR will be discussed. In addition, data from animal experiments which have attempted to determine mechanisms to explain the adverse responses identified in the human trials will be presented. Finally, we will suggest new avenues for investigation into how amino acid supplementation might be used safely to treat and/or prevent IUGR.

Prolonged maternal amino acid infusion in late-gestation pregnant sheep increases fetal amino acid oxidation

Protein supplementation during human pregnancy does not improve fetal growth and may increase small-for-gestational-age birth rates and mortality. To define possible mechanisms, sheep with twin pregnancies were infused with amino acids (AA group, n = 7) or saline (C group, n = 4) for 4 days during late gestation. In the AA group, fetal plasma leucine, isoleucine, valine, and lysine concentrations were increased (P < 0.05), and threonine was decreased (P < 0.05). In the AA group, fetal arterial pH (7.365 +/- 0.007 day 0 vs. 7.336 +/- 0.012 day 4, P < 0.005), hemoglobin-oxygen saturation (46.2 +/- 2.6 vs. 37.8 +/- 3.6%, P < 0.005), and total oxygen content (3.17 +/- 0.17 vs. 2.49 +/- 0.20 mmol/l, P < 0.0001) were decreased on day 4 compared with day 0. Fetal leucine disposal did not change (9.22 +/- 0.73 vs. 8.09 +/- 0.63 micromol x min(-1) x kg(-1), AA vs. C), but the rate of leucine oxidation increased 43% in the AA group (2.63 +/- 0.16 vs. 1.84 +/- 0.24 micromol x min(-1) x kg(-1), P < 0.05). Fetal oxygen utilization tended to be increased in the AA group (327 +/- 23 vs. 250 +/- 29 micromol x min(-1) x kg(-1), P = 0.06). Rates of leucine incorporation into fetal protein (5.19 +/- 0.97 vs. 5.47 +/- 0.89 micromol x min(-1) x kg(-1), AA vs. C), release from protein breakdown (4.20 +/- 0.95 vs. 4.62 +/- 0.74 micromol x min(-1) x kg(-1)), and protein accretion (1.00 +/- 0.30 vs. 0.85 +/- 0.25 micromol x min(-1) x kg(-1)) did not change. Consistent with these data, there was no change in the fetal skeletal muscle ubiquitin ligases MaFBx1 or MuRF1 or in the protein synthesis regulators 4E-BP1, eEF2, eIF2alpha, and p70(S6K). Decreased concentrations of certain essential amino acids, increased amino acid oxidation, fetal acidosis, and fetal hypoxia are possible mechanisms to explain fetal toxicity during maternal amino acid supplementation.

Intrauterine growth restriction increases fetal hepatic gluconeogenic capacity and reduces messenger ribonucleic acid translation initiation and nutrient sensing in fetal liver and skeletal muscle

Expression of key metabolic genes and proteins involved in mRNA translation, energy sensing, and glucose metabolism in liver and skeletal muscle were investigated in a late-gestation fetal sheep model of placental insufficiency intrauterine growth restriction (PI-IUGR). PI-IUGR fetuses weighed 55% less; had reduced oxygen, glucose, isoleucine, insulin, and IGF-I levels; and had 40% reduction in net branched chain amino acid uptake. In PI-IUGR skeletal muscle, levels of insulin receptor were increased 80%, whereas phosphoinositide-3 kinase (p85) and protein kinase B (AKT2) were reduced by 40%. Expression of eukaryotic initiation factor-4e was reduced 45% in liver, suggesting a unique mechanism limiting translation initiation in PI-IUGR liver. There was either no change (AMP activated kinase, mammalian target of rapamycin) or a paradoxical decrease (protein phosphatase 2A, eukaryotic initiation factor-2 alpha) in activation of major energy and cell stress sensors in PI-IUGR liver and skeletal muscle. A 13- to 20-fold increase in phosphoenolpyruvate carboxykinase and glucose 6 phosphatase mRNA expression in the PI-IUGR liver was-associated with a 3-fold increase in peroxisome proliferator-activated receptor-gamma coactivator-1 alpha mRNA and increased phosphorylation of cAMP response element binding protein. Thus PI-IUGR is-associated with reduced branched chain amino acid uptake and growth factors, yet up-regulation of proximal insulin signaling and a marked increase in the gluconeogenic pathway. Lack of activation of several energy and stress sensors in fetal liver and skeletal muscle, despite hypoxia and low energy status, suggests a novel strategy for survival in the PI-IUGR fetus but with potential maladaptive consequences for reduced nutrient sensing and insulin sensitivity in postnatal life.

Effects of chronic hypoglycemia and euglycemic correction on lysine metabolism in fetal sheep

In this study, we determined rates of lysine metabolism in fetal sheep during chronic hypoglycemia and following euglycemic recovery and compared results with normal, age-matched euglycemic control fetuses to explain the adaptive response of protein metabolism to low glucose concentrations. Restriction of the maternal glucose supply to the fetus lowered the net rates of fetal (umbilical) glucose (42%) and lactate (36%) uptake, causing compensatory alterations in fetal lysine metabolism. The plasma lysine concentration was 1.9-fold greater in hypoglycemic compared with control fetuses, but the rate of fetal (umbilical) lysine uptake was not different. In the hypoglycemic fetuses, the lysine disposal rate also was higher than in control fetuses due to greater rates of lysine flux back into the placenta and into fetal tissue. The rate of CO2 excretion from lysine decarboxylation was 2.4-fold higher in hypoglycemic than control fetuses, indicating greater rates of lysine oxidative metabolism during chronic hypoglycemia. No differences were detected for rates of fetal protein accretion or synthesis between hypoglycemic and control groups, although there was a significant increase in the rate of protein breakdown (P<0.05) in the hypoglycemic fetuses, indicating small changes in each rate. This was supported by elevated muscle specific ubiquitin ligases and greater concentrations of 4E-BP1. Euglycemic recovery after chronic hypoglycemia normalized all fluxes and actually lowered the rate of lysine decarboxylation compared with control fetuses (P<0.05). These results indicate that chronic hypoglycemia increases net protein breakdown and lysine oxidative metabolism, both of which contribute to slower rates of fetal growth over time. Furthermore, euglycemic correction for 5 days returns lysine fluxes to normal and causes an overcorrection of lysine oxidation.

Maternal obesity downregulates myogenesis and beta-catenin signaling in fetal skeletal muscle

Skeletal muscle is one of the primary tissues responsible for insulin resistance and type 2 diabetes (T2D). The fetal stage is crucial for skeletal muscle development. Obesity induces inflammatory responses, which might regulate myogenesis through Wnt/beta-catenin signaling. This study evaluated the effects of maternal obesity (>30% increase in body mass index) during pregnancy on myogenesis and the Wnt/beta-catenin and IKK/NF-kappaB pathways in fetal skeletal muscle using an obese pregnant sheep model. Nonpregnant ewes were assigned to a control group (C; fed 100% of National Research Council recommendations; n=5) or obesogenic (OB; fed 150% of National Research Council recommendations; n=5) diet from 60 days before to 75 days after conception (term approximately 148 days) when fetal semitendenosus skeletal muscle was sampled for analyses. Myogenic markers including MyoD, myogenin, and desmin contents were reduced in OB compared with C fetal semitendenosus, indicating the downregulation of myogenesis. The diameter of primary muscle fibers was smaller in OB fetal muscle. Phosphorylation of GSK3beta was reduced in OB compared with C fetal semitendenosus. Although the beta-catenin level was lower in OB than C fetal muscle, more beta-catenin was associated with FOXO3a in the OB fetuses. Moreover, we found phosphorylation levels of IKKbeta and RelA/p65 were both increased in OB fetal muscle. In conclusion, our data showed that myogenesis and the Wnt/beta-catenin signaling pathway were downregulated, which might be due to the upregulation of inflammatory IKK/NF-kappaB signaling pathways in fetal muscle of obese mothers.