Chronic Hyperinsulinemia Increases Myoblast Proliferation in Fetal Sheep Skeletal Muscle

Abstract

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.

Dual effects of obesity on satellite cells and muscle regeneration

“…Because muscle regeneration is a complex process which involves a hierarchy of cellular events, including but not limited to SCs, it is reasonable to speculate that the negative impacts of HFD on SCs may be somehow buffered in vivo as compared to in vitro. For example, HFD mice have insulin resistance and exhibit hyperinsulinemia with a high level of circulating insulin, a well-known myoblast proliferation and differentiation enhancer (Allen & Rankin, 1990;Brown et al, 2016;Prentki & Nolan, 2006), which may boost SC function and thus compensate HFD-caused SC harm in vivo, at least in a temporary manner. Thus, future studies focusing on the signaling pathways emanating from the HFD niche will help expound the molecular mechanism responsible for SC homeostasis in a given pathological setting, which, in our case, is obesity.…”

Myoblast replication is reduced in the IUGR fetus despite maintained proliferative capacity in vitro

“…Isopentane-preserved biceps femoris, tibialis anterior, and soleus muscles were removed from corkboard and 14 μm sections were prepared as previously described (Brown, et al 2016). Muscle sections were incubated with the following antibodies: anti-Pax7 mouse monoclonal IgG (1:100; Developed by Tokyo Institute of Technology and obtained from the Developmental Studies Hybridoma Bank, created by the NICHD of the NIH and maintained at The University of Iowa, Department of Biology, Iowa City, IA), anti-Ki-67 rabbit monoclonal IgG (1:250; Cell Signaling Technology, Danvers, MA), anti-laminin rabbit polyclonal IgG (1:100, Sigma-Aldrich, St. Louis, MO), anti-myosin heavy chain (MHC) Type I (slow) mouse monoclonal IgG (1:20; Vector Laboratories, Burlingame, CA), and anti-MHC Type IIa mouse IgG (1:2; a gift from Dr. Leslie Leinwand, Boulder, CO).…”

“…NE-PER ™ Nuclear and Cytoplasmic Extraction Reagents (Thermo Fisher) were used on ground biceps femoris muscle biopsies according to manufacturer’s instructions and as described previously (Brown et al 2016). 15μg of protein was separated by gel electrophoresis and transferred to membranes as described above.…”

ASAS-SSR Triennnial Reproduction Symposium: Looking Back and Moving Forward—How Reproductive Physiology has Evolved: Fetal origins of impaired muscle growth and metabolic dysfunction: Lessons from the heat-stressed pregnant ewe1

“…This explains altered myoblast populations and reduced fiber sizes in IUGR hindlimb muscles near term (Yates et al, 2014 and at 28 d of age (Camacho et al, 2016b), as postnatal muscle stem cells (satellite cells) originate from fetal myoblast precursors (Greenwood et al, 1999;Messina and Cossu, 2009). Insulin promotes proliferation and differentiation of myoblasts (Allen et al, 1985) via AKT-mediated signaling pathways (Sumitani et al, 2002), and insulin infusion into uncompromised sheep fetuses increases myoblast proliferation (Brown et al, 2016b). However, skeletal muscle AKT content is reduced in IUGR sheep fetuses (Thorn et al, 2009) and adult rats (Camm et al, 2011).…”