Sheep recombinant IGF-1 promotes organ-specific growth in fetal sheep.

IGF-1 is a critical fetal growth-promoting hormone. Experimental infusion of an IGF-1 analog, human recombinant LR3 IGF-1, into late gestation fetal sheep increased fetal organ growth and skeletal muscle myoblast proliferation. However, LR3 IGF-1 has a low affinity for IGF binding proteins (IGFBP), thus reducing physiologic regulation of IGF-1 bioavailability. The peptide sequences for LR3 IGF-1 and sheep IGF-1 also differ. To overcome these limitations with LR3 IGF-1, we developed an ovine (sheep) specific recombinant IGF-1 (oIGF-1) and tested its effect on growth in fetal sheep. First, we measured in vitro myoblast proliferation in response to oIGF-1. Second, we examined anabolic signaling pathways from serial skeletal muscle biopsies in fetal sheep that received oIGF-1 or saline infusion for 2 hours. Finally, we measured the effect of fetal oIGF-1 infusion versus saline infusion (SAL) for 1 week on fetal body and organ growth, in vivo myoblast proliferation, skeletal muscle fractional protein synthetic rate, IGFBP expression in skeletal muscle and liver, and IGF-1 signaling pathways in skeletal muscle. Using this approach, we showed that oIGF-1 stimulated myoblast proliferation in vitro. When infused for 1 week, oIGF-1 increased organ growth of the heart, kidney, spleen, and adrenal glands and stimulated skeletal myoblast proliferation compared to SAL without increasing muscle fractional synthetic rate or hindlimb muscle mass. Hepatic and muscular gene expression of IGFBPs one to three was similar between oIGF-1 and SAL. We conclude that oIGF-1 promotes tissue and organ-specific growth in the normal sheep fetus.

Pulsatile hyperglycemia increases insulin secretion but not pancreatic ß-cell mass in intrauterine growth-restricted fetal sheep.

Impaired ß-cell development and insulin secretion are characteristic of intrauterine growth-restricted (IUGR) fetuses. In normally grown late gestation fetal sheep pancreatic ß-cell numbers and insulin secretion are increased by 7-10 days of pulsatile hyperglycemia (PHG). Our objective was to determine if IUGR fetal sheep ß-cell numbers and insulin secretion could also be increased by PHG or if IUGR fetal ß-cells do not have the capacity to respond to PHG. Following chronic placental insufficiency producing IUGR in twin gestation pregnancies (n=7), fetuses were administered a PHG infusion, consisting of 60 min, high rate, pulsed infusions of dextrose three times a day with an additional continuous, low-rate infusion of dextrose to prevent a decrease in glucose concentrations between the pulses or a control saline infusion. PHG fetuses were compared with their twin IUGR fetus, which received a saline infusion for 7 days. The pulsed glucose infusion increased fetal arterial glucose concentrations an average of 83% during the infusion. Following the 7-day infusion, a square-wave fetal hyperglycemic clamp was performed in both groups to measure insulin secretion. The rate of increase in fetal insulin concentrations during the first 20 min of a square-wave hyperglycemic clamp was 44% faster in the PHG fetuses compared with saline fetuses (P0.23). Chronic PHG increases early phase insulin secretion in response to acute hyperglycemia, indicating that IUGR fetal ß-cells are functionally responsive to chronic PHG.

Quantitative genomics: exploring the genetic architecture of complex trait predisposition.

Most phenotypes with agricultural or biomedical relevance are multifactorial traits controlled by complex contributions of genetics and environment. Genetic predisposition results from combinations of relatively small effects due to variations within a large number of genes, known as QTL. Well over 200 QTL have been reported for growth and body composition traits in the mouse, which likely represent at least 50 to 100 distinct genes. Molecular biology has yielded significant advances in understanding these traits at the metabolic and physiological levels; however, little has been learned regarding the identity and nature of the underlying polygenes. In addition to the significantly poor precision inherent to QTL localization, it is very difficult to differentiate between co-localization and coincidence when comparing QTL with other QTL and with potential candidate genes. The wide gap between our knowledge of physiological mechanisms underlying complex traits and the nature of genetic predisposition significantly impairs discovery of genes underlying QTL. Identification and genetic mapping of key transcriptional, proteomic, metabolomic, and endocrine events will uncover large lists of significant positional candidate genes for growth and body composition. However, integration of experimental approaches to jointly evaluate predisposition and physiology will increase success of QTL identification by merging the power of recombination with functional analysis. Measuring physiologically relevant subphenotypes within a structured QTL mapping population will not only facilitate pathway-specific prioritization among candidate genes, but may also directly identify genes underlying QTL. This would advance our understanding of the genetic architecture of complex traits by testing the central hypothesis that genes controlling predisposition to a quantitative trait are primarily involved in trans-regulation of the primary physiological pathways that regulate the trait.