Isolation of an insulin-like growth factor II cDNA from guinea pig liver: expression and developmental regulation

How to Make a Rodent Giant: Genomic Basis and Tradeoffs of Gigantism in the Capybara, the World's Largest Rodent

Gigantism results when one lineage within a clade evolves extremely large body size relative to its small-bodied ancestors, a common phenomenon in animals. Theory predicts that the evolution of giants should be constrained by two tradeoffs. First, because body size is negatively correlated with population size, purifying selection is expected to be less efficient in species of large body size, leading to increased mutational load. Second, gigantism is achieved through generating a higher number of cells along with higher rates of cell proliferation, thus increasing the likelihood of cancer. To explore the genetic basis of gigantism in rodents and uncover genomic signatures of gigantism-related tradeoffs, we assembled a draft genome of the capybara (Hydrochoerus hydrochaeris), the world's largest living rodent. We found that the genome-wide ratio of nonsynonymous to synonymous mutations (ω) is elevated in the capybara relative to other rodents, likely caused by a generation-time effect and consistent with a nearly neutral model of molecular evolution. A genome-wide scan for adaptive protein evolution in the capybara highlighted several genes controlling postnatal bone growth regulation and musculoskeletal development, which are relevant to anatomical and developmental modifications for an increase in overall body size. Capybara-specific gene-family expansions included a putative novel anticancer adaptation that involves T-cell-mediated tumor suppression, offering a potential resolution to the increased cancer risk in this lineage. Our comparative genomic results uncovered the signature of an intragenomic conflict where the evolution of gigantism in the capybara involved selection on genes and pathways that are directly linked to cancer.

Maternal insulin-like growth factors-I and -II act via different pathways to promote fetal growth

The placenta transports substrates and wastes between the maternal and fetal circulations. In mice, placental IGF-II is essential for normal placental development and function but, in other mammalian species, maternal circulating IGF-II is substantial and may contribute. Maternal circulating IGFs increase in early pregnancy, and early treatment of guinea pigs with either IGF-I or IGF-II increases placental and fetal weights by mid-gestation. We now show that these effects persist to enhance placental development and fetal growth and survival near term. Pregnant guinea pigs were infused with IGF-I, IGF-II (both 1 mg/kg.d), or vehicle sc from d 20-38 of pregnancy and killed on d 62 (term = 69 d). IGF-II, but not IGF-I, increased the mid-sagittal area and volume of placenta devoted to exchange by approximately 30%, the total volume of trophoblast and maternal blood spaces within the placental exchange region (+29% and +46%, respectively), and the total surface area of placenta for exchange by 39%. Both IGFs reduced resorptions, and IGF-II increased the number of viable fetuses by 26%. Both IGFs increased fetal weight by 11-17% and fetal circulating amino acid concentrations. IGF-I, but not IGF-II, reduced maternal adipose depot weights by approximately 30%. In conclusion, increased maternal IGF-II abundance in early pregnancy promotes fetal growth and viability near term by increasing placental structural and functional capacity, whereas IGF-I appears to divert nutrients from the mother to the conceptus. This suggests major and complementary roles in placental and fetal growth for increased circulating IGFs in early to mid-pregnancy.

Adaptive evolution of the insulin gene in caviomorph rodents

Insulin is a conservative molecule among mammals, maintaining both its structure and function. Rodents that belong to the Suborder Hystricognathi represent an exception, having a very divergent molecule with unusual physiological properties. In this work, we analyzed the evolutionary pattern of the insulin gene in caviomorph rodents (South American hystricomorph rodents). We found that these rodents have higher rates of nonsynonymous:synonymous substitutions (d(N)/d(S)) than nonhystricomorph rodents and that values are heterogeneous inside the group. We estimated codons under positive selection, specifically the second binding site (A13 and B17) and others related with hexamerization (B18, B20, and B22). In the monomer structure, all selected sites formed a single patch around the second binding site. In the hexamer structure, these amino acids were grouped into three major patches. In this structure, contacts between B chains involved all selected sites (except B18), and between faces in the center of the molecule, all contacts were among selected sites. While there is no clear hypothesis regarding the cause of this drastic change, experimental evidence does show that this group of rodents has some peculiarities in growth function, and, whether coincidental or not, these changes appeared together with important changes in life-history traits.

Insulin-like growth factor (IGF) binding proteins and their mRNAs in connective tissues of fasted guinea-pigs

Fasting (with vitamin C-supplementation) and vitamin C-deficiency in guinea-pigs are associated with decreased collagen gene expression in connective tissues. Recently we presented evidence that circulating insulin-like growth factor binding protein (IGFBP)-1 and-2 that are induced during both nutritional deficiencies may be responsible for this inhibition by interfering with IGF-I action. The present objective was to determine whether circulating IGFBPs are accumulated in bone, skin and cartilage during fasting, which would support an endocrine role for them. IGFBP-1 mRNA was not detected in any of the connective tissues. The protein, as measured by ligand blotting, was not present in tissues of normal animals but accumulated early during fasting in all of the tissues. Bone and cartilage from normal animals contained IGFBP-2 and its mRNA, but only in bone did their levels increase during fasting. IGFBP-3 mRNA was not detected in connective tissues from normal or fasted guinea-pigs. Little or no IGFBP-3 was detected in normal tissue extracts, but protein accumulated during fasting and presumably was derived from the circulation. IGF-I and-II mRNAs were expressed in bone and cartilage but in skin, only IGF-II mRNA was detected. Affinity cross-linking revealed that in skin, IGFBP-3 contained relatively few unoccupied IGF-I binding sites compared to IGFBP-1 while in bone and cartilage, only IGFBP-1 contained unoccupied binding sites. IGFBP-1, acting by endocrine action, is probably the major factor responsible for inhibition of IGF-I-dependent collagen gene expression during fasting.