Distinct post-transcriptional regulation of Igfbp1 gene by hypoxia in lowland mouse and Qinghai-Tibet plateau root vole Microtus oeconomus

Our previous study revealed the particular expression patterns of insulin-like growth factor 1 (IGF1) and insulin-like growth factor binding protein 1 (IGFBP1) in the Qinghai-Tibet plateau root vole (Microtus oeconomus) under hypoxic challenge. Here we report the molecular mechanisms of Igf gene regulation associated with adaptation to hypoxia. M. oeconomus IGF1 and IGFBP1 were shown to be highly conserved. Hypoxia (8.0% O2, 6h) did not change the liver-derived Igf1 expression in either M. oeconomus or mouse. Hypoxia significantly upregulated hepatic Igfbp1 gene expression and IGFBP1 levels in the liver and plasma of the mouse, but not in M. oeconomus. A functional U-rich element in the 3' untranslated region was found in mouse Igfbp1 mRNA, which was associated with Igfbp1 mRNA stabilization and upregulation under hypoxia, and this U-rich element was eliminated in the M. oeconomus Igfbp1, resulting in blunted Igfbp1 mRNA upregulation, which might be understood as a sequence variation modified during molecular evolution under hypoxia.

Rates of molecular evolution vary in vertebrates for insulin-like growth factor-1 (IGF-1), a pleiotropic locus that regulates life history traits

Insulin-like growth factor-1 (IGF-1) is a member of the vertebrate insulin/insulin-like growth factor/relaxin gene family necessary for growth, reproduction, and survival at both the cellular and organismal level. Its sequence, protein structure, and function have been characterized in mammals, birds, and fish; however, a notable gap in our current knowledge of the function of IGF-1 and its molecular evolution is information in ectothermic reptiles. To address this disparity, we sequenced the coding region of IGF-1 in 11 reptile species-one crocodilian, three turtles, three lizards, and four snakes. Complete sequencing of the full mRNA transcript of a snake revealed the Ea-isoform, the predominant isoform of IGF-1 also reported in other vertebrate groups. A gene tree of the IGF-1 protein-coding region that incorporated sequences from diverse vertebrate groups showed similarity to the species phylogeny, with the exception of the placement of Testudines as sister group to Aves, due to their high nucleotide sequence similarity. In contrast, long-branch lengths indicate more rapid divergence in IGF-1 among lizards and snakes. Additionally, lepidosaurs (i.e., lizards and snakes) had higher rates of non-synonymous:synonymous substitutions (dN/dS) relative to archosaurs (i.e., birds and crocodilians) and turtles. Tests for positive selection on specific codons within branches and evaluation of the changes in the amino acid properties, suggested positive selection in lepidosaurs on the C domain of IGF-1, which is involved in binding affinity to the IGF-1 receptor. Predicted structural changes suggest that major alterations in protein structure and function may have occurred in reptiles. These data propose new insights into the molecular co-evolution of IGF-1 and its receptors, and ultimately the evolution of IGF-1's role in regulating life-history traits across vertebrates.

Class 2 IGF-1 isoforms are dispensable for viability, growth and maintenance of IGF-1 serum levels

Insulin-like growth factor 1 (IGF-1) is a pleiotropic factor involved in growth, cell survival and cellular differentiation. It exerts its functions through endocrine, paracrine or autocrine mechanisms. Circulating IGF-1 is essential for normal fetal and postnatal growth, although the published phenotypes of IGF-1 null animals have been only partially penetrant, presumably due to mixed genetic backgrounds. Molecular dissection of IGF-1 action is complicated by the existence of at least nine different IGF-1 isoforms, generated in both humans and rodents by usage of alternate promoters, differential splicing and different post-translational modifications. Several lines of evidence suggest that the Class 2 IGF-1 isoform is specifically destined for circulation, supporting an endocrine role of IGF-1 in normal growth processes. Using Cre/LoxP conditional gene targeting of exon 2 of the IGF-1 gene, we have generated a Class 2 IGF-1 knockout mouse line in a pure C57/Bl6 genetic background, where the specific removal of exon 2 ablated Class 2 IGF-1 isoform. Class 2 IGF-1 knockout mice exhibited normal development and postnatal growth patterns and had normal IGF-1 circulating levels, due to compensatory upregulation of Class 1 transcripts. In contrast, progeny of a total IGF-1 knockout line lacking exon 3 in the same genetic background were predictably smaller, displayed dramatically reduced IGF-1 receptor phosphorylation and all died perinatally, apparently due to respiratory failure. These results confirm that Class 2 signal peptide is not necessary for systemic circulation of IGF-1, revealing an internal compensation system for maintaining IGF-1 serum concentrations. We also uncover a vital requirement of IGF-1 for perinatal viability, previously obscured by modifiers in heterogeneous genetic backgrounds.

Growth hormone and insulin-like growth factor of naked carp (Gymnocypris przewalskii) in Lake Qinghai: expression in different water environments

Here, we report the cloning and characterization of growth hormone (GH), insulin-like growth factor-I (IGF-I) and IGF-II from naked carp (Gymnocypris przewalskii), a native teleost fish of Lake Qinghai in the Qinghai-Tibet Plateau of China. The GH of naked carp encodes for a predicted amino acid sequence showing identities of 63%, 63%, 91% and 94% with cherry salmon, rainbow trout, zebrafish and grass carp, respectively. Compared to common carp and goldfish, evolutionary analysis showed that genome duplication has had less influence on the relaxation of purifying selection in the evolution of naked carp GH. Sequence analysis of naked carp IGF-I (ncIGF-I) and ncIGF-II showed a high degree of homology with known fish IGF-I and IGF-II. To investigate effects of salinity and ionic composition of the aquatic environment on the GH-IGF axis in naked carp, male fish held in river water were assigned randomly to 4 groups: RW (river-water), RW+Na (NaCl in RW), RW+Mg (MgCl(2) in RW) and LW (lake-water) groups. The concentrations of Na(+) in RW+Na and Mg(2+) in RW+Mg were equal to the concentrations of these ions in lake-water. After 2 days of exposure, the plasma IGF-I levels in the RW+Na and LW groups were significantly higher than the control group (RW), and the plasma GH levels of the LW group were also significantly higher than the RW group. The somatostatin (SS) levels in the hypothalamus significantly increased in the RW+Na group. After 5 days of exposure, these hormone levels did not differ significantly among groups. These results indicate that while the plasma GH and IGF-I levels are osmosensitive, the absence of a change in GH secretion in RW+Na might be partly due to a transiently increased release of hypothalamic SS induced by the stress of neutral-saline water. This is the first report of a salinity-induced increase of GH-IGF-I circulating levels in Cypriniformes.

Interaction of gender and dietary protein on renal growth and the renal growth hormone-insulin-like growth factor axis

The female kidney tends to be smaller, have a lower glomerular filtration rate, and be less susceptible to glomerulosclerosis than the male kidney. Insulin-like growth factor-I (IGF-I) is a peptide growth factor that appears to be important for normal and adaptive kidney growth. The purpose of this study was to compare the kidney growth response of the male and female rat kidneys to increased dietary protein intake and to see whether differences in IGF-I production or receptor expression might underlie any gender differences seen. Male (M) and female (F) Munich-Wistar rats (6 to 9 weeks of age) were randomized to isocaloric diets containing either 20% (NP) or 50% (HP) protein and studied after 3 and 14 days. In the male rat, wet kidney weight was significantly increased with HP at both day 3 (M-HP 1028+/-21 mg vs M-NP 891+/-19 mg, p < 0.01) and day 14 (M-HP 1499+/-41 mg vs M-NP 1246 +/-37 mg, p < 0.01). In contrast in the female rat, while there was evidence of initial increased growth at day 3 in the kidneys of F rats fed HP (F-HP 788+/-39 mg vs F-NP 650+/-23 mg, p < 0.01), this difference was not sustained at 14 days (F-HP 961+/-67 mg vs F-NP 931+/-71 mg, p = NS). At day 3, kidneys of both male and female rats fed HP exhibited an increase in total protein but not DNA content. The kidneys of male rats showed increased protein/DNA ratios in the medulla and inner cortex, whereas in the kidneys of female rats, the increase in protein/DNA ratio was confined to the cortex. After 14 days of HP ingestion, the kidneys of male rats showed increases in total kidney content of both DNA and protein, and protein/DNA ratios returned to control values in whole kidney, inner cortex, and medulla. In contrast, in the kidneys of female rats, not only was overall growth response reduced, but neither total kidney protein content nor DNA content was increased. Increased protein/DNA ratios were seen in inner cortex and in outer and inner medulla, similar to that seen at day 3 in the kidneys of male rats. Neither baseline plasma (M-NP 793+/-10 ng/ml, F-NP 704+/-32 ng/ml, p = NS) nor kidney IGF-I content (M-NP 520+/-55 ng/gm tissue, F-NP 506+/-54 ng/gm tissue, p = NS) differed between male and female rats fed NP diets. Both male and female rats showed a comparable increase in kidney IGF-I after 3 days of HP ingestion, and kidney IGF-I returned to control values by 14 days. There was no significant difference in the number or affinity of glomerular IGF-I receptors between male and female rats. In conclusion, we have shown that in the adult male rat, an increase in dietary protein ingestion results in a sustained increase in kidney size that is initially consistent with a hypertrophic response but subsequently shows elements of hyperplasia. In contrast, in the female rat, although there was evidence of the initial hypertrophic (and IGF-I) responses to increased dietary protein, the increase in kidney size was not sustained. However, these profound gender-based differences in the growth response to dietary protein did not appear to be due to differences in kidney expression of IGF-I or its receptors.