Frog brain and liver show evolutionary conservation of tissue-specific differences among insulin receptors

Cocaine and amphetamine regulated transcript peptide (CART) in the tadpole brain: Response to different energy states

Cocaine- and amphetamine-regulated transcript peptide (CART) is an anorexigenic neuropeptide known to play a key role in energy homeostasis across the vertebrate phyla. In the current study, we have investigated the response of the CART immunoreactive system to varying energy states in the brain of a tadpole model. The pro-metamorphic tadpoles of Euphlyctis cyanophlyctis were fasted, or intracranially injected with glucose or 2-deoxy-d-glucose (2DG; an antagonist to glucose inducing glucoprivation) and the response of the CART containing system in various neuroanatomical areas was studied using immunohistochemistry. Glucose administration increased the CART immunoreactivity in the entopeduncular neurons (EN), preoptic area (POA), ventral hypothalamus (vHy) and the Edinger Westphal nucleus (EW) while CART positive cells decrease in response to fasting and glucoprivation. A substantial decrease in CART was noted in the EW nucleus of tadpoles injected with 2DG. These regions might contain the glucose-sensing neurons and regulate food intake in anurans. Therefore, we speculate that the function of central CART and its antagonistic action with NPY in food and feeding circuitry of anurans is evolutionary conserved and might be responsible for glucose homeostasis.

Insulin-family peptide-receptor interaction at the early stage of vertebrate evolution

This is an overview of our studies on insulin and insulin-like growth factor-I (IGF-I) interactions with their own and each other's receptors in the lamprey (Lampetra fluviatilis L.), an extant representative of the ancient vertebrate group of Agnathans as compared to mammal (rat). Lamprey insulin receptor shows species specificity, namely, it binds its own insulin with higher affinity than mammalian hormone. Nevertheless, and unlike mammalian insulin receptor, lamprey receptor discriminates relatively poorly between insulin and IGF-I. Autophosphorylation patterns are identical for both receptors. In contrast, IGF-I receptors in lamprey tissues are very similar to mammalian IGF-I receptors confirming known evolutionary conservatism of IGF receptor system. Presumed common evolutionary origin of insulin and IGF-I receptors and poor ability of lamprey insulin receptor to discriminate between two ligands, implies that lamprey insulin receptor is closer to putative ancestral protein that IGF-I receptor. Contrary to the common belief, ambient temperatures for lampreys (4-15 degrees C) put no constraints on either downregulation of receptors or the endocytosis of hormone-receptor complexes.

Insulin receptors in Xenopus laevis liver and forelimb regenerates and the effects of local insulin deprivation on regeneration

As forelimb regeneration in Xenopus laevis is mainly a cell proliferative event which results in a spike-shaped appendage, we set out to examine the possibility that insulin is a growth-promoting factor in this process. The objectives were 1) to detect the presence of insulin receptors (IRs) in the liver (a specific target organ for insulin) and IRs in the forelimb regenerates of X. laevis, 2) to determine whether the receptor is similar to IRs identified in other organisms, and 3) to absorb insulin locally by implanting anti-insulin antibody-soaked hydrolyzed polyacrylamide beads into regenerating forelimb outgrowths in order to assess the effects of insulin deprivation on regeneration. The results show that IRs are present in Xenopus liver plasma membranes (XLPM) as well as in plasma membranes of 21 day forelimb regenerates. Insulin binding to this receptor is time-dependent and specific, as unlabeled bovine insulin competes with radioiodinated insulin for binding to XLPM more effectively than insulin-like growth factor-I, guinea pig insulin, or glucagon. Scatchard analysis of insulin binding to XLPM describes a two binding site receptor possessing a low affinity (0.16 nM-1), high capacity (3.2 +/- 0.9 pM/mg) binding site and a high affinity (2.7 nM-1), low capacity (0.5 +/- 0.3 pM/mg) binding site. The holoreceptor has a molecular mass of 380 kDa. The reduced receptor has subunits of 130 kDa and 95 kDa. The 95 kDa subunit undergoes autophosphorylation following insulin stimulation. Implantation of hydrolyzed polyacrylamide beads, saturated with anti-insulin antibody, into regenerating Xenopus forelimbs significantly impeded development of the regenerates and, therefore, demonstrates that insulin is required for growth of Xenopus forelimb regenerates.

Detection of insulin receptors in newt liver and forelimb regenerates and the effects of local insulin deprivation on epimorphic regeneration

Previous in vivo and in vitro studies indicate that insulin is required in adult newt forelimb regeneration. The objectives of the current study were 1) to detect insulin receptors in the liver (a classical target organ for insulin) and once verified, detection of insulin receptors in the adult newt forelimb regenerate; and 2) to determine whether locally implanting insulin antibody-soaked hydrolyzed polyacrylamide beads (hypa beads) into a regenerating forelimb blastema would affect its growth and/or differentiation. The results show that insulin receptors are detectable in the plasma membranes of newt liver and forelimb regenerates. Radioiodinated bovine insulin binding is time-dependent and specific; unlabeled bovine insulin competes with labeled insulin for binding to NLPM more effectively than does insulin-like growth factor-I, guinea pig insulin, and glucagon. The newt hepatic insulin receptor binds insulin with high affinity (1.1 nM-1) and low capacity (63 +/- 8 fmoles/mg). The size of the alpha subunit of the newt insulin receptor is 130 kDA and that of the beta subunit is 95 kDa. The beta subunits undergo insulin-stimulated phosphorylation in response to insulin. An autoantibody against the human insulin receptor recognizes the newt receptor protein. Insulin receptors are also detectable in 15 and 20 day newt forelimb regenerates. Specific immunogold labelling of the receptor-bound antibody appears to be restricted to the cellular processes of the regenerate. Implanting hypa beads soaked with purified insulin antibody into regenerating adult newt forelimbs results in abnormal growth and differentiation of the regenerates, confirming that insulin plays an essential role in adult newt forelimb regeneration.

Annual variations in the binding of insulin to hepatic membranes of the frog Rana esculenta

Amphibia undergo regular annual cycles of metabolic activity that are influenced by both exogenous factors and hormones. Insulin binding to crude frog hepatic membranes was studied throughout the year. The general character of insulin binding was similar to that in other vertebrates; the maximum specific binding was achieved after 4 hr at 4 degrees, the optimum pH was 7.8, half-maximal displacement of bound insulin was from 9 x 10(-10) to 1 x 10(-9) M, and insulin analogs competed for the insulin receptor in line with their relative biological potencies. A biphasic Scatchard plot and negative cooperativity of the receptor were also observed in frog liver membranes. Affinity constants from Scatchard plots revealed high and low affinity binding sites which were unchanged during the year. The seasonal cycle, however, markedly affected the binding capacity for both sites. Maximum binding occurred in May-June and the minimum in November-December for both classes of receptors. Binding capacities ranged from 1.71 to 11.33 fmol/mg protein for the high affinity sites and from 432 to 3171 fmol/mg protein for the low affinity sites. It is concluded that annual cycles of insulin binding reflect modulation of receptor number rather than receptor affinity.

Insulin and IGF I receptor-mediated Na+ transport in toad urinary bladders

We compared the concentration dependence of insulin- and insulin-like growth factor I (IGF I)-stimulated Na+ transport with ligand-receptor affinities in the urinary bladder of the toad Bufo marinus. Threshold, half-maximal, and maximal natriferic concentrations of both peptides were approximately 0.1, 1, and 10 nM, respectively. Amiloride, but not ethyl isopropyl amiloride, (10(-5) M), abolished Na+ transport. Maximal responses to either peptide rendered the tissue insensitive to challenge with the other. Separate insulin and IGF I receptors were identified by equilibrium binding and polyacrylamide gel electrophoresis of cross-linked ligand-receptor complexes. For both peptides, half-maximal binding occurred at 3-10 nM; crossover binding to the other receptor occurred with 10- and 100-fold lower affinity. Thus, in this model "high-resistance" renal epithelium, 1) ligand binding to specific insulin and IGF I receptors stimulates transcellular Na+ flux, 2) the natriferic effects of insulin and IGF I apparently depend on activation of apical Na+ channels rather than Na+-H+ antiporters, and 3) the natriferic pathways activated by insulin and IGF I appear to converge subsequent to ligand-receptor binding but before the final transport ("effector") step(s).

Insulin and insulin-like growth factor receptors in the nervous system

Insulin and the insulin-like growth factors (I and II) are homologous peptides essential to normal metabolism as well as growth. These peptide hormones are present in the brain, and, based on biosynthetic labeling studies as well as evidence for local gene expression, they are synthesized by nervous tissue as well as being taken up by the brain from the peripheral circulation. Furthermore, the presence of insulin and IGF receptors in the brain, on both neuronal and glial cells, also suggests a role for these peptides in the nervous system. Thus, these ligands affect brain electrical activity, either as neurotransmitters or as neuromodulators, altering the release and re-uptake of other neurotransmitters. The insulin and IGF-I and -II receptors found in the brain exhibit a lower molecular weight than corresponding receptors on peripheral tissues, primarily caused by alterations in glycosylation. Despite these alterations, both brain insulin and IGF-I receptors exhibit tyrosine kinase activity in cell-free systems, as do their peripheral counterparts. Brain insulin and IGF-I receptors are developmentally regulated, with the highest levels appearing in fetal or perinatal life. However, the altered glycosylation of brain receptors does not appear until late in fetal development. The receptors are widely distributed in the brain, but especially enriched in the circumventricular organs, choroid plexus, hypothalamus, cerebellum, and olfactory bulb. These studies on the insulin and IGF receptor in brain, add strong support to the suggestion that insulin and IGFs are important neuroactive substances, regulating growth, development, and metabolism in the brain.