The influence of purified somatomedins and insulin on foetal rat brain DNA synthesis in vitro

Effect of kainic acid treatment on insulin-like growth factor-2 receptors in the IGF2-deficient adult mouse brain

Insulin-like growth factor-2 (IGF2) is a member of the insulin gene family with known neurotrophic properties. The actions of IGF2 are mediated via the IGF type 1 and type 2 receptors as well as through the insulin receptors, all of which are widely expressed throughout the brain. Since IGF2 is up-regulated in the brain after injury, we wanted to determine whether the absence of IGF2 can lead to any alteration on brain morphology and/or in the response of its receptor binding sites following a neurotoxic insult. No morphological differences were observed between the brains of IGF2 knockout (IGF2(-/-)) and wild-type control (IGF2(+/+)) mice. However, our in vitro receptor autoradiography results indicate that IGF2(-/-) mice had lower endogenous levels of [(125)I]IGF1 and [(125)I]insulin receptor binding sites in the hippocampus and cerebellum as compared to IGF2(+/+) mice, while endogenous [(125)I]IGF2 receptor binding showed a decrease only in the cerebellum. Seven days after kainic acid administration, the [(125)I]insulin receptor binding sites were significantly decreased in all brain regions of the IGF2(+/+) mice, while the levels of [(125)I]IGF1 and [(125)I]IGF2 binding sites were decreased only in select brain areas. The IGF2(-/-) mice, on the other hand, showed increased [(125)I]IGF1 and [(125)I]IGF2 and [(125)I]insulin receptor binding sites in selected regions such as the hippocampus and cerebellum. These results, taken together, suggest that deletion of IGF2 gene does not affect gross morphology of the brain but does selectively alter endogenous [(125)I]IGF1, [(125)I]IGF2 and [(125)I]insulin receptor binding sites and their response to neurotoxicity.

Subpopulations of stromal cells from long-term human bone marrow cultures: ontogeny of progenitor cells and expression of growth hormone receptors

Long-term culture of bone marrow derived stromal colony forming cells (S-CFC) in matrix and nutrient defined agar medium resulted in stromal cell colonies that pass sequentially through three distinct morphological stages: firstly, aggregated loose syncytium of round to avoid cells (stage I), a second developmental stage of large branching colonies in which the cells become enlarged, elongated with cytoplasmic projections forming a loosely anastomized network with adjacent cells (stage II), and finally cells become dissociated, loosing their long, thin cytoplasmic filaments and breaking their contacts with one another, but remain large and retain a bi-polar nature (stage III). Cells were also grown in liquid medium in a culture microenvironment closely resembling conditions of haemopoiesis in vitro. Using a panel of well defined monoclonal antibodies reactive against the rat, rabbit and human growth hormone receptors, this study found immunochemical evidence of the presence and localization of binding sites of growth hormone (GH) in the cell membrane and extra-nuclear Golgi area of long-term bone marrow derived human stromal cells in liquid and semi-solid nutrient agar mediums. GH-receptor immunoreactivity was present in small proliferating progenitor cells, myofibroblast-like cells, large reticular fibroblast cells, adipocytes and endothelial cells. Only MAb known to be reactive against human tissue resulted in strong immunoreactivity. The expression of GH-receptors not only on small proliferating, but also on the well differentiated cells, indicates a role for growth hormone on non-progenitor cells. GH-receptor immunoreactivity on differentiating and/or differentiated cells suggests that GH is also necessary for, or has a trophic function in differentiation. We propose that direct GH action is necessary not only for differentiation of progenitor cells as implied by the dual effector hypothesis, but also their subsequent clonal expansion, differentiation and maintenance.

Insulin-like growth factors and insulin stimulate erythropoietin production in primary cultured astrocytes

Erythropoietin (EPO) is established as a major regulator of erythropoiesis. However, we and others have shown that neurons express erythropoietin receptor (EPO-R), that astrocytes produce EPO and that EPO may act as a neurotrophic factor in the CNS. We also found that EPO production is activated by insulin and insulin-like growth factors (IGFs) in astrocytes in a dose-dependent manner and that IGF-I was the most potent activator. The concentrations required for half-maximal activation were 3 nM IGF-I, 10 nM IGF-II and 100 nM insulin. The oxygen concentration regulates EPO production; hypoxia stimulates EPO production in astrocytes. The stimulatory effect of IGFs and insulin on EPO production in astrocytes was not affected by the oxygen concentration of astrocyte culture. Insulin and IGFs did not increase the total protein synthesis of astrocytes but increased EPO mRNA levels, indicating that EPO production is stimulated at the mRNA level. It appeared that the growth factor-induced accumulation of EPO mRNA in astrocytes was caused by activation of the tyrosine kinase-signal transduction pathway, because tyrosine phosphorylation of receptors for IGF-I and insulin was activated when astrocytes were stimulated by these growth factors.

Expression of insulin-like growth factor-1 (IGF-1) and IGF-binding protein 2 (IGF-BP2) in the hippocampus following cytotoxic lesion of the dentate gyrus

Receptor binding and gene expression of several members of the IGF gene family were examined in the rat brain following lesion of the hippocampal dentate gyrus granular cells by intradentate colchicine injection. Dentate granular cell loss was accompanied by extensive reactive gliosis in the lesioned hippocampus and damaged overlying cortex, as verified by the increase in GFAP mRNA and BS-1 lectin binding. At 4 days post-lesion, 125I-IGF-2 binding was dramatically increased within the lesioned dentate gyrus and damaged overlying cortex, and corresponded temporally and anatomically with increased IGF-BP2 gene expression following the lesion. Increased IGF-BP3 gene expression was only observed in the overlying cortex at 10 days post-lesion, and corresponded with an increase in 125I-IGF-1 binding at the injured surface of the cortex. Type-2 IGF receptor mRNA expression was reduced to background levels in the lesioned dentate gyrus, suggesting that IGF-BP2 was a major component of the observed increase in 125I-IGF-2 binding. In situ hybridization also revealed a prominent increase in IGF-1 mRNA expression by 4 days post-lesion, which was localized within the lesioned dentate gyrus and damaged cortical areas, and was shown to be expressed by microglia. While no IGF-2 mRNA expression was observed within the CNS, either prior to, or following the lesion, IGF-2 mRNA expression was observed in the choroid plexus, meningeal membranes, and in blood vessel endothelium, providing a potential source for the transport of IGF-2 into the CNS. In the injured CNS, increased IGF-BP2 expression may act to maintain or transport IGF-1 or IGF-2, as well as modulate the local autocrine and paracrine actions of the IGFs. Increased microglial IGF-1 expression following colchicine treatment correlates with the timing of a number of post-traumatic events within the CNS, suggesting that IGF-1 may have a role as a neuroprotectant for surviving neurons and signal for local neuronal sprouting, as well as a role in reactive astrogliosis.

Evidence of a direct role for growth hormone (GH) in mammary gland proliferation and lactation

A panel of monoclonal antibodies to the growth hormone (GH) receptor/binding protein was used to demonstrate the existence and detail the expression of GH receptors in ductal and alveolar epithelial cells from rat and rabbit mammary glands by immunohistochemistry. Intense immunoreactivity was present in membrane, cytoplasm and some nuclei of epithelial cells during proliferation and lactation. Receptor expression decreased during weaning and was absent or weak in regressive mammary glands. Immunoreactivity was weak in ductal epithelial cells from virgin adult animals. Pronounced expression of GH receptor/binding protein was observed with two monoclonal antibodies and lesser reactivity was seen with others, paralleling their affinities for the receptor. The cytoplasmic presence of this putatively plasma membrane located GH receptor is accounted for by the existence of a soluble form on the GH receptor, namely the growth hormone binding protein derived from the membrane receptor by cleavage. Primary localization of the receptor in proliferating and lactating epithelial cells suggests that the rat and rabbit mammary gland is a GH target tissue. This finding is in contradiction to both classical GH action and the somatomedin hypothesis and challenges the widely held view that GH has no direct influence on mammary growth and function.

Insulin-like growth factor-1 stimulation of protein synthesis is attenuated in cerebral cortex of aging rats

It has been postulated that brain aging and the accompanying neurodegenerative processes associated with aging result from a deterioration of mechanisms that regulate the maintenance of basic cellular processes. In the present study, it was hypothesized that decreased availability and/or diminished responsiveness of tissues to growth factors such as insulin-like growth factor-1 may be partly responsible for decreases in total protein synthesis previously observed in aging animals. Male Brown Norway rats (5-7 and 27-28 months old) were used to determine (1) whether in vivo protein synthesis in cortex, hippocampus, hypothalamus and cerebellum decreases with age and (2) whether these deficiencies are associated with age-related alterations in response to insulin-like growth factor-1, des (1-3) IGF-1 or insulin. Analysis of in vivo protein synthesis rates revealed a decline of 20% in cortex of old rats (P < 0.05) but no changes were observed in hippocampus, hypothalamus, or cerebellum. Stimulation of cortical slices in vitro with insulin-like growth factor-1, des (1-3) insulin-like growth factor-1, or insulin increased protein synthesis rates in young animals, but the response to these growth factors was blunted in old animals. Analysis of type 1 insulin-like growth factor receptor densities by quantitative autoradiography demonstrated age-related decreases in receptor levels in cerebellar cortex and dentate gyrus of the hippocampus but no changes in cortex. Regional distribution of type 1 insulin-like growth factor receptors within each of these tissues did not appear to change with age.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin-like growth factor II is induced during wound repair following hypoxic-ischemic injury in the developing rat brain

Recent evidence suggests that insulin-like growth factor-I (IGF-I) acts as a neurotrophic factor in the injured CNS. The role of the related peptide IGF-II is unclear. Therefore, we compared the induction of IGF-II in the developing rat brain following mild or severe hypoxic-ischemic (HI) injuries. Ligation of the right carotid artery of 21 day old rats followed by either 15 or 60 min exposure to 8% oxygen led to mild or severe unilateral damage respectively. Brains were collected at 1 day, 3, 5, 7 and 10 days, post-hypoxia. In situ hybridization showed that the 15 min injury (which produced selective neuronal loss) produced no change in basal IGF-II gene expression. However, the 60 min injury, which resulted in cortical infarction and severe neuronal loss in other regions, led to the induction of IGF-II mRNA mainly in the infarcted cortex, from 5-7 days post-hypoxia. Immunohistochemical analysis of brains collected 10 days after the 60 min injury showed that IGF-II immunoreactivity (IR) was also increased, predominantly in damaged regions, but also in the contralateral hippocampus. IGF-II IR was associated with non-neuronal cells that appeared to be microglial-like cells and astrocytes. Together these data suggest that IGF-II may modulate the response of glial cells during recovery from cerebral infarction.

Effect of in utero ethanol exposure on the postnatal ontogeny of insulin-like growth factor-1, and type-1 and type-2 insulin-like growth factor receptors in the rat brain

There is convincing evidence that alcohol consumption during pregnancy causes major CNS abnormalities; however, the molecular and cellular basis of these dysfunctions is currently not understood. This study examined the effects of prenatal ethanol exposure on the expression of insulin-like growth factor-1 messenger RNA and type-1 and type-2 receptor protein and messenger RNA expression in the developing rat brain. Mothers were maintained on an ethanol containing liquid diet from day 2 of pregnancy through parturition and the offspring were killed at birth, 10, 20 and 40 days of age. Insulin-like growth factor-1 messenger RNA, and insulin-like growth factor receptors demonstrated developmentally dependent expression in specific brain regions throughout the postnatal period of CNS maturation. Insulin-like growth factor-1 gene expression in the brain, as analysed by dot-blot hybridization, was greatest at birth, and decreased 61% in ad libitum and pair-fed animals by 20 days of age. In contrast, ethanol-treated animals exhibited only a 25% decrease in insulin-like growth factor-1 messenger RNA levels during the same period. This delay in insulin-like growth factor-1 messenger RNA maturation may be related to a developmental delay in CNS development in the prenatally ethanol exposed offspring. Prenatal ethanol exposure did not alter the observed localization of insulin-like growth factor-1 messenger RNA. While alterations were observed in long-term insulin-like growth factor-1 messenger RNA regulation, quantitative receptor autoradiography and in situ hybridization demonstrated no alterations in either type-1 or type-2 insulin-like growth factor receptor populations in ethanol-treated animals. Changes in hepatic and plasma insulin-like growth factor-1 and insulin-like growth factor-binding protein regulation have also been observed in these animals, suggesting changes in protein translation and the autocrine/paracrine actions of this peptide. The present study demonstrated that insulin-like growth factor-1 messenger RNA and insulin-like growth factor receptors are regionally expressed during early postnatal development and that ethanol administration influenced the long-term regulation of insulin-like growth factor messenger RNA levels in the brain without affecting either its localization or insulin-like growth factor receptor populations.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunohistochemical localization of growth hormone receptor binding protein in the mammalian cerebellum

A panel of monoclonal antibodies to the growth hormone (GH) receptor/binding protein was used to demonstrate the existence and detail the expression of GH receptors in the cerebellum of 2, 10, 28 days old postnatal and adult rats and 10, 20 days old and adult rabbits by immunohistochemistry to define potential targets for endogenous GH action in the cerebellum. Receptors were localized in membrane and cytoplasmic components of neurons and glial cells and expression decreased with age. Intense immunoreactivity was observed in the cytoplasm and dendrites of Purkinje cells and in cells of the cerebellar nuclei. Glial cells also showed receptor expression. Strong immunoreactivity was observed with two monoclonal antibodies and lesser reactivity was seen with others, paralleling their affinities for the receptor. The cytoplasmic presence of this putatively plasma membrane located GH receptor is accounted for by the high receptor content of endoplasmic reticulum and the existence of a soluble form of the GH receptor, namely the GH binding protein (BP) derived from the membrane receptor by cleavage, and receptor localization reported here correlate well with the distribution of insulin-like growth factor 1 (IGF-1) mRNA and immunoreactivity in cerebellar Purkinje cells and glial cells. Primary localization of the receptor in the cerebellum is in direct contradiction to both classical GH action and the somatomedin hypothesis and supports and extends the theory of genetically regulated macroneuronal maturation.

Insulin-like growth factor II promotes in vitro cholinergic development of mouse septal neurons: comparison with the effects of insulin-like growth factor I

We investigated the effect of insulin-like growth factors II and I (IGFII and IGFI) on septal primary cultures from mouse embryonic day 15 brains. The addition of IGFII to septal cultures enhanced total choline acetyltransferase (ChAT) activity in a dose-dependent manner. Maximal stimulation of ChAT activity was observed at 10 ng/ml IGFII. The effect of IGFII on ChAT activity was completely blocked by anti-IGFII/M-6-P receptor antibodies, whereas the antisera alone had no effect on the enzyme activity. Double-labeled immunohistochemical studies revealed that most ChAT-positive neurons expressed IGFII/M-6-P receptor immunoreactivity. These results indicate that the trophic effect of IGFII results from the direct action of this molecule through the IGFII/M-6-P receptor in septal cholinergic neurons. IGFI also stimulated ChAT activity, but with less potency than IGFII. Antibodies against the IGFII/M-6-P receptor inhibited approximately 50% of the IGFI response, suggesting that the effect of IGFI is mediated in part by the IGFII/M-6-P receptor. Thus, it appears that IGFII and IGFI are potent trophic factors for central cholinergic neurons and could potentially play a significant role in the differentiation, maintenance and regeneration of these neurons.

Insulin-like growth factors and insulin-like growth factor binding proteins in cerebrospinal fluid and serum of patients with dementia of the Alzheimer type

After acid gel-chromatography cerebrospinal fluid and serum levels of immunoreactive insulin-like growth factor 1 and 2 (IGF-1 and IGF-2) were determined in patients with dementia of the Alzheimer type (AD) and in healthy subjects. The AD CSF levels of immunoreactive IGF-1 did not differ from the subjects but the levels of immunoreactive IGF-2 was significantly elevated in both serum and CSF in the AD patient group. Additionally immunoreactive IGF-1 in AD serum was found to be significantly elevated. To characterize the CSF IGF binding protein activity (IGFBP), ligand blotting was performed on whole CSF from AD patients and subjects. The results demonstrate two major forms of IGFBP in CSF with approximate molecular weights of 33 KDa and 30 KDa. The two IGFBP forms are suggested to represent IGFBP-2 and IGFBP-6. A highly significant increase in both the IGFBPs was observed in the CSF of the AD patients compared to the healthy subjects.

Quantitative autoradiographic localization of [125I]insulin-like growth factor I, [125I]insulin-like growth factor II, and [125I]insulin receptor binding sites in developing and adult rat brain

Insulin-like growth factors I and II (IGF I and IGF II) and insulin itself, which are structurally related polypeptides, play an important role in regulating brain growth and development as well as in the maintenance of its normal functions during adulthood. In order to provide a substrate for the better understanding of the roles of these growth factors, we have investigated the anatomical distribution as well as the variation in the density of [125I]IGF I, [125I]IGF II, and [125I]insulin receptor binding sites in developing and adult rat brain by in vitro quantitative autoradiography. The distributional profile of [125I]IGF I, [125I]IGF II, and [125I]insulin receptor binding sites showed a widespread but selective regional localization throughout the brain at all stages of development. The neuroanatomic regions which exhibited relatively high density of binding sites with each of these radioligands include the olfactory bulb, cortex, hippocampus, choroid plexus, and cerebellum. However, in any given region, receptor binding sites for IGF I, IGF II, or insulin are concentrated in anatomically distinct areas. In the cerebellum, for example, [125I]IGF II receptor binding sites are concentrated in the granular cell layer, [125I]insulin binding sites are localized primarily in the molecular layer, whereas [125I]IGF I receptor binding sites are noted in relatively high amounts in granular as well as molecular cell layers. The apparent density of sites recognized by each radioligand also undergoes remarkable variation in most brain nuclei, being relatively high either during late embryonic (i.e., IGF I and IGF II) or early postnatal (i.e., insulin) stages and then declining gradually to adult levels around the third week of postnatal development. These results, taken together, suggest that each receptor-ligand system is regulated differently during development and thus may have different roles in the process of cellular growth, differentiation, and maintenance of the nervous system. Furthermore, the localization of [125I]IGF I, [125I]IGF II, and [125I]insulin receptor binding sites over a wide variety of physiologically distinct brain regions suggests possible involvement of these growth factors in a variety of functions associated with specific neuronal pathways.

The developmental pattern of rabbit brain insulin and insulin-like growth factor receptor expression

To examine the effect of development on rabbit brain insulin and insulin-like growth factor (IGF) receptor expression, we characterized and quantitated receptor mRNAs by Northern blot analysis and affinity-labeled ligand bound receptors by SDS-PAGE and autoradiography. At various stages of development ranging from 23 to 30 day gestational (term approximately 31 days), 1 to 10 day postnatal ages and the adult, no change in the whole brain insulin receptor mRNA (7.0, 6.0 and 5.5 kb) and affinity-labeled receptor protein (approximately 125 kDa) levels was observed. The IGF-I receptor mRNA (11.5, 6.5 and 4.5 kb) and affinity-labeled receptor (approximately 125 kDa) protein levels declined during the neonatal stages of development. In the case of the IGF-II receptor, while the mRNA levels (9.0 and 4.5 kb) remained constant, the corresponding affinity-labeled receptor protein (approximately 230 kDa) declined with maturation. We conclude that a differential regulation of brain insulin, IGF-I and IGF-II receptor expression occurs during development.

The effects of pituitary stalk transection, hypophysectomy and thyroid hormone status on insulin-like growth factor 2-, growth hormone releasing hormone-, and somatostatin mRNA prevalence in rat brain

We have used in situ hybridization histochemistry to determine the effects of pituitary stalk transection, hypophysectomy and drug-induced changes in thyroid status on mRNA levels encoding insulin-like growth factor 2, somatostatin, and growth hormone-releasing factor in the choroid plexus, hypothalamic periventricular nucleus, and arcuate nucleus, respectively. Pituitary stalk transection and hypophysectomy in Sprague-Dawley rats decreased insulin-like growth factor 2 and somatostatin mRNA and increased growth hormone-releasing factor mRNA. In each case, the effect of hypophysectomy exceeded that of pituitary stalk transection. Treatment with propylthiouracil for 10 days decreased somatostatin mRNA, markedly increased growth hormone-releasing factor mRNA but had no significant effect on insulin-like growth factor 2 mRNA. Treatment with triiodothyronine had no effect on the mRNAs measured. These findings corroborate the clinical observation of abnormal somatic growth in disturbances of thyroid and growth hormone status and provide further evidence of the effects of these metabolic disturbances and of pituitary disconnection and hypophysectomy on insulin-like growth factor 2 mRNA prevalence.

Localization of insulin-like growth factor-1 mRNA in murine central nervous system during postnatal development

Insulin-like growth factor-1 (IGF-1) is believed to play a role in the regulation of brain growth. The identity of cells responsible for its synthesis in the immature brain, however, has not been established. To identify potential sites of IGF-1 synthesis, in situ hybridization has been utilized to localize IGF-1 mRNA in the murine brain during the first postnatal month. Although IGF-1 mRNA was detected in all regions of the neonatal brain, there was considerable regional variation in the level of expression. Neurons were the principle sites IGF-1 mRNA expression and expression was typically restricted to one or two neuronal cell types within each region. In the cerebellar cortex, for example, only Purkinje cells hybridized to the IGF-1 probe. In contrast to gray matter, IGF-1 labeled cells were rarely found in presumptive white matter tracts of the forebrain. The hybridization signal was most prominent in regions where neurogenesis persisted after birth, including the cerebellum, olfactory bulb, and hippocampal complex. The timing of IGF-1 mRNA expression appeared to be temporally related to local neuronal proliferation. The number of labeled cells and intensity of hybridization signal was greatest during the first 2 postnatal weeks, a period of rapid neuronal proliferation in these regions. At the end of the first month, when neurogenesis had essentially ceased, IGF-1 signal strength had declined to background levels. The temporal and spatial pattern IGF-1 mRNA expression in the immature CNS was consistent with a role for locally produced IGF-1 in the regulation of brain development.

Increase of extracellular insulin-like growth factor I (IGF-I) concentration following electrolytical lesion in rat hippocampus

The change of extracellular insulin-like growth factor I (IGF-I) level following electrolytical lesion in rat hippocampus was studied by intracerebral microdialysis technique. The microdialysis probes were inserted into the lesioned and normal side of hippocampus on the day and each week after making the lesion for a period of four weeks. The content of IGF-I in the dialysis perfusate was measured by a radioimmunoassay. One week after surgery, IGF-I level (mean +/- S.E.M.) increased from 235.6 +/- 25.4 pg/100 microliters perfusate (day 0) to 305.5 +/- 15.6 pg/100 microliters perfusate (day 7), and then decreased gradually. This suggests that there is accumulation or increase in secretion of IGF-I at the lesioned site, and that IGF-I might have an important role in vivo in the brain tissue recovery from damage.

Insulin-like growth factor I mRNA levels are developmentally regulated in specific regions of the rat brain

The expression of mRNAs encoding insulin-like growth factor I (IGF-I) and the IGF-I receptor in the developing rat brain from embryonic day 16 to postnatal day 82 was analyzed using solution hybridization-RNase protection assays. Four distinct developmental patterns in the steady-state levels of IGF-I mRNA were seen. Specifically, the olfactory bulb showed a high perinatal level of IGF-I mRNA which declined dramatically by postnatal day 8. In contrast, cerebral cortex displayed maximal levels of IGF-I mRNA at postnatal day 8 and 13, which subsequently declined to adult levels (P82). A third developmental pattern was seen in the hypothalamus, where IGF-I mRNA increased from E16 up to postnatal day 3 and remained elevated thereafter. Finally, IGF-I mRNA levels in brainstem and cerebellum remained unchanged throughout the time period studied. We conclude that there are specific regional patterns of IGF-I gene expression in the developing rat brain. In contrast, IGF-I receptor gene expression did not exhibit any region-specific developmental changes. The developmental patterns of IGF-I gene expression seen in this study further substantiate the potential role of IGF-I in normal brain development.

Fibroblast growth factor-mediated proliferation of central nervous system precursors depends on endogenous production of insulin-like growth factor I

Fibroblast growth factor stimulates proliferation and subsequent differentiation of precursor cells isolated from the neuroepithelium of embryonic day 10 mice in vitro. Here we show that fibroblast growth factor-induced proliferation is dependent on the presence of insulin-like growth factors (IGFs) and that IGF-I is endogenously produced by the neuroepithelial cells. Blocking of endogenous IGF-I activity with anti-IGF-I antibodies results in complete inhibition of fibroblast growth factor-mediated proliferation and in cell death. IGF-I alone acts as a survival agent. These observations correlate with the detection of transcripts for IGF-I and basic fibroblast growth factor in freshly isolated neuroepithelium and are consistent with an autocrine action of these factors in early brain development in vivo.

Distribution of insulin-like growth factor 1 (IGF-1) and 2 (IGF-2) receptors in the hippocampal formation of rats and mice

This study demonstrated species differences in IGF-1 and IGF-2 receptor binding and localization in the hippocampus of the rat and mouse. Competition binding studies indicated that there were no differences in the relative binding affinities for the type 1 or type 2 receptors between the brains of these animals. These results suggested that the observed species differences were not attributable to alterations in IGF receptor kinetics. Receptor autoradiographic analyses demonstrated that IGF-1 binding differed in both the localization and overall receptor densities observed, with the rat demonstrating more specific localization and greater receptor density in the hippocampus than the mouse. The rat also exhibited a greater density of IGF-2 receptors in the hippocampus than the mouse. Despite differences in IGF receptor populations, both species exhibit similar hippocampal structure and lamination. Therefore, these results demonstrate a disparity in the localization of IGF receptor binding in the rat and mouse, suggesting that IGFs in these species are differentially regulated, with distinct neuromodulatory, neurotrophic, and/or developmental roles in this region of the brain. Previous comparative anatomical studies of the hippocampal formation of rats and mice fail to offer an explanation for the absence or reduction of binding of IGF-1 in the mouse. Although the mouse has a greater cell density in the s. granulosum than the rat, and both species exhibit similar glia and synaptic contact densities in the s. moleculare of the dentate gyrus, the mouse exhibits a complete absence of IGF-1 binding in this region. The lack of anatomical differences in the hippocampal formation of these species suggests that the patterns observed in IGF binding result from alterations in either neurochemical modulation of these neurons or specific neurotrophic requirements of the cells in this region. Differences have been reported on the concentrations and binding of various neurotransmitters in the hippocampus of these species, however these differences do not easily account for the variations observed in IGF binding in this study. IGFs are known to influence acetylcholine neurotransmission in the hippocampus as well as other brain areas in the rat. Recently, a truncated form of IGF-1, in which a tripeptide is cleaved from the N-terminus of the peptide, has been reported in brain. The cleaved tripeptide has been shown to activate glutamate receptors, which may dramatically influence excitatory neurotransmission in this region. Therefore, in addition to the possible neurotrophic actions of the peptide itself, subsequent processing of IGF-1 may be an important aspect of IGF-1 activity in the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin-like growth factor I (IGF-I) distribution in the tissue and extracellular compartment in different regions of rat brain

The regional distribution of insulin-like growth factor I (IGF-I) was examined in the tissue and extracellular compartment of rat brain. The tissue content of IGF-I was the highest in the pituitary gland, followed by the olfactory bulb, upper brainstem, cerebellum, striatum, hippocampus, lower brainstem, and cerebral cortex. The extracellular concentration was studied by intracerebral microdialysis technique, and the highest content was found in the hippocampus, followed by the olfactory bulb, hypothalamus, cerebellum, striatum, and cerebral cortex. The tissue and extracellular contents were significantly correlated in the olfactory bulb, hypothalamus, cerebellum, striatum, and cerebral cortex. IGF-I might act by paracrine and/or autocrine regulatory mechanisms in these regions.

Insulin-like growth factor II effects mediated through insulin-like growth factor II receptors

Insulin-like growth factor II (IGF-II) resembles the homologous peptide insulin-like growth factor I (IGF-I) in that it stimulates cellular growth in vitro. This effect is generally believed to be mediated through IGF type 1 receptors; the role of the IGF type 2 receptor remains, as yet, unknown. IGF-II has been shown to stimulate clonal expansion in cells from the human erythroleukaemia cell line K562, which displays binding of IGF-II and insulin but not IGF-I. This IGF-II effect was dose-dependent and correlated to the amount of specific binding; IGF-I did not stimulate growth. A similar effect on clonal growth was observed in the human T-cell line Jurkat. Furthermore, IGF-II was found to stimulate the cytotoxic activity of natural killer cells (as does interleukin 2). This effect was not inhibited by addition of IGF binding protein 1. Thus, it can be concluded that IGF-II, besides demonstrating standard IGF properties, exhibits unique biological effects in certain cells.

Localization and Characterization of Insulin-Like Growth Factor-I Receptors in Rat Brain and Pituitary Gland Using in vitro Autoradiography and Computerized Densitometry* A Distinct Distribution from Insulin Receptors

Abstract In order to identify likely sites of action of insulin-like growth factor-I (IGF-I) in rat brain and pituitary gland, we have used the technique of in vitro autoradiography and computerized densitometry to map, characterize and quantify its receptors in coronal and sagittal sections. A discrete and characteristic distribution of IGF-I receptor binding was demonstrated, with specific binding representing 85% of total binding. Displacement and specificity competition curves in the olfactory bulb were typical for authentic IGF-I receptors and computer analysis indicated a single class of binding site with a dissociation constant (K(d)) of 13 nM for the choroid plexus and 5.1 nM for the olfactory cortex. IGF-I receptor density was very high in the choroid plexus in ail ventricles, but the binding in other circumventricular organs was variable, with high levels in the median eminence and the sub-fornical organ, and low levels in the organum vasculosum of the lamina terminalis. Highest binding was seen in the glomerular layer of the olfactory bulb and its associated regions the taenia tecta and anteromedial olfactory nucleus. The preoptic and septal regions showed moderate binding, while the hypothalamus, with the exception of the median eminence, showed low IGF-I binding. The pituitary gland showed very high binding density in both anterior and posterior lobes, similar to the median eminence. The thalamus had high IGF-I binding density, while it was low in basal ganglia. In the limbic system the hippocampal CA2, CAS, CA4 layers showed high binding, with little in CA1, while binding was high also in the adjacent amygdala. Binding was low in the mid and hindbrain, with the exception of the geniculate bodies, and the sensory nucleus of the trigeminal nerve. Binding was high in the primary olfactory and endopyriform cortex and in specific superficial layers. Cerebellar binding was also high in the molecular layer. Fibre layers showed no binding. Comparison with insulin receptors revealed common distribution in the choroid plexus, paraventricular nucleus, cerebellum, entorhinal cortex and amygdala, with receptor density three- to five-fold higher for IGF-I than for insulin. In contrast, in the hippocampus, insulin binding was high in the CA1 field, and low in CA2, CA3, CA4 while for IGF-I binding the converse was seen. The arcuate nucleus showed prominent insulin labelling and minimal IGF-I binding, while the median eminence showed low insulin and high IGF-I binding. The hypothalamus was more widely labelled with insulin, while in the thalamus the converse was true. Olfactory bulb laminae were labelled with differing intensity by insulin and IGF-I. In common with insulin receptor distribution was the high density of IGF-I receptors over areas of extensive dendritic arborizations which receive rich synaptic inputs, in the cerebellum, hippocampus and olfactory bulb. We conclude that IGF-I receptors are widespread throughout rat brain and pituitary gland, with concentration in regions concerned with olfaction, autonomie and sensory processing, as well as in regulation of growth hormone release, via feedback at the median eminence and pituitary gland. Many of these regions have in common high rates of metabolic and synthetic activity, which may be mediated by IGF-I and its receptors.

Insulin growth factors regulate the mitotic cycle in cultured rat sympathetic neuroblasts

While neuronal mitosis is uniquely restricted to early development, the underlying regulation remains to be defined. We have now developed a dissociated, embryonic sympathetic neuron culture system that uses fully defined medium in which cells enter the mitotic cycle. The cultured cells expressed two neuronal traits, tyrosine hydroxylase [L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating); EC 1.14.16.2] and the neuron-specific 160-kDa neurofilament subunit protein, but were devoid of glial fibrillary acidic protein, a marker for non-myelin-forming Schwann cells in ganglia. Approximately one-third of the tyrosine hydroxylase-positive cells synthesized DNA in culture, specifically incorporating [3H]thymidine into their nuclei. We used this system to define factors regulating the mitotic cycle in sympathetic neuroblasts. Members of the insulin family of growth factors, including insulin and insulin-like growth factors I and II, regulated DNA synthesis in the presumptive neuroblasts. Insulin more than doubled the proportion of tyrosine hydroxylase-positive cells entering the mitotic cycle, as indicated by autoradiography of [3H]thymidine incorporation into nuclei. Scintillation spectrometry was an even more sensitive index of DNA synthesis, revealing a 4-fold insulin stimulation with an ED50 of 100 ng/ml. Insulin-like growth factor I was 100-fold more potent than insulin, whereas insulin-like growth factor II was less potent, suggesting that insulin growth factor type I receptors mediated the mitogenic responses. In contrast, the trophic protein nerve growth factor exhibited no mitogenic effect, suggesting that the mitogenic action of insulin growth factors is highly specific. Our observations are discussed in the context of the detection of insulin growth factors and receptors in the developing brain.

Insulin-like growth factors and somatomedin B in the cerebrospinal fluid of patients with dementia of the Alzheimer type

Cerebrospinal fluid levels of radioreceptor assayable insulin-like growth factors (RRA-IGFs) and immunoreactive somatomedin B (SMB) (RIA-B) were determined in apparently healthy individuals and in patients with dementia of the Alzheimer type (AD). The CSF levels of RIA-B and RRA-IGFs did not alter from the healthy controls. After being acidified, the CSF from the controls and from the presenile ADs were separated over a G-50 fine Sephadex . The RRA-IGFs activity eluted in three peaks. The results indicate that the major constituent of CSF RRA in both AD patients and controls is an IGF binding protein. The two minor peaks eluted at approximately 9 K and 6 K, corresponding to the elution positions of "big" IGF-2 and IGF-2.

Insulin-like growth factor 1 stimulates the release of acetylcholine from rat cortical slices

The effect of somatomedin, or insulin-like growth factors (IGF-1 and IGF-2), on the basal and potassium induced release of [3H]acetylcholine ([3H]Ach) from rat cortical slices, previously preincubated with [3H]choline ([3H]Ch), was studied in vitro. IGF-1 (1.4 x 10(-9) to 1.4 x 10(-8) M) had no effect on the basal release of [3H]ACh, while IGF-1 (1.4 x 10(-9) to 4.3 x 10(-8) M) increased the potassium induced release of [3H]ACh from rat brain slices in a concentration-dependent manner. However IGF-2 (1.4 x 10(-8) M) had no effect. Insulin (1.8 x 10(-8) to 5.3 x 10(-8) M), similarly, did not have any influence on the release of [3H]ACh, demonstrating that the facilitatory effect of IGF-1 on [3H]ACh release is not mediated via insulin receptors. This report demonstrates for the first time that IGF-1 has an effect on neurotransmission in the adult brain.

Expression of the insulin-like growth factor II gene in the choroid plexus and the leptomeninges of the adult rat central nervous system

The rat insulin-like growth factor II gene, encoding a fetal somatomedin, expresses a multitranscript family in embryonic/fetal tissues and in the adult brain and spinal cord. By performing in situ hybridization on tissue sections of adult brain and spinal cord, we have found that these transcripts are not expressed in neural or glial cells but are expressed in the epithelium of the choroid plexus of each cerebral ventricle and in the leptomeninges. We propose that the choroidal epithelial cells synthesize and secrete insulin-like growth factor II into the cerebrospinal fluid.

Regulation of insulin-like growth factor II receptors by growth hormone and insulin in rat adipocytes

The acute and long-term effects of growth hormone (GH) on the binding of insulin-like growth factor II (IGF-II) were evaluated in adipose cells from hypophysectomized rats given replacement therapy with thyroxine and hydrocortisone and in cells from their sham-operated littermates. After the cells were incubated with insulin and/or GH, the recycling of IGF-II receptors was metabolically inhibited by treating the cells with KCN. IGF-II binding was 100 +/- 20% higher in cells from GH-deficient animals when compared with sham-operated controls. These GH-deficient cells also showed an increased sensitivity for insulin as compared with control cells (the EC50 for insulin was 0.06 ng/ml in GH-deficient cells and 0.3 ng/ml in control cells). However, the maximal incremental effect of insulin on IGF-II binding was reduced approximately 27% by hypophysectomy. GH added to the incubation medium increased the number of IGF-II binding sites by 100 +/- 18% in cells from hypophysectomized animals. This increase was rapidly induced (t1/2, approximately 10 min), but the time course was slower than that for the stimulatory effect of insulin. Half-maximal effect of GH on IGF-II binding was obtained at approximately equal to 10 ng/ml. Thus, GH added in vitro exerted a rapid insulin-like effect on the number of IGF-II receptors. GH also appears to play a regulating role for maintaining the cellular number of IGF-II receptors and, in addition, modulates the stimulatory effect of insulin on IGF-II binding.

Characterization of somatomedins from human fetal brain: identification of a variant form of insulin-like growth factor I

A fetal form of somatomedin (insulin-like growth factor) that crossreacts in the fetal brain radioreceptor assay has been proposed to exist in humans. Using this assay to monitor activity during purification, we have isolated a variant form of insulin-like growth factor I (IGF-I) from human fetal brain tissue. The variant IGF-I showed potent crossreaction in the fetal brain radioreceptor assay and stimulated DNA synthesis in fetal brain cells in vitro. Structural analysis revealed the variant IGF-I to have a truncated NH2-terminal region compared to IGF-I isolated from serum. An additional peptide, which displayed less potent crossreaction in the fetal brain radioreceptor assay, was also isolated from the human fetal brain. Partial amino acid sequence analysis revealed identity to insulin-like growth factor II.