Localization of insulin-like growth factor I (IGF-I)-like immunoreactivity in the developing and adult rat brain

Interaction of insulin-like growth factor-I and estradiol signaling pathways on hypothalamic neuronal differentiation

Neurotrophic effects of estradiol and insulin-like growth factor-I were assessed in primary cultures from fetal rat hypothalamus. Cultured neurons were immunostained with an antibody for the microtubule-associated protein-2. While both estradiol and insulin-like growth factor-I increased the number of microtubule-associated protein-2-immunoreactive neurons and the extension of immunoreactive processes, the effect of these two factors was not additive. The estradiol-induced increases in neuronal numbers and extension of neuronal processes were blocked by either the estrogen receptor antagonist ICI 182,780 or by an anti-sense oligonucleotide to the estrogen receptor. Furthermore, incubation of the cultures with an anti-sense oligonucleotide directed against the insulin-like growth factor-I messenger RNA also blocked the effect of estradiol. In turn, the effects of insulin-like growth factor-I were blocked by the estrogen receptor antagonist ICI 182,780 and by the anti-sense oligonucleotide to the estrogen receptor. These findings suggest that estradiol-induced activation of the estrogen receptor in developing hypothalamic cells requires the presence of insulin-like growth factor-I, and that both estradiol and insulin-like growth factor-I use the estrogen receptor as a mediator of their trophic effects on hypothalamic neurons.

In vivo actions of insulin-like growth factor-I (IGF-I) on cerebellum development in transgenic mice: evidence that IGF-I increases proliferation of granule cell progenitors

The in vivo actions of insulin-like growth factor-I (IGF-I) on cerebellum development have been investigated in transgenic (Tg) mice (IGF-II/I Tg mice) in whom an IGF-II promoter-driven IGF-I transgene is highly expressed in cerebellum. Compared to normal littermates, the brains of IGF-II/I Tg mice exhibited overgrowth beginning from the second week of postnatal life. Among the brain regions examined, cerebellum exhibited the greatest increase in size, such that by 50 days of age cerebellar weight and DNA content were increased by 90% and 143%, respectively, compared to littermate controls. Morphological studies of adult IGF-II/I Tg mice showed that the total number of granule and Purkinje cells was increased by 82% and 20%, respectively, findings consistent with the increased cerebellar DNA content and indicating that the increased cerebellar weight was due in part to an increase in cell number. The thickness of the molecular layer also was increased in IGF-II/I Tg mice. During early postnatal development the number of external granular layer cells, as well as the number of BrdU labeled external granular cells, was increased. These data strongly indicate that IGF-I increases granule cell number by a mechanism that involves the stimulation of granule cell progenitor proliferation. Our findings also indicate that IGF-I influences the growth of Purkinje cells and possibly of other cell types in the cerebellum.

Immunohistochemical localization of insulin-like growth factor binding protein-5 in the developing rat nervous system

The insulin-like growth factors (IGF-I and IGF-II) are peptides with both growth-promoting and insulin-like metabolic effects. The IGFs interact with and are modulated by a group of six IGF-binding proteins (IGFBP-1 through IGFBP-6). Previous studies have characterized IGFBP-5 and IGF-I gene expression in the developing nervous system. In the current study, cellular and tissue-specific distribution of IGFBP-5 protein was examined in the developing rodent nervous system using immunohistochemistry. Beginning with embryonic stage E12, IGFBP-5 immunoreactivity was observed in peripheral nerves. This pattern persisted through adulthood and was detected within Schwann cells and axons after postnatal day 16 (P16). IGFBP-5 immunoreactivity first appeared in the CNS at P16. Purkinje cells of the cerebellum were immunostained at P16, P32 and in the adult. IGFBP-5 immunoreactivity was also detected in several brain stem nuclei and their corresponding tracts as well as neuroglia. Nerve tracts and glia in the postnatal spinal cord were also immunopositive, however, spinal cord neurons were not stained. The current results, coupled with the known profile of IGF-I expression during nervous system development demonstrates the colocalization of IGF-I and IGFBP-5 in PNS, cerebellum, and brain stem.

The insulin-like growth factor I system in cerebellar degeneration

Brain insulin-like growth factor I (IGF-I) and its related molecules may be involved in neurodegenerative processes in which IGF-I-containing pathways are compromised. Since IGF-I is present in the olivocerebellar circuitry, two types of late-onset cerebellar ataxias (olivopontocerebellar and idiopathic cerebellar cortical atrophy) were chosen to test this hypothesis. The following significant changes in the peripheral IGF-I system of these patients were found: low IGF-I levels, and high IGF-binding protein 1 (BP-1), and BP-3 affinity for IGF-1. Sixty percent of the patients also had significantly low insulin levels. Patients suffering from other neurological diseases with cerebellar dysfunction and ataxia not involving the olivocerebellar pathway also had low IGF-I levels, while IGFBPs and insulin levels were normal. Our data indicate that degeneration of an IGF-I-containing neuronal pathway produces significant changes in the peripheral IGF system. This suggests strongly that the endocrine (bloodborne) and the paracrine/autocrine (brain) IGF systems are linked functionally. We propose that alterations in the blood IGF-I system may constitute a marker of some cerebellar diseases.

Involvement of protein kinase C and nitric oxide in the modulation by insulin-like growth factor-I of glutamate-induced GABA release in the cerebellum

Insulin-like growth factor-I elicits a long-term depression of the glutamate-induced GABA release in the adult rat cerebellum that lasts at least several hours. We studied whether protein kinase C and nitric oxide may be involved in this effect of insulin-like growth factor-I on GABA release since both signalling pathways have been implicated in other forms of neuromodulation in the cerebellum. By using microdialysis in the adult rat cerebellum, we found that either an inhibitor of protein kinase C (staurosporine) or of nitric oxide synthase (Nw-nitro-L-arginine methyl ester) counteracted the long-term, but not the acute effects of insulin-like growth factor-I on glutamate-induced GABA release. On the contrary, when either an activator of protein kinase C (phorbol ester), or an nitric oxide donor (L-arginine), were given with glutamate, they mimicked only the acute effects of insulin-like growth factor-I on glutamate-induced GABA release. Finally, when both protein kinase C and nitric oxide-synthase were simultaneously inhibited by conjoint administration of staurosporine and Nw-nitro-L-arginine methyl ester, a complete blockage of both the short and the long-term effects of insulin-like growth factor-I on GABA release was obtained. These results, indicate that: (i) activation by insulin-like growth factor-I of either the protein kinase C or nitric oxide-signalling pathways is sufficient for the short-term inhibition of glutamate-induced GABA release; and (ii) simultaneous activation of both the protein kinase C and the nitric oxide signalling pathways is necessary for insulin-like growth factor-I to induce a long-term depression of GABA responses to glutamate. Thus, long-term depression of glutamate-induced GABA release by insulin-like growth factor-I in the cerebellum is mediated by simultaneous activation of both protein kinase C and nitric oxide-signalling pathways.

Insulin-like growth factor-1 is a potent neuronal rescue agent after hypoxic-ischemic injury in fetal lambs

This study was designed to determine the potential of IGF-1 as a neuronal rescue agent after cerebral ischemia. Unanesthetized late gestation fetal sheep were subjected to 30-min cerebral ischemia by inflation of carotid artery occluder cuffs. 2 h later either 0.1 microgram rhIGF-1, 1 microgram rhIGF-1, 10 micrograms rhIGF-1, or vehicle was infused into a lateral cerebral ventricle over 1 h. Histologic outcome was assessed 5 d later. Overall neuronal loss was reduced with 0.1 microgram (P < 0.05) and 1 microgram (P < 0.002) rhIGF-1, but treatment with 10 micrograms was not effective. With 1 microgram rhIGF-1 neuronal loss scores were significantly lower in brain regions examined including cortex, hippocampus, and striatum, whereas with 0.1 microgram rhIGF-1 the parietal cortex and thalamus were not improved and the improvement seen in other regions was less than with 1 microgram rhIGF-1. Treatment with 1 microgram rhIGF-1 also delayed the onset of seizures and reduced their incidence. Moreover, the secondary phase of cytotoxic edema was reduced and delayed in onset. We conclude that low dose rhIGF-1 therapy promotes neuronal rescue after cerebral hypoxic-ischemic injury in utero, but the effect is dose dependent. Importantly, rhIGF-1 is effective and nontoxic when administered 2 h after the hypoxic ischemic insult. This distinguishes IGF-1 from most other neuroprotective therapies and suggests clinical application may be possible.

Developing oligodendroglia express mRNA for insulin-like growth factor-I, a regulator of oligodendrocyte development

Insulin-like growth factors IGF-I and IGF-II are potent inducers of oligodendrocyte development. Because IGF-I is produced, in some cases, by the same cells that respond to it (autocrine/paracrine action), we examined the possibility that IGF-I is expressed by developing oligodendroglial cells. We employed a sensitive method, reverse transcriptase-polymerase chain reaction (RT-PCR), to detect IGF-I mRNA in purified populations of oligodendroglial cells isolated from rat brain during the period of oligodendrocyte development. Cells were purified by fluorescence activated cell sorting (FACS), using antibodies to the cell surface antigenic markers O4 and galactocerebroside (GC). RNA was isolated from the sorted cells, reverse-transcribed, and PCR-amplified, using a strategy that recognizes IGF-I mRNA but not DNA. The amplified band was identified as IGF-I by size, hybridization to an IGF-I-specific antisense probe, and restriction analysis. IGF-I mRNA was detected in O4-positive/GC-negative oligodendrocyte precursors and, more weakly, in GC-positive oligodendrocytes. IGF-I mRNA could be detected reproducibly in RNA extracted from 100-cell samples of O4-positive cells, making it unlikely that the mRNA was derived from contaminants in the FACS-sorted cell populations. We conclude that IGF-I is expressed by developing oligodendroglia. Autocrine expression of IGF-I by developing oligodendroglial cells suggests that oligodendrocyte development is, in part, autoregulatory.

Insulin-like growth factor I is an afferent trophic signal that modulates calbindin-28kD in adult Purkinje cells

Recent evidence suggests that Purkinje cells are specific targets of insulin-like growth factor I (IGF-I) through their entire life span. During development, Purkinje cell numbers and their calbindin-28kD content increase after IGF-I treatment in culture. In the adult, part of the IGF-I present in the cerebellum is transported from the inferior olive, and modulates Purkinje cell function. We investigated whether IGF-I produced by inferior olive neurons and transported to the contralateral cerebellum through climbing fibers may modulate the levels of calbindin-28kD in the cerebellum of adult animals. Twenty-four hr after injection of an antisense oligonucleotide of IGF-I into the inferior olive, both IGF-I and calbindin-28kD levels in the contralateral cerebellar lobe were significantly reduced, while the number of calbindin-positive Purkinje cells was unchanged. The effect of the antisense on IGF-I levels was fully reversed 3 days after its injection into the inferior olive, with a postinhibitory rebound observed at this time, while calbindin-28kD levels slowly returned to control values. A control oligonucleotide did not produce any change in either IGF-I or calbindin-28kD content in the cerebellum. These results indicate that normal levels of IGF-I in the inferior olive are necessary to maintain appropriate levels of IGF-I in the cerebellum and of calbindin-28kD in the Purkinje cell. These results also extend our previous findings on the existence of an olivo-cerebellar IGF-I-containing pathway with trophic influence on the adult Purkinje cell.

Insulin prevents apoptosis of external granular layer neurons in rat cerebellar slice cultures

Using the slice culture system of 9-day-old rat cerebellum, effects of insulin on cell death of developing granule neurons were examined. Apoptotic cells were observed after 3 days culture by the in situ nick end labeling technique. Insulin deprivation induced apoptosis of granule neurons largely in the external granular layer, but scarcely in the internal granular layer. Proliferation of external granular layer neurons during the early culture period was not affected significantly by the insulin deprivation. These results suggest that insulin may prevent apoptosis of premigratory granule neurons during the development of the cerebellar cortex.

Developmental changes in the expression of gamma-aminobutyric acidA/benzodiazepine receptor subunit mRNAs in the murine inferior olivary complex

The pharmacological and physiological properties of ligand-gated ion channels are dependent on their subunit composition; spontaneously occurring changes in subunit composition during neuronal development may result in dramatic functional differences between embryonic and adult forms of the receptor complex. In the present study, in situ hybridization with antisense cRNA probes was used to examine the subunit composition of the gamma-aminobutyric acidA/benzodiazepine (GABAA/BZ) receptor in the developing inferior olivary complex. This receptor is thought to be a pentameric chloride channel comprised of selected alpha, beta, gamma, delta, and rho subunits, the majority of which have several isoforms: alpha 1-6, beta 1-4, gamma 1-4, and rho 1,2. Among the 13 subunit variants present in the mammalian central nervous system, alpha 2-5, beta 3, and gamma 1,2 mRNAs are expressed at significant levels in the inferior olivary complex. Two clearly different temporal patterns of GABAA/BZ receptor subunit mRNA expression were observed: The expression of alpha 3, alpha 5, beta 3, and gamma 2 mRNAs was at a peak during embryonic and early postnatal development followed by rapid down-regulation thereafter. Conversely, alpha 2, alpha 4, and gamma 1 mRNA expression was very low or absent during early development, and a pronounced increase was observed at the end of postnatal week 1. These studies suggest that there are developmental changes in the subunit composition of the GABAA/BZ receptor in inferior olivary neurons. These changes in subunit expression, which occur during a period of major alterations in afferent and efferent synaptic connections, may subserve a change in the role of GABA from its function as a neurotrophic factor to that of an inhibitory neurotransmitter.

The insulin-like growth factor signaling system and ALS neurotrophic factor treatment strategies

Because of its multi-faceted potential as a neurotrophic factor, insulin-like growth factor I (IGF-I) has been given to hundreds of ALS patients world-wide. Unlike some patients with post-polio syndrome and fragile elderly males, it is unclear whether any of these patients possess disturbances in IGF signaling. We found that about 25% of ALS patients in a controlled trial of human growth hormone (hGH) had lower or higher than normal IGF-I serum levels. Many ALS patients do have some of the characteristics of type II diabetes mellitus, where IGF-I therapy is also under way. In addition, in type I diabetes significant increase in a circulating molecule that binds IGF-I, IGF-I binding protein 1 (IGFBP-1), occurs along with reduced IGF-I, when neuropathic complications are prominent. We have studied the response of IGFBPs in ALS patients to subcutaneous rhIGF-I and found transient induction of IGFBP-1. Studies related to the IGFBPs have not been done in familial ALS (FALS) patients. However, the gene for another IGFBP, BP-2, co-localizes with the gene for juvenile ALS (ALSJ) on chromosome 2. IGF-I has been given to several models of motor neuron degeneration in the mouse, including motor neuron disease and wobbler, with beneficial effects. However, it is also not known whether any accepted genetic mouse model of motor neuron degeneration possesses any disturbance in the IGF signaling system.

Igf1 gene disruption results in reduced brain size, CNS hypomyelination, and loss of hippocampal granule and striatal parvalbumin-containing neurons

Homozygous Igf1-/- mice at 2 months of age had reduced brain weights, with reductions evenly affecting all major brain areas. The gross morphology of the CNS was normal, but the size of white matter structures in brain and spinal cord was strongly reduced, owing to decreased numbers of axons and oligodendrocytes. Myelinated axons were more strongly reduced in number than unmyelinated axons. The volume of the dentate gyrus granule cell layer was reduced in excess of the decrease in brain weight. Among populations of calcium-binding protein-containing neurons, there was a selective reduction in the number of striatal parvalbumin-containing cells. Numbers of mesencephalic dopaminergic neurons, striatal and basal forebrain cholinergic neurons, and spinal cord motoneurons were unaffected. Cerebellar morphology was unaltered. Our findings suggest cell type- and region-specific functions for IGF-I and emphasize prominent roles in axon growth and maturation in CNS myelination.

Nutritional insult and recovery in the neonatal rat cerebellum: insulin-like growth factors (IGFs) and their binding proteins (IGFBPs)

Alterations in growth caused by neonatal malnutrition may be mediated in part by changes in insulin-like growth factor (IGF) and IGF binding protein (IGFBP) expression. Since the neonatal rat cerebellum undergoes a transient, proliferative growth phase in the first two weeks of life, this structure was used to determine whether alterations in circulating and tissue IGFs and IGFBPs may mediate effects of impaired nutrition on the developing central nervous system. Gravid rats were placed on a 4% (protein-calorie deprived, D) or 20% (control, C) protein diets one day prior to delivery and allowed to nurse their pups postpartum. Pups nursing from D mothers received a limited volume of milk and were calorically deprived. Some litters of D pups were foster fed by C mothers from day 8 to day 13 to constitute a recovery group (R). Cerebellar weight, protein, and DNA content in D pups were less than C, p < 0.001. In R pups, DNA and protein returned to C levels by day 13. Between days 6 and 13, serum IGF-I levels rose from 158 +/- 18 to 210 +/- 18 ng/ml in C but remained low in D (47 +/- 6 ng/ml and 25 +/- 3 ng/ml), respectively. In R pups, serum IGF-I partially recovered during this time, and increased from 49 +/- 5 to 110 +/- 7 ng/ml. In cerebellar extracts, IGF-I levels in both C and D were lower at 13 days than at 6 days, p < 0.05 and p < 0.005, respectively. IGF-I levels in C were similar at day 9 and day 11 and were consistently higher than D (11.84 +/- 0.83 vs 8.56 +/- 0.92 ng/g, p < 0.02 C vs D). In R, IGF-I was reduced on day 11, but was similar to C on day 13. Serum IGF-II in D was lower than C, p < 0.01, and did not increase in the R group. Cerebellar IGF-II was virtually undetectable in either group. Immunoprecipitation and ligand blotting studies of serum demonstrated that circulating levels of 32-34 K IGFBPs were increased 3-4 fold in D vs C, reflecting high levels of IGFBP-1 and/or -2, while levels of 24 K IGFBP-4 were lower in D vs C. By contrast, immunoprecipitation and ligand blotting of cerebellar extracts detected IGFBP-2 and -4, but did not detect IGFBP-1. Further, tissue levels of IGFBP-2 were not increased in D vs C, and levels of IGFBP-4 also were not markedly affected by nutritional deprivation. These results suggest that alterations in tissue content and the availability of IGF-I only modestly contributed to the effects of impaired nutrition in the developing central nervous system.

An immunohistochemical and in situ hybridization study of insulin-like growth factor I within fetal neuron cell cultures

Fetal neuron cell cultures (NCC) from 22 day gestation and 18 day gestation fetal rabbit brain were studied for the presence of insulin-like growth factor I (IGF I). The 22 day gestation NCC were incubated in an IGF I free/insulin free/serum free medium. The 18 day gestation NCC were incubated in: (1) IGF I free/insulin free/serum free medium, (2) IGF I containing medium (100 ng)/serum free medium, and (3) serum containing medium. The 22 day gestation NCC survived in the IGF I free/insulin free/serum free medium. Furthermore, IGF I was detected in the medium by RIA from day one to day ten of incubation. In contrast, the 18 day gestation NCC did not survive in the IGF I free/insulin free/serum medium, but survived in the serum medium. When the 18 day gestation NCC were incubated in the serum free medium containing 100 ng IGF I the cells survived for a period of 2-3 days. Immunoreactive IGF I was found within the 22 day gestation NCC incubated in the IGF I free/insulin free/serum free medium and 18 day gestation NCC in serum medium. Likewise, IGF I mRNA was found only within the 22 day gestation NCC. Internalization studies of IGF I have shown that the peptide was internalized from the medium by the two different gestational age NCC's studied. IGF I receptors were found in both 22 day gestation and 18 day gestation NCC. In conclusion IGF I may promote cell survival in early stages of brain development, and may be of exogenous origin. In contrast the 22 day gestation NCC are capable of producing and secreting IGF I, and indeed appear to respond to this growth factor in an autocrine fashion.

Brain growth retardation due to the expression of human insulin like growth factor binding protein-1 in transgenic mice: an in vivo model for the analysis of igf function in the brain

Three lines of transgenic (Tg) mice carrying a fusion gene linking the mouse metallothionein-I promoter to a cDNA encoding human insulin-like growth factor binding protein-1 (hIGFBP-1) were found to express the transgene in brain. As judged by comparing Tg brain weights to those of non-transgenic littermates, adult hemizygotic Tg mice of each line exhibited brain growth retardation (16.2%, 14.4% and 8.1% reductions in weight, respectively in each line). In two lines, total brain DNA and protein content were decreased. Further analysis indicated that the brain growth retardation was manifested in the second week of postnatal life. Given that the insulin-like growth factors (IGFs) stimulate cell proliferation and/or survival in neural cultures and that hIGFBP-1, when present in a molar excess, inhibits IGF interactions with their cell surface receptors, the brain growth retardation in hIGFBP-1 Tg mice likely results from hIGFBP-1 inhibition of IGF-stimulated growth-promoting actions. These hIGFBP-1 Tg mice should prove useful in defining IGF actions during postnatal brain maturation.

The insulin-like growth factor I system in the rat cerebellum: developmental regulation and role in neuronal survival and differentiation

The developmental regulation of insulin-like growth factor I (IGF-I), its receptor, and its binding proteins (IGFBPs) was studied in the rat cerebellum. All the components of the IGF-I system were detectable in the cerebellum at least by embryonic day 19. Levels of IGF-I receptor and its mRNA were highest at perinatal ages and steadily decrease thereafter, although a partial recovery in IGF-I receptor mRNA was found in adults. Levels of IGF-I and its mRNA also peaked at early ages, although immunoreactive IGF-I showed a second peak during adulthood. Finally, levels of IGFBPs were also highest at early postnatal ages and abruptly decreased thereafter to reach lower adult levels. Since highest levels of the different components of the IGF-I system were found at periods of active cellular growth and differentiation we also examined possible trophic effects of IGF-I on developing cerebellar cells in vitro. We found a dose-dependent effect of IGF-I on neuron survival together with a specific increase of the two main neurotransmitters used by cerebellar neurons, GABA and glutamate. Analysis of cerebellar cultures by combined in vitro autoradiography and immunocytochemistry with cell-specific markers indicated that both Purkinje cells (calbindin-positive) and other neurons (neurofilament-positive) contain IGF-I binding sites. These results extend previous observations on a developmental regulation of the IGF-I system in the cerebellum and reinforce the notion of a physiologically relevant trophic role of IGF-I in cerebellar development.

Expression of the raf protooncogene in glial cells of the adult rat cerebral cortex, brain stem and spinal cord

The raf protooncogenes encode cytoplasmic serine/threonine-specific protein kinases which can be activated from different growth factor receptors by phosphorylation. Our previous immunohistochemical studies proved that raf kinases are present in neurons of the mammalian central nervous system. The present study describes the immunohistochemical localization of raf kinases in glia-like cells of the rat cerebral cortex, spinal cord and brain stem. Small glia-like cells measuring 8-12 microns were observed in the neocortex, the entorhinal and prepiriform allocortical areas and the subcortical white matter. In the hippocampus, the immunoreactive cells were most numerous in the fimbria, the alveus and the molecular layer of the dentate fascia. Ultrastructural studies following preembedding immunohistochemistry revealed that in the cerebral cortex only astrocytes contained raf-protein-like immunoreactivity. Our immunofluorescence studies showed, that the white matter of the spinal cord, the pyramids of the medulla and the basis of the pons contained small glia-like cells, too. No electron microscopic investigations were performed, but the location (white matter tracts) and size (6-12 microns) of these cells suggested their glial nature. On the basis of data from other cell systems we expect that raf kinases participate in growth factor- and cytokine-regulated glial functions of the mammalian central nervous system.

Insulin-like and fibroblast growth factors in spinal cords, nerve roots and skeletal muscle of human controls and patients with amyotrophic lateral sclerosis

Insulin-like growth factors (IGF-I and IGF-II) and fibroblast growth factors [acidic FGF (aFGF) and basic FGF (bFGF)] are trophic for motor neurones in vitro and (in laboratory animals) in vivo. An immunohistochemical investigation was performed on the distribution of these factors in the neuromuscular system of control patients and patients with amyotrophic lateral sclerosis (ALS). Comparisons were made with rat tissue. IGF-I immunoreactivity (IGF-I-IR) was seen in motor neurone cell bodies and axons, astroglia and Schwann cells, and in muscle fibres. IGF-II-IR was weak in all these cells. aFGF-IR was present in motor neurone cell bodies and axons, oligodendroglia and muscle fibres, but was not demonstrable in Schwann cells. bFGF-IR was present in motor neurone cell bodies and axons, and in astroglia, but was not seen in Schwann cells or muscle fibres. The distribution of the IGFs and FGFs in material from motor neurone disease (MND) and controls was similar. A role for any of these factors in the etiology of MND is, therefore, unlikely. IGF-I-IR and aFGF-IR were stronger in type II than in type I muscle fibres and were increased in denervated fibres. Species differences were found for IGF-I and bFGF. The function of these factors is apparently not entirely similar in humans and rats.

Insulin-like growth factor I-immunoreactive peptide in adult human cerebellar Purkinje cells: co-localization with low-affinity nerve growth factor receptor

It has been proposed that Insulin-like growth factor I is involved in the development, growth and maintenance of the central nervous system possibly interacting with other trophic factors. High levels of insulin-like growth factor I have been detected in the cerebellum during development and adulthood suggesting a specific role for insulin-like growth factor I in this brain area. While there is ever increasing data regarding the cell types containing endogenous insulin-like growth factor I in the rat brain, no information on the human brain is yet available. In the present study we sought to analyse the precise location of insulin-like growth factor I peptide in the adult human cerebellum using a specific antiserum against recombinant human insulin-like growth factor I. After immunocytochemistry, numerous Purkinje cells exhibited intense positive staining occupying the cell soma, dendrites and dendritic spines as well as axons. Occasionally, immunoreactive Purkinje cell axons were arciform and exhibited bulbous dilatations along their proximal length. Putative recurrent collaterals of Purkinje cell axons were also insulin-like growth factor I reactive. Double-staining immunocytochemistry in the same sections consistently showed, as expected, co-expression of insulin-like growth factor I and calbindin, although a few calbindin containing Purkinje cells lacked insulin-like growth factor I immunostaining suggesting there are insulin-like growth factor I positive Purkinje cell subsets in the human cerebellum. In addition, co-expression of insulin-like growth factor I and low-affinity nerve growth factor receptor-immunoreactive protein was found in a subpopulation of insulin-like growth factor I positive Purkinje cells. The results of this study prove the presence of insulin-like growth factor I immunoreactivity in a Purkinje cell subpopulation of the adult human cerebellum suggesting that insulin-like growth factor I may participate in paracrine or autocrine regulatory systems in the adult human brain.

Orthograde transport and release of insulin-like growth factor I from the inferior olive to the cerebellum

Insulin-like growth factor I (IGF-I) and its receptor are expressed in functionally related areas of the rat brain such as the inferior olive and the cerebellar cortex. A marked decrease of IGF-I levels in cerebellum is found when inferior olive neurons are lesioned. In addition, Purkinje cells in the cerebellar cortex depend on this growth factor to survive and differentiate in vitro. Thus, we consider it possible that IGF-I forms part of a putative trophic circuitry encompassing the inferior olive and the cerebellar cortex and possibly other functionally connected areas. To test this hypothesis we have studied whether IGF-I may be taken up, transported, and released from the inferior olive to the cerebellum. We have found that 125I-IGF-I is taken up by inferior olive neurons in a receptor-mediated process and orthogradely transported to the cerebellum. Thus, radioactivity found in the cerebellar lobe contralateral to the injection site in the inferior olive was immunoprecipitated by an anti-IGF-I antibody, co-eluted with 125I-IGF-I in an HPLC column, and co-migrated with 125I-IGF-I in an SDS-urea polyacrylamide gel electrophoresis. Time-course studies indicated that orthograde axonal transport is relatively rapid since 30 min after the injection, radiolabeled IGF-I was already detected in the contralateral cerebellum. Furthermore, transport of IGF-I from the inferior olive is specific since when 125I-neurotensin was injected in the inferior olive or when 125I-IGF-I was injected in the pontine nucleus, no radioactivity was found in the contralateral cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term depression of glutamate-induced gamma-aminobutyric acid release in cerebellum by insulin-like growth factor I

We tested the possibility that insulin-like growth factor I (IGF-I) acts as a neuromodulator in the adult cerebellar cortex since previous observations indicated that IGF-I is located in the olivo-cerebellar system encompassing the inferior olive and Purkinje cells. We found that conjoint administration of IGF-I and glutamate through a microdialysis probe stereotaxically implanted into the cerebellar cortex and deep cerebellar nuclei greatly depressed the release of gamma-aminobutyric acid (GABA), which normally follows a glutamate pulse. This inhibition was dose-dependent and long-lasting. Moreover, the effect was specific for glutamate since KCl-induced GABA release was not modified by IGF-I. Basic fibroblast growth factor, another growth-related peptide present in the cerebellum, did not alter the response of GABA to glutamate stimulation. In addition, electrical stimulation of the inferior olivary complex significantly raised IGF-I levels in the cerebellar cortex. Interestingly, when the inferior olive was stimulated in conjunction with glutamate administration, GABA release by cerebellar cells in response to subsequent glutamate pulses was depressed in a manner reminiscent of that seen after IGF-I. These findings indicate that IGF-I produces a long-lasting depression of GABA release by Purkinje cells in response to glutamate. IGF-I might be present in climbing fiber terminals and/or cells within the cerebellar cortex and thereby might affect Purkinje cell function. Whether this IGF-I-induced impairment of glutamate stimulation of Purkinje cells underlies functionally plastic processes such as long-term depression is open to question.

Expression of neuropeptide processing enzymes and neurosecretory proteins in ependyma and choroid plexus epithelium

Recent studies suggest that brain ependyma and choroid plexus produce neuropeptide processing enzymes. To facilitate the understanding of these cells and their ability to produce biologically active peptides, we developed cultures of defined cell type. Ependymal cells were characterized by morphological criteria, and choroid plexus epithelial cell lines were characterized by the presence of the mRNA for IGF-II and transthyretin, a thyroxine binding protein produced in liver and choroid plexus. The ependymal cells and the choroid plexus epithelial cell lines were then examined for the presence of mRNAs for various neuropeptide processing enzymes. Northern blot analysis revealed high levels of furin, carboxypeptidase E, and peptidyl glycine alpha-amidating monooxygenase mRNAs, with levels in ependymal cells comparable to those in brain or pituitary. Carboxypeptidase E activity was detected in medium from cultured ependymal cells; this activity was identified as carboxypeptidase E based on the acidic pH optimum and sensitivity to various inhibitors. The mRNAs for other neuropeptide processing enzymes, such as prohormone convertases 1 and 2, were not detected on Northern blots of RNA from ependyma or choroid plexus epithelium. Since ependyma and choroid plexus epithelium express a subset of processing enzymes, we suggest that these cells have the capacity to produce biologically active peptides. Initial screening by reverse transcriptase-polymerase chain reaction assays has demonstrated the presence of mRNA for the neurosecretory proteins chromogranin B and secretogranin II in both ependyma and choroid plexus epithelium.

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.

Natural fluctuation and gonadal hormone regulation of astrocyte immunoreactivity in dentate gyrus

The number and the surface density of cells immunoreactive for the specific astrocytic marker glial fibrillary acidic protein (GFAP), were evaluated in both the hilus of the dentate gyrus and the granular layer of the vermis of the cerebellar cortex of adult female rats during the different phases of the estrous cycle, after ovariectomy and after the pharmacological administration of estradiol and/or progesterone to ovariectomized rats. Although no significant differences were detected in the number of immunoreactive cells among the different experimental groups studied, their surface density showed significant changes in the hilus of the dentate gyrus. The surface density of immunoreactive cells was increased in the afternoon of proestrus and on the morning of estrus compared to the morning of proestrus, diestrus, and metestrus, was decreased after ovariectomy, and showed a dose-dependent increase in ovariectomized rats injected with 17 beta estradiol (1, 10, or 300 micrograms/rat), alone or in combination with progesterone (500 micrograms/rat). In contrast, it was not affected by the administration of 17 alpha estradiol (300 micrograms/rat). The surface density of immunoreactive cells was significantly increased over control values by 5 h after the injection of 17 beta estradiol (300 micrograms/rat) and as early as 1 h after the administration of progesterone. The separate injection of either 17 beta estradiol or progesterone had smaller effects on the surface density of immunoreactive cells than did the administration of both hormones together. The surface density of GFAP-immunoreactive cells reached maximal values by 24 h after the administration of 17 beta estradiol and/or progesterone and returned to control levels by 48 h after the combined injection of progesterone and 17 beta estradiol, while in the rats that were injected with only one of the two hormones, the surface density of immunoreactive cells remained over control values for at least 9 days. No such hormonal effects on GFAP-immunoreactive cells were observed in the cerebellar cortex.

Localization of insulin-like growth factor binding protein-4 expression in the developing and adult rat brain: analysis by in situ hybridization

We have previously isolated insulin-like growth factor binding protein-4 (IGFBP-4) from media conditioned by a neuronal cell line and have detected IGFBP-4 mRNA in selected regions of the developing and adult rat brain by Northern blot analysis. In this study, the ontogeny and regional distribution of IGFBP-4 expression were determined by in situ hybridization histochemistry. While IGFBP-4 mRNA expression at embryonic day 15 was restricted to choroid plexus primordium and meninges, by embryonic day 20 IGFBP-4 mRNA was also localized in the basal ganglia. In the postnatal rat, at days 1 and 5, IGFBP-4 was also present in the meningeal cell layer surrounding the developing cerebellum and in the hippocampal formation. The distribution of IGFBP-4 mRNA in the adult brain was considerably more widespread. The principal areas where IGFBP-4 mRNA was detected were the cerebral cortex (layers II and IV), olfactory peduncle (anterior olfactory nuclei), limbic system (hippocampus and amygdala), thalamus and basal ganglia, as well as choroid plexus and meninges. The widespread and persistent expression of IGFBP-4 is in marked contrast with IGFBP-2, the other IGFBP in the brain, whose localization by in situ hybridization is reported to be restricted to choroid plexus and meninges. The spatial pattern of IGFBP-4 expression in areas known to either overlap, be adjacent to, or project to regions that express the IGFs or their receptors may reflect a role for IGFBP-4 as a modulator of IGF action in the brain.

Expression of insulin-like growth factor I by astrocytes in response to injury

Astrocytes are known to express several growth factors in response to injury and neurological disease. Insulin-like growth factor I (IGF-I) induces astrocytes to divide in vitro and is expressed by developing, but not adult astrocytes both in vivo and in vitro. We tested whether IGF-I is re-expressed by reactive astrocytes in response to injury. We found that astrocytes surrounding the lesioned parenchyma after introduction of a cannula through the cerebral cortex, hippocampus and midbrain contain high levels of immunoreactive IGF-I, as determined by immunocytochemistry using a highly sensitive and specific anti-IGF-I monoclonal antibody. Interestingly, the contralateral hippocampus also contained IGF-I positive astrocytes although in substantial lower numbers. Intact animals showed no detectable IGF-I immunoreactivity in astrocytes. IGF-I was detected at the first time point tested after the lesion was made, 1 week, and for at least 1 month thereafter. Reactive astrocytes expressing high levels of glial fibrillary acidic protein were found in a much wider distribution all along the lesioned area and beyond. We conclude that mechanical injury of the brain induces a specific pattern of expression of IGF-I by a subpopulation of astrocytes. These findings suggest that IGF-I is participating in the response of astrocytes to injury.

Cytokines in neural regeneration

Growth factors with already established multiple effects on non-neural cells continue to be of considerable interest to researchers with regard to the nervous system, where regulation of cell maintenance and plasticity in relation to lesion and regeneration is part of their functional repertoire. Fibroblast growth factors, interleukins, and type beta transforming growth factors are prominent representatives of such proteins. Ciliary neurotrophic factor is another multifunctional neurokine. The proposed role of this molecule as a 'lesion factor', however, is still not firmly settled.

Climbing fiber deafferentation reduces insulin-like growth factor I (IGF-I) content in cerebellum

The presence of insulin-like growth factor I (IGF-I) and its mRNA in adult rat cerebellum has recently been documented. Previous immunocytochemical studies showed prominent IGF-I-like staining in fibers around Purkinje cell somas. To determine the origin of this IGF-I input to the Purkinje cell we destroyed the inferior olivary complex by either 3-acetylpiridine administration or electrolytical lesions. In both types of lesions we found a similar significant depletion of IGF-I levels in cerebellum (40-50% of controls). No changes were found in cerebellar IGF-I receptors. These results suggest that almost half of the IGF-I content in cerebellum is provided by climbing fiber afferents arising from the inferior olivary complex.