Insulin-like growth factor II in the rat choroid plexus

VEGF/VEGFR-2 changes in frontal cortex, choroid plexus, and CSF after chronic obstructive hydrocephalus

Chronic hydrocephalus (CH) is often associated with decreased cerebral blood flow (CBF) and oxygen levels. While the exact pathophysiology is not clear, vascular endothelial growth factor (VEGF) and its receptor-2 (VEGFR-2) may be involved. Because the choroid plexus (CP) is involved in cerebrospinal fluid (CSF) production and secretes numerous growth factors including VEGF, it is important to understand VEGF/VEGFR-2 levels in the CP-CSF circulatory system. Our results showed significant decreases in CBF and VEGFR-2 levels in frontal cortex (FC) in CH compared with SC; there were no significant changes in VEGF levels. CBF change in FC was positively correlated with VEGFR-2 levels (P=0.024). Immunohistochemistry (IHC) showed robust expression of VEGF/VEGFR-2 in CP. After CH induction, ventricular CSF volume and VEGF levels significantly increased. These results suggest that the decreased VEGFR-2 levels in FC may be contributed to decreased CBF and increased ventricular CSF-VEGF levels possibly reflected a hypoxic response and/or accumulation of VEGF from CP secretion after blockage of CSF outlet. Further investigation into CSF-VEGF levels in different sites may provide a better understanding of VEGF/VEGFR-2 modulation in the normal and hydrocephalic brain, and may represent a feasible approach to potential therapeutic options for hydrocephalus.

Conditioned medium of the primary culture of rat choroid plexus epithelial (modified ependymal) cells enhances neurite outgrowth and survival of hippocampal neurons

The choroid plexus epithelial (modified ependymal) cells (CPECs) are specialized for cerebrospinal fluid (CSF) production and serve as blood-CSF barrier. It is suggested that, in addition to CSF production, the CPECs may regulate CNS function through expression of secretory factors into CSF. There have been reports that the CPECs express various types of factors including growth factors. However, the actual effects of the molecules produced and secreted from the CPECs on the central nervous system (CNS) are virtually unknown both in vivo and in vitro. With the use of pure culture of CPECs, we demonstrated that the conditioned medium (CM) from CPECs can enhance neurite outgrowth and survival of cultured neurons derived from rat hippocampus on postnatal day 1 in 24-h cultures. The effect of the CM was retained in fractions that contains complex of molecules larger than 50kDa in native condition with ultrafiltration method and disappeared by trypsin digestion. The results of the present study indicate that CPECs can support the survival and function of neurons in vitro by secreting factors that are likely to be of peptide/protein nature rather than small chemicals.

Human choroid plexus growth factors: What are the implications for CSF dynamics in Alzheimer's disease?

The choroid plexus plays a key role in supporting neuronal function by secreting cerebrospinal fluid (CSF) and may be involved in the regulation of various soluble factors. Because the choroid plexus is involved in growth factor secretion as well as CSF dynamics, it is important to understand how growth factors in CSF interact with the brain parenchyma as well as with cells in direct contact with the flowing CSF, i.e., choroid plexus and arachnoid villi. While the existence of growth factors in the choroid plexus has been documented in several animal models, the presence and distribution of growth factors in the human choroid plexus has not been extensively examined. This study describes the general distribution and possible functions of a number of key proteins in the human choroid plexus and arachnoid villi, including basic fibroblast growth factor, FGF receptor, and vascular endothelial growth factor. FGF and VEGF could both be readily demonstrated in choroid plexus epithelial cells. The presence of FGF and VEGF within the choroid plexus was also confirmed by ELISA analysis. Since Alzheimer's disease (AD) is known to be associated with a number of growth factor abnormalities, we examined the choroid plexus and arachnoid villi from AD patients. Immunohistochemical studies revealed the presence of FGF and VEGF within the AD choroid plexus and an increased density of FGFr in both the choroid plexus and the arachnoid villi of AD patients. No qualitative changes in the distribution of FGF and VEGF were observed in the AD choroid plexus. The appearance of FGFr in AD arachnoid was associated with robust amyloid and vimentin immunoreactivity. These findings confirm the presence of FGF and VEGF within the normal and AD choroid plexus and suggest that the alteration of growth factors and their receptors may contribute to the pathogenesis of the hydrocephalus ex vacuo that is characteristically seen in AD.

Uptake of circulating insulin-like growth factor-I into the cerebrospinal fluid of normal and diabetic rats and normalization of IGF-II mRNA content in diabetic rat brain

Brain injury has been prevented recently by systemic administration of human insulin-like growth factor-I (hIGF-I). It is widely believed that protein neurotrophic factors do not enter the brain from blood, and the mechanism by which circulating hIGF-I may be neuroprotective is uncertain. This investigation tested the hypothesis that hIGF-I is taken up into cerebrospinal fluid (CSF) from the circulation. (125)I-hIGF-I was injected subcutaneously into rats. The (125)I-IGF-I recovered from CSF and plasma were indistinguishable in size from authentic (125)I-hIGF-I on SDS-PAGE. An ELISA was used that detected immunoreactive hIGF-I, but not rat IGF-I, rat IGF-II, human IGF-II, or insulin. Osmotic minipumps were implanted for constant subcutaneous infusion of various hIGF-I doses. Uptake into CSF reached a plateau at plasma concentrations above approximately 150 ng/ml hIGF-I; the plateau was consistent with carrier-mediated uptake. The plasma, but not CSF, hIGF-I level was significantly reduced in streptozotocin diabetic vs. nondiabetic rats, and uptake of hIGF-I into CSF was nonlinear with respect to plasma hIGF-I concentrations. Nonlinear uptake excluded leakage or transmembrane diffusion of IGF-I from blood into CSF as a dominant route for entry, but the site and mechanism of uptake remain to be established. The IGF-II mRNA content per milligram brain (P < 0.02) as well as per poly(A)(+) RNA (P < 0.05) was significantly increased towards normal in diabetic rats treated by subcutaneous administration of hIGF-I vs. vehicle. This effect of circulating hIGF-I may have been due to regulation of IGF-II gene expression in the choroid plexus and leptomeninges, structures at least in part outside of the blood-central nervous system barrier. These data support the hypothesis that circulating IGF-I supports the brain indirectly through regulation of IGF-II gene expression as well as by uptake into the CSF.

Patterns in expression of insulin-like growth factor-II and of proliferative activity in the normal human first and third trimester placenta demonstrated by non-isotopic in situ hybridization and immunohistochemical staining for MIB-1

The expression of insulin-like growth factor-II (IGF-II) in normal human first and third trimester placental tissue was investigated by non-isotopic in situ hybridization (ISH). This is the first ISH study on IGF-II expression in placenta using an alkaline phosphatase-labelled probe. The expression was correlated with the proliferative activity of the cells using the proliferative marker MIB-1. In first trimester tissue, IGF-II was expressed in the cytotrophoblast, the extravillous trophoblast, the fetal endothelial cells and the mesenchymal fetal cells in the villi. In third trimester tissue, IGF-II expression was found in the amnion, the extravillous trophoblast and the mesenchymal fetal cells especially in the endothelial cells and the outer contractile sheet in the stem villi. In areas with perivillous fibrin deposits, strong expression of IGF-II was found in the cytotrophoblasts invading the fibrin. In first trimester tissue, the proliferative activity of the villous cytotrophoblast correlated well with the degree of IGF-II expression whereas in third trimester tissue, there was a discrepancy between MIB-1 positivity and the IGF-II expression. Expression of IGF-II does not seem to be correlated exclusively to the mitogenic activity of cells.

Synthesis and secretion of insulin-like growth factor (IGF)-II and IGF binding protein-2 by cultivated brain meningeal cells

The meninges (the pia mater and the arachnoid) covering the surface of brain parenchyma are known to participate in the regulation of blood circulation and the blood-cerebrospinal fluid barrier. In the present study, we isolated and characterized some of the major proteins secreted into the conditioned medium of the meningeal cell cultures prepared from 1-2-day-old rats. Five protein molecules (7 kDa, 16 kDa, 18 kDa, 27 kDa and 32 kDa) were recognized as major proteins in the conditioned medium by SDS-PAGE. The major proteins were isolated and purified to homogeneity, respectively. Their N-terminal sequences, except that of 27 kDa protein, were successfully determined. Homology search has revealed that the N-terminal sequences of the 7 kDa protein and the 32 kDa protein were identical with those of insulin-like growth factor-II (IGF-II) and IGF-binding protein-2 (IGFBP-2), respectively. The N-terminal sequences of the 16 kDa and 18 kDa molecules were identical with those of the corresponding fragments of IGFBP-2. The present study demonstrates that cultured meningeal cells produce and secrete large amounts of IGF-II and IGFBP-2. The meninges may, therefore, be one of the main sources of these proteins in the cerebrospinal fluid and involved in the modulation of neuronal and/or glial cell survival or functioning.

The cerebral expression of plasma protein genes in different species

The cerebrospinal fluid (CSF) contains the same proteins as blood plasma, but with a different pattern of concentrations. Protein concentrations in CSF are much lower than those in blood. CSF proteins are derived from blood or synthesized within the brain. The choroid plexus is an important source of CSF proteins. Transthyretin is the protein most abundantly synthesized and secreted by choroid plexus. It determines the distribution of thyroxine in the cerebral compartment. Synthesis of transthyretin first evolved in the brain, then later it became a plasma protein synthesized in the liver. Other proteins secreted by choroid plexus are serum retinol-binding protein, transferrin, caeruloplasmin, insulin-like growth factors, insulin-like growth factor binding proteins, cystatin C, alpha 1-antichymotrypsin, alpha 2-macroglobulin, prothrombin, beta 2-microglobulin and prostaglandin D synthetase. Species differences in expression of the genes for these proteins are outlined, and their developmental pattern, regulation and roles in the cerebral extracellular compartment are discussed.

Role of insulin-like growth factors in peripheral nerve regeneration

Prolonged denervation results in atrophy of target organs and increased risk of permanent paralysis. A better understanding of the mechanism responsible for nerve regeneration may one day lead to improved rates of nerve regeneration and diminished risk of loss of function. Neurobiologists have known for decades that soluble neurotrophic activity is present in nerves and nerve targets. Until recently, the soluble molecules that regulate the rate of nerve regeneration have eluded identification. Insulin-like growth factor (IGF) gene expression is correlated with synapse formation during development and regeneration. IGFs are now identified as the first soluble nerve- and muscle-derived neurotrophic factors found to regulate the rate of peripheral nerve regeneration. The roles of IGFs and other neurotrophic factors in peripheral nerve regeneration, motor nerve terminal sprouting and synapse formation are reviewed.

Gene expression and secretion of insulin-like growth factor-II and insulin-like growth factor binding protein-2 from cultured sheep choroid plexus epithelial cells

The gene expression of insulin-like growth factor II (IGF-II) and insulin-like growth factor binding protein-2 (IGFBP-2) has previously been demonstrated in rat and human choroid plexus by in situ hybridization analysis. In the present study we have characterized IGF-II and IGFBP-2 transcripts and proteins in primary cultures of epithelial cells from lateral choroid plexus of sheep brain. Northern blot analysis of total RNA showed one major IGF-II mRNA of 4.8 kb and four minor IGF-II transcripts of 1.5, 2.0, 3.0 and 6.0 kb as well as one IGFBP-2 transcript of 1.7 kb. Radioreceptor assay of conditioned medium from the cultured choroid plexus epithelial cells showed inhibition of [125I]IGF-I and [125I]IGF-II binding to mouse NIH 3T3 fibroblasts, the displacement curves being identical to that of unlabelled IGF-II. The conditioned medium was fractionated by gel filtration on a Bio-Gel P-60 column, and analysis by IGF-II radioreceptor assay showed two peaks of IGF-II-binding inhibitory activity of M(r) 7.5-10 and 25 kDa, suggesting the presence of both IGF-II, and an IGFBP. Western immunoblot analysis of conditioned medium with antibodies toward IGF-II and IGFBP-2 demonstrated proteins with M(r) 6 kDa and 32 kDa, respectively. Protein binding assays of the conditioned medium with [125I]IGF-I or [125]IGF-II demonstrated that the IGFBP present in the conditioned medium preferentially binds IGF-II. In conclusion, cultured sheep choroid plexus epithelial cells synthesize and secrete IGF-II and IGFBP-2, suggesting that the choroid plexus epithelium is the main source of these polypeptides in the cerebrospinal fluid.

Insulin-like growth factor-I (IGF-I) production by astroglial cells: regulation and importance for epidermal growth factor-induced cell replication

The insulin-like growth factors are postulated to play a role during brain development. Because they are believed to act in a paracrine/autocrine manner, the production of insulin-like growth factor-I (IGF-I) by cultured astroglial cells was examined. Quantities of IGF-I in conditioned media were determined by RIA after separation of IGFs from IGF-binding proteins by high-pressure liquid chromatography. Astrocytes from 1-day-old rats and the rat glioma cell line (C6) both secreted 7.5-kDa IGF-I. A peak of immunoreactivity with an apparent mol wt of 12,000 was additionally present in media conditioned by C6 cells. Exposure to epidermal growth factor (EGF) increased media content of immunoreactive IGF-I slightly (60%) in C6 cells but more than 2-fold in normal astrocytes. Fibroblast growth factor also increased the amount of IGF-I contained in media conditioned by normal astrocytes. To determine whether the secreted IGF-I was biologically active, media IGFs were immunoneutralized with a monoclonal antibody (Sm 1.25). In the presence of the antibody, EGF-stimulated astrocyte replication was blocked. These data indicate that IGF-I secretion by rodent astrocytes is stimulated by factors thought to be important for brain growth and development and that the IGFs are likely intimate participants in EGF-induced astrocyte growth.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

Species specificity and developmental patterns of expression of the β amyloid precursor protein (APP) gene in brain, liver and choroid plexus in birds

1. Human APP cDNA hybridized to a 3.5 kb mRNA in liver and brain RNA from chickens, pigeons, quail and ducks as well as in RNA from choroid plexus of chicken and quail. In contrast to all other species hitherto examined a 1.6 kb mRNA hybridizing to APP cDNA was found in abundant amounts in RNA from chicken and quail livers. 2. In the chicken, before hatching, the levels of APP mRNA in total RNA from liver and choroid plexus were higher than those in RNA from liver and choroid plexus of adults. However, RNA from the rest of the brain of chicken embryos contained less APP mRNA than RNA from brain of adults. 3. In the chicken, between 10 and 40 days after hatching, APP mRNA levels in RNA from liver were higher than adult levels, APP mRNA levels in RNA from choroid plexus were similar to adult levels and APP mRNA levels in RNA from the rest of brain were below the adult levels.

Gene expression and receptor binding of insulin-like growth factor-II in pig choroid plexus epithelial cells

To elucidate the function of insulin-like growth factor-II (IGF-II) in the choroid plexus, the gene expression and receptor binding of IGF-II were studied in isolated epithelial cells from the porcine choroid plexus. The choroid plexus expressed multiple IGF-II transcripts of 1.2, 1.6, 2.4, and 4.4 kb, at levels higher than those found in porcine liver and kidney. These data suggest that IGF-II is synthesized by the choroid plexus. Choroid plexus epithelial cells contained high levels of IGF-I receptors on the cell surface whereas very low levels of receptor binding were found for 125I-IGF-II and 125I-insulin. Solubilization of epithelial cells showed that a large proportion of the IGF-I receptors were present in the detergent-insoluble fraction whereas IGF-II receptors and insulin receptors were concentrated in the detergent-soluble fraction. These results suggest that IGF-I receptors are located in clathrin-coated pits of the plasma membrane whereas IGF-II receptors and insulin receptors are present in endosomal vesicles. The tyrosine kinase activity of the IGF-I receptor beta-subunit was stimulated by IGF-I, IGF-II, and insulin, in order of potency, suggesting that these peptides exert a regulatory function in the choroid plexus epithelium. In conclusion, we propose that the IGF-I receptor tyrosine kinase on the surface of the epithelial cells in the pig choroid plexus mediates effects of IGF-I and IGF-II, whereas IGF-II receptors are down-regulated due to the synthesis and secretion of IGF-II in these cells.

Dihydrotestosterone (DHT) regulation of insulin-like growth factor II mRNA in neonatal rats

Insulin-like growth factors (IGFs) are well known as peptide mitogens and important growth factors in fetal as well as in early postnatal development. In particular, IGF II is strongly expressed during fetal life and in neonatal animals. Very little is known about the regulation of IGF II gene expression. In order to study in detail the regulation of IGF II mRNA levels in the liver by the potent nonaromatizable androgen dihydrotestosterone (DHT), we have used quantitative in situ hybridization to detect the mRNA encoding for this growth factor. Pups were separated into 4 groups and injected twice a day immediately after birth with 3 different doses of DHT: 0.1 mg DHT/day, 0.25 mg DHT/day, 0.5 mg DHT/day for 4 and 7 days, and the control groups were injected with the vehicle alone. Animals were perfused with 4% paraformaldehyde and sections from the liver, heart, kidneys and brain were cut with a cryostat. A [35S]-labeled cDNA probe was used to detect IGF II mRNA levels. After hybridization, sections were autoradiographed with X-ray films and then coated with liquid photographic emulsion. Densitometric measurement revealed that, at 4 days of age, IGF II mRNA levels were lower in DHT-treated rats than in control animals. No statistically significant differences in IGF II mRNA levels were observed among the three groups treated with the different doses of DHT, thus revealing that even the lowest dose of DHT (0.1 mg/day) used was sufficient to inhibit IGF II gene expression in neonatal rats. Moreover, at 7 days of age, DHT-treated rats showed the same levels of IGF II mRNA as those observed in rats treated with DHT for 4 days. These results suggest that DHT may play an important role in the regulation of IGF II gene expression in the rat liver during the neonatal period.

Chromaffin cells express two types of insulin-like growth factor receptors

The receptor binding, internalization and tyrosine kinase activation of insulin-like growth factors, IGF-I and IGF-II have been investigated in cultured adult bovine chromaffin cells. IGF-I receptor alpha-subunits (Mr approximately 130,000) bound IGF-I and IGF-II with identical affinity (Kd approximately 1 nM) and insulin with about 1000 times lower affinity. IGF-II receptors (Mr approximately 250,000) bound IGF-II with a Kd of 0.5 nM, IGF-I with about 10 times lower affinity and insulin with greater than 10,000 times lower affinity. The amounts of IGF-I and IGF-II receptors on the cell surface were 8 x 10(4) and 4 x 10(4) sites per cell, respectively. Insulin bound to a specific receptor with Kd approximately 2 nM and the amount of receptors was 1.5 x 10(4) sites per cell. IGF-I and IGF-II stimulated tyrosine kinase activity and autophosphorylation of the IGF-I receptor beta-subunit (Mr approximately 94,000) with equal potency (ED50 approximately 1 nM), whereas insulin was approximately 5 times less potent. Both IGF-I and IGF-II were internalized after their binding to cell surface receptors. Mannose-6-phosphate, which binds to the IGF-II receptor, did not alter the binding or internalization of IGF-II. It is concluded that IGF-I and IGF-II can exert their biological effects in chromaffin cells by activation of the IGF-I receptor tyrosine kinase or by interaction with the IGF-II receptor.

Expression of NGF receptor in the rat forebrain detected with in situ hybridization and immunohistochemistry

The expression of nerve growth factor (NGF) receptor mRNA and NGF receptor protein was examined in the adult rat basal forebrain using in situ hybridization and immunohistochemical techniques. NGF receptor mRNA and protein were detected within cells in the medial septum, diagonal band of Broca, and nucleus basalis of Meynert. Controls showed that the hybridization signal was not due to nonspecific binding of the probe to heterologous RNAs or other molecules. As expected, the distribution of NGF receptor mRNA-containing cells correlated nicely with the distribution of NGF receptor immunoreactive cells in each of these areas. These data extend previous work which suggests that neurons in these areas express the NGF receptor mRNA and manufacture functional NGF receptors. NGF receptor immunoreactivity was also detected in the arcuate nucleus of the hypothalamus, in the leptomeninges at the base of the brain and overlying the tectum, and within ependymal regions along the lateral walls of the cerebral ventricles. A few weakly stained neurons in the lateral hypothalamus and ventrolateral striatum were also consistently observed. In contrast, NGF receptor mRNA was not detected within any meningial, ependymal, or hypothalamic tissues using in situ hybridization. A cross-linking/immunoprecipitation assay demonstrated normal, membrane-bound NGF receptors within extracts of dorsal superior colliculus, ventromedial hypothalamic, and overlying meningial tissues, proving that the staining observed in these areas was not a non-specific artifact associated with the immunohistochemistry. The lack of hybridization in these areas may reflect levels of NGF receptor mRNA which are too low to be detected by the in situ hybridization methods being used. Alternatively, the staining may represent innervation of these areas by afferents whose cell bodies are located elsewhere, and whose terminals contain the NGF receptor protein.

Function of neurotrophic factors in the adult and aging brain and their possible use in the treatment of neurodegenerative diseases

This review summarizes the current knowledge of characterized neurotrophic factors, including nerve growth factor (NGF) which serves as paradigmatic example when studying novel molecules. Special consideration is given to the function of neurotrophic factors in the adult and aging brain. Strategies are discussed for the eventual development of pharmacological applications of these molecules in the treatment of neurodegenerative diseases.