Isolation and characterization of variant IGF-1 as well as IGF-2 from adult human brain

The Role of Insulin-Like Growth Factors and Insulin-Like Growth Factor-Binding Proteins in the Nervous System

The insulin-like growth factors (IGF-I and IGF-II) and their receptors are widely expressed in nervous tissue from early embryonic life. They also cross the blood brain barriers by active transport, and their regulation as endocrine factors therefore differs from other tissues. In brain, IGFs have paracrine and autocrine actions that are modulated by IGF-binding proteins and interact with other growth factor signalling pathways. The IGF system has roles in nervous system development and maintenance. There is substantial evidence for a specific role for this system in some neurodegenerative diseases, and neuroprotective actions make this system an attractive target for new therapeutic approaches. In developing new therapies, interaction with IGF-binding proteins and other growth factor signalling pathways should be considered. This evidence is reviewed, gaps in knowledge are highlighted, and recommendations are made for future research.

The complexity of the IGF1 gene splicing, posttranslational modification and bioactivity

The insulinlike growth factor-I (IGF-I) is an important factor which regulates a variety of cellular responses in multiple biological systems. The IGF1 gene comprises a highly conserved sequence and contains six exons, which give rise to heterogeneous mRNA transcripts by a combination of multiple transcription initiation sites and alternative splicing. These multiple transcripts code for different precursor IGF-I polypeptides, namely the IGF-IEa, IGF-IEb and IGF-IEc isoforms in humans, which also undergo posttranslational modifications, such as proteolytic processing and glycosylation. IGF-I actions are mediated through its binding to several cell-membrane receptors and the IGF-I domain responsible for the receptor binding is the bioactive mature IGF-I peptide, which is derived after the posttranslational cleavage of the pro-IGF-I isoforms and the removal of their carboxy-terminal E-peptides (that is, the Ea, Eb and Ec). Interestingly, differential biological activities have been reported for the different IGF-I isoforms, or for their E-peptides, implying that IGF-I peptides other than the IGF-I ligand also possess bioactivity and, thus, both common and unique or complementary pathways exist for the IGF-I isoforms to promote biological effects. The multiple peptides derived from IGF-I and the differential expression of its various transcripts in different conditions and pathologies appear to be compatible with the distinct cellular responses observed to the different IGF-I peptides and with the concept of a complex and possibly isoform-specific IGF-I bioactivity. This concept is discussed in the present review, in the context of the broad range of modifications that this growth factor undergoes which might regulate its mechanism(s) of action.

Neurodevelopmental effects of insulin-like growth factor signaling

Insulin-like growth factor (IGF) signaling greatly impacts the development and growth of the central nervous system (CNS). IGF-I and IGF-II, two ligands of the IGF system, exert a wide variety of actions both during development and in adulthood, promoting the survival and proliferation of neural cells. The IGFs also influence the growth and maturation of neural cells, augmenting dendritic growth and spine formation, axon outgrowth, synaptogenesis, and myelination. Specific IGF actions, however, likely depend on cell type, developmental stage, and local microenvironmental milieu within the brain. Emerging research also indicates that alterations in IGF signaling likely contribute to the pathogenesis of some neurological disorders. This review summarizes experimental studies and shed light on the critical roles of IGF signaling, as well as its mechanisms, during CNS development.

Insulin-like growth factor-I peptides act centrally to decrease depression-like behavior of mice treated intraperitoneally with lipopolysaccharide

Centrally administered insulin-like growth factor (IGF)-I has anti-depressant activity in several rodent models, including lipopolysaccharide (LPS)-induced depression. In this study we tested the ability of IGF-I and GPE (the N-terminal tri-peptide derived from IGF-I) to alter depression-like behavior induced by intraperitoneal (i.p.) administration of LPS in a preventive and curative manner. In the first case, IGF-I (1 μg) or GPE (5 μg) was administered i.c.v. to CD-1 mice followed 30 min later by 330 μg/kg body weight i.p. LPS. In the second case, 830 μg/kg body weight LPS was given 24 h prior to either IGF-I or GPE. When administered i.p., LPS induced full-blown sickness assessed as a loss of body weight, decrease in food intake and sickness behavior. None of these indices were affected by IGF-I or GPE. LPS also induced depression-like behavior; assessed as an increased duration of immobility in the tail suspension and forced swim tests. When administered before or after LPS, IGF-I and GPE abrogated the LPS response; attenuating induction of depression-like behaviors and blocking preexistent depression-like behaviors. Similar to previous work with IGF-I, GPE decreased brain expression of cytokines in response to LPS although unlike IGF-I, GPE did not induce the expression of brain-derived neurotrophic factor (BDNF). LPS induced expression of tryptophan dioxygenases, IDO1, IDO2 and TDO2, but expression of these enzymes was not altered by GPE. Thus, both IGF-I and GPE elicit specific improvement in depression-like behavior independent of sickness, an action that could be due to their anti-inflammatory properties.

Growth factors and plasticity

Neuroprotective strategies can prevent lesions from getting worse but agents that have neurotrophic properties can also affect repair in a developing brain. Although prevention and treatment in the early stages of brain lesions are desirable, delayed cell death or improved post-lesion plasticity are the only realistic targets in many cases. Several trophic factors can limit delayed cell death in animal models of perinatal brain damage. In addition, melatonin and brain-derived neurotrophic factor have been shown to promote post-lesion plasticity following neonatal excitotoxic white-matter damage in newborn mice. Despite these promising results, additional preclinical data are required for most of the trophic factors that have been tested, although some candidate drugs, e.g. melatonin or erythropoietin, might reach clinical trials in the near future.

Reconciling data from transgenic mice that overexpress IGF-I specifically in skeletal muscle

Transgenic mice that overexpress insulin-like growth factor-1 (IGF-I) specifically in skeletal muscle have generated much information about the role of this factor for muscle growth and remodelling and provide insight for therapeutic applications of IGF-I for different pathological states and ageing. However, difficulties arise when attempting to critically compare the significance of data obtained in vivo by using different genetically engineered mouse lines and various experimental models. Complications arise due to complexity of the IGF-I system, since multiple transcripts of the IGF-I gene encode different isoforms generated by alternate promoter usage, differential splicing and post-translational modification, and how IGF-I gene expression relates to its diverse autocrine, paracrine and endocrine modes of action in vivo has still to be elucidated. In addition, there are problems related to specification of the exact IGF-I isoform used, expression patterns of the promoters, and availability of the transgene product under different experimental conditions. This review discusses the factors that must be considered when reconciling data from cumulative studies on IGF-I in striated muscle growth and differentiation using genetically modified mice. Critical evaluation of the literature focuses specifically on: (1) the importance of detailed information about the IGF-I isoforms and their mode of action (local, systemic or both); (2) expression pattern and strength of the promoters used to drive transgenic IGF-I in skeletal muscle cells (mono and multi-nucleated); (3) local compared with systemic action of the transgene product and possible indirect effects of transgenic IGF-I due to upregulation of other genes within skeletal muscle; (4) re-interpretation of these results in light of the most recent approaches to the dissection of IGF-I function. Full understanding of these complex in vivo issues is essential, not only for skeletal muscle but for many other tissues, in order to effectively extend observations derived from transgenic studies into potential clinical situations.

Synthesis and neuroprotective activity of analogues of glycyl-l-prolyl-l-glutamic acid (GPE) modified at the α-carboxylic acid

The synthesis of nine GPE* analogues, wherein the alpha-carboxylic acid group of glutamic acid has been modified, is described by coupling readily accessible N-benzyloxycarbonyl-glycyl-L-proline 2 with various analogues of glutamic acid. Pharmacological evaluation of the novel compounds was undertaken to further understand the role of the glutamate residue on the observed neuroprotective properties of the endogenous tripeptide GPE.

Synthesis and pharmacological evaluation of glycine-modified analogues of the neuroprotective agent glycyl-l-prolyl-l-glutamic acid (GPE)

The synthesis of 10 G*PE analogues, wherein the glycine residue has been modified, is described by coupling readily accessible dibenzyl-L-prolyl-L-glutamate 2 with various analogues of glycine. Pharmacological evaluation of the novel compounds was undertaken to further understand the role of the glycine residue on the observed neuroprotective properties of the endogenous tripeptide GPE.

Synthesis and pharmacological evaluation of side chain modified glutamic acid analogues of the neuroprotective agent glycyl-l-prolyl-l-glutamic acid (GPE)

The synthesis of eight GPE* analogues, wherein the gamma-carboxylic moiety of the glutamic residue has been modified, is described by coupling readily accessible N-benzyloxycarbonyl-glycyl-L-proline with various analogues of glutamic acid. Pharmacological evaluation of the novel compounds was undertaken to further understand the role of the glutamate residue on the observed neuroprotective properties of the endogenous tripeptide GPE.

Brain tissue volume segmentation in patients with anorexia nervosa before and after weight normalization

OBJECTIVE

To examine whether gray and white matter volumes are preferentially reduced and cerebral spinal fluid (CSF) increased with starvation in patients with anorexia nervosa compared with healthy controls and to determine what changes occur with weight normalization.

METHOD

Whole intracranial volumes of patients and controls were segmented into gray matter, white matter, and CSF volumes and results compared. A subgroup of patients were rescanned after weight normalization.

RESULTS

Total white matter and several regional white matter volumes were significantly reduced and total and regional CSF volumes were significantly increased in patients versus controls whereas gray matter was not significantly reduced. Total and regional CSF volumes were significantly decreased in patients upon weight normalization whereas white and gray matter volumes increased.

DISCUSSION

These changes in brain tissue may be related to a variety of pathophysiologic mechanisms. We hypothesize that insulin-like growth factor-1 may be involved.

Neuroprotective effects of the N-terminal tripeptide of IGF-1, glycine-proline-glutamate, in the immature rat brain after hypoxic-ischemic injury

Insulin growth factor 1 (IGF-1) has an important role in brain development and is strongly expressed during recovery after a hypoxic-ischemic injury. Some of its central actions could be mediated through the N-terminal tripeptide fragment of IGF-1: Gly-Pro-Glu (GPE). The neuroprotective properties of GPE given after a moderate injury in the developing rat brain were evaluated and the binding sites of [(3)H]GPE characterised by autoradiography. After right unilateral injury, GPE or vehicle (V) was injected in the right lateral ventricle (i.c.v.) or in the peritoneal cavity (i.p.) of 21-day-old rats. The percentage of surviving neurons in CA1-2 of the hippocampus was higher in the animals treated with 30 microg of GPE i.c.v. (V: 7.7+/-4.9%, GPE: 26.4+/-7.5%, P=0.02) and 300 microg i.p. (V: 30.2+/-9.1%, GPE: 68.8+/-10.6%, P=0.02) than in animals receiving vehicle. I.p. injection of 300 microg of GPE (V: 78.4+/-7.5%, GPE: 88.4+/-3.2%, P=0.04) was also neuroprotective in the lateral cortex. I.c.v. injection of [(3)H]GPE suggested binding to glial cells in the white matter tracts, the cortex and striatum as opposed to neurons. Although the precise mode of action of GPE is unknown, this study suggests that local administration of GPE is neuroprotective after brain HI injury via glial cells. In addition, systemic administration of GPE showed a more widespread neuroprotective effect. GPE may represent a complementary pathway for central and systemic IGF-1's antiapoptotic effects.

Uptake of circulating insulin-like growth factors (IGFs) into cerebrospinal fluid appears to be independent of the IGF receptors as well as IGF-binding proteins

Peripheral administration of human insulin-like growth factor (hIGF) results in both uptake of hIGF into the cerebrospinal fluid (CSF) and amelioration of brain injury. We tested the hypotheses that IGF uptake into CSF is independent of IGF receptors and IGF-binding proteins (IGFBP). Adult rats were injected sc with various concentrations of hIGF-I or structural analogs, and serum and CSF were withdrawn for assay 90 min later. An enzyme-linked immunoassay was used that detected immunoreactive hIGF-I and its analogs, but not rat IGF-I, IGF-II, or insulin. Plasma hIGF-I levels increased linearly (r = 0.97) with hIGF-I dose between 25-300 microgram/rat. By contrast, uptake into CSF reached saturation above 100 microgram, suggesting carrier-mediated uptake. hIGF-II reduced the uptake of hIGF-I into CSF (P < 0.02). Des(1-3)hIGF-I is a hIGF-I analog missing the N-terminal tripeptide, resulting in greatly reduced affinity for IGFBP-1, -3, -4, and -5. Nevertheless, des(1-3)hIGF-I was taken up into CSF. [Leu(24)]hIGF-I and [Leu(60)]hIGF-I have 20- to 85-fold reduced affinity for the type I IGF receptor, yet both were taken up into CSF in amounts similar to hIGF-I. In addition, hIGF-I and des(1-3)hIGF-I were taken up into CSF, although binding to the type II receptor is extremely weak. These data suggest that uptake of circulating IGF-I into CSF is independent of the type I or II IGF receptors as well as IGF sequestration to IGFBP-1, -3, -4, or -5.

Blood-brain barrier for human growth hormone and insulin-like growth factor-I

There is a blood-brain barrier (BBB) for GH. A certain, unknown amount of GH passes the BBB, acts on the neuronal GH receptors and directly influences the brain mechanisms serving the feedback and ultradian secretion of GH. The high density of GH receptors in the choroid plexus suggests a possible receptor-mediated transcytosis transport. The effects of GH on brain development, neuronal plasticity and neuroprotection seem to be mediated by IGFs. GH and IGFs are also synthesized in the brain. The relative contributions to brain functions of GHs produced inside and outside the BBB are unknown. The cerebrospinal fluid (CSF) space is the compartment inside the barrier accessible to clinicians. High GH levels in CSF were reported in acromegaly and also a small increase was reported after chronic administration of hGH in GH-deficiency syndromes. For the practitioner it is necessary to determine the normal range of hGH levels in CSF.

The phylogeny of the insulin-like growth factors

The insulin-like growth factors are major regulators of growth and development in mammals and their presence in lower vertebrates suggests that they played a similarly fundamental role throughout vertebrate evolution. While originally perceived simply as mediators of growth hormone, on-going research in mammals has revealed several hierarchical layers of complexity in the regulation of ligand bioavailability and signal transduction. Our understanding of the biological role and mechanisms of action of these important growth factors in mammals patently requires further elucidation of the IGF hormone system in the simple model systems that can be found in lower vertebrates and protochordates. This review contrasts our knowledge of the IGF hormone system in mammalian and nonmammalian models through comparison of tissue and developmental distributions and gene structures of IGF system components in different taxa. We also discuss the evolutionary origins of the system components and their possible evolutionary pathways.

High-molecular weight IGF-2 expression in a haemangiopericytoma associated with hypoglycaemia

Spontaneous hypoglycaemia is usually caused by an insulin-producing islet-cell tumour of the pancreas. Rarely, it can be caused by non-islet cell tumours. Most of the tumours are of mesenchymal type, large, and slowly growing. One representative is haemangiopericytoma (HAP). The present report describes a case of a large recurrent retroperitoneal HAP associated with severe hypoglycaemia. Blood serum insulin and proinsulin concentrations were low. By means of acid-gel chromatography and dot-blot techniques, an increased amount of a high-molecular-weight IGF-2 peptide was found. By using antigen retrieval procedures, IGF-2-immunoreactive tumour cells were found in specimens of the recent tumour recurrence-but not in the original. When the in situ hybridization technique was used it could be shown that IGF-2 mRNA labelling had already occurred in the original tumour specimen, 11 years before the onset of hypoglycaemic symptoms. These observations confirm the hypothesized hypoglycaemic effects of high-molecular-weight (HMW) IGF-2, but also point to the presence of a prolonged compensation of this effect. A literature review, based on 17 similar cases of haemangiopericytoma with hypoglycaemia, is presented. Our observation and findings in the literature review support the idea that non-islet-cell tumour hypoglycaemia is caused by an overproduction of a HMW IGF-2 peptide. The insulin-like effect is mediated via non-specific binding to the insulin receptors. To anticipate patients at risk of developing this kind of hypoglycaemia, the histopathological investigation should include not only immunohistochemical analyses of the presence of IGF-2 peptide, but also in situ hybridization of the IGF-2 mRNA expression.

Insulin-like growth factors (IGF) enhance three-dimensional (3D) growth of human glioblastomas

Human glioblastomas (gliomas) are characterized as rapidly growing brain tumors which are highly invasive but rarely metastatic. Human gliomas synthesize and secrete increased levels of insulin-like growth factors (IGFs) as well as expressing increased numbers of IGF receptors when compared to normal brain tissue. These observations suggest the existence of an IGF-mediated autocrine mechanism for glioma growth regulation. The purpose of this study was to examine the effect of human recombinant IGF (hrIGF) treatment on the in vitro growth of human glioma monolayer and three-dimensional (3D) multicellular spheroid cultures. The data demonstrate that hrIGF-I treatment of glioma cell lines slightly enhanced tumor monolayer proliferation as measured by [(3)H]thymidine incorporation. In contrast, treatment of glioma spheroids with hrIGF-I or hrDes(1-3)IGF-I, the truncated brain form of IGF-I, dramatically enhanced 3D tumor growth with a 1.5-2-fold reduction in spheroid doubling time (FRSDT). In addition, IGF-treated glioma spheroids were more densely packed than spheroids grown in media alone with no observed necrosis. These data suggest that IGFs will dramatically enhance glioma proliferation when 3D cell-cell contact occurs. This observed enhancement suggests that IGFs both synthesized in the brain and systemically support rapid proliferation of gliomas in vivo.

Insulin-like growth factors and inflammatory bowel disease

Hallmarks of IGF-I action include synergy with other hormones and growth factors and the ability to stimulate proliferation or differentiated cell function dependent on physiological or pathophysiologial context. A complete understanding of IGF action in IBD will require analyses of mechanisms of IGF interaction with other growth factors, hormones and cytokines. GH and IGF-I may be administered to children over prolonged periods to correct growth disorders. The definition of the benefits and problems of GH/IGF-I therapy in IBD needs to distinguish between long-term and short-term effects. Short-term administration of GH and IGF-I to animal models of IBD such as the PG-PS and TNBS models, which share features of Crohn's disease (Sartor, 1992), and a recently developed murine model of ulcerative colitis induced by ingestion of dextran sulphate (Okayasu et al, 1990; Sartor, 1992; Cooper et al, 1993) could address the beneficial or detrimental consequences of short-term GH/IGF-I therapy. Adaptation of the PG-PS, TNBS and dextran sulphate models of inflammation to available transgenic mouse lines that over-express GH and IGF-I (Behringer et al, 1990; Ulshen et al, 1993), especially if over-expression is inducible, could help to define the potential benefits and problems of long-term GH/IGF-I therapy or the effects of GH/IGF-I on immune cell function and cytokine production during intestinal inflammation. It will be useful to study intestinal inflammation and complication in animal models of GH or IGF-I deficiency. In this regard, mice with targeted ablation of the IGF-I gene could be useful (Liu et al, 1993) although neonatal mortality in these models currently poses problems for in vivo studies. Development of mesenchymal cell lines from such animals could, however, provide a useful in vitro system to study the role of IGF-I in altered cell function in response to pro-inflammatory cytokines.

N-terminal deletion mutants of insulin-like growth factor-II (IGF-II) show Thr7 and Leu8 important for binding to insulin and IGF-I receptors and Leu8 critical for all IGF-II functions

To define the role of the N-terminal region of insulin-like growth factor-II (IGF-II) in its binding to insulin and IGF receptors, deletion mutants des-(1-5)-, des-(1-7)-, and des-(1-8)-recombinant (r) IGF-II, and the Gly8 for Leu substitution mutant of rIGF-II were prepared by site-directed mutagenesis, expressed in Escherichia coli, and purified. The binding affinity and mitogenic activity of these rIGF-II mutants as well as commercially available des-(1-6)-rIGF-II were analyzed. While the relative affinity of des-(1-5)- and des-(1-6)-rIGF-II for purified human insulin and IGF-I receptors remained at > or = 50% levels of that of rIGF-II, the affinity of des-(1-7)-rIGF-II decreased to approximately 10% and approximately 3%, respectively, of that of rIGF-II. When the octapeptide including Leu8 was removed prior to the Cys9-Cys47 intrachain bond, the relative affinity of this deletion mutant, des-(1-8)-rIGF-II, for these receptors dramatically decreased to < 1% of that of rIGF-II. Substituting Gly8 for Leu in rIGF-II decreased the affinity of this mutant for the IGF-I and insulin receptors to about the same extent. These results suggest that the side chains of Thr7 and Leu8 may play an important role in retaining all of the IGF-II functions. Decreases in the relative affinity for binding of the mutants to these receptors paralleled the decreases in their mitogenic potency for cultured Balb/c 3T3 cells. Although the relative affinity of des-(1-8)- or [Gly8]rIGF-II for rat IGF-II/CIM6-P (cation-independent mannose 6-phosphate) receptors was also < 1% of that of rIGF-II, the relative affinities of des-(1-5)-, des-(1-6)-, and des-(1-7)-rIGF-II for these receptors was significantly greater than that of rIGF-II. These results clearly demonstrate that Thr7 and Leu8 are important for binding to insulin and IGF-I receptors and Leu8 is critical for expression of all IGF-II functions.

Truncation of IGF-I yields two mitogens for retinal Müller glial cells

In the CNS only a truncated form of insulin-like growth factor-I (IGF-I) is detected. Although truncated IGF-I (t-IGF-I) retains mitogenicity, growth promoting activities have not been detected for the tripeptide that is cleaved from IGF-I during truncation. Here, we asked whether the tripeptide is itself a growth factor. Using cultured Müller glial cells from the adult human retina, we found that the cleaved tripeptide, glycine-proline-glutamate, stimulated the proliferation of these cells. Pharmacological experiments indicated that this proliferative effect involves activation of N-methyl-D-aspartate (NMDA) receptors. In addition, t-IGF-I was also mitogenic in our culture system and had an EC50 markedly less than that for IGF-I. Thus, truncation of IGF-I may be a mechanism to augment the mitogenic effect of this gene product by creating a more potent variant and a cleaved tripeptide that is itself a mitogen.

Purification and characterization of insulin-like growth factor II (IGF II) and an IGF II variant from human placenta

In order to purify variant IGF II peptides from human placenta, we have developed a purification procedure combining heparin affinity chromatography and cation-exchange, reversed-phase and size-exclusion HPLC. Two peptides were purified, both having apparent M(r) values of ca. 7300 Da as evaluated by SDS-PAGE. N-Terminal sequencing revealed IGF II and an IGF II variant in which Ser29 was replaced by the tetrapeptide Arg-Leu-Pro-Gly. The final yield of variant IGF II was about eight-fold lower than that of IGF II. Both pure peptides were functionally active as they bound to type I and type II IGF receptors from ovine and human placental membranes, as determined by crosslinking experiments and displacement curve studies.

Excessive production of insulin-like growth factor-I by silicotic rat alveolar macrophages

The supernatant of silicotic rat alveolar macrophages can stimulate fibroblast growth. The present study demonstrates that this activity is mainly attributed to insulin-like growth factor-I. Partial purification of the supernatant of alveolar macrophages, which were from silica-exposed 5 to 6-week-old rats, revealed a protein peak (peak 5) eluted from a molecular-sieve HPLC column, corresponding to a MW of 6-9 kDa. Activity assay and radioimmunoassay indicated that this peak is more potent with regard to stimulation of fibroblast growth and has higher insulin-like growth factor-I immunoreactivity, but there was no detectable activity of interleukin-1 or tumor necrosis factor. Quantification of insulin-like growth factor-I also manifests elevated insulin-like growth factor-I levels in silicotic rat bronchoalveolar lavage fluids which tend to increase with prolongation of silica exposure in vivo, but no alteration in insulin-like growth factor-I level can be found in sera. These findings suggest that excessive production of insulin-like growth factor-I by alveolar macrophages locally may play a pivotal role in silica-induced pulmonary interstitial fibrosis.

Insulin-like growth factor-I receptors in human brain and pituitary gland: an autoradiographic study

Insulin-like growth factor (IGF)-I receptors were studied in adult human postmortem brain and pituitary gland using quantitative autoradiography with human recombinant [125I]IGF-I. The highest densities were found in the choroid plexus, pituitary gland--where IGF-I receptors were mainly concentrated in the anterior lobe, pineal gland, glomerular layer of the olfactory bulb, and the molecular layer of the cerebellar cortex. Moderate densities were present in cerebral cortex, caudate nucleus, putamen, accumbens, the CA1, CA2, CA3 fields and dentate gyrus of the hippocampus, the dentate nucleus of the cerebellum, amygdala, thalamus, pontine nuclei, and substantia nigra. All other brain areas, including white matter, contained low densities of IGF-I receptors. The finding that several well-defined brain structures are enriched with IGF-I receptors suggests a neurotrophic/survival or neuromodulatory role of insulin-like growth factors on specific neuronal systems. IGF-I receptors observed in the white matter may be associated with oligodendrocytes.

Growth factors in Parkinson's disease

The etiology of Parkinson's disease, one of the most frequent neurodegenerative disorders in human, is unknown. New hopes concerning satisfactory therapies include transplants of autologous adrenal medullary chromaffin tissue, fetal mesencephalic dopaminergic neurons, and local application of growth factors with a neurotrophic capacity. A large body of evidence supports the notion that neurons require trophic support not only during a limited period of ontogenesis, but during their whole lifespan. Relevant molecules promote survival, transmitter synthesis and other differentiated properties, and become crucially important when a neuron is metabolically or toxically impaired. Several molecules, most of which occur in the striatum and the substantia nigra, have been identified that protect lesioned dopaminergic nigrostriatal neurons in culture or in animal models of Parkinson's disease. These include members of the neurotrophin, fibroblast growth factor, and insulin-like growth factor families as well as epidermal growth factor/transforming growth factor alpha, interleukins and ciliary neurotrophic factor. Whether their effects are merely pharmacological, or reflect a physiological role in the nigrostriatal system, is unclear as yet. This article reviews experiments that document the trophic effects of these factors on dopaminergic neurons and discusses their possible physiological and therapeutic relevance.

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 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.

The nature of the trophic action of brain-derived neurotrophic factor, des(1-3)-insulin-like growth factor-1, and basic fibroblast growth factor on mesencephalic dopaminergic neurons developing in culture

Brain-derived neurotrophic factor, basic fibroblast growth factor and des(1-3)-insulin-like growth factor-1, a brain specific form of insulin-like growth factor-1, were analysed, in the rat, for their influence on survival, morphological growth, and transmitter-specific differentiation of dopaminergic neurons in vitro. Brain-derived neurotrophic factor, des-insulin-like growth factor-1, and basic fibroblast growth factor were found to differentially regulate development of dopaminergic cells. Brain-derived neurotrophic factor stimulated survival, the formation of primary neurites and dopamine uptake activity. des-Insulin-like growth factor-1 was most effective in promoting survival, stimulated dopamine uptake less effectively than brain-derived neurotrophic factor and did not alter the morphology of dopaminergic cells. Basic fibroblast growth factor produced comparatively mild increases in survival and dopamine uptake, and slightly reduced neurite growth of the cells. None of the factors stimulated the expression of the tyrosine hydroxylase gene. These findings suggest that (i) effective growth factors may stimulate different, but partially overlapping, molecular pathways during developmental differentiation, (ii) none of the factors stimulates dopaminergic cell differentiation comparable to the pronounced trophic action of nerve growth factor on peripheral sympathetic or basal forebrain cholinergic neurons, and (iii) localization and effects of none of the factors are compatible with a role as target-derived survival-regulating neurotrophic factor.

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.

Microcephalic cerebrum with hypomyelination in the pygmy mouse (pg)

To determine whether somatomedin has a direct action on cerebral development instead of an indirect action of a growth hormone, we examined the central nervous system of the pygmy mouse (pg), a mutant with normal somatomedin activity. Our findings are: (A) the weights of the pg/pg cerebrum and cerebellum weighted were significantly less than those of the normal controls (pg/+), 14 and 15% less, respectively; (B) the total DNA content was reduced by 17% in the cerebrum and cerebellum of the pg/pg mouse; (C) the total RNA content was reduced in the cerebrum and cerebellum, proportional to the reduction in DNA; (D) CNPase activity was reduced selectively in the cerebrum of the pg/pg mouse by 25%; and (E) the pg/pg mice exhibited a strikingly reduced level of activity with an indistinct diurnal periodicity. Therefore, the present findings suggest that the action of somatomedin on the proliferation and maturation of glial cells might be a necessary precondition to myelin formation.

Granulosa cell-derived insulin-like growth factor (IGF) binding proteins are inhibitory to IGF-I hormonal action. Evidence derived from the use of a truncated IGF-I analogue

An increasing body of information now suggests that insulin-like growth factor (IGF) binding proteins (BPs) may serve as antigonadotropins at the level of the ovary. It is the objective of the present communication to evaluate the functional role of endogenous (granulosa cell-derived) IGFBPs by exploiting the unique properties of des(1-3)IGF-I, a naturally occurring IGF-I analogue characterized as a weak ligand of IGFBPs but not of type I IGF receptors. Given IGFBP-replete circumstances, des(1-3)IGF-I proved more potent (10-fold) than its intact counterpart in promoting the follicle stimulating hormone (FSH)-stimulated accumulation of progesterone by cultured rat granulosa cells. In contrast, des(1-3)IGF-I proved virtually equipotent to the unmodified principle under IGFBP-deplete circumstances. Taken together, these findings are in keeping with the notion and that the apparently enhanced potency of des(1-3)IGF-I (under IGFBP-replete conditions) is due to its diminished affinity for endogenously generated IGFBPs and that rat granulosa cell-derived IGFBPs are inhibitory to IGF (and thus inevitably to gonadotropin) hormonal action. Accordingly, the reported ability of gonadotropins to attenuate IGFBP release by granulosa cells may be designed to enhance the bioavailability of endogenously generated IGFs in the best interest of ovarian steroidogenesis.

Immunocytochemical demonstration of IGF-II immunoreactivity in human phaeochromocytoma and extra-adrenal abdominal paraganglioma

Insulin-like growth factor (IGF)-II immunoreactivity was demonstrated in 14 adrenal glands, six retroperitoneal paraganglia, 18 phaeochromocytomas, and six extra-adrenal abdominal paragangliomas through the use of a monoclonal antibody against rat IGF-II which cross-reacts with human IGF-II. A subpopulation of parenchymal cells in normal adrenal medulla and in retroperitoneal paraganglia was positively immunolabelled. Eighteen cases with phaeochromocytoma including two malignant and four multiple endocrine neoplasias all contained numerous IGF-II-immunoreactive tumour cells. All six extra-adrenal abdominal paragangliomas contained numerous immunoreactive tumour cells. Almost all the other normal human adult tissues examined failed to immunolabel. These results indicate that IGF-II-like immunoreactivity is widely expressed in normal and neoplastic human paraganglionic tissues, although its biological significance in these tissues has not been determined.

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.

Mutants of human insulin-like growth factor II (IGF II). Expression and characterization of truncated IGF II and of two naturally occurring variants

Insulin-like growth factor II (IGF II) and four structural analogs, constructed by site-directed mutagenesis, were expressed as protein A fusion proteins in Escherichia coli BL21pLysS cells, cleaved with cyanogen bromide and purified by affinity chromatography and HPLC. Two mutants (Ser29 substituted by Arg-Leu-Pro-Gly, and Ser33 substituted by Cys-Gly-Asp) represent two naturally occurring variants of IGF II. The other two mutants, (7-67)IGF II and (9-67)IGF II, are truncated at the amino-terminus in analogy to the naturally occurring des(1-3)IGF I ('truncated IGF I'). These mutants were tested for their binding affinities to type-1 and type-2 IGF receptors, to IGF binding protein-3 (IGFBP-3) and for their stimulation of thymidine incorporation into DNA. The affinities of the Ser29 and Ser33 mutants to the type-1 IGF receptor were 85% and 39%, respectively, compared to wild-type IGF II, those of (7-67)IGF II and (9-67)IGF II 96% and 15%, respectively. The potencies of the Ser33 and the (9-67) mutant to stimulate thymidine incorporation into DNA correlated closely with the affinities to the type-1 IGF receptor, whereas the bioavailability of the Ser29 mutant was lower and that of the (7-67) mutant higher than the type-1 receptor binding, possibly due to interferences with endogenously secreted IGFBPs. The affinities of the Ser29 and Ser33 mutants to the type-2 IGF receptor were 110% and 71%, respectively, those of the two truncated mutants 25% and 23%, respectively. The affinity of the Ser29 mutant to IGFBP-3 was increased to 171%, whereas those of the Ser33 mutant and the two truncated mutants were reduced (34%, 10% and 19%, respectively).

Glucose, insulin, and insulin-like growth factor I regulate the glycogen content of astroglia-rich primary cultures

The glycogen content of astroglia-rich primary cultures derived from the brains of newborn rats depends on the concentration of glucose in the culture medium. After administration of culture medium lacking glucose, the glycogen content decreases with a half-time of 7 min. Readdition of glucose results in replenishment of the glycogen stores within 2-3 h, but fully only if glucose is present in a concentration of at least 4 mM. Insulin, or the more potent insulin-like growth factor I, increases the content of glycogen approximately 1.7-fold, with the half-maximal effects being attained at concentrations of 10 and 0.5 nM, respectively. These results suggest that (a) glucose or a metabolite of it and (b) insulin-like growth factor I or a closely related peptide, but not insulin, are likely to be physiological regulators of the level of glycogen in astrocytes.

Characterization of two cDNAs encoding insulin-like growth factor 1 (IGF-1) in the human fetal brain

Insulin-like growth factor 1 (IGF-1) regulates growth of the brain. In order to characterize the variant IGF-1 present in human fetal brain we have determined the cDNA sequence for human fetal brain IGF-1. Using PCR to amplify cDNA obtained from isolated human fetal brain mRNA, two cDNA sequences encoding precursor proteins which correspond to IGF-1a and IGF-1b were obtained. This is the first characterisation of IGF-1 and its IGF-1a and IGF-1b precursors in the nervous system.

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.

Truncated IGF-1 exerts trophic effects on fetal brain tissue grafts

Truncated IGF-1 (tIGF-1), a form of IGF-1 identified in the human brain, has been suggested, from in vitro experiments, to exert neurotrophic effects on developing fetal brain tissue. We studied the effects of tIGF-1 and IGF-1 on small defined areas of the developing central nervous system by using the in vivo model of intraocular transplantation which allows for direct observations of graft survival and growth. Truncated IGF-1 was found to significantly enhance the growth of fetal spinal cord (Embryonic Day (E) 14) and parietal cortex (E16-17) grafts transplanted to the anterior chamber of the eye of adult rats. tIGF-1 increased the volume of cerebral cortex grafts by approximately 100% and of E14 spinal cord grafts by approximately 50%. E18 spinal cord grafts and hippocampal grafts were not stimulated by tIGF-1 as compared to controls given HSA. Effects in cortex were seen with tIGF-1 using concentrations down to at least 10 ng/microliters. Interestingly, intact IGF-1 had no effect on cortical grafts. These findings show for the first time, using an in vivo system, that tIGF-1 is a potent stimulator of growth of grafted fetal cortex cerebri and spinal cord and suggest a possible role for endogenous tIGF-1 in cortical and spinal cord development.