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

The neurobiology of insulin-like growth factor I: From neuroprotection to modulation of brain states

After decades of research in the neurobiology of IGF-I, its role as a prototypical neurotrophic factor is undisputed. However, many of its actions in the adult brain indicate that this growth factor is not only involved in brain development or in the response to injury. Following a three-layer assessment of its role in the central nervous system, we consider that at the cellular level, IGF-I is indeed a bona fide neurotrophic factor, modulating along ontogeny the generation and function of all the major types of brain cells, contributing to sculpt brain architecture and adaptive responses to damage. At the circuit level, IGF-I modulates neuronal excitability and synaptic plasticity at multiple sites, whereas at the system level, IGF-I intervenes in energy allocation, proteostasis, circadian cycles, mood, and cognition. Local and peripheral sources of brain IGF-I input contribute to a spatially restricted, compartmentalized, and timed modulation of brain activity. To better define these variety of actions, we consider IGF-I a modulator of brain states. This definition aims to reconcile all aspects of IGF-I neurobiology, and may provide a new conceptual framework in the design of future research on the actions of this multitasking neuromodulator in the brain.

Novel model of cortical-meningeal organoid co-culture system improves human cortical brain organoid cytoarchitecture

Human cortical organoids (hCOs), derived from human induced pluripotent stem cells (iPSCs), provide a platform to interrogate mechanisms of human brain development and diseases in complex three- dimensional tissues. However, current hCO development methods lack important non-neural tissues, such as the surrounding meningeal layer, that have been shown to be essential for normal corticogenesis and brain development. Here, we first generated hCOs from a single rosette to create more homogenous organoids with consistent size around 250 µm by day 5. We then took advantage of a 3D co-culture system to encapsulate brain organoids with a thin layer of meningeal cells from the very early stages of cortical development. Immunostaining analysis was performed to display different cortical layer markers during different stages of development. Real-time monitoring of organoid development using IncuCyte displayed enhanced morphology and increased growth rate over time. We found that meningeal-encapsulated organoids illustrated better laminar organization by exhibiting higher expression of REELIN by Cajal-Retzius neurons. Presence of meningeal cells resulted in a greater expansion of TBR2 intermediate progenitor cells (IPCs), the deep cortical layer (CTIP2) and upper cortical layer (BRN2). Finally, meningeal-encapsulated organoids enhanced outer radial glial and astrocyte formation illustrated by stronger expression of HOPX and GFAP markers, respectively. This study presents a novel 3D co-culture platform to more closely mimic the in vivo cortical brain structure and enable us to better investigating mechanisms underlying the neurodevelopmental disorders during embryonic development.

Meninges: A Widespread Niche of Neural Progenitors for the Brain

Emerging evidence highlights the several roles that meninges play in relevant brain functions as they are a protective membrane for the brain, produce and release several trophic factors important for neural cell migration and survival, control cerebrospinal fluid dynamics, and embrace numerous immune interactions affecting neural parenchymal functions. Furthermore, different groups have identified subsets of neural progenitors residing in the meninges during development and in the adulthood in different mammalian species, including humans. Interestingly, these immature neural cells are able to migrate from the meninges to the neural parenchyma and differentiate into functional cortical neurons or oligodendrocytes. Immature neural cells residing in the meninges promptly react to brain disease. Injury-induced expansion and migration of meningeal neural progenitors have been observed following experimental demyelination, traumatic spinal cord and brain injury, amygdala lesion, stroke, and progressive ataxia. In this review, we summarize data on the function of meninges as stem cell niche and on the presence of immature neural cells in the meninges, and discuss their roles in brain health and disease. Furthermore, we consider the potential exploitation of meningeal neural progenitors for the regenerative medicine to treat neurological disorders.

The insulin-like growth factor-1 system in the adult mammalian brain and its implications in central maternal adaptation

Our knowledge on the bioavailability and actions of insulin-like growth factor-1 (IGF-1) has markedly expanded in recent years as novel mechanisms were discovered on IGF binding proteins (IGFBPs) and their ability to release IGF-1. The new discoveries allowed a better understanding of the endogenous physiological actions of IGF-1 and also its applicability in therapeutics. The focus of the present review is to summarize novel findings on the neuronal, neuroendocrine and neuroplastic actions of IGF-1 in the adult brain. As most of the new regulatory mechanisms were described in the periphery, their implications on brain IGF system will also be covered. In addition, novel findings on the effects of IGF-1 on lactation and maternal behavior are described. Based on the enormous neuroplastic changes related to the peripartum period, IGF-1 has great but largely unexplored potential in maternal adaptation of the brain, which is highlighted in the present review.

Temporal expression patterns of insulin-like growth factor binding protein-4 in the embryonic and postnatal rat brain

Background

IGFBP-4 has been considered as a factor involving in development of the central nervous system (CNS), but its role needs to be further clarified. In present study, the localization of IGFBP-4 expression in the embryonic forebrain, midbrain and hindbrain was determined using immunohistochemistry, and the levels of IGFBP-4 protein and mRNA were semi-quantified using RT-PCR and Western blot in the embryonic (forebrain, midbrain and hindbrain) and postnatal brain (cerebral cortex, cerebellum and midbrain).

Results

A clear immunoreactivity of IGFBP-4 covered almost the entire embryonic brain (forebrain, midbrain, hindbrain) from E10.5 to E18.5, except for the area near the ventricle from E14.5. The change of IGFBP-4 mRNA level was regularly from E10.5 to E18.5: its expression peaked at E13.5 and E14.5, followed by gradual decreasing from E15.5. The expression of IGFBP-4 protein was similar to that of mRNA in embryonic stage. After birth, the pattern of IGFBP-4 expression was shown to be rather divergent in different brain areas. In the cerebral cortex, the IGFBP-4 mRNA increased gradually after birth (P0), while the protein showed little changes from P0 to P28, but decreased significantly at P70. In the cerebellum, the IGFBP-4 mRNA decreased gradually from P0, reached the lowest level at P21, and then increased again. However, its protein level gradually increased from P0 to P70. In the midbrain, the IGFBP-4 mRNA first decreased and reached its lowest level at P28 before it increased, while the protein remained constant from P0 to P70. At P7, P14, P21, P28 and P70, the levels of IGFBP-4 mRNA in the cerebral cortex were significantly higher than that in the cerebellum or in the midbrain. Differently, the protein levels in the cerebellum were significantly higher than that either in the cerebral cortex or in the midbrain at P14, P21, P28 and P70.

Conclusions

The temporal expression pattern of IGFBP-4 in the embryonic brain from E10.5 to E18.5 was consistent with the course of neurogenesis in the ventricular zone, suggesting an important role of IGFBP-4 in regulating differentiation of neural stem cells. A strikingly higher abundance of the IGFBP-4 protein observed in the cerebellum from P14 to P70 suggests that IGFBP-4 may participate in the maintenance of cerebellar plasticity.

The many faces of insulin-like peptide signalling in the brain

Central and peripheral insulin-like peptides (ILPs), which include insulin, insulin-like growth factor 1 (IGF1) and IGF2, exert many effects in the brain. Through their actions on brain growth and differentiation, ILPs contribute to building circuitries that subserve metabolic and behavioural adaptation to internal and external cues of energy availability. In the adult brain each ILP has distinct effects, but together their actions ultimately regulate energy homeostasis - they affect nutrient sensing and regulate neuronal plasticity to modulate adaptive behaviours involved in food seeking, including high-level cognitive operations such as spatial memory. In essence, the multifaceted activity of ILPs in the brain may be viewed as a system organization involved in the control of energy allocation.

Nestin- and doublecortin-positive cells reside in adult spinal cord meninges and participate in injury-induced parenchymal reaction

Adult spinal cord has little regenerative potential, thus limiting patient recovery following injury. In this study, we describe a new population of cells resident in the adult rat spinal cord meninges that express the neural stem/precursor markers nestin and doublecortin. Furthermore, from dissociated meningeal tissue a neural stem cell population was cultured in vitro and subsequently shown to differentiate into functional neurons or mature oligodendrocytes. Proliferation rate and number of nestin- and doublecortin-positive cells increased in vivo in meninges following spinal cord injury. By using a lentivirus-labeling approach, we show that meningeal cells, including nestin- and doublecortin-positive cells, migrate in the spinal cord parenchyma and contribute to the glial scar formation. Our data emphasize the multiple roles of meninges in the reaction of the parenchyma to trauma and indicate for the first time that spinal cord meninges are potential niches harboring stem/precursor cells that can be activated by injury. Meninges may be considered as a new source of adult stem/precursor cells to be further tested for use in regenerative medicine applied to neurological disorders, including repair from spinal cord injury. Stem Cells 2011;29:2062–2076.

Toward a comprehensive neurobiology of IGF-I

Insulin-like growth factor I (IGF-I) belongs to an ancient family of hormones already present in early invertebrates. The insulin family is well characterized in mammals, although new members have been described recently. Since its characterization over 50 years ago, IGF-I has been considered a peptide mostly involved in the control of body growth and tissue remodeling. Currently, its most prominent recognized role is as a quasi-universal cytoprotectant. This role connects IGF-I with regulation of lifespan and with cancer, two areas of very active research in relation to this peptide. In the brain, IGF-I was formerly considered a neurotrophic factor involved in brain growth, as many other neurotrophic factors. Other aspects of the neurobiology of IGF-I are gradually emerging and suggest that this growth factor has a prominent role in brain function as a whole. During development IGF-I is abundantly expressed in many areas, whereas once the brain is formed its expression is restricted to a few regions and in very low quantities. However, the adult brain appears to have an external input from serum IGF-I, where this anabolic peptide is abundant. Thus, serum IGF-I has been proven to be an important modulator of brain activity, including higher functions such as cognition. Many of these functions can be ascribed to its tissue-remodeling activity as IGF-I modulates adult neurogenesis and angiogenesis. Other activities are cytoprotective; indeed, IGF-I can be considered a key neuroprotective peptide. Still others pertain to the functional characteristics of brain cells, such as cell excitability. Through modulation of membrane channels and neurotransmission, IGF-I impinges directly on neuronal plasticity, the cellular substrate of cognition. However, to fully understand the role of IGF-I in the brain, we have to sum the actions of locally produced IGF-I to those of serum IGF-I, and this is still pending. Thus, an integrated view of the role played by IGF-I in the brain is not yet possible. An operational approach to overcome this limitation would be to consider IGF-I as a signal coupling environmental influences on body metabolism with brain function. Or in a more colloquial way, we may say that IGF-I links body "fitness" with brain fitness, providing a mechanism to the roman saying "mens sana in corpore sano."

A novel combination of factors, termed SPIE, which promotes dopaminergic neuron differentiation from human embryonic stem cells

Background

Stromal-Derived Inducing Activity (SDIA) is one of the most efficient methods of generating dopaminergic (DA) neurons from embryonic stem cells (ESC). DA neuron induction can be achieved by co-culturing ESC with the mouse stromal cell lines PA6 or MS5. The molecular nature of this effect, which has been termed “SDIA” is so far unknown. Recently, we found that factors secreted by PA6 cells provided lineage-specific instructions to induce DA differentiation of human ESC (hESC).

Methodology/Principal Findings

In the present study, we compared PA6 cells to various cell lines lacking the SDIA effect, and employed genome expression analysis to identify differentially-expressed signaling molecules. Among the factors highly expressed by PA6 cells, and known to be associated with CNS development, were stromal cell-derived factor 1 (SDF-1/CXCL12), pleiotrophin (PTN), insulin-like growth factor 2 (IGF2), and ephrin B1 (EFNB1). When these four factors, the combination of which was termed SPIE, were applied to hESC, they induced differentiation to TH-positive neurons in vitro. RT-PCR and western blot analysis confirmed the expression of midbrain specific markers, including engrailed 1, Nurr1, Pitx3, and dopamine transporter (DAT) in cultures influenced by these four molecules. Electrophysiological recordings showed that treatment of hESC with SPIE induced differentiation of neurons that were capable of generating action potentials and forming functional synaptic connections.

Conclusions/Significance

The combination of SDF-1, PTN, IGF2, and EFNB1 mimics the DA phenotype-inducing property of SDIA and was sufficient to promote differentiation of hESC to functional midbrain DA neurons. These findings provide a method for differentiating hESC to form DA neurons, without a requirement for the use of animal-derived cell lines or products.

Increasing the levels of insulin-like growth factor-I by an IGF binding protein inhibitor produces anxiolytic and antidepressant-like effects

The present studies were conducted to determine if increasing central levels of the neurotrophic factor insulin-like growth factor-1 (IGF-I) either directly or indirectly produces anxiolytic and antidepressant-like effects in the mouse. Central levels of IGF-I can be increased directly, by administering IGF-I, or indirectly by blocking the insulin-like growth factor binding proteins (IGFBPs). The IGFBP family has the unique ability to regulate IGF-I levels by sequestering IGF-I into an inactive complex. Therefore, an IGFBP inhibitor increases the level of IGF-I available to bind to its receptor. Intracerebroventricular (icv) administration of the nonspecific IGFBP inhibitor NBI-31772 (10-30 microg) increases the number of punished crossings in the four-plate test and NBI-31772 (0.3-10 microg) increases time spent in the open quadrant of the elevated zero maze (EZM), indicative of anxiolytic-like effects. NBI-31772 (3-30 microg) also decreases immobility time in the tail suspension test, indicative of antidepressant-like effects. Similarly, icv administration of IGF-I (0.1 microg) produces anxiolytic-like effects in the four-plate test and IGF-1 (0.3-1 microg) produces anxiolytic-like effects in the EZM. IGF-I (10 microg) also produces antidepressant-like effects in the tail suspension test. Coadministration of the IGF-I receptor antagonist JB1 with NBI-31772 or IGF-I blocks the anxiolytic-like and antidepressant-like effects of these compounds. These results suggest that NBI-31772 produces behavioral effects by increasing levels of IGF-I that in turn activate the IGF-I receptor. The present studies demonstrate that an IGFBP inhibitor mimics the behavioral effects of IGF-I and that IGFBP inhibition may represent a novel mechanism by which to increase IGF-I to treat depression and anxiety.

Expression profiles of Insulin-like growth factor binding protein-like 1 in the developing mouse forebrain

Insulin-like growth factor binding protein-like 1 (IGFBPL-1) has a domain structure that resembles one in the insulin-like growth factor binding proteins (IGFBPs), which is a domain known to be bound by insulin-like growth factors (IGFs) with high affinity and to thereby affect cell growth and metabolism. However, the distribution and functions of IGFBPL-1 in the brain are not known. Thus, to clarify IGFBPL-1 expression profiles during development, we have investigated the expression patterns of IGFBPL-1 mRNA in developing mouse brains using in situ hybridization, focusing especially on the forebrain. On embryonic day (E)12, IGFBPL-1mRNA expressions were localized in the preplate of the neopallium, dorsal thalamus, the differentiating field of the epithalamus, and the hippocampus. On E14, strong expression was recognized in the subventricular zone (SVZ) and intermediate zone (IMZ) in the neopallium, hippocampus, the differentiating field of the epithalamus, and the dorsal thalamus. E14 expressions were generally the strongest among all the stages examined and signals decreased thereafter. At E16 and E18, IGFBPL-1mRNA was strongly detected in the hippocampus and the SVZ of the neopallium. In postnatal stages, mRNA was found only in the granule cell layer of the dentate gyrus (DG). Notably, at P20, expression was restricted to the deep fraction of the granule cell layer, the location of late-born DG neurons.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

Insulin-like growth factor binding protein-4 interacts with centrosomes and microtubules in primary astrocytes

Insulin-like growth factor-1 and -2 (IGFs) are important for CNS development and have implications in pathological situations of the brain. Insulin-like growth factor binding protein-4 (IGFBP-4) regulates the biological effects of IGFs. We examined the expression of IGFBP-4 in primary rat and human astrocytes. IGFBP-4 mRNA was detectable by reverse transcription-polymerase chain reaction (RT-PCR) and protein expression was verified by Western blotting of cell lysates as well as conditioned culture medium. When astrocytes were immunostained for IGFBP-4 we detected an intracellular structure that did not derive from organelles involved in cellular trafficking. Ingestion of fluorescein isothiocyanate-labeled transferrin excluded detection of IGFBP-4 within vesicles of endosomal nature, in which the protein might have been incorporated. Double staining with gamma-tubulin demonstrated co-localization of IGFBP-4 with centrosomes of these cells. Treatment with nocodazole resulted in absence of IGFBP-4 signal on centrosomes, indicating a dependency on intact microtubules. Immunoelectron microscopy revealed IGFBP-4 localization not only at the centrioles but also a direct interaction with microtubules. There was no binding of IGFBP-4 to centrioles in primary rat oligodendrocytes, microglia or meningeal cells. The association of IGFBP-4 with centrioles and microtubules in astrocytes suggests an involvement of this molecule in microtubule functions of these cells.

Cell death in the nervous system: lessons from insulin and insulin-like growth factors

Programmed cell death is an essential process for proper neural development. Cell death, with its similar regulatory and executory mechanisms, also contributes to the origin or progression of many or even all neurodegenerative diseases. An understanding of the mechanisms that regulate cell death during neural development may provide new targets and tools to prevent neurodegeneration. Many studies that have focused mainly on insulin-like growth factor-I (IGF-I), have shown that insulin-related growth factors are widely expressed in the developing and adult nervous system, and positively modulate a number of processes during neural development, as well as in adult neuronal and glial physiology. These factors also show neuroprotective effects following neural damage. Although some specific actions have been demonstrated to be anti-apoptotic, we propose that a broad neuroprotective role is the foundation for many of the observed functions of the insulin-related growth factors, whose therapeutical potential for nervous system disorders may be greater than currently accepted.

Growth hormone action on proliferation and differentiation of cerebral cortical cells from fetal rat

To define the role of GH during central nervous system development, we performed studies in cultured rat cerebral cortical cells from 14- (E14) and 17-d-old embryos (E17). The expression of GH receptor, IGF-I receptor, and IGF-I mRNAs was confirmed. In E17, GH increased total cell number (3.9-fold), [(3)H]-thymidine incorporation (3.5-fold), proliferating cell nuclear antigen levels (2.5-fold), and bromodeoxyuridine (BrdU)-positive cells (2.5-fold). GH action on nestin/BrdU-positive cells was increased in E14 cells at 3 d in vitro (80-fold) but not at 7 d in vitro. In E14 cells, GH increased (9.5-fold) beta-tubulin/BrdU cells. In E17 cells, GH induced neuronal differentiation, as indicated by the absence of beta-tubulin/BrdU-positive cells and the 5.9-fold increment of beta-tubulin protein, and increased glial fibrillary acidic protein/BrdU-positive cells (2.5-fold) and glial fibrillary acidic protein expression (4.5-fold). GH-induced proliferation and differentiation was blocked by IGF-I antiserum. GH increased IGF-binding protein-3 (IGFBP-3), IGF-I receptor protein and its phosphorylation. This study shows that GH promotes proliferation of neural precursors, neurogenesis, and gliogenesis during brain development. These responses are mediated by locally produced IGF-I. GH-induced IGFBP-3 may also have a role in these responses. Therefore, GH is able to activate the IGF-I/IGFBP-3 system in these cerebral cells and induce a physiological action of IGF-I.

Tumor necrosis factor-alpha regulation of insulin-like growth factor-I, type 1 IGF receptor, and IGF binding protein expression in cerebellum of transgenic mice

Tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, has been implicated in the pathogenesis of several disorders and injuries in the central nervous system (CNS). Unlike IGF-I, which promotes CNS growth, TNF-alpha causes brain growth retardation and neural damage. Recently TNF-alpha has been shown to inhibit IGF-I signaling and actions in non-neural tissue. To investigate whether TNF-alpha deleteriously influences brain growth by altering the IGF-I system in vivo, we examined the expression of IGF-I, the type 1 IGF receptor (IGF1R) and IGF binding proteins (IGFBPs) in the brain of transgenic (Tg) mice with murine TNF-alpha overexpression. We show that overexpression of TNF-alpha reduces the weights of whole brain and all brain regions examined during development. In adult TNF-alpha Tg mice, cerebellum (CB) exhibited the greatest reduction in weight among the five brain regions examined, being approximately 77% of that in wild-type (WT) mice. IGF-I abundance was decreased in the CB, as well as in cerebral cortex and diencephalon, of TNF-alpha Tg mice. When compared to those in WT mice, CB IGF-I abundance in Tg mice was reduced by approximately 35%, approximately 45%, and approximately 40% at 2, 6, and 9 weeks of age, respectively. Of the IGFBPs studied the abundance of IGFBP-3 and IGFBP-4 was increased by 2-3.7-fold, and the abundance of IGFBP-5 was decreased by approximately 3-fold (as judged by Western immunoblot analysis). Histological analysis and immunocytochemical staining confirmed that TNF-alpha specifically increases IGFBP-3 and IGFBP-4 immunoreactivity, as well as that of the IGF1R, in radial glial and Purkinje cells. In addition, TNF-alpha alters CB cytoarchitecture, apparently by influencing granule cell migration. Our data indicate that TNF-alpha alters the expression of IGF-I system proteins in vivo, and suggest that altered expression of IGF-I system proteins may in part explain TNF-alpha deleterious actions on brain growth.

Effects of early undernutrition on the brain insulin-like growth factor-I system

Undernutrition reduces circulating concentrations of insulin-like growth factor (IGF)-I, but how it affects the brain IGF system, especially during development, is largely unknown. We have studied IGF-I, IGF-II, IGF receptor and IGF binding protein (BP)-2 mRNA expression in the hypothalamus, cerebellum and cerebral cortex of neonatal rats that were food restricted beginning on gestational day 16. One group was refed starting on postnatal day 14. Rats were killed on postnatal day 8 or 22. Undernutrition did not produce an overall reduction in brain weight at either age but, at 22 days, both the cerebellum and hypothalamus weighed significantly less. At 8 days, no change was detected in the central IGF axis in response to undernutrition. However, in 22-day-old undernourished rats, IGF-I and IGF receptor mRNA expression were increased in both the hypothalamus and cerebellum, while IGFBP-2 was decreased, but only in the hypothalamus. Refeeding had no effect on any of these parameters. These results suggest that the hypothalamus and cerebellum respond to malnutrition and the decrease in circulating IGF-I, a peptide fundamental for growth and development, by increasing the local production of both the growth factor and its receptor in attempt to maintain normal development.

Selective developmental regulation of gene expression for insulin-like growth factor-binding proteins in mouse spinal cord

STUDY DESIGN

Prospective, randomized experimental study in mice.

STUDY OBJECTIVE

To determine whether insulin-like growth factor binding proteins (IGFBPs) are present in mouse spinal cord and, if so, what role they play in its development.

SUMMARY OF BACKGROUND DATA

Insulin-like growth factors are well recognized hormonal effectors of growth hormone and are expressed in the mammalian spinal cord. The IGFBPs are a group of six genetically distinct proteins that bind IGFs and modulate their bioactivity. They appear in the brain during development, localize to the neuromuscular junction, and promote motor neuron survival. The benefit of IGF-I in amyotrophic lateral sclerosis ALS and its potential use in preventing motor neuron apoptosis in spinal cord injury dictates that studies of the presence and response of IGFBPs in that tissue be performed.

METHODS

The IGFBPs in mouse spinal cord were analyzed by Western ligand blot, Western immunoblot, and reverse transcription-polymerase chain reaction at various time points from embryonic day 14 to postnatal day 30.

RESULTS

Three IGFBPs with molecular masses of 24, 28, and 32 kDa were found, the latter two being the most prominent. The data indicate that these are IGFBP-4, -5, and -2.

CONCLUSION

Both IGFBP-2 and BP-5 are developmentally regulated in mouse spinal cord, with higher levels of those at early embryonic stages indicating their potential role in development of the mouse spinal cord.

Insulin-like growth factor binding proteins in cerebrospinal fluid during human development and aging

We analyzed samples of insulin-like growth factor binding proteins (IGFBPs) in human cerebrospinal fluid (CSF) in neurologically normal patients from one day after birth to age 76 years. CSF samples were separated on SDS-PAGE and then transferred to nitrocellulose membranes where IGFBPs were detected by Western ligand blot using [(125)I]-IGF-II, confirming other reports where we found the presence of IGFBP-2, 3, 4, 5. The 34 kDa IGFBP-2 was present in all samples, and progressively decreased with age. A broad 28- to 30-kDa IGFBP band, having the appearance of IGFBP-5, was triphasic: faint during infancy, barely detectable at 6 months, but intense in adult and aged individuals. The 24-kDa IGFBP-4 band was only seen in neonatal CSF samples, while the IGFBP-3 doublet gradually increased during aging. Thus, these present results show that IGFBP-2, 3, 4 and 5 in CSF are developmentally regulated, suggesting roles for these molecules in the development of the nervous system.

Cellular aspects of trophic actions in the nervous system

During the past three decades the number of molecules exhibiting trophic actions in the brain has increased drastically. These molecules promote and/or control proliferation, differentiation, migration, and survival (sometimes even the death) of their target cells. In this review a comprehensive overview of small diffusible factors showing trophic actions in the central nervous system (CNS) is given. The factors discussed are neurotrophins, epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, insulin-like growth factors, ciliary neurotrophic factor and related molecules, glial-derived growth factor and related molecules, transforming growth factor-beta and related molecules, neurotransmitters, and hormones. All factors are discussed with respect to their trophic actions, their expression patterns in the brain, and molecular aspects of their receptors and intracellular signaling pathways. It becomes evident that there does not exist "the" trophic factor in the CNS but rather a multitude of them interacting with each other in a complicated network of trophic actions forming and maintaining the adult nervous system.

Distinct sites of insulin-like growth factor (IGF)-II expression and localization in lesioned rat brain: possible roles of IGF binding proteins (IGFBPs) in the mediation of IGF-II activity

Although expression of the IGF-II has been demonstrated within the central nervous system (CNS), past studies have failed to reveal its precise roles or responses subsequent to a traumatic injury. To demonstrate that IGF-II, IGFBP, and IGF receptor (-R) expression alters in response to a penetrating CNS injury, we used the techniques of ribonuclease protection assay, in situ hybridization, immunohistochemistry, Western blotting, and RIA. Under normal physiology, IGF-II expression is restricted to the mesenchymal support structures of the brain, including the choroid plexus, where its expression is coincident with that of IGFBP-2. Between 1-7 days post lesion (dpl), in the acute phase following a penetrant wound to the CNS, IGF-II and IGF-IIR protein, but not messenger RNA, were colocalized, with IGF-I, IGF-IR, and IGFBP-1, -2, -3, and -6, to neurons, macrophages, astrocytes, and microglia within the damaged tissue. Within the cerebrospinal fluid (CSF), levels of IGF-II peptide increased to peak at 7 dpl. IGFBP-2, -3, and -6 were also observed within the CSF, with IGFBP-2 predominating and exhibiting an increase in binding efficiency from 7-10 dpl. In the chronic phase of injury (7-14 dpl), an increase in both IGF-II, IGF-IIR and IGFBP-5 messenger RNA and protein was observed specifically and focally in the marginal astrocytes forming the limiting glial membrane of the wound. Thus, our evidence suggests that there are two mechanisms of action for IGF-II within the injured rat brain. During the acute phase, the secretion of IGF-II from the choroid plexus into the CSF is up-regulated, resulting in increased transport of the peptide to the wound. In the CSF, transported IGF-II is complexed to IGFBP-2 and essentially demonstrates an endocrine mode of action with a balance of locally produced IGFBPs modulating its bioactivity in the wound. Later in the wounding response, levels of IGF-II decline in the CSF and the wound neuropil, possibly with the aid of increased IGFBP-5 levels that may help to locally sequester and down-regulate IGF-II activity. Hence, in the chronic phase of the injury response, IGF-II reasserts itself to a predominantly autocrine/paracrine role restricted to the mesenchymal support structures, including the glia limitans, which may help reestablish and maintain tissue homeostasis.

Insulin-like growth factor binding protein-2 binds to cell surface proteoglycans in the rat brain olfactory bulb

A family of six insulin-like growth factor binding proteins (IGFBPs) bind IGF-I and modulate its biological activity. IGFBPs may bind to macromolecules on the cell surface or pericellular extracellular matrix, and this interaction may modulate their effect on IGF activity. To date, little is known about the specificity of IGFBPs in the regulation of IGF action in the brain. We therefore explored whether IGFBPs were associated with cell membrane or extracellular matrix components in the rat brain. IGF-I binding sites with the characteristics of an IGFBP were found in the olfactory bulb mitral cell layer. This IGFBP was identified as IGFBP-2 by immunoprecipitation of both solubilized membrane preparations and cross-linked 125I-IGF: IGFBP complexes. While binding of IGFBP-2 to cell membranes was unaffected by RGD-containing peptide, it was inhibited by high salt concentration, suggesting interaction with proteoglycans. IGFBP-2 bound in vitro to the glycosaminoglycans chondroitin-4 and -6-sulfate, keratan sulfate, and heparin. IGFBP-2 also bound the proteoglycan aggrecan, an effect reduced by digestion of its glycosaminoglycans. Binding of IGFBP-2 to chondroitin-6-sulfate decreased the binding affinity of IGFBP-2 for IGF-I approximately 3-fold. Finally, an IGFBP-2 antibody coimmunoprecipitated IGFBP-2 and an approximately 200 kDa proteoglycan containing chondroitin-sulfate side chains from the rat olfactory bulb, providing definitive evidence for IGFBP-2 binding to olfactory bulb proteoglycans. These findings indicate that IGFBP-2 binds to proteoglycans in cell membranes of the rat olfactory bulb. Because we have previously shown that IGFs are highly expressed in the rat olfactory bulb, cell associated IGFBP-2 may have an important role in directing IGFs to specific sites in this brain region.

Spatial and temporal changes in the insulin-like growth factor (IGF) axis indicate autocrine/paracrine actions of IGF-I within wounds of the rat brain

A precise role for insulin-like growth factors (IGFs), IGF-binding proteins (IGFBPs), and IGF-receptors (IGF-Rs) in damaged central nervous system (CNS) tissue has not been elucidated, although their expression in the ischemic brain has been demonstrated. However, little is known of IGF responses after CNS trauma. In this study, we have used ribonuclease protection assay, in situ hybridization, and immunohistochemistry to demonstrate that IGF-I, IGFBPs, and IGF-1R expression alters in response to a penetrating CNS injury. Within penetrant cerebral wounds in the acute phase of the response (1-7 days post lesion; dpl), increased levels of IGF-I, IGFBP-1, -2, -3, -6, and IGF-1R protein were localized to injury responsive astrocytes, neurons and cells of the monocyte lineage. IGF-I, IGFBP-2, and 3 showed a congruency in sites of messenger RNA (mRNA) and peptide expression, with IGF-I and IGFBP-2 mRNA expression predominating. IGF-I, IGFBP-1, and IGFBP-3 protein were also associated with the microvascular endothelium, which was accompanied by increased levels of IGFBP-3 mRNA. These early changes in IGFBP expression probably facilitate IGF-I action. Later in the wounding response (7-14 dpl), the expression of IGFBP-4 and IGFBP-5 peaked within astrocytes and neurons, with IGFBP-5 mRNA being specifically localized to the glia limitans within the wound, suggesting an inhibitory role for these proteins, down-regulating the effects of IGF-I chronically. Our evidence suggests that within penetrating CNS wounds, IGF-I acts in an autocrine/paracrine manner to regulate cellular responses, with its spatial and temporal availability being modulated by the differential presence of stimulatory vs. inhibitory IGFBPs.

The effect of insulin-like growth factors on brain myelination and their potential therapeutic application in myelination disorders

Degenerative disorders of the cerebral white matter, leukodystrophies and demyelination diseases, are characterized by the faulty formation or excessive breakdown of myelin. Insulin-like growth factors (IGFs) promote the proliferation of oligodendrocytes as well as their myelin synthesis. IGF-I overexpressing mice show a significant increase in brain weight associated with increased myelin content. In contrast, the brains of IGF-binding protein-1 transgenic mice show a dramatic decrease in myelination. Furthermore, IGFs and IGF-binding proteins are among the factors that are induced by brain injury and have neuroprotective effects. IGFs also induce neurite growth and survival, in particular in glial cells of the peripheral nervous system. In demyelinating diseases, IGF-I may be useful for reducing myelin breakdown and promoting myelin regeneration. These observations may lead to new therapeutic applications for IGFs, for example promoting remyelination or limiting damage following brain injury.

The role of the insulin-like growth factors in the central nervous system

Increasing evidence strongly supports a role for insulin-like growth factor-I (IGF-I) in central nervous system (CNS) development. IGF-I, IGF-II, the type IIGF receptor (the cell surface tyrosine kinase receptor that mediates IGF signals), and some IGF binding proteins (IGFBPs; secreted proteins that modulate IGF actions) are expressed in many regions of the CNS beginning in utero. The expression pattern of IGF system proteins during brain growth suggests highly regulated and developmentally timed IGF actions on specific neural cell populations. IGF-I expression is predominantly in neurons and, in many brain regions, peaks in a fashion temporally coincident with periods in development when neuron progenitor proliferation and/or neuritic outgrowth occurs. In contrast, IGF-II expression is confined mainly to cells of mesenchymal and neural crest origin. While expression of type I IGF receptors appears ubiquitous, that of IGFBPs is characterized by regional and developmental specificity, and often occurs coordinately with peaks of IGF expression. In vitro IGF-I has been shown to stimulate the proliferation of neuron progenitors and/or the survival of neurons and oligodendrocytes, and in some cultured neurons, to stimulate function. Transgenic (Tg) mice that overexpress IGF-I in the brain exhibit postnatal brain overgrowth without anatomic abnormality (20-85% increases in weight, depending on the magnitude of expression). In contrast, Tg mice that exhibit ectopic brain expression of IGFBP-1, an inhibitor of IGF action when present in molar excess, manifest postnatal brain growth retardation, and mice with ablated IGF-I gene expression, accomplished by homologous recombination, have brains that are 60% of normal size as adults. Taken together, these in vivo studies indicate that IGF-I can influence the development of most, if not all, brain regions, and suggest that the cerebral cortex and cerebellum are especially sensitive to IGF-I actions. IGF-I's growth-promoting in vivo actions result from its capacity to increase neuron number, at least in certain populations, and from its potent stimulation of myelination. These IGF-I actions, taken together with its neuroprotective effects following CNS and peripheral nerve injury, suggest that it may be of therapeutic benefit in a wide variety of disorders affecting the nervous system.

Increased expression of type 1 insulin-like growth factor receptor messenger RNA in rat hippocampal formation is associated with aging and behavioral impairment

Insulin-like growth factor messenger RNAs are expressed in adult rat brain. However, little is known about the effects of aging on the expression of the insulin-like growth factors, their receptors, and their binding proteins in different regions of rat brain. The goal of the current study was to assess whether there is altered expression of the insulin-like growth factor system during normal aging in the hippocampal formation, a region particularly vulnerable to the aging process. A spatial learning task in the Morris water maze was used to assess the cognitive status of young (7-8-month-old) and aged (28-29-month-old) male Long-Evans rats. Sites of expression and abundance of insulin-like growth factor-I, type 1 insulin-like growth factor receptor, and insulin-like growth factor binding protein-4 messenger RNAs were then examined by in situ hybridization histochemistry and solution or northern blot hybridization assays. In situ hybridization histochemistry revealed no qualitative differences in the regional distribution of insulin-like growth factor-I, type 1 receptor, and insulin-like growth factor binding protein-4 messenger RNAs within the hippocampal formation of young and aged rats. However, quantitative analysis of messenger RNA abundance in hippocampal tissue homogenates showed a significant age-related increase in type 1 receptor messenger RNA (n = 25; t = -2.5; P < 0.02). Furthermore, linear regression analysis indicated that type 1 receptor messenger RNA abundance was significantly correlated with spatial learning impairment in the water maze (r = 0.44; P < 0.03) such that greater behavioral impairment was associated with higher type 1 receptor messenger RNA levels in the hippocampal formation. Neither insulin-like growth factor-I nor insulin-like growth factor binding protein-4 messenger RNA abundance was related to age or behavior. However, linear regression revealed a negative correlation between insulin-like growth factor-I messenger RNA abundance and type 1 receptor messenger RNA abundance in aged hippocampus (r = -0.72, P < 0.01). These data indicate that increased hippocampal expression of type 1 receptor messenger RNA is associated with aging and cognitive decline. The correlation between type 1 receptor and insulin-like growth factor-I messenger RNA abundance in the hippocampal formation of aged rats suggests that insulin-like growth factor availability may influence type 1 receptor expression. However, because no overall age difference was found in the amount of insulin-like growth factor-I messenger RNA in the hippocampal formation, decreased insulin-like growth factor from other sources such as the cerebrospinal fluid and the peripheral circulation may be involved in up-regulating type 1 receptor messenger RNA. Alternatively, type 1 receptor messenger RNA regulation may be part of a trophic response to the degenerative and regenerative events that occur within the hippocampal formation during aging.

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.

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.

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.

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.

The developing CNS: a scenario for the action of proinsulin, insulin and insulin-like growth factors

The multifunctional cytokines of the family of insulin and insulin-like growth factors (IGFs) have not yet gained general recognition as essential cell signals for the development of the vertebrate nervous system. This is, in part, a consequence of previous constraints in our thinking, focused for many years on the endocrine roles of these factors in late mammalian development and postnatal stages. The cellular distribution of the components of the insulin and IGFs signalling system in the developing mammalian and avian CNS is remarkably conserved. While receptors are widespread, the much less abundant factors and modulatory proteins are highly regulated in time and space. Progression of neural development through the steps of cell proliferation, differentiation, maturation and survival is stimulated, at least in culture, by proinsulin and insulin and the IGFs. Thus, these factors might be important autocrine and paracrine signals during development of the CNS.

Cell membrane association of insulin-like growth factor binding protein-2 (IGFBP-2) in the rat brain olfactory bulb

Identification of sites of expression of IGF, IGF receptors and IGFBPs in the olfactory bulb of the rat brain suggested the presence of a paracrine IGF system. Since cell association of IGFBPs has been suggested as an important factor in their modulation of IGF action, we investigated whether IGFBPs are cell associated in olfactory bulb (OB). This was supported by des(1-3)IGF-I only partially competing for [125I] IGF-I binding to rat OB membrane, suggesting the presence of a cell associated IGFBP. Affinity cross-linking of [125I]IGF-I to rat OB membrane demonstrated a 39-kDa complex which was reduced by IGF-I and IGF-II, but not by des(1-3)IGF-I or insulin. Western ligand blotting of solubilised membrane showed a 38-kDa IGFBP which was immunoprecipitated by anti-IGFBP-2 antiserum but not by anti-IGFBP-5 antiserum. We conclude that in the rat IGFBP-2 is associated with membranes from OB. Whether the cell membrane association is due to integrin binding via its RGD sequence or glycosaminoglycan binding is currently under investigation. Cell associated IGFBP-2 may modulate IGF action in the neonatal rat OB.

Insulin-like growth factor binding protein-1 is pre-synaptic at mouse neuromuscular synapses and is transported in nerve

In a previous study, we localized insulin-like growth factor binding protein 1 (IGFBP-1) to mouse neuromuscular junctions, and intramuscular nerves. To determine if pre-synaptic accumulation of IGFBP-1 occurred, we used double ligation of sciatic nerve in adult mice at different time points. IGFBPs were detected by Western ligand blot (WLB) with 125I-IGF-I. WLB and Western immunoblot (WIB) analysis of extracts from double-ligated nerves showed a delayed (6 days) increase of IGFBP-1 in the soluble fraction between the ligatures and distal to the distal ligature. For comparison we evaluated transport of neurofilament components, using WIB and confirmed the primarily anterograde transport of these intraaxonal proteins. These data suggest that expression of IGFBP-1 is both by activated Schwann cells as well as retrograde axonal transport with likely entry into the axon at the synapse.

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.

Expression of insulin-like growth factor binding protein-4 and -5 mRNAs in adult rat forebrain

Accumulating evidence indicates that the insulin-like growth factors (IGFs) can act as neurotrophic factors. A family of at least six IGF binding proteins (IGFBPs) has been characterized. The IGFBPs prolong the half-life of IGFs in plasma and may modulate IGF action in a cell- or tissue-specific fashion. Two recently characterized IGFBPs, IGFBP-4 and -5, have been shown by northern blot hybridization to be expressed in rat brain, but their cellular sites of synthesis are poorly characterized. Because IGFBP-4 and IGFBP-5 could potentially modulate IGF actions in the brain, we used in situ hybridization histochemistry and 35S-labeled IGFBP-4 and IGFBP-5 riboprobes to localize sites of IGFBP-4 and -5 mRNA expression in adult rat brain. The two IGFBP mRNAs are abundantly expressed within discrete regions of brain. The expression patterns of the two genes are largely nonoverlapping. Notably, IGFBP-4 mRNA is highly expressed within hippocampal and cortical areas, whereas IGFBP-5 mRNA is not detected above background in these areas. Within the hippocampus, abundant IGFBP-4 mRNA expression is detected in pyramidal neurons of the subfields of Ammon's horn and the subiculum and in the granule cell layer of the anterior hippocampal continuation. In the cortex, IGFBP-4 mRNA is widely expressed in most areas and layers. In contrast, IGFBP-5, but not IGFBP-4, mRNA is detected within thalamic nuclei, leptomeninges, and perivascular sheaths. The distinct expression patterns of IGFBP-4 and -5 mRNAs within the brain suggest that these IGFBPs may modulate paracrine/autocrine actions of the IGFs in discrete brain regions or compartmentalization of the IGFs within the brain.

Des (1-3) IGF-I potently enhances differentiated cell growth in olfactory bulb organ culture

We recently provided evidence that newborn rat olfactory bulb (OB) could be maintained in serum-free organ culture with combinations of insulin-like growth factor-I (IGF-I) and basic fibroblast growth factor (bFGF), both of which are locally synthesized. Des (1-3), or truncated, IGF-I is a potent analog of IGF-I isolated from rat and human brain. We proposed in this study to examine the effects of des (1-3) IGF-I on cell function, morphology and on neuronal and glial cell differentiation in our cultured OB model, using cell-specific immunostains for neurons (150 kDa neurofilament) and glial cells (glial fibrillary associated protein--GFAP). OB were cultured in Iscove's serum-free medium containing IGF-I or des (1-3) IGF-I both alone or in combination with bFGF. Dose dependent responses of 14C amino acid uptake showed des (1-3) IGF-I to be 3-5 fold more potent than IGF-I with a half maximal response at about 20 ng/ml in comparison to 100 ng/ml of IGF-I. The maximum response to IGF-I +/- bFGF was seen at 150 ng/ml; a ten-fold higher dose of insulin +/- bFGF was required to achieve the same response. While morphology was close to fresh 6 day OB following culture with IGF-I (150 ng/ml) and bFGF (25 ng/ml), the substitution of des (1-3) IGF-I at 50 ng/ml markedly improved morphology. Neurons were identified following culture in IGF-I or bFGF alone, but showed greater organisation in the mitral layer following combined IGF-I/bFGF culture. However, in contrast to IGF-I (150 ng/ml), des (1-3) IGF-I (50 ng/ml) supported marked neuronal expression. Furthermore, when des (1-3) IGF-I (50 ng/ml) was substituted for IGF-I, in combination with bFGF, the pattern of enhanced neuronal expression in the mitral layer was very close to that seen in the fresh 6 day bulb, with dendrites projecting to the glomerular layer. In OBs treated with no growth factors, or either IGF-I, des (1-3) IGF-I or bFGF alone, glial expression was widespread and poorly organised, suggesting an injury response. In contrast, following treatment with combinations of bFGF with IGF-I or des (1-3) IGF-I, a more ordered, though enhanced glial response was seen in glomerular and granule cell layers.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of insulin-like growth factor binding protein-4 (IGFBP-4) by rat neural cells--comparison to other IGFBPs

We recently isolated and characterized the 24 kDa and N-glycosylated 28 kDa insulin-like growth factor binding protein-4 (rIGFBP-4) from the B104s rat neuronal cell line (Endocrinology, 129 (1991) 1009-1115). To examine the prevalence of IGFBP-4 secretion by cells of neural origin, we assessed the expression of IGFBP-4 in different neural cell types using ligand blotting, immunoblotting and blot hybridization with relevant cDNAs. A specific IGFBP-4 antibody raised against a synthetic 20 amino acid peptide was used for immunologic recognition. In all the neural cells tested (B104s, C6 astrocytoma, primary neonatal astrocytes and primary fetal neurons), IGFBP-4 was definitively identified by immunoblotting. Blot hybridization using a rat cDNA probe revealed expression of IGFBP-4 mRNA transcripts by all these cells. Using a combination of the same techniques, expression of IGFBP-1, -2, and -3 were also examined. The B104s cells secreted primarily IGFBP-4; C6 cells secreted predominantly IGFBP-3 and small amount of IGFBP-4; both primary neonatal astrocytes and fetal neurons secreted IGFBP-2 as the major IGFBP accompanied by a small quantity of IGFBP-4. IGFBP-1 was not identified in any of the cell media. When probed with the respective IGFBP cDNAs, the mRNA abundance generally reflected the media IGFBP content. The expression of IGFBP-4 mRNA in vivo was examined as well and compared to that of IGFBP-1 and IGFBP-2. Transcripts for both IGFBP-2 and IGFBP-4 were found in all gross anatomical divisions of the rat brain from embryonic day 15 until adulthood, whereas IGFBP-1 was not detected at any time. IGFBP-4 mRNA tended to be more abundant at the youngest ages whereas IGFBP-2 increased during development. These data indicate that IGFBP-4 is produce by a variety of neural cell types and suggest that it may play a role in brain development.