Autocrine role of insulin-like growth factor II secretion by the rat choroid plexus

Insulin and IGF receptor signalling in neural-stem-cell homeostasis

Neural stem cells (NSCs) are found in two regions in the adult brain: the subgranular zone (SGZ) in the hippocampal dentate gyrus and the subventricular zone (SVZ) adjacent to the lateral ventricles. Similarly to other somatic stem cells, adult NSCs are found within specialized niches that are organized to facilitate NSC self-renewal. Alterations in stem-cell homeostasis can contribute to the consequences of neurodegenerative diseases, healthy ageing and tissue repair after damage. Insulin and the insulin-like growth factors (IGFs) function in stem-cell homeostasis across species. Studies in the mammalian central nervous system support essential roles for IGF and/or insulin signalling in NSC self-renewal, neurogenesis, cognition and sensory function through distinct ligand-receptor interactions. IGF-II is of particular interest as a result of its production by the choroid plexus and presence in cerebrospinal fluid (CSF). CSF regulates and supports the development, division and migration of cells in the adult brain and is required for NSC maintenance. In this Review, we discuss emerging data on the functions of IGF-II and IGF and/or insulin receptor signalling in the context of NSC regulation in the SVZ and SGZ. We also propose a model for IGF-II in which the choroid plexus is a major component of the NSC niche.

Transplantation of cultured choroid plexus epithelial cells via cerebrospinal fluid shows prominent neuroprotective effects against acute ischemic brain injury in the rat

Choroid plexus (CP) epithelial cells (CPECs) produce cerebrospinal fluid (CSF) to provide the CNS with a specialized microenvironment. Our previous study showed that the conditioned medium of cultured CPECs enhanced the survival and neurite extension of hippocampal neurons. The present study examined the ability of cultured CPECs to protect against ischemic brain injury when transplanted into the CSF. Rats were subjected to a transient occlusion of the middle cerebral artery, followed by an injection of cultured CPECs into the fourth ventricle. The injection markedly reduced neurological deficits and infarction volume within 24h. Other beneficial effects were (1) a reduction in number of apoptotic and inflammatory cells, (2) an up-regulation of the mRNA expression of an anti-apoptotic effecter, cAMP-response element binding protein, and (3) a down-regulation of the production of pro-inflammatory factors such as interleukin-1 beta and inducible nitric oxide synthase. The injected CPECs were located within the ventricles and on the brain's surface, not in the ischemic foci, suggesting that they exert their effects by releasing diffusible neuroprotective factors into the CSF. The transplantation of CPECs via CSF is a potential new strategy for protecting against ischemic brain injury.

Fgf2 is expressed in human and murine embryonic choroid plexus and affects choroid plexus epithelial cell behaviour

Background

Although fibroblast growth factor (Fgf) signalling plays crucial roles in several developing and mature tissues, little information is currently available on expression of Fgf2 during early choroid plexus development and whether Fgf2 directly affects the behaviour of the choroid plexus epithelium (CPe). The purpose of this study was to investigate expression of Fgf2 in rodent and human developing CPe and possible function of Fgf2, using in vitro models. The application of Fgf2 to brain in vivo can affect the whole tissue, making it difficult to assess specific responses of the CPe.

Methods

Expression of Fgf2 was studied by immunohistochemistry in rodent and human embryonic choroid plexus. Effects of Fgf2 on growth, secretion, aggregation and gene expression was investigated using rodent CPe vesicles, a three-dimensional polarized culture model that closely mimics CPe properties in vivo, and rodent CPe monolayer cultures.

Results

Fgf2 was present early in development of the choroid plexus both in mouse and human, suggesting the importance of this ligand in Fgf signalling in the developing choroid plexus. Parallel analysis of Fgf2 expression and cell proliferation during CP development suggests that Fgf2 is not involved in CPe proliferation in vivo. Consistent with this observation is the failure of Fgf2 to increase proliferation in the tri-dimensional vesicle culture model. The CPe however, can respond to Fgf2 treatment, as the diameter of CPe vesicles is significantly increased by treatment with this growth factor. We show that this is due to an increase in cell aggregation during vesicle formation rather than increased secretion into the vesicle lumen. Finally, Fgf2 regulates expression of the CPe-associated transcription factors, Foxj1 and E2f5, whereas transthyretin, a marker of secretory activity, is not affected by Fgf2 treatment.

Conclusion

Fgf2 expression early in the development of both human and rodent choroid plexus, and its ability to modulate behaviour and gene expression in CPe, supports the view that Fgf signalling plays a role in the maintenance of integrity and function of this specialized epithelium, and that this role is conserved between rodents and humans.

Insulin like growth factor-1 and insulin like growth factor binding proteins in the cerebrospinal fluid and serum from patients with Alzheimer's disease

Insulin like growth factor-1 (IGF-1) is ubiquitously expressed growth factor that has profound effects on the growth and differentiation of many cell types and tissues, including cells of the central nervous system (CNS). IGF-1 is produced by a wide variety of cells and is found in many biological fluids including cerebrospinal fluid (CSF). IGF-1 plays important role during CNS development and repair. IGF-1 has broad range neuroprotective effects and is a therapeutic candidate for Huntington's disease (HD). IGF-1 protects striatal neurons from the toxicity of mutated Huntington in vitro and improves neuronal survival in vivo in a phenotypic model of HD. Alzheimer's disease (AD) is an age-dependent dementia characterized by progressive loss of cognitive functions and by characteristic pathological changes in the brain: the formation of aggregates extracellularly by beta-amyloid (AB) peptide and intracellularly by tau proteins. Since cerebrospinal fluid (CSF) is in contact with the extracellular space of the brain, biochemical brain modifications could be reflected in the CSF. IGFs in circulation and other physiological fluids are associated with a group of high-affinity binding proteins insulin like growth factor binding proteins (IGFBPs) that specifically bind and modulate their bioactivity at the cellular level. The aim of this study was to determine the level of CSF and serum IGF-1 and IGFBPs concentrations in the patients with AD. CSF was obtained by lumbar puncture. The presence of IGF-1 and IGFBPs in the CSF and serum samples was confirmed by Western blot using anti-IGF-1 and IGFBPs antibodies. Using enzyme linked immunosorbent assay (ELISA), it was shown that the concentration of CSF and serum IGF-1 and IGFBPs in the patients with AD is higher than in normal control. The data from this study indicate that IGF-1 is a constant component of human CSF. It is also concluded that high levels of CSF IGF-1 may be partly related to AD pathophysiology.

A novel role for the choroid plexus in BMP-mediated inhibition of differentiation of cerebellar neural progenitors

Cerebellar granule cells, the most abundant neurons in the mammalian brain, arise in the rhombic lip located at the roof of the brain's fourth ventricle. Bordering the rhombic lip is the choroid plexus, a non-neuronal structure, composed of blood vessels enveloped by epithelial cells. Here, we show a striking decrease in neural differentiation of rhombic lip-derived cells, which failed to extend neuritic processes and attenuate Math1 promoter activity, when co-cultured with choroid plexus cells. Moreover, a blocking antibody against BMP7, a morphogenetic protein expressed in the choroid plexus, blocked the inhibitory effect of the choroid plexus, whereas purified BMP7 mimicked this effect, demonstrating causal involvement of BMP. On the other hand, the BMP antagonist NBL1 promoted neurogenesis in rhombic lip cultures from Math1 null mice displaying arrested differentiation. Our data indicate that besides its secretory and barrier functions, the choroid plexus has a novel role in attenuating the differentiation of adjacent neural progenitors.

Insulin-like growth factor signaling in fish

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

The Choroid Plexus‐Cerebrospinal Fluid System: From Development to Aging

The function of the cerebrospinal fluid (CSF) and the tissue that secretes it, the choroid plexus (CP), has traditionally been thought of as both providing physical protection to the brain through buoyancy and facilitating the removal of brain metabolites through the bulk drainage of CSF. More recent studies suggest, however, that the CP-CSF system plays a much more active role in the development, homeostasis, and repair of the central nervous system (CNS). The highly specialized choroidal tissue synthesizes trophic and angiogenic factors, chemorepellents, and carrier proteins, and is strategically positioned within the ventricular cavities to supply the CNS with these biologically active substances. Through polarized transport systems and receptor-mediated transcytosis across the choroidal epithelium, the CP, a part of the blood-CSF barrier (BCSFB), controls the entry of nutrients, such as amino acids and nucleosides, and peptide hormones, such as leptin and prolactin, from the periphery into the brain. The CP also plays an important role in the clearance of toxins and drugs. During CNS development, CP-derived growth factors, such as members of the transforming growth factor-beta superfamily and retinoic acid, play an important role in controlling the patterning of neuronal differentiation in various brain regions. In the adult CNS, the CP appears to be critically involved in neuronal repair processes and the restoration of the brain microenvironment after traumatic and ischemic brain injury. Furthermore, recent studies suggest that the CP acts as a nursery for neuronal and astrocytic progenitor cells. The advancement of our knowledge of the neuroprotective capabilities of the CP may therefore facilitate the development of novel therapies for ischemic stroke and traumatic brain injury. In the later stages of life, the CP-CSF axis shows a decline in all aspects of its function, including CSF secretion and protein synthesis, which may in themselves increase the risk for development of late-life diseases, such as normal pressure hydrocephalus and Alzheimer's disease. The understanding of the mechanisms that underlie the dysfunction of the CP-CSF system in the elderly may help discover the treatments needed to reverse the negative effects of aging that lead to global CNS failure.

Insulin-like growth factor II mRNA is expressed in neurones of the brain of the bony fish Oreochromis mossambicus, the tilapia

The physiological meaning of insulin-like growth factor II (IGF-II) is still enigmatic. IGF-II occurs in the adult mammalian brain where it is expressed in the mesodermal portion of the choroid plexus and the meninges, but results on its presence in cells of neuroepithelial origin are controversial. However, IGF-II mRNA is transiently expressed in neurones during mammalian early development. In bony fish, IGF-II mRNA is also present in the adult brain but nothing is known about its synthesis sites. Thus, the present study using in situ hybridization with digoxigenin-labelled RNA species-specific probes investigates the cellular distribution of IGF-II mRNA in the adult brain of a bony fish, the tilapia (Oreochromis mossambicus). As in mammals, IGF-II mRNA was strongly expressed in the choroid plexus and meninges. Thus, IGF-II synthesis by choroid plexus and meninges seems to have a long evolutionary history and may be common to all vertebrates. However, as shown by the detailed investigation of landmark nuclei and regions, IGF-II mRNA occurred also in numerous neurones at all levels of the tilapia brain. The distinct localization of IGF-II mRNA in neurones might indicate that neuronal IGF-II acts as transmitter or modulator. However, the widespread occurrence of the IGF-II-producing neurones argues against this assumption and most probably suggests that IGF-II plays a role in the differentiation, maintenance and regeneration of neurones. It is further assumed that the sustained neuronal IGF-II expression in the brain of the adult tilapia correlates with continued post-embryonic up to life-long brain growth as has been shown in many teleost fishes.

GDF-15/MIC-1 a novel member of the TGF-beta superfamily

We have cloned, expressed, and raised antibodies against a novel member of the TGF-beta superfamily, growth/differentiation factor-15 (GDF-15). The predicted protein is identical to macrophage inhibitory cytokine-1 (MIC-1), which was discovered simultaneously. GDF-15 is a more distant member of the TGF-beta superfamily and does not belong to one of the known TGF-beta subfamilies. In the CNS, GDF-15/MIC-1 mRNA is abundantly expressed by the choroid plexus. In addition we have preliminary evidence that GDF-15/MIC-1 is a potent trophic factor for selected classes of neurons in vitro and in vivo. Thus, GDF-15 is a novel neurotrophic factor with prospects for the treatment of disorders of the CNS.

Grafting of choroid plexus ependymal cells promotes the growth of regenerating axons in the dorsal funiculus of rat spinal cord: a preliminary report

Nerve regeneration in the central nervous system has been studied by grafting various tissues and cells. In the present study, we demonstrated that choroid plexus ependymal cells can promote nerve regeneration when grafted into spinal cord lesions. The choroid plexus was excised from the fourth ventricle of adult rats (Wistar), minced into small fragments, and grafted into the dorsal funiculus at the C2 level in adult rat spinal cord from the same strain. Electron microscopy and fluorescence histochemistry showed that ependymal cells of the grafted choroid plexus intimately interacted with growing axons, serving to support the massive growth of regenerating axons. CGRP-positive fibers closely interacted with grafted ependymal cells. HRP injection at the sciatic nerve showed that numerous HRP-labeled regenerating fibers from the fasciculus gracilis extended into the graft 7 days after grafting. This regenerating axons from the fasciculus gracilis was maintained for at least 10 months, with some axons elongating rostrally into the dorsal funiculus. Evoked potentials of long duration were recorded at a level ca. 5 mm rostral to the lesion in the rats 8 to 10 months after grafting. These findings indicate that choroid plexus ependymal cells have the ability to facilitate axonal growth in vivo, suggesting that they may be a promising candidate as graft for the promotion of nerve regeneration in the spinal cord.

Comparative immunohistochemical study of insulin-like growth factor II and insulin-like growth factor receptor type 1 in pediatric brain tumors

Insulin-like growth factor (IGF)-II is an important growth factor in development of the central nervous system. The purpose of this study was to evaluate expression of IGF-II and IGF receptor type 1 (IGFR1) in various pediatric brain tumors. Immunohistochemistry for IGF-II and IGFR1 was performed on 15 choroid plexus papillomas (CPPs) including 1 atypical CPP, 2 choroid plexus carcinomas (CPCs), 5 anaplastic ependymomas, 7 nonanaplastic ependymomas (simply referred to as "ependymoma"), 5 medulloblastomas, 1 cerebral neuroblastoma, and 1 atypical teratoid/rhabdoid tumor (ATRT) along with 10 non-neoplastic choroid plexus and 3 non-neoplastic ependymal linings. All non-neoplastic choroid plexus, CPPs, CPCs, anaplastic ependymomas, ATRT, 71% of ependymomas, and 67% of non-neoplastic ependymal linings showed cytoplasmic positivity for IGF-II, whereas all medulloblastomas and the cerebral neuroblastoma were negative for IGF-II. In addition to cytoplasmic positivity for IGFR1, membranous positivity was observed in 73% of CPPs, both CPCs, the ATRT, 22% of non-neoplastic choroid plexus, 80% of anaplastic ependymomas, and 29% of ependymomas, but not in any medulloblastoma, cerebral neuroblastoma, or non-neoplastic ependymal lining. IGF-II and IGFR1 may play roles in the pathogeneses of CPP, CPC, anaplastic ependymoma, ependymoma, and ATRT. Immunohistochemical testing for IGF-II and IGFR1 may be useful in differentiating ATRT, CPC, and anaplastic ependymoma from medulloblastoma and cerebral neuroblastoma.

Choroid plexus: target for polypeptides and site of their synthesis

Choroid plexus (CP) is an important target organ for polypeptides. The fenestrated phenotype of choroidal endothelium facilitates the penetration of blood-borne polypeptides across the capillary walls. Thus, both circulating and cerebrospinal fluid (CSF)-borne polypeptides can reach their receptors on choroidal epithelium. Several polypeptides have been demonstrated to regulate CSF formation by controlling blood flow to choroid plexus and/or the activity of ion transport in choroidal epithelium. However, many ligand-receptor interactions occurring in the CP are not involved in the regulation of fluid secretion. Increasing evidence suggests that the choroidal epithelium plays an important role in hormonal signaling via a receptor-mediated transport into the brain (e.g., leptin) and helps to clear certain CSF-borne polypeptides (e.g., soluble amyloid beta-protein). Thus, impaired choroidal transport or insufficient clearance of polypeptides may contribute to pathogenesis of systemic or central nervous system (CNS) disorders, such as obesity or Alzheimer's disease. CP epithelium is not only a target but is also a source of neuropeptides, growth factors, and cytokines in the CNS. These polypeptides following their release into the CSF may exert distal, endocrine-like effects on target cells in the brain due to bulk flow of this fluid. Distinct temporal patterns of choroidal expression of several polypeptides are observed during brain development and in various CNS disorders, including traumatic brain injury and ischemia. Therefore, it is proposed that the CP plays an integral role not only in normal brain functioning, but also in the recovery from the injury. This review attempts to critically analyze the available data to support the above hypothesis.

Excretory plugs from the choroid plexus in the cerebrospinal fluid of dogs with neurological disease: possible role in the formation of corpora arenacea

Cytological specimens of cerebrospinal fluid (CSF) from 13 (6.5%) of 200 dogs with various neurological diseases contained excretory plugs (EPs) originating from choroid plexus cells (ChPCs). The presence of EPs was not related to age, breed or sex, or to any particular neurological disease. EPs, which ranged from 3 to 25 microm in diameter, had a sharp outline and contained granular material. Many such bodies were also found within ChPCs of dogs without neurological disease, which were examined to determine the origin of EPs. The results suggest that EPs found in the CSF are excretory products originating in a small population of ChPCs, and that when they reach a certain size they are eliminated into the CSF. It is possible that they serve as cores around which corpora arenacea ("brain sand") are subsequently formed.

Choroid plexus in the central nervous system: biology and physiopathology

Choroid plexuses (CPs) are localized in the ventricular system of the brain and form one of the interfaces between the blood and the central nervous system (CNS). They are composed of a tight epithelium responsible for cerebrospinal fluid secretion, which encloses a loose connective core containing permeable capillaries and cells of the lymphoid lineage. In accordance with its peculiar localization between 2 circulating fluid compartments, the CP epithelium is involved in numerous exchange processes that either supply the brain with nutrients and hormones, or clear deleterious compounds and metabolites from the brain. Choroid plexuses also participate in neurohumoral brain modulation and neuroimmune interactions, thereby contributing greatly in maintaining brain homeostasis. Besides these physiological functions, the implication of choroid plexuses in pathological processes is increasingly documented. In this review, we focus on some of the novel aspects of CP functions in relation to brain development, transfer of neuro-humoral information, brain/immune system interactions, brain aging, and cerebral pharmaco-toxicology.

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.