|[Frontiers in Bioscience 2, d88-125, March 1, 1997]|
CROSS-TALK SIGNALS IN THE CNS: ROLE OF NEUROTROPHIC AND HORMONAL FACTORS, ADHESION MOLECULES AND INTERCELLULAR SIGNALING AGENTS IN LUTEINIZING HORMONE-RELEASING HORMONE (LHRH)-ASTROGLIAL INTERACTIVE NETWORK|
Department of Pharmacology, Medical School, University of Catania, 95125 Catania, Laboratory of Biotech. Neuropharmacology, OASI Institute for Research and Care (IRCCS) on Mental Retardation and Brain Aging (IRCCS) Troina, (EN), Italy.
Received 8/2/96; Accepted 2/20/97; On-line 3/1/97
In the dynamic flow of informational molecules participating in neuron-glia dialogue, the neurotransmitters, peptides and growth factors (GFs), represent the major bi-directional signaling agents. The importance of GFs in the coordination of developmental and adult physiological processes of both neurons and astrocytes is well recognized (36, 41-44, 51-60). A large number of proteins with growth promoting activity has been identified in recent years (Table 1). Substantial data are available for some members of the protein families of neurotrophins, fibroblast growth factors (FGFs), epidermal growth factor (EGF), and insulin-like growth factors (IGFs) (for comprehensive review see 36, 59, 60). A number of GFs have been shown to stimulate survival and differentiation of neurons and are often referred to as neurotrophic factors (36). Most of the characterized actions of growth and neurotrophic factors on brain cells relate to developmental processes, however, GFs also appear to be important in the function of the adult nervous system and for maintenance of structural integrity and regulation of synaptic plasticity (36).
An important factor in neuron-astroglial cell interactions is that glia in different brain regions express region-specific properties, including ion channels, neurotransmitter uptake and receptor systems, GFs production, and cell-surface adhesion systems (57). Then, the particular nature of the neuronal-glial interaction may depend on the specific neuronal and glial systems involved in a specific brain region (41). Thus, the dynamics of the cross-talk between neurons and astrocytes appears to be very complex. Neurons and glial cells are likely to be exposed to a number of different extracellular signaling molecules that may vary from moment to moment, and as a function of the particular physiological status (sex, age, stressful situation...). Therefore, a sophisticated regulatory network is likely to orchestrate the final appropriate response of both cell types.
The family of fibroblast growth factors (FGFs) include bFGF and aFGF, two mitogenic proteins originally purified based on their ability to bind to heparin, FGF-5, FGF-6, keratinocyte growth factor, and the oncogene product int-2 and hst (36, 61-63, for review). These proteins have different lengths but share a core of approximately 120 aminoacids, with a 50% aminoacid identity. Most FGFs, but not aFGF and bFGF, contain hydrophobic signal sequences believed to be necessary for secretion. The location of aFGF to the inner side of the neuronal membrane supports the speculation that it represents a sequestered protein with intracellular function (64). However, export has been observed in fibrosarcoma cells, suggesting a new type of releasing mechanism (65). Three members of a protein family of high affinity FGF receptor proteins have been characterized (61, 62). These proteins contain an intracellular tyrosine kinase domain believed to mediate signal transduction. FGF's bind to proteoglycans, although with a lower affinity than the tyrosine kinase receptor proteins (36). It has be reported that proteoglycans are necessary for binding FGFs to the high affinity receptor, and it has been speculated that they induce a conformational change of FGF necessary for binding to the high affinity receptor (66). FGFs occur in many peripheral tissues, and they are potent mitogens for a large number of cell types. The brain and pituitary are rich sources of bFGF. The biological function of these peptides in the brain is limited to aFGF and bFGF. Basic FGF and its mRNA are extensively distributed in the cells of the circumventricular organs and a neuroendocrine function of this factor in the pituitary-portal system has been suggested (62, 63, 67). Moreover, the association of bFGF mRNA and bFGF-R with specific loci of cells in the hypothalamus, with selective populations of neurons (such as magnocellular neurons (PVN) and the supraoptic neurons (SON) has been recently demonstrated. The median eminence (ME) shows intense hybridization signal for bFGF mRNA, which is mainly associated with neuronal fibers, glial and endothelial cells (67). The ependymal and subependymal cells lining the 3rd ventricle contain bFGF and FGF-R mRNAs (67). Acidic and basic FGF are abundantly expressed in the developing and adult CNS of chick, mouse, rat, monkey, and human (see 59).
Most of the immunohistochemical staining experiments have revealed that aFGF and bFGF are associated with neurons in vivo and vitro. However, aFGF and bFGF and their mRNAs have also been found in astroglial cells. Basic FGF is expressed at high levels by astrocyte and the CA2 hippocampal neurons in rats and mice (68), while aFGF has been shown to be expressed in motor, sensory and retinal neurons (62). Acidic FGF has been localized in a subpopulation of ependymal cells and tanycytes, as well as in some glial cells of adult rat brain. Basic FGF can be internalized by neurons and astrocytes and anterogradely transported by retinal ganglion cells after internalization (69). Both bFGF and aFGF influence development, survival and differentiation of various neuronal populations (70-72). Basic FGF promotes the survival and differentiation of both cultured cholinergic neurons of the rat basal forebrain, mesencephalic dopaminergic neurons (70-75), neostriatal GABAergic neurons (76) and immortalized hypothalamic neurons (44). Acidic FGF stimulates cholinergic, glutamatergic and GABAergic differentiation of spinal cord neurons in culture (77).
In culture, rat and bovine brain astrocytes synthesize bFGF and to a lesser extent aFGF (see 59). Of major importance, the release of endogenous astroglial bFGF has been shown to be influenced by other growths factors and lymphokines (59). In particular, Casper and coworkers (78), have reported that in dissociated cultures of dopaminergic neurons obtained from rat mesencephalon, bFGF mRNA is synthesized during time in culture, and its level increases according to the increased total RNA level and as a result of EGF treatment. These authors have, however, demonstrated that EGF and bFGF exert their trophic actions independently. Moreover, the addition of aFGF or bFGF to astrocytes in culture has been shown to induce them to synthesize and secrete NGF (79). Fibroblast growth factors have shown to elicit strong morphologic effects on astroblast, characterized by a shrinkage of their cell bodies, and give rise to extended processes. Besides other GFs, FGFs have been found to stimulate the proliferation of quiescent astroblasts. The fact that an antisense bFGF oligodeoxyribonucleotide inhibits DNA synthesis in rat astrocytes (80), suggests that bFGF in addition to its classical effect as a growth factor, is also an endogenous growth signal in developing astrocytes (59). Basic FGF can also delay the differentiation of O-2A progenitor cells probably as a result of its mitogenic effect (81). In the olfactory epithelium, FGF appears to promote neurogenesis by delaying terminal differentiation of a committed neuronal precursor and supporting proliferation of a possible stem cell, thereby expanding the initial pool of committed precursors (82). Petroski et al. (83) have shown that bFGF acts on astrocytes, not on the hypothalamic neurons growing on them, arresting the proliferation of embryonic astroblasts and inducing morphological differentiation of astrocytes. Neurons, subsequently differentiate well on astrocytes treated with this growth factor.
Epidermal growth factor and transforming growth factors (TGFs) are related acidic proteins of approximately 50 aminoacids, and are members of a larger protein family that includes certain viral proteins (36, 84). Recently discovered growth factors belonging to this family are amphiregulin, heparin binding growth factor, and schwanoma-derived growth factor (SDGF) (see 36). Both EGF and TGF-alpha stimulate a protein kinase receptor structurally related to v-erb-B oncogene (84).
EGF and TGFs are trophic polypeptides that stimulate the proliferation and differentiation of different cell types. EGF is also shown to be a potent stimulator of astrocytic proliferation. EGF is not synthesized by the developing neuronal cells, but its homolog, TGF-alpha , is expressed in the brain. During gliogenesis, immunoreactive EGF is detected in tissues and blood. EGF is also a neurotropic agent in neuronal cell cultures, acting as a neurotrophic factor for a number of neurons including rat embryo mesencephalic dopaminergic neurons in culture (36, 78), and immortalized hypothalamic peptidergic secreting neurons (44). Both EGF and TGFs stimulate the proliferation of primary astrocyte cultures obtained from rat cerebral hemispheres (54, 55). EGF is known to strongly affect the morphology of astrocytes and induce upregulation of the glutamine synthase and of the level of S-100 (see 54, 55, and 59 for review). In the peripheral nervous system (PNS) studies have indicated that individual growth factors act as critical determinants of transmitter type . In the brain, however, the initiation of neurotransmitter specific genes appears to involve more complex mechanisms, requiring the obligatory interaction of multiple signal molecules (see 85, 86).
Insulin and the structurally-related insulin-like growth factors I and II (IGF-I and IGF-II), are another family of proteins involved in the regulation of metabolism and cellular growth of different type of cells. They exhibit overlapping receptor specificity, with IGF-I and insulin being the most potent ligands, respectively for the IGF-I receptor and the insulin receptor (87, 88). Studies in "in vivo" and "in vitro", clearly demonstrate that IGF-I is a potent mitogen through interaction with its specific type I IGF receptor, which belongs to the family of transmembrane signal-transducing tyrosine kinases (87). This receptor prefers IGF-I over IGF-II and binds insulin with low affinity. The cellular actions of IGF-II are not clarified. The type II IGF receptor preferentially binds IGF-II over IGF-I and does not bind insulin. IGF-I and IGF-II associate with multiple high affinity binding proteins that can modify peptide-receptor interactions (89). Six distinct IGF-binding proteins (IGFBPs) have been characterized and designated IGFBP-1 to IGFBP-6. These IGFBPs are present in extravascular fluids and are secreted by a variety of cells in culture, suggesting their ability to critically modulate local actions of the IGFs (89, 90).
IGF-I and IGF-II expression in the brain is prominent during early development but its expression is limited decreased in the adult brain (91). In contrast Insulin and type I IGF receptor's proteins seem to be expressed by many neurons in the adult brain, particularly in cortical, hippocampal, and cerebellar structures (see 36). Insulin, IGF-I and IGF-II promote the survival and stimulate neurite outgrowth from cultured central and peripheral neurons, including the forebrain cholinergic and mesencephalic dopaminergic neurons (72, 92), as well as cultured immortalized hypothalamic cells (44). The fact that IGF-I is transiently expressed in projection neurons during synaptogenesis has led to the idea that IGF-I has a functional role in synapse formation or stabilization (36). In support of this idea, IGF-I and its receptor are permanently expressed in the olfactory bulb, where the process of synaptogenesis persists during adult life (91). Since IGF-I stimulates myelin formation, it has been suggested to act as a myelinization signal.
Insulin and the IGFs, have been found to stimulate the proliferation of quiescent astroblasts. Primary astroglial cells possess IGF-R, have the capability of synthesizing IGFs and IGF binding proteins, and exhibit a growth response to IGFs (93, 94). Expression of IGFs is developmentally regulated: IGF is present, at relatively low levels, during fetal growth in rodents, monkeys and humans, and rises post-natally to reach a peak during adolescence (94). Receptors for insulin and IGFs have been characterized both on astrocyte-enriched fractions from rat brain (95), as well as on cultured astrocytes (93, 96). There is also a considerable body of evidence pointing to insulin and IGFs being glial mitogens. Ins and IGFs have been reported to promote precursor incorporation into DNA, or to increase cell number in glial cultures (see 54, 55, 59). Insulin may also be involved in regulating glial differentiation. For example, it has been shown that insulin is capable of influencing the phenotypic appearance of astrocytes and the expression of mRNA for GFAP and its encoded protein in cerebellar organotypic cultures (96). Moreover, Avola and associates have clearly shown that the effect of EGF, IGFs and Ins on astroglial DNA and RNA as well as cytoskeletal protein labeling in primary astroglial cultures may change as a function of the changing micro-environment (see 54, 55).
In peripheral tissues, insulin is well known to be involved in the regulation of cell metabolism. Studies on cultured cells suggest that glial glucose utilization may be similarly controlled by insulin and IGFs, and this metabolic response has been shown to be developmentally regulated (see 59).