CNS glial cells express neurotrophin receptors whose levels are regulated by NGF

Design, synthesis and anticancer evaluation of novel arylhydrazones of active methylene compounds

Nerve growth factor (NGF) and its receptor, tropomyosin kinase receptor kinase type A (TrkA) is emerging as an important target for Glioblastoma (GBM) treatment. TrkA is the cancer biomarker majorly involved in tumor invasion and migration into nearby normal tissue. However, currently, available Trk inhibitors exhibit many adverse effects in cancer patients, thus demanding a novel class of ligands to regulate Trk signaling. Here, we exploited the role of TrkA (NTRK1) expression from the 651 datasets of brain tumors. RNA sequence analysis identified overexpression of NTRK1 in GBM, recurrent GBM as well in Oligoastrocytoma patients. Also, TrkA expression tends to increase over the higher grades of GBM. TrkA protein targeting hydrazone derivatives, R48, R142, and R234, were designed and their mode of interaction was studied using molecular docking and dynamic simulation studies. Ligands' stability and binding assessment reveals R48, 2 2-(2-(2-hydroxy-4-nitrophenyl) hydrazineylidene)-1-phenylbutane-1,3-dione, as a potent ligand that interacts well with TrkA's hydrophobic residues, Ile, Phe, Leu, Ala, and Val. R48- TrkA exhibits stable binding potentials with an average RMSD value <0.8 nm. R48 obeyed Lipinski's rule of five and possessed the best oral bioavailability, suggesting R48 as a potential compound with drug-likeness properties. In-vitro analysis also revealed that R48 exhibited a higher cytotoxicity effect for U87 GBM cells than TMZ with the IC50 value of 68.99 μM. It showed the lowest percentage of cytotoxicity to the non-cancerous TrkA expressing MEF cells. However, further SiRNA analysis validates the non-specific binding of R48, necessitating structural alteration for the development of R48-based TrkA inhibitor for GBM therapeutics.

Differential contribution of TrkB and p75NTR to BDNF-dependent self-renewal, proliferation, and differentiation of adult neural stem cells

Alterations in adult neurogenesis are a common hallmark of neurodegenerative diseases. Therefore, understanding the molecular mechanisms that control this process is an indispensable requirement for designing therapeutic interventions addressing neurodegeneration. Neurotrophins have been implicated in multiple functions including proliferation, survival, and differentiation of the neural stem cells (NSCs), thereby being good candidates for therapeutic intervention. Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family and has been proven to promote neurogenesis in the subgranular zone. However, the effects of BDNF in the adult subventricular zone (SVZ) still remain unclear due to contradictory results. Using in vitro cultures of adult NSCs isolated from the mouse SVZ, we show that low concentrations of BDNF are able to promote self-renewal and proliferation in these cells by activating the tropomyosin-related kinase B (TrkB) receptor. However, higher concentrations of BDNF that can bind the p75 neurotrophin receptor (p75NTR) potentiate TrkB-dependent self-renewal and proliferation and promote differentiation of the adult NSCs, suggesting different molecular mechanisms in BDNF-promoting proliferation and differentiation. The use of an antagonist for p75NTR reduces the increment in NSC proliferation and commitment to the oligodendrocyte lineage. Our data support a fundamental role for both receptors, TrkB and p75NTR, in the regulation of NSC behavior.

Olfactory Ensheathing Cells for Spinal Cord Injury: Sniffing Out the Issues

Olfactory ensheathing cells (OECs) are glia reported to sustain the continuous axon extension and successful topographic targeting of the olfactory receptor neurons responsible for the sense of smell (olfaction). Due to this distinctive property, OECs have been trialed in human cell transplant therapies to assist in the repair of central nervous system injuries, particularly those of the spinal cord. Though many studies have reported neurological improvement, the therapy remains inconsistent and requires further improvement. Much of this variability stems from differing olfactory cell populations prior to transplantation into the injury site. While some studies have used purified cells, others have used unpurified transplants. Although both preparations have merits and faults, the latter increases the variability between transplants received by recipients. Without a robust purification procedure in OEC transplantation therapies, the full potential of OECs for spinal cord injury may not be realised.

Expression pattern of high-affinity tyrosine kinase Aduring the development of human fetal spinal cord

High-affinity tyrosine kinase A (TrkA) is responsible for the biological activities of nerve growth factor. Most studies of the molecular mechanisms of TrkA that underlie the development of the spinal cord have been conducted in animals and the expression pattern of TrkA during the development of the human fetal spinal cord is not well characterized. We investigated 45 3-28-week-old (G3W-G28W) human fetuses. We assessed the expression pattern of TrkA in the human fetal spinal cord using immunohistochemistry, western blot and reverse transcription polymerase chain reaction to clarify the spatiotemporal developmental changes and to determine the role TrkA plays in development. TrkA immunoreactive products were detected widely in the alar and basal plates, ependyma, glial cells, gray and white matter, internal limiting membrane, mantle layer, marginal layer, neuroepithelium and neurons during this period of development. Expression levels of TrkA mRNA and protein peaked at G12W and G16W, respectively. The strong expression of TrkA was closely related to the formation of the dorsal and ventral horns, and the differentiation of somatic motor neurons during late embryonic development. Our findings suggest that TrkA receptors play crucial roles during the development of human fetal spinal cord. The characteristic expression patterns may clarify the developmental characteristics of the human spinal cord.

Tumor Necrosis Factor-stimulated Gene-6 (TSG-6) Is Constitutively Expressed in Adult Central Nervous System (CNS) and Associated with Astrocyte-mediated Glial Scar Formation following Spinal Cord Injury

Tumor necrosis factor (TNF)-stimulated gene-6 (TSG-6) binds to hyaluronan and can reorganize/stabilize its structure, also enhancing the binding of this glycosaminoglycan to its cell surface receptor, CD44. TSG-6 is rapidly up-regulated in response to inflammatory cytokines protecting tissues from the damaging effects of inflammation. Despite TSG-6 treatment having been shown to improve outcomes in an experimental model of traumatic brain injury, TSG-6 expression has not been extensively studied in the central nervous system (CNS). We hereby analyzed the expression profile of TSG-6 in the developing CNS and following injury. We show that TSG-6 is expressed in the rat CNS by GFAP(+) and CD44(+) astrocytes, solely in the mature brain and spinal cord, and is not present during the development of the CNS. TSG-6(-/-) mice present a reduced number of GFAP(+) astrocytes when compared with the littermate TSG-6(+/-) mice. TSG-6 expression is drastically up-regulated after injury, and the TSG-6 protein is present within the glial scar, potentially coordinating and stabilizing the formation of this hyaluronan-rich matrix. This study shows that TSG-6 is expressed in the CNS, suggesting a role for TSG-6 in astrocyte activation and tissue repair. We hypothesize that within this context TSG-6 could participate in the formation of the glial scar and confer anti-inflammatory properties. Further studies are required to elucidate the therapeutic potential of targeting TSG-6 after CNS injury to promote its protective effects while reducing the inhibitory properties of the glial scar in axon regeneration.

BDNF deletion or TrkB impairment in amygdala inhibits both appetitive and aversive learning

Brain-derived neurotrophic factor (BDNF) is known to have an integral role in establishing stable memories after learning events. The neuroplasticity induced by Pavlovian fear conditioning has likewise been shown to rely on interactions between BDNF and its principal receptor, tyrosine kinase receptor B (TrkB), in the amygdala after training. Although the necessity of amygdala bdnf expression and TrkB activation for associative learning within aversive contexts has been explored, it is unclear to what extent this interaction is involved in appetitive learning. It is also unclear whether the noted increases in amygdala BDNF after fear conditioning are due to local gene transcription and translation or anterograde transmission from cortical regions. To address both of these questions, we used two lentiviral approaches in mice, using both fear conditioning and cocaine-conditioned place preference (CPP), during acquisition and extinction. First, we decreased expression of bdnf mRNA in the amygdala of homozygous floxed mice with a Cre-expressing virus. In a second set of studies, we infused a virus that expressed a dominant-negative TrkB isoform into the same region. These approaches significantly impaired consolidation of fear conditioning and cocaine-CPP, as well as extinction of CPP. Together, these data suggest that BDNF-TrkB signaling is critical for amygdala-dependent learning of both appetitive and aversive emotional memories.

Neuroprotective role of nerve growth factor in hypoxic-ischemic brain injury

Hypoxic-ischemic brain injuries (HIBI) in childhood are frequently associated with poor clinical and neurological outcome. Unfortunately, there is currently no effective therapy to restore neuronal loss and to determine substantial clinical improvement. Several neurotrophins, such as Nerve Growth Factor (NGF), Brain-Derived Neurotrophic Factor (BDNF), and Glial Derived Neurotrophic Factor (GDNF), play a key role in the development, differentiation, and survival of the neurons of the peripheral and central nervous system. Experimental animal studies demonstrated their neuroprotective role in HIBI, while only a few studies examined the neuroprotective mechanisms in patients with severe HIBI. We report two cases of children with HIBI and prolonged comatose state who showed a significant improvement after intraventricular NGF administration characterized by amelioration of electroencephalogram (EEG) and cerebral perfusion at single-photon emission computed tomography (SPECT). The improvement in motor and cognitive functions of these children could be related to the neuroprotective role exerted by NGF in residual viable cholinergic neurons, leading to the restoration of neuronal networks in the damaged brain.

Characterization of trkB immunoreactive cells in the intermediolateral cell column of the rat spinal cord

The objective of the present study was to characterize the trkB receptor immunoreactive (-ir) cells in the intermediolateral cell column (IML) of the upper thoracic spinal cord. Small trkB-ir cells (area=56.1+/-4.4 microm(2)) observed in the IML showed characteristics of oligodendrocytes and were frequently observed in close apposition to choline acetyltransferase (ChAT)-ir cell bodies. Large trkB-ir cells (area=209.3+/-25.2 microm(2)) showed immunoreactivity for the neuronal marker NeuN, indicating their neuronal phenotype, as well as for ChAT, a marker for preganglionic neurons. TrkB and ChAT were co-localized in IML neurons primarily in cases that had received in vivo administration of nerve growth factor (NGF). These findings reveal two different cell types, oligodendrocytes and neurons, in the IML of the spinal cord that show trkB immunoreactivity, suggesting their regulation by brain derived neurotrophic factor (BDNF) and/or neurotrophin-4 (NT-4). In addition, there is evidence that NGF may play a role in the regulation of trkB-ir preganglionic neurons in the IML.

Estrogen effects on high-affinity choline uptake in primary cultures of rat basal forebrain

Basal forebrain cholinergic neurons (BFCNs) degenerate in aging and Alzheimer's disease. It has been proposed that estrogen can affect the survival and function of BFCNs. This study characterized primary rat BFCN cultures and investigated the effect of estrogen on high-affinity choline uptake (HACU). BFCNs were identified by immunoreactivity to the vesicular acetylcholine transporter (VAChT) and represented up to 5% of total cells. HACU was measured in living BFCN cultures and differentiated from low-affinity choline uptake by hemicholinium-3 (HC-3) inhibition. A HC-3 concentration curve showed that 0.3 muM HC-3, but not higher concentrations that inhibit LACU, could distinguish the two transport activities. 17-beta-Estradiol treatment increased HACU in some culture preparations that contained non-neuronal cells. Elimination of dividing cells using antimitotic treatments resulted in a lack of estrogen effects on HACU. These results suggest that estrogen may have indirect effects on BFCNs that are mediated through non-neuronal cells.

Transient ischemia-induced expression and changes of tyrosine kinase A in the hippocampal dentate gyrus of the gerbil

The present study examined ischemia-related changes in tyrosine kinase A (trkA) immunoreactivity and its protein content in the dentate gyrus after 5 min of transient forebrain ischemia in gerbils. One day after ischemic insult, cresyl violet-positive polymorphic cells showed ischemic degeneration. The ischemia-induced changes in trkA immunoreactivity were found in the polymorphic layer (PL) and granule cell layer (GCL) of the dentate gyrus. In the sham-operated group, trkA immunoreactivity in the dentate gyrus was very weak. From 30 min after ischemia, trkA immunoreactivity was increased in the dentate gyrus and peaked in the dentate gyrus at 12 h after ischemia-reperfusion. Thereafter, trkA immunoreactivity was decreased time-dependently after ischemia-reperfusion. Four days after ischemic insult, trkA immunoreactivity was similar to that of the sham-operated group. In addition, it was found that ischemia-related changes in trkA protein content were similar to the immunohistochemical changes. These results suggest that the chronological changes of trkA in the dentate gyrus after transient forebrain ischemia may be associated with ischemic damage in polymorphic cells of the dentate gyrus.

Tyrosine kinase A but not phosphacan/protein tyrosine phosphatase-ζ/β immunoreactivity and protein level changes in neurons and astrocytes in the gerbil hippocampus proper after transient forebrain ischemia

In the present study, ischemia-related changes in tyrosine kinase A (trkA) and phosphacan/protein tyrosine phosphatase-zeta/beta (PTP-zeta/beta) immunoreactivities and protein contents were examined in the hippocampus proper after transient forebrain ischemia for 5 min in a gerbil model. Our investigations showed that ischemia-induced changes occurred in trkA immunoreactivity in the hippocampal CA1 region, but not in the CA2/3 region of the hippocampus proper. In the sham-operated group, trkA immunoreactivity was barely detectable. trkA immunoreactivity increased from 30 min after ischemia and peaked at 12 h. Four days after ischemic insult, trkA immunoreactivity was observed in GFAP-immunoreactive astrocytes in the strata oriens and radiatum. In addition, we found that ischemia-related changes in trkA protein content were similar to immunohistochemical changes. On the other hand, PTP-zeta/beta immunoreactivities in the hippocampus proper were unaltered by forebrain ischemia. These results suggest that chronological changes of trkA after transient forebrain ischemia may be associated with an ischemic damage compensatory mechanism in CA1 pyramidal cells.

Neurotrophic factors and the pathophysiology of schizophrenic psychoses

The aim of this review is to summarize the present state of findings on altered neurotrophic factor levels in schizophrenic psychoses, on variations in genes coding for neurotrophic factors, and on the effect of antipsychotic drugs on the expression level of neurotrophic factors. This is a conceptual paper that aims to establish the link between the neuromaldevelopment theory of schizophrenia and neurotrophic factors. An extensive literature review has been done using the Pub Med database, a service of the National Library of Medicine, which includes over 14 million citations for biomedical articles back to the 1950s. The majority of studies discussed in this review support the notion of alterations of neurotrophic factors at the protein and gene level, respectively, and support the hypothesis that these alterations could, at least partially, explain some of the morphological, cytoarchitectural and neurobiochemical abnormalities found in the brain of schizophrenic patients. However, the results are not always conclusive and the clinical significance of these alterations is not fully understood. It is, thus, important to further neurotrophic factor research in order to better understand the etiopathogenesis of schizophrenic psychoses and, thus, potentially develop new treatment strategies urgently needed for patients suffering from these devastating disorders.

Remyelination in multiple sclerosis: a new role for neurotrophins?

Multiple sclerosis (MS) is a common neurological disease, which affects young adults. Its course is unpredictable and runs over decades. It is considered as an autoimmune disease, and is neuropathologically characterized by demyelination, variable loss of oligodendroglial cells, and axonal degeneration. Demyelination provides a permitting condition for axonal degeneration, which seems to be causative of permanent neurological deficits. Hence, the current treatment, which works preferentially immunmodulatory, should be complemented by therapeutics, which improves remyelination not only for restoring conduction velocity but also for preventing an irreversible axonal damage. One strategy to achieve this aim would be to promote remyelination by stimulating oligodendroglial cells remaining in MS lesions. While central nervous system neurons were already known to respond to neurotrophins (NT), interactions with glial cells became apparent more recently. In vitro and in vivo studies have shown that NT influence proliferation, differentiation, survival, and regeneration of mature oligodendrocytes and oligodendroglial precursors in favor of a myelin repair. Two in vivo models provided direct evidence that NT can improve remyelination. In addition, their neuroprotective and anti-inflammatory role would support a repair. Hence, a wealth of data point to NT as promising therapeutical candidates.

Concomitant up-regulation of astroglial high and low affinity nerve growth factor receptors in the CA1 hippocampal area following global transient cerebral ischemia in rat

We have examined the effect of global transient cerebral ischemia, evoked in rat by 10 min of cardiac arrest, upon the changes in the cellular expression of two nerve growth factor (NGF) receptors (TrkA and p75) in the hippocampus. We have used immunocytochemical procedures, including a quantitative analysis of staining, along with some quantitative morphological analyses. We have found, under ischemic conditions, a decrease of TrkA immunoreactivity in degenerating CA1 pyramidal neurons and in neuropil. On the other hand, a strong, ischemia-induced up-regulation of TrkA and p75 immunoreactivity was observed in the majority of reactive astroglia population in the adjacent CA1 hippocampal region. The colocalization of the two receptors in the same reactive astroglial cells was evidenced by double immunostaining and further supported by quantitative morphological analysis of TrkA and p75 immunoreactive glial cells. Our data implicate the involvement of NGF receptors in the postischemic regulation of astrocytic function; however, the lack of NGF receptor expression on some astrocytes suggests heterogeneity of astroglia population. Our results also indicate that the lack of neuroprotective action of astroglial NGF induced in the ischemic hippocampus [J Neurosci Res 41 (1995) 684; Acta Neurobiol Exp 57 (1997) 31; Neuroscience 91 (1999) 1027] is not caused by a paucity of NGF receptors but may rather be due to the counteraction of some proinflammatory substances, released simultaneously by glia cells. On the other hand, the up-regulated astroglial TrkA receptor may be an important target for exogenous NGF, which, as previously described [J Neurosci 11 (1991) 2914; Neurosci Lett 141 (1992) 161], exerts a neuroprotective effect in ischemia.

Neurotrophins reduce degeneration of injured ascending sensory and corticospinal motor axons in adult rat spinal cord

Spinal cord regeneration in adult mammals is limited by neurite outgrowth inhibitors and insufficient availability of outgrowth-promoting agents. Formation of degenerative swellings at the proximal ends of severed axons (terminal clubs), which starts early after injury, also may hinder recovery and their rupture may contribute to secondary spinal cord damage. We investigated whether neurotrophins would reduce these degenerative processes. Adult rats received a transection of the dorsal column sensory and corticospinal motor tracts at T9 and anterograde tracing of the axons from the sciatic nerve and motor cortex, respectively. The highest number of terminal clubs was found at 1 day and approximately half remained present until at least 28 days. A single injection immediately after injury of a mixture of nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 into the lesion site, reduced the number of terminal clubs in the sensory system by approximately half at 1 and 7 days (but not 14) after the lesion. Individual or combinations of two neurotrophins were as effective, suggesting that the neurotrophins protected similar axonal populations. The injected neurotrophins did not affect degeneration of corticospinal motor axons. A 7-day continuous intrathecal infusion of neurotrophin-3 was more effective and also reduced terminal club formation of corticospinal axons by approximately 60%. Spinal tissue loss was not affected by the neurotrophin treatments, suggesting that terminal clubs are not major contributors to the pathogenesis of secondary spinal degeneration during the first two weeks. Thus, neurotrophins can reduce axonal degeneration in the spinal cord after traumatic axonal injury.

Nerve growth factor plays a divergent role in mediating growth of rat C6 glioma cells via binding to the p75 neurotrophin receptor

Dysregulation of proliferation, differentiation and cell death play a major role in glial tumors, and there is evidence for regulatory mechanisms involving nerve growth factor (NGF) and its receptors in various CNS-derived tumor cell lines. The aim of our study was to observe the effect of exogenous recombinant NGF on C6 rat glioma growth, to characterize the role of endogenous NGF and the p75 neurotrophin receptor (p75) and to rule out whether p75 is necessary to mediate the effect of exogenous NGF. Recombinant exogenous NGF (1-100 ng/ml) was applied under different serum conditions (0%, 1%, 5%) and knockdown of endogenous NGF and p75 was achieved by lipid-mediated antisense oligonucleotide treatment. In presence of serum, NGF had a positive whereas in absence of serum NGF produced a negative effect on C6 cell number. A knockdown of NGF or p75 increased cell numbers and enhanced BrdU incorporation. In p75-knocked down cells NGF did not enhance C6 glioma growth in presence of serum. We conclude that (1) exogenous recombinant NGF enhances C6 glioma growth under serum conditions but decreases cell number in absence of serum, that (2) the effect of exogenous NGF is mediated by p75 alone or by heterodimers containing p75 and that (3) either basal levels of endogenous NGF or basal levels of p75 receptor moderate C6 glioma growth and represent an autoregulatory potential of C6 glioma cells.

Enhanced expression of NGF receptors in multiple sclerosis lesions

The receptor for nerve growth factor (NGF) comprises a 75-kDa (NGFRp75) and a tyrosine kinase A (TrkA) subunit. In view of conflicting opinions on the identity of glial targets of NGF in human central nervous system (CNS), we examined the cellular distribution of both NGF receptor subunits in normal CNS and in chronic multiple sclerosis (MS) lesions. For this, we compared the pattern of recognition of 2 monoclonal antibodies (mAbs) and a polyclonal antiserum to NGFRp75. Only the 2 mAbs specifically recognized NGFRp75, while the polyclonal antiserum showed widespread reactivity. In normal CNS and silent MS lesions, immunohistochemistry with anti-NGFRp75 mAbs and for TrkA revealed perivascular cell reactivity. At the edge of chronic active MS lesions, selective NGFRp75 staining was prominent on reactive astrocytes, while throughout the lesion, NGFRp75 was expressed on microglia/macrophages. The vast majority of mature or precursor oligodendrocytes did not express NGFRp75. Both NGF receptors were co-expressed on a subset of inflammatory cells. Immunoreactivity for NGFRp75 on glial and immune cells did not correlate with the distribution of apoptotic figures, as detected by TUNEL. Thus, expression of NGF receptors in active MS lesions suggests a role for NGF in regulating the autoimmune response at both immune and glial cell levels.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Hippocampal tyrosine kinase A receptors are restricted primarily to presynaptic vesicle clusters

Adult septohippocampal cholinergic neurons are dependent on trophic support for normal functioning and survival; these effects are largely mediated by the tyrosine kinase A receptor (TrkA), which binds its ligand, nerve growth factor (NGF), with high affinity. To determine the subcellular localization of TrkA within septohippocampal terminal fields, two rabbit polyclonal antisera to the extracellular domain of TrkA were localized immunocytochemically in rat dentate gyrus by light and electron microscopy. By light microscopy, TrkA immunoreactivity was found mostly in fine, varicose fibers primarily in the hilus and, to a lesser extent, in the granule cell and molecular layers. By electron microscopy, the central and infragranular regions of the hilus contained the highest densities of TrkA-immunoreactive profiles. Most TrkA-labeled profiles were axons (31% of 3,473), axon terminals (20%), and glia (38%); fewer were dendrites (6%), dendritic spines (5%), and granule cell and interneuron somata (<1%). TrkA immunolabeling in axons and axon terminals was discrete, often concentrated in patches of small synaptic vesicles that were adjacent to somatic and dendritic profiles. TrkA-labeled terminals formed both asymmetric and symmetric synapses, primarily with dendritic shafts and spines. TrkA-immunoreactive glial profiles frequently apposed terminals contacting dendritic spines. The findings that presynaptic profiles contain TrkA immunolabeling in sites of vesicle accumulation suggest that NGF binding to TrkA may influence transmitter release. The presence of TrkA immunoreactivity in somata, dendrites, and glia further suggests that cells within the dentate gyrus may take up NGF.

Rat oligodendrocytes express the vitamin D(3) receptor and respond to 1,25-dihydroxyvitamin D(3)

The present study investigates the presence of vitamin D receptor (VDR) in cells of the rat oligodendrocyte (OL) lineage. VDR transcripts were detected by in situ hybridization in a fraction of rat OL in secondary cultures. The VDR protein was shown to be co-localized in cells that are also recognized by an anti-myelin basic protein (MBP) antibody. Likewise, in vivo, VDR-positive cells were found in the brain white matter, such as the internal capsule of the striatum or the corpus callosum but also in the spinal cord. At least part of these positive cells in vivo correspond to OL, since they were co-stained by an anti-carbonic anhydrase II antiserum. Northern blot analyses of the CG-4 OL cell line demonstrated that the VDR transcripts are already found in the O-2A precursors. There was a two-fold increase in the relative abundance of these transcripts in differentiated OL or in type-2 astrocytes. 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] increased the pool of transcripts encoding its own receptor, the VDR. The hormone also enhanced the abundance of the mRNA of the nerve growth factor (NGF) and of its low-affinity receptor, the p75(NTR) protein. By contrast, the hormone had no effect on the levels of MBP or proteolipid protein (PLP) mRNA. This finding suggests that unlike retinoic acid (RA) or thyroid hormone, 1,25-(OH)(2)D(3) has no regulatory action on the synthesis of myelin proteins.

Glial cells as targets and producers of neurotrophins

Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

Mitogenesis in glioblastoma multiforme cell lines: a role for NGF and its TrkA receptors

Neurotrophins have definitive roles in the growth/maintenance of neuronal populations, but their function in malignant gliomas is unknown. The ability for nerve growth factor (NGF) to serve as a mitogenic agent was investigated in several human glioblastoma multiforme (GBM) cell lines, including U251, U87, and U373. In a serum-free medium, the addition of NGF (200 ng/ml) to these cell lines increased cell counts over controls, after 3 days in culture by 9%, 16%, and 33%, respectively. Dose-dependent increases in cell counts and [3H]thymidine uptake were found in the more rigorously investigated U373 cell line. Proteins for both the high affinity NGF-specific tyrosine kinase binding site (p140TrkA; TrkA) and the low affinity neurotrophin (p75NTR) receptor were present in all three GBM cell lines. TrkA mRNA was identified in U373 (only cell line studied). NGF-stimulated proliferation was inhibited in a dose-dependent fashion by K252a, a blocker of Trk-induced receptor kinases. NGF, measured by ELISA, was detectable in all GBM cell lines even after 7 days of growth in serum-free medium. These data suggest that GBM cell growth can be enhanced by NGF acting via Trk receptor phosphorylation. Future studies of antiproliferative therapies should consider agents directed against intracellular Trk signaling cascades.

p75NTR immunoreactivity in the rat dentate gyrus is mostly within presynaptic profiles but is also found in some astrocytic and postsynaptic profiles

To localize neurotrophin binding sites within the rat dentate gyrus, the distribution of low-affinity p75 neurotrophin receptor (p75NTR) immunoreactivity (IR) was examined by using antiserum raised against the cytoplasmic domain of the receptor. Semiquantitative electron microscopic examination of p75NTR-labeled sections showed that most p75NTR-labeled profiles were axons and axon terminals (72% from a total of 3,975); p75NTR-IR was observed throughout the extent of these structures and was not limited to the plasmalemmal surface. Axons and axon terminals containing p75NTR-IR were distributed in approximately equal proportions across the hilus, infragranular zone, and the inner, middle, and outer molecular layers; significantly fewer p75NTR-labeled profiles were observed in the granule cell layer. Axon terminals containing p75NTR-IR, which made synapses (296 of 552), formed equal proportions of symmetric and asymmetric synapses, primarily with the shafts and spines of dendrites. The remainder of the p75NTR-labeled terminals apposed unlabeled somata and dendrites without forming synapses in the single sections analyzed. In addition, p75NTR-IR was contained within some astrocytes (17.5% of 3,975) and dendritic shafts (3%) and spines (5%). Within dendritic spines, p75NTR-IR was most often associated with the plasmalemmal surface near postsynaptic densities; in dendritic shafts, p75NTR labeling was associated with microfilaments distant from the plasmalemma. Most p75NTR-labeled dendritic profiles were located in the molecular layer, and some originated from granule cells. Moreover, in some granule cell somata (<1% of 3,975), p75NTR-IR was associated with endosomes. The primary localization of p75NTR-IR to presynaptic structures in the dentate gyrus, presumably arising from medial septal/diagonal band neurons, agrees with previous reports. However, p75NTR-IR within some astrocytes, somata, and dendritic structures suggests that this receptor may also be involved in controlling local neurotrophin levels and possibly modulating the viability of local hippocampal cell populations.

Signal transduction of stress via ceramide

The sphingomyelin (SM) pathway is a ubiquitous, evolutionarily conserved signalling system analogous to conventional systems such as the cAMP and phosphoinositide pathways. Ceramide, which serves as second messenger in this pathway, is generated from SM by the action of a neutral or acidic SMase, or by de novo synthesis co-ordinated through the enzyme ceramide synthase. A number of direct targets for ceramide action have now been identified, including ceramide-activated protein kinase, ceramide-activated protein phosphatase and protein kinase Czeta, which couple the SM pathway to well defined intracellular signalling cascades. The SM pathway induces differentiation, proliferation or growth arrest, depending on the cell type. Very often, however, the outcome of signalling through this pathway is apoptosis. Mammalian systems respond to diverse stresses with ceramide generation, and recent studies show that yeast manifest a form of this response. Thus ceramide signalling is an older stress response system than the caspase/apoptotic death pathway, and hence these two pathways must have become linked later in evolution. Signalling of the stress response through ceramide appears to play a role in the development of human diseases, including ischaemia/reperfusion injury, insulin resistance and diabetes, atherogenesis, septic shock and ovarian failure. Further, ceramide signalling mediates the therapeutic effects of chemotherapy and radiation in some cells. An understanding of the mechanisms by which ceramide regulates physiological and pathological events in specific cells may provide new targets for pharmacological intervention.

Expression of the low affinity neurotrophin receptor, P75NGFR, in the rat forebrain, following unilateral bulbectomy

It has been hypothesized that the main olfactory bulb, with its relatively rich source of neurotrophins, may provide trophic support for neurons that project to the bulb. We monitored expression of the common, low affinity receptor for neurotrophins, p75NGFR, in the olfactory bulb and basal forebrain of unilaterally bulbectomized and sham-treated rats, 1-16 weeks post-surgery, using the monoclonal antibody MAb192. An induction of p75NGFR-immunoreactivity was observed in both the glomerular and olfactory nerve layers of the right, contralateral main olfactory bulb of lesioned animals. The naturally occurring regeneration taking place in the olfactory neuroepithelium is known to be altered by olfactory bulbectomy, with subsequent changes in the sensory input to the remaining bulb. These changes in expression of p75NGFR in the olfactory bulb support the hypothesis we have developed in previous papers, that changes in the extent of the peripheral input from the olfactory neuroepithelium to the main olfactory bulb regulate p75NGFR expression in both the glomerular and the olfactory nerve layers. Expression of p75NGFR in the basal forebrain of bulbectomized animals was found to be no different than sham-treated controls and does not support the hypothesis that the olfactory bulb provides trophic support to this region of the central nervous system.

Competitive signaling between TrkA and p75 nerve growth factor receptors determines cell survival

In addition to its role as a survival factor, nerve growth factor (NGF) has been implicated in initiating apoptosis in restricted cell types both during development and after terminal cell differentiation. NGF binds to the TrkA tyrosine kinase and the p75 neurotrophin receptor, a member of the tumor necrosis factor cytokine family. To understand the mechanisms underlying survival versus death decisions, the TrkA receptor was introduced into oligodendrocyte cell cultures that undergo apoptosis in a p75-dependent manner. Here we report that activation of the TrkA NGF receptor in oligodendrocytes negates cell death by the p75 receptor. TrkA-mediated rescue from apoptosis correlated with mitogen-activated protein kinase activation. Concurrently, activation of TrkA in oligodendrocytes resulted in suppression of c-jun kinase activity initiated by p75, whereas induction of NFkappaB activity by p75 was unaffected. These results indicate that TrkA-mediated rescue involves not only activation of survival signals but also simultaneous suppression of a death signal by p75. The selective interplay between tyrosine kinase and cytokine receptors provides a novel mechanism that achieves alternative cellular responses by merging signals from different ligand-receptor systems.

Immunohistochemical distribution of neurotrophins and their receptors in the rat retina and the effects of ischemia and reperfusion

1. Neurotrophins are molecules that regulate the survival, development and maintenance of specific functions in different populations of nerve cells. 2. In the present work, we studied the localization, at the cellular level, of the different neurotrophins and their receptors within the rat retina in control and after ischemia-reperfusion of the retina. We found variations in the localization of some of these molecules depending on the reperfusion time of the retina after the ischemic lesion. 3. Thus it is suggested that the changes in the distribution and concentration of neurotrophins and their receptors caused by ischemia are protective reactions related to neuronal damage and synaptic reorganization.

A role of the thymus and thymosin-alpha1 in brain NGF levels and NGF receptor expression

Using neonatal rats we investigated the role of the thymus and thymosin-alpha1 (T-alpha1) in brain NGF levels, NGF receptor (p75NGFr) expression, as well as the activity of choline acetyl-transferase, a cholinergic enzyme regulated by NGF. It is shown that early postnatal thymectomy causes a decrease in NGF in the hippocampus and cortex and p75NGFr distribution in the basal forebrain cholinergic neurons (FBCN). Intracerebral T-alpha1 injection in thymectomized animals induces a recovery, albeit not complete, of both NGF and p75NGFr. These findings indicate that thymectomy affects both the brain NGF producing and responding cells and that T-alpha1 may be one of the thymic hormones involved in the regulation of cerebral NGF synthesis.

Nerve growth factor signal transduction in mature pig oligodendrocytes

It has previously been shown that nerve growth factor (NGF) is of functional significance for mature pig oligodendrocytes (OLs) in culture. The present data give evidence for the expression of TrkA, the so-called high-affinity NGF receptor, and of p75NTR, the so-called low-affinity NGF receptor. TrkA is upregulated during culturing, in contrast to the p75 receptor. Exposure of OLs to NGF induces an autophosphorylation of TrkA via its intrinsic tyrosine kinase. K-252a inhibits the TrkA autophosphorylation, which reduces the OL process formation to control levels. To the tyrosine-phosphorylated sites of TrkA several proteins, such as phospholipase C-gamma1, the adaptor protein SHC, the phosphotyrosine phosphatase SH-PTP2 (SYP) associate via their SH2 phosphotase SH-PTP2 domain. The association of SHC to TrkA is shown by co-immunoprecipitation. Indirect evidence for a possible activation of PLC-gamma1 is given by an NGF-induced increase of oligodendroglial [Ca2+]i. Downstream from TrkA, a mitogen-activated protein kinase cascade, which includes Erk1 and Erk2, is operating. An in-gel myelin basic protein kinase assay revealed that NGF activates predominantly Erk1. Finally, it is shown that NGF stimulates expression of c-fos.

Expression of full-length trkB receptors by reactive astrocytes after chronic CNS injury

Growth factors, including members of the neurotrophin family, are expressed by neuronal and glial elements following injury to the CNS. In order to assess the capacity for glial cells to respond to neurotrophins at sites of chronic injury, full-length trkB receptors were localized following implantation of a nitrocellulose filter into the cerebral cortex for 30 days. Northern analysis demonstrated that filter implants contained cells expressing transcripts for full-length and truncated trkB receptors, in contrast to the predominant expression of truncated trkB receptors by cultured astrocytes. In situ hybridization and immunohistochemistry using probes to the trkB kinase domain colocalized full-length receptors with GFAP-immunopositive reactive astrocytes adjacent to and within the filter implant. In contrast, OX-42-immunopositive microglia/macrophages were not stained for full-length trkB. These data indicate that reactive astrocytes can express functional trkB receptors following a chronic insult to the cerebral cortex and support the hypothesis that neurotrophins may regulate astrocytic responses to injury.

Mouse astrocytes store and deliver brain-derived neurotrophic factor using the non-catalytic gp95trkB receptor

A study of the brain-derived neurotrophic factor (BDNF)-binding capacity of pure astrocytes demonstrated that these cells bind and endocytose [125I]BDNF rapidly using the gp95trkB truncated receptor. A linear Scatchard plot indicated the presence of only one type of receptor that bound the ligand, with a low Kd of 1.24 x 10(-8) M. There were an average of 36,468 copies of this receptor on untreated astrocytes. Interestingly, the neurotrophin was not degraded intracellularly, as demonstrated by HPLC experiments. Furthermore, the stored molecule was released by a mechanism regulated by the extracellular BDNF concentration as a bioactive neurotrophic molecule that supports neuron survival, in a time- and temperature-dependent manner. The data demonstrate that astrocytes exert an active role in the bioavailability of this neurotrophin, which is further enhanced in an inflammatory-like situation induced experimentally in culture using interferon-gamma.

Nerve growth factor and neurotrophin-3 differentially regulate the proliferation and survival of developing rat brain oligodendrocytes

There is increasing evidence that the neurotrophins, particularly nerve growth factor (NGF) and neurotrophin-3 (NT-3), play a role in the regulation of glial development in the CNS. Recent studies have shown that the proliferation of optic nerve-derived O2A progenitors (OLPs) is potentiated by NT-3 in combination with platelet-derived growth factor, whereas NT-3 alone supports the survival of their differentiated progeny (Barres et al., 1994). In this study, we have examined the expression of the high-affinity neurotrophin receptors (trks) and the low-affinity nerve growth factor receptor p75 in developing oligodendrocytes (OLs). In addition, we have examined the effects of NGF and NT-3 on proliferation and survival of OLPs and OLs, respectively. TrkC, the high-affinity NT-3 receptor, and trkA, the high-affinity NGF receptor, are both expressed from the early OLP through the mature OL stage. The truncated form of trkB, lacking the tyrosine kinase domain, and the low-affinity neurotrophin receptor p75 are expressed at low levels in OLPs and are upregulated in mature OLs. NGF and NT-3 both induced the phosphorylation of mitogen-activated protein kinase (MAPK) in OLPs and in OLs. In both OLPs and OLs, NT-3 sustained the activation of MAPK more than NGF. NT-3 enhanced the proliferation of OLPs and supported the survival of OLs. By contrast, unless coadministered with FGF-2, NGF did not exhibit mitogenic effects on OLPs but did enhance the survival of differentiated OLs. Our data demonstrate the presence of functional trkA and trkC in developing OLs and indicate that both NGF and NT-3 have a broad spectrum of developmental actions on cells of the OL lineage.

Death of oligodendrocytes mediated by the interaction of nerve growth factor with its receptor p75

Members of the nerve growth factor (NGF) family promote the survival of neurons during development. NGF specifically activates the receptor trkA, initiating a signal transduction cascade which ultimately blocks cell death. Here we show that NGF can have the opposite effect, inducing the death of mature oligodendrocytes cultured from postnatal rat cerebral cortex. This effect was highly specific, because NGF had no effect on oligodendrocyte precursors and astrocytes. Other neurotrophins such as brain-derived neurotrophin factor (BDNF) and neurotrophin-3 (NT-3) did not induce cell death. NGF binding to mature oligodendrocytes expressing the p75 neurotrophin receptor, but not trkA, resulted in a sustained increase of intracellular ceramide and c-Jun amino-terminal kinase (JNK) activity, which are thought to participate in a signal transduction pathway leading to cell death. Taken together, these results indicate that NGF has the ability to promote cell death in specific cell types through a ligand-dependent signalling mechanism involving the p75 neurotrophin receptor.

Analysis of low affinity neurotrophin receptor (p75) expression in glia of the CNS-PNS transition zone following dorsal root transection

Peripheral nerves exit from the brain through the transition zone where oligodendroyctes and astrocytes of the central nervous system (CNS) and Schwann cells of the peripheral nervous system (PNS) are in close proximity. In this zone, the same axons are ensheathed by oligo-dendrocytes and Schwann cells. We examined, in adult rats, the expression of the low affinity neurotrophin receptor (p75) in central glia and Schwann cells in response to lesion of lumbar dorsal roots. In normal rats, scattered p75-immunoreactive glial cells were present in the CNS-PNS transition zone. A marked increase of p75 immunoreactivity occurred in Schwann cells near the transition zone from 4 days to at least 3 weeks after dorsal root transection. In contrast, the p75 immunoreactivity remained unchanged in central glia. The differential expression of p75 in the two types of glial cells was sharply demarcated at the CNS-PNS border. Our results are consistent with earlier observations that axon damage is less potent in its ability to induce central glial expression of p75, and further, suggests a possible mechanism for the failure of regenerating dorsal root axons growing into the spinal cord.

Neurotrophin activates signal transduction in oligodendroglial cells: expression of functional TrkC receptor isoforms

The role of the NT-3 has been implicated in the survival of progenitor oligodendrocytes in culture. The object of this study was to investigate the expression of the TrkC receptor and its responsiveness in glial cells. We report the expression of two TrkC receptor isoforms in rat primary oligodendrocyte cultures, a glial progenitor cell line, CG-4, and in C6 glioma cells. The reverse transcription-polymerase chain reaction-aided amplification of glial trkC with specific primers from the kinase domain, followed by its cloning and sequencing, shows the presence of two trkC transcripts. The sequence of one of the transcripts is homologous to a previously identified trkC isoform which encodes a functional receptor. The other transcript contains a 42-bp insert in the kinase domain. A Western blot of CG-4 and C6 probed with antibody to a TrkC revealed the presence of gp145-kDa protein band. The investigations revealed a rapid autophosphorylation of gp145TrkC in CG-4 and C6 cells in the presence of its specific ligand, NT-3. Furthermore, K252a, a neurotrophin-specific inhibitor, abolishes the NT-3-mediated receptor autophosphorylation. We also examined other NT-3-dependent phosphorylation of cellular substrates in oligodendroglial cells. Interestingly, we observed phosphorylation of phospholipase C gamma-1 in CG-4 and C6 cells, and phosphorylation of phosphatidylinositol 3-kinase in C6 cells in the presence of NT-3. Both the NT-mediated phosphorylation of phospholipase C gamma-1 and phosphorylation of phosphatidylinositol 3-kinase are blocked in the presence of K252a. The detection of the NT-3-mediated early signal transduction events demonstrates that TrkC receptor exhibits NT-3-mediated intracellular response in oligodendroglial cells.

Interleukin-6 released in human cerebrospinal fluid following traumatic brain injury may trigger nerve growth factor production in astrocytes

Cytokines are involved in nerve regeneration by modulating the synthesis of neurotrophic factors. The role played by interleukin-6 (IL-6) in promoting nerve growth factor (NGF) after brain injury was investigated by monitoring the release of IL-6 and NGF in ventricular cerebrospinal fluid (CSF) of 22 patients with severe traumatic brain injuries. IL-6 was found in the CSF of all individuals and remained elevated for the whole study period. NGF appeared in the CSF if IL-6 levels reached high concentrations and was often detected simultaneously with or following an IL-6 peak. The amounts of NGF correlated with the severity of the injury, as indicated by the clinical outcome of the patients. The functional relationship of IL-6 and NGF was investigated utilizing cultured mouse astrocytes. The CSF of 8 patients containing IL-6 induced NGF production in astrocytes, whereas control CSF without IL-6 had no effect. The induction of NGF was inhibited up to 100% by adding anti-IL-6 antibodies. These results were corroborated when astrocytes were exposed to recombinant IL-6 at different concentrations resulting in NGF production. Thus, the production of IL-6 within the injured brain may likely contribute to the release of neurotrophic factors by astrocytes.

Ciliary neurotrophic factor selectively protects human oligodendrocytes from tumor necrosis factor-mediated injury

Oligodendrocytes (OLs) and their myelin membranes are the apparent injury targets in the putative human autoimmune disease multiple sclerosis. The basis for this selective injury remains to be defined. OLs in vitro have been shown to be susceptible to both tumor necrosis factor (TNF) and non-TNF-dependent immune effector mechanisms. The former involves initial nuclear injury (apoptosis); the latter, when mediated by activated T cells, involves initial cell membrane injury (lysis). In the current study, we determined whether human adult CNS-derived OLs could be protected from the above immune effector mechanisms by selected neurotrophic factors (CNTF, BDNF, NGF, NT-3, and NT-4/5) or cytokines demonstrated to protect from human or experimental autoimmune demyelinating diseases (beta-interferon [IFN], IL-10, and TGF-beta). Nuclear injury was assessed in terms of DNA fragmentation using a DNA nick-end-labelling technique; cell membrane injury was assessed by lactate dehydrogenase or chromium 51 release. MTT and cell counting assays were used to assess cell viability and cell loss, respectively. Amongst the neurotrophic factors and cytokines tested, only CNTF significantly protected the OLs from TNF-mediated injury. CNTF also protected the OLs from serum deprivation-induced apoptosis. CNTF, however, did not protect the OLs from injury induced by activated CD4+ T cells. CNTF also did not protect human fetal cortical neurons from serum deprivation or TNF-induced DNA fragmentation, nor did it protect the U251 human glioma cell line from DNA fragmentation induced by a combination of TNF and reduced serum concentration in the culture media. Our results indicate that potential protective effects of neurotrophic factors or cytokines on neural cell populations can be selective both for cell type involved and mechanism of immune-mediated injury. CNTF is the protective factor selective for nuclear-directed injury of OLs.

Neurotrophins and their trk receptors in cultured cells of the glial lineage and in white matter of the central nervous system

Previous studies have analyzed the expression of different members of the neurotrophin family and their trk receptors in glial cultures composed mainly or exclusively of type-1 astrocytes, whereas only partial data have been published on other cultured glial types. In this article we compare the mRNA levels for neurotrophins (NGF, BDNF, NT-3, NT-4) and their high-affinity receptors (trkA, trkB, trkC) in cultures enriched in specific glial types, such as microglia, type-1 astroglia, and cells of the O/2A lineage (type-2 astroglia and oligodendroglia). Relatively high levels of NGF mRNA (comparable to those observed in adult rat cerebral cortex) are present in all types of cultured glial cells, except for a low level of expression in cultures enriched in microglial cells. In contrast, BDNF mRNA is undetectable in all cultures examined. NT-3 and NT-4 mRNA molecules, at a level equal to that observed in adult rat cerebral cortex, are easily detected in type-1 astrocyte cultures, whereas their hybridization signals are undetectable in cells of the O/2A lineage and in microglial cultures. The analysis of neurotrophin receptor mRNAs confirms the absence of trkA mRNA, the presence of relatively high levels of trkB mRNA (70-100% of cerebral cortex values), and low levels of trkC mRNA (10-18% of cerebral cortex values) in both cultured astroglial and oligodendroglial cells. Only very low levels of trkB and trkC mRNAs are observed in microglial cultures. Although cultured glial cells express mainly mRNAs encoding for the truncated form of trkB and trkC, a low level of mRNA encoding for the full-length catalytic form of these receptors is detected by the sensitive ribonuclease protection assay.

Selective regulation of TrkA and TrkB receptors by retinoic acid and interferon-gamma in human neuroblastoma cell lines

Trk receptors are a family of genes implicated in the survival, differentiation, and growth of certain neurons and tumors of the nervous system. A better understanding of the regulation of Trk receptors is relevant for developmental and oncological studies. Human neuroblastoma (NB) cell lines constitutively express low levels of TrkA mRNA, while TrkB mRNA is not readily detectable. Differentiation of NB cells is accompanied by a differential modulation of Trk expression in human NB cells. Nanomolar concentrations of RA induce a stable increase of TrkB mRNA. A transient induction of TrkA mRNA levels requires micromolar concentrations of RA. Induction of both TrkA and TrkB mRNA does not require new protein synthesis. However, RA-induced TrkB mRNA expression is transcriptionally regulated, while the transient RA-induced increase of TrkA mRNA is a consequence of extended mRNA stability. Interferon gamma (IFN-gamma) selectively increases TrkA mRNA without affecting TrkB mRNA levels. Similar to RA, IFN-gamma does not modify the transcriptional rate of TrkA mRNA, but rather increases TrkA mRNA stability. Thus, RA and IFN-gamma differentially regulate TrkA or TrkB expression in the same cell type by predominantly transcriptional (TrkB) or post transcriptional (TrkA) mechanisms. Such experiments indicate the complexity of Trk mRNA regulation and also indicate compounds that may affect neurotrophin responsiveness in developing neural cells.

Neurotoxic injury in rat hippocampus differentially affects multiple trkB and trkC transcripts

In the present work we determined, by Northern blotting, ribonuclease assay and in situ hybridization, the level of multiple trkB and trkC transcripts at different times after ibotenic acid-induced neuronal injury in the rat hippocampus. All the transcripts (7.0-7.5, 2.4 and 1.8 kb) encoding the truncated TrkB receptor are coordinately up-regulated following neurotoxic injury, with a time-course similar to that observed for the glial fibrillary acidic protein mRNA, a molecular marker of reactive astrocytes. The highest level of induction was observed for the 2.4 kb mRNA level. The 1.8 kb mRNA, whose relative level is higher in astroglial cultures compared to normal brain tissue, is detectable only in the gliotic hippocampus. The 9 kb trkB mRNA, which encodes the full-length TrkB receptor, rapidly decreases with a time-course similar to that previously observed for other neuronal markers. In situ hybridization studies show that the increased mRNA level per cell is a major determinant in the up-regulation of truncated trkB expression. A decrease of truncated and full-length trkC mRNA was observed in the neuron-depleted astroglia-enriched hippocampus, suggesting that this mRNA is mainly localized in the neuronal layers and that no induction of its expression occurs in reactive astrocytes.

192IgG-saporin immunotoxin-induced loss of cholinergic cells differentially activates microglia in rat basal forebrain nuclei

To characterize the specificity of a novel cholinergic immunotoxin (conjugate of the monoclonal antibody 192IgG against the low-affinity nerve growth factor receptor with the cytotoxic protein saporin), coronal sections through the basal forebrain of adult rats, that received a single intracerebro-ventricular injection of 4 micrograms of 192IgG-saporin conjugate, were subjected to histochemical and immunocytochemical procedures to evaluate cholinergic (choline acetyltransferase (ChAT)-immunoreactive, acetylcholinesterase-positive, NADPH-diaphorase-positive) and GABAergic structures (parvalbumin-immunoreactive, labeling of perineuronal nets with Wisteria floribunda agglutinin) as well as microglia (visualized with Griffonia simplicifolia agglutinin) and astrocytes (immunostaining for glial fibrillary acidic protein). Seven days following injection of the immunotoxin, ChAT-immunoreactive cells nearly completely disappeared throughout the magnocellular basal forebrain complex, including globus pallidus, as compared to vehicle-injected controls. However, there was no significant difference in the number of ChAT-positive cells in the adjacent ventral pallidum and in the caudate-putamen of immunolesioned and control animals. NADPH-diaphorase-containing cells, including a significant subpopulation of cholinergic cells, also strikingly decreased in number by more than 90% in the magnocellular basal forebrain complex following immunolesion, and only a few noncholinergic diaphorase-positive cells survived in the medial septum, vertical and horizontal diagonal band, and nucleus basalis of Meynert. In contrast, the number of parvalbumin-containing GABAergic projection neurons in the septum-diagonal band of Broca complex and nucleus basalis of Meynert from immunolesioned rats was not different from that of vehicle-injected control animals. Immunolesioning also did not result in any change in either number or shape of cells surrounded by perineuronal nets, which are frequently associated with parvalbumin-containing GABAergic neurons. Seven days following injection of the immunotoxin, a very strong activation of microglia with an identical distribution pattern was observed in all experimental animals. Large numbers of activated microglia were found in all magnocellular basal forebrain nuclei, corresponding to the distribution of degenerating cholinergic cells. Additionally, immunolesioning also resulted in a dramatic activation of microglia in the lateral septal nuclei, which are known to be almost free of cholinergic cells, but not of penetrating cholinergic dendrites in adjacent zones, and in the ventral pallidum, where there was no observed loss of cholinergic cells. There was no significant increase in microglia activation in striatum and cortical areas, and no astrocytic response in any of the basal forebrain nuclei at this particular time point of survival. These results suggest that 192IgG-saporin specifically destroys basal forebrain cholinergic neurons and does not suppress their neuronal activity.(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro glial responses to nerve growth factor

Nerve growth factor (NGF) stimulates expression of the low affinity neurotrophin receptor p75NGFR mRNA in primary cultures of neonatal rat cortical type I astrocytes. Nerve growth factor treatment altered glial morphology in glial fibrillary acidic protein positive (GFAP+) cell cultures derived from newborn (P0) and 3-day-old (P3) rat pups. When P0- or P3-derived primary glial cultures were serum-deprived, in the presence of 200 pM NGF for 5 days, the flat polygonal glia present in culture assumed a fibrous morphology, an effect not seen in the untreated serum-deprived controls. The NGF effect on astrocytic morphology was blocked by continuous serum treatment. Nerve growth factor did not stimulate astrocytic proliferation under these culture conditions, as assayed by cell cycle analysis using 3H thymidine autoradiography. P0-derived primary glial cultures expressed the signal transducing neurotrophin receptors p145trkB and p140trkA as determined by reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR products were identified by sequencing or restriction enzyme analysis. Astrocytes internalized 125I-NGF at 37 degrees C but not at 4 degrees C, consistent with energy requirements for internalization. Also, internalization of 125I-NGF was abolished by the addition of a 300-1,000-fold excess of unlabeled NGF. Thus, astroglial cells in culture internalize NGF through a specific receptor-mediated process, express trkA and full-length trkB mRNAs at low levels, and respond to exogenous NGF by expressing a fibrous morphology under serum-free culture conditions.

Developmental regulation of neurotrophin-3 and trk C splice variants in optic nerve glia in vivo

Extensive evidence indicates that the survival and development of neurons is dependent on neurotrophins. However, the factors potentially required for glial development and function, and the sites of synthesis, are not well defined. To investigate the potential role of neurotrophins in glial development in vivo, we studied the trk family of receptors and their cognate neurotrophins in the postnatal rat optic nerve using reverse transcription-polymerase chain reaction. Our results indicate that trk A, B and C messenger RNAs are expressed throughout development, and in adulthood. Both trk B and trk C expression decreased during development. However, trk C expression decreased most markedly, reaching barely detectable levels by day 90. These findings suggest that neurotrophins can affect both immature and mature glial function and that their actions may be regulated through the modulation of putative receptors. To determine whether alternatively spliced forms of trk C potentially mediate neurotrophin-3 actions, we assessed expression of the different trk C isoforms. We employed reverse transcription-polymerase chain reaction using primers that selectively amplify the extracellular or intracellular domains. Optic nerve expressed both the full-length receptor and one form containing an insertion in the tyrosine kinase domain. In addition, the expression of the insert splice variant was developmentally regulated. Our observations suggest that, in glia, actions of neurotrophin-3 are probably mediated through the full-length receptor and that selected alternatively spliced forms may also be involved in trk C receptor function. To determine whether glia at different stages of differentiation elaborate neurotrophins, we analysed expression of nerve growth factor, brain-derived neurotrophic factor, neurotrophins-3 and -4/5 in the optic nerve during development. Messenger RNAs for all the neurotrophins were detected at all postnatal ages, suggesting that progenitor cells, immature and mature glia are potential sources of neurotrophins. However, neurotrophin expression was not developmentally regulated. The invariant neurotrophin messenger RNA levels, and the changing expression of trk B and trk C during ontogeny, suggest that trophic regulation of glial development is primarily governed through modulation of receptor expression.

Changes in expression of the low affinity receptor for neurotrophins, p75NGFR, in the regenerating olfactory system

We have disrupted the integrity of the rat olfactory neuroepithelium using intranasally applied TX-100, a procedure known to reversibly eliminate the sensory neuron input from the neuroeithelium to the olfactory bulb [Margolis et al. (1974) Denervation in the primary olfactory pathway of mice: biochemical and morphological effects. Brain Res. 81, 469-483]. One week after TX-100 exposure, we observed a disruption of the pseudo-stratified organization of the neuroepithelium which was accompanied by a 60% reduction in neuroepithelial width, compared to saline-treated controls. Full recovery of the neuroepithelium was not observed until 16 weeks post-lesion. During this post-lesion period, we monitored the expression of the low affinity receptor for neurotrophins, p75NGFR, in the olfactory bulb of saline- and TX-100-treated animals, using the monoclonal antibody, MAb192. In saline-treated animals, p75NGFR-immunoreactivity (p75NGFR-ir) was localized to individual glomeruli in the olfactory bulb, with little or undetectable p75NGFR-ir in the olfactory nerve layer. We have previously reported that pre-lesioned levels of p75NGFR-ir in the glomerular layer were dramatically reduced while an induction of p75NGFR-ir was observed in the olfactory nerve layer, one and two weeks after intranasal exposure to TX-100 [Turner & Perez-Polo (1992) Regulation of the low affinity receptor for nerve growth factor, p75NGFR, in the olfactory system of neonatal and adult rat. Int. J. Devl Neurosci. 10, 343-359]. In this paper, we demonstrate that this previously reported reduction in glomerular p75NGFR-ir took 16 weeks to fully recover and was, thus, coincident with the post-lesion recovery of the neuroepithelium. In the olfactory nerve layer, the return of p75NGFR-ir to pre-lesioned levels took only four weeks. No changes in neuroepithelial width and integrity or alterations in p75NGFR-ir in the olfactory bulb were observed in saline-treated animals. Thus, the TX-100-induced removal of the peripheral input to the olfactory bulb resulted in a reversible change in expression of p75NGFR-ir in the bulb. We believe that these changes are a reflection of the regenerative capacity of the olfactory system.

Nerve growth factor released by transgenic astrocytes enhances the function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease

Previous studies have demonstrated that astrocytes genetically modified to express recombinant nerve growth factor (NGF) support the survival and neuronal transdifferentiation of intrastriatal adrenal chromaffin cell grafts at 2 weeks post-transplantation [15]. The present study was performed to determine whether these effects would be maintained at longer times post-transplantation and, if so, whether the co-grafts would reduce rotational behavior in the unilateral 6-hydroxydopamine-lesioned rat. In the present study, we have demonstrated that primary type I rat astrocytes infected with a replication-defective retrovirus conferring expression of a mouse beta-NGF cDNA sequence secrete NGF at a rate that is approximately 40-fold higher than that of controls (i.e., 8.0 vs. 0.2 pg NGF/h/10(5) cells, respectively). The genetically modified astrocytes were also found to express recombinant NGF following intrastriatal transplantation, as indicated by a 23% increase in striatal NGF content compared with controls, measured at 4 weeks post-transplantation. When NGF-producing astrocytes and adrenal chromaffin cells were co-grafted into the dopamine-denervated striatum of the unilateral 6-hydroxydopamine-lesioned rat, the chromaffin cells displayed extensive neurite outgrowth and a 5-12-fold increase in survival compared to controls at 10 weeks post-grafting. These effects were paralleled by a 60% reduction of apomorphine-induced rotational behavior, suggesting a partial normalization of striatal function. These results suggest that genetically modified astrocytes promote the prolonged survival and function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease.

Evidence for an NGF-induced autocrine neurotrophic potential of glial cells in nervous system development

NGF receptor bearing peripheral glial cells, purified from chicken embryo dorsal root ganglia, were found to secrete neurite growth promoting activity (NGPA) but no NGF (detection limit 1 pM). If, however, minute concentrations of NGF (2 pM) were added to the glial cultures, an autocrine response was observed. Levels of NGF-like activity in the medium rose up to 50-fold. Induction of this autocrine response occured within a narrow range of NGF concentrations. NGPA production was not affected by addition of NGF. Coculture with neurons influenced the neurotrophic factor production. The data suggest that glial cells can "sense" traces of NGF and regulate its availability during neural development.