The effects of nerve growth factor on the development of septal cholinergic neurons in reaggregate cell cultures

Microfluidic engineered high cell density three-dimensional neural cultures

Three-dimensional (3D) neural cultures with cells distributed throughout a thick, bioactive protein scaffold may better represent neurobiological phenomena than planar correlates lacking matrix support. Neural cells in vivo interact within a complex, multicellular environment with tightly coupled 3D cell-cell/cell-matrix interactions; however, thick 3D neural cultures at cell densities approaching that of brain rapidly decay, presumably due to diffusion limited interstitial mass transport. To address this issue, we have developed a novel perfusion platform that utilizes forced intercellular convection to enhance mass transport. First, we demonstrated that in thick (>500 microm) 3D neural cultures supported by passive diffusion, cell densities <or=5.0 x 10(3) cells mm(-3) were required for survival. In 3D neuronal and neuronal-astrocytic co-cultures with increased cell density (10(4) cells mm(-3)), continuous medium perfusion at 2.0-11.0 microL min(-1) improved viability compared to non-perfused cultures (p < 0.01), which exhibited widespread cell death and matrix degradation. In perfused cultures, survival was dependent on proximity to the perfusion source at 2.00-6.25 microL min(-1) (p < 0.05); however, at perfusion rates of 10.0-11.0 microL min(-1) survival did not depend on the distance from the perfusion source, and resulted in a preservation of cell density with >90% viability in both neuronal cultures and neuronal-astrocytic co-cultures. This work demonstrates the utility of forced interstitial convection in improving the survival of high cell density 3D engineered neural constructs and may aid in the development of novel tissue-engineered systems reconstituting 3D cell-cell/cell-matrix interactions.

Distinctive morphological features of a subset of cortical neurons grown in the presence of basal forebrain neurons in vitro

Basal forebrain cholinergic neurons (BFCNs) provide the major subcortical source of cholinergic input to cerebral cortex and play an important role in regulating cortical activity. The present study examined the ability of BFCNs to influence neocortical neuronal growth by examining effects of the presence of BFCNs on certain cortical neurons grown under the controlled conditions of dissociated cell culture. Initial experiments demonstrated distinctive morphological features of a population of neurons (labeled with SMI-32, a monoclonal antibody to nonphosphorylated neurofilament proteins that labels pyramidal neurons in vivo) in cocultures containing basal forebrain (BF) and cortical cells. These neurons (large neurons immunoreactive for SMI-32 [SMI-32(+) neurons]) were characterized as having extensive axons, greater soma size, and more dendritic growth than did most SMI-32(+) neurons in the cultures. Staining for SMI-32 in cocultures in which the cortical neurons were labeled with a fluorescent marker before adding the BF cells indicated that virtually all large SMI-32(+) neurons were of cortical origin. Eliminating BFCNs with the selective cholinergic immunotoxin 192 IgG-saporin resulted in a >80% decrease in the number of large SMI-32(+) neurons, although causing little damage to other cells in the treated cultures; this suggests that survival or maintenance of large SMI-32(+) neurons may depend on ongoing trophic support from BFCNs. Thus, present findings suggest that BFCNs may provide powerful growth- and/or survival-enhancing signals to a subset of cortical neurons.

Decreased choline acetyltransferase activity in nerve growth factor-transgenic mice during brain development

Activity of the synthetic enzyme for acetylcholine, choline acetyltransferase was investigated during development and in adult nerve growth factor-transgenic mice. A conspicuous reduction of choline acetyltransferase activity was observed in the anterior brain of nerve growth factor-transgenic embryos from embryonic days 13 to 16 (E13 to E16). Choline acetyltransferase activity levels subsequently resumed to normal levels, with the exception of a 15% increase in the adult hippocampus. Nerve growth factor contents followed a similar time-course and regional distribution in normal and nerve growth factor-transgenic animals and displayed significantly higher values from E14 to the early postnatal period. Nerve growth factor contents were normal in the adult brain. In vitro experiments confirmed the involvement of nerve growth factor in the decrease of choline acetyltransferase activity levels observed in transgenic neurons during development. These results suggest a role for nerve growth factor in the initial phase of the phenotypic differentiation of cholinergic neurons. They show that nerve growth factor may, under specific development conditions, lead to a paradoxical down-regulation of choline acetyltransferase activity.

Enhancement of choline acetyltransferase activity in coculture of rat septal and hippocampal neurons

We report that choline acetyltransferase (ChAT) activity and neuronal survival were enhanced in rat septal neurons cocultured with hippocampal neurons. The enhancement of ChAT activity also occurred as a result of the addition of hippocampal conditioned medium (HpCM). When septal neurons from embryonic day 17 (E17) rats were cocultured with hippocampal neurons, ChAT activity was increased 2-fold compared with homogeneous culture of septal neurons. By contrast, no increase in ChAT activity was observed in coculture of septal and neocortical neurons. Treatment with HpCM obtained from cultured E19 rat hippocampal neurons enhanced the ChAT activity of E17 rat septal neurons. The enhancement of ChAT activity caused by coculture with hippocampal neurons and that caused by the addition of HpCM were not blocked by the addition of anti-nerve growth factor (NGF) antibody, suggesting that NGF, which is known to increase the ChAT activity of septal neurons both in vivo and in vitro, did not participate in the increase of ChAT activity. These findings indicate that possible target-derived neurotrophic factor(s), other than NGF, from hippocampal neurons enhance(s) the ChAT activity of septal neurons.

The neurotrophins and their receptors: structure, function, and neuropathology

The neurotrophins are a family of polypeptides that promote differentiation and survival of select peripheral and central neurons. Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, neurotrophin-4, and neurotrophin-5 are included in this group. In recent years, tremendous advances have been made in the study of these factors. This has stimulated our review of the field, characterizing the neurotrophins from initial isolation to molecular analysis. The review also discusses their synthesis, localization, and responsive tissues, in both the periphery and CNS. The complex receptor interactions of the neurotrophins are also analyzed, as are putative signal transduction mechanisms. Discussion of the observed and postulated involvement in neuropathological disorders leads to the conclusion that the neurotrophins are involved in the function and dysfunction of the nervous system.

Early nerve growth factor-induced events in developing rat septal neurons

A culture system enriched for nerve growth factor (NGF) receptor bearing cells was developed to investigate signal transduction events activated by NGF in postmitotic central nervous system neurons. Cells from the septal region of embryonic rats at 16 days of gestation were grown on glass coverslips above a glial cell layer established from postnatal rat cortex. The separation of glial and neuronal planes in this "bilaminar" system permits the diffusion of glial-derived factors required by septal neurons for survival yet allows the investigation of NGF responses in a pure neuronal population. Approximately 15% of the neurons in this culture system were immunoreactive for the low affinity NGF receptor. NGF rapidly increased MAP kinase activity (2-5 min) and transiently induced expression of c-fos in septal neurons. NGF treatment also increased choline acetyltransferase activity, while the number of cholinergic neurons remained constant. Septal neuron survival depended on the presence of glial cells, but neuronal viability in the bilaminar system was unaffected by anti-NGF antiserum, indicating that glial-derived neurotrophic support is not mediated by NGF alone. These data suggest that the bilaminar culture system is a useful system for the study of early events in NGF-activated signal transduction and the nature of glial-derived trophic support of developing basal forebrain neurons.

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

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

Expression of neurotrophins and the low-affinity NGF receptor in septal and hippocampal reaggregate cultures: local physiologic effects of NGF synthesized in the septal region

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are members of a family of trophic factors designated the neurotrophins, each of which can bind to the low-affinity NGF receptor (LNGFR). To investigate the mechanisms that regulate the expression of the neurotrophins and the LNGFR in the developing brain, we grew cells from the embryonic mouse septum and hippocampus in reaggregating cell culture and compared neurotrophin and LNGFR expression in developing reaggregates with that seen in the developing septum and hippocampus in situ. NGF, BDNF, NT-3 and LNGFR were each expressed in septal and hippocampal reaggregates as well as the native septum and hippocampus. Additionally, the temporal expression profiles observed in reaggregates were generally similar to those seen in the respective brain regions in situ. In order to determine whether NGF can modulate neurotrophin or LNGFR expression, reaggregates were cultured in the continual presence of either exogenous NGF or anti-NGF antibodies. NGF-treated septal cultures expressed twice the level of LNGFR mRNA as was seen in untreated septal cultures; on the other hand, septal cultures grown in the presence of anti-NGF antibodies, to neutralize endogenously synthesized NGF, displayed a 3-fold decrease in LNGFR mRNA expression compared to untreated cultures. No effects of NGF or anti-NGF were observed on LNGFR expression in hippocampal reaggregates, or on neurotrophin mRNA expression in either reaggregate type. These results suggest that regulatory mechanisms intrinsic to the septal and hippocampal regions control neurotrophin and LNGFR expression. NGF is likely to be one of these regulatory cues since it acts locally in septal reaggregates to control the developmental expression of LNGFR mRNA. The possible roles of locally synthesized NGF and other neurotrophins in the development of septal neurons are discussed.

Specific modulation of dopamine expression in neuronal hybrid cells by primary cells from different brain regions

MN9D is an immortalized dopamine-containing neuronal hybrid cell line. When MN9D cells were coaggregated with primary embryonic cells of optic tectum, a brain region that does not receive a dopaminergic innervation, there was a marked reduction in their dopamine content, tyrosine hydroxylase immunoreactivity, and tyrosine hydroxylase mRNA. Similar reductions in dopamine content were produced by coaggregation with cells from embryonic thalamus, another brain region devoid of dopaminergic innervation. Coaggregation of MN9D cells with dopaminoceptive cells from the corpus striatum or the cortex did not have a demonstrable stimulatory effect on the dopamine content of MN9D cells. The decrease in MN9D dopamine content produced by optic tectum cells was not reversed by addition of corpus striatum cells. Thus, the MN9D hybrid cells are able to respond to an inhibitory factor(s) from cells derived from brain areas that are not targets for dopaminergic neurons. Catecholamine-producing PC12 cells did not respond in a similar manner, suggesting that the response of MN9D cells is a function of their mesencephalic origin. Given the selective response of MN9D cells to different brain cell populations, this hybrid cell line should facilitate investigations of cell-cell interactions in the central nervous system that may be involved in the expression of neurotransmitter phenotype and establishment of specific neuronal connections.

Coexistence of cholinergic, catecholaminergic, serotonergic, and glutamatergic neurotransmitter markers in mouse clonal hybrid neurons derived from the septal region

Two clonal immortalized neurons designated SN6.1b and SN6.2a were isolated by limiting dilution from a mouse embryonic septal cholinergic neuronal hybrid cell line SN6 (Hammond et al., 1986). In the serum-containing medium without extra differentiating agents, one-third of SN6.1b cells stably exhibited a morphology of differentiated neurons with extensive elaborate neurites, while a majority of SN6.2a cells, along with the parent cell line SN6, were round in shape with poorly branched short processes. Neurochemical studies showed that both clones synthesized choline acetyltransferase (ChAT), dopamine, norepinephrine, serotonin, and glutamate. Immunocytochemically, they expressed a number of neuronal antigens, such as 200-kDa neurofilament protein, neuron-specific enolase, microtubule-associated protein 2, tau protein, tubulin, neural cell adhesion molecule, Thy-1.2, saxitoxin-binding sodium channel protein, ChAT, tyrosine hydroxylase, serotonin, and glutamate. The coexistence of cholinergic, catecholaminergic, serotonergic, and glutamatergic neurotransmitter markers in the clonal hybrid septal neurons that express a variety of immunocytochemical properties of differentiated neurons suggests that embryonic septal cholinergic neurons are potentially multiphenotypic with respect to neurotransmitter synthesis.

Stability of septohippocampal neurons following excitotoxic lesions of the rat hippocampus

The present study examined the effects of removing hippocampal nerve growth factor (NGF)-producing neurons upon cholinergic and noncholinergic septohippocampal projecting neurons. To deplete septal/diagonal band neurons of their intrinsic source of NGF, rats received unilateral intrahippocampal injections of ibotenic acid and were sacrificed 2-24 weeks later. Choline acetyltransferase and parvalbumin immunohistochemistry failed to reveal changes in the number of cholinergic or gamma-aminobutyric acid-containing neurons, respectively, within the septal/diagonal band region ipsilateral to the hippocampal lesion at any time point examined. Additionally, immunocytochemical localization of nonphosphorylated and phosphorylated neurofilament proteins did not reveal abnormal staining characteristics within the septal/diagonal band complex, suggesting that this lesion does not alter cytoskeletal features of neurons which project to the hippocampus. Selected rats received unilateral hippocampal lesions and 3 months later were injected with fluorogold into the remaining hippocampal remnant and with wheat germ agglutinin conjugated to horse radish peroxidase into the intact contralateral hippocampus. Both retrograde tracers were predominantly transported to their respective ipsilateral septum and vertical limb of the diagonal band. This indicates that following the lesion, septal/diagonal band neurons still project ipsilaterally and sprouting to the NGF-rich contralateral side does not occur. RNA blot analysis revealed a decrease in NGF mRNA expression within the lesioned hippocampus with a maximum reduction of approximately 70%. In contrast, no change in NGF mRNA expression was observed within the ipsilateral septum relative to the contralateral side. The present study demonstrates that removal of hippocampal target neurons does not alter the number, morphology, or projections of both cholinergic and noncholinergic septal/diagonal band neurons.

Initial cholinergic differentiation in embryonic chick retina is responsive to insulin and cell-cell interactions

Previous work [Kyriakis et al., Proc. Natl. Acad. Sci. U.S.A., 84 (1987) 7463-7467] had shown that insulin, when added during a window of binding from embryonic days 9-11, stimulates the normal developmental increase in choline acetyltransferase (ChAT) activity (a marker for cholinergic differentiation) in cultured embryonic chick retinal neurons. Here, we investigated the effect of insulin and IGF 1 on embryonic chick retinal neurons at the stage of development (embryonic day 6) when ChAT activity is first expressed. We investigated insulin peptide effects in retinal tissue developing in vitro as well as in cultures of retinal cells. We show that insulin also stimulated the initial embryonic increase in ChAT activity but had no stimulatory effect on glutamic acid decarboxylase activity (a marker for GABAergic differentiation), an enzyme whose activity also increases developmentally in the same retinal neurons. In fact, insulin inhibited the expression of GAD activity in the retina. The insulin-mediated increase in ChAT activity was independent of normal cell-cell interactions but could not replace them. Insulin also stimulated choline uptake but only after a two day delay, suggesting that the normal program for cholinergic differentiation in the chick retina was induced by insulin. IGF 1 did not have any effect on either cholinergic or GABAergic differentiation. We conclude that cholinergic differentiation in chick embryo retinal neurons is dependent on both insulin- and cell contact-mediated signals.

Survival-promoting effect of NGF on in vitro septohippocampal neurons with cholinergic and GABAergic phenotypes

Recently, we demonstrated a survival-promoting effect of nerve growth factor (NGF) on cultured hippocampus-projecting neurons from developing septum/diagonal band region using fluorescent latex microspheres as a retrograde neuronal marker (Arimatsu et al., 1989). In the present study, we characterized these projection neurons by combining the retrograde cell labeling and histochemical stainings for acetylcholinesterase (AChE) activity and NGF receptor-, choline acetyltransferase- (ChAT-) and gamma-aminobutyric acid- (GABA-) immunoreactivities. The surviving microsphere-labeled neurons were, for the most part, immunoreactive for NGF receptor in the culture. A great majority (about 90%) of the microsphere-labeled neurons showed strong AChE activity and ChAT-immunoreactivity. The number of strongly AChE-positive neurons and that of ChAT-immunoreactive neurons in the culture supplemented with NGF was much greater with than without exogenous NGF. In addition, a major part (about 70%) of the microsphere-labeled neurons exhibited GABA-immunoreactivity in the presence of NGF. The number was also much greater than that without NGF. A considerable portion of cultured septal cholinergic neurons were shown to express GABA-immunoreactivity by a two-color immunofluorescence labeling experiment for ChAT and GABA. These findings are consistent with the assumption that NGF plays an important role in the development and organization of the cholinergic and GABAergic septohippocampal systems by supporting the neuronal survival, and raise a possibility that cholinergic and GABAergic fractions of the septohippocampal neurons may be developmentally correlated.

In vitro cell cultures as a model of the basal forebrain

The basal forebrain has attracted considerable attention because of its putative role in complex functions such as learning, memory and behavioral state control as well as its vulnerability in neurological disorders such as Alzheimer's Disease (AD). The finding that nerve growth factor provides trophic support for the cholinergic basal forebrain neurons has stimulated further interest in understanding trophic interactions of basal forebrain neurons as well as in possible trophic factor therapeutic strategies for disease states. Our laboratory has utilized primary cell cultures and developed immortalized central nervous system cell lines to study the trophic interactions that establish and maintain the septohippocampal pathway, a basal forebrain component which plays an essential role in cognitive function and is prominently affected in AD. The results of our primary cell culture studies have demonstrated the importance of trophic signals elaborated by the hippocampus in mediating the development of septal cholinergic neurons. Nerve growth factor plays an important role in this process, but it cannot account for all of the trophic signals elaborated by authentic hippocampal target cells. The development by this laboratory of clonal cell lines of septal and hippocampal lineage offers the prospect of investigating both the response to and elaboration of neural trophic signals at a more precise level of resolution than can be achieved with primary cultures. The technology and information that is generated from the engineering of such cell lines will also serve as a strategy to study trophic interactions in other brain circuits in future years, and to investigate possible changes or dysfunctions that occur neurological diseases.

Long-term administration of mouse nerve growth factor to adult rats with partial lesions of the cholinergic septohippocampal pathway

Nerve growth factor (NGF), a neurotrophic factor acting on cholinergic neurons of the basal forebrain, has been proposed as a treatment for Alzheimer's disease. Experimental support for its pharmacological use is derived from short-term studies showing that intraventricular administration of NGF during 2-4 weeks protects cholinergic cell bodies from lesion-induced degeneration, stimulates synthesis of choline acetyltransferase, and improves various behavioral impairments. To investigate the consequences of long-term NGF administration, we tested whether cholinergic cell bodies are protected from lesion-induced degeneration and whether cholinergic axons are stimulated to regrow into the denervated hippocampus following fimbrial transections. We found that intraventricular injections of NGF twice a week for 5 months to adult rats resulted in extended protection of cholinergic cell bodies from lesion-induced degeneration and did not produce obvious detrimental effects on the animals. NGF treatment mildly stimulated growth of cholinergic neurites within the 2-mm area directly adjacent to the fimbrial lesion but it failed to induce significant homotypic growth of cholinergic neurites into the deafferented hippocampus.

Development and characterization of clonal cell lines derived from septal cholinergic neurons

Studies employing primary cells to determine the molecular basis of neuronal development and selective synaptogenesis in the central nervous system are limited by cellular heterogeneity. Clonal hybrid cell lines derived from a particular region of brain, which express differentiated characteristics typical of the cells of origin, offer a potentially powerful alternative approach. We previously demonstrated the feasibility of deriving such cell lines from septal cholinergic cells. We now delineate the methods employed, and describe the development of additional cholinergic cell lines expressing neuronal and cholinergic features from later developmental stages. One cell line has been studied in detail and found to form neurites, express choline acetyltransferase (ChAT) and neurofilament protein (NFP), and display typical neuronal ultrastructural characteristics, including puncta adherens, neuritic varicosities, vesicles, and growth cones.

Immortalized young adult neurons from the septal region: generation and characterization

Studies of the development of the central nervous system would be greatly facilitated by the ability to immortalize neuronal tissue from a broad range of ages. We have previously used somatic cell fusion techniques to generate neuronal cell lines from embryonic mice. To immortalize older neuronal cells, a cell isolation technique was developed to obtain viable septal cells from postnatal day 21 mice. The septal cells were fused to N18TG2 neuroblastoma cells and then cultured in selective medium to isolate septum x neuroblastoma cell lines. The hybrid nature of the lines was verified by chromosome analysis and electrophoretic analysis of glucosephosphate isomerase isozymes. The lines express phenotypes typical of differentiated septal neurons. Many lines morphologically resemble neurons and express the high molecular weight neurofilament protein. Several lines express high levels of choline acetyltransferase activity; others synthesize nerve growth factor. These results demonstrate that young adult neuronal tissue can be immortalized and that hybrid cells express properties of the neuronal parent.

Neurotrophic factors and Alzheimer's disease

Defects in essential trophic interactions represent one possible explanation for the systems degenerations that occur in Alzheimer's as well as other neurodegenerative diseases. Since a multiplicity of neural pathways are affected in Alzheimer's disease, it is likely that more than one neurotrophic factor may be implicated. Through modern approaches in cell and molecular biology, it may be possible to identify such factors and to more precisely study their role in both sustaining neural connections, and in diseases involving those connections.