Nerve growth factor modulates tubulin transcript levels in pheochromocytoma PC12 cells

Temporal and functional dynamics of the transcriptome during nerve growth factor-induced differentiation

The rat pheochromocytoma cell line (PC12) is an extensively used model to study neuronal differentiation. The initial signaling cascades triggered by nerve growth factor (NGF) stimulation have been subject to thorough investigation and are well characterized. However, knowledge of temporal transcriptomal regulation during NGF-induced differentiation of PC12 cells remains far from complete. We performed a microarray study that characterized temporal and functional changes of the transcriptome during 4 subsequent days of differentiation of Neuroscreen-1 PC12 cells. By analyzing the transcription profiles of 1595 NGF-regulated genes, we show a large diversity of transcriptional regulation in time. Also, we quantitatively identified 26 out of 243 predefined biological process and 30 out of 255 predefined molecular function classes that are specifically regulated by NGF. Combining the temporal and functional transcriptomal data revealed that NGF selectively exerts a temporally coordinated regulation of genes implicated in protein biosynthesis, intracellular signaling, cell structure, chromatin packaging and remodeling, intracellular protein traffic, mRNA transcription, and cell cycle. We will discuss how NGF-induced changes may modulate the transcriptional response to NGF itself during differentiation.

Essential role of NKCC1 in NGF-induced neurite outgrowth

The Na(+)/K(+)/2Cl(-) cotransporter (NKCC) mediates electroneutral transport of 2Cl(-) coupled with Na(+) and K(+) across the plasma membrane, and plays crucial roles in Cl(-) uptake into the cells, homeostasis of cellular Cl(-), and cell volume regulation. However, we have very limited information on the roles of ion transporters in neurite outgrowth in neuronal cells. In the present study, we report the role of NKCC1 (an isoform of NKCC) in NGF-induced neurite outgrowth of rat pheochromocytoma PC12D cells. The expression level of NKCC1 protein was increased by NGF treatment. Knock-down of NKCC1 by RNA interference (RNAi) drastically diminished the NGF-induced neurite outgrowth. Transfection of enhanced green fluorescent protein (EGFP)-tagged rat NKCC1 into cells for clarification of intracellular localization of NKCC1 revealed that the EGFP-rNKCC1 was mainly localized in the plasma membrane at growth cone during neurite outgrowth. These observations suggest that NKCC1 plays a fundamental role in NGF-induced neurite outgrowth of PC12D cells.

Quantification of neurotrophin mRNA expression in PMN mouse: modulation by xaliproden

Compounds possessing neurotrophic properties may represent a possible treatment for neurodegenerative disorders such as amyotrophic lateral sclerosis. Xaliproden (SR57746A), an orally-active non-peptide compound, which has been found to exhibit neurotrophic effects in vitro and in vivo, increased the lifespan and delayed the progression of the motor neuron degeneration in PMN mice. We have used a quantitative reverse transcription/polymerase chain reaction amplification technique to study the regulation of neurotrophin mRNA and trk mRNA expression in PMN mice. NGF and NT-3 mRNA are downregulated in PMN mice. These deficiencies can be overcome by a treatment with xaliproden. Such an effect could contribute to neurotrophic effects of xaliproden in vivo and in vitro.

Estrogen effects on neurite outgrowth and cytoskeletal gene expression in ERalpha-transfected PC12 cell lines

The potential of gonadal steroids like estrogen (E) to promote neurite sprouting is of interest in development and aging, as well as after neural trauma. The specific roles of the two main estrogen receptors, ERalpha and ERbeta, in neuronal sprouting are not yet well understood. We examined the hypothesis that E can enhance nerve growth factor (NGF)-stimulated neurite sprouting in an ERalpha-dependent manner. PC12 cells that were stably transfected with the full-length rat ERalpha gene (PCER) and a control line of cells transfected with vector DNA alone (PCCON) were compared. Both cell lines vigorously differentiate neurites when treated with NGF. We determined that both lines show basal expression of ERbeta mRNA, but only the PCER cells express ERalpha mRNA. Estrogen treatment markedly enhanced NGF-stimulated neurite outgrowth from PCER but not from PCCON cells. Significantly larger proportions of PCER cells (34 and 53% at 24 and 48 h, respectively) had neurites than did the PCCON cells (17 and 26% at 24 and 48 h) after E plus NGF treatment. We also examined the effects of E and NGF treatment of PCER and PCCON cells on peripherin, alpha-tubulin, and tau mRNA expression. In undifferentiated PCER cells, E treatment increased peripherin, reduced alpha-tubulin, and did not alter tau mRNA levels. No changes in these mRNAs were observed in the controls (undifferentiated PCCON cells) after E treatment. NGF treatment markedly stimulated expression of peripherin, alpha-tubulin, and tau mRNAs in both PCER and PCCON cells. From these observations we conclude that E synergizes with NGF and stimulates neurite sprouting and also modulates expression of several cytoskeletal mRNAs through ERalpha.

Hormonally induced neuronal plasticity in the adult motoneurons

Sex steroids are known to play a crucial role in reproductive neuroendocrine functions in adulthood. A number of neurons in the neuroendocrine brain contain sex steroid receptors, and are thought to be a key element of functional neural circuits that are regulated by sex steroids. Motoneurons in the spinal nucleus of the bulbocavernosus in adult male rodents are one of the androgen-sensitive neural substrates. In the spinal nucleus of the bulbocavernosus, castration of adult male rats results in a significant decrease in the somatic size and dendritic length of the motoneurons, and in the number and size of chemical and electrical (gap junction) synapses onto these motoneurons. Androgen treatment of castrates reverses these changes. Furthermore, androgen has been reported to be involved in regulation of androgen receptor expression and gene expression of structural proteins such as beta-actin, beta-tubulin and gap junction channels in these motoneurons. The findings suggest that androgen induces morphological and molecular changes in the motoneurons that reflect their neural functions, and may provide evidence for the mechanisms of hormonally induced neuronal plasticity in the motoneurons in adulthood.

Developmental regulation of alpha-tubulin mRNAs during the differentiation of cultured cerebellar granule cells

T alpha 1 and T26 alpha-tubulin mRNA expression was examined during the differentiation of rat cerebellar granule cells in vitro and in situ. High levels of T alpha 1 transcript correlated with neurons extending processes and hence may implicate T alpha 1 with neuritogenesis. In comparison, T26 labeling was much less prominent, appeared more constitutive and was possibly associated with cell proliferation. Such profiles indicate that the different isotypes play different roles in the assembly and function of microtubules during neuronal differentiation.

Effect of sustained physiological hyperinsulinaemia on hypothalamic neuropeptide Y and NPY mRNA levels in the rat

Neuropeptide Y (NPY) synthesized in the arcuato-paraventricular projection in the rat hypothalamus is thought to play an important role in controlling energy homeostasis. The factors that regulate hypothalamic NPY are not known but, amongst others, insulin has been postulated as an inhibitory modulatory agent. To test this hypothesis, normal male rats were given either insulin (2 units/day) or saline via subcutaneous osmotic minipumps for 3 days. Euglycaemia was maintained by a concomitant glucose infusion in insulin-infused rats which had peripheral insulin levels 5-8 times higher than saline-infused controls. Hyperinsulinaemic rats ate 42% less than controls, but their total energy intake (food intake plus glucose infusion) was higher than that of controls, and they gained more weight than controls during the experimental period. Hyperinsulinaemia had no significant effect on hypothalamic NPY mRNA or NPY levels in the arcuate nucleus. NPY concentrations in the paraventricular nucleus were, however, significantly increased by 73% in hyperinsulinaemic rats, but were closely similar to controls in all other areas. Insulin may act as a satiety factor in that hyperinsulinaemic rats ate less, but the fact that these animals had increased total energy intake and gained excessive weight suggests that insulin may not function as an overall regulator of energy balance. In addition, physiological hyperinsulinaemia does not apparently inhibit NPY gene expression in the arcuate nucleus. Due to the lack of effect of hyperinsulinaemia on NPY synthesis in the arcuate nucleus, the elevated NPY concentrations in the paraventricular nucleus could result from a reduction of its release, which would be in keeping with the reduction in food intake.

Nerve growth factor and neurotrophin-3 enhance neurite outgrowth and up-regulate the levels of messenger RNA for growth-associated protein GAP-43 and T alpha 1 alpha-tubulin in cultured adult rat sensory neurones

The effect of nerve growth factor (NGF) and neurotrophin-3 (NT-3) on neurite outgrowth was related to the level of mRNA for the growth-associated protein GAP-43 and the neurone specific alpha-tubulin, T alpha 1, in dissociated cultures of adult rat sensory neurones. Treatment with NGF or NT-3 for 3 days enhanced neurite outgrowth in a dose-dependent manner and by 7-fold and 5-fold, respectively, over control at the highest concentrations used. NGF and NT-3 elevated the level of mRNA encoding GAP-43 by 2.3-fold and T alpha 1 alpha-tubulin by 3.2-3.5-fold. The estimated ED50 values were 0.1-0.3 ng/ml for NGF and 2 ng/ml for NT-3 for both neurite outgrowth and mRNA up-regulation.

Androgenic regulation of expression of beta-tubulin messenger ribonucleic acid in motoneurons of the spinal nucleus of the bulbocavernosus

Expression of beta-tubulin mRNA was examined in androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in adult male rats by in situ hybridization histochemistry using cDNA encoding mouse beta-tubulin. Hybridizable beta-tubulin mRNA was localized in the somata and proximal dendrites of SNB motoneurons. Removal of androgen by castration significantly reduced the expression level of beta-tubulin mRNA in the SNB motoneurons, whereas the change was prevented by testosterone treatment. On the contrary, castration or testosterone treatment did not induce any changes in the expression level of beta-tubulin mRNA in the androgen-insensitive motoneurons of the retrodorsolateral nucleus. These results suggest that androgen regulates the expression of beta-tubulin gene in the SNB motoneurons and may provide evidence for the molecular mechanisms of hormonally-induced neuronal plasticity in the SNB motoneurons.

Insulin and insulin-like growth factor I enhance regeneration in cultured adult rat sensory neurones

Insulin and the insulin-like growth factors (IGFs) may directly affect the growth, development, and maintenance of the vertebrate nervous system. Previous in vitro studies have focused on embryonic nervous tissue. In this study the effects of insulin, IGF-I, IGF-II and nerve growth factor (NGF) on regeneration and neuronal survival were studied in cultured adult rat sensory neurones in a cell culture environment that limited non-neuronal cell mediated effects. Regeneration, as assessed by neurite outgrowth, was significantly enhanced by insulin and IGF-I in a dose-dependent manner. The half-maximally effective concentrations, ED50's, were approximately 1 nM and 0.1 nM for insulin and IGF-I, respectively. Concentrations of IGF-I as low as 10pM were active. There was some evidence that IGF-II stimulated regeneration, although this failed to reach statistical significance. NGF also promoted regeneration, confirming previous studies, exhibiting an ED50 of approximately 0.3 ng/ml and inducing a maximal response 2-fold greater than that observed with insulin or IGF-I. Combined treatment with NGF and insulin had an additive effect. Specific anti-NGF antiserum inhibited the regenerative response to NGF but failed to block the response to IGF-I, supporting the view that IGF-I was acting directly on sensory neurones rather than stimulating NGF production by non-neuronal cells. Insulin, IGF-I and NGF had no effect on neuronal survival in this culture system. These results show that adult sensory neurones can respond with enhanced regenerative growth to insulin and IGF-I, in addition to NGF although the response to IGF-II was less clear.

The mammalian iris-ciliary complex affects organization and synthesis of cytoskeletal proteins of organ and tissue cultured lens epithelial cells

A water soluble growth inhibitor was isolated from the mammalian ocular iris-ciliary complex. The molecular weight of this protein is 10 kD or lower as determined by ultrafiltration fractionation. The iris-ciliary (IC) complex water soluble protein(s) significantly inhibits synthesis of lower molecular weight proteins of the epithelial cells of the organ cultured mammalian ocular lens. It was also found that this inhibitory effect of IC is mediated via the structural organization of the lens. Monolayer cultures of the lens epithelial cells exposed to IC did not manifest any inhibition of their protein synthesis. Moreover, these tissue cultured lens epithelial (TCLE) cells showed a significant increase in their protein synthetic activities in response to the presence of IC factors in the culture medium. It is postulated that the IC activity is modulated via either the lens capsule, an extracellular matrix, or due to the specific organization of the intact lens. The specific effects of IC on the cytoskeletal organization and synthesis in the organ cultured lens epithelial (OCLE) and TCLE cells were also examined. Both groups, treated with IC factors, manifested significant alterations in their protein synthetic activities and cytoskeletal architecture. The 3H-leucine incorporation experiments showed that alpha-actin and alpha-tubulin synthesis is partially inhibited by IC factors in OCLE cells but vimentin synthesis is not, whereas in TCLE cells all of them showed increased synthesis in response to IC factors. Turnover rates of these proteins in both OCLE and TCLE cells were also computed. The immunofluorescence and microscopic evaluation of OCLE and TCLE cells exposed to IC factors illustrated significant alteration in the cytoarchitecture of the filaments. We demonstrate that an inhibitor(s) molecule of 10 kD or lower size isolated from IC inhibited protein synthesis of OCLE cells and stimulated protein synthesis in TCLE cells. The IC factor also affects the synthesis and organization of cytoskeletal filaments of both the OCLE and TCLE cells.

Cytoskeletal events underlying dendrite formation by cultured pigment cells

In contrast to neurite outgrowth, pigment cell dendrite formation is relatively unstudied. Keratinocyte-conditioned medium (KCM) induces a striking dendricity in human melanocytes and B16 melanoma cells that is detectable within 30 min, maximal in 24-48 hr, and quantifiable by computerized image analysis. Cytochalasin B (CB), known to disrupt actin microfilaments, completely blocks dendrite formation if added to cultures before or with KCM. This effect is rapidly reversible, and dendrites appear within 1 hr after refeeding with KCM alone. In contrast, CB treatment fails to disrupt existing dendrites previously induced by KCM. Agents known to cause microtubule disassembly (colchicine, nocodazole, or vinblastine) do not inhibit dendrite formation if added before or with KCM. In contrast, these agents disrupt established dendrites. Inhibition of protein synthesis with cycloheximide or actinomycin D completely blocks dendrite formation, but if cultures are provided fresh KCM lacking protein synthesis inhibitors, dendrites reappear within 24 hr. Actin microfilaments visualized with a monoclonal antibody or rhodamine-phalloidin are poorly organized in untreated cells, but form numerous fibers localized along dendrites in KCM-treated cells. Microtubules visualized with a monoclonal anti-tubulin antibody are localized in the center of dendrites. These cytoskeletal changes occur without altering beta actin or beta tubulin mRNA levels. Taken together, these data implicate actin microfilaments in dendrite outgrowth, but not in maintenance, and conversely microtubules in dendrite maintenance but not in formation. These keratinocyte-induced changes involving beta actin and beta tubulin polymerization appear to require both new protein synthesis and post-translational regulation. The observed similarities between melanocytes and other neural crest-derived cells suggest that cutaneous pigment cells might serve as an alternative model for studies of neurite outgrowth.

Effects of insulin and insulin-like growth factors on neurofilament mRNA and tubulin mRNA content in human neuroblastoma SH-SY5Y cells

Insulin-like growth factors (IGFs) are implicated in the development of the vertebrate neural circuitry, and increase neurite growth in vitro and in vivo. The construction of the cytoskeleton is necessary for growth of axons and dendrites, and the neurofilament (NF) 68 kDa and 170 kDa proteins assemble to help form major fibrillar elements of the neurite cytoskeleton. We report that physiological concentrations of insulin, IGF-I or IGF-II increased the contents of 68 kDa NF, 170 kDa NF, alpha-tubulin, and beta-tubulin mRNAs, relative to total RNA, in cultured human neuroblastoma SH-SY5Y cells. In contrast, the relative contents of histone 3.3 mRNA, and poly(A)+ RNA were not increased. Ligand concentrations which increased NF mRNAs were very similar to those which increased neurite outgrowth. Although each gene was evidently independently regulated, the 68 kDa NF, 170 kDa NF, alpha-tubulin, and beta-tubulin mRNAs were nevertheless all transiently elevated over approximately the same time interval in response to insulin. These data, when considered together with studies by others with nerve growth factor, show that the 68 kDa and 170 kDa NF mRNAs are elevated in a biochemical pathway activated in common during neurite outgrowth directed by insulin, IGF-I, IGF-II, and nerve growth factor.

Thyroid hormone modulates the development of cholinergic terminal fields in the rat forebrain: relation to nerve growth factor receptor

Hyperthyroidism, induced in rat pups by the daily intraperitoneal administration of 1 microgram/g body weight triiodothyronine, facilitated the development of ChAT fiber plexuses in brain regions innervated by basal forebrain cholinergic neurons, leading to an earlier and increased expression of cholinergic markers in those fibers in the cortex, hippocampus and amygdala. A similar enhancement was seen in the caudate-putamen complex. This histochemical profile was correlated with an accelerated appearance of ChAT-positive telencephalic puncta, as well as with a larger total number of cholinergic terminals expressed, which persisted throughout the eight postnatal week, the longest time examined in the present study. Hypothyroidism was produced in rat pups by adding 0.5% propylthiouracil to the dams' diet beginning the day after birth. This dietary manipulation resulted in the diminished expression of ChAT in forebrain fibers and terminals. Hypothyroid treatment also reduced the quantity of ChAT puncta present during postnatal weeks 2 and 3, and, from week 4 and continuing through week 6, the number of ChAT-positive terminals in the telencephalic regions examined was actually less than the amount extant during the former developmental epoch. Immunostaining for nerve growth factor receptor (NGF-R), which is associated almost exclusively with ChAT-positive somata and fibers in the basal forebrain, demonstrated a different time course of postnatal development. Forebrain fibers and terminals demonstrating NGF-R were maximally visualized 1 week postnatally, a time at which these same neuronal elements evinced minimal ChAT-like immunopositivity. Thereafter and correlated with increased immunoreactivity for ChAT, fine details of NGF-R stained fibers were observed less frequently. Although propylthiouracil administration decreased NGF-R immunodensity, no alteration in the development of that receptor was observed as a function of triiodothyronine treatment. Cholinergic terminals in the ventrobasal thalamus, which derive from ChAT-positive neurons in the pedunculopontine and laterodorsal tegmental nucleus, were unaffected by either hyperthyroid or hypothyroid conditions. These cells also do not demonstrate NGF-R. We conclude from these experiments (1) that cholinergic fiber plexuses eventually exhibiting ChAT positivity in the telencephalon demonstrate NGF-R prior to the cholinergic synthetic enzyme, (2) that susceptibility to thyroid hormone manipulations may involve sensitivity to NGF, at least in some forebrain cholinergic systems and (3) that the effects of thyroid hormone imbalances on brain cholinergic neurons are regionally selective.

Neuritogenesis in cerebellar granule cells in vitro: a role for protein kinase C

We have used short-term (8 h) cultures of week-old rat cerebellar granule cells to examine the effects on neuritogenesis of activation and down-regulation of protein kinase C by phorbol esters. We have previously demonstrated that endogenously released glutamate promoted neurite outgrowth in the same system acting via N-methyl-D-aspartate receptors. Low levels (0.1-1 nM) of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) evoked increases in the number of granule cells which extended neurites; higher levels (10-250 nM) which caused a down-regulation of total protein kinase C, inhibited outgrowth in a dose-dependent manner. N-Methyl-D-aspartate by itself also stimulated process outgrowth but could not reverse the inhibition evoked by either TPA or the protein kinase C inhibitor sphingosine. Stimulation of protein kinase C with 0.1 nM TPA resulted in a general increase in the incorporation of 32P-labelled inorganic orthophosphate into granule cell polypeptides. The results indicate that the activation of protein kinase C is involved in neuritogenesis in granule cells and are consistent with the idea that N-methyl-D-aspartate receptor activation may exert its effect on neuritogenesis through protein kinase C.

Stabilization of tubulin mRNAs by insulin and insulin-like growth factor I during neurite formation

Neurotrophic factors may increase axon and dendrite growth in part by regulating the content of cytoskeletal elements such as microtubules, which are comprised of tubulin subunits. The mechanism by which insulin, insulin-like growth factors (IGFs), and nerve growth factor (NGF) can increase the relative abundance of tubulin mRNAs as a prelude to neurite formation was studied. Insulin significantly increased the abundance of tubulin mRNAs relative to total RNA in cultured human neuroblastoma SH-SY5Y cells. This increase was not the result of a generalized elevation of all transcripts, because tubulin mRNAs were elevated relative to poly(A)+ RNA as well. Moreover, whereas polymerases I and III were elevated in activity, polymerase II was not. Tubulin mRNAs were stabilized against degradation in the presence of actinomycin D by both insulin and IGF-I. In contrast, actin and histone 3.3 mRNAs were neither increased nor stabilized. Insulin did not alter alpha- or beta-tubulin gene transcription rates in nuclear run-off experiments, and did increase the relative synthesis of tubulin proteins. These results suggest that tubulin mRNA levels are increased mainly through selective stabilization by insulin and IGFs. Because NGF is known to stabilize tubulin mRNA levels also, stabilization of tubulin mRNAs is suggested to be a common event in the pathway leading to neurite elongation directed by neuritogenic polypeptides.

Neurotrophic agents may exacerbate the pathologic cascade of Alzheimer's disease

The thesis is advanced that Alzheimer's disease is triggered by alterations in the regulatory mechanisms governing the patterns of cytoskeletal protein expression in structurally plastic neurons in the mature nervous system. As a consequence, polypeptide species acting to stabilize the cytoskeleton are preferentially affected, and neuronal architecture becomes increasingly determined by proteins involved in labile structural states. A cascade of interdigitating pathologies is then postulated to develop characterized by nerve terminal aberrancies, subsequent extrusion of atypical polypeptide species and their conjugates, reactive gliosis, abnormal neuronal growth, and degeneration. Within this context, growth factors promote and accelerate the pathologic cascade. Based on this model, a treatment strategy is suggested that the most effective management of Alzheimer's disease, particularly during earlier stages, is to delay its projected normal onset and to control the aberrant neuronal growth that is a hallmark of the malady.

Effect of nerve growth factor on the elongation of neurites from axotomized rat embryonic septal-basal forebrain neurons: an in vitro analysis

The administration of nerve growth factor (NGF) into the brain of a fornix-fimbria lesioned rat can rescue many cholinergic, septal-basal forebrain (SBF) neurons from imminent cell death. Unfortunately, it is unclear if NGF can stimulate regenerative growth from axotomized, SBF neurons. In the present study, we used an in vitro model system to determine if NGF could affect neurite outgrowth from nonaxotomized and/or axotomized, embryonic SBF neurons. Axotomized neurons were obtained by severing the neuritic fields surrounding embryonic day (E) 15 SBF explants maintained in primary culture. Acetylcholinesterase (AChE) histochemistry was used to assess the effects of NGF on cholinergic neurites. We report that 1) neurite outgrowth on type I collagen from E15 SBF neurons in primary culture (nonaxotomized neurons) was not affected by NGF. 2) NGF enhanced the outgrowth (regeneration) of axotomized, SBF neurons on a collagen substratum; however, neurons had to be treated with NGF both before and after axotomy to stimulate regeneration effectively. Application of NGF either before or after axotomy did not enhance regenerative neurite outgrowth. 3) SBF neurons had to be axotomized for NGF to facilitate neurite outgrowth. This is supported by the observation that SBF explants, initially maintained in NGF-supplemented medium in suspension culture, did not demonstrate enhanced neurite outgrowth in the presence of NGF when plated onto a substratum. 4) The regenerative growth of AChE-negative, as well as AChE-positive, neurites was facilitated by NGF treatment. In addition to data concerning neurite outgrowth, we also found that the NGF receptor, as recognized by the antibody 192-IgG, expands its distribution as time in culture progresses; i.e., staining, originally confined to cell bodies and proximal processes within the explant, later included neurites that emanated from the explant. Thus, our results demonstrate that NGF can stimulate regenerative growth from axotomized, but not nonaxotomized, embryonic SBF neurons. We hypothesize that, given the appropriate substratum for axon elongation in vivo, NGF can stimulate the regeneration of SBF neurons in the injured adult brain.

NGF induction of NGF receptor gene expression and cholinergic neuronal hypertrophy within the basal forebrain of the adult rat

Chronic infusion of nerve growth factor (NGF) into the forebrain of the adult rat produced increases in NGF receptor (NGF-R) mRNA hybridization, NGF-R immunoreactivity, choline acetyltransferase (ChAT) mRNA hybridization, and neuronal hypertrophy, when compared with vehicle infusion or noninfused rat brain. In situ hybridization showed NGF induction of NGF-R gene expression, documented by increases in the number of NGF-R mRNA-positive cells within the medial septum, diagonal band, and nucleus basalis magnocellularis. NGF also produced hypertrophy of ChAT mRNA-positive neurons. These results suggest that NGF produces cholinergic neuronal hypertrophy through induction of NGF-R gene expression within the basal forebrain.

Nerve growth factor receptor is associated with cholinergic neurons of the basal forebrain but not the pontomesencephalon

Sequential immunohistochemical demonstration of nerve growth factor receptor and cholinergic acetyltransferase on the same tissue section in the rat revealed that approximately 92% of all cholinergic neurons in the basal forebrain possessed that receptor. Only 0.9% of the neurons demonstrating nerve growth factor receptor in the basal nuclear complex lacked the cholinergic synthetic enzyme, and a similarly small percentage of cholinergic cells, 7.1%, were choline acetyltransferase-positive but nerve growth factor receptor-negative. Affiliation of nerve growth factor receptor with structural entities morphologically indistinguishable from those demonstrating choline acetyltransferase on separate but corresponding tissue sections was also observed in the telencephalic fiber tracts and terminal fields of basal forebrain cholinergic neurons, including cholinergic puncta in the reticular nucleus of the thalamus. Nerve growth factor receptor was not found in association with choline acetyltransferase-positive somata of the pedunculopontine and laterodorsal tegmental nuclei, however, nor were fibers immunoreactive for nerve growth factor receptor observed originating from those cell bodies. These results suggest that nerve growth factor receptor, which is probably synthesized in cholinergic basal forebrain somata and transported throughout their dendritic and axonal arbors, has a physiologic role in those cells in the adult nervous system. This does not appear to be the case for phenotypically similar neurons of the pontomesencephalotegmental cholinergic complex.

Insulin and insulinlike growth factor receptors regulating neurite formation in cultured human neuroblastoma cells

The functional role of brain insulin and insulinlike growth factor (IGF) receptors is being sought. Recently it has been found that these ligands are members of a newly identified family of neuritogenic polypeptides. We studied the relationship between 125I-insulin and 125I-IGF binding and their capacity to enhance neurite formation in cultured human neuroblastoma SH-SY5Y cells. The binding of 125I-insulin was temperature-dependent and heterogeneous. The Scatchard plot and dissociation rate were both consistent with the presence of two types of sites. There appeared to be about 900 high affinity sites per cell with a Kd of about 3 nM. This compared favorably with the half-maximal concentration of 4 nM for enhancement of neurite formation. The type I IGF sites were also present. Physiologic concentrations of insulin clearly enhanced neurite formation through the insulin sites, whereas physiologic concentrations of IGF-I and IGF-II enhanced through the IGF sites. Cross-occupancy of sites was observed at supraphysiologic concentrations, providing a reasonable explanation for the broad dose-response curves for these ligands. These results support the suggestion that one function of insulin and IGF receptors in neural tissues may be to modulate neurite formation.