Subcellular localization of nerve growth factor receptors. Thirteen-day chick embryo brain

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.

Ontogeny of high- and low-affinity nerve growth factor receptors in the lumbar spinal cord of the developing chick embryo

The binding of 125I-labeled nerve growth factor-beta (NGF) to soluble extracts of intact or dissociated embryonic chick lumbar cords was used to investigate the kinetic properties and to quantify the levels of NGF receptors (NGFRs) in the developing chick between Embryonic Day 6 (E6) and E10. Both high-affinity (type I; Kd = 7.4 x 10(-11) M) and low-affinity (type II; Kd = 2.4 x 10(-9) M) NGFRs were detected by Scatchard analysis of 125I-NGF binding to E6 spinal cord extracts. A total of 4 x 10(9) type I and 5 x 10(10) type II receptors/cord were found in extracts of E6 cords. As development progressed there was a decline of both types of NGFRs; however, the decline of type I receptors occurred more rapidly than that of type II. Between E6 and E8 greater than 90% of the type I but only 25% of the type II receptors were lost. These relative rates of loss were maintained over the next week of development, with type I receptors no longer detectable by E12, and type II receptors reduced to 0.025% of their E6 numbers by E15. Analyses of NGFR levels in subpopulations of E6 and E8 lumbar cord cells, prepared by metrizamide density gradient centrifugation, showed that during this period there is an enrichment of both types of NGFRs in the motoneuron-containing subpopulation, relative to other cell populations. The loss of NGFRs does not appear to be influenced by those peripheral-trophic interactions which control other aspects of motoneuron development: curarization of the embryos between E6 and E9 increased motoneuron number in E10 embryos by 30%, but did not significantly affect the loss of NGFRs. These results provide the first quantitative evidence that type I and type II NGFRs are differentially regulated in the spinal cord during embryonic development and raise the possibility that distinct cellular mechanisms may govern their expression.

Localization of high-affinity and low-affinity nerve growth factor receptors in cultured rat basal forebrain

Previous work has indicated that nerve growth factor specifically and selectively increases choline acetyltransferase and acetylcholinesterase in organotypic cultures of rat basal forebrain-medial septal area. To determine whether these actions are potentially receptor-mediated, organotypic and dissociated basal forebrain-medial septal area cultures were examined. Two independent methods, [125I]nerve growth factor binding and immunocytochemistry with a monoclonal nerve growth factor receptor antibody (192-IgG), detected specific receptors. The nerve growth factor receptors were localized to two different cellular populations: flat, large, non-neuron-like cells, and small, round, process-bearing, neuron-like cells. Dissociation studies with [125I]nerve growth factor suggested that high-affinity receptors were localized to the neuron-like population, while only low-affinity receptors were localized to the non-neuron-like cells. We tentatively conclude that nerve growth factor may elicit cholinergic effects by directly binding to high-affinity receptors on neurons. To begin examining receptor regulation, cultures were exposed to exogenous, unlabeled nerve growth factor continuously for 10 days before binding studies were performed. Prior exposure to nerve growth factor did not alter binding characteristics of the receptor, using the present methods.

A nerve growth factor-induced gene encodes a possible transcriptional regulatory factor

Nerve growth factor (NGF) is a trophic agent that promotes the outgrowth of nerve fibers from sympathetic and sensory ganglia. The neuronal differentiation stimulated by this hormone was examined in the NGF-responsive cell line PC12. Differential hybridization was used to screen a complementary DNA library constructed from PC12 cells treated with NGF and cycloheximide. One of the complementary DNA clones that was rapidly induced by NGF was found to have a nucleotide sequence that predicts a 54-kilodalton protein with homology to transcriptional regulatory proteins. This clone, NGFI-A, contains three tandemly repeated copies of the 28- to 30-amino acid "zinc finger" domain present in Xenopus laevis TFIIIA and other DNA-binding proteins. It also contains another highly conserved unit of eight amino acids that is repeated at least 11 times. The NGFI-A gene is expressed at relatively high levels in the brain, lung, and superior cervical ganglion of the adult rat.

Cholinergic neurons of fetal rat telencephalon in aggregating cell culture respond to NGF as well as to protein kinase C-activating tumor promoters

Serum-free aggregating cell cultures of fetal rat telencephalon treated with the potent tumor promoter phorbol 12-myristate 13-acetate (PMA) showed a dose-dependent, persistent stimulation of the enzymes choline acetyltransferase (ChAT), glutamic acid decarboxylase and glutamine synthetase. After elimination of the proliferating cells by treatment of the cultures with Ara-C (0.4 microM) only the cholinergic marker enzyme, ChAT, could be stimulated by tumor promoters. The non-promoting phorbol ester, 4 alpha-phorbol 12,13-didecanoate proved to be inactive in these cultures, whereas the potent non-phorbol tumor promoter, mezerein, produced an even greater stimulatory effect than PMA. Since PMA and mezerein are potent and specific activators of protein kinase C, the present results suggest a role for this second messenger in the development of cholinergic telencephalon neurons. Stimulation of ChAT required prolonged exposure (48 h) of the cultures to PMA and the responsiveness of the cholinergic neurons to the tumor promoters decreased with progressive cellular maturation. The cholinergic telencephalon neurons showed the same pattern of responsiveness for tumor promoters as for nerve growth factor (NGF). However, the combined treatment with NGF and either PMA or mezerein produced an additive stimulatory effect, suggesting somewhat different mechanisms of action.

The contribution of citrate to the synthesis of acetyl units in synaptosomes of developing rat brain

The activities of pyruvate dehydrogenase, citrate synthase, and choline acetyltransferase in rat brain synaptosomes increased during ontogenesis by 3 and 14 times, respectively. Activity of ATP-citrate lyase decreased by 26% during the same period. Pyruvate consumption by synaptosomes from 1-day-old animals was 40% lower than that found in older rats; however, citrate efflux from intrasynaptosomal mitochondria in immature synaptosomes was over twice as high as that in mature ones. The rates on production of synaptoplasmic acetyl-CoA by ATP-citrate lyase were 1.03, 1.40, and 0.49 nmol/min/mg protein in 1-, 10-day-old, and adult rats, respectively. 3-Bromopyruvate (0.5 mM) inhibited pyruvate consumption by 70% and caused a complete block of citrate utilization by citrate lyase in every age group. Parameters of citrate metabolism in cerebellar synaptosomes were the same as those in cerebral ones. These data indicate that production of acetyl-CoA from citrate in synaptoplasm may be regulated either by adaptative, age-dependent changes in permeability and carrier capacity of the mitochondrial membrane or by the inhibition of synthesis of intramitochondrial acetyl-CoA. ATP-citrate lyase activity is not a rate-limiting factor in this process. Metabolic fluxes of pyruvate to cytoplasmic citrate and acetyl-CoA are presumably the same in both cholinergic and noncholinergic nerve endings. The significance of citrate release from intrasynaptosomal mitochondria as a regulatory step in acetylcholine synthesis is discussed.

Nerve growth factor (NGF) stimulation of cholinergic telencephalic neurons in aggregating cell cultures

The addition of nerve growth factor (2.5S NGF) to serum-free aggregating cell cultures of fetal rat telencephalon greatly stimulated the developmental increase in choline acetyltransferase activity. Two other neuronal enzymes, acetylcholinesterase and glutamic acid decarboxylase, showed only slightly increased activities after NGF treatment whereas the total protein content of the cultures and the activity of 2',3'- cyclic nucleotide phosphodiesterase remained unchanged. The stimulation of choline acetyltransferase was dependent on the NGF media concentrations, showing a 50% maximum effect (120% increase) at approximately 3 ng/ml (10-10 M 2.5S NGF). NGF treatments during different culture periods showed that the cholinergic neurons remained responsive for at least 19 days. The continued treatment was the most effective; however, an initial treatment for only 5 days still caused a significant stimulation of choline acetyltransferase on day 19. The observed stimulation appeared to be specific to NGF. Univalent antibody fragments (Fab) against 2.5S NGF completely abolished the NGF-dependent increase in choline acetyltransferase activity, whereas Fab fragments of control IgG were ineffective. Furthermore, angiotensin II, added in high amounts to the cultures, showed no stimulatory effect. The present results suggest that certain populations of rat brain neurons are responsive to nerve growth factor.

Regional and subcellular distribution of ATP-citrate lyase and other enzymes of acetyl-CoA metabolism in rat brain

The activities of ATP-citrate lyase in frog, guinea pig, mouse, rat, and human brain vary from 18 to 30 mu mol/h/g of tissue, being several times higher than choline acetyltransferase activity. Activities of pyruvate dehydrogenase and acetyl coenzyme A synthetase in rat brain are 206 and 18.4 mu mol/h/g of tissue, respectively. Over 70% of the activities of both choline acetyltransferase and ATP-citrate lyase in secondary fractions are found in synaptosomes. Their preferential localization in synaptosomes and synaptoplasm is supported by RSA values above 2. Acetyl CoA synthetase activity is located mainly in whole brain mitochondria (RSA, 2.33) and its activity in synaptoplasm is low (RSA, 0.25). The activities of pyruvate dehydrogenase, citrate synthase, and carnitine acetyltransferase are present mainly in fractions C and Bp. No pyruvate dehydrogenase activity is found in synaptoplasm. Striatum, cerebral cortex, and cerebellum contain similar activities of pyruvate dehydrogenase, citrate synthase, carnitine acetyltransferase, fatty acid synthetase, and acetyl-CoA hydrolase. Activities of acetyl CoA synthetase, choline acetyltransferase and ATP-citrate lyase in cerebellum are about 10 and 4 times lower, respectively, than in other parts of the brain. These data indicate preferential localization of ATP-citrate lyase in cholinergic nerve endings, and indicate that this enzyme is not a rate limiting step in the synthesis of the acetyl moiety of ACh in brain.

Short-latency drinking and increased Na appetite after intracerebral microinjections of NGF in rats

NERVE GROWTH FACTOR (NGF) is a polypeptide trophic factor for peripheral sympathetic and sensory neurones(1,2). Apparent NGF(3-5) and NGF receptors(6,7) have also been identified in the brain, and intracerebral administration of NGF in the adult rat produces marked biochemical(8) and morphologica(9,10) changes in brain tissue. These findings, taken together with the observations that central injections of NGF facilitate behavioural recovery from brain damage(11,12), indicate that this polypeptide may have an important role in brain function. It has been observed that rats given intraventricular injections of up to 2.3 microg NGF drink copiously (M.E.L. and G. Guroff, unpublished observations). Perkins et al.(13) reported that diencephalic application of crystalline NGF (1-15 microg) resulted in an intense polydipsia. The present report confirms the observations of M.E.L. and Guroff, and extends the findings of Perkins et al.(13) by using solutions of NGF instead of crystals. It also describes for the first time a second phenomenon produced by intracranial administration of NGF, namely an intense appetite for aversive concentrations of sodium solutions.

Trisialoganglioside synthesis by a chicken brain sialyltransferase. Comparative study with the similar reaction for the synthesis of disialoganglioside

An enzyme preparation from embryonic chicken brain catalyzes the transfer of sialic acid from CMP-N-acetylneuraminic acid to ceramide-Glc-Gal(NeuAc-NeuAc)-GalNAc-Gal (GDlb) to form ceramide-Glc-Gal(NeuAc-NeuAc)-GalNAc-Gal-NeuAc (GTlb). The sialyltransferase activity was measured during the development of the embryo, the subcellular distribution of this activity was determined and several kinetic properties of the reaction were examined. A comparative study with the similar reaction involved in the transfer of sialic acid to the terminal galactose in ceramide-Glc-Gal(NeuAc)-GalNAc-Gal (GMl) was made. The results obtained in this comparative study suggest that the transfer of sialic acid in both reactions is catalyzed by the same enzyme.