Induction of apoptosis by the low-affinity NGF receptor.

Nerve growth factor (NGF) binding to cellular receptors is required for the survival of some neural cells. In contrast to TrkA, the high-affinity NGF receptor that transduces NGF signals for survival and differentiation, the function of the low-affinity NGF receptor, p75NGFR, remains uncertain. Expression of p75NGFR induced neural cell death constitutively when p75NGFR was unbound; binding by NGF or monoclonal antibody, however, inhibited cell death induced by p75NGFR. Thus, expression of p75NGFR may explain the dependence of some neural cells on NGF for survival. These findings also suggest that p75NGFR has some functional similarities to other members of a superfamily of receptors that include tumor necrosis factor receptors, Fas (Apo-1), and CD40.

Safety assessment of aqueous olive pulp extract as an antioxidant or antimicrobial agent in foods.

The olive fruit, its oil and the leaves of the olive tree have a rich history of nutritional, medicinal and ceremonial uses. Olive oil, table olives and olive products are an important part of the Mediterranean diet, the greatest value of which may be due to olive polyphenols that contribute to the modulation of the oxidative balance in vivo. The objective of this review is to examine the available safety/toxicity literature on olive polyphenols, particularly hydroxytyrosol, to determine the safety-in-use of a standardized aqueous olive pulp extract (HIDROX). Among the polyphenols found in the extract, the major constituent of biological significance is hydroxytyrosol (50-70%). In oral bioavailability studies, urinary excretion of hydroxytyrosol and its glucuronide was found to be associated with the intake of hydroxytyrosol. Oral bioavailability of hydroxytyrosol in olive oil and in an aqueous solution was reported as 99% and 75%, respectively. In comparative studies, urinary excretion of hydroxytyrosol was greater in humans than in rats. The LD(50) of the extract and hydroxytyrosol was reported to be greater than 2000 mg/kg. In a subchronic study, the no observed adverse effect level (NOAEL) of the extract in rats was found to be 2000 mg/kg/day. In developmental and reproductive toxicity studies, HIDROX did not cause toxicity at levels up to 2000 mg/kg/day. In an in vivo micronucleus assay, oral exposure of rats to HIDROX at dose levels up to 5000 mg/kg/day for 29 days did not induce increases in polychromatic erythrocytes in bone marrow. Based on the available studies of the extract and polyphenols, and a history of exposure and use of components of the extract through table olives, olive products and olive oil, the consumption of HIDROX is considered safe at levels up to 20 mg/kg/day.

Desensitization of melanotropin receptors in COS-7 cells.

Non-transfected COS-7 cells have been found to possess functional melanotropin receptors on their cell surface. These receptors, and the properties of the melanocyte stimulating hormone (MSH) peptides can be characterized by measuring melanotropin stimulation of cAMP accumulation in the cells. In these cells we studied the ultra-long lasting super agonist [Nle(4)-D-Phe(7)]-alpha-MSH (NDP-alpha-MSH), and compared it with the endogenous MSH peptides with respect to potency, maximal activity, duration of action, and rate of desensitization. Surprisingly, NDP-alpha-MSH did not act as a full agonist in COS-7 cells. In multiple experiments, it could stimulate cAMP accumulation to approximately 50% of the level of alpha-MSH, beta-MSH and adrenocorticotropic hormone (ACTH). The MSH receptor mediating this activity is unknown. The time course of cAMP accumulation, and the duration of receptor activation was also investigated. In contrast to other systems NDP-alpha-MSH did not induce prolonged activity, with respect to cAMP accumulation, in COS-7 cells. The MSH receptors present in COS-7 were found to desensitize rapidly subsequent to pretreatment by any of the MSH peptides. As expected for a partial agonist, the activity of NDP-alpha-MSH desensitized more rapidly than any of the full agonists. Surprisingly, desensitization induced by pretreatment with NDP-alpha-MSH also occurred more rapidly than desensitization induced by the other MSH analogs.

Dopamine metabolism in hypoxanthine-guanine phosphoribosyltransferase-deficient variants of PC12 cells.

Lesch-Nyhan syndrome results from a deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT). It is manifest by behavioral abnormalities, including self-mutilation, and evidence of abnormal 3,4-dihydroxyphenylethylamine (dopamine) metabolism. To assess whether an HPRT deficiency in a dopaminergic cell can adversely affect dopamine metabolism in that cell, dopamine metabolism was examined in HPRT-deficient variants of PC12 pheochromocytoma cells and in cells that had regained HPRT activity by virtue of transformation with a recombinant retrovirus containing the human gene for HPRT. There was no correlation between HPRT activity and endogenous dopamine levels, dopamine uptake, dopamine release, or monoamine oxidase activity. Transformation with the HPRT retrovirus did not adversely affect dopamine metabolism.

PC12 variants deficient in catecholamine transport.

We have isolated PC12 cell variants deficient in transporter-mediated uptake of 3,4-dihydroxyphenylethylamine (dopamine). The variants either were obtained nonselectively, or they were selected by resistance to guanethidine or N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Dopamine uptake into guanethidine-resistant cells occurred with a decreased Vmax; the Km for dopamine and inhibition by guanethidine were normal. MPTP-resistant cells lacked the capacity to take up dopamine. Most of the variants resembled wild-type PC12 in their response to nerve growth factor and the storage and secretion of dopamine. MPTP-resistant cells exhibited several deficiencies in addition to dopamine transport, i.e., no measurable storage of dopamine or acetylcholine and no observable response to nerve growth factor. Wild-type and variant cells were compared with respect to the labeling of cell proteins with [3H]xylamine, which binds covalently to certain proteins apparently only after entering PC12 via the catecholamine transporter. When intact variant cells were used, there was markedly reduced labeling of the proteins by [3H]xylamine. Almost all of these proteins were readily labeled when cell homogenates were exposed to [3H]xylamine. However, MPTP-resistant cells were missing three of these proteins. Northern blot analysis with cDNA clones revealed that the MPTP-resistant cells had markedly reduced levels of several specific mRNA species.

Differential modulation of cell death proteins in human brain cells by tumor necrosis factor alpha and platelet activating factor.

Programmed cell death contributes to the morbidity and mortality of several neurological disorders including stroke, Alzheimer's disease and human immunodeficiency virus (HIV)-associated dementia. Patients with HIV dementia show evidence of programmed cell death in brain. In vitro data demonstrates several neurotoxic products of macrophage infection that cause neural cell death, including tumor necrosis factor alpha (TNFalpha) and platelet activating factor (PAF). We treated human brain aggregate cultures with these cytokines and determined their effect on the mRNA and protein levels for Bcl-2, Bcl(x) and Bax alpha. TNFalpha and PAF differentially regulate the Bcl-2 family of proteins at a post-transcriptional level. Following TNFalpha treatment, Bcl-2 protein is significantly decreased, and at least one additional Bax isomer emerges. Bcl(xL) protein is slightly increased after treatment with either cytokine. We demonstrated that overexpression of Bcl-2 in brain aggregate cultures protects cells from TNFalpha-induced damage but has no effect on cell damage induced by PAF. We conclude that Bcl-2 and Bax alpha proteins play significant roles in modulating neural cell death from TNFalpha- but not from PAF-induced cell damage.

Studies on the structure and binding properties of the cysteine-rich domain of rat low affinity nerve growth factor receptor (p75NGFR).

The binding domain of p75NGFR contains four "repeats" of a 6-cysteine pattern. To test whether these repeats have any structural or functional independence, each repeat has been separately deleted. In each case, deletion led to the loss of most nerve growth factor (NGF) binding activity. The epitopes of two monoclonal antibodies, MC192 and 271c, could be distinguished, however. Repeat IV was found to be unnecessary for binding MC192, whereas Repeat I was not required for binding 217c. This suggests that either terminal repeat can be removed without loss of native-like structure in the remaining repeats. Trp155 in the fourth repeat forms an essential part of the 217c epitope but is not required for either MC192 or NGF binding. Deletion of the linker region between the membrane-spanning domain and the cysteine-rich domain does not affect the binding of NGF or MC192, and has only a slight, if any, effect on 217c binding. Cyclic permutation of the four repeats failed to yield protein capable of binding NGF or MC192.

Nerve growth factor binding domain of the nerve growth factor receptor.

A structural analysis of the rat low-affinity nerve growth factor (NGF) receptor was undertaken to define the NGF binding domain. Mutant NGF receptor DNA constructs were expressed in mouse fibroblasts or COS cells, and the ability of the mutant receptor to bind NGF was assayed. In the first mutant, all but 16 amino acid residues of the intracellular domain of the receptor were removed. This receptor bound NGF with a Kd comparable to that of the wild-type receptor. A second mutant contained only the four cysteine-rich sequences from the extracellular portion of the protein. This mutant was expressed in COS cells and the resultant protein was a secreted soluble form of the receptor that was able to bind NGF. Two N-terminal deletions, in which either the first cysteine-rich sequence of the first and part of the second cysteine-rich sequences were removed, bound NGF. However, a mutant lacking all four cysteine-rich sequences was unable to bind NGF. These results show that the four cysteine-rich sequences of the NGF receptor contain the NGF binding domain.

Molecular approaches to nerve regeneration.

Current research into regeneration of the nervous system has focused on defining the molecular events that occur during regeneration. One well-characterized system for studying nerve regeneration is the sciatic nerve of rat. Numerous studies have characterized the sequence of events that occur after a crush injury to the sciatic nerve (Cajal 1928; Hall 1989). These events include axon and myelin breakdown, changes in the permeability of the blood vessels, proliferation of Schwann cells, invasion of macrophages, and the phagocytosis of myelin fragments by Schwann cells and macrophages. The distal segment of the injured sciatic nerve provides a supportive environment for the regeneration of the nerve fibres (Cajal 1928; David & Aguayo 1981). Within a period of weeks, the injured sciatic nerve is able to regrow and successfully reinnervate the appropriate targets. Some of the molecules that provide trophic support for the regrowing nerve fibres have been identified, including nerve growth factor (NGF) (Heumann et al. 1987) and glial maturation factor beta (Bosch et al. 1989). Another class of molecules show changes in their rates of synthesis during regeneration, including both proteins (Skene & Shooter 1983; Muller et al. 1986) and mRNA species (Trapp et al. 1988; Meier et al. 1989). To better understand nerve regeneration, we have taken two, parallel molecular approaches to study the events associated with regeneration. The first of these is to study in detail the mechanism of action of a molecule that has been implicated in the regeneration process, nerve growth factor. The second approach is to identify novel gene sequences which are regulated during regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the low-affinity nerve growth factor receptor enhances beta-amyloid peptide toxicity.

The low-affinity nerve growth factor receptor (NGFR) p75NGFR induces apoptosis in the absence of nerve growth factor (NGF) binding but enhances neural survival when bound by NGF. Basal forebrain cholinergic neurons express the highest levels of p75NGFR in the adult human brain and are preferentially involved in Alzheimer disease, raising the question of whether there may be a functional relationship between the expression of p75NGFR and basal forebrain cholinergic neuronal degeneration in Alzheimer disease. The expression of p75NGFR by wild-type and mutant PC12 cells potentiated cell death induced by beta-amyloid peptide. NGF binding to p75NGFR inhibited the toxicity of beta-amyloid peptide, whereas NGF binding to TrkA, the high-affinity NGFR, enhanced it. These results suggest a possible link between beta-amyloid peptide toxicity and preferential degeneration of cells expressing p75NGFR.

A novel splice variant of the cell death-promoting protein BAX.

Cell death plays an important role in a number of physiological processes in all complex multicellular organisms. One of the molecules that regulates this process is BAX, an integral membrane protein, that promotes apoptosis. The function of BAX is countered by BCL-2 and BCL-XL. The differential expression of these proteins can influence the ability of the cell to die or survive. In this paper, we describe the cloning, biochemical, and functional characterization of a novel splice isoform of BAX, called BAX-omega. Transient overexpression of BAX-omega protein potentiates cell death at levels comparable to that of BAX-alpha overexpression.