Louis E. Burton

The neurotrophic factors brain-derived neurotrophic factor and neurotrophin-3 are ligands for the trkB tyrosine kinase receptor

Neurotrophic factors are essential for neuronal survival and function. Recent data have demonstrated that the product of the tyrosine kinase trk proto-oncogene binds NGF and is a component of the high affinity NGF receptor. Analysis of the trkB gene product, gp145trkB, in NIH 3T3 cells indicates that this tyrosine kinase receptor is rapidly phosphorylated on tyrosine residues upon exposure to the NGF-related neurotrophic factors BDNF and NT-3. Furthermore, gp145trkB specifically binds BDNF and NT-3 in NIH 3T3 cells and in hippocampal cells, but does not bind NGF. Thus, the trk family of receptors are likely to be important signal transducers of NGF-related trophic signals in the formation and maintenance of neuronal circuits.

Recombinant human nerve growth factor prevents retrograde degeneration of axotomized basal forebrain cholinergic neurons in the rat

Cholinergic neurons in the basal forebrain magnocellular complex (BFMC) respond to nerve growth factor (NGF) during development and in adult life, and it has been suggested that the administration of NGF might ameliorate some of the abnormalities that occur in neurological disorders associated with degeneration of this population of neurons. A prerequisite for the introduction of NGF in clinical trials is the availability of active recombinant human NGF (rhNGF). The present investigation was designed to test, in vivo, the efficacy of a preparation of rhNGF. Axons of cholinergic neurons of the BFMC in the rat were transected in the fimbria-fornix; this manipulation alters the phenotype and, eventually, causes retrograde degeneration of these neurons. Our investigation utilized two lesion paradigms (resection and partial transection of fibers in the fimbria-fornix), two different strains of rats, and two delivery systems. Following lesions, animals were allowed to survive for 2 weeks, during which time one group received intraventricular mouse NGF (mNGF), a second group received rhNGF, and a third group received vehicle alone. In animals receiving vehicle, there was a significant reduction in the number (resection: 70%; transection: 50%) and some reduction in size of choline acetyltransferase- or NGF receptor-immunoreactive cell bodies within the medial septal nucleus ipsilateral to the lesion. Treatment with either mNGF or rhNGF completely prevented these alterations in the number and size of cholinergic neurons. The rhNGF was shown to be equivalent in efficacy with mNGF. Thus, rhNGF is effective in preventing axotomy-induced degenerative changes in cholinergic neurons of the BFMC. Our results, taken together with the in vitro effects of rhNGF (42), indicate that an active rhNGF is now available for further in vivo studies in rodents and primates with experimentally induced or age-associated lesions of basal forebrain cholinergic neurons. These investigations provide essential information for the consideration of future utilization of rhNGF for treatment of human neurological disorders, including Alzheimers disease.

K‐252b Selectively Potentiates Cellular Actions and trk Tyrosine Phosphorylation Mediated by Neurotrophin‐3

Abstract: K‐252b, a protein kinase inhibitor, has been shown earlier to inhibit nerve growth factor actions on cholinergic neurons of the basal forebrain. In the present study, K‐252b was found to prevent trophic actions of two other neurotrophins, brain‐derived neurotrophic factor, and neurotrophin‐3, on central cholinergic and dopaminergic neurons, peripheral sensory neurons, and PC 12 pheochromocytoma cells, when used at >2 μM concentration. Comparable actions of nonneurotrophin growth factors were not affected. Surprisingly, at 0.1‐100 nM, K‐252b selectively enhanced the trophic action of neurotrophin‐3 on central cholinergic neurons, peripheral sensory neurons, and PC 12 cells. In PC 12 cells, K‐252b potentiated the neurotrophin‐3‐induced tyrosine phosphorylation of trk, a protein kinase responsible for transmitting neurotrophin signals. Of the three structurally related nerve growth factor inhibitors, K‐252a, K‐252b, and staurosporine, only the first two also mediated neurotrophin‐3 potentiation. These findings indicate that K‐252b generally and selectively potentiates the neurotrophic action of neurotrophin‐3 and suggest that this action involves trk‐type neurotrophin receptors.

Trophic actions of recombinant human nerve growth factor on cultured rat embryonic CNS cells

NGF is a neurotrophic factor for basal forebrain cholinergic neurons and may serve to counteract the cholinergic deficits that are observed in Alzheimers disease. Prior to the introduction of clinical trials, it is essential that recombinant human NGF (rhNGF) be produced and that its actions on target cells in the CNS be demonstrated. We prepared rhNGF and examined its actions on fetal rat brain neurons in culture including, in particular, the cholinergic neurons of the basal forebrain. rhNGF was more potent in increasing choline acetyltransferase (ChAT) activity in septal cultures than NGF purified from mouse salivary glands (mNGF). ED50s of the beta-NGF dimers were 4.9 pM for rhNGF and 12.4 pM for mNGF. The maximal ChAT activity response was achieved at approximately 35 pM with both NGFs and their efficacies were not significantly different. The two NGFs were not additive in effect. Identical to the results with mNGF, rhNGF strongly enhanced the intensity of ChAT immunostaining in septal cultures. Neither rhNGF nor mNGF affected the appearance of the cultures under phase-contrast illumination. Survival of cells at very low plating density on polyornithine/laminin-coated culture dishes was not affected by rhNGF or mNGF. Protein content and the uptake of GABA were also unaffected. At concentrations of up to 10 micrograms/ml, rhNGF did not significantly increase uptake of dopamine into cultures of ventral mesencephalon. We conclude that rhNGF produces potent and selective actions on cholinergic neurons of the basal forebrain as previously shown for mNGF.

Purification and characterization of recombinant human activin B

Recombinant human activin B has been isolated to more than 95% purity from a mammalian kidney cell line. Activin B is a covalently-linked homodimer with an apparent molecular mass of 25.9 kDa (unreduced) and 15.2 kDa (reduced) as determined by SDS-polyacrylamide-gel electrophoresis. On gel filtration in 6 M guanidine hydrochloride, activin B chromatographs with an apparent molecular mass of 11 kDa, whether reduced or not. The amino-terminal sequence of the purified protein is consistent with the expected sequence derived from the beta subunit of inhibin B. The amino acid composition of the purified molecule agrees with the expected theoretical composition of the beta subunit of inhibin B. Activin B has an apparent pI of 4.6 as determined by isoelectric focusing in 6 M urea and 4.7 as determined by chromatofocusing in 6 M urea. The extinction coefficient is 1.8.

Biochemical Characterization of Recombinant Human Nerve Growth Factor

Abstract: Recombinant human nerve growth factor (rhNGF) was expressed and secreted by Chinese hamster ovary cells and purified to homogeneity using ion‐exchange and reversed‐phase (RP) chromatography. The isolated product was shown to be consistent with a 120‐amino‐acid residue polypeptide chain by amino acid composition, sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE), RP‐HPLC, and mass spectrometry and with an N‐terminal sequence consistent with that expected from the cDNA for human nerve growth factor. By size‐exclusion chromatography, rhNGF behaves like a noncovalent dimer. Limited enzymatic digests of the 120‐residue monomer produced additional species of 118 (trypsin, removal of the C‐terminal Arg119‐Ala120 sequence) and 117 (trypsin plus carboxypeptidase B, removal of the C‐terminal Arg118‐Arg119‐Ala120 sequence) residues. Each of these species was isolated by high‐performance ion‐exchange chromatography and characterized by amino acid and N‐terminal sequence analyses, SDS‐PAGE, RP‐HPLC, and mass spectrometry. All three species were present in the digests as both homodimeric and heterodimeric combinations and found to be equipotent in both the chick dorsal root ganglion cell survival and rat pheochromocytoma neurite extension assays.

Activity and Biospecificity of Proteolyzed Forms and Dimeric Combinations of Recombinant Human and Murine Nerve Growth Factor

Abstract: Purified recombinant human nerve growth factor (rhNGF) and submaxillary gland‐derived murine NGF (muNGF) were characterized by amino acid composition, polyacrylamide gel electrophoresis (PAGE), reversed‐phase HPLC (RP‐HPLC), and high‐performance ion‐exchange chromatography (HPIEC). Limited tryptic digest of the N and C termini of the 120‐residue form of rhNGF produced a species of 109 residues (10–118). The previously observed natural murine analogue of this variant, muNGF lacking the first eight N‐terminal amino acids, was also isolated as a homodimer. Both species were purified using HPIEC and characterized by amino acid analysis, N‐terminal sequence, PAGE, and RP‐HPLC analysis. Each of the four homodimeric species was evaluated in some or all of the following biological assays for NGF: chick dorsal root and sympathetic ganglion assays and rat pheochromocytoma‐12 cell line neurite extension assay. The 118‐residue homodimeric versions of both rhNGF and muNGF displayed equivalent bioactivity, whereas the N terminal‐modified molecules presented activity reduced by 50‐ to 100‐fold. Utilizing HPIEC, we have examined the ability of the monomeric forms of any two of the homogeneous dimeric species of rhNGF to recombine. We have shown that not only can all of the previously described species form dimers by recombination, but an interspecies dimer can be created between muNGF and rhNGF.

Purification and partial characterization of recombinant human differentiation-stimulating factor

Recombinant human differentiation-stimulating factor (rhD-factor) has been isolated to greater than 95% purity from Chinese hamster ovary cells. RhD-factor is a glycoprotein with an apparent molecular weight of 45.6 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. On gel filtration in 6 M guanidine-hydrochloride, rhD-factor elutes with an apparent molecular weight of 21.5 kDa; it elutes with an apparent molecular weight of 44.8 kDa under neutral pH (native) conditions. The amino-terminal sequence (12 residues) is consistent with the expected sequence derived from the genomic DNA sequence. Recombinant D-factor is heavily glycosylated with 30% by weight neutral sugar and 12% sialic acid. The ED50 for rhD-factor was 0.25 ng/ml. Trifluoromethanesulfonic acid-deglycosylated rhD-factor has a biological activity comparable to that of the native recombinant protein (ED50 = 0.40 ng/ml). The biological activity of rhD-factor was stable at pH 1 for 40 h, in 6 M guanidine-HCl containing buffers with or without reducing agent, and in 1% SDS. Carboxymethylation of D-factor after reduction totally destroyed biological activity.

Crystallization and preliminary characterization of three crystal forms of human recombinant transforming growth factor-α

Three crystal forms of human recombinant TGF-α have been grown from solutions containing 2-methyl-2,4-pentanediol. One of the forms belongs to the orthorhombic space group C2221 and the other two belong to the monoclinic space group C2. Two of the crystal forms diffract to approximately 2.3A Bragg spacings. X-ray diffraction data has been collected for all three forms. These data appear to be suitable for crystal structure determination, using either heavy atom isomorphous replacement methods or molecular replacement, for phase determination.

Crystallization and preliminary X-ray diffraction studies of recombinant rabbit interferon-gamma

Two different crystal forms of recombinant rabbit IFN-gamma were obtained under different crystallization conditions. The first, a tetragonal form with space group P43212 or P41212, was obtained through vapor phase equilibration using the sitting drop rods technique with ammonium citrate as the major precipitating agent. The unit cell dimensions of this crystal form are a = b = 82.1 A and C = 116.3 A. These crystals diffract to 2.8 A resolution and contain a dimer in the asymmetric unit. A second crystal form was obtained by the batch method at pH 8.0 using sodium chloride as the precipitating agent. The crystals are hexagonal, space group P6122 or P6522, and with unit cell dimensions of a = b = 58.0 A and c = 169 A. This form contains monomer in the asymmetric unit and diffracts to greater than 2.7 A resolution. Both forms appear to be eminently suitable for further analyses and crystal structure solution.