Calcitonin gene-related peptide- and substance P-immunoreactive axons in the nucleus gracilis of the rat with special reference to axonal dystrophy: light and electron microscopic observations

Calcitonin gene-related peptide (CGRP) and substance P (SP)-immunoreactive (IR) axons in the nucleus gracilis of normal rats (1-15 months of age) were studied by light and electron microscopy. Besides many CGRP-IR and SP-IR varicosities with normal appearance, we found a few swollen (nearly round or oval) varicosities with either CGRP or SP immunoreactivity. Swollen CGRP-IR varicosities were more frequently seen than SP-IR ones, appearing from 3 months of age and increasing in number and size (up to approximately 25 microns in diameter) with advancing age. At the electron microscopic (EM) level, CGRP-IR and SP-IR swollen varicosities showed dystrophic changes, i.e., many membranous dense bodies, and proliferation of microtubules and neurofilaments. CGRP-IR or SP-IR dystrophic axons also contained many mitochondria and sometimes made synaptic contacts with nonreactive dendrites (occasionally with non-IR axons). These findings suggest that the dystrophic CGRP and SP axonal profiles represent a functionally distinct subpopulation of axonal dystrophy in the nucleus gracilis and use CGRP or SP as a neuroactive substance. Using a double-immunostaining method, many of normal CGRP-IR axons were identified to be SP-IR. However, no single dystrophic varicosity was found to contain both CGRP and SP immunoreactivities. These findings suggest that CGRP and SP afferents are independently affected and progress to dystrophic changes.

A new strategy for enhancing the stability of lyophilized protein: the effect of the reconstitution medium on keratinocyte growth factor

PURPOSE

Protein stabilization during lyophilization has previously focused on optimization of the formulation as well as the freezing and dehydration process parameters. However, the effect of the reconstitution medium has been largely neglected. We have investigated its effect on aggregate formation using recombinant keratinocyte growth factor (KGF).

METHODS

The protein was lyophilized under suboptimal conditions to induce aggregation and precipitation upon reconstitution with water. A series of additives were examined by UV spectrophotometry and size exclusion chromatography (SEC-HPLC) for their effects on decreasing the degree of KGF aggregation and precipitation by the increase in recovery of soluble monomer.

RESULTS

Several additives resulted in a significant reduction of aggregation, including sulfated polysaccharides, surfactants, polyphosphates, and amino acids. A similar effect was achieved by adjusting the ionic strength of the reconstitution medium. SEC-HPLC indicated that the amount of soluble monomer was also increased by these additives suggesting that the recovery of the soluble protein correlates with the native, monomeric protein.

CONCLUSIONS

These results suggest that optimization of reconstitution conditions will be a useful methodology for increasing the recovery of soluble, active proteins and that for KGF, the recovery of the soluble protein correlates with the native, monomeric form.

Stability of recombinant consensus interferon to air-jet and ultrasonic nebulization

The stability of recombinant consensus alpha-interferon (rConIFN) to air-jet and ultrasonic nebulization was evaluated. Volumes of 10 mL of 0.5 mg/mL rConIFN in phosphate-buffered saline (PBS) at pH 6.3 were nebulized with a Collison three-jet nebulizer at 40 psig (10 L/min) for up to 25 min. The effects of pH (3.0, 6.3, and 9.0), additive (0.1% w/v Tween 80, 0.1% w/v Tween 20, and 1% w/v PEG 8000), and ionic strength (0, 0.25, and 1.0) were examined. The effects of ultrasonic nebulization were studied using three devices (DeVilbiss "Aerosonic"; Mountain Medical "Microstat", and Medix "Easimist"). Stability of rConIFN was assessed by size exclusion chromatography and native and sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE). Structural effects were examined by circular dichroism spectroscopy and bioactivity was assessed by an in vitro mitogenic inhibition bioassay. rConIFN is destabilized by air-jet nebulization. Insoluble noncovalent aggregates are produced rapidly, and only approximately 25% of the initial monomeric protein remains after 25 min of nebulization. This correlates with a decrease in in vitro bioactivity. Aggregation during nebulization is influenced by pH (9.0 < 6.3 < 3.0) but even at the highest pH, > 25% aggregation is observed. Ionic strength does not appear to influence aggregation. rConIFN is also seen to adhere to glass after nebulization. Samples from a rinse of the emptied reservoir with 0.1% w/v SDS, after thorough rinsing with water (three times), show a strong rConIFN band on SDS-PAGE gels. The use of PEG 8000 and Tween mitigate aggregate formation and adhesion (< 20%). The cumulative output collected as a wet or dry aerosol is not aggregated to the same extent as the residual protein remaining in the nebulizer. Ultrasonic nebulization also results in aggregation, but the extent of denaturation is dependent upon the nebulizer used and is related to the heating of nebulizer solutions. Cooling of the nebulizer solution during operation (< 30 degrees C) minimizes aggregation (< 5%), and bioactivity is retained.

Optimization of lyophilization conditions for recombinant human interleukin-2 by dried-state conformational analysis using Fourier-transform infrared spectroscopy

PURPOSE

Examination of the dried-state conformation of interleukin-2 (IL-2) was used to determine the pH conditions and stabilizers that provide optimal storage stability for the lyophilized product.

METHODS

Fourier-transform infrared spectroscopy and accelerated stability studies which examined solubility, aggregate formation, and covalent cross-linking were used.

RESULTS

Varying the pH in the absence of excipients resulted in dramatic differences in the dried state conformation of IL-2. At pH 7, IL-2 unfolds extensively upon lyophilization while at pH below 5 it remains essentially native. Additional unfolding was observed upon incubation at elevated temperatures. A strong direct correlation between the retention of the native (aqueous) structure during freeze-drying and enhanced stability is demonstrated. IL-2 prepared at pH 5 is approximately an order of magnitude more stable than at pH 7 with regard to formation of soluble and insoluble aggregates. A similar pH profile was observed in the presence of excipients, although the excipients alter the overall stability profile. Additional accelerated stability studies examined the stabilizers necessary for optimal stability.

CONCLUSIONS

Excipients with the capacity to substitute for water upon dehydration better preserve the native structure resulting in enhanced stability. Those that have high glass transition temperatures provide the highest level of stability during storage, although they do not prevent dehydration induced unfolding.

Idiopathic colonic phlebitis with massive ascites

We describe a 35-year-old woman with colonic phlebitis of unknown origin accompanied by effusion of serum protein into the peritoneal cavity. Abdominal ultrasonography and computed tomography showed massive ascites and edematous wall thickness of the colon. Laboratory examination of the peritoneal fluid showed a high concentration of protein, probably due to nonselective efflux of serum protein. The main histopathological finding was extensive edema of the submucosa with vasculitis in the colon. This kind of phlebitis with massive ascites, but without systemic involvement, and with the ascitic fluid almost identical to the serum protein level, has not been reported previously.

The importance of Arg40 and 45 in the mitogenic activity and structural stability of basic fibroblast growth factor: effects of acidic amino acid substitutions

High-affinity binding of basic fibroblast growth factor (bFGF) to the tyrosine kinase receptor requires cell-surface heparan sulfate proteoglycan or exogenous addition of heparin. The crystal structure of bFGF shows Arg40 and 45 on the surface opposite to the heparin-binding region, suggesting that these charged residues may be involved in the receptor binding. Therefore, these amino acids were mutated to aspartic acid separately or simultaneously, and also a simultaneous mutation to glutamic acid was introduced. These mutants displayed a mitogenic activity decreased greater than tenfold compared to the wild-type protein. Addition of heparin had no effect on the activity, while these mutants showed heparin-binding characteristics resembling those of the native sequence protein. The mutants exhibited decreased stability compared to the native sequence protein. Gradual changes in conformation were observed by circular dichroic and infrared spectroscopy. Heparin chromatography also showed the presence of denatured form for these mutants. However, in the presence of multivalent anions such as citrate, sucrose octasulfate, and heparin, the conformation of the mutants resembled that of the wild-type protein, as revealed by X-ray crystallography and circular dichroism spectra of the mutant with a Arg40-->Asp substitution.

Zinc deficiency delays gastric ulcer healing in rats

Zinc is an important element in wound healing. Zinc compounds hasten the healing of gastric ulcers, by an unknown mechanism(s). We studied the effect of the induction of zinc deficiency on gastric ulcer healing. Rats were given a control or zinc-deficient diet for six weeks and then subjected to the induction of acetic acid-induced chronic gastric ulcers. Four days later, zinc-deficient rats were divided into two groups. In the first group, the zinc-deficient diet was continued. In the second group, the diet was changed to the control diet. Zinc-deficient rats had a mean serum zinc concentration approximately 70% of that in controls. Zinc deficiency did not affect the formation of gastric ulcers; however, it reduced cell proliferation by day 4 and delayed ulcer healing. Zinc supplementation brought zinc to control levels within a week, but failed to reverse the delay in ulcer healing. We conclude that zinc is crucial for healing of gastric ulcers, especially at the early stage.

Antimicrobial response of a T cell-deficient host to cytokine therapy: effect of interferon-gamma in experimental visceral leishmaniasis in nude mice

To determine if interferon (IFN)-gamma can enhance intracellular antimicrobial defense in a T cell-deficient host, nude BALB/c mice were infected with Leishmania donovani and treated with IFN-gamma. IFN-gamma induced killing of L. donovani in livers of euthymic mice but had no effect in nude mice despite activating peritoneal macrophages in vivo. Transfer of CD4+ or CD8+ T cells permitted nude mice to respond to IFN-gamma; treatment with T cell-regulated antileishmanial cytokines (interleukin-2, granulocyte-macrophage colony-stimulating factor, or tumor necrosis factor-alpha) could not substitute for T cells. NK cells played no apparent role. In reconstituted nude mice, the antileishmanial effect of IFN-gamma correlated with markedly enhanced mononuclear cell recruitment to infected liver foci. Thus, although IFN-gamma activates macrophages in the absence of host T cells, a T cell mechanism is required for antileishmanial activity in tissue. Provided one T cell subset is adequately preserved, IFN-gamma may prove useful in intracellular infections in the T cell-deficient host.

The transcriptional regulation of human arachidonate 12-lipoxygenase gene by NF kappa B/Rel

As examined by the luciferase assay, a negative regulatory region including the NF kappa B motif was found in the 5'-flanking region of the 12-lipoxygenase gene in human erythroleukemia cells. The negative control was abolished by a site-specific mutation of the NF kappa B motif. Probes including the NF kappa B region gave positive bands upon a gel-shift assay. The bands were super-shifted by antibodies for NF kappa B p50, NF kappa B p65 and c-Rel, and were lost by a NF kappa B competitor DNA. Furthermore, the NF kappa B sequence was protected in DNase I footprinting. Thus, two kinds of heterodimer (p50 and p65; p50 and c-Rel) seemed to control the over-expression of the human 12-lipoxygenase gene.

Insulin nonattenuation of vasoactive agent-induced responses in mesangial cells from spontaneously hypertensive rats

We recently found that insulin attenuates intracellular calcium transients and cell contraction caused by vasoactive agents in cultured rat mesangial cells. Because altered glomerular function may be causally related to the evolution of hypertension, we examined in the present study the effects of insulin on the functions of mesangial cells derived from spontaneously hypertensive rats (SHR) of 4- and 8-weeks of age. Age-matched Wistar Kyoto rats (WKY) were used as controls. Intracellular calcium concentration ([Ca2+]i) was measured with Fura-2 method in suspended mesangial cells. Pretreatment of mesangial cells with 5 micrograms/ml insulin for 120 minutes did not affect basal [Ca2+]i in either WKY or SHR mesangial cells. However, insulin pretreatment significantly attenuated [Ca2+]i transients to vasoactive agents in WKY mesangial cells. In contrast, [Ca2+]i transients to these agents were not attenuated by insulin in SHR mesangial cells. Additionally, SHR mesangial cell contraction in response to angiotensin II (Ang II) was not altered by insulin, while WKY mesangial cell contraction to Ang II was, as in normal Wistar rats, significantly reduced by insulin. Since we previously showed the possibility that the attenuation of calcium signal by insulin is via insulin-like growth factor I (IGF-I) receptor, we also examined the effect of IGF-I. In contrast to WKY mesangial cells, IGF-I-induced attenuation of [Ca2+]i responses to platelet activating factor was absent in SHR mesangial cells. [125I]-IGF-I binding in SHR mesangial cells was not significantly different from that in WKY mesangial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and activity of granulocyte colony-stimulating factor derived from CHO cells containing cDNA coding for alternatively spliced sequences

Two different cDNAs have been isolated, coding for two forms of granulocyte colony-stimulating factor (G-CSF): one for a polypeptide of 174 amino acids and the other for a polypeptide of 177 amino acids. In this paper, we have expressed these two forms in Chinese hamster ovary cells and characterized the purified proteins for activity and conformation. In vitro mitogenic assay showed a 50-fold lower activity for the 177 form than for the 174 form. In vitro receptor binding assay showed that binding of the 177 form to the purified extracellular domain of G-CSF receptor was also diminished, while the 174 form complexed with the receptor. Circular dichroic spectra showed that both forms are similar in the secondary structure, but are slightly different in the tertiary structure. Infrared spectra also showed a slight difference between the two forms. Both techniques also demonstrated differences in stability; i.e., the 174 form is more stable than the 177 form during storage or against heat denaturation.

High level expression of human leukemia inhibitory factor (LIF) from a synthetic gene in Escherichia coli and the physical and biological characterization of the protein

LIF is a multi-functional cytokine that elicits effects on a broad range of cell types. In this report, we present the high level expression of human LIF (hLIF) from a chemically synthesized gene template in Escherichia coli where it comprises up to 25% of the cellular protein. The recombinant hLIF, after purification and folding, was examined using CD, FTIR spectroscopy and light scattering. CD and FTIR spectra showed that the hLIF is an alpha-helical protein and has a distinct tertiary structure. The IFTR spectrum resembles that of other four helical bundle proteins including G-CSF and IL-6. Light scattering analysis indicated that it is a monomeric protein, distinguishing it from M-CSF and interferon gamma, which also belong to the class of four helical bundle proteins but are dimeric. Recombinant hLIF was assayed for its activity on the murine leukemic cell line, M-1 as well as on human leukemic cell line, ML-1. It inhibited the growth of M-1 cells and differentiated them towards macrophages. However, it did not have any differentiation inducing effect on human leukemic cell lines alone or in combination with other cytokines.

Possible involvement of an enzymatic system for superoxide generation in lepidopteran larval haemolymph

The superoxide generative system in Pseudaletia separata larval haemolymph plasma was shown to consist of at least two components, low (LMF) and high (HMF) molecular weight factors using ultrafiltration, dialysis, and gel filtration. The LMF was concluded to be a molecule(s) smaller than 5 kDa while the HMF to be a protein(s) larger than 100 kDa. The total amount of superoxide produced depended on the amount of LMF and was independent of that of HMF added to the reaction mixture. Both the LMF and HMF were required for O2- generation. From results in the present article, it was hypothesized that the LMF was a substrate(s) discharging electrons and HMF was an enzyme(s) to mediate the electron transfer to O2 forming O2-. Superoxide production was detected in the haemolymph plasma of 5 lepidopteran species in addition to P. separata. It was concluded that superoxide production is a common phenomenon at least in these lepidopterans.

Effects of lansoprazole on ethanol-induced injury and PG synthetic activity in rat gastric mucosa

Lansoprazole, a proton pump inhibitor, has been reported to inhibit ethanol-induced injury in rats. However, the mechanism of the protective effect is not known. This study was carried out to investigate the role of prostaglandin (PG) in the protective effect of lansoprazole. Male Wistar rats were given either 30 mg/kg of lansoprazole or vehicle alone intragastrically 30 min before administration of ethanol. They were killed to measure the gross mucosal lesions in the stomach as the ulcer index. In another experiment, 5 mg/kg of indomethacin was injected 30 min before administration of lansoprazole. The effects of 16,16-dimethyl PGE2 (dmPGE2) on ethanol-induced injury over time were compared with lansoprazole. PGE2 synthesis in gastric mucosa after administration of lansoprazole was measured by radioimmunoassay. Lansoprazole reduced gastric mucosal injury by lansoprazole. DmPGE2 significantly reduced gastric mucosal injury from 5 min after exposure to ethanol. Lansoprazole did not affect PGE2 synthesis in the gastric mucosa. These results suggest that PG may not be involved in the protective effect of lansoprazole.

Indomethacin interferes with epidermal growth factor binding and proliferative response of gastric KATO III cells

Indomethacin induces gastric ulcerations and decreases cell proliferation in the gastric ulcer margin. Since epithelial cell proliferation is under control of epidermal growth factor (EGF), we studied whether indomethacin may affect specific binding of [125I]-EGF to its receptors in cultured human gastric KATO III cells. To assess effects of EGF, indomethacin and their combination on cell proliferation, KATO III cells were incubated for 24 h with either (a) vehicle (b) indomethacin (doses from 10(-5) to 10(-3) M), EGF (doses 0.01, 0.05 or 0.1 microgram/ml) or (d) a combination of b and c, and the bromodeoxyuridine labeling index was determined. Indomethacin in a dose which did not affect cell viability significantly (by 21.5%) decreased [125I]-EGF binding to the KATO III cells and decreased the bromodeoxyuridine labeling index. Epidermal growth factor significantly increased cell proliferation and increased the labeling index from 28.9 +/- 0.6% in the vehicle group to 36.2 +/- 0.5%. Co-treatment with indomethacin significantly reduced the proliferative response of KATO III cells to EGF. In conclusion, indomethacin, in a dose which does not affect cell viability, decreased binding of EGF to cultured gastric KATO III cells and decreased their proliferative response to EGF.

Role of extracellular matrix in attachment, migration, and repair of wounded rabbit cultured gastric cells

The aim of this study was to determine whether extracellular matrix components affect attachment, proliferation, migration, and repair of wounded rabbit gastric cells in primary culture. Gastric cells were inoculated on culture plates coated without or with collagen type I, type IV, laminin, or fibronectin. The number of attached cells was counted at 3, 5, 24, 48, and 72 h after inoculation. Cell proliferation was assessed with [3H]thymidine incorporation. To assess cell migration and repair after wounding, a confluent monolayer was cut in a standardized manner and the size of the wound area was measured with a computerized analysis system. The number of attached cells increased significantly in plates coated with the extracellular matrix components within 3 h after inoculation. Collagen type I was the most potent stimulator of attachment. Extracellular matrix components did not affect thymidine incorporation. Wound healing was significantly accelerated in the plates coated with collagen type I, type IV, and laminin (70.2 +/- 1.0%, 68.0 +/- 1.0%, and 69.2 +/- 0.9% of initial wound area, respectively, vs. 74.0 +/- 0.6% of initial wound area in controls). Extracellular matrix components play an important role in attachment, migration, and repair of wounded cultured gastric epithelial cells.

Some factors associated with the ultrasonic nebulization of proteins

Ultrasonic nebulization of lactate dehydrogenase (LDH) was investigated using a DeVilbiss "Aerosonic" nebulizer. The enzyme (8ml, 0.025 mg/ml Na2HPO4, pH 7.0) was completely inactivated after 20 minutes of operation. However, the inactivation profile observed during ultrasonic nebulization was different from that previously observed using air-jet nebulization. At least two mechanisms are involved, one associated with heating and the other with aerosol production. By preventing heating of the nebulizer fluid during operation, the denaturation profile was dramatically altered. By additionally including 0.01% w/v Tween 80 or 1% w/v PEG 8000, almost all activity was retained. Similar results were obtained by preventing aerosol production and heating. However, 100% of activity was lost when heating was allowed to occur without aerosol formation. The results demonstrate that cooling in conjunction with a surfactant is one approach that could be used to stabilize proteins to ultrasonic nebulization. However, cooling also significantly reduced solute output from the nebulizer. When operated at 10 degrees C output was negligible. At 50 degrees C the output was 5x greater than that found at room temperature. The median droplet size (micron(s)) was not significantly influenced by the operating temperature of the nebulizer fluid (3.6 +/- 0.4, 21 degrees C; 3.9 +/- 0.2, 50 degrees C, p = NS (n = 6)) although the size distribution was noted to increase at the higher temperature.

Contribution of the surface free energy perturbation to protein-solvent interactions

Surface tension measurements were carried out at 20 degrees C by a capillary drop-weight method on aqueous solutions of sodium glutamate (NaGlu), lysine hydrochloride (LysHCl), potassium aspartate (KAsp), arginine hydrochloride (ArgHCl), lysylglutamate (LysGlu), argininylglutamate (ArgGlu), guanidinium sulfate, trehalose, trimethylamine N-oxide (TMAO), dimethyl sulfoxide, 2-methyl-2,4-pentanediol (hexylene glycol), and poly(ethylene glycol)s of molecular weights 200, 400, 600, and 1000. All of the salts and the sugar increased the surface tension of water, while the last four compounds decreased it, with 2-methyl-2,4-pentanediol lowering it most effectively and TMAO being the least effective. The preferential hydration of bovine serum albumin (BSA) and lysozyme was measured in KAsp, ArgHCl, LysGlu, and ArgGlu. The high values of preferential hydration found in all cases, except for BSA in ArgHCl, suggest that they should stabilize protein structure, as had been found for lysine hydrochloride and monosodium glutamate [Arakawa, T., & Timasheff, S. N. (1984) J. Biol. Chem. 259, 4979-4986]. A correlation was found for both BSA and lysozyme in KAsp, NaGlu, LysHCl, ArgGlu, and LysGlu between the surface tension effect and the observed preferential interactions, indicating that the change in the surface free energy of the protein-containing cavity due to the surface tension increase for water by these amino acid salts contributes dominantly to the observed increase in the chemical potential of the protein by their addition. The lack of a correlation observed for BSA, but not lysozyme, in ArgHCl at low concentrations where preferential binding is close to zero suggests, however, that the surface tension effect is not the sole factor involved in the protein-solvent interactions in these amino acid salts. Binding of ArgHCl to BSA, probably through hydrogen bonds between the Arg guanidinium group and peptide bonds, was proposed to occur, the affinity of Arg+ being reduced by electrostatic repulsion when proteins carry a net positive charge, such as is the case with lysozyme. Since the four organic solvent additives also lead to protein preferential hydration, no correlation exists between their preferential interactions and the surface free energy perturbation. Therefore, in their case, the preferential hydration must be ascribed to other factors that overcome the preferential binding expected from the Gibbs adsorption isotherm. The surface tension results, however, are consistent with the binding of the organic solvents to proteins through hydrophobic interactions, explaining, at least in part, the observed concentration dependence of the interactions.

Strategies to suppress aggregation of recombinant keratinocyte growth factor during liquid formulation development

Recombinant human keratinocyte growth factor (rhKGF) is a fairly unstable protein, posing a challenging problem for long-term storage. During storage, the protein unfolds at relatively low temperatures and the unfolded proteins aggregate rapidly, leading to the formation of large visible precipitates. Thermal unfolding of rhKGF displays a similar pattern, i.e., unfolding is followed immediately by aggregation as the temperature is increased. As the unfolding and aggregation (precipitation) of rhKGF limit the storage life of the protein, a search for stabilizers to suppress rhKGF unfolding and aggregation has been done by examining the effects of excipients on thermal melting temperature and on the rate of protein aggregation during storage. Sulfated polysaccharides and citrate are found to be effective in increasing the melting temperature of rhKGF or preventing its aggregation. In particular, 0.5% (w/v) heparin and high molecular weight dextran sulfate, and 0.5 M citrate are highly effective, decreasing the rates of rhKGF aggregation by about 50-fold. Other negatively charged small ions, such as phosphate, also have moderate stabilizing effects on rhKGF. A mechanistic study of the aggregation pathway of rhKGF has led to a better understanding of the stabilizing effects of these molecules. Molecules which enhance rhKGF conformational stability are capable of effectively suppressing rhKGF aggregation.

Interactions of neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and the NT-3.BDNF heterodimer with the extracellular domains of the TrkB and TrkC receptors

Interactions of three neurotrophin dimers, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and a NT-3.BDNF heterodimer with extracellular, soluble TrkB and TrkC receptors were studied using native gels, light scattering, and sedimentation equilibrium. These three neurotrophins showed binding of two TrkB receptors per neurotrophin dimer, with a tendency to dissociate into one TrkB per dimer for NT-3 and the heterodimer, as determined by native gels, light scattering, and sedimentation equilibrium. For TrkC, native gels suggested binding of NT-3, heterodimer, and BDNF but not of nerve growth factor. Sedimentation equilibrium revealed that all three neurotrophin molecules bind to TrkC at two receptors per dimer but that BDNF binds much more weakly and that the heterodimer has an intermediate binding strength. Light scattering/size exclusion chromatography showed complexes with two TrkC receptors per NT-3 dimer and one TrkC per heterodimer but did not detect binding of BDNF to TrkC. This latter result is not inconsistent with the sedimentation data, because the weak binding of BDNF to TrkC may be easily dissociated by nonspecific interactions of BDNF with the size exclusion column. The relative binding constants for these neurotrophins and the soluble receptor extracellular domains, as determined by sedimentation equilibrium, are correlated with their biological activity. However, the magnitude of these binding constants is insufficient by approximately 3 orders of magnitude to promote receptor dimerization at physiologically active concentrations.

Formation of heterodimers from three neurotrophins, nerve growth factor, neurotrophin-3, and brain-derived neurotrophic factor

Three neurotrophic factors, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and nerve growth factor (NGF) form noncovalent homodimers in solution. Since they are highly homologous proteins, it seemed probable that two monomers of these proteins might associate together to form a heterodimer. This was tested by denaturing the two different proteins together in 6 M guanidine HCl and refolding them in phosphate-buffered saline. When the refolded mixture of BDNF and NT-3 was subjected to Mono S cation exchange chromatography, a new peak was observed eluting between NT-3 and BDNF, which accounted for about 30% of the protein used. This new protein species migrated as a single band upon native gel electrophoresis with mobility between that of the NT-3 homodimer and the BDNF homodimer, indicating that a complex had been formed. Sedimentation equilibrium data show that the dissociation constant of this heterodimer is < 3 x 10(-10) M. The heterodimer was stable upon incubation at 37 degrees C in phosphate-buffered saline over 11 days. Having determined that the heterodimer is highly stable, it was subjected to various biological assays. Autophosphorylation assay using TrkB receptor showed that the heterodimer is indistinguishable from the BDNF or NT-3 homodimer in the ability to induce phosphorylation of the receptor. It was also indistinguishable from the homodimers in the neurotrophic activity using chick dorsal root ganglion explant. In the sympathetic neuron survival assay, the heterodimer behaved more similarly to NT-3, whereas in the dopamine uptake assay, it was intermediate between the two homodimers. In addition, the heterodimer was shown to be retrogradely transported in the dorsal root ganglion neurons. A heterodimer between NGF and BDNF is formed but much less effectively than the NT-3.BDNF heterodimer, and it is not stable even at 4 degrees C. These results indicate that BDNF and NT-3 have an intersubunit contact surface for dimerization resembling each other's but different from the contact surface of NGF.