High affinity, specific binding of [125I] beta nerve growth factor to glass beads

Reversible sedimentation and masking of nerve growth factor (NGF) antigen by high molecular weight fractions from rat brain

Nerve growth factor (NGF) was recently found to be largely associated with sedimentable fractions of adult rat brain and treatments of the fractions by alkaline pH increased the measurable amount of their NGF antigen as well as its solubilization [M.C. Hoener, E. Hewitt, J.M. Conner, J.W. Costello and S. Varon, Nerve growth factor (NGF) content in adult rat brain tissues is several-fold higher than generally reported and is largely associated with sedimentable fractions, Brain Res., 728 (1996) 47-56; M.C. Hoener and S. Varon, Effects of sodium chloride, Triton X-100, and alkaline pH on the measurable contents and sedimentability of the nerve growth factor (NGF) antigen in adult rat hippocampal tissue extracts, J. Neurosci. Res., in press (1997); C. Zettler, D.C.McL. Bridges, X.-F. Zhou and R.A. Rush, Detection of increased tissue concentrations of nerve growth factor with improved extraction procedure, J. Neurosci. Res., 46 (1996) 581-594]. We have further investigated the reversibility of these pH effects. Reversal of the pH of an adult rat hippocampal tissue extract from 10.5 to 7.4 led to an almost complete transfer of NGF back from nonsedimentable to sedimentable fractions and to a remasking of the previously unmasked portion of NGF antigen. Thus, molecules causing masking and sedimentation of NGF at pH 7.4 were likely to be present in the alkaline extract. A gel filtration column in PBS, pH 10.5 was used to separate such putative binding molecules from the NGF. All of the NGF antigen from rat hippocampal alkaline extract was found to elute with 19 kDa fractions. The same apparent molecular weight was found for mouse submaxillary beta-NGF and recombinant human beta-NGF. Masking and sedimentation no longer occurred when newly generated 19 kDa rat brain NGF was returned to pH 7.4. When high molecular weight fractions derived from the same gel filtration (in PBS, pH 10.5) were added back to the 19 kDa NGF pool at pH 7.4 and the mixture incubated and centrifuged, the measurability of 19 kDa rat brain NGF antigen was markedly reduced and half of the antigen was recovered in sedimentable fractions. Similar but less dramatic results were obtained when mixing the same high molecular weight fractions with 19 kDa mouse or human beta-NGF. These findings provide new opportunities to identify molecules to which NGF may be bound within intact brain tissues.

Interaction of nerve growth factor with murine alpha-macroglobulin

The murine nerve growth factor, when injected i.v. or, combined in vitro with plasma, was found largely associated with the mouse alpha-macroglobulin (a homologue of human alpha 2-macroglobulin). The nerve growth factor-alpha-macroglobulin complex produced is sufficiently stable to resist separation by gel filtration in 1.0 M sodium chloride, polyacrylamide gel electrophoresis, and immunoprecipitation by antibodies against alpha-macroglobulin. As determined by equilibrium binding studies and computer generated Scatchard analysis, alpha-macroglobulin apparently possesses two types of binding sites with the apparent dissociation constants of 1.2 x 10(-6) and 2.9 x 10(-9) M, respectively, saturable by 3.7 and 0.03 moles of nerve growth factor. Hence, about one mole of nerve growth factor is bound to each of the four subunits of alpha-macroglobulin. Nerve growth factor can be readily dissociated from alpha-macroglobulin in sodium dodecyl sulfate gel electrophoresis in the absence of a reductant. Procedures that affect the proteinase-binding or methylamine- activities of alpha-macroglobulin do not affect the binding of nerve growth factor, and the binding is unaffected by the presence of zinc ions or EDTA. Hence, nerve growth factor is noncovalently associated with alpha-macroglobulin at a site separate from that of the proteinase-, methylamine-, and zinc-binding sites of alpha-macroglobulin. Mouse alpha-macroglobulin can protect the nerve growth factor from inactivation by trypsin. Even in the presence of trypsin, alpha-macroglobulin-nerve growth factor complexes still can stimulate the neurite outgrowth by dorsal root ganglia of 9-day-old chicken embryos. Since alpha-macroglobulin can specifically and noncovalently carry nerve growth factor, one important role of this alpha-macroglobulin in the circulation and extracellular spaces may be to protect the nerve growth factor from proteinase inactivation.

Uptake and transepithelial transport of nerve growth factor in suckling rat ileum

Nerve growth factor (NGF) is necessary for the development of sympathetic and some sensory neurons. Milk may be a source of NGF for suckling young, but sites of intestinal absorption of the protein have not been identified. To determine whether NGF is transported across the absorptive epithelium of suckling rat ileum, we assessed binding, uptake, and transport of 125I-NGF by light microscopy and EM autoradiography. Blood and tissue extracts were analyzed by biochemical and immunological methods to determine whether NGF was taken up structurally intact. NGF binding sites were identified on microvilli and apical invaginations of ileal absorptive cells in vitro. Injected into ileal loops in vivo, NGF radioactivity retained by fixation was evident after 20 min in apical regions of absorptive cells, in endocytic tubules (which mediate the uptake of membrane-bound ligands), in vesicles (which mediate nonspecific endocytosis), and in the supranuclear lysosomal vacuole. At 1 and 2 h, radiolabel in these compartments increased and silver grains were evident at the basal cell surface, and in cells, matrix, and vessels of the lamina propria. In blood and liver, radiolabeled molecules that were immunologically and electrophoretically indistinguishable from NGF and that co-eluted with NGF on gel filtration columns were detected, confirming that some NGF was transported across the epithelium structurally intact. Thus, absorptive cells of suckling rat ileum can take up NGF by both receptor-mediated and nonspecific endocytosis, and direct NGF either to the lysosome for degradation, or into a transepithelial transport pathway.

Binding constants of isolated NGF-receptors from different species

It is known that NGF-responsive cells bind NGF at cell surface receptors in a specific and saturable fashion and there are two separate kinds of receptor-ligand binding interactions as judged by Rosenthal analyses. Following isolation of nerve growth factor receptors from embryonic chicken sensory ganglia, rat pheochromocytoma cells and human neuroblastoma cells, equilibrium binding studies were carried out and two different equilibrium binding constants similar to that described for whole cells were determined. This evidence is consistent with the hypothesis that there are two different receptors for NGF which have been conserved.

Sequestration requirements for the degradation of 125I-labeled beta nerve growth factor bound to embryonic sensory neurons

Nerve growth factor interacts with responsive cells by binding to cell surface membrane receptors. There are two different receptors on both embryonic sensory and sympathetic neurons, a high-affinity (type I) receptor and a lower-affinity (type II) receptor. Sequestration, which we have defined as bound nerve growth factor that becomes inaccessible to the external milieu with time, occurs through the type I receptor on both sensory and sympathetic neurons. We describe here a process subsequent to sequestration involving internalization and degradation of bound nerve growth factor and showing that bound nerve growth factor is not degraded under the following conditions: (1) low temperature, ie 4 degrees C; (2) when a large excess of unlabeled nerve growth factor is added concomitantly with the labeled nerve growth factor and the temperature is raised from 4 degrees C to 37 degrees C; (3) when metabolic inhibitors sodium fluoride and dinitrophenol are added concomitantly with the labeled nerve growth factor and the temperature is raised from 4 degrees to 37 degrees C. On the other hand, conditions that allow bound nerve growth factor to be degraded are the following: (1) incubation of the sensory nerve cells at low temperature (ie, 4 degrees C) only in the presence of labeled nerve growth factor, then raising the temperature to 37 degrees C; (2) when sodium fluoride and dinitrophenol are added when the temperature is raised to 37 degrees C; (3) when excess unlabeled nerve growth factor is added when the temperature is raised to 37 degrees C. These studies are consistent with the idea that nerve growth factor has to bind to the cells in order to be degraded; however, binding is not sufficient for degradation to occur. Second, the bound nerve growth factor must be sequestered in order to be degraded. Third, the process of internalization of the bound nerve growth factor, unlike sequestration, is not an energy-dependent process. Thus, it seems reasonable to suggest the following steps for the interaction of nerve growth factor with responsive cells: binding to a cell surface membrane receptor, followed by sequestration of the bound nerve growth factor, and finally, internalization of the sequestered nerve growth factor.

Decrease in the number of lower affinity (type II) nerve growth factor receptors on embryonic sensory neurons does not affect fiber outgrowth

Nerve growth factor binds to two different specific receptors on responsive cells. The relationship of these two receptors is not fully understood at this time. We have studied the binding of labeled NGF to a different strain of white leghorn chicken embryo dorsal root ganglionic cells. The equilibrium dissociation constants for the two sites (KdI = 4.1 +/- 1.8 x 10(-11) M, KdII = 1.0 +/- 0.8 x 10(-9) M) are identical to those obtained previously. Also, the number of type I sites per cell (3.8 +/- 1.3 x 10(3)) is the same as that previously determined. However, the number of type II sites per cell (1.9 +/- 1.3 x 10(4)) is significantly different than that previously determined. This 2.5-fold decrease in the number of type II sites does not affect the concentration of NGF needed to obtain maximal fiber outgrowth from explanted sensory ganglia. The rate of association (1.2 +/- 0.2 x 10(7) M-1 sec-1 at 22 degrees C) of labeled NGF with receptors on sensory neurons from this different strain of chickens is identical to that previously obtained. The rate of association of NGF with its receptors on sensory neurons was also determined at 4 degrees C. This rate constant (2.1 +/- 1.1 x 10(6) M-1 sec-1) along with the rate constants obtained at 22 degrees and 37 degrees C were used to determine an activation energy for the binding of NGF to its receptors. The activation energy obtained (16.2 kcal/mole) suggests that binding is not a diffusion-controlled process.

Specific binding of nerve growth factor to normal and denervated canine heart

Preparations of membranes from normal and surgically denervated canine heart were tested for binding of radiolabeled nerve growth factor. Specific binding was detected in both normal and denervated hearts. Binding was nonsaturable and complex for normal atria and ventricles and denervated atria but appeared to be saturable for denervated ventricles. Nerve growth factor bound per microgram of protein was lower in denervated ventricles than normal ventricles, whereas slightly higher binding to denervated atria than normal atria was observed. This binding could not be displaced by cytochrome c, insulin, or epidermal growth factor. The data indicate that a large part of binding to normal ventricles could be due to nerve terminals attached to the heart, but specific binding detected in the denervated ventricles may be an intrinsic property of the tissue itself. These sites may serve as storage or uptake sites to direct sympathetic innervation in the developing or reinnervating myocardium.

Sequestration of 125I-labeled beta nerve growth factor by embryonic sensory neurons

Nerve growth factor (NGF) binds to two specific receptors on sensory nerve cells. These two receptors are characterized by different equilibrium dissociation constants. The higher affinity (type I) receptors have an equilibrium dissociation constant of 3.3 X 10(-11) M. The lower affinity (type II) receptors have an equilibrium dissociation constant of 1.7 X 10(-9) M. These two receptors are not a result of negative cooperativity, but apparently are different receptors. At 22 degrees C the rate of association is 1 X 10(7) M-1 S-1 and the rates of dissociation are 6.5 X 10(-4) S-1 (type I) and 3.2 X 10(-2) S-1 (type II). After binding, a time-dependent process occurs that makes that NGF inaccessible to the external milieu (sequestered). The sequestration process is energy-dependent, but apparently temperature-independent. The data suggest that only the type I receptors are involved in the sequestration process. This process is similar to that observed on sympathetic neurons and may be the first step in the internalization of NGF by responsive cells.

Interaction of [125I] beta nerve growth factor with acidic proteins

We have demonstrated that the beta nerve growth factor will interact with various acidic proteins apparently nonspecifically. When 125I-labeled beta nerve growth factor at a concentration of 3.8 X 10(-10) M is incubated with an acidic protein at 2 mg/ml (4.5 X 10(-6)-4.4 X 10(-5) M), a complex is formed. This complex changes the isoelectric point of the 125I-labeled beta nerve growth factor sufficiently so that the 125I-labeled beta nerve growth factor migrates anomalously in polyacrylamide gel electrophoresis. The interaction between beta nerve growth factor and bovine serum albumin, which appears to be complex, may be the cause of the previously reported activation of the beta nerve growth factor when bovine serum albumin is present in a typical bioassay.