Receptor binding and biological activity of mammalian expressed sensory and motor neuron-derived factor (SMDF).

Sensory and motor neuron-derived factor (SMDF) is a member of the neuregulin family of proteins. SMDF is structurally characterized by a novel N-terminal domain. Using the signal sequence and N-terminal 28 amino acids (the "epitope") of herpes simplex virus type 1 glycoprotein D (gD), we have expressed SMDF as an epitope-tagged protein (gD-SMDF) in 293 cells, and purified it to > 98% homogeneity on a monoclonal anti-gD column. gD-SMDF stimulates human Schwann cell growth and 3H-thymidine incorporation in MCF-7 and T47D human breast tumor cells in vitro. The biological activity of gD-SMDF is consistent with its ability to compete with 125I-labeled heregulinbeta1 peptide (rHRGbeta1(177-244)) to bind to soluble dimeric ErbB receptor-IgG fusion proteins. gD-SMDF binds with low affinity to homodimeric ErbB3-IgG and ErbB4-IgG but with higher affinity to heterodimeric ErbB2/ErbB3-IgG and ErbB2/ErbB4-IgG. Using a SMDF-IgG(Fc) fusion protein we generated a monoclonal antibody (3G11) which binds SMDF, crossreacts with rHRGbeta1(177-244), and neutralizes the in vitro activities of gD-SMDF and rHRGbeta1(177-244) in human Schwann cells.

Binding and cross-linking of 32P-labeled human relaxin to human uterine cells and primary rat atrial cardiomyocytes.

Relaxin is known for its function in parturition and has been suggested to participate in the regulation of blood pressure, heart rate, and the release of neuropeptides such as oxytocin and vasopressin. Consistent with the physiological roles of relaxin, high affinity relaxin receptors have been demonstrated in the rat uterus, brain, and cardiac atrium. Here we report the binding and cross-linking of a biologically active, 32P-labeled human relaxin to a human uterine cell line and primary rat atrial cardiomyocytes. Relaxin binding to the human uterine cells consisted of a single class of high affinity sites (Kd = approximately 0.44 nM) with approximately 1082 +/- 62 binding sites/cell. Binding and cross-linking of relaxin to the human uterine cells and rat atrial cardiomyocytes followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the putative relaxin receptor showed a major component with an apparent M(r) greater than 220 kilodaltons and a minor component of approximately 36 kilodaltons, and was not disulfide linked. The binding and cross-linking of [32P]relaxin could be displaced by unlabeled relaxin in a concentration-dependent manner, but not by a 1000-fold molar excess of insulin, insulin-like growth factor I (IGF-I), or IGF-II. These data suggested that the relaxin receptor was similar in size but distinct from the insulin, IGF-I, and IGF-II receptors.

Sensory and motor neuron-derived factor. A novel heregulin variant highly expressed in sensory and motor neurons.

The heregulin family of polypeptides arise as splice variants from a single gene and share a conserved epidermal growth factor (EGF)-like domain thought to be the major determinant of their biological activities. We report here the cloning of a novel member of this family, termed sensory and motor neuron-derived factor or SMDF, which is highly expressed in sensory and motor neurons in human and rodent species. It contains a C-terminal beta-type EGF-like domain and an unique N-terminal sequence which lacks an Ig-like domain and is distinct from all known heregulin variants. Mammalian cell-expressed SMDF activates tyrosine phosphorylation of a 185-kDa protein in cell lines expressing p185erbB2, indicating that it is biologically active. Analyses of expression patterns suggest that, unlike other heregulin variants, SMDF is expressed mainly in the nervous system. In situ hybridization signals with the unique SMDF sequence probe and with a probe to the conserved EGF-like domain are comparable, suggesting that SMDF is the predominant isoform expressed in sensory and motor neurons. Expression of SMDF is maintained in both adult motor neurons and dorsal root ganglion neurons. These findings suggest that SMDF may mediate biological responses such as Schwann cell proliferation and acetylcholine receptor induction in the peripheral nervous system.

Expression of relaxin mRNA and relaxin receptors in postnatal and adult rat brains and hearts. Localization and developmental patterns.

Relaxin is a polypeptide hormone best known for its role in parturition. However, high affinity relaxin receptors have been localized in the rat brain and heart in addition to the uterus. Several lines of evidence also suggest that relaxin may be involved in the regulation of blood pressure, heart rate, and the release of oxytocin and vasopressin. We now show by Northern analysis that a 1-kilobase relaxin transcript is detected in the rat brain as well as the ovary of pregnant rats. Using in situ hybridization, relaxin mRNA is localized in discrete regions of the male and female brains, including the anterior olfactory nucleus, tenia tecta, pyriform cortex, neocortex, and hippocampus. Developmental studies show that relaxin mRNA is present in the 1-day postnatal brain, while relaxin receptors are not detectable until 7 days after birth. The relaxin receptor binding affinity was similar in the developing brains, but there was a steady increase in relaxin binding sites during postnatal days 7 to 29, suggesting that relaxin may play a role in brain maturation. While relaxin mRNA is not detected in the heart, high levels of relaxin receptors are detected in the cardiac atrium as early as 1 day after birth. These atrial receptors remained at similar levels throughout postnatal development, suggesting an important role for relaxin in cardiovascular function.

Relaxin binding in the rat heart atrium.

Relaxin is a member of the insulin family of polypeptides that is best known as a reproductive hormone. In an effort to elucidate the mechanism of action of relaxin we previously localized the specific binding sites of a 32P-labeled relaxin in the rat uterus and brain. These studies suggested that, in addition to its classical role in pregnancy, relaxin might have other physiological functions. In the present paper we describe the specific and high-affinity binding of relaxin to the cardiac atrium of both male and female rats. The relaxin binding could not be displaced by peptides belonging to the same family [insulin, insulin-like growth factor I (IGF-I)] or by peptides that were identified in the atrium or were known to have cardiovascular functions (atrial natriuretic peptide, angiotensin II). The dissociation constant for relaxin in the atrium was estimated to be 1.4 nM, which was similar to that found in the uterus (1.3 nM) and the brain (1.4 nM). In view of the close association of relaxin with reproduction, an experiment was also performed to compare the relaxin binding in the uterus and heart after gonadectomy and sex steroid treatment. It was found that the relaxin binding in the rat uterus was diminished by 53% overall following ovariectomy but was restored to 90% of normal levels when treated with estrogen (but not with testosterone). In contrast, the relaxin binding in the rat heart was not affected by castration or sex steroid treatment. We conclude that specific and high-affinity relaxin receptors exist in the atrium of both the male and female rat heart and that these are regulated differently than the relaxin receptors in the uterus.

Autoradiographic localization of relaxin binding sites in rat brain.

Relaxin is a member of the insulin family of polypeptide hormones and exerts its best understood actions in the mammalian reproductive system. Using a biologically active 32P-labeled human relaxin, we have previously shown by in vitro autoradiography specific relaxin binding sites in rat uterus, cervix, and brain tissues. Using the same approach, we describe here a detailed localization of human relaxin binding sites in the rat brain. Displaceable relaxin binding sites are distributed in discrete regions of the olfactory system, neocortex, hypothalamus, hippocampus, thalamus, amygdala, midbrain, and medulla of the male and female rat brain. Characterization of the relaxin binding sites in the subfornical organ and neocortex reveals a single class of high-affinity sites (Kd = 1.4 nM) in both regions. The binding of relaxin to two of the circumventricular organs (subfornical organ and organum vasculosum of the lamina terminalis) and the neurosecretory magnocellular hypothalamic nuclei (i.e., paraventricular and supraoptic nuclei) provides the anatomical and biochemical basis for emerging physiological evidence suggesting a central role for relaxin in the control of blood pressure and hormone release. We conclude that specific, high-affinity relaxin binding sites are present in discrete regions of the rat brain and that the distribution of some of these sites may be consistent with a role for relaxin in control of vascular volume and blood pressure.

Preparation of biologically active 32P-labeled human relaxin. Displaceable binding to rat uterus, cervix, and brain.

Relaxin is a member of the insulin family of polypeptide hormones and is known to exert its biological effects on various parts of the mammalian reproductive system. Biologically active human relaxin has been chemically synthesized based on the nucleotide sequence obtained from an ovarian cDNA clone. In the present study synthetic human relaxin was radiolabled by phosphorylation with cAMP-dependent protein kinase and [gamma-32P]ATP to a specific activity of 5000 Ci/mmol. The phosphorylated relaxin was purified on cation exchange high performance liquid chromatography and was shown to co-migrate with relaxin on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Mass spectrometry revealed a single phosphorylated site on the B chain of relaxin. The 32P-relaxin was able to bind to a goat anti-relaxin antibody, and this binding could be displaced by unlabeled relaxin in a concentration-dependent manner. A comparison of the concentration responses of cellular cAMP production stimulated by relaxin and phosphorylated relaxin in a primary human uterine cell line showed that phosphorylation did not affect the in vitro biological efficacy of relaxin. This made it suitable for in situ autoradiographic localization of relaxin binding sites in rat uterine, cervical, and brain tissue sections. Displacement of the binding of 100 pM 32P-relaxin by 100, 10, and 3 nM unlabeled relaxin, but not by 100 nM insulin, insulin-like growth factor-I, and an insulin-like growth factor-I analog, demonstrated the high affinity and specificity of such binding. We conclude that 32P-labeled human relaxin is biologically and immunologically active and that this novel probe binds reversibly and with high affinity to classical (e.g. uterus) and unpredicted (e.g. brain) tissues.

Human interleukin-2 anti-idiotypes.

We describe the generation of a monoclonal anti-idiotypic (anti-id) antibody directed against affinity purified rabbit antibodies (id) to recombinant human IL-2. This monoclonal antibody, 6A12, has the ability to inhibit the neutralization of IL-2 activity by the id, suggesting that it may bind at or near the IL-2 neutralizing site on the id. 6A12 exhibits IL-2 agonist activity on PHA-activated human T cells. The purified IgG of 6A12 is also shown to bind to a purified soluble recombinant p55 subunit of the IL-2 receptor. Furthermore, purified 6A12 shows inhibition of IL-2 activity in an IL-2 dependent mouse T cell line (CTLL) and this inhibition can be reversed by excess IL-2. These results suggest that although 6A12 may not be an exact 'internal image' of the receptor binding site of IL-2, it may bind to at least the P55 subunit of the ligand binding site on the high affinity IL-2 receptor complex.

Interferon-like activity in an anti-interferon anti-idiotypic hybridoma antibody.

We describe here an anti-idiotypic hybridoma antibody directed against affinity-purified rabbit idiotypic antibodies (Rb-Id) to a homogeneous protein, the recombinant human leukocyte A interferon (rIFN-alpha A). The supernatant of the hybridoma, designated 3-1B, was able to inhibit the neutralization of rIFN-alpha A activity by the idiotypic antibodies. An in vivo passage of uncloned 3-1B cells yielded hybridoma cells (presumably a subclone), designated 3B1, the supernatant of which exhibited interferon-like antiviral activity with both bovine kidney (MDBK) cells and human amnion (WISH) cells. This activity could be absorbed by polymer-bound goat anti-mouse immunoglobulin serum and by Rb-Id coupled to Affi-Gel 10, and could be partially eluted from the latter at pH 2.5. The anti-idiotypic hybridoma antibody was able to compete with 125I-rIFN-alpha A for binding to the Rb-Id and also to interferon receptor-bearing MDBK cells. The clinical significance of an interferon-like anti-idiotypic antibody is discussed.

Monoclonal antibodies to a recombinant human leukocyte interferon (rIFN-alpha B).

Recombinant human leukocyte B interferon (rIFN-alpha B) was purified to approximately 60% on a column of monoclonal antibody to IFN-alpha. This partially purified rIFN-alpha B was in turn used as immunogen in obtaining four monoclonal antibodies to this recombinant interferon. All four antibodies, while showing varying degrees of cross-reactivity to some other rIFN-alpha subtypes, showed specificity to monomeric and dimeric forms of rIFN-alpha B only and not to the contaminants in the partially purified preparation as demonstrated in the Western blot immunoassay.

Antigen stimulation of prostaglandin synthesis and control of immune responses.

Within 2 min following the intravenous injection of sheep erythrocytes (sRBC) there occurs 20 to 80-fold increase in prostaglandin (PG) F2alpha in the spleen. This burst of synthesis is followed by a slow decline to control levels over the next 1-4 hr. No increase in splenic PGF2alpha levels is observed between 24 and 72 hr after injection. Injection of colloidal carbon results in a small increase, approximately 20% of the increase in PGF2alpha observed with sRBC. The early increase in splenic PGF2alpha levels stimulated by sRBC is also dependent upon thymus-derived (T) cells, since the increase is small or nonexistent in athymic mice and NZB mice. Also, the elevation of splenic PGF2alpha levels is blocked by the administration of indomethacin or Ro 20-5720, both of which block the synthesis of prostglandin. A small increase (2-fold) in PGF2alpha levels occurs in the thymus. A soluble antigen, bovine gamma globulin, stimulated a bimodal increase in splenic PGF2alpha levels, the early peak occurring at 2 hr and the later increase occurring at 48 hr. Using inhibitors of prostaglandin synthesis, it is possible to enhance the appearance of cells forming 19S antibody against sRBC, both in vivo and in vitro. Furthermore, inhibition of prostaglandin synthesis enhances DNA synthesis induced in a two-way mixed-lymphocyte reaction only in whole spleen cell cultures and not in cultures of spleen cells purified by passage over glass wool. Based on this evidence, it is proposed that the prostaglandins represent a major soluble mediator in the control of T cell-T cell interactions and also play an important part in T-B (bone-marrow derived) cell interactions.