Application of ELISA for assessment of antiserum immunoreactivity in endocrine immunocytochemical studies

Enzyme-linked immunosorbent assay for the quantitative/qualitative analysis of plant secondary metabolites

Immunoassays are antibody-based analytical methods for quantitative/qualitative analysis. Since the principle of immunoassays is based on specific antigen-antibody reaction, the assays have been utilized worldwide for diagnosis, pharmacokinetic studies by drug monitoring, and the quality control of commercially available products. Berson and Yalow were the first to develop an immunoassay, known as radioimmunoassay (RIA), for detecting endogenous plasma insulin [1], a development for which Yalow was awarded the Nobel Prize in Physiology or Medicine in 1977. Even today, after half a century, immunoassays are widely utilized with some modifications from the originally proposed system, e.g., radioisotopes have been replaced with enzymes because of safety concerns regarding the use of radioactivity, which is referred to as enzyme immunoassay/enzyme-linked immunosorbent assay (ELISA). In addition, progress has been made in ELISA with the recent advances in recombinant DNA technology, leading to increase in the range of antibodies, probes, and even systems. This review article describes ELISA and its applications for the detection of plant secondary metabolites.

Specific antibody recognition of rat pituitary adenylate cyclase activating polypeptide receptors

Pituitary adenylate cyclase activating polypeptide (PACAP) is a new member of the secretin/VIP family of peptides. The specific receptor for PACAP has been cloned in rat, human, and bovine tissues. The distribution of the transcripts of PACAP receptor genes has been studied in various tissues using in situ hybridization. However, the unavailability of a specific antibody against the PACAP receptor has hampered further study of the expression of receptor proteins. In the present study, rabbit antisera were generated against a synthetic 25-residue peptide corresponding to the C-terminal intracellular domain of the rat PACAP receptor. To validate the specificity of the antisera, CHO cells and cells stably transfected with rat PACAP receptor cDNA were prepared. Using one of these antisera, the membrane and soluble fractions of the transformants were examined by Western blot analysis. Three bands were observed in subcellular fractions from the transfected CHO cells, but no bands were found in similar preparations from the nontransfected cells. A distinct 57-kDa band, which corresponds to the size of cloned rat PACAP receptor, was detected. In addition, a less intense band, larger than 57 kDa, and a very weakly stained band, smaller than 57 kDa, were demonstrated. All of these bands disappeared or were considerably diminished when the antiserum was preabsorbed with the synthetic immunogen peptide. This suggests that these bands are PACAP receptor-related proteins. The membranes from the transfected CHO cells bound to [125I]PACAP27. The size of the ligand/protein crosslinked product approximated 60 kDa, corresponding to the combined size of the PACAP receptor and PACAP27. No additional bands were observed, indicating that the immunopositive proteins larger or smaller than 57 kDa do not bind to the ligand and are not functional. Unlabeled PACAP27 and PACAP38, but not VIP, displaced the binding, suggesting that the receptors expressed in CHO cells are specific for PACAP. Solubilized membrane fractions prepared from rat brains were used for an immunoprecipitation study with [125I]PACAP27 and [125I]VIP. The PACAP receptor antiserum recognized [125I]PACAP-, but not [125I]VIP-bound proteins in the solubilized brain membrane fractions. Immunohistochemistry using this antiserum showed a distribution of PACAP receptor-like immunoreactivities similar to the distribution of the mRNA of PACAP receptor in the rat brain. Thus, the PACAP receptor antiserum is sufficiently specific to be used as a tool for studying the expression of PACAP receptors and related proteins.

Localization and gene expression of the receptor for pituitary adenylate cyclase-activating polypeptide in the rat brain

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a recently identified member of the secretin/vasoactive intestinal polypeptide (VIP) family. There are at least two types of receptor for PACAP: type I (PACAPR), which specifically binds PACAP; and type II (VIP/PACAPR), which binds both PACAP and VIP. The localization of PACAPR in the rat brain was determined by in situ hybridization and immunocytochemistry. We raised antisera against a synthetic peptide that corresponds to the carboxy-terminal cytoplasmic domain which is found in all subtypes of PACAPR in order to localize PACAPR-like immunoreactivity (PACAPR-LI) in the rat brain. In general, the distribution of PACAPR-LI correlated well with the distribution of PACAPR transcripts. Particularly strong PACAPR mRNA expression was detected in the olfactory bulb, hippocampus, cerebellum and hypothalamus and moderate labeling was detected in other scattered regions. At the cellular level, PACAPR-LI appeared to be concentrated predominantly in neuronal perikarya and dendrites. At the ultrastructural level, strong immunostaining for the PACAPR was found in plasma membranes, rough endoplasmic reticulum, cytoplasmic matrix, and at synapses. This study provides the basis for a better understanding of the functions of PACAP in the rat brain.

Immunohistochemical localization of PACAP in the ovine digestive system

The localization of immunoreactive PACAP (PACAP-IR) in the entire length of the sheep gastrointestinal tract and the pancreas was studied by an immunohistochemical method. PACAP-IR-containing nerve fibers innervated the longitudinal smooth muscle layer of the mucosa in the esophagus, stomach, and small and large intestines, the muscular layer of the stomach and intestine, Brunner's gland of the duodenum, and the walls of small arteries. PACAP-IR fibers also innervated the exocrine acini, islets of Langerhans, and the small arteries in the connective tissue septa of the pancreas. These findings suggest a regulatory role of PACAP in the digestive system.

Immunochemical investigation of insulinomas for islet amyloid polypeptide and insulin: evidence for differential synthesis and storage

An affinity purified antibody to fragment 14-29 of islet amyloid polypeptide (IAPP) has been prepared. This antibody, which does not cross-react with the related molecule calcitonin gene-related peptide, was used to investigate immunochemically the presence of IAPP in normal and neoplastic human pancreatic endocrine tissue. The pattern of IAPP staining in normal pancreas mirrors that of insulin, although slight differences were observed. In neoplastic tissue, IAPP was found in 16 out of 19 tumours that were positive for insulin, and was absent from one tumour negative for insulin. In some cases there were differences in the staining patterns of IAPP and insulin. These results suggest that the synthesis and secretion of IAPP and insulin are not inter-dependent and support the concept that IAPP has a discrete biological function. Islet amyloid polypeptide was found in six out of six insulinoma amyloid deposits, suggesting that the peptide is an invariable component of these deposits. Over-expression of IAPP, with aberrant processing and/or secretion, may be the causative factor for amyloid deposition in insulinomas and in the islets of type 2 (non-insulin dependent) diabetic patients. Investigation of patients with insulinomas and of insulin cells in culture and tissue sections may help to clarify the biological function of IAPP.

Adenohypophysis hormone gene products in 14 pituitary adenomas: analysis by immunohistochemistry and northern blotting

We investigated 14 pituitary adenomas (10 silent adenomas; 3 prolactinomas and one GH-secreting tumor) for the presence of hormone gene transcripts (Northern blot) as well as for translation products (immunohistochemistry). The GH-secreting tumor was shown to express the genes coding for GH and PRL and to synthesize the corresponding hormones. In the cases of prolactinomas, immunohistochemical data demonstrated the synthesis of prolactin only. In addition to the PRL gene, Northern blot analysis revealed the transcription of the alpha-subunit gene in one case. Hormone genes were found to be expressed in 7 out of the 10 silent tumors, whereas no hormone synthesis was detected in any of these tissues. LH-beta mRNA was found in 3 cases, FSH-beta mRNA in 5 cases and alpha-subunit gene was shown to be expressed in one case. Surprisingly, the level of expression of the FSH-beta gene was higher than in normal tissue. This study confirms that some so called "silent" adenomas are expressing alpha- and/or beta-subunit glycoprotein hormone genes, even if no hormone is synthesized. The therapeutic action of bromocriptine described in some "silent" adenomas cases could be related to that hormone gene expression potentiality.

Immunohistochemical localization of the neuropeptide, pituitary adenylate cyclase activating polypeptide (PACAP), in human and primate hypothalamus

A 38 residue neuropeptide was recently isolated from ovine hypothalamus in our laboratory, and named pituitary adenylate cyclase activating polypeptide (PACAP38) based on its biological activity. Rabbit antisera against synthetic PACAP27 were characterized by ELISA for immunohistochemical use. PACAP-immunoreactive neuronal elements having similar distributions were demonstrated in both human and spider monkey hypothalami. Many PACAP-immunoreactive cell bodies were present in the supraoptic and paraventricular nuclei. Immunopositive nerve fiber networks were stained throughout the hypothalamus, including in both external and internal zones of the tuber cinereum, close to the transition of the pituitary stalk (median eminence). These results suggest that PACAP plays multifunctional roles as a hypophysiotropic hormone, neurohypophysial hormone, neurotransmitter or neuromodulator in higher vertebrate species including man.

FMRF-amide immunoreactivity in the mammalian gastroenteropancreatic neuroendocrine system

The presence of FMRF-amide, a cardioactive tetrapeptide, was studied by immunocytochemistry in human and rat gastric antrum and pancreas, and in the ovine, bovine, canine and rabbit pancreas. In human and rat gastric antrum, numerous cells contained FMRF-amide immunoreactive material. By staining of serial sections and by double staining, colocalization of immunoreactivity for gastrin and FMRF-amide was observed in part of the gastrin cells. In the pancreas of these and the other species, immunoreactivity for FMRF-amide was located both in acinar and islet endocrine cells. Colocalization of FMRF-amide and pancreatic polypeptide was found in a proportion of pancreatic polypeptide cells in the pancreas. FMRF-amide immunoreactivity never colocalized with the other neurohormonal peptides which occur in the gastric antrum and the pancreas. Our observations show that neuroendocrine cells occur in the gastric antrum and pancreas which are exclusively immunoreactive or gastrin and for pancreatic polypeptide respectively. In addition cells occur which show immunoreactivity for FMRF-amide as well as for gastrin in the gastric antrum and with antiserum to FMRF-amide as well as for pancreatic polypeptide in the pancreas. It is concluded that FMRF-amide antibodies probably recognize a substance in G and PP cells which is not identical but may be structurally related to gastrin and pancreatic polypeptide.

CRF immunoreactive peptides in the human hypophysis: a cautionary note

By monitoring with a non competitive enzyme linked immunosorbent assay (ELISA), corticotropin releasing factor (CRF)-like immunoreactive material was isolated from the human hypophysis. After acid extraction of peptides from frozen human hypophyses, the purification was achieved by affinity chromatography using purified anti-ovine-CRF IgG bound to a solid phase and then by two HPLC steps using an alkylsilane-bonded large pore size silica. Two CRF-like peptides were purified: discrete immunoreactive peaks coinciding with an optical density peak at 215 nm. Although these peptides were recognized by ELISA, they were not recognized in an RIA using the same anti-ovine-CRF serum and ovine CRF-41 as tracer. Neither of these CRF-immunoreactive peptides had any effect on either the spontaneous or stimulated ACTH release in the perfused isolated anterior pituitary cell bioassay.