Oliver J. Kreuzer

Cellular and subcellular localization of gastrointestinal glutathione peroxidase in normal and malignant human intestinal tissue

The gastrointestinal glutathione peroxidase (GI-GPx) is believed to prevent absorption of hydroperoxides. GI-GPx is expressed in the intestine together with the other three glutathione peroxidase isoenzymes, raising the question of the physiological role of the different GPx types. We therefore studied the cellular and subcellular distribution of GI-GPx in normal and malignant tissue obtained from patients with colorectal cancer or familial polyposis by immunohistochemistry. In healthy ileum epithelium GI-GPx was preferentially enriched in Paneth cells. In unaffected crypts of colon and rectum, it decreased gradually from the ground to the luminal surface. In crypt ground, GI-GPx was uniformly distributed, whereas in cells at the luminal surface it was concentrated in structures capping the nuclei at the apical pole. In colorectal cancer, GI-GPx expression depended on the stage of malignant transformation. In early stages, GI-GPx was increased and pronouncedly associated with the vesicular structures. In progressed stages of malignancy, structures disintegrated and GI-GPx distribution became more diffuse. These observations support the hypothesis that GI-GPx, apart from being a barrier against hydroperoxide absorption, might be involved in cell growth and differentiation.

Functional peptide microarrays for specific and sensitive antibody diagnostics

Peptide microarrays displaying biologically active small synthetic peptides in a high‐density format provide an attractive technology to probe complex samples for the presence and/or function of protein analytes. We present a new approach for manufacturing functional peptide microarrays for molecular immune diagnostics. Our method relies on the efficiency of site‐specific solution‐phase coupling of biotinylated synthetic peptides to NeutrAvidin (NA) and localized microdispensing of peptide‐NA‐complexes onto activated glass surfaces. Antibodies are captured in a sandwich manner between surface immobilized peptide probes and fluorescence‐labeled secondary antibodies. Our work includes a total of 54 peptides derived from immunodominant linear epitopes of the T7 phage capsid protein, Herpes simplex virus glycoprotein D, c‐myc protein, and three domains of the Human coronavirus polymerase polyprotein and their cognate mAbs. By using spacer molecules of different type and length for NA‐mediated peptide presentation, we show that the incorporation of a minimum spacer length is imperative for antibody binding, whereas the peptide immobilization direction has only secondary importance for antibody affinity and binding. We further demonstrate that the peptide array is capable of detecting low‐picomolar concentrations of mAbs in buffered solutions and diluted human serum with high specificity.

Agonist-Mediated Endocytosis of Rat Somatostatin Receptor Subtype 3 Involves β-Arrestin and Clathrin Coated Vesicles

Agonist‐induced endocytosis of somatostatin receptors determines subsequent cellular responsiveness to peptide agonist and influences somatostatin receptor scintigraphy, a technique to image various tumours. We examined the internalization of sst3HSV, an epitope‐tagged type 3 somatostatin receptor, in transfected rat neuroendocrine insulinoma cells. Stimulation of these cells with somatostatin induced trafficking of coexpressed enhanced green fluorescence protein/β‐arrestin1 fusion protein and sst3HSV to colocalize in the same endocytic vesicles. Coexpression of a dominant negative mutant of the arrestin fusion protein with the receptor blocked the internalization of sst3HSV. Stimulation with somatostatin also induced the transient translocation of α‐adaptin, a component of the adaptor protein complex 2, to the plasma membrane. α‐adaptin and clathrin colocalized with the receptor. By electron microscopy, we observed internalized sst3 in clathrin coated pits, endosomes and at the limiting membrane of multivesicular bodies, a location typical for receptors being recycled. Concordantly, we observed sst3HSV colocalized with Rab11 in a perinuclear compartment which is likely to correspond to the pericentriolar recycling endosome. Thus, agonist‐induced endocytosis of sst3 depends on its interaction with β‐arrestin, involves the adaptor protein complex 2 and proceeds via clathrin coated vesicles to the recycling compartment.

Cloning, expression, pharmacology and tissue distribution of the mouse somatostatin receptor subtype 5.

The gene encoding the mouse somatostatin receptor subtype 5 has been isolated from a genomic library and the mRNA start point mapped to position −95 relative to the translational start codon. The promoter region is devoid of TATA and CAAT boxes but contains putative binding sites for AP‐1, AP‐2 and SP1 and response elements for glucocorticoids (GRE) and phorbol esters (TRE). The encoded receptor protein with a predicted molecular weight of 42.5 kDa is comprised of 385 amino acids and thus contains 22 and 21 amino acids more than rat and human counterparts. The extra amino acids are caused by another translational initiation codon located further upstream. In the region of overlap the mouse somatostatin receptor subtype 5 displays 96.7% sequence identity to the rat and 81.7% to the human homologue. Application of somatostatin‐14 and −28 to human embryonic kidney cells expressing the recombinant receptor resulted in the inhibition of forskolin‐stimulated adenylyl cyclase with comparable EC50 values. Consistent with the observed sequence relationship, the mouse somatostatin receptor subtype 5 displays a pharmacological profile that resembles the rat homologue more closely than the human counterpart. mRNA for the mouse somatostatin type 5 receptor has been detected in pituitary, kidney, spleen and ovary and, to a lesser extent, in brain, stomach, intestine and thymus but was not observed in heart, pancreas and liver.

Peptide microarrays with site-specifically immobilized synthetic peptides for antibody diagnostics

Abstract Peptide microarrays bear the potential to discover molecular recognition events on protein level, particularly in the field of molecular immunology, in a manner and with an efficiency comparable to the performance of DNA microarrays. We developed a novel peptide microarray platform for the detection of antibodies in liquid samples. The system comprises site-specific solution phase coupling of biotinylated peptides to NeutrAvidin, localized microdispensing of peptide–NeutrAvidin conjugates onto activated glass slides and a fluorescence immuno sandwich assay format for antibody capture and detection. Our work includes synthetic peptides deduced from amino acid sequences of immunodominant linear epitopes, such as the T7 phage capsid protein, Herpes simplex virus glycoprotein D, c-myc protein and three domains of the Human coronavirus 229E polymerase polyprotein. We demonstrate that our method produces peptide arrays with excellent spot morphology which are capable of specific and sensitive detection of monoclonal antibodies from fluid samples.

Intracellular degradation of somatostatin-14 following somatostatin-receptor 3-mediated endocytosis in rat insulinoma cells

Somatostatin receptor (SSTR) endocytosis influences cellular responsiveness to agonist stimulation and somatostatin receptor scintigraphy, a common diagnostic imaging technique. Recently, we have shown that SSTR1 is differentially regulated in the endocytic and recycling pathway of pancreatic cells after agonist stimulation. Additionally, SSTR1 accumulates and releases internalized somatostatin‐14 (SST‐14) as an intact and biologically active ligand. We also demonstrated that SSTR2A was sequestered into early endosomes, whereas internalized SST‐14 was degraded by endosomal peptidases and not routed into lysosomal degradation. Here, we examined the fate of peptide agonists in rat insulinoma cells expressing SSTR3 by biochemical methods and confocal laser scanning microscopy. We found that [125I]Tyr11‐SST‐14 rapidly accumulated in intracellular vesicles, where it was degraded in an ammonium chloride‐sensitive manner. In contrast, [125I]Tyr1‐octreotide accumulated and was released as an intact peptide. Rhodamine‐B‐labeled SST‐14, however, was rapidly internalized into endosome‐like vesicles, and fluorescence signals colocalized with the lysosomal marker protein cathepsin D. Our data show that SST‐14 was cointernalized with SSTR3, was uncoupled from the receptor, and was sorted into an endocytic degradation pathway, whereas octreotide was recycled as an intact peptide. Chronic stimulation of SSTR3 also induced time‐dependent downregulation of the receptor. Thus, the intracellular processing of internalized SST‐14 and the regulation of SSTR3 markedly differ from the events mediated by the other SSTR subtypes.