br Material and methods br Results The search

Material and methods
Results The search for proteins selectively expressed in pancreatic islet OPP in the Human Protein Atlas (HPA) yielded 27 hits, including well-known islet cell markers such as insulin, glucagon and pancreatic polypeptide (Table 1). The search algorithm excluded islet cell markers expressed in other abdominal organs as for example VMAT2, somatostatin and the glucose transporter type 2 (GLUT2). Twelve proteins which showed a clear membranous staining pattern, or according to bioinformatics prediction models displayed transmembrane regions and epitopes located at the cell surface (information is available at http://www.proteinatlas.org), were selected for further analysis (Table 2).
Discussion In the present study, four novel proteins specifically expressed in human beta cells have been identified. All proteins, i.e. DGCR2, GBF1, GPR44 and SerpinB10 showed strong beta cell-specific cytoplasmic immunoreactivity accentuated to the plasma membrane in islets of non-diabetic individuals, subjects with T2D and in insulin-positive islets in sections from individuals with recent onset T1D. Expression of these proteins was not detected in insulin-negative islets of T1D subjects. DGCR2 is a putative adhesion receptor that is expressed during embryogenesis and believed to play a role in neural crest cell migration [19]. Mutations in this gene are associated with the DiGeorge syndrome with phenotypic features as cardiac anomalies, immune deficiencies and palatal defects, reviewed in [20]. The HPA antibody against DGCR2 showed distinct expression only in pancreatic beta cells and not in any of the other normal cell types examined, and is therefore considered as highly specific, a criterion necessary for a biomarker. GBF1 is known as a Golgi guanine nucleotide exchange factor that activates ARF GTPases, which are involved in regulation of membrane dynamics and protein transport [21], [22]. GBF1 has thus a key role in protein secretion processes. In the screening of different normal tissues, GBF1 was expressed in erythrocytes and in a subset of cells in white pulp of spleen and urinary bladder. In addition, endothelial cells in pancreas, but not in any other examined tissues, were distinctly positive. Due to the expression in erythrocytes and endothelial cells in the pancreas it may not be suitable for beta cell imaging because of possible background signals. However, considering the beta cell specificity within islets of Langerhans as well as endothelial specificity for pancreas, it still constitutes an interesting target for further understanding of beta cell biology. The G protein-coupled receptor GPR44, a receptor for prostaglandin D2 (PGD2), has previously shown to be expressed in CD4 positive T2-cells, basophils and eosinophils where it mediates pro-inflammatory chemotaxis during allergic inflammation [23], [24]. In the present investigation, GPR44 was strongly expressed only in pancreatic beta cells, with no or very low expression in all other cell types examined, including abdominal organs and lymphoid tissues. Antagonists to GPR44 are currently being developed for treatment of allergic asthma and allergy since the receptor is believed to be a key player in these conditions. Importantly, small molecule antagonists, e.g. 3-sulphur-substituted indole derivatives, 2,3 substituted pyrazine sulfonamides and tricyclic spiro derivatives, are already available and currently tested in clinical trials. Labeled small molecule antagonists could therefore readily be tested for their usefulness as ligands for assessment of beta cell mass. SerpinB10 is a member of the Serpin super family, which consists of peptidase inhibitors that regulate both intracellular and extracellular processes [25]. Despite the name of the family, some members are non-inhibitory and involved in various other functions including storage, hormone transport and tumor suppression. SerpinB10 has previously shown to be expressed in hematopoietic cells [26]. In the present study, distinct expression of SerpinB10 was observed in a few normal cell types, including a subset of cells in the adrenal gland, placenta and tonsil. However, positivity of these cell types is unlikely to interfere with the results in beta cell imaging experiments because of the distance between the organs. In addition, the expression was low or absent in abdominal organs except for beta cells in the pancreas. SerpinB10 is therefore considered as a potential beta cell marker.