Isolation and characterization of novel intestinal polypeptides of the enteroinsular and brain-gut axes and of macrophages

Abstract

The gastrointestinal tract has been recognized as our largest endocrine organ, fromwhich many peptide hormones, neuropep- tides and growth factors have been isolated.There is also increasing evidence that the gastrointestinal tract can be consideredas our largest immune organ, from which many antibacterial peptides, cy tokines,cytotoxins and chemokines have been characterized. Different entero-endocrine cells, peptidergic nerve terminals and immune cells arefound in this organ. This thesis describes the isolation and characterization, from intestinal tissues,of seven polypeptides, five porcine [daintain/AIF1, diazepam binding inhibitor (DBI),dopuin, pancreatic polypeptide (PP), vimentin-37] and two from chicken (DBI1-86 andDBI35-86). These peptides were first detected in chromatographic peptide fractionsthat influenced the glucose-induced secretion of insulin from rat, isolated pancreaticglands or islets. Purification was guided by monitoring this effect or chemicallyby analysis for the presence of cyst(e)ine peptides. The polypeptides dopuin (do, many; pu, proline) and daintain- /AIF1 (da, big; in,influencing; tai, peptide; in, insulin secretion /allograft inflammatory factor 1)represent novel structures. The multifunctional peptide DBI and the peptide hormonePP have now been isolated from intestinal tissues, while chicken DBI and its novel,processed form DBI35-86 represent peptides now isolated from avian sources. Bothof the purified chicken DBI forms inhibit glucose-induced insulin release in vitro,and were not known in structure earlier. Dopuin is a 62-residue polypeptide with a high content of pro-line in its N-terminalpart and a high proportion of histidine in its C-terminal part. Both proline andhistidine are important residues for specific structures and specialized functions.Six half-cystine resi-dues are present and shown to be involved in three intrachaindi-sulphide bridges (Cys22-25, 23-54, 35-44) which contribute to a tightly foldedcore segment in the molecular structure. At 10 nM concentration, the peptide hasan inhibitory tendency on glucose induced insulin release in vitro. Daintain/AIFI consists of 146 amino acid residues and is N-ter- minally acetyl blocked.An internal 44-residue segment with a sequence pattern (---KR--- KK---GKR---) resemblesthat found in the structural theme of peptide hormone precursors and of the GKR segmentfor C-terminal amide formation during the processing of peptide hormones. Daintain/AIFIis immunohistochemically localized to microglial cells in the central nervous system,to macrophages, and to tissue-specialized macrophages such as dendritic or Kupffer'scells in the immune system of different tissues. Its weakly insulin-suppressing activityin vitro, dual influence on glucose-stimulated insulin secretion in vivo and relativeabundance in macrophages of the pancreatic islets of prediabetic and newly diabeticBB rats raise the possibility that daintain/AIFI has a role in the pathogenesis ofinsulin dependent diabetes mellitus and other autoimmune diseases.