VGF precursore ed eterogenei VGF-peptidi e loro risposta ad aspetti nutrizionali

vgf is an inducible gene implicated in the regulation of energy balance. Its derived gene product, VGF, acts as a multi-functional precursor, where from a modulated diversity of VGF peptides are differentially produced by post-translational processing. In the present context, it is relevant to note that chronic administration of the VGF derived peptide TLQP-21 delayed the onset of overt diabetes in Zucker diabetic fatty rats, and potentiated glucose stimolated insulin secretion from rat and human islets, resulting in improved glucose tolerance in vivo. The extended form TLQP-62 stimulated insulin secretion from insulinoma cell lines. Nonetheless, the precise localizations and in vivo response of several diversely acting VGF-peptides have been scarcely clarified. My aim has been to address the response to glucose of a number of VGF peptides, with a focus on: 3 peptide stretches within the C-terminal domain of the precursor (TLQP-21; NAPP-19 and VGF C-terminus), and the QQET-30 peptide. Thus, I have carried out an immunohistochemical and image-quantification study on the endocrine pancreas and the hypothalamic Suprachiasmatic Nucleus (SCN). In parallel, the same peptides were measured in plasma by ELISA. Specific antibodies for peptides related to the VGF C-terminus, and to the known VGF cleavage products: TLQP-21, NAPP-19, and QQET-30 were used. Antibodies were directed to the relevant N- or C-terminal end of the mentioned cleaved peptides. Affinity purification procedures (sequential and differential steps) were used to optimise antibodies and their applications, as well as to characterise their molecular specificity under non-competitive conditions. Mice (Group1, male, 12 weeks) underwent a period of starvation (18h), hence received (i.p.) a glucose load, or saline, and were sacrificed 30/120 min later. A pilot study (Group2) was perfomed on high-fat diet obese mice, similarly exposed to a glucose load. As expected, the glucose load resulted in a glicemia peak at 30 min, with return toward basal values at 120 min. TLQP-21 like peptides showed a progressive increase in plasma 30 and 120 min after the glucose load, in parallel to a major pancreatic islets degranulation at 30 min, and a reconstitution of their tissue stores at 120 min. Such peptides were not produced in the SCN, at least in their known, N-terminally cleaved form. Conversely, VGF C-terminus, QQET-30 and NAPP-19 related peptides were found to be reduced in plasma at 30 min, and returned toward basal values after 120 min. QQET-30 related peptides were not revealed in pancreatic islets, while showing a negative modulation (reduced immunoreactivity) in SCN 120 min after the glucose load. A similar trend was revealed for NAPP-19 related peptides. As to obese mice (Group2), TLQP-21 related peptides showed a clear cut increase vs. non-obese animals, both under basal and glucose-load conditions, associated with a blunted response to glucose in both pancreatic islets and plasma. In the same mice, NAPP-19 like peptides were somewhat more abundant in pancreatic islets and reduced in plasma, with little response to glucose. On the whole, VGF peptides appeared to show a significant response to the acute administration of glucose, with distinct differential patterns across the peptides studied. Of especial interest were TLQP-peptides, which showed a profile suggestive of prompt release to plasma upon the glucose load (30 min). Such pattern seemed to be maintained in the later phase studied (120 min), probably in connection with the biosynthesis and continuing release to plasma. In the obesity model studied, TLQP peptides were distinctly accumulated in pancreatic islets, in parallel with a absent response to glucose in plasma, suggestive of their deranged release. Taken together with known evidence of insulinotropic action of TLQP-peptides, the above results may be of relevance to the neuroendocrine response to an acute glucose load, hence to the pathophysiology of obesity.