Figure 5.
Metabolic and signaling roles of the invertebrate BBB. The Drosophila BBB can sense and respond to various systemic signaling inputs. This includes the lipoprotein-like receptor (LRP) ligand lipophorin (LPP), the cytokine-like molecule Unpaired-2 (Upd2) and other unidentified signaling molecules (??) that are released in response to metabolic/nutritional triggers. This leads to localized secretion of Drosophila insulin-like peptide (dILP) by the subperineurial glia (SPG) and/or cortex glia (CG) to coordinate neuroblast (NB) proliferation. The coordinated response of the SPG is thought to occur through synchronous calcium (Ca2+) oscillations via gap junction proteins. In addition to localized growth regulation, the Drosophila BBB can also regulate systemic growth (whole organism growth). The SPG cells both express and constitutively secrete a negative regulator of insulin signaling: the secreted decoy of insulin receptor (SDR); SDR mutants lead to excessive growth of the whole animal.