Transthyretin binds to glucose-regulated proteins and is subjected to endocytosis by the pancreatic ß-cell.

Transthyretin (TTR) is a functional protein in the pancreatic ß-cell. It promotes insulin release and protects against ß-cell death. We now demonstrate by ligand blotting, adsorption to specific magnetic beads, and surface plasmon resonance that TTR binds to glucose-regulated proteins (Grps)78, 94, and 170, which are members of the endoplasmic reticulum chaperone family, but Grps78 and 94 have also been found at the plasma membrane. The effect of TTR on changes in cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) was abolished if the cells were treated with either dynasore, a specific inhibitor of dynamin GTPase that blocks clathrin-mediated endocytosis, or an antibody against Grp78, that prevents TTR from binding to Grp78. The conclusion is that TTR binds to Grp78 at the plasma membrane, is internalized into the ß-cell via a clathrin-dependent pathway, and that this internalization is necessary for the effects of TTR on ß-cell function.

Positioning High-Throughput CETSA in Early Drug Discovery through Screening against B-Raf and PARP1.

Methods to measure cellular target engagement are increasingly being used in early drug discovery. The Cellular Thermal Shift Assay (CETSA) is one such method. CETSA can investigate target engagement by measuring changes in protein thermal stability upon compound binding within the intracellular environment. It can be performed in high-throughput, microplate-based formats to enable broader application to early drug discovery campaigns, though high-throughput forms of CETSA have only been reported for a limited number of targets. CETSA offers the advantage of investigating the target of interest in its physiological environment and native state, but it is not clear yet how well this technology correlates to more established and conventional cellular and biochemical approaches widely used in drug discovery. We report two novel high-throughput CETSA (CETSA HT) assays for B-Raf and PARP1, demonstrating the application of this technology to additional targets. By performing comparative analyses with other assays, we show that CETSA HT correlates well with other screening technologies and can be applied throughout various stages of hit identification and lead optimization. Our results support the use of CETSA HT as a broadly applicable and valuable methodology to help drive drug discovery campaigns to molecules that engage the intended target in cells.

Transthyretin constitutes a functional component in pancreatic beta-cell stimulus-secretion coupling.

Transthyretin (TTR) is a transport protein for thyroxine and, in association with retinol-binding protein, for retinol, mainly existing as a tetramer in vivo. We now demonstrate that TTR tetramer has a positive role in pancreatic beta-cell stimulus-secretion coupling. TTR promoted glucose-induced increases in cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) and insulin release. This resulted from a direct effect on glucose-induced electrical activity and voltage-gated Ca(2+) channels. TTR also protected against beta-cell apoptosis. The concentration of TTR tetramer was decreased, whereas that of a monomeric form was increased in sera from patients with type 1 diabetes. The monomer was without effect on glucose-induced insulin release and apoptosis. Thus, TTR tetramer constitutes a component in normal beta-cell function. Conversion of TTR tetramer to monomer may be involved in the development of beta-cell failure/destruction in type 1 diabetes.

Apolipoprotein CIII promotes Ca2+-dependent beta cell death in type 1 diabetes.

In type 1 diabetes (T1D), there is a specific destruction of the insulin secreting pancreatic beta cell. Although the exact molecular mechanisms underlying beta cell destruction are not known, sera from T1D patients have been shown to promote Ca(2+)-induced apoptosis. We now demonstrate that apolipoprotein CIII (apoCIII) is increased in serum from T1D patients and that this serum factor both induces increased cytoplasmic free intracellular Ca(2+) concentration ([Ca(2+)](i)) and beta cell death. The apoCIII-induced increase in [Ca(2+)](i) reflects an activation of the voltage-gated L-type Ca(2+) channel. Both the effects of T1D sera and apoCIII on the beta cell are abolished in the presence of antibody against apoCIII. Increased serum levels of apoCIII can thus account for the increase in beta cell [Ca(2+)](i) and thereby beta cell apoptosis associated with T1D.