Serotonin Regulates Pancreatic β-Cell Mass during Pregnancy

During pregnancy, the energy requirements of the fetus impose changes in maternal metabolism. Increasing insulin resistance in the mother maintains nutrient flow to the growing fetus, while prolactin and placental lactogen counterbalance this resistance and prevent maternal hyperglycemia by driving expansion of the maternal population of insulin-producing β-cells1–3. However, the exact mechanisms by which the lactogenic hormones drive β-cell expansion remain uncertain. Here we show that serotonin acts downstream of lactogen signaling to drive β-cell proliferation. Serotonin synthetic enzyme Tph1 and serotonin production increased sharply in β-cells during pregnancy or after treatment with lactogens in vitro. Inhibition of serotonin synthesis by dietary tryptophan restriction or Tph inhibition blocked β-cell expansion and induced glucose intolerance in pregnant mice without affecting insulin sensitivity. Expression of the Gα(q)-linked serotonin receptor Htr2b in maternal islets increased during pregnancy and normalized just prior to parturition, while expression of the Gα(i)-linked receptor Htr1d increased at the end of pregnancy and postpartum. Blocking Htr2b signaling in pregnant mice also blocked β-cell expansion and caused glucose intolerance. These studies reveal an integrated signaling pathway linking β-cell mass to anticipated insulin need during pregnancy. Modulators of this pathway, including medications and diet, may affect the risk of gestational diabetes4.