Molecule of the Month: Incretins

Your body must carefully maintain the levels of sugar in your bloodstream within a narrow range. Too much sugar can damage organs, while too little sugar can cause confusion or even unconsciousness. When you eat a meal, carbohydrates are broken down into sugars and enter the bloodstream, increasing blood sugar levels. Within minutes of starting to eat, however, cells in your gut also rev into action and start secreting two peptide hormones--glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). Together, these hormones are known as incretins. Incretins travel to the pancreas, where they activate cells to secrete insulin, another small peptide hormone. Insulin is carried to different tissues throughout the body and instructs cells to take up glucose from the bloodstream.

GLP-1 and GIP (shown on the right) can activate cells by binding to GLP-1 receptors and GIP receptors on the plasma membrane. Shown below on the left is GLP-1 binding to the GLP-1 receptor, PDB 6B3J. On the right is GIP binding to the GIP receptor, PDB 7RA3. Both GLP-1 and GIP receptors are members of the large and diverse G protein-coupled receptor family. Members of this family are responsible for responding to diverse extracellular cues, such as odorants, neurotransmitters, and hormones (such as adrenaline, which binds to adrenergic receptors).

In the inactive state, GPCRs are bound to a G protein. When bound by their specific target ligand, GPCRs undergo a conformational change that facilitates recruitment and activation of a G protein through the release of GDP and subsequent exchange for a GTP. The G protein, which is made up of three subunits, called alpha, beta and gamma, then breaks apart into two pieces. The GTP-bound alpha subunit can move away and activate downstream pathways, and the beta and gamma subunits can also dissociate and trigger complementary pathways. Many GPCRs, including GLP-1R and GIPR, couple to the stimulatory G protein, Gs, to activate adenylyl cyclase and produce the second messenger cyclic AMP. You can read more about G proteins in a previous Molecule of the Month article.

Many different cell types express GLP-1R and GIPR and utilize G proteins to relay signals, but the impact of incretin binding can have vastly different results in different tissues. While binding to GLP-1 causes pancreatic cells to secrete insulin, in the brain, GLP-1 activates neurons in the hypothalamus, resulting in a feeling of satiation, or fullness. In the stomach and gut, GLP-1 binding impacts the smooth muscle and nervous system, acting to reduce muscle contractions and allowing food to stay in the stomach longer. While incretins have significant and widespread impacts on different organs, their actions are not long-lasting; GLP-1 and GIP are only able to circulate for a few minutes before they are inactivated. The enzyme dipeptidyl peptidase 4 (DPP4) is responsible for breaking down incretins.

Gila monsters are venomous, slow moving reptiles found in the American southwest. Scientists observed that, despite only consuming a few large meals a year, Gila monsters have no trouble maintaining their blood sugar levels. Subsequent studies in the 1990s showed that a peptide-based component of Gila monster venom could trigger insulin synthesis and release from the pancreas. The specific compound was named exendin-4.

In addition to being expressed in venom, exendin-4 is also produced endogenously in the Gila monster, where it slows down digestion and helps maintain blood glucose despite long periods of fasting. Interestingly, exendin-4 is similar in structure and function to GLP-1, but is resistant to degradation by DPP4, allowing exendin-4 to remain in circulation for hours rather than just a few minutes. Exendin-4 was eventually developed into exenatide, a treatment for diabetes and the first GLP-1 receptor agonist approved for medical use. On the right, exendin-4 is shown bound to the GLP-1 receptor (PDB 7LLL).