Messages from the inside
Tracking cell signaling proteins reveals how receptors keep sending messages after they are no longer on the surface of the cell.
Cells sense changes in their chemical environment using proteins called receptors. These proteins often sit on the cell surface, detecting molecules outside the cell and relaying messages across the membrane to the cell interior. The largest family of receptors is formed of ‘G protein-coupled receptors’ (or GPCRs for short), so named because they relay messages through so-called G proteins, which then send information into the cell by interacting with other proteins called effectors. Next, the receptors leave the cell surface, travelling into the cell in compartments called endosomes. Researchers used to think that this switched the receptors off, stopping the signaling process, but it is now clear that this is not the case. Some receptors continue to signal from inside the cell, though the details of how this works are unclear.
For signals to pass from a GPCR to a G protein to an effector, all three proteins need to be in the same place. This is certainly happening at the cell surface, but whether all three types of proteins come together inside endosomes is less clear. One way to find out is to look closely at the location of effector proteins when GPCRs are receiving signals. One well-studied effector of GPCR signaling is called adenylyl cyclase, a protein that makes a signal molecule called cAMP. Some G proteins switch adenylyl cyclase on, increasing cAMP production, while others switch it off.
To find out how GPCRs send signals from inside endosomes, Lazar et al tracked adenylyl cyclase proteins inside human cells. This revealed that a type of adenylyl cyclase, known as adenylyl cyclase 9, follows receptors as they travel into the cell. Under the influence of active G proteins, activated adenylyl cyclase 9 left the cell surface and entered the endosomes. Once inside the cell, adenylyl cyclase 9 generated the signal molecule cAMP, allowing the receptors to send messages from inside the cell. Other types of adenylyl cyclase behaved differently. Adenylyl cyclase 1, for example, remained on the cell surface even after its receptors had left, and did not signal from inside the cell at all.
Which cell behaviors are triggered from the membrane, and which are triggered from inside the cell is an important question in drug design. Understanding where effector proteins are active is a step towards finding the answers. This could help research into diseases of the heart, the liver and the lungs, all of which use adenylyl cyclase 9 to send signals.
Originally published at https://elifesciences.org/digests/58039.
