Persistent cAMP-signals triggered by internalized G-protein-coupled receptors

Area of Science:

• Cell Biology
• Molecular Endocrinology
• G-protein-coupled receptor signaling

Background:

• G-protein-coupled receptors (GPCRs) traditionally signal from the cell surface to second messengers like cyclic AMP (cAMP).
• Upon stimulation, GPCRs are internalized and generally thought to cease second messenger signaling.
• Intracellular GPCRs were believed to primarily trigger non-classical signals, such as mitogen-activated protein kinase (MAPK) activation.

Purpose of the Study:

• To investigate the signaling capacity of internalized GPCRs.
• To determine if internalized GPCRs can continue to produce cAMP.
• To explore the distinct cellular consequences of cell surface versus internalized GPCR signaling.

Main Methods:

• Generated transgenic mice with ubiquitous expression of a fluorescent cAMP sensor.
• Isolated native, 3-D thyroid follicles from these mice.
• Studied cAMP responses to thyroid-stimulating hormone (TSH) stimulation and receptor internalization.

Main Results:

• TSH stimulation led to the internalization of TSH receptors into a pre-Golgi compartment.
• Internalized TSH receptors remained associated with G-protein alpha(s)-subunits and adenylyl cyclase III.
• Internalized GPCRs continued to stimulate sustained cAMP production, distinct from cell surface receptor signaling.

Conclusions:

• GPCRs can continue to produce cAMP signals from intracellular compartments after internalization.
• Classical paradigms of GPCR signaling require revision to include intracellular cAMP production.
• Intracellular GPCR signaling may lead to cellular responses distinct from those initiated at the cell surface.