Nucleotide-based signaling
Second messengers play an important role in cell signaling, relaying and amplifying signals from receptors on the cell surface to downstream targets. These messengers commonly include calcium ions (Ca2+), inositol triphosphate (IP3), diacylglycerol (DAG) and nitric oxide (NO), in addition to the cyclic nucleotides cAMP and cGMP. Nucleotide signaling pathways are numerous and have diverse effects, coordinating a wide range of cellular processes and physiological responses.
AMPK Signaling cAMP-mediated signaling G Beta Gamma Signaling G Protein Signaling Mediated by Tubby G-Protein Coupled Receptor Signaling GPCR-Mediated Integration of Enteroendocrine Signaling Exemplified by an L Cell GPCR-Mediated Nutrient Sensing in Enteroendocrine Cells Gα12/13 Signaling Gαi Signaling Gαq Signaling Gαs Signaling Protein Kinase A Signaling Rac Signaling RhoA Signaling RhoGDI Signaling Signaling by Rho Family GTPases Thrombin Signaling
Nucleotide signaling
While individual components differ from one nucleotide signaling pathway to another, they generally follow the same mechanism (1). A seven transmembrane G-protein coupled receptor (GPCR) receives a signal at the cell surface, undergoing a conformational change. The GPCR is then able to interact with an associated G-protein composed of three subunits: alpha (α), beta (β), and gamma (γ). The GPCR promotes the exchange of GDP for GTP on the α-subunit, causing it to dissociate from the βγ-dimer and freeing both to interact with downstream effectors.
Of the four families of G-proteins, G-αs and G-αi/o both signal through cAMP with G-αs proteins stimulating adenylate cyclase (AC) to produce cAMP from ATP. The increase in cAMP results in activation of protein kinase A (PKA) which goes on to phosphorylate a variety of target proteins. In contrast, G-αi/o proteins inhibit adenylate cyclase to block the production of cAMP from ATP, resulting in lower activation of PKA and reduced downstream signaling. G-αs and G-αi/o pathways are involved in processes such as metabolism, growth and neurotransmission.
As a precursor to cAMP, ATP availability can have a significant impact on G-αs mediated signaling. AMP activated protein kinase (AMPK) is a master metabolic regulator that activates pathways that generate ATP and inhibits pathways that consume ATP, thereby influencing nucleotide signaling via cAMP.
G-αq/11 proteins signal through phospholipase C (PLC) which goes on to hydrolyze phosphatidylinositol 4,5-bisphosphate (PIP₂), producing second messengers IP3 and DAG which can trigger the release of Ca2+ from intracellular stores and activation of the ERK1/ERK2 cascade via protein kinase C (PKC) respectively. G-αq/11 plays an important role in cellular processes such as muscle contraction and neurotransmitter release. In specialized intestinal enteroendocrine cells (IECs), it plays a role in nutrient sensing and the subsequent secretion of gut hormones peptide tyrosine tyrosine (PYY) and glucagon-like peptide-1 (GLP-1) which are both targets of anti-diabetic therapies.
G-α12/13 proteins signal through the activation of small GTPases, typically RhoA, Rac or Cdc42 which all impact the cytoskeleton and are therefore involved in a wide array of cellular processes ranging from development to cell migration and metastasis. GTPase activation occurs by the exchange of GDP for GTP at its binding site, enabling signal transmission through interaction with downstream effectors until the GTPase eventually hydrolyzes GTP to GDP.
The tightly regulated GTP-binding/GTPase cycle requires the coordinated action of three types of regulatory proteins. Guanine nucleotide exchange factors (GEFs) stimulate the GTP-GDP exchange reaction by inducing a conformational change in the GTPase that favors GTP binding. In contrast, GTPase activating proteins (GAPs), stimulate the GTP-hydrolytic reaction. Lastly, GDP-dissociation inhibitors (GDIs), antagonize the actions of both GEFs and GAPs.
At the same time that the α-subunit is free to interact with downstream effectors, the βγ-dimer is also free to interact with downstream effectors. Similar to G-αq/11 proteins, the βγ-dimer uses PLC as one of its mediators, activating the ERK1/ERK2 cascade via PKC.
While most GPCR pathways follow the canonical model of a receptor activated by ligand binding, usually a hormone or neurotransmitter, thrombin signaling is mediated via a unique class of GPCRs known as protease-activated receptors (PARs). PARs are trimeric GPCRs activated through cleavage by a serine protease like thrombin. Cleavage exposes a new N-terminus that acts as a tethered ligand, activating signaling primarily through G-αq/11, G-α12/13 and G-αi/o.
Through a combination of their unique seven transmembrane structure, diverse signaling partners and implication in numerous diseases, including diabetes, obesity, and cancer, GPCRs have been, and continue to be, a frequent, successful target of drug discovery efforts (3).
References
- Weiss WI, Kobilka BK. The molecular basis of G protein-coupled receptor activation. Annu Rev Biochem. 2018;20:87:897–919.
- Heuberger DM, Schuepbach RA. Protease-activated receptors (PARs): mechanisms of action and potential therapeutic modulators in PAR-driven inflammatory disease. Thromb J. 2019;29:17:4.
- Yang DH, et al. G protein-coupled receptors: structure- and function-based drug discovery. Signal Transduct Target Ther. 2021;6(1):7.