Browsing Pathways

The 5-HT4 receptor is primarily found in the CNS, GI tract and PNS. Peripheral receptors have important roles in the function of many organ responses (alimentary tract, urinary bladder, heart and adrenal gland). Alimentary tract receptors have a role in smooth muscle tone, mucosal electrolyte secretion, and the peristaltic reflex. Urinary Bladder receptors control cholinergic/purinergic transmission. Atrial heart receptors produce positive inotropy and tachycardia that can precipitate arrhythmias. This receptor is also thought to have roles in Anxiety, Appetite, GI Motility, Learning, Memory, Mood, and Respiration. The 5-HT4 receptor activates G(s) proteins which lead to the activation of adenylyl cyclase which produces the secondary messenger cAMP. cAMP activates PKA (protein kinase A) which phosphorylates downstream effectors that lead to a specific cellular response.

In the nervous system, dopamine acts as a neurotransmitter with roles in motor control, motivation, arousal, cognition, and reward. The mesolimbic pathway is the main pathway associated with reward, and the dopaminergic neurons of this pathway are found in the substantia nigra (SNc) and ventral tegmental area (VTA) of the midbrain. Dopamine acts on different G protein-coupled receptor subtyes. The D1-class (D1 and D5) receptors stimulate cAMP production by activating adenylyl cyclase, which activates the reward pathway. The D2-class (D2, D3, and D4) subtypes act oppositely, inhibiting cAMP production by inhibiting adenylyl cyclase. The differing distributions of the receptor subtypes mean that complex outputs often produce a synergistic effect, despite the receptor subtypes having opposite molecular effects.

Guanosine nucleotide-binding proteins (G proteins) are a class of proteins involved in transmitting extracellular stimuli to the inside of a cell. G proteins are activated by G protein-coupled receptors (GPCRs) on the membrane, which activate intracellular G proteins in respond to extracellular signaling factors such as hormones and neurotrasmitters. Activated GPCRs act as guanine nucleotide exchange factors, exchanging the GDP on the alpha subunit of inactive G proteins for GTP, thus turning the G proteins "on". G proteins may be mononmeric of trimeric. Monomeric G proteins consist of only the alpha subunit. Trimeric G proteins also have beta and gamma subunits, which dissocate from the alpha subunit together after activation. Once activated, both the alpha and beta/gamma subunits can then activate different signaling cascades. The alpha subunit eventually hydrolyzes the attached GTP to GDP using its inherent enzymatic activity, allowing it to re-associate and start a new cycle. Regulatory proteins may accelerate this hydrolysis to speed up signal termination. G proteins regulate numerous metabolic enzymes, ion channels, transporters, and other parts of the cell machinery.

G protein-coupled receptors sense stimuli outside the cell and transmit signals across the plasma membrane. Activation of protein kinase C (PKC) is one of the common signaling pathways. When a class of GPCRs are activated by a ligand, they activate Gq protein to bind GTP instead of GDP. After the Gq becomes active, it activates phospholipase C (PLC) to cleave the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacyl glycerol (DAG). IP3 can bind Ins3P receptor to open calcium channel by diffusion from cytoplasm to ER. Activated calcium channel will release the calcium from ER into cytoplasm. Calcium can activate the kinase activity of PKC.

The branched chain amino acid (BCAA) leucine is able to signal transduction pathways that modulate translation initiation for protein synthesis in skeleton muscles. In the presence of leucine, hyperphosphorylation of 4E-BP1 causes its affinity for eIF4E to be lowered. This allows eIF4F protein complexes to recognize, unfold and guide the mRNA to the 43S preinitiation complex thereby increasing translation initiation. In addition, leucine has a transient affect on the release of insulin and/or enhances sensitivity of muscle cells to insulin. A culmination of both signals at the mammalian target of rapamycin (mTOR) and perhaps other signaling, such as PKCδ, are needed for maximum translation initiation to occur.

Cyclooxygenase is the major producer of prostacyclin. Prostacyclin binding to its receptor increases vasodilation and decreases platelet aggregation. The receptor is a G-protein coupled receptor, upon its binding it activates G proteins causing the activation of adenylyl cyclase and production of cAMP messenger molecules. cAMP activates PKA (protein kinase A) which phosphorylates downstream effectors that lead to a specific cellular response.In vasodilation, the PKA activity causes phosphorylation of MLCK, decreasing its activity, resulting in dephosphorylation of MLC of myosin. This leads to smooth muscle relaxation resulting in vasodilation.

PGD is the major prostanoid released from human mast cells upon immunological challenge. Prostaglandin D2 (PGD2) can trigger asthmatic responses by acting as a cell-derived mediator in mice. The PGD receptor activates G(s) proteins which lead to the activation of adenylyl cyclase which produces the secondary messenger cAMP. cAMP activates PKA (protein kinase A) which phosphorylates downstream effectors that lead to a specific cellular response.

Histamine is an organic nitrogenous compound that is involved in local immune responses and it is a neurotransmitter for brain. Histamine can mediate various actions by interacting with histamine receptors (H1, H2, H3 and H4). The H2 receptor activates G(s) proteins which lead to the activation of adenylyl cyclase which produces the secondary messenger cAMP. cAMP activates PKA (protein kinase A) which phosphorylates downstream effectors that lead to a specific cellular response.

In the nervous system, dopamine acts as a neurotransmitter with roles in motor control, motivation, arousal, cognition, and reward. The mesolimbic pathway is the main pathway associated with reward, and the dopaminergic neurons of this pathway are found in the substantia nigra (SNc) and ventral tegmental area (VTA) of the midbrain. Dopamine acts on different G protein-coupled receptor subtyes. The D1-class (D1 and D5) receptors stimulate cAMP production by activating adenylyl cyclase, which activates the reward pathway. The D2-class (D2, D3, and D4) subtypes act oppositely, inhibiting cAMP production by inhibiting adenylyl cyclase. The differing distributions of the receptor subtypes mean that complex outputs often produce a synergistic effect, despite the receptor subtypes having opposite molecular effects (PMID: 20925949, 21303898).