Cell signaling and signal transduction: Signal transduction is the process of signal transmission from the cell surface receptors to intracellular targets. In most cases, the signal is amplified while it is transmitted intracellularly. The initial signal will be in the form of a hormone, growth factor, cytokine, neurotransmitter, drug, etc. The following patterns of signal transmission are seen at a cellular level:
i) G protein coupled receptors or GPCR: G proteins are intracellular signaling proteins which can bind GTP (hence the name). They have inherent GTPase activity, which can convert GTP to GDP. Heterotrimeric or three-component G-proteins consist of an alpha subunit G alpha, that binds and hydrolyzes GTP and beta and gamma subunits that inhibit G alpha, but can also participate in signaling reactions.
GPCR are plasma membrane receptors that are associated with heterotrimeric G proteins and have seven transmembrane spanning regions. When not stimulated, G alpha is bound to GDP and to G beta and gamma to form the inactive heterotrimer. Ligand binding to GPCR causes a conformational change leading to G alpha subunit to dissociate from GDP and bind to GTP. The G alpha-GTP complex now dissociates from the rest of the G protein complex.
Three types of Galpha subunits are present - Gs, Gq and Gi. Gs and Gq activate downstream enzymes while Gi inhibits the activity of downstream enzymes. The three subunits utilize specific enzymes to produce second messengers which further transmit the signal.
G alpha subunit: Gs
G alpha subunit: Gq
G alpha subunit: Gi
Eventually, the enzyme phosphodiesterase (PDE) converts cAMP to AMP, hence terminating the effects of increased cAMP. Methylxanthines like theophylline and caffeine are PDE inhibitors which cause a sustained increase in intracellular cAMP levels.
ii) Catalytic receptors: Most catalytic receptors are a group of single transmembrane protein chains with the cytoplasmic tail having catalytic activity. Thus, the receptor itself has catalytic activity. Catalytic receptors with intrinsic tyrosine kinase activity are used by insulin, TGF, EGF, and PDGF. Ligand binding causes autophosphorylation of tyrosine residues on the tail, followed by docking of adaptor proteins like Ras and JAK/STATs (signal transducers and activators of transcription).
In some cases, phosphatidylinositol kinase (PIK) pathway is activated by the catalytic receptor. It is important in mechanisms involved with cell growth and survival. PIK binds to phospho tyrosines in the tail and acts on membrane phospholipid phosphatidylinositol di-phosphate or PIP2 and converts it to phosphatidylinositol triphosphate or PIP3. It further docks Akt or Protein Kinase B, which then phosphorylates and inactivates pro-apoptotic factor Bad, hence preventing apoptosis. PIP3 is finally inactivated by PTEN, hence terminating the action.
Insulin receptor specifically causes tyrosine phosphorylation of IRS (Insulin Receptor Substrates) 1, 2,3 and 4. Downstream signals are transmitted using multiple pathways (signal-splitting) via Ras, MAPK, JAK/STATs, and PIK, leading to widespread effects. IRS 3 is expressed in fat cells, beta cells of the pancreas, and liver.
iii) MAP kinase pathway: This pathway utilizes a series of kinases that phosphorylate the next kinase in the pathway. The series begins with activation of MAP-kinase kinase kinase, which then activates MAP -kinase kinase, which then activates MAP kinase. MAP kinase (ERK) acts on several intracellular targets affecting cell growth and differentiation, immune systems, etc. Ras signal transduction protein uses the MAP kinase pathway for its effects. Ras is a small GTP-binding protein that resides in the plasma membrane. Receptors with tyrosine kinase activity, like Insulin receptors, activate Ras by GTP binding. Activated Ras then activates MAP-kinase kinase kinase.
iv) Ligand gated ion channels: Some ion channels in the plasma membrane are receptors for signaling molecules. Upon binding the signaling molecule, the ion channels open, allowing the passage of ions. This causes membrane depolarisation and generates an action potential. Examples are nicotinic, GABA-A, 5HT3 receptors, etc.
v) Steroid receptors: Steroid effects result from membrane initiated steroid signalling or MISS and nuclear initiated steroid signalling or NISS.
MISS activity involves Ras, G Proteins, tyrosine kinase, growth factor receptors, etc., culminating in phosphorylation of target proteins by kinases. Effects are seen within seconds to minutes.
NISS involves the steroid ligand binding to its intracellular or intranuclear receptor. Steroid binding activates the receptor, and the complex acts as a transcription factor to initiate transcription. Effect is seen within hours to days.
Intracellular steroid receptors may be cytoplasmic or nuclear. Nuclear receptors are seen with Vitamins A and D, retinoids, thyroid hormone, progesterone, and estrogen. Cytoplasmic steroid receptors are seen with testosterone, glucocorticoids, and mineralocorticoids. Cytoplasmic steroid receptors translocate to the nucleus after steroid binding. Activated hormone receptor complexes bind to HREs ( Hormone Response Elements) on DNA in the promoter or enhancer region with the help of zinc-finger motifs. The DNA-binding domain of the steroid hormone receptors contains zinc finger motifs which contain cysteine.
Sign up for free to take 5 quiz questions on this topic