Signal Transduction

The binding of an agonist (e.g. norepinephrine or oxytocin) to the surface receptor of smooth muscle induces a signal that spreads from the outside to the inside of the plasma membrane and activates several effectors that ultimately initiate contraction. There are three components of this system that we discuss: 1) Inositol 1,4,5-trisphosphate, 2) G-proteins, 3) Phosphoinositide-specific phospholipase C.

Inositol 1,4,5-trisphosphate: The inositol ring contains six hydroxyl residues, most of them can be phosphorylated by specific kinases. Inositol 1-monophosphate is the constituent of phosphatidylinositol (PI) one of the phospholipids in animal cell membranes. PI 4-kinase and PI (4) P 5-kinase to generate PI (4) P and PI (4,5) P2, respectively, sequentially phosphorylate PI. Inside the cell membrane resides a phosphoinositide specific phospholipase C, one of its hydrolytic product is inositol 1,4,5-trisphosphate (IP3), (see Fig. SM 8).

0P03 0P05

OPO3

Fig. SM8. D-myo-inositol 1,4,5-trisphosphate. (Barany and Barany, 1996b,with permission from Biochemistry of Smooth Muscle Contraction, 1996, Academic Press). The arrow indicates the site of the ester link with diacylglycerol in phosphatidylinositol. The negative charge of the phosphate group is not indicated.

OPO3

G-proteins: The guanine nucleotide binding proteins (G-proteins) are heterotrimers consisting of a-, p- and y-subunits. The a-subunits appear to be most diverse and are believed to be responsible for the specificity of the interaction of different G-proteins with their effectors. Fig. SM9 depicts a simple model for the activation of G-proteins. In the basal state, the a-subunit contains bound GDP and association of a- and py-subunits is highly favored, keeping the G-protein in the inactive form. Stimulation of the G-protein results when it binds GTP rather than GDP. Receptors interact most efficiently with the heterotrimeric form of the G-protein and accelerate activation by increasing the rate of dissociation of GDP and enhancing the association of GTP. Activation of G-protein coupled receptor results in the dissociation of heterotrimeric G-proteins into a-subunits and py-dimers. Finally, the G-protein a-subunit has an intrinsic hydrolytic activity that slowly converts GTP to GDP and returns the G-protein to its inactive form.

Phosphoinositide-specific phospholipase C: This term refers to a family of enzymes all specific for the phosphoinositide moiety of the phosphatidylinositol, but differing in their specificity depending on the number of the phosphoryl groups on the inositol ring. The p-, y- and 5-isoforms of PI-phospholipase C (PI-PLC) show the greatest specificity for the trisphosphorylated phospholipid (PIP2)). There are two basic mechanisms by which agonists activate PIP2 hydrolysis (Fig. SM10). In case of hormones, neurotransmitters, and certain other agonists, the signal is transduced to p-isozymes of PI-PLC. The upper left row of Fig. SM10 shows the most common pathway for PI-PLCp-isoform activation, initiated by stimulation of a ai-adrenergic receptor (ai-R) with norepinephrine (NE), and involving Gaq-proteins. The lower left row shows the activation of PI-PLC-p isoforms, initiated by acetylcholine (ACH) stimulation of M2-muscarinic receptor (M2-R), and mediated by the p y-subunit of the pertussis toxin-sensitive G-protein (GI). Concerning the other basic activating mechanism, e.g. in the case of growth factors, activation of their receptors results in enhanced tyrosine kinase activity. The right part of Fig. SM10 shows the activation of PI-PLC-y isoforms, initiated by the binding of epidermal growth factor (EGF) to its receptors, and executed by the tyrosine phosphorylation (YP) of PI-PLC-y . In all three examples, the activated PI-PLC hydrolyzes PIP2 to form the messengers IP3 and diacylglycerol (DAG). IP3 releases Ca2+ from the sarcoplasmic reticulum and thereby initiates smooth muscle contraction. DAG activates protein kinase C, the exact result of this activation is not known at the cellular level.

Fig. SM9. Model for the activation of G-proteins. ( Barany and Barany, 1996b, with permission from Biochemistry of Smooth Muscle Contraction, 1996, Academic

Press).

Fig. SM 10. Pathways for activation of PI-PLC isoforms. (Barany and Barany, 1996b, with permission from Biochemistry of Smooth

Muscle Contraction, 1996, Academic Press).

Fig. SM 10. Pathways for activation of PI-PLC isoforms. (Barany and Barany, 1996b, with permission from Biochemistry of Smooth

Muscle Contraction, 1996, Academic Press).

Was this article helpful?

0 0

Post a comment