Stimulation of Inositol 1,4,5-Trisphosphate (IP3) Receptor Subtypes by Analogues of IP3

Most animal cells express mixtures of the three subtypes of inositol 1,4,5-trisphosphate receptor (IP3R) encoded by vertebrate genomes. Activation of each subtype by different agonists has not hitherto been examined in cells expressing defined homogenous populations of IP3R. Here we measure Ca2+ release evoked by synthetic analogues of IP3 using a Ca2+ indicator within the lumen of the endoplasmic reticulum of permeabilized DT40 cells stably expressing single subtypes of mammalian IP3R. Phosphorylation of (1,4,5)IP3 to (1,3,4,5)IP4 reduced potency by similar to 100-fold. Relative to (1,4,5)IP3, the potencies of IP3 analogues modified at the 1-position (malachite green (1,4,5)IP3), 2-position (2-deoxy(1,4,5)IP3) or 3-position (3-deoxy(1,4,5)IP3, (1,3,4,5)IP4) were similar for each IP3R subtype. The potency of an analogue, (1,4,6)IP3, in which the orientations of the 2- and 3-hydroxyl groups were inverted, was also reduced similarly for all three IP3R subtypes. Most analogues of IP3 interact similarly with the three IP3R subtypes, but the decrease in potency accompanying removal of the 1-phosphate from (1,4,5) IP3 was least for IP(3)R3. Addition of a large chromophore (malachite green) to the 1-phosphate of (1,4,5)IP3 only modestly reduced potency suggesting that similar analogues could be used to measure (1,4,5)IP3 binding optically. These data provide the first structure-activity analyses of key IP3 analogues using homogenous populations of each mammalian IP3R subtype. They demonstrate broadly similar structure-activity relationships for all mammalian IP3R subtypes and establish the potential utility of (1,4,5)IP3 analogues with chromophores attached to the 1-position.