Methods for the Analysis of Phosphoinositides and Inositol Phosphates

There is now compelling evidence that stimulation of inosito1 phospholipid metabolism via phosphoinositidase C (MC) is the signal transduction pathway for a wide variety of receptors in the mammalian nervous system. A by no means comprehensive list is shown in Table 1 . Receptors are thought to be coupled to PIC through a guanine nucleotide-binding protein (G protein—see Chapter 1 by Milligan) termed G , by a mechanism analogous to coupling of receptors with adenylyl cyclase, though as yet the identity of this G protein has not been established.* (For general reviews, see Hawthorne and Pickard, 1979 ; Downes, 1986 ; Fisher and Agranoff, 1987 ; Watson and Godfrey, 1988 ; Berridge and Irvine, 1989 ; Nahorski and Potter, 1989 ). The initial receptor-coupled event is a PIC-mediated breakdown of phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P₂ ) to give the two intracellular messengers inositol 1,4,5-trisphosphate (Ins(1,4,5)P)₃ which mobilizes intracellular Ca²⁺ (see Chapter 3 by Taylor et al.), and 1,2-diacylglycerol (DG), which activates protein kinase C (see Chapter 6 by Burgess). Ins (1,4,5)P₃ is then either phosphorylated to inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P₄ ) or dephosphorylated to Ins(1,4)P_3′ , the reactions catalyzed by a 3kinase or 5-phosphatase, respectively.