- Email: [email protected]
Inositol lipids and calcium signalling
drugs really exist which fit into this new class of pharmacologic answer to both questions might be “no” for several reasons: ties within this group of drugs and they are chemically quite tive functions have not clearly
n established as yet neither in animal nor in human
clinical trials then the clinical relevance of those effects has to
FRC Utsilofhsecr
dagv olwl Pticologv,
Deparlment of Zaolqqv, Downing Street, Cambridge CB2 3EI. U.K.
cells respond to external signals they usually have to encode the information into a form which y an internal effector system. One of the major signal pathways used by a great variety of are cleaved as part of the transduction process to generate second lipid used is phosphatidylinositol4,Sbisphosphate (PtdIns4,5P,) and inositol 1,4,5-&phosphate (Ins(l;Q,S)P,) which are the two mechanism. The substrate used by the receptor mechanism is a minor orylation of phosphatidylinositol (PtdIns). A specific kinase phosphoryis then phosphorylated to PtdIns4,5P,. When an agonist binds to its nformational change w activates a GTP-binding protein (Gp) which, in turn, stimulates the ns(4,5)P,. This transduction mechanism can be activated indepenm combination with aluminium form an ALF,-complex which can formation of IX and Ins(l,4,5)P, even in intact cells. e Ins(1,4,5)P, released from the membrane is dephosphorylated to free inositol e simplest is a stepwise dephosphorylation sequence with Ins(l,rl)P, and Ins4P as is more complex in that it begins with a phosphorylation step whereby a kinase 3position to give Ins (1,3,4,5)P,. The tetrakisphosphate is then dephosphorylated and Ins3P as the major intermediates. agonists exert their effects by raising intracellular levels of calcium ate a variety of events including contraction, secretion and various ow most about bow calcium is released from specialized regions of the endoplasmic response to a rise in s(l,4,5)P3. This calcium-mobilizing action of Ins(1,4,5)P, has been permeabilized and int cells. The precise identity of the non-mitochondrial pool of calcium is setmuve to Ins(I.4,5)PS is still uncertain but current evidence seems to indicate that it is part of the
endoplasmic reticulum (ER). Of the calcium taken up by the non-mitochondrial pool of most cells, only 50% is mobilized by lns(1,4,5)P3 thus giving rise to the concept that there are Ins (1,4,5)P,-sensitive and in~~sitive calcium pools. These two pools may interact with each other, in both space and time, to gen patterns such as calcium waves and calcium oscillations. Calcium oscillations, which have now been described in many different cell types, have mtr~4;lced ; new way o thinking about how the calcium signalling system is organized. Based on the ~n~~,4,5~~~-indu~ oscillations w have been studied in Xenopus oocytes, a model has been developed to account for this oscillatory activity based two pools described earlier (Berridge and Galione ‘1988). The idez is the: lnti:1,4,5)P, provides a constant s calcium which then primes and ultimately triggers the lns(1.43) P,-insensitive pool to release calcium into tit Once all the stores have been emptied the calcium is pumped out of the cell and the intracellular level o returns ciose to basal thus generating a characteristic calcium transient. In the continued presence of agonist. especially at low concentrations, this transient begins to repeat itself thus setting up calcium oscillations. Oscilhttions based on this mechanism are very dependent on having a continuous influx of calcium from the bathing medium. Image analysis of single cells has begun to reveal that each of the transients making up an o~iilato~ sequence a precise spatial organization (Cheek et al., 1989). Certain cells appear to have distinct initiation sites from whence t signal then spreads &rough the cell in the fotm of a wave propagating at rates of lo-lOQPm/s. The same mechanisms used to describe oscillatory activity appear to be responsible for generating this wave-like behaviour. The onset of each calcium transient depends upon a carefully orchestrated sequence of events ~~~~ng with an 1~~~.4,5~P~-i~d~ mobilization of calcium which is amplified by a subsequent release of calcium from the Ins(l.4.5)P3-insensitive 1 of calcium. In effect, each transient reiterates a spatially organized programme of events. This unified spatiotemporal model provides a useful framework to explore further the relationship between Ins(lAS)P, and calcium si~alli~g-
Berridge, M.J. and GaBone. A. (1988) FASEB J. 2.3074-3082. Berridge, M.J. and Irvine, RF. (1989) Nature 341, 197-205. Cheek. T.R., O’Sullivan, A.J., Moreton, R.B.. Berridge, M.J. and Burgoyne. R.D. (1989) FEBS Letters 247.429-434.
Pharmacological knowledge comes from experiments. Experiments cannot be conducted with a blank mind. The choice of preparation, procedure and experimental design predicates idea. expectation, rationale. Pharmacological ideas are generated within the prevailing knowledge of biochemistry and physiology. inter alia. bi~he~st~ ident~fi~ the molecular components - enzymes, hormone receptors and so on - which pharmacologists can imagine as specific chemical targets. On the other hand, physiology, the study of organic systemi helps pharmacologists to anticipate the functional consequences of expe~mental molecular interactions. In ph~r~~a~~logy, the known ex~e~menta~ elements. the chemical input and functional or physiological output, are now rationally related to each other using molar models based on the hypothesized underlying molecular interactions at the biochemical level. Therefore. pharmacological experiments and related expectations are initiahy wholly rational. During an expe~ment, serendipity, fo~unate discovery ~cu~ng by accident, can happen at any time. However. the circumstances, that is the narrow window of experimental observation, which determined where the accident could occur, has been restricted by rationale. Moreover, serendipity is recognised because of surprise, surprise that the rationally-expected .xrtcome had not occurred. Therefore in pha~a~logy, rationale both determines and defines serendipity; rationale is p~rna~.
n established as yet neither in animal nor in human
clinical trials then the clinical relevance of those effects has to
FRC Utsilofhsecr
dagv olwl Pticologv,
Deparlment of Zaolqqv, Downing Street, Cambridge CB2 3EI. U.K.
cells respond to external signals they usually have to encode the information into a form which y an internal effector system. One of the major signal pathways used by a great variety of are cleaved as part of the transduction process to generate second lipid used is phosphatidylinositol4,Sbisphosphate (PtdIns4,5P,) and inositol 1,4,5-&phosphate (Ins(l;Q,S)P,) which are the two mechanism. The substrate used by the receptor mechanism is a minor orylation of phosphatidylinositol (PtdIns). A specific kinase phosphoryis then phosphorylated to PtdIns4,5P,. When an agonist binds to its nformational change w activates a GTP-binding protein (Gp) which, in turn, stimulates the ns(4,5)P,. This transduction mechanism can be activated indepenm combination with aluminium form an ALF,-complex which can formation of IX and Ins(l,4,5)P, even in intact cells. e Ins(1,4,5)P, released from the membrane is dephosphorylated to free inositol e simplest is a stepwise dephosphorylation sequence with Ins(l,rl)P, and Ins4P as is more complex in that it begins with a phosphorylation step whereby a kinase 3position to give Ins (1,3,4,5)P,. The tetrakisphosphate is then dephosphorylated and Ins3P as the major intermediates. agonists exert their effects by raising intracellular levels of calcium ate a variety of events including contraction, secretion and various ow most about bow calcium is released from specialized regions of the endoplasmic response to a rise in s(l,4,5)P3. This calcium-mobilizing action of Ins(1,4,5)P, has been permeabilized and int cells. The precise identity of the non-mitochondrial pool of calcium is setmuve to Ins(I.4,5)PS is still uncertain but current evidence seems to indicate that it is part of the
endoplasmic reticulum (ER). Of the calcium taken up by the non-mitochondrial pool of most cells, only 50% is mobilized by lns(1,4,5)P3 thus giving rise to the concept that there are Ins (1,4,5)P,-sensitive and in~~sitive calcium pools. These two pools may interact with each other, in both space and time, to gen patterns such as calcium waves and calcium oscillations. Calcium oscillations, which have now been described in many different cell types, have mtr~4;lced ; new way o thinking about how the calcium signalling system is organized. Based on the ~n~~,4,5~~~-indu~ oscillations w have been studied in Xenopus oocytes, a model has been developed to account for this oscillatory activity based two pools described earlier (Berridge and Galione ‘1988). The idez is the: lnti:1,4,5)P, provides a constant s calcium which then primes and ultimately triggers the lns(1.43) P,-insensitive pool to release calcium into tit Once all the stores have been emptied the calcium is pumped out of the cell and the intracellular level o returns ciose to basal thus generating a characteristic calcium transient. In the continued presence of agonist. especially at low concentrations, this transient begins to repeat itself thus setting up calcium oscillations. Oscilhttions based on this mechanism are very dependent on having a continuous influx of calcium from the bathing medium. Image analysis of single cells has begun to reveal that each of the transients making up an o~iilato~ sequence a precise spatial organization (Cheek et al., 1989). Certain cells appear to have distinct initiation sites from whence t signal then spreads &rough the cell in the fotm of a wave propagating at rates of lo-lOQPm/s. The same mechanisms used to describe oscillatory activity appear to be responsible for generating this wave-like behaviour. The onset of each calcium transient depends upon a carefully orchestrated sequence of events ~~~~ng with an 1~~~.4,5~P~-i~d~ mobilization of calcium which is amplified by a subsequent release of calcium from the Ins(l.4.5)P3-insensitive 1 of calcium. In effect, each transient reiterates a spatially organized programme of events. This unified spatiotemporal model provides a useful framework to explore further the relationship between Ins(lAS)P, and calcium si~alli~g-
Berridge, M.J. and GaBone. A. (1988) FASEB J. 2.3074-3082. Berridge, M.J. and Irvine, RF. (1989) Nature 341, 197-205. Cheek. T.R., O’Sullivan, A.J., Moreton, R.B.. Berridge, M.J. and Burgoyne. R.D. (1989) FEBS Letters 247.429-434.
Pharmacological knowledge comes from experiments. Experiments cannot be conducted with a blank mind. The choice of preparation, procedure and experimental design predicates idea. expectation, rationale. Pharmacological ideas are generated within the prevailing knowledge of biochemistry and physiology. inter alia. bi~he~st~ ident~fi~ the molecular components - enzymes, hormone receptors and so on - which pharmacologists can imagine as specific chemical targets. On the other hand, physiology, the study of organic systemi helps pharmacologists to anticipate the functional consequences of expe~mental molecular interactions. In ph~r~~a~~logy, the known ex~e~menta~ elements. the chemical input and functional or physiological output, are now rationally related to each other using molar models based on the hypothesized underlying molecular interactions at the biochemical level. Therefore. pharmacological experiments and related expectations are initiahy wholly rational. During an expe~ment, serendipity, fo~unate discovery ~cu~ng by accident, can happen at any time. However. the circumstances, that is the narrow window of experimental observation, which determined where the accident could occur, has been restricted by rationale. Moreover, serendipity is recognised because of surprise, surprise that the rationally-expected .xrtcome had not occurred. Therefore in pha~a~logy, rationale both determines and defines serendipity; rationale is p~rna~.