Phospholipid Signaling Protocols

371 Pages·1998·21.284 MB·English

by  Ian Bird

1
Phosphoinositidase C Activation Assay I
Cell Labeling, Stimulation, and Recovery of Cellular
pH]Phosphoinositides and PHjPhosphoinositols
Ian M. Bird
1. Introduction
1.1. Background
The minor mosttol-contammg membrane phosphohpids, the phospho-
mositides, play a central role m cell signal transduction. Activatron of a
hormone-sensitive phosphohpase C (phosphoinositidase C) at the plasma mem-
brane results in the rapid catabolism of the polyphosphoinosrtides to form the
two second messengers inositol 1,4,5-trtsphosphate (Ins( 1,4,5)P,), a water
soluble phosphomositol that promotes the release of Ca2+ from intracellular
stores, and diacylglycerol (DG), which remains in the plasma membrane and
activates protein kinase C (1-3). The metabolic pathways involved m the syn-
thesis of phosphattdylinositol 4&bisphosphate, and the metabolic fate of the
DG and Ins( 1,4,5)Ps formed on activation of phosphoinositidase C, are sum-
marized m Fig 1.
7.2. Experimental Sfrafegy
Hormone stimulation of phosphoinositidase C causes a rapid (within sec-
onds) loss of PIP2 and PIP, but slower loss of PI, together with a correspond-
ingly rapid (within seconds) formation of IP, and IP2 (and possibly IPJ, but
delayed rise in IP,. A complication in monitoring changes in the phospho-
inositides alone is the ability of cells to resynthesize PI rapidly, and therefore
PIP and PIP2 (see Fig. 1). However, inositol monophosphate phosphatases are
inhibited by Li+; thus if cells are premcubated in medium containing 10 rnM
From Methods m Molecular B/o/ugy, Vol 105 Phospholipid S/gna/mg Protocols
E&ted by I M Bird D Humana Press Inc , Totowa, NJ
1
2 Bird
A
J
Membrane
Ptdlns - Ptdlns4P - DG - PtdOH
CYUWJ/
lns4P - lns(1,4)P2 c-- lns(1,4,5)P,
InslP t-- Ins(lJIP2 InsP, B InsP,
*
.*
l
lns(l,3.4,61P,b
B
J
PI - PIP - PIP2 DG - PtdOH
_______________--_----------
Ins
LI+
I
\ IP, - IP, - IPI - lP4 4 * h/s
Fig. 1. Metabolic pathways activated as a consequence of phosphomosztidase C
action (A) Major metabolic pathways activated by phosphomositidase C action on
PtdIns(4,5)P, are shown with solid arrows Some of the additional pathways that may
be activated are shown by broken arrows. Abbreviations: PtdIns, phosphatidylinosztol;
PtdIns4P, phosphatidylinositol 4-phosphate; PtdIns(4,5)P,, phosphatidylinosztol
4,5-bzsphosphate; DG, diacylglycerol, PtdOH, phosphatidic acid; CDP-DG, CDP-
diacylglycerol; Ins, Inositol. For phosphoinositols, abbreviations are in the form
Ins(x,y,z)P,, where x, y, and z refer to the positions of the phosphate groups on the
myo-mositol ring and n refers to the total number of phosphates (B) A simplified
outline of the metabolic pathways in (A) also showing alternative abbreviations. PI,
phosphatidylinositol; PIP, phosphatidylinositol phosphate; PIP,, phosphatidylinositol
bu-phosphate; DG, PtdOH, CDP-DG and Ins as above. Phosphoinositols are referred
to as IP,, where n refers to the number of phosphates on the mositol rmg. In both
panels, sites of Li+ mhibztton are also shown.
LiCl, the water-soluble phosphoinositol products can accumulate over a longer
stlmulatlon time (minutes), predominantly in the form of IP, and IP2. Such
accumulation is a highly sensitive indicator of phosphomositldase C activation.
Phosphoinositldase C Activation Assay I 3
1.2.1. Cell Prelabeling
Phosphoinositides (with the exception of PI) and phosphomositols in the
small numbers of cells usually available are barely detectable by conventional
means. Therefore most studies use radiolabels for “quantification.” Radio-
labeled glycerol or fatty acids label all phospholipids, including phospho-
mositides, but not phosphoinositols; 32P,o n the other hand, labels not only all
phospholipids and phosphoinositols, but also nucleotide and sugar phosphates.
An alternative and widely used approach is to prelabel cells with myo-
[3H]inositol. Both phosphoinositides and phosphoinositols become labeled so
all metabolites can be monitored and, since myo-mositol is not rapidly
metabolized through other pathways, a labeled product indicates an inositol-
based structure. The only disadvantage is that it takes several days to label
phosphomositides to isotopic equilibrmm, or at least a steady state; only under
these conditions can changes m radioactivity be interpreted as changes in mass.
Nevertheless, detection of phosphoinositidase C activation by increased for-
mation of phosphoinositols can be successful with prelabelmg for several
hours. However, the attendant problems of increased phosphomositide label-
ing due to increased specific activity during stimulation and the nonlinear
increase in labeling of phosphoinositols that results means that long-term
labeling is the method of choice.
1.2.2, Cell Stimulation Conditions
The Li+ block technique requires preincubation of cells in a physiological
medium containing Li+ for at least 15 min prior to stimulation, and Li+ should
remain present for the stimulation period. It is also preferable to use medium
free of any pH indicators, since phenol red binds to anion exchange resins. The
volume of mcubatlon medium should be small (4 mL if possible), as salts
present m the medium are recovered m the final extracts and may interfere
with the subsequent chromatographic analysis (see Chapters 24).
7.2.3. Extraction of Labeled Products from Cells
Three extraction procedures are commonly used for maximum recovery of
highly charged radiolabeled products, namely the Bligh and Dyer acidified
solvent extraction procedure (451, and the perchloric acid (PCA) and trichlo-
roacetic acid (TCA) procedures. An advantage of the Bhgh and Dyer proce-
dure (4,5) is that it allows simultaneous and efficient recovery of both
phosphoinositols and phosphoinositides. However, if the extraction is to be
carried out on plastic culture dishes, or if samples are required for high-perfor-
mance liquid chromatography (HPLC) analysis, the PCA or TCA extraction
methods should be used. In these cases, the phosphoinositides can be recov-
ered from the protein/membrane pellets of PCA (or TCA) lysates by the acidi-
fied Bligh and Dyer method (see Subheadings 2.3. and 3.3.).
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1.2.4. Stability and Storage of Recovered Samples
Products are reasonably resistant to acid degradation, but only when kept at
0-4”C, so all samples should be processed immediately and kept on ice during
the extraction procedures. Phosphoinositides in membrane pellets from PCA
or TCA precipttation are only stable for several hours at -2O”C, because of the
presence of residual acid, Provided the aqueous extracts are neutrahzed, how-
ever, they can be stored frozen at -2OOC for several weeks. Phosphoinositides
extracted using the acidified Bligh and Dyer method can be stored for several
hours (overnight) at -2O”C, provided they have been dried down (so removing
acid) and redissolved in chloroform. To minimize oxtdatton of the unsaturated
fatty acids, samples should be stored in stoppered tubes with a mmimum an
space above flushed with nitrogen gas. If the phosphoinosmdes are deacylated
(see Chapter 2) the neutral glycerophosphoinositol products can be stored fro-
zen at -2O’C for several months.
2. Materials
General note: Purchase all solvents to analytical grade. Wear eye protection
and use a fume hood when performing extraction procedures. Use standard
radioactivity containment and disposal procedures.
2.1. Prelabeling of Cells in Culture
1. myo-[3H]Inositol: Aqueous solution (-20 Wmmol, 1 mCi/mL, Amersham, Ar-
lington Heights, IL) with anion exchange bead (to adsorb radiolytic degradation
products) (see Note 1). Store and use under sterile conditions.
2. Cells: Prepare usmg appropriate conditions for cells, and preferably plate m 12-
or 24-well plates at near confluence m growth medium (see Note 2).
3. Cell labeling medium: Cell “growth” medium supplemented with 10 pC!i/mL
myo-[3H]mositol (see Notes 1 and 2).
4. Sterile tissue culture supplies including pipet tips and 12- or 24-well culture
plates.
2.2. Preparation of Labeled Cells for Stimulation
1. Ml99 (basic physiologic medium or equivalent; see Note 3), 0.2% bovine serum
albumin (BSA).
2. Ml 99 or equivalent, 0.2% BSA, 10 mM Ins, 10 mM LiCl (see Note 8).
3. Agonist stocks prepared to at least 100X cont., and diluted to 10X cont. m Ml99
or equivalent, 0 2% BSA, 10 mMIns, 10 mMLiC1 (see Notes 4 and 8).
2.3. Acidified Nigh and Dyer Extraction
1. Chloroformmethanobconcentrated HCl (CHC13:MeOH:HCl), (100.200: 1 [v/v/v]).
2. Chloroform.
Phosphoinositidase C Activation Assay I 5
3. 0.1 M Hydrochloric acid.
4. 1 M Sodmm hydroxide.
5 Solvent resistant tubes (5 and 10 m.L).
6. Oxygen-free nitrogen gas.
7. Bench centrifuge.
8 Positive displacement or an displacement pipets (see Note 5)
2.4. PCA Extraction
1, 10 or 15% (w/v) Perchloric acid, as appropriate (see Subheading 3.4.).
2. DistIlled water
3. 1,1,2-Trichlorotrifluoroethane (freon):tri-n-octylamine (1: 1 [v/v]) (see Note 6).
4. MicrocentrifUge tubes (1 S mL).
5. Glass tubes (13 x 100 mm).
6. l-mL Syringe.
7. Microcentrifuge
8 Vortex mixer
2.5. TCA Extraction
1. 10 or 15% (w/v) Trlchloroacetlc acid, as appropriate (see Subheading 3.5.)
2. Distilled water
3. Water saturated diethyl ether.
4. Sodium hydrogen carbonate (100 mM).
5. Microcentrifuge tubes.
6. Glass tubes (13 x 100 mm).
7. 1- mL Syringe.
2.6. Recovery of Phosphoinositides from Acid-Insoluble Pellet
1. Chloroform:methanol:concentrated HCl (CHC&:MeOH:HCl), (100*200: 1 [v/v/v]).
2. Chloroform.
3. 0.1 M Hydrochloric acid.
4. Distilled water.
5. Solvent resistant tubes.
6. Oxygen-free nitrogen gas
7. Vortex mixer.
8. Bench centrifuge.
3. Methods
Procedures are described for the extraction of 0.5 mL of a cell suspension or
for extraction of a cell monolayer. These can be scaled up or down as appropri-
ate. Alternative procedures scaled for extraction of whole tissue are also
described in detail in Chapters 4 and 6 (see also Note 5).
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3.1. Labeling of Cells in Culture
1 Cells prepared and plated in 12- or 24-well plates are incubated for 24 h to allow
attachment.
2. Growth medium is removed and replaced with 0.5 mL of the same medium with
added myo-[3H]inositol (10 pCi/mL). Cells are preferably left to incorporate label
for 48 h before use (see Notes 1 and 2).
3.2. Preparation of Labeled Cells for Stimulation
3.2.1. Cells in Culture
1. Remove the labeled medium from each well (to a container in which it can be
stored safely for disposal) and wash once and replace with 0.5 mL of M199/BSA.
Incubate the cells for 15 min This washes away extracellular inositol.
2. Remove the medium from each well and replace it with 0 45 mL of M199/BSA
with added inositol (unlabeled, 10 mM) and LiCl(l0 mM). (Overfill the 1-mL tip
and dispense to resistance point only to eliminate large au bubbles m wells.)
Incubate the cells for a further 15 mm (to allow the cold inosltol to enter the cells
and start to chase out the labeled mositol, and to allow the LP to inhibit the
inosltol phosphate phosphatases).
3. At the end of the 15-mm incubation period, make additions as required in a vol-
ume of 50 pL and incubate as required.
4. Terminate stimulation as described in Subheadings 3.3.-3.6.)
3.2.2. Cells In Suspension
1. Label en mass as for plated cells (1.e , 10 pCl/mL medium).
2. Spin cells at 400g for 5 mm and resuspend in M199lBSA. Incubate for 15 min.
3. Spin as in step 2 and resuspend m Ml 99/BSA/LiCl/Ins
4 Spin as in step 2 and resuspend in M199/BSA/LiCl/Ins at a density of 200,00&
250,000 cells per 0.45 mL. Dispense to microfuge tubes or glass tubes (0.45 mL/
tube) as appropriate to extraction procedure (see Subheadings 3.3~3.6.)
5. Incubate for 10 mm before adding agonists (50 pL)
6 Incubate as required and extract as described (see Subheadings 3.3.-3.6.).
3.3. Acidified Bligh and Dyer Extraction
1 Add 1.88 mL of CHC13:MeOH:HCl to 0.5 mL of cell suspension; mix and allow
to stand for 5-10 mm The sample should form a single clear phase (see Note 7).
2 Add a further 0.625 niL CHC13 followed by 0.625 mL 0.1 MHCl, and mix gen-
tly. Two phases will form and any protein will precipitate.
3. Centrifuge the samples for 10 min in a bench centrifuge (16Og) to complete phase
separation. Both upper and lower phases should be clear, with protein at the interface.
4. Remove 1.8 mL (of approx 2.25 mL total) of the upper aqueous phase (contam-
ing inositol and phosphoinosltols) and neutralize to pH 7.0 using 1 A4 NaOH
(approx 70 pL). Store frozen at -20°C.
Phosphoinositidase C Activation Assay I 7
5. Transfer 1 mL (of approx 1.3 mL total) of the lower organic phase (contammg
phosphoinosrtides) to a solvent-resistant tube (5-mL tube if deacylatton is to be
carried out; see Chapter 2) and dry under a stream of nitrogen gas (warming the
tube to 3540°C tf necessary). Redissolve the dried matenal m chloroform as
reqmred
3.4. PCA Extraction
1. To cells (0.5 mL) incubated in solvent resistant (microcentrrfuge) tubes, add
0.5 mL of 10% PCA (Ice cold).
2 Alternatively, if cells are adherent to culture dishes/multiwell plates during stimu-
lation, add 0.25 mL of 15% PCA, and scrape the substratum with a syringe
plunger. Transfer all material to a solvent-resistant (preferably microcentrifuge)
tube; rinse each well with a further 0.5 mL H,O and transfer these washings to
the same (microcentrifuge) tube.
3. Pellet the precipitate by centrifugation (3 min at 3300g) and transfer all the
supernatant to a separate tube for neutralization. Complete transfer can be carried
out by decanting, provided the pellet is firm (see Notes 8 and 9).
4 Add 1 5 mL of freshly prepared freon:octylamine mixture (see Note 6) to the
aqueous extracts and mix thoroughly by vortexmg for 10 s, until the mixture takes
on a milky appearance Centrifuge samples for 2-3 min at 1300g. Three phases
should form. water (top), octylamine perchlorate (middle), and freon:octylamine
(bottom)
5. Remove 0.7 mL of the top phase, or 0.9 mL for samples from multrwell plates.
Check the sample pH; it should be neutral. Store samples frozen at -2O’C
3.5. TCA Extraction
1 Carry out steps l-3 of Subheading 3.4., substrtuting TCA for PCA.
2. Mix the aqueous extracts with 2 mL water-saturated diethyl ether (see Note 10).
After phase separation (using brief centrifugation if necessary to obtain a clean
interface), discard the bulk of the ether and repeat the extraction four trmes.
3. Evaporate the remaining ether by standing samples in a stream of an m a fume
cupboard. Neutralize each sample to pH 6.0-7.0 by addition of 100 rnUNaHC0,
(approx 50 )&/sample) Store samples frozen at -20°C.
3.6. Recovery of Phosphoinositides from Acid-insoluble Pellet
(see Notes 9 and 11)
1. Add 200 pL Hz0 to each pellet from 0.5 mL of cell suspension prepared as in
Subheadings 3.2. or 3.3. and freeze at -2O’C (this softens the pellet), Thaw
samples to room temperature.
2. Break up the pellet by vortexmg.
3. Add 750 & of CHCls:MeOH:HCl to each tube. Allow samples to stand for 5-10
min. A single clear phase should form.
4. Add 250 & CHCls, and 250 pL 0.1 M HCl to each tube. Centrifuge samples at
75g for 5 mm to separate the phases completely.
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5. Carefully remove and discard 600 pL of the upper aqueous phase
6. Carefully transfer 400 I.~L (83%) of the lower organic phase to a solvent-resistant
tube (5-mL tube if deacylatton is to be carried out, see Chapter 2). Remove sol-
vent and residual acid under a stream of nitrogen and redissolve in chloroform as
required.
4. Notes
1. As a general rule, for any phosphoinositidase C assay based on myo-[3H]mositol
labeling to be sensitive, cell labeling of the phosphoinositides after 48 h should
achieve -100,000 dpmwell in a 12-well dish (200,00&250,000 cells) This is
because, at basal level, the phosphoinositols are usually labeled to -0.1-l% of
the total phosphomosmde (lipid) labeling, and sttmulation may only hberate a
small percentage of lrptd label m a weakly respondmg tissue. Thus, if poor label-
mg is achieved, more radioactive tracer can be added to the labeling medium and/
or the 20-Wmmol preparation can be replaced wtth a higher spectfic activity
form (45-80 C&nmol, NEN DuPont) with labeling at 100 uCt/mL in growth
medium
2. Other factors that influence labeling efficiency are the “cold” inositol concentra-
tion of the basic medium, as well as the percent serum present, since serum also
contains inositol. Generally 10% serum m a balanced salt-nutrient medium with
-10 @4 or less mositol will give good results (see also Note 1).
3 Indicator-free medium should be used as a rule, since phenol red binds to amon-
exchange resins.
4. Many agomsts/pharmacological agents are poorly soluble m water and so must
be made up in solvents such as ethanol or DMSO. However these agents can also
have effects, at least in part, through changes in membrane fluidity. As a general
rule, such vehicle effects are mimmtzed or absent by making agents up to at least
100 times the final desired concentration in vehicle, and then diluting to 10 ttmes
in M199, 0.2% BSA, 10 mM inositol, 10 mM LiCI. The diluted agent is then
added as a 50 I.~L volume in a final total of 500 pL to give 1X concentration.
5. If an-displacement ptpets are being used to dispense volatile solvents or recover
the lower organic phase, then they should first be well-primed with the organic
solvent so the air inside the pipet becomes saturated with the vapor; otherwise the
first few samples will be short-measured.
6. Freonloctylamine should be prepared immediately before use This mixture will
react slowly on standing for more than 30 min.
7. If cells are attached to culture plates, the CHCl+MeOH:HCl can be added directly
and then rapidly transferred to a solvent-resistant tube for subsequent phase sepa-
ration. This procedure, however, is not recommended smce it may dissolve some
of the plastic (6,7).
8. A firmer membrane pellet is obtamed on centrifugatton of the acid lysate if the
cell incubation medium contams protein. If the incubation medium lacks protein,
it may be added (50 pL of 2% [w/v] BSA) after the acid.
Phosphoinositidase C Activation Assay I 9
9. When PCA or TCA lysates are pelleted and the acid supernatant decanted from
the membrane-protem pellet, tt is most important to remove as much of the
supernatant as possible Otherwise, too much water will remain to allow a single
phase to form on subsequent phosphoinositide extractton (see Subheading 3.6.,
step 3; see also Note 8).
10 An alternative to using diethyl ether to extract TCA is to carry out the freon/
octylamme procedure described for PCA extraction. Samples should be made
2 rr& wrth respect to ethylene diamme tetra-acetic acid (EDTA) before neutral-
ization is carried out.
II. If large numbers of samples are extracted by the PCA (or TCA) method, there
can be a considerable delay between decanting the supematant from the acid
lysate and extractmg the phosphomosrtides from the pellet. Under such circum-
stances, 200 pL H,O should be added to each pellet (Subheading 3.6, step 1)
after the supernatant is removed and the pellets frozen immediately. The
phosphoinositides are stable under these conditions for several hours only,
allowing time to complete neutrahzation of the aqueous extracts; but they should
be processed as soon as possible
Acknowledgments
I would hke to thank my former colleagues A. D. Smith, D. Sculster, S. W.
Walker, and B. C. Wllllams, with whom I performed these studies, and to
acknowledge the support of awards from the NIH (HL56702) and the USDA
(9601773) to IMB.
References
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(1984) The calcium signal and phosphattdylinositol breakdown m 2H3 cells,
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