
* The Phagocyte Research Laboratory, Department of Medical Microbiology and Immunology, University of Göteborg, S-413 46 Göteborg, Sweden
Department of Immunology and Parasitology, Yamagata University School of Medicine, Yamagata, Japan
Correspondence: Claes Dahlgren, Department of Medical Microbiology and Immunology, University of Göteborg, Box 435, 405 30 Göteborg, Sweden. E-mail: Claes.Dahlgren{at}microbio.gu.se
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Key Words: granulocytes subcellular localization granule cell adhesion
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The neutrophil contains several types of subcellular organelles: peroxidase-positive granules (termed primary or azurophil granules) that in many respects (but not all) resemble classical lysosomes [8 , 9 ] and at least two types of peroxidase-negative granules/vesicles (secondary or specific granules and secretory vesicles, respectively) [7 , 10 , 11 ]. The membrane of the specific granules contains a variety of different receptors such as CD11b/CD18 (Mac1), CEACAMs (CD66a and b), the formylpeptide receptor (FPR), the fibronectin receptor, and the laminin receptor. These are all up-regulated on the cell surface as a consequence of granule fusion with the plasma membrane. However, several neutrophil receptors, including Mac-1 and FPR, can be mobilized to the cell surface without corresponding exocytosis of specific granule contents. This is explained by the fact that these receptor molecules are stored not only in the membrane of the specific granules but also in the membrane of the most easily mobilizable neutrophil organelle, the secretory vesicle [7 , 10 , 11 ]. The aim of the present study was to determine the subcellular localization of GPI-80 in human neutrophils. We found that the secretory vesicles are the intracellular reservoir for this novel, adhesion-regulating protein.
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Isolation of cells
Human polymorphonuclear leukocytes (neutrophil granulocytes)
were isolated from buffy coats as described by Bøyum et al.
[13
]. The purified neutrophils were washed twice and
resuspended in Krebs-Ringer medium (120 mM NaCl, 4.5 mM KCl, 1.2 mM
MgSO4, 1.0 mM CaCl2, 1.7 mM
KH2PO4, 8.3 mM Na2HPO4,
10 mM glucose, pH 7.3). Cells used for subcellular fractionation were
resuspended in physiological saline and treated with the serine
protease inhibitor DFP (5 mM, 10 min on ice). The cells were then
washed and resuspended in relaxation buffer [10 ml, 100 mM KCl, 3 mM
NaCl, 3.5 mM MgCl2, 10 mM Pipes, 1 mM ATP(Na)2,
0.5 mM phenylmethylsulfonyl fluoride (PMSF), pH 7.4].
Subcellular fractionation
Neutrophils (5x108 cells) in relaxation
buffer were disrupted in a nitrogen bomb (375 psi; Parr Instruments,
Moline, IL). To separate azurophil granules, specific granules, and
plasma membrane/secretory vesicles, the postnuclear supernatant was
layered on top of two-step (1.05 and 1.12 g/ml), Percoll-density
gradients and fractionated according to the technique described by
Borregaard et al. [14
] and Sjölin
et al. [15
]. A modification of the flotation
technique described earlier [8
] was used to separate
secretory vesicles from plasma membranes. In short, the postnuclear
supernatant was mixed with twice the vol of a heavy Percoll solution
(1.12 g/ml), and this mixture (15 ml) was underlaid 15 ml of a light
(1.04 g/ml) Percoll solution. Relaxation buffer (5 ml) was applied on
top of the gradient, which was then centrifuged at 37,000 g
for 30 min (4°C) using a fixed-angle Beckman JA-20 rotor. Gradient
fractions (1.5 ml) were collected from the bottom of the gradients.
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis
(SDS-PAGE) and Western blotting
SDS-PAGE was performed essentially according to Laemmli
[16
] using homogenous 10% polyacrylamide gels. The
gradient fractions were applied to the gels in vol corresponding to the
fractionated content of 2.5 x 105 cells. The gels
were blotted electrophoretically onto PVDF membranes, and the membranes
were blocked and incubated with antibodies as described
[15
]. The blots were then developed with substrate
buffer containing NBT (0.3 mg/ml) and BCIP (0.15 mg/ml). The blots were
replicated using a UMAX C12 scanner, and analysis of the images was
performed using the public domain NIH Image program (developed at the
U.S. National Institutes of Health and available on the Internet at
http://rsb.info.nih.gov/nih-image/).
Quantification of granule markers
The Percoll fractions were assayed for myeloperoxidase
(MPO) [17
], vitamin B12 binding protein
(B12BP) [18
], and alkaline phosphatase (ALP)
in the presence and absence of Triton X-100 as described
[19
]. The molecules analyzed constitute markers for
azurophil granules (MPO), specific granules (B12BP),
secretory vesicles (latent ALP), and plasma membranes (nonlatent ALP
activity), respectively. ALP latency was determined by subtracting the
activity measured in the absence of Triton X-100 (nonlatent pool) from
the activity obtained in the presence (total pool) of detergent.
Surface up-regulation of complement receptor 3 (CR3) and GPI-80
Cells were kept at 4°C (resting cells) or stimulated
with formylmethionyl-leucyl-phenylalanine (fMLF;
10-7 M final concentration) at 15°C for 10
min followed by incubation at 37°C for another 10 min. In one set of
experiments, the expression of GPI-80 on the cell surface was analyzed
by adding 10 µl mouse mAbs (3H9 diluted 1/100) to cell pellets
(around 106 cells). The cells were incubated at 4°C for
30 min, washed twice, and incubated with a FITC-labeled secondary
antibody, washed again, and examined in a flow cytometer (FACS Scan;
Becton Dickinson, Mountain View, CA). Phycoerythrin-conjugated mAbs
against CD11b (Becton Dickinson) were used to determine mobilization of
CR3 [20
]. In another set of experiments, PLC (50 ng/ml,
final concentration) was added to the cells that were then incubated
for 10 min at 37°C. The proteins released from the cell surface by
PLC were isolated by brief centrifugation of the cells in a microfuge.
The resulting supernatants were analyzed with respect to ALP activity
and content of GPI-80.
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![]() View larger version (18K): [in a new window] |
Figure 1. Mobilization of secretory vesicles. Surface up-regulation of Mac-1 (a)
and GPI-80 (b) on human neutrophils, expressed as mean fluorescence
intensity of cells labeled with antibodies against CD11b (a) and GPI-80
(b). Cells were kept at 4°C (control; solid lines) or preactivated
with fMLF (see Materials and Methods; 10-7 M
final concentration; dotted lines). A representative experiment out of
six is shown, and the increased expression of Mac-1 and GPI-80,
respectively, calculated as the ratio, and absolute difference between
the mean fluorescence value, the fMLF, and control cell populations
from the six experiments is given. Panel (c) shows the release into the
medium of markers for specific granules (vitamin
B12-binding protein; B12bp) and azurophil
granules (MPO) after fMLF-induced activation of the cells. The release
is expressed as percent of the total amount in control cells.
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![]() View larger version (44K): [in a new window] |
Figure 2. Cleavage of GPI-80 and alkaline phosphatase by a PI-specific PLC.
PI-PLC was added to neutrophils that were preactivated with fMLF
(10-7 M final concentration) as well as to
nontreated control cells. The cells were removed by centrifugation, and
the amounts of ALP (determined by the enzyme activity in the samples)
and GPI-80 (determined by Western blotting and quantification by
densitometric measurements of the blots), respectively, were
determined. The figure shows an immunoblot for GPI-80 from one
representative experiment out of four. The figures represent the mean
ratios obtained for ALP and GPI-80, respectively, between amounts in
the supernatants of the fMLF and control cell populations in the four
experiments.
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-band),
specific granules (the ß-band), and azurophil granules (the
-band;
Fig. 3a ). No GPI-80 was detected in the S2 fraction, which is what
was to be expected because GPI-80 is a membrane protein. GPI-80 was not
found in the azurophil-granule fraction either (the
-band) or the
specific-granule fraction (the ß-band). A modified, two-step
gradient, allowing an even better separation of the plasma
membrane/secretory vesicles from the gelatinase granules, revealed that
no GPI-80 was present in any of the granule subtypes. Instead it was
found exclusively in the light membrane fraction (the
-band),
enriched for secretory vesicles and plasma membranes
(Figs. 3 and 4
).
![]() View larger version (39K): [in a new window] |
Figure 3. Identification of GPI-80 in neutrophil subcellular fractionations. A
postnuclear supernatant of disrupted neutrophils was fractionated on a
discontinuous, two-layer, Percoll-density gradient (14 ml 1.12 g/ml and
14 ml of 1.05 g/ml). Three bands were visible, which were denoted ,
ß, and in order of decreasing density. Cytosol components were
located in the upper portion of the gradient (S2). a)
Fractions were analyzed for myeloperoxidase (a marker for azurophil
granules; ), B12BP (marker for peroxidase
negative-specific granules; ), nonlatent ALP (marker for the plasma
membranes; ), and latent ALP (marker for plasma the secretory
vesicles; ). Each fraction was also analyzed by SDS-PAGE under
nonreducing conditions in 10% (w/v) polyacrylamide gels and
immunoblotting with anti-GPI-80 antibodies (b). The results of one
representative experiment of six are shown in (b). The arrowheads
designate the peak fractions of the , ß, and bands,
respectively.
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![]() View larger version (35K): [in a new window] |
Figure 4. Localization of gelatinase and GPI-80 in neutrophil-subcellular
fractionations. A postnuclear supernatant of disrupted neutrophils was
fractionated on a discontinuous, two-layer, Percoll-density gradient (3
ml 1.12 g/ml and 25 ml 1.05 g/ml Percoll, respectively). The peak
fractions for MPO and vitamin B12BP are marked and ß,
respectively. The fractions (125) were analyzed by SDS-PAGE under
nonreducing conditions in 10% (w/v) polyacrylamide gels and
immunoblotting with anti-gelatinase antibodies (upper panel) and
anti-GPI-80 antibodies (lower panel), respectively.
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-fraction, the postnuclear material was
fractionated on a flotation gradient. Two light membrane bands were
seen in the gradient after centrifugation, and based on the
distribution profiles for MPO (azurophil-granule marker) and vitamin
B12BP (specific-granule marker), we conclude that the
modified gradient permits a clear separation of the light membranes
from the granules. The nonlatent ALP activity and latent ALP activity
were also clearly separated. Most of the alkaline phosphatase
present in the
1-fraction (Fig. 5
) was latent (80.6±4.7%; mean±SD, n=5) and
thus derived from the secretory vesicles, whereas most of the activity
present in the
2-fraction (Fig. 5)
was nonlatent (84±10%;
mean±SD, n=5) and thus derived from the plasma
membrane. We conclude that our gradient system permits a clear
separation of the secretory vesicles (
1) from the plasma
membranes (
2). GPI-80 was found in the secretory-vesicle
fraction and the plasma-membrane fraction, respectively, in similar
amounts (Fig. 6
).
![]() View larger version (20K): [in a new window] |
Figure 5. Separation of secretory vesicles and plasma membranes. A postnuclear
supernatant of disrupted neutrophils was fractionated on a modified,
Percoll-density gradient. Fractions were analyzed for nonlatent ALP
activity (plasma membrane marker; solid symbols) and latent ALP
activity (marker for secretory vesicles; open symbols). The positions
in the gradient of the azurophil granules ( ) and the specific
granules (ß) are indicated by arrows. Fractions enriched in secretory
vesicle ( 1) and plasma membrane ( 2), respectively, were pooled,
concentrated, and analyzed further by SDS-PAGE and immunoblotting (Fig. 6)
.
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![]() View larger version (52K): [in a new window] |
Figure 6. Identification of GPI-80 in plasma membrane and secretory
vesicles. Fractions enriched in secretory vesicles ( 1) and plasma
membrane ( 2), respectively, were prepared as described (see the
legend to Fig. 5
) and analyzed by SDS-PAGE under nonreducing conditions
in 10% (w/v) polyacrylamide gels. Immunoblotting was performed with
anti-GPI-80 antibodies. The results of one representative experiment
out of five are shown, together with the ratio (mean±SD)
between the amount GPI-80 present in the secretory vesicles and the
plasma membranes calculated from five experiments.
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The secretory vesicles are of endocytic origin. Accordingly, the
vesicle matrix contains plasma proteins [22
], and the
membrane is derived from the plasma membrane. During vesicle formation,
the plasma membrane proteins are sorted to become included (and
sometimes even enriched) or excluded from the invagination that forms
the vesicle. The membrane of the secretory vesicles has subsequently
been shown to contain a number of different proteins including
CR3/Mac-1, the b cytochrome of the neutrophil nicotinamide adenine
dinucleotide phosphate (NADPH) oxidase, CR1, FPR, Fc receptor for IgG
(Fc
R)III, and the UPA receptor, and we can now add GPI-80 to this
group of membrane proteins. The presence in the secretory vesicles of
this array of functionally important proteins implies that these
organelles, when incorporated into the plasma membrane in response to
inflammatory mediators, furnish the cell surface with new proteins.
This alters the functional repertoire of the neutrophil markedly and
enables it to interact with endothelial cells, extracellular matrix,
and microorganisms. Secretory vesicles are therefore believed to be of
prime importance in the early neutrophil activation, leading to
endothelial adhesion and transmigration. The subcellular localization
of GPI-80 is in line with its proposed function as a regulator of
neutrophil adhesion and migration events. Although the secretory
vesicles resemble the so-called caveolae, in that they contain several
GPI-anchored molecules, the two are different in nature. This is
illustrated clearly because the secretory vesicles are not
primary-located to the subplasmalemmal region, and they lack caveoline
[23
].
GPI-80 is probably involved in the regulation of neutrophil adhesion, but very little is known about the precise mechanisms involved in the GPI-80-mediated sequential up- and down-regulation of ß2-integrin-dependent neutrophil functions. A GPI-induced modulation of the avidity of the ß2-integrin for its ligands has been suggested [24 ], and this gains support from the fact that another GPI-anchored protein (UPAR) has been shown to be of importance for leukocyte recruitment via the ß2-integrin [25 ]. It is worth noting that UPAR is also present in the secretory vesicles [26 ]. Taken together, the presented data show that GPI-80 is present in the membrane of the secretory vesicles and in the plasma membranes of resting neutrophils. GPI-80 may, when present on the surface of activated/extravasating phagocytes, have a potentially important role as modulator of neutrophil ß2-integrin-mediated adhesion, but further investigations are needed to clarify the precise role of GPI-80 in neutrophil adhesion, the functional significance of a 50% increase in surface GPI-80, and the mechanism(s) by which it affects the properties of other adhesion molecules.
rdalstiftelsen, the Fredrik and Ingrid
Thuring Foundation, the Swedish Society for Rheumatological Research,
the Swedish Society for Medical Research, and the Swedish Society for
Medicine supported the work. The technical assistance of Lisbeth
Björck is greatly appreciated. Received May 30, 2000; revised September 15, 2000; accepted September 20, 2000.
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