Vol. 274, Issue 1, F26-F33, January 1998
Distinct localization of renin and GLUT-4 in juxtaglomerular
cells of mouse kidney
Timothy J.
Anderson1,
Sally
Martin1,
Jennifer L.
Berka2,
David E.
James1,
Jan W.
Slot3, and
Jennifer L.
Stow1
1 Centre for Molecular and
Cellular Biology, University of Queensland, Brisbane, 4072 Queensland;
2 Department of Physiology,
University of Melbourne, Parkville, 3052 Victoria, Australia; and
3 Department of Cell Biology,
State University of Utrecht, 3511 HG Utrecht, The
Netherlands
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ABSTRACT |
The insulin-responsive glucose transporter, GLUT-4, is found
primarily in adipocytes and skeletal muscle cells, where it is sequestered in a specialized recycling compartment, from which it can
be recruited to the cell surface following insulin stimulation. Lower
levels of GLUT-4 are also expressed in other tissues, including the
kidney, where it is present particularly in cells of the afferent arteriole and juxtaglomerular apparatus (JGA). The exact nature of
GLUT-4-containing compartments and their relationship to other regulated trafficking pathways in different cells are not yet well
defined. The trafficking of GLUT-4 has been studied in different cells
with regulated secretory pathways, and a recent study shows that, in
cardiomyocytes, GLUT-4 is sorted and packaged into multiple regulated
pathways (J. W. Slot, G. Garruti, S. Martin, V. Oorschot, G. Pshuma, E. W. Kraegen, R. Laybutt, G. Thibault, and D. E. James. J. Cell Biol. 137: 1243-1254,
1997). In the kidney, cells of the JGA synthesize and secrete their
major product, renin, via a well-established, regulated, secretory
pathway. These cells also express GLUT-4 and thus offer the potential
to directly compare the localization and trafficking of GLUT-4 and
renin in a unique cell type. The present study was undertaken to
investigate the intracellular distribution of GLUT-4 in mouse kidney
cortex and to determine whether GLUT-4 and renin are trafficked in the
same or in separate regulated pathways. Ultrathin cryosections of mouse
kidney were labeled by the immunogold technique and viewed by electron
microscopy, demonstrating the distribution of GLUT-4 in cells of the
JGA, afferent arteriole, and distal tubule. In granular cells of the JGA, renin was localized in secretory granules of the regulated secretory pathway, whereas GLUT-4 labeling in the same cells was found
in a distinct tubulovesicular compartment located adjacent to the
trans-Golgi network. We show that granular cells have separate, morphologically distinct compartments for the sequestration of renin
and GLUT-4, providing evidence that there may be distinct pathways for
the sorting and trafficking of these two proteins.
insulin-regulated glucose transporter; granular cell; regulated
secretion
 |
INTRODUCTION |
RENIN IS A KEY ENZYME in the generation of the
vasoactive hormone, angiotensin II, and in this role it is essential
for maintaining blood pressure and electrolyte balance. Circulating
renin in the plasma is primarily derived from the juxtaglomerular
apparatus (JGA) of the kidney. A group of modified smooth muscle cells
in the wall of the terminal portion of the afferent arteriole, known as
granular cells, are responsible for the synthesis and secretion of
renin within the JGA (8). Renin is initially synthesized as an inactive
precursor, prorenin, which is trafficked through the Golgi complex and
packaged into immature secretory granules or protogranules. During
maturation of the granules, the prosequence is cleaved to form active
renin, which is then stored in the mature secretory granules (8).
Active renin is secreted via a stimulus-coupled, regulated secretory
pathway in which stored renin is released into the surrounding
interstitium from individual granules or by compound exocytosis from
multiple granules (4, 16). In some circumstances, prorenin may also be
secreted from immature granules (4, 25) or in smaller vesicles via a
constitutive pathway (7).
Renin-containing granules are somewhat atypical secretory granules,
having some features in common with lysosomes; myeloid bodies and
vesicular membranes are found as inclusions within the granules, and
they contain proteolytic enzymes, including cathepsin B for processing
prorenin, cathepsin D, and other acid hydrolases (26). To date,
however, many aspects of the renin secretory pathway remain poorly
understood; for instance, it is not known how prorenin is sorted and
packaged into immature granules at the level of the trans-Golgi network
(TGN). One approach to further investigating renin secretion is to
compare the sorting and trafficking of renin with that of other
proteins trafficked in the same, or in distinct, regulated pathways in
granular cells.
Glucose transporters are widely and variably distributed in different
cells, where they reside constitutively at the cell surface to mediate
the facilitative uptake of glucose into cells (1). One member of this
family, the insulin-regulated glucose transporter, GLUT-4, is
sequestered within the cell and is recruited to the cell surface to
augment glucose uptake in response to insulin (reviewed in Ref. 13).
GLUT-4 is expressed in adipocytes and muscle cells, which have acute
and frequent requirements for increased glucose uptake (19, 20). In the
unstimulated state, GLUT-4 in these cells is concentrated in a
specialized tubulovesicular compartment located just beneath the plasma
membrane (19, 20). This reservoir of GLUT-4 is then available for rapid
recycling to and from the cell surface upon insulin stimulation (21). GLUT-4 is thus trafficked, within specialized cells, in an
insulin-responsive, customized, recycling, regulated pathway. The exact
nature of the tubulovesicular compartment and the GLUT-4 recycling
pathway has not yet been fully defined. It is not yet clear, for
instance, whether the GLUT-4 pathway is unique or whether it is a
subset of other regulated trafficking pathways. Similarly, is the
sorting of GLUT-4 handled by unique mechanisms or is it sorted by
mechanisms common to other regulated secretory proteins?
To address these questions, GLUT-4 distribution has been studied in
cells with multiple regulated secretory pathways. In two separate
studies, GLUT-4 was expressed in PC12 neuroendocrine cells, and its
distribution was compared with that of other proteins serving as
markers for defined secretory pathways in these cells. One of these
studies (12), but not the other (11), found evidence for GLUT-4
packaging into large, dense core granules of the regulated secretory pathway, in addition to its presence in smaller vesicles. Recently, localization of GLUT-4 in atrial cardiomyocytes showed that
GLUT-4 was partitioned into both regulated secretory granules and into
tubulovesicular elements (17). Questions about the sorting and
trafficking of GLUT-4 and about the origins of the tubulovesicular
compartment in different cell types therefore still persist.
GLUT-4 is expressed at relatively low levels in a variety of other
tissues, including the kidney (1), where its intracellular distribution
and modes of trafficking are not yet known. Members of the glucose
transporter family are expressed throughout the kidney, where their
distribution varies with glucose transport activity and metabolic
requirements of cells along the length of the nephron (5, 6, 9, 22).
Previous studies, using a combination of immunocytochemical techniques
to detect GLUT-4 mRNA or protein at the light microscopy level, have
demonstrated relatively high expression of GLUT-4 in smooth muscle
cells of the afferent arterioles (5) or in cells of the thick ascending limb (TAL) of the nephron (6, 9). These studies vary in detection of
GLUT-4 in the glomerulus, although GLUT-4 has been shown to be
expressed by cultured mesangial cells (5, 27). The ultrastructural
localization of GLUT-4 within different renal cells has not yet been
determined, yet such information is key for understanding the possible
physiological role of GLUT-4 in the normal kidney and also in the
diabetic condition.
The intracellular distribution of GLUT-4 in kidney cells, particularly
those with regulated secretory pathways, is of interest in further
defining the pathway(s) for GLUT-4 trafficking. In this study, we
investigated the distribution of renin and GLUT-4 in granular cells of
the JGA. Our aim was to compare the sorting and packaging of these two
proteins in this unique cell type, which has at least one established
regulated pathway for renin secretion.
 |
MATERIALS AND METHODS |
Fixation and tissue processing. BALB/c
mice weighing 15-20 g were perfused with phosphate-buffered saline
(PBS) followed by fixative (4% paraformaldehyde in 0.1 M sodium
phosphate buffer, pH 7.4) for 5 min. Both kidneys were dissected out,
and 1-mm3 cubes of cortex were
immersed in fixative for a further 1-2 h, then stored at 4°C.
Immunogold labeling. Tissue blocks
were washed in PBS, infiltrated with 2.3 M sucrose in PBS overnight,
mounted on cryostubs, and frozen in liquid nitrogen. Thin sections were
cut using a Reichert Ultracut S cryoultramicrotome and labeled with
antibodies specific for GLUT-4 and renin. The polyclonal GLUT-4
antibody was raised against a carboxy-terminal peptide of GLUT-4 and
has been shown to give specific labeling of this protein in a variety of cells (19, 20). A polyclonal rabbit anti-mouse renin
serum has been shown to recognize renin in mouse and rat tissues (4). This renin antibody recognizes both prorenin and active renin, lacking
all or part of the propeptide. All primary antisera were diluted to 5 µg/ml with PBS-1% bovine serum albumin (BSA). Protein A-10-nm gold
and protein A-5-nm gold conjugates were produced by the method of Slot
et al. (18). Ultrathin cryosections on grids were immunolabeled by
successive flotation on drops of PBS containing the following major
additives: 2% gelatin, 0.02% glycine, 1% BSA, primary antiserum,
protein A-gold (5- or 10-nm particles), and finally 1% glutaraldehyde.
Sections were then stained and mounted in 1.8% methyl cellulose
containing 0.3% uranyl acetate and viewed in a JEOL 1010 electron
microscope.
 |
RESULTS |
Examination of immunolabeled, serial cryosections of mouse kidney
cortex revealed that renin labeling was present only in granular cells
of the JGA or the afferent arteriole. GLUT-4 labeling was present at
several sites in the mouse renal cortex, including 1) cells of the JGA,
2) smooth muscle cells along the
length of the afferent arteriole wall, and
3) distal tubule epithelial cells. No GLUT-4 labeling was seen in proximal tubule epithelial cells in this
study. Within the glomerulus, we found no specific labeling over the
mesangium.
Colocalization of GLUT-4 and renin in juxtaglomerular
cells. Serial ultrathin cryosections of the JGA in
renal cortical tissue were single or double labeled with renin and
GLUT-4 antibodies. Renin labeling was confined to a small group of
cells, identifiable even at low magnification as granular cells by the
presence of prominent secretory granules (Fig.
1). At higher magnification, it was evident
that gold particles representing renin labeling were concentrated in
the lumens of secretory granules (Fig. 2). Labeling was found in a range of granules with different sizes and
densities, representing sequential stages of maturation (Figs. 2 and 3,
A-C).
In these sections, there was also labeling of (pro)renin in newly
forming granules associated with the TGN, but no labeling of prorenin
in earlier parts of the Golgi or in the endoplasmic reticulum was
seen.

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Fig. 1.
Juxtaglomerular apparatus (JGA) of mouse kidney cortex. Low-power view
of the JGA showing the area bordered by the macula densa (md), afferent
arteriole (aa), and glomerulus (gl). Granular cells are identified by
the presence of large electron-dense secretory granules (arrowhead).
Labeling found in marked cells (1 and
2) is shown at higher magnification
in Fig. 3. Bar = 5 nm.
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Fig. 2.
Renin labeling of a granular cell in the JGA. Mature renin granules (R)
are heavily labeled with renin. Smaller, newly forming granules
(arrowheads), some adjacent to Golgi stacks (g) with less dense
labeling, can also be seen. Bar = 0.25 µm.
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Within the JGA, GLUT-4 labeling was present in granular cells, in
adjacent cells of the afferent arteriole without prominent renin
granules, and in cells of the adjacent macula densa (described below).
Over the extraglomerular mesangium there were occasional sparse gold
particles, at the level of background labeling and not indicative of
specific GLUT-4 labeling. The intracellular distribution of GLUT-4
within granular cells is shown in Fig. 3.
The labeling was concentrated in distinct clusters of vesicles, including clathrin-coated vesicles and interconnected ramifications, which were often adjacent to the TGN. The TGN was identified in these
sections by its position adjacent to stacked Golgi cisternae and its
proximity to the centriole (Fig. 3). These GLUT-4-labeled structures had the appearance of tubulovesicular elements found in
other cells for the specialized sequestration and recycling of GLUT-4
(20, 19, 17). There was no labeling of GLUT-4 on the plasma membranes
of granular cells.

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Fig. 3.
Double labeling of renin and the insulin-responsive glucose
transporter, GLUT-4, in JGA cells. Serial sections of a granular cell
are shown in A,
B, and
C. These frozen sections appear
somewhat overstretched as a result of the weak fixative used to
preserve antigenicity; this often causes the Golgi stack to split. A
large granule is intensely labeled for renin and smaller, newly forming
granules are less densely labeled for renin (r) (arrows). A small
amount of labeling for renin is also found associated with the
trans-Golgi network (TGN) adjacent to the stacked Golgi (g). Clusters
of GLUT-4-positive tubulovesicular elements (arrowheads) are adjacent
to stacked Golgi cisternae in the area of the TGN (t) which can be
identified by its relationship to the centriole (c). Scattered
tubulovesicular elements and clathrin-coated vesicles in this region
are labeled for GLUT-4 (arrowheads).
C: a slightly reduced field of view
shows that, although tubulovesicular elements and the TGN directly abut
the renin granule at this level, there is no overlap in renin and
GLUT-4 labeling; n, nucleus. Bars = 100 nm.
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Double labeling of sections with antibodies to both GLUT-4 and renin
confirmed that both proteins were found in separate locations within
granular cells. Most notable was the complete absence of GLUT-4
labeling in the renin-containing secretory granules (Fig. 3,
A-C).
Gold labeling representing GLUT-4 or renin was in separate post-Golgi
compartments, either the tubulovesicular elements or the secretory
granules, respectively. Even within the TGN, the labeling for GLUT-4
and (pro)renin was found in separate areas (Fig.
3C), consistent with the sorting and
segregation of the two proteins into distinct pathways or compartments.
Distribution of GLUT-4 and renin along the afferent
arteriole. Granular cells, identified by the presence
of electron dense granules, were found along the wall of the afferent
arteriole, even at a distance from the JGA. These cells contained
multiple secretory granules labeled for renin (Fig.
4). GLUT-4 labeling was also found in these
granular cells, and additionally, in other non-renin-labeled, smooth
muscle cells in the walls of the afferent arteriole and interlobular
arteries. The labeling of GLUT-4 in cells of the arteriolar walls was
thus more widespread than that of renin. Both proteins exhibited
intracellular distributions in these cells similar to those seen in the
JGA cells. GLUT-4 was abundant on tubulovesicular elements near the TGN
(Fig. 4), and renin labeling was seen in typical secretory granules.
Thus the distributions of renin and GLUT-4 remain separate and
consistent with respect to one another in cells along the length of the
arterioles.

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Fig. 4.
Arteriolar renin and GLUT-4 labeling distal to the JGA. A
renin-secreting cell in the wall of the afferent arteriole at a
distance from the JGA is double labeled for GLUT-4 (10-nm gold) and
renin (5-nm gold). GLUT-4 labeling (arrowheads) is on coated vesicles
and tubulovesicular elements adjacent to the TGN. Renin labeling
(arrows) is shown here in one fully formed granule and one newly formed
granule (top). Bar = 0.25 µm.
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Localization of GLUT-4 in epithelial cells of the
distal tubule. Significant concentrations of GLUT-4
labeling were also found in renal tubule epithelial cells within the
cortex (Figs. 5 and 6). There was no GLUT-4
labeling in proximal tubule cells nor in cells of cortical collecting
ducts. Viewing of several tissue blocks, serial-sectioned through this
region of cortex, showed GLUT-4 labeling in macula densa cells,
providing a potentially interesting link to the GLUT-4 in the
renin-secreting granular cells of the JGA. A cluster of macula densa
cells with a typical compressed columnar appearance was identified by
its position adjacent to the JGA (see Fig. 1). GLUT-4 was localized in
a perinuclear position, near the Golgi complex in these cells (Fig.
5A). The labeling was concentrated
in classic tubulovesicular elements at this position (Fig.
5B). Prominent staining of GLUT-4
was seen in the cells at the outer edges of the macula densa, whereas
the equivalent peri-Golgi areas of the more central macula densa cells were not in these sections.

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Fig. 5.
GLUT-4 labeling in the macula densa.
A: low-power view showing a cross
section of the macula densa. One of the cells on the outer edge of the
macula densa is shown at higher magnification in
B. B:
area outlined in A shows gold labeling
of GLUT-4 in perinuclear tubulovesicular elements. There was no
labeling near the cell surface. Golgi areas are not observed in the
more central macula densa cells in these sections. Bars: 2 µm
(A) and 0.25 µm
(B).
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Specific GLUT-4 labeling was also present in cells along the length of
the TAL of the distal tubule, outside the area of the macula densa
(Fig. 6). In contrast to all of the other
cells labeled in these mouse kidney sections, these TAL epithelial
cells showed prominent labeling of GLUT-4 in tubulovesicular elements
close to the cell surface (Fig. 6). GLUT-4 was also found in
tubulovesicular elements at the level of the TGN in a supranuclear
position. GLUT-4 was also present among the infoldings of the
basolateral plasma membrane, where there was labeling on the
basolateral plasma membrane itself; however, no specific labeling was
found on the apical (luminal) plasma membrane (Fig.
6B). The peripherally located clusters of GLUT-4 in TAL cells are more typical of GLUT-4 distribution in cells such as adipocytes, which most actively deploy and recycle this transporter.

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Fig. 6.
GLUT-4 labeling in an epithelial cell of the thick ascending limb (TAL)
of the distal tubule. A: GLUT-4 is
present on tubulovesicular elements in the region of the TGN in a
supranuclear (n) position within this TAL cell. There is no labeling
along the apical plasma membrane (ap); sporadic gold particles can be
seen near the lateral plasma membrane below the level of the tight
junction. B: clusters of gold
particles representing GLUT-4 labeling on small cross sections of
tubulovesicular elements and scattered membrane staining can be seen
among the basal infoldings of the TAL cells. Base of this cell is
denoted by the adjacent basement membrane (bm). Bars: 1 µm
(A) and 0.5 µm
(B).
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 |
DISCUSSION |
We have demonstrated that the insulin-regulated glucose transporter,
GLUT-4, is expressed in smooth muscle cells and in restricted populations of epithelial cells in the mouse renal cortex. The most
prominent concentrations of GLUT-4 we detected were in afferent arteriole cells and smooth muscle cells of the larger arterioles. Both
GLUT-4 and renin were localized in the granular cells of the JGA. The
presence of GLUT-4 in the same cells that secrete renin is significant,
because it shows that these unique cells, the granular cells, contain
two, distinct secretory compartments for the separate sequestration of
renin and GLUT-4.
The intracellular processing and storage of renin in distinctive
secretory granules in granular cells of the JGA and distal afferent
arteriole has been well documented in the literature (reviewed in Ref.
24). Our data showing localization of renin in secretory granules at
different stages of maturation in granular cells is consistent with
previous observations at the electron microscopy level (4, 23). The
antibody used here for labeling recognizes both prorenin and renin. The
concentrated labeling seen in mature renin granules most likely
represents labeling of the cleaved, active form of renin, whereas
smaller amounts of labeling detected in the TGN represent prepackaged
prorenin. The sections of granular cells observed in this study were
from untreated mouse kidneys and thus had little evidence of active renin secretion. Morphological features associated with active secretion, such as discharging granules, widespread labeling of renin
and prorenin in a variety of post-Golgi structures, and concentrations
of secreted renin in extracellular interstitium, are usually only seen
following treatment with angiotensin converting enzyme inhibitors or in
hydronephrotic kidney cells, where renin secretion is greatly enhanced
(2-4). Interestingly, under stimulated conditions, extra cells in
the afferent arteriole wall can be recruited for de novo renin
production, seen as the emergence of renin granules in increased
numbers of cells in this region (2, 8). Renin can also be produced by
other cortical cells, particularly, it seems, under stimulated
conditions or in cell culture where gene expression is turned on.
Proximal tubule cells in culture for instance express renin (10, 22)
and have also been reported to express GLUT-4 (6), although the mouse
kidney sections showed no labeling for either of these proteins. Thus arteriole smooth muscle cells continuously express GLUT-4, as shown
both previously (5) and in the present study, but only initiate the
expression and trafficking of renin under stimulated conditions.
In this study we found that the only significant concentrations of
GLUT-4 in mouse kidney cortex were seen in cells of the JGA, in
arterioles, and in epithelial cells of the TAL. Previous studies have
also reported the presence of GLUT-4 mRNA or protein in each of these
cell types (5, 6, 9). In addition, GLUT-4 mRNA has been detected in
situ in glomerular cells and in cultured mesangial cells (5, 27). We
found no significant labeling over areas of the intra- and
extraglomerular mesangial cells. The relative sensitivity of techniques
used in different studies or differences in expression between species
or differences in the state of physiological stimulation may account
for the inconsistent detection of GLUT-4 in glomeruli. Similar
explanations may apply to the absence of GLUT-4 labeling in proximal
tubule cells, which is at odds with some of the previous reports
showing GLUT-4 expression in these cells (9). Although the presence of
very low amounts of GLUT-4 in other cortical cells cannot be ruled out,
the consensus of this study, together with previous data, suggests that
the major sites of GLUT-4 expression in the kidney cortex are in the renal arterioles and in distal tubule TAL epithelial cells.
We found that GLUT-4 in TAL epithelial cells was localized in the
region of the TGN and was also found more peripherally in the cells in
tubulovesicular elements and on the basolateral plasma membrane. This distribution resembles the pattern of insulin-responsive GLUT-4 accumulation in adipocytes and muscle cells (19,
20). GLUT-4 has previously been described in cells of the
medullary TAL, where it was noted that the same cells express
insulin-like growth factor-I (IGF-I) (6). This fact, along with the
finding that GLUT-4 mRNA expression increases with stimulation of TAL metabolic activity, led to the speculation that IGF-I-responsive GLUT-4
might be involved in providing the high levels of glucose typically
utilized by TAL cells for oxidative metabolism (6). Our data showing
clusters of GLUT-4 near the cell surface are consistent with such an
active utilization of GLUT-4 for glucose uptake in TAL cells. The
distribution of GLUT-4 in these TAL cells is similar to that found in
adipocytes and muscle cells, where GLUT-4 is typically concentrated in
a prominent tubulovesicular compartment from which it is recruited to
the cell surface upon insulin stimulation (19).
The other contents of the tubulovesicular elements and the nature of
the recycling pathway for GLUT-4 have not yet been fully characterized,
even in adipocytes (13). It is not clear, for instance, to what degree
the GLUT-4 pathway overlaps with the trafficking of other
secreted proteins. There is now evidence to suggest that GLUT-4 may be
trafficked by divergent pathways in some cell types (11, 12, 17).
Heterologous expression of GLUT-4 in PC12 neuroendocrine cells resulted
in sorting of some of the GLUT-4 into dense core secretory granules
where it colocalized with secretogranin (12). Another
study in PC12 cells, however, reports that the expressed GLUT-4 was
excluded from the regulated secretory pathways but was concentrated in
a unique, small vesicle population, which is likened to the
tubulovesicular compartment in adipocytes (11). These somewhat
conflicting results may be due in part to the heterologous expression
of the GLUT-4.
Endogenous GLUT-4 has also recently been localized in atrial
cardiomyocytes, which have a major regulated secretory pathway for the
release of atrial natriuretic factor (ANF) from stored secretory
granules (17). In atrial cardiomyocytes, GLUT-4 was found in
tubulovesicular elements that were often adjacent to the TGN,
similar to the distribution we report here in granular cells. However,
in the cardiomyocytes, a significant proportion of GLUT-4 was also
found within the dense secretory granules, where it is packaged along
with the soluble ANF (17). The pool of GLUT-4 in ANF granules was
derived from a recycling pathway and was not responsive to insulin,
whereas the GLUT-4 found in the tubulovesicular elements was
translocated upon insulin challenge (17). Thus atrial cardiomyocytes
have two distinctly regulated pools of GLUT-4 in separate, regulated
secretory pathways.
In the current study our approach was to compare the localization of
endogenous GLUT-4 with that of an endogenous regulated secretory
protein, renin, in JGA cells. Our findings in granular cells show that
GLUT-4 is completely excluded from the regulated secretory pathway and
is sequestered in the tubulovesicular compartment. Granular cells in
the kidney, while having an active regulated secretory pathway for
renin release, do not package any GLUT-4 into regulated secretory
granules. Thus the sorting and trafficking of endogenous GLUT-4 is
different in two types of modified smooth muscle cells, granular cells
and atrial cardiomyocytes. Our results suggest that granular cells have
a highly efficient sorting mechanism for routing GLUT-4 into its own
compartment, whereas the overall sorting of GLUT-4 in atrial
cardiomyocytes is simply less efficient or occurs by different
mechanisms.
The studies here describe the steady-state distribution of GLUT-4 in
tubulovesicular compartment in granular cells. It is not currently
known how GLUT-4 in granular cells is regulated. There was no
significant labeling of GLUT-4 on the plasma membrane in granular cells
in the kidneys of unstimulated mice, which is consistent with the
intracellular sequestration of GLUT-4 in other cells in the absence of
insulin. By comparison with analogous pools of GLUT-4 in other cells
(17, 20), it is likely that this pool of GLUT-4 (i.e., the whole
complement of GLUT-4 in granular cells) is available for
translocation to the cell surface in response to a stimulus such as
insulin. Further studies are required to test the trafficking of GLUT-4
in granular cells in response to a range of stimuli. In diabetes
mellitus, GLUT-4 expression and translocation in adipocytes are
defective, resulting in decreased glucose uptake (14). Similarly,
GLUT-4 expression and resulting glucose uptake are also decreased in
glomerular and vascular cells from rats with streptozotocin diabetes
(15). The coordinate presence but distinct compartmentalization of
GLUT-4 and renin in granular cells raises the possibility that these
cells have multiple hormonally responsive pathways that may contribute
to the pathophysiology of diseases such as diabetes and hypertension.
 |
ACKNOWLEDGEMENTS |
We thank Darren Brown for excellent technical help with electron
microscopy. The electron microscopy was carried out in the Centre for
Microscopy and Microanalysis, University of Queensland. The Centre for
Molecular and Cellular Biology is a Special Research Centre of the
Australian Research Council.
 |
FOOTNOTES |
J. L. Stow and D. E. James are Wellcome Trust Senior Medical Research
Fellows. This work was supported by funds from the Ramiacotti Foundation and the National Health and Medical Research Council (to J. L. Stow; postdoctoral fellowship to J.S. Anderson)
Address reprint requests to J. L. Stow.
Received 14 November 1996; accepted in final form 19 August 1997.
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