Tissue-specific role of glycogen synthase kinase 3beta in glucose homeostasis and insulin action.
ABSTRACT Dysregulation of the protein kinase glycogen synthase kinase 3 (GSK-3) has been implicated in the development of type 2 diabetes mellitus. GSK-3 protein expression and kinase activity are elevated in diabetes, while selective GSK-3 inhibitors have shown promise as modulators of glucose metabolism and insulin sensitivity. There are two GSK-3 isoforms in mammals, GSK-3alpha and GSK-3beta. Mice engineered to lack GSK-3beta die in late embryogenesis from liver apoptosis, whereas mice engineered to lack GSK-3alpha are viable and exhibit improved insulin sensitivity and hepatic glucose homeostasis. To assess the potential role of GSK-3beta in insulin function, a conditional gene-targeting approach whereby mice in which expression of GSK-3beta was specifically ablated within insulin-sensitive tissues were generated was undertaken. Liver-specific GSK-3beta knockout mice are viable and glucose and insulin tolerant and display "normal" metabolic characteristics and insulin signaling. Mice lacking expression of GSK-3beta in skeletal muscle are also viable but, in contrast to the liver-deleted animals, display improved glucose tolerance that is coupled with enhanced insulin-stimulated glycogen synthase regulation and glycogen deposition. These data indicate that there are not only distinct roles for GSK-3alpha and GSK-3beta within the adult but also tissue-specific phenotypes associated with each of these isoforms.
- [show abstract] [hide abstract]
ABSTRACT: The c-Jun N-terminal kinase (JNK)/stress activated protein kinase is preferentially activated by stress stimuli. Growth factors, particularly ligands for G protein-coupled receptors, usually induce only modest JNK activation, although they may trigger marked activation of the related extracellular signal-regulated kinase. In the present study, we demonstrated that homozygous disruption of glycogen synthase kinase 3beta (GSK-3beta) dramatically sensitized mouse embryonic fibroblasts (MEFs) to JNK activation induced by lysophosphatidic acid (LPA) and sphingosine-1-phosphate, two prototype ligands for G protein-coupled receptors. To a lesser degree, a lack of GSK-3beta also potentiated JNK activation in response to epidermal growth factor. In contrast, the absence of GSK-3beta decreased UV light-induced JNK activation. The increased JNK activation induced by LPA in GSK-3beta null MEFs was insufficient to trigger apoptotic cell death or growth inhibition. Instead, the increased JNK activation observed in GSK-3beta-/- MEFs was associated with an increased proliferative response to LPA, which was reduced by the inhibition of JNK. Ectopic expression of GSK-3beta in GSK-3beta-negative MEFs restrained LPA-triggered JNK phosphorylation and induced a concomitant decrease in the mitogenic response to LPA compatible with GSK-3beta through the inhibition of JNK activation, thus limiting LPA-induced cell proliferation. Mutation analysis indicated that GSK-3beta kinase activity was required for GSK-3beta to optimally inhibit LPA-stimulated JNK activation. Thus GSK-3beta serves as a physiological switch to specifically repress JNK activation in response to LPA, sphingosine-1-phosphate, or the epidermal growth factor. These results reveal a novel role for GSK-3beta in signal transduction and cellular responses to growth factors.Journal of Biological Chemistry 01/2005; 279(49):51075-81. · 4.65 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Glycogen synthase (GS) activity is reduced in skeletal muscle of type 2 diabetes, despite normal protein expression, consistent with altered GS regulation. Glycogen synthase kinase-3 (GSK-3) is involved in regulation (phosphorylation and deactivation) of GS. To access the potential role of GSK-3 in insulin resistance and reduced GS activity in type 2 diabetes, the expression and activity of GSK-3 were studied in biopsies of vastus lateralis from type 2 and nondiabetic subjects before and after 3-h hyperinsulinemic (300 mU x m(-2) x min(-1))-euglycemic clamps. The specific activity of GSK-3alpha did not differ between nondiabetic and diabetic muscle and was decreased similarly after 3-h insulin infusion. However, protein levels of both alpha and beta isoforms of GSK-3 were elevated (approximately 30%) in diabetic muscle compared with lean (P < 0.01) and weight-matched obese nondiabetic subjects (P < 0.05) and were unchanged by insulin infusion. Thus, both basal and insulin-stimulated total GSK-3 activities were elevated by approximately twofold in diabetic muscle. GSK-3 expression was related to in vivo insulin action, as GSK-3 protein was negatively correlated with maximal insulin-stimulated glucose disposal rates. In summary, GSK-3 protein levels and total activities are 1) elevated in type 2 diabetic muscle independent of obesity and 2) inversely correlated with both GS activity and maximally insulin-stimulated glucose disposal. We conclude that increased GSK-3 expression in diabetic muscle may contribute to the impaired GS activity and skeletal muscle insulin resistance present in type 2 diabetes.Diabetes 02/2000; 49(2):263-71. · 7.90 Impact Factor
- Nature 07/2000; 406(6791):86-90. · 38.60 Impact Factor
MOLECULAR AND CELLULAR BIOLOGY, Oct. 2008, p. 6314–6328
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Vol. 28, No. 20
Tissue-Specific Role of Glycogen Synthase Kinase 3? in Glucose
Homeostasis and Insulin Action?†
Satish Patel,1Bradley W. Doble,1,2Katrina MacAulay,1Elaine M. Sinclair,1Daniel J. Drucker,1
and James R. Woodgett1*
Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada,1
and McMaster Stem Cell and Cancer Research Institute, McMaster University, 1200 Main Street West, Hamilton,
Ontario L8N 3Z5, Canada2
Received 12 May 2008/Returned for modification 7 July 2008/Accepted 31 July 2008
Dysregulation of the protein kinase glycogen synthase kinase 3 (GSK-3) has been implicated in the devel-
opment of type 2 diabetes mellitus. GSK-3 protein expression and kinase activity are elevated in diabetes, while
selective GSK-3 inhibitors have shown promise as modulators of glucose metabolism and insulin sensitivity.
There are two GSK-3 isoforms in mammals, GSK-3? and GSK-3?. Mice engineered to lack GSK-3? die in late
embryogenesis from liver apoptosis, whereas mice engineered to lack GSK-3? are viable and exhibit improved
insulin sensitivity and hepatic glucose homeostasis. To assess the potential role of GSK-3? in insulin function,
a conditional gene-targeting approach whereby mice in which expression of GSK-3? was specifically ablated
within insulin-sensitive tissues were generated was undertaken. Liver-specific GSK-3? knockout mice are
viable and glucose and insulin tolerant and display “normal” metabolic characteristics and insulin signaling.
Mice lacking expression of GSK-3? in skeletal muscle are also viable but, in contrast to the liver-deleted
animals, display improved glucose tolerance that is coupled with enhanced insulin-stimulated glycogen syn-
thase regulation and glycogen deposition. These data indicate that there are not only distinct roles for GSK-3?
and GSK-3? within the adult but also tissue-specific phenotypes associated with each of these isoforms.
Glycogen synthase kinase 3 (GSK-3) is a highly conserved,
ubiquitously expressed serine/threonine protein kinase that ex-
ists as two isoforms, GSK-3? (51 kDa) and GSK-3? (47 kDa),
which are encoded by separate genes that produce highly ho-
mologous proteins that differ significantly only in their N- and
C-terminal regions (50). GSK-3 is highly active under resting
conditions and is rapidly inactivated by insulin through phos-
phorylation of an N-terminal domain serine residue (Ser 21 of
GSK-3? and Ser 9 of GSK-3?) (43). Insulin inhibition of
GSK-3 requires the activation of phosphatidylinositol 3-kinase
and protein kinase B (PKB/Akt), with PKB/Akt phosphorylat-
ing both isoforms of GSK-3 on these regulatory serine residues
(11). GSK-3 was originally identified as a regulator of glycogen
synthase (GS), a rate-limiting enzyme that promotes glycogen
deposition (17). In the absence of insulin, active GSK-3 phos-
phorylates four serine residues in the C-terminal domain of GS
and negatively regulates its activity, reducing the capacity of
cells to synthesize and store glycogen (25). Inhibition of GSK-3
by insulin (via Ser 9/21 phosphorylation) results in the dephos-
phorylation and activation of GS, leading to increased rates of
glycogen synthesis (15, 35).
A major feature of type 2 diabetes mellitus (T2DM) is im-
pairment of both basal and insulin-stimulated glucose metab-
olism in insulin-responsive, peripheral tissues, including skel-
etal muscle (SM) and liver (2, 37, 41). While there are no
known disease-associated mutations in the two GSK-3 genes
(21), there are data that show elevation of GSK-3 expression
and activity in the SM of patients with T2DM and in adipose
tissues of obese diabetic mice (16, 33). Furthermore, trans-
genic overexpression of GSK-3? in the SM of mice results in
impaired glucose tolerance, elevated plasma insulin levels, and
reduced glycogen content (36). Conversely, GSK-3 inhibitors
can mimic insulin action in cell lines and tissues (8, 27, 29, 34),
while administration of GSK-3 inhibitors to rodent models of
obesity and T2DM improves insulin sensitivity and glucose
homeostasis by increasing glycogen synthesis and coordinately
reducing glucose output by inhibiting hepatic gluconeogenesis
(9, 14, 20, 24; reviewed in reference 40).
Given that chemical inhibitors of GSK-3 are unable to dis-
criminate between the two isoforms, it is not possible to eval-
uate isoform-specific functions of GSK-3? and GSK-3? by
using these drugs. However, evidence for isoform-specific roles
has emerged from mouse models. Mice engineered to lack
GSK-3? (but still retaining GSK-3?) die during embryogenesis
(23). Interestingly, mice that lack GSK-3? (retaining GSK-3?)
are viable and display improved whole-body glucose tolerance
and hepatic insulin sensitivity (28). Furthermore, insulin reg-
ulation of GS is completely abolished in the SM of homozy-
gous “knock-in” mice expressing an insulin-insensitive mutant
(S9A) of GSK-3?, whereas in GSK-3? (S21A) knock-in mice,
GS activity is unaltered (30). These findings suggest that there
are GSK-3? and GSK-3? tissue- and isoform-specific roles in
regulation of glucose metabolism.
Due to the embryonic lethality of global GSK-3? knockout
(KO) mice, we employed a conditional gene-targeting ap-
proach to derive mice that selectively lack GSK-3? expression
in two major insulin-sensitive tissues. Here, we report that
* Corresponding author. Mailing address: Samuel Lunenfeld Research
Institute, Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario
M5G 1X5, Canada. Phone: (416) 586-8811. Fax: (416) 586-8839. E-mail:
† Supplemental material for this article may be found at http://mcb
?Published ahead of print on 11 August 2008.
liver-specific and SM-specific GSK-3? KO mice display differ-
ent phenotypes with respect to glucose metabolism. While the
liver-specific GSK-3? KO (L?KO) mice exhibited normal met-
abolic characteristics, SM-specific GSK-3? KO (M?KO) mice
displayed improved glucose tolerance and more-effective GS
activation. The GSK-3? tissue-specific KO mice provide new
genetic tools for the analysis of isoform- and tissue-specific
roles of GSK-3 in cellular regulation and enable the therapeu-
tic potential of GSK-3 inhibitors in the treatment of T2DM to
be more thoroughly assessed.
MATERIALS AND METHODS
Construction of the GSK-3?-targeting vector. The genomic region flanking
exon 2 of the GSK-3? gene was isolated from a lambda phage clone library
containing genomic DNA from the mouse strain 129/Ola. The “arms” of homol-
ogy for the targeting vector were obtained by PCR amplification of the GSK-3?
genomic regions by use of Platinum Pfx (Invitrogen) and inserted into the
backbone targeting vector pSPUC. LoxP sites were introduced by PCR into the
intronic region flanking exon 2 of the GSK-3? gene, while the neomycin resis-
tance cassette was inserted and flanked by FLP recombination target (FRT)
sites. The fragments were sequenced for any PCR-generated errors.
Generation of GSK-3?-floxed targeted mice and tissue-specific KO mice. The
R1 mouse embryonic stem (ES) cell line was obtained from Andras Nagy (Sam-
uel Lunenfeld Research Institute [SLRI]) and maintained in “KO” Dulbecco’s
modified Eagle’s medium containing a high concentration of glucose supple-
mented with 15% fetal bovine serum (HyClone), 0.1 mM nonessential amino
acids, antibiotics (100 units penicillin G, 100 ?g/ml streptomycin), 2 mM L-
glutamine, 1 mM sodium pyruvate, 0.1 mM 2-mercaptoethanol, and 1,000
units/ml ESGRO (Chemicon).
Thirty micrograms of the GSK-3?-targeting vector was linearized and electro-
porated into R1 ES cells (6 ? 106cells/ml) by using a Bio-Rad Gene Pulser II
instrument (250 V, 500 ?F), and the cells were then plated onto gelatinized
plates in the presence of 250 ?g/ml of G418. One hundred twenty G418-resistant
colonies were screened by PCR, and 15 ES cell clones that had undergone
correct homologous recombination were identified. Two ES cell clones with
optimal morphologies were microinjected into C57/B6J blastocysts, which were
then reimplanted into recipient female mice. Chimeric mice that had high de-
grees of ES cell contribution were identified by coat color and were then crossed
with C57/B6J, and germ line transmission of the GSK-3?-floxed allele was ver-
ified by PCR (with only one of the ES cell lines achieving germ line transmis-
sion). These GSK-3?-floxed mice were subsequently crossed with B6 mice that
carried the FLPe recombinase (JAX Lab) to remove the neo cassette that resides
within the intronic region of the GSK-3?-targeting vector. Resultant interbreed-
ing of these mice yielded GSK-3?-floxed (FL/FL?) mice that were viable,
healthy, and born at the expected Mendelian frequency.
To generate liver-specific GSK-3? KO animals, the GSK-3?-floxed mice were
crossed with the B6.Cg-Tg(Alb-Cre)21Mgn/J (Jackson Laboratory) strain, which
carries Cre recombinase under the control of the albumin promoter. These
GSK-3? Fl/FL AlbCre?mice (B6/129 background, F3 backcross) are designated
L?KO, denoting them as liver-specific GSK-3? KO.
With a similar breeding strategy, SM-specific GSK-3? KO animals were gen-
erated by breeding the conditionally targeted GSK-3? KO mice with mice ex-
pressing Cre under the control of the myosin light chain 1f (MLC1f) promoter
(kindly provided by Steve Burden, Skirball Institute, NY) (5). These GSK-3?
Fl/FL MLC1f-cre?mice (B6/129/ICR background, F3 backcross) are denoted
M?KO, indicating an SM-specific GSK-3? KO. Genotypes for the various tissue-
specific deletions were confirmed by PCR for the presence of Cre and for the
detection of the deleted, floxed GSK-3? allele.
The GSK-3? FL/FL lines demonstrated expression levels of GSK-3 protein as
well as other components of the insulin-signaling pathway similar to those for
wild-type (WT) animals (data not shown). We performed additional experiments
to demonstrate that the presence of the Alb-cre or MLC-cre transgene in WT
animals with respect to GSK-3 had no significant effects on the metabolic profiles
(as assessed by glucose tolerance tests [GTT] and insulin tolerance tests [ITT])
analyzed in these studies (data not shown). In addition, all studies were per-
formed with respective littermate controls to control for possible environmental
influences. GSK-3? FL/FL control mice and the tissue-specific KO mice were
housed five per cage under a light/dark cycle of 12 h each in the Ontario Cancer
Institute animal facility, with free access to food and water except where noted.
All procedures were conducted according to protocols and guidelines approved
by the Ontario Cancer Institute Animal Care Committee.
GTT and ITT. Following an overnight fast (16 to 18 h for the GTT) or a 5-h
fast (for the ITT), mice were injected intraperitoneally (i.p.) with 1 mg/g of
glucose (for the GTT) or 0.75 mU/g insulin (for the ITT), and blood glucose (tail
vein blood) was measured using a OneTouch UltraSmart blood glucose moni-
toring system (Lifescan, Canada) at the indicated times. For certain studies
involving fasted and fed animals, measurements were taken between 8:00 and
10:00 a.m. for both fasted (overnight, 16 to 18 h) and fed (ad libitum) cohorts.
Determination of plasma insulin, adiponectin, and triglycerides. Blood sam-
ples were collected from the tail veins of mice fasted overnight (16 to 18 h) and
either treated or not treated with 1 mg/g glucose (i.p) for 15 min and centrifuged
at 6,000 ? g for 10 min, and the plasma supernatant was collected. The plasma
insulin and adiponectin levels were measured using a Mercodia ultrasensitive
insulin enzyme-linked immunosorbent assay kit (Alpco Diagnostics) and an
adiponectin enzyme-linked immunosorbent assay kit (Linco Research), respec-
tively. Plasma triglycerides were determined using a Wako L-type triglyceride H
kit (Wako Chemicals, USA, Inc.).
In vivo SM glucose uptake. After an overnight fast, mice were injected i.p. with
2-deoxy-D-[1,2-3H]glucose mixed with 20% dextrose (2 g/kg body weight; 10
?Ci/mouse), and the accumulation of radiolabeled 2-deoxy-D-glucose in SM was
measured as described previously (53).
Glucose transport assay with isolated muscle. After an overnight fast, animals
were deeply anesthetized using Avertin, intact whole-extensor digitorum longus
(EDL) and soleus muscles were isolated and incubated in the absence or pres-
ence of 2 mU/ml insulin, and the uptake of 2-deoxyglucose (2DG) was measured
as described previously (49).
Determination of glycogen content. Fifty milligrams of gastrocnemius muscle
or 20 mg liver tissue was acid hydrolyzed in 2 N HCl by heating it at 95°C for 2 h
and neutralized using 2 N NaOH. The liberated free-glycosyl units were assayed
spectrophotometrically by using a glucose reagent hexokinase-dependent-assay
kit (Amresco, OH) as previously described (4). Glycogen was also visualized in
paraffin-embedded tissue sections by using a periodic acid-Schiff (PAS) staining
kit (Sigma Aldrich). Tissue sections were also stained with standard hematoxylin
and eosin for general cell morphology.
Isolation of primary hepatocytes. Eight- to 12-week old L?KO and GSK-3?
FL/FL littermate control animals were anesthetized and primary hepatocytes
isolated by a retrograde, nonrecirculating in situ liver perfusion with collagenase
as described previously (28).
Tissue lysate preparation. Following an overnight fast, mice were either
treated with 100 mU/g insulin, 1 mg/g glucose or refed (by placing standard chow
pellets back into cages) for various times. After cervical dislocation, mouse
tissues were dissected and quickly frozen in liquid N2and stored for subsequent
processing or were lysed immediately. Fresh or snap-frozen tissues were lysed in
50 mM NaCl, 25 mM PIPES [piperazine-N,N?-bis(2-ethanesulfonic acid)], pH 7,
0.5% NP-40, 5 mM EGTA, 25 mM NaF, 1 mM Na-orthovanadate, 10 mM
Na–?-glycerophosphate, 1 mM sodium pyrophosphate, phosphatase inhibitor
cocktail (Sigma), and a protease inhibitor cocktail tablet (Roche) by using a 2-ml
Kontes tissue grinder. Tissue homogenates were cleared by centrifugation at
16,000 ? g for 15 min at 4°C, and protein concentration was determined by a
Bio-Rad DC protein assay. The supernatants were stored at ?80°C until analysis.
Immunoblotting. Twenty micrograms of protein lysates was denatured in so-
dium dodecyl sulfate (SDS) sample buffer and resolved by SDS-polyacrylamide
gel electrophoresis, and proteins were transferred onto polyvinylidene difluoride
membranes. Membranes were blocked with 5% nonfat dry milk for 1 h at room
temperature and incubated with antibodies directed against the following anti-
gens: phospho-GS Ser 641, phospho-GSK-3?/? Ser9/Ser21, phospho-PKB/Akt
Ser 473, and PKB/Akt (Cell Signaling Technologies); muscle GS, GLUT1, and
GLUT4 (Chemicon); GSK-3?/? (Biosource) and ?-catenin (BD Transduction
Laboratories); liver GS (gift from J. Guinovart, University of Barcelona);
glycogen phosphorylase and phosphorylase phosphoserine 15 (gift from Tricia
Cohen, University of Dundee); and glyceraldehyde-3-phosphate dehydrogenase
(GAPDH) or caveolin-1 (Santa Cruz). Following antibody incubation, the mem-
branes were washed five times in Tris-buffered saline-0.1% Tween-20 prior to
incubation with horseradish peroxidase (HRP)-conjugated anti-rabbit immuno-
globulin G (IgG), HRP-conjugated anti-mouse IgG, or HRP-conjugated anti-
sheep IgG secondary antibodies. The bands were visualized using SuperSignal
Western Pico chemiluminescence reagent (Pierce) and quantified using an Alpha
Innotech FluorChem HD2 documentation system.
GS assays. Frozen SM and liver tissue samples were minced using 1 ml
homogenization buffer (50 mM Tris, pH 7.8, 100 mM NaF, 10 mM EDTA, 5%
glycerol, and a protease inhibitor cocktail tablet) and homogenized using a
Brinkman Polytron. The tissue homogenates were cleared by centrifugation at
VOL. 28, 2008 TISSUE-SPECIFIC ROLES OF GSK-3?
3,000 ? g for 10 min at 4°C, and GS activity from the supernatants was assayed
according to the method of Thomas et al. (46) based on incorporation of UDP-
[3H]glucose into glycogen. Immunoblot analysis performed under these condi-
tions demonstrated that regulated phosphorylation of GS was maintained (data
Glycogen phosphorylase assays. Glycogen phosphorylase activity was deter-
mined by measuring the incorporation of [14C]glucose-1-phosphate into glycogen
in the absence or presence of 2 mM AMP as previously described (7).
GSK-3 kinase assays. Assays of GSK-3 were performed on mouse tissue
extracts as described previously (13), with the following modifications. Briefly,
0.1 g of muscle or liver tissue was lysed in 3 ml of detergent-free lysis buffer (50
mM Tris-HCl, 4 mM EDTA, 2 mM EGTA, 10 mM Na–?-glycerophosphate, 5
mM NaF, 1 mM phenylmethylsulfonyl fluoride, 1 mM Na-orthovanadate, and
protease and phosphatase inhibitor cocktails, pH 7) and homogenized using a 2
ml Kontes tissue grinder. The tissue homogenate was centrifuged at 4,000 ? g for
30 min at 4°C and the supernatant applied to a chilled, carboxymethyl (CM)-
Sepharose fast-flow resin column (Amersham) preequilibrated with 25 mM Tris-
HCl, 2 mM EDTA, 1 mM EGTA, 10 mM Na–?-glycerophosphate, 2.5% glyc-
erol, 5 mM NaF, 1 mM phenylmethylsulfonyl fluoride, 1 mM orthovanadate, and
protease and phosphatase inhibitor cocktails, pH 7 (buffer A). The column was
washed with 5 column volumes of low-salt wash buffer (buffer A plus 20 mM
NaCl) and GSK-3 eluted using buffer A plus 250 mM NaCl. Fractions containing
the highest concentrations of protein were pooled and used in a kinase assay to
assess GSK-3 activity as described in reference 13.
Extraction of total muscle membrane. Muscle tissue (0.1 g) was minced using
0.5 ml homogenization buffer (20 mM NaHCO3, 250 mM sucrose, 5 mM NaN3,
1 mM EDTA, and a protease inhibitor cocktail tablet), homogenized using a
Brinkman Polytron, and clarified by centrifugation at 1,300 ? g for 10 min. The
supernatant was saved, and the pellet was resuspended in 0.5 ml of homogeni-
zation buffer, rehomogenized, and centrifuged. The resulting supernatant was
pooled with the first supernatant and recentrifuged at 9,000 ? g for 10 min at
4°C. The resultant pellet was resuspended in 0.5 ml homogenization buffer and
centrifuged at 229,000 ? g for 90 min at 4°C, and the total membrane pellet was
resuspended in 0.3 ml homogenization buffer and the protein concentration
Statistical analysis. For multiple comparisons, statistical analysis was per-
formed using either two-way analysis of variance followed by a Bonferroni
posttest or unpaired Student t tests. Data analysis was performed using Graph-
Pad Prism or Microsoft Excel.
Generation of mice harboring conditional alleles of GSK-
3?. Mice expressing a conditional allele of GSK-3? were gen-
erated as described in Materials and Methods (Fig. 1). GSK-3?
FL/FL (FL/FL?) or GSK-3??/FL (data not shown) mice ex-
pressed GSK-3? levels equivalent to those in WT animals
(GSK-3??/?or GSK-3??/?AlbCre?) as judged by immuno-
blotting (Fig. 2C), indicating that the LoxP sites flanking exon
2 did not interfere with expression of the gene (Fig. 2D).
Liver-specific GSK-3? KO mice are viable and develop nor-
mally. The liver plays a central role in glucose homeostasis,
responsible for regulating postprandial glucose uptake and
production/output (6). To generate mice lacking GSK-3?
within the liver, GSK-3?-floxed mice were crossed with a trans-
genic strain expressing Cre under the control of the hepato-
cyte-specific albumin promoter. The resultant tissue-specific
(GSK-3? FL/FL Alb Cre) KO mice, termed liver-specific
GSK-3? KO (L?KO), were viable, fertile, and born at the
expected Mendelian frequency and displayed whole-body (Fig.
2A) and liver (data not shown) weights similar to those of their
GSK-3? FL/FL (FL/FL?) littermates. PCR (Fig. 2B) and im-
munoblot (Fig. 2C) analyses demonstrated that the deletion of
the GSK-3?-floxed allele with consequent loss (?90%) of pro-
tein expression occurred specifically within the liver and not in
other tissues of the KO animal (Fig. 2D). The residual expres-
sion in the KO tissue most likely reflects GSK-3? protein
expression in nonhepatic cells (e.g., endothelial cells or
Kupffer cells), which account for approximately 10% of the cell
population within the liver and do not express alb-Cre (39).
Liver-specific GSK-3? KO mice display normal metabolic
characteristics. Mice treated with GSK-3 inhibitors or engi-
neered to lack GSK-3? display improved hepatic glucose ho-
meostasis and insulin sensitivity (20, 28). We therefore inves-
tigated whether loss of liver GSK-3? affected whole-body
glucose homeostasis. GTT and ITT performed on L?KO and
GSK-3? FL/FL littermates at either 8 weeks or 6 months of
age revealed no statistically significant differences in the rate of
glucose clearance upon i.p. administration of glucose or insulin
(Fig. 3A). Furthermore, no significant differences were found
in blood glucose levels between fasted or fed L?KO and
GSK-3? FL/FL animals (see Fig. S1 in the supplemental ma-
Liver and SM are two well-characterized tissues responsible
for the disposal of the majority of a glucose load and for the
storage of this glucose as glycogen. We determined the glyco-
gen levels in fasted and fed tissues from L?KO and GSK-3?
FL/FL control mice. Upon examination of PAS reagent-
stained, paraffin-embedded tissue sections, no significant dif-
ferences were observed in accumulation of glycogen in liver
(Fig. 3B) or SM (data not shown) between the two groups. This
finding was independently confirmed by biochemical quantifi-
cation of glycogen with an acid hydrolysis technique applied to
frozen tissue samples (Fig. 3C).
Analysis of insulin signaling in liver-specific GSK-3? KO
mice. Insulin modulates the activities of PKB/Akt, GSK-3, and
GS by phosphorylation. Phosphorylation of PKB/Akt at Thr
308 and Ser 473 results in its activation, whereas phosphory-
lation of GSK-3 (Ser 21 of GSK-3? and Ser 9 of GSK-3?) and
GS (Ser 641, Ser 645, Ser 649, and Ser 653) inhibits these
enzymes. We investigated whether lack of hepatic GSK-3?
affected the activation of the insulin-signaling pathway. Levels
of insulin-induced activation of PKB/Akt were similar in the
livers of both L?KO and GSK-3? FL/FL mice, as judged by
quantification of immunoreactivity to the phospho-Ser 473
PKB/Akt antibody (Fig. 4A). Indeed, levels of insulin-induced
phosphorylation and activation of other signaling proteins,
such as S6 kinase (S6K) and ribosomal S6 protein, were also
similar in primary hepatocytes isolated from L?KO and
GSK-3? FL/FL mice (Fig. 4B). Furthermore, the levels of
expression and insulin-induced phosphorylation of GSK-3? at
Ser 21 remained equal for the two groups, indicating that there
was no compensatory regulation of the remaining GSK-3?
isoform in the L?KO mice (Fig. 4A and B and 2C). GSK-3
assays derived from partially purified liver extracts revealed
that total GSK-3 activity was reduced by approximately 50%
(55.9% ? 5%) in the L?KO mice compared to that in FL/FL?
control animals (Fig. 4C).
It is well established that in SM and liver tissue, GS is
regulated by covalent phosphorylation, which inhibits the en-
zyme and by the allosteric coactivator glucose-6-phosphate
(G6P) (3, 30). Using a phospho-specific antibody directed
against one of the four GSK-3-specific sites, Ser 641, we found
that administration of glucose (15 min) or 2 h of feeding after
fasting induced marked degrees of dephosphorylation of GS
that were similar in the livers of the control FL/FL? and L?KO
mice (Fig. 4A). Furthermore, the levels of activation of GS
6316 PATEL ET AL.MOL. CELL. BIOL.
FIG. 1. Generation of conditional GSK-3?-floxed (FL), targeted mice. (A) Schematic outline describing the targeting strategy for generating
conditional GSK-3?-floxed (FL) mice and the subsequent tissue-specific KO mice. After homologous recombination with the targeting vector, exon
2 (E2) of the GSK-3? gene was replaced with a LoxP-flanked (floxed) exon 2 and an FRT-flanked neomycin resistance cassette. Mice carrying the
GSK-3?-floxed allele were bred to FLPe recombinase-expressing mice to remove the neo cassette. Deletion of exon 2, and hence deletion of
GSK-3? expression, is achieved by crossing GSK-3? FL mice with strains expressing Cre recombinase under the control of tissue-specific
promoters. Triangles, Lox P recombination sites; ovals, FRT recombination sites; B, BglII. Asterisks represent hybridization sites for Southern blot
probes. (B) Southern blot analysis. ES cells and mouse tail genomic DNA were digested with BglII and probed with the 5? GSK-3? or neomycin
DNA probe (?), confirming proper targeting of the GSK-3?-floxed allele in R1 mouse ES cell (mES) clones 1B9 and 2D12 and germ line
transmission of the GSK-3?-floxed allele (by use of the 1B9 clone) in mice. The 11.8-kb band corresponds to the WT GSK-3? allele, whereas the
13.6-kb band corresponds to the correctly targeted GSK-3? FL allele. (C) PCR genotype analysis of genomic DNA isolated from the progeny of
the GSK-3??/FL F1 cross. The WT GSK-3? band is 886 bp, whereas the GSK-3? FL band is 1,095 bp.
were similar for the two groups when liver extracts were as-
sayed in the presence of low (0.1 mM) or high (10 mM) con-
centrations of G6P (Fig. 4D). Analysis of insulin signaling in
SM extracts from the control FL/FL? and L?KO mice re-
vealed no significant differences in insulin-mediated regulation
of PKB/Akt, GSK-3, or GS (Fig. 4E).
Deletion of GSK-3? in SM. Glucose uptake into SM ac-
counts for the disposal of 70 to 90% of a glucose load. To
evaluate the role of GSK-3? in regulation of glucose ho-
meostasis in SM, we generated SM-specific GSK-3? KO
(M?KO) mice by crossing the GSK-3?-floxed (FL), targeted
mice with transgenic animals that expressed Cre-recombinase
under the control of the SM-specific mlc1f promoter. Like the
liver KO animals, M?KO mice were viable, fertile, born at the
expected Mendelian frequency, and similar in weight to their
FL/FL? control littermates (Fig. 5A). PCR (Fig. 5B) and im-
munoblot (Fig. 5C) analyses demonstrated selective and effi-
cient deletion (?90%) of GSK-3? only in SM tissue types, not
in cardiac muscle or nonmuscle tissues, demonstrating the
tissue specificity of the mlc1f gene. As previously reported, the
FIG. 2. Generation and characterization of liver-specific GSK-3? KO (L?KO) mice. (A) Weight analysis. Male GSK-3? FL/FL (termed
FL/FL?) control and GSK-3? FL/FL AlbCre?(termed L?KO) mice were weighed every 2 weeks between the ages of 4 and 24 weeks. Each point
represents the mean ? standard error of the mean (SEM), with n indicating the number analyzed in each group. No statistical difference in the
mean weights was observed between the groups by Student’s t test. (B) PCR analysis. Genomic DNA was isolated from the tissues stated and
analyzed by PCR for Cre-mediated excision of GSK-3?. PCR for GSK-3? (top) demonstrates that exon 2 of the GSK-3? FL allele is excised only
in the livers of the L?KO mice, not in those of the control FL/FL? or the WT AlbCre?/?(littermate control) mice. The bottom panel displays
genomic detection of the Cre transgene. (C) Western blot analysis. Tissue extracts from liver, muscle (quadricep) and brain were prepared from
8-week-old male mice of the genotypes indicated and immunoblotted with an antibody that specifically recognizes GSK-3? and GSK-3?. Equal
loading of the lanes was assessed using an anti-GAPDH antibody, and the blots shown are representative of three independent experiments.
(D) Quantification of GSK-3. The densitometries of GSK-3? and GSK-3? were measured and normalized to GAPDH levels, and the expression
of each isoform is displayed relative to the total (GSK-3? plus GSK-3?) expression level in the respective WT (or AlbCre?) tissue.
6318PATEL ET AL.MOL. CELL. BIOL.
use of the mlc1f locus restricts Cre expression to fast/glycolytic
fibers, which, in mice, are the overwhelming muscle type, with
the exception of the soleus, which is primarily composed of
slow/oxidative fibers (5).
SM-specific GSK-3? KO mice exhibit improved glucose ho-
meostasis and enhanced insulin sensitivity. We examined
whether selective loss of GSK-3? from SM affected glucose
metabolism. Initially, we found no significant differences in
blood glucose levels between fasted or fed M?KO and GSK-3?
FL/FL animals at 6 to 12 weeks of age (Fig. 6A). However,
upon i.p. injection of glucose, M?KO (but not heterozygous
GSK-3??/Fl MLCf Cre?) mice (see Fig. S2 in the supplemen-
tal material) exhibited a significantly enhanced ability to clear
circulating blood glucose, demonstrating improved glucose
sensitivity compared to that of their GSK-3? FL/FL counter-
parts (Fig. 6B). The enhanced glucose tolerance in the M?KO
mice was not accompanied by any changes in glucose-stimu-
lated insulin release (Fig. 6B, inset); however, we observed an
FIG. 3. L?KO mice display “normal” glucose homeostasis and insulin sensitivity. (A) Glucose metabolism profile. GTT and ITT were
performed on 8-week- or 6-month-old male FL/FL? control and L?KO mice as described in Materials and Methods. Values are the means ?
SEMs, with n indicating the number of mice in each group. (B) Glycogen staining. Liver sections from fasted (overnight, 16 h) or fed (ad libitum)
8-week-old male FL/FL? control and L?KO mice were stained with PAS to detect glycogen accumulation (dark purple staining; top) or
hematoxylin and eosin (H?E) for general cell morphology (bottom). The images are representative of results from five animals. (C) Glycogen
content in liver. Liver tissues were extracted from fasted (overnight, 16 h) or fed (ad libitum) 8-week-old male FL/FL? control and L?KO mice,
and glycogen content was determined by acid hydrolysis and is expressed as ?mol of glucose units per gram of liver tissue. Data are presented as
means ? SEMs, with each liver sample assayed in triplicate, and n indicates the number of mice in each group. NS, not significant.
VOL. 28, 2008TISSUE-SPECIFIC ROLES OF GSK-3?
FIG. 4. Analysis of insulin signaling in L?KO mice. (A and E) Insulin signaling in liver (A) and muscle (quadricep) (E) samples. Eight-week-old
male FL/FL? control and L?KO mice were fasted overnight and injected i.p. with 100 mU/g insulin for 15 min or with 1 mg/g glucose for 20 min
or were allowed to refeed ad libitum for 2 h. Following treatment, liver (A) and SM (E) tissues were extracted, lysed as described in Materials and
Methods, and immunoblotted with the indicated antibodies. Equal loading of the lanes was assessed using an anti-GAPDH antibody, and a
representative blot is shown (left) with the quantification of results from four independent experiments displayed (right). Data are presented as
a bar graph showing signal intensities compared to that for fasted FL/FL? mice, which is set as 1. NS, not significant. (B) Insulin signaling in
hepatocytes. Primary hepatocytes were isolated from 10-week-old male FL/FL? control and L?KO mice and serum starved overnight prior to
6320PATEL ET AL.MOL. CELL. BIOL.
enhanced ability in the M?KO mice to clear blood glucose
upon i.p. administration of insulin (Fig. 6B), which was more
pronounced in younger M?KO mice (6 weeks), an effect ob-
served by others (e.g., reference 12).
We also observed improved insulin signaling in SM extracts
from M?KO mice. More specifically, insulin-induced dephos-
phorylation and activation of GS were significantly increased in
M?KO mice compared to those in the FL/FL? controls (Fig.
7A [quantified in panel B, left]). The enhanced GS regulation
was in line with the measured enzymatic activity of GS from
muscle extracts (Fig. 7B, right). Levels of insulin regulation of
PKB/Akt and other signaling components of the pathway, such
as phosphorylation of extracellular signal-regulated kinase
(ERK) and S6K, remained similar in both liver and SM ex-
tracts from both the M?KO and the FL/FL? control mice (Fig.
7C). Interestingly, however, we observed a significantly ele-
vated basal phosphorylation of GSK-3? in the SM of M?KO
mice. Furthermore, kinase assays performed on SM extracts
from the M?KO mice revealed a larger-than-expected reduc-
tion in total GSK-3 activity compared to the level for FL/FL?
control animals (Fig. 7D). GSK-3 activity was reduced by ap-
proximately 70% (to 28.6% ? 8% of FL/FL? levels), and this
was not accompanied by changes in GSK-3? expression (Fig.
7A). Together, these findings suggest that GSK-3? activity
plays a greater role than GSK-3? in regulating GS in SM.
Kinase assays performed on SM and liver extracts from
GSK-3? KO mice revealed that total GSK-3 activity was re-
duced by approximately 40 to 50% in each tissue for liver (to
58.0% ? 8% of WT levels) and for SM (to 62% ? 7% of WT
levels) (see Fig. S3 in the supplemental material).
In addition to the lower total GSK-3 activity and more-active
GS, we also observed elevated glycogen storage within the SM
of the M?KO mice (Fig. 8A). To assess whether changes in
components of the glucose transport machinery accounted for
the increase in glycogen storage observed in the SM of the
M?KO mice, the relative expression levels of glucose trans-
porters that promote the uptake of glucose into SM were
quantified. No significant changes were observed in the total
cell membrane expression of the GLUT1 or the insulin-regu-
lated GLUT4 glucose transporters in SM homogenates from
the M?KO mice (Fig. 8B). We next determined the rates of
glucose transport in isolated EDL and soleus muscles. While
there was a trend for the rate of glucose transport to be higher
in the insulin-stimulated EDL muscles of the M?KO mice than
in those of the FL/FL? control mice, this did not reach statis-
tical significance. Using [3H]2DG as a tracer, we performed in
vivo glucose uptake measurements during an i.p. GTT and
determined that the uptakes of 2DG in muscle tissue were
similar in both M?KO and FL/FL? control mice (Fig. 8D). We
next examined whether alteration in the glycogen degradation
process could account for the differences observed in the
M?KO mice. No significant changes were observed in the
expression, phosphorylation (Fig. 8E, left), or activity (right)
levels of glycogen phosphorylase, the enzyme that catalyzes the
rate-limiting step of glycogenolysis.
Here, we describe the generation and characterization of
tissue-specific genetic deletions of GSK-3? and the impacts of
inactivation of this enzyme in two tissues (SM and liver) on
glucose metabolism and insulin sensitivity. A central finding of
this conditional KO mouse analysis is that deletion of GSK-3?
from SM results in improved glucose tolerance and enhanced
insulin sensitivity. In contrast, specific inactivation in liver had
no effect on glucose metabolism or insulin responsiveness. The
effects in the SM KO of GSK-3? were associated with poten-
tiation of insulin-induced dephosphorylation and activation of
GS and increased glycogen deposition in SM. These data are
consistent with previous studies demonstrating that inhibitors
of GSK-3 (that inactivate both GSK-3? and GSK-3?) can
stimulate GS activity and glycogen deposition within SM (10,
22, 29, 31). Our genetic data show that GSK-3? is the major
regulator of GS in SM. Indeed, transgenic overexpression of
GSK? in the SM of mice leads to glucose intolerance, together
with a reduction in both GS activation and muscle glycogen
content (36). Furthermore, McManus et al. demonstrated that
insulin regulation of GS was completely abolished in the SM of
homozygous “knock-in” mice expressing a point mutation in an
insulin-insensitive phosphorylation site (serine 9) of GSK-3?,
whereas in the equivalent GSK-3? knock-in mutant mice
(serine 21), GS activity was unaltered (30). The tissue-specific
effect observed for GSK-3? over GSK-3? on GS regulation in
murine SM could be explained in part by the higher relative
levels of expression of the GSK-3? isoform in muscle. Indeed,
for human SM it has been reported that GSK-3? is more
abundant than GSK-3?, implicating GSK-3? as the major reg-
ulator of glucose metabolism in human SM (30).
In our studies, we find that GSK-3? and GSK-3? protein
levels in murine SM are essentially equivalent (44.9% ? 0.9%
for GSK-3? versus 54.6% ? 1.3% for GSK-3?). That said,
there is a greater-than-expected reduction (?75%) in total
GSK-3 activity in SM extracts from the M?KO mice, compared
with a 50 to 55% reduction in total GSK-3 activity observed
when GSK-3? is deleted within the liver or in tissues that lack
GSK-3? (see Fig. S3 in the supplemental material). Further-
incubation with 20 nM insulin. At the times indicated, cells were lysed and the lysates subjected to SDS-polyacrylamide gel electrophoresis and
immunoblotted with the indicated antibodies. A representative blot is shown (left), with quantification of results from four independent
experiments displayed (right). NS, not significant. (C) GSK-3 kinase assays. Liver tissues were extracted from 8-week-old male FL/FL? control and
L?KO mice and partially purified through a CM-Sepharose column as described in Materials and Methods. GSK-3 kinase activities were
determined using a quantitative peptide phosphorylation assay and are expressed relative to that for the FL/FL? control (which is set at
100%). Values are the means ? SEMs of results for five different livers, with each assayed in triplicate. (D) GS activity. Liver tissues were
extracted from 8-week-old male FL/FL? control and L?KO mice and homogenized, and GS activity was measured in the presence of 0.1 mM
(low) or 10 mM (high) G6P as described in Materials and Methods. Values are the means ? SEMs of results from four different livers, with
each assayed in duplicate. ??, P ? 0.01 (for comparison to fasted mice for each genotype); NS, not significant (for comparison to fasted
VOL. 28, 2008TISSUE-SPECIFIC ROLES OF GSK-3?
FIG. 5. Generation of SM-specific GSK-3? KO (M?KO) mice. (A) Weight analysis. Male FL/FL? control and GSK-3? FL/FL mlc1f Cre?
(termed M?KO) mice were weighed every week between the ages of 4 and 24 weeks. Each point represents the mean ? SEM, with n indicating
the number analyzed in each group. No statistical differences in mean weight were observed between the groups by Student’s t test. (B) PCR
analysis. Genomic DNA was isolated from the tissues stated and analyzed by PCR for Cre-mediated excision of GSK-3?. PCR for GSK-3? (top)
demonstrates that exon 2 of the GSK-3? FL allele was excised in all SM types of the M?KO mice but not the control FL/FL?. The bottom panel
indicates genomic detection of the Cre transgene. Gastroc, gastrocnemius. (C) Western blot analysis. Tissue extracts were prepared from
8-week-old male mice of the genotypes indicated and immunoblotted with an antibody that recognizes GSK-3? and GSK-3?. The loading of the
lanes was assessed using an anti-GAPDH antibody, and the blots are representative of three independent experiments. Gastro, gastrocnemius.
6322 PATEL ET AL.MOL. CELL. BIOL.
FIG. 6. M?KO mice display improved glucose homeostasis and insulin sensitivity. (A) Blood glucose profile. Six- or 12-week-old male FL/FL?
control and M?KO mice were either fasted overnight for 16 h (fasted) or fed ad libitum (random fed), and blood glucose levels were measured.
The data are presented as means ? SEMs, with the number (n) of mice in each group indicated. (B) Glucose metabolism profile. GTT and ITT
were performed on 6-week- or 12-week-old male FL/FL? control and M?KO mice as described in Materials and Methods. The inset depicts the
plasma insulin concentrations in the FL/FL? and M?KO mice following a 15-min i.p. injection of glucose. Values are means ? SEMs, with
n indicating the number of mice in each group. Asterisks signify statistically significant changes compared to levels for the FL/FL? control mice.
?, P ? 0.05; ??, P ? 0.01; ???, P ? 0.001.
VOL. 28, 2008TISSUE-SPECIFIC ROLES OF GSK-3?
Fastedinsulin 10' insulin 15'
Relative signal intensity
Relative signal intensity
Relative signal intensity
Fastedinsulin 10' insulin 15'
Relative signal intensity
Fastedinsulin 10' insulin 15'
Relative signal intensity
Fastedinsulin 10' insulin 15'
Fasted insulin 10'insulin 15'
Relative signal intensityRelative signal intensity
phospho PKB/Akt phospho glycogen synthase
phospho glycogen synthase S641
phospho PKB/Akt S473
Signal intensity (%)
GS activity ratio (low/high [G6P])
phospho ribosomal S6 protein S235
Phospho GSK-3 S9/21 α
10 min 15 min
phospho PKB/Akt S473
phospho glycogen synthase S641
Total GSK-3 kinase activity (%)
Relative signal intensity
Fasted insulin 10'insulin 15'
phospho glycogen synthase S641
FIG. 7. Analysis of insulin signaling in M?KO mice. (A and C) Insulin signaling in muscle (A and C, top) and liver (C, bottom). Six-week-old
male FL/FL? control and M?KO mice were fasted overnight and injected i.p. with 100 mU/g insulin or with 1 mg/g glucose for 20 min or for the
times indicated. Following treatment, different SM types (top panels) and liver tissue (bottom left) were extracted, lysed as described in Materials
and Methods, and immunoblotted with the indicated antibodies. A representative blot is shown (left), with the quantifications of results from at
least four independent experiments displayed (right). ?, P ? 0.05; NS, not significant. (B) GS regulation. (Left) Signal intensities of the
6324 PATEL ET AL.MOL. CELL. BIOL.
more, the insulin-induced changes in GSK-3 activity were sim-
ilar in muscle extracts from FL/FL? control and M?KO mice
(see Fig. S4 in the supplemental material). In this study, we
also find that as the M?KO mice age, their capacity to main-
tain improved glucose tolerance and insulin sensitivity dimin-
ishes. While glucose tolerance becomes less pronounced by age
(6 months) (see Fig. S4 in the supplemental material), insulin
sensitivity and glycogen accretion are lost by 12 weeks (insulin
tolerance is lost at 6 months) (see Fig. S4 in the supplemental
material). We investigated the possibility that there was an
age-dependent compensatory increase in GSK-3? that over-
came the sensitization effects in the M?KO mice. However,
while there was increased expression of GSK-3? in aged mice,
levels of this were comparable in both M?KO mice and FL/
FL? controls (see Fig. S5 in the supplemental material). Thus,
we speculate that the apparent loss in glucose and insulin
tolerance in the M?KO mice may be explained by the com-
monly observed decrease in insulin responsiveness that occurs
with aging (12). Indeed, young animals as well as mildly (but
not severely) diabetic animal models show a greater respon-
siveness in lowering blood glucose when treated with GSK-3
Previous studies have demonstrated that application of
GSK-3 inhibitors to isolated SM enhances insulin signaling (14,
22, 32, 40). In addition to its effects on GS regulation, these
GSK-3 inhibitors also potentiated insulin-stimulated phosphor-
ylation of PKB/Akt. This observation was explained by previ-
ous findings demonstrating that GSK-3 can phosphorylate and
negatively regulate insulin receptor substrate 1 (IRS-1), im-
pairing downstream insulin signaling. While we do not see
significant differences in PKB or S6K phosphorylation in the
M?KO mice, there is a significant increase in basal serine
phosphorylation of GSK-3?, in addition to enhanced GS reg-
ulation. The mechanism by which this occurs may not be due to
upregulation of IRS1/PKB but may be explained by the fact
that, in addition to PKB, several other kinases (PKC, PKA,
p90rsk, and S6K) can phosphorylate GSK-3 (18, 19, 43, 52). It
is possible that increases in the activities of these kinases may
mediate GSK-3 phosphorylation. Indeed, there is a trend,
though not statistically significant, for upregulation of ERK in
the M?KO mice (Fig. 7C). GSK-3 is a known negative regu-
lator of ERK/Jun N-terminal protein kinase signaling (1, 26,
48), and there is evidence to suggest that GSK-3 regulates the
activity of a protein phosphatase, which in turn regulates
GSK-3 itself (51).
Net glycogen content is coordinately and simultaneously reg-
ulated by both synthesis and breakdown. We addressed
whether changes in these processes accounted for the elevated
glycogen deposition in the M?KO animals. From this study, it
is clear that glycogen synthesis is, in part, mediating glycogen
accretion in the M?KO mice. Insulin-stimulated GS activity is
consistently elevated in the SM of M?KO mice. While this
form of GS regulation is most likely the significant contributor
toward glycogen status in fed mice, the mechanism by which
glycogen is elevated in fasting mice is unclear (since GS activity
remained similar to that for FL/FL?). We tested whether the
glucose transport machinery was altered in the M?KO mice.
The expression of the insulin-sensitive GLUT4 transporter re-
mained unchanged. In addition, there were no significant dif-
ferences in the rates of basal and insulin-stimulated glucose
transport in isolated muscle tissues or in vivo. Furthermore,
there appeared to be no alteration in the glycogen breakdown
pathway as assessed by glycogen phosphorylase regulation.
Thus, at present, the mechanism underlying the elevated gly-
cogen in fasted M?KO mice is unresolved. One explanation
for the higher glycogen levels is that the M?KO mice display
lower muscle activity. Anecdotal observation of these mice has
not revealed an obvious difference in movement compared to
levels for WT littermates, although more-subtle differences in
activity would require more-extensive study to reveal. It is also
possible that the M?KO mice have adapted to utilizing alter-
native fuels, such as free fatty acids, thereby “shunting” glucose
toward glycogen. However, we observed no differences in ad-
iposity (as determined by fat pad weights, adiponectin, and
triglyceride levels) (see Fig. S6 in the supplemental material) in
our studies that would account for improved glycogen storage,
glucose tolerance, and insulin sensitivity. It will be of interest to
examine whether any changes occur in the insulin signaling
profile or glucose metabolism in M?KO mice that are sub-
jected to a high-fat diet to promote insulin resistance or that
are bred onto a genetically obese background.
Deletion of hepatic GSK-3? resulted in a minimal pheno-
type with regard to glucose regulation and insulin signaling,
although we cannot exclude the possibility that certain meta-
bolic phenotypes may manifest themselves only under condi-
tions of insulin resistance (high-fat diet or ob/ob background).
Based on the finding that the global GSK-3? KO mice die in
embryogenesis from massive liver apoptosis, our original hy-
pothesis was that the L?KO mice would present a similar
phenotype (23). However, L?KO mice are viable and show no
abnormalities within the liver or whole body. As the albumin
gene (and hence Cre) is not efficiently expressed during em-
bryogenesis (38), we were unable to determine whether em-
bryonic deletion of hepatic GSK-3? would recapitulate the
lethality of the global GSK-3? KO. However, hepatocytes iso-
lated from 8- to 12-week-old L?KO mice (when Cre is maxi-
insulin-stimulated dephosphorylation of GS from muscle (gastrocnemius) tissues of the FL/FL? control and M?KO mice were quantified, and the
data are presented as a bar graph showing signal intensities as percentages of the WT level (100%). Values are the means ? SEMs from at least
four independent experiments. ?, P ? 0.05 (FL/Fl? versus M?KO, insulin treated). (Right) Muscle tissues were extracted from 6-week-old male
FL/FL? control and M?KO mice and homogenized, and GS activity was measured in the presence of 0.1 mM (low) or 10 mM (high) G6P as
described in Materials and Methods. Values are the means ? SEMs of results from six different muscle samples, with each assayed in duplicate.
?, P ? 0.05 (FL/Fl? versus M?KO, insulin treated). (D) GSK-3 kinase assays. Muscle (gastrocnemius) tissue was extracted from 6-week-old male
FL/FL? control and M?KO mice and partially purified with CM-Sepharose chromatography as described in Materials and Methods. GSK-3 kinase
activity was determined using a quantitative peptide phosphorylation assay. The inset shows an immunoblot of GSK-3 that was used in the kinase
assay reaction. GSK-3 kinase activities are expressed relative to that for the FL/FL? control (which is set at 100%) and are the means ? SEMs
of results from five different muscle samples, with each assayed in triplicate.
VOL. 28, 2008 TISSUE-SPECIFIC ROLES OF GSK-3?
mally expressed) displayed increased sensitivity to tumor ne-
crosis factor alpha-induced apoptosis (data not shown),
consistent with previous data that have shown that GSK-3?
KO mouse embryonic fibroblasts have elevated susceptibility
to tumor necrosis factor alpha-mediated cell death and defects
in NF-?B signaling (23, 42, 44).
The tissue-selective effects of GSK-3 isoform inactivation
indicate the existence of isoform- and tissue-specific roles for
FIG. 8. Analysis of glycogen deposition and glucose transport in M?KO mice. (A) Glycogen content in muscle. Muscle tissue (gastrocnemius) was
by acid hydrolysis and expressed as ?mol of glucose units per gram of muscle tissue. The data are presented as means ? SEMs, with each muscle sample
assayed in triplicate, and n indicates the number of mice in each group. ?, P ? 0.05; NS, not significant. (B) GLUT4 expression analysis. Muscle tissue
(gastrocnemius) was extracted from 6-week-old male FL/FL? control and M?KO mice and total cell membrane isolated as described in Materials and
Methods and immunoblotted with the indicated antibodies. Equal loading of the lanes was assessed using an anticaveolin antibody, and the blots shown
are representative of at least four independent experiments. (C) In vitro glucose transport measurement. Intact soleus and EDL muscle fibers were
isolated from deeply anesthetized FL/FL? control and M?KO mice and were incubated with or without 2 mU of insulin per ml for 30 min.
2-Deoxy-D-glucose uptake was measured over a 20-min period as described in Materials and Methods. The data are presented as means ? SEMs, with
the number (n) of mice in each group indicated. NS, not significant. (D) In vivo glucose uptake. Six-week-old male FL/FL? control and M?KO mice were
fasted overnight and injected i.p. with a mixture of unlabeled dextrose and [3H]2DG, and the accumulation of the 2DG in muscle tissue (gastrocnemius)
was determined as described in Materials and Methods. Data are presented as means ? SEMs, with the number (n) of mice in each group indicated.
(E) Glycogen phosphorylase regulation. (Left) Muscle tissue (gastrocnemius) was extracted from 6-week-old male FL/FL? control and M?KO mice and
immunoblotted with the indicated antibodies. A representative blot is shown (below), with quantification of results from three independent experiments
displayed as a bar graph (above). Total glycogen phosphorylase expression was normalized to GAPDH levels and expressed as signal intensities, while
the signal intensities from the phosphorylated form of the protein are expressed relative to the WT level (100%). (Right) Muscle tissues were extracted
from male FL/FL? control and M?KO mice and homogenized, and glycogen phosphorylase activity was measured in the absence or presence of AMP
as described in Materials and Methods.
6326PATEL ET AL.MOL. CELL. BIOL.
GSK-3 in the regulation of glucose metabolism. Indeed,
Tanabe et al. demonstrate that mice heterozygous for GSK-3?
preserve ?-cell integrity and reduce the onset of diabetes in
two genetic models of this disease (45). Furthermore, using the
GSK-3?-floxed lines described here, Tanabe et al. generated
islet-specific GSK-3? KO mice. These animals show a reduc-
tion in the age-dependent hyperglycemia normally observed in
IRS-2 KO mice, suggesting that islet GSK-3? is required for
the chronic elevation of blood glucose in this model (45).
Furthermore, we have recently generated mice that lack
GSK-3? expression in all tissues. Unlike their global GSK-3?
KO counterparts, GSK-3?-null mice are viable but display
enhanced hepatic insulin sensitivity and glucose homeostasis
(28). However, the SM of these animals exhibited normal in-
sulin signaling, GS regulation, and glycogen deposition.
Therefore, the findings presented in this study, along with
the phenotypes associated with the GSK-3? and GSK-3? KO
mice, demonstrate that the two mammalian isoforms of GSK-3
each have distinct as well as overlapping biological roles. On
the one hand, GSK-3? and GSK-3? appear to be equally
important in regulating Wnt/?-catenin signaling (13). How-
ever, GSK-3? has a more critical role than GSK-3? in regu-
lating hepatic glucose metabolism and insulin sensitivity (28),
and GSK-3? is the predominant regulator of GS in SM. We
speculate that the functional dominance of either GSK-3? or
GSK-3? within individual tissues is determined by selective
association with targets through their noncatalytic domains,
possibly via adaptor or scaffolding proteins. Our studies also
suggest that intermediate inhibition of both forms of GSK-3 is
necessary for insulin resensitization effects in muscle and liver
tissues in T2DM.
We thank T. Hansotia (SLRI) and G. Wiggin (SLRI) for critical
review of the manuscript and discussion and M. Woo, A. Klip, A.
Tung, and K. Sakamoto for valuable technical advice and assistance.
We thank J. Guinovart (University of Barcelona), P. T. Cohen (Uni-
versity of Dundee), and S. Burden (Skirball Institute, NY) for re-
Funding for this work was provided by the Canadian Institutes of
Health Research (J.R.W. [MOP 74711], S.P., and B.W.D.), the Cana-
dian Diabetes Association (J.R.W.), and the Banting and Best Diabe-
tes Centre (J.R.W., S.P., and K.M.).
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