Leptin resistance contributes to obesity and hypertension in mouse models of Bardet-Biedl syndrome
Bardet-Biedl syndrome (BBS) is a heterogeneous genetic disorder characterized by many features, including obesity and cardiovascular disease. We previously developed knockout mouse models of 3 BBS genes: BBS2, BBS4, and BBS6. To dissect the mechanisms involved in the metabolic disorders associated with BBS, we assessed the development of obesity in these mouse models and found that BBS-null mice were hyperphagic, had low locomotor activity, and had elevated circulating levels of the hormone leptin. The effect of exogenous leptin on body weight and food intake was attenuated in BBS mice, which suggests that leptin resistance may contribute to hyperleptinemia. In other mouse models of obesity, leptin resistance may be selective rather than systemic; although mice became resistant to leptin's anorectic effects, the ability to increase renal sympathetic nerve activity (SNA) was preserved. Although all 3 of the BBS mouse models were similarly resistant to leptin, the sensitivity of renal SNA to leptin was maintained in Bbs4 -/- and Bbs6 -/- mice, but not in Bbs2 -/- mice. Consequently, Bbs4 -/- and Bbs6 -/- mice had higher baseline renal SNA and arterial pressure and a greater reduction in arterial pressure in response to ganglionic blockade. Furthermore, we found that BBS mice had a decreased hypothalamic expression of proopiomelanocortin, which suggests that BBS genes play an important role in maintaining leptin sensitivity in proopiomelanocortin neurons.
1458 The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008
Leptin resistance contributes to
obesity and hypertension in mouse
models of Bardet-Biedl syndrome
Melissa A. Fath,
Daniel R. Thedens,
Christopher J. Berry,
Darryl Y. Nishimura,
and Val C. Sheffield
Department of Internal Medicine,
Department of Pediatrics,
Howard Hughes Medical Institute, and
Department of Radiology, University of Iowa Carver College of Medicine, Iowa City, Iowa, USA.
Bardet-Biedl syndrome (BBS) is a heterogeneous genetic disorder characterized by many features, includ-
ing obesity and cardiovascular disease. We previously developed knockout mouse models of 3 BBS genes:
BBS2, BBS4, and BBS6. To dissect the mechanisms involved in the metabolic disorders associated with BBS, we
assessed the development of obesity in these mouse models and found that BBS-null mice were hyperphagic,
had low locomotor activity, and had elevated circulating levels of the hormone leptin. The effect of exogenous
leptin on body weight and food intake was attenuated in BBS mice, which suggests that leptin resistance may
contribute to hyperleptinemia. In other mouse models of obesity, leptin resistance may be selective rather than
systemic; although mice became resistant to leptin’s anorectic effects, the ability to increase renal sympathetic
nerve activity (SNA) was preserved. Although all 3 of the BBS mouse models were similarly resistant to leptin,
the sensitivity of renal SNA to leptin was maintained in Bbs4
mice, but not in Bbs2
mice had higher baseline renal SNA and arterial pressure and a greater reduction
in arterial pressure in response to ganglionic blockade. Furthermore, we found that BBS mice had a decreased
hypothalamic expression of proopiomelanocortin, which suggests that BBS genes play an important role in
maintaining leptin sensitivity in proopiomelanocortin neurons.
Bardet-Biedl syndrome (BBS) is a pleiotropic autosomal recessive
disorder with the primary clinical features of obesity, retinopathy,
polydactyly, learning disabilities, and hypogenitalism (1, 2). BBS
is also associated with an increased susceptibility to hypertension
and cardiovascular disorders (1, 3, 4). Although BBS is rare in the
general population, there has been considerable interest in iden-
tifying the genes and determining the pathological mechanisms
involved in BBS because some components of the phenotype
are very common in the general population. Also, a recent study
showed that polymorphisms in certain BBS genes might increase
the risk of obesity and hypertension in non-BBS individuals (5).
BBS is genetically heterogeneous, and 12 BBS genes have been iden-
tified to date (6–17). Much evidence suggests that BBS genes play a
role in cilia function and/or maintenance (18, 19). Most BBS genes
are expressed in ciliated organisms and not in nonciliated organisms
(20). Many of the BBS proteins have been shown to localize to basal
bodies, which are modified centrioles found at the base of cilia and
flagella (19). We developed 3 different BBS-knockout mouse models
, and Bbs6
) and found that all 3 knockout mice have
defects in flagella formation during spermatogenesis, although these
animals do develop other motile and primary cilia (21–23). Although
animals are able to form cilia, sensory neuronal cilia of the
cochlea and olfactory bulb are abnormal (24).
Obesity is a central feature of BBS, but the pathophysiological
pathways leading to excessive body fat in this syndrome remain
largely unknown. Obesity results from energy imbalance between
ingested and expended calories. Energy balance is largely regulated
by the central nervous system, which senses metabolic status from
a wide range of hormonal and neural signals (25). Several brain
regions ranging from the cortex to the brainstem are known to be
involved in energy homeostasis, with the hypothalamus playing a
key role. The identification of leptin has helped unravel the architec-
ture of neuroendocrine circuitry that controls appetite and energy
homeostasis. This hormone is expressed mainly by adipocytes and is
released in the blood in proportion to the amount of adipose tissue
(26). The severe obesity and the hyperphagia caused by the absence
of leptin or its receptor in rodents and humans indicate the impor-
tance of this hormone for the control of energy homeostasis.
In lean subjects, leptin circulates at low levels (5–15 ng/ml). Plas-
ma leptin is transported to the central nervous system by a satu-
rable, unidirectional system (27) that involves binding of leptin to
the short form of the leptin receptor located at the endothelium
of the vasculature and the epithelium of the choroid plexus (28).
Leptin suppresses appetite and increases energy expenditure by
activating leptin receptors on specific neurons. At least 2 classes
of neuronal pathways are known to account for leptin sensitivity
within the arcuate nucleus of the hypothalamus (29): (a) a catabolic
pathway, which is represented essentially by the proopiomelanocor-
tin (POMC) neurons and is activated by leptin, and (b) an anabolic
pathway, which is represented principally by neuropeptide Y (NPY)
neurons (which also express agouti-related protein [AgRP]) and is
inhibited by leptin. In POMC neurons, leptin is known to increase
the gene expression of the Pomc gene. In contrast, leptin inhibits the
expression of the Npy and Agrp genes in NPY neurons.
Nonstandard abbreviations used: AgRP, agouti-related protein; BAT, brown adi-
pose tissue; BBS, Bardet-Biedl syndrome; CSF, cerebrospinal fluid; HR, heart rate;
MAP, mean arterial pressure; NPY, neuropeptide Y; POMC, proopiomelanocortin;
SNA, sympathetic nerve activity.
Conflict of interest:
The authors have declared that no conflict of interest exists.
Citation for this article:
J. Clin. Invest. 118:1458–1467 (2008). doi:10.1172/JCI32357.
The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008 1459
Other effects of leptin, besides the modulation of energy
homeostasis, include activation of the sympathetic nervous sys-
tem and an increase in arterial pressure (30). Interestingly, in
several mouse models of obesity, the ability of leptin to increase
cardiovascular sympathetic nerve activity (SNA) is preserved,
despite resistance to leptin’s effect on food intake, body weight,
and thermogenic sympathetic tone (31, 32).
Here, we sought to use mouse models to further explore and dis-
sect the mechanisms involved in the metabolic and cardiovascu-
lar disorders associated with BBS. We hypothesize that defects in
energy balance and central neurogenic mechanisms play a major
pathophysiological role in obesity and in hypertension associated
with the deletion of Bbs genes in mice. The pivotal role of leptin
in energy homeostasis led us to assess potential alterations in the
action of this hormone on energy homeostasis and sympathetic
and cardiovascular function.
Obesity in BBS-knockout mice. We created mutant models of 3 of the
known BBS genes: Bbs2
(22), and Bbs6
knockout mice share many fea-
tures, a number of which are
observed in human BBS patients,
including obesity. Indeed, dele-
tion of any of the 3 BBS genes
leads to increases in body weight
and fat mass (Figure 1 and Table
1) and is associated with hyper-
phagia, decreased locomotor
activity, and high circulating
levels of leptin (Table 1). To test
whether the hyperleptinemia
associated with BBS mice occurs
early in life, we measured plas-
ma leptin levels in 3 groups of
5–6-week-old mice with no significant difference in body weight:
wild-type (14 ± 1 g; n = 4), Bbs2
(15 ± 1 g; n = 3), and Bbs4
(12 ± 2 g;
n = 3). We found that plasma leptin levels were elevated 3.1- and
2.3-fold, respectively, in Bbs2
To assess the relative contributions of hyperphagia and decreased
energy expenditure to the obesity associated with deletion of the
BBS genes, we performed a pair-feeding experiment in which 5- to
mice were given the same amount of
food consumed by their wild-type littermate controls for 13 weeks.
Despite pair-feeding, both Bbs2
mice continued to
have increased adiposity (2–3-fold higher visceral adipose tissue:
reproductive, omental, and perirenal fat depots) compared with
controls (Table 2). Taken together, these results indicate that BBS
mice have low energy expenditure, which may contribute to obe-
sity in these animals.
Exogenous systemic leptin failed to alter body weight and appetite in BBS-
knockout mice. The increased circulating level of leptin in BBS-knock-
out mice prompted us to investigate the role of leptin in the obesity
associated with these animal models. To test whether BBS-knockout
mice have an altered sensitivity to leptin, we compared the effect of
Representative T1-weighted MRI dem-
onstrating greater fat mass in Bbs2
, and Bbs6
mice than in wild-
type littermate controls. Coronal (top)
and axial abdominal (bottom) sections
of wild-type and BBS mice are shown.
With this pulse-sequence, fat appears
white and muscle and water appear gray
or black. The plus signs indicate subcu-
taneous fat, and the asterisks indicate
Characteristics of wild-type control and BBS mice
Parameter Wild-type Bbs2
Body weight (g) 28.7 ± 0.7 40.6 ± 2.4
41.1 ± 2.1
40.3 ± 1.9
Food intake (g) 3.65 ± 0.05 4.73 ± 0.24
4.46 ± 0.12
4.28 ± 0.11
Locomotor activity (AU) 66.8 ± 2.8 35.2 ± 5.6
35.1 ± 1.6
37.4 ± 1.5
BAT (g) 0.09 ± 0.01 0.29 ± 0.05
0.30 ± 0.04
0.28 ± 0.04
Reproductive fat (g) 0.59 ± 0.07 3.24 ± 0.59
3.01 ± 0.39
2.35 ± 0.34
Omental fat (g) 0.22 ± 0.04 1.81 ± 0.28
1.39 ± 0.30
1.42 ± 0.17
Perirenal fat (g) 0.18 ± 0.02 1.25 ± 0.26
1.00 ± 0.16
0.88 ± 0.16
Plasma leptin (ng/ml) 4.1 ± 1.5 50.5 ± 17.0
43.2 ± 10.0
43.5 ± 10.0
Data are mean ± SEM; n = 7–43 mice per group.
P < 0.05 compared with wild-type controls.
1460 The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008
exogenous leptin on body weight and food intake between wild-type
and BBS-knockout mice. Administration of mouse leptin i.p. (1 μg/g
body weight, twice a day for 4 days) caused a significant decrease
in body weight and food intake in the wild-type mice (Figure 2).
In contrast, i.p. leptin failed to significantly decrease body weight
or food intake in Bbs2-, Bbs4-, and Bbs6-knockout mice. These data
demonstrate that the obesity and hyperleptinemia observed in the
BBS-knockout mice are associated with leptin resistance.
Defects in blood-brain transport do not account for leptin resistance in
BBS-knockout mice. To gain insight into the mechanism of leptin
resistance in BBS-knockout mice, we evaluated the role of the
blood-brain barrier in the resistance
to leptin in BBS-knockout mice. A
decreased ability of leptin to cross the
blood-brain barrier has been suggested
to be an important mechanism of leptin
resistance (33). Therefore, we first com-
pared leptin concentrations in the cere-
brospinal fluid (CSF) of wild-type and
BBS-knockout mice to test whether high
levels of circulating leptin translate into
elevated CSF leptin levels. We found that
CSF leptin levels were significantly great-
er (P < 0.05) in Bbs2
(2.09 ± 0.57 ng/ml;
n = 8), Bbs4
(3.90 ± 1.39 ng/ml; n = 8),
(3.76 ± 2.29 ng/ml; n = 5) mice
than in wild-type mice (0.17 ± 0.10 ng/ml;
n = 4). Furthermore, the CSF/serum
leptin ratio was not different between
BBS-knockout mice and their wild-type
littermates (P = 0.827). These data dem-
onstrate that high circulating levels of
leptin are associated with elevated CSF
leptin levels, which suggests that endog-
enous leptin crosses the blood-brain bar-
rier in BBS-knockout mice.
Next, we compared the effect of
i.c.v. administration of leptin between
wild-type and BBS-knockout mice. In
wild-type mice, i.c.v. administration
of leptin (5 μg) caused a significant
decrease in body weight and food intake
(Figure 3). However, the appetite- and
weight-reducing effects of leptin were
significantly attenuated in Bbs4- and
Bbs6-knockout mice as compared with
wild-type animals. Furthermore, the
effects of leptin on appetite and body weight were completely
blunted in Bbs2-knockout mice. Administration of vehicle i.c.v.
did not significantly alter food intake or body weight in BBS-
knockout and wild-type mice (Figure 3).
Finally, we compared the effect of i.c.v. administration of leptin
on the weight of different fat depots between BBS-knockout mice
and wild-type controls. As shown in Table 3, i.c.v. administration
of leptin caused a significant decrease in brown adipose tissue
(BAT), omental fat, and perirenal fat in wild-type mice. In contrast,
i.c.v. administration of leptin did not significantly alter any of the
fat depots in BBS-knockout animals (Table 3).
Decreased hypothalamic Pomc gene expression in BBS-knockout mice. To
test whether the resistance of BBS-knockout mice to the catabolic
actions of leptin is associated with alterations in the expression
of downstream neuropeptides involved in the control of energy
homeostasis, we compared the mRNA levels of POMC, AgRP, and
NPY in the hypothalamus between BBS-knockout mice and wild-
type controls. As shown in Figure 4, the expression of hypotha-
lamic POMC mRNA was significantly reduced in Bbs2
mice. In contrast, the mRNA levels of AgRP and NPY
were not significantly altered in BBS-knockout mice as compared
with wild-type controls. These data indicate that there is a defect
specifically in the POMC neurons that may account for the obesity
phenotype of BBS-knockout mice.
Effect of pair-feeding on body weight and fat depots in BBS mice
Parameter Wild-type Bbs2
Body weight (g) 24.3 ± 1.5 27.1 ± 3.2 24.7 ± 3.0
BAT (g) 0.13 ± 0.03 0.18 ± 0.06
0.17 ± 0.03
Reproductive fat (g) 0.42 ± 0.09 0.97 ± 0.39
1.00 ± 0.50
Omental fat (g) 0.27 ± 0.04 0.65 ± 0.19
0.52 ± 0.15
Perirenal fat (g) 0.24 ± 0.07 0.49 ± 0.10
0.42 ± 0.15
Data are mean ± SEM; n = 4–7 mice per group.
P < 0.05 compared
with wild-type controls.
Change in body weight (A–C) and food intake (D–F) after intraperitoneal administration of vehicle
or leptin (1 μg/g body weight, twice daily) in Bbs2
, and Bbs6
mice as compared with
wild-type littermate controls. Data are mean ± SEM; n = 9–22 mice per group.
P < 0.05 compared
with BBS-knockout mice.
The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008 1461
Renal sympathetic nerve response to leptin is differentially altered in BBS-
knockout mice. Recently, we demonstrated in 2 mouse models of
obesity (agouti obese mice and mice with diet-induced obesity) that
leptin’s ability to activate renal SNA is preserved despite resistance
to the anorectic and weight-reducing effect of leptin (31, 32). To test
whether obese BBS-knockout mice also have a preserved renal sym-
pathetic nerve response to leptin, we compared the effect of i.c.v.
administration of leptin on renal SNA between wild-type and BBS-
knockout mice. Interestingly, as shown in Figure 5 (A and B), base-
line renal SNA was significantly greater in Bbs4
than in wild-type controls. In contrast, baseline renal SNA in Bbs2
mice was not significantly greater than that in wild-type animals.
As expected, i.c.v. administration of mouse leptin (5 μg) caused a
robust increase in renal SNA in wild-type mice (Figure 5C). Surpris-
ingly, the renal SNA response to leptin was differentially affected by
deletion of the Bbs genes. Indeed, Bbs4
animals have a
preserved renal sympathetic activation in response to leptin because
the increase in renal SNA induced by leptin in these mice was compa-
rable with that in the wild-type littermates. In contrast, Bbs2
have an absent renal SNA response to leptin (Figure 5C). Because
of the higher baseline renal SNA in Bbs4
mice, we also
calculated the percentage change in renal SNA in response to leptin.
When expressed as percentage change, leptin-induced renal sympa-
thetic excitation tends to be attenuated in Bbs4
(Figure 5D), but there was no statistical difference in the responses
of these knockout mice compared with wild-type animals. In BBS-
knockout and wild-type mice, i.c.v. administration of vehicle caused
no significant change in renal SNA (data not shown).
Contrasting baseline arterial pressure in BBS-knockout mice. To test
whether the obesity observed in BBS-knockout mice is associ-
ated with hypertension, we compared arterial pressure between
wild-type and BBS-knockout mice (12–14 weeks old). Consistent
with our previous report (23), we found that arterial pressure in
mice was approximately 12 mmHg higher than that in
their wild-type littermate controls (Figure 6A). Arterial pressure
was also greater, by approximately 13 mmHg, in Bbs4
in their wild-type littermates. However, Bbs2
mice had normal
arterial pressure compared with the wild-type controls (Figure 6A).
Heart rate (HR) was comparable between Bbs2
and wild-type controls (Figure 6B; P = 0.91).
To examine whether the elevation of blood pressure is delayed in
mice, relative to Bbs4
mice, we compared arte-
rial pressure between older wild-type, Bbs2-, Bbs4-, and Bbs6-null mice
(6–8 months; n = 6–7 per group) in subsequent experiments. As with
younger mice, an increase in mean arterial pressure (MAP) of approx-
imately 15 mmHg (P < 0.001) was observed in Bbs4- and Bbs6-knock-
out mice, but no significant increase was detected in Bbs2-knockout
mice (116 ± 2 mmHg) relative to wild-type controls (115 ± 1 mmHg).
mice do not became hypertensive, even at later stages,
despite the presence of obesity. Whereas all 3 of our BBS mouse mod-
els exhibited obesity, only 2 (Bbs4
) were hypertensive.
Bbs2-knockout mice did not develop high arterial pressure.
Change in body weight (A and B)
and food intake (C and D) after
i.c.v. administration of vehicle or
leptin (5 μg) in Bbs2
mice as compared with
wild-type littermate controls. Data
are mean ± SEM; n = 8–12 mice
per group. *P < 0.05 compared with
P < 0.05 compared with
Effect of i.c.v. leptin (5 μg) on different fat depots in BBS-knockout mice
Wild type Bbs2
Parameter Vehicle Leptin Vehicle Leptin Vehicle Leptin Vehicle Leptin
BAT (g) 0.07 ± 0.01 0.05 ± 0.01
0.24 ± 0.02 0.25 ± 0.03 0.21 ± 0.01 0.21 ± 0.01 0.25 ± 0.02 0.24 ± 0.02
Reproductive fat (g) 0.35 ± 0.15 0.19 ± 0.06 3.10 ± 0.36 3.54 ± 0.23 2.62 ± 0.32 2.29 ± 0.31 2.74 ± 0.39 2.04 ± 0.25
Omental fat (g) 0.26 ± 0.05 0.10 ± 0.03
1.39 ± 0.14 1.54 ± 0.25 1.76 ± 0.17 1.58 ± 0.16 1.55 ± 0.36 1.31 ± 0.17
Perirenal fat (g) 0.17 ± 0.03 0.08 ± 0.03
1.04 ± 0.12 0.82 ± 0.17 1.10 ± 0.09 1.16 ± 0.17 1.19 ± 0.15 1.05 ± 0.26
Data are mean ± SEM; n = 5 mice per group.
P < 0.05 compared with vehicle.
1462 The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008
Exaggerated arterial pressure response to ganglionic blockade in Bbs4-
and Bbs6-null mice. The finding of elevated renal SNA in Bbs4- and
Bbs6-knockout mice described above prompted us to examine the
effects of inhibition of ganglionic transmission on arterial pres-
sure. We compared the effect of ganglionic blockade with hexa-
methonium (1 μg/g body weight, i.p.) on arterial pressure between
BBS-knockout mice and wild-type controls. A summary and
comparison of the peak of hexamethonium-induced arterial pres-
Hypothalamic mRNA levels of POMC, AgRP, and NPY in Bbs2
, and Bbs6
mice as compared with wild-type littermate controls. Data
are mean ± SEM; n = 3 per group. *P < 0.05 compared with wild-type controls.
Comparison of renal SNA, at baseline and in response to leptin, between Bbs2
, and Bbs6
mice and wild-type littermate controls.
(A) Segments of original records of renal SNA at baseline and 4 hours after leptin (5 μg, i.c.v.). (B) Comparison of baseline renal SNA between
wild-type and BBS mice (n = 13–37 mice per group). (C) Average renal SNA response 4 hours after leptin administration expressed as a function
of time (n = 10–16 mice per group). (D) Average renal SNA response 4 hours after leptin administration expressed as the percentage change
from baseline. Data are mean ± SEM. *P < 0.05 compared with wild-type controls.
The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008 1463
sure responses in wild-type and BBS mice are shown in Figure 7.
Ganglionic blockade with hexamethonium caused equivalent
depressor responses in Bbs2
and control mice; MAP decreased
by approximately 17 mmHg. In contrast, hexamethonium caused
a significantly greater reduction in MAP in Bbs4
(Figure 7A). The enhanced depressor response to hexamethoni-
um in Bbs4
relative to wild-type controls was also
observed when the results were expressed as percentage change
(Figure 7B). These results suggest that there is an exaggerated
contribution of sympathetic tone to the maintenance of elevated
arterial pressure in Bbs4
Relevance of hyperleptinemia to the elevated arterial pressure in Bbs4
mice. To test the role of hyperleptinemia in the elevated
arterial pressure observed in Bbs4
mice, we exam-
ined the effect of modulating circulating leptin levels on arterial
pressure in these animals. The decrease in plasma leptin induced
by fasting was associated with a significant decrease in MAP in
mice (Figure 8). Reversing the decrease in
plasma leptin during fasting with exogenous i.p. administration
of leptin prevented the decrease in MAP in Bbs4
(Figure 8), which indicates the importance of hyperleptinemia
in maintaining arterial pressure elevation in these mice. Of note,
the weight loss induced by fasting in Bbs4
similar regardless of whether the mice were treated with vehicle
(–12.2% ± 1.5% and –11.5% ± 0.8%, respectively) or leptin (–13.1% ± 1.2%
and –12.0% ± 1.6%, respectively), which further demonstrates the
resistance of these mice to the metabolic actions of leptin.
Suppression of hyperleptinemia by a 48-h fast also normalized
baseline renal SNA in Bbs4
mice, as indicated by the finding that
renal SNA was comparable between wild-type controls (1.33 ± 0.15
volts × s/min; n = 4) and fasted Bbs4
mice (1.24 ± 0.08 volts ×
s/min; n = 3). As for the arterial pressure, renal SNA remained ele-
vated in fasted Bbs4
mice given exogenous leptin i.p. (1.99 ± 0.07
volts × s/min; n = 3). These results indicate that the increased renal
SNA in Bbs4
mice was due to hyperleptinemia.
Normal cardiac phenotype in BBS-knockout mice. Cardiac abnor-
malities have been reported in BBS patients (4, 34). To determine
whether BBS-knockout mice have cardiac defects, we performed
echocardiography in conscious mice (14–20 weeks of age). As
shown in Table 4, heart weight and echocardiographic data were
not different between BBS mice and wild-type mice. In agreement
with our radiotelemetry data, HR measured by echocardiography
was also comparable between BBS-knockout mice and wild-type
controls (P = 0.286). To assess whether cardiac abnormalities may
be slow in onset and develop at later stages, we performed echo-
cardiography in older (6–12 months) Bbs6
mice and wild-type
control littermates (n = 5 each). No significant difference in the
echocardiography data was observed between older Bbs6
wild-type mice (data not shown).
Obesity and cardiovascular disease are common in BBS. Howev-
er, limited data on the mechanisms underlying the pathogenesis
of these conditions in BBS are available. In the present study, we
showed in 3 mouse models of BBS (Bbs2
, and Bbs6
obesity was associated with hyperleptinemia, resistance to the ano-
rectic and weight-reducing effects of leptin, and decreased hypo-
thalamic POMC mRNA levels. Furthermore, we demonstrated that
Comparison of MAP (A) and HR (B) between Bbs2
mice and wild-type littermate controls. Data are mean ± SEM
of MAP and HR recorded continuously for 24 hours over 7 days
(n = 5–6 mice per group). Compared with wild-type controls, Bbs4
mice had significantly elevated MAP (P < 0.001), but
mice did not (P = 0.678).
Change in MAP induced by intraperitoneal administration of hexame-
thonium (1 μg/g body weight) in Bbs2
, and Bbs6
and wild-type littermate controls expressed in mmHg (A) and as the
percentage change from baseline (B). Data are mean ± SEM; n = 5–10
mice per group. *P < 0.05 compared with wild-type controls.
1464 The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008
Bbs4- and Bbs6-knockout mice also develop elevated blood pressure,
whereas Bbs2-knockout mice do not. Interestingly, the mice with
hypertension exhibited an increase in renal sympathetic nerve drive,
a preserved renal sympathetic activation in response to leptin, and
a greater reduction in arterial pressure in response to ganglionic
blockade. The elevated arterial pressure observed in Bbs4
mice was reversed when hyperleptinemia was suppressed by
fasting, but was maintained when hyperleptinemia was sustained in
fasted mice treated with exogenous leptin. Thus, whereas all 3 BBS
mice were resistant to the metabolic actions of leptin, Bbs4
mice remained responsive to the effects of leptin on renal
SNA and arterial pressure, which led to hypertension.
Despite the predominance of obesity in BBS (34), including mor-
bid obesity (body mass index > 40 kg/m
), little is known about
the biochemical and physiological abnormalities leading to energy
imbalance in this syndrome. Our data indicate that the develop-
ment of obesity in BBS mice is associated with increased food
intake and decreased locomotor activity. Interestingly, the decrease
in locomotor activity (44%–53%) in BBS-null mice seemed to be
more pronounced than in non-BBS obese mice, although body
weight was comparable in both groups of mice. Indeed, the body
weight (39 ± 1 g) of C57BL/6J mice that consumed a high-fat (45%)
diet for 20 weeks was comparable with that in BBS-null mice (Table
1), but their locomotor activity was decreased by only 30% com-
pared with the lean controls (K. Rahmouni, unpublished observa-
tions). In agreement with our results, Grace et al. (35) showed a
lower level of physical activity in subjects with BBS than in healthy
control subjects, despite comparable body mass indexes. In the
present study, we also showed that pair-feeding is not sufficient to
normalize fat mass in BBS mice. Taken together, these data suggest
that the obesity associated with BBS is caused by a combination of
increased energy intake and decreased energy expenditure.
Our findings also indicated that defects in leptin action are
involved in the obesity associated with BBS. Indeed, we found
that BBS mice have high circulating levels of leptin, even at an
early age before any difference in body weight can be detected.
Hyperleptinemia is usually considered to reflect a state of leptin
resistance, which often predicts a loss of metabolic responsive-
ness to leptin (33). However, emerging evidence indicates that
obesity can develop independently from defects in leptin action
(36, 37). For instance, mice null for neuromedin U are obese
and hyperleptinemic, but have a preserved metabolic response
to the exogenous administration of leptin (37). In BBS mice, we
found that systemic administration of leptin failed to decrease
body weight and food intake, which indicates that these animals
are resistant to the appetite-suppressant and weight-reducing
actions of exogenous leptin.
The high circulating levels of leptin in the CSF of BBS mice
indicate that endogenous leptin does cross the blood-brain bar-
rier in these animals and argue against a meaningful role for
transport defects in the resistance to leptin. This would seem to
indicate that leptin resistance is due to defects in the action of
leptin at the receptor level and/or in the downstream pathways.
Our finding of a reduced expression in the Pomc gene, but not in
the Agrp and Npy genes, supports the notion that the obese phe-
notype observed in BBS-knockout mice is likely due to a defect in
POMC neurons. In the hypothalamus, many of the leptin-respon-
sive neurons are ciliated (38), and immunoreactivity of the leptin
receptor has been shown to be enriched in the cilia membranes
of neurons of the olfactory mucosa (39). Given the importance
of BBS proteins to ciliary function, their absence could lead to
defects in the neuronal cilia and alter the signaling machinery
associated with the leptin receptor. Alternatively, an effect on
intracellular transport resulting from a defect in Bbs genes could
lead to abnormal intracellular trafficking of the leptin receptor
or other signaling machinery. In support of this, intracellular ves-
Effect of a 48-hour fast, with or without leptin administration, on plasma
leptin and MAP in Bbs4
mice. MAP during fasting (with
or without leptin administration) represents the average of the last
24 hours of the fasting period. Data are mean ± SEM; n = 8 mice per
group. *P < 0.05 compared with baseline and fasting+leptin.
Heart weight and echocardiography findings in BBS mice
Genotype HW (g) EDV (μl) ESV (μl) LVm (mg) Vol/LVm (μl/mg) SV (μl) CO (μl/m) EF (%)
Wild type 0.15 ± 0.10 45 ± 3 12.2 ± 1.6 93 ± 5 0.49 ± 0.03 33 ± 3 14,497 ± 1,416 0.74 ± 0.02
0.14 ± 0.10 43 ± 3 13.5 ± 2.9 81 ± 4 0.54 ± 0.05 29 ± 3 13,323 ± 1,906 0.69 ± 0.05
0.14 ± 0.10 42 ± 7 8.6 ± 2.0 104 ± 7 0.39 ± 0.04 33 ± 6 17,142 ± 3,568 0.78 ± 0.05
0.14 ± 0.10 52 ± 9 14.8 ± 5 90 ± 17 0.69 ± 0.18 37 ± 5 13,015 ± 2,696 0.75 ± 0.05
Data are mean ± SEM; n = 7–20 mice per group. HW, heart weight; EDV, left ventricular end-diastolic volume; ESV, left ventricular end-systolic volume;
LVm, left ventricular mass; Vol/LVm, left ventricular end-diastolic volume/LVm ratio; SV, left ventricular stroke volume; CO, cardiac output; EF, left ventricu-
lar ejection fraction.
The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008 1465
icle trafficking has been shown to be compromised in zebrafish
BBS-knockout models (40). Additional experiments are required
to test these hypotheses.
In Bbs4- and Bbs6-knockout mice, i.c.v. administration of leptin
was also able to increase sympathetic nerve drive to the kidney.
This finding suggests that, at least in Bbs4- and Bbs6-null mice, the
leptin receptor is present, because the absence of the long form of
the leptin receptor, as observed in db/db mice, leads to complete
loss of renal sympathetic activation in response to leptin (41). The
finding of preserved leptin-induced renal sympathetic activation
in Bbs4- and Bbs6-knockout mice agrees with our previous find-
ings in agouti obese and diet-induced obese mice, i.e., that leptin
resistance is selective, sparing the renal sympathetic response (32).
Interestingly, preservation of leptin’s ability to activate renal SNA
in Bbs4- and Bbs6-knockout mice is associated with higher base-
line sympathetic nerve tone. In addition to exhibiting elevated
renal sympathetic outflow, Bbs4- and Bbs6-knockout mice exhib-
ited increased arterial pressure just like the agouti obese and diet-
induced obese mice. Of importance, Bbs2-knockout mice that did
not have elevated arterial pressure also did not have elevated renal
SNA and were resistant to the renal sympathetic action of leptin.
The preserved renal sympathetic activation in response to leptin,
in association with high CSF leptin levels, indicates that leptin
may be the main cause of hypertension and enhanced sympathet-
ic drive, which are associated with Bbs4 and Bbs6 genotypes. This
notion is supported by the finding that suppression of hyperlep-
tinemia after fasting normalized arterial pressure and renal sym-
pathetic tone in these mice, but sustaining the hyperleptinemia in
the fasted mice with exogenous leptin administration prevented
the decrease in arterial pressure and renal sympathetic outflow.
The exaggerated depressor response to hexamethonium in Bbs4-
and Bbs6-knockout mice indicates that the elevated arterial pressure
observed in these animals is sympathetically mediated. This finding
is consistent with the elevated renal sympathetic tone observed in
these mice. The kidney is known to play a major role in the control
of cardiovascular function and blood pressure. The renal effects of
an increased renal sympathetic drive lead to renal sodium reten-
tion, a decrease in renal blood flow and the glomerular filtration
rate, renal vasoconstriction, and an increase in renin release (42).
These alterations are known to promote increases in arterial pres-
sure. Diet-induced obesity is accompanied by an elevation in renal
sympathetic outflow (43), and renal denervation attenuated the
increase in arterial pressure associated with dietary obesity (44).
Our current finding of an elevated arterial pressure in Bbs4- and
Bbs6-knockout mice but not in Bbs2-knockout mice is particular-
ly interesting because Moore et al. (34) reported that, in humans,
all BBS genotypes (including BBS4 and BBS6) are accompanied by
hypertension, except for BBS2. This supports the appropriateness
of our mouse models for the study of hypertension in human BBS.
In another study, non-BBS individuals with particular variants of
BBS4 and BBS6, but not BBS2, were found to be at higher risk of
developing hypertension (5). Our BBS mouse models thus provide
the opportunity to gain insight into the mechanisms involved in the
pathophysiological processes of obesity-induced hypertension. Elu-
cidation of the mechanisms that protect some obese patients from
developing hypertension, as in BBS2, will further the understanding
of the pathophysiology of obesity-associated hypertension.
It was recently shown that 7 of the 12 known BBS proteins,
including BBS2 and BBS4, form a core complex known as the
BBSome (45). BBS6 is not part of the BBSome, but rather has
homology to chaperonins. It is currently only speculative whether
BBS6 is involved in BBSome formation. It is of interest that both
Bbs2- and Bbs4-knockout mice would be expected to have abnor-
mal BBSome function, yet have different phenotypes with respect
to elevated arterial pressure, renal sympathetic nerve tone, and
resistance to the renal sympathetic action of leptin. These findings
indicate a difference in BBSome function, depending on whether
the BBS2 or BBS4 component of the complex is missing. Alter-
natively, some components of the BBSome could have functions
separate from their function as a component of the BBSome.
With respect to cardiac function evaluated by echocardiography,
we found that Bbs2
, and Bbs6
mice did not have any
cardiac alterations. Few studies have analyzed the cardiac func-
tion in BBS patients. In one study, 32% of BBS patients showed
some type of abnormalities on echocardiography (4). In another
study, congenital heart defects were noted in 3 of 46 BBS patients
(34). However, in both studies, the genotype of BBS patients with
a heart defect was not reported. Whether the deletion of Bbs genes,
other than those considered here, in mice would recapitulate the
cardiac abnormalities observed in BBS patients remains to be
determined. Also, we cannot exclude cardiac defects as a cause of a
less than predicted number (based on Mendelian ratios) of homo-
zygous BBS-knockout mice (21, 22).
In conclusion, BBS-knockout mouse models provide an appro-
priate tool for studying BBS. Further study of these animals may
help elucidate the pathophysiological mechanisms of the diseases
associated with BBS, including obesity and hypertension.
Animals. For each genotype, homozygous knockout (–/–) and littermate con-
trol (+/+) mice were produced by crossing heterozygous mice with a mixed
genetic background of C57BL/6J and 129/SvEv mice. Genotyping was
performed by PCR as described previously (21, 22). Animals were housed
in a room maintained at a constant temperature (23°C) and a 12-hour
light/12-hour dark cycle (lights turned off at 6 pm) and had free access to
standard mouse chow (Harlan Teklad) and tap water. For the pair-feeding
studies, mice were housed in individual cages, and a knockout animal was
given the amount of food consumed the day before by a sex-matched wild-
type littermate. Imaging of fat by MRI was performed after the animals
were anesthetized (91 mg/kg ketamine and 9.1 mg/kg xylaxine, i.p.) using a
Varian Unity/Inova 4.7 T small-bore MRI system (Varian Inc.). The acquisi-
tion consisted of a T1-weighted fast spin-echo sequence (repetition time/
echo time = 625/12 ms) with an in-plane resolution of 0.13 × 0.25 mm
and a slice thickness of 1 mm acquired in the axial and coronal planes.
To quantify the fat tissues of BBS and wild-type mice, different fat depots
were dissected at sacrifice and weighed. Implantation of cannulae i.c.v. was
performed as described previously (32). Mice were given at least 1 week to
recover from surgery before the experiments began. The University of Iowa
Animal Research Committee approved all protocols.
Effects of leptin on food intake, body weight, and fat depots. The feeding and
body weight responses to leptin were compared between wild-type and
BBS-null mice. Each mouse was housed individually starting at least 1
week before the study. The body weight and food intake of each mouse
were measured daily between 9 and 10 am for 4 consecutive days before the
vehicle treatment began (1 μl/g body weight i.p., twice daily for 4 days). One
day after the end of vehicle treatment, mouse leptin (1 μg/g body weight)
was injected i.p. into the same mice twice daily for 4 days. Other groups of
mice were assigned to receive i.c.v. administration of single injections of
vehicle (1 μl) or leptin (5 μg/mouse). Body weight and food intake were
measured every day throughout the treatments and 5–6 days thereafter. To
1466 The Journal of Clinical Investigation http://www.jci.org Volume 118 Number 4 April 2008
test the effect of i.c.v. administration of leptin on the weight of fat depots,
wild-type and BBS-null mice were treated with either vehicle or leptin
(5 μg/mouse) i.c.v. For each genotype, the mice treated with vehicle and
leptin were weight-matched. Mice were sacrificed 24 hours after i.c.v. treat-
ments, and the weight of individually dissected BAT, reproductive fat,
omental fat, and perirenal fat depots was measured.
Recording of renal SNA. Mice were anesthetized by i.p. injection of ketamine
(91 mg/kg) and xylaxine (9.1 mg/kg). Catheters were introduced into a
carotid artery and jugular vein for the measurement of hemodynamic vari-
ables and the administration of anesthetic (α-chloralose), respectively. To
measure direct multifiber renal SNA, a nerve fascicle to the left kidney was
carefully isolated. A bipolar platinum-iridium electrode (Cooner Wire) was
suspended under the nerve and secured with silicone gel (Kwik-Cast; WPI).
The nerve signal was amplified and filtered as described previously (32).
Baseline renal SNA and hemodynamic variables were recorded for
10 minutes. An average of 2 separate measurements during the 10-minute
control period was considered the baseline value for each animal. Leptin
(5 μg) or vehicle (2 μl) was then administered i.c.v. The renal SNA response
to leptin or vehicle was recorded continuously for 240 minutes. Anesthesia
was sustained with the administration of α-chloralose (25 mg/kg/h) intra-
venously. Body temperature was maintained at 37.5°C with the assistance
of a lamp and a heating pad. At the end of the study, mice were euthanized
with a lethal dose of ketamine and xylazine. The integrated voltage after
death (background noise) was subtracted from the total integrated voltage
to calculate real renal SNA.
Measurement of arterial pressure. Arterial pressure was recorded in con-
scious mice using continuous radiotelemetric measurement. Mice were
anesthetized i.p. with a ketamine and xylaxine cocktail. The left common
carotid artery was isolated, and the catheter was inserted and tied securely
using silk. The transmitter was slipped under the skin and down into a dis-
sected free “pocket” along the flank, as close to the right hind limb as pos-
sible. The neck incision was closed using silk and then was further sealed
with tissue adhesive. Mice were kept warm on a heating pad and monitored
closely until they fully recovered from anesthesia.
The animals were allowed to recover for several days before arterial pres-
sure, HR, and locomotor activity were recorded continuously in the con-
scious unrestrained state for 7 days. The effect of ganglionic blockade (hexa-
methonium, 1 μg/g body weight, i.p.) on basal arterial pressure was then
examined. Measurements were recorded for 10 seconds every 5 minutes and
stored on a personal computer using Data Science Dataquest software.
To study the relevance of hyperleptinemia to the hypertension associ-
ated with BBS, Bbs4
mice were equipped for radiotelemetry
measurement and allowed to recover for several days as described above.
Baseline MAP was recorded over 3 days, after which the mice were fasted
for 48 hours. During fasting, mice were treated i.p. (twice daily) with vehi-
cle or leptin. Leptin was administered in incremental dose (0.5, 1, 1.5, and
2 μg/g body weight). In a pilot study, we found that such i.p. leptin treat-
ment maintained hyperleptinemia in fasted Bbs4
week later, during fasting again for 48 hours, the Bbs4
received the inverse treatment, i.e., the mice treated with vehicle and leptin,
respectively, in the first fasting period were given leptin and vehicle, respec-
tively, in the second fasting period. Blood for plasma leptin assay was col-
lected from the tail vein of each mouse at baseline (just before the record-
ing of baseline MAP began) and at the end of each fasting period.
Echocardiography. Two-dimensional echocardiography was performed as
previously described (46). Mice were lightly sedated with midazolam (0.15 mg,
s.c.), which allowed the mice to remain conscious but docile. The following
indices of cardiac function were measured: left ventricular end-diastolic vol-
ume, left ventricular end-systolic volume, left ventricular mass, left ventricular
end-diastolic volume divided by the left ventricular mass ratio, left ventricular
stroke volume, cardiac output, and left ventricular ejection fraction.
Analysis of hypothalamic neuropeptide expression. Mice were sacrificed by CO
asphyxiation, and mediobasal hypothalami were excised and mRNA extract-
ed using Tri reagent. Real-time RT-PCR was used to compare the mRNA lev-
els of POMC, NPY, and AgRP between the control and BBS-knockout mice.
The primers used were as follows: 5′-CTGCTTCAGACCTCCATAGATGTG-
3′ (forward POMC), 5′-CAGCGAGAGGTCGAGTTTGC-3′ (reverse POMC);
5′-TCAGACCTCTTAATGAAGGAAAGCA-3′ (forward NPY), 5′-GAGAA-
CAAGTTTCATTTCCCATCA-3′ (reverse NPY); and 5′-CAGAAGCTTTG-
GCGGAGGT-3′ (forward AgRP), 5′-AGGACTCGTGCAGCCTTACAC-3′
(reverse AgRP). RPL19 was used as an internal control.
Leptin measurements. Leptin was measured in plasma and CSF. Plasma was
obtained by centrifuging the blood collected from the mice at 2,040 g for
8 minutes. For the collection of CSF, each mouse was anesthetized with a
ketamine and xylazine cocktail and then positioned in a stereotaxic appara-
tus with the neck flexed. An incision was then made to expose the membrane
located in the area between the occipital notch and the first cervical verte-
brae. Once the membrane was exposed, a 20-μl Hamilton syringe was used to
puncture the membrane and, immediately, the CSF was carefully suctioned
out; only clear CSF was used. Mice were sacrificed after the blood was col-
lected. Murine leptin concentrations were measured by radioimmunoassay
using a commercially available kit (Crystal Chem Inc.).
Statistics. Results are expressed as mean ± SEM. Data were analyzed using
1- or 2-way ANOVA. When ANOVA indicated significance, Fisher’s test was
used to compare the mean values between the different groups of mice.
P < 0.05 was considered significant.
We thank Mike Andrews and Donald A. Morgan for assisting
with mouse maintenance and SNA experiments, respectively, and
Allyn L. Mark and Ruth Swiderski for critically reading the manu-
script and making helpful comments. This work was supported
by grants from the American Heart Association (to K. Rahmouni)
and the NIH (to K. Rahmouni and V.C. Sheffield). V.C. Sheffield is
an investigator of the Howard Hughes Medical Institute.
Received for publication April 10, 2007, and accepted in revised
form January 16, 2008.
Address correspondence to: Kamal Rahmouni, Center on Func-
tional Genomics of Hypertension, Department of Internal Medi-
cine, University of Iowa Carver College of Medicine, 3135C MERF,
Iowa City, Iowa 52242, USA. Phone: (319) 353-5256; Fax: (319)
353-5350; E-mail: firstname.lastname@example.org.
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