Endocrinology Vol. 142, No. 8 3421-3425
Copyright © 2001 by The Endocrine Society
Leptin-Deficient Mice Backcrossed to the BALB/cJ Genetic Background Have Reduced Adiposity, Enhanced Fertility, Normal Body Temperature, and Severe Diabetes
J. Qiu,
S. Ogus,
K. Mounzih,
A. Ewart-Toland and
F. F. Chehab
Department of Laboratory Medicine, University of California, San
Francisco, California 94143-0134
Address all correspondence and requests for reprints to: Dr. Farid F. Chehab, 505 Parnassus Avenue, Department of Laboratory Medicine, University of California, San Francisco, California 94143-0134. E-mail:
chehabf{at}labmed2.ucsf.edu
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Abstract
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A deficiency of leptin synthesis in mice results in a complex
phenotype characterized by morbid obesity, diabetes, sterility, and
defective thermogenesis. To determine whether the genetic background
could alter the pleiotropic effects of leptin deficiency, we
backcrossed the ob mutation for 10 generations from the
C57BL/6J to the BALB/cJ genetic background. Compared with C57BL/6J
ob/ob mice, BALB/cJ ob/ob mice showed at
27 wk of age a 3540% reduction in body weight attributed to a 60%
decrease in white adipose tissue mass. Food intake was not
significantly different between the two obese strains, suggesting
distinct utilization of energy intake. In the fed state, BALB/cJ
ob/ob mice had elevated insulin and triglycerides
levels, demonstrating a worsening effect on diabetes. At the
reproductive level and in contrast to sterile C57BL/6J
ob/ob mice, male and female BALB/cJ ob/ob
mice were capable of reproducing after a mating period of 16 and 32 wk,
respectively. At thermoneutrality, the body temperature of BALB/cJ
ob/ob mice was 2.9 C higher than that of C57BL/6J
ob/ob mice, whereas exposure of both groups to 4 C
demonstrated a prolonged cold tolerance of BALB/cJ ob/ob
mice. These studies show that the abnormalities caused by leptin
deficiency can be genetically dissected and separated from each other,
suggesting discrete pathways controlled by leptin modifier
genes.
 |
Introduction
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THE PHENOTYPE OF the ob/ob mouse
is characterized by a morbid obesity, hyperphagia, transient
hyperglycemia with late-onset diabetes, and life-long sterility
(1). Despite intense investigations over half a decade
aimed at uncovering its underlying physiological defect, it was only
when positional cloning strategies came of age that a nonsense mutation
(R105X) in a new gene was unveiled. The protein encoded by this gene
was found to be a secreted protein named leptin (2).
Administration of the recombinant hormone leptin to ob/ob
mice rescues their metabolic and reproductive abnormalities
(3, 4, 5, 6, 7) establishing an important role for this hormone in
body weight homeostasis and reproductive biology. The search for leptin
gene mutations among obese individuals led to the uncovering of
rare homozygous cases that exhibited phenotypes similar to that of
ob/ob mice (8) and responded well to
recombinant leptin therapy, as evidenced by a decrease in food intake,
reduction in body weight, and onset of normal reproductive function
(9).
Most of the experiments carried out on ob/ob mice were
largely on mutant mice maintained on an inbred C57BL/6J genetic
background. Hybrid vigor and genetic heterogeneity have long been known
to confer a selective advantage to an organism, due mainly to the
overall effects of modifier genes. Although the ob mutation
has been mostly studied on the C57BL/6J background, a congenic line of
ob/ob mice on the C57BL/Ks background was also generated.
C57BL/Ks ob/ob mice exhibit initially an obese phenotype,
but then start to lose weight progressively before dying prematurely of
severe diabetes resulting from exhaustion and death of the islets of
Langerhans (10). We recently described the phenotypes of
F2 ob/ob mice bred on a mixed
C57BL/6J/BALB/cJ genetic background (11) and demonstrated
that modifier genes on the BALB/c genome ameliorate the reproductive
defect of ob/ob males and significantly contribute to an
alleviation of their obese phenotype. In this communication we describe
the characteristics of a new congenic line of ob/ob mice
backcrossed for 10 generations on the BALB/cJ genetic background.
BALB/cJ mice were previously found to be resistant to
streptozotocin-induced diabetes (12) and have not been
reported to exhibit a susceptibility to obesity. We found in this study
that interaction of the BALB/cJ genome with homozygosity for the
ob mutation results in a drastic decrease in fat
accumulation, increased insulin resistance without a life-threatening
condition, normal thermogenesis, and improved fecundity of the obese
mice.
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Materials and Methods
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Derivation of the congenic line
Inbred mice on the BALB/cJ background and ob/ob mice
on the C57BL/6J background were purchased from The Jackson Laboratory (Bar Harbor, ME) and maintained at the University of
California-San Francisco mouse facility under a standard regimen of
alternating 12-h light and dark periods. All procedures were approved
by the University of California-San Francisco committee on animal
research. All mice were fed ad libitum a basic chow diet
(FormuLab 5008, Ralston Purina Co., St. Louis, MO) with
unrestricted access to water. Obligate heterozygous ob mice
were generated by treating C57BL/6J ob/ob males with
recombinant leptin as previously described (7), followed
by mating with normal BALB/cJ females. F1
ob/+ males were then backcrossed to pure +/+ BALB/cJ females
to yield a 1:1 ratio of homozygous normal +/+ and ob/+
heterozygous mice in the F2 generation.
Heterozygous ob +/- mice were identified by PCR genotyping
of the R105X mutation using a PCR assay (6). The process
of backcrossing the ob +/- males to inbred BALB/cJ females
was repeated for 10 generations to yield a congenic line of
ob/ob mice on the BALB/cJ genetic background.
During this process, 1 round of backcrossing involving a female
ob +/- with a +/+ BALB/cJ male was carried out to ensure
transfer of the BALB/cJ Y chromosome to the new congenic line.
Body weights, food intake, and adiposity
Body weights of ob/ob males and females on the
BALB/cJ and C57BL/6J backgrounds were determined regularly until 27 wk
of age. Food intake was monitored daily for 18 d in C57BL/6J
ob/ob (n = 5) and BALB/cJ ob/ob (n = 5)
females housed in individual cages. Obese mice from each sex and group
were killed at 2731 wk of age, and the liver, white and brown adipose
depots, and remaining carcasses devoid of any organs were weighed on an
analytical balance.
Metabolic assays
Blood was collected by retroorbital sinus bleeding from five
ob/ob males from each genetic background at 1517 wk of
age. Plasma glucose, cholesterol, triglycerides, and insulin levels in
24-h fasted and ad libitum-fed mice were determined,
respectively, on a Hitachi 747 Clinical Chemistry Analyzer
(Hialeah, FL) and by a rat insulin RIA (Linco Research, Inc., St. Charles, MO).
Cold challenge
C57BL/6J ob/ob (n = 3) and BALB/cJ
ob/ob (n = 7) females were housed individually and
placed at 4 C. Rectal temperature recordings were determined before and
hourly after exposure to cold with a precision thermometer (YSI, Inc., Yellow Springs, OH) equipped with a 0.2-mm probe
(YSI, Inc., model 451).
Fertility of BALB/cJ ob/ob mice
To investigate the fertility of this new ob/ob line,
6- to 12-wk-old BALB/cJ ob/ob males (n = 10) and
BALB/cJ ob/ob females (n = 4) were housed with proven
wild-type breeding littermates over a period of 16 wk for the obese
males and 32 wk for the obese females. As we regularly monitored the
body weights of the obese mice and their mates, it was possible to
retroactively determine their body weights at mating, considering a
gestation period of 19 d.
Statistics
All values are reported as the mean ± SEM.
Significance levels were determined by unpaired t test for
independent groups using the Statistica 4.1 software package (Statsoft,
Tulsa, OK) for the Macintosh computer.
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Results
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Body weights, adiposity, and food intake
The foremost striking difference between BALB/cJ ob/ob
and C57BL/6J ob/ob mice was a reduction in body weight in
the former group. At 5 wk of age, the body weights of C57BL/6J and
BALB/cJ ob/ob mice were, respectively, 35.2 ± 2.9
vs. 21.4 ± 1.2 g for males (n = 11 each
group; P = 6 x
10-7) and 30 ± 2.9
vs. 20.7 ± 1.3 g for females (n = 10 each
group; P = 0.009). Differences in body weight between
the two obese strains were highly significant throughout the monitoring
period (Fig. 1
), such that at 27 wk of
age, the body weights of male and female BALB/cJ ob/ob mice
were 27 and 31.5 g less than those of their C57BL/6J
ob/ob counterparts. Figure 2
shows a photograph of two age-matched ob/ob females on the
BALB/cJ and C57BL/6J backgrounds, depicting the impressive reduction of
body weight in BALB/cJ ob/ob mice.

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Figure 1. Body weights of ob/ob males and
females on the C57BL/6J or BALB/cJ genetic background from 427 wk of
age.
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Figure 2. Typical photograph of two ob/ob
females on the C57BL/6J or BALB/cJ genetic background depicting the
reduced adiposity on the BALB/cJ background.
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To determine whether this reduction in body weight was due to a
decrease in adiposity, we weighed the white and brown adipose tissue
masses, liver, and carcasses of ob/ob mice from both strains
at 2731 wk of age (Fig. 3
). Compared
with their C57BL/6J ob/ob counterparts, BALB/cJ
ob/ob mice had their white and brown adipose tissue masses
reduced, respectively, by 2.5-fold (P = 0.00002) and
1.5-fold (P = 0.05) in males and by 2.2-fold
(P = 0.0000001) and 1.3-fold (P = 0.06)
in females. Furthermore, the liver was 1.6-fold (P =
0.004) and 1.2-fold (P = 0.05) smaller in male and
female BALB/cJ ob/ob mice, respectively, vs. that
in age- and sex-matched C57BL/6J ob/ob mice. Weights of the
carcasses, which included muscle, bone, and brain, remained significant
in both males (P = 0.002) and females
(P = 0.0002), suggesting additional effects of the
BALB/cJ genetic background.
Food consumption by ob/ob mice on both genetic backgrounds
was measured during an 8-d period to determine whether the decrease in
food intake contributed to the noted differences in adiposity. We found
that the cumulative food consumption of ob/ob mice from both
strains was nearly identical and was not statistically significant
(Fig. 4
), demonstrating that food intake
is not a contributing factor to the alleviation of the obese phenotype
in BALB/cJ ob/ob mice.
Thermogenesis
At the initiation of this experiment, the body weights of the
C57BL/6J ob/ob and BALB/cJ ob/ob females were
65.2 ± 3.0 and 51.4 ± 0.8 g, respectively
(P < 0.001). The ambient body temperatures of the
C57BL/6J ob/ob and BALB/cJ ob/ob females were
determined with a rectal probe and were 35.0 ± 0.3 and 37.9
± 0.1 C, respectively (P =
10-6). At 4 C, the
temperature of C57BL/6J ob/ob and BALB/cJ ob/ob
mice fell, respectively, to 29.0 ± 1.9 and 35.8 ± 1.1 C
(P = 0.01) after 1 h and to 21.1 ± 3.9 and
35.3 ± 1.2 C (P = 0.001) after 2 h (Fig. 5
) In compliance with an animal care
regulation that prohibits animal death as the end point of an
experiment, the C57BL/6J ob/ob mice had to be removed from
the cold room after 1 h when their body temperature reached
critically low levels, whereas the BALB/cJ ob/ob mice
withstood the cold for up to 34 h but eventually succumbed to it.

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Figure 5. Body weights and core temperature of
ob/ob females on the C57BL/6J and BALB/cJ genetic
backgrounds during a 4 C cold challenge. *, P =
0.01; ***, P < 0.0001.
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Serum chemistries
Plasma levels of glucose, insulin, triglycerides, and cholesterol
were determined in the fasted and fed states in ob/ob males
from both strains at 1517 wk of age (Fig. 6
). In the fasted state there was no
significant difference in any measurement between groups except for
glucose (P = 0.04). However, in the fed state,
triglycerides and insulin levels were, respectively, 3-fold
(P = 0.00003) and 2.5-fold (P =
10-7) more elevated in obese BALB/cJ than in obese
C57BL/6J mice. Glucose and cholesterol levels were not significantly
different between the two groups.
Fertility
Six of 10 ob/ob males and all 4 females on the BALB/cJ
genetic background were fertile, as determined by the 13 pregnancies
induced by the obese fathers and the 12 pregnancies of the obese
mothers (Table 1
). The body weight range
around the time of mating was 30.752.1 g for ob/ob males
and 35.763.6 g for ob/ob females. There were no
correlations between the number of pregnancies and the initial or at
mating body weights of BALB/cJ ob/ob mice.
 |
Discussion
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The genetic makeup of an organism often accounts for the variation
in penetrance associated with the phenotype of a particular mutation.
To determine this effect in the context of an obese phenotype, we
previously generated an F2 intercross segregating
the ob mutation on the mixed C57BL/6J and BALB/cJ genetic
backgrounds (11). The resulting F2
ob/ob mice showed variability in the expression of obesity,
diabetes, and fertility, which was attributed to modifier genes from
both genetic backgrounds. In this study we followed up on these
experiments by backcrossing the ob mutation for 10
generations on the BALB/cJ genetic background. Intercrossing of the N10
ob heterozygotes resulted in a new congenic line of
ob/ob mice on the BALB/cJ genetic background. Except for a
differential C57BL/6J chromosomal segment of approximately 20 cM, which
is retained from selection of the ob mutation at each round
of backcrossing (13), the contribution of modifier genes
from the BALB/cJ genetic background onto the ob/ob phenotype
can now be assessed independently of the C57BL/6J genome.
One of the major findings in BALB/cJ ob/ob mice was their
decreased adiposity, which could not be attributed to decreased
hyperphagia, because food intake was similar in both strains. Most
likely, mechanisms that increase energy expenditure will be the first
candidates to investigate the basis of their reduced adiposity. As
C57BL/6J ob/ob mice have defective thermogenesis and low
body temperature (14), the finding that BALB/cJ
ob/ob mice have normal ambient body temperature and are more
tolerant to the cold challenge than C57BL/6J ob/ob mice
suggests that cold-stimulated sympathetic outflow to brown fat in the
absence of leptin can be corrected with the appropriate modifier genes.
This observation further suggests that increased sympathetic activity
of BALB/cJ ob/ob mice may be a contributing factor to their
reduced obese phenotype. Although not yet determined, the general
activity of BALB/cJ ob/ob mice appears to be increased
compared with the well known passive nature of C57BL/6J
ob/ob mice.
The elevated serum insulin levels of fed BALB/cJ ob/ob mice
demonstrate an insulin resistance state that is more severe than that
of ob/ob mice on the C57BL/6J background and is quite
distinct from that of the ob/ob strain bred on the C57BL/Ks
background, which succumbs to the diabetic state as a result of
pancreatic islet exhaustion (10). Thus, when coupled to an
obese phenotype, the BALB/cJ background expresses modifier genes that
are deleterious to glucose homeostasis. The elevated triglycerides
levels of fed BALB/cJ ob/ob, but not C57BL/6J
ob/ob, mice may be explained by decreased activity of
endothelium-bound lipoprotein lipase, which normally is stimulated by
insulin (15, 16). However, an insulin resistance
environment such as that in obese mice, may explain at least in part
the reduced synthesis and release of lipoprotein lipase leading to
impaired triglycerides breakdown and accumulation in the circulation.
Genetically, these hypotheses would imply that the modifier genes
brought into the ob/ob phenotype by the BALB/cJ background
greatly influence these pathways and that the uncovering of such
factors, for example, by analysis of differentially expressed genes or
microarray hybridization panels, will undoubtedly lead to the
manipulation and understanding of these pathways.
The sterility of ob/ob mice can be rescued with leptin
treatment (6, 7) or through modifier genes brought into
the ob phenotype by the mixed C57BL6J/BALB/cJ genetic
background (11). In the present study the BALB/cJ
ob/ob males exhibited a similar fertility as the
F2 C57BL6J/BALB/cJ ob/ob males.
However, previously (11) we had found that the mixed
C57BL/6J-BALB/cJ genetic background rescued the reproductive defect of
only ob/ob males. In this study only 60% of BALB/cJ
ob/ob males were fertile, suggesting that the mixed genetic
background is more beneficial to male reproduction than the BALB/cJ
inbred background alone. These differences may be attributed to the
interaction of both genomes on reproductive function. Furthermore, the
fertility of BALB/cJ ob/ob females, but not of
C57BL/6J-BALB/cJ ob/ob females, demonstrates a stimulatory
effect of BALB/cJ modifier genes on the female reproductive system of
ob/ob mice. On the other hand, the idea that the obese state
imposes a physical hindrance that might interfere with fecundity is not
founded by our studies. For example, the body weights at mating of the
BALB/cJ ob/ob mice ranged from 30.762.3 g. At these body
weights, the C57BL/6J ob/ob mice remain sterile and fail to
reproduce. Thus, adiposity is not a major physical factor for the
failure of ob/ob mice to reproduce. Instead, BALB/cJ
modifier genes, which remain to be unveiled, control the firing of the
reproductive system and must play a critical role in reproductive
pathways associated with obesity.
Overall, the novel BALB/cJ ob/ob phenotype demonstrates the
existence of modifier genes that can rescue a defective leptin pathway
without leptin. The elucidation of the factors encoded by these
modifier genes will not be an easy task and will require complex
genetic and genomic approaches in central and peripheral tissues, but
it will ultimately reveal whether these genes are part of the leptin
pathway or of other dominant pathways. In either case, they will lead
to an increased understanding of the mechanisms that govern body weight
homeostasis, energy expenditure, and the reproductive system.
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Footnotes
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This work was supported by NIH Grant HD-35142.
Received February 27, 2001.
Accepted for publication April 11, 2001.
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