Nhlh2: a basic helix-loop-helix transcription factor controlling physical activity.

In mice, targeted deletion of the basic helix-loop-helix transcription factor, nescient helix-loop-helix 2 (Nhlh2), leads to adult-onset obesity and reduced physical activity. We propose the novel hypothesis that transcriptional activity by Nhlh2 (NHLH2 in humans) controls either the ability or the motivation for exercise.

Reduced activity without hyperphagia contributes to obesity in Tubby mutant mice.

The Tub gene was originally identified as a spontaneous mutation in C57Bl/6J mice, and associated with adult-onset obesity (Tub MUT mice). Although the original Tub MUT mouse was identified over 15 years ago, there have been few reports on the animal's food intake, body fat percentage or energy expenditure. In this study, we report food intake, body weight from 5-20 weeks, body fat, body temperature and three different measures of physical activity behavior. Tub MUT mice display reduced food intake, uncharacteristic of many obese mouse models, and reduced voluntary wheel running with normal home cage ambulatory behavior. We conclude that motivation for food and exercise is an underlying defect in TUB MUT mice.

Reduced voluntary activity precedes adult-onset obesity in Nhlh2 knockout mice.

Targeted deletion of the neuronal basic helix-loop-helix transcription factor Nhlh2 results in adult-onset obesity in mice. Measurement of body weight and body composition in animals aged 3-25 weeks indicates that while male and female Nhlh2 knockout (N2KO) animals both show adult-onset obesity, the time frame for development of obesity is different, with females becoming obese by 7 weeks of age and males becoming obese by 10 weeks of age. Heterozygous (HET) animals also become obese but with a slower onset, indicating a dosage effect for the activity of the Nhlh2 transcription factor. Food intake, body temperature, and voluntary activity were measured in both preobese and obese N2KO, HET, and wild-type (WT) animals to determine which factors contributed to weight gain. While increased food intake and decreased body temperature were found in older obese N2KO animals, only reduced physical activity preceded the onset of obesity in N2KO mice. N2KO animals had no deficit in either circadian rhythm or balance and motor control, indicating that reduced voluntary activity is the result of a behavioral change. These data demonstrate a role for the Nhlh2 transcription factor in controlling genes important to energy expenditure, and more specifically voluntary physical activity of animals.

The Nhlh2 transcription factor is required for female sexual behavior and reproductive longevity.

Nhlh2 is a member of the basic helix-loop-helix (bHLH) transcription factor family and is expressed in developing and adult neuroendocrine tissues such as the pituitary and hypothalamus. Targeted deletion of Nhlh2 (N2KO) in mice results in hypogonadism and obesity. While gonadally intact male N2KO mice are infertile and lack male sexual behavior, female N2KO mice can become pregnant and carry litters to full term. Unlike normal females in which fertility averages 8-12 months with approximately one pregnancy per month, N2KO females have a shorter reproductive span with most females supporting only three to four pregnancies in a 9-month period. In addition, N2KO females exhibit abnormal estrous cycles characterized by a truncated estrus and a prolonged proestrus. We have found that while young female N2KO mice ovulate the same number of oocytes as normal females in response to exogenous hormones, the number of oocytes released by aged N2KO females is reduced over 50%. Interestingly, oocytes from N2KO females are equally competent for in vitro fertilization assays when compared to oocytes from similarly aged normal and heterozygous mice. We have further demonstrated that both young and old N2KO females show at least a 50% reduction in hormone-stimulated sexual behavior as measured by their lordosis quotient. This suggests that N2KO females show a lifelong behavioral hyporesponsiveness to exogenous steroid hormones accompanied by a reduction in reproductive longevity via reduced ovulation with aging. Potential gene regulatory mechanisms that involve the action of the Nhlh2 transcription factor on female fertility and sexual behavior are discussed.