Insulin-like growth factor 1 (IGF-1) and insulin exert biological effects through highly homologous tyrosine kinase receptors, which are ubiquitously expressed in rodents. During the last two decades, substantial progress has been made in understanding the role of IGF-1 and insulin signaling in the brain. Major progress has been made in identifying differences of IGF-1 and insulin signaling in the brain and understanding the phenotypic discrepancies of disruptions of the IGF-1 receptors (IGF1Rs) and insulin receptors (IRs) in the brain. Metabolic diseases such as obesity and diabetes are global public health crises. Moreover, perturbations of metabolism cause various reproductive diseases such as abnormal puberty onset, irregular estrus cycle, altered ovarian function, infertility and reproductive system cancers. Thus, understanding and deciphering brain IGF1R and IR signaling are crucial to current research and crucial for potential therapeutic interventions for metabolic and reproductive diseases.
Neurons are the fundamental units of the brain and carry out distinct functions, which raises another challenge -- understanding the role of a given subset of neurons. Two subsets of neurons-leptin receptor (LepRb) neuron and kisspeptin (Kiss1) neuron have drawn my attention due to their distinct activities in metabolism and reproduction respectively. Current technique Cre/loxP system enables conditional suppression of gene expression in distinct subsets of neurons of interest. We used this technique to generate transgenic mice to study the role of IGF1R and IR signaling in LepRb neurons and Kiss1 neurons. Chapter 1 gives a review of metabolic and reproductive function of IGF1R and IR, and a central control of metabolism and reproduction by LepRb neurons and Kiss1 neurons.
By characterizing reproductive and metabolic phenotype of mice lacking IGF1Rs and/or IRs exclusively in LepRb neurons (IGF1RLepRb mice and IGF1R/IRLepRb mice), we found that IGF1RLepRb and IGF1R/IRLepRb mice experienced growth retardation, delayed puberty and impaired fertility. Male mice had decreased gonadotropin and testosterone levels, impaired testicular histology, suggesting direct disruptions of hypothalamic-pituitary-gonadal axis. Interestingly, female reproductive hormones were normal at 4 weeks of age, while IGF1R/IRLepRb showed elevated gonadotropin and decreased follicle counts compared to female IGF1RLepRb or control mice. The decreased serum growth hormone (GH) and IGF-1 levels in male IGF1RLepRb mice demonstrates communication between LepRb neurons and the GH/IGF-1 axis. Our findings highlight the importance of IGF1R in LepRb neurons in the regulation of body growth, puberty and fertility (Chapter 2).
In Chapter 3, we found female IGF1RLepRb mice had decreased body weight and food intake accompanied by increased VO2, physical activity, and thermogenic gene expression in brown adipose tissue (BAT). These effects were sexually dimorphic; IGF1R signaling was not critical in regulating body weight, food intake or glucose homeostasis in male mice. Interestingly, IGF1R/IRLepRb mice showed dramatically increased fat mass percentage and insulin insensitivity compared to either IGF1RLepRb or control mice. In sum, loss of IGF1R in LepRb neurons confers resistance to obesity due to increased energy expenditure (EE), showing IGF1R signaling is obesogenic. These effects diminished in IGF1R/IRLepRb mice due to decreased EE and physical activity and increased lipid storage in BAT, suggesting IR signaling in LepRb neurons has an overall protective effect against obesity. Thus, our findings provide novel evidence that IGF1R and IR signaling in LepRb neurons interact and provide counterbalancing effects on the regulation of body composition and insulin sensitivity.
Kiss1 neurons express LepRbs; however, we have shown that leptin’s effects on puberty in mice do not require Kiss1 neurons. But we do not know whether the effects of IGF1R and IR signaling require Kiss1 neurons. To answer this question, we have now generated transgenic mice lacking IGF1Rs and/or IRs exclusively in Kiss1 neurons (IGF1RKiss1 mice and IGFR/IRKiss1 mice) (Chapter 4). We found that IGF1RKiss1 mice experienced decreased body weight, body length, delayed pubertal development and decreased litter size. Surprisingly, these parameters were comparable between the IGF1RKiss1 and IGF1R/IRKiss1 mice. These results indicate IGF1R signaling in Kiss1 neurons is the major driver of effects on body weight, body length and adult fertility. Notably, IGF1R/IRKiss1 mice had significantly increased fat mass, decreased physical activity and disrupted glucose homeostasis, which suggest IGF1R and IR may have compensatory effects in the regulation of fat mass, physical activity and glucose homeostasis. In summary, IGF1R and IR signaling in Kiss1 neurons have unique and cooperative roles in regulating metabolic and reproductive functions.
The reproductive axis is linked to nutritional status. Previous chapters demonstrated that undernutrition (decreased food intake and body weight) was associated with reproductive dysfunctions, in Chapter 5 we discussed the role of overnutrition in reproduction. Recent work shows that gut microbial dysbiosis contributes to the risk of obesity in children whose mothers consume a high fat diet during both gestation and lactation or gestation alone. Obesity predisposes children to developing early puberty. However, to date, no study has examined how maternal high-fat-diet (MHFD) during lactation regulates pubertal timing, and fertility of children. Here, we found MHFD during lactation markedly altered the gut microbiota of dams and offspring. This outcome was associated with juvenile obesity, early puberty, and irregular estrous cycles. We also found that MHFD induced early puberty may be mediated by increased IGF-1 signaling. Moreover, MHFD during lactation disrupted glucose and energy homeostasis. Remarkably, permitting coprophagia between MHFD and maternal normal chow offspring successfully reversed early puberty and insulin insensitivity. Our data suggest that microbial reconstitution may prevent or treat early puberty and insulin insensitivity.
In summary, this dissertation 1) dissected the crucial role of IGF1R and IR signaling in metabolism and reproduction in distinct subsets of neurons; 2) demonstrated IGF1R signaling in both LepRb and Kiss1 neurons plays a dominant role in the regulation of puberty, fertility and growth; 4) IGF1R and IR signaling in both LepRb neurons and Kiss1 neurons have compensatory roles in the regulation of body composition and glucose homeostasis; 5) finally, demonstrated MHFD during lactation is crucial to metabolic and reproductive health of offspring, which is mediated via modulation of gut microbiota.