Abstract
Leptin and insulin are key peripheral signals that act as positive indicators to the brain of the body’s nutritional reserve. Chronic overstimulation of these hormones may result in leptin and insulin resistance, which are common hallmarks underpinning obesity, type II diabetes, and dietinduced infertility. However, leptin signaling is multi-faceted, and has many regulators influencing its action. This project focused on the role of the STAT3 phosphatase, protein tyrosine phosphatase non receptor type 2 (PTPN2), a leptin and insulin signaling inhibitor.
My first aim was to establish the degree of involvement PTPN2 had in the onset of leptin resistance in mice, by generating forebrain-wide Ptpn2 knockouts (KO) and challenging their metabolic function on a high calorie diet (HCD). HCD-fed male mice showed significant weight gain, fat accumulation, peri-renal brown fat reduction, and insulin resistance after 2 months on the diet in comparison to the low calorie diet (LCD) fed group. Surprisingly, HCD-fed Ptpn2 KO males also developed a significant obesity and diabetes phenotype in spite of loss of Ptpn2, demonstrating that it does not confer an obvious protective benefit to metabolic syndromes in males. Ptpn2 KO female mice, however, trended towards a lower body weight on average than HCD fed controls. Though not significant (P = 0.1), and with some data collection still in process at the time of submission, loss of neuronal Ptpn2 appears to offer a mild increase in resistance to weight gain in females.
My second aim was to assess the degree of leptin and insulin-responsiveness of the arcuate nucleus (ARC) in the hypothalamus to an exogenous leptin and insulin challenge. Male mice were sacrificed for perfusion after 2 months on their respective diets. Immunohistochemical analysis of the mid-rostral ARC, staining for either pSTAT3 (phosphorylated STAT3) or pAKT (phosphorylated AKT) showed that the male Ptpn2 KOs had reduced leptin and insulin sensitivity, having a similar number of immunoreactive cells than HCD-fed controls. This showcases that loss of Ptpn2 is not sufficient in preventing the onset of leptin and insulin resistance in males.
My final aim was to characterise the role of PTPN2 in the onset of diet-induced infertility. This data however is only preliminary due to low sample numbers at the time of submission. After 3 months on the diet, estrous cyclicity of female mice was determined with vaginal cytology over a 14 day period. HCD KOs showed a promising rescue from diet-induced infertility, averaging 6 days in metestrus/diestrus (M/D) compared to the HCD controls which were almost completely arrested at the M/D phase, averaging 10.6 days. Estrous cycle length was also improved in HCD KOs, averaging 4-day cycles in comparison to the average 7-day cycles of the HCD control.
To conclude, there is preliminary evidence that Ptpn2 is sexually dimorphic in mice, as forebrain-wide deletion resulted in an almost significant increased resistance to weight gain for females, but not for males. Metabolic and fertility data from females are still preliminary due to difficulties with breeding and time constraints. In the future, I would like to include a fecundity study, as well as assessing glucose tolerance, physiological response of the brain to leptin and insulin, and fat pad dissection for the females as had been done for the males to draw a complete comparison between both sexes. I would also like to gain anatomical evidence of neuronal Ptpn2 deletion using in situ hybridisation.