Growth hormone is required for normal linear growth during childhood and the maintenance of body composition throughout life. Adult-onset growth hormone deficiency (GHD) is associated with increased adiposity, decreased muscle mass, reduced energy, and decreased cognitive function. Leptin receptor (LEPR) signaling controls appetite and energy expenditure, and leptin also regulates somatotropes. The goal of this project was to determine the role of leptin receptors in somatotrope function by deleting leptin receptors specifically in somatotropes. CreLoxP technology was used to selectively ablate Lepr exon 1 in somatotropes, which removes the signal peptide, causing the loss of all isoforms of LEPR. Using GH assays and immunolabeling, adult deletion mutants had decreased serum GH and reduced GH stores. Metabolic cages, MRI imaging, and other hormone assays indicated that loss of Lepr leads to metabolic dysfunction and reductions in TSH and prolactin in females. Developmental studies revealed that excision of Lepr happens as early as PND 1, and GH stores are reduced at PND 5 and throughout life. Despite reduced GH stores, the developing mice secreted normal levels of GH until day 21 (in the male). They also grew normally in length, however young male mice weighed less than controls. This is correlated with lower lean body mass. In vitro tests showed that GH secretion from somatotrope specific Lepr exon 1 deletion mutant cells was low, and it could not be restored by GHRH with or without ghrelin. Detection of GHRH receptors with biotinylated GHRH showed significantly reduced GHRH binding sites in mutants, which correlates well with the lack of responses to GHRH. Ghrelin did not restore biotinylated GHRH binding sites. Tests of inhibitors of transcription (actinomycin D) and the JAK/STAT3 pathway indicate that leptin does require transcription and phosphorylated STAT3 for GH expression. This correlates with lower GH mRNA levels in mutant females, but not males. This study indicates that the role of leptin in somatotropes is complex, and the exact pathway behind leptin’s regulation of somatotropes may vary with the sex. Leptin may act at the level of transcription for males and females and also potentially post-transcriptionally in males.
|Advisor:||Childs, Gwen V.|
|Commitee:||Benes, Helen, Chang, Jason, Kineman, Rhonda, MacNicol, Melanie|
|School:||University of Arkansas for Medical Sciences|
|Department:||Neurobiology and Developmental Science|
|School Location:||United States -- Arkansas|
|Source:||DAI-B 76/09(E), Dissertation Abstracts International|
|Keywords:||Crelox, Growth hormone, Leptin receptor, Metabolic dysfunction|
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