AbstractsBiology & Animal Science

Stabilisation of β-catenin in hypothalamic neurons through feeding related stimulation and the role this may have on synaptic function in the control of metabolic homeostasis.

by Hayden John Leonard McEwen

Institution: University of Otago
Year: 0
Keywords: Beta-catenin; metabolism; diabetes; tcf7l2; neuroendocrinology; schizophrenia; clozapine
Record ID: 1313538
Full text PDF: http://hdl.handle.net/10523/5556


Polymorphisms in the gene T-cell factor 7-like 2 (TCF7L2) carry one the biggest genetic linkage to the development of type 2 diabetes. However the exact mechanism through which this occurs is still under debate, although these polymorphisms are suspected to be somehow due to increases in TCF7L2 gene expression. The TCF7L2 transcriptional co-factor, β-catenin, is regulated by several metabolic hormones in peripheral tissues and has been shown to be important for the formation and function of neuron synapses. Overexpression of TCF7L2 causes the translocation of β-catenin out of the cytoplasm and into the nucleus, potentially affecting its functions. It is possible that modulated TCF7L2 expression is affecting β-catenin function at the synapse in hypothalamic neurons and contributing to the metabolic dysregulation in individuals with the TCF7L2 polymorphisms. We aim to determine if various feeding related stimuli are able to increase β-catenin stabilisation in hypothalamic nuclei that control glucose homeostasis and if this stabilisation of β-catenin is affecting neuron function. The stabilisation of β-catenin in fasted versus fed rats was assessed in the hypothalamus through immunohistochemistry. The paraventricular (PVN), dorsomedial (DMH) and arcuate (Arc) hypothalamic neuclei all showed increases in the number of cells stained positive for β-catenin. These regions also showed increased expression of the β-catenin responsive gene markers of Axin2, CyclinD1, Cyclin D2 and c-myc. To investigate which feeding related stimuli was inducing the changes observed after feeding the hypothalamic neuronal model of GT1-7 cells was used. GT1-7 cells were stimulated with leptin, glucose, insulin, the glucagon-like peptide-1 (GLP-1) analogue Exendin-4 and alpha-melanocyte stimulating hormone (alpha-MSH). Glucose, insulin, Exendin-4 and alpha-MSH all had the capacity to stabilise β-catenin through increasing phosphorylation at the serine 552 and 675 residues on β-catenin. In the case of Exendin-4, this was shown to be a PKA dependent effect. Glucose and Exendin-4 were administered to rats in an attempt to replicate the observations made in cell culture. Despite glucose not showing an effect on the stablisation of β-catenin, Exendin-4 increased the number of positively stained cell for β-catenin in the PVN, DMH and Arc using immunohistochemistry. This was further supported by elevations in Axin2, CyclinD1, cyclinD2 and c-myc transcripts. Finally, β-catenin expression was suppressed using siRNA transfection, or stabilised via inhibition of GSK-3β in GT1-7 cells prior to stimulation with Exendin-4. Decreased β-catenin expression reduced the secretary capacity of the cells when stimulated with Exendin-4, while increasing β-catenin stability had the opposite effect. This indicated that β-catenin was important for neurosecretion. Additionally, there was also a further study that sought to determine if the phosphorylation of Akt in the brain observed after administration of the atypical antipsychotic medication, clozapine, is…