Abstract
The placenta is an important endocrine organ, secreting various placental-derived mediators which influence the maternal milieu to support the demands of pregnancy. A physiological metabolic consequence of pregnancy is a progressive increase in maternal insulin resistance midlate gestation, thus facilitating a constant supply of glucose to the developing foetus. The insulinsecreting β-cells in the pancreatic islets of Langerhans undergo adaptations by enhancing insulin secretory responses to elevations in plasma glucose and increasing β-cell mass to compensate the insulin resistance. In some pregnancies, these adaptations fail and chronic hyperglycaemia and overt gestational diabetes mellitus (GDM) ensues. The corticotropin releasing hormone (CRH) system (including structurally related peptides, urocortin (UCN)1, -2 and -3 and the two cognate receptors, CRHR1 and CRHR2) is well known as the neuroendocrine system regulating the stress response as part of the hypothalamic-pituitary-adrenal (HPA) axis. However, extra-hypothalamic roles for this system in peripheral tissues have been reported, including within the endocrine pancreas. Increasing evidence also suggests that the CRH peptide family may be involved in peripheral metabolic control via direct actions on β-cells. Generally, circulating levels of CRH is low. However, we have previously reported that during mouse pregnancy, expression of CRHlike peptides increase in the placenta suggesting that these peptides may have pregnancy specific roles. Therefore, the aim of this project was to investigate whether there is a physiological role for the CRH system in the pancreatic islet adaptations to pregnancy.Functional in vitro studies using isolated mouse islets and either static incubation or dynamic perifusion methodologies confirmed that activation of both CRHRs, using selective receptor agonists could enhance insulin release in response to elevations in glucose. Expression of CRH and all urocortin peptide mRNAs were confirmed in the mouse placenta along with both types of CRHR in mouse islets. Intriguingly, pregnant islets displayed a significant decrease in CRHR1 expression and maintained CRHR2 expression levels when compared to non-pregnant control islets. Moreover, mouse pregnancy is associated with a significant increase in circulating UCN2 levels with CRH, UCN1 and UCN3 levels unchanged.
Pharmacologically blocking endogenous CRHR signalling using selective and non-selective CRHR antagonists chronically administered via subcutaneously implanted osmotic minipumps in vivo during pregnancy, revealed a pregnancy–and receptor-specific phenotype in mice. Intraperitoneal glucose tolerance tests revealed a significant, albeit mild and transient, impairment to glucose tolerance with selective CRHR2 blockade but not with CRHR1 blockade which was associated with a decrease in glucose-stimulated plasma insulin levels, but not basal fasted insulin. No detectable effects on insulin sensitivity or the proliferative capacity of β-cells were observed.
Furthermore, no impairment to overall glucose homeostasis was observed with total (i.e. nonselective) CRHR blockade outside of pregnancy.
Modelling the beneficial effects of endogenous UCN2 during pregnancy by exogenously administering the peptide in an alternative animal model of impaired glucose homeostasis (i.e. the insulin resistant obese ob/ob mouse) highlighted the variable effect of UCN2 on glucose homeostasis between acute (i.p.) and chronic (osmotic minipump) administration. Overall, studies mimicking physiological pregnant levels of UCN2 revealed no significant alterations to glucose homeostasis indicating that the positive effects of UCN2 on β-cell function is confined to pregnancy and may suggest something unique about the pregnancy environment, perhaps synergy with other pregnancy mediators, is important for its functional benefit.
Preliminary clinical studies in pregnant women revealed a positive association between plasma UCN2 and insulin responses to oral glucose along with HOMA2-%b (index of b-cell function), consistent with the theory that higher levels of maternal UCN2 can directly enhance islet b-cell function and suggests a conserved mechanism between mouse and human pregnancy. However, no significant differences in plasma levels of UCN2, UCN3 or CRH between healthy women and women diagnosed with GDM were observed, in line with the milder phenotype displayed in vivo in mice with endogenous UCN2 blockade during pregnancy.
In summary, the studies presented in this thesis suggest that an endogenous ligand, most likely UCN2, signalling via CRHR2, contributes to maintaining maternal normoglycaemia during pregnancy. The current data suggests that UCN2 may support β-cell adaptations by amplifying the insulin secretory response to the metabolic demand. Data from the clinical cohort appear to support the pregnancy-specificity of this signal which may be conserved in mouse and human gestation. Therefore, UCN2/CRHR2 signalling represents a novel signal involved in the pancreatic islet adaptations to pregnancy.
| Date of Award | 1 Apr 2021 |
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| Original language | English |
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| Supervisor | James Bowe (Supervisor) & Peter Jones (Supervisor) |