Kisspeptin upregulates β-cell serotonin production during pregnancy

During pregnancy the maternal pancreatic islets of Langerhans undergo adaptive changes to compensate for gestational insulin resistance. The lactogenic hormones are well established to play a key role in regulating the islet adaptation to pregnancy, and one of the mechanisms through which they act is through upregulating β-cell serotonin production. During pregnancy islet serotonin levels are significantly elevated, where it is released from the β-cells to drive the adaptive response through paracrine and autocrine effects. We have previously shown that placental kisspeptin (KP) also plays a role in promoting the elevated insulin secretion and β-cell proliferation observed during pregnancy, although the precise mechanisms involved are unclear. In the present study we investigated the effects of KP on expression of pro-proliferative genes and serotonin biosynthesis within rodent islets. Whilst KP had limited effect on pro-proliferative gene expression at the time points tested, KP did significantly stimulate expression of the serotonin biosynthesis enzyme Tph-1. Furthermore, the islets of pregnant β-cell-specific GPR54 knockdown mice were found to contain significantly fewer serotonin-positive β-cells when compared to pregnant controls. Our previous studies suggested that reduced placental kisspeptin production, with consequent impaired kisspeptin-dependent β-cell compensation, may be a factor in the development of GDM in humans. These current data suggest that, similar to the lactogenic hormones, KP may also contribute to serotonin biosynthesis and subsequent islet signalling during pregnancy. Furthermore, upregulation of serotonin biosynthesis may represent a common mechanism through which multiple signals might influence the islet adaptation to pregnancy.

The use of mice in diabetes research: The impact of physiological characteristics, choice of model and husbandry practices

Diabetes mellitus is characterised by hyperglycaemia, which results from an absolute or relative lack of insulin. Chronic and acute hyperglycaemia are associated with a range of health complications and an overall increased risk of mortality. Mouse models are vital in understanding the pathogenesis of this disease and its complications, as well as for developing new diabetes therapeutics. However, for experimental questions to be suitably tested, it is critical that factors inherent to the animal model are considered, as these can have profound impacts on experimental outcome, data reproducibility and robustness. In this review, we discuss key considerations relating to model choice, physiological characteristics (such as age, sex and genetic background) and husbandry practices and explore the impact of these on common experimental readouts used in preclinical diabetes research.

Activation and expression of endogenous CREB-regulated transcription coactivators (CRTC) 1, 2 and 3 in the rat adrenal gland

The activation and nuclear translocation of cAMP-response element binding protein (CREB)-regulated transcription coactivator (CRTC)2 occurs in the rat adrenal gland, in response to adrenocorticotrophic hormone (ACTH) and stressors, and has been implicated in the transcriptional regulation of steroidogenic acute regulatory protein (StAR). We have recently demonstrated the activation of CRTC isoforms, CRTC1 and CRTC3, in adrenocortical cell lines. In the present study, we aimed to determine the activation and expression of the three CRTC isoforms in vivo in relation to Star transcription, under basal conditions and following a robust endotoxic stress challenge. Rat adrenal glands and blood plasma were collected following i.v. administration of either an ultradian-sized pulse of ACTH or administration of lipopolysaccharide, as well as under unstressed conditions across the 24-hour period. Plasma ACTH and corticosterone (CORT) were measured and the adrenal glands were processed for measurement of protein by western immunoblotting, RNA by a quantitative reverse transcriptase-polymerase chain reaction and association of CRTC2 and CRTC3 with the Star promoter by chromatin immunoprecipitation. An increase in nuclear localisation of CRTC2 and CRTC3 followed increases in both ultradian and endotoxic stress-induced plasma ACTH, and this was associated with increased CREB phosphorylation and corresponding increases in Star transcription. Both CRTC2 and CRTC3 were shown to associate with the Star promoter, with the dynamics of CRTC3 binding corresponding to that of nuclear changes in protein levels. CRTC isoforms show little variation in ultradian expression or variation across 24 hours, although evidence of long-term down-regulation following endotoxic stress was found. We conclude that co-transcription factors CRTC2 and, more clearly, CRTC3 appear to act alongside phosphorylated CREB in the generation of ultradian pulses of Star transcription, essential for the maintenance of basal StAR expression. Similarly, our findings suggest CRTC2 and CRTC3 mediate Star transcriptional initiation following an endotoxic stressor; however, other transcription factors are likely to be responsible for the long-term up-regulation of adrenal Star transcription.

UCN2: a new candidate influencing pancreatic β-cell adaptations in pregnancy

The corticotropin-releasing hormone (CRH) family of peptides, including urocortin (UCN) 1, 2 and 3, are established hypothalamic neuroendocrine peptides, regulating the physiological and behaviour responses to stress indirectly, via the hypothalamic-pituitary-adrenal (HPA) axis. More recently, these peptides have been implicated in diverse roles in peripheral organs through direct signalling, including in placental and pancreatic islet physiology. CRH has been shown to stimulate insulin release through activation of its cognate receptors, CRH receptor 1 (CRHR1) and 2. However, the physiological significance of this is unknown. We have previously reported that during mouse pregnancy, expression of CRH peptides increase in mouse placenta suggesting that these peptides may play a role in various biological functions associated with pregnancy, particularly the pancreatic islet adaptations that occur in the pregnant state to compensate for the physiological increase in maternal insulin resistance. In the current study, we show that mouse pregnancy is associated with increased circulating levels of UCN2 and that when we pharmacologically block endogenous CRHR signalling in pregnant mice, impairment of glucose tolerance is observed. This effect on glucose tolerance was comparable to that displayed with specific CRHR2 blockade and not with specific CRHR1 blockade. No effects on insulin sensitivity or the proliferative capacity of β-cells were detected. Thus, CRHR2 signalling appears to be involved in β-cell adaptive responses to pregnancy in the mouse, with endogenous placental UCN2 being the likely signal mediating this.

Involvement of CREB-regulated transcription coactivators (CRTC) in transcriptional activation of steroidogenic acute regulatory protein (Star) by ACTH

Studies in vivo have suggested the involvement of CREB-regulated transcription coactivator (CRTC)2 on ACTH-induced transcription of the key steroidogenic protein, Steroidogenic Acute Regulatory (StAR). The present study uses two ACTH-responsive adrenocortical cell lines, to examine the role of CRTC on Star transcription. Here we show that ACTH-induced Star primary transcript, or heteronuclear RNA (hnRNA), parallels rapid increases in nuclear levels of the 3 isoforms of CRTC; CRTC1, CRTC2 and CRTC3. Furthermore, ACTH promotes recruitment of CRTC2 and CRTC3 by the Star promoter and siRNA knockdown of either CRTC3 or CRTC2 attenuates the increases in ACTH-induced Star hnRNA. Using pharmacological inhibitors of PKA, MAP kinase and calcineurin, we show that the effects of ACTH on Star transcription and CRTC nuclear translocation depend predominantly on the PKA pathway. The data provides evidence that CRTC2 and CRTC3, contribute to activation of Star transcription by ACTH, and that PKA/CRTC-dependent pathways are part of the multifactorial mechanisms regulating Star transcription.

Generating Beta-Cell-Specific Transgenic Mice Using the Cre-Lox System

Beta-cell-specific transgenic mice provide an invaluable model for dissecting the direct signaling mechanisms involved in regulating beta-cell structure and function. Furthermore, generating novel transgenic models is now easier and more cost-effective than ever, thanks to exciting novel approaches such as CRISPR.Here, we describe the commonly used approaches for generating and maintaining beta-cell-specific transgenic models and some of the considerations involved in their use. This includes the use of different beta-cell-specific promoters (e.g., pancreatic and duodenal homeobox factor 1 (Pdx1), rat insulin 2 promoter (RIP), and mouse insulin 1 promoter (MIP)) to drive site-specific recombinase technology. Important considerations during selection include level and uniformity of expression in the beta-cell population, ectopic transgene expression, and the use of inducible models.This chapter provides a guide to the procurement, generation, and maintenance of a beta-cell-specific transgene colony from preexisting Cre and loxP mouse strains, providing methods for crossbreeding and genotyping, as well as subsequent maintenance and, in the case of inducible models, transgenic induction.