Probes for INSULIN

Thioredoxin-interacting protein (Txnip) has emerged as a key factor in pancreatic beta cell biology and its upregulation by glucose and diabetes contributes to the impairment in functional beta cell mass and glucose homeostasis. In addition, beta cell deletion of Txnip protects against diabetes in different mouse models. However, while Txnip is ubiquitously expressed, its role in pancreatic alpha cells has remained elusive. We therefore now generated an alpha cell Txnip knockout (aTKO) mouse and assessed the effects on glucose homeostasis. While no significant changes were observed on regular chow, after a 30-week high-fat diet, aTKO animals showed improvement in glucose tolerance and lower blood glucose levels compared to their control littermates. Moreover, in the context of streptozotocin (STZ)-induced diabetes, aTKO mice showed significantly lower blood glucose levels compared to controls. While serum insulin levels were reduced in both control and aTKO mice, STZ-diabetes significantly increased glucagon levels in control mice, but this effect was blunted in aTKO mice. Moreover, glucagon secretion from aTKO islets was >2-fold lower than from control islets, while insulin secretion was unchanged in aTKO islets. At the same time, no change in alpha cell or beta cell numbers or mass was observed and glucagon and insulin expression and content were comparable in isolated islets from aTKO and control mice. Thus, together the current studies suggest that downregulation of alpha cell Txnip is associated with reduced glucagon secretion and that this may contribute to the glucose-lowering effects observed in diabetic aTKO mice.

Systemic insulin sensitivity shows a diurnal rhythm with a peak upon waking1,2. The molecular mechanism that underlies this temporal pattern is unclear. Here we show that the nuclear receptors REV-ERB-α and REV-ERB-β (referred to here as 'REV-ERB') in the GABAergic (γ-aminobutyric acid-producing) neurons in the suprachiasmatic nucleus (SCN) (SCNGABA neurons) control the diurnal rhythm of insulin-mediated suppression of hepatic glucose production in mice, without affecting diurnal eating or locomotor behaviours during regular light-dark cycles. REV-ERB regulates the rhythmic expression of genes that are involved in neurotransmission in the SCN, and modulates the oscillatory firing activity of SCNGABA neurons. Chemogenetic stimulation of SCNGABA neurons at waking leads to glucose intolerance, whereas restoration of the temporal pattern of either SCNGABA neuron firing or REV-ERB expression rescues the time-dependent glucose metabolic phenotype caused by REV-ERB depletion. In individuals with diabetes, an increased level of blood glucose after waking is a defining feature of the 'extended dawn phenomenon'3,4. Patients with type 2 diabetes with the extended dawn phenomenon exhibit a differential temporal pattern of expression of REV-ERB genes compared to patients with type 2 diabetes who do not have the extended dawn phenomenon. These findings provide mechanistic insights into how the central circadian clock regulates the diurnal rhythm of hepatic insulin sensitivity, with implications for our understanding of the extended dawn phenomenon in type 2 diabetes.

The nucleus tractus solitarii (NTS) is the primary central station that integrates visceral afferent information and regulates respiratory, gastrointestinal, cardiovascular, and other physiological functions. Leptin receptor b (LepRb)-expressing neurons of the NTS (NTSLepRb neurons) are implicated in central respiration regulation, respiratory facilitation, and respiratory drive enhancement. Furthermore, LepRb dysfunction is involved in obesity, insulin resistance, and sleep-disordered breathing. However, the monosynaptic inputs and outputs of NTSLepRb neurons in whole-brain mapping remain to be elucidated. Therefore, the exploration of its whole-brain connection system may provide strong support for comprehensively understanding the physiological and pathological functions of NTSLepRb neurons. In the present study, we used a cell type-specific, modified rabies virus and adeno-associated virus with the Cre-loxp system to map monosynaptic inputs and outputs of NTSLepRb neurons in LepRb-Cre mice. The results showed that NTSLepRb neurons received inputs from 48 nuclei in the whole brain from five brain regions, including especially the medulla. We found that NTSLepRb neurons received inputs from nuclei associated with respiration, such as the pre-Bötzinger complex, ambiguus nucleus, and parabrachial nucleus. Interestingly, some brain areas related to cardiovascular regulation-i.e., the ventrolateral periaqueductal gray and locus coeruleus-also sent a small number of inputs to NTSLepRb neurons. In addition, anterograde tracing results demonstrated that NTSLepRb neurons sent efferent projections to 15 nuclei, including the dorsomedial hypothalamic nucleus and arcuate hypothalamic nucleus, which are involved in regulation of energy metabolism and feeding behaviors. Quantitative statistical analysis revealed that the inputs of the whole brain to NTSLepRb neurons were significantly greater than the outputs. Our study comprehensively revealed neuronal connections of NTSLepRb neurons in the whole brain and provided a neuroanatomical basis for further research on physiological and pathological functions of NTSLepRb neurons.

The majority of mammalian genome is composed of non-coding regions, where numerous long non-coding RNAs (lncRNAs) are transcribed. Although lncRNAs have been identified to regulate fundamental biological processes, most of their functions remain unknown, especially in metabolic homeostasis. Analysis of our recent genome-wide screen reveals that Gm15441, a thioredoxin-interacting protein (Txnip) antisense lncRNA, is the most robustly induced lncRNA in the fasting mouse liver. Antisense lncRNAs are known to regulate their sense gene expression. Given that Txnip is a critical metabolic regulator of the liver, we aimed to investigate the role of Gm15441 in the regulation of Txnip and liver metabolism.We examined the response of Gm15441 and Txnip under in vivo metabolic signals such as fasting and refeeding, and in vitro signals such as insulin and key metabolic transcription factors. We investigated the regulation of Txnip expression by Gm15441 and the underlying mechanism in mouse hepatocytes. Using adenovirus-mediated liver-specific overexpression, we determined whether Gm15441 regulates Txnip in the mouse liver and modulates key aspects of liver metabolism.We found that the expression levels of Gm15441 and Txnip showed a similar response pattern to metabolic signals in vivo and in vitro, but that their functions were predicted to be opposite. Furthermore, we found that Gm15441 robustly reduced Txnip protein expression in vitro through sequence-specific regulation and translational inhibition. Lastly, we confirmed the Txnip inhibition by Gm15441 in vivo (mice) and found that Gm15441 liver-specific overexpression lowered plasma triglyceride and blood glucose levels and elevated plasma ketone body levels.Our data demonstrate that Gm15441 is a potent Txnip inhibitor and a critical metabolic regulator in the liver. This study reveals the therapeutic potential of Gm15441 in treating metabolic diseases.