β Cell Stress and Endocrine Function During T1D: What Is Next to Discover?

Canonically, type 1 diabetes (T1D) is a disease characterized by autoreactive T cells as perpetrators of endocrine dysfunction and β cell death in the spiral toward loss of β cell mass, hyperglycemia, and insulin dependence. β Cells have mostly been considered as bystanders in a flurry of autoimmune processes. More recently, our framework for understanding and investigating T1D has evolved. It appears increasingly likely that intracellular β cell stress is an important component of T1D etiology/pathology that perpetuates autoimmunity during the progression to T1D. Here we discuss the emerging and complex role of β cell stress in initiating, provoking, and catalyzing T1D. We outline the bridges between hyperglycemia, endoplasmic reticulum stress, oxidative stress, and autoimmunity from the viewpoint of intrinsic β cell (dys)function, and we extend this discussion to the potential role for a therapeutic β cell stress-metabolism axis in T1D. Lastly, we mention research angles that may be pursued to improve β cell endocrine function during T1D. Biology gleaned from studying T1D will certainly overlap to innovate therapeutic strategies for T2D, and also enhance the pursuit of creating optimized stem cell-derived β cells as endocrine therapy.

Inflammation targets CDK11 in autoimmune diabetes as a protective mechanism against beta cell apoptosis

Autoimmune diabetes (T1D) is caused by the immune-mediated destruction of insulin-producing beta cells in the pancreas leading to hyperglycemia. Apoptosis is the main mechanism responsible for beta cell demise, including Fas-receptor engagement and Perforin release as essential death executers. However, the chronic inflammatory milieu pervading pancreatic islets prior to diabetes onset, alters beta cells function and primes beta cells for apoptosis long before the debut of the disease. To identify those genes that are targeted by inflammation and are causally related to beta cell fitness and viability we have used the microarray technology. We have identified several candidate genes being downregulated by inflammation, one of this genes is Cdk11, a cyclin-dependent kinase which is involved in transcription (CDK11p130), mitosis and apoptosis (CDK11p58). L-type cyclins are the partners for Cdk11p130, and Cyclin D3 is the partner of CDK11p58. The Cdk11 gene, when is transcribed, generates a single mRNA form, that can be translated either into CDK11p130 or CDK11p58 by a differential IRES (Internal Ribosome Entry Site) usage, being CDK11p58 only expressed during mitosis, while CDK11p130is expressed during all cell cycle phases. We have addressed the role of CDK11 in autoimmune diabetes using the T1D-prone NOD mouse model hemideficient in CDK11, since the CDK11 null mutation is embryonically lethal. First of all, we have confirmed the effect of inflammation on Cdk11 mRNA expression in islet cells by submitting NODSCID mice to adoptive transfer of increasing amounts of diabetogenic NOD splenocytes, and, observed that Cdk11 expression in islet endocrine cells is downregulated by inflammation in a dose-response dependent manner.

CDK11 Promotes Cytokine-Induced Apoptosis in Pancreatic Beta Cells Independently of Glucose Concentration and Is Regulated by Inflammation in the NOD Mouse Model

Background

Pancreatic islets are exposed to strong pro-apoptotic stimuli: inflammation and hyperglycemia, during the progression of the autoimmune diabetes (T1D). We found that the Cdk11(Cyclin Dependent Kinase 11) is downregulated by inflammation in the T1D prone NOD (non-obese diabetic) mouse model. The aim of this study is to determine the role of CDK11 in the pathogenesis of T1D and to assess the hierarchical relationship between CDK11 and Cyclin D3 in beta cell viability, since Cyclin D3, a natural ligand for CDK11, promotes beta cell viability and fitness in front of glucose.

Methods

We studied T1D pathogenesis in NOD mice hemideficient for CDK11 (N-HTZ), and, in N-HTZ deficient for Cyclin D3 (K11HTZ-D3KO), in comparison to their respective controls (N-WT and K11WT-D3KO). Moreover, we exposed pancreatic islets to either pro-inflammatory cytokines in the presence of increasing glucose concentrations, or Thapsigargin, an Endoplasmic Reticulum (ER)-stress inducing agent, and assessed apoptotic events. The expression of key ER-stress markers (Chop, Atf4 and Bip) was also determined.

Results

N-HTZ mice were significantly protected against T1D, and NS-HTZ pancreatic islets exhibited an impaired sensitivity to cytokine-induced apoptosis, regardless of glucose concentration. However, thapsigargin-induced apoptosis was not altered. Furthermore, CDK11 hemideficiency did not attenuate the exacerbation of T1D caused by Cyclin D3 deficiency.

Conclusions

This study is the first to report that CDK11 is repressed in T1D as a protection mechanism against inflammation-induced apoptosis and suggests that CDK11 lies upstream Cyclin D3 signaling. We unveil the CDK11/Cyclin D3 tandem as a new potential intervention target in T1D.

Redox control of yeast Sir2 activity is involved in acetic acid resistance and longevity

Yeast Sir2 is an NAD-dependent histone deacetylase related to oxidative stress and aging. In a previous study, we showed that Sir2 is regulated by S-glutathionylation of key cysteine residues located at the catalytic domain. Mutation of these residues results in strains with increased resistance to disulfide stress. In the present study, these mutant cells were highly resistant to acetic acid and had an increased chronological life span. Mutant cells had increased acetyl-CoA synthetase activity, which converts acetic acid generated by yeast metabolism to acetyl.CoA. This could explain the acetic acid resistance and lower levels of this toxic acid in the extracellular media during aging. Increased acetyl-CoA levels would raise lipid droplets, a source of energy during aging, and fuel glyoxylate-dependent gluconeogenesis. The key enzyme of this pathway, phosphoenolpyruvate carboxykinase (Pck1), showed increased activity in these Sir2 mutant cells during aging. Sir2 activity decreased when cells shifted to the diauxic phase in the mutant strains, compared to the WT strain. Since Pck1 is inactivated through Sir2-dependent deacetylation, the decline in Sir2 activity explained the rise in Pck1 activity. As a consequence, storage of sugars such as trehalose would increase. We conclude that extended longevity observed in the mutants was a combination of increased lipid droplets and trehalose, and decreased acetic acid in the extracellular media. These results offer a deeper understanding of the redox regulation of Sir2 in acetic acid resistance, which is relevant in some food and industrial biotechnology and also in the metabolism associated to calorie restriction, aging and pathologies such as diabetes.

Molecular Targets of inflammation in pancreatic endocrine cells in Type 1 Diabetes (T1D)

T1D (Type 1 Diabetes) is an autoimmune disorder caused by the immune-mediated destruction of insulin-producing beta cells in the pancreas. There are direct mechanisms that have been demonstrated to cause final beta cell demise, such as the pathways triggered by Fas engagement and Perforin release. However, focus should be placed on other indirect mechanisms, such as pro-inflammatory cytokines present in the beta cell niche early on, that progressively debilitate and alter beta cells from earlier phases of T1D progression, rendering them as helpless targets of the immune system at the end. Though beta cells may resist adverse stimuli in early phases of the autoimmune process, the complicity of both, direct and indirect deleterious mechanisms, leads to beta cell death. In an effort to identify those indirect mechanisms that chronically besiege beta cells from the T1D-prone NOD mouse model, we used the microarray technology. We have identified several candidate genes the expression of which is altered in the NOD islet prior to T1D onset. Several of these genes are related to cell cycle progression and are downregulated due to the insulitic attack to the islet. One of these genes is cyclin D-3, a Dtype cyclin that interacts with either Cdk4 or Cdk6 to promote G1/S cycle progression. We have reported that cyclin D3 protects beta cells against cytokine-induced apoptosis and is required for proper beta cell function in a cell-cycle independent fashion. Cdk11 is also affected by inflammation in islet cells and is a cyclin-dependent kinase which is involved in transcription, mitosis and apoptosis. The natural partners of Cdk11 are L-type cyclins. We have examined whether the immune function is affected in the relative absence of Cdk11.

Reversible glutathionylation of Sir2 by monothiol glutaredoxins Grx3/4 regulates stress resistance

The regulatory mechanisms of yeast Sir2, the founding member of the sirtuin family involved in oxidative stress and aging, are unknown. Redox signaling controls many cellular functions, especially under stress situations, with dithiol glutaredoxins (Grxs) playing an important role. However, monothiol Grxs are not considered to have major oxidoreductase activity. The present study investigated the redox regulation of yeast Sir2, together with the role and physiological impact of monothiol Grx3/4 as Sir2 thiol-reductases upon stress. S-glutathionylation of Sir2 upon disulfide stress was demonstrated both in vitro and in vivo, and decreased Sir2 deacetylase activity. Physiological levels of nuclear Grx3/4 can reverse the observed post-translational modification. Grx3/4 interacted with Sir2 and reduced it after stress, thereby restoring telomeric silencing activity. Using site-directed mutagenesis, key cysteine residues at the catalytic domain of Sir2 were identified as a target of S-glutathionylation. Mutation of these residues resulted in cells with increased resistance to disulfide stress. We provide new mechanistic insights into Grx3/4 regulation of Sir2 by S-deglutathionylation to increase cell resistance to stress. This finding offers news perspectives on monothiol Grxs in redox signaling, describing Sir2 as a physiological substrate regulated by S-glutathionylation. These results might have a relevant role in understanding aging and age-related diseases.

Impaired PLP-dependent metabolism in brain samples from Huntington disease patients and transgenic R6/1 mice

Oxidative stress has been described as important to Huntington disease (HD) progression. In a previous HD study, we identified several carbonylated proteins, including pyridoxal kinase and antiquitin, both of which are involved in the metabolism of pyridoxal 5´-phosphate (PLP), the active form of vitamin B6. In the present study, pyridoxal kinase levels were quantified and showed to be decreased both in HD patients and a R6/1 mouse model, compared to control samples. A metabolomic analysis was used to analyze metabolites in brain samples of HD patients and R6/1 mice, compared to control samples using mass spectrometry. This technique allowed detection of increased concentrations of pyridoxal, the substrate of pyridoxal kinase. In addition, PLP, the product of the reaction, was decreased in striatum from R6/1 mice. Furthermore, glutamate and cystathionine, both substrates of PLP-dependent enzymes were increased in HD. This reinforces the hypothesis that PLP synthesis is impaired, and could explain some alterations observed in the disease. Together, these results identify PLP as a potential therapeutic agent.