A novel cardioprotective agent in cardiac transplantation: metformin activation of AMP-activated protein kinase decreases acute ischemia-reperfusion injury and chronic rejection.

The main cause of mortality after the first year from cardiac transplantation is cardiac allograft vasculopathy (CAV), which leads to chronic rejection of the heart. To improve long-term outcomes in cardiac transplantation, treatments to prevent or diminish CAV are actively being researched. Ischemia-reperfusion (I-R) injury has been shown to be the strongest alloantigen-independent factor in the development of CAV. Here, we investigate the use of metformin in murine cardiac transplantation models as a novel cardioprotective agent to limit acute I-R injury and subsequent chronic rejection. We show that metformin treatment activates AMP-activated kinase (AMPK) in vitro and in vivo. In the acute transplantation model, metformin activation of AMPK resulted in significantly decreased apoptosis in cardiac allografts on postoperative day (POD) 1 and 8. In the chronic transplantation model, metformin pretreatment of allografts led to significantly improved graft function and significantly decreased CAV, as measured on POD 52. Taken together, our results in the acute and chronic rejection studies suggest a potential cardioprotective mechanism for metformin; we demonstrate a correlation between metformin-induced decrease in acute I-R injury and metformin-related decrease in chronic rejection. Thus, one of the ways by which metformin and AMPK activation may protect the transplanted heart from chronic rejection is by decreasing initial I-R injury inherent in donor organ preservation and implantation. Our findings suggest novel therapeutic strategies for minimizing chronic cardiac rejection via the use of metformin- and AMPK-mediated pathways to suppress acute I-R injury.

Mammalian polynucleotide phosphorylase is an intermembrane space RNase that maintains mitochondrial homeostasis.

We recently identified polynucleotide phosphorylase (PNPase) as a potential binding partner for the TCL1 oncoprotein. Mammalian PNPase exhibits exoribonuclease and poly(A) polymerase activities, and PNPase overexpression inhibits cell growth, induces apoptosis, and stimulates proinflammatory cytokine production. A physiologic connection for these anticancer effects and overexpression is difficult to reconcile with the presumed mitochondrial matrix localization for endogenous PNPase, prompting this study. Here we show that basal and interferon-beta-induced PNPase was efficiently imported into energized mitochondria with coupled processing of the N-terminal targeting sequence. Once imported, PNPase localized to the intermembrane space (IMS) as a peripheral membrane protein in a multimeric complex. Apoptotic stimuli caused PNPase mobilization following cytochrome c release, which supported an IMS localization and provided a potential route for interactions with cytosolic TCL1. Consistent with its IMS localization, PNPase knockdown with RNA interference did not affect mitochondrial RNA levels. However, PNPase reduction impaired mitochondrial electrochemical membrane potential, decreased respiratory chain activity, and was correlated with altered mitochondrial morphology. This resulted in FoF1-ATP synthase instability, impaired ATP generation, lactate accumulation, and AMP kinase phosphorylation with reduced cell proliferation. Combined, the data demonstrate an unexpected IMS localization and a key role for PNPase in maintaining mitochondrial homeostasis.

Decreased hepatic futile cycling compensates for increased glucose disposal in the Pten heterodeficient mouse.

Despite altered regulation of insulin signaling, Pten(+/-) heterodeficient standard diet-fed mice, approximately 4 months old, exhibit normal fasting glucose and insulin levels. We report here a stable isotope flux phenotyping study of this "silent" phenotype, in which tissue-specific insulin effects in whole-body Pten(+/-)-deficient mice were dissected in vivo. Flux phenotyping showed gain of function in Pten(+/-) mice, seen as increased peripheral glucose disposal, and compensation by a metabolic feedback mechanism that 1) decreases hepatic glucose recycling via suppression of glucokinase expression in the basal state to preserve hepatic glucose production and 2) increases hepatic responsiveness in the fasted-to-fed transition. In Pten(+/-) mice, hepatic gene expression of glucokinase was 10-fold less than wild-type (Pten(+/+)) mice in the fasted state and reached Pten(+/+) values in the fed state. Glucose-6-phosphatase expression was the same for Pten(+/-) and Pten(+/+) mice in the fasted state, and its expression for Pten(+/-) was 25% of Pten(+/+) in the fed state. This study demonstrates how intra- and interorgan flux compensations can preserve glucose homeostasis (despite a specific gene defect that accelerates glucose disposal) and how flux phenotyping can dissect these tissue-specific flux compensations in mice presenting with a "silent" phenotype.

The TCL1 oncoprotein binds the RNase PH domains of the PNPase exoribonuclease without affecting its RNA degrading activity.

TCL1 is an AKT kinase coactivator that, when dysregulated, initiates mature lymphocyte malignancies in humans and transgenic mice. While TCL1 augments AKT pathway signaling, additional TCL1 interacting proteins that may contribute to cellular homeostasis or transformation are lacking. Here, an exoribonuclease, PNPase, was identified in a complex with TCL1. The AKT interaction domain on TCL1 bound either RNase PH repeat domain of PNPase without influencing its RNA degrading activity, which was compatible with predicted docking models for a TCL1-PNPase complex. Our data provide a novel protein interaction for mammalian PNPase that may impact TCL1 mediated transformation.

Loss of Ubr2, an E3 ubiquitin ligase, leads to chromosome fragility and impaired homologous recombinational repair.

The N-end rule pathway of protein degradation targets proteins with destabilizing N-terminal residues. Ubr2 is one of the E3 ubiquitin ligases of the mouse N-end rule pathway. We have previously shown that Ubr2-/- male mice are infertile, owing to the arrest of spermatocytes between the leptotene/zygotene and pachytene of meiosis I, the failure of chromosome pairing, and subsequent apoptosis. Here, we report that mouse fibroblast cells derived from Ubr2-/- embryos display genome instability. The frequency of chromosomal bridges and micronuclei were much higher in Ubr2-/- fibroblasts than in +/+ controls. Metaphase chromosome spreads from Ubr2-/- cells revealed a high incidence of spontaneous chromosomal gaps, indicating chromosomal fragility. These fragile sites were generally replicated late in S phase. Ubr2-/- cells were hypersensitive to mitomycin C, a DNA cross-linking agent, but displayed normal sensitivity to gamma-irradiation. A reporter assay showed that Ubr2-/- cells are significantly impaired in the homologous recombination repair of a double strand break. In contrast, Ubr2-/- cells appeared normal in an assay for non-homologous end joining. Our results therefore unveil the role of the ubiquitin ligase Ubr2 in maintaining genome integrity and in homologous recombination repair.

Live cell interferometry reveals cellular dynamism during force propagation.

Cancer and many other diseases are characterized by changes in cell morphology, motion, and mechanical rigidity. However, in live cell cytology, stimulus-induced morphologic changes typically take 10-30 min to detect. Here, we employ live-cell interferometry (LCI) to visualize the rapid response of a whole cell to mechanical stimulation, on a time scale of seconds, and we detect cytoskeletal remodeling behavior within 200 s. This behavior involved small, rapid changes in cell content and miniscule changes in shape; it would be difficult to detect with conventional or phase contrast microscopy alone and is beyond the dynamic capability of AFM. We demonstrate that LCI provides a rapid, quantitative reconstruction of the cell body with no labeling. This is an advantage over traditional microscopy and flow cytometry, which require cell surface tagging and/or destructive cell fixation for labeling.

High throughput cell nanomechanics with mechanical imaging interferometry.

The dynamic nanomechanical properties of a large number of cells (up to hundreds), measured in parallel with high throughput, are reported. Using NIH 3T3 and HEK 293T fibroblasts and actin depolymerizing drugs, we use a novel nanotechnology to quantify the local viscoelastic properties with applied forces of 20 pN-20 nN, a spatial resolution of <20 nm, and a mechanical dynamic range of several Pa up to ~200 kPa. Our approach utilizes imaging interferometry in combination with reflective, magnetic probes attached to cells. These results indicate that mechanical imaging interferometry is a sensitive and scalable technology for measuring the nanomechanical properties of large arrays of live cells in fluid.